Handbook of Magneto-Optical Data Recording Materials Subsystems Techniques

Materials, Subsystems, Techniques Edited by Terry W. McDaniel IBM Storage Systems Division San Jose, California Rand...

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Materials, Subsystems, Techniques

Edited by

Terry W. McDaniel IBM Storage Systems Division San Jose, California

Randall H. Victora Eastman Kodak Company Rochester, New York

lnpl

NOYES PUBLICATIONS Westwood, New Jersey, U.S.A.

Copyright 8 1997 by Noyes Publications No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission in writing from the Publisher. Library of Congress Catalog Card Number: 95-30333 ISBN: 0-8155-1391-7 Printed in the United States Published in the United States of America by Noyes Publications 369 Fairview Avenue, Westwood, New Jersey 07675 10 9 8 7 6 5 4 3 2 1

Library of Congress Cataloging-in-Publication Data Handbook of magneto-optical data recording : materials, subsystems, techniques / edited by Terry W. McDaniel and Randall Victora. p. cm. Includes bibliographical references and index. ISBN 0-8155-1391-7 1. Computer storage devices. 2. Magnetooptical devices. 3. Data disk drives. 4. CD-ROMs. I. McDaniel, Terry W. 11. Victora, Randall. TK7895.M4H346 1996 62 1.3972-dc20 95-30333 c1P

MATERIALS SCIENCE AND PROCESS TECHNOLOGY SERIES

Series Editors Rointan F. Bunshah, University of California, Los Angeles Gary E. McGuire, Microelectronics Center of North Carolina Stephen M. Rossnagel, IBM Thomas J. Watson Research Center

Electronic Materials and Process Technology HANDBOOK OF DEPOSITION TECHNOLOGIES FOR FILMS AND COATINGS, Second Edition: edited by Rointan F. Bunshah CHEMICAL VAPOR DEPOSITION FOR MICROELECTRONICS: by Arthur Sherman SEMICONDUCTOR MATERIALS AND PROCESS TECHNOLOGY HANDBOOK: edited by Gary E. McGuire HYBRID MICROCIRCUIT TECHNOLOGY HANDBOOK: by James J. Licari and Leonard R. Enlow HANDBOOK OF THIN FILM DEPOSITION PROCESSES AND TECHNIQUES: edited by Klaus K. Schuegraf IONIZED-CLUSTER BEAM DEPOSITION AND EPITAXY: by Toshinori Takagi DIFFUSION PHENOMENA IN THIN FILMS AND MICROELECTRONIC MATERIALS: edited by Devendra Gupta and Paul S. Ho HANDBOOK OF CONTAMINATION CONTROL IN MICROELECTRONICS: edited by Donald L. Tolliver HANDBOOK OF ION BEAM PROCESSING TECHNOLOGY: edited by Jerome J. Cuomo, Stephen M. Rossnagel, and Harold R. Kaufman CHARACTERIZATION OF SEMICONDUCTOR MATERIALS, Volume 1: edited by Gary E. McGuire HANDBOOK OF PLASMA PROCESSING TECHNOLOGY: edited by Stephen M. Rossnagel, Jerome J. Cuomo, and William D. Westwood HANDBOOK OF SEMICONDUCTOR SILICON TECHNOLOGY: edited by William C. O’Mara, Robert B. Herring, and Lee P. Hunt HANDBOOK OF POLYMER COATINGS FOR ELECTRONICS, 2nd Edition: by James Licari and Laura A. Hughes HANDBOOK OF SPUTTER DEPOSITION TECHNOLOGY: by Kiyotaka Wasaand Shigeru Hayakawa HANDBOOK OF VLSl MICROLITHOGRAPHY: edited by William 8. Glendinning and John N. Helbert CHEMISTRY OF SUPERCONDUCTOR MATERIALS: edited by Terrell A. Vanderah CHEMICAL VAPOR DEPOSITION OF TUNGSTEN AND TUNGSTEN SILICIDES: by John E. J. Schmitz ELECTROCHEMISTRY OF SEMICONDUCTORS AND ELECTRONICS: edited by John McHardy and Frank Ludwig HANDBOOK OF CHEMICAL VAPOR DEPOSITION: by Hugh 0. Pierson

V

vi

Series

DIAMOND FILMS AND COATINGS: edited by Robert F. Davis ELECTRODEPOSITION: by Jack W. Dini HANDBOOK OF SEMICONDUCTOR WAFER CLEANING TECHNOLOGY: edited by Werner Kern CONTACTS TO SEMICONDUCTORS: edited by Leonard J. Brillson HANDBOOK OF MULTILEVEL METALLIZATION FOR INTEGRATED CIRCUITS: edited by Syd R. Wilson, Clarence J. Tracy, and John L. Freeman, Jr. HANDBOOK OF CARBON, GRAPHITE, DIAMONDS AND FULLERENES: by Hugh 0. Pierson MOLECULAR BEAM EPITAXY: edited by Robin F. C. Farrow HANDBOOK OF COMPOUND SEMICONDUCTORS: edited by Paul H. Holloway and Gary E. McGuire HANDBOOK OF VACUUM ARC SCIENCE AND TECHNOLOGY: edited by Raymond L. Boxman, Philip J. Martin, and David M. Sanders HIGH D E N S I N PLASMA SOURCES: edited by Oleg A. Popov DIAMOND CHEMICAL VAPOR DEPOSITION: by Huimin Liu and David S. Dandy HANDBOOK OF MAGNETO-OPTICAL DATA RECORDING: edited by Terry McDaniel and Randall H. Victora HANDBOOK OF REFRACTORY CARBIDES AND NITRIDES: by Hugh 0. Pierson ULTRA-FINE PARTICLES: edited by Chikara Hayashi, R. Ueda and A. Tasaki

Ceramic and Other Materials-Processing and Technology SOL-GEL TECHNOLOGY FOR THIN FILMS, FIBERS, PREFORMS, ELECTRONICS AND SPECIALTY SHAPES: edited by Lisa C. Klein FIBER REINFORCED CERAMIC COMPOSITES: edited by K. S. Mazdiyasni ADVANCED CERAMIC PROCESSING AND TECHNOLOGY, Volume 1: edited by Jon G. P. Binner FRICTION AND WEAR TRANSITIONS OF MATERIALS: by Peter J. Blau SHOCK WAVES FOR INDUSTRIAL APPLICATIONS: edited by Lawrence E. Murr SPECIAL MELTING AND PROCESSING TECHNOLOGIES: edited by G. K. Bhat CORROSION OF GLASS, CERAMICS AND CERAMIC SUPERCONDUCTORS: edited by David E. Clark and Bruce K. Zoitos HANDBOOK OF INDUSTRIAL REFRACTORIES TECHNOLOGY: by Stephen C. Carniglia and Gordon L. Barna CERAMIC FILMS AND COATINGS: edited by John B. Wachtman and Richard A. Haber CERAMIC CUTTING TOOLS: edited by E. Dow Whitney

Related Titles CODE COMPLIANCE FOR ADVANCED TECHNOLOGY FACILITIES: by William R. Acorn SEMICONDUCTOR INDUSTRIAL HYGIENE HANDBOOK: by Michael E. Williams and David G. Baldwin

Preface

Optical data storage devices utilizing magnetic media and magneto-optic (MO) readout became commercially available in the last half of the 1980’s. This technology has now progressed through three generations ofinternational standard format products. Today it is firmly positioned as a robust, highly reliable and extendible method of rewritable (erasable), high density opticalstorageof not only computerdata, but also entertainment (audio, video) formats and other information. As with other forms of optical storage, this has been enabled by the development of low cost, compact solid state laser devices. An additionalkey to the emergenceof MO recording was the discovery anddevelopmentfi-omtheearly 1970’s of amorphousmagnetic alloy film materials with adequate perpendicular magnetic anisotropy and thermomagnetic properties compatible with thermally-assisted magnetic recording. These materials provided acceptable signal-to-noisecharacteristics that madethermomagneticrecording feasible. MO recording technology offers options for high performance and quality combined with media removability, a prime attribute of optical storage. This handbook brings together, in a single volume, expert contributions on the many aspects of MO data recording, including materials in use, techniques for achieving the recording function, and storage device subsystems. As a multiple author treatment, it brings perspective from many viewpoints and institutions. The insights delivered should be valuable to a wide audience from students to practitioners in all areas of information storage.

vii

viii

Preface

1 he organization ofthis book mirrors the synthesis of a disk drive from its components. Following a short introduction, a detailed discussion of the major subassemblies, including media, is provided in Chs. 2-6. The interaction of these elements, from both experimental and theoretical viewpoints is then examined in Chs. 7-11. System-wide integration of these components into a durable and reliable package is the focus of Chs. 12 and 13. The book ends with a discussion of the outlook for MO recording. The editors wish to recognize the fine contributions of the many authors and their sources. We thank our employers (T.W.M., International Business Machines Corporation; R. H. V., Eastman Kodak Company) for their ongoing support of the publication of this book. We are also grateful to our families for their toleration of the commitment required of us to produce the handbook. San Jose, California Rochester, New York April 1996

Terry W. McDaniel Randall H. Victora

Contributors

Blair I. Finkelstein Brian J. Bartholomeusz Mass Memory Research Laboratory IBM Storage Systems Division Tucson, AZ Eastman Kodak Company Rochester, NY Edward C. Gage Mass Memory Division Bernard W. Bell, Jr. Eastman Kodak Company MOST, Inc. Rochester, NY Monument, CO Charles Brucker Mass Memory Division Eastman Kodak Company Rochester, NY David K. Campbell Hewlett Packard Company Greeley, CO Marvin B. Davis MOST, Inc. Monument, CO

Kurt W. Getreuer MOST, Inc. Monument, CO Leo J. Grassens MOST, Inc. Monument, CO Scott B. Hamilton Apex Systems, Inc. Boulder, CO

ix

x

Contributors Terry W. McDaniel Quinta Corporation San Jose, CA

Dennis G. Howe Optical Sciences Center University of Arizona Tucson, AZ

William A. McGahan Nan0 Metrics Inc. Sunnyvale, CA

Jerry E. Hurst, Jr IBM Almaden Research Center San Jose, CA

Thomas D. Milster Optical Sciences Center University of Arizona Tucson, AZ

David B. Kay Mass Memory Division Eastman Kodak Company Rochester, NY Yoshimitsu Kobayashi Mitsubishi Chemical Corporation Yokohama, Japan Mark H. Kryder Department of Electrical & Computer Engineering Carnegie Mellon University Pittsburgh, PA

Randall H. Victors Mass Memory Research Laboratory Eastman Kodak Company Rochester, NY C. David Wright Electrical Engineering Laboratories The University of Manchester Manchester, England

NOTICE To the best of our knowledge the information in this publication is accurate; however the Publisher does not assume any responsibility or liability for the accuracy or completeness of, or consequences arising from, such information. Thisbook is intended for informational purposes only. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the Publisher. Final determination of the suitability of any information or product for use contemplated by any user, and the manner of that use, is the sole responsibility of the user. We recommend that anyone intending to rely on any recommendation of materials or procedures mentioned in this publication should satisfy himself as to such suitability, and that he can meet all applicable safety and health standards.

Contents

1 Magneto-Optical Data Recording: Introduction and Overview Bernard K Bell. Jr., and David K Campbell 1.0 INTRODUCTION TO OPTICAL STORAGE ......................... 1.1 Optical Data Storage .........................................................

..................................

1.2 Magneto-Optical Data Storage ........................................... 1.3 Rotating Mass Memories ................................................... ....................................................................... 1.4 History 1.5 Other Methods of Erasable Optical Storage ........................

1 1 1 2 2 4 6

2.0 THE ADVANTAGES OF OPTICAL STORAGE.................... 9 2.1 Removability ..................................................................... 9 2.2 Areal Density .................................................................... 9 2.3 Archivability ................................................................... 12 3.0 THE STORAGE HIERARCHY AND

OPTICAL LIBRARIES ...................................................... 12 4.0 THE MO DRIVE .................................................................. 13 4.1 Optical Heads .................................................................. 13 4.2 MO Readout .................................................................... 15 4.3 Servos and Actuators ....................................................... 20 4.4 Magneto-Optical Media Standards ................................... 22

xi

xii

Contents 4.5 Magneto Optical Disks and Cartridges ............................. 4.6 Drive Electronics ............................................................. 5 .0 MAGNETO-OPTICAL STORAGE PRODUCTS AND SUCCESS IN THE MARKETPLACE ...............................

REFERENCES

24 25

.....................................................................

27 32

..................................................

33

2 Heads and Lasers

David B Kay and Edward C Gage 1.0 INTRODUCTION ................................................................. 1.1 Overview of Optical Head Functions ................................ 1.2 Layout of an MO Optical Head ........................................ 1.3 Erasing, Writing. and Reading ......................................... 2.0 LASER DIODES ................................................................... 2.1 Laser Diode Design ......................................................... 2.2 Operating Characteristics ................................................. 2.3 Laser Diode Beam Properties ........................................... 2.4 Laser Noise ..................................................................... 2.5 Laser Diode Lifetime and Reliability ................................ 3.0 INCIDENT LIGHT PATH .................................................... 3.1 Laser Beam Collimating and Shaping ............................... 3.2 Optical Head Efficiency................................................... 3.3 Lenses .....................................................................

33 33 33 35 36 38 39 43 50 55 59 59 64 69 3.4 Optical Stylus .................................................................. 72 4.0 RETURN LIGHT PATH ....................................................... 76 4.1 Polarization State ............................................................ 76 79 4.2 Signal Detection .............................................................. 98 4.3 Servo Signal Detection ..................................................... 5.0 EXAMPLES OF PRODUCTION MO OPTICAL HEADS .. 110 5.1 Unified Optical Heads .................................................... 110 113 5.2 Split Optical Heads ........................................................ 6.0 FUTURE IMPROVEMENTS.............................................. 115 115 6.1 Shorter Wavelength Light Sources ................................. 6.2 Head Miniaturization ..................................................... 117 ACKNOWLEDGMENTS .......................................................... 120 REFERENCES ................................................................... 120

..........................................

3 Servos and Actuators Kurt W Getreuer and Leo . I Grassens

1.0 INTRODUCTION ............................................................... 2.0 THE SERVO LOOP ............................................................

125 125 126

Contents xiii 2.1 Loop Elements in Optical Disk Drives ............................ 2.2 Servo Control Basics ..................................................... 2.3 Optical Disk Runouts .................................................... 2.4 Servos in Optical Recording ........................................... 2.5 Track Capture ............................................................... 3.0 ACTUATOR TECHNOLOGY ............................................ 3.1 Architectures ................................................................. 3.2 Actuator Dynamics ........................................................ 3.3 Suspension Systems ....................................................... 3.4 Voice Coil Design .......................................................... LIST OF SYMBOLS ................................................................. REFERENCES ...................................................................

4 Media Substrates And Format

...........................

126 128 133 134 138 148 148 157 178 190 205 208

209

YoshimitsuKobayashi 1.0 DISK LAYOUT AND FUNCTIONAL AREAS .................. 209 1.1 I S 0 Conventional CCS Format ...................................... 210 1.2 Banded Format .............................................................. 218 2.0 PRE-MASTERING ............................................................. 225 2.1 Formatting ................................................................... 225 2.2 Editing ................................................................... 228 3.0 MASTERING ................................................................... 232 3.1 Process ................................................................... 232 3.2 Pit and Groove Characteristics of CCS Media ................ 241 243 3.3 Mastering for Next Generation Media ............................ 4.0 MOLDING AND STAMPING ............................................ 245 4.1 Molding ................................................................... 246 4.2 Stamping ................................................................... 250 5.0 PROTECTION COATS; LIFETIME .................................. 253 5.1 Coating Method ............................................................. 253 5.2 Hard-Coat (Laser-beam Incident Side) ........................... 254 5.3 Over-Coat (Film Side) ................................................... 255 5.4 Lamination ................................................................... 258 5.5 Life Test and Lifetime Estimation .................................. 259 6.0 HUBBING ................................................................... 262 6.1 Structure and Basic Requirements for Hubs ................... 262 263 6.2 Hubbing Process ............................................................ ................................................................... 264 6.3 Testing 7.0 CARTRIDGING ................................................................. 264 7.1 Structure and Basic Requirements for Cartridge Case ..... 264 266 7.2 Cartridging Process ....................................................... 7.3 Testing ................................................................... 267

xiv

Contents 8.0 OPTICAL AND MECHANICAL PROPERTY CONTROL . 267 8.1 Optical Properties .......................................................... 267 8.2 Mechanical Properties .................................................... 274 ACKNOWLEDGEMENT ......................................................... 276 ................................................................... 277 REFERENCES

5 Magneto-Optical Thin Film Recording Materials 279 in Practice

........................................................

Charles Brucker 1.0 INTRODUCTION............................................................... 279 2.0 DESIGN CONCEPTS ......................................................... 280 2.1 Issues and Opportunities ................................................ 280 283 2.2 Practical Design Methodology ....................................... 3.0 FILM DEPOSITION AND MANUFACTURING METHODS ................................................................... 288 3.1 Batch vs . Integrated Processing ...................................... 288 294 3.2 Sputter Deposition of Dielectric Materials ...................... 3.3 Sputter Deposition of Multi-Component RE-TM Materials ....................................................... 301 3.4 Sputter Deposition of Co/Pt Multilayer Materials ........... 306 311 4.0 Magneto-Optical Thin Film Materials .................................. 4.1 Rare Earth Transition Metal .......................................... 311 4.2 CoPt Multilayer Materials ............................................ 319 4.3 Garnets and Other Iron Oxide Based Materials ............... 329 4.4 Other High Kerr Rotation Materials ............................... 336 5.0 REFLECTOR THIN FILM MATERIALS .......................... 339 6.0 ENVIRONMENT AND LIFETIME .................................... 342 7.0 SUMMARY AND OUTLOOK ........................................... 349 ACKNOWLEDGMENTS .......................................................... 351 ABBREVLATIONS AND SYMBOLS ....................................... 352 ................................................................... 353 REFERENCES

6 Materials Characterization

.

................................

362

WilliamA McGahan 1.0 INTRODUCTION ............................................................... 362 2.0 OPTICAL CHARACTERIZATION .................................... 363 2.1 Techniques for Optical Characterization ........................ 367 2.2 Theory of Optical Reflection from Multilayers ............... 371 2.3 Example of Optical Characterization .............................. 379

Contents

xv

3.0 MAGNETO-OPTICAL CHARACTERIZATION................ 381 3.1 Physical Origins of the Polar Kerr Effect ....................... 384 3.2 Magneto-Optical Measurements ..................................... 388 3.3 Phenomenology of the Polar Kerr and Faraday Effects ... 392 3.4 Multilayer Formalism for Optical and Magneto-Optical Calculations ............................................................... 397 3.5 Examples of Magneto-Optical Characterization of Materials................................................................ 403 3.6 Examples of Combined Optical and Magneto-Optical Characterization.............................. 405 4.0 THERMAL CHARACTERIZATION.................................. 414 5.0 SUMMARY ................................................................... 420 ACKNOWLEDGMENTS.......................................................... 420 REFERENCES ................................................................... 420

7 Writing and Erasing in Magneto-Optical Recording

.

.........................................................

425

Jerry E Hurst. Jr. and Terry u! MeDaniel 1.0 INTRODUCTION ............................................................... 1.1 Requirements for the Write Strategy ............................... 2.0 WRITING REGIMES AND LIMITS .................................. 2.1 The Adiabatic Writing Limit .......................................... 2.2 The Stationary Media Writing Limit .............................. 2.3 The Steady State Thermal Writing Limit ........................ 3.0 WRITING ISOLATED MARKS ......................................... 3.1 Effect of Laser Power and Pulse Width .......................... 3.2 Effect of Bias Field ........................................................ 3.3 Effect of Media Velocity ................................................ 4.0 Writing and Calibrating Data Sequences .............................. 4.1 Writing PPM Sequences ................................................ 4.2 Writing PWM Sequences ............................................... 5.0 ERASING DATA SEQUENCES ......................................... 6.0 SPECIAL TOPICS .............................................................. 6.1 Tip Writing ................................................................... 6.2 Read While Write .......................................................... 6.3 Laser Modulation Direct Overwrite ................................ 7.0 CONCLUSION ................................................................... REFERENCES ...................................................................

425 426 430 430 433 436 438 438 443 445 448 448 453 469 469 469 472 473 474 474

mi Contents

8 The Magneto-Optical Readout Process C David Wight

..............476

1.0 INTRODUCTION ............................................................... 2.0 ORIGINS OF THE MAGNETO-OPTICAL

476

READOUT SIGNAL ........................................................

477 477 480 3.0 OPTICAL PROPAGATION IN THE READOUT PATH .... 486 3.1 The Readout System as a Scanning Laser Microscope .... 486 3.2 DifFraction Theory for the Optical Disc Player ............... 487 3.3 The Readout Path in Detail ............................................ 495 4.0 OPTICAL SYSTEM CHARACTERIZATION.................... 504 4.1 Linear Incoherent Systems ............................................. 504 511 4.2 Partially Coherent Optical Systems ................................ 5.0 NOVEL READOUT TECHNIQUES ................................... 525 5.1 Superresolution Techniques ........................................... 527 5.2 Apodization Techniques ................................................. 531 5.3 Confocal Detection ........................................................ 535 5.4 Edge Detecting Readout Heads ...................................... 542 ACKNOWLEDGMENTS .......................................................... 547 REFERENCES ................................................................... 547 2.1 Polarized Light .............................................................. 2.2 Magneto-Optical Kerr Effects ........................................

9 Sources of Noise in Magneto-Optical Readout

.. 550

Blair I. Finkelstein 1.0 INTRODUCTION ............................................................... 1.1 The MO Readout System ............................................... 2.0 SHOT NOISE ................................................................... 3.0 ELECTRONIC NOISE ....................................................... 4.0 LASER NOISE ...................................................................

550 551 552 553 556 5.0 DIFFERENTIAL DETECTION AND MISBALANCE ....... 564 566 6.0 INTRODUCTION TO MEDIA NOISE ............................... 7.0 DISK REFLECTIVITY FLUCTUATIONS AND DEPOLARIZATIONNOISE ........................................... 568 8.0 WRITE NOISE ................................................................... 576 9.0 JITTER AND SIGNAL-AMPLITUDE FLUCTUATIONS .. 579 9.1 Effects of Finite Beam Size on Signal and Noise Spectra . 585 10.0 EQUALIZATION ............................................................... 588 11.0 SNRAND JITTER ............................................................. 591 11.1 Peak Shift ................................................................... 593 11.2 Figure of Merit ............................................................ 593

Contents xvii 11.3 Bit Error Rate (bER) ................................................... 11.4 Phase Margin .............................................................. 12.0 SUMMARY ...................................................................

ACKNOWLEDGMENTS .......................................................... REFERENCES ...................................................................

594 595 596 596 597

10 Modeling the Magneto-Optical Recording 598 Processes Teny FK McDaniel and Brian J. Bartholomeusz 1.0 INTRODUCTION ............................................................... 598 2.0 THE ROLE OF MODELING .............................................. 603 2.1 General ................................................................... 603 605 2.2 MO Recording Processes ...............................................

..................

...................

607 3.0 OPTICAL MODELING ...................................................... 3.1 Scope ................................................................... 607 3.2 Write and Read Stylus ................................................... 608 3.3 Light Collection Optics .................................................. 613 3.4 Light Interaction with Domains (Magneto-Optics) .......... 617 3.5 Light Interaction with Multilayer Films .......................... 622 3.6 Light Interaction with Features ...................................... 627 3.7 Multilayer Design .......................................................... 628 632 3.8 Birefringence Effects ..................................................... 4.0 THERMAL MODELING .................................................... 636 4.1 Overview ................................................................... 636 4.2 Background ................................................................... 637 4.3 Multilayered MO Media ................................................ 643 5 .0 THERMOMAGNETIC MARKING .................................... 662 5.1 Introduction ................................................................... 662 5.2 Simple Adiabatic Thresholding Model ............................ 663 5.3 Generalized Thresholding Model .................................... 669 674 6.0 MAGNETIC MODELING .................................................. 6.1 Mean-Field Modeling ..................................................... 674 6.2 Bubble Model ................................................................ 674 681 6.3 Micromagnetics ............................................................. 6.4 Nanomagnetics .............................................................. 685 6.5 Exchange-Coupled Systems and Direct Overwrite ..........689 692 7.0 SYSTEM MODELING ....................................................... 7.1 Noise Modeling ............................................................. 693 8.0 SUMMARY ................................................................... 696 REFERENCES ................................................................... 698

xviii Contents

11 Testing

.................................................................

.

706

Tom D Milster and Scott B. Hamilton 1.0 OVERVIEW ................................................................... 706 2.0 INFLUENCE OF TESTING CONDITIONS ON TEST RESULTS ....................................................... 708 2.1 Definitions ................................................................... 710 2.2 Numerical Aperture and Wavelength ............................ 710 2.3 Overfill of the Objective Lens ....................................... 711 2.4 Aberrations .................................................................. 713 2.5 Read Channel Bandwidth ............................................. 716 2.6 Polarization of Read Beam ........................................... 717 2.7 Spindle Runout ............................................................ 718 2.8 Laser Noise .................................................................. 719 2.9 Relative Intensity Noise ................................................ 720 2.10 Other Considerations .................................................... 720 3 .0 MECHANICAL TESTS ...................................................... 721 3.1 Definitions ................................................................... 721 3.2 Testing Techniques for Displacement ............................. 722 3.3 Testing Techniques for Acceleration .............................. 725 3.4 Open Loop Techniques .................................................. 725 3.5 Tilt ................................................................... 726 3.6 Calibration ................................................................... 727 4.0 OPTICAL TESTS ............................................................... 728 4.1 Definitions ................................................................... 728 4.2 Static Tests ................................................................... 729 4.3 Dynamic Tests ............................................................... 734 5.0 PRERECORDED CHARACTENSTICS TESTS ............... 738 6.0 RECORDING FUNCTION TESTS .................................... 740 6.1 Narrow-Band Carrier-to-Noise Ratio (CNR) .................. 741 6.2 Cross-Talk Ratio ........................................................... 742 6.3 Wide-Band Carrier-to-Noise Ratio ................................. 744 6.4 Eye Patterns .................................................................. 745 6.5 Timing Jitter .................................................................. 746 6.6 Byte Error Rate (BER) .................................................. 749 6.7 Defect Mapping ............................................................. 750 7.0 STANDARDS DOCUMENTATION .................................. 751 7.1 Media Standards ............................................................ 751 7.2 Test Methods ................................................................. 751 8.0 TESTING ISSUES WITHNEXT GENERATION MEDIA . 752 REFERENCES ................................................................... 754

Contents

12 Drive Packaging Marvin R. Davis

..................................................

xix

756

1.0 INTRODUCTION ............................................................... 756 2.0 FORM FACTORS .............................................................. 757 3.0 MEDIA CARTRIDGES AND STANDARDS ..................... 758 4.0 BASEPLATE DESIGN CONSIDERATIONS ..................... 763 4.1 Spindle Motor ............................................................... 763 4.2 Carriage Issues .............................................................. 764 4.3 Optics and Thermal Issues ............................................. 765 4.4 Other Baseplate Features ............................................... 766 5.0 MECHANISMS AND PACKAGING .................................. 768 5.1 Dust and Contaminants .................................................. 769 5.2 Bias Magnetic Field Generators ..................................... 770 5.3 Thermal Cooling Issues ................................................. 771 5.4 Electronic Considerations .............................................. 772 5.5 Chassis and Bezel Design .............................................. 773 6.0 ENVIRONMENTAL AND AGENCY REQUIREMENTS .. 774 6.1 Electromagnetic Interference (EMI) ................................ 774 6.2 Laser Safety .................................................................. 775 6.3 UL Recognition ............................................................. 777 6.4 Miscellaneous Other Specifications ................................ 778

13 Data Reliability and Errors

................................

780

Dennis G.Howe 1.0 INTRODUCTION ............................................................... 780 2.0 OVERVIEW OF THE DIGITAL OPTICAL 781 RECORDING CHANNEL ............................................... 2.1 User Data and Channel Data .......................................... 781 2.2 Write Waveforms .......................................................... 785 2.3 Read Signals and Data Detection ................................... 788 2.4 Channel Data Decoding and Error Correction ................ 791 3.0 DATA RELIABILITY ASPECTS OF THE RECORDINGFORMAT .................................................. 794 797 3.1 The CCS Recording Format ........................................... 3.2 Full Density Data Sectors .............................................. 802 3.3 Error Detection and Error Correction ............................. 815 3.4 Defective Sector Management ........................................ 842 4.0 THE NATURE OF DIGITAL ERRORS ............................. 844 4.1 Shift Errors, Dropouts and Drop-ins .............................. 845 4.2 Synchronization Errors .................................................. 847

xx

Contents 4.3 Soft Errors and Channel Data Error Probability ............. 850 4.4 Hard Errors ................................................................... 852 4.5 Shift Errors and Their Multiplication During Channel Data Demodulation ............................ 852 5.0 DATA RELIABILITY ESTIMATION ................................ 859 5.1 Error Statistics .............................................................. 861 5.2 RS ECC Output Data Reliability Estimation .................. 873 6.0 ERROR CONTROL IN FUTURE MO STORAGE SYSTEMS ............................................. 886 6.1 RS Codes with Increased dPi,,............................................................. 887 6.2 Two-Dimensional Interleavlng of Two ECCs ................. 888 6.3 Enhanced Decoding ....................................................... 889 REFERENCES ................................................................... 893

14 Outlook for Magneto-Optical Recording

.

...........895

Mark H Kryder 1.0 TRENDS IN INFORMATION PROCESSING SYSTEMS . 896 2.0 TRENDS IN MAGNETIC STORAGE ................................ 898 3.0 TRENDS IN MAGNETO-OPTICAL DRIVES ................... 900 4.0 ADVANCED MAGNETO-OPTICAL MEDIA .................... 904 5.0 ADVANCED MAGNETO-OPTICAL HEADS ................... 905 6.0 DIRECT OVERWRITE ...................................................... 908 7.0 MAGNETIC SUPER-RESOLUTION, OPTICAL SUPER-RESOLUTION AND NEAR-FIELD OPTICS ..... 912 8.0 CONCLUSIONS ................................................................. 916 ACKNOWLEDGMENTS .......................................................... 917 REFERENCES ................................................................... 917

Index

.........................................................................

920

Magneto-Optical Data Recording: Introduction and Overview Bernard ?K Bell, Jr., and David K Campbell

1.0 INTRODUCTION TO OPTICAL STORAGE

1.1 Optical Data Storage Compact discs (CD's) bring music into our lives. CD-ROM's bring software and information to our computers. Photo CD's allow us to archive pictures and display and manipulate them on our televisions and on our computers. Write-once optical drives store our credit card transactions and m&icai remr&. ReAbloie opiicai &SIKS mirid ~ . " e sai;o-w. .us to craie terabytes of information, whether it be video, audio, or digital. With magneto-optical MiniDiscE we can record music and carry it in our shirt pocket. Fifteen years ago, optical storage was something thought about and carried out in research labs, today it permeates our lives. In 1995 alone, worldwide sales of optical storage media and devices exceeded 50 billion dollars, with more than 150 million optical disk drives being sold. There is hardly a home in America, Europe, or Japan that doesn't have at least one CD player and dozens of discs. In the future, as storage capacities continue to grow and device sizes continue to shrink, optical 1

2

Magneto-Optical Data Recording

storage of information will continue to play an ever more important role in people’s lives. The next generation promises f i l l length digital movies and backward compatible players for your first generation CD’s.

1.2 Magneto-Optical Data Storage This handbook is about magneto-optical data storage. Magnetooptical storage is rewriteable storage. Compact Discs are read-only storage, they allow the customer to only read the data. Photo CD’s and CDRecordable disks are WORM (write once, read many) disks, they allow a user to write the data once, then read it as many times as desired. Magnetooptical disks allow a customer to read, write, erase, and rewrite the same disk a nearly infinite number of times. They provide the same functionality as other forms of magnetic storage such as magnetic tape and magnetic hard disks. 1.3 Rotating Mass Memories Optical Readout of Information. Optical data storage in the context of this handbook will mean the storage of data on a rotating medium where the readout is done by optical means. The information may be written optically or by optically assisted techniques. In general, this means a beam of light is focused to a spot of radiant energy which interacts with a storage The data on the disk is generally medium in the form of a rotating organized into tracks. The information is stored serially along the track in a binary fashion and might represent video, audio, or computer data. It may be e n d e d via a variety of recording techniques. The interaction between the light and the medium can occur in a number of ways but must m d @ some detectable property of the light, that is, it’s amplitude, phase, or polarization. In almost all cases, the light is reflected from the media, reenters the focusing lens where it is directed to a set of photodetectors which convert the optical signals into electrical signals. These electronic signals are decoded to produce the information of interest. Readout from magneto-optical disks” 51 follows the above description. For a magneto-optical disk, the storage medium is a thin magnetic film (or films) deposited onto a transparent substrate. Data is recorded in a binary form as submicron-sized magnetic domains arranged serially in tracks on the disk. Readout takes place by reflecting light from the film, with the interaction between the light and the medium producing a change in the polarization of the light, the magneto-optical effect. The polarization

Introduction and Overview

3

change between alternating bits is converted to electrical signals and decoded in the magneto-optical head and drive electronics. Magneto-Optical Writing. Thermomagnetic recording is the process used to write data on magneto-optical disks, and as its name implies, requires both laser heating and a magnetic field. A beam of laser light is focused to a submicron spot of radiant energy which increases the temperature of the magnetic thin film in a very localized area. Simultaneously, a magnetic field is applied to the heated area and remains present after the radiant energy is removed. As the film cools, a magnetic domain with magnetization determined by the applied field is frozen in place. Because the heating is very localized, this domain will have dimensions similar to the dimensions of the focused beam. If the same area is heated a second time, and a magnetic field of opposite polarity is applied, a domain of opposite magnetization is produced. This process allows data in the form of domains to be written, erased, and rewritten to the same area on the disk a nearly infinite number of times. Magneto-optical thin films have very large vertical anisotropy which means that they only support magnetization in a direction that is perpendicular to the plane of the thin film. Hence only two magnetization orientations can be recorded on the disk. These are domains with magnetization direction pointing towards the laser beam entrance face, and domains with magnetization pointing away from this face. There are several variations to the basic thermomagnetic recording scheme. The laser power can be held at a DC level while the magnetic field is switched at high rates, the magnetic field can be held fixed while the laser power is switched, or both can be switched. In all cases, the outcome is that, as the disk rotates under the focused laser spot, magnetic domains of varying spacing and or length are recorded representing the encoded data. The particular variation of thermomagnetic recording used depends upon such factors as the recording density and recording data rates that are required. The most commonly employed thermomagnetic recording scheme is the DC field with switching laser power. This is known as light power modulation. The field for this type is supplied by either a large electromagnet whose polarity can be changed by reversing the current, or a large permanent magnet that switches the polarity by rotating the magnet to present either the north pole or the south pole of the magnet toward the disk. The field from either type is approximately perpendicular to the recording films and the dimensions are chosen so that it extends over the band of tracks. Thus the magnetic field is coincident with the focused spot regardless ofwhich track the laser spot is on. Recording requires two passes

4

Magneto-Optical Data Recording

or rotations of the disk. During the first pass, the magnetic field is on and the laser power is raised and held at a power so that an entire block is erased. On the second rotation, the magnetic field is reversed and the laser power is modulated with the encoded data pattern between a low power (sufficient for readins but not alterins the magnetic domains) and a high power The resulting recording is a track with a series of domains of one magnetization, with spaces between them of the opposite magnetization. The primary advantages in light power modulation recording are that high (>10 MHz) data rates can be achieved and that the magnet can be spaced a long distance (several millimeters) from the disk allowing double-sided disks. An enhancement to this scheme known as light intensity modulation direct overwrite (LIMDOW)[161which eliminatesthe first erase pass began to appear in 1996 in 2.6 GB drives marketed by Nikon and others. These MO disks incorporated additional film layers which allowed the magnetic domains to freeze into either of the two directions depending on the temperature they had reached when heated by the laser spot. By pulsing the laser power to heat the media above the high threshold, a domain is written. A laser pulse of sufficient power to heat the media above the low threshold but not enough to raise the temperature above the high threshold produces an erased domain. Drives using LIMDOW media effectively doubled the write transfer rate while maintaining the use of double sided disks. See Ch. 7 for a more detailed discussion of LIMDOW writing. Another recording technique is magneticjield modulation (MFM) in which the laser power is held constant at a DC level while a small magnetic head modulates the magnetic field at a high rate. To achieve a fast switching speed, this magnetic head needs to be small and in close proximity to the magneto-optical thin films. This requires that it move with the laser spot to access different tracks on the disk. The most significant advantage of MFM recording is that single pass recording can be achieved using somewhat simpler media than with the LIMDOW technique. The disadvantages of MFM recording in current implementations are that data rates are usually slow, limited by the switching time of the magnetic field, and the magnetic head must be located in very close proximity to the magneto-optical films. The close proximity of the magnetic head precludes double-sided disks when a 1.2 mm protective cover type substrate is used.

1.4

History

Published acc~unts[~l[~l of the modem concept of recording information using laser light date to the 1960’s. Optical recording using laser light

Introduction and Overview

5

developed as an application of lasers. Patents covering some of the fundamental concepts were filed in the early 1970’s. Optical recording has been an active area of research in corporate and university laboratories since those times. Interestingly, some of the earliest work in optical recording was in the area of magneto-optical recording rather than in write-once, or readonly recording systems. Cheni’J in the 1960’s demonstrated reading and writing of magneto-optical data on a thin film of MnI3i in a fashion very similar to what is today commercialized in magneto-optical drives. Unfortunately, MnBi could only be deposited as a polycrystalline film rather than as an amorphous film. During readout, the high noise from the crystal grain boundaries resulted in an unacceptable signal-to-noise ratio. This problem did not lend itself to an easy solution in the 60’s or early ~ O ’ S and , most researchers abandoned MO recording and turned their attention towards development of read-only and writeance optical recordmg. In the early 1980’s, researchers at IBM, Xerox, and KDD[21-[41solved the signal-to-noise ratio problems of magneto-optical recording by developing rare-earth transition metal (RE-TM) alloy films of materials such as terbium iron, terbium iron cobalt, gadolinium iron and even something as exotic as europium oxide. These materials (in particular TbFeCo) gave high signal-to-noise ratios, allowed small stable domains to be written, and could be fabricated using sputtering techniques. The only drawback these materials showed was a very rapid propensity to oxidize. A several hundred angstrom film of bare TbFe would completely oxidize in a matter of minutes once it was exposed to normal laboratory environments. Materials scientists discovered that these films could be made to provide acceptable, if not exceptional lifetimes, by a combination of the following techniques: 1. Alloying elements such as Si, Cr, and others with TbFeCo. 2. Sputter depositing the RE-TM materials as dense high molecular weight films. 3. Encapsulating the RE-TM films between transparent, nonreactive, dielectricfilms. (Chapter 5 will describe how these dielectric films also provide optical enhancement of the magneto-optic effect.) 4. Depositing the RE-TM and dielectric films on transparent plastic or glass substrates then overcoating these films with an aluminumreflectivelayer followed by an organic overcoat. This effectively encapsulatesthe RE-TMfilms with a window for the laser beam to focus through.

6

Magneto-Optical Data Recording

These, techniques solved the last fundamental problem facing magneto-optical storage and led to the commercialization of magnetooptical media and drives. Today’s commercial media can provide lifetimes up to hundreds of years.

1.5 Other Methods of Erasable Optical Storage Magneto-optical data storage is clearly the most successful form of erasable optical storage. It is by no means, though, the only erasable storage technique that has been investigated, or commercialized. Practically every optically reversible and optically assisted reversible effect has at one time been investigated, and proposed as the best possible method for data storage. Data has been stored with phase-change optical materials. Dye polymer materials using multiple lasers with differentwavelengthsto record, erase, and read the data have been used. Thin layers of liquid crystal materials allow optically rewritable data storage, as do other electrochromic, photochromic and thermochromic materials. Except for magneto-optics and phase-change recording, these attempts at rewritable optical storage have never gotten out of the laboratory. Phase-change on the other hand has received considerable research and development, (perhaps even more than magneto-optics). However, due to technical limitations, to date rewritable phase-change optical storage has only been commercialized by one company, while magneboptics has been commercialized by dozens. Phase-Change Recording. Phase-change optical recording stores data in a thin metallic layer that is heated using a focused laser beam. After the heating and cooling process, the heated area enters either an amorphous or crystalline phase depending on the temperature reached and the time it was held at that temperature. The optical reflectivity for these two phases differs, allowing a low power laser beam to read written data by detecting a change in optical reflectivity. Phase-change optical recording was originally investigated as a write-once or permanent optical recording means. Amorphous films were used, which upon writing became crystalline in the regions that were written. Because of the composition and structure of the films, these crystalline marks were very permanent, allowing the media to be used as a write-once media. Matsushita, Kodak, and other companies commercialized products using this write-once method. Later researchers discovered that altering the composition and structure of phase-change films allowed the same focused laser beam to write either crystalline or amorphous marks. This is done by operating the laser at a lower power and for a longer pulse period to heat the films to a lower temperature than used to create the amorphous state, but still high enough to cause them to crystallize.

Introduction and Overview

7

The Ideal Solution. Phase-change recording appears to be the ideal solution to rewritable optical data storage. Unlike magneto-optical storage, which needs a magnetic field plus a laser source, phase-change recording requires only a s e e focused laser beamfor writmg and r e . Compared to the small magneto-optic effect, the difference in reflectivity between amorphous and crystalline states is very large giving rise to large signals. Optics which convert the polarization changes for magneto-optid readout to intensity signals are not required. Before the commercialization of LIMDOW type MO, one of the most significant advantages of phase-change recording was the ability to do single pass direct overwriting. When writing, the laser power is modulated with the encoded data between two power levels creating either a crystalline mark or an amorphous mark on the disk. In one pass, old data is overwritten with new data. Limitations. With all of the significant benefits to phase-change recording, why has magneto-optics become the dominant force in the rewritable optical storage market? The reason is because of the limitations of phase-change recording. The first and most critical limitation is the problem of cyclability. Magneto-optics allows an almost infinite number of read, write, and erase cycles of the media. The best phase-change media allows only somewherebetween lo3and lo5cycles. Two effects occur after a number of cycles: submicron areas of the films remain crystalline and will not switch to amorphous states, or permanent damage occurs to either the substrate materials or the dielectric encapsulating materials surrounding the phase-change thin film. Recrystallization Time and Data Retention. Another limitation to phase-change recording is the time required to convert the amorphous marks back to crystalline. Ideally this recrystallization time (when heated to elevated temperature with the writing beam) would be on the order of hundreds of nanoseconds. This is the dwell time for a micron-sized focused laser spot on a rotating disk. Current phase-change materials require longer times; this requires the disks to spin at slower rates with corresponding lower data rates than comparable MO products. Through material changes, the recrystallization times can be reduced, but this often leads to another limitation of phase-change, that of data retention. An ideal data storage medium will retain the data for an infinite period of time under the most severe storage conditions. Phase-change materials designed for rapid recrystallization, however, often allow the written amorphous data to also crystallize at room temperature in a matter of minutes. Progress has continued in phase-change materials and drive media solutions to the problem exist.

8

Magneto-Optical Data Recording

Power Margins. One final limitation to phase-change optical recording is one of laser power levels and laser power margins. A delicate balance is needed to achieve the correct temperature required when writing amorphous marks. A temperature which is too high damages the substrate. Power which is too low results in amorphous marks that are not well written. Tight control is also required when writing crystalline marks. These requirements translate into carefully controlled laser power levels in order to control the power densities at the recording films. As described more fully in Ch. 2, Sec. 3, power density is influenced by disk substrate thickness and tilt tolerances, and by numerous optical component tolerances. To keep from having to control all of these beyond reasonable manufacturing tolerances, phase-change media has been tailored to use very high power lasers. These higher power lasersare more expensive, thus cancelling some of the economic benefits due to the simple optics used in phase-change drives. Commercialization of Phase-Change. Matsushita[l31and others have commercialized rewritable phase-change media and drives. Their products have achieved capacities similar to magneto-optical drives, with performance levels that are lower than magneto-optics (with the exception of the single pass overwrite). To solve the cyclability problem, the drives monitor the number of cycles, and a subsequent wear-out mechanism substitutes fresh areas of the disk for ones which are nearing wear-out and alerts the user when a new disk is needed. For low usage applications, this is a very acceptable solution. Future. As to the future of magneto-optical recording versus phasechange recording, much debate continues. Currently magneto-optics holds the lead in terms of number of product offerings, installed base, and performance levels. The capacity of phase-change drives has not kept pace with MO, probably owing to difficulties with pulse-width modulation recording. Research continues, though, at a rapid pace in phase-change with continuing improvements. Most phase-change materials lose about 30% of their contrast at blue wavelength. This is not much worse than TbFeCo but substantially inferior to Copt superlattices. Shorter wavelengths can be focused to a smaller difiaction-limited spot, thus future higher density optical recording will require the use of blue or blue-green lasers. The response of TbFeCo materials in the blue is still under development, with the possibility that new materials or alloys will be needed. In addition, land and groove recording areal density improvements look promising and may be easier to implement in phase-change recording systems than in MO systems. These advantages may give phase-change the boost it needs to overcome magneto-optics current dominant role.

Introduction and Overview 2.0

9

THE ADVANTAGES OF OPTICAL STORAGE

2.1 Removability Optical storage techniques offer three primary advantages over other methods of storing information. These are removability, density, and archivability. Optical storage stands alone in its ability to interact with the storage media from a distance. This is due to the imaging property of optical systems which allows the image of an object to be formed in space. In the case of the opticaZ sfyIus (as the focused spot is sometimes called), this image is of the beam waist of a laser, e.g., at or near the exit facet of a laser diode. This image is formed in the focal plane of the objective lens of the “optical head” and is usually a millimeter or two from the lens itself. Thus the media can be protected by a transparent layer through which the beam is focused. The “far field focusing behavior of the lens and the media’s protective layer are the basis of the media’s removability and make it practical to change the pieces of media in an optical disk drive in a noncontrolled environment. The focused spot at the information layer, which is typically 0.6-1.2 mm beyond the entrance surface, has micrometer dimensions. The size of the converging beam at the entrance surface of the protective layer is on the order of a millimeter. This reduces, by several orders of magnitude, the sensitivity to defects and contamination on the transparent cover layer. Usually, there is a cartridge with a sliding door to allow access for the beam which houses the media to further protect it. The media thus encased in its protective cover is very robust and portable and does not require a carefully sealed environment. In fact, an optical cartridge with the disk inside may be dropped off the desktop without harm. MO disks share this removable characteristic with magnetic floppy disks, but have vastly more information capacity (250-2000 times). This storage density advantage puts MO disks into a different class of computer peripheral.

2.2 Areal Density Another advantage of optical storage techniques are their high data density. This is due mainly to an advantage in track density. Optical track spacings are on the order of a micrometer. Standard MO disk track pitches range from 1.6-1.15 micrometers per track (15,000-22,000 tracks per inch). Current inductively read magnetic rigid disk track densities are roughly five times less.

10

Magneto-Optical Data Recording

This density is achievable because the energy of interaction, either reading, erasing or writing, comes from an offdisk source. In inductive magnetic recording techniques, the read energy is furnished by the rotational energy of the disk itself. The readout signal is produced when the flux lines from a magnetic domain in the media pass through a conductive loop in the head thereby inducing signal current in the loop. The amplitude of the signal is proportional to the rate at which the flux lines cross the loop, and thus is tied to the rotation rate of the disk, the sizes of the loop and the recorded domain, and the number of loops of the readout coil. Increasing the number of turns increases the signal but cannot be increased indefinitely as this limits the bandwidth of the readout due to increased capacitance. In optical storage, the readout signal is proportional to the laser energy supplied by the laser and is independent of the rotation rate of the disk or the size of the recorded domain above the focused spot size. The readout laser power cannot be increased indefinitely, either, as it must be low enough to insure that the heating of the media during reading does not begin to write thus degrading the stored information. MO heads and media are designed such that the signal-to-noise ratio["] fiom the reading of a narrow domain on an optical disk is comparable to the signal-to-noise ratio from a several-times-wider domain on an inductively read magnetic disk. The narrower track spacings on MO media provides an areal density advantage but this comes at the cost of more precise servo control requirements as discussed in Ch. 3. Separate readout energy techniques have recently (approximately 1991) found their way into rigid disk magnetic recording systems via the use of magneto-resistive (MR) type read heads.[l41 Figure 1 illustrates the challenge facing MO recording technology as the areal density increases due to MR and giant MR (GMR) threaten to equal MO densities.[l41 The wavelength and numerical aperture (NA) of an optical recording system determine the dimension of the optical spot. The focused spot or optical stylus can be characterized by its f i l l width at half maximum (FWHM)value via:

where K = 0.56 for Gaussian beam aperture filling, h is the optical wavelength and NA is the numerical aperture defined as the sine of the converging cone angle (marginal ray angle) on the disk side of the objective

Introduction and Overview

11

lens. Thus shorter wavelengths and larger numerical apertures increase areal density proportional to the square of their ratio.

IS0 STANDARD

lE10 1E9 1ES

lE7 lE6 1E5 lE4 1000 100

1957 1973 1982 1992 1995 Year Figure 1. Areal density of magnetic recording and MO recording versus year of product introduction. Units are bits per square inch.

12

Magneto-Optical Data Recording

2.3 Archivability Perhaps archivability of data is MO’s strongest advantage over other storage mediums. MO disks have very high coercivities requiring, at room temperature, very large magnetic fields to flip the magnetic domains from one state to the other. In order to reverse the polarities with reasonable field strengths, the media is locally heated by the laser spot. This is the reason MO is sometimes called optically assisted magnetic storage, or thermomagnetic storage. In effect, the information is frozen into the active layer at room temperature which contributes to its long life. The perception of archivabilityrequiresjudgment. Thisjudgment should be tempered by data. Techniques of assessing the stability and lifetimes of MO disks is an active area of research and are discussed in Ch. 5 in this volume. However, as more data on stability is collected and interpreted, MO media is proving to be a superb archival storage medium.

3.0

THE STORAGE HIERARCHY AND OPTICAL

LIBRARIES The modern digital computer has created a hierarchy of data storage needs. These range from fast to slow in access, expensive to cheap, and temporary to permanent. Examples are very fast (nanoseconds) storage registers accessible by only the CPU itself, fast (tens of nanoseconds) random access memories (RAM), much slower (milliseconds) fixed magnetic hard drives, comparable but removable rotating memory devices (magnetic, MO, and some WORM drives), and slow (seconds) removable backup devices which are used to restore information lost in the event of a disaster (tape) or to archive information for historical purposes (tape, MO, and WORM optical drives). The concept of an optical library makes use of optical’s removability. An optical library is similar to a magnetic tape library in that large amounts of information are stored in an off-line but quickly accessible manner. The definition of “large amounts” of information depends on the application and must be a trade-off between amount of information and the time to access it. The architecture of a storage device which uses removable media is highly flexiblebut must be optimized with the target system in mind. Similarly,the concept of “quickly” also fits into a range of “more or less” depending on the

Introduction and Overview

13

methad of finding and “mounting” the volume of information that is requested. This type of storage has been traditionally used to backup information that was too infrequently used to justify the use of main computer memory. If a person had to physically find the storage medium, mount it on the redwrite device and initialize the unit before the computer had access to it, “quick” could be defined in minutes or fraction of an hour providing that the human operator was immediately available for the task. In recent years, these processes have been automated resulting in a much shorter and less expensive operation. “Quick” can now be defined as a few seconds and can be performedjust as easily at 2:OO AM or at 2:OO PM. Optical libraries are actually very simple robots relieving mankind of some of the most monotonous tasks relating to computer data storage. Like tape libraries, optical libraries offer low cost-per-gigabyte solutions since both offer multiple media cartridges per unit. An optical library does differ from a tape library in an important way. Magneto-optical disks are random access devices with performance levels comparable to magnetic hard drives; that is to say both MO drives and magnetic hard drives have access times in milliseconds and transfer rates in the range of several megabytes per second. Tape drives are much slower with typical access times measured in seconds and transfer rates in the submegabyte per second ranges. Hence tape drives and optical drives, while both capable of large storage capacities, fill different niches in the computer data storage hierarchy.

4.0

THEMODRIW

4.1

Optical Heads

Introduction. The MO head must perform the basic functions of illuminating the disk with a focused spot and collecting and manipulating the return light after its interaction with the spinning disk. Thus, a head in an MO drive consists of all the optical components from the source to the photodetector. In addition, a head usually includes the necessary electronic components to drive current through the laser diode and to amplifythe photo current from the detectors used to convert the light signals into electronic signals for the data and servo channels. Heads typically fall into two basic classes. Either the entire head is translated, or the head is split into a fixed

I4

Magneto-Optical Data Recording

and a moving part. The latter method allows for a smaller, lighter moving mass and thus for faster actuation across the disk. The moving portion of a split type head typically consists of two optical components: the objective lens and a beam-turning component (mirror or prism) to change the beam propagation direction from parallel to the disk surface to perpendicular to it. In addition to these, the moving part or carriage assembly usually carries a focus actuator to move the objective lens in order to maintain a focused spot on the spinning disk. It may also include afine trachng actuator to move the lens in the radial direction in order to keep the focused spot on the track. Optical Wavefront Quality. From a purely optics point of view, the requirements on a MO optical recording head are extraordinary. An MO head must produce a difraction-limited spot while focusing and tracking on a spinning piece of plastic. This is opticaljargon for as nearly asperfect as the laws of physics will allow. Information must be written and read at densities of roughly 2 bits per micrometer (50 kilobits per inch) while the disk is going past at up to 50 mph (22.6 meters per second) assuming a 3600 rpm 130 mm disk. In practice, this is quantified by wavefront aberration distributionson the assembled head with means of 0.04 waves rms, standard deviations on the order of 0.01 waves, and worst-case limits of approximately 0.06 waves rms. Included in this optical system is a piece of molded plastic as the cover layer and a substrate. For the non-optically trained reader, this is an order of magnitude better than your 35 mm camera and on a par with the best astronomical telescopes in the world. Polarization Properties. An MO head must provide this superb wavefront quality plus measure the f 1' polarization rotation produced by the 1's and 0's states of the media magnetization. This means that the optical components and coatings must be controlled in polarization properties in addition to the stringent geometrical parameters responsible for attaining excellent optical wavefront requirements. This polarization control technology is new with MO and has required new classes of thin film coating design and measurement technology to be developed. Typical component tolerances are on the order of 10" (0.027 waves) of phase shift between orthogonal polarization states. The polarization readout of MO recording is its most novel aspect, and is what differentiates it from other optical recording schemes. The next section presents the important concepts for understanding MO readout using a graphical device invented by the French mathematician Henri PoincarP] in the late 19th century.

Introduction and Overview 4.2

15

MO Readout

A Graphical Introduction. One of the functions of an MO head is the polarization manipulation of the return light. This is done to amplify the small rotation caused by the media to produce a larger rotation for detection by the optical data channel. The most general state of polarized light is an elliptical state. An ellipsometer is a device for measuring the state of polarizationof a beam of light. An MO head is a limited-functionellipsometer. The data signal used in MO disk drives is based on detecting orientational changes in the polarization state of light reflected from the disk. This change is typically small (= 1”) and often must be detected in the presence of undesired birefringence in the disk substrate and phase retardation in the readout optics. The mathematics used to model the propagation of light through the optical system does not typically produce intuition or insight. The Poincare sphere description provides a graphic representation of the polarization state on the surface of the sphere as well as a set of rules for transforming the polarization state as it interacts with various system component^.^^] This visualization develops intuition and provides a conceptual tool for understanding the behavior of system components and their effects on system performance. The next sections illustrate the evolution of the polarization state through an MO head by the use of the Poincare sphere. A generalized MO optical head consists of two components as shown in Fig. 2e: 1. A “leaky beam splitter”to amplifLthe small rotation caused by the media. 2. An analyzer to generate a differential signal. This is more fully discussed in the next chapter. The PoincarCSphere. The Stokes (I, Q, U, V)form a vector which provides a complete description of any state of polarization and when describing totally polarized light, Z2= Q2+ U 2+ V . Dividing each parameter by Z produces the normalized Stokes vector used in the following discussions. Linearly polarized light of horizontal or vertical orientation is represented by Q = +1 or -1, respectively, and the Stokes vector (1, *l, 0, 0). Linearly polarized light oriented along the *45” directions is represented by U = *1 or the Stokes vector (1, 0, *l, 0). Any other orientation is represented by a combination of these two parameters. For example, linearly polarized light at an orientation of +22.5” has the normalized Stokes vector (1,1/al@, , 0). Circularly polarized light, either right or left handed, is represented by V = *1 or the Stokes vector (1, 0, 0, il).

16

Magneto-Optical Data Recording

The Poincare sphereL51maps each point on its surface to a unique polarization state whose Cartesian x, y, z coordinates are Q, U,V. Points on the equator are linear polarization states, (1, Q,U,0), the northern and southern hemispheres contain right and left handed elliptical states, (1, Q, U,fV), respectively. The poles represent the right and left circular states (lj0; Oi kl). Every diameter has orthogonal polarization states as its endpoints. Two rules govern retarder and polarizer effects on the sphere:l81 1. An optical element with phase shift 6 moves a polarization state along an arc formed by rotating the sphere counter clockwise 6 degrees about a diameter whose endpoints are the eigenstates of the optical element. 2. The fraction of light transmitted by an analyzer is given by the cosine squared of half the arc length between the analyzer’s transmission state and the polarization state being analyzed. Light Incident on the Disk. For illustration purposes, assume a linear polarization state in the horizontal plane ((3, U, V) = (1, 0,O) as the incident state on the disk substrate. This is represented by the point on the equator of the sphere where the equator and the solid longitude line cross near the center of Fig. 2a. Further, assume the index of refraction ellipsoid which describes the MO substrate is vertically oriented and rotates as the disk spins. Thus the endpoints of the diameter of rotation in rule #1, moves around the equator as the disk spins and the point is transformed into an “8” shape. The “8” shape is traced out twice as the disk rotates. Thus the light reaching the MO thin films after travelling through the substrate is linearly polarized only four times per disk rotation, that is, at the points on the “8” which cross the equator. After interaction with the MO films, the polarization state is shifted along the equator (assuming zero ellipticity media, i.e., no phase shift between the reflected and MO-generated components) either clockwise or counter clockwise depending on the state of magnetization of the films. Both magnetizations are shown in Fig. 2b. Light in the MO Detection Arm. After reflection from the MO films and the return passage through the substrate the figure-eights are taller and wider. See Fig. 2c. After reentering the optical head and interacting with the “leaky beam splitter,” the polarization states are rotated through an angle which corresponds to the phase shift encountered on the return path through the head according to rule #1 above and the media rotation angle amplified. Single-endeddetection with an analyzer corresponds to measuring the arc length from the longitudinal position on the equator which corresponds to the analyzer angle to the point on the figure-eight which

Introduction and Overview

17

corresponds to the position in the track where the magnetic domains (either up or down) are written. See Fig. 2d. The optical signal is proportional to the cosine squared of half that arc length per rule #2 above. Differential detection using analyzers at f 45” to the horizontal corresponds to measuring the difference in arc lengths from the (Q, U, y) = (0, 1,O) and (Q, U, V) = (0, -1, 0) positions on a diameter of the equator of the sphere to the point on the figureeight which corresponds to the domain under the readout spot. The arc length increases and decreases four times as the “8” figure is traced out twice.This causes an oscillationin the MO signal which is four times the disk rotation rate. However, since the “8” figures are not aligned longitudinally on the sphere but are rotated due to the phase shifts in the optical head, there will be a two times spin rate oscillation in the arc lengths and hence the MO signal. There is little change in the difference of the arc lengths and hence little change in the differential MO signal amplitude. Note that a 90” rotation of the “8” figures causes the MO signal to go to zero as there is no difference in the arc lengths to the different “8” figures (the up or down magnetization states.)

Figure 2a. Light incident on the MO films becomes a figure “8” after tranversing the substrate. Linearly polarized light (the point at the intersectionof the equator and the solid longitude line) becomes elliptically polarized light (the equatorial point is translated to a point on the figure “8”) after traveling through a plastic MO disk substrate. The figure is mapped out twice per disk rotation.

18

Magneto-Optical Data Recording

Figure 2b. Poincare sphere showing the light reflected from the MO films. The figure “8” described in Fig. 2a is translated clockwise or counterclockwise along the equator depending on the magnetization state (up or down) of the domain that the light interacts with on the medium.

Figure 2c. Poincarb sphere showing the light returning from the disk. Light after the return passage through the substrate becomes more elliptically polarized as indicated by the figure “8’s” growing larger after the return trip though the substrate.

Introduction and Overview

19

Figure 2d Light in the head detection path. The figures grow even larger after interaction with a “leakybeam splitter” used to ampllfy the small MO-caused rotation. The figures are tiltedby the phase shift in the beam splitter. The larger figures indicate even more elliptically polarized states in the detection arm of the MO optical head. The signal fi-om a differential type MO detection head is proportional to the difference in the lengths of the arcs fi-om the equator of the sphere. Note a 90” rotation of the figures would produce no signal.

LERKY

USER

BEAM SPLITTI3

DIODE

/

R

MO MEDIR

ANALYZER

Figure 2e. A generalizedMO head consisting of a laser diode, a leaky beam splitter, and an analyzer.

20

Magneto-Optical Data Recording

Thus we see how lateral substrate birefringence causes an “8” shaped polarization state trajectory on the Poincare sphere. Retardation in the head causes an orientation change of the “8” shape. Both effects contribute to bias in the MO signal. This bias or offset can be a substantial problem in designing an error-free read channel and can be caused by local or global stresses in plastic substrates. This Poincark sphere representation serves to provide a graphical overview to introduce the reader to the concepts of polarization readout in general. MO readout is discussed in greater detail in Ch. 2, “Heads and Lasers,” Ch.8, “MO Readout,” and Ch. 9, “Sources of Noise in Magneto-Optical Readout” in this handbook. 4.3

Servos and Actuators

Background. An MO drive has many servo control systems. In addition to the standard rotating memory servos such as spindle speed control, seek and tracking position, MO drives must control the laser powers at read, write, and erase, and the position of the laser spot. Generally there are three spot positioning servo loops: focus, fine and coarse tracking. Both fine and coarse tracking systems are required due to the micrometer track pitches and the two orders of magnitude larger radial runouts on MO disks. These servos require a few kilohertz bandwidth. As mentioned earlier, the actuator technology and beam positioning servos used in MO drives is rooted in audio and video consumer product technologies. This should not be interpreted as “simply borrowed” since the challenges faced in MO recording systems push the performance limits of actuator and servos. Typical MO systems use 680 nm wavelength laser diodes with an NA of 0.55. Typical Compact Disc (CD) parameters are 830 nm with an NA of 0.47. This translates into a focused spot which is 30% smaller than a CD spot. Tracking Servo. Optical track spacings on the order of a micrometer, and allowable errors of about a tenth of that, coupled with peak to peak radial track runouts of 50 micrometers, impose severe requirements on the track following servo system. Normally two actuators are used to maintain the spot on track and to control seeking to a desired track. These two tracking actuators are often driven in a masterhlave configuration. The master is the fine position actuator while the coarse one is slaved to it. The fine actuator follows the small amplitude track runout errors. The coarse actuator follows the larger and slower frequency runouts and is used to access different tracks on the disk. The accelerations scale as the square of the spin rate; a two times increase in rpm results in a four times increase in required acceleration. Thus the required accelerations on a 3600 rpm MO disk can be 60 times that of a CD.

Introduction and Overview

21

Focus Servo. The focus servo system must follow the disk axial runout to within the depth of focus tolerance. This tolerance scales as h/NA2 When both NA and wavelength affects are scaled, the allowable focus tolerance decreasesto roughly 60% of the CD values. Thus going from a CD spin rate of 600rpm to a MO disk spin rate of 3600rpm would require 36 times the acceleration for the same axial disk runout. Fortunately, MO disks are flatter than CD’s. Nonetheless, the required accelerations for MO drives are roughly an order of magnitude higher for focus servo systems as compared to CDs. This results in focus servo system bandwidths of several kilohertz and allowable focus errors of less than a micrometer. Tilt Tolerance. While not controlled by a servo system, tilt is an optical system issue having to do with the optics and actuators in the optical head, the disk, the spindle, and the hub/disk mounting interface. A focus actuator has to move the objective in one axis of motion. This sounds deceptively simple when expressed this way; it is equivalent to saying that the other five degrees of freedom must be held within some tolerance while the sixth is actuated The requirements for tilt between the disk and the objective lens are governed by the optical wavefront aberration which can be tolerated. The Seidel aberration known as coma is the main culprit in the case of tilt. It is called coma because its effect is to cause a tightly focused spot to flare out into a comet-shaped spot complete with a tail. While never growing a full blown tail in a functional optical head, the first signs of coma are an asymmetry in the spot profile causing a side lobe to occur in the direction of the relative tilt. This energy side lobe causes a decrease in the central spot energy but its main effect is to spread out the area of interaction between the spot and the information layer. One classic symptom is the increase of signal from a track adjacent to the track one is attempting to read. This is a form of crosstalk and is known as adjacent track crosstaIk (ATC). The effect of coma scales as the ratio of the wavelengths and the third power of the NA ratio. Hence for a comparable wavefront tolerance, the allowable tilt decreases to (680/830)(0.47/0.55)3= 0.51 of the CD value, and is typically on the order of a few milliradians. These numbers are not to be taken literally since CDs and MO systems employ different types of optical interactions resulting in very different inherent signal modulation. This, in itself, will impose different servo tolerances on the systems. However, the numerical comparisons are meant to illustrate that while servos systems and actuators for MO systems may have similar functions with those of CD’s, they are often fundamentally different in their performance requirements. Chapter 3, “Servos and Actuators,” discusses these important subsystems and explores many of the associated issues.

22

Magneto-Optical Data Recording

4.4 Magneto-Optical Media Standards Introduction. A number of organizations worldwide have cooperated to produce standards for the characteristics of optical disk cartridges which provide for information to be written, read and erased many times using the MO effect. In the international community, the IS0 (International Organization for StandarMon) and the IEC (International Electrotechnical Commission) have a joint technical commim-ISO/IEC JTC 1. Draft International Standards (DIS) are circulated to national bodies for approval. For a proposal to become a published Standard, 75% of the national bodies must approve it. The Technical Committee X3B11 of the American Standards Committee X3 of ANSI (American National Standards Institute) and other bodies such as ECMA (European Computer Manufacturers Association) and JNB (Japanese National Body) participate in the preparation and approval of these ISO/IEC Standards. It is interesting to note that while writing domains in MO media is a reversible process, indeed it can be erased and rewritten many millions of times, a standardhas been approved (ISO/IEC 11560, ANSI X3.220-1992, ECMA -184, ISO/IEC 13549) which uses MO media in a WORM (writeonce read-many) mode in which the rewriting of information is prohibited by firmware in the optical disk drive. In general, these standards specify the following: Conditionsfor conformancetesting and the reference drive used in this test Environments in which the disk cartridges are used and stored Mechanical,physical and dimensional characteristiwofthe cartridge Physical format of the disk for both embossed and user-written information ECC and modulation methods and codes used Characteristics of the embossed information Characteristics of the MO information layers to insure write interchangeability Minimum quality of user-written information to insure read interchangability In addition to these physical characteristics, a standard for the logical volume and file system structures used has been approved. Together, these two Standards provide for the full interchange of information between different data processing systems (drives) across host operating systems environments (UNIX, DOS, Macintosh, etc.).

Introduction and Overview

23

In practice, the ranges of the physical specifications spelled out in these standards are rather broad for many parameters and may not be sufficientto guarantee that all optical disk drive processing systems will be able to read, write and erase from any disk which falls in the specified ranges. Rather the standards form the necessary set of physical ranges that must be met to allow for interchange. Most drives do not operate well with media at both lower and upper extremes of the ranges specified. Indeed, lot to lot manufacturing variations of most MO media being manufactured are smaller than specified in the standards. Thus most optical drives are only guaranteed to work with certified media from specific media manufacturers. Other media which meet the standards may work, but are not guaranteed by the drive manufacturer. Media manufacturers typically use drives from one or more drive vendors to certifir their media. Certification criteria are not mandated in the standards and are determined by each drive manufacturer. This is one of the parameters that give each make and model of optical disk drive its unique characteristics. Recording Parameters. Shown below are some of the recording parameters for the major rewriteable MO standards on 130 mm and 90 mm MO disks.

Table 1. Selected Parameters for Different Capacities on Major 130 mm Rewriteable MO Media Standards Parameter/Capacity

1X

2x

3x

4x

Capacity (MEVcartridge) Bit Density ( w i t ) Track Density (pdtrack) Stylus FwHM (pn) Format Recording Method Modulation Scheme Min. Feature Size (pn) Timing Window (pn) Year Standard: ANSI: X3B11.XX-XX ECMA-XXX ISO/IEC DIS-XxxXX

650 1.02 1.60 0.88 CAV PPM RLL(2,7) 1.53 0.5 1 1992

1300 0.86 1.39 0.80 ZCAV PPM RLL(2,7) 1.29 0.43 1993

2000 0.56 1.34 0.80 ZCAV PWM RLL(l,7) 0.74 0.37 1994

2600 0.50 1.15 0.70 ZCAV PWM RLL( 1,7) 0.66 0.33 1996

212-1992 na 10089A

na 184 13549

na 195 13842

na ?

15417

24

Magneto-Optical Data Recording

Table 2. Selected Parametersfor Different Capacitieson 90 mm Rewriteable MO Media Standards

Parameter/Capacity

1X

2x

Capacity (MB/cartridge)* Bit Density ( w i t ) Track Density (@track)

128 1.04 1.60 0.88 CAV PPM RLL(2,7) 1.53 0.51 1990?

230 384 0.55 0.86 1.39 1.39 0.80 0.80 ZCAV ZCAV PWM PPM ~ ~ 1 . 4 2 ~ 7 GcR/RZ;L(O,3) ) 1.29 0.49 0.49 0.43 1994 1994

na 154 10090

na 20 1 13963

Stylus FWHM (pn) Format Recording Method Modulation Scheme

Min. Feature Size (pm) Timing Window (pn) Year Standard: ANSI: X3Bll.XX-XX ECMA-XXX

Iso-xxxxx

3x

na na na

5x 640 0.50 1.15 0.70 ZCAV PWM RLL(1.7)

0.66 0.33 1995? na na 15041

*Using512 byte sector size except for 3X which is 1024.

4.5 Magneto Optical Disks and Cartridges Typical disk structures have the actual recording films protected on one side by various protective layers and on the other side by a 1.2 mm thick substrate. There is a metal hub bonded to the substrate in order to magnetically clamp the disk on the spindle in the disk drive. The larger 130 mm disks are double-sided and are composed of two such structures bonded together back to back. Smaller form factor disks (90 mm diameter and less) are composed of a single such structure. This final structure is surrounded by a cartridge to keep it free from dust and to protect the substrate from handling damage such as scratches. The physical features and dimensions of this cartridge are called out in the same standards as mentioned above. These features include holes for sensing information about the media, slots for grasping it by a mechanical picker, and a spring loaded door which slides open as the cartridge is inserted to allow access to the media by the read write head and the bias coil.

Introduction and Overview

25

Cartridge and Media Handling. MO drives typically employ a mechanism which loads and locates the cartridge into the drive, opens the cartridge door and places the metal hub onto the drive spindle. This must be accomplished before the drive can begin to access information on the disk. Normally the sequence of operation is to start spinning the disk, move the objective lens to acquire a focus error signal and close the focus servo loop, close the tracking servo loop and begin to read format information about the disk which is e n d e d in various ways in different functional areas of the disk. This could include standard r d w r i t e laser powers and defect management information recorded during formattingand certification of the media. The drive is now nearly ready to accept media access commands from the host computer but may first perform some signal level checking and perhaps some r d w r i t e testing to initialize itself. This whole process happens in a couple of seconds. Sooner or later, once the desired information is written or read from a piece of media, access to another piece will be desired. The sequence of operation is reversed, the spindle is stopped, the media is lifted off the spindle and ejected from the drive. A drive may load and unload different cartridges many thousands of times during it’s life on the desktop and many hundreds of thousands of times in a library. A particular cartridge may be loaded and unloaded many tens of thousands of times. For this reason, considerable care in the design and testing of the cartridges, hubs and media must be used to guarantee reliable operation. In addition, packaging of the loader and ejector mechanism into ever shrinking form factors requires increasingly clever mechanical design and use of space and materials.

4.6 Drive Electronics Introduction. Drive electronics usually fall into two functional areas: drive control and data control. Drive control electronicshandle “low level” things like current drivers and servo loops for spindle motors, coarse and fine actuators, cartridge load and eject mechanisms, laser power control, etc. Data control electronics handle communication with the host computer, waveform modulation encoding and decoding of the digital data, and error correction d i n g . Data error recovery algorithms and drive exception condition handling typically involve both areas. SCSI Controller. The vast majority of MO drives use the Small Computer System Interface (SCSI) to communicate with the host computer. Several ODC (Optical Drive Controller) chipsets are commercially

26

Magneto-Optical Data Recording

available which combine SCSI bus control protocols along with ENDEC (Encode DECode) algorithms in hardware. Similar proprietary custom chipsets have been developed for use in drives by various companies. In either case, a microprocessor and its associated program memory are used to execute embedded control software (usually called Jirmware) which orchestrates the sequence of events and sets up and initiates appropriate tasks such as erasing, writing, and reading. This firmware is often fieldupgradeable by the host computer by downloading replacement code over the SCSI bus. It is this combination of hardware and firmware which gives a drive its personality, that is, describes how it hnctions in given situations. Error Correction Coding. Error Correction Coding (ECC) is needed in MO drives because of the extremely high areal density recording. This translates into very small written domains and their commensurate sensitivity to media defects either in the thin films or substrates. The uncorrected bit error rate on state-of-the-art MO media is typically on the order of 1 error in 104-106 bytes. The distribution of these errors is heavily weighted toward 1 to 2 bit errors. Error burst lengths longer than this are much rarer but do occur. The ECC scheme employed must be able to handle both types and correct this raw bit error rate to something less than 1 error in 10l2bytes. Prior to correction, however, errors must first be detected. In order to detect errors, a number of check bits is added to the original number of information bits during the encoding process to form a code word. The check bits are computed using a code generator polynomial plus the original information bits. The check bits are appended to the information bits in such a manner that the code word thus formed is also a valid code word when the code bits are cyclically shifted in a linear feedback shift register. This allows for a simple hardware implementation to determine the code word. On readback, a similar process of shifting bits leads to the calculation of the syndrome of errors. A nonzero value of the syndrome means that an error has been detected in the readback code word and a correction is required. In practice, CRC (Cyclic Redundancy Codes) operating along the same principals just described but using 8-bit binary bytes as symbols instead of bits, are used to detect errors. These CRC check bytes are appended to the data bytes. Further processing by the ECC is required in order to correct the errors. In practical MO systems, long distance Reed-Solom~n[~] cyclic codes are used. These codes have a maximum code word size of 255 bytes. Interleaving is another important part of a robust ECC strategy. Consecutive bytes going to the waveform

Introduction and Overview

27

modulation encoder are taken from different code words. Thus each code word’s bytes are interleaved when they are written to the media. Consequently, long-burst media errors are spread across code words, thereby lessening their effect on error propagation. This subject is treated in detail in Ch. 13 authored by Dennis Howe. Currently, there are two popular sectors sizes used in the standards: 512 and 1024 byte. The 5 12 byte sectors use five code words of length 106 bytes for a total of 530 bytes (5 12 user bytes + 4 CRC + 14 control bytes). Another 80 bytes of ECC is appended to this. Including embossed headers information and buffer bytes, the grand total is between 746 bytes for 1X and 2X capacities and 799 bytes for 3X and 4X capacities. The 1024 byte sectors use 10 code words of 104 bytes for a total of 1040 bytes (1024 + 4 CRC + 12 control bytes). To this are added 160 bytes of ECC. The total for 1024 byte sector disks is 1360 bytes for 1X and 2X capacities and 1410 for 3X and 4X capacities. Consequently, the total overhead is smallest for the larger sectors (33-38%) and larger for the smaller sectors (4656%). This fact is pushing the optical community to consider larger sector sizes in the future and momentum is building for a 2048 byte sector size as is used in CD-ROMs. This has been adopted in the 90 mm 5X standard and will be part of the 8X 130 mm standard.

5.0 MAGNETO-OPTICAL STORAGE PRODUCTS AND SUCCESS IN THE MARKETPLACE

Videodisk. The first optical storage products brought to market were videodisk players. These were introduced in the late 1970’s and are readonly devices. Videodisks are 300 mm diameter, double-sided, and provide up to one hour full-motion video with two channel stereo sound per side. The video and audio information are stored as analog signals rather than digital. A low power HeNe laser was used in the first products, while later videodisk players employ semiconductor diode lasers. Low cost video storage onto helical scanning magnetic tape provided significant competition, and optical read-only videodisks were only moderately successful in the market. Later generation videodisks are however still on the market, providing high quality, random access video information.

28

Magneto-Optical Data Recording

Compact Disc. The first truly successful product and clearly the most successful rotating optical storage production in the commercial marketplace, has been the storage of high quality digital audio information on compact discs (CDs). These disks store information in the form of depressions (pits) in a plastic substrate which is covered by thin reflective and protective layers. These pits are replicated into the plastic when the disk is molded and hence are read-only information devices. This optical storage medium was commercially introduced in about 1983 by Philips and Sony. In the last fay years, the computer storage medium known as CD-ROM (CD Read-only Memory) which closely resembles the audio CD has experienced remarkable commercial growth. First MO Products, 5.25 Inch Drives. The first magneto-optical drives were brought to market in 1989 by Sharp, Sony, Olympus, Maxtor, and others. These rewritable drives were 5.25 inch form factor products. They all adhered to the I S 0 and ANSI standard disk cartridge, and disk format giving 650 MB user storage capacity. This allowed customers to buy media for these drives from a variety of sources and to exchange data and disks between drives from various manufacturers. Performance of these products varied. A typical drive had a rotation rate of 2400 RPM, an average seek time of 60 milliseconds, and a read data rate of 5.3 Mbitshec. Writing data rates were half of this since all first generation drives took two passes or rotations to write the disk. One pass was to erase the sector or track and the second to write the data. (Some drives employed specialized controller algorithmsto allow the use of preerased disks, so as to give write data rates equal to read data rates.) The majority of these drives were sold either as subsystems (a drive, power supply and controller all contained in a desktop box), or into an optical library. Prices for these subsystems were from $5000 to $10,000 in the 1989 through 1991 time frame. Disk cartridges were around $250. Later Generation 5.25 Inch MO Drives. First generation MO drives enjoyed moderate market success. They proved to be usefbl to customers as both on-line and as backup devices. In addition, MO disks provided an easy to use, compact, and reliable archival storage medium. This success fueled the development of later generation drives with higher capacity, better performance, and lower prices. In 1993, I S 0 and ECMA approved a double capacity 5.25 inch standard boosting the user capacity to 1.3 Gbytes per cartridge. A typical drive developed around this standard is shown in Fig. 3. This drive, from Hewlett Packard Company, operates with both first generation (1X) 650 Mbyte capacity media and second generation

(2X) 1.3 Gbyte capacity media. Performance levels for this drive were improved; with disk mtatiod speeds of 36W W M ,an average seek time of 23.5 milliseconds,ared readdata rates of 13 Mbits/sec. Writing for this and all second gmmtion drives is still accampfishedwith two passes; m e to erase and one to write data, Prices in 1994 for s m n d gemration 5.25 inch drives w m from $2000 to $3000, disk cartridges were from $60 to $100. These decreases h price, and impmvments in capacity and p e & m hseased the market demand for MO drives. Estimatesare that upwards of 130,000five and it quarter inch MO drives were sold in 1995.

Figure 3. Hewlett Pacbard Madel 1300T.A typical second generation magneboptical drive with a capacity of I .3 Gbytes. (phoro courtesy of Hewlett P d a r d Co.)

In 1996,4X capacity, or 2.6 gigabyte, drives began shipping. Write perfonnanc3e caught read pwflDmaance as m e ofthese drives inclwdgld the 4X IS0 Standard LMDOW option as shown in Fig. 4. The design of these drives allows them to read and mt'emrlisr generation MO disks as well as rfme new 2.6 GB MO standard. They pushed the read and write transfer rate t0 4 Mb@s/'Sec. P h % dropped t0 $lo0&$1500. majority Of fQUrth generation drives are half-high form factor drives.

30

Magnet+Opticd Duta Recording

Figure 4. The first 130 mm L m W 4X capacity (2.6 GB) MO drive. These drives appeared in the markerplace in 1996. Their advent pushed the wrik transfer to equal the read transfermte via the use.of Ihe IS0 standard LIMDOW media option. Most 4X drives were backward compatible with the earlier 2X and 1X capacity disks. (Photo courtmy of MOST Inc.)

3.5 Inch M O Drives. As the physical size of computers has decreased the space for a mass storage device such as an optical drtve has also decreased. To help satis@ the d& fbr s d l e r =rage devices, 3.5 inch magneto-optical prducts were developed and commercialrzed in the early 1990's with Fujitsu as the dominant drive m a n a m . (These are also o h called 90 mm products.) As the name implies, 3.5 inch drives fit into a 3.5 inch form factor computer bay. Because of the smaller size of the disk these products have lower capacities. First generation capacities started at 128 Mbytes, were followed by 230 Mbytes and 3 84 Mbyte capacity products. To provide the smallest possible Cartridge, only single-sided disks are used in 3.5 inch products. In 1995, prices were b e e n $400 and $700 for these drives. Upwards of 1,400,000 drives are estimated to have shipped in 1995. In 1996, LIMDOW compatible 230 MB drives and media began shipping. MiniDisc@Audio Drives. During the 1980's, the compact disc almost completely replaced the vinyl record as the music distribution m&urn. It seemed very appropriate in 1992 for Sony to announce the development a 65 mm rewritable magneto-optical media and drive for recording dig~talaudio. MiniDisc@ is the name trademarked for this

Introduction and Overview

31

prduct, and the first models were small, portablc, pocket sized, recordmg devices. The data version of MiniDisc* stores 140 Mbytes of digital data, The audm version provides up to 74 minutes of bgital audio recordmg. Mignctic field modulation direct ovennnite is used in MiniDisc@to prevent the need for two-pass recordings. A small magnetic head slides on the back of the disk as it rotates. In 1993, prices for MmrDisc@recorders were from $400 to $600, blank recording disks were $12. It is anticipated that these prices will approach the price of read-only compact d w piayers inthe years to come. Magneto-Optical Libraries. Magnetoqtid libraries come in all sizes W n g fiom libraries with a single drive with 6 cartridges,to libraries with up to 6 dtlvcs and hundreds of &sb. This gives computers storage capacities of up to a terabyte, with any piece of data accessible in less than 10 seconds time, at a cost of less than ten cents per Mbyte. One example of a magneto+ptical library is shown in Fig. 5 . This library is from Hewlett Packard Co. and uses 144 5.25-inch MO cartridges along with up to four dnves. It provides up to 200 Gbytes of on-line storage.

Figure 5. Hewlett Packard Optical Library. This library can hold up to four 5.25 inch magnetooptical drives and 144 wtridges and provides access to any one of them in 6 seconds or less. (Photo courtqy of Hewleft Packard Uo.)

32

Magneto-Optical Data Recording

REFERENCES 1. Chen, D., Ready, J., Bernal, E., MnBi Tlun Films: Physical Properties and Memory Applications, Journal ofApplied Physics, 39:3916 (1968) 2. Maydan, D., Mircromachining and Image Recording on Thin Films by Laser Beam, The Bell System Technical Journal, 50:1761 (1971). 3. Gambino, R J. and McGuire, T. R., Enhanced Magneto-opticProperties of Light Rare Earth Transition Metal Amorphous Alloy, J. Magn. andMag. Mat., 54-57:1365 (1986) 4. Imamura, N., Tanaka, S., Tanaka, F., and Nagao, Y., Magneto-optical Recording on Amorphous Films, IEEE Trans. Magn., MAG-21:1607 (1985) 5. Poincad, H., Thtorie Mathtmatique de la Lumitre, Vol. 2, Ch. 12, Gauthiers-Villars, Paris (1892) 6. Bell, B. W., Jr., Mueller Matrix: an Experimental and Analytical Tool for Magneto-optics, Optical Engineering, 28:141 (1989) 7. Bell, B. W., The Poincard SphereDescription of Magneto-Optic Read Out, Joint International Symposium on Optical Memory and Optical Data Storage 1993, P12 (1993) 8. Theocaris, P. S., and Gdoutos, E. E., Matrix Theory of Photoelasticity, Springer-Verlag, Berlin, Heidelberg (1979) 9. Reed, I. S. and Solomon, G., Polynomial Codes over Certain Finite Fields, J. Siam, 8:300 (1960) 10. Choudhari, P., Cuomo, J., Gambino, R., and McGuire, T., “Beam Addressable Film Using Amorphous Magnetic Material,” U. S. Patent #3,949,387 (1976) 11. Connell, G. A. N., Treves, D., Allen, R., Mansuripur, M., Signal-to-Noise Ratio for Magneto-Optic Readout from Quadrilayer Structures, Appl. Phys. Lett., 42:742, (1983) 12. Marchant, A. B., Optical Recording: A Technical Overview, AddisonWesley Publishing Company (1990) 13. Takenaga, M., Yamada, N., Ohara, S.,Nishiuchi, K., Nagashima, M., Kashihara, T., Nakamura, S.,and Yamashita, T., New Optical Erasable Medium Using Tellurium SuboxideThin Film, SPIE Proceeding, 420: 173 (1983) 14. Covault, M., What’s driving hard disk storage cost?, SPIE Proceeding, 25 14:4 (1995) 15. Mansuripur, M., The Physical Principles of Magneto-optical Recording, Cambridge Univ. Press (1995) 16. Saito, J., Sato, M., Matsumota, H., and Akasaka, H., Direct overwrite by Light Power Modulation on Magnetooptic Multilayer Media, Digest of International Symposium on Optical Memory (Sept 1987)

Heads and Lasers David B. Kay and Edward C. Gage

1.0 INTRODUCTION 1.1 Overview of Optical Head Functions The MO optical head is the transducer in a MO optical disk drive. It must provide a small laser spot (-4.O pm FWHM diameter) at the recording media surface, maintain that spot at focus to < *0.5 pm and on the data track to < *O. 1 pm. It must sense the very small Kerr rotations of the linear polarized light reflected from the magnetized domains, within the data track with a high signal-to-noiseratio (SNR). It must also record domains with alternating magnetization into the data track. The challenge is great, but it is met by a combination of a high-power single spatial mode laser diode, a high-performance voice-coil actuator, high quality optics, and detectors packaged together and supported with preamplifiers, laser driver, servo, and channel electronics.

1.2 Layout of an MO Optical Head Figure 1 shows the optical layout of a typical uniJed M O optical head. The incident light path consists of the laser diode source, collimator lens, a beam shaping prism (i.e., achromatic beam expansion prism), a first partial polarization beam splitter (PPBS l), a fine focus/tracking actuator 33

34

Magneto-Optical Data Recording

(not shown) with objective lens, and the optical disk. A front facet detector is shown for the laser power servo. The return optical path consists of the same fine focus/tracking actuator with objective lens, the first PPBS1, a second PPBS2, and two sensors. The first sensor consists of a waveplate, a lens, a Wollaston prism, and two detectors to provide the differential RF data signal. The second sensor consists of a quadrant prism and detector array to provide the servo signals-the focus error signaZ (FES) and tracking error signal (TES). BIAS FIELD

DETECTOR B

Figure 1. Optical path of a representative unified M O head.

MO “split” optical heads (as compared to the above unified head) are now very popular in 3.5” and 5.25” disk drives and consist of two parts; a fixed part and amovingpart. Themovingpart contaitlsatumingmirrorand the fine focus/tracking actuator with objective lens, and is a part of the radial access carriage that travels between the ID and the OD of the disk. The remaining components are stationary in a housing attached to the baseplate. The advantages of a split optical head include: a lower mass radial access carriage, good heat-sinking of the laser and the head electronics, and a

Heads and Lasers

35

reduction in electrical leads to the moving carriage. Section 5 shows examples of split MO optical heads as well as unified heads. On the opposite side of an MO disk is one of two components: (a) an electromagnetto produce a bias field of -300-400 Oe for systems that erase full sectors of data before writing new data, or (b) a magnetic recording head flying very close to the back surface of the optical disk that performs direct overwrite with new data, without the need to first erase.

1.3 Erasing, Writing, and Reading Erase-Then-Write (2-Pass) MO Optical Head. This type of optical head was historicallythe first to be commercializedand is used in many MO disk drives. The recording process consists of two steps and takes two rotations of the disk. In the first step, the bias electromagnet,which is on the opposite side of the disk from the MO head, is switched on continuously with a polarity that is termed the erase biasjield,and the laser diode is turned on at high power continuously in the sectors to be erased. The sectors that pass under the laser spot become magnetized with the same polarity as the erase bias field. In the second step, the electromagnet polarity is reversed to the write biasjield. The laser is then pulsed to write power in the erased sectors with new digital data resulting in magnetic domains that are near-replicas of the laser current digital waveform and are magnetized with the polarity of the write bias field. Hence the recorded data consists of domains magnetized with the write bias field polarity embedded in the erased track of opposite magnetization. The laser pulse is often shaped to improve the morphology of the recorded domains. Direct Overwrite (1-Pass) MO Optical Head This type of MO optical head is used in several new MO disk drives (for example the Sony MiniDisc@). It can overwrite prewritten data tracks in the same fashion as is performed in magnetic disk drives. On the opposite side of the optical disk from the MO head is a small magnetic recording head that fliesjust opposite the laser spot at a distance of 50-100 pm above the disk surface. This magnetic head is modulated with the digital data and produces strong magnetic fields (-75 Oe) needed in MO recording. The laser is either turned on continuously or pulsed synchronouslywith the data clock when magnetic domains are to be written. In this manner, the laser spot defines the domain size to be magnetized by heating it to above the Curie temperature while the

36

Magneto-Optical Data Recording

flying magnetic head defines the domain polarity. This recording process is a form of thermally assisted magnetic recording. Reading. Section 4.2 discusses in detail the readout process. The polarity of the magnetization of each domain along the data track affects the linear polarization of the reflected laser beam. The reflected polarization is sensed by a differential technique that provides high SNR with good common mode noise rejection. The pages that follow discuss each portion of the MO optical head in some depth, and present options available to the head designer. 2.0

LASER DIODES

The maturation of semiconductor laser technology over the past twenty years has been the enabling technology for optical recording. Conversely, the success of a consumer read-only optical recording product, the compact disc player (CD Audio), has made the laser diode the best selling laser of all time. Our goal here is not to review the history or state of the art in semiconductorlasers as there are a number of excellent books that review this subje~t.[~l-[~] Here we concentrate on the laser diode characteristics from the point of view of designing an MO recording system. In the magneto-optical head designs that we discuss, one laser diode provides the light for reading, writing, and erasing the magneto-optical media. Each of these optical head fbnctions places requirements on the laser. The laser diode’s properties are almost ideal for magneto-optical recording. The laser diode has a number of advantages over other laser types.i51 Modulation. The most important feature of the laser diode for optical recording is that it is a very fast transducer of electrical current to coherent radiation. Laser diodes can be directly modulated to GHz frequencies with rise and fall times of under 1 ns. This greatly exceeds the requirements of current optical recording systems. Long life. Today’s laser diodes are manufactured with very high crystal purity and structural quality. This has resulted in mean time to failures (MTTF) exceeding 10,000 hours in the optical recording environment. Section 2.5 discusses laser diode lifetime and reliability.

Heads and Lasers Small size. An example of a laser diode structure is schematically shown in Fig. 2. The laser itself is typically 350 pm long, 200 pm wide, and 100 pm thick. The actual laser diode is dwarfed by its accompanying sealed package, which also contains a rear facet monitoring photodiode (RFM), a heatsink, and the required electrical connections, as shown in Fig. 3. Cost. In high volume, laser diodes appropriate for magnetooptical recording are near or below $30 apiece. This is considerably less expensive than any other laser type, yet the laser is often the most expensive element in the optical recording head. Single spatial mode. Because the laser diode emission must ultimately be focused to a very small spot, it is important that the laser have a very good wavefront. This requires that the laser cavity remains a single-mode waveguide over its operating power range; however, as we will discuss, the laser still contributes to one wavefront aberration, astigmatism. High power. Single transverse laser diodes with output powers at or above 100mW are commerciallyavailable. The optical irradiance at the front facet of the laser diode is on the same order as that of the focused spot at the optical recording layer, lo6 W/cm2. The impressive ability for the laser to continuously tolerate these power densities and not sustain catastrophic optical damage (COD) is a testimony to the remarkable progress in material growth technology. Ed’ciency. Today’s laser diodes convert 10-40% of their input electrical power to optical power. This is a high conversion efficiency compared with other laser types. This allows the design of high speed, low noise, compact laser driving circuitry and minimizes the heat generated by the laser diode. Because the laser drive dircuitry includes fast current pulses for writing (rise times on the order of 5 ns) and high frequency injection, see Sec. 2.4, electromagnetic interference considerations often dictate that the laser and laser driver circuitry be in closeproximity. Any heat generated by the laser diode and its associated circuitry must be efficiently removed in order to avoid reducing laser lifetime, see Sec. 2.5.

37

38

Magneto-Optical Data Recording

2.1 Laser Diode Design Before we discuss the disadvantages of laser diodes, we need to review the laser diode design. An example of a laser diode structure is schematicallyshown in Fig. 2. The laser shown was discussed by Kagawa[6] and is a good example of a device designed for optical recording. The layered structure is grown on the substrate in an ultrahigh vacuum.

a) End View

Anetal contact -p-GaAs contact layer I I n - G a A s current blocking layer buffer layer

-AIGaAs cladding layer

-AIGaAs active layer n-AIGaAs cladding layer \n-GaAs substrate -metal

b) Side View

Wire bond to laser cathode

contact

Front Facet

Rear Facet Coating

350 prn

Figure 2. An example of a laser designed for optical recording; the inverted inner stripe laser with a p-GaAs buffer layer is shown.[6]The laser is shown with the growth direction up in (a) and the typical junction down mounting in (b). Objects are not to scale in the growth direction.

Heads and Lasers

39

Upon applying a current to the laser diode structure, electron holes are injected from the pdoped material and electrons are injected from the ndoped material into the active region. The potential barriers in the cladding structure cause the holes and electrons to remain localized in the active layer. This leads to a population inversion in the electronic band structure, which is necessary to produce gain. In order to improve the efficiency of the device, the mobile charges are c~nfinedin the lateral direction by a higher resistance current blocking layer. The region of gain is on the order of a micron in both dimensions down the length of the structure. The structure is also designed to act as an optical waveguide, so that the light is confined to approximately the same volume as the population inversion. Optical confinement is obtained by using material in the cladding layers with lower index of refraction thanthe active region. Lasers with this type of optical confinement in the plane of the junction are called index guided lasers. (Early laser diodes used the gain to localizethe optical mode in the lateral direction. These gain-guided lasers were less efficient and had poor beam quality.) The end mirrors of the laser oscillator are formed by cleavingthe wafers to the appropriatecavity length and applying coatings to modify the reflectivity. Typicallya highly reflective coating is applied to the rear facet and a much lower reflectivity on the front facet so that most of the light (96% for the coatings of R = 90% and R = 12% in Fig. 2) is emitted toward the optical disk. Devices with this simple structure to provide feedback are called Fabry-Peroflasers. The laser chip is mounted into a laser package with a glass window that also contains the laser heatsink, the rear facet monitor, the required electrical connections, and provides the laser a hermetically sealed environment. An example of the common “g7’package is shown in Fig. 3.

2.2 Operating Characteristics With an increasing forward bias current applied across the laser diode structure, the device reaches a population inversion, which increases until the gain of the laser medium is large enough to overcome the losses in the cavity and at the facets. Above this threshold currentIrh,the device is lasing and the light output is a linear h c t i o n of applied current with a slope efficiency77. Figure 4 shows a plot of optical power emitted from the front facet ofthe laser versus applied current. Typical values of& are 20-80 mA and of 77 are 0.4-1 .OmW/mA for index guided lasers. If the applied current is increased further, the power curve will eventually start to roll over

40

Magneto-Optical Data Recording

because of thermal effects in the laser and the laser may suffer damage. The power level where damage or nonlinearities occur should be well above the instantaneous power regions required by the optical head design.

Glass Cap Heatsink Laser Chip RFM photodiode Stem

Figure 3. A cutaway view of a laser diode package is shown. (Courtev of S h a v C O ~ . ) [ ~ ]

The wavelength of the laser is determined first by the material composition, and secondly by the cavity structure, length, and facet coatings. AlGaAs lasers such as that shown in Fig. 2 typically lase in the wavelength range from 770-850 nm. Devices that lase in the 670-695 nm range are also on the market and will be an important consideration for fiture generation optical recording systems. Because of the difficulty of obtaining strong optical and electrical confinement, these shorter wavelength InGaAlP lasers require a more complex laser structure to provide adequate power (220 mw) for most MO head designs (see Ch. 9 of Ref. 4). Hence these devices will initially be applied to high performance optical recording applications, where the additional cost is justified.

Heads and Lasers

0

50

100

150

200

41

250

Injection Current (mA) Figure 4. Output power is shown as a function of injection current for a 780 nm laser rated for powers up to 35 mW. The insets show the optical spectrum at (a) 2 m W and (b) 30 mW.

At threshold, the gain becomes equal to the loss at the peak of the gain curve as shown in Fig. 5 . The Fabry-Perot resonator provides constructive feedback at wavelengths that obey

where L is the laser cavity length, q is an integer, and nefis the effective index of refraction. At wavelengths where there is constructive feedback and the gain exceeds the loss, the laser modes will begin to lase. An example of low power multimode operation is shown in the inset (A) of Fig. 4. Forthe laser shown in Fig. 2 with A = 785 nm, L = 350 pm, and ng= 4.0, the laser modes will be separated by 0.2 nm as calculated from Eq. 1. As the laser injection current is increased, mode competition and saturation effects modify the laser mode structure. For index guided lasers, the longitudinal

42

Magneto-Optical Data Recording

mode (i.e., laser wavelength) is typically a single or nearly single-mode at higher powers as seen in inset (B) in Fig. 4.

t MODES \

\

/LASING

d

F R EOUENC Y Figure 5. A schematic of gain curve, cavity loss, and longitudinal modes for a semiconductor laser.

Because temperature modifies both the cavity length and the laser gain curve, the laser wavelength is a strong function of temperature. At higher powers where the laser is predominantly single-mode,the wavelength is a staircase function of temperature (see Fig. 6). Typical average rates of change are 0.2-0.3 d o c .This is caused by the laser gain peak moving to longer wavelength at higher temperatures. Within each relatively flat step, M A T is about 0.07 d o c ,caused by the index of refraction and the laser length changing with temperature. As we discuss in Secs. 2.4 and 3.1, a more serious consideration for the optical head design is that the laser wavelength is also a function of laser injection current or power. Injection current modifies the index of refraction as well as the gain curve. The 3 nm shift for the 28 mW power change in Fig. 4 is typical for most AlGaAs lasers (0.1 nm/mW). Thus, as the laser is modulated from read to write or erase powers, the laser wavelength will be changing on the order of 1-3 nm. These wavelength variations associated with laser diodes are a disadvantage that places limitations on the selection of optical elements and head designs.

Heads and Lasers

43

2.3 Laser Diode Beam Properties As seen in Fig. 2, the waveguide structure is quite different in the direction parallel and perpendicular to the growth direction. At the output facet of the laser, the transverse optical mode is approximately 1 by 3 pm. This asymmetry in the waveguide leads to different divergence angles in the far field, astigmatism of the output beam, and different effective indices for TE and TM polarizations. The different reflectivity of the polarization states at the facets is responsible for lasers typically being preferentially polarized in the plane of the junction (TE). 795

.'

n

E

x a

790

0

E a

h 785

---

-

-.-

A -

Po = 2 mW

-R.-

780

Figure 6. Average wavelength versus temperature at output powers of 2 mW and 30 mW.

Typical far field distributions for the structure of Fig. 2 are shown in Fig. 7. In the direction perpendicular to the junction (direction of growth), the optical mode is well confined in the waveguide. This small near field leads to a large divergence angle in the far field. Typical values for fill width half maximum (FWHM)divergence angles 0, are 20" to 35O. Confinement is weaker parallel to the junction leading to a larger near field

44

Magneto-Optical Data Recording

and a tighter far field with FWHM angles of 4,= 8" to 15". In most cases, the fkr field distributions are well represented as Gaussian beams. The laser output beams aspect ratio is defined by

Eq. (2)

AR =eL/ell

with typical values of AR from 2 to 4. This asymmetry in the laser emission leads to trade-off in head design (Sec. 3.1) between adding additional complexity in the optical head or accepting lower head efficiency, or working with a larger spot size at the disk in one direction.

Figure 7. Typical far field patterns are shown for emission parallel and perpendicular to the heterojunction. (Courtesy of Mitsubishi C0rp.)[~1

Heads and Lasers

45

Another disadvantage of laser diodes is that they exhibit a wide variation of operating characteristics from device to device. This can be seen by looking at the specifications in any laser diode handbook. An example of these types of specifications are shown in Table 1. Accommodating these ranges of device characteristics leads to a trade-off in the optical head design. This is of particular concern with variation in divergence angles. As we discuss in Sec. 3, this requires that the optical path severelytruncatesthe laser beam to avoid excessive spot size variation at the disk. Table 1. Example of Laser Manufacturer's Specification

Peak power Wavelength Threshold current Slope efficiency

30 mW

830 *20 nm 60 *30 mA 0.5k0.2mWlmA 1 1 *3" 25 k7" 0.08 d m W 0.3 nm/"C

New laser growth techniques are significantly reducing these variations as shown by Kagawaf6]and Nakata.f91Most new laser structures are being grown by molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). A comparison of the scatter plots of divergence angles grown by LPE and MOCVD is shown in Fig. 8. Both of the newer techniques, MOCVD and MBE, offer better structure control and allow the growth of thinner layers, such as required for quantum well lasers that are described in Z0ry.[~1These techniques also allow novel new structures that may be able to reduce the aspect ratio of the output beam. Cockerill et a1.[l01and Kahen et al.f1l1have shown that a depressed index cladding structure can reduce the aspect ratio. For fiture optical recording applications,new structuresare desired that address the requirements of low aspect ratios and high powers.

46

Magneto-Optical Data Recording

The asymmetry in the waveguide also leads to astigmatismin the laser emission. If the laser beam is focused with a simple lens as shown in Fig. 9, the minimum spot size occurs at different positions. It appears as if the diverging laser diode beam is emitted from different points parallel and perpendicular to the emission. Figure 10 shows the beam width as a function of laser position for the setup in Fig. 9. The application note “Astigmatism Measurements Made Easy”[121 from Photon Inc. provides detailon the experimental setup. 29

el

(deg.)

28

-

27

-

Conventional LPE Growth

26 25 24 23

-

21 22 20

..

.

.

.

0

0

-* 00

O

0

0

0

8> ‘8

-888

.:t

.a.

. . o

I

.?...8 0

0

o . g ( O

0

0

-

.

0. 0 Q .

*

n = 100

I

P

o .l

I

I

I

29 28 27

-

26 el(deg.)

New MOCVD Growth ..a O

“

“pglpD.

n = 100

8

25-

23 24

22 21 20

-

1g9

I 10

I

I

I

71

12

13

1

4

Figure 8. Beam divergence angle distributions are shown for devices similar to the structure in Fig. 2 grown by LPE and MOCVD from Kagawa.t61 (Used with permission.)

Heads and Lasers Lens Laser

47

Scanning Slit Beam Profiler

Photdetector

Figure 9. A simple setup to measure laser diode astigmatism is shown.

The gain and therefore the complex index of refraction varies spatially in the waveguide, particularly parallel to the junction. Casey and describe how this leads to nonplanar phase fronts in the laser emission. In gain-guided lasers, it is this complex index variation that confines the optical mode in the lateral direction and hence astigmatism can be quite large. The apparent emission points are separated by 10-50 pm. For index-guided lasers, the apparent separation is typically much smaller, 110 pm, but is still of concern for optical head design, as discussed by Milster et al.[l4] The measurement of Fig. 10 shows an astigmatism of 2.5 pm for the device illustrated in Fig. 2. Values less than 5 pm typically do not significantly reduce the spot quality at the optical disk.

Laser Diode Position Cum) Figure 10. The beam’s full width half maximum is plotted as a function of laser diode position, with arbitrary origin, for the setup in Fig. 9. This device shows 2.5 pn of astigmatism.

48

Magneto-Optical Data Recording

In optical heads like that shown in Fig. 1with beam expansion prisms, a slight decollimation of the beam at the inclined surface can compensate a small amount of astigmatism. This would seem to allow much larger amounts of astigmatism in optical heads with beam expansion prisms. Unfortunately, index guided lasers that show a significant amount of astigmatism, 25 pm, are sufferingfrom some gain-guiding at low powers, so that as the power is increased, the gain coefficient flattens spatially, and the devices are more index-guided. This results in a lower astigmatismat higher powers as seen in distributions of astigmatism measurements shown in Fig. 11. This change of astigmatism with power, an average of 4.4 pm for the 350 m W change with the devices in Fig. 11, is a major concern if the head design used decollimation at an inclined surface to remove astigmatism. At either high or low power, the spot quality at the disk will be degraded. I

N

I 3 0

Minimum Maximum

10 8

*T-H+wUn 2

0 0

10

4

6

~

8

10

-

8--

6--

# of Lasers 4

--

2

--

0

-

Maximum 7

-

-

-

Std Deviation

-

-

I I I I

I I I I I I I I 1 I 1 I - L I I I

Figure 11. Distributions of measured astigmatism for 30 index-guided lasers are shown for output powers of 3 mW and 50 mW.

Heads and Lasers

49

The polarization ratio of the laser diode r, given by the ratio of TE intensity to TM intensity, is typically about 10 at low powers and can exceed 100 at high powers. Figure 12 shows the polarization ratio as a function of power for two different numerical aperture (NA) collimating lenses. Because the TM light is predominantly spontaneous emission, which is not as directional as the TE light and is relatively constant above threshold, we see that r increases with power and lower NA In the magnetooptical head shown in Sec. 1.2, the polarization state to the disk is cleaned up by the partial polarization beam splitter (PPBS 1 in Fig. 1). Hence, the presence of a nearly constant TM intensity should not cause any problems. Thus the polarization ratio of the laser is not a major concern for most optical head designs.

.-c0.

e

C

.c9 0 N

'C

-0 a.

0.5

1.0 2 5 10 20 Optical output power, PO(mW)

Figure 12. The polarization ratio for a 15 m W 830 nm laser is shown as a function of output power for two different numerical aperture collimating lenses. (Courtey of Hituchi Corpomtion.)['s]

50

Magneto-Optical Data Recording

2.4

Laser Noise

Both the intensity and wavelength of the laser fluctuate. Intensity noise can reduce the signal-to-noise of the readback process and introduces noise into the servo signals. Wavelength fluctuations place strict requirements on an achromatic head design. Even in a welldesigned MO head, these laser fluctuations can be the limiting hctor on system pefiormance. At read powers in the MO head design, the laser diode is only about 10% above threshold (1-3 mw). At these low powers, an index-guided laser is typically multimode, with typical wavelength spreads on the order of a nanomekr. As the device is switched to a write or erase power, the laser moves to longer wavelengths with fewer participating modes, as shown in Fig. 4. The randomness associated with this wavelength change is shown in Fig. 13. The wavelength from pulse to pulse in the write train can also vary leading to degradation in the recorded data if dispersion is present in the oytical head .[l61f1 71 2.5 MHz write pulsa train (12 mW write. 0.8 mW read)

optical spectra in 10 ns window of the write pulse train

Figure 13. Typical optical spectra are shown for three different times in the write pulse train: (a) read level, @), (c) leading edges, and (4 trailing edge of the write pulse.

Heads and Lasers

51

This combination of systematicand random wavelength changes with power and temperature lead to broadband requirements on optical coatings in the optical heads and an achromatic incident and read path. Dispersion of the optical elements in the optical head must be minimized so that the optical spot is at the same position as the laser switches from read to write or erase. The read path must be carefully designed so that wavelength fluctuations do not degrade the SNR. The intensity noise is quantified as the laser’s relative intensity noise (RIN) as given by

where ([AP(f)12)is the average square intensity variations at frequencyf in a bandwidth B and (P) is the average intensity. For an isolated laser with no optical feedback (OFB), the intensity noise can be quite low even at low powers and decreases further at higher powers. Figure 14 shows the RIN of an isolated laser vs power. At read power in the MO head, the laser power is on the order of 1-3 mW. For isolated lasers, typical RIN values at read powers are -120 to -130 dB/Hz. When the laser is powered by a constant current, the laser noise is typically white (no frequency dependence) at frequencies in the range of interest for readback (0.5-10 MHz). The impact of this white noise on the signal-tonoise ratio (SNR) of the read channel is treated in Ch. 8. Typically, RIN levels of -1 10to -120 db/Hz can be tolerated before the system performance is significantly impacted. For comparison, in write-once systems, RIN values of less than -120 to -130 db/Hz are desired. In an optical recording environment, reflections from the optical media are fed back into the laser cavity. This optical feedback (OFB) can make the laser unstable and increase the laser RIN by more than 20 dB, as shown in Fig. 14. For MO heads, typically 2-10% of the laser output is reflected back to the laser as OFB. OFB levels are determined by the head efficiency and the media reflectivity. An example of RIN as a function of OFB is shown in Fig. 15. The effects of OFB depend on the quantity of OFB, the optical path length, the laser structure, temperature, and laser power. Petermad21has catalogued the OFB effects into five regimes. In an MO head, the laser dynamics are described by the three regimes of mode hopping, coherence collapse, and locking to the external cavity. The

52

Magneto-Optical Data Recording

former two regimes result in large increases in the laser RIN,while the latter results in very quiet operation. This type of low RTN operation as seen at 2% OFB in the solid curve in Fig. 15(a) is very sensitive to OFB level and phase. With certain combinations of temperature, power, and feedback, the laser operates very unstably near this rezime leadin2 to high noise and irreproducibleresults. This makes the external cavity regime very unstable and undesirable for optical recording. This behavior as well as the RIN increase with OFB has been shown to be caused by chaotic windows of laser dynamics by Gray and cow0rkers.[~~1[191

0

1

Power (mW)

2

3

Figure 14. Laser FUN measured at 5 MHz is shown as a function of laser power with and without OFB.

The OFB also changes the efficiency of the laser so that the laser needs to be operated in a constant power mode in the optical head. OFB can also disturb the relative outputs between the front and rear facets of the laser, so for precise laser power control the front facet emission is often monitored. This constant read power servo should have a bandwidth of a few KHz, so that the focus and tracking servo signals are not impacted by laser intensity fluctuations. Extending the bandwidth of this servo to quiet the laser to frequencies above the data channel has been demonstrated in

Heads and Lasers

53

principle by DeCaro[201and Satoh.f2l1The difficulty of producing a servo with bandwidth on the order of 10 MHZ and very low noise has prevented this technique from being used in commercial products.

(a)

........................

-100

-

h

N

L

m‘

-110

E

z

...........

a -120 -130

--).

@>

h

-C

-300 MHz 480 MHz -600 MHz

-.

--- - -.._ - 700 MHz 1000 MHz -&

-1 10

-115

N

s

E

-120

zU

-125

-130

Figure 15. Averages of experimental measurements of FUN at 5 MHz as a function of OFB is shown at 1.6 mW and an optical path length of 100 mm for the laser structure shown in Fig. 2. (a) With a fixed 480 MHz modulation frequency and four modulation depths. (b) Five different HFI modulation frequencies with 40 mA peak-to-peak depth of modulation.

54

Magneto-Optical Data Recording

To suppress the feedback-induced FUN, a technique known as high frequency injection (HFI) is often empl0yed.[181[1~1With this method, a sinusoidal current modulation at a frequency much higher than the data rate is summed with the laser injection current. (Other HFI waveforms have also been studied by Hendricks et a1 )[221 The experimental results show that the RTN increase does not occur if the modulation fiequency is suitably optimized and if the modulation amplitude is large enough to ensure that the laser is below threshold during part of the HFI modulation cycle. An example of the RIN improvement with optimized HFI parameters is shown inFig. 15. The ultimate goal of HFI technique is to make the laser intensity noise resilient to changes in OFB. There have been different interpretations of the mechanism by which HFI achieves this goal. Some explain that HFI works because, at the time the feedback returns to the laser cavity, the modulation has turned the laser oE[221-[261Others argue that HFI changes a laser operating in a single predominant mode, but suffering from mode hopping made worse by OFB, into a stable multimode laser.[271[281Yamada and HigashP91argue that HFI acts similar to spontaneous emission to weaken the competitionbetween modes; HFI is thus effective because it suppresses mode hopping. All of these methods have provided useful qualitative information on understanding HFI, but none have been successful at predicting optimum HFI parameters for the range of conditions in the optical head. These optimum parameters, frequency and depth of current modulation, are found empirically in the conditions of the optical head, laser read power, temperature, and optical feedback levels for the particular laser device being considered. An example of this experimental search for the HFI parameters that minimize the laser noise is shown in Fig. 15. For this path length (100 mm) and laser power (1.6 mw), the optimum condition for this laser seem to be an injection frequency of 480 MHz and 50 mA peak-to-peak modulation. These values produce a stable RIN of less than -125 dB/Hz for less than 10% OFB. Recent work[181[191has demonstrated that the HFI-induced noise suppression can be understood via a rate equation model that includes spontaneous emission, shot noise of the camers, optical feedback, and HFI. Figure 16 shows an example of the model’s predictions for the optimum frequency for the conditions of Fig. 15. In agreement with the experiment, 480 M H z appears to be a suitable modulation frequency. This work explains that HFI suppresses the onset of OFB-induced chaos. This

Heads and Lasers

55

suppression restores the RIN to the case for the isolated laser and produces a multimode laser with mode partition noise pushed to frequencies well above the concern of the data channel. In agreement with the experiments, this numerical integration technique also predicts that increased HFI modulation depth increases the OFB level at which RIN suppression is incomplete. Thus for an MO drive with 1.6 mW of laser power at read, with a 100 mm optical head path length and 4 0 % OFB, this laser would have 480 MHz and 50 mA peak-to-peak for optimum HFI conditions. This model holds promise of being able to provide quantitativetheoretical predictions of optimum HFI parameters.

-118

-120 -122

8a 3 n

-124

W

-126 -128 -130

200

400

600

800

lo00

1200

Modulation Frequency Figure 16. Dependence of low-frequency RIN on HFI modulation frequency for three levels of OFB external reflectorE,, from the model of Gray et a1.[1*1[191Modulation depth is 20 mA peak-to-peak.

2.5 Laser Diode Lifetime and Reliability Because of the desire of the optical head designer to maximize the availablewrite or erase power, the laser reliability is an important MO drive

56

Magneto-Optical Data Recording

design consideration. It determinesthe maximum available laser power and tolerable temperature rise in the optical head. Because laser failure mandates the optical head be replaced or reassembled, very reliable and longlived lasers are required. Newer growth techniques and laser designs have resulted in some lasers with mean time tofailures (MTI'F) of greater than 100,000hours at 100 mW and 25°C.[301Mean time tofailure is the average time at which the lasers under study reach a failure condition. As the laser degrades with age or misuse, the device's optical properties at the facets or in the waveguide, or the thermal or electrical properties of the structure become Two examples of misuse that can result in sudden device failure include a diode being damaged by electrostatic discharge (ESD) or the device being overdriven to catastrophic optical damage (COD). ESD demands that lasers be h d l e d carehlly in optical head assembly and are protected by a shorting circuit when not being operated. The laser device should only be operated at currents or optical powers well below the occurrence of COD. As a laser diode ages under normal operation, the dominant change of device characteristics is a loss of conversion efficiency. Benedict and have shown that the optical properties remained relatively unchanged as the laser threshold increases and slope efficiency decreases. ?hemulbngkreaseinoperatingcurratmpmdtoreachagivenoutputpowerand temperature is used to define the laser lifetime. Figure 17 shows lop as a functionof time for a typical 780nm laser diode designedfor optical recording. When the operating current has increased by a fixed percentage (manufacturers' use values between 20 and 50%) the device is said to have failed.

330

2

310 ;

Tc- 6D' C Po =30mW/CW

290; 270 y

260

-

lopes

5 230:

5 " .-P

"

!

O

210: 190-

f

170 150130 110 90 7 70b

-

sot '

0

1

1

lo00

2000

3000

4Ooo

t

500(

Operating TKne lhr.1

Figure 17. ZOp vs time at a constant power of 30 mW at 60°C. (Used with permission.)

Heads and Lasers

57

Laser failures are described as occurring at three different periods: infant mortality, random failure, and wear-out. Infant mortality is considerably reduced by burning-in the lasers at an elevated temperature for some period of time from 10-200 hours and then screening the Experimentally it is found that the failure rate for random failures and wearout is well described by log-normal Figure 18 shows an example of cumulative failures vs time for a 30 mW, 830 nm laser. The failures are well described by log-normal statistics with an MTTF of 13.5 kHr and a median life tmof 10 kHr. Median lifetime correspondsto the time at which 50% of the lasers have failed. MlTF alone does not adequately describethe failure rate. It is usually desired to have a quantitative measure of early failures. A time tl equal to the time when 15.9% of the lasers have failed (see Fig. 18) is convenient, because then the MTTF is given by

where cT= h(tm ltl)

describes the slope of the log-normal fit. In many life tests, insufficient failures occur to predict the MlTF. An estimate of M l T F can be obtained to the failure condition or a lower bound to by extrapolatingthe change in lop M"TF can be set from random failure statistics.[31] Temperature and laser power are important parameters in detennining laser degradation rate. As a h c t i o n of temperature, lifetimes are found to scale by an Arrhenius type equation as given by

MTTF(T,) = MTTF(T,) e

where E, is the empiricallydeterminedactivation energy and k is Boltzmann's constant. Typical values for E, are 0.4-2.0 eV. The laser power scales the MTTF as

where n is experimentally determined with common values being 2-4. If these effects scale the degradation rate, is expected to be unchanged. All

58

Magneto-Optical Data Recording

of these lifetime scalings require that the changed conditions do not bring in new degradation mechanisms. An example of M'ITF as a contour plot of power and temperature is shown in Fig. 19 for the case of Fig. 18 with E, = 0.8 eV and n = 3.2.

Id

103

104

l@

Time (hours)

Figure 18. Cumulative failures are shown vs time as a log-normal plot for a 30 mW, 830 nm laser.

40

T ("C) 60

ab

Figure 19. A contour plot of log M'ITF in hours as a function of laser power P and temperature T.

Heatls and Lasers

59

The above information, MTI'F as a function of CW power and constant temperature and possibly COD levels, is information typically provided by laser diode manufacturers. But for the optical head design additional questions remain. Because the current supplies for read and writderase are often different, the degradation of Ithand are each important and not just the change of lop.A laser model of efficiency is often employedto estimatethe separate degradations. If HFI is on when the laser is at the higher powers of write or erase, how will this effect laser lifetime? We usually assume that if HFI does not cause the laser to approach COD levels it will not effect lifetime. How does lifetime scale for pulsed operation as occurs during writing to the disc? The appropriate scaling here depends on the frequency and duty cycle of the laser pulses. For typical data rates, the cumulative time that the laser is at the higher power causes degradation, so that lifetime scales as the duty cycle for writing. Even with these assumptions, it is difficult to answer what the expected lifetime of the laser is in the optical drive because of the uncertainty of the time the laser will spend reading, writing, and erasing. Also the laser temperature will depend on the laser's operating environment. The complete drive design must anticipate operating conditions and laser failure statistics to determine the required MTTF and acceptable early failure rate for the laser.

3.0

INCIDENT LIGHT PATH

3.1

Laser Beam Collimating and Shaping

Collimator Design. The beam emerging fiom the laser diode is typically collimated by a 0.3-0.4NA lens with -7 mm effective focal length. This lens is chosen to collect the laser diode beam which is then directed to a beam expansion surface like the one shown in Fig. 22. The distance between the laser and the collimator lens must stay constant over the operating temperature range of the head so that astigmatism is not introduced through beam decollimation on the beam expansion prism (BEP) refractive surface. Figure 20 shows the R M S wavefront error of the collimated beam as the temperature was changed for two different laser collimators plus beam expansion prisms.

60

Magneto-Optical Data Recording

Collimator #l

- - - - Collimator #2

Temp (Deg. C)

Figure 20. Collimated and expanded beam wavefiont error vs head temperature.

Figure 2 1 shows a schematic of the laser-collimator subassemblies, only a different metal was used for the collimator barrel and lens barrel in each case and hence the thermal coefficients of expansion were different. When thermal compensation is achieved (i.e., the lens back focus is maintained at the laser emission point) the beam remains collimated over temperature changes. Beam Shaping/Expanders. Many optical heads use a beam expansion prism to produce a beam that is round in cross section. The laser diode beam from the laser, and hence the collimator lens assembly, is typically elliptical in cross section. This beam ellipticity is due to different values of 6, and (see Sec. 2.3). This beam is refracted by the prism's inclined surhce and emerges round in cross section from the output surface of the prism. Older heads would achieve the expansion with two Littrow prisms, newer heads use a single expansion prism (Fig. 22). The beam expansions (Exp.) achieved in the two cases are given below:

q,

Eq.(8)

Exp. = n2, (Littrow prism pair)

Eq. (9)

Exp. = cos(y)/[l

- n2sin2(y)]",

(standard prism)

Heads and Lasers

61

where n is the index of refraction of the glass of the prisms, and y is the y = cot?(n) and a pvertex angle. In the case of the Littrow polarized beam is transmitted, with no reflection losses, at the entrance surface of the prisms. In the case of the standard prism, a multilayer thinfilm coating must be used to reduce reflection losses at the entrance surface and y is dependent upon both Exp. and n. Marchant discusses several versions of single beam expansion prisms (USPatent 4,759,616).

/

Head Body

Barrel

Laser Diod Collimator Barrel

- .

Figure 21. Schematic of laser-collimator subassembly.

Standard Prism

Figure 22. Beam expansion prisms.

Littrow Prisms

62

Magneto-Optical Data Recording

Achromatic Beam Expanders. Today most optical heads that record data use achromatic beam expansion prisms as discussed by Kay et al.[l71 When the laser wavelength shifts a few nanometers because of the change from read to write power, and also when the laser is mode hopping, significant beam pointing changes will occur unless the prism is chromatically corrected. The prism can be achromatized by combining it with the The beam polarization beam splitter prism element to form a prism is first incident on a crown prism that is followed by a flint prism. Figure 23 shows such a design to be used with a 785 nm beam that provides a 2.21 expansion ratio.

(crown glass) Collimator Lens Laser Diode

Figure 23.

Achromatic beam expansion prism system.

Figure 24 shows beam pointing as a function of wavelength for the design of Fig. 23. Beam pointing error is very small over a wavelength range of *lo nm,with a pointing error of 50.3 prad/nm. Two additional achromaticbeam expanders are shown in Figs. 25(b) and 25(c). These two prism are derivatives of the one shown in Fig. 25(a) as can be seen when their internal reflections are unfolded as shown in Fig. 26. In these figures, C indicates the crown glass, F indicates the flint glass.

Heads and Lasers

63

0.00090

0.00020

4.00020 785

no

ns

780

78s

780

m

800

806

Luer Mode Wavelength

Figure 24. Beam pointing angle as a function of wavelength for the achromatic beam expansion prism shown in Fig. 23.

Figure 25. Three types of achromatic beam expansion prisms.

cpF

PEy

Figure 26. Achromatic beam expansion prisms (b) and (c) are optically unfolded to show how they are derivatives of (a).

64

Magneto-Optical Data Recording

3.2

Optical Head Efficiency

Efficiency Calculation. The efficiency of the p-polarized light path from the laser to the disk (Efl{L + d}) depends on several factors. These factors are best presented with the aid of an example. Referring to the optical path of the head in Fig. 1, the following efficiency formula applies:

where (Eff. of laser diode collimator)refers to the fraction of the laser diode beam that is collimated out of the collimator lens and specular and usefid to the writing and reading process vs scattered light and losses, GEsrefers to the light transmitted by the beam expansion surface, TppBsl is the ppolarized transmission of the partial polarization beam splitter surface, Tmcod is the transmission of each AR-coated surface in the incident beam path with n surfaces not including collimator and objective lens, Gbjis the transmission of the objective lens, and Trunc. is the fraction of the beam truncated by the objective lens pupil. The beam truncation is dependent upon the beam apodization and the spot size required at the disk surface and is discussed in detail in the next section. Using a laser diode collimator efficiency of 0.87, GEs= 0.94, TppBsl = 0.80, Trunc. = 0.20, Tmcoat=0.995, n = 1 surface, and Gbj= 0.97, the Eff.(L + d) = 0.505. This is a typical throughput efficiency for an MO head. WORM heads are usually higher in throughput efficiency (-0.62) because of a very high TppBsl = 0.99. These figures may be increased if the BES surface is also AR coated. The partial polarization beam splitter coating is optimized to reflect 99.9% ofthe s-polarized light returning from the disk along with a sufficient quantity of p-polarized light (here 20%) for the data signal detection process, while still providing a reasonably high throughput of p-polarized light from the laser diode to the disk for writing. The PPBS 1 optimization is discussed in Sec. 4.1. Truncation and Beam Expansion. The beam that is emitted from a laser diode is nearly Gaussian in the X and Y meridonal intensity profiles (see Sec. 2.2) and deviations from a Gaussian profile primarily show up in the tails of the distributions, that is, beyond the e-2 intensity levels. The pupil of the objective lens always truncates a portion of the Gaussian beam in a manner that results in a compromise between spot size and head throughput efficiency (see Fig. 27). Using the beam equations in the article

Heads and Lasers

65

by Mar~hant,[~~] a beam’s irradiance emerging from the collimator lens and incident on the objective lens is given by:

Es. (11)

I v , r ) = I, exp(-&/R2) exp(-aP/p2R2)

where R is the radius of the objective lens R = (f.l.obj)(NAobj),a is an apodization parameter wherein the beam is truncated at the exp(-a) intensity level in the X dimension of the objective lens aperture, and the beam ellipticityis parametized by p (the ratio of the Y diameter of the beam to the Xdiameter of the beam), and where f.l.obj= focal length and NAobj= lens numerical aperture. The value of p 1 is achieved by using a beam expansion prism as discussed in Sec. 3.1.

-

Figure 27. Gaussian intensity distribution at the objective lens pupil in the incident path (a=1.2,p=1>.

The beam emerging from the objective lens is focused to a spot on the disk recording layer. Spot size is measured in terms of the full width at the half maximum irradiance levels in the two meridians, FWHM,and FWHM,. The FWHM,is shown in Fig. 28 along with the isometric plot of the irradiance distribution in an aberration-free spot. Figure 29 presents a plot of the on-axis (optical axis) normalized irradiance of the focused spot Ir/Io at x = 0, y = 0 for p = 1.O vs a. As pointed out by Mar~hant,[~~] a round beam truncated at a= 2.5 provides the maximum on-axis (on the optical axis of the objective lens) irradiance at the

66

Magneto-Optical Data Recording

recording layer. This is important for using the diode laser light efficiently in marking the recording media.

1

0

X Spot Meridional Profile

Figure 28. Focused spot irradiance distribution at the surface of the recording media along with an x-axis meridianal profile and width.

Heads and Lasers

67

- - - FWHM . . . ...... On-Axis lrradiance

Truncated Power

1

0.6

0.95

0.5

0.9 0.4

0.85 0.8

0.3

0.75

0.2

0.7

0.1

0.65

0

0.6 0.5

I

1.5

2

2.5

3

3.5

4

Alpha

Figure 29. The relationship of truncated power, spot FWHM,and on-axis irradiance to a parameter for a rotationally symmetric Gaussian beam,NA = 0.55, = 785 nm.

the

However, achieving the smallest spot FWHM with a given lens NA and laser wavelength is also very important in order to maximize the data density in the disk. This second consideration leads head designers to truncate the collimated beam more strongly than a = 2.5. Figure 29 shows the spot FWHM, on-axis normalized irradiance, and truncated power vs a. Truncation values of a = 1+ 1.5 are typically used. If the laser diode beam divergence tolerances A@, and A@,,are small, then a larger value of a can be used and head throughput efficiency can be increased. A smaller a value is used when a laser diode has large beam divergence tolerances from one unit to the next in order to reduce the spot size variation from head to head at the expense of head throughput efficiency. Truncation Without Beam Expansion. Several recent MO heads do not use a beam expansion prism in order to simplifjr the head design, its alignment, and reduce the number of parts. However, the throughput efficiency is reduced and therefore a higher power laser is needed for

68

Magneto-Optical Data Recording

recording. By using an increased level of truncation (smaller a)a reasonably round and small spot can be achieved without a beam expansion prism. Figure 30 shows truncated power vs beam ellipticity p for a particular optical head layout that uses a 0.5 NA (4.35 mm f.1.) objective lens and a 0.2 NA (14 mm fl.) collimator lens. In general, a slightly elliptical spot less than 0.92 mm is achieved. Also with semimajor dimension (FWHM,,) shown in Fig. 30 is the ellipticity of the spot (FWHM,lFWHM& for this system as a fimction of p. For a laser diode with a p= 2.5, this system required 37% more laser power than the equivalent system using a beam expansion prism. Cost trade-offs are necessary to determine whether to use or not to use a beam expansion prism.

-FWHMy/FWHMx

- - - TruncatedPower (%)

1.2

1.15

1 0.95 0.5

1

1.5

2

2.5

3

3.5

Beta

Figure30. The relationshipof spot ellipticity(FWHM,,/FWHMx)and truncated power vs

p for a non-rotationally symmetric Gaussian beam.

Heads and Lasers 3.3

69

Lenses

In the 1980’s, most lenses in optical recording heads were multielement lenses (except those used in some CD player heads). Eastman Kodak Company and Corning Glass Company pioneered the field of high quality molded glass lenses for optical data storage and now most heads employ molded lenses. Other manufacturers now include Matsushita and Hoya. Figure 31(a) shows a conventional 0.5 NA objective lens, model #AV4350-3 (designed by Olympus Corporation), made up of three optical elements vs a molded aspheric lens pig. 3l(b)].

U

OBJECTIVE LENS

OPTICAL DISK c

b)

OBJECTlVE LENS

-

Figure 31. An (a) Olppus triplet vs (b) a singlet asphere objective lens. (Used with permissionfrom Ol’pus Optical Co., Ltd.)

The advantages of the molded lens are many: 1. The molding process is well-suited to volume manufacturing with most of the cost existing in the tool (or mold) and molding machine and the labor intensive steps of blocking, grinding, and polishing are eliminated. 2. The lens worlung distance (air spacebetween the objective lens and the disk) is increased relative to the multielement designs as shown in Fig. 32.

70

Magneto-Optical Data Recording

3. The optical field performance of a molded aspheric lens is equivalent to or greater than a t~iplet.[~~l[~'] 4. The objective lens mass is lower than a conventional lens. This reduces the actuator forces needed for auto focusing and tracking functions. (A conventional triplet lens in its barrel will weigh 300-350 mg, whereas the low dispersion molded asphere counterpart will weigh 100130 mg or less and does not use a barrel.) +,

(Conventional Lenses)

--D- (Molded Aspheric Lenses)

2.2

2 n

E 1.8

E

W

1.6 1.4

1.2 1 0.8 0.6 0.48

0.5

0.52 0.54

0.56

0.58

0.6

0.62

Obj. Lens NA

Figure 32. Objective lens working distance vs design type.

Figure 33 shows the field performance of a triplet lens and its molded asphere counterpart. The abscissa presents the field angle of off-axis points at the disk recording layer, in fractions of a degree from the optical axis of the lens, and the ordinate presents the R M S wavefront aberration of the spot in wavelengths of light. (The disk is refocused at each off-axis point to correct for field curvature.)

Heads and Lasers

A

71

Conventional Lens

Objectives at 780 nm

+Molded Glass Asphere 0.05 0.045 0.04 0.035 0.03 0.025 0.02

0 -0.2

0

0.2

0.4

0.6

0.8

1

1.2

Field of View (Degrees)

Figure 33.

Focused beam wavefiont error vs objective lens type.

Molded aspherical collimator lenses are also used and replace multielement lenses. Collimator lenses must be made from glass when used with beam expansion prisms in order to adequately compensate for environmental conditions (see Sec. 3.1). Low dispersion molded glass lenses are also now to handle the wavelength shifts encountered with higher power lasers being used in higher performance MO heads. Table 2 shows the focal shift/nm for a conventional triplet objective lens, a high index molded aspheric objective lens, and a low dispersion aspheric objective lens. When high power lasers -50 mW are used in an MO head, wavelength shifts 'of -5 nm are possible that force the head designer to use more achromatic designs. As mentioned in Sec. 2.1, wavelength shifts of -0.1 nm/mW are typical of laser diodes used in optical recording.

72

Magneto-Optical I h t a Recording

Objective Lenses Conventional

j

Moldcd Asphcric

Triplet

High Dispersion Flint Glass

Low Dispersion Crown Glass

-0.13 p d n m

-0.2 p d n m

<0.1 p d n m

If and when higher power lasers, 2100 mW, begin to be used in optical data storage, lenses with a higher degree of chromatic correction will be needed. One potential solution being studied by Kodak and the Univeris a hybrid achromatic objective lens, U.S. Patent sity of #5,349,471. The lens consists of a high index molded glass asphere lens combined with a very weak diffkactive optical element to form an achromatic doublet lens. This so called “hybrid achromatic objective lens” has the size, mass, performance, and working distance advantages of a molded aspheric lens. The lens has been designed and toleranced and the polychromatic Strehl ratio vs field curves calculated. Its chromatic correction is < 0.03 pn/nm. Molded aspheric plastic objective lenses are being used in a few MO writehead systems. The designer must pay very close attention to not only dispersion properties, but environmental stability since many plastics are hygroscopic and have much higher coefficients of thermal expansion. Thinfilm protective coatings have improved the environmental stability. 3.4

Optical Stylus

Spot RMS Wavefront Error and Strehl Ratio. The spot at the disk recording surface, or the optical stylus as coined by Marchant,[’] is the marking and readback optical interface to the disk. Its quality is very important to both the accurate marking and readback of the optical disk to

Headv and Lasers

73

achieve sufficicnt phase margin. The measures of optical spot quality can be found in Born and W0lfl4"I and are repcated here for the reader. The RMS wavefront error, in wavelengthsof light, is die square root of the variance of the actual wavefront compared to a perfect spherical wavefront converging to focus in the disk substrate at the recording layer. It is given by:

where 0 is optical path difference (OPD) between the actual wavefront and the spherical wavefront and integrations have been performed over the pupil to obtain @ and There is another useful measure of spot quality termed Strehl ratio. Consider the spot distribution focused by a perfect lens, that is, with no wavefront aberration or a 0.0 RMS wavefront error. This spot is referred to as a dij'raction limited spot because only diffraction by the edge of the pupil contributes to the finite size of the spot. After normalization, the on-axis irradiance value becomes 1.O, and the distribution falls off in each x and y dimension. Next, add optical aberration to the wavefront and one will find that the on-axis intensity drops to some new value that is always less than 1.0. This new value is called the Strehl ratio. In other words, it is the ratio of the on-axis irradiance of the aberrated spot divided by the on-axis irradiance of the diffraction limited unaberrated spot. For wavefront errors less that 0.07 RMS there is a good approximation between R M S wavefront error and Strehl ratio, which is given by the following expression: Eq. (13)

Strehl = 1 - (~Z/A)~[RMS Error @)I2

where the RMS Error is in wavelengths of light. Figure 34 plots Strehl vs RMS Error (A). Note that an RMS wavefront error of 0.07 waves is known as the Marbchal criteria and is used as the minimum required quality of the spot for a CD player under worst case optics and disc tilt conditions. An MO writerheader optical head requires a spot which is 5 0.05 RMS error or quivalently a minimum of 0.9 Strehl ratio excluding disk effects. Aberrations. Aberrations in the spot are quantified by the amount of coma, astigmatism, and spherical in the wavefront converging to the focal distribution. Coma is the worst offender in optical recording and is a consequence of

74

Magneto-Optical Data Recording Wedge or tilt between lens surfaces that result from the lens manufacturing process Tilts of lenses in the head

The tilt of the thick optical disk substrate relative to the axis of the focused beam. The effect of coma from disk tilt is indicated in the increase of RMS wavefront error in the plot of Fig. 35. Spot distributions with -h/4 coma, astigmatism, and spherical are shown in Figs. 36 (a)-(c), respectively.

I

7

0.96

........ ............ '............

.g 0.92 m LL 0.88 t;

........ ............ '............ ........ ............ '............

0.84

....... ............ ;. .........

0.8

0

+

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

RMS Error (Waves)

Figure 34. The relationship between Strehl ratio and RMS wavefront error for small errors (4.07 h Rh4S or less). -+- 0.5 N.A. Molded Glass Lens

Figure 35. The effect of disk tilt (introductionof coma) on the RMS wavefront error (CD substrate, 1.2 mm thick).

Ilea& and 1-asers

75

Figure 36. Isometric plots of focused spot irradiance distributions at the media showing the aberrations of (a) coma, @) astigmatism, and (c) spherical aberration.

76 Magneto-Optical Data Recording Polarization Effects. When an unaberrated, spatially coherent, linearly polarized, Gaussian beam is focused by a -0.55 NA objective lens to the disk, vector diffraction theory should be used to obtain the exact irradiance distribution. When the linearly polarized beam electric vector is parallel to the tracks and focused through a homogeneous disk substrate to the recording media, the focused spot irradiance distribution is somewhat narrower in the cross-track dimension compared to the in-track dimension. This is caused by the vector addition required to determine the irradiance distribution. Vector diffraction is discussed by Richards and Sugaya and Mansurip~r[~~] have calculated a typical elliptical spot size for a typical MO head working with a grooved polycarbonate disk.

4.0

RETURN LIGHT PATH

The return optical path is designed to maximize the signal-to-noise ratio of the data detection and provide robust focus and tracking error signals. These goals must be traded off with optical head efficiency, cost, and complexity. We begin by discussing the polarization properties of the reflected beam from the MO media.

4.1 Polarization State In most magneto-optical detection systems, linearly polarized light is focused onto the MO recording layer. During readout, the reflected beam contains the information in the resulting elliptical polarization state. The polarization eigenstates for magneto-optical data storage media are rightand left-hand circular polarization Upon reflection from the magneto-optical media, the circularly polarized states remain unchanged except for being multiplied by the complex amplitude reflection coefficient for the media, which will be slightly different for the two circularly polarized states. Examples of complex amplitude reflection coefficients for magnetized TbFeCo are rr= 0.70 + 0.2% and rl = 0.71 + 0.24i, (complex scalar numbers are bold) where Y and 2 refer to right- and left-handed circular polarization When the state of magnetization is reversed in the media, the reflection coefficients 2 and Y are reversed. The Jones for reflection from an MO film is given by:

Heads and Lasers

-[ ][ 1 1

Eq.(14)

'M-O

=

2

o

0

rr+rl

- 1 i(rr -rl)

i(rl-rr) rr +rl

77

1

where the brackets indicate matrices. The first matrix in Eq. 14 preserves the sense of handedness for our coordinate system after reflection. If we reflect a linearly polarized state from the media, the field returning to the head is given by:

We will not consider the modifications to Eq. 16 because of the focusing of the beam on the media. This has been shown to be a small effect that requires a more complicated If we rewrite r,. and rl in polar coordinates

and ignore an overall phase of the field, then

where (rr+ r1)/2is often identified as rll,the parallel reflection amplitude,

78

Magneto-Optical Data Recording

and the f sign depends on the state of magnetization of the recording media. Because of the f sign in the expression with 6 in Eq. 19, we can limit the range of p to - d 2 I p l d 2 , which is convenient for the arctangent. The quantity S2 is the ratio of intensity in the y-polarization to the x- (incident) polarization and p is the phase differencebetween the two field components. The reflected field is elliptically polarized with the angle between the major axis of the ellipse and the incident polarization given by the Kerr rotation angle 0,: fan(2ek)=

26COS(p) s2-1

A sketch of the resulting ellipse is shown in Fig. 37, with a and b representing the major and minor axes of the ellipse.

Ey

t

-

a-I b 6 sinP

Figure 37. A sketch of the polarization state of the reflected beam

Heads and Lasers

79

For the small Kerr rotations seen with TbFeCo,

and the ellipticity of the reflected polarization state is

In many of the efforts to optimize the MO media, it is the Kerr angle that is considered. As we show in Sec. 4.2, if the polarization properties of the other optics are designed to compensate for P, S= (6$z + E ~ ) should ~ ' ~ be . . maxuI1Iz6d to produce the highest signal-to-noise ratio. From the reflection coefficients for thick film TbFeCo, 6= 0.00685, )r,,1=0.75, and p = 37' for the linearly polarized incident state. This yields a Kerr rotation angle of -0.31' (-0.00547 radians) and an ellipticity of 0.0041. This very small angle and the large phase shift p complicate the signal detection. By sandwichingthe MO recording layer in an optical film stack, 6 or 6, can be Alternative media configurations (quadrilayers, superlattices, etc.) may have larger values of p, hence it is important to view 6 and not @k as a figure of merit. For purposes of illustration, we will use these values from bare TbFeCo in our calculations.

4.2 Signal Detection In a magneto-optical recording system, the smallness of the polar magneto-optical (MO) Kerr effect mandates a careful optimization of the data detection system in the optical head. A number of readout techniques have been proposed, including edge d e t e c t i ~ n , [ ~ detection ~ l [ ~ ~ ] of the media and the use of the magneto-optical media as an external reflector for a polarization bistable By far the preferred method for MO readout is differential detection of the polarization state of the reflected light from the media. We will limit our discussion to differential detection with a single polarization We will discuss the role of the partial polarizing beam splitter and examinethe use of wave plates in the detection path to maximize the differential detected signal and minimize the DC offset, which only contributes to the noise.

Magneto-Optical Data Recording

80

The layout for the optical head was shown in Fig. 1. The partial polarizing beam splitter reflects a portion of the return beam towards the differential detection system and modifies the polarization state of the beam. The waveplates firther modie the polarization state of the light. The differential detection system converts the information in the polarization state of the beam to an analog waveform. The field returning from the disk is split off with the partial polarizing beam splitter (see Fig. 1 and Fig. 38).

4 , to waveplates and detectors

rs

I'

'P

I=

to laser 4

444

1 1 1

444

444 tP

to d i s r

Figure 38. A sketch of the fields at the partial polarization beam splitter.

The field reflected by the beam splitter, given by

Erbs, to the waveplates is

Heads and Lasers

81

The ideal partial PBS has Ir,l= 1 and ts = 0. We will leave rp and tp arbitrary for the moment with the constraint lrJ2 + [$,I2 = 1. The phases of the complex coefficients are also arbitrary. Then,

1

+-

6

ij

where Rp = lrJ2 and fl is the phase delay between the s- andp-polarizations after the partial PBS, which has been generalized to include the phase delay from the MO media and the partial PBS, as well as any other retardation in the system up to the waveplates. (In order for media birefringence to only alter fl and not 6, the incident polarization must be along the fast or slow axis of the media birefringence. Because these axes and the incident polarization are usually along the disk's r or 8 axis, this is often the case.) The beam splitter is not the only element that determines the head efficiency. The beam will be truncated by the optical head, and the other optical elements introduce loss. Let us define qm as the fraction of the laser intensity that would arrive at the disk in the absence of the beam splitter attenuation, so that the head efficiency is given by:

Eq. (28)

Head efficiency = qm(l - Rp)

The intensity lost in the collection process back to the polarization analyzer (1 will hrther reduce the light available for signal detection. Then Eq. (27) becomes

The value of qm is typically -50% and qDLis -70%, with the difference caused by the truncation in the optical head. It is the signal information in this field that we wish to detect with maximum signal-tonoise ratio.

82

Magneto-Optical Data Recording

This polarization state reflected from the beam splitter needs to be conditioned for the differential detectors to obtain maximum signal and minimumnoise. The noise is minimized by ensuring that, in the absence of a data signal, the differential signal is zero. This maximizes the common mode noise rejection. We also wish to maximize the signal. Toward these ends various polarization optics may be used. Examples are a general waveplate and fixed PBS ~ornbination,[~~1[~*1 a compensator plate and rotating PBS combination,[451[521[53] and a half-wave, quarter-wave plate and fixed PBS Because the return beam may be sampled via different beam splitter locations for the focus and tracking detectors, we will not consider the effects on polarization or intensity on the return beam because of this additional beam splitter.

A

Partial Polarization ,D ,',

1

Analyzer

w

Figure 39. A schematic of the differential detection system with a single waveplate to optimize the signal-to-noise ratio.

General Waveplate/Fixed PBS (Case I). In order to introduce terminology, let us start by considering a general waveplate (a waveplate with arbitrary retardance and rotation angle) between the partial polarization beam splitter and the analyzer polarization beam splitter (see Fig. 39).

Heads and Lasers

83

The Jones matrix for a general waveplate is

where k is the phase delay ( 2 a d n &A), modulo 2 a , between the fast and the slow axis of the waveplate and is the angle between the x-axis of the optical system and the waveplate’s slow axis (k > 0) (thep-axis of the partial polarizing beam splitter defines the x-axis of the optical system). If our signal optimization is done with a single waveplate, the signal incident on the differential detection system is given by:

+

In order to gain some understanding of this field, we need to calculate the differential signal, DS (see Fig. 39). If the eigen axis of an ideal polarization analyzer and partial polarization beam splitters are aligned, the differential signal current can be expressed as

Eout

2

[

0 0

-K,

2

where K, and Kb are the responsivities of the individual detectors in amps per watt of optical power. From trigonometry, Eq. 32 can be simplified to

84

Magneto-Optical Data Recording

where

K=

Ko -k K b

2

and

AK=

Ka

- Kb 2

and we have only kept terms to the first order in S and AK, the detector asymmetry. The measured analog waveform from the optical head after the differential amplifier should be DS times the transimpedance gain of the differential amplifier. The measured signal then has a component independent of Sthat we will call DC and a term proportional to Sthat we will call Sig. The DC term is the differential offset that contributes to the noise. To calculate the shot noise from the differential detection system, we also need to know the s u m of the two photocurrents, SS, given by 2

0 0'

+Kb[ 0

-

2

Eout

1

Because 6 changes sign in the data track, the average sum signal will be almost independent of Sto first order. For a 50% duty cycle track (equal up (+) and down (-) magnetized media) SS is given by Eq. (37)

S S = i W { K + AK [c0s2(2@+ sin2(2+)cos(k)])

The first thing we want to be sure of is that DC is zero. This insures that the intensity is split evenly between the two detectors so that the noise associated with the laser or media will largely cancel in the differential signal. Hence, the common mode noise rejection is maximized. A contour plot of DCI(KM2) is shown as a function of the general waveplates parameters, k and 4 in Fig. 40. We see that the DC term, independent of S, is most sensitive to 4 when k = n(a half-waveplate) and is most sensitive to k when 4 = n/4. We

Heads and Lasers

85

shall return to the discussion of sensitivities. Although the solution is not obvious from the figure, the DC term in Eq. 33 vanishes if

Eq. (38)

tan2(24) =-

-1

cosjk j

From Eq. 38, we see that cos(k) must be negative or

for Eq. 38 to have a solution for real 4. Even if k is not exactly as designed or the laser wavelength varies fiom nominal (hence changmgk), the angle 4 can be adjusted to compensate for this change and eliminate the DC component as long as k 2 xf2.

32s 2 n:

k IL 2

0

i

8

h

I

8

lJ

$

4

: &E

8

Figure 40. A contour plot of the normalized DC portion of the differential signal is shown as a function of k and 4, the general waveplates retardance and angle. The shaded region indicates where DC is negative.

86

Magneto-Optical Data Recording

If Eq. 38 determines the orientation of the general waveplate, k can be chosen to maximize the signal as a function of the phase shift fl. Substituting Eq. 38 into Eq. 33 (and recalling that -7d2 I P I ;rc/2), we find that the normalized differential signal as a function of k and P is given by

where sign@) is positive for fl in the first quadrant and negative for fl in the fourth quadrant. The differential signal DS is plotted in Fig. 41.

?I:

3.a 4

k Il

2

4

Figure 44. The normalized signal from Eq. (40) is shown as a contour plot as a function of k and p . The shaded region indicateswhere the signal is degraded by less than 5% from its maximum value.

Heads and Lasers

87

The maximum zero-to-peak signal amplitude is given by

with the optimum retardation for the waveplate given by Eq. (42)

k

=

arccos[-cos2@)]

~ 1I 2k I?r

and the optimum angle for the waveplate is

Because of the properties of the arctan, 4 only needs to cover the range 4 4 I 4 I ~ 1 4 The . nice feature of this design is that if the k value of the waveplate corresponds to p ’ instead of fl, the DC term will vanish at an angle dependmg only on p ’ and the signal will only be degraded as ws@ - p ’). This is seen from the large area of the contour plot in Fig. 4 1 for which the signal is reduced by less than 5%. Because waveplates of any retardation are easily produced, this should represent a low-cost method of maximizing the common-mode noise reduction and the signal for a given head-media interface. Let us now estimate the peak-to-peak differential signal after the photodetectors from a 50% duty cycle track. If we take the media parameters from the estimates in Sec. 2, S= 0.00685 and fl = 37O, we find that k should be 2.26 radians or 0.36 waves and 4 of 25.7”is optimum. Then, with xD= 0.5, %D = 0.7, Ilmer= 4.3 mW, Rp = 0.3 (so that the intensity at the disk is 1.5 mw), and a detector responsivity of 0.5 mA/mW, the peak-topeak signal before amplification is 4.3 PA. Because each detector has a photocurrent of -44 PA, the importance of minimizing the DC term is evident. Also the shot noise from the 5’5’term may be an important contributor to the total noise. Half-Wave PlatdFixed PBS (Case I, with k = 4. Now we will look at a specific example, the half-wave plate. Here k = ?r, thus from Eq. 38 the DC term is zero when Eq. (44)

4

7 t 7 t

= -+n-

8

4

88

Magneto-Optical Data Recording

for integer n. Thus at 4 = 22.5' our signal amplitude will be

fiom Eq. 40. For smaii j7,the haif-wave piate does a good job ofmaximizing the signal. Equation 45 gives the maximum signal possible from this media and beam splitter times cos@). From Eq. 45 or Fig. 41, we see that the signal will be degraded by less than 5% for f l < d o . For our media example, where fl = 37', the signal is down 20%, which will reduce the carrier-to-noise ratio by about 2 dB. It should be noted that the fixed PBS, half-wave plate combination is equivalent to a rotating analyzer PBS, except the rotating PBS will not show a wavelength sensitivity. Therefore an analyzer PBS with its axis of transmission at an angle of 7r/4 from the p-axis of the system, and no waveplate, yields the differential signal in Eq. 45. This is the simplest form of differential detection. Looking at Eq. 24, we see that the signal of Eq. 45 from this simple detection scheme depends on the media parameters as lrl,126'k (the media's average reflectivity times the Kerr rotation), except for the possible modification of fl caused by the beam splitter. Because many commercial MO products are designed for low phase shift from the media and optical head, this detection system is quite popular and lrI1l261, is often taken as a figure of merit for the media. Compensator Plate/Rotating PBS (Case 11). Another possibility is to include a compensator plate to set fl to zero and then use a half-wave plate or a rotating PBS to match the intensity on the detectors, as shown in Fig. 42. This method is discussed in Ref. 45. Reference 53 uses the same idea with the phase plate between the objective lens and the partial polarization beam splitter, where the effect is the same. The idea here is to use a phase plate (i.e., a quartz waveplate, a Soleil-Babinet compensator, or a liquid crystal variable retarder) with the slow axis of the optic aligned to the p- or s-axis, that is, 4 = 0' or 90', to change the elliptical polarization to linear. Thus a rotating analyzer PBS would provide zero DC and the maximum signal as given by Eq. 41. After carefblly measuring fl, a waveplate with k near -fl would be chosen and oriented at 4 = 0'. Because the signal would then degrade as cos@ - k), the variations in phase delay from head to head or disk to disk would have a small impact on the signal.

Heads and Lasers

89

For research, it would be desirable to have an MO head that could work with a variety of media and thus be able to compensate for any Such a head is shown in Fig. 42. The liquid crystal variable retarder would provide a voltage-tunable retardation to compensate for (Ref. 55 provides a good discussion of commercial liquid crystal variable retarders). Figure 43 is a plot of measured k vs rms voltage for a liquid crystal variable retarder (Meadowlark Optics model LVR-0.7). Also, by calibrating the liquid crystal device, the voltage for maximum signal could provide a measurement of fl. This detection system in a high performance optical head could compensate for variations in phase shift from disk to disk or within a track on one disk.

F.

Square wave Generator

I

Partial Pnlnri. ~-~ti nn .-_--

1 Beamsditter

DS

Detectors

Rotating CGstal Wollaston Variable Polarization Retarder Beamsplitter

~

~

Figure 42. A differential detection system is shown comprising a voltage tunable liquid crystal variable retarder oriented at 4 = 0" and a rotating Wollaston prism.

90

Magneto-Optical Data Recording

k

0

4

2

,v,

6

8

(volts)

Figure 43. The retardance of a liquid crystal variable retarder is shown as a function of applied RMS voltage (1 kHz square wave).

Quarter and Half-Wave PlatelFmed PBS (Case 111). A quarterwave plate, half-wave plate combination with a fixed PBS (see Fig. 44) can also provide a null DC term and maximum signal. This detection system is of particular interest because with realignment, it can be optimized for any phase shift. With the quarter-wave plate at an angle a and the halfwaveplate at an angle @, the DC portion of the differential signal is shown as a contour plot in Fig. 45. For the half-wave plate, it is only the relative angle to the quarterwave plate ( a- 2 0 ) that is relevant. The DC term vanishes in two cases:

Eq. (46)

7r

7r

4

4

a = - or a=-+2@

which we will call the fixed a case and the variable a case, respectively. These two cases can be seen as the horizontal (fixed case) and inclined (variable case) lines of DC = 0.

Heads and Lasers

91

Partial Polarization

Figure 44. A differential detection system is shown comprising a rotatable quarter- and half-wave plate with a polarization analyzer aligned with the partial polarization beam splitter.

I€ 2

a ZI 4

Figure 45. A contour plot of the normalizedDC portion of the differential signal is shown as a function of a and @, the angles of the quarter- and half-wave plates. The shading indicates the region where DC is negative.

92

Magneto-Optical Data Recording

ZE 4

0

Figure 46. The normalized Sig portion of the differential signal is shown as a contour plot of F and s”. The shaded region indicates where the signal is above 95% of the maximum value given by Eq. 4 1.

The differential signal amplitude in the variable case becomes

This is very similar to the result for just a half-wave plate except that the sine of the phase delay appears insteadzf the cosine. This arrangement of waveplates would be appropriate for p near * d 2 . The unique thing about this solution is that it does not depend on the absolute position of either waveplate but only on their relative orientation. Now for the fixed a case, the signal is shown as a contour plot as a function of @and p in Fig. 46. DS will be maximum when

Heads and Lasers

93

with the maximum signal again given by Eq. 4 1. Thus, this system should also be capable of producing the maximum carrier-toznoise ratio. Although this system can provide a maximum signal for any p by rotating the halfwave plate, the concern is that the interaction between the two plates will yield a system that has tight tolerances and has a more complex alignment. Carrier-to-Noise Ratio. In order to compare the polarization manipulation schemes, a figure of merit for the detection performance is required. Often the carrier-to-noise ratio is chosen because of its simplicity to measure and model. (Chapter 9 and Refs. 45,52,53,56, and 57 provide additional information on CNR models and a more in-depth discussion of noise sources in magneto-optical readout.) The CNR depends on the sum signal SS, the differential signal offset DC,and the differential signal term Sig. The noise terms that contribute to the total noise power are often uncorrelated and have a quadratic dependence on laser power intensity. The constant term represents electronic noise that is independent of read power. The term linear in laser power is the shot noise. The term proportional to laser power squared contains laser noise (see Sec. 2), media noise, polarization noise, and written-in noise contributions. Because the signal power is proportional to the read laser power squared, in general, higher read powers yield a higher CNR as long as we are not saturating the detectors or any electronics in the channel or overheatingthe media. The read power may be limited by thermal heating of the media, which reduces the Kerr effect and decreases & and may degrade the recorded information. Partial Polarizing Beam Splitter. An important part of the detection system design is the partial polarization beam splitter reflectivity Rp. To achieve a maximum signal (for example, from Eq. 41) we find that Rp should be 1/3. But, this partial derivative maintains the optical power constant at the laser when it seems more reasonable to maintain constant power at the disk and let the laser power be set by considerations of head efficiency, required write power, and the laser's power rating. As discussed above, higher read powers are preferable so read power should be chosen by consideration of thermal effects on the signal strength and data integrity after a large number of read cycles. Read power or the intensity at the disk (Idrsk) is VLLI ( l - Rp) Ihw. Then Ds goes as

Eq. (49)

94

Magneto-Optical Data Recording

That is, the signal increases as the square root of the beam splitter’s reflectivity for thep-polarization. By increasing Rp, we increase the signal, decrease the optical feedback to the laser, which is proportional to (1 - Rp)2, and increase the power of the laser for the same read power at the disk. All of these things should improve the signal and decrease the laser noise, but the shot and laser noise terms also depend on Rp. The shot noise and the laser noise caused by the signal are linear in Rp for constant power at the disk. The DC term’s contribution to the laser noise goes as Rp2for constant read power. Thus for different values of Rp, different noise terms may dominate. An interesting endeavor would be to empirically determine how the laser noise varies with feedback and power, and thus predict the ideal Rp for a given head-media interface. In any head design, head efficiency, factors that influence laser noise, expected DC offsets, and CNR requirements will have to be considered in the choice of Rp. It is clear that Rp has no universal optimum. Tolerances and Sensitivities. In this section we will discuss the effects of head component assembly tolerances and the sensitivity of CNR to variations of parameters that contribute to CNR loss. Examples of variables for which tolerances are important include alignment, waveplate retardation, and detector mismatch. The head media interface, especially GNR, will be sensitive to changes in laser wavelength and variations in P from one disk to another. To understand the effects of AK on CNR we have to know something about how the optics are aligned. Assume that is known and that the optical elements are chosen as described for a general waveplate or compensator plate chosen with a specific retardance and two detectors are chosen with a AK value of less than 0.02 mW/mA. For the general waveplate (case I), the head will be focused onto a mirror and the waveplate is rotated until the DC term is minimized. For the rotating PBS and compensator plate combination (case 11), the PBS is installed, the head is focused onto a mirror, and the PBS is rotated to minimize the DC term. Then, the compensator plate is installed, the head is focused onto a data track and the compensator plate is rotated to give maximum signal. For the quarter-wave plate, half-wave plate, fixed PBS combination (case 111), the quarter-wave plate is installed, the head is focused onto a mirror, and the quarter-wave plate is rotated to minimize the DC term. Then the half-wave plate is installed, the head is focused onto a data track, and the half-wave plate is rotated to give maximum signal.

Heads and Lasers

95

For the rotating PBS and compensator plate combination (case 11), there is no loss in CNR for a small detector For cases I and 111, the CNR degradation depends on both fl and the sign of AK. Case I shows almost no CNR loss until p approaches 90". At this p value, the general waveplate description calls for a waveplate with a retardance approaching a quarter of a wave. A quarter-wave plate, regardless of rotation, cannot result in more than half the intensity being converted into the orthogonalpolarization. So if K, is greater than Kb,the system with the quarter-wave plate cannot steer more light to detector b to null the DC term. Thus a positive dK causes a more severe CNR drop. The same situation exists for case I11 and hence the two methods give the same CNR at p= 90" for AK= 0.02 N m W . With AK = -0.02 mAJmW, the quarter-wave plate can be rotated to null the DC term, but then any rotation of the half-wave plate severely changes the DC term and we see a large drop in CNR. Obviously our alignment scheme is inappropriate for the method in case 111. If we had fixed the quarter-wave plate at 45O, which was found to be the best solution for AK 2 0, the solution for AK = -0.02 mA/mW would be identical to that for dK = 0.02 mA/mW. Experience in the lab has shown that trial and error is the only alignment technique for case 111. Now we wish to turn our attention to wavelength sensitivity. We will assume that the head is initially aligned perfectly, but the laser wavelength changes, caused by temperature or device aging. We assume that the waveplates are zero order and the Kerr effect is independent of wavelength over the small range we will consider. To illustrate the wavelength sensitivity, we will discuss how the DC term changes with A. Experimental investigations of an MO head with the detection scheme described in case I11 showed that the DC term was a linear function of ~avelength.1~~1 Figure 47 shows the DC term as a function of laser wavelength for two beta values, fl = -5" and fl = 80". Again, case I1 is largely insensitive to wavelength variations. Case I shows a very different sensitivity to A change depending on fl. Case I11 shows a linear dependence on A in agreement with experimental observation and for small wavelength changes the DC term can shift by a sizable fraction of the signal. This will increase noise in the differential signal and may lead to problems in other portions of the read channel. This can be especially troublesome when the laser mode-hops.

96

Magneto-Optical Data Recording

3

I

!

\ i ........................... 2 ........'\........... * :

i...........................

I-

1

q

......................... I.+..

I -

.......................................... i

i........................

i

1 ._----------- \ I

0

-- - - -- - -

i-

I

j\

-, j

..............................................................................

-1

-2 --.........................

-3--

-

i i .........................

j

I

..........................

x

;i ...........................

i...........................

. I: \ :.........................

1

I

1

!

I

6 ...........................

i...........................

\

j I

--3 2

-C

........................

I

.........................

m-

- 0 '

1

-D

........................

i........................... j

WCLA) 0

"

,

-1

j

0

.j.+..* ........--0;

&: 7

+=-i .........rl.......................................................... 4**

I r

i..........................

q

i

j i........................--

f*-L

-2 ........................

i ...........................

i ...........................

i..........................

-3

I

I

I

-D

Figure 47. DC is shown as a function of 1,for the three detection methods. and p" = 80" in (b).

-

p" = -5" in (a)

Heads and Lasers

97

The three detection methods show quite different tolerances to parameters in the build process and different sensitivities to changing media or laser parameters. The detection systems are qualitatively compared in Table 3. The three methods each have an advantage. The phase compensator plate rotating PBS (case 11) is the most tolerant and the least sensitive, but unless a liquid crystal retarder is used as a phase plate it can only be optimized for a single fl value. The general waveplate, fixed PBS (case I) has the advantagethat alignment is fairly simple; therefore it may be a good choice for a low-cost head. The quarter-wave plate, half-wave plate, fixed PBS combination (case m)has the advantage that by rota- the half-wave plate it can be optimized for any fl value, but it was found to be the least tolerant of variations in the head or media. The simple alignment scheme we discussed previously did not always produce optimum CNR for this head. Our experience with a head that used this differential detection method described in case I11 indicated that trial and error alignment was required to maximize CNR.

Table 3. Comparison of Detection Methods.

Case I HWFJ Fixed PBS

Case I1 Fixed CP Rotating PBS

Case I1 LCVR Rotating PBS

Case I11 QWP/HWP Fixed PBS

simple over mirror

simple over mirror

simple over mirror

difficult head and media

cost

low

moderate

high

moderate

hYP

no

no

yes measure j

yes with realignment

Tolerances CNR on AK

good

best

best

poor

CNR on A 1

good

best

best

fair

CNR on AS

good

good

best

poor

Dc on A 1

faidpoor

best

best

poor

high performance with standardized media

media tester or universal head

Alignment

Application volume production with standardized media

98

Magneto-Optical Data Recording

The appropriate detection system depends on the intended usage of the head. Although we have not explored all the possible combinations of detection systems, we can make some recommendations for different applications. For a high-volume MO head, the general waveplate may be a good choice because of its simple alignment. For a high performance MO product, the compensator plate and rotating PBS would be a good choice. For a research tester head, the compensator plate should be replaced with a liquid crystal variable retarder. This head could then be optimized for any MO media and in the optimization process, could be measured.

4.3 Servo Signal Detection Focus Sensing Systems. The focus sensor in an optical head must detect disk defocus with a very high resolution (< 0.1 pm) and low noise for the servo electronics. There are several popular focus sensing systems. A good focus sensor will meet the following requirements: 1. Provide a high sensitivity focus error signal in terms of detector photocurrent ( d p )with sufficiently low focus error noise (<0.001 pRMS noise).

2. Be sufficiently insensitive to disk birefringence. 3. Be sufficiently insensitive to wavelength shifts of the laser. 4. Have low ( < O S p) tracking into focus optical crosstalk or feedthrough. 5 . Be sufficiently insensitive to disk tilt. 6. Be sufficientlyinsensitiveto detector movement because of environmental changes (i.e., operating thermal range and shock).

The term “sufficiently insensitive” above, depends upon the total focus error budget for the head media interface (HMI), the contribution of each item to that budget, and the drive’s ability to detect and compensate or correct for the effect. All these factors should be considered in developing a total focus error budget for an MO writerheader that can maintain the disk at focus to within about *0.5 p.This is difficult to acheve under all environmental conditions, and so some drives test for best focus and apply correction to the

Heads and Lasers

99

optical and electronic offsets that are sensed. Be that as it may, the head designer works to meet the above requirements in a passive sense in the opto-mechanical system, as best he can, while minimizing complexity. In the following discussion, we categorize focus sensors into two categories-near field and f3.rfield. NearJield means the focus detector is in a plane that is image conjugate to the disk. FarJield means the focus detector is in a plane that is not image conjugate to the disk and the light distribution at the detector is a scaled version of that in the objective lens pupil. Near Field Focus Sensors. The obscuration focus sens0r,[~*1[~~1 sometimes referred to as the dual halfaperture focus sensor, places the detectors in a plane that is essentially image conjugate to the disk, that is, in the near field of the beam returning from the disk. This type of focus sensor evolved from the Foucault knife-edge The most common implementations use a prism pair to divide the far field light returning from the disk in half and then each half-beam is focused to a split photodetector as shown in Fig. 48. This figure shows the use of two prisms with their wedges in opposite directions. Philip's European Patent 0 177 108 shows this type of sensor system. (The sensor system can also be implemented with a roof prism.) The implementation of Fig. 48 uses a pair of split detectors wherein the focus error signal (FES), tracking error signal (TES), and the data signal are obtained. Here, the sensor configuration is insensitive to wavelength shifts of the laser diode due to laser power changes or mode hopping that cause spot movement along the split detector gaps, rather than across them. (Note that in Fig. 48, as well as in the following figures, the FES, TES, and data signal equations contain letters that refer to each detector, but represent detector currents that are being added and subtracted.) There is an orientation of the dual half-aperture focus sensor that reduces tracking ( T ) + focus ( F ) optical crosstalk or feedthrough. The prisms and detectors must be rotated by 90" about the optical axis. This new orientation implements the focus sensing process orthogonally to the process of sensing the push-pull tracking signal. Hence the push-pull light distribution changes in the beam occur in a direction that is orthogonal (at 90") to the light distribution changes that indicate a focus error. This orthogonal relationship works to decouple the tracking and focus servo systems and reduces the T + F crosstalk or feedthrough levels. To implement the low crosstalk version of the dual-half aperture sensor system, the push-pull tracking signals are usually detected by a separate detector by Dual half-aperture focus sensors are also reasonusing a beam ably tolerant of disk birefringence and small detector displacement.

100 Magneto-Optical Data Recording

FES = (A+D) - (B+C) TES = (A+B) - (C+D) RF = A + B + C+ D C

/

FOCUS SENSOR LENS

\

,TRACK

/’ Figure 48. Beam obscuration or dual half-aperture focus sensor system.[58]

NEC Corporation has used a QUADRANT prism to implement their own version of a near field focus sensor. Light from two quad apertures is used for sensing the focus error signal, and light from the other two quad apertures is used for sensing the push-pull tracking error signal (Fig. 49).[621 Far Field Focus Sensors. The astigmaticfocus sensor discussed by Bouwhuis et al.[581is a very popular focus sensor that places the detector in the far field region of the beam returning from the disk. (The push-pull tracking signals are also available in the far field of the beam and can be detected by the same detector array.) The astigmatic focus sensor is shown in Fig. 50 and was used in the early video disk players by Philips in the 1 9 7 0 ’ ~and ~ is currently used in many CD players, and in some MO writer/ readers. It has proved to be a robust focus sensor, but can have high levels of F -+ T crosstalk or feedthrough when astigmatism and disk birefringence are p r e ~ e n t . [ ~ ~The l [ ~more ~ ] circular distributionof light exists between the line foci, as shown in the sagittal and tangential planes in Fig. 50, and is

Heads and Lasers 101 detected as the condition for the disk at best focus. As the disk moves out of focus, one of the line foci, depending on polarity, moves towards the detector and an imbalance of the differenced detector signals results.

c

RETURN BEAM

-

-

ir

I

LENS

QUAD PRISM

I

DETECTOR

FES = (CAI)-(d+c’) TES = a-b RF = a+b+c+c’+d+d‘ Figure 49. NEC quad-prism servo sensor sy~tern.I~~]

Spot Size Focus Sensor. This sensor detects the increase or decrease of the return beam size by placing a detector with three parallel sensitive regions in the f%r field beam that is convergingto a focus (Fig. 5 1). Yamamoto et al.[651and Ukita et a1.[661discuss this type of sensor. It was modified by Verbatim and used in a prototype 3.5” MO head (US Patent 5,113,386). The spot size sensor implements the orthogonality principle discussed above and is also reasonably insensitiveto detector displacement. In the form shown in Fig. 5 1, the sensor is sensitive to in-track diffraction from prefonnatted Verbatim used a mask to reduce this data and a focus error will effect and the effects of changes in the zero-order intensity that occurs from

102 Magneto-Optical Data Recording diffraction of the light by the groove. Verbatim also derived a traclang error signal by dividmg the center detector element in half. This FES sensor is somewhat sensitiveto disk birefnngencein the form shown in Fig. 5 1.

SAGITTAL PLANE

OBJECTIVE LENS. CYL. LENS

*

Figure 50. Astigmatic focus sensor.[58]

FES=(a + c) - b

x==u

RETURN BEAM

a b c

Figure 51. Spot size focus sensor.

Heads and Lasers 103 Critical Angle. This focus sensor was first introduced to commercial optical recording by Olympus[681and is shown in Fig. 52. It is also discussed by Bouwhuis et al.B8] The return collimated beam from the disk enters the prism and reflects from an internal glass-to-air interface at the critical angle. When the return beam deviates from collimation caused by a disk defocus, a portion of the beam will leak out the total internal reflection (TIR)su&ace and is sensed by a split detector. A diverging beam causes light to leak out so that the B signal is reduced. The opposite happens for the converging beam and the A signal is reduced. The sensor detects changes in the light distribution that occur in the in-track dimension and therefore implements the orthogonality principle.

-

*Sin

DISK AT FOCUS

A

B

1 -

n

FES

Figure 52. Critical angle focus sensor.

Differential Focus Sensors. There are differential forms of the far field focus sensors that were discussed above. The differential versions have lower crosstalk or feedthrough and are less sensitive to pattern noise, in general, because of the common mode rejection that occurs in the differential detection p r o c e s ~ . [ ~ ~The l [ ~ ~differential ] techniques can also reduce Figure 53 shows a differential TIR sensitivity to disk focus sensor discussed by and Fig. 54 shows a differential spot

104 Magneto-Optical Data Recording size focus sensor discussed by Oka et al.r7l] Milster et al.[72]discuss a differential spot size focus sensor that is based on a wax-wane sensor and uses a pair of quad cells.

FES = (A1 - A2) + (B1 - B2) Figure 53. Differential critical angle focus sensor.[69](Used with permission.)

MIRROR

-------

PBS MICRO PRISM

\

PIN PHOTO-DIODE

Figure 54. Differential spot size focus sens0r.[~1](Used with permission.)

Heads and Lasers 105 Tracking Sensing Systems. A good tracking sensor system will have the following attributes: 1. Provide a high sensitivity tracking error signal in terms of detector photocurrent with sufficiently low tracking error (< O.OO! PR??-?S mise).

2. Be sufficiently insensitive to disk tilt. 3. Have a small phantom tracking error signal (see Fig. 56 and accompanying text.). 4. Be sufficiently insensitive to disk birefringence. 5. Be sufficiently insensitive to disk defocus. 6. Be sufficiently insensitive to wavelength shifts of the laser. 7. Be sufficiently insensitive to detector movement because of environmental changes (i.e., operating thermal range and shock). Again, the term “sufficiently insensitive” depends upon the total detracking budget for the head media interface (HMI), the contribution of each of the above, and the drive’s ability to detect and compensate or correct for the effect. The tracking error signal is derived in most MO heads from the pushpull signals diffracted by the disk pregrooves that are molded into the disk substrate (Thompson, US Patent 5,182,743). Figure 55 shows a pregrooved substrate and the incident and undifiacted zero order and diffracted *lst order beams returning from the disk. The equation for the push-pull signal from an unwritten pregrooved disk

where RL and R, are the reflectivity of the disk land and groove, respectively, AGL is the phase difference between the light reflected from the groove and the light reflected from the land, yresults from the integration of the apodized pupil distribution, and W, P,and care shown in Fig. 55. (The symbol cis the radial coordinateof the disk.) TES is a sinusoidal signal that has zero crossings at the center of the grooves and lands. Far field focus sensors can also detect the push-pull tracking signal, along with the focus signal, if the detector elements are configured correctly. However, one must check that tracking into focus crosstalk is within acceptable limits.

106 Magneto-Optical Data Recording

TOP VIEW

LuuLuLuuuJ-wc groove width = W

groove pitch = P

Figure 55. Pregrooved substrate and the reflected zero-order light and the diffracted light that interfere to give the push-pull tracking error signal.

A problem with the push-pull tracking method is the presence of a phantom tracking error signal that is produced by the radial motion of the objective lens with respect to the rest of the optical head. (The tracking actuator moves the lens in a radial or cross-track direction, Ch. 3, Sec. 3.0.) Figure 56 shows the cause of the phantom tracking error signal. The head is initially aligned with the split detector (5’1,5’2) centered on the return beam. When the lens is moved radially by a distance S, the beam center returning to the split detector is shifted by 2Sand a TES offset results. This effect must be compensated or designed out in a working drive. The design must also deal with the offset in the push-pull TES introduced by disk tilt.

TES = (Sl-S2)

+ 6

Figure 56. Tracking error offset vs objective lens decenter (phantom TES).

Heads and Lasers 107 A very robust form of tracking that was implemented by Philips Corporation in an early video disk player and can also be implemented in a MO return path for pregrooved disks of the correct phase depth, is called and is shown in Fig. 57. A satellite spot or outrigger spot grating is used in the incident path of the optical head to diffract - 10- 15% of the primary beam into each +lst and -1st diffracted orders. The diffracted beams are focused to the disk in front and behind the main write/ read spot and straddle the left and right side of the data track. The return light in each spot is returned to its own detector and low-pass filtered. The difference between the two outrigger detector signals indicates a tracking error. Alternately, for grooves with a phase depth for push-pull signals, the outrigger detectors may be placed in the far field of the return beam and each detector divided into a split detector. In this implementation, push-pull signals detected from the outrigger detectors are reversed in polarity from that detected from the main central beam. By adjusting the gain of the outrigger signals and subtracting their sum from the central beam push-pull signal, a push-pull tracking signal is obtained without a phantom error signal. That is to say, the phantom tracking signal on the main beam is compensated by the push-pull tracking signals from the outrigger beams. (European Patent 0 392 775 ). This is called a diflerential push-pull tracking system. This system is also fairly insensitive to disk tilt.

E

DISK PLANE

E

M

F

DETECTOR PLANE

Figure 57. Outrigger or satellite spot tracking sy~tern.I~~1

108 Magneto-Optical Data Recording Another tracking sensor that is attracting interest is the sampled servo system. Here offset tracking pads are used as shown in Fig. 58 and placed at regular intervals. A tracking error signal is sampled only when the pads are present and held until the next set of pads pass under the focused spot. Men the reflected !i&t signs! from each pad is equal9the read spot is on the center of the track. An unequal signal indicates a tracking error signal with a polarity related to the direction of read spot detracking. Sampled tracking and even sampled focus servo systems have the advantage that the error signals can be collected at particular times to reduce the influence of preformat marks or grooves on the servo error signals and therefore eliminate data-servo crosstalk. This sensor system also simplifies the head design. READ SPOT

/

.-.om.e

Pad 2

-=n -o.e-ae.a.. Pad 1

DATA TRACK

Figure 58. Offset tracking pads for sampled tracking servo system

Feedthrough or Optical Crosstalk. Feedthrough, or tracking into focus (T +F)optical crosstalk, is shown in Fig. 59. This plot was obtained by monitoring the open loop focus error signal while scanning the read spot across a grooved disk. The open loop focus error signal has perturbations imposed on it. Focus sensors that employ the orthogonality condition mentioned in the earlier parts of this section have, in general, lower levels of this feedthrough or T + F optical crosstalk. P r ~ k r y l [has ~ ~ performed ] a detailed study of fxdthrough for many of the different focus sensor systems. This subject is also addressed in Refs. 59,67, and 70. Effects of Disk Substrate Birefringence. Plastic disk substrates made of polycarbonate have significant levels of birefringence. The term optical birefringence refers to a difference in the optical index of refraction (n) along the three principle axes of the substrate material. The three orthogonal coordinate axes of the disk are: the radius of the disk (r),the track direction (t), and the perpendicular to the surface of the disk (z). Sugaya and Man~uripur[~~] note typical values of; n,- n, k 2.7 x which is referred to as the lateral birefringence, and n, - n, = 6 x lo-" which is ~~] termed the vertical birefringence, and n, = 1.58 1. M a r ~ h a n t [recognized that the vertical birefringencewas much higher than the lateral birefringence and responsible for most of the adverse optical effects.

Heads and Lasers 109

Figure 59. TES into FES optical crosstalk or feedthrough.

Sugaya and Mansurip~r[~~] have performed a study of the effects of birefringence on the focused spot at the recording layer. They note that without aberrations, the disk birefringence introduces a small amount of astigmatism (- W8) to a 0.55 NA beam that is converging to the focused spot. The resulting line foci are separated by -1.5 p. Sugaya and Mansuripur also studied the effect of disk birefringence on an astigmatic FES and push-pull TES. Without optical aberrations, the effects were small for typical polycarbonate birefringence levels. However, when optical aberrations were included, and in particular astigmatism, the TES amplitude was decreased and then maximized at defocused disk positions, and significant FES offsets were found at detracked positions of the spot (i.e., high levels offeedthrough were found). Feedthrough was also studied by Oka et al.r7l] for astigmatic and obscuration focus sensors in typical MO head architectures. It is therefore very important to evaluate each focus and tracking sensor system in its MO head return path configuration, with typical levels of disk birefringence, disk tilt, and optical aberrations, to determine robustness and performance. One of the more intractable issues with MO drives is the so-called “hard sector noise” problem. The 1X capacity MO media standard defined hard sectors which are radially aligned. However, the 2X standard called for sliding sectors. This gives the situation where hard sectors can be directly adjacent to an MO data area causing a noise on the data signal. The cause of the noise is thought to be a local tipping of the index ellipsoid due to local stresses in the hard sector region. Drive designers have had some success in mitigating the effects of this noise through waveplate alignment algorithms. The 4X MO media standard has reverted back to radially aligned hard sectors to avoid this problem.

5.0

EXAMPLES OF PRODUCTION M O OPTICAL =ADS

5.1 Unified Optical Heads

Figure 60 shows an MO unified optical W-camage combination from Sony Corporatiun for their 5.25" hull-height MO drive. The entire head moves on the radial access taiIs to access dffkmtradii of the MU disk. Figure 6 1 shaws the optitxi1 pathof the optical head. The head uses an astigmatic fbcus sensor, outrigger spot tmkng system, and differentid data signal detection via a half-waveplateand PBS. To reduce complexity, the head does not use a beam expansion prism, as discussed in Sec. 3.2,

Figure 62 shaws the Sony MiniDisc@optical head-carriage cornbinatim, As in the previous case,the entire h d moves on the radial access rails to accless diffkmt radii of the MO disc. The head is quite miniature and fits into the small portable drive form-factor required fbr the new Smy MieriDisc* System. Figure 63 shows the optical path ofthe head. Tbe head use an astigmatic fwws sensor, outrigger spot trackin& and a Wollaston

Heads and Lmem I1 f prism p o l d o n h splitter. This head also does without a bearn expansion prim in the incident light path.

Figure 61. optlcal p a h t of the Sony 5.25" MO drove optical head where u)and PI3 are the laser diode and photodetectors. (Used with pennissim fm Smy. )

Figure 6 2 Photo ofthe Sony MiniDis~" optical h e a d d a g e . (Usedwith pemissim

f m sony.1

112 Mugnet+Opt$cd Data Recording

F4guurc 64. photo of the IBM 3.9" MO opbrnechanical subasmbly showing a split optical head configuration. (Used m'fh pennidonfmm IM.

Heads and Lasers 113 5.2 Split Optical Heads Figure 64 shows IBM’s 3.5”MO disk drive opto-mechanical subassembly which implements a split optical head architecture. The fixed portion of the optics provides the collimated laser beam to the moving camage which contains a turning mirror and fine focus/tracking actuator with objective lens. The beam reflected from the disk returns to the fixed optics which also contains the focus, tracking, and FW data sensors. The moving mass is reduced in a split optical head architecture and excellent heat sinking is provided for the laser and electronics. Figure 65 showsthe optical path of the IBM split optical head system. The head implements separate paths for the astigmatic focus sensor, push pull tracking, and the differential data signal detection.

WRITE BLOCK

AUXILIARY DETEC

.LASER DIODE ASM BEAM BENDER

Fgure 65. The optical path of the IBM 3.5” MO drive split optical head. (Used with permissionfrom IBM.)

114 Magneto-Opn;CalDutu Recording

Figure 66 shows the MOST Mmuf&cturhgopto-mechanical subassembly for a 5 25’’ MO disk drive which also implenmentsa split optical head architect~~m.This unit can be used in a half-height or fidI-kight drive. Figure 67 p m t s the optical path of the MO split optical head. The head uses a differential l’lR f m s sensor, push-puII tracking, and differential data signal dewtion via a micro PBS at the data detector

Figure 66. Photo of the MOST 5.25” MO OpbMcchanical-Assembly. (Used with pemissim fmMOST Manufaciuring. 1

Figure 67. optical path of the MOST 525” M O split optical head.

pmissimfrom MOSTMmfocfutfng.)

(Usedwith

Heads and Lasers 115 6.0

FUTURE IMPROVEMENTS

Design and development for future magnetosptical heads is a very active area of research. The efforts can be roughly grouped into the development of new components and the development of new head designs. Examples of components that would offer advantages for the future include compact blue laser sources, small Faraday isolators, low-mass objective lenses, and round-beam laser diodes. Examples of new head designs include miniaturization, integration, and multiple spot heads. Each of the abovementioned topics is an active subject of research, and as such, a proper discussion could fill its own book. Some of these advances will be discussed in Ch. 15. Here we will focus on compact, shorter wavelength light sources and the trends in new optical head designs.

6.1 Shorter Wavelength Light Sources A shorter wavelength light source would result in a smaller spot size at the disk which would support a higher storage density. This goal of higher capacity as well as other uses of blue light has driven the research for a compact blue light source. For example if the wavelength were reduced from 780 nm to 400 nm, the spot size would be reduced by almost a factor of 2. Without changing code or detection strategies (i.e., same resolution), the disk capacity could be increased by almost a factor of 4. An example of this capacity increasewith a blue light source has been demonstrated in Ref. 76. A number of other issues need to be addressed to realize this capacity increase and include: tighter tolerances on the focus and tracking servos, improved detector responsivity, lower read power, reduced Kerr rotation, and new mastering or formatting techniques. Because of the smaller spot size, the required optical power for thermomagnetic recording is lowered, hence the acceptable read power which preserves data integrity may also be lowered. This, combined with the lower responsivity of silicon photodetectors, the smaller magneto-optical effect for most materials at the shorter wavelength, will lead to more difficulty in obtaining the required signal-tonoise for this high capacity channel. A number of methods for developing compact blue light sources have been investigated (for a thorough review of the state of this subject see the Proceedings from the Compact Blue Light Source Annual Meeting, Ref. 77). These methods include blue/green laser diodes, frequency-doubled laser diodes, frequency-doubled diode pumped Nd:YAG lasers, and up-

116 Magneto-Optical Data Recording conversion lasers. Because of some long-lived lifetimes in states involved in the lasing action, the latter two methods are not directly modulatable and therefore are more appropriate for read-only systems. The first demonstration of a blue green laser has rekindled a great deal of interest in 11-VI semiconductors for laser materials. These ZnSe devices have been shown to be capable of relatively high powers (>lo0 mW/facet) but a great deal of development remains before these devices are a commodity Remaining issues include device lifetime, growth technology, doping levels, ohmic contacts, and defect density. These issues are being pursued for a host of short wavelength laser diode semiconductors. Very recently, Nichia Company has demonstrated pulsed generation, at room temperature, of a GaN semiconductor laser at 410 nm.[971 Frequencydoubled laser diodes are an attractivemethod of producing a compact blue laser source because they leverage the existing technology for high power, long life, 780-830 nm laser diodes. Resonant external cavity frequency doubling has been shown to be capable of very efficient The doubling (m50 mW of blue light from 125 mW of infrared complexity of this method makes its use in any but the highest performance, highest cost magneto-optical head doubtfil. It requires a precise temperature control and laser frequency servo control to keep the laser resonant with the external cavity. A number of issues including mechanical tolerances and laser aging, resulting in a mode-hop near the laser resonant frequency, remain to be addressed. Another method that is being vigorously pursued is quasi phase matching (QPM)in a nonlinear waveguide. This allows 'relatively long interaction lengths without the tight frequency control of the such as a resonant doubling.[811[821 With a single frequency laser distributed feedback or distributed Bragg reflector device, only a temperature servo may be required. The major issues include waveguide fabrication with appropriate phase matching and nonlinear properties and the tight mechanical tolerances associated with coupling into a singlemode waveguide. Although high power frequencydoubled laser diodes have been demonstrated in the laboratory, a number of issues must be addressed before they could be incorporated into a reliable commercial product. With the previously-mentioned system issues that arise from the incorporationof a shorter wavelength source in the optical recording system and the remaining progress to make a reliable compact blue source, it is unlikely that blue light sources will be responsible for any near term capacity increases in a consumer magneto-optical product.

-

Heads and Lasers 117 6.2 Head Miniaturization Hiroshi Nishihara has discussed studies of miniaturization of WO and MO optical disk pickups in Japan in Ref. 84. The following is a brief overview of approaches that have been taken or are under development to miniaturize only MO optical heads for the optical storage industry. Compact Designs. Conventional MO optical heads have been reduced in size by using smaller diameter optics, eliminating the beam expansion prism, using a single return path detection system, and through ingenuity in layout and packaging. Two MO heads that are representative are the Sony MiniDisc@head shown in Sec. 5.0 and an NEC head discussed by Yamanaka et al.ls5] Ohba et a1.[861discusses a compact MO pickup that uses a holographic optical element that performs all beam splitting functions and is a step towards the designs discussed in the next paragraph. LDGU Designs. Laser Diode Grating Units (LDGU) as described by Ophey[871from Philips or Hologram Laser Unit as marketed by appear to be the next practical step in optical head miniaturization. These units are already produced commercially for read-only heads-in particular for CD players. The concept of adding a beam splitter and detector array to a laser diode package was first introduced by Lee,Is91 (for example, US Patent 4,731,772). There are several benefits to putting the laser, beam splitter, and detector array into a single package. They include: miniaturization, lower costs, and improved stability of the focus error and tracking error sensor systems. A holographic beam splitter for optical pickups is also discussed by Kimura et a1.[901 Writelread WO versions of these heads are more challenging than playback heads due to the need for a good incident path efficiency and laser isolation for laser noise suppression. MO versions are also challenging due to the need for differential data signal detection with high SNR. Ophey[871 discusses how he expects MO heads to evolve from read-only LDGU to a HP-LDGU where HP means high power to provide a recording capability. Figure 68 (from Ophey[sfl) shows an LDGU providing the laser source and servo signal detection hctions, and a separate return path for the differential data signal detection. A recent example of a very compact MO-LDGU given by Kataya~na[~l] is shown in Fig. 69.

I I8 Magneto-Optical Data Recording

Figure 68. MO HP-LDGU optical head (less actuator).

Figure 69. Compact MO HP-LDGUreported on by Katayama et a1.[911 (Used with pemtission. )

Heads and Lasers 119 Integrated Optics Designs. Professor Nishihara, Professor Suhara, and Dr. Ura at Osaka University have been developing optical integrated circuits for optical pickups since 1985 and H. Sunagawa et al.ig2]discusses such an integrated optic detection device for MO disks (see Fig. 70). Even though a significant amount of good work has been performed in this technology area, it is likely that HP-LDGU type units will be commercialized first since there are yet significant practical issues to solve in integrated optics circuits that include: efficient coupling of the laser to the integrated circuit, high incident and return path efficiencies, and wavelength sensitivity. Perhaps when distributedfeedback (DFB) lasers are grown on the same substrate along with the integrated optical circuit, we will see practical implementations of these type of devices. In Ref. 84, Nishihara, Suhara, and Ura review several of the integrated-optic disc pickup devices.

DISK TRIFOCAL FOCUSING GRATING COUPLER

w AVEGu I

D~\F, rY

SUBSTRATE

7

$59 LENS

LASER DIODE

I I 92

@

Figure 70. Integrated optic detection circuit for an MO optical head.

120 MagnebOptical Data Recording Fiber Optic Designs. The use of single mode optical fibers to reduce the size and weight of WO optical heads was first proposed by Barnes et al.[=] in ’86 for the case of a flying optical head. Renard et al.rg41discuss an MO version of a flying optical head. Opsasnick et al.Ig5]have recently studied the use of a single mode polarization preserving fiber for detecting the MO Kerr rotation signal. This work was aimed at flying heads. Further development is needed to collect focus and perhaps push-pull tracking error signals should the technology be applied to conventional removable disk drives, perhaps by using several fibers.

ACKNOWLEDGMENTS The authors wish to acknowledge the assistance of Bob Metzger for preparing Figs. 24, 33, and 35 and the assistance of Scott Beckens in collecting the data presented in Figs. 10, 14, and 15. We also wish to acknowledge Professor Masud Mansuripur, of the Optical Sciences Center (Optical Data Storage Center) at the University of Arizona, for the use of his program “DIFFRACT” which was used to calculate and produce Figs. 27, 28, and 36. Finally, we wish to acknowledge Tim Gardner for information in Sec. 4.3on the hard sector noise problem.

REFERENCES 1. Thompson, G. H. B., Physics of Semiconductor Laser Devices, John Wiley, New York (1980) 2. Petennann, K., Laser Diode Modulation and Noise, Kluwer Academic Publishers, Boston (1988) 3. Agrawal, G. P., and Dutta, N. K., Long-WavelengthSemiconductorLusers, Van Nostrand Reinhold, New York (1986) 4. Zory, P. S.,Jr., (ed.), Quantum Well Lasers, Academic Press, San Diego, California (1993) 5. Marchant, A. B., Optical Recording: A Technical Overview, AddisonWesley Publishing (1990) 6. Kagawa, H., JEE, p. 108, (October, 1991) 7. Sharp Corporation, Laser Diode User’sManual (1988)

Heads and Lasers 121 8. Mitsubishi Electric Corporation, Optoelectronics Data Book (1992) 9. Nakata, I., JEE, pp. 3840, August (1988) 10. Cockerill, T. M., Honig, J., DeTemple, T. A., and Coleman, J. J., Appl. Phys. Lett., 59:2694 (1991) 11. Kahen, K. B., Shantharama, L. G., Shepherd, J. P., and Peterson, D. L., Appl. Phys. Lett., 62:1317 (1993) 12. Astigmatism Measurements Made Easy, Application Note 210, Photon Inc. (1992) 13. Casey, H. C., Jr., and Panish, M. B., Heterostructure Lasers Part B, Academic Press, New York (1978) 14. Milster, T. D., Benedict, M. K., and Stahl, R. P., SPIE Proc. 1316:143 (1990) 15. Hitachi Corporation, Optodevice Data Book (1989) 16. Gage, E. C., and Bartholomeuesz, B. J., J. Appl. Phys., 69569 (1991) 17. Kay, D. B., Chase, S. B., Gage, E. C., and Silverstein, B. D., SPIE Proc., 1499:281 (1991) 18. Gray, G. R., Ryan, A. T., Agrawal, G. P., and Gage, E. C., Opt. Eng., 32:739 (1993) 19. Ryan, A. T., Agrawal, G. P., Gray, G. R., and Gage, E. C., IEEEJ. Quant. Elec., 30:668 (1993) 20. DeCaro, J., “Characterizationand Suppressionof Noise in Optical Recording using Negative Electrical Feedback,” private communication (July, 1989) 21. Satoh, H., Kinoshita, Y., Okao, K., Tanaka, M., Nagatani, H., and Honguh, Y., SPIE Proc., 1499:324 (1991) 22. Hendricks, H. D., Cook, A. L., Shull, T. A., andMack, T. L., “Relative Intensity Noise Measurements of Laser Diodes and Laser Diode Arrays for NASA’s Space Optical Disk Recorder,” LEOS Annual Meeting (1992) 23. Arimoto, A., Ojima, M., Chinone, N., Oishi, A., Gotoh, T., and Ohnuki, N., Appl. Opt., 251398 (1986) 24. Call, D. E., and Finkelstein, B. I., SPIE Proc., 1078:272 (1989) 25. Ojima, M., Arimoto, A., Chinone, N., Gotoh, T., and Aiki, K., Appl. Opt., 25:1404 (1986) 26. Gage, E. C., and Beckens, S. B., SPIE Proc., 1316:199 (1990) 27. Stubkjaer, K., and Small, M. B., Electron. Lett., 19:388 (1983) 28. Kanada, T., Trans. IECEJpn., 68:180 (1985) 29. Yamada, M., and Higashi, T., IEEEJ. Quantum Electron., 27:380 (1991) 30. Scifres, D. R., Major, J., Jr., Zucker, E. P., Craig, R. R., Geels, R. S., and Welch, D. F., Optical Data Storage Topical Meeting ‘94, Dana Point, CA, paper MDl-1, SPIE, (1994)

122 Magneto-Optical Data Recording 31. Fukuda, M., Reliability and Degradation of Semiconductor Lasers and LEDs, Artech House, Boston (1991) 32. Benedict, M. K., Morrison, C. B., Tzou, A. J., Gleckler, A. D., Fried, D. W., and Giewont, K. J., SPIE Proc., 1043:318 (1989) 33. Fritz, W., SPIE Proc., 1043:368 (1989) 34. Land, E. H., “Collimating System,” US Patent 2,405,960 (1946) 35. Marchant, A. B., Appl. Opt., 23:670 (1984) 36. Maschmeyer, R O., SPIE Proc., 679:49 (1986) 37. Tanaka, Y., Nagaoka, Y., and Ueda, M., Jpn. J. Appl. Phys., 26:121 (1987) 38. Tanaka, Y., Murata, J., Takano, T., Shimizu, Y., Sunohara,M., Shirafuji, Y., and Aoyama, T., Natl. Tech. Rpt., 35: 138 (1989) 39. Kubalac, D., Morris, G. M., and Kay, D. B., Joint ISOWODS Meeting, paper F3.5,Maui, Hawaii (1993) 40. Born, M., and Wolf, E., Principles of Optics, Pergamon Press, Oxford ( 1970) 41. Richards, B., and Wolf, E., Proc. R. Soc. Ser. A, 253:358 (1959) 42. Sugaya, S., and Mansuripur, M., “Effects of Substrate Birefringence on Focusing and Tracking Servo-Signals in Magneto-Optical Disk Data Storage,”Appl. Opt., 335073 (1994) 43. Clark, J. R.,J.O.S.A., 31:488 (1941) 44. Mansuripur, M., LEOS Annual Meeting, paper E08.1, Boston (1990) 45. Mansuripur, M., Connell, G. A. N., and Goodman, J. W., J. Appl. Phys., 53:4485 (1982) 46. Lynch, R. T., Jr., and Levenson, M. D., SPIE Proc., 13 16:168 (1990) 47. Mansuripur, M., Appl. Phys. Lett., 55:716 (1989) 48. Schlichting, W., Ulster, T. D., Brucker, C., and Wang, M. S., “Signal and Noise with a Magnetic Circular Dichroism Detection System in Optical Data Storage,” Optical Data Storage Center Quarterly Report, (December 15, 1993) 49. Fukushima, N., and Sawaki, I., SPIE Proc., 1078:90 (1989) 50. Kurz, C. N., and M i d , J. J., “Magneto-Optic Readout Method and Apparatus Using Polarization Switching of Readout Beam,” US Patent 5,235,570 (1993) 51. Challener, W. A., and Rinehart, T. A., Appl. Opt., 26:3974 (1987) 52. Gage, E. C., Joint ISOWODS Meeting, paper F2.2, Maui, Hawaii (1993) 53. Nakao, T., Arimoto, A., and Takahashi, M., SPIE Proc., 1499:433 (1991)

Heads and Lasers 123 5 4. Olympus Optical Co., “Apparatus for Reproducing Information from

55. 56. 57. 58.

59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77.

Opto-Magnetic Recording Medium-Uses Differential Polarization Analysis Involving Quarter Wave Plate and Polarizing Beam Splitting Wollaston Prism,”European Patent 385-701-A (1991) Meadowlark Optics, Precision Optics Catalog, Longmont, Colorado (1990) Futamata, A., Moritsugu, M., Abe, F., and Ogawa, K., SPIE Proc., 1316:136 (1990) Finkelstein, B. I., and Williams, W. C., Appl. Opt., 27:703 (1988) Bouwhuis, G., Braat, J., Huijser, A., Pasman, J., van Rosmalen, G.,and Schouhamer Immink, K., Principles of Optical Disc Systems, Adam Hilger Ltd., Bristol and Boston (1985) Irie, M., Fujita, T., Shinoda, M., and Kondo, M., Jpn. J. Appl. Phys., 26:183 (1987) Bouwhuis, G., and Braat, J. J. M., Appl. Opt., 17:1993 (1978) Kay, D. B., US Patent 5,406,541 (1995) Katayama, R., Yoshihara, K., Yamanaka, Y., Tsunekane, M., Yoshida, K., and Kubota, K., SPIE Proc., 1078:98 (1989) Latta, M. R., Strand, T. C., and Zavislan, J. M., SPIE Proc., 1663:157 (1992) Bernacki, B. E., and Mansuripur, M., SPIE Proc., 1663:150 (1992) Yamamoto, M., Watabe, A., and Ukita, H., Topical Meeting on Optical Data Storage, paper ThCC2, Washington D.C. (1985) Ukita, H., Watabe, A., and Katoh, K., A Digest of Technical Papers of Conference on Lasers and Electro-Optics, paper WR-2, May (1983) Getreuer, K. W., SPIE Proc., 1663:128 (1992) Olympus Optical Co. Ltd., TAOHS-LC3 Optical Head Assembly. Marshall, D. R., SPIE Proc., 1499:332 (1991) Bernacki, B. E., Ph.D. Thesis, Optical Sciences Center of the University of Arizona (1992) Oka, M., Fukumoto, A., Osato, K., and Kubota, S., Jpn. J. Appl. Phys., 26:187 (1987) Milster, T. D., Wang, M. S., Froehlich, F. F., Kann, J. L., Treptau, J. P., and Emin, K. E., SPIE Proc., 1499:348 (1991) Durnin, J., Eastman Kodak Company, private communication (1993) Velzel, C. H. F., Appl. Opt., 17:2029 (1978) Pnkryl, I., SPIEProc., 1078:244(1989) Hmt, J. E., Jr., andKozlovsky, W. J., Jpn. J.Appl. Phys., 32:5301(1993) Proceedings from the Compact Blue Light Source Annual Meetings

124 Magneto-Optical Data Recording 78. Haas, M. A., Qiu, J., DePuydt, J. M., and Cheng, H., Appl. Phys. Lett., 59:1272 (1991) 79. Cheng, H., Joint ISOM/ODS Meeting, paper Tul 2, Maui, Hawaii (1993) 80. Kozlovsky, W. J., Lenth, W., Latta, E. E., Moser, A., and Bona, Appl. Phys. Lett., 56:2291 (1990) 81. Lim, E. J., Fejer, M. M., andByer, R. L., Electron. Lett., 25:74 (1989) 82. Webjorn, J., Laurell, F., and Arvidsson, G., in TopicalMeeting Nonlinear Guided-Wave Phenomena: Phys. Appl., Feb 2-4, (1989), Houston, TX, Tech. Dig. Series, vol. 2, Opt. Soc. Am., Washington, D.C., pp. 6-9 (1989) 83. Jens Buus, Single Frequency Semiconductor Lasers, SPIE Optical Engineering Press,Volume ' I T 5 in Tutorial Texts in Optical Engineering, Bellingham, Washington (1991) 84. Nishihara, H., Suhara, T., and Ura, S., SPIE Proc., 1663:26 (1992) 85. Yamanaka, Y., Katayama, R., Komatsu, Y., Ishikawa, S., Itoh, M., and Ono, Y., SPIE Proc., 1499:263 (1991) 86. Ohba, A., Sugama, S., Katayama, R., Urino, Y., and Ono, Y., SPIE Proc., 13 16:159 (1990) 87. Ophey, W. G., Jpn. J. Appl. Phys., 32:5252 (1993) 88. Sharp Corporation, i.e., Model LTOH30P. 89. Lee, W. H., Opt. Eng., 2866:650 (1989) 90. Kimura, Y., Sugama, S., and Ono, Y., Appl. Opt., 27:668 (1988) 91. Katayama, R., Ohba, A., Komatsu, Y., and Ono, Y., Optical Data Storage Topical Meeting '94, Dana Point, CA, paper WB3-l(l994) 92. Sunagawa, H., Ura, S., Suhara, T., andNishihara, H., Int. Symp. Optical Memory, paper FA7 (1987) 93. Barnes, F. S., Lee, K. S., and Smith, A. W., Appl. Opt., 25:4010 (1986) 94. Renard, S., and Valette, S., SPIEProc., 1499:238 (1991) 95. Opsasnick, M. N., Stancil, D. D., White, S. T., and Tsai, M. H., SPIE Proc., 1499:276 (1991) 96. Marchant, A. B., SPIE Proc., 695270 (1986) 97. Nakamura, S., Senoh, M., Nagahama, S., Iwasa, N., Yamada, T., Matsushita, T., Kiyoku, N., and Sugimoto, Y., Jpn. J. Appl. Phys, (Jan 1996)

Servos and Actuators Kurt I;t: Getreuer and Leo J. Grassens

1.0 INTRODUCTION In optical disk systems, the laser beam must be focused accurately on the disk track. Typical systems require the beam to be focused within one micrometer on the information layer and to be positioned on track center within one tenth of a micrometer. Meanwhile the axial and radial runout of optical disks are two to three orders of magnitude larger than the allowable focus and tracking errors and the optical head cannot be guided mechanically or aerodynamically by the disk. As a result the disk needs to be actively followed. This is done with feedback control systems, also referred to as servo systems. Although spot or beam positioning in principle would only require a focus and a tracking servo, tracking is almost always performed with two servo loops working interactively. These loops control two actuators, called theJine and course tracking actuators. Since the tracking servo system is usually not capable of performing random track access, the fine and coarse tracking actuators are driven over multiple tracks by another servo system, called the seek loop. Due to the tight spot position requirements and large disturbances, high gain focus and tracking servos are used with bandwidths of multiple IrHz. These bandwidths can only be achieved with dynamically well-behaved

I25

126 Magneto-Optical Data Recording actuators. This usually requires attention to any actuator resonance below 30 kHz. This can be challenging, because complicated interactions may exist between the three servos, the three actuators (focus, fine tracking and coarse tracking), and the servo loop input signals. Since the servo inputs for focus and tracking are derived optically, coupling between optics and mechanics needs to be considered. For example, we will show that the observed dynamic performance of a mechanical structure, that is, as perceived by the servo channel, depends on the choice of relay optics and sensing method. Consequently the central servo control issues in optical recording are: Robust and reliable position error sensing The development of dynamically well-behaved actuators and structures A solid understanding of interactions between servo, optics and mechanics In this chapter, we discuss the servo system and actuator technology used in the beam positioning servos in optical recording. Many texts are available on servo control, optics, dynamics and electromagnetics. On the other hand, not much literature exists that links these four fields to actuator technology. Therefore, the section on actuator technology covers more detail than the servo system section.

2.0 THE SERVO LOOP

2.1

Loop Elements in Optical Disk Drives

A basic position or seek control loop contains the following elements: sensor, actuator and loop controller. Optical Head Sensors. Optical head technology enables the generation of focus and tracking sensor signals with incredible resolution. Displacements in the order of ten nanometers can easily be picked up. Popular examples of focus methods are astigmatic, pupil obscuration, critical angle, Foucault knife-edge, spot size detection and wax-wane. Track sensing is usually achieved with either: (radial) push-pull, three-spot (twin-spot) or di-bit (wobble bits), or some derivation or combination of these methods. Most of these focus and track signal generation methods are explained by Braat"] and Marchant.[*]

Servos and Actuators 12 7 The sensor is an integral loop element and can significantly affect available margins. For example, sensor offsets can cause a defocus or offtrack condition. If sensor offsets cannot be corrected, then they need to be accounted for in an overall servo error budget, resulting in tighter requirements for the other loop elements. One sensor implementation may pick up an actuator resonance mode and reduce the servo loop stability, while another implementationmay not sense the same actuator mode.* At the end of Sec. 3.2, we discuss how the use of diflerential focus methods can improve the observed frequency response of an actuator. Actuator. Optical disk drives usually contain voice coil motors. These motors are similar to those found in loudspeakers, where coils are suspended in magnetic fields. Lorentz forces are created by applying current through coils (F = I x B 0. Section 3.4 discusses common motor configurations. Ideally the motor forces drive the dynamic mass with a predictable acceleration (following Newton's law F = mu) or deflect a spring suspension (F = kx) in a predetermined direction. This desired response occurs with a point mass suspended on a spring with one degree of freedom. Motion of a real world mass involves a minimumof six degrees of freedom (three translations and three rotations or combinations of these). Consequently, a rigid body on a suspension will display six rigid body modes, each potentially at a different natural resonance frequency. The rotational modes are often called pitch, yaw and roll. If the mass is not suspended, then all the rigid body modes occur at 0 hertz. Practical actuators are not rigid, so the actual structural rigidity is a key parameter in any actuator design. But it is as important to the actuator design that resonances (including unwanted rigid body modes) do not get driven. This requires careful attention to the effect of all forces (actuator motor, suspension, inertial, and external forces) on a structure and to the effect of a resonance on the sensor elements. Examples of sensor elements are a focus objective, a lens aperture, and a beam routing mirror or prism. A good design rule is that forces should act through the center of gravity (CG) of a structure, although this often must be compromisedin practical designs. We will show that it is also beneficial to mount sensor elements at the CG and to implement structural symmetry around the CG. Controller. The controller complements the above servo elements in such a way that the loop is stable and that the loop reduces given disturbance This should not come as a surprise: the perceived dynamic performance of many structures seems worse if sensing is done via a camera mounted on that structure (e.g., camera on skis, motorcycle, car, human).

128 Magneto-Optical Data Recording levels to acceptableservo errors. Part ofthe design task is to figure out what disturbance levels can be expected (like disk accelerations, external vibration levels) and how much of a total focus or tracking error budget can be allocated to these. For a seek servo; fast access involves large transients, so the loop response to a fast seek trajectory with large acceleration and decelerations needs to be determined. Besides its fundamental servo system fbnction, the controller typically performs many other functions such as initialization, enabling of the servo loops (acquisition), servo signal normalization and optimization, loop status and error detection, error handling, failure recovery and powerdown.

2.2

Servo Control Basi~sI~1[~1

Transfer Function. Linear systems can be characterized by a transfer function which describes the response of the system to an input. For example, for an ideal non-suspended voice coil actuator, the acceleration is proportional to the motor current by an acceleration constant Ka, since F = ma and F = I x B 1. Therefore the actuator position x(t) is proportional to the double integral of the current i(t):

This leads, after Laplace transformation, to the transfer fhction H,,(s) of position over current

where s is referred to as the Laplace operator, and can be thought of as a complex frequency s = c + iw. Position X(s) and current I(s) are the Laplace transforms of x(t) and i(t). Loop Stability and Damping. A closed-loop system is obtained by subtractingthe outputx(t) of an open-loop system H(s)from a control input r(t) and feeding the resulting difference e(t)= r(t)- x(t) back to the system input as shown in Fig. 1.

Servos and Actuators 129

Figure 1. Closed-loop system with open-loop system H(s).

From the closed-loop transfer function

Eq. (3) we see that the loop will be unstable when the open-loop transfer function H(s) = -1 (avoiding this condition is not sufficient-for stability, HC&) should not have any poles in the right half of the complex plane). Sensing the position of a voice coil actuator and feeding it right back into a motor driver creates a loop without any damping. Assuming other loop elements contribute only a frequency-independentgain G, then the loop will oscillate at an angular frequency

which is called the natural loop frequency, To stabilize the loop, a compensation filter needs to be added, which moves the phase of the open loop transfer function H(s) away from - 1 8 0 O for frequencies around the gain-crossoverfequency where the magnitude of H(s) is unity (0 dB). A common type of compensation is the lead network (Fig.2), which has a transfer function

130 Magneto-Optical Data Recording Voice coil and gain

Figure 2. System with lead compensation.

Stability margins and damping of a servo loop can be investigated with a Bode diagram or a Nyquist diagram. It is common to determine the phase margin from 180" at the crossover frequency and the gain margin from 0 dB when the phase is at 180". The peak responseMpof the closedloop servo (i.e., the maximum value of the magnitude of the closed-loop ranges of frequency response) can be estimated for phase margin 30-45"by

The peak response is about 1.4 with a phase margin of 45", and 2.0 for 30". Generally one strives for peaking values of less than 1.5 in order to avoid large overshoots after a step input and ringing after a transient. For a loop with an ideal actuator and a lead network, the optimum phase margin is obtained when the crossover frequency is set at 1 fodB = 2 4 %

In this case, it is convenientto write the open loop transfer function in terms of the angular crossover frequency ooa (= 2 z f O a )

1+-

H(s) = 1+ "OdB

Jar

Servos and Actuators 131 where a = tlrad/thg is the span of the lead network. In this case, the (the/natural ~ crossover is related to the natural loop frequencyas w,= ~ , , d loop fiequency of the loop in Fig. 2 is w,, = and the phase margin equals

a)

Eq. (Sj

Span values of nine or ten are popular. These spans create a maximumphase advance of 53.1" and 54.9". Fig. 3 gives an overview of loop variables in a Bode diagram. 20 log

10

-1

a OdB

Figure 3. Bode diagram of the open-loop transfer function of a voice coil with and without lead network.

The above lead network (Eq.5) provides phase margin over a large frequency range. A disadvantage is that it often creates a loop with not enough gain at low frequencies (<500 Hz) and too much gain at high frequencies (>15 kHz). This may be solved by using multiple leads with shorter span a. Another approach is to use a compensation with a complex lead (conjugate zero pair) and complex pole (conjugate pole pair). This gives more design flexibility since one damping ratio can be selected for the lead and one for the lag. Of course, improvements in gain will come at the expense of a narrower frequency range with phase margin. Phase Lag with Sampled Servo Signals. In some disk formats, the tracking signal is sampled from di-bits or wobble bits. The sampling introduces a frequency dependent phase lag. The transfer function of the sampler can be approximated with a zero-order hold

132 Magneto-Optical Data Recording

where T is the time between samples. Substitutionof s = io and rearrangement of terms (using sin[x]= [+- e-’”]/2i)results in the frequency response

HZOH

60)=

Tsin(O.5oT) e-i0.5cuT 0.5oT

The exponential term on the right hand side introduces a phase lag of 0.5wT. Example: with a sample frequency of 90 kHz and a crossover frequency of 3 kHz, we lose (3/90)x 180 = 6” of phase margin. Residual Servo Error. The difference between the desired loop response r(t) and the actual loop output x(t) is the servo error e(t), sometimes called the residual error (see Fig. 1). Its response to the input r(t) is given by

In optical drives, the focus and tracking error signals correspond directly to the residual error, since these signals sense the relative position between disk and laser beam spot. The desired spot position with respect to a global or actuator coordinate system is not available. The steady state error of a loop described by Eq. 7 to a position or velocity step input is zero. But for an acceleration step input A (where R(s) = A / s 3), the servo will respond with a finite position error

which follows from the final-value theorem

For frequencies well below the crossover frequency, sinusoidalaccelerations with amplitude A cause the same steady state error. Assume that

Servos and Actuators 133 we allow a steady state tracking error of 0.05 pm due to a track acceleration of 10 m/s2, then the crossover frequencyof the tracking loop needs to be 3.9 kHz (with a lead network span a of nine). The same crossover frequency is required in a focus servo if we allow 0.5 pm of focus error due to a disk acceleration of 100 m/s2. Bandwidth. Often the crossover frequency value is equated with the servo bandwidth. This is not correct, because the servo bandwidth BW is defined as the frequency where the magnitude of the closed-loop transfer For the loop of Eq. 7 with a lead network span h c t i o n is 0.707 (or -3 a). aofnine,theBW= 1.64j&.

2.3

Optical Disk Runouts

Specificationsof optical disk runouts are available for compact disks and severalgenerations of 3.5" and 5.25" rewritable disks. Specificationsof first generation disk standards are listed in Table 1. Because disk accelerations change with spindle speed, the standards call out a test spindle speed. Table 1. Runouts of optical disks

Disk Standard

Compact Disk"

Spindle speed referenced in standard Runout amplitude in focus Total runout in tracking Acceleration in focuse Acceleration in trackinge

1.3 m/s 0.4 % 140 cun, 10 m / s 2 0.4 m/s2

5.25" 3.5" Rewritableb Rewritable" 1800 rpm

1800 rpm

0.22m m

0.3 mm

50 W P

50 tu"pp

10 m / s 2 3 m/s2

20 m / s 2 6 m/s2

Notes: aECMA-130:Data Interchange on Read-only 120 mm Optical Data Disks (CD-ROM) bISO/IEC 10090: 90 mm Rewritable Optical Disk Cartridge. cISO/IEC 10089: 130 mm Rewritable Optical Disk Cartridge dThe spindle speed is varied such that the linear track velocity is 1.3 d s . This is called constant linear velocity (CLV) as opposed to constant angular velocity (CAV). The spindle rotation changes from approximately 600 rpm at the inner radius to 240 rpm at the outer radius. CNote that the accelerations are higher than what would be calculated from the runout amplitudes and the reference spindle speed.

134 Magneto-Optical Data Recording Because higher data rates and shorter latency times improve the product performance, there is a continuous push for increased spindle speeds (the latency is half the rotation period and is typically added to the random seek time to obtain the random access time). As usual there is no gain without pain: doubling the spindle speed quadruples the disk accelerations. To follow these accelerationswithout any increase in servo error, the servo bandwidth needs to be doubled. Also the actuator will dissipate sixteen times more power unless its motor constant is quadrupled (without increasing the coil resistance). In Fig. 4,these parameters are graphed as a function of spindle speed. All parameters are normalized to one at 1800 rpm. Presently 5.25" drives are on the market with spindle speeds of 3600 rpm, so accelerationsof first generation IS0 disks can be expected up to 80 m / s 2 in focus and 24 m/s2 in tracking. Y

lo00

100

--' +latency - +data BW rate/ nquimd servo 1-I - +disk

accelerations

10

1

0.1

Figure 4. Optical drive parameters as function of spindle speed. All parameters are normalized to one at 1800 rpm.

Focus Loop. The focus loop is usually a straigh~onvardposition loop. Often a proportional integrator-with transfer function (1 + s z)/s z- is added

Servos and Actuators 135 to the loop to cancel gravity and actuator flexure forces. Even though the servo system is quite simple, it is usually a challenge to stay within acceptable defocus limits, since many error sources exist which lead to defocus. Table 2 is an example of a simple focus error budget. (A more rigorous approach would take the probability distribution of each error source into account. Without focus signal offset correction, the worst case focus error is usually unacceptable, as in the example in Table 2. Fortunately, several of these focus error sources are (semi) stationary, and their error contribution can be reduced with focus optimization routines. Table 2. Example of focus error allocation budget for system without focus signal optimization.

Defocus Source

Defocus urn

Disk accelerations External vibration Internal vibration Focus offset at alignment Focus shift over temperature Focus shift between read and write power Electrical offset Disk defects Disk noise Laser Noise Crosstalk of groove into focus

0.2 0.2 0.2 0.3 0.4 0.4 0.1 0.1 0.05 0.05 0.1

Worst case RMS

2.1

0.75

Defocus due to focus crosstalk may occur when using disk formats with continuous pregroo~e.[~] The diffraction of the spot at the pregroove generates a “push-pull” pattern, which is used to derive the tracking signal. Often this diffraction also causes unwanted variation in the focus error signal. This effect is called focus crosstalk or feedthrough. Especially during track initializationor seeking, the focus loop can severely distort the tracking signal by following the crosstalk.

136 Magneto-Optical Data Recording Figure 5 shows a time domain simulation of the tracking signal distortion for a system with a closed focus loop before track acquisition. Radial disk runout makes the tracks cross the spot at varying frequencies. This sweeps the frequency of the focus crosstalk. Due to its finite bandwidth,the focus servo responseto the focus crosstalk is frequency dependent: At low track cross frequencies, the focus servo is able to drive the focus signal to 0 volts, thus driving the actuator out of focus At intermediate frequencies around the crossover frequency of the focus servo, even more defocus may occur due to low loop damping At frequencies well above the servo bandwidth, the loop is not able to follow the crosstalk and no additional defocus occurs (although it is possible that the focus crosstalk introduces saturation in some of the loop electronics, which may change the focus position). 9

1

I

I

I

I

I

I

I

I

8

I

I

I

I

I

I

I

I

I

I

I

1 0

TIME in ms

I 20

Figure 5. Distortion of tracking signal due to focus crosstalk. Top: focus error signal. Bottom: radial push-pull tracking signal.

Servos and Actuators 13 7 Tracking Loop. Common practice in optical disk drives is the use of a dual tracking system. One popular scheme is a master-slave system, where two basic position loops are tied together. The master loop controls the position of a fine tracking actuator using the tracking error signal (generated with the disk and laser optics). The slave loop controls a coarse actuator such that it follows the fine tracking actuator. The feedback in the slave loop is usually provided by a position sensor which measures the relative distance between the fine and coarse motor. Figure 6 shows an example of a master-slave tracking-seek servo. Low cost drives such as compact disc players do not have a position sensor. Instead,they feed a signal corresponding to the fine motor current into the coarse loop. The coarse loop then tries to reduce the average fine motor current. This implementation assumes a spring relation between the average fine actuator position and the spring force (F = kr) and a Lorentzforce motor (F = K f l ) . The position is then proportional to the fine motor current by a constant K/k.

opto-mechanics

Figure 6. Block diagram of master-slave tracking-seek servo.

Seek Loop. In some fast seek systems, the master-slave approach is used again. In its simplest form, the fine tracking actuator is driven to the new track position by controlling its velocity according to a desired velocity

138 Magneto-Optical Data Recording profile. Meanwhile, the coarse actuator just follows the fine actuator, driven by the same slave loop used during tracking. Although the coarse actuator is not directly controlled by the seek loop, seek will only be successful if the seek velocity profiles have been designed such that the coarse loop can keep the fine and coarse motor positions together.

2.5

Track Capture

The tracking error signal is a periodic function of the distance between track center and spot position. As a result, the servo loop response is nonlinear when the spot is away from track center, which is the case during track acquisition. Even though most tracking servo loops have large bandwidths and are capable of generating high accelerations, the margins for a direct lock on to the track are small. Below we discuss the capture behavior of an ideal and a practical servo loop using phase plane plots and time domain analysis. Since it turns out that the velocity of the head with respect to the track at the time of capture is important, we will look into the velocity error of a seek velocity servo. Phase Plane Analysis of Track Capture. The pull-in behavior of phase-lock loops is often analyzed with phase plane techniques.r61 These methods are usefbl in the investigation of the capture behavior of a simple tracking servo loop as shown in Fig. 7. For many optical drive systems, we can crudely approximate the tracking error as 4 2r e(t) = -sin(-[r(t) 2r 4

Eq.(13)

-x(t)]}

where q is the track pitch, r(t)the track position and x(t) the spot position.

e(t) =

Lead

Voice coil and gain

~~

Figure 7. Tracking position loop with nonlinear error signal e(t).

Servos and Actuators 139 The closed-looptransfer functionwhen the spot is near a track center, is

Eq. (14) or, written in terms of damping ratio z and natural loop frequency on,

which is identical to that of a second order phase-lock loop with an active filter (after phase lock). The spot position as function of the tracking error after Laplace transformation is

Suppose the disk does not move, so r(t) = 0 ,then combining Eq. 13 through Eq. 16 and transforming back to the time domain yields

Introducing the state variables

reduces EQ.17 to

140 Magneto-Optical Data Recording Finally, division of Eq. 18 by y2 = dyl/(dt w,,) and rearranging of terms gives

a dyl

= -2
sin@, 1 -Y2

Observe that the left hand side of Eq. 19 is proportional to the derivative of spot velocity to spot position. By calculatingthis derivative for multiple combinations of spot position and velocity, aphase plane plot can be constructed. Phase Plane Examples of Track Capture. Figure 8 shows phase plane trajectories over an area with five tracks for a loop with 0.5 damping. For low velocities, the tracking loop locks onto the nearest track center. This could be called direct capture. For higher velocities, the tracking servo skips one or more tracks before locking onto a track center (indirect capture).

1

2

3

4

POSITION [ c e n t e r o f t r a c k Figure 8. Phase plane plot of track capture for a loop with 0.5 damping.

5

#I

Servos and Actuators 141 Figure 9 gives a closer look of track capture for a loop with 0.7 damping. When closing the tracking loop one fifth to one fourth before track center, velocities can be captured up to

This corresponds to 12.5 mm/s for a loop with a 3 kHz crossover fiequency and a track pitch of 1.6 pm. A good rule of thumb is to use capture velocities of

V

0 W

>

- track/2

center

+ track/2

POSl TI ON Figure 9. Phase plane plot of track capture for a loop with 0.7 damping. Each vertical division equals qw,,4 2 4 .

142 Magneto-Optical Data Recording Track Capture with Practical Loops. The simple loop of Fig. 7 did not include any real-world parameters like limited available accelerations, additional lags, limited supply voltages, L/R time constants,fiction, flexure forces and resonances. An illustration is given in Figs. 10 and 11 using numerical h e domain d y s i s for a !mp with the fo!!ownz C W f i S t i c s : Lead network with a qeudof 0.3 ms and a qugof 0.01 ms

Voltage to current power amp with a maximum of 5 V over the motor coil Voice coil with 10 ohm resistance, a LIR time constant of 0.01 ms and a motor constant of 200 d s 2 A Loop gain K = 9-10' rad/s2; (natural loop frequency = 2 ~ 1 5 1 0rad/s). Figure 10 shows track capture with a capture velocity of 10.5 mm/s. One track is skipped before lock is accomplished. Doubling of only the lag time constant from 0.01 ms to 0.02 ms changes the capture dramatically as shown in Fig. 11. The loop cannot capture a track and will move the actuator into a crash stop.

-_ TRACKING ERROR SIGNAL

..................

................,

.*

* .... .* *

,,..a * .

..........

VELOCITY ERROR 10 mm/s div

_l_____._

--

I

I 1

1 I

0.4

I I

I

I

I

I

0.0

I

1.2

I I

I I

L TlnE

I I

lrns 1

Figure 10. Track capture for a loop with a lag time constant of 0.01 ms.

Servos and Actuators 143

TRACKING ERROR SIGNAL

VELOCITY ERROR 10 mm/s div

ACCELERATION 100 m/s 2 div

I

8

I I

0.4

I I

I

I I

0.0

1.2

I

I

I I

I I

I

I

L TIME ~ m s l

Figure 11. Track capture for a loop with a lag time constant of 0.02 ms.

Velocity Control During Seek. Since every seek needs to be completed with a track acquisition, we will now investigate how well a seek servo can control the track capture velocity. A typical seek loop controls the actuator velocity relative to a desired velocity profile. If we ignore the sampler in the seek velocity loop of Fig. 12, the velocity error response to an input velocity is

where E,,(s) is the Laplace transform of the velocity error evez(t), R(s) the Laplace transform of the input r(t), and A the open-loop gain.

144 Magneto-Optical Data Recording

Voice coil and gain

Figure 12. Velocity seek loop.

With a loop gain A of 2000 r d s , an acceleration of 10 m/s2 of the input r(t)will create a steady state velocity error of 5 mm/s . Of course, this error can be reduced by increasing the loop gain A. However, in many systems, the velocity is sampled by timing the interval between zero crossings of the tracking signal. This sampling creates a lag in the loop, and as a result the loop gain has to be restricted in order to guarantee stability and damping. The velocity sampler can be approximated with a zero-order hold in combination with a delay

where T is the time between zero crossings and Tdelwa delay time. The derivation of the velocity from successive zero crossings causes a delay of half the time between zero crossings. Usually there is some additional delay time Tcahafter the zero crossing before the loop updates the velocity error (Tdelw= 0.5 T + Tcab). The loop damping can be analyzed with a root locus diagram, which we create by mapping the poles of the z-transform of the closed-loop transfer function for various sample frequencies in the zdomain. The closed-loop transfer function of the seek loop including sampler after z-transformation is

Eq.(24)

H ( z ) = G(z,m) 1+ G(z,m)

in which G(z,m) is the modified z-transform of the open-loop transfer function

Servos and Actuators 145

Parameter m is related to the delay as m the denominator of Eq.24 are

~ q(26) .

=1

- Tdeh/T. The roots of

z1,2 = 05 1- AmT f ,/(AmT - 1)2 - 4AT(1- m)

1

Figure 13 shows the locations of the roots zIfor several values of the sampling fi-equency (1/T) and two loop gain values A. Figure 13 also contains contours of constant loop damping. The bottom half of the z-plane is not displayed: it is a mirror image of the top half. Note that for the loop with a gain of 4,000 r d s , the loop is unstable for zero crossing frequencies below 2.5 kHz. This corresponds to velocities below 2 m m / s for systems

Z PLANE sampling frequency +i 1 .0\'

- 1 .o

kHz

I

*1.0

Figure 13. Root locus plot of sampled seek loop with varying zero crossing frequencies.

146 Magneto-Optical Data Recording Improving Track Capture and Seek Velocity Control. We have seen that for successful track capture with a simple position servo, the actuator velocity relative to the track needs to be below about ten millimeters per second. Meanwhile, the gain of a seek velocity loop that samples velocity by timing track crossing intervals cannot be arbitrarily set. Low track capture velocities at the end of the seek require a limited gain in order to warrant sufficient loop damping. Due to the limited gain, the seek servo cannot maintain the capture velocity within a few millimeters per second in the presence of disk runout, external vibrations, or when fast seek times are required. As a result, track capture problems may occur. To guarantee acceptable seek error rates, various methods have been devised to improve margins either during seek or during track capture. The seek loop gain can be increased by adding signals to the sampled track cross velocity. Examples are Adding an external position, velocity, or acceleration transducer signal. Using an estimator for the head velocity. For ideal voice coil motors, the current is directly related to the acceleration. By integrating the voice coil current the head velocity can be estimated."1 Derive additional count signals while crossing tracks to double the sample frequency. In some cases, it is possible to generate signals that are in quadrature with the tracking error signal (cosine signal). Zero-crossing detection of this signal and the regular tracking signal creates four counts per track (rather than the usual two). Correction of the Loop Damping. Improving the pull-in behavior can be done as follows. From the phase plane plot in Fig. 8, one can see that the tracking servo accelerates the actuator when the spot is more than a quarter track away from any track center. In these areas, the tracking signal slope reverses sign and, as a result, the loop damping becomes negative (since the loop damping is created by differentiatingthe tracking signal with a lead network). In Fig. 14, a tracking loop is depicted which keeps the damping positive by switching the lead compensation when crossing quarter-track boundaries. Figure 15 shows the improved track capture behavior in a phase plane plot.

Servos and Actuators 147 Voice coil

Tracking signal generation

I 'disk

<

Figure 14. Tracking loop with controlled damping by switching lead networks at quartertrack boundaries.

POSITION [center o f track '1

Figure 15. Phase plane plot of track capture for the loop of Fig. 14.

I48 Magneto-Optical Data Recording Doubling of the Trackmg Signal Period. In some sampled servo disk formats, the track capture range is doubled by alternating the offset of the di-bits from track center (even-odd track format). Radially aligned sets of di-bits (wobble bits) are offset to the left side of a track on even numbered tracks and to the right on odd numbered tracks. The trackmg signal goes through only halfa cycle per track. The polarity of the trackmg signal needs to be switched when trackmg on collsecufivetracks (see Fig. 16 a, b).

wobble bit

Figure 16a. Standard sampled servo format.

Figure 16b. E vdodd sampled servo track format.

3.0 ACTUATOR TECHNOLOGY

3.1

Architectures

The actuator function for an optical storage system is to have controlled access to all the storage locations on the storage medium. Known optical technologies today are compact disk, optical tape, optical card, and optical disks. Presently disk drive systems are available with disk diameters from 2.5" to 14". We will only discuss disk-based systems. Different actuator architectures can be selected to accomplish the access functions described above. A typical product engineering trade-off has to be made between cost, performance, and complexity. Required performance should be weighed against cost and complexity.

Servos and Actuators 149 The performance requirements are controlled by: Disk runout and disk rotation speed Required access time Recording density in the data direction and distance between tracks Allowable focus error Allowable radial error External shock and vibration requirements Operational temperature range All of these lead to more or less demanding requirements for the actuator Allowable focus and radial errors should be part of an error budget. The parameters that control actuator performance are bandwidth of the focus and radial servos and the accelerations that the actuators can perform. The dynamics are a function of the construction of the actuator and have to do with the mass and the rigidity of the moving system. The acceleration is constrained by the power dissipation of the motors and the maximum temperature allowed. Looking at an optical recording architecture from an actuator standpoint, the followinggenealogy could be used:

' * * OPTICAL SYSTEM

SPLIT OPTICS

MOVING OPTICS LINEAR

ROTARY

LINEAR

ROTARY

Split Optics and Moving Optics. Some configurationshave all the optics moving along with the radial access system. This we call a moving optics design (Fig. 17). These systems are usually less complex and have cost benefits over split optics designs that are addressed later (Fig. 18). They can be found in earlier designs and they are still extensively used in lower performance systems such as CD players and CD-ROM.

150 Magneto-Optical Data Recording Moving Optics. Advantages are: Possibility of using finite conjugate optics. (No need for a collimated beam and possibly reducing component count.) No relay mirrors are required. Relatively simple alignment. Easier to use a rotary arm concept. A drawback is the large mass of the coarse actuator. This is where systems with split optics have an advantage. As the mass of the optoelectronics gets smaller, such as with integrated optics designs that use wafer fabrication technology, moving optics design may become more attractive from a cost/performance standpoint.

Figure 17. Moving optics design.

Split Optics. In split optics designs, the majority of the optics is not moving, but is mounted stationary into a base plate or optical module. The beam is reflected into the moving actuator using a relay mirror system. The actuator typically has one mirror or prism and an objective lens as optical elements (Fig. 18).

Servos and Actuators 151 Advantages are: The actuator has a low moving mass allowing faster disk access times. There are fewer constraints on optics complexity, mass and space. Better electrical signal conditioning for the detector signals is possible becausethere are h e r mass and geometryconstraints for the signal amplifiers, laser drivers, and electromagnetic shielding. In moving optics, the signals typically have to be transmitted over a rolling flex cable. The optical alignment of this system has to be maintained over the radial access stroke. This alignment can easily be accomplished with moderate tolerances.

Figure 18. Split optics design.

Linear or Rotary Actuators. Linear Actuators. A linear actuator system moves in a straight linear motion across the access area of the disk. The linear actuator can have a direct drive motor or can be driven by a rotational DC motor or stepper motor with a transmission. The carriage

152 Magneto-Optical Data Recording system needs a guidance or bearing system that makes it move in the radial axis direction and that controls the necessary degrees of freedom. A big advantage of the linear actuator is that the image on the disk and the returned image on the signal detectors remains constant over the radial access stroke. This is advantageous, for example, in the case of a radial push-pull servo system. A linear actuator has advantages in the case of a split optics design because the beam can easily be relayed with a mirror or prism. Rotury Actuators. There are several important advantages when using a rotating actuator concept: The construction of the bearing system is simpler. It is not coincidental that all the joints in the human body are rotation type joints. A rotary arm can be built with a shaft and two bearings or with two balls on either end of the rotation axis. A linear actuator typically requires two rails and six roller bearings or a slider bearing schemewith careful consideration for wear and mechanical constraints. Rotary arms have lower friction. One can use the Zeverage arm ratio (Fig. 19) to optimize the motor performance. It gives an additional design parameter that can be used to optimize the design for faster seeks.

Motor coil

mass Figure 19. Rotary actuator leverage arm ratio.

Servos and Actuators 153 A linear actuator could be compared to a car with a stick shift transmission that is broken. Only one gear ratio is working. The rotary actuator also has only one gear ratio working but the driver can select the gear ratio that is best for fast access times. The highest accelerationof the payload mass for a certain power input is achieved when

L1 -~2

I coilmass - payload mass

as derived in Sec. 3.4. The payload mass could consist of part of the arm and the objective lens and a turning mirror. Rotary coarse actuator systems with split optics (Fig. 20) are complex because the beam has to be mirrored with a periscope construction. The dual mirror system has to be mounted in the center of rotation. Center of rotation

optical module

Figure 20. Rotary actuator with split optics.

154 Magneto-Optical Data Recording Single-Stage, Dual Stage, and Galvo Systems. In most optical drives, the tracking accuracy can only be met with a two-stage tracking servo system. Tracking with a coarse and a fine actuator allows the fine tracking actuator to be light and small. This results in better dynamics, which is required for higher bandwidths. Typical servo bandwidths for the fine tracking servo are 1-6 kHz. Because of the low mass of the fine tracking actuator, higher accelerations with less power dissipation are possible in a dual-stage system compared to a single stage system. Several types of fine tracking actuators exist as shown in the tree below: Fine tracking actuator

Two-axes focus

2D-linear

2D-rotary

Upstream focus

Galvo

with carriage

on baseplate

2-0 Actuators. Most often, the fine tracking actuator is part of a two-axis motor that drives the objective in focus and tracking direction. The fine tracking actuator motion in these two-dimensionalor 2-0 actuators can be either linear or rotary. The suspension is very different between 2-D linear and 2-D rotary actuators as discussed in Sec. 3.3. Gafvo System Another type of fine tracking actuator is the galvo, which deflects a beam with a mirror. This results in a spot displacement at the disk after the deflected beam passes through the focus objective. The spot displacement equals the angle of the beam times the focal length of the objective lens. The galvo system can be moved along with the carriage assembly or it can be mounted on the baseplate, as is the case in some split optics designs. Three types of mirror rotation axes that one could select are shown in Fig. 2 1.

Servos and Actuators 155

Figure 21. Three galvo axes choices.

Note that each alternative has a different optical angle amplification. The factor for alternative (a) applies for small excursions around the nominal angle. Option (b) results in a beam angle that is twice the mirror rotation angle. Option (c) has no optical angle amplification. Fine tracking with a galvo has several advantages: More design freedom with less constraints because of separated functions. Separatingfunctions, in the case of a galvo that is stationary, simplifies the assembly. Although there are more subassemblies, the subassembliestend to be less complicated with fewer components and process steps for assembly. Because the functions are separated, the power dissipation in the motors is spatially distributed, making it easier to get rid of the dissipation energy. Mirrors can typically reach high g-levels. Issues with the application of galvos: Beam Walk The beam does not only make an angle that leads to spot displacement on the disk, it also moves relative to the aperture. The angle of the returned beam is canceled after its reflection on the same galvo. However, the beam will be offset on the detector systems in proportion to the mirror angle.151

a

156 Magneboptical Data Recording

Moving the galvo with the coarse carriage reduces the beam walk but does have some disadvantages such as increase in actuator mass and assembly complexity. Upstream Focus and Fine Tracking. An alternative technique for focus and fine tracking is the upstream focus or upstreamjne tracking techniquewhich uses a telescope lens pair, A pair of convex lenses is placed in a collimatedbeam. In the nominal position, both the lenses are centered in the beam. The distance between the two lenses is such that the beam, after passing through both lenses, is collimated again (see Fig. 22). By moving one of the lenses in the direction of the beam, the output beam becomes either convergent or divergent. After this beam passes through an objective lens, this affects the focus condition of the spot on the disk. Moving one of the lenses perpendicular to the beam axis introduces beam angle. This causes spot displacementon the disk after the deflected beam passes through the focus objective. Motor hnctions can be separated and mounted away from other parts of the design. In the case of a split optics design, it reduces the moving mass of the carriage if the upstream focus or fine tracking is mounted on a baseplate and not part of the moving carriage or arm. Separating the functions spatially allows higher power dissipation and higher g-levels. The dynamical performance of the servo system also benefits from separated functions. There are also drawbacks. Two additional optical elements are required with increased complexity and cost. If one wants to use inexpensive long focal length spherical lenses, larger excursions and higher g-levels are required than with a conventional focus or tracking actuator. To focus objective

Radial servo

c=s/

Focus servo

Figure 22. Upstream focus and fine actuator.

Servos and Actuators 157 3.2

Actuator Dynamics

Any mechanical structure has a least six degrees of freedom resulting in six resonance modes, called rigid body modes (three translations and three rotations or a combination of these). A typical camage assembly with a focus actuator on top requires the analysis of twelve rigid body modes. Fortunately dynamic behavior can often be understood with simple models with only a few degrees of freedom. Single Degree of Freedom System. In its simplest form, the actuator consists of an ideal voice coil motor with mass m and one degree of freedom x(t). The motion x(t) as function of motor current i(t) is given by Newton’s and Lorentz’s law

ma(?)= F(t)

Es. (27)

F(t) = i(t) x BZ

3

F ( t ) = Kfi(t)

where Kf is the force constant of the motor. After Laplace transformation, we find the actuator transfer function

The Bode plot of this actuator has a magnitude which drops 40 dI3 per frequency decade and a phase of -180’ for all frequencies. If the above actuator is suspended on a (onedimensional)spring with viscous damping, then the equation of motion changes to Eq. (29)

m q f ) +&(I)

+ la(?=)F ( t )

and the actuator transfer function to

Eq. (30) Many text books contain examples of the time and frequency response of second order systems which are described by a transfer function of the same f~rm.[~l[*] Table 3 lists parameter definitions used with second order systems.

158 Magneto-Optical Data Recording

Figure 23. Single degree of freedom mass-spring system with damper.

Table 3: Definitions for second-order system and single degree of freedom mass-spring system with damping ratios 5 < 1.

General second order transfer function

Natural fresuency q

Q(iiaIity)factor

Damping ratio 5

Damped natural fiequency

Frequency at which the phase of H(s = i@,Jis -90"For a mas5spring system: r ~ =, 4

K

Magnitude of H(s) at cv,, Qfactor = 1426) Fraction of critical damping

= 1). For a mas5

springsystem:

Frequency of free vibration, observed in the time domain after an impulse or step input. The response to a unit step is

Servos and Actuators 159 Table 3. (cont'd)

Resonance frequency m Ilnr...n..l

.\

Frequency where maximum response occurs during

..--.. ...A+A:..

, n1 U.1,

I,. :..I..,.. - LWdl 1-

+..

\l\G~UlUUlL U ~ U G l l b y )

c A IUlbGU

wr =wJ-

the Q-$actor and sometimes is called Peak resonance MPor Peaking

C*,.,...?...".

Maximum overshoot

GAblI(IL1UlI.

c . - ,r KU1 b

1

I=/-

pJ[J

11

blUX LU

Maximum deviation after a unit step response overshoot = e- MJ1-s.

Bandwidth

Frequency at which the magnitude drops 3 dB of its zero-frequency level

Two and Three Degree of Freedom Systems. Some actuator issues can be analyzed with simple two or three degree of freedom models. Here we will study a small dangling mass, a nonrigid motor connection, actuator pitch, and coupling between the fine and course tracking actuator. Actuator with Parasitic Mass. The two degrees of freedom system of Fig. 24 represents an actuator with a parasitic mass m2attached to its main structure ml.

160 Magneto-Optical Data Recording

-I

Figure 24: Actuator with a parasitic mass m2hanging off the main structure m,.

The equations of motion are

Eq.(31)

+)I

q z 2 ( r )+ d[i2 ( r ) - il(r)] + k[x2(I-)

=

o

Laplacetransformation and rearranging of terms, results in the actuator transfer function

-X , (s) 6(S)

+

2

s2 s d / q k/m, - s2 + 2C2W2S W 2 2 2 2 2 2 4 s (s + s d / M + k / M ) q s (s + 2 ~ , w , s + o , )

+

+

where A4 = mlm2/(ml+m2). Figure 25 shows an example of a transfer function with m2/m1= 0.5. (More precisely; shown is the Bode plot of the frequency response. The frequency response is obtained by substituting s = iw in the above equation.) Note the magnitude jump of the mass line after the resonance frequency. The jump equals (ml + m2)/m1. Characteristic for parasitic mass resonances is that the phase disturbance is always positive, because the antiresonance frequency q is smaller than the resonance frequency 0,. Most servos can handle this type of resonance if the parasitic mass is small (m2/m1< 0.1).

Servos and Actuators 161

-50

IHI

shift of mass line

-

dB

I I

0

I

Phase

degr. -360

I

I

Figure 25. Transfer function of actuator with parasitic mass of Fig. 24. The shift of the mass line equals (m,+m,)lm,.

Actuator with Compliant Motor Connection. Often a limited stiffness exists between the motor coil and the sensor. This can be analyzed with the same model (Fig. 24). In this case, we assume that mass m 1is the motor and mass m2 is a lens. The actuator transfer function is

Figure 26 shows an example (parameter values are different from those in Fig. 25). Note the 180" phase loss above frequency q,,sometimes called the decouphng resonance (the motor decouples from the sensor).

162 Magneto-Optical Data Recording -50

IHI dB

resonance

-

-

-

-2m

1

I

Phase degr.

Frequency in lcHz Figure 26. Transfer function of actuator with compliant motor connection. At very high frequencies the slope of the magnitude goes to -60 dB per decade and the phase to -270”.

Most servo systems require that this type of resonance occurs about a factor of ten above the crossover frequency. If the damping ratio & is low, then a lead network is not sufficient to stabilize the servo loop as shown in Fig. 27a. The loop will oscillate near the decoupling frequency. The instability is illustrated again in the Nyquist diagram in Fig. 27b. In a Nyquist diagram, the imaginarypart of the open-loop transfer functionH(s) is plotted versus the real part of H(s) for various frequencies s; or stated differently, the magnitude and phase of H(s) are plotted as polar variables. In Fig. 27b, the “-1” point is encircled by the decoupling resonance. It can be proven that this leads to a closed-loop transfer function that has poles with negative One way to stabilize the loop is by adding a second order filter

Es. (34)

Servos and Actuators 163 which rotates the phase away fkom -180’ near the resonance fkequency w,. The Bode diagram of the loop after adding a second order filter-with its natural frequency set somewhat below the resonance frequency w, and damping ratio 0.3-is depicted in Fig. 27c. The relative stability of this loop is better visible in the Nyquist diagram of Fig. 27d, where constant M circles are added. These circles correspond to loci-of-constant-peakingMp (Mpis the maximummagnitude of the closed-loop frequencyresponse). The peaking of the loop will be between 1.3 and 1.5, since the Nyquist diagram of the loop follows a path between the 1.3 and 1.5M circle.

z/ 7 1 -340 0.1

-4

1

Real(H,)

10

100

4

(b) Figure 27 a & b. Open-loop Bode plot (a) and Nyquist diagram (b) of servo With actuator with compliant motor connection and a lead network. The loop is unstable, since it encircles the “-1” point.

164 Magneboptical Data Recording

I

0

0.1

-4

1

10

1

100

Realm)

Figure 27 c & d. Bode plot (c) and Nyquist diagram (d) of loop with additional second order low pass filter. Included are constant M circles, which indicate the maximum ~gMp.Inthiscase,thepealungisbehKeen1.3and 1.5.

Actuator wirh pirch. Usually fine tracking actuators with flexure suspensions show a pitch mode in their transfer function. This effect can be modeled with the two degrees of fieedom system of Fig. 28, where pitch is the rotation angle 4.

Servos and Actuators 165

lens u

Figure 28. Actuator with two degrees of freedom suspension (one translation and one rotation)

Referring to the parameters indicated in Fig. 28 and assuming a mass m m d a mass moment of inertia J, we can write the motion {xcg(t), K t ) } of the center of gravity as

%g(t)

Eq.(35)

+

%&)

J a t ) + d,

+ &=(I)

=

qf)

81)+ 4t)= M(I ) C,

M ( t ) = @(I) where k and d are the translational stiffness and damping, and c, and d, the rotational stifhess and damping. The translation of the lens follows from x(t) = xcg(t) + uf(t). After combining the lens translation with Eq. 34 and Laplace transformation, we find the transfer function

which can be rewritten as

166 Magneto-Optical Data Recording

The translational mode is at frequency o1= (klm)’” with damping

5;= dl(2mq).The rotation or pitch mode is at frequency q? = (c,.IJ)~” with & = dJ(2JcuJ. An antiresonance occurs at frequency 0, = ,/(a&

+cr)/(uSm+ J )

with damping 5; =(a@+dr)/[2w,(aSm+J ) ] Figure 29 shows Bode plots with a positive and negative offset 8. The phase disturbance around the pitch frequency q can be positive or negative (since the antiresonance frequency w,can be smaller or larger than q). The sensitivity to pitch can be avoided altogether by mounting the lens at the CG (a = 0).

10

100

lo00

lo000

Frequency in Hz

Figure 29. Bode diagram for the actuator with pitch of Fig. 28. Solid line: the tracking motor force acts below the center of gravity ( d is negative). Dotted line: the force acts above the center of gravity (d is positive).

Servos and Actuators 167 Coupling between Fine and CoarseActuator. Cartridge designs for optical computer disks o h provide limited space for head access. Therefore, the fine tracking actuator is usually mounted on top of the coarse actuator. Unfortunately, reaction forces of the fine actuator motor now will have a moment arm Z, as shown in Fig. 30. The moment generated by the reaction force pitches the coarse actuator on its suspension and rotates the routing prism mounted in the coarse actuator. This causes a displacement of the spot at the disk.

........................

Figure 30. Fine actuator reaction forces driving a pitch mode of the coarse actuator.

The transfer function can be simulated with a three degrees of freedom model. Using variables as indicated in Fig. 30 and ignoring motor and suspension friction forces of the coarse actuator, we find for the equations of motion Fine translation: m A ( t ) + d [ ~ 1 ( t ) -X2((t)]+k~1(t)-x*(t)]=

Eq. (37)

4(t)

Coarse translation: m2Z2( t ) + u p 2( t ) - il(t)]+ k[x2( t ) - xl( t ) ] = -4 (t) Coarse pitch: J2bi, + dr&

+ c,&

Spot motion: xsPot= XI - 2&F

= F,(t)Zr

168 Magneto-Optical Data Recording where &,, is the focal length of the objective, k and d the translational stif€ness and damping of the fine actuator suspension, and c, and d, the rotational stifhess and damping of the coarse actuator suspension. Note that the reaction force F,,,li,, is equal to the fine tracking motor forceFl and creates a moment on the carriage body via an arm 1,. From this set of equations, we derive the transfer function

whereM= m1m2/(ml+m2).Figure 31 shows the Bode diagram.

10

100

1000

10000

Frequency in Hz Figure 31. Transfer function of the fme tracking actuator in Fig. 30. Solid line: the reaction force of the fme actuator drives a pitch mode of the coarse actuator. Dashed line: transfer function without reaction force.

In a design which is symmetric around the carriage CG, the fine actuator reaction force would line up with the carriage CG (1, = 0), and the sensitivityto carriage pitch would be avoided. Without a symmetric design,

Servos and Actuators 169 the sensitivity to carriage pitch can be eliminated by exchanging the rightangle prism with a penta-prism. Multiple Degree of Freedom System. The equation of motion of structures with many degrees of freedom x,, xz, ..., x, can be expressed in terms of mass, damping, and stif€ness matrices M, C and K Eq. (39)

MX(t ) + CX(t ) + Kx(t ) = F(t )

The displacement of degree of freedom xi due to a force acting at degree of freedom x,. is

where the right hand side is element ij of the matrix between brackets. Multiple-Input-Multiple-Output Systems. Because optical drives have several servos, interaction may exist between loops which alters the transfer function of an actuator. An approach is to describe the system as a multiple-input-multiple-output system in the following form

x(t) = Ax(t) + Bu(t)

y ( t ) = Cx(t)+ Du(t) The response Hnm of system output variable y,(t) to system input variable u,(t)

can be derived after Laplace transformation of Eq. 4 1 and rearrangement of terms from x(S) =

(SI - A)-'Bu(~)

[

Y(S) = ~ ( S I A)-'B

1

+ D U(S)

170 Magneboptical Data Recording where I is the unity matrix, and initial conditions are implied to be zero. The transfer function H,,(s) is element (n,m) of the transformation matrix between Y(s) and U(s). Coupling Between Focus and Tracking. As an example of a multiple-input-multiple-output system, we will discuss how the focus loop affects the tracking transfer function of the two-axes focus and tracking actuator shown in Fig. 32.

"4 6

Figure 32. Focus and tracking actuator with three degrees of freedom (two translations and one rotation).

The motion of the center of gravity {xcg(f),zcg(t)and f(t)}is described by

where m is the actuator mass, J its mass moment of inertia, k, and 4 the stifhess in X and 2 direction, c, the rotational stifhess and dithe damping ratios. The tracking and focus motor forces F, and F, are misaligned relative to the CG by d and e. The lens motion {xdt),zdt)} in the tracking and focus direction is

Servos and Actuators 171 With the focus loop open, the focus motor force F, is zero, and the tracking actuator transfer function H,= XJF, is the same as the one found earlier for a two degrees of W o m actuator with pitch. Now assume a closed focus loop in the presence of focus crosstalk. Traclang motion will modulate the focus error signal, to which the focus loop will respond with a focus motor force Fz(t). If the focus motor force is not in line with the lensholder CG, then the lensholder will pitch and move the lens in the traclung direction. For the focus loop of Fig. 33 and v(t) = 0 ,the focus motor force is

where

Gel

Kf

Gz Gxz z

- gain of loop electronics (AN) - focus motor force constant (N/A) - sensitivity of focus signal (V/m) - Sensitivity of focus crosstalk (V/m) lead time constant (s)

I

L

I

Focus detection

FOCUS

signal

...

Figure 33. Focus loop including focus crosstalk.

The system with closed focus loop can be written in the form of Eq. 41 as

Eq. (46)

1 72 Magneto-Optical Data Recording The matrices A, B and C are 0

1

-klfm -d,fm 0

A=

0

0

0

0

0

0

0

0

1

0

0

aC, -

aC, 7 -

C, -

C,-k, C,r m 0

m 0

Czr-d2 m 0

cc,

Q

Ec,

&,T

J

J

J

J

m 0

B=

0

0

and

c

=

m 0

m 1 ~ C , - C , mC,r-d,. J J

[ l o 0 0 a 01

where C,= -G,G,,K. and C,= -GzGeZKfThe tracking actuator transfer function of lens position over tracking motor force is

Two Bode plot examples with opposite polarities of the focus crosstalk sensitivityare shown in Fig. 34. The parameter values are CJC, = +0.5 and -0.5, S= 0 mm, E = 0.4 mm, m = kgm2, k,/m = (22c.30 kg, J = Hi)2, c,/J= (27~200 Hi)2,dJm = 0.2 (2r30Hi),d,/J= 0.04(2n*200B),a = 3 mm, focus loop gain CJm = lo8 rad/s2, and lead z = 0.1 ms. The disturbanceat the pitch frequency is clearly visible even though the tracking motor is centered (S=0). Also apparent is a phase deviation from -180" for frequencies around 1-2 kHz.

Servos and Actuators 173 In an actual drive, one can easily reverse the sign of the crosstalk slope by switching the desired tracking position from “track center” to and “between track centers.” For a system with a crosstalk amplitudeAxtuLk track pitch q, a crude model of the focus crosstalk signal is

in which lyis the phase angle between the crosstalk and tracking signal. For small deviations around a track position xo, the slope G,, is

w

xo = nq : Gxz = Axtalk (2 cos( ry) track center between track centers x, = (2n + 1) q/2 : G, = -Axtulk(2dq) cos( ry)

The above coupling between the focus servo and the tracking dynamics can be avoided by using a focus method without focus crosstalk, or a focus motor which acts through the lensholder CG, or by mounting the lens at the CG. The coupling can be reduced near track center with a crosstalk phase angle ryof 90’. Other examples of interaction between the focus, fine and coarse tracking servos can be found in Re. 18.

0

-1M

I

I

I

I

Phase degr. -360

10

1000

100

10000

Frequency in M

Figure 34. Tracking actuator transfer function affected by focus crosstalk. Solid line: the crosstalk slope G, is positive. Dotted line: the crosstalk slope G, is negative.

174 Magneto-Optical Data Recording Finite Element Analysis. Many texts are available on Finite Element Analy~is.[~l[~~] Here we will show how to derive transfer functions from FEA results. Ignoring damping, the equation of motion of a structure can be written in terms of mass and stiffhess matrices M and K

Eq.(49)

Mx(t) + Kx(t) = F(t)

If the external force vector F(t) is zero, then the equation of motion reduces to an eigenvalue problem, which is solved by Finite Element Analysis programs as

Eq. (50)

[-q2M+K]h ={O}

where mi - eigenvalue i or resonance frequency of mode i - eigenvector of mode shape i

oi

Because the mass matrix M is symmetric and the eigenvectors +iare orthogonal, a diagonal matrix can be created. Typical Finite Element programs scale each eigenvectorsuch that an identity matrix I is obtained with

in which F is the weighted modal matrix and @ its transpose. The columns of the modal matrix are the eigenvectors +i. Since stifiess matrix K is also symmetric, a diagonal matrix A can be formed containing 4as elements

The symmetry of the mass and stifiess matrices follows from the reciprocity theorem, which states that, for linear systems, the work done by forces Fiand F/ is independent of the order in which Fiand 5 are applied. If we introduce principal or normaZ coordinates q(t) = @lx(f) and rewrite the equation of motion as M@@-lX(t )

+ K@@-'x(t ) = F(t )

then after multiplying both sides by @ and using Eqs. 50 and 5 1, we obtain

Servos and Actuators 175

Frequency Response from FEA Results. For the derivation of the transfer function, we add viscous damping to the equation of motion Eq. (55)

M i ( t ) + CX(t ) + Kx(t ) = F(t )

In case ofproportional damping, the damping matrix relates to M andKas

As a result @CQ, will be diagonal and the equation of motion in principal coordinates is

Eq.(57)

q(t)+ Aq(t) + Aq(t) = mTF(t)

where A is a diagonal matrix with elements 25;.wi.Laplace transformation yields Eq. (58)

+

Q(s) = [s21 sA+ A]' OTF(s)

where initial conditions are implied to be zero. The matrix [S*I+SA+A]-~is diagonal with elements [s~+~&,s+o/]-~. Finally with X(s) = Q,Q(s), the displacement at nodej due to a force at node k or transfer function of position 4.over force F' becomes

in which N is the number of modes and 4Pnm is element nm of the modal matrix a). Perceived Actuator Dynamics and FEil. The observed actuator performance may depend on the optics implementation. Therefore, deriving only mechanical transferfitnctions is not relevant. System transfer functions can be studied by combining optical beam path models with Eq. 59.

I76 Magneto-Optical Data Recording Figures 35 and 36 show an example. The Finite Element model represents a two-axes focus-and-fine tracking actuator mounted on a coarse tracking actuator. The transfer function of focus signal over focus motor current is modeled for two cases: 1. Focus crosstalk is present. The focus error signal is sensitive to spot translations across the track. 2. No focus crosstalk. The focus error signal is not sensitive to spot translations across the track. The model includes reaction forces of the focus motor, because they may drive coarse actuator modes which lead to spot translations across the track.

Figure 35. Finite Element model of a two-axes focus-and-fine tracking actuator mounted on a coarse tracking actuator.

Servos and Actuators 177 -30

IHI dB -80

0 Phase &gr.

1

Frequency in kHz

10

(a) No focus crosstalk.

-30

IHI dB -80

0

Phase degr.

-360 1

Frequency in kHz

10

(b) With focus crosstalk. Figure 36. Focus actuator transfer functions synthesized for the above Finite Element model. The transfer function is H(s) = focus error signal /focus motor current.

178 Magneto-Optical Data Recording Note from the Bode plots in Fig. 36 that many more resonances show up in the presence of focus crosstalk. Focus crosstalk can be reduced by using dzflerential focus methods.[l11 The diffraction of the beam at the pregroove, which introduces the unwanted focus crosstalk, can be thought of as a form of pattern noise. Pattern noise is suppressed in differential focus methods by relaying the return-beam from the disk to two focus detector arrays. The detectors are combined in such a way that corresponding regions in the return beam are subtracted from each other. Because the observed performance of a mechanical structure depends directly on the selection of the sensor, we could state: “A good mechanical design is an optical illusion.” Besides the sensing method, proper placement of sensor and optical elements within the mechanics are important in creating this illusion. Critical is the location of the objective lens with respect to the actuator assembly. Unwanted motion of the spot at the disk due to actuator resonances and forces can be minimized by aligning the optical axis of the objective lens through the center of gravity (CG) of the actuator, especially if the actuator design is symmetric around the CG. Symmetry of the design ensures that all forces act through the CG and that the effective CG does not shift over the frequency range of interest. In an asymmetric design, the decoupling of parasitic masses above certain frequencies causes shifts ofthe effective CG may shift. Lens motion, specifically pitch and roll, can be suppressed even more if the center of the lens pupil coincides with the CG (as shown earlier for the actuator in Fig. 28). This can be thought of as placing the lens in the “eye of the storm,” where the “eye” is the actuator CG, and the “storm” the excitation due to resonances. Time Domain Analysis and FEA. Transient conditions of servo systems with complex dynamic structures can be simulated by combining Finite Element results with numerical time domain analysis as shown by Ernst in Ref. 12. This technique is for example usefbl to study actuator pitch during seeks.

3.3

Suspension Systems

Every mechanical object in space has six degrees of freedom: three translations in the X, Y and 2 directions and three rotations. We will use the notation system of Fig. 37 for these coordinates.

Servos and Actuators 179

Translations

Rotations

Figure 37. Definition of coordinates.

It is the task of a suspension system to restrain those degrees of freedomthat we want to be fixed, and to leave the other degree@)of freedom free. The restraining function is important so that alignment angles and positions can be maintained. It is important to realize that, even though these degrees of freedom are restrained, they are not infinitely stiff and an object could still be subject to resonances in the direction of the restraint. Attention needs to be paid to the number of restraints. Too often, systems are overconstrained without a designer realizing the implications. Overconstraining of a construction can be done if careful consideration is given to the construction detail, and if the designer realizes what the consequences are. In most cases, overconstraining will lead to increased mechanical hysteresis and backlash. This causes uncertainty in the positioning of features and can cause position inaccuracy. When subjected to mechanical forces or changes in temperature, position or angle alignments may change. A change of temperature may introduce thermal expansion differences and with overconstrained constructions this leads to micro creep. Overconstrainedconstructions should be avoided when high accuracies and good thermal stabilities are required. Figure 38 shows a cube in which every coordinate of the cube is controlled once by one truss element.

180 Magneto-Optical Data Recording A truss element can be thought of as a bar with two ball joints on each end. It can only transmit a force in the longitudinal direction of the truss element. Thus for this cube, exactly six degrees of freedom are controlled.

X

Figure 38. Example of a six-degrees-of-keedom-restrainedobject.

Fine Motor with Four Wire Suspension. The 2-Dactuator in Fig. 39 can move in the focus and the radial direction. The four wires are parallel and remain parallel over the radial and focus stroke. The lens will maintain its angle over the focus and radial excursion. The four wires can be used to provide current to the motor coils. Each wire controls one degree of freedom in the Y direction: Wire1 Wire2

Y1 Y2 togetherwith YI

controls: controls:

Y Rx

(tangential) (roll)

Wire 3 Wire4

Y3 togetherwithY1 Y4

controls: controls

Rz

(Yaw) overconstrained

Rx,Rz

The system is overconstrained for the roll and yaw rotations Rx and Rz. The pitch mode Ry is not controlled. This overconstrained situation caused by wire 4, can work because of the limited stifFnessof the wires and the assemblies they are mounted to. The pitch frequency is a function of the mass moment of inertia and the stiflhess of the wires and the assemblies the wires are mounted to. This example can be compared with a table. A table has four legs each trying to dictate the height of the table top. Since a plane is defined in space by three points, there could be a conflict. The fourth table leg makes the table overconstrained. It is only because table tops are usually

Servos and Actuators 181 torsion compliant, and because in many cases they rest on a compliant carpeted floor, that tables are not wobbly. A small four legged table on a garage floor is usually wobbly.

Figure 39: Actuator with four-wire suspension.

More Elaborate Four Arm Suspensions. To overcome the limitations of the four-wire suspension, and also to restrain the pitch mode, a system with wires or flexures at an angle can be used. The introduction of an angle avoids the overconstraint of the design. See Fig. 40. The motion in the radial direction is no longer exactly linear. For small excursionsthe armature rotates in an arc with the center of rotation at the pole P. The followingexplains the pitch stifhess increase or constraint. The wires are no longer acting purely in the Y direction. To understand how

182 Magneto-Optical Data Recording the pitch mode is now constrained, we can look at the virtual work theorem.[l31 If the 2-D actuator with parallel wires is subject to a certain pitch angle, the amount of work in the deflected wires is low because of the low mechanical energy that it takes to bend the wires. With wires at an angle, introducing a certain pitch angle requires two wires to become shorter and two wires to become longer. This elongation and shortening of the wires takes more energy or work than bending only. The rotation stifhess S for the pitch mode follows from

where Wis the work, T is the torque, and v,is the pitch angle. For a single suspension wire in tension or compression, the work equals

where E is the elasticity modulus, A is the cross sectional area of the wire, x is the elongation of the wire and I is the wire length. This will give a much higher stiflhess for pitch rotation than a system with parallel wires or suspension arms. Often damping material is added to the four-wire suspension to improve the dynamics.

Figure 40: Four-wire actuator with angled wires.

Servos and Actuators 183 Linear Actuator with Slide Bearings (Fig. 41). Slide bearings are attractive because of their low cost and low mass. A typical slide bearing system uses two rails with three bearings. Two round bearings slide on one rail and one slotted bearing slides on the other rail. The single bearing #3 is slotted to avoid overconstraining and possible lockup of the system. It will also make it tolerant to dimensional variations, in particular the distance between the rails and the angle between the two rails over the radial stroke.

slotted

I

bearings

Figure 41. Slide bearing system.

TheX axis is along the rails, Z is vertical, and Y is perpendicular to X and 2. Bearings 1,2, and 3 control these degrees of freedom: Bearing1 Y1, Z1 Bearing2 Y2, 22 Bearing3 23

together with Y1 andZl together with Z1 and 22

controls: Y, Z controls: Ry, Rz controls: Rx

This leaves the X coordinate free so that the actuator can slide along the rails.

184 Magneto-Optical Data Recording

An issue with this kind of bearing system is the clearance necessary to allow free motion with relatively low friction. The clearance poses two problems for optical recording systems: If the clearance is too large, the system will be on the opposite side of its clearancezone when it is held in differentorientations such as sideways or upside down. This could lead to unacceptable tilt angles for the objective lens and could cause unacceptable aberrations, in particular coma. When subject to either internal forces such as actuator reaction forces, or when subject to external system accelerations, the system will bounce through its clearance zones. Typical bearing materials that can be found in printers, floppy disk drives and CD players are porous bronze and special plastics with low wear rates. The porous bronze contains a lubricant such as a mineral oil. The rail material is usually a stainless steel with a hardness of at least 58 Rockwell C. Linear Actuator with Roller Bearings. A commonly found bearing system in optical drives uses ball bearings and rails. The friction is relatively low compared to slide bearing systems. Most implementations use six bearings, of which five control the five degrees of freedom that need to be constrained and one bearing provides preload. Figure 42 shows an example of a linear carriage with roller bearings. The carriage body is shown as transparent for clarity. The bearing at the lower right is the preload wheel and is equipped with a spring. We use the same coordinate system as with Fig. 4 1. The bearing system constrains the degrees of freedom as follows: Top Right front TopRightrear Top Left middle Lower Left front Lower Left rear

Z1 22 23 Y 1,Zl Y2,Z2

together with Z1 together with Z1 together with Z1 and 22 together with Y

controls: controls: controls: controls: controls:

Z Ry

Rx Y

Rz The last two bearings are not orthogonal to the coordinate system. It is the combination of these two bearings with the other bearings that makes the Y and Rz degrees of freedom restrained. The preload bearing at the lower right provides the loading of the system. It makes the system more tolerant for geometric changes over the stroke of the actuator, for example, when the rails are not exactly parallel. It is spring-loaded and assures that all the bearings remain in contact with the rails without play or “rattle.” It also compensates for wear.

Servos and Actuators 185

Figure 42. Perspective view of a carriage with six ball bearings.

The diagram of Fig. 43 shows various roller bearing schemes. The perspective view carriage shown in Fig. 42 corresponds with option h. In the diagram, the preload spring is shown with a symbolic spring and the symbol F. The numbers indicate the numbers of bearings in each location along the rail direction. As a rule of thumb, the preload force is about ten times the weight of the moving mass. This is a reasonable compromise that results in friction forces that are not too high, yet provides sufficient robustness against shock and vibration. Rotary Actuator Suspensions. Rotary actuators can be used in optical recording systems for both the fine and the coarse actuator system. The suspension for a rotary actuator system can be just like a magnetic disk drive rotary arm using two ball bearings. Less expensive schemes with balls or shafts with sleeve type bearings are an alternative. Careful consideration has to be given to the drawbacks of these less expensive schemes such as reduced stiflhess of the suspension or even rattle or play. Suppose the axis of rotation is the Z axis, then analysis of the degrees of freedom gives: Topbearing X1, Y1, Z1 controls: X, Y, Z Lowerbearing X2, Y2, 22 controls: Ry, Rx, Z

186 Magneto-Optical Data Recording

2X

1x

2x

2X

a: All orthogonal

b: Does not work 2X

2x

2x

2X

1x F

d: Same bearing orientation as c

c: Three rail scheme

1x

ZX

ZX

1x

2x

F f Often seen in Japanese designs

e: Same as d, mirrored

1x

2x

IX

2X

2x

F g: As e, with low building height

h: MMI actuator

Figure 43. Various roller bearing schemes.

In total there are six constrained coordinates and one, namely Rz,left free to move. Rz should be left free because it is the servo axis. The system is overconstrainedby one in the Zdirection. The conflict is with 2 1 and 22. They both control the vertical Z direction. To make the system work, one has to introduce compliance in the vertical direction. This can be

Servos and Actuators 187 accomplishedwith a design based on the limited stifiess of the bearings or the actuator. A better controlled solution is to build compliance in explicitly. Let one of the bearings move freely in the Zdirection and provide a spring that maintains the bearing load while restricting the motion of that bearing in the X and Y direction. This can be accomplished with the use of a belleville washer or an O-ring. Attention needs to be paid to the dynamics aspects of the rotary arm. A concept similar to that used in magnetic disk drives is likely to cause difficulties. Optical systems have a track spacing that is an order of magnitude smaller than magnetic disk drives. Secondly, because optical disks are generally removable, one has to cope with large runouts. This means higher bandwidth requirements for the radial servo than with magnetic disk drives. In most magnetic disk drives, the driving force of the actuator and the magnetic head are on opposite sides relative to the center of rotation. This means that, when driving the arm with a voice coil, the bearing has to produce a reaction force equal to the driving force of the voice coil but opposite in direction. The bearing is compliant and will deflect as a result of this reaction force. This affects the displacement of the magnetic head and will show up as resonance that limits the attainable servo bandwidth. Pin Type 2-DRotary Actuator (Fig. 44). Many optical disk drives and CD players use a pin type actuator. It is mounted on a carriage system €orthe coarse access. The design is simple and straightforward. The actuator rotates around a shaft or pin for the fine tracking motion. It can slide up and down along the shaft for focus. The actuator design is balanced to offset the mass of the objective. It is done such that a torque produces a pure rotation around the axis of rotation without reaction forces to the pin. Similarly the focus driving force will produce a pure translational motion in the focus direction without introducing tilt. Most designs have features built in that will keep the actuator loaded against the shaft to eliminate play. There are a couple of major drawbacks though. For high performance actuators with fast access requirements, high accelerationlevels will be reached during seek. This will result in high reaction forces from the shaft to the moving part. As a result the friction at the shaft will increase. Example: Supposethe 2-D actuator has a moving mass of 2 grams. The carriage does a seek with a 10 g acceleration. The friction coefficient between the shaft and the bearing is 0.1.

188 Magneto-Optical Data Recording The reaction force of the shaft equals Fh = m a = 0.002 kg x 100 m l s 2 = 0.2 N The friction force in the focus direction will be F,

= 0.1 Fh = 0.02 N

This is equivalent to a focus acceleration of af= F,/m

=

10 m/s2

-_

.

Figure 44. Pin type actuator.

If, during a seek, the disk height changes, friction will introduce a focus servo error. Another issue is that due to the limited shaft stifkess, reaction forces will excite a resonance. This limits the attainable servo bandwidth. Making the shaft thicker would help. Unfortunately, a thicker shaft causes a higher friction torque which negatively impacts the radial servo system. Galvo Mirror Suspensions. The design of a good suspension system is not trivial. Suspensions with ball bearings, sleeve bearings or jewel type bearings have friction or backlash. Friction directly results in servo errors and backlash could cause limit cycZing type resonances. Spring suspensions avoid friction issues and give the galvo a home position if no current is applied to the motor.

Servos and Actuators 189 Spring suspensions, however, often require more attention to rigid body mode shapes, frequencies, and their damping. Every solid body in space has six degrees of freedom. We want, for a galvo, one degree of fkdom to be free and controlled by the servo and the other five restrained by the suspension system. These degrees of freedom introduce six rigid body modes. Figure 45 gives an example (where we want only Rx to be free).

Y

X

Rx:0.2 kHz Y:9.2 kHz

X:4.6 kHz ............... j~" ..........,.... :: :

.........._.___

Rz: 11.3 ltHz

:::

~~

x 1

Z 9.1 kHz

Ry: 11.7 kHz

Figure 45. Rigid body modes of galvo actuator with flexure suspension.

190 Magneto-Optical Data Recording A good galvo design has the following characteristics: Only the rotation axis of the mirror is driven. Other possible modes of vibration are not excited. This is accomplished by carefid balancing and driving the galvo mirror at its center of rotation only. Ifthe mirror does get excited, it should be an excitation that does not affect the optical system. A rotation of a mirror, for example, with the axis perpedcular tothe reflectivesurfke is not noticeable. The second design rule requires that the rigid body modes couple as little as possible, meaning in this case that the translational modes X Z) and rotational modes Ry and Rz do not introduce any rotation Rx. Note that in Fig. 45, the Y rigid body mode introduces a slight rotation Rx. On a system level, this will cause a resonance-antiresonance pair in the tracking servo transfer fbnction.

v,

3.4

Voice Coil Design

Magnetic Circuit Configurations. Closed and Open-Magnet Circuits. The design of an efficient motor with permanent magnets involves creating a field with high flux density and inserting current carrying conductors perpendicular to this field. Two basic types of motor designs exist: 1. Closed-magnet systems. Magnetic steel is used to guide as much flux as possible across a small air gap. The coil wires are placed in the air gap. 2. Opn-magnet systems. No steel is used, although in some cases the magnet has a steel plate on its back to enhance the field. The coil wires are placed in the fringe fields of the magnet. Figure 46 shows examples of both type of circuits. Included is one other motor type with a moving iron armature, which is used in the wellknown Olympus TAOHS actuator. More rigid structures are possible with open-magnet circuits because coils do not have to be wrapped around yokes. Relative to closed-magnet designs, however, open-magnet designs require more attention to issues such as Stray fields at the disk Attraction to steel parts (rails, spindle motors, pole pieces) Fringe fields creating moments or unexpected force vectors (especially with an actuator which is not in its center position) Interaction between motors.

Servos and Actuators 191

t

t

t

t

l

I

I

t

t

I

t

t

t

s

Figure 46. Examples of voice coil motors with closed and open-magnet circuits. Arrows indicate the force on the coil. The top three and half cows show closed-magnet designs. The bottom two and half rows show open-magnet circuits.

192 Magneto-Optical Data Recording Moving-Coil and Moving-Magnet Designs. With open-magnet systems, the motor can be constructed with Moving coil(s) and stationary magnet(s) Moving magnet(s) and stationary coil(s) Movingcoil actuators usually are more power efficient because of a lower dynamic mass for a given motor constant. On the other hand, movingmagnet actuators allow larger dissipation, have superior dynamics and are easier to build, mostly because of a simpler coil wire termination . Simple Motor Models. Since electromagnetic modeling programs are relatively expensive, especially for three-dimensional problems, we discuss two simple modeling methods. For more background on electromagnetics and modeling, see Refs. 14 and 15. Lumped Parameter Method The magnet field in the air gap of a closed-magnet circuit can be approximated with simple hand calculations. For the circuit of Fig. 47, we assume that the permeability of the pole is much higher than that of air. This requires that the pole is not saturated. The flux through the magnet equals that through the air gap except for some leakage expressed by a leakage factor p

4= B,A,

= pBgAg

where B, is the flux density in the magnet with cross sectional area A, and Bg is the flux density in the air gap with area A,.

Am

magnet

pole piece

Figure 47. Closed-magnet circuit with air gap.

Servos and Actuators 193 The magnetomotive forceW a c r o s s the magnet is the same as that across the air gap except for some losses through the circuit expressed by a loss factor q

where H, is the magnetic field intensity at the magnet. The leakage and loss values are typically based upon experience with similarly shaped magnetic circuits. Using

where po is the permeability of free space, and combining the above equations leads to

This defines a line which intersects the demagnetization curve for the given magnet material at the workingpoint as illustrated in Fig. 48. Now the flux density Bg in the gap can be found from Eq. 62.

Bfi

load ' 1

line

I

B

T

H Figure 48. Demagnetization curve. The magnet working point is determined by the loadline with slope BIH.

194 Magneto-Optical Data Recording If the demagnetization curve is straight (the curve has “no knee,” or has been linearized), then for rare earth magnet materials we can approximate the demagnetization curve by

where B, is the residual induction of the magnet material. Now Bg can be expressed directly as

Ribbon Current Model. The fields of an open-magnet system can be derived by modeling the magnet as a ribbon-of-current on the magnet surface. For rare earth magnets, the relative permeability pr is close to unity, which allows the magnet to be modeled as a ribbon-current in air. The fields generated by a current camer follow from Biot-Savart’s law

B=-j-dZ & %xr 4n

iri3

The linear current densityj, on the magnet face is j,,, =B,& [Mm]. A benefit is that fields can be analyzed three dimensionally with a simple numerical program. Motor Optimization. Thermal considerations typically limit the motor accelerations. The temperature rise AT of a coil is, to first approximation, linear with the continuous power dissipation P in the coil

where h is a convection constant and Acoolinga cooling surface area. Therefore, one design approach is to optimize the acceleration for given power. This can be done by maximizing the followingfigure of merit:

Eq. (70)

figure ofmerit

=

K~/*

Servos and Actuators 195 where R is the coil resistance and K,the accelerationconstant in ms2/A(K, = Kf/m, the force constant divided by the dynamic mass). In the SI system, this figure of merit has the units m/(s2@). Actuator acceleration capabilities can be readily compared by using this figure of merit and assuming a dissipation of 1 watt. This exercise has only value when comparing similarly sized designs, since the above figure of merit favors smaller motors. Less powerefficient motor designs with large cooling areas may have better acceleration performance. Optimum Coil Muss For a single axis linear voice coil motor, the accelerationfor given power can be maximized by optimizing the coil mass. Suppose a “payload mass” mp is to be moved by a motor coil with mass m,iz. The coil has N turns-which turns out to be irrelevant for this analysis-and a cross-sectional area A. The current through A equals NI. Part of the coil-over the length L&s placed in a magnetic field B.

Figure 49. Coil in magnetic field.

The current through the coil in terms of power P and coil resistance R is

196 Magneto-Optical Data Recording The coil resistance follows from the coil material resistivity r and fill factory:

Eq. (72)

N'L R=r-

YA

and so where L is the coil inductance. The fill factor y determines how much of the coil cross section A carries current. For a coil wound with round coil wire and accounting for insulation and bonding layers, the fill factor is usually equal to or less than 0.6. From the motor force F = BNILeffand the acceleration f = F/m ,it follows that

where p is the density of the coil material. Note that the denominator equals the total dynamic mass m = mp + m,, and that the acceleration for given power P is independent of the number of turns N. Finally, from dfldA = 0 we find the optimum coil mass for maximum acceleration at a given power for a single axis linear motor

Sincethis optimum is flat, one can deviate from this condition and still have an adequate performance that fits the cost and performance expectations. For a two-axis focus and tracking motor, the above design rule does not apply. Optimization of Rotary Actuator. The acceleration of a rotary actuator for given motor force F can be optimized with the rotary arm lengths. Suppose a coil mass mcoilis at a distance a, from the center of rotation, and a payload mass mp is at an arm length up from the center of rotation, then the system is given by

Servos and Actuators 19 7

Jd =T

Equation of motion

:

Payload acceleration

: i=up+

Moment from motor

: T =u,F

..

Mass moment of inertia : J =mcojpi +mpu; From this set of equations, we find the payload acceleration

with 5 = up/um. The optimum length of the rotary arms for given force follows from d f / a{= 0 ,which leads to

-_ aupm

-d"-.'

mp

Optimum Coil Material. Coils can be wound with copper or aluminum wire. Copper has a higher density than aluminum, but a lower resistivity as shown in Table 4.

Table 4. Magnet wire parameters (at 20°C).

Copper

Density Resistivity at 2OoC Resistivity change with temperature

8.96 1.72 0.39

Aluminum

unit

2.70 2.79

1O3 kgm"

0.41

Yd°C

ohmem

198 Magneto-Optical Data Recording Which coil material leads to the best performance depends on the relative coil mass. For a given coil volume Vc0,,switching from copper to aluminum wire helps, when the ratio of figure of merits

is larger than 1. This is the case if the mass of the copper wound coil is larger

than: 3 1% of the total mass (total mass m = mp + mc0& or 44% of the payload mass. Similarly, the figure of merit increases when switching from aluminum to copper wire, if the mass of the aluminum wound coil is less than: 1 1.8% of the total mass,or 13.4% of the payload mass. Scaring Laws. It is often of interest to predict the performance improvement for a scaleddown version of an existing actuator design. We will discuss two scaling examples: 1. The actuator acceleration is power limited 2. The actuator acceleration is thermally limited Power Limited Acceleration. The acceleration a, for given power dissipation (Pis constant) is

a, =-

Eq. (79)

m

If all actuator dimensions in all directions are multiplied by a factor

v, then ‘scaled

- v3Leff - vm

dg,

= v -312 a,

For n = 0.5, the power limited accelerationimproves a factor 2.8. The If the seek time was power seek time is a function of the acceleration limited, then scaling changes the seek time as

Servos and Actuators 199 u4

Since the mass scales as v3,we could state that tS& = m . Lowering the mass by 50% reduces the seek time by 16%. Thermally Limited Acceleration. The acceleration a, for a constant temperature rise AT is

After scaling the design, the thermally limited acceleration is -112

ascaled

=

For v = 0.5, the thermally limited acceleration improves a factor 1.4. The thermally limited seek time changes with scaling as

1/12

In this case, the relation between seek time and mass is tseek= m . Lowering the mass by 50% reduces the seek time by 5.6%. These and other scaling examples are listed in Table 5 . Table 5. Effect of scaling actuator dimensions. All parameters are normalized at a scale factor of one.

Parameter

Power coefficient 0.9

0.8

0.7

0.5

-312 314

1.17 0.92

1.40 0.85

1.71 0.77

2.83 0.60

-112 114 3

1.05 0.97 0.73 0.73 1.17 0.81

1.12 0.95 0.51

1.20 0.92 0.34

1.41 0.84

0.51

0.34 1.71 0.49

0.125 2.83 0.25

VpOWH

Power limited: acceleration seek time Thermally limited: acceleration seek time Dynamic mass Power dissipation Figure of merit L/R time constant

Linear scale factor v

3 -312 2

1.40 0.64

0.125

200 Magneto-Optical Data Recording Shorted Turn. For fast seeks, large currents need to be quickly generated in the motor coil. This may require unreasonably large voltages over the coil, if the coil has a sizable LIR time constant due to a high selfinductance. The effective coil inductance can be decreased with a shorted turn, which consists of a copper sleeve wrapped around the pole piece as shown in Fig. 50.

=

copper sleeve

Figure 50. Copper shorted turn around pole piece to improve current to voltage response.

When applying current through the coil, a current is induced in the shorted turn, which counters some of the flux generated by the motor coil. This reduces the net flux change through the pole piece and lowers the permmce seen by the motor coil. The shorted turn can be analyzed with a transformer model, with the motor coil as the primary winding and the Using the subscripts c for the voice coil shorted turn as the and st for the shorted turn, the currents i, and is, are given by

where A4 is the mutual inductance, Kemf the back-emf constant, and V, the voltage over the coil. For now, we assume the coil does not move (no backemfvoltage)and we set the mutual inductancewith a coupling coefficient a to

Servos and Actuators 201

This leads, after Laplace transformation, to the current-to-voltage response

in which z, and ,z are the L/R time constants of the voice coil and the shorted turn. Although this equation has an exact analytical solution for the step response in the time domain, we simplify its denominator using s 2 a + s b + 1= (Sa/b+l)(sb+l) assuming b2>> a. This condition is met when the coupling coefficient a is close to 1, The current response to a voltage step over the coil is now

where rm = z,z, (1-&)/( z, + qt). A fixtion of the finalcurrent value (&,JR,) flows quickly into the coil with this time constant. Because zm << (r, + qt) and L,, k: L,/NL, this fraction is

paction

x

R C

R,

+N ~ R , ,

The above model assumesthat the shorted turn inductance and mutual inductance can be derived from the coil inductance. Wagner calculates all the inductances with a magnetic model in Ref. 16. The following example illustrates the shorted turn effect: If the shorted turn time constant is twice the time constant of ~ the coil and the coupling a is 0.95, then r, = 0 . 0 6 5 ~and fraction = 0.66. Figure 51 shows the current response for this example.

202 Magneto-Optical Data Recording Coil Impedance. Figure 51 compares the shorted turn current response with that of a coil with an impedance consisting of a pure selfinductance and a series resistance. The impedance of a coil around a steel pole piece is more complicated due to a shorted turn effect in the steel (eddy currents). For frequencies of interest, the resistance of the steel shorted turn is much higher than that of a copper shorted turn and is very frequency dependent. The frequency dependency can be modeled by calculating the effective thickness of the shorted turn from the skin depth. For good conductors, the skin depth S is

with r the resistivity andfthe frequency. The resistivity for certain magnetic steels is eight times that of copper and its relative permeability might be 1000 (depending on the flux density). At 1 kHz,the skin depth of copper is 2.1 mm, while only 0.19 mm for the steel.

F

1 0.8

a

0.6

0.4 1

0

0.2

"

0 0

1

2

3

4

5

Time divided by time constant of coil Figure 51. Current response to a voltage step. Solid line: with a shorted turn. Dashed line: coil without shorted turn (assuming simple inductance).

The time constant of the shorted turn will decrease with frequency, unless the shorted turn is thinner thanthe skin depth. The current-to-voltage response can be simulated with a frequency dependent time constant zSt(s)in

Servos and Actuators 203 Eq.83. Figure 52 shows examples of the transfer function of current over voltage for both a coil around a steel pole and a coil with a copper shorted turn. Note that the coil around the steel pole does not behave as a simple inductor plus resistor.

Figure 52. Transfer function of current over voltage for a voice coil. Solid line: coil around steel pole. Dashed line: with copper shorted turn.

Moving Shorted Turn. If the armature of an actuator is made of conductive material that couples inductively with the motor coil, then the actuator transfer function will be affected. This case can be thought of as a moving shorted turn. The moving shorted turn changes the second part of Eq. 81 to:

and the equation of motion for a simple voice coil actuator to

204 Magneto-Optical Data Recording Kcic(t)+ Ksrist(I)= mP(f)

Eq. (87)

where K, is the force constant of the voice coil and K,, is the force constant of the shorted turn. The number of variables can be reduced by working in SI units, in which case the b a c k e d constant value equals the force constant. Using the previous relationships between inductances and assuming that K,, = PK, / N (where p is a constant), we find the actuator transfer function

case P or ,z, is zero, the response is that of an with on= /~,/,/Zji . ~n ideal voice coil. Otherwise the transfer h c t i o n is altered as shown in the example in Fig. 53 (z,, = 0.18 ms, on= 2 r 160 r d s , and ap= 0.8 1). This phase dip decreases the phase margin of typical servos.

Phase

-

phase dip

I

-

degr. -360

I

I

Figure 53. Transfer function of position over current of voice coil actuator with moving shorted turn.

Servos and Actuators 205 LIST OF SYMBOLS acceleration flux density flux density in air gap flux density in magnet residual inductance bandwidth damping matrix rotational stiffness viscous damping constant rotational damping constant residual error signal velocity error force fkequency gain-crossover fkequency acceleration of gravity (9.8066 d s 2 ) gain sensitivity of focus crosstalk sensitivity of focus signal convection constant transfer function; magnetic field intensity closed-loop transfer finction actuator transfer finction transfer finction of zero-order hold electrical current in time domain unity matrix mass moment of inertia electrical current after Laplace transform

206 Magneto-Optical Data Recording spring constant; stifhess DC open-loop gain acceleration constant backemf constant force constant stifhess matrix length reaction force arm inductance; length mass; delay related parameter in modified z-transform modal mass; mutual inductance payload mass maximum magnitude of closed-loop frequency response mass matrix magneto motive force track pitch; magnetic leakage factor principal coordinates quality factor magnetic loss factor power dissipation servo loop input. Desired output. resistivity resistance Laplace operator time sample period delay time

Servos and Actuators 207 X

X W z

displacement in time domain displacement after Laplace transformation work displacement; eXP(-sl?

a

lead network span; mutual inductance coupling coefficient phase margin eigenvector element nm of modal matrix Q, modal matrix fill factor diagonal matrix with modal frequencies w? as elements permeability of free space relative permeability scaling factor density time constant lead time constant lag time constant time constant of WR of shorted turn angular frequency damped natural frequency natural frequency resonance frequency angular gain-crossover frequency damping ratio

208 Magneto-Optical Data Recording

REFERENCES 1. Bouwhuis, G., Braat, J. J. M., Huijser, A., Pasman, J., van Rosmalen, G., Schouhamer Immink, K., Principles of Optical Disc Systems, Adam Hilger, Bristol(l985) 2. Marchant, A. B., OpticalRecording, Addison-Wesley Publishing Company (1990) 3. Kuo, B. C., Automatic Control Systems, Prentice-Hall, Englewood Cliffs, NJ (1982) 4. Kuo, B. C., Digital Control Systems, Holt-Saunders, Tokyo (1981) 5. Getreuer, K. W., Optical Data Storage, SPIE Vol. 1663, pp. 128-135 (1992) 6. Gardner, F. M., Phaselock Techniques, 2nd edition, pp. 55-58, John Wiley & Sons, New York (1979) 7. Ogawa, M., Nakatsu, K., Hayashi, S.,Ito, O., Watanabe, I., Tanaka, K., Proc. Optical Memory Symposium, pp. 191-196, Optoelectronic Industry and Technology Development Association, Japan (1986) 8. Harris, C. M., Crede, C. E., Shock and Vibration Handbook, McGrawHill, New York (1987) 9. Kardestuncer, H., Nome, D. H., Finite Element Handbook, McGraw-Hill, New York (1987) 10. Thomson, W. T., Theory of Vibration with Applications, Prentice Hall (1972) 11. Marshall, D. R., Optical Data Storage, SPIE Vol. 1499, pp. 332-339 (1991) 12. Ernst, C. H., Optical Data Storage, SPIE Vol. 1499, pp. 129-135 (1991) 13. Rothbart, H. A., Mechanical Engineering Essentials Reference Guide, McGraw-Hill, New York (1988) 14. Paul, C. R, Nasar, S. A., Introduction to Electromagnetic Fields, McGrawHill, New York (1987) 15. Hoole, S. R., Computer-Aided Analysis and Design of Electromagnetic Devices, Elsevier, New York (1989) 16. Wagner, J. A., IEEE Transactions on Magnetics, MAG-18, No. 6 (Nov. 1982) 17. Grassens, L. J., Optical Data Storage, SPIE, Voi. 2338, pp 289-294 (1994) 18. Getreuer, K. W., Optical Memory and Neural Networks, Allerton Press, 3(4):369-376 (1994)

Media Substrates And Format Yoshimitsu Kobayashi

In this chapter, we discuss the basic concept of the substrate with format for the use of magneto-optical data recording, and technical details of mastering and molding. We also discuss the so-called “finishing process,” that is, protective coating, hubbing, and cartridging, together with optical and mechanical properties. All parts of the process of the media, except thin films of the recording layers are reviewed in this chapter. Mainly the 130 mm and 90 mm magneto-optical (MO) disks will be discussed because currentlycommerciallyavailable disks are of that dimension. This discussion of the 130 mm and 90 mm MO disks frequently refers to the IS0 standardizationdocuments ISO/IEC 10089r’l and DIS 10090.[21 1.0

DISK LAYOUT AND FUNCTIONAL AREAS

Disk formats currently used in the magneto-optical data recording are categorized into two types. One is based on the continuous composite servo tracking method. The other is based on the sampled servo tracking method.

209

21 0 Magneto-Optical Data Recording In the continuous composite servo (CCS) format, tracking is continuously centered between adjacent grooves that are preformed on the disk as shown in Fig. l(a). On the other hand, in the sampled servo format, the tracking error signal is derived from the signal amplitude difference at the two wobble marks in the servo area as shown in Fig. l(b). Here we will mainly discuss the CCS format because it is used more widely in the current optical recording.

Pregrooves Pregrooves Pregrooves Laser beam

-

(a)

0

0.-..-.--..Track center

~.~~..~.~~..~.~~..~.~~..

0

Wobble mark

I

(b)

Mark

Figure 1. Schematic diagrams of CCS format and sampled servo format.

1.1 IS0 Conventional CCS Format Here we show the CCS format for 130 mm rewritable optical disk cartridges specified by International Standard ISOAEC 10089.[I] The CCS format for 90 mm optical disk cartridges specified by International Standard ISOAEC 10090[*]is almost the same except for the recording area and the control track PEP zone as shown in Table l(b).

Media Substrates and Format 211 Table 1 (a). Formatted Zone for 130 mm Optical Disk Cartridges Zone ReflectiveZone Control Track PEP Zone Transition Zone for SFP Inner Control Track SFP Zone Inner ManufacturerZone User Zone Outer Manufacturer Zone Outer Control Track SFP Zone Lead-Out Zone

Radius (mm)

Track Number

27.00-29.00 29.00-29.50 29.50-29.52 29.52-29.70 19.70-30.00 30.0040.15 60.0040.15 60.15-60.50 60.50-61.00

-300 to -188 -187 to -1 0 to 18750 18751 to 18845 18846 to 19063 19064 to 19375

*Dimensionsand track numbers are given as reference only.

Table 1 (b). Formatted Zone for 90 mm Optical Disk Cartridges Zone Lead-in Zone Initial Zone Acquire Zone Lead-in tracks Focus tracks Inner Test Zone For manufacturer For drives Inner Control Zone User Zone Lead-out Zone Outer Control Zone Outer Test Zone For drives For manufacturer Buffer Zone *Dimensionsare given as reference only.

Radius (mm)

Track Number

22.60-22.90 22.90-23.53

-688 to -297 -296 to -293

23.53-23.75 23.75-23.97 23.97-24.00

-292 to -155 -154 to -17 -16 to-1

24.00-40.00

0 to 9999

40.00-40.02

10000 to 10015

40.02-40.24 40.24-40.46 40.46-41.OO

10016 to 10153 10154 to 10291 10292 to 10624

212 Magneto-Optical Data Recording Track Geometry. Grooves for continuous tracking and embossed pits for representing sector addresses are prerecorded on optical disks as the CCS format shown in Fig. 2. Except for the offset detectionflag (ODF) marks for detecting offset in the tracking servo caused by disk skew, all areas have continuous grooves spiraling outwards and user data is recorded on the center of the lands between the grooves.

Figure 2. Track geometry of the CCS format disk.

Formatted Zone. The formatted zone is divided as shown in Table l(a). The control track PEP zone is prerecorded using low frequency phaseencoded modulation. This zone does not include any servo information. The marks in all tracks of the PEP zone are radially aligned as shown in Fig. 3, so as to read without radial tracking being established by a drive. This zone includes the information necessary for reading the control track SFP zones (format, modulation code, baseline reflectance, maximum read power for the SFP zone, etc.). The transition zone for SFP is an area in which the format changes from the control track PEP zone without servo information to a zone including servo information.

The inner control track SFP zone consists of a band of tracks recorded by the same modulation method and format as are used in the user zone. This zone includes a duplicate of the PEP are used for reading and writing on the user zone by the drive.

Media Substrates and Format 213 The inner manufacturer zone is provided to allow the media manufacturer to perform tests on the disk, including Write operation in an area located away from recorded information. The user zone comprises 18751 tracks. The outer manufacturer zone is also provided for tests of the media manufacturer. The outer control track SFP zone includes the same information recorded in the same format as the inner control track SFP zone.

0 2 ~ 0 0 0 0 0 0 oooc3ooooo

6

xp:,, 0 0 0 0 0 0 0 0 ................ : .... .::.::,.= ,.:.:':.":.:.. ....:.:.:.:.. 0 00 0 00 .:.x.:

v

a

OOP

Laser beam path

Figure 3. An example of pit recording in PEP.

Sector Format. Sectors have one of the two layouts shown in Fig. 4, dependingon the number of user bytes in the data field. There are 17 sectors per track for 1024 user bytes per sector; there are 3 1 sectors per track for 5 12 user bytes per sector. Figure 5 shows a schematicdiagram of the sector layout for 1024 user bytes per sector. The pre-formatted area of 52 bytes, the header, is the same for both sector formats. Table 2 shows a summary of the sector structure. The sector mark (SM) is used for detecting the start position of the header. The sector mark pattern is shown in Table 2, where T corresponds to the time length of one channeZ bit. This pattern can be detected without using synchronized data clock because this pattern does not exist in data. There are four areas designated VFO 1, VF02,and VF03 to lock up the variablefiequency oscillator, VFO. The recorded

214 Magneto-Optical Data Recording information for VFOl and VF03 is identical in length and pattern. Since there are three ID fields and run-length Zimited (RLL)(2,7) modulation coding is used, the pattern chosen for each VF02 depends on the last byte of the CRC recorded in the preceding ID field. The address mark (AM) is used for detecting the decoding timing of the ID field. The AM is a channel bit pattern not used in RLL(2,7) and is a run-length violated for RLL(2,7). The ID and CRC field consists of 5 bytes. The first and second bytes of the ID and CRC field represent the track number, the third byte represents the ID number and the fourth and fifth bytes represent a 16-bits CRC computed over the first three bytes of this field. The postamble (PA) allows the last byte of the CRC in the ID3 field to achieve closure and permits the ID field to end always in a predictable manner. The ofset detectionflag (ODF) has neither grooves nor preformatted data, and is used for detecting offset in the tracking servo. The GAP field consists of an unrecorded area. The content of the flug fieZd is ignored in interchange for rewritable disks. The auto laser-power control (ALPC)field consists of an unrecorded area. It is intended for testing the laser power level by the drive. me duttzjeh’consisis ofeiifer 1259 bytes comprising 1U24 user bytes, 223 bytes for CRC, ECC, and resync, and 12 bytes for control information, or 650 bytes comprising 512 user bytes, 124 bytes for CRC, ECC, and resync, and 12 bytes for control information and 2 (FF)-bytes. The buffer field is used for completion of the RLL(2,7) coding scheme, and also used for motor speed tolerances and other electrical andor mechanical tolerance. The 8-bits bytes in the three ID fields and in the data field, except for resync bytes, are converted to channel bits on the disk according to the runlength limited code known as RLL(2,7) shown in Table 3. Each one channel bit is recorded as a mark produced by a write pulse of the appropriate power and width.

-

Media Substrates and Format 215

E

c

Pre-formattedHeader: 52 bytes

J

1274byles

1360bytes

0 M VFOl M

Pre-formattedHeader: 52 bytes

J

665bytes

J

746bytes

(b) Figure 4. (a) Sector format for 1024 user bytes, (b) sector format for 512 user bytes.

21 6 Magneto-Optical Data Recording

Figure 5. Schematic diagram of sector layout for 1024 user bytes per sector.

Table 2. Summary of Sector Structure Area

Function

SM

Sector Mark ( 5 bytes) Lock up field for PLL

Pattern

Note: VF02 varies depending on previous in CRC V F O 1: 0100 100100 1...o10010 VF02: 10010010010...o 10010 VF02: 00010010010...010010 VF03: 01001001001...010010

VFO 123

(12 bytes)

AM

Address Mark (bit/bytes sync) (1 byte)

0100 100000000100

ID and CRC

Track Number (2 bytes) Sector Number (1 byte)

Most Significant Byte Least Significant Byte bits 7-6: ID Number bit 5: 0 bits 4-0: Sector Number CRC polynomial seed = 1's

(8 bytes) (8 bytes) ( 12 bytes)

ID field check bytes (2 bytes)

Media Substrates and Format 21 7 Table 2. (Cont’d) Area

Function

Pattern

PA

Postamble (1 byte)

CRC RLL closure

ODF

Offset Detection Flag (1 byte)

Not Written, no groove

GAP

Splice (3 bytes)

Unformatted area

FLAG

Not specified ( 5 bytes)

Unformatted area

ALPC

Auto Laser Power Control (2 bytes)

Unformatted area

Redundant sync for data (3 bytes)

010000100100001000100010 01000100 1000001001001000

DATA

User Data, Control CRC, ECC, and Resync (1259/650 bytes)

Resync: 0010 0000 0010 0100

BUFFER

Postamble and RPM timing margin (20/15 bytes)

SYNC

ECC RLL closure and not written area

Table 3. Conversion of Input Bits to Channel Bits

Input bits 10 010 0010 11 01 1 001 1

000

Channel bits 0100 100100 00100100

1000 00 1000 00001000 000 100

21 8 Magneto-Optical Data Recording 1.2 Banded Format Banding Methodology and Capacity. Recently some new proposals have been made for the next generation of optical disk cartridges and some of them are used on the market. In these proposals, the disk capacity is extended by using banded format as well as increasing linear recording density and using narrower track pitch. In the banded format, the user zone is divided into bands. The number of sectors per track increases from band to band moving from the inner radius to the outer radius. This is because the data recording capacity per track increases from the inner radius to the outer radius with successive bands so that the spatial recording density becomes uniform in every band. Within each band, the recording capacity per track is constant. The capacity per surface increases as the number of bands increases.

Table 4. Formatted Zone for ZCAV Format of 1024 User Bytes per Sector.

Zone No. Radius (mm) Sectornrack 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

30.00-3 1.77 31.77-33.53 33.53-35.30 35.30-37.06 37.06-3 8.83 3 8.83-40.59 40.59-42.36 42.36-44.12 44.12-45.89 45.89-47.66 47.66-49.42 49.42-5 1.19 5 1.19-52.95 52.95-54.72 54.72-56.48 56.48-58.25 5 8.25-60.0 1

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

TracWZone 1177 1177 1177 1177 1177 1177 1177 1177 1177 1177 1177 1177 1177 1177 1177 1177 1177

Media Substrates and Format 21 9 Here we discuss some examples of the banded formats for 130 rnm rewritable optical disk cartridges which are currently used or will be used in the near future. ZCAV Format. Some vendors proposed ZCAV format (see Ref. 3). Table 4 shows the configuration of the formatted zone for this format. In this format, almost the same sector structure as the conventional format is adopted and the track pitch is set to 1.5 pm. The user zone is divided into 17 zones. The number of sectors per track increases from 17 to 33 moving from the inner radius to the outer radius. Sectors are lined up in each zone as shown in Fig. 6. As can be calculated from the total number of sectors, the capacity per surfacebecomes 5 12 Mbytes, which is about 1.5 times as large as the conventional format disk.

Figure 6. Appearance of ZCAV format disk.

2X Format (Extended Capacity). Table 5 shows the configuration of this format (see Ref. 4). Here, the track pitch is set to 1.39 pm. The user zone is divided into 16 bands. Each band contains the same number of revolutions of the spiral groove. These revolutions are organized into groups called revolution groups. Each band has the same number of revolution groups, namely 50. The number of tracks per revolution increases from group to group moving from the inner radius to the outer radius

220 Magneto-Optical Data Recording (from 32 to 62 trackshevolution group). This is because the data recording capacity per revolution increases from the inner radius to the outer radius with successive band. Within each band, the recording capacity per revolution is constant. Every revolution group comprises an integer number of tracks and it begins and ends on the same radial line. The hierarchy is: 17 or 3 1 sectors = I track (logical track) 32 to 62 tracks (27 revolutions) = 1 revolution group 50 revolution groups = 1 band 16 bands = the user zone Here, the sector structure is exactly the same as the conventional format. In this format, sectors are generally not radially aligned as shown in Fig. 7. As one can calculate from the total number of sectors, the capacity per surface becomes 654 or 596 Mbytes depending on the number of user bytes per sector, which is about double the capacity of the conventional format disk.

Figure 7. Schematic diagram of sector layout for 2X format.

Media Substrates and Format 221 Table 5. Formatted Zone for 2X Format Zone of Band Label Reflective

Radius Revolution Tracks per h n b e r Start-End Groups Revolution of Tracks Group

Start-End

Revolution NUmber Start-End

Track NUmber

17.50-27.00 27.00-29.00

NIA NIA

NIA NIA

NIA NIA

NIA NIA

NIA NIA

29.50-29.52 29.52-29.81 29.81-30.00 30.00-3 1.88 3 1.88-33.75 33.75-35.63 35.63-37.5 1 37.51-39.38 39.38-41.26 41.26-43.14 43.14-45.01 45.01-46.89 46.8948.77 48.77-50.64 50.64-52.52 52.52-54.40 54.40-56.27 56.27-58.15 58.15-60.02 60.02-60.14 60.1460.50 60.50-6 1.00

NIA NIA

NIA NIA 32 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 62 NIA NIA

14 209 160 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 186 26 1 360

-383-370 -369-161 -16-1 0-1599 1600-3299 3300-5099 5 100-6999 7000-8999 9000-11099 11100-13299 13300-15599 15600-17999 18000-20499 20500-23099 23100-25799 25800-28599 28600-31499 31500-34499 34500-37599 37600-37785 37786-38046 38047-38406

-358-345 -344-136 -135-1 0-1349 1350-2699 2700-4049 4050-5399 5400-6749 6750-8099 8100-9449 9450-10799 10800- 12145 12150-13495 13500-14845 14850-16195 16200- 17545 17550-1 8895 18900-20245 20250-21 595 21600-2168( 21681-21941 21942-22301

SFP Transitioi Inner SFP Inner m g Band 0 Band 1 Band 2 Band 3 Band 4 Band 5 Band 6 Band 7 Band 8 Band 9 Band 10 Band 11 Band 12 Band 13 Band 14 Band 15 outer M f g Outer SFP Lead Out

5 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 3 N/A NIA

Note: NIA indicatesNot Applicable

3X Format (2 Gbyte per Cartridge Capacity). In this format, the track pitch is set to 1.34pm and RLL( 1,7) mark edge recording is adopted to achieve 1 Gbytes per surface. Table 6 and 7 show the configurations of this format for 1024 and 5 12 user bytes per sector, respectively (see Ref. 5). Track 0 is located at the outer radius and the user zone is divided into 30 and 55 bands depending on the number of user bytes per sector. At boundaries between the bands, the last 12 tracks of a band and the first 4 tracks of the next band shall not be used to record user data in the user area.

222 Magneto-Optical Data Recording Table 6. Formatted Zone for 3X Format of 1024 User BytedSector leyolutiol

racks pe

Jumber

Gmups

lW0lUtiOl

of

59 59

1298 885

8o.itm.m

22 15 7

59

413

Band 0

6O.0059.W

44

59

2596

Band 1 Band 2 Band 3

59.0058.00

44

58

2552

58.0058.99

44

57

2508

56.99.55.99

44

58

Band 4 Band 5 Band 6

55.9954.99 54.9063.99

44 44

53.9S52.98

44

Band 7

52.8651.98

Band 8 Band 9 Band 10 Band 11 Band 12 Band 13

zone of Band

Radius Start-End

Lead-In

81.00.00.50

Outer SFP

80.5040.16

Outer Mlg

sectors pel RevoluUon

Track Number Stan-End -2596-1299

Revolutlon

-1298-41 4

-374-120

59

-413-1

-119-1

59

02595

0-747

59

2596.5147

7 4 8 1495

58

5148-7655

1496-2243

57

2464

7656-10119

2244-2991

56

55

2420

1012012539

2992-3739

55

54

2378

12540 14915

3740-4487

64

53

2332

t4916-17247

4400-5235

53

44

52

2280

17248-19535

5236-5983

52

51.gam.w

44

51

2244

19536-21779

59846731

51

50.9a49.98

44

50

2200

21 78023979

6732-7479

50

49.9a48.97 48.97-47.97 47.97-46.97

44

40

2156

49

48

82286975 8976-9723

48

47

21 12 2068

23980-26135 26 136-28247 28248-30315

?4806227

44 44

46.97-45.97

44

46

2024

30316-32339

9724-10471

46

Band 14

45.97-44.97

44

45

1980

3234034319

10472-11219

45

Band 15 Band 16 Band 17

44.97-43.96

44

44

1936

34320-36255

11220-1 1967

44

43.96-42.96

44

43

1892

36256-38147

ii96ai2715

43

42.8641.96

44

42

1848

38148-39995

~18-13483

42

Band 18

41.96-40.96 40.96-39.95

44

41

1804

39996-41799

1346614211

41

44

40

1760

41800-43559

14212-14959

40

39.9538.95

44

39

1716

43460-45275

14960- 15707

39

Band 21

38.95-37.95

44

38

1672

45276-46947

i~70~-16455

38

Band 22

37.9536.65

44

37

1628

46948-48575

16466-17203

37

Band 23 Band 24

36.9535.94

44

36

1584

48576.501 59

17204- 17951

38

35.94-34.94

44

35

1540

5O160-51699

17952- 18699

35

Band 25 Band 26

34.94-33.94

44

34

1496

18700 19447

34

33.94-32.94

44

33

1452

51700-53195 53 196-54647

19448-20195

33

Band 27

32.94-31.94

44

32

1408

54648-56055

20196-20943

32

Band 28

31.94-30.93

44

31

1364

56056-57419

20944-21691

31

Band 29

30.9129.93

44

1320

57ezo-sm9

21692.22439

30

Inner Mlg

29.93-29.70

300

5874059039

29.7029.52

30

240

5904059279

22440-22609 2261022745

30

Inner SFP

FP Transitio

29.52-29.50

PEP

29.5029.00

10 8 WA NIA NIA

rl 30

(mm)

Band 19 Band 20

Rdledive

d e :NIA im

29.00-27.00

- Grwp

-

Ues 'No( 4 1 abw

Number Stan-End -748-375

59

47

30

Media Substrates and Format 223 Table 7 (a). Formatted Zone for 3X Format of 512 User BytedSector ,Band 29) Zone of Band Lead-In Outer SFP Outer Mlg Band 0 Bud1 Band 2

80.5060.17 80.1760.00 80.M).50.48 59.46-58.92

Eiziz

Numbel

bvolutior Group

of

58.92-58.38

13 13 13

105 105 105 105 104 103

6 4

Sectors per Revolution

Tracks 1260

105 105 105 105

420 1365 1352 1339

-1260-421 420- 1 01364 1365-2718 27174055

-372-125 .124--1

8081208

104 103

840

(wo2

403-805

Band 3

58.30.57.84

13

102

1326

40565381

12041611

102

Band4 Band 5 Band 6 Band 7

57.84-57.30 57.30-56.76 56.7656.22

13

101 100

1313 1300 1287 1274

53828691

101

7985-9261

1612-2014 20152417 2418-2820

Band 6 Band 9

55.68-55.14 55.14-54.60 54.60-54.06 54.08-53.52 53.52-52.98

9262-10555 105S11616 11817-13064

2821-3223 3224-3626 36274029

1235 1222 1209 1196 1183 1170

1306514299 1430015521 15522-16730 16731-17926 17927-19106

40304432 4433-4835 4836-5238

52.44-51.90

13 13 13 13 13 13 13 13 13 13

Band 15 Band 16

51.9051.36

13

91 90

1911020279

5239-5641 5842-8014 6045-6447

51.36-50.62

13

89

1157

2028021236

6448-6850

Band 17 Band 16

50.82-50.28 50.28-49.74

13 13

88

1144 1131

21237-22580 22561-23711

8851-7253 7254-7658

Band 19 Band 20 Band 21

49.7449.20 49.M-08.66 46.66-46.12

1118 1105 1092 1079 1066

23712-24829 2483025934 2593527026 27027-28105 281W29171

86

48.1247.58 47.5847.M

8866-9268

Band 24

47.M46.50

86 85 84 83 82 81

7657-8059 -8462 6463-8865

Band 22 Band 23

13 13 13 13 13 13

1053

29172-30224

972-10074

83 82 81

Band 25 Band 26

46.W5.96

13 13 13 13 13

80

1040 1027 1014 loo1 988

30225-3i-m 3126532291 3229243305 3330634306 34307-35294

irn7~i04n io47e10880 10881-11283 11284-11688 11687-12089

80 79 78 77 76

Band 10 Band 11 Band 12 Band 13 Band 14

Band 27 Band 26 Band 29

56.22-55.68

52.98-52.44

45.9645.42 45.42-44.88 44.w.34 44.3443.80

I

86

98 97 96

m 94 93 92

67

79 78 77 76

1261 1248

-

X95-7994

92689671

The hierarchy of 1024 user bytes per sector is: 17 sectors = 1 track (logical track) 59 to 30 tracks (17 revolutions) = 1 revolution group 44 revolution groups = 1 band 30 bands = the user zone

100

99 98 07 96 95 94 93 92 91

90 89 88 87 85

a4

224 Magneto-Optical Data Recording Table 7 (b). Formatted Zone for 3X Format of 512 User BytedSector (Band 30-) izGiizG

Zone of Band

Radius Start-End

Band 30 Band 31 Band 32 Band 33 Band 34

43.8043.26 43.2642.72 42.7242.18 42.1841.64 41.6441.10

13 13 13 13

Band 35

41.10-40.56

13

Band 36 Band 37 Band 38

40.56.40.02 40.02-39.48 39.48-38.94

13 13 13

Band 39 Band 40

38.94-38.40 38.4637.86

Band 41

'racks pe levolutioi Group 75 74 73 72 71 70 69

Uumber

68

884

67

871

13 13

66 65

858

37.86.37.32

13

64

832

Band 42

37.32-36.78

13

819

Band 43

36.78-36.24

13

Band 44 Band 45

36.24-35.70 35.7035.16

13 13

63 62 61 80

780

Band 46

35.16-34.62

13

59

767

Band 47 Band 48

34.62-34.08

58

34.08-33.54

13 13

57

754 741

Band 49

33.54-33.00

13

56

728

Band 50

33.00-32.46

13

Band 51 Band 52

32.48-31.92 31.92-31.38

13

13

55 54 53

715 702 689

Band 53

31.38.30.84

13

52

676

Band 54 Inner Mlg

30.84-30.30 30.30-29.72

13 14

51 51

663

714

Track Number Start-End 3529536289 3627037231 37232-38180 38181.39116 39117-40039 40040-4mM 4095041846 41847-42730 42731-43601 43602-44458 4446044530546138 46137746956 4695647761 47762-48554 4855548334 4933550101 50102-50855 508S51596 51597-52324 5232553038 5304053741 53742-54430 54431-55106 55107.55788 5577056483

Inner SFP FP Transilia

29.72-29.51

5

51

255

564W56738

29.51-29.50

NIA

NIA

Groups

(mm)

PEP

29.5029.00

Adbc(ivL3

29.0027.00

do : NIA ind

IDS 'Nd &p

13

Of

Tracks 975 962 949

936 923 910 897

845

806 793

Revolution

Sectors poi

Start-End 12090124%?

13702-141W

74 73 72 71

1410514541

m

1450814810 14919-15313 1531615716

69

124- 126% 12806.13298 13299 13701

68

15717-16118 1612016522

67 66 65

1652316925

64

1692S17328

63

1732S 17731

62 61 80

17732-18134 1813 5 18537 1853a18940

59

18941-la43 193u-19746

58

19747-20149

56

2015020552

55 54

2055320955 209S21358

57

53

216522W

52 51 51

225992753

51

2135921761 21762-22164

N/A

Be'

The hierarchy of 5 12 user bytes per sector is: 3 1 sectors = 1 track (logical track) 105 to 5 1 tracks (3 1 revolutions)= 1 revolution group 44 revolution groups = 1 band 55 bands = the user zone.

In both of the two cases of the sector size, since the number of revolutions per revolution group is selected as equal to that of sectors per track, the number of sectors per revolution becomes the same number of

Media Substrates and Format 225 tracks per revolution group, that is, an integer. Consequently, sectors are radially aligned as in the ZCAV format. The user zone data capacities of 1024 and 5 12 user bytes per sector are 1023 and 885 Mbytes, respectively. These capacities include the spare sectors and the non-usable sectors. In this format, the sector structure is a little modified as shown in Fig. 8. 4X Format (2.6 Gbyte per Cartridge Capacity). In this format, the track pitch is set to 1.15 pm and the same mark edge recording as the 3X format is adopted to achieve 1.3 Gbytes per

2.0

PRE-MASTERING

2.1 Formatting Role of Formatter. The equipment which electrically controls the formation of pits and grooves on the optical disk is called the formatter. The formatter controls the track layout of the optical disk, the address information called the header, and the control track information. The formatter outputs electric signals to the optical modulators of the LBR (laser beam recorder), then the laser light modulated by the optical modulators forms pits and grooves onto the photoresist layer coated on the glass plate (resist master). Those pits and grooves will be stamped on the surface of the optical disk. The formatter also generates the clock of the turntable rotation, which controls the rotation of the turntable of the LBR, because it is necessary to synchronize both the signals of pits and grooves and the rotation of the turntable. Structure of Formatter. An exampleof the structureof the formatter is shown in Fig. 9. The formatter generates the master clock from the crystal oscillation circuit. The clock of pit signal and the clock of turntable rotation are generated from this master clock, and those are synchronized. This is because the frequency of the master clock is chosen among the common multiple of both clock frequencies. The clock of turntable rotation is supplied for the turntable driver of the LBR. The rotation of the turntable is controlled uniformly by the PLL (phase-locked loop) circuit. The clock of pit signal is supplied to the pit signal generator to make both pit signals and groove signals.

226 Magneto-Optical Data Recording

S M

VFOl

A ID A ID P ALPC M 1 VF02 M 2 A and Gaps

VF03

27

c

S

DataField

c

UserData, DMP,CRC, ECC and Resync

Buffer

-4.-

1278

.,

24

Sb~tes

(a) Sector format of 3X standard for 1024 user bytes, (3) sector format of 3X standard for 5 12 user bytes.

Figure 8.

I

Formatter

LBR

EL! Modulator

F l Modulator

Figure 9. An example of structure of the formatter

228 Magneto-Optical Data Recording The pit signals consist of two parts, one is for the header which shows the address, the other is for the control track which shows the control information of the optical disk. The method to construct signals of the header is as shown below. The address information consisting of track addresses sector nxmbers are bput xnd tb_e CRC (cyclic redimdmr.y code) is calculated over the address information. These information are converted by the RLL(2,7) modulation. After this conversion, sector marks, VFOs, and address marks are added. The method of creating signals of the control track is as shown below. The control information of the optical disk are input and the CRC and ECC (error correction code) are calculated over the control information. This information is converted by RLL(2,7) modulation. After this conversion, VFOs and sync or resync patterns are added. The constructed pit signals are synchronizedwith the clock of the pit signal. The clock fiequency of the pit signal is the reciprocal of the length of one channel bit of pits. The pit signals are amplified and output to the optical modulator of the LBR. The clock of pit signal is also used for the generation of groove signals. The grooves are formed continuously, but have short breaks called ODFjust after the header in all tracks. The intervals of ODFs are controlled using the clock of pit signal. The groove signals are amplified and output to the optical modulator of the LBR. To get good read characteristics of the optical disk, the formatter makes a modification to the length of the constructed pit signals. The length of the pit signals changes the volume of the pits formed on the optical disk, which significantly affects the read characteristics. 2.2

Editing

In addition to the 90 mm rewritable magneto-optical disk (abbreviated to “MO” in this section), there is an optical disk called the 0-ROM which contains a read-only region. There are two types of 0-ROM. The fill-ROM contains only the read-only region, and the partial-ROM comprises both the read-only and the rewritable regions (see Fig. 10). The 0-ROM can record a large amount of data only by the injection molding process. This allows the data such as an operating system, application program, and image data, to be duplicated cheaply and fabricated on a large scale.

Media Substrates and Format 229

Figure 10. Disk layouts of 90 mm MO, hll-ROM, and partial-ROM.

230 Magneto-Optical Data Recording The production of 0-ROM differs from the production of MO in that an editing process is necessary before the formatting stage. In this section, we describe this process. Unless specified otherwise, 0-ROM refers to both the fbll-ROM and the partial-ROM throughout this text. Data Acceptance. For the read-only region, the phase pit must be recorded during the laser beam recording stage of the stamper production. It is necessary to obtain data from a user for the initial recording. Although an MO disk is mainly used as the media for the data acceptance, other media such as a magnetic tape, a cartridge tape, and floppy disks will do. But the data must be once recorded on the MO in order to produce the 0-ROM. The data treatments and the resultant data forms vary to some extent depending on the computer system which the user selects, including the input device, the operating system (OS), and its version. After being adjusted to an appropriate form, the data is recorded on the MO and sent to the editing stage. Figure 11 illustrates an example of the hardware system used for the data receiving, the data adjusting, and the data recording to the MO.

1Hinch

Work station

Magnetic Tape Drive Personal Computer

90mm, 130mm WO Drive

CD-ROM Drive

Figure 11. A hardware system for data acceptance.

Media Substrates and Format 231 Source MO Producing. The following paragraphs describe some important points in producing the source MO which is used in the next editing stage. Difference in Data Capacity Between MO and 0-ROM. An MO disk has spare sectors in addition to the data sectors, in order to manage defects. The spare sector is used when a rewritable sector is damaged by an “acquired” defect, for example, adhesion of dust particles. It is possible to allocate up to 1024 spare sectors in one 90 mm MO disk. On the other hand, an O-ROM has parity sectors to check and correct errors, besides CRCECC. Parity sectors contain the results of the “exclusive or” operation on data bits located at the same position of each sector. In general, one parity sector is made every 25 data sectors. Since spare and parity sectors have different capacities, the actual user-available capacities of the MO and the O-ROM under DOS are 128 Mbytes and 122 Mbytes, respectively. If there is more than 122 Mbytes of data on the source MO, all data cannot be recorded on one O-ROM disk. Data Placemennt. The data placements on disks also vary depending on the OS, and the data is ofken recorded discontinuously. If the data is placed discontinuously on the O-ROM, the access time and other criteria are negatively affected. Especially, this critically affects some applications that require a fast playback of images or sounds. Thus, the data is allocated continuously on the MO disk in advance. This process is called the defragmentation. Grouping. Grouping is the operation of dividing the user area into a certain number of groups and placing parity and/or spare sectors in each group so that a drive may quickly perform a defect management. When an error occurs in a data sector, an optical head moves between the data sector and the correspondent parityhpare sector. There is no problem with usage of the predetermined grouping value for the MO and the full-ROM. But, for the partial-ROM, there is a limitation on the grouping value which depends on the 0s and other factors, since it has both rewritable and read-only regions. For example, under the DOS environment, four (4) sectors make up one cluster, and rewritable area and read-only area may be mixed in the same cluster depending on the grouping value. Data Editing. Using the source MO produced in the former stage, the data editing is performed as follows. Storing Parity Sectors. In Fig. 10, the physical layouts of the 0ROM are illustrated. A parity sector consists of the error detection/

232 Magneto-Optical Data Recording correction check bits correspondent to the read-only region, which assist the conventional CRCECC. As a result, the error rate decreases when the rate is compared with only using the conventional CRCLECC. Actually, the parity data is produced during the laser beam recording process, from the data of several sectors. Storing CRU?ICC. For the 0-ROM, different from the MO, it is necessary to produce and record the error detectiodcorrectioncheck bits for the user data on the disk. The amount of computation required to calculate the CRCECC for all the data of an 0-ROM disk is so much that a hardware system for the calculation may be used. DDS Information. The DDS (disk dejnition sector) is the area that includes the information on the physical format of the disk. For the MO, DDS is the region to be used for the defect management information which is recorded by a disk drive during the c e r t i w g process. For the full-ROM, DDS includes the grouping information which is described before. For the partial-ROM, the grouping information is recorded once on the other readonly area of the disk during the laser beam recording process, and, when the partial-ROM is formatted with a drive, the drive should copy the grouping information to the DDS area.

3.0 MASTERING

3.1 Process In the mastering process, a metal (nickel) copy (mold) which is called a stamper is fabricated to be used for duplication of optical disk substrates in large quantities. This process flow is shown in Fig. 12. Key technologies of this process are composed of the following four items: 1. Defect management 2. Prerecorded signal quality 3. Format signal generation 4. Process control

Media Substrates and Format 233

I

I

glass polishing

.1

laser beam recording .1

I

I

I I

developing

I

Ni metallizing I

I

I

electroforming

I

finishing

I

1 .I.

Figure 12. Mastering process flow chart.

This process requires class 100 clean environment and ultrapure water which is used for glass cleaning, developing, and electroforming. Resist Preparation. Before laser beam recording, a photoresistcoated circular glass plate needs to be prepared. For example, a glass plate of 200 mm diameter and 6 mm thickness is used. For the material of the glass, soda-lime is widely used because it is easily polished. The surface of the glass is required to have good flatness, low roughness, and no defects, because the numerical aperture of recording lens is high (about 0.90)and focal depth is very shallow. So, the surface of the glass plate is polished before photoresistcoating. The polished glass plate is cleaned by ultrasonic cleaner and then photoresist is coated by the spin-coating method. In this methad, photoresist dropped on the glass surface is spread by centrifugal force during the spinning. The advantage of this method is the controllability of the thickness with high precision.

234 Magneto-Optical Data Recording As the adhesion force between glass and photoresist is weak, an organic primer is usually coated on the glass prior to photoresist coating. A silane coupling reagent is used as the primer, e.g., (CH,),SiNHSi(CH,), . Positive-type photoresist is adopted in the optical disk mastering process because it has a high resolution as compared with negative-type photoresist. The lightexposed area of positive-type photoresist is dissolved in developing reagent. On the other hand, negative-type photoresist remains following light exposure. The thickness of photoresist depends on spinning speed, concentration of photoresist, viscosity, air exhaust, and environmental conditions of temperature and humidity. By optimization of these conditions, desired thickness and good uniformity can be obtained. Figure 13 shows examples of the uniformity of photoresist thickness measured by ellipsometer. Many kinds of positive-type photoresist are available now. Each photoresist has different characteristics of resolution, photosensitivity, and g-value (see Sec. 3.2). Selection of photoresist used in the mastering process must be carefully done so that the signal levels of finished optical disks may meet the requirements of specifications.

Radius(mm) Figure 13. An example of thickness uniformity of photoresist. The variation of thickness is controlled within 100 A.

Media Substrates and Format 235 A photoresist-coated glass is baked in a clean oven to completely evaporate solvent remaining in the photoresist thin film. For baking temperature, 80-90°C is appropriate. Defect scan and thickness measurement follow baking. Recording. In this process, a photoresist-coated glass, which is called a resist master is exposed by the focused laser beam of a precise lightexposure machine (called a laser beam recorder (LBR) or a laser cutting machine). The intensity of the laser beam is modulated by an optical modulator (electro-optical modulator or acoustic-optical modulator) with the information signals which are controlled by a format signal generator (formatter). The functions of the formatter are described in Sec. 2.1. Recorded pits and grooves appear from the exposed area in the following developing process. The LBR consists of an optical system, a mechanical system, and an electric system. The optical system consists of a recording gas laser, relay optics, optical modulators, focus optics, and an objective lens which can focus the recording laser beam up to diffraction limit. The mechanical system consists of a rotational spindle and a translation slider. The preciseness of these mechanical parts directly affects track pitch accuracy and servo signal quality. Figure 14 shows a block diagram of the LBR. Recording power and beam profile should be adjusted in order to optimize recording conditions. Developing. The relief patterns of pits and grooves appear on the surface of the recorded master in this stage. For a naphthoquinone diazide type photoresist, which is a representative positive photoresist, a photoactivated compound in the light exposed area is converted to carboxylicacid (R-COOH). The developing reagent is alkaline, the exposed area is dissolved into the developing solutions. As the sensitivity of photoresist is varied due to several factors, including production lot, temperature, humidity, developing conditions, and so on, it is difficult to keep the developing speed constant for each recorded master. However, it is possible to compensatethe variation of these factors by monitoring the progress of development, detecting an end point, and stopping automatically. The principle of construction of this monitoring system of the development is shown in Fig. 15. In this apparatus, a laser beam for monitoring goes through a resist master and gradually appearing grooves act as a grating. The intensity of each diffracted order of light is monitored, and when detectingthe end point, in which monitoring IILovalue is equal to the set value of desired Il/Io, developing is automatically stopped by changing to rinse water from developer. By using this method, it is possible to obtain the desired groove depth reproducibly.

236 Magneto-Optical Data Recording

1-

3 ln

0 0

L

L

0

cr

Media Substrates and Format 23 7

Figure 15. Principle of construction of automatic developing equipment.

The relationship between grating shape and each diffracted order intensity has been well studied by scalar theory by Hopkins,['I Sheng,[*]and Boi~in.[~] The intensity of each diffracted order (I,) is expressed as,

I =-

1

P2 where, F(x) is a periodic hnction which expresses the shape of grating and one period is -p/2to p12, where:

p is a track pitch, and k is a wavenumber that is equal to 2plh.

238 Magneto-Optical Data Recording If one assumes that groove shape is rectangular (U-shaped) (see Fig. 16a), then,

Calculating Eq. 1,

where,

w is a groove width d is a groove depth

N is refractive index of photoresist

If one assumes that groove shape is triangular (V-shaped) (see Fig. 16b), then F(x) =

--2dw

d

(-:SxSO)

W

-

-2dx -d W

= o

(05x2)

Media Substrates and Format 239

-d

r

Figure 16. Examples for periodic function of grooves.

Calculating Eq. (1)

where

a = x -W- ,

P

P = 4 r -d

a

Using the above relationship, the approximate groove depth and width (U-shaped or V-shaped) of the developed master can be calculated by each difiacted order intensity. Those calculated groove shapes are used for process control.

240 Magneto-Optical Data Recording Metallization. As the surface of the developed master is electrically nonconductive, metallizing treatment is required on the surface for the following electroformingprocess. There are two methods which are adopted now. One is a dry method which is vacuum evaporation or sputtering. Silver has been conventionally used since audio anal02 records (EP and LP record) were developed. However, silver is sensitive to oxygen and, consequently, the surface immediately becomes rough. This causes high surface noise on an optical disk and has disadvantages especially for a magneto-optical disk. Nowadays, a nickel sputtering method is widely adopted because of the chemical stability and low surface noise. Especially, DC sputtering is adopted because of non-damageto photoresist. The other metallization method is a wet one, that is, non-electrolytic pZating method (NEP). In this method, metal ion mi2+)which is reduced by a chemical agent is deposited on the master in a plating bath. This method does not need vacuum system and has an advantage of ease in construction of an in-line type mastering system. However, careful control of the quality of the plating bath is required. Electroforming. In this process, the metallized master is electroformed as the cathode in a nickel sulphamate bath using nickel pellets, which confain nickel sulfide, as the anode. The thickness of the electroplated film is about 300 pm and this process takes about two hours to be completed. The concentration of nickel sulphamate is about 600 gA, and the temperature of the bath is kept in 4O-5O0C so that the amount of the roughness of plated surface may be small. The bath is kept contaminationfree by continuous filtration and pH is also controlled within 4.0 *0.1. Electrolytic cleaning is also carried out in order to eliminate metal impurity ions from the bath. Stifhess, back side roughness, flatness, and thickness uniformity of the electroformedstamper depend on plating bath conditions. The following parameters should be carefully controlled-bath composition, additives, bath temperature, pH, and electric current density. In order to control bath composition, the concentration of nickel ion, chloride ion, ammonium ion, and hydrogen ion are constantly monitored. Increase of ammonium ion in the bath results in high stifhess of the stamper. The electroformed stamper is separated from the master and any remaining photoresist on the stamper is removed by alkaline reagent. A stamper family-making technique has been developed to save the stamper cost. In this method, the first nickel copy from the metallized master is called a father, which is a negative copy of the master surface.

Media Substrates and Format 241 After a surface treatment of the father, several mothers are electroformed. By the same process, several sons are plated from each mother. It is possible to duplicate several tens of sons from a father. Those sons are used as stampers for disk duplication. In this family process, stamper quality de-wnds on the technique of surface passivation treatment. There are two methods for passivation treatment. One is a chromate treatment and the other is an electrolytic (anode) treatment. The latter method is preferred because chromate has a toxic property and makes the metal surface rough, causing high surfhce noise. After the electroforming process, the back side of the stamper is polished, if necessary, and then, the stamper is punched out at the inner and outer radius to fit the molding machine.

3.2 Pit and Groove Characteristics of CCS Media The disk layout of CCS format media is described in detail in Sec. 1.1. CCS media has pre-grooves for tracking servo and seek of a desired position by counting the number of crossing grooves. The characteristics of the grooves are specified in IS0 specifications.['][*] Major parameters which determine the quality of fiibricated grooves are as follows: 1. The y-value of photoresist 2. Profile of recording laser beam 3. Power of recording laser beam The response of photoresist film to the exposing light power is indicated by the y-value. For the laser beam with a certain power and a certain profile, large y-value photoresist gives a steep slope of groove or narrow groove width, while small y-value one gives a dull slope or wide groove width. Figure 17 shows the experimental results for two kinds of photoresists which have different y-value by Takeshima.[lo] Smaller y photoresist gives a larger push-pull signal value. Groove width can be also varied by changing the recording beam profile. Laser beam profile is controlled by modification of optical systems of the LBR. The focused recording beam diameter can be varied by changing the incident beam diameter to the objective lens. The focused beam diameter D is expressed as follows:

Eq. (4)

242 Magneto-Optical Data Recording where,

o is the incident beam diameter to lens fis focal length R is wavelength of recording laser K is a constant After adjustment of beam profile, the recording power is selected to have good groove signals. Of course, the recording power is closely connected with developing conditions. Recording power and developing conditions determine the groove shape.

Figure 17. Groove read signals dependency on groove shape and y-value.

Media Substrates and Format 243 CCS media has embossed pre-pits which are used to identi@ the sector addresses (track number and sector number). The characteristics of these pre-pits are also specified in IS0 specification.[11[21In order to meet IS0 specification, the recording conditions for pits must be optimized by changing the thickness of photoresist or by the same methods as groove optimization. On a recordable CCS media (write-once or rewritable), total pit density in a whole disk is not as high and the pre-pits hardly affect groove signals. However, the pit density is comparatively high in the read-only area of an 0-ROM disk, where user data are put as embossed pits at the stage of mastering. As the pre-pits in read-only areas seriously affect groove signals, the margin of groove depth and width to meet I S 0 specifications becomes small. Figure 18 shows the degradation of groove signals when prepits are installed in user tracks. Thus, recording conditions of pre-pits and grooves are optimized in order to have wide margins to the specifications. The surface of the CCS format stamper is shown in Fig. 19 which is observed by a scanning tunneling microscope (STM).

3.3 Mastering for Next Generation Media The format of second generation media is stated in Sec. 1.2. In the second generation media, track pitch becomes narrower than the 1.6 pm of first generation media, and also one channel bit length becomes shorter to obtain higher linear bit density. In order to catch up with the demands for high density and high capacity, the technologies on the mastering process should be improved in priority. Firstly, the LBR has to be improved for high density recording. As the sizes of pits and grooves become smaller, more accuracy of fabrication is required. It is necessary to upgrade the mechanical parts to achieve a higher accuracy of track pitch, and it is also necessary to have a high speed optical modulator for higher frequency recording. Currently, the 4579 A line of Ar+ laser or 44 1 1 A line of He-Cd+laser is usually used for the laser beam recording stage. Gas lasers with shorter wavelength will be adopted on the LBR in the near future. A Kr+ laser (3375-4067 A line) or an Ar+ laser (3336-3638 A line) will replace the current laser. If the ultraviolet lasers are adopted on the LBR (e.g., excimer lasers), it is necessary to use the materials for lens and optical modulator which are non-absorbing at that wavelength.

244 Magneto-Optical Data Recording

DC

8T

3T

!I

0.6

I

Groove (DC) Pattern

I

I

8T

3T

100000001

0

@

H pit Density Q

8T D.125

-

1001

G D @ @

m 3T 3T

0.333 T = 1 channel bit

Figure 18. Degradation of groove signals in read-only area, where recording modulation method is (2,7)pitposition modulation code. In this code, 3T pattern is the most dense and 8T pattern is the least dense.

Media Substrates and Formui 245

Figure 19. SIU photo of surface structurc of CCS formatted stllmplr

Secondly, a new f o m t generator has to be prepared bccausc the track layout, modulation codc, and zoning of scctor structure are changed. In the mastering prwess, the format generator should be modified or newly prepared each time the format changes. Thirdly, mastering process control technologies should be improvcd for higher density media. Especially, defect managcmcnt will be still more important because recorded bit sizes become smaller.

4.0

MOLDING AND STAMPlNG

An optical disk mainly comprises a substrate with pits and grooves which is replicated from a stamper, a recording layer, and somc protcctivc layers. The purpose of this section is to discuss the substrate and the manufacturing process, including molding technology and stamping. For the optical disk substrate, the replication fidelity and the optical and mechanical characteristics are most important. The fimdamental requirements for the substrate arc shown as follows:

246 Magneto-Optical Data Recording High transmittance Good uniformity of refractive index Low birefringence (small coefficient of photoelasticity) High glass transition temperature and high heat deformation temperature Low moisture absorption High modulus of bending elasticity High resistance to impact Good dimensional stability Good adhesion to inorganic layers Furthermore, it is necessary to manufacture disks in large quantities at lower cost. 4.1

Molding

Molding is the method to shape the substrate in the mold cavity from a thermoplastic resin. The resin is melted in the cylinder and is injected into the cavity. Molding is widely applied for the optical disk substrate manufacturing due to the advantage in productivity. But there are some problems to be addressed in this process. In the resin filling stage of molding process, high pressure in the mold cavity and large mold clamping force are necessary for the precise replication because of high viscosity of the resin. Consequently, residual stress in the resin deterioratesmold releasing ability, optical characteristics including birefringence, and mechanical characteristics of the substrate. Materials for Substrate. For the optical disk substrate, contaminant-free special grade resins will be used because contaminants in the substrate cause defects and give poor reliability. Principal properties of typical materials for the substrateare shown in Table 8. Currently, polycarbonate (PC) is most widely used because of high heat resistance, low moisture absorption, and low price. But the large photoelasticity of PC causes an increase in birefringence even with slight stress. The birefringence unfavorably affects the optical disk performance.[l11 PC of optical disk grade has low average molecular weight of 14,000-16,000 to decrease the melting viscosity, although that of general grade has an average molecular weight of 24,000-26,000. The molding of

Media Substrates and Format 247 the optical disk is carried out at high temperature, so that high heat resistance is required. Because the molding of PC is relatively difficult, optimization of molding conditions and molding methods should be investigated in detail. Poly (methyl methacrylate) (PMMA) is a rather better material concerning the optical properties, but deformation caused by moisture absorption and poor heat resistance are disadvantages, especially when used as a single-sided disk. Recently, amorphous polyolefin (cyclic polyolefin, CPO) has become one candidate for optical disk material.[l21 In particular, it is expected to demonstrate better heat- and wear-resistance and lower birefringence compared to PC. Injection Machine. The injection machine has some special specifications to prevent dust. As the examples shown in Fig. 20 indicate, there are several types of mold and injection machines. Around the mold and stocker of the replica, air cleanliness of class 100-300 (class 100 means less than 100 particles of 0.5 pm per 1 cf), is required in order to prevent replicated defects from the dust. The screw design of the machine is optimized for eliminating the resin burning or yellowing. High speed injection and quick response of the mold clamping with closed-loop control are required to obtain the disks with low birefringence and good replication uniformity. Disk characteristics are mostly dependent on the mold design, particularly, dimensional precision, structures of cooling system, and melt flow path. If mold dimensions do not fit well with each other, the mold flush occurs at the outer edge and the gatecutting area of the inner edge. The mold cooling and melt flow structures have significant effects on the birefringence uniformity and other characteristics. Molding. Figure 2 1 shows the typical molding process outline. The resin which was dried for a few hours is fed into the injection machine and then is melted. This melted resin is injected into the mold, then groove and pit shapes on a stamper are replicated to the surface of the substrate. After the gate-cutting around the inner diameter of the replica and cooling of the replica in the mold, the mold is opened for removing the replica. For the conventional injection, high holding pressure and long holding time are used in order to obtain the precise replication. For injection compression molding, the cavity volume can be changed during injection and cooling stages. At the initial stage of injection compression molding, the mold is opened partially, then the mold is compressed gradually with cooling. Currently, the injection machine and the mold have become more complicated in order to reduce the birefringence and also control the various characteristics.

Table 8. Material Properties for Optical Disk Substrates I

h,

8

I

I

I

Glass

PMMA

PC

CPO"

Transmittance (%)

92

92

89-90

92

Refractive index

1.52

1.49

1.59

1.55

-

2.7-3.8. 10-7

7.4*10-7

-6.6. lom7

Photoelastic coefficient (cm2/kgf)

-

Flexural rigidity (kgfkm2) Glass transition temperature ("C) Heat deformation temperature ("C) Thermal conductivity (caVs*cm*"C) Water absorption degree (%) Rockwell hardness *CPO = Cyclic Polyolefin

I

3.31010~

2.51*104 I

I

I

80-105

140

<300

70-95

125

1.2. lo-'

4.6010~

3.21010~ I

-300

4?

Ps R1 Y

145

I

F ! i P

Da

130

4.601O4

4.701O4

<0.01

1.6

0.3

0.01

-

82-90

45

75

&

Media Substrates and Format 249 ,Tie

Ram

Bar

Movable Mold

(a)

Bar

3

+ Movable Mold

+xed Mold

(b)

Figure 20. Examples of structure of injection machines, (a) straight hydraulic system, (b) toggle hydraulic system.

Q

Jtter

Injection Process

Gate Cut and Cooling Process

Figure 21. Typical molding process flow.

Mold Opening and Releasing

B

250 Magneto-Optical Data Recording Figure 22 illustrates the effect of compression on birefringence. Although the birefringence uniformity can be improved by using the injectioncompression method, improvement of vertical birefringence is also Birefringence depends on the molding conditions, for example, mold temperature, cooling time, mold clamping force, and the injection rate. If one changes the mold temperature to improve birefringence, it will result in replica deformation. There is a trade-off relationship for the optimization of the various requirements. In compact disc molding, the cycle time has been reduced to five or six seconds. To shorten the cycle time of the optical disk molding, will require development of not only the resin but also the mold design and equipment.

20

30

40

50

60

Disk Radius [mm] Figure 22. Birefringence uniformity of replicas.

4.2

Stamping

There is another method for the production of optical disk substrate. UV curable resin is coated onto the flat substrate, then grooves and pits are

Media Substrates and Format 251 formed on the resin, and finally the resin is cured by UV irradiation. This production process is called 2P (photopolymer) method. Materials for Substrate. The 2P method enables us to fabricate the disk made of glass as well as polymers such as PC and PMMA. Glass has many advantages for the substrate of the optical disk; excellent optical properties, dimensional stability, less volatile matter, and low water permeability. The UV curable resin requires the following characteristics:

Good releasability from the mold and the stamper High adhesion force to the substrate Neither residual monomer nor volatile materials after polymerization

Typical components of UV curable resin are shown in Table 9.

Table 9. Examples of UV Curable Monomers UV monomer

1

Chemical equation

2.Ethylhexyl acrylate

CH,=CHCOOCH,CH(C,H,)CC,H,

2-Hydroxyethyl acrylate

CH,=CHCOOCH,CH,OH

24Ethoxyethoxy) ethyl a q l a t e

CH2=CHCOOCH,CH20CH,CH~OCHzCH3

N-Vinyl-2-pyrrolidone

2

36

1,6-Hexanediol diacrylate

CH,=CHCOO(CH,),OCOCH=CH,

Ethylene glycol diacrylate

CH,=CHCOOCH,CH,OCOCH=CH,

Neopentylglycol diacrylate

CH,=CHCooCH,C(CH,),CH,OCOCH=CH~

Polyethylene glycol diacrylate

CH,=CHCOO(CH,CH,O),COCH=CH,

Penhqthritol diacrylate

(CH,=CHCOOCH,),C(CH,OH~

Trimenthyloipropanetriacrylate

(CH,=CHCOOCH,),CC,H,

Penheqthritol triacrylate

(CH,=CHCOOCH,),CCH,OH

Penheqthritol t e t r q l a t e

C(CH,OCOCH=CH,),

Dipentaerythrrtol hexaacrylate

(CH~=CHCOOCH,),CCH,OCH,C(CH,OCOCH=CH&

*F.G. = Number of Functional Groups

252 Magneto-Optical Data Recording Stamping. The 2P method has some advantages compared with injection method, as follows: High quality substrates (e.g., glass) Precise replication of pits and grooves Lower cost of the equipment Ease of elimination of contaminants However, it has a disadvantage in productivity compared with the injection molding. In the 2P method, the photopolymer is coated or filled between the substrate and stamper, then UV light is irradiated for curing. The substrate with UV resin on which pits and grooves are replicated is released from the stamper. Figure 23 shows the 2P process schematically.

Ni Stamper

1 1

2P resin

/ +

Supply of 2P resin

UV curing 1

f Figure 23. Typical 2P process flow.

Peeling off

Media Substrates and Format 253 5.0 PROTECTION COATS; LIFETIME

5.1 Coating Method Definition and Method. brd-coat and over-coat (top coatj cover on the laser-beam incident side and the film side of an optical disk, respectively, in order to protect the media. The hard-coat layer and the over-coat layer are generally prepared by the spin coating method followed by an appropriate curing process. There are a few examples of the preparation of the hardcoat (over-coat) layer by a vacuum deposition of an inorganicmaterial such as SiO,. The spin coating method is mostly employed because it is able to prepare the hard-coat layer and the over-coat layer separately and to give a good layer with a uniform thickness. The dip coating method can be employed if both the material of the hard-coat and the over-coat are identical, and it is able to give an excellent layer with a uniform thickness. Other coating methods such as spray coating are difficult to employ to prepare the hard-coat (over-coat) layer because of the difficulties in obtaining good layers. In the lamination process, the spin coating method can be adopted for an adhesive which is liquid at room temperature. The roll coating method by melting an adhesive at high temperature is employed for a resin which is solid at room temperature. Materials. As the hard-coat (over-coat) material, ultraviolet (UV) curable resins are widely used because of their advantages in high productivity, low cost, and capability of obtaining high hardness layer. Thermosetting resins are sometimes used. The materials for the spin coating usually have viscosity of 3-1000 CPin order to facilitate the coating. A high degree of air cleanliness and careful handling of the substrate are required. The material with small particulate content should be filtrated just before the coating because it is unacceptable to introduce foreign materials onto the substrate. Reliability of the disk is strongly related to the content of the particulate component in itself.

254 Magneto-Optical Data Recording

5.2 Hard-Coat (Laser-beam Incident Side) Purpose and Requirement. The hard-coat layer is prepared on the laser-beam incident side of the substrate in order to prevent a scratch or a dust adhesion which could cause an error in read or write operation. High hardness and low surface resistivity are required for the hardcoat layer to prevent scratches and dust adhesion, respectively. It is also required that the hard-coat layer maintain its surface characteristics against Wiping of the surface by a cleaner. Two types of cleaner are used, that is, a dry type and a wet one. For the dry cleaner, it is sufficient for the hard-coat to have high hardness and low resistivity initially. But, for the wet cleaner, in addition, it is necessary to keep the surface resistivity at a low level on exposure to water and/or alcohol, because the liquid for the wet cleaning mainly consists of water and alcohol such as ethanol or 2-propanol. Preparation. The hardness and the surface resistivity of the hardcoat layer generally depend on the thickness and the curing condition of the layer. The hard-coat layer usually has the thickness of 0.5-10 pm. An increase in the thickness of the layer generally brings an increase in the hardness, but is unfavorable because a crack is easily generated in the layer. Pencil hardness is usually used for the estimation of the hardness of the layer. A harder layer is usually more favorable for the hard-coat, however, in the case of the plastic substrate, the pencil hardness above HB is considered to be sufficient and that above 2H is hard to achieve due to the softness of the substrate itself. The requirement for the surface resistivity is 1010-1014Wsq., but is slightly dependent on the environment in which the disk is placed. The hardcoat material usually consists of a resin containing an antistatic agent which is effective for lowering the surface resistivity. One of the most important properties for the hard-coat is high transparency in the wavelength of the laser beam. Therefore, it is quite unfavorable that the hard-coat layer has an uneven surface or includes a foreign material. Any change in the appearance of the hard-coat layer such as coloration should not be allowed. The hard-coat layer should not peel off from the substrate and not cause any change in the properties of the substrate. Typical materials for the hard-coat and their characteristics are shown in Table 10. As a resin with high hardness, W curable resin or siloxane resin is favorable. As the antistatic agent, surfactant (cationic,

Media Substrates and Format 255 anionic, nonionic, or amphoteric), polyelectrolyte, or fine powder of metal oxide is employed. For the deposition of a metal oxide on the substrate, SiO,, SnO,, or IT0 (In,O,-SnO,) is used. 5.3

Over-Coat (Film Side)

Purpose and Requirement. The over-coating is done on the film side in order to prevent corrosion and scratches. The MO disk usually has a protective layer made by metal oxide or nitride in order to protect the corrosion of the recording layer such as oxidation by oxygen or water. However, complete coverage is difficult to achieve by the protective layer alone because of a small defect such as a pin hole in the layer or a crack due to a large stress of its own. Therefore, the over-coat layer needs to be prepared to protect the recording layer completely. Components such as alkaline metal ion, alkali earth metal ion, free chloride ion, and free acid (acrylic acid or sulhric acid used at synthesis of resin) which can possibly corrode the film should be minimized in the over-coat resin. Preparation. There are several items which should be taken care of in order to prevent the degradation of the disk. The choices of the material, the thickness, and the curing conditions of the overcoat are most important for the protection of the film. The thickness is usually 0.5-20 pm. The protection is not sufficient if the thickness is too thin or the curing incomplete. Too thick an over-coat layer (insufficient U V cure) may contain unreacted components which are poisonous to film, or generates a crack due to the difference in expansion coefficient between the recording layer and the over-coat layer. High hardness of the over-coat is required to prevent scratching of the film. However, relatively low hardness is sufficient for the over-coat because wiping of the film side is unlikely to occur, and a slight scratch is considered to cause little damage. The pencil hardness above 2B is sufficient for the plastic substrate. Adhesion force between the film and the over-coat layer is also important. Poor adhesion force causes peel-off of the overcoat layer from the disk, and excessively strong adhesion may cause a crack of the film during the usage due to the difference in expansion coefficient. The characteristics of the materials used in the overcoat are shown in Table 11.

Table 10. Typical materials for hardcoat and their characteristics. Materials

Methods

Components

Antistatic agent

Remaining unreacted monomer (initiator) which is poisonous to the substrate and the film

(Meth)acrylate monomer (Meth)acrylate oligomer Photoinitiator

Siloxane resin

Alkkyl silicate oligomer

Super high hardness Hgh weather resistancc Low toxic

Cationic Anionic

Surfactant

Large effect by a small amount Cheap

Easily Hnped off by water or alcohol Likely gives a harmful influence on the substrate and the film

Chemically stable

Small effect even by a laroe amount

Nonionic

I

Polyol, etc.

I

Easily peeled from subshte Relatively low productivity

Polyelectrolyte

Polymer with dissociative goup

Large effect by appropriate molecular design

Poor solubility to resin

Metal oxide

SnO,, ln20,, etc.

Large effect Chemically stable

Difficult to disperse to resin

Large effect Chemically stable

Unable to obtain sufficient thickness only by deposition High cost Low adhesive strength to the substrate

I

Deposition of metal oxide

Hgh productivity Hgh hardness layer

Disadavantaaes

UV curable resin Resin

Coating of mixture or resin and antistatic agent

Advantages

Metal Oxide

I

SiO, Sn02. ITO, etc.

Table 11. Typical materials for over-coat and their characteristics.

Methods

Materials

Cornponents

UV curable resin

(Meth)aaylate monomer (Meth)aaylate oligcuner Photoinitiator

Siloxane resin

Alkkyl silicate oligomer

Advantages High productivity High hardness layer

Coating

Deposition or sputtering

Inorganic compound

Good weather resistivity Super high hardness Low toxic

Large effect Chemically stable

Disadavantages Remaining unreacted monomer (initiator) and acid (sulfuric and a a y l i used at synthesis of the components) which are poisonous to the film Easily peeledrfom subsbate Relatively low productivity Unable to obtain sufficient layer only by deposition or sputtering

258 Magneto-Optical Data Recording

5.4 Lamination Lamination is a process employed in the production of 130 mm MO and 130 mm WO. The characteristicsof the adhesive are shown in Table 12. Table 12. Typical Materials for Adhesive and Their Characteristics Forms

Hot melt

Components

Advantages

Disadvantages

Polyolefm

High productivity

Low heat-resistivity

Polyurethane

High productivity High heat-resistivity

Dificulty in operation

Acryl Two-part adhesive Epoxy Urethane

Low productivity High heat-resistivity Corrosion of film by unreacted componenb

UV curable

High productivity High heat-resistivity

Acryl

Corrosion of film by unreacted componenb

The adhesives are formally classified into three types, that is, hot melt adhesive, two-part, and W curing. As the hot melt adhesive, a conventional thermoelastic polyolefin and a reactive polyurethane are examined. They have an advantage in high productivity. They have disadvantages in low heat-resistivity of the thermoelastic adhesive and in the complicated operation of the reactive hot melt one. As the two-part adhesive, thermosetting resins such as acrylic resin, epoxy resin, and urethane resin are examined. They have good heat-resistivity. They also have disadvantages in low productivity and relatively high possibility of corrosion of the film such as pin hole formation by the remaining unreacted monomer, oligomer, andor curing agent. The W curing adhesive contains acrylic monomers and a photoinitiator. In addition to the independent UV curing system, the mixture of W curing and hot melt is also proposed. This system has advantages in high productivity and good heat-resistivity, but has the same disadvantages as the thermosetting adhesives.

Media Substrates and Format 259 The thermoelastic hot melt adhesive is mainly employed for the commercially produced MO by the advantage in productivity and reliability. The thermoelastic adhesive has an advantage in ease of the lamination process; however, it has disadvantages in the relatively bad mechanical characteristics and in a slip of the disks under high temperature. A striking example is that the adhesive is softened by the heat of the irradiated laser beam. It is considered to be difficult for the thermoelastic adhesive to have both high glass transition temperature (nevertheless, high temperature is necessary to soften the adhesive) and relatively low viscosity at the lamination temperature. The temperature of the media rises in the adhesion process using the thermoelastic adhesive or the thermosetting one, but high temperature should be prevented because mechanical characteristics of the substrate are changed and the film is affected. A large difference in the expansion coefficient between the adhesive and the substrate badly affects the mechanical characteristics of the disk. Uneven thickness of the adhesive or uneven pressure on the disks can also have a h a h l influence on the mechanical characteristics. 5.5

Life Test and Lifetime Estimation

As the long-term reliability of the data stored in the optical disk is one of the most important concerns, it is necessary to estimate the disk lifetime. Like other electrical devices, various kinds of environmental tests are undertaken to confirm the optical disk reliability.[l4I Major environmental tests are: high-temperature-high-humidity test (stress test), corrosive gas test, and temperature-humidity cyclic test. 1. In the stress test, disks are stored in severe temperature and humidity conditionsto acceleratethe disk degradation. In an ordinary case, temperature and humidity of the chamber are changed at a limited rate, as mentioned in IS0 standard.i21 One should pay attention not to expose the disks in a test chamber to too rapid a change of temperature or humidity. Since contaminants in the water supplied to the chamber may condense on the surface of disks and produce defects, water should be deionized and filtered before supplying to the chamber. For example, there is a possibility that a solute in the

260 Magneto-Optical Data Recording water will be deposited on a disk surface during the operation under high temperature and high humidity. One should observe the conductivity of the water in the chamber to detect the Occurrence of the contamination in the chamber during the operation.

2. In corrosive gas test, disks are stored in a chamber in which air containing corrosive gas such as sulphur oxide (SOX) and nitrogen oxide (NOx) is circulated. A check is made to determine whether corrosive gas affects the disk performance. One should pay attention not to leak the corrosive gas that is hazardous to both disks and humans. 3. In temperature-humidity cyclic test, disks are stored in a chamber in which both temperature and humidity rise and fall sharply in several hours. The point to remember is that the operation is similar to that of a stress test. A temperaturehumidity diagram of a typical cyclic test, the Z-ADtest, is shown in Fig. 24.[151 One should change temperature and humidity in such a way that condensation on the disk surf' does not occur.

80- r I

-200

--------I

I

5

I

I

1

I

1

r---------I

I

100

1 I

I

1

10 15 20 Program Step

Figure 24. Temperature and humidity pattern of Z-AD test.

25'

Media Substrates and Format 261 Lifetime estimation is based on the assumption that degradation proceeds according to a single difision phenomena. In that case, reaction rate K at temperature T ( K ) is described below (Arrhenius formula):

K

=

- Ea Koe kT

where KO is reaction rate at T = 0, E, is an activation energy and k is Boltzmann's constant. In the Eyring model, lifetime L at temperature T and humidity H is estimated as: h ( L ) = ln(&)+---+nH 4 kT where Lo and n are the constants which are determined on the basis of data. A multivariant analysis is applied to the data at various temperature and humidity conditions to derive the acceleratingfactor. In most case, lifetime at room temperature is linearly extrapolated from several points drawn in the In (L) vs. 1/T chart, the Arrhenius plot, using the data at various temperatures and constant humidity, such as 85"C/85%RH, 75OC/85%RH, and 65"C/85%RH. In the ordinary case, BER (byte error rate) is used for the indicator of disk degradation. Intrinsic defect growth caused by thin film corrosion is estimated after categorizing the defect. Lifetime is the time required to reach the specified BER limit at a given environmental condition. The BER A at time = t is estimated from the equation below.[l6]

where A, is BER at time = 0 and C is a constant which is determined on the basis of data. Lifetime in a certain condition is obtained by the mean value of each disk's lifetime. In other cases, behavior under a certain condition is estimated using the cumulative hazard rate. A typical Arrhenius plot of commercial disks is shown in Fig. 25.

262 Magneto-Optical Data Recording

Figure 25. Arrhenius plot of commercial MO disks.

6.0 HUBBING 6.1

Structure and Basic Requirements for Hubs

A hub plays an important role as an interface between an optical disk and a drive spindle. Magnetic clampings were introduced to professional disks like 130 mm write-once and rewritable disk, and 90 nun rewritable and read-only disks. Figure 26 shows the structure of typical magnetic clamping hubs. The hub has a permeable ring on it to provide the attractive force with a magnet in the spindle of a drive. The important items of the requirements for the hub are the following: Accurate dimensions for mechanical interchangeability Abrasion resistance of the center hole against lodunload cycle Adequate magnetic force and corrosion resistance of the permeable ring High bonding strength between the hub and the substrate with high temperature and high humidity resistance

Media Substrates and Format 263

!

A hub for 90mm disk

I !

A hub for 130mm disk

I

i A click hub for 130mm disk Figure 26. Structure of typical hubs for optical disks.

6.2

Hubbing Process

Adhesive Bonding. For the adhesive bonding, high bonding reliability is achieved by carefblly selecting proper U V curing type adhesives. However, a bonding machine is required to have the precise centering mechanism, which employs groove image detection with a CCD camera or groove tracking function, to set the hub at the center of the spiral or the concentric circles of grooves. This mechanism increases the cost of the bonding machine and lowers the machine throughput.

264 Magneto-Optical Data Recording Ultrasonic Bonding. This type of bonding exceeds the other method in productivity. However, so far, there seems to be no commercially available machine which does not degrade the optical characteristics of plastic substrates. Mechanical Bonding. Mechanical bonding is the method by which a pair of hubs are applied from both upper and lower side and connected mechanically by hooking their nails on the other side of the disk. This mechanism ensures high throughput compared with adhesive type above, however it requires high accuracy of centering in the injection molding process because of the unavailability of centering h c t i o n during hubbing process.

6.3 Testing Dimensions. Center hole diameter and outer diameter chamfer of the hub are important for interchangeability. Position of the permeable ring to the disk reference plane is also important, because it decides clamping force together with the structure of the ring. Clamping Force. Clamping force is given by the attraction force between the magnet in the spindle and the permeable ring of the hub. For example: Less than 14N: 130 mm disk (by ISOAEC 10089[11) 3.0-4.5N: 90 mm disk (by ISOAEC 10090[*1) Center Hole Durability. Some applications like jukeboxes (optical libraries) require high wear resistance to lodunload cycles. The center hole diameter should be kept within 4.016-4.004 mm even after 40,000 cycles of lodunload test. Corrosion Resistance. The permeable ring should have the same level of corrosion resistance as the recording layer has. 7.0

CARTRIDGING

7.1

Structure and Basic Requirements for Cartridge Case

A cartridge case is a rigid, protective enclosure which consists of rectangular shape shells generally made of plastics and a shutter which covers access windows for an optical head (and a bias magnet for magneto-

Media Substrates and Format 265 optical disk) and automatically uncovers them upon insertion of the cartridge into the drive. The case has a means for positioning itself in the drive, identifymg the mediatype, and protecting the write operation. I S 0 standard cases for 90 mm and 130 mm optical disks form factor are shown in Figs. 27 and 28.

Slot for

shutter opener

Side B

k Label area

Mis-insert protection I

c

autoloading

ADetent for autoloading

9"'

Slot

'Location hole CI

Side A

nctional Area Label area Spindle and head windows

Figure 27. Structure of 90 mm optical disk cartridge.

266 Magneto-Optical Data Recording

Slot for shutter opener User label area

Shutter sensor notch Insertion direction

Hub

holes for side B

hole for side B

Figure 28. Structure of 130 mm optical disk cartridge.

Basic requirements for cartridge case are the following: Accurate dimensions for mechanical interchangability Impact resistance (material and structure) Abrasion resistance (shells, shutter, shutter holder, and contacting parts of shells with the disk) Antistatic characteristics (shell).

7.2 Cartridging Process Case Shell Molding. The types of the materials for shells are restricted by the requirements such as dimension stability, impact resistance, and heathumidity resistance. Polycarbonate resin is a typical

Media Substrates and Format 267 material for the shells. To prevent electrostatic discharging andor dust attraction, an antistatic agent may be mixed into the material when the shells are injection molded. Shutter, Spring, and Shutter Holder. Selection of material and structure of shutter, spring, and shutter holder are important to withstand l d u n l o a d cycle test, drop test, and environmental test. Cartridge Assembly. At present, screw fixing type and click shell type are used in mass production.

7.3 Testing Dimensions. Both initial dimensions and dimensions after environmental tests should be tested. ISO/IEC 10090r21defines practical tests with special tool for edge distortion in its annex A, and for flatness and flexibility of the case in annex B. Drop Test. The optical disk cartridge should withstand the impact on each corner when it is dropped from 760 mm high to a concrete floor covered with a vinyl layer of 2 mm thickness. LoadUnload Test. After lodunload test, the following items should be checked: Abrasioddust generation (shell, location hole, shutter, shutter holder, and contacting parts of shells with the disk) Shutter opening/closing force (initial and after cycle test).

8.0

OPTICAL AND MECHANICAL PROPERTY CONTROL

Optical, servo, and mechanical properties are the major control parameters in optical disk substrate Each parameter or measuring method is described in this section.

8.1

Optical Properties

Optical transmittance of polymer resin used for the optical disk can vary from 90-92% (at thickness of 1.2 mm). Optical transmittance is affected by several factors such as substrate thickness, surface characteristics (roughness), and material (refractive index and impurity contents).

268 Magneto-Optical Data Recording Measuring the optical transmittanceis a usehl method to monitor the above mentioned factors. To measure the optical transmittance, one uses a laser and measures the laser power with photodetector when a disk is put into the optical path and removed. The ratio of the laser power when a disk is put into the optical path to that removed is optical transmittance. The light wavelength and disk thickness dependency of the optical transmittance of polycarbonate, which is widely used for optical disk substrate, are shown in Figs. 29 and 30.[181

Wavelength [nm]

Figure 29. Wavelength dependence of optical transmission.

Figure 30. Thickness dependence of optical transmission.

Media Substrates and Format 269 The MO disk uses rotation of polarized light to detect the recorded mark (KerrRaraday effects). Substrate birefringence is the most important optical control parameter. Birefringence is affected by polymer orientation, molecular weight of polymer, and injection conditions. Birefringence can be measured by the following method (see Fig. 3 1): Expose the linearly polarized laser beam on the disk Rotate the polarizer in front of the detector and measure the * maximum and the minimum laser power (Imarand Imi,,) Birefringence An can be calculated from the equation below:

An=2*arctan

(m -

Optical Disk

He-Ne Laser

I

Polarizer

I

Figure 31. Schematic diagram of birefringence measurement.

In another method, using the optical pickup, birefringence is measured from the ratio of differential output of a polarized beam splitter (PBS). In ISODEC 10090,[*1measuring birefringence using the optical pickup is mentioned as MO Signal Imbalance. The measuring setup depicted in ISODEC 10090 is shown in Fig. 32. Because birefringence is affected by the polymer orientation, the value of the substrate birefringencejust after molding and that after annealing are different from each other. Figure 33 shows the radial distribution of birefringence of the disk before and after the anneal measured with a collimated laser beam.

--_.----

B C D E F G

Collimator lens Optical shaping prism Beam splitter Polarizing beam splitter Objective lens Optical disk

--------

H Optical half-wave plate I Phase retarder J Polarizing beam splitter (PBS, p-s ratio > 1 .OO) K1 2 Photodiodes for channels 1 and 2 K 3 Split photodiode L1 2 DC-coupled amplifiers M ’ Tracking channel

Figure 32. Schematic diagram of measuring birefringence with reference servo.

Media Substrates and Format 2 71

- 0 3 n

E C

Y

C

.-0

B

(d

10 -

20

0-

c

-10

-

-*So

30

40 50 Radius [mm]

60

Figure 33. Radial distribution of retardation.

Servo characteristics of the substrate are examined by an optical disk tester to evaluate whether the injection molding conditions are proper or the stamper used for the molding is within the specifications. Items to be measured, which are mentioned in ISOAEC 10090,[*1 are listed below (see Figs. 34 and 35). Servo and prerecorded characteristics which are required for 90 mm MO disk are: Servo characteristics: Cross-track signal [(I1 + I,) peak-to-peak] Cross-track minimum signal [(I1 + 12) min] Push-pull signal [(I1 I, ) peak-to-peak] Divided push-pull signal On-track signal (13 Prerecorded characteristics: Sector mark modulation (IJIo) VFO modulation &do) VFO resolution (Id,,/&)

-

272 Magneto-Optical Data Recording Cross-track signal is observed when only the focus servo loop is closed and the optical beam crosses the grooves. The cross-track signal is defined as the ratio of the peak-to-peak value of s u m signal of tracking channel to sum signal (I,) at the area where neither embossed signal nor ~~~ermve . . is ~-recorded. ~ . _ Sigm! _ ~ at the ODF is used for I, value. The cross-track minimum signal is the ratio of minimum value of sum signal of tracking channel to b. Thepush-pull signal is the ratio of the peak-to-peak value of the difference signal in the tracking channel when the beam crosses the grooves to I,. The divided push-pull signal is the ratio of the difference signal in the tracking channel divided by the sum signal when the beam crosses the grooves. The on-track signal is the ratio of the signal at the grooved area without embossed signal when tracking loop is closed to I,. Servo characteristics reflect the width and depth of the groove. Since measurement of servo characteristics with the tracking servo loop open are affected by the eccentricity of the disk and embossed ID marks, another method is proposed in which the servo characteristics measurement is carried out by jumping to the opposite direction in every sector.

Light beam

(11+12)min

Figure 34. Signals from grooves in the tracking channel.

$.

Media Substrates and Format 2 73

track

\-. I

1

0 Level

1

Figure 35. RF sum signals from headers.

Prerecorded characteristics are obtained by the RF s u m signal when embossed marks of the header are observed. Both modulations are defined as the peak-to-peak value of RF sum signal modulation caused by the corresponding embossed mark. VFO resolution is the ratio of signal amplitude in the W O field where the interval of the signal is the shortest to the maximum signal amplitude in the ID field that has the longest interval such as 8T. Prerecorded characteristics reflect the molding fidelity. To evaluate the injection molding conditions, especially the molding fidelity, a readback waveform of longer pits (i.e., sector marks) are observed. The difference in the readback waveform that depends on the molding fidelity is shown in Fig. 36. This figure indicates that observartion of sector mark signal waveform is a usefbl method to estimate the molding condition qualitatively.

Figure 36. Comparison of readback waveform at sector mark. (a) Normal waveform, (b) deformed waveform vofding).

2 74 Magneto-Optical Data Recording It is often assumed that particles in the substrate or debris on the substrate would have little affect on the read-write performance because the optical beam is defwused except at the recording layer. However, defects on the substrate or in the substrate cause the scatter of optical beam, and affect the servo performance. It is necessary to detect and eliminate these kinds of defects in the early stage of the manufacturing process. To detect these kinds of defect, the laser beam is scanned over the whole surface and any change of reflected andor transmitted light is observed. Applying defect testing to the substrate enables detection of defects caused by particles in the substrate or debris on the hardcoat as well as the stamper. Quality control of substrate material is done by detection of the foreign material in the solution of the polymer tip before molding. The polymer solution is obtained by dissolving the polymer tip with the appropriate solvent (as for polycarbonate, methylene dichloride). The apparatus that categorizes the defect by detecting the reflected and transmitted light of the laser beam is available commercially.

8.2 Mechanical Properties Mechanical properties of the substrate are measured after the injection moldmg. Through the measurement, substrate eccentricityand flatness are determined. Mechanical characteristics are measured with a mechanical tester of the substrates sampling from the manufacturing line product. There are two methods to measure the mechanical properties: (i) Disk displacementis detected by the driving current of the objective lens actuator. The acceleration is obtained by differentiating the displacement. (ii) Attaching the sensor that detects the displacement of the lens, disk displacement is obtained by the sensor and differentiation of the sensor output results in the acceleration. Typical block diagram of mechanical tester is shown in Fig. 37. There is another acceleration measuring method in which the reference servo mechanism is used and frequency dependence of displacement is taken into consideration. Acceleration is defined as the residual error signal of the reference servo. This method is proposed in IS0 standard of 90 mm optical disk, but no mechanical tester implemented with the reference servo is available. To obtain quick feedback, a simplified detachable hub is used for measuring mechanical properties of molded substrates. The eccentricity of

Media Substrates and Format 275 the substrate with the simplified hub is a kind of parameter that is varied after assemblingthe glued hub. The eccentricityof the disk is determinedby contributions which come from substrate and hub assembly accuracy. It should be measured before assembling the cartridge case. Along with the substrate flatness that is obtained by the fringe when the expanded laser beam is radiated onto the substrate surface, the axial runout of the substrate indicates whether the substrate is molded properly. Axial runout of the disk is determined by not only substrate deflection but also the factors such as stress of the sputtered film and stress of hardcoat shrinkage. Axial runout should be measured before assembling the case.

Optical Disk I

Static Deflection Axial I Radial Runout

i

b

Lens Movement Lens Displacement (x) Lens Actuator

Filter

* Actuator Currents

($-) d'

. I

J.

V

Band Pass Filter Servo Electronics

1 Axial I Radial Acceleration I

I

Figure 37. Block diagram of mechanical characteristics measuring system.

Tilt angle, which indicates the small local slope on the disk surface, is obtained from the difference of axial runout among the adjacent measuring points. The following describes how tilt angle is calculated:

Eq. (9)

2 76 Magneto-Optical Data Recording (rl r r 2 )

Tilt =,/Tilt(r)’ + Tilt (0)’ D,, D,: axial runout at the point 1,2 Rl(rl,Ol), R2(r2,eJ: position at the point 1, 2 (radius, angle) As an alternative to monitoring tilt angle with mechanical tester, there is a method of obtaining tilt angle from the aberration of the reflected beam spot ofthe collimated laser beam that is irradiated to the disk surface. As for the substrate, this method is not optimal because reflectivity of the substrate is low. At the stage of molding, measuring of the substrate parameters listed below is important, resulting in higher manufacturing yield: Defect Servo and prerecord signal characteristics Birefringence.

Molding conditions greatly affect the property of the disk with respect to these parameters, and disk properties can be predicted from the substrate properties. However, there is some possibility that the process variation after molding causes the fluctuation ofthe disk properties. Thus, one should verify the properties of the finished disk, consisting of a substrate, a recording layer, a protective coating, and a hub.

ACKNOWLEDGEMENT The author would like to express his great gratitude to the following scientists and engineers of Mitsubishi Chemical Corporation (Aobadai Research Center and Mizushima Plant) who have prepared a part of this article, Mr. H. Takeshima, Mr. S. Harada, Mr. H. Kubo, Mr. T. Kobayashi, Mr. S. Yokota, Dr. M. Aoyama, Mr. T. Nakamura, and Mr. K. Ebashi. In addition, the author is very grateful to Ms. C. Ueda and Mr. Y. Saitoh, who have assisted with chapter revisions.

Media Substrates and Format 277

REFERENCES 1. International Standard ISO/IEC 10089, Information Technology-I30 mm Rewritable Optical Disk Cartridgesfor Information Interchange, First Edition (1991) 2. International Standard ISO/IEC 10090,Information Technology--90 mm Optical Disk Cartridges,Rewritable and Read Only,for Data Interchange (1992) 3. International Standard ISO/IEC 13481, Information Technologv--Data Interchange on 130 mm Optical Disk Cartridges-Capacity: I Gigabytes per Cartridge (1993) 4. International Standard ISO/IEC 13549, Information Technologv--130 mm Optical Disk Cartridge-Capacity: I . 3 Gigabytesper Cartridge-for Data Interchange (1993) 5. International Standard ISO/IEC DIS 13842,Information Technology130 mm Optical Disk Cartridges-Capacity: 2 GigabytesPer Cartridgefor Data Interchange (1995) 6. International Standard ISO/IEC CD 14517, Information Technology130mm Optical Disk Cartridges-Capacity: 2.6 GigabytesPer Cartridgefor Data Interchange (1995) 7. Hopkins, H. H., Diffraction Theory of Laser Readout Systems for Optical Video Disk, J. Opt. SOC.Am., 69(1):4-24 (1979) 8. Sheng, P., Theoretical Considerations of Optical Diffraction from RCA Video Disk Signals, RCA Review, 39512-555 (1978) 9. Boivin, L. P., Multiple Imaging Using Various Types of Simple Phase Grating,Appl. Opt., 1 l(8): 1782-1792 (1972) 10. Takeshima, H., Kobayashi, T., and Ohmori, T., Study on Fabrication of IS0 Formatted Stampersfor Optical Disk, Mitsubishi Kasei R&D Review, 3(3):87-93 (1989) 11. Takahashi, A.,Mieda, M., Murakami, Y., Ohta, K., and Yamaoka, H., Influence of Birefringence on The Signal Quality of Magnetooptic Disks Using Polycarbonate Substrates,Appl. Opt., 27( 14):2863-2866 (1988) 12. Dunn, M. R.,Introduction to The Moulding of Optical Disc Substrates, Eng. Plast., 2(21):115-126 (1989) 13. Kanai, T.,Shimizu, K., and Uryu, Y., Prediction of Birefringence for Optical Memory Disk, Intern. Polymer Processing, 4(3):132-142 (1989) 14. Okino, Y.,Reliability Measurement, in: Measuring and Evaluation Technique of Optical Disk Part HI, (T. Kubo, ed.), pp. 233-252, Japan Industry Engineering Center, Tokyo (1 988)

2 78 Magneto-Optical Data Recording 15. ISO/IEC 68-2-38, Basic Environmental Testing Procedures, Part II: Tests. Test Z-AD:Composite Temperature/Humidity Cyclic Test (1974) 16. Okazaki, H., Sasaoka, T., Nakane, Y., Kiyomiya, T., Makino, H., and Aoki, T., International Society for Optical Engineering-Optical Data Storage Topical Meeting, 1078:51-57 (1989) 17. Matsui, T., Yoshizawa, A., Mikuriya, K., andTanaami, K., Measurement for The Substrate, in Measuring and Evaluation Technique of Optical Disk Part I., (T. Kubo, ed.), pp. 11-105, Japan Industry Engineering Center, Tokyo (1988) 18. NOVAREX Technical Bulletin, Mitsubishi Chemical Industry (1976)

Magneto-Optical Thin Film Recording Materials in Practice Charles Brucker

1.0

INTRODUCTION

At the heart of the magneto-optical (MO) disk is a thin film layer stack that gives the storage medium its name, many of its attributes, and some of its shortcomings. Current generation media are based exclusively on rare earth-transitionmetal (RE-TM) amorphous alloys for the active MO layer. Due to the high chemical reactivity of the rare earth component, the MO layer is protected between chemically stable dielectric and reflector layers, which also provide optical and thermal enhancement functions. Properly designed and fabricated, the stack provides virtually unlimited rewritability and an operational lifetime of at least a decade. On the other hand, it is the major source of fabrication yield loss and represents the ratelimiting production step, contributing significantly to cost. The materials and fabrication processes used to build the stack are the subject of this chapter. The author has attempted to provide a fairly comprehensive discussion of design, fabrication, and performance issues for each layer individually and/ or as part of the complete stack. Some key enabling technologies have been singled out for emphasis, namely arc-free sputter deposition of dielectric materials, compositionallyuniform sputter deposition from multicomponent RE-TMtargets, and modem disk handling techniques.

279

280 Magneto-Optical Data Recording

As for future generation media, disk recording performance for promising MO materials is compared as a function of wavelength in anticipation of future green and blue light sources. The use of more chemically stable MO materials such as Co/Pt multilayers and gamets removes some of the design constraints imposed by RE-TM type materials. Much simpler stack structuresbecome possible with implications for reducing media manufacturing cost in addition to providing improved short wavelength response. An update is provided on the continuing search for very high Kerr rotation materials with potential for MO recording. Among these new materials, significant hurdles remain in satisfying other requisite attributes for a successful MO material such as adequate perpendicular anisotropy, low processing temperature, and low media noise. Modem fabrication techniques, however, offer unprecedented flexibility and control of material deposition parameters and progress is rapid. Sections are also devoted to reflector materials, and to lifetime testing for the complete disk structure. No single section is devoted to dielectric materials, rather, discussions related to dielectrics have been incorporated in Sec. 3.2 on dielectric film deposition methods, and Sec. 6.0 on lifetime.

2.0 DESIGN CONCEPTS 2.1 Issues and Opportunities The evolution of MO disk media has been shaped largely by the choice of material for the functional MO layer and by the perceived need to be performance-competitive with magnetic rigid disk storage solutions. Current generation MO media uses amorphous rare earth-transition metal (RE-TM) material for the functional MO layer, most often an alloy based on Tb, Fe, and Co. Generally, the RE-TMmaterial is chemically unstable due to the high affinity of the rare earth element for oxygen, and structurally metastable. As a consequence, the issue of long term chemical and physical stability has been foremost in MO media design. Since MO disk storage solutions are coming of age in the face of competing high performance and rapidly evolving magnetic disk systems, a premium is also placed on those attributes which can close the performancegap vs magnetic systems (Table 1).

Magneto-Optical Thin Film Recording Materials in Practice 281 Table 1. Attributes of MO Disk Media

Prerequisite attributes Chemical and physical stability for archival storage, write/erase cyclability, and manufacturing process compatibility Appropriate thermomagnetic switching characteristicsfor the specific application. Inherent attributes Reliability (non-contacting) Large areal storage density Removability Rewritability Attributes driven by rigid magnetic disk performance Reduced latency (e.g. direct overwrite) for faster access Higher linear bit density (e.g. super-resolution or short wavelength light) for faster data transfer rate Comparable SNR

An example of an MO disk media structure embodying the conventional quudriluyer structure['] is shown in Fig. la. The quadrilayer structure forms an optical resonant cavity that provides near-optimum readback performance. A consensus seems to have developed that the manufacturing complexity, if not amenableto a truly low-cost disk media, is tolerable. The dielectric layers sandwiching the RE-TMlayer are of necessity very stable chemically, usually oxygen-free compounds such as nitrides of Si or Al, and are deposited with utmost care to eliminate permeation pathways for oxidation and corrosion agents. The metal reflector layer, typically a corrosionstabilized alloy of aluminum, must also provide environmental protection in addition to a measure of optical and thermal tuning. Opportunities for future generation MO media arise from the needs not only to improve performance but also, just as importantly, to reduce cost. New schemes for direct overwrite and super-resolution based on exchange-coupled RE-TM layers[*] promise significant performance enhancements, but at the expense of increased media complexity. New MO

282 Magneto-Optical Data Recording materials based on Co/Pt multilayers and garnets, which exhibit excellent environmental stability and strong short wavelength Kerr rotation, have been the subject of many publications in recent years. For example, a proposed simple disk structure based on a Co/Pt m~ltilayer[~] is compared with the more conventional quadrilayer structure in Fig. lb. Note the use of an oxide dielectric, the complete absence of a second dielectric layer, and the reduced total thickness of deposited layers in the case of the CORY multilayer. Largely because of its excellent environmental stability, Co/Pt was the material of choice for the recent demonstration of 45 Gbits/in2 data density using near-field scanning optical micro~copy.[~~

Figure 1. (a) Commercial quadrilayer film stack structure based on RE-TM as the functional MO layer."] (b) Proposed simple bi-layer film stack structure based on a Copt multilayer as the functional MO layer.L31 Layer thicknesses are drawn to scale.

Magneto-Optical Thin Film Recording Materials in Practice 283 MO disks with garnets as the active MO layer have also been fabricated with exceedingly simple stack structures, although special substrates are required (Sec. 4.3). Each new MO material has unique processing and performance implications, but certainly the superior chemical stability of many of these materials compared to RE-TM will allow greater flexibility in the choice of stack materials, and permit thinner or less complex stack structures for reduced media manufacturing cost.

2.2 Practical Design Methodology The design of MO film structures to accommodate the required optical, thermal, and magnetic function of thermomagnetic writing and MO reading is a complex task. The list of desired attributes on a layer-by-layer basis is indeed formidable at first glance (Table 2). As pointed out by McDaniel and Sequeda,l51a tractable approach in the case of RE-TM is to focus on optimizing optical and thermal performance of the layer stack under the assumption that the magnetic quality of the RE-TM layer can be independently controlled. The magnetic quality of polycrystalline MO materials such as CoPt multilayers, on the other hand, can depend sensitively on the nature of the substrate, and this must be anticipated in the choice of material and deposition conditions for the first dielectric layer. Fortunately, a good variety of suitable dielectric seed layers and surface treatments have already been found for CoDt (Secs. 3.4, 6.0), so that the above design philosophy is not prohibitively constrained. The principals involved in optimizing the optical response of optically absorbing MO media are quite straightforward and result in systems that have the maximum signal-to-noise ratio associated with differential readout of the stored information using the normal incidence Kerr effect.[61-[101 The enhancement is brought about by a simultaneous optimization of the Kerr rotation &and reflectance R. Minimizationof the phase difference S = tan-'(+/&), where E- is the Kerr ellipticity, is preferably accomplished by proper stack design. Otherwise, unwanted effects of parasitic Kerr ellipticity can only be reduced by the use of an external phase correcting optic (usually a W2 retarding plate), which adds weight, size, and cost to the readout head. It is possible to achieve 0, values over 25" for a RE-TM stack, but R becomes vanishingly In practice, R is determined by the need to provide sufficient tracking and focus signal for the readout beam. Optimal system performance is usually achieved for intermediate values of''6 and R (ISO/ANSI Standards['*] speci@ 10 5 R 5 34%), and vanishing E-. Given

284 Magneto-Optical Data Recording Table 2. Magneto-optical disk materials and their desired attributes. Representative indices of refraction n + ik (A= 800 nm) and thermal conductivities K (watts/meter-Kelvin)indicated for materials correspondingto Fig. 1. Layer

Desired MDU~S

..

1

Mareriais

m

.

InI.

I ypicai upacai

wd

Thermal Parameters Substrate (See also Ch. 4) optical, thermal d d d u s t defocus, protection high optical transparency and uniformity low birefringence minimal light scatteringdefects other rigidity and mechanical strength precision replication film processing compatibility (high T& dimensional stability, minimal water absorption low cost (injection molding) Dielectric optical, thermal re6active index n + ik: nrvb < ndk, < n- k + 0 optically homogeneous (low noise) thermal conductivityK Kdje,<< K, other environmental encapsulation of RE-TM oriented growth of Corn good adhesion, low stress low deposition temperature low cost (high deposition rate) MO optical, thermal strong signal (large MO effects) low noise (amorphous or small grain size) write/erase cyclahility ( lo6 cycles) other adequate perpendicular anisotopy, coercivity, squarenes environmental stability low deposition temperature, high deposition rate Reflector optical, thermal high, uniform reflectance thermal conductivityK: Km,>> Kdie,,K, other film processing compatibility environmental stability good adhesion to dielectric Protective (see also Ch. 4) Overcoat durability against mechanical abrasion environmental stability good adhesion to reflector, low stress low cost (spin coat, W cure)

-

polycarbonate n = 1.50 + i 0.00 K=0.1

n = 1.58 + i 0.00 K= 1

Si-N

n = 2.0 + i 0.02 K = 1.5

Zn-0

n = 1.9 + i 0.01

Tb-Fe-Co

n+ = 3.20 i 3.50 n- = 3.25 i 3.55 K =7

corn

n+= 2.57 + i 5.13 n- = 2.71 + i 5.19

-

K=25

-

Al alloy

n = 2.7 i 8.3 K m 25-100

organic resin

n = 1.5 + i 0.00

Magneto-Optical Thin Film Recording Materials in Practice 285 this constraint on R,performance can only be improved through the h d a mental optical and magneto-optical constants of the magnetic medium itself. Ideally, the design of a phase-optimized structure would be done systematically with complete freedom to choose dielectric materials, which satis@analyticallydetermined indices. An elegant example of this approach is provided by Atkinson et al.,p]one result of which is shown in Fig. 2. In this example a semi-infinite aluminum substrate is used with air-incident light (front-surfacerecording),the only other constraints being the use of TbFeCo for the MO layer, an arbitrarily chosen reflectance of 16.7%, and an operating wavelength of 633 nm (for experimental convenience).

(4

A1 reflector (semi-infinite)

2nd dielectric layer

lbF&

M O layer

1st dielectric layer

n = 1.39 + 17.65

= 1'46

=126nm

t

n+ = 2.65 + i3.27

n- = 2.69 + 13.33 t =13nrn

= 2.36

h = 633 nm, air incident

L

0

0

20

40

60

80

100

1st dielectric layer thickness (nrn)

Figure 2. Analytically designed quadrilayer showing principle of phase adjustment with constant reflectance. (a) The layer structure shows calculated refractive indices for the first and second dielectric layers, which in practice can be satisfied using ZnS and SO,, is controlled by the thickness of the first respectively. (b) The phase 6 = tan-'(.Qek) dielectric layer for pure Kerr rotation (6 = O", desired for differential detection) or pure ellipticity (6= 90'). Reflectance is constant at 16.7% thro~ghout.[~]

286 Magneto-Optical Data Recording The structure shown has the appealing feature that the phase S can be continuously adjusted between pure Kerr rotation (S= 0') and pure Kerr ellipticity (S=90') simply by changing the thickness of the first dielectric layer, while maintaining constant reflectance. These predictions were verified experimentally using ZnS (n = 2.35) for the first dielectric layer. Although practical material compatibility issues are not addressed, nor is front-surface recording widely practiced, this work is recommended to the reader who wishes to gain clear intuitive insights into the mechanism of optical enhancement beyond those provided in more heavily numerical analyses. For through-substrate recording the author is not aware that a solution as elegant as that shown in Fig. 2 for front-surface recording has been shown. Rather, the dielectric material is chosen for reasons of chemical stability and process compatibility first and index of refraction second. Optimization of reflectance and phase is then accomplished by iterative variation of the thicknesses of the dielectric and MO layers. In the course of actual MO media design and development, a user-friendly computer algorithm to predict the MO response of trial stack str~ctures,[~1[~~1 as well as an organized method of displaying the multidimensional matrix of optical characteristics and layer are invaluable tools. Having mapped the space of the optical response of the quadrilayer, further tuning of the stack must be addressed for thermal considerations. Compared to optical methodology, this typically involves somewhat more empirical adjustments based on previous knowledge and engineeringjudgment. In a properly designed quadrilayer, power delivered to the stack by the laserthat is not reflected is largely absorbed in the MO layer. Thereafter, the balance between radial and axial heat flow can be controlled to some extent by adjusting film thicknesses andor thermal conductivities.[151-[171The primary degrees of fieedom are the dielectric film thickness, the reflector film thickness, and the reflector the& conductivity. Temperature gmhents thus produced at the leading and trailing edge of the written mark are fundamentalto freezing the domain wall position repeatedly fiom mark to mark. Successful thermal performance based purely on educated judgment becomes increasingly fortuitous as one deviates from well-known structures and materials. More likely than not, problems such as domain edge jitter or thermally induced degradation will be encountered, and thermal modeling can be an indispensable tool. As with optical modeling, accurate material parameters for the stack materials are required as inputs. In regard to this, it is noted that there is a glaring lack of hard thermal conductivity data for thin metal, dielectric, and polymer films. Table 3 lists a sampling of

Magneto-Optical Thin Film Recording Materials in Practice 287 measured thermal conductivity data for some representative thin film and bulk materials to illustrate the range of values and sensitivityto microstructure. First it can be observed that the thermal conductivity of thin films for a large variety of materials can be as much as two orders of magnitude lower than that of the correspondingbulk solid, i.e., bulk thermal conductivity data are not applicable to thermal thin film stack design. The basic reason for reduced thermal conductivity of thin films is phonon scattering from point defects, dislocations, internal and external boundaries, impurities, and other phonons.[181 In the case of AN, Shaw-Klein et al.Il9]have discussed the large effect of boundary scattering and inclusions, manifest in markedly lower thermal conductivity for thinner films. The data for amorphous TbFe films shows the high degree of heat flow anisotropy, which can result A more comprehensive compilation from columnar film of knownthermophysicaldata for typical MO layer stack materials is provided by McDaniel and Bartholomeusz in this book.

Table 3. Thermal Conductivity K for Various Thin Film and Bulk Materials Material Diamond (I and KI) cu

Al Si (single crystal) AlN (dense, polycrystalline) Tb-Fe alloy (dense, amorphous) Tb-Fe alloy (columnar, amorphous)

COPt Garnet Many oxides and fluorides Glass Polycarbonate Lacquer

Air

190

0.03

0.5

< 0.15 0.25-1.O 0.25-1 .O

16 5.3 (in plane of film) 7.0 (perpendicular) 0.3 (in plane of film) 4.3 (perpendicular) 25 0.05-1 .o

1000-2300 400 240 120 70- 180

21 21

30-40

20

0.1-2

21 19

20

0.25-1.0 0.2

24

80

6 1-50 1 0.1 0.6 0.03

23 26 18 21

24 21

288 Magneboptical Data Recording

A good example of practical thermostructural design optimizationfor a quadrilayer organic/Si-N/Tb-Fe-Co-WSi-N/Al-alloy stack is provided by Ogthara et a1..[221Desired write/erase sensitivityas well as repetitive write/ erase stability was achieved by layer thickness optimization, and by the use of an Al alloy to control the thermal conductivity of the reflectorkat sink layer (Sec. 5.0). Some sacrifice in optical enhancement was made to ensure thermal performance and corrosion stability. For a trilayer glass (2p)/Al-N/ Dy-Fe-Co/Al stack, Raas~h[*~] found that the write and erase power thresholds increased about 1 mW per 40 A increase in Al reflector thickness for are the disk velocities up to 22.6 m/s. The quantity ST = ,where T,and GOmp Curie and compensation temperatures, respectively, is a useful parameter that appears in many discussions of media thermal design. T,-Tcomp,which is roughly proportional to the saturation magnetization, can be chosen by proper composition (Sec.4.1), and is typically in the range 100 to 150°C based on considerations of CNR and recording For high quality domain edge definition, Mergel et al.[251have emphasized the importance of a steep dependence of H,on T and moderate M, at temperatures near the switching temperature. For stability of recorded domains, high coercivity and low M, at room temperature and at readout temperature are desired. Mergel also provides a concise summary of the bubble-like domain formation theory, which can be used to understand the correlation between the thermomagnetic properties of the MO layer and the switching parameters in dynamic recording experiments. Mouchot et a1.[26Jhave presented thermal design calculations for a glass/Cr/garnet disk structure intended for front surface recording. In the case of a garnet film with very low optical absorption, the “reflector” layer must also serve the purpose of absorbing laser beam energy. 3.0 FILM DEPOSITION

AND MANUFACTURING

METHODS 3.1 Batch vs. Integrated Processing Two basically different approaches to the production of MO disks, batch vs integrated processing, are shown in Fig. 3. In batch processing, the different production steps such as substrate injection molding, sputtering, protective overcoating, hubbing, etc. are grouped and connected by transfer systems or operators. The step with the largest throughput (as

Magneto-Optical Thin Film Recording Materials in Practice 289 drawn in Fig. 3, the sputtering step) is accomplished by loading many disks onto a large transport pallet. Integrated processing consists of stand-alone production islands with local transfer systems in which all downstream equipment uses single-substrate processing matched to the output of the injection molding. Both concepts, and hybrid versions thereof, are used for high-volume production. (a) Batch Processing

Hub, Case, Test

Injection Sputtering

Protection

(b) Integrated Processing Injection

Sputtering

Protection

Hub, Case, Test

Figure 3. Schematic machine layout and process sequence for production of MO disk media using batch (top)and singledisk (bottom) substrate handling.[34]

290 Magneto-Optical Data Recording Highly automated batch processing, because it avoids equipment redundancy, will ultimately give the lowest fabrication cost per disk, but only for long uninterrupted production runs of uniform product. Integrated processing, on the other hand, is attractive from the point of view of low start-up investment, ease of automation, flexibility of process flow, and lowsubstrate inventory. These and other attributes are compared in Table 4.

Table 4. Comparison of Batch vs Integrated MO Disk Production Processes Batch (Multiple Disk Handling)

Integrated (Single Disk Handling)

Advantage

start-up capital investment

high equipment costs large space requirements high support costs slow return on investment

low per line fast return on investment

integrated

Labor

labor intensive or custom handling systems

readily automated

integrated

Thtoughput

high not easily expanded

relatively low per l i e expandable by adding lines

batch

Productive time

large, complex equipment limits reliability maintenance, cleaning difficult

small, redundant equipment offers flexibility and improved reliability maintenance and cleaning relatively straightforward

integrated

Yield

vulnerable to production flow bottleneck if any part of system fails large inventory loss upon equipment failure

loss of output due to unit failure can often be accommodated by other lies low inventory loss upon equipment failure

integrated

Product uniformity

potential process variation from center to edge of multiple substrate pallet

all substrates receive identical treatment

integrated

wear of moving parts

one wear cycle per pallet

one wear cycle per disk

batch

Product monitoring

spot monitoring possible but requires single disk handling

in-line monitoring easily accommodated by additional station

integrated

Substrate size

change in substrate sue accommodated by pallet redesign no lower limit to substrate size

change in substrate size may require substantial retooling lower limit to substrate sue

batch

ID and OD Masking

self-aligning masks can be attached to each substrate

masks difficult to alien batch and prone to debris accumulation

probably limited to one

mask size can be different from station to station

Mask Sue

mask s& for all layers

Y

integrated

Magneto-Optical Thin Film Recording Materials in Practice 291 The historical evolution of other “high technology” mass produced

thin film products such as integrated circuits or compact disks has proceeded from evaporation-batch processing at first to sputtering-integrated processing in the current state of the art. A similar trend to more integrated processing can be discerned for MO disk manufacture, although the difficulty of single-disk sputter processing of substrates much smaller than 120mm diameter (CD size) may still require buffering and multi-substrate handling at least for some process steps. Whether batch, integrated, or hybrid manufacturing is used, minimization of handling is key to reducing cost. Unsophisticated handling in the manufacturing process has contributed to the relatively high retail cost of MO disks (currently on the order of $50-$loo), which in turn has limited the sales volume (< 2 million in 1992). Ambitious cost reduction has been undertaken for rewritable 2.5”MiniDisc@[*]production. Fully automated processing promises a manufacturing cost on the order of $2 for the disk itself (excluding hub and enabling a final retail cost on the order of $10-$20. Pilot line projections of step-by-step cost and yield factors are shown in Fig. 4. The major cost factor is the sputtering of the MO layer stack, which, in turn, is dominated by the cost of the difficult-tofabricate RE-TM MO target (Sec. 3.3). Costs incurred by yield loss are minimized by an inspection step just prior to the expensive sputtering step. After lacquering, the disks are relatively immune to handling damage and can be moved off-line for hubbing and cartridging. Sputtering is one of the core technologies for the mass production of magneto-optical disks. Several companies have developed machinery and sputter process technology for the production of multilayer MO disk media.[281-[341Machine layouts typical of batch and single-disk sputtering systems are shown in Fig. 5, where for illustration a cathode arrangement suitable for the deposition of the quadrilayer SiN/RE-TM/SiN/Al media of Fig. la is assumed. Several variations of these basic layouts are found in current production facilities, including planetary substrate motion during RE-TM deposition. Whatever the equipment details, any production system for MO media production must satis@ the following requirements: 1. The system concept must ensure high throughput. Rate-

limiting steps, such as deposition of the relatively thick first dielectric layer, can be brought into balance with subsequent steps by the addition of a second sputter cathode as in Fig. 5 .

292 Magneto-Optical Data Recording 2. The system must be versatile to accommodatechanges in the individual layers or stack architecture. Modular design in the case of linear systems simplifies the rearrangement or addition of process stations. Circular a d closed Imp layouts are less flexible in this regard9 unless designed with the foresight of spare and interchangeable stations. 3. The system must have feedback process control to ensure reproducibility within strictly defined limits (Table 5). 4. All processes must be carefilly separated from each other for individual control without cross-contamination. Dynamic buffers have been demokrated with a pressure separation factor of 10-5 between active stations, permitting a continuous flow of substrates through the 5 . Mechanical components must be designed for minimal

debris generation.

Table 5. Key parameters that can be monitored to provide process feedback regulation to maintain the indicated tolerance specifications in an MO production

Layer

dielectric

MO

Regulation

Monitor

Cathode I-V(reactiveDC) N2flow N2pressure

Transmittance Refractive index Thickness (900 A) Disk warpage

Ar flow Ar pressure Magnetron magnetic field Cathode power

Hc Tromp

& Thickness (800 A)

Specification

*2Yo *2%

15% *lo% *2% &2%

Magneto-Optical Thin Film Recording Materials in Practice 293

~~

~

1 Injection molding 2 Inspection 3 Sputtering of MO-layer stack 4 Lacquering 5 lnsoection

(a)

Administrative costs Variable costs

1.5

7 Cartridge assembly 8 Final inspection 1.2

0.9 0.6

0.3 0.0 1

2

3

4 5 Process step

6

7

8

Yield variation at sputter process step

A Nominal example value 85% inieclion molding yield 95% sputter coaling yield

1.2

-0

1.1

-

0 tf3

.-C

0

A

fn

fi

0

A

c1

Bz:

A "O

-

0.9

-

0.8

Yield variation a1 inieclion molding step

1 h h

I

I

I

.

I

A

I

1

294 Magneto-Optical Data Recording

degassed substrates

(a) Batch sputtering process 1

1st dielectric

f

MO 2nd dielectric =lo m

reflector

single substrate

-Am-

Figure 5. MO sputtering concepts based on (a) batch vs @) integrated processing.[33]

3.2 Sputter Deposition of Dielectric Materials One of the most critical tasks in MO disk fabrication is a fast and stable process for deposition of the dielectric layers. Sputtered dielectrics

Magneto-Optical Thin Film Recording Materials in Practice 295 are widely used in the optics and microelectronics industries, and there are several good references for general background information on the h d a mentals of s p ~ t t e r i n g . [ ~ ~The J - [ ~desire ~ ] to use low-temperature polymer substrate materials in the optical recording industry have provided the impetus to develop stable deposition processes with high deposition rates and low-substrate heating. Sputter deposition technology has responded to these needs and can now provide the vacuum production engineer with several reliable methods to deposit insulating materials. Three of the more important methods are radio frequency (RF), reactive DC, and reactive midfrequency (MF) sputtering. There is a tremendous engineering richness in this rapidly evolving field,I3*I too extensive to review here, but some fundamental principals can be laid out. Conceptually, the simplest way to sputter a dielectric material is to bond it to a metal backing plate to which a large negative voltage is applied. A plasma results if an inert gas atmosphere at the appropriate pressure is supplied. Inert gas ion bombardment of the target surface dislodges atomic and molecular species, which condense on the substrate. If a DC voltage is used, the target surface quickly acquires a net positive charge until the full power supply voltage appears across the target rather than between the target and the plasma. This occurs in fractions of a microsecond for typical dielectric target capacitances and resultmg in plasma extinction. One solution is to alternate the voltage on the backing plate so that electrons are periodically drawn to the target to neutralizethe build up of ionic charge, at frequencies on the order of several megahertz given the charging times involved. RF sputtering is almost universally done at 13.56 M H z , the FCC allocated industrial radio frequency. RF powered magnetrons provide stable operations, but suffer severe limitations for MO media production. Since usefbl sputtering only occurs during part of the power cycle, the deposition rate normalized to the power density (the specific rate) is about half that obtained with DC power. The additional RF power that must be applied for deposition rates comparableto those of DC sputtering, contributes to undesirable substrate heating. Also, distributingRF power uniformly to large targets is difficult in practice, and requires expensive and mechanically cumbersome impedance matching networks. The most commonly used alternative to RF sputtering for dielectrics is DC reactive sputtering. In this technique, DC power is applied to a conducting metallic (or suitably doped semiconducting) target and a reactive gas (e.g., N,, 0,)is mixed with Ar, which reacts with the metal (or

296 Magneto-Optical Data Recording semiconductor) to form the desired compound. Advantages are that the specific deposition rates are greater than those for RF, substrate heating is reduced, and the problems of RF power distribution are eliminated. Disadvantages of DC reactive sputtering are the difficulty of control and, more seriously,target arcing. Both of these problems arise from the fact that the interaction between metal and reactive gas atoms is not limited to the substrate, but also takes place on the target surface. The control problems are a consequence of the need to operate most DC reactive sputtering process in a state of unstable equilibrium,especially when the reaction product is a dielectric. Figure 6 shows schematically the competition between erosion of the magnetron “race track,” where the sputter yield is higher than the growth rate of recaptured material, and the build up of nonconducting reacted material where the opposite situation prevails. Process instability arises from any perturbation leading to greater compound formation on the target, which if unchecked leads to a rapidly cascading progression of decreased sputtering rate + decreased reactive gas consumption + increased reactive gas partial pressure + hrther compound formation on the target +and eventual “poisoning” of the entire target surface.

Conducting target Erosion race track Dielectric

film

Plasma

Figure 6. Enlarged cross-section of the target-plasma system showing voltage gradients between a plasma and a conducting target partially covered with a dielectric film.

Magneto-Optical Thin Film Recording Materials in Practice 29 7 One solution is to adjust the partial pressure of the reactive gas for the best compromise between proper film stoichiometryand minimal compound formation on the target. This can be done using feedback control of the reactive gas flow rate in response to changes in the gas consumption rate or the target condition. For DC operation. the target condition (degree of coverage) is well reflected in the voltagecurrent characteristic of the discharge. If this characteristic is hysteretic (Fig. 7a), initial and final equilibrium states will be different after a perturbation, so it is imperative that the size of these perturbations be minimized. Hysteresis behavior can be eliminated by increasing the ratio of pumping speed to target size (Fig. 7b), making the requirements for process stabilization much less critical, but as coating system size increases there is a practical limit on pumping system enlargement. Regulation of DC reactive sputtering would simply be a challenging control problem were it not for the second and more serious difficulty, target arcing. Just as with DC sputtering of thick dielectric targets, the reacted target surfacetends to charge up, inducing a voltage drop across the dielectric layer. Before the full target voltage is realized, however, the dielectric strength of the insulating layer is inevitably exceeded. For many dielectric materials the breakdown field is on the order of lo6 Vkm, which for a target voltage of 500 V is exceeded for film thicknesses less than about 1 pm. The resultant low-voltagehigh-current arc can seriously disrupt plasma stability, eject macroscopic particulates from the target, which create defects in the growing film, and induce cracking of the target. The onset and frequency of arcing can be controlled to some extent by cathode and shield design, by supplying additional anode structures to enhance ionization of reactive gas atoms in the vicinity of the substrate, and by sophisticated on-line control. Rotating magnets designed to scan the plasma over the complete surface of a ring-shaped dielectric target have been shown to be effective in reducing a r ~ i n g , [ ~ ~but 1[~ only ~ 1 for a limited range of target geometries. Another approach is to remove the substrate from the target region, either by increasing the throw distance or by the use of opposing targets such that arc-induced particles travel predominantly toward the other target instead of the substrate. A significant breakthrough in dielectric deposition has occurred recently based on the realization that the impedance of the poisoned target surf' is low enough such that intermediate frequencies can be used to prevent the charge accumulation that leads to arcing. Since the charging

298 Magneto-Optical Data Recording

I

400

.

.

.

.

I

.

.

.

Increasing flow Decreasing flow

0

350

.

~

>

h

v

al

Transition region

0)

(d

c.

8

300

250

-

I

1

400

350 -

--

I

.

-

(a) 100 I/s pumping speed

200

0)

"c

-.

.

1

.

.

.

1

.

.

.

1

.

'\

1

'

.

.

1

.

.

Decreasingflow Increasing flow

-

9

v

0 0)

-

(d c.

8

300

-

a

c.

a

%\.;

F

t-"

250:

. 200

(b) 450 Vs pumping speed 1

10

-

20

-

1

30

-

1

40

-

1

50

-

1

60

.

1

70

-

ao

Figure 7. Target voltage vs nitrogen gas flow for AlN deposition at two different pumping speeds, (a) 100 literisec and (b) 450 literlsec, and otherwise identical operating conditions (constant target current control mode). In (a), only the transition region permits the deposition of stoichiometric layers with high productivity and sufficient process stabilityP1

Magneto-Optical Thin Film Recording Materials in Practice 299 time for typical dielectricjlm capacitances and currents is on the order of milliseconds, as opposed to microseconds for bulk (:. 5 mm thick) targets, the build up of charge can be neutralized using mid-frequencies (MF) in the 10-100 kHz range. Cormia et al.l4l]first demonstrated the effectiveness of MF sputtering for arc suppression during reactive TiO, deposition. A key ~~1 developmentwas the use of balanced targets by Este and W e s t ~ d , [who studied the sputtering rate as a function of frequency from DC to RF. They found that the deposition rate increased with decreasing frequency below RF, reaching the DC rate at about 30-50 kHz (Fig. 8).

I 1

0.5 MHz

dc

ac(6OHz)

1

1

80kHz

ff(l3.rMHz)

Figure 8. Variation with power supply fresuency of the relative sputtering rates from a doubleended Al target in Ar (o),Kr (x), and N, ( 8 )dischargesat 0.3 Pa and in Ne (A) at 0.6Pa(l Pa=7.3 mtorr). TheN,valuesareplottedontheassumptionthatthe 13.56MIIz value is 0.5.[421

300 Magneto-Optical Data Recording Below about 50 kHz,the relative mobility of ions and electrons in a sputtering plasma makes the process more typical of DC than RF.[431 Balanced magnetron geometries have subsequently evolved such as shown in Fig. 9, which reduce the otherwise objectionably large positive target voltage during the dielectric discharge cycle. Glockerr41 has characterized the substrate heat flux for Al-N deposition with a sinusoidal MF voltage split between a pair of identical magnetrons and found it to be greater than for reactive DC. It is not necessary that the applied voltage be sinusoidal or have a symmetrical waveform, however. The technique can be optimized for highdeposition rate and low-substrate thermal loading by tailoring the relative AC/DC content andor AC duty cycle to provide only as much arc suppression as needed. Moreover, power distribution in this frequency range is simple and efficient using DC feedthroughs and audio transformers, of considerable practical importance.

Disk

Masks

Figure 9. Balanced magnetrongeometries for mid-frequency reactive sputter deposition using (a) side-by-side rectangular targets for pass-by coating, and (b) nested circular targets for static single-disk

Magneto-Optical Thin Film Recording Materials in Practice 301 Mid-frequency reactive sputtering and variants thereof, e.g., DC plus superimposed AC, are increasingly finding applications for MO film production. The process is reliably arc-free with outstanding long-term stability and high rates, and good quality Si-N and Si-0 dielectricfilms have been produced with acceptable heat loading of PC s ~ b s t r a t e s . [ ~ ~ lIt[ ~is~noted l[~~] that arcing problems can be avoided altogether if the conductivity can be increased, either through compositional control, or doping, to allow straightforward DC sputtering. This has been demo& for example, inthe case of Sic dielectric layers for use with RE-TM,[451 but the range of dielectric materials that can be deposited in this h h i o n is rather limited.

3.3

Sputter Deposition of Multi-Component RE-TM Materials

The use of an alloy target for deposition of the RE-TM MO layer is highly desirable for process simplicity and chemical passivation of the target RE constituent(s). A fundamental difficulty in sputter deposition from alloy targets, in general, arises from differences in the net angular distribution of constituent atoms amving at the substrate, resulting in film composition gradients if no counter measures are taken. Substratemotion in pass-by pallet coaters somewhat compensates for this effect in the transport direction, but does not prevent a composition gradient along the film thickness direction. A composition gradient transverse to the transport direction can also result for substratespassing near the target ends. Angular effects are most severe for stationary coating of single substrates using small targets. Planetary substrate motion can be employed to minimize angular effects, but adds considerable mechanical complexity. Transient and steady state sputtering phenomena also contribute to angular irregularities. Transient changes in film composition can result from differential resputtering of the deposited film because of energetic bombardment and redeposition onto the target surface, producing an altered deposited film, target surface composition, and morphology. Even after care has been taken to properly condition the target, the steady state back and forth transport of material is still subject to angular effects arising not only from the sputter ejection process but also from scattering in the inert gas ambient. The angular distribution of sputtered atoms for normal incidence ion bombardment is a strong hnction of incident ion energy. For incident energies > 10 keV, the angular distribution is sharply peaked in the direction normal to the target In the 1-10 keV range, the angular

302 Magneto-Optical Data Recording distribution is well described by Knudsen’s cosine (originally applied to small area evaporation sources), still peaked in the normal direction, but more gently than at higher energies. For energies less than 1 keV, the angular distribution tends to be decidedly under cosine, i.e., more material is geed tn the side thm in_ the rlireC.tinn nnrrr-ai tn the arge.t surfaCe[481 (Fig. 10). Furthermore, for multicomponenttargets, the degree of sideward ejection depends strongly on matrix eflects (e.g., relative mass, size, and chemical reactivity of the target constituents), first observed for Fe-Ni and Ni-Cu single-phase alloy targets by Olson and Weht~er.[~~] For incident energies below about 300 eV, lighter atoms are preferentially ejected in the a phenomenon explainable by simple reflective collinormal sion arguments (lighter atoms are more likely to be backscattered in a shortrange collision cascade). In the intermediate 300-1000 eV energy range, precisely that employed in typical plasma magnetron operation, deeper ion penetration produces more complex collision cascades, such that prediction of ejection profiles becomes less than straight&omrd.

150

Figure 10. Angular distributionsof sputteredMo particles for Hg+ions normally incident on a plycrystal Mo target, showing progressivelyunder-cosine behavior with decreasing incident ion energy. The contours representrelative deposited film thickness as a function of ejection angle.[48]

Once ejected from the target surface, differential scattering effects due to collisions with ambient inert gas atoms hrther compound the problem. Depending on relative mass and scattering cross-section, one

Magneto-Optical Thin Film Recording Materials in Practice 303 species may be more likely to be scattered out of the deposition zone and onto surrounding shield surfaces as opposed to the substrate. This can be particularly significant at pressures high enough to produce difisionlimited transport over distances comparable to the target-substrate separation. An instructivetheoretical overview of angular ejection and scattering phenomena for Tb-Fe is provided by Bartholomeusz and I-Iat~ar.[~~] These angular effects can have dramatic consequences for MO film properties. Chen et al.[5210bserveda strong variation in magnetic hysteretic properties of amorphous Tb-Fe thin films due to process-induced changes in film composition (Fig. 11). The target consisted of a circular Fe plate into which smaller Tb disks were inset in a regular hexagonal pattern. The value of H, and the morphology of the Tb target surface showed a strong dependence on Ar pressure but a lesser dependence on power. Intermetallic TbFe, compound formation was detected on the Tb disk surface, especially at higher pressure.

at. % Tb

0

0

I

I

20

I

I

40

I

I

I

60

I

I

80

Ar pressure (rnTorr) Figure 11. Dependence of coercivity and loop sense on Ar pressure for 300 A thick TbFe films RF sputtered at 0.5 kW (A), 1.0 kW (0), and 1.7 kW (0)power input to the same target. Inset: H, and loop sense as a function of Tb content for 1 jtm thick films produced from targets of different composition. The films were deposited from an 8" diameter composite target (small Tb disks inset into an Fe target) of nominal area composition 33% Tb at a target-to-substrate separation of 2".[521

304 Magneto-Optical Data Recording Hatwar et al.[531observed a sensitive dependence of Tb/Fe ratio, but no measurable change in Co concentration vs Ar pressure for Tb-Fe-Co films DC sputtered from a “homogeneous multi-phase” melt-cast target (Fig. 12). Interestingly, the trend of increasing Tb content with increasing pressure in_ FiS- 12 is opposite to the trend show in Fig, 11 (inset)j only emphasizing the capricious nature of RE-TM angular effects.

-4

-3

-2

-I

0

1

2

3

4

Radial distance (cm)

Figure 12. Radial distribution of Tb and Fe in Tb-Fe-Co films DC-sputtered in 3 and 15 mtorr Ar pressure. No variation was measured in the Co concentration (12 at%). The films were deposited from a 2“ diameter melt-cast alloy target of nominal composition Tb,,Fe,,Co,, at a target-tesubstrate distance of 6”.[531

Finding a practical means of compensating for these angular effects has been a challenging and largely empirical undertaking given the complexity of the sputtering process and number and dissimilarity of constituents found in commercial MO targets. Using hot-pressed powder techniques, RE-TM targets have been fabricated with controlled metallurgical structure and magnetization propertie~[~~1-[~~1 (Table 6). In this way, for example, the

Magneto-Optical Thin Film Recording Materials in Practice 305 angular distribution of Tb atoms sputtered from Tb metal, which tends to be cosine, can be balanced with that from Tb-Fe intermetallic compound, which tends to be under-co~ine.[~~] At the same time, target permeability and saturation magnetization can be kept as small as possible to enhance magnetic field penetration of thick targets for longtime target usage. In experimental sputtering systems, compositional uniformity on the order of *l at% Tb has been demonstrated over substrate sizes comparable to the target size, and compositional stability of the same order has been demonstrated over the life of the target.[541-[561

Table 6. Comparison of deposited film compositional uniformity and stability for long-term usage of RE-TM targets made by pressed powder metallurgy techniques. An “x” denotes a target constituent containing intermetallic compound (IMC), rare-earth (RE) only, or transition metal (TM) only. Targets with high, medium, or low IMC content are designated as compound, intermediate, or composite, respectively. A “+’, or “-” denotes advantageous or disadvantageous. IMC

Designation

RE

TM

Composition

4xM,

uniformity

Stability

1. Natelsl

Hot pressed powder RE-TM targets by reductioddffision method Startingmaterial: RE oxide powder (inexpensive) + TM powder Compound target shows best long term compositional stability Intermediate and composite targets show best compositional uniformity compound intermediate composite

X

x x ~~~

Tb17~17Fe58C08

x x

x

~~~~~~~~~

kG 5kG 17 kG

+ +

+ -

~

2. Murakami[”l Hot pressed targets with low oxygen content for good ductility, no cracks Startingmaterial: RE metal powder + TM powder Best compositional uniformity for intermediate target: combination of TbFe, (underashe for Tb) and Tb (cosine) Intermediate target stable after 7 h (80 h target lifetime) compound intermediate composite

~(40%) x ~(25%) x ~(16%) x

x x

Tb Fe

23,

77

x

<9 kG 9kG >9 kG

-

+

+

-

3. Maki[%l Further development of intermediate type target Improved magnetic field configuration for long-term compositional stability 45 vol% target usage with 7 mm erosion depth intermediate-I intermediate-2 intermediate-3

x x

x

x x x

TbpFe72C08 Tb,Fe,Co8 Tb31Fe62C08

5 kG

+ + +

+ +

+

306 Magneto-Optical Data Recording Properly designed and fabricated RE-TM targets have performed extremely well in production coaters, both pallet pass-by and single-disk type, with uniform and stable RE concentration within =t1 at% or better.[281[291[311[321 The compositional control appears to be more than adequate for reprducihle M O properties such as Hc and TCU,,,>and is maintained for high deposition rates. While remarkable progress has been made regarding RE-TMtarget performance, difficulties still remain in that alloy brittleness often requires complicated mosaic block construction of large targets, and that a potentially major new target development effort is required for each new desired film composition. We note that whereas the great bulk of effort has been devoted to planar target geometries to minimize fabrication complexity, novel cathode geometries have also been described as an alternative approach to achieving compositional uniformity and lowsubstrate heat

3.4 Sputter Deposition of Co/Pt Multilayer Materials The discovery and early demonstrations of spontaneous perpendicular anisotropy in Corn and CoPd multilayered materials by Carcia et al.l5*] and Zeper et al.[591occurred in the mid to late 1980’s. By this time the commercialization of rewritable MO media based on RE-TM materials was already established following the discovery of perpendicular anisotropy in Gd-Co and Gd-Fe films by Chaudhari et a1.L6O1in 1973. In spite of this, the Co/Pt multilayer class of materials has continued to grow in interest. From a fabrication point of view, some key attributes of C O Bare the chemical and structural stability, the simplicity and flexibility of composition control afforded by singleelement sputtering targets, and room temperature processing compatibility. A number of investigations have concluded that the measured CNR for Co/Pt is comparable to that of RE-TM in the 600-800 nm wavelength range and superior to RE-TM at 400 nm (Sec. 4.2). Nearly ideal single-crystal CoPd and CoPt superlattices have been prepared by ultra high vacuum evaporation onto epitaxial seeding layers, with large perpendicular anisotropy (greater than that for a bulk singlecrystal of hcp or fcc Co) and H, well in excess of 3 kOe.[611[621Experimentally, the effective magnetic anisotropy & - p e runit volume of Co with thickness t,, can be described by a phenomenological expression including a volume and interface type anisotropy,

Magneto-Optical Thin Film Recording Materials in Practice 307

Contributing sources to the volume anisotropy K,, per unit volume of Co are at least threefold: shape anisotropy, magneto-crystalline anisotropy, and magnetoelastic anisotropy (magnetostriction). The factor of two arises from the f h t that each Co layer has two Pt interfaces. For CoPd, electronic structure calculations by MacLaren and V i c t ~ r a [are ~ ~in] good quantitative agreement with the measured interface anisotropy K,. The most strongly perpendicular superlattices are (1 11) oriented fcc with a favorable K,, . The anisotropy behavior of polycrystalline (1 11) CoPd multilayers displays i.e., the introduconly a very small perpendicular volume tion of grain boundaries and less than ideal (1 1 1) texture compromises some of the magneto-crystalline contributions. The total anisotropy of these multilayers is therefore dominated by the interfhce and shape contributions. This presages some of the key materials goals and hbrication challenges in the developmentof a practical sputter deposition process: (i) a high degree of (111) polycrystahe texture, (ii) properly defined interfaces for large K,,and (iii) many such interfaces per unit film thickness for large net perpendicular anisotropy. Early work on sputtered Co/Pt multilayers achieved coercivities up to about 1.5 kOe (compared to 2 5 kOe for RE-TM media). Hashimoto et a1.fa1 explored the use ofhigh Ar sputtering pressure and metallic underlayers. High sputtering pressure is objectionable, however, in that it leads to reduced film density, and metallic underlayers are impractical for throughsubstrate recording. Weller et al.[651achieved square loop coercivities over 3 kOe on etched SiN, underlayers, but again the high Ar pressures employed produced porous and granular films. More recently, Pitcher et a1.[66]have reported coercivities approaching 6 kOe using Ar sputtering on etched glass substrates with thin Pt underlayers. Carcia et al.[671[6sIachieved square-loop coercivities up to about 2 kOe without compromising film density using heavy inert sputtering gas (Kr, Xe) and oxide dielectric seed layers. The problem of thermalization of sputtered atoms and reflected neutrals has been considered with respect to growth of other layered coherent structures and metastable superconduct o r ~ . [ Carcia ~ ~ ] pointed out that thermalization not only of sputtered Co and pt atoms, but also of reflected neutral inert gas atoms should be considered during the formation of the CoPt interface. The calculated distribution in initial energy of such reflected neutrals is shown in Fig. 13. Because of the greater initial average energy of reflected neutrals, up to 200 eV vs 10-20

308 Magneto-Optical Data Recording eV for sputtered atoms, this source can easily dominate the energy flux to the growing film. The final energy upon arrival at the substrate can be estimated for a given target-to-substrate distance and pressure. For example, Ar atoms reflected from a Pt target at a target-to-substrate distance of 15 cm; arrive at the substrate with most probable energies of about 68 eV for 5 mtorr pressure or 19 eV for 10 mtorr pressure (Table 7). These energies are much greater than the binding energy of a metal (a5 eV) and sufficient to induce atomic rearrangement in the growing film. By reducing the atomic mass imbalance between the sputter gas and target atoms using heavier Kr or Xe, the final energy can be controlled with respect to surface binand -ion energies. Considerationsof surface mobility and negative heat of mixing for the C& multilayer system on intehial roughness and consequences for magnetic anisotropy have been dis~ussed.[7~1[~~1

Energy (eV)

Figure 13. Distribution in initial energy of sputter gas atoms (Ar, Kr, Xe) reflected from a Pt target, as calculated by a sputtering simulation code (TRIM[186]). The incident ion (Ax+,Kr+, Xe+)energy was 500 eV, typical of actual experimental conditions, and 10,000 events were considered for each gas. The calculated fraction of reflected atoms per incident ion was 0.39 (Ar), 0.31 (Kr), and 0.19 (Xe).f6’1

Magneto-Optical Thin Film Recording Materials in Practice 309 Table 7. Initial and final energies for neutral inert gas atoms with 500 eV incident energy reflected from Pd and Pt targets. Initial energies immediately after reflection are calculated assuming hard sphere momentum transfer. For a Pt target, the initial energies indicated correspond to the peak energies in Fig. 13. Final energies upon arrival at the substrate are estimated using the self-scattering formalism of S ~ m e k h [for ~ ~a] 15 cm target-to-substrate distance and indicated sputtering pressures.

InertGas

+

Target

Initial Recoil Energy (eV)

Final Energy (eV) 10 mtorr

5 mtorr 32

9

<1 68

<1 19

9

1

4

4

Magnetic hysteresis loops for a Kr sputtered CoRt multilayer deposited on a ZnO dielectric are shown in Fig. 14. The ZnO films were polycrystalline hcp and grew with preferred c-axis orientation normal to the film plane. An epitaxial relationship has been confirmed for this system, i.e., 2qlll) = 5.54 8, for Pt and ~ u ( , , , ,=~ 5.63 8, for ZnO, only a 1.6% mismatch, where ql 1) and cl(oool) are nearest neighbor distances in the (1 1 1) plane of Pt and the (000 1) plane of ZnO, respectively. The recording noise level is affected by the morphology of the ZnO ~urface.[~*1[~~1 The effect of residual gas pressure during Co/Pt deposition on 0, has been studied by Sumi et al.[741and compared to that for TbFe (Fig. 15). They found no decline in &of Co/Pt films even for background pressures as high as lo4 Torr. In contrast, for TbFe films saturated 0, could only be obtained for background pressures below 6 x torr. This represents a substantial process advantage for Co/Pt. Many schemes for depositing a multilayer can be imagined, e.g., rotary or oscillatory substrate motion, actuated shutters, multiplicity of targets, etc., but a practical manufacturing process capable of rapid deposition and free of machine generated debris remains to be demonstrated.

31 0 Magneto-Optical Data Recording

I!

I

F

I -5.0

-10.0

5.0

0

10.0

H (kOe) -t

Figure 14. Magnetic hysteresis loops for a 1OX (3.4 A Co + 11.3 A Pt) CoPt multilayer grown on a loo0 A thick ZnO layer. The ZnO layer was deposited at 8 mtorr (Ar+ 25% 0,) by RF sputtering, and the Co and Pt layers were DC magnetron sputtered in 7 mtorr Kr at 7.5 cm target-to-substrate

0)

0.4

Q)

n

v

-0) Q)

0.3

1

TbFe

c

([I

g .c

0.2

([I

+

2

5

0.1

Y

0 10-7

10-6

10-5

Background pressure

10-4

(Torr)

Figure 15. Effect of residual background gas pressure on Kerr rotation of CoPt and TbFe films."41

Magneto-Optical Thin Film Recording Materials in Practice 311 4.0 MAGNETO-OPTICAL THIN FILM MATERIALS 4.1 Rare Earth Transition Metal

w)

Amorphous i E I J i i thin fib, with RE seiected from (Ib,Gd, and TM selected from (Fe, Co) are one class of materials satisfjmg nearly all the material requirements in Table 2 except long-term chemical stability. RE-TMalloys exhibit either ferromagnetic (light rare earth) or antiferromagnetic (heavy rare earth) coupling between the RE and TM moments. In the latter case sperrimagneticor femmagnetic order is present, and the ratio of RE/"M ratio can be adjusted to control Tcompfor desired thermomagnetic switching characteristics. The temperature dependence of magnetic properis shown in Fig. 16. At T,,,, the ties for a 500 A thick Gd2.,Tb1Fq5 RE and TM subnetworks compensate each other and H, diverges. Alloys for practical use exhibitmg a T,,, are limited to a narrow compositional range of about 0.7 < x < 0.85. For Dy,,(Fe,Co), alloys, T,,, decreases linearly with increasing TM content with a slope of 27OC per atm% TM.[241Thus a few percent change in the RE/TM ratio can shift Tcompover 100°C, putting strong demands on process composition control. Control of T, is also important with respect to limited available laser power, although T, need not be exceeded for successful recording. Fe-base alloys exhibit a low T, due to the low T, of amorphous Fe, whereas Co rich alloys exhibit a T,, which can be too high for magnetosptical recording. For example, T, ranges from 135OC for TbFe to temperatures higher than the crystallization temperature (4OOOC) for T ~ C O . [ The ~ ~ ]highest T, values within the RE-Fe series are found for RE = Gd, Tb corresponding to their RE-RE and RE-TM exchange coupling constants. For TM = (Fe, Co), tailoring of T, can be achieved with ~] small Co additions leading to a T, rise of roughly 7 K per atm% C O ; [ ~for GdTbFe alloys, T, can be adjusted by the Gd/Fe ratio. Comprehensive reviews of magnetic and magnetosptical properties, write strategies, and resultant domain stability for a variety of RE-TM films have been provided by Mergeliz5land H a n ~ e n Iet ~ ~al.] In amorphous films prepared by evaporation or sputtering, an anisotropic structural arrangement along the film normal induces K,, perpendicular to the substrate The small difference in K, for films with and without a substrate suggests that the stress-induced contribution to K,, is min0r.[7~1The constant K,, arises essentially from the RE single-ion anisotwhich is largestfor Tb and Dy correspondingto their strong spin-orbit coupling. The high K,, and low M, of RE-TM materials gives rise to

312 Magneto-Optical Data Recording excellent squareness of the hysteresis loops and good switching characteristics. Both high room temperature H, and its temperature profile can be controlled by proper choice of composition and fhbrication conditions. Practical MO RE-TM alloys are usually ternary-Gd-Tb-Fe, where Tb increases out-of-plane anisotropy K,, Tb-Fe-Co and Dy-Fe-Co, where Co increases T, and OK,Gd-Fe-Co for high OK,etc. Tb-Fe-Co based films are used in the majority of commercial MO disks, usually with one or two additional dopant elements to enhance chemical stability and/or improve recording sensitivity (Sec. 6.0). Alloys containing Dy andor Gd provide special hctionalities required for direct overwrite and super-resolution structures (see below).

-E 600

0.6

\

a 500

Y

400

0

100

200 300 400

500

T (K) Figure 16. MagnetizationM,,coercivity H,, anisotropy energy K", and Kerr rotation 0, as a fimction of temperature T for G&4Tb,Fe7,.[7Sl

It is difficult to obtain RE-TM films that exhibit all the desired magneto-opticaland micromagnetic properties. In most cases, optimization of one property affects another property adversely. However, the ferrirnagnetic ordering and consequent Tcompbehavior of RE-TM alloys, while presenting challengingfhbrication problems, can be exploited for hctional separation of layers based on exchange-coupled multiluyers (ECML). The

Magneto-Optical Thin Film Recording Materials in Practice 313 ability to use ECML sandwich structures of two or more RE-TM films (or other MO materials) with complementary properties has opened up new and exciting possibilities for this class of materials. Exchange coupling can be characterized by an effective exchange

where t is the layer thickness and a, is the intehce wall energy density. This exchange field can be used to stabilize read enhancement l a y e r ~ [ ~ ~ l [ ~ ~ ] or m d @ switching some examples of which are shown in Table 8. The most spectacular applications of ECMLs, however, have been the achievement of direct overwrite with laser modulation and magnetically induced super-resolution.

Table 8. Examples of exchange coupled multilayer structures for read enhancement and recording sensivity improvement. Exchange Coupled Multilayer

Comment

Ref

Col,p1, alloy or TbFeCo (cap) TbFeCo (storage)

Cap layer with in-plane (Co-Pt) or low perpendicular (Tb-Fe-Co) anisotropy coupled to storage layer with high perpendicular anisotropy for dramatic reduction in switching field tw50 Oe (for magnetic field modulation over-writing). Nd-Co alloy with good short wavelength 4 but in-plane anisotropy sandwiched between two strongly perpendicular TbFeCo films. Readout layer with good short wavelength 4 but weak perpendicular anisotropy coupled to storage layer with strong anisotropy. Thickness of exchange coupled Co-Pd limited to about 60-80 .& Storage function separated 6om readout hnction by exchange coupling two layers, one with high 61,but low H,(readout layer) and the other with low Ok but high H, neceSSafy for stable bits (storage layer).

86

Tbl,Fe,,Co,, (stabilization) Nd,,Co, (readout) Tb,,Fe,,Co,, (storage) CoJ'ds alloy (readout) ~lSC08S(storage)

PtMnSb (readout) TbFe (storage) Gd-Fe Tb-Fe

Readout layer with very good 4 but no perpendicular anisotropy coupled to storage layer with strong perpendicular anisotropy. Energy density of interface wall c estimated by comparison of experimental and theoretical switching fields to be 1-2 erg/Cm?

85

84

83

82

81

314 Magneto-Optical Data Recording Direct Overwrite. In general, techniques for achieving direct overwrite (DOW) in MO recording can be classified as media approaches and drive approaches. Two media approaches have been demonstrated using single-layer media and exchange-coupled multilayers. Best DOW performance at tbe disk level has been obtained for the latter approach. Saito et al.i8fl first demonstrated good direct overwrite (DOW) performance for ECML bilayer disks of TbFe/TbFeCo and TbFdGdTbFe, shown schematically in Fig. 17. This scheme, however, required an impracticably large initialization field of about 5 kOe to overcome the energy of the domain wall between the two layers.

Figure 17. Principle of direct overwrite in exchange-coupled bi-layers. Moving from right to left, the initialization layer is DC erased downwards by the initialization magnet, with no effect on previously written domains in the memory layer. Under high laser power, both layers are heated near their T,, and upward domains are first frozen into the initialization layer by the recording magnet and subsequently copied to the memory layer by exchange couplingupon further cooling. Under low laser power, only the memory layer is heated near its T,,and downward domains are copied from the unchanged initialization layer to the memory layer upon cooling.[ml

Aratani et a1.[88] showed that H,, could be reduced to about 3.5 kOe by interposing a thin intermediatelayer to moderate the exchange coercivity, remains a concern for the long-term stability of but even this reduced Hinrt memory layer domains over a typical drive operating temperature range of 10-5OoC, and for downward compatibility with existing media. Further decrease in H,, can come from reducing Ho,IL(Fig. 17), butthis may cause an increase in media writing noise. Fukarni et al.[891[901 have proposed a four-layer scheme, which eliminates H,,,altogether by incorporating the

Magneto-Optical Thin Film Recording Materials in Practice 315 initialization function into the media itself. A TbCo layer with very high T, is aligned once during disk fabrication and never reverses during subsequent recording or reading because its reversing field is always larger thanthe bias field. H o s o k a ~ a [ and ~ ~ ]Saito[921et al. have shown that a GdFeCo read enhancement layer can be coupled to the tri-layer ECML DOW structure with excellent fidelity in copying recorded domains fiom the memory layer. This approach resulted in a bit cell window margin adequate to maintain a raw BER of 10" for 0.54 pm bit length, but the reduction in CNR and increase in media complexity remain. A comparison of layer structures and measured recording performance for various light intensity modulation DOW schemes is shown in Table 9. In addition to the ECML schemes, the approach using a simple is included. The ECML approach can single-layer media str~cture[~~1-[~~1 meet or exceed the IS0 CNR specification of 2 45 dB at 0.75 pm mark length. The four-layer approach with no external initialization magnet meets desired practical constraints of downward compatibility with first generation media and minimal drive space. The chief objection to the ECML approach is the media complexity. The ECML structure is also rather thick to be used in the conventional quadrilayer enhancement structure. Overall, Lid%]has estimated a penalty of several dB in CNR for various ECML schemes compared to optimized conventional recording structures. For completeness we note two drive approaches that have been proposed for MO DOW. Of all DOW schemes, magnetic field modulation DOW, i.e., so-called laser assisted magnetic recording, is the most obvious and equally applicable to all types of MO media. The length of written chevron-like domains is insensitive to laser power variation and media nonunifo~mity,[~~l-[~l and can be smaller than the optical beam size. Field modulation DOW is thus attractive for high density mark-length modulation recording. The key issue in field modulation DOW is power consumption, i.e., energy loss during bias field switching at several M H z , which is limited by the available power supply or by the allowed temperature rise in the magnetic head. For operation in the 1-2 M H z range, LidW]has estimated a maximum bias coil-media spacing of =0.5 mm for a non-resonant ferrite core magnetic head. Battery powered operation at 1.4 Mbitlsec has been achieved in a consumer drive by a combination of flying or sliding the magnetic head in close proximity to the protective lacquer layer (and, hence, the MO media layer) and tuning the thermomagnetic MO characteristicsfor minimal required switching field."] Note that this particular implementation of field modulation DOW does not permit the use of double-sided MO

31 6 Magneto-Optical Data Recording media. Multiple light beam approaches to single pass overwrite have also been d e sc r ii using twd1Ooo1r thrd1O11optical beams.

Table 9. Comparison of magneto-optical direct overwrite schemes using light intensity modulation. CNR measured for various mark lengths and disk velocities as indicated. Where reported, light wavelength is between 780 and 800 nm and recording bandwidth is 30 kHz. Note that, in some cases, increased CNR is obtained by increasing n while maintaining the bandwidth of 30 kHz. MO Layer(s)

Magnetic Field(s)

Laserpower Levels

CNR f v (dB) (MHz) ( d s )

Mark

Ref

Length (PI

(mW)

initialize 1 write 10 erase variable rewrite 10 write8.5 erase 3.5

30

1.8

7.2

2.0

94

42

2.8

11.3

2.0

95

H,,,,~200 H,,7000

write8.6 erase4.6 read 1.6

54 52

1 2

13.4

6.7 3.4

87

Si,N, GdTbFeCo 700 A (initialization) GdFeCo 300 &writing) TbFeCo 300 A (memory) Si,N, glass with 2p groove

H,,950 Himi,3500

write 14.4 erase 4.0-7.2 read 2.0

51

9

13.5

0.75

88

Si-N Tbco 400 A (initialiition) TbFe 200 A (exchange) GdDyFeCo 1400 A (writing) TbFeCo 600 A (memory) Si-N glass with pregroove

H,,,,350

write 13

47

6.2

9.4

0.75

90

53

7.5

11.3

0.75

92

SiO, Gd,2Tb,,Fe;, 900 A SiO, glass with 2p groove Ai alloy 600 A AlSiN 350 A GdTbFeCo AlSiN 1100 A (etched) substrate

Hn,,,,,20O

dielectric TbFe 670 A ( i t i d i i o n ) TbFeCo 520 A (memory) dielectric glass with 2p groove

Si-N 700 A DyFZO 500 A (htialization) G d F e ~ o100 A (exchange) T ~ F ~ C 200 O A (memo61 GdFeco 500 A (readout) SIN 700 A glass with 2p groove

f= carrier firequmcy,

Hnc,,,+500

'I

erase5 read ?

H,cold ? Hinil?

v = linear disk velocity

write ? erase ? read ?

Magneto-Optical Thin Film Recording Materials in Practice 31 7 Super-Resolution. In conventional MO recording, the limitation in linear density due to the finite size of the readout beam spot is described by a spatial cutoff frequency 2NA/h, where NA is the numerical aperture of the objective lens and h is the light wavelength. For NA = 0.55 and h = 830 nm, this correspondsto a minimummark len-@h of 0.75 um, below which mutual interference of recorded marks sharply reduces readout signal modulation. The principle of super-resolution in however, indicates that a resolving power exceedingthe classical limit can be obtained by putting an aperture directly against the object which intercepts all light outside a certain region. Arata,ni, Fukumoto et al. demonstrated experimentally[103] and the application of this technique to MO recording using a dynamic or “active” aperture, induced magnetically in an exchangecoupled readout layer. The principle of this technique, so-called magnetically induced super-resolution (MSR), is shown in Figs. 18 and 19. MSR as depicted in Fig. 18, where the aperture is located on the upstream side of the light spot, is referred to asfront aperture detection (FAD). Incorporaenables front tion of an intermediate switching layer with low Tc,swrtch aperture creation at a temperature well removed from Tc,readof the read layer, critical for strong carrier level. Rear aperture detection (RAD) is also possible using an upstream magnet to DC initialize the readout layer (Fig. 19). The resulting MSR apertures in FAD and RAD can be considerably smaller than the real focused area, resulting in super-resolution. CNR performance for these two schemes is summarized in the first two rows of Table 10. In the case of RAD, note that two intermediate layers were interposed in the structureof Fig. 19to provide adequate operatingmargins. The CNR of 47-48 dB for 0.4 pm mark length shows that MSR has the potential to roughly double the linear density of a conventional MO disk. Crosstalk for RAD was found to be very low even for 0.8 pm track pitch recording, a benefit of the narrow aperture (Fig. 19), indicating the potential for MSR to double the track density as well. Of course, the same concerns about media thickness and complexity mentioned above for direct overwrite apply as well for MSR, as well as difficulties in maintaining laser power level and media switching field tolerances. Subsequently a simplified two-layer MSR media structure was demonstrated by Murakami et al.,[lo5]who used a GdFeCo readout layer whose magnetization direction changed from in-plane to perpendicular with increasing temperature. No external field was required for readout. CNR performance for this scheme is summarized in the third row of Table 10,

318 Magneto-Optical Data Recording where again substantiallyimproved short-mark length performance is indicated compared with a conventional MO disk structure (bottom row, Table 10).

Disk moving dimtion ain

Figure 18. Principle of magnetically induced super-resolution (MSR) in exchangecoupled multilayersusing front aperture detection (FAD). In MSR-FAD, an intermediate switching layer with low Curie temperature Tc,sw.,dris used to control the effective exchange field Hwhw, between recordmg and readout layers. Domains are written in the recordq layer by conventional light intensity modulation and transferred to the readout layer during cooling below Tc,syTldl when the condition Hwhwe > Hrcd + Hc,readis reached. During readout, motion of the disk to the left results in a heated region downstream of the irradiated spot, and (since when Tc,sw.,dris exceeded, the readout layer magnetization is aligned with Hread HwhW = 0 and Hmd > Hc,md). This effective Dc erases the heated region, which thus does not contribute to signal modulation and acts as a magnetic mask. The signal is modulated by unswitched domains in the remaining chevron-shaped aperture under the irradiated spot. M e r readout, recorded domains are restored in the readout layer by Hw+,., just as in the original recording process.[~03]

In principal it should be possible to append a super-resolutionreadout layer onto a direct overwrite structure to provide both super-resolution and direct overwrite functionalities in a single ECML structure using light intensity modulation. Although the author is not aware that this has been demonstrated, the combination of direct overwrite by magnetic field modulation and magnetically-induced super-resolution for high density readout has been shown by Ohta.[lo61 A CNR of x 40 dB was achieved for 0.3 pm mark length using an external field of 2 60 Oe for readout.

-

Magneto-Optical Thin Film Recording Materials in Practice 319 masked area

disk moving direction

domam

Figure 19. In MSR-RAD, erasureof the readout layer to mask domains in the recording layer is accomplished by an upstream initiabng magnet. Domains are exchangeapied during readback in the heated region downstream of the light spot. The aperttm in RAD has a truncated oval shape and is somewhat smaller, especially in the cross-track direction, than that for FAD.[I03I

4.2 Co/Pt Multilayer Materials Multilayer CoRt films are another class of materials satisfjmg nearly all of the material requirements in Table 2, save demonstration of a production-worthy deposition process. Control of thermomagnetic properties for Co/Pt,with only two constituent elements and ferromagnetic ordering, is relatively straightforward compared to RE-TM materials. By the same token, material versatility is less due to the reduced number of degrees of freedom. Basically, the magnetic properties can be tailored by adjusting the thicknesses of the individual Co and Pt layers and the total thickness of the layered structure. Perpendicularanisotropy and a square hysteresis loop are obtained when very thin Co layers (about 3-6 8,) are alternated with slightly thicker Pt layers (about 8-20 8,). Optimum magnetic properties tend to be obtained for a total thickness in the 100-300 8,range. The values ofM,, OK,and T, all increase with increasing C o r n ratio at 800 nm. Results from Zeper et al.[1071-[10gl are shown in Fig. 20. The deviation of Keflfrom linear behavior at low t , thicknesses (Fig. 20a) was ascribed by these authors to interface roughness being on the same order as tco. The temperature T, z 390°C was estimated for a 4 8, Co/9 8, Pt multilayer by extrapolation of the temperature dependence of OK,which is proportional to M, (Fig. 20b). Note that H, reaches zero at a temperature below T,, allowing switching at temperatures below T,. The temperature T, can be

320 Magneboptical Data Recording lowered by increasing the Pt layer thickness (Fig. 20c) or by decreasing the Co layer thickness (Fig. 20d). These trends are qualitativelyconsistentwith reduced coupling between Co layers (for thick Pt layers) or crossover from a three- to a two-dimensional system (for thin Co) for which T, should be strongly reduced zccmhg to t h m l y . [ l l l l Th_e value of OK i!lcr~seswith increasingCo content and with decreasing wavelength(Fig. 2Oe). The large & at short wavelength can be partially attributed to Pt (or Pd)

Table 10. Comparison of super-resolution schemes. The first and second rows show results for magnetically induced super-resolution using front aperture detection (FAD)and rear aperture detection (RW),respectively, corresponding to Figs. 18 and 19. The third row shows results for superresolution readout using an in-plane magnetization layer (see text). The bottom row shows data for a conventional (single MO layer) reference disk for comparison. CNR measured for a wavelength of 780 nm,an objective lens with 0.53-0.55 NA, and other conditions as indicated. MO Layer StruCtUre

Magnetic Field(s) We)

LaserRead Power

CNR (dB)

f (Mhz)

v Mark Ref ( d s ) Length

(run)

(mw)

Si3N4 H,,-300 TbFeCo 400 A (record) TbFeCoAI 100 A (switching) GdFeCo 300 A (readout) Si,N, Pcsubstrate

2.5

50 47 42

6.7 10 13.3

8

0.6 0.4 0.3

103

Si3N4 Hnd+3OO TbFeCo 400 A (record) GdFeCo 150 A (switching-2) hi^, -3000 TbFeC0Al100 A (switching-1) GdFeCo 300 A (readout) Si,N4 Pc substrate

2.5

50 48 43

6.7 10 13.3

8

0.6 0.4 0.3

103

AI-N200 A DyFeCo 400 A (record) GdFeCo 400 A (readout) AI-N 800 A glass substrate

2.3

44

4.2 6.3 7.1

5

0.6 0.4 0.35

105

6.7 10 13.3

8

0.6 0.4 0.3

94

conventional reference disk

HmdO

34 30

HmadO

f= carrier frequency, v = linear disk velocity

2.5

48 28 =O

Magneto-Optical Thin Film Recording Materials in Practice 321

T('C)

-

Figure 20. Magnetic and magneto-optical properties of Co/Pt multilayer films. (a) Product of the effective anisotropy constant K,# and the cobalt layer thickness ta as a function of tco. (b) Temperature dependence of the coercivity H,and Kern rotation 0, for a 3 lx (4 A C d 9 A Pt) multilayer. (c) Curie temperature ,'Z as a function of the Pt layer thickness tpl with tco = 4 A. (4 Temperature dependence of the magnetization (normalized to that at 77 K) as a function of Co layer thickness with t,, = 13.1 A. (e) Absolute values of Kerr rotation BK for thick (2500 A) Copt and CoPd multilayers and Gd,,Tb,Fe,, alloy films as a function of wavelength.[~07]-[~~1

322 Magneto-Optical Data Recording

450 400

oe

h

$ 350

300 250

4 15

10

5

tP,N

1.2

I

I-

-

I

I

I

0

0

A

1

(d)

A

r'

20

1.0

C

.-0 .-N

.. (TI

0,

0.8 A

C 0)

r" \

0.6

t

xACo113.1 Apt A

.-

a 0,

C

9

*

0

6.1

N

0)

m

0

7.4

0.4

a

0

8.4

C

.-0

*

A

0.2

5.0

3.6 0

u 0

2.4

0.0 0

100

200

300

400

Temperature ( K )

Figure 20. (Cont'd)

500

600

700

Magneto-Optical Thin Film Recording Materials in Practice 323

I

0.5

---

GdTbFe n x (4A co + 9A Pt) -. -.. n x (4A CO + 18A PI) ............. n x (4A CO + 9A Pd)

(el 0.4

h

0)

a,

Y

0.3

9

0.2

..

"..

-

...... ........ .2:-...................... 0.1

0.0 1

300

- .- ._

-*

I

I

I

1

400

500

600

700

I

-

800 h(nm)

900

Figure 20. (Cont'd)

Not surprisingly, the overall optimum MO recording structure generally involves some compromises. For strong perpendicular anisotropy, high coercivity, square hysteresis loop, and low Curie temperature, tco (or Co/Pt ratio) is generally smaller than that for which the largest 0, is obtained. Co/ Pt has a relatively high and unadjustable T, compared to RE-TM, which has Van led to criticism of its recording sensitivity at higher disk Kesteren and Zeper[log1have shown that the addition of several percent 0s or Re to Co is more effective than reducing the Co/Pt ratio in lowering T, with minimal loss in inherent 0, and Kerr hysteresis loop squareness. Similarly, Ochiai et al.I1lo] have investigated CoM/Pt superlattices, where M includes a large number of transition metal, rare earth, and semiconductor elements, for the purpose of reducing T,. It is also noted that the extreme thinness of the Co/Pt multilayer is conducive to switching at low laser power or at high-media velocity where the recording speed might otherwise be limited by the power available from semiconducting laser light sources. The most extrinsic (i.e., fabrication process dependent) of the above properties is H,. Weller[ll31 and S ~ z u k i [et~ ~al.~ observed ] a lack of correlation between H, and columnar diameter or vertical grain size for sputtered Co/Pt multilayers. Rather, these authors concluded, based on TEM cross-section and x-ray diffraction data, that structure on a very fine

324 Magneto-Optical Data Recording length scale of 0.4-1 nm within columnar grains played a key role in domain wall pinning and nucleation. These authors suggested coercivity contributions from defects such as pinning sites of nonmagnetic inclusions and thickness fluctuations in tc,, which enters the expression Kef= K, + 2K& and hence that for the domain wall energy,

where A is the exchange constant. Carcia et al.[ll5]observed a correlation between columnar boundary sharpness and coercivity. The importance of nanometer-scale thickness and boundary variations and other nanometerscale defects seems well established, and progress is being made in the difficult task of characterizingthem. The measured optical constants of thick Co/Pt and CoPd multilayer films as a fhction of wavelength are shown in Fig. 21 from work by Hashimoto et al..[641The measured reflectance R is ~ 7 0 %at 800 nm and decreases monotonically to =60% at 400 nm. CoPd and Co/Pt multilayers can be described by a single effective nefwithout taking the individual layers into account. This is expected since tco and tpt << h and has been using the method of Hotslag et a1.[ll61 The large Co/Pt reflectance (R for TbFeCo is about 10-15% lower over the same wavelength range) compensatesfor the modest Kerr rotation at 800 nm. For very thin multilayers less than about 100 A, however, it should be noted that a degradation of measured''6 has been observed based on calculations using 1[~ attributed ~~] to a disordering andor thick film optical c o n ~ t a n t s [ ~ ~and island formation during initial film growth. Special attention must therefore be paid to substrate preparation and film fabrication procedures to minimize such initial layer effects. Zeper et a1.[ll81 presented a comparative study of the recording properties of Co/Pt vs GdTbFe media at three different wavelengths: 820, 647, and 458 nm, corresponding to present and fbture generation optical recorders. Some features of the test recorders and media are shown in Tables 11 and 12.

Magneto-Optical Thin Film Recording Materials in Practice 325

Y

i

0.15

-00

mu

Y

0.10

C048iIPt104,i Co4.8iIPd104A

-

Q

e

-4

0.05 Ak I 0 I (b)400

(b) I

Y

II

II

500

600

II \I\ I

700

800

Wavelength (nm) Figure 21. Measured optical constants of2000 A thick4.8 A Co110.4 A Pt and 4.8 A Col 10.4 A Pd multilayers: (a) n and k, (b) An and Ak. Here n and k are the complex refractive indices n + ik, and An = n+ - n-and Ak = k+ - k , where the signs correspond to right (+) and left (-) circularly polarized light for magnetic saturation of the sample (n+ and n- are related to e, and ev, the diagonal and offdiagonal elements of the complex dielectric tensor,by nf = (e,fi%)l/2, and to Q, the gyroelectric constant, by nf = n( 1 f Q/2).)[641

326 Magneto-Optical Data Recording Table 11. Features of Three Magneto-Optical Test

No.

I

I1 I11

h e r

AlGaAs Krypton Argon

h (nm)

820 647 45 8

NA

0.53 0.46 0.60

cutoff U2NA

i d

Detection BEUll Spiiiter

0.77 0.70 0.38

PBS + U 2 Wollaston PBS + U 2

Comparisonsbased on static MO merits are given (Table 12) with the proviso that they can be misleading, especially regarding the so-called shotnoise limited FOMRO,' when that limit is not yet reached (as was the case in this study). Identical 3.5"glass disks were used with aphotopolymerizable (2P) polymer containing a continuous 1.6 pm pitch spiral groove. Reflectance R above 20% and high figure of merit ReK* at each wavelength were maintained by tuning the thickness fAw of the first dielectric layer. In the case of Co/Pt, a sputter etch treatment of the AlN layer before depositing the CoRt multilayer improved the structural and magnetic properties of the Co/ Pt. The Corn was left unprotected while the GdTbFewas protected by three additional optically tuned layers (Table 12). In all disks, the signal was written using laser modulation with optimized writing fields and laser powers. Carrier and noise levels as a function of domain period are shown in Fig. 22. It can be concluded from this work that: (i) at 820 nm, the CNR of GdTbFe is about 3 dB higher due to the lower disk plus written noise, presumably related to the difference between an amorphous and nanocrystalline material, and (ii) at 458 nm, the CNR of Co/Pt (48 dB for 1.5 pm period or 0.75 pm mark length) is about 3 dB higher due to the higher carrier level.

Magneto-Optical Thin Film Recording Materials in Practice 327 Table 12. Coercivity H,, reflectance R, and Kerr rotation 6, at a given wavelength for GdTbFealloy and Corn multilayer based disks with AlN enhancement layer thickness tAw. Disk structures shown below." l81 Disk

tAw

(4

H, R(%) OtO4 OtOe)

(deg)

RBK2 (deg2)

@A (W

6,

Re,

(deg)

GdTbFe-1

900

6.5

23

0.76

0.18

0.130

820

GdTbFe-2

700

4.0

22 23

0.67 0.70

0.15 0.16

0.099 0.110

647 820

GdTbFe-3

500

4.8

22

0.49

0.11

0.053

458

corn-1

900

1.2

31 31

0.45 0.51

0.14 0.16

0.063 0.081

820 647

corn-2

700

1.0

33

0.48

0.16

0.076

647

CQrn-3

500

1.2

30

0.60

0.18

0.110

458

Corn disk structure:

GdTbFe disk structure: lacquer 200 8,A1 200 8,AlN

(14 x

252 8, Corn [4 8, Co + 14 8, Pt])

450

Gd,,Tb,Fe,, _

tA&

AlN (etched)

tAw

AlN

2P polymer

2P polymer

glass

glass

_

~

~

328 Magneto-Optical Data Recording

41

rec I : @ 820 nm s GdTbFe-2

cort.1

,

-40 -

1

I

:

-50 . 10

I

-

v-5mJs P,.,d 0.9 rnW

N

0"

........... ................

...............

I

bl

E -10

m.

rec II : @ 647 nrn GdTbFe-2

.

;-20 ?

tort-1

-30

' -40 -50

1

Pread I

................................................................... decteclor

-f

t

laser noise

-70 01 C)

-10

E 0-

if

m -20

'

-30

/

rec Ill : @ 458 nm GdTbFe-3 Copt-3

I

-

.

U

u

1.0 mW

.........-.................. .

-60-

1

-

v = 5 m/s

v-5rnfs P,ead 0.8 m W

-40

I

8

- 6 0 1cut !l/

0

N .

. . -............................

..................................................................... 1

2

3

4

5

domain period (pm)

Figure 22. Canier (C) and noise (N)levels for Corn (dashed lines) and GdTbFe (solid lines) MO disk structures as a function of the written domain period (twice the mark length). The sum of the detector and laser noise levels was measured with the disk stopped. N is the total noise level including disk written noise. Note that comparison of the CNR between test recorders was imprecise due to differences in optical and electronic components.[lI81

Magneto-Optical Thin Film Recording Materials in Practice 329 These authors observed that the total noise Nconsisted mainly of disk noise, defined as the unwritten noise level, and that this noise increased at shorter A. This, combined with the smaller read power, may explain why the CNRs at 458 nm were 1-3 dB lower than at the other wavelengths. They attributed this mainly to the influence of substrate quality and groove structure on a smaller spot, emphasizingthe critical role the substrate plays in achieving low noise media at short wavelength. For example, undesirable phase shifts due to the birefringence of the polycarbonate substrate degrade the performance of the servo and readout systems,[' 191 and these phase shifts increase with decreasing wavelength. Surface roughness noise has also been shown to increase with decreasing

4.3 Garnets and Other Iron Oxide Based Materials Within the group of magneto-optical memory materials, the ferrimagnetic crystallinecubic garnets are important contenders. Garnets are oxides of rare earth metals and iron, expressed as R3Fe,012or 3R203.5Fe203. Ferrimagnetic ordering arises from the presence of different sites for the metal ions with tetrahedral, octahedral, and dodecahedra1symmetry. Garnets feature outstandingstructural and chemical stability and very favorable MO properties in the visible and near-ultraviolet spectrum. Much is known about the garnet material system through work in bubble memories and microwave devices. The intrinsic optical and magnetic properties can be tailored over a fairly wide range by chemical substitution. Thin films on glass substrates invariably have shown high media noise, however, thus most work has centered around reducing the media noise rather than finding the optimal disk structure or best fabrication process. A brief background on material characteristics is provided here followed by a review of film fabrication issues and a comparison of measured CNR values. The origin of the magneto-optical activity in garnets and the phenomenological theory describing the effects of various dopant ions on saturation magnetization, compensation temperature, uniaxial anisotropy, refractive index, optical absorption, and Faraday and Kerr rotation and ellipticity have been reviewed by Hansen, Krumme et al.[1211-[1231 Some key points from their reviews are summarized here. Briefly, garnets of composition Dy,-&Fe,-,,G$O12 with A = Bi,Ce are well suited for MO recording media. Dy3' ions with high single-ion anisotropy improve the wall coercivity in polycrystalline films by magnetostriction. Bi3+ and Ce3+ ions cause a

330 Magneto-Optical Data Recording significant increase in T,and greatly enhanced MO effects, originatingfrom mediated super-exchange interaction and spin-orbit coupling of the the 02F$+ ions through structural and electronicmechanisms. Ga3+(or A13+)ions cause a reduction in T, and permit control of Tcompover a wide temperature rmze at c o n m t Bi3+ Content; It is worth notbs that optia! transmittance can be very high in garnet films, so high, in fact, that multilevel recording can be c o n ~ e i v e d . [ ~ ~ ~Co2+ 1 [ ~ ~ions ~ 1 can be used to increase optical absorption in the visible spectrum. The main structural and magnetic properties are summarized in Table 13, together with a schematic indication of the influence of various ionic substitutions on the different lattice sites. The quantitative influence of some of these ions on the temperature dependence of magnetization, anisotropy, and coercivity is shown in Fig. 23. The cubic symmetry of garnets gives rise to cubic anisotropy, which can be understood by single-ion For many garnet films, the cubic anisotropy is much smaller than the uniaxial part consisting of a stress-induced and a growth-induced contribution. For garnet films deposited by sputtering, the uniaxial anisotropy constant K, is essentially stressinduced, expressed by

where A, is the saturation magnetostriction constant. The film stress o,(not , used previously to denote domain wall energy to be confused with a density), is given by expressions of the type

Q. ( 5 )

o= (9as)YAT/(l-p)

where Y is Young’s modulus, p is Poisson’s ratio, ?and asare the thermal expansion coefficients of the film and substrate, respectively, and AT is the difference between Crystallizationand room temperatures. The value of K,, can primarily be influenced via A,, AT, and q-a,,however, only modest values have been achieved. The constant, A,, which is maximized for Dybased garnets, has very limited influence on K, . Similarly, increasing K,, via and high cror ATis restricted because large aleads to cracks in the crystallization temperatures are undesirable. Therefore, small anisotropies must be contended with, which is acceptable as long as the condition K, > 2&: is satisfied. This condition can be met for Tcompclose to room temperature, but the stability of net anisotropy to operating temperature

Magneto-Optical Thin Film Recording Materials in Practice 331 excursions remains a concern. Reinforcing this concern, Kenny et a1.[1261 observed that the size of static domains written at constant laser power increased when the sample was heated, although comparisons were not provided for other media types.

Figure 23. Temperature dependence of (a) saturation magnetization for garnet film of differing compositions, and (b) uniaxial anisotropy constant and coercivity for a sputtered and annealed on a garnet film of composition DY,,,SSm,,,,Bia83Fe~,~lG~,~Ol~ glass substrate. The lines in (a) represent calculated molecular field theory results for @yxB,,),(FeGa),O,, garnets. In (a) refer to the left vertical axis for T < TcOmp and the right axis for T > T,,,,,p.[l~l

332 Magneto-Optical Data Recording Table 13. Structure and magnetic properties of Fe-garnets, showing influence of ionic substitutions on their magnetic and magneto-optical

0.065

0.1

3 x dodeca

8

2

X

OCt8

6

0.049

3 x tetra 4

magnetic moments of Gd3RsOiz ( lf total FC momeat)

Bi +

In-

Ga

-

Magneto-Optical Thin Film Recording Materials in Practice 333 The CNR for garnet disks is limited by noise sources which appear to be primarily process dependent. Garnet thin films have been deposited on glass and single-crystal garnet substrates by many techniques, including liquid phase epitaxy, chemical vapor de?osition, RF magnetron sputtering, pyrolysis, ion beam deposition, sol-gel, facing targets sputtering, and electron cyclotron resonance sputtering, each with its own advantages and disadvantages. RF magnetron sputtering from sintered ceramic targets has been preferred for material studies since compositions can be realized that are difficult to obtain using other methods. Sputtering process improvements aimed mainly at reducing the polycrystallinegrain size and improving film morphology have been pioneered by Gomi,[l2v Itoh,[1281Shon0,['~~1 and Suzuki et a1..[1301Significant progress has been reported including a reducing sputtering atmosphere of Ar + H, to improve grain boundary doping with W[1271or Cu[1331to enhance domain wall pinning and coercivity, pyrolysis deposition of a Rb-doped seed layer to reduce grain rapid thermal annealing (RTA) to reduce grain and multilayering size (Fig. 24) and improve surface morphol0gy,[~~~1[~~~] using spacer layers of Cr or SiO, to decrease grain size and enhance

700

5

600

0 0;

.? 500

ffl

E

400 300

.

200

I

I 1

I

I

I

5

10

50

Heating Ramp Rate,

r

I

('C/s)

Figure 24. The dependence ofgrain size onthe heating ramprate for Dy,,J3i2,$q~,Ga,,,0,2 garnet films sputtered on gadolinium-gallium-garnet substrates. T, is the substrate deposition temperature.11301

334 Magneto-Optical Data Recording A comparison of static MO data for garnets, RE-TM, and Co/Pt is shown in Fig. 25, and dynamic recording data for garnets is summarized in Table 14. On single-crystal gadolinium-gallium-garnet (GGG) substrates, a figure of merit Re, of 2.1' at 3L = 5 14 nm has been measured for an "optimized" disk (GGG substrate/0.23 pm garnetl0.1 um Al reflector), more than ten times that of a typical RE-TM disk, with a corresponding CNR as high as 60 dJ3.[1291

Table 14. Structure, Fabrication, and CNR (Large Mark) for Garnet-based Magneto-Optical Recording Disks MO Fabrication Technique

MO Microstructure

CNR (B)

Ref

Al or Cr BIG glass or GGG

sputtering + anneal

polycrystalline

45 (glass)

131

Al BIG or CIG GGG

sputtering + anneal

Al-Ti BIG seed layer glass

pyrolysis for seed layer, sputtering for recording layer + anneal

polycrystalline (40 nm)

48

128

reflector BIG Rb-doped seed glass

pyrolysis for seed layer, sputtering for recording layer + programmed anneal

polycrystalline (30-40nm)

54

132

Disk Structure

BIG CIG GGG

60 (GGG)

bismuthdoped iron garnet ceriumdoped iron garnet gadolinium-galium-garnet

polycrystalline 50 nm(B1G) 10 nm (CIG)

57(BIG)

129

48 (CIG)

Recording = 5 14 nm CNR @ 30 kHz bandwidth

Magneto-Optical Thin Film Recording Materials in Practice 335

400

Mo

6w

700

ow

Wavelength, h(nm)

Figure 25. Dependence of magnetooptical figures-of-merit on wavelength for Bisubstituted garnet, Tb-Fe-Co amorphous, and CoPt mutilayer films. The conventional figure of merit Re,, where R is the reflectance and '6' is the Kerr rotation, is used for Co/ Pt and Tb-Fe-Co. The figure of merit 2dOexp(-2ad) is used for garnet, where d is the film thickness, 8 is the Faraday rotation, and a is the absorption coefficient of the garnet layer. The term 2dBresults from the assumption of a perfect reflector, while the term exp(-2ad) accounts for absorption in the garnet

Recording sensitivity issues remain to be clarified, but again the dependence of write/erase sensitivityon the reflectivity and thermal conductivity of the disk reflector material and other possible stack materials has hardly been studied. On glass substrates, the CNR is approaching 55 dB and steady progress is being shown, especially regarding microstructure and crystallization kinetics. A grain size less than =1/10 the diameter of the recorded domain has been achieved (Table 14), yet noise is still problematic, suggesting that optical inhomogeneities may be principally responsible. Notably absent from Table 14, as of this writing, is any recording data from films processed using the above mentioned RTA or multilayering techniques. We note that other crystalline iron oxides, i.e., the cubic Co-based spinel f e r r i t e ~ [ ~ ~ ~ 1and - [ ~hexagonal ~*1 Ba ferrites,[1391[1401 are also being investigated for MO recording, but have yet to show recording performance CNR on glass substrates comparable to that of the garnets.

336 Magneto-Optical Data Recording Perhaps the chief materials challenge remaining for garnets and other oxides is to find a technique that can produce films on glass to avoid the prohibitive cost and limited size of single-crystal substrates. Practical problems of economical deposition and groove formation must also be solved given the hi& annealins temperatures and larse film thicknesses typically employed. One possibility is to form after-grooves in the lacquer/ reflector following the high temperature processing required for the MO layer. The use of sputtering fiom sintered ceramictargets suffers from lowdeposition rate and difficult control of stoichiometry and compositional as~ mentioned previously in Sec. 3.3 regarding RE-TM unif~nnity,[~ ~l materials. The chemical solution approaches such as pyrolysis, sol-gel or organometallic decomposition are both rapid and inexpensive, however, suitable precursors for necessary dopant ions may be difficult to synthesize and films of adequate quality remain to be demonstrated using these methods.

4.4 Other High Kerr Rotation Materials One starting point in the search for better MO materials is within well-known families of materials known to have high Kerr rotation, some of which are listed in Table 15. Most of these also suffer rather severe shortcomings for practical MO application, such as inadequate perpendicular anisotropy, inappropriate Curie temperature, or high-processing temperature. In some cases, reported values have not been independently confirmed or conflicting data has been reported. Nevertheless, there is considerable research activity in these materials based on the hope that doping or multilayering techniques will remedy largely intrinsic deficiencies (e.g., K,, T,) and processing solutions can be found for largely extrinsic properties (e.g., grain structure, H,).One such class of materials is based on intermetallic alloys and compounds of 3d Co and Fe metals. For Pd and pt alloys with Co (and Ni), it is well established that the 4d or 5d bands become spin-polarized by the Co moments, just as with multilayers (Sec. 3.4). The strong contribution to OK is a result of the spin-orbit splitting, which is two to three times higher in pt than in Pd.[1421The value of OK of pt binary alloys with Co and Fe (and Ni) can be higher than those of the pure When the Co and of their multilayer concentration is fairly high, an approximately linear relationship is observed between OK and the corresponding M,. Rather high processing temperatures in excess of 2OOOC are required to develop perpendicular anisotropy (Table 15).11451[1461

Magneto-Optical Thin Film Recording Materials in Practice 33 7 Table 15. Intrinsic (Bulk or Thick Film) Ken Rotation for High Kerr Rotation Materials

Material

comment

Ref

0.39

for comparison

143

0,

633 nm

830 nm

1. Co and Fe-based materials co 0.35 Fe

0.41

0.54

for comparison

143

Co-Pt alloy

0.40

0.44

200-300°C processing temperature for perpendicular anisotropy

143, 144

Fe-Pt alloy

0.5-0.6

0.62

450675°C processing temperature for perpendicular anisotropy

143, 145, 146

Fe-Ag bilayer

0.50

perpendicular anisotropy not demonstrated

150

cubic anisotropy; strong wavelength dependence of 0,; optical constants sensitive to degree of crystalline order

143, 187

2. Mn-based materials

0.74

PtMnSb

0.93

MnBi

0.56-0.7

~-5300°Cprocessing temperature; magnetic phase unstable to temperature cycling

153, 154

Aldoped MnBi

conflicting data

375°C processing temperature; T, = 380°C; 105 writelerase cyclability demonstrated at 15 mW laser power, 600 Oe bias field

153, 154

338 Magneto-Optical Data Recording The enhanced MO effects in Fe/X (X = Cu, Ag, Au, TiN) bilayers[1471[1481 have been e ~ p l a i n e d [ ~ ~as~an 1 [effect ~ ~ ~ 1of the anomalously low optical constants of the constituentX, which can be thought of as a reflector which modifies the effective optical constants of the bilayer system. At wavelengths where the optical constants of the reflector are suitably low, e.g.,forAgn+ik=0.135 +i3.99at633 nm, Q'isenhancedbythereduced denominator in the (approximate)

where E, and Sare the diagonal and offdiagonal elements of the dielectric tensor, respectively, and E, = (n + ik)*. We note that other materials have also been shown to exhibit this so-calledplasma-edge enhancement when used as reflectors, e.g., RB, compounds (R = rare earth element), which exhibit a steep edge in the reflectance spectrum in the 600-800 nm wavelength The Mn class of materials includes MnBi and Heusler alloy PtMnSb type materials, with very high room temperature QK (Table 15). PtMnSb was mentioned previously as a read enhancement layer in an exchangecoupled multilayer scheme (Table 8). No clear correlation between 0, and M, exists for Mn-based materials, in part due to the tendency of Mn to order antiferromagnetically. High '6' in PtMnSb results from a reasonably high value of the offdiagonal element 8 of the dielectric tensor combined with a rather low value of the diagonal element E,, according to Eq. 6. Detailed band structure calculations by Wijngaard et al.[ls2]indicatethe MO activity in PtMnSb arises from the peculiar half-metallic nature of the bands for majority vs minority spin electrons; the spin orbit interaction splits the band and pushes one state above the Fermi energy. MnBi derives its large MO effect from the large spin-orbit interaction of the Bi 6p level. Indeed, the spin-orbit interaction in Bi is ten times larger than in a typical transition metal, which poses the question as to why the MO response isn't even stronger. M i ~ e m e r " ~has ~ ] attributed this to small matrix elements for transitions between Bi 6p and Mn 3d states. Quite a number of additives to MnBi have been investigated for the purpose of enhancing Q,, reducing the grain size, and eliminating a phase transition that occurs on temperature cycling. MnBiAl films are of considerable current interest due to their perpendicular anisotropy, large polar Kerr rotation, reduced grain size, and structural stability." 531-~1541 Conflicting data has been reported as to the intrinsic Kerr rotation of MnBiAl, possibly

Magneto-Optical Thin Film Recording Materials in Practice 339 due to the extreme sensitivity of OK to the Mn/Bi ratio. Self-consistent spin polarized electronic structure suggest that the Kerr rotations of MnBi and MnBiAl should have similar magnitudes. For completeness, we mention some materials with truly enormous OK (and t$), some of which have been known since the 1960’s. These are the “giant MO rotators” based on the europium chalcogenides (e.g., Eu-S) and the uranium chalcogenides (U-S, U-Se) and pnictides (U-As, U-Sb).[1561 These materials exhibit OK values as high as = 8 O and R”*OK figure of merit values up to ~ 3 ” .Presently these materials are far from usehl for MO recording because their Curie temperature is below room temperature, anisotropy and coercivity are low, and their MO activity is strongly wavelength dependent.

5.0 REFLECTOR THIN FILM MATERIALS Materials for the reflector layer in quadrilayer MO disks have generally been based on A1 and its alloys for their high reflectance, ease of deposition, and engineering versatility. The reflector layer is the primary determinant of recording sensitivity and strongly influences the corrosion stability and disk reliability. Aluminum is a reactive metal that owes its resistance to environmental attack to the formation of an inert passive film on its surface. The presence of certain aggressive anions, most notably C1-, results in localized attack of the passive film and metal substrate to form pits. Many of the alloying additions responsible for the passivation of stainless steel can also be used to enhance the passivity of aluminum, provided that these elements remain in solid solution in the alloy. Because of the very low solid solubility of these additions (e.g., < 1 atm% for Cr, Mo, Ta, Zr, or W) in Al, however, efforts to eliminate pitting by conventional ingot metallurgy approaches have been unsuccessful. Additions beyond the solubility limit generally lead to preferential nucleation of pits at precipitates, presumably due to local action galvanic cell effects. The solubility limit can be increased well beyond the equilibrium limit, with concomitant increase in corrosion performance, if rapid solidification deposition techniques such as ion implantation[l5’1 or s p ~ t t e r i n g ~ ~are ~ ~employed. l-[~~~l Onishi et al.[l6l1studied the effect of a number of metal additives to sputtered Al alloy reflector films on the sensitivity and reliability of MO disks.

340 Magneto-Optical Data Recording All alloy films showed no detectable precipitate formation as judged by TEM diffraction and x-ray diffraction. Figure 26 shows the effects of these additives on stability, thermal conductivity, and MO recording sensitivity. Effective passivation was obtained for group IVB (Ti, Zr, Hf),VB (Nb, Ta), VIB (Cr, Mo), and VIIB (Mn) elements. Group VIII (Ni, Pd, Pt) and IB (Cu) metals and Si were found to be less effective, and Mg was actually detrimental. The reflectance behavior shown in Fig. 26 a was directly correlated with passivation currents measured by solution anodic polarization techniques. Surface analysis of polarized IVB-VIIB and VIII alloy films showed exclusively Al-oxide with no alloying element.

Figure 26. (a) Reflectance reduction of Al alloy thin films afier environmental testing vs atomic number of alloying element (1-4 atm%). The disk structure was PC substratel500 A Al alloy110 pm lacquer. The laser beam ( h= 780 nm, 4 = 0.2 mm) was incident through the substrate. Environmentalconditionswere 105"C, 1.2 atm., and 100% RH for 60 h. (b) Thermal conductivity (1 cakm-sec-K = 435 Wlm-K) of Al alloy thick films vs alloying element content. Sample structure was glass substratell0 pm Al alloy. (c) Optimum recording power for MO disks vs alloying element content. Disk structurewas PC or glass substratel980 A SiNI 250 A Tb-Fe-Co1330 A SiN1500 A Al alloy/lO pm lacquer. Recording conditions were 5.6 d s e c linear velocity, 3.7 MHz recording frequency, and 33.3% duty

Magneto-Optical Thin Film Recording Materials in Practice 341 0.6

*cu -m-

pt

0.4 -0- Ta -0-

Ti

0.2

-0

n

5

10

15

Metal element content (at%)

Tb4A-7 A

2

0

1

I

5

10

Metal element content (at%) Figure 26. (Cont'd)

15

342 MagnebOptical Data Recording The thermal conductivity of Al-alloy films was most strongly influenced by the same M3-VIIB elements most effective in providing passivation. The thermal conductivity behavior shown in Fig. 26b was directly correlated with electrical resistivity measurements and with the optimum recording power behavior show. in Fig: 26c: n e averqe grain size in the alloy films was 350-470 A compared to 1500 A for pure Al. The reduction in thermal conductivity and electrical resistivity was attributed to grain boundaries, lattice distortions,lattice def’ects, and electronicstructurechanges. It was possible to double the recording sensitivity with no change in CNR using Ti, Ta, or Cr additives. In commercial or preproductionRE-TM disk products, Al alloys such as N C r and AVI’i/Ni have been used for the reflectorbeat sink layer. For highly transparent garnet media, the reflector must also serve the function of light absorption, as already mentioned in Sec. 2.2, thus metals such as Cr, Ti, Ni, and W with moderate reflectance in the 50-70% range may be more appropriate.

6.0 ENVIRONMENT AND LIFETIME At the outset it should be recognized that media lifetime is difficult to define, and, realistically, impossible to measure. Manufacturers of MO media warrant disk lifetimes in the 10-25 year range, and quote longer, even “lifetime” stability under controlled conditions. Generally, the more benign the specified operational or storage environment, the longer the estimated lifetime, so the scientifically rigorous accelerated aging projections on which the estimates are based must be carefully understood. Difficulties arise since the testing is often performed by the media manufacturers themselves and the results may, or may not be, published in scientific journals. Where they are, it may be difficult to relate the published findings to specific products when the tests are performed on laboratory development disks rather than commercially available media. Also, the perceived market requirements and customer operating habits are subject to frequent redefinition. The perception of archivability, for example, is quite different if the data represents computer backup (on the order of 10 years), or precious book, or family images (on the order of 100 years or more). Marchant[162J has provided a thoughtful critique of lifetime estimates and supporting

Magneto-Optical Thin Film Recording Materials in Practice 343 environmental testing methodology for the complete family of optical storage media (i.e., read only, write-once, and rewritable). During the period 1987- 1989, the Japanese Standard Committee for Optical Digital Data Disks conducted reliability tests of 130 mm MO disks supplied by 25 companies, including environments of corrosive Recently, the National Institute of Standards and Technology (NIST) has sparked by begun to develop its own optical disk testing mandates to reduce the use of paper in government agencies at all levels and by the loss of some data stored for about ten years on magnetic tape. Lifetime testing is also being conducted in the Optical Storage Testing and Preservation within the Library of Congress (USA). Hopehlly such non-vendor sponsored testing programs will help to define the meaning of archivability and to improve the standardization and objectivity of lifetime estimation. Not making the task any easier is the quandary that system obsolescence may render the recorded bit patterns unreadable long before they deteriorate. Hardware, software, and documentation likewise will require extensive standardization in order to qualify optical storage systems as truly archival. The ANSI standard environmental restrictions on 130 mm rewritable MO disks are summarized in Table 16. The ANSI standards specify a benign test environment in which the detailed performance specifications must be demonstrated in addition to the progressively more stressful operating, long term storage, and short term storage environments. Formal standards normally do not specify a necessary lifetime within the operating or storage environments; such numbers are treated as quality features specific to individual products. Manufacturers typically provide separate lifetime estimates for recording and playback stability (Table 17). Here, Zfetime denotes the expected duration between the time of disk manufacture and the time one of its critical parameters degrades to a point where the disk becomes unusable, or (preferably) to a predefined and measurable “end-of-life” (but still usable) value for that parameter.

344 Magneboptical Data Recording Table 16. ANSI Environmental Specifications for 130 mm Rewritable Optical Disk Cartridge Testing

Temperature ("C)

23 f 2

Operation

Long Term Storage

Short Term Storage (I 14 Days), shipping

10 to 50 -10 to 50 -20 to 55

Relative humidity (%)

45 to 55

10 to 80

1Oto90

5to90

Air cleanliness(class )

1OO,o0o

1,ooo,o0o

1,000,000

1,000,000

Temperature gradient ("Ch) -

10

15

20

-

10

10

20

Relative humidity gradient (%/h) General:

No condensation on or within disk Wet bulb temperature I29OC Atmospheric pressure 75 to 105 kPa (0.74 to 1.04 atm) Magnetic field strength in volume of cartridge - 48 kA/m (603 Oe) 2 48 h of conditioning prior to testing 2 2 h of conditioning prior to operation

An example would be the disk byteerror-rate (BER) deg5 x lo4, a defined end-of-life point for 5.25-inch MO Recording lifetime, sometimes referred to as shelf life, denotes the period of time during which new information can reliably be recorded. Most rewritable media are not heated as strongly by the optical stylus as write-once media, which in part explains the longer recording lifetimes estimated for MO media. Playback lifetime, sometimes referred to as storage life, denotes the period of time during which previously recorded information can reliably be retrieved. MO disk manufacturers for the most part claim a recording lifetime of 10 years and a playback lifetime of 10 years for media with plastic substrates and 15-25 years for media with glass substrates. Rewritable media are also expected to support at least 1O6 write/erase cycles.

Magneto-Optical Thin Film Recording Materials in Practice 345 Table 17. Recording and Playback Lifetimes for Rewritable and WriteOnce Optical Disk

Media Technology

Recording Lifetime (Years)

Playback Lifetime

magneboptical

10

10-25

3M, Hitachi, Kyocera, Maxoptix, Nakamichi, Panasonic, Ricoh, Maxtor, Sharp, Sony, Verbatim

rewritable phase change

10

10

Panasonic

write-once phase change

5

15-30

Eastman Kodak Company Panasonic

dye

5

15

Eastman Kodak Company, Pioneer, Ricoh, Maxtor

ablative tellurium

5

1040

Fujitsu, Hitachi, Literal, LMSI, Mitsubishi, Toshiba

50

Plasmon

ablative F’t

MediaiDrive Manufacturer or Distributor

(Yew

thennal bubble

5

10-60

ATG

dual alloy

5

100

Sony

The harmonious consistency of longevity specifications for MO media is startling consideringthe diversity of media designs and materials used, the variety of manufacturing techniques employed, and the differences in manufacturing discipline for production lines at various stages of cleanliness and maturity. For the typical user, it may be difficult or impossible to substantiate these estimates based on the scant technical literature available on accelerated aging, thus inviting the same criticism that has been levied against other types of optical media: namely, published lifetimes are market driven and more a measure of company aggressiveness than actual media lifetime. The lack of objectivity and/or incorrect methodology in lifetime testing is a problem endemic to the recording industry in general, including magnetic media.

346 Magneto-Optical Data Recording The statistical merit of accelerated life expectancy methods used for guidance are of crucial importance. If a single chemical or physical process is responsible for media aging, the Arrhenius expression

where k is the rate parameter, A is a preexponential factor, E is the activation energy, R is the universal gas constant, and T is the temperature, is useful to predict byteerror-rate (BER) lifetime for an average disk. Arrhenius analyses with temperature as the sole variable are widespread in the acceleratedtesting literature, probably because of ease of use. Realistically, however, it is recognized that relative humidity (RH)plays a role in most media degradation mechanisms. Furthermore, the concern is over the probability that a particular disk will survive for a given time. Podi0['~~1 and have pioneered the use of confidence intervals in lifetime estimation. Murray has applied a modified Eyring model for which

where f(M,T) is a function of relative humidity and temperature, to the analysis of accelerated aging data for 3M MO disk media. In this work, the degradation in BER was monitored for a number of disks exposed to designed combinations of temperature and humidity. Although impossible to predict the lifetime of any individual disk, the experimental design allows a confidence interval to be determined for a specified level of product compliance over a given period of time. Based on data such as shown in Fig. 27, for example, 3M is confident that data can be stored safely on their MO media for time fi-ameson the order of decades. This statistical approach to lifetime testing methodology combined with variation of the relative humidity provides archivists a means of obtaining lifetime estimates under conditions that are realistically useful. Of course, as with any form of accelerated testing, the danger in extrapolating a lifetime value for a different set of conditions is that the degradation mechanism itself may change with temperature-humidity . Stress resistance tests such as the Z/AD a standard test ofthe International Electrotechnical Commission involving temperature and humidity cycling, are used to simulate harsher (but temporary) environments that might be encountered, for example, during shipping. B o ~ l e y [ ' ~has ~] discussed the subtle difference between using stress resistance tests to

Magneto-Optical Thin Film Recording Materials in Practice 347 investigate disk failure mechanisms (appropriate) and to estimate disk lifetimes (inappropriate).

0

mm

1-

YEARS

TD

xyy)

RFACH EfR S X E-4

EMS 10 R U M BER 5E-4

Figure 27: (a) Predicted survival function based on an end-of-life BER = 5 x for 3M MO disk media assuming "worst-case" archival storage conditions of 30°C and 90% relative humidity. Dashed lines indicate 95% confidence intervals. (b) Expanded view of upper leA comer of (a). This data can be interpreted as indicating that, with 97.5% confidence, 99.5% of media will require at least 10 years to reach a BER of 5 x lo4 (or, with the same confidence, 95% of media will survive 30 years). The MO media tested consisted of double-laminated sides of polycartmnate/SiClTbFeCoTdSiC/reflector/ lacquer."6'1

348 Magneto-Optical Data Recording Although not addressed by existing standards, it is also essential in some applications to evaluate the ability of recording media to withstand atmospheric corrodants. Sony evaluates its media in a simulated smog atmosphereconsisting of 30°C: 90% RH with 2 ppm each of SO,, H,S, and NO, and 1 ppm C1, as well as atmospheres containins4% ammonia or 5 pprn ozone (see Ref. 162). Although such testing environments are perhaps unrealistically harsh, they provide a gauge of media resistance to common industrial and urban pollutants. The evolution of commercial MO products and processes continuesto show improvements in quality and reliability in accelerated aging studi e ~ . [ l The ~ ~ ]dominant failure mode in welldesigned and well-manufactured MO disks is no longer macroscopic oxidation of the MO layer but rather have localized “pitting” corrosion of the MO layer. Several studies[1701[171] shown the strong benefit of addug an additional active (Ti, Zr, Nb Ta, Cr, Al) and/or noble (Pd, Pt, Au) metal element to RE-TMto improve oxidation, corrosion, and pitting resistance. Proposed passivation mechanisms include the formation of an impervious oxide layer for an active metal such as Zr, and a retardation of oxidation kinetics for a noble element such as Pt.[1711 Such alloying additives can provide effective stabilization at concentrations low enough to avoid significant degradation of MO properties, and some have beneficial synergistic effects on other properties such as recording sensitivity.[l711 For Co/Pt based media, reports of BER based accelerated aging studies are just beginning to appear. Sumi et al.i3] have constructed Arrhenius plots for the simple disk structure shown in Fig. l b based on BER testing of temperature-humidity stressed disks. Their analysis indicates an activation energy E for defect formation of 1.5 eV, higher than the activation energy for the formation of pitting defects in RE-TMor for grain boundary diffusion of Co and Pt. This limited but encouraging test data indicates a room temperature lifetime substantially greater than 10 years. Regarding dielectric materials, not only their intrinsic chemical stability but also their compatibility with the active MO layer must be considered for a stable media package. The tendency of RE-TMto reduce most metal oxides to form rare-earth oxides is well d o c ~ m e n t e d . [ ~ ~ There~l-[~~~] fore, although oxides have been explored as protective layers, oxygen-& or oxygen deficient dielectrics are usually recommended. Quite a variety of dielectric materials have been used to protect RE-TM,a sampling of which is listed here: Si-N,[l] Al-N,[1751Si-C,[451Si-C-N,[1761amorphous ~irconia,[~~] amorphous C,[ln1 Si02,[1781[1791 SiOx,[1801 Si-0-X (X =N, Al-Y-N, Mo),[lw1

Magneto-Optical Thin Film Recording Materials in Practice 349 Ta.@5,[301 and ZnS.[301Among these, at least Si-N, Al-N, SiO,, and Al-0 have appeared in commercial or preproduction MO disk media products. It is also noted that multilayer optical edge filters of exceptional stability in humid atmosphereshave been fabricated with Si-N and Si-0 thin In the case of CoPt, chemical reactivity is less of a concern but, as mentioned previously, magnetic properties and recording noise performance depend sensitively on the nature of the substrate material on which the CoPt is deposited. A sampling of dielectric underlayer materials that have been found to enhance Corn coercivity includes: Zn-0,[31[681In-0,[681 In-Sn-0,[6gland Si-N.[651[1821Metallic underlayers of Pt, Pd, Au, Ag,[641 and CoZrNb[lg31have also been found to increase H,. Even if the MO recording stack itself is perfectly reversible, degradation can occur if the substrate is damaged by heat or the environment. For example, Shimazaki et al.[ls4Ihave correlated microscopiccorrosion defects with the presence of low-weight molecular oligomers in the PC resin, thought to segregate to the surface leading to localized adhesion failure at the interface with the thin film stack. They improved the disk corrosion resistance substantially by narrowing the molecular weight distribution of the PC material. Nugent,[lg51a leading archivist, offers the caution that all plastics will eventually deform, warp, and succumb through porosity to airborne contaminants. In his opinion, the preferred preservation medium is an uncoated glass substrate with an etched bit pattern, which can be appropriately coated with a reflector at the time its data is to be extracted. This perception may be right or wrong, but the challenge to change it will clearly involve materials and processing improvements in both the substrate and deposited materials. A1203,[173]

7.0

SUMMARY AND OUTLOOK

The current generation of MO drives and media combine bit densities on the order of 107/cm2(for a total capacity of =l Gbyte for 130 mm disks) with data transfer rates of about 0.5 Mbytdsec. The media are, without exception, disks with RE-TM materials of general composition GdTbDyFeCo for the active MO layer. RE-TM materials meet all of the requirements for thermo-magneto-optical recording:

350 Magneto-Optical Data Recording 1. Thennomagnetic properties can be well tailored to practical needs by proper control of the composition. 2. Good domain regularity can be achieved owing to their fine amorphous structure and large perpendicular anisotropy. 3. High coercivity enables stable domains. 4. Lowdeposition temperature insures compatibility with injection molded polymer substrates. 5 . For fbture generation MO media, the great versatility of exchange-coupled multilayers will likely have strong impact on increasing capacity and transfer rate (superresolution) and in reducing access time (direct overwrite). The main deficiencies of RE-TMmaterials are: 1. Their chemical instability, which necessitates the use of thick protective encapsulating layers. 2. The difficulty of achieving sufficient compositional uniformity. 3. For fbture generation MO media, deficient blue light response, which may necessitate a specialized readout layer. 4. For future generation MO media, high light absorption, which probably will preclude multiple-level recording. Alternative materials that might overcome some of these deficiencies are multilayers and femmagnebc oxides. MO materials based on CoPt multilayers and garnet oxides have the distinct advantage of good chemical stability. This opens the way to simpler disk designs yet maintaining long lifetimes. Their short wavelength response is good. Co/Pt multilayers are compatible with room temperature processing, and compositional control is relatively straightfonvard. The transparency of garnets offers the possibility of multiple recording levels, which can be individually addressed. The chief difficulties with garnets are high processing temperature which precludes the use of organic materials for pre-grooves, and large grain size which leads to high media noise. Magneto-optical storage solutions have been criticized for performance shortcomings in comparison to rigid magnetic disks, but the biggest

Magneto-Optical Thin Film Recording Materials in Practice 351 barrier to greater market penetration, particularly for home consumer use, is the high cost of media and drives. Reducing the cost of the media, the consumable part of the system, is just as important as improving performance in the race against competing magnetic disk and tape solutions and in encouraging development of new applications. Toward this end, streamlined fabrication processes with minimal disk handling have been conceived, implemented, and are constantly being improved. Simpler disk structures made possible by the superior chemical stability of alternativeMO materials promise further reductions in media fabrication costs. In the search for even better MO materials, those based on Mn compounds and alloys show the highest room temperature Kerr rotation but present the major challenges in meeting other requisite attributes for practical MO disk media. If the past is any guide to the future, major media breakthroughs have occurred only after long and diligent searches, witness MnBi in the late 1950’s, GdCo in the early 1970’s, and CoDt in the late 1980’s. However modem fabrication techniques and sophisticatedtheoretical tools are accelerating the pace and sharpeningthe focus of new materials exploration. In conclusion, the rapid progress on all fronts of MO media development, from materials research to disk manufacture, leaves the author with the feeling that much of this chapter will quickly become dated. The hture appears rich and exciting.

ACKNOWLEDGMENTS It is a pleasure to acknowledge the members of the Optical Recording Materials Lab at Eastman Kodak Company during the period 1990-1992 for fostering the stimulating technical environment that prompted this review: M. Culver, J. Farruggia, T. K. Hatwar, A. Palumbo, D. Stinson,Y. S. Tyan, and R. Victora. Several of these members also provided critical readings of the manuscript.

352 Magneto-Optical Data Recording ABBREVIATIONS AND SYMBOLS MO

RE-TM PC H C

% ‘?k eF, ‘?F

a TCOmp

Tc hCP fcc

K, R

K A FOM

Re, R’” 6)) CNR SNR RF DC MF BER RH ANSI IS0

magneto-optical rare earth-transition metal polycarbonate coercivity Kerr rotation and ellipticity Faraday rotation and ellipticity optical absorption coefficient compensation temperature Curie temperature hexagonal close packed face centered cubic uniaxial magnetic anisotropy constant optical reflectance thermal conductivity magnetic exchange constant figure of merit camer FOM shot-noise limited CNR FOM carrier-to-noise ratio [slot (usually 30 kHz)signal-tonoise ratio] broadband signal-to-noise ratio radio fiequency direct current mid-frequency byteerror-rate relative humidity American National Standards Institute International Standards Organization

Magneto-Optical Thin Film Recording Materials in Practice 353

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360 Magneto-Optical Data Recording 147. (a) Katayama, T.,Suzuki, Y., Awano, H., Nishihara, Y., and Koshizuka, N., Phys. Rev. Lett., 60:1426(1988); (b) Suzuki,Y., and Katayama, T., J. Magn. Soc. Jpn., 17:29 (1993) 148. (a) Balasubramanian, K., Opt. Engr.,3 1 :2674(1992); (b) Balasubramanian, K., SPIE, 1782521 (1992) 149. Reim, W., and Weller, D., Appl. Phys. Lett., 53:2453 (1988) 150. Zhou, S. M., Lu, M., and Zhai, H. R., Phys. Status Solidi (B), 168:651 (1991) 151. Argyres, P.N.,Phys. Rev., 97:334 (1955) 152. Wijngaard, J. H.,Haas, C., and de Groot, R A., Phys. Rev. B, 40:9318 (1989) 153. Wang, Y. J., Luo, C.P., Kong, L. C., Qi, X. S.,Huang, D., and Chen, Y., J. Magn. SOC.Jpn., 17:294(1993) 154. Di, G.Q.,Iwata, S.,and Uchiyama, S.,J. Magn. Magn. Muter., 131:242 (1994) 155. Jaswal, S.S.,Shen, J. X., Kirby, R. D., and Sellmyer, D. J, AbstractEB-01, Proc. 38th Con$ Magn. and Magn. Materials, November, Minneapolis (I 993) 156. Gambino, R J., Fumaglai, P., andRuf, R. R., J. Magn. SOC.Jpn., 17:276 (1 993) 157. Al-Saffer, A. H., Ashworth, V., Bairamov, A. K. O., Chivers, D. J., Grant, W. A., and Proctor, R P. M., Corros. Sci., 20:127(1980) 158. Moshier, W. C., Davis, G. D., Ahearn, J. S., and Hough, H. F., J. Electrochem. SOC.,134:2677(1987) 159. Frankel, G. S.,Russak, M. A., Jahnes, C.V., Mirsamaani, M., andBrusic, V. A.,L Electrochem. SOC.,136:1243(1989) 160. Shaw,B. A., Fritz., T. L.,Davis, G. D., and Moshier, W. C., J. Electrochem. Soc., 137:1317(1990) 161. Onishi, T.,Yamamoto, S.,Yoshikawa, K., and Koga, Y., J. Magn. SOC. Jpn., 17:106 (1993) 162. Marchant, A. B., Optical Recording: A Technical Overview, AddisonWesley, New York (1990) 163. Makino, H., Jpn. J. Appl. Phys., 28:97(1989) 164. Podio, F. L.,SPIE, 1663:447(1992) 165. Bouley, R. J., CD ROM Librarian, p. 10 (Jan., 1992) 166. Document ImageAutomation Update (W. W a d y , ed.), Meckler Publishing Corp. (Feb., 1992) 167. Murray, W. P.,J. Magn. SOC.Jpn., 17:Suppl.S1,309 (1993)

Magneto-Optical Thin Film Recording Materials in Practice 361 168. Industrial Electrochemical Commission Publication 68-2-38 UAD. See also Ref. 161, p. 382 169. Wong, J. S.,Fortunati, L. J., Hong, B., Kluska, A. P, N e b e d , L. L., Mercado, M. F., Rodriguez, Y., and Yen, Y. S.,IEEE Truns. Instrum. Meus., 41:304 (1992) 170. Niihara, T., Kirino, F.,Ogihara,N., Koyama, E., Sudou, R., Shigematsu, K., and Ohta, N., IEEE Truns. Mugn., MAG-24: 2437 (1988) 171. (a) Hatwar, T. K., and Majumbdar, D., IEEE Truns. Mugn., MAG24:2449 (1988); @) Hatwar, T.K., J. Appl Phys., 70:6335 (1991) 172. Anthony, T. C., Brug, J., Naberhuis, S.,and Birecki, H., J. Appl. Phys., 59:213 (1986) 173. Luborsky, F.E., IEEE Truns. Mugn., MAG-22:937 (1986) 174. Yang, M., and Reith, T., J. Appl. Phys., 71:3945 (1992) 175. Hatwar, T. K.,Appl. Phys. Commun., 11:471 (1992) 176. Palumbo, A. C., and Genova, D., Appl. Phys. Commun., 11:447 (1992) 177. Robinson, C., Payne, R., andBell, A., Tech. Dig. Ser. 1987,10:131(1987) 178. Bernstein, P., and Gueugnon, C.,IEEE Truns. Mugn., 21:16 (1985) 179. Sato, M., Onisawa, Y., Niwa, H., Tsukane, N., and Toba,H., IEEE Truns. J. Mugn. Jpn., 2:395 (1987) 180. Watanabe, N., Tushima, K., Malmhall, R., and Rao, K. V., Muter. Res. SOC.S p p . Proc., 150:239 (1989) 181. Martin, P. J.,J. Muter. Sci., 21:l (1986) 182. Greidanus, F.J. A. M., Zeper, W. B., Jacobs, B. A. J., Spruit, J. H. M., and Carcia, P. F.,Jpn. J. Appl. Phys., 28:37 (1989) 183. Yusu, K., Hashimoto, S.,and Inomata, K., J. Mugn. Soc. Jpn., 1756 (1993) 184. Shimazaki, K., Furisho, H., Goto, T., Kuwarahara, K., Kikuchi, N., Iida, T., and Ohta, N., J. Mugn. Soc. Jpn., 15:369 (1991) 185. Nugent, W. R, as quoted by Bouley in Ref. 163 186. Biersack, J. P., and Haggmark, L. G., Nucl. Instrum. Methods, 174:257 (1980) 187. Carey, R, Jenniches, H., Newman, D. M., and Thomas, B. W., J. Mugn. SOC.Jpn., 17:290 (1993)

6

Materials Characterization William A. McGahan

1.0 INTRODUCTION Materials characterization is a crucial issue in the field of magnetooptical (MO) data storage. MO data storage involves thermomagnetic writing of small domains in a thin magnetic film embedded in a multilayered disk structure, and readout of these domains via the detection of the polar Kerr rotation of a beam of monochromatic light reflected from the disk. Also, the reflected beam is monitored to control the position of the heads when reading or writing the disk. These three processes impose stringent requirements on the materials used to fabricate MO disks, as well as on the structure of the disk itself. Knowledge of the physical properties of the materials used in fabricating MO media coupled with accurate models for the processes involved in reading and writing the disk allows accurate prediction of the performance of a given disk structure, and calculations can be performed to optimize disk structures for maximum signal-to-noise performance. Also, hdamental studies of the material properties which strongly effect the read and write processes are of great benefit in the search for better materials for MO recording media. There are several properties of the thin films which are important for their application in MO storage media. The optical properties of all films in the disk as well as the substrate affect the intensity of the reflected beam and 362

Materials Characterization 363 the size of the Kerr rotation and ellipticity, as do the thicknesses of the films. The magneto-optical properties of the magnetic film are important as the readout of the disk is performed via detection of the magneto-optical polar Kerr rotation from the disk. The magnetic properties of the magnetic film are important for the optimization of the thermomagnetic writing process, as thin film materials which exhibit an easy magnetic axis normal to the film plane and square hysteresis loops are required. The thermal properties of all materials in an MO disk structure determine how quickly heat will diffuse away from a small spot on the disk when heated by a laser pulse. This cooling rate is important for the optimization of the thermomagnetic writing process, and can be predicted if the thermal conductivity and diffisuvity of each material in the disk and the thicknesses of the films on the disk are known. In this chapter we review the optical, magneto-optical, and thermal characterization of materials used for MO media, with emphasis on the measurement of these properties for thin films. The magnetic properties of bulk materials and thin films are commonly measured via vibrating sample magnetometry (VSM), SQUID magnetometry, or torque magnetometry. Structural observations are performed directly via electron microscopy and atomic force microscopy (AFM), and somewhat indirectly by x-ray dif€raction (XRD). The measurement of these structural and magnetic properties is well described in the literature and will not be further discussed in this chapter.

2.0

OPTICAL CHARACTERIZATION

The optical characterization of a material involves the measurement of the complex index of refraction or complex dielectric constant of the material:

Eq.(1) Eq.(2)

Z=n+ik = E’ + i ~ ”

complex index of refraction complex dielectric constant

where n is the (real) index of refraction of the material, k is the extinction coefficient, and E’ and E” are the real and imaginary parts of the complex

364 MagneteOptical Data Recording

dielectricconstant. These quantities are commonly referred to as the optical constants of a material, and are related as follows:

2

&=n -k

Eq. (4)

E"

2

= 2nk

Optical constants have been measured for a wide variety of material~,[~]-[~] and some typical values of the optical constants for dielectric, semiconducting, and metallic materials at 632.8 nm are given in Table 1. Unless otherwise noted, the values are obtained fiom Palik's Handbook of Optical Properties of Solids, vol. S11or vol. II.r21

Table 1: Optical Properties of Materials.

Material SiO Si02 Si,N4 A1203

a-C:H, E e 2 . 2 eV41 a-C:H, E g 1 . 2 eV41 MgF2 Si Ge

GaAs Aluminum Copper Silver

Platinum

n

k

1.96 1.46 2.02 1.77 1.67 2.07 1.38 3.87 5.47 3.86 1.37 0.25 0.13 2.33

0.010 0.000 0.000 0.000 0.002 0.20 0.000 0.017 0.82 1 0.198 7.62 3.41 3.99 4.15

E'

3.86 2.12 4.08 3.12 2.79 4.25 1.89 14.98 29.28 14.84 -56.15 -11.58 -15.89 -11.75

Ell

0.043 0.00 0.00 0.00 0.008 0.83 0.000 0.128 8.987 1.531 20.92 1.686 1.07 19.34

The optical constants of three important MO materials are given at 450 nm and 800 nm in Table 2. The TbFeCo constants were measured fiom

a thin filmof Tbo,203Feo~,,,Coo,,odeposited by nonbiased DC magnetron

Materials Characterization 365 sputtering in a trilayer stack between films of silicon nitride.l51 The Copt multilayer constants were measured from a thin sputtered film consisting of 15 bilayers of Co (3 angstroms)/Pt (10 angstroms) with a total measured thickness of 219 angstroms.[6]The garnet optical constants were measured which was annealed at from a 320 nm film of (Bi2.0Dy,.,)(Fe,.SGa,,~)Ox 65OOC for 30 Other constants will be presented for these materials, and the designations TbFeCo, Co/Pt, and “garnet” will refer to them unless otherwise noted.

Table 2: Optical constants of MO materials.

TbFeCo

450 800

2.10 3.49

3.10 4.09

-5.22 -4.54

12.99 28.49

corn

450 800

1.50 2.49

3.41 5.42

-9.37 -23.13

10.24 26.96

Garnet

450 800

2.72 2.37

0.31 0.00

7.32 5.62

1.71 0.00

Note that the complex index of refraction and the complex dielectric function represent the same information, and each requires the specification of two numbers, the real and imaginary parts. It is for this reason that two experimentally measurable quantities (reflectance and transmittance, ellipsometric y and A) are required to determine the optical constants of a given material. Both the complex dielectric fimction and the complex index of refraction have direct physical significance. The complex dielectric function relates the displacement field at a point in the given material to the applied electric field at that point, and is a measure of the degree of electrical polarization which occurs in the material in response to the applied field:

Eq. ( 5 )

366 Magneboptical Data Recording where 6 is the displacement field, I? is the applied electric field, and is the polarization field. The complex index of refraction determines the amplitude and phase of an electromagnetic wave travelling in the material. Plane waves propagating in the material in any direction will have the form:

E ( F , ~=)Eo exp

r2: -4

1

- - 7 exp(- iwr)

where s‘ is a unit vector along the direction of wave propagation, and A is the wavelength in vacuo. As E0 is a (complex vector) constant, the dependence of the amplitude and phase of the plane wave on position is completely governed by the complex index of refraction of the material and the wavelength of the propagating wave. As a result, the optical constants n and k or d a n d d’completely determinehow an isotropic material will effect a beam of light reflecting from a planar surface of the material or transmitting through the material. Modeling of the optical and magneto-optical response of multilayered disk structures requires knowledge of the optical constants of all materials in the stack as well as the thickness of the individual films. Also, the optical constants and layer thicknesses must be known in order to predict where in the stack heat is absorbed from the writing laser beam as this distribution of absorbed energy is the starting point for more sophisticated calculations of the thermal diffusion of heat in the disk structure during the thermomagnetic write Note that the choice of the sign of the argument of the exponential in Eq. 6 dictates whether the extinction coefficient should be positive or negative, as the amplitude of the wave cannot increase as it propagates. For our choice of a positive sign in Eq. 6, the extinction coefficient (and the imaginary part of the dielectric function) must be positive. If the opposite sign is assumed, the extinction coefficient and dielectric function must be negative. Note that the magnitude of these quantities is independent of this choice of sign. This will not be the case for the MO constants, as their magnitudes depend on the choice of sign employed. In addition to the measurement of optical constants for materials used in disk structures, optical experimentscan be very useful for nondestructively determining the thicknesses of the individual layers in multilayered struct u r e ~ . [ ~ ] -In [ ~ fact, ~ ] as many of the magnetic materials used as storage layers are not chemically stable in atmosphere; protective coatings are required such that the measurement of the optical constants of such films

Materials Characterization 367 involves multilayered modeling procedures. In this section, we discuss the two most common optical measurements for materials and structures relevant to MO storage: combined reflectance and transmittance and ellipsometry. The formalism describing the optical response of bulk materials and multilayers is also discussed, and several examples of the optical characterization of materials and multilayer structures are given.

2.1 Techniques for Optical Characterization It is generally not possible to directly measure the complex index of refraction or complex dielectric function of a given material, so most methods of optical characterization rely on the measurement of some physical quantity which is a strong function of the index of refraction of the material under test. The measured data must then be modeled in order to determine the unknown optical and structural properties of the system. The two most common methods of optical characterization of materials are combined intensity reflectance and transmittance measurements and ellipsometry. An intensity reflectance and transmittance measurement is shown schematically in Fig. 1.

Sample

Figure 1. Schematic of reflectandtransmittance measurements. The intensity of the light beam is measured with and without the sample in place.

A monochromaticbeam of light of known intensity strikes the sample at normal (or near-normal) incidence. The reflected or transmitted beam is

368 Magneboptical Data Recording then incident on a detector, such that the intensity of the reflected or transmitted beam is measured as a function of the wavelength (or photon energy) of the incident beam. This measured intensity is divided by the intensity of the incident beam, such that the percentage of the power of the incident beam which is reflected or transmitted is measured. Reflectance andor transmittance measurements can also be performed at oblique angles of incidence. Reflectance and transmittance experiments have the advantage of being inexpensive and easy to perform, but also have several inherent disadvantages. First, any light which is scattered nonspecularly from the surface (or interfaces) of the sample will decrease the accuracy of the measurement, as this leads to an apparent decrease in the reflectance or transmission of the sample. Second, the measured data can be very sensitive to fluctuations in intensity of the light source. Third, if the sample is opaque, the transmission will be zero, which precludes thick absorbing films or opaque substrates. In spite of these difficulties, combined reflection and transmission measurements have been used by several authors to determine the optical constants of thin films. Examples of optical and combined MO and optical characterization involving the use of reflectance and transmittance measurements are given in Secs. 3.4 and 3.5. Another popular experiment for optical characterization of materials is ellipsometry. In an ellipsometric experiment (depicted schematically in Fig. 2), a monochromatic beam of light of known polarization state strikes the sample under test an a known (oblique) angle of incidence bo. The polarization state of the reflected (or transmitted) beam is then measured, usually as a function of the wavelength of the incident beam and/ or the incident angle. The incident beam is usually linearly polarized, with the polarization direction lying at a known azimuthal angle P from the pplane. In the most common experimental configuration, the polarization state of the reflected beam is analyzed by passing the reflected beam through a rotating polarizer prior to the detector such that the detector output at any given moment is proportional to the square of the component of the electric field of the reflected beam along the axis of the rotating polarizer. The axis of a polarizer is defined as the direction along which the beam transmitted through the polarizer is polarized. This signal is measured as a function of time, and will have the following general forrn:[l21

Eq.(7)

I D ( t )cc 1+ acos(2A) + psin(2A)

Materials Characterization 369 where A is the angle between the fast axis of the rotating polarizer and the plane of incidence. The ellipsometric parameters psi (Y) and delta (A) are defined as the ratio of the p- and s-plane pseudo-Fresnel reflection coefficients (defined later in Eq. 25) of the sample:

tanY exp('A)

I

4 R S

and can be shown to be simple functions of the Fourier coefficients of the detector signal a and pand the angle P between the input polarizer fast axis and the plane of

Note that the sign of the cosA equation is positive if the polarizer angle P lies between 0 and IC,and negative otherwise. The measured psi and delta data can be modeled for a given multilayer structure by calculating the pseudo-Fresnel coefficients Rp and Rs for the structure, and adjusting parameters (thicknesses, optical constants) in the model until the calculated data matches the experimental data as closely as possible. Sophisticated regression schemes based on the Levenberg-Marquardt nonlinear minimization algorithm have been shown to be very useful for this fitting process.[13] There are many advantages to ellipsometric techniques. First, the measured quantities are not dependent on the intensity of the light source provided enough light is present at each wavelength of interest to overcome the inherent noise in the experimental apparatus. Second, the ellipsometric parameters are generally very sensitive to the layered structure and optical properties of the materials in multilayered systems. Third, ellipsometric experiments can be performed in reflection or transmission modes, in-situ or ex-situ. The technique is particularly powerful in-situ when combined with real-time data analysis algorithms which provide layer thicknesses and

3 70 Magneto-Optical Data Recording optical constants as a function of time during the deposition The drawbacks of the ellipsometric technique are that the experiment’al apparatus tends to be more complicated (and expensive) than the equipment required for reflectance and transmittance measurements, and the calibration and a!ip-ment prd-ures rquired tn acquire accurate data with the ellipsometer may be somewhat involved. Also, ellipsometric data are very sensitive to both the texture of the sample surface and the presence of oxide or contaminant layers at the surface of the ~ample.[~~1-[~~1 As a result, oxidation of an unprotected reactive sample surface or contamination of the sample s u h c e will have a large effect on the ellipsometric data, and hence the optical constants of the material calculated from these data will not be accurate unless the effects of the thin surface layer are modeled correctly.

Light source

Sample

Figure 2. Schematic of an ellipsometric measurement. A beam of light of known polarization is reflected from (or transmitted through) the sample. The polarization state of the reflected (transmitted) beam is then measured, usually by rotating the second polariZer (analyzer) and measuring the resultant detector signal as a function of time.

Materials Characterization 3 71 2.2 Theory of Optical Reflection from Multilayers In order to model ellipsometric, reflectance, or transmittance data acquired from a bulk or multilayered sample, it is necessary to be able to predict these quantities given the thicknesses and optical constants of each film and the optical constants of the substrate. Layer thicknesses andor optical constants can then be determined from the measured data by varying these quantities to find the values which yield the best match to the measured experimental data. To develop such a model, we first consider a monochromatic electromagnetic plane wave propagating in an isotropic and homogenous medium:

r2:-

E(7,t) = Eo exp -q

-1

.Y

exp(- iw t)

where q is a unit vector along the direction of wave propagation, n is the complex index of refraction n + ik, A is the wavelength of the light in vacuum, w is the angular frequency of the wave, and E, is a complex vector constant specifjmg the amplitude and polarization of the plane wave. Note that if the extinction coefficient (imaginary part of the index of refraction) k is nonzero, the wave decays exponentially as it propagates according to

which provides a measure of how far the beam will penetrate into a given material, as the amplitude of the wave will decay to l/e of its original value after propagating a distance given by:

il 27rk

due =-

Eq. (13)

Also, in an isotropic material with no free charges, the Maxwell equation ? 6 = 47rp reduces to - I? = 0 ,which requires that the electric field vector E lie in the plane perpendicularto the direction of propagation of the wave q. This is known as a transverse wave, and we can completely describe a beam of any polarization state by specifjmg the components of

-

3 72 Magneto-Optical Data Recording the electric field l? along any two orthogonal directions which lie in the plane perpendicular to the wavevector. These basis directions are chosen along thep- and s-directions, as shown in Fig. 3.

Ambient m

Sample

!

Trans&tted Figure 3. Geometry for light scattering at a single interface.

The p-direction lies in the so-called “plane of incidence,” which contains the incident and reflected wavevectors as well as the sample normal. The s-direction is perpendicular to the pdirection and parallel to the sample surface. Note that q, p, and s are chosen to form a right-handed Cartesian coordinate system, and that at normal incidence the pdirection is reversed upon reflection from the sample. We can then describe any propagating beam by specifling the elements of the two component Jones vector (see Ref. 12 for a thorough discussion of Jones vectors and matrices):

Materials Characterization 3 73

where g p and Esare the (complex) components of the electric field vector along the p- and s-directions, respectively. The following examples (u)-(d) will clarifl the above discussion:

A plane wave of unit amplitude polarized linearly along the p-direction is written as shown in (a). In this case, if we look into the beam as it propagates towards us, we would see the tip of the electric field vector move back and forth on a line in thep-direction. Similarly, an s-polarized wave of unit amplitude is written as in (6). These two Jones vectors are a complete basis for transverse waves, hence we can describe any arbitrary polarization state as a linear combination of these two Jones vectors. A right circularly polarized wave is described as shown in (c) and a left circularly polarized wave appears as in (d). A fortuitous consequence of this decompositionof arbitrary polarizations into a linear combinationoftwo simple basis vectors is that, for a given sample structure, we need only calculate the response of the system to excitation by the p- and s-polarized plane waves given in the previous paragraph, and the response to any arbitrary polarization can be found by taking the appropriate linear combinationof the results. In other words, the problem is separable from any arbitrary polarization into the calculation of the p- and s-direction responses. The above formalismapplies to the propagation of an electromagnetic wave in an arbitrary material. The next step is to determine what will happen to this wave when it encounters an interfacebetween materials. This will complete the optical model for optically thick substrates with no films, and provides a key building block for the development of a multilayer optical model. Consider the geometry shown in Fig. 3. In this case, we have a plane wave of known amplitude and polarization incident at some angle &

3 74 Magneto-Optical Data Recording (measured from the sample normal) on the sample surface. The subscript 0 refers to the incident medium, and the subscript I will be used to refer to the medium which the light strikes. Part of the wave is reflected from the sample surface at some angle 4r, and part of the wave is transmitted into the sample at some angle 4; which will be complex if the sample is absorbing. If we write the incident wave as a Jones vector, the Jones vectors representing the amplitude and polarization of the reflected and transmitted beams can be written as follows: rejected

Eq. (15)

transmitted

incident

incident

At this point, the 2 x 2 matrices (known as Jones matrices) relating the incident and reflected (or transmitted) fields are simply arbitrary transfer matrices describing the effects of the interface on the incident beam. One of the consequences of the isotropy of the ambient medium and the sample is that there are no offdagonal elements in these transfer matrices, such that regardless of the angle of incidence a purely p- or s-polarized beam will retain the original linear polarization upon reflection or transmission, with only the amplitude and phase (not the direction) of the electric field vector being modified. This will not be the case in magneto-optical materials, as off'agonal elements occur and polarization mixing ensues. The elements 5, T , and ?; known as Fresnel coefficients, can be calculated by writing the expressions for the incident, reflected, and transmitted fields (both electric and magnetic) and enforcing the continuity of the tangential components of?!, and fi and the normal components of d and 3. This calculation yields four usefil results:[l21

5,

The reflected angle is identical to the incident angle.

Materials Characterization 3 75 Zosin /o = El sin

Eq.(17)

which is Snell’s law, where KO is the index of refraction of the incident medium (KO = 1 for air), E,is the complex index of refraction of the sample, the transmitted angle, can be complex. and

+,,

Eq.(18)

Eq. (19)

‘P=

-t , =

Elcos40-Eocos41 E l c o s ~+oE o c o s ~ l 2E0 cos (60 E, cos40 +EOC0S(dl

-

r, =

Eo cos@o- El cos$41 Eocos $40 + El cos 4,

-t, =

2E0 cos 4o

E o c o s ~+El o COSA

Note that the Fresnel coefficients (Eqs. 18, 19) are functions only of the indices of refraction of the ambient medium and the sample and the incident and transmitted angles. Also, Snell’s law can be used to eliminate the transmitted angle from the above equations, such that the Fresnel coefficients depend only on the ambient medium and sample indices of refraction and the incident angle, all of which are presumably known quantities. The above equations are derived for the simple case of an interface between two regions of differing optical constants, and describe the effect of any interface on a propagating light beam. They represent a complete optical model for reflection experiments from optically thick samples, as they completely specifL the polarization and amplitude of an arbitrarily polarized beam of light reflected from the surface of the sample given the amplitude and polarization of the input beam. If two optical measurements are performed on an optically thick sample (ellipsometric y and 6,for example), Snell’s law and Eqs. 18, 19 can be used to determine the two unknown parameters n and k (or the real and imaginary parts of the dielectric function) directly from the measured data. This is one of few exampleswhere the analysis of ellipsometric data does not require nonlinear multiple regression techniques. It is now possible to extend the above formalism to describe multilayers, as each reflection of the beam within the multilayer must obey Eqs. 18, 19. There are several means by which the multilayer calculations can be performed. The most straightforward is to trace all of the multiple

3 76 Magneto-Optical Data Recording reflections of the beamas it is reflected in the films, and sum the components of the beam leaving the sample.[l21This is depicted schematicallyin Fig. 4.

Transmitted

Transmitted Transmitted

Transmitted

Figure 4. Multiple reflections in a single film on an optically thick substrate. The multiple reflected beams must be summed to calculate the optical response of the fildsubstrate SY-.

Each time a component beam strikes an interface, it splits into a reflected and transmitted component, each of which must be accounted for and will subsequently be split. Analytic solutions for arbitrary numbers of layers may be obtained by summing the reflected components of the beam, but it is very difficult to derive these solutions for samples with more than a few films. The most generally useful approach is to develop a 2x2 transfer matrix for each film and the substrate which relates the electric fields at the top and bottom of the film, and to multiply these matrices together to describe the multilayer.[121 This approach is used for the simultaneous calculation of the optical and magneto-optical response of multilayers (which requires 4x4 transfer matrices), and is described in detail in Sec. 3.3. Another useful approachrl71is based on the matching of the field components at all interfaces in the system.

Materials Characterization 3 77 Figure 5 depicts an arbitrary multilayer, with the (complex) incident and reflected fields at each interface labeled A, B, C,and D. The incident beam (and hence all waves in the multilayer) is assumed to be eitherp- or spolarized, and the substrate is assumed to be optically thick such that the component C does not exist at the substratehottom film interface. As the Eqs. 18, 19 are valid for each interface, we can write each of the field quantities in terms of two others:

Bi = F,,+ Ai +

<+

1,

Eq. (23) where C,, is the p- or s-polarized Fresnel reflection coefficient for a wave incident on material j from region i, C j is the p- or s-polarized Fresnel transmission coefficient for a wave passing from region i into regionj, and

mi = exp(T)i27rdiq where di and 4 are the thickness and complex index of refraction of the ih region, respectively. The above Eqs. 20-24 must be simultaneously satisfied at all interfaces in the multilayer, and there are two means for finding the (complex) values of the fields A-D everywhere in the sample. The fields can be evaluated analytically by choosing a value for the transmitted field (one, for example) and back-calculating the field components through the stack. This is often termed the method of successive removal of interfaces, and is very easy to implement numerically for arbitrarily large numbers of films. Unfortunately, if any film in the structure is optically thick or nearly so, huge values of the incident field will result due to the assumption of a nonzero transmitted field, and numerical precision can be lost in the determination of the ratio of the reflected (B,) to incident (A,) fields. A more general approach to the calculation involves assuming a known value (one) for the incident field A, and numerically iterating through

3 78 Magneto-Optical Daia Recording the successive evaluation of Eqs. 20-24 at each interface. This can be thought of as a transient calculation of the response of the multilayer to the incident wave. Both approaches have the added benefit of yielding, as part of the calculation, the complete electric field distribution in the multilayer (see Fig, 5 I j which is particularly useful for determining where heat is absorbed in the multilayer when it is illuminated by a laser beam.['*]

A0

BO

Ambient (Region 0)

Film #1

Film #2

A2

Substrate

Figure 5. Crook's method for the determination of the electric field distribution in multilayers when illuminated at normal or oblique angles of incidence. The field quantities at the interfaces must obey Fresnel's reflection and transmission equations for a single interface (Eqs. 18 and 19) and Snell's law.

Materials Characterization 3 79 The scattering of a plane wave of arbitrary polarization from a multilayer can be modeled by decomposing the incident polarization into pand s-components, and performing the above calculation for both thep- and s-polarized cases. We can easily identi@the pseudo-Fresnel reflection and transmission coefficients in terms of the fields in the ambient and the substrate:

Eq.(25)

where the subscriptsp and s on the pseudo-Fresnel coefficients refer to the choice of the incident (or transmitted) polarization when calculating the fields. The reflectance, transmittance, and ellipsometric parameters for a given multilayer can be calculated given the pseudo-Fresnel coefficients for the structure, such that this formalism is sufficient for the modeling of optical data. A more complicated approach is required, however, when magneto-optical effects are to be modeled or have significant effects on the measured optical quantities, and a more general formalism is developed for this purpose in Sec. 3.4.

2.3 Example of Optical Characterization Two examples of optical characterization of thin films are presented in this section, both performed with variable angle of incidence spectroscopic ellipsometry (VASE). First, measured and best-fit calculated VASE data for a thin film of thermal silicon dioxide on a silicon wafer are shown in Figs. 6 and 7. These data were fit by using literature optical constants to model the substrate and film and varying the silicon dioxide layer thickness in order to best fit the experimental data. The thickness of the silicon dioxide layer was determined from this fit to be 23.7 f 0.1 nm. This is an example of the very simplest analysis of ellipsometric data, requiring only one fit parameter. This analysis can be performed with ellipsometric data at a single wavelength and angle of incidence if the substrate and film optical constants are known.

380 Magneto-Optical Data Recording

-ModelFit ----. Exp E loo Exp E 7 5 O _ _ Exp E SOo

200

I

I

I

I

400

600

800

1000

Wavelength in nm

Figure 6. Experimental and best-fit calculated ellipsometric data as a function of wavelength and angle of incidence, measured from a 23 nm film of thermal silicon dioxide on silicon.

-Model

Fit

20

200

I

400

I

600

I

800

1000

Wavelength in nm Figure 7. Experimental and &-fit calculated ellipsometric data as a function of wavelength and angle of incidence, measured from a 23 nm film of thermal silicon dioxide on silicon.

Materials Characterization 381 Figure 8 shows the optical constants obtained from the analysis of VASE and normal incidencetransmission data for a thin amorphous hydrogenated carbon film deposited by plasma enhanced CVD on a microscope ~lide.1~1 The film was determined to be 30.5 f 0.2 nm thick. Note that the film is absorbing (k is greater than zero) over the entire spectral range. As a result, VASE data alone were not enough to uniquely determine the thickness and optical constants of the film, but the addition of transmittance data to the fit decorrelates these parameters such that they can be uniquely determined.

Figure 8. Optical constants of a thin a-C:H film deposited on glass. The constants were part of the best-fit solution to both VASE and transmission data, with the optical constants and a-C:H film thickness as

3.0 MAGNETO-OPTICAL CHARACTERIZATION The magneto-optical characterization of a material consists of the measurement of either the complex Voigt parameter or the offdiagonal elements of the complex dielectric tensor

Es.(27)

0 = Q' + iQ"

(complex Voigt parameter)

382 Magneto-Optical Data Recording Eq.(28)

rw=

+ Z&&

(offdngonalcomplex dielectrictensor elements)

These parameters are related as follows:

Eq. (29) where Zaxdenotes the diagonal element of the complex dielectric tensor. Recall that we have assumed the sign of the extinction coefficient to be positive. While this assumption only determines the sign of the extinction coefficient or imaginary part of the dielectric hnction and does not affect the magnitude of any of the optical constants, it does effect the value of the MO constants. This can be a problem when attempting to use constants derived under one sign convention with equations which have been derived using the other convention. The relation between values of 0 or Zw for the differing sign conventions can be calculated most easily by equating the expressions for the complex Kerr effect and solving for one set of constants in terms of the other. The results are:

Eq. (30)

1 - (n - ik)2)( n +ik)Q+ 1-(n+ik)2 n-ik

where a plus (+) subscript denotes the quantity appropriate to models assuming ii = n +ik , and a minus (-) subscript pertains to models with ii=n-ik. Typical values at room temperatureof the real and imaginary parts of and gv for some common MO materials are given in Table 3. Note that these coflsfants were measured with the sample magnetization saturated, and that all samples exhibited square hysteresis loops such that they are also valid for the remanent state. Also, the Q values tabulated in Ref. 19 for these materials have been transformed using Eq.30 for model calculations with i i = n - i k .

a

Materials Characterization 383 Table 3. Magnetooptical Constants of MO Materials[1s]

€4

E4.

Material

qnm)

Q'

Q

TbFeCo

45 0 800

-0.017 -0.027

-0.0054 0.0062

-0.19 -0.80

-0.16 0.054

corn

450 800

-0.011 -0.0088

0.012 0.013

-0.23 -0.54

0.02 0.15

Garnet

450 800

-0.0022 -0.0009

-0.022 -0.0003

-0.16 -0.002

-0.021 0.005

N

As was the case for the optical constants of materials, there are two magneto-optical constants, and two measured quantities are required for the complete MO characterization of a material. The most common measured quantities are the Kerr rotation and ellipticity and the Faraday rotation and ellipticity. Knowledge of the optical constants and layer thicknesses for all materials in a multilayer, as well as of the MO constants of any magnetic materials in the structure is required for the calculation of the Kerr rotation and ellipticity which will result fiom the multilayer. Also, fundamental studies of the size and spectral dependence of the magneto-optical constants are useful for the understanding of the physical processes which lead to large MO effects for some materials and in the search for better materials for MO media.[Zol-[261 In this section we discuss the physical origins of magneto-optical effects, as well as the formalism required to model these effects in both bulk materials and multilayers. Methods of measuring magneto-optical effects are reviewed, and specific examples of magneto-optical characterization of materials are presented. Finally, the last part of the section is devoted to examples of the combined optical and magneto-optical characterization of materials and multilayers.

384 Magneto-Optical Data Recording

3.1 Physical Origins of the Polar Kerr Effect The physical origins of the magneto-optical effects can be understood by examining the dielectric response of a material in the presence of a mq-& field. m-e&ipIipcfrjc fipns~ris &$A& a th. rec~fid_ r2n-k t e f i g ~ y which relates the displacement field to the electric field at any point in the given material:

Es.(32) where d is the displacement field vector and j j is the polarization field vector at a point in the material. The dielectrictensor can then be thought of as measuring the response of the charges in the material to a driving electric field E . In an isotropic, nonmagnetized material, all directions are equivalent in the material, such that the response of the charges in the material is independent of the direction of the driving electric field vector. This can be understood classically by considering the force on an electron (or other charge) in a uniform electric field, given by qg ,where q is the charge. This vector force always lies along the direction of the applied field, so the displacement and polarization fields are collinear with the applied electric field, and the dielectric tensor in Eq. 32 can be effectively replaced by the previously described scalar dielectric constant 2. The form of the dielectrictensor in the presence of magnetization can be deduced from simple symmetry considerations. Neumann's principle states that any physical property of a crystal must exhibit at least the symmetry group of the crystal Thus, the dielectric tensor for a given material must be invariant under symmetry operations which leave the crystal structure of the material unchanged, and the symmetry group of the crystal can be used to deduce the symmetry of the tensors describing physical properties of the crystal. If we consider a cubic material, the dielectric tensor must be diagonal and the three diagonal elements must be equal, as this is the only form of the dielectric tensor which remains invariant under rotations by 90' and 180' about each of the Cartesian axes. This is the case described in the previous section, where the dielectric tensor may be replaced by a single dielectric constant, equal to the square of the index of refraction. If a magnetic field exists in the crystal, however, the cubic (or other symmetry) is broken, and at most cylindrical symmetry about the axis of the

Materials Characterization 385 magnetic field will exist. In this case, assuming the field to lie along the zaxis and applying rotations about the z-axis leads to the following form for the dielectric tensor:

[."I = Eq.(33)

where the Voigt parameter 0 is defined in Eq. 29. Also, invariance under inversion of the magnetic field requires the diagonal elements of the dielectric tensor to be even functions of the magnetic field and the offdiagonal elements to be odd functions of the magnetic field. We can then expand these elements in power series:

,2 Eq.(34)

= Z&(l+aA42

+pM4 + . . .)

Zv = Z.(yM+
These series fall off rapidly after the first term, such that the diagonal elements are effectively independent of the magnetization, while the offdiagonal elements are effectively linear in the magnetization. The Kerr effect for a bulk sample is linear in zv, and as a result will be proportional to the magnetization as well. Note that for materials with more than one magnetic sublattice, contributions to the offdiagonal dielectric tensor elements will occur from each magnetic sublattice, such that zv is generally not proportional to the total magnetization of the material. To illustrate this point, consider a ferrimagnetic material with two magnetic sublattices at its compensationtemperature. At this temperature, the sublattice magnetizations exactly cancel, such that the total sample magnetization is zero. Since the sublattices are still magnetized, however, the total Kerr effect for a bulk sample of the material will not vanish unless the contributionsto Zv from each sublattice are equal and no contributions to Zv exist which are not linear functions of one of the sublattice magnetizations. Neither of these conditions are very likely to be satisfied in any

386 MagnebOptical Data Recording material, and nonzero Kerr effects are commonly observed for ferrimagnetic materials when the total sample magnetization is zero. The existence of the off-diagonal elements in the dielectrictensor (Eq. 33) for magnetized materials (with fi along 2) has several interesting consequences. First, the polarization, displacement, and applied electric fields are no longer collinear. This can be understood in terms of the Lorentz force law as previously described. Second, the plane electromagneticwaves propagating in the material are no longer necessarily transverse waves, as there is, in general, some small component of the electric field along the directionof wave propagation. This can be seen by examiningthe Maxwell’s equation V .d = V .[.“],I? = o (assuming no free charges in the material). When the dielectricresponse is isotropic, the dielectric constant can be taken to the left of the gradient symbol, with the result ? I? = 0 , which is the transversality condition. However, when the dielectric tensor has the form shown in Eq. 33, we find that 9 * f 0 , and wave is not, in general, transverse. At this point, we have seen that the existence of magnetization in an otherwise isotropic material leads to a dielectric tensor for the material which is not diagonal. The next step would seem to be to solve Maxwell’s equations for propagating plane waves in such a material. Fortunately, for the normal incidence case, we can deduce the effects of the off-diagonal elements of the dielectric tensor in a much simpler manner. Consider the case of isotropic materials in the absence of magnetization. The dielectric tensor has three nonzero diagonal elements,all of which are equal. Since the tensor is diagonal, the three principle axes are simply the x, y, and z axes, and the three eigenpoZarizations (allowed polarizations) in the medium are linearly polarized waves with the electric field vector lying along the x-, y-, or z- directions. Each of the three eigenpolarizations propagates in the material with an index of refraction equal to the square root of the corresponding diagonal element of the dielectric tensor. Since the material is isotropic, all three principle indices are equal, and a plane wave will propagate identically regardless of the direction in which it propagates. When a magnetic field is present, this is not the case. If we diagonalize the dielectric tensor (Eq. 33), we find that the principle axes correspond to rightcircularly polarized light, leftcircularly polarized light, and linearly polarized light along the z-direction. If a wave of arbitrary polarization propagates in the medium, we must resolve it into its principle components, in this case its left- and right-circularlypolarized components and its z-component. Note that the left- and right-circular polarization and the z-polarization are

-

Materials Characterization 387 a complete and orthogonal basis for describing any arbitrary polarization. Now, the key difference is that each of these components propagates with a different index of refraction:[**] Eq.(35j

Eq.(37)

z-polarized:

Z O

where Zo denotes the isotropic (in the absence of magnetization) complex index of refraction. Now, if there is no magnetization, a linearly polarized beam incident upon the material will always be linearly polarized along the same direction after reflection (at normal incidence). The x- and y- polarized components of the incident beam are reflected identically as both polarizations exhibit the same index of refraction in the medium, so the relative amplitude and phase of the x- and y- components is preserved and the reflected wave has the same polarization as the incident wave. When the material is magnetized, this is not the case. The incident linear polarization must be resolved into its (equal) left- and right-circularly polarized components, and each component obeys the Fresnel reflection equation appropriate for its principle index. The right-and lefi-circularly polarized reflected components are given by:

Eq. (38)

Eq. (39)

As a result, if Z+ # Z- ,the amplitude and phase of each component is altered differently upon reflection, and the resulting beam does not have the linear polarization of the incident beam. The reflected beam is instead ellipticallypolarized, with the major axis of the polarization ellipse slightly rotated from the original linear polarization axis. Thus the effect can be

388 Magneto-Optical Data Recording considered to be a rotation of the polarization state of the incident beam, with a slight ellipticityinduced as well. Rotation of the polarizationaxis and induced ellipticityare also observed when a linearly polarized beam of light is transmitted through a magnetized material and there is some component of the magnetization along the direction of propagation of the beam. This is known as the Faraday effect, and its physical origins are identical to those of the Kerr effect. In the next section, means of measuring the Kerr and Faraday effects are discussed.

3.2 Magneto-Optical Measurements There are many types of MO measurements which are commonly performed to characterize materials. The type of magneto-optical measurement performed generally depends on the purpose of the experiment. Kerr rotation measurements at a single wavelength are often performed to determine whether a material exhibits significant magneto-optical activity (see Ref. 29 for example). Also, the Kerr rotation can be measured at a single wavelength as a function of the applied magnetic field in order to determine the magnetic hysteresis loop for the material, as shown for a series of Co/Pt multilayers in Fig. 9.16JThese loops are particularly interesting in that they demonstrate the strong dependence of the magnetic properties of these multilayers on the thickness of the magnetic layer (cobalt, in this case). A schematic of a typical apparatus for the measurement of the polar Kerr effect is shown in Fig. In this experiment, a monochromatic beam of light first passes through a polarizer such that the resulting linearly polarized beam strikes the sample at near-normal incidence. The beam usually passes through a hole bored in the pole face of a magnet as shown, although Helmholtz coils are sometimes used instead to provide a magnetic field in the normal orientation at the sample. The magnet must be designed such that the applied magnetic field is uniform over the area of the sample which is illuminated by the incident beam. The reflected beam passes through a Faraday cell, another polarizer and then strikes the detector. The fast axis of the second polarizer is usually fixed at nearly 90’ away from the fast axis of the input polarizer, and the polarization of the reflected beam is modulated such that synchronous detection can be used to improve the signal-to-noise ratio of the measured detector signal, which is proportional to the Kerr rotation. Insertion of a quarter-wave plate (“optical phase shifter” in Fig. 9)

Materials Characterization 389 into the path of the beam yields a detector signal which is proportional to the Kerr ellipticity. Note that a similar apparatus can be used for measurement of the Faraday effect.

0.4

0.0'

0.4

/--

-0.4

0.0

17E

-0.4-

0.4.

0.0

J-J-' -a

a

Co(7)/Pt(lO)xl8

0.4

0.0

0

g

-0.4

-8

0

8

Figure 9. Kerr rotation hysteresis loops for six Co(m)/Pyn)xN multilayer samples evaporated on glass substrates, where rn and n are layer thicknesses in angstroms and N is the total number of bilayers. The x axes represent the applied magnetic field, in kG and they axes represent measured Kerr rotations, in degrees.F61

390 Magneto-Optical Data Recording

Audio

Lock -in

osc

Amplifier

Figure 10. Experimental apparatus for measurement of the Kerr rotation and ellipticity as functions of wavelength, applied magnetic field, and

A second approach is to rotate the second polarizer in Fig. 9 continuously. In this case, the MO measurement apparatus is identical to a rotating analyzer ellipsometer operating at near normal incidence (assuming the polarization or amplitude of the beam are not modulated). When the Fourier coefficients of the detector signal are measured (aand pin Eq. 7), the Kenrotation exhibited by the sample is given by Ref. 3 1:

and, when a quarter-wave compensator is inserted into the beam path the ellipticity is given by the measured Fourier coefficients as

Eq.(41)

Materials Characterization 391 Spectral measurements of the Kerr rotation are often used to locate optimal wavelengths for use of a material as an MO media. Such measurements are sometimes misleading as the performance of the material in an MO disk will depend on its optical and MO constants, which represent four As a result, to accurately predict the parameters of the performance of a material in an optimized disk structure, two optical constants and two MO constants for the material must be known. The Kerr rotation is a function of all four constants, but knowledge of the Kerr rotation (one parameter) is not sufficient to determine all four MO and optical constants. Measurements of the Kerr rotation and ellipticity can be used to calculate the MO constants of a material if the optical constants of the material are known. This is the case even for multilayers, provided there is only one magnetic film (or substrate) in the structure and that the layer thicknesses and optical constants of all materials in the structure are known. This is the basis of most combined optical and MO studies of materials: the optical model for the given sample is usually derived first, and then the MO constants of the magnetic film in the sample are calculated from the known optical model and the measured Kerr effect (or Faraday effect). It is possible to simultaneouslyperform optical and MO characterizations of a sample by adding a magnet to a variable angle of incidence ellipsometer, such that magnetic fields can be applied to the sample in the normal direction. A standard ellipsometer can be used for this experiment by measuring \y and S with the sample magnetization saturated in both direction~.[~*l-[~~] The observed differences in the ellipsometricparameters due to the change of magnetization direction are manifestations of the Kerr rotation and ellipticity of the sample. The ellipsometricparameters can be measured at both magnetization orientationsas functions of wavelength and angle of incidence, and these measurements provide both optical and MO data which are equivalent to both standard ellipsometric and Kerr effect data. This approach suffers from low signal-to noise for the MO data, however, as the observed differences in the ellipsometric parameters when the sample magnetktion is reversed are usually quite small, and often do not exceed the accuracy of the ellipsometer. Perhaps the most powerful instrument for the simultaneous optical and MO characterization of materials has been developed by Ruane, Mansuripur, and co-worker~.[~~l-[~~] This instrument is similar to a variable angle of incidence ellipsometer, but relies on the positioning of quarter- and half-wave retarders to provide seven different measurable quantities which

392 Magneto-Optical Data Recording are functions of the diagonal and offdiagonal pseudo-Fresnel coefficients of the sample. These seven quantities are measured as a function of angle of incidence and are functionsof the pseudo-Fresnel coefficients of the sample, and provide essentially the same information as both variable angle of incidence ellipsometry and variable angle of incidence Kerr effect (or Faraday effect) measurements. This measurement can also be performed spectroscopicallyprovided retarders appropriateto the measurement wavelengths are employed, and has the advantage of providing the most optical and MO infbrmationabout a sample of any singleexperiment. Results obtained for materials using this technique are presented in Sec.3.5.

3.3 Phenomenology of the Polar Kerr and Faraday Effects There are a number of observable magneto-optical effects, but the most important for storage applications are the Faraday and polar Kerr effects. The Faraday effect is manifested as an induced rotation and ellipticity of the plane of polarization of a beam of light when propagating through a magnetized material. There must be a nonzero projection of the magnetization along the direction of propagation of the beam for this effect to occur. There are three observable Kerr effects (polar, longitudinal, and transverse), depending on the orientation of the sample magnetization with respect to the direction in which the light is propagating. The polar Kerr effect occurs when the projection of the sample magnetization along the normal to the reflecting surface is nonzero, and is by far the largest effect of the three. The polar Kerr effect occurs at all angles of incidence, but is usually measured at normal or near-normal (
Materials Characterization 393

Reflected

Incident

Figure 11. The incident and reflected polarization states of a beam of linearly polarized light which reflects from a magnetooptically active sample at normal incidence. The rotation of the major axes of the reflected polarization ellipse away from the axis of the incident linear polarization is the Kerr rotation angle k,and the ratio of the minor to major axes of the reflected polarization ellipse is the Kerr ellipticity k.

The Kerr ellipticity 7is defined as the ratio of the minor to major axes of the polarization ellipse. Both the Kerr rotation and ellipticitychange sign when the direction of magnetization in the material is reversed, hence this effect can be used to discern the direction of magnetization at a point in the material, which naturally leads to the application of sensing magnetic domains in an optical data storage system. The complex polar Kerr effect is then normally defined as

The polar Kerr effect can be described phenomenologicallyby the use of Jones matrices, defined in the previous section. The effect of the dielectric tensor anisotropy is to induce off-diagonal elements into both the

394 Magneto-Optical Data Recording reflection and transmission Jones matrices, such that for a single interface, the reflection and transmission Jones matrices are now:

Note that the Jones matrices for reflection and transmission are no longer diagonal. The offdiagonal terms and V are odd hnctions of the magnetization, usually taken to be linear in It is these small offdiagonal componentswhich allow energy to be transferred from the incident linear polarization to the perpendicular one upon reflection or transmission by the sample. We can,by means of basic geometry, calculate the Kenrotation and ellipticity in terms of the matrix elements in Eq. 43.

a.

which is valid at normal incidence. At oblique incidence, the material will exhibit differing Ken rotations depending on whether the incident beam ispor s- polarized. Even though this result is derived for a single interface, it is actually general in that a single Jones matrix can always be found which will describe the reflection of light from a given sample, multilayered or otherwise.We describe a procedure for finding this single effective Jones matrix for multilayer systems in the next section. For the case of an optically thick sample, the diagonal elements of the Jones matrices are given in Eq. 18, and the offdiagonal elements, which like the diagonal elements may be found by enforcing the continuity of normal and tangential field components, are

Materials Characterization 395 Insertion ofthis expression into Eq. 45 yields, at normal incidence,the following well known expression for the polar Kerr effect of bulk materials:

where we have also assumed the index of refraction of the ambient medium to equal one. A similar equation for the Faraday effect observed when a linearly polarized beam passes through a slab of material of thickness d (neglecting multiple reflections) can be

where A is the wavelength of the light. Note that the Kerr and Faraday effects are linear in the Voigt parameter (or off-diagonal dielectric tensor element) which is linear in the magnetization. As a result, reversal of the direction of the sample magnetization leads to a change in sign of the observed Kerr or Faraday effects, both rotation and ellipticity. This is the principle by which domains of up and down magnetization are resolved during readout from an MO disk. Also, it can be shown from Eqs. 47 and 48 that bulk transparent materials will exhibit no Kerr rotation (only ellipticity) and no Faraday ellipticity (only rotation). Before proceeding to the calculation of optical and magneto-optical scatteringfrom multilayers, it is important to note that the Kerr (or Faraday) rotation exhibited by a material is not a fundamental physical constant of the material. Inspection of Eq. 45 shows that the Kerr effect is basically defined as a ratio of two quantities: 7 ,which is nonzero for all materials and represents the optical reflection from the sample, and which is assumed to be linear in the magnetization, but which also depends on the optical properties of the system. If we also inspect the polarkation ellipse shown in Fig. 2, we see that there are two means by which the Kerr rotation can be increased. First, the amount of energy transferred from the original linear

z,

396 Magneto-Optical Data Recording

polarization to the perpendicular direction can be increased, corresponding to an increase in E , and hence in or Ew. This is difficult in principle, as 0 and Zv are fundamental properties of the given material. Second, the optical reflection from the sample can be decreased by decreasing 7.The use of antireflectioncoatings to suppress the isotropic component in reflection can lead to very large values of the Kerr rotation, but very little light is then reflected from the sample, making detection and disk tracking control very difficult. A useful figure of merit for the case in which antireflection coatings are used to maximize the Kerr signal from a disk structure

Note the direct physical significance of this figure of merit. The magnitude of the offdiagonal dielectric tensor element is indicative of the amount of energy the material can transfer from the original linear polarization to the perpendicular polarization. It is this perpendicular component which is detected by disk readout systems. The denominator of Eq. 49 is the absorptive part of the dielectric function, such that lower absorption leads to better performanceby the material. This is partly due to the fact that low absorption means that the photons in the readout beam will generally interact with more of the MO storage material. It would seem that transparent materials would offer the best performance, however difficulties in heating transparent films with the writing laser and in controlling the microcrystallinity of transparent magnetic materials (c.f., garnets) have prevented their use as commercial MO media to date. Values of this figure of merit are tabulated for several materials in Table 4. Also, the Kerr rotation and reflectance calculated for bulk samples of the material, along with the signal-to-noise ratio calculated by McDaniel et. al.[l9]for optimized disk structures based on thin films of the materials are shown. Note that the SNR values calculated for the garnet structures were penalized 4.7 dB for media noise due to the grain structure of the films. Note also that the garnet material is transparent and exhibits a very small Kerr rotation as expected. Correspondingly,the Faraday ellipticity is expected to be very small and the Faraday rotation nonzero for the garnet.

Materials Characterization 39 7 Table 4. FOM and Kerr Effect Comparison of Materials.

Material

A(m)

FOM

ek

R

x 100

Re,

SNR

x 100

dB

TbFeCo

450 800

1.89 2.8 1

-0.17 -0.27

0.56 0.62

9.52 16.74

23.11 24.37

corn

450 800

2.2 1 2.07

-0.21 -0.13

0.66 0.76

13.86 9.88

23.37 22.45

Garnet

450 800

9.58

0.50 -0.01

0.22 0.17

11.00 0.17

22.82 22.24

Singular

3.4 Multilayer Formalism for Optical and Magneto-Optical Calculations In order to characterize MO films with protective coatings or model multilayer MO structures it is necessary to develop a model for both the optical and MO response of multilayers. There are a number of multilayer formalisms for MO calculations in the literature, and all yield equivalent results. The 4x4 characteristic matrix approach of Lissberger, Gamble, Keay, and Parker[391-[421 is reviewed here; it provides a straightforward means for simultaneously calculating both the optical and magneto-optical response of multilayers. This method is based on solving the wave equation for the electric field of the light beam:

where b = [E ] E . The solution of this equation with the dielectric tensor as given in Eq.33 is reviewed in detail in Refs. 39-42, so we present only the main results here. It is possible to define a 4-vector of tangential field components (required to be continuous at the interfaces in the system) such

398 Magneto-Optical Data Recording that a single 4 x 4 transfer matrix can be found which relates the tangential field components at the top of a film to those at the bottom of the film. Mathematically, we have, for a single layer: Q. (Sij

1.-

(prop

\

r-11.-

= [C J(4m.n

)

"

Eq. (52)

EY

Hx

and

- iMs

2R S = -n

a

-cos4d

LisSQ 2

f Q N ( c S - S)

is N

C

Materials Characteiization 399 where d is the thickness of the film, given in the same units as the wavelength A, F is the permittivity of the material, and 4 is the complex angle of propagation in the given layer which may be calculated from Snell's law (Eq. 17). Given the complex index of refraction, Voigt parameter, and thickness of a given film (or the complex diagonal and off-diagonal dielectric tensor elements and the film thickness) all elements of this characteristic matrix for the film can be easily calculated. The total characteristicmatrix for a multilayer can be calculated by finding the Characteristic matrix for each layer and the substrate and multiplying them together in sequence:

From the total characteristic matrix for the system, the effective 2x2 Jones matrix which describes the reflection ofp- and s-polarized light from the sample can be calculated. This matrix relates the incident p- and selectric field components to the reflectedp- and s-polarized components: reflected

Eq. (58)

The pseudo-Fresnel coefficients R and K are given in terms of the elements of the total characteristic matrix for the sample as follows:

Eq.(59)

-

R, =

Ap-Dp AP 'DP

-

B,-F, R, =B, + F ,

400 Magneto-Optical Data Recording

B, =-(C13 1

-%),

CQSA

C 3 1 ~ ~+-'32) 4, , m,

)

C 4 1 ~ ~+-+ " , , m S

Note that the subscript 0 refers to the ambient medium (usually air) and the subscript s refers to the substrate. Many important experimentally measurable quantities can then be calculated from the pseudo-Fresnel coefficients:

Materials Characterization 401 Ellipsometric parameters, neglecting magneto-optical effects:[l21

Ellipsometric parameters, including magneto-optical Use Eqs. 8 and 9, with the Fourier coefficients given by

P=

S e ( R p r ) c o s 2P + Se(ii,X')sin2 P + Se(iT,R:)sinPcosP

P+~R~I'

l ~ ~ l ~ c o s ~sin2P + Z [ S ~ ( R ~ R '~) + e(ii,~)]sin~cos~

where P is the polarizer azimuthal angle with respect to the plane of incidence and we assume zp= zs. Intensity reflectance, normal incidence:

Kerr rotation and ellipticity, normal incidence:

Note that Eq. 73 expresses the Kerr rotation and ellipticity of a multilayer as the ratio of two quantities, the pseudoPresne1coefficients and z. The square of the magnitude of the complex Kerr effect is then given by the ratio of the anisotropic reflectance to the isotropic reflectance of the sample:

402 Magneto-Optical Data Recording

There are obviously two factors which determine the size of the Kerr effect for a given sample. First, the magnitude of the Kerr effect will scale linearly with the magnitude of the offdiagonal reflectance pseudo-Fresnel coefficient ?l ,which is, in general, a function of the layered structure of the sample as we!! as the optica! and magletosptica! constants ofthe materials in the sample. The magnitude of the Kerr effect is inversely proportional to the magnitude of the diagonal pseudo-Fresnel coefficients, which at normal incidence equal the square root of the ratio of the intensity of the reflected beamto that of the incident beam. We then expect the Kerr effect to become very large when the sample reflectance is very low. This effect is entirely independent of the MO constants of the materials in the structure, and depends only on the layer thicknesses and optical constants of the materials in the structure. This method of enhancement is used in the current commercially available disk structures via antireflecting coatings, and has also been demonstrated in thin films of MO materials on thick metal films which have sharp reflectance edges at their plasma energy. Above the plasma energy, the reflectance of the metal film drops to near zero, and the corresponding sample reflectance exhibits a minimum. This leads to a much larger Kerr effect for the film/metal system than would otherwise be observed for the MO film. This enhancement can be modeled accurately with the multilayer formalism presented in this section. Unfortunately, although arbitrarily large Kerr effects can be achieved by depositing antireflecting coatings on MO materials, these large effects are accompanied by near-zero optical reflectance, such that the reflected beam is very weak. This is troublesome in recording applications as the signal-to-noise ratio for readout from a MO disk drops with decreasing reflected intensity from the disk. Optimal design of MO disk structures involves a trade-off between the reflectance and Kerr rotation of the multilayered structure, such that the general goal is to maximize quantities such as R e , or &, subject to the condition that very small values of R will cause tracking difficulties. The interested reader is referred to Ch. 10 by McDaniel in this work for a more complete discussion. The formalism presented to this point allows the calculation of many experimentallymeasurableoptical (reflectance, transmittance, ellipsometric parameters) and magneto-optical (Kerr effect, Faraday effect) properties. The ability to accurately model multilayer structures allows measured data to be fit in order to determinethe physical structure of the sample and/or the optical and magneto-optical properties of the materials in the sample.

Materials Characterization 403 3.5

Examples of Magneto-Optical Characterization of Materials.

The literature abounds with measurements of the Kerr rotation of different materials at single wavelengths (often 632.8 nm). Measurements of the both the Kerr rotation and ellipticity for materials at single wavelengths are somewhat scarcer, and spectroscopic Kerr rotation and ellipticity data for materials are uncommon. Calculation of the MO constants from the Ken rotation and ellipticityof a given material requires knowledge of the optical constants of the material as well, and as a result data for the MO constants of materials are somewhat rare. The MO constants are required for modeling MO films in multilayers and for study of the electronic processes leading to the observed MO effects, but are very difficult to accurately determine. As a result, the Kerr rotation is most often measured and used as a means of identifjmg materials which are potential MO recording candidates.[431-[531 A good example of the use of Kerr rotation and ellipticity measurements for the systematic study of materials is the work of Buschow et. a1.[291[541 on intermetallic compounds. They fabricated all known magnetic intermetallic compounds and measured their room-temperature polar Kerr rotation at 633 nm and 830 nm and saturation magnetization. Table 5 contains the results of these measurements on several iron-based intermetallic compounds. Table 5. Kerr Rotation of Fe-Based Intermetallic Compounds

Material

Kerr Rotation, 633 nm

Kerr Rotation, 830 nm

Saturation Magnetization

(h2W Fe3Al Fe,Ga, Fe3Ga4 Fe3C FqSi Fe& F%B FeB FeSn FeV

-0.39 -0.45 -0.16 -0.2 1 -0.32 -0.17 -0.45

-0.26 -0.03 0.00

-0.48 -0.56 -0.19 -0.24 -0.36 -0.25 -0.47 -0.22 -0.03 0.00

156 147 24 100 138 76 166 84 0.6 0.3

404 Magneto-Optical Data Recording Spectroscopic measurements of both the polar Kerr rotation and ellipticity were then undertaken on the compounds which showed large rotations at either wavelength. Figure 12 shows the polar Kerr rotation obtained for the Fe/Sn compounds listed in Table 5 , with the spectrum for bulk iron shown for comparison. The measured Ken ellipticity is also shown for FeSn. 0.04

0.02

- '\ \

'. .. \-

0 -0.02

Fe Sn

-\

-

---* -99

n

B

z 5

-Oa20 -0.40

-0.60 0 -0.02

-0.04 -0.06

-0.80

1

I

1.o

1

I

2.0

I

1

3.0

I

I

4.0

I

5 .o

Energy (eV) Figure 12. Measured K e n rotation spectra for bulk Fe and several FelSn compounds, from Ref. 29. The Kerr ellipticity is shown as the dashed line for FeSn.

Materials Characterization 405 Buschow and coworkers compared measured spectra for a large number of intermetallic compounds to study systematic trends associated with the constituents and stoichiometry of the compounds, and were able to draw useful conclusions as to the electronic processes associated with the variations in the Kerr rotation observed for different compounds. Many authors have used spectroscopic Kerr measurements to study MO absorption processes in various m a t e r i a l ~ , [ ~ ~but l - [ ~such ~ ] study is dangerous unless optical characterization of the same materials is performed such that the diagonal and off-diagonal components of the dielectric tensor can be separated. The Kerr effect can show spectral features which are not related to magneto-optical electronic transitions, as it is a function of both the optical and MO constants for a given material. The off-diagonal dielectrictensor elements can be interpreted in terms of electronic structure as they can be shown to be simple functions of the difference in the dipole matrix elements for right- and lefi-circularly polarized summed over all possible initial and final electronic states.

3.6 Examples of Combined Optical and Magneto-Optical Characterization. The simplest combined optical and magneto-optical characterization of a sample constitutes the measurement of its Kerr rotation and reflectivity, both at normal incidence, and usually at a single wavelength. Unfortunately, while simple figures of merit based on the product of the Kerr rotation and the reflectance are useful for rough comparisons of the ability of a material to h c t i o n as an MO storage layer, realistic comparison of different materials requires the evaluation of a more accurate FOM appropriate for multilayer recording structures, such as Eq. 49 or the more sophisticated merit functions described in the chapter in this work by McDaniel (Ch. 10). Any combination of at least two independent optical measurements and two independent MO measurements is sufficient to determine both the optical and MO constants of a bulk sample of any material. They are also sufficient for the determination of optical constants of the substrate or any one film in a multilayer if all other optical and MO constants and layer thicknesses are known for the multilayer. Reflectance and transmittance measured at normal incidence constitute exactly two measured parameters

406 Magneto-Optical Data Recording such that all other optical parameters for the sample must be known in order to calculate the optical constants of the film under study. Once the optical constants are calculated from the reflectance and transmittance data, Kerr or Faraday rotation and ellipticity data are used to calculate the MO constants. An interesting variant of this approach is found in the MO work of ReimI3O1 and optical measurements of S c h o e n e ~ [ on ~ ~ ]uranium monochalcogenides and pnictides. Schoenesused measurements of the ratio ofp- to s-reflectance at two oblique angles of incidence to determine the optical constants n and k of single crystals of US, Use, W e , UAs, and USb. Reim then used these optical constants with measurements of the Kerr rotation and ellipticity to determine the elements of the optical conductivity tensor, related to the dielectric tensor elements as:

Eq. (75) Figure 13 shows the measured Kerr rotation and ellipticity of Use at 15 K in an applied field of 4 T. Figure 14 shows the spectra for the offdiagonal optical conductivity tensor element calculated from the Kerr measurements shown in Fig. 13 and the optical constants measured by Schoenes for this material. Note the lack of correspondence between spectral features in the Kerr data and spectral features in the offdiagonal conductivity tensor element. In this case, the optical constants show considerable dispersion, and analysis of the Kerr rotation and ellipticity without eliminating the effects of the optical constants could be misleading. Variable angle of incidence measurements yield multiple measured data points at each wavelength, and for multilayered samples the dependence of the measured data on the angle of incidencecan depend strongly on both the thicknesses and optical constants of the materials in the sample. Variable angle of incidence ellipsometry was demonstrated in Sec. 2.5 to provide the ability to characterize multilayers, and these measurements have been used in conjunction with Kerr rotation and ellipticity or Faraday rotation and ellipticity meas~rernents[~l-[~[~~] to provide complete optical and magneto-optical characterizationof a given material. It is also useful to combine variable angle measurementswith reflectance and/or transmittance measurements for the same sample.

Materials Characterization 40 7

1

2

3

4

5

Photon Energy [eV] Figure 13. Measured Kerr rotation and ellipticity spectra for

1

2

3

4

Photon Energy

5

(&I

Figure 14. Offdiagonal optical conductivity tensor element spectra calculated from the data in Fig. 13 and the measured spectra of the optical constants of USe.[S0I Postulated free electron contributions to these spectra are shown as the solid (real part) and dashed (imaginq part) fines.

408 Magneto-Optical Data Recording Spectroscopicreflectanceand variable angle of incidence ellipsometric measurements were simultaneously analyzed by the auth0rC51to determine the thickness and optical constants of thin TbFeCo t l m s deposited between silicon nitride dielectric layers. Figure 15 shows the structure which was analyzed.

Silicon Nitride

26 nm

TbFeCo

10-80 nm

Silicon Nitride

40 nm

Silicon Wafer

Substrate

Figure 15. Nominal sample structure for TbFeCo thin films characterized in trilayer structures between silicon nitride dielectric films.[51

The optical constants of the silicon wafer were evaluated from a bare substrate, and several single films of silicon nitride on silicon wafers were characterized by variable angle ellipsometry in order to determinethe nitride optical constants. The bottom nitride layer thickness and the TbFeCo layer thicknesses were held fixed at their nominal values in the analysis, as unique solutions which included these parameters as variables could not be obtained. Even with these parameters fixed, it is not possible to find a unique best-fit solution for the top nitride thickness and TbFeCo optical constants from ellipsometric data alone, as these parameters are very strongly statistically correlated. When reflectance and ellipsometric data were simultaneously fit, unique solutions were obtained, and the optical constants of the TbFeCo thin films were found not to depend significantly on the TbFeCo film thickness, as shown in Fig. 16. The Kerr rotation and ellipticity were also measured for these samples, and since the optical model was determined for the samples from the combined reflectance and ellipsometry fits, the MO constants were directly calculated fmmthe measured Kerr data,with the results shown in Figs. 17 and 18.

TbFeCo Optical Constants

1.oo 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 WaveI en gth (An gsttom s) Figure 16. Optical constants obtained from fitting ellipsometric and reflectance data simultaneously to determinethe top dielectric layer thickness and TbFeCo optical constants for samples based on the structure in Fig. 15.[51 The error bars on this figure represent the spread in optical constants obtained for six samples with TbFeCo films ranging from 100-800 angstroms thick.

3 0'

2 Y,

TbFeCo Real Part of t h e Voigt Parameter -0.01 O r 4

-0.01 2

b

P 6 -0.020 -0.022

o -0.024 -0.026 -0.028

A

0

100 Angstroms 300 Angstroms 500 Angstroms 800 Angstroms

-

Figure 17. Real part of the Voigt parameter calculated from saturationKerr rotationand ellipticity measurements on four TbFeCo thin optical model for the multilayer structures was determined from modeling of ellipsometric and reflectance data.

The

TbFeCo

Imaginary Part of the Voigt Parameter

0.01 0

0.008 0.006

i

0*004[7

0

0 0 A

100 Angstroms 300 Angstroms 500 800 Angstroms

0.002

-o.oooh\

-0.002

-0.004

-0.006

-0.008 -

Figure 18. Imaginary part of the Voigt parameter calculated from saturation Ken- rotation and ellipticity measurements on four TbFco thin

412 Magneto-Optical Data Recording The magneto-optical ellipsometer developed by Ruane, Mansuripur, and coworkers which was described previously has also been employed to estimate the diagonal and offdiagonal elements of the dielectric tensor of optically thick samples, but could potentially be analyzed in a manner similar to that commonly used for ellipsometric data in order to characterize multilayered samples as well. Figure 19 shows experimental data for seven quantities based on the sample pseudo-Fresnel coefficients as fhctions of angle of incidence. The experimental quantities shown as curves 1-7 are defined as follows:

curve

Quantity

I 2

In the above table, Rp and R, are the complex pseudo-Fresnel reflection coefficients of the sample, and K is the complex offdiagonal pseudoFresnel reflection coefficient of the sample. The solid and dashed lines in Fig. 19 were calculated using the estimated value of the dielectric tensor for the Co/Pd multilayer under study. Note that there is considerablenoise in the MO data (curves 4-7), but the acquisitionof variable angle of incidence data allows the overdeterminationof the optical and MO constants such that reasonably accurate results can be expected from this technique.

Materials Characterization 413

\

'4 D

0

Figure 19. Combined optical and MO dataP1 Seven quantities are measured versus angle of incidence, all functions of the diagonal and off-diagonal psebFresne1 coeficients of the sample. C w e s 1-3 depend only on the diagonal coefficients, and 4-7 depend on both the diagonal and offdiagonal coeficients. See text for definition of the individual curves.

414 Magnetcl-Optical Data Recording 4.0

THERMAL CHARACTERIZATION

The thermal properties of the materials used in multilayer MO storage systems are of great interest, as the rate of diffusion of heat away from a locally heated spot on the disk is critical for the success of the thermomagnetic recording process. This process can be modeled, and sophisticated disk optimization approaches have been developed (see Ch. 10) which simultaneously optimize the readout (optical and magneto-optical) and recording (thermal) performance of the structure. Unfortunately, while general formalisms exist for the thermal modeling of multilayer structures, the actual measurement of the thermal properties of thin films is very difficult. Required parameters for modeling thermal conduction in a material are the thermal conductivity K and the thermal diffisivity a! of the material. The thermal diffisivity equals the thermal conductivity divided by the product of the mass density and heat capacity of the material, such that measurement of either the thermal conductivity or diffisivity is sufficient for the thermal characterization of a material if the mass density and heat capacity of the material are known. The thermal conductivity of thin films of many materials have been found to differ by orders of magnitude from the thermal conductivity of bulk samples of the same m a t e ~ i a l , [ ~such ~ - ~that ~ ] any usefbl measurement of the thermal conductivity of a material to be used in a MO recording structure should be performed on a film of roughly the same thickness as the film to be used in the MO disk. The thermal conductivity of thin films is also known to depend strongly on the microstructure of the film and its such that films of the same material deposited by different methods or under different conditions may exhibit very different thermal conductivities. The first approach to estimating the thermal conductivity of thin metallic films is based on the measurement of the electrical conductivity of the given film.The Wiedemann-Franz law then relates the thermal conductivity of a metal to its electrical conductivity (assuming all electrical and thermal conduction is due to free electrons):

Eq.(76)

Materials Characterization 415 where K is the thermal conductivity, c is the electrical conductivity, T is the temperature, kB is Boltzmann’s constant, and e is the electronic charge. The above equation also serves to define the Lorentz number L:

Eq. (77)

7c2 ’-‘ 2 L = - - ( s ) =2.45x10-* watt-ohm/deg2

3

e

Measurementsof thin-filmelectrical conductivityand the WiedemannFranz law have been used to estimate the thermal conductivity of thin films of amorphous TbFeCo[611via Eq.77. The calculated thermal conductivities were corrected for additional thermal conduction by phonons. While this approach provides a reasonable estimate for the thermal conductivity of metallic materials, a more direct measurementof the thermal conductivity is desirable, and other techniques which provide information about the thermal properties of the dielectric materials used in MO recording structures would be usekl as well. Anderson’s data for the electrical conductivity and calculated thermal conductivity of several rare earth/transition metal based amorphous films are summarized in Table 6.i6l1

Table 6. Electrical and Thermal Conductivity of RE/Iu Films

Material

Deposition Thickness (nm) Method

Electrical Thermal Conductivity Conductivity (S/cm) (W/cm K)

Gd23Co77

Planar magnetron sputtering

28

3.9~10’

0.036

~24Co,Fe68

Ion beam sputtering

51

5.3x lo3

0.049

Tb2,Co,$e,

Triode magnetron sputterins

100

6.9~10’

0.064

Gd,,Tb,,Co&e,

Triode magnetron sputtering

70

4.6~10’

0.043

416 Magneto-Optical Data Recording A second more direct approach to the measurement of the thermal properties of thin films is the thermal comparator techniq~e.[~*l[~~] In this experiment, the thermal conductivity of a filmhubstratesystem is measured by detectingthe temperature drop of a metal probe brought into contact with the surface of the film and comparing it to the temperature drnps r w r d d when the probe is brought into contact with bulk samples of materials of known thermal conductivity. The substrate thermal properties must also be known,as the measured data are modeled in order to eliminate the substrate effects and determine the thermal conductivity of the film. Some values obtained for thin films by the thermal comparator technique by Lambropoulo~[~~] are listed in Table 7. The most promising technique for the thermal characterization of thin films is the measurement of photothermally induced laser beam deflections (PTD,for photothermal deflection). The general concept of a photothermal deflection experiment is illustrated in Fig. 20.

SIDE VIEW

n

Argon-Ion Laser Acousto4j1tic

Modulator Lens on X-Y Table

Hc-Nc

Lock-in

Quadcell

TOP VIEW

Hating Beyn Foul Spot

Figure 20. Side and top views of a typical photothermal deflection

A focused high power laser heats the sample, and the thermally induced deflections of a second laser beam passing near the heated region are measured.

Materials Characterization 41 7 In a PTD experiment, a small region of the sample is heated by a focused laser beam, which is normally incident on the sample surface. The (gaussian) radius of the heating laser beam at the focal point (sample surface) is typically -30-60 microns. The sample absorbs energy from the heating laser in the form of heat, which then diffises towards the cooler regions of the sample and into the ambient medium around the sample. The heating laser beam is periodically modulated, such that the diffusion of the absorbed heat can be treated in the form of thermal waves propagation. Modulation frequencies from 5-2500 Hz are typical for systems in which the heating beam is mechanically chopped, and much higher modulation frequencies are obtained by the use of photoacoustic modulators. Some of the heat absorbed by the sample escapes into the ambient region above the sample such that thermal waves propagate in a small area above the heated portion of the sample as well. The index of refraction of the ambient changes with temperature such that any gradient in the temperature of the ambient above the sample is accompanied by a similar gradient in the index of refraction of the ambient. This temperature (or index of refraction) gradient is measured by passing another focused laser beam (the “probe” beam) above the surface of the sample, as this beam will be deflected upon propagating through a medium which exhibits a gradient in its index of refraction. This is commonly known as the mirage effect. The deflection of the probe beam is then synchronously measured in the directions parallel (tangential) and perpendicular (normal) to the sample surface as a h c t i o n of the distance between the center of the probe beam and the center of the heating beam, which is scanned across the surface of the sample. Also,the componentsof the deflection which are both in and out of phase with the modulation of the heating beam are measured. In principle, these data can then be analyzed in terms of a suitable optical and thermal model for the sample in order to determine the thermal conductivity of the sample, or of the film or films which are deposited on the sample. PTD measurementshave been used very successfullyto determinethe thermal conductivity of bulk m a t e r i a l ~ , [ ~ ~and l - [with ~ ~ ]some success for the determination of thin film thermal conductivities.[601~681-[701 The sensitivity of the PTD experiment to the thermal properties of a thin film on a thick substratedecreases rapidly with decreasing film thickness, and also depends on the degree of mismatch between the film and substrate thermal conductivities.[l81 For thinner films, most of the heat conduction takes place in the substrate, and the thermal distribution in the ambient above the sample is not strongly affected by the film.

418 Magneto-Optical Data Recording Figure 21 shows measured and calculated PTD data acquired for a 144 nm film of silver deposited on fused

-

150

Theory

Mld: W I z Low: 32kHz

100

50

0 -200

-1 00

0

100 Relative Beam Position (prn)

200

Figure 21. Measured and calculated PTD profiles for a 144 nm film of Ag on fused Only the magnitude of the transverse deflection of the beam is shown as a function of the distance between the probe and heating beams. The magnitude of the normal deflection and the phase of both deflections are measured and fit as well.

The thermal properties of a number of thin films as measured by the various techniques, including PTD, are reproduced in Table 7 from the review of WU,[~OIwhich may be consulted for a thorough discussion of physical mechanisms which effect thermal conduction in thin films. Of the three types of characterization reviewed in this chapter, thermal characterizationis by far the most difficultto perform, and the results of thermal experiments are the least meaningful for modeling purposes as the thermal properties of thin films are strongly dependent on the thickness and microstructureof the film, as would seem apparent from the followingtable.

Materials Characterization 419 Table 7. Themal Conductivity of Some Thin Films

Materia

Miragt

Deck&

-

Si02

Ti4

1 .o 0.25 0.5-2.0 7

Hfo2

Au

-

-

T G Bulk

-

-

0.17

0.25

1.18

0.45,0.61 0.1

0.128 0.42 0.76 1.05 1 .o 1.24 0.52-2.1 7

PTR‘

RiStaU‘

10.4 1.42 2.06 0.59 0.018

0.256 0.08 3.26-0.50

0.31

1.63

0.052

0.02 0.1 0.2 0.5 1 .o 2.5

25.2 31.5

Ag

0.144

345

427

Al

0.046

99.8

237

W

0.094

11.5

173

a

138 248 31 1

-

0.6 31 1 7.9 :2.0 87.8 I00

126

-

All electron beam evaporated. Free standing for on fused silica for Ristau,rn] and on silicon (all others) for Lambropoulos.[621 Photothermal reflectance (F’TR),[nl, photothermal deformation (PD),rnI, and transient thermal grating (TTG)[741techniques.

420 Magneto-Optical Data Recording 5.0 SUMMARY Optical, magneto-optical, and thermal characterization of thin films for MO recording media have been discussed. Experimental methods for the optical, MO, and thermal characterization of materials and films have been discussed, as well as the modeling of experimental data for each case. The physical constants which are determined for materials under study are useful for fundamental study of the materials as well as for accurate modeling of MO media containing them. A complete formalism for predicting the optical and MO response of arbitrary multilayers was presented, and several examples of each type of characterization were discussed.

ACKNOWLEDGMENTS The author would like to thank the NSF (Grant #DMR 8918889), NASA Lewis (Grants NAG-3-95 and NAG-3-154), and the Air Force (Grant F08630-9 1-C-0047) for financial support.

REFERENCES 1. Handbook of Optical Constants of Solids, vol. I , (E. D. Academic Press, Orlando, FL (1 985)

Palik, ed.),

2. Handbook of Optical Constants of Solids, vol. 11, (E. D. Palik, ed.), Academic Press, Orlando, FL (1 985) 3. Lichtenstein, T., Handbook of Thin Film Materials, M. S.Thesis, College of Engineering and Applied Science, University of Rochester (1979) 4. McGahan, W. A., Makovicka, T., Hale, J., and Woollam, J. A., Surface and Coatings Technology, 62:707 (1993) 5. McGahan, W. A., He, P., Chen, L. Y., Bonafede, S., Woollam, J. A., Sequeda, F.,McDaniel, T., and Do, H., J.Appl. Phys., 69(8):4568 (1991) 6. He, P.,McGahan, W. A., Woollam, J. A., Sequeda,F., McDaniel, T., and Do, H., J. Appl. Phys., 69(7):4021 (1991) 7. McGahan, W. A., Chen, L. Y., Woollam, J. A., Sequeda, F., andDo, H., Appl. Phys. Comm.,11(4):549 (1992)

Materials Characterization 421 8. McDaniel, T. W., Handbook of Magneto-Optic Data Recording, Noyes Publications, Park Ridge, NJ (1996) 9. Woollam, J. A., and Snyder, P. G., Mat. Sci. andEngr. B, 5:279 (1990) 10. Aspnes, D. E., J. Vac. Sci. Technol., 18:289 (1981) 11. Vedam, K.; Muter. Res. SOC.Bull.; 12:21 (1987)

12. Azzam, R M. A., and Bashara, N. M., Ellipsometry and Polarized Light, North Holland Publishing, Amsterdam (1977) 13. Woollam, J. A., Snyder, P. G., and Rost, M. C., Muter. Res. SOC.Proc. 93, p. 203 (1987) 14. Johs, B., Meyer, D., Cooney, G., Yao, H., Snyder, P. G., Woollam, J. A., Edwards, J., and Maracas, G., Mat. Res. SOC.,216:459 (1991) 15. Ellipsometry in theMeasurement ofsurfaces and Thin Films, (E. Pussaglia, R. R Suomberg, and J. Kruger, eds.), Natl. Bur.Std. Misc. Publ. 256, U. S. Govt. Printing Mice, Washington (1964) 16. Aspnes, D. E., Handbook of Optical Constants of Solids, vol. I, Academic Press, Orlando, FL (1985) 17. Crook, A. W., J. Opt. SOC.Amer., 38:11, 954 (1948) 18. McGahan, W. A., and Cole, K. D., J. Appl. Phys., 72(4):1362 (1992) 19. McDaniel, T. W., Sequeda,F. O., McGahan, W. A., and Woollam, J. A., Proc. of Magneto-Optical Recording International Symposium 1991, J. Magn. SOC.Jpn., Supplement S1, 15:361 (1991) 20. Brandle, H., Schoenes, J., Hulliger, F., and Reim, W., J. Magn. Magn. Mat., 83:29 (1990) 21. de Groot, R. A., Mueller, F. M., van Engen, P. G., and Buschow, K. H. J., J.Appl. Phys., 55(6):2151 (1984) 22. Hansen, P., J. Magn. Magn. Mat., 83:6 (1990) 23. Katayama,T., and Hasegawa, K., Proc. 4th Int. Con$ on Rapidly Quenched Metals, Sendai (1981) 24. Mukimov, K. M., Sharipov, S.M., and Emazarova, L. A., Phys. Stat. Sol. @) 127:K129 (1985) 25. van Engelen, P. G., de Mooij, D. B., and Buschow, K. H. J., IEEE Trans. Magn. 24:1728 (1988) 26. Schoenes, J., andReim, W., J. Less CommonMet., 112:19 (1985) 27. Burns, G., Solid State Physics, Academic Press, Orlando Fl(l985) 28. McGahan, W. A., and Woollam, J. A., Appl. Phys. Comm.,9(1&2):1 (1989) 29. Buschow, K. H. J., FerromugneticMuterials, vol. 4, (E. P. Wohlfarth and K. H. J. Buschow, eds.), Elsevier Science Publishers, Amsterdam (1988)

422 Magneto-Optical Data Recording 30. Reim, W., J. Magn. andMagn. Mat. 58:l (1986) 31. McGahan, W. A., He, P., Woollam, J. A., and Sequeda, F. O., Appl. Phys. Comm. 11(4):375 (1992) 32. McGahan, W. A., M.S. Thesis, Department of Electrical Engineering, Univ. of Nebraska-Lincoln (1989) 33. McGahan, W. A., Shan, 2. S., Massengale, A. M., Tiwald, T. E., and Woollam, J. A., J. Vac. Sci. Technol.A, 7(3):1271 (1989) 34. Ruane, M., Mansuripur, M., and Rosenvold, R, Appl. Opt., 25(12):1946 (1986) 35. Zhou, A. F., Erwin, J. K.. Brucker, C. F., and Mansuripur, M., Appl. Opt., 31(29):6280, (1992) 36. Zhou, F. L., Erwin, J. K., and Mansuripur, M., J. Appl. Phys. 695091, (1991) 37. Sokolov, A. V., Optical Properties ofMetals, Elsevier, New York (1967) 38. Mansuripur, M.,Appl. Phys. Lett., 49(1):19 (1986) 39. Gamble, R., andLissberger, P. H., J. Opt. SOC.Amer., 5:1533 (1988) 40. Parker, M. R., SPIE 31st Annual International Technical Symposium on Optical and OptoelectronicApplied Science and Engineering, San Diego CA, (August 18, 1987) 41. Lissberger, P. H., Rep. Prog. Phys. 33:197 (1970) 42. Keay, D., and Lissberger, P. H., Optica Acta, 15:373 (1968) 43. Di, G. Q., Iwata, S., Tsunashima, S., and Uchiyama, S., J. Magn. Magn. Mat., 104-107:1023 (1992) 44. Reim, W., and Weller, D., Appl. Phys. Lett., 53(24):2453 (1988) 45. Nakamura, H., Ohmi, F., Kaneko, Y., Sawada, Y., Watada, A., and Machida, H., J. Appl. Phys., 61:3346 (1987) 46. McGuire, T. R., Gambino, R. J., McElfresh, M. W., and Plaskett, T. S., IEEE Trans. Magn. 26(5):1349 (1990) 47. Hansen, P., Hartmann, M., and Witter, K., Advances in Magneto-Optics, Proc. Int. Symp. Magneto-Optics,J. Magn. SOC.Jpn. Vol. 11, Supplement S1, p. 257 (1987) 48. Choe, Y. J., Tsunashima, S.,Katayama, T., and Uchiyama, S., Advances in Magneto-Optics, Proc. Int. Symp. Magneto-Optics, J. Magn. SOC.Jpn., Vol. 11, Supplement Sl, p. 273 (1987) 49. Hansen, P., Clausen, C., Much, G., Rosenkranz, M., and Witter, K., J. Appl. Phys., 66:756 (1989) 50. Weller, D., Reim, W., and Schrijner, P., IEEE Trans. Magn., 24(6):2554 (1988)

Materials Characterization 423 51. Zeper, W. B., Greidanus, F. J. A. M., Carcia, P. F., and Fincher, C. R., J. Appl. Phys. 65:4971 (1989) 52. Katayama,T., Awano, H., and Nishihara, Y., J. Phys. SOC.Jpn., 55(8):2539 (1986) 53. Hashimoto, S., Ochiai, Y., and Aso, K., J.Appl. Phys., 67(9):4429 (1990) 54. Buschow, K.H. J., van Engen, P. G., and Jongebreur, R., J. Magn. Magn. Mat., 38:l (1983) 55. Bennett, H. S., and Stem, E. A., Phys. Rev. 137(2A):A448 (1965) 56. Schoenes, J., Phys. Rep. 66:187 (1980) 57. Nath, P., and Chopra, K. L., Thin SolidFilms, 18:29 (1973) 58. Nath, P., and Chopra, K. L., Thin Solid Films, 20:53 (1974) 59. Boiko, B. T., Pugachev, A. T., and Bratsychiv, V. M., Thin Solid Films, 17:157 (1973) 60. Wu, Z. L., Proceedings of the 1993 International Conference on Metallurgical Coatings and Thin Films, San Diego, CA (1993) 61. Anderson, R. J.,J. Appl. Phys., 67(11):6915 (1990) 62. Powell, R W., J. Sci. Instrum., 34:485 (1957) 63. Lambropoulos,J. C., Jolly, M. R., Amsden, C. A., Gilman, S. E., Sinicropi, M. J., Diakomihalis, D., and Jacobs, S.D., J.Appl. Phys., 66:4230 (1989) 64. Shaw-Klein,L. J., Burns, S.J., Kadin, A. M., and Jacobs, S.D., Supercond. Sci. Technol., 5:368 (1992) 64. Kuo, P. K., Lin, M. J., Reyes, C. B., Favro, L. D., Thomas, R. L., Kim, D.

S.,Zhang, Shu-Yi, Inglehart, L. J., Fournier, D., andBoccara, A. C., Can. J. Phys., 64:1165 (1986) 65. Anthony, T. R., Banholzer, W. F., Fleischer, J. F., Wei, Lanhua, Kuo, P. K., Thomas,R. L., and Pryor, R. W., Phys. Rev. B, 42(2):1104 (1990) 66. Salazer, A., Sanchez-Lavega, A., and Fernandez, J., J. Appl. Phys., 69(3):1216 (1991) 67. Boccara, A. C., Fournier, D., and Badoz, J., Appl. Phys. Lett., 36:130 (1980) 68. Favro, L. D., Kuo, P. K., and Thomas, R. L., Photoacoustic and Thermal Wave Phenomena in Semiconductors, (A. Mandelis, ed.), Elsevier, New York, p. 69 (1987) 69. Machlab, H., McGahan, W. A., and Woollam, J. A., Thin Solid Films, 215:103 (1992) 70. Decker, D. L., Koshigoe, L. G., and Ashley, E. I., NBSSpec. Publ., vol. 727, @ E.Bennett, I. A. H. Guenther, D. Milam, and B. E. Newman, eds.), Government Printing Office, Washington, 291 (1980)

424 Magneto-Optical Data Recording 71. Ristau, D., and Ebert, J., Appl. Opt., 254571 (1986) 72. Wu, Z. L., Reichling, M., Groenbeck, H., Fan, Z. X., Schaefer, D., and Matthias, E.,Proc. SPIE, vol. 1624, (H. E.Bennet, L. L. Chase,A. H. Guenther, B. E.Newnam, and M. J. Soileau, eds.), p. 33 I (1992) 73. Wu,Z. L., Jauregui, J., Schaefer, D., and Matthias, E.,Proc. SPIE, vol. 1624, (H. E.Bennet, L. L. Chase, A. H. Guenther, B. E.Newnam, and M. J. Soileau, eds.), p. 13 (1992) 74. Scott, M. L.,NZSTSpec. Publ., vol. 688, (H. E.Bennet, A. H. Guenther, D. Milam, andB. E.Newnam, eds.), p. 329 (1986) 75. American Institute ofphvsics Handbook, @. E. Gary, ed.),McGraw-Hill, New York (1982)

7 Writing and Erasing in Magneto-Optical Recording Jerry E. Hurst, Jr. and Terry M? McDaniel

1.0

INTRODUCTION

Thermomagneticrecordmg, magneto-optical(MO) recording, or “writing” as it is referred to more commonly, is the process whereby magnetic domains are created in a medium which has been previously magnetized in the opposite direction, i.e., “erased.” As the name implies, writing and erasing are thermally driven processes with the energy supplied by a focused laser spot and the final magnetization direction defined by an external bias field generated either by an external magnet or by exchange coupling to an “internal” magnetic layer. One then has only to specifjr the timedependent energy dose supplied to the MO medium by the laser, i.e., the write algorithm or strategy, and the (possibly) timedependent external bias field to completely define the write or erase processes. In the typical case, the bias field is not time dependent and is simply oriented in the “write” or “erase” direction as required, and one has only to specifjr the magnitude of the field. Given this scenario, it is somewhat surprising that such a topic could deserve having an entire book chapter devoted to it! However, a brief perusal of the technical and patent literature on the topic should convince the overly optimistic engineer of the difficulties associated with optimizing the write/erase strategy. In this chapter, we attempt to describe what is required 425

426 Magneto-Optical Data Recording of the write/erase strategy, how external factors such as medium velocity and code data rate affect the strategy, and provide a summary of the current state of the art and product. From the above description, it could be inferred that writing and erasing are two distinct processes which occur at different times. In fact this is true for many products based on MO media. Writing is usually referred to as a three-pass process wherein previously recorded data stored on the media is first erased, new data is then written, and finally verified by reading. Occasionally, the read process (which is carried out as essentially an ineffectual erase) is eliminated for the sake of expediency, and writing is reduced to a two-pass operation. Although it is possible to hrther improve the system performance by “hiding” the time required for erasure in the system overhead and preerasing unwanted areas of the disk, this is not suitable for real-time applications such as audio recording on M O media. Because of this distinction between erasing and writing evident in normal MO recording, both topics are initially treated separately in this chapter. Alternatively, because of the special requirements of real time applications, several MO recording technologies have been developed which eliminate this distinction by allowing for direct overwriting (DOW)of previous data. The most obvious technique is to use the optical spot to locally heat the media and thereby confine the written domain while modulating the external magnetic field to define the data pattern. This is essentially magnetic recording and may be appropriately termed laserassisted magnetic recording. In the MO storage community, this has been alternatelytermed magneticjeld modulateddrect overwrite (MFMDOW). A second approach is to take advantage of exchange coupling and create a medium which has the overwrite hnction built in. This media solution to the problem in usually termed light intensity modulation direct overwrite (LIMDOW) since the desired data pattern is defined by the optical stylus. This solution is more closely related to normal MO recording and the combined write/erase strategy is essentially a write algorithm superimposed on an erase power. As such, the separatedescriptions of writing and erasing described below can be applied to LIMDOW as well.

1.1

Requirements for the Write Strategy

Before describing the various write strategies, it is instructive to review what specificpattern@)must be recorded on the MO media and what bounds exist in choosing a write strategy to generate these patterns. The medium considered here is a rotating disk comprised of a grooved substrate,

Writing and Erasing 427 usually constructed of either injection molded plastic or glass having a patterned polymer overlayer, onto which has been deposited a series of layers, at least one of which is magnetic, that can support an isolated magnetic domain of the desired size. Although MO recording is not limited to this application, it is the most common geometry for computer data storage and illustrates most of the important details of the recording process. In order to take advantage of the key aspects of MO recording systems, namely removability and rewritability, it has been necessary to develop standards which ensure interchangeability. These standards not only define what is to be written on the medium, but can limit the range of write strategies available to the engineer. The existence of these standards places rigorous demands on the writing process, so much so that the concept of a reference drive or channel is emerging in the standardization community. This reference drive represents a minimal drive and read channel which can read data written according to the particular standard. In addition, the most recent drive standards have defined what is referred to as implementation independent mark qualify determination tests (IIMQD), which define a series of particular data patterns which must be written such that they can be read back correctly by the “standard” drive. Both of these machinations are attempts to relax the requirements on the write process while ensuring interchangeability. The underlying assumption is that it is undesirable to completely define the write process in the standard. This view excludes the possibility of correctingfor errors in the write process by, for instance, altering the read process accordingly. The write and read processes are made distinct and the engineer is obligated to design a write strategy which places the magnetic domains such that the standard channel correctly reconstructs the desired pattern. As described below, it would be possible to obviate this distinction by making the standard read channel “adaptive.” What patterns are to be written? The answer, as one may have guessed by now, is usually defined by a standard! In most MO computer data storage applicationstoday, user data is encoded according to a particular (d,k) RLL code. These codes constrainthe binary sequences which must be written to having a minimum of d+l and a maximum of k+l zeros between ones. The standard also defines the type of modulation which is used to record these coded and constrained sequences on the medium, namelypulseposition modulation (PPM) orpulse width modulation (PWM; also called mark edge recording). These are indicated in Fig. 1 . In PPM modulation, a magnetic domain is formed coincident with a one in the desired data sequence. Essentially circular domains are written on the

428 Magneta-Optical Data Recording

medium with a dmnctcr that can be optimized for other purposes and with spacings of (d+l ) h ,to (k+ 1)A, where A s Is the length of a c d d bit. Here we are interested only in the position of the magnetic domain. In contrast, in PWM the magnetizationstate of thc medium is changed coincidently with a onc in the desired data saquenoe. In this case, magnetic domains must be written and spaced by the full range of sizes (d+ 1 )Asto ( k + l ) h , , Here both the domain position and size must be correct because information i s encoded at the “edges” of domains. As should bc cvidcnt, writing PPM sequences is similar to witing isolatd marks and both are discussed below in Secs. 3 .O and 4.1. Writing PWM soquenccs is a much more difficult task and will be discussed in Scc.4.2.

Figure 1. Illustration of a PPM- and P W - m d u l a t e d (2,7) RI.1, coded data sequence (3 mark, 5 gap. 8 mark).

In addition to the mark placement (PPM)and edge placement (PWM) requirements described above, the write stratcgy must be as robust and insensitive as possible to variations in the many cxtcrnal variables which affect the write proccss and dcfine the environment of the MO medium and dnve. Examples are the change in the domain size with lascr power and external magnetic field, rcferrd to as mark lengrhpower sensitivity (MLPS) and mark length bias sensifivig (MLBS), respectively. Although these may

Writing and Erasing 429 at first glance seem to be solely media requirements, it is knownthat they are greatly affected by the choice of write and erase strategy. These “sensitivities” should not be confused with the media writing sensitivity, which is a measure of the optical power required to write the desired pattern. The former represent derivatives or slopes of the dependence fhnctions at the design point, whereas the latter represents the design point itself. Another important sensitivity parameter is the shift in the media sensitivity with erase cycling. This is the result of relaxation in the magnetic media driven by the many erase cycles that the media must be designed to handle (usually 106-107). Such relaxation is inherent in the amorphous phase which comprises all RE-TM media to one degree or another, depending on exact composition and processing. Although cycling relaxation can be eliminated by preannealing/prerelaing the media, the temperaturesand times required would damagethe polycarbonate substrate. Another way to reduce erase-cycling sensitivity changes to manageable levels is throughjudicious choice of the erase and write strategies. Erasure should be camed out to reduce the cycling exposure, and writing to “hide” the effect by making the writing less dependent on the details of the relaxation. Another expectation of the write and erase strategies is that there be a technique for customizing the various degrees of freedom present in the strategy for a particular disk sample. For instance, if a particular write strategy requires that laser power be adjusted in defining the written domain, some procedure should exist whereby the drive can optimally determine this power for a particular medium. This is necessary not only because of variations in media properties from disk to disk and manufacturer to manufacturer, but because of variations in drive ambient temperature, objective lens transmission, and any other parameter which affects the write process. The more complex the write strategy with multiple degrees of freedom, the more difficult and time consuming the “calibration” procedure. This time could ultimately impact the end user by increasing system overhead. Finally, there are those obvious constraints on the write and erase strategies resulting from limitations in the components needed to implement the technology, (i.e., limitations on available laser power, detector responsivities at the wavelength of interest, servo technology, electronics for the laser driver which define pulse width and amplitude, media reflectivities, magnetic properties, and so on). Although these constraints are very important in defining the write and erase strategies, they are

430 Magneto-Optical Data Recording dynamic, and as such are not really suitablefor the general considerationsof this chapter. Similarly, these arguments apply to the system cost, which should be minimized. One could consider write strategies which involve multiple lasers, laser drivers, detectors, heads etc. and while such strategies may solve a particular problem, they ultimately would increase cost and would be preferred only if all else failed. For instance, it has been suggested that a direct-read after write optical head (DRAW)using two laser spots could solve most write problems by having the first laser write a mark, while the second reads the mark and adjusts the writing process (not the read channel because such marks could not be read by the standard read channel unless it too was adaptable!) until the domains are formed correctly. In fact, although many patents exist describing such dynamic/adaptivewrite channels, the authors are not aware of any in existing products. It is possible that in the fbture the write channel will become dynamically adaptive and will not be calibrated as done today. Hopefblly, the concepts presented here will still be applicable in choosing the particular degrees of freedom such write channels will need to accommodate. Ultimately, write and erase strategies are integration issues. Optimally, they should incorporatea balance between system resources required for implementationagainst gains obtained in system performance by accurately placing domains of the correct size on the medium while minimizing “sensitivities”and masking deficiencies in other system components.

2.0 WRITING REGIMES AND LIMITS 2.1

The Adiabatic Writing Limit

When heating of the magneto-optic film layer@)by absorption of the laser light occurs on a time scale much shorter than that associated with cooling of the heated area by thermal diffusion, the thermomagnetic writing process is referred to as adiabatic. Here the term adiabatic has the usual meaning; heat is neither absorbed from or transferred to the surrounding media from the heated volume during the time the laser irradiation is being absorbed by the film. There is some ambiguity in this definition in that it may be possible to select some subset of film layers within which heating is confined and the temperature is rapidly equilibrated during the laser pulse time. Such a “subsystem”would appear adiabatic on the time scale of the laser pulse even though significant thermal diffusion has occurred.

Writing and Erasing 431 In practice, this picture does not pertain to MO recording using relatively short writing pulses because of the relatively large thermal diffusivities and spatial extent of all the film layers involved. Therefore, we can consider the adiabatic limit to refer to the case where heat is confined in the layer in which the light is initially absorbed. In this limiting case, the spatial profile of the incident laser light is transformed directly into a thermal profile, which ultimately defines the resulting written mark. This thermal profile is defined both in the plane of the MO film and perpendicular to it, the latter profile being determined by the optical absorption properties of the MO film. A phenomenological manifestation of the adiabatic limit is that the energy required to heat the system to the desired temperature is minimized and is independent of the laser pulse width. This criterion encompasses both scenarios described above, and could be checked experimentally since the following relationship will be obeyed:

Eq. (1)

E{Joules} = P,z, / (1- a)0 Constant

where E is the energy required to form the desired mark, P, {Watts} is the required write power, ,z {seconds} is the write pulse duration, and a is the disk reflectivity. The authors are not aware of any experimental data demonstrating Eq. (1) in the case of MO recording (see Eq. 12 below). Experimentaldata relating to short pulse writing (3 ns < ,z < 70 ns) has been given by Otsuki et. a"]and Wakabayashi et. al.[*]. In this latter work, measurements ofthe dependence of the write threshold energy on write pulse duration for commercially available MO media were carried out and are shown in Fig. 2. Although the interpretation of this data is complicated by the presence of a continuous 1.5 mW tracking power onto which was superimposed the write pulses, it is apparent that there is a strong dependence of E on .7, The observed ,z dependence is similar to a square root dependence expected for simple one-dimensional diffusion. An inflection point or region of relatively fast decrease in write threshold energy is observed around ,z = 10 ns, which was interpreted by the authors as an indication of an approach to the adiabatic limit. A more likely explanation is thatthe heating is being confined in the direction normal to the film plane to fewer layers and a smaller volume, thereby requiring less energy to heat."] It is probably not surprising that there are no experimental verifications of Eq. (1) since, as shown below, the expected pulse widths are very short indeed.

432 Magneto-Optical Data Recording

Figure 2. Dependence of the write threshold energy on write pulse duration for 830 nm recording on conventional MO media. Data is shown at various disk rotation speeds and Write pulses are superimposedon a continuous 1.5 mW tracking level. WriWerase field = f 500 Oe.

It is simple to show that writing of magnetosptical media, as it is practiced today in rotating disk applications, is far from the adiabatic limit. Consider the energy required to heat the active layer in a region defined by the optical stylus to the Curie point as given by the expression:

where 7 is the volume heat capacity (J/cm3K), Vis the volume to be heated (cm3), and AT is the temperature increase. Assuming a layer thickness of 200 A, spot size of 0.68 pm, reflectivity of 0.2, and heat capacity of 3 J/ cm3K,the estimated energy required to raise the layer temperature 25OOC to the Curie point is 27 fJK, or some 7 pJ for this example. In actuality, the energy requirement is higher because the thermal profile will be Gaussian in

Writing and Erasing 433 the short pulse limit and the peak temperature at the center of the profile will be twice that at the FWHM (full-width at half-maximum) radius. This correction amounts to a factor of = 2.1, increasingthe estimate to 14 pJ. Recent data presented by one of the authors (J.H.) indicates that about 20 mW of optical power incident on the disk is required to write marks under these conditions when using a pulse width of 10 ns and currently available quadrilayer optical media. This represents 200 pJ of energy, about a factor of 14 larger than the estimated value. Most of this “excess” energy is transported via thermal diffusion into the media layers surrounding the active layer as discussed in Sec. 3.1.

,/m

2.2

The Stationary Media Writing Limit

When the width of the laser pulse used for heating of the magnetooptical film layer@) is much shorter than that associated with the time required for the optical media to move a distance equal to the FWHM of the optical stylus, the writing process is said to occur in the stationary media limit. This is sometimes referred to as the static marking limitL31 or the LaBudde model for static The relative distance moved by the storage medium during the time the laser pulse is on is given by: R,

= VZ,/S

where v is the medium velocity. This ratio will be indicative of the velocity sensitivityof the write process. For typical velocitiesin the range 10-20 m/s, a 7 laser pulse width of 10 IS,and a FWHM ofthe optical stylus of S ~ 0 .pm, the ratio R,, is about 0.15-0.30. Although at first glance this limit may seem similar in consequenceto the adiabatic writing limit, the two are clearly different in the way in which energy is distributed. As writing moves away from the stationary media limit for a fixed write pulse width, the same energy is deposited over a larger volume of the medium and the peak temperature is reduced independently of thermal diffusion. This result is shown in Fig. 3 which illustrates the dependence of the relative on-axis energy dose,r31(i.e., energy per unit area, on the medium velocity (5-40 d s ) for pulse widths of 10 and 100 ns assuming a beam 5’of 0.78 pm). The dose function, D(x,y), is calculated from the integral:

434 Magneto-Optical Data Recording

where x andy representdistances along and perpendicularto the direction of motion, respectively, P is the laser power, v the disk velocity, and CT is relatedtothebeamFWHM S by:

Q. ( 6 )

1

.. .

0.9

. ..

0.8

~-

0.7

0.6

Bg

-

0.5

'f:

P

-

0.4

.

03

.

.

0.2

..

0.1

0

2

0

2

L 4

6

0

10

Reduced Distance

Figure 3. (a) Relative energy dose vs. reduced distance for rw = 10 ns and media velocities of5,10,15,20,25,30,35, and 40 m/s going from left to right. (b) Same as (a) but rw= 100 ns. (c) Relative mark length change vs. velocity assuming writing occurred at a relative dose of 50%. All calculations done assuming a Gaussian spot with FWHM S = 0.78 pm.

Writing and Erasing 435

4

1. z

f

Figure 3. (Contil)

436 Magneto-Optical Data Recording Here we have assumed a Gaussian intensity profile and a laser pulse that starts at t = 0 and is initially centered at (0,O). The figure represents ranges for R,, of = 0.05 e 0.50 at ,z = 10 ns and R,, = 0.50 e 5 at ,z = 100 ns. In the absence of thermal diffusion, the dosing hnction is expected to define the resulting magnetic domain. It is apparent that as R,, increases, the resulting domain is first shifted and then stretched along the direction of motion, i.e., noncircular in shape, until finally the peak temperature reached would be insufficient to form a domain in the medium. Despite this stretching,the domain would otherwise be symmetric about its center. In the presence of thermal diffusion, the symmetry of the thermal profile is broken and the mark becomes asymmetric. An interesting aspect of writing in the stationary media limit is that the velocity dependence of the written domain size is eliminated as shown in Fig. 3c. At ,z = 10 ns, there is virtually no change in apparent mark length over a wide range of velocities. This is a desired result as most MO data storage applications use constant angular velocity (CAV) drive motors because of the improved performance. This means the recording linear velocity changes with recording radius and a write strategy which eliminated the sensitivity to medium velocity could be desirable over one that required calibration or adjustment with velocity.

2.3

The Steady State Thermal Writing Limit

When the laser pulse duration is long compared to the time scale for thermal equilibration of the medium, writing occurs in the steady state thermal limit, This is sometimes called the scanned marking limit.L31 In this situation, the temperature distribution in the medium as viewed from the laser spot is stationary and asymmetric. The temperature gradient looking toward the direction of travel is much steeper than that viewed back toward where the spot has already traversed. This temperature profile is depicted in Fig. 4. The asymmetry is not a property of the dosing function D(x,y),but is caused by thermal diffusion. The dosing function for the case of steady state writing or erasing can be derived from Eq. (4) by setting ,z = 00:

Writing and Erasing 43 7

I

I

I

I

I

4

Figure 4. Steady state temperature profile generated by a continuous laser illumination calculated using a finite element model.

The on-axis dose reduces to the following in the limit of x/a >> 1: D(c0,O)= P(&m)-l

Although this would suggest that the power required for a fixed dose during continuous writing or erasing should increase in proportion to velocity, thermal diffusion alters the velocity dependence significantly. A detailed mathematical description of the case of static marking is given by Mar~hant[~] wherein a discussion is given of the relationship among the NA of the focusing objective, the FWHM of the optical stylus, and the energy required to write a long mark.

438 Magneto-Optical Data Recording 3.0 WRITING ISOLATED MARKS There are two phenomenological rules of thumb which, when used in conjunction with the considerations above, are useful in predicting the performance of different write strategies. The first involves the idea of a critical writing temperature or threshold. The assumption is made that there is a certain critical temperature, determined by the particulars of the recording medium, above which a domain magnetized parallel to the external field is formed, and below which no reverse domain is formed. The area ofthe medium which is heated above this temperature sometimeduring the laser illumination would correspondto the final written mark. Although it is tempting to liken this temperature to the Curie temperature of the material and the written mark to the area of the Curie disk, such an assumption is unnecessary and probably incorrect. Even if the initial domain were formed at the Curie radius, forces acting on the domain wall during the subsequent cooling of the media could result in motion of the domain wall. Depending on the sign of these forces, the domain could either expand or contract. Despite these effects, the concept of MO writing as a thresholding phenomena can be applied nonetheless. This idea was implicitly assumed in Sec. 2.2 above. The second general rule is, in some sense, a consequence of the picture of writing as a thresholdingphenomenon. One might expect that to the extent that writing occurs at a particular temperature, it would be advantageous to maintain a steep thermal gradient in the material at the particular location where the temperature is equal to the critical writing temperature. In so doing, one can accurately locate the domain wall and minimize the effect of local variations in the value of the critical temperature (see Eq. 18), or account for the case where the critical temperature corresponds to a range of temperatures for whatever reason (see Sec. 4.2 for further details).

3.1

Effect of Laser Power and Pulse Width

In order to understand the laser power/pulse width requirements for magneto-optical recording of isolated marks in detail, complex thermal, magnetic, and optical factors have to be considered. Some of these are detailed in Ch. 10. However, given the discussion above, consideration of the thermal diffusion problem in conjunction with the concept of a writing or threshold temperature gives some insight to the write process. Excellent

Writing and Erasing 439 treatments of the thermal diffusion problem as it relates to the typical quadrilayer disk structure used in optical storage are presented in the books by Mar~hant[~] and Mee and Danielr4]and in many contributed papers.I61 Following these analyses, the energy to write a mark of the desired size (given by Eq. 1) can be estimated by summing the energy which is used to directly heat the active layer (given by Eq.2), with that which diffises into the surroundingmedium during the laser pulse. Considerthe quadrilayer structure shown in Table 1 and writing conditions assumed in Sec. 2.1 : a mark of diameter 0.68 pm is formed by raising the temperature of the medium by 250°C. For convenience, the MO layer has been divided into two sublayers of equal thickness since the maximum temperature will be reached near the center of the MO layer. Given the volume heat capacities and thicknesses in Table 1, the energy required to heat the quadrilayer is ~ 7 pJ. 0 Assuming a 200 pJ energy requirement, we can estimate the thickness of the substrate and organic layers that would need to be heated: 0.3 pm! Alternately, assuming that no heat is lost into the substrate or organic, we can estimate the diameter of the heated spot: 1.4 pm, or twice the Gaussian FWHM and the desired mark size. Clearly the substrate and overcoat play an important role in the write process, and control of their thermal properties is required to ensure adequate mark size control and minimize the related writing noise. The film thickness which is heated by thermal diffusion during a laser pulse of duration r, can be estimated by:

Eq. (9) where A is the thickness of the heated layer and K is the thermal diffusivity. Alternately, the characteristicdiffusion time to traverse a layer of thickness A can be estimated by:

The diffusivity and heat capacity can be replaced by effective values in the case of an isothermal multilayer stack (see Ref. 3). For the case of a 0.02 pm metal alloy active layer with K 2 0.1 cm2/sand a relatively short writing pulse width of r, = 10 ns, A is estimated to be ~ 0 . 6pm, much

440 Magneto-Optical Data Recording Table 1. Thennophysical Parameters for a Typical Quadrilayer MO Disk Structure

Substrate

0.001

1.5

>O. 175

0.011

Dielectric

0.010

2.0

0.025

0.044

0.070

5

MO Layer

0.100

3.0

0.010

0.100

0.003

3

MO Layer

0. 100

3.0

0.010

0.100

0.003

3

Dielectric

0.010

2.0

0.010

0.064

0.016

2

Metal

0.100

3.0

0.050

0.091

0.135

16

organic

0.001

1.50

>0.278

0.030

10.0

10.0

27

43

thicker than the 0.02 pm active layer thickness. We can therefore assume that the entire active layer thickness is uniformly heated during the 10 11s laser pulse. In fact, using Eq. (10) to calculate the laser pulse width required to achieve thermal equilibration in the active layer gives =lo ps! Therefore considerable heat begins to flow into the dielectrics shortly after the laser pulse starts. Again using Eq. (9), heat will travel through a dielectric layer with K k: 0.01 cm2/s, a distance 0.2 pm (i.e., the dielectrics between the active layer and metal reflector and between the active layer and the substrate will be traversed in =25 ps and m150 ps, respectively). After the first 100-200 ps, the temperature rise of the system increases much more slowly as the metal reflector, which has a relatively large heat capacity and thermal conductivity, begins to heat. A metal reflector with K k: 1 cm2/swill heat to a thickness of A k: 2 pm during the remaining laser pulse and the characteristic time to heat the 0.06 pm thickness reflector considered here is

Writing and Erasing 441 9 ps! In reality the metal reflector thin film is typically doped with Cr or some other anticorrosion agent which serves to decrease K by an order of magnitude and increase the characteristictime to 90 ps, which is still very rapid. Obviously, the metal reflector and intervening dielectric thicknesses are critical parameters which determinethe thermal properties of the stack. Clearly, all of the characteristic diffision times discussed above are much shorter than the 10 ns pulse width. This implies that the quadrilayer stack acts as a tightly coupled thermal system during the pulse and can be thought of as a single film, at least in the vertical direction. Temperature gradients in the quadrilayer film stack vertical direction occur during the laser pulse only, decaying rapidly after the pulse ends. Temperature gmbents drive the flow of heat away from the directly heated MO film quadrilayerboth laterally in the quadrilayer and vertically into the substrate and organic overcoat. The discussion so far has centered on the vertical cooling of the structure, and comparing this rate to that expected for heat input into the media. There is certainly lateral diffusion occurring simultaneously which can also account for a significant increase in required energy. For instance, suppose the region heated by the laser pulse is localized to a disk shaped region with diameter equal to the optical stylus FWHM and thickness determined by arguments given above to be about 0.2 lm. The relative volume increase due to thermal diffision can be estimated from:

where Arand A, are the diffision distances laterally and vertically, and L is the effective thickness of the heated disk-shaped region. These factors express the geometric effects regarding cooling in the typical structures used today. In this case, R, = 0.3 for S = 0.7 pm, meaning the lateral cooling is about 30% of the vertical cooling. Vertical diffusion dominates during the initial stages of cooling in these structures because of the geometry of the initial heated spot. An interesting empirical formula which relates the laser power required for writing and the pulse duration can be derived from the considerations in Sec. 2.1 above. This formula

442 Magneto-Optical Data Recording

Pw=C

1.- t+ -

A

with C a constant, has appeared in every international standard for MO media interchange since the first generation.['O] The formula is found to be a good fit to experimentaltone pattern writing data over a linite range 10 ns < zw < 70 ns for a fixed velocity. Some physical insight can be gained by multiplying Eq. (12) by zw to form the energy in a rectangular pulse:

Eq. (13)

P,zw = E , = C + C&

From the discussion in Sec. 2.1 and earlier in this section, it is tempting to identie the constant term in Eq. (13) as the adiabatic component, and the second term as a diffisive term. The occurrence of the same constant C in both terms of Eq. (13) seems puzzling (see below). A fit of experimental pulse power versus

usually results in a linear plot with a nonzero Pw-intercept. This suggests that Eq. (13) might be generalized further to

Eq.(14)

P,

7,

=Ew= A + B &+

Dz,

where D becomes the nonzero P,-intercept, and the constants in the adiabaticand diffisive terms are allowed (required?) to differ. It was clear from both Eqs. (12) and (13) that a single constant C was inappropriate from dimensionality arguments. Equation (12) only works when P, is measured in mW and ,z in ns! Interestingly, it can be shown that the ratio of the constant in the diffusive term to the adiabatic term constant is approximately (l/dns) for typical M O thin film materials when simple physical expressions for each term are used. The fact remains that the successful use of a singre constant C in Eq.(12) when using a particular set of units is simply a fortuitous accident! The Dzw term in Eq. (14) can be viewed as a second-order correction to the diffusion heat loss in writing.

Writing and Erasing 443 A final observation concerningthis empiricism may be of interest. We have observed that plots of (zw,Pw)data involving fitting of P,,,versus l/drw result in a straight line with zero P,-intercept (within experimental error). Consequently,

P, zw =E,

=

B ‘ z

appears to be a valid relationship. This implies that the energy deposited in a writing pulse can be interpreted as that needed to heat an effective volume determined by a diasion length to an average writing temperature. No additional heat assignments to an adiabatic or higher order difisive loss need be invoked. Another application of Eq.(12) is to characterize media sensitivityfor writing via specification of the constant C. By placing an upper bound on allowable C values, one limits the required laser writing power, thereby controlling media sensitivity. Recent standards have seen the limit on C drop from 75 to 50. An alternate means of characterizing media sensitivity is by monitoring the threshold writing power. This power is defined as the power at which the MO signal (carrier) emerges from the background noise level as writing power is increased; that is, it defines the onset of writing.

3.2

Effect of Bias Field

Typical magnetosptical media in use today exhibit strong perpendicular anisotropy and an easy axis of magnetization perpendicular to the film plane. This is desired because the preferred read mechanism is differential detection of the polar Kerr effect. By convention, the nominal write magnetic field is also defined as being perpendicular to the film plane (*15O) and pointing from north to south when lookmg from substrate to magnetic film,thereby insuring interchangeability between different drive manufacturers. Figure 5 shows typical behavior of the MO signal (carrier) and media noise power as a function of applied bias magnetic field. With regard to MO signal, the plot illustrates that some threshold of applied field in the writing direction is necessary to aid in the formation of “clean” magnetic domains. The HB position of this transition can be shifted considerably along the horizontal axis by adjusting the MO alloy composition or multilayer magnetic The peak in medium noise power correlates with the position of “incomplete” domain It is important that the minimum magnitude of DC bias field utilized in conventional MO writing or erasing in the recording device exceed the onset of magnetization “saturation” (the base of the noise peak).

70 4 c Initial Noise 60

4 m ~ h i t ew i s e

50

+ D

carrier

40

30

20 10 0

-lo -20 -30

-40 -50

do -70 -1000

-800

600

-400

-200

0

200

400

Write B i a s (Oe)

Figure 5. Measured MO signal and noise levels versus applied bias magnetic field strength. Notice how noise level rises at the onset of the signal, indicating a regime of poor or incomplete Writing.

Writing and Erasing 445 Details concerning the role of applied bias field have been treated in the nanomagnetic dynamics simulationsof Mansuripur and Giles (see Ch. 10, Sec. 6.4). 3.3

Effect of Media Velocity

In this section we address velocity effects in MO thermomagnetic writing beyond those introduced in Sec. 2.0. Velocity effects are nicely captured in thermal recording characteristic curves discussed below. For an optical disk recording at CAV, it is advantageousto find writing methods that show the least dependence on medium linear speed. Such an algorithm would probably adhere closely to the stationary media writing limit described in Sec. 2.2 above. This would simplifLthe development of a write calibration procedure which is workable at all disk radii, or at higher values of disk angular speed. A Useful Model. A model further described in Ch. 10 has been used to generate thermal recording characteristic curves discussed below. The model is a combined finite element (FE) and Green’s function (GF) thermal conduction model. It has been used to study the evolution of thermal fields for moving beam optical recording configurations. A two-dimensional axisymmetric FE thermal impulse solution is determined through the depth of the thin film multilayer stack on an optical disk substrate. After providing the FE solver the stack geometry and the thermophysical constants for the film materials, one obtains the temperature field T(r,z,t), where (r,z) are the cylindrical coordinates (for axisymmetry) denoting a disk spatial position relative to the instantaneous laser spot center, and t is the elapsed time from the “instant” of the laser irradiation impulse, t = 0. A superposition integral of the GF yields the full field solution for a moving beam: f

E¶.(16)

Jjd.’&’P(t’) G(x-x’,y-y’,z,t-ty

T(XJ,Z,V,~) = Id’ -00

x’,y’

where P(t3 is the laser power versus time profile, and G is the GF for the medium structure. In the numerical model, this integration is approximated with a finite summation combining an input laser power versus time profile with the GF impulse solution and the linear velocity of the moving spot. Thermal Recording Characteristic Curves. The optical recording thermal response of a particular disk structure can be determined using the

446 Magneto-Optical Data Recording FE/GF thermal model as follows. One calculates the laser power of a rectangular pulse of duration % t o form a recorded mark of a fixed maximum width (cross-track direction) for a range of linear speeds. A simple temperature threshold writing mechanism is assumed. For pulses of ~ a plot like Fig. 6 is constructed. In this duration =,z 10,20,40, 8 0 , ns, case, the written mark width criterion was 0.67 pm. In Fig. 6a, the writing pulses were added on a zero power background. For Figs. 6b-e, a baseline CW power of 1-4 mW was chosen. Notice how the required pulse power using a short writing pulse (z, = 10 ns) with zero baseline power is nearly independent of disk speed as suggested in Sec. 2.0. With short pulsing, the deposition of the writing energy occurs closer to the limit of an impulse, so little spreading of the energy due to spot displacement during irradiation occurs. When the baseline power rises to 4 mW in Fig. 6e, the incremental power needed to reach the written mark width criterion rises rapidly. This reflects the fact that a 4 mW baseline power is inadequate to write the mark and a significant incremental pulse power must be added to the CW power as the speed increases.

FEM THERUN. MODEL (At-3nl)

(P,-OmW)

KLoclTy (m/4

(a) Figure6. Thermal characteristic curves for writing MO media; (0) PB= 0; (b) PB= 1mW, (c) PB= 2 mW, (d) PB= 3 mW, (e) PB= 4 mW. Power versus velocity to write a fixed width mark (0.67 pm) for various duration rectangular writing power pulses.

Writing and Erasing 447

FEU lHERMbL MODEL (At-*)

r

Figure 6. (Cont 'd)

I

(Pr-lmW) I

FRI lHERuU Y W U (At-31s) (PpZrnW)

448 Magneto-Optical Data Recording The previous discussion suggests that linear superposition arguments may be adequate to predict the curves of Fig. 6b-e given the curves of Fig. 6a. In fact, the thermal field for the media model used is linear (temperature independent thermal properties for the film stack materials), so superposition would be expected to hold. The only difficulty is that the temperature superpositionmust be done point by point over the (x,y) coordinates of the MO film plane. For a mark width writing criterion, the maximum width location x along the direction of writing spot displacement occurs at different x values for different values .,z A more successful prediction for superposition occurs if the writing criterion is simply finding the power necessary to first reach the writing temperature at any point under the beam. In this case, the peak temperature occurs on they = 0 axis at an x location only weakly dependent on z;, (only lateral heat spreading along the track centerline is involved). Figure 7 shows characteristic curves for the criteTambient at any (x,y). The superposition rion of first attaining AT = T ~ - k relation:

Eq. (17) allows calculation of the pulse power at a general nonzero bias power Pw,b using that bias power Pb and the information from Fig. 7a (the zero bias power Pw,oand the CW power for reaching the writing temperature Pcw,o). We have verified that curves computed using Eq. (17) match very closely curves computed directly with the FE/GF model in Figures 7b-e. The use of Eq. (17) to compute the writing width criterion curves of Figs. 6b-e provides the qualitative characteristics, but only fair quantitative agreement for the reason cited above. As one would expect, the approximation of Eq. (17) for Figs. 6b-e is better for low speed and long z;,,since in those limits the difference in the two-dimensionalmap (in the x-y plane) between the CW and pulsed temperature isotherms is small.

4.0 Writing and Calibrating Data Sequences 4.1

Writing PPM Sequences

Thermal Intersymbol Interference. We define thermal intersymbol interference (TISI) as a time-sequence effect whereby laser power input at t, perturbs a later writing event at tz. A simple illustration of a TISI test pattern is discussed in Ref. 11 for an isolated pattern of the two closest

Writing and Erasing 449 written PPM marks in (2,7)RLL encoding (see Fig. 8). A more recent example from the 2.6 GB 130 mm rewritable optical standard[**] is the “2646” pattern for thermal error (Eth)measurement in PWM writing. Both of these patterns in Fig. 8 are intended to expose the effect of TISI in a modulation coding setting. TISI is detrimental in thermal recording because of patterndependent perturbation of writing which causes unwanted displacement of data-bearing pattern features. These effects are sometimes called “thermal peak (edge) shift” in PPM (PWM)recording. They arise largely due to lateral heat flow from the t, event to the event. It becomes more severe at high linear density and high linear velocity due to increased spatial crowding of written features and reduced intersymbol cooling time, respectively. Ultimately, without suitable precautions, TISI can limit achievable recording density and data rates.

FEM THERMAL MOD= (At-*)

( P c O mw)

Figure 7. Thermal characteristic curves for writing MO media; (a) PB = 0; @) PB = 1 mW, (c) PB = 2 mW; (d) PB = 3 mW; (e) PB = 4 mW. Power versus velocity for medium to fust reach a fixed temperature elevation (175°C) corresponding to the onset of writing for various duration rectangular writing power pulses. Panels (b)-(e) (see nextpage) could be calculated from (a) with a formula based on superposition (Eq. 17).

450 Magneto-Optical Data Recording

Figure 7. (Cont’d)

Writing and Erasing 451 WRITE POWER PROFILE I

'

0

I

I

I

100

I

200

I

I

300

TIME (ns)

CUMULATNE AT CONTOURS (MARK=1750; A=250)

Figure 8. Thermal intersymbol interference (TISI) illustration; v = 18.3 d s . (a)-@) Pulse power vs. time profile and resultant mark pattern (...2Tm,6Ts,4Tm,6Ts....),s = space, m = mark, for PWM TISI test. (c)-(d) Power vs. time profile and resultant mark pattern (....6Ts,3Ts,6Ts,....) for PPM TISI test.

452 Magneto-Optical Data Recording

WRITE POWER PROFILE I

-s

I

I

I

I

I

I

1

m

E

v

z

g 0 zt *

g

N

0

0

loo

200

300

TIME (ns)

(4 PPM CUMULATIVE AT CONTOURS (MARK=175.;

Figure 8. (Cont’d)

A=25.)

Writing and Erasing 453 Two antidotes for excessiveTISI are available to the system designer. First, one can minimize thermal energy deposition in writing events, for as we have seen, excessive thermal energy is dissipated by difiion in the medium structure. Therefore, given a choice, it seems preferable from a TISI minimization standpointto write with efficient utilization of thermal energy input. Efficient writing is also preferable from media write/erase cycling and thermal perspectives. Secondly, the medium structure could be designed to control and minimize lateral heat diffusion in the storage layer. This usually is accomplishedby including a heat-sinking layer or cooling luyer in the thin film stack design to direct writing heat diffusion vertically from the deposition spot. Both of these TISI reduction antidotes tend to raise the requirement for laser pulse power for writing. The reader should be aware that an alternate philosophy on TISI is sometimes practiced, apparently to limit the demands on peak laser power from commercially available diode sources. In this approach, one tries to use lateral heat flow advantageously, but control it so that unwanted TISI is limited. Media designed for this approach has been called heat accumulation because it encapsulates the MO film between low thermal conductivity films to help hold deposited thermal energy in the storage layer, and minimize axial diffusion (see the next section). 4.2

Writing PWM Sequences

Power Biasing. Power biasing is used in writing PWM sequencesfor many of the same reasons described above for PPM sequences; namely, to reduce the peak power requirements of the laser and in some cases to ensure adequate signals in the tracking/focusing channel of the drive during writing. As a special instance, in the case of LIMDOW media, the bias power level is required to perform the erase or initialization hction. Serration. Serration is broadly described as the subdivision of the laser intensity profile used to write a particular PWM run-length into a series of shorter, usually higher intensity, pulses. A conceptual representation of this technique is shown in Fig. 9. This strategy represents a step away from the steady state thermal writing limit and acts to reduce the total energy required for writing a mark, increase the thermal gradients both at the edges of a PWM run-length and in the cross-track direction during formation of the mark, and limit the spread of heat in the cross-track direction. This latter point is especially important as control of the width of the mark transverse to the data track is critical for ensuring adequate ontrack signal amplitudeand uniformity while limiting adjacent track crosstalk. As described below, the increased thermal gradient leads to improved robustness in the write process.

454 Magneto-Optical Data Recording I

I

0 Q

P,

1

time

Figure 9. A general example of writing power sewation in time. Various power levels Po through P, over intermediate respective time intervals 1 through 6 represent a general pulse power variation. Pulse ridfall times are shown to be zero for simplicity.

To illustrate, Fig. 10 compares cumulative maximum temperature profiles in the storage film of a typical quadrilayer MO disk structure for rectangular and serrated pulse writing of a periodic 8T (T = clock period) mark run-length, 8T gap run-length. The contour at the writing temperature corresponds to the shape of the written mark. These results were obtained from the numerical Green’s fbnction thermal model described in Sec. 3.3 of this chapter and Ch. 10 of this volume. The temperature gradient along the track was calculated at the leading edge and trailing edge of the marks for the two writing methods. In Fig. l l a is shown the cumulative maximum temperature profiles down the track center, and Fig. l l b shows the leading and trailing edge gradients in these temperature fields over the writing events shown in Fig. 10. This particular profile never actually exists at any instant of time, so these gradients are fictitious. Nevertheless, they do suggest what profile determines the written mark shape and can be shown to be a faithful representation of the instantaneous temperature gradient profiles for the writing events of Fig. 10. Note that the gradients in serrated writing are systematically larger in magnitude than those for rectangular pulse writing.

Writing and Erasing 455 WRITE POWER PROFILE

-

RECTANGULAR

T-----l

0

TIME (ns)

(4 CUMULATIVE AT CONTOURS-RECTANGULAR (MARK=l750;

A=250)

6

Figure 10. Comparison of simple rectangular and serrated pulse thermomagnetic writing using the FE/GF model; v = 9.4 d s ; (a) and (b) show the power versus time and cumulative peak isotherms, respectively, including the presumed PWM 81' mark profile for the threshold temperature isotherm; (c) and (d) show the corresponding results for a simple serrated pulsing in which the 8T mark is formed with four distinct narrow pulses. Notice the difference in the temperature gradients at the leading and training edges of the mark in (b), and compare with the situation in (4.

456 Magneto-Optical Data Recording

WRITE POWER PROFILE - SERRATED

TIME (ns)

(4 CUMULATIVE AT CONTOURS-SERRATED (MARK=I

0

2

4

x (urn)

Figure 10. (Cont’4

750; A = Z ~ O )

6

Writing and Erasing 457 CENTERLINE CUMULATlVE MAXIMUM TEMPERATURE PROFILES I

I

I

I

I

I

I

I

I

-

,

r-..

I

I

I

0

2

I

I

I

I

I

I

2

1

x

I

3

4

Oun)

(4 GRADIENT OF CUMUlATlVE MAXIMUM TEMPERATURE I

I

.;'

..;

i

....._..,

.. .

:

I

:

I RECTANGULAR PULSE WRITE ................. I

I

I

I

SERRATED PULSE W R E :. .. .... ; ".: .

I

I

-

h

E

i

'4 0 v

X

3u

Figure 11. (a) Plots of the mark centerline cumulative maximum temperature profiles for the marks in Figure 10. (b) Plots of the gradient of the temperature in (a),dT,/dx. Notice the relative symmetry in the leading and trailing edge gradients for the two types of pulse writing.

'

458 Magneto-Optical Data Recording Figure 12 shows another comparison of a pair of power serration mark writing approaches. Figures 12a and 12b show a power-time pulse sequence and its corresponding written mark profile for a writing strategy developed for one commercial implementation of the IS0 3X capacity (1.0 GB/sudace) 130 m-m MO standard Notice how a cnmh p i h e serratinn results in the formation of a reasonably uniform width long mark [8T in a (2,7)RLL code] with a pulse power that does not exceed 8 mW in the FE/GF thermal model. This writing strategy has been associated with the use of heat accumulation type media mentioned above. The writing method and medium design depend on managing a considerablelevel of lateral heat flow to form a mark of desired size and shape, but it cares little about achieving high lateral thermal gradients. The peak values of dT,/dx at the leading and trailing edges of the mark during its formation were +O. 180 and -0.277 Wnm. Contrast this with the pair of Figs. 12c and 12d showing a sparse sequence of high power pulses to form an 8T mark from a chain of adjacent near-circular marks. It is apparent from the contour plots of Figs. 12b and 12d (25 K interval between isotherms) that a much higher gradient dT,/dx is achieved with the high power pulsing method. In fact, the peak gradients corresponding to Fig. 12d are +0.337 and -0.342 Wnm at the leading and trailing edges respectively. It is also interesting to compare the integrated energy (averagepower) in the pulse sequences of Figs. 12a and 12c. The average power in the serration of Fig. 12c is only 64% of that of Fig. 12a. This more efficient writing corroborates the claims made earlier in Sec. 3.1. Although the sparse pulsing approach requires higher instantaneous pulse power from the laser, the total thermal energy deposited in the medium is considerably lower, and this is believed to be advantageous in reducing adjacent track thermal crosstalk[15]and write cycling stress in MO media.[l71 High temperature gradients in writing are desirable for the following fundamental reason. A goal in mark edge writing in optical recording is accurate placement of edges with low jitter (time domain variance in edge detection relative to a data clock). Jitter as a function of several system parameters might be expressed as:

dr = (dT, /&)-'{dT, + [dT,/dP] dP + [dT,/dv] dv + (d~,/dT)l(dH,+ [dH,/dC] dC + [dH,/dr] dr + [dHc/dO]dO))

Writing and Erasing 459 where k = spatial jitter along track, T, = medium temperature, To = ambient temperature, P = power, v = velocity, H, = medium coercivity, HB = bias magnetic field, C = write/erase cycle count, and (r, 8)= disk position polar coordinates. This chain rule relation has a single coefficient on the r.h.s. of inverse downtrack temperature gradient, so clearly, a high value of dT,/klT=Twrite is beneficial for low jitter (k).The bias field, cycling, and disk position factors have a common coefficient of inverse temperature slope of the coercivityH, (T). Equation (18) is helpful for identifLingwhich variations contribute to jitter, what needs to be controlled, and how to separate media and system factors,

WRITE POWER PROFILE I I 7 -

I

I

I

-1

I

-111111

TIME (ns)

Figure 12. A FE/GF model comparison of writing with a “comb” serration (a) and (b) with a sparse, narrow pulse serration (c) and (4. The writing is forming the 8T PWM marks shown in (b) and (d). The leading and trailing edge maximum gradients in (b) are M.180Wnm and -0.277 Wnm, respectively. In (4, the values are +0.337 Wnm and -0.342 Wnm, respectively.

460 Magneto-Optical Data Recording

CUMULATIVE AT CONTOURS (WRITE=1750; A=250) .:

:.

.I 1 ........... ...'

.. . .i

. . ... :.. ._. ...

I ........................................ '..

. ....

.......-.

I-.

'

i

i

?:

.

. . ..,'-'-' ; i ; '

. :. ......

: : .. ... ... ... i

.................... .......................................... .. .. . . .. . ......................... .

.

...............................

:

I

I

':

.. .. .. ........... ... ... ... ... . .

........ ................................ .. . . .. ........... ,.I I . . .

2

0

4

x

(4

(b) WRITE POWER PROFILE

0

100

200

TIME (ns)

Figure 12. (Cont'd).

300

Writing and Erasing 461

CUMULATIVE AT CONTOURS (WRITE=l750;

A=250)

Figure 12. (Cont’d)

LeadingTrailing Edge Emphasis. Next to serration, leading edge emphasis is probably the most commonly used technique to ensure accurate PWM recorded sequences. The purpose of leading edge emphasis is several-fold. First and most importantly, leading edge emphasis ensures rapid heating of the medium at the beginning of a PWM mark. This is required since the disk may have cooled substantially during the relatively long time between marks when only the DC bias power is applied. In addition, any phenomena such as nucleation, etc., which may have an activation time (lunetics) associated with them can be initiated by applying additional energy at the beginning of mark formation. Second, leading edge emphasis can be used to compensate to some degree for intra-symbol TISI caused by thermal buildup during the formation of long PWM symbols. Conceptually,the idea is to overwrite the beginning of a long PWM symbol

462 MagneteOptical Data Recording by an amount equal to that which results at the end of the symbol because of thermal buildup. The net result is a recorded mark which looks more symmetric at the leading and trailing edges. Finally, leading edge emphasis is useful in controlling the effects of high medium velocity at high data rates by minimizing the phase delay associated with writing which occurs Ear from the stationary media limit in this case. In the absence of leading edge emphasis, translation of the medium during the initial stages of writing a PWM symbol leads to a slow temperature increase, which leads to less robust writing and a long lag between the time power application is initiated and mark formation begins. Leading edge emphasis can be implemented by any combination of power andor pulse width. The most common implementationis to utilize a different power level at the beginning of a P W M symbol for a specified duration as shown in Fig. 13a. Alternately, when used in conjunction with serration, a different pulse duration at a fixed power or a different pulse power at a fixed duration can be used as in Fig 13 b,c. Finally, in some cases both techniques are used together; i.e., a different power level, either higher or lower, is used in conjunction with a longer pulse width as in Fig 13d. The decision as to which technique to use depends on the design of the laser driver used to generate the write sequences, cost constraints, and effectiveness. With regard to this latter point, increasing power with fixed minimum pulse duration is usually the preferred choice since this ensures writing as close to the stationary media limit as possible and maximizes thermal gradients. In contrast to leading edge emphasis, trailing edge emphasis has seen little use in optical storage applications mainly because of the intra-symbol TISI present in most magneto-optical medium designs. Trailing edge emphasis would tend to aggravate the intra-symbol TISI problem by providing excess heat at the end of the mark. However, if heat is removed from the center section of the P W M symbol and the thermal buildup is reduced prior to writing the trailing edge of the symbol, trailing edge emphasis can be used to advantage in increasing thermal gradients and improving cyclability. An example of such a waveform has been shown to reduce sensitivity shifts due to media and is given in Fig. 14.

Writing and Erasing 463

ri

1

L

n

1

Figure 13. Example leading edgdtrailing edge write strategies for (a) long pulse writing, and short pulse serrated writing with (b) initial pulse width modulation, (c) initial pulse amplitude modulation, and (4 combined initial pulse width and amplitude modulation.

Figure 14. Example leading edgdtrailing edge write strategy which has been shown to decrease media erase cycling sensitivities.

464 Magneto-Optical Data Recording

Power Cutting. Power cutting is a technique used by many write strategies to provide rapid cooling of the media when appropriate. In keeping with the above discussion, the most obvious time to implement rapid cooling is immediately before and after a P W M symbol ishas been vdten. Reducing fie merlillm temperamre just prior to writing a symho! allows the steepest thermal gradients at the leading edge of the symbol since most of the temperature rise required for writing is generated by the write pulse and not by the DC preheat level. However, such cutting usually increases the power requirements for writing. Similarly, power cuttingjust after a P W M symbol has been written ensures the steepestthermal gradients at the trailing edge of the symbol. This is especially necessary since considerable thermal buildup may have occurred during the writing of the symbol. The additional coolingtime defined by the power cutting allows this heat to dissipate (diffuse) into the surrounding material. To illustrate the effect of power cutting on thermal writing, we return to the method of thermal characteristic curves discussed in Sec. 3.3. Figure 15 shows characteristic curves derived from the FE/GF model as follows: Again the power in a rectangular pulse of duration Tw needed to form an isolated mark ofmaximum width 0.67 prn (equal to 2vTfor the 2.6 GB IS0 MO standard) is determined. The difference from the situation previously described in Fig. 6 is that here a rectangular power cut to zero mW has been placed immediately prior to and following the writing power pulse. The cut power is a power decrease fromthe given bias power (1-4 mW in Figs. 15ad) or the writing pulse power to zero, and the duration of the cut is the time correspondingto a distance of vT, namely 0.333 p;that is, the cut duration is 0.333 j.un/v. Notice that the zero bias power case is not present in Fig. 15 since it does not allow power cutting. Also, the curves do not extend down to v = 0 since the cut power duration diverges in that limit. (One expects, however, that the powers for the v + 0 limit match the v = 0 powers in Fig. 6a where the bias power was zero.) In comparing Figs. 15a-d with the corresponding plots in Figs. 6b-e, we note that the slopes of the power versus velocity curves are markedly reduced, more so for increasing bias power. This arises physically because the power cutting, particularly for longer times at low speed, allows more medium cooling, and so the writing pulse must heat up “cold” media; hence the low velocity writing powers are increased the most and the curves are flattened. The shallower sloped curves indicate that power cutting is beneficial in reducing the velocity sensitivity of thermo-optical writing, and thus making the writing process more robust to radial variation in CAV recording.

Writing and Erasing 465

PB=2mW C U PULSES; ATWrlt.=l75K

Figure 15. Thermal characteristic curves for writing MO media with power cutting: (a) Pb =1 mW, (b) Pb = 2 mW, (c) Pb = 3 mW, (4 Pb = 4 mW. See legend in (4. Power versus velocity to write a fixed width mark (2vT = 0.67 pn) for various duration rectangular writing power pulses. Power is cut to zero for a duration 0.333 pn/v just prior to and following the writing power pulse. Cutting is done &om the preceding bias power or writing power level.

466 Magneto-Optical Data Recording

PB=3mW CUT PULSES; ATw,=175K

Figure 15. (Cont ’d.)

Writing and Erasing 467 In practice, power cutting at the leading edge of a symbol has not been used as extensively as power cutting at the trailing edge of a symbol for reasons similar to those which have caused leading edge emphasis to be more popular than trailing edge emphasis (i.e., power cutting at the leading edge would seem to contradict the idea of leading edge emphasis since the cutting would reduce the temperature at the leading edge of the mark). Certainly, the combination of leading edge emphasis and trailing edge cutting is very consistent with the idea of a write algorithm which includes a large degree of preheating by a DC bias level. A perfect example of such an algorithm is that described for DOW media where a large DC preheat is required for the overwrite function. However, as is evident from the growing DOW literature,[lo1the many advantages related to the steeper thermal gradient at the leading edge of the P W M symbol has led to a series of newer write algorithms which utilize leading edge power cutting. Example PWM Writing Strategies. Two example PWM writing strategies are presented here as representative of the two extremes depicted by the characteristic curves described in Sec. 3.3 above: First, pulsed writing[’][*] shown in Fig. 16 which attempts to approach the stationary media limit; second, thermal accumulation ~ r i t i n g [ ~ 1 [ ~shown ~ 1 [ ~in 1 Fig. 17 which attempts to reduce laser pulse power requirements while maintaining accurate writing. Both algorithms utilize the techniques described above and bothalgorithmsrequire a multitude of pulse powers to achieve accuratewritmg. The improved pulsed writing algorithm depicted in Fig. 16 utilizes trailing edge emphasis by minimizing the heat input into the media during the middle part of the mark formation. Note the use of a modified serration algorithm combined with leadingkrailing edge emphasis to improve thermal gradients at both the leading and trailing edges of the PWM symbols. This algorithm is one in a class of write strategies referred to as edge writing algorithms because they focus on accurate formation of leading and trailing edge submarks which define the edges of a particular P W M symbol while creating “filler” submarks between the edges which serve only to ensure accurate readback. These filler submarks are located so as not to disturb writing of the all important edge submarks. The filler pulses are purposely shifted toward the leading edge of the P W M symbol so as to minimize thermal interaction with the trailing edge writing pulse. In addition, power cutting is used extensively as the tracking or bias power level is maintained throughout whenever a writing pulse is not present. In fact, the bias power level is usually set equal to zero (the laser is biased at its threshold current) and the write pulses themselves provide the light necessary to maintain tracking and focus during a mark writing event.. This algorithm, although

468 Magneto-Optical Data Recording

requiring high laser pulse powers, is very efficient in that a minimum amount of energy is used in forming the PWM symbols. Short pulses are used to maximize these effects and reduce medium velocity effects.

4Tc

5Tc

7Tc

Figure 16. Example PWM pulse writing strategy applied to (2,7)lUL coded data which utilizes serration, power cutting, and edge emphasis. No power is applied during the P W M gap symbols.

Writing and Erasing 469 The thermal accumulation algorithm shown in Fig. 17uses extensive power biasing in the P W M gaps to reduce the laser power pulse requirements and to control TISI. The "IS1 is controlled by purposely increasing the ambienttemperature of the media through the bias power level to such an extent that a much smaller write power (and hence write temperature increase) is required to form the P W M symbol. In a way, writing appears as a relatively smaller perturbation on the ambient (preheated) condition, and, in the scanned mark limit, acts to reduce TISI at the expense of much shallower thermal gradients and a much increased velocity sensitivity. The algorithm takes advantage of serration and leading edge emphasis combined with trailing edge power cutting as described above.

5.0 ERASING DATA SEQUENCES Since it is desired that erasure should not require any knowledge of what has been previously written, it is generally performed using continuous heating with a CW laser and a continuous bias field. This ensures complete erasure of any recorded information regardless of the particular pattern of written domains. Conceptually, reading is the same process as erasing, except that the laser power levels are low enough that the previously written data remains intact. However, in practice, reading is performed using a rapidly modulated RF laser which is required for feedback noise suppression in diode lasers. Writing, on the other hand, typically requires creating a pattern of magnetic domains on the medium. This is usually performed using a pulsed laser and a continuous bias field aligned oppositeto that used in erasing.

6.0

SPECIAL TOPICS

6.1

Tip Writing

Figure 18 illustrates writing in which the readback signal amplitude (carrier) increases rapidly with power. The small carrier in this region is caused by the formation of subresolved marks; that is, the written marks are so small that much of the region illuminated by the read beam does not contain a mark and this provides no signal. These very small marks formed by the tip of the Gaussian-shaped write beam can be quite usehl. For example, the small marks readable by magnetic superresolution may be formed by this tip writing. Note the values of write and read beam

470 Magneto-Optical Data Recording

FWHM S used in the simulation of Fig. 18. In addition, low power read lasers are typically available at shorter wavelengths before their more powerful counterparts used for writing. Tip writing with a longer wavelength writer may make a higher density product available sooner than otherwise possible.

3Tc

4Tc

Figure 17. Example PWM thermal accumulation Writing strategy applied to (1,7)lUL coded data which utilizes serration, relatively high bias power level, leading edge emphasis, and trailing edge power cutting. The bias power level is applied in PWM gap symbols.

W

n 3

k

_1

a

E 6 Y

---

(Y

Q

5

n W

4

+

N

0

a a W

3T TONE pp AMPLITUDE

z

0 * b -

o b -

o m

0

v,

0

0

6.8

7.2

7.6

10ns PULSE POWER (mW)

Figure 18. Tip writing is illustrated using the FE/GF model. A highest frequency 3T tone in (2,7)RLL PPM was written. The mark diameter was set to be about half the writing spot FWHM. For the 3T tone pattern, the peak-peak amplitude versus writing pulse power curve is very narrow, indicating that extremely accurate control on the laser output power would be required to write with the peak intensity tip ofthe laser beam.

2

472 Magneto-Optical Data Recording The drawback to this procedure arises from the steep slope of low frequency carrier versus power. It indicates potentially large variations of mark size in response to small variations in write condition. This is obviously a more substantial problem for PWM recording, where the position of the mark edge is crucial, than for PPM. However, PWM recording will typically be used in the high density systems that would most value short wavelengths. This problem will likely generate future searches for reliable methods of subresolved recording. 6.2

Read While Write

In optical-thermal recording data storage systems (MO as well as write-once types), a recording throughput penalty is often encountered when a write-verifjl step is added to the data writing step. The normal implementation in disk recording systems with a single writehead light beam is to wait the latency period of an additional disk revolution after the writing to read the newly written information on the next pass of the data under the beam to verifL the information. Clearly, if the reading for write-verie could be accomplishedon the same revolutionas the writing, valuable system throughput could be gained. Obviously, this could be achieved in a direct way by adding a separate reading beam downstream from the writing beam, a method called direct read after write (DRAW).However, a more economical approach is to attempt to read the newly written data with the SAME beam as used for writing on a single pass over the data. This method is called read while write (RWW). A couple of obvious issues present themselves, raising questions of whether this is technically feasible. First, is the newly written mark sufficiently stabilized to allow meaninghl reading? This depends on the details of the mark formation process for the mode of optical recording employed. For MO thermomagnetic recording, this mark formation is essentially determined by the rate of cooling of the MO layer below the writing temperature. By this time the magnetic moment reversal dynamics are complete since they occur on time scales 1-2 orders of magnitude faster than thermal conduction. Therefore, if the writing beam linear speed is not too high, the writing power can be dropped down to the reading power before the beam position moves from its overlap location on the newly formed mark. (Beam/mark width transit times for typical disk velocities lie in the range 30-150 ns, much longer than laser power fall times. These times are also long enough by a factor of 2-3 for the peak MO temperatures

Writing and Erasing 473 during writing to fall below the mark formation temperature when the writing power is dropped.) A second implementation issue has to do with whether the data reading channel can respond when the beam power undergoes a rapid transition from the writing level to the reading level, a power level drop by a hctor of perhaps 5-10. A reading detector and preamplifier combination would have to deal with signal transients in which preamp outputs are expected to drop suddenly from a saturation state to a stable readback level. At a minimum, the read channel must be “hardened”to withstand these sorts of transient conditions and still return meaninghl information. This raises the question of what level of write verification is required. In all likelihood, a RWW verification can be at some reduced level of rigor compared to a normal data reading operation. It may be sufficient to simply confirm that writing is occurring, and to distinguish that condition from a gross failureto write due to the presence of a medium defect, for example. 6.3

Laser Modulation Direct Overwrite

One of the most challenging system development tasks in rewritable optical storage is the design and implementation of light intensity modulated direct overwrite (LIMDOW) function on an exchange-coupled multilayer (EMCL) structure of MO films. As discussed in Chs. 5 and 10 , DOW is achieved by writing new information (a string of binary 1’s and 0’s) in the memory layer, directly overwriting old information in a single pass of the laser beam by modulating the writing power between “low” and “high” These two power levels impose thermal profiles in the structure which alternately engage and disengage an exchange coupling between the initializing and memory layers. When the coupling is engaged (“low” temperature), the memory layer magnetic polarity replicates that of the permanently magnetized initializing layer. When the coupling is removed (“high” temperature), the polarity of the bias magnet determines the state written to the memory layer. The mode of magnetization pattern imprinting is similar to the shape of the dynamic writing temperature contour as it moves out from under the influence of the heating beam on the moving disk. Therefore, the length of the “up” or “down” magnetization regions (domains) in the storage layer is proportional to the duration of the “high” and “low” power modulation pulses. Because the MO ECML for DOW is possibly 3- 10times thicker than the single MO layer in a standard quadrilayer structure, there is much less

474 Magneboptical Data Recording flexibility in medium design for high thermal gradients (see Ch. 10). Consequently, LIMDOW recording has been forced to employ strategies such as power cutting (see Sec. 4.2) to improve the control of heat flow for better recording performance.

7.0 CONCLUSION The basic concept of magneto-optical data recording is relatively simple: a small volume of the recording medium is initially heated by a focused laser beam, then allowed to cool under an appropriately directed magnetic field. The complications arise from the effort to make this process robust under the demands of ever higher density and other performance constraints. PWM, in particular, has increased the demand for write schemes that include serration and minimal bias power levels. New techniques to increase performance such as magnetic superresolution or direct overwrite through exchange coupled layers promise to extend the challenge of write scheme development for the foreseeable hture.

REFERENCES 1. Ohtsuki, T., h a , S., and Yamada, F., Optical Data Storage, SPIE, 1316:2805 (1990) 2. Wakabayashi, H., and Yamada, F., Proceedings of Magneto-Optical Recording International Symposium 92, J.Magn. SOC.Jpn., SI(17):218221 (1993) 3. Marchant, A., Optical Recording, A Technical Overview, pp. 354-356, Addison-WesleyPublishing Company (199O) 4. M e , C. Denis, andDaniel, E. D.,MagneticRecording: Volume11Computer Data Storage, pp. 348-350, McGraw-Hill Book Company (1988) 5. LaBudde, E., LaBudde, R., and Hazel, R., O U Topical Meeting on Optical Data Storage, WdD3 (1985) 6. Hurst, J. E., Jr., Cheng, D., Davis, C. R, IBM Technical Disclosure Bulletin, 37(7):283-284 (July 1994) 7. Hurst, J. E., Jr., Belser, K.,andMadison, M., Optical Datastorage Pulse WidthModulation System andMethod, US Patent 5,400,313 8. Schouhamer, K., Aarts, R., Opheij, W., Method of Recording a Digital Information Signal on a Record Carrier having a Radiation Sensitive Layer, Apparatus for Carrying out the Method, and Optical Recording Carrier Provided with Such a Digital Information.Signal, U S Patent 4,473,829

Writing and Erasing 475 9. Ide, H., TOQ, T., Kirino, F., Ma& T., Kugiya, F., Mita,S.,and Shigematsu, K., Jpn. J. Appl. Phys., 325342-5348 (1993) 10. Miamoto, H., Ojima, M., Toda, T., Niihara, T., Maeda, T., Saito, J., Matsumoto, H., Hosokawa, T., Akasaka,H., Jpn. J.Appl. Phys., 3254575458 (1993) 11. Fujita, G., Kawashima, T., Watanabe, T., and Aoki, Y., Jpn. J. Appl. PAYS., 28-3:329-333 (1989) 12. Saito, J., Sato, M., Matsumoto, H., and Akasaka, H., Jpn. J. Appl. Phys., 26-4:155-159 (1987) 13. Aratani, K., Fukumoto, A., Ohta, M., Kaneko, M., and Watanabe, K., SPIE, 1499:209 (1991) 14. Ohno, E., Nishiuchi, K., Ishibashi, K., Yamada, N., and Akahira, N., Jpn. J. Appl. Phys., 30:677481 (1991) 15. Finkelstein, B. I., and Wagner, G. J.,J. Mugn. SOC.Jpn., S1(17):213-214 (1993) 16. Ide, H., Toda, T., Kirino, F., Maeda, T., Kugiya, F., Mita, S., and Shigematsu, K., Jpn. J. Appl. Phys., 325342-5348 (1993) 17. Sukeda,H., Ojima, M., Takahashi, M., and Maeda, T., Jpn. J. Appl. Phys., 26-4:243 (1987) 18. H. Sukeda, T., Tsuchinaga, H., Tanaka, S.,Niihara, T., Nakamura, S., Mita, S., Yamada, Y., Ohta, N., and Fukushima, M., SPIE, 1499:419 (1991) 19. Iwanaga, T., and Inada, H., Jpn. J. Appl. Phys., 31:580-583 (1992) 20. Otter, E. L., Finkelstein, B. I., Madison, M.R., and McDaniel, T. W., J. Appl. Phys., 675325 (1990) 21. McDaniel, T. W., and Finkelstein, B. I., J. Mugn. SOC.Jpn., S1(17):209212 (1993) 22. 2.6 GB, 130mm MO Standard:ISO/lECJTC1.23.14517 23. Tabata, M., Jpn. J. Appl. Phys., 335811-5816 (1994) 24. Cheng, D. C., McDaniel, T. W., and Davis, C. R., IEEE Trans. Mugn., 26:1903 (1990) 25. Miyamoto, H., Toda,T., Ide, H., Saito, A., Andoo, K., Niihara, T., and Ojima, M., J. Mugn. SOC.Jpn. S1(17):179-182 (1993) 26. Saito, J., Sato, M., Matsumoto, H., and Akasaka,H., Jpn. J. Appl. Phys.., 26-4:155 (1987); Lin, C. J., J. Appl. Phys,. 67:4409 (1990); Ito, M., Nakaki, Y., Tsutsumi, K., and Ito, O., J. Mugn. SOC.Jpn., S1(17):155 (1993); Mihara, M., Tanaka, T., Kitade, Y., Namba, Y.,and Hashimoto, Y, Ibid: 159 27. Yoshihim, M, S W , K., Ohta, N., Toda, T., Awano, H., and Qima, M., Jpn. J. Appl. Phys., 32( 1):5441 (1993)

The Magneto-Optical Readout Process C. David Wright

1.0 INTRODUCTION

In this chapter we review the processes which determine the form of the readout signal in magneto-optical (MO) recording. Although the primary interest is, of course, MO systems, many of the techniques for calculating and characterizing the output signal can be applied to other forms of optical recording, including phase-change, write-once and compact disc type formats. A description of the magneto-optical effect, discovered by the Scottish physicist John Kerr in 1876, upon which the generation of the MO readout signal relies, is given in the following section. The treatment is phenomenological, and for completeness other forms of MO contrast are very briefly discussed. After this look at the origins of the MO signal, which essentially reveals the maximum detected signal amplitude that can be expected, we concentrate on describing how the optical system of the readout head affects the detailed shape of detected waveform. This necessitates an excursion into the realms of the theory of optical diffraction and, perhaps more surprisingly to the uninitiated, the theory of image formation in scanning optical microscopy. Such topics are covered in Secs. 3 and 4. Armed with this knowledge, we can begin to make predictions of the effects various readhead optical components on signal characteristics in both the 476

The Magneto-Optical Readout Process 477 time and frequency domains. This leads naturally to an investigation of ways in which the performance of the readout system can be improved, particularly with respect to enhancing the high frequency output by way of improved optical resolution. A variety of novel head designs and detection techniques are discussed, with this aim in mind, in Sec. 5 .

2.0

ORIGINS OF THE MAGNETO-OPTICAL READOUT SIGNAL

The first report of the interaction of light with magnetized materials was given by Michael Faraday in 1845, who observed the rotation of linearly polarized light induced by passage through lead glass in a magnetic field."] Some thirty years later, in 1876, John Kerr observed that the plane of polarization of linearly polarized light was rotated upon reflection from the poles of an This effect, named after its discoverer, is at the heart of the magneto-optical readout process. In quantum mechanical terms, such MO effects can be viewed as the interaction of light with an effective magnetic field due to the so-called spin-orbit interaction of atomic We shall, however, not be concerned with such descriptions here, but rather we shall adopt a phenomenological approach. Before we discuss the magneboptical effects themselves in any detail, it may prove worthwhile to review briefly some of the properties of polarized light.

2.1 Polarized Light Electromagnetic waves propagating in free space or within an isotropic medium are characterized by oscillating electric and magnetic fields oriented in planes perpendicularto the direction of propagation. Plane wave solutions to the well-known wave equation

v2 W=&/l-- a'w dt2

where ry represents either electric or magnetic field vectors E or B (or H), can be written

478 Magneto-Optical Data Recording

where we have assumed that the wave is monochromatic, uniform and p p g & q g Lq ?hez &re&n iq a Cade~izqcmr&nate syste~.acd -x zqd j are unit vectors. Since the photodetectors used in readout heads respond to the time-averaged value of the square of the electric field amplitude, we shall, in this work, be primarily concerned with the value of the electric field E which we will also refer to as the optical field. Thus, the terms optical field, electric field, E and y can generally be used interchangeably in what follows. The polarization state of the optical field is determined by the relative amplitudes of t,q, and yq and their relative phase difference (4x - 4y). In general, Eq. (2) represents an electric field vector which lies in the ( X J ) plane, orthogonal to the direction of propagation z, with the tip of the electric field vector describing an ellipse rotating through one complete cycle as the wave progresses one wavelength in the z direction. Linear and circularly polarized states are special cases of such elliptical polarization. Linear and Circular Polarizations. With (bX- 4y)taking the value zero or =k2nn7where n is an integer, the wave is said to be linearly polarized since the E field oscillates along a line, tilted at an angle given by arctan (*yo,,/ya) to the x axis. As before, the E field progresses through one complete cycle as the wave propagates by one wavelength. For the w e y, = yOyand a phase difference of (-7d2 f 2n74 the wave is A

which represents an E field of constant scalar amplitude at any particular value of z (the amplitude will still, of course, vary sinusoidally in the propagation direction), with the tip of the E vector describing a circle in the (x,y) plane. The minus (-) sign in Eq.(3) represents the situation where E rotates in a clockwise fashion, looking back at the source, and is referred to as right circular polarization (RCP).The plus (+) sign describes left circularly polarized light (LCP),which rotates in an anticlockwise sense looking back at the source. From Eq. (3), it is apparent that we can describe circularly polarized light in terms of two linear components with a specific amplitude and phase relationship. It should come as no surprise, then, that we can think of

The Magneto-Optical Readout Process 479 linearly polarized light as combinations of RCP and LCP components. For example, adding in-phase RCP and LCP waves of equal amplitude yields

which describes linear, horizontally (x) polarized light. Likewise, subtracting equal amplitude RCP and LCP components results in linear, vertically (y) polarized light. Adding or subtracting equal amplitude RCP and LCP components with an arbitrary phase difference *$4 between them yields a linear polarization at an angle *4/2 to the x axis. Elliptical Polarization. As already discussed, linear and circular polarizations can be viewed as special cases of the more general elliptically polarized state represented by Eq. (2). Taking the real part of the expression of Eq. (2), which represents the physical field, it is straightforward to show that we can write (e.g., see p. 273 of Ref. 4)

where $4 = ($4x - $4y). Equation ( 5 ) should now be recognizable as that of an ellipse whose major axis makes an angle 0 with the ( w, ,wy) coordinate system, where

wawoy

tan(28)=2

wm

2

cos $4 = tan(2a)cos $4

- woy

Eq.(6) tan(a) = VOY wm

Again we find that we can depict elliptical polarizations as appropnate combinations of circularly polarized components. For example, adding in-phase RCP and LCP components having different amplitudes yo-and %tyields

Eq.(7) w ( x , y m ) = [(Yo+ +

wo-p+ j(wo+- w0-)Y]exp[j(~- 41

480 Magneto-Optical Data Recording which, by comparison with Eq. (2), we recognize as elliptically polarized light with the major axis aligned in the x direction. We can tilt the major axis away from the horizontal by introducing an arbitrary phase difference between the RCP and LCP components.

2.2 Magneto-Optical Kerr Effects The form of the magneto-optical interaction between a magnetized material and polarized light depends, among other things, on the direction of magnetization relative to both the plane of incidence and the sample surface. Three primary configurations are illustrated in Fig. 1 and are known in reflection as the longitudinal, polar and transverse (or equatorial) Keneffects. If the incident light is linearly polarized parallel or perpendicular to the plane of incidence (we usually term these p and s polarizations respectively), the polar and longitudinal effects induce a small, magnetizationsensitive rotation of the plane of polarization in the reflected wave, along with a certain amount of ellipticity. For the transverse configuration, the interaction is somewhat different. Here, no effect is observed if the incident polarization is perpendicular to the plane of incidence, but a p-polarized wave undergoes a small, magnetization-sensitive change in reflectance amplitude. longitudinal Kerr effect

polar Kerr effect

transverse Kerr effect

Figure 1. Majpeto-optical K m effects

The Polar Kerr Effect. For MO storage applications, it is the polar Kerr effect that is our main concern. From a phenomenological Viewpoint

The Magneto-Optical Readout Process 481 we can describe this polar effect in terms of the propagation in the magnetized medium of left and right circularly polarized components (remember that in Sec. 2.1 we showed that any arbitrary polarization state can be considered as appropriate combinations of RCP and LCP waves). Under normal incidence it can be sh0wn[~1[~] that the RCP and LCP components propagate at different speeds given by c/n*where c is the free space speed of light and n* is the rehctive index of LCP and RCP waves respectively, given by n* = ( E f ~ ' ) l ' * where E and E' are the complex diagonal and offdiagonal elements of the permittivity tensor. For isotropic, nonmagnetic materials the off-diagonal components of this tensor are zero and so n+ = n= & ,the well-known relationship between refractive index and permittivity. For magnetic materials, the fact that RCP and LCP waves display different refractive indices will mean that their Fresnel amplitude reflection coefficients, r*, (see Ref. 4, p. 94) will also be different and we can write

where for simplicity we have assumed the incident medium to be free space. If the incident light is linearly polarized, i.e., it comprises RCP and LCP components of equal magnitude, then Eq. (8) identifies two limiting cases. The first is when It-+/= Ir-1 but 14+1z 14-1. This is known as magnetic circular birefringence (MCB), and results in a reflected wave that is also linearly polarized but rotated by an angle 0, = (4+ - 43/2. The second case, known as magnetic circular dichroism (MCD), is where the reflected RCP and LCP components maintain their incident phase relationship, but the magnitudes of their reflection coefficients differ, i.e., Ir+l# Ir-1 but /4+/= 14-1. This yields an ellipticallypolarized reflected wave with the major axis of the ellipse aligned with the incident polarization direction. In reality there is usually a mix of MCB and MCD behavior such that a linearly polarized incident wave has elliptical polarization upon reflection, with tbe major axis ofthe ellipse tilted at an angle 0, to the incident polarization direction and an ellipticity (ratio of minor to major axes) of (Ir+l - ~ r - ~ ) / ( ~ r + ~ + ~ r - ~ ) . In analyses of the signal induced as a result of the polar Kerr effect, it is common to work with the Cartesian reflection coefficients r, and r,, as opposed to those referring to circular polarizations. It is, however, straightforward to switch from one representation to another. Remembering from

482 Magneto-Optical Data Recording Sec. 2.1 that addition of RCP and LCP components of unity strength yields a linear polarized wave of twice unity strength, we write (see Ref. 6).

rxi + r y j = r+(i+ j j ) + r-(? - j j )

Eq.(9)

+

r+ + r -

rY

r, = 2

.r - r 2

-

=J-

For an incident linear polarization parallel to the x axis, the reflected light will generallybe ellipticallypolarized as shown in Fig. 2 and described by the relations

tan(2e.4 = ~EUI(~CZ)COS+= 2

lr~llrYl lrxI2

Eq. (10)

tana = -, Iryl Ir.

I

cos+

- l'Y12

S i n ( 2 E K ) = sin(2a)sinp= 2

Ir,lM kX12

+kYlZ

Y

X

I I

I

Figure 2. Ellipticallypolarized reflected light showing the Kerr angle, OK, Kerr ellipticity, cK, and reflection coefficients r, and r,,.

The Magneto-Optical Readout Process 483 The angle 0, is the so-called Kerr rotation angle, 4 is the phase difference between the reflected x and y components and E~ is the Kerr ellipticity, the tangent of which is the usually defined ellipticity (ratio of minor to major axes). It is apparent from Eq. (10) that the Kerr rotation angle is maximized for zero phase difference between the x and y components (4 = 0), in which case we have pure rotation. We shall examine ways to ensure this condition shortly. For small angles the relations of Eq. (10) reduce to

Detection Strategies. The polarization rotation induced via the polar Kerr effect is relatively small, with intrinsic Kerr angles for rare earthtransition metal (RE-TM)media of around 0.2 to 0.4 degrees and Kerr It is, therefore, ellipticities of the order 0.1 to 0.4 degrees being important that the detection strategy employed attempts to maximize the available signal and ensure that the maximum possible signal-to-noise ratio (SNR) is achieved. Noise processes in MO systems are discussed in Ch. 9 of this book, so here we shall concentrate on signal amplitude. The simplest detection technique would be to employ a single analyzer and photodiode, as shown in Fig. 3(a). If we assume that the incident light is polarized in the x direction and that the analyzer makes an angle p with this direction, then it follows that the irradiance at the detector is

where the f sign refers to the situation for positive and negative magnetizations respectively, P, is the light power incident on the disc, and for the present we have assumed that the ellipticity is zero. We see that the photocurrent generated in the detector, given by multiplying Eq. (12) by the detector responsivity, has a small signal component superimposed on a relatively large DC level (which is independent of magnetization direction). The signal is maximized for an analyzer setting of p= 4 5 O , but at this setting the DC component is large and the SNR, due to the influence of media, laser and shot noise, is reduced from its optimum value. It can be shown that an analyzer setting a few degrees from the crossed position, (p = 80°-85") maximizes the SNR in this case.[*] Note that if we had chosen not to ignore the effects of ellipticity, the signal component in Eq. (12) would have been reduced by the factor C O S ~ .

484 Magneto-Optical Data Recording

LD

Disk

PD

Figure 3. Single (a) and differential (b) detector arrangements showing the laser diode (LD), collimating lens (CL), beamsplitter (BS), objective lens (OL), analyzer (A), detector lens @L), photodiode (PD), leaky polarizing beamsplitter (LPBS), phase compensator (PC), and polarizing beamsplitter (PBS).

The Magneto-Optical Readout Process 485 In most commercial MO drives, a differential detection system is employed, along the lines of that shown in Fig. 3(b). Here, light from the collimated laser source first passes through a "leaky" polarizing beam splitter (PBS) before being focused onto the disc by the objective lens. Upon reflection from the disc, the light acquires, via the polar Kerr effect, a Y component to its electric field, assuming the incident light to be linearly polarized in the x direction. The return beam again passes through the leaky PBS which directs a fraction of the x component and all of the y component towards the detectors. The PBS in the detector arm of the head is oriented so that it acts as an analyzer set at p = + 4 5 O for one detector, while for the other it acts as if p= -45". The signal at each detector is thus still given by Eq. (12) (after multiplying the lrJ2term by p and the lrxllryl term by 5 to account for the effects of the leaky PBS) but with p= * 4 5 O . The result is that the two detectors pick up signals of opposite polarity superimposed on identical DC components and so the peak-to-peak amplitude of the differential signal is twice as large as that of the single-ended system. The differential system also has the advantage that it can remove some of the common mode noise, so improving SNR (see the Ch. 9 in this volume). Note that in some head arrangements the detector arm PBS is oriented along the x axis, and a half wave plate, with its axis oriented at 22.5" to x, is introduced prior to the PBS to attain the same balanced differential configuration. The phase compensator plate in the return beam path in Fig. 3b serves, ideally, to remove the ellipticity of the reflected wave by eliminating any phase difference between x and y components. This has the effect of increasingthe amplitude of detected photocurrent, as already indicated. The compensatorplate itself may be a variable retarder, such as a Babinet-Soleil compensator or, a more recent development, a liquid crystal retarder.L9] These have the advantage of being able to compensate for any phase difference between x andy components. However, a more usual approach is to use a quarter-wave plate oriented at 45". In this case, the analyzer must be retuned for specific values of and E ~ . [ (These ~ ~ ] considerations are discussed further in Ch. 2.) It should also be noted that the parameter 5 of a leaky PBS will also impinge on the detected signal and SNR level, a point that is covered more fully in Ch. 9. Finally, it must be pointed out that the use of dielectric enhancement layers for the recording medium can have a large influence on the detected signal by modification of the reflectance amplitudes, elimination of ellipticity, or both. These matters are discussed in more detail in Ch. 6 of this book.

<

486 Magneto-Optical Data Recording

3.0 OPTICAL PROPAGATION IN THE READOUT PATH In the previous sections, we investigated the mechanisms of signal generation in MO recording systems, paying particular attention to factors influencing the amplitude of the readout signal. We now turn our attention to understanding the processes that affect the detailed shape of the readout waveform.

3.1 The Readout System as a Scanning Laser Microscope The optical readout system of most optical disc players currently on the market is analogous to that of a scanning laser microscope. This similarity was realized in work leading up to the development of the Philips Laser VisionTMplayer in the 19707s,and the Compact Disc player in the early 1980'~.["1-[~~1 Theoretical treatments of the process of image formation in the scanning laser microscope are well developed (see, for example, the work of Wilson and Sheppard and their co-workers),[141~[161 and can be applied directly to the analysis of the form of the optical replay signal in a variety of optical disc players. It may at first seem strange that we talk of optical disc players in terms of imaging systems or microscopes. After all, the output from the optical replay head is a one-dimensional time-varying signal, not a twodimensional image? However, this output signal is, in fact, a onedimensional line scan through a virtuaZ image of the optical disc. To produce a true image we would simply have to capture signals arising not only from scan lines through the center of a track, but also from a set of closely spaced, parallel, adjacent scan lines. This fact has been demonstrated for magnetooptical systems by the author and his co-workers using X-Y based scanning system^.[^^-[^^] In similar vein, Benschop et al.[*O]have used a compact disc head to construct a scanning optical microscope sensitive to ordinary reflectance and phase objects. Types of Scanning Optical Microscope. Two distinct classes of scanning optical microscope have been identified, named prosaically Type 1 and Type 2. The two classes have different imaging properties; Type 1 being, under certain conditions, linear in light intensity (in the detector plane), whereas Type 2 is linear in electric field amplitude. Put another way, the Type 1 microscope behaves essentially as an incoherent microscope, while the Type 2 or confocal microscope, as it is frequently called, is

The Magneto-Optical Readout Process 487 coherent. The physical difference between a Type 1 and a Type 2 microscope lies primarily in the form of the detector, assuming both to be illuminated by a point source. For the Type 1 case, the detector effectively collects all the light from each scanned object point (hence its linearity in intensity). This can be achieved by having a detector of infinite extent (and uniform sensitivity), or by focusing onto a detector of finite extent using a collector lens. This latter arrangement is that of disc player systems. In the Type 2 microscope, the detector is effectively point-like. This can be achieved by a detector of truly infinitesimal extent, by a coherent detector, or more realistically by placing a physical aperture (pinhole) in the back focal plane of the collecting lens. The confocal nature of the Type 2 microscope confers upon it some interesting imaging effects, which we discuss in Sec. 5 . For the present we concentrate on imaging in Type 1 systems, which resemble more closely the optical layout in read heads. The basic optical arrangement of the Type 1 scanning microscope is shown in Fig. 4a. A scanning point source illuminates, via the objective lens, a very small region of the object and the power transmitted by the collector lens is measured by the detector. The system shown operates in transmission. However, we could equally well imagine a reflection mode, and in this mode it is most likely that the objective lens and the collector lens are one and the same. Furthermore, the point source in Fig. 4a could be removed to infinity, resulting in collimated, plane waves incident on the objective. This latter arrangement is just that arising from the use of a collimated laser as the light source. Also, instead of scanning the point source, we could achieve the same effect by scanning the object itself. A Type 1 scanning microscope modified in these three respects is shown in Fig. 4b. The similarityto the arrangement of an optical disc readout system, with scanning achieved by the disc’s rotation, is straightway apparent.

3.2

Diffraction Theory for the Optical Disc Player

The optical layout of a more realistic readout head to that of Fig. 4b was shown in Fig. 3b. (Since we are primarily concerned with the readout signal itself, as opposed to tracking and focus servo systems, the focus and tracking optics were omitted from this layout). Unfolding the optical path of this readout head yields an optical system as shown in Fig. 5 where for simplicity only a single detector path is illustrated. Here we have, for the sake of generality, included explicitly the limiting aperture stops of the

488 Magneto-Optical Data Recording system. In practice, the limiting aperture of the readout head is usually determined simply by the clear aperture of the objective lens and is identical in both incident and reflected paths. However, as we shall show later (in Sec. 5.2), the optical response of the readout system can be altered significantly by the inclusion of real, physical apertures in the optical path.

objective

obiective and

collector

Figure 4. (a) Scanning microscope in transmission and @) modified to the disc readout head layout.

collector

I

L

Figure 5. Unfolded optical path of a typical readout head.

detector

The Magneto-Optical Readout Process 489 The ultimate aim of this present section is to calculatethe form of the signal current that is generated by the photodetectors. This in turn requires knowledge of the light intensity, i.e., the square magnitude of the electric field (in strict terminology, the irradiance), in the detector plane (discussed in Ch. 10). The problem, therefore, reduces to one of calculating electric field amplitudes throughout the optical system of Fig. 5 . In particular, we are concerned with the electric field existing at various important planes in the system; namely the incident aperture plane, the focal plane (the disc surface), the reflected aperture plane and, finally, in plane of the detectors. Kirchoff's Diffraction Formula. To calculate the electric field amplitudes at various points in the optical path of the readout head, we can make use of the well known Kirchoff diffraction formula (see e.g., p. 394 of Ref. 4) which gives the amplitude in one plane {x2,y2} in terms of the electric field distribution in a previous plane {xl,yl}, (see Fig. 6). The formula itself is usually written as +m +m

Eq. (13) describes the spatial variation of the electric field, in where yl(xl,yl) amplitude and phase, in the plane {xl,yl}, y2(x2,y2)describes the field in the plane {x2,y2},k is the wavenumber given by 2 d A ,A is the wavelength of the radiation and R is the distance between two points of interest in the two planes. This expression assumes that ylis slowly varying compared to the wavelength, and that both yl and y2 are only appreciable in a region around the optic axis which is small compared to the axial separation, z, of the two planes. It is not the intention here to derive Eq. (13), which arises from a solution to the Helmholtz equation (see p. 462 of Ref. 4). However, it will be instructive at this stage to review briefly the physical origins of this important relationship, which lie in Huygen's principle. Huygen's principle, first published in 1690, states in essence that every point on a primary wavefront serves as the source of spherical secondary wavelets, such that the primary wavefront at some later time is the envelope of these wavelets. In other words, we can represent an arbitrary wavefront y as a collection of spherical point sources, and the field at any other point in space is simply the sum of these spherical waves. Mathematically, we represent a spherical wave disturbance emanating from a point source as

490 Magneto-Optical Data Recording

Eq. (14) where the exponential term in Eq. (14) describes how the phase varies as a function of distance r from the source, the llr term relates to the observation that the energy falls off as 1 h 2 , and the f term indicates a diverging or converging wave. (Note that we here use I to denote radial distance from a pointyas opposed to reflectivity amplitude as in Sec. 2.2). Thus, if the disturbance on a particular plane is given by yl(xl,yl)ythe Huygen's Principle development gives the field at some second plane as

planel

where r is now a function of the separation, z, between the two planes and the position (x2,yJ in the second plane. We can see that this simple approach yields a result very similar to the Kirchoff formula of Eq. (13). The missing l/jA factor in the Huygen's expression arises from the application of Green's theorem to the solution of the Helmholtz equation (see p. 462 of Ref. 4), the l/j term implying that the phase of the secondary wavelets leads that of the primary by 90".

Figure 6. Coordinate system for diffraction calculations.

The Magneto-Optical Readout Process 491 The Fresnel, Far-Field and Fraunhofer Approximations. The Kirchoff diffraction formula can be simplified under certain conditions, many of which are applicable to optical disc readout systems. The first simplification is the so-called Fresnel approximation. Consider the two points P and Q on planes 1 and 2 of Fig. 6. The coordinatesof these points are (xl,yl, 0) and (x2,y2,z) respectively. We can therefore write the separation, R, of points P and Q as

R~ =(x2

+(y2-yJ2 + z 2

i.e., R = Z

If we assume that z is large compared to the distances (x2 - xl) and (y2 yl), then Eq.(16) can be expanded using the binomial series to yield 2

Eq. (17)

(x2 - x1)

1

+ (Y2 - Y d 2 +. ..... 22

Ignoring terms higher than the second power, the diffraction formula of Eq. (13) now becomes

where we have set R = z in the amplitude factor multiplying the integral. Equation (18) can often be evaluated using tables of Fresnel integrals or using the elegant Cornu spiral (see p. 449 of Ref. 4). The region of applicability for Fresnel diffraction is usually referred to as the near-field region, which mathematically we can represent as z (( k(x12+ y12)max. A more stringent approximation to Eq. (13) is the fur-field case. Here, strictly, it is assumed that the second plane is removed to infinity. However, the condition z )) (k/2)(x?+y?), is usually considered sufficient. The argument of the integrand in Eq. (18) can then be simplified to give the far-field or Fraunhofer approximation

492 Magneto-Optical Data Recording

So fkr we have considered the propagation of the optical field from one arbitrary plane to another. What happens if a lens is introduced to focus the propagating beam? For an ideal lens, under the paraxial approximation, we can consider the presence of the lens to be equivalent to multiplication of the incoming wavefront by a phase factor

1:;

4 x . y ) = exp

+ -(xz

+ y ’)

1

This accounts for the fact that the ideal lens will collimate a spherical wavefront originating from a point a distanceffrom the lens, or, alternatively, convert an incident plane wave into a spherical wave and bring this to a perfect focus at a distancefaway. Inserting this phase factor into the Fresnel diffraction formula of Eq. (18) and simplifying the argument of the exponential in the integrand, we find that the field at the focal point of the lens (i.e., at z =f)is given by

where we have assumed that the lens is placed in the { x , j l } plane. It should be noted that the above expression is identical to the Fraunhofer approximation of Eq.(19), with the distance z replaced by$ The field at the focal point of an ideal lens therefore obeys Fraunhofer diffraction, and it is this Fraunhofer equation that we will find most useful in the study of optical disc readout systems. However, we will first rewrite the Fraunhofer integral into a slightly different, but most useful form. Fourier Transform Methods. Inspection of Eqs. (19) and (21) reveals them to be nothing more than Fourier transforms, albeit in two dimensions. Defining the two dimensional Fourier transform pair as

The Magneto-Optical Readout Process 493

-OD

-m

enables us to recast the Fraunhofer expression of Eq. (19) as

where Iy(x2//zz,y2/;zZ)is the two dimensional Fourier transform ofthe field vl(xl,yl) in the {xl,y,} plane. Strictly the Fourier transform relationship between the two fields ly, and y2 is satisfied only on a spherical surface centered on the axial point of the {xl,yl} plane, and this is the origin of the exponential term on the left hand side of the above expression. The simple and elegant result of Eq. (23), which tells us that the optical field on an arbitrary plane can be calculated as the two dimensional Fourier transform of the distribution on some prior plane, will enable us to calculate the form of the optical beam as it propagates throughout the optical system of the readout head. If one of the planes contains a (ideal) focusing or collimating lens, we can again use Eq. (23) to describe its effects, but with z replaced by the focal lengthfof the lens. Diffraction at a Circular Aperture. A case of some importance in the treatment of optical readout systems is that of diffraction at a circular aperture. Consider an aperture placed in the { x l , y , }plane. The optical field immediately after the aperture is given, provided that the dimensions of the aperture are large compared to the wavelength, simply by the product of the incident wavefront, yq(xl,yl), and the so-called aperture (or pupil) function,p,(x, J,), which describesthe transmission properties of the pupil. The wavefront at some distant plane away from the aperture is, accordingto the far-field approximation of Eq. (23), given by the 2-D Fourier transform of ~l(xl,y,)p,(xl,y,). If the incident wavefront is a uniform plane wave, then ry,(x, ,yl)is a constant and the far-field diffractionpattern will be the 2D Fourier transform of the aperture finction itself. For a clear, circular aperture we have

494 Magneto-Optical Data Recording

or, sincep,(x,y,) is radially symmetric

i

1, r, I a

Eq. (25)

Plkl) =

0, r, >a

where r1 = (x, + yf)”*. The solution of the .D Fourier translvrm of the circle function of Eq. (25) can be evaluated by various means and is well known (see e.g., p. 416 of Ref. 4). Here we take advantage of the radial symmetry of the problem and note that in general, for a radially symmetric function, p(r), the 2-D Fourier transform is equivalent to the Hankel transform, j5 (u), of&) (defined by Ch. 5 of Ref. 21),

where J, is a Bessel function of the first kind of zero order and u is a real constant. Explicitly the relationship between the 2-D Fourier transform, P(m,n),and the Hankel transform, p” (u), of the radially symmetric function p(r) can be written as

For the simple clear circular aperture described by Eq. (25), the Hankel transform is (p. 145 of Ref. 21)

The Magneto-Optical Readout Process 495 We can therefore write the f5r-field diffraction pattern in the plane {x2,y2}as

'Y,('2

3

[3

Y 2 )exp

- x2 + y 2 = 2,]

The diffracted irradiance, which is the square modulus of the diffracted amplitude, is then given by the well known Airy formula (p. 419 of Ref. 4).

Equation (30) describes the irradiance that would be observed on a screen placed some large distance z from the plane containing the circular aperture. The irradiance distribution in the focal plane of an ideal, clear, circular lens is also described by the Airy distribution of Eq. (30), but with z replaced by the lens focal l e n a j

3.3 The Readout Path in Detail We are now in a position to begin calculating the form of the optical wavefront as it propagates through a typical readout head. Consider first a simple head comprising the objective lens with its associated pupil function pl(xl,yl),the disc, the collector lens with its associated pupil function p2(x2,y2),and the detector, as shown in unfolded form in Fig. 7. Of course, in currently available optical disc systems, the objective and collector lenses and apertures are one and the same. However, for the present we will assume that they can be different.

496 Magneto-Optical Data Recording

The Focused Spot. If the objective is illuminated by a plane wave of unit strength, then, as already shown, the amplitude distribution, tyo(xo,yo), in the focal plane of the objective (i.e., in the object plane {xo,yo}),is given by the 2-D Fourier transform ofpl(x,,yl). Following Eqs. (22) and (23) we can write

where we have ignored any phase factors and hl(xo,yo)is known as the amplitudepoint spreadfinction of the lens. For a clear, circular objective aperture the point spread hnction will therefore be as given in Eq. (29) (with all subscripts 2 replaced by subscript o and z =f),and the irradiance distribution I(xo,yo)in the focal plane is simply given by the Airy formula of Eq. (30). The Airy distribution is plotted in Fig. 8 as a b c t i o n of the reduced optical coordinates (kalf)(x,2 +y,2)" = RNA(x,2 +y,2)%where NA is the so-called numerical aperture of the objective lens. The distribution reveals a large central maximum, with subsidiary maxima or rings at larger radii. The width, WAjY,of the Airy disc is usually taken to be the distance between the first two minima (actually zeros) located either side of the main peak, and is easily shown to be (Ref. 4, p. 419) WAiY= 1.22-fa = 1.22- a a NA

The Magneto-Optical Readout Process 49 7 Airy disc

untruncated 9aussian

rp E

c

0.4 0.2

0 -10

-8

-6

-4

0 2 reduced distance

-2

4

6

8

1

0

Figure 8. hadiance distributions in the focused spot. Top shows 3-D profiles for the Any disc and untruncated Gaussian. Bottom shows cross-sectional profile for the Airy disc (solid line), untruncated Gaussian (dashed line) and e-* truncated Gaussian (dotted line). All plots extend to +10 in reduced distance units of [ka(x2+3)”]lJ:

498 Magneto-Optical Data Recording For current MO disc systems, this would yield a typical spot size of around 1.9pm (A= 780 nm, NA = 0.5). An alternative definition of spot size often used by engineers is that offill width at halfmaximum (FWHM). The FWHM value for the Airy disc is approximately 0.6 A/NA, which reduces the s p ~size t estLmate to just under 1 pm in the above case. In most practical readout heads, the distribution of the optical field incident on the objective aperture is, in fact, Gaussian and not the simple uniform distribution assumed in calculationof the Airy disc (see Ch. 2). We represent the incident Gaussian profile as

where w defines the spatial extent of the Gaussian distribution. The effect of the objective aperture on this incident field is to truncate it to zero outside the aperture radius, i.e., for rl a. The amount oftruncation ofthe distribution of Eq. (33) depends on the relative sizes of the aperture radius and the width parameter w. If the term w is very small as compared to the aperture radius, then the Gaussian field is essentially untruncated since the field amplitude will be very small at the edge of the aperture. For example, a value of w = a12 will give a field amplitude at the edge of the aperture of em4times the peak amplitude vl(O,O). Under these circumstances the field in the focal plane of the lens is given, to a very good approximation, as the 2-D Fourier transform of the untruncated distribution of Eq. (33). Again, using tabulated Hankel transforms (Ref. 21, p. 145), we can show that the focused spot amplitude distribution is normalized to unity incident amplitude (i.e., n(0,O) = l), thus, ignoring phase factors,

The irradiance in the focal plane is given by the squared magnitude of the above expression, and is shown, for the case w = aI2,alongside the Airy profile in Fig. 8. It can be seen that in this so-called under-jUed situation,

The Magneto-Optical Readout Process 499 the irradiance in the focused spot is itself Gaussian, and it exhibits an e-2 diameter of

4fil - 4a we-2= -Ica

zNA’

a for w = 2

which coincides approximately with the width of the Airy profile given in Eq.(32). If the Gaussian beam is greatly expanded prior to being incident on the objective aperture, such that w )) a, then the amplitude distribution across the aperture can be considered uniform and the focused spot reverts to the Airy disc form. This is usually referred to as the overjlled case. As is evident from Fig. 8, overfilling of the objective aperture results in a narrower focused spot, as compared to the under-filled case, and is often the arrangement utilized in read-only type disc formats such as the compact disc. However, overfilling will also drastically reduce the available light power incident on the disc’s surface, which will limit its usefulness as far as recordable technologies are concerned. For MO, and indeed other recordable formats, a compromise between achieving the narrowest (Airy) read spot and ensuring sufficient write power is therefore made. A commonly adopted approach ensures that the e-l amplitude of the Gaussian beam is coincident with the edge of the objective aperture (i.e., w = a), equivalent to saying that the irradiance (power) in the incident beam has fallen to c2of its central, peak value at the aperture’sedge. The irradiance distribution in the focused spot for such a fill condition cannot be expressed in a simple analytical form, but may be calculated quite straightfonvardly using numerical 2-D fast Fourier transform (FFT) routines. The result of such a calculation is also shown in Fig. 8 and, as we might expect, for the e-l fill arrangement, the focused spot is of a form intermediate between the true Gaussian and the Airy profiles. The Detected Signal. We now move on to the process of calculating the detected signal. Immediatelyafter reflection from the surface of the disc, which here we assume to be in perfect focus, the field, v0’(xo,yo),is given by the product of the incident amplitude and the disc reflectance

500 Magneto-Optical Data Recording where we have now used y to represent the complex amplitude reflectance of the disc (to avoid conhion with radial coordinate) and (x,,y,) represents the position of the rotating disc (or in other words the scan coordinates of the focused spot). Equation (36) enables modelling of the interaction between the incident optical field and objects with variations in reflectance magnitude, reflectance phase or a combination of both. This kind of treatment is well suited to the cases of phase change type media and compact disc systems. However, it will be noted that Eq. (36) does not explicitly take into account the polar Kerr effect that, as we have seen, is responsible for generating the signal in MO disc systems. In essence, for the MO case, we are incorporating the combined effects of the Kerr interaction and the detector arm analyzer arrangement into a pseudo-reflectance change. Treatments which include the Kerr interaction ab-initio do not produce significantly different results from those derived here (see Sec. 4.2 and Ref. 22). In any case, as we shall shortly show (p. 5 13), it is possible to separate out the influence on the readback signal of the disc fi-omthat of the optical system itself. After reflection from the surface of the disc, the optical field propagates back towards the collector lens. The reflected field in the plane of the , y ~ be ) , the far-field diffraction pattern, i.e., collection lens, ~ y ~ ( x ~ will ,~~), after the 2-D Fourier transform, of the field, ~ y ~ ’ ( x ~ immediately interaction with the disc. Thus

The effect of the collector lens on the return beam is, as previously discussed (p. 492), to multiply the wavefront by the phase factor of Eq. (20), which merely collimates the reflected spherical wave. The field that is propagated back to the detector plane is, therefore, given by the field at the collector lens multiplied by the collection lens pupil function. The signal measured by the detector will be the irradiance (intensity) integrated over the detector area. Assuming that the detector has a uniform responsivity (set to unity), the measured signal will be

The Magneto-Optical Readout Process 501

Rewriting Q. 38 as

and then substituting for ly, from Eq. (37) into Eq. (39) yields

where dummy variables x, ' and yo' have been introduced to deal with the product of y2 and its conjugate. Replacing the integral inside the curly brackets in Eq. (40) by the spread function associated with the squared modulus of the collector pupil function, p2(x2y2),i.e.,

yields the detector signal in terms of the amplitude point spread function of the objective lens, the disc reflectance and the collection pupil spread function, i.e.,

502 Magneto-Optical Data Recording The Role of the Collection Aperture. It is instructive at this stage to look at two limiting cases for the detected signal expressed by Eq. (42). Consider first the case for which the collection aperture is very large, such that its pupil functionp2(x2,y2)is a constant, which we set to unity. This is equivalent to an associated spread function which is a delta hnction, g2(xo,ya)= S(xo,y,>and it follows from Eq.(42) that the detected signal will be W

which we write in short form notation as

The minus sign in the argument of y in Eq. (44) arises because strictly Eq. (43) is a form of correlation integral, and the similarity of the latter to the reciprocity used to calculate the readout signal in magnetic recording systems should not go unnoticed. The signal generation process described by Eq.(44) is seen to be one that is linear in the square modulus of the electric field amplitude (i.e., the irradiance), and is usually termed incoherent imaging. The second limit to consider is when the collection aperture is very small, such that it can be described by a delta function. The corresponding collector spread hnction is simply g,(xo,yo) = 1 and Eq. (42) becomes

The process is now linear in field amplitude and is usually termed coherent imaging. In reality the signal generation will be neither wholly incoherent nor wholly coherent. In particular, in optical disc readout heads, it is invariably the case that the objective lens and the collection lens are one and the same. Furthermore, since, at least in current systems, the objective lens also defines the limiting aperture, the incident and collection apertures are also one and the same. The imaging in such so-called “partially coherent” systems is slightly more complicated than as described by Eqs. (44) and (49, and we shall return to it in Sec. 4.2.

The MagnetolOptical Readout Process 503 The Convolutional Model. The expression of Eq. (43) has been u d by several authors to calculate the form of the readout signal in a variety of optical disc formats, and in such works is often referred to as the convulufional model (see Ref. 8, [p. 1221, and Refs. 17, 18, 24, and 25). The trament ofthe optical system as a simple incoherent imaging configu-

ration yields results that agrce quite wcll with experimental observations. In Fig. 9, for example, the readout signal from a TbFeCo disc sample is compared to that prdicted by the convolutional m d c l . The agreement is g d , at feast within the noise level of this particular measurement.

Figure 9. (Top) Polar Kerr image of bits stsltically recorded into a TbFeCo film.f”J (8oHm) The s i p 1 from the trnck center for an isolated bit (solid line) and the theoretical prediction based on the convolutional madel ( h h e d line).

504 Magneto-Optical Data Recording

4.0 OPTICAL SYSTEM CHARACTERIZATION 4.1 Linear Incoherent Systems We saw in the previous section that, under certain conditions, optical systems of the kind used in readout heads behave in a linear fashion. We might, therefore, imagine that we can apply many of the well-known results of linear system theory developed in other fields, such as for treating electrical and electronic systems, to the optical case, and this is indeed true. In linear electrical or electronic systems, for example, it is commonplaceto characterize the behavior of the system in terms of its impulse response, step response or frequency response (transfer fbnction); three descriptions that are all interrelated. Since we have shown that the readout system can be modelled to a first approximation as incoherent, we concentrate initially on that case. Impulse Response. The optical equivalent of an electrical impulse is a single point object that we can represent as a delta function, i.e.,

Substituting this reflectance in the expression for the detected signal for the incoherent imaging case given by Eq. (43) yields an impulse response of the form

If the system is using an unobscured circular aperture with constant illumination, then the impulse response is given by the well-known Airy disc pattern of Eq. (30) and Fig. 8. For an untruncated Gaussian beam, the impulse response is that given by the square magnitude of Eq. (34) and also shown in Fig. 8, with the effects of truncation on the impulse response being as discussed previously. Once the impulse response is known, the output of the system for any arbitrary input (i.e., any arbitrary disc reflectance pattern) can be calculated quite simply using the convolution relation of Eq. (44). The Step Response. Let us now consider the step object defined by

Eq.(48)

Y(X0?YO)=

{

1, x ,

>o

0, x ,

The Magneto-Optical Readout Process 505 This yields, again from Eq. (43), a detected signal of X.

m

-m -m

As we might expect, the impulse response and the step response are related to one another. For one-dimensional systems, such as linear electronic systems with time dependent inputs and outputs, it is well known that the step response is, in fact, simply the running integral of the impulse response. However, in the two-dimensional case, it is necessary to first integratethe impulse response entirely in the direction orthogonal to the step (the y direction in the above equation), before performing the running integration. This process yields the So-called line spreudJitnction. For a uniformly illuminated circular objective aperture, Eq. (49) becomes

where p is the reduced coordinate equal to ka(x,2 +y,2 )ll2Kand multiplicative constants have been ignored. Equation (50) can be solved analytically in terms of Struve functions and the Struve However, since the latter only exists in tabular or series from, it is now more straightforward to evaluate the double integral numerically, using a series expansion for the Bessel function. The resulting step response is shown in Fig. 10, along with that for untruncated Gaussian illumination [calculated by numerical integration of the square of the spread function of Eq. (34)]. Also shown in the figure is an experimental step response obtained by recording a pseudo-step It object, such as that shown in Fig. 23, into a TbFeCo disc ~ample.[~~I[~*] is clear that the experimental and theoretical results are in close agreement. Note that the Gaussian step response is sometimes approximated as an error functionin studies of the performance of detector and equalizationchannels.[2q The Incoherent Optical Transfer Function. For linear electrical systems, it is often most useful to describe the system in the frequency domain, as opposed to using the time domain descriptions of step and impulse response. We therefore talk of a frequency response, or transfer function, in which the variable is temporal frequency (i.e., l/time), and we

506 Magneto-Optical Data Recording know that the impulse response and the frequency response form a Fourier transform pair. Again the same approach may be used for linear optical systems, where we talk of the optical transfer function, or OTF, in which the variable of interest is spatial frequency (i.e., Vdistance) (see p. 508 of Ref. 4). In effect, the transfer function approach considers the imaging of sinusoidal objects, and the spatial frequency is the reciprocal of the crest to crest distance of a sinusoidal waveform used as a basis function in the analysis of the object and image. The units of spatial frequency are simply (meter)-', however, for optical systems it is more common to talk of spatial frequencies in terms of lines per millimeter. For example, a crest to crest distance of 1 pm in the object has a spatial frequency lo6 m-l, or equivalently 1000 lines per mm.

1.21

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Figure 10. Theoretical step responses for Airy disc (solid line), untruncated Gaussian illumination (dashed fine). Also shown are experimental points from recording a pseudostep in a TbFeCo film (open circles). Distance units as for Fig. 8.

The incoherent OTF is given by the 2-DFourier transform of the incoherent impulse response, i.e., the 2-D transform ofthe square magnitude of the amplitude point spread function

The Magneto-Optical Readout Process 507

where v, and v,, are spatial fiequency coordinates in the x and y directions and O(v,,v,,) is the OTF.However, in Sec. 3.3, Eq. (3 l), we showed that the amplitude point spread function is simply the transform of the objective pupil function, therefore

which, using the convolution and scaling theorems for Fourier transforms, yields

Eq. (53)

00

-m

The incoherent OTF is therefore given by a 2-D convolution of the aperture function with its (reversed) complex conjugate. This is equivalent, as shown in the above equation, to correlation of the aperture function and its conjugate. For circular apertures under uniform illumination and in the absence of aberrations, the incoherent OTF is purely real and Eq. (53) tells us that for any particular spatial frequency pair (v,, vv) it is given by the area of overlap of two circles with their centers displaced by an amount v,Af in the x direction and v,Afinthey direction, as shown in Fig. 1l(a). The result of performing this operation in two dimensions yields the OTF shown in Fig. 1l(b), where the usual procedure of normalizing the OTF to unity at the origin has been adopted. It is clear fiom Fig. 1l(a) that the OTF will be zero for spatial frequencies above the critical value 2 a l g where a is the radius of the objective aperture. In terms of the numerical aperture of the objective lens, this cutoff frequency occurs at 2 N N A . For objects with detail only in

508 Magneto-Optical Data Recording one direction, e.g., along the x axis only, the OTF can be expressed analytically as (see p. 35 of Ref. 21)

where P is a normalized spatial frequency given by vAf/a. incoherent otf

Figure 11. (a) Correlation of pupil functions with rn = v,A f and n = vu;lf and (b) the resulting incoherent OTF plotted against spatial frequency in x and y directions (to m = *2a and n = *2a).

In general, the OTF is a complex hnction having both magnitude and phase. The magnitude of the OTF is usually referred to as the modulation transfer jinction (MTF), whilst the phase part is the phase transfer firnction (PTF). In the example above the OTF is purely real and so the MTF in this case is simply given by Eq. (54) and the PTF is zero. This would not be the case in general when, for example, aberrations or defocus are present, or when the impulse response is asymmetric (we will meet an example of this latter case when we treat magnetically induced superresolution imaging in Sec.5.1). The MTF is also often described in the literature as being a measure of the visibility or contrast in an optical system. In this respect we usually talk of the MTF as being the ratio of the modulation depth in the image to that in the object, assuming a sinusoidal type object, i.e.,

The Magneto-Optical Readout Process 509

Eq.(55)

M F ( vx,Vy)=

(vx ) Mdj ( ) M i n t

7

vy

v x 9 vy

where M =

AC component DC component

This description is however, just another way of saying that the MTF is the magnitude of the OTF. It is worthwhile to note at this point that, by the introduction of the concept of the OTF, we have removed from our description of the imaging problem any dependence on the form of the medium itself. The OTF therefore provides a measure for the performance only of the optical system, and as such can be used to comparethe merits of various, alternative optical designs. Measurements of the MTF. A wide variety of experimental techniques have been devised for the measurement of the OTF, or more fiequently the MTF, in all manner of optical systems. We have shown above that, in principle at least, the MTF can be obtained by measuring the impulse response and taking its 2-D Fourier transform, by measuring the line spread fbnction and take its 1-Dtransform (this generates a single contour of the MTF along a direction orthogonal to the orientation of the line), or by recording the step response of the system, differentiatingto yield the line spread fbnction and then l-D transforming the result. All these methods are in common use. However, impulse response testing is difficult if the spatial extent of the impulse response is small, which for optical disc systems means if the focused spot size is small (which indeed it is!). Step response testing also has a drawback in so much as the process of differentiation inevitably increases the noise in the measurement, particularly in the high spatial frequency region where the MTF itself is generally small. This can lead to large measurement errors in this important high frequency region. The MTF can also be obtained by the system’s response to a sinusoidal object, where the image modulation depth is measured as a h c t i o n of spatial frequency. Unfortunately sine wave targets suitable for testing optical disc heads (particularly MO sine wave targets!), are not readily available. Yet another approach commonly used in the general optics field is to measure the square wave response of the system, which is sometimes called the contrast transfer function (CTF). The uncorrected CTF overestimates the MTF because of the influence of harmonics other than the fundamental in the square wave. However, it is staightforward, theoretically, to convert from the CTF to the MTF and vice-versa using

510 Magneto-Optical Data Recording simple Fourier series Furthermore, if the spectrum of the CTF is obtained by some method and the relative amplitude of the fundamental harmonic is recorded as a hnction of spatial frequency, then this will reveal the MTF. Of a!! the MTF measurementtechniques discussed above, the one that is most easily adapted to the situation of an optical disc player is the square wave response approach. For magneto-optical recording systems (and indeed for phase-change systems or any write-once type technology), it is a straightforward task to record a square wave pattern onto the disc, over a wide range of spatial frequencies. The amplitude of the hndamental harmonic in the readout signal is then measured, for example by using a real-time spectrum analyzer or by capturing the signal using a storage oscilloscope and performing a fast Fourier transform in software, to reveal the MTF. The result ofjust such an approach is shown in Fig. 12.[291The wavelength used in this study was 780 nm, and the objective lens had a numerical aperture (taken to be equal to alf ) of 0.53. Such values are fairly typical of systems currently available, and result in a spatial cutoff fiequency of 1.36 x lo6 m-l, or 1360 lines per millimeter. Also shown in Fig. 12 is the theoretical result of Eq. (54), and it can be seen that the agreement is good. The theoretical MTF for an e-* truncated Gaussian is .also shown for comparison.

0

0.5

1 1.5 norwlised spatial frequency

2

2.5

Figure 12. Modulation transfer hctions for Airy disc illumination (solid line), Gaussian illumination (dashed line) and from experiment (circles[*gl).Reduced spatial frequency units are vkf/a.

The Magneto-Optical Readout Process 511 In some interesting work, McDaniel and Finkel~tein[~~] used a slightly different noise-based technique to reveal the MTF. Their approach relies on the fsct that if the media noise in a magneto-optical disc system is truly white then, after subtraction of the electronic noise component, the measured noise spectrum should be identical in shape to the MTF (this is in effect an application of the well-known white noise testing method). Again good agreement with incoherent imaging theory was obtained. Once the experimental MTF has been determined it is straightforward, if desired, to make the comparisons with theory in the spatial domain, simply by Fourier transformation. As discussed in the previous section, the (inverse) transform of the MTF will yield the line spread function for the system. Just such a comparison is shown in Fig 13rz51and the agreement with theory is, once more, considerable. 1.21

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Figure 13. Line spread hctions for Auy disc illumination (solid fine), Gaussian illumination (dashed line) and from experimental noise measurements

4.2 Partially Coherent Optical Systems In the previous section, we modelled the performance of the optical disc readout head in terms of an incoherent imaging system and presented experimental results that showed that indeed, it appears that such a theoretical descriptionaccurately predicts many important featuresof head

512 Magneboptical Data Recording

behavior. However, the assumption of incoherent imaging is only true if, as pointed out in Sec. 3.3, the collection aperture is assumed to be infinitely large such that all the light reflected from the disc's surface reaches the detectors. In reality the size of the collection aperture is limited. The optical system is; therefore, not truly incoherent but partially coherent: The theory of imaging in partially coherent systems is a little more complex than the coherent or incoherent cases, and was perhaps first addressed in detail by ~1 approach was applied to scanning Hopkins in the 1 9 5 0 ~ . [ ~Hopkins' microscopes by Wilson and Sheppard and their co-workers in the late 1970s and 1 9 8 0 ~ . [ ~ ~ 1 - [ ~ ~ 1 We begin our study of partially coherent systems by returning to Eq. (42), that provides a spatial domain description for the signal detected in a scanning microscope of the configuration of Fig. 7 for arbitrary incident and collection apertures. This equation is reproduced below and we remind ourselves that h, is the amplitude point spread function of the objective lens, yis the disc's amplitude reflectance and g, is the point spread function associated with the square modulus of the collection aperture hnctionp,.

We would like to recast this equation into a form which describes the imaging in terms of a transfer function for this general, partially coherent case. To achieve this we first replace the expressionsfor the disc reflectance by their transforms defined by

The Magneto-Optical Readout Process 513 where v, and vyare spatial frequencies in the x and y directions respectively and v,' and vy' are dummy spatial frequency variables introduced for convenience in treating the product of yand y*. Substituting Eq. (57) into Eq. (56) yields the expression for the detected signal

Replacing the t e r n in square brackets by the hction C(v,,y,; v,' 3') gives

-m

exp(2@[ (v,

+ (vv - v;)ys])d",d";dvvdv;

- vx')xs

Thus we can represent the detected signal intensity for any arbitrary object by Eq. (59) in which the hnction C(v,,vy;v,'vy') depends only on the optical system [in fact, from Eq. ( 5 8 ) we see that it depends only on the point spread functions of the incident and collection apertures h, and g,]. C(v,, vy;v,'vy') is a form of transfer function, and is usually termed the partially coherent transfer jimction. It describes the intensity of the components in the image with spatial frequencies equal to ( v, - v, ') in the x direction and ( vy - vy') in the y direction. The introduction of the transfer function concept as defined by Eq. (59) provides a unified mathematical framework within which we can treat incoherent systems, which were previously defined by Eqs. (53) and (54) in terms of an incoherent transfer function describing the intensity of image components arising from an intensity object, coherent systems which can be described in terms of a coherent transfer hnction describing the amplitude of image components

514 Magneto-Optical Data Recording arising due to an amplitude object, and the partially coherent case which is linear neither in amplitude nor in intensity. From Eq.(59) we can see that if C( v,, vr;vx'vr')is everywhere equal to unity, then the detected intensity is simply

This might be described as perfect imaging, since the detected signal is just a replica of the object's intensity reflectance. However, it should be pointed out that such so-called ''perf& imaging" may in some circumstances be undesirable; for example, purely phase objects would be undetectable! Just as for incoherent imaging where we were able to express the transfer h c t i o n in terms of the system pupil functions, so we can recast the expression for the partially coherent case. From Eq. (58) and (59) we have

Remembering that the point spread fhctions h, and g, are just the Fourier transforms of their respective pupil fhctions [see Eqs. (31) and (41)] then substituting for h,, h,* and g2 into Eq. (61) and manipulating, yields

For circular, aberration-free pupils, this expression represents the area of overlap between two circles representing p1 centered on (v,AJ vyilf)and

The Magneto-Optical Readout Process 515 ( v,%< v,,.'A.) which also falls within the circle defined by p,. This result is very s m l a r to that obtained for the incoherent case in Eq. (53) except that now we have included explicitly the effects of the collection aperture p2. In general C( v,, v,,; v,'~,,') is a function of four variables, which makes graphical representation difficult. However, if we restrict our analysis to objects which are functions of one direction only, for examplethe x direction, then the imaging properties of a system may be studied by plotting C( v,, v,') as a function of v, and v, '. Under these circumstances Eq. (62) reduces to

-m

that for a particular pair of spatial frequencies v, and v, 'is represented by the area shown shaded in Fig. 14, again assuming circular, aberration-free pupils. For currently available disc players, the case of most interest is that of equal incident and collection apertures. Evaluating Eq. (63) over all (v,;~,') space for this particular case yields a 3-D representation of the transfer function as shown in Fig. 15a. An alternative visualization is as a contour plot, as shown in Fig. 15b. It should perhaps be pointed out that although v, and v, 'are plotted in Fig. 15 in orthogonal directions they do in fact represent spatial frequencies in the same direction. This may at first seem confusing and obscure the interpretation of C( v,, v,'), but it should be remembered that v, is associated with the object's reflectance amplitude spectrum while v,' is associated with the complex conjugate ofthis spectrum. Furthermore, for a large class of objects it is the form of C( v,;O) and C(0;v,'), i.e., along the v, and v,' axes, that defines the imaging process. These points can perhaps be best illustrated by consideration of a few simple examples. Consider first an object with a cosinusoidal amplitude reflectance defined by

516 Magneto-Optical Data Recording The spectrum of such an object consists of impulses at spatial frequencies (k vo;iv,) in (v,; v,’) space, thus the values of the transfer function that are of concern are C(+ vo;+ v,) which lie on the lines v, = k v,‘. In other words the intensity of the components in the image with spatial frequencies equal to (v,-v,’), i.e., 2v, in this case, will therefore be determined by C(kv,;Tv,). From Eqs. (57), (59) and (64) we can write the image intensity as

which we can write as

r ( q )= --[C(v,;v,)+ 1

c(-v,;-vo)+

C(v,;-v,)

.exp(4ljv0x,)+ c(-v,;vo)exp(-4~vOx,)]

Figure 14. Correlation of pupil functionsto reveal the partially coherent transfer function where p = v 2f and q = viAJ:

The Magneto-Optical Readout Process 51 7

(b)

J

-3

-2

-1

0

1

2

3

normalised spatial frequency

Figure 15. (a) Partially coherent transfer function for equal incident and collection apertures (axes are reduced spatial frequencies v$ f la and v x ' fl la and extend to *3). @) Contours at values of 0.8, 0.6,0.4,0.2 and 0 for the transfer function of (a) normalized to a maximum value of unity.

For a system with the transfer function shown in Fig 15, symmetry reduces the above expression to

I(XJ = -[C(v,; 1 2

V")

+ C(v,;-v0)c0s(4nv0x,)]

where as we might expect a reflectance amplitude of cos(2;rcv,x) has led to an image intensity component of cos(4zv,x) plus a DC term. The performance of the partially coherent system for this simple object is, therefore,

518 Magneto-Optical Data Recording the same as we would expect from a completely incoherent system that would image an object reflectance intensity of cos2(2q,x) into a DC term and an intensity component of cos(4lcv,x) (this is easy to see if we remember that cos2x = [ 1 + cos(2x)/2]. In fact the form of the partially coherent transfer function along the (v, - v,') axis as a function of (v, - v,') is, assuming equal, circular, aberration-free objective and collection apertures, of the same form as given by Eq. (54). The simple cosinusiodal reflectivity amplitude variation described above is perhaps of limited practical application. As a second example therefore, consider an object described by

-[2

y(x,) = 1 1+ bcos(21cvoxo)]=

-2!{

1+ -[exp(j2zvoxo) b

2

+ exp(- j21cvox,)]}

where b is, in general, complex and of small magnitude. A purely real value for b would represent a change in reflectivity amplitude, whereas a purely imaginary b would represent a periodic phase object. Following the procedure adopted for the simple cosine object, the image intensity due to the object of Eq. (68) can be written

Eq. (69)

1 b J(xJ = -c(o;o)+ -[c(v,;o) + c(0;v~)]cos(2zv,x~) 4 4

where for simplicity we have assumed that b is purely real, that b is small such that terms in b2can be ignored and that C(v, ,v, ') is an even function of vx and v,'. We see that in this case the imaging is effectively determined by the form of the transfer function along the v, = 0 and vx'= 0 axes. For the readout head system defined by the transfer function of Fig. 15, it is apparent that C(v,,O)= C(O,v,') and so Eq. (69) can be reduced still further to

'(XJ

=

1 b -c(o;o) + -C(v,;O)cos(2nv,xs) 4

2

and the detected signal depends only on the form of the partially coherent transfer function along the v,' = 0 axis. Under these circumstances the partially coherent imaging system again results in exactly the same image as would be obtained for a completely incoherent optical system.

The Magneto-Optical Readout Process 519 The kind of weakly modulating object such as that described above

m.(68) with b small] is, in fact, just the kind of object that we come across

in optical recording. For example, the readout process in phase-change systems relies on a relatively small modulation in the reflectivity of the sample, with reflectivities of 55% and 45% for crystalline and amorphous phases respectively being fairly typical. Likewise, in magneto-optical systems, we can view the polar Kerr component as a weak modulation of the ordinary reflection amplitude. This makes interpretation of the partially coherent transfer function more straightforward since imaging is effectively determined by a one-dimensional version of the transfer function, for example C(v,,O;O,O). For the case of equal incidence and collection apertures, C(v,,O;O,O) has the same form as the incoherent transfer function, and this helps to explain why the incoherent model of the readout head can often yield results in close agreement with experimental observations. The 1-D version of the partially coherent transfer function is used by many authors to compare the performance of different optical readout head d e s i g n ~ . [ ~ ~lIt[should, ~~ ] . however, be remembered that,in the general case, we must consider the full form of C( v,, vr; v,'~,'), the I-D approximation only being valid for certain optical arrangements and certain classes of objects. Computational Calculations. In the examples of partially coherent image calculations given above, simple object structures were chosen to enable analytical solutions to Eq. (59). For more complicated and realistic objects we need to resort to numerical computations. Numerical solution of Eq.(59) for 2-D objects is cumbersome. However, for 1-D line type objects, Eq.(59) can be rearranged in a particularly simple form as

which can be evaluated via FFT-based matrix techniques. In detail, a (square) numerical matrix representing a given transfer function C( v,, vx'), calculated according to Eq. (63), is first array multiplied by a matrix representing the object's spatial frequency spectrum r(v,) and a transposed version of that spectrum, F(v,'). The 1-D FFT of each row of the resultant matrix is then formed, followed by the 1-D inverse FFT of each column of the result of the forward FFT process. The image intensity I(x,)

520 Magneto-Optical Data Recording is revealed by taking the values along the diagonal (xs' = x,) of the final matrix. As an example of this process some of the steps in the calculation of the image of a typical two bar resolutiontarget, using under-filled Gaussian illumination in an optical system with equal, circular, aberration-free, incident and collection apertures, are shown in Fig. 16.

Figure 16. Steps in the computation of partially coherent images. (u) Shows the object, a two bar reflectance type resolution target of length I6h/NA, while (b) shows the corresponding image. The (absolute value of the) object's spectrum matrix TT*is shown in (c) while (4 is the partially coherent transfer function assuming under-filled Gaussian illumination and circular, aberration-free apertures (axes in (c) and (4 are reduced spatial frequencies and extend to *2).

The Magneto-Optical Readout Process 521

Figure 16. (Cont’d.)

To calculate the partially coherent image of a truly two-dimensional object, it is more convenient to revert to the representation of the image intensity as expressed in Eq. (38). This equation can again be evaluated by FFT techniques, as indeed was shown in the early 1980’s by Jipson and However, it should be pointed out that in Eq. (38) the effects of the optical system and the object properties are inextricably combined. We therefore lose the benefit of the transfer function approach in which the properties of the optical system or readout head are separated out from those of the disc itself, allowing detailed and meaningful comparisons of various readout head designs.

522 Magneto-Optical Data Recording Evaluation of Eq.(38) first begins with calculation [via Eq. (3 l)] of the amplitudepoint spread function, h,, ofthe objective Iens. The product of this point spread function and the object's amplitude reflectance for a particular scan position (x,,y,) is then formed and the resulting amplitude distribution, yo',is propagated back to the collector aperture by taking its 2D FFT. The field amplitude at the collector is then multiplied by the collector pupil function and the square modulus of the result is integrated (summed) over the detector area to reveal the image intensity for the scan position (x,,yJ. The process is repeated for all scan positions until the complete image is generated. The computations involved are straightforward but relatively lengthy, since each scan position requires the calculation of a 2-D FFT. As an example ofthis method, Fig. 17 shows the two-dimensional partdly coherent image of an array of circular bits, again imaged using underfilled Gaussian illumination [see Eq. (35)] and circular apertures. Evaluation of this image involved computation of 16 K , 128 x 128 2-D FFTs. Magneto-OpticalEffects. The explicit inclusion of MO effects into the framework of partially coherent imaging has been described by Milster As we might expect, the partially coherent transfer function and C(v,, vy;v,' 5')remains identical whether we consider a magneto-optical or a straightforward reflectance type object. However, the influence of the transfer function on the resultant image intensity is subtly different. For magneto-optical objects then, we replace the product of reflectance spectra TT* in Eq. (59) with a so-called medium fitnction M( v,, vy;vx'vy') whose form depends on the object's MO properties (i.e., Ken rotation and phase shift between x and y reflected components), its ordinary reflectance properties and on the form of polarization detection employed. In the single detector case (Fig. 3a), with the analyzer aligned at an angle p to the incident x polarization direction, the medium function is given by

1

- r*(vi,v,,'

)

cos p + A*(vi,v,,') sin p]

The Magneto-Optical Readout Process 523

Here, the variation as a function of position of the Kerr angle, a(x,,y,) as given by Eq. (lo), and phase difference between x and y reflected polarization components, +(xo,yo),are explicitly included and the function A( v,, vr) is interpreted simply as the Fourier transform of the distribution of these magneto-optical properties multiplied by the ordinary reflectance r,. Note that for the case a(x,,y,) = 0 and ,8= 0 (i.e., zero Kerr rotation and analyzer aligned with incident polarization axis), the medium hnction becomes simply equal to I F and we revert to the case of ordinary reflectance imaging.

Figure 17. (u) An may of reflectance type circular bits and (b) the corresponding twodimensional partially coherent image. Under-filled Gaussian illumination and circular incident and collector apertures were used. The size of the arrays correspond to 16X/(NA) square.

524 Magneto-Optical Data Recording For the more common MO differential detector arrangement (see Fig. 3b), the medium function is modified slightly such that

with A( v,, v,,) taking the same form as in Eq.(72), except for a multiplying factor of 46 which arises due to the effects in the optical path of the leaky polarizing beamsplitter (see Sec. 2.2 and Fig. 3b). Now, ideally, the ordinary reflectance y(x,,y,) of a MO disc is homogeneous everywhere over the disc surface and so the reflectance spectra I‘and r* in Eq.(73) are replaced by impulses centered on v, = vv = 0 and v,’ = v; = 0 and the medium function reduces to (considering for simplicity only 1-D line objects)

Eq.(74)

M(v,;v,’) = S(v,) A*(vJ + S(v,’) A (v,)

Thus, substitutingthe above expression forM(v,;v,’) in place of r(v,)r*(v,’) in (the 1-D)version of Eq.(59) yields a magnetosptical image intensity of

m

+ ~C(O;~~)A*(v~)exp(-2~v,’x,)dv,’ This result tells us that, for MO objects and differential detection, it is purely the value of the transfer function along the v, = 0 and v,’ = 0 axes that is important in determining MO image intensity. This is subtly different from the straightforward reflectance and single detector MO case where, in general, the value of the transfer function over all (v,; v i ) space is important. Note that numerical evaluation of Eq. (75) is staightforward since it represents the sum of a forward and reverse 1-DFourier transform. The fact that differential MO readout depends purely on axial values of the partially coherent transfer function reveals some interesting features. For example, if the MO spectrum A(v,,v,) is “white” (Le., equal to unity everywhere) then, for uniformly illuminated, circular, aberration-free

The Magneto-Optical Readout Process 525 pupils, Eq. (75) predicts a line-spread function that is identical to that for the straightforward incoherent imaging case discussed in Sec. 4.1. Such a similarity has already been observed experimentally (see Ref. 25 and Fig. 13). Likewise, the MO step response predicted by Eq. (75), again assuming uniformly illuminated, circular, aberration-free pupils, is also identical to that for incoherent imaging (see Eq. 50 and Fig. 10). Once again these revelations help to explain why the simple, reflectance-based, incoherent “convolutional” model described in Sec. 3.3, and often adopted to describe the MO readout process, provides such a useful approximation. Where the incoherent reflectance model falls down is in its inability to model accurately cases in which the incidence and collection apertures are not identical nor aberration-free. In Fig. 18 for example, we show both the partially coherent MO image and the incoherent reflectance image of a two bar target {with tan[a(x,,y,)] or y(x,y,) exhibiting the values *1 respectively} for the case when the collection aperture is of the obscured annular type (see Sec. 5.2) and under-filled Gaussian illumination is used. The shortcomings of the incoherent model, since it ignores entirely the effects of the collection aperture, are now all too obvious!

5.0 NOVEL READOUT TECHNIQUES In the preceding sections we have discussed the origins of the magneto-optical readout signal and examined the way in which the optical system of the readout head itself affects the signal properties. The readout mechanism so far described is usually termed fillaperture polar Kerr detection. This is because the limiting apertures in the system are determined by the clear, unobscured objective lens’ aperture,and straightforward polar Kerr detection is used to sense the direction of magnetization in the disc. There are, however, alternative readout mechanisms and detection techniques that can be used. For example, a range of superresolution techniques with the capability for increasing the spatial frequency cutoff beyondthediffraction limit, such as the magnetically inducedsuperresolution approach pioneered by Sony,[291[311[321 are currently generating immense interest. Obscured aperture optical systems[141[221[331 are also an area of substantial research since, while not capable of a resolution beyond the difhction limit, they do offer the possibility of an increased e f f i i v e resolution by altering the shape of the optical transfer function. Confocal

526 Magneto-Optical Data Recording and semi-confocal detection, in which the readout head is modified so that the photodetector effectively behaves as a point detector, have also been shown to be usehl for reducing intersymbol interference.[181[221[261[341Yet another approach, the so-called magneto-optical phase contrast or edge detection technique, relies on detection of the light diffracted by magnetic transitions between adjacent bits, rather than the polar Kerr signal fi-om the In this section, we review these novel recorded bits themselve~.[~~][~~~-[~~~ approaches and discuss some of their advantages and disadvantages.

Figure 18. (a) Partially coherent MO image of a two bar MO resolution target for a differential readout head assuming under-filled Gaussian illumination, an unobscured circular incident aperhwe and an annular collector (width of annulus is one tenth of the aperture width). The image predicted by the incoherent reflectance model, which ignores entirely the effects the collection aperture, is shown for comparison in @). Images are 16X/NA in length.

The Magneto-Optical Readout Process 527 5.1 Superresolution Techniques There are various commonly used criteria for defining optical resolution. One of the most widely used is the Rayleigh criterion for a two-point object, which rather arbitrarily states that the two points are resolved when the intensity at the midpoint of the resulting image is 0.735 times (or less) that of the peak intensity. From a frequency domain viewpoint, on the other hand, we might define resolution as the spatial cutoff frequency in the optical transfer function. In this chapter we follow this frequency domain approach and define superresolution as an increase in the spatial cutoff frequency beyond that achieved conventionally. As we shall see, it is in fact possible to design optical systems with an improved resolution according to the Rayleigh criterion, but which do not exhibit an increased cutoff fiequency. We shall, to avoid confksion, call this latter phenomenon enhanced-resolution. The spatial frequency cutoff in conventional optical readout heads is, as we showed in Secs. 4.1 and 4.2, determined by twice the numerical aperture of the objective lens divided by the wavelength of illumination. To increase the cutoff frequency using a conventional optical arrangement would therefore necessitate an objective with increased numerical aperture, an illumination source of decreased wavelength, or both. Objectives in current use typically have NA values between 0.45 and 0.55. It is hard to imagine these values increasing dramatically, due to the strong NA dependence of focus servo tolerances, although using values of around 0.65 may be feasible. Turning to the wavelength of illumination, a typical value for current systems is 780 nm to 830 nm which is in the near infrared. Visible red laser diodes that emit at around 670 nm are now available with sufficient powers for use in recording systems, and their adoption would give a slight increase in the cutoff frequency achievable. A tremendous effort is also currently being expended in the development of solid state lasers that emit in the green or the blue. For example frequency doubled lasers operating in the green at 532 nm have been demonstrated by several laboratories, while blue lasers at around 430 nm have also been demonstrated, albeit at low operating temperatures. When such devices are able to be produced at a suitably low cost and with reliable performance we should see a quite dramatic increase in the spatial fiequencies attainable. A 0.5 NA system with 780 nm illumination, for example, yields a cutoff at 1280 lines per millimeter (correspondingto a bit period of approximately 0.8 pm), whereas a 0.6NN530 nm system gives a spatial

528 Magneto-Optical Data Recording frequency cutoff of 2260 lines per millimeter (equivalent to a bit period of 0.44 pm). Magnetically Induced Super-Resolution. The improvements in resolution discussed in the previous paragraph occur as a direct result of redncbg the size ofthe fnr.uspA spot in_ the $me, i.e.,at the surface of the disc. This raises the question as to whether it is possible to reduce the size of the focused spot by other mechanisms than the conventional approach of increased numerical aperture or decreased illumination wavelength? One possible approach is to introduce a physical aperture stop of sub-wavelength dimensions into the focal plane of the objective lens. The feasibility of this technique was first demonstrated by Ash and Nicholls in 1972 who achieved a resolution of W60 using microwave radiation (seep. 152 of Ref. 14). Extension of this method to the optical domain using nanometer sized, evanescent, fiber-optic apertures has spawned a new class of socalled near-feld imaging To implement such an approach in an optical recording system where the focal plane is the active layer of a rapidly spinning disc is, however, a daunting task. Researchers at Sony have circumvented this problem through an ingenious arrangement in which the focal plane aperture is not a true physical aperture, but rather is a magnetically induced effective aperture. This is the so-called magnetically induced superresolution (MSR) technique.[291[311[321 The development of MSR arose from the experimental observation that the amplitude of a signal from a recorded bit pattern with a period near the spatial cutoff frequency could be substantially increased by using a relatjvely high readout power in association with a small, magnetic, readout field. This increase in signal amplitude occurs because the high power readout beam heats the recording layer, which, coupled with the application of a magnetic field, erases the recorded bit pattern during the readout process. By manipulation of the readout power in tandem with the strength of the readout field, it is possible to control the size of the area subject to erasure. This is illustrated in Fig. 19a where it is apparent that the entire process is equivalent to placing a crescent shaped physical aperture, or alternatively an elliptically shaped mask, in the focal plane. The result is to reduce the area of the focused spot, and hence increase the resolution of the system. The method so far described does however have one major drawback; the information is erased immediately after it has been read! This problem has been overcome by the use of an exchange-coupled triple layer recording film comprising readout, switching and recording layers. For this system, the readout layer is still erased during readout in the manner

The Magneto-Optical Readout Process 529 described above, but the original data pattern is retained in the recording layer and copied back to the readout layer away fiom the influence of the readout beam and field. This slightly more involved process, which Sony have termedfront aperture detection (FAD), thus retains the optical masking feature without the permanent erasure of stored

/

recorded bit

j

:

mask

t yo

erased bit

+ readout field

magnetic layer

(a)

Figure 19. (a) The principle behind magnetically induced superresolution and (b) the optical mask used to model this process.

It is fairly straightforward to include the effects of the MSR optical masking process into the mathematical description of the readout process developed in Sec. 4. For simplicity we assume that the mask m(x0 ,yo)is of simple circular form, having a radius u and displaced a distance v from the scanning spot center

The form of this mask is shown in Fig. 19b, and it would be a straightforward task to model a more realistic mask shape, such as that shown in Fig. 19a, assuming we had knowledge of the temperature and magnetic field distribution in the readout layer. We can view the effects of this optical

530 Magneto-Optical Data Recording mask as modifjrlng the amplitude point spread function, h,(x,,y,), of the objective lens by multiplying by the function (l-m(x,,y,)). This is equivalent to defining an effective incident pupil function, ple& J,), given by Eq. (77)

We can now calculate the optical transfer function for this system using Eq. (63) [or for the incoherent case Eq. (53)] with the effective pupil functionplefreplacingp,. Some typical results are shown in Fig. 20.[221It is immediately apparent that the effect of the mask is to dramatically improve the high frequency performance of the optical system, yielding a response to well beyond the conventional cutoff frequency. In this respect the system truly exhibits superresolution. Also in Fig. 20 we show some experimental results taken from the work of Kaneko et al.[32]using the FAD technique. It can be seen that a response to 2500 lines per millimeter, corresponding to a bit length of 0.2 pm, is obtained for a system with a conventional spatial cutoff frequency of 1360 lines per millimeter. Sony have also developed an alternative MSR technique, known as rear aperture detection or RAD, in which the masked and clear regions shown in Fig. 19 are This approach has the added benefit of reducing the extent of the effective read spot in the lateral, cross-track direction. This greatly reduces the effects of cross-talk and opens the way for the systems with extremely narrow tracks and intertrack spacings. The technique of magnetically induced superresolution offers great promise for overcoming the diffraction limit of conventional optical heads. One potential drawback is the complex structure required of the active layer of the recording medium itself, with trilayer and quadrilayer active films needed for FAD and RAD systems respectively. Such complexity obviously has an impact on the difficulty and costs of media manufacture. It is perhaps interesting to note that Sony has recently announced yet another self-masking, superresolution readout technique in which the mask is produced in a phase-change type readout layer. This opens up the possibility of superresolution in compact disc and phasechange optical disc systems. Other researchers are working on superresolution systems in which the aperture is centralized in the readout spot (so-called central aperture detection, CAD for short).

The Magneto-Optical Readout Process 531

1.2

I

I

I

I

I

I

I

A Ip

0.2 -

....

- - - _...._

0..

0.5

0

1

1.5

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2.5

............ 0

I

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noraalised spatial frequency Figure 20. Theoretical modulation transfer functions for conventional system (solid line) and MSR system with circular mask (dashed line). Also shown are experimental results (circles).[32]

5.2

Apodization Techniques

In all of the optical arrangements so far discussed the system apertures have generally assumed to be clear and unobscured. It has long been known, however, that the use of obscured or apodized apertures can affect the optical performance in imaging systems. A common form of apodization involves the use of the so-called annular aperture with a central obscuration such that the pupil function is described by

532 Magneto-Optical Data Recording The amplitude point spread function associated with such an aperture is given by

assuming uniform illumination and (1 - e) << 1 (i.e., effectively a ring aperture). If this form of apodizationwere to be inserted in the incident path

of the optical readout head, the irradiance distribution in the focal plane would be given by the square magnitude of Eq. (79), as opposed to the Airy disc pattern. The resulting focused spot has a narrower central peak, but increased sidelobes, as shown in Fig. 2 la. The influence of apodization on the optical transfer function can be calculated, as before, by application of Eq. (63) for the partially coherent case but with the aperture functions modified to account for obscuration. (Note that the position of the apodized aperture in the optical path can affect the form of both incident and collector pupil functions. An aperture placed prior to the first "leaky" beamsplitter of Fig. 3b, for example, affects only the incident pupil function, whereas an aperture inserted between the leaky beamsplitter and the objective lens affects both incident and collector fbctions. An aperture insertedjust prior to the detector lens affects only the collector pupil function). As an illustration, we here choose to use the incoherent approximation to calculate the transfer function for the system described by the point spread function of Eq. (79). The resulting W F can be calculated via the Hankel transform (see p. 145 of Ref. 21)

0 , v>-

2a

Af

The Magneto-Optical Readout Process 533 which is plotted in Fig. 21b. It is apparent that although apodization has altered the form of the MTF, the spatial frequency cutoff remains the same as for the unobscured aperture system. This is in spite of an increase in the Rayleigh two-point resolution resulting from a focused spot with a narrower main lobe. This is a general feature of all obscured aperture systems, and so we have a form of enhanced-resolution,as opposed to true superresolution, as previously discussed. 1.21

I

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I

-10

-8

-6

-4

-2

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2

4

6

1

6

10

reduced distance 1.21

0

I

0.5

I

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I

1

1.5

2

2.5

normalised spatial frequency Figure 21. (a) Focused spot irradiance for unobscured circular aperture, i.e., Airy disc (solidfine)and for a ring type, annular aperture (dashedline). (b) MTFs for the above two cases (ring aperture case is normalized to unity at a reduced spatial frequency of 0.05).

534 Magnet*Optical Data Recording Several researchers have investigated the use of aperture obscuration techniques in optical disc systems, both from an experimental and theoretical s t a n d p ~ i n t . [ ~ ~ Alcommon [ ~ ~ ] approach is to use a rectangular form of shading band, as shown in Fig. 22. Yamanaka et al.[331chose to insert such an aperture in the illumination path, but the work of Milster and has shown that similar results can be obtained with the aperture in the collection path. Also in Fig. 22, we show the effects on the MTF of rectangular shading bands of widths equal to 20% and 75% of the clear aperture diameter ( s / h equal to 0.2 and 0.75), inserted in the collection path. These results are from Ref. 22 and are for the general, partially coherent case (so the plot is really of C(v,,O;O,O)) and assumes Gaussian illumination. The changes induced in the transfer function due to the shading band are quite dramatic, with an apparentlymuch-improved performance in the all-importanthigh frequency region being possible. However, as already pointed out for the incoherent annular aperture case, the spatial frequency cutoff remains the same as for the unobscured system. However, since the efsective resolution of the readout system will be determined not only by the cutoff frequency but also by the system noise floor, the possibility of a high frequency optical signal boost is attractive. Although the results of this section seem to indicate that the introduction of shading bands in the optical head readout path offer some advantages, care should be exercised in the interpretation of transfer function responses such as those of Fig. 22. This is because it is usual practice to normalize the MTF response to unity at DC. However, it is obvious that the introduction of a shading band into the illumination andor the collection optics will reduce the amount of light incident on the disc surface and/or that falling on the photodetectors. For example, a rectangular shading band with s/2a of 0.75 effectively obscures approximately 85% of the available aperture area. If this were placed in the illumination beam it would reduce the incident irradiance by a factor of approximately UO.15, assuming a uniform beam profile. We might therefore rescale the MTF for such a system such that the DC value is equal to 0.15 of the unobscured value, resulting in the MTF response similar to that shown by the dash-dotted line in Fig. 22. The advantage of the insertion of the shading band is now not so obvious! So, does the shading band improve performance or not? The answer lies in the form of the system noise, in particular whether the noise power scales with the signal power. For example, we could overcome the reduction in signal level as a result of inserting the shading band by increasing the incident light power. However, this would give us no real

The Magneto-Optical Readout Process 535 improvement in the effective resolution if the noise level scaled by the same amount. If this were not the case however, we can see that a real improvement in high frequency performance would result. 1.2

I

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I

I-I

0.2 -

2a

-.-.-.

\.-./'

U

0

1

0.5

1.5

2

2.5

nornalised spatial frequency

Figure 22. MTFs for unobscured circular aperture (solid line) and rectangular shading bands with sl2a equal to 0.2 (dashed line) and 0.75 (dotted line). Also shown is the sl2a = 0.75 case scaled by the reduction in incident irradiance (dashed-dot line).

The process of modifying the MTF via obscuration techniques can also be viewed as an optical domain equivalent to the electronicequalization that is often used in data storage detection channels. It may well be that performing this equalization in the optical domain has some benefits as far as SNR is concerned, since electronic equalization invariably involves boosting the system noise in at least part of the spectrum. Obscuration techniques may therefore offer more subtle benefits over and above those of an increased effective resolution.

5.3 Confocal Detection

In the discussions of the readout process so far presented, we have generally assumed that all sf b e light emergkg from the collection aperture

536 Magneto-Optical Data Recording is incident upon the detectors and contributes to the generation of the readout signal. What happens, however, if the detector is limited in its spatial extent so that not all the light incident on its surface contributes to signal generation? A particular case of interest is when the detector can be considered point-like; LJnder these circumstances the i m q e intensityj for the optical arrangement of Fig. 4(a) and reflectance type objects, can be

-m

I

where h2(x0,y0)is the amplitude point spread function of the collection aperture. Comparison with Eq. (45) reveals that this is a form of coherent imaging, but with a point spread function which is the product of that of the objective and collection apertures. The optical system described by Eq. (8 1) is thus one which is linear in electric field amplitude, and is usually termed a confml, or more prosaically a Type 2, scanning laser microscope, as discussed briefly in Sec. 3.1. It is apparent from Eq. (8 1) that simply by using a point detector we can change the imaging properties of the optical system that comprises a typical readout head. It is pertinent to ask whether such changes are beneficial in terms of either improved signal characteristics, increased spatial fiequency cutoff, or both. We can answer this question to some extent by investigating the impulse response, step response and transfer function for the system. Considerthen the reflectance impulse object of Eq. (46). Substituting into Eq. (83) yields an expression for the impulse response in the confocal case of

where we have assumed the objective and collection lenses are one and the same. We see that the impulse response is simply the square of that of the incoherent Type 1 system described by Eq. (47). Turning to the step response, by substitution of the step reflectance of Eq. (48) into Eq. (8 1) we find that again the confocal step response is simply the square of that of the

The Magneto-Optical Readout Process 537 incoherent Type 1 case.[181[261The confocal step response will therefore be sharper than for the non-confocal Type 1 case, and this could prove beneficial in reducing the effects of readback induced intersymbolinterference (ISI). The influence of confocal detection on the form of the step response has been examined experimentally by several authors for a range of nonmagnetic and magnetic imaging system~.[~~1[~~1 In Fig 23a, for example, a polar Kerr image of a step in a TbFeCo film recorded and imaged using a scanning laser microscope with a confocal detection system is shown.[’*] In this work, as in most practical implementations of the confocal scheme, the detectors were not truly point detectors but were made to appear point-like by placing an auxiliary pinhole aperture in the return beam. Changing the size of the auxiliary pinhole changesthe form of the imaging. For very small pinholes we approach a truly confocal arrangement, whereas for relatively large pinholes we revert to the conventional Type 1 imaging. This is illustrated in Fig. 23b where step responses for both 12 pm and 100 pm auxiliary pinholes are shown. As we would expect, the response for the 12 pm pinhole is the sharper of the two. Although the use of a confocal detection scheme improves the system step response, it leads to a large reduction in the detector photocurrent amplitude, a point that we discuss in a little more detail on p. 54 1. In Sec. 4.2 we introduced the concept of the general optical transfer h c t i o n C(v,, vy;v,’, vy’) that provided a unified description for the performance of coherent, incoherent and partially coherent optical systems. We can include confocal imaging into this framework by following the same procedures outlined in that section. Replacingthe reflectance expressionsin Eq. (81) by their Fourier transforms [as per Eq. (57)] leads to a transfer function of

c(v, ,Vy ;v,‘, vy’) = +,, vy) - (v,#, vy‘) C*

-m

-m

538 Mapwto-Uptical Data Recording

Figure 23. (‘u) Polar Kerr image o f a step recorded into a TbFeCo film (see Ref. 1s). (55) Experimental step responses for 100 pm (solid line) and 12 pm (dashed line) dctector pinholcs corresponding approximately to nonconfocal and confocal detection schemes.

The Magneto-Optical Readout Process 539 As before, this transfer function may be represented in terms of the aperture functions of the system. Remembering that the amplitude point spread function and pupil functions are transform pairs we can show that for line structures Eq. (83) can be expressed as m

-a m

and we recognize that the confocal transfer fimction is given by the correlation of the incident and collection pupil functions, multiplied by the corresponding complex conjugate. For circular, aberration-free pupils under uniform illumination the confocal transfer function is illustrated in Fig. 24. It should be noted that the response cuts-off, as for the nonconfocal case, at spatial frequency of 2NA/L This means that although confocal detection has been shown to lead to a narrower impulse response and a sharper step response, it does not yield true superresolution. Another feature of confocal imaging, that accounts for its popularity in the field of general microscopy and in particular in biological microscopy, is the so-called depth discrimination property. In the confocal arrangement, only light emanating from the focal plane contributes significantly to the detected signal. This is because the pinhole at the detector effectively blocks light from out of focus planes, and, as a result, only the object detail from the focal plane is present in the confocal image. that the confocal intensity For a perfect reflector it can be I(u) of a point object varies as a function of displacement from focus as

where u is the normalized displacement from focus given by z.(8dh).sin2(c/2), where z is the real displacement and sin(c) is the numerical aperture in the detector space. We might ask the question as to whether this depth discrimination property of confocal systems can be employed for any usefhl purpose in magnetosptical disc heads? Could it be used, for example, to

540 Magneto-Optical Data Recording provide a focus servo mechanism? From Eq. ( 8 5 ) we see that the form of the confocal response as a function of axial position is unipolar and, as such, is not particularly attractive for a focus servo, since the focus error signal would not reveal at which side of focus, too near or too far, the disk was situated. We could, however, generate a bipolar focus error signal by an arrangement using two detectors and two pinholes placed either side of the true m n f d position, as shown in Fig. 25. This kind of focus detection system is attractive since it relies only on the totd detected irradiance, not on the irradiance distribution as in conventional astigmatic or knife-edge techniques. This should make this type of confocal arrangementless susceptible to false focus error caused by scattering from tracking grooves etc. A potential drawback is that separate servo and signal channels are necessary.

-3

-2

-1

0 1 nornalised spatial frequency

2

3

Figure 24. (a) The confocal transfer function (axes are reduced spatial frequencies vxhj7aand vx'hj7a and extend to 1 3 ). (b) Contours at values of 0.8,0.6,0.4,0.2and 0 for the transfer function of (a) normalized to a maximum value of unity.

The Magneto-Optical Readout Process 541

detector lens

BS

pin-hole

detector

-;I

I I

pin-hole

detector

-e

-6

-4

-2 0 2 4 displacel~ent frm focus (Microns)

6

6

Figure 25. (a) Confocal focus detection system and (b) resulting focus m r signal for 35 pn pinhole (solid fine)and 50 pm pinhole (dotted line) with a 0.5 NA objective.

542 Magneto-Optical Data Recording It was mentioned previously in this section that one drawback of confocal detection is that the level of the detected photocurrent is substantially reduced due to the presence of the detector pinhole. This has led some authors to propose a semi-confocal arrangement in which the confocal pinhole is replaced with a slit apert~re:[331[~~1 The result is an optical system that exhibits confocal imaging properties in the track direction, but more conventional Type 1 properties in the radial direction.

5.4 Edge Detecting Readout Heads Conventional readout systems generate the readout signal via an optical arrangement designed to convert the polarization rotation, due to the polar Kerr effect, into an intensity variation. Such systems result in a maximum signal output at the center of a recorded bit (ignoring the effects of ISI) and are often termed level detection systems. An alternativecontrast mechanism which relies on light diffracted by magnetizationtransitions has, however, been identified by several authors.[181~351-[381 This alternative approach leads to a maximum signal occurring at the location of the recorded bit edge, and is usually referred to as edge detection. Since the contrast mechanism is a essentially a form of phase contrast, the alternative name of mugneto-opticul phase contrast (MOPC) is also used by some authors. An image of recorded bits obtained using the edge detection technique (in a scanning laser micro~cope)[~~1[~~1 is shown in Fig. 26, and corresponds to the same sample imaged using polar Kerr detection in Fig. 9. It is clear from Fig. 26 that the transition on one side of a written domain appears white, while the transition on the other side appears dark. The interior regions of the written domains have the same contrast, irrespective of whether they are magnetized upwards or downwards. The differences between the polar Kerr detection and edge detection are also shown in Fig. 26 for bits recorded using magnetic field modulation Again, as we would expect from the preceding discussion, the edge detection image consists of a series of bright and dark lines located at the position of the up/ down and down/up magnetic transitions. The intensity variation along the track centers in Fig. 26 is equivalent to the readout waveform in a disc system. Such a waveform is shown in Fig. 27 where the differences between the polar Kerr and edge detection signals are immediately apparent (an example of confocal detection is also given in the figure).

The Magneta-Optical Readout Process 543

Figure 26. (Top)Recorded bits of Fig. 9 readout (imagd) using edge detection tcchRique. (Middle)MFM recorded bits imaged using polar Kcrr detection. (Botrom) MFM bits imaged using edge detection tcchniquc.

544 Magneto-Optical Data Recording

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E 10 distance i n aicrons

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Figure 27. Readout waveforms from the MFM bits of the previous figure using conventional polar Kerr detection (dashed line), confocal detection (solid line) and edge detection (dotted line).

Perhaps the simplest form of edge detecting readout head is shown in Fig. 28. Linearly polarized light emerging from the laser collimator is converted to circularly polarized light by the quarter-wave plate, and is then incident on the disc. On reflection from the disc surface linearly polarized light, as explained in Sec. 2, is subject to a Kerr rotation off$ for up/down magnetization states. Such a rotation for a linear polarization corresponds to aphase shvt of *OK for circularly polarized light. A magnetic transition located at the center of the focused, circularly polarized beam will therefore cause wavefronts on one side of the transition to undergo a phase shift of +eK,while wavefronts to the other side will suffer a phase shift of -BK. The resulting phase mismatch in wavefronts across the transition leads to diffraction at the transition, which in turn results in an asymmetric intensity distribution in the reflected beam. In the head arrangement of Fig. 28, this reflected beam asymmetry is sensed by using a single, split photodiode detector, the read signal being the dif&rence signal between the two halves.

The Magneto-Optical Readout Process 545

Ken mtation+O

1

@

signal = A - B

i

Kerrmtation6

RCP phase shift 6

RCP phase shift +B

Cb) Figure 28. Edge detection with circularly polarized incident light showing (a) the head arrangement and (b) the diffraction effect at a magnetic transition.

The readout arrangement of Fig. 28 has several advantages over conventional MO designs, not least of which is the relative simplicity of the optics. Furthermore, as in compact disc and write-once type systems with which this head is compatible, the combination of the quarter-wave plate and the polarizing beam splitter ensures that all the reflected beam is directed away from the laser, so reducing laser feedback noise. The detection of the transition region can also eliminate the need for any electronic differentiation used in the signal recovery channel. There are, however, drawbacks with this technique. Firstly, the readout signal peakpeak amplitude at low frequencies is approximately half that of conventional systems, although the amplitudes at high frequencies are comparable.[361-[381Secondly, fluctuations of the position of the beam on the photodiode surface lead to an additional noise source not present in conventional heads. Such fluctuations can arise due to stochastic changes in the direction of the beam emitted from the laser diode source itself, or due to small scale roughness and other defects of the storage medium. It does however appear that edge detecting systems are less sensitiveto laser power fluctuations than conventional systems .[181[351[371 The above implementationof edge detection relies for its operation on circularly polarized incident light. It is, however, possible to generate edge contrast using linearly polarized incident light, and a head arrangement for so doing is shown in Fig. 29. It should be noticed that the optical layout of

546 Magneto-Optical Data Recording this design, excepting the use of split detectors, is virtually identical to that of conventional systems. Indeed, for media with little or no ellipticity, such a head could generate both level signals, using the detector combinations (Al+Bl)-(A2+B2), and the edge signals, via the combinations (Al-Bl)-(M-B2). The operation of this head can again be most easily understood by decomposingthe incident linearly polarized light into its constituent left and right circularly polarized components (LCP and RCP). When incident upon a magnetic transition, the LCP and RCP components are diffracted, as explained earlier in this section, but in opposite directions. The combination of quarter-wave plate, half-wave plate and polarizing beam splitter direct the light correspondingto the LCP and RCP components to separate split detectors, which sense the asymmetry in the reflected beam due to the presence of the transition. This process is illustrated schematically in Fig. 29.

from disk

t

-

linear x polarisation

=

Ex lags Ey by 90

Ex leads Ey by90

T

i

’*

horizontal

Ex and Ey in

Ex leads @y by 180

P b

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HWP axis

Ex +

@ 22.5 deg ,

a

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.

**_-- -.

PBS

RCP component

EY

w

t

i

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component

Signal = (Al-Bl)

- (A2-B2)

Figure 29. Edge detection with linearly polarized incident light showing (a) the detection optics and (b) an analysis of the polarization states.

The Magneto-Optical Readout Process 547 Alternative edge detecting heads have been proposed by various authors, including designs using unsplit photodiodes and a semicircular half-wave ~ l a t e , [ ~ ~ and l [ those ~ ~ ] which require only a quarter-wave plate All of these twodetector designs and PBS in the detection ameliorate to some extent the ureviously discussed noise problems that occur with single detector designs. In a recent work, Yamaguchi et al.[381 used the edge readout system shown in Fig. 29 and compared performance with conventional level detecting readout. They obtained a slightly lower value of CNR using edge detection, which they attributed to the enhanced effects of media noise. The spatial frequency cutoff of both level and edge detecting systems was identical, as expected. It would appear, therefore, that until media of very high quality, particularly in terms of surface roughness, can be produced, edge detecting heads offer slightly poorer performance than conventional level detecting designs. However, as magneto-optical recording moves towards the use of pulse width modulation techniques, i.e., the information is coded in bit edges as opposed to bit centers, the ability to detect magnetic transitions by direct optical means will become most attractive. Furthermore, the ability to implement an edge detection system using particularly simple optical arrangements, such as that of Fig. 28, may result in its use in some noncritical, low performance applications.

ACKNOWLEDGMENTS I would like to thank Paul W. Nutter and Philip W. M.Filbrandt for performing the computational calculations presented in this chapter. It is also a pleasure to thank my colleague Dr. Nick A. E. Heyes for his help in obtaining the scanning laser microscopy images used in the chapter.

REFERENCES 1. Faraday, M., Proc. Roy. SOC.,5567-569 (1845) Kerr, J., Phil. Mag., 3:321-343 (1877) 3. Argyres, P. N., Phys. Rev., 97:334-345 (1955) 4. Hecht, E., Optics (2nd ed.), Addison-Wesley Publishers (1987) 2.

548 Magneto-Optical Data Recording 5. Lissberger, P. H., Thin Film Magneto-Optics, in Applied Magnetism, (R. Gerber, C. D. Wright and G. Asti, eds.),Kluwer Academic Publishers (1994)

6. Mansuripur, M., and Connell, G. A. N., J. Appl. Phys., 53(6):4485-4494 (1982) 7. Kryder, M. H., Magnetic Information Storage, in AppliedMagnetism, (R. Gerber, C. D. Wright and G. Asti (eds.), Kluwer Academic Publishers (1994) 8. Marchant, A. B., Optical Recording, A Technical Overview, AddisonWesley Publishers, (1990) 9. Gage, E., Comparison of Polarization Readout Techniques for MagnetoOptic Systems, paper F2.2, Digests of ISOM/ODS 93 Conf; Maui, Hawaii (July 1993) 10. Challener, W. A., and Rinehart, T. A., Appl. Optics, 26(18):3974-3980 (1987) 11. Hopkins, H. H., J. Optical Society ofAmerica, 69(1):4-24 (1979) 12. Jipson, V. B., and Williams, C. G., Applied Optics, 22:2202-2209 (1983) 13. Bouwhuis, G., Braat, J., Huijser, A., Pasman, J., Van Rosmalen, G., and Schouhamer Immink, K., Principles of Optical Disc Systems, Adam Hilger Ltd (1985) 14. Wilson, T., and Sheppard, C., Theory and Practice of Scanning Optical Microscopy, Academic Press Inc condon) Ltd (1984) 15. Sheppard, C. J. R., and Choudhury, A., Optica Acta, 24:1051-1073 (1977) 16. Sheppard, C. J. R., and Wilson, T., OpticaActa, 25:315-325 (1979) 17. Wright, C. D., Heyes, N. A. E., and Clegg, W. W., Proc. SPIE, 13 16:122-133 (1990) 18. Wright, C. D., and Heyes, N. A. E., IEEE Trans. Magn., 27:5127-5129 (1991) 19. Wright, C. D., Heyes, N. A. E., and Clegg, W. W., J. Appl. Phys., 69:4942-4944 (199 1) 20. Benschop, J. P. H., Ossekoppele, M., and Van Rosmalen, G. E., Proc. SPIE, 1139:llO-116 (1989) 2 1. Papoulis, A., Systems and TransformswithApplications in Optics,McGraw Hill Publishers (1968) 22. Milster, T. D., and Curtis, C. H., Applied Optics, 31(29):62724279 (1992) 23. Mallinson, J. C., and Minuhin, V. B., IEEE Trans. Magn., 20(2):456-457 (1984)

The Magneto-Optical Readout Process 549 24. Tanaka, Y., Tanabe, T., and Arai, R., Jpn. J. Appl. Phys., 31:590-595 (1992) 25. McDaniel, T. W., and Finkelstein, B. I., IEEE Trans. Magn., 2751185120 (1991) 26. Wright, C.D., Heyes, N. A. E. and Clegg, W. IEE Con$ Proc., 319:7-14 (1990) 27. Maeda, T., et al., Jpn. J. Appl. Phys., 32:5335-5341 (1993) 28. Boreman, G. D., Transfer Function Techniques, Handbook of Optics, McGraw Hill Publishers (1978) 29. Fukumoto, A. et al., Proc. SPIE., 1499:216-225 (1991) 30. Hopkins, H. H., Proc. Roy. SOC.,217:408-432 (1953) 31. Fukumoto, A., andKubota, S., Jpn. J. Appl. Phys., 31:529-533 (1992) 32. Kaneko, M., Aratani, K., and Ohta, M., Jpn. J. Appl. Phys., 31:568-575 (1992) 33. Yamanaka, Y. et al., Applied Optics, 29:3046-3051 (1990) 34. Suhara, T., andNishihara, H., Jpn. J. Appl. Phys., 31:534-541 (1992) 35. Heyes, N. A. E., Wright, C. D., and Clegg, W. W., J. Appl. Phys., 69:5322-5324 (1991) 36. Mansuripur, M., Appl. Phys. Lett., 55:716-717 (1989) 37. Levenson, M. D., Lynch, R. T., and Tan, S. M., Applied Optics, 30:232252 (1991) 38. Yamaguchi, E. et al., Jpn. J. Appl. Phys., 325349-5353 (1993) 39. Betzig, E. et al., Appl. Phys. Lett., 61:142-144 (1992) 40. Brakenhoff, G. J., ImagingModesof Confocal Scanning LightMicroscopy, J. Microsc., 117:233-242 (1979)

9 Sources of Noise in Magneto-Optical Readout Blair I. Finkelstein

1.0

INTRODUCTION

The signal-to-noise ratio, along with the media intrinsic defect level, determines the realizable capacity and the data rate of an optical disk storage system. In this chapter we analyze the various sources of noise encountered in optical storage and discuss the spectral nature of the noise. Also, we will develop the relationship between noise, jitter, and system byte error rate (BER). Although the focus is on magneto-optical (MO) disk storage, most of these sources are known to occur in other optical recording systems as well. The arguments of the present discussion, therefore, may be adapted and applied to other media and systems. We can improve system performance by understanding and quantifying the various noise sources. This chapter has three objectives: 0) to serve as a brief tutorial on the nature of noise in optical recording, (ii) to quantify the noise sources associated with MO recording media, and (iii) to discuss the limits of reducing noise and improving jitter and BER. Several items that degrade system performance are not covered in this chapter. Correlated noise terms, such as adjacent track pickup and 550

Sources of Noise in Magneto-Optical Readout 551 incomplete erasure, are not discussed. While both of these can seriously reduce effective signal-to-noise ratio (SNR), they cannot be considered classical noise. Optical aberrations (coma, astigmatism, defocus) also degrade signal quality, but are outside the scope of this chapter. The MO readout scheme considered in this chapter is briefly reviewed. Then we describe the characteristics of the shot noise arising from random fluctuations of photon arrival times in Sec. 2, followed in Sec. 3 by a considerationof noise originating in the electronic circuitry. In Sec. 4, we describe a model for the laser noise and present measurement results that yield the strength of laser power fluctuations. In Sec. 5 , we discuss the importance of differential detection for MO recording. An introduction to disk noise is found in Sec. 6. In Sec. 7, spatial variations of the disk reflectivity and random depolarization phenomena contribute to the overall level of noise in readout. Write noise is another important source of noise that needs to be understood and is be examined in Sec. 8. Section 9 is devoted to numerical simulation results describing some frequently encountered sources of noise accompanying the recorded waveform itself, namely, jitter and signal-amplitude fluctuation noise. In the last section, the relationships among noise, jitter, and BER are explored.

1.1 The MO Readout System Throughout this chapter, we frequently reference the readout system depicted in Fig. 1. The light source in this system is a semiconductor diode laser operating at constant output power, that is, in CW mode. (In practice, for noise reduction purposes, the laser’s drive current is modulated at a frequency in the range of several hundred M H z . Generally, the modulation frequency is well beyond the bandwidth of the detection system and is ignored by the detectors. Signal and noise analysis, therefore, may proceed by ignoring the modulation of the laser and treating its beam as CW.) The light reflected from the MO disk consists of a polarization component parallel to the direction of incident polarization, X , and a perpendicular component, Y, which cames information about the pattern of recorded data. For the purpose of monitoring the laser power fluctuations, the leaky polarizing beam-splitter (PBS) directs a small fraction v2 of the emitted light onto a photodetector, the output of which is then fed back to the laser in an effort to stabilize the level of output radiation. The light transmitted by the leaky PBS is linearly polarized along X but, upon reflection from the disk, it acquires a Y-component also. This Y-component is the magneto-

552 Magneto-Optical Data Recording optically generated signal. In the return path, the leaky PBS provides the detection module with a fraction y of the X-component and the entire Ycomponent. The X-component, although barren of information, is needed in the scheme of differential detection for proper extraction of the recorded data.

To eliminate the ellipticity of the beam, we use a phase compensator (either quarter-wave plate or adjustable retarder). A regular PBS, with its axes at 45" to X and Y, and detectors 1 and 2, comprise a balanced differential detection module. These detectors are low-noise photodiode/ preamplifier integrated circuits whose outputs are fed to a differential amplifier for subtraction and fbrther amplification.

2.0

SHOT NOISE

Direct detection of optical signals usually entails their conversion into electrical signals by a photodetector. Photodetection is a quantum process whereby incident photons cause the release of electrons, which then go on to produce an electric current. Unfortunately, the photon arrival times are random. The random arrival of photons at the detector creates fluctuations in the resulting photocurrent, which is generally known as the shot noise.[*I Shot noise, which is a fundamental property of photodetection, can be practically reduced only by decreasing the amount of light incident on the detectors. (We say practically because techniques, such as quantum nondemolition detection, can, in theory, reduce shot noise while maintaining light levels. However, such techniques could not be applied to optical storage. In terms of noise current per hertz of bandwidth, we can write shot noise as follows:

where e is the charge on the electron, h, is the quantum efficiency of the detector& is the power incident on the detector, and Idc is the DC current from the detector. Shot noise is white, has constant noise power per hertz of bandwidth, and is directly proportional to laser power on the photodetectors, thus depends on disk reflectivity, read power, and path efficiency. While shot noise is proportional to power, signal power is, to first order, proportional to

Sources of Noise in Magneto-Optical Readout 553 laser power squared. Reducing shot noise by decreasing laser power does not improve system performance. Measuring shot noise (Ish) directly is extremely difficult because of laser noise. However, one can measure the DC current through the diode and calculate the shot noise from the two detectors. In most MO storage systems, shot noise and electronic noise are roughly equivalent. Given the current state of preamplifier design, reducing the electronic noise by an appreciable amount is extremely difficult. Replacing the PIN photodetectors with avalanche photodiodes (APDs) does not improve the signal-to-noise ratio because the shot noise gain in an APD is higher than signal gain.

3.0 ELECTRONIC NOISE In an optical recording system, electronic noise is similar to that in any electronic system. Electronic noise consists of dark current noise, thermal noise, and base current noise, which are white (flat with frequency), and have l/f and amplifier noise voltage that are colored noise sources. Ideally, electronic noise should be less than noise from all other sources in the recording system. In reality, this depends heavily on the semiconductor manufkcturing process, the preamplifier and detector scheme chosen, and the system bandwidth. The fluctuating conduction electrons within resistors and transistors of an electronic circuit create a random voltage or current at the circuit’s output terminal. When these fluctuations are caused by thermal motion of the electrons, the resulting noise is referred to as Johnson noise. A resistor R at an equilibriumtemperature Texhibits a noise whose spectrum is flat for practically all frequencies of interest. If the Johnson noise of this resistor is measured within a bandwidth B, the resulting magnitude of the noise current will be as follows

4kBTB)

R In the above equation, the brackets ( ) indicate statistical averaging or averaging over h e , kB is the Boltzmann constant (1.38 x joule/K), T is the absolute temperature (in Kelvins), B is the bandwidth (in hertz), and R

554 Magneto-Optical Data Recording is the resistance (in ohms). For the sake of simplicity we shall denote the root-mean-square value of the noise current by i,h instead of (Ith2)”. Because the frequency spectrum of thermal noise is flat, we usually specie the strength of the noise in a 1 Hz bandwidth. Thus, the noise amplitude of a 1 kW resistor at room temperature is about 4 p A l ( H ~ ) If ~ .the bandwidth of the measuring system is B = 1 MHz, then the measured noise current is 4 nA. The w r r e s p o rms ~ noise voltage is then given by vth = & = 4 mV. Another nlanifestationof the thermal noise is the dark current noise of photodetectors. Within the bulk of the semiconductor crystal constituting a photodiode, some valence electrons are thermally excited into the conduction band, giving rise to a current similar to that generated by photoinduced electrons. Unlike Johnson noise, however, the average value of the dark current is nonzero. Fluctuations of the dark current around its average value are known as dark current noise. These fluctuations are very similar to the shot noise. Their similarity arises from the fact that dark-current electrons, which are thermally generated, are produced randomly and independentlyof each other. Both dark current noise and base current noise of the transimpedance amplifiers have the following form:

In2 = 2eZxB where I, is the DC base current or dark current, depending on which is being measured. Note the similarity of this equation to the shot noise equation. If the temperature ofthe semiconductor photodiode is kept constant, the rate of generation of the dark-current electrons is constant also. For the purpose of analysis, dark current and its associated noise may be treated by adding an equivalent (constant) current to the photocurrent from the optical detector. An amplifier, in addition to multiplying the input noise amplitude by a gain factor, introduces a noise of its own. This noise is due to the fact that in transistors, random generation and recombination of electrons and holes, and the random arrival of these carriers at the collector, create current and voltage fluctuations. The termsjlicker noise and Zynoise are often used to refer to these fluctuations. The Yfnoise is limited to the low frequency range of the spectrum and in most systems can be ignored. The excess noise of an amplifier is generally characterized by the noise factor F, which is the ratio of the available output noise (including contributions by the amplifier) to the available output noise arising from the input alone.[’] Although every stage of the amplificationprocess contributes to the overall noise level, it is usually the case that the preamplifier (that is,

Sources of Noise in Magneto-Optical Readout 555 the first stage) makes the most significant contribution, simply because the noise from this stage is amplified more than that from the stages that follow. Amplifier noise voltage depends on the input capacitance and is a strongly increasing h c t i o n of frequency. Because of this strong frequencydependence, amplifier noise voltage can be a factor in both code selection and target detection channel. At the high frequencies required by high performance systems, this noise source can be dominant. In terms of noise current, this noise can be written as follows:

Eq. (4)

1

where E, is the voltage noise figure for the amplifier, C is the capacitance, R is the load resistor and f is the frequency of interest. Thus, the integrated noise goes as the cube of the highest frequency. To have low electronic noise, the noise figure for the amplifier and the total capacitance of the preamplifier must be minimized. In Fig. 1, the currents from the diodes are first amplified and then subtracted, This configuration has at least two advantages: (i) more flexibility in diode biasing andor placement is achieved, and (ii) the gain for each leg can be independentlyadjusted. Another, lower noise configuration exists. In a totem-pole configuration,the currents from the photodetectors are first subtracted directly, and secondly they are amplified. In this case, E,' is halved (one amplifier, not two), but the capacitance of the circuit may be slightly higher. The choice of configuration is another trade-off in drive design. In any event, one may obtain an accurate picture of the total electronicnoise (En)contributionby measuring the spectrum of the differential detector output while the laser beam is blocked. Because the electronic noise is independent of the signal, there is no need to analyze its various components separately; instead, it shall be treated in its entirety as one source of noise. Another consequence of electronic noise being independent of the optical system is that any step that improves signal amplitude & h e r Kerr effect, smaller spot size, and so on) improves the signal-toelectronic noise ratio.

556 Magneto-Optical Data Recording

Photo Detector (Laser Monitor)

R

Disk

Collimator

%-

(Laserk

Photo Detector I

YI

z

Photo Detector 2

Figure 1. Schematic diagram of a magnetooptical readout system. In the forward path, the leaky PBS provides a fraction y2 of theX-polarized light to the monitor of laser power. In the retum path, 100%of the magnetooptically generated Y-component plus the kction yz of the returning X-polarized light are deflected towards the differential detection module.

4.0

LASER NOISE

Laser diode noise can be a major contributor to total noise during readout. Effects such as mode-hopping, spontaneous emission, optical feedback, and line broadening, cause additional noise in the recording system. Furthermore, power supply noise and laser driver noise can appear

Sources of Noise in Magneto-Optical Readout 557 as laser noise. Functionally, laser noise depends on laser power, temperature, external cavity length, feedback power, and laser design. Variations in laser power, polarization, phase, and wavelength, constitute laser noise. These variations manifest themselves as amplitude noise in a recording system, or in the case of wavelength shifts, as spot position shifts. Random mode-hopping induced from optical feedback into the laser cavity is the largest source of laser noise. Light reflected into the cavity causes instabilities and mode competition. Fluctuations in the number of electrons and photons on emission are the source of the noise, which is amplified by mode competition. When the laser jumps from one mode to another, the energy balance in the laser cavity changes abruptly, and this change causes the intensity fluctuations that result in amplitude noise. The major problem with uncontrolled laser noise is not only the average noise power (which can be quite high or quite low), but rather with its popcornlike nature. These short, intense bursts of noise can degrade BER more than the average noise power would imply. Total laser noise is only equal to the quantum noise when the laser is operating stably in purely single mode. Optical feedback occurs when some portion of the laser light is reflected into the laser cavity. Light emitted from the laser can be reflected into the laser cavity fi-om elements in the optical path, such as lenses, beam splitters, and the optical disk. Semiconductor lasers are very sensitive to laser feedback. Feedback changes the PI curve and increases the noise level. Small amounts of feedback can have beneficial effects by mode-locking the laser, reducing line widths, and spontaneous emissions. However, under the higher feedback conditions typical of optical storage, competitive modehopping induces noise. We can discuss laser noise using perturbation analysis. The laser light amplitude E is not constant, but varies randomly with time. Its fluctuationsare rooted in the instabilities of the laser cavity, which results in mode-hopping and mode competiti0n.[~l-[~1 We can write the following expression for the amplitude of the light beam:

Es.( 5 )

E(t) = Eo[l + S,(t)]exp(-iwt)

where S,(t) is a dimensionless, complex quantity with IS,(t)l << 1 for all times t. Figure 2 is a diagram showing 1 + S,(t) as the sum of two vectors in the complex plane. Assuming that the complex phase of S,(t) can assume all values between 0 and 2n,we observe that 1 + S,(t) is confined to a small disk centered at (1,O) in the complex plane.

558 Magneto-Optical Data Recording

imaginary

.t. Complex Plane

I

locus 1 + 6, (t)

Figure 2. Complex-plane diagram shows the locus of 1 + S,(t), where SJt) is a small complex quantity whose phase can assume arbitrary values in the interval [0,2n].The radius of the circle, which is the upper bound on the magnitude of SJt), is exaggerated.

The laser power is proportional to the electric field intensity and is therefore given by the following equation:

Eq. (6)

P(t) 0tIE,1~[1+ Se(t)+ Se*(t)+ IS,(t)12]~Po{ 1 +2Real[S,(t)]}

The rms fluctuationsof the laser power are thus equal to 2 P d ewhere,

Unlike the spectra of shot noise and Johnson noise, the spectrum of Se(t) is not necessarily flat. In general, the frequency response of any measurement system has a finite range and a nonuniform magnitude, thus requiring that the spectral content of Se(t)be properly trimmed and adjusted, before using it in Eq. (7) to calculate the rms noise value.

Sources of Noise in Magneto-Optical Readout 559 Despite the fact that laser noise is not flat with frequency, the standard method to quantify laser noise is to quote a relative intensity noise (RIN) value. RIN is the laser noise in a specified 1 Hz band normalized by the incident power, that is,

RIN = 20 log (ilaser/Idc) where iher is the noise as measured by a spectrum analyzer. Typical RIN values for laser in a recording system (without any suppression) range from 80dBto lOOdBat5MHz. When we measure the spectrum of the laser power fluctuations using a photodetector (that is, intensity detection), we obtain a trace on the spectrum analyzer which, in addition to the desired spectrum, contains contributions from the shot noise and the thermal noise of the detection circuitry. The thermal noise, however, may be measured independently by observing the detector output when the beam is blocked, and the shot noise may be estimated from Eq. (1) using the average incident light power P, It may also happen that the fluctuations of Se(t) have a limited frequency content, in which case the noise outside the frequency range of S,(t) is identified as shot plus electronic noise. Because both the shot noise and the thermal noise have flat spectra, we can proceed to subtract their contributions from the total and obtain the spectrum of the laser power fluctuations. Laser output variation with wavelength can deteriorate the system performance beyond just power fluctuations. The wavelength shifts can change spot size, focal point, and/or beam position. Changes in spot size and focal point are similar because they affect the resolution of the system. Changes in wavelength can also displace the beam either along the track or across the track, depending on the amount and direction of the lateral magnification of the system. Displacement across the track decreases the signal slightly and may increase crosstalk from the adjacent track. Displacement along the track adds a form of frequency modulation to the signal, which degrades reliability. Four techniques are available to minimize laser noise: (i) modify the laser diode, (ii) use an optical isolator, (iii) provide an electrical feedback system to control the power fluctuations, or (iv) force the laser into multimode operation with a high-frequency current to reduce mode competition. Proper application of any of these techniques reduces the laser noise by several orders of magnit~de.[~l[~~]

560 Magneto-Optical Data Recording You can reduce laser noise by changing either the diode structure or the facet reflectivities. For example, index-guided laser diodes are less prone to feedback effects than gain-guided structures. Also, high facet reflectivity decreases spontaneous emission and reduces susceptibility to feedback by decreasing the probability of mode-hopping. However, increasing the facet reflectivity decreases laser output power and life. Another way to eliminate laser-feedback noise is to eliminate laser feedback. CD and WORM system use an optical isolator consisting of a polarizing beam splitter and a quarter-wave plate before the objective lens. This combination is extremely effective in reducing feedback to the laser. However, the light incident on the disk from such a system is circularly polarized and thus cannot be used in present MO systems. The readback signal from this system is small and may have insufficient SNR. For the current detection methods, the light needs to be linearly polarized on incidence at the disk. Using a Faraday rotator instead of the quarter-wave plate for isolation, adapts this technique to optical recording. (Faraday rotators are non-reciprocal optical devices that use MO interactions to produce rotation.) Two passes (to the disk and back) through a rotator set for 45" causes a net rotation of 90'. At this rotation, the light is not reinjected into the laser cavity to cause mode competition. Faraday rotators have four problems: cost, size, insertion loss, and isolation ratio. However, over time, these problems can be solved. Laser noise is characterized as power fluctuations in laser output. With fist detectors and servo electronics, it is possible to reduce the magnitude of the fluctuations. As noise spikes cause the power to vary, electronic feedback compensates to maintain constant power. This is a proven method. The challenge with such a system is to achieve the several hundred MHz servo system required to control the noise. The system must not oscillate and must always provide the proper amount of negative feedback over laser, head, and temperature variations. High-frequency modulation (HFM) of the laser is the most commonly used technique for reducing laser-feedback noise in MO recording systems. In this technique, a high-frequency (200-1000 MHz) modulation current is superimposed on the DC bias current that drives the laser. By driving the laser through the threshold, the modulation current forces the laser to operate multimode, which reduces mode competition. A lag exists between the time the laser is switched on and when it achieves single-mode operation. When the laser is in single mode, it is most vulnerable to laser feedback. If

Sources of Noise in Magneto-Optical Readout 561 the laser is modulated at a highenough repetition rate so that it never reaches single-mode operation, laser feedback noise is minimized. However, total noise will still be higher than quantum noise. Also, because the laser is operating at several different, closely-spaced wavelengths, the spot will not be as small as if it were single-mode because of chromatic errors introduced by the optical components. Reducing the spot size penalty in controlling laser noise will be critical in fiture high-capacity systems. After applying suppression, FUN values are typically reduced from the range of -80 to -100 dE3 to a range of -110 to -135 dE3, which is a substantial reduction. Without some form of laser noise control, laser noise spectra within the data band is primarily low frequency, rolling off as (l/fn) where n is between 0.1 and 2. With laser noise control, the noise is essentially flat, with just a small amount of roll-off. In Sec. 1, we described the read beam as continuously on, whereas in reality, the laser's drive current is modulated at a frequency in the range of several hundred MHz. Generally, the modulation fiequency is well beyond the bandwidth of the detection system and is ignored by the detectors. Signal and noise analysis, therefore, may proceed by ignoring the modulation ofthe laser and treating its beam as CW. However, the HFM should be turned off during writing or additional write noise is generated. We discuss the effects of laser noise on writing later in this chapter. Example 1: Figure 3 shows the measured noise spectra at the output of the laser monitor in a typical experiment (see Fig. 1). The lower tracecorrespondsto the electronicnoise that is monitored in the absence of the light beam, and the upper trace shows the total noise with an incident light power ofPo=380 mW. The PIN photodioddamplifierused for these measurementshad a sensitivity of qs= 0.42 A N at the laser wavelength of 680 nm, and a gain (that is, w e n t to voltage conversion factor) of G = 104V/A. The electronic noise level read from Fig. 3 is -84 dE3m. This is the power deliveredto a 50 !2 resistor in a bandwidth of 30 kHz; thus, the rms thermal noise voltage at the amplifier output is vth= 80 nV/ &,and the corresponding noise current at the output of the photodiode prior to amplification is ith= 8 PA/&. The shot noise current density is calculated from Eq.(1) as follows: ih=Jm-=J2x

1.6~10-'~ ~ 0 . 4 2 ~ 3 lo-'' 8 0 ~=7.2 PA/&

562 Magneto-Optical Data Recording

n

-78

0

2

4

6

8

10

Frequency (MHz) Figure 3. Spectrum of the combined electronic + shot + laser noise, and the electronic noise alone at the output of a laser power monitor.[6] The operating wavelength is 1 = 680 nm and the incident light power is 380 mW.

Because the total noise power in Fig. 3 is about 14 dB (that is, a factor of 25) above the thermal noise level, we have the following:

'th

from which we find ilarer=38.5 PA/&. This is the rms current fluctuation caused by the laser noise at the photodiode output (prior to amplification). The laser noise may be normalized by the average laser power and stated as a relative intensity noise 0, that is,

Because the laser noise spectrum in Fig. 3 is relatively flat through the frequency range f = 0 to 10 MHz, we can readily integrate the FUN over this bandwidth and obtain the figure of -62.5 dB. Comparison with Eqs. (6) and (7) shows that the rms value of Real [8,(t)], confined to a 10 MHz bandwidth, is Ae x: 3.75 x 104,

Sources of Noise in Magneto-Optical Readout 563 Example 2: The spectra shown in Fig. 4 correspond to the output of a laser monitor, which is a low-noise, hybrid PIN photodioddamplifierwith a sensitivity of qs= 0.5 A/W at h = 830 nm.[V The detector's current-to-voltageconversionfactor is G = 3.64 x 104 VIA. The lower trace shows the spectrum of the detector output in the absence of light. The -88 dBm electronic noise level corresponds to vlh = 51 nV/& at the amplifier output and to i,= 1.4 pAI& at its input. We obtained the upper trace when 39 pW of laser light was incident on the detector. From Eq. (l), the shot noise current density (prior to amplification) is found to be ish= 2.5 PA/&. Thus, ( i s + i$,)Ii~,,= 4.2 = 6.2 dB Therefore, the thermal-plus-shot noise level must be approximately 6.2 dE3 above the thermal noise level in Fig. 4. The remaining noise is due to fluctuations of the laser power, which may now be separated. Let us approximate the frequencydependence of the total noise density in Fig. 4 with a linear function, as follows: lOlog

= 23 - 1.3f

(finMHz)

This expression leads to the following frequencydependence for the laser noise: (ihe,/i1J2

= -4.2 + 200 exp (-0.3 f)

(fin M H z )

The above function is now averaged over the frequency range fiomf= 0 tof= 10 MHz, yielding the average value of 59 for (ilae,/i1h)2. Thus, the average noise current arising from the laser power fluctuations (within a 10 MHz bandwidth) is ilmer= 10.75 PA/&. Normalization by qSp, yields an average RTN of -125.2 dE3, and the rms value of the E-field fluctuations over the 10 MHz bandwidth of interest is Ae x 8.7 x lo4.

564 Magneto-Optical Data Recording

I

II

I

0

I

Thermal + Shot + Laser

2

4

6

8

10

Frequency (MHz) Figure 4. Spectrum of the combined electronic + shot + laser noise and the electronic noise alone at the output ofa laser power monitor. The operating wavelength is h = 830 nm and the incident light power is 39 mW. (The spikes are generated by the electronic circuitry of the laser power supply and are ignored in the noise analysis.)

5.0 DIFFERENTIAL DETECTION AND MISBALANCE Any of the laser noise reduction schemes we have discussed suppress laser noise enough for most CD-ROMand WORM recording systems. The high signal modulation level in these systems ensures adequate BER. However, even though these suppression techniques reduce laser noise power by up to a factor of 1000 (30 dB), MO systems still lack SNR because of the inherently low Kerr effect signal levels. What makes MO storage systems possible at all is differential detection. The output signal in an MO system is the light signal falling on one detector minus that Glling on the other. This subtraction and amplification combination represents a classic differential detection system. If the detectors and the preamplifiers were perfectly matched in gain and frequency response, and the light levels incident on each detector were equal, the effect of laser noise (or any intensity noise, such as reflectivity variations as discussed in the next section) would be zero. Any change in laser power

Sources of Noise in Magneto-Optical Readout 565 would appear at both photodetectors and thus, would be canceled. If the light levels on both detectors were always equal, there would be no data signal either. But the noise from the DC component of the light (which for MO systems is significantly greater than the signal component) is in fact greatly reduced, by another two or three orders of magnitude,giving RINs in the range of -1 17 to -135 dB. Many other factors besides the MO signal itself can upset the balance between the detectors. Disk birefringence and ellipticity, waveplate misalignment, optical path transmission differences between detectors, differences in detector responsiveness and bias, and gain differences S e c t the balance between the two detectors. (The misbalance is sometimes referred to as common mode rejection ratio or CMRR, after a similar electrical expression.) We must carehlly design and manufacture drives and media to minimize these effects. Otherwise, high-performance, high-capacity MO recording systems will not be possible. All MO recording products shipping today require high-frequency laser modulation and carehlly balanced differential detection to achieve satisfactory results. Other than minimizing misbalance, improving the signal-to-laser noise ratio is difficult. Most steps to increase signal also increase the apparent laser noise by the same amount. Decreasing the spot size improves signal resolution without appreciably increasing laser noise at the preamplifier output. For a recording system where the amount of misbalance is known, and one detector can be blocked, shot noise and laser noise as a function of frequency can be determined. With the disk kept in focus, we can measure the noise with a spectrum analyzer differentially with one detector blocked. The noises in differential detection are:

where N, is electronic noise, Ns is shot noise, Nl is laser noise, and m is the misbalance ratio for the system. Because Ne can be measured separately and m is known, these are now two equations in two unknowns at each frequency, which yield shot noise and laser noise as hnctions of frequency. The spectrum of shot noise may not be flat because the gain of the

566 Magneto-Optical Data Recording preamplifier may not be flat. Section 7 contains a more detailed, mathematical discussion of misbalance.

6.0 INTRODUCTION TO MEDIA NOISE The noise in an optical recording system (as measured at the output of the preamplifier) increases when the disk rotates. Variations in absorption, Kerr rotation, reflectivity, and scattering, are intrinsic to the disk and contributeto classical disk noise. These variations can be caused by surface roughness, tracking grooves/servo pits, and dust, or by inhomogeneities in the thickness, magnetization, structure, or composition of the MO film. Write noise is a type of media noise, which we discuss in Sec. 8. Media noise is a major contributor to total noise in any optical recording system, and is the dominant noise source in today’s commercial, high-performance, MO storage systems. Early attempts at creating MO systems failed because the media noise swamped the signal. In the 1970’s, the materials used were crystalline in nature and suffered from grain boundary noise. The amorphous RE-TM materials used today lack grain boundaries; therefore, they have higher signal-to-noise ratios. However, fkture systems operatingwith wavelengths in the blue may use multilayers such as Copt,which are polycrystalline. Without care in design and manufacture, these disks may also suffer from grain boundary noise and from noise produced by the magnetic coupling between the grains. Media noise differs from the previously discussed noise sources in that it is not truly random. Shot noise, electronic noise, and laser noise, are stochastic, whereas media noise is spatially linked to the disk. The temporal frequency distribution of media noise is a deterministic consequence of the spatial distribution of the noise on the disk and the disk linear velocity. To first order, the magnitude of the noise at a given point on the disk is the same every time the beam scans this point, so the noise is identical. Because these noise sources are intrinsic to the disk, the total integrated noise is independent of disk velocity, but the noise spectrum scales with velocity. Spectrally, media noise should be white, as modified by the optical transfer fimction. However, on some disks, the noise is slightly colored. This coloring arises from two possible sources: (i) the noise is built into the disk; for instance, variations in groove width from the mastering process can

Sources of Noise in Magneto-Optical Readout 567 cause variations in reflectivity with a spectral shape; fii) the correlation distance for micromagnetic patches (see Sec. 6.4 of Ch. 10) can be long enough to change the shape of the noise. Extrinsic disk noise is caused by changes in the optical path resulting from the motion of the disk. For instance, laser noise is affected by axial runout, which changes the optical path length of the feedback, and by reflectivity variations during disk rotation, which change the amount of light fed back into the laser cavity. Noise levels can also shift because of spot size variations resulting from shock, vibration, or axial acceleration-induced focus variations. Many substrate materials, such as PMMA and polycarbonate (PC), exhibit anisotropy in their manufacture: the optical properties in one direction on the disk are not the same as the other directions, thus optical birefringence occurs. For systems using polarizationdependent components (such as with MO), changes in polarization caused by birefringence can lead to signal loss and increased noise. Birefringence in the plane of the disk by itself cannot be considered as a noise source, but it affects noise power because it increases the magnitude of the misbalance, thereby increasing the effect of intensity noise sources. The in-plane birefringence can also cause baseline shifts (signal imbalance), which degrade the ability of the system to distinguish between digital 1 and 0. Uncompensated vertical birefringence causes astigmatism, which increases the amount of crosstalk and groove noise pickup at the point of best focus where the signal amplitude is at a maximum. Media noise can be divided into two general types: 2() light intensity noise, which would fall equally on both detectors if the system were perfectly balanced, and fii) polarization rotation noise, in which the light hlls preferentially on one detector over the other. Intensity noise is simple variation in reflectivity. Rotation noise can arise from Kerr rotation variations or scattering in which some of the light changes polarization. The same method for separating the shot noise from the laser noise can be applied to separating reflectivity-type noises from rotation-type noises. To accomplish this, with the disk spinning and the drive in focus and tracking, measure the noise with a spectrum analyzer differentiallyand with one detector blocked. The noises in differential detection are:

Eq. (lOa) Nt(dlfferent'aO= N, + 2N, + 4Nth + p(Nl + NJ

568 Magnet+Optical Data Recording

where Ne is electronic noise, Ns is shot noise, Nl is laser noise, Nthis rotation (A& noise), N,. is reflectivity noise, and ,D is the misbalance ratio for the system. BecauseN,, Ns, and N! can be measured separately and ,u is known, these are now two equations in two unknowns at each frequency. Solving the two equations yields rotation noise and reflectivity noise versus frequency. Experimentally, we found that rotation noise is white (that is, flat spectrum) as modified by the optical transfer function of the system. In fact, using rotation noise from a poorly erased track is an excellent method to determine the optical transfer function of the system. Intensity noise has a more complex structural shape, differing dramatically among disk vendors. In an MO storage system using differential detection, rotation noise is significantly larger than intensity noise. The drive designer has some control over the shot noise, the electronic noise, the laser noise, and the amount of misbalance in the system. Whereas shot, electronic, and (to a lesser extent) laser noise can all be modeled and predicted, media noise is too process-dependentto predict in advance. All useful media noise models are empirically Quadrilayer media design serves as an illustrativeexample. As the optical layers are tuned, the signal amplitude increases in agreement with optical thin film modeling. However, in this structure, media noise increased faster than signal because of increases in stress and defects in the dielectrics. One optical technique that shows promise for reducing the effects of media noise is the use of a shading band in the readback path. This technique improves resolution (by reducing the low-frequency signals), and reduces the low-frequency noise and the noise from grooves and the adjacent tracks. In well designed systems, the BER is limited by the signal-to-media noise ratio and the defect level. Because of this, the achievable data rate and the capacity depend on the media design and manufacture.

7.0 DISK REFLECTIVITY FLUCTUATIONS AND DEPOLARIZATION NOISE In the followinganalysis, we assumed that the disk spins at a constant speed, and that the noise is measured on an erased track, that is, one with no reverse-magnetized domains. The effective light power incident on the disk

Sources of Noise in Magneto-Optical Readout 569 is denoted by Po. This effective power is usually less than the actual incident power because it incorporates the reflection and the transmissions losses of the various optical elements. The three sources of noise we considered are: (i) amplitude and phase variations of the X-polarized re- (it) random depolarizationof the incident (X-polarized) beam; flected liPht; and (iii) amplitude and phase fluctuations of the magneto-optically generated component of polarization along Y.[81-[111The physical mechanisms responsible for these noises are described in the following sections. For various reasons,there are fluctuations in the e&ctive reflectivity rx for theX-component of polarization. First, the material composition and/or structure varies over the disk surface. Second, if there are grooves on the disk, their edge roughness is likely to scatter the light in a random fashion. Third, there are residual amounts of off-track and defocus errors that vary with time and, therefore, cause the reflectivity to fluctuate. Some of these noise sources are fixed on the disk and, consequently, their timedependencies scale with the disk velocity; others, such as those due to mechanical vibrations or focushrack errors, are caused by fluctuations elsewhere in the system and do not scale with the velocity. Assuming that these fluctuations are small, we write:

Eq. 11 where 4(t)is a small, dimensionless, complex coefficient representing the fractional variations of rx. The possibility exists of conversion of some of the X-polarized incident light into Y-polarized reflected light due to depolarization, as distinct from the magneto-optic conversion. This phenomenon is partly due to substrate birefringence, but is also caused by the various scattering mechanisms at work on the disk surface. Perhaps the best way to characterize the depolarization contribution to noise is by defining a disk reflection coefficient r, as follows:

where, as before, S,(t) is a small, dimensionless, complex coefficient. Finally, there is the magneto-optic contribution to the Y-polarized reflected light. For a completely erased track, we may write the Ycomponent of the reflectivity as follows:

570 Magneto-Optical Data Recording Some of the fluctuations in ry originate from the structuraVmagnetic variations of the disk material along the track, but there is also a correlation between the fluctuations in ry and those in r,. For example, if more Xpolarized light is reflected, less light is available to interact with the magnetic material and to produce the Y-polarized component. For simplic.'8 and 4, and treat these ity, we ignore all possible correlationsamong S,, parameters as independent random variables. The phase compensator in the data detection branch of the readout system of Fig. 1 eliminatesthe nominal ellipticity of the beam by bringing r, and r in phase with each other. Consequently,we assume at the outset that YO. r,,,,/r, is real and, to emphasize its realness, we write it as ~ryo/rxo~. The compensator has no influence on the noise coefficients S,, 4, and ' 8 other than adding to them an irrelevant constant phase. The power incident on individual detectors may be written as follows:

where the plus (minus) sign applies to the incident power P,(PJ on detector 1 (2). Expanding the above expression and ignoring the second order terms in the noise variables, we find the photocurrents S, and S, of the detectors as follows:

4, s 2

1

,%Po

2

[vI r,, If Irp I]

The sum of the two detector signals is given by

y2Real(ae+ax)+

Sources of Noise in Magneto-Optical Readout 571 The first term on the right side of Eq. (15) is the photocurrent signal of the two detectors combined. The second term represents the fluctuations of this signal, and consists of three contributions: The contributions due to the laser noise 6, and the noise in the X-component of reflectivity S, are proportional to y2. The second contribution comes from depolarization noise Sv and is proportional to ~ryo/rm~. Note that individual channels have a larger noise component arising from depolarization, but in adding the two channels, much of this noise cancels out. The third contribution is a combination of the laser noise 6, and the noise in the magneto-optic component of the reflected light 3; it is proportional to ~ryo/r,o~2. In practice, Ir /r I is usually smaller than y 2 and, therefore, the second and To '" third contnbutions to the noise may be neglected. Under these circumstances, the total fluctuations of the sum signal arise from electronic noise, shot noise, laser noise, and the r, noise. Because all but the last of these fluctuations can be measured when the disk is stopped, the excess noise observed upon spinning the disk is attributable to the r, contribution. The differential output is the difference of the two detector signals, namely,

Eq. (16)

S,- S2 z 277,P0 y I rxoI IryoI 1 + Real(26, + 6, + 6y)+ The first term on the right side of this equation is the MO signal, observed in the erased state of the disk. The second term is the combined contribution of the laser noise 8, and the reflectivity noises 8, and 4; depolarizationnoise Sv constitutes the third term. Note that the depolarization term has a larger coefficient than the other noise term, and may, therefore, be the dominant noise in MO readout. We observe that in going from the sum signal of Eq. (15) to the difference signal of Eq.(16), the contribution of (6,+ 8,)is attenuated by a factor of lryo/yml.The reason is that the laser noise and the r, noise are identical in the two channels and, for a balanced system, they cancel out. The residual noise observed in this instance is due to interference with the magneto-optic component, ryo. The same thing happens to the rY noise, but there the interferencewith r, creates a term larger than the original rynoise, which is why the coefficient of (S,+ 3)in the difference signal is larger than that in the sum signal by a factor of ~yr,o/ryo~. Similar considerations apply to the depolarization noise SV.

572 Magneto-Optical Data Recording The signals S, and S, of the individual detectors may themselves be written in terms of the sum and difference signals in the following way : S1,S2= %(S1+ S,)

Eq.(17) TA

1-

-I---AL_AAL_

i~ IS clear

LIIC

f

!h (S, - S2)

-t _.. 1_ z- AL-1---1 -L---1 L-. s u m signal IS u~t; common-mwt; signai, SKIUGU uy

the two detectors and rejected only by the differential amplifier. Practical differential amplifiers, unfortunately, have a finite CMRR, and the noise accompanying the sum signal in Eq. (15) also appears at the differential output, albeit after a substantial attenuation. In the remainder of this section, we assume that the differential amplifier has a large CMRR, and proceed to ignore the common-mode noise. The nns values of the various fluctuating parameters (within the bandwidth of the system) are now defined as follows:

The total signal and the rms noise at the differential output (including thermal and shot noises, but excludingthe residual common-mode noise) are written as follows:

Eq. (20)

i2noise

{ 2% + 2 e v . v ~1rxo1~[1+ ~~ 1 r y ~ ~ r x )o B I~1 + (2 r 7 s p , irx0i ~ iryo1)2(4~2,+A: + A; +

irxo/ryoi2~&)

From a practical system design standpoint, there is an advantage in choosing a small yso that the most light can get through to the disk during recording and erasure. However, Eqs. (19) and (20) indicate that ycannot be made too small either, because in that case the thermal noise becomes dominant and the overall SNR suffers. Therefore, ymust be large enough to bring the total noise level well above the level of the thermal noise. In the

Sources of Noise in Magneto-Optical Readout 573 ideal case, when yis sufficiently large to make the thermal noise negligible, the SNR becomes independent of yand can be expressed as follows:

Although the ideal SNR is never attained, in practice it may be approached closely. From Eq. (20), the critical value of at which the thermal noise strength equals the combined strength of all the other noises, is given by,

+ 2( q)eB)POlry,J2[4A2, + A: + A$ + Irxo/ry0)* A$) When y= E, the SNR is only 3 dB below its ideal value; when y = 2y, the gap shrinks to 1 dB,and with y= 3y,, the ideal SNR is only 0.5 dB away. Further increases of y beyond these values are unjustifiable; in fact, for large values of the common-mode noise begins to appear in the differential output, which causes the SNR to deteriorate. According to Eq. (22), beyond the shot noise limit we are in a regime where the dominant noise is proportional to the signal; therefore, increasing the laser power Po does not enhance the SNR. In this regime, if the depolarizationnoise Sv is to be an important contributor, increasing ryoalso increases the SNR. However, if the laser noise 8, andor the reflectivity noises Sxand % are dominant, increases in ryohave no effect on the SNR. These are some of the issues to contemplate when optimizing the readout system. Example 3: The system of Fig. 1 operates under the following conditions: laser wavelength h = 780 nm, objective’s numerical aperture NA = 0.55, effective laser power at the disk P o = 0.92 mW, disk reflection coefficientr,= 0.42, magneto-opticreflection coefficient r,,= 0.006, leakage parameter of the beam splitter y = 0.46, photodetector sensitivity qs = 0.44 A/W, preamplifier conversion factor G = 6400 V/A, and differentialamplifier gain = 44. The amplifier’s CMRR is better than 100. Figure 5a shows the measured spectra of the differential output under various circumstances. With the light blocked from both detectors (lower

574 Magneto-Optical Data Recording trace), the integrated electronicnoise in the 20 MHz bandwidth of the system is -37.7 dBm or 2.9 mV. Referred to individual photodiode currents prior to amplification,this thermal noise has a density of i, z 1 . 6 4 p A l G . The shot noise current density for the 17.2mW of incident power on each photodiode is calculated from Eq.(1) as is, ; 1.55pA/&. Thus, the rms thermal plus shot noise voltage at the differential output (over the entire bandwidth) is 4 mV or -34.9 dBm, which is near the measured value of the total (integrated) noise when the disk is stopped, and the light is allowed to reach both detectors (see the middle trace in Fig. 5a). For this measurement, we demagnetized the area of the disk under the light spot to reduce ry0to zero. As a result, the contribution of the laser noise at the differential output is negligible (see Eq. 20). In the same experiment, when one of the detectors was blocked, we obtained the upper trace in Fig. 5a. The integrated noise power under this trace is -32 dBm or 5.6 mV, which, when the 2.9 mV contribution of the thermal noise and the 1.95 mV contribution of the shot noise (one detector only) are subtracted, yields the figure of 4.4 mV for the laser noise. Dividing this by the amplifier’s gain and using Eq. (14) as an expression for the noise at each detector, we find the rms fluctuations of the beam amplitude Ae in the 20 M H z bandwidth of the system to be 1.03 x 1O-3. (Becausethe track is demagneked, rp must be set to zero in Eq. 14.) Figure 5a translates into a RIN of -126.7 dB for the laser in a 1 Hz bandwidth. Figure 5b shows the output spectra with the disk spinning (linear track velocity = 14.6 d s e c ) and the track fully erased in one of two possible states (that is, magnetized up or down). We obtained the upper trace with one detector blocked. The total noise integrated from 0-20 MHz in this case is -29.5 dI3m or 7.5 mV. When we subtract the 5.6 mV contribution of thermal, shot, and laser noise from this figure, we are left with 5 mV of erased media noise. From Eq. (14) we find that if reflectivity fluctuations were entirely responsiblefor this noise then Ax would be around 1.17 x lo5. (In this calculation we ignored ryo/rm because it is small compared to 7). Similarly, if fluctuations of ry alone produce the noise, then Ay must be around 3.8 x 10-2,and if depolarization is the sole contributor, then Av 2 5.4 x lo4. Without further information, it is impossible to decide what fraction of the noise is contributed by each of these three sources. The lower trace in Fig. 5b shows the output spectrum with neither detector blocked. The integrated noise in this case is -33.2 dBm or 4.9 mV. Previously the thermal plus shot noise contributions were 4 mV. The laser noise may be calculated from Eq. (20) (with the aid of Ae estimated earlier) and is 0.54 mV. Thus, the media noise is about 2.8 mV. We now have two

Sources of Noise in Magneto-Optical Readout 575 shultaneous equations in A%A,, and AV from the two measurements on the spinning disk; they yield the following:

rmal

F

+ ahot + laser (one ddector

v

blocked)

w WHz)

t Thermal

h

E

+ shot + laser (one detector blocked)

S?

!i

p f

Figure 5. Measured noise spectra at the differential output of a magnetooptical readout system operating at 1 = 780 run; the various system parameters are given in Ex. 3 and the curves are discussed in the text. In (a) the disk is stopped and the area under the focused spot is demagnetized. In (b) the disk is moving and the track is fully erased (that is, placed in a uniformly magnetized state).

576 Magneto-Optical Data Recording Clearly, the singledetector noise is dominated by the media reflectivity fluctuations, Ax, which is a common-mode noise and does not figure prominently in the differential output. The differential output noise, however, is a mixture of A, and Av, although the individual contributions of these terms are impossible to ascertain from the measurements described so far. It appearsthat fluctuationsof the magneto-optic signal need to be fairly large (that is, A, lo-*) if they are to account for the observed noise, whereas small amounts of depolarization (that is Av z 1.5 x lo4) can readily explain the data.

8.0

WRITENOISE

The noise level increased on writing the media. This additional noise created by writing on MO media is the combination of variations in mark size, shape, position, and modulation. Also, writing MO media alters the balance between the detectors, affecting the leakage noise. The largest contribution to write noise and edge-placement variation is due to the jaggedness of the mark wall, which is intrinsic to the media. The write noise can also have contributions from variations in the write power, bias field, laser wavelength or clock instability. However, these contributions are small relative to the media contribution. We can reduce the influence of thermal andor magnetic variations by minimizing laser noise and using short, high-power laser pulses to induce sharp thermal gradients. Proper selection of optical components minimizes the effect of wavelength variations. Clock variation control is a parameter intrinsic to the system design. In MO recording, the magnetically active material is an amorphous alloy, similar in many aspects to the metal films used in conventional perpendicular magnetic storage. These conventional films exhibit a large amount of writing noise that results from irregularities and distortions: (i) caused by variations in magnetic anisotropy, (ii) in the domain walls separating oppositely magnetized regions, or (iii) caused by the presence of defects in the magnetic films. The same physics applies to MO films. Conventional thin film recording systems and MO recording devices share another attribute: write noise level increases as the density of transitions increase because we can think of writing noise as residing in the magnetization transitions. The shape of the spectrum remains constant, but the

Sources of Noise in Magneto-Optical Readout 577 magnitude of the noise is a function of transition density. This linear increase in noise with increase in transition density continues until a superlinear regime is entered. At this point, the transitions are close enough to influence each other. The amount of noise increases dramatically and the peak in the write noise spectra shifts towards lower frequency. The peak shifts because the interaction between transitions allows a longer correlation distance. However, leakage noise from changes in detector balance compounds any writing noise measurement. If the optical system is adjusted for minimum noise with no net Kerr rotation, at low densities the leakage noise is the highest. As linear density increases, the system becomes more balanced and leakage noise decreases. In this example, as linear density increased, intrinsic write noise also increased, while leakage noise decreased. Magnetic field modulation (MFM) is a means of direct overwrite in MO recording. In MFM, the temperature of the medium is raised by the laser to the point where changing the bias field direction determines the polarity of the recorded mark, just as in hard disk, floppy disk, and tape systems. The magnet floats close to the active layer, allowing the required field strength to be generated at the write frequencies. MFM writing is noisier than the standard, light-modulation, writing. However, it remains unclear whether MFM is inherently more noisy or if the problem is the rise time of the bias field or the difference in mark shape between the oval of standard writing and the crescent shape of MFM. If the problem is caused by the shape, then changes in media thermal structure may reduce the differences in noise. Again, the same method for separating noise sources can be applied to separating reflectivity-type write noises from rotation-type write noises. With the disk spinning and the drive in focus and tracking on a written track, measure the noise with a spectrum analyzer differentially and with one detector blocked. The noises in differential detection are as follows:

= N, + N, + NI + Nth + N, iNwth+ Nwr Eq. (23b) Nt(singlesnded)

578 Magneto-Optical Data Recording where N, is the electronic noise, N, is the shot noise, Nl is the laser noise, Nth is rotation (A&) noise, N,. is the reflectivity noise, NWthis rotation-type write noise, Nw is additional reflectivity-type write noise, and p is the misbalance ratio for the system. Because N, can be measured separately (Sec. 3) as can N, and N: (Sec. 5 ) and Ni;iand N, (Sec. 6)9and p is known,these are now two equations in two unknowns at each frequency. Solving the two equations yields rotation noise and reflectivity noise as a h c t i o n of frequency. In MO recording, N,,. is extremely low, to the point that if any is determined, it is really due to changes in p. WORM materials also suffer from write noise because of variations in edge placement. This variation is driven by thermal variations in the disk without any magnetic contributions. However, ablative and bubble-forming materials also have an additional noise source: scattering and reflectivity variations from the rim of the pit. This additional noise source does not mean that the SNR is poorer for ablative disks than for phase-change disks. In many cases, the total performance of ablative disks is significantly better than phase-change ones. In Sec. 6, we showed that media noise was inherently white, modified by the optical transfer function. For write noise, the situation is slightly more complex. If the edge distribution of the marks is Gaussian and noncorrelated, then the spectral shape of write noise is also white, modified by the optical transfer hction. If, however, the edge distributions are correlated positively so that the centers of the marks do not move, only mark lengths change, then the noise rolls off as the optical transfer function times a cosine-squared function in mark size. But, if the edges are correlated negatively (constant length but varying position), then the write noise appears as the optical transfer h c t i o n times a sine squared function in This FM noise has no power at DC. In one of the cases mark explored by Y. Honguh (constant length and position, but varying width), it showed a spectral shape similar to the optical transfer function, but rollingoff somewhat faster. Repeated reading on a written track can also cause the noise to increase. This increase in noise is not apparent in erased tracks, only in written ones. Two different data degradation phenomena can occur under the influence of the increase in media temperature caused by the read beam. The marks may change size uniformly, either growing or shrinking, depending on the media magnetidthermal characteristics and on the polarity of any external field. In this case, we can expect changes in signal amplitude. The other effect that occurs is the random degradation of the marks.

Sources of Noise in Magneto-Optical Readout 579 Microdomains nucleate andor the mark edges become jagged. The random degradation of the marks increases the noise and can be an important cause of system performance degradation. Proper selection of read power, consistent with operating temperature and bias field, can eliminate this degradation threat. Reducing writing noise requires increasing both disk homogeneity and mark uniformity while reducing misbalance and leakage noise sources. The composition and magnetization of the MO thin film affects the amount of write noise. The composition controls the demagnetization and wall roughness of the marks, both of which influence write noise. Also, steep magnetic gradients with temperature can minimize the importance of steep thermal gradients on writing. On the drive side, writing with short pulses with fast rise-times minimizes disk variations because of the steep thermal gradients. In general, once write noise exists, any readback technique to reduce its contribution to total noise will also reduce signal amplitude.

9.0 JITTER AND SIGNALAMPLITUDE FLUCTUATIONS In this section we present computer simulation results pertaining to the noise arising from fluctuations of the information-carrying signal. We make a distinction betweenjitter, which is caused by random variations of the zero-crossings of the signal waveform, and signal-amplitude-fluctuation noise. The basic waveform we consider is a 4 MHz, 50% duty-cycle camer signal whose amplitude alternates periodically between the values of +1 and -1, with transitions between the two levels of the signal being infinitely sharp. We computed numerically the power spectrum of this waveform, and we show the result in Fig. 6. The sampling intervals in the time and frequency domains were 0.5 ns and 30 kHz,corresponding to a total number of samples N,, = 65536. The spectrum contains the first and the third harmonics of the waveform at 4 MHz and 12 MHz. With the unity power of the square-wave signal corresponding to 0 dB, the fundamental and the third harmonic power levels are at -0.912dB and -10.455 dB, respectively. In Fig. 6, the spikes at frequencies other than 4 and 12 M H z are numerical noise due to truncation errors, with their very small magnitudes attesting to the basic accuracy of the numerical routine. Next, we introduced jitter in the transition times of the signal. A random number generator selected values in the interval [-At, At] with uniform distribution.These random numbers were then added independentlyto

580 Magneto-Optical Data Recording individual transition times of the ideal signal, creating a wavdonn with jitter magnitude of At. As an example, Fig. 7 shows a section of the signal between t = 0 and t = 2 ps, with At = 25 ns ofjitter. We show the computed power spectra of the resulting noisy signals in Figs. 8 a 4 , which correspond, respectively, to jitter magnitudes of At = 3, 5, 15 and 25 ns. (The spectral power densities are computed in a 30 kHz bandwidth.) Note that within the bandwidth of interest the spectra are relatively flat and that, understandably, their levels rise with the increasing magnitude of jitter.

-60 0

2

4

6

8

1

0

1

2

1

4

f (MHd

Figure 6. Computed power spectrum of a 4 MHz, 50% duty cycle, square-wave signal in the absence ofjitter and other signal fluctuations. We used a total of 65536 samples in this fast Fouriertransform(FFT) computation. The exact duration ofthe signal waveform is 33.25 p, which is an integer multiple of the carrier period (0.25 p).The approximate values of the sampling intervalsin time and fresuency are, therefore, 0.5 11sand 30 kHz, respectively. (The exact values of the sampling intervalsare slightly different from those quoted above; these are adjusted to llfill tworequirements: (i) the duration of the signal in time is an integer multiple of the carrier period, (ii) the total number of samples is an integer power of 2.)

Sources of Noise in Magneto-Optical Readout 581

1.5

1.0

0.5

0

-0.5

-1.0

-1.5 0.4

0.8

1.2 Time (ps)

1.6

2.0

Figure 7. Section of a square-wave signal in the time interval [0, 2 p],with jitter magnitude At = 25 ns. A random number generator is used to select the deviation of each zerocrossing point from its nominal position. The deviations are uniformly distributed in the interval [-At,At], and the random number corresponding to any given point is independent of that for any other point.

582 Magneto-Optical Data Recording

-00

0

2

4

3

O

M

12

14

-"I

-20

.OOt

.O0t

Figure 8. Spectra of squarewave signals (as shown in Fig. 7) with different amounts of randomjitter. The noise density is computedwithin a 30 kHz bandwidth. (a) At = 3 ns;(b) At = 5 ns; (c) At = I5 ns; (4 At = 25 ns.

In Fig. 9 we show the effects of amplitude fluctuations on the signal waveform and its power spectrum. Figure 9a shows a section of a jitter-free waveform, exhibiting random amplitude variations in its upper half. We randomly and independently chose the values of these signal amplitudes from the interval [0.85, 1.001 with a uniform probability distribution. The corresponding power spectrum in Fig. 9b shows a significant noise contribution only at low frequencies. We reached a similar conclusion upon

Sources of Noise in Magneto-Optical Readout 583 inspecting the spectrum of Fig. 9c, which corresponds to a signal whose positive amplitude fluctuates within [0.70,1.001and has, in addition, ajitter noise component with At = 3 11s. Comparison with Fig. 8a clearly indicates that the high frequency noise is due solely to jitter, whereas at low fiequencies the noise may be attributed to the amplitude fluctuations.

, I.)

IS

,

,

,

,

,

,

,

,

10

0.1

0

Q.5

-10

I

I

Figure 9. Section of a square-wave signal in the time interval [0,5ps]. The waveform is free from jitter, but its positive amplitude fluctuates randomly within the interval [0.85, 1.001. (b) Spectrum of the signal in (a), showing that the noise appears only at low frequencies. (c) Spectrum of a signal waveform with AI = 3 ns of jitter in addition to positive amplitude fluctuations in the interval [0.70,1.001.

584 Magneto-Optical Data Recording Finally, Fig. 10a shows a section of a signal waveform with positive amplitude fluctuations within [0.70, 1.OO], negative amplitude fluctuations within [-1.00, -0.701, and jitter magnitude of At = 3 ns. The corresponding spectrum in Fig. 10b shares its general features with that in Fig. 9c, but it has a larger noise content in the low fiequency regime, which is expected. The spectrum in Fig. 1Oc is obtained when the jitter magnitude is increased to At = 15 ns while amplitude fluctuations are maintained at their previous level. The noise in this instance is due almost exclusively to jitter, as a comparison with Fig. 8c reveals. I6

,

,

,

,

,

,

,

,

,

(4

Figure 10. (a) Section of a square-wave signal in the time interval [0, 5 ps]. The waveform has At = 3 ns of jitter, its positive amplitude fluctuates randomly within the interval [0.70,1.OO], while its negative amplitude fluctuations are confined to the interval [-1.00, -0.701.(b) Spectrum of the signal in (a). (c) Spectrum of the same signal when jitter is increased to At = 15 ns.

Sources of Noise in Magneto-Optical Readout 585 9.1

Effects of Finite Beam Size on Signal and Noise Spectra

Up to this point, we have ignored the fact that the focused spot on the disk surface has a finite diameter. The effects of the finite spot size are readily incorporated into the preceding results if one ignores the diffracton effects. The signal at the output of the readout system is then obtained by correlating the intensity profile of the spot with the recorded pattern of information on the disk. For simplicity, we assume a Gaussian intensity distribution for the spot, as follows:

Here x is the spatial coordinate on the disk surface along the track, and p is a measure of the spot size (FWHh4 = 1.66 p). The function in Eq. (24) is properly normalized so that the integrated intensity is unity. If the actual pattern of recorded data along the track is denoted by y,(x), and if vo is the disk velocity, then the output signal is as follows:

Eq. (25) The function Hx), the result of correlation between Z(x) and vo(x), is scaled along the horizontal axis by the factor v, to yield the time dependence of the observed signal. Upon Fourier transformation, the convolution turns into the product of the Fourier transforms of the individual functions, that is,

The transform of the Gaussian function is readily calculated, as follows:

If the spectra of v/,(vot) and H v d ) are denoted by S&) respectively, we will have the following:

and S@,

586 Magneto-Optical Data Recording

In Eq. (28), we show that the spectrum of the signal (including jitter and amplitude noise) is attenuated by a multiplicative factor that is the Fourier transform of the spot intensity distribution. In the literature, this effect is usually attributed to the modulation trunsferfinction (MTF) of the optical system. The fiequency at which the spectrum is attenuated by a factor of 2 is a measure of the width of the MTF. According to this definition, the width of the Gaussian fimction in Eq. (28) is approximately equal to v0/5p. Example 3 (continued): We recorded a single-frequencytone at

f = 11.34 MHz on the track. Figure 1la shows the spectrum of the readback signal along with the erased-track spectrum we described previously. The overall integrated noise from the recorded track is -31.3 dBm or 6 mV. Subtracting from this figure the erased-track noise of 4.9 mV, we obtain the rms noise voltage of 3.5 mV at the differential output, contributed by the recorded signal itself (that is, jitter and amplitude fluctuation noise). The signal amplitudecalculated from Eq. (19) is 264 mV or 1.4 dBm. The recorded tone is read at the constant velocity of vo= 14.6 m/s. Assuming a Gaussian profile for the focused spot withFWHM=0.8pm(i.e.,r=0.48pm), wefindfromEq. (28) an MTF attenuation of 11.9 dB. The signal amplitude is thus estimated at -10.5 dBm, which is 1.9 dB above the measured value in Fig. 1la. The difference is probably due to the fact that in the cross-track direction, the focused spot is somewhat wider than the recorded marks. In the same experiment when one detector was blocked, we obtained the spectrum of Fig. 1lb. The signal amplitude is now down by 6 dB (that is, a factor of 2), simply because each detector contributes half of the total signal. The noise trace, however, is almost indistinguishable from the corresponding measurement on the erased track (that is, the upper trace in Fig. 5b). This is due to the fact that individual detectors, with their large share of laser noise and reflectivity noise, drown the noise that accompaniesthe data signal.

Of the various noises we discussed in this, the spectra of the electronic, shot, and laser noise, are obviously unaffected by the MTF. The spectrum of the disk noise, however, is attenuated in a manner similar to that of the information signal as described by Eq. (28). Figure 12 shows an example of this phenomenon where the spectrum of an erased track is shown for three different disk velocities.[l21 In each case, the tail of the trace corresponds to thermal + shot + laser noise, which is velocity-independent.

Sources of Noise in Magneto-Optical Readout 587 The low frequency magnitudes of the spectral density functions drop by 3 dB each time the velocity is doubled. This reflects the fact that the noise power is constant; in other words, when the velocity is doubld (causing the spectrum to stretch over a range of frequenciestwice as large)the magnitude of the spectral density must drop by a factor of 2 (i.e., 3 dB)to maintain the total power constant. If we assume that the underlying sources of disk noise have flat spectra, then the curves of Fig. 12 are simply plots of the MTF.

0

2

4

6

Frequency (MHz)

a

10

Figure 11. Measured noise spectra at the differential output of the system described in

Ex.5. The disk is spinning and the linear track velocity is 14.6 d s . Neither detector is blocked in (a), whereas in (b) only one detector received the light. The upper trace is read from a track recorded with a single tone at 11.34 MHz; the lower trace is from the same track but erased. (The spectra of the erased track are the same as those in Fig. 5b.)

588 Magneto-Optical Data Recording

Frequency (MHz) Figure 12. Measured noise spectra from an erased track on a magneto-optical disk. The traces are obtained from the same track under identical conditions, with the exception that

the disk velocity is different for different traces. The velocities corresponding to these measurements are vo = 3 m/s, 6 d s , and 12 m/s.

Example 4: In Fig. 12 the curve corresponding to v,,= 6 m / s drops by about 12 dB betweenf= 0 andf= 4 MHz. Assumingthat the curve represents the actual MTF of the system and that the MTF has the Gaussian form of Eq. (28), we find p = 0.56 pm, corresponding to FWHM = 0.93 pm.

10.0 EQUALIZATION Optical data storage involves writing and reading information from the disk. In writing, sequences of 1’s and 0’s are encoded and passed to the laser driver, which then writes the encoded bits on the disk. On readback, the signal is corrupted by noise and distortion. The distortion causes bits to interfere with each other making the detection of individual bits difficult. This effect is known as intersymbol interference. Equalization attempts to

Sources of Noise in Magneto-Optical Readout 589 compensate for signal degradation. In optical storage, equalization boosts the higher frequencies to improve resolution and to reduce intersymbol interference. Equalization also restricts the bandwidth to reduce noise. The improved signal-to-noise ratio at the data detector allows significant improvements in capacity. One advantage of optical recording over magnetic recording is that the phase of the transfer function is linear. Thus, equalization in optical recording need only correct deficiencies in the magnitude of the transfer function. In an ideal, noiseless system, the equalizer varies the gain as a function of fiequency so that the combination of equalizer transfer function and the optical transfer function achieves a target detection channel (for example, cosine squared channel). Then, the amplitude of the high density signals are nearly equal to that of the low density signals, and the average feature is centered in the timing window. These theoretically ideal channels only produce optimum results in the absence of noise or defects. In reality, equalization is a balance between improving signal resolution and decreasing noise. Another issue (besides the relative signal and the noise spectra) in equalization is the detection technique required for the modulation used. In pulse position modulation (PPM), the information is stored in the distance between mark centers. All marks are the same size, but vary in position. The current MO standards (90 mm 1X and 2X, 130 mm 1X and 2X) use this type of modulation. These systems use PPM because variations in write power, media thermal sensitivity, and defocus, do not change the location of the center of the mark, only the SNR of the system. To detect a 1 or an 0, the centers of the marks are detected by differentiating the signal, converting peaks into zero crossings. Differentiatingin the time domain is equivalent to multiplying by ‘tf” in the fiequency domain. This multiplication greatly reduces low-frequency noise, but increases high-fiequency noise. Figure 10a shows typical noise traces at the output of the preamp. Selectively increasing boost while decreasing bandwidth further improves SNR. As an example of how detection technique and equalization changes the relative amounts of noise in PPM recording, consider the following table:

590 Magneto-Optical Data Recording

Noise Source

Preamp output

Differentiator output

After Equalization

Electronic

1.0 (1.0)

1.0 (1.4)

1.0 (0.8)

Shot

0.96

0.76

0.82

Laser

0.06

0.05

0.05

Media

6.7

1.46

2.7

Preamp output is the relative integrated noise powers measured at the preamp over a bandwidth equal to twice the frequency of the highest density tone. Drferentiator output is the noise over the same bandwidth after differentiation. After equalization shows the noise powers with the equalizer boost and the bandwidth settings optimized for minimumbit error rate. The noises at each stage are normalized to the electronic noise at that stage. The value in parenthesis is the relative electronic noise, normalized to the electronic noise at the output of the preamp. Note that the total electronic noise increasesthrough the differentiator. This noise increases because electronic noise increases with frequency. However, when the bandwidth of the system is reduced for best performance, electronic noise at the equalizer is less than the noise at the preamp output. At each stage, media noise decreases, but the amount normalized to electronic noise appears to increase after equalization only because electronic noise decreased by a greater amount. Media noise dominates at the preamp output, but because media noise rolls off with frequency, differentiation decreases its relative value. The 130 mm and 90 mm 4X standards use pulse width modulation (PWM). In PWM, information is stored in the position of the edges of the marks, as opposed to the centers. Because a mark has two edges, but only one center, the coding efficiency is significantly higher for PWM recording, ifwe can write the marks satisfactorily. The shape of equalizationfor PWM depends on which detection method we choose. If we use a threshold channel (DC channel), the edges are detected as threshold crossings. The

Sources of Noise in Magneto-Optical Readout 591 channels need DC response and, therefore, the noise prior to equalization is the same as the noise out of the preamp. (Another difference between an optical head and a magnetic head is that an optical system passes DC.) An alternative detection scheme is to differentiate the signal twice and then detect zero crossings. This detection method (AC channel) has a tremendous amount of high-frequency boost, which can degrade system SNR, especially in devices with appreciable electronic noise. The advantage of an AC channel is that the system is less susceptible to baseline wander, birefringence variation, and certain types of defects. Carrier-to-noise ratio (CNR) or narrow-band SNR is a parameter of limited usefilness. CNR is often measured using the noise from a 30 lcHz band around the signal. If the noise in optical recording is flat, then an SNR can be computed from the CNR value to predict performance. However, we have shown that the noise components do not have a flat power-spectral density. Electronic and laser noises are functions of temporal frequency; disk motion and write noises are functions of spatial frequency. Only shot noise is truly white, but after equalization, even shot noise is not white. As an example, consider the case of a track written once but read at two different velocities. The CNR for the higher speed case is higher (if media noise dominates), and yet the SNR is worse because the system bandwidth is greater. Spectral shaping helps achieve optimal performance. Equalization must provide the best detection, not necessarily the best SNR. Equalization optimization finds the best compromise between residual distortion, noise, and defect performance that produces the best system performance, performance usually being measured as probability of error or error rate. In the final analysis, equalization improves data reliability, which allows the high linear density of current products.

11.0 SNR AND JITTER Signal-to-noise ratio is the standard metric of performance for academics and other researchers in fundamental studies. SNR has several advantages because it is closely related to the physics of optical recording and does not require channel electronics. Also, SNR is a frequency domain measure taken at the preamp, and does not directly reflect drive data

592 Magneto-Optical Data Recording detection. For those involved in drive development, timingjitter is a superior metric. While measuring timing jitter requires a channel, the effects of detection methodology (for example, threshold detection, peak detection, and so on), and equalization are taken into account. As a side benefit, measuring jitter is extremely fast compared to measuring SNR, and lends itself to automated testing. Jitter is defined as one standard deviation of either the detected edgeto-edge (PWM) or center-to-center (PPM) timing distributions. Jitter can be measured as a function of just about any parameter of interest (focus, spot size, read power, and so on) in an optical storage device. When combined with peak shift to create data figure of merit (FOM) values, jitter provides reasonable estimates of the random error rates. While jitter is the better predictor of data reliability, under ideal conditions, the relationship between SNR at the data detector and jitter can be established. Starting with the equations defining SNR and jitter. Jitter

=

Noise (rms -at data detector) Signal slope at detection

SNR = 20 Log

Signal (rms) Noise (rms)

Using the sinewave approximation for the signal, the slope of the signal at detection is equal to the frequency times the signal amplitude. Slope = wA = 2zfA wherefis the frequency of the high density signal and A is the zero-to-peak signal amplitude. Using the above equation and the relationship between signal zero-to-peak and signal rms,jitter can be rewritten as

Jitter =

Noise (rms) 2JZlrfsigna1 (rms)

Jz.1o-sm’20

Sources of Noise in Magneto-Optical Readout 593 In many cases,jitter is normalized by the window width, which allows comparison between different systems and different operating points. The relationship between frequency and timing window depends on the modulation code. For instance, (2,7)PPM, llf= 3 t, (1,7) PWM, llf= 4 t,

(2,7) PWM, llf= 6 f,

Thus, for a (1,7) PWM system operating with a 20 dB SNR at the data detector, we expect a jitter of approximately 4.5% of the timing window.

11.1 Peak Shift Peak shift is the difference between the center of the timing window and the mean of the timing distribution, normalized by the width of the timing window. Peak shift differs from noise in that it is systematic in nature. In an optical system, pattern dependent peak shift (ISI) is a function of spot size, equalization, and data pattern. In addition, the thermal interaction inherent with the write process is another source of peak shift. Peak shift is measured after the effects of adaptivethresholds, dual clocks or any other similar compensation technique used by the system. Peak shift can be a major determiner of byte error rate (BER), depending on the relative amount of peak shift and jitter. Equalization, adaptive thresholding, and a robust write algorithm greatly reduce the induced peak shift.

11.2 Figure of Merit The best metric for predicting data reliability is the data communication figure of merit (FOM). Mathematically, FOM can be expressed as follows: FOM- = [(t,/2) - offset]/jitter

FOM, = [(t,/2) + offset]/jitter

594 Magneto-Optical Data Recording where tw is the timing window, offset is the combination of all the system offsets, such as peak shift, andjitter is one standard deviation of the detected timing distribution. While we show two expressions for FOM, only the smaller (worse case) FOM is ever plotted because it dominates the BER. FOM is the number of standard deviations from the mean of the timing distribution to the edge of the window. Through the use of the complementary error function, a random error rate can be predicted. When FOM is measured as a function of write power, the optimal FOM does not occur at the same write power as minimum jitter. Peaks shift increases with increased write power. FOM can also be normalized with respect to the timing window by dividingthe denominatorand the numerator by tw . The above equationscan be rewritten as follows:

FOM* = (0.5 f offset ’)/jitter’ where offset’ and jitter‘ are normalized with respect to the timing window.

11.3 Bit Error Rate (bER) The random bit error rate (bER) varies functionally with jitter and offset as the complementaryerror function (erfc). (See Ch. 11, Sec. 6.6 for a discussion of byte and bit error rates.) This model of bER has been used extensively in predicting error rates in fiber optic communication systems. The model’s predictive value is somewhat less in storage systems because of the presence of defects. However, the model serves as a lower bound on the bER expected of an optical storage system. The bER can be approximated as follows: bER = Q(F0M-) + Q(FOM+) The bER as a function of jitter for four different offkets was measured.

As jitter increases, the bER differences between offsets shrinks. At a 5% jitter, the difference projected between no offset and 15% offset is an incredible ten orders of magnitude in bER vs 10-l2).At 10%jitter, the difference shrinks to a little more than two orders of magnitude (1Oa vs lo4). With no offsets, a bER of >lo4 is reached at jitters under 10%; at an offset of 15%, the same bER is reached at jitters under 6.5%. Both of these cases correspond to an FOM of approximately 5.

Sources of Noise in Magneto-Optical Readout 595 A dataset used for margin characterizationdata of a DVT-level drive shows bER as a function ofjitter. All four bands tested have about the same FOM, which proves that the system is basically media-noise dominated. The noise bandwidths for electronic shot and laser noises doubles from ID to OD, with little effect on jitter as a hction of the timing window. The ulots of jitter versus bER for the four bands are almost identical: a flat region below 9%jitter where the system bER is defect- and weak-bit limited, and a rising region above 11% where the system bER is jitter-limited. Extrapolating jitter as a fhction of bER in the jitter-limited region to the bER = 1O6 point yields a FOM of approximately 5 . From a bER modeling viewpoint, the conclusions are the same in that 1O6 random error rate requires a FOM near 5 . While a bER model based on the complementary error function may appear simplistic, such a model has real utility in system design. A FOM of 5 appears to be the lower bound on FOM if 10" random error rate is required. 11.4 Phase Margin

Some companies use phase margin (PM) as the measure of performance rather than either SNR or FOM. The definition for phase margin at 10" reliability is

PM=I--

4.90

ost,

When the phase margin is zero, the system should be operating at a random error rate of 10". Thus, a phase margin of zero corresponds to a FOM of 4.9. Hewlett-Packard has a target phase margin of 50% for a system under nominal conditions. Phase margin is usually measured in random data, therefore, peak shift is a small contributor. Under the conditions of small peak shift random data, the relationship between PM and FOM can be determined. The above equation can be rewritten as follows: 4.9 - 0.5tWFOM I-PM CT

596 Magneto-Optical Data Recording Thus, a PM of zero is equivalent to a FOM of 4.9 and ajitter of 10% of the full window. A PM of 50% translates to a FOM of 9.8 and ajitter of 5% of the window. From a practical point of view, measuring jitter and peak-shift remains the most informative means of quant@mg performance.

12.0 SUMMARY

The system SNR determines the random error rate for an optical data storage device and, therefore, the capacity of the disk. The major noise sources in optical recording are electronic, shot, laser, media, and write noise. Modeling allows accurate predictions of the relative amounts of these noises. Equalization changes the signal-to-noise ratio and the amounts of the different noise sources. Predictions for BER can be made from the SNR at the data detectors.

ACKNOWLEDGMENTS The author acknowledges the work of his colleagues at IBM, especially D. Call, M. Madison, T. McDaniel, M. Tayefeh, and W. Williams. Much of the discussion in this chapter, particularly on the mathematical aspects of noise in MO recording, as well as several of the figures, were generously provided by Professor Masud Mansuripur of the University of Arizona. This is gratefidly acknowledged. The author also acknowledges Anthony Dewey (IBM Almaden Research Center) for the use of measured noise spectra.

Sources of Noise in Magneto-Optical Readout 597 REFERENCES 1. Smith, J., Modern Communication Circuits, McGraw-Hill, New York (1986) -uy"u-.y, .*., A . Prnhnhilih, ., -.-..--... Rnndnm Vnrinhlor nnd -..- Stnrhnrtir -Prnrurwr -------, -. 3 Panniilic 2nd ed.,McGraw-Hill, New York (1984) 3. Arimoto, A., Ojima, M., Chinone, N., Oishi, A., Gotoh, T., and Ohnuki, N., Applied Optics, 25: 1398-1403 (1986) 4. Ojima, M., Arimoto, A., Chinone, N., Gotoh, T., and Aiki, K., Applied Optics, 25:1404-1410 (1986) 5. Acket, G. A., Lenstra, D., Den Boef, A. J., and Verbeek, B. H., J. Quant. Elect., QE-20: 1163-1 169 (1984) 6. Dewey, A. G.,"Optimizing signal to noise ratio from a magneto-optic head," paper E08.2, presented at the meeting of the Optical Society of America, Boston, Massachusetts (1990) 7. Dewey, A, G., SPIE, 1078:279-286 (1989) 8. Beck, J. W., Applied Optics, 9:2559-2564 (1970) 9. Heemskerk, J. P. J., Applied Optics, 17:2007-2012 (1978) 10. Inoue, F., Maeda, A. Itoh, A., and Kawanishi, K., IEEE Trans. Magn., 21: 1629-163 1 (1985) 11. Treves, D., and Bloomberg, D. S., Optical Engineering, 25:881-891 (1986) 12. Dewey, A. G., SPIE, 69372-78 (1986) 13. Mansuripur, M., Connell, G. A. N., and Goodman, J. W., J. Appl. Phys., 53:4485 (1982) 14. Kubota, S.,Appl. Opt., 26:3961 (1987) 15. Finkelstein, B. I., and Williams, W., Appl. Opt., 27:703 (1988) 16. Finkelstein, B. I., and Call, D. E., SPIE, 899:69 (1988) 17. Yardy, R., Finkelstein, B. I., and McDaniel, T. W., Proc. SPIE, 1316:106 (1990); J. Appl. Phys., 675325 (1990) 18. Finkelstein, B. I., and Childers, E. R., Proc. SPIE, 1499:438 (1991) 19. McDaniel, T. W., and Finkelstein, B. I., IEEE Trans. Magn., 275118 (1991) 20. McDaniel, T. W., and Finkelstein, B. I.. J. Appl. Phys., 69:4966 (1991) 21. McDaniel, T. W., Rubin, K. A., and Finkelstein, B. I., IEEE Trans.Magn., 30:4413 (1994) 22. Madison, M., J. Appl. Phys., 735782 (1993)

I---..---.I

10 Modeling the MagnetoOptical Recording Processes Terry H? McDaniel and Brian J. Bartholomeusz

1.0

INTRODUCTION

Our intent in this chapter is to present a practical summary of the application of modeling in the description or simulation of magneto-optical (MO) recording systems or subsystems. Modeling has been applied to MO recording technology in ways consistent with conventional engineering and scientific practice. By this we mean that while some of the models themselves have exhibited novelty in the application of particular analysis tools to circumstances unique to MO recording technology, probably little or no new science or mathematics has emerged from modeling MO (or magnetic) recording. This is definitely the realm of engineering and applied science. It is our belief that modeling MO recording systems is particularly interesting, primarily because of the richness of the technology with its optical, thermal, and magnetic components. In addition, as in modeling any recording system, there are electrical, electromechanical, mechanical, and communication (digital channel signaling) issues to be considered. MO 598

Modeling the Magneto-Optical Recording Processes 599 recording is highly crossdisciplinary, touching many subdisciplines in applied physics, in addition to chemistry, materials science, electrical and mechanical engineering, communications theory, and computer science and engineering. We have focused our attention in this discussion on the aspects of MO recording that set it apart from magnetic recording, namely the optical and thermal considerations, and to a lesser degree, the magnetics issues. Magneto-optical phenomena are discussed in the optical modeling section, while the magnetics issues covered are generally independent of the optical recording context. What is Modeling? Our use of the word modeling adheres to the following: Noun-A theoretical and mathematical representation of a physical system; a simulation of a physical system using established theory and mathematical tools Verb-The application of theoretical and mathematical methods to simulate the fhction of a physical system The implication of these definitions is that a worthwhile model is an assemblage of mathematical statements that expresses some set of physical fhctions in terms of their theoretical representation. A good model is structured in such a way that it is flexible in accepting input data and handling various experimental scenarios. It should provide output (results) that can be compared with physical experimental data. Thus, at the very least, the model is a “black box” that accepts input and provides output. Even a black box model can be extremely valuable in engineering development. For example, such a model may allow the user to establish bounds on inpudoutput parameters, and perhaps develop f h c tional specifications. Of course, the limitations of black boxes are wellknown. A user is restricted to inferring the structure of the system by studying the relation between input and output data. An analog from physical science is a scattering experiment in nuclear or high energy physics. Inferences arrived at in this way may not be unique, and may not be completely general. As a practical matter, the models with which we are concerned are obviously man-made constructs, and therefore the “black box” interior is accessible in principle. Often, a particular user may not wish to probe the model structure, and is content to operate the model in black-box mode. However, the full potential of a model is realized when we go beyond a black-box exercise, and become involved in analysis and possible modification of the structure. Only at this level are we interacting with the physics of

600 Magneto-Optical Data Recording the model, and we open the possibility of optimizing the model structure against some objective standard, such as its accuracy as an approximation to a physical system. It is apparent that we are considering operation at completely different levels of system understanding when we compare “black box” operation with model optimization. What is the Physical Realization of a Model? At the present stage of technology development, a model normally Consists of executable mathematical statements and instructions, e.g., a computer program Utilizes a calculator or computing machine (personal computer, workstation, minicomputer, mainframe) to execute programs Involves auxiliary hardware and software for writing and running programs, handling input and output data, and presenting results Consequently, when modelers discuss their craft, one hears discussions not only of the theory underlying the technological applications, but also the merits of various techniques of applied mathematics and numerical methods,as well as the issues associated with choice of programming language and computing machinery. There are obvious analogies here to the way experimentalists converse about measurement equipment and techniques, sample preparation, measurement automation, data analysis, and the like. Where does Modeling Fit in the Approaches of Science and Engineering? A classical view of the structure of scientific activity (and, to a lesser degree, engineering) has been that the work of the enterprise is divided into theoretical and experimentalparts. A practitioner in one of the disciplines of the physical or natural sciences is usually classified as a theorist or an experimentalist. A small minority of working scientists manage to operate in both camps. However, the tools, techniques, training, approach, and infrastructure of the two sectors are generally quite distinct. Indeed, most academic research departments in the scientific disciplines are purposely partitioned according to the experimental or theoretical content of the work and background of the members of the faculty and staff. With the recent rapid growth of computer technology, a third distinct sector of the scientific community has arisen because it is not compatibly classified as either theory or experiment. This newly classified group of scientificpractitioners utilizes computing machinery to merge many aspects of theory and experiment. Essentially, computational scientists do experimentation via the computer; that is, they model or simulate real systems with

Modeling the Magneto-Optical Recording Processes 601 more or less elaborate theoretical constructs on a computer. They do not usually create theories (although they have a role in testing, modifying, and extendingthem); they do experimental exercises of theories on the computer which search for implications and seek validation of underlying theory against experimental observations. Hence, computational science is a novel activity facilitated by computing technology, which borrows from and serves both theory and experiment, and consequently it advances the scientific enterprise. It is probably worthwhile to step back and inquire about the relation among design, modeling, and experiment. Figure l(a) can guide our thinking. The two anchors in the diagram are theory and the physical system. In this discussion, the physical system is the optical media we are engineering, plus its surrounding optical disk drive or optical tester hardware which effects the physical interaction with the media. The physical system is the goal of our development effort. From the standpoint of engineering design, we can view theory as a “given” body of knowledge of the pertinent physical principles associated with optical data storage media and its use. We will very likely draw from theory in our work (mainly to build models), but we will almost certainly not contribute to the theory (since we are working in the realm of engineering and applied science rather than basic research). Next, we can fill in the interconnections of Fig. l(a) by noting that interaction with the physical system occurs in just two ways-experiments performed on the system and the “build” operation of modifying the system. The model is a mathematical construct based on known theory which attempts to accurately simulate the behavior of the physical system. To do so, the model requires parameter inputs characteristicof the physical system that are obtained from experimental measurements on the system (testing and characterization), or knowledge of the build specifications of the system. A well-developed and validated model will be usefid in the process of design optimization. A model attains an acceptable state of veracity through an active interchange of measurement results and predictions with the experiment. This interchange is wholly analogous to comparisons of theory and experiment that are a hallmark of science. However, for our complex engineering system, the model is a more appropriate representation of theory than a textbook collection of first principles. We view the kind of modeling being described in this chapter as a natural outgrowth of the traditional modeling advocated in engineering, and also as part of the rapid development of computational science that has accompanied the explosion of computing technology since World War 11.

602 Magneto-Optical Data Recording The strong ties of modeling into theory and experiment are evident from the discussion already presented, and are reinforced with the remaining material of the chapter. In summary, theory is the basis for the structure of a model or simulation, and an experimental approach is the paradigm for the utilization of a model. Modelers do experimental exercises of theory on a computing machine-they perform computational experiments.

Customer Data In

Analog Signal Processing

-

Write Encoder Hardware (ECC h 7 h Modulation)

-

1 i

Data Clocking

-

ECC Decade

-

Storage Media

Data Decoder

Read Hardware

L

Process

1

2 i f

Customer Data Out

Figure 1. (a) Schematic diagram of the relationship among design, modeling, and experiment. @) Block diagram of generic digital data recorder. The portion inside the dotted rectangle includes the magneto-optic write and read transducers interacting with the media, which comprise the data storage subsystem treated in this chapter.

Modeling the Magneto-Optical Recording Processes 603 2.0 THE ROLE OF MODELING

2.1 General Modeling or simulation is an enterprise in building and exercising an approximation to a real physical system. It is an approximation because a description is chosen that is incomplete and therefore incorrect to some degree. An approximation is made not because we purposely wish to model incorrectly, but because we are willing to compromise as a practical necessity in order to make progress in technology development. Approximations are used in science and engineering for several reasons: Our knowledge of the nature of technological systems (e.g., an MO recorder) may be incomplete; a complete theory of every subsystem may not exist. The application of rigorous theory, even if it exists, to every aspect of a system is not practical because the resulting complexity would reduce model functionality below an acceptable minimum. Conversely, approximations may yield quite acceptable accuracy while markedly increasing model simplicity and functionality. Much of the art of engineering involves knowing how to arrive at suitableapproximations of reality, and modeling is a faithful prototype of this process. A model serves us well in instances where the expense and effort involved in its construction and use is significantly less than a comparable experimental study utilizing hardware. Commonly, the greatest advantages of a modeling approach lie in these areas: The economy (expense, time) in carrying out a study of a large number of cases. Avoidance of the creation of numerous wellcharacterized measurement samples with precisely controlled parameters. Therelatively easy accesstofundamentalparameters andmetrics associated with the physics of a system under study; frequently these quantities are not readily accessible experimentally. A model, being an object whose principal constituent is software, may be more amenable to automation and remote operation than a hardware experiment.

604 Magneto-Optical Data Recording The aggregate of these attributes of models suggests that simulation is an approach to technology system development that offers the potential for high productivity and the possibility of deep insights. It is apparent that by its very nature a simulation has an inherent artificiality. This characteristic can be prevented from becoming a detriment by maintaining an appropriate link between reality (physical experiment) and simulation. Models must be validated by checking their behavior (input/output) against comparable experimental test cases. Comparable means the experiment must be sufficiently simple and wellcontrolled to be accurately modeled. Experience shows that model validation is often a process of iterative debugging and modification of both the experiment and the model until consistent agreement is achieved over the widest possible range of input and output parameters. The adjustment of a model during validation can occur on different levels. The least extensive alterations are those that leave the physical content of the simulation intact, and are restricted to modifications of physical parameters (e.g., time steps; spatial sampling intervals) and material “constants” (e.g., thermal conductivities; refractive indices). These parameter adjustments are somewhat unique to theoretical and computational work; the closest analogies in experiment may be the setting of sampling rates (measurement intervals), and altering the physical materials employed in the experiment. An intermediate level of model adjustment is appropriate when the parameter settings and physics content are correct, but the computational implementation has shortcomings. Perhaps a numerical method is unsuitable because it fails under some conditions. The repair could involve substitution of a more robust subroutine or computational utility, possibly one coded in a different language than the main program. The deepest level of modification involves adjustment of the physics represented in the model. An example might be the discovery that in modeling a particular regime of heat flow in a thin film structure that the classical Fourier equation is breaking down.[’] This might require installation of a more general theory, or at least a means to automatically switch to a suitable subcase of the more general theory. Clearly, a model can be only as good as the correctness of the physics it embodies, and modeling is a convenient testing ground of our understanding of the physics of the system being studied.

Modeling the Magneto-Optical Recording Processes 605 The process of model validation, when completed, serves as a rigorous confirmation of our understanding of the operation of the system under development. It is a convincing illustration that we can assemble the relevant physics and associated computational function to accurately simulate the behavior of the actual physical system. There can be no more meaninel demonstration of one's understanding of a system than to correctly simulate its essential function with a model based on the applicable theory. With a validated model in hand, one can proceed to exercise it to achieve its full complement of potential benefits. We previously discussed the speed and economy that modeling can bring to the design and development cycle. This in an extremely valuable contribution to the potential for economic success of a product. In addition, a model should aid the design process by enabling optimization and establishing the parameter limits for acceptable function of the system or subsystem. Finally, modeling can provide the initial survey of the operating parameter space, and identify the appropriate subspace for experimental confirmation of hardware component performance.

2.2 MO Recording Processes In this section we discuss how modeling can be profitably applied to MO recording technology. We preview the following sections that detail the utility of modeling the optical, thermal, and magnetic aspects of the MO recording system. We do this with an eye toward eventual assembly of the models of the various subsystems and individual components into manageable models of the larger system. We describe in this chapter the modeling of a sequence of steps in the MO recording process including the elements shown in Table 1. We arbitrarily restrict the set of processes considered here to those enclosed inside the dashed region in Fig. l(b). Notice that subsystems A-E in Table 1 can be overlaid on function blocks of Fig. l(b). Our principal decomposition of the recording processes delineated in Table 1 will be according to the physical domain of the subsystem functions, which are indicated in parentheses in the Function column of the table. Therefore, it is clear that in order to effectively treat these processes, we must have models that include optical, thermal, and magnetic physics. In addition, to the extent that we wish to model the data channel, we must enter the electronic and signal processing domains to estimate the error rate in

606 Magndo-Optical Data Recording data processing. Alternatively, one could process the servo signals and use this as input to models of the tracking and focus servo subsystems. Channel modeling is discussed in Ch. 13, while servo modeling is addressed in Ch. 3. Before embarking on a treatment of the central topics of this chapter, it may be enlightening to consider the analogs to Fig. l(b) and Table 1 for magnetic recording, a technology considerably older than MO recording, but an erasable data storage approach with many apparent similarities to MO recording.[2b1[261

Table 1. MO Recording Process Subsystems

Subsystem

Input ~~~

A. Writing and eras-

~~~~

output ~

~~

~~

Function (Domain) ~

~~~

~~~

~~

Channel code bit stream of input data to write driver; erase instructions

Focused beam a t media

Media irradiation for thermomagnetic write and erase; (optical, electronic)

B. Media-write/ erase process

A output

Final state of media along track after writderase radiation absorbed; reflectance, transmission not tracked

Thermomagnetic write/emse process; direct overwrite; (optical+thermal +magnetic)

C. Reading transducer (head read Path)

Reflectedldiffracted light from written media - D output

Detector output - data Process reflected/dif(IAJB) and swo Chmfracted light from menels FES, llJ2) dia back to detectors to yield output currents; (optical, electronic)

D. M e d i a - r e a d process

Focused read beam at media - A output

R , O,, +, 6, diffracted beam from features

Interaction of read beam with written media; magnetic super resolution; (optical, magneto-optical, thermal)

E. Read channeldata; focus and tracking servo

c output

Data detection events; focus or tracking actuator response

Signal processing; equalization; BER; servo function; (electronic)

ing transducer (head illumination path)

ms,

Modeling the Magneto-Optical Recording Processes 607 Figure l(b) is somewhat generic in that it can equally well represent nearly any digital recording system in which a transducer interacts with a storage medium in a writing, reading, or erasing mode. However, now consider the appearance of Table 1 for a magnetic recording system. Every appearance of “optical” and “thermal” is removed, or replaced with “magnetic.” There is little or no optical or thermal physics involved in a conventional magnetic recording system. This is why we maintain that the MO recording process is a far “richer” system in terms of its physical content than the magnetic recording process. And because the magnetics of thennomagnetic recording involve large thermal excursions, a case can be made that even the magnetics of MO recording is technically more interesting than its magnetic recording counterpart.

3.0

OPTICAL MODELING

3.1

Scope

Optical modeling is central to describing MO recording, playing a large role in subsystems A-D of Table 1. Examination of the associated fimctions in the table reveals that the physical processes of interest include: Beam conditioning and propagation from the output of the laser diode to arrival of the focused beam (stylus) at the medium-this is the illumination path. Interaction of the focused beam with the optical disk (the storagemedium and its “packaging’- substrate and coatings that surround the thin films). Propagation of the reflectddihcted beam from the disk back through the collection optics to the light detectors (head read path, including data and servo channels). Because we are primarily concerned with modeling the MOW in this chapter, we will not treat in any detail much of the beam conditioning of the illumination path; that is addressed in Ch. 2. We are concerned with the characteristics of the focused stylus, since that is crucial to the interaction of the laser beam with the media. There are essentially two physical processes to understand in our optics problem: (a)the propagation of a beam of electromagnetic (EM) radiation through a classical optical system; and (b) the interaction of a focused beam with a multilayer MO medium.

608 Magneto-Optical Data Recording Process (a), which covers subsystems A and C of Table 1, is standard in classical optical analysis. For an MO head, we are concerned with the behavior of the polarization states of the radiation as we account for the diffraction, refraction, reflection, transmission, and absorptionof the propagating beam. Process (b) (subsystems B and D of Table 1) is perhaps more challenging optically, particularly if we strive for maximum rigor in the modeling. Fortunately, experience has shown that first-order phenomena, and probably most second-order effects as well, are accounted for (at least qualitatively) by relatively simple modeling (theoretical) approachesscalar diffraction and plane-wave decomposition, in which one assumes complete coherence of the interacting EM waves (2b). This is a clear example where tractable approximations enable more extensive engineering investigation of a problem than if one were bound to a more rigorous treatment of each constituent of the analysis. It is a trade-off that often pays dividends. It does not preclude the possibility of subsequently revisiting the more questionable stages of the approximate analysis with an approach of greater depth and accuracy. In this way, a piecewise treatment of a complex system shows how varying levels of theoretical analysis may need to be brought to bear for a fully satisfying treatment.

3.2 Write and Read Stylus (Subsystem A) The recording stylus, the focused laser beam (spot), which interacts with the storage medium, is the transducer in the MO recording process. It is analogous to the read head geometrical structure in magnetic recording. The details of the light amplitude and phase distributions at and near focus over the depth of the storage medium structure may be critical for some applications. Further, we are interested in each vector component of the R(F) distribution around the region of focus. Knowledge of this distribution is, in principle, prerequisite for the modeling of subsystems B and D of Table 1. Briefly, the relevant optical physics of subsystem B is the conversion of electromagnetic energy of the focused beam to heat by the absorption of photons in the thin film stack of the medium. As we shall see, the dominant heating is in the metallic MO film. The pertinent optical physics of subsystem D is richer than that of subsystem B. During readout, we may consider a much milder film heating of B, but the first-order effects are diffraction of the beam fiom the disk topography and the MO Kerr effect during beam reflection. All these beam-media interactions will be considered later; in this section we concentrate on modeling the spot.

Modeling the Magneto-Optical Recording Processes 609 The theory of focused light beams is well establi~hed.[~l-[~] There has been controversy from time to time about the adequacy of approximations and the limits of numerical aperture (NA) over which a particular theory is effective. The examples shown in Figs. 2(a)-(c) were produced with the program DIFFRACT, and the supporting theory is discussed in Refs. 4 and 7. This program produces results in excellent agreement with widely referenced resultd6Iand we cite DIFFRACT results frequently in this chapter. It is useful to consider the set of variables that are important in determining E(T) near the beam focus. Figure 3 shows these variables schematically. It is simplestto initially assume that the beam incident on the objective lens aperture is perfectly collimated and has a circularly symmetric Gaussian intensity distribution. It is the role of the beam conditioning optics from the laser to the objective lens to prepare the beam in the desired state (see Fig. 7). A very simple scalar diffraction theory[lOl[lllfor the ill give the spot intensity profile at focus. circularly symmetric case of Fig. 3 w Reference 10 assumes no wavefront aberrations, and Fig. 4 shows the spot intensity distributions for several values of aperture overfill 4cAvSs/~ and lens NA. Given the light wavelength A, we observe that a considerable range of spot sizes (full width at half-maximum intensity-FW€€M) and side lobe energy distributions can be obtained. As we shall discuss shortly, trade-offs between central lobe FWHM (Fig. 5 ) and the relative energy in the side lobes are particularly important for readback considerations. Central lobe FWHM determines readback resolution (the ratio of high-to-low signal frequency response), and side lobe characteristics are critical for adjacent track crosstalk. Another important illumination beam design consideration is shown in Fig. 5 . Overfilling of the objective aperture has a very strong influence on power transmission efficiency of the objective lens, as well as the focused beam energy distribution. When diode laser output power is constrained (usually the case, particularly for shorter wavelength diodes), use of overfill to achieve a spot geometry target may be difficult. Another option for focused beam shaping is to use asymmetric overfill, (e.g., different overfill ratios for the directions parallel and perpendicular to the media pre-groove directions). This requires use of a noncircular beam for illumination of the objective aperture, or a noncircular aperture. A potential benefit is adjusting the writehead beam profile for an optimum combination of writelerase function and readback crosstalkhesolutionperformance. This is somewhat analogous to the geometrical design of a magnetic recording writehead head. Figure 6 shows an asymmetric spot achieving improved central lobe readback resolution along track, while suppressing crosstalk-inducing side lobes in the cross-track direction.

610 Magneto-Optical Data Recording

The attainment of improved focused spot characteristics through special aperture apodization or obscuration has been propo~ed.[~*1-[~~1 While this approach is conceptually straightforward, practical realization could prove challenging, particularly for a manufacturable optical head. Nevertheless, redistribution of focused and diffracted beam energy can be an effective means of tailoring the recording stylus. One might predict that rigorous vector diffraction analysis (with boundary conditions) of the focused stylus for the case of aperture apodization would be more critical when detailedknowledge of the polarization state of the spot is paramount. This is because edge effects around the obscuration are occuning in the interior regions of the original aperture where the beam intensity is relatively high.

-2

-1

0

I

7

(4 Figure 2. (a) Contours of I,+xf ;from the center outward, the levels are 0.9,0.8,0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.03, 0.003, 0.0003, 0.00003. Incident beam was polarized along x; li. = 780 nm; NA = 0.55. A Gaussian incident beam overfilled the circular lens aperture, with the ryio of 8 diameter to aperture diameter being 1.09. See Fig. 3 also. (b) Contours of Iy=lEyl for the spot of (a). Note the fourfold symmetry arising from refraction through a circularly-symmetric Intensity levels, from highest to lowest, are ...., 10-5.5,10". (c) Contours of I, =lE,r for the spot of (a). This spot has twofold symmetry arising from refraction through a circularly-symmetric Intensity levels are the same as in (b).

Modeling the Magneto-Optical Recording Processes 611

Figure 3. An illustration of a Gaussian beam incident on a circular aperture hm i ng a focusing lens, which, in turn,forms a diffraction-limitedspot in the lens focal plane. Ifthe incident beam intensity is l oexp[- (r/~oAm)lwe can define aperture overfill as a a v s s / ~ . The final focused spot FWHM is determine by both overfill and the ratio of m A , where NA = G/f = sin 0 is the lens numerical aperture, andf is the lens focal length.

d

612 Magneto-Optical Data Recording

Radius

(prn)

Figure 4. Radial intensity distributions for focused spots in systems of the type illustrated in Fig. 3 with h = 780 nm and various values of overfill and NA. The method of Ref. 10 was used to produce the curves.

Figure 5. Focused spot FWHM and fractional optical power transmitted through the aperture vs overfill ( u a v s / ~ )computed by the methodology of Ref. 10.

Modeling the Magneto-Optical Recording Processes 613

Figure 6. (a) Contours of I , .clE,I' for li = 780 nm with asymmetric overfill; the overfill along the polarization direction x is 1.40, while perpendicular to the polarization along y the underfill is 0.90. The intensity levels are as in Fig. 2(a). (b) The intensity along the xandy-axes from (a). The different spot FWHM values are indicated.

3.3 Light Collection Optics (Subsystem C) A typical MO write-read head layout is schematically shown in Fig. 7. In modeling the MO recording process, our principal interest in representing the behavior of the light collection optics is to make predictions of the MO signal constituents 1, and lB (fromdetectors A and B) and the focusing and tracking error signals derived from a quadrant detector:

There are essentially two approaches in use to compute these signalsa simple algebraic and a diffractive method. The algebraic is usually cast in terms of Jones matrices, a compact means of analyzing polarized light optical systems. In this method, one uses an idealized representation of the function of each optical element in Fig. 7. The Jones matrix formulation involves 2 x 2 matrices to represent

614 Magneto-Optical Data Recording the propagation of a transverse electromagnetic wave (E,,E,) along the local z-coordinate direction through some optical processing element. The matrix links the output and input fields:

ELECTROMAGNET SPINDLE

STORAGE FILM

DISK OBJECTIVE

$'

U

-LENS

"4 SPLITTER PARTIAL POLARIZING BEAM SPLITTER (PPBS)

BEAM CIRCULARIZER COLLIMATING LENS

L

1

:

1

,

--

-- I--

"2

--

POLARIZING BEAM SPLITTER DETECTOR B

DETECTION LENS

-

il

6

&El -

QUADRANT DETECTOR DUTPUTS I(,12,1s.14 L

LASER DIODE

Figure 7. Layout of MO writehead head, including servo and data branches.

Matrix elements of some commonly used Jones matrices are listed in Table 2. The Jones calculus ignores diffraction, but does a trace of a particular ray (plane wave) through an optical system. Good illustrations of the use of Jones calculus in an MO read head are given in Refs. 17, 18, and 19. This analysis is an appropriate choice in the parts of the optical path where difFractive effects are negligible, and 1:1 ray mapping is effective.

Modeling the Magneto-Optical Recording Processes 615 Table 2. Commonly Used Jones Matrices OpticalElemd

Mi,

MI,

M,,

M,,

Horizontal linear polarizer

1

0

0

0

E-field polarized along xaxis; propagation along zaxis

Righaeft circular polarizer

0.5

MSi

MSi

0.5

E-field polarized along xaxis; propagation along z-

Kerr effect mirror

rpp

kPS

1

0

Comments

axis

kq

= kps

r8 = rPP

kpl - l o 3 ‘pp; Fresnel reflection coefficients

Phase plates

+ = phase delay CW coordinate transformation

Polarizingbeam splitter analyzer

P,

0

0

PY

amplitude fractions passed

A scalar diffraction analysis of the collection path is more complete than the Jones calculus, as it treats the distribution of light energy over the aperture of the various optical elements. Therefore, losses due to overfill and variations in sensitivity over an element aperture (e.g., a light detector) can be accounted. Aberrations across the beam wavefront can also be incorpohandled. A diffractive analysis, as in the program rates the fundamentals of Fourier Fourier transforms are used to map the propagation of a bundle of rays through lenses. For read-path design work where knowledge of the light distribution at locations along the light path is required, dieactive analysis is mandatory. This can be demonstrated with an example. Figure 8a illustrates an intensity distribution in the plane of a quadrant detector of a servo path like that depicted in Fig. 7. This computed result from DIFFRACT shows the intensity profile at the midpoint between the principal foci of an astigmatic lens. Subtleties in the distribution arise from the dieactive effects of light interaction with the pre-grooves in the disk. Nevertheless, there is meaningful structure in this intensity pattern when the TES of Eq. (1) is calculated

616 Magneto-Optical Data Recording for a spot scan perpendicular to the tracking pre-grooves by one-half the track pitch as shown in Fig. 8b. Similarly, a FES (Fig. 8c) is obtained by displacing the disk surface *2h from the position of best focus. The variation of the peak intensity at the MO data detector during this defocus excursion is shown in Fig. 8d. Clearly, diffraction analysis is essential to give the intensity distribution over the detector, which is required to compute the TES and FES from the integrated intensity.

I'

"

"

"

"

a c

Figure 8. (a) Light intensity distribution on the servo quadrant detector (Fig. 7) from the program DIFFRACT when the focused spot is positioned midway between the land center and groove center. The same level of substratebirefringence as used in Figs. 15 and 16 has been included. (b) Tracking error signal for a spot scan perpendicular to tracks from a land center to an adjacent groove center. (c) Focus error signal for a spot centered on the land, but with objective lens scanned over +2 1 displacement for defocus. (d) Strehl ratio vs defocus from DIFFRACT.

Modeling the Magneto-Optical Recording Processes 61 7 3.4 Light Interaction with Domains (Magneto-Optics) (Subsystem D) The polar magneto-optic Kerr effect (MOKE) is the physical phenomenon at the heart of MO readout. As the first row of Table 2 and Eq. (2) indicate, a phenomenological picture of the MOKE is of linearly polarized light reflected from a magnetic mirror. The reflected light has a reduced amplitude due to Fresnel reflection, but also has a weak orthogonal polarization induced due to the interaction of the electromagneticwave with the electronic structure of the magnetic mirror. Although the Kerr effect on linearly polarized light is conceptuallyclear, the theoretical analysis is more naturally presented in terms of circularly polarized light. States of circular polarization are eigenstates for the solution of Maxwell’s equations that yield electromagnetic wave modes in magnetic materials.[211[221This gives rise to the dual phenomena of magnetic circular dichroism (differential absorption of left and right hand circularity polarized light-LCP and RCP) and magnetic circular birefringence (phase retardation between LCP and RCP light). The physics of the MO effects has been thoroughly reviewed in the l i t e r a t ~ r e [ ~ ~and l - [ we ~ ~ ]will not discuss this basic theory further. Our objective in this section is to develop a model for computing the MO readout signal in the MO recording process. The supporting theory is presented in Ch. 8 on “The MO Readout Process.” To recap quickly, readout in an optical disk drive is an example of the optical function of a scanning The focused beam (stylus) interacts with the thin film stack that covers a surface having topographical features (pits and grooves). The interaction is best represented as reflectioddiffraction from an amplitude/phasegrating. The reflected light is collected by the focusing has summarized the essential result of several objective. Mar~hant[’~~] papers[2q-[291 covering the scalar difli-actiontheory of MO readout. Marchant identifies two regimes for modeling the readout signal: 1. The signal is a simple correlation of the optical stylus intensity I(x,y)with the disk reflectance pattern.

so, -JJ

dx‘dy’I(x’,y’)a(r‘+ v t , y ‘ )

2. Form the signal from the light intensity by squaring the diffracted light amplitude, which turns out to be E ; ~ ~ ~ ‘ (*Xf () 7 ) ,a convolution of the truncated incident field with the 2-D Fourier transform fof the disk reflection coefficient pattern:

618 Magneto-Optical Data Recording

Regime 1 is a limiting case of the more general result of regime 2. Result (3a) is a good approximation when (i, aperture truncation is small and (ii) difFraction effects from the interaction of the stylus with the written mark pattern are weak. Condition (ii) holds if the marks do not introduce significant phase shifts, only reflection modulation, and if the diffractive features are not too small or the transitions too sharp. Of course, the reflectivity contrast at the mark edge can be abrupt, and it is well knownthat subtle differences in the signal waveform arise in modeling the readout from a step transition with Eq. (3a) and Eq. (3b).[311[311 Marchant[l91has indicated how Eq. (3a) drops out of Eq. (3b) if the mark reflection coefficient is slowly varying and if aperture truncation is negligible. Then simple analysis from Fourier optics corresponding to the system in Fig. 9 gives

where

andfis a 2-D Fourier transformation operator. Applying the convolution theorem for Fourier transforms,

where denotes a 2-D convolution. Combining Eq. (5) with the first quantity on the r.h.s. of Eq. (6a) results in

(a repeated application of a Fourier transformation returns the original function with the coordinate argument negated). If E,(x) = E,(-x), (i.e., E,(x) is a symmetric function), then Eq. (6a) becomes

Modeling the Magneto-Optical Recording Processes 619

DIFFRACTING OBJECT INPUT LENS

r (. x ).

/r ,

EXIT LENS

Figure 9. Unfolded schematic of readout system illustrates diffraction theory of readOUt.“41

This establishes the integrand of Eq. (3b). The integrand of Eq. (3a) is essentially the square of E(x).; from Eq. (4), the electromagnetic field propagating toward the light collection objective immediately after reflection diffraction from the disk. Squaring Eq. (7) to get point intensity and integrating over the stylus yields the output intensity. So Eq. (3a) is equivalent to Eq. (3b) in the limit of the conditions stated in the paragraph following Eq. (3b). It would be helpful to demonstrate what system variables govern the MO readback signal. We will give this derivation for the most popular read head configuration,the differential detection approach shown in Fig. 7. A phenomenologicalanalysis of the MO Kerr effect is gotten from considering the interaction of the incident with the disk. This has previously been described using a Jones matrix, as in Eq. (2) and Table 2. Figure 10 indicates that if linearly polarized light (along x) is incident on a magnetized disk, the Kerr effect results in the induction of a small y-component E,, upon “reflection”. A well-designed read head should exhibit optimal sensitivityto detection of this effect.

620 Magneto-Optical Data Recording

-\\ \

ANALYZER AXIS B

Figure 10. Ellipticallypolarized light in the Kerr effect with respect to the analyzer axes. E rotates at optical fiequency.

A differential scheme (Fig. 7) utilizes a polarizing beam splitter or a Wollaston prism that passes orthogonal components of linear polarization. One adjusts these “analyzer angles” to a and a -90 relative to x shown in Fig. 10. Calling the resulting analyzer axes A and B results in the following light amplitudes on detectors A and B:

Eq. ( 8 4

EA = E x cos a+E, sin a

Eq. (8b)

EB= Ex cos (90-a) - E, sin (90-a)

Using E, = kE,e” ,where k = kpsis the Kerr rotation coefficient from Table 2 and S is phase shift of the Kerr component E, relative to the Fresnel component Ex, the intensities on the detectors are:

Modeling the Magneto-Optical Recording Processes 621 Eq.(8c)

I , = E A E A * =E ~ ( ~ o s ~ a + k ~ s i n ~ a + k c o s S s i n 2 a )

Eq. ( 8 4

I , = EBEB* =

2

2

a + k cos

Q - kcosSsin2a

As shown in Fig. 7, the differential detection system forms the sum and difference signals

and Eq. (8f)

A =I,

cos2a + 2kcosssin2a

1

These results show why a i s nominally set to 45",in which case Eq. ( S f ) reduces to the simple result

Eq. (9)

SMo E A = 21,Rk cos 6 = IoR sin 20, cos 6

where IoR = E:. Using the relations for elliptically polarized one can show that the identity sin 2 0,cos 6 = sin 2 @K,,, cos 2&, holds, where OK,,,and &K are, respectively, the phase-corrected Kerr rotation and the Kerr ellipticity. Since 0,oc M ywe find that the MO readout signal is linear in the domain magnetization. The MO signal is proportional to a pseudo-reflectivity R@ (since 0,<< 1 radian typically). This explains why Eq. (3a) works when 72 = ROK, that is, 72 is varied between two levels for two magnetization states. The DC content of the signal, some of which can arise from the choice of a and imperfections in the balance of the differential detection, is irrelevant for the detection of modulation stored in the medium. Similarly i in Eq. (3b) can be interpreted as essentially the pseudo-reflectivity ROK for MO readout. Equation (9) is quite sensitive to the adjustment of the phase plate(s) in the head. These elements allow phase compensation for the media and optical elements of the head so that the cos S term in Eq. (9) can be adjusted

622 Magneto-Optical Data Recording to 1 to maximize signal. The phase shift S (and hence E ~ can ) arise from phase shifts in the disk thin films, the optical elements in the head, and from substrate birefringence. As a prelude to the next section where we discuss the connection between disk thin film design and the optimization of the readback MO signal, we should note that a wavelength dependence of Eq. (9) is anticipated. Each of the variables R, OK,and S will depend on A,through both multilayer thin film geometry considerations and intrinsic frequency dependence of material optical properties. We focus on the former in the next section, and simply note the latter here. With the growth of MO recording technology over the past ten years, the optical properties of most media Over the wavecandidate materials have been spectrally length range of most practical interest in MO recording (400-850 nm), one is interested in the spectral dependenceof the dielectric Ei/(A). The diagonal terms give the square of the refractive index, while the off-diagonal terms describe MO effects (OK,E,, 6).Candidate materials for MO media show significant spectral dependence over 400-850 nm. However, design optimization of the film stack can counteract some of the loss due to material spectral One other issue somewhat relevant to this section, particularly Eqs. (3a) and (3b), is the implication that we can neglect any dependence of R and 0, on angle of incidence of the readout rays. Data on the angular dependence of R and 0,has been Generally, one finds that nonpolar contributionsto the MOKE diminish the total,phasecompensatedrotation. Equations (3a) and (3b) assume that use of the value of the polar (4 = 0) Kerr effect can be used with little error, and Ref. 33 confirms that this is reasonable due to the weighting effect of the intensity profile across a focused beam. Naturally, there is an NA-dependence to the accuracy of this approximation, but for practical values of NA (< 0.70),the error in neglecting a sum over all angles of incidence is small.

3.5

Light Interaction with Multilayer Films

This section continues our consideration of modeling the readout light interaction with the storage medium. Here we concern ourselves with the physical issues associated with light reflection from a multilayer thin film. The analysis of plane waves interacting with multilayer film stacks is well This classical analysis is readily adapted to a structure establi~hed.[~~l-[~~] containing one or more MO layers as in Fig. 11. In this instance, it is

Modeling the Magneto-Optical Recording Processes 623 advantageous to treat the polar Kerr effect working with the polarization eigenstates. We decompose a linearly polarized incident beam into an equal admixture of right- and left-hand circularity polarized light.[211[381[391

Substrate Dielectric 1 Magneto-Optic

Alloy

Dielectric 2 Ref l e c t o r

Sealcoat

Figure 11. Generic quadrilayer structure with MO film. Light is incident through the substrate, and focused onto the thin films. Figure not to scale-the substrate is roughly B thousand times thicker than the whole thin film stack.

The MO physics is accounted for in the optical constants in several equivalent ways. Most fundamentally, it arises from a nonzero offdiagonal dielectric tensor element, such as E~~= ] e n 2 ,where Q is the Voigt parameter. We can write[*l]

624 Magneto-Optical Data Recording

n* = n ( l T jf) As Ref. 19 illustrates, all of the optical and MO responses can be calculated from the reflection coefficients for RCP and LCP light. Figure 12 illustrates the behavior of the optical and MO responses of a quadrilayer disk structure (Fig. 1 1) when modeled by the approach of Ref. 38. Although normally incident rays were considered, the difference from a summation This figure over rays in a focused, moderate NA beam is illustrates how the optical and MO response of an MO film stack can be tailored not only with the physical properties of the film materials, but with the geometry of the film stack. Numerous papers in the literature have treated the enhancementof MO disk readout through film stack design, and we treat this in Sec. 3.7. The goal is clear-find the geometrical structures that optimize the MO readout figure of merit from Eq. (9): R sin 2% cos 8. It is not only the value of Eq. (9) in absolute terms that is important, but also its tolerance to film thickness variations. Furthermore, there are usually constraints on the film thickness ranges within which the design must perform. All of these considerations can be handled with a thin film optical model. There may be other characteristics of importance beyond readout. For instance, in writing we are concerned with the absorption of light in the MO film since that governs the heat generation in the stack. There may be applications where other optical responses such as transmission are pertinent. A key ingredient of multilayer stack modeling is reliable knowledge of film optical constants. At least two approaches are evident to determine these optical constants. One is direct measurement of refractive indices and MO parameters, often through ellip~ometry.[~~1[~~1 A second approach, particularly useful for determining optical and MO properties of a magnetic layer, is to perform a nonlinear regression fit of a multilayer optical model to In the latter approach, a measured optical and/or MO response unique, determinant solution is available if the number of measured re, &,, etc.) equals or exceeds the number of sponse values (for example, R, ,@ opticaVMO property unknowns, counting real and imaginary parts of all refractive indices and Voigt parameters separately. To illustrate, this technique was applied to spectral to determine the wavelength dependence of complex refractive index n - jk and Voigt parameter Q = Q,+jQz of a TbFeCo MO alloy film. These constants were not provided in the reference, but were deduced by the model regression

Modeling the Magneto-Optical Recording Processes 625

REFLECTANCE

p

7 8

-

C

C

v

V

I

P n

c

c

n v

n V + X

.5+

I! n

4

X

E

i-

v

(4

V

s n

s

n c

c

n v

0 v

+a

4%

0

t(D2)lm(nrn) '%

w

o

0 t(D2)'m(nm)

'=

W

Figure 12. Optical and magnetooptical responses of a quadrilayer structure vs the variation of thickness of the encapsulating dielectrics, D1 and D2. The variation is over one optical period. 4pis the same as 6,and is the phase difference between the x- andycomponents of polarization. Read power was 2.0 mW, reflector layer thickness was 50 nm,refractive indices for substrate/Dl/MO/D2/refl/overcoatwere 1.58;2.0;3.38j3.86, 3.51-J3.97; 2.0;2.0-j7.1;1.58,respectively. (a) f,, = 10 nm;(b) f,, = 20 nm.

626 Magneto-Optical Data Recording technique from the sample geometry and MO response data disclosed. Figure 13 shows plots of n,k, and Q, , and Q, vs A. We wanted to determine the four optical and MO parameters at each of six measurement wavelengths shown in Eq. (8). Thus, a total of 24 data points were required. Using Figs. 2,3, and 4 from Ref. 44, we could determine eight independent response values for the three wavelengths 400, 600, and 830 nm. This enabled accurate determinations of n, k, Q,, and Q2at these three wavelengths. Seeing an apparent smooth and monotonic variation of these parameters, a cubic spline interpolation of n, k, Q,, and Q2at the “missing” wavelengths of 500, 700, and 800 nm could be done. A check on the accuracy of the interpolated values was done by “predicting” the given values of Ken rotation and ellipticity in Fig. 2 of Ref. 44. Iterative adjustment of the missing wavelength parameter could be done against the available data for a best overall fit. Recently, with considerable apparent impetus from MO recording technology, the analytical understanding of MO multilayer structures has progressed ra~idly.1~~1 Subtle computational implications of the usual firstorder expansion of MO effects in the Voigt parameter Q in the magnetic layer characteristic matrices were revealed and analyzed. The limitations of the usual approximations have been clarified.

Figure 13. Determination of magneto-optical and optical constants from the data of Fuji, et a1.[441 The lower right panel shows a final fit to the data for 0, and E, vs wavelength using the optical constants found in the other 3 panels.

Modeling the Magneto-Optical Recording Processes 62 7 3.6

Light Interaction with Features

We discussed the MO interaction between the beam and the disk in the last two sections. Now we turn to the important issue of the light interaction with topographical features on the disk-tracking pre-grooves and embossed pits. We note that these are the primary readout phenomena encountered with read-only (ROM) and some types of write-once (WORM) optical media. The simplest way to model topographical features beneath the reflecting thin films is to consider the non-smooth substrate as a phase grating of nearly uniform intensity reflectivity. The appropriate physics is expressed as the scalar diffraction integrals of Eq.(3b). Although scalar theory is understood to be an approximation, experience in modeling optical disks has shown that it can be quite accurate if care is Probably the most important procedures are sampling the topographical features with sufficient sampling frequency and accurately describing the true feature depth It is known that seemingly subtle inaccuracies in feature profile description can have significant impact on the TES and FES of Eq. (1). Even local reversals of slope in the TES can occur for this reason, and this can cause radically erroneous conclusions to be reached. It appears to be critical when applying scalar diffraction to optical disks to maintain close coupling with experimental analysis of both surface features and measured ROM and servo signals. Vector diffraction analysis has occasionally been employed for optical disk appli~ations.[~~1[~~1 Although extremely powerful and rich in its implications, there are two drawbacks to its wide application. First, it is considerably more complicated to implement computationally, usually reis quiring a pure numerical solution technique. Coupled wave well suited for this sort of electromagnetic wave scattering problem. The second difficulty revolves around enforcing correct boundary conditions on the field vectors in the solution of Maxwell’s equations. Little is known about the optical conductivity of the reflecting metallic thin films, particularly with the topographical complexity of embossed grooves and pits. Nevertheless, even in the context of idealized assumptions, vector diffraction theory has shown a richness of results that scalar theory is incapable of Diffraction modeling of the beam interacting with ever smaller disk topographical features is an area where advances in modeling technology would be welcome. Reliance on scalar theory, arguably a consequence of compu@tionalexpediency, causes a mild discomfort characteristic of hopeful application of a theory deficient in rigor to a problem near the presumed

628 Magneto-Optical Data Recording limit of its applicability. And yet scalar theory, applied with sufficient care, continues to show capability of delivering physical insight seemingly beyond the range of its assumed applicability. Clearly, simplicity and productivity combine to keep scalar diffraction theory in one of the well-used pockets of the modeler's kit bag.

3.7 Multilayer Design This section is closely allied with Sec. 3.5. Here we discuss how the approach of the previous section can be profitably applied to the design of the thin film stack for optimum readout performance. Stack design has been a popular topic over the years in the literature. However, we should emphasize that there are many important considerations besides simply maximizing the readout signal or signal-to-noise response of a film stack. At minimum, there are co-existing thermal and magnetic performance issues to accompany the optical and MO response. McDaniel and Seq~eda[~l] discussed strategies and methodology for joint optical, thermal, and magnetic design of MO disks. A fortuitous circumstance often prevails that decouples many aspects of the three performance attributes from one another. Thus, in a simple case, the magnetic design can be undertaken independently from the optical and thermal aspects. Further, there are means available for control of thermal behavior that, to first order, leave unaffected the parameters which determine optical performance. We discuss some specific examples later in this section. An additional design issue of importance is the choice of film thickness regimes consistent with Eabrication processing and environmental stability. This consideration effectively provides constraints on the design space, walling off some regimes of the multidimensional manifold from practical consideration. As a simple example, consider a case in reference to Fig. 11 where optical design might call for an extremely thin to,. Since this dielectric material serves a joint function as an optical phase adjustment layer and is an environmental barrier against the migration of water or it corrosive chemical species into the proximity of the reactive MO is clearly inadvisable to sacrifice either of the critical functions. What we have outlined is a fairly standard engineering problem in design optimization with constraints. There is an extensive literature on the formulation and treatment of such The standard elements of optimization modeling include:

Modeling the Magneto-Optical Recording Processes 629 Cataloging of the sets of input and output (response) parameters. Modeling software that describes system performance. This processes input into output accordingto the permitted physical (and mathematical) !aws gnvemhg the system. Definition of an objectfinction or penalfyfinction, which specifies an optimization condition. Description of the constraints on the allowed performance of the system; constraints are often merged into a penalty hction. Indeed, this prescription has been applied to the design of MO multilayers.[331[391[511 Most of the literature that discusses MO multilayer design has focused on specificmethodologieswhich handle elements of the second bullet above. Atkinson and coworkers[91[451[541 have described some general physical principles for realizing optimal MO response from multilayer film systems. This is the familiar topic of Kerr effect enhancement. One theme that is important is full utilization of the Kerr effect for readout signal optimization. Although a standard readout head (Figs. 7 and 10) is sensitive to Kerr rotation, with Kerr ellipticity a signal degrader (see Eq. 9), it is possible to However, in either approach, design a head with the opposite it is important to design the system for phase compensation to effectively deliver 100%ofthe available Kerr effect in the desired mode. Conversion of Kerr ellipticity gK to rotation 0,has been thoroughly treated.[431[561 Integration of the full set of stack design considerations into a prescription for global MO readout system optimization (that is, application of a system-wideobjective function) goes beyond the scope of most papers in the These authors discuss a broad modeling literature, but not a11.[331[391[511 approach that cames out a full exploration of the MO film stack geometrical design space to optimize a multiparameter objective function with embedded constraints. In the process, maps of the optical and MO performance over the film thickness space were generated, enabling easy identification of desirable design subspaces. The central physical model forming the engine of this direct search optimization scheme is a model of the kind presented in Sec. 3.5. Inputs include the film stack sequence, a set of thicknesses, complex refractive indices of all materials, and the light wavelength. Outputs include full optical and MO responses of the thin film stack (and the corresponding responses at the top of a thick cover ~heet[~Ol[~~] or

630 MagneteOptical Data Recording substrate), plus computed MO signal, system noise components, and system SNR. Thus a desired subset of responses can be assembled into a system object function such as

Each of the four terms in Eq. (1 1) has assigned weights, which determine their relative influence o n 5 Suppose the mathematical intent is to search for a maximum value off: The weights are expressed in units compatible with their cofactors to yield terms of reasonable relative magnitudes. The cofactors of the weights in terms 1, 3, and 4 are logical expressions equal to 0 or 1 if false or true, respectively. In words, when Eq. (11) is maximized, we might expect to simultaneously satisfy the inequalities of terms 1,3, and 4 (dealing with disk reflectivity, disk MO phase shift, and top dielectric thickness, respectively) and to maximize system SNR. A wide variety of optimum designs are accessible by manipulation of a relation like Eq. (1 1). Figure 1 4 illustrates optical and MO response surfaces as described in Refs. 32 and 5 1. These contour plots represent slices through a potentially multidimensional parameter space. These maps along with an objective function like Eq. (1 1) are indispensable when attempting design optimization. The contours show behavior in the first optical period of the dielectric film thicknesses, but solution for thicker dielectric films exist in thickness increments of At = W2n, which follows from equating the optical phase change upon double passage through a thin film (index n, thickness t )to 2x. Exact periodicity holds in the limit of zero absorbance (k= 0), while a small, nonzero k gives an expected slow degradation of optical performance with increasing t into higher order periods. Multilayer design for MO recording media that exploit optical enhancement has probably reached the stage of becoming routine engineering procedure. Fundamental new insights into the optical aspects of MO readout are becoming rare. Recently, a different approach for optimizingreadout performance has increased in popularity. A thin film readout layer of optimum composition for high Kerr response at the system wavelength is deposited adjacent to a storage layer, with exchange coupling between the films transferring the stored magnetic information to the readout layer.[57]The magnetic or MO properties of each layer can be separately

Modeling the Magneto-Optical Recording Processes 631 tailored to its primary function, reading or writing. Differentiating optimization by function is a logical development from the original approach of combining storage and readout in a single magnetic layer. From a modeling viewpoint, the readout problem takes on a greatly increased magnetic character. Now the double magnetic-MO layer is probably too thick to allow much light transmission, so the associated optical enhancement strategy of Kerr and Faraday effect utilization in an optical resonant cavity is less beneficial. We will consider models of exchange coupling layers (ECL) in a later section of this chapter.

SNR

(dB)

Figure 14. Contour maps of the MO signal and S N R responses of the structures presented in Figs. 12(a) and (b). S N R was derived from a noise model determined semi-empirically using experimental noise measurementsfrom a test stand. (a) and (b) are for t,, = 10 nm, while (c) and (d) are for t,,,, = 20 nm.

632 Magneto-Optical Data Recording 3.8 Birefringence Effects

In this final section on optical modeling topics, we return to a disk substrate issue. Optical birefringence in disk substrates, which form the fiLm3, un h3ve sipificant deleterinus effects on cover plate over the readout perfonnan~e.[~~1[~~1 Birefringence has the largest impact in MO storage (relative to other forms of optical recording) because the readout detection utilizes light polarization phenomena, and birefringence is essentially anisotropy in light ray propagation velocity with respect to wave vector direction and the state of linear polarization. Among the best known phenomena arising from anisotropy in the substrate refractive indices are: Imbalance in the differential MO signal Reduction of MO signal amplitude due to phase retardation (see Eq. 9) Introductionof (variable) wavefront aberration in the focused beam, especially Modeling the effects of birefringence in MO recording systems has been approached essentially in two ways. One is via a ray tracing approach, usually combined with a Jones matrix analysis ofthe entire optical head.[171[181 Rays comprising a moderate NA converging beam are traced individually to determine the phase retardation between s- and p-polarization components, and the cumulative effects for the full beam are determined by summing. A second approach uses the scalar diffraction program such as and estimates substrate retardation effects by replacing the thick substrate with an optically anisotropic thin film. A characteristic matrix approach treats the “thin film” substrate. In either method, one can represent either homogeneous substrate optical properties, or inhomogeneity along the substrate thickness direction (at least). Inhomogeneity along the film normal direction can be readily implemented by subdividing the substrate into homogeneous sublayers. The composite retardation is then accounted for with matrix products. Inhomogeneity in the disk plane over the dimension of the light spot could also be modeled, particularly in the ray tracing approach. A way of doing the same for the diffraction model is less obvious. We will present examples of each modeling approach applied to MO recording. Birefringence arises from anisotropy of optical properties. In a homogeneous material, this anisotropy is conveniently represented with the refractive index ellipsoid, or indicutrzx.[601 A ray trace computer model based on Refs. 17 and 18 was generalized to include an index ellipsoid with

Modeling the Magneto-Optical Recording Processes 633 arbitrary orientation using Euler angles. With such a model, one can study the contribution of individual rays to the MO signal, or alternatively one can sum over all rays in a converging beam to simulate the impact of varying retardation on MO signal quality. Figures 15(a)-(d) show single-ray dependence of the MO and unbalance signals for an untipped and a tipped index ellipsoid. For differential detection (Fig. 7,detectors 1 and 2) and two possible medium magnetization states, + and -,the MO signal is

while the unbalance signal is

The symmetry and periodicity of the signals in Fig. 15 exposesthe effect of indicatrix tipping. Figures 16(a) and 16(b) show the effect of indicatrix tipping on S, and U for the full-focused light beam on the disk. The ray trace model appears capable of describing the effect of phase retardation on signals derived from integrated intensity at the detectors. Additional subtleties arising from induced aberration and diffraction phenomena are not incorporated. The scalar diffraction model DIFFRACT[41[591[601 is capable of including substrate birefringence in an approximate manner. In this model, the birefringent properties of an optically thick substrate are concentrated into an optically thin (t = 25 A) layer. The variation in refractive indices is scaled upward in inverse proportion to the thickness change to give the same optical thickness as the physical substrate. In this approach, spot aberration imparted by birefringent phase retardation is a primary result, thus enabling accounting of geometrical effects of spot-mark overlap upon readout, something the ray trace model ignores. By defining varying properties through several sublayers, composite retardation effects can be estimated using products of characteristic matrices. Birefringence effects are difficult to isolate because they are often masked or modified by attendant readout complexities. The tool DIFFRACT is powerful for dealing with this reality because it treats concomitantly substrate topography, beam aberration, and spot-mark correlation to

634 Magneto-Optical Data Recording compute the readout signal. By varying the amount of birefringence, one can separate out its contributionto readout and servo effects. Reference 59 gives illustrations of how astigmatic aberration induced by substrate birefringence gives rise to offsets in focus servo systems. Figure 8 was ,producedwith the model DIFFRACT while including substrate birefnngence. (a) UO Siinol and Unbalonce

(b)

UD Siqnd

ond Unbalonc.

,'

AZIMUTH ANDLE 1d.p)

-3

ANGLE of INCiUENCE (dq)

Figure 15. Behavior of S,, and U (Eqs. 12 and 13) for single rays propagating through a birefringent optical disk substrate. For (a) and (c), the angle of incidence is that of a peripheral ray for anNA = 0.55 system, about 20.4" within the substrate. For (b) and (4, the azimuthal angle is 0" (the plane of incidence tangent to the disk tracks). The assumed level ofbirefiingence is An,, = 20 x 10"and An,,, = 500 x 10". For (a) and (b), the index ellipsoid principal axes are aligned with the disk polar axes. For (c) and (4, the index is tipped using Euler angles (a,ply) = (20", 2", 0'). (We assume Euler angle p is a rotation about the already rotated y-axis, and that the Cartesian x-axis is initially parallel to the disk circumferential direction, with the y-axis parallel to the disk radial direction.)

Modeling the Magneto-Optical Recording Processes 635

(a) YO Signal and Unbalance

0

100

2m E v l r angle II ( d q )

(b) MO Signal and Unbalance

0

U N W C E SIGNAL

100

200

xa

Eulw angle a (dog) (d) LID Signal and Unbalance

Figure 16. Behavior of S , and U(Eqs. 14 and 15) for a full cone of rays propagating through a birefringent optical disk substrate. (a) (a,P,y) = (a,0", 0'). (b) (a,P,y) = (a, 2", 0"). (4 (a,P,r)= (45", P, 0'). (4 (a,P,v) = (O",P. 0").

Most modeling of optical disk birefringence to date has assumed homogeneity of substrate optical properties. There is considerable evidence[611[621 that spatially varying birefringence exists in injection molded plastic disks. This raises concerns that the molding of surface topographical features (grooves, pits) on the disk surface may, through inhomogeneous local stress-strain fields, induce "local" variations in birefringence over a length scale of a few microns. Experimental data on the nature of such local birefringence has not been published. This is a subject where fluid modeling of molded polymers could make a useful contribution to our understanding of optical property inhomogeneity. An accurate simulation of the complete disk injection molding process would appear to be quite challenging. has applied the model DIFFRACT to 1-10 modeled Recent sublayers to fit extensive birefringence data for varying angles of incidence

636 Magneto-Optical Data Recording and initial polarization states. The model could achieve excellent fits to the data, usually with five or less sublayers, each having determined values of Anl2 and An12-3and index ellipsoid tilt angle. While the modeling approach could not establish uniqueness of the best fit structure, it did confirm that inhomogeneous birefringence through the substrate thickness is probably necessary to account for the optical properties of injection molded substrates.

4.0

THERMAL MODELING

4.1

Overview

In optical data storage the recording and retrieval of information is accomplished by the action of a scanning, focused laser spot and detailed modeling of the laser-storage-medium interaction has understandably received a lot of attention. Apart fiom providing insights into the fbndamental physics associated with the processes of data recording and retrieval, modeling has proven invaluable in establishing the linkages between material properties and performance in various operational scenarios. (i) Most experimental methods used to characterize the bdamental storage capability and recording performance of various storage media are undertaken using a probe laser beam. Modeling provides the framework for interpreting and evaluating the results of these experiments and for devising additional test procedures if necessary. In short, it is a valuable media characterization (iz) In almost all conventional optical storage media, the mark formation process is thermally induced. Manifestations of thermal effects abound and range fiom mark shape nonuniformities to the interaction of adjacent marks. Modeling has proved invaluable in the pursuit of higher storage densities and provides a convenient, rapid, cost-effective vehicle for testing the usefblness and feasibility of various novel writing schemes and strategies and of various media configurations to this end.[511[721-[791

Modeling the Magneto-Optical Recording Processes 63 7 (iii)Modeling also provides a convenient vehicle to evaluate media performance in conditions far from ideal. For example, it can be used to study the impact of ambient temperature excursions on media performance. It can also readily estimatethe effect of changes in parameters such as laser wavelength, laser mode-hopping, media nonuniformity, and in general serves as a valuable tolerancing tool for evaluating and characterizing the system-media interface.[801-[821 4.2

Background

The heating of homogeneous solids by static and scanning focused-laser (and electron) beams has received a great deal of attention due to its wide ranging technological implication~.[~~1-[~~1 We will, in the following sections, sample liberally from this literature due to the basic similarities of the problems under consideration and the broad applicability of the results that have been generated in disparate fields and applications. The heat flow problem is customarily posed in the framework of the Fourier theory of heat conduction. One of the cornerstones of this theory, the “fundamental hypothesis for the Mathematical Theory for the Conduction of Heat,” is the assumption that the heat flux at a point is directly proportional to the local temperature gradient, i.e.,

where K is the thermal conductivity. However, the very steep local temperature gradients that can develop at, or near, the surface of a laser-irradiated, strongly-absorbing material may necessitate the incorporation of higher (second) order terms that are routinely neglected. Harrington[881used a kinetic model to show, for strongly absorbing metals, that the first-order expression held to within 2% if the local temperature gradient was more or less constant over a minimum range of *5A, where A represented the electron mean free path in the metal. Harrington derived the condition of applicability of the conventional heat diffusion equation as:

638 Magneto-Optical Data Recording He showed that for cases where this condition was violated, the use of the usual equation of heat conduction overestimated the effect of thermal conduction and gave incorrect temperature distributions within the first few mean-fiee paths of the surface. For very short-time laser irradiation of strongly ahsorhhg m d a , the temperame distribution closely follows the Beer's law absorption and the above condition can be explicitly framed as:

where a is the absorption coefficient. For metals, the electron mean-free path (A) and absorption depth (l/u) are often very similar; typically a few hundred angstroms[861[881 and we could very well be in violation of the above condition. Although this could lead to underestimates of peak surface temperatures for marking events of very short duration, our use, in subsequent sections, of thickness averaged temperatures and temperature dependent material properties serves to de-emphasize such concerns. For dielectric materials where heat transport is primarily phonon based, these issues are of greater significance. The limiting parameter in this instance is the phonon scattering length, which can be on the order of microns. In extreme cases, this can lead to the invalidation of the heat diffusion equation and require an alternative, more appropriate formulation."] Another issue to consider is the rate of energy conversion during the irradiation process. For the case of metals, quanta of light are absorbed by the internal photoeffect that promotes electrons to higher energy states in the conduction band. The electrons very rapidly give up this energy through collisions with themselves and with lattice phonons, thus transforming the absorbed optical energy into lattice vibrations, i.e., heat. The process of heat generation is thought to occur on time scales of 1O-'* s or less for pure metal~.[~~1[*~1 Therefore, for most of the relatively long laser irradiation events characteristic of optical data storage, heat generation has been treated as instantaneous. We should point out that in the case of transition metals, if scattering occurs into a d-band with a high density of states and from which phonon de-excitation is slow, the rate of transfer of optical energy to heat could be slower than previously stated. Accounting for the finite thermal relaxation, Eq. (14) can be rewritten as:

Eq.(17)

Modeling the Magneto-Optical Recording Processes 639 where q.is the relaxation time, which is typically on the order of 10-l'~for metals. The governing equation now becomes:

where K = K/pc is the thermal diffisivity. Solving this hyperbolic equation is somewhat more involved than the conventional parabolic one, but numerous successfid methods and strategies for doing so in particular scenarios have been pre~ented.[~1[~~1 The salient feature of the solutions are marked discontinuities in the transient temperature profiles. Most analyses of the laser heating problem ignore the temperature dependence of thermophysical properties. Accounting for them, the governing equation for (say) a laserirradiated half-space can be written:

dT

Eq. (19)

V[K(T)VT]- p ( T ) c ( T ) X =

Ae-pZrZ

e

-a(+

The quantity /3 = 1/ (fioef) where oefis the irradiance radius of the focused Gaussian spot. The variables K ( 0 , a(T), p(t), c(T) are, respectively, the temperature dependent thermal conductivity, optical absorption coefficient, density, and specific heat of the irradiated solid. Methodologies for rigorously solving the nonlinear problem represented by Eq. (19) have been outlined previously and, in specific instances, include the use of linearizingtechniques such as the Kirchhofftransf0rm.[~~I-[~~1 For example, let us consider the nonlinear governing equation for a system with heat generation at rate, i.e.,

Assuming that pc is independent of temperature, we define a transformed temperature 8 by:

640 Magneto-Optical Data Recording

where KO = K(TO). The goveming Eq. (20) can then be reduced to the standard, linear form:

Various other techniques can be used to simplifl the problem. For example, as a first approximation, one could define a mean value of a given material property (P,)for the temperature range Tl to T,

The next level of sophisticationwould be to assume a linear temperature dependence of the thermophysical properties, which is in reasonable agreement with experimental observations over limited temperature range~.[*~1[~~1[~~] Methods for solving Eq. (19) with this added refinement have been outlined by various investigators and rely on judiciously chosen variable transformations. For the case of a metal thin film whose absorption coefficient displayed a marked temperature sensitivity, C h e ~ n g [ pre~~] sented an approximate, iterative method that gave results in agreement with those obtained from a finite element computation (within 5%). On the other hand, the iterative, approximate model provided a stable solution in a few orders of magnitude less time than the finite element calculation, highlighting one of the attractive features of the semi-analytical approach. These concerns notwithstanding, all the thermal modeling described in subsequent sections are based on the first-order formulation of the heat diffusionequation and as we shall see, their predictions, for the most part, are in reasonable agreement with experimental observations. We should note that another commonly made assumption is the neglect of radiative heat losses. This is deemed permissible since the heat losses associated with radiation have been estimated to be many orders of magnitude (3 to 6) less than the incident For the case of semi-infinite solids or freestanding thin films, the solutionsto the thermal problem can be readily obtained in fairly convenient formats, sometimes even in c l o s e d - f ~ r m . ~ ~For ~ I -example, [ ~ ~ ~ ~ for a thick film irradiated by a scanning, focused, circular, laser-beam, the surface

Modeling the Magneto-Optical Recording Processes 641 temperature rise at location (x,y) at time t after the laser is switched-on can be derived as:[lo61

where

The upper limit of integration t* = t for t I z, the duration of the write pulse. If t > zthen t* = z. The position where the laser is switched on is defined to be the spatial and temporal origin. The value of x is measured along the scan direction andy perpendicular to it. The variable P is the laser power and v the scanning velocity; R is the reflectance of the irradiated solid. The above expressions, with slight modifications can be used to treat irradiation by elliptical beams. For example, if the spot radii are 08;r and D$- in the x andy directions, respectively, Eq. (25) can be rewritten in the form:

where

Using the same notation as before, P, = 1/(&0>) and Pr = l/\fi0&-). For the simple case of static irradiation of (say) an amorp ous rare material like TbFeCo with a circular laser earth-transition metal (RE-TM) beam, the peak temperature rise attained at the surface is given, to a very good approximation,

642 Magneto-Optical Data Recording

T""(O,O,t)

c

24 a

J X- Jat - J Z + m-'(Jat)

where a = 4p2K, b = 2 / a 2 K , and all other quantities have the same significance as before. Note that the precise form of this expression is dependent on whether the product ab is less than or greater than unity. In this particular instance, ab < 1.0. For extended irradiation (i.e., t + m) the peak (steady state) temperature rise is given by:

Eq. (29)

-F[

T(O,O,oo)= 2aA K n - J=tan-'[ a an 2

Job) 1-ab

- ab

In the preceding expressions, apart from the laser and thermophysical properties, notice that the major influence on the surface temperature rise is the ratio of the absorption depth to the laser-spot radius. We can utilize Eq. (28) to gauge the impact of the commonly made simplifying assumption of surface absorption. For example, the ratio of the steady-state temperatures rise achieved at the surface of a continuously irradiated solid under the assumptions of distributed [T(O,O,m)]and surface absorption [TS(O,O,m)], respectively, is given by:[lo71

where, as before, ab = 8 P 2 / a 2 . Figure 17 clearly illustrates the weakened validity of the surface absorption assumption as the ratio of the absorption depth to the spot-radius increases. For example, when ab = 0.4the surface absorption assumption leads to a 20% overestimate of the peak temperature rise. For depths in excess of the absorption length, the temperature distribution is understandably insensitive to the precise mechanism of energy

Modeling the Magneto-Optical Recording Processes 643

I .L

1.o

0.8 0.6 0.4

0.2 0.0

-5

-4

-3

-2

-1

0

1

2

log,o[abl Figure 17. The ratio of the steady state temperature rise at the surface of a continuously laser-irradiated solid with a Beer's Law absorption profile T(O,O,m) to that derived assuming purely surface absorption Ts(O,O,m) as a function of the factor ub.[Io71

Current MO data storage structures typically are comprised of a stack of dissimilar thin films on a thick glass or plastic substrate. The inhomogeneity that this introduces increases the complexity of the thermal problem to the point where its rigorous solution requires numerical or semi-analytical methods at best. In the following sections we explore the various techniques that have been utilized to model the thermal response of multilayered MO media packages and compare their predictive capabilities against experimental observations wherever possible.

4.3 Multilayered MO Media In conventional RE-TM media, the active layer is often sandwiched between dielectric films (e.g.,. AlN, SiN, SiO) for both corrosion protection and optical enhancement. This active optical stack is deposited on a relatively thick glass or polycarbonate substrate (typically 1.2 mm thick) and capped with a polymeric protective coating (tens of microns thick) both

644 Magneto-Optical Data Recording of which can be treated as semi-infinite from a thermal standpoint. Some representativemedia configurationsare depicted in Fig. 18. In general, we would expect the thermophysical properties of the various layers to differ appreciably. Table 3 is a compilationof thermophysicalpropertiesfor some materials commonly encountered in MO media configurations.

I

I

Protective Overcoat AIN, SiN, SiO MO AIN, SiN, SiO

1 (a)

Substrate

Q MO AIN, SiN, SiO

(b)

Substrate

Q Protective Overcoat MO (superlattice) Seed Layer Substrate

I (c)

Figure 18. Schematic depiction of (a) conventional RE-TM media package. (b) Quadrilayer package, and (c) possible media package for superlattice MO media.

Modeling the Magneto-Optical Recording Processes 645 Table 3. Thermophysical Properties Material

GdzCO, Tb24c08Fe68 c018e69

Gd,S T b , OCO!@66 GdTbFe GdTbFe GdTbFe GdFeCo TbFeCo TbFeCo TbFeCo TbFeCoZr C04& co4pt,8

Al Ti AlN AlN AlN SiN, SiN, SiO, TiO, glass p l ycarbonate lacquer

Thermal Conductivity (W/m-K)

Specific Heat (J/m3-K)

3.6" 4.9" 6.48 4.3" 4.0b 17.0b 9.0-17.0b 5.9"

8. 1" 7.0b 14.0b 4.3-7.3" 22Sb 26Sb 190.0b 8.0b

15.0b 20.0b 0.5-16.0' 3.9 lSb 0.41-1.05" 0.48' 0.72' 1.05 0.15 0.56

a Calculated from measured electrical resistivity using

Reference

108 108 108 108 109 110 70, 111 112 112 113 114 112 115 115 110 110 110,111 17 116 113 117 117 117 118 82 111

Wiedemann-Franz law. Derived from measurements of domain size and thermomagnetic marking model. Derived from thermal comparator measurements.

646 Magneto-Optical Data Recording There has been a great deal of uncertainty associated with the assignment of thermal properties, particularly thermal conductivity, to vacuumdeposited thin films. what is universally accepted is that their thermal conductivities differ from those of the corresponding bulk materials. To a ereat extent, this stems from the recowenitionthat thermal conductivity depends on details of long-range atomic ordering which can be a very strong function of both physical dimensions and fabrication conditions. Properties such as specific heat, on the other hand, are influenced primarily by shortrange ordering (packing density), which is not very different from thinfilm to bulk. The impact of fabrication conditions on the microstructureand properties of MO materials has been extensively documented.[1201-[121Recently, it has been s h 0 ~ n [ ~ ~ ~ that 1 [ ~not ~ 1only [ ~ can ~ ~t lhin films possess anisotropic conductivities as a result of microstructural anisotropy but that their measured effective, through-thickness conductivites possess an appreciable interfacial resistance (heat transfer) component. Measurement of thermal properties of thin films, particularly those that are encapsulated for one reason or the other, is nontrivial and easily confounded by such effects. The thermal conductivities of some of the thin films used in optical data storage have been estimated from experimental data using two pathways. The more direct methods rely on experimental conditions that replicate a standard heat flow problem.[1161[1171[1241[1251 Figure 19 provides a schematic depiction of the thermal comparator used in the direct measurements of Lambropoulos et al.[ll7] Schultz et al.[lo9]utilized a variation of this technique in that they monitored, with very precise temporal resolution, the temperature-induced change of an observablematerial property namely, the Kerr rotation. Figure 20 depicts the high speed MO imaging apparatus used by Schultz et al.1' l91 to directly measure temperature distributions during laser irradiation. For MO data storage in particular, the more indirect methods are based on fitting conventional data derived from laser marking scenarios with a thermomagnetic marking model. The latter techniques are obviously sensitively dependent on the values assigned to various material and laser parameters. Shieh[ll4Ireported a remarkable sensitivity of the results to parameters such as the thickness of the MO layer itself. In the interest of thermal efficiency and noise suppression, media design is undertaken to concentrate absorption of optical energy in the MO layer. For very thick metallic MO layers the energy absorption profile or energy input distribution (EID) is exponentially attenuated and given by Beer's law. For thinner films, as would be encountered in (say) a quadrilayer structure,the EID is more complex due to the increasingly significant return

Y

Modeling the Magneto-Optical Recording Processes 647 beam. In such instances, the EID is rigorously derived as the gradient of the Poynting vector (the energy flow) through the optical Even this socalled rigorous derivation of the EID (as applied to conventional MO disk structures) is approximate in that it is derived on the basis of the plane-wave solution to the thin film stack uroblem. which is then modulated by the input beam intensity profile. The extension of these results to the case of focused laser beams is commonly justified by the relatively large depth of focus associated with the Gaussian spot. In grooved structures, on the other hand, complex departures from the thin film solution are revealed by more rigorous formulations including vector diffraction methods,I1261-[1 281 Moderate to long irradiation events in strongly absorbing, conductive films would render many of these concerns of more aesthetic than practical significance.

Probe

4--5

Substrate

Sample

Film Layer Sensing Tip (0.25 mm Dia.)

Copper Heating Block Heater Control Thermocouple

Figure 19. Schematic of the thermal comparator used by Lambropoulos et al."I7] to measure thin film thermal conductivity.

648 Magneto-Optical Data Recording

Silicon Intensified

I

Image

I

Half-Silvered Mirror

Dielectric Mirror Collimating Optics

Figure 20. Schematic depiction of the high speed MO imaging apparatus employed by Schultz et a1.f1l91for real-time monitoring of laser heating.

The effects of heat transport would effectively serve to blur details of the EID and reasonably good results can and have been obtained by using accurate estimates of the total energy absorbed. Simple Models. Much effort has been expended in the detailed thermal modeling of laser irradiated solids and thin films. However, some simple models have been developed which, in a limited number of cases, have proven to be extremely accurate and useful. Incorporatingthe basic physics of the laser marking process, as they do, these models have provided valuable insight into the thermal response of laser-irradiated solids and have greatly aided in the interpretation of experimental observations. Adiabatic Model. The simplest marking models are the so-called need rely on the adiabatic ones that ignore heat flow all principle of linear superposition. Their success can be attributed directly to the inherent linearity of many operational phenomena. Let us examine the

Modeling the Magneto-Optical Recording Processes 649 adiabatic temperature profile that results in a moving disk due to the write power train. We assume, as in an IS0 standard drive (say), that the writeto enable laser drops to a bias power level (Pb) between write pulses (P,,,) servo functions and also stabilize laser operation. We distinguish this from read power (P,)even though they are identical in most practical situations. Integrating the energy deposited by the translated Gaussian spot, we obtain the resulting temperature

where A is a constant, L the write pulse length, and v the scanning velocity. This resultant temperature distribution together with the separate contributions due to the bias and write powers are depicted in Fig. 21.

-

1.0

1.5

2.0

2.5

30

Position (microns)

Figure 21. Temperature distributionsassociated with bias and write power components of the Write stylus. Adiabatic, loss-free medium, 2.0 pn write pulse length andP, = 0.2Pw.[i29]

650 Magneto-Optical Data Recording

Approximate Models. The use of physically realistic, approximate models have proven extremely successful in describing the laser marking process. What is often gained at the expense of mathematical rigor is a tremendous decrease in computational complexity. These models, which allow for heat diffusion in the film and substrate, incorporate the dominant, first-order effects and often result in a degree of accuracy no worse than the limits of confidence of many of the thermophysicalproperties of the systems under consideration. The concept of diffirsion length permits very crude estimates to be made of the extent of heat flow in various time frames. For example, Corcoran and Ferrier[l3l1postulated that three time constants essentially delineated the marking response regimes of a laser-irradiated thin film on a semiinfinite substrate.

They interpreted and modeled the static heating and melting of thin metallic films on thick, poorly conducting substrates with some success using the following guidelines. (i) When t > zg,the through-thickness temperature gradient in the thin film is negligible. Using values from Table 2 for a 250 nm thick AlN encapsulated RE-TMfilm (K z 4 x 10" m2/s) we obtained zg on the order of 10-20 ns. (ii) When t > radial heat diffusion from the focused spot dominates optical diffraction in determining written mark size. For irradiation by a 1.Opm FWHM laser spot zf is on the order of 20-30 ns. (iii)When t > z,, heat losses to the substrate account for a substantial portion of the absorbed energy. For the above composite film on polycarbonate and glass substrates, zsis on the order of 2 ps and 150 ns, respectively. In fact, this result is compatible with the finding by Abraham and H a l l e ~ [ ' ~that ~ ] as Kf or sf increase the substrate plays a lesser role in controlling and influencing the diffusion of heat.

v,

Modeling the Magneto-Optical Recording Processes 651 For times in excess of z, one could justifiably use the relatively simple expressions derived for the laser irradiation of a semi-infinite solid. In fact, many investigators have, with varying degrees of success, used the following simple representation for the peak temperature rise under static irradiation conditions.[651[%1[1331-[1351

The variable Pa represents the fraction of the incident laser-power absorbed in the thin film. The short and long time asymptotic solutions 4p2 K,t <<1 and >> 1 can be readily derived as:

using the relation tan-l(X) = 7d2 - tan-l(l/x). The time-dependence of temperature predicted by the Eq. (33) can exhibit a distinct slope-change at a well-defined time which Aamodt et al.[1361related to the thermal mismatch between film and substrate. They found that the slope increased or decreased for relatively insulating or conductive substrates, respectively. For various special cases, relatively compact and convenient expressions have been derived for the temperature distributions in laser irradiated thin film structures. For example, Abraham and H a l l e ~ [ ’ ~obtained ~] a solution for the temperature distribution in a thin absorbing film on a semiinfinite, non-absorbing substrate. They used a thickness-averaged energy input and assumed that the thermal difisivities of film and substrate were identical whereas their thermal conductivities were not. Doing so they obtained:

4{--

z,(l -t’/.,)

I’ 1 +

652 Magneto-Optical Data Recording and

C" \J"I /ZC\ Mi-

B ( k / 4 ) ; A + , B(t - t')e

'-

T,(R,z,.t' -

l + k !u'

'+*''To

z,(l+t'/ro)

where k = K/K,, B(t') is the time-dependent heat source term, Kf = K, = K, R = p,.,Z = p,, L is the film thickness, and z, = 1/4p2~. Ghez and L d w 1 solved for the peak temperature rise during the static laser-irradiation of a thin film on a semi-infinite substrate. They assumed a uniform incident flux locally, which would be only strictly applicableto relatively long irradiation events or for regions along the axis of the beam. Theyjustified the use of the following boundary condition that facilitated the derivation of a convenient analytical solution to the heat flow problem.

Using this relation, they derived the peak temperature rise in the thin film (in our notation) as:

where z= (Pscs/p~f8f)2icst and F, is the incident power density (flux). The next step is to treat the MO and encapsulating layers (for the trilayer configuration) as a single, composite layer from a thermal standpoint. This would be permissible if the thermophysical properties of constituent layers are similar as would be the case for (say) amorphous TbFeCo and AN.The absorption of energy is confined to the MO layer and we allow for heat transport in the composite thin film and the substrate. A similar analysis would hold true for a single absorbing thin film on a semi-infinite The model neglects thermal gradients through the relatively

Modeling the Magneto- Optical Recording Processes 653 conductive composite layers and, from a heat balance of the shaded element of film in Fig. 22, we obtain the following goveming equations for static laser-irradiation:

subject to the boundary conditions,

where Tr, T, are the temperatures in the film and substrate, respectively. Using the above results we obtain the following estimate for the net heat flux resident in the film.

Applying Duhamel’s theorem to the solution of the static marking problem, we can derive the temperature distribution in the film caused by a scanning laser beam

654 Magneto-Optical Data Recording where

The subscripts s, f refer to the substrate and film, respectively. The variable Sfis the MO film (absorbing layer) thickness whereas 6; is the composite film thickness and all other terms have the same significance as before. The assumption of no through-thickness thermal gradients obviously weakens if the film conductivity is not greatly in excess of that of the substrate, or if we have marlang events of very short duration.[83*[1391[1401 For example, Fig. 23 depicts the simulated through-thicknessmark profiles for a low conductivity thin film on an thennophysically similar ~ubstrate.[~~~1 Irrespective of the laser incidence direction, it was found that the mark appeared longer at the surface than at the film-substrate interface. These results were consistent with experimental measurements.

Substrate

I I

to Substrate

Figure 22. Schematic description of the thin film-substrate configuration irradiated by a static laser beam.

Modeling the Magneto-Optical Recording Processes 655

n

€ Y

I

I I

0

4

I

I

I

I

iaser incidence Direction

I

I

AiR A id

I

-(4 3 0.04tn E 0.06E! 0.08 -

a,

0 0.02 a

y.

L.

Y-

a, 0 C

0.I

a 0.12 tn

c,

.-

n

~

-0.25

SUBSTRATE I

0

I

1

I

I

0.25

0.5

0.75

I

1.25

1.5

1.75

n

*

E

W

a 0 a

w-L

3 v)

E

2

Y-

a, 0

c

a

c,

.-v)

n

In-track distance ( p m )

Figure 23. Calculated cross-sections of marked regions for front surface and throughsubstrate laser incidence. Laser power = 3 mW, scanning velocity = 13 d s , and pulse duration = 100 ns.[1391

656 Magneto-Optical Data Recording Using the values pfcf = 3.0 x lo6 J/m3-K and Kf = 7 W/m-K, we compared the predictions of Eq.(42) with those of the simple Eq. (3 1) for the static marking of an AlN encapsulated RE-TMfilm on a polycarbonate substrate. Figure 24 depicts the normalized peak temperature rise as a function of irradiation time. As seen from the figure (even though the absolute values of predicted temperature differed appreciably), the functional timedependence is quite similar. In fact, given our neglect of the temperature dependence of thermophysical properties and the uncertainties associated with the specific values themselves, this discrepancy is fbrther reduced in significance. This explains why extremely simple expressions such as Eq. (31) can possess predictive capabilities of more complex formulations provided they are normalized, in some appropriate fashion, to the experimental data.

-eq. 42 - - eq.31

200

400

600

800

1( 00

Time (ns) Figure 24. Normalized peak temperature due to static laser irradiation predicted by Eqs. (31) and (42). Kf= 7 W/m-K, K, = 0.15 W/m-K, Sf= 100 nm, Sf*= 250 nm, and spot FwHM= 1 pm.

Increasing in sophistication, the Effective Layer Model developed by H~ltslag['~~] explicitly accounted for the dissimilar thermophysical properties of the MO layer and its surroundingsthrough a time-dependent thermal conductivity term. The model closely resembled the composite layer model in its physically justified neglect of through-thickness thermal gradients in

Modeling the Magneto-Optical Recording Processes 65 7 the thin films comprising the “active layer,” i.e. the MO layer and the adjacent thin film encapsulants. The temperature distribution in the “active layer” due to a circular, scanning laser-beam was derived by Holtslag (in our notation) as:

The variable PA represents the timedependent incident laser power in (say) pulsed-marking scenarios. The suffixes 1,2 refer to the substrate and thick protective overcoat, respectively. The other parameters in the above expression are given by:

I

Z=

jDpyt‘ = owl+ 2QJ -Dm [fi- ln(1 + fi)] 0

Y

Note that in the above expressions c represents the heat capacity, not simply specific heat. The suffix oi refers to the thin film constituents of the “active layer.” Very good agreement has been obtained between the predictions of this expression and those of more rigorous and complex numerical simulations. Holtslag reported a very interesting result from the rigorous numerical simulations he undertook to qualify the effective layer model. For the case of a highly conductive active layer sandwiched between

658 Magneto-Optical Data Recording lower conductivity films, heat initially flowed from the active layer to the surroundings. However, at some later time, due to the enhanced conductivity of the active layer, the direction of heat flow reversed and heat flowed from the surroundings back into the active layer. Despite the fact that the effective layer model does not replicate this behavior, he pointed out that the self-consistent energy balance that it incorporated would effectively reduce the cooling rate of the active layer to partially replicate this phenomenon. Rigorous Models. These models explicitly allow for the existence of individual, dissimilar thin films on a semi-infinite substrate. The most obvious method of solution is purely numerical utilizing finite element or difference schemes. However, techniques based on Green’s functions and the method of images can be employed with a significant reduction in computational requirements. Green’s FunctionApproach. Using the technique advanced by Bellman et al.[1421to deal with an analogous diffusion problem in two dissimilar halfspaces, Burgener and outlined a method to derive the Green’s functions associated with a thin absorbing film on a thick substrate. For the half-space geometry under consideration, they assumed the source Green’s function (Gi) to be the sum of the Green’s function solution for a point source in a semi-infinite medium (pi) and the solution to the homogeneous linear heat equation (wi),such that Gi= pi+ w isatisfied the initial and boundary conditions. The fimction piis of the standard form

for a half plane geometry and the problem, in effect, reduces to the derivation of wi.The basic formalism they developed permitted the inclusion of arbitrary absorption profiles. However, to reduce algebraic complexity, they assumed that all the energy was absorbed at the surface. This weakened the generality of their results, particularly for thin or poorly conducting films Madison and M ~ D a n i e l k ~extended ~~I the analysis of Burgener and Reedy and developed a formalism to treat multilayered structures while maintaining the flexibility of including arbitrary energy absorption profiles. An attractive feature of their analysis was the grouping of terms in a fashion

Modeling the Magneto-Optical Recording Processes 659 that permitted the identification and interpretation of their origins and physical significance. They considered a general N-layer medium with L layers above the source layer andM-L below it where N = M +1. The basic problem consisted of setting up expressions for N Green’s functions and solving then with the 2N boundary conditions. The Laplace transformed Green’s function for the energy absorbing layer in the N-layer structure was derived (in our notation) as:

+[

4fL.L e

-tl/ (26,

-2.)

+ 4f*...L 4fL+I...Me

1- 4f 1...L 4jL+l...M

-7, 26,

e

where the coupling terms resulting from the BC’s and given by:

Q. (49)

qi =

-/,

-v,(26,+z*)

I

4 is recursive by nature

R2 = (x - x ’ ) ~+ ( y - y r ) 2

Equation (49) represents the Laplace transformed Green’s hnction solution to the heat flow due to an instantaneouspoint source located in the source layer, which is part of an N-layer medium. The three integrals in Eq. (49) can be physically identified as the source term and the multiple infinite series of point images on either side of the source layer, respectively.

660 Magneto-Optical Data Recording Applying the previously derived Green's fbnction in the standard fashion to the static irradiation of an N-layer medium, we obtain the following general expression for the temperature.

o

-m

-m

o

. where th is the length of the heating pulse, r2 = x2 + y, = x ' +~Y ' ~ The constant u is large enough so that all singdarities are to the left of the contour of integration. The heat source term is given by:

where Psis the peak power density of the irradiating laser-beam, H(I) is the laser-heating time profile, and F(z') describes the EID in the source layer. Integrating over x' and y', the above expression reduces to the following, more compact, generalized form

Modeling the Magneto-Optical Recording Processes 661 where

Madison and McDaniel were able to further simplify this expression for certain specific cases, the inverse Laplace transform reducing to integration

along straightforward contours. An alternative strategy would be to use a computationally efficient numerical inversion strategy such as that developed by Kant[1451in his treatment of a similar problem. Despite the apparent algebraic and numerical complexity of the above expressions, they are considerably easier to evaluate numerically than a full-blown finiteelement, or finite difference technique. The above analysis can be readily extended to treat the case of a scanning laser beam through an obvious coordinate transfonnation. Given the physical interpretation of the terms in Eq.(47), Madison et a1.[141subsequently established that this solution was analogous to the infinite-series solution obtained by the application of the method of images. obtained a rigorous solution of the multilayer problem by assuming a linear variation of temperature through individual layers. He justified this simplification on the grounds that individual films could, without too much increase in computational complexity, be subdivided into smaller segments to provide the requisite degree of accuracy. Purely Numerical Modeling. In an earlier section, we alluded to the fact that the rigorous solution of the laser marking in inhomogeneous thin film structures is most often dealt with purely numerically. While the Green’s Function approach utilized by Madison et al.[1461in the previous section falls into this category, it is somewhat distinct in that they expended considerable effort to transform the governing equations into a computationally efficient format. Many numerical treatments use the base governing equations directly in a finite element or finite difference scheme. In many cases, the high conductivity and strongly localized nature of the heat source terms require the use of very fine time and distance steps to

662 Magneto-Optical Data Recording For example, for onedimensionalheat ensure stability and flow, the time (At)and distance (h) steps have to satisfy:

found that if attention were focused on the Schvan and temperature rise along the center of the laser beam, the fill three-dimensional problem could be split into two, coupled, one-dimensionalequations, accounting for radial and vertical heat flow, respectively. Waechter et al.[1491used this result to reduce the heat flow problem in a laser-irradiated, multilayer thin film structure to three coupled partial differential equations that they subsequently solved using the backward implicit Euler method with Newton iteration. Kivits et al.[ls0]treated the heat flow problem using finite center difference equations and integrated the governing equation using the Alternating Direction Indicator Method. A common feature of many of these studies was the use of meshes, which gradually increased in size with increasing z and r. Apart from reflecting the physical characteristics of the problem geometry itself, this strategy increased both computational efficiency and accuracy.

5.0 THERMOMAGNETIC MARKING 5.1

Introduction

Over the past thirty years there has been considerable activity devoted to the modeling of the laser-induced thermomagneticwriting process. These ranged, for example, from the relatively straightforward Curie point writing model[1s11-[1s31 to those based on variations of the approach originally advanced for the stability of cylindrical domains in bubble [ls61 with allowances made for localized, nonuniform temperature~.[~~~l[~~~][1621 The thermomagnetic marking problem itself can be decomposed into three distinct, interdependent components: 1.Absorption and conversion of optical energy to thermal energy. 2. Heat flow during and after irradiation. 3. The interactionoftemperaturedependent material properties to effect “domain” or “mark” formation.

Modeling the Magneto-Optical Recording Processes 663 Mansuripur et al.[1631-[1651 outlined what is perhaps the most rigorous numerical framework for treating thermomagnetic marking and integrated, in a comprehensivefashion, the constituent optical, thermal, and magnetic problems. However, in many instances,very good results can and have been obtained using simpler, approximate treatments. Let us. in thls section, consider the mechanistic aspects of domain formation and stability and examine the various models that have been used for this purpose.

5.2 Simple Adiabatic Thresholding Model The simplest representation of thermomagneticmarking would be to use the adiabatic thermal model in conjunction with a thresholding marking assumption; i.e., we ignore heat flow and assume that any region whose temperature exceeds Tthis instantly “marked.” If we refer back to the adiabatic temperature distribution derived previously (Eq. 33), it is apparent that two writing powers are of particular significance. These are the threshold power Pth where the medium first undergoes marking and the optimum recording power (OW,,,) at which the written mark length coincides with the write pulse length. These can be derived explicitly as:

From Eqs. (42) and (43) we obtain:

664 Magneto-Optical Data Recording For long marks (say in excess of 5 oef), the error function terms in the above expressions are all nominally unity and we obtain the simpler relations:

Applying a Taylor expansion or simple perturbation analysis to Eq.(3 l), we obtain the length error .sL (with respect to the expected mark length L) of a (long) mark written at a write power P , (which differs from OW,) as:

Simple density readout schemes have proved capable of very accurate ~ ~further ~1 been estabreplications of the MO readout p r o c e ~ s . [ ~ ~It1 [has lished that the curvature of the leading and trailing mark edges causes the read back length to be less than the physical length.[1301[1671 Let us assume that the length discrepancy is some fraction 4 of the spot radius creF Then, from Eq. (59) the optimum record power that is determined by optical is given by: readout (ORP,.)

Allowing for the readout-induced length discrepancy, it can be shown, for long marks, that Eq. (59) should be rewritten

where 6;. is the optically (readout) detected mark length error. Figures 25-29 compare some of the predictions of these expressions with experimental measurements.

Modeling the Magneto-Optical Recording Processes 665 Velocity Dependence. Figure 25 depicts the measured velocity dependence of Pthand OW,for a representative MO medium. The write pulse length was held fixed at 2.0 pm. We observe the linear dependence on velocity predicted by Eq. (57). In fact, as seen in Fig. 26, a pronounced linear dependence of O W , on v is observed for quite an extensive velocity range. The latter data pertains to a TbFeCo film whose compensation point is at or near room temperature, as is often chosen. The marking responses of such films are well described by thresholding marking models due, perhaps, to the major impact of coercivity in determiningthe final domain configurations. Other compositions, particularly those with compensation points above room temperature, could exhibit nonthresholding marking responses due to more complex domain equilibrium considerations. Equations (57) and (58) imply that a/av(ORP,) should be ~ 2 . 5x ~lav(P,). From Fig. 25 we obtained a value of approximately 2.14. Equation (57) further implies that Pth+0 as v + 0. However as in Fig. 25, we commonly, if not always, observe a finite intercept which is a reflection of the heat loss and can be related to the steady state thermal response of the package. In the same vein, the functional dependence of OW,on v implies a negative intercept as v + 0. It is not difficult to envision how the incorporation of bias power into the adiabatic model leads to these seemingly incongruous results.

o Threshold

10-

-

ORP

-

E 0-

Y

write pulse length = 1.8 microns

00

I

5

I

I

10 15 Scanning velocity (m/s)

-

I

20

25

Figure 25. Measured velocity dependence of OW,,and P,,, for a representative AlN encapsulated TbFeCo thin film on a polycarbonate Write pulse length = 1.8 pn, applied field = 300 Oe, and bias (= read) power = 1.5 mW in all cases.

666

0 Linear Velocity (mk) Figure 26. Measured OW, vs velocity for a representative, A N encapsulated, TbFeCo film in a glass sandwich structure.[1~7]

Bias Power Dependence. Figure 27 compares the predicted and measured dependence of Pth and OW, on P , from Eqs. (58) and (60). The agreement is reasonable and can be rationalized on the following basis. Bias power contributes little to the peak temperature at the center of a moderateto-long mark. Hence it should have little or no impact on threshold power. On the other hand, it significantly impacts the temperatures at the leading and trailing edges of the mark and so, unlike for Pth, we would expect that increasingPb decreases O W , (as defined). Mark Length Error. Figure 28 compares the predictions of Eq. (6 1) with experimentally measured mark length errors for a range of scanning velocities. The model used the measured OW, at each velocity and the overall agreement is seen to be excellent. Figure 29 depicts similar data plotted in a somewhat different fashion. Analogous to the media transfer h c t i o n concept advanced by LaBudde et al.,[1691we obtain a convenient linear media marking response fbnction that holds over an extremely wide write power range. The importance of incorporating the bias power is clearly illustrated by the dotted line that is essentially the prediction of Eq. (61) with P , set to zero.

Modeling the Magneto-Optical Recording Processes 667 In summary, the surprisingly good agreement obtained by this extremely simplistic adiabatic model in the limited instances we have examined is due to fi) the effective factoring out of the heat loss terms and (ii) a reflection of the thermally thresholding marking response of the RE-TM materials under consideration. Their utility is obviously limited but they provide a great deal of insight into the observed marking responses of a wide variety of optical data storage materials.

Figure 27. The predicted (dashed lines) and measured (symbols) dependence of P,,,and OWr on Pb for a representative AlN encapsulated TbFeCo thin film on a polycarbonate substrate. Solid lines represent least squares fit to the dataF"1

668 Magneto-Optical Data Recording

I

80 -

1

I

I

2

- 6oI

h

v)

C

40

-

-

I

0 5m/s 0 I0 m/s 0 15 m/s .20m/s

.

-

Figure 28. Predicted (solid fines - Eq. 61) and measured (symbols) mark length error as a function of write power at various velocities. The measured OW,, at each velocity was utilized in the rn0del.['2~]

,-*--..a

0.50 E

a

v

........... .....

0.25

-J

L

2 L

a,

0

r,

F -0.25

a, Y 2

-0.50

2 -0.75 I 0.6

-..-... -...-........

3 Exoeriment ead Power it Read Power I

oa

L I h

\ I

I.o

I

1.2

I 1.4

PSHM/P Figure 29. The predicted mark length error with and without allowance made for bias power (solid and dashed lines, respectively) and measured mark length error (symbols) as a function of OW,/P, for a representative A N encapsulated TbFeCo thin film on a polycarbonate substrate."29]

Modeling the Magnett+OpticaI Recording Processes 669 5.3

Generalized Thresholding Model

Apart from possessing poor predictive capabilities. in gcncral, the simple adiabatic modcl is incapable of rcplicating many important features observed in thermomagnctic marking. This can bc accomplished by using a more rcalistic t h c m l m d c l in conjunction with the same thermally thresholding marking mechanism. For example, Eq. (42) was capable of replicating, to a fair degree of accuracy, the thermal broadening that is characteristic of domains written in amorphous TbFeCo media under typical marhng conditions (sm Fig. 30). This broadening is a consequence of the flow of heat ahead o f the scanning h e r spot that results in a cumdative increast: in peak temperature with scanning time as depictcd in Fig. 3 1.

Figure 30. Calculated(using Eq. 42) and actual, digitized image ofdomairis written in an AN encapsulated '1'bFeCo film.11381

670 Magneto-Optical Data Recording 400

o^

-%

300

._

[r

2 200 3

c

cal

g 100 t-" n " -1

0

+

1 Distance (microns)

t

pulse off

pulse on

300

3

2

-

n "l -1

I

I

0 f

pulse on

I

I

I

1 Distance (microns)

I

2

I

3

t

pulse off

Figure 31. In-track temperature at various times during a dynamic laser marking event."381

There has been much discussion regarding the assignment of thermophysical parameters in the modeling of laser marking as applied to optical data storage. It is now generally accepted that the thermal conductivity of the amorphous RE-TM layers is significantly lower than for the bulk. For 101[1401[1701 had to significantly reduce example, some investigator~[~~~l[~ thermal conductivity from the conventionally assumed value of 30-40 W/ m-K (up to a factor of ten in some instances)to fit measured data. However, reasonably good agreement between model predictions and experimental data was also obtained using the higher values of c o n d ~ c t i v i t y ~ ~ ~ ~ ~ ~ ~ often by appropriately normalizing or calibrating the model to the data under consideration.

Modeling the Magneto-Optical Recording Processes 6 71 Using the weighted average for the specific heat of a composite AlN encapsulated TbFeCo film, we used Eq.(42) to calculate threshold power as a function of velocity assuming various values for the thermal conductivity of the composite film. As depicted in Fig. 32, the best fit was obtained with hk:5 W/m-K and pf cf = 2.5 x lo6 J/m3-K, which is comparable to some values reported in Table 2. However, the calculated marking threshold k:190°C was appreciably higher than the temperature =160"C at which the coercivity and applied field (300 Oe) were equal. In fact, the predicted marking threshold was on the order of the Curie temperature for the given composition 195°C.When we assumed that pycf equaled 3.0 x lo6 J/m3-K, the best fit to the data gave Kfk: 7.0 W/m-K and Trhk: 165°C). It is reasonable to expect that the actual marking temperature lies somewhere between these two limits and, therefore, the composite film thermal conductivity is between 5 and 7 W/m-K. Note that,for ppf = 3.5 x lo6 J/m3-K the corresponding values were Kf = 10.0 W/m-K and Trhk: 135"C, which provides a measure of the sensitivity of this approach.

Threshold Power (mW)

0

5

10

1s

20

25

30

Velocity ( m / d Figure 32. Predicted (solid lines)and measured (datapoints) threshold power (P,,, as a function of scanning velocity.[1181 Write pulse length = 2 pm, reflectance = 30%, irradiance radius of focused laser spot = 0.408 jm. TbFeCo thickness = 1000 A, composite film thickness = 1000 A, and p , y 2.54 x lo6 J/m3-K.

672 Magneto-Optical Data Recording The excellent agreement between the predictions of the simple adiabatic model and measured data depicted in Figs. 28 and 29 is somewhat surprising given the generally unrealistic assumptions made in its derivation. Figure 33 compares the normalized temperature distributions obtained w;t?? a d wit!lol!t s!!nwaces m d e for hsit !QSS @qs. 31 a d 42) esch normalized so that the written mark length coincides with the write pulse length of 2 p.There are similarities and obvious differences between the two profiles. On the one hand, the difference between the peak and threshold levels is an indication of different relationships between Pth and OW, in each instance. On the other hand, the averaged slope of the temperature distributions(leading and trailing edges together) at threshold is very similar and explains why the simple adiabatic model is capable of predicting the write-power dependence of mark length error as well as it does. Essentially, the dependence of mark length on temperature (and hence incident power) is comparable.

Figure 33. Normalized temperature distributions obtained from adiabatic model (Eq.31darhed line) and one that incorporates heat losses during and after the marking event (Eq. 42-solid line). P, = 8 mW, Pb = 1.5 mW, v = 10 d s , all other parameten similar to those inFig. 32.

Modeling the Magneboptical Recording Processes 673 Expressionsanalogousto those derived for a laser-irradiated solid @q.28) were used by Inoue et al.[1711to model the thermomagnetic marking of thick S~Er3,Ga,,Fe,~,,012 garnet films on Gd3Ga50,, substrates since, to a very good approximation, the film-substrate combination could be treated as a semi-infinite continuum. They assumed that the Curie radius dictated the finaldomain configuration. Figure 34 compares their experimentally measured bit diameters (data points) with those predicted by the thermomagnetic model (solid lines). The better agreement for lower powers and/or shorter write-pulse durations was attributed by them to a weakening of some of the assumptions made in the marking model.

01

0

I

50

I

I

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100

150

200

Laser Power (mW) Figure 34. Measured (datapoints) and calculated (solid lines) diameters of statically Written bits in Sm,Er,,Ga,,Fe,,O,, garnet films on Gd,Ga,O,, substrates as a function of laser power for various Write-pulse

For example, their neglect of the temperature dependence of the absorption coefficient which, they postulated, was significant. Despite the fact that the thermally thresholding marking model has been used with great success to treat a variety of issues surroundingthermomagnetic marking, it provides no insight into the dynamics of domain formation, which becomes increasingly significant in direct overwrite type scenarios.

674 Magneto-Optical Data Recording 6.0 MAGNETIC MODELING

6.1 Mean-Field Modeling A mean-field model (MFM) is a relatively simple approach for obtaining temperature dependent magnetic properties of an alloy system. References 166,173 and 179 provide accounts of the background and a formulation suitable for MO recording. The essence of the method is the establishment of atomic (or ionic) magnetic dipole moments for “average” elemental species in the alloy, in addition to mean values for exchange interactions between atoms. This is done in accord with a criterion that the resultant computed temperature dependence of one or more magnetic parameters (for example, magnetization, uniaxial magnetic anisotropy) match experimental data. Thus, a mean-field model is a phenomenological construct that ignores real variance in alloy structures and local properties, but does provide mean, macroscopic magnetic behavior. In the MO recording application, the MFM provides the temperature dependence of magnetic parameters that may be difficult to obtain experimentally (domain wall energy density, exchange coefficients). The MFM has frequently been used to generate inputs for the magnetic to be discussed in the following sections. The limitations on the predictive power of the MFM follow directly from the artificial premise at the core of its construction. Nevertheless, the MFM can do a good job of fitting the temperature dependence of average bulk magnetic properties.

6.2 Bubble Model As alluded to in the previous sections,much effort has been expended in

modeling the dynamic domain marking process. The simple, conventional approach begins with the stability criterion of a single reverse domain in an otherwise homogeneous medium, which is then perturbed to account for the variable temperature associated with laser marking. The reader is directed to Sec. 6.3, “Micromagnetics,” for a more complete treatment of the magnetic aspects of domain formation and stability. The total energy associated with such a single reverse domain (relative to the domain free film)ETotis given by:[1541-[1571

Modeling the Magneto-Optical Recording Processes 675 where AI!?~AI!?d, and Maare the incremental changes in domain wall energy, internal magnetostatic energy (also referred to as serfor demugnetizution energy), and energy due to externally applied fields (Zeemun energy), respectively. Assuming through-thickness homogeneity we obtain, for a cylindrical reverse domain, that

Es. (63)

AEw = 2nr6’cW(r) and

AE_= 4n6’ Ha 0

wheresf is the magnetic layer thickness, a , is the domain wall energy density, M,is the saturation magnetization, and H, is the normal component of the externally applied field. We shall, for the present, avoid explicit representation of the demagnetization energy term. The radial force on the domain wall can be derived in a perturbative fashion by considering its displacement. In the limit, this corresponds to differentiating the domain energy (AETot)with respect to the domain radius (r)giving:

This can be simplified to

where r?,( I ) represents a thickness averaged “demagnetization field” at the domain wall and Ms(r) can be thought of as a complex, averaged,

676 Magneto-Optical Data Recording “effective magnetization.” The above expression pertains to a cylindrical reverse domain but, with some loss of brevity, one can readily derive the generalized energy variation (force) associated with a reverse domain of arbitrary shape.“ 551[1 561 The negative terms in the previously derived expression for domain energy act to shrink the domain whereas the positive ones tend to expand it. Domain equilibrium is conventionallyderived from the balance between the previously calculated force Fd(r) and the coercive, frictional force F, in the The stability criterion can then be magnetic layer, i.e., &(r) explicitly written as:

c(r).

The absolute value of the left-hand side dictates domain stability whereas its sign indicates if the domain shrinks (negative) or expands (positive). The so-called bubble-model of thermomagnetic marlung utilizes Eq. (66) in conjunction with the solution to the thermal problem and a knowledge of the temperature dependence of magnetic properties. At any instant in time, the force on the domain wall is calculated to obtain an estimate of its “stability” and then a determination is made if it shrinks or expands. Doing so, for the duration of a laser pulse, permits an estimate to be made of the final domain configuration under various conditions. The general applicability of this approach is questionable given the strongly varying temperature in the domain itself, which could radically alter the instantaneous numerical values of Eu and Ed. Despite these reservations, extensions of the bubble model have, in general, proven capable of qualitatively and quantitatively replica- experimental observations[6flas depicted in Fig. 35. Suits et al.[641found that the bubble model gave good results when applied to RETM media that exhibited single domain behavior and in which wall motion dominated the writing process (Fig. 36a). In principle, these are the compositions traditionally employed in data storage scenarios. For media that exhibited multiple domain writing behavior, the assumptions inherent in the bubble model were violated and the agreement was lessened (Fig. 36b).

Modeling the Magneto-Optical Recording Processes 677 In this instance, a nucleationjeld model was found to give better agreement with the data (Fig. 36c). In this model, a determination was made as to where the action of the externally applied field was just sufficient to effect magnetization reversal. For dynamic thermomagnetic marking, the resulting noncircular domains, strictly speaking, preclude the use of the simple stability criterion derived for cylindrical domains. However, the somewhat cavalier application of this criterion has provided results compatible with experimental observations. To some degree this can be attributed to the extremely steep temperature gnubents and sometimes even steeper gradients in material properties, which tend to overshadowthe inadequacies and spatial inconsistency of the model. For example, Fig. 37 depicts the reasonable agreement between predicted and measured mark lengths in a representative AlN encapsulated TbFeCo media Savage et presented a variation of the conventional bubble model for dynamic marking scenarios that preserved the continuity of the domain through a semidiscretizationof its boundary. The method, in effect, ensured the physically realistic collective, coupled behavior of adjacent domain segments, which is absent in the simple, quasi-static extensions of the cylindrical model. Weng et a1.[681incorporated an additional refinement in their thermomagnetic marking model in that they explicitly accounted for domain wall mobility and motion through the Landau-Lifshitz-Gilbert formalism. Despite their use of assumed thermal parameters, they obtained very good agreement between their predictionsand the experimental data of Suits et al.[641as seen in Fig. 38. The bubble model basically treats the domain wall as being infinitely thin and devoid of structure. This obviously is a limited representation and leadsto singular behavior as the domain diameter tends to zero. Mansuripur and C ~ n n e l l [circumvented ~~~] this problem by allowing for a finite domain wall width and permitting magnetization tilt in the plane of the magnetic film. This served to break down the wall energy term into the separate contributions arising from anisotropy (K,)and exchange (Ax),

E , =jK,,(r)sin*[[S(r)~V and

E, =~ ~ A x ( r ) ( V a i ) 2 d V i

678 Magneto-Optical Data Recording where o(r) is the deviation (from the normal) of the magnetkition at radius r fi-omthe domain center and aiare the direction cosines of the magnetization at the given location.

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Figure 35. Comparison of the predictions of thermomagnetic models (derived from the simple bubble model) with experiment.[67](a) Optimum values of the externally applied switching field for a given domain length in GdTbFe. (b) Measured and predicted domain lengths as a h c t i o n of Curie temperature for TbFeCo and GdTbFe films. Ha = 16 kAlm and writing energy = 0.6 nJ pulse.

Modeling the Magneto- Optical Recording Processes 6 79

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Modeling the Magneto-Optical Recording Processes 681 6.3 Micromagnetics An alternate method for treating thermomagnetic writing has been advanced and consists of treating the magnetic film as a discrete distribution of magnetic dipoles (cells). By allowing small random variations in material properties from cell to cell, Mansuripur and collaborators[631[721[1~l[1741 were able to not only demonstrate coercivity in this class of materials but also replicate many of the observed features of domain reversal and formation. Despite the fundamentally different emphasis placed on the extent and demonstratedthat scale of thermomagneticinteractions,Perlov et al.[1661[1751 both methods gave rise to almost identical predictionsof written domain size and shape. There were some differences in the predictions near threshold that they attributed to a weakening of some of the primary assumptions. A thin film is subdivided into a mosaic of cells that will interact magnetically. Cells are randomly assigned temperaturedependentphysical properties from specified distributions; 2 is generally constrained to align parallel with the film normal. A temperature-dependent expression for the change in system energy upon magnetization reversal of cell i is developed:

Equation (68) is computed assuming all other cells in the model are held fixed while cell i undergoes a trial reversal. At each time step of a solution sequence, each cell of the model is interrogated as to its probability of switching. The Metropolis rule is employed to establish the switching probability: Pi= exp(-AEi/kT). This is the Boltzmann factor of classical statistical (thermal) physics. A particular detail of the switching protocol is important in this type of model, and if implemented without care, artifactual behavior can result. A choice can be made at time step m about which states of the N-1 cells surrounding cell i to reference in computing the present AEdemag,i.One choice would be to use the state at the (m-1)* time step, in which case the order of cell interrogation and switching is immaterial. Another possibility is to use serial interrogation and switching, querying the state at time m for the n cells already interrogated,and to use the state at time m- 1 for the (N-1)-n cells not yet interrogated at time m. In the latter case, the order of interrogation and switching is certainly important, and the state of the set {Mi} at the completion of time step m is highly dependent on the order of interrogation and switching.

682 Magneto-Optical Data Recording The energetics of the magnetic system are composed of the modeled expressions for the three terms on the right-hand side of Eq. (68). The Zeeman term is simply the magnetic dipole energy determined by the angular orientation of the moment relative to an external applied field. c . e fie map-& field ARde,,,,,is &Q a zemzn-!&e tern-, hut influencing the dipole is the stray magnetostatic field due to the volume of magnetized material surrounding the switching cell. The domain wall term is the net change in intercell energy stored in walls separating cells with opposite M states when cell i reverses. The demagnetizationterm is by far the most laborious to compute as one must sum the contributions at cell i of stray field from all other cells in the model. This fact is a reflection of the fact that the magnetostatic field from a collection of dipoles is a long-range field, so the summation is one with many terms. The summed contribution to the energy falls off as l/r2, so in a practical sense one can approximatethe full summation over all cells with a truncated sum out to some maximum radius centered on the ith cell. This will introduce a fixed percentage error that one can make arbitrarily small. The trade-off is against computation time. The model reported in Ref. 72 uses a magnetostatic field surrounding a uniformly magnetized right circular cylinder as the stray field from each cell, and a numerical integration over the volume of cell i is approximated with a sum over 100 subvolumes to estimate the magnetostatic coupling of any pair of cells i and j . A micromagnetic thermomagnetic recording model uses the above magnetic model in combination with a heat transfer model and the output of a mean field model (MFM). Typically, as in Ref. 72, one models the wribng of a sequence of a few MO domain reversals (marks) by supplying a thermal model involving a moving, focused laser beam with the correspondmg laser power pulse sequence. An effective“writing temperature” is establishedby the temperature dependence of the MFM coupled with the physics of Eq. (69) and the Metropolis rule. One way to interpret the thermomagnetic domain reversal is that a temperaturedependent coercivity is established by this physics-mercivity is never a primary magnetic property, but a secondary one determined by an interplay of magnetic “forces” or energies (anisotropy, exchange, magnetostatics) combined with the existence of local energy barriers due to material inhomogeneities and defects. A temperaturebut dependent coercivity has been derived from a micromagnetic it is a very coarse and rudimentary representation of H,(T). Nevertheless, it has the essenceof the explanationofH,(T) in an amorphousferrimagneticalloy such as a RE-TM system. The nanomagnetic model of the following section

Modeling the Magneto-Optical Recording Processes 683 discusses the next level of refinement in a first principles explanation of the origin of coercivity in the RE-TMmaterials. We should note that the wall energy term is a derivative and condensation of the separate fundamental physical effects of exchange, anisotropy, and magnetostatics. In as much as the boundaries between model cells are “artificial” domain walls (zero thickness), there is a tendency for these wall effects to become overly intrusive in the larger model. This means, for example, that qualitative estimates of domain nucleation or wall motion contributions to coercivity may be distorted. This is undoubtedly a consequence ofthe oversimplification (coarseness) of the model. Perlov et al.[1751 attempted to correct some of the artificiality of the intercell domain wall by replacing the domain boundary with fitted sectors of a bubble model wall. It was claimed that this approach offered domain growth characteristics more in accord with experiment. This improvement was gained at the cost of a somewhat cumbersome hybridization of the model. Figure 39 shows some cases of modeled thermomagnetic writing and erasing. The model shown was composed of a mosaic of hexagonal closepacked cells with edge length of about 25 nm. This resolution is quite adequate for modeling domain patterns involving mark diameters of one micron or a bit less. The top two images in Fig. 39 show writing of a moderately long MO mark in pulse-width modulation (PWM) writing where the digital information is encoded in the beginning and ending domain edge location. This figure illustrates a striking difference between two simple methods of pulsing the writing laser power. The simplest conceivable pulsing in the top image results in a gradual warm up of the storage film, so the initial domain edge is poorly placed. The middle image showsthe change when the laser power is given a sharp pre-emphasis power to compensate for the fact that the writing event begins on “cold media.” After the initial pre-emphasis, the thermal buildup takes hold, and so the power level can be reduced. The net result is that power pre-emphasis can provide better overall shaping of the domain to accurately locate the domain edges. Also visible in these images is domain edge (wall)jaggedness, which is a result of cell to cell inhomogeneity of magnetic properties of the modeled media, as well as the details of the writing method. The bottom image in Fig. 39 shows the middle domain after model erasure with a reversed bias field and grossly insufficient continuous erase power. Sizable mark remnants remain after the erase pass, and this incomplete erasure would result in noisy background upon rewrite.

684 Magneto-Optical Data Recording

,.,,....... .....,. ....,.....,.. .........................................

Figure 39. The top and middleimages show thennomagneticwriting with the micromagnetics model. Different laser power pulsing schemes are illustrated. The preemphasis writing results in better domain shaping for PWM writing. The bottom image illustrates modeled incomplete erasure when the continuous laser power accompanying the reversed bias field is too low.

Modeling the Magneto-Optical Recording Processes 685 These models show, at least in their global behavior, that the domain reversal process is driven largely by temperature variations, and that indeed one can estimate a “writing temperature” that follows from the imposed magnetic properties. Apart from the appearance of some degree of local jaggedness in domain walls (boundaries between reversed and unreversed regions), walls form along a “cumulative isotherm” during the thermomagnetic writing process. These observationslead to two conclusions about the predictions of micromagnetic thermomagnetic writing models: At a “global” level, the magnetic physics could be replaced reasonablywell by a single “marking temperature”for simple, single-layer MO film recording structures. The degree of domainjaggedness is somewhat affected by the interacting terms in Eq.(68), but these effects are ultimately constrained by the cell size that governs the resolution limit of the model. One comes away with the feeling that the micromagnetic model teaches that at the macro level it is almost superfluous (thermal effects dominate), while at the micro level it is evidently too coarse to account for fundamental origins of coercivity or media noise, and thus somewhat inadequate overall. 6.4

Nanomagnetics

As a response to the shortcomingsof micromagnetic models in accounting for the origins of coercivity and the detailed role of domain wall dynamics in RE-TM alloy films, and facilitated by the appearance of massively parallel computing hardware, nanomagnetics (NM) simulations appeared in the mid to late 1 9 8 0 ~ ~ . ~ ~ ~ ~ 1 ~ ~ ~ ~ 1 ~ ~ * ~ 1 - ~ ~ * ~ 1 In this numerical approach to modeling magnetic behavior arising from submicron inhomogeneity in RE-TM alloy physical properties, the relevant physics is imparted to interacting point dipoles arranged on a 2-D grid (rectangular or hexagonal). A typical grid spacing is 10 A, while the arrays studied have usually been at least 256 x 256 grid points in extent. Each grid point is assigned a set of physical properties drawn randomly from distributions. In a full ConnectionMachine simulation, one grid point per processor can be accommodated. The full interconnectivity among the processors can readily mimic the magnetostatic coupling of the dipoles, thus greatly

686 Magneto-Optical Data Recording

speeding the heretofore serial computation of the demagnetization effects. In addition, a fast Fourier transform scheme for evaluating the magnetostatic interaction was developed,[1801-[1821 which further alleviates the previous computational bottleneck. The formulation of the NM problem is cast in terms of the LandauLifshitz-Gilbert (LLG) equation for the temporal evolution of a dynamic system of interacting magnetic dipoles. One develops expressions for the effective magnetic field at the site of each of the N dipoles due to the present state of the other moments. Both long-range magnetostatic and short-range anisotropy and exchange interactions accompany a Zeeman term in comprising the effitive field. The LLG differential equation includes a dissipation term to phenomenologically account for the interactive loss of moment precessional energy to the thermal bath of the metal ion network and the electron gas of the material. The spatial resolution of this simulation is high enough to reproduce the internal structure of domain walls. Further, it can easily demonstrate a continuum of reversal dynamics from nucleation to wall motion, depending on the anisotropy and exchange characteristics imparted to the interacting dipoles. Simulations have shown that the assignment of dipole properties such that these parameters are uniform over regions of spatial extent 100 A (called patches) best reproduces the hysteretic behavior of RE-TM alloy films (see Fig. 40). To date there has been no direct experimental confirmation of the physical manifestation of patches, but materials experts believe it plausible that variations of homogeneity on this scale could arise in the fabrication of the RE-TM alloy films. Mansuripur, Giles, and coworkers have demonstrated that variations in coercivity consistent with experiment can be generated with the patch structure combined with reasonable choices of physical parameter values. Thus, the evidence for the existence of patches is indirect. Nevertheless, the NM model described represents the closest approximation to a coercivity model that we have for amorphous RE-TM alloys. Again, as in the micromagnetic model approach, coercivity is an output from the model, and it is derived from primary magnetic parameters (contrast this with the bubble model). Figure 40 shows a sequence in the time evolution of magnetic reversal. The spatial extent of the model is about 0.25 p.Solution time steps are adjusted according to the rate of change occurring, but can be as small as 1 ps. The duration of a complete solution will usually not exceed a few

-

Modeling the Magneto-Optical Recording Processes 68 7 nanoseconds. These solutions are not very extensive in time or spacc, and yet are very intensive due to the extreme resolution they achieve. (The corresponding sampling increments, Ar and At, for the micromagnetic solutions of the previous section were about 25 nm and 1-10 ns, respec-

tively.) T h e compactness of the NM solution in space and time precludes realistic modeling of a recording sequence with a practical beam size. Present computing hardware limits the NM approach to detailed studics of a submicron regime. Nevertheless, this model has proven very adequate for probing the detailed mechanrsms of domain reversal in RE-TM alloys. A recent study did extcnd thc tochquc to a situation where thermal gradients were imposed, as in themmagnetic writing.11*31

Figure 40. Results of the nanomagnelics modeling of RE-TMfilms by Mailsuripur ad Giles.[lmJol-[1mlIn (a&#), we see the structure of patches imposed in the model, where these patches are subregions of locally unifm proptrlcs, which can change more or less abruptly acToss II pakh border. Also shown in this qwnce is the development of a revcrsed domain over a time of a few hundred picoxconds. (d) This shows a mdeltd magnetix~tiontransition. which is a part oTa complete. rather quare hysteresis log.

688 Magneto-Optical Data Recording

Modeling the Magneto-Optical Recording Processes 689

Figure 40. (Cont'd)

6.5 Exchange-Coupled Systems and Direct Overwrite A significant increase in the complexity of MO recording media occurs when multiple magnetic films are used. As introduced in Ch. 8, Sec. 5.1 and in Sec.3.7of this chapter with dual MO layer structures for enhanced readout, the media design and function can be differentiated and further refined with this approach. Separately optimized read, erase, and write functions, including direct overwrite (DOW) and magnetic super resolution (MSR), can be incorporated. The price for this differentiated function is increased complexity of operation and function. Multilayer designs with ] - [ ~ *several ~ I years time, from 2-6 MO films have been r e p ~ r t e d . [ ~ ~ ~ Over the functionality has improved so that excellent DOW or MSR function has been achieved with simplified magnetic bias field requirements.[1881-[190] Modeling exchange-coupled Zayers (ECL) is still relatively primitive. Most early models dealt with the interlayer coupling by considering the energetics of an additional domain wall between the l a y e r ~ . [ ~ ~ lThese -[~~~1 studies were generally phenomenologicaland were characterized by simple, idealized analytic equations to account for interlayer coupling. For example, the expressions in Eq. (69) below correspond to the two-layer system shown in Fig. 4 1 From the fiee energy difference between states 1 and 2, one can very simply derive the switching field Hlznecessary to switch the net magnetization of film 1 with an applied field. For example, for state 2 which has an interfacial domain wall, the energy is:

690 Magneto-Optical Data Recording

where4, is 1 or 0 when the adjacent film net magnetizations are antiparallel from Eq.(69)[1881that or parallel, respectively. It

HI2

==-

Qw12 H C I

sl I

A pleasing result of this class of model is an ability to deduce interlayer wall energy densities and to predict, at least qualitatively, the different regimes of switching behavior.

domain wall

T STATE 2

STATE 1

Figure 41. A simple schematic diagram of a two-layer, exchange4oupledfilm structure. A magnetization reversal in layer 1 (top layer) under the action of the applied field occurs in a transition tkom state 2 to state 1, accompanied by the disappearance of the interlayer

domain wa11P"I

Numerical modellng of a simpleEC structurehas also been attempted.[lw1 A solution of the LLG was found for a dual ECL structure (memory and readout layers of limited spatial extent). Coercivity was introduced via an anisotropy variation, which was chosen to be sinusoidal. This choice introduced some oscillatory artifacts into magnetization and other distributions, but the model appeared to account for the main features of ECL structures. For more complexDOW and MSRdesign (4-6 MO films), the practitioner is presently restricted to model through-thicknesstemperatureprofiles, and to try to link these with static magnetic properties of the constituent EC layers. This kind of piecewise analysis is capable of accounting for ECL media

Modeling the Magneto-Optical Recording Processes 691 behavior, at least in qualitative terms. Figure 42 shows some results of applying a numerical Green's h c t i o n thermal model to a thick (1450 A), multilayer EC MO structure, in which the temperature profile at three depths through the MO stack were examined. The velocity dependence of three parameters are considered in Fig. 42-peak temperature rise, the increasing positional lag of the location of peak medium temperature through the depth of the MO stack, and the MO film area in the x-y plane at a particular z with T > Twitch.This thermal analysis can aid the modeler in understanding the behavior of both DOW and MSR structures. For example, the area inside a T = Twitchisotherm can be compared with the reading spot profile to get quantitative estimates of aperture size and shape for front, central, and rear aperture MSR

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692 Magneto-Optical Data Recording 7.0

SYSTEM MODELING

We have listed the following system modeling topics that may consider the role of MO recording in their full treatment: rm I nermomagnetic write process Noise, especially in media Digital channel Errors Some of these topics are treated in considerable depth in other chapters, and in those cases we will just make some general remarks. Our interpretationof system modeling is that it must use an assemblage of models to simulate a larger entity than that described by a simple model treating one physical phenomenon. A good example is thermomagnetic writing which, as we have seen in Secs. 5.0 and 6.3 of this chapter, may consist of a combination of optical, thermal, and magnetic models. A system (or subsystem) model is sufficiently complex to call on more than one of the models that we have presented. Noise modeling is potentially a very broad subject, and it is addressed in Ch. 9. In the next subsection, we consider three approaches to modeling MO media noise specifically. Most of this is handled within the framework of models previously described. Digital channel performance generally consists of analyzing the handling of an analog recording signal as the input to a digital channel. Our primary interest here is on modeling the analog signal output of an MO recorder. Sections 3.4 and 3.5 of this chapter discussed the calculation of MO readout signal waveforms. The most direct and practical way to generate simulated readout signals is to use Eqs. (3a) and (3b) to give signals for various isolated features, and then superpose them in an appropriate sequence corresponding to an allowed pattern from a particular digital code. Error modeling is a specific applicationof channel performance modeling. Here, the focus is on the relation between signal “stresses” (dropins, dropouts, variance [noise], shifts and offsets, etc.) and channel errors. Chapter 13 considers this in detail. Error modeling can be helphl for understanding the robustness of a channel design against media imperfections. It is also useful for development of error recovery procedures in a digital signaling device.

Modeling the Magneto-Optical Recording Processes 693 7.1

Noise Modeling

In this section, we consider only models of MO media noise that differ from those discussed in Ch. 9. Our aim is to illustrate some methods of representing stochastic processes that affect the erasing, writing, and reading process in MO recording. We will give a brief overview of three models, each of which results in quantitative output concerning MO media noise. Frequency Content. Reference 195 discusses an idealized model of MO readout that examines the spatial frequency response limits of a scanning MO head (that is, a scanning microscope; see Sec. 3.2). To check on the detailed modulation transfer function (MTF) exhibited by a Gaussian or Airy beam scanning spatial variations in reflectance in the sense of the overlap model of Eq. (3a), McDaniel et al.[l%] scanned a hypothetical medium with 2-D periodicity. Both rectangular (checkerboard) and sinusoidal modulation were treated. The MTF could be explored in the context of the overlap readout model as a function of Wu (A = light wavelength; u = period of the medium reflectance modulation) when the details of the spot intensity profile was altered. Checks against the textbook expressions for the cutoff frequency of an MTF were made. Model predictions were compared with experimental media noise spectra for erased and tone-written MO media. One interesting application of this analysis is determination of the size of the laser stylus by fitting the computed and measured erased noise spectra. An explicit assumption of the “white” media noise over some frequency band facilitates this analysis, and the validity of the assumption can be checked by the self-consistencyof the results. Comparison of the shape of the tone pattern media noise floor with that for erased media will exhibit the spectral content of MO “writing noise.” By removing the spectral shaping due to the stylus (as confirmed by the model), one finds the true writing noise spectrum, and this in turn is helpful in establishing the dominant ranges of correlation lengths in the written media. Simulation of M and Noise. Chapter 9 provides discussion of the MO media noise arising from variance in reflectivity and Kerr effect. A numerical overlap readout model can be used to gauge the impact of different spatial distributions of R and/or 0, variations on the readout signal. An issue arising in this simulation is the choice of a scheme for assigning “random” variations in media parameters, and what (if any) correlations to impose among the assignments.

694 Magneto-Optical Data Recording An approach that can be used to simulate the effect of media defect size distributions on MO or servo signal waveforms is described here. We need to clarifLour meaning of the term “defect” at this stage, for we are about to discuss a model that could be used to simulateeither the random occurrence nfsiahle film &fm.&f=sQ&ed --tb_ dAt2 emor$, a tandnm va-atinn in_ film optical or other propertiesusually associated with the normal processes of film fabrication. Because our interest here is MO film noise mechanisms, our “defects” represent the latter interpretation. In our simplified view, the distinction between true film defects and media noise sources is mainly a matter of feature size and spatial frequency of occurrence. Whether the issue is defects or noise, the feature does corrupt an ideal signal and interferes with the ability of the electronic channel to accurately process digital information. Further, the corruption tends to be a random process, and should be modeled as such. Figure 43a shows schematically how circular defects of distributed size placed randomly on a model film can be used in an overlap signal Calculation to assess signal impairment. It is well-known that particulate defects in a clean-room environment are often found to follow a simple inverse power distributionof frequency of occurrence vs defect size. For simplicity, defect size distributions from experiment [N(r)oc rn,n = 2-31 have been adopted, whether they are interpreted as error-causing defects or film property variations (a media noise source). In Fig. 43(b), we see the result of an overlap scan of a reading spot over modeled media with an r3relative distribution of reflectance variations down to a circle radius of 0.5 pn, and then dropping linearly to zero at zero radius. The circle sizes were chosen at random from this size distribution, and reflectance values were assigned randomly from a normal distributionwith mean 0.20 and standard deviation 0.01. A number, N, circular defects were placed randomly (with uniform probability) within the rectangular strip in Fig. 43(a). The quantity N was taken to be 200 for this example. Figure 43(c) shows an autocorrelation calculation for the reflectance variation depicted in Fig. 43(b). This is a measure of the spatial correlation of the “defects” imposed on the strip media model. We see that the correlation drops to zero at a distance of about 2.9 p.An estimated power spectral density (PSD),which is a Fourier transform of the autocorrelation function, is shown in Fig. 43(d) and closely resembles the input “defect” size distribution. This demonstrates a degree of closure in the model.

Modeling the Magneto-Optical Recording Processes 695

STRIP of MEDIA (b)

REFLECTANCE VARUTlON v8. POSmON

(c)

M O W R R M T I O N of REFLECTANCE

Figure 43. (a) A modeled strip of MO media with presumed reflectance variations represented by circular areas of varying size and reflectance. (b) Reflectance variation sensedby a scanning read spot in a model of the sort shown in (a). The size distributionof circles was proportional to W. (c) An autocorrelation of the trace in (bJ. (4 This shows a Fourier transform of the autocorrelation in (c), an estimate of the power spectral density of the reflectance variation in (b). We note the resemblance of the truncated 119 distribution assumed for the spatial characteristicof the reflectance.

696 Magneto-Optical Data Recording

Jitter Simulation. The micromagnetics model described in Sec. 6.3 combined with an overlap readout model has been used to estimate the role of several potential contributors to perceived media noise and signal Separatecases of repeated writing and repeated reading were used to distinguish among several contributionsto jitter. The readout variances were assessed by first writing a nominal small reversed domain “mark” with the micromagneticsmodel, and then reading it several hundred times with a scanning spot whose FWHM was varied according to random selection from a Gaussian distribution of spot sizes. Such a variation could easily arise from focus servo misregistration. From reading the mark with each spot, a distribution of readback mark widths could be developed and a jitter extracted. A writing-inducedjitter that might arise from variance in writing power was simulated by writing many marks with identical write pulse widths and media properties, and reading the marks out with a fixed width and power read spot. While this process might represent variation in laser power output with actual hardware, identical results would be achieved if the writing process were done with a fixed power and width write pulse, but with the marking temperature of the medium varied. A final jitter type, which is perhaps the closest to classic mediajitter due to material inhomogeneity on a submicron scale, was simulated by writing a large number of marks with the distribution variance/width of medium magnetic properties for each mark itself varied randomly from mark to mark. Again, readout was done with a fixed parameter read spot. These simulations helped establish the sensitivity of media jitter to a variety of potential contributors. These sensitivities for the first two mechanisms discussed could be estimated by simple estimates of read spot variation in a correlation (overlap) readout and by the relation between power, temperature, and writing spot size. The third model contained more complex physics that could not easily be accounted for by simpler means.

8.0

SUMMARY

This chapter opened with a discussion of the role of modeling in engineering and applied science, both in a general sense and as specifically applied to MO recording. We catalogued the type of physical models needed to treat the MO recording process, considering inputs and outputs

Modeling the Magneto-Optical Recording Processes 69 7 for each hnction to cover the process-the recording stylus, the head optics, light interaction with domains, multilayers, and topographical features. Multilayer optical design for optimization of media SNR performance was summarized, and optical considerations were concluded with a discussion of the degradation in MO recording performance due to substrate birefiingence. Next, a detailed treatment of thermal modeling was presented. Models ranging from simple to rigorous for the multilayered media case were reviewed. Next, several approaches to marking were covered, with emphasis on simplified but accurate thresholding approximations. This section closed with consideration of domain stability criteria in thermomagnetic writing and media heating effects during MO readout. Magnetic modeling was the subject of the next section, beginning with relatively simple mean field and bubble domain models. Next, we reviewed some usehl numerical approaches to describing the thermomagnetic write process and the details of magnetization switching dynamics. A nanomagnetics model with simulated material inhomogeneity yielded an accurate prediction of hysteretic behavior for RE-TM alloy materials. More primitive models of exchange coupled multilayers are beginning to emerge to account for dual layer optimized writehead, laser modulated direct overwrite, and thermally apertured magnetic super resolution. The final section provided a brief overview of noise models that have been developed for MO recording media. Models have considered media noise spectral content and response as perceived by a scanning optical head. Other simulations have dealt with representations of reflectivity and Kerr effect variations. A micromagnetics model of thermomagnetic writing with overlap readout assessed several contributions to signal jitter arising from the drive and media. Modeling and simulation has been extremely usefd in the broad field of MO recording, both for the recording process, as described in this chapter, as well as for other aspects of the complete system as other chapters in this volume explain. The literature of applied science and engineering has been greatly enriched by this work over the past fifteen years or so. Modeling has proven its worth many times over in this field by showing the way for physical realization of systems that are now enjoying commercial success. A technology with such a firm base of understanding and mathematical expression is well positioned for continued rapid advance in the future.

698 Magneto-Optical Data Recording

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704 Magneto-Optical Data Recording 147. Anderson, R.J., J.Appl. Phys., 64:6639-6645 (1988) 148. Schvan, P., and Thomas,R.E., J. Appl. Phys., 57:4738-47 (1985) 149. Waechter, D., Schvan, P., Thomas, R. E., and Tarr, N. G., J. Appl. Phys., 59:337 1-3 374 (1986) 150. Kivits, P., de Bont, R., and Zalm,P., Appl. Phys., 24:273-278 (1981) 151. Chen, D., Ready, J. F., and Bernal, E., J. Appl. Phys., 39:3916-3927 (1968) 152. Bernal, E., J. Appl. Phys., 42:3877-3887 (1971) 153. Schuldt, S.,and Chen, D., J. Appl. Phys., 42:1970-1976 (1971) 154. Bobeck, A. H., BellSyst. Tech. J., 46:1901-1925 (1967) 155. Thiele, A. A., Bell Syst. Tech. J., 48:3287-3335 (1969) 156. Thiele, A. A.,J. Appl. Phys., 41:1139-1145 (1970) 157. Huth, B. G., IBMJ. Res. Dev., 18:lOO-109 (1974) 158. Kryder, M. H., Meiklejohn, W. H., and Skoda, R.E., Proc. SPIE, 420:236241 (1983) 159. Mansuripur, M., and Connell, G. A. N., Proc. SPIE, 420:222-230 (1983) 160. M a n ~ u r i p M., ~ , and COMell, G. A. N., J. Appl. Phys.., 55:3049-3055 (1984) 161. Hansen, P., J. Appl. Phys., 62:216-230 (1987) 162. Han~en,P., J. Appl. Phys., 63:2 364-2371 (1988) 163. Mansuripur, M., and Connell, G. A. N., and Goodman,J. W., Appl. Opt., 27:1106-1114 (1982) 164. Mansuripur, M., and Connell, G. A. N., Appl. Opt., 21:666-670 (1983a) 165. Mansuripur, M., and Connell, G. A. N., J. Appl. Phys., 54:4794-4798 (1983) 166. Perlov, C. M., and Birecki, H., Jpn. J. Appl. Phys., Suppl. 28-3, 28:349351 (1989) 167. Ichihara, K., Mizusawa, Y., Yasuda, N., and Suzuki, T., J. Appl. Phys., 63:5 154-5 158 (1988) 168. Bartholomeusz,B. J., and Gupta, M. C., Appl. Opt., 31:48294833 (1992) 169. LaBudde, E. V., LaBudde, R. A., and Shevlin, C. M., Proc. SPIE, 382: 116-148 (1983) 170. Niihara, T., Takahashi, M., Miyamoto, H., Kirino, F., Ogihara,N., and Ohta, N., J. Mugn. Mugn. Mat., 88:177-182 (1990) 171. Inoue, F., Itoh, A., and Kawanishi, K., Jpn. J. Appl. Phys., 19 2105-2114 (1980) 172. Savage, C. M., Watson, M., and Meystre, P., J. Appl. Phys., 66: 1789-1792 (1989)

Modeling the Magneto-Optical Recording Processes 705 173. Mansuripur, M., and Ruane, M. F., IEEE Trans. Magn., MAG-22:3343 (1986) 174. Mansuripur, M., Giles, R. C., and Patterson, G., J. Mag. SOC.Jpn., 15:1730 (1991) 175. Perlov, C. M., Della Torre, E., and Birecki, H., J. Appl. Phys., 67:44444446 (1990) 176. Perconti, J., unpublished results. 177. Bartholomeusz, B. J., “The Thermal Conductivity of Amorphous Rare Earth-Transition Metal Thin Films,” unpublished results. 178. Perlov, C. M., J. Appl. Phys., 69:4945 (1991) 179. Gangulee, A., and Kobliska, R. J., J. Appl. Phys., 49:4896 (1978) 180. Mansuripur, M., and Giles, R., Comput. Phys., 4:291 (1990) 181. Giles, R, and Mansuripur, M., Comput. Phys., 5:204 (1991) 182. Fu, H., M. Mansuripur, M., and Giles, R., Comput. Phys., 8:80 (1994) 183. Giles, R., and Mansuripur, M., J.Magn. SOC.Jpn., 17(Sl):255 (1993) 184. Saito, J., Sato, M., Matsumoto, H., and Akasaka, H., Jpn. J. Appl. Phys., 26(4):155 (1987) 185. Lin, C. -J.,J. Appl. Phys., 67:4409 (1990) 186. Ito, M., Nakaki, Y., Tsutsumi, K., and Ito, O., J. Magn. SOC.Jpn., 17(S1): 155 (1993) 187. Mihara, M., Tanaka, T., Kitade, Y., Namba, Y., and Hashimoto, Y., ibid. 188. Tsutsumi, K., IEEE Transl. J. Magn. Jpn., 7:645 (1992) 189. Kaneko, M., IEEE Trans. Mugn., 28:2494 (1992) 190. Ohnuki, S., Shimazaki, K., Ohta, N., Inagoya, O., and Sakemoto, A., J. Magn. SOC.Jpn., 17(S1):205 (1993) 191. Kobayashi, T., Tsuji, H., Tsunashima, S., and Uchiyama, S., Jpn. J. Appl. Phys., 20:2089 (1981) 192. Gambino, R J., Science and Technology of Nanostructured Magnetic Materials, Plenum Press, New York (1991) 193. Hu, X.,Yorozu, T., Honma, S., Kawazoe, Y., Ohnuki, S., and Ohta, N., IEEE Trans. Magn., 29:3790 (1993) 194. Hasegawa, M., Moroga, K., Okada, M., Okada, O., and Hidaka, Y., J. Mugn. Soc. Jpn., 17(S1):249 (1993) 195. Kaneko, M., Ohta, M., and Fukumoto, A., IEEE Transl. J. Magn. Jpn., 7:685 (1992) 196. McDaniel, T. W., and Finkelstein, B. I., IEEE Trans. Mugn., 275118 (1991)

Testing Tom D. Mihter and Scott B. Hamilton

1.0

OVERVIEW

Magneto-optical recording systems are comprised of optical, mechanical, and electrical subsystems that often contain state-of-the-art components. The advanced technology is necessary because the tolerances required in positioning the light spot on the recording surface are typically on the order of a few tenths of a micron. As in most complicated systems, it is important to understand the behavior of individual components of the system as well as end-toend performance. A great deal of understanding comes from modeling and simulation, but due to the complexity of the system and manufacturing variabilities, both individual component and endtoend performance tests must be implemented. An additional complication is that magneto-optic media are removable, so interchangeability between disks is an important testing issue. A functional description of a magneto-optical disk drive and various signals is shown in Fig. 1. As the optical disk spins on the motor, the optical head tightly focuses a light spot onto the recording surface. The micronsized spot is used to erase, record, and read information. The optical head has several signals that interface to the electronics. Some signals are used only for control functions, which are usually low-frequency logic waveforms (i.e., I T L signals). One high frequency input signal is dedicated to 706

Testing 707 the data pattern that is written on the medium. Analog servo signals and read channel signals also interfaceto the optical head. The servo signals are used to control the position of the focused spot on the medium. The read channel contains information derived from the reflected light about the data pattern on the disk. Testing procedures are either static or dynamic. Static tests involve evaluating components of the system separately or evaluating some aspect of the drive while the disk is not rotating. Dynamic tests involve evaluating the function of the drive or the quality of the disk as it is spinning. Several useful dynamic measurements can be derived from the servos and read channels, as shown in Fig. 1. The servo signals yield information about runout and acceleration for both tracking and focus directions, as well as information about the push-pull tracking signal. The data channel yields information useful for cross talk measurements, carrier-to-noise ratio (CNR), resolution, byte error rate (BER), andjitter. Procedures for testing media as well as procedures for calibrating (setting up) the test equipment are explained more fully in the following sections.

I

....

-

U

disk I

Tracking radial runout push-pull radial acceleration FOCUS axial runout axial acceleration

Figure 1. Functional descriptionof a magnetooptical disk drive and various signalsused in testing

708 Magneto-Optical Data Recording A detailed drawing of the head optics in a generic disk tester is shown in Fig. 2.['1 Light from a laser diode (A) is collimatedby the lens (B). The laser beam is polarized in the plane of the drawing. The beam shape is made circular by .the prism (C) and passes through beamsplitter (D). The collimated and c i r c u l d light is transmitted through another beamsplitter (E) and is focused to the recording layer by an objective lens (F). The focusing light is transmitted through the disk substrate (G) of the optical disk before it reaches the recording layer. The reflected light from the recording layer, which is recollimated by the objective lens, contains both servo and data information. The data information is reflected by the beamsplitter (E) and is processed by the polarization components (H, I, and J) onto the split detectors (K, and KJ. The sum signal is from the two detectors is labeled Channel 1 and yields signals from preformatteddata. The difference signal i,, is labeled Channel 2 and yields signals from magnetooptic data. Some light is transmitted through beamsplitter (E) that is then reflected off the beamsplitter @) and is usedfor trackmg signal detection with split detector K3. The focus servo scheme is not shown. This chapter concentrates on several optical and mechanical testing procedures. In Sec. 2, we discuss the influence of testing conditions on the testing results. This includes optical parameters like numerical aperture, wavelength, polarization, overfill of the objective lens and aberrations. Other factors, such as read-channel bandwidth and laser noise, are also considered. In Sec. 3 we describe mechanical tests that relate to actuators and disk imperfections. In Sec. 4 we discuss optical tests (both static and dynamic) that relate to readout signal quality. In Sec. 5 we include a short section on testing preformatted information such as sector marks and grooves. In Sec. 6 we describe recording function tests that include narrowband carrier-to-noise ratio (CNR), cross-talk ratio, wide-band carrier-tonoise ratio, eye patterns, jitter, byte error rate (BER), and defect mapping. In Sec. 7 we briefly discuss standards. Finally, in Sec. 8 we discuss testing issues with next generation media.

2.0

INnUENCE OF TESTING CONDITIONS ON TEST RESULTS

An important consideration when dynamicallytesting optical media is the performance of the optomechanicalsystem used to write, read, and erase

L

.Y

.U

- -

_a

hc

4

xi

.... ....... ..................

.................

E

Testing 709

710 Magneto-Optical Data Recording data. Inappropriate system parameters can affect spot size, noise levels or writing characteristics. An increase in the spot size reduces contrast for high-fiequency data signals. Increased noise levels can arise from many sources, such as the laser diode or electronic circuitry. Reduced signal contrast or increased noise levels degrade the carrier-to-noise ratio (CNR) during readout. Other parameters affect writing characteristics, which can increasejitter. The following paragraphs describe a few system parameters and why they are important.

2.1 Definitions Limiting aperture: The limiting aperture in the optical system is the surfacethat defines the maximumextent of the light beam passed through to the objective lens. The limiting aperture is more commonly referred to as the stop of the system. Overfill: Overfill refers to the distribution of light energy that illuminatesthe limiting aperture. Overfill is usually specified in terms of the ratio between the width of an illuminating Gaussian beam and the diameter of the aperture. Polarization: Polarization is the orientation of the electric field vibrations of a light beam. Polarization is oriented in a plane perpendicular to the direction of any light ray traveling through the optical system. Aberrations: Aberrations refer to imperfectionsin the light beam that cause a redistribution of the light energy in the focused spot. Aberrations increase the size and shape of the focused spot, and are thus undesirable.

2.2 Numerical Aperture and Wavelength The numerical aperture (NA) of an objective lens is displayed in Fig. 3. The limiting aperture defines a maximum ray angle that focuses to the disk plane. The sine of the angle that this ray makes with the optical axis in air is the NA. Due to the wave nature of light, the NA determines the focused spot size at the recording layer. A rough approximation to the physical full-width-at-half maximum (FWHM) irradiance of the focused spot is given by

Testing 711

where A is the free-space wavelength of the laser diode. For A = 0.78 pm and NA = 0.5, FWHM = 0.94 pm. When substrate material is inserted between the objective lens and the recording layer, the angle B in the coverplate reduces due to Snell's Law, but the effective wavelength also reduces, which results in a zero net change in the focused-spot FWHM. For consistent results in testing applications, it is necessary to keep the ratio of UNA within some range (usually f 2-3%) in order for the spot size not to vary significantly. The free-space wavelength, h, is also specified separately for optimization of the readout performance from the recording layer.

2.3 Overfill of the Objective Lens Figure 3 displaysthe irradianceprofile of the laser beam incident onto the limiting aperture as a Gaussian shape, which is typical for laser diode sources. The hll-width-at-l/e2 (FWl/e2) irradiance of the beam is cu The focused spot size is a function of the ratio WID,where D is the diameter of the limiting aperture. For the approximation in Eq.(l), w/D = 0.9. This effect is illustrated in Fig. 4, where the spot irradiance profile is shown for uniform illumination (Airy disk), a truncated Gaussian o / D = 0.9, and a Gaussian distribution without a limiting aperture. The minimum spot size is obtained with uniform illumination,which is equivalent to specifjrlng o / D )) 1. However, this condition would result in wasting a significant portion of the laser power. An optimum overfill ratio of w/D = 0.89 was defined by HaskaU21 in order to give maximum peak irradiance at the recording layer. A special consideration when using laser diodes is the irradiance pattern at the objective lens, which may not be circularly The focused spot will be elongated in the direction that corresponds to narrow filling in the limiting aperture. If the spot is longer in the direction parallel to the track, signal contrast will degrade. If the spot is longer in the direction perpendicular to the tracks, data from neighboring tracks will be detected, and a significant amount of unwanted cross talk results. A circularizer in the form of prism (C) in Fig. 2 is typically used to round out the irradiance pattern in the limiting aperture, but care must be used in the design because of the variations found in divergence angles of laser diodes.["]

712 Magneto-Optical Data Recording

laser beam irradiance

limiting aperture

objective recording lens layer rl

optical axis

..I.............x.B

-

overfill o/D

-

NA sin 8

Figure 3. The laser beam irradiance on the limiting aperture is often Gaussian in shape. The overfill ratio is w 0 , where w is the FW l/e*of the Gaussian and D is the diameter of the limiting aperture. The sine of the angle in air of the marginal angle through the limiting aperture is the numerical aperture (NA).

1

B

48 8

w

0

0.5

*i

A

Testing 713 2.4

Aberrations

If we consider the light beam to be composed of a collection of rays, a perfect-focus condition exists when all of the rays focused by the objective lens are coincident at the recording layer. Differences in surface shapes or thicknesses other than design values can cause the rays to deviate from their ideal paths and hence not be in register at the recording layer. The aberrated rays redistribute energy in the focused spot. Common ray errors introduced into an optical storage device include astigmatism, coma, and spherical aberrations. Each type of aberration distributes energy in the focused spot differently. Figure 5 displays several aberrated irradiance patterns.

Figure 5. Several aberrated focal spots (a)best focus with astigmatism, @) coma, and (c) a combination of spherical aberrations.

The readout signal is distorted due to the redistribution of energy in the focused spot. Astigmatism manifests itself as an elongation of the spot in the direction of the aberration. It is similar to the condition of underfilling the limiting aperture, except that a slight adjustment of the recording-layerto-objective-lens focus distance can reduce the eccentricity of the spot. Even at the best focus position shown in Fig. 5(a), the spot profile is slightly elongated in two directions near its base. The FWHM of the aberrated best-

714 Magneto-Optical Data Recording focus spot profile is nearly equal to that of the unaberrated spot profile, but the peak power is reduced and some energy is redistributed into a broader base of the spot ~attern.[~l If a focus distance that is too large is used to compensate the astigmatism, the eccentricity of the spcrt reverses. The presence of astigmatism in the optical system can be observed experimentally. The procedure consists of varying focus offset while observing the open-loop tracking error signal (TES)and the data signal. As focus offset is varied, the TES and the data signal will not be maximizedat the same setting if astigmatism is present. This effect is displayed in Fig. 6 for a simulated optical system. Some mount of astigmatism is unavoidable due to birefiingence of plastic

(a)

Data Track

Not Compensated

(4 Compensated

(c) Too much Compensation

...

11111111111111111111llllllllll

Figure 6. A profile of the optical spot, the TES and the data signal for astigmatism in the direction perpendicular to the tracks that is compensated by various amounts of defocus. (a) Not compensated, (b) properly compensated, and (c) too much compensation.

Introduction of coma into the system is most commonly due to tilt of the substrate, but it can be caused by misalignments of the optical components. The most important feature of the spot profile in the presence of coma is that the leading and trailing edges of the spot have different slopes. The leadingedge ripple shown in Fig. 5(b) is present only for large amounts on coma. The coma can be oriented in any direction, which depends on the source of the aberration. If the coma is aligned in the direction parallel to the

Testing 715 tracks, pulses in low-frequency data signals (which correspond to recorded marks) exhibit differing leading-edge and trailing-edge responses. For highfrequency data signals, the pulse amplitude is reduced. The presence of spherical aberration is an unavoidable consequence of focusing a high NA beam through the substrate material. The objective lens is designed to compensate for the spherical aberration produced by the nominal substrate thickness and index of refraction, but variations in these parameters due to normal production practices can lead to small amounts of spherical aberration during testing. As shown in Fig. 5(c), spherical aberration causes redistribution of the energy within the central lobe to concentric rings surrounding the central lobe. At the best focus position, the FWHM of the central lobe is nearly equal to the FWHM of the unaberrated spot profile for relatively large amounts of spherical aberration. The effect of spherical aberration on low-frequency data signals is a reduction in the slope of the pulse edges. For high-frequency data signals, the pulse amplitude is reduced. The best way to determine the amount of aberration in the optical system is to measure it with a phase-shifting interferometer.['] The measurement must be performed in several steps in order to isolate different parts of the optical system. In the first step, the optical path from the laser diode to the objective lens is tested with the objective lens removed, as shown in Fig. 7 at measurement plane A. The most common aberration found in this path is astigmatism from improper alignment of the laser diode.[*] For the second step, a highquality mirror is placed in plane A to reflect light back into the optical system. A measurement is taken in plane B, which determines the amount of astigmatism introduced by the beam splitter in reflection. The mirror is then removed. For the third step, the objective lens should be tested separately with a laser beam of good quality. For the fourth test, the objective lens is replaced and the measurement is repeated at plane B. The focus loop is closed so that the spot is well-focused onto the recording layer. The spherical aberration introduced by the coverplate may now be determined by subtracting the results of the first, second, and third step from the fourth step. Odd aberrations, like coma can't be measured accurately with these measurements, but it is assumed that the coma is minimized by proper alignment of the disk. In this way, tester optics can be characterized sufficiently well to predict their performance with diffraction modeling

716 Magneto-Optical Data Recording

Laser Diode

0

Collimating Lens

Measurement Plane B

Figure 7. A section of the optical head assembly shown with different measurement planes that are used when measuring aberrations with an interferometer.

A very usehl method for testing the significance of various aberrations in the servo circuits is to use both the focus and tracking error signals to form a Schell pattern[lo1in which the TES is plotted as a function of the FES as the disk is moved in and out of focus and across tracks (i.e., all servo loops open). The resulting pattern is a superposition of Lissajous figures. Schell pictures can be generated experimentally by supplying the FES and TES to an oscilloscope in x-y mode. Certain wavefront aberrations and servo detector misalignments produce characteristic features in the Schell picture. For example, Fig. 8a shows the Schell picture for no aberrationsand Fig. 8b shows the Schell picture for onequarter wave of coma.

2.5 Read Channel Bandwidth The bandwidth of the readout channel is an important consideration in byte error rate (BER) measurements. Inadequate bandwidth can cause unnecessary errors. This is true of both the cutsn frequency and the cutoff frequency. Although the specificbandwidth for any application depends on the channel design and the coding scheme,rl1la general rule of thumb is that the read channel bandwidth should be set to

Testing 717

where T, is the width of the timing window. For example, with 2,7 PWM data at a radius of 25 mm and a disk spinning at 3600 rpm, T, = 28 nsec and BW= 12.5 M H z . For PPM data Eq. (2) must be multiplied by a factor of two. Track I n9 Signal

Tr ackiog Signal

t

60%

60%

40 " Focus

t

4p-" Focus

Signal

Sigoal

t

t

Figure 8. Lissajous patterns showing (a) unaberratedsystem and (b) a system with coma.

2.6

Polarization of Read Beam

The detection mechanism for magneto-optical recording is based on determining small differences in the state of polarization of the beam reflected from the recording layer, so the polarization characteristics of the optical system must be characterized. The popular detection method is diftkmtial detection,as explained in Chs. 2 and 9. Beam splitters E and J in Fig. 2 are designed for differential detection. Their polarization properties are specified in terms of the reflectance of s and p polarization states, as shown in Fig. 9. The light from the laser illumination optics is specified to have an extinction ratio of PJPP < 0.01, where P, and Pp are the optical powers (in watts) of the s and p polarization states, respectively. Beam splitter E is specified so that R, > 0.90 and Rp 0.30, where R, and Rp are the s andp reflectivities,respectively. Beam splitkrJ is specified sothatR,/Rp> 100. The recording layer has a bulk reflectivity of R 0.20 and K = tan 8, is the tangent of the Ken angle. Because the extinction ratio of the light from the

-

-

718 Magneto-Optical Data Recording laser illumination optics is not zero, some light from the laser can leak through to the data detection optics. The leak light reduces the contrast in the data signal after differential detection, and it can introduce additional noise in the system. With the specifications listed above, the maximum leakage current is about 15% of the data signal current. From Laser

I

J

EL* ,100 Ro

Figure 9. Portion of the optical head showing reflectivity requirements on various polarization components.

2.7 Spindle Runout Two types of spindle errors are repeatable runout and nonrepeatable runout. Repeatable runout is caused by incorrect placement of the hub on the disk, incorrect centering of the hub onto the spindle chuck or slight misalignment of the spindle motor shaft. Nonrepeatablerunout is caused by motor bearing noise or vibration between the disk and optical head. When testing blank media, i.e. media with no pregroove information,nonrepeatable runout is a problem because, for each revolution of the disk, the optical beam may not be in the same place as the previous revolution. This causes a modulation of the data signal as the beam moves on or off the previously written track. Normally, an air or fluid bearing spindle motor is used to eliminate nonrepeatable runout by reducing motor bearing noise.

Testing 719 2.8

Laser Noise

The effects of laser noise are described in Ch. 9. With proper highfrequency modulation of the laser diode during readout, laser noise can be reduced to an insignificant level.[l31 In dynamic testing applications, an estimate of the laser noise can be found with the following procedure. First, stop the disk from rotating and, with the data detectors blocked, record the electronic noise floor. Then, turn on the laser and lock focus while the disk remains stationary. The power level at the recording layer is set to the same value as when reading data. The high frequency modulator is turned on. Record the noise floor again. This is the laser + shot noise + electronicnoise floor. Usually, the electronic noise floor is more than 6 dB below the combined noise floor. The shot noise can be calculated separately from the DC current in each detector, as described in Ch. 9. The shot noise and electronic noise can then be subtracted from the combined noise floor to yield laser noise. In experiments where the read power is the independent variable, the noise values are usually normalized by the read power. A way to characterize laser noise separately from disk noise is to obtain data from the auxiliary power detector, as shown in Fig. 10. First block the light on the detector to obtain the electronic noise floor, then unblock the detector with the focus locked and the disk not spinning. The resulting noise floor is an indication of laser noise, and it is especially usefbl in order to determining the source of structure in the noise floor.

Laser Diode

Auxiliary Detector

0

To disk

Figure 10. Portion of the optical head showing placement of the auxiliary detector for laser noise measurements.

720 Magneto-Optical Data Recording 2.9 Relative Intensity Noise Relative intensity noise (FUN) usually refers to measurement of laser noise in a narrow bandwidth due to feedback of light into the laser cavity.[14] The fixxibackis due to transmissionof the leaky beamsplitter (E) in Fig. 2 after light reflects from the recording layer. RIN is given by

where Pnis the noise power, P, is the DC signal power when the laser is operating at normal playback levels, and @is the bandwidth of the noise measurement. Usually d f = 30 kHz. If P,,and P, are measured on a spectrum analyzer, the dBm units must be converted into watts incident on the detector. Usually, RIN noise is measured around 5 M H Z .

2.10 Other Considerations Several other factors must be considered in the setup of testing experiments. For example, the method of detecting data will affect the bottom-line byte error rate. Differentiator topologies, threshold settings, zero-crossing detectors, automatic gain control, etc. can be implemented in a variety of ways, resulting in varying degrees of robust data recovery. The servo system bandwidths are also extremely important, especially at high rotation speeds. Lack of focus servo bandwidth will cause amplitude modulation of the data. Lack of tracking servo bandwidth will cause track jump and track acquisition problems and increase cross talk. Still another consideration is that poor write pulse rise/fall time can introduce uncertainties in the length of the marks on the medium. Advanced techniques apply write pulse premmpensation (usually increase write power at the front end of the pulse) to insure correct length of written Slow rise times will spread the readback pulse for PPM data and put the edges in the wrong place for PWM data due to incorrectly recorded mark lengths.

Testing 721 3.0 MECHANICAL TESTS Mechanical characteristics of the medium determine how much error the servo system is required to compensate in order to maintain adequate focus and tracking. Total runout, acceleration and tilt of the substrate are three parameters of concern. Total runout dictates the required dynamic range of the actuators, or how far they must travel. Acceleration determines servo system bandwidth. Tilt affects the shape of the optical spot on the recording layer by introducing coma.

3.1

Definitions

Disk referenceplane: The disk reference plane is the plane perpendicular to the optical axis of the objective lens in which the recording layer would rotate in the absence of any mechanical errors, i.e., perfect spindle mounting and a perfectly flat disk. Axial runout: Axial runout is the deviation of any point on the recording layer from its nominal position, in a direction normal to the disk reference plane which is measured as the disk is spinning. Radial runout: Radial runout is the difference between the maximum and minimum radial distances of any track from the reference axis of rotation, measured along a fixed radial line as the disk is spinning in the disk reference plane. Radial acceleration: Radial acceleration is the acceleration of any track along a fixed radial line, measured within specified bandwidth while the disk is spinning at a specified rate. Tilt: Tilt is the angle between the normal to the top surface of the disk and the normal to the disk reference plane. Error signal: An error signal is an electrical voltage that is proportional to the distance between the actual location of the focused spot and its ideal location. The focus error signal (FES) defines the axial direction, and the tracking error signal (TES) defines the radial direction. A more complete explanation of TES and FES is presented in Ch. 3. ResonancejFequency: The resonance frequency refers to the primary mechanical resonance of the actuator, which is usually between 30 Hz and 45 Hz. (A more complete description is presented in Ch. 3). Actuator transferjimction: The actuator transfer function describes the relationship between the displacement of the actuator and the frequency

722 Magneto-Optical Data Recording of a constant amplitude driving current. (A more complete description is presented in Ch. 3). Open-loop transferfinction: The open-loop transfer function is the relationship between the displacementof the focused spot and the frequency of a constant amplitude driving voltage applied to the focus actuator. It is different thanthe actuatortransfer function because it includes the effects of electronic gain and conditioning on the input voltage signal. (A more complete description is given in Ch. 3). Servo bandwidth: The servo bandwidth is defined as the frequency at which the open-loop transfer function crosses the point of unity gain. It is also the point where the closed-loop transfer function begins rolling off. Typical servo bandwidths are 2 W to 4 W . (A more complete description is given in Ch. 3).

3.2 Testing Techniques for Displacement All displacement measuring techniques rely on the fact that the servo systems of any optical tester are designed to keep a focused beam on a

particular track. This gives a built-in way to measure mechanical characteristics without designing special test equipment. Actuator Signal Method. It is possible to calculate both runout and acceleration of the medium surface by examining the current through the focus and tracking actuators. Since most actuators are a mass or springmass system, the current is proportional to the displacement below the resonance fiequency and proportional to the acceleration above the resonance frequency. If the current waveform is filtered by the polynomial that describes the actuator transfer function, the result is proportional to displacement. While this technique is relatively easy to implement, as shown in Fig. 11, it has the disadvantage that the actuator transfer function must be well known and constant. The latter is not always the case, as the transfer function can vary with respect to the orientation of the other axis. For example, the tracking actuator transfer fbnction may change depending on where the lens is within its focus travel range, and visa versa. A second disadvantage is that runout and acceleration are only measured within the servo bandwidth. Displacements above the servo bandwidth are not followed by the actuator, and they are not accurately reflected in the actuator current.

coil drive current residual error signal

actuator

positii

signal

position displacemen sensor 4 method

1

displacement error signal methorl

(s+ C)(s + A) s (s+B) s2+2zoos+o~ servo open loop transfer function

1-

actuatortransfer function

Figure 11. Block diagram of various techniques far measufing displacement.

displacement

724 Magneto-Optical Data Recording Error Signal Method. The error signals are used by the servo systemto drive the actuator. As the focus error signal begins to deviate from zero, which indicates that the recording sufice is moving away from the focal point of the beam, the actuator is driven by the error signal to move in the direction that forces the error signal back toward zero. If the open-loop servo transfer functions are known, the error signals can be used to measure the deflectionof the recording surface from the ideal in-focus position. As the disk spins, the error signal is passed through the polynomial that describes the open-loop transfer function, as shown in Fig. 11, and then added to result in an estimate of displacement. The summed signal accurately represents the displacement if the polynomial coefficients are accurate. If an integrator is present in the servo loop, it is necessary to initialize it prior to making the measurement. The disadvantage of this technique is that the open-loop servo has tremendous gain within the servo bandwidth, particularly at low frequencies. Gains of 60 dB (1000) at 30 Hz are common, which can mean that a displacement of 100 pm results in an error signal of 0.1 pm when the loop is closed. Multiplying the error by 1000 can yield inaccuracies due to noise problems. The large low frequency gains can also yield offset problems, so AC coupling the error signal with a very low cutoff frequency is usually required. Position Sensor Method. This technique requires a specially instrumented optical pickup, but it is the most accurate method for measuring deflection of the recording surface. The pickup is modified so that sensors detect positions of the tracking and focus actuators with respect to the body of the head. Thus, as the lens follows the recording surface, the position sensor will give a signal proportional to the lens movement in each axis, and thus the disk movement. The advantage is that neither the actuator transfer function nor the open-loop servo transfer h c t i o n need to be known. Normally, each signal is combined with its respective error signal to give a wide-band indication of recording surface displacement, since the position error signal is only good within the servo bandwidth (the lens will not follow the disk outside of the servo bandwidth). A gain constant K1 is used to equalize the gains of the position error signal and the residual error signal before they are combined, as shown in Fig. 11. The major disadvantage of this technique is the special instrumentation required. While many pickups are made with a radial position sensor (useful during a fast seek in an optical drive), it is difficult to also incorporate a vertical sensor.

Te.dng 725 3.3

Testing Techniques for Acceleration

Any of the above displaccment signals may be doubly differentiated to yicid acceleration. As with any differentiated signal, excessive noise can result. The morc usual techque is to use the actuator signal prior to filtering, since it is nominally proportional to acceleration above the resonancc frequency. In fact, frqucncics below the resonance frequency arc not of interest, since it can bc shown that the disk must significantly violate the displacement spccification before the acceleration specifications are violated at normal rotational froqucncies and below. The resonance frequency and rotational frequency are tyically within a factor of two or lcss from each other.

3.4

Open Loop Techniques

The radial runout of thc disk can be rneasurcd by opening the tracking loop and counting the track crossings during a disk rotation. The focus loop remains locked. The track crossings appear as S-shaped curves in the TES, as shown in Fig. 12. The total radial runout is the number of crossings divided by two times the track pitch.

Figure 12. Opcn-loop trackirlg error signal. showing track crossings.

726 Magnetcl-Optical Data Recording It is theoretically possible to measure axial runout by turning on the laser, moving the lcns so that thc focal point is in the nominal center of the media, and measuring the dcviation ofthc FES. This is usually impractical, bccause the dynarmc range of the FES is typically 20 vrn or less, and nodinar, as shown in Fig. 13. Therefore, only extremely flat media, such as disks that are glass substrates, are amenable to this technique.

Figure 13. --loop

3.5

fwus m o r signal, showing the fmus S-curve

Tilt

Substrate tilt specified by angle a is typically calculated using the measured disk axial acceleration aM and the known disk medium radial velwicity v a t the point of measurement. Given zero initial displacement and axial disk surface velocity, the instantaneous tilt as a function of time is given by

Eq. (4)

Testing 727 It is difficult to perform the integration of Eq. 4 due to the unknown initial conditions and electrical implementation. A more useful parameter is the change in tilt A a over a specified time interval At, which is given by

Eq.( 5 ) Usually the time interval A t = t2 - t, is on the order of millisecondsto indicate the change in tilt over a sector length. If the acceleration is slowly varying over this time interval, a good approximation to da is

The calculation in Eq. (6) does not take into account static tilt, which is the angle between the optical beam and the normal to recording layer due to misalignment of the spindle motor or optical head. Static tilt can be measured and removed by using an autocollimator to determinethe tilt of a reference surface on the optical pickup, or better yet the beam from the optical pickup itself.

3.6

Calibration

The tester must be calibrated by finding the transfer functions of the actuators. To obtain the transfer function of the focus actuators (amps/ meter) at DC, the lens is focused and locked onto a track. The disk is not spinning. The optical head is then moved with respect to the media by a known number of microns, and the resulting current is noted. To obtain the dynamic transfer function of the actuators, a two-channel spectrum analyzer can be used. With channel A monitoring the error signal and channel B monitoring the current, a swept sinusoid from the tracking generator is injected into the actuator current. The resulting AfB signal is the open-loop transfer function of the actuator. To obtain the transfer function of the focus error signal (volts/mcter), the laser is turned on but not focused. By moving the lens or entire head assembly by a known amount from the center of focus, the error signal can be measured and the transfer fknction determined.

728 Magneto-Optical Data Recording All of the above techniqueswork best on a nonspinning disk with pregrooves. On some write-once media this is difficult because the material ablates when read power levels are applied to stationary media. An alternative is to spin the disk and measure the DC component of the focus current. To obtain the transfer fhnction of the tracking error signal (volts/meter), the beam is focused on pregrmved media, and the disk is spun up. A series of Scurves will be present due to the slight eccentricity of the disk with respect to the hub. The peak-peak amplitude of the Scurve is one-half of the track pitch, since one complete S-curve represents a track crossing. The normal technique is to take half the peak-peak amplitude and equate it to one-sixth of the track pitch.

4.0

OPTICAL TESTS

Optical tests are divided into two categories. The first category concerns static testing of disk substrates. The substrate must be of the proper thickness and index of refraction in order to limit the effects of aberrations in the focused spot. Also, substratebirefringencemust be below some critical value so that imbalance does not occur in the data signal. The second category concerns dynamic testing of disks. The dynamic tests described in this section involve basic properties of the data signal amplitude.

4.1 Definitions Index of re#action: Index of refraction is the ratio of the speed of light in the substrateto the speed of light in a vacuum. Higher index materials cause the light ray direction to be deviated more strongly at interfaces where the incident ray direction is not normal to the plane of the interface. Birefringence: Birefringence is the variation of the index of refraction in a substratethat depends on the orientation of the polarization and the direction of propagation through the substrate. Horizontal or lateral birefringence refers to differences between the indices of refraction for two directions of light polarization in the plane of the disk, for example at 0"and 90° with respect to a radial reference line. Vertical birefringence refers to the difference between the index of refraction for light polarized in the plane of the disk and light polarized in the focus direction. (A more complete description of birefringence is given in Ch. 10).

Testing 729

Reflectance: Reflectance is the ratio of the reflected power in a light beam to the incident power in the beam. Kerr signal: The Kerr signal definition used in this chapter is the signalamplitudeofthe analog data signal as measured &r the photodetection circuitry. Optical transfer@nction: The optical transfer function is the relative Kerr signal amplitude versus frequency of the data pattern. 4.2

Static Tests

Index of Refraction. The index of refraction of a disk substratemust be between specified limits in order for the objective lens to provide a difEadon-limitedspot through the substrateon the recording layer. Spherical aberration is introduced when an improper index or thickness is used. Since the amount of aberration is a complicated fhction of both index and thickness, an acceptable range of both is specified in optical disk standards. Figure 14 shows the acceptable range of index of refraction and substrate thickness for most disk specifications.

1.46

1.48

1.5

1.52

1.54

1.56

1.58

1.6

index.ofrefraction Figure 14. Acceptable ranges of index of refraction and substrate thickness for typical optical disks.

730 Magneto-Optical Data Recording Many index of refraction measurement systems require that the sample be of a certain size and shape.[l61This complicates measurement of plastic optical disk substrates because the index of refraction can vary depending on how the plastic is formed. Critical-angle refractometers use only one surface of the optical disk, and they require no special modifications. The measurement accuracy of the Abbe critical-angle refractometer is better than An = 0.00001. However, critical-angle refractometers only measure the index of refraction close to the surface of the disk, which may be different than the bulk index. A way to test bulk index of optical disk substrates is to use an oil-immersion technique. The disk is placed in a liquid whose index of refraction is a strong function of temperature. The index of the disk must not vary appreciably over the temperature measurement range. As the temperature changes, the index of the liquid becomes closer to the index of the disk. When the index of the liquid is equal to the index of the disk, the liquiddisk interface vanishes. By knowing the relationship between index of the liquid and temperature, the disk index can accurately be determined. Birefringence. (This section contributed by Dr. Hong Fu, University of Arizona, Tucson, Arizona 8572 1.) The magneto-optical (MO) recording disk usually has an injection-molded polycarbonate (PC) plastic substrate. Because the molding process introduces preferential molecular orientations and stress into the plastic, the substrate commonly exhibits birefringence. Undesirable phase shifts due to the birefringence degrade the performance of the servo channels as well as that of the MO readout signal. Therefore, the substrate birefringence is an important characteristic for MO recording. One way to measure the birefringence is with ellipsometry, which is an optical measurement technique in which the change of the state of polarization of light upon reflection from or transmission through a surface is measured. With adequate modifications, ellipsometry has proven to be an effective tool in the characterization of magneto-optical recording substrates and active layers.[''] Figure 15 shows a variable-angle ellipsometer.['*] The He-Ne laser ( h= 632.8 nm) passes through a polarizer and a quarter-wave plate (QWP) whose fast axis is at 45" to the transmission axis of the polarizer. The beam becomes circularly polarized at this point. The second polarizer selects the linear polarization direction vpo1. to be incident upon the sample. A hemispherical glass lens is placed m optical contact (using index-matching fluid) with the grooved side of the substrate. The light enters normally into the hemisphere, goes through the substrate, and is reflected from the back

Testing 731 side of the substrate. The hemispherical lens eliminates refraction, enables a much larger propagation angle (up to 70') within the substrate, and thus increases the measurement sensitivity. It also eliminates the polarization effects of grooves. The reflected beam is usually elliptically polarized as characterized by the orientation angle ed of the major axis of the ellipse and the ellipticityE. The beam then passes the detector channel consisting of a QWP, a Wollaston prism and two detectors. By rotating the QWP and the Wollaston prism until the light is extinguished, the values of €)axis and E can determined.

Plastic Substrate

lemiphere

A14 Plate Woiiaston Prism

Y3

fP 1

Detectors

Figure 15. The system setup for the variable-angle ellipsometer with

v = 632.8 tun.

The measurement of birefringence usually needs two steps. First, the ellipsometer is used to measure the values of eds and E. To achieve high accuracy and obtain complete information, it is valuable to perform measurements with different ypl: different polar angles of incidence ahC and different azimuthal angles of mcidence OhC(i.e., angle between the projection of the incident beam onto the substrate and the radial direction of the

732 Magneboptical Data Recording substrate). Specifically, E as a function of yPl at normal incidence is s E as important for determining the in-plane birefringence An,,,and O ~ and functionsof Oh at Oh are necessary for determining the orientationsof the ellipsoid of birefringence (EOB) and the variation of birefringence parameters through substratethickness. The second step is to fit the measured O&s and E with theoretical solutions for given sample, thus determining the unknown parameters. A computer program MULTILAYER which solves the Maxwell’s equations for a plane-wave propagating in a multilayer structure has been developed for this The ellipsometer has been applied to measure various aspects of substrate birefringence. It is found that Anll for most PC substrates is less than 2 x lo”, and is a decreasing function of the distance from the disk center. The vertical birefringence Anl is around 6 x lO4.[l3] The principal axes of EOB usually exhibit deviations (for example, loo) around the radial, track and perpendicular directions of the substrate, and the birefringence parameters vary through the thickness of the substrate. These deviations have significant effect on the polarization state of beam incident at large O;1c.[201The wavelength dependence of the birefringence has also been In atypical case, Anll goes from 1.7 x loq5to 1.2 x lom5 and Anl drops from 7.5 x lo4 to 5.7 x lo4 in the wavelength range from 360 nm to 860 nm. Reflectance. The optical power level at the detectors is directly proportional to the power incident on the disk and the reflectivity of the recording surface. Since the detector amplifiers must be designed within some range of gain values, the reflectance of the recording surface is standardized and must therefore be tested. The tester is first calibrated with samples of known reflectivity. The samples consist of flat transparent coverglass material that are the proper thickness to minimize errors due to aberrations. At least two samples are required. One sample is a low-reflectance reference, which can be an uncoated coverglass. The second sample is a high-reflectance reference, which can be made by coating a coverglass on one side with Al. The reflection properties of the samples should be determined before the measurement with standard techniques. The calibration procedure consists of placing the samples in the objective lens focus so that the converging beam shines through the uncoated surface and focuses on the reference surface. The sample need not rotate, but the focus servo should be locked during testing. The optical power of the reflected beam is measured at the data or servo detector for each sample. Since the detector current changes linearly with recording

Testing 733 layer reflectance, a graph like that shown in Fig. 16 describes the relationship between detector current and reflectivity, where R, and RH are previously determined values of low and high sample reflectivity, respectively; and iL and iHare the correspondingvalues of low and high detector current, respectively. For measurement of an unknown disk sample, the detector current i, is measured and plotted on the graph in Fig. 16 to find the measured disk reflectivity RM The disk may be either stationary or rotating, but the measurement with the disk rotating will generally yield a lower value of RM due to the finite ability of the focus actuator to hold perfect focus. There is a difference in reflectivity of the recording layer between a low NA system and a high NA system due to the Fresnel reflection properties. However, with NA < 0.6, the correction factor is much less than 0.1%, which is better than the accuracy of the measurement. In both the calibration and the measurement, the detector circuitry must be designed such that there is a linear relationship between optical power and amplifier output.

'L

i

M

'H

dc detector current Figure 16. Relationship between measured detector current and sample reflectance. RH, and iL are calibration values, and iM is a measured value of an unknom sample.

RL, i,,

Kerr Rotation and Ellipticity. The state of polarization reflected from a magnetosptical medium is shown in Fig. 17 when linearly polarized light along the x axis is incident onto the medium. The angle 0,is the Kerr

734 Magneto-Optical Data Recording rotation angle, ,E is the ellipticity, and Irk[and IrJ are reflectivity coefficients. The signal amplitude from a magneto-optical recording layer is directly proportional to OK,which is a function of the material properties and thin-film structure of the disk. Some amount of ellipticity E, is also introduced, which reduces the signal There are several ~ ] instruments have methods to experimentallydetermine 0,and E ~ . [ Some been designed especially for magneto-optical materials, such as the wavelengthdependence measurement of @, and &K.[241 Other specialty instruments can measure the angular dependence of the recording

Figure 17. The state of polarization resulting from an x-polarized laser beam reflecting off of a magnetooptic medium.

4.3

Dynamic Tests

Kerr Signal Figure of Merit. The electrical signal pulse amplitude V that results when reading an isolated mark (e.g., a single, well-resolved reversed domain that is well-separated from its neighboring reversed domains on an MO medium) is given by the fimctional

Eq. (7)

V = f ( R sin 0,cosp)

where R is the recording layer reflectivity, 0,is the Kerr angle in radians, and p i s related to the ellipticity E by

Testing 735

The definitions of &and0, are given in Fig. 17. A figure of merit for optical disks is given by

The procedure to measure FOM on a dynamic testbed is as follows. First, the tester must be equipped with a spindle motor that has an accurate tachometer reference pulse. The reference pulse is used to gate the laser driver circuitry so that long domains can be written in the same angular position on the disk across several tracks, as shown in Fig. 18. For best results, marks are written in both lands and grooves. The marks create a low-frequency sequence of complete up and down magnetization. Dif€i-action effects from domain edges in the radial direction are insignificant. Therefore, the signal voltage obtained from the data detectors is given by

where C is a constant. The tester is calibrated by forming the mark pattern on a medium with known R, 0,and p. The calibration medium must also exhibit low birefringence, so a glass substrate is recommended. The signal measurement from the calibration disk is used to determine C. It is important for the calibration and test conditions, e.g., laser readout power, spindle velocity, etc., to be identical. The measured value of FOM from the test disk is determined from

FOM =

(3

Kerr Signal Imbalance. The analog data signal amplitude can be reduced because of birefringencein the The effects of birefiingence can be determined by measuring the imbalance of the analog data signal when reading mark patterns of opposite magnetization. Ideally, the analog data signal amplitude is the same when reading either sense of magnetization. The imbalance is not desirable because it produces additional multiplicative noise in the channel. Imbalance can be measured dynamically with a well-characterized optical head assembly.

736 Magneto-Optical Data Recording

Data Marks

Track n + 2

Track n + 1 focused laser spot

groove

+ h

Tradcn

h

Figure 18. Distribution of syncbnkdmarks used for testing Kerr signal figure of merit.

For the dynamic measurement, a radial annulus on the recording layer consisting of several tracks is first completely erased. On the center track of the annulus, a low frequency data signal is recorded. Both the sum current i, and the difference current i, (as shown in Fig. 2) are measured during readout of the center track. The ratio

is determined. The test area is then completely erased with the opposite sense of the magnetic field. Again, the center track is recorded with a lowfrequency data signal, but the sense of magnetization is opposite that used in the first measurement. The value of S, is then determined as in Eq. 12. The difference A, = S, - S, is the Kerr signal imbalance. For standards purposes, the A, measurement is referred to a standard value of beamsplitter ratios by computing

Eq. (13)

Testing 737 where R, = 0.95, Rp = 0.30, and R,' and Rp' are the measured values corresponding to beamsplitter (E)in Fig. 9. Resolution (Optical Transfer Function). The finite size of the focused spot determines the minimum resolvable feature on the disk. The highest recording frequency resolvable on the disk is called the cut08 frequency. The signal amplitude is highest when long marks and spaces are recorded. The signal amplitude gradually decreases with decreasing mark and space lengths, i.e., higher frequencies display reduced signal amplitude. The functional dependence of the electrical signal amplitude versus recording frequency is called the optical transferhnction. The shape of the optical transfer function depends greatly on the specific optical system used to read out data. High NA systems (NA > 0.5) have higher cutoff frequenciesthan low NA systems (NA < 0.5). Aberrations can dramatically affect the shape of the optical transfer function, but they do not affect the cutoff frequency. For example, Fig. 19 displays simulated optical transfer functions for unaberrated systems with NA = 0.4 (Fig. 19a) and NA = 0.5 (Fig. 19b)and an aberrated NA = 0.5 system with a small amount of defwus (Fig. 19c). A laser wavelength of 0.83 pm was used to compute Fig. 19. The cutoff frequency for the NA = 0.4 system is 5.9 MHz, while for the NA = 0.5 system the cutoff frequency is 7.8 MHZ. Note that defwus aberration significantly decreases the optical transfer hction.

0

1

2

3 4 5 Mark Fresuency (Mk)

6

7

Figure 19. Simulated optical transfer functions for (a) unaberrated NA = 0.4, (b) unaberrated NA = 0.5, and (c) defocused NA = 0.5.

738 Magneto-Optical Data Recording There are several ways to measure the optical transfer hnction. A straightforward method is to record the signal level from spectrum analyzer traces of single-tone data recorded at a series of frequencies from low frequency to high frequency. The data are converted from dBm to signal power with the procedure outlined in the next section. Spectrum analyzer data are useful because data are averaged over a significantlength of track. A second method for measuring the optical transfer function is to observe the timedomain difference current i, of Fig. 2 on an oscilloscope and measure the peak-to-peak amplitude for a series of single tones from low frequency to high frequency. The signal amplitude is a h c t i o n of the mark and space lengths on the recorded track. Since the inner-diameter tracks have lower linear velocity (on constant angular velocity CAV systems) than the outer-diameter tracks, the mark and space lengths are shorter on the inner-diameter tracks for any given data frequency. Therefore, the effects of the optical transfer function are more pronounced on innerdiameter tracks. It is common practice to specifjl the ratio of the value of the optical transfer function at the highest user data frequency, e.g., 3.7 MHz, to the value of the optical transfer function at a lower data frequency, e.g., 1.4 MHz. As observed in Fig. 19, the ratio depends greatly on the type and quality of the optical system used to read back the data.

5.0 PRERECORDED CHARACTERISTICS TESTS

Part of the standard informationon a track is a sequenceof prefonnatted pits (e.g., the sector headers) that form the beginning of each sector. A detailed description of each information field in the prefonnatted area is given in Ch. 4. The electrical signals that correspond to the information in the sector header are derived from the is detector shown in Fig. 2 as the sector headers are read. Due to design requirements for the associated electronic circuitry, the is detector signal is specified to exhibit certain contrast levels and must therefore be tested. Testing of the characteristics of the prerecorded marks that correspond to the sector headers are not as demanding as for data fields because only signal amplitudes and offsets are examined. As shown in Fig. 20, different areas of the prerecorded sector header will have different signal

Testing 739 amplitudes and offsets. Usual specifications require the calculation of the signal contrast, which is defined by

Eq. (14) where I,, is the signal modulation peak-to-peak amplitude and I,, is the maximum signal amplitude referenced from the zero level in area i.t The

contrast SC, will vary depending on the orientation of the polarized light with respect to the grooves. Therefore, SC, is measured with polarization both parallel and perpendicular to the grooves. ODF

71DrnW*-~~-.'lArea 1

1

..............

I'

It-,I

Area2

I-11-1 ...................

Area3

0 Level

f-+

1/71

T

Figure 20. Prerecorded signal characteristics from the sum channel i,.

A very usefhl area of each prerecorded sector header is the offset detection flag (ODF) area. This field contains no prerecorded marks and no tracking grooves in the region adjacent to the track. Thus, the ODF fields are relatively large, featureless (without any marks or grooves) radial spokes on the disk. The ODF area is most commonly used to null electronic and optical trackmg oikts. Because tracking information from the pregrmves is absent at this point, other data,such as medium reflectivity,are also measured signal is shown in Fig. 20. accurately in the ODF area. The IoDF ?Note that this definition of contrast is different from other definitionscommonly used in optical testing.

740 Magnet~Opfical Data Recording

Similar m i n g requirements apply to the TES,which is derived from the split detector signals I , - I, as shown in Fig. 2 and Fig. 2 1. The relevant tune-domaiasignal definitions are shown in Fig. 21 with the tracking Imp unlocked. The push-pull ratio is defined by

where (Il + Iz)m is the maximum sum signal in an ungrooved area and (Il - IJpp is the. peak-to-peak diiTermce signal from the p h o t d e t m r with the focus servo locked and the tracking servo unlocked. Ught beam

\

*re 21. Rerecwdedsignal chamchistics from the grooves when the m b g loop is unlocked. Thc tracking signals are generated by the split photodiode.

6.0

RECORDING FUNCTION TESTS

Several tests can be performed to characterize the khavior of disks under operating coorlltions. These tests include narrow-band carrier-tunoiseratio (CNR),cross talk ratio, wide-band carrier-to-noise ratio, eye patterns, jitter, byte error rate (BER),and defect mapping. The foHoWing

Testing 741

paragraphs describe each test. To completely characterize write and erase attributes, it is necessary to perform measurements while varying read power, write power, erase power, write magnetic field, erase magnetic field, write pulse width and write frequency.

6.1

Narrow-Band Carrier-to-Noise Ratio (CNR)

Carrier-to-noise is a usefbl characteristic of a recording medium because it indirectly relates to reliability of data recovered from the storage medium. The CNR test is performed at the maximum recording frequency, fo. The spectrum analyzer used for testing should be set with a center frequency off0 and a resolution bandwidth of 30 kI-Iz. The signal and noise powers are recorded from the instrument trace as shown in Fig. 22, where So is the signal power, Nl and N2are the upper and lower noise floor values, respectively. The computed noise floor value, No, is given by No = (Nl + N . / 2 . The values of Nl and N2should be determined atfo f where Af is just beyond the signal envelope. A typical value of Af is 170 kHz. The CNR is computed by

where Soand No are dBm units. The requirement for reliable recording is that CNR > 45 dB for all Before taking CNR measurements one should verify that the electronic noise floor measured with the detectors blocked (or laser turned of€) is at least 10 dBm below the medium noise floor value N,. Also, only laser diode sources using high-frequency modulation or some other noise reduction technique should be used. Several items must be considered during the measurement. First, the mean noise floor power measured on each side of the carrier contain small statistical fluctuations that vary from measurement to measurement. To be more accurate in the reporting of test results, several noise-floor measurements should be averaged. The worst-case value should be reported as well. Secondly, optical media exhibit writing noise, which means that the noise floor on an erased track is lower than the noise floor on a written track. Therefore, the track on which the signal is recorded should be used for the noise measurement,not an unrecorded reference track. Lastly, preformatted data and grooves can affect the CNR measurement by 1 or 2 dBm. Thus, it is normal practice to gate out the preformatted area on the disk when measuring the CNR.

742 Magneto-Optical Data Recording

f 0

Frequency

Figure 22. Signal and noise as observed from a single-tone pattern spectrum analyzer trace. The values So, N , , N2 and No are used to calculate the narrow-band carrier-to-noise ratio.

After the measurement, the noise may require a correction This is due to two effects. First, the envelope detector of the spectrum analyzer may or may not measure the true root-mean-square (rms) level of noise. Secondly, the resolution bandwidth filter is not rectangular, which is the shape assumed when calculating the equivalent noise bandwidth. The factor that corrects for these effects, which usually adds about 1 dBm or 2 dBm to each noise floor sample, must be applied when older model spectrum analyzers are used for CNR measurements. Newer units factor in the correction automatically. 6.2

Cross-Talk Ratio

A small amount of the signal from adjacent tracks can usually be detected when scanning a data pattern. Most of the cross talk is due to the interaction of the light in the tails of the focused optical spot and the mark patterns on adjacent tracks. Additional cross talk can be due to residual tracking errors. The procedure for quantifymg cross talk is to first erase five adjacent tracks. Write on the center track (track n) with a mid-range frequency. For example, a good choice isf, = 2 MHz. Write on the two adjacent tracks with

Testing 743 different frequencies such that the fundamental or harmonics of the patterns do not overlap. For example, a good choice isf,-, = 1.5 M H z for one adjacent track andL+, = 2.5 M H z for the other adjacent track. Return to the centertrack and measure S,, S,, and Sn+,in dBm units as shown in Fig. 23. The cross talk ratio (CTR) is defined as the larger of

Eq. (17) or

where the Si are in dBm units. Usually, the CTR is smaller than -25 dB.

Figure 23. Signal and noise as observed from multiple-track signals used for the cross

talk measurement.

Cross talk can be severely affected by several factors. For example, CTR can be affected by the optical transfer function, so care should be taken to insure that mark lengths equal to or greater than two microns are used. Cross talk can also be affected by tilt in the radial direction due to imperfect disk mounting, which manifests itself as asymmetrical cross talk where the residual carrier level when reading track n-1 is different than when reading track n+l, that is S,-, f Sn+,.

744 Magneto-Optical Data Recording 6.3 Wide-Band Carrier-to-Noise Ratio Although narrow-band CNR is a useful parameter for storage medium characterization, it is limited to providing information over a narrow range of fiequencies. A measurement that is similar to narrow-band CNR but contains noise information over a broad bandwidth is the wide-band carrier-to-noise ratio CNR,, which is defined by

Eq. (19)

CNR,

(3

= lolog,, =

where Sois the signal power at the test frequencyfo and

is the integrated noise power between frequenciesf, andf, that are defined by the bandwidth of the data channel. In Eq. (19) and Eq.(20), So and Bare in power units, not dBm. To convert spectrum analyzer signal dI3m data to mW, simply take

and likewise for noise data. Since we integrate the noise floor we must take into account the sampling of data. The data value at the ifh sampling point is approximately

where Afis the resolution bandwidth settingon the spectrum analyzer. If the noise is sufficiently flat over AJ

Testing 745

Eq. (23) An estimate for Nis calculated by summing the Pivalues like M

Eq.(24)

i-1

i-1

YJ

where As is the frequency step between sample points. The signal must be treated differently thanthe noise because its bandwidth is small comparedto the resolution bandwidth setting on the spectrum analyzer. In most cases, the signal power is simply So as calculated directly from Eq.(21).

6.4 Eye Patterns Most channel codes consist of multiple discrete run lengths, as described in Ch. 13. In optical recording this translates into discrete distances between marks for PPM data or discrete mark lengths for PWM data. In either case, if the readback waveform corresponding to random data is displayed on an oscilloscopethat is triggered off the data waveforms, an eye puttern will result, as shown in Fig. 24 for a PPM recording of 2, 7 run-length limited data. The eye pattern is due to the fact that the oscilloscope triggers off pulse edges in the readback waveform that are generated by marks on the disk, and these marks can have any of the six discrete lengths allowed by the 2, 7 code. Each trace of the oscilloscope may show a different set of these mark lengths, but since they are discrete, the edges should always fall at quantized discrete distancesfrom the edge that triggered the oscilloscope. If enough traces (i.e., pulses due to individual marks) appear at the same time due to the persistence of the scope display phosphor, all possible mark lengths are represented, and there are dark holes between each edge. The holes are called eyes. The quality of the eye, i.e., the size or area of the eye, gives a qualitativemeasurement of the caliber of the data recovery. Sincethe ability to decode data depends on the edges falling where they are theoretically supposed to, fuzzy edges or closed (small area) eyes are bad and sharp edges or open (large area) eyes are good. An equivalentquantitative measurement is done via a time-interval analysis of the data. This measurementgenerates a histogram of the detected mark length or interval between marks by

746 Magneto-Optical Darn Recording logging (counting)the nurnkr of pulscs in thc readback waveform that havc various lengths, as shown in Fig. 25. Note that there are seven distinct pulsc (marWspace) lengths indicated in Fig 25. The six longcst correspond to thc six discrete run lengths that comprise the 2,7 RLL data. The shortcst corresponds to a spurious measurement because the sector mark was not gatedautduring the test. If the discrete pulse distributions overlap, the rcad channel will have trouble d e d i n g the data. If the distributions do not overlap, the read channel performs well.

Figure 24. Eye pattern of 2,7 PPM data. The clack channcl frequency is 14.8 M I k , and the pxid is 67.6ns. The shortest run is 200 ns.

6.5

Timing Jitter

Whilc not specified in any standard,channcl data timing jitter (hcrmfter called p e r ) is perhaps the most important recovered data reliability parameter. Jitter is essentially a measurement of how accurately the marks are written on the medium. The more accurate the placement ofthe centroid of a PPM mark (or placement of the leading and trailing edges of a P W M mark), the smaller the timing bit cell that can be used, and the higher the linear density. Jitter is usually specified in nanoseconds, and it can be determinedfrom the pulse distribution obtained via timing interval analysis (c.f., Fig. 25).

Testing 747

Figure 25. Distribution of mark lengths in PWM recorded 2,7 data. (The first hump is spurious becausc the sector mark was not gated-out during the measurement.) The clock channcl pCrid I s 67.6 ns. The smallest run i s three channel clocks or 203 ns, which is the location of the first major distribution. The longest run is eight channel clocks or 541 ns.

The most common jitter measurement for PPM encoded data is referend to the mark-to-mark spacing. A tone with a specific pulse width is written on the medium. In thrs case, jitter is defined as the standard deviation of the time correspondingto the mark spacing, and is tqpically less than 6 ns for a medium velocity of 5.6 ms-l at thc inner diameter. For PWM encoded data, a randomly encoded data pattern is written and the resulting mark lengths measured, which will fall into bins comparable to the eye patterns seen in the time domain. Here, jitter is defined as the standard deviation of mark lengths. A g o d result exhibits clear areas between each bin with no data, as shown in Fig. 25, just as the eye pattern exhibits holes between each transition. The major difficulty with this measurement is setting the pulse detection threshold. If the channcl is DCcoupld, birchngence of the disk causes an offset to bc added to thc data waveform, which vanes at a lowfiquency around the track as shown in Fig. 26. Some time-interval analyzers (llh), wtuch are used to make this measurement, can compensate for this.

Other techniques include DC restoring the signal by sampling

748 MagnebUptical Darn Recurding at the sector header ODF flag, setting the threshold by measuring the peakto-peakamplitudc of the signal and subtracting out thc DC component, and AC coupling the signal. The latter is asiest, although the optimum threshold will change with duty cycle of the data. When jitter measurements are made on prefonnatted, pregrooved media, the preformattad data areas must be gated out. If the TIA cannot sample every mark sequentially, the sample point must be moved relative to the beginning of the sector so that each sample is taken in a different place. Jitter measurements made on blank media will be slightly better than on grmved media due to noise induccd by thc grwve. Jitter measurements on blank media need to be made with an extrcmcly stable spindle motor. Typically a fluid or air bearing is required, otherwise nonrepeatable runout of the bearing will cause amplitude modulation of the waveform and degrade the results.

Figure 26. Effect orbircfringcncc in the data c h m e l . The trace shows the data envclope with one sector erased. Spindle rotation s p e d is 2400 rpm.

Testing 749 6.6

Byte Error Rate (BER)

Raw byte error rate (BER) testing is a systems-level test for any type of storage media, in that it includes effects of the particular channel code write channel pre-emphasis and read channel equalization that is implemented. In this test, data are written to the disk, then read back and compared to what was written. Nominal error rates for magneto-optic media are 1 in lo5 bytes, which is typically written as The 2,7 FUL code as implemented on optical disks is a self-clocking code. This means that the phase-locked loop (PLL) is designed to lock onto the data and generate a synchronous system clock from it. Clocking problems occur when there are missing data due to defects, so synchronization bytes are inserted into the written data to allow the PLL to recover and reset the decoder. Many times a defect will cause subsequently recorded bytes that occur prior to the next synchronization byte to be erroneous, since the channel clock and decoder may both be confused. Also, a defective synchronization byte can cause the entire next section, up to the next synchronization byte, to be erroneous. All ofthis can cause confusion when reporting byte error rate, unless the method of counting bad bytes is specified with the test. A measurement that is not clearly defined is the test for soft errors. Soft errors are nonrepeatable, and they are typically caused by poor mark definition, low CNR, or any other phenomenon that affects the overall margin of the channel. Most byte error measurements are made with retries, so that if an error is detected, that particular area will be reread. The philosophy behind this is that commercial drives follow this procedure, and since soft errors are hard to quantify it is best to eliminate them. There is some confusion between byte error rate and bit error rate. Byte error rate is based on user data bytes, which are mapped into the channel code, or series of laser pulses that are actually written to the disk. When data are retrieved, they are decoded back into user data. Therefore, quantization is at the byte level. For example, assume that hex FF was written to the disk. Reading back a hex FE (one bit in error) is no more significant than reading back a hex 00 (eight bits in error). In both cases, the channel code may have been wrong by only one bit, but in the user data domain that one bit can result in all eight bits in error. A more complete description of error is presented in Ch. 13. Channel bit error rate, on the other hand, is a measure of the defect rate on the disk. These defects may or may not translate into byte errors at

750 Magneto-Optical Data Recording the user data level. Typically, pulses at the shortest run length of the channel code (the closest together that two pulses are allowed) are written as a tone and then read back. Missing pulses are considered bit errors. 6.7

Defect Mapping

Defects can be defined as either drop-ins or drop-outs on the RF readback signal. Drop-ins are extra pulses and drop-outs are missing pulses. Drop-ins are typically measured on an erased or blank track, and drop-outs are measured after writing a pattern, usually a tone, and rereading the same pattern. Because there are two channels (is and iD) in magnetooptic recording, there are three possibilities, which are drop-ins on the sum channel and drop-ins and drop-outs on the difference channel. Drop-outs are not usually measured on the sum channel because no user data are recovered from is. Studies show that a correlation exists between drop-in defects and byte error Nearly 95% of byte errors correlate to hard drop-ins (errors that repeat at least five times). This is true for both the sum and the difference channels. Other hard drop-ins correlate to bit errors that do not produce a byte error, i.e., even though the bit sequence was contaminated with an error, it was demodulated correctly with the error correction code. These bit errors are called sop bit errors. For the study described in Ref. 28, 22% of the soft bit errors correlate to hard drop-ins in the sum channel, and 15% of the soft bit errors correlate to hard drop-ins in the difference channel. The advantage of drop-in measurements on the s u m channel is that no write or erase passes need to be made, so the measurement time per side is the number of tracks times the rotation speed. Drop-in measurements on the difference channel require an initial erase pass, so the test time is doubled. Another advantage of drop-in measurements is that no complex data detection is required. A threshold is set, and the location of any drop-in crossing the threshold is measured. Defects in the preformatted address mark information can cause an entire sector to be unreadable. In order to prevent this, the VFO synchronization clock and address marks are repeated three times. It is important when testing for media defects to read the header information and report any errors that occur.

Testing 751 7.0 STANDARDS DOCUMENTATION Because optical media are interchangeable, and one manufacturer’s disk must play in another’s drive, standards are very important. The American National Standards Institute (ANSI) has several committees dealing with media standards. The resulting ANSI standards are used as input to the International Standards Organization (ISO), which also generates media standards. The European Computer Manufacturers Association (ECMA) represents the European Community, and it has its own standards as well.

7.1 Media Standards The media standards specify many parameters that are important to both the media and drive engineer. The major part of each standard consists of the following four sections: 1. Mechanical and physical characteristics 2. Format information 3. Characteristics of the recording layer 4. Characteristics of the embossed information The first section deals with mechanical attributesof the media, as well as the media cartridge. The second section specifies attributes important to the logical formatting of the disk, such as the number of tracks, the number of sectors, the format of each sector header, error detection and correction algorithms, and defect management strategies. The third section specifies the recording layer characteristics, such as read, write and erase power, magnetic bias field, carrier-to-noiseratio, cross talk,etc. The fourth section specifies the required quality of the preformatted header information and the pregroove itself.

7.2 Test Methods The standards do not completely define the method for making a particular measurement. For example, the carrier-to-noiseratio is specified as 45 dB at a certain frequency, using a resolution bandwidth of 30 kHz. In addition, the measurement is shown on a qualitativegraph, with no numbers on either axis. It is left to the measurer to determine where and how to

752 Magneto-Optical Data Recording measure the noise, how to convert to a true 30 kHz bandwidth, and how to compensate for the preformatted areas. To mitigate the definition problem, various informative (i.e., implementation not required) annexes are appended to guide the user through some of the measurements. In addition, both ANSI and IS0 are developing Test Methods documents that standardize the way each measurement is performed to verig conformance with the media standard.

8.0 TESTING ISSUES WITH NEXT GENERATION MEDIA Next generation optical media will have higher track densities and higher linear densities. This will be accomplished by using variable (disk radius dependent) channel clocks, smaller focused spot sizes, higher rotational speeds, and more sophisticated read and write channels. In this section we discuss how test equipment and techniques must be modified to stay abreast of these rapidly evolving technologies. The rotational speed directly affects the average seek time latency and the data rate. Next-generation drives will spin at 3000 to 4800 rpm, and in fact there are already drives on the market that spin at 4800 rpm. Test systems will need wider bandwidths for both focus and tracking servos, which in turn may require more sensitive focus and tracking actuators in the optical head assembly, as well as the minimization of unwanted resonances in both actuators. There are several types of channel codes currently in use. The I S 0 standard is 2,7, while others use a 4,15 or MFM implementation. A 1,7 code is being extensively investigated by IBM and others as a possible alternative. Because of the uncertainty in the code to be used for next generation machines, the ideal test system should have some flexibility in channel code realization. The simplest technique is to let the computer controlling the tester calculate the channel code and then download it to the hardware. Another technique is to use programmable gate arrays to implement the encoder, thus allowing the hardware to be changed by changing the gate array programmable read only memory (PROM). Pulse-width modulation (PWM) effectively doubles the linear data density when compared to pulse position modulation (PPM), since both edges of the mark are used. However, data detection using P W M is more difficult, since the detection threshold is more critical, and jitter margins are

Testing 753 smaller. The test system’s read channel must adequately detect data without using too much adaptive circuitry, such as automatic gain controls and DC restoration techniques. This is always a fine line that the tester designer must be aware of, because the motivation is to test the quality of the storage medium, not the test system. One approach is to make as many parameters as possible programmable through the computer, so that the user can selectively program the characteristics of the channel. Zoned-bit recording is the process of keeping the linear data density relatively constant as one moves toward the outside of the disk. Typically this is done in discrete intervals called bands. Thus, a band of tracks will be recorded at a given channel clock frequency, while the next band out is recorded at a higher frequency, etc. Since the linear velocity of the medium increases toward the outer diameter, it is necessary to increase the channel clock proportionately to keep the same data density (markdmm). The test system must be able to accept a varying channel clock and number of sectors. Direct overwrite can be accomplished using either light intensity modulation or magnetic field modulation. Light intensity modulation requires that the system contain an initialization magnet, as well as multilevel laser driver and special media. Magnetic field modulation requires a magnet fist enough to be modulated with the channel data,as well as synchronously pulsing the laser to make well-formed marks. The ideal tester will allow the user to change the phase and pulse width of the laser modulation with respect to the data. Write precompensation shapes the write pulse (i.e., varies the laser power and time used to write a mark) to allow the best mark formation. Typically the leading part of the pulse is given a higher write power than the body of the pulse. To allow maximum flexibility, the tester’s laser driver circuit needs to allow various levels of write current (and hence write power), which can be switched between levels in a very short amount of time. Shorter laser diode wavelengths are desirable because, as shown in Eq.(l), the spot size decreases with decreasing wavelength, which permits smaller data patterns to be written. The most common lasers in use today have h = 780 nm and h = 680 nm. In the not-todistant future, most experts believe that blue lasers, with h = 420 nm will be incorporated into products. The difficulties of testing with shorter wavelengths are similar to those experienced by the optical head design engineer. For example, tolerances on mechanical movements scale with the wavelength, making optical

754 Magneto-Optical Data Recording components and actuators more precise. Thin-film coatings on sensitive components must also be readjusted to the shorter wavelengths. A third difficulty that is especially important with blue wavelengths is that the responsivity of silicon detectors used in the drive decrease by a factor of five or more from their responsivity in the near infrared. As spot sizes decreases, the track separation will also decrease. While not difficult in principle, manuhturing of reduced-pitch disks is a formidable problem. The reason for the increased difficulty is that the mastering machines used to write the disk molds are based on blue wavelengths. If the disks are used in the infrared, there is a large tolerance difference between the mastering machine and the user drive. However, when the user drive also uses a blue wavelength, there is little tolerance leeway. Most microscopic imperfections on the recording surface that contribute to disk noise are of a constant size or correlation length. When scanned with shorter wavelengths, these imperfections can induce a larger change in the signal, and, hence, increase disk noise.

REFERENCES 1. Doc.10089:1991(E), (InternationalStandards Organization/IEC),Annex A (1991) 2. Haskal, H. M., Appl Opt., 18:2143-2146 (1979) 3. Marchant, A. B., Appl. Opt., 23:670-673 (1984) 4. Milster, T. D., Benedict, M. K., Stahl,R. P., SPIE, 1316:143-149 (1990) 5. Milster, T. D., and Walker, E. P., SPIE, 1834:79-85 (1992) 6. Bernacki, B. E., and Mansuripur, M., Appl. Opt., 32:65474555 (1993) 7. Greivenkamp, J. E., and Bruning, J. H., in Optical Shop Testing, 2nd edition, @. Malacara, ed.), pp. 501-548, John Wiley, New York (1992) 8. Froehlich, F. F., Wang, M. S., and Milster, T. D., Appl. Opt., 30:44814483 (199 1) 9. Wang, M. S., and Milster, T. D., Jpn. J.Appl. Phys., 325277-5283 (1993) 10. Grove, S. L., Getreuer K. W., and Schell, D. L., SPIE, 1499:354-359 (1991) 11. Howe, D. G., SPIE, 695255-261 (1986) 12. Howe, D., private communication (Feb. 23, 1994) 13. Finkelstein, B. I., and Call, D. E., Proc. SPIE, 899:69-76 (1988)

Testing 755 14. Gage, E. C., and Beckens, S., SPIE, 13 16:199-204) 15. Mallinson, J. C.,Magnetic Recording, VolumeI: Technology, (C. D. Mee, and E. D. Daniel, eds.), p. 366, McGraw-Hill, New York (1987) 16. Goodman, D. S., Geometrical and Instrumental Optics, @. Malacara, ed.),pp. 183-190, Academic Press, San Diego (1988) 17. Mansuripur, M., Physical Principles ofMagneto-Optical Recording, Ch 5 and 6, Cambridge, University Press, London (1994) 18. Fu, H., Sugaya, S.,Erwin, J. K., and Mansuripur, M., “Measurement of Birefringencefor Optical Recording Disk Substrates,”Appl. Opt., 33: 1938 (1994) 19. Mansuripur, M., J.Appl. Phys., 67:6466 (1990) 20. Fu, H., Sugaya, S. and Mansuripur, M., “Measuring Distribution of the Ellipsoid of Birefringence through the Thickness of Optical Disk Substrates,”Appl. Opt., 335994 (1994) 21. Fu, H., Yan, Z., and Mansuripur, M., “Measurement of the Wavelength Dependence of Birefringence for Optical Disk Substrates,” Appl. Opt., 33:7406 (1993) 22. Challenger, W. A., and Rinehart, T. A., Appl. Opt., 26:3974-3980 (1987) 23. Azzam,R. M. A., and Bashara, N. M., Ellipsometry and Polarized Light, North-Holland Publishing, Amsterdam (1987) 24. Mansuripur, M., Zhou, F., and Erwin, J. K. Appl. Opt., 29:1308-1311 (1990) 25. Ruane, M., Mansuripur,M., andRosenwold,R.,Appl. Opt., 25:1946-1951 (1986) 26. Hallam, K. J., and Yamashita, K., Doc. No. X3B11/90-003-R1 American National Standard (1990) 27. Better Noise Measurements with the HP 3588A and 3589A, HewlettPackard, Inc., Application note #1213 (1989) 28. O’Reilly, J., Media Certzjkation Techniques,Optical Data StorageTesting Workshop, University of Arizona (1990)

12

Drive Packaging Marvin B. Davis

1.0

INTRODUCTION

The packaging of a magneto-optic (MO) drive is a glue that ties the efforts of several involved disciplines together into a cohesively designed product. As a result, the discussion of package design will, of necessity, include interfaces to other types of engineering activities and requirements. In this discussion we survey many of the features that must be addressed in the design of a baseplate, the cartridge loading mechanism, and the package for the drive. Other criteria are placed on the package design by regulatory agencies, international standards, and industry standards, some of which are referred to here. In addition, many patents have been issued for various mechanical implementations which need to be surveyed to assess possible infringement problems or to point out desirable license opportunities. Specific company standards, marketing inputs, manufacturing techniques, and customer desires also drive the packaging design, but are beyond the scope of this discussion, with a couple of common exceptions. First, most current drive designs use an electric eject system which allows the drive to power down in a controlled manner. Rather than giving the user the ability to interrupt a read or write sequence, logic is used to complete the activity, turn off the laser, spin down the disk, and park the camage before 756

Drive Packaging 757 eject is carried out. Due to the possibility of power failure or, in some cases, the failure of the eject mechanism in the drive, users usually require a feature that will let them eject the cartridge manually. This is usually done with a straightened paper clip, but in some cases a special tool has been provided by the manufacturer. A second user interface feature that is usually provided is an activity light on the front bezel of the drive so the user has a sense of what is being done by the drive. Both these features should be considered early in the design so the enclosure and the baseplate will have provisions for them.

2.0

FORM FACTORS

The first issue to be considered in the package design is the form factor of the drive being developed. In MO drives there are two standard sizes being driven by accepted media standards at this time. The first is the 5.25 inch and the second is the 3.25 inch. In the 5.25 inch drives, there are two sizes of product, the first being what is called aJitl1 height drive which measures 3 1/4 inches high, and a second that measures 1 5/8 inches high and is called a half-high drive. In the 3 1/4 inch form factor drives, there are also two heights that are becoming the common sizes. First is the full height 3 1/4 drive which measures 1 5/8 inches and second is a newly emerging standard that measures 1 inch tall. This latter size is being developed to fit the one inch slots that are common in most personal computers. In each of the drive form factors, the width of the drives are standardized, and received some of their standard dimensions from previously determined requirements for hard drive and floppy drive mounting requirements. The standardized dimensions for the 5 1/4 inch form factor are; Width...................................... 146.0 mm (5.7 inches) Length.................................... 203.2 mm (8.0 inches) (It should be noted that there is some variation in the actual total length of these drives, and there is some variation in the number and placement of mounting holes that are provided.)

758 Magneto-Optical Data Recording The standardized dimensions for the 3 1/2 form factor are: Width...................................... 101.6 mm (4.0 Inches) Length.................................... 146.05 mm (5.75 Inches) (It should be noted again that some drives have exceeded this dimension with the addition of fans and extra length circuit boards at the rear of the drive. There is a variety of mounting hole patterns in these drives also.) The dimensions given above are exclusive of the decorative bezels that are typically added to the front of drives that are used as stand alone devices, but not necessarily used on drives that are installed in jukebox (library) systems.

3.0 MEDIA CARTRIDGES AND STANDARDS Media cartridges for use in the small form factor drives are fklly defined in standards that are available from ANSI (American National Standards Institute), ECMA (European Computer Manufacturers Association), and from IS0 (International Standards Organization). Pictures of these cartridges, with labels for the different features are shown in Fig. 1 and 2. These standards define the mechanical dimensions of the cartridge with the features that have been agreed upon for the location of the disk in the drive. These features include locating pins for centering the cartridge (which, in turn, presents the center of the disk hub within the capture range of the spindle nose), as well as landing pads to position the cartridge and disk at the proper elevation above the optics system and spindle while providing adequate clearance for the disk in the cartridge. The standards also depict predetermined locations for write protect switches (or write inhibit holes) and media I.D. devices when they are used. Positions for media I.D. switches or detectors have been included in most cartridge standards proposals for the 5.25 versions, but the features have not been included in cartridges or drives so far. The need for these to be included in drive designs will evolve as new higher capacity drives are developed, and the intelligence from these sensors is required to give the drive information about media types, direction of spin, and spindle speeds duringthe initialization of the drive.

Drive Packaging 759

ShutterShutter Sensor Notch Slot for Shutter Opener

Side B

Label Area

\

Detent for Autoloading

,Mis-Insertion Protection

-

Detent for Autoloading

Refe

Reference Surface

Reference Surface

Side A

Figure 1. Features of a 3.5-inch cartridge.

760 Magneto-Optical Data Recording Slot For !he Shutter Opener h

/

User Label Area Disk Side B

Jukebox Gripper Feature

Figure 2. Features of a 5.25-inch cartridge.

The 5.25 cartridges are designed for two-sided media, with door openings that are identical top and bottom. The cartridge can be inserted in the drive in either orientation which requires door opening mechanisms that are bidirectional. The 3.5 inch cartridges are designed for single-sided media and are inserted in only one orientation. The door opening sizes in the 3.5 cartridge are not the same sizes on the top and bottom and therefore require features in the mechanism to prevent upside down insertion which could cause damage to the focus lens or actuator. Another difference between the two types of cartridges is the way the requirementto load a bias mechanism is handled. Since the bias system must run close to the media, the design of the 5.25 cartridge requires this to be loaded down into the cartridge. The 3.5 inch design has been provided with a very thin wall at the front end that will not require the bias mechanism to be moved down, but rather just slide straight in to the opening that is provided as the cartridge is inserted into the drive. This does create a problem of its own because in

Drive Packaging 761 most implementations, especially the 1-inch high versions, the cartridge door must be completely open prior to the time the front of the cartridge reaches the spindle and bias mechanism. In many designs this will create some difficulty in producing a force to eject the cartridge since the cartridge door spring is not part of the loop during the first half of the eject cycle. A problem that occurs often is the interaction of the spring forces on the cartridge doors with the ejection mechanism, which can create a system that ejects the cartridge completely out of the drive. Good design practice will prevent this, but is complicated with the desire to have the drive completely reliable in several orientations, with several degrees of tilt possible in each case. Care should be used here to eliminate a system that ejects the cartridge too far in one orientation and not far enough in another. One thing that can impact this phase of design is the fact that the cartridges are multiple piece plastic assemblies that have some built in warp. The thickness and flatness of these cartridges is defined in the standards along with the definition of checking fixtures to veriQ their compliance, but the fiiction caused by a warped cartridge can greatly affect the way the eject system with its limiters will fbnction with both form factors; the cartridges are longer than they are wide, which prevents inserting the cartridge into a drive sideways, but in each case they could be installed is the drive backwards. This requires the designerto provide features in the mechanism, using features on the cartridges, to prevent backwards insertion and the possibility of damage to the lens or actuator. Although most cartridge features are defined in the standards, there are two areas which are not controlled that can create problems because differences are seen between different media and cartridge manufacturers. The first is the inte&ce provided for opening of the doors. Even though the feature sizes are the same, and in the same locations, all interfaces are not implemented in the same manner. There are cases when a door link system for opening the cartridge door will work fine with one cartridge but will hang up on the cartridge of another manufacturer. To avoid this problem, it is wise, early in the design, to do a survey of cartridges you expect to be used. The second area of variation in cartridges is seen in the way the surface of the parts are textured in the area that slides in the cartridge guide or receiver. Some cartridges have a smooth surface in these areas while others are highly textured. This difference in the finish of the parts can cause large differences in the way the eject system functions.

762 Magneto-Optical Data Recording In the cases of both 3.25 inch cartridges and the 5.25 inch cartridges, additional features are defined that can be used with parts of a mechanism to limit or control ejection from the drive, or to provide for features to pull the cartridge into the drive. In both cartridge sizes, the shapes and locations of the door opening features are defined which can control the options available in designing door opening mechanisms. The dimensions for the openings in the cartridges that permit access for the optical head, the spindle, and the bias coil are defined in the standards as are the essential dimensions for the design of the hub on the disk. The acceptable ranges of magnetic clamp forces to the spindles are defined with tolerances. It should be noted that, with a fixed range of clamp forces and the availability of both glass and plastic media, the ability to accelerate and decelerate disks within the desired speed budgets is limited by the interface between the disk surface and the top of the spindle. Of special concern to the mechanism designer is the loading technique used to get the disk center hole properly located over the spindle nose. When the drive is used in library or jukebox systems, the wear at this interface may be accelerated due to rapid loading speeds and must be controlled to prevent an eventual enlargementof the center in the hub, thus losing the centering of the disk. To complicate matters further, there is the requirementto load the media onto the spindle with the drive on its side, or in the vertical direction, as will happen if the end user places the computer or stand-alone box in a different orientation. Somejukeboxes designed for both 3.5 and 5.25 drives require the drive to be oriented vertically. The standards definitions provide features inside the cartridge which are intended to presenter the disk by means of the hub features or the outside diameter of the disk. In all these schemes, clearance is provided to allow for tolerances and to assure that there is running space for the disk. It is these clearances that cause the maximum displacement in the vertical orientation, and create additional wear on the centering of the disk with repeated loading. As disk capacities continue to increase, the requirement for more load cycles will increase as well, creating more of a concern for ultimate usable disk lifetimes. The design of the mechanism should take advantage of the cartridge locating pin features to pre-locate the cartridge on the baseplate prior to the time the disk center is to be lowered onto the spindle nose.

Drive Packaging 763 4.0 BASEPLATE DESIGN CONSIDERATIONS As part of package design, the basis of the system is the design of the baseplate. This baseplate must tie the optics system, moving camage, mechanism, and the outer package to the media in a manner that provides for the required precision and stability of the overall system. 4.1

Spindle Motor

With the fine track pitches that are in use and being contemplated, specifications for spindle motors are critical. Although there are a few offthe-shelf spindles available, most designs have used custom motor assemblies. Some cautions are required to assure that the spindle supplier consistently meets the tolerances, especially for the motor side of the disk clamping forces as spelled out in the cartridge and media standards. Typical motor specifications of interest when having a motor designed for these applications are as follows: 1. Bearing stifFness, radial and axial 2. Torque constants 3. Inertia 4. Torque ripple and motor cogging 5. Orientations 6. Stadstop cycles over life 7. Continuous running life in hours 8. Rotational speeds of operation 9. Range of operating and storage temperatures 10. Operating voltage range 11. Maximum thermal coefficients in terms of resistance and torque 12. Operating current range 13. Winding break down voltage 14. Maximum stadstop duty cycle 15. Shock and vibration requirements 16. Direction of rotation 17. Hardness of spindle nose (should be harder than disk hubs.)

764 Magneto-Optical Data Recording When having a custom spindle developed, it is best to provide the supplier with the primary media you primarily intend to use in the development of the media clamp forces, along with a copy of the applicable media standards. Another specification that is sometimes used is the surface finish on the top surface of the spindle that comes in contact with the media to enhance the ability of the interface to transmit torque to the disk, especially for jukebox applications where spin-up and spindown speeds are important. To control dynamics which are internal to the motor and then might create excitation frequenciesat the media, it may be important to specie the number of poles, selection of bearings, and winding configurations. Both radial and axial acceleration limits may be important in these specs as well. In developing the motor start-up specification, don’t overlook the effect of the aerodynamics of the disk spinning in the cartridge. The difference in motor load between an enclosed disk and one running in open air is minimal, but may affect your calculations.

4.2 Carriage Issues There are two types of optics and moving head designs commonly in use. One type has the optics and laser on board the moving head, and is referred to a moving optics system. These systems are often being replaced with systems which have the optics stationary in the baseplate (and are calledfixed or split optics systems) in order to reduce the moving mass and thereby lower the access and seek times of the system. In these designs, the only components which move under the disk are those required to achieve coarse tracking, fine tracking, and focus. There is a variant to this approach that uses what may be called a galvo mirror or tracking mirror which is mounted as part of a fixed optics system, and is used to dither the beam to achieve fine tracking. This has the advantage of potentially hrther reducing the mass that must be moved as part of the head. There is yet one other potential type of optics head, known as rotary actuator, that has been studied but not as yet put into products for the market. There has been developmentwork and patents have been granted on some of the technology required. One of the current problems with rotary actuator schemes lies in the interface to the standard cartridges. All the cartridge standards as of this writing are tailored to work best with linear actuators, and most bias mechanisms are designed to be in place above linear systems in the cartridges.

Drive Packaging 765 In the case of the linear motor approach, there are two methods in use. The first employs a stepper motor and a lead screw to move a carriage that is guided on hardened and ground rails. The second method is the voice coil motor powering a carriage on rails. At the rail interface, different designs are used to achieve low friction guiding. Most fast drives are now using ball bearings on the carriage which roll on the rails, but there are systems which use bushings of various materials to do the samejob. Each approach has its own attendant problems. With ball bearings, there are accelerationprofiles in which the ball bearing no longer rolls, but starts to slide on the rail. With bushing systems, the problem becomes one of creatingthe proper preload to prevent chatter or allowing the carriage to be mispositioned due to the clearancesbetween the bushing and the rails. Which system is chosen helps to determine the requirements for space and accuracy that are needed for the baseplate design. 4.3

Optics and Thermal Issues

The choice of optics system to be used determines many of the important design guidelines for the baseplate. For example, in a design which has a fixed (or split) optics system, the requirements for precision in the baseplate are much more stringent due to the alignment accuracies needed to position the optics beam that must be followed by the moving head. Thermal stability of the baseplate and the optics mounting system become more critical to maintain the needed alignments over the intended operating temperature ranges of the product. The optics elements are usually mounted in a separate module to aid in manufacturing,which has to be attached to the baseplate with fasteners. This joint can cause thermal drift problems if not treated with caution. The torque required to tighten screws at such a joint often creates forces that cause the joint to slip during thermal cycling. Thermal instability is one of the major concerns in MO drive development, and comes in several forms. First is the thermal drift which is caused by the normal expansion of materials and joints as mentioned above. These can normally be compensated in the electronics of the system if they return to the original position. The drift that is difficult to handle is that which shifts with temperature changes and does not return to the as-built state. These kinds of drifts are seen in glue joints as well as mechanical joints, especially when attempting to anchor materials together that have widely different coefficients of thermal expansion. Adhesive

766 Magneto-Optical Data Recording joints are particularly subject to this kind of movement if not properly designed, and if the adhesive is not completely cured. In a system which uses a voice coil type of linear motor, there are usually relatively large steel pole pieces, magnets, and carriage guide rails mounted in the baseplate. This will create problems if the baseplate is an aluminum or zinc material and the steel parts are firmly anchored to them. If care is not taken at these joints, thermal differential expansion can cause a warpage of the baseplate and a permanent nonrepeatable movement of the joints that secure the parts, often displacing the optical path that the moving head must follow. Similar phenomena can occur at the interface of the spindle motor and the baseplate. Another optical consideration in a split optics system is that there are more optical elements exposed to the environment inside the drive, and additional care should be taken to seal them off, if possible. This is not of as much concern in a moving optics system where the optical elements move as part of the carriage and can be more easily enclosed.

4.4 Other Baseplate Features In systems that use voice coil motors (either linear or rotary), there is a requirement to have the head parked at a known position to aid in the initialization of the drive, either the inner or outer tracks of the media. Information on the location for these tracks is given in the cartridge and media standards. In addition, this kind of feature is used to protect the head from damage due to shock when the drive is powered down or during shipping and handling. For most designs, this feature is also needed to position the head in a safe place during the insertion and loading of the cartridge. There may be a requirement for voice coil actuators to have crush stops at the end of travel to protect the fine actuator from damaging deceleration due to uncontrolled voltage spikes andor power failures that cause runaway coarse motor operations. Crash stops also will serve to protect the head if the drive sees a severe handling shock while it has a cartridge installed, and thereby has the parking latch disabled. It will fall to the baseplate engineer to provide for these features. Some designs have these crash stops on the carriage, with areas on the baseplate for them to engage, while others have been placed in the baseplate to minimize the moving mass of the carriage. In either case, they must be long enough to decelerate the camage safely and accurate enough to allow the carriage to

Drive Packaging 767 reach the inner and outer tracks on the media. In most stepper motorAead screw designs these features are not needed, and adequate protection is provided within the fine actuator itself. The heads that are common to CD use a pin or shaft around which rotates the focus and fine tracking actuator, and may be referred to as pin type actuators. These systems are apparently less subject to the kinds of damage referred to here than fine actuators which use a four-bar flexure system to support the focus and fine tracking motors. An interesting problem for the packaging engineer is the development of the shock mount system that is used to isolate the drive from external perturbances. Since most drives are designed to be used primarily as stand-alone devices or mounted in a computer, there is a requirement for them to operate successfblly through some level of applied shock and vibration. The ideal system would provide adequate excursion and damping to isolate the moving camage from reasonable shocks and vibrations. The industry standards for both the drive sizes and the cartridge sizes that exist don’t provide that luxury. With the sizes so constrained, the amount of allowable sway space for the shock mounted mass is very limited. Complications for shock mount design occur because the drive may be optimized for horizontal use and then be used in a vertical application when the user lays his computer on its side, or it is used in a jukebox that requires vertical mounting. Due to the multiple directions of mounting, the shock mount calculations become complex and probably need to be modeled and looked at in time domain simulations or in terms of radial and focus transfer functions. In doing these calculations or simulations, the designer will need to be able to make some assumptions or have data on the damping levels of the material used. Given that, for various input frequencies of shock and vibration, sway (and therefore required sway space inside the chassis to the mechanism) can be modeled at the different frequencies. In addition, given a known mass for the suspended system (including cartridge and media), droop due to gravity can be modeled in each of the mounting orientations that are anticipated. The difficulty with shock mounts lies in the likelihood that at some spindle speeds with the normal unbalance that exists in all disks, you may have a condition that continuously excites the shock mount system, and what is worse, it will be very different between horizontal and vertical mounting of drive. In jukeboxes, we see multiple drives with the requirementto have one drive performing a read or write operation while the jukebox mechanism is

768 Magneto-Optical Data Recording rapidly (in many cases) installing or removing a cartridge from an adjacent drive. This can create additional concern for the capability of the shock mount system to isolate these inputs adequately. Before the shock mount design can be analyzed, the total suspended mass of the system with the mechanism and the cartridge must be approximated or known, and at the same time the space allocations must be made for the shock mounts to allow the baseplate to be h l i z e d . A primary consideration for the shock mount system is symmetry in all directions. Most actuator and carriage systems have difficulty with shock and vibration inputs that result in yaw, pitch, or roll modes at the fine actuator. These motions can cause unseating of the carriage bearing systems and instability in the fine actuator, thus displacing or tilting the spot on the disk. When voice coil types of motors are used, they are susceptible to shocks in the direction of travel, which the shock system should help to damp out. This happens to be the direction some of the shock pulses will be generated from in the jukebox environments. As you consider the problem, you can see that the ideal place for any gyration to occur in the area of the focus lens would be at the spot where the light beam is focused on the sensitive layer of the media. Since this is not possible, some drives have additional buffer memory added to help overcome the inability of a shock mount system to deal with the problem. In cases where drives are used in military or other rugged applications, additional shock mounting is provided when package constraints are not as tight.

5.0 MECHANISMS AND PACKAGING

This section discusses some other issues and design requirementsthat are required in the development of a complete drive mechanism and the package. Typical lifetime specificationsfor the MO drives are being advertised at between 30,000 and 50,000 cycles of cartridge load and unload for desktop or stand-alone units, and between 650,000 and 1 million cycles for jukebox systems. This latter figure presents the designer with a potential problem of wear and dust particle generation inside the drive, plus the problem of engineering mechanisms that will be reliable for this many operations.

Drive Packaging 769 5.1 Dust and Contaminants A problem with optical drives of all types is the collection of dust and contaminant films on elements of the optical path, especially the focus lens. It is the nature of optical drives to be insensitive to a particle of dust on the focus lens, but the problem comes when there is a layer of dust to the point that the optical beam is diffised as it leaves the lens, and can no longer create the spot quality that is needed. Most drives which are designed for use in normal environments are designed with doors at the decorative bezel to minimize intrusion of dust fiom the outside environments. If you open up a drive of any kind that has been in service for a while, including your CD player or CD ROM drive, you will find a very layer of very fine dust everywhere. It is the case that in most computer installations, or in stand-alone boxes, there is a fan in the enclosure which draws air fiom the fiont and blows it out the back. In this arrangement, any airborne particulates are pulled into and over all the surfaces of the drive that are in the flow of moving air. The first thought may be to seal the drive so this airflow doesn’t go over the componentsin the drive, but we have our own heat generators internally that need cooling, so this approach is often not practical. Drives such as the MO devices we are working on are likely to be installed near printers of one type or another, and a subject to collection of paper dust which is one of the most difficult to keep out of the drive. The particles are very small and stick to any surface that has a surface charge of static electricity. Since all of our media cartridges are plastic, and tend to be left laying on the surfaces in offices, they are good carriers for such dust into the drives. To aid in controlling the problem, some drives have been designed that have dust doors that close behind the cartridge after it has been inserted in the drive. For the drive engineer, an equally important consideration is the dust and contaminant problem that is created inside the drive itself, especially with the long lifetimes and large number of load cycles these systems will see. Depending on the design of the mechanism, the most likely generator of particulates in the drive is the action of inserting the cartridge, and the static charged particles that are created at the interface of the cartridge and the cartridge receiver. There is a second possible problem in terms of lubricants we might want to use in the mechanisms. If not chosen carehlly, some of them will have a tendency to migrate onto optical surfaces, and being wet

770 Magneboptical Data Recording

will attract and hold any particulates which may be in the drive. The warning is, do not use lubricants that contain silicones, nor should any of the plastic parts you may use in your mechanism be molded with the use of silicone mold releases. When these drives are used in library orjukebox systems, the problem with dust can be increased significantlyover a normal desk top environment. There are cases where the continuous and rapid cartridge insertions have caused the cartridge receiver to become an electret, and the resultant static charge has created dust retention problems and an electrical problem. In this case, a plastic was chosen for the cartridge guide to minimize wear, but the plastic built up a charge which could still be there more than 24 hours later. The electrical result was some blown components, with the resultant change to a conductive plastic to bleed offthe charge. Not to be ignored is the dust and wear that is created by the jukebox itself. It is sometimes the case that drives to be used in a jukeboxes will not have the bezel nor the door to aid in dust control, and usually they have their own air movement systems which should be considered. The degree of the dust control problem is being understood as the industry, due in part to customer demand, is making available special head deaning cartridges to periodically clean the focus lenses in these drives while they are in service. Users are starting to do preventative maintenance just as on VCRS, tape drives, and floppies.

5.2 Bias Magnetic Field Generators MO drives require the presence of a bias field generator in close proximity above the media with fields that are shaped and directly opposite the focus lens on the moving carriage. These bias generators have been done in several forms including simple coils with central pole pieces, bar magnets which rotate (sometimes called flipping magnets), a row of small coils, and a very clever one by Fujitsu in their 1-inch high, 3.5 inch drive that uses a small flexure system to steer the bias field. The simpler coil type systems are probably less expensive to implement, but have the disadvantage of possibly running hot and requiring heat sinking of some type. The flipping magnet approaches do not have the heat problem, but may require additional servo control to keep them positioned properly in addition to needing precise bearings for rapid rotation.

Drive Packaging 771 In the 3.5 inch form factor drives, it is not necessary to lower this device into the cartridge to achieve the required spacing since the cartridge as defined in the standards permits a straight in to the drive insertion with adequate clearance for the bias generator. The 5.25 inch cartridge standard was developed when the only optical drive systems were WORM (write-once read-mostly) and there was no bias generator to be concerned with. As a result, the shape of the cartridge opening is such that the bias generator must be lowered into the cartridge to achieve the proper spacing to the disk. This presents the mechanism designer a set of criteria that are challenging, especially in the half-high form factor drive. For the 5.25 inch drive, the cartridge will need to be lowered about 5 mm down onto the spindle (dependent on spindle and cartridge locating pin design), and the bias generator will need to be lowered into the cartridge about an additional 3 mm, depending on the design of the bias generator. 5.3

Thermal Cooling Issues

Cooling of the elements in the drive which dissipate thermal power is a problem when combined with the desire to keep the drive closed to control dust as has been mentioned previously. If the bias generator is of the large coil-and-pole-piece type, the heat generated by this system can be large, especially if the device is used for a long duration write or erase sequence. Obviously, as the temperature of the coil rises, the efficiency drops. Since this is also true of the coils used in voice coil coarse and fine actuators, the need for some cooling is evident with these systems. There is some cooling help in the area of the fine actuators with the proximity of turbulent air due to the disk spinning in the cartridge, even though it is minimal. It is troublesome to consider a long cycle when a bias coil is in use that is interrupted near the end by a power failure. The latent heat which is built up in the coil(s) could then put the user’s media and data at risk, if he is using plastic media, and there is no adequate cooling or some method of retracting the bias coil away from the disk. In most of the electric cartridge eject systems, there is no easy method to do this, but it needs to be considered. The electronics in the drive are a contributor to thermal dissipation, and placement of some of the components may be critical for adequate cooling. Drives which have very efficient electronics may be able to live with ambient air cooling while others will need the addition of fans and

772 Magneto-Optical Data Recording possibly directed air flow over some components. The difficulty with designing too close to specs in this area is the lack of control on how the drive may be put into service by the end user. If installed in a computer which has inadequate cooling, or obstructed airflow, normal convection cooling could prove inadequate.

5.4 Electronic Considerations A concern for both the baseplate and the outer package (chassis) is adequate provision for electrical hookups, in particular the flexlead that provides power and signal lines to the moving head. Most often this is a Kapton or polyimide flexlead that must roll or follow the moving carriage. The requirement is often for this flexlead to provide a relatively constant force to the carriage through its entire travel. This flexlead is usually very thin to minimize these forces and must follow the motion of the carriage through the lifetime of the product, or several billion seeks. Usually there are special considerations given to guiding this flexlead and making provision in the system for it to have the largest rolling radius possible. Aside from making provisions for the various electronic components that are part of the drive, an issue for the engineer is the definition of the grounding scheme that is to be used at a system level, and it is helphl to have this deiined early in the design cycle to prevent changes and problems later on. There may be several different current sources that share the same return path and, as a result, can cause EM1 problems with the drive, and it is possible to create ground loop problems if there are analog circuits in the drive. With these and other considerations, it can be seen that developing the requirements for system grounding should not be left to chance, but must be determined with some care. Any areas that require special consideration in packaging to provide for elements of the design that need critically short lead lengths for the circuits should be addressed early for the same reasons. In many cases, the laser and its mount needs to be isolated electrically from other parts of the drive, and sometimes the baseplate and mechanism needs to be isolated from the chassis. The circuit boards may need to be at least partially isolated, and there may be a customer requirement to have a separate ground lug accessible somewhere on the chassis. There may also be requirements for special SCSI terminators available at the back of the chassis. As was mentioned previously, there may be a need to provide ground straps or leads to the cartridge receiver and mechanism.

Drive Packaging 773 A feature that deserves special attention in the electromechanical design is the enclosure for the RF modulator at the laser. These are now being run in frequency ranges of 400 to 500 megahertz, and create emissions that are virtually impossible to contain in the drive if the issue is not dealt with effectively at the modulator. This is usually done with small cans that are designed and sealed as tightly as possible to keep the emissions isolated. One of the major destroyers of shielding is to pass an unterminated wire or flexlead through the shield, and in this case, dimensions do not matter. A small hole with a wire running through it can carry large amounts of energy through the shield. The enclosures can be aluminum or steel as either are good shielding materials. In observing various drive designs, there have been cases where RF modulators were mounted at the laser (due to the desire for short lead lengths), a steel can soldered to the laser mount, and then finally wrapped with copper tape when it was discovered that the system leaked. Better designs tend to use feed-through capacitors to get the leads out of the can,which work better since these are terminated leads where they are soldered to the housing. A problem comes in protecting these two shielding materials, in that steel rusts and the aluminum develops a nonconductive film. Both metals must be treated, with chromate treatments being used on steel and iridites used on aluminum, but in either case the coatings must be thin to work. Stainless steel has been used for shielding applications to get around the coating problems, but stainless is much less conductive than either aluminum or conventional steel, and may require the use of gaskets or welded joints to do the job.

5.5

Chassis and Bezel Design

As can be seen from the previous comments, shielding the drive from undesirable emissions is important, and it doesn’t stop at the RF modulator at the laser. There are other circuits in a drive that can cause problems with EMI, and the important thing is the length of a slot or opening, not the area. If you look at MO drives, the first thing of note is the shape of the cartridge entrance opening, which is a slot. The higher the frequencies generated (if unshielded) in the drive, the better this slot acts as an antenna. For these and other reasons, the chassis design should enclose the drive as completely as possible. On most drives, use of a decorative bezel at the front is required. To complete the shielded enclosure, this needs to be conductive as well. Work is being done by many suppliers to develop metal loaded plastics, but so far with only limited success. These kinds of materials would also need

774 Magneto-Optical Data Recording to be painted for use on the front of a drive. The approach most manufacturers are using now is to coat the plastic parts where it is possible. The ones used most are conductive paints, electroless plating, and vacuum (thin film) deposition. One of the paints is silver and is effective, but is considerably more expensive than ones that contain nickel or passivated copper. While discussing the bezel, it needs to be pointed out that the plastic materials chosen for these parts, as well as any plastic parts that are used in the mechanism, need to be capable of passing the UL flame retardancy test. In this case, not all plastics that are used need to be flame retardant, but the volume of those that are not is limited, and this determination is made on a design by design basis. A final consideration for the chassis design is finding a place to put all the labels that are required for agency approvals, plus whatever may be required for internal use and by customers. The labels required by T W , UL, CSA, and FDA can use an area of about 13 square inches, depending on the sizes chosen.

6.0 ENVIRONMENTAL AND AGENCY REQUIREMENTS This section addressesthe more common requirementsthat are placed on a product design by various governmental agencies along with some of the key points of concern for the product designer. CSA (Canadian Standards Association), is a Nationally Recognized Testing Laboratory (NRTL) by OSHA, and will work with a manufacturer to obtain product approvals for either the American or Canadian marketplace. Other agencies such as FCC (Federal Communications Commission), VDE (Verband Deutcher Elecrotechniker), FDA (Food and Drug Administration), and UL (Underwriters Laboratories) have specific areas for which they are primarily concerned.

6.1 Electromagnetic Interference (EMI) EMI, or electromagnetic interference is a problem being stated as a piece of electronic equipment that doesn’t work as it should because there is unwanted electrical energy in the wrong place, at the wrong time, and doing the wrong things. The primary problem was created with the development of the computer market and the growing complaints of television and radio interference, with the result that the FCC introduced regulations to limit

Drive Packaging 775 both conducted and radiated emissions from computers to protect the radio spectrum. The most familiar mandatory limits on EM1 are imposed by the FCC, W E , and if it is a military requirement, there are requirements in MIL-

STD-461. Europe has somewhat more stringent regulations than either the United States or Japan, and usually allow fewer exceptions. For example, The European Community (EC) is making immunity to RF, ESD, and power disturbancesmandatory while the US and Japan do not address these issues. It is expected that the EC regulations will replace the VDE regulations soon. The contents of another specification, from the IEC (International Elecrotechnical Commission), have been incorporated into the European Community (EC) requirements. If the drive being developed is to be marketed in Europe, the IEC standard is the one to meet. Most of the commercial EM1 regulations are mandatory, and the limits are not negotiable. You either pass or fail. The limits are divided into class A, which applies to products that are used solely in business environments, and class B, which applies to products used in residential environments. Since many of the MO disk drives can be used in the home, they must meet class B. Agency testing is expensive, running in the range of $20,000 and up, so the costs of re-engineering, redesign, and retesting are significant if the drive is not designed to pass the first time. 6.2

Laser Safety

The FDA has been given the responsibility of regulating the safety of use for devices that contain lasers. These regulations are under the Department of Health and Human Services, Center for Devices and Radiological Health. These regulations cover the certification of lasers, require registration for laser products that are used by other manufacturers to complete a laser product, and certification of a completed product for sale, in our case probably a completed MO drive. The registration process primarily is an activity to inform the FDA that you are producing a product that uses a laser, a description of the product, a list of model numbers you use and the type of laser (wavelength and power). When this is done you are granted an access number which is a record keeping means for FDA to compile records of laser products that are being sold and provides a level of traceability.

776 Magneto-Optical Data Recording If you are producing a complete drive, the entire task of record keeping and certification rests on you as the supplier to the end user of a laser-wntaining device. The requirements for a drive are more extensive since the regulations include definitions for safety interlocks in the drive to prevent accidental injury for the end user and service personnel that could be working on the drive. Basically, the latest requirement states that a user shall not be able to put a rod of any diameter or shape into a collimated beam with the ability to redirect and/or refocus that beam. As a result, there are requirements for interlocks designed into the product to prevent powering the laser without over riding or disabling the interlocks. Even in these cases, the labels mentioned earlier must be visible, even to a qualified technician. In addition to the constraints that are added to the drive, there is much more involved record keeping required in the manufacturing to provide traceability for the products. A copy of the regulations is available from: Food and Drug Administration Center for Devices and Radiational Health 1390 Picaard Drive Rockville, MD 20850 Some of the additional requirements involve having warning labels on the literature that accompanies the product and on the shipping containers. With the advent of lower wavelength lasers, these requirements may change, and it is best to stay up to date on FDA regulations, and possibly retain the services of one of the laser safety consultants that are available. Most of the time, a certified consultant will need to fill out a report which contains an assessment of the design from the standpoint of compliance to the requirements. If your product is for sale in foreign countries, some of them have requirements that are different, and will require the labels to be in different languages, quite often in German and French. As the EC continues to advance the regulations they are working, there will probably be requirements we have not seen yet. Two publications that help in keeping up with the EC regulation and pending actions are: “EUROPE NOW,” and “BUSINESS AMERICA” U. S. Department of Commerce International Trade Administration Room 3036 14th Street and Constitution Avenue, N.W. Washington, D.C. 20230

Drive Packaging 777 6.3 UL Recognition

UL recognizes products for their end use requirements for products of their type especially in terms susceptibility to ignition and behavior in fire, smoke generation, and chemical hazards. In addition, when providing recognition for a material, many things are tested in addition to the flammability such as impact strength, tensile strength, heat deflection temperatures, and dimensional stability. Because of this extensive testing, it saves much effort if in specifjmg plastic material in the design of the drive, the materials that have already been granted the UL Yellow Card. All material manufacturers have their engineering materials UL recognized, but you may have to choose materials which have flame retardants added to them. UL has four U.S.locations where the testing has to be done: Melville, Long Island, New York Northbrook, Illinois Santa Clara, California Raleigh, North Carolina The normal procedures for obtaining UL listing of a device are as follows: 1. The manuficturer opens an assignment with UL by describing the device and requesting a review of the product and the application forms. 2. UL will respond by identifjmg the device standard to which the drive will be tested, outliningthe test programs, providing cost limits, and providing the required forms. 3. The applicants will submit the required number of test units along with the filled out forms. 4.The samples will be tested by UL. 5 . If the product passes, the device will be listed by UL, at which point the manufacturer is authorized to attach the UL label, and follow-up procedures are established. 6. Ifthe product fails, corrections must be made and samples resubmitted and retested. There are several categories that UL will test to (known as 94V-0 through 94-5V),but the common requirement for a disk drive will be 94V-0, which is common for devices such as like computer housings, telephone

778 Magneto-Optical Data Recording equipment, data terminals, etc. The flammability ratings of various materials are determined using relatively large test bars and controlled methods of heating and burning. As has been stated previously, a thing that determines the amount of certain materials in a drive which might sustain flame (not selfextinguishing) is determined by total volume of that material as compared to other materials in the product. It is best to keep these materials to a minimum, but in the description of the product to UL,it will become clear if there will be a problem in receiving recognition. Simply, what UL is looking for is a product, that if ignited, will self-extinguish when the source of the ignition is removed, and do so with a minimum of hazardous smoke and chemicals.

6.4 Miscellaneous Other Specifications Some other types of specifications that are common to MO drives are shown below, but are shown only for reference, as your product may be better in its ability to live through some of specifications, or more sensitive to others. Actual figures are up to you, with competitors specs available on their data sheets. 1. Shock 5 . Humidity Operating Operating Non-Operating Non-Operating 6. MTBF (Mean time between Failure) 2. Vibration Operating Non-Operating 3. Altitude limits 7. MTTR (Mean time to Repair) Operating Non- Operating 8 . MSBF or MEOL 4. Temperature (Mean swaps before failure or Operating Non-Operating mechanical end of life)

One last issue of interest is the design of the shipping packages if your product is likely to be sold and shipped to Europe There are regulations coming into play that will require containers and shipping packages to be largely if not totally recyclable for the product to cross some international

Drive Packaging 779 borders without problems. When you set out to design the packaging for the product, don’t overlook this item and then have to redo it with the additional expense later. Information on the standards as they develop are being tracked by the Department of Commerce at the addresses given previously, and as standards are finalized, will be available from them.

Data Reliability and Errors Dennis G.Howe

1.0 INTRODUCTION Digital data is extracted from the analog signal that emanates from an optical recorder’s read head via an electrical circuit called the data detector. Techniques used to reduce analog read signal impairments (noise and distortion) that cause errors to occur in the recovered digital data stream output by the data detector, or to reduce the signal quality needed to achieve a reasonably low level of errors in the recovered data include: selecting a modulation code to control the length and spacing of the data-canying marks that are written on the disk; using equalizatiodfiltering of the playback signal to improve signaVnoise ratio and maintain synchronization with the data detection clock; employing an analog/digital conversionmethod that is optimized for use with the chosen modulation code and equalization; etc. In this ckpter, we discuss how digital data errors propagate through the digital part of the recorder’s read channel and review the digital data processing techniques that are used .to deliver reliable data (i.e., data having a specified statistical error rate) to the user. Major topics that we cover are: An overview of the digital optical recording channel The recording format and data synchronization The nature and characteristics of various types of digital errors Error correction codes (ECCs) and codingldecoding The statistical description of digital data errors Estimation of recovered user data reliability 780

Data Reliability and Errors 781 We treat these topics from a systems engineering point of view. Rigorous mathematical derivations will be avoided; in their place we shall try to impart a high level understanding of how digital data errors are created in and by the data storage system, how they propagate through the data channel and how they are ultimately managed. Particular attentionwill be paid to the statistical definition of data reliability and methods for estimating it. Although the discussion in this chapter is specific to digital optical data storage, much of the coding and signal processing that will be described is commonly used in both optical and magnetic disk drives. In particular, hard decision data detection, run-length-limited modulation coding and linear, block errorcorrectioncodes are widely used in both the optical disk drive and the hard magnetic disk drive industry. 2.0

OVERVIEW OF TEE DIGITAL OPTICAL RECORDING

CHANNEL Many of the topics that are discussed in this section are treated indepth elsewhere in this book. Their treatment here is qualitative and is focused on how the various components of the digital recording channel may influence the reliability of data that is recovered from the storage medium. 2.1

User Data and Channel Data

A high level block diagram of a conventionaldigital optical recording channel is given as Fig. ( 1 ) . In this system, binary user (source) data is subjected to two encoding processes prior to its being recorded on the storage medium. Both the source data and the data emerging from the first (ECC) encoder are unconstrained, i.e., a randomly selected bit in the data stream may be either a “one” or a “zero” with equal probability and arbitrarily long sequences of all “ones” or all “zeros” may occur. The data emerging from the second (modulation) encoder, on the other hand, is usually d,k constrained, i.e., there must be at least d, but no more than k, “zeros” between any two “0nes”.[ll-[~1Such d,k constrained data is said to be run-length-limited (RLL) and it is referred to as channel data. RLL sequences consist of catenated segments of binary data that have length d + l , d+2, ...., k+l bits; each segment, or run, consists of a ‘‘~ne”that is followed by at least d, but not more than k, contiguous “zeros”.

782 Magneto-Optical Data Recording

DATA SOURCE

ECC ENCODER

MOWLATION ENCODER

WRITE SIGNAL GENERATION

I UNCONSTRAINED

BINARY DATA

DATA SINK

ECC DECODER

BINARY RLL SEQUENCES

MODULATION DECODER

AID&READ ELECTRONICS

Figure 1. High level block diagram of aconventional recordmg system’s writdread channel.

The International Standard (ISOLIEC 10089; “Information Technology: 130-mm rewritable optical disk cartridgefor information interchange”) that describes the first generation, 325 Mbyte/disk-side, 130-mm magnetooptical (MO) disk storage system specifies two different recording formats, which employ very different modulation codes. Drives and storage media that use either of the formats are covered in the International Standard. One of the specified modulation codes, known as four-out-ofjjteen (FOOF) code, maps groups of eight sequentially occurring, unconstrained, data bits into fixed length blocks of fifteen channel bits that each contain exactly four “ones” and eleven “zeros.” FOOF code produces fixed-length channel data sequences, i.e., 8n data bits are always mapped to 15n channel bits. However, the resulting channel data sequence is essentially unconstrained (d = 0 and k = 16 for FOOF channel sequences). Codes such as FOOF that are based on fixed length data blocks, or words, are known as block codes. For various reasons, among them the technical opinion that extended recording density can more easily be achieved with RLL modulation coding that maps fixed-length data blocks into variable-length channel data sequences than with block modulation (e.g., see the discussion on migrating from a pulse-position modulation write waveform to a pulse-width modulation write waveform in Sec. 2.2), the second recording format specified in ISO/IEC 10089, which employs 2,7 RLL modulation code, has become the

Data Reliability and Errors 783 format of choice (d = 2 and k = 7 for this modulation code). This format is widely used in available first generation, 325 Mbyte/disk-side, 130-mm MO optical drives. Moreover, only RLL modulation is specified in the working draft standard ISO/IEC JTC l/SC 23, “Information Technology,” which covers the second generation, extended capacity (654 Mbyte/diskside) 130-nun MO optical drive that is commercially available today. The coding table for the 2,7 RLL modulation code specified by ISOOEC 10089 is given as Table 1. Table 1. The 2,7 Modulation Code

Input Data

Channel Data

10

0100

11

1000

010

100100

01 1

00 1000

000

000 100

0010

00 100100

001 1

0000 1000

The 2,7 RLL modulation code specified by Table 1 is a variable length modulation code. Here, the term variable Zength refers to encoding or decoding blocks of data in a way that causes the size of the block to depend on the actual data contained in it. Variable length 2,7 RLL code maps contiguous segments, or phrases, of unconstrained data that are either 2,3 or 4 bits in length to segments of channel data having respective lengths of 4, 6 or 8 bits in such a way that the channel segments, when catenated, obey the 2,7 constraints. Overall, however, each data bit is mapped to exactly 2 channel bits, i.e., 2,7 RLL code has a codingrate of 0.5. Ifa block

784 Magneto-Optical Data Recording of data that is input to the 2,7 encoder does not end with one of the exact data phrases shown on the left hand side of Table 1, encoding cannot take place until extrapad data bits are added to the input block in order to cause it to end with a complete 2,7 phrase. As many as three of these padding bits may be required (see Table 1). The resulting block of 2,7 RLL channel data will therefore have length that varies by as many as six bits, depending upon the data that is encoded. In all of the examples and illustrations given in this chapter we shall use 2,7 RLL sequences to represent the channel data. Additional details of 2,7 RLL code will be presented later (in Sec. 4.0). Variable length 2,7 RLL modulation code is also used in 5.25-inch form factor write-once-read-many times (WORM) optical disk drives that conform to the so-called continuous-compositeservo recording format. An alternative recording format known as the sampled-servo recording format is employed in some 5.25-inch WORM optical drives (FOOF modulation code is used in this recording format). The Compact Disc (CD) family of optical disk systems, i.e., CD-Digital Audio (CD-DAD), CD-Read Only Memory (CD-ROM) and CD-Recordable (CD-R), employ a block modulation code known as eight-to-fourteen (EFM) that maps blocks containing eight data bits to 14-bit channel blocks. These channel blocks are subsequently catenated using three link channel bits per block to form d= 2, k= 10 RLL channel data sequences that have fixed length, i.e., n user bytes are mapped to exactly 17n channel High density magnetic hard disk drives, which had been using variable length 2,7 RLL modulation code almost universally, are currently migrating to variable length 1,7 RLL modulation code (d = 1 and k = 7 for this modulation code). System studies generally have shown that when new magnetic hard drive recording components, such as thin film storage media and playback heads are used, 1,7 modulation code can provide a higher data storage density in conjunction with an acceptable level of recovered data reliability than can be obtained by using 2,7 The magnetic recording industry has also been experimenting with the direct recording of unconstrained data, or loosely constrained (i.e., d = 0 and k >> d ) channel sequences to increase storage density. For example, read channels that incorporate partial response equalizationand maximum-likelihood sequenceestimation (MLSE) detection of d = 0, k >> d channel sequences have been developed for magnetic disk ~torage.[’J[~] (Note: Partial response equalization means that the playback channel’s bandwidth is such that the smallest features recorded on the storage medium cannot be resolved. A direct result of this is that a multilevel analog playback signal, e.g., a signal having five

Data Reliability and Errors 785 distinct amplitude levels, is obtained when the medium is read. This is quite different from the two-level playback signal that is obtained when a d > 0 RLL modulation code is used in conjunction with a full response playback channel that has the bandwidth required to resolve the smallest features that are recorded on the medium. MLSE detectors consider a length of playback signal that corresponds to a multibit segment of data before deciding what bits should be assigned to the segment. Full response read channels usually employ hard decision detectors that synchronously interrogate a length of playback signal that corresponds to only one channel bit; decisions about what data is being received are made one bit at a time.) The use of 1,7 RLL modulation coding with hard decision detection, as well as partial response equalization with MLSE detection of unconstrained data, in MO storage drives is also under investigation. It is not yet clear whether these techniques can yield significantperformance increases over what is obtainablewith 2,7 modulation and hard decision data dete~tion.[~1[~1 (Note: 1,7 RLL modulation coding and hard decision detection is used in the 1.3 Gbyte/disk-side MO drives that became commercially available in 1995.) 2.2

Write Waveforms

The RLL data stream is converted by the write signal generation electronics (see Fig. 1 ) into a pulse waveform that is used to turn the writing laser odoff, thereby causing the light beam that is focused on the storage medium to create a track of optically detectable marks along the path that it scans on the medium. This write-pulse waveform may be either of two types: (i) a pulse-length-modulated (PLM) waveform in which the lengths of the succession of equal height pulses and intervening spaces corresponds to the exact lengths of the runs in the RLL channel data (see Fig. 2a), or (ii) a pulse-position-modulated (PPM) waveform in which the separations between the centers of equal height pulses of uniform length correspond to the exact lengths of the runs in the RLL channel data (see Fig. 2b). Thus, the length of each of the equal height pulses and spaces in a PLM waveform derived from a d,k RLL sequence is equal to one of the kd+l discrete lengths (d+l)T,, (d+2)TW,...., (k+l)T,, where T, is the temporal length of a single channel bit. Similarly, the center-tocenter spacing of any pair of the equal height, uniform length pulses in the PPM waveform derived from a d,k RLL sequenceis equal to one of the k-d+ 1 lengths (d+l)Tw,(d+2)TW, ...., (k+l)T,.

786 Magneto-Optical Data Recording Assuming ideal recording, a PLM write waveform will cause marks and intervening spaces, each having one of k-d+l discrete lengths, to be written on the medium, and the occurrence of a channel data “one” will correspond to an edge (i.e., the start or end) of a written mark. Ideal recording with a PPM write waveform will cause identical marks that have length roughly proportional to one channel bit time T, to be written to the storage medium; pairs of these short marks will have one of k-d+ 1 discrete center-to-center spacings and the center of each mark will correspond to the occurrence of a channel data “one.”

>

CHANNEL CLOCK

2,7RLLDATA SEQUENCE

...0 0 1 0 0 0 1 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 0 0 0...

CORRESPONDING PLM WAVEFORM

8 WRllTEN MARKS (a)

4k

2 fllESs3 3

CORRESPONDING PPM WAVEFORM

T w

- -

-

GssB

- - -

Figure 2 gives examples of the ideally recorded mark patterns that would result if either PLM or PPM write waveforms were used with 2,7 RLL modulation code. The shortest and longest of the ideally recorded marks and spaces written using the PLM waveform have length (d+l)vT, and (k+l)vT, respectively, where v is the velocity at which the focused writing laser beam scans the medium. All of the ideally recorded marks written using the PPM waveform will have length that is about equal to the full-width-at-half-maximum (FWHM) intensity (i.e., irradiance or power) diameter of the focused playback light beam to insure that these marks are well-resolved during playback. This size criterion insures that the recorded marks are large enough to cause appreciable modulation of the light reflected from the medium as it is scanned by the focused playback light spot. Thus, to insure that agood playback signal is attained, i.e., that the playback signal has a useful depth of modulation, one must insure that

Data Reliability and Errors 787

where FWHMR is the intensity diameter of the focused playback (read) spot on the medium. The pulse waveform emerging from the write signal generating electronics may be considered to be an analog signal. In fact, the length and height of certain of the pulses in both the PLM and PPM waveforms may be systematicallyvaried by a slight amount (usually less than 15%) to account for known nonlinear characteristics of the optical mark writing process. For example, as a consequence of the low thermal diffisivity of a practical (useful recording sensitivity), thermally-activated optical storage medium and the somewhat broad spatial extent of the focused write spot on the medium, the maximum temperature that a given microregion on the medium will experience during the mark writing process depends on the distances of a short PLM waveform pulse the nearest neighbor m a r k ~ . [ ~ l -Therefore, [~~] (of length (d+l)T,,,or (d+2)Tw)that is meant to turn the write laser “on” might be increased in height andor length whenever such a short pulse occurs between two long spaces that hold the write laser “off ” in order to insure that enough heat is supplied to record a short optical mark that has proper length. Conversely, if a short space that is meant to turn the writing laser off is nested between two very long pulses that hold the laser on, then the lengths of the long pulses may be somewhat shortened in order to insure that the length of the short space that will separate the corresponding two long marks on the storage medium will have the correct length, i.e., to insure that the space is not too short. This may be done by prematurely terminating the first of the long pulses and delaying the start of the second long pulse, for example. Similarly, when a PPM write waveform is used, the width of those pulses that are spaced far away, say at distance kT,, or (k+l)T,, from their nearest neighbor pulses may be increased in order to insure that the corresponding written marks are not too short. This process of slightly altering (in a quasi-analog fashion) the pulse write waveform to affect the proper relative lengths of the marks and intervening spaces written to the medium is known as write pre-emphasis or write precompen~ation.[~~[~I The first generation MO drives employed 2,7 modulation code and the PPM write waveform. This modulatiodwrite strategy was chosen because many first generation MO storage materials had the tendency to produce written marks, i.e., small domains of reversed magnetization in a uniformly magnetized thin film, that vary in size by a significant fraction of their average diameter in response to a series of identical writing exposures.

788 Magneto-Optical Data Recording More specifically, such marks tend to elongate, or bloom, in a way that depends on, among other things, the local thermal history of the recording layer, which in turn, depends on the pattern of written data.[131[141 Sincethis behavior alters the physical location of the geometric centers of the written marks much less than it does the physical locations of any of their edges, the combination of a PPM write waveform, which associates the centers of identical, short written marks with the occurrence of channel data “ones,” and 2,7modulation code, which causes the centers of closest-spaced neighbor marks written via a PPM write waveform to be separated by at least three channel bits, represents a robust medium-marking technique (from a data reliability point of view).”] The price of this is storage density since only a single channel data “one” is carried per written mark. PLM write waveforms, which cause two channel “ones” to be carried per mark (a first “one” locates the leading edge of a given mark while the next “one” locates the trailing edge of the mark), offer a means to double the channel data density if both the smallest written mark length and modulation code are unchanged. Third generation 130-mm MO disk drives, which have a storage capacity of 1.3 Gbytddisk-side, use 1,7 FUL modulation code together with a PLM write waveform as key ingredients of a minimallychanged system that exhibits both increased storage density and data rate. That is, minimum overall redesign was required to realize the increased performance since the basic continuous-composite servo recording format was retained. 2.3

Read Signals and Data Detection

Earlier (see Sec. 2.1) we mentioned full response playback channels that employ d > 0 RLL modulation code and hard decision data detection. The MO storage systems that are considered in this chapter use this playback channel architecture. In such systems, under ideal recording and playback conditions, the electronic signal output by the playback photodetector when the recorded optical storage medium is read is a low-pass filtered version of the write waveform that was used to record the information on the medium. In real systems, however, the read signal is contaminated by various types of noise and some of the pulses contained in it are distorted in the sense that their FWHM length, and possibly the location of their peaks (i.e., the points of maximum amplitude in each pulse), are different than the length and center position of corresponding pulses in the write waveform. A pulse in a PPM playback waveform is said to be shlfted

Data Reliability and Errors 789 from its correct location if the position of its peak, as determined by the data detector, does not coincidewith the center of the correspondingwritten mark on the storage medium. A pulse in PLM playback waveform is said to be shifted if the position of either its leading or trailing edge, as determined by the data detector, does not coincide with the position of the respective edge of the corresponding mark on the storage medium. Indeed, in some cases many ofthe pulses in the read signal are asymmetric (i.e., skewed) and may have small amplitude oscillationsor ringing superposed on them. Thus, the pulses in real read signals may only vaguely resemble the pulses in the corresponding write waveform. The read electronics (see Fig. 1) consist of noise filters, equalization filters and other circuits (that perform DC restoration, etc.) to reduce the effect of the read signal impairments on the data detection process.[l51That is, after passing through the read electronics, the playback signal will exhibit reduced noise and the pulses contained in it will ideally have FWHM lengths (when a PLM write waveform is used) that coincide with the lengths of the marks written on the storage medium (see Fig. 3). When a PPM write waveform is used, the center positions of the pulses in the playback signal that exits the read electronics will ideally align with the centers of the corresponding marks on the storage medium. Data detection consists of two main activities: (i) the conversion of the analog playback signal to a two-level pulse waveform similar to its corresponding write waveform by an A/D-like circuit and (iz) the synthesis of a clock, i.e., a 50% duty cycle pulse waveform that is synchronous with the channel data frequency and correctly phased to the two-level pulse waveform mentioned just above. We shall refer to the two-level pulse waveform that is output by the data detector as the data waveform or signal. Thus, the data detector generates the data waveform and synthesizes a data clock. An ideally operating data detector would process the playback signal shown at the bottom of Fig. (3) to obtain a data waveform that is very similar to the PLM waveform shown at the center of Fig. (2) and a clock such as the one shown at the top of Fig. (2). It would then establish timing windows of width T, that are centered on each transition (edge) of the clock waveform, and would interrogate the data waveform once in each of these timing windows in order to determine when transitions (edges) in the data waveform occur. Channel data “ones” are assigned to each timing window in which a data waveform edge is found; “zeros” are assigned to all other timing windows. The resulting sequence of “ones” and “zeros” is the detected, or recovered, channel data sequence. Note that, to insure optimum operation of such a data detector, any and all data waveform pulses must

790 Magneto-Optical Data Recording correspond to an integer number of clock waveform pulses and the centers of the clock waveform pulses must be well-aligned with edges in the data waveform, i.e., the clock waveform must be synchronous with, and phased to, the data waveform.

2,7 RLLDATA SEQUENCE

...0010001001000000010000001001000.

+I

W

W

CORRESPONDING PLM WAVEFORM 8 WRITTEN MARKS

ELECTRONICS

Figure 3. PLM playback signals before and atter processing by the read electronics.

When a PLM write waveform is used, the data waveform can be generated by locating the times at which the playback signal output by the read electronics crosses an intermediate slicing level, such as the dashed horizontal line in the lower part of Fig. (3).[’1[l61 If a PPM write waveform were used, the data waveform could be generated by finding those times at which the corresponding playback signal pulse peaks are located, e.g., by taking a derivative of the playback signal that is output by the read electronics and then finding where it crosses a zero-value slicing level. The data waveforms recovered from either PLM or PPM written marks will, in principle, be identical. We shall assume that the data waveform is a nonreturn-to-zero (NRZ) signal such as the PLM write waveform shown in Figs. (2) and (3). Once the data waveform is obtained, the channel data sequence can be derived in the same manner, regardless of which type of write waveform (PLM or PPM) was used.

Data Reliability and Errors 791 The recovered (synthesized) channel clock is usually obtained by using the output of a phase-locked-loop (PLL) that follows the locations of pulse edges in the recovered data waveform to vary the phase of a variable fiequency oscillator (VFO) that nominally operates at the channel data frequency. Because channel data errors do not occur if the edges in the data waveform Ml withinthe appropriate channel timing window, small amounts of channel clock dephasing, say up to 0.3Tw,is permitted if the noise and distortion in the analog playback waveform output by the read electronics is sufficiently low. It is the job of the PLL to maintain the channel clock phase precision to within such aphuse margin. The time constant that govern the rate at which the channel clock’s phase may be shifted by the PLL must be carefully chosen; the PLL must track changes in bit timing that result from normal playback perturbations such as disk RPM variations, or slight mistracking of the recorded marks by the optical head, but it should not be significantly affected by spurious data waveform pulses, or isolated data waveform edge shifts, that are caused by noise spikes or individual marks that were poorly written on the medium. In addition, the clock phase cannot be allowed to drift when a long segment of data waveform that has no pulse edges is encountered (this will occur whenever a long medium dropout causes the playback signal to be interrupted for a time). The data detectorjust described is a hard decision detector, so-called because it bases its decision about whether a given channel bit is a “one” or a “zero” on the information (i.e., the recovered data waveform amplitude) contained in only a single channel bit timing window. Data detectors that base their decisions on multiple-bit sequences, e.g., majority-logic-vote or maximUm-likelihood sequence estimation detectors, are not widely used in data storage drives, largely because inexpensive hard decision data detectors exhibit adequate data recovery reliability at today’s high playback data rates and storage densities when used with the coding and read channel architectures that were mentioned earlier (see Sec. 2.1).

-

2.4

Channel Data Decoding and Error Correction

Next, (see Fig. 1) the recovered RLL channel data is processed (demodulated) by the modulation decoder (demodulator) which outputs error-correctionded, unconstrained binary data. Because some of the channel data that enters the demodulator is erroneous, the unconstrained binary sequence that is output by the demodulator also contains errors. However, both the fraction of bits in error and the average length of an error

792 Magneto-Optical Data Recording event will be larger in the unconstrained sequence output by the demodulator

than in the RLL channel data sequence. This occurs for two reasons: first, errors are usually multiplied during RLL sequence demodulation, i.e., a channel sequence error of length b bits will always produce an error event of and second, length 2b bits in the sequence output by the dernodulat~r;[’~] RLL codes have a coding rate < 1, i.e., a constrained binary sequence of length m must be used to represent the number of messages that are carried by an unconstrained sequence of length n < m. The multiplication of errors by the channel data demodulator when 2,7 modulation code is used is discussed at length in Sec. 4.5. There we show that a single erroneous channel bit can cause an error of length five bits in the demodulator output sequence. Much longer errors will result if the channel data sequence is contaminated in a way that causes the data waveform and clock derived by the data detector to lose synchronization. This occurs when the channel clock phase error is greater than 180 degrees; in effect, the bit-timing is then “slipped’ by one (or more) channel bits. If bit-timing is slipped by an odd number of channel bits, the demodulator will incorrectly phrase the corresponding segments of input channel data (see Table 1). Most data that is output while this internal (to the demodulator) sync-loss condition persists will, with high probability, be incorrect. If bit-timing is slipped by one or more channel bits, the demodulator may lose synchronization with the ECC decoder that follows it. That is, the demodulator will incorrectly determine where, i.e., at which bit in the channel data stream, the ECC codeword block actually begins. The recording format specified in ISOAEC 10089 does not provide sync patterns that can be used by the ECC decoder to independently establish ECC codeword sync. Thus, the demodulator must correctly determine the logical ECC codeword boundaries within the stream of channel data to enable subsequent ECC decoding to take place. Internal demodulator synchronization, as well as external synchronization of the demodulator and ECC decoder, is initially established and maintained by having the demodulator search for, and locate, special patterns of channel data that are periodically inserted during recording by the modulation encoder. If an error event causes one of these sync patterns to be entirely missed, or be found at the wrong place within the stream of recovered channel data that is processed when the demodulator attempts sync pattern detection, errors due to incorrect bit-timing may result. The ECC decoder could still fbnction when channel sync is lost if the unconstrained data stream output by the demodulator contained synchronization

Data Reliability and Errors 793 patterns that the ECC decoder itself could use to determine ECC codeword boundaries. Synchronization errors are discussed in Sec. 3.2. The ECC decoder processes the data that is output by the demodulator; it detects and corrects any and all of those data error types that it is designed to recognize and handle. The type of ECC codes used with optical data storage systems are known as Reed Solomon (RS)codes. These are block codes; encoded data consists of codewords, or blocks, that contain a fixed number of bits. Encoding consists of organizing the block of binary data that is to be encoded into a succession of multibit symbols called information symbols, computing a number of additional symbols (of the same length) called pan'@ checks and appending the parity checks to the information symbols to form a codeword. (Eight-bit information symbols are used in the RS codes employed by optical data storage systems.) An RS code will correct up to a certain number of erroneous symbols in a given codeword. A symbol is incorrect if only one, or more, of its constituent bits are in error. A single wrong bit will cause only one wrong symbol, but an error burst of b > 1 contiguous incorrect bits, which can start at an arbitrary point within a code symbol, will cause a burst of, at most, int[(b-l)/q]+2 incorrect symbols, where q is the number of bits per symbol and int[x] indicatesthe largest integer contained in x. The RS code used in MO optical storage systems has codewords of length about 120 bytes (i.e., 120 eight-bit symbols) and it can correct up to eight erroneous bytes per word. If the correctable number of erroneous symbols (errors), or less, occur in a codeword, all of the errors will be corrected! If more than the correctable number of errors appear in a given ECC codeword, one of the following will occur: either (i) the ECC decoder will recognize that the received block is contaminated with an uncorrectable error pattern and subsequently will inform the system controller that the data being returned (i.e., the data contained in the information symbols of the corresponding codeword) is contaminated, or (ii) the decoder will mistakenly interpret the received block as containing a correctable error pattern which contaminates some codeword that is different than the one that actually was written to the storage medium. When the latter occurs the decoder corrects the errors it believes it has found and passes the data contained in the information symbols of the hlse codeword (i.e., the wrong data) to the system controller without informing the controller that anythmg out of the ordinary has O C C U K ~ ~ .This event is known as misdecoding or catastrophic decoding. Suppose the RS code can correct 2 t errors in a codeword, and that the words have length N = K + 2t symbols, where K is the number of

794 Magneto-Optical Data Recording information symbols in the word and 2t is the number of added parity checks. In case (i) above, the system controller is aware that > t of the N symbols that constituted the codeword were incorrect, and therefore that > t erroneous symbols may be contained in the K symbols returned to it by the ECC decoder (one or more of the > t errors in the contaminated codeword may have been in the parity check symbols, which are not output by the decoder). But, which symbols are in error is not known . In case (ii), the controller is unaware that the block of K symbols it received from the ECC decoder contains any errors at all, when in fact, for RS codes, the block may contain 2t + 1 errors since any two codewords of an Rs code have at least 2t + 1 different symbols. Estimation of the reliability of data output by the ECC decoder consists of computing the statistical probability that an uncorrectable number of erroneous symbols will occur in an arbitrary codeword received by the decoder and, in addition, calculating the conditional probability that a misdecode will occur when the decoder processes an uncorrectable word. Methods for estimating data reliability at both the input to, and output of, an RS ECC decoder are covered in Sec. 5.0.

3.0 DATA RELIABILITY ASPECTS OF THE RECORDING FORMAT A detailed description of the recording format used in MO data storage systems is given in this section. The discussion is aimed at disclosing how the recording format contributes to the control of errors. In particular, we shall describe how one specific recording format, namely the continuous composite servo (CCS) recording format specified in ISO/IEC 10089, contributes to the reliability of the data writing and recovery processes. Information to be stored by a disk storage system is usually organized into contiguous, uniformly sized blocks prior to its being recorded. Each of these blocks is written onto a portion of the storage medium that is referred to as a sector. Each sector contains user data and any ECC parity information related to it, i.e., calculated from it. Sectors also contain ancillary system information such as the sector address which usually specifies the sector’s physical location on the disk, e.g., the track number and azimuthal position in the track at which the sector will be written. For example, unconstrained user data that is generated by the data source of Fig. 1 might be parsed into contiguous 512-byte, or 1024-byte, segments

Data Reliability and Errors 795 before introducing it to the ECC encoder; after ECC encoding, additional (unconstrained)data that identie the information block and which provide a unique header that can be used to synchronize an ECC decoder may be added. Just prior to writing a block of information into a sector on the disk, i.e., during the transformation of the user data and its related ECC parity data into channel data by the modulation encoder, certain special patterns of channel data are embedded into the channel data representation of the user and parity data. These patterns, which are known as sector marks, sync fields, VFO fields, address marks, etc., cause sequences of marks to be written onto the storage medium which (during playback) will produce playback signal segments that are used by the drive’s channel data detector/ demodulator to adjust the frequency and phase of the read channel clock (see Sec. 2.3), determine the correct phrasing of the recovered channel data sequence (see Sec. 2.1) and establish the exact boundaries of the user data and ECC parity data within the information that is recovered from the sector (see Sec. 2.4). In some cases, additional special patterns will be added when writing the sector to provide playback signal segments that can be used by the drive to obtain the magnitude and direction of any residual tracking and/or focus servo error. The information that constitutes a sector is usually written onto the medium in two parts. The first part of the sector, which contains from 3 to 12 percent of the information in the sector, depending upon the recording format, is known as the sector header. The header consists entirely of multiple fields of the above-mentioned special patterns together with sector address data, and it is prerecorded, i.e., the headers of 211 sectors are placed on the disk either when it is manufactured or when it is certifidformatted for use. The sector headers are used by the drive when it records new user data on the disk; the header is read in order to adjust the drive’s channel clock, verie the current position of the writehead head on the disk and initialize/start the write sequence that places the remainder of the sector’s data adjacent to (i.e., directly following) the header. In effect, the prerecorded headers act as fiducials that are used by the disk drive to accurately and reliably position recorded data on the disk. The remainder ofthe sector, i.e., the part that follows the header, will consist of user (input) data, information that is dependent on the user data (e.g., the ECC parity symbols that were mentioned earlier) and some additional user data-independentspecial patterns. In some sectors on the disk, data that describes the specific storage medium/ disk being used is substituted for the user data. Note that entire nonheader

796 Magneto-Optical Data Recording parts of the sector are always completely written in a single write operation, i.e., the nonheader portion of a sector is completely filled when data is recorded in a sector; if there is not enough input data to completely fill the sector then an appropriate number ofpudding bits, e.g., a contiguous string of “zeros”, are added to the data to ensure that the entire sector is filled when it is recorded. This ensures that each written sector contains the same amount of information which, in turn, causes the various data fields and special patterns of the sector to always occupy identical relative physical locations in each sector. This simplifies, and enhances the reliability of, the process of locating these fields and patterns during playback. The channel data sequence that represents a sector is written on the storage medium as a succession of small marks and intervening spaces that cover the disk’s surface. Optical disk drives use a single spiral of such written marks while magnetic disks use concentric circles of written marks, but in either case the recorded information on a single turn of the disk is referred to as a truck. Before any data can be recovered from the recorded storage medium, the read head must first access the medium and find the data that is requested by the storage system controller. This usually involves moving the writehead head to a particular radius of the disk and locating the track(s) and sector@)that contain the requested data. If the storage system was just started, e.g., just turned on, or if a new disk was just inserted into a removable-medium storage system prior to the data request, the drive must also synchronize its channel data clock to the disk since spindle RPM may have changed, or some of, or all of, the removable disk may have been written on a different drive. In the case of a removable disk drive, the system would also have to learn some important details about the current disk (e.g., what sector size is being used on the disk, the nominal amplitude and polarity of signals that are obtained from prerecorded sectors and headers, etc.). The disk drive can accomplish these tasks because specific information is always recorded at particular radii on the disk using various defined sector types. Each of these sector types have specific structures which enable the information recorded in them to be reliably read. Thus, in order to make a storage medium usable, i.e., to enable data to be reliably written on and read from it, the recorded data must be embedded in a recording format that (i) causes certain system information to be prerecorded in specific types of sectors at specific locations on the disk and (ii) provides for the writing of user data in its appropriate sector type at particular locations on the disk.

Data Reliability and Errors 797 3.1

The CCS Recording Format

In the following discussion of recording formats we concentrate on the continuouscompositeservo (CCS) format that is specified in the International Standard ISODEC 10089. Note that the CCS format is one of two recording formats specified in ISODEC 10089; the CCS format specifies the use of 2,7 RLL modulation and PPM write waveforms. Although this format is designed for an MO disk storage system, many of its features are common to WORM optical disk and magnetic disk systems. The other format defined by ISODEC 10089, which is known as the sampled-servo (SS) format; specifies the use of constant block length FOOF modulation code and PPM write waveforms. Because the CCS format is exclusively specified in the second generation, extended capacity 130-mm MO optical recording system working standard (see Sec. 2. I), we shall not consider the SS format in this chapter. The extended capacity MO recording format is very similar to the CCS format described in this section. A high level map of the information that is contained in various radial zones of an ISODEC 10089-compliantCCS format disk is given as Fig. 4. The information contained in the entire Phase Encoded Part (PEP) zone and in the header portion of every sector found in all of the other zones of the disk (except the Reflective zone, which contains no recorded information) is preformatted, i.e., this information is camed by marks, or pits, that are pressed into the surface of the disk. This preformatted information is concurrently copied to the disk at the time of its manufacture; it is not serially written by a focused laser beam. Starting at a radius of 29.52 mm, i.e., at the first track of the inner Standard Formatted Part (SFP) zone, and extending to a radius of 61.00 mm (the maximum recorded radius of the disk), the entire surface of the disk is covered with a narrow (0.5 microns wide), continuous, spiral groove having a nominal pitch of 625 turns/mm (1.6 microns spacing between turns). This tracking groove is followed by the drive’stracking servo system while the disk is being written andor read. In all disk zones other than the Reflective and PEP zones, information is serially written by using a focused laser beam to mark the featureless regions of the storage medium (called lands) that lie between the turns of the spiral tracking groove. However, such serial laser-writing of information is not done in the land regions located in the header part of any sector that lies at a radius 2 29.52 mm; these header regions contain preformatted marks that represent the multiple fields of special patterns that make up the headers.

798 Magneto-Optical Data Recording

Starting Physical Radius Track

Zone Name

Information Contained in Each Track

(mm) Number 27.00 29.00 29.50 29.52 29.70 -OOO1

30.00

60.15 60.50

PEH1

I

PEP#2

I

PEH3

SFP#l SFP#2 SFP#3 SFP#4 ...... SFp#17 or #31

m 0002 0003

60.00

Reflective PEP Transition Inner SFP Manufacturer’s her

N-3 N-2 N N+1

UserDMA

DMAs

User Data

3 1 or 17 sectors of User Data (depending upon whether 5 12 B or 1024 B sectors are used)

User DMA Manufacturer’s Outer Outer SFP Leadat

DMAs

SFP#l SFH2 SFP#3 SFP#4 ......

SFP#17 or #31

Figure 4. High level organization of information on a CCS-formatted disk.

The Reflective zone has no specified recording within it, but it must contain the same recording layer, i e., it must exhibit the same writehead characteristics, as all other zones of the disk. The featureless, mirrorlike, reflective surface provided by this zone could (at the drive manufacturer’s option) be used to acquire focus when the disk is initially accessed by the drive. The PEP zone found at the inner recorded radius of the disk is used to provide information about the format of the disk (i.e., whether the CCS or sampled servo format is used and the number of bytes per sector), the

Data Reliability and Errors 799 precise physical location at which certain information is located on the disk (e.g., the track number at which the outer SFP zone starts) and to specifjl some of the disk’s writdread characteristics (i.e., the baseline reflectivity of non-preformatted land portions of the disk, the maximum allowed read power and the nominal amplitude and polarity of the waveform that should be obtained when preformatted sections of the disk are read). As was mentioned above, no tracking groove exists in the PEP zone. To insure that a disk can be initially accessed and read by the drive, the essential systeddisk information contained in the PEP zone is formatted in a way that allows it to be read without track following and without the use of a precise, synchronous channel clock. To accomplish this, the information in the PEP zone is organized into sectors that each contain from 187 to 189 bits, but each bit has length equal to 6561; along the arc defined by the local disk track, i.e., each PEP-bit is 656 times longer than one of the fulldensity channel bits that result from 2,7 RLL modulation. A PEP-bit is designated as a “zero” or “one” by causing either the first half or last half of a PEP-bit cell on the disk to be filled with preformatted pits having length 2Twand separation 2Tw along the tangential direction of the disk, i.e., along the disk track direction. There are exactly three PEP sectors per track in the PEP zone, and the track spacing is maintained at the nominal value of 1.6 microns. Moreover, each PEP sector contains exactly the same information; all PEP-bits (and therefore all PEP sectors) in the PEP zone are aligned radially. Thus, when the drive reads the disk at an arbitrary radius in the PEP zone, it “sees” angular track segments that are filled with preformatted pits and other angular segments that are specularly reflecting lands, regardless of whether or not the focused read spot crosses multiple tracks in the PEP zone. The recording format used in the PEP zone enables reading of the PEP zone data without tracking or precise read channel clocking. In addition, the regular pattern of preformatted marks makes it possible to coarsely synchronize the 2,7 RLL channel clock while the PEP zone is being read. The data fields that make up a single PEP sector are as shown in Fig. 5 . The Preamble consists of 16 “zero” PEP-bits; Sync is a single “one” PEP-bit; the Sector Number is the 8-PEP-bit binary-codeddecimal (BCD) representation of either 0, 1 or 2; CRC contains the parity of an 8-bit cyclic redundancy check (CRC)error detection codeword that extends over the Sector Number and Data fields (CRCerror detecting codes are discussed in Sec. 3.3); and the Gap consists of 10 to 12 “zero” PEP-bits. The Gap’s

800 Magneto-Optical Data Recording variable length allows a rather loose tolerance on the length of the PEP-bits. The Data field wntains information about the recording format, the outer SFP zone starting track number and the disk’s writehead characteristics.

S Field Content

Preamble

y n

Sector Number

Data

CRC

Gap

8

144

8

10-12

C

Field Size (PEP bits)

16

1

Figure 5. Structure of PEP zone sector (CCS format).

The Transitionzone is an area in which the recording format changes from that of the PEP zone, which contains no continuous spiral tracking groove, to a format that contains a tracking groove. Information is recorded in the Transition zone as fulldensity 2,7 RLL channel sequences, i.e., each bit of user information is represented by a length of track that corresponds to 2Tw(versus the 656Twper recorded PEP-bit). Recording at all disk radii larger than 29.52 mm is done using a PPM write waveform that is derived (see Sec. 2.2) from fulldensity 2,7 RLL channel sequences. The storage medium is marked on the land areas that occur between the turns of the spiral tracking groove. In addition, the 2,7 RLL modulated data is organized into either 3 1 or 17 sectors per track depending on whether the 5 12 user byte per sector or the 1024 user byte per sector form of the CCS format specified in ISO/IEC 10089 is used (one or the other of these must be used globally over a given disk, but the drive will handle either of them). The storage medium is never marked in the header part of any sector since preformatted information resides there. The specific structure of the fulldensity recording sector format is discussed later in this section.

Data Reliability and Errors 801 Each sector of data in the SFP zones (inner and outer) replicates the information that was recorded in the PEP zone (as the first 18 user bytes of each of these sectors) and contains additional storage medium information (366 bytes) and system information (64 bytes). These sectors are mostly filled if the 5 12 user byte per sector format is used; the final 5 12 user bytes of the SFP sectors are “all zeros” if the 1024 user byte per sector format is used. The user data area in each sector of the Manufacturer ’s zones (inner and outer) are reserved for the use of the storage medium manufacturer. The Lead-Out zone is also reserved for the medium manufacturer’s use. The tracks of the User zone are numbered fkom 0 through N. Tracks 0000 to 0002 and tracks N-2 to N hold defect management information in the user bytes of their sectors. These six tracks constitute the Defect Management Areas (DMAs) of the disk; they are discussed in the last part of this section. Tracks 0003 through N-3, inclusive, are used to store arbitrary user data in the user bytes of their sectors. Thus, up to 3 1x5 12x(N-5) or 17x 1024x(N-5) user bytes can be stored on one side of the 130-mm MO disk, depending on whether 512-byte or 1024-byte user data sectors are employed. Since first generation MO disks specified in standard ISOAEC 10089 have a nominal spacing of 1.6 microns between tracks in the User zone, N = 18,750 for these disks. Thus, the first generation 5.25-inch MO disks will store a maximum of about 325 Mbytes of user data on one side when 1024-user byte sectors are used. In the first half of 1994, the second generation 5.25-inch diameter MO disk storage systems became commercially available. The recording format ofthis system differs only marginally from that specified in ISOAEC 10089. In particular, PPM recording and 2,7 RLL modulation, as well as the ECC and full density recorded sector structure that are defined in ISOAEC 10089 were retained (the structure of full density sectors will be described in the next subsection). Two significant changes were made to the first generation M O recording format to arrive at the recording format used in the second generation system. First, the intertrack spacing was decreased to 1.39 microns. Secondly, zoned constant angular velocity (ZCAV) was used. Via ZCAV, the disk is divided into a number of radial bands, or zones, that each have a different channel bit clock frequency (the channel bit clock frequency is fixed in each zone, however). The channel bit clock is varied in step-wiseincrements with disk radius in an attempt to hold the spatial length that correspondsto T, essentially fixed over the entire User zone of the disk. The first generation MO system, which has the spatial extent of T, linearly increasing with radius over the entire disk, can be considered to be a singlezone ZCAV disk. The second generation 5.25-inch MO disk that employs

802 Magneto-Optical Data Recording 1024 user bytes per sector has 16 ZCAV zones distributed over its User zone area, which extends radially from 30 mm to 60 mm. The ZCAV zone at the inner radius of the User zone holds 20.2 sectors per track and the ZCAV zone at the outer radius holds 39.03 sectors per track. Thus, the length of T, is maintained at a more or less constant value (- 0.4 1 microns to 0.45 microns) over the entire User zone of the second generation MO system’s ZCAV disk. (The first generation system has T, 0.5 1 microns at a 30 mm disk radius and T, linearly increases with radius to twice this value at the outer portion of the User zone.) Second generation, 5.25-inch diameter MO system disks that employ 1024-userbyte sectors achieve twice the user storage capacity of 1024-user byte sectored first generation disks; they store a maximumof 654 Mbytes per disk side. Second generation 5.25inch MO disks that employ 5 12-user byte sectors have a similar relationship to first generation 512-user byte sectored disks as do the 1024-user byte sectored disks and their first generation brethren. It should be mentioned that the small decreases in track spacing and minimum T, that are needed to realize the 2x capacity second generation 5.25-inch diameter MO system were somewhat enabled by requiring the use of a shorter wavelength (780 nm) laser to effect writing and reading in the second generation system (830 nm wavelength lasers could be used in the first generation system). In mid-1994, one manufacturer began commercial shipments of 5.25inch diameter MO drives with a maximum storage capacity of more than 1,000 user Mbytes per disk side. This 3x system utilized PLM recording, 1,7 RLL modulation and ZCAV with 30 zones (1024 user byte sectors) or 27 zones (5 12 user byte sectors). The structureof the full density recorded sectors of this system is similar to that described in the next subsection. MO disk drives that use disks that store 1.3 Gbyteddisk-side became commercially available in 1995. The 4x capacity of these disks was achieved by using ZCAV, 1,7 RLL modulation, PWM recording, and by reducing the nominal length of T, to 0.31 pm and decreasing track spacing to 1.19 pm (680 nm lasers are specified to accommodate the reduced feature sizes on the disk.

-

3.2

Full Density Data Sectors

The individual data bits contained in the PEP sectors discussed above (i.e., the PEP-bits) had length correspondingto 656T,. PEP-bitsare read as “zero” or “one” depending on whether the mean amplitude of the playback signal corresponding to the first half of the PEP-bit is lower or higher than the signal amplitude obtained when reading the last half of the PEP-bit. This means that each PEP-bit is represented by the pair of channel bits

Data Reliability and Errors 803 {0,1} or { l,O},i.e.,dataismappedtothesePEP-bitsviaa d=O,k=2 RLL modulation code knownasphase modulation (PM) code. Each PM channel bit has length equal to one-half of a PEP-bit, i.e., PM is a rate 0.5 modulation code. There are from 187 to 189 PEP-bits per PEP sector (144 of these are user data) and there are three PEP sectors per track. And, all sectors in the PEP zone of the disk contain exactly the same information. Clearly, in the PEP recording zone, data reliability is achieved by a combination of very low data density and very large redundancy. In effect, the aggregate storage capacity of the entire PEP zone is only 144 bits! Full density sectors, on the other hand, contain channel bits that correspond in length to T,, and ECC-encoded user data is mapped to them via 2,7 RLL modulation code. When the 1024-user byte sectors defined in ISOiIEC 10089are used, each full density sector on the disk will contain the equivalent of 1360 bytes of information, 1024 of which may be unique user data, and there will be 17 such fulldensity sectors per track. When the 5 12user-byte sectors are used, each fulldensity sector contains the equivalentof 746 bytes of information,5 12 of which may be unique user data, and there are 3 1 sectors per track. The sectors recorded at full density, i.e., the sectors of all disk zones at radii 2 29.52 mm, have the structure illustrated in Fig. 6. The number of user bytes in all of the full density sectors on the disk (5 12 or 1024) is specified by the first data byte of every sector in both the PEP and SFP zones. The header parts of each fulldensity sector (which are comprised ofthe equivalent of 52 bytes of preformatted user information) are the same for both 5 12 and 1024 user byte sectors. Each fulldensity recorded sector clearly contains special channel data patterns and other information in addition to user data;this overhead co~lsumesabout 25% of the real estate that correspondsto each 1024-userbyte sector (3 1% in the case of 5 12-user byte sectors). The sole purpose of this overhead is to insure that user data camed by fulldensity sectors can reliably be written to and read from the storage medium. In the remainder of this subsection, we describe the data reliability aspects of each sector part, orjeld, that is identified in Fig. 6. Sector Mark The Sector Mark (SM) consists of the preformatted mark (pit) pattern lop, 6L, 6P, 14L, 6P, 6L, 6P, 6L, lop, OOXOOlOOlO where the notation nP,mL indicates a pit of length nT, followed by a land of length mT, and 00x0010010 represents a track segment of length 1OT, that contains pits of length -T, only at the positions corresponding to the “ones” (theXcould be either a “one” or a “zero”, the choice is left to the disk manufacturer). The SM thus has total length correspondingto 80T,, which means that this field occupies storage medium real estate that could have been used to store 5 bytes of user information (via 2,7 RLL modulation).

804 Magneto-Optical Data Recording The SM is unique since it is the only sector field located at a disk radius 229.52 mm that contains marks (pits) longer than T, (pits in the marked half of PEP-bits in the PEP zone of the disk have length 2T, and all other pits on the disk have length T,); this fact allows the SM to be used as an easily found (by the drive)Jag that unambiguously indicates the beginning of a sector.

Field Contenl

Field Size (bytes)

1 f

User Data

650

c

Pre-formatted Header (52 bytes)

t

rial

c Re?; (1 2741665

Entire Sector (1360/746 bytes)

1

j +

Figure 6. Structure of fulldensity 1024/5 12 user byte sectors (CCS format).

ID Fields. The three sector Identification Fields (IDl, ID, and ID,) of a given sector are identical; each consists of five bytes of information (recorded, of course, as PPM-written 2,7 RLL modulated channel data). The first and second bytes are the most signlficant byte (MSB) and least signijicant byte (LSB) respectively of the binary coded decimal (BCD) representation of the number of the physical disk track in which the sector is located. The third byte specifies (in BCD) the sector number (i.e., it has numerical value 0 through 17 in the case of 1024 user byte sectors) and whether the specific ID field is the first, second or third ID field of the header. The fourth and fifth bytes contain the parity check bits of a 16-bit cyclic redundancy check (CRC) codeword that extends across all five bytes of the given ID field, i.e., all 40 bits of a given ID field belong to a single CRC codeword. Because of this coding of the ID field, many of the errors that may contaminate the codeword, i.e., that occur within a single ID field when it is read from the disk, can be reliably detected. For example, all single errors with length 216 bits and more than 99.995% of all single errors of length > 16 bits are detected via this coding structure. A more detailed

Data Reliability and Errors 805 description of the ID field CRC code used in this recording format is given in Sec. 3.3. Note that because the track and sector numbers vary, the bit pattern of the fourth and fifth ID field bytes will also vary (because the content of the CRC parity bytes is dependent on the value of the first three ID field bytes); this causes the channel data representation of these bytes to vary as well. Thus, the channel data representation of an ID field might not end with one of the entire channel phrases given on the right hand side of Table 1, i.e., it could end at either the second bit, fourth bit, sixth bit or last bit of any one of the seven possible 2,7 channel phrases. Address Marks. The three Address Mark (AM) fields are identical and consist of the PPM-written (i.e., pits of length T, occur only at the “ones”) 16-bit channel data pattern 0100 1000 0000 0100, which is not a legal 2,7 RLL sequence since it contains eight contiguous “zeros.” The purpose of the AM field(s) is to establish the phrasing of the 2,7 RLL data that is processed by the 2,7 demodulator during playback. This is easily accomplished if the demodulator recognizes the AM because the unique AM field channel sequence has two contiguous “ones” (i.e., the “ones” separated by the eight “zeros”) that occupy even and odd channel data positions respectively in the 16-bit pattern (AM bit numbering is taken to run from bit 0 through bit 15). The AM field is designed to preserve the 2,7 RLL constraints at the boundary between itself and any of the VFO fields that may precede it in the sector header (see the following discussion of the VFO fields). The AM field is also designed to preserve the 2,7 RLL constraints when it is catenated with the arbitrary 2,7 channel phrase that could start the channel data representation of any ID field (because the AM ends with a valid 2,7 channel phrase). VFO Fields. The fields WO#1 and VFO #2 consist of the preformatted pit pattern that results from placing pits of length -T, at each “one” in the following 2,7 RLL sequences:

-

VFO#l: 192 channel bits 3 01001001001....010010 VF0#2: 128 channel bits 3 10010010010....010010 VF0#2: 128 channel bits

* 00010010010....010010

The first of the two VF0#2 fields given above is used if the ID field that precedes it ends with any complete 2,7 RLL user data phrase (where the user data phrases are those given on the left hand side of Table 1) or with a complete user phrase followed by the orphan bit “0;” the second VF0#2 field is used if the ID field that precedes it ends with a complete user phrase

806 Magneto-Optical Data Recording that is followed by the orphan bit “1 ,”or one the partial user phrases “00,” “01,” or “001.” This catenation of ID fields that have variable ending bit patterns and one of the two VF0#2 fields given above is illustrated in Fig. 7. Note that the complete phrases referred to in the first column of Fig. 7 could be any of the user phrases shown on the left hand side of Table 1, while the complete phrases mentioned in the second column of Fig. 7 are the corresponding 2,7 channel data as given on the right hand side of Table 1.

I ID Field-Ending User Data

Corresponding ID FieldEnding Channel Data

Complete Phrase Complete Phrase Complete Phrase + 00 Complete Phrase + 0 Complete Phrase + 1 Complete Phrase + 0 1 Complete Phrase + 00 Complete Phrase + 000 1 Complete Phrase + 0 1 Complete Phrase + 1001 Complete Phrase +001 Complete Phrase + 00 1001

Following W0#2 Field 100100100100100100......10010 100100100100100100...... 10010 000 100100100100100.....,10010 000 100100100100100......lo0 10 000 100100100100100......lo010 000100100100 100100......10010

Figure 7. Illustration of the catenation of 1D fields and W0#2 fields.

From Fig. 7 it can be seen that regardless of the ending of the ID field, by selection of the appropriate one of the two possible VF0#2 fields, a 2,7 RLL channel data sequence that decodes properly, i.e., produces the correct ID field data upon 2,7 RLL demodulation and contains an invariant VF0#2 field pattern (after the first three VF0#2 bits) is obtained. The VFO#l pattern, which is designed to preserve the 2,7 RLL constraints when catenated with the trailing bits of the SM, is identical to the two possible VF0#2 patterns in its ending 125 bits. Both VFO#1 and VF0#2 are used to lockup, i.e., establish the proper frequency and phase of, the drive’s writehead channel clock when the sector header is read. More specifically, these fields establish the write channel clock’s frequency and phase when the sector is being written, and they are used to obtain a preliminary establishment of the read channel clock’s frequency and phase when the entire sector is being read. Final frequency and phase lock of the read detection clock is established via VFO#3 and the Sync field that

Data Reliability and Errors 807 follows it since those fields are actually written by the drive when it records the portion of the sector that follows the header. VF0#3 is identical to VFO#l. VFO#1 is longer than VF0#2 as it is the first VFO field that is encountered when the sector is read. Postamble. The above discussion of VFO fields explainedthat one of two possible W0#2 patterns is used so that the fields ID, and ID,, which have variable content over the various sectors of a disk, can always be catenated with a constant length VFO field in a way that preserves the 2,7 FUL constraints, regardless of the information content of the particular ID field (see Fig. 7). The Postamble (PA) is a field of length 16 channel bits that serves a similar purpose with respect to the field ID,, i.e., the preformatted channel data segment found in the PA is identical to the first 16 bits of the appropriate one of the channel data patterns listed in the third column of Fig. 7. The PA is the final field in the preformatted header portion of the sector. All of the fields that follow the PA are meant to be totally, or partly, written by the drive. Offset Detection Flag. The Offset Detection Flag (ODF) field has length equivalentto 14 user bytes. This field begins with a mirror section of track that has length equivalent to 16Tw. This mirror segment of track has neither adjacent tracking grooves nor any preformatted pits (the continuous spiral tracking groove is not actually continuous over all zones of the disk having radius > 29.52 mm as was stated earlier; it is interrupted for a 16Tw interval only within the mirror portion of every ODF field). The mirror segment may be used by the drive to check focus. The smooth, featureless surface of the mirror segment enables the drive’s focuserror signal detector to “see” a focus servo signal that is not affected by light that is diffracted by the groove or pit structure-this ultimately contributes to the robustness of the focus maintenance process and thus indirectly improves recovered data reliability. This mirror segment of the ODF is directly followed in sequence by a 48Twlong track segment called the Gap, an 80Twlong segment called the Flag and another 48Twlong Gap. The contents of these three subfields are not specified in ISOAEC 10089. Moreover, these subfields are usually not used, i.e., they contain no preformatted channel data pattern(s) and are not written to by the drive; they are included in the MO recording format only to preserve compatibility with the sector format that is used with WORM optical disks (the WORM optical recording format provides the Flag subfield as a means to indicate whether the portion of the sector following the header has been previously written by the drive). The final

808 Magneto-Optical Data Recording segment of the ODF, which has length 32TW,is known as the Auto Laser Power Control (ALPC) area. The ALPC is an unrecorded length of track that is intended for testing the write laser power level. Optionally, any test channel data pattern that is written here should be designed to catenate with the W 0 # 3 pattern that will eventually be written in the immediately succeeding field, i.e., the test pattern should maintain the 2,7 RLL constraints when it is joined with the W 0 # 3 pattern. Sync. The Sync pattern consists of the 48-bit channel data sequence 01000010010000100010001001000100100000100100 1000. This pattern, which is written by the drive when the nonheader portion of the sector is recorded, cooperates with the W 0 # 3 pattern to (i) synchronize and phase the read channel clock to the playback waveform (see Sec. 2.3), (ii) align, or phrase, the 2,7 RLL demodulator to the playback channel data sequence (see Sec. 2.4) and (iii) reliably locate the exact start of, i.e., the first channel bit of, the User Data field (also see Sec. 2.4). Fine adjustment of the frequency and phasing of the read channel data clock are performed when W 0 # 3 is read (coarse adjustment is already obtained via W O # l and W 0 # 2 ; reading W 0 # 3 allows the drives read clock to adjust to any clock drift or clock offset relative to the timing established using the WO#l and VFO#2 fields, that occurred in the write clock of the drive that actually recorded W0#3). The drive’s demodulator determines exact phrasing of the playback channel data by determining when the Sync pattern is exactly matched with a copy of itself that is stored in a comparator. When such a pattern match occurs, the drive has also found the start of the User Data Field portion of the sector. The effect of channel data errors on the reliability of using this method to establish the boundary of the User Data field is discussed in Sec. 4.2. User Data Field. When 1024-user-byte sectors are employed, this field consists of 1259 bytes as follows: 1024 user data bytes; 12 bytes for defect managementhntrol; 4 bytes of CRC parity, 160 bytes of ECC parity; and 59 resynchronization bytes. When 512-user-byte sectors are employed, this field consists of 650 bytes as follows: 5 12 user data bytes; 12 bytes for defect managementkontrol; 2 all “zero” bytes; 4 bytes of CRC parity, 80 bytes of ECC parity; and 40 resynchronization bytes. Diagrams of the byte-level layout of the Sync and User Data field portions of 1024user-byte sectors and 5 12-user-byte sectors are given in Fig. 8 and Fig. 9, respectively.

Data Reliability and Errors 809 RowNo. I Column No. j

+

0

1

2

3

4

s

6 -

7

D7

D8

8

-

D27

D28

I I I

D37

D38

I

D17

9

t

DQ

D18 DlQ I D2Q D39

D47

D57

D67

DQQ7

4

01007

3

D1017 -

2

P1.3

I 0

-1

-2 -3

-I 4

-IS -1 6

Figure 8. Data fields of 1024-byte(user data) sector. Rows -1 through -16 contain parity check bytes of depth -10 interleaved RS error correcton code.

810 Magneto-Optical Data Recording

Row No. i

T 105 104 103 102 101 100 99 I

I

I

I

I

1

4 3

P1,4

I P2,l I P2,2 I P2,3 I I

I

2

I

1

0

-1

-2 -3

-1 4

-15 -16

Figure 9. Data fields of 5 12-byte (user data) sector. Rows -1 through -16 contain parity check bytes of a depth -5 interleaved RS error correcton code.

Data Reliability and Errors 811 The discussion in the remainder of this section pertains to 1024-userbyte sectors, but the comments made apply directly to 5 12-user-byte sectors as well; the main difference in these two sector types is simply in the number of bytes that make up each of the subfields that will be described below and in the number of interleaves of the RS ECC that is specified by ISO/IEC 10089. This RS ECC is used for detection and correction of errors in the User Data field (the same RS ECC is used in the 1024-user-byte and 5 12user-byte sectors, except that the codewords employed differ in length by two bytes). Referring to Fig. 8, the 1024 user data bytes are labeled D1, D2, ...., D1024; the 59 resynchronization bytes are labeled RS 1, ...., RS59; the 12 defect management bytes are labeled Pl,l, P1,2, ...., P3,4; the four CRC parity bytes are labeled C 1, ...., C4; and the 160 bytes of ECC parity information are labeled El, 1, E2,1, ...., E10,16. The three bytes labeled SBl, SB2 and SB3 that appear at the upper left hand of the figure, i.e., the first three bytes shown in the figure, make up the 48-channel-bit (3-userbyte equivalent) Sync field that precedes the User Data field. (Note: the Sync field is physically recorded on the disk as a particular 48-bit long segment of channel data, but since 2,7 RLL code maps each unconstrained user bit into exactly 2 channel bits, the 48-channel bit Sync field occupies the same real estate, or sector area, as would three bytes of user data.) The bytes SB1, SB2 and SB3 are not part of the User Data field; they are included in Fig. 8 only to illustrate their relationship to the resynchronization bytes that are embedded with a 20-byte spacing throughout the User Data field. To see that a resync byte occurs every 20 bytes, one must understand that the bytes specified in Fig. 8 are written to the disk sector row by row, from left to right. More specificallythe bytes labeled D 1, D2, ...., D 10 of row number 103 (i.e., the top row) are written first, immediately following the last byte (SB3) of the Sync field. These are directly followed by bytes D 11, ...., D20 of row number 102 (the second row) and byte D20 is directly followed by bytes RS1, D21, D22, ...., D30 of row number 101, etc. Resync Subfield. The Resync subfields are recorded as the channel data segment 0010 0000 0010 0100. Although we refer to this pattern as a Resync byte, it really consists of a unique, 16-bit, 2,7 RLL channel data segment, i.e., it normally cannot occur at the output of the 2,7 RLL modulator (see Table 1). (Recall that 2,7 RLL code is a rate 0.5 code; there are exactly two channel bits for every user data bit. The Resync field is not related to user data, i.e.., the 16 Resync field bits are not 2,7 encoded user information. Rather, they are a special 16-bit channel data pattern that occupies the same amount of disk track real

812 Magneto-Optical Data Recording estate as would be occupied by a single 2,7 RLL-encoded user byte-hence the name resync byte is generally used to when referring to this field.) This channel data segment is spliced into the channel data stream that results at the output of the 2,7 RLL modulator as it encodes the data. The structure of the Resync pattern allows such splicing to take place in a way that maintains the d,kconstraints ofthe 2,7 channel sequence. The 2,7 demodulator simply ignores the Resync patterns when it decodes channel data back to a user data stream. These Resync patterns are used to checwmaintainphrasing of the 2,7 RLL demodulator during playback. This is important in an optical recording system since defects in the disk recording layer, such as digs andor scratches, which may cause lengthy interruptions, or very noisy portions, of the playback signal, could cause the playback channel clock to become dephased with respect to the desired (i.e., noiseless) playback waveform. Since such defects may occur with nonnegligible probability, the Resync fields play an important role in maintaining data reliability. This is due to the fact that, if a storage medium defect causes the channel data clock to dephase by 180 degrees, or more, relative to the channel playback signal, the demodulator could lose “count” of the bits in the channel data stream. This would effect the phrasing of the received channel data by the demodulator and all succeeding data output by the demodulator in the current sector would (with high probability) be erroneous. When Resync bytes are used as shown in Fig. 8, the demodulator has an opportunity to regain clock/ playback signal phasing every 20 bytes. It accomplishes this by detecting the begkindend of a Resync pattern (by shifting the recovered channel data stream through a comparator that stores a copy of the Resync pattern) and resetting its channel bit count to the value that corresponds to the position of the detected Resync pattern in the User Data field. Defect Management Subfield. The twelve defect management pointer (DMP) bytes that constitute the Defect Management subfield, i.e., the bytes labeledPn,m in Fig. 8, are included in the ISODEC 10089 recording format specification principally to maintain consistency with the recording format used in WORM optical disk systems. When a sector is declared to be defective, it may be retired by rewriting its contents to one of the “spare” sectors in the User Data zone of the disk that are reserved for this purpose (see the discussion of defective sector management in Sec. 3.4 below). When a sector is retired, these DMP bytes can optionally be used to point to the spare sector to which the retired sector’s data has been moved, i.e., the DMP bytes will contain the track number and sector number of the specific

Data Reliability and Errors 813 spare sector that was used. Note that, although only four bytes are required to specify whether a sector has been remapped (1 byte) and identie the location of the rewritten sector (3 bytes), the format provides twelve DMP bytes; this 3-fold redundancy is used in the interest of data reliability. Regardless of whether the DMP subfield bytes are used, ISO/IEC 10089 specifies that information about replaced sectors must be written to the defect management area (DMA) of the disk. We discuss the DMA and its use in Sec. 3.4. ECC Parity Subfield. The 160 bytes labeled El,l, E2,1, ...., E10,16 in Fig. (8) are the parity bytes of ten 120-byte RS codewords that extend over all the bytes in each of the ten columns of that figure, i.e., columns 0, 1, 2, ...., 9. For example, RS ECC codeword #O contains the bytes D1, D l l , D21, ...., D1021, P2,3, El,l, ....,E1,16 and codeword #5 contains the bytes D5, D15, ...., E5,16. Since the User Data field bytes contained in Fig. 8 are sequentially written to the disk sector row by row (see the discussion in the preceding subsection entitled “User Data Field”), the column by column orientation of the RS ECC codewords causes any two contiguously recorded bytes to occur in different codewords. Such a data recording/coding structure, which causes the data that comprises a single ECC codeword to be periodically spaced throughout the sector, is referred to as an interleaved ECC coding system. The User Data field format illustrated in Fig. 8 employs adepth-I0 interleave, i.e., every tenth recorded byte falls in a given ECC codeword. Interleaving of codewords is a technique that is used to enhance an ECC system’s ability to handle long error events (i.e., burst errors). To illustrate this, consider a 24-byte long error burst that begins at byte D22 and extends to byte D45 in Fig. 8. This burst will contaminate two bytes in RS ECC words #0, #5, #6, #7,#8 and #9 while contaminating three bytes in each of words #1, #2, #3 and #4. Thus, due to depth-10 interleaving, when a 24-byte burst error occurs, it will cause a maximum of only three errors to appear in an individual codeword. Clearly, the depth-10 interleaved RS ECC is able to handle fairly long burst errors, e.g., if the RS ECC decoder were configured to correct the maximum of eight byte errors in each of the ten interleaved codewords, this format could correct any single error burst having length 84 bytes (four of these 84 bytes are resync bytes). Further discussion of the depth-10 interleaved RS ECC will be found in Sec. 3.3. The depth-10 RS ECC codewords are decoded immediately after channel data demodulation; the 160 ECC parity bytes are discarded after this processing occurs. The remaining data, namely the data bytes, the

814 Magneto-Optical Data Recording DMP bytes and the four RS CRC parity bytes (but not the Resync bytes as they are discarded by the demodulator) in the rows numbered from 0 through 103 in Fig. 8 are then processed by the RS error detecting decoder that is described in the next subsection. This is done to catch any errors that are not w m or detected,by the depth- 10 interleaved RS ECC. CRC Subfield. The four bytes of this subfield comprise the parity of a Reed Solomon code that is used only to detect the presence of any unmarked errors (i.e., erroneous data that is thought to be good) in the user data, DMP bytes and User Data field CRC bytes that appear at the output of the depth-10 interleaved RS ECC decoder. If the RS ECC decoder is designed to report detected uncorrectable error patterns, e.g., by raising a flag when a codeword that contains an uncorrectable error pattern is detected, then such unmarked errors will only occur due to mis-decodings of the depth-10 interleaved Reed Solomon ECC (see Secs. 1.1 and 1.2). The errordetecting RS codeword we are discussing here is formed prior to forming the depth-10 interleaved RS ECC codewords by first summing (xor) the ten bytes of each of the rows numbered from 1 through 103 and the six bytes in columns 0 through 5 in row 0 of Fig. 8 to form 104 new byte values (one corresponding to each row), and then using those 104 bytes to calculate 4 parity bytes, i.e., the bytes labeled C 1, ...., C4 in Fig. 8. These latter bytes, i.e., the four User Data field CRC bytes, are thus the parity bytes of a codeword that indirectly encodes the sector’s 1024 user data bytes and the twelve DMP bytes (we say that these bytes are indirectly encoded because sums of 104 subsets of their values, rather than the byte values themselves, are taken to be the information symbols of the code). Although these four parity bytes are referred to in ISO/IEC 10089 as CRC parity, this is not strictly correct. A CRC code is one that is designed to reliably detect only a single burst of erroneous bits having some maximum length, i.e., all the erroneous bits in the codeword must lie within a randomly placed segment of the codeword that has a specific maximum length. A RS code can either locate and correct a specific number of erroneous bytes that are randomly distributed throughout the RS codeword, or alternatively it can detect when a limited number of additional erroneous bytes are present somewhere (at unknown locations) in the codeword. The so-called CRC bytes that are contained in the User Data field are the parity bytes of a single 108-byte long RS code that is not used for error correction; it is used only to detect when the codeword contains erroneous bytes. That is, the decoder for this RS code is configured such that t, = 0 and td = 4, where t, and td are the maximum number of guaranteed correctable and

Data Reliability and Errors 815 detectable byte errors, respectively, in a codeword. If more than four of the 108 bytes of this codeword are erroneous, as is likely when a mis-decode of one of the depth-10 Rs ECC codewords occurs, there exists a finite probability (roughly one in 2564, or -2.33 x 10-lofor this RS code) that the errors will go undetected. Further details of the User Data field CRC code are given in Sec. 3.3. Buffer. The preformatted sector headers are always uniformly spaced along the recording track since they are written in a single recording session on a mastering machine that has a precision spindle and wellcontrolled rotational velocity. Because the 1274, or 655, bytes that comprise the nonheader portions of the disk's sectors (depending on whether 1024-userbyte or 5 12-user-bytes sectors are used) are recorded by the user, i.e., by the disk drive, which has less precise mechanical tolerances and greater spindle RPM jitter than the mastering machine that recorded the sector headers, these user-written portions of the sectors will have variable physical length. The Buffer field has nominal length equal to 320 channel bits (for 1024-user byte sectors), or 240 channel bits (for 512-user byte sectors), when the physical length of the entire user-written portion of the sector is precisely correct. Up to 16 of these channel bits are allotted to enable the user-written portion of the sector to end with a complete (i.e., demodulatable) 2,7 RLL phrase. That is, the first 16 bits of the Buffer field will be comprised of the first 16 bits of the appropriate one of the channel data sequences listed in the third column of Fig. 7. The remaining length of the Buffer allows for drive spindle motor speed tolerances and other electrical and mechanical tolerances that may affect the physical length of the user-written portion of a sector, i.e., the physical length of the Buffer varies in order to insure that the total physical length of each disk sector is uniform. The trailing part of the Buffer field is usually left unwritten since it adjoins the Sector Mark field of the next sector. 3.3

Error Detection and Error Correction

This subsection is meant to be a high level tutorial on the engineering aspects of the error detection and error correction coding specified in ISO/ IEC 10089. Accordingly, we shall not attempt to teach the mathematical foundations of error control coding here, or review the various classes of error control codes. Rather, we shall attempt to provide the reader with a general understanding of the functional aspects and limitations of the cyclic redundancy check (CRC) codes used for error detection and the Reed

816 Magneto-Optical Data Recording Solomon error correcting codes (RS ECCs) used for error correction/ detection in M O optical recording systems. Formal mathematics are used only to the degree necessary to accomplish these goals, e.g., to impart a feeling for the geometric structure of the codes of interest and to facilitate an understanding of why certain simple hardware configurationscan be used to implement the processes of error control encoding and error detection. Detailed information on the subject of error detection, error correction, the many classes of codes that are useful for controlling errors, hardware for the encoding and decoding of these codes, and the mathematical formalism that underlies such codes can be found in any of the many texts that treat this subject, e.g., Ref. 18. In what follows, many points about error control coding will be made without formal proof of them; virtually all of these points can be verified and amplified via Ref. 18. This general citation of Ref. 18 is made here to avoid cluttering the following presentation with a multitude of individual citations. In the discussion of full density sectors in the preceding subsection, we stated that the sector ID fields are protected by a 16-bit CRC code that is used to detect errors. Also, in the earlier discussion of PEP-sectors, it was mentioned that the Sector Number and Data fields of the PEP-sectors are protected by an 8-bit CRC error detecting code. An r-bit CRC code has r redundant parity bits per codeword (such bits are calledpurity bits because their value is determined from that of the “information” bits contained in the codeword; they are called redundant because they are used only for error control and thus do not add to the user information content of the codeword). A CRC code is used to detect the presence of a single long error event, i.e., a burst error, in a specified block of data. The length of a single burst error is defined by the number of bits in the shortest data segment that contains all the erroneous bits, even though some of the bits in that segment may not be in error. For example, if we send the 24-bit binary data stream 100000111010001110000000 and we receive 1000 01 11 1010 1001 0100 0000 we say that the received data is contaminated by an error burst of length 13 bits because the received data differs from the transmitted data only within the 13-bit segment that begins at the sixth bit and ends on the eighteenth bit. An r-bit CRC code will detect a single error burst of length I r bits that occurs anywhere in the data block spanned by the CRC codeword; the reliability of this error detection is loo%, i.e., such burst errors will always

Data Reliability and Errors 81 7 be detected via the CRC code. Single burst errors that have length > r bits will not always be detected. The fraction of single error bursts of length ( e l )bits that are not detected by such a code is 142r-1) and the fraction of bursts with length > ( e l ) bits that go undetected is 142r). In our discussion of the User Data field in Sec. 3.2, we stated that the user data and DMP bytes are the information that is protected by a 108symbol RS code that is used for error detection only and that the user data, DMP bytes and User Data field CRC parity bytes are the information protected by a depth-10 interleaved, 120-symbol RS code that can be used for error detection and/or correction. This latter code, which uses 8-bit bytes as its symbols, can correct t,; 0 I t, I 8 randomly located byte errors, or alternatively detect a total of t,+td ; 0 I td I 16-2tCerroneous bytes, in a given 120 byte long codeword. The value of t, (and the dependently related value of td) are established by the design of the disk drive’s RS ECC decoding hardwarehirmware. If the codeword contains e errors (i.e., e erroneous bytes) and e I t, ,then all e errors will be corrected. If the number of errors in the codeword is in the range t, < e I 16-t,, then the decoder will recognize that an uncorrectable error pattern has occurred and the codeword’s information bytes, i.e., all user data, DMP and User Data field CRC bytes contained in the codeword, will be marked as unreliable when they are returned to the controller. On the other hand, if the codeword contains e errors, such that e > 16-t,, then, with finite probability, the decoder will interpret the uncorrectable error pattern contained in the codeword to be a correctable error pattern and it will attempt to correct it. This latter event is known as misdecoding; additional errors are created by it. Erroneous data is inadvertently passed to the controller when a mis-decode occurs, i.e., the controller is not informed that some of the bytes delivered to it are erroneous. The probability of always detecting uncorrectable codewords decreases as t, increases. The conditional probability of experiencing a misdecode, given that an uncorrectable codeword occurs, increases with t,. These probabilities will be discussed in Sec. 5 . The magnitudes of t, and td are jointly constrained by

where the code’s minimum (Hamming) distance d,,,, = 17 for the 120symbol RS code being considered here. The choice of how many byte errors to correctldetect per RS code interleave (i.e., per RS codeword) is left to the drive manufacturer. These values must be carefblly chosen to optimize a

818 Magneto-Optical Data Recording given disk drive’s recovered data reliability (see Sec. 5 and the subdivisions of this section that treat syndrome generation and decoding). For this RS code, the work that must be done to locate and correct errors increases roughly as the cube of the number of errors being corrected. Thus, the amount of time needed to perform error correction, which impacts the maximum continuously sustainable data recovery rate, is affected by the choice of tc. Drive manufacturers can therefore favorably differentiatetheir products via fast ECC decoder implementation. The 108-symbol RS code mentioned above is used to detect residual errors in the user data, DMP and User Data field CRC bytes that appear at the output of the depth-10 interleaved, 120-symbol RS decoder. Thus, this 108-symbol RS code is meant to detect any errors that were not detected by, or were created by, the decoder of the interleaved, 120-symbol RS code that is layered beneath it. In general, such errors will only occur when the interleaved, 120-symbol RS code is mis-decoded, i.e., when the 120-symbol RS code’s decoder attempts to correct an error pattern that it is incapable of correcting. Polynomial Representation of Data Blocks. Using polynomials to represent blocks of digital data provides several advantages relative to the error control coding of data. This (polynomial) representation provides the basis for a formal mathematical structure that is used to readily accomplish both parity generation (i.e., encoding) and error detection; it also enables simple hardware that performs both of those functions to be realized. Moreover, the algorithms used to “invert” the encoding process in a computationally efficient manner, i.e., to enable practical error-correcting decoding of long, complex binary codewords, also rely on the polynomial representation of data. The degree (N1) polynomial N-1

Es.(3)

p(.) = p i x ’ = b, + blx + b2x2+. * .+bN-,xN-’ i=O

which has coefficients b, that take on binary values (i.e., they are either 0 or 1) is used to represent binary sequences of length N bits. For example, the 7bit sequence 0 110111 (most significant bit first) can be represented by the degree-6 polynomial

Eq. (4)

P(x)=x5 + x4 + x2+ x + 1

Data Reliability and Errors 819 (Note: Although the polynomial p(x) in Eq. (4) has degree = 5 , we understand that is has virtual degree = 6 since a total of seven terms is needed to represents a 7-bit data block; i.e., the coefficient b, of the term b& must be equal to zero.) Such polynomials can be divided in the normal way, except that binary field arithmetic must be used when doing this, i.e., addition is equivalent to subtraction and both are accomplished via the exclusive-or (xor) operation; multiplication is the same as it is in the field of real numbers. Thus,

and

As an example of dividing of two polynomials that both have binary coefficients, we shall compute p(x)/g(x),where g(x)=x3+ x + 1 and p(x) is given by Eq.(4):

7 + x2 + x + l

x3 + x + l x + x 5

3

x +

x +x 4

x + x

3

2

+

x+l 2

x4 +

x +x 3

2

x + x + 3

x +

1

x+l 2

x + x

That is,

820 Magneto-Optical Data Recording The remainder in Eq. (6) is Eq. (7)

p(x) mod g(x) = x* + x

This remainder also can be expressed as a segment of binary data. To do this we recognize that the degree of the remainder polynomial must always be exactly one less than the degree of the divisor and take the individual elements (bits) of the binary data segment to be the same as the coefficients of the remainder polynomial (the coefficient of the highest degree term becomes the MSb, but remember that there may one or more leading “zeros”). In our example above, the divisor has degree = 3 and therefore the binary representation of the remainder is 110. If binary digits, i.e., the field comprised of the numbers 0 and 1, are the information symbols utilized by the error control code, then the coefficients of the polynomials that are used to represent data blocks are the individual bits that constitute the data blocks, as in Eqs. (4) through (7) above. The 8-bit and 16-bit CRC error detecting codes that are used in the CCS recording format’s PEP sectors and full density sector ID fields use bits as their information symbols. Alternatively, the information symbols of the code may be contiguous segments, or q-tuples, of bits; in this case the binary data will be partitioned into contiguous q-bit words, each having one of 24 possible values, for the purpose of error control coding. The q-bit words are ordered such that they comprise the 24 sequential elements of an algebraic field. The field elements which correspond to the q-bit words in a data block serve as the coefficients of the polynomial that represents the data block. As an example, the RS codes used in MO recording utilize 8-bit words, i.e., bytes, as their information symbols. Using a mapping technique that is described below, each of the 256 possible byte values are placed in a one-to-one correspondence with the 256 elements 0, uO,ul,a*,....., u254 of a finite field known as Galoisjeld 256, or GF(256), which is also written as GF(2*). The elements 0 and a’; 0 I i I 254 are then used as the coefficients of the polynomial that represents a data block for the purposes of error control coding via RS codes. In essence, this is just a technique that allows each coefficient of the polynomial to correspond to eight bits, rather than only one bit. Irreducible Polynomials and Primitive Polynomials. A polynomial p(x) of degree-m is said to be irreducible if p(x) cannot be factored in the algebraic field of its coefficients (9-1) mod&) = 0 for N = P - 1

Data Reliability and Errors 821 An irreducible polynomial of degree > 1 cannot have any root that is an element (member) of the field that contains its coefficients. The irreducible polynomials of degree = m can be determined by completely factoring d2-'1-1. For example, for m = 4,the polynomial x15-l, which has binary coefficients, can be factored in the binary field [i.e., in the field comprised of the numbers 0, 1 which is also known as GF(2)] as

The factors with degree = 4, the degree-2 factor and the factor x+l in Eq. (8) are irreducible polynomials of degree = 4,2 and 1 respectively. An irreducible polynomial g(x) of degree = m is said to beprimitive if, and only if, (d-1) mod g(x) f 0 for all integers J < N = 2"-1 The degree-4 polynomials jz1+x+l and x4+9+1 in Eq. (8) are primitive polynomials of degree = 4;the polynomial x2+x+1is primitive in degree = 2. There are primitive polynomials of every degree. The Generator Polynomial for Primitive Cyclic Codes. The CRC and RS codes specified in ISODEC 10089 both belong to a class of codes known asprimitive cyclic codes. These codes have codewords that are N = 2"-1 symbols in length, where m is some integer > 0. If the codewords contain K < N information symbols and the remaining N-K symbols carry parity information, we say that the codewords belong to an N,K code. A degree N-K polynomial g(x) is said to be the generator polynomial for a N,K primitive cyclic error control code if, and only if, (x?+')l mod g(x) = 0 for N = 2"-1, where m is an integer > 0, and

g(x) is the product of irreducible polynomials that each have degree = m,or some integer that is a factor of m. As an example, the first and last factors on the right hand side of Eq. (8) can be used to form g(x)=(x4+ x + l)(x + l)=x5+ x4 + x2 + 1 which is the m = 4, degree-5 generator polynomial of an N = 15 bit, K = 10 bit primitive cyclic code. Encoding (Parity Calculation). The generator polynomial is used to obtain the parity symbols of an N,K codeword as follows: First, a degree-K-1 information polynomial is formed from a K symbol long data sequence b,,, b,, ...., b, by writing

822 Magneto-Optical Data Recording

The polynomial r(x) that represents the N-K parity check symbols is then obtained via the degree N-K generator polynomial by taking

= - [ i ( x ) P Kmod g(x)]

That is, theparitypolynomial is the negative of the degree-N-K-1 remainder that results when i ( x ) P K is divided by g(x). (Negation has no effect in the binary field of numbers; the minus sign appears on the right hand side of Eq. (10) only because some fields contain negative elements. For example, the ternary field consists of the numbers 0, 1,- 1 and ternary algebra is such that 1 + 1 = -1; -1 + -1 = 1; 1 + -1 = 0 and -1 x 1 = -1). The N-K parity symbols are the coefficients ri; 0 Ii IN-K- 1 of r(x). Thus, division by g(x) generates the code parity check symbols. The complete codeword polynomial is then obtained by catenating the shifted (i.e., multiplied by P Kinformation ) polynomial and the parity polynomial, i.e., by

and the correspondingencoded data block is obtained simply by catenating the coefficients of Eq. (1 1) as follows:

Recall that each element of the data sequence Eq. (12) represents a bit if the code's symbols are bits, i.e., elements of GF(2); the sequence Eq. (12) elements are q-bit words if the code's symbols are elements of the field GF(29). From Eq.(1 1) and the definition of r(x)given by Eq. (lo), we see that any codeword polynomial c(x) will satis6

Data Reliability and Errors 823 Eq. (13)

c(x) mod g(x) = 0

In fact, any polynomial having degree I N-1, which has its coefficients drawn from the field that containsthe code’s symbols, represents a codeword of the code generated by g(x) if it satisfies Eq. (13). There are exactly 2K such codewords for binary codes (i.e., codes that use bits as symbols) and 2qK such codewords for codes that use q-bit words as their symbols. Syndrome Calculation. Data that has been encoded to form codewords is subsequently “transmitted.” In the case of optical data storage this transmission of the data is done by writing it to a sector on the optical storage medium. The codewords are eventually read from the storage medium and “received” by the decoder. If the received codewords contain no errors, they will be identical to the original transmittedcodewords. If there are errors in the received codeword, it will differ from the original codeword only at the error locations, i.e., only those symbols that are erroneous will have values that are different from the corresponding symbols in the original codeword. Thus, an arbitrary received codeword can be represented by the polynomial t ( x ) = c(x) + e(x) where c(x) is given by Eq. (11) and e(x) is a degree-N- 1 polynomial that has “zero” coefficientsfor all terms exceptthose that correspond to the locations of erroneous symbols, i.e., e(x) represents the N-symbol error pattern that contaminates the received codeword. The first action of the decoder is to calculate the syndrome of the received codeword. The polynomial representation of the syndrome s(x) is the remainder that is obtained via division of the polynomial that represents the received codeword by the generator polynomial. Thus,

yields the degree-N-K-1 polynomial whose coefficients comprise the N-K symbol sequence that is the syndrome of the received codeword . From Eq.( 13) through Eq.(15), we see that s(x) is identically zero if there are no errors in the received word, i.e., if e(x) = 0. In fact, Eqs. (13) through (15) fbrther

824 Magneboptical Data Recording suggest that syndromes correspond not to received codewords, but to received error patterns. This is, in fact, the case. However, there are 2N, or 2qN, possible error patterns and only 2N-K, or 2dN-Q, possible syndromes, respectively, for primitive cyclic codes of length N symbols on the field GF(2), or GF(24). It would appear then, that a particular syndrome cannot be uniquely associated with, i.e., be generated by, a particular error pattern. This surmise is only partially correct; it turns out that there is a unique mapping among a subset of the syndromes and an equal-sized subset of the possible error patterns. And, the fraction of the total number of possible syndromes and error patterns that are uniquely related is dependent on the values of N, K, q and d,,,,, for the code. The mapping of syndromes to error patterns will now be explained. [The explanation will make use of the following facts: (i) the codewords of every block code (i.e., a code which produces codewords having a fixed number N of symbols) are spaced apart by some minimum distance d,,,,, ,which is just the minimum number of symbols by which any pair of codewords can differ, (ii) every block code has the all “zeros” block, i.e., the binary data block comprised of N all “zero” symbols, as one of its codewords, (iii) the weight of a codeword is equal to the number of q-bit symbols in the word that are not all “zero” symbols; possible values for codeword weight are w = 0, d,,,,,, d,,,,, + 1, d,,, +2, ...., N, and (iv) primitive cyclic codes are linear, i.e., any sum (symbol-by-symbolxor) of two or more codewords is also a codeword.] Consider a very large 2dimensional array that has 2qKcolumns and 2dN-0 rows, where q is an integer 2 1. Let the elements of this array be unique, each element being one of the possible binary sequences of length qN bits, i.e., the 2@ elements of the 2-D array comprise the total set of possible received codewords of a N,K code on GF(24). Now, the first row of this 2-D array contains all the 2qKerror-free codewords of the code, ordered such that the lowest weight codewords occupy the leftmost columns of the row. That is, the all “zeros” codeword, which has w = 0, resides in the first column (i.e., in column 0); the second column houses the next-lowest weight codeword, etc. Thus, if the code has 1 codewords that have weight d,,, ,the next 1 columns that are adjacent to the first column of the first row of our 2D array, i.e., columns 1 through I, will be occupied by error-free codewords of weight d,,,. Since the elements of the first row of our 2-D array are the codewords of the code, we shall denote them by the symbols c ~ , c~ ~, , c~ ~, , ~ , ....., co,2qK-1 ,where coJ;0 I j I 2qK-1 , indicates the element (codeword) in the O* row and]* column of our 2-D array.

Data Reliability and Errors 825 To fill the second row of our 2-D array, we first place an N-symbol error pattern of weight w = 1 symbols in the first column, i.e., in column number 0 of the second row. Usually, the w = 1 error pattern selected is that one which is most likely to occur. The remaining 2qK-1 columns of the second row are then filled by the N-symbol blocks that are obtained by adding (xor) the w = 1 error pattern in column 0 to each of the error-free codewords that occupy columns number 1 through 2qK-1in the first row. Thus, the element in thej* column of the second row is formed by adding the error pattern in the O* column of the second row to the codeword co,, . Next, we select a different weight w = 1 error pattern and use it to fill the third row (i.e., row number 2) of the array in the same manner. Succeeding rows are filled in this fashion until all of the possible weight w = 1 error patterns have been used. Since there are

possible error patterns with weight w,each of rows 1 through n,( 1) = N x (24-1) is populated by adding an N-symbol, weight w = 1 error pattern to the appropriate error-free codewords of row number 0. Any set of 2qK binary data blocks (each block consists of N, q-bit symbols), that can be formed by adding the lowest weight block in the set to each of the error-free codewords of an N,K code on GF(29, is said to be a coset of that code. The lowest weight element(s) of a coset is called the coset Zeuder(s). The leader of the coset comprised of the 2qKerror-free codewords of the code is, of course, the all “zeros” codeword. Note that the contents of cosets that occupy rows 1through N(24-1)of our 2-D array are all of the possible received codewords that are contaminated by a weight w = 1 symbol error pattern. The w = 1, N-symbol blocks in the O& column of each of these rows result when the all “zeros” codeword is contaminated by each of the possible w = 1 error patterns. The next highest weight N-symbol block in each of these rows must have weight w 2 dmjn-l.If d,, > 2, the w = 1 pattern in the O* column of each of these rows will be the lowest weight N-symbol block in each row. Therefore, the leaders of the n,( 1) cosets that occupy rows 1through n,( 1) of our 2-D array are all of the possible w = 1 error patterns that may contaminate codewords of the code. Thus, the distance between any of the weight w = 1 coset leaders and any of the other N-symbol blocks in the leader’s coset is at least d,,,-2.

826 Magneto-Optical Data Recording There are

possible error patterns that have weight w = 2 symbols. Each of these error patterns is placed in the O* column of one of the next ne(2) unfilled rows of our 2-D array. The remainder of the columns of each of these rows are filled via the same method used to fill the n,( 1) rows that comprise the cosets having weight w = 1 leaders. Thus, if d,,,,, > 4,these ne(2) rows of our array contain those cosets of the code that have all possible w = 2 error patterns as their leaders. The distance between any of these coset leaders and any element of that leader’s coset is at least d,,,,,-4; so long as the code has d,,, > 4,the w = 2 coset leaders will be the sole lowest weight element in their respective cosets. The elements of the cosets that have sole weight w = 2 leaders will comprise all possible received codewords that are contaminated by weight w = 2 error patterns. From the above discussion, we can deduce that an arbitrary element of our 2-D array (say, the]* element of the row), that belongs to a coset whose leader is the sole minimum weight element of its coset, is the received codeword C,,,,k = c0,, + e,,k where co,, is the error-free codeword that occupies the]* column of the O* row and eskis the weight w = k error pattern that is the leader of the coset that occupies the *r row of the 2-D array. Clearly, if i = 0, so that the selected array row contains the coset of error-free codewords, then esk=eo,o= 0 (i.e.7the coset leader is the all “zeros” error pattern) and the received codewords are exactly the error-free codewords. That is, A

= co,, . Eventually, the weight of the error pattern will be large enough to cause some of the array rows that are formed from it to contain elements that have weight equal to, or lower than, the weight of the error pattern itself. If the d,,, of the code is an odd number (which is the case for the primitive cyclic codes used in optical data storage), this can occur when the weight of the error pattern is w 2 (d,,,,, + 1)/2. When the error pattern has this high weight, some of the rows formed by it will have elements that have weight w < (d,,,,, + 1)/2. However, all of the possible elements with w < (kh + 1)/2 already are sole minimum weight leaders of a different coset, i.e., a coset that occupies a lower index (upper) row of our 2-D array. As an example, suppose we are in the process of forming the I* row of our 2-D array using C0,J.O

Data Reliability and Errors 82 7 the error pattern e,4d-+l)/2 and find that the . I element * of the row has weight (dmin-1)/2and further that it is identical to the O* element of row I’ < I which has a sole minimum weight coset leader. That is,

where the equality on the far right follows because the error pattern el,(d--1)/2 is the coset leader of row number I‘. Then, since any sum of two codewords is a codeword, an arbitrary element if the P row of our 2D array can be expressed as

wherej # j ’ . Thus, every element of the I * row is already contained in row I’
4-

828 Magneto-Optical Data Recording element in the coset; in this case each one of these minimum weight elements is a leader. Every coset leader is one of the possible N-symbol long error patterns that may contaminate codewords of the code, but not every such error pattern is a coset leader. In this regard, there are four types of error patterns: (i) Every error pattern that has weight w I (d,,,,, - 1)/2 is a sole leader of a coset in the array, i.e., each of these error patterns is the lowest weight element in its respective coset. (ii) Some of the error patterns with weight w > (d,,,,, - 1)/2 are sole leaders of cosets in the array. (iii) Other error patterns having weight w > (d,,, - 1)/2 are one of multiple minimum weight elements of the coset in which they reside; these error patterns are one of the multiple leaders of their respective coset. And (iv) another M i o n of the error patterns of weight w > (d,,,,, - 1)/2 are not leaders of any coset in the array, but they are elements of some coset that has a lower weight leader. Each coset (row) of the array has a unique syndrome associated with it and every element (received codeword) of a particular coset will generate the same syndrome. Therefore, of the four types of error patterns identified immediately above, types (i) and (ii) are uniquely mapped to syndromes while type (iii) and type (iv) are not. That is, error type (iii) belongs to a coset that must associate only one of a number of equal weight error patterns with its syndrome and error type (iv) belongs to a coset that associates some different, lower weight error pattern with the coset syndrome. The preceding discussion was meant to show that only those error patterns with weight w I (d,,,,, - 1)/2 are guaranteed to generate unique syndromes, i.e., a different syndrome is assigned to every error pattern that - 1)/2. Only some of the error patterns with weight has weight w I (dmi, w > (d,,,,, - 1)/2 will generate (be assigned to) unique syndromes. For example, error patterns that have weight w = (d- - 1 +u can generate syndromes that are identical to those generated by o er error patterns having the same weight, as well as those generated by error patterns having w <X(d- - 1)+ u . ~n approximation that weights X(dh -1) -u+ 1I is commonly used is that a randomly selected error pattern of weight w > (d,,,,, - 1)/2 will generate any of the 2dN-Qpossible syndromes randomly, i.e., with uniform probability. Reference 24 shows that, for RS codes, the fraction of error patterns with a given weight w > (d,,,,, - 1)/2 that generate one of those syndromes that are associated with (i.e., generated by) any of the error patterns of weight w 5 (d,,,,,-1)/2 is well-approximated by

X

1

Data Reliability and Errors 829 (d&-1)/2

Cne(w> w=o

=

2q(N-K)

which is the same as Eq. (58) in Sec. 5.2. Decoding (Error DetectiodCorrection). An error control decoder can be thought of as a giant read-only memory (ROM) that is addressed by the syndromes and which stores a different N-symbol error pattern in each of its storage locations (i.e., addresses). These stored error patterns are leaders of the cosets of the code, i.e., all of the sole coset leaders and one leader from each of those cosets that have multiple leaders. For example, the 120-byte RS code used for error correctioddetection in the User Data field of the CCS recording format has N = 120 bytes and N-K = 16 bytes; this code therefore has a total of 25616 = 3 . 4 ~ syndromes. Clearly, an unrealizable ROM, which stores 3.4 x 120-byte data blocks (perhaps compressed in some way) and which is addressed by a 16 byte word, would be needed to accommodate such a code. In real systems, the error pattern is calculated from the syndrome by computationally inverting the encoding process. A description of the algebraic steps involved in this operation is beyond the scope of this chapter; our goal here is to convey to the reader the notion that all the information that the code derives about the nature of an error pattern that contaminates a received codeword is contained in the syndrome. The concept of using syndromes to address a giant ROM in order to retrieve an error pattern will serve herein as an adequate picture of the decoding of a received codeword. Once the error pattern is known, it is simply added to, i.e., xor-ed with, the received codeword to correct the error(s). From the above picture (i.e., using a giant ROM to effect decoding) and the discussion of syndrome generation in the previous subsection, it is clear that d,,,, ,which is dependent on the N-K value of the code, determines the maximum weight of error patterns the code can correct. In order to perform error correction, a given syndrome can address, or correspond to, only one error pattern, which must be taken to be the error pattern that is extant in the received codeword. But, as we have seen, all error patterns having weight w I (d,,,,, - 1)/2 are sole leaders of one of the cosets of the code; each such error pattern will therefore correspond to a unique syndrome. For example, the all “zeros” error pattern is assigned to the all

830 Magneto-Optical Data Recording “zeros” syndrome and the decoder assumes that no errors are present in any received codeword that generates an all “zeros” syndrome. We have also seen that only some of the error patterns having weight w > (dmin- 1)/2 are sole coset leaders; decoding when such a higher weight error pattern occurs could be ambiguous (in the event the error pattern is one of the leaders of a coset that has multiple leaders) or erroneous (if the error pattern is a member of a coset that has a different, lower weight error pattern as its sole leader). Thus, only error patterns having weight w 5 (dmi,- 1)/2 are guaranteed correctable. This works well, since in most cases the occurrence in a received codeword of only a few errors is a relatively high probability event versus the occurrence of a large number of errors. It is also important to note that when any syndrome that does not correspond to a weight w I (dmi,- 1)/2error pattern occurs, the presence of an uncorrectable error pattern of unknown weight w > (dmin- 1)/2 is detected. Moreover, since type (iv) error patterns (see the discussion near the end of the previous division of this subsection that dealt with syndrome generation)of weight w = (dmin+ 1)/2 can only appear in a coset that has a weight w = (d,,,,, - 1)/2 error pattern as its leader, we can guarantee that all weight w = (d,,, + 1)/2 errors are detectable by declaring that only weight w 5 %(dmi,- 1) - 1 errors are correctable. This would free-up the - 1)/ substantial number of syndromes associated with the weight w = (dmin 2 error patterns so that they could be used for detection of both weights w = (dmin- 1)/2 and w = (timin+ 1)/2 error patterns [note that weight w = (dmin + 1)/2 and weight w = (dmin - 1)/2 error patterns would then be guaranteed to be detectable because none of these error patterns are elements of cosets that have leaders with weight w I %(dm, - 1) - 11. This illustrates why the maximum weight of error patterns that are guaranteed to be detectable can be increased by reducing the maximum weight of the error patterns that are guaranteed to be correctable, i.e., it provides the basis for Eq. (2). Shortened Primitive Cyclic Codes. We mentioned earlier that the length of a primitive cyclic code is N = 2m-1symbols, where m is such that the code’s generator polynomial can be factored completely into polynomials of degree m,or some integer factor of m. In most applications, it is not convenient to have codewords that have exactly one of these discrete lengths. To overcome this limitation, shortened codes are usually used. Any N,K primitive cyclic code can be shortenedby U symbolsto form an N-U,KU code, which has only K-U information symbols, but still retains N-K parity symbols. Shortenedcodes are formed from their parent codes simply

Data Reliability and Errors 831 by specifjllng that the U most significant information symbols, i.e., the .....,,,b in Eq.(g), are “zero”. This has the effect of coefficients b,, ,,b making the leading U terms of all codeword and received codeword polynomials equal to “zero” as well. It also reduces the total number of possible error patterns by a fixtor of 24,. ID Field CRC Code. The 16-bit CRC error detection code is a binary, rn = 15 primitive cyclic code, i.e., its symbols are bits and it has native length N = P - 1 = 32,767 bits. The generator polynomial for this code is

Thus, N-K = 16 bits, i.e., there are 16 parity bits. The codewords of this 16-bit CRC code span the entire 5 bytes (40 bits) ofthe ID field. The code is thus shortened by U = 32,767-40 = 32,727 bits. The last 16 bits are the code’s parity bits; the first three bytes of the ID field comprise the information bits. The information polynomial of the ID field CRC code is given by

Eq. (17)

i=8

i=O

where b, represents the most significantbit (MSb) of the first ID field byte, b, is the least significant bit (LSb) of that field’s third byte and indicates that the i bit is inverted. This information polynomial is multiplied by x16 and then divided by the degree-16 binary generator polynomial GC&) to obtain the codewords. For example, suppose the initial three bytes of a givenIDfieldhavethebinaryvalues 1111 1111; 1111 1111; 00000101. Then, the corresponding information polynomial is (after inverting the l e h o s t sixteen bits)

6

and after multiplication of Eq. (18) by x16 and division by GCRc(x), we obtain

832 Magneto-Optical Data Recording

X

18

+X

x X

16

14

+x

+ X

14

16

+X +X

x

14

+x

I

I

12

12

+ x + X

+x

2

5

+1 + x + x +1 + x

I

2

2

5

That is,

Es.(19)

x

14

+x

12

I

5

2

+ x + x + x +1

The 16 binary coefficients of the degree-15 remainder of this division are the 16 bits that comprise the 16-bit CRC parity (note that if the remainder has degree < 15, we take “zero” to be the coefficient of each of the missing high degree terms). The coefficient of the highest order term of the remainder is the MSb of the first CRC parity byte, while the lowest order coefficient is the LSb of the second CRC parity byte. In the case of our example, the 16-bit parity is given by the coefficients of the polynomial in the numerator of the second term on the right hand side of Eq. (19), i.e., 0101 0000; 1010 0101. These 16 bits are catenated to the three ID field information bytes to form the CRC encoded (5-byte) ID field 1111 1111; 1111 1111; 0000 0101; 0101 0000; 1010 0101. Decoding an r-bit CRC codeword is accomplished simply by dividing the polynomial that represents the received, possibly error-contaminated, codeword by the degree-r code generator polynomial GC&. The remainder of this division operation is a degree-(r-1) binary polynomial that represents the syndrome of the received codeword, i.e., the syndrome is the r bit long sequencecomprised of the r binary coefficients of this polynomial. If the coefficients of the syndrome are all “zeros” the decoder declares the received codeword to be error-free. Conversely, if one or more of the coefficients is a “one,” an error burst has been detected. For example, suppose the sequenceof bits that corresponds to a specificCRC codeword is read from the disk and used (as coefficient values) to form the received

Data Reliability and Errors 833 codeword polynomial & ( x ) . codeword is obtained via

Then the syndrome for that received

S(x) is thus a degree-(r-1) binary polynomial and its r binary coeffi-

cients constitute the syndrome. All r bits of the (binary) syndrome are guaranteed to be “zero” if there is no error in the received codeword and at least one bit is guaranteedto be a “one” if the received word is contaminated by a singleerror burst of length I r bits. (We saw earlier, in Eqs. 13 through 15, that the syndrome is zero only if the received error pattern is identical to one of the codewords of the code; because no codeword of an r-bit CRC code has all of its nonzero symbols confined to a contiguous segment of length r bits or less, an error of length Ir bits cannot be a codeword.) On average, only a small fraction, i.e., 2-@4,of any of the length (r+l)-bit error bursts that may possibly contaminate the received codeword will cause an all “zeros” syndrome to be generated; 2-‘ of the total number of possibly contaminating error bursts having length > r+l bits will produce this same result. That is, the fractions 2-<‘-’) and 2-‘ are the fractions of arbitrarily located (within the codeword) error patterns of length r+l and length > r + l , respectively, that are exactly the same as one of the valid codewords of the CRC code. Thus, the probability of not detecting any single long (> 16 bits) error event that could contaminate an ID field, given that such an error event occurs, is about 1.5 x which may be an uncomfortably high value (depending on the probability of encountering a error burst with length > 16 bits). This, in addition to the fact that the preformatted ID fields are not protected by an error correcting code of any type, explains why three copies of the ID field are placed in the header portion of each sector. A real benefit of using binary polynomials to represent codewords and using a generator polynomial to define a code is that it provides a means to implement parity calculation and syndrome generation with simple hardware. More specifically, division by a degree-(N-K) binary generator polynomial can be implemented using an N-K-1stage binary shift register wired in a linear feedback configuration. Such a shift register, which implements division by the generator in Eq. (16) is shown in Fig. 10; the sixteen storage cells of this shift register correspond, from left to right, to the terms 9,xl, ....., x15 of the generator polynomial, the output at the right corresponds to the x16 term and feedback connectionsexist only at the inputs

834 Magneto-Optical Data Recording to those storage cells that correspond to the generator polynomial terms that have nonzero binary coefficients, namely, xo, x5, x12 and x16. The summing junctions in Fig. 10 perform the xor operation.

Figure 10. Hardware for parity generation and syndrome calculation; 16-bit CRC code.

To see that the Fig. 10 hardware performs division by the generator polynomial, consider the example of polynomial long division given just above in Eq. (19). Note that the division is actually carried out by multiplying the divisor (ie., the generator polynomial) by various powers of x and then xor-ing the result with the dividend polynomial until the dividend is reduced to a polynomial of degree less than that of the divisor. A single shift of the register contents in the Fig. 10 hardware is equivalent to multiplication of the polynomial representation of the register contents by x. If the register contents are initialized to all “zeros” and then a single “one” is shifted in at the parity generation input (withthe feedback gate closed), the contents of the registers will be exactly

where the equality in Eq. (21) is easily demonstrated by dividing x16 by the polynomial G,,(X) given by Eq. (16). Thus, when the K information bits b,,, b,, ...., b,, b, are shifted, in sequence @gh order bit first), into the right hand input of the Fig. 10 register, the contents of the register at the end of each successive shift will be equivalent to

Data Reliability and Errors 835

The last of the equations in Eq.(22) shows, of course, that the register ultimately (after K input/shifi operations) contains the parity bits that correspond to the K-bit information sequence that was processed, see Eqs. (9) and (10). The feedback gate is held open during the 16 shifts that are required to clock the parity bits out of the register; this serves to re-initialize the register contents to all “zeros.” As soon as the last parity bit has left the register, the feedback gate is closed and the first (h~ghorder) bit of the next K-bit information sequence is shifted in. The input for syndrome calculation is held open during parity generation. To calculate a syndrome, the N bits of the received codeword are (x) sequentially shifted (the high order bit of the codeword polynomial is input first) into the low order end of the register, as shown in Fig. 10. The feedback gate is held open only for the first 16 input/shift operations; this has the same effect as initializing the register to all “zeros” prior to starting the input of the received codeword data. On completion of all N inputs/ shifts of the received word data,the register will contain the syndrome, see Eq. (15). Thereafter, the feedback gate is opened and the 16-bitsyndrome is clocked out of the register while the initial bits of the next received codeword are clocked in (the bits of the next word directly follow, i.e., they are only one storage celVshift behind). The input-for-parity-generation is held open during syndrome calculation. User Data Field Interleaved RS ECC. The symbols used by RS codes are binary q-tuples, i.e., q-bit words. The RS code specified for use in the User Data field of the CCS recording format has q = 8 and the symbols are 8-bit bytes. Before describing the code, we must first describethe code’s symbol field and the algebra that is defined on this field. The specific symbol field used by the code of interest to us here, which is a finite field known as Galois field 256, or GF(2*), is generated via the binary (ie., the coefficients are “0” or “1”) primitive polynomial

eCRc

836 Magneto-Optical Data Recording Eq. (23)

P(x) = x8 + x5

+ x3 + x2 + I

(this polynomial is specified in ISO/IEC 10089). We use Eq. (23) to extend the 2element binary field to a field that has 256 elements, each corresponding to one of the possible 256 values of a byte of binary data. One of these elements will be the “0” element and another will be the “1” element. These will correspond to the specificbytes 0000 0000 and 0000 000 1 respectively. The other 254 elements will be denoted by the symbols pi;1 S i S 254 . Also, we will use the notation Po= 1 and p1= p. We shall see that these elements will form a finite algebraic field, such that p255 = 1 = Po. The extension field which is of interest to us is formally defined by declaring p to be the root of the primitive polynomial Eq. (23) (note that this polynomial has no roots in the binary field). Thus, we can write

or equivalently, p8=p5+p3+p2+

1

[Note:there are no minus signs in Eq. (24) because addition in GF(28) is defined as the bit by bit xor of the eight binary constituentsof the elements pi and thus, since -1 = 1 in the binary field, -pi = pi.] Eq. (24) is functionally equivalent to the assignment Eq. (25)

pz(x)= xzmod P(x); 0 I i I 254

where P(x) is given by Eq. (23). Equation (25) states that the polynomial representation of the extension field element p iis the degree-7 remainder obtained by dividing xi by the primitive polynomial Eq. (23). Now, pi(x)is a binary polynomial (because xi and P(x) are binary polynomials) and its eight binary coefficients are the eight bits of the 8-tuple (i.e., byte) that constitutes the value of the extension field element p i. The use of Eq. (25) to calculate the first twelve extension field elements is illustrated in Table 2 below. From Table 2 we see that the value for the extension field element p obtained via Eq. (25) is exactly the same as that specified by Eq. (24). That is, the byte value of psis given by the bit-by-bit sum (xor) of the byte values of p5,p3,p 2and Poas follows:

Data Reliability and Errors 83 7

00 100000

+ 00001000 =[00101101] = p8 + 00000100 + 00000001 Table 2 also demonstratesthat the polynomial representation of p9is given by the product of the polynomial representations of p8and pl, which indicatesthat p9= pax p1= p8+l.Finally, using Eq.(25) we find that p255 = x255mod P(x) = 1 because P(x) is a degree-8 primitive polynomial, which means that [x(’’-’) - I] mod ~ ( x =) o . Table 2. Illustration of Generation of GF(28)via P(x) = x8 + 9 + 9 + x2 + 1 1 mod P(x)=l x mod P(x)=x x2 mod P(x)=x2

po = 0000 0001 p1 = 0000 0010 p2

= 0000 0 100

x3 mod P(x)=x3 x4 mod P(x)=x4

p4

x5 mod P(x)=x5

p5 = 0010 0000

x6 mod P(x)=x6 x7 mod P(x)=x7 x8 mod P(x)=x5+x3+x2+ 1 x9 mod P(x)=X6+x”+x3+x x10 mod P(x)=x7+x5+x4+x2 x12 mod P(x)=x6+x2+ 1

p6

p3 = 0000

1000

= 0001 0000 = 0100 0000

1000 0000 p8 = 0010 1101 p9=0101 1010 p10= 1011 0100 p7 =

p12

= 0100 0101

Thus, either Eq. (24) or (25) also defines addition and multiplication in the extension field. That is, addition is performed by taking the bit-by-bit xor of the individual byte values of the corresponding elements and multiplication is defined by

838 Magneto-Optical Data Recording Division is handled by making use of the fact that example,

p255= 1.

For

One final transformation is required before we actually obtain the extension field that comprises the symbol field of the RS codes used in the CCS recording format. This final extension field has elements 0 and a’;0 I i I254, such that 0 has the byte value 00000000 and the a’are related to the previously enumerated elements pJby

Eq.(27)

Now that we have defined the extension field that contains the symbols of the RS code of interest, we can proceed with the definition of the RS code. The generator polynomial of the RS code is defined on GF(29, i.e., its coefficients are drawn fiom the extension field specified by Eq. (27). The specific generator for the CCS recording format’s User Data field error correcting/detecting code is given by

n(..ni) 135

Eq. (28)

G~~(x)=

i=120

Data Reliability and Errors 839 where the ai in Eq. (28) are defined by Eq. (27). All sixteen of the factors of Eq.(28) are degree-1 polynomials in the extension field comprised of the elements ai. It is also true that each of these polynomials x + a i evenly divides the extension field polynomial when m = 8, that is

Eq.(29)

(aO~*~~ mod +a (Ha1) ~ ) = 0; 0 I i I 254

Thus, Eq. (28) generates an RS code that has length N = 2*-1 = 255 bytes (2,040 bits). Since Eq. (28) has degree = 16, this code has N-K= 16 parity bytes. The actual code specified for use in the CCS recording format is a shortened version of this code; it has length N = 120 bytes, K = 104 information bytes and N-K = 16 parity bytes. Encoding (parity calculation) and syndrome generation for this code are as described by Eqs. (9) through (19, keeping in mind that the symbols used throughout, i.e., the polynomial coefficients in Eqs. (9) through (15), are those defined by Eq. (27) and the generator polynomial is given by Eq. (28). Thus, the information polynomial for thej* interleave of the 120-byte RS code is given by 103

Eq. (30)

lm, (x) = x ( a k , j ) x k ; OI j 5 number of interleaves- I k=O

where akJ. is the extension field element that corresponds to the value of the byte in the P row of column in either of Fig. 8 or Fig. 9, and the number of interleaves is ten (1024-user-byte sectors) or five (5 12-user-byte sectors). The parity bytes for the information polynomial Eq. (30) are then the coefficients of the degree-15 polynomial

Eq. (31)

'RS,

(')

=6 IIR. S) ,. (

mod G R S

(')

and the corresponding complete RS codeword is comprised of the 8-bit coefficients of the RS codeword polynomial

Eq. (32)

'RS,

(')

= 6 I I 'R)S ., (

+ RRS,

(.)

840 Magneboptical Data Recording Finally, the syndrome of any 120-byte long error pattern which may contaminate a received word of this code is given by the 8-bit coefficients of

em,

Eq.(33)

'RS,

(.)

= 'RS,

(.)

Od GRS

(.)

The hardware used to perform encodingand syndromegeneration for this RS code is the linear feedback shift register illustrated in Fig. 11. The operation of the Fig. 11 hardware is identical to that of the hardware illustrated in Fig. 10. The main differences between the Fig. 10 and Fig. 11 hardware is that the RS hardware shown in Fig. 11 has 8-bit wide data paths (since it processes bytes, rather than bits) and its feedback connections contain circuitry that multiplies input byte values by a specific extension field element at, where the rules of this multiplication are given by Eq. (26). A possible implementation for such a multiplier would be an 8-bit by 256-byte ROM which uses the input byte value in the feedback path as an address; the output is the byte value of the product of the extension field element that corresponds to the input byte value and the specific multiplier extension field element in the relevant branch of the feedback loop. The summingjunctions in the Fig. 11 circuit perform bitby-bit xor-ing of the input bytes.

ab,a',..... ,a\-,

input for parity generation

Figure 11. Hardware for parity generation and syndrome calculation; 120-byte RS code.

Data Reliability and Errors 841

User Data Field RS Error Detecting Code (CRC). The four CRC bytes in the User Data field comprise the parity information of a Reed Solomon code that is used only to detect errors that were not corrected, or detected, by the interleaved, 120-byte RS ECC. The CRC parity byte values are computed from the specific values of the user data and twelve DMP bytes contained in the User Data field of each sector, i.e., these are the information bytes of the RS-CRC codewords. In the case of the 1024 user bytes per sector recording format, the information polynomial for this code is obtained by first summing (bit-by-bit xor) the ten bytes in columns 0-9 in each of rows #1 through #103, and the six bytes in columns 0 through 5 of row #O in Fig. 8 to obtain a total of 104 information bytes. In the case of the 5 12 user bytes per sector format, the five bytes in columns 0 through 4 in each of rows # 1 through # 105 and the single byte in column 0 of row #O in Fig. 9 are summed to obtain 106 information bytes Thus, there is only a single RS-CRC codeword which spans all the user data and DMP bytes, as well as the codeword's parity checks, in a given User Data field. The extension field elements that comprise the symbol field of this RS-CRC code are identical to those used by the 120-byte RS errorcorrectingldetecting code described in the previous division of this subsection. The generator polynomial, however, is different; it is given by

Thus,'this RS-CRC code has only four parity bytes. The information polynomial for these codewords is given by

+cq,jfor 5

Ik(x)=~[&ajk)xk) k=O j=O

;

j=O

1024-byte sectors

842 Magneto-Optical Data Recording where the notation is identical to that used in Eq. (30). The parity bytes of this RS-CRC code are the four 8-bit &cients of the degree-3 polynomial Eq. (36)

RRScRc

(.)

= 'RSCRC (.)x4

mod GRScRc

(.)

The codewords of this code are obtained in the normal way, i.e., by catenating the 8-bit coefficients of IRSmc(.)x4 with the four parity bytes determined via Eq. (36). And the syndromes are comprised of the four 8-bit coefficients of the remainder obtained by dividing received codewords by the generator polynomial Eq. (34). The hardware for parity calculation and syndrome generation is similar to that shown in Fig. 11, except that only a five-stage shift register with feedbackconnectionsspecifiedby Eq. (34) is used. 3.4

Defective Sector Management

In the preceding two subsections, we establishedthat the highest level of error correction and detection in a data storage peripheral is accomplished over a data block that encompasses the entire User Data field of a sector (this is the data block that is protected by the interleaved RS errorcorrectingldetecting code and the RS-CRC errordetecting code). This means that a sector is the smallest data block on which meaningful, quantitative data reliability targets can be placed. Such targets can be expressed as a sector error rate, i.e., as the probability of experiencing any error in the data recovered at the output of the ECC decoder from a randomly chosen sector. In general, when an uncorrectable ECC codeword occurs at the input of an ECC decoder, each byte in the data block output by the decoder has identical probability of being erroneous. Thus, when an interleaved ECC is used in conjunction with a second code (such as the RSCRC code specified in ISO/IEC 10089) that is designed to detect errors at the output of the interleaved ECC's decoder, the sector error rate will be equal to the probability of occurrence of an uncorrectable codeword times the number of ECC interleaves per sector. The units of sector error rate are erroneous sectors per recorded sector. If one insists on estimating a byte error rate, the correct units to be applied to it are erroneous bytes/recorded byte; the byte error rate estimate can be calculated as the sector error rate times the mean probable fraction of erroneous bytes in an uncorrectable interleaved codeword divided by the number of user information bytes contained in a sector. Methods of estimating mean decoded block error rates

Data Reliability and Errors 843 at the output of the interleaved RS ECC decoder and the RS-CRC decoder are discussed in Sec. 5 . The CCS recording format provides further means (in addition to ECC) for controlling errors, namely, it provides for removing fiom use those sectors that are found to contain more than a maximum (threshold) number of certain types of errors. If such a sector already holds recorded data, that data is relocated to a new, previously unused sector. Sectors that are "retired" fiom use through this process are called defective sectors. If defective sectors were left in use, they would be the main contributors to high sector error rates. ISO/IEC 10089 does not specify when a sector is defective. However, it gives the following guidelines for declaring a sector to be defective: The sector has only one reliable ID field in its header, i.e., errors were detected in two of the ID fields via the ID field CRC error detection code The Sector Mark cannot be read A single long burst error of length > 30 bytes is detected in a 1024-user-byte sector (length > 15 bytes in a 5 12-user-byte sector) The total number of defective bytes exceeds 40 in a 1024user-byte sector (15 bytes in a 5 12-user-bytesector), or 5 or more defective bytes are found in a single interleaved codeword of a 1024-user-byte sector (3 or more in a single interleave of a 5 12-user-byte sector) A sector may be found to be defective either when the disk is certified (although disk certification is not required), or during usage. Defective sectors found during disk certification are handled by a sector slipping algorithm; the physical addresses of such sectors is removed from the logical sector map of the disk and the physical address of each higher valued logical sector is slipped by one sector. Defective sectors that are found during normal write/read operations on the disk are handled via a linear replacement algorithm, i.e., the defective physical sector is removed from use by rewriting its contents to a spare sector on the disk. There are 2048 such spare sectors distributed throughout the User Data zone of the disk (comprised of physical disk tracks 3 through N-3, see Fig. 4). Thus, the maximum number of remapped sectors per disk side is 2048. All of the sectors in the six physical disk tracks numbered 0, 1,2 and N-2, N-1 and N are reserved for defect management information; these

844 Magneto-Optical Data Recording sectors comprise the Defect Management Area (DMA) of the disk. The DMA holds four identical defect management information groups, each group consisting of 25 sectors when 1024-user-byte sectors are used (46 sectors per group when 512-user-byte sectors are used). This 4-part redundancy is solely for the sake of data reliability (even though each sector of every defect management information group is fblly protected by the RS ECC and Rs-CRC coding). The defect management information recorded in these groups consists of the locations of the spare sectors on the disk, the number of slipped sectors and the physical addresses of each slipped sector, and the number of linearly remapped sectors as well as the physical addresses of both the replaced sector and of the sector that replaced it. Clearly, the CCS recording format, like all data storage peripheral formats, has many levels of pattern recognition and coding to insure data reliability. The retiring and replacement of sectors that are declared to be defective is the last, i.e., highest level, ofthese measures. The availability of a feature such as dynamic defective sector replacement demands that one put as much emphasis on error detection as on error correction when choosing the overall error correctiotddetection coding strategy.

4.0

THE NATURE OF DIGITAL ERRORS

All digital errors that occur in a digital optical data storage system are created during the data detection process, i.e., at, or by, the data detector. The data detector may produce a channel data sequence that is differentthan the one used to create the write waveform if any of the various impairments that were mentioned in subsection 2.3 cause the widths and/or center positions of any of the pulses in the recovered playback signal that is generated when the disk is read to be measurably different than the widths and/or center positions of the corresponding pulses in the write waveform that was used to mark the storagemedium. These impairments, in turn,have many sources such as nonuniformityof the storage medium, laser noise (that may occur during either writing or reading), focus and tracking subsystem perturbations, electrical noise, mechanical motor RF’M instabilities, etc., to name a few. Hard decision data detectors, which determine whether a given channel bit is a ”one” or a “zero” based on a single interrogation (or sampling) of the playback signal within each channel bit cell of width T,, are used exclusively in MO storage systems today. Thus, in this section, we shall only consider error types that are generated by hard decision data

Data Reliability and Errors 845 detection. We shall not consider error types that are exclusively generated by sequenceestimation or majority-logic type data detectors which sample the playback signal several times (at time intervals T,) before assigning a “most likely” corresponding channel data sequence (e.g., detectors that determine which of the possible written channel data segments is a best fit, in a minimum mean-squareerror sense, to the channel data segment obtained from the sampled playback waveform). Errors are manufactured at the hard decision data detector in either of two ways. First, impairments, i.e., distortion and noise, in the pulse waveform recovered by reading the storage medium may cause “ones” to be assigned to the wrong channel clock time windows (due to a recovered pulse having incorrect FWHM amplitude or peak position location). These same impairments, as well as local defects in the storage medium, might also cause some channel “ones” to go entirely undetected (due to pulses that have an inordinately low amplitude, or that are missing entirely), or they may cause a spurious “one” to occur (e.g., due to large noise spike). Second, since the read channel clock is derived from the pulse waveform that is recovered from the storage medium, the impairments may cause the read channel clock to become dephased, or otherwise become unsynchronized with the pulse waveform, thereby causing some of the “ones” to be assigned to incorrect time window locations. 4.1

Shift Errors, Dropouts and Drop-ins

The assignment of a channel “one” to the wrong time window is known as a shift error.[”] In most cases, a “one” affected by a shift error will be positioned (detected) either one channel bit to the left or right of its proper location. Very high levels of noise and distortion in the recovered pulse waveform are required to cause a shift error of two channel bits in magnitude; two-bit shift errors usually have a negligible probability of occurrence relative to that of a single-bit channel bit shift error. A missing (deleted) channel data “one” is known as a dropout. A large defect on the storage medium may cause all the “ones” of a contiguous series of channel data runs to become deleted, thereby producing a long dropout. (Recall that a channel run is comprised of a single channel data “one” together with all the contiguous channel data “zeros” that directly follow it; for 2,7 RLL code, channel runs can have one of six discrete lengths from 3 bits through 8 bits). In effect, a dropout decreases the number of runs in a sequence of channel data. Insertion of a single (or multiple) spurious channel data “one(s)” constitutes a drop-in. A drop-in will increase the number of runs in

846 Magneto-Optical Data Recording a sequence of channel data. The effect of these various channel data error types on a 2,7 RLL sequence is illustrated below in Table 3.

Table 3. Types of Channel Data Errors A B C D E

Original RLL sequence Sequence A with a shift error Sequence A with a dropin Sequence A with a dropout Sequence A with a long dropout

1001001000010o0o000100 10010001oO01 m 0 0 0 1 0 0 1001001000010001000100 100100OOOOO1OOOOOOO100 100000o0oooooo0o000100

Long dropouts (see sequence E in Table 3) are usually caused by defects in the optical disk’s storage layer (i.e., the thin material layer in which the detectablemarks are written) that are larger than a few microns in size. Large (tens to hundreds of microns in size) specs of dust, dirt, scratches and fingerprints located on the transparent surface of the disk substrate opposite the storage layer can also lead to long dropouts by attenuating the light beam transmitted through the disk substrate during writingheading. Tracking andor focus servo instabilities, which result in marginal tracking andor focus conditions that diminishes the read signal amplitude for some period of time, can also lead to a long dropout. Short (smglee-bit) dropouts and short drop-ins (see sequences D and C in Table 3) are generally caused by noise from one source or another. (Small defects, such as micro-pinholes, in the storage layer that affect only a single channel “one” may be considered to be a type of medium noise.) At times when the wideband (i.e., bandwidth channel clock frequency) signal-tonoise ratio (SNR) is low, e.g., when the read laser spot becomes aberrated (spread-out andor defocused) due to dust, a scratch, or a fingerprint on the disk surface opposite the storage layer, these types of channel data error may occur with high probability. However, data storage drives are designed to nominally operate with high wideband SNR (the wideband SNR is usually > 20-25 dB); when the SNR of these systems is not substantially degraded shift errors will be the dominant error type.[171[191[201 Factors that contribute to the creation of shift errors when the SNR is high are: small, unintended (spurious) written mark size variations,

-

Data Reliability and Errors 847 read channel clock edge (timing) jitter and intersymbol interference (ISI). ISI, which is also known as pulse-crowding or peak-shift, is a measure of the extent to which the width and spacing of the pulses in the read signal to fail to exactly reproduce the width and spacing of the marks recorded on the medium; this phenomenon becomes increasingly important as the size of the smallest written marks and spaces approach the diameter of the laser spot focused on the storage medium, i.e., when the smallest recorded features approach the limit of resolution of the optical head. 4.2

Synchronization Errors

The data reliability functions served by the Sync field, i.e., the 48-bit channel data pattern that corresponds to the bytes labeled “SBl,” “SB2,” and “SB3” in Figs. 8 and 9, were discussed in subsection 2.4 and in the division of subsection of 3.2 titled “Sync.” Here we discuss how errors in the channel data stream impact the ability to correctly synchronize the recovered data. This synchronization is accomplished by detecting the location of the Sync field pattern in the stream of channel data recovered from the disk. The Sync field is written by the drive. Since channel data errors may be very likely when some sectors of an optical disk are read (perhaps 1 bit in 10,000 or more, may be erroneous) and since the data pattern that constitutes the Sync field is not protected by any ECC, the 48bit sync pattern is chosen such that it is defect tolerant, i.e., so that it can be accurately located, with high reliability, within the stream of channel data even when many of the 48 bits of the pattern are incorrect. The Sync pattern specified in ISO/IEC 10089 is the 48-bit sequence that is obtained when user data bytes “89h, EM, CBh” are input to the 2,7 RLL modulator, i.e., 0100 0010 0100 0010 0010 0010 0100 0100 1000 0010 0100 1000. Some features of this pattern are: (i) it consists of twelve 4-bit symbols, each comprised of a “one” and three “zeros,” (ii) it obeys the 2,7 RLL constraints and (iii) it has an autocorrelation pattern that is highly peaked (this last point is explained below). The technique that is normally used to detect the sync pattern is as follows: First, a certain length (say, 68 channel bits) of the recovered channel data stream is synchronouslyfed (once per sector) through the sync detection circuit. The length of the channel data block sent to the sync detector is called the sync detection window; its size is left to the drive manufacturer. Second, once the initial 48 bits of the received data in the

848 Magneto-Optical Data Recording sync detection window are loaded into the sync detector, a majority logic circuit groups them into twelve 4-bit sync symbols and compares these with a copy of the twelve 4-bit symbols that comprisethe error-free sync pattern. This comparison is performed each time a new channel bit is shified into the sync detector. Lastly, a majority logic detector signals when, i.e., during which timing-bit window, the number of matches among the twelve received and stored 4-bit symbols first exceeds some threshold number. A variation on this would be to store all the majority logic detection values and flag the timing-bit window that corresponds to the maximum value (however, that would require some buffering of the playback channel data). The reliability of this sync detection technique hinges on the autocorrelation properties of the sync pattern. If we assume a 68-bit-wide sync detection window, that is centered on the 48-bit sync pattern given above, the error-free channel data block that is fed through the sync detector will consist of the last 10 channel bits of the W0#3 field, the 48-bit sync pattern and any length 10 channel bit sequence that could result at the output of the 2,7 RLL modulator when arbitrary user data is input, i.e., 00100 10010 + 48-bit sync + xxxxx xxxxx ,where “x” is a bit that obeys the 2,7 RLL constraints. Using a worst case 68-bit pattern, i.e., varying the last 10 bits of the above 68-bit pattern such that they cause a maximum possible number of 4-bit sync symbol matches to occur at each of the 21 sync detection shift positions that are possible when a 68-bit sync detection window is used (recall that 48 bits must be first loaded into the sync detector before any detection operation can take place), we find the worst case autocorrelation pattern to be

Thus, a maximum of 12 matches will occur, as expected, exactly when the 48-bits of the sync pattern are loaded into the sync detector. It can also be seen that a secondary maximum of 5 matches occurs at an offset of +3, i.e., after three of the trailing worst case channel bits are shifted into the sync detector.

Data Reliability and Errors 849 Due to the asymmetry in the autocorrelationpattern presented above, i.e., the fact that a maximum of four symbol matches occur prior to the exact framing of the sync pattern in the detector, while a maximum of five matches may occur after the exact framing event, a good sync detection strategy might be to set the sync detection threshold level at 8 sync symbol matches and declare the sync pattern to be located at the earliest time at which 8 or more symbol matches occur. If this strategy is used, an error burst of length > 13 channel bits (which could alter a maximum of five sync symbols), located entirely within the 48-bit sync pattern, would be required to cause a sync detection failure, i.e., the sync detector would report that sync could not be found (which might cause the controller to initiate a sector reread). On the other hand, a highly improbable number of serendipitously located shift errors, dropouts and drop-ins would be required, especially in the trailing bits of the channel data sequence that is contained in the sync detection window, to cause sync to be detected at a wrong timing-bit location. This last point can be illustrated by comparing the 48-bit sync pattern with the error-free, received 48-bit pattern that would be resident in the sync detector at an offset = -3, i.e., just after the first 45 bits of the 48-bit sync pattern are shifted into the sync detector. This comparison is given by the first two lines of the following list (note that exactly four of the 4-bit sync symbols contained in the two compared bit sequences are identical): 48-bit sync pattern: 010000100100001000100010010001001000001001001000

received bits, -3 offset: 010010000100 10000100010001001000 1001000001001001

error pattern # 1 : 0oO010100000 1010011001100000xxxxxxxxxxxx0000xxxx

error pattern #2: 00ooxxxxoooo 10100110011000001100xxxxxxxx0000xxxx

error pattern #3 : 0000xxxx0000xxxx01100110000011000001xxxx0000xxxx

Also shown in the above list are three specific 48-bit error patterns that would cause the received sync detection window pattern to produce a total of 8 sync symbol matches at offset = -3, if the error pattern is aligned appropriately with the sync pattern written on the disk. The notation xxxx in these patterns means that the corresponding error pattern bits can have

850 Magneto-Optical Data Recording any value. Also, an error that consists of two adjacent “ones”, i.e., “1 l”, can be produced either by a two-bit drop-in, or by a single, isolated shift error. Clearly, there are a limited number of very specific error patterns that can cause erroneous detection of sync. Therefore, if the sync detection threshold is set at 8 matches, as discussed above, the probability of a sync detection fkilure (ie., the event in which no sync location is determined by the sync detector) is equal to the probability that of a burst error having length > 13 channel bits is wholly contained within the recovered 48-bit sync pattern. If the sync detection threshold is lowered to 7 sync symbol matches, the length of the burst error required to cause a sync detection failure would be extended to > 17 bits. A robust sync detection strategy might be to use a relatively high nominal sync detection threshold and lower it only during sector read retries that are initiated by a sync detection failure. The probability of encountering one of the few combinations of shift, drop-in and dropout errors that could cause a incorrect sync location to be detected is judged to be very small relative to the probability of occurrence of a long dropout that could produce a sync failure. The probabilities of occurrence for any of these error types can be estimated once recovered data error statistics have been obtained for the optical storage system(s) of interest (see Sec. 5.1). Finally, it should be noted that the MO optical recording format does not employ sync patterns in the user data stream, i.e., in the data that is input to the 2,7 RLL modulator (and output by the 2,7 RLL demodulator). Such user data stream sync patterns could be utilized to synchronize the ECC decoder. In the MO storage system, both channel bit synchronization, i.e., 2,7 RLL phrasing for the purpose of demodulation, as well as user data block synchronization, i.e., codeword boundary determination for the purpose of ECC decoding, are dependent on the correct detection of the 48-bit Sync field and 16-bit Resync fields that are provided by the CCS recording format. The CD-ROM recording format is an example of a format that uses sync patterns in both the channel data and user data streams. 4.3

Soft Errors and Channel Data Error Probability

A soft error is any error that would vanish in the absence of noise. If a segment of a recorded disk is read many times, and errors are sometimes present and sometimes not, then those transient errors are soft. Accordingly, the number of soft errors will increase as SNR falls. In effect, if a particular channel bit is susceptible to soft error, e.g., because the written

Data Reliability and Errors 851 mark on the storage medium that corresponds to the particular channel bit in question is not properly formed, then whether or not an error is made when that bit is detected depends upon the exact amplitude and phase of the composite noise signal that is impressed on the portion of the read signal that corresponds to the bit during the particular read event. Sometimesthe noise will cause the data detector to locate a “ ~ n e ”in a bit-window that should contain a “zero” (or vice versa), and sometimes it will not. Alternatively, noise may, or may not, perturb the data detector’s PLL, thereby causing the recovered channel clock to locallyjitter with a large enough magnitude to produce a shift error. If the recorder is operating with nominal SNR, noise alone will not cause a significant number of errors. (ISOAEC 10089 specifies that the playback signal, obtained when a long sequence of length T, marks spaced at distance 3T, is read, shall have a minimumsignal-to-noise ratio of 45 dE3 when measured using a noise slot bandwidth of 30 kHz that is centered on the fundamental fiequency of the narrow-band recovered signal. We shall take this value for the 30-KHz carrier-to-noise ratio that applies to playback of the highest modulation code carrier fiequency as the nominal SNR.) For example, if the channel clock is properly phased to the read signal, the marks written on the medium have the correct positionsAengths, there is negligible IS1 and the SNR is nominal, the probability that noise will cause a sufficiently large perturbation of the read signal to cause a channel data error is very small. In fact, data storage drive read channels are usually designed such that the probability of occurrence of a channel data error during errors per channel bit. A storage nominal operation is much less than system’s operational and component tolerances are usually chosen such that, when anomalies such as composite noise, channel clock placement jitter, tracking and focus perturbations, etc., are degraded to their 30 design values, the conditional probability of occurrence of a soft channel data error only degrades to a value such as lo6 per channel bit, or so. Then, since the probability of the system operating at its 30 tolerances is about 0.001, the absolute probability of a soft channel data error occurring in a system that is per channel bit. This operating at its 3a tolerances would be about value does not decrease in proportion with the lower probability that the system will operate outside of its 3 0 tolerances That is, a system operating at looser, say 40, tolerance values (the probability of this is -10”) of noise, clock jitter, etc., may exhibit an absolute probability of occurrence of a soft channel data error that is very much higher than per channel bit. This occurs because the conditional probability of occurrence of a channel data

852 Magneboptical Data Recording error, given a certain level of noise, clock jitter, etc., increases very rapidly as noise, clock jitter, etc., are increased (see the discussion of the timing jitter distribution and shift error probability in Sec. 4.5). This latter characteristic is such that the absolute probability of occurrence of a soft channel data error can be maintained at a more or less constant low value, e.g., I 10-8 or per channel bit, for systems that operate within their 3a design tolerances, but this error probability usually increases precipitously as the system’s operational integrity degrades. 4.4

Hard Errors

Any error that is not related to a random process, such as noise, is considered to be a hard error. A hard error will always manifest itself whenever the particular portion of the disk that contains the error is read. Hard errors that take the form of long burst errors, which are due to defects such as digs, scratches and birefringence in molded plastic disk substrates (as well as handling-related scratches and fingerprints) are the dominant error source in the read-only Compact Disc and lower recording density WORM optical disk systems that operate at high signal to noise ratio. In W,hard burst errors may cause raw byte error rates, i.e., the error probability seen at the input of the RS error correctioddetectiondecoder, as high as lo4 or erroneous byteshyte. The powerful (eight error correcting), highly-interleaved (to depth five or ten) RS error correcting/ detecting coding that is widely used in optical recording today was designed primarily to handle these types of hard errors. Ngh density magnetic disk systems, and even some magnetic tape systems enjoy the ability to certify the storage medium prior to its use; this process enables sectors that are contaminated by long error bursts to be removed from use during the medium formattinghtification process. MO optical storage systems, via their defective sector handling strategies, share this capability [see Sec. 3.41.) Highly interleaved RS error correction systems are just now beginning to appear in small-format, very high density magnetic storage peripherals. 4.5

Shift Errors and Their Multiplication During Channel Data Demodulation

Channel data errors that result from the combination of an incipient hard error source and a moderate level of noise, e.g., a poorly written mark that is not so badly distorted that it will produce an error when the SNR is

Data Reliability and Errors 853 high, are perhaps the most troublesome type of error. These will usually manifest themselves as randomly located, isolated shift errors which, as we describe below, lead to short (one or two byte long) error events at the input of the RS ECC decoder. An important, if not the predominant, source of data errors in MO optical data storage is the inability to write marks (i.e., inverted magnetic domains) that have precisely the correct size in the MO storage medium, especially when storage densities are very high (i.e., when the smallest written domains and unwritten spaces between them become less than one micron in length along the direction of the recording track).[’*] This problem has been given the name mark blooming. In fact, PPM recording (see the discussion of PPM write waveforms in Sec. 2.2) was used by the first generation MO disk storage system mainly as a means of dealing with, i.e., minimizing the deleterious effects of, mark blooming. PLM recording (again,see Sec. 2.2) is used as a means of increasing data storage density and writehead data rate in the current (third generation) high performance MO storage systems. Thus, the discussion in the remainder of this section is specific to PLM recording (apply the comments regarding the detection of mark edges in the sequel to the detection of mark centers to specialize the discussion to PPM recording). StatisticalDescription of Shift Errors (Timing Jitter Distribution). Figure 3 illustrates how PLM recording uses the leading and trailing edges of written marks to store the locations of channel data “ones.” The physical location of a particular mark’s edges determines the temporal locations of the edges of the playback signal pulse that corresponds to that mark. If the written mark has precisely the correct length and there is only a small amount of noise and distortion present in the playback signal, the edges of the pulse corresponding to the written mark will be found (by the data detector) to occur within the appropriate channel timing windows. These timing windows, which have width T,, are positioned relative to the mark edges by the read channel clock when the disk is read. However, noise in the playback signal will lead to fluctuations in the exact times at which the data detector locates mark edges. Similarly, noise can induce perturbations in the correct placement in time of the timing windows. The variability in the precise times at which a particular mark edge is detected relative to the position of the timing window that corresponds to it, over many instances of playback of the mark, is described by a statistical distribution called the timing jitter distribution. When a written mark has the precisely correct length and only noise affects the playback signal i.e., distortion (ISI) is

854 Magneto-Optical Data Recording negligible, the jitter distributions that correspond to the detection of the channel “ones” associated with the edges of that mark are centered in their appropriate timing windows. Various jitter distributions can be used to statistically characterize the detection of channel data. For example, jitter distributions are generally obtained that correspond to playback of (i) long sequences of modulation code carriers (i.e., length nT, marks that are separated by length nT, spaces, where d < n < k+2); (ii) maruspace patterns that produce playback signals that exhibit high levels of ISI; and (iii) maruspace patterns that correspond to pseudorandom user data. Jitter distributions are usually measured by using a time interval analyzer (TIA) to determine the distribution of pulse lengths in the playback signal that occurs when a long sequence of the maruspace pattern of interest is recovered (usually >lo5 pulses are processed in a single measurement). Furthermore, since the TIA can be configured to trigger at arbitrary voltage levels, or on either the leading or trailing pulse edges, or on both edges of the pulses, jitter distributions that apply to different data detection strategies can be investigated. The statistical aspect of mark edge detection is illustrated in Figs. 12 and 13. In these figures, the statistical distribution that describes the timing jitter caused by noise and channel clock drift is depicted as a Gaussian probability distribution with standard deviation 0,. In Fig. 12 the jitter distribution is centered within the timing window of width T,; the table in this figure shows how E(X), the probability that a channel “one” will be detected outside the appropriate timing window, decreases as the ratio TJa, increases. The probability E(X)is identical to the fractional area ofthe jitter distribution that lies outside the correct timing window (depicted by the shaded areas in Fig. 12). Detecting a channel “one” outside the appropriate timing window, i.e., in a neighboring timing window, is a shift error; E(X) therefore specifies the probability of Occurrence of a shift error. Ifthe written mark has incorrect length (e.g., due to mark blooming), or if the corresponding pulse in the playback waveform has incorrect width (e.g., due to ISI), the jitter distribution will not be precisely centered withm the timing window. The result ofthis is a precipitous increase in shift errors, i.e., there is a substantially higher probability that the channel “one” which corresponds to the affected mark edge will be detected in the wrong (neighboring) timing window. Figure 13 exhibits the shift error probability versus T,,,/a, when the jitter distribution is not centered in the timing window (the fractional offset ofthe center of the jitter distribution is specified by the parameter fkctional peak shift, or FPS, which is defined in this figure).

Data Reliability and Errors 855 Figure 13 shows that, if a system has T,,,/afl= 12, which is a reasonable operating point for a storage system that has nominal SNR, then an FPS < 0.3 is needed to maintain a shift error rate < loq5.Thus, to insure FPS < 0.3, a maximum mark edge position error of about 0.15Twwould be allowed. And since there are 3Tw per shortest written mark, or space, when 2,7 FUL modulation is used in conjunction with PLM recording, this corresponds to a mark length accuracy specificationof O.05Tm,,where T,,,, is the length of the shortest recorded mark or space on the storage medium. (Note that T, 2 12a should be achieveable with a system that operates with a wideband SNR of 20-25 dl3.[l61 In this context wideband means that the noise is measured in a bandwidth commensurate with the operation of the data detector, i.e., bandwidth (2TW)-l. The wideband SNR can be estimated from the measured 30 kHz camer-to-noise ratio by simply reducing it by 10 log1,(30,000 x 2TW).[l91

-

-X

+X

2x10-1

1.282

2x 10-2

2.326

zX10-3

3.090

2x10-4

3.719

~~10-5

4.265

2x 10-6

4.753

2x10-7

5.199

2x104

5.612

2x10-9

5.998

2x10'10

6.361

2x10-1 1

6.706

2x10-12

7.035

Figure 12. Channel bit shift error probability for various amounts of Gaussian timing jitter.

856 Magneto-Optical Data Recording

x,=T. (1 IFPSI) 20" where FPS (Fractional Peak Shift) is specified as a fraction of

Td2,that is, (FPS( = 1 when the center

of the timing jitter distribution is located at *T&.

Here, the error probabilty is

1 4 -

X.

E ( x ~ )=

x+

0.1

0.2

X [ erfc(x+/Ji)+erfc(l~-l/fi)]. 0.3

0.4

0.5

I

Figure 13. Channel bit shift error probability vs Gaussian timing jitter and fractional peak shift.

The discussion in the preceding paragraph was designed to illustrate that shift errors, which are precipitated by noise, IS1 and recorded mark geometry inaccuracy, will be the dominate type of errors in high performance MO storage systems. Indeed, this is the case even in high performance magnetic disk drives that were made commercially available ten years ago.[201 Error Multiplication During Channel Data Demodulation. From the 2,7RLL coding table (Table 1) we see that a demodulator that converts 2,7 RLL channel data back to (ECC encoded) user data must consider a string of no less than eight channel bits. Moreover, since 2,7 RLL code has rate 0.5, i.e., each user bit is mapped to two channel bits, a 2,7 RLL demodulator will ingest channel bits in pairs and one user bit will be output for each pair of channel bits that is taken in. Thus, a typical 2,7 RLL

Data Reliability and Errors 857 demodulator will simultaneously process eight sequential channel bits, which are input in pairs. This means that a given channel bit will reside in the demodulator’s internal register for four input/output cycles. If a single, isolated channel bit error occurs, such as in the case of either the drop-in or dropout error illustrated in Table 3, it can affect as many as four contiguous user bits. An isolated shift error (cf, sequence B in Table 3), on the other hand, causes two contiguous channel bits to be erroneous; a shift error can therefore affect as many as five consecutive output user bits. Reference 17 computes the probabilities that a user data error burst having a length of either 1,2,3,4 or 5 bits is caused by the demodulation of an arbitrary segment of 2,7 RLL channel data which contains a randomly located, isolated shift error. The conditional probability of occurrence of a user error burst having any of these lengths, given that a randomly located, isolated shift error has occurred in a 2,7 RLL channel data stream is derived for two interesting recording scenarios: case (i) in which peak shift is negligible; and case (ii) in which peak shift is high. Case (i) corresponds to the situation in which the MO recording density is sufficiently low that mark bloom and playback IS1 are negligible. Here, the jitter distribution is essentially centered in the timing window (see Fig. 12) and all channel “ones” have the same shift error probability. In case (iz), the levels of playback IS1 and mark bloom are assumed to be large whenever a 2,7 RLL “worst-case pattern” occurs. Such worst-case patterns are produced by a PLM write waveform segment that consists of the sequence of pulses: LT,, 3T, or 3T,, LT,; where L = 4, 5 , 6, 7 or 8. Thus, the channel “one” that occurs between the two pulses has the minimum allowable number of “zeros” on its right and more than this number of “zeros” on its left, or vice versa. When PLM recording is used, these pulse patterns will cause a minimum length mark (space) to occur adjacent to a longer space (mark). In either of these markhpace configurations,the position of the recovered (via playback) channel “one” that corresponds to the end of the first pulse and the beginning of the second, will be affected by (i.e., its location will be shifted by) mark bloom and playback ISI. Mark bloom and playback IS1 will be maximized (due to underlying physics that causes these phenomena) when such worst-case patterns having L = 8 are written and read. The playback of worst-case segments of channel data, i.e., case (ii), are described by a jitter distribution that is shifted from the center of its respective timing window (see Fig. 13), while playback of the non-worst-case segments of channel data, i.e., case (i), will be described by a jitter distribution that is essentially centered within its respective timing window (see Fig. 12).

858 Magneto-Optical Data Recording

The shift error probability for case (i) can be experimentally estimated by using a time interval analyzer VIA) to measure the statistical distribution that describes the jitter in the location of the trailing edges of pulses contained in the playback waveform when the TIA is triggered by the pulses’ leading edges and vice versa; the standard deviations of these measured distributionsare then used to represent 0, in the equationsgiven in Fig. 12. The shift error probability for case (iz) can be estimated by first measuring the jitter distribution width as described above, then determining a value for FPS by using the TIA to measure the average mark length error in segments of the playback waveform that correspond to worst-case patterns of written marks and thereafter using the measured jitter distribution standard deviation and FPS value in the equations given in Fig. 13. On average, 22.6% of the channel bits in a random 2,7 RLL sequence are “ones” and 43.4% of these “ones” occur in a worst-case pattern.[l71 The data reliability ramifications of the multiplication of shift errors via 2,7 RLL demodulation are summarized in Fig. 14, which relates Pq, the probability of encountering a burst error of length q bits in the user data at the output of the demodulator, to the average (overall) channel data shift error probabilityp for cases (i) and (ii) described above. Figure 14 states that the user byte (q = 8) error probability at the output of the demodulator is about four times higher than the input channel data shift error probability for case (i) and somewhat less than twice the input shift error probability for case (ii). The absolute user byte error rate at the demodulator output will very likely be higher for case (ii), however, since the absolute channel shift error rate will be higher for that case. This informationis relevant since user bytes are the symbols of the RS ECC and RS-CRC codes used in the CCS recording format. That is, Fig. 14 shows us how to relate the error probability at the data detector, i.e., in the channel data that is input to the 2,7 RLL demodulator, to error probability that occurs at the input to the RS ECC decoder. In both cases (i) and (ii), Pq/p is linear versus q because shift errors occur only at channel data “ones” and, on average, the number of channel “ones” that correspond to q user bits increases linearly with q. Because user burst errors greater than one bit in length can occur due to the error multiplication associated with 2,7 demodulation, some of these error bursts will span user byte boundaries, i.e., they will contaminate two consecutive user bytes and thereby cause two consecutive RS ECC symbols to be erroneous. The fraction of user byte errors that belong to 2-byte long bursts is 14% for case (i) and about 7% for case (ii).[171

Data Reliability and Errors 859

I

I

I

I

I

2,7 RLL Demodulation Error Multiplier 4.0

4

P

3.0

2 .o

1

.o q=4

q=4

q=6

q=7

q=8

Figure 14. 2,7 RLL demodulation error multiplier for q-bit symbols. (i) Any channel “one” may be. shifted with equal probabilityp. (ii) Only worst-case “ones” may be shifted with uniform probabilityp.

5.0

DATA RELIABILITY ESTIMATION

In this section we discuss data reliability and describe some methods for estimating it. Often, one hears data reliability described as a dimensionless error rate value, such as lo-’*. What does such a number mean? After some thought, one might conclude it means that, on average, if 10l2bits are received (from the communications channel, storage medium, or whatever), then one of them will be erroneous. Although this interpretation of data reliability is sensible when applied to data received at the input to the ECC decoder, it misrepresents what really happens at the output of an errorcorrectingldetecting decoder. ECC decoders process codewords, which contain user information and parity check symbols. The decoder outputs only the user information (the redundant parity is discarded after decoding is completed). If a correctable (by the decoder) number of erroneous synbols were extant in the received codeword, the output user information will be absolutely reliable, i.e., it will contain no errors. If more than the correct-

860 Magneto-Optical Data Recording able number of errors, but less than the detectable number of errors occurred in the received codeword, the output user information block will be flagged as erroneous, but there is absolutely no knowledge of which symbols (i.e., data bytes) are erroneous. For example, a decoder of the 120-byte Rs ECC used in the CCS recording format, which contains 104 information bytes, might be designed to either correct all the errors in any received codeword that is contaminated by tc = 4, or fewer errors, or to detect when any received codeword is contaminated with td = 8, or less, additional erroneous bytes. If six erroneous bytes occur in a received codeword, this decoder will not attempt to correct any errors; it will strip the parity bytes and pass the information block to the controller, optionally flagging the corresponding block of 104 output information bytes as unreliable. What error rate describes this event? Possibly tc + td = 12 of the 104 output bytes are incorrect (some of the errors may have occurred in the parity bytes that were discarded, but that is not known). None of the 104 output information bytes can be trusted a priori (although subsequent processing of the file by an external agent may allow the error locations to be determined, e.g., spelling or formattingerrors could be spotted if the informationwere part of a word processor document file, and if the errors did not contaminate system bytes and thereby prevent the recovered file from being loaded by the word processing application). A third possibility is that more than the correctable and detectable number of errors (i.e., more than t, + td errors) occur in the received codeword. In this instance, there is a nonnegligible probability that the decoder will proceed with error correction processing, i.e., it will output a valid codeword that is different from the actual codeword that was transmitted. The (incorrect)output codeword will most likely be a nearest neighbor codeword to the codeword that was actually transmitted; in that case the output information block will differ from the correct information block in or d,, + 1 symbol locations, where d,, is the minimum distance about dmin, of the code (dmin= 17 bytes for the RS ECC used in the CCS recording format and most neighbor codewords are 17,18 or 19 bytes apart). There is a slight probability that a non-nearest neighbor codeword would be output by the decoder when more than the detectable and correctable number of errors occurs in the received codeword; if this occurs, more than d,, symbols in the output data block will be erroneous. The main point of the previous discussion is this: the block of data that constitutes the ECC codeword is thefindamental unit of data integrity. A meaningful data reliability estimate (for data at the output of the ECC

Data Reliability and Errors 861 decoder) will state the probability that this findamental unit contains no errors. Thus, an error probability (error rate) estimate should have the units erroneous codewords per codeword; a data reliability value of 10" erroneous codewords/codeword would mean that if lo" received codewords are decoded, on average, the information content of one of them will be unreliable. Moreover, since there are two distinct types of ECC decoding failure modes, two separate reliability estimates are needed The first estimate will specifir the probability that the decoder will encounter an uncorrectable received codeword, that it determines to be uncorrectable. The information block that is delivered when such an event occurs will contain more than t,, but fewer than tc + td i1 symbol errors. However, the decoder can flag the delivered, uncorrectable information block as being unreliable. The second data reliability estimate will specifLthe conditional probability of decoding failure, i.e., the probability that the decoder will mistakenly output a codeword that is different than the codeword that was actually intended, given that a received codeword having > t, + td errors has occurred. The information block that is delivered when this latter event occurs can contain at least d,,,,,,or more, erroneous symbols and the decoder cannot flag the information block as being unreliable. Finally, if need be, a byte error probability at the output of the ECC decoder can be estimated by multiplying the erroneous information block probability by the probable mean fiaction of erroneous bytes that can be expected in an erroneous information block that is output by the ECC decoder. Thus, for example, an estimate of the worst-case byte error probability due to the occurrence of error patterns that are uncorrectable, but detectable, by the interleaved RS ECC is given by the probability of encounteringand detectinga single uncorrectable interleaved ECC codeword times the number of interleaved codewords per sector times t, + td divided by the number of ECC information bytes per sector. We note that this estimate ignores the occurrence of more than one uncorrectable interleaved codeword;the probability of encounteringmultiple uncorrectablecodewords per sector is very small relative to the probability that one or more uncorrectable codewords will occur. In the remainder of this section, we discuss methods for arriving at these data reliability estimates. 5.1 Error Statistics

The essential ingredient in data reliability estimation is measured error statistics. The two most common, and most valuable, error statistics are the

862 Magneto-Optical Data Recording error burst length distribution [i.e., P(B), the probability of occurrence of an error burst of length B symbols versus B] and the good data gap length distribution [i.e., P(G), the probability of occurrence of G contiguous nonerroneous symbols versus GI. Examples of these statistics, which were compiled from byte-symbol error data measured on prototype, dye-polymer WORM optical disks having non-grooved glass substrates, are given in Figs. 15 and 16. A total of N,,, = 1.3109 bytes were measured to produce these statistics; 57,377 erroneous bytes in 41,343 error burst events (which were distributed as shown in Fig. 15) and 41,342 individual gaps (which were distributed as shown in Fig. 16) were found. Thus, the average byte error probability, average burst length and average gap length are given by

Eq. (37)

- =57377 = 4.4 x 10-5 erroneous byteslbyte pB Nm

Eq. (374

B = - 57’377 = 41,343

1.388 erroneous byteshrst

and 57,377) c=( N m4-1,342 = 3 1,445 byteslgap

Eq. (3%)

Since the absolute probability of encounteringan error burst of length B bytes is equal to the probability that any error event (i.e., error burst) will occur times the conditional probability that, if a burst does occur, it will have length B bytes, the values of P(B) given in Fig. 15 are obtained via P(B) =

(-)B B+ G

N(B) E - N ( B ) -= 41,343 1 . 3 109 ~

where N(B) is the measured number of burst errors that have length B. Similarly, because the probability of encountering a gap of length G bytes is equal to the probability of occurrence of a gap (i.e., a contiguous segment of nonerroneous data) times the conditionalprobability that the gap has length G, the values ofP(G) given in Fig. 16 are obtained via P(G) =

c (_=) B+G

+

N ( G ) / ~ O O ON ( G ) / ~ O O O k 41,343 4 1,342

Data Reliability and Errors 863 where N(G) is the total measured number of gaps having any length that falls in the 1000 byte range extending from G-1 kbytes to G kbytes, G = 1, 2, 3, ..., etc. Note that measured gaps are grouped into 1000-byte bins in order to limit the data points in plots such as Fig. 16 to a reasonable number.

Figure 15. Burst error probability,P(B),vs burst error length B for prototype dye-polymer disks. The distribution has been smoothed for B > 15 bytes (see text).

“T

l 5 5 2 5 3 5 4 6 5 6 8 6 7 5 8 5 9 5 G (kbytes)

Figure 16. Gap occurrence probability, P(G), vs gap length G for prototype dye-polymer disks.

864 Magneto-Optical Data Recording

Burst error distributions such as Fig. 15 are immediately useful; they provide information on the interleave depth I that is required to insure that a correctable or detectable number of errors will occur in a given codeword. For example, if an RS ECC that can correct up to eight erroneous bytes per interleaved codeword is employed to handle error events that are distributed according the error burst length distribution given by Fig. 15, interleaving to at least depth I = 9 would be required since error events having length up to 71 bytes could be encountered. The fairly uniform probability of occurrence for the longer error bursts seen in Fig. 15 is due largely to the fact that the underlying measured N(B) data for B > 15 bytes has been smoothed by averaging several contiguous values (such averaging is needed because the total measured number of burst errors in this length range is small, i.e., fewer than five error burst events of length B were measured for each length value in the range 15 < B I 71 and for many values of B in this range no bursts having that length were observed). The error burst length distribution can be altered byfencing-out sectors (i.e., removing them from the logical sector map of the disk) that contain either a large number of errors, or any error burst that exceeds a certain length. For example, an MO optical storage system could use the sector slipping algorithm described in Sec. 3.4 during disk certificatiodformatting to remove any sector that contains an error burst longer than B* from use. This would have the effect of truncating the error burst length distribution, e.g., P(B) in Fig. 15 would be forced to zero for all bursts having length greater than B*. The average byte error probability FB, which is the probability that a randomly chosen byte at the input of the RS ECC decoder is erroneous, is used in making first order estimates of data reliability; more sophisticatedestimates would utilize the individual P(B) values presented in the error burst length distribution. The average byte error probability (measured on data recovered from the disk, i.e., at the input of the ECC decoder) is often improperly used as an overall descriptor of the quality of the storage medium. If the error events are not uniformly distributed on the disk & can seriously underestimate the worst-case error environmentthat may exist in local regions on the storage medium. Local (on the disk) byte error probability values can vary by an order of magnitude or more; the error control techniques used must be capable of handling the worst-case local conditions that may reasonably be encountered during the useful lifetime of the data storage system. The main use of the gap length distribution is to indicate whether the errors are randomly distributed with a single uniform probability of

5

Data Reliability and Errors 865 occurrenceover the entire disk, or whether they are “clumped” or otherwise correlated in some way. If the error events (bursts) are truly globally independent and uniformly distributed,their generation could be modeled by the 2-state Markov process depicted in Fig. 17a.f2l]In this model, when the system is in state “G” (the gap, or good state), it creates erroneous symbols with probability PG;when in state “B” (the burst, or bad state), the system generates erroneous symbols with probability PB.A new symbol is emitted by the system each time it cycles from its current state to a successor state; the probability that the newlyemitted symbol will be erroneous is PG if the successor state is “G ” and PB if the successor state is “B. ” When it cycles to a successor state, the state machine shown in Fig. 17atraverses one of the paths that connect the previously occupied state to either the other state, or to itself. The probabilities of traversing a particular path are given by 3: 1-y, pand 1-p,as shown in the figure. If the symbols are binary digits (i.e., bits), then PG = 0 and PB = 0.5. When used to describe the error bursts and good data gaps that are relevantto a RS code over GF(29, the system of Fig. 17a emits q-bit symbols; for the RS codes of interest here, these symbols are 8-bit bytes. In this case, reasonable choices for the symbol error probabilities are PG = 0 and PB = 1.[221

(b)

Figure 17. (a) Gilbert Model; (b) modified Gilbert Model.

866 Magneto-Optical Data Recording Using the state transition probabilities indicated in Fig. 17a, the conditional probability of occurrence of a gap of length G bytes is

and the mean gap length is

The mean burst length is computed in a similar fashion to be

Eq. (40)

E = (1- p)-’

The steady state probabilities of being in state “G’ or state “B” are then

Finally, the average byte error rate of the Fig. 17a system is given by

where we have used PG = 0 and PB = 1. Substitutingthe measured values of G and (37b) into Eqs. (39) and (40), we find that Eq. (43)

B from Eq. (37a) and Eq.

y = 0.9999682 and P= 0.27945

Substituting from Eq. (43) into Eq. (42) then yields

Data Reliability and Errors 867

FB = 4.4 x 10-5 erroneous byteshyte which is in agreement with the measured value in Eq. (37). This indicates that Fig. 17a provides a reasonable model for the generation of the mean burst and gap values embodied in the measured statistics that underlie Figs. 15 and 16. The solid line in Fig. 16 is a plot ofthe functionP(G)=ltGp(G)given by Eqs. (38) and (41) when y= 0.9999682 and p= 0.27945 as prescribed by Eq. (43). The fact that this line is a reasonable fit to the measured gap distribution indicates that the measured errors are essentially randomly distributed (the deviations in the fit of the straight line to the measured data in Fig. 16 at values of G I 6 kbytes and at values G 2 40 kbytes are discussed later in this section). On the other hand, the fact that a single straight line is not a good fit to the measured error burst distribution (Fig. 15) indicates that more than one error generation mechanism exits. This, of course, is sensible; the bulk of the long errors having length B > 5 bytes, or so, are most likely long dropouts (see Sec. 1.3) due to defects in the storage medium, while most of the shorter bursts (especially those of length one or two bytes) are probably caused by noise, IS1 and imperfectlywritten marks. A 3-state Markov process that can be used to model the random generation of error bursts from two mutually exclusive error sources (i.e., only one error source can be generating an error event at any given instant) is illustrated in Fig. 17b.rul As before, there is a gap state “G”, but now two bursts states are present. When in state “B1” the system will generate error events that have mean length El < Bywhere Bis the overall mean error burst length; error events having mean length B2 > Byare generated when the system is in state “B2. ” As was the case with the 2-state model of Fig. 17a, we assign the symbol error probabiltiesPG= 0 and PBl= P B 2 = 1. The intrastate transition probabilities for states “BI ” and “B2 ” are given by p1 and p2respectively and the intrastate and interstate transition probabilities associated with the state “G ” are related by

which can be rewritten as

Eq. (44)

868 Magneto-Optical Data Recording Following Eqs. (38) through (40) we can therefore write

pel (B) = h"-'(l-

4);8 = (1 - A)-'

Eq. (45)

where the last equation is obtained by using Eq. (44). Since the steady state probability ~r, for both of the systems illustrated in Figs. 17a and 1% is given by the first of Eq. (4 l), the last of equations, Eq. (45) asserts that the gap distributiongenerated by the 3-state system is identical to that generated by the 2-state system. However, the (composite) conditional probability of generation of an error burst of length B by the 3-state system is given by

p( B) = -fly pi A +h

Eq. (46)

4

= -h"-'(l-

1+ 5

1- 4 )+ -&B-'(h A +pz 1 4 )+ -pf-'(l1+ 5

The mean composite burst length is therefore

which upon rearrangement yields

-a)

1

4)

Data Reliability and Errors 869

Using Eq. (49, the steady state occupancy probabilities of the states “BI ” and “B2” are found to be

XBl

6

PI PI = ---

h +pz

i7+B

h +P2 (l-$

+(l-$

?7+B

PI

+(l-#

Eq. (49) =B2

=----

PI +P2

+P2

(l-$

Finally, using Eqs. (44) through (46) and Eqs. (48) and (49), we find that the error burst length probability distribution generated by the 3-state system of Fig. 17b is given by

Eq. (50)

P(B)given by Eq. (50) is plotted in Fig. 18as the smooth solid curve for the ca~einwhi~hyandparegi~enbyEq.(43)andB, =1.372 B (since B = 1.388 bytes). The corresponding measured error burst length distribution from Fig. 15 is also plotted in Fig. 18 as the histogram outline Px(B). We note that there is good agreement between the model, i.e., Eq. (50), and the measured data for bursts with length B < 30 bytes. The poor fit at longer B values is due in part to the averaging that was done on the measured data plotted in Fig. 15. Moreover, in a practical system the long bursts with length B > 15 or 20 bytes would most certainly be eliminated by the defective sector retirement strategies described in Sec. 3.4. The entire set of 3-state error generating machine (see Fig. 17b) parameters that apply to the solid curves in Figs. 18 and 16 is listed below:

870 Magneto-Optical Data Recording

PG = 0; PSI = 1; Pm = 3 1,445 bytes;

=

1

5 = 1.372 bytes; B2 = 7.0 bytes; B = 1.388 bytes ;

~ = 0 . 2 7 1 1 4 ; ~ = 0 . 8 5 7 1 4 ; p , = 3 . 1 7 1x le5;p,=8.941 x lo-*;

- -

y= 1 pl fi = 0.9999682; ,TG

= 0.9999559; ~ 5 =4.351 1 x

“52 = 6.259 x

XG

+ XBl + z B 2 = 1

-

-7

IoJ(WB))

c= 354.672;

-

-8-

-

9-

-

-10

-I 1

0

I

I

I

I

I

I

I

I

I

5

10

15

20

25

30

35

40

4s

50

B

Figure 18. Comparison of computed, P(B), and measured, Px@’, error burst length distributions. -4

-4.5

-5

-5.5

-6

-6.5

0

10

20

30

40

M

60

70

80

90

100

G

Figure 19. Comparison of Computed, P(G), and measured, Px(G), gap length distributions.

Data Reliability and Errors 871 Upon closer inspection of Fig. 16, we find that a multiple personality is also exhibited in that figure's gap statistics; there are a larger number of gaps with length < 6 kbytes than is predicted by Eq. (38) and, in fact, the measured gap statistics in the region 0 < G < 6 kbytes can be fit by a straight line segment having higher slope thanthat of the straight line segment shown in the figure (i.e., yfor this latter line segment is less than 09999682). This is demonstrated by Fig. 19, in which we compare the experimentally determined gap length distribution(plotted as the outline histogramPx(G)in Fig. 19) with the function

which is plotted (as the solid line) in Fig. 19 using y' = 0.999969 and y"= 0.99945. There is a good fit of Eq. (51) to the measured gap length distribution data. The interpretation to be applied to Eq. (5 1) is that 95.5% of the error burst events on the storage medium are distributed randomly with a mean spacing of = (1 - y')-l = 32,260 bytes, while 4.5% of the bursts are randomly distributed with mean spacing of G" = (1 - y")-' = 1,8 18 bytes. If we assume that the burst events that comprise both of these subsets are distributed according to the solid line curve in Fig. 18, i.e., that both subsets of error events are generated by 3-state machines that are identical to the one shown in Fig. 17b except that the parameter y is replaced by y' and y" respectively, we find that the mean occupancy probabilities of the states "BI," "B2," and "G" for the two subsets of error bursts and associated gaps are

c'

These mean state occupancy probabilities were calculated using Eqs. (49) with pl, f i replaced by y: pi = {pi, p; = (1 - y')/( 1 - {) and y", p'i = {p'i, p'; = (1 - y ")/( 1 {) respectively, while maintaining the values ofthe 354.7. Next, parameters p= 0.27945, PI = 0.27114, = 0.85714, and following Eq.(42) we find that the average byte error rate of the composite (two 3-state machines) error generation system is

-

c=

872 Magneto-Optical Data Recording erroneous bytesibyte, which is within a factor of two of 4.4 x erroneous bytesibyte, the value for obtained from the experimental error event data. Finally, we note that the mean byte error probabilities within the two subsets of error events are pi = (nil+ nh)= 4.302 x and P’;, = (nGl + = 7.627 x lo4, i.e., the mean symbol error probability within the smaller of the two subsets of error events is more than an order of magnitude greater than the mean symbol error rate that describes the entire population of measured errors. It turns out that the excess number of measured short gaps in the range G < 6 kbytes (see Fig. 19) actually occurred in regions of the prototype WORM disk that contained “streaks” in the spin-coated dyepolymer recording layer. Thus, these spatial disk regions, which housed long hard error burst events, also were very rich in shift errors (see the discussion in Sec. 4.5 on incipient hard errors). Such “correlated” error events are especially difficult to deal with. The relative values of Fi and Fi cited above provide an indication of the additional error correction power that is required to manage errors that will occur when data is recovered from these correlated-error disk regions. Eliminating such regions from the storage medium, either by stringent manufacturing process control, or by retirement of the data storage sectors that are located in the affected regions, is the prudent course of action. The byte-symbol error generation model, which is comprised of Eqs. (50) and (5 I), provides a good fit to the measured error burst and good data gap distributions (see Figs. 18 and 19) and reasonably good agreement with the measured mean byte error rate. A model that gives even better agreement with all of these experimentallydetermined statisticsmight be obtained by adjusting one or more of the values of the state symbol error generation probabilities PG Ps,,and PB3 (recall that we set PG = 0 and PB, = Pb2= 1 in our model), by adding a third burst “B3,” or by allowing the two subsets of error bursts to be generated by state machines that have different values of the parameters 3/, and i.e., the two machines would have intra-burst state return probabilities pi, p; and P’,’, p;‘ respectively. The importance of obtaining an error event generation model such as the one constituted by Eqs. (50) and (5 l), which generates error events having statistical characteristics that are identical to those of the measured error events, lies in the fsct that it can be used to obtain the quantity P(m,N;I) which is the probability that exactly m erroneous symbols will contaminate a randomly chosen block comprised of N2 m symbols when such N-symbol blocks are interleaved to depth I symbols along the data track of the storage medium under study. A correct form for P(m,N;I)is difficultto obtain when

5

n‘a

a,

Data Reliability and Errors 873 error events occur as bursts and/or they are not randomly distributed; this function is easily calculated when error events occur as randomly distributed, single-symbol events [see Eq. (52) in the following subsection]. It should be noted that the exact form or type of state machine used as the basis for the error event generation model is unimportant; it only matters that the resultant model generate error events that are statistically equivalent to the error events that actually contaminate the storage medidsystem of interest. Techniques for computing P(m,N;I) from a model such as the one constituted by Eqs. (50) and (51) are given in Ref. 22. A few additional comments on Fig. 16 are in order. The large number of length 40 kbyte gaps (there are about 2,350 excess gaps of this length) is due to the 2-dimensional effects of defects on the disk; there were 40 kbytes of data per disk track and the 40 kbyte gaps predominantly occurred at bands of contiguous tracks which had only a single error event that was caused by a defect that spread radially over all of the tracks in the band. The measured gap distribution could be modified to account for this phenomenum by proportionally distributing the excess 40 kbyte gaps over the region 40 kbyte < G < 100 kbyte using Eq. (38). A decrease in the measured number of gaps having length 40 kbyte to 60 kbyte is due to a similar 2dimensional phenomenon, namely, two defects with multitrack radial extent that are separated azimuthally by, say, 45 degrees on the disk would create a number of 5 kbyte gaps, but only one or two 45 kbyte gaps. Clearly, the influence of two-dimensional defects on error data measured from disks must be considered when interpreting the burst and gap distributions that result.

-

-

5.2 RS ECC Output Data Reliability Estimation We are ultimately interested in the reliability of data that is delivered by the storage system to its controller. For the MO optical storage system, the integrity of this data depends solely on how the decoder of the interleaved, 120-byte RS ECC interprets the codewords it receives as well as the subsequent action of the 108-byte RS-CRC decoder. Before we can estimate the reliability of data output by these decoders, we must acquire two types of knowledge. First, the probabilities of encountering more than the correctable number of errors, and more than the detectable number of errors, in a received codeword must be understood. And second, the probability of misdecoding when more than the detectable number of errors appears in a received codeword must be known. In the remainder of this

874 Magneto-Optical Data Recording section, we compute these probabilities for the case of the CCS recording format’s RS codes working against randomly distributed, single-byte errors. In the sequel, the symbols P d i T ,P?,, and Pe,,,a shall respectively indicate the probability of occurrence of an uncorrectable error pattern that is detectable by the code, the probability of an error pattern that is neither correctable or detectable, and the conditional probability that an error pattern that is neither correctable or detectable causes a misdecode. Calculating PdIZ and Pc,i. Suppose that the errors input to the q = 8-bit Rs decoder are statistically independent and randomly distributed and that & is the probability that a randomly chosen byte at the input to the decoder is erroneous. Ideally, 5 would be obtained from measured error statistics. Furthermore, assume that the RS code has length N bytes and that its codewords are interleaved to depth I, where I is large enough such that no single error burst can contaminate more than t,, the correctable number of bytes, in any one ofthe interleaved codewords. So long as NI is sigmficantly less than G , the mean gap length between error events, the process of interleaving will effectively randomize the occurrence of errors in a given codeword. Ideally, I and t, would be chosen based on the measured burst distribution and desired level of recovered data reliability (see the discussion in the following subsections). Given independent and randomly distributed single-byte error events (a condition which may have been achieved, in part, via the process of interleaving as discussed above), the probability that exactly m errors will occur in a block of length N symbols is given by

where

):(

N! = m!(N - m)!

is the total number of ways of selecting m items from N items. The probability of between t,+l and t,+td errors occurring in the Nsymbol block is then

Data Reliability and Errors 875 and the probability of more than t,+td errors occurring is

The relationship between t, and td is discussed in Sec. 3.3 and is quantified by Eq. (2). As indicated, Eq. (53) is PdF,the probability that an uncorrectable, but detectable, number of errors wll occur in a received codeword, while Eq. (54) is PTa ,the probability that the received codeword will contain enough errors to possibly cause the decoder to misdecode. P and P,,a are plotted versus t, in Figs. 20 and 2 1, respectively, for an = 120 byte RS code on GF(2*) that has N-K = 16 parity bytes. These plots assume randomly distributed, single-byte errors with 5 = lo-', and

$

0

1

2

3

- Byte err prob. = 1Oe-9 --%- Byte err prob. = 10e-7 -a- Byte err prob. = 1Oe-5 Byte err prob. = 10e-3

4

5

6

7

8

tC

-*

Figure 20. Pdb vs t, for N = 120 byte RS code on GF(28);N-K = 16 bytes. Errors are randomly distributed with = 10-X;x = 9,7,5, 3.

876 Magneto-Optical Data Recording

10-

0

1

2

3

- Byte err prob. = 10e-9 -%- Byte err prob. = 1 Oe-7

4

5

6

7

8

tc

Byte err prob. = 1 Oe-5

-o. Byte err prob. = 10e-3 Figure 21. PC,a vs t, for N = 120 byte RS code on GF(29; N-K = 16 bytes. Errors are randomly distributed with = 10-X;x = 9 , 7 , 5 , 3 .

Figure 20 demonstrates that, for the CCS recording format's 120-byte RS code, the probability of encounteringan uncorrectable error pattern that is detected as being uncorrectabledecreases rapidly with t,. This behavior is driven by the facts that 6) more of the most probable (i.e., lower weight) error patterns are corrected and (iz) the total number of uncorrectable, but detectable, error patterns decreases as t, increases. In fact, Pdle= 0 when t, = 8 since detection of error patterns that have weight greater than d,,,, -I - t, is not possible since d,,,, = 17 for this code. (Note: The fact that tc+td, the maximum number of errors that can be detected, decreases to its minimum value of 8 as tc+8, since td= 16 - 2t, +0 as t, + 8, causes P',J to increase precipitously with tc, as is shown in Fig. 2 1. The discontinuity in the rightmost curve in Fig. 20 is due to the fact that Pdle= 0 when t, = 8.

Data Reliability and Errors 877

5,

As an alternative to using the random byte error value, i.e., to compute PN(tc< n, I t, + td) and P d n , > t, + td), one could utilize the full error burst length and good data gap length distributions in a Monte Carlo analysis. This would involve alternately selecting error bursts and good data gaps from their respective distributions at random until 2 NI bytes were obtained, and then determining the number of codewords in the length NI symbol block that experienced t, < n, I t, + td errors and the number that experienced n, > t, + td errors. This process would be repeated enough times to produce good statistical estimates of P d t , < n, I t, + td) and P d n , > t, + td). This method can, of course, be used even if there is correlation among the error events. Reference 22, on the other hand, presents an analytical technique for calculating the P(m,N) that applies to interleaved RS codes working against burst error events that are not completely random. This latter method requires the construction of a model ofthe error event generation process (see the discussion in Se., 5.1) If either the Monte Carlo numerical method, or the analytical method of Ref. 22 is used to arrive at the various P(m,N), both PdlFand P,,? will depend on the interleave depth I, as well as on the error burst probability distribution, t,, td and N. Calculating P31. Here we shall calculate the conditional probability of occurrence of a sdecode, given that an uncorrectable and undetectable error pattern has occurred. Once this quantity is known, the probability of a misdecode is obtained via Eq.( 5 5 ) In the subsections of Sec. 3.3 that discussed syndrome calculation and decoding of N,K codes on GF(29, we stated that every correctable error pattern could be associated with a unique syndrome and that all the information needed to correct or detect the correctable or detectable error patterns is contained in the syndromes that are generated by such error patterns. It was also pointed out that, because a one-to-one association among syndromes and correctable error patterns is needed to effect error correction, and since there are only 2qfl-K)syndromes, the maximum fraction of possible error patterns that can be corrected or detected is 2-qK,which isjust the reciprocal of the number of codewords in the code. The weights of those error patterns that are guaranteed to be correctable or detectablewhen a particular decoder co&guration is used (i.e., by decoding with a specific choice o f t , I (dmi,,-1)/2) is given by Eq. (2), which states that only the lowest weight error patterns, that have weight w I t,, are correctable while error patterns with somewhat

878 Magneto-Optical Data Recording higher weights, namely, t, < w Idmjn-l- t, are detectable. In the earlier discussion of syndrome generation in Sec. 3.3, we also showed that Eq. (2) is in agreementwith the performance of a decoder that operates by associating one of the possible 2qm-Q syndromes with each of the unique cosets of the code. Our first task in the sequel of this subsection is to give a geometrical picture of decoding which also supports the unique association of each of the lowest-weight error patterns with a code syndrome and which is fully consistent with Eq.(2). Consider an N,K RS code that has symbols in GF(28). Such a code has N-K parity bytes and it has d,,,, = N-K+l bytes. It can correct any error pattern containing It, errors or detect any error pattern containing up to t,+td errors, where t, and td are defined by Eq. (2). The number of error patterns with a given weight w 2 0 (bytes) that can contaminate a codeword of this code is given by

Eq. (56)

[

ne(w)= :)(256-1)1

where the term “-1” is included to eliminate the all “zeros” error pattern when w = 1 and patterns of weight less thanw when w > 1. The total number of error patterns with weight w It, that can contaminate a codeword is then

w=o

Now consider an integer lattice in a Kdimensional space (i.e., the coordinates of each lattice point are given by a sequence of K integers that have one of the values 0, 1,2, ..., 2%1). A block of K information bytes (q = 8) can be thought of as corresponding to a point of such an integer lattice. If the value of any byte in the K-byte block is changed, the block would then correspond to a different lattice point in K-space. Next, let every pair of lattice points in this K-space, which correspond to two K-byte blocks that have only one different information symbol (byte), be connected by an path that has unit length. Each lattice point is connected to exactly (28 - l)K other lattice points via such unit length paths. Every one of the 28Klattice points found inside, and on the boundary of, a region of K-space that contains lattice points that are interconnected to exactly 255K other lattice points by unit length paths will then correspond to one of the possible 28K

Data Reliability and Errors 879 different K-byte blocks. We shall refer to such a region in K-space as a sphere of volume 28K. This sphere is centered on the origin (which corresponds to the block comprised of K“zeros”, i.e., the all “zeros” block). Distance is measured in this K-space in a non-Euclidean “city block” sense, i.e., a path between any arbitrary pair of lattice points is constructed by catenating a number of unit length path segments that connect adjacent lattice points which correspond to K-byte blocks that differ by only the value of one byte; the total length of the constructed path is equal to the number of connected unit length segments and the minimum distance between any pair of lattice points is the shortest path formed in this manner. From the preceding discussion, we can discern that the lattice point at the origin is connected to each of the 255K lattice points that correspond to the information blocks that have exactly one nonzero byte by a single unit length path and, in turn, each of these latter lattice points is connected to 254K lattice points that correspond to other information blocks that have only a single nonzero byte and to 255(K-1) lattice points that corresponds to information blocks which have exactly two nonzero bytes by a single unit length path; the minimum distance from the origin to any lattice point that corresponds to one of the information blocks having weight w = 2 is two. After encoding by the RS code, this sphere of points in K-space is remapped into an integer lattice in N-space, i.e., into a region of N-space that contains 28N lattice points which are interconnected (as explained above) by unit length paths and that is centered on the origin (the lattice point at the origin corresponds to the all “zeros” codeword and we shall refer to the region of lattice points in N-space that are connected by unit length paths as a sphere of volume 2 9 . However, because there are only 28K codewords, the integer lattice inside and on the surface of the sphere with volume 2@’ in N-space is not compactly filled. In fact, points that correspond to any two codewords in N-space cannot be closer than distance dmin. This situation is illustrated in Fig. 22, which shows the region of N-space that contains the lattice points correspondingto the three codewords labeled “A, ” “B” and “C” (these points are at the centers of the three spheres shown in the figure). Codewords are spaced apart by distance 2 d,, because the N-K unique parity bytes added to each K-symbol information block during encoding are designed to space codewords in this manner. Note that a spacing of unity in N-space corresponds to the spacing that exists between any pair of lattice points that represent two N-byte blocks that differ in only one of their byte locations. Similarly, a spacing of two is the distance which occurs between any pair of lattice points that represent two N-byte blocks that differ in

880 Magneto-Optical Data Recording exactly two of their byte locations. Thus, integer lattice points in N-space that represent N-byte blocks that differ from a codeword in just a few byte locations are very close to the lattice point that corresponds to a codeword. These are exactly the lattice points in N-space that correspond to received codewords that are contaminated by low-weight error patterns. Thus, all received codewords that contain only a single byte error are represented by lattice points in N-space that are on the surface of spheres of radius = 1 that surround (are centered on) the integer lattice points in N-space that represent codewords. And all received codewords that contain exactly t erroneous bytes are represented by lattice points that fall on the surface of spheres of radius = t surrounding each codeword lattice point. We shall call the number of N-space integer lattice points contained within,or on, such a sphere, the “volume” of the sphere. Thus, the volume of a sphere of radius t in N-space that is centered on a codeword’s lattice point is equal to the number of weight w It error patterns that can contaminate a codeword of the Nzcode. That is, n,,,, as given by Eq.(57) is the volume of such a sphere of radius t in N-space. Also, the total volume of N-space is 28N

Figure 22. Codewords “A, ” “B*’ and “C” in N-space. Decoding spheres that surround the codewords have radius r, I(d,,,,n-l)/2.

Data Reliability and Errors 881 Every integer lattice point within, and on, the sphere of volume 28Nthat is centered on the origin in N-space will correspond to a possible received codeword of the N,K code. A decoder of the code essentially draws spheres of equal radius around each integer lattice point in N-space that corresponds to a valid, error-free codeword. A received codewordrepresented by a lattice point that M s within one of these spheres will be decoded to the codewordthat corresponds to the lattice point at the respechve sphere’s center. Clearly, the largest radius these decdng spheres can have is (d,,,,,-1)/2; spheres of larger radius would either touch or overlap and unique decoding could then not be guaranteed. This picture of decoding explains why the maximum weight of a correctableerror pattern is limited to t, = (dm,,-1)/2. Received codewords that are represented by integer lattice points that do not fall inside of, or on the surfaceof, any one ofthe decoding spheres, i.e., by lattice points that lie in the space between spheres, are not correctable, but the decoder will detect that such received codewords contain errors. Thus, if t, < (d,,,,,-1)/2, in addition to correcting all error patterns of weight w I t,, the code will also detect (guaranteed) any error pattern having total weight tc+t,, where td I(dmi,- 1) - 2t,. If the decoder draws spheres of radius zero around the integer lattice points that represent error-free codewords, no errors will be corrected, but any error pattern with weight w < d,,,,, is guaranteed to be detected (in fact, only error patterns that are themselves codewords, i.e., they are represented by one of those lattice points in N-space that represent codewords, will not be detected). Finally, if the error pattern has weight w 2 (d,,,,, 1) - t,, there is a finite probability that the lattice point which represents a received codeword that is contaminated with such an error pattern will fall into the wrong decoding sphere, i.e., into a decoding sphere that surrounds a lattice point that represents a different codeword than the one that should have been received. This, of course, will cause a misdecode, i.e., the wrong codeword will be output by the decoder. The probability of this happening when a highweight error pattern occurs can be minimized only by reducing the radius of the decoding spheres, i.e., by reducing t,. Now, the volume of each decoding sphere of radius t, is given by Eq.(57). Also, there are 2qK such spheres (one for each codeword) and 2@ total integer lattice points in the spherical region of N-space that is of interest. Thus, the fraction of the code’s N-space sphere that is occupied by lattice points that correspond to correctable received codewords, (i.e., the fractional volume of N-space occupied by the decoding spheres) is

-

882 Magneto-Optical Data Recording

However, 24m-K)is exactly the total number of syndromes for the N,K code and n,(w) is the total number of error patterns of weight w. Thus,Eq.(58) also represents the fractional number of syndromes that are associated by the decoder with correctable error patterns. And, since it is also equal to the fractional volume of N-space occupied by the decoding spheres which represent the low-weight error patterns, the syndromesused to perform error correction are associated with the low-weight error patterns. What syndromes, then, are generated by uncorrectable error patterns (accordingto our geometrical picture of decoding)? The n,,,c lowest weight error patterns (which have weight w I t,) will be uniquely mapped to the same number of syndromes. And error patterns with weight w ,such that t, w < d,,,-t,, are detected and therefore do not cause misdecoding, which implies that such error patterns are mapped to syndromes that are different from those that are associated with error patterns of weight w I t,. However, since error patterns having a given weight in the range tc < w < d,,,-tc cannot be corrected, a number of such error patterns having the same weight must be assigned to some syndromes, i.e., there is not a unique, one-to-one mapping among syndromes and error patterns having weights in this range. On the other hand, some of the error patterns that have weight w 2 d,,,-tc may possibly cause a misdecode. Thus, some fraction of these error patterns will be associated with syndromes that have been assigned to error patterns that have weight w I t,. We shall now illustrate this with an example: The 120-byte RS code on GF(28)that has d,,, = 17can correct all error patterns with weight w I8. This code has 25616= 3.4 x syndromes, but there are only nes8= 1.5 x 1031 length 120 byte error patterns with weight w I 8 (see Eq. 57). In fact, there are only n,,,=l.35 x length 120 byte error patterns with w I 10. Why doesn’t this code correct 10 errors instead of only 8? The answer lies in the code’s geometry; some codewords are spaced at a distance d,,, from their nearest neighbor codewords, while other codewords have closest neighbors that are much farther away. In general, a weight w = 9 error pattern will contaminate a codeword that has nearest neighbors with distance > 17. Such contaminated received codewords will generate syndromes that are

Data Reliability and Errors 883 different from any of those syndromes generated by error patterns with weight w I 8, but those syndromes will not be unique (i.e., some of the syndromes will correspond to a number of &fferent weight w = 9 error patterns). However, when a weight w = 9 error pattern contaminates a codeword that is only distance 17 away from its neighbor codeword(s) and the contaminated received codeword falls on the boundary of the decoding sphere of a distance 17 neighbor codeword, the w = 9 error pattern appears to be a weight w = 8 error impressed on the neighbor codeword. This causes such a w = 9 error pattern to generate a syndrome that is assigned to one of the weight w = 8 error patterns. For RS codes,the fraction of error patterns of any weight w 2 dmjn - t, that cause misdecodes can be approximated by Eq. (5 8).[241 Clearly, the geometrical picture of decoding gives results identical to those obtained via the technique of enumerating cosets of the code. Namely: (i) the lowest weight error patterns, i .e., those having weight w I t, I (dmin - 1)/2, are correctable while those with weight t, < w < d,, - t, are detectable, (zz) the highest-weight error patterns that have weight w 2 d,, - t, may cause misdecodes and (iii)syndromes are uniquely associated with correctableerror patterns only. These points can be taken to mean that all the information needed to correct or detect a received codeword that is contaminated by a correctable or detectableerror pattern is contained in the syndrome that is generated by the error pattern that contaminatesthe erroneous received word. Also, identical estimates of the fraction of highest-weight error patterns that cause misdecodes are obtained by the geometrical and coset pictures of decoding. It is widely assumed that high-weight (i.e., w 2 d,, - to)error patterns generate syndromes randomly and uniformly. Reference 24 indicates that this is a reasonable approximation for RS codes. Based on this assumption, we reason that the conditional probability of a mis-decode, given that a suitably high-weight error pattern has occurred, is equal to the probability that the integer lattice point in N-space, which represents an arbitrary received codeword that is contaminated by the high-weight error pattern, is in, or on the surface of, an incorrect decoding sphere. Then,

884 Magneto-Optical Data Recording where the “-1” term in the numerator accounts for the possibility that the lattice point which represents the undecodable error pattern may occur in the correct decoding sphere, andf, is given by Eq. (58). In Fig. 23 we plot for RS codes on GF(29 that have 16 parity bytes (i.e., N-K= 16 bytes) d four different codeword lengths; N = 30, 60, 120 and 240 bytes (recall that the maximum possible length for an RS code on GF(28) is N = 255 bytes). The length N = 120 byte code in this figure is, of course, the RS code used in the CCS recording format. Note that the conditional probabilwhen tc is ity of misdecoding for that code increases to about 4.4 x increased to its maximum value of eight.

cP-

0

1

2

- N = 240 bytes N = 120 bytes

3

4

5

6

1

8

tC

-*-

N=60bytes

- 6 . N = 30 bytes

Figure23. Pe,c,a~st,forN=30,60,120and240byteRScodesonGF(2~);N-K= 16bytes.

RS Output Information Block Reliability. The probability of encountering an information block that is known to be unreliable at the output

of the RS decoder is given by

[see Eq. (53) and Fig. 201 for the case of

Data Reliability and Errors 885 randomly distributed byte errors at the input of the decoder. Each such unreliable output information block could contain from t, + 1 to t, + td erroneous bytes. Fewer errors may occur since one or more of the input parity bytes, which are not output by the decoder, may have been erroneous. F'ela,a depends on the code's length N, distance dmin,the number of errors being corrected by the decoder t, and the details (statistics) of the errors input to the decoder. P,,the probability of encounteringan information block that has suffered a misdecode, is given by Eq. (55); its constituent factors &,a and Pelr,a are given by Eq. (54) and Fig. 21, and Eq. (59) and Fig. 23, respectively. When a misdecode occurs, the informationblock from one of the nearest neighbor codewords, in N-space, is usually output by the decoder. If this neighboring word has distance d,,,, from the correct codeword, then up to d,, errors will occur in the output information block. Fewer than d,, errors may occur since the neighbor codewords may have different panty bytes, which are not output. More errors will occur if the neighbor codeword is at distance > d,,,, fiom the correct codeword. KT in mind that Pele,a as given by Eq. (53) and P,,a as given by Eq. (54) apply to the case of randomly distributed byte errors at the input of the decoder. If there is correlation among the error events that amve at the input to the ECC decoder, or if the depth of interleaving I is not sufficiently large to completelyrandomize all of the possible burst error events, then Pela,aand &,a are not given by Eqs. (53) and (54) respectively, since then they will depend on Z as well as on N, t,, dmi,,.[221When errors are correlated andor interleavingis used, Eqs. (53) and (54) yield lower bounds for Pele,aand &,a. Each sector consists of I interleaved codewords. Therefore, to account for the fact that the data integrity of the sector as a whole is compromised if any of the information blocks obtained by decoding the I interleaved, received codewords contain erroneous data, the reliability estimates for the entire recovered sector are obtained by multiplying both &,a and P, by I. Lastly, since the information blocks output by the RS ECC decoder constitute a received codeword of the RS-CRC error detecting code on GF(2*) that has a length of 108 bytes, one additional decoding is necessary before the final information is delivered to the controller (the four RS-CRC parity bytes are used in this decoding process; they are not contained in the information block that is output by the RS-CRC decoder). The RS-CRC code has d,,,, = 5 and is meant to be decoded using t, = 0. Thus, td = 4 and up to four byte errors in the received codeword will always be detected, regardless of whether they are contained in an N = 120 byte RS ECC information block that was flagged as being unreliable, or are part of an

886 Magneto-Optical Data Recording i&ormation block that came from a misdecoded 120-byte received codeword. If more than four errors occur in an RS-CRC received codeword, the probability of the error pattern not being detected by the RS-CRC decoder is

Eq. (60)

p, =-- 1 - 2.32 x 10-l’ 2564

since the error pattern can only escape detection if it is identical to a codeword (i.e., it generates the syndrome that correspondsto the all “zeros” error pattern).

6.0 ERROR CONTROL IN FUTURE MO STORAGE SYSTEMS The values of $,a and P, that result when error events are randomly distributed with 6 < 10-5,and when an N =120 symbol RS code on GF(28) with tc I 8 is used against such errors, are reasonably low (see Figs. 20,21 and 23). And, the burst and gap distributions of Figs. 15 and 16 illustrate that the assumption of randomly distributed errors is not entirely unreasonable for current optical storage systems. However, these error data are relevant to low data density recording on a good quality WORM optical storage medium which realizes high SNR when used in an appropriate disk drive. The reader should also note, that even in this case, some correlation among error events was seen. It is likely that the statistical nature of error events that will be experienced in future, high density, high data rate MO optical storage systems will change from those illustrated by Figs. 15 and 16 in two major ways: (i) the mean symbol error rate, i.e., ,will increase to lo4, or 5 x lo4, or perhaps even higher (due to the lower wideband SNR and an increase in the length of burst errors caused by dropouts) and (ii) error events will exhibit some degree of correlation (due to the increase in shift errors that are synergistic with hard errors on the storage medium, see Secs. 4.1,4.5). These changes will cause Pdlpto decrease and Pc,a and P, to increase relative to their respective values 111 Figs. 20, 2 1 and 23. There are three means of dealing with higher average symbol error rates and correlated error events: Use RS codes with larger dmin,to increase t, without decreasing PdlP and increasing $.a and P,

5

Data Reliability and Errors 887 Use two error-correcting codes that are twodimensionally interleaved Use enhanced decoding techniques such as cooperative decoding of interleaved codewords, andor decoders that utilize “erasures” and “soft-decision information” to effectively increase t, in a manner that does not decrease Pdlaand increase PT,zand P,. The first and second of these is incompatible with the CCS recording format, i.e., they would require a change in the data organization and size of the User Data field (although they could be implemented in the context of the CCS recording format under the penalty of about a 10-20% reduction in the amount of user data per sector). The third suggestion, namely, employing enhanced decoders, could be implemented with the CCS format, but advanced decoding hardwarehirmware would be required.

6.1 RS Codes with Increased dIncreasing the RS code’s d,, enables it to handle higher error rates and correlated errors because the decoding spheres will be spaced farther apart (see Fig. 22). Current optical storage systems use RS codes with dmin=17; this is mostly due to the commercial availability of decoding chips for such codes. Chip cost, of course, is driven by the required size and speed of the ECC chipset. A rough idea of the required hardware complexity can be obtained fiom the table below, which lists the number of computations that are required to decode an N,K RS codeword. The table shows that the total work required is strongly driven by the magnitude of tc, and therefore by dm,,,.[251

Decoding Operation

No. Required Multiply-adds

Syndrome generation Determine error locator polynomial

2Ntc 4t,2

Find error locations (via Chien search)

NtC

Determine error values

3tc (tc-l)

Error correction Total work

C t *7t:

+ 3Ntc

888 Magneto-Optical Data Recording The number of multiply-adds required per ECC symbol time is obtained by dividing the total work value in the last row of the above table by N, the number of symbols per codeword. Note that the work related to finding the error location may be reduced if a Galois field arithmetic unit is used to solve for the roots of the error locator polynomial, instead of locating them via a search algorithm. Also, the multiply-adds referred to here are in the 8bit arithmetic of GF(28) (see Sec. 3.3). Decoders for RS ECCs on GF(28)that have d,,,, as high as thirty-three have been recently developed for specialty government applications; the cost/performance of VLSI technology has improved to the point where decoders for RS ECCs that have dmin = 33 might be made available as quasicommodity chips if demand is sufficient. 6.2 Two-Dimensional Interleaving of Two ECCs

This strategy is used by the Compact Disc optical storage system to achieve a relatively large effectived,,,, from two ECCs that each have small dmjfl. The CD system employs two length N 30 byte, RS ECCs on GF(28) that are 2dimensionally cross-interleaved, i.e., the data output by the first code’s decoder is input to the second code’s decoder, but thirty received codewords must be decoded by the first code’s decoder before one received word of the second code is obtained. That is, each byte of the second code is carried in a different codeword of the first code. A similar system could be implemented with the CCS recording format as follows: Suppose the bytes in columns 1 through 4 in each of rows 1,3, 5 , 7, ...., 101, 103 in Fig. 8 are the parity bytes of multiple N = 20, K = 16 RS ECC codewords that extend over the 20 bytes contained in the pairs of rows numbered 0 and 1; 2 and 3; 4 and 5; etc. This, of course, would reduce the number of user data bytes from 1024 to 8 16, but this is only a tutorial example so disregard that for the moment. This 20-byte RS ECC, which has d,,,, = 5, would be decoded after decoding of the regular depth-10 interleaved, 120-byte RS ECC codewords, which span each of the columns 0 through 9 in Fig. 8. Thus, the information bytes that are output by the 120byte RS decoder would constitute the 52 received codewords of the 20-byte RS ECC. An even more robust system could be obtained by staggerinterleaving the 20-byte ECC code as well, e.g., by letting pairs of rows that are spaced by 4 rows constitute the 20 bytes of a given codeword. That is, 52 codewords would be formed from row pairs 103 and 100; 102 and 99; ....., 1 and 102; 0 and 101 (this is essentially what is done in the CD’s ECC

-

Data Reliability and Errors 889 system). Thejob of the 20-byte RS ECC is to “clean up” errors (i.e., correct residual errors) that are contained in the information blocks that are output by the 120-byte RS ECC decoder (such errors, of course, will be caused by uncorrectable, or misdecoded, 120-byte RS ECC received codewords). Such a 2-dimensionally interleaved ECC has an effective dmmthat is equal to the product of the d,,,, values of the two 2-D interleaved RS ECCs. Our example 2-Dinterleaved coding system would therefore have an effective d,,,, = 5 x 17 = 85, but to correct all error patterns ofweight w = 42 bytes, or less, that could contamhate a given sector, e n h a n d decoding techniques would have to be used (e.g., the d,,,, = 17 code’s decoder would have to send “erasures” to the d,,,, = 5 code’s decoder, see Sec. 6.3). Note that the function of the RS-CRC remains unchanged in this error control system. Another method for achieving 2dimensional interleaving of error control codes involves using one of the two codes to protect channel data (i.e., to encode blocks of channel bits), while the other code is a conventional interleaved RS ECC that encodes blocks of user data. In this scheme, blocks of channel data would be processed by an error-correcting decoder prior to channel data demodulation; this technique is especially effective against shift errors and significantly reduces demodulation related error multiplication.[261 6.3 Enhanced Decoding

This option may be useful if it is desired to preserve the configuration of the CCS recording format in future MO optical disk drives, e.g., in order to maintain backward compatibility of recorded disks. The CD optical storage system has used this technique to enhance the error control capability of its cross-interleaved ECC system, i.e., it has capitalized the on the increased computational power made possible by modern, low cost VLSI technology to realize advanced ECC decoders. For example, in the initial CD disc drives that were offered for sale in the early 1980s, only one byte error was corrected in each of the two cross-interleavedcodes. This was done because the high mean byte error rate (experienced on discs that had suffered the fingerprints and scratches associated with consumer handling) lead to a high Pc,a, which caused an unacceptably high P, value when two byte errors were corrected by one, or both, of the cross-interleaved code’s decoders (this is due to the fact that, for the 30-byte RS code used in the CD system, P2,d 1.78 x 10“ when t, = 1 and Peje,a -0.0066 when t, = 2). The ore powerful decoders (which were enabled by the new generations of VLSI)

-

890 Magneto-Optical Data Recording provided the ability to correct up to four errors (that are flagged as erasures) in each ofthe two RS ECCs, thereby realizing a tremendous improvementin data reliability. (Note that this reliability improvement is due solely to enhanced decoding, i.e., the CD system’s error correcting code has not been changed.) The subject of erasures is taken up next. The geometrical picture of error correction decoding (see Fig. 22) can be used to illustrate erasure decoding as well. Consider and N,K RS code having distance dmj,= N - K + 1. Suppose that a codeword of this code is contaminated by an error pattern of weight w and thatfs (d,,,,,- 1) symbols of that codeword are erased by some external agent (such erasure is actually realized byflagging the erased symbols in some manner). Assume thatj I w of the codeword’s erroneous symbols are erased; then ( f - j ) 2 0 nonerroneous symbols are erased as well. Let such a codeword (that contains w errors andferasures, j of which fall on actual errors) be presented to an erasure correctingdecoder that is designed to correct up to txerased symbols as well as an additional t, nonerased, erroneous symbols. An erasurecorrecting decoder considers every erased symbol to have an incorrect value, even if the symbol value is correct (as would occur when a nonerroneous symbol is flagged). Unlike erroneous symbols, however, the locations of erased symbols in the received codeword are obviously known to the decoder. To correct an erasure, only its value must be calculated. To correct an error, the decoder must locate its position in the codeword and compute its correct value. Iff5 t,, the erasure correcting decoder will: (i) initially set thefflagged symbols aside, (ii) transform the remaining N-fsymbols into a received word of an N-JK RS code having distance d’,,, = N-K +l-f= d,,, - f, (iii) attempt to correct up to t, errors in this new received codeword, and (iv) use the N-fsymbols that result after this error correction process is completed to calculate new (correct) values for theferased symbols that were initially set aside. Iff> t, and the decoder has fixed values oft, and t, (e.g., they are hardwired in the decoder circuitry, as is normally done) the decoder will usually declare the received codeword to be uncorrectable. (Note that enhanced erasurecorrecting decoders may adaptively reconfigure their error handling strategy, i.e., change the values t, and t, that they use in real time. Such reconfiguration is based on externallyprovided information that works in concert with the internal logic in the decoder; the goal of this is to always handle the maximum number of erasures and errors that can be reliably corrected, i.e., to reliably correct all the errors in the maximum possible number of received codewords). The error correctioddetection

Data Reliability and Errors 891 process mentioned in step (iii) above is done in the normal way, i.e., the number of errors that may be reliably corrected and detected are constrained by the inequality 2tc + td < (d,,,,, -f)as prescribed by Eq. (2) and the probabilities of encountering an uncorrectable, but detectable, number of errors, or of misdecoding, are given by Eq. (53) and Eqs. (54) through (59) respectively. These probabilities are illustrated by Fig. 22 (except that d,,, in that figure must be replaced with d,,,,, -f).The combined erasurderror correction and error detection capabilities of a decoder such as the one described immediately above are jointly constrained by

where d,,,,, is the minimum distance of the N,K RS code that is used to encode the received codewords that are processed by the erasure-correcting decoder. Eq.(61) is analogous to Eq. (2). All of the w errors which contaminate the codeword described above will be successhlly corrected by the normal (non-enhanced) erasure correcting decoder so long a s f l tx and w - j It,. If this latter inequality does not hold, then either the error correction process described in (iii) above will detect that the w - j unflagged errors comprise an uncorrectable error pattern, or a misdecode will occur. If misdecoding occurs, then about d,,,,, ferrors will contaminatethe N-fsymbols returned by the decoding step (iii) and incorrect values for all of theferased symbols will be computed as well [via step (iv)]. Thus, erasurecorrection decoding may handle a larger number of errors per received codeword than error-correctiondecoding, but it can only do this reliably if the external erasure-flagging process is robust (a large fraction of the errors must be flagged and flagging of non-erroneous symbols must occur with low probability). Lastly, the amount of work [multiply-adds in GF(2q)I that is required to effect erasure correction is significantly increased versus error-correction decoding, due mostly to the fact that the erasure-correcting decoder must continually reconfigure to decode words of one of a number of different N-JK RS codes, depending upon which symbols in the received N,K RS codeword are actually erased [see the above discussion of decoding step (ii)]. The above discussion illustrates that a decoder which is capable of performing erasure correction can correct up to d,,,,, -1 erasures in a received codeword, which is twice the maximum number of "docated" errors that can be corrected by an error-correcting decoder that does not handle erasures. The essence of enhanced decoding is to enable the erasure-

892 Magneto-Optical Data Recording correcting decoder to correct up to dmin- 1 erasures per codeword with extremelyhigh reliability. Techniques for realizing such enhanced decoding are discussed next. The reliability of erasure correction is 100% so long as Eq. (61) is satisfied. Note that, if a perfectly correct symbol is identified as an erasure, the decoder will simply proceed to correct it; the original correct value of the symbol will be returned. The power of erasure correction lies in the fact that more than (dmin-1)/2errors may be corrected, ifthey are correctly identified as erasures, i.e., located. If the real errors in the received codewords cannot be consistently located by the erasing agent, the usefulness, as well as the reliability of erasure correction is lost. Perhaps the most reliable method of providing erasure information is to generate it via another code. This technique in used in the CD error control system found in many of today’s CD players; the first decoder adaptively configures itself to correct erasures if any are sent to it by the channel data demodulator, which appends erasure flags to any user data bytes that are obtained from RLL code segments that are contaminatedby runlength constraint violations, i.e., corresponding user bytes are erased when too few, or too many, “zeros” are found to occur between pairs of “ones” in the channel data input to the demodulator (the modulation code employed in the CD optical storage system is a 2,lO RLL code). Erasures are “hard” in the sense that they are communicatedthroughout the error control signal processing system via one-bit flags, i.e., a symbol is either considered to be good, or it is erased. Softdecision information is essentially a means of providing for a multibit erasure. This type of symbol quality information is conveyed by attaching a multibit reliability flag to the data symbols. When such flags are attached to the symbols of a received codeword, an intelligent decoder can use a maximum likelihood or majority vote algorithm to determine which symbols it will accept as being erased prior to attempting erasure decoding. Again, the most advance CD players utilize this technology; the first ECC decoder attaches a 2-bit soft decision flag to the information blocks that it outputs (to the second ECC decoder). The flags are assigned by the first decoder based on the number of bytes in an input received codeword that are erased by the demodulator and the number of errors/erasures it attempts to correct (essentially, the first decoder estimates the values for P+ and P, that apply to a decoded received word and assigns identical 2-bit reliability flags to every byte of the corresponding information block that it outputs). The second decoder adaptively configures itself, i.e., chooses values for t,, t,, and td based on the

Data Reliability and Errors 893 mix of 2-bit flags that are attached to the symbols of a given received codeword, together with the syndrome that it calculates for that codeword. Reference 27 contains a thorough discussion of soft decision decoding of a binary primitive cyclic code; it describes a decoding algorithm that utilizes a soft decision data detector to determine the least reliable bit of a received codeword and provides an analysis of the reliability of this soft decision based error correction technique as well as a description of the required decoder hardware. Cooperative decoding by the decoders of interleaved codewords is another technique for generating and using erasure/softdecision information. Basically, as the successive interleaves of an interleaved code are decoded, erasurehoft decision information is iteratively produced and exploited in some way. Symbols in a given interleaved codeword might be erased based on the number of errors found during the decoding of preceding interleaved codewords; such erasure information is purely digital, i.e., it is not based on a soft decision data detector operating on an analog waveform. This type of enhanced error control decoding is used in some magnetic tape systems to effectively combat long dropout errors. The power of the enhanced decoding techniques mentioned above is practically constrained by the computational speed that is needed for flag processing, adaptive decoder reconfiguring, etc. The low data rate (125 kbytes/sec) of the CD system has enabled very powerful decoding to be accomplished in real time. As VLSI speed and complexity increase, these enhanced decoding options will be practical in high data rate storage systems.

REFERENCES 1. Siegel, P., IEEE Trans. Magnet., MAG-21(5):1344-1349 (1985) 2. Immink, K. A. S , Proc. IEEE, 78(11):1745-1758 (1990) 3. Immink,K. A. S., Coding Techniquesfor Digital Recorders, Prentice Hall International, UK Ltd. (1991) 4. Koren, N., IEEE Trans. Magnet., 27(6):4594-4599 (1991) 5 . Kobayashi, H., andTang, D., IBMJ. Res. Develop., 14(1):368-375 (1970) 6. Moon, J. J., and Carley, L. R., IEEE Trans. Magnet., 24(6):2973-2975 (1988) 7. Lynch, R T., Jr., IEEEJ. Select. Areas in Comm.,10(1):57-72 (1992)

894 Magneto-Optical Data Recording 8. Ha&, R, andLynch,R. T.,Jr., IEEEJ. Select. Areasin Comm., 10(1):182190 (1992) 9. Howe, D. G., and Marchant, A. B., SPIE Vol. 382:Optical Data Storage, pp. 103-1 15, Societyof Photo-Optical InformationEngineers, Bellingham, WA (1983) 10. Ehrtholomeusz, B. J., Bowers, P. and Genova, D. J., J. Appl. Phys., 66(10):4635 (1989) 11. Bartholomeusz, B. J., Genova, D. J. and Stinson, D. G., Appl. Optics, 29(20):3030 (1990) 12. Yoneyama, Y., Tanaka, H., Satoh, T., Takatsuka, Y., and Yorozu, T., IEEE Trans. Magnet., MAG-25(5):4042-4044 (1989) 13. Suits, J., Rugar, D., and Lin, C. J., J. Appl. Phys., 64(1):252-261 (1988) 14. Mansuripur, M., and Connell, G. A. N., J. Appl. Phys., 54(9):4794-4798 (1978) 15. Mee, C. D., and Daniel, E. D., Magnetic Recording - Vol. II: Computer Data Storage, pp. 85-103, McGraw-Hill, New York (1988) 16. Howe, D. G., SPIE Vol. 421: Optical Disk Systems and Applications, pp. 3 1-42, Society of Photo-Optical Information Engineers, Bellingham, WA (1983) 17. Howe, D. G., and Hilden, H. M., IEEE J. Select. Areas in Comm., 10(1):223-232 (1992) 18. Petersen, W., and Weldon, E. J., Jr., Error Correcting Codes, MIT Press, Cambridge, MA (1981) 19. Howe, D. G., SPIE Vol. 695: OpticalMass Data Storage Il, pp. 255-261, Society of Photo-Optical Information Engineers, Bellingham, WA (1986) 20. Howell, T. D., IBMJ. Res. Develop., 28(2):206 (1984) 21. Gilbert, E. N., Capacity of a Burst-Noise Channel, Bell Sys. Tech. Jour.: 1253 (1960) 22. Yee, J., and Weldon, E. J., Jr., Evaluation of the Performance of Errorcorrecting Codes on a Gilbert Channel, IEEE Trans. Comm., 43(8):23162323 (Aug. 1995) 23. Marchant, A. B., Optical Recording: A Technical Overview. pp. 282-288, Addison-Wesley, Reading, MA (1990) 24. Cheung, K-M., IEEE Trans. Comm., 40(5):857-859 (1992) 25. Weldon, E. J., Jr., Seminar Notes: Error Correcting Codes withApplications to Communications and Computer Systems, Chapter 9, Copy-right by E.J. Weldon, Jr., Honolulu, HI (1987) 26. Hilden, H. M., Howe, D. G. and Weldon, E. J., Jr., IEEE Trans. Magnet., 27(6):4600-4605 (1991) 27. Berlekamp, E. R, IEEE Trans. Info. Theory, IT-29(3):372-377 (1983)

14 Outlook for MagnetoOptical Recording Mark H. KIyder

Magneto-optical recording is still a relatively new technology and is not well established. Applications for it are still developing. Recent market studies forecast growth rates of 26% per year for the next five years,"] but any number of changes in the technology or marketplace could make this growth rate greatly change in either the positive or negative directions. Making projections of the future for a technology so early in its development is subject to large errors. Nevertheless, in the sections which follow, such projections are made. The purpose in making these projections is to provide others with a perspective of what some of us working on the technology expect in the future, to point out the strengths and weaknesses of the present technology and to highlight possible new developments which may alter the role which the technology plays in the future. The future trends in magneto-optical recording, are dependent not only upon the capabilities of magneto-optical recording technology, but also upon future trends in information processing systems and other data storage technologies. Therefore, before considering what may occur in magnetooptical recording technology, trends in information processing and other data storage technologies are considered.

895

896 Magneto-Optical Data Recording 1.0

TRENDS IN INFORMATION PROCESSING SYSTEMS

The processing speed, main memory size, data storage capacity and input/output rate of data storage devices in computer systems are necessarily interrelated. As the speed of processors increases, larger capacity main memory is used so that more complex problems involving more data may be addressed. This in turn means that the capacity of the data storage device needs to be increased. A computer system typically has one to two orders of magnitude more storage capacity than it has main memory. Similarly, as the processing speed of computers increases, it is necessary that the input/output rate from storage devices be increased; otherwise the storage device would limit the number of instructions which the processor could handle in a given amount of time, rather than the processor speed itself. By the year 2000, it is expected that computer systems will be interlinked by fiber optic networks operating with 100 Mbyte/sec data rates, such as utilized in the High Performance Parallel Interface (HIPPI). To not limit the performance of these systems, data storage systems will also provide data at such data rates, probably through parallelism in disk arrays.i2I The historical rate of progress in semiconductor memory is indicated in Fig. Simple curve fitting to the data indicates that the density of semiconductor memory is advancing at a rate given by

Eq.(1)

bits/mm2= 90 x 1.43'-lg71

while the chip area used for memory is given by

Q. (2)

chip size = 10 x 1.11'-lWo m2

Extrapolations of these data suggest that by the year 2000 semiconductor memory will reach approximately 1 gigabit per chip with a chip area of about one square inch. Whether this rate of progress will be sustained over the next decade is not clear. Progress in the semiconductor industry has, in the past, always been tied to advances in optical lithography. At either 64 Mbit or 256 Mbit chip size, a change to some other form of lithography is expected to be necessary. X-ray lithography appears to be a likely candidate. Semiconductor experts suggest that the change in lithography techniques may slow the rate of progress. It seems likely that 64 Mbit or 256 Mbit chips will be heavily utilized in the year 2000.[4]

Outlookfor Magneto-Optical Recording 897

Projections I00000

r

10000 N

Memory Capacity

E E

3;. + .n 1000

total chip area r = memory array area 67

70

73

76

79

82

85

88

Year

Figure 1. A plot of the areal density versus year in which semiconductor chips were introduced. Memory array areas are shown at each capacity level.

With 256 Mbit chips, it is reasonable to expect a personal computer to have of the order of 1 Gbyte (32 chips) of memory. The number of chips would fit on a practical board size, and the expected cost would be a reasonable fiaction of the total cost of a personal computer. Since the typical personal computer today has only about 8 Mbytes of main memory and 500 Mbytes of magnetic storage, semiconductor proponents frequently assert that with 1 Gbyte of semiconductor memory, some portion of which may be nonvolatile or have battery backup, magnetic storage will not be needed. These people are very aware of the capabilities of semiconductortechnology, but are not aware of the growth in demand for inexpensive storage technology or the expected advances in both magnetic and magneto-optical storage devices. What they fail to recognize is that, although the semiconductor memory market is indeed growing rapidly, the market for lower cost storage is growing at an exponential rate, and it is

898 Magneto-Optical Data Recording unlikely to stop growing in the foreseeable future. Recent estimates are that the total amount of recorded information is about 10’’ bits, but that, based upon extrapolations of the growth rate over the past 50 years, by year 2000 the total will be 1020bits-a 100,000 X increase.[5]A digital audio compact disk today contains about 600 Mbytes of information. Clearly one would not want to store his library of digital audio recordings in semiconductor memory. Furthermore,the data rate required for High Definition Television (HDTV) is about 100 Mbytes/sec. Even with data compression, it would not take long to fill 1 Gbyte of memory with high quality images such as those for HDTV, and it is highly likely that we will be storing vast quantities of such data by the year 2000. The increased processing speed of semiconductor devices is, in fact, partially responsible for the increased demand for storage. Only with high speed processors and large random access memories can one process and utilize digital audio or image data which consume such large volumes of storage. Those working on magnetic and magneto-optical storage should hope semiconductor memory and processing speed continues its rapid advance. This will only increase the demand for advanced storage technology.

2.0 TRENDS IN MAGNETIC STORAGE Before considering magneto-optical storage, it is important to assess not only the progress which is expected in computers and semiconductor technology, but also in magnetic storage devices. In late 1989, IBM announced that they had demonstrated a magnetic recording density of 1 Gbit/in2on a thin film rigid di~k.[~l[’1[*1 The headdisk flying height was less than 50 nm and the raw bit error rate was about Subsequentlyin 1991 Hitachi announced they had achieved a storage density of 2 Gbit/in2.[91[10] Although these were laboratory demonstrations, not products, it is highly likely that storage density on rigid disk drives will reach about 4 Gbit/in2by the year 2000. A recording density of 4 Gbit/in2would enable a range of drives to be made. Using the standard 3.5 inch form factor which is popular today, a 60 gigabyte drive could be made, as shown in Table 1. This drive is projected to have 12,000 bits per mm (300,000 bits per inch) and 560 tracks per mm (14,000 tracks per inch) with 12 disks on a spindle. Data rates from asmgle

Outlookfor Magneto-Optical Recording 899 head d d b e 15 Mbytes/sec, with adisk rotation rate of 5,400 rpm. New high performance head materials, such as multilayer iron nitride,["] make it possible to achieve reasonable permeabilities at frequencies of several hundred megahertz. Therefore, the data rate limitation is likely to be the speed of the semiconductor devices used for the signal processing and detection circuitry in the drive. Again, advances in semiconductors enable advances in disk drive technology.

Table 1. 3.5 Inch Magnetic Disk Drive in 2000

Capacity: 60 Gbytes Twelve 3.5 in. disks 300,000 bits per inch 14,000 tracks per inch 2.8 Gbytedside Zone bit recording

Data Rate: 15 Mbyteshec @ 5,400 rpm Access time: 11.2 msec Seek: 5.6 msec @ lOOG, 1/3 stroke Latency: 5.6 msec @ 5,400rpm

At the low end, one might expect one-inch disk drives to be plugged into a board like semiconductor memory chips are today. As indicated in Table 2, a one-inch disk drive could store 600 Mbytes on a single disk with an areal density of 4 Gbit/in2. With a disk rotation rate of 10,800 rpm, which is quite reasonable with a one-inch disk, the data rate could be of the order of 10 Mbyteslsec. Multiple one-inch drives could be plugged into a board like semiconductor chips are today. In fact, given the rapid decrease in disk size and the increase in semiconductor chip size which has been occurring, the two are very likely to be in similar size packages before the year 2000. Assuming 1 Gbyte of semiconductor memory is used in a personal computer, the disk storage might be 10 Gbytes. A 4 x 4 array of one-inch drives would provide this capacity on a board using RAID architectures[21to provide faster access times, higher data rates and better reliability than a single drive.

900 Magneto-Optical Data Recording Table 2. 1 Inch Magnetic Disk Drive in 2000

Data Rate: 10 Mbytedsec Capacity: 600 Mbytes 1 in. disk Inner track @ 10,800 rpm 300,000 bits per inch 14,000 tracks per inch 300 Mbytedside Access time: 7 msec Seek: 4.2 msec Zone bit recording @ 50G, 1/3 stroke Latency: 2.8 msec @ 10,800 rpm

Obviously other configurations of rigid magnetic disk drives are possible, but the above examples serve to illustrate a range of capabilities which may be expected. It should be noted that the 3.5-inch drive is probably as large a disk drive as will be made with a 4 Gbit/in* storage density. A capacity of 60 Gbytes is a lot for one spindle and larger disks would make the data rate too high for the semiconductor channel electronics. Two and one-half inch drives are likely to be the high volume products when such storage density is available.

3.0 TRENDS IN MAGNETO-OPTICAL DRIVES Assuming the aboveprojections for processors, semiconductormemory and rigid magnetic drives are met, what are the opportunities for magnetooptical recording? To answer that question, let us consider what type of progress might be made in magneto-optical recording by the year 2000. It can be expected that the wavelength of lasers will decrease by about a factor of two to 400 nm. In addition, some increase in the numerical aperture of the objective lenses

Outlookfor Magneto-Optical Recording 901 used to focus the light to about 0.65 is likely. This would make possible a spot size of the order of 0.33 micrometers and a recording density of about 3,000 flux changes per mm (75,000 flux changes per inch). Using a run length limited code which encodes 1.5 bits per flux change, a linear bit density of 4,500 bits per mm (1 12,500 bpi) results. The track density could also be increased by a large factor. The reduced laser spot size could reduce the recorded track width to 0.33 micrometer. This would enable a track pitch of the order of 0.7 micrometer using continuous composite servo with 0.37 micrometer grooves. This continuous composite servo, on the other hand, is rather wasteful of disk surface. The groove between tracks is nearly as wide as the recorded tracks, yet the tracking servo accuracy is of the order of 0.05 micrometer. An alternative approach would be to use a sector servo which would enable one to pack recorded tracks much more closely together. A sector servo scheme is illustrated in Fig. 2. In this case, the position error signal (PES) is derived from the difference in detected signal amplitudes as the laser scans between two pits in the disk surfkce. With such a servo, space between tracks need not be given up to a groove. It would, for example, be possible to use the upper edge of a pit for the PES of one track and the lower edge of the same pit for the PES of an adjacent track. In principle, such a sector servo could produce nearly twice the track density. It is suggested that with a 0.33 micrometer spot size, a track pitch of 0.4 micrometer. A linear bit density of 4,500 bits per mm and a track pitch of 0.4micrometers yields an areal density of 11 Mbits/mm2or 7 Gbits/in2. By using zone bit recording, a storage capacity of 4 Gbytes could then be achieved on a double-sided 2 inch disk, as shown in Table 3. Even with a high disk rotation speed of 5,400 rpm, the data rate would be about 3.75 Mbytes/sec, slower than the one inch rigid magnetic drive discussed above. Thus, the data rate of magneto-optical drives is not only lower than in magnetic drives today, but is likely to remain lower in the future. The linear density in magneto-optical recording, using lenses to focus the light, is limited by the laws of diffraction of light, whereas in magnetic recording it is not. Since at constant rotation rate, the data rate increases with linear bit density, the data rate of magneto-optical drives is expected to lag that of magnetic drives.

902 Magneto-Optical Data Recording

Sector Servo

A

B

Figure 2. A diagram illustrating a means of providing sector servo information on a magneto-optical disk.

Table 3. 2 Inch Magneto-Optical Disk Drive in 2000

Capacity: 4 Gbytes 2 in. disk 112,500 bits per inch 0.33 mm spot 1.5 bitdtransition 63,500 tracks per inch 0.33 pm spot 0.07 pm guard band 2 Gbytedside Zone bit recording

Data Rate: 3.75 Mbyteslsec Inner track @ 5,400 rpm Access time: 12.5 msec Seek: 5.9 msec @ 50G, 1/3 stroke Latency: 5.6 msec @ 5,400 rpm

Outlookfor Magneto-Optical Recording 903 In addition to having a lower data rate than magnetic drives, magnetooptical drives today have a longer access time and fewer disks on a spindle. This is because optical heads are considerably larger and more massive than magnetic heads. A significant effort is being made to reduce the size and mass of the optical heads by moving the larger and more massive components off the actuator arm and coupling the light to the objective, either directly with carefully aligned optical components or with optical fibers. Furthermore, some researchers are pursuing integrated optical heads in which integrated optics are used to build a full optical head on a chip. Assuming these efforts are successful, magneboptical drives could approach the access time and number of disks per spindle of magnetic drives. The fact that the number of disks per spindle on a magnetic drive exceeds that of magneto-optical drives is a major reason that magnetic drives today offer a lower cost per bit and larger capacity than magnetooptic drives. Although magneto-optical drive costs are likely to come down, it is unlikely they will be cheaper than magnetic drives. Thus it is likely the cost per bit for a magnetic drive will continue to be less than that of a magneto-optical drive in the future. What magneto-optical recording does offer, which a rigid magnetic disk drive does not, is a removable medium. Magneto-optical disks are therefore likely to be used for data interchange between computers, backup and archival storage. It is because magneto-optical disks are expected to be used primarily as removable media that Table 3 describes a 2 inch disk drive as an example. Anythmg significantlysmaller would be too easily lost when it was removed from the drive. Compared to floppy disks, which today fill some of the same roles, magneto-opticaldisks have much higher areal density and capacity per disk, higher data rates and shorter access times. Because the capacity and data rate of a magneto-optical disk more closely matches that of rigid magnetic drives, they are more suitable as a backup storage medium than floppies. Comparedto magnetic tape, magneto-optical disks offer faster access times, but lower volumetric storage density and lower data rates. Hence, it is likely that magneto-optical disks will be preferred where access time is a significant concern, but that magnetic tape will likely still be used extensively because of its large volumetric storage density, low cost per bit and high data transfer rate through the use of helical scan recording andor parallel recording heads.

904 Magneto-Optical Data Recording

4.0 ADVANCED MAGNETO-OPTICAL MEDIA To meet the above projections, improved magneto-optical media are needed. Rare earth-transition metal alloys, which are the presently accepted media, can be made to operate with 400 nm wavelength light; however, their magneto-optical coefficients decrease with the wavelength of light and do not provide adequately high signal to noise ratio (SNR) even at 820 nm to enable data rates comparable to magnetic disks. Other magneto-optical materials show higher magneto-optical coefficients, but typically do not have adequately low noise. Bismuth and ceriumdoped garnets have been shown to offer large magneto-optic coefficients in the blue and green wavelength region.[12] In single crystal form, such garnets have been demonstrated to provide a carrier to noise ratio (CNR) of 54 dB.[l31 In addition, since the garnets are very stable oxides, they have excellent resistance to corrosion and long term annealing. The problem with single crystal garnets is that their cost is high, because they have only been successfully grown on single crystal gadolinium gallium garnet substrateswhich are quite expensive. Polycrystalline garnets can be fabricated on glass substrates by sputtering[13]or pyrolysis,[14]but the noise level in these materials is typically intolerably high. The reasons the noise level is high are that the grain size is comparable to the written domain size and the domain walls to tend to follow the grain boundaries producing very irregularly shaped domains, and also because there is optical scattering from the grain boundaries. Recent results show progress towards solving this problem. S~zuki['~l reported that the use of rapid thermal annealing reduces the grain size, and experiments by W. Eppler[l6Ihave confirmed this finding and show that domains written into rapidly thermally annealed garnets are much more regular than those in conventional furnace annealed samples. Multilayers of WCo or Pd/Co also show promise as potential short wavelength MO media.["]. They offer higher magneto-optical coefficients than rare earth-transition metals at short wavelength and are also relatively corrosion resistant. Although they are polycrystalline, the grain size is adequately small that additive media noise is not a major problem. Carrier to noise ratios of 64 dl3 have been reported with these materials.['*] These multilayers typically consist of 3 angstrom thick layers of Co interleaved with 10 angstrom thick layers of Pt or Pd. The perpendicular anisotropy is believed to be caused by interfacial anisotropy at the Co surfaces, although stress induced magnetostrictive effects may also play a role.

Outlookfor Magneto-Optical Recording 905 Yet another very promising material for blue wavelength MO recording is MnBi. After garnets, MnBi has one of the largest known magnetooptical e f f i . Furthermore, it exhibits strong perpendicular magnetic anisotropyand a relatively high coercivity which is easily controlled through minor composition variations or doping. Like the garnets, MnBi has suffered from a high noise level due to large grain size and has required a relatively high growth temperature to form the ferromagnetic compound. An additional problem is that the Curie temperature of MnBi is 750 K, but it undergoes a structural phase transition at 630 K. This distorted phase is retained upon quenching to room temperature and can be a problem during the magneto-optical write process. Researchers are currently investigating various dopants which can be added to MnBi to reduce the grain size and either to reduce the Curie temperature below the structural phase transition temperature, or to stabilize the low temperature structure to temperatures above the Curie temperature. Heusler alloys of PtMnSb offer a large magnetosptical effect, but have not been shown to possess the requisite perpendicular anisotropy. Researchers are investigating the possibility of introducing the anisotropy through epitaxial strain with a substrate or underlayer material. Although garnets and Co/Pt multilayers show promise for high density magneto-optical recording with blue wavelength light, significant improvements are still required if the areal bit density and data rate indicated in Table 3 are to be achieved. The 64 dB CNR reported by Lin[l8Ion Co/Pt multilayers corresponds to about 30 dB in a 60 MHZ bandwidth, which could be used with advanced coding and signal processing to achieve the 3.75 Mbytdsec data rate called for in Table 3. The carrier which Lin used was 2.5 MHz. Several dB in signal-to-noise ratio would likely be lost if the data rate were increased to 3.75 Mbytekec, because of the increased bandwidth. This does not leave much margin for variability in media and drive characteristics.

5.0 ADVANCED MAGNETO-OPTICAL HEADS As noted above, improvements in access time and the number of disks which may be placed on a spindle are limited by the mass and size of the magneto-optical heads. There is considerable effort being directed at reducing both quantities.

906 Magneto-Optical Data Recording One approach which has met with some success is to build a split head in which the more massive components are either located off the arm of a linear actuator or nearly over the rotational axis of a rotary actuator so that they do not significantly add to the inertia which the actuator must overcome. The laser beam is coupled out to a small mirror or prism and deflected into an objective lens mounted in a small voice coil assembly which enables tracking and focussing to be controlled.[19]Although further improvements are expected, thus far, split head optical actuators provide seek times which are still slightly slower than actuators on magnetic drives. An additional drawback of split head actuators is that they require stability and precise optical alignment over the carriage movement range. To further reduce the size and mass of optical heads, a number of alternative technologies are under consideration. These include the use of optical thin film holographicoptical e ~ e m e n t ~ , [nonrnechanical ~~][~~] scanners,[23]and integrated head structures.[231-[26] One possibility for significantly reducing the size and weight of an optical head is to use optical fibers to convey light to and from the d i ~ k . [ ~ ~When I [ ~ * ]combined with a suitable microlens to focus the light onto the recording medium, a modified split-head configurationis formed. In this case, however, the moving parts are small enough to fly on a slider like a magnetic recording head but with a higher flying height. Heads using optical fibers have been proposed for both compact disks and magneto-optical disks. The use of fibers is more difficult for magnetooptical recording, however, because rotations of the polarization state of the light must be preserved. This need cannot be automatically satisfied by the use of single-mode polarization preserving fibers. The difficulty arises from fluctuations in the relative phase between the orthogonal fiber modes with temperature and flexure. Because of these fluctuations, only linear polarization aligned with one of the fiber axes will be preserved; the rotated polarization state reflecting from the medium will be significantly altered. Detection schemes have been proposed, however, that are insensitive to these phase fluctuations.[20]These schemes show sigmlicant promise provided suitablemicrolenses and sliders can be realized. Further progress is also neededto develop servo signals in fiber+ptic heads. Holographic optical elements (HOES) are formed by photolithographically depositing or etching patterns of varying thickness on a transparent substrate. The regions of varying thickness introduce appropriate phase retardations needed to perform optical functions such as beam splitting, focusing, and formation of multiple beams. Suitable patterns for some

Outlookfor Magneto-Optical Recording 907 applications can be formed by exposing a photosensitive film on the membrane with interfering laser beams. A more flexible technique, however, is to computer generate the desired pattern. Computer-generated holograms (CGHs) have the advantage of being able to combine several optical functions into a single element. Since HOEs are smaller and lighter than conventional optical elements and multiple optical functions can be combined into a single element, the use of these elements could also significantly reduce the size and weight of magneto-optical heads.[21][22] Significant challenges remain, however, for holographic elements to be used in this application. These include achieving high diffraction efficiencies and satisfactory polarization performance at high numerical apertures. In addition, the performance of HOEs is sensitive to any wavelength variation from the laser. Such variations can result from temperature fluctuations, power modulation, or optical feedback. Another improvement to the split-head configuration is to replace the voicecoil h e servo actuator with a nonmechanical beam scanning device. Beam deflectioncould be accomplished, for example, using diffraction from surface acoustic waves (SAWS), or voltage-controlled electro-optic elements. Since there would be no moving parts, such a device could result in seek times several orders of magnitude shorter than with mechanical actuators. However, the deflection range of these devices would probably be limited to a few hundred tracks around the current position of the coarse actuator. A significant performance improvement could still result, however, if the data is organized on the disk in such a way that a large fraction of the required seeks are within this range. An optical disk tracking system using a SAW deflector has been demonstratedthat exhibits a 10 ms track switching time, and a range of 100 tracks.[23] Potential advantages of using an electro-optic deflector include lower power consumption, higher optical throughput, and the absence of an unwanted undifiacted beam. Electro-optical scanners are unlikely to achieve scan ranges as large as SAW devices, however. A number of improvements would result if the various discrete optical elements mentioned above could be combined on a single integrated subIn addition to size and weight advantages, it is likely that cost ~trate.["]-[~~I and reliability improvements would also be realized. Ura, et al.,[24]demonstrated an integrated device concept for reading compact disks based upon a planar silicon dioxide waveguide formed on a silicon substrate. The laser was buttcoupled to the device, but photodetectors were formed directly in

908 Magneto-Optical Data Recording the silicon. Light was coupled to and from the device using focusinggrating couplers. An integrated device for sensing polarization has also been demonstrated,[25]but has not yet been combined with a complete optical inkgrated circuit for readmg magneto-opticaldisks. Although these concepts are promising, a number of challenges remain to be met before a practical integrated device is available. Focusing grating couplers have many of the same advantages and problems as HOES, and schemes that are capable of both writing and reading need to be developed. Accepting that there will be steady progress toward lighter and smaller optical heads via the above methods, the most likely scenario is neverthelessthat magneto-optical drives will only approach magnetic drives in terms of seek time or number of disks on a spindle. It is unlikely magnetooptical recording will offer superior performance compared to rigid magnetic drives. This does not mean that improving the performance of magneto-optical drives should not be addressed, however. Given that there is a market for magneto-optical drives based on their removability, faster access times will be desirable, and will likely lead to a larger market.

6.0

DIRECT OVERWRITE

Another drawback of magneto-optical recording as compared to magnetic recording has been the absence of direct overwrite capability. Magneto-optical drives used for computer data storage today use a two-step overwrite process. First a magnetic field is applied and a continuous laser beam is applied to a sector of the disk to align all the magnetization with the magnetic field, thus erasing any previously recorded information. Then, during a second rotation of the disk, the magnetic field is again reversed, and the laser is pulsed to write new information. This two-step overwrite technique causes magneto-optical drives to be slower than magnetic drives when previously recorded information is to be updated. At least three different approaches to achieving direct overwrite in magneto-optical recording have been proposed. The first to be proposed was a laser beam modulated between two different energy levels to selectively record or erase domains in a single magnetic layer.[29]This technique utilizes specially designed materials in which a high energy laser pulse writes a domain, whereas, a low energy pulse destabilizes the domain and

Outlookfor Magneto-Optical Recording 909 causes it to collapse. Critical to the operation of this technique are a high domain wall mobility and a significant difference in the thermal gradients achieved during writing and erasing. This single-layer laser modulation technique is the most attractive to implement from a systems point of view, because it utilizes a single magnetic layer, not substantially more difficult to manufacture than present media, and because it also could use present magneto-optical Double-sided media would be allowed and data collected thus far indicate the technique does not restrict the data rate. In fact, the technique presently works better at higher data rates than at low data rates.[31] The problem with this technique is that the signal-to-noise ratio has been marginal. The accepted standard calls for a 45 dB carrier-to-noise ratio (CNR) in a 30 kHz slot bandwidth. A 46 dB CNR has been demonstrated at 4.7 MHz with a disk velocity of 11.3 dsec, and further improvements are expected.[31] Another approach is to use a magnetic recording head flying close to the disk to provide the rapidly modulated magnetic field required to overA diagram of an apparatus to do this is write data at the full data shown in Fig. 3. The magnetic head is flown directly above the magnetic thin film which is overcoated with a protective layer. The laser beam is focused on the magnetic layer through the transparent substrate. With this field modulation technique, the laser beam is normally left on continuously and only the field from the recording head is modulated. The result is that the domains are crescent shaped as indicated. This technique reportedly works well at the data rates of present drives. Reportedly, the jitter in the position of the domain walls is actually smaller with this technique than when the laser is pulsed. This has led to the realization that such field modulation overwrite could utilize run length limited codes which pack more than one bit per transition. The disadvantages of this technique include the fact that only singlesided disks may be used, since the magnetic head must be close to the magnetic layer. In addition, the double-sided head is more complex and adds to the cost of the drive, and the inductance of the head places limitations on the frequency at which this technique may be applied. The third direct overwrite technique uses laser modulation rather than field modulation, but it utilizes multilayer exchange-coupled media.[33]-[35] To understand this technique, consider Fig. 4, which shows two exchangecoupled layers. The memory layer is selected to have a low Curie temperature, but a very high room temperature coercivity. The reference layer has

910 Magneto-Optical Data Recording a higher Curie temperature, but a lower room temperature coercivity. The initializing magnet produces a field Hjniwhich is adequate to saturate the reference layer, but inadequate to alter the magnetization in the storage layer. As shown, it magnetizes the reference layer downward. The bias magnet applies a small field Hb opposite in direction to the reference magnet to both films, but it is too weak to change the magnetizationin either layer without the help of a laser pulse. When the laser is pulsed with low energy, the memory layer is heated above its Curie temperature, but the reference layer is not. Consequently, when the memory layer cools, exchangeaupling with the reference layer causes the magnetization in the memory layer to assume the orientation of the reference layer. On the other hand, ifthe laser is pulsed with high energy, both the memory and reference layers are heated above their Curie temperatures and the bias magnet, which is opposite to the initialized magnetization in the reference layer, determines the direction of the final magnetization. Hence, by controlling the energy level of the laser either upward or downward pointing magnetization may be written.

r I J

Magnetic Head _ Z-

-

1

I -

84pm

I

Protective Coatina

I

Figure 3. A diagram illustrating an apparatus for performing direct oveMnite by field moddation.[321

Outlookfor Magneto-Optical Recording 911

New Data

Pre-recorded Data

Figure 4. A diagram illustrating an apparatus for performing direct overwrite by laser modulationusing two exchangecoupledmagnetic layers.

When using only two layers as diagrammed in Fig. 4, it has been found that the initializing magnet causes some reduction in the size of the written domain, resulting in added noise during readout. To overcome this, researchers have added third [331 and, in some cases, fourth layers[34]to the media. With three- and four-layer media, CNR in excess of 50 dB has been achieved. Because of the multiple layers and the criticality of their parameters, this technique is expectedto add to the cost and degrade fiom the yield of manufactured media. The multiple layers also make a relatively thick medium which requires somewhat higher laser power for writing. Furthermore, although 50 dE3 CNR at 1 Mbyte/s does meet the ANSI specification of today, it is not necessarily adequate to enable reliable operation at the 3.75 Mbyte/s data rate suggested in Table 3, and no one has yet reported this technique operating with 400 nm wavelength light. On the other hand, if an adequate medium can be achieved, the technique does not require any other changes to the drive. The only direct overwrite technique thus far implemented in a commercial drive is field modulation. This technique is used in the Sony minidisk system used for digital audio recording. It is noted that the data rate required for digital audio is low compared to that desired for data storage on computers, and that the field modulation technique is more difficult to apply at higher data rates. Hence, it is not yet clear that this

912 Magneto-Optical Data Recording

technique will, in fact, be extended to use in high data rate drives for computers. Work is continuing in laboratories throughout the world on both the single layer and multilayer laser modulation schemes as well. None of the overwrite techniques have yet been demonstrated to work well with 400 nm wavelength light and data rates of several megabytes per second. Nevertheless, with the progress being made, it seems likely that hture magneto-optical drives for computer data storage will offer direct overwrite as the Sony minidisk does today.

7.0 MAGNETIC SUPER-RESOLUTION, OPTICAL SUPER-RESOLUTION AND NEAR-FIELD OPTICS The bit density of conventionaloptical recording technology is limited by the resolving power of the objective lens used to focus the readout laser beam. The minimum spot size which may be resolved is generally taken to be

where A is the wavelength of the light used and sin O is the numerical aperture of the objective lens. However, there is currently considerable research on methods which can be used to exceed this limit. One method, referred to as magnetic superresolution, makes it possible to write and read marks smaller than the diffraction limit, while still using large head-to-media spacing. Another method, known as optical super-resolution, makes it possible to achieve optical spot sizes smaller than can be achieved with spherically ground lenses. A third method is to use near-field techniques to exceed the resolution limit of diffraction limited optics. Magnetic super-resolution is achieved by using multilayer exchange coupled media to form magnetic apertures, which are smaller than the optical beam size, in the top readout layer of the Two forms of magnetic super-resolution have been described and are known as front aperture detection and rear aperture detection. The principles of front aperture detection are shown in Fig. 5. The properties of the three layers are summarized in Table 4.

Outlookfor Magneto-Optical Recording 913

Light Spot

Aperture Disk Motion ____)

RecordedMark

:

\ :

Laser Beam

\I

Hr&Y Readout Layer Switching Layer Recording Layer

Figure 5. A diagram illustrating how front aperture detection magnetic super-resolution may be used to read recorded domains which are smaller than the laser spot size.

Table 4. Magnetic Properties of the Films Used to Make Up the Front Aperture Detection Magnetic Super-Resolution

Readout Switching Recording

GdFeCo TbFeCoAl TbFeCo

30 10 40

>300 -140 -250

c0.5 >10 >10

In this technique, recording is performed with a magnetic head flying close to the media to provide a magnetic field which is modulated at the recording frequency of the laser. Since the magnetic field, and not the laser spot size, determines whether the magnetization is directed upward or downward, mark size can be smaller than the laser spot. The information to be stored is recorded into all three exchange-coupled layers shown in Fig. 5 . To readout the information, a magnetic field H, is applied downward as shown in Fig. 5 . Sufficient energy is applied from the laser to reach the Curie temperature T, of the switching layer. This breaks the exchange coupling between the readout and recording layers and allows any downward directed domains recorded in the readout layer to switch. Since the light spot encompasses the switching domain, when the domain switches,

914 Magneto-Optical Data Recording there is a change in the net magneto-optical rotation of the light, which may be detected. Hence, domains smaller than the beam diameter can be detected. The readout mechanism is nondestructive, because when the switching layer cools below its Curie temperature, it again exchange couples the recording and readout layers. Since the recording layer has a very large coercivity and the readout layer has a small coercivity, the exchange coupling forces the readout layer to replicate the pattern in the recording layer. In the rear aperture detection method, a quadrilayer structure is used as shown in Fig. 6 . The properties of the four layers are summarized in Table 5 . In this technique, an initializing field Hjnjis used to erase the information in the readout layer before readout. In the vicinity of the readout laser beam, a field H, is applied upward as shown in Fig. 6. In the hottest region of the readout beam, the readout and subsidiary layers switch their magnetization upward forming an up-spin mask. However, in the cooler crescent-shaped region just outside the up-spin mask where the temperature is raised above a critical value, the data retained in the recording layer are copied into the readout layer by exchange coupling between the layers. Outside the aperture, there is a down-spin mask region where all the spins in the readout layer remain in the downward direction. Thus, the effective aperture is sandwiched between the up-spin and down-spin masks and is much smaller than the beam diameter.

Down-Spin Mask

Figure 6. A diagram illustrating how rear aperture detection magnetic super-resolution may be used to read recorded domains which are smaller than the laser spot size.

Outlookfor Magneto-Optical Recording 915 Optical super-resolution uses filters in the exit pupil and beam shaping in the illumination aperture of the optical system to achieve smaller optical resolution than can be achieved with sphericallyground lenses alone. Several researcher~[~~1[~~] have shown the spot size may be significantly a decrease in effective spot size reduced by these techniques. In one from 0.8 to 0.3 micrometers was achieved using an infrared laser. These techniques do not require changes in wavelength or the objective lens numerical aperture. Thus, the expected gains from reductions in laser wavelength and increases in the numerical aperture will multiply the improvement kctor gained from optical super-resolution.

Table 5. Magnetic Properties of the Films Used to Make Up the Rear Aperture Detection Magnetic Super-Resolution Layer Readout Subsidiary Intermediate Recording

Material GdFeCo TbFeCoAl GdFeCo TbFeCo

Thickness (nm) 30 10 15 40

Tc

H C

("C)

(KOe)

>300

-140

<4 combined

-250 -250

<0.5 >10

Near-field optical recording is achieved by defining a very small aperture and bringing the magneto-optic medium into the near-field region of the aperture. In such a case, the resolvable spot size is determined primarily by the aperture size, which can be much smaller than can be obtained with focusing optics. This technique was used by Betzig et al.[39] to demonstrate recording and readback of an array of magnetic domains having a density of 45 Gbit/in2. Even higher densities are believed possible. Betzig et al.[39]used an optical fiber which had been drawn down so it had approximatelya 20 nm diameter at the end to define the aperture. The sides of the fiber were coated with an aluminum reflector in the region near the aperture to prevent the evanescent light energy from escaping where the fiber was too small to support an optically guided wave. By using polarizing optics and scanning the fiber above the surface of a CoPt

916 Magneto-Optical Daia Recording multilayer film,magnetic domains as small as 60 nm were written and read back. By using a flying head this technique could, in principle, be used to achieve storage densities above 100 Gbit/in2. Although the recording and readout of an array of domains at very high density has been demonstrated with this near-field optical recording technology, there is a large amount of work yet required before a practical storage device can be made. The demonstration conducted thus far was at a data rate of 10 KHz, which is clearly several orders of magnitude too low. Furthermore, there have been no measurements of bit error rate. Nevertheless the demonstration does indicate that magneto-optical recording potentially can achieve much higher storage density than the drives of today. Assuming the technology continues to be developed, such storage densities may eventually indeed be realized.

8.0

CONCLUSIONS

The need for low-cost, high density storage is extremely large and growing at such a pace that it is not likely to be hlfilled by any technology in the next several decades. Although 1 gigabyte semiconductor memory boards may be available by year 2000, magnetic disk drives will offer tens to hundreds of gigabytes of storage in that time-fiame. A single 3-1/2 inch disk drive could offer 60 gigabytes of storage, while an array of sixteen oneinch drives is expected to provide 10 gigabytes on a board with improved access time and higher reliability. Magneto-optical recording needs to advance rapidly to keep pace with these other technologies. The main advantage of magneto-optical recording today is its removable media. The storage density of magneto-optical media must keep pace with advances in density of magnetic recording media. This is expected to occur as shorter wavelength laser sources, improved servo techniques, more advanced modulation codes and improved signal processing are utilized. In addition, although magneto-optical drives are expected to lag magnetic drives in terms of access time or data rate, magneto-optical drives will likely close the gap in performance. This will be achieved through smaller, lighter weight heads, higher disk rotation speeds, direct overwrite and improved actuators. With high storage density, low access time and high data rates, magneto-optical recording could capture a large portion of the low-end data storage market.

Outlookfor Magneto-Optical Recording 91 7 Ultimately, by using near-field optical techniques the storage density limits imposed by the diffraction of light may be exceeded. In that case, the ultimate limits will be set by the minimum size of a stable magnetic domain as determined by superparamagnetism. This is the same ultimate limit faced by magnetic recording and is estimated near 10l2bits/cm2 for today’s materials. To utilize such near-field optical techniques, however, it will be necessary to bring the optical head extremely close to the media, just as one does in magnetic recording. Thus, a key advantage of current magnetooptical recording devices, removability of the media, will have to be sacrificed.

ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. ECD-8907068. Portions of this material have been previously published in J. Mogn. Soc. Jpn., Vol. 15, Suppl. No. S 1, pp 139-144 (1991).

REFERENCES 1. Freeman Reports: Optical Data Storage Outlook, Santa Barbara, CA, Freeman Associates (1992) 2. Patterson, D. A., Gibson, G. A., and Katz, R. H., Proc. of the 1988 ACM SIGMOD Con$ on Management of Data, Chicago, IL: 109-1 16 (1988)

3. Faggin, F., Trends in Microcomputers, Camputer Structures: Principles andExumples, Ch. 36, @. Siewiorek, G. G. Bell and A. Newell, eds.), McGraw Hill, New York (1982) 4. Meindl, J. D., Scientific American, 257(4):78-88 (1987) 5. Pelton, J. N., Higher Education in 2021, Com-Forum Cod., Phoenix, AZ, (Mar. 14-15, 1991) 6. T a g , C., Chen, M. M., Yogi, T., and Ju, K., IEEE TransMagnet., MAG26:1689-1693 (1990) 7. Yogi, T., et al., IEEE Trans. Magnet., MAG-26:2271-2276 (1990) 8. Howell, T. D., et al., IEEE Trans. Magnet., MAG-26:2298-2302 (1990)

918 Magneto-Optical Data Recording 9. Futamoto, M., Kugiya, F., Suzuki,M., Takano,H., Matsuda, Y., Inaba, N., Miyamura, Y., Akagi, K., Nakao, T., Sawaguchi, H., Fukuoka, H., Munemoto, T., and Takagaki, T., IEEE Trans. Magnet., MAG-27:52805285 (1991) 10. Takano, H. Fukuoka, H., Suzuki, M., Shijki, K., and Kitada, M., IEEE Trans. Magnet., MAG-27:4678-4683 (1991) 11. Kryder, M. H., Wang, S., and Rook, K., FeAlN/Si02 and FeAlN/A1203 Multilayersfor Thin FilmRecordingHeads,J. Appl. Phys., 10:6212-6217 (1993) 12. Gomi, M., and Abe, M.,Mat. Res. Soc..p’S Series, 150:121-130 (1989) 13. Shono, K., Kuroda, S.,Kano, H., Koshino, N., and Ogowa, S.,Mat. Res. Soc..p’S Series, 150:131-135 (1989) 14. Itoh, A., Jpn. J. ofAppl. Phys., 28(3): 15-20 (1989) 15. Suzuki,T.,J. Appl. Phys., 69:47564760 (1991) 16. Eppler, W. R., andKryder, M. H., Garnets for Short WavelengthMagnetoOptic Recording, J. Phys. Chem. Solids, 56(11)1479-1490 (1995) 17. &per, W. B., Greidanus, F. J. A. M. and Carcia, P. F., IEEE Trans. Magnet., MAG-25:3764-3766 (1989) 18. Lin, C. J., and Do, H. V., IEEE Trans. Magnet., MAG-26:1700-1702 (1990) 19. Komura, K., Takizawa, F., Iwanaga, T., Inoda, H., Yamanaka, Y., and Yoshida, K., Proc. SPIE, 1078:239-243 (1989) 20. Opsasnick, M. N., Stancil, D. D., White, S. T.,and Tsai, M. H., SPIE, 1499:276-280 (1991) 21. Sincerbox, G. T., Proc. SPIE, 1136:80-91 (1989) 22. Ono, Y., Kim,Y., and Nishida, N., Proc. SPIE, 1052:150-157 (1989) 23. Arimoto, A., Muraoka, K., Shimono, T., Send, K., and Ishikawa, S., Applied Optics, 29:247-250 (1990) 24. Ura, S., Suhara, T., Nishihara, H. and Koyama, J., IEEE J. Lightwave Technology, LT-4:913-918 (1986) 25. Valetta, S.,Reading Head in Integrated Opticsfor Reading Information Recorded on a Magnetic Support, U.S.Patent #4,796,226 (1989) 26. Ura, S.,Sunagawa, H., Suhara,T.,and Nishihara, H., Focusing Grating Couplers for Polarization Detection, IEEE J. Lightwave Technology, LT6:1028-1033 (1988) 27. Barnes, F. S.,Lee, K. S.,and Archibald, W. S.,Applied Optics, 25:40104012 (1990) 28. Kuwayama, T., andMatsumoto,K., OpticaI HeadandMethodofDetecting the Focus ThereoJ U.S.Patent #4,626,679 (1986)

Outlookfor Magneto-Optical Recording 919 29. Shieh, H. P. D., and Kryder, M. H., Appl. Phys. Lett., 49:473-474 (1986) 30. Kryder, M. H., and Schultz, M. D., Jpn. J. Appl. Phys., 28(3):3-9 (1989) 31. Schultz, M. D., Kryder, M. H., Sekiya, M., and Chiba, K., IEEE Trans. Magnet., MAG-29:3772-3777 (1993) 32. Nakao, T., Ojima, M., Miyamura, Y., Okamine, S.,Sakeda, H., Ohta, N., and Takeuchi, Y., Jpn. J. Appl. Phys., 26(3):149-154 (1987) 33. Saito, J., Sato, M., Matsumoto, M., and Akasaka,M., Jpn. J.Appl. Phys., (26):155-159 (1987) 34. Kaneko, M., Aratani, K., Mutoh, Y., Nakaoki, A., Watanabe, K., and Makino, H., Jpn. J. Appl. Phys., (28):27-31 (1989) 35. Fukami, T., Nakaki, Y., Tokumagu, T., Tagachi, M., and Tsutsumi, K., J. Appl. Phys., 67:4415-4416 (1990) 36. Kaneko, M., Aratani, K., Fukumoto, A., and Miyaoka, S., Proc. IEEE, 82~544-552(1994) 37. Yamanaka, Y., Hirose, Y., andKubota, K., Jpn. J. Appl. Phys., 28-3:197200, Kobe, Japan, 26-28 (Sept. 1989) 38. Fukumoto, A., Aratani, K., Yoshimura, S., Udagawa, T., Ohta, M., and Kaneko, M., SPIE Proc. on Optical Data Storage, 1499:216-218 (1991) 39. Betzig, E., Trautman, J. K., Wolfe, R., Gyorgy, E. M., Finn, P. L., Kryder, M. H., and Chang,C. H., Appl. Phys. Lett., 61:142-144 (1992)

Index

A Aberration free spot 65 Aberrations 73, 710, 713,729 wavefront 2 1 Ablative disks 578 Absorption 396 Absorption coefficient 673 Absorption depth 642 Absorption of light 624 Absorption profile 646, 658 AC channel 591 Accelerated testing 346 Acceleration 20, 721, 725 Acceleration constant 128, 195 Acceleration measuring method 274 Acceleration of IS0 disks 134 Access time 13 Achromatic beam expansion prisms 62 Achromatic objective lens 72 Acquisition 128 Activity light 757 Actuator 2-D 154, 180 moving-magnet 192 pintype 187 rotary 196 Actuator architectures 148 Actuator design 127 Actuator performance 149, 175

Actuator pitch 178 Actuator resonance 126, 127 Actuator technology 20 Actuator transfer function 157, 160, 161, 204, 721, 722, 723 Actuator velocity 146 Adaptive Write channels 430 Address 228 Addressmark 214, 805 Adhesion force 255 Adhesive bonding 263 Adhesives 258 Adiabatic limit 430-433 Adiabaticmodel 648, 663, 667, 669 Adjacent track crosstalk 21 Adjacent track pickup 550 Afier-grooves 336 Air cleanliness 247, 253 Airgap 190, 192, 193 Airydisc 496, 504 Airy distribution 495, 496 A1 alloy 288, 342 AllCr 342 AYTiMi 342 AlGaAs lasers 40 Algebraic method 6 13 Alignment technique 95 Alloy target 30 1 Alloying additives 348

920

Index 921 AlN 287 AlNdeposition 298 AlN encapsulated RE-TM 656 ALPC (seeAuto laser-power control) Alternating Direction Indicator Method 662 Aluminum 339, 773 Alumhumwire 197, 198 AM 214 AMfield 805 Amorphousfilm 5 Amorphousphase 6 Amorphous polyolefm 247 Amplifier 554 Amplifier noise 555 Amplitude difference 2 10 Amplitude distribution 496, 498, 499 Amplitude fluctuations 582, 586 Amplitude point spread function 496, 512, 522, 532, 539 Amplitude reflection coefficient 76, 481 Analyzer angles 620 Angular crossover frequency 130 Angular distribution of sputtered atoms 301 Angular ejection 303 Anisotropy 3, 307, 323, 329, 905 Annularaperture 531 ANSI 22, 343, 751, 758, 911 Antenna 773 Antiferromagnetic 31 1 Antireflection coatings 64, 396, 402 Antiresonance 160, 166 Antistatic agent 254, 267 APD 553 Aperture 317, 528, 710 circular 493 Aperture apodization 6 10 Aperturefunction 493, 539 Aperture obscuration 534 Aperture truncation 618 Apodization 531, 610 Apodized apertures 531 Apodized pupil distribution 105 Approximatemodels 650 Arsputtering 307 AR coated (see Antireflectioncoatings) Arc suppression 300

Archivability 9, 12, 342, 343 Arcing 297, 301 Arrhenius 261, 346 Aspheric lens 72 Astigmatic aberration 634 Astigmatic Focus Sensor 100 Astigmatism 37, 43, 47, 48, 73, '74, 100, 109, 551, 567, 713 Asymmetric spot 609 ATC (see A&acent tmck crosstalk) Atmospheric corrodants 348 Atomic force microscopy 363 Audiodrives 30 Audio recording 426 Auto laser-power control 214, 808 Autocorrelation 848 Avalanche photodiodes 553 Average power 458

B Bafemtes 335 Babinet-Soleil compensator 485 Back-emf 204 Backlash 188 Balanced magnetron 300 Ball bearings 765 Banded format 2 18, 2 19 Bands spin-polarized 336 Bandwidth 125, 720, 722 readout channel 716 recording 316 servo 154 Baseplate 763 Batch processing 288, 290 BCD (see Binaly coded decimal) Beam apodization 64 Beam conditioning 607, 609 Beam ellipticity 68 Beam expansion 59, 62 Beam expansion prism 48, 59, 60, 65 Beam positioning 20, 125 Beam profile 242 Beam shaping 915 Beam truncation 64 Beam walk 155 Beamsplitter 16, 33, 81 polarizing 79

922 Magneto-Optical Data Recording Bearings 184 Beer’slaw 638, 646 BEP (seeBeam expansion prim) bER (see Bit error rate) BER (see Byte error mte) BERmodel 595 BERtesting 749 Bessel function 505 Bezel 773 Bi-substituted garnet 335 Bias 20 Bias current 39 Biasfield 35 Biasmagnet 35, 770, 771, 910 Biasmechanism 760 Biaspower 453, 464, 467, 666 Bilayer system 338 Binary coded decimal 804 Binary digits 820 Binary field arithmetic 819 Binary polynomials 833 Binding energy 308 Biot-Savart’s law 194 Birefringence 15, 20, 81, 100, 103, 108, 109, 246, 269, 329, 565, 567, 617, 632, 633, 635, 728, 730, 735 measurement of 73 1 uniformity 250 wavelength dependence 732 Bitdensity 349, 901, 912 Bit error rate 594, 749 Bit errors soft 750 Bittiming 792 Black box model 599 Blockcodes 782 Block error rates 842 Block synchronization 850 Bloom 788 Bluelasers 8, 115, 527, 753 Bluelight 115, 280, 904, 905 Bludgreen laser diodes 115 Bodediagram 130, 131, 163, 168 Bodeplot 157, 160, 172, 178 Boltzmann factor 681 Boltzmann’s constant 26 1, 41 5 Bonding 264 Bonding machine 263

Breakdown field 297 Bubble materials 662 Bubble model 683 Bubble-like domain 288 Bubble-model 676 Buffer field 214, 815 Build process 97 Bulkindex 730 Burnix-in 57 Bursterror 813, 816, 850, 852 Burstlength 862, 866, 868 Bus control protocols 26 Bushing systems 765 Byte error probability 86 1, 862, 864 Byte errorrate 261, 315, 344, 346, 550, 568, 593, 594, 749, 842, 866 Byte error measurements 7 16 Bytes per sector 2 13

C Calibration 727, 732, 735 procedure 429 Capacitances dielectric 299 Capture behavior 138 Capture velocity 142, 146 Carbonfilm 381 Carboxylic acid 235 Carriage assembly 14 Carriagebody 184 Carriage pitch 168 Carrier level 326 Carrier-to-noise ratio 88, 93, 97, 315, 326, 333-335, 591, 744, 851, 904 Cartridge 25,133,210 mechanical dimensions 758 Cartridgecase 264 Cartridge doors 76 1 Cartridge entrance opening 773 Cartridging 209, 266 Catastrophic decoding 793 Catastrophic optical damage 37, 56 CAV (see Constant angular vefocity) CCS (See Continuous composite servo) CCS format 241, 797, 835, 839, 843, 844, 887, 888 CCS format stamper 243

Indm 923 CD (see Compact discs) CD-ROM 1, 28, 133 Center hole diameter 264 Center of gravity 127, 178 Certification 843 Certifying process 232 CG (seeCenter of gmviq) CGH (see Computer generated hologmms) Chalcogenides 339 Channel bit 213 Channel bit clock frequency 801 Channel bit error rate 749 Channelblocks 784 Channelclock 791, 795, 845 Channel codes 752 Channeldata 781, 795 Channel data errors 791, 847, 851 Channel performance modeling 692 Channelruns 845 Characteristiccurves 445,464 Characteristic matrix 399 Characterization of materials 363 Chassis design 773 Checkbits 26, 232 Checkbytes 26 Chemical vapor deposition 333 Chromate 241, 773 Chromatic correction 72 Circle function 494 Circularaperture 493, 495, 504 Circular polarization 373, 386, 546, 617 Cladding structure 39 Clampingforce 246, 264 Cleaner 254 Cleanliness 247 Clearance 184 Clock 789, 791, 806 master 225 Clockdrift 808 Clock frequency 228, 801 Clockperiod 454 Clock phase error 792 Clock waveform pulses 790 Closed-loop frecluency response 163 Closed-loop system 128 Closed-loop transfa h c t i o n 129, 133, 139, 144, 162

Closed-magnet circuit 192 Closed-magnet systems 190 CLV (see Constant linear velocity) CMRR 565, 572 CNR (see Carrier-to-noisemtio) CNR for garnet 333 CNR measurements 74 1 CNRmodels 93 CNRperformance 317 CNR test 74 1 Co concentration 304 CoPdmultilayers 306, 307, 324, 412 Co/Pt 280, 282, 283, 306-309, 319, 323, 324, 326,334, 335, 348, 905, 915 Coarse actuator 167 Coarse loop 137, 138 COD 37, 59 Codeword polynomial 822 Codewords 26, 813, 842, 861, 873, 879 coding modulation 2 14 Codingrate 783, 792 Coercivity 12, 303, 307,314, 323, 324, 329, 333, 349, 665, 671, 681, 682, 683, 905, 910, 914 Coercivity model 686 Coherence collapse 5 1 Coherent imaging 502 Coilimpedance 202 Coil inductance 200 Coil mass 196, 198 Coil material 197, 198 Coil resistance 195, 196 Collection aperture 502, 512 Collection optics 6 13 Collector lens 500 Collector pupil function 532 Collimating lens 493 Collision cascades 302 Coloring 566 Columnar grains 324 CoM/Pt superlattices 323 Coma 21, 73, 74. 551, 713, 714, 715 Combpulse 458 Commonmodenoise 82, 84, 485, 573

924 Magneto-Optical Data Recording Common mode rejection 103, 565 Compactdiscs 1, 2, 28, 133 Compensation filter 129 Compensation technique 593 Compensation point temperature 288, 665 Compensatorplate 94 Complementary error function 594 Compliance 187 Compliant motor connection 162 Composition gradient 301 Compositional uniformity 305 Compression 250 Computer-generated holograms 907 Conductive paints 774 Conductivity thermal 414 Conductivity tensor 406 Confocal detection 526, 535 Confocalimaging 537, 539 Confocal microscope 486 Confocal step response 537 Confocal transfer function 539 Constant angular velocity 133, 436 Constant linear velocity 133 Constant read power servo 52 Constrained data 78 1 Contaminantfilms 769 Contaminants 246 Continuous composite servo 209, 2 10, 784, 794, 797, 901 Continuous grooves 2 12 Contrast 508 Contrast transfer fhction 509 Control electronics 25 Control input 128 Control track information 225 Controller 127 Conversion efficiency 56 Convolution theorem 504, 618 Convolutional model 503 Cooling 771 Copper sleeve 200 CoPt 365, 566 Cornuspiral 491 Correctable error patterns 883 Correlation integral 502 Corrosion 348

Corrosion resistance 264 Corrosion-stabilized alloy 28 1 Corrosive gas test 260 Coset 825 Coset leader 828, 829 Coset-forming error pattem 827 Cosine law 302 Cosine signal 146 cost 37 ofMO disks 291 Coupled wave analysis 627 Coupling constants 3 11 CPO 247 Crash stops 766 CRC (see Cyclic redundanq) CRCbytes 841 CRC error detection code 831 CRC parity 814 CRC parity byte 832 CRCECC 232 Critical angle 103 Critical angle refractometer 730 Critical writing temperature 438 Crook’s method 378 Cross-interleaved 888 Cross-track signal 272 Crossover frequency value 133 Crosstalk 21, 99, 100, 108, 173, 711, 742, 743 Crystal 5 Crystal oscillation 225 Crystal structure 384 Crystalline phase 6 CSA 774 CTF 509 Cubic spline interpolation 626 Cubic symmetry 330 Cumulative hazard rate 26 1 Cumulative maximum temperature 454 Curie point writing model 662 Curie temperature 35, 288, 323, 330, 432,438, 671, 874, 905 Current-to-voltageresponse 20 1, 202 Cutoff frequency 737 CWmode 551 Cyclability 7, 8 Cyclic redundancy 26, 228, 799, 804, 815

Index 925 Cyclic Redundancy Code 26, 228 Cycling relaxation 429 Cylindrical domains 662

D Damped natural frequency 158 Damping 146, 162 Dampingratio 146, 162 Dark current 554 Datachannel 707 Data communication figure-of-merit 593 Datacontrol 25 Data degradation phenomena 578 Datadensity 9, 67, 282 Datadetection 788, 844 Datadetector 780 Dataediting 231 Dataerrors 694 Datafield 213, 214 Dataforms 230 Dataintegrity 885 Data placement 23 1 Data rate limitation 899 Datarates 4, 7, 28, 29, 909 Datarecording 209 Data recording capacity 218, 220 Datarecovery 745, 818 Data reliability 812, 859-861 Dataretention 7 Datasignal 714 Datasource 794, 895 Datastorage 362 hierarchy 12 optical 2 Data track density groove 107 Data transfer rate 349 Data waveform 789 DCchannel 590 DC reactive sputtering 295 DDS 232 Decoder 793, 860, 873, 881, 888 Deccdingfailure 861 Decoding sphere 881, 882 Decoding techniques 887 Decollhnation 48 Decorative bezel 773 Decoupling resonance 161 Defect “aquired” 23 1

Defect management 231, 232, 245, 843, 801

Defect ManagementAreas 801, 813-815, 844

Defect management pointer 812 Defect rate 749 Defect size distributions 694 Defect testing 274 Defective sectors 843 Defects 274, 287, 694, 812, 846 Defocus 135, 551, 616, 737 Dehgmentation 231 Degradation 59, 346,579 thermally induced 286 Degrees of freedom 127, 152, 159, 164, 165, 169, 170, 171, 178, 179, 184,185,189 Demagnetization curve 194 Demagnetization effects 686 Demagnetization energy 675 Demagnetization term 682 Demodulator 791, 856

Density areal 9 Density limitation 3 17 Depolarization 569, 571 Depolarizationnoise 573 Deposition processes 295 Depth discrimination 539 Depthoffocus 21 Depth of modulation 786 Depth- I 0 interleave 813 Design MO film structure 283 Design concepts 280 Design optimization 628 Detect errors 841 Detection device 119 Detection methods 97 Detection performance 93 Detection systems 79, 80, 93, 97, 98 Detector combinations 546 Detracking budget 105 Developing reagent 235 Di-bits 126, 131, 148 Diagonal pseudo-Fresnel coefficients 402 Dielectric capacitance 299

926 Magneto-Optical Data Recording Dielectricconstant 386, 392 complex 363 Dielectric encapsulating materials 7 Dielectric function complex 365, 367 Dielectric layers 5, 281,280, 286, 301, 348, 485, 643 Dielectrictensor 338, 384, 386, 395, 406, 412, 622 complex 382 Dielectric tensor anisotropy 393 Differencecwent 738 Differential amplifier 84, 572 Differentialdetection 88, 97, 114, 564, 565, 577, 619, 621, 633, 717 of polarization state 79 Differential detection module 552 Differential focus sensors 103 Differential push-pull tracking system 107 Differential signal 86 Differential signal offset 84, 93 Differential system 485 Differential TIR focus sensor 103, 114 Differentiator output 590 DIFFRACT 609, 615, 632, 633, 635 Diffracted amplitude 495 Diffractedbeams 107 Diffraction analysis 610, 615, 616, 627 Diffraction formula 491, 492 Diffractionlimited 14, 73 Diffraction of the beam 608 Diffraction theory 76 Diffision lateral 441 Diffusion length 650 Diffusion limited transport 303 Difhiontime 441 Diffusivity 439 thermal 414 Digital audio recording 3 1, 9 11 Digital channel performance 692 Digital errors 844 Dimensions 267 Dip coating 253 Dipole properties 686 Direct capture 140

Direct drive motor 152 Directoverwrite 4, 7, 35, 281, 314, 316, 318, 426, 467,472, 577, 908, 911, 912 Directreadafterwrite 115, 430, 472 DIS 22 DIS 10090 22, 210 Discontinuous data 23 1 Discrete optical elements 907 Disk acceleration 128 Disk capacity 115, 218 Disk certification 843 Disk definition sector 232 Disk degradation 26 1 Disk formats 209 Disk media 28 1 Disknoise 566, 567, 586 Disk reference plane 72 1 Diskstructure 24 Disk tilt 106 Dispersion 50 Displacement field 366, 384 Displacement measuring 722 Displacement signals 725 Distributive feedback lasers 119 Dither 764 Divergence angle 43 Divided push-pull signal 272 DMA (see DeJect management area) Domain 436 Domain boundary 683 Domain energy 675, 676 Domain formation 288, 663 Domain jaggedness 685 Domain marking process 674 Domain period 326 Domain reversal 681, 685 Domain stability 676 Domainwall 314, 682, 683 Domain wall energy 324 Domain wall mobility 677, 909 Domains 2, 3, 473 Doorlink 761 Door opening mechanisms 762 Door opening size 760 Dopant ions 329 Doped material 39 DOS environment 23 1 Dosing function 436 DOW (see Direct overwrite)

Index 927 Down-spin mask region 914 Downloading 26 DRAW (seeDirect read a f t r m’te) Drivecontrol 25 Drive electronics 25 Drivestandards 427 Drives 3.5 inch 30 5.25inch 28 audio 30 Drop test 267 Dropins 750, 845 Dropouts 750, 845 Drycleaner 254 DualECLstructure 690 Dualhalfaperture 99 Dual stage servo 154 Duhamel’s theorem 653 Duplication of optical disk substrates 232 Dust 254, 769 Dynamicmarking 677 Dynamic measurements 707 Dynamic recording data 334

E ECC 26, 214 ECC codeword sync 792 ECCdecoder 793, 818, 889 Eccentricity 272, 274 ECL (see Exchange coupled multilayers) ECMA 22, 751, 758 ECMA-130 133 ECML (seeExchange coupled multilayers) Eddycurrents 202 Edge defition 288, 853 Edgedetection 79, 542, 545, 854 Edgejitter 286 Edge recording 22 1 Edgewriting 467 Editing 228, 230, 231 Effective layer model 656, 658 Effective light power 568 Effective pupil function 530 Effective reflectivity 569 Efficiency 59, 64

Efficiency formula 64 EFM (see Eight-to-fourteen) E m (see Energy input distribution) Eigenpolarizations 386 Eigenstates 617 Eigenvector 174 Eight-to-fourteen 784 Ejectsystem 756, 761 Electric field 478 Electric field amplitudes 489 Electric field distribution 378 Electrical hookups 772 Electrical polarization 365 Electmchromic 6 Electrofoming 240 Electroless plating 774 Electrolytic psivation 24 1 Electromagnetic interference 774 Electromagnetic radiation 607 Electron mean free path 637 Electronic band structure 39 Electronicnoise 553, 555, 590 Electronic noise floor 719 Electronics 25 Electrostatic discharge 56, 267 Ellipsometer 391 Ellipsometric experiments 369 Ellipsometric parameters 401 Ellipsometry 367, 368, 391, 406, 730 Elliptical polarization 76, 478 Elliptically polarized 387, 621 Ellipticity 68, 383, 552, 565, 734 Embossed marks 273 Embossedpits 212, 627 Embossed pre-pits 243 E M (see Electromagnetic interference) End point 235 Endsf-life 343, 344 ENDEC 26 Energyflow 647 Energy input distribution 646, 660 Energy of interaction 10 Enhanced decoding 890 Enhanced resolution 527, 533 Environment of the MO medium 428 Environmental stability 72, 282 Equalization 589, 590, 780, 784 Equalizer transfer function 589 Erasable optical storage 6

928 Magneto-Optical Data Recording Erase bias field 35 Erase-then-write 35 Erasing 425, 469, 683, 892 Erasure correction 892 Erroneous bytesfrecorded byte 842 Erroneous codewords per codeword 861 Erroneous sectors per recorded sector 842 Erroneous symbols 865 Error burst 813, 867 Error burst distribution 867 Error burst length distribution 862, 864, 869 Error control coding 8 16 Error control decoder 829 Error correction 228, 791, 815, 829, 860 Errordetection 128, 231, 815, 817, 818 Error generating machine 869 Error generation mechanism 867 Error locator polynomial 888 Error modeling 692 Error patterns 824,826, 828, 830, 874, 881, 883, 886 uncorrectable 876 Error probability 858, 861 Errorrate 26, 232, 594, 859, 866 Error signal 721, 724 Error statistics 861, 874 Error-free codeword 881 Errors 26, 793 ESD (see Electrostatic discharge) Estimating data reliability 794 Euler angles 633 Europium chalcogenides 339 Europiumoxide 5 Even-odd track format 148 Exchange coercivity 3 14 Exchange coupling 426, 914 Exchange-coupled layers 689 Exchange-coupled media 909 Exchange-coupled multilayers 312, 314, 318, 338, 473 Excimer laser 243 Extended capacity format 2 19 External variables 428

Extinction coefficient 363, 366, 371 signof 382 Extinction ratio 717 Eye pattern 745 Eyring model 261, 346

F Fabrication cost 290 Fabrication yield loss 279 Fabry-Perot lasers 39 Fabry-Perot resonator 4 1 FAD (see Front aperture detection) Failure recovery 128 Farfield 99, 100, 107 Far-field diffraction pattern 495, 500 Far field focus sensors 103 Far-field region 49 1 Faraday effect 388, 389, 392 Faraday isolators 115 Faraday rotation 383 Faraday rotator 560 Fast axis 388 Fast seek 128, 137, 200 Father 240 FCC 774 FDA 774, 775 FE 445 FUGFmodel 446, 458, 464 Fe/X bilayers 338 FEA 174, 178 Feedback control systems 125 Feedback gate 835 Feedback shitt register 840 Feedthrough 99, 108, 109, 135 Feedthrough capacitors 773 Fencing out 864 Ferrimagnetic 311, 329, 385 Ferrites 335 Ferromagnetic 3 11 FES see (Focus error signall FeSn 404 Fiber optic networks 896 Fiber optics 120 Fiducials 795 Field modulation (see Magneticfield modulation) Figure ofmerit 194, 198, 593

Index 929 Figureeights 16 Figure-of-merit data communication 593 Fill factor 196 Filler submarks 467 Film defects 694 Film morphology 333 Filmstack 628 Film thickness variations 624 Final-value theorem 132 Fineactuator 137, 138, 154, 156, 167, 168 Fine tracking actuator 14, 154 Finishing process 209 Finite center difference 662 Finite element 445 Finite Element Analysis 174, 176, 640 Fixed optics system 764 Flag 807 Flagfield 214 Flame retardant 774 Flatness 274 Flexlead 772 Flicker noise 554 Flipping magnets 770 Flying optical head 120 Focal plane aperture 528 Focusactuator 14 Focus crosstalk 135, 178 Focuserror 101, 125 Focus error budget 98, 135 Focuserrorsignal 25, 34, 99, 100, 109, 171, 721, 724, 807 Focus loop 134, 171 Focusmethods 126 Focus motor force 17 1 Focus offset 714 Focussensor 98, 99, 103, 109 Focus servo response 21, 108, 136 Focus signal offset correction 135 Focused beam energy distribution 609 Focused beam shaping 609 Focused laser beam 608 Focused light beam 609 Focused stylus 607 Focusing grating couplers 908 FOM (see Figure of merit) FOOF 782 Force constant 195, 204

Form factor disks 24 Form factors 757 Format, disk 2X 219 3 x 221 4X 225 Format generator 245 Formatter 225, 235 Foucault knife-edge sensor 99 Four-bar flexure system 767 Four-layer structure 3 15 Four-wire suspension 182 Fourier coefficients 369, 390 Fourier theory of heat conduction 637 Fouriertransform 492, 499, 615, 618 Fractional peak shift 854 Fraunhofer approximation 492 Frequency domain 505 Frequency-doubled laser diodes 115 Fresnel amplitude reflection coefficients 481 Fresnel approximation 49 1 Fresnel coefficients 374, 375, 402, 412 Fresnel reflection coefficient 369, 377, 379, 733 Fresnel reflection equation 387 Friction 188 Front aperture detection 317, 318, 529, 912 Front-surface recording 285 Full aperture polar Kerr detection 525 Full density sector 803 Full height drive 757 Full response 785 Full ROM 228, 231, 232 Full width at half maximum 10, 65, 498, 609, 710 Fundamental unit of data integrity 860 Future generation media 280 Future trends 895 FWHM (see Full width at half maximum)

G “g” package 39 Gadolinium gallium garnet 333, 334, 904

930 Magneto-Optical Data Recording Gadoliniumiron 5 Gaincurve 42 Gainmargin 130 Gain-crossover frequency 129 Gain-guidedlasers 39, 47 Galois field 256 820, 835, 888 Galvo design 190 Galvomirror 764 Galvo system 154 Gammavalue 241 Gap 799, 807 Gap distribution 864, 873 GAPfield 214 Gaplength 862, 866 Garnet 280, 282, 283, 329, 333, 334, 396, 904 Garnetfilms 288, 673 Garnet optical constants 365 Gas flow rate 297 Gbytes per surface 221 %Ga,O,, substrates 673 GdFeCo readout layer 3 17 GdTbFe 324, 326 Generator polynomial 821, 830, 833, 839, 841 GGG (see Gadolinium-gallium garnet) Giant MO rotators 339 Gianth4R 10 GilbertModel 865 Glass 251, 336 Glass plate 233 Good data gap length distribution 862 Grainboundaries 5 , 307, 904 Grain boundary noise 566 Grainsue 333, 342, 904 Green’sfunction 445, 658, 659, 661 Green’s function thermal model 454, 691 Green’stheorem 490 Grid point 685 Groovedepth 235 Grooveshape 238, 239, 242 Groovesignals 228, 243 Groovewidth 241 Grooves 212 asagrating 235 Groundloop 772 Ground straps 772

Grounding 772 Grouping 231 Grouping information 232 Gyroelectric constant 325

H Half-high drive 757 Half-metallic bands 338 Half-wave plate 87 Handling cartridge and media 25 Handling damage 29 1 Handling during manufacturing 291 Hankeltransform 494, 498, 532 Hard decision 785, 791, 844, 845 Harderror 852 Hardcoat 253, 254 Hardened 473 Hardness 254 Harmonic power levels 579 Head (See Optical head) Head media interface 98, 105 Headpark 766 Head throughput efficiency 67 Head-carriage combination 110 Header 213, 225, 228, 795, 807 Heat accumulation type 453 Heat capacity 414, 439, 657 Heat diffusion equation 637 Heat flow 637, 652, 659, 662 Heatflux 653 Heat generation 624, 638 Heat sinking layer 453 Heattransfer 646, 682 Heattransport 638 Helmholtz coils 388 Helmholtz equation 489 Heusleralloy 338, 905 HFI (see High frequency injection) Hierarchy 220, 223, 224 High frequency injection 54 High frequency modulation 560 High frequency noise 589 High Performance Parallel Jnterface 896 High-volume production 289 HMI (see Head media intersace) HOE (see Holographic optical element) Hologram Laser Units 117

Index 931 Holographic elements 906, 907, 117 Hot melt adhesive 258, 259 HP-LDGU 117, 119 Hub detachable 274 Hubfeatures 762 Hubbing 209, 262, 263 Humidity 259 Huygen's principle 489 Hybrid achromatic objective lens 72 Hysteresis 179 Hysteresisloop 309, 310, 319, 323, 382, 388 Hysteretic 297, 303

I IDfields 214 IDnumber 214 Identification Fields 804 IEC (SeeInt'l Elec. Comm.) IIMQD 427 Illumination path 607 Image conjugate 99 Imageintensity 524 Imaging systems Type1 487 IMC (see Intermetallic compound) Impedance 202 Implementation independent mark quality determination 427 Impulseresponse 504, 509, 536 In-track diffraction 101 In20,-Sn0, 255 Incoherent imaging 502, 512 IncoherentOTF 507 Index ellipsoid 633 Indexguidedlasers 39, 48 Index of refraction 16, 42, 108, 728, 729 complex 363, 365, 367, 371, 375, 399 Index-guidedlasers 47, 50, 560 Indicatrix 632, 633 Indirect capture 140 Inductance 196, 201, 909 Inductive magnetic recording 10 Inertgas 295 Infantmortality 57

Information blocks 885 Information bytes 841 Information polynomial 831, 839 Information processing 895 Information symbols 793, 820 Infrared 527 InGaAlPlasers 40 Initialization 128 Injection compression 250 Injection compression molding 247 Injection current 42 Injection machine 247, 249 Inner Control Track SFP Zone 2 12 Inner Manufacturer Zone 2 13 Instability 162 Integrated optics 150, 903 Integrated processing 288, 290, 291 Intensitynoise 50, 51, 54 Interchangeability 22, 264, 427,706 Interlayer coupling 689 Interleaved codewords 813, 852, 861, 874, 885 Interleaving 26, 888, 889 Intermetallicalloys 336 Intermetalliccompound 305 International Electrotechnical Commission 22, 346, 775, 782 Intersymbol interference (ISI) 537, 588, 593, 847,857 Intertrack spacing 801 Ion beam deposition 333 Iridites 773 Ironnitride 899 Irradiance 489, 710 Irradiance distribution 65, 76, 495, 499 Irreducible polynomials 820 IS1 (see Intersymbol inte$erence) Island formation 324 IS0 22, 751, 758, 782 IS0 CNR specification 3 15 IS0 specification 241 ISOstandard 209, 752 ISO/ANSI Standards 283 ISO/IEC 10089 133, 210, 264, 792, 794, 797, 800, 801, 807, 812, 815, 836, 843, 847, 851 ISO/IEC 10090 133

932 Magneto-Optical Data Recording lT0 (see Inlo,SnO,) NB-VIIBelements 342

J Japanese Standard Committee 343 Jitter 458, 459, 579, 580, 584, 586, 592, 593, 696, 746, 747, 815, 847, 851, 854, 909 Jitter distribution 854, 857, 858 Johnsonnoise 553 Jones calculus 6 14 Jonesmatrix 76, 83, 393, 394, 399, 613, 619, 632 Jonesvector 372, 373, 374 Jukebox 264,758, 762, 764, 767, 770

K Kerrangle 79, 483,717 Kerr detection 525 Kerr effect (see Magneto-optic Kerr effect) Kerrellipticity 389, 393, 483, 621 Kerr interaction 500 Kerrrotation 33, 78, 79, 280, 282, 286, 310, 324, 336, 337, 363, 383, 390, 391, 394, 396, 401, 404, 483,522, 621 Kerr signal figure of merit 734 Kerr signal definition 729 Kerr signal imbalance 736 KerrFaraday effects 269 Kirchhoff transform 639 Kirchoff diffraction formula 489 Kirchoff formula 490 Knudsen’s cosine law 302

L Ufnoise 554 LaBudde model 433 Lamination process 253, 258 Landau-Lifshitz-Gilbert (LLG) equation 686 Lands 797 Laplace operator 128 Laplace transformation 128, 139 Laseraging 116

Laser assisted magnetic recording 3 15, 426 Laser beam focus 125 Laser beam profile 24 1, 7 11 Laserbeamrecorder 225, 230, 235 Laser cutting machine 235 Laser degradation rate 57 Laser diode 36, 38, 115 operating characteristics 45 Laser Diode Grating Units 1 17 Laserdynamics 51 Laser failures 57 Laserheating 3 Laser lifetime 57, 59 Laser marking 648, 650, 66 1, 674 modeling 670 Laser materials 116 Laser mode-hops 95 Lasernoise 50, 93, 556, 557, 559, 561, 564, 565, 567, 573, 719 Laserpower 8, 51, 214, 438, 441, 683,808 Laser pulse width 440 Laser reliability 55 Lasersafety 775 Laser wavelength 42 Laser-feedback noise 560 Lasers blue or blue-green 8 Latency 134 Lateral birefringence 108 Lattice points 878, 879, 880 LBR 225, 235, 243 LCP (see Lejl circularly polarized light) LDGU (see Laser diode grating unit) Leadnetwork 129, 162 span 131 Lead network span 133 Lead-Out zone 80 1 Leading edge emphasis 461, 462, 467 Leakage current 718 Leakage factor 192 Leakage noise 577 Leaky beamsplitter 16 LeakyPBS 485, 551 Least significant byte 804 Let? circularly polarized light 478, 617 Length discrepancy 664

Index: 933 Lengtherror 664 Lens 492,493

Lenses moldedglass 69 Level detection 542 Levenberg-Marquardt algorithm 369 Leverage arm ratio 152 Library optical (see Optical l i b n y ) Library of Congress 343 Life test 259 Lifetime 6, 12,259, 279, 342 Lifetime estimation 261 Lifetime specifications 768 Lifetime testing 343, 345 Light intensity modulation 3 15 Light intensity modulation direct overwrite 426, 453, 473 Light power modulation 3 LIMDOW (seetight intensity modulation direct overwrite)

Limit cycling 188 Limitingaperhue 710, 711 Line spread function 505, 509 Linear actuator 152 Linear bit density 243 Linearcarriage 184 Linearmotor 765 Linear optical systems 506 Linear system theory 504 Liquid crystal materials 6 Liquid crystal retarder 88, 485 Liquid phase epitaxy 45, 333 Lithography 896 Littrowprism 61 Loadunload test 267 Loading technique 762 Lockup 183 Long-term reliability 259 Longevity specifications 345 Loopdamping 144, 145, 146 Loopgain 144 Loopsense 303 Loopstability 127, 128 Loopstatus 128 Lorentz force law 386 Lorentzforces 127, 137 Lorentz number 4 15 Lorentz’s law 157 Loss factor 193

Low-fiequency noise 589 LPE 45 LSB 804 Lubricants 769

M Magnet working point 193 Magnetic behavior 674 Magnetic circular birefringence 481, 617 dichroism 481, 617 Magnetic clamping hubs 262 Magnetic dipoles 68 1, 682 Magnetic disk drives 187 Magnetic domain 2,3 Magnetic field modulated direct overwrite 4, 7, 426, 476, 577, 674, 682, 905 Magnetic films multiple 689 Magnetic model 674, 682 Magnetic properties 3 1 1, 3 19 Magnetic steels 202 Magnetic storage 2 Magnetic sublattice 385 Magnetic super resolution 317, 318, 528, 530, 689, 912 Magneto-optic absorption processes 405 Magneto-optic characterization 383, 391 Magneto-optic constants of materials 403 Magneto-optic data 334 Magnetooptic disks 209 Magneto-optic interaction 480 Magneto-optic head 13, 15, 33 Magneto-optic Ken effect 2, 3, 7, 93, 95, 283, 338, 388, 392, 402, 426, 476, 480, 522, 564, 608, 617, 619, 629, 693, 905 measuring 383 Magneto-optic libraries 3 1 Magneto-optic materials 336, 382 Magnetooptic measurements 388 Magnetooptic phase contrast 526, 542 Magneto-optic pickup 117 Magneto-optic polar Kerr detection 525 Magneto-optic polar Kerr effect 392, 480, 623

934 Magneto-Optical Data Recording Magneto-optic readout 477, 6 17 Magnebopticrecording 5, 427 Magneto-optic storage trends 898 Magneboptic thin films 3 Magnebopticwriting 3 Magneto-optical characterization 381 Magneboptical effect (seeMagnetooptic Kerr eflect) Magneboptical recording futureof 8 Magnetometry 363 Magnetomotive force 193 Magnetostatic coupling 682, 685 Magnetostriction 329 Magnetostrictive effects 904 Magnetron 296 Majority-logic-vote 791 Manufacturer’s zones 80 1 Manufacturing cost 291 Manufacturing process 245 Mapping 824 Mapping technique 820 Markbloom 853, 857 Mark edge detection 854 Mark edge recording 225, 427 Mark edge writing 458 Markedges 853 Marklength 318 Mark length accuracy 855 Mark length bias sensitivity 428 Marklengtherror 666, 672 Mark length power sensitivity 428 Mark reflection coefficient 6 18 Marksize 913 Markspacing 747 Markwidthcriterion 446, 448 Mark writing process 787 Marketdemand 29 Markingmodel 646 Markingprocess 650 Marking responses 665 Markovprocess 865, 867 Massline 160 Massmatrix 174 MasS-Spring 158 Masterclock 225 Masterloop 137 Master-slave system 20, 137

Mastering 232, 245 Materials characterization 362 Matrixeffects 302 Maximum overshoot 159 Maximum-likelihood sequence 784,79 1 Maxwell’s equation 386, 617 MBE (seeMolecular beam epitaq) MCB 481 MCD (see Magnetic circular dichroism) Meanfreepath 637 Mean time to failures 36, 56, 57, 59 Mean-field model 674 Measuringmethod 267 Mechanical bonding 264 Mechanical errors 72 1 Mechanical properties 274 Mechanical tester 274 Mechanical tolerances 116 Mechanical transfer functions 175 Media design 280 Medianoise 93, 329, 566, 568, 574, 578, 590, 693, 694, 846 Medium fhction 522 Melting viscosity 246 Metal films thick 402 Metal oxide 255 Metal reflector layer 281 Metallization 240 Metropolis rule 681, 682 MFM (see Magneticfield modulation direct overwrite) MFMDOW (see Magneticfield modulation direct overwrite) Micro creep 179 Micromagnetic thermomagnetic recording model 682, 696 Microprocessor 26 Microscope 486 Mid-Frequency Reactive Sputtering 295, 299 MIL-STD461 775 Miniaturization 1 17 MiniDisc 30 Mirage effect 4 17 Misbalance 565 Mis-decode 793, 817,877, 881, 885 MLBS 428 MLPS 428

Index 935 MLSE 784 MMF (seeMagneto-motiveforce) Mnmaterials 338 MnBi 5, 338, 905 MnBiAl 338 MO-LDGU 117 MOCVD 45 Modalmatrix 174, 175 Modehopping 51, 54, 62, 99, 557, 637 Mode partition noise 55 Modeshape 174 Modeling 598, 601, 603, 613, 650 efficiency 59 magnetic behavior 685 motor 192 recording process 637 rigorous 658 validation 605 Modified z-transform 144 Modulation 36 amplitude 54 lightpower 3 magnetic field 4 Modulationcode 214, 780, 782, 783 Modulation frequency 54 Modulation transfer function 509, 586, 693 measurement techniques 510 MOKE (see Magneto-optic Kerr effect) Molding 245,246,247,249,250,273 Molecular beam epitaxy 45 Molecular weight distribution 349 Moment of inertia 180 Monochalcogenides 406 MOFC 542 Most significant byte 804 Mother 241 Motion equation 157, 169, 174 Motor acceleration 194 Motor constant 134, 142 Motor forces 127 Motor specifications 763 Motor start-up 764 Moving coil 192 Moving magnet 192 Movingoptics 149, 150, 764 Moving shorted tum 203

MSB (see Most signipcant byte) MSR (seeMagnetic super-resolution) MTF (see Modulation transferfunction) MllTF (see Mean time tofailures) Multibit erasure 892 MULTILAYER 732 Multilayer constants 365 Multilayer design 283, 622, 629 Multilayer films (see Multilayer thin films) Multilayer formalism 397 Multilayer stack 279, 283, 624, 628, 629 isothermal 439 Multilayer stack modeling 624 Multilayer thin film 279, 306, 324, 335, 375, 379, 622, 643, 904 Multilayer tuning 286 Multilevel recording 330 Multiple layers 91 1 Multiple light beam 3 16 Multiple magnetic films 689 Multiple-input-multiple-output system 169, 170 Multiply-adds 888

N Nanomagnetic model 682 Nanomagnetics 685 NmoW-band S N R 59 1 National Institute of Standards and Technology (NIST) 343 Natural frequency 158, 163 Natural loop frequency 129, 142 Nd:YAG lasers 115 Ne,Ns, 568 Near field 99 Near-field imaging techniques 528 Near-field optical recording 915 Near-field region 491 Nearest neighbor 860 Negative-type photoresist 234 NEP 240 Neumann’s principle 384 Newton iteration 662 Next generation optical media 752 Nickel 240 Nickel sulphamatebath 240

936 Magneto-Optical Data Recording Nitride 255 Nitride optical constants 408 Nitrogen oxide 260 NM (seeNanometncs) Noise 5 , 335, 550, 553, 559, 566, 571, 574, 576, 590 coloring 566 isminimized 82 Noise control 561 Noise factor 554 Noisefloor 741 Noiselevel 326, 329 Noise modeling 692 Noisepower 587 Noise reduction 564 Noisesources 93, 94, 567, 569, 577, 694 Noisespectrum 511 Noise suppression 54, 646 Noise terms (seeNoise sources) Non-electrolytic plating 240 Non-return-to-zero 790 Normal coordinates 174 NRTL 774 NRZ (see Non-return-to-zero) Nucleation field model 677 Numericalaperture 10, 317, 496, 527, 609, 710, 900 Numerical computations 5 19 Nyquistdiagam 130, 162, 163

0 0-ROM 228, 231, 243 Object function 629 Objective lens 609 Obscuration 6 10 Obscuration focus sensor 99 Obscuration techniques 534 O K (see Optical drive controller) ODF (see Offset defectionflag) OFB (see Opticalfeedback) Offdiagonal dielectric tensor element 394-396 offset 20 Offset detection flag 212, 214, 228, 739, 807 On-track signal 272 Open-loop system 128

Open-loop transfer function 130, 131, 162, 722 Open-magnet systems 190, 192, 194 Operating current 56 Operating systems 22 Optical aberrations 109 Optical angle amplification 155 Optical birefringence (see Birefn’ngence) Optical characterization of thin films 363, 367, 379 Optical conducitivity tensor 406 Optical confinement 39 Opticalconstants 324, 338, 371, 391, 406, 624 ofthinfilms 368 Optical crosstalk 108 Optical disk tester 271 Optical drive 30 Optical Drive Controller 25 Optical feedback 51, 557 Optical fibers 120, 903, 906, 915 Optical field 478 Optical head 13, 33, 42, 80, 117, 764, 903 I-pass 35 2-pass 35 direct overwrite 35 split 14, 34 Optical head access 167 Optical head cleaning cartridges 770 Optical head efficiency 67, 81 Optical head miniaturization 117 Optical integrated circuits 119 Optical libraries 12, 13, 264 Optical lithography 896 Opticalmodeling 373, 375, 391, 607, 632 Optical modulator 235 Optical path readout head 487 Optical path difference 73 Optical pickup 119, 269 Optical power 732 Optical readout (see Readout) Optical recording (see Recording) Optical reflectivity 6 Optical resonant cavity 281 Optical response 283

Index 937 Optical spot dimension 10 Optical spot quality 73 Opticalstack 643 Optical storage advantages 9 Optical storage system 148 Optical Storage Testing and Preservation Laboratory 343 Opticalstylus 9, 72, 433 Optical super-resolution 912 Opticalsystem 14 characterization 504 Opticaltests 728 Optical transfer hction 506, 530, 568, 578, 589, 729, 737, 738 Optical transmittance 267, 268 Optical wavefront requirements 14 Optical waveguide 39 Optically assisted magnetic storage 12 Optically thick samples 375, 394 Optimization 628 Optimum parameters 54 Organic overcoat 5 Organometallic decomposition 336 Orthogonal polarization 95 Orthogonal relationship 99 Orthogonality principle 101, 103 OSHA 774 OTF 509 Outer Manufacturer Zone 2 13 Output beam aspect ratio 45 Outputpower 37 Outrigger spot 107 Overcoat 253, 255 Overconstrained 180 Overconstraining 179 Overfill 499, 609, 710, 711 Overhead 803 Overlap readout model 696 Overshoots 130 Overwrite (seeDirect overwrite) Oxidation 255 Oxide dielectric 282 Oxide dielectric seed layers 307 Oxides as protective layers 348 Oxidize 5 Oxygen-free compounds 28 1

P 2P method (see Photopolymer) P-i-n photodetectors 553 PA (see Postamble) Paddatabits 784, 796 Parasitic mass 159, 160, 178 Parity bits 831, 835 Paritybytes 813, 814, 860 Parity calculation 833, 839 Parity checks 793 Parity generation 8 18 Parity sectors 231 Park head 766 Partial polarization beamsplitter 33, 49, 64, 93 Partial response equalization 784 Partial-ROM 228, 231, 232 Partially coherent 502, 5 12, 522 Partially coherent transfer function 513, 519 Particulate 253, 694 Passivation 241, 340, 348 Passivity of aluminum 339 Patches 686 Pattern noise 178 Payloadmass 153, 195, 198 PBS (see Polarizing beamsplitter) PC (see Polycurbonate) PdCo 904 Peak response 130 Peakshift 593, 847, 857 Peak surface temperatures 638 Penalty function 629 Pencil hardness 254, 255 PEP zone (see Phase-encodedpart) Perfect imaging 5 14 Performance tests 706 Perturbation analysis 557 Phantom error signal 106, 107 Phase-change disks 578 Phase-change recording 6, 7 futureof 8 limitations 7 Phase change type media 500 Phase compensation 621, 629 Phase compensator 97, 485, 552 Phase contrast 542

938 Magneto-Optical Data Recording Phasedelay 92 Phasedifference 283 Phasedisturbance 160, 166 PhaseEncodedPart 210, 212, 797, 798, 803 Phasegrating 627 Phaselag 131, 132 Phase-lockedloop 139, 225, 749, 791 Phasemargin 130, 132, 595,791 Phase modulation 803 Phase-optimized structure 285 Phase pit 230 Phaseplane 140 Phaseplaneplots 138, 146 Phase retardation 617, 633 Phase shift 89, 329,522, 544 Phononscattering 287, 638 Photo CD 1, 2 Photoacoustic modulators 417 Photo-activated compound 235 Photochromic 6 Photodetection 552 Photodetectors 489 Photoeffect 638 Photoelasticity of PC 246 Photopolymer 251, 252, 326 Photoresist 225, 233, 234, 235, 241 Photothemal deflection 4 1 6 418 Phrasing 812 Physical specifications 23 PIN photodioddamplifier 563 Pin type actuator 187, 767 Pinholes 846 Pinning 324, 333 Pit position modulation code 244 Pit recording 2 13 Pit signal 225 Pitch 127 Pitch frequency 180 Pitchmode 164 Pits 212, 243, 627, 797 Pitsandgrooves 225, 235 Pitting 339, 348 Planartargets 306 Plane of incidence 372 Plane-wave decomposition 608 Plasma 295 Plastic substrate 255 Plating bath conditions 240

Playback channel architecture 788 Playback lifetime 344 PLL 225 PLM (see Pulse width modulation) PM code (see Phase modulation) PMMA 247, 567 Pnictides 339, 406 Poincard sphere 15 Point spread function 496, 512, 522, 532 Poisoning 296 Polarization 2, 43, 320, 551, 567, 632, 710, 906 circular 617 Polarization analyzer 79 Polarization changes 7 Polarization effects 76 Polarization eigenstates 76 Polarization ellipse 387 Polarization field 366 Polarization manipulation 15 Polarization manipulation schemes 93 Polarization noise 93 Polarization optics 82 Polarization ratio 49 Polarization rotation 14 Polarizationstate 16, 79, 80, 368, 478, 608, 636, 717, 733 Polarized light 269, 477 Polarizing beamsplitter 82, 551, 620 Polycarbonate 108, 109, 246, 268, 567, 730 Polycarbonate resin 266 Polycarbonate substrate 329 Polycrystalline 5 , 566, 904 Polyelectrolyte 255 Polymer 2P 326 Polymer tip 274 Polymeric protective coating 643 Polynomials 818, 819, 833, 839 Polyolefin 247, 258 Polyurethane 258 Population inversion 39 Positionerror 132, 724, 901 Position sensor 724 Positive-type photoresist 234 Postamble 214, 807 Potential barrier 39

Index 939 Powerbiasing 453, 469 Powercutting 464, 467 Powerdensities 37 Power dissipation 194 Power limited acceleration 198 Powermargins 8 Power spectral density 694 Power transmission efficiency 609 Powerdown 128 Poyntingvector 647 PPBS (see Partial polarization beamsplitre4 PPM 427, 453, 589, 752, 785 Pmemphasis 683 Preerasing 426 Pre-formattedarea 213 Pre-gmoves 241, 627 Pre-pits 243 Preamble 799 Preamp output 590 Precenter 762 Precompensation 720, 753 Preformatted mark 803 Preformatted sector headers 8 15 Pregrooves 105 Continuous 135 Preloadbearing 184 Preloadspring 185 Preloadwheel 184 Prerecorded characteristics 273 Primer 234 primitive cyclic code 821, 830 Primitivepolynomial 821, 835, 836 Principal coordinates 174, 175 Production costs 293 Production of MO disks 288 Production requirements 291 Proportional damping 175 Protectivecoatings 209, 397 Protectivelayers 9, 24, 255 Pseudo-reflectance 500 Pseudo-reflectivity 621 PTD (see photothennal deflection) PtMnSb 338, 905 Pull-in behavior 146 Pulse position modulation 427. 589, 785 Pulse width 431, 438, 440, 441

Pulse width modulation 8, 253, 427, 428, 453, 461, 464, 467, 472, 590, 683, 752, 785 Pulse-crowding 847 Pulse-length-modulated (see Pulse widrh modulation) Pulsedwriting 467 Punpingspeed 297 Pupil distribution 105 Pupil function 493, 502, 530, 532,539 Push-pull 126, 135 Push-pull ratio 740 Push-pull MIVO 152 Push-pull signal 105, 107, 241, 272 Push-pull TES 109 Push-pull track@ 99, 100, 106, 114 Pyrolysis 333, 336

Q Q(ua1ity) factor 158 Q-bitwords 820, 835 QUADRANTprism 100 Quadrilayer structure 28 1, 286, 315, 439, 441, 914 Quantumwelllasers 45 Quarter-wave plate 388, 485 Quasi phase matching 1 16

R R-bit 816 RAD (see Rear aperture detection) Radial runout 20 Radial tracking 212 Radio frequency 295 RAID architectures 899 Rail interface 765 Random access time 134 Randomerrorrate 594, 595 Random failure 57 Random seek time 134 Rapid thermal annealing 333 Rapidly thermally annealed garnets 904 Rareearth 415 Rare earth magnets 194 Rare earth transition metal 5, 279, 280, 283, 301, 311, 312, 323, 334, 342, 349, 483, 641, 643, 686

940 Magneto-Optical Data Recording Rate equation model 54 Ray trace model 633 Raytracing 632 Rayleigh criterion 527 Rayleigh limit 73 RCP (seeRight circularpolarization) RE-TM (seeRear earth transition metal) RE-TMtargets 306 Reaction force 188 ReactiveDC 295 Reactive mid-frequency 295 Read channel clock 795 Read electronics 789 Readpower 93 Read signal impairment 789 Readwhilewrite 472, 473 Read-only 228, 230, 231, 627 Read-write performance 274 Readback resolution 609 Readback signal 6 19 Readback waveform 273, 745 Readout 2, 414, 486, 493, 617 polarization 14 Readout head 629 Readout laser power 10 Readout light 622 Readout model 696 Readout phenomena 627 Readout process 36 Readout signal 476, 617, 629 Readout system 486, 55 1 Readout techniques 79 Readout waveform 486 Rear aperture detection 317, 530, 912, 914 Rear facet monitoring 37 Reciprocity integral 502 Reciprocity theorem 174 Record keeping 776 Record power optimum 664 Recordable disks 2 Recorded marks 786 Recorded master 235 Recording 5, 35, 51, 209, 414, 425, 476, 550, 589 singlepass 4 thermally assisted 36

thermomagnetic 3 twopasses 3 Recording beam profile 24 1 Recording capacity per track 2 18 Recording density 898, 901 Recording format 782, 784, 794, 801 Recording layers 209 Recording lifetime 344 Recording parameters 23 Recording power 242 Recording properties 324 Recording sensitivity 323, 335, 342 Recording structure 323 Recording stylus 608, 610 Recording thermal response 445 Recovery 745 Recrystallization time 7 Red laser diodes 527 Redundant parity bits 816 Reed Solomon code 26, 793, 814, 815, 817, 835, 839, 841 Reference drive 427 Reference servo mechanism 274 Reflectance 283, 324, 365, 368, 396, 402, 729, 732 Reflectance behavior 340 Reflection coefficient 79, 61 8, 624 Reflections 5 1 Reflective zone 797, 798 Reflectivity 93, 105, 569, 717, 732 difference 7 Reflectivity contrast 6 18 Reflectivity noise 568, 573, 578 Reflector layer materials 339 Refraction 728, 729 Refractive index ellipsoid 632 Refractometers 730 Regulations 775 Relative humidity 346 Relative intensity noise 51, 54, 559, 561, 562, 565, 574, 720 Relaxation 429 Reliability 36, 55, 788, 844, 858, 873 long-term 259 Reliability estimates 861 Reliability of data output 794 Reliability tests 343 Remainder polynomial 820 Remanent state 382

Index 941 Removability 9, 12 Removable disk drive 796 Removable medium 903 Replacement algorithm 843 Replication 247 Replication fidelity 245 Residual error 132 Residual gas pressure 309 Residual induction 194 Residual servo error 132 Resin 246, 251 Resist master 235 Resistivity 254 Resolution 527 Resonance frequency 127, 159, 160, 163, 174, 721 Resonances 127 Resonant cavity (see Optical resonant caviq) Resynchronization 214, 228, 808, 811, 812 Retardation 20, 81,632, 633 Retarders 89, 391 Retarding plate 283 Return optical path 76 Reverse domain 676 Revolution group 2 19, 224 Rewritable 1,2, 28, 133, 210, 228, 231, 243 Rewriteable MO standards (see Standards) RF modulator 773 RFsputtering 295, 333 RF sum signals 273 RFM (see Rearfacet monitoring) Ribbon-of-current 194 Right circular polarization 478, 617 Rigidbodymodes 127, 189 RIN (seeRealtive intensiq noise) Ringaperture 532 Ringing 130 Ripple 714 RLL (see Run length limited) Roll 127, 253 Rollerbearing 184, 185 ROM 228, 627 Rootlocus 144, 145 Rotary actuator 152, 196, 764

Rotaryarm 187, 196 Rotating analyzer PBS 88 Rotating mass memories 2 RotatingPBS 94 Rotating polarizer 368 Rotationnoise 567, 568, 578 Rotationrate 10, 28 Rotational modes 127 Rotational speed 29 Rotational stiffness 168, 182 Roughness 319, 329 RS code (see Reed Solomon) RS decoder 888 RSECC 811, 813, 814, 816 RS error correcting 852 RSCRC decoder 873 Run 845 Run-length limited 214, 221, 228, 427, 449, 781-783, 792, 815 Run-length violated 2 14 Runout 125, 133, 187, 721 axial 726 radial 725 RWW (see Read while write)

S Sales worldwide 1 Sales volume 29 1 Sampledservo 108, 131, 148, 209, 210, 784, 797 Sampling frequency 145 Satellite spot 107 Saturation 443 Scalar diffraction 608, 609, 615, 627, 633 Scaling 198, 199 Scaling laws 198 Scanned marking limit 436 Scanning laser microscope 486, 536 Scanning microscope 5 12 Scanning optical microscopy 476 Scatter of optical beam 274 Schell pattern 716 Scratches 254 Scratching 255 SCSI (see Small Computer System IntetjCace)

942 Magneto-Optical Data Recording Secondorderfilter 162, 163 Second order systems 159 Sector 794 Sectoraddresses 243 Sectorerrorrate 842 Sector formats 213 Sectorheaders 738, 795 Sectormark 213, 803 Sector mark signal waveform 273 Sectornumber 799 sectorservo 901 Sectorslipping 843, 864 Sectorstructure 213, 220, 225 Sector types 811 Sectorsperttack 213, 218, 219 Sectorssize 27 Seedlayer 283, 333 Seekerrorrates 146 Seekloop 125, 138, 144 Seekservo 128 Seek time 28, 198, 199 Seek velocity 138, 146 S e i d e l a h t i o n 21 Selfenergy 675 Self-clocking code 749 Sensitivity 443 Writing 429 Sensor elements 127 Sensoroffsets 127 serratedpulse Writing 454 Serration 453, 462 Senation algorithm 467 Serration mark writing 458 Servobandwidth 53, 133, 720, 722 Servo characteristics 271, 272 Servocontrol 20, 126 Servoerror 132 Servo error budget 127 Servo signal normalization 128 Servo signals 707 Servosystems 125 Servo tolerances 21 SFP zones (see Standardformatted Part) Shadingband 568 Shelf life 344 Shells 266 Shielding 773

Shift error probability 858 Shifterrors 845, 846, 853, 854, 856 Shiftregister 833, 840 Shipping packages 778 Shock mount design 768 Shock mount system 767 Short wavelength 904 Short-mark length 318 Shorted turn 200, 201, 202 Shorter wavelength light source 115 Shorting circuit 56 Shotnoise 84, 93, 551, 552, 559, 591, 719 Shot noise current density 563 Shot-noise limited FOM 326 Shutter 267 Side lobe energy 609 Sign convention 382 Signal imbalance 735 Signal amplitude 483, 734, 735, 737, 738 Signal detection 79 Signal jitter 696 Signal voltage 735 Signal-amplitude-fluctuation noise 579 Signal-to-noiseratio 5 , 10, 33, 283, 396, 550, 551, 565, 566, 573, 591, 630, 851, 904 Silane coupling reagent 234 Silicon nitride 408 Silicon wafer 408 Silicones 770 Siloxane resin 254 Silver 240 Simulation 598, 603 Sinewave approximation 592 Single-crystal substrates 336, 904 Single pass direct overwriting (see Direct overwrite) Single-ion theory 330 Single-mode waveguide 37 Single pass recording 4 Single-phase alloy targets 302Single polarization analyzer 79 Single stage 154 Single-substrate processing 289 SiO, 253, 255

Index 943 Skew 212, 789 Skindepth 202 Slaveloop 137 Slidebearing 183 Slot 773 Slowaxis 83, 88 SM 213 Small Computer System Interface 25 Smog atmosphere 348 Sm$r3,Gi+.Fe5,0,, 673 Snell’s law 375 SnO, 255 SNR (see Signal-to-noise ratio) Soft bit errors 750 Softerror 749, 850 Sol-gel 333, 336 Soleil-Babinet compensator 88 Son 241 SourceMO 231 Spacerlayers 333 Spanofleadnetwork 131, 133 Sparesector 231, 812 Spatial recording density 2 18 Specific heat 671 Spectral density functions 587 Spectral shaping 591 Spectroscopic measurements 404 Spectnunanalyzer 738, 741 Sperrimagnetic 311 Sphere 879, 880, 881 Sphericalaberration 73, 74, 715 Spherical wave 492 Spincoating 233, 253 Spin-orbit 330, 336, 338, 477 Spin-polarized 336 Spindle 764 Spindle motor 763 Spindlerunout 7 18 Spindlespeed 133, 134 Spinel ferrites 335 Spiral tracking groove 797, 807 Split detector 107 Splithead 34, 149, 150, 152, 764, 766, 906, 907 Spontaneous emission 49, 54 Spot 608 Spot intensity distribution 586 Spot intensity profile 609 Spot irradiance profile 71 1

Spotposition 125, 132 Spotquality 73 Spotsize 115, 585, 711, 901, 912, 915 Spot size focus sensor 103 Spot size sensor 101 Spray coating 253 Spread function 502 Sputtering 279, 291, 295, 301, 333 Sputtering simulation code 308 Square wave response 5 10 SQUID magnetomew 363 SS format (see Sampled servo) Stack (seeMultilayer stack) Stagger-interleaving 888 Stamper 230, 232, 240 Stamping 245, 250, 252 Standard formatted part 212, 797, 801 Standardsizes 757 Standards 22, 23, 427, 751 Static charge 770 Static marking limit 433 Static tilt 727 Stationary media limit 433, 436 Statistics 862 Steady state thermal limit 436, 453 Steel 773 Steel pole 202, 203 Stepresponse 504, 505, 509, 525, 537 Stepper motor 765 Stiffness 170 Stiffness matrix 174 Stokes parameters 15 Stop 710 Storage on-line 31 rewriteable 2 Storage capacity 90 1 Storage density 9, 1 15 Storage life 344 Storage medium 148 Storage technology 897 Strehl ratio 72, 73, 616 Stress 330 Stress in the resin 246 Stress resistance tests 346 Stress test 259 Struve functions 505 Sublattices 385

944 Magneto-Optical Data Recording Substrate 245, 246, 632, 643 Substrate thickness 729 Sulphur oxide 260 Sumchannel 750 Super-exchange interaction 330 Super-linear regime 577 Super-resolution 317, 318, 320, 469, 527, 530, 912, 915 Superposition 448 Surface acoustic wave 907 Surface resistivity 254 Surface roughness 547 Surfactant 254 Suspension f 0 U r - h 181, 182 Suspension system 188 Swayspace 767 Switching domain 913 Switching protocol 681 Symbol error probabilities 865 Symmetrygoup 384 sync 799 Sync detection 848, 849 Sync detection threshold 850 Sync detection window 847 Sync field 811 Syncpattem 228, 847 Sync-loss condition 792 Synchronization 792, 847 Synchronizationbyte 749 Synchronizationerrors 793 Syndrome 823, 824. 828, 829, 842, 877, 882 Syndrome calculation 835 Syndrome generation 833, 839 Syndrome of errors 26 System modeling 692 System transfer functions 175

T Tape libraries 13 Target arcing 296, 297 Target detection channel 589 Target-to-substrate distance 308 TbFe 5, 287, 303, 304, 309, 335 TbFeCo 5, 8, 79, 285, 324, 364, 408, 415, 624, 641, 665 T, (see Curie tenipsrature)

Temperature 42, 259, 346 change 179 writing 438 Temperature control 116 Temperature dependence 330, 674, 676 Temperature distribution 651, 653, 672 Temperature gradients 458, 637 Temperature rise 642 Temperature superposition 448 Temperature variations 685 Temperature-humidity cyclic test 260 Terbium iron (see TbFe) Ternary alloys 3 12 TES (seee Tracking error signao Tester 753 Testing 264, 267, 706 Testing environments 348 Testing laboratory 343 Testing procedures 708 Thermal accumulation 469 Thennal accumulation writing 467 Thermal analysis 691 Thermal buildup 461, 464 Thermal characterization 416 Thennal comparator 646 Thermal conduction model 445 Thennal conductivity 287, 414, 415, 646, 670, 671 Thennal conductivity data Thermal conductivity of Al-alloy 342 Thennal crosstalk 453 Thennal design 288 Thermal diffusion 438 Thermal diffusivity 414 Tliermal drift 765 Thennal effects 93 Thennal efficiency 646 Thennal energy 453 Thennal expansion 179 Thermal gradients 464, 579, 909 Thennal history 788 Thermal intersymbol interference 448 Thermal modeling 636, 648,669 Thermalnoise 554, 559 Thennal problem 640, 643 Thermal properties 414, 418 Thennal writing 464 Thennalization 307 Tliennally limited acceleration 199

Index 945 Thennally-activated 787 Thermochromic 6 Thennoelastic adhesive 259 Thennoelastic polyolefm 258 Thermomagnetic domain reversal 682 Thennomagnetic erasing (see Erasing) Thennomagnetic marking (see Writing) Thennomagnetic recording (see Writing) Thermomagnetic storage 12 Thennomagnetic switching (see Writing)

Thennomagnetic writing (see Writing) Thennophysical parameters 670 Thennophysical properties 644 Thennoplasticresin 246 Thermosettingresins 253 Thickness substrate 729 Thickness Uniformity of photoresist 234 Thin film optical model (see Optical model)

Thin film stack (see Muftilayer thin films) Thinfilms 3, 209, 362 Three-pass process 426 Three-spot 126 Threshold channel 590 Threshold current 39 Threshold power 443, 671 Thresholding 669 Thresholdingphenomenon 438 Through-substraterecording 286 Through-thickness mark profiles 654 Tilt 21, 74, 721, 726, 727 Tilt angle 275, 276 Time domain analysis 138, 142, 178 Timing interval analysis 746, 854, 858 Timingjitter 592, 746, 854 Timing jitter distribution 853 Timingwindow 593, 848, 853 Tipwriting 469 TIR (see Total internal reflection) TIR focus sensor 103 TISI 448, 453, 461, 469 Tolerances 706 Top coat 253 Topographical features 627 Torque 187 Torque magnetometry 363

Total internal reflection 103 Totem-pole configuration 555 Track 2, 796 Track acquisition 138 Trackcapture 138, 142, 146, 148 Track crossings 725 Track density 901 Track layout 225 Tracknumber 214 Trackpitch 9, 146, 219, 221, 225, 243, 901 Trackrunouts 20 Track sensing 126 Track spacing 20, 187, 799 Track velocity 133, 574 Tracking 107 Tracking actuators 125 Tracking error budget 128 Tracking error signal 34, 99, 102, 108, 109, 125, 138, 210, 714, 721 Tracking groove 797, 807 Tracking into focus 108 Trackingloop 137, 140, 141, 146, 147 Tracking mirror 764 Tracking position 173 Tracking pre-grooves 627 Tracking sensor 105, 108 Tracking servo 20, 154 Tracking signal 131, 136, 137 Tracking systems 20, 148 Tracking transfer function 170 Tracks per inch 9 Tracks per revolution 2 19 Transducer 33 Transfer function 168, 506, 727 Transfer matrix 376 Transfer rate 13 Transformer model 200 Transimpedance gain 84 Transition density 577 Transition metal 4 15 Transition zone 212, 800 Translational stiffness 168 Transmission coefficients 379 Transmittance 330, 365, 368 Transparent materials 396 Transversality condition 386 Transverse electromagnetic wave 6 14

946 Magneto-Optical Data Recording Transverse wave 371 Trilayer glass 288 TRIM 308 Truncation 64, 68 Turntable 225 Twin-wt 126 Two-level pulse waveform 789 Two-part adhesive 258 Two-sided media 760 Type 1 scanning microscope 487

U UL 774, 777 Ultrasonic bonding 264 Ultraviolet lasers 243 Uncorrectable ECC codeword 842 Uncorrectable error patterns 8 14 Under-filled 499 Underlayers 349 Uniaxial anisotropy 330 Upconversion lasers 115 Upspinmask 914 Upstream fine tracking 156 Upstream focus 156 Uranium chalcogenides 339 Uranium monochalcogenides 406 User bit 856 User byte errors 858 Userdata 243, 801 User data error burst 857 User data field 808, 84 1 User data phrase 805 Userzone 212, 219, 221, 801 User zone data capacities 225 W curableresin 250, 251, 253, 254 W curing adhesive 258, 263

V Variable angle spectroscopic ellipsometer 391, 379, 406, 730, 791 Variable frequency oscillator 2 13, 791, 806 VASE (see Variableangle spectroscopic ellipsometer) VDE 774 Vector diffraction analysis 627

Vector diffraction theory 76 Velocity dependence 436, 437, 665 Velocity effects 445 Velocityemor 138, 143, 144 Velocity profile 137, 143 Velocity sensitivity 433 Vertical birefringence 108 VFO (see Variablefrequency oscillator) VFOfields 805 VFO resolution 273 Vibrating sample magnetometq 363 Vibration levels 128 Videodisk 27 Virtual work theorem 182 Viscosity 246, 253 Voice coil 766, 906, 907 Voice coil actuator 128, 129 Voice coil motor 127, 157, 191, 195, 765 Voigtparameter 381, 385, 395, 399, 623, 624, 626 Voltage-current characteristic 297

W Wall energy 683 Water ultrapure 233 Wave equation 397 Wave plates 79 Waveform pulsewrite 787 Wavefront 495 Wavefront aberration 14, 21, 37 Wavefronterror 59, 73 Wavefront quality 14 Waveguide 39 Waveguide fabrication 116 Wavegidde structure 43 Wavelength 10, 40, 268, 316, 324, 338, 711, 753, 900, 904 blue 566 LBR 243 Wavelength changes 5 1 Wavelength dependence 622 of birefringence 732 Wavelength noise 50 Wavelength of illuinination 527

Index 947 Wavelength sensitivity 95 Wavelengthshifts 71, 559 Waveplate 82, 90, 94, 98 Wavevectors 372 Wax-wanesensor 104 Weakly modulating 5 19 wear-out 57 Wetcleaner 254 Whitenoise 51 White noise testing method 5 11 Wideband 855 Wiedemann-Franz law 4 14, 4 15 Wire parameters 197 Wobblebits 126, 131, 148 Wobble marks 2 10 Wollaston prism 620 Working point 193 WORM (see Write-onceread-many) WORMheads 64 WORM materials 578 WORMmode 22 WORM optical disk 797 Worst-case pattern 857 Write bias field 35 Writenoise 576, 577, 578, 579 Write pre-emphasis 787 Write precompensation 787 Writeprocess 3, 439 Write pulse 431, 753 Write signal 787 Writestrategy 426, 429 Write verification 472, 473 Write-once 243, 627 Write-once optical drives 1 Write-once read-many 22, 627, 771, 784, 807, 812, 852 Write-power dependence 672 Write-read head layout 6 13 Writderase cycles 344 WriWerase cycling stress 453 WriWerase sensitivity 335 Write/erase strategy 425 Writdread characteristics 799 Writing 3, 31 1,425, 426,662, 663, 673, 677,683, 692 Writing criterion 448 Writing crystalline marks 8 Writing methods 445 Writingnoise 93, 693

Writingpower 443, 663 Writingpulse 443, 446, 464 Writing sensitivity 429 Writing strategy 458 Writing temperature 438 Writing-induced jitter 696 Writtenmarks 796 Written-in noise 93

X X 448 X-ray diffraction 363 X-ray lithography 896 X-Y 486

Y Yaw 127 Z Z-AD test 260, 346 Z-domain 144 Z-transfom 144 ZCAV (see Zoned constant angular velocity)

Zeeman energy 675 Zeemanterm 682 Zero crossings 144 Zero ellipticity 16 Zeroorder hold 131 ZnOfilms 309 ZnSe devices 116 Zone bit recording 753, 901 Zoned constant angular velocity 219, 801, 802

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