Laser Applications (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series Advanced Materials and Technologies)

Landolt-Börnstein GROUP VIII: Advanced Materials and Technologies VOLUME 1 Laser Physics and Applications SUBVOLUME C Laser Applications Title Pages Contributors, Preface, Contents Contributors Preface Contents 1

Fundamentals

1

1.1 1.1.1

Fundamentals of laser-induced processes (H. HÜGEL, F. DAUSINGER) Introduction

3 3

1.1.2

Energy coupling

4

1.1.2.1

Fundamentals

4

1.1.2.2

Optical properties of metals

6

1.1.2.2.1

Temperature effects

9

1.1.2.2.2

Chemical effects

14

1.1.2.2.3

Roughness effects

14

1.1.2.3

Optical properties of ceramics

16

1.1.2.4

Scattering and absorption by particles

18

1.1.2.5

Non-linear absorption

24

1.1.3

Thermophysical and dynamical “response”

25

1.1.3.1

Condensed matter

25

1.1.3.1.1

Heat conduction

25

1.1.3.1.1.1

Fourier heat conduction

25

1.1.3.1.1.2

Two-temperature model

28

1.1.3.1.2

Phase transitions

30

1.1.3.1.2.1

Melting

30

1.1.3.1.2.2

Evaporation

32

1.1.3.1.3

Melt dynamics

34

1.1.3.1.3.1

Origin of driving forces

34

1.1.3.1.3.2

Resulting effects in laser machining

35

1.1.3.2

Interaction mechanisms in the gas and plasma phase

37

1.1.3.2.1

Basic ionization and absorption mechanisms

38

1.1.3.2.1.1

Bound electrons

38

1.1.3.2.1.2

Free electrons

40

1.1.3.2.2

Absorption and refraction effects in laser-induced plasmas

41

1.1.3.2.2.1

Plasma composition and temperature

41

1.1.3.2.2.2

Absorption

47

1.1.3.2.2.3

Refraction

50

1.1.3.2.3

Dynamical effects

52

1.1.4

Simplified dependences in laser processes

56

1.1.4.1

Energy coupling in laser processes

56

1.1.4.1.1

Coupling rate in laser cutting

56

1.1.4.1.2

Coupling rate in laser welding

57

1.1.4.2

Process windows

58

1.1.4.2.1

Power threshold

59

1.1.4.2.2

Factors determining process velocity

60

1.1.4.2.3

Factors determining efficiency

61

References for 1.1

62

2

Production engineering

73

2.1 2.1.1

Surface treatment (H.W. BERGMANN) Laser macro processing

75 75

2.1.1.1

Solid-state hardening

76

2.1.1.1.1

Physical basics

76

2.1.1.1.2

Material science basics

77

2.1.1.1.3

Production-related aspects

78

2.1.1.1.4

Time scheme of the irradiation

79

2.1.1.1.5

Observed degradations and their reasons

79

2.1.1.2

Remelting

81

2.1.1.2.1

Remelting of cast iron

83

2.1.1.2.2

Remelting of aluminum alloys

84

2.1.1.2.3

Remelting of titanium alloys

86

2.1.1.2.4

Remelting of magnesium alloys

88

2.1.1.2.5

Observed degradations of surface-remelted components and their reasons

89

2.1.1.3

Laser cladding

92

2.1.2

Thin-layer technologies

95

2.1.2.1

Laser cleaning

96

2.1.2.2

Laser cleaning and smoothing of cast iron

97

2.1.2.3

Surface alloying

97

2.1.3

Laser shock hardening

98

References for 2.1

101

2.2 2.2.1

Rapid prototyping (A. GEBHARDT) Layer manufacturing

105 106

2.2.1.1

Rapid-prototyping process chain

106

2.2.1.2

Prototypers

107

2.2.1.3

Characteristics of rapid-prototyping processes

108

2.2.1.4

Materials

108

2.2.1.5

Post-processing

113

2.2.1.6

Finishing

113

2.2.1.7

Functional metal parts

113

2.2.1.8

Rapid tooling

113

2.2.1.8.1

Indirect rapid tooling

115

2.2.1.8.2

Direct rapid tooling

116

2.2.2

Application of rapid-prototyping models

117

2.2.2.1

Model characteristics and model properties

117

2.2.2.2

Criteria for the use of rapid-prototyping models

117

2.2.2.3

Examples

119

2.2.2.4

Rapid manufacturing

121

2.2.3

Recent developments and future trends

121

References for 2.2

123

2.3 2.3.1

Thermal bending (M. GEIGER, F. VOLLERTSEN) Principle of laser forming

125 125

2.3.2

Mechanisms

125

2.3.3

Influence parameters

126

2.3.3.1

Threshold energy

127

2.3.3.2

Processing parameters

127

2.3.3.3

Material parameters

128

2.3.3.4

Geometric parameters

130

2.3.4

Bend radii

131

2.3.5

State of the art and trends

132

References for 2.3

133

2.4 2.4.1

Joining (H. HAFERKAMP) Introduction

137 137

2.4.2

Conduction welding

139

2.4.3

Deep-penetration welding

140

2.4.3.1

Capillary formation

142

2.4.3.2

Plasma formation

143

2.4.3.3

Humping effect

144

2.4.4

Material weldability

145

2.4.5

Thermal distortion

146

2.4.6

Tailored blanks

147

2.4.7

Soldering and brazing

149

2.4.8

Diode-laser applications

150

2.4.9

How to avoid quality degradation

151

References for 2.4

156

2.5 2.5.1

Laser separating (W. O'NEILL) Introduction

159 159

2.5.2

Cutting

160

2.5.2.1

Fusion cutting

160

2.5.2.2

Reactive-gas cutting

162

2.5.2.3

Sublimation cutting

163

2.5.3

Cleaning

163

2.5.4

Machining

167

2.5.4.1

Oxidation processes

168

2.5.4.2

Liquid-phase machining

168

2.5.4.3

Vapor-phase machining

168

2.5.5

Drilling

170

2.5.5.1

Piercing

172

2.5.5.2

Multiple-pulse drilling

172

2.5.5.3

Trepanning

173

2.5.5.4

High-speed drilling

175

2.5.6

Non-thermal ablation

176

2.5.7

Marking

177

2.5.8

Comparison with conventional processes

178

2.5.9

How to avoid quality degradation

180

References for 2.5

184

2.6 2.6.1

Cutting: Modeling and data (W. SCHULZ, C. HERTZLER) Diagnostics, monitoring and modeling

187 188

2.6.2

Experiments and diagnostics

189

2.6.3

Mathematical formulation

191

2.6.4

Inertial manifolds

192

2.6.5

Dimension in phase space

193

2.6.6

Spatial one-dimensional model

194

2.6.7

Spatial two-dimensional model and diffusive eikonal

196

2.6.8

Iterative refinement

199

2.6.9

Cutting data

202

2.6.9.1

Laser power

202

2.6.9.2

Modulation of the laser output (gating frequency)

203

2.6.9.3

Beam quality and power density distribution

203

2.6.9.4

Spatial and temporal beam stability

205

2.6.9.5

Polarization

205

2.6.9.6

Output mirrors of the laser unit

205

2.6.9.7

Beam alignment

205

2.6.9.8

Astigmatism

206

2.6.9.9

Deflection mirror

206

2.6.9.10

Focusing lens

206

2.6.9.11

Beam to nozzle alignment

206

2.6.9.12

Shape of nozzle exit

207

2.6.9.13

Cutting speed

207

2.6.9.14

Type of assist gas

207

2.6.9.15

Gas pressure

207

2.6.9.16

Focal position

208

2.6.9.17

Material type and composition

208

2.6.9.18

Thickness

208

2.6.9.19

Surface condition of the sheet metal

208

2.6.9.20

Cut shape

209

2.6.9.21

Kerf width

209

2.6.9.22

Dross formation

210

2.6.9.23

Mean roughness

210

2.6.9.24

Perpendicularity and slant tolerance

211

2.6.9.25

Drag lines

211

2.6.9.26

Pitting

211

2.6.9.27

Heat-affected zone

211

2.6.10

Machining data tables for cutting

212

References for 2.6

215

2.7 2.7.1

Laser systems for materials processing (G. SEPOLD, M. GRUPP) Laser macro systems

219 220

2.7.1.1

Laser sources

221

2.7.1.1.1

CO2-laser

221

2.7.1.1.2

Nd:YAG laser

222

2.7.1.1.3

High-power diode lasers

223

2.7.1.1.4

Fiber lasers and thin disc lasers

224

2.7.1.2

Laser beam guiding

224

2.7.1.2.1

Beam-guiding systems for CO2-lasers

224

2.7.1.2.2

Beam guiding for Nd:YAG-lasers

225

2.7.1.3

Beam-forming elements

226

2.7.1.3.1

Focusing optics

227

2.7.1.3.1.1

Laser cutting heads

228

2.7.1.3.1.2

Welding heads

228

2.7.1.3.1.3

Working heads for surface treatment

229

2.7.1.4

Handling devices

230

2.7.1.4.1

System concepts

230

2.7.1.4.1.1

1-dimensional systems

230

2.7.1.4.1.2

2-dimensional systems

230

2.7.1.4.1.3

3-dimensional systems

232

2.7.1.4.2

New developments

234

2.7.1.4.3

Special systems

235

2.7.1.4.4

Actuation and control of laser systems

236

2.7.1.4.5

Clamping devices

236

2.7.2

Laser microtechnology

237

2.7.2.1

Beam sources

238

2.7.2.2

Beam-guiding and -forming techniques

238

2.7.3

Conclusions and outlook

239

References for 2.7

241

2.8 2.8.1

Process monitoring and closed-loop control (W. WIESEMANN) Introduction

243 243

2.8.2

Basics of process monitoring and closed-loop control

244

2.8.2.1

General

244

2.8.2.2

Process-surveillance objectives and strategies

245

2.8.2.2.1

Support for scientific research by monitoring of process output parameters

245

2.8.2.2.2

On-line treatment fault probability assessment and documentation during serial production

246

2.8.2.2.3

Closed-loop control during serial production

246

2.8.2.3

Treatment quality indicators, span of surveillance

247

2.8.2.4

Process output parameter detection

249

2.8.2.4.1

Theoretical introduction

249

2.8.2.4.2

Radiation emission from the interaction zone

250

2.8.2.4.3

Radiation reflection and transmission at the interaction zone

250

2.8.2.4.4

Radiation detection and sensor arrangement

251

2.8.2.4.5

Two-dimensionally resolved radiation emission

252

2.8.2.4.6

Sound detection

253

2.8.2.4.7

Electrical-charge detection

253

2.8.2.4.8

Multiple-sensor fusion

253

2.8.2.5

Signal assessment methods

254

2.8.2.6

Control actions

256

2.8.3

State of the art of process monitoring and control technology

258

2.8.3.1

Cutting and drilling

259

2.8.3.1.1

General

259

2.8.3.1.2

Scientific research

259

2.8.3.1.2.1

Cutting

259

2.8.3.1.2.2

Drilling, piercing

260

2.8.3.1.3

Industrial applications

261

2.8.3.2

Welding

261

2.8.3.2.1

General and historical

261

2.8.3.2.2

Recent scientific research

262

2.8.3.2.2.1

Optical emissions, photo-diode detection

263

2.8.3.2.2.2

Optical emissions, camera detection

264

2.8.3.2.2.3

Reflected laser radiation

264

2.8.3.2.2.4

Electrical charge collection

265

2.8.3.2.2.5

Multiple sensor schemes

265

2.8.3.2.2.6

X-ray and visible-light shadowgraphy, holography, laser beam probe, ultrasonic inspection

266

2.8.3.2.3

Industrial applications

267

2.8.3.2.3.1

Monitoring systems

267

2.8.3.2.3.2

Closed-loop control systems

268

2.8.3.3

Transformation hardening

268

2.8.3.4

Cladding, alloying

269

2.8.3.5

Cleaning, caving

269

2.8.4

Outlook

270

References for 2.8

272

3

Life science, biological and chemical processing

277

3.1 3.1.1

Lasers in biology and medicine (O. MINET, K. DÖRSCHEL, G. MÜLLER) Light and heat transport in tissue

279 279

3.1.1.1

Light transport and optical parameters

279

3.1.1.2

Heat transport and thermal parameters

282

3.1.2

Laser-tissue interactions

285

3.1.2.1

Laser diagnostics by transillumination and induced fluorescences

285

3.1.2.2

Laser-induced photochemistry

287

3.1.2.3

Photothermal effects

287

3.1.2.3.1

Coagulation

288

3.1.2.3.2

Evaporation of tissue

288

3.1.2.4

Effects of short-pulsed laser radiation

288

3.1.2.4.1

Photoablation

289

3.1.2.4.2

Optical breakdown and plasma formation

290

3.1.2.4.3

Laser-induced shock waves and cavitations

291

3.1.2.5

Laser biostimulation

291

3.1.3

Medical laser systems

292

3.1.3.1

Typical medical lasers

292

3.1.3.2

Laser light delivery

292

3.1.3.2.1

Articulated arm

292

3.1.3.2.2

Light guide

292

3.1.3.2.3

Optical glass fiber

295

3.1.3.2.4

Hollow wave guide

296

3.1.3.3

Applicator systems

296

3.1.4

Laser applications in medicine

296

3.1.5

Medical laser safety

298

3.1.5.1

Medical laser pyrolysis products

298

3.1.5.2

Regulatory requirements for medical laser systems

301

3.1.5.3

Specific aspects of medical laser safety

302

References for 3.1

303

3.2 3.2.1

Laser chemical processing (D. BÄUERLE) Introduction

311 311

3.2.2

Pulsed-laser ablation

313

3.2.2.1

Surface patterning

314

3.2.2.2

The threshold fluence

317

3.2.2.3

Ablation rates

317

3.2.2.4

Material damage

319

3.2.2.5

Influence of an ambient atmosphere

320

3.2.2.6

Instabilities, structure formation

320

3.2.3

Materials etching

320

3.2.3.1

Etching of metals

321

3.2.3.2

Etching of semiconductors and insulators

321

3.2.4

Laser-induced chemical vapor deposition (Laser-CVD)

325

3.2.4.1

Microstructures

325

3.2.4.2

Thin-film formation

327

3.2.4.3

Adsorbed layers, hybrid techniques

329

3.2.5

Deposition from liquids

331

3.2.5.1

Electroless plating

332

3.2.5.2

Electrochemical plating

333

3.2.6

Pulsed-laser deposition (PLD)

333

3.2.6.1

Overview of materials and film properties

334

3.2.6.1.1

Inorganic materials

334

3.2.6.1.2

Organic materials

336

3.2.6.2

Nanocrystalline films

338

3.2.6.2.1

Nanocomposite materials

338

3.2.6.2.2

Size-selective ablation

339

3.2.6.3

Hybrid techniques

339

3.2.6.4

Laser-induced forward transfer

339

3.2.7

Chemical surface transformations

339

3.2.7.1

Doping

340

3.2.7.2

Alloying and synthesis

341

3.2.7.3

Oxidation, nitridation, reduction

341

3.2.7.4

Transformation of organic materials, laser lithography

342

References for 3.2

345

4

Optical data processing

353

4.1 4.1.1

Communication (M. MÖHRLE, H. VENGHAUS) Introduction

355 355

4.1.2

Heterostructures

356

4.1.3

Material systems

359

4.1.4

Diode laser structures

359

4.1.4.1

Ridge-waveguide (RW) lasers

360

4.1.4.2

Buried-heterostructure (BH) lasers

360

4.1.4.2.1

Conventional BH-lasers

361

4.1.4.2.2

Semi-insulating BH-lasers

362

4.1.4.2.3

Buried-Ridge-Stripe (BRS) lasers

362

4.1.5

Laser types

362

4.1.5.1

Multimode devices (Fabry-Perot (FP) lasers)

362

4.1.5.2

Single-mode devices

364

4.1.5.2.1

Distributed-feedback (DFB) lasers

364

4.1.5.2.2

Distributed-Bragg-reflector (DBR) lasers

367

4.1.6

Characteristics of 1.3 µm and 1.5 µm lasers

367

4.1.6.1

Fabry-Perot (FP) lasers

368

4.1.6.2

Distributed-feedback (DFB) lasers

371

4.1.6.2.1

General

371

4.1.6.2.2

Linewidth

373

4.1.6.2.3

HF characteristics

373

4.1.6.3

Lasers for uncooled operation

374

4.1.6.4

Lasers for supervisory channels

375

4.1.7

Fiber-based lasers

375

4.1.7.1

Fiber lasers

375

4.1.7.2

Hybrid Fabry-Perot fiber Bragg grating lasers

376

4.1.8

Integrated laser-modulator

376

4.1.9

Tunable lasers

378

4.1.9.1

Standard devices

378

4.1.9.2

Semiconductor lasers with enhanced tuning range

379

4.1.9.3

Commercial tunable single-chip (SC) lasers

380

4.1.9.4

Linewidth of widely tunable lasers

380

4.1.9.5

Wavelength tunable/selectable fiber laser

380

4.1.9.6

External-cavity laser

381

4.1.10

Monolithic integrations

381

4.1.10.1

Integrated spot size converter

381

4.1.10.2

Integrated multi-wavelength sources

382

4.1.10.3

Integrated mm-wave source

383

4.1.10.4

Transceiver

384

4.1.11

Lasers for advanced optical systems

384

4.1.11.1

Short-pulse sources

384

4.1.11.2

Self-pulsating lasers

384

4.1.12

Pump lasers for optical amplification

385

4.1.12.1

Pump lasers for erbium-doped fiber amplifiers (EDFAs)

385

4.1.12.2

Pump lasers for Raman amplification

386

4.1.13

Vertical-cavity surface-emitting lasers (VCSELs)

386

4.1.13.1

Short-wavelength VCSELs

387

4.1.13.2

Long-wavelength (1.3 µm, 1.55 µm) VCSELs

388

4.1.13.2.1

1.3 µm VCSELs

389

4.1.13.2.2

1.55 µm VCSELs

389

4.1.13.3

Tunable VCSELs

392

4.1.14

Reliability

392

References for 4.1

395

5

Metrology

403

5.1 5.1.1

High-precision optical metrology for surfaces (H.J. TIZIANI, M. TOTZECK) Introduction

405 405

5.1.2

Microstructure metrology

407

5.1.2.1

Resolution in optical imaging

407

5.1.2.2

Improving the optical resolution

410

5.1.2.2.1

Decreasing the wavelength

410

5.1.2.2.2

Increasing the numerical aperture (NA)

411

5.1.2.2.3

Decreasing the prefactor κ

411

5.1.2.3

Usage of a-priori information: From model-based imaging to threshold criteria

411

5.1.3

Methods and instrumentation

412

5.1.3.1

High-NA lenses

413

5.1.3.2

Field-measuring microscopy

414

5.1.3.2.1

Intensity microscopy

414

5.1.3.2.2

Microscopy with pupil filters

417

5.1.3.2.3

Interference microscopy

417

5.1.3.2.4

Polarization interferometry

420

5.1.3.3

Confocal microscopy

421

5.1.3.4

Near-field microscopy

425

5.1.4

Large-field metrology

428

5.1.4.1

Interferometry for spherical surfaces

428

5.1.4.2

Aspherics and their testing methods

429

5.1.4.3

Interferometry for aspherical surfaces

429

5.1.4.3.1

The computer-generated hologram (CGH) null

429

5.1.4.3.2

Computation and fabrication of CGH’s

431

5.1.4.3.3

CGH application and error reduction

432

5.1.4.4

Heterodyne interferometry

433

5.1.4.4.1

Principle of external reference

433

5.1.4.4.2

Scanning differential heterodyne interferometry

435

5.1.4.5

Shack-Hartmann sensors

437

5.1.5

A look into the future

438

References for 5.1

439

5.2 5.2.1

Environmental control (M. ULBRICHT) Introduction

443 443

5.2.2

Tunable diode laser spectroscopy (TDLAS)

443

5.2.3

Cavity ring-down spectroscopy (CRDS)

444

5.2.4

Photoacoustic spectroscopy (PAS)

445

5.2.5

Lidar

446

5.2.5.1

Backscatter lidar

446

5.2.5.2

Differential absorption lidar (DIAL)

447

5.2.5.3

Raman lidar

450

5.2.5.4

Fluorescence lidar

451

5.2.5.5

Doppler lidar

451

5.2.5.6

Lidar using intense femtosecond laser pulses

452

References for 5.2

453

6

Laser safety and ecology

457

6.1 6.1.1

Laser safety (H. WELLING) Hazard potentials

459 459

6.1.2

Norms and standards for laser safety

460

6.1.3

Effects of laser radiation and safety measures

461

6.1.3.1

Effects of laser radiation on biological tissue

461

6.1.3.2

Threshold limit values and laser classification

464

6.1.3.3

Safety measures

465

6.1.3.4

Hazard distances

465

6.1.3.4.1

Specular-reflected beam

466

6.1.3.4.2

Diffuse-reflected beam

466

6.1.4

Secondary hazard potentials and safety measures

466

6.1.4.1

Laser system and components

466

6.1.4.1.1

Electrical safety

466

6.1.4.1.2

Optical components

467

6.1.4.1.3

Laser gases

467

6.1.4.1.4

Handling devices

468

6.1.4.2

Secondary radiation

468

6.1.4.3

Explosive atmospheres and fire hazards

469

6.1.4.4

Emission of gases and fumes

469

6.1.4.4.1

Characteristics of laser-generated air contaminants

469

6.1.4.4.2

Extraction systems

473

6.1.4.4.3

Filtration

474

6.1.5

Risk assessment

475

6.1.6

Training and education

476

References for 6.1

477

Index

481

http://www.springer.com/978-3-540-00105-8

Ref. p. 62]

1.1 Fundamentals of laser-induced processes

3

1.1 Fundamentals of laser-induced processes H. HÜGEL, F. DAUSINGER

1.1.1 Introduction Laser applications in industrial production are based on definite interactions of the beam with the workpiece. Since the electro-magnetic energy transported by the laser beam is providing the heat source for all the thermal processes, the mechanisms governing the conversion of these energy forms play a decisive role and represent the fundamentals of any laser-induced process. To make material processing an efficient and widely accepted technology, it is of great importance to utilize as much as possible of the energy radiated onto the workpiece. A sound knowledge of the basic phenomena involved and their dependences on laser beam and workpiece properties is decisive for a proper choice of the laser source to be used for a particular application and for the way the process itself should be conducted. The thermal processes of heating, melting or evaporation that will occur in the interaction zone – the area at, in, or above the workpiece where the laser beam directly interacts with material – basically depend on a balance of the heat fluxes released there and those being transported away from there by conduction, convection, or radiation. As a result, the geometry and the thermo-mechanical as well as optical properties of the interaction zone will adjust themselves accordingly. In other words, the phenomena of energy coupling and “the response” of the workpiece are closely interrelated. Consequently, in treating interaction mechanisms the effects of laser beam and material properties as well as geometrical conditions of the interaction zone have to be considered at the same time. Against this background, the fundamentals of energy conversion in laser-induced processes will be treated by answering the central questions: • • •

How much of the energy irradiated onto the workpiece will be released as heat? How does the workpiece “respond” to this energy supply? Are there – on the basis of answers to the first two questions – some generalized conclusions that directly predict the energy coupling in real processes?

Along these lines, this contribution is organized following the order of events generally occurring when a laser beam is focused onto the surface of a workpiece: In the very first instant, the electro-magnetic wave interacts with the electron system of the material. Some fraction of the radiation will be reflected, the rest will propagate into the workpiece experiencing an attenuation. It is the corresponding loss of the wave’s energy that is gained as heat by the material; this aspect is briefly reviewed in Sect. 1.1.2.1 together with methods to calculate reflectivity resp. absorptivity for condensed matter (solid and liquid). Pertinent theoretical and experimental data and figures for metals and ceramics are presented in Sects. 1.1.2.2 and 1.1.2.3. In processes such as cladding or alloying, the interaction of particles with the laser beam will modify the intensity reaching the workpiece surface and the absorption there; this aspect is addressed in Sect. 1.1.2.4. A short reference to non-linear effects concludes this section on energy coupling (Sect. 1.1.2.5). Section 1.1.3 deals with the “response” of the workpiece to heating. The phenomena are addressed according to the state of the material the radiation interacts with. In the first place, therefore, mechanisms governing the energy flux and balance in condensed matter will be treated in Sect. 1.1.3.1. The discussion includes heat conduction (Sect. 1.1.3.1.1), phase transitions (Sect. 1.1.3.1.2), and melt dynamics (Sect. 1.1.3.1.3). Whenever the electro-magnetic energy flux (intensity) is very high, e.g. in drilling or welding processes, the evaporated material will become ionized. In this extended interaction zone along the beam axis absorption and refraction occur (Sect. 1.1.3.2) leading to modifications of the energy cou-

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4

1.1.2 Energy coupling

[Ref. p. 62

pling. Basic ionization mechanisms are reviewed in Sect. 1.1.3.2.1 and typical plasma properties, absorption and refraction data are presented in Sect. 1.1.3.2.2. Dynamical effects of laser-induced plasmas such as the formation of shock waves are briefly dealt with in Sect. 1.1.3.2.3. In Sect. 1.1.4 an attempt is made to summarize the basic implications of the discussed mechanisms in some generalized formulations.

1.1.2 Energy coupling A description of the propagation of electromagnetic radiation and its interaction with matter can be given by the physical models of particles (photons) and electromagnetic waves. The latter is used in this chapter to discuss reflection, absorption and refraction mechanisms. The influence of various parameters that are of particular interest especially in laser materials processing will be shown.

1.1.2.1 Fundamentals The propagation of electromagnetic waves can be described theoretically on the basis of Maxwell’s equations, see e.g. [86Bor]. A general solution of the wave equation for the time-dependent electric field strength E in the case of a plane wave propagating in z direction is i

ω~ nz

E ( z, t ) = E0 eiω t e c0

where n~ = n + i k .

(1.1.1)

Herein, E0 denotes the amplitude of the electric field, ω the angular frequency, c0 the speed of light in vacuum, and ñ the complex refractive index with n and k representing the index of refraction and extinction, respectively. For k = 0 the amplitude of the electric field and hence the energy contained within the wave remain constant over time which corresponds to the free propagation in vacuum. If, however, k is larger than zero, the wave will be damped and will lose energy upon its propagation within an absorbing medium. This absorbed energy eventually will be transformed into heat and thereby will be available for the laser treatment process. The energy flux of the wave is determined by the Poynting vector whose time-averaged value corresponds to the intensity and can be expressed by the averaged square of the field strength

I = c0 ε 0 E 2 ,

(1.1.2)

with ε0 denoting the electrical permittivity. In absorbing materials, the light intensity decreases along a distance from z = 0 to z according to Beer’s law:

I ( z ) = I 0 e −α z .

(1.1.3)

The absorption coefficient α, which is a function of the vacuum wavelength λ0, can be derived by comparing the exponents resulting from (1.1.1) in conjunction with (1.1.2) with that of Beer’s law (1.1.3):

α=

4 k (λ0 ) . λ0

(1.1.4)

The distance after which the intensity is reduced by a factor of 1/e is called the absorption length or optical penetration depth lα , which is the reciprocal value of the absorption coefficient α :

lα = 1 / α .

(1.1.5)

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

5

The question whether a medium can be regarded as transparent or absorbing is answered by comparing lα with its geometrical dimension in direction of the laser beam propagation. A survey of values for the absorption length and complex refractive index for metals and ceramics at room temperature is given in the following sections. For metals k is larger than unity in the infrared and visible spectral range. Consequently, the absorption length for laser light with such wavelengths is smaller than the wavelength λ0. With lα being many orders of magnitude lower than the typical dimensions of the workpiece (and even that of the interaction zone), the absorbed energy acts as a surface heat source. On the other hand, in cases where lα is comparable to the thickness of the workpiece, e.g. for thin metal films or ceramic plates, volume heating has to be considered. In laser treatment processes the incident intensity I will not be coupled completely into the workpiece. The incident beam intensity is split up into a part reflected at the surface, a part absorbed within the bulk, another one, which is scattered by the material, and a part, which is transmitted through the workpiece. Those fractions can be described by the reflectivity R, the absorptivity A, the scattered fraction S and the transmissivity T, respectively. Due to conservation of energy these contributions have to fulfill the following relationship:

R + A+ S +T =1.

(1.1.6)

The starting value I0 for Beer’s law (1.1.3) is given by the intensity, which actually enters the workpiece, i.e. (1−R) I. The absorbed intensity A⋅I will contribute to the heating of the material and will thereby have a direct impact on the process. If scattering can be neglected this fraction can be evaluated by integrating (1.1.3) over the thickness of the workpiece while the transmissivity T corresponds to the remaining fraction of intensity at the bottom of the workpiece. Otherwise the attenuation and deflection of the beam due to scattering has to be taken into account which will be discussed in Sect. 1.1.2.4. If, additionally, the thickness of the material is much larger than the optical penetration depth lα transmission can be neglected such that A = 1–R. The reflectivity and beam deflection at the interface between two media can be computed as a function of incident angle and polarization by the so-called Fresnel equations. Their general form is given in various optics books (see e.g. [93Ped] or [75Jac]). For the special case of perpendicular incidence on the surface of a beam propagating from an optically thin medium (e.g. air with n ≅ 1) into matter with a complex index of refraction, the reflectivity R can be computed by the following equation: R=

2

2

2

2

( n − 1) + k (n + 1) + k

.

(1.1.7)

For oblique incidence on metals (n2+k2 >>1) and laser light with wavelengths λ longer than 0.5 µm the Fresnel equations can be simplified according to [90Pro] R|| =

(n cos ϕ − 1)2 + k 2 cos 2 ϕ (n cos ϕ + 1)2 + k 2 cos 2 ϕ

R⊥ =

(n − cos ϕ)2 + k 2 (n + cos ϕ)2 + k 2

,

,

(1.1.8)

(1.1.9)

where ϕ is the angle between the surface normal and the incident beam. R|| describes the reflectivity for the electric field vector parallel (p-polarization) to the plane of incidence (defined by the incident beam and the surface normal) whereas R⊥ denotes the reflectivity for its orientation perpendicular (s-polarization) to it. It is obvious that in the limit of perpendicular incidence (ϕ = 0) these expressions reduce to (1.1.7). In the following sections these theoretical findings will be further examined and compared to experimental data. Due to the different optical behavior for opaque and transparent materials this survey is split into separate sections for metals and ceramics.

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6

1.1.2 Energy coupling

[Ref. p. 62

1.1.2.2 Optical properties of metals Various methods have been developed to determine n and k describing the optical behavior of materials and their interfaces [91Pal]. Pure materials at room temperature with clean and smooth surface conditions were preferentially used in most investigations. A rather complete data base, [91Pal] and [81Wea], is at hand for such samples. A survey of data for these optical constants at different laser wavelengths is given in Table 1.1.1. It can be observed that the extinction index k usually is much larger than unity which leads to absorption lengths smaller than the wavelength, see Table 1.1.2.

Table 1.1.1. Optical constants n and k at room temperature for various metals. Ref.

Laser / Wavelength [µm] CO2

CO

Nd:YAG

Diode

Excimer

9.5…11

4.96…5.7

0.992…1.064

0.8…0.826

0.248…0.257

n

k

n

k

n

k

n

k

n

k

Aluminum 25.3

89.8

8.67

48.6

1.21

10.6

2.8

8.45

0.19

2.94

91Pal

25.5

60.9

7.84

35.7

0.98

7.65

1.12

6.0

0.183 2.88

62Lan

31.2

104

8.59

48.2

1.99

7.05

30.4

4.49

15.4

83Ord

Iron 5.81

3.23

4.35

6.26

26.8

4.96

14.58

3.24

4.26

5.81

30.4

4.59

15.4

3.23

4.35

3.19

4.43

0.36

7.22

0.54

6.53

3.05

3.77

1.31

1.95

91Pal

1.01

0.88

62Lan

1.14

1.87

81Wea 85Ord

Steel 1.38

1.87

62Lan

Copper 10.8

47.5

3.81

27.5

9.89

62.4

3.07

32.8

11.6

49

3.6

26.6

8.31

63

3.26

33

0.26

5.26

1.47

1.78

91Pal

0.29

5.18

1.37

1.783

62Lan

0.12…0.26

5.07…5.26

83Ord 85Ord

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

7

Table 1.1.2. Absorption length lα for metals at different wavelengths and room temperature. lα [nm]

Laser / Wavelength [µm] CO2

CO

Nd:YAG

Diode

Excimer

9.5…11

4.96…5.7

0.992…1.064

0.8…0.826

0.248…0.257

8.9

8.2

8.1

7.5

6.7

91Pal

91Pal

91Pal

91Pal

91Pal

26

28.2

19.3

17.4

10.6

85Ord

91Pal

81Wea

81Wea

81Wea

13

15

12

12.7

11.1

85Ord

91Pal

62Lan

62Lan

91Pal

Aluminum

Iron

Copper

Table 1.1.3 summarizes absorptivity values of metals and some alloys at different wavelengths and various surface conditions found in literature. Table 1.1.3. Experimental absorptivity values of metals and some alloys at room temperature for various surface conditions. They are either obtained from measured n and k-data (I) or from direct absorptivity (II), emissivity1) (III), or reflectivity (IV) measurements. A [%]

Ref.

Laser / Wavelength [µm] CO2

CO

Nd:YAG

Diode

Excimer

9.5…11

4.96…5.7

0.992…1.064

0.8…0.826

0.248…0.257

1.41

4.13

13.05

7.6

1.65

3.33…4.03

13.5…15.3

7.55…8.0

Aluminum and Alloys Pure Al

1.15 1.05

1.42 1.7

13.23…13.57 6

12

91Pal (I) 62Lan (IV) 83Ord (I) 80Dec (IV)

1.2

89Bru (I)

2

91Yil (IV)

oxidized, 3 µm rough

5

6

72Tou (III)

oxidized, 115 µm rough

25

25

72Tou (III)

anodized

20…97

polished

2

6.25

93Ste (II)

milled

2.45

7.8…11.25

93Ste (II)

sandblasted

15.5

22.4

93Ste (II)

9…49 2

26…60

72Tou (IV)

(continued) 1

) According to Kirchhoff’s law [93Mod] absorptivity is equal to the spectral emissivity.

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8

1.1.2 Energy coupling

[Ref. p. 62

Table 1.1.3 continued. A [%]

CO2

CO

Nd:YAG

Diode

Excimer

9.5…11

4.96…5.7

0.992…1.064

0.8…0.826

0.248…0.257

7.2

13.4

99Sei (II)

7.8

14.0

99Sei (II)

AlMg5Mn AlMgSi1 AlCu33

Ref.

Laser / Wavelength [µm]

3

91Fre (II)

Al alloy 6061-T6 chem. milled

1.0

1.0

5.0

9.0

72Tou (IV)

sandblasted

22.5

35.0

58.5

46.0

72Tou (IV)

2.4

6.7

35.3

8.5

35

38.5

83.8

62Lan (ns)

3.2

8.0

35.9

39.8

56.5

81Wea (I)

2.4

6.8

35.1

Iron and Alloys Pure Fe

3.5 5

91Pal (I)

85Ord (I)

33.0

47Pri (III)

33.2

95Ste (II)

35 polished

57.3

38

99Sei (II)

9

72Tou (IV)

2 µm rough

40

46

72Tou (IV)

15 µm rough

49

54

72Tou (IV)

oxidized

90.5

90

72Tou (IV)

35CD4 35NCD16

4.4

29.9

5.2

8.5

11

7

St1403/FeP04

95Ste (II)

29…31

95Ste (II) 00Sch1 (II)

38

45

99Sei (II)

Copper and Alloys Pure Copper

1.8

2.0

3.6

63.4 62.3

1.8

1.98

2.67

1.78…4.0

1.32

0.87…1.98

2.4

1.78

0.82

1.18

4.78

91Pal (I) 62Lan (IV) 83Ord (I) 85Ord (I)

0.8

89Bru (I) 4.5

5.5

11.0

17.0

72Tou (IV)

47.0

77.5

72Tou (IV)

CuZn37

5.2

5.8

99Sei (II)

CuSn6

5.3

6.2

99Sei (II)

2.52 polished oxidized

2.5

99Sei (II) 91Yil (IV)

3.0

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

9

In Fig. 1.1.1 the calculated absorptivity of iron, aluminum, and copper at room temperature for perpendicular incidence following (1.1.7) is shown. With increasing wavelength a nearly steady decrease is observed, interrupted only by some local maxima. According to (1.1.8) and (1.1.9), for light which is not normally incident the effect of the orientation of polarization becomes important. In Fig. 1.1.2 this dependence is shown for iron. For higher angles p-polarization leads to an increasing absorptivity with a maximum at the so-called Brewster angle, whereas s-polarization shows a steady decrease. The Brewster maximum is shifted towards larger angles for longer wavelengths. For circular polarization the absorptivity values correspond to the arithmetic mean value of p- and s-polarization, i.e. Ac = (A||+A⊥)/2.

II

ϕ Fig. 1.1.1. Dependence of absorptivity A on wavelength for Fe (I), Al (II) and Cu (III) at normal incidence; n, k-data from [91Pal, 81Wea].

Fig. 1.1.2. Angular dependence of absorptivity A for linearly polarized laser light for Fe at different laser wavelengths; n, k-data from [91Pal].

1.1.2.2.1 Temperature effects An early attempt to calculate absorptivity at elevated temperatures on the basis of the close relation of electronic and optical properties of the materials was made in [1903Hag]: A= 2

4 ⋅ f ⋅ ε0 , σ 0 (T )

(1.1.10)

where σ0(T) denotes the temperature-dependent dc electrical conductivity and f the optical frequency. This so-called Hagen-Rubens approximation is restricted to the long wavelength range. The limit of validity is shifted towards shorter wavelengths with decreasing conductivity. In the case of iron, (1.1.10) yields acceptable values at the wavelength of CO- and CO2-lasers, see e.g. [95Dau]. Since in metals the conductivity decreases with increasing temperature an even better coincidence of (1.1.10) with experimental data can be expected at higher temperatures such as the melting temperature. Calculations of optical constants or absorptivity as functions of temperature from elementary electron theory of metals were made by [1900Dru, 55Rob, 65Seb, 59Rob, 93Dau, 94Hue]. A short historical survey of this development is given in [93Dau]. The various approaches differ in the way by which the two mechanisms relevant for the absorption of laser radiation in metals are taken into account. First, the electrons in the conduction band are accelerated by the electric field of the wave and their movement is damped by collisions with other electrons or lattice vibrations and imperfections. Here, the interdependence of electrical conductivity and optical properties becomes obvious. This absorption mechanism is Lando lt -Börnst ein New Ser ies VIII/1C

10

1.1.2 Energy coupling

[Ref. p. 62

often referred to Drude [1900Dru]. The acceleration can also be described as an increase of the energy of the electron respectively a lift to a higher energy level within an electronic band and, therefore, this absorption mechanism is called intraband absorption. Second, at higher excitation energies, the electron can overcome the bandgap and can be lifted from the valence band into the conduction band. This mechanism shows a resonance character [71Ash] with local maxima at definite wavelengths and is called interband absorption (see e.g. absorption peak at 800 nm for Fe in Fig. 1.1.1). Upon irradiation by a laser beam the surface temperature of a workpiece will undergo a considerable increase. Depending on the process, the surface may reach values just below melting temperature (martensitic hardening) to melting temperature (cutting, remelting) or up to vaporization temperature (welding, drilling). As a consequence, the thermophysical properties of the material, including absorptivity, will vary. With respect to a reliable description of energy coupling the temperature dependence of absorptivity has to be known. Temperature influences the intraband as well as the interband absorption. Generally speaking, the first mechanism increases with temperature because of an increase of the collision frequency of the free electrons. On the other hand, the interband absorption leads via an increase of damping due to the rise in temperature to a decrease of the maximum and to a broadening of the local absorptivity peak. At the melting point, band structures can change which can result in a vanishing of the interband absorption. Detailed investigations on the wavelength dependence of the optical constants for the liquid phase of several metals have been described in [69Mil] and [72Com]. In [69Mil] a vanishing of the 840 nm absorption peak for Al upon melting was observed, whereas in [94Hue] an intraband behavior was calculated even for temperatures far above the melting point. The temperature dependence of absorptivity for Fe and steel at 1.06 µm (Nd:YAG laser), 5 µm (CO laser) and 10 µm (CO2 laser) is shown in Fig. 1.1.3. At Nd:YAG-wavelength a negative gradient with temperature was obtained which is due to the influence of an interband transition. At higher wavelengths, absorptivity can be described by a pure intraband characteristic, which causes the positive temperature dependence. In case of Al a predominant influence of the interband absorption at 1.06 µm is not observed, which yields a positive temperature dependence for Nd:YAG and CO2-wavelength up to the melting point, as shown in Fig. 1.1.4. 10

35 30

8

25 6

20

4

15 10

2

5 0

200

600 1000 1400 Temperature T [°C]

1800

Fig. 1.1.3. Measured temperature dependence of absorptivity A at Nd:YAG- (squares), CO- (triangles) and CO2-wavelength (circles) of polished iron (I, V) and polished steel 35NCD16 (II, III, IV); exp. data [95Ste].

0 0

100

200 300 400 500 Temperature T [°C]

600

700

Fig. 1.1.4. Measured temperature dependence of absorptivity A at Nd:YAG- (squares) and CO2wavelength (circles) for polished Al 99.5; exp. data [95Ste].

The temperature dependence of absorptivity for alloys is influenced by the fact that, even at low alloying contents, the collision frequency of the “free” electrons is changed. It is expected that the interband absorption is less influenced than the intraband part. At higher alloying grade the band structure is changed distinctively and, hence, a simplified theoretical calculation of the absorptivity cannot describe the experimental findings very well [93Dau] (see Fig. 1.1.5). Calculated temperature dependences for different steels and Al-alloys reveal that, in case of a low alloying grade, absorptivity is not remarkably

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

11

influenced. For higher alloying grade (see Fig. 1.1.5, stainless steel 304) a coincidence of experimental and calculated data could not be obtained. An extension of Drude’s theory was performed in [76Wie] being in good coincidence with experimental values of stainless steel at wavelengths longer than 10 µm. Towards shorter wavelengths an increasing deviation was observed. In Fig. 1.1.6 the calculated temperature dependence for pure Al and some alloys at 1.06 µm and 10.6 µm wavelength is shown. For the solid as well as for the liquid phases the absorptivity of alloys is increased compared to the pure material. 15 c b a

13 Absorptivity A [%]

Absorptivity A [%]

15

10

5

11 9 7 5

c b a

II

3 1

0 0

600 200 400 800 Temperature T [°C]

1000

Fig. 1.1.5. Temperature dependence of absorptivity A at 10.6 µm; experimental (symbols) [90Ste] and calculated (lines) [93Dau] values for Fe (I), steel Ck45 (II), steel 35NCD16 (III) and stainless steel 304 (IV).

I 0

200 400 600 Temperature T [°C]

800

Fig. 1.1.6. Temperature dependence of absorptivity A at 10.6 µm (I) and 1.06 µm (II) for Al (a), AlMgSi1 (b) and AlMg5 (c), calculated data [95Dau].

Experimental and theoretical data for the temperature dependence of absorptivity of Al, Fe, Cu and steel of various authors are shown in Tables 1.1.4 and 1.1.5. The figures for the temperature dependence (∆A/∆T) represent a linear fit of the measured or calculated dependences and merely give a rough indication of the change in absorptivity for the indicated temperature range. The experimental results agree reasonably with each other for all materials shown. In [91Yil] values for Al and Cu have been obtained, which deviate – due to the influence of oxidation during the experiment – about one order from that of other authors. Theoretical models yielded values, which are in agreement with each other and with experimental values in case of CO2 wavelength. For Nd:YAG wavelength the values spread slightly. The only exception is the approach in [72Uji] considering a Drude term with a temperature dependence of the electron-phonon collision; for Al it yields a strong deviation from other theoretical and experimental data (except for Nd:YAG values in [91Yil]).

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12

1.1.2 Energy coupling

[Ref. p. 62

Table 1.1.4. Experimental results for the temperature dependence of absorptivity. Material

T-range

∆A/∆T

[°C]

[% / 100°C]

CO2

25…630

CO2

Laser

Surface

Method

Ref.

0.11

polished

calorimetric

93Ste

25…544

1.55

polished to 2.5 µm

reflectometric

91Yil

CO2

25…660

0.28

polished

ellipsometric

89Bru

Nd:YAG

25…630

0.16

polished

calorimetric

93Ste

CO2

25…1200

0.875

polished

calorimetric

95Ste

CO2

600…950

1.036

not specified

not specified

62Lan

Nd:YAG

25…700

–0.457

polished

calorimetric

95Ste

Nd:YAG

1200…1580

–0.526

2 µm RMS

emissivity

72Tou

CO2

25…550

0.27

polished

reflectometric

76Wie

CO2

25…1000

0.3

polished

calorimetric

95Ste

CO2

25…1000

0.55

polished

calorimetric

95Ste

CO

25…1000

0.65

polished

calorimetric

95Ste

Aluminum

Iron

Steel SS304 35NCD16

Ck45

Nd:YAG

25…1000

–0.35

polished

calorimetric

95Ste

CO2

25…1100

0.36

polished

calorimetric

93Ste

Nd:YAG

25…1100

–1.05

polished

calorimetric

93Ste

CO2

25…790

3.27

polished to 2.5 µm

reflectometric

91Yil

CO2

25…1100

0.16

polished

ellipsometric

89Bru

Nd:YAG

1160…1456

0.0005

liquid

ellipsometric

90Kri

Copper

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

13

Table 1.1.5. Theoretical results of the temperature dependence of absorptivity. Material

Laser

T-range

∆A/∆T

[°C]

[% / 100°C]

25…660

1.27

Model

Remarks

Ref.

Aluminum CO2

Nd:YAG

25…660

3.6

CO2

25…660

0.3

CO2

Tm…1600

0.11

Drude + temp.dep. el.-phonon coll. frequency

72Uji

72Uji good agreement with [95Dau, 89Bru] and [84Arn]

94Hue

94Hue

Nd:YAG

25…660

0.38

94Hue

Nd:YAG

Tm…1600

0.235

94Hue

CO2

25…Tm

0.3

CO2

Tm…1900

0.13

Nd:YAG

25…Tm

0.455

95Dau

Tm…1900

0.23

95Dau

CO2

25…660

0.326

CO2

660…1600

0.246

CO2

0…700

0.638

see above

95Dau

CO2

700…Tm

0.439

see above

95Dau

temp.-dep. Drude + temp.-dep. interband absorption

95Dau

95Dau

Drude + temp.dep. conductivity

84Arn 84Arn

Iron

CO2

Tm…3000

0.1

see above

95Dau

Nd:YAG

0…700

–0.98

see above

95Dau

Nd:YAG

700…Tm

0.082

see above

95Dau

Nd:YAG

Tm…3000

0.18

see above

95Dau

35NCD16

CO2

25…1000

0.687

see above

95Dau

35NCD16

CO

25…1000

0.547

see above

95Dau

35NCD16

Nd:YAG

25…1000

–0.57

see above

95Dau

Ck45

CO2

25…1000

0.698

see above

95Dau

Ck45

CO

25…1000

0.579

see above

95Dau

Ck45

Nd:YAG

25…600

–0.586

see above

95Dau

CO2

25…1000

0.39

see above

84Arn

CO2

25…1100

0.9

see above

72Uji

Nd:YAG

25…1100

2.45

Steel

Copper

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72Uji

14

1.1.2 Energy coupling

[Ref. p. 62

1.1.2.2.2 Chemical effects In most practical applications of laser processing, materials are not pure elements with smooth and clean surfaces but alloys with oxidized and, in general, rough surfaces. The more the technical surface conditions deviate from the physically clean ones, the less information about optical constants is available. In literature only few investigations are concerned with the influence of oxidation as the most important chemical process modifying surface properties and absorptivity. Oxidation processes can be divided into two groups, the natural oxidation and the laser or temperature induced oxidation. Metals kept in air are always covered with a so-called native oxide layer with a thickness of 10…50 nm [97Nan]. On the other hand, as oxidation is a time as well as a temperature-controlled process, with elevated temperatures caused by e.g. direct laser irradiation or heat conduction from the interaction zone, the oxide layer grows faster. As reported in [90Pro] for copper foils and steel, oxidation starts when the temperature is high enough for activation. Absorption then increases strongly and, moreover, shows an “oscillatory” character (see Fig. 1.1.7). This effect can be explained as follows. According to [90Pro, 97Nan] there is a direct effect of oxide layer thickness ξ on absorptivity due to interference of the incident and reflected beams at the metal-oxide interface. The change in absorptivity with oxide layer thickness dA/dξ can have a negative as well as a positive sign, which results in “oscillations”. Oxide layers in most cases do not consist of just one single oxide type. The oxide layer on copper is made up of two components, CuO/Cu2O, whereas on iron a three-component layer of FeO/Fe3O4/Fe2O3 is present. Data for optical constants of these oxides are given in [91Pal, 79Tan, 82Kar] and for emissivity values in [80Ise, 79Neu]. In [95Ste] the change in absorptivity during the growth of oxide layers on a ground steel (35NCD16) by heating the sample with a CO2-laser in atmospheric ambience is described. Applying simultaneously an Nd:YAG- and a CO2-probe laser, oscillations have been observed. In [94Jun] a detailed description of the influence of oxidation in the laser hardening process of C60 steel is given. A review of the interdependence of oxide formation on metal surfaces and roughness is given in [62Eub]. 1200

Cu

Absorptivity A [%]

0.6 0.5

1000 T

0.4

800 A

600

0.3

400

0.2

200

0.1 68

70 72 74 Irradiation time t [s]

Temperature T [°C]

0.7

Fig. 1.1.7. Influence of oxidation on absorptivity of Cu at CO2-wavelength [90Pro].

1.1.2.2.3 Roughness effects Although roughness of surfaces plays an important role in laser materials processing, not much information is found concerning this effect. A theoretical investigation on the influence of random and regular roughness for cases when the Root Mean Square (rms) of the roughness is much smaller than the wavelength of the laser radiation is given in [82Els]. The contribution of the so-called Roughness-Induced Absorption (RIA) is compared to the intrinsic absorptivity of the material. For p-polarization the RIA for

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

15

randomly rough surfaces contributes to 1…2 % of the intrinsic value whereas for periodically rough surfaces it can reach the same order as the intrinsic value. The absorptivity of p-polarization is characterized by peaks following sin ϕ = 1 – N λ/∆, where ∆ is the peak-to-peak periodicity of the roughness. This indicates the stimulation of surface plasmon2) resonances which was described by various authors [82Els, 82Bar]. In case the wavelength is of the order of or below the dimensions of characteristic surface structures, multiple reflection respectively multiple absorption within such a groove or depression occurs which increases energy coupling. A model calculating the absorptivity of a depression considering multiple reflection of radiation is presented in [68Aga] and yields

A=

RS 1 1 + ( − 1) Ai

(1.1.11)

with Ai being the intrinsic absorptivity of the material (the value for a smooth surface), RS the surface roughness factor, which is defined by the ratio of the surface of a depression and its projected area. For a single cavity at given width the absorptivity increases with RS, i.e. with increasing depth of the depression. A survey of absorptivities (which are according to Kirchhoff’s law equal to emissivities) in dependence of roughness calculated using (1.1.11) for various forms of cavities (cylindrical, conical, hemispherical) is given in [68Aga, 79Neu]. Some experimental results of absorptivity of randomly rough samples of steel and aluminum alloys are shown in Figs. 1.1.8 and 1.1.9. A tendency of increasing absorptivity with increasing roughness can be recognized with peaks occurring around particular values of roughness. Comparing the absorptivity of steel and Al-alloys in Fig. 1.1.9 the latter is characterized by a stronger influence of roughness, which is in accordance to [80Loe] showing a stronger influence of roughness for metals with low absorptivity. A varying influence of roughness depending on temperature is reported in [95Ste] and shown in Fig. 1.1.10.

Absorptivity A [%]

Absorptivity A [%]

60 55 50 45 40 35 II 30 I 25 20 15 10 5 0 0.5

III III III

1.0 1.5 2.0 2.5 3.0 Roughness Ra [µm]

3.5

4.0

Fig. 1.1.8. Dependence of absorptivity A on roughness Ra (average peak to valley height) for 35NCD16 at Nd:YAG- (squares), CO- (triangles) and CO2wavelength (circles), polished (I), ground (II) and milled (III) samples; exp. data [95Ste].

2

50 40 30 20 10 0

2

4

10 12 6 8 Roughness Rz [µm]

16

Fig. 1.1.9. Dependence of absorptivity A on roughness Rz (peak to valley height) at 808 nm for steel St1403 (squares), AlMg0.6Si1.2 (full circles) and AlMg5Mn (open circles); exp. data [99Sei]; all samples ground.

) Plasmons are collective longitudinal vibration states of an electron gas.

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14

16

1.1.2 Energy coupling

Absorptivity A [%]

50 40

[Ref. p. 62

VI V

30 IV 20 10

III

Fig. 1.1.10. Temperature dependence of absorptivity A for Ck45 at Nd:YAG- (squares) and CO2wavelength (circles); polished (I, IV), ground (II, V) and sandblasted surfaces (III, VI); exp. data [93Ste].

II I 0

200

400 600 800 1000 1200 Temperature T [°C]

The influence of roughness on the dependence of absorptivity at λ = 808 nm on the angle of incidence was investigated in [98Sei] for steel and aluminum samples. Even polished samples with only a few remaining scratches showed a reduced or less pronounced angular dependence compared to the one calculated with given n, k data. This can be explained by the fact that on the wall of a depression the angle of incidence and the polarization state (s- or p-polarization) differ from the macroscopically adjusted one. A manifold of angles and polarizations occur which lead to an averaging effect of the polarization influence. Theoretical considerations on the influence of roughness on polarization effects are given for example in [83Pop], showing a decrease of the angular dependence with increasing roughness.

1.1.2.3 Optical properties of ceramics Some data of optical properties of pure ceramics can e.g. be found in [85Pal] and are summarized in Tables 1.1.6 and 1.1.7. The reflectivities obtained from these data are typically much lower than those of metals. As can be seen from Fig. 1.1.11 its value for parallel polarization reaches, in contrast to metals, almost zero at the Brewster angle. Another important difference compared to metallic materials is found concerning the absorption lengths which are usually much larger for ceramics. Figure 1.1.12 shows the dependence of this optical penetration depth on wavelength for two ceramic materials. It is remarkable that in the spectral range between 0.5 and 10 µm the penetration depth can be up to three orders of magnitude larger than the wavelength. Table 1.1.6. Optical constants n, k at room temperature. Laser

Ref.

CO2 n

CO k

n

Nd:YAG k

n

Diode k

n

Excimer k

n

k

2.598

3.16

0.259

91Pal

2.08

2.278

0.0048

91Pal

SiC-crystal 0.0593

1.21

2.467 8.75×10–4

non-crystalline Si3N4

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

17

Table 1.1.7. Absorption length for ceramics at different wavelengths and room temperature. lα [nm]

Laser CO2

Ref. CO

AlN (Aluminum nitride)

Nd:YAG

Diode

208 000

182 000

Excimer

99Rai

SiC crystal

31

455

76

91Pal

4028

91Pal 99Rai

Si3N4 non crystalline technical

40 000…160 000

37 000…130 000

I

I

II

Fig. 1.1.11. Angular dependence of the Fresnel reflectivity of Si3N4 at different wavelengths. For 0.5 µm wavelength both parallel (||) and perpendicular (⊥) polarizations are shown, for all other wavelengths only parallel polarization. Calculation based on pure material data [85Pal].

Fig. 1.1.12. Optical penetration depth of SiC and Si3N4 (pure materials) in dependence on the wavelength, derived from experimental data in [85Pal].

For a detailed description of the energy coupling of ceramics, besides the absorptivity of pure crystalline material, the structure of the ceramic has to be taken into account. Technical ceramics consist of grains embedded in a glass phase both having different optical properties. Radiation thus is multiply scattered at the grain boundaries as well as at impurities. This results in smaller absorption lengths compared to those in pure materials. Hence, the scattered fraction in (1.1.6) can no longer be neglected. Scattering therefore decreases transparency and penetration depth but does not necessarily increase absorption. The reflectivity and the penetration depth of some technical ceramics are shown in Fig. 1.1.13 [99Rai]. The values are measured at low intensity and ambient temperature with a Fourier spectrograph. The various species of Si3N4 differ in grain size and sinter-additives. Compared to the values of reflection and penetration depth shown in Fig. 1.1.12, a marked reduction in penetration depth is observed.

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1.1.2 Energy coupling

[Ref. p. 62

α

18

a

b

Fig. 1.1.13. (a) Reflectivity and (b) penetration depth of some technical ceramic materials measured at ambient temperature [99Rai]. Sinter-additives (MgO for 1225 whereas Al2O3+Y2O3 for 1253 and 1254) and mean grain size (0.59 µm, 0.82 µm and 0.36 µm for 1225, 1253 and 1254, respectively) vary for the three different Si3N4 ceramics.

These values, measured at room temperature, are only valid and relevant at the beginning of the heating process. At process temperature the optical properties can be totally different. Optical properties such as transmission and reflection at high temperature are reported in [97Gar]: In the case of AIN reflectivity and optical penetration depth decrease with increasing temperature, whereas the values for Si 3N4 are nearly constant over a broad range of temperatures (see Fig. 1.1.14). At a certain temperature, a steep increase in reflection is observed. It has been shown that this temperature corresponds to the value where decomposition of the ceramic starts [99Rai].

Fig. 1.1.14. High-temperature reflectivity and penetration depth of technical AlN and Si3N4 ceramic samples in air atmosphere [97Gar].

1.1.2.4 Scattering and absorption by particles In most practical cases of material processing the laser beam propagates through a more or less well defined “atmosphere” above the workpiece before it reaches the solid or liquid surface. Depending on the process itself as well as on the way it is being conducted, this atmosphere consists of e.g. pure gas, gas mixtures or gases containing particles. These particles can be produced by condensation of clusters within a plasma plume [94Kar, 96Sch1, 98Cal, 98Luk], by melt expulsion [97May], phase explosion [96Kel] or can be provided to the interaction zone by an external flow of powder, e.g. in the case of laser cladding. Here, the influence of particles will be reviewed.

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

19

If multiple scattering is negligible the intensity of a beam is exponentially attenuated according to a relation similar to (1.1.3) with a coefficient resulting from the absorption and scattering in all directions by the particles. Although both processes occur simultaneously, one or the other may dominate, depending on the density of particles and their properties like chemical composition, size, shape and orientation, on the ambient atmosphere and on the properties of the incident beam. The extinction is the combined effect of absorption and scattering, the extinction coefficient αext is expressed as the sum of absorption coefficient αabs and scattering coefficient αsca :

αext = α abs + αsca = N (Cabs + Csca ) = NCext

(1.1.12)

with N denoting the number of particles per unit volume. Equation (1.1.12) also defines Cabs and Csca , the absorption and scattering cross sections, and Cext the resulting “effective” extinction cross section. The cross sections indicate the amount of energy absorbed or scattered by a particle per unit irradiation intensity and area. These cross sections are normalized to the geometrical cross section G of the particle, generally, which leads to dimensionless values:

Qext =

Cext C C , Qabs = abs , Qsca = sca , G G G

(1.1.13)

which are called the efficiency factors for extinction, absorption and scattering and indicate the absorption and scattering capability of an irradiated particle. The efficiency factors of spherical particles of arbitrary size and refractive index can be calculated by the Mie theory [57Hul, 83Boh]. They depend on two parameters: the complex refractive index n~ and the size parameter x x=

⋅d , λ

(1.1.14)

where d is the diameter of the particles and λ is the wavelength of the beam. The intensity of the scattered light I at a large distance R from the particle must be proportional to the intensity of the incident light I0 and R–2: I = I0

λ2 F (θ , ϕ ) , 4 2R2

(1.1.15)

where θ is the scattering angle measured from the forward direction of the light and ϕ is an azimuth angle. The dimensionless function F(θ,ϕ) defines the angular distribution of the scattered light and can be written for the special case of incident linearly polarized light and spherical particles:

F (θ ,ϕ ) = S 2 (θ ) cos 2 ϕ + S1 (θ ) sin 2 ϕ , 2

2

(1.1.16)

where Si(θ ) are the amplitude functions for the two lateral electric field components. For spherical particles the two complex amplitude functions depend only on the scattering angle θ and are functions of the size parameters x and the complex refractive index n~ [57Hul]. The efficiency factors can be calculated from:

Qsca =

π

1 dθ x 2 ∫0

{ S (θ ) 1

4 Qext = 2 Re{S (0)} . x

2

+ S 2 (θ )

2

} sin θ

,

(1.1.17)

(1.1.18)

For forward scattering the amplitude functions have the same value, i.e. S1(0) = S2(0) = S(0). Using the amplitude function F(θ ) = 1/2{⎜S1(θ )⎜2 + ⎜S2(θ )⎜2} the angular distribution of unpolarized light scattered by a particle can be calculated as well. In Fig. 1.1.15 examples of F(θ) for different size parameters according to [94Han] are shown; the incident light is at 180°. It is seen that the angular de-

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20

1.1.2 Energy coupling

[Ref. p. 62

pendence of the scattering is a strong function of the particle size parameter x. The angular dependence of the scattering becomes very strongly forward-directed for the range of x >> 1. This suggests that strong forward scattering of laser radiation may occur during laser cladding with blown particles, where powders with typically d > 40 µm are applied. Small particles, which appear during penetration welding, will scatter laser radiation away from the beam axis and in some cases, back towards the laser [94Han]. 400 300 200

120

90

60

150

x = 5.928

30

100 0 100 200

180

0

330

210

300 400

240

120

270

90

300

60

x = 1.186

60

30

0

180

330

210 240

120

270

90

300

x = 0.593 60

0.075 150

30

0.050

150

30

0.025 180

0

0

180

0

0.025

1 2

90

150

0.100

1 0

0.25 0.50 0.75 1.00 1.25

120

x = 1.778

3 2

1.25 1.00 0.75 0.50 0.25 0

330

210

3 240

270

300

0.050 0.075 0.100

330

210 240

270

300

Fig. 1.1.15. Angular distribution F(θ) of the light scattered by Fe-particles at λ = 10.6 µm for four different size parameters according to [94Han].

For particular values of the size parameter x and the refractive index n~ , the Mie theory can be simplified. In Fig. 1.1.16 the two main limiting cases are shown. For very small particles (x << 1), the Rayleigh approximation can be applied and the power of the scattered light occurs in terms of order d 6 . For large particles (x >> 1) the power of the scattered light is proportional to the geometrical cross section of the particle and the geometrical-optics approach can be applied. In between these two limiting cases the more complex Mie theory has to be applied [87Ruc].

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

~d

21

2

log ( PS )

Mie theory Rayleigh approximation

~d

geometrical optics approximation

6

small

medium

Fig. 1.1.16. Power of the scattered light PS =

very large

∫ I dA versus A

particle diameter [87Ruc].

log ( d )

For welding and cutting with Nd:YAG- and CO2-lasers the particles fulfill the conditions underlying the Rayleigh approximation. In that case the absorption and scattering cross sections are given by [83Boh]

⎧⎪ m 2 − 1 ⎡ 4 x 3 n~ 3 ⎛ m 2 − 1 ⎞⎤ ⎫⎪ 1 ⎟⎟⎥ ⎬ ⇒ Cabs ∝ , Im ⎜⎜ 2 Cabs = d 2 x n~ Im⎨ 2 ⎢1 + λ 3 ⎪⎩ m + 2 ⎣ ⎝ m + 2 ⎠⎦ ⎪⎭ C sca

2 d 2 4~4 m2 −1 = x n 3 m2 + 2

2

⇒ C sca ∝

1 λ4

(1.1.19)

(1.1.20)

with the relative refractive index m

n~ , m= ~ nM

(1.1.21)

where n~ and n~M are the refractive indices of the particle and the medium, respectively. The scattering cross section in (1.1.20) is proportional to d 6 as shown in Fig. 1.1.16. If (m2–1)/(m2+2) is only weakly dependent on wavelength (which is not true e.g. for metallic particles) then the radiation scattered by a sphere under the condition of small x is proportional to 1/λ4. This approximation can be used for sufficiently small spheres with a refractive index not strongly dependent on wavelength over the region of interest [83Boh]. As can be seen from (1.1.19) and (1.1.20), scattering is increasing stronger than absorption for decreasing wavelengths. Depending on the properties of the particles the dominance of one mechanism over the other can change. For particle sizes in a region in between the two limiting cases, Rayleigh and geometrical optics, rather complex formulas have to be applied. Near the transition to the geometrical-optics approach the Mie theory yields the efficiency factor for extinction (the limiting value)

Qext = 2 ,

(1.1.22)

which corresponds to a cross section twice as large as its geometrical area. This paradox that, according to the Mie theory, a large particle removes twice the energy that is incident on it, is a consequence of some simplifying assumptions [83Boh]. Geometrical optics leads to value of exactly Qext = 1. In the process of laser cladding and alloying the laser beam may interact with a stream of powder in some region above the workpiece surface. This interaction leads to an attenuation of the energy flux

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22

1.1.2 Energy coupling

[Ref. p. 62

within the beam, a change in absorptivity as particles come to lie on the surface and to a heating of the particles due to the absorbed part of energy before they hit the workpiece surface. For the process of laser cladding and alloying depending on the particle diameter, refractive index and wavelength the Mie theory or geometrical-optics approach can be applied. The efficiency factors for Fe and Cu particles are calculated e.g. in [89Lag] according to the Mie theory. The considered wavelength is 10.6 µm and the powder diameters are between 5 and 115 µm. With this data one can estimate the extinction during the laser cladding process where usually powder diameters are between 40 and 150 µm. The assumed refractive index for Fe is n~ = 5.95 – 32i and for Cu n~ = 9 – 65.9i, respectively. As shown in Table 1.1.8 the scattering at Fe and Cu particles is dominating over absorption, in this case. Table 1.1.8. Efficiency factors for absorption and scattering for Fe and Cu particles at λ = 10.6 µm [89Lag]. Iron (Fe) d [µm] 5

15

25

35

45

55

65

75

85

95

105

115

Qabs

0.0468 0.0377 0.0350 0.0330 0.0330 0.0326 0.0322 0.0320 0.0318 0.0317 0.0316 0.0316

Qsca

2.2311 2.1534 2.1049 2.0779 2.0617 2.0505 2.0435 2.0377 2.0328 2.0290 2.0270 2.0270

Copper (Cu) d [µm] 5

15

25

35

45

55

65

75

85

95

105

115

Qabs

0.0169 0.0135 0.0126 0.0121 0.0119 0.0326 0.0116 0.0115 0.0114 0.0114 0.0113 0.0113

Qsca

2.1810 2.1393 2.0953 2.0718 2.0568 2.0505 2.0409 2.0358 2.0319 2.0287 2.0271 2.0271

1.0

1.0

0.9

0.9

0.8

0.7

0.6 0.5

a

Transmission T [%]

Transmission T [%]

Using the extinction factors for Fe following from Table 1.1.8 the transmission through a powder stream applied using a lateral nozzle was calculated in [93Gas], the results are presented in Fig. 1.1.17a. In Fig. 1.1.17b, the transmission through a stellite powder stream is seen which has been calculated according to [92Mar] by a geometrical approach. The attenuated power is calculated by assuming that the laser beam and powder stream are two intersecting cylinders. The results show that the attenuation is proportional to the powder feed rate and inversely proportional to the powder size. One can see in Fig. 1.1.17 that both methods yield similar results of the transmission.

40

5 g/min 10 g/min 15 g/min 20 g/min 25 g/min

100 60 80 Particle diameter d [µm]

0.8

0.7

0.6 0.5 40

120

b

5 g/min 10 g/min 15 g/min 20 g/min 25 g/min

100 60 80 Particle diameter d [µm]

120

Fig. 1.1.17. Dependence of the transmission through a powder gas stream on the particle size and powder feed rate at λ = 10.6 µm by using (a) Fe powder [93Gas] and (b) stellite powder [92Mar].

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

23

In [03Kin] and [96Haa] approaches were made for studying the absorption of particles, primarily. Starting from the described dependence of the Fresnel absorption on the angle of incidence and the orientation of the polarization of the laser light in Sect. 1.1.2.1, the average absorptivity of a sphere has been calculated in [03Kin]. For the calculation it has been assumed that a collimated laser beam is incident on the spherical particles. Taking into account the angle of incidence at the surface of the sphere, the total absorptivity of the sphere can be calculated. In Table 1.1.9 the absorptivity of a plane surface for perpendicular incidence of the beam is compared to the average absorptivity of a sphere. It can be seen, that the absorptivity is higher in the case of the sphere than at a plane surface [03Kin]. Table 1.1.9. Calculated absorptivity of Fe and Al for wavelengths of 1.064 µm and 10.6 µm [03Kin]. Wavelength

1.064 µm (Nd:YAG)

10.6 µm (CO2)

Substrate

Fe

Fe

Al

Al

Temperature [°C]

25

1227

25

1227

25

1227

25

1227

Absorptivity on a plane surface [%]

36.0

31.2

5.7

11.3

3.3

12.1

2.3

5.6

Absorptivity on a sphere [%]

37.7

33.1

6.6

13

4.1

14.4

3.0

7.0

Percentile changing [%]

4.9

6.1

15.6

15.6

25.6

19.0

28.3

24.8

In [96Haa] the absorption characteristic of a metal powder bed under CO2-laser irradiation was investigated. Calorimetric absorption measurements were carried out for different powder materials Al, Cu, Fe and Ti3Al with a particle size range between 75 µm and 200 µm. The experimental results showed that no dependence between absorption and particle size could be observed. The measured absorptivity at low laser intensities of the order of 1 to10 W/cm² ranged between 28 % and 43 %. The Ti3Al powders lead to the highest absorptivity, the Al powders to the lowest. As laser intensity was increased (laser power up to 230 W), the Cu and Fe powders showed strong signs of oxidation when irradiated in air (Fig. 1.1.18). Along with the oxidation, absorptivity of these powders rose up to 70 %. Neither oxidation nor increased absorptivity was observed when helium was used as shielding gas. 100

100

Copper

Iron 90

80

Absorptivity A [%]

80

Absorptivity A [%]

90

air argon helium

70 60 50

70 60 50

40

40

30

30 20 0

20 0

a

50

100 200 150 Laser power P [W]

250

b

air argon helium

50

100 200 150 Laser power P [W]

250

Fig. 1.1.18. Absorptivity at 10.6 µm versus laser power for (a) Fe and (b) Cu particles with a particle size < 100 µm [96Haa].

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24

1.1.2 Energy coupling

[Ref. p. 62

1.1.2.5 Non-linear absorption In the previous sections only “linear” absorption has been considered where the index of refraction is basically independent of laser intensity. With increasing electric field in the material also non-linear absorption mechanisms may become important. In this case the energy coupling will be a function of intensity. Two basic non-linear absorption mechanisms are impact ionization and photoionization like multiphoton and tunnel ionization. Impact ionization is the inverse process of Auger recombination. Collisons of high energetic free or conduction electrons with valence electrons can yield additional low-energetic conduction electrons. In a sufficiently high electric field these electrons will then be accelerated again such that upon the next collision the process can be repeated. This will increase the density of conduction electrons exponentially if losses due to recombination can be compensated. The resulting electron avalanche leads to a heavily absorbing microplasma and finally to an explosive material damage known as “optical breakdown” even in the case of initially weakly absorbing dielectrics. Impact ionization is almost independent of laser wavelength [87All1]. Such an avalanche is only possible if initially a sufficient density of seed electrons is available. For metals this is always true whereas in dielectrics conduction electrons have to be generated first. Such seed electrons can be provided e.g. either by energetically unfavorable but statistically possible highly excited electrons or by defects and impurities [98Shi]. This would result in a stochastic nature of material damage concerning both threshold and localization, which, in fact, has been observed experimentally [97Liu]. Another possible mechanism for seed electron generation in dielectrics is multiphoton absorption. Due to the lack of conduction electrons, insulators can only absorb photons by the electronic system at deep temperatures if the provided energy is larger than the bandgap. For typical laser wavelengths this is only possible if several photons are simultaneously absorbed. Since the probability for this process decreases rapidly with the number of necessary photons, multiphoton absorption is only important for very high intensities. In contrast to impact ionization it shows a strong dependence on laser wavelength and is nonstochastic. Furthermore, this absorption mechanism is basically instantaneous in time whereas typical build-up times for electron avalanches in insulators are of the order 0.1…1 ns [87All1]. For pulse lengths longer than about 1…10 ps, impact ionization alone is sufficient to explain the onset of material damage in insulators and plasma breakdown [99Tie, 96Stu, 98Ret, 96Du, 93Maz]. If pulse lengths are too short to induce optical breakdown by impact ionization from background carriers alone, multiphoton absorption can provide the necessary additional seed electrons. This behavior is confirmed in Fig. 1.1.19, which shows calculated values of fluence where material damage occurs (this threshold is a measure for the onset of effective absorption) as a function of pulse duration together with the contributions of impact and photoionization only. The observation of significant multiphoton absorption can be shifted towards shorter pulse lengths for higher background carrier density or more frequent electron collisions [99Tie, 96Du]. These theoretical predictions are in good agreement with experimental observations as shown in Fig. 1.1.20 for SiO2. For longer pulses the ablation threshold is roughly proportional to the square root of the pulse duration, which has also been predicted by simplified models [95Nie].

Lando lt -Bö rnst ein New Ser ies VIII/1C

1.1 Fundamentals of laser-induced processes impact ne = 1015 cm-3 impact ne = 2.2 × 1012 cm-3 impact ne = 10 8 cm -3 multiphoton total

Fluence [J cm-2 ]

10

1 10 -14

10 -11 10 -13 10 -12 Pulse duration tH [s]

10 -10

Fig. 1.1.19. Damage threshold as a function of laser pulse length τH (measured at FWHM) for different background electron densities ne in SiO2 [99Tie]. Contributions of impact and multiphoton ionization have been denoted by symbols and dashed line, respectively.

Landolt-Börnstein New Series VIII/1C

100 Fluence [J cm-2 ]

Ref. p. 62]

25

Du et al. (800 nm) Stuart et al. (1053 nm) Lenzner et al. (800 nm) Tien et al. (800 nm)

10

1 10 -15 10 -14 10 -13 10 -12 10 -11 10 -10 10 - 9 10 - 8 Pulse duration tH [s]

Fig. 1.1.20. Experimental results for damage threshold in SiO2 taken from various references [99Tie, 96Stu, 96Du, 98Len].

62

References for 1.1

References for 1.1 (1/2) 1900Dru

Drude, P.: Ann. Physik 1 (1900) 566.

1903Hag Hagen, E., Rubens, H.: Ann. Physik 11 (1903) 873. 47Pri

Price, D.J.: Proc. Phys. Soc. London 47 (1947) 131.

55Rob

Roberts, S.: Phys. Rev. 100 (1955) 1667.

57Hul

Hulst, H. van de: Light Scattering by Small Particles. New York: Wiley, 1957.

59Rob

Roberts, S.: Phys. Rev. 114(1) (1959) 104.

62Eub 62Lan

Eubanks, A.G., Moore, D.G., Pennington, W.A.: J. Electrochem. Soc. 109 (1962) 211. Landolt, H., Börnstein, R.: 6. Aufl., Berlin: Springer, 1962.

65Seb

Seban, R.A.: J. Heat Transfer (1965) 173.

68Aga

Agababov, S.G: High Temperatures – High Pressures 6(1) (1968) 76.

69Mil

Miller J.C.: Phil. Mag. 20 (1969) 1115.

71Ash

Ashcroft, N.W., Sturm, K.: Phys. Rev. 3 (1971) 1898.

72Com 72Tou 72Uji

Comins, N.R.: Phil. Mag. 25(4) (1972) 817. Touloukian, Y.S., DeWitt, D.P.: The Macmillan Company, New: IFI/Plenum, 1972. Ujihara, K.: J. Appl. Phys. 43 (1972) 5.

75Jac

Jackson, J.D.: Classical Electrodynamics, 2nd ed., New York, NY: John Wiley & Sons, 1975.

76Wie

Wieting, T., Schriempf, T.: J. Appl. Phys. 47 (1976) 4009.

79Neu 79Tan

Wörner, B., Neuer, G.: High Temperatures – High Pressures 11(4) (1979) 383. Tanaka, T.: Jap. J. Appl. Phys. 18(6) (1979) 1043.

80Dec

Decker, D.L., Hodgkin, V.A.: Laser Induced Damage in Optical Materials 1980, Proceedings of a Symposium (NBS-SP-620), Washington, DC: NBS (1981) 190. Isetti, C., Nannei, E.: High Temperature 12 (1980) 307. Löfving, S.: Appl. Phys. Lett. 36(8) (1980) 632.

80Ise 80Loe 81Wea

Weaver, J.H., Krafka, C., Lynch, D.W., Koch, E.E.: Karlsruhe: Fachinformationszentrum Energie – Physik – Mathematik, 1981.

82Bar

Barbarino, S., Grasso, F., Guerriera, G., Musumeci, F., Scordino, A., Giulitti, D., Lucchesi, M.: Lettre al Nuovo Cimento 33(13) (1982) 417. Elson, J.M., Sung, C.C.: Appl. Opt. 21(8) (1982) 1496. Karlsson, B., Ribbing, C., Roos, A., Karlsson, T.H.: Physica Scripta 25 (1982) 826.

82Els 82Kar

References for 1.1 83Boh

63

83Pop

Bohren, C.F., Huffmann, D.R.: Absorption and Scattering of Light by Small Particles, New York: Wiley, 1983. Ordal, M.A., Long, L.L., Bell, R.J., Bell, S.E., Bell, R.R., Alexander, R.W., Jr., Ward, C.A.: Appl. Opt. 22(7) (1983) 1099. Popova, O.R.: High Temperature 21(2) (1983) 199.

84Arn

Arnold, G.: Appl. Opt. 23(9) (1984) 1434.

85Ord

Ordal, M.A., Bell, R.J., Alexander, R.W., Jr., Long, L.L., Querry, M.R.: Appl. Opt. 24(24) (1985) 4493. Palik, E.D.: Handbook of Optical Constants of Solids, Vol. 1, Orlando, FL: Academic Press, 1985.

83Ord

85Pal 86Bor

Born, M., Wolf, E.: Principles of Optics, 6th ed., Oxford: Pergamon Press, 1986.

87All1

Allmen, M. van: Laser-Beam Interactions with Materials, Materials Science 2, Berlin: Springer, 1987. Ruck, B.: Laser-Doppler-Anemometrie: Eine berührungslose optische Strömungsgeschwindigkeitsmeßtechnik, Stuttgart: AT-Fachverlag, 1987.

87Ruc 89Bru 89Lag

Brückner, M., Schäfer, J., Uhlenbusch, J.: J. Appl. Phys. 66(3) (1989) 1326. Lagain, P.: Contribution experimentale aux traitements de surface par laser avec apport de poudre, Ph.D. thesis, Université Aix-Marseille, France, 1989.

90Kri

Krishnan, S., Hansen, G.P., Hauge, R.H., Margrave, J.L.: High Temperature Science 29 (1990) 17. Prokhorov, A.M., Konov, V.I., Ursu, I., Mihailescu, I.N.: Laser heating of metals, Bristol: Hilger, 1990. Stern, G.: Proc. 3rd European Conf. on Laser Treatment of Materials ECLAT '90 (Erlangen, Germany). In: Bergmann, H., Kupfer, R. (eds.): Coburg: Sprechsaal Publ. Group (1990) 25.

90Pro 90Ste 91Fre 91Pal 91Yil

Frenk, A., Hoadley, A., Wagniere, J.: Metallurgical Transactions B 22B (1991) 139. Palik, E.D.: Handbook of Optical Constants of Solids, Vol. 2, Orlando, FL: Academic Press, 1991. Yilbas, B.S., Danisman, K., Yilbas, Z.: Meas. Sci. Technol. 2 (1991) 668.

92Mar

Marsden, C. F., Frenk, A., Wagniere, J.-D.: In: Mordike, B.L. (ed.): Laser Treatment of Materials, ECLAT '92, Oberusel: DGM Informationsgesellschaft Verlag (1992) 375.

93Dau 93Gas

Dausinger, F., Shen, J.: ISIJ International 30(9) (1993) 925. Gasser, A.: Oberflächenbehandlung metallischer Werkstoffe mit CO2-Laserstrahlung in der flüssigen Phase, Ph.D. thesis, TH Aachen, Germany, Wissensschaftsverlag, 1993. Mazhukin, V.I., Gusew, I.V., Smurov, I., Flamant, G.: In: Denney, P., Miyamoto, I., Mordike, B.L. (eds.): Proc. Laser Materials Processing Conf. ICALEO '93 (Orlando, FL), Orlando, FL: LIA 77 (1993) 213. Pedrotti, F.L., Pedrotti, L.S.: Introduction to optics, 2nd ed., Englewood Cliffs, NJ: PrenticeHall International, 1993. Stern, G.: Absorption kontinuierlicher CO2- und Nd:YAG-Laserstrahlung durch verschiedene Metalllegierungen, Anwendungen im Bereich der Laserbehandlung, Report for European Project EUREKA EU194, 1993.

93Maz 93Ped 93Ste

64 94Han 94Hue 94Jun 94Kar 95Dau 95Nie 95Ste

References for 1.1 Hansen, F., Duley, W.W.: J. Laser Appl. 6(3) (1994) 137. Hüttner, B.: J. Phys.: Condens. Matter 6 (1994) 2459. Jung, R.: Untersuchungen zur Erzeugung gleichmässiger Oxidschichten und deren Einfluss auf den Absorptionsgrad beim Festphasenhärten mit Laserstrahlung, Diploma thesis, Fraunhofer Institut für Lasertechnik, RWTH Aachen, Germany, 1994. Kar, A., Mazumder, J.: Phys. Rev. E 49(1) (1994) 410. Dausinger, F.: Strahlwerkzeug Laser: Energieeinkopplung und Prozeßeffektivität. Habilitation thesis, Univ. of Stuttgart, Germany. Stuttgart: B.G. Teubner Verlag, 1995. Niemz, M. H.: Appl. Phys. Lett. 66 (1995) 1181. Stern, G.: Société Française des Mécaniciens SFM (ed.): Proc. Deutsch-Französische Tagung Mechanik und Optik: Hochleistungslaser im Maschinenbau (Saint-Louis, France), Courbevoie: SFM (1995) 393.

96Du 96Haa 96Kel 96Sch1 96Stu

Du, D., Liu, X., Mourou, G.: Appl. Phys. B 63 (1996) 617. Haag, M., Hügel, H., Albright, C.E., Ramasamy, S.: J. Appl. Phys. 79(8) (1996) 3835. Kelly, R., Miotello, A.: Appl. Surf. Sci. 96–98 (1996) 205. Schittenhelm, H., Callies, G., Berger, P., Hügel, H.: J. Phys. D: Appl. Phys. 29 (1996) 1564. Stuart, B.C., Feit, M.D., Hermann, S., Rubenchik, A.M., Shore, B.W., Perry, M.D.: Phys. Rev. B 53 (1996) 1749.

97Gar

Garnov, S.V., Konov, V.I., Tsarkova, O.G., Dausinger, F., Raiber, A.: Proc. SPIE 2966 (1997) 149. Liu, X., Du, D., Mourou, G.: IEEE J. Quant. Electron. 33 (1997) 1706. Mayerhofer, R.: Mikromaterialbearbeitung mit Kupferdampflasern: Prozeßcharakterisierung und Werkstoffabhängigkeit des Abtrags, Ph.D. thesis, Univ. of Erlangen-Nürnberg, 1997. Nanai, L., Vajtai, R., George, T.: Thin Solid Films 298 (1997) 160.

97Liu 97May 97Nan 98Cal 98Len 98Luk 98Ret 98Sei 98Shi 99Rai 99Sei 99Tie

Callies, G., Schittenhelm, H., Berger, P., Hügel, H.: Appl. Surf. Sci. 127–129 (1998) 134. Lenzner, M., Krüger, J., Sartania, S., Cheng, Z., Spielmann, C., Mourou, G., Kautek, W., Krausz, F.: Phys. Rev. Lett. 80 (1998) 4076. Luk’yanchuk, B.S., Marine, W., Anisimov, S.I.: Laser Physics 8(1) (1998) 1. Rethfeld, B., Kaiser, A., Vicanek, M., Simon, G.: Proc. SPIE 3343 (1998) 388. Seibold, G., Brandner, M., Dausinger, F., Hügel H.: In: Mordike, B.L. (ed.): Proc. European Conf. on Laser Treatment of Materials ECLAT '98 (Hannover, Germany), Frankfurt: Werkstoff-Informationsgesellschaft (1998) 189. Shirk, M.D., Molian, P.A.: J. Laser Appl. 10 (1998) 18. Raiber, A.: Grundlagen und Prozeßtechnik für das Lasermikrobohren technischer Keramiken, Ph.D. thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1999. Seibold, G., Dausinger, F., Hügel, H.: In: Kujanpää, V., John, I. (eds.): 7th Nordic Conference in Laser Processing of Materials NOLAMP (Lappeenranta, Finland), Acta Universitatis Lappeenrantaensis 84 (1999) 526. Tien, A.-C., Backus, S., Kapteyn, H., Murnane, M., Mourou, G.: Phys. Rev. Lett. 82 (1999) 3883.

00Sch1

Schellhorn, M.: CO-Hochleistungslaser: Charakteristika und Einsatzmöglichkeiten beim Schweißen, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2000.

03Kin

Kindler, H.: Optische und gerätetechnische Entwicklungen zum Laserspritzen, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2003.

Ref. p. 62]

1.1 Fundamentals of laser-induced processes

25

1.1.3 Thermophysical and dynamical “response” 1.1.3.1 Condensed matter 1.1.3.1.1 Heat conduction As has been described in Sect. 1.1.2 in detail, laser energy is initially absorbed only by electrons leading to a non-thermal electronic distribution. Due to subsequent electron-electron collision the electrons then return to a thermal Fermi distribution within a characteristic relaxation time of the order τe ≈ 100 fs [98Ret]. Energy transfer to the lattice by electron-phonon interaction is usually much slower. Only for timescales considerably longer than the electron-phonon relaxation time τph, the system can be described by just one temperature value and classical heat conduction. For shorter laser pulses, at least the temperatures for electrons and phonons have to be regarded separately and, for pulse durations of the order of electronic relaxation times or shorter, it is not possible to characterize the system by any thermal distribution. 1.1.3.1.1.1 Fourier heat conduction The classical Fourier heat conduction can be applied for times of interest longer than about 100 ps because in such cases temperature equilibrium between electrons and phonons is reached (see also Table 1.1.12). The continuity equation for the heat flux can then be written in terms of the local material temperature T(x, t):

ρcp

∂ T ( x, t ) − ∇ ⋅ (λ th ∇T ( x, t ) ) = q& ( x, t ) . ∂t

(1.1.23)

Here ρ, cp, and λth represent density, specific heat capacity, and thermal conductivity respectively whereas q& has been included to account for a volumetric heat source. Together with the boundary condition

λ n ⋅ ∇TS = (1 − R ) I 0 − hc (TS − T∞ ) − ε σ (TS4 − T∞4 ) Landolt-Börnstein New Series VIII/1C

(1.1.24)

26

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

at the surface and an initial temperature distribution, this equation is sufficient to describe the transient temperature within a material. Herein n denotes a unit surface normal vector, ε the emissivity factor, σ the Stefan-Boltzmann constant, and T∞ is the ambient temperature. In this equation a surface heat source of intensity I0 has been included taking a reflectivity R at the surface into account. Typical values of intensities used in laser materials processing are in the order of 104…1014 W cm–2. Losses due to heat convection and re-radiation at the surface as described by the last two expressions in (1.1.24) can be roughly estimated: Using a typical convective heat transfer coefficient for a sonic jet of hc ≈ 200 W m–2 K–1, a difference of 3500 K between surface temperature TS and the flow would lead to a surface cooling intensity of 70 W cm–2 [97Mod]. On the other hand a black body of the same surface temperature would emit about 850 W cm–2 [70Chu]. Both fluences are considerably lower than the absorbed values and can, therefore, be neglected for most applications. Since (1.1.23) is linear in temperature for constant thermophysical properties it can be solved for many cases analytically [90Car, 72Dab, 96Eng]. A frequently used expression is the one-dimensional case for constant laser intensity, which can be either applied as a surface heat source as in (1.1.24) or a volumetric source according to Lambert-Beer’s law of absorption: q ( z , t ) = α (1 − R ) I 0 e −α z .

(1.1.25)

Due to the small values of lα both descriptions, the surface and the volume heat source, yield basically the same result for metals while for insulators (1.1.25) should be used. Depending on the ratio between the thermal diffusion length lth = 2 κ t and the absorption length lα = α–1 (where κ = λth/(ρcp) represents the thermal diffusivity) l th ≈ 2α κ t lα

(1.1.26)

one obtains for the time-dependent surface temperature: ⎧ 2 (1 − R) I 0 κ t ⎪ T∞ + ⎪ λth TS (t ) = ⎨ α ( 1 − R) I 0 ⎪ T∞ + t ρ cp ⎪⎩

for lth >> lα

,

for lth << lα

.

(1.1.27)

Here, the two important limiting cases for metals and insulators have been considered. For metals and pulse lengths of at least several nanoseconds the ratio of thermal to optical penetration depth (1.1.26) is usually much larger than unity since typical values of thermal diffusivities κ are of the order of 10–5 m2 s–1. Values for the absorption length lα for metals can be found in Table 1.1.2. On the other hand, the temperature distribution in dielectrics is dominated by Lambert-Beer’s law rather than heat conduction for timescales smaller than some 10 µs. In this case the surface temperature is initially only a function of absorbed energy density. In Tables 1.1.10 and 1.1.11 typical thermophysical data of selected metals and semiconductors and of selected ceramics, respectively, at room and melting temperature can be found. A one-dimensional description of the heating process is only valid if the lateral dimensions of a rather homogenous heat source are much larger than the penetration depth. Otherwise the heat conduction equation (1.1.23) has to be solved including lateral heat losses. If laser beams with Gaussian or top-hat spatial distribution of radius w and peak intensity I0 are incident on a metal surface the maximum temperature in the center of the laser spot is given by [00Sch1] where ierfc(x) denotes the indeterminate integral over the complementary error function: TSGauss (t ) = T∞ + TSTop − Hat (t ) = T∞ +

⎛ 2 2κ t ⎞ (1 − R) I 0 w ⎟ arctan⎜ ⎜ w ⎟ λth 2 ⎝ ⎠ 2 (1 − R) I 0 λth

κ t ⎧⎪ ⎨1 − ⎪⎩

,

⎛ w ⎞ ⎫⎪ ⎟ ierfc⎜ ⎜ 2 κ t ⎟ ⎬⎪ ⎝ ⎠⎭

(1.1.28) .

Lando lt -Bö rnst ein New Ser ies VIII/1C

Ref. p. 62]

1.1 Fundamentals of laser-induced processes

27

Table 1.1.10. Typical thermophysical data of selected metals and semiconductors at room and melting temperature taken from [92Smi, 88Wyb]. Material

Al

Cu

Fe

Si

melting temperature Tm [K]

933

1356

1809

1683

boiling temperature Tb[K]

2793

2833

3133

–1

–1

5

5

3543 5

1.8×106

latent heat of melting Lm [J kg K ]

3.9×10

latent heat of evaporation Lv [J kg–1 K–1]

1.1×107

4.8×106

6.1×106

1.4×107

2700

8960

7870

2340

2385

8000

7015

2510

238

394

78

139

density ρ [kg m ] at RT –3

density ρ [kg m ] at Tm –3

thermal conductivity λth [W m K ] at RT –1

–1

2.1×10

2.7×10

thermal conductivity λth [W m K ] at Tm

94

166

30

22

spec. heat capacity cp [J kg–1 K–1] at RT

904

389

450

729

spec. heat capacity cp [J kg–1 K–1] at Tm

1086

495

750

–1

–1

viscosity η [N s m ] at Tm –2

1.3×10

surface tension σ [N m ] at Tm –1

–3

0.914

4×10

–3

1.285

5.5×10

1040 –3

1.872

9.4×10–4 0.865

Table 1.1.11. Typical thermophysical data of selected ceramics at room and melting temperature taken from [92Smi, 61Bar, 90Kos, 70Stu, 99Rai, 97Whi]. In general, data of these materials may vary considerably with composition. Material

AlN

Al2O3

SiC

decomposition temperature Td [K]

2500

2320

2970

–1

–1

latent heat of decomposition Ld [J kg K ] density ρ [kg m ] at RT –3

1.7×10 3260

6

1.1×10 3970

6

1.6×10 3170

Si3N4 2170 6

3200

thermal conductivity λth [W m K ] at RT

90

39

42

35

spec. heat capacity cp [J kg–1 K–1] at RT

737

781

676

713

spec. heat capacity cp [J kg–1 K–1] at Td

1246

1385

1400

1325

–1

–1

Figure 1.1.21 shows the dependence of absorbed laser intensity on the time necessary to reach boiling temperature for metals. For Gaussian and top-hat beams a deviation to the one-dimensional model can be observed for small radii w or long interaction times. Due to the assumption of temperature-independent material properties and the uncertainty in some of these data, values obtained by (1.1.28) should only be regarded as estimates. Since for typical beam radii the deviations occur only for pulse lengths longer than several microseconds, the one-dimensional case yields good results for dielectrics within the validity of the assumption that thermal diffusion can be neglected compared to optical penetration. Here, heating times for a given absorbed intensity are typically two or three orders of magnitude longer than in the case of metals.

Lando lt -Börnst ein New Ser ies VIII/1C

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

A

R I

28

−7

−8

−9

−6

−5

H

−4

−3

−2

t

Fig. 1.1.21. Absorbed laser intensity as a function of time to reach boiling temperature for Gaussian and top-hat beams with different beam radius w and onedimensional limit. For Cu and Al only the situation for a Gaussian profile and w = 10 µm is shown; all other curves correspond to Fe. Constant material properties at room temperature have been assumed.

A general solution to (1.1.23) in an infinite medium can be expressed in terms of Green’s function for a point heat source [90Car]: T (x, t ) = T∞ + ∫

t t ’= 0

∫∫∫ 8ρ c

2 ⎞ ⎛ ⎜ − x − x’ ⎟ d 3 x’ dt ’. exp 3 ⎜ 4κ (t − t ’) ⎟ κ (t − t ’) ⎠ ⎝

q (x’, t ’) p

(1.1.29)

Isothermal boundary conditions can be introduced by applying the method of image heat sources, further simplifications are possible for symmetric heat sources [72Pae]. Equation (1.1.29) has been used by many authors e.g. to describe heat distribution for welding [41Ros, 73Swi, 88Ste, 00Cha]. It is a very powerful method to solve heat conduction for arbitrary spatial heat sources. This is demonstrated in Fig. 1.1.22, which shows a comparison of experiment and calculation (in the same scale) for the case of welding with the dual focus technique in AlMgSi1. 0

Depth [mm]

1 2 3 4 5 -4 a

-3

-2

-1 0 1 Distance [mm]

2

3

4 b

Fig. 1.1.22. (a) Cross section of calculated heat affected zone and melt pool geometry and (b) experimental result for laser welding of AlMgSi1 with dual focus technique (Nd:YAG Laser with two foci of 2 kW 1 mm apart and welding speed 2 m/min) [00Cha].

1.1.3.1.1.2 Two-temperature model As mentioned above, the results of the previous section are only valid as long as lattice and electrons are in thermal equilibrium with each other. With growing interest in shorter laser pulses for precise machining, several models adequate for the description of picosecond laser pulse interaction have been proposed [74Ani, 93Qiu, 92Kar1, 97Koe, 97Maz, 96Hue]. Common among these is the description of the solid by separate temperatures Tph and Te for lattice and electrons respectively. Both subsystems itself are assumed to be thermalized which is only justifiable if the pulse duration is much longer than the electronic relaxa-

Lando lt -Bö rnst ein New Ser ies VIII/1C

Ref. p. 62]

1.1 Fundamentals of laser-induced processes

29

tion time τe. After introduction of a coupling constant G for the two subsystems, heat conduction can be described by a set of coupled rate equations C e (Te )

∂ Τe = −∇ ⋅ Qe − G (Te − Tph )+ q ∂t

∂ Τph

C ph (Tph )

∂t

= G (Te − Tph )

,

(1.1.30)

,

with Ce and Cph being the electron and the phonon heat capacity, respectively, and Qe the electronic heat flux. The lattice heat conduction has been neglected compared to the electronic one. Typical coupling constants for metals are listed in Table 1.1.12. Table 1.1.12. Relaxation times and transport properties for selected metals taken from [96Hue] and estimated using data from Table 1.1.10 and [86Kit, 76Ash, 87All2, 87All3, 83Zie]. Due to some crude assumptions on data these values should only be regarded as a rough orientation. Metal

Al

Cu

Fe

Pb

Nb

electron-electron relaxation time τe [ps]

0.067

0.467

0.030

0.377

0.0135

electron-phonon relaxation time τph [ps]

4.27

57.5

0.5

11.2

–3

5.7 × 10

–1

coupling constant G [W m K ] electron heat capacity to temperature ratio Ce/Te [J m–3 K–2] phonon heat capacity Cph [J m–3 K–1] electron thermal conductivity λe [W m K ] –1

–1

17

6 × 10

16

7 × 10

18

1.3 × 10

1.33 17

1.7 × 1018

127

93

700

163

767

2.43 × 106

3.45 × 106

3.5 × 106

1.45 × 106

2.26 × 106

238

401

78

38

54

There have been many discussions on the question of a proper choice for the electronic heat flux Qe [89Jos, 90Jos]. A diffusion-like expression

Qe = −λ th ∇Te

(1.1.31)

will lead to a parabolic differential equation which can have an unphysical infinite propagation speed. Models based on this assumption are usually called parabolic two-step models. Another approach uses a kind of relaxation formulation

Qe = − ∫

t

t ’=0

⎛ t − t ’⎞ λth ⎟⎟ ∇Te dt ’ exp⎜⎜ − τe ⎝ τe ⎠

(1.1.32)

guaranteeing finite heat propagation in a wave-like differential equation and is called hyperbolic two-step model. Both descriptions reduce to the classical one-step model if coupling constants are large and hence electronic relaxation times τe =

Ce G

(1.1.33)

short. In Fig. 1.1.23 experimental transient surface temperatures for femtosecond laser pulses are compared with predictions of the different theoretical models. The deviation from the classical approach is clearly observable.

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1.1.3 Thermophysical and dynamical “response”

N

30

T

t

[Ref. p. 62

Fig. 1.1.23. Transient surface temperature after irradiation of Au with a 96 fs laser pulse of 10 J m–2 fluence. Experimental results are compared to results obtained by parabolic (PTS) and hyperbolic (HTS) two-step and classical one-step (POS) heat conduction [93Qiu].

1.1.3.1.2 Phase transitions As the material is heated to high temperatures, it usually passes through several phase transitions like structural changes, melting and evaporation. Usually these are first order transitions with discontinuities in thermodynamic potentials leading to a latent heat L. Typical values for this quantity are listed in Tables 1.1.10 and 1.1.11.

1.1.3.1.2.1 Melting When crystalline material is heated beyond the melting point the long range atomic order is lost which can result in a considerable change in material properties such as absorptivity, thermal conductivity and heat capacity. A collection of thermophysical material data for temperatures beyond the melting respectively decomposition points can e.g. be found in [92Smi, 70Stu] and Tables 1.1.10 and 1.1.11. If constant material properties are applied, (1.1.27) can be used for an estimate of the time necessary to reach the melting point, and in a similar way also cooling rates can be deduced [90Car, 92Bas]. Figure 1.1.24 shows measured and calculated heating and melt duration times for silicon and germanium as a function of the deposited energy density. Here, a numerical code including temperature-dependent material properties has been employed. Depending on energy input, melt has sometimes been detected for several hundreds of nanoseconds after the 38 ns laser pulse. A simple estimate for the melt thickness in metals can be obtained from the thermal diffusion length lth (see (1.1.26)) with the laser pulse duration τH = t. As can be expected from this formula, melt thickness decreases for shorter laser pulses which is also confirmed in the numerical calculation of Fig. 1.1.25 for silicon where nano- and femtosecond pulses are compared. Again, for ultrashort pulses the considerations of the previous section have to be employed. Calculations show that contrary to the suggestion of (1.1.26) melt thickness can not be decreased infinitely by using shorter pulses which is depicted in Fig. 1.1.26 [97Koe].

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes 350

40 Si Ge

300

Melt duration [ns]

30

Melt onset [ns]

31

20

250 200 150 100

10

50 0 0

a

0.5 1.5 1.0 Energy density [J/cm 2 ]

0 0

2.0

b

0.5 1.5 1.0 Energy density [J/cm 2 ]

2.0

M

Fig. 1.1.24. Time for (a) melt onset and (b) duration for Si and Ge as a function of deposited energy density. Symbols correspond to measured data whereas lines represent results of a numerical simulation using temperaturedependent material properties (KrF-laser; 38 ns pulse width) [86Jel].

Fig. 1.1.25. Calculated dependence of melt thickness in Si on laser fluence for 7 ns and 100 fs pulse duration [97Liu].

M

F

P

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Fig. 1.1.26. Calculated melt depth in Cu as a function of pulse duration at a fluence value of H = 1 J cm–2 [97Koe].

32

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

If, simultaneously, material is ablated with a velocity u, then for constant laser intensity and long interaction times the temperature decreases in the one-dimensional case exponentially within the material yielding a melt thickness of d=

κ ⎛ TS − T∞ ⎞ , ln ⎜ ⎟ u ⎝ Tm − T∞ ⎠

(1.1.34)

where TS, Tm and T∞ are surface, melting and room temperature, respectively [76All]. Most industrially used ceramics decompose at high temperatures into a metallic melt layer and an evaporated volatile component. Si3N4 for example forms a silicon rich surface layer at about 2150 K, thereby loosing its characteristic dielectric material properties [98Ohm, 99Rai].

1.1.3.1.2.2 Evaporation When the lattice temperature, i.e. atomic velocity, is high enough to overcome cohesion, single atoms or clusters can be evaporated. Under the simplifying assumptions of an ideal gas, negligible volume of the liquid phase compared to the gaseous and temperature-independent latent heat of evaporation, the equilibrium vapor pressure above the material can be expressed according to the integral form of ClausiusClapeyron’s law [74Gob]

⎛ L p(T ) = pamb exp ⎜ − v ⎜ R Sp ⎝

⎡ 1 1 ⎤⎞ ⎢ − ⎥ ⎟⎟ , ⎣ Tb T ⎦ ⎠

(1.1.35)

where Tb and Lv represent boiling temperature at ambient pressure pamb and latent heat of evaporation, respectively. The specific gas constant RSp corresponds to the ratio of universal gas constant and molar mass. Under the assumptions of a thermal Maxwellian distribution of evaporated atoms on the surface and an ideal gas law the continuity of evaporated mass m per area A yields the Hertz-Knudsen formula for the mass flux density j [74Mar]: j (T ) =

p (T ) − pamb m . = ρ u =α A 2 RSpT

(1.1.36)

In most calculations the ambient pressure pamb is taken to zero corresponding to evaporation into an ideal vacuum. The assumption of a thermal Maxwellian distribution for the velocities of evaporated atoms just above the surface is, in general, not true since they will need some collisions to reach equilibrium first. This non-equilibrium zone of several mean free paths’ width is called „Knudsen layer“. After collisions some atoms will be scattered back to the surface reducing thereby the net evaporated mass flux which is accounted for in (1.1.36) by introducing a factor α smaller than unity. Macroscopic models have been proposed to estimate changes in thermodynamic properties across the Knudsen layer [68Ani, 79Kni]. Although this is, strictly speaking, not possible with an equilibrium based thermodynamics and some assumptions have to be introduced, nevertheless, the predictions show reasonable agreement with microscopic Monte Carlo simulations [91Sib]. Contrary to macroscopic models, molecular dynamical simulations are well suited for the description of the evaporation process itself. Although these calculations require still long computational times, they are at least applicable to ablation of small geometries on short time scales. Several results for ablation characteristics in various materials have been published [97Zhi1, 98Her1, 96Ohm]. Figure 1.1.27 shows a calculated example for the desorption of an organic solid after a laser pulse of 15 ps duration. At a certain threshold fluence a transition from pure surface evaporation yielding individual molecules to a volume process in which clusters are desorbed can be observed.

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

33

10

10

Small clusters (4 molecules or less)

80

3

Fraction [%]

Number of desorbed molecules

100

60

40

2

20

total yield monomers

Large clusters (10 molecules or more)

0 0.4

a

0.6

1.0 0.8 Fluence [mJ/cm 2 ]

1.2

0.4

b

0.6

1.0 0.8 Fluence [mJ/cm 2 ]

1.2

Fig. 1.1.27. (a) Desorption yield and (b) plume composition vs. fluence in a molecular dynamical simulation of the evaporation process for an organic solid (laser pulse length 15 ps) [97Zhi2].

Equation (1.1.36) describes evaporation for a superheated surface which sometimes is called normal vaporization. For slow heating it is also possible that within the melt bubbles form at special nucleation sites like inclusions. In the case of this so-called heterogeneous bubble nucleation or normal boiling the surface temperature remains almost fixed at boiling temperature [96Kel]. For shorter laser pulses this process is unlikely since bubble nucleation is slow at low superheating and the number of such nucleation sites is finite. Nevertheless, for very fast and high energy input a considerable superheating of the liquid is possible leading to homogeneous bubble nucleation: At temperatures of about 90 % of the critical temperature, instabilities within the liquid may produce very fast a significant number of vapor bubbles leading to what is called “phase explosion” or spinodal decomposition [74Mar, 96Kel]. Since for evaporation the attractive energy barrier corresponding to the latent heat has to be overcome by the atoms they will be slowed down and the surface is thereby cooled. This cooling is sometimes included by theoretical models in the surface boundary condition (1.1.24) which leads especially for transparent materials to a considerable superheating within the material compared to the surface [94Maz1]. Other authors have pointed out that extending the cooling over a distance as small as a few interatomic spacings would reduce this superheating considerably thereby making it, at least for metals, a minor effect [96Kel]. For long laser pulses the temperature distribution within the solid will reach, after the initial heating period, a steady state with respect to the moving ablation front, finally. In this case the absorbed power density AI is balanced by the one removed with evaporated atoms: A I = ρ u (TS ) {Lv + c p (TS − T∞ )} .

(1.1.37)

The expression in parenthesis on the right hand side represents the specific enthalpy of the evaporated atoms and can be extended to include latent heats for other phase transitions, kinetic energy of atoms or, when writing it as an integral over specific heat capacity cp, also temperature-dependent material properties. Figure 1.1.28 shows the dependence of this upper limit of ablation velocity u on absorbed intensity for several materials. The differences between them are rather small which is a consequence of (1.1.37) being independent of absorption length and thermal conductivity. This formula can be solved directly for the ablated mass flux if in a first order approximation enthalpy is evaluated at boiling temperature Tv rather than at the actual surface temperature TS [65Rea]. The result is depicted in Fig. 1.1.28 for the case of aluminum by a dashed line.

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1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

A

u

34

A

Fig. 1.1.28. Calculated stationary ablation velocities for Al, Cu, Fe and Si using (1.1.37). The dashed line denotes the linearized case for aluminum where the enthalpy in this equation has been evaluated always at boiling temperature.

1.1.3.1.3 Melt dynamics The existence of molten material in or around the interaction zone has consequences on both the energy balance and the resulting quality of the particular laser processing technique: For example in welding, the flow in the melt pool will primarily affect the energy flow by transporting thermal energy and modifying the keyhole geometry (as a consequence, of course, the seam geometry will be directly influenced). On the other hand, in drilling, especially at shorter pulse lengths, the primary concern will be questions regarding the exactness of the bore. Therefore, the actual role of melt dynamics has to be discussed in connection with the particular process; here, only few general remarks will be given.

1.1.3.1.3.1 Origin of driving forces During laser machining melt will be accelerated, deformed or expulsed due to several process forces acting on it. The most important among these are ablation or gas dynamical pressure as well as surface tension. The balance of these forces with hydro-dynamical pressures determines the characteristics of the melt flow. Several authors have measured and calculated the momentum transferred to the material during ablation [74Low, 73Het, 99Sia, 76Ste]. Figure 1.1.29 shows an example of experimental values for ablation of aluminum with a CO2 laser. Values for the peak pressure in the order of some 107 Pa depending on laser intensity are typical for this process which can also be deduced from the vapor pressure as described by (1.1.35). For a detailed theoretical description of momentum transfer a thorough examination of evaporation and gas expansion beneath the surface including backscattering and shock formation is necessary. Estimations of the ablation pressure may also be obtained by theoretical models describing the shock formation during ablation (see Sect. 1.1.3.2.3). Shear forces induced e.g. by assist gas are in the case of drilling probably negligible whereas for cutting they are necessary for melt removal [92Kar2, 98Zef]. Frictional forces due to ejected vapor jets from a weld keyhole are likely to accelerate the surface melt layer considerably [98Mat, 93Ber]. According to Laplacian’s formula, pressure due to surface tension is inversely proportional to the main local radius of curvature. In laser micromachining structures can be smaller than some 10 µm leading to considerable pressures of the order of 105 Pa. Typical curvatures occurring during laser welding are usually larger leading to of at least one order of magnitude smaller pressures [95Dum]. Since surface tension also shows considerable variations with alloy composition and temperature, such local gradients can induce additional acceleration which is called Marangoni effect and has been shown to be of importance for laser welding [99Fuh, 98Mat]. Due to the continuity of mass within a stationary melt pool, resulting hydrodynamic forces can be fairly large close to geometric obstacles like keyholes in welding. Here the flow of melt around the vapor capillary will lead to high fluid velocities at its side walls which has been demonstrated by calculations [95Dum, 92Bec]. Lando lt -Bö rnst ein New Ser ies VIII/1C

Ref. p. 62]

1.1 Fundamentals of laser-induced processes

35

220

Peak pressure [bar]

180

140

100

60

20 0

1

2 8

3 2

Initial power density [10 W/cm ]

Fig. 1.1.29. Peak pressure on aluminum target as a function of laser power density (CO2 laser with 100…700 J) [73Het].

In contrast to the so far mentioned forces, buoyancy and radiation pressure are for most laser applications negligible [98Mat, 96Olf].

1.1.3.1.3.2 Resulting effects in laser machining In the case of cutting and drilling a thorough melt removal is usually desirable for reasons of efficiency and precision. Melt ejection will even exceed material evaporation for laser drilling with longer pulses of low intensity. Figure 1.1.30 shows analytically calculated ablation rates for aluminum and iron as well as the contributions of melt and vapor using the stationary model of [76All]. Liquid expulsion will dominate at low intensities whereas vaporization takes over at higher. Similar results have also been obtained by other authors [87Cha]. Nevertheless, for shorter pulses melt acceleration cannot be neglected which is illustrated in the experiment depicted in Fig. 1.1.31. For short pulses evaporation dominates due to the fast heating compared to slower fluid acceleration whereas, here, for more than about 100 µs pulse duration almost 90 % of material is ejected by melt. Sometimes substantial melt ejection has not been observed experimentally for pulses shorter than milliseconds [92Bas]. In laser drilling melt from the hot central spot is accelerated towards the crater boundary forming recast which can been seen in the simulation result of Fig. 1.1.32. If the acceleration of melt near the surface is large, instabilities can lead to droplet formation [97Wil]. Experimentally observed droplets have typically a minimum speed of 1…10 m s–1 and appear after a time delay of some 100 µs [78Zhi, 71Kar]. In laser deep welding the ablation pressure is responsible for creating and maintaining an open vapor capillary. Several authors have examined the pressure balance between vapor, surface tension and hydrodynamic fluid flow around the vapor cavity to determine its form [95Dum, 94Kap, 76Kle, 92Bec]. Due to the material feed the keyhole is slightly asymmetric and inclined with respect to the beam axis. Figure 1.1.33 shows a calculated example for the transient evolution of the vapor capillary in pulsed welding [96Gri, 98Hue]. In cw-welding, the pressure inside the keyhole is of the order of several 100 mbar and can reach up to 1 bar during the formation of the capillary [93Ber, 95Dum, 98Fab]. The evaporative losses in common laser welding applications are rather small [96Bec1]. Simulations for high speed welding demonstrate that melt velocity is largest at the side of the keyhole where fluid has to flow around the cavity and that it can be considerably larger than welding speed itself. An example for such a calculation is shown in Fig. 1.1.34. Above a certain welding speed a fast jet towards the end of the weld pool is formed which is assumed to be responsible for process instabilities like humping [92Bec].

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36

1.1.3 Thermophysical and dynamical “response”

Removal rate [m s −1]

80

total vapor liquid

60

Al Fe

40

20

1×10 7 1×10 8 Absorbed intensity Al [ W cm−2 ]

Fraction of melt to total removal [%]

100

100

0 1×10 6

[Ref. p. 62

80

60

40

20

0

Fig. 1.1.30. Calculated total removal rate together with contributions from melt (dashed) and vapor (dotted) for Al and Fe.

Al Cu Mo Ni

100 300 400 200 Laser pulse duration [µs]

Fig. 1.1.31. Fraction of melt to total removal as a function of pulse duration (Nd-Glass laser with 107 W cm–2) [70Chu].

Depth [µm]

50

0

− 50

−100

0

50

200 150 100 Hole radius R [µm]

250

Fig. 1.1.32. Numerical simulation of a hole profile for drilling of Hastelloy-X with a single microsecond pulse of Gaussian temporal and spatial distribution after 200 µs (3.5×109 W m–2 peak intensity, 70 µs FWHM) [96Gan].

In addition to the keyhole shape and feed rate also local variations of the surface tension due to gradients in temperature and melt composition will determine the melt flow for laser welding. It has been shown that already small amounts of surface active alloying components can alter by this so-called Marangoni effect the resulting weld geometry significantly [99Fuh]. Figure 1.1.35 shows typical 3dimensional flow field simulations for two different surface tension temperature gradients.

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

37

t

a

b

Fig. 1.1.33. Calculation of transient keyhole formation for welding (P = 4 kW, w = 0.19 mm) [96Gri, 98Hue]. Arrow t denotes increasing time.

a

Fig. 1.1.34. Streamlines for laser welding simulation of Fe with (a) 5 m/min and (b) 15 m/min feeding rate [92Bec].

b

Fig. 1.1.35. Melt pool geometry and velocity field for (a) negative and (b) positive temperature gradient of surface tension (feeding rate 2 m/s). Low-alloyed steel [99Fuh].

1.1.3.2 Interaction mechanisms in the gas and plasma phase At high incident intensities a plasma can develop close to or above the interaction region at the workpiece. Depending on the particular process, ionization occurs in the evaporated material, in the ambient gas or in both. The resulting interaction between radiation and plasma as well as gas-dynamic effects have consequences on central issues of materials processing, namely on energy coupling and the quality of the process results. Therefore, they were investigated by many groups around the world applying a variety of theoretical models [70Rai, 73Ani, 78Pir, 79Kni, 93Ani, 99Arn] and experimental techniques: Imaging techniques such as shadow and Schlieren photography [68Bas, 85Mat, 90Ven, 95Cal, 97Jan], resonance

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38

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

absorption [91Gil, 92Ven, 98Sch2], streak photography [69Hoh, 90Fab, 91Sok], luminescence and LIF photography [73Ste, 91Sap, 93Geo, 95Wit, 96Al, 98Pur], and interferometry [68Bas, 88Aut, 94Lu, 94Lin, 97Hug, 97Sch1, 98Vog, 98Sch3] were used as well as spectroscopic [87Pop, 88Sok, 89Sok, 90Ber, 93Meh, 95Her, 96Sch2, 97Hug, 98Doy] or time-of-flight techniques [87Lar, 91Che, 91Wie, 92Koo, 98Pin].

1.1.3.2.1 Basic ionization and absorption mechanisms A plasma can interact with laser radiation via several mechanisms, such as reflection, absorption, refraction, or scattering [65Rai, 86Miy, 87All1, 93Ade, 93Bou, 93Maz, 96Mat], which in detail depends mainly on the temperature and the electron number density of the plasma. These properties in turn are governed by the characteristics of the laser beam, i.e. pulse duration, pulse intensity, laser power and wavelength. The “starting point” of all these mechanisms is the interaction between electrons and the electric field of the radiation which leads to an energy transfer from the radiation field to the plasma. As this energy transfer is both a dominant source of plasma generation and heating and – on the other hand – a cause for attenuation of a laser beam during materials processing, the ways by which energy can be absorbed in plasmas are of primary interest. Since the electron number density and the temperature are the key parameters, the absorption mechanisms will be discussed in order of increasing electron density or temperature of the plasma. •

•

For low temperatures, line absorption (bound–bound), multiphoton absorption (bound–free I) and photoionization from excited states (bound–free II) have to be considered [86Miy, 95Bul, 98Lun, 98Sch1]. For high temperatures or high electron densities, inverse bremsstrahlung is of special importance [70Hor, 73Mul, 84Pop, 85Bey, 90Mat].

1.1.3.2.1.1 Bound electrons Bound–bound absorption (line-absorption). Within the material vapor, line absorption can occur if the incident laser photon energy is in resonance with the energy equivalent of an electronic or vibrational transition within the molecules, atoms, or ions. Therefore, this absorption mechanism strongly depends on the energy level distribution of the considered species. The corresponding absorption bands are typically narrow and rarely coincide with the wavelengths used in laser materials processing, so this mechanism does not play a very dominant role. For further details regarding bound–bound absorption it is referred to [66Zel]. Bound–free absorption I (Multi-Photon Ionization, MPI). Multiphoton ionization or multiphoton absorption can occur if a number of photons is available for interaction within the lifetimes of virtual energy levels of a particle species such that the sum of the energy of these photons is sufficient to ionize the species. This mechanism is important for very high photon fluxes, e.g. for ultra-short laser pulses in the ps- and fs-pulse duration range. The probability of a multiphoton ionization process depends on the number of photons which is necessary for an ionization act. Resonant as well as non-resonant MPI is possible. If a particular level within the energetic system of the species can be excited resonantly by one photon, the multiphoton ionization probability increases drastically due to the longer lifetime of the excited level compared to the virtual energy level in the non-resonant case. A comparison of resonant and non-resonant MPI, e.g. for aluminum and iron [84Pop], shows an eightfold higher absorption coefficient for 248 nm radiation (KrF-laser) in iron vapor than in aluminum vapor. For very high laser intensities direct field ionization is also possible. A comparison between the MPI and field ionization regimes is shown in Fig. 1.1.36 and indicates that for “traditional” laser treatment processes this mechanism will not occur.

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Ref. p. 62]

10

15

1.1 Fundamentals of laser-induced processes Wavelength λ [ µm] 10

100

39

1

Intensity I [W/cm2 ]

Fieldionization KrF-laser 10

13

Nd-YAG-laser

10

11

CO2-laser

Multiphoton Ionization

10

9

10

13

14

15

10 10 Frequency ω [ 1/s]

10

16

Fig. 1.1.36. Comparison of MPI and field ionization versus radiation frequency [84Pop].

Bound-free absorption II (PhotoIonization, PI). At higher temperatures excited states in the species become more and more populated (Boltzmann distribution) so that a single photon may provide enough energy for an ionization process. The absorption coefficient for PI can be described on the basis of Kirchhoff’s laws. The absorption coefficient for hydrogen-like atoms is given by [66Zel] ∞ Z 4e10 me 1 N1 λ03 ∑ 3 e α PI = 5 6 4 j 48 3 ε 0 h c0 j = j*

1

−

E1 ⎛⎜ 1 ⎞⎟ 1− k bT ⎜ j 2 ⎟ ⎝ ⎠

,

(1.1.38)

Absorption coeff. α PI

where e and me denote the elementary charge and the electron mass, h and kb are Planck’s and Boltzmann’s constants. Z is the effective charge number of the atomic species and N1 and E1 are the neutral atoms’ number density in the ground state and their ionization energy. Finally, j* denotes the number of the lowest ionizable electronic level. Due to the quantization of the energy level the absorption coefficient shows a saw-tooth-like dependence on wavelength as shown in Fig. 1.1.37. Towards shorter wavelengths, whenever the energy of the photons is sufficient to liberate electrons from the next lower – and more densely populated – energy level, the number of excitable electrons increases all of a sudden and consequently a sharp increase is observed for the absorption coefficient.

3

∝λ 0

Wavelength λ 0 Lando lt -Börnst ein New Ser ies VIII/1C

Fig. 1.1.37. Schematic dependence of the photoionization (PI) absorption coefficient on the wavelength. Contributions of each single electronic level (dashed) and total absorption coefficient (solid) [66Zel]. The PI absorption follows a λ30 -dependence within the contribution of each electronic level.

40

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

1.1.3.2.1.2 Free electrons For high temperatures or high electron number densities of the plasma, inverse bremsstrahlung is an important mechanism. It is initiated by the presence of free electrons in the plume which are accelerated in the electrical field of the incident radiation. Via collisions with partners such as neutral atoms or ions their energy is transferred to the heavy particle system. The absorption coefficient for inverse bremsstrahlung based on a semi-classical analysis of free electron motion in the Coulomb field of an ion and on the principle of detailed balancing (Kirchhoff’s law) is given by [54Mae, 66Zel]: 1

⎞ 2 Z 2e6 3 + ⎛ 2 4 ⎟ ⎜ α ib = λ0 N N e 3 ⎜ 3( 4 ε 0 ) ⎝ 3me k b T ⎟⎠ h c04 me

⇒ α ib

∝ N e λ03 ,

(1.1.39)

with N+ and Ne denoting the number densities of the single ionized ions and the free electrons. Using the Saha equation for the first stage of ionization which describes the dependence of the electron density Ne on temperature in thermal equilibrium: 1

E

1 N + N e ⎛ 2 m e k b T ⎞ 2 u + − k bT ⎟ e , = ⎜⎜ ⎟ u N h2 ⎝ ⎠

(1.1.40)

the inverse bremsstrahlung absorption coefficient can be further simplified when for moderate plasma temperatures multiple ionization can be neglected:

α ib =

1

Z 2 e 6 k bT

12 3 ε 0 h 4 c04 3

Nλ e 3 0

−

E1 k bT

(1.1.41)

.

The neutral atom number density is given by N and u, u+ are the electronic partition functions of the atom and of the ion, respectively. The absorption coefficients described by (1.1.39) through (1.1.41) are purely atomic properties. Socalled effective absorption coefficients α ib ’ that include the effect of re-emission of radiation due to − hν k T

b induced emission are obtained by multiplication of αib by a factor of 1 − e [66Zel, 80Wen]. Comparing photon energy with kinetic energy, two limiting cases can be developed for the effective absorption coefficient which exhibit different wavelength dependence. While the behavior indicated by (1.1.39) and (1.1.41) holds true also for α ib ’ for hν >> k bT , a weaker dependence is found for hν << k bT

where α ib ’∝ λ20 . The inverse bremsstrahlung absorption coefficient can also be calculated on the basis of the classical Lorentz oscillator model where collisions between electrons and heavy particles are considered to be responsible for the damping thus causing the energy transfer [73Mul, 87All1]. The absorption coefficient is then given by 1

⎡ 2 ⎞ ω 2 2 ⎢ ⎛⎜ − 1− 2 p 2 ⎟ + α ib = ⎢ ⎜ λ0 ω + ν c ⎟⎠ ⎢⎣ ⎝

2

⎛ ω p ⎞⎟ ⎛⎜ ν c ω p ⎞⎟ ⎜1 − + ⎜ ω 2 +ν 2 ⎟ ⎜ ω ω 2 +ν 2 ⎟ c ⎠ c ⎠ ⎝ ⎝ 2

2

2

⎤2 ⎥ , ⎥ ⎥⎦

(1.1.42)

with νc being the total collision frequency consisting of contributions by electron–atom (νea) and electron–ion collisions (νei) [67Tan]:

ν c = ν ea + ν ei , ν ea =

8 3 π

σc N

(1.1.43)

2 k bT , me

(1.1.44)

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

ν ei =

(

e 4 ln 12 N e−1 2 e −3 (ε 0 k b T )

32

3ε

2 0

me

) (2

(

41

)

k b T ) −3 2 N + + N + + +  .

(1.1.45)

Here, σc describes the classical collisional cross section between particles with radii r1 and r2 according to c = (r1 + r2 ) 2 and N+, N++, ... denote the number densities of singly-, doubly-, ... -ionized ions. Electron–atom collisions dominate for low-temperature plasmas whereas collisions with ions prevail at higher temperatures. The so-called plasma frequency ωp, finally, which is an important figure characterizing plasmas, is determined by the free electron density Ne according to:

ω p2 =

N ee2 . ε 0 me

(1.1.46)

For λ << 2 c ω p which holds for all practical cases of materials processing with lasers the inverse 0

bremsstrahlung absorption coefficient according to (1.1.42) exhibits a λ 20 -dependence.

1.1.3.2.2 Absorption and refraction effects in laser-induced plasmas 1.1.3.2.2.1 Plasma composition and temperature The plasma properties are varying in a broad range and primarily depend on the characteristics of the laser radiation and on the properties of the irradiated material. The most important property characterizing a plasma is the free electron density Ne which depends strongly on the plasma temperature. This is shown exemplarily in Fig. 1.1.38 for plasma conditions that are typical for laser beam welding, i.e. for mixtures of metal vapor and shielding gases. The influence of the wavelength on the resulting plasma temperature – and consequently on its electron density – is demonstrated by the experimental data presented in Fig. 1.1.39 [97Sch3, 00Sch1].

1E18

Number density N [cm −3 ]

Number density N [cm −3 ]

1E17

1E18

Ntotal

He I

Ne

Al I CO-laser

1E16

Al II

1E15

Al III

CO2-laser He II

Ntotal

Ar I 1E17

1E16

Ne

Al I

CO-laser CO2-laser

Al II Al III Ar II

1E15

Al IV 1E14 5000

a

15000 20000 10000 Temperature T [K]

Al IV 1E14

25000

5000

b

15000 20000 10000 Temperature T [K]

25000

Fig. 1.1.38. Number densities of species in (a) Al/He and (b) Al/Ar plasmas in dependence on temperature. Shielding gas content (He resp. Ar) 80 % [00Sch1].

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1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

20000

Temperature T [K]

Aluminum 16000

12000

Fig. 1.1.39. Spectroscopically measured temperatures within welding plasmas for aluminum and steel at different laser wavelengths. The effect of a variation of the shielding gases lies within the error bars, temperatures measured with Ar as shielding gas range in the upper part of the margins, with He shielding in the lower range. Nd:YAG: 1.06 µm, 5×106 W/cm2; CO: 5 µm, 5×107 W/cm2; CO2: 10.6 µm, 5×106 W/cm2 [97Sch3, 00Sch1].

Steel 8000

4000 0

2

4 8 10 6 Laser wavelength λ [µm]

12

25 Temperature

2.0

2+

from Al -lines

3.5

+

from Al -lines

1.5

3.0

1.0 0.5

2.5

Ne Laser pulse

0

100 200 Time t [ns]

2.0 1.5

300

Fig. 1.1.40. Temporal evolution of plasma temperature and electron density determined by spectroscopy of the spatially integrated luminance of Nd:YAG-laser induced plume: 1064 nm, 4 mJ, 120 ns pulse width, 2×108 W/cm2 [96Wit].

Electron density Ne [10 16 cm − 3 ]

2.5 Electron density Ne [10 17 cm − 3 ]

Electron temperature Te [eV ]

The knowledge of the temporal evolution of the electron density in a laser-induced plasma during and after the pulse can be used to optimize the process. This is particularly important for high repetition rate machining where subsequent laser pulses may experience absorption in the plasma plumes induced by previous pulses. The temporal evolution of electron densities has been determined experimentally for different processing conditions. Examples for various timescales and wavelengths are shown in Figs. 1.1.40–43 [96Wit, 98Her2, 99Agu, 00Sch2].

Air Ar He

20

15

10

5

0

5

10

20 15 Time t [µs]

25

30

35

Fig. 1.1.41. Spectroscopically determined evolution of integral electron density of Nd:YAG-laser induced plume in different atmospheres at atmospheric pressure: steel target, 1064 nm, 4.5 ns pulse width, 3.8×1010 W/cm2 [99Agu].

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1.1 Fundamentals of laser-induced processes

Electron density Ne [ cm − 3 ]

Ref. p. 62]

43

10 19

0.5 mm 1.0 mm 1.5 mm 2.0 mm 3.0 mm 4.0 mm 6.0 mm

10 18

10 17

a

10 16

4 1.0 mm 2.5 mm 3.5 mm 5.5 mm

Ne

kTe [eV ]

3 2 1

b

0

400

0

Fig. 1.1.42. Temporal evolution of electron densities at various locations within an excimer-laser induced plume: aluminum target, 248 nm (KrF), 25 ns pulse duration, 1.5×109 W/cm2 [00Sch2].

800 Time t [ns]

1200

1600

Fig. 1.1.43. Temporal evolution of (a) electron density and (b) temperature of an excimer-laser induced plume in 100 Pa nitrogen atmosphere at various distances from a titanium target: 308 nm (XeCl), 25 ns pulse duration, 5×108 W/cm2 [98Her2].

Table 1.1.13 presents a summary of experimentally obtained electron densities and temperatures for plasmas generated during pulsed and cw laser material treatment, showing the effects of laser wavelength and power density, ambient gas species and pressure, and of work piece material (see also [98Dul]). Table 1.1.13. Experimentally determined plasma properties. h is the height above the surface of the target; t is the time after the begin of the laser irradiation. Laser (cw/pulsed)

Material

Power / Pulse energy

Power density [W/cm²]

Gas (flow [l/min])

Ne [cm–3]

Te [K]

Remarks

Ref.

CO2 (cw)

Fe

15 kW

5×106 max. 1×106 max. 5×106 max.

He (50)

3…8.2×1016

5500…6700

92Pou, 93Pou

Ar/air

3…7×1016

5200…6250

Ar

1.7…2×1017 max. 0.7…4×1016

15100

1.2 m/min, h = 5…0 mm 1.2 m/min, h = 1…0 mm 1.2 m/min, h = 5…0 mm 25…800 hPa

1 kW 15 kW CO2 (cw)

Fe

CO2 (cw)

mild steel

CO2 (cw)

steel

Ar

10.5 kW

He

20 kW

5450…6200

17

1.4×106

air

1…1.5×10 max. values 2.8×1015

4×106

air

1.2×1016

7650

air

1…31×1017

9600…12100

6400

95Ver

95Ohj 0.5…10 m/min spatially res. deg. of ionization 88Sok 48 % deg. of ionization 89 % in keyhole

(continued)

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44

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

Table 1.1.13 continued. Gas (flow [l/min])

Ne [cm–3]

2 kW

Ar (10)

2 kW

Ar (30)

0.7…2.8 ×1017 0.7…2×1017

Laser (cw/pulsed)

Material

Power / Pulse energy

CO2 (cw)

steel

CO2 (cw)

CO2 (cw)

CO2 (cw)

steel

steel

stainless steel

CO2 (cw)

stainless steel

CO2 (cw)

Al

CO2 (pulsed, 0.2 µs)

Al

Power density [W/cm²]

CO2 (cw)

AlMgSi1

Ti

Remarks

Ref.

0.5 m/min spatially res. 0.5 m/min spatially res.

92Miy

2 kW

Ar

3 kW

Ar (30)

1…2×1016

3 kW

Ar (30)

2×1017

13000

0.6 m/min inside 95Mai plume in keyhole

10 kW

Ar (20)

1.0×1017

12900

h = 0…10 mm

10 kW

Ar (20)

1.3×1017

13500

h = 30…40 mm

10 kW

Ar (20)

6.5×1016

12000

h = 60…70 mm

2.5 kW

He (15)

3…6.1×1016

6900…8800

h = 2…0 mm

2.5 kW

He (40)

1.8…4×1016

6100…8000

h = 2…0 mm

2.5 kW

Ar (40)

8000…12000

7200…8600

h = 2…0 mm

2 kW

Ar (10)

9000…10800 (Fe I and II) 6400…7800 (Fe I) 17400

2 m/min h= 1.25…0 mm

9500

4×106

N2 (60)

3.5…8.5 ×1016 9…11×1016 (Ar lines) 3.5…7×1016 (Fe lines) 6.5×1016

4×106

He (30)

4.6×1016

16100

4×106

O2 (40)

6.2×1016

17100

70 mJ

17

94Sei

94Szy

97Szy

89Sok

Ar

2.7×10

He

1×1017

He (30)

18100…18700

6 m/min

Ar (20)

3.3…3.2 ×1016 7.4…7×1016

19400…20000

6 m/min

He (40)

4×1015 max.

6000…6800

h = 2…0 mm

2.5 kW

Ar (40)

3.2…3.8 ×1016 1…1.2×1017 (Ar lines)

7000…8700

h = 2…0 mm

8800…11400 (Ti I and II) 13500 (Ti II) <8000 (Ti I)

4 m/min h = 1…0 mm

97Szy

6 m/min spatially res.

95Ohj

70 mJ CO2 (cw)

Te [K]

2.5…4 kW 2.5…4 kW 2.5 kW

6

6×10 max. 6×106 max.

h = 1.2 mm

90Mic

h = 1.2 mm 97Sch2

94Szy

CO2 (cw)

Ti

2 kW

Ar

CO2 (cw)

Sn

2.5 kW

Ar (2)

0.5…1.5 ×1017

TEA CO2 (pulsed) (80 ns + 8 µs) TEA CO2 (pulsed) (90 ns + 2 µs) TEA CO2 (pulsed) (80 ns + 8 µs)

Al

750 J

1.25×109 + 3.5×108

vacuum

1017 …5×1017

19000…35000

88Aut

Al

5.5 J

5×109 + 8×108

vacuum

3×1017

22700

96Wit

C

750 J

1.25×109 + 3.5×108

vacuum

2×1017 …2.5×1017

60000…100000

88Aut

(continued)

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

45

Table 1.1.13 continued. Laser (cw/pulsed)

Material

Power / Pulse energy

Power density [W/cm²]

Gas (flow [l/min])

Ne [cm–3]

Te [K]

TEA CO2 (pulsed) (90 ns + 2 µs) TEA CO2 (pulsed, 0.9…5 µs)

C

5.5 J

5×109 + 8×108

vacuum

1.8×1018

33100

metals, semiconductors

1J max.

1×109 max.

air

1×1017 max.

8000…19000

var. of height above target

77Mal

CO (cw)

AlMgSi1

2.5…4 kW 2.5…4 kW

7×107 max. 7×107 max.

He (30)

4.7…4.8 ×1016 4.3…5.4 ×1016

14400…15300

6 m/min

97Sch2

15100…18600

6 m/min

5300 4800

h = 0 mm h = 1.3 mm

Nd:YAG 1.06 µm (pulsed, 5 ms) Nd:YAG 1.06 µm (cw) Nd:YAG 1.06 µm (pulsed, 5 ms) Nd:YAG 1.06 µm (pulsed, 7 ms) Nd:YAG 1.06 µm (pulsed, 120 ns) Nd:YAG 1.06 µm (pulsed, 21 ms) Nd:YAG 1.06 µm (pulsed, 3 ns) Nd:YAG 1.06 µm (pulsed, 120 ns) Nd 1.06 µm (pulsed, 12 ns) Nd:YAG 1.06 µm (pulsed, 12 ns) Dye 583 nm (pulsed, 2 µs)

Nd:YAG 532 nm (pulsed, 8 ns)

Ar (20)

Fe

2 kW

air

steel

3 kW

4.2×106

3 kW

4.2×106

He

3×1016

Ref.

96Wit

3500

97Lac

97Sch3

Ar

3700

air

6700 5300

h = 0 mm h = 1.3 mm

97Lac

1.6×106

Ar

3400

weak intens. dependence

87Pee

4 mJ

2×108

vacuum

2×1017

14500

80 J

1×106

air

1.85×1013

3280

h = 1 mm

95Mat

0.4…1.2 ×109

–

5000…12000

water confinement

92Fab

stainless steel

2 kW

Al

400 W

Al

Al–Mg alloy Cu

C

4 mJ

2×108

vacuum

2.8×1018

C

10 J

8×1011

vacuum

0.4…2×1019

Si3N4

5 mJ

1×1011

air

2…4.5×1019

Al 2024

105 J/cm2

5.3×107

vacuum

3.3×1017

2

7

Al

Remarks

16

96Wit

28400

8100

96Wit

spatially resolved spatially resolved

68Bas

h = 1.27 mm

87Knu

105 J/cm

5.3×10

8×10

7000

h = 5 mm

26 J/cm2

1.3×107

3.5×1017

7660

h = 1.27 mm

880 mJ

4×1010

0.8×1017

12800

vacuum

99Bre

96Wit

(continued)

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46

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

Table 1.1.13 continued. Laser (cw/pulsed)

Material

Power / Pulse energy

Power density [W/cm²]

Gas (flow [l/min])

Ne [cm–3]

Te [K]

Nd:YAG 532 nm (pulsed, 40 ps) Nd:YAG 532 nm (pulsed, 8 ns) Nd:YAG 532 nm (pulsed, 40 ps) Nd:YAG 532 nm (pulsed, 12 ns)

Al

25 mJ

5×1011

vacuum

0.5×1017

13400

96Wit

C

880 mJ

4×1010

vacuum

1.5×1018

19200

96Wit

C

25 mJ

5×1011

vacuum

1.1×1018

22600

96Wit

Si3N4

2 mJ

9.3×1010

air

2.2…3.5 ×1019

spatially resolved

99Bre

Nd:YAG 355 nm (pulsed, 12 ns)

Si3N4

1.1 mJ

9.5×1010

air

1…2×1019

spatially resolved

99Bre

Excimer 308 nm (pulsed, 40 ns) Excimer 308 nm (pulsed, 20 ns)

Al

2J

1×109

vacuum

1.2×1018

18600

Al

0.5 J max.

9×108

vacuum

4.5×1018 …2×1017

29000…9000

tempor. res. t = 50…150 ns

91Gai

0.5 J max.

6×108

vacuum

23000…27000

3.0…9.5 ×108

vacuum

16000…29000

h= 1.25…0.1 mm h = 0.2 mm

91Gai

0.5 J max.

4×1016 …5×1017 2×1017 …1.8×1018

2…3.8 J/cm2

0.7…1.3 ×108

vacuum

2.5 ×1017…1019

23000…46000

h < 0.5 mm

93Meh

2…3.8 J/cm2 12 mJ

0.7…1.3 ×108 1.2×109

vacuum

~1018

23000…28000

h > 1 mm

air

4…9×1019

spatially resolved

12 mJ

1.2×109

He

2…7×1019

spatially res.

12 mJ

9

1.2×10

Ar

6…8×1019

spatially res.

Al

600 mJ

1×109

vacuum

5×1017

18600

C

600 mJ

1×109

vacuum

1.1×1019

25500

Excimer 248 nm (pulsed, 30 ns)

Excimer 248 nm (pulsed, 30 ns)

Excimer 193 nm (pulsed, 20 ns)

Al

Al

Remarks

Ref.

96Wit

91Gai

00Sch2

96Wit

The spatial distribution of the electron density within the plasma plume reveals regions where the incident laser energy is absorbed and where refraction will occur predominantly. This information is therefore essential for any quantitative description of the interaction mechanisms. As an example, a typical distribution of Ne for pulsed laser processing is shown in Fig. 1.1.44 [99Bre]. Under these particular conditions the regions with the highest absorption are close the target surface and at the tip of the induced shock wave.

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

47

Fig. 1.1.44. Electron density distribution in a Nd:YAG-laser induced plasma plume: silicon nitride, 5 mJ, 1200 J/cm², pulse width 12 ns, 15 ns after pulse beginning [99Bre].

1.1.3.2.2.2 Absorption For cw-laser processing, e.g. laser beam welding, the electron densities in the plasmas are in general lower than those in plasmas induced by pulsed laser irradiation in ablation and drilling applications due to the lower intensity values and lower evaporation rates that result therefrom. Furthermore, a relatively free expansion of the material vapor away from the interaction zone allows better dilution of the plumes. Nevertheless, even the drastically lower electron density of cw-laser induced plasmas can cause considerable absorption due to the wavelength dependence of inverse bremsstrahlung absorption when mid-IR lasers are used (CO, CO2). In Fig. 1.1.45 calculated values of the absorption coefficient are shown. Typical values of the total absorption in welding plasmas are summarized in Table 1.1.14. In most cases of pulsed interaction a combination of several of the mechanisms discussed in Sect. 1.1.3.2.1 is responsible for the absorption. A comparison of the wavelength dependence of these phenomena is plotted in Fig. 1.1.46, calculated in [00Sch2] for a special set of parameters using a model described in [90Mat]. The relative importance of the involved absorption mechanisms, inverse bremsstrahlung and photoionization is presented in Fig. 1.1.47 for a calculation performed by [96Cha] on the basis of experimentally measured values for a copper-laser induced plasma.

Absorption coefficient α [cm −1 ]

1 10 10 10 10 10 10

−1

−2

−3

−4

−5

Fe-Ar plasma p total = 1 atm pure Ar p Fe = 0.001 atm p Fe = 0.01 atm pFe = 0.1 atm pure Fe

−6

5000

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10000 Temperature T [K]

15000

Fig. 1.1.45. Calculated absorption coefficients in dependence of the temperature for Fe/Ar plasmas for CO2-laser welding, λ = 10.6 µm [94Szy].

48

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

Table 1.1.14. Typical values of overall absorption in welding plasmas. Laser (cw/pulsed)

Material

Power [kW]

CO2 (cw)

steel, titanium

2

Overall absorption

Conditions

Reference 94Szy, 97Szy

6%

Ar 40 l/min

2%

He 40 l/min

CO2 (cw)

steel

2

up to 23 %

Ar 10 l/min, spatially resolved

92Miy

CO2 (cw)

steel

15

20…25 %

He atmosphere

92Fab

up to 50 %

Ar atmosphere

CO2 (cw)

iron

15

20±10 %

He atmosphere

15

50…65 %

Ar atmosphere

92Pou, 93Pou

1

8±5 %

Ar/air atmospheres

CO2 (cw)

aluminum

5

21 %

Ar atmosphere

7

31 %

CO2 (cw)

–

2.8…4.3

58…68 %

freely burning Ar-plasma 96Tsu

CO2 (cw)

steel

0.6

42…15 %

Ar 1…8 l/min

0.4

–3

Nd:YAG (pulsed)

aluminum

<10 %

Absorption coefficient α [cm −1 ]

α

3

10 2

Photoionization

10

1

10

10

Inverse bremsstrahlung −1

−2

0

Fig. 1.1.46. Comparison of the order of magnitude of absorption coefficients due to different mechanisms. The parameters have been chosen to match the start of plasma formation for typical processes of pulsed laser ablation with 30 ns pulses of a KrF-laser at 109 W/cm2: T = 8000 K, N = 4×1019 cm–3, Ne = 4×1018 cm–3; particle/cluster radius 5 nm and particle/cluster number density 3×1014 cm–3 [90Mat, 00Sch2].

90Mil 87Pee

10

λ

87Roc

2 6 4 8 2 Laser intensity I [GW/cm ]

10

Fig. 1.1.47. Comparison between IB and PI absorption for an aluminum vapor plume generated by a pulsed Cu-vapor laser: 511 and 578 nm, 40 ns pulse duration, 40 mJ max. pulse energy [96Cha].

Photoionization, as well as Mie absorption by particles within the plume, can act as a starting mechanism for producing free electrons at the beginning of the interaction between laser-induced material vapor and laser radiation. This initial production of free electrons may lead to the start of the absorption via inverse bremsstrahlung [94Ade, 98Amo, 98Lun, 99Cal, 00Sch2]. Using (1.1.39) or (1.1.42) the inverse

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

49

α

α

bremsstrahlung absorption coefficient can be calculated for a variety of conditions. An example for this procedure based on the Lorentz oscillator model in (1.1.42) is given in Fig. 1.1.48 for a regime typical in drilling and ablation applications. Experimental values were obtained by transmission and reflection measurements [00Sch2] and are shown in Fig. 1.1.49 for several materials and energy densities. Very high values are reached that can lead to an absorption within the plume of up to nearly 100% of the incident laser pulse energy.

Fig. 1.1.48. Calculated absorption coefficients versus temperature at 10 MPa and for different wavelengths in an aluminum plasma during drilling/ablation [99Cal].

Fig. 1.1.49. Absorption coefficients in an excimer-laser induced plume during ablation of several materials in air at 0.1 MPa; estimated absorption length within plume 150 µm, λ = 248 nm (KrF-laser) [00Sch2].

For very high intensities, the combination of the absorption mechanisms described above can induce a considerable amount of absorption within the material vapor or the ambient gas leading to a gas breakdown [66Zel]. Such a breakdown may cause a partial or complete shielding of the workpiece. A theoretical treatment of this phenomenon is to be found in [01Maz]; the main result is given in Fig. 1.1.50. The consideration of the electronic level system yields a considerable decrease of the breakdown threshold compared to the classical approach dominated by inverse bremsstrahlung. One of the chief mechanisms is photoionization from excited states within the aluminum vapor. The drastic drop in the threshold (for instance at 394 nm) is caused by a resonant ionization process of the aluminum atoms (see also (1.1.38) and Fig. 1.1.37).

Laser intensity I [W/cm2 ]

10

9

Ar

10

8

10

7

10

6

10

5

10

4

10

3

10

2

XeCl

KrF

ArF

1

HeNe GaAs Nd II Nd Rubin

HD CO 2

10

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2

1 Wavelength λ [µm]

0.1

Fig. 1.1.50. Intensity necessary to initiate breakdown in an aluminum vapor under consideration of inverse bremsstrahlung (1) and further mechanisms such as photoionization (2) [01Maz].

50

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

1.1.3.2.2.3 Refraction In addition to the absorbing effect of laser induced plasmas a refracting influence is observed that, in view of conducting a well defined process, might be even more detrimental. Due to a generally inhomogeneous density distribution of the interacting particles the refractive index inside the plasma also shows an inhomogeneous distribution which may act like a lens for the incident radiation. Two mechanisms contribute to the resulting index of refraction: the polarizability of heavy particles and the presence of free electrons. The latter will be dominating for laser radiation with wavelengths in the infrared region and for high electron densities. The index of refraction, which is an important property of the plasma, can be calculated according to [65Wha, 86Kit]:

n ( λ0 ) = 1 +

1 2ε 0

∑ α k ( λ0 ) N k − k

e2 2 λ Ne , 2 2 0 8 ε 0 me c 0

(1.1.47)

with Nk and αk(λ) beinLieg the number density of the heavy particles of species k and the corresponding polarizability. The dependence of the refractive index on temperature and composition is exemplarily shown in Fig. 1.1.51 for various plasmas of pure gases or vapors at CO2-laser wavelength [95Bec].

Fig. 1.1.51. Calculated index of refraction for plasmas consisting of various pure gases or metal vapors in dependence of the plasma temperature at constant pressure for the CO2-laser wavelength λ = 10.6 µm [95Bec].

The way how an inhomogeneous distribution of the index of refraction affects the beam propagation through a plasma is demonstrated in Figs. 1.1.52 and 1.1.53. For the welding process with high-power CO2-lasers, in addition to the change of intensity by a varying spot diameter on the workpiece, the laser beam will be laterally deflected when an asymmetry occurs in the plume. In view of the high practical relevance of these effects, they were extensively investigated theoretically [92Duc, 92Pou, 95Bec, 00Sch1], see e.g. Fig. 1.1.52, as well as experimentally [87Roc, 97Jue, 99Ker, 99Sch, 00Sch2], Fig. 1.1.53. It should be emphasized that, depending on the particular electron density distribution, the beam might also experience a focussing [01Sud].

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

51

Fig. 1.1.52. Modeled defocusing effect of a plasma plume in dependence of the plume length zpl for Nd:YAG, COand CO2-laser radiation at 3 kW laser power and for two different plume temperatures Tpl. Plasma composition: 20 % Al vapor, 80 % He or Ar shielding gas [00Sch1].

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52

1.1.3 Thermophysical and dynamical “response”

a

[Ref. p. 62

b

.

c

.

Fig. 1.1.53. Calculated beam deflection and defocusing on the basis of an experimental refractive index distribution obtained by interferometry. CO2-laser at 3.6 kW on steel, beam diameter in the focus 200 µm [99Sch, 00Sch2].

1.1.3.2.3 Dynamical effects At sufficiently high intensity the workpiece material starts to evaporate very fast (see Sect. 1.1.3.1.2.2). As the vapor emits from the surface it displaces the ambient gas resulting in a shock wave which propagates in the surrounding atmosphere [68Ani, 73Ani, 90Ade, 92Ade, 93Smu, 94Maz2]. The strength of the shock is given by the evaporating mass flux and can be calculated by the Rankine–Hugoniot relations. In the case of strong evaporation, where the vapor leaves the Knudsen layer with sonic velocity, a centered rarefaction fan appears additionally, connecting the shock wave and the Knudsen layer. In a by now “traditional” theoretical approach of laser-supported absorption waves (LSA), the laser induced shock waves are classified in two different regimes: the laser-induced combustion waves (LSC) and the laser-induced detonation waves (LSD) as shown in Fig. 1.1.54. The distinctive feature is the location of the energy absorption: in the case of LSC waves the energy is deposited in a region close to the surface whereas in LSD waves most of the incident laser energy is absorbed within a narrow region behind the shock front shielding the workpiece from energy supply [73Ste, 75Fow, 78Pir, 80Mah, 84Dan, 84Woo]. Recent investigations, however, indicate that mixtures of these pure forms of LSA waves occur that require a more detailed description according to the particular processing parameters. A main parameter determining the character of the shock waves is the processing wavelength. As shown in Fig. 1.1.55 for Nd:YAG- and excimer-laser induced plumes a LSD-type shock wave is formed in the case of IRradiation, representing the absorption of the laser radiation at the very tip of the shock wave. With shorter wavelengths the plumes are changing their expansion behavior towards a more spherical shock wave shape. This change in morphology correlates to a shift in the regions where the main absorption takes place [01Bre], close to the surface for short wavelengths and more towards the wave’s front for longer wavelengths (see also Fig. 1.1.44). The development of a LSD wave in dependence on intensity is displayed in Figs. 1.1.56 and 1.1.57. Numerical simulations of the expansion based on the Euler equations provide the flow field, the shape and other properties of the shock, see Fig. 1.1.58. This approach identifies details such as areas of different particle density or areas where condensation of particles can occur [68Ani, 73Ani, 73Pro, 79Kni, 87Cha]. On the other hand, classical gasdynamic theories do provide useful insight into the basic behavior of laser-induced shock waves [59Sed, 65Rai, 69Hoh, 73Pir, 94Lu, 94Pra, 95Cal, 96Nic, 97Ade, 99Jeo], Lando lt -Bö rnst ein New Ser ies VIII/1C

Ref. p. 62]

1.1 Fundamentals of laser-induced processes

53

see for example Fig. 1.1.59. In particular, they allow to calculate the high pressures at the workpiece’s surface, see Fig. 1.1.60. Experimentally deduced values are presented in Fig. 1.1.61. They are of practical interest insofar as they may cause severe damage to the workpiece during an ablation process. On the other hand, these high values can also be utilized in gaining higher efficiency of the treatment process, for example by melt expulsion during drilling. Another effect caused by the increase in pressure is the increase of the evaporation temperature which is described by the Clausius-Clapeyron theory (see (1.1.35)).

a

Fig. 1.1.54. Characteristic features of the simplified models of (a) LSC and (b) LSD waves [73Ste, 75Fow, 78Pir, 80Mah, 84Woo, 84Dan].

b

Fig. 1.1.55. Morphology of laserinduced shock waves for different processing wavelengths. Aluminum target. Nd:YAG (1064 nm, 532 nm, 355 nm): 12 ns pulse duration, 1011 W/cm2; KrF-Excimer (248 nm): 25 ns pulse duration, ca. 109 W/cm2 [01Bre, 00Sch2]. 100 LSD

Evaporation

Shock front velocity [km/s]

LSC

ν LSD

20 Pa 10 2 Pa 3 10 Pa

10

5

Shock wave

10 Pa 5

10 Pa 1 3

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30 5 2 Power density [10 7/cm ]

50

Fig. 1.1.56. Development of laser-induced shock wave expansion for excimer-laser ablation. Evaporation, LSC and LSD regimes: 308 nm (XeCl), copper target [93Sch1].

54

1.1.3 Thermophysical and dynamical “response”

[Ref. p. 62

Fig. 1.1.57. Shadowgraphs of laserinduced shock waves in ambient air atmosphere at different intensities 30 ns after pulse beginning: Si3N4 target, 532 nm (Nd:YAG), 12 ns pulse duration [01Bre].

Fig. 1.1.58. Calculated pressure distribution and flow field in an LSD-wave sustained by a CO2-laser beam at (a) 0.66 µs (b) 1.63 µs (c) 2.19 µs. Pressure scale in 106 erg/cm3: 1: 1.8; 2: 3.6; 3: 7.2; 4: 14.4; 5: 28.8; 6: 57.6; 7: 115. Power density: 10 MW/cm2, spot diameter 0.22 cm [75Nie].

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

55

1.6 original Sedov theory modified Sedov theory measured values

Distance [mm]

1.2

0.8

0.4

0 10 2

10

10 3

Time t [ns]

Fig. 1.1.59. Comparison of shock front expansion data (copper target, λ = 248 nm (KrF), 30 ns (FWHM), 30 J/cm², 8.4 mJ/pulse) [95Cal] to models based on the Sedov theory of gasdynamic shock waves [59Sed]. The modification of the Sedov theory accounts for the temporal evolution of the laser pulse. Energy content of the shock wave 80 %.

Surface pressure p [bar]

10 4

10 3

10 2 Pirri Sedov Raizer

10

1 0

10 2

10

10 3

Time t [ns]

Fig. 1.1.60. Calculated temporal evolution of the surface pressure during pulsed laser ablation of aluminum with H = 47 J/cm2 [93Cal]. Comparison of the shock wave expansion described by a modified Sedov theory (dashed) [59Sed] (compare also Fig. 1.1.59), a 1DLSD wave theory (dash-dotted) [77Rai], and a twodimensional LSD wave model (solid) [73Pir] where a transition occurs from a 1D- to a 2D-expansion in dependence on the illuminated spot diameter (190 µm) at 9 ns.

Lando lt -Börnst ein New Ser ies VIII/1C

0.1

1

10

Fig. 1.1.61. Vapor pressure at various locations of a laser-induced shock wave deduced from experimental shock and vapor velocities via hydrodynamic relations of adiabatic shock front expansion. Pulsed Cu-vapor laser: 511 and 578 nm, 40 ns pulse duration, 40 mJ max. pulse energy. Target: aluminum [96Cha].

56

1.1.4 Simplified dependences in laser processes

[Ref. p. 62

1.1.4 Simplified dependences in laser processes 1.1.4.1 Energy coupling in laser processes The energy coupling to a workpiece during a laser process is defined by the coupling rate: ηA =

PA , PL

(1.1.48)

i.e. by the ratio of power absorbed and released as heat to the laser power that is supplied to the workpiece. It can be determined straightforwardly by measuring the heat input calorimetrically. If one, on the other hand, wants to calculate it from the optical constants reviewed in Sects. 1.1.2.2 and 1.1.2.3, one has to know in addition to the beam properties the • • •

geometry of the interaction zone, e.g. keyhole shape, cutting front, intensity distribution in the interaction zone (generally modified by reflections there) including angles of incidence and polarization, temperature distribution of the surface hit by the laser beam. In cases where plasma effects play a significant role like welding and drilling

• •

the interaction of the laser beam with the laser induced plasma above and in the workpiece and the energy transfer from plasma to workpiece

have to be taken into account, additionally. Complex process models are required to calculate energy coupling in a self-consistent way. For first estimations and for demonstration of the most important dependences simplified models as described in the following sections are sufficiently informative, however.

1.1.4.1.1 Coupling rate in laser cutting The interaction zone hit by the laser beam in cutting can be simplified by a half cylinder with diameter df inclined with an angle α relative to the beam path. For estimating α it is a useful choice to relate it to the focal diameter df and the sheet thickness s:

tan α =

s . df

(1.1.49)

Calculations in [95Dau] for a laser beam assumed to have a top hat intensity profile and to impinge on the half cylinder totally and only once yield results that are depicted in Fig. 1.1.62. The following trends become obvious: • • •

higher coupling efficiency for linear polarization parallel to cutting direction, higher coupling rate for shorter wavelengths at low aspect ratios (s/df), higher coupling rate for longer wavelengths at high aspect ratios.

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Ref. p. 62]

1.1 Fundamentals of laser-induced processes

57

100 Fe CO2 , par. CO2 , circ.

Coupling rate ηA [%]

80

Nd:YAG, par. Nd:YAG, circ.

60

40

20

10 20 Aspect ratio s /d t

0

Fig. 1.1.62. Energy coupling efficiency in laser cutting.

1.1.4.1.2 Coupling rate in laser welding In welding the keyhole produced by the recoil pressure of vaporization acts as an efficient “absorber” by forcing the impinging laser beam to multiple reflections (and absorptions) before parts of it may escape. The total absorption in a keyhole can be estimated with a formula deduced by [45Gou] for “black cavities”. Assuming a conical keyhole with opening diameter df and depth s one obtains approximately [95Dau]: 2 ⎛d ⎛d ⎞ ⎞ 1 + (1 − A) ⎜ f − ⎜ f ⎟ ⎟ ⎜ 2s ⎝ 2s ⎠ ⎟ ⎝ ⎠ . ηA ≈ A df ⎞ df ⎛ A ⎜1 − ⎟ + ⎝ 2s ⎠ 2s

(1.1.50)

By taking values for the intrinsic absorptivity A at boiling temperature averaged over all possible angles of incidence and polarizations the results shown in Fig. 1.1.63 have been obtained. They are in good agreement with results from a self-consistent keyhole model [95Dau, 96Bec2]. It is obvious that the coupling rate strongly depends on the aspect ratio, especially if it is below 10. As in cutting, aluminum shows a lower coupling rate, compared to iron. At very high aspect ratios the differences vanish. The influence of plasma on energy coupling in a keyhole was investigated in [96Bec2]. Figure 1.1.64 shows the following main points for CO2-laser wavelength: • • •

the influence of plasma absorption increases with welding depth, the contribution of plasma absorption is larger at materials with lower absorptivity, e.g. Al, up to a quite high penetration depth Fresnel absorption is the dominating mechanism.

Towards lower wavelengths the interaction beam/plasma decreases strongly following approximately a λ2 to λ3 behavior, see Sect. 1.1.3.2.1.2. It can be expected, therefore, that in welding with Nd:YAGlasers the influence of plasma on energy coupling will be less important.

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1.1.4 Simplified dependences in laser processes

[Ref. p. 62

ηA

58

Fig. 1.1.63. Energy coupling efficiency in keyhole welding with lasers.

Fig. 1.1.64. Influence of plasma absorption on keyhole welding with CO2-lasers.

1.1.4.2 Process windows An exact modeling of laser processes could not be achieved, up to now. For making the choice of laser types, layout of systems, and for process development with respect to a particular application it is sufficient, however, to have simple formulas at hand describing the basic dependences. A prerequisite for starting a laser process is to choose the power level beyond a threshold. Above it the yield of process (velocity, depth, width) increases with power, generally. This is true until a region is reached where nonlinear effects start to disturb the process either by reducing the output or the quality. Figure 1.1.65 illustrates the situation for keyhole welding. The lower limit of the process window is caused by the onset of deep penetration, the upper one by plasma effects. Inside the process window it is useful to estimate the maximum achievable velocity, the efficiency of the process and the most important parameters determining these values. In the following, simple formulas are collected describing threshold, velocity and thermal efficiency.

λ

λ

λ

2

Fig. 1.1.65. Processing window in keyhole welding. The lower limit, i.e. the threshold, is determined by the onset of deep penetration. The threshold value for the ratio power P over focus diameter df increases with thermal conductivity λth, boiling temperature Tb, and decreasing absorptivity A of the material to be welded. The upper limit of the process window is caused by the onset of plasma effects, which are governed by the ionisation potential Ei of the atmosphere and the metal vapor, respectively. Due to the influence of the laser wavelength on the absorptivity of the workpiece material and the absorption coefficient of the plasma the process window gets wider with decreasing wavelength. Lando lt -Bö rnst ein New Ser ies VIII/1C

Ref. p. 62]

1.1 Fundamentals of laser-induced processes

59

1.1.4.2.1 Power threshold The minimum power Pth – the threshold value – required for a laser process is determined by a geometrical factor (G), a numerical factor (B) influenced by the beam intensity distribution as well as by material (M) and velocity / interaction time (V) related terms through a general expression:

Pth = B ⋅ M ⋅V . G

(1.1.51)

The various terms can be calculated from approximations and are collected in Table 1.1.15. A set of material properties appears in the combination

M =

TP ⋅ λth ηA

(1.1.52)

in all cases. The process temperature TP is characteristic for the processes under consideration and is, e.g., the melting temperature for cutting. Whenever in the η A column the absorptivity A is addressed, it is recommended to take values for perpendicular incidence averaged between room and process temperature (see Sects. 1.1.2.2 and 1.1.2.3). In other cases the coupling rate values calculated by coupling models have to be taken (see Sect. 1.1.4.1.2). An example may illustrate the general dependences: For keyhole welding (1.1.51) was deduced in the following form [96Bec2]:

P = df

⋅

Tb ⋅ λ th A

v ⋅ df + 1.1 . 4κ

(1.1.53)

In this case, the relevant geometrical factor G is the focus diameter df. The factor B having the means that a Gaussian beam is considered. Since for deep penetration welding the metal vapor value is needed to keep the keyhole open, the boiling temperature Tb is the appropriate choice for the process temperature. The velocity-related expression shows a weak dependence on welding velocity v which can be neglected in a first approximation, especially at low velocities (related to temperature diffusivity κ). Table 1.1.15. Constituent terms of (1.1.51) and (1.1.52). The cases of ablation and welding correspond to Gaussian intensity distributions, the expressions for cutting have been derived for line sources.

Process

Case

G

B

TP

ηA

V

ablation

One-temperature model (τ >> τph)

df

1.97 Tb

A

1/arctan( 2 2 lth / df)

0.89 Tb

A

1 / lth

d f2 / 4

df

Tb

A

low velocity

df

Tb

A

~1

2.5

Tm

ηA

~1

95Dau

2

Tm

ηA

~1

88Pet

0.5

Tm

ηA

~1

95Dau

s s df

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96Bec2

general

inert gas cutting

oxygen cutting

(1.1.28) (1.1.27)

One-temperature model, one-dimensional limit: df >> lth >> lα keyhole welding

Reference

steel

s

v d f / 4κ + 1.1

60

1.1.4 Simplified dependences in laser processes

[Ref. p. 62

For ablation, the onset of vaporization is taken as the start of material removal. Vaporization can be the dominant material removal process at very high pulse power, but even at processes dominated by melt expulsion is it necessary to create a driving force by the recoil pressure of vapor production and expansion. It is important to note, however, that in drilling the threshold increases with penetration depth [99Dau]. In most cases the threshold shows up in terms of power divided by focus diameter. For cutting, the minimum power scales with the sheet thickness s, mainly. Only in the case of ablation with short pulses, the intensity is the right measure for the threshold. If for the cases of ablation and welding other intensity distributions than Gaussian are considered, the factor B needs to be modified. Table 1.1.16 gives correction factors for top hat and Airy profiles, the latter being representative for unstable resonators, the first for beams homogenized e.g. transmitted by glass fibers. The correction factors depend on the dimensionality of heat flux and are given for the two limiting cases characterized by the ratio between the thermal diffusion length and the focal diameter. Table 1.1.16. Correction factors for B in case of non-Gaussian beams [00Sch1].

l th / d f >> 1

l th / d f << 1

Gaussian

1

1

Top hat

1.25

2

Airy

0.61

Profile

1.1.4.2.2 Factors determining process velocity Above the threshold the velocity v of laser processes follows an approximately linear behavior of the form [95Dau]: v ≈ −a1

η P κ + a2 A ⋅ L . df H P s df

(1.1.54)

The first term takes into account heat losses, the second one is a result of the energy balance between absorbed laser power (according to (1.1.48)) and the specific enthalpy H P required to heat the material volume to be treated to the required process temperature. The latter can be calculated from density ρ , latent heat values of phase transitions L, specific heat cp, and the process and room temperatures Tp and T∞: H P = ρ (∑ L +

TP

∫c

p

(1.1.55)

dT ) .

T∞

Table 1.1.17. Coefficients of (1.1.54). Process

a1

a2

Reference

ablation

stationary

~0

1

Sect. 1.1.3.1.2.2

keyhole welding

0.6 < vb / κ < 12

1.05

0.53

95Dau

v b / κ > 12

0

0.483

95Dau

~2

~0.76

95Dau

inert gas cutting

In ablation processes the calculation of the process enthalpy is impeded by the difficulty to know the phase composition and temperature of the removed material ranging from just molten to superheated at temperatures above boiling point, see Sects. 1.1.3.1.2.2 and 1.1.3.1.3.2. Lando lt -Bö rnst ein New Ser ies VIII/1C

Ref. p. 62]

1.1 Fundamentals of laser-induced processes

61

Table 1.1.17 gives summaries of the coefficients a1 and a2 taken from approximative models. The values for keyhole welding have been obtained from a two-dimensional model [73Swi, 95Dau]. It has to be distinguished between two velocity regimes, the confining value being determined by the so-called Peclet number which relates the velocity v to the melt width b and the thermal diffusivity κ . For inert gas cutting a linear approximation to the models of [93Sch2, 93Ros] was used to calculate the coefficients.

1.1.4.2.3 Factors determining efficiency The efficiency of laser processes is determined by the coupling rate η A (1.1.48) discussed in Sect. 1.1.4.1.2 and by the thermal efficiency η th according to:

η P = η A η th .

(1.1.56)

The thermal efficiency describes to what extent the absorbed power (released heat) produces the desired effect on the workpiece, e.g. the melt in welding:

η th ≡

vbsHP . ηA P

(1.1.57)

An analysis on the basis of analytic approximations [95Dau] showed that, for a given process, the thermal efficiency depends on the following parameter combination, mainly,

X ≡

η A PL ⋅ . λth TP s

(1.1.58)

It is worth to note, that the combination of material parameters in (1.1.58) is simply the inverse of the factor M in (1.1.52) which determines the threshold value. Figure 1.1.66 shows for four different metals the values of thermal efficiency calculated on the basis of [73Swi]. It demonstrates that e.g. for welding of aluminum at least 2 kW per millimeter of welding depth are necessary to achieve a satisfying efficiency level.

Thermal efficiency η th [%]

100

80

Experimental Al 6110; sandblasted Al 6110; untreated Al 5182; untreated

Calculated Ti Fe Al Cu

60

40

20

0

3 5 1 2 4 Power/welding depth P/s [kW/mm]

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6

Fig. 1.1.66. Thermal efficiency in 2D-keyhole welding. Experimental data for several Al-alloys together with curves calculated according to [73Swi] using the following values of C = ηA/L⋅Tm [kW/mm]. Ti: 24, Fe: 10, Al: 4, and Cu: 1.5.

62

References for 1.1

References for 1.1 (2/2) 41Ros

Rosenthal, D.: Welding J. 20 (1941) 220s.

45Gou

Gouffé, A.: Rev. Optique 24 (1945) 1.

54Mae

Maecker, H., Peters, T.: Z. Physik 139 (1954) 448.

59Sed

Sedov, L.I.: Similarity and Dimensional Methods in Mechanics. London: Cleaver–Hume Press, 1959.

61Bar

Bartels, J., Borchers, H., Ten Bruggencate, P., Hausen, H., Hellwege, K.-H., Schäfer, K., Schmidt, E. (eds.): Landolt-Börnstein II/4, Berlin: Springer, 1961.

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Raizer, Yu P.: Sov. Phys. JETP 21(5) (1965) 1009. Ready, J.F.: J. Appl. Phys. 36 (1965) 462 Wharton, C.B.: Microwave Techniques. In: Huddlestone, R.H., Leonard, S.L. (eds.): Plasma Diagnostic Techniques. New York, NY: Academic Press, 1965.

66Zel

Zel’dovich, Ya B., Raizer, Yu P.: Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena I, New York, NY: Academic Press, 1966.

67Tan

Tanenbaum, B.S.: Plasma Physics, New York, NY: McGraw-Hill, 1967.

68Ani 68Bas

Anisimov, S.I.: Sov. Phys JETP 27(19) (1968) 182. Basov, N.G., Gribkov, V.A., Krokhin, O.N., Sklizkov, G.V.: J. Sov. Phys. JETP 27(4) (1968) 575.

69Hoh

Hohla, K., Büchl, K., Wienecke, R., Witkowski, S.: Z. Naturforsch. 24a (1969) 1244.

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Chun, M.K., Rose, K.: J. Appl. Phys. 41 (1970) 614. Hora, H., Wilhelm, H.: Nuclear Fusion 10 (1970) 111. Raizer, Yu P.: Sov. Phys. JETP 31(6) (1970) 1148. Stull, D.R., Prophet, H.: JANAF Thermochemical Tables, US Dept. of Commerce, 1970.

71Kar

Karasev, I.G., Kirillov, V.M., Norskii, V.E., Samoilov, V.I., Ulyakov, P.I.: Sov. Phys. Techn. Phys. 15 (1971) 1523.

72Dab 72Pae

Dabby, F., Paek, U.-C.: IEEE J. Quant. Electron. 8 (1972) 107. Paek, U.-C., Gagliano, F. P.: IEEE J. Quant. Electron. 8 (1972) 112.

73Ani 73Het 73Mul

Anisimov, S.I., Rakhmatulina, A.Kh.: Sov. Phys. JETP 37(3) (1973) 441. Hettche, L.R., Schriempf, J.T., Stegman, R.L.: J. Appl. Phys. 44 (1973) 4079. Mulser, P., Sigel, R., Witkowski, S.: Physics Reports (Section C of Physics Letters) 6(3) (1973) 187. Pirri, A.: The Physics of Fluids 16(9) (1973) 1435. Prokhorov, A.M., Batanov, V.A., Bunkin, F.V., Fedorov, V.B.: IEEE J. Quant. Elec. 9(5) (1973) 503. Stegman, R.L., Schriempf, J.T., Hettche, L.R.: J. Appl. Phys. 44(8) (1973) 3675.

73Pir 73Pro 73Ste

References for 1.1

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73Swi

Swift-Hook, D.T., Gick, A.E.F.: Welding J. 52 (1973) 492s.

74Ani 74Gob 74Low 74Mar

Anisimov, S.I., Kapeliovich, B.L., Perel’man, T.L.: Sov. Phys. JETP 39 (1974) 375. Gobrecht, H.: In: Bergmann-Schaefer: Lehrbuch der Experimentalphysik, Vol. I, Berlin: de Gruyter (1974) 712 Lowder, J.E., Pettingill, L.C.: Appl. Phys. Lett. 24 (1974) 204. Martynyuk, M.M.: Sov. Phys. Tech. Phys. 19 (1974) 793.

75Fow 75Nie

Fowler, M.C., Smith, D.C.: J. Appl. Phys. 46(1) (1975) 138. Nielson, P.E.: J. Appl. Phys. 46(10) (1975) 4501.

76All 76Ash

Allmen, M. van: J. Appl. Phys. 47 (1976) 5460. Ashcorf, N.W., Mermin, N.D.: Solid State Physics, Fort Worth: Saunders College Publishing, 1976. Klemens, P. G.: J. Appl. Phys. 47 (1976) 2165. Steverding, B., Dudel, H.P.: J. Appl. Phys. 47 (1976) 1940.

76Kle 76Ste 77Mal 77Rai

Malz, D., Vogler, K.: Exp. Technik Physik 25(6) (1977) 555. Raizer, Yu P.: Laser-induced Discharge Phenomena, Studies in Soviet Science, New York: Consultants Bureau, 1977.

78Pir 78Zhi

Pirri, A.N., Root, R.G., Wu, P.K.S.: AIAA Journal 16(12) (1978) 1296. Zhiryakov, B.M., Popov, N.I., Samokhin, A.A.: Sov. Phys. JETP 48 (1978) 247.

79Kni

Knight, C.J.: AIAA J. 17(5) (1979) 519.

80Mah 80Wen

Maher, W.E., Hall, R.B.: J. Appl. Phys. 51(3) (1980) 1338. Wende, B.: Das Plasma. In: Gobrecht, H. (ed.): Bergmann-Schaefer: Lehrbuch der Experimentalphysik Vol. IV: Aufbau der Materie, Part 2, 2nd ed., Berlin: de Gruyter, 1980.

83Zie

Ziesche, P., Lehmann, G.: Ergebnisse in der Elektronentheorie der Metalle, Berlin: Springer, 1983.

84Dan 84Pop

Dan‘shchikov, E.V., Lebedev, F.V., Ryazanov, A.V.: Sov. J. Plasma Phys. 10(2) (1984) 225. Poprawe, R.: Materialabtragung und Plasmaformation im Strahlungsfeld von UV-Lasern, Ph.D. thesis, Germany: TH Darmstadt, 1984. Woodroffe, J.: In: Onorato, M. (ed.): Proc. Fourth Intl. Symposium on Gas Flow and Chemical Lasers GCL '82 (Stresa, Italy), New York, NY: Plenum Press (1984) 97.

84Woo 85Bey 85Mat

Beyer, E.: Einfluß des laserinduzierten Plasmas beim Schweißen mit CO2-Lasern, Ph.D. thesis, TH Darmstadt, Germany, Düsseldorf: DVS, 1985. Matsunawa, A.: In: Albright, C. (ed.): Proc. Laser Welding, Machining, and Materials Processing, Proc. ICALEO '85 (San Francisco, CA), Berlin: Springer (1986) 41.

86Jel 86Kit 86Miy

Jellison, G.E., Lowndes, D.H., Mashburn, D.N., Wood, R.F.: Phys. Rev. B 34 (1986) 2407. Kittel, C.: Introduction to Solid State Physics, 6th ed., New York, NY: Wiley, 1986. Miyamoto, I., Maruo, H., Arata, Y.: Proc. SPIE 668 (1986) 11.

87All1

Allmen, M. van: Laser-Beam Interactions with Materials, Materials Science 2, Berlin: Springer, 1987.

64 87All2 87All3 87Cha 87Knu 87Lar 87Pee 87Pop 87Roc

References for 1.1 Allen, P.B.: Phys. Rev. Lett. 59(13) (1987) 1460. Allen, P.B.: Phys. Rev. B 36(5) (1987) 2920. Chan, C.L., Mazumder, J.: J. Appl. Phys. 62(11) (1987) 4579. Knudtson, J.T., Green, W.B., Sutton, D.G.: J. Appl. Phys. 61(10) (1987) 4771. Larciprete, R., Stuke, M.: Appl. Phys. B 42 (1987) 181. Peebles, H.C., Williamson, R.L.: In: Arata, Y. (ed.): Proc. Intl. Conf. on Laser Advanced Materials Processing LAMP '87 (Osaka, Japan), Osaka: High Temperature Society of Japan (1987) 19. Poprawe, R., Wehner, M., Brown, G., Herziger, G.: Proc. SPIE 801 (1987) 191. Rockstroh, T.J., Mazumder, J.: J. Appl. Phys. 61(3) (1987) 917.

88Aut 88Pet 88Sok 88Ste 88Wyb

Autric, M., Vigliano, P., Astic, D., Bournot, P., Dufresne, D.: Proc. SPIE 1020 (1988) 103. Petring, D., Abels, P., Beyer, E., Herziger, G.: Feinwerktechnik & Meßtechnik 96 (1988) 364. Sokolowski, W., Herziger, G., Beyer, E.: Proc. SPIE 1020 (1988) 96. Steen, W.M., Dowden, J., Davis, M., Kapadia, P.: J. Phys. D: Appl. Phys. 21 (1988) 1255. Wybourne, M.N.: In: Properties of Silicon, EMIS Data Review Series 4, London: INSPEC (1988) 37.

89Jos 89Sok

Joseph, D.D.: Rev. Mod. Phys. 61 (1989) 41. Sokolowski, W., Herziger, G., Beyer, E.: Proc. SPIE 1132 (1989) 288.

90Ade 90Ber 90Car 90Fab

Aden, M., Beyer, E., Herziger, G.: J. Phys. D: Appl. Phys. 23 (1990) 655. Bermejo, D., Fabbro, R., Sabatier, L., Leprince, L., Orza, J.M.: Proc. SPIE 1279 (1990) 118. Carslaw, H.S., Jaeger, J.C.: Conduction of heat in solids, 2nd ed., Oxford: Clarendon, 1990. Fabbro, R., Bermejo, D., Orza, J.M., Sabatier, L., Leprince, L., Granier, V.: Proc. SPIE 1276 (1990) 461. Joseph, D.D., Preziosi, L.: Rev. Mod. Phys. 62 (1990) 375. Kosolapova, T.I.: Handbook of high temperature ceramics, New York: Hemisphere Publishing Corporation, 1990. Matsunawa, A.: Proc. Laser Materials Processing ICALEO '90 (Boston, MA). In: Ream, S.L., Dausinger, F., Fujioka, T. (eds.): Orlando, FL: LIA 71 (1991) 313. Michaelis, A., Schäfer, J.-S., Uhlenbusch, J., Viöl, W.: Proc. SPIE 1276 (1990) 231. Miller, R., DebRoy, T.: J. Appl. Phys. 68(5) (1990) 2045. Ventzek, P.L.G., Gilgenbach, R.M., Sell, J.A., Heffelfinger, D.M.: J. Appl. Phys. 68(3) (1990) 965.

90Jos 90Kos 90Mat 90Mic 90Mil 90Ven 91Che 91Gai 91Gil 91Sap 91Sib 91Sok 91Wie 92Ade 92Bas 92Bec

Cheung, N.H., Zheng, Q.Y., Kwok, H.: J. Appl. Phys. 69(9) (1991) 6349. Gaidarenko, D.V., Leonov, A.G., Chekhov, D.I.: Sov. J. Plasma Phys. 17(8) (1991) 534. Gilgenbach, R.M., Ventzek, P.L.G.: Appl.. Phys. Lett. 58(15) (1991) 1597. Sappey, A.D., Gamble, T.K.: Appl. Phys. B 53 (1991) 353. Sibold, D., Urbassek, H. M.: Phys. Rev. A 43 (1991) 6722. Sokolowski, W.: Diagnostik des laserinduzierten Plasmas beim Schweißen mit CO2-Lasern, Ph.D. Thesis, TH Aachen, Germany, Aachen: Verlag der Augustinus-Buchhandlung, 1991. Wiedeman, L., Helvajian, H.: J. Appl. Phys. 70(8) (1991) 4513. Aden, M., Beyer, E., Herziger, G., Kunze, H.: J. Phys. D: Appl. Phys. 25 (1992) 57. Basu, S., DebRoy, T.: J. Appl. Phys. 72 (1992) 3317. Beck, M., Berger, P., Nagendra, P., Dantzig, J.A.: In: Waidelich, W. (ed.): Laser in der Technik – Proc. 10th Intl. Congress Laser '91 (Munich, Germany), Berlin: Springer (1992) 429.

References for 1.1 92Duc 92Fab 92Kar1 92Kar2 92Koo 92Miy 92Pou 92Smi 92Ven 93Ade 93Ani 93Ber 93Bou 93Cal 93Geo 93Maz 93Meh 93Pou 93Qiu 93Ros 93Sch1 93Sch2 93Smu

94Ade 94Kap

65

Ducharme, R., Kapadia, P.: In: Farson, D., Steen, W.M., Miyamoto, I. (eds.): Proc. Laser Materials Processing Symposium ICALEO '92 (Orlando, FL), Orlando, FL: LIA 75 (1992) 188. Fabbro, R.: In: Matsunawa, A., Katayama, S. (eds.): Proc. Intl. Conf. on Laser Advanced Materials Processing LAMP '92 (Nagaoka, Niigata, Japan), Osaka: High Temperature Society of Japan (1992) 305. Kar, A., Chan, C.L., Mazumder, J.: Trans. ASME 114 (1992) 14. Kar, A., Rockstroh, T., Mazumder, J.: J. Appl. Phys. 71 (1992) 2560. Kools, J.C. , Baller, T., Zwart, T.D., Dieleman, J.: J. Appl. Phys. 71(9) (1992) 4547. Miyamoto, I., Maruo, H.: In: Matsunawa, A., Katayama, S. (eds.): Proc. Intl. Conf. on Laser Advanced Materials Processing LAMP '92 (Nagaoka, Niigata, Japan), Osaka: High Temperature Society of Japan (1992) 311. Poueyo, A., Deshors, G., Fabbro, R., de Frutos, A.M., Orza, J.M.: In: Matsunawa, A., Katayama, S. (eds.): Proc. Intl. Conf. on Laser Advanced Materials Processing LAMP '92 (Nagaoka, Niigata, Japan), Osaka: High Temperature Society of Japan (1992) 323. Smithells, C.J.: In: Brandes, E.A. (ed.): Smithells metals reference book, 7th ed., Oxford: Butterworth-Heinemann, 1992. Ventzek, P.L.G., Gilgenbach, R.M., Ching, C.H., Lindley, R.A.: J. Appl. Phys. 72(5) (1992) 1696. Aden, A., Kreutz, E.W., Voss, A.: J. Phys. D: Appl. Phys. 26 (1993) 1545. Anisimov, S.I., Bäuerle, D., Luk’yanchuk, B.S.: Phys. Rev. B 48(16) (1993) 12076. Berger, P., Griebsch, J., Beck, M., Hügel, H.: In: Geiger, M., Hollmann, F. (eds.): StrahlStoff-Wechselwirkung bei der Laserstrahlbearbeitung: Ergebnisse des Schwerpunktprogramms der DFG 1991 bis 1992, Bamberg, Germany: Meisenbach (1993) 15. Boulmer-Leborgne, C., Hermann, J., Dubreuil, B.: Plasma Sources Sci. Technol. 2 (1993) 219. Callies, G.: Experimentelle Untersuchungen der physikalischen Wechselwirkung zwischen Material und Laserstrahl beim Abtragen mit gepulsten Hochleistungslasern, Diploma thesis, Inst. f. Strahlwerkzeuge, Univ. of Stuttgart, Germany, 1993. Geohegan, D.B., Puretzky, A.A., Hettich, R.L., Zheng, X.Y., Haufler, R.E., Compton, R.N.: Trans. Mat. Res. Soc. Jpn. 17 (1993) 349. Mazhukin, V.I., Gusew, I.V., Smurov, I., Flamant, G.: In: Denney, P., Miyamoto, I., Mordike, B.L. (eds.): Proc. Laser Materials Processing Conf. ICALEO '93 (Orlando, FL), Orlando, FL: LIA 77 (1993) 213. Mehlman, G., Chrisey, D.B., Burkhalter, P.G., Horwitz, J. , Newman, D.A.: J. Appl. Phys. 74(1) (1993) 53. Poueyo-Verwaerde, A., Fabbro, R., Deshors, G., de Frutos, A.M., Orza, J.M.: J. Appl. Phys. 74(9) (1993) 5773. Qiu, T.Q., Tien, C.L.: J. Heat Transfer 115 (1993) 835. Rossmann, A., Simon, G.: Theorie zum Laserstrahlschneiden. In: VDI-Technologiezentrum Physikalische Technologien (ed.): Schneiden mit CO2-Lasern, Düsseldorf, Germany: VDIVerlag, 1993. Schutte, K.: Prozeßdiagnostik und technologische Untersuchungen zur Materialbearbeitung mit Excimerlasern, Ph.D. thesis, Univ. of Erlangen-Nürnberg, 1993. Schulz, W., Becker, D., Franke, J., Kemmerling, R., Herziger, G.: J. Phys. D: Appl. Phys. 26(9) (1993) 1357. Smurov, I., Aksenov, L., Flamant, G.: In: Denney, P., Miyamoto, I., Mordike, B.L. (eds.): Proc. Laser Materials Processing Conf. ICALEO '93 (Orlando, FL), Orlando, FL: LIA 77 (1993) 242. Aden, M.: Plasmadynamik beim laserinduzierten Verdampfungsprozeß einer ebenen Metalloberfläche, Ph.D. thesis, TH Aachen, Germany, Aachen: Shaker Verlag, 1994. Kaplan, A.: J. Phys. D: Appl. Phys. 27 (1994) 1805.

66 94Lin 94Lu 94Maz1 94Maz2 94Pra 94Sei 94Szy 95Bec 95Bul 95Cal 95Dau 95Dum 95Her 95Mai 95Mat 95Ohj 95Ver 95Wit 96Al 96Bec1 96Bec2 96Cha 96Eng 96Gan 96Gri 96Hue 96Kel 96Mat 96Nic 96Ohm 96Olf

References for 1.1 Lindley, R.A., Gilgenbach, R.M., Ching, C.H., Lash, J., Doll, G.L.: J. Appl. Phys. 76(9) (1994) 5457. Lu, J., Ni, X.-W., He, A.-Z.: Proc. SPIE 2004 (1994) 100. Mazhukin, V.I., Smurov, I., Flamant, G., Dupuy, C.: Thin Solid Films 241 (1994) 109. Mazhukin, V.I., Samarskii, V.V.: Surv. Math. Ind. 4 (1994) 85. Prat, C., Sarnet, T., Autric, M., Inglesakis, G.: Proc. SPIE 2502 (1994) 701. Seidel, B., Beersiek, J., Beyer, E.: Proc. SPIE 2207 (1994) 290. Szymanski, Z., Kurzyna, J.: J. Appl. Phys. 76(12) (1994) 7750. Beck, M., Berger, P., Hügel, H.: J. Phys. D: Appl. Phys. 28 (1995) 2430. Bulgakov, A.V., Bulgakova, N.M.: J. Phys. D: Appl. Phys. 28 (1995) 1710. Callies, G., Berger, P., Hügel, H.: J. Phys. D: Appl. Phys. 28 (1995) 794. Dausinger, F.: Strahlwerkzeug Laser: Energieeinkopplung und Prozeßeffektivität. Habilitation thesis, Univ. of Stuttgart, Germany. Stuttgart: B.G. Teubner Verlag, 1995. Dumord, E., Jouvard, J.M., Grevey, D.: In: Mazumder, J., Matsunawa, A., Magnusson, C. (eds.): Proc. Laser Materials Processing Conf. ICALEO '95 (San Diego, CA), Orlando, FL: LIA 80 (1995) 951. Hermann, J., Thomann, A.L., Boulmer-Leborgne, C., Dubreuil, B., Giorgi, M.L.D., Perrone, A., Luches, A., Mihailescu, I.N.: J. Appl. Phys. 77(7) (1995) 2928. Maiwa, T., Miyamoto, I., Mori, K.: In: Mazumder, J., Matsunawa, A., Magnusson, C. (eds.): Proc. Laser Materials Processing Conf. ICALEO '95 (San Diego, CA), Orlando, FL: LIA 80 (1995) 708. Matsunawa, A., Kim, J.-D., Takemoto, T., Katayama, S.: In: Mazumder, J., Matsunawa, A., Magnusson, C. (eds.): Proc. Laser Materials Processing Conf. ICALEO '95 (San Diego, CA), Orlando, FL: LIA 80 (1995) 719. Ohji, T., Shiwaku, T., Kirimura, K., Hirata, Y.: In: Mazumder, J., Matsunawa, A., Magnusson, C. (eds.): Proc. Laser Materials Processing Conf. ICALEO '95 (San Diego, CA), Orlando, FL: LIA 80 (1995) 729. Verwaerde, A., Fabbro, R., Deshors, G.: J Appl. Phys. 78 (1995) 2981. Witke, T., Lenk, A., Schultrich, B., Schultheiss, C.: Surface Coatings and Technology 74/75 (1995) 580. Al-Wazzan, R.A., Hendron, J.M., Morrow, T.: Appl. Surf. Sci. 96–98 (1996) 170. Beck, M.: In: Dausinger, F., Bergmann, H.W., Sigel, J. (eds): Proc. 6th European Conf. on Laser Treatment of Materials ECLAT '96 (Stuttgart, Germany), Wiesbaden, Germany: AWT (1996) 61. Beck, M.: Modellierung des Lasertiefschweißens, Ph.D. thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1996. Chang, J.J., Warner, B.E.: Appl. Phys. Lett 69(4) (1996) 473. Engin, D., Kirby, K.W.: J. Appl. Phys. 80 (1996) 681. Ganesh, R.K., Bowley, W.W., Bellantone, R.R., Hahn, Y.: J. Comp. Phys. 125 (1996) 161. Griebsch, J.: Grundlagenuntersuchungen zur Qualitätssicherung beim gepulsten Lasertiefschweißen, PhD thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1996. Hüttner, B., Rohr, G.: Appl. Surf. Sci. 103 (1996) 269. Kelly, R., Miotello, A.: Appl. Surf. Sci. 96–98 (1996) 205. Matsunawa, A., Kim, J.D., Katayama, S., Semak, V.V.: In: Duley, W.W., Shiabata, K., Poprawe, R. (eds.): Proc. Laser Materials Processing Conf. ICALEO '96 (Detroit, MI), Orlando, FL: LIA 81 (1996) B58. Nicolas, G., Autric, M.: Appl. Surf. Sci. 96–98 (1996) 296. Ohmura, E., Fukumoto, I.: Int. J. Jap. Soc. Prec. Eng. 30 (1996) 128. Olfert, M., Duley, W.W.: J. Phys. D: Appl. Phys. 29 (1996) 1140.

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97Ade 97Hug 97Jan 97Jue 97Koe 97Lac 97Liu 97Maz 97Mod 97Sch1 97Sch2 97Sch3 97Szy 97Whi 97Wil 97Zhi1 97Zhi2 98Amo 98Doy 98Dul 98Fab 98Her1 98Her2 98Hue 98Lun 98Mat 98Ohm 98Pin

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Schellhorn, M., Eichhorn, A.: Optics & Laser Technology 28(5) (1996) 405. Tsukamoto, S., Hiraoka, K., Asai, Y., Irie, H.: In: Duley, W.W., Shiabata, K., Poprawe, R. (eds.): Proc. Laser Materials Processing Conf. ICALEO '96 (Detroit, MI), Orlando, FL: LIA 81 B76. Witke, T.: Optische Plasmaspektroskopie und Kurzzeituntersuchungen an gepulsten Laser-, Bogen und Kanalfunkenplasmen, Ph.D. thesis, Univ. of Dresden, Germany, Aachen: Shaker Verlag, 1996. Aden, M., Kreutz, E.W., Schlüter, H., Wissenbach, K.: J. Phys. D: Appl. Phys. 30 (1997) 980. Hugenschmidt, M., Schmitt, R., Althaus, M.: Proc. SPIE 2869 (1997) 1017. Jandeleit, J., Russbüldt, P., Urbasch, G., Hoffmann, D., Treusch, H.G., Kreutz, E.W.: Proc. SPIE 3092 (1997) 481. Jüptner, W., Franz, Th., Sikau, J., Sepold, G.: Laser Physics 7(1) (1997) 202. Körner, C.: Theoretische Untersuchungen zur Wechselwirkung von ultrakurzen Laserpulsen mit Metallen, PhD thesis, Univ. of Erlangen-Nürnberg, 1997. Lacroix, D., Jeandel, G.: J. Appl. Phys. 81(10) (1997) 6599. Liu, X., Du, D., Mourou, G.: IEEE J. Quant. Electron. 33 (1997) 1706. Mazhukin, V. I., Nosov, V. V., Callies, G., Schittenhelm, H., Berger, P.: Proc. 15th IMACS World Congress (Berlin) Vol. III, Berlin: Wiss. u. Technik Verlag (1997) 271. Modest, M. F.: J. Laser Appl. 9 (1997) 137. Schittenhelm, H., Callies, G., Berger, P., Hügel, H.: Appl. Surf. Sci. 109/110 (1997) 493. Schellhorn, M., Eichhorn, A.: Proc. SPIE 3092 (1997) 522. Schellhorn, M., Eichhorn, A., Hohenberger, B.: Frühjahrstagung der DPG (Mainz, Germany), Verhandl. DPG (VI) 32 (1997) 221. Szymanski, Z., Kurzyna, J., Kalita, W.: J Phys. D: Appl. Phys. 30 (1997) 3153. White, G.K., Minges, M.L.: Int. J. Thermophysics 9 (1997) 1269. Willis, D.A., Xu, X., Poon, C.C., Tam, A.C.: In: Fabbro, R., Kar, A., Matsunawa, A. (eds.): Proc. Laser Materials Processing Conf. ICALEO '97 (San Diego, CA), Orlando, FL: LIA 83 (1997) A71. Zhigilei, L.V., Kodali, P.B.S., Garrison, B.J.: J. Phys. Chem. B 101 (1997) 2028. Zhigilei, L.V., Kodali, P.B.S., Garrison, B.J.: Chem. Phys. Lett. 276 (1997) 269. Amoruso, S., Armenante, M., Berardi, V., Bruzzese, R., Velotta, R., Wang, X.: Appl. Surf. Sci 127–129 (1998) 1017. Doyle, L.A., Martin, G.W., Al-Khateeb, A., Weaver, I., Riley, D., Lamb, M.J., Morrow, T., Lewis, C.L.: Appl. Surf. Sci. 127–129 (1998) 716. Duley, W.W.: Laser Welding, New York, NY: John Wiley & Sons, 1998. Fabbro, R., Chouf, K., Sabatier, L., Coste, F.: In: Beyer, E., Chen, X., Miyamoto, I. (eds.): Proc. Laser Materials Processing Conf. ICALEO '98 (Orlando, FL), Orlando, FL: LIA 85 (1998) F179. Herrmann, R.F.W., Gerlach, J., Campbell, E.E.B.: Appl. Phys. A 66 (1998) 35. Hermann, J., Vivien, C., Carricato, A.P., Boulmer-Leborgne, C.: Appl. Surf. Sci. 127–129 (1998) 645. Hügel, H., Berger, P., Dausinger, F.: In: Beyer, E., Chen, X., Miyamoto, I. (eds.): Proc. Laser Materials Processing Conf. ICALEO '98 (Orlando, FL), Orlando, FL: LIA 85 (1998) G141. Lunney, J.G., Jordan, R.: Appl. Surf. Sci. 127–129 (1998) 941. Matsunawa, A., Seto, N., Mizutani, M., Katayama, S.: In: Beyer, E., Chen, X., Miyamoto, I. (eds.): Proc. Laser Materials Processing Conf. ICALEO '98 (Orlando, FL), Orlando, FL: LIA 85 (1998) G151. Ohmukai, M., Takigawa, Y., Kurosawa, K.: J. Appl. Phys. 83 (1998) 3556. Pinho, G.P., Schittenhelm, H., Duley, W.W., Schlueter, A., Jahani, H.R., Mueller, R.E.: Appl. Surf. Sci. 127–129 (1998) 983.

68 98Pur 98Ret 98Sch1 98Sch2 98Sch3 98Vog 98Zef 99Agu 99Arn 99Bre 99Cal 99Dau 99Fuh 99Jeo 99Ker 99Rai 99Sch 99Sia 00Cha 00Sch1 00Sch2 01Bre 01Maz 01Sud

References for 1.1 Puretzky, A.A., Geohegan, D.B.: Appl. Surf. Sci. 127–129 (1998) 248. Rethfeld, B., Kaiser, A., Vicanek, M., Simon, G.: Proc. SPIE 3343 (1998) 388. Schittenhelm, H., Callies, G., Straub, A., Berger, P., Hügel, H.: J. Phys. D: Appl. Phys. 31 (1998) 418. Schittenhelm, H., Callies, G., Berger, P., Hügel, H.: J. Thermophys. Aeromech. 5(2) (1998) 255. Schittenhelm, H., Callies, G., Berger, P., Hügel, H.: Appl. Surf. Sci. 127–129 (1998) 922. Vogel, N., Kochan, N.: Appl. Surf. Sci. 127–129 (1998) 928. Zefferer, H.: Dynamik des Schmelzschneidens mit Laserstrahlung, PhD thesis, RWTH Aachen, Germany, Aachen: Shaker Verlag, 1998. Aguilera, J.A., Aragon, C.: Appl. Phys. A 69 (1999) S475. Arnold, N., Gruber, J., Heitz, J.: Appl. Phys. A 69 (1999) S87. Breitling, D., Schittenhelm, H., Berger, P., Dausinger, F., Hügel, H.: Appl. Phys. A 69 (1999) S505. Callies, G.: Modellierung von qualitäts- und effektivitätsbestimmenden Mechanismen beim Laserabtragen, Ph.D. thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1999. Dausinger, F., Abeln, T., Breitling, D., Radtke, J., Konov, V., Garnov, S., Klimentov, S., Kononenko, T., Tsarkova, O.: LaserOpto 31(3) (1999) 78. Fuhrich, T., Berger, P., Huegel, H.: In: Christensen, P., Denney, P., Miyamoto, I., Watkins, R. (eds.): Proc. Laser Materials Processing Conf. ICALEO '99 (San Diego, CA), Orlando, FL: LIA 87 (1999) E166. Jeong, S.H., Greif, R., Russo, R.E: J. Phys. D: Appl. Phys. 32 (1999) 2578. Kern, M.: Gas- und magnetofluiddynamische Maßnahmen zur Beeinflussung der Naht-qualität beim Laserstrahlschweißen, Ph.D. thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1999. Raiber, A.: Grundlagen und Prozeßtechnik für das Lasermikrobohren technischer Keramiken, Ph.D. thesis, Univ. of Stuttgart, Germany, Stuttgart: B.G. Teubner Verlag, 1999. Schittenhelm, H., Müller, J., Berger, P., Hügel, H.: In: Christensen, P., Denney, P., Miyamoto, I., Watkins, R. (eds.): Proc. Laser Materials Processing Conf. ICALEO '99 (San Diego, CA), Orlando, FL: LIA 87 (1999) E195. Siano, S, Pini, R., Salimbeni, R.: Appl. Phys. Lett. 74 (1999) 1233. Chang, C.-L.: Berechnung der Schmelzbadgeometrie beim Laserstrahlschweißen mit Mehrfokustechnik, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2000. Schellhorn, M.: CO-Hochleistungslaser: Charakteristika und Einsatzmöglichkeiten beim Schweißen, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2000. Schittenhelm, H.: Diagnostik des laserinduzierten Plasmas beim Abtragen und Schweißen, Ph.D. thesis, Univ. of Stuttgart, Germany, Munich: Herbert Utz Verlag, 2000. Breitling, D., Schittenhelm, H., Berger, P., Dausinger, F., Hügel, H.: Proc. SPIE 4184 (2001) 534. Mazhukin, V.I., Nosov, V.V., Smurov, I., Nickiforov, M.G.: In: German Scientific Laser Society WLT (ed.): Proc. First Intl. WLT-Conf. on Lasers in Manufacturing 2001 (Munich, Germany), Stuttgart, Germany: AT-Fachverlag (2001) 199. Sudnik, W., Erofeew, W., Radaj, D., Berger, P., Hügel, H.: Simulation der Fügetechniken – Potentiale und Grenzen: Beiträge zum DaimlerChrysler-Technologiekolloquium 2001 (Wiesensteig, Germany). In: Pollmann, W., Radaj, D. (eds.): DVS-Berichte 214, Düsseldorf, Germany: Verl. für Schweißen und Verwandte Verfahren, DVS (2001) 94.

Ref. p. 101]

2.1 Surface treatment

75

2.1 Surface treatment H.W. BERGMANN

Based on the fundamentals of interaction between laser radiation and substrate material, which are described in Part 1, Chap. 1, the present chapter reports on the technical application of laser surface treatment. The great advantage of the laser beam compared to other heat sources for surface treatment is its flexibility which allows an effective and locally defined treatment with good process control. Generally, these processes can use gaseous, liquid or solid additives and supporting media like shielding gases and coatings for improvement of the absorption. Laser surface treatment is used to change the material properties (e.g. surface hardening or impulse strengthening), to change the material and the geometry of the workpiece (e.g. coating, thermal spraying or LPVD/LCVD – laser physical/chemical vapor deposition – processes) or to change the system properties (e.g., cleaning and smoothing, changing the stress state and structuring). Depending on the desired effects, the mechanisms are either thermal, mechanical or chemical. However, an exact separation of mechanisms is difficult, because all of them interact somehow (Fig. 2.1.1). The different laser surface techniques can be divided into laser macro processing and thin-film technologies (Fig. 2.1.2).

Laser surface treatment Laser macro processing

Thin layer technology

CO2-lasers, Nd:YAG-lasers

Q-switched Nd:YAG, Excimer

cw / ms-pulsed / 0.1 ms-pulsed layer thickness: 0.05 − 10 mm

pulse duration 0.1 − 200 ns layer thickness: 0.05 − 50 µm

Surface hardening Remelting / alloying Coating

Cleaning and smoothing Alloying Coating Shock hardening

Fig. 2.1.1. Different mechanisms of influence to the material.

Fig. 2.1.2. Subdivision of laser surface treatments.

2.1.1 Laser macro processing The technique of laser macro processing is limited to thin-layer techniques with an affected depth of typically 50 µm and is usually performed with CO2- or Nd:YAG-lasers, operating either in the cw or

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2.1.1 Laser macro processing

[Ref. p. 101

millisecond pulsed mode (see Fig. 2.1.2). Examples for these processes are surface hardening of ironbased materials (solid-state hardening), remelting of cast iron or alloying techniques using a liquid phase.

2.1.1.1 Solid-state hardening Surface hardening of steels using laser radiation is a multipurpose process for generating hard, wearresistant layers on tools and components and is applicable to transformable iron-based materials [86Bra]. The surface of the workpiece is heated up to a temperature above the Ac3 temperature (end of austenization for hyper- and hypoeutectique steels) using the laser beam as a heat source (short-time austenitization, see also Sect. 2.1.1.1.2). The cold workpiece behaves as a very efficient heat sink and therefore no external quenching is necessary (self-quenching). Due to the rather short dwell time in the austenite phase region (several seconds), the peak temperature (control temperature) has to be higher than the austenitizing temperature used for conventional hardening to guarantee a complete transformation and a sufficient homogenization of the austenite. One of the great advantages of the laser beam hardening process is the relatively low distortion of the workpiece. A schematic drawing of the process is given in Fig. 2.1.3.

Fig. 2.1.3. Schematic drawing of the laser hardening process.

2.1.1.1.1 Physical basics When laser light is incident on a metallic workpiece a proportion of the radiation is reflected while the rest is absorbed by the surface and transformed into heat. This leads to heating of the workpiece in the absorption layer and, through heat conduction, to a heating of deeper layers [83Kou, 83Rot, 88Gei]. Controlling factors are the laser beam intensity, the interaction time and the material properties of the workpiece as already discussed in Part 1, Chap. 1. This can be summarized in two basic equations which describe on the one hand energy transfer and on the other mechanical changes which occur through thermal expansion: ∂T – ∇ (λ ∇ T) = Q ∂t with

(2.1.1)

T: temperature, λ: thermal conductivity, Q: heat source,

Landolt-Börnstein New Series VIII/1C

Ref. p. 101]

ρ with

2.1 Surface treatment

∂ 2ui ∂t

2

–

∂ σij = ρ Fi ∂x j

77

(2.1.2)

ρ: density, ui: movement of a volume element, σij: component of the stress tensor, Fi: force acting on a volume element.

The material's reaction is thus a function of both, temperature and stress fields, as long as the heating rates are relatively moderate (power densities < 105 W/cm2). The changes caused by the temperature field and the thermodynamic properties of the material are bound to intrinsic time constants [94Kör]. The phase reactions that occur during the thermal cycle of the hardening process of steels are summarized in TTT (Time-Temperature-Transformation) and TTA (Time-Temperature-Austenization) diagrams [54Wev, 72Ros, 73Ohr, 76Ohr]. The efficiency of optical to thermal energy transformation depends on the optical properties of the material for the wavelength of the incident radiation. Within the range of wavelengths commercially available high-power lasers provide (CO2 lasers 10.6 µm, Nd:YAG lasers 1.064 µm, diode lasers 800…940 nm), metals normally show relatively low absorption coefficients (see e.g. Part 1, Chap. 1, Table 1.1.2 with the absorption coefficient α = 1/lα according to Part 1, Chap. 1, (1.1.5)). The absorption coefficients vary with increasing temperature and depend on the surface structure of the workpiece. Several possibilities exist to enhance the absorption [88Dau, 88Rud, 92Jas]: -

thermochemical reactions between the substrate and the surrounding gas atmosphere, absorbing coatings, use of polarized laser light.

2.1.1.1.2 Material science basics Heating of a steel above the Ac1 temperature (begin of austenization) leads to a phase transformation from the iron bcc (body centered cubic) crystal structure, called ferrite, into a fcc (face centered cubic) structure, called austenite. Between Ac1 and Ac3 (see above) (and Acm – complete austenization for hypereutectique steels, dissolution of carbides) the iron carbide decomposes and the carbon dissolves into the austenite which has a high solubility for carbon. The carbon concentration gradient established after a certain time is defined by diffusion coefficients which are both temperature and concentration dependent. At high heating rates, a maximum temperature well above the Ac3 or Acm point is necessary to get a homogeneous distribution of carbon. Due to the temperature gradient, this can only be reached within the outer surface layers. With increasing distance from the surface, decreasing effects of austenitization can be observed [91Fri, 91Lep]. When the laser irradiation ends, the surface layer is cooled down rapidly by heat conduction into the core of the workpiece, which remains cold. This effect is called self-quenching. The resulting transformed microstructure depends on the cooling rate, the chemical composition and the final temperature of the surface [85Wis]. Figure 2.1.4 shows calculated time-temperature curves for laser beam hardening with typical CO2 lasers. Typical hardness-depth profiles for laser beam hardening of a 0.45 % carbon steel are displayed in Fig. 2.1.5 [91Bac].

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2.1.1 Laser macro processing

Fig. 2.1.4. Calculated time-temperature curves for laser beam hardening with a typical CO2 laser (cross section of the laser beam: 10 × 10 mm2, feed rate: 0.3 m/min.) [96Mor], z: depth below the surface, 1: z = 0 mm, 2: z = 0.5 mm, 3: z = 1.1 mm, 4: z = 2.0 mm, 5: z = 3.7 mm, 6: z = 6.6 mm, 7: z = 20 mm.

Fig. 2.1.5. Hardness-depth profiles hardening of 0.45 % carbon steel. temperatures (feed rate: 0.3 m/min.) 1200 °C, 2: Tc = 1250 °C, 3: Tc = 1350 °C.

[Ref. p. 101

for laser beam Different control [91Bac]: 1: Tc = 1300 °C, 4: Tc =

In addition to microstructural and property changes, the temperature field causes thermal expansion (according to the different expansion coefficients of ferrite and austenite) and volumetric changes due to phase transformation. This results in a transient stress field. The temperature dependent material properties (particularly the flow stress) and the hysteresis of the α → γ, γ → α phase transformation (α – body centered cubic (ferrite), γ – face centered cubic (austenite) iron) lead to elastic and plastic deformations and cause dimensional changes, distortions and residual stresses. A functional design and a locally defined heat treatment help to minimize distortions and to generate compressive residual stresses within the critical, high-strained areas of the workpiece. This helps to improve fatigue strength and therefore leads to an extended fatigue lifetime [96Mül].

2.1.1.1.3 Production-related aspects According to DIN EN ISO 11145, a system for laser beam hardening comprises a laser source, an optical arrangement for beam guiding and shaping, and a machine for manipulation of the workpiece (or the laser beam). For surface hardening of larger areas, high-power lasers in the multi-kilowatt range are necessary. The electrical overall efficiency and other properties for common laser types are given in Table 2.1.1. The laser-beam-hardening process can be performed either power- or temperature-controlled. In the first case, the output power of the laser and the time of irradiation is controlled (kept constant). In the second case, the temperature of the surface is measured within the hot zone of the laser spot (for example by pyrometric measurement) and, via a closed loop system kept at a predetermined level through control of the output power [88Ber, 93Gei]. To achieve an appropriate depth of the hardened zone, a power density of 104...105 W/cm2 and irradiation times between 0.1 s and 3 s are required. Different optical systems and components are available to generate a beam profile with the desired shape and intensity distribution [88Jüp, 90Pie, 90Rud, 90Tön1, 90Tön2, 92Ber, 94Agr]. Large areas of the surface can be hardened by movement of the laser spot (hardened tracks). If those tracks are overlapping, the first track is tempered by the second one, accompanied by changes in microstructure and properties. This should be avoided by choosing suitable hardening strategies [91Gei, 95Mül].

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Ref. p. 101]

2.1 Surface treatment

79

Table 2.1.1. Properties of common laser types for surface hardening. Laser type CO2 lasers

Properties gas laser, very high output powers available wavelength: 10.6 µm (absorption coating is necessary) beam guiding and shaping via cooled mirrors electrical overall efficiency < 12 %

Nd:YAG lasers

solid-state laser wavelength: 1064 nm beam guiding via glass fibers, beam shaping via lenses electrical overall efficiency < 5 %

Diode lasers

semiconductor laser, arrays of many single diodes wavelength: 800…940 nm electrical overall efficiency > 20 % small in size and weight

2.1.1.1.4 Time scheme of the irradiation The effect of pulsed irradiation on the resulting microstructures, properties and hardness depths has been discussed in [97Ber]. Different hardness-depth profiles attained using either cw or pulsed radiation with various frequencies are shown in Fig. 2.1.6. The results are generally similar, but using the pulsed mode, a slight increase of the hardening depth can be achieved. On the other hand, by using pulsed radiation with frequencies below 50 Hz, the risk of surface melting increases, while the desired compressive residual stress is not achieved [95Dom].

Fig. 2.1.6. Hardness-depth profiles for laser beam hardening in the cw and pulsed mode with different pulse frequencies (Nd:YAG-laser, feed rate: 0.3 m/min.) [97Ber]: 1: cw, 2: 100 Hz, 3: 50 Hz, 4: 25 Hz, 5: 12.5 Hz.

2.1.1.1.5 Observed degradations and their reasons Table 2.1.2 gives possible degradations which can be observed occasionally (also in classical surface hardening) and their metallurgical and/or process-related reasons.

Landolt-Börnstein New Series VIII/1C

80

2.1.1 Laser macro processing

[Ref. p. 101

Table 2.1.2. Degradations of laser-surface-hardened parts and their metallurgical and process-related reasons [96DIN1]. Characteristic feature 1 surface hardness too low

Reason conditional on the heat treatment Relevant fault during surface hardening 1.1

untransformed ferrite, pearlite, or too less carbides dissolved

a) b)

1.2 proportion of martensite too low 1.2.1 due to formation of bainite and/or a) pearlite and/or ferrite b) c) d) e) 1.2.2 due to retained austenite a) b) c) d)

1.3

e) martensite too soft (also locally a) defined) b) c)

austenitizing temperature too low time of irradiation too short

austenitizing temperature too low time of irradiation too short (self-)quenching effect too low severe oxidation of the surface decarburization austenitizing temperature too high (overheating) time of irradiation too long (oversoaking) not, too late or not adequately tempered not, too late or not adequately subzero-treated carburization due to coatings tempering temperature too high tempering time too long tempering because of overlapping tracks not tempered or too late tempered tempering temperature too low tempering time too short

1.4

proportion of retained austenite a) too high and/or too less carbide b) precipitated c)

2 surface hardness too high

2.1

martensite within the surface a) layer too hard b) c)

not tempered tempering temperature too low not tempered a sufficient number of times

3 insufficient depth of hardening

3.1

spatially incomplete austenitiza- a) tion b) proportion of martensite too low a)

austenitization temperature too low time of irradiation too short insufficient quenching rate

4 depth of harde- 4.1 ning too high

depth of austenitization too high

austenitizing temperature too high time of irradiation too long

5 too much distortion

(thermal and/or transformation a) induced) internal stresses too high or too asymmetric b) c)

3.2

5.1

a) b)

too rapidly or too unsteadily heated and austenitized unsuitable fixture of the workpiece unfavorable size/shape of the heattreated area

Landolt-Börnstein New Series VIII/1C

Ref. p. 101]

2.1 Surface treatment

81

Table 2.1.2 continued. Characteristic feature 6 cracks

Reason conditional on the heat treatment Relevant fault during surface hardening 6.1

(thermal and/or transformation a) induced) internal stresses too high (fracture toughness ex- b) ceeded) c) d) e)

too rapidly or too unsteadily heated and austenitized not tempered tempering temperature too low tempering time too short overlapping tracks

7 deviations in the shape of edges and/or surfaces

7.1

partial melting

temperature too high time of irradiation too long power flux density of the beam too high overheating of an edge

a) b) c) d)

2.1.1.2 Remelting In contrast to solid-state hardening, the surface is heated up to a temperature above the melting point. The aim of laser surface melting is to generate a surface layer with better properties than the substrate material resulting from rapid solidification. The quenching rates that can be obtained can be as high as those achieved in completely different techniques, like melt spinning or powder atomization. These extremely high cooling rates determine the nature and stability range of phases, their distribution, size and morphology [96Mor]. The solidification process can be characterized by three important parameters: -

the quenching rate ε = dT/dt, the solidification velocity R = υ • n (velocity of the solidification front), the temperature gradient G = n ∇ T across the liquid/solid interface.

These parameters are related by the equation

ε = R G .

(2.1.3)

As Fig. 2.1.7 illustrates, these parameters depend on the shape of the melt pool [93Her], which is determined by the laser parameters, the material properties and the melt dynamics due to convection within the pool [84Böt, 86Rap, 90Kre]. Laser v C

A R

α

Gx

α

L

time of melt pool existence: τ = L/υ, temperature gradient: Gz = dT , Gx = dT ,

v

Gz

Fig. 2.1.7. Schematic drawing of the melting pool and definition of the characteristic parameters of the solidification process:

dz

dx

solidification velocity: R = υ cos α, B

TM

cooling rate: dT = ε = R G

dt

(cooling rate corresponds to quenching rate).

Landolt-Börnstein New Series VIII/1C

82

2.1.1 Laser macro processing

u eq

ia

x

d

rit

ic

10 =

4

ε

10 2

ed

d en

[Ref. p. 101

10

2

K/

s

0

rit de

nd

10 0

−2

10

Solidification velocity R [mm/s]

ic

10

10

−2

r

la

lu el

10

10

c −4

pl

an

ar

−4

10 −1

10 0 10 1 10 2 Temperature gradient G [K/mm]

10 3

Fig. 2.1.8. Influence of the solidification velocity R and the temperature gradient G on the solidification morphology [86Kur].

The actual values of the variables ε , R and G determine the resulting microstructure [81Kur, 83Cly, 83Sah], which can be epitaxial, dendritic, planar, cellular or even amorphous. The relationship is given in Fig. 2.1.8 [86Kur]. Remelting can be performed without additives on cast iron and steels and with or without additives on several nonferrous metals. If additives are used, they can either be pre-deposited on the surface of the workpiece (two-step process), see Fig. 2.1.9a, or they can be supplied continuously during the melting process (one-step process), see Fig. 2.1.9b. In the one-step process, the additives can either be supplied in backhand or forehand direction (Fig. 2.1.9c and d).

Landolt-Börnstein New Series VIII/1C

Ref. p. 101]

2.1 Surface treatment

83

Fig. 2.1.9. Different methods of laser surface remelting: (a) two-step process, (b) one-step process, (c) backhand remelting, (d) forehand remelting.

2.1.1.2.1 Remelting of cast iron Iron-carbon alloys can solidify according to the metastable Fe-Fe3C phase diagram or the Fe-graphite equilibrium phase diagram. The relative stability of cementite (Fe3C) and graphite is effected by alloying elements. Generally, the stability of graphite increases with increasing content of carbon and silicon (gray cast iron). Rapid cooling favors the formation of cementite. This effect is used in laser surface treatment of cast iron. If the surface of gray cast iron is melted, the extraction of the heat into the bulk substrate leads to resolidification and a self-quenching effect of the surface layer. The melt solidifies as ledeburite, an eutectic mixture of Fe3C and austenite. The mechanical properties of this layer differ dramatically from those of the gray cast iron, especially the hardness and wear resistance is improved. Figure 2.1.10 gives an example for a hardness-depth profile of a laser-remelted camshaft made of GGL 25. The hardness of the resolidified layer reaches between 800 and 900 HV 0.1. A remarkable spread of the values within the transition area is due to microstructural effects (primary Fe-dendrites and ledeburite) [92Mül]. Typical hardness values for different alloyed cast irons obtained after laser surface melting are given in Table 2.1.3. Table 2.1.3. Hardness of various remelted cast irons in the as-quenched state [85Ber]. Cast-iron composition [wt.-%] C Si Mn 3.5 2.0 ... 3.5 0.5 ... 2.5 1.7 ... 3.0 2.4 6.0 2.1 5.0 ... 2.7 0.3 ...

Landolt-Börnstein New Series VIII/1C

Hardness [HV] P 1.0 2.0 ... ... ... ...

Cr ... ... ... 12.0 ... 28.0

Ni ... ... 33.0 ... ... ...

1100...1200 1100...1200 450...550 450...550 600...650 600...650

84

2.1.1 Laser macro processing

Fig. 2.1.10. Hardness-depth profile of a laser surface remelted camshaft of lamellar gray cast iron (GGL25) [92Mül]; material: lamellar gray cast iron (GGL25), laser output power: 8 kW, intensity profile: line (16 mm × 1.5 mm), feed rate: 0.8 m/min., shielding gas: CO2, pre-heating: 480 °C, after-process treatment: 580 °C/30 min.

[Ref. p. 101

Fig. 2.1.11. Influence of the laser output power and the feed rate on the depth of the molten zone [92Mül]; material: ductile cast iron (GGG60), intensity profile: line (15 mm), surface preparation: phosphate, initial surface roughness: Rz = 15 µm.

The depth of the melt typically varies between 0.5 and 1.5 mm. Figure 2.1.11 demonstrates the influence of the laser output power and the feed rate on the depth of the molten zone [92Mül]. The effect of the surface roughness before surface melting on the melting depth can be neglected. The roughness of the surface after laser surface melting is in the magnitude of about 10 µm and depends on process parameters and supply of the shielding gas [88Gil]. The properties of the heat-affected zone between the surface layer and the unaffected substrate depend on the microstructure of the matrix. This is an important factor in deciding upon the layer specifications to meet the requirements of the component in service.

2.1.1.2.2 Remelting of aluminum alloys One of the main difficulties in surface remelting of aluminum and aluminum alloys is the poor absorption of infrared laser light (see e.g. Part 1, Chap. 1, Table 1.1.3 and Fig. 1.1.1). The absorption can be improved by grinding or sand blasting of the surface, providing higher roughness, but usually coatings are used to get satisfactory results. Furthermore, use of shielding gas is necessary to prevent reactions between the melt and the atmosphere (for example oxygen in the atmosphere). A significant increase of the surface hardness and therefore of wear resistance can be achieved by the following mechanisms: -

grain refinement, precipitation hardening, dual phase hardening / dispersion hardening.

An example for laser surface remelting of nonferrous metals without using additives is the surface treatment of eutectic or near eutectic Al-Si-alloys (e.g. pistons for automotive engines). Due to rapid quenching, laser surface remelting results in a very close-grained microstructure, which has better wear resistance than the cast structure, although the hardness of the surface layer is not enhanced significantly [96Mor]. The range of the solid solubility in binary aluminum alloys can be extended significantly by rapid quenching. This leads to a combination of solid solution hardening and/or precipitation hardening. The strength of Al-alloys at room temperature and at elevated temperatures can be improved by precipitation

Landolt-Börnstein New Series VIII/1C

Ref. p. 101]

2.1 Surface treatment

85

hardening. For an efficient precipitation hardening process a high solid solubility of the 2nd element in the Al-rich crystal at higher temperatures is desired, simultaneously with almost no solid solubility at room temperature. If in addition high-temperature strength is required, it is necessary that stable intermetallic phases with high melting point precipitate. Table 2.1.4 gives various examples for alloying elements, their solid solubility and the melting point of the intermetallic compounds, which can be precipitated.

Table 2.1.4. Solid solubility of transition metals in binary aluminum alloys and some of their intermetallic compounds. Alloying elements

Solid solubility Intermetallic Melting in rapid compound temperature of solidification the inter[at. %] metallic compound [°C] 1.9 5.0 6.0 CrAl7 725 940 Cr2Al11 1010 CrAl4 1170 CrAl3 4.4 FeAl3 1160 9.0 MnAl6 710 822 MnAl4 880 MnAl3 7.7 NiAl3 854 1133 Ni2Al3

Ce Co Cr

Solid solubility in conventional solidification [at. %] 0.01 0.01 0.38

Fe Mn

0.03 0.70

Ni

0.02

Ti Zr V

0.74 0.08 0.20

2.0 3.0 2.0

W Mo

0.02 0.07

1.9 1.0

TiAl3 ZrAl3

1340 1580

Comments

rarely used improvement of corrosion resistance

suitable for Al-Si-alloys, formation of extremely hard Mn-silicides very fine eutectic with good strength and ductility suitable alloying element suitable alloying element extremely high hardness, but inhomogeneous layers extremely high hardness, but inhomogeneous layers

Laser remelting allows to disperse hard particles into the surface layers. These particles have to meet the following conditions: -

thermal stability within the temperature range of application, chemical stability against the liquid metal (e.g. no oxides that can be reduced by aluminum), higher hardness than the wear partner of the component, no porosity (explosion under the laser irradiation), within a suitable density range (otherwise sedimentation or floating of the powders).

Table 2.1.5 gives examples for hard particles which can be dispersed in aluminum alloys.

Landolt-Börnstein New Series VIII/1C

86

2.1.1 Laser macro processing

[Ref. p. 101

Table 2.1.5. Properties of ceramic components for dispersion in an Al-matrix. Density [g/cm3]

Hardness [GPa]

Melting / decomposition temperature [°C]

Price / availability

Comments

3.97 5.8 2.3

18...23 10...15 10...13

2050 (melt.) 2700 (melt.) 1723 (melt.)

++ + +++

not chemically attacked not chemically attacked

SiC

3.2

20...30

2300 (decomp.)

++

dissociated, if the particle size is too small

TiC Cr2C3 B4C diamond

4.9 6.7 2.51 3.5

approx. 28 10...18 approx. 30 –

3140 (melt.) 1980 (melt.) 2450 (melt.) decomp. dep. on heating rate

0 – + ––

Si3N4

3.18

8...19

1900 (decomp.)

+

TiN AlN

5.4 3.2

16...20 approx. 12

2930 (melt.) 2235 (decomp.)

– ++

4.5

15...28

2900 (melt.)

–

Material

Oxides Al2O3 ZrO2 SiO2 Carbides

Nitrides reaction products around the particles

Borides TiB2

The appropriate particle size is between 5 µm and 50 µm (for a layer thickness of some 100 µm). If the particles are too large, they sometimes contain thermal cracks. Smaller particles are inefficient barriers to plastic deformation, hence wearing out of the surface can occur. A volume fraction of 10...30 % of hard particles within the surface layer has shown good results. 2.1.1.2.3 Remelting of titanium alloys Titanium alloys have inherent advantages of high strength combined with low density. Their tribological behavior is characterized by high friction coefficients and poor wear resistance. The hardness values are typically 300...350 HV. TiN, which is formed by the reaction Ti + ½ N2 → TiN, has a hardness of about 2400 HV. Thus it can be used to generate hard and wear-resistant surface layers on titanium and titaniumalloy components. Conventional solid-state treatments (e.g. annealing within a nitrogen atmosphere or plasma heat treatment) depend on diffusion processes and therefore on a parabolic law for the increase of the layer thickness. Within reasonable times, a layer thickness of some 10 µm can be reached. Laser remelting of the surface within a reactive atmosphere (“gaseous laser alloying”) is an alternative treatment for titanium and Ti-alloys. It can provide a layer thickness of typically 0.5...1 mm. If the layer is too thick, it tends to crack. The hardness of the surface depends on the laser parameters and on the composition of the atmosphere (Fig. 2.1.12). With pure nitrogen, a surface hardness of ∼900 HV can be reached [86Ber]. Table 2.1.6 Landolt-Börnstein New Series VIII/1C

Ref. p. 101]

2.1 Surface treatment

87

gives examples of the properties of gaseous laser-alloyed TiAl6V4 (IMI318) for different thermochemical treatments. Dispersing of particles (e.g. TiC) into titanium alloys is also possible [90Gas], but the extreme reactivity of titanium melts restricts the number of appropriate powders.

Fig. 2.1.12. Hardness-depth profiles of laser-nitrided TiAl6V4: (a) Influence of the laser output power; CO2-laser, cw-mode, feed rate: 1 m/min., pre-heating: none, atmosphere: N2, 1: 700 W, 2: 600 W, 3: 500 W, 4: 400 W, 5: 300 W, 6: 200 W. (b) Influence of the atmosphere (nitrogen content); CO2-laser, cw-mode, feed rate: 1m/min., preheating: none, atmosphere: 1: 100 % N2, 2: 90 % N2, 3: 75 % N2, 4: 50 % N2, 5: 25 % N2, 6: 10 % N2.

Table 2.1.6. Properties of gaseous laser-alloyed TiAl6V4 (IMI318) for different thermochemical treatments. Treatment

Color

Microstructure

Phases

as received

silver

equiaxial grains

α', β-Ti

remelted under N2

golden

TiN-dendrites in α'-matrix

α'-Ti + TiN

dense layer

remelted under CH4 + Argon

gray

TiC-precipitates in α'-matrix

α'-Ti + TiC

inhomogeneous layer

repeatedly remelted under CH4 + Argon

silver

TiC-dentrides in α'-matrix

α'-Ti + TiC

homogeneous, dense layer

remelted under O2

dark gray

massive oxide

TiO + TiO2

inhomogeneous layer thickness

Landolt-Börnstein New Series VIII/1C

Comments

88

2.1.1 Laser macro processing

[Ref. p. 101

2.1.1.2.4 Remelting of magnesium alloys Due to their very low specific densities (1.75...1.85 g/cm3), magnesium alloys are of great interest for some technical application, but the poor wear and corrosion properties cause some restrictions. Table 2.1.7 summarizes the chemical composition of some commonly used Mg-alloys and the microhardness that can be reached by laser surface remelting without additives, due to grain refinement and microstructural changes [94Gal]. A significant increase of the surface hardness can be obtained by laser alloying with powders of elements that can form hard and stable intermetallic phases with magnesium. Table 2.1.8 gives some examples. The results of laser-alloying experiments are summarized in Table 2.1.9 [94Gal, 95Gal]. Table 2.1.7. Chemical composition (wt.%) of common Mg-alloys and their microhardness after laser surface remelting. Alloy

Condition Al

cp Mg Al 80 AZ 61 AZ 91 WE 54

as cast extruded extruded

Zn

0.8 6.0 9.0

Mn

1.0 1.0

Y

Zr

RE

Mg

2.9

99.9 rem rem. rem. rem.

0.3 0.3

cast, T6

5.1

0.5

Hardness of substrate [HV 0.1] 35

Hardness of laser track [HV 0.1] 45

60 70 95

75 115 75 (85 after precipitation hardening)

Table 2.1.8. Alloying elements and their intermetallic compounds with magnesium. Alloying element

Melting point [°C]

Max. solubility [at. %]

Intermetallic compound

Al Cu

660 1083

11.5 0.013

Ni Si

1453 1410

0.04 0.01

Mg17Al12 Mg2Cu MgCu2 Mg2Ni Mg2Si

Melting point of the compound [°C] 402 568 819 760 (peritectic) 1085

Table 2.1.9. Results of laser-alloying experiments on Mg-alloys. Substrate

Alloying element Al Al Al

Alloying content [at. %] 29 16 26

cp Mg cp Mg AZ 91

Hardness HV 0.1 200 100 180

cp Mg Al 80 AZ 61 AZ 91 WE 54

Cu Cu Cu Cu Cu

41 33 30 45 31

180 230 220 180 230

WE 54

Cu

38

220

Structure of the intermetallic compound compact eutectic fibers compact

Comments

compact oriented dendrites dendrites compact needle shaped dendrites needles

resulting microstructure strongly dependent on the alloying elements of the substrate

relatively low increase of hardness, above 34 at. % Al completely intermetallic compound due to extended solubility after rapid quenching

Landolt-Börnstein New Series VIII/1C

Ref. p. 101]

2.1 Surface treatment

89

Table 2.1.9 continued. Substrate

Alloying element Ni Ni Ni Ni

Alloying content [at. %] 32 28 36 32

cp Mg Al 80 AZ 91 WE 54

cp Mg AZ 61 AZ 61 WE 54

Hardness HV 0.1 300 250 320 300

Si Si Si Si

23 22 28 27

260 220 300 280

Structure of the intermetallic compound oriented dendrites oriented dendrites dendrites needle shaped dendrites

Comments

dendrites dendrites dendrites dendrites

hardness values up to 750 HV are possible with a Si content of ∼55 at. % (primary Si with Si-Mg2Sieutectic)

hardness can be enhanced by higher Ni contents, but the resulting embrittlement leads to cracking

2.1.1.2.5 Observed degradations of surface-remelted components and their reasons Table 2.1.10 gives possible degradations which can be observed occasionally on surface-remelted components of cast iron and their metallurgical and/or process-related reasons. An overview of the degradations on surface-remelted components (with additives) of nonferrous alloys is given in Table 2.1.11. Table 2.1.10. Occasionally complained degradations on surface-remelted components of cast irons and their metallurgical and process-related reasons [96DIN2]. Characteristic feature 1 too many pores within the surface layer

Reason conditional on the heat treatment 1.1 overheating of the melt

Relevant fault during surface hardening a) time of irradiation too long or intensity too high

2 graphite within the 1.2 incomplete melting of the surface a) molten layer layer

time of irradiation too short or intensity too low

3 melted depth too low

3.1 line energy too low

a) b)

energy flux density too low feed rate too high

4 melted depth too high

4.1 line energy too high

a) b)

energy flux density too high feed rate too low

5 surface hardness too low

5.1 carbide content within the surface layer too low

a)

5.2 content of retained austenite too high within the surface layer

a)

solidification rate too high, due to too high energy flux density and too short time of irradiation inadequate cooling rate below the Ac1-temperature not or insufficiently tempered

b) 6 surface hardness too high

Landolt-Börnstein New Series VIII/1C

6.1 carbide content within the surface layer too high

a)

6.2 content of martensite too high within the surface layer

a)

solidification rate too low, due to too less energy flux density and too long time of irradiation cooling rate below the Ac1temperature too high

90

2.1.1 Laser macro processing

[Ref. p. 101

Table 2.1.10 continued. Characteristic feature 7 too much distortion

Reason conditional on the heat treatment 7.1 thermal and transformation induced stresses too high or too asymmetric

7.2 displacement of the melt too large

8 cracks

8.1 thermal and transformation induced stresses too high or too asymmetric (fracture toughness exceeded)

Relevant fault during surface hardening a) too rapidly or too unsteadily heated and remelted b) unsuitable fixture for the workpiece c) unfavorable size/shape of the heat-treated area a) inhomogeneous energy distribution b) inadequate position of the workpiece a) b) c) d) a)

8.2 too much deformation in the incompletely solidified state (hot cracking) b) c)

9 deviations in the shape of edges and/or surfaces

9.1 surface of the melt deforms too much

a) b) c) d) e)

pre-heating temperature too low too rapidly or too unsteadily heated and remelted too rapid or too unsteady cooling not or insufficiently tempered inadequate energy distribution of the beam unsuitable fixture of the workpiece unfavorable size/shape of the heat-treated area time of irradiation too long energy flux density too high inhomogeneous intensity distribution inadequate position of the workpiece unfavorable size/shape of the heat-treated area

Table 2.1.11. Occasionally complained degradations on surface-remelted components of nonferrous metals (with additives) and the metallurgical and process-related reasons [96DIN2]. Characteristic feature 1 too many pores in the surface layer

Reason conditional on the heat treatment 1.1 overheating of the melt

Relevant fault during surface hardening a) time of irradiation too long or intensity too high

2 washing-in of the substrate material

1.2 incomplete melting of the surface a) layer

time of irradiation too short or intensity too low

3 melted depth too low

3.1 line energy too low

energy flux density too low feed rate too high insufficient pre-deposition or feed rate of the additive material too low

3.2 content of additives too low

a) b) a)

Landolt-Börnstein New Series VIII/1C

Ref. p. 101]

2.1 Surface treatment

91

Table 2.1.11 continued. Characteristic feature 4 melted depth too high

Reason conditional on the heat treatment 4.1 line energy too high 4.2 content of additives too high

5 surface hardness too low

5.1 content of hard particles within the surface layer too low

Relevant fault during surface hardening a) energy flux density too high b) feed rate too low a) excessive pre-deposition or feed rate of the additives too high a)

b) c)

6 surface hardness too high

6.1 content of hard particles within the surface layer too high

a)

b) c) 7 too much distortion

7.1 thermal and transformation induced stresses too high or too asymmetric

a) b) c)

7.2 displacement of the melt too large

a) b)

8 cracks

8.1 thermal and transformation induced stresses too high or too asymmetric (fracture toughness exceeded) 8.2 too much deformation in the incompletely solidified state (hot cracking)

a) b) c) a) b) c)

9 deviations in the shape of edges and/or surfaces

9.1 surface of the melt deforms too much

a) b) c) d) e)

Landolt-Börnstein New Series VIII/1C

inadequate solidification rate due to unsuitable choice of energy density and irradiation time inadequate cooling rate after the resolidification insufficient pre-deposition or feed rate of the additive material too low inadequate solidification rate due to unsuitable choice of energy density and irradiation time inadequate cooling rate after the resolidification excessive pre-deposition or feed rate of the additives too high too rapidly or too unsteadily heated and remelted unsuitable fixture of the workpiece unfavorable size/shape of the heat-treated area inhomogeneous energy distribution inadequate position of the workpiece too rapidly or too unsteadily heated and remelted too rapid or too unsteady cooling not or insufficient tempered inadequate energy distribution of the beam unsuitable fixture of the workpiece unfavorable size/shape of the heat-treated area time of irradiation too long energy flux density too high inhomogeneous intensity distribution inadequate position of the workpiece unfavorable size/shape of the heat-treated area

92

2.1.1 Laser macro processing

[Ref. p. 101

2.1.1.3 Laser cladding Laser cladding is used to generate a surface layer of an alloy different from the substrate material to improve the wear and/or corrosion properties of a component [86Maz, 89Wu]. Laser cladding can be performed either in a one-step process [90Lug1, 92Lug, 94Lug] or in a two-step process (pre-deposited material method) [90Mar1] (compare with Fig. 2.1.9a and b). Furthermore, the two-step process can be performed either in a free-cladding or in a shape-determined way using a crucible (Fig. 2.1.13) [94Lan, 95Lan].

Fig. 2.1.13. Different methods of laser cladding: (a) free cladding, (b) shape-determined cladding using a crucible.

In contrast to the remelting processes like alloying and dispersing (Sect. 2.1.1.2), the amount of molten substrate material is very low only ensuring a metallurgical bond between the substrate and the layer. The major part of the clad is built up by the deposited material. Therefore final shape of the workpiece is defined by the thickness and shape of the layer. Using the two-step process, different forms of pre-deposition can be taken into account, depending on the application, like -

powders, pastes, foils, wires, thermally sprayed coatings.

In the application, the one-step process is often preferred, because it is easier to control. The most common method of laser surface cladding is to inject the material in form of powder or wire [89Bur] through a nozzle into the laser/material interaction zone. If a powder is used, a certain amount of the powder, which does not hit the molten pool, gets lost. Powder efficiency and surface quality are very sensitive to the geometrical arrangement. The influence of the incident direction of the powder stream on the powder efficiency is discussed in Fig. 2.1.14. Alternatively, a coaxial powder feed nozzle can be used, promising best results and avoiding directional effects on the shape of the clad bead [96Lin].

Landolt-Börnstein New Series VIII/1C

Ref. p. 101]

2.1 Surface treatment

93

100

N

oz

zl

e

−

Position of the nozzle

80

+

Efficiency [%]

Laser

40

10

θ1

60

Y axis

20

Sample displacement

a

0

10 20

60 50

−

θ 2-0°

Position of the nozzle

+

+θ 2

b

X axis

Efficiency [%]

− θ2

θ 2 -180°

30 40 50 60 70 80 90 θ 1 [degrees]

40 30 20 10 0 -180

-90

0 θ 2 [degrees]

90

180

Fig. 2.1.14. Influence of the incident direction of the powder stream on the powder efficiency [90Mar1]: (a) influence of angle θ1 (θ2 = 0°), (b) influence of angle θ2 (θ1 = 55°). Substrate: mild steel (St37), powder: Stellite 6, laser output power: 1500 W, beam diameter: 2 mm, speed: 0.8 m/min., step: 0.6 mm, powder flow rate: 6 g/min., gas flow rate: 1.3 l/min.

Using a mixture of different powders, in which one component has a high melting point and low absorption coefficient, a dual phase layer containing hard particles can be generated [84Tuc] (e.g. for cutting tools). Taking into account that both components may differ severely in density, it has to be considered that floating or sedimentation of the particles may occur. A vertical cladding position is an alternative method to prevent this effect [95Lan]. Crack-free cladding is essential for satisfactory corrosion resistance. The different thermal expansion coefficients of the substrate and the coating often result in tensile residual stresses within the cladded coating. As a consequence, cracking can occur even after the process but prior to the application of any additional load. Pre-heating of the component and/or post-annealing can help to reduce the residual stresses [86Dek]. Examples for laser cladding on different substrate materials and possible applications are given in Table 2.1.12.

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94

2.1.1 Laser macro processing

[Ref. p. 101

Table 2.1.12. Examples of laser cladding on different substrate materials. Substrate material Cladding various NiCrBSi

mild steel

NiCrBSi NiCrTaB 50 vol.-% TiC, 50 vol.-% Stellite 6 Co-base alloy with TIC or WC FeCrAlY

CoCrAlY

mild steel (St37) mild steel (St37) mild steel (St37)

steels

Stellite 6 45Fe-55Cr Fe-12.5Cr-9.5P1.7Y Fe-12.5Cr-9.5P1.7Y Ni-25Cr-5.2Al2.7Si-0.5Y NiCrBSi

steels

NiCr(Nb,Ta)B

45NiCr6

Stellite 21

GGG40

Ni

14CrMoV6-9 NiCr20AlTi

Mo-alloy NiCrAlY

pure Ni IN738

mild steel (St52)

Al2O3/TiO2 (97/3) Al2O3/TiO2 (87/13) mild steel (St52) + Al2O3/TiO2 (97/3) bondcoat Ti (laser remelting) mild steel (St52) ZrO2/Y2O3 (92/8) X6CrNiTi18-10 Al2O3/TiO2 (97/3) X6CrNiTi18-10 Al2O3/TiO2 (87/13) X6CrNiTi18-10 PSZ: ZrO2/Y2O3 (92/8) X6CrNiTi18-10 Al2O3 X6CrNiTi18-10 Cr2O3 X6CrNiTi18-10 Cr2O3/SiO2/TiO2 (90/5/3)

Method two-step (pre-placed paste) blown powder blown powder blown powder

Comments Ref. superior wear properties to 79Bel plasma-sprayed coatings, esp. at high pressures

good metallurgical bonding WC superior to TiC

two-step (pre-placed powder) two-step (pre-placed powder) blown powder blown powder blown powder

84Tuc

excellent oxidation resistance 83Liu at 1000...1200 °C excellent oxidation resistance 83Liu at 1000...1200 °C, but cracking problems 90Mar2 90Mar2 90Mar2

blown powder

90Mar2

blown powder

90Mar2

one-step / two-step better corrosion resistance than austenitic steel and Nimonic 75 one-step / two-step better corrosion resistance than austenitic steel Nomonic 75 and NiCrBSi coatings two-step, repairing of dies coaxial-nozzle remelting of transport containers for plasma-sprayed nuclear application layer one-step extruder parts, 780...1130 HV one-step exhaust valves of large diesel engines one-step no sufficient bondage! one-step no sufficient bondage! one-step improved wettability of metal to ceramic

90Lug2

one-step one-step one-step one-step one-step one-step one-step

cracks and pores free of pores and cracks free of pores and cracks Y freely soluble in Zr2O3 cracks and pores <100 µm crack free wide range of parameters possible

90Lug2

96Gas 96Gas

96Gas 88Ame 94Lug 94Lug 94Lug

94Lug 94Lug 94Lug 94Lug 94Lug 94Lug 94Lug

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95

2.1.2 Thin-layer technologies Thin-film technologies require either q-switched Nd:YAG-lasers or excimer radiation. With a pulse duration between 0.1 and 200 ns, a layer thickness of typically 0.05 up to 50 µm can be generated. In the following examples different thin-film technologies, such as cleaning, smoothing and surface alloying are reported. According to the surface characteristics of a technical material, different absorption mechanisms are involved, e.g. for organic contaminations, oxide films, deformed layers or the bulk substrate. Varying the energy density a different response of the material is observed. This is illustrated in Fig. 2.1.15, where the typical change in properties (e.g. the surface roughness) as a function of the excimer laser intensity is shown for metal surfaces as an example. Low intensities do not change the substrate, whereas a slight increase in energy density causes cleaning by evaporation of organic or inorganic (desoxidation) contaminants. Higher intensities cause etching effects followed by smoothing or roughening (in case of exceeding plasma level). The highest densities may be used for alloying [89Sch] or for thermal removal of thin layers. This behavior can be explained by the typical structure of a technical metal surface (Fig. 2.1.16). Above the non-deformed base material a whole series of layers with different deformation or oxidation states follows. The surface region contains the fingerprints of each production step, so that organic contaminations, oxides or residues of grinding materials will be present, followed by regions of different deformation caused by turning, grinding or polishing. This detailed description of a technical surface is necessary, because the short interaction time and the high pulse energy allows only modifications in depth regions between 0.1 and 1 µm in single-pulse experiments (with multiple-exposure treatments up to 20 µm are possible). The temperature gradient produced does not lead to any thermal influence in a substrate depth of 1 µm while the surface is at plasma temperature [90Ber]. The necessary intensity to produce plasma formation is different for organic adsorbates, oxide layers and the real substrate. This explains the different processing results obtained with various energy densities.

Fig. 2.1.15. Typical change in properties (e.g. the surface roughness) as a function of the excimer laser intensity for metal surfaces [90Ber].

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2.1.2 Thin-layer technologies

[Ref. p. 101

Fig. 2.1.16. Typical structure of a technical metal surface [90Ber].

Treating ceramics, a sealing effect combined with reduced roughness occurs at energy densities above the plasma level. The strategy in ceramics processing is completely different to the machining of metals. Due to their brittleness ceramic materials are extremely sensitive to tensile and shear stresses, because they cannot diminish them by plastic deformation. Another crucial factor for the lifetime is the sensitivity for cracks. Once a crack is present, it will propagate very fast and result in brittle failure of the component, so that crack initiation determines lifetime. To improve ceramics, the size of the largest occurring crack or defect has to be reduced. Crack propagation will occur if a crack is already present (micro-cracks) or has been initiated at a surface defect, like machining marks, pores or simply grain boundaries. Such defects can be reduced by remelting the surface using an excimer laser. This positive effect however is accompanied by the generation of internal stresses during irradiation. If no phase transformation occurs, the thermal cycle by laser heating will always result in tensile stresses. This effect is adverse to the lifetime prolongation caused by smoothing and may overweigh the improvement caused by the excimer laser treatment of the ceramic device.

2.1.2.1 Laser cleaning A review of laser cleaning in the application of art restoration is given in [96Wat]. Laser cleaning has the following advantages over conventional mechanical or chemical methods: -

physical process which ends shortly after the laser pulse, selective process which can be tuned for the removal of specific substances, non-contact process that can be automated, surface relief is preserved, versatile process by selection of operating conditions, controllable process, since a specific thickness can be removed.

The appropriate laser parameters (wavelength, pulse length, power density) and the cover fluid (air, argon, water nitrogen etc.) depend on the substrate material and the surface impurities that have to be removed. The following mechanisms of interaction are proposed: -

selective evaporation, scouring of the surface by the action of rapidly expanded vapors, thermal and photo decomposition of superficial layers, removing of small adherent particles as a result of selective excitation of the particles of the substrate, delamination of superficial deposits as a result of thermal expansion mismatch with the substrate, spallation induced by laser induced shock-wave.

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97

2.1.2.2 Laser cleaning and smoothing of cast iron Excimer lasers can be used for the surface finishing treatment of cast iron, for example within the cylinder walls of automotive combustion engines. The irradiation causes a cleaning and smoothing effect while the surface becomes nitrided within a 1 µm thin surface layer and the graphite lamellae of the cast iron are opened. Using this technique, friction and wear of the cylinder surface, which determine oil consumption, energy loss, airtightness and lifetime of the engine, can be improved. For the treatment of large surfaces it is necessary to move either the laser beam or the workpiece. In the case of cylindrical holes, rotating of the workpiece results in much better stiffness of the beam handling system. While the whole cylinder crankcase is rotating with up to 120 rpm, a carriage is used to move the rotating axis from one cylinder to another so that only one acceleration is necessary for each engine, resulting in a reasonable process speed [98Ber].

2.1.2.3 Surface alloying As an example of thin-film laser surface alloying, the nitriding of aluminum alloys is reported. The resistance of aluminum-alloy substrates against abrasive wear and corrosion can be improved by generating a thin layer containing aluminum nitride (see Table 2.1.13). This can be reached by UV exposure within a nitrogen atmosphere [96Bar]. The process consists of several steps: -

removal of the Al2O3 diffusion barrier, formation of atomic nitrogen within the laser-induced plasma, melting of a thin surface film (up to 20 µm), dissolution of atomic nitrogen within the melt, quenching-in of the nitrogen during resolidification of the melt (quenching rate 107...1011 K/s), precipitation of aluminum nitride.

The influence of the process parameters energy density, number of pulses and nitrogen pressure on the percentage of aluminum nitride within the surface layer is given in Fig. 2.1.17. A significant increase of the surface hardness is reached due to -

dispersion hardening, solid-solution hardening, grain refinement.

This results in a significant reduction of the friction coefficient for dry gliding, reducing the wear rate up to one third, compared to the plain substrate material [98Bar].

Table 2.1.13. Comparison of the properties of AlN and Al [98Bar]. Property thermal conductivity λ [W/m K] thermal expansion coeff. [K–1] melting point Tm [°C] density [kg/m3] electrical resistivity [Ωm] Vickers hardness [N/mm2]

Landolt-Börnstein New Series VIII/1C

AlN 285 4.9 ⋅ 10–6 1750 (decomposition) in vacuum 2750 (decomposition) in 100 atm. N2 3.26 ⋅ 103 1013 ... 1015 1230

Al 238 23 ⋅ 10–6 660 2.7 ⋅ 103 2.6 ⋅ 10–8 < 100

98

2.1.3 Laser shock hardening

[Ref. p. 101

Fig. 2.1.17. Influence of the process parameters on the phase proportion of aluminum nitride within the surface layer: (a) influence of the energy density, (b) influence of the number of pulses, (c) influence of the nitrogen gas pressure.

2.1.3 Laser shock hardening Laser shock hardening (laser impulse strengthening) is an almost cold (non-thermal) process to increase the hardness and wear resistance of metals in the surface region, generating compressive residual stresses even in deeper zones underneath the surface [96Keu]. The effect of impulse strengthening is based on mechanical deformation and can be compared to mechanical treatments like shot peening. Two different methods of performing the laser shock hardening process are shown in Fig. 2.1.18 [98Eis]. Using a free-expanding plasma method, a dense plasma is ignited on the surface of the workpiece. Due to the hydrodynamic flow of the ionized metal vapor a momentum transfer to the surface is implemented. As the power flux densities are very high, thermal effects damaging the surface cannot be avoided [95Mas]. The second method, using a confined plasma, is carried out with a double-layer system on the surface. This layer system consists of a confining film which is transparent for the irradiating laser wavelength, inhibiting the hydrodynamic flow of the plasma, and an additional absorbing layer which is used to separate the thermal from the mechanical effects preventing damaging the surface. Using the confined-plasma method, the applied pressure is much higher (up to one magnitude) than for freeexpanding plasma techniques [90Fab, 90Kre, 93Dev, 95Pey, 96Eis]. This is demonstrated in Fig. 2.1.19, where the resulting pressure, measured on the backside of the specimen, is compared for both cases (note the different scales for both curves!).

Landolt-Börnstein New Series VIII/1C

Ref. p. 101]

2.1 Surface treatment

99

Fig. 2.1.18. Different methods of performing the laser shock hardening process: (a) freeexpanding plasma, (b) confined plasma.

Fig. 2.1.19. Measured pressure at the backside of the sample: 1: free-expanding plasma technique, 2: confined-plasma method.

The resulting pressure pulse at the surface induces elastic and plastic shock waves (supersonic pressure pulses), which travel into the material. An extremely short rise time of the laser pulse is necessary to produce a sufficient pulse pressure (several kilobars, i.e. several 108 Pa, minimum), whereas the pulse duration has no substantial influence on the development of the microstructural properties of the material (Fig. 2.1.20). If the resulting pressure exceeds the flow stress of the substrate material, plastic deformation occurs, generating microstructural changes like twinning and resulting in compressive residual stresses. Figure 2.1.21 displays measured residual stress-depth profiles for two different steel substrates and different numbers of pulses. The maximum values observed reached up to 70...80 % of the yield strength of the substrate material. The depth of the zone with compressive residual stresses increases with the total number of pulses until a saturation is reached. For substrates with higher yield strength more pulses are necessary to reach saturation.

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100

2.1.3 Laser shock hardening

[Ref. p. 101

Fig. 2.1.20. Signal of the laser pulse and pulse pressure at the backside of the sample [98Eis]: (a) long rise time, (b) short rise time; 1: signal of the laser pulse, 2: pressure measured at the backside of the sample.

Fig. 2.1.21. Residual stress-depth profiles for two different steel substrates and different numbers of pulses; power density: 1 GW/cm2, confined plasma with absorption layer, excimer laser XP 2020. (a) 34CrAlNi7 (yield strength 650 MPa), (b) X2Cr11 (yield strength 380 MPa); 1: 4 pulses, 2: 16 pulses, 3: 64 pulses.

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References for 2.1

101

References for 2.1 54Wev

Wever, F., Rose, A.: Atlas zur Wärmebehandlung der Stähle, Teil 1, Stahleisen, 1954.

72Ros

Rose, A., Haugardy, H.: Atlas zur Wärmebehandlung der Stähle, Bd. 2, Stahleisen, 1972.

73Ohr

Ohrlich, J., Rose, A., Wiest, P.: Atlas zur Wärmebehandlung der Stähle, Bd. 3, Stahleisen, 1973.

76Ohr

Ohrlich, J., Pietrzeniuk, H.-J.: Atlas zur Wärmebehandlung der Stähle, Bd. 4, Stahleisen, 1976.

79Bel

Belmondo, A., Castagna, M.: Thin Solid Films 64 (1979) 249.

81Kur

Kurz, W.: In: Sahm, P.R.: Proc. of the workshop on solidification, Gießerei Institut, Aachen, Germany (1981) 57.

83Cly

Clyne, T. W.: In: Sahm, P.R.: Proc. of the workshop on solidification, Gießerei Institut, Aachen, Germany (1983) 96. Kou, S., Sun, D.K., Lee, Y.P.: Metallurgical Transactions 14A (1983) 643. Liu, C.A., Humphries, M.J., Krutenat, R.C.: Thin Solid Films 197 (1983) 269. Rothe, R., Chatterjee-Fischer, R., Sepold, G.: In: 3rd Int. Colloquium on Welding and Melting by Electrons and Laser Beams, CIS FFEL, Lyon, 1983. Sahm, P.R.: In: Sahm, P.R.: Proc. of the workshop on solidification, Gießerei Institut, Aachen, Germany (1983) 5.

83Kou 83Liu 83Rot 83Sah

84Böt 84Tuc

Böttinger, W.J., Coriell, S.R.: Mat. Sc. Eng. 65 (1984) 27. Tucker, T.R., Clauer, A.H., Wright, I.G., Shopky, J.T.: Thin Solid Films 118 (1984) 73.

85Ber 85Wis

Bergmann, H.W.: Surface Engineering 1(2) (1985) 137. Wissenbach, K.: Ph.D. Thesis, TH Darmstadt, Germany, 1985.

86Ber 86Bra 86Dek 86Kur 86Maz 86Rap

Bergmann, H.W.: Opto Elektronik Mag. 2(3) (1986) 224. Bransden, A.S., Gazzard, S.T., Inwood, B.C., Megaw, J.H.P.C.: Surface Eng. 2(2) (1986) 107. Dekumbis, R.: 6th Int. Conf. on Lasers in Manufacturing, Birmingham, 1989. Kurz, W., Fisher, D.J.: Fundamentals of Solidification, Trans Tech Publ., 1986. Mazumder, J., Singh, J.: In: Laser Surface Treatment of Metals, Boston (1986) 287. Rappaz, M.: Proc. of ECLAT'86.

88Ame 88Ber 88Dau 88Gei 88Gil 88Jüp 88Rud

Amende, W.: Laser Mag. 3 (1988) 8. Bergmann, H.W., Geissler, E.: Proc. of ECLAT'88 (1988) 109. Dausinger, F., Rudlaff, T.: Techn. Rundschau 37 (1988) 44. Geissler, E., Bergmann, H.W.: Opto Electronic Magazin 4 (1988) 4. Gillner, A., Wissenbach, K., Kreutz, E.W.: Opto Elektronik Mag. 4(1) (1988) 57. Jüptner, W.P.O., Becker, R., Sepold, G.: SPIE 957 (1988) 82. Rudlaff, T., Dausinger, F.: Proc. of ECLAT'88, 1988.

89Bur 89Sch 89Wu

Burchards, D., Hinse, A., Mordike, B.L.: Mat.-wis. u. Werkstofftech. 20 (1989) 305. Schubert, E., Bergmann, H.W.: Opto Elektronik Mag. 5 (1989) 6. Wu, W., Streiff, R., Wang, M.: Mat. Sc. Eng. A 121 (1989) 499.

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102 90Ber 90Fab 90Gas

References for 2.1

90Kre 90Lug1 90Lug2 90Mar1 90Mar2 90Pie 90Rud 90Tön1 90Tön2

Bergmann, H.W., Schubert, E.: Proc. of ECLAT'90 (1990) 813. Fabbro, R.: J. Appl. Phys. 68(2) (1990) 775. Gasser, A., Kreutz, E.W., Krönert, W., Lohmann, K., Wissenbach, K., Zografou, C.: Proc. of ECLAT'90 (1990) 651. Kreutz, E.W., Pirch, N.: SPIE Proc. 1276 (1990) 343. Lugscheider, E., Bolender, H.: Proc. of ECLAT'90 (1990) 111. Lugscheider, E., Oberländer, B.C., Meinhardt, H.: Proc. of ECLAT'90 (1990) 555. Marsden, C.F., Frenk, A., Wagnière, J.-D., Dekumbis, R.: Proc. of ECLAT'90 (1990) 535. Marsden, C.F., Houdley, A.F.A., Wagnière, J.-D.: Proc. of ECLAT'90 (1990) 543. Pierce, R.L.: J. of Laser Appl. 1 (1990) 18. Rudlaff, T., Dausinger, F.: Proc. of ECLAT'90 (1990) 251. Tönshoff, H.K., Meyer-Kobbe, C.: Laser Mag. 4 (1990) 23. Tönshoff, H.K., Meyer-Kobbe, C., Homburg, A.: Laser Mag. 5 (1990) 29.

91Bac 91Fri 91Gei 91Lep

Bach, J., Damaschek, R., Geissler, E., Bergmann, H.W.: HTM 46(2) (1991) 97. Fritsch, H.U., Bergmann, H.W.: HTM 46(3) (1991) 145. Geissler, E., Bergmann, H.W.: HTM 46(2) (1991) 91. Lepski, D., Luft, A., Reitzenstein, W.: Opto Elektronik Mag. 7(4) (1991) 304.

92Ber

Bergmann, H.W., Endres, T., Anders, R., Freisleben, B., Müller, D.:Proc. of ECLAT'92 (1992) 363. Jaschek, R., Taube, R., Schutte, K., Bergmann, H.W.: Proc. of ECLAT'92, 1992. Lugscheider, E., Bolender, H., Krappitz, H.: Proc. of ECLAT'92 (1992) 369. Müller, D., Domes, J., Bergmann, H.W.: HTM 47(2) (1992) 123.

92Jas 92Lug 92Mül 93Dev 93Gei 93Her

Devaux, D.: J. Appl. Phys. 74(4) (1993) 2268. Geissler, E.: Ph.D. Thesis, Univ. Erlangen-Nürnberg, 1993. Herziger, G., Loosen, P.: Werkstoffbearbeitung mit Laserstrahlung, Hanser-Verlag, München, 1993.

94Agr 94Gal 94Kör

Ågren, J.: Mat. Sc. Forum 163–165 (1994) 3. Galun, R., Weisheit, A., Mordike, B.L.: Proc. of ECLAT'94, DVS (1994) 421. Körner, C., Müller, K., Domes, J., Damaschek, R., Bergmann, H.W.: Proc. 15th Int. Symp. on Mat. Sc., Roskilde (1994) 381. Lang, A., Waldmann, H., Bergmann, H.W.: Proc. of ECLAT'94 (1994) 456. Lugscheider, E., Bolender, H., Hochmuth, K., Herziger, G., Gasser, A., Wissenbach, K. Bitterlich, R., Pirch, N.: Proc. of ECLAT'94 (1994) 213.

94Lan 94Lug

95Dom

95Pey

Domes, J.: Die Ausbildung von Makroeigenspannungen beim Randschichthärten mit Laserstrahlung und anderen Verfahren der Lasermaterialbearbeitung, Ph.D. Thesis, ErlangenNürnberg, 1995. Galun, R., Weisheit, A., Mordike, B.L.: Proc. of ICALEO'95, LIA 80 (1995) 69. Lang, A.: Laserstrahlspritzen von Schneidbelägen, Ph. D. Thesis, Univ. Erlangen-Nürnberg, 1995. Masse, J.-E., Barreau, G.: Surface and Coatings Technology 70 (1995) 231. Müller, D.: Beitrag zum Randschichthärten für kraftübertragende Komponenten im Motorenbau, Shaker Verlag, Aachen, 1995. Peyre, P., Fabbro, R.: Optical and Quantum Electronics 27 (1995) 1213.

96Bar 96DIN1 96DIN2 96Eis

Barnickel, J., Schutte, K., Bergmann, H.W.: Proc. of ECLAT'96 (1996) 547. E DIN 17022-5: Verfahren der Wärmebehandlung – Teil 5: Randschichthärten, 1996. E DIN 30950: Schmelzhärten, 1996. Eisner, K., Lang, A., Schutte, K., Bergmann, H.W.: SPIE 2789 (1996) 274.

95Gal 95Lan 95Mas 95Mül

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References for 2.1 96Gas 96Keu 96Lin 96Mor 96Mül 96Wat

103

Gasser, A. Wissenbach, K., Hoffmann, E., Schulte, G., Beyer, E.: Proc. of ECLAT'96 (1996) 287. Keusch, C., Hintz, G., Tkotz, R., Negendanck, M., Christiansen, J., Bergmann, H.W., Eisner, K., Schutte, K.: Proc. of ECLAT'96, 1996. Lin, J., Stehen, W.M.: Proc. ICALEO'96 (1996) 27. Mordike, B.L.: Proc. of ECLAT'96 (1996) 253. Müller, K., Körner, C. Bergmann, H.W.: HTM 51(1) (1996) 19. Watkins, K.G., Larson, J.H., Emmony, D.C., Stehen, W.M.: In: Mazumder, J. et. al.: Laser Processing – Surface Treatment and Thin Film Deposition, Kluver Publishing, Netherlands (1996) 907.

97Ber

Bergmann, H.W., Bohner, F., Stiele, H.: HTM 52(2) (1997) 108.

98Bar

Barnickel, J.: Nitrieren von Aluminiumwerkstoffen mit UV-Laserstrahlung, Ph.D.-Thesis, Univ. Erlangen-Nürnberg, 1998. Bergmann, H.W.: Proc. of EKLAT'98, 1998. Eisner, K.: Prozeßtechnische Grundlagen zur Schockverfestigung von metallischen Werkstoffen mit einem kommerziellen Excimerlaser, Ph.D.-Thesis, Univ. Erlangen-Nürnberg, 1998.

98Ber 98Eis

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105

2.2 Rapid prototyping A. Gebhardt

The method of building scaled physical models by layer manufacturing, also called generative manufacturing or additive manufacturing, is commonly named “Rapid Prototyping (RP) (technology)”. In contrast to traditional manufacturing such as grinding or milling, the desired shape is obtained by adding material instead of removing it. Using so-called prototypers, RP models are generated in a couple of hours directly from 3D CAD. Only 3D data but no model-dependent tools are needed. Since 1987 when the first prototyper, the Stereolithography Apparatus (SLA), was introduced by 3D-Systems (CA, USA), more then 50 different machines have been presented using more then 10 basic processes. Most of the models are made of plastic materials, but also foundry sands, ceramics and metals are available. Due to the direct interaction of the RP and CAD process, RP models are just facsimiles of the 3D-data set and consequently can be obtained at all stages of product development. For many years making prototypes has been the main application of the RP technology but since the middle of the 90’s RP has also been used for making tools and even final products. This is why an appropriate nomenclature is under discussion. Today (2003) people tend to name the layermanufacturing technology “rapid technologies” or “rapid technology” while the main applications are named rapid prototyping (including solid imaging and functional prototyping), rapid tooling and rapid manufacturing. Figure 2.2.1 describes this relation. Rapid prototyping is used to obtain samples for the evaluation of selected product characteristics mainly in the area of geometry, surface, fit and function. The major fields are: – Solid imaging or concept modeling enables men to “touch” a virtual 3D data set. The models are sometimes called “show-and-tell” models as well to indicate that they are important for communication especially during the very first stage of product development. – Functional prototypes support the evaluation of one or more isolated properties (fit and function) of the later series product.

Technology

Rapid Technology

Manufacturing of Concept Models

Solid Imaging Rapid Prototyping Application / Technique / Strategy

Manufacturing of Functional Prototypes

Functional Prototyping

Manufacturing of Tools and Tool Inserts

Rapid Tooling

Manufacturing of Final Parts

Rapid Manufacturing great

small Distance to Series Quality

Fig. 2.2.1. Rapid prototyping in the context of rapid prototyping technology and its applications.

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2.2.1 Layer manufacturing

[Ref. p. 123

Rapid tooling is called the making of tools either directly by using a prototyper or from RP masters. Rapid tooling therefore bridges the gap between prototyping and manufacturing. Rapid manufacturing means the use of layer-based processes for making final parts – (series-) manufacturing. In order not to confuse the reader by a too sophisticated nomenclature in the following text only the commonly accepted and the widely spread name “Rapid Prototyping” is used to name the layer-manufacturing technology and the basic process as well as its application for making prototypes.

2.2.1 Layer manufacturing 2.2.1.1 Rapid-prototyping process chain To obtain physical models by rapid prototyping, the whole process chain has to be taken into account. Figure 2.2.2 shows the entire rapid-prototyping build process and its adjacent process steps. The first one is to create a 3D data set which completely describes the whole volume. In the field of industrial product development it is the result of a 3D CAD design but it can also be obtained from measurements or scanning procedures. In the field of medical applications mostly Computerized Tomography (CT) scanners (CT-scans) and a subsequent 3D reconstruction are used. The 3D data are transmitted to the so-called “front end” of the RP system using standard interfaces, such as STL SLC IGES VDA STEP

Stereolithography interface based on triangles, Stereolithography interface based on Contours, Initial Graphics Exchange Specification – CAD data exchange interface, German Association of Automotive Manufacturers CAD data exchange interface, Standard for the Exchange of Product model data

and others. Even if the contour-orientated SLC format leads to better results, the much more easy-to-handle STL format, which is based on a triangulation of the model’s surface (tessellated surface), has gained some kind of a quasi-industrial standard. The front end mainly consists of the

Parameters for Machine Control

Generating the CAD-Model l

Interface (e.g. STL, SLC)

Process-related Transformation of Geometric Data

Generating Physical Model

PostProcessing

Finishing

Auxiliary Geometries ( Supports, ...) CAD Software

Rapid-Prototyping Software

Build Process

Manual Finishing

Fig. 2.2.2. Rapid-prototyping-process data flow.

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107

so-called rapid-prototyping software, that comes either from the manufacturer or from third parties. Beside the data handling it allows simple operations such as positioning, orientation, cutting and rearranging of models as well as the design of support structures, if needed. The build data set that controls the entire build process is derived from the geometric data and some process-related data to be added by the operator. From the build process a physical model is obtained, that has to be finished due to the process’s special requirements. Stereolithography for example needs cleaning, post-curing and the removal of the supports. Finally, all RP processes need a non-process-dependent surface treatment that is called finishing. In most cases RP models do not meet the customers’ requirements because of their lack of surface quality, material properties or quantity. The results can be improved by using the RP models as masters for copying (or cloning) procedures such as Room Temperature Vulcanization (RTV), see Sect. 2.2.1.8.1.

2.2.1.2 Prototypers Rapid-prototyping processes are characterized by the layer-orientated manufacturing of the models, as can be seen in Fig. 2.2.3. The volumes defined by the 3D data set are sliced into layers with even thickness (internal cross sections, virtual layers). The contour data of each layer is transmitted to the prototyper. The entire RP build process consists of two main steps: 1. manufacturing of each single layer, e.g. by cutting it from sheet material or by cladding discrete volumes (voxels) adjacent to each other, 2. joining each layer on top of the partly finished model to obtain a solid, thus growing the model from the bottom to the top. These process steps can be seen very clearly in Fig. 2.2.3 that shows the principle in comparison to a special rapid-tooling process (see Sect. 2.2.1.8) based on laser cutting and sheet or diffusion welding.

3D-CAD-Model

Slicing of Internal CrossSections (Layers)

Fig. 2.2.3. Rapid-prototyping process steps. Landolt-B¨ ornstein New Series VIII/1C

RP-Process

Physical Model (Solid)

108

2.2.1 Layer manufacturing

[Ref. p. 123

Rapid-prototyping processes consequently differ mainly from each other by: 1. 2. 3. 4.

the the the the

physical principle of converting virtual into physical layers, characteristics of the contour generating element (i.e. laser, extruder), material the layers are made of, physical principle how the adjacent layers are joined to a solid.

Frequently, the generation and the joint of the layers are based on the same physical process and therefore both steps are done simultaneously. It is a common practice to classify rapid-prototyping procedures due to the state of the build material. This leads to fundamental physical principles how to make solid layers (Fig. 2.2.4) and to the basic RP processes listed below. Beneath the bold horizontal line the most established industrial processes and their manufacturers are listed. Beneath the thin line some more are mentioned that are either under development or on the threshold of their industrial application. They will be discussed briefly in Sect. 2.2.3 below. Further details can be found in [03Geb].

2.2.1.3 Characteristics of rapid-prototyping processes Based on the systematics of Fig. 2.2.4, in more then 15 years that passed since the introduction of the first prototyper a huge amount of different systems has been presented, but only 5 families of prototypers could establish themselves as industrial applicable systems due to their good performance and widely spread acceptance. Table 2.2.1 gives a short description of the most important industrial-used RP processes. It describes each basic process including the working principle, its pros and cons, the materials that can be used and the follow-up processes that are needed to obtain short-run series of different materials. Table 2.2.1 also contains the name of some manufacturers and of the commercialized process (abbreviation). For further details see [03Geb] and the internet addresses listed in Table 2.2.2. Figure 2.2.5 shows a typical stereolithography machine (3D-Systems) and Fig. 2.2.6 shows a laser-sintering machine (EOS).

2.2.1.4 Materials There is a very limited number of materials available for RP applications. The reason is that RP materials are process-related in two ways: On the one hand, they first have to serve the process. For example, stereolithography materials must be photo-curable. All other properties, such as durability, elasticity or thermal stability have to be put into the background. On the other hand, the materials are mostly related to one machine and consequently to the manufacturer. The upcoming laser-based metal processes, such as “laser cusing” (Concept Laser, D) “selective laser melting”, (Fockele&Schwarze, D), “LENS” (Optomec, USA) or even the onecomponent EOS laser-sintering process have the potential to overcome these limits and open up the possibility to use any kind of industrial available metal powder [03Ove]. RP materials simulate the properties of the corresponding engineering materials, even if the brand name or the chemical composition are identical the same. Table 2.2.3 shows a selection of plastic and metal RP materials in comparison to engineering materials.

Landolt-B¨ ornstein New Series VIII/1C

Landolt-B¨ ornstein New Series VIII/1C

Wax Process, Generis.

FDM, Stratasys. ModelMaker, Solidscape.

(Selective) Laser Sintering (SLS)

Local Melting and Solidification

Layer Laminate Manufacturing (LLM)

Cutting and Joining

Foil

Lamp

GS 1500 (Sand), Generis.

Laser Lamp

Eden 300, Objet. Invision, 3D Systems.

Opto 15/400 ...90/800, 3D Systems.

Paste Polymerization

PhotoPolymerization

Resin-Flour Mixture

Paste

Printing and PhotoPolymerization

Stereo- Solid Ground Polyjet Curing Lithography Droplet (SGC) Polymerization (SL)

Laser

Direct PhotoPolymerization

Liquid Resin

LOM, Z3..., Z 8..., Vanguard, 3D Systems. SLA, 3D-Systems. EOSint, EOS. Cubic Technologies. Z-Corporation. Perfactory, SLM, F&S. Stratoconception, Prometal, Envision Tec. M3 linear, Concept. CIRTES. Extrude Hone. LENS, Optomec. LMC, Zimmermann.

3D-Printing (3DP)

Fused Deposition Modeling (FDM)

Ballistic Particle Manufacturing (BPM)

Solidification by Binder

Local Melting and Solidification

One- or MultiComponent Powder

Fluid

Fig. 2.2.4. Physical principles of rapid-prototyping layer generation due to the state of the build material, industrial rapid-prototyping processes, industrial (commercial) prototypers and future industrial prototypers.

Future Prototypers

Industrial Prototypers

Industrial Process

Physical Principle of Layer Generation

Filament

Solid

Rapid Prototyping - Layer Manufacturing Techniques

Ref. p. 123] 2.2 Rapid prototyping 109

Description

Cutting layer from foil and laminating.

Layer-laminate manufacturing (LLM)

) RTV – Room-Temperature Vulcanization.

In case of models with high volume: Quick and cheap.

Quick and cheap. Local solidification of Colored models powders by injecting possible. binder fluid. Post-process infiltration required.

3D-Printing (3DP)

Wider range of materials. Acceptable thermal and quite good mechanical properties.

Extrusion of thermoplastic material by heated nozzles. Solidification by cooling down.

1

Poor thermal and mechanical properties.

Cons

Short-run series made of plastic by

Less details than stereolithography. Anisotropic properties.

Very rough surfaces; poor details.

Cubic Techn. / LOM

RTV possible. Paper, (plastic) (Metal) (Ceramics)

Investment casting from wax copies.

Not recommended. Investment casting ZCorporation RTV possible. after wax infiltration. / 3DP

Investment casting Stratasys from directly / FDM extruded wax parts.

3D Systems / SLS EOS / LS

3-D Systems / SLA

3D-Keltools. Investment casting.

Direct sintering of metal parts. Investment casting.

Manufacturer / industrial process

Short-run series made of metal by

Starch Plaster

Rougher surface Polyamide Direct extrusion Polyester of plastic parts. and less details ABS, PPSF RTV possible. than stereolithography.

Direct sintering of plastic parts. RTV possible.

Epoxy resin Vaccum casting Acrylate (RTV1 )). Composite tools.

Materials

Local melting of plastic Acceptable thermal Rougher surface Polyamide than Metal powder. Solidification and quite good stereolithography. Sand by cooling down. mechanical properties.

Very good surface quality. Fine details.

Pros

Extrusion / fused-layer manufacturing (FLM)

Laser sintering (LS / SLS)

Stereolithography Solidification of (SL, SLA) liquid resin by photopolimerization.

Process

Table 2.2.1. Industrial rapid-prototyping processes – overview.

110 2.2.1 Layer manufacturing [Ref. p. 123

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Ref. p. 123]

2.2 Rapid prototyping

111

Table 2.2.2. Manufacturer and supplier of prototypers and related products. Company

Products

URL

3D Systems DSM SOMOS Cubic Technologies CP-GmbH EOS GmbH FhG-IPT FhG-ILT Fockele & Schwarze Generis Materialise MCP-HEK GmbH, Germany

Manufacturer of prototypers Supplier of SL materials Manufacturer of prototypers Rapid-prototyping service bureau Manufacturer of prototypers Research institute Research institute Manufacturer of prototypers Manufacturer of prototypers Supplier of RP software Supplier for materials for RTV and low-melting-point alloy molding; SLM-realizer metal prototyper Manufacturer of prototypers Manufacturer of prototypers Manufacturer of prototypers Manufacturer of prototypers Manufacturer of prototypers Layer-milling sofware and systems Manufacturer of pharmaceutica Supplier of SL materials Manufacturer of prototypers Manufacturer of milling machines for model shops

www.3dsystems.com www.dsmsomos.com www.cubictechnologies.com www.cp-gmbh.de www.eos-gmbh.de www.ipt.fhg.de www.ilt.fhg.de www.fockeleundschwarze.de www.generis.de

MicroTech Objet Optomec Roeders Stratasys Stratoconception Therics Vantico Z-Corporation Zimmermann

Fig. 2.2.5. Stereolithography prototyper (3DSystems, USA).

Landolt-B¨ ornstein New Series VIII/1C

Fig. 2.2.6. GmbH, D).

www.mcp-group.de

www.microtec-d.com www.objet.co.il www.optomec.com www.roeders.com www.stratasys.com www.stratoconception.fr www.therics.com www.tooling.vantico.com www.zcorp.com www.zimmermann. de

Laser-sintering

prototyper

(EOS-

Unit MPa

MPa % MPa MPa

kJ/m

Property Tensile strength

Tensile modulus Elongation at break Flexural modulus Flexural strength

Impact strength

2

2400-2600 6-15 2400-2600 83-104

42-62

0.5

3296 5.4 3054 99

77

Renshape SL5510 SL7560

Epoxy resin

Stereolithography

Process

Material Type Brand name

Vantico

Company

3D Systems

13

2413 91

2345

69

PPSF

2

2517 >10 2657 66

35

P400

ABS

Fused deposition modeling

Stratasys EOS

1600 22 1430

4.8

1700 20

45

Dura-form PA2200 PA

Polyamide

Laser sintering

3D Systems

Rapid-prototyping material

Plastics

ABS

15-20

8-12

1600-2000 2000-2600 50-200 20 2400 88

50-65

PA66

Injection molding

Engineering material

Table 2.2.3. Rapid-prototyping and engineering materials – samples for plastics and metals.

Laserform ST100

bronze infiltrated

326

12

587

EOS

1 without; 2 with infiltration

182*10 14 300-400 (1,2) 327 + / - 53

3

120-200 (1,2) 495 + / - 135

Direct-metal 50-V2

Steel 1.4404

Fockele&Schwarze MCP-HEK Selective laser melting

Metal powder

Laser sintering

3D Systems

Rapid-prototyping material

Metals

>190

3

200*10 > 40

490 - 690

Steel 1.4404

Milling, turning, ...

Engineering material

112 2.2.1 Layer manufacturing [Ref. p. 123

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 123]

2.2 Rapid prototyping

113

2.2.1.5 Post-processing Rapid-prototyping models are semi-finished products when they leave the prototyper. They show the exact geometry, but they lack surface quality. Due to the RP process additional work has to be done, e.g. supports have to be removed. This process-related procedure is called “post-processing”. It requires additional skilled staff and equipment for manual after-treatment (post-processing). After post-processing one obtains a 3D physical model (facsimile) of the 3D data set. It shows process-dependent properties, such as accuracy, surface quality, physical properties and others (see also Table 2.2.4).

2.2.1.6 Finishing In most cases the quality of RP models does not meet the user’s particular requirements. In such cases additional finishing by blasting, sanding, varnishing and so on is required. Finishing is a RP-process-independent procedure: It has to be done in any case but it is not an entire element of rapid prototyping. Anyway, an excellent finishing is a precondition for subsequent molding and casting processes (see Sect. 2.2.1.8) as well.

2.2.1.7 Functional metal parts Although many basic rapid-prototyping processes are not related to a specific material, until today the majority of the industrial applications has been working with plastic materials. Applications or process modifications for manufacturing metal models are developing rather slow. This is because the high melting temperature and the high surface tension of metals limit the transfer of the results valid for one material to another. This is also the reason why RP processes for “functional metal parts” as well as for metal tool inserts and tools are discussed separately (see Sect. 2.2.1.8). Metal processes are divided into “indirect metal processes” and “direct metal processes”. Indirect metal processes use plastic master models obtained from RP processes. Metal parts are obtained either from investment casting processes (which are based on wax copies made from RP masters) or from sand-casting procedures. Sand casting can be done with cores made from polymer-coated casting sands which are manufactured directly in prototypers which have been designed for plastic materials. Direct metal processes use laser-sintering processes with either one- or more-component metal powders or even with polymer-coated powders. Alternatively, 3D printing processes are known that inject a binder fluid into metal powder (Extrude Hone). Except for the one-component steel powder processes (F&S, Concept, Optomec, EOS) all metal processes are multi-stage ones. After the green part has been produced in the prototyper, de-binding, sinter and infiltration processes are used to receive a fully dense metal part. Until today the main problems of all metal processes have been rough surfaces and that the parts tend to distortion. A different method is the “laser-cutting and sheet-welding” procedure which uses engineering sheet metal (see Sect. 2.2.1.8).

2.2.1.8 Rapid tooling Any method that either allows to produce molds and dies using rapid-prototyping processes or to make copies from RP master models is called “rapid tooling”. Methodically, there is no difference Landolt-B¨ ornstein New Series VIII/1C

60 80 80 0.5 0.13 0.1 1, 2, 3, seldom more than 10 related to RP-process some isolated functions completely guaranteed monochrome as RP-material Z-Corp.: fully colored FDM: one color per part

Accuracy (% relative to SL = 100 %) Details (abs. [mm]) Quantity Material

Number of evaluable functions

Consistency of data set

Color

) Prices are only good for orientation.

50

80

ca. 55–70 < 15 < 70 brittle > 2600 80

< 50 > 1400 100

< 60 ca. 2500

100 0.05

100

SL

some days (except mold design) some minutes (> 5 min., < 30 min.) 4–8 weeks

Composite mold / hard tooling

part: EUR 50 mold: EUR 500 (for 15 parts)

55–85 / 120 same as PU resin standard: approx. 2600

part: EUR 5 mold: EUR 7500 (for some 100 parts)

same as series material same as series material same as series material

colored, translucent or transparent models are possible one color per part /casting one color per injected part

more but limited many or all functions number of functions not guaranteed because of manual finishing

according to the quality of the RP master (preferably SL master) according to the quality of the RP master (preferably SL master) 15 per mold 100, seldom more than 1000 polyurethane (PU) resin thermoplastic (series) material

mold: some hours part: approx. 1 h 2 weeks

Follow-up process (indirect application) RTV / soft tooling

2.2.1 Layer manufacturing

1

Relative price / part (%)1 ) (100 % = EUR 500) Price per part, fitting the build chamber1 )

Mechanical Properties Temperature stability [◦ C] Tensile strength [MPa] Flexural strength [MPa]

< 5 days

Delivery time

80

50

Build time (%, 100 % := 16 h)

75

Rapid-prototyping process (direct application) 3DP FDM SLS

Criterion / property

Table 2.2.4. Characteristic properties of parts obtained from rapid-prototyping and follow-up processes.

114 [Ref. p. 123

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Ref. p. 123]

2.2 Rapid prototyping

115

between rapid tooling and rapid prototyping but because of two important facts, rapid tooling is treated as an independent process: 1. While rapid prototyping just leads to a scaled facsimile of the 3D data set, rapid tooling requires additional mold design, such as parting line, sliders, demoldability, feed point(s) and ejecting pins. 2. Making molds for plastic injection molding and metal die casting is directly related to metal RP processes (see Sect. 2.2.1.7) although until today some non-metallic tooling processes have been widely used in industry. In practice rapid tooling is divided into indirect rapid tooling and direct rapid tooling:

2.2.1.8.1 Indirect rapid tooling Copying rapid-prototyping master models by using different kinds of easy-to-make molds is called “indirect rapid tooling”, which often is also called “cloning”. The basic principle is the separation of the model’s characteristics: The complex geometry including the freeform surfaces is made very quickly using rapid prototyping while the desired physical properties as well as the quantity result from the molding process. Many different methods are known. All of them, like the best-known called “Room-Temperature Vulcanization (RTV)”, do not relate directly to rapid prototyping but are used for many years. They have all had some kind of a renaissance since rapid prototyping allowed to make complex 3D masters in a couple of hours. The molds are made from soft material like silicon rubber (RTV) or from filled liquid or pasty material that can be hardened according to the RP master’s shape. The main procedures are: Room-temperature vulcanization (RTV). The rapid-prototyping master is equipped with runners put in a frame and covered with silicon rubber. After hardening, the solid block of silicon rubber is cut according to the parting line and the master is removed. In a manual process, the resulting cavity is cast with polyurethane resin which leads to about 15 copies per mold. Because of the soft material that allows de-molding even of slight overhangs and the compensation of shrinkage by pre-heating the mold, no mold design is needed. Composite molds are hard molds that allow to produce up to some hundred shots of thermoplastic material like PA, using cavities made from aluminum-filled epoxy resin instead of silicon rubber. Because the cavity hardens to a solid, a complete mold design including the compensation of shrinkage is required. The final parts are manufactured on an injection-molding machine after having fit the cavity into a mold attachment. For making this kind of molds and for the production of parts using these molds different suppliers offer various silicon-rubber materials and PU resins. 3D-Keltool (3D-Systems) is a proprietary process similar to composite mold making, including an RTV-like procedure to obtain a positive high-temperature silicon cavity which is filled with a tartan-based metal powder. The powder is brought to semi-solid state to cover every detail and finally solidifies at the end of a multi-step process that includes infiltration and thermal treatment. Due to the stereolithography master model the mold is very precise in terms of surface quality and accuracy up to an overall mold size of approximately 100 mm. Metal deposition. This process uses stereolithography master models which are coated with a layer of approximately 5 mm thickness made by metal spraying of aluminum. The obtained shell is backed with filled epoxy resin and treated like a composite mold to become a tool for plastic injection molding. Similar processes work with electro-chemically applied coatings. High-Speed Machining (HSM). Although HSM is not a layer-oriented manufacturing method it has to be considered as an alternative to rapid prototyping because of the newly available direct data conversion from the CAD system to the milling machine and because of the latest improvements of milling technologies which enable men to make molds of series materials with excellent surface qualities and tolerances.

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116

2.2.1 Layer manufacturing

[Ref. p. 123

Therefore, every mold that can be milled should be milled. Consequently, in general the direct competitor of rapid tooling is not milling but Electric-Discharge Machining (EDM).

2.2.1.8.2 Direct rapid tooling The manufacturing of preferably metallic inserts for molds and dies within a prototyper is called “direct rapid tooling”. The aim of most of the processes is rather to make a series mold than a prototyping one. As a precondition metallic materials have to be manufactured. Consequently, all procedures that can fabricate metallic prototypes (see Sect. 2.2.1.7) are principally suitable for rapid tooling. Figure 2.2.7 shows a plastic injection tool, that has been manufactured using the EOS metal sintering process for making mold inserts and a conventional mold-making procedure to assemble the tool. Laser cutting and diffusion welding is a direct RP procedure to produce preferably internalcooled molds. The cross sections of the parts are cut precisely from any kind of tool-making material using lasers. The sheets are filed and joint by diffusion welding to obtain a fully dense and high-loadable mold insert [01Sae]. Finally, the parts have to be machined to reach the quality requirements on their outer surfaces. Together with the SLM process this is the only procedure known until now that leads to mold inserts made from tooling steel. The lower part of Fig. 2.2.3 shows the main steps of this process (from left to right): – – – –

CAD model with conformal cooling channels, cutting the layers using lasers, joining the layers for diffusion welding, diffusion-welded and machined tool insert.

Fig. 2.2.7. Plastic-injection-molding tool based on EOS’s metal laser-sintering process (Source: EOS-GmbH, D).

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 123]

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117

Since there is no rule without exception, the AIM process (ACES Injection Molding, while ACES is a 3D Systems build style called “Accurate Clear Epoxy Solids”) uses stereolithography models directly as mold inserts [92Jac].

2.2.2 Application of rapid-prototyping models 2.2.2.1 Model characteristics and model properties Rapid-prototyping models are not only used at any stage of industrial product development of any branch, but for medical applications as well as in the field of history of arts, art, architecture and others. To obtain a rough rating what prototyper can be used to fabricate what kind of model, a model classification was defined as shown in Fig. 2.2.1. For special purposes sometimes more detailed classifications are published [03Geb]. Based on Table 2.2.1 the main characteristics such as speed, complexity, physical properties and economic aspects of RP processes and follow-up casting and molding processes are listed in Table 2.2.4. It gives rough information what results can be expected from what process. It cannot, however, be a substitute for a detailed discussion of the pros and cons of each specific RP application before use.

2.2.2.2 Criteria for the use of rapid-prototyping models Criteria for the use of rapid-prototyping models are (as mentioned in Table 2.2.4): – – – –

build speed, complexity, accuracy, fit and function, material properties.

The main criterion is speed. The use of rapid prototyping is only advantageous, if speed is very important for the product’s success on the market. This is because one can’t get back lost time – not even if spending more money. The advantage of RP processes to make models of unlimited complexity only counts if the required models are quite complex. The materials used in RP processes should be either directly applicable or follow-up casting and molding techniques have to be used. The criteria are listed according to the processes in Table 2.2.4. The users mostly want to do some kind of simulations with the model. This can be the control of the data set or the evaluation of ergonomics, assembly, design details as well as the prove of certain physical parameters such as maximum strength. Table 2.2.5 shows which RP process and which follow-up process is preferably used to achieve the desired model characteristics.

Landolt-B¨ ornstein New Series VIII/1C

+

+

–

– ––– – ––

Ergonomics

Geometric details Fit, assembly, disassembly

Mechanical details Snap-fits, film hinge, locking device

Physical parameters Strength Transient parameters, e.g. strength Force, pressure, temperature Resistance against media, EMC

) Rating: – – –, – –, –, o, +, ++, +++.

+++ –

Evaluation of the design Control of data set Design/styling, appearance

1

3DP

++ ++ –

++ +

++

++

++

+

SLS

++

++

++

++

+

FDM

SL

+ o

+

+

++

++

+ –– ++ +

++

++

+++

possible, but not economical ++ ++

Soft tooling

+++ – +++ ++

+++

+++

+++

++

Hard tooling

Composite tooling

RTV

Concept modeling

Functional prototyping

Copying or cloning procedures Indirect

Rapid-prototyping process Direct

Suitable for to investigate: 1 )

Process or Application

Table 2.2.5. Criteria for the selection of rapid-prototyping processes.

118 2.2.2 Application of rapid-prototyping models [Ref. p. 123

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 123]

2.2 Rapid prototyping

119

2.2.2.3 Examples Typical applications are shown in: Fig. Fig. Fig. Fig.

2.2.8: 2.2.9: 2.2.10: 2.2.11:

Fig. 2.2.12:

concept model made by 3D-printing (Z-Corp process), functional prototype made by laser sintering (3D Systems process), stereolithography model (Objet PolyJet process), stereolithography model (3D Systems SLA process), to be used as a master model for RTV (top) and as an RTV copy (below), cast resin tool made of composite material (aluminum-filled epoxy resin) based on a stereolithography master (to be seen on top of the tool). Parts can be seen on the left.

Fig. 2.2.8. Concept model made by 3D-printing (CP-GmbH, D).

Landolt-B¨ ornstein New Series VIII/1C

Fig. 2.2.9. Functional prototype made by laser sintering (CP-GmbH, D).

120

2.2.2 Application of rapid-prototyping models

[Ref. p. 123

Fig. 2.2.10. Stereolithography, PolyJet process model (Objet, IL/FH-Aachen, D).

Fig. 2.2.11. Stereolithography model to be used as a master model for RTV (CP-GmbH, D).

Fig. 2.2.12. Indirect rapid tooling: Cast tool made of composite material (aluminum-filled epoxy resin) based on a stereolithography master (on top of the tool). Parts can be seen on the left (CP-GmbH, D).

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 123]

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121

2.2.2.4 Rapid manufacturing Rapid manufacturing is the application of the rapid-prototyping technology for the manufacturing of final parts or (series) products. It is gaining raising interest because of the general trend to reduced product life time and decreasing quantities that tend to converge to a “one-of-a-kind” production. This trend underlines the need for new production technologies. Today’s prototypers in general are not suitable for the production of final parts because of their lack of material properties and because of the rather slow build speed if higher quantities are requested. In particular, some applications are visible in the field of sintered parts from polyamide and metal. There are various approaches to rapid manufacturing: Indirect rapid manufacturing. Investment-casting processes based on RP masters and sandcasting processes using laser-sintered cores are suitable for the production of small series of final parts. Rapid-tooling techniques mostly produce mold inserts that can be used with series materials and series injection parameters. This leads to final plastic parts. In this context one has to realize that direct rapid tooling is not for making prototypes but for making final parts. Direct rapid manufacturing. If the design is based on RP material, the RP part is a final product [02Ove]. But mostly the design is based on engineering material which consequently leads to prototypes instead of final products. Therefore all published examples of rapid manufacturing do have more the character of feasibility studies than of a series production report. One true rapid-manufacturing process was established by Therics, a pharmaceutical company, which uses a proprietary-modified 3D printing process to manufacture about 20.000 tablets per hour.

2.2.3 Recent developments and future trends While recent and future developments try to overcome today’s prototypers’ disadvantages, they force a trend to more specialized machines. On the one hand this is an advantage for the user who can realize much more sophisticated applications. But on the other hand he has to know much more details about each entire process. The major trends are directly correlated with today’s disadvantages: Stair-stepping effects. To reduce stair stepping, the layer thickness of all prototypers and the grain size of sintering and 3D-printing materials are reduced (down to 0.025 mm). Today the only approach to overcome stair stepping completely is (5-axis) layer milling, which is a combined subtractive (milling of the contours of each layer) and additive (joining adjacent layers to a solid) method. One strategy is to mill all layers and join them afterwards (Stratoconception, F) using liners, another is to mill every new layer after gluing it onto the partly finished model (Zimmermann, D). All processes which are based on gluing are not suitable for metals. These can be processed e.g. by diffusion welding (see Sect. 2.2.1.8). A different approach, preferably for metallic mold inserts, is the Controlled Metal Build-up (CMB) process [FhG-IPT, Aachen and RoedersTec, D), a combined laser-cladding and contour-milling method. Build speed. Advanced 3D printing techniques, multi-head, multi-laser (EOS) or multi-nozzle processes are used to speed up the processes. A different approach is to use solid material (layer milling) or to generate just the contours using an accurate but slow process and fill up the volume by high-speed deposition of material (Generis, D). Material properties. The users require models made of engineering or series materials. Therefore, all manufacturers improve their materials. Besides, new processes, such as Optoform’s “paste

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122

2.2.3 Recent developments and future trends

[Ref. p. 123

polymerization process” open up the range of materials including metals and ceramics. A different approach are improved joining technologies such as diffusion welding (see Sect. 2.2.1.8). Dimensions. Because of the available build space RP models are limited to a max. size of approximately 600 mm × 500 mm × 600 mm. Bigger models can be made by joining parts. Multilaser-sintering processes (EOS) slightly increase both, speed and build space (720 mm × 380 mm × 400 mm), at least in one (x) direction while Materialise’s proprietary machine “Mammuth” uses three lasers and builds models up to 2150 mm×620 mm×500 mm. But only layer-milling techniques open up new dimensions, because they can use almost any milling machine available on the market. On the other hand, small dimensions are limited by the spot size of the laser beam, thus producing models with details within the range of some hundredth of a mm. New systems claim to reach the sub-micron range, using alternatively both, laser and mask technology (MicroTech). Economy. Prototypers cause investments between EUR 30,000 and EUR 700,000, which are significantly higher then for comparable advanced traditional machines including high-speed milling machines. Such costs are allowable for “functional parts” and especially for rapid-tooling applications but they are much to high for solid images obtained by “concept modelers”. Therefore, many manufacturers focus on even cheaper concept modelers (3D Systems: Invision; Stratsys: Prodigy; Z-Corp: Z 310; etc.) and there are some more under development. The strategic goal is a prototyper for about EUR 10,000 that can be used on the CAD designer’s desk in the office, opening up a market for some 100,000 units per year.

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References for 2.2

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References for 2.2 92Jac

Jacobs, P.: Rapid Prototyping and Manufacturing – Fundamentals of Stereolithography, Dearborn, MI: Society of Manufacturing Engineers, 1992.

01Sae

S¨andig, S., Leutbecher, T., Wiesner, P.: Laminated Tool Manufacturing by Laser Cutting and Diffusion Bonding. In: 20th International Congress on Applications of Laser & Electro-Optics ICALEO’2001, Jacksonville (USA), 15.–18.10.2001, 10 pages, Proc. LIA Volume and CD-ROM ISBN: 0-912035-73-0.

02Ove

Over, C., Meiners, W., Wissenbach, K., Lindemann, M., Hutfless, J.: Laser Melting: A new approach for the direct manufacturing of metal parts and tools, Proceedings of Euro U-Rapid, Frankfurt, 2002.

03Geb

Gebhardt, A.: Rapid Prototyping, 1st edition, Cincinetti, OH: Hanser-Gardner Publications Inc., 2003. Over, C.: Generative Fertigung von Bauteilen aus Werkzeugstahl X38CrMoV5-1 und Titan TiAl6V4 mit “Selective Laser Melting”, Ph.D. thesis, RWTH Aachen, Germany, 2003.

03Ove

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2.3 Thermal bending M. Geiger, F. Vollertsen

2.3.1 Principle of laser forming Thermal bending, i.e. laser forming, is a method of metal forming by kinematic shaping, which means that the shape is generated by the relative movement between the tool (laser beam) and the workpiece. The result of the laser-forming process is mostly characterized by the bend angle αB and sometimes by the bend radius ra (see also Fig. 2.3.7). Besides, surface melting, unwanted changes of the microstructure, generation of residual stresses and distortion of the workpiece have to be accounted for. These effects are described in [96Vol1]. In some cases, three-dimensional shapes are realized. The basic principle of thermal laser forming is the following: In order to change the shape of a workpiece, which is usually a metallic sheet or profile, a sample surface is heated by laser irradiation. The resulting thermal field introduces thermal stresses which in turn generate a plastic strain gradient. The process is run by guiding a laser spot along a straight or curved bending edge or by heating discrete points by laser pulses. In line heating, normally defocused beams are used to avoid melting of the surface, while in spot heating surface melting is often accepted. In contrast to flame bending (see e.g. [80DIN, 89Pfe]) laser heating enables the generation of steep temperature gradients even in thin sheets of materials having a high thermal conductivity, e.g. in 0.05 mm Cu foil. While the first publications [79Mar, 83Gut, 86Nam, 87Fra, 89Ros, 91Gei] concentrated on point or line heating for bending using the temperature gradient mechanism, 5 thermal mechanisms are known up to now which are analyzed in [96Vol1, p. 67 ff.] and [98Vol].

2.3.2 Mechanisms Three mechanisms are characterized by the temperature gradient along the thickness direction as shown in Fig. 2.3.1, part A: the Temperature Gradient Mechanism (TGM), the Point Source Mechanism (PSM) and the Residual stresses Relaxation Mechanism (RRM). For the TGM the sheet metal is irradiated along a straight or curved line using parameters (laser power Pl , processing speed vl , coupling coefficient A) which lead to the formation of a temperature gradient with maximum temperature at the heated and nearly room temperature at the unheated side. The heated material tends to expand according to its coefficient of thermal expansion, but it is obstructed by the surrounding cold material. Thermal stresses develop, which have to reach the flow stress of the heated material to generate plastic compression. After cooling, the material near the heated surface is shorter than that at the unheated one, yielding a bending towards the laser beam. Typical bend angles are in the order of 0.1 ◦ to 3 ◦ [96Vol1, p. 75]. The bend angle can be increased up to 180 ◦ by repetition of the irradiation. The PSM is very similar to the TGM with one difference: Heating occurs not along a line but only at one point of the surface. This results in higher internal stresses after cooling and in lower Landolt-B¨ ornstein New Series VIII/1C

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[Ref. p. 133

Fig. 2.3.1. Mechanisms of laser forming [96Vol1, p. 69].

bend angles, typically in the order of 0.02 ◦ to 0.2 ◦ [96Vol1, p. 71]. An increase of the bend angle is possible by repeating the process at a position different from that of the previous irradiation. The RRM is based on the asymmetric relaxation of residual stresses, thus it cannot be observed in stress-free samples. The induced thermal field enables stress relaxation at one surface. Stresses are redistributed leading to the formation of a maximum bend angle of 0.5 ◦ [96Vol1, p. 79], which cannot be increased by repetition of the irradiation at the same line on the surface. If parameters are used for irradiation which lead to a very smooth temperature gradient along the sheet thickness, e.g. if the processing speed is low or if a material with a high thermal conductivity is used, only a lateral temperature gradient develops (Fig. 2.3.1, part B). If, in addition, the diameter of the heated area is large compared to the sheet thickness, the Buckling Mechanism (BM) [94Hol, 95Arn, 96Kal] may occur. It can lead to bending towards or away from the laser beam. The typical bend angles are in the order of 0.1 ◦ to 15 ◦ per pass of the laser. In the case that the sheet thickness is large, yielding a high stiffness against buckling, a smooth temperature gradient leads to local upsetting by the Upsetting Mechanism (UM) (Fig. 2.3.1, part C). This local shortening may be used for extrusion bending or in-plane aligning operations. The mechanisms explained above are described in various analytical models [93Gei2, 94Vol2, 96Vol1, 96Vol2, 97Yau], which were verified by experiments conducted in a way that only one mechanism was active. Normally, a superpositioning of two or more mechanisms takes place, yielding virtually contradictionary results. Therefore, for the precise analysis of the meaning of various influence parameters the Finite Element Method (FEM) has to be imployed. Generally, this analysis is made in two steps. Firstly the temperature field is calculated and in a second uncoupled step the plastic strains are determined [93Kit, 94Alb, 94Kop]. The optimum choice of FEM element types is discussed in [94Gei2, 95Vol2]. The influence of the counterbending, i.e. the bending away from the laser beam during the heating phase of the TGM, on the bending result is calculated in [95Vol2]. In [95Gei, 97Kra2] the FEM is used for an optimization of extrusion bending. Some of the numerous influence parameters are analyzed in [94Kit, 96Hol].

2.3.3 Influence parameters The parameters influencing the primary result, the bend angle, can be divided into three groups: the processing parameters, the material parameters and the geometry parameters. Due to elastic effects, which are reversible and dependent on all three groups of these parameters, a threshold energy exists, below which no bending is observed. There are two kinds of elastic effects: the elastic deformation of the heated material and that of the surrounding workpiece. The latter can

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be measured during the forming operation as a bending away from the heated surface. Therefore, it is called counterbending [91Gei, 93Gei2, 96Vol1]. It was analyzed analytically in [97Yau].

2.3.3.1 Threshold energy If the thermal strain of the locally heated material is lower than the elastic strain at the flow stress, the process is reversible, no forming occurs after a heating and cooling cycle. The boundary temperature Tth above which plastic strains will occur can be calculated as a first approximation from the material parameters kf (flow stress), E (elastic modulus) and αth (linear coefficient of thermal expansion) [96Vol1, p. 115], if counterbending is strictly suppressed. As the counterbending cannot be suppressed in most real cases, measured values are more practical. They are given as a threshold Lth for the coupled line energy Lc (see below), as the line energy is the primary parameter in experiments, which lead to a certain surface temperature. Values are given in Table 2.3.1. Beside the parameters listed in Table 2.3.1 Lth is dependent on the sample width b, the forming mechanism, the beam mode and others. For small bend angles there is a linear relationship between the coupled line energy Lc (determined from Pl A/vl with Pl : laser output power, A: coupling coefficient, vl : processing speed) and the bend angle: αB = me (Lc − Lth )

for Lc > Lth

.

(2.3.1)

Values for the slope of efficiency me are also given in Table 2.3.1.

2.3.3.2 Processing parameters The processing speed vl , the coupling coefficient A (a material parameter), and the laser power Pl determine the coupled line energy Lc which may be used to control the laser forming process, as there is a strong relationship between Lc and the bend angle. We have already seen, see (2.3.1), that it is linear for small angles. Usually, the laser output power is varied to adapt the desired angle. When using pulsed irradiation the line energy might be controlled by variation either of the pulse energy or of the number of pulses. An increase of the processing speed has two effects which affect the bend angle: The coupled line energy Lc decreases, decreasing the bend angle. In addition, the temperature gradient increases, resulting in an increasing bend angle as shown in Fig. 2.3.2 for bending with constant line energy and increasing vl in the TGM-range. Decreasing the processing speed can lead to a change in the forming mechanism as demonstrated in Fig. 2.3.3. Slightly precurved samples were bent. At high processing speeds the TGM is active, yielding always positive angles, i.e. bending towards the laser beam. At low speeds the BM is dominant and the bend direction is determined by the orientation of the precurvature. A second important factor which determines the direction of bending are the residual stresses [95Vol1]. The bend angle is usually not constant along the width of the sample. This increases the distortion, called banana effect [97Muc]. The magnitude of the banana effect is dependent on the processing speed. It has been experimentally investigated [97Mag], analytically modeled [97Muc], and analyzed using FEM [96Gei].

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[Ref. p. 133

Table 2.3.1. Threshold energy Lth and slope of efficiency me for different materials. Plc is the coupled max laser power; αB is the bend angle below which (2.3.1) is valid; s0 is the sheet thickness; dl is the laser-spot diameter; n.d. = no data, n.s. = not further specified, empty field = same data as in previous line. Material Steel DC04

s0 Laser type dl vl [mm] [mm] [mm/s]

Plc [W]

2

120. . . 480

Steel (n.s.) X8CrNi18 12

1 6.35 12.7 19.1 25.4 2 0.3

AISI 304

1

AISI 1018

Aluminum Al AlMg3

CO2

5 5 5 5 5 5.3 6 n.d. 4 4 n.d. n.d. n.d. n.d. simulation 2.5 Nd:YAG 0.3 simulation 3.2 (excimer) CO2 5.4

me Ref. [ /(J/mm)]

1.1 1.0 0.5 0.25 0.1 120. . . 360 1.0 720. . . 1050 1.6 420. . . 700 0.4 210. . . 280 1.0 66. . . 140 1.2 (35) 1.5 1.0 0.8 0.5 100. . . 180 0.3 3.5. . . 21 0.7 1) 0.09

4.4 3.5 1.6 1.2 0.87 5.9 2.8 2.8 5.1 3.0 44 82 113 136 2.6 3.9 × 10−4 2.8 × 10−4

5.0 8.5 15 22 16 5.3 7.4 5.3 8.6 24 2.4 0.82 0.40 0.28 5.8 362 486

167. . . 250

900

1.5

n.d.

15

[86Nam]

25 58 12 11 50

[86Nam] [96Vol1] [93Kit] [95Fra] [97Mag]

33.3 180 1)

[96Vol1]

[97Hen2] [93Gei1] [93Kit] [87Scu]

[95Fra] [97Hov] [97Vol]

AA 2014 AlCuMg n.s.

1.2 1 2 2 0.8

5.4 n.d. 4 simulation 2.5 CO2 10

167 25 33.3 33.3 35 . . . 60

600. . . 900 120 . . . 300 225. . . 300 120. . . 180 480

1.3 5 1.0 0.4 1.8

2.2 6.5 3.3 3.5 6.6

Copper Cu

0.05

excimer 308 nm

2)

2)

1.0

1.3 × 10−2 5.7 × 103

[97Gei]

1.6 × 10−2 2.5 × 103 9.4 1.4

[95Fra]

12.7

[97Mag]

0.1 2 Titanium α-β-Ti n.s. 1) 2)

0.8

CO2

10 20 80 160 200 20 66.6 83.3. . . 147 16.7 16.7. . . 25 (3. . . 14)

max αB Lth ◦ [ ] [J/mm]

2

simulation 2.5

33.3

320. . . 600

3.9 0.2

CO2

30 . . . 40

480

1.1

10

61

Pulse duration 10. . . 20 ns, energy density 0.1. . . 0.6 J/cm2 . Pulse duration 45 ns, energy density 0.2. . . 10 J/cm2 .

2.3.3.3 Material parameters The coupled laser power is determined by the beam-material interaction, where the absorption has an important meaning. The dependencies of the coupling coefficient/absorptivity A are discussed in Part 1, Chap. 1, Sect. 1.2. The results of a variation of the coupling coefficient A with respect to laser forming are the same as a variation of the laser power. Concerning the bend angle, the material parameters αth , the linear thermal expansion coefficient, cp , the specific heat, and ρ, the density, have a strong influence on it, see Fig. 2.3.4. The data Landolt-B¨ ornstein New Series VIII/1C

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Fig. 2.3.2. Bend angle for different processing speeds at constant coupled line energy; steel, s0 = 2 mm; A: carbon steel SAE 1035, Lc = 28 J/mm [95Vol2]; B: Lc = 12.5 J/mm [94Kit].

Fig. 2.3.3. Bend angle for different processing speeds; carbon steel; s0 = 0.5 mm, Pl = 750 W, dl = 4.2 mm. Negative angles due to buckling mechanism at low processing speeds [95Arn].

were measured for conditions of the BM, at which the thermal conductivity λ is of less importance [96Vol1, p. 154]. In the TGM λ has a strong influence as shown in Fig. 2.3.5. The effects of a variation of λ on the temperature field are the same as a variation of the processing speed for constant line energies (compare Fig. 2.3.2). Figure 2.3.5 shows calculated instead of measured values for the variation of λ without a simultaneous change of other material parameters. This is not possible in experiments. The single points were calculated using temperature-independent values for λ except in one case. In this case, the natural dependence of λ on the temperature, given by the width of the horizontal bar, was used. It can be seen that it has no influence on the result for the bend angle. A variation of the bend angle is also generated by a variation of the material strength. As the strength increases the bend angle decreases. For the UM, if the strength is increased from 265 MPa to 385 MPa by cold work before the laser treatment, the plastic strain after laser irradiation decreases from 1.2 % to 0.8 %. It was shown that this increase is due to the increased share of the reversible elastic work [97Kra1], i.e. an increase of the threshold energy. The same influence of the strength can be deduced from the influence of the strain-hardening coefficient n on the degree of degression of αB -i-curves (i: number of passes in line heating), see Table 2.3.2. Working with multiple irradiations shows often that the incremental increase of the bend angle decreases from step i to step i + 1. For a large strain-hardening coefficient the exponent q, which quantifies the curvature of the αB -i-curves, is small, as the strength increases strongly, reducing the incremental bend angle. The effect is less pronounced for materials having lower strain-hardening coefficients.

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[Ref. p. 133

Fig. 2.3.4. Bend angle for different materials; s0 = 1 mm, Pl = 820 W, graphite-coated samples, vl = 25 mm/s, dl = 10.6 mm [93Vol].

Fig. 2.3.5. FEM-calculated bend angles for different thermal conductivities λ; s0 = 2 mm, A: Pl = 1850 W, vl = 83 mm/s, dl = 7.2 mm [96Gei]; B: Plc = 250 W, vl = 20 mm/s, dl = 2.5 mm [94Kit].

Material

Thickness [mm]

n

q

DC04

2.8 4 5 7 3.4 2.8

0.27

0.65 0.62 0.47 0.31 0.49 0.80

AISI304 Ti6Al4V

0.48 0.04

Table 2.3.2. Influence of the strain-hardening coefficient n on the curvature of αB -i-curves for a fit like αB = const. iq [94Spr].

2.3.3.4 Geometric parameters For bending sheets along straight lines the sheet thickness is the most important geometric influence parameter. It can be described by a power law. The exponent is −2 for the TGM and −2/3 for the BM, see Fig. 2.3.6. Other important features are the width to thickness ratio b/s0 and the shape of the irradiation path. Bending along curved lines yields smaller bend angles than equivalent bending along straight lines [97Hen1]. Decreasing b/s0 reduces the bend angle when keeping the other parameters constant [94Kop, 95Vol3, 97Hen1]. Landolt-B¨ ornstein New Series VIII/1C

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Fig. 2.3.6. Influence of the sheet thickness s0 on the bend angle αB for the TGM (except H, where the BM was active). A: mild steel, line energy L = 2.0 kJ/mm [87Scu]; B: DC04, Pl = 1 kW, vl = 10 mm/s [91Gei]; C: carbon steel (St37), Pl = 825 W, vl = 16.7 mm/s [96Hol]; D: carbon steel (St37), Pl = 1 kW, vl = 50 mm/s [96Vol1, p. 165]; E: AISI 304, Pl = 1.5 kW, vl = 167 mm/s [86Nam]; F: copper, pulse energy HE = 5 J/cm2 [97Gei]; G: carbon steel (C75), HE = 5 J/cm2 [97Gei]; H: X5CrNi18 10, Pl = 57 W, vl = 80 mm/s [96Kal].

2.3.4 Bend radii One of the advantages of laser forming is the generation of various bend radii. This may be realized by adaptation of the beam diameter or variation of the path distance when working with multiple irradiations. Figure 2.3.7 shows variations of the beam diameter, using a focused beam for the smallest bend radius, yielding an inner bend radius ri in the order of the sheet thickness. Due to the high intensity, surface melting occurred in that case. Bending with larger beam diameters avoids surface melting and results in higher radii. However, if large radii are requested it is more promising to irradiate the sample with parallel paths, while the single paths have a distance a. Figure 2.3.8 shows the linear correlation between the path distance and the bend radius. The number of irradiations necessary to adapt a bend angle of 30 ◦ increases with decreasing distance a due to the interaction of the internal stresses induced by the previous and the actual irradiation.

Fig. 2.3.7. Outer bend radius ra as a function of the beam diameter dl ; steel, s0 = 2 mm, αB = 30 ◦ [97Hen2].

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[Ref. p. 133

Fig. 2.3.8. Outer bend radius ra as a function of the distance between laser paths a; i is the number of irradiations necessary for αB = 30 ◦ ; beam diameter dl = 5.3 mm, steel, s0 = 2 mm, samples graphite-coated [97Hen2].

2.3.5 State of the art and trends There are only few industrial applications of laser forming which are published. A supplier of electronic components uses it for the aligning of relay springs [84Stei, 90Ham]. One company promotes a system called laser hammer for the sub-micron alignment of small assemblies. The potentials of laser forming are investigated in the following fields: – aligning of sheet metal structures, partly after welding [94Dei, 94Gei1, 95Vol4, 95Vol5, 98Eng, 99Eng, 99Wal], – aligning of microelectronic components [97Han, 97Hov, 97Olo, 97Vol], – straightening of flat cutting tools including the generation of advantageous residual stresses [88Toe, 94Toe], – rapid prototyping for sheet metal parts [93Koe, 94Gei3, 94Pri, 94Vol1, 95Vol3, 97Hen1, 97Tho], – rapid prototyping for bent profiles [94Gei4, 94Gei5, 95Gei], – rapid prototyping for shaped tubes [92Fra, 93Vac], – bending of wires [97Gei]. These works demonstrate the capability of the process for the production of car door panels, spoons, lamp housings and so on. The main advantages of the process are the following: – controllable contactless forming, – very small deformations can be realized (displacements of less than 1 µm, angles of less than 0.1 ), – scalability (workpieces with wall thicknesses between 50 µm and 25.4 mm were processed), – potential for automatization, – high accuracy. They can be especially used for: – aligning of small parts after assembly, – forming of materials which must not be contaminated by other materials (not even by the tool materials), – rapid prototyping for bent profiles. Therefore, the current aim is the realization of closed-loop-controlled processes [93Gei1, 96Hol, 97Hen2, 97Tho, 01Hut, 02Hut], to achieve high accuracy in automated processes, and the automated generation of the irradiation pathes from CAD data of the desired geometry [03Che].

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References for 2.3

133

References for 2.3 79Mar

Martin, R.: Deutsches Patentamt, Offenlegungsschrift 29 18 100, 1979.

80DIN

DIN 8522, 1980.

83Gut

von Gutfeld, R.: IBM Technical Disclosure Bulletin 25(9) (1983) 4585.

84Stei

Steiger, E.: Siemens Components 22(3) (1984) 135.

86Nam

Namba, Y.: In: Wang, C.P. (ed.): Int. conf. on lasers ’85 (1986) 403.

87Fra 87Scu

Frackiewicz, H., Mucha, Z., Trampczynski, W., Baranowski, A., Cybulski, A.: European Patent 0 317 830 A2, 1987. Scully, K.: Journal of Ship Production 3(4) (1987) 237.

88Toe

T¨onshoff, H., Rosenthal, A.: Blech Rohre Profile 35(10) (1988) 822.

89Pfe 89Ros

Pfeiffer, R.: Richten und Umformen mit der Flamme, D¨ usseldorf: DVS-Verlag, 1989. Rosenthal, A.: Patentschrift DE 39 24 262 C2, 1989.

90Ham

Hamann, C., Rosen, H.G.: In: Waidelich, W. (ed.): Laser/Optoelektronik in der Technik, Berlin: Springer, 1990, 661.

91Gei

Geiger, M., Vollertsen, F., Amon, S.: Blech Rohre Profile 38(11) (1991) 856.

92Fra

Frackiewicz, H., Trampczynski, W., Przetakiewicz, W.: ISATA 25th (1992) 373.

93Gei1 93Gei2 93Kit 93Koe

Geiger, M., Vollertsen, F., Deinzer, G.: SAE Paper 930 279, 1993. Geiger, M., Vollertsen, F.: CIRP ANNALS 42(1) (1993) 301. Kittel, S., K¨ upper, F.: B¨ ander, Bleche, Rohre 34(3) (1993) 54. K¨ onig, W., Weck, M., Herfurth, H.J., Ostendarp, H., Zaboklicki, A.K.: VDI-Z 135(4) (1993) 14. Vaccari, J.A.: American Machinist June 1993 (1993) 36. Vollertsen, F., Geiger, M.: Blech Rohre Profile 40(9) (1993) 666.

93Vac 93Vol 94Alb 94Dei 94Gei1 94Gei2 94Gei3 94Gei4 94Gei5 94Hol 94Kit 94Kop

Alberti, N., Fratini, L., Micari, F.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering, Bamberg: Meisenbach (1994) 327. Deinzer, G., Vollertsen, F.: Laser und Optoelektronik 26(3) (1994) 2. Geiger, M.: Annals of the CIRP 43(2) (1994) 1. Geiger, M., Holzer, S., Vollertsen, F.: In: Kr¨ oplin, B., Luckey, E. (eds.): Metal Forming Process Simulation in Industry, M¨ onchengladbach (1994) 335. Geiger, M., Arnet, H., Vollertsen, F.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering, Bamberg: Meisenbach (1994) 81. Geiger, M., Kraus, J., Vollertsen, F.: B¨ ander Bleche Rohre 11 (1994) 26. Geiger, M., Kraus, J., Vollertsen, F.: B¨ ander Bleche Rohre 12 (1994) 18. Holzer, S., Arnet, H., Geiger, M.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering, Bamberg: Meisenbach (1994) 379. Kittel, S.: Simulation der thermischen Blechumformung mittels der Finiten Element Methode, D¨ usseldorf: Verlag Stahleisen Umformtechnische Schriften, Band 55, 1994. Kopp, R., Kittel, S., Scholl, C.: B¨ ander Bleche Rohre 35(10) (1994) 34.

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134 94Pri 94Spr 94Toe 94Vol1 94Vol2 95Arn 95Fra 95Gei 95Vol1 95Vol2 95Vol3 95Vol4 95Vol5 96Gei 96Hol 96Kal 96Vol1 96Vol2 97Gei 97Han 97Hen1 97Hen2 97Hov 97Kra1 97Kra2 97Mag

References for 2.3 Pridham, M.S., Thomson, G.A., Menon, U., Koch, M.: In: Marcus, H.L., et al. (eds.): Solid Freeform Fabrication Symposium, Austin, Texas (1994) 25. Sprenger, A., Vollertsen, F., Steen, W.M., Watkins, K.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering, Bamberg: Meisenbach (1994) 361. T¨onshoff, H.K., Berndt, A.H., Rosenthal, A.R.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering, Bamberg: Meisenbach (1994) 337. Vollertsen, F., Holzer, S.: VDI-Z 136(1/2) (1994) 35. Vollertsen, F.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering, Bamberg: Meisenbach (1994) 345. Arnet, H., Vollertsen, F.: Proc. Inst. Mech. Engrs. 209 (1995) 433. Fratini, L., Micari, F.: In: Griethuysen, J.-P.S. van, Kiritsis, D. (eds.): Proceedings of the International Symposium for Electro Machining, April 17-21, 1995, EPFL, Lausanne, Switzerland, Presses polytechniques et universitaires romandes (1995) 827. Geiger, M., Vollertsen, F., Kraus, J., Hoffmann, P.: In: Soliman, J.I., Roller, D. (eds.): Proceedings 28th ISATA (Stuttgart) (1995) 467. Vollertsen, F., Komel, I., Kals, R.: Modelling Simul. Mater. Sci. Eng. 3 (1995) 107. Vollertsen, F., Holzer, S.: In: Shen, S.-F., Dawson, P. (eds.): Simulation of Materials Processing: Theory, Methods and Applications (NUMIFORM 95), Balkema Rotterdam (1995) 785. Vollertsen, F., Geiger, M.: Trans. NAMRI/SME XXIII, SME Dearborn/MI (1995) 33. Vollertsen, F.: In: Geiger, M. (ed.): Key Technology Laser: Challenge for the Factory 2000, (Laser 95), Bamberg: Meisenbach (1995) 151. Vollertsen, F., Geiger, M.: Forming and Fabricating 2(6) (1995) 38. Geiger, M., Holzer, S., Vollertsen, F.: Production Engineering, Ann. of the WGP III 2 (1996) 39. Holzer, S.: In: Geiger, M., Feldmann, K. (eds.): Ber¨ uhrungslose Formgebung mit Laserstrahlung, Fertigungstechnik Erlangen, Bamberg: Meisenbach 57 (1996). Kals, R.T.A., Vollertsen, F.: MB Produktietechniek 62(4) (1996) 100. Vollertsen, F.: Laserstrahlumformen – Lasergest¨ utzte Formgebung: Verfahren, Mechanismen, Modellierung. Bamberg: Meisenbach, 1996. Vollertsen, F.: In: Kals, H.J.J., Shirvani, B., Singh, U.P., Geiger, M. (eds.): Sheet Metal 1996, Proc. of the Int. Conf., University of Twente I (1996) 411. Geiger, M., Becker, W., Vollertsen, F.: Laser und Optoelektronik 29(4) (1997) 67. Hanebuth, H., Hamann, C.: In: Weidelich, W. (ed.): Lasers in Engineering, Proc. of the 15th Int. Congress, Berlin: Springer (1997). Hennige, T.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering 2, Bamberg: Meisenbach (1997) 409. Hennige, T., Holzer, S., Vollertsen, F., Geiger, M.: J. Materials Processing Technology 71 (1997) 422. Hoving, W.: In: Weidelich, W. (ed.): Lasers in Engineering, Proc. of the 15th Int. Congress, Berlin: Springer (1997). Kraus, J.: In: Geiger, M., Feldmann, K. (eds.): Laserstrahlumformen von Profilen, Fertigungstechnik Erlangen, Bamberg: Meisenbach 69 (1997). Kraus, J.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering 2, Bamberg: Meisenbach (1997) 431. Magee, J., Watkins. K. G., Steen, W.M., Calder, N., Sidhu, J., Kirby, J.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering 2, Bamberg: Meisenbach (1997) 399.

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Mucha, Z., Hoffman, J., Kalita, W., Mucha, S.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering 2, Bamberg: Meisenbach (1997) 383. Olowinsky, A., Gillner, A., Poprawe, R.: Proceedings of the Sensor’97 4 (1997) 133. Thomson, G., Pridham, M.S.: Rapid Prototyping Journal 3(4) (1997) 137. Vollertsen, F., Geiger, M.: In: J¨ uptner, W. (ed.): Laser – Von der Wissenschaft zur Anwendung, BIAS Bremen (1997) 309. Yau, C.L., Chan, K.C., Lee, W.B.: In: Geiger, M., Vollertsen, F. (eds.): Laser Assisted Net Shape Engineering 2, Bamberg: Meisenbach (1997) 357. Engler, I., Walz, C., Schubert, E., Sepold, G.: In: Proc. Eclat’98, Hannover, Frankfurt: DGM-Verlag (1998) 357. Vollertsen, F.: In: Schu¨ ocker, D. (ed.): Handbook of the EuroLaser Academy, London: Chapman & Hall (1998) 357. Engler, I.: Verfahrenskombination Laserstrahlschweißen und -richten am Beispiel einer Titan-Leichtbaustruktur, Sepold, G., J¨ uptner, W. (eds.), Strahltechnik, Vol. 12, Bremen: BIAS, 1999. Walz, Ch., Schubert, E., Sepold, G.: In: Proc. European Symposium on Assessment of Power Beam Welds, Geesthacht: GKSS (1999) Paper 25.

01Hut

Hutterer, A., Otto, A., Menzel, T.: In: Duflou, J.R., Geiger, M., Kals, H.J.J., Shirvani, B., Singh, U.P. (eds.): Sheet Metal 2001 (2001) 459.

02Hut

Hutterer, A., Otto, A., Geiger, M.: In: Proc. of the 7th Intern. Fall Workshop, Vision, Modelling and Visualisation (2002) 355.

03Che

Cheng, J.G., Yao, Y.L.: In: Lasers in Manufacturing, Stuttgart: AT-Fachverlag (2003) 281.

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2.4 Joining H. Haferkamp

2.4.1 Introduction Welding has become the most important joining method in manufacturing. The German standard DIN 1910 defines welding as the joining of materials in the welding zone by applying heat and/or force with or without filler material. The work affecting the welding zone is supplied externally by energy carriers [95Bey]. Laser welding belongs to the fusion-welding category. The necessary heat is generated through transformation of focused high-energy radiation when hitting or penetrating the workpiece (DIN 1910). During laser welding, material is continuously melted by a focused laser beam. Simultaneously, part of the material is vaporized. The melted material subsequently solidifies, constituting the welding seam, which is ribbed on its top bead, and root. Next to the fusion zone, towards the base metal, is what is known as the Heat-Affected Zone (HAZ). During welding, process gas is supplied onto the surface and root. Lasers are high-quality energy sources. The higher the energy quality, the more costly is its generation, which also holds true for laser radiation. Efficiency, with regard to electric energy input, is currently between 1 % and 30 %. What distinguishes the laser from all other light sources, is, apart from other features, the minimum divergence of its beam. This makes it possible to deliver radiation energy to the workpiece with a comparatively low deviation of the beam diameter. Due to this insignificant divergence, welding can take place at a distance z of several meters from the laser source (e.g. z > 40 m). Furthermore, the slight divergence allows the laser beam to be focused to very small beam diameters, which leads to extremely high intensities (energy flux density) generated on the surface of the workpiece. These high intensities allow even metals to be locally heated to vaporization temperature, and thus vaporize. Laser-welding systems essentially consist of five main components [95Bey], see Fig. 2.4.1: 1. laser and laser periphery, consisting of a laser-radiation source, gas supply, and cooling system, 2. beam guidance system, consisting of a beam delivery system, including cooling device for components, beam switches, and beam forming device, 3. handling system for the workpiece, consisting of a loading and unloading device, workpiece or beam positioning system, movement unit, as well as process and shielding gas supply, 4. controlling unit, consisting of a laser control, handling control, as well as a process and system monitoring device, 5. safety devices, consisting of a protective tube, in which the laser beam is guided, a safety cabin, beam absorber, and process gas exhaust system with corresponding filters. Laser welding depends on material temperature, laser power, laser beam radius, as well as absorption and material properties of the workpiece. Only a narrowly focused laser radiation interacts with the workpiece, which results in narrow dimensions of the welded joints and heat-affected zone. Furthermore, the joined workpiece shows usually insignificant distortion. In laser welding, there are two different processes that may be distinguished: Landolt-B¨ ornstein New Series VIII/1C

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If the portion of laser radiation absorbed by the material surface and transformed into heat energy “enters” the workpiece by mere heat conduction, the process is known as conduction welding. Energy is transported by means of heat conduction, which is supported by convection. The maximum fusion zone depth (weld depth) is typically in the range of half the fusion zone width (weld width). As a result of the deformed fusion surface, the fusion zone depth may also become slightly larger than the width. During laser welding, similar to electron-beam welding, the deep-penetration effect occurs. Energy is mainly transported optically from the surface into the workpiece, i.e. laser radiation penetrates the workpiece through a capillary. The radiation is absorbed by the capillary walls and transformed into heat. Fusion geometry results from energy being transported from the capillary walls, and not from the workpiece surface. This allows for slender and deep welds with depth/width ratios larger than 10 [95Bey]. CO2 and Nd:YAG laser welding in the kilowatt power range has become increasingly important for the automotive industry. CO2 lasers emit radiation at a wavelength of λ = 10.6 µm, whereas Nd:YAG lasers emit at λ = 1.06 µm. Laser welding enhances the rigidity of auto-body constructions and allows weight reduction at the same time. Continuous welding seams may be processed fully automated and error-free, at speeds of several meters per minute.

Laser Welding System

Laser / Periphery

Beam Guidance

Workpiece Handling

Controlling Unit

Safety Devices

Radiation Source

Beam Delivery

Loading Unloading

Laser Control

Protective Tube

Gas Supply

Beam Forming

Positioning Unit

Handling Control

Safety Cabin

Movement Unit

System Monitoring

Beam Absorber

Cooling System

Process and Shielding Gas

Gas Exhaust Filter System

Fig. 2.4.1. Main components of a laser-welding system.

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2.4.2 Conduction welding The physical principle of laser welding is based on laser radiation being focused on the joining spot and the portion of radiation absorbed by the workpiece being intense enough to locally melt the material. This leads to both components being joined, after solidification of the molten metal. In metals, laser radiation is absorbed by a very thin surface layer (d < 10−5 cm), which results in the formation of a surface heat source. The resulting temperature distribution in the workpiece, and also the fusion geometry, are determined by the source geometries and power flux from it. This makes it possible to calculate the fusion geometry, applying the heat-conduction equation, and taking the absorbed radiation power into consideration [95Bey], see also Part 1, Chap. 1. Figure 2.4.2 shows the principle of laser welding through heat conduction. The fusion geometry is usually wider than that of the heat source, the latter corresponding to the cross-sectional area of focused laser radiation. As a function of time, the fusion isotherm penetrates the workpiece and reaches a maximum. During conduction welding, laser radiation is absorbed by the material surface, as illustrated in Fig. 2.4.2. Longer irradiation results in a three-dimensional heat flux.

Laser beam

Workpiece t1

< t2 < t3 t = Duty cycle of laser beam

Fusion geometry t4

Fig. 2.4.2. Conduction welding [95Bey].

If the portion of laser radiation absorbed by the workpiece surface and transformed into heat energy is transported into the workpiece by mere heat conduction, the process is known as conduction welding. Here, only small fusion depths with lenticular fusion volumes can be achieved, which are solely determined by the absorbed energy density. Maximum fusion widths and depths are reached when the surface temperature of the source increases further, as a result of heat-conduction losses. When heat conduction into the workpiece is limited by the finite material thickness, heat accumulates at the bottom side of the workpiece, and the fusion geometry is changed, as illustrated in Fig. 2.4.2 at the right side.

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2.4.3 Deep-penetration welding If the laser-radiation intensity is increased during conduction welding, the vaporization temperature tV can be locally achieved at the surface of the workpiece (tV (W) = 5500 ◦ C, tV (Al) = 2447 ◦ C, tV (St 52-3)= 2860 ◦ C) [89Bla]. As a consequence, the fusion surface continues to deform and the laser beam penetrates the workpiece with a small diameter. This results in the formation of a vapor capillary, whose geometry depends on laser, process and material parameters. The diameter of the vapor capillary is typically in the range of the focal diameter (0.2 . . . 1 mm). Its depth corresponds approximately to the welding depth. Through a vapor capillary, laser radiation can easily penetrate the workpiece. The vapor capillary is surrounded by a liquid phase. The pressure of the vaporizing material protects the capillary from blocking up. The capillary moves through the material like a fine tube, due to feed motion. Part of the molten metal flows around the capillary, another part vaporizes and escapes out of the capillary as ionized metal vapor, and a further part condenses on the colder back wall of the capillary. The vapor capillary model for laser welding corresponds to that of electron-beam welding. The vapor capillary makes deep-penetration welding possible, and therefore allows for deep and slender seams, which are characteristic for laser welding. In Fig. 2.4.3 deep-penetration welding with a laser beam is schematically drawn.

Laser beam

Metal vapor Melting zone Vapor capillary Solidified melt Focus position

Fig. 2.4.3. Schematic drawing of deep-penetration welding with a laser beam.

Laser-welding systems currently may be purchased up to an average power of 45 kW. These are CO2 lasers (wavelength λ = 10.6 µm) that allow welding depths in steel of up to 40 mm. Solid-state lasers (λ = 1.06 µm), however, may only be obtained up to a power of approx. 6 kW. Despite higher absorption rates using solid-state lasers, welds are not as deep, in comparison to welds performed with CO2 lasers. On the other hand, solid-state lasers can use optical fibers for energy transport, which is of great technical benefit, so that the lower efficiency of solid-state lasers becomes less important for certain applications. Figure 2.4.4 illustrates possible penetration depths for various laser power densities at different laser power outputs of a CO2 laser at λ = 10.6 µm using steel St 52-3 at a feed rate of v = 10 mm/s.

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10 P ro cessi ng a re a

P la sma th re sho l d in ten si ty I c

Wel d- i n depth [m m]

8

Sh i el di ng ar e a

6 Ste el S t 52 -3 v = 1 0 mm/s l = 1 0.6 m m 4

3 kW

2 kW

1 kW

2

0 5 10

10

6

10

7

10

8

2

La ser p owe r de nsi ty E [W/cm ] Fig. 2.4.4. Weld-in depth as a function of laser power densities.

At power densities below a critical intensity Ic , the plasma threshold intensity, of about 106 W/cm2 , penetration depths of several millimeters can be achieved, which corresponds to laserbeam conduction welding. At power densities above 106 W/cm2 , absorption conditions change to such an extent that the penetration depth highly increases. Comparatively high penetration depths can be achieved at power densities up to approximately 107 W/cm2 , depending on the laser power. This makes it possible to weld constructional steel, e.g. in the ship-building industry, with a thickness of more than 20 mm. At power densities above approximately 107 W/cm2 , absorption conditions change again, which then leads to a shielding of the process by the laser-induced plasma, and thus to a decrease in penetration depth. Due to comparatively high power densities produced during laser material processing, high welding speeds can be achieved. At the same time, processing is locally limited, which results in a low absolute energy input, or energy per section, compared to other welding methods. This allows for a distortion-minimized processing. Closed and self-consistent model solutions for deep-penetration welding with lasers can scarcely been found, so far, due to the complexity of the processes (heat conduction, fusion movement, multiple reflections, plasma absorptions, etc.). The individual phenomena, however, may be regarded separately, and are described in [95Bey], using approximation models. They provide an extensive overview of what happens during laser welding and outline correlations. No applied engineering programs for modeling laser-beam welding with a personal computer were known until recently. In [97Rad] the development of an engineering method of modeling is presented, using the experience obtained from numerical modeling of consumable electrode welding in active gases. The dynamic model of laser-beam welding is designed for determining the geometric characteristics of the weld and temperature fields in the welding zone of components of various shapes made of metal alloys.

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2.4.3.1 Capillary formation Figure 2.4.5 shows a schematic drawing of capillary and secondary radiation during laser welding. First of all, a weld pool is formed at the workpiece surface by local input of laser-beam power. Then, a capillary starts to form, and the molten surface deforms (depression). The capillary has a diameter that can be compared to that of the focused beam. The fusion in front of the depression starts to flow around the capillary. If the laser intensity is higher than a critical intensity Ic of an order of magnitude of approximately 106 W/cm2 , see Fig. 2.4.4, the vaporization temperature is reached at the workpiece surface. The pressure of the escaping metal vapor causes the fusion-zone surface to further deform. This deformation and the vaporization of material result in capillary formation. The incident laser radiation is no longer perpendicular to the metal surface, but strikes a curved or inclined surface towards the welding direction. This results in an increased absorption and vaporization rate. At the same time, there are multiple reflections, which increase energy incoupling. Laser-induced plasma may form, depending on the metal vapor density, which further increases absorption. At the beginning, the “drilling process” caused by vaporization is perpendicular to the surface, which, however, becomes perpendicular to the capillary front after capillary formation. Pulse transmission of the escaping vapor atoms (vaporization pressure) supports fusion movement around the capillary. Vapor pressure in the capillary keeps it from blocking up. So far, absorption processes in the vapor or in the plasma capillary can only be described quantitatively by approximation, because of the dynamic processes during welding. In literature, absorption mechanisms, such as multiple reflections and plasma absorption, are treated separately, and refer to stationary cases. This allows a deeper knowledge about absorption processes and effects on dynamic processes. A self-consistent description of capillaries, in view of absorption and heat conduction, is only in the initial stage at present. A self-consistent description of the deep-penetration-welding process is given in [98Sud].

Metal vapor radiation

Reflected laser radiation Laser beam

Weld pool heat radiation

Weld pool

Plasma radiation

Metal vapor plasma Workpiece

Fig. 2.4.5. Schematic drawing of capillary and secondary radiation during laser welding.

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2.4.3.2 Plasma formation This section describes the time-resolved formation of plasma in laser technology. First of all, metal vapor is formed, which reduces the reflected laser radiation. After several 100 µs, a laser-induced plasma forms in the escaping vapor. Along with plasma formation, the portion of reflected radiation decreases. After approx. 0.1 ms, plasma fluctuations become visible and a phase opposition to the reflected radiation can be observed. Micrographs show that, already at this point in time, a deeppenetration effect as well as a vapor capillary have been formed [95Bey]. Nevertheless, there are high reflections of laser radiation at times, during which no laser-induced plasma is being formed. This clearly shows that plasma formation increases the fusion geometry and at the same time energy incoupling. The deep-penetration effect shows that as soon as the vaporization temperature is reached at the workpiece surface a vapor capillary forms, which can be seen from the escaping metal vapor. Fusion geometry and energy incoupling are, however, not increased before a laserinduced plasma is formed. One of the preconditions for laser-induced plasma formation by inverse Bremsstrahlung in the metal vapor is that sufficient free starting electrons are present in the interaction volume. Free electrons move almost inertia-free with the electric field strength of laser radiation. However, on a time average they are not able to consume energy from the electric interaction field. This effect can be described using the model of the oscillating dipole, which emits energy in a phase shift. Radiation is therefore completely reflected. The electron gas is heated, as a result of inverse Bremsstrahlung. Part of the laser radiation is absorbed, due to electrons not being able to move in an ideal manner. They collide with other electrons, and may therefore be accelerated in the interaction field. Consequently, energy incoupling into the material (absorption) is characterized by elastic collisions of electrons with atoms or ions. In the vicinity of an ion, a free electron absorbs a photon, which leads to an increase in its kinetic energy, and thus its speed. This process is termed inverse Bremsstrahlung [93Ber], see Fig. 2.4.6. The energy levels correspond to the discrete energy levels of the bound electron. The hatched area marks the continuous energy field of the free electron. The black dot stands for the state of energy of the electron under observation. V2

V1 0 E2

0 E2

V1 < V2

E1 Fig. 2.4.6. Inverse Bremsstrahlung.

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2.4.3.3 Humping effect When the critical process-parameter-dependent welding speed is exceeded, weld-pool movement becomes unstable. This instability is marked by drops being formed, partly periodically, on the upper surface of the seam. In thin sheets, holes may form in front of the drops, whereas in thick sheets increased pore formation may occur. This type of fusion build-up (drop formation) takes place at high feed rates, and is termed “humping effect”, see Fig. 2.4.7.

Fig. 2.4.7. Hole and drop formation [95Bey].

The humping effect has not only been observed during laser welding, but also during electronbeam and arc welding. There are different mechanisms described in literature which cause the humping effect. These, however, will not be discussed further at this point [95Bey]. Figure 2.4.8 shows the principle of capillary and weld-pool formation as well as flow processes during welding with or without the humping effect. The weld-pool geometry and flow conditions can be determined by longitudinal cross-section grindings. Increased bulging of the weld pool can be observed in the range of parameters in which the humping effect occurs. If the focal geometry is adapted appropriately (e.g. double focus), the humping effect may be suppressed. Laser beam

Laser beam

Humping drop Weld pool

Feed rate Fig. 2.4.8. Humping effect [95Bey].

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Two different humping effects can be distinguished: 1. The seam has periodic build-ups on its upper side, along with some depressions on its lower side, as well as several holes. 2. The seam has a reinforcement, overlaid by several build-ups, along the entire upper side. The lower side is characterized by a significant concave root surface.

2.4.4 Material weldability The high number of variables that must be taken into consideration is a problem with regard to improving welding quality by choosing the right parameters. Correlations become clear when the influencing variables are categorized, which has been done in [73DIN], by elaborating simplified structures for welding processes. The importance of general influencing variables, introduced as a main connecting thread in [73DIN], is further emphasized by describing the multitude of influencing variables and processes during laser welding in more detail. Defining weldability comprehensively with regard to the process, as is done in [73DIN], gives an overview of correlations during welding and makes clear just how complex welding processes are. A component is defined as weldable if a joint can be formed by welding, using a given welding process, and if the welded joint meets the requirements with regard to its effect on the entire construction of the component and to quality. This description relates to components with a defined product profile and a known stress of the joints. The weldability equally depends on three main influencing variables, which are the material, the construction, and the manufacturing. Influencing variables and weldability correlate indirectly because of the following properties: – welding suitability of the material, – welding reliability of the construction, – welding possibilities in manufacturing. These properties depend on the material as well as on the construction and manufacturing. The significance of these three properties is, however, different. Figure 2.4.9 shows the correlation of influencing variables and weldability as described in the German standard DIN 8528. The three influencing variables are basically of similar significance and determine the welding result. In [94EN], quality assurance of welded joints in steels using electron-beam and laser welding is described. This standard is valid for quality monitoring during steel-products manufacturing. It outlines assessment categories for imperfections in – electron-beam-welded butt joints in workpieces of a thickness between 0.5 and 50 mm, – laser-welded butt joints in workpieces of a thickness between 0.5 and 10 mm. There is no general guarantee for the weldability of parts, since production conditions (including weather conditions at construction sites) as well as construction design must be considered along with material properties. The welding suitability of a material may, however, be guaranteed to a certain extent. Therefore, details on weldability are given in the corresponding material instructions. However, material manufacturers do not give a general guarantee.

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Material Welding suitability

Weldability of a

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rel ity

ing

ibil

tur

We

fac

oss

nu

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Ma

ldin

iab

We

component

Fig. 2.4.9. Weldability of a component.

2.4.5 Thermal distortion During welding, the heat source used causes several material-based, chemical, and physical changes in the welded joints, as well as deviations in the component’s dimensions. Large-scale material changes only occur in fusion-welded joints. All changes are not advantageous for the component. This is the result of the extreme thermal cycle, which is characterized by: – high heating speed, amounting to several 100 K/s, – high cooling speed, amounting to 600 K/s, – minimum austenization duration, in the range of several seconds. The thermal cycle during welding results in: – – – –

material changes, dimensional changes of the welded joint (construction), reduced corrosion resistance due to residual stress and structure changes, various chemical and physical processes.

The thermal cycle is determined by the power density of the welding method, which is given in Table 2.4.1. Under similar conditions, with an increasing power density, cooling speed increases, the heat-affected zone (HAZ) narrows, and the maximum hardness of the HAZ increases. The highest possible hardness is reached if the HAZ structure is 100 % martensitic. Table 2.4.1. Power density range of several welding methods [96Sch]. Welding procedure

Power density [W/cm2 ]

Manual electrode welding Inert gas welding Submerged melt welding Laser-beam welding Electron-beam welding

104...5 105...6 105...6 106...7 107...8

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Methods with high power densities, such as laser welding, produce a very narrow HAZ, and lead to very low distortion of the component. A decisive disadvantage, however, is the very high maximum hardness in the HAZ of steel weldings, as a result of very high cooling speeds. The temperature-time field depends on the power density of the heat source. The thermal effect on the workpiece can sufficiently be described with heat input, which, apart from power density, is influenced by the feed rate of the heat source. The reason for changes in material and the different types of changes are due to the thermal cycle during welding. The point-shaped and power-dense heat source causes residual stress and dimensional changes in the component. Residual stress is not caused by external forces, as is the case with load stress, but by several different influencing factors that lead to irregularly distributed plastic deformations in the component: – – – –

temperature differences, material transformations, plastic deformation, change in material flow.

Temperature differences are the most important reasons for residual stress or deformations occurring in the workpiece. With regard to thermal distortion of the workpiece, laser welding is furthermore favorable, because of the possibility to mechanize the method, which allows fluctuations of heat input to be minimized in the course of the welding seam.

2.4.6 Tailored blanks Tailored blanks are components that are adapted to technical requirements with regard to material and workpiece geometry. These tailor-made sheets are a combination of individual, laser-welded sheets of different thicknesses or materials. This makes it possible for soft materials with good deepdrawing properties to be used for critical forming zones and high-tensile sheet steels in areas where high rigidity and stability is more important. Light metal, such as aluminum and magnesium, can also be applied, welded together as wrought and cast alloy. Using inner-door sheet of auto bodies as an example, [96Fue, 96Har] show that tailored blanks can not only improve production, but also reduce costs. Tailored blanks are applied in wheel housings, bumpers, or longeron spars. Almost 80 million tailored blanks are produced in 2003 in Western industrial countries. Process-adapted sheet blanks have emerged from the development of tailored blanks. Sheets are welded together, so that they do not overly deform during the subsequent deformation process [97Vol]. Oftentimes, this makes non-linear seams necessary, which can be welded close to the contour, and in a flexible manner, using laser welding. In [96Hei], a cupping test is described, in which the bottom of the cup, made of higher-tensile steel (H340), was joined to the upper part using laser welding. An improved limit drawing ratio was observed for a sample made of deep-drawing steel (DC04). The diameter of the higher-tensile steel welded was 1.5 times larger than the stamp diameter of the cupping test. Laser welding of high-tensile, micro-alloyed steel (H300) with deep-drawing steel (DC04) is introduced in [97Haf]. It must be emphasized that the fatigue strength of the welding seam is higher than that of deep-drawing steel. This results in failure of the non-affected base metal DC04. The ability to deform tailored blanks of SPCC (mild steel with a carbon content lower than 0.12 %) is being investigated in the Japanese automotive industry [97Kus]. The mild steels under investigation are welded together using CO2 laser radiation. The forming behavior of the welding

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2.4.6 Tailored blanks

[Ref. p. 156

seams as well as of the base metal is being examined, applying spherical punch stretch forming and hole-widening tests. Depending on the direction of stress, failure was observed either in the welding seam or in the weaker base metal. If the stress direction is parallel to the welding seam, it must take up total stretching up to the point of damage. If, however, stress direction is perpendicular to the welding seam, the latter is only slightly deformed, due to its higher strength. The main stretching takes place in the weaker base metal, which is damaged. Examinations of the joints of different sheet thicknesses and materials further show that plastic deformations are limited to the weaker partner, when the sheet thickness or strength ratio is larger than 1.5 and the pull direction is perpendicular to the joining zone. Depending on hardness, the maximum degrees of deformation of the welding seam are εmax = 0.2 for HV= 350 and εmax = 0.3 for HV= 260. Maximum deformation of the mild steel under investigation is approx. εmax = 0.5 at shear stress. Tailored blanks are also manufactured using mash seam welding (Fig. 2.4.10). This, however, involves weld reinforcements that have to be reduced by post-smoothing.

a

a

b

a

Fig. 2.4.10. Mash seam welding with similar and dissimilar sheet thickness.

Deformation characteristics are compared in [97Sie]. These can be explained by a larger weldingseam cross-section and a higher hardness increase in the heat-affected zone. There are only minimum differences in the forming behavior. The forming ability for laser-welded seams, however, is slightly increased, as opposed to mash-welded seams. The use of tailored blanks opens new dimensions in steel auto-body manufacturing. Tailored blanks make the future development of an ultralight steel vehicle, which allows for a reduction in fuel and exhaust gases due to its low weight. 32 steel manufacturers from 15 nations are working on the ULtralight Steel Auto Body project (ULSAB) to meet the requirements. The project mainly aims at reducing weight by exclusively using steel. This involves using new steel products such as higher-tensile steel sheets, premanufactured blanks of different steels, as well as innovative joining methods for steel such as laser-beam welding. Ultralight steel autobodies may be reduced in weight by approximately 25 %, and at the same time they are lower in costs by approximately US$ 150 [95NN], see Table 2.4.2. Table 2.4.2. Parameter comparison of a reference car (middle-of-the-market sedan) with an ultralight steel auto body (ULSAB).

Weight [kg] Rigidity against torsion [Nm/grad] Flexural strength [N/mm] Natural frequency [Hz] Costs [US$]

Reference

ULSAB

Aim

271 11,531 11,902 38 1,116

205 19,056 12,529 51 962

200 13,000 12,200 40 –

From a constructional point of view, all goals with regard to rigidity against torsion, flexural strength, and vibrational behavior of the auto body are significantly excelled. Crash simulations prove that this auto body meets high safety standards. Landolt-B¨ ornstein New Series VIII/1C

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2.4.7 Soldering and brazing The German standard DIN 8503, part 3 categorizes laser soldering under “soldering method with a beam”. The energy carrier is the high-energy and monochromatic light radiation generated in a beam source. Radiation is generated up to temperatures necessary for soldering by the radiation which is absorbed by the workpiece. Three soldering methods can be applied using laser radiation as a thermal tool: soft, hard, and high-temperature soldering [80Lug]. An essential difference between laser soldering and other soldering methods is the fact that energy penetrates the material locally, which protects the components from heating up. This is advantageous for quite a few components, since different component structures may be stressed in a thermally-adapted manner. Figure 2.4.11 illustrates the principle of laser soldering [97Kre]. The laser beam is imaged onto the workpiece using beam-guiding optics. The portion of laser radiation that is absorbed heats the material up to the soldering temperature and melts the solder metal simultaneously. Solder may either be supplied manually before the process or automatically during the process. Wire or powdered solder metal is continuously provided by a supply unit, which is time-dependent on the laser beam. To assist the soldering process, soldering flux may be manually or automatically supplied, and, as an option, process gas can be provided. Solder, molten as a result of radiation absorption, wets the surface that has heated up to soldering temperature, and displaces the soldering flux. As a result of relative motion towards the workpiece and processing head, a closed or interrupted solder bead forms, depending on the application. The quality-relevant influence of parameters affecting the soldering process is complex and cannot be considered alone, but interactively. Laser soldering is basically determined by energyincoupling phenomena, heat-conduction effects, and solid-liquid-interface reactions. The influencing variables to the joining process may be distinguished into beam- and process-related variables, as well as workpiece- and material-specific parameters.

Focusing optics

Laser beam

Soldering agent supply Workpiece Soldering seam bead

Feed direction

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Fig. 2.4.11. Schematic drawing of laser soldering.

150

2.4.8 Diode-laser applications

[Ref. p. 156

In the electrical industry, mostly CO2 or Nd:YAG lasers are used for soft soldering, and for innovative applications also diode lasers with a beam power of up to 25 W are applied [93Toe]. The use of Nd:YAG solid-state lasers with beam guidance through an optical fiber leads to a certain homogenization of the intensity profile, which is advantageous for soldering, because a local temperature increase can be avoided. This application is mostly used in the field of forming conducting links in electronic components. Temperature-controlled laser-soldering systems are already available. The soldering temperature is controlled by IR signals, and its curve may be forwarded to quality-assurance systems for documentation [93Jaf]. Laser-based brazing as well as high-temperature soldering with CO2 or Nd:YAG lasers are of increasing interest, and new areas of application are emerging, as shown by research and development activities [95Haf]. Hard soldering with laser radiation offers new perspectives for cost-effective joining of parts with relatively large cross-sections, as in the automotive sub-supplier industry or autobody manufacturing. High process reliability can be achieved, which is the most important advantage of this method [93Haf]. An important application of laser soldering is joining ceramics and metals, because the combination of these two materials is oftentimes the only possibility to exploit the advantages of ceramics, since the ceramic workpieces must be integrated into the existing structures.

2.4.8 Diode-laser applications Diode lasers are semiconductor lasers. In diode lasers, the laser beam is generated by microelectronic diodes that directly transform electric current into laser energy. Semiconductors function as light guide and resonator. The most striking feature of this type of laser is its efficiency when transforming electric into optical energy, amounting to 30 %. This allows for a considerable reduction in operating costs. High-power diode lasers have an increasing importance in material processing. In the early 1990’s, this type of laser was only known to be used in entertainment and communications technology. The reason that this type of laser has entered the field of production technology are its numerous advantages: – – – – – – –

small size, higher efficiency (30 . . . 60 %), little or no maintenance (lifetime between 10,000 and 20,000 operating hours), broad range of wavelengths due to different material systems (λ = 0.4 . . . 30 µm), fiber coupling possible, power increase by combining several laser bars to form stacks, steady increase in output power of new diode laser generations while prices drop [97Pop].

The power output required for material processing is achieved by putting several laser bars together to form stacks. This leads to power outputs in the range of 4 kW at power densities in the focused beam of up to 105 W/cm2 . This means, that no deep-penetration welding is possible, see Fig. 2.4.4. Investigations on hardening 42 CrMo 4 steel with a polished surface (uncoated) have shown that, using 650 W laser power and a feed rate of 650 mm/min., hardness penetration depths and widths of approx. 0.4 mm and 4.5 mm, respectively, can be achieved. The short wavelength of the laser beam (λ = 940 nm or λ = 810 nm), improved absorption, as well as a homogeneous intensity distribution, allow optical energy to be transformed efficiently into a hardened volume. In addition, diode lasers can be used for conduction welding of thin sheets. CrNi sheets with a thickness of 1 mm may be butt welded, using a feed rate of 450 mm/min. Studies will have to

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prove the technological and economic importance of this high-power-diode-lasers application, by comparing it with other conduction-welding methods and performing tension and stability tests. Lap welding of plastics using high-power diode lasers has already established itself in joining technology. For this process, the upper part of the object to be joined is transparent to the laser beam, while the lower part is absorbent. Dynamic and intense research activities in high-power-diode-laser development conducted worldwide will result in their further miniaturization, broader power ranges, and increased lifetime. The state of development and experience gained in material processing have already opened entirely new perspectives in production technology. An improved efficiency of processes in use, such as laser hardening, in combination with the miniaturization of machines may give way to process and concept innovations in industry.

2.4.9 How to avoid quality degradation Mastering production and processing to obtain high-quality products is one of the most important measures to be taken in quality assurance. Processes which allow on-line process monitoring and control must be developed to assure good quality and cost-effectiveness as well as productivity through less rejects. During transformation hardening of steels using laser radiation, the quality of the hardening result is closely related to the thermal cycle that the material passes through. Temperature in the interaction zone of the workpiece is an important parameter that has to be kept in close tolerances to guarantee consistent high-quality processing results. If the emission behavior is known, the temperature at the workpiece surface can be derived from heat radiation and used to control the hardening process. For this method, an adaptive temperature control is used, which relates the thermal cycle of the workpiece to the laser power. The measuring device transfers the actual value of the workpiece temperature to the temperature control unit, so that controlled processing is made possible. Using a controlling device, the temperature of the workpiece surface can be kept in close tolerances: – – – –

overheating and melting of the workpiece surface can be avoided, reproduction of the processing result is improved, uniformity of processing quality can be guaranteed, the process can thus be automated.

For a non-contact surface-temperature measurement, radiation pyrometers have been used for many years. To determine the temperature, temperature-dependent heat or temperature radiation is taken into account. By applying different detectors, filters and lenses, the measurement range of radiation pyrometers can be delimited, which leads to the following distinction: Total radiation pyrometer: Partial radiation pyrometer: Spectral pyrometer:

This device reacts to the total spectral range of temperature radiation. The measurement takes place in a wide spectral range. This pyrometer only reacts to a very narrow spectral range.

Oftentimes, the workpiece surface is covered with a thin absorbing layer to facilitate an increase in energy input into the material. This is due to a higher degree of absorption of the coating as opposed to the material. Consequently, the coating absorbs electromagnetic radiation from a wide range of the IR spectrum and is not transparent. Therefore, a pyrometric measurement of the

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Laser

[Ref. p. 156

Workpiece

Power Monitor

Regulator

Radiation Thermometer

Fig. 2.4.12. Closed-loop structure of adaptive temperature control [91Dre].

workpiece temperature below the coating is not possible, so that merely the temperature of the coating surface can be measured. This calls for an adaptive measurement method, which can detect the workpiece temperature below the coating, determining the correlation of the coating temperature and laser power and providing the temperature-control unit with the actual value (Fig. 2.4.12). A model of the hardening process is the basis for the measurement, which allows a simplified on-line heat-conduction calculation on a process computer. Processing the signals for laser power and coating temperature, the process computer is the core for the applications of adaptive measurements to control the workpiece temperature. The conventional electronic PID (Proportional plus Integral plus Derivative) control is replaced by the adaptive temperature control system, which evaluates the signals sent by a radiation thermometer and laser power screen and processes them to become single variables. Process control during laser welding means that quality-relevant process parameters must be recordable. This involves light emission being recorded during the process [02Spec]. Light signals carry information on different process phases, energy incoupling, fusion and vaporization [03Hil]. While carrying out advanced investigations on plasma density fluctuations during laser welding, broadband light emission is measured with a photodiode and sound emission above the joint is measured using a microphone. The plasma above the workpiece is subject to extreme density fluctuations, depending on the parameters set and the current process status. The welding efficiency and quality is significantly dependent on laser-induced plasma. The power density of the plasma illumination is dependent on the material’s vaporization rate and the plasma density. The buildup time of the light signal is a measurement for the speed of change in plasma density. This in turn is determined by the mass-flow rate out of the capillary and therefore by the vaporization rate [88Gat]. A quickly increasing vaporization rate generates pressure higher than the ambient pressure in the effective area. The plasma coming out expands and becomes a sound wave [98Far]. Light emission reduces with decreasing plasma pressure. Since the degree of absorption of the incident laser radiation is dependent on plasma density, a greater portion of laser radiation passes through to the material. This involves a greater portion of metal being vaporized and ejected out of the capillary. This results in oscillating plasma density, which is similar to oscillating light emission. At the same time, sound emission confirms oscillation of the mass flow which results from the ejecting plasma. An increased feed rate of the workpiece leads to decreased light intensity, which can be derived from a reduced portion of vaporization. The oscillation frequency increases simultaneously. Similar investigations are being conducted on the influence of plasma during CO2 laser welding. Special attention is drawn to the distribution of the different spectral lines which plasma emission consists of. Apart from the spectral lines of each metal, also lines of the surrounding gases, such as nitrogen or oxygen, can be observed. If the beam power density during welding is too high,

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it results in an additional strong plasma formation of Argon, used as assist gas, above the joint. Consequently, laser radiation is shielded, depending on the density. Energy incoupling into the workpiece is therefore greatly reduced, which leads to a short-term change from deep penetration to conduction welding. This effect can be observed due to the reduced emission of process radiation. If helium is used as an assist gas, the excitation-energy input through laser radiation is not sufficient for ionization. Process radiation may be used as a process parameter for quality control during welding, as investigations on the process behavior have shown [95Hey, 99Sun, 96Gri1, 01Haf]. There are typical process errors that may occur during laser welding. Welding seam faults can be the reason for a change in process parameters, such as the decrease in laser-beam power, or a process error resulting from a fault in the component’s joining area. Table 2.4.3 illustrates essential errors that may arise during welding. Table 2.4.3. Essential errors during laser welding [96Ove]. Type of error

Reason for error

Change of process

Signal characteristics (plasma charge)

No error

Good conditions

Steady plasma and constant weld-pool size

Holes

Defective workpiece edge, weld burnthrough as a result of extreme beam power Change of surface tension causes the fusion to sink.

Breakdown of plasma, substantial change in weld-pool geometry

Reduced laser power, change in surface reflectivity, faulty focal position Laser-power breakdown, change in incoupling conditions Extreme energy per area causes changes in weld pool and flow, mainly at extreme feed rates. Mechanical problem due to the clamping device or beam guiding system Turbulences in the weld pool

Reduced plasma temperature

Signal with a constant amplitude swing (±10 % change in level) Short-term breakdown of signal, minimum detectable hole diameter 0.2 mm Reduction of amplitude swing, grooves up to a diameter of 0.2 mm may be detected. Reduced signal amplitude

Top bead depression at the end, groove

Low penetration depth

Seam interruption

Wide and rough weld surface

Weld distortion

Material accumulation

Edge misalignment

Mechanical problems, when welding edges are misaligned vertically

Partial breakdown of plasma, reduced weld-pool size

Extremely lower plasma formation

Longer-term breakdown of signal

Higher plasma formation

Rise of the signal amplitude for the plasma charge sensor

Reduced plasma temperature

Reduced signal amplitude

High fluctuations Increasing and changes in amplitude fluctuation weld-pool geometry Reduced plasma intensity Reduced signal amplitude and weld-pool size

The portions of process radiation emitted during welding are primarily radiation, heat radiation of the weld pool, and metal vapor radiation [02Spec]. These three kinds of radiation differ significantly from each other. An essential portion of the radiation emitted by the plasma is in the UV range. Emissions that result from the weld pool and metal vapor do not differ from each Landolt-B¨ ornstein New Series VIII/1C

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[Ref. p. 156

other with regard to wavelength, but with regard to their signal frequency spectrum. Metal vapor emission is typically in the signal frequency spectrum of 10. . . 20 kHz, while radiation from the weld pool fluctuates significantly slower. With regard to these facts, a system for process control has been developed which detects seam depressions and misalignments. In [96Gri2] a method for quality control during laser welding is described that is based on the measurement of the welding plasma’s UV emission, the IR radiation of glowing spatter from the weld pool, and the welding seam surface temperature. Since the single phases may be recognized from the welding plasma during laser welding, this measurement signal can be evaluated for process control. When welding metals using CO2 laser radiation, UV emission has a characteristic spectral range of 200. . . 400 nm, which is recorded with the help of a Si photodiode in connection with an optical filter. A plasma charge detector, which directly measures free charge carriers in the plasma, has been developed [97Hil, 98Mue], and combined with sound-emission analysis, can be used as a process control for tube welding. The plasma charge detector measures the electrical field strength which forms at the joint as a result of laser-induced plasma. Electric charge results from the ionization of metal vapor and assist gas as well as free electrons generated at the joint. Due to the fact that the amount of free charge carriers is a measurement for the degree of ionization in the flame, its measurement may serve as a process control. Sound Emission Analysis (SEA) is suitable for spot welding with laser radiation. During spot welding of spring connectors made of copper alloys, SEA allows determination of the penetration depth into the material, focusing, contact of the parts to be joined, and the degree of fusion. Table 2.4.4 gives an overview of sensors for process monitoring during laser welding [96Ove].

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Table 2.4.4. Sensors for process monitoring during laser welding [96Ove]. Sensor

Measuring method

Measurable variable

Measuring range

Typical time units

Costs per sensor (guide number) [EUR]

Pyroelement Pyroelectric

Electromagnetic Spectral: radiation, circular 0.1 nm . . . 100 µm

Typical build-up time: 2 ns

150

Photodiode

Photoelectric

Electromagnetic Spectral: radiation, circular 200 . . . 1100 nm

Typical build-up time: 10 ns

50

HgCdTe detector

Photoelectric

Electromagnetic Spectral: radiation, circular 2 . . . 22 µm

Typical time constant: 0.5 µs

7,500

Thermocamera

Pyroelectric

Thermal radiation, areal

Spectral: 3 . . . 5 µm and/or 8 . . . 12 µm

Image repetition frequency: 50 Hz

40,000 to 100,000

Inductive sensor

Eddy-current principle

Distance, e.g. between processing head and workpiece

Typical distance: 0.1 . . . 20 mm, resolution: 0.1 mm

Cutoff frequency: 2 . . . 3 kHz

500

Piezoelement

Piezoelectric

Ultrasound

Signal frequency: typically 20 kHz . . . 10 MHz

Typical build-up time: 10 ns

500

Microphone

Sound pressure Electrostatic, electrodynamic, piezoelectric, contact converter

Signal frequency: typically 10 Hz . . . 30 kHz

Typical build-up time: 30 µs

250

CCD camera

Photoelectric, bucket-chain principle

Image, areal

Spectral: 400 . . . 1100 nm

Exposure time: 25 . . . 0.1 ms

1,000

Thermopile

Seebeck effect

Temperature

Spectral: UV. . . far IR

Typical time constant: 32 ms

< 500

Plasma charge detector

Capacitive

Electric field

Unknown (lab assembly)

Unknown (lab assembly)

Unknown (lab assembly)

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References for 2.4

References for 2.4 73DIN

DIN 8528-1: Schweißbarkeit, metallische Werkstoffe, Begriffe, Ausgabe 1973-06, 1973.

80Lug

Lugscheider, E., Lison, R.: Verbindungstechnik 12(12) (1980) 47.

88Gat

Gatzweiler, W., Maischner, D., Beyer, E.: Laser und Optoelektronik 20(5) (1988) 64.

89Bla

Blanke, W.: Thermophysikalische Stoffgr¨oßen, Berlin: Springer-Verlag, 1989.

91Dre

Drenker, A., Sturm, V., Oebels, H., Loosen, P., R¨ uhl, F.: Laser und Optoelektronik 23(5) (1991) 48.

93Ber

Bergmann, L., Schaefer, C.: Lehrbuch der Experimentalphysik Band 3, Optik, 9. Auflage, Berlin: Walter de Gruyter, 1993. Haferkamp, H., Marquering, M., Kinzel, A., Kreutzburg, K.: Laser Magazin 3 (1993) 12. Jafari, S., Dorn, L.: L¨ oten mit dem Laserstrahl, DVS-Berichte 119, Fachbuchreihe Schweißtechnik, Materialbearbeitung durch Laserstrahl, D¨ usseldorf: DVS-Verlag GmbH, 1993. T¨onshoff, K., Berndt, A., St¨ urmer, M.: Maschinenmarkt 35 (1993) 72.

93Haf 93Jaf

93Toe 94EN

EN 729-1: Schmelzschweißen metallischer Werkstoffe, Teil 1: Richtlinien zur Auswahl und Verwendung, Ausgabe 1994.

95Bey 95Haf

Beyer, E.: Schweißen mit Laser, Berlin: Springer-Verlag, 1995. Haferkamp, H., Bach, F.-W., Alvensleben, F. von, Kreutzburg, K.: Laser Magazin 4 (1995) 43. Heyn, H.: Prozeߨ uberwachung und -diagnose beim Schneiden und Schweißen mit CO2 Hochleistungslasern, Dissertation, Braunschweig, Techn. Univ., 1995; Aachen: Verlag Shaker, 1995. N.N.: Die neue Dimension im Karosseriebau – Das ultraleichte Stahl-Auto der Zukunft, Stahl 5 (1995) 22.

95Hey

95NN

96Fue 96Gri1 96Gri2 96Har

96Hei 96Ove 96Sch

97Haf

F¨ unders, P.: Einsatz von hochfesten St¨ ahlen zur Gewichtsreduzierung im Karosseriebau, Praxis-Forum Karosserie-Rohbau, 1996. Griebsch, J.: Grundlagenuntersuchung zur Qualit¨ atssicherung beim gepulsten Lasertiefschweißen, Dissertation, Stuttgart, Univ., 1995; Stuttgart: Teubner, 1996. Griebsch, J., Schlichtermann, L., Jurca, M., Hoving, W., Nillesen, C.: Proc. ICALEO 1996, Orlando, FL: LIA, Laser Institute of America (1996) 164. Hartmann, G., M¨ uschenborn, W., Schneider, C., Simon, R.: H¨ oherfeste Feinbleche und neue Blechkomponenten f¨ ur den Automobilleichtbau, Neue Entwicklungen in der Blechumformung, 1996. Hein, P.: Maschinenmarkt 46 (1996) 36. Overmeyer, L.: Beitrag zur Prozeßkontrolle und -regelung beim Schneiden und Schweißen mit CO2 -Laserstrahlung, D¨ usseldorf: VDI Verlag, 1996. Schulze, G., Krafka, H., Neumann, P.: Schweißtechnik, VDI-Buch, Berlin: SpringerVerlag, 1996. Haferkamp, H., Bach, F.W., Burmester, I., H¨ofemann, M., Niemeyer, M., Kreutzburg, K.: Werkstattstechnik 87(6) (1997) 299.

Landolt-B¨ ornstein New Series VIII/1C

References for 2.4 97Hil 97Kre

97Kus 97Pop 97Rad 97Sie 97Vol 98Bey

157

Hillerich, B., Schumacher, J., Alvensleben, F. von: Schweißen und Schneiden 49(4) (1997) 226. Kreutzberg, K.: Aspekte des Laserstrahll¨otens von metallischen und metall-keramischen Verbindungen, Dissertation, Hannover, Univ., 1997; VDI Fortschrittberichte, Reihe 2: Fertigungstechnik, Vol. 421, D¨ usseldorf: VDI-Verlag, 1997. Kusuda, H., Takasago, T., Natsumi, F.: Journal of Materials Processing Technology 71(1) (1997) 134. Poprawe, V.: Lasertechnik I und II, Vorlesungsskript des Lehrstuhls f¨ ur Lasertechnik RWTH Aachen, 1997. Radaj, D., Sudnik, V.A., Erofeey, V.A.: Welding International 11(3) (1997) 243. Siegert, K., Knabe, E., Possehn, T., Glasbrenner, B.: Werkstatttechnik 87(6) (1997) 304. Vollertsen, F., Geiger, M.: VDI-Z Integrierte Produktion 139(6) (1997) 26.

98Sud

Beyer, E., Wissenbach, K.: Ober߬achenbehandlung mit Laserstrahlung, Berlin: Springer-Verlag, 1998. Farson, D.F., Kim, K.R.: Optical and Acoustic Emissions in Laser Welding. In: Proc. ICALEO 1998, Orlando, FL: LIA, Laser Institute of America, 1998. Mueller, R.E., Duley, W.W.: Proc. ICALEO 1998, Orlando, FL: LIA, Laser Institute of America (1998) 262. Sudnik, W., Radaj, D., Erofeew, W.: J. Phys. D: Appl. Phys. 31(24) (1998) 3475.

99Sun

Sun, A., Kannatey-Asibu, E., Jr., Gartner, M.: J. Laser Appl. 11(4) (1999) 153.

01Haf

Haferkamp, H.D., Alvensleben, F. von, Niemeyer, M., Specker, W., Zelt, M.: Welding. In: T¨ onshoff, H.K., Inasaki, I. (eds.): Sensors in Manufacturing, Weinheim: Wiley-VCH Verlag, 2001.

02Spec

Specker, W.: Prozeߨ uberwachung beim Schweißen mit Nd:YAG-Lasern, Dissertation, Hannover, Univ., 2002; VDI-Fortschrittberichte, Reihe 2, D¨ usseldorf: VDI-Verlag, 2002.

03Hil

Hillers, O.: Fehlerklassifizierende Prozesskontrolle mittels multivarianter Statistik beim Laserstrahlschweißen, Dissertation, Hannover, Univ., 2003; Berichte aus dem IFW, Band 07/2003, Hannover: Produktionstechnisches Zentrum GmbH, 2003.

98Far 98Mue

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2.5 Laser separating W. O’Neill

2.5.1 Introduction In general, manufacturing processes can be classified in terms of the action that is applied to the material being processed. Processes tend to change the material in order to render it useful for a particular application. There are three generic process classifications: subtractive processes which reduce bulk material to a required size and shape, e.g. cutting, milling, grinding; formative processes which involve the deformation of a correct amount of material to the required shape, e.g. moulding, casting, pressing and additive processes which selectively add material where and when required to produce the correct shape, e.g. cladding, selective re-melting, laminated object manufacture etc. Laser removal is a generic term that describes the removal of material by laser to produce the required effect in the case of laser cleaning and marking, and the required shape in the case of laser cutting, drilling, and machining. It is not a commonly used term but it is useful in this case to describe the collection of processes that make up the subject of this section. This section presents selective data sets for a range of laser processes. Laser cutting (see Part 2, Chap. 6) has been widely used by industry since the early 1970’s to process a range of materials utilizing the intense thermal effects that can be induced by high-power industrial laser sources. Commonly used laser sources for cutting are CO2 and Nd:YAG laser systems, these have been developed extensively for industrial use and as such have been accepted widely by almost every industrial sector. Laser machining processes have only recently been employed by industry with little or no penetration into traditional machining markets such as those satisfied by mechanical milling, turning and grinding. The reason for this is the poor tolerances and control that can be achieved compared to traditional routes. Despite this slow take-up laser machining processes are finding their applications in tool finishing and fine sculpting. Laser marking processes have sustained high levels of market penetration due to their versatility and flexibility. Both CO2 and Nd:YAG laser systems are employed in marking applications to process a broad range of materials. The aerospace sector has employed Nd:YAG drilling systems to improve the performance of turbine blade components for many years. Much effort has been made to develop this particular application due to the distinct advantages of laser-drilling processes. Laser ablation using excimer and Copper Vapor Laser (CVL) is finding useful roles in the production of polymeric microsystems as a cheap alternative to the x-ray-based LiGA process in addition to a number of microelectronic applications in the processing of circuits, via-holes and interconnects. The application of lasers in the selective removal of material is extremely widespread and easily accounts for the greatest number of industrial laser installations at the present time. Whilst it is not possible to provide explicit data for every material and laser system, the author has provided data that is representative of optimal processing conditions. It is possible to use different lasers with the same material and produce a differing set of optimum process parameters. This is not unusual, as most of the data presented in this section has been generated by various workers in the field over the last 30 years. The author asks the reader to consider these data sets as guidelines for operation and not explicit parametric landmarks.

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2.5.2 Cutting

[Ref. p. 184

2.5.2 Cutting Laser cutting was the first widespread industrial application of the laser. It was first used to process die-boards for the packaging industry in the early seventies and went on to be a very effective process for the cutting of metals, plastics, fabrics and wood-based products. Figure 2.5.1 presents a schematic of the laser-cutting process. A high-power laser is focused to a point on or below the surface of the material. The products generated by the intense interaction, the liquid or gaseous phase, are subsequently blown away by a strong coaxial gas jet. Various gases are employed to enhance thermal energy input, e.g. reactive gases such as oxygen, or to minimize oxidation reactions, e.g. nitrogen assist gas or inert gases such as argon. Laser

Lens

Gas input

Gas nozzle

Laser:

CO2:Nd:YAG

Gas:

oxygen, nitrogen

Pressure:

0.5 bar - 25 bar

Power:

100 W - 6 kW

Materials:

metals,polymers, ceramics,organics

Spark shower

Fig. 2.5.1. Typical gas-assisted laser-cutting set-up.

2.5.2.1 Fusion cutting Laser fusion cutting is the term used to describe the cutting process where a laser is used to melt the material which is subsequently removed by a coaxial gas jet through melt shear and pressure forces. Fusion cutting has particular benefits in that it eliminates the deleterious effects of oxidecoated cut edges when processing stainless steels and other materials where clean edge cuts are required. Processing speeds are lower than those obtained with reactive gases and gas-delivery pressures are usually higher, a typical pressure range is 12 . . . 22 bar. Due to the low beam quality of Nd:YAG lasers, the main choice of laser for fusion laser cutting is the industrial CO2 laser with power levels typically over 1.5 kW. Operating rates are typically 50 . . . 70 % of those obtained with oxygen. Fusion cutting is used to process a wide variety of metals and plastics that undergo a transition to a molten product when exposed to high-intensity laser radiation. In Figs. 2.5.2–2.5.5 the maximum cutting speed for different materials is given as a function of sheet thickness. In Tables 2.5.1 and 2.5.2 the typical cutting speed for selected polymers and for selected ceramics, respectively, is listed. Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 184]

2.5 Laser separating 6

16

5

Cutting speed v [mm min -1 ]

1500 W

12

Cutting speed v [mm min -1 ]

Ti

Ni Cr steel

14

161

10 8 6

1000 W

4

3

2

4

1

2 500 W 0

0

2

4

6 8 10 Thickness t [mm]

12

14

16

Fig. 2.5.2. Maximum cutting speed for Ni-Cr steel against sheet thickness for incident CO2 laser power levels of 500 W, 1000 W, 1500 W [98Pow, p. 60].

0

0

1

2 Thickness t [mm]

3

4

Fig. 2.5.3. Maximum cutting speed for Ti against sheet thickness for an incident CO2 laser power level of 1 kW using an Ar gas jet at 8 bar [98Pow, p. 75].

Table 2.5.1. Typical cutting speed for selected polymers at 500 W CO2 laser power, gas pressure in the range 1 . . . 4 bar, glossy edge formed at gas pressures less than 0.25 bar [98Pow, p. 102]. Thickness [mm]

Polyethylene [m/min.]

Polypropylene [m/min.]

Polystyrene [m/min.]

Polyamide [m/min.]

ABS [m/min.]

1 2 3 4 5 6 7 8 9 10 12

11.0 4.0 2.2 1.5 1.2 1.0 0.8 0.6 0.5 0.4 0.3

17.0 7.0 4.0 2.8 2.0 1.6 1.3 1.1 0.9 0.7 0.4

19.0 7.4 4.2 3.0 2.3 1.8 1.6 1.2 1.0 0.9 0.7

20.0 8.0 4.8 3.5 2.6 2.0 1.6 1.2 1.0 0.8 0.5

21.0 8.2 5.0 3.6 2.7 2.1 1.7 1.3 1.1 0.9 0.6

Landolt-B¨ ornstein New Series VIII/1C

162

2.5.2 Cutting

20

[Ref. p. 184 5

C Mn steel

18

4

Cutting speed v [mm min -1 ]

Cutting speed v [mm min -1 ]

16 14 12 10 8 6

3

2

1

4 2 0

0

1

2 Thickness t [mm]

3

4

Fig. 2.5.4. Maximum cutting speed for mild steel against sheet thickness for an incident CO2 laser power level of 2.5 kW using an N2 gas jet at 15 bar [84Coh].

0

2

4

6

8 10 Thickness t [mm]

12

14

Fig. 2.5.5. Maximum cutting speed as a function of sheet thickness for polymeric materials using a CO2 laser at 500 W. Filled circles: polystyrene, open circles: polypropylene, filled triangles: polyamide [93Kul].

Table 2.5.2. Typical cutting speed for selected ceramics using a CO2 laser, air assist gas in the range the range 1 . . . 2 bar [98Pow, p. 113]. Material

Thickness

Cutting speed [m/min.]

Laser power [W]

Crown glass

1 2 3 1 2 2 1

1.5 1.0 0.5 1.4 0.6 2.0 0.6

500 500 500 500 500 1000 1200

Al2 O3 Silica

2.5.2.2 Reactive-gas cutting Reactive-gas cutting is similar to fusion cutting except that additional energy is generated through the use of a reactive-gas assist. Cutting speeds are generally 30 . . . 60 % faster when using reactive gases. Mild steels are almost exclusively cut using oxygen gas assist in contrast to stainless steels which are usually cut with high-pressure nitrogen gas at about 15 bar. Cutting Ni-Cr steels using oxygen produces a burnt edge due to the formation of CrO2 oxide film which disturbs the stability of the cutting process and lowers edge quality. There are many parameters that affect the performance of reactive-gas cutting, two of the most influential parameters are: the composition of the material, specifically the percentage of non-reactant elements; the concentration of the oxygen gas. Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 184]

2.5 Laser separating 10

14

Ni Cr steel

Low carbon steel

1500 W 8

1500 W

10

Cutting speed v [mm min -1 ]

Cutting speed v [mm min -1 ]

12

163

8 6 1000 W 4

6

1000 W

500 W 4

2 2 0

0

500 W 2

4

10 6 8 Thickness t [mm]

12

14

16

Fig. 2.5.6. Maximum oxygen-assisted CO2 lasercutting speed as a function of sheet thickness for low carbon steel at various laser powers, gas pressure 3.0 . . . 0.5 bar [98Pow, p. 26].

0

0

2

4 6 Thickness t [mm]

8

10

Fig. 2.5.7. Maximum oxygen-assisted CO2 lasercutting speed as a function of sheet thickness for Ni-Cr steel at various laser powers, gas pressure 2.0 . . . 10.0 bar [98Pow, p. 75].

In Figs. 2.5.6–2.5.10 the maximum cutting speed for different materials in oxygen- or airassisted laser-cutting processes is given as a function of sheet thickness. Figure 2.5.11 shows the laser-cutting velocity as a function of the oxygen concentration.

2.5.2.3 Sublimation cutting Cutting data is presented in Tables 2.5.3–2.5.5 for a number of materials cut using sublimation as the primary process mechanism. Whilst it is relatively easy to cut these materials with a laser, the by-products of the interaction can be particularly hazardous to the health of the operator. Care must be taken to ensure that a suitable closed-bed fume extraction system is operating during processing.

2.5.3 Cleaning The operation and reliability of laser-based cleaning systems are slowly becoming established in an industrial context. The various international government regulations have restricted the use of CFC-based cleaning solvents due their deleterious environmental impact. Aqueous-based cleaning is commonly used although this approach still exhibits environmental affects due to the high volume

Landolt-B¨ ornstein New Series VIII/1C

164

2.5.3 Cleaning 8

Al

1000

A B C D E

4

Cutting speed v [mm min -1 ]

Cutting speed v [mm min -1 ]

6

2

0

[Ref. p. 184

Cu

100

10

1

0

2

4 6 Thickness t [mm]

8

10

Fig. 2.5.8. Cutting velocity of Al versus thickness; oxygen-assisted CO2 laser: A: 0.75 kW cw [84Coh]; B: 1.5 kW cw [84Spe]; C: 1.4 kW pulsed [90Mic]; D: 2 kW cw [84Spe]; E: 4 kW cw [82Rot].

0.1 0.01

0.1

1 Thickness t [mm]

10

100

Fig. 2.5.9. Cutting velocity of Cu versus sheet thickness. Filled circles: 47 W CVL; open circles: 1.8 kW Nd:YAG; filled triangles: 2.5 kW CO2 ; open triangles: 12 kW CO2 ; filled squares: 15 kW CO2 ; open squares: 18 kW CO2 [96Bar].

Table 2.5.3. Cutting speeds of thermoset plastics, rubbers and fiber-reinforced materials with a CO2 laser [98Pow, p. 103]. Material

Thickness [mm]

Cutting speed [m/min.]

Laser power [W]

Formica

1.6 1.6 3.0 3.0 6.0 3.0 6.0 9.0 12.0 3.0 6.0 9.0 12.0 1.6 1.6 3.2 3.0

7.8 14.0 2.8 2.9 1.1 4.0 1.6 0.9 0.6 3.0 1.2 0.7 0.4 5.2 15.0 2.4 2.6

400 1200 400 400 400 400 400 400 400 400 400 400 400 450 1200 400 400

Phenolic resin Rubber

Rubber (carbon filled black)

Glass reinforced carbon fiber resin Glass filled nylon

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 184]

2.5 Laser separating

165

Table 2.5.4. CO2 laser cutting speeds of polymers using air as an assist gas (0.25 . . . 4 bar), 500 W [98Pow, p. 102]. Thickness [mm]

PMMA [m/min.]

Polycarbonate [m/min.]

Polyvinylchloride [m/min.]

1 2 3 4 5 6 7 8 9 10 12

35.0 15.0 8.0 5.5 4.5 3.5 3.0 2.3 1.9 1.5 1.2

21.0 8.2 5.0 3.6 2.7 2.1 1.7 1.3 1.1 0.9 0.6

28.0 11.0 6.4 4.3 3.2 2.5 2.0 1.7 1.4 1.2 1.0

Table 2.5.5. CO2 laser cutting speeds of wood-based products using air as an assist gas (3 . . . 10 bar) [98Pow, p. 110]. Material

Thickness [mm]

Cutting speed [m/min.]

Laser power [W]

Poplar Douglas fir Yellow pine Walnut Cherry Scottish pine Beech Teak Mahogany Oak Ash Ebony Pine Pine Pine Plywood Plywood Plywood Medium density fiberboard Medium density fiberboard Medium density fiberboard Hardboard Hardboard Corrugated card Paper

10 10 10 10 10 10 10 10 10 10 10 10 6 12 20 6 12 20 6 12 20 3 4 3 0.1

5.0 3.5 3.2 3.8 4.3 3.3 4.0 3.5 3.1 2.9 2.6 1.2 8.0 3.2 1.6 7.0 3.0 1.5 9.0 4.0 2.0 10.0 7.0 25.0 > 500

500 500 500 500 500 500 500 500 500 500 500 500 500 1000 1000 1000 1000 1000 1000 1000 1000 500 500 500 500

Landolt-B¨ ornstein New Series VIII/1C

166

2.5.3 Cleaning

12

[Ref. p. 184 5

Ti

Cutting speed v [mm min -1 ]

Cutting speed v [mm min -1 ]

10

8

6

4

4 Low carbon steel 1 kW

3 Ni Cr steel 1.5 kW

O2

2 Air 0

Ar 0

1

2

3 4 5 Thickness t [mm]

6

7

Fig. 2.5.10. Cutting velocity of Ti versus sheet thickness for air assist and oxygen assist at 1 kW CO2 laser power [98Pow, p. 75].

2 99.3

99.4

99.5

99.6 99.7 99.8 Oxygen purity %

99.9

100.0

Fig. 2.5.11. CO2 laser cutting velocity of 3 mm mild steel and Ni-Cr steel versus oxygen concentration [93Gab].

of waste that is produced. There have been wide-ranging investigations into laser cleaning systems and almost every laser has been applied to remove contaminants such as oxides, oils, varnishes, paints, fingerprints, particulates etc. There are several mechanisms that are employed to effect laser cleaning as shown in Fig. 2.5.12: pulsed laser irradiation of particulates using a thermo-mechanical effect; interface boiling when contaminant absorption is low; surface boiling when contaminant absorption is high; volumetric whole film boiling; direct film ablation; vaporization; photochemical degradation and differential thermal expansion. Tables 2.5.6 and 2.5.7 present a range of data for contaminant removal from surfaces. Table 2.5.6. Laser-based cleaning of contaminants. Material

Contaminant

Source

Wavelength

Pulse width Fluence [J/cm2 ]

Ref.

Limestone Limestone Limestone Limestone Limestone Canvas Marble Silicon Glass SS 316

Encrustation Encrustation Encrustation Encrustation Encrustation Varnish Encrustation Al2 O3 Oil Al2 O3

TEA-CO2 Dye Excimer Nd:YAG Nd:YAG Excimer Nd:YAG Excimer Excimer Nd:YAG

10600 nm 590 nm 248 nm 1064 nm 1064 nm 248 nm 1064 nm 248nm 248 nm 248 nm

100 ns – 30 ns 100 µs 10 ns 15 ns 15 ns 20 ns 20 ns 20 ns

[92Coo] [92Coo] [92Coo] [92Coo] [92Coo] [98Geo] [97Mar] [92Tam] [94Lu] [96Olt]

< 15 3 7 7 1 1 6.3 0.35 0.5 0.4

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 184]

2.5 Laser separating Pulsed laser irradiation

Pulsed laser irradiation Water film Substrate vibration

Particle vibration

Pulsed laser irradiation

Pulsed laser irradiation Water film

Pulsed laser irradiation Water film

Interface boiling

Surface boiling

Pulsed laser irradiation

Pulsed laser irradiation

Plasma

167

Whole film boiling

Pulsed laser irradiation

Vapor

Ablation

Vaporization

Photochemical degradation

Differential thermal expansion

Fig. 2.5.12. Laser-based cleaning methods. Table 2.5.7. Laser-based cleaning of contaminants; Q-switched Nd:YAG laser, 1064 nm, pulse width smaller than 250 ns [95Eng]. Material

Contaminant

Energy flux/ pulse [J/cm2 ]

Average energy flux [J/cm2 ]

Power flux/pulse [MW/cm2 ]

Quartz

Paint, fingerprints Particles and haze Rustoleum Rust Oxide Oxide Oxide Oxide Oxide Oxide Oxide Oxide Paint, particles Paint, particles Paint, particles Fingerprints Paint, particles

0.31 . . . 1.63

2.44 . . . 20.56

7.8 . . . 47.8

0.41 . . . 0.80 0.14 . . . 0.36 0.95 . . . 2.06 0.24 . . . 0.31 0.50 . . . 0.73 1.14 . . . 1.38 1.23 . . . 2.30 0.35 . . . 0.85 0.22 . . . 0.30 0.84 . . . 0.93 0.24 . . . 0.27 0.16 . . . 0.33 0.26 . . . 0.31

7.97 . . . 24.54 2.93 . . . 12.87 7.40 . . . 18.51 0.70 . . . 2.73 0.50 . . . 0.73 1.28 . . . 4.32 19.44 . . . 22.99 1.31 . . . 2.58 0.88 . . . 4.19 3.58 . . . 4.94 0.96 . . . 3.12 1.30 . . . 10.34 2.56 . . . 3.27

12.0 . . . 23.4 4.2 . . . 9.6 60.5 6.90 . . . 8.90 14.8 . . . 21.4 33.6 . . . 40.6 57.2 . . . 67.1 9.60 . . . 19.70 6.40 . . . 8.90 26.9 . . . 27.3 3.25 . . . 4.93 2.93 . . . 4.67 4.04 . . . 5.16

0.26 . . . 0.27

2.57 . . . 2.58

4.54 . . . 4.70

Paint Paint

0.77 . . . 1.16 0.21 . . . 0.53

1.85 . . . 2.75 7.09 . . . 17.27

2.93 . . . 4.39 3.12 . . . 4.80

Stainless steel Steel Molybdenum Erbium Tantalum Tungsten Nickel alloy Nickel/iron alloy Zirconium Chromium HDPE Polypropylene Acrylic Polycarbonate (Lexan) Nylon Teflon

2.5.4 Machining The ability to remove material from depths of nanometers to several millimeters makes laser machining techniques particularly attractive. Bulk metals can be machined with a variety of approaches depending on the surface finish and machining rate that is required. The processes of laser machining fall into four main categories (Fig. 2.5.13): oxidation-based processes (oxide chip caving), liquid-phase machining (fusion caving), vaporization and photochemical degradation or non-thermal ablation. Large volume removal rates in liquid-phase machining require high-energy Landolt-B¨ ornstein New Series VIII/1C

168

2.5.4 Machining Laser irradiation Oxygen

[Ref. p. 184 Laser irradiation

Laser irradiation

Oxygen

Pulsed laser irradiation Vapor

Oxide chip melt

Oxide chipping

Non thermal ablation

Liquid phase

Vaporization

Fig. 2.5.13. Laser-based machining process configurations.

pulses in order to overcome the effects of conduction. In addition to the natural recoil forces from the expanding debris, gas assist is usually applied to increase the melt removal rates. Vaporization machining requires considerable pulse energies and is often difficult to control due to the unpredictability of the molten surface. The melt film has a tendency to splash out of the interaction zone causing resolidified burrs and surface structures. Non-thermal ablation is essentially restricted to polymeric materials for nanosecond pulses operating in the UV at wavelengths less than 355 nm and can be demonstrated on most materials when using pulse widths smaller than 150 fs (see Sect. 2.5.6).

2.5.4.1 Oxidation processes Laser oxide chip caving of steel utilizes a focused laser beam to produce a localized temperature rise in an oxygen atmosphere. The material that is removed is almost completely oxidized [92Pet, 93Ebe]. The process is capable of produced good-quality surfaces such that Rz < 10 µm, where Rz is the average peak to trough height variation on the sample surface. Typical parameters are: power: 100 . . . 200 W; feed rate: 0.3 . . . 1 m/min.; gas: oxygen; layer thickness: 50 . . . 500 µm; ablation rate: 3 . . . 15 mm3 /min. Figure 2.5.14 shows the volumetric ablation rate of steel versus CO2 laser power for laser oxide chip caving at different velocities.

2.5.4.2 Liquid-phase machining Liquid-phase machining relies on the production of laser-generated melt volume that is subsequently blown away by an assist gas jet. In fusion caving a simple off-axis jet is used. In vortexassisted laser machining a horizontal gas vortex is created using two jets that enhances the melt removal rate through the production of narrow slots of varying depth. Figure 2.5.15 shows typical removal rates for roughing (fusion caving) and finishing (oxide chip caving) in laser-based machining of steels. Figure 2.5.16 shows the slot depth versus speed for vortex-assisted laser machining.

2.5.4.3 Vapor-phase machining Vapor-phase machining requires intense short-pulse illumination with incident intensity above the ablation threshold of the material. Although melting is produced on most materials the dominant

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 184] 40

2.5 Laser separating

Steel

0.5 m min-1

35

Volumetric ablation rate A [mm 3 min-1]

169

30 25

0.75 m min-1

20 15 1.0 m min-1

10 5 0 160

180

200

220 240 Power P [W]

260

280

300

Fig. 2.5.14. Volumetric ablation rate of steel versus CO2 laser power for laser oxide chip caving at velocities of 0.5 m/min., 0.75 m/min., and 1.0 m/min. [96Pen].

4

Steel

Steel R Z < 50 µm fusion caving

100

10

Machining depth d [mm]

Volumetric removal rates A [mm 3 min-1]

1000

3

2

R Z < 10 µm oxide chip caving 1 10

100 Power P [W]

1000

Fig. 2.5.15. Typical removal rates for roughing (fusion caving) and finishing (oxide chip caving) versus laser power in laser-based machining of steels using a CO2 laser [94Bey].

Landolt-B¨ ornstein New Series VIII/1C

1 0

500

1000 1500 2000 2500 Processing speed v [mm min-1]

3000

Fig. 2.5.16. Slot depth versus speed for vortexassisted laser machining, slot width 300 µm. CO2 laser powers: filled circles: 333 W; open circles: 666 W; filled triangles: 1000 W [95ONe].

170

2.5.5 Drilling

[Ref. p. 184

0.20

180

Al2O3

160 140

Ablation rate A [×10 4 mm 3 min -1 ]

Ablation rate A [mm pulse -1 ]

0.15

0.10

0.05

120 100 80 60 40 20 0

0 0

5

10

15

20 25 30 35 Fluence I [ J cm-2 ]

40

45 50

Fig. 2.5.17. Ablation rate for alumina Al2 O3 (96 %) versus fluence; excimer laser operating at 308 nm, pulse width approx. 60 ns (peak power calculable from fluence and pulse width, although not usually described explicitly). Filled circles: [89Ton]; open circles: [90Lut]; filled triangles: [86Sri].

-20 0

5

10 15 20 Fluence I [ J cm-2 ]

25

30

Fig. 2.5.18. Ablation rate versus fluence for different metals; excimer laser operating at 248 nm, pulse width approx. 60 ns (peak power calculable from fluence and pulse width, although not usually described explicitly). Filled circles: Ag; open circles: Cu; filled triangles: Ta; open triangles: Mo [92Hue].

product of the machining process is a vapor. Ablation thresholds for most ceramic and metal targets are around a fluence of several J/cm2 . The short pulse width at UV wavelengths causes steep thermal gradients in the workpiece which prevents the formation of large molten volumes, even when working metals. Figures 2.5.17 to 2.5.19 show the ablation rate versus fluence for alumina, different metals and different ceramics, respectively. Figure 2.5.20 shows the ablation depth in graphite versus laser power for three operating speeds.

2.5.5 Drilling Material removal in laser-drilling processes involves the generation of pressure gradients to effect the removal of molten material. This can be achieved by delivering high-pressure gas jets coaxial with the laser beam or by using intense laser pulses to vaporize the material. The basic mechanism of laser drilling is shown in Fig. 2.5.21. An intense laser beam interacts with a substrate causing local melting and vaporization. As molten material is ejected from the interaction front through pressure gradients, the molten front is driven into the material producing a void of diameter close to that of the incident laser beam. Plasma effects can enlarge the hole considerably and a resolidified lip often appears on the entrance perimeter. There are a number of laser systems that are employed to drill holes, the most common ones being Nd:YAG (λ, λ/2, λ/3), CVL, CO2 and excimer lasers. The low-power CVL and Nd:YAG systems employ no gas as the pressure gradients are produced through intense vaporization. Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 184]

2.5 Laser separating

0.25

171

1400

Graphite 1200

0.20

1000

Ablation depth d [µm]

Ablation rate A [µm pulse -1 ]

1 m min-1

0.15

0.10

2 m min-1 800 3 m min-1

600 400

0.05 200

0

0

5

10

15

20 25 30 35 Fluence I [ J cm-2 ]

40 45

50

Fig. 2.5.19. Ablation rate versus fluence for different ceramics; excimer laser operating at 248 nm, pulse width approx. 60 ns (peak power calculable from fluence and pulse width, although not usually described explicitly). Filled circles: Si3 N4 ; open circles: Al2 O3 -SiO2 ; filled triangles: Al2 O3 -TiO2 ; open triangles: ZrO2 ; filled squares: SiC [92Hue].

0

0

100

200

300 400 Power P [W]

500

600

Fig. 2.5.20. Ablation depth in graphite versus laser power for three operating speeds; Nd:YAG laser operating at 1064 nm continuous wave (cw) [96Pen].

Laser irradiation Gas

Melt

Vapor

Drilling speed Laser drilling

Landolt-B¨ ornstein New Series VIII/1C

Fig. 2.5.21. Schematic of the laser drilling process.

172

2.5.5 Drilling

[Ref. p. 184

2.5.5.1 Piercing Piercing with single- or multiple-pulse exposure is carried out by plate cutters at the start point of a part within a nested array. It is a rapid process which produces clean holes with a minimum of adherent dross. Oxygen gas assist is usually used to provide exothermic energy and to drive the melt from the hole. In Tables 2.5.8 and 2.5.9 the process parameters for piercing mild steel plate and stainless steel plate, respectively, using a pulsed CO2 laser are given. Table 2.5.8. Process parameters for piercing mild steel plate using a pulsed CO2 laser [96Pen]. Plate thickness [mm] Parameter

1

2

3

4

5

6

8

10

12

Output mode Duty cycle [%] Frequency [Hz] Ramp-up time [s] Dwell time [s] Power [W] O2 gas [psi]1

Gated 10 100

Gated 20 200

Gated 35 200

0.1 600 15

0.3 1000 15

0.5 1000 12

Gated 35 200 0.5 0.8 1300 12

Gated 30 250 0.5 0.8 1500 12

Gated 35 2510 0.5 1.0 1500 12

Gated 35 400 2.5 4.0 1600 10

Gated 35 350 5.0 7.0 1300 10

Gated 35 150 7 10 1700 10

1

1 psi = 6.89476 kPa.

Table 2.5.9. Process parameters for piercing stainless steel plate using a pulsed CO2 laser [96Prc]. Plate thickness [mm] Parameter

1

2

3

4

5

6

8

Mode Frequency [Hz] Duty cycle [%] Dwell time [s] Power [W] Focal point [mm] O2 gas [psi]1

Super 200 20 0.2 600 −0.5 14.5

Super 200 25 0.2 800 −1 14.5

Super 200 25 0.5 800 −2 14.5

Super 250 25 1.0 1100 0.0 29

Super 250 25 1.0 1100 0.0 14.5

Super 250 25 1.0 1350 0.0 14.5

Super 250 25 2.0 1350 0.0 14.5

1

1 psi = 6.89476 kPa.

2.5.5.2 Multiple-pulse drilling Multiple-pulse drilling utilizes a static free-running pulsed laser to drill through the target material until breakthrough occurs. Percussion drilling is another term for multiple-pulse drilling. This drilling technique is commonly used for drilling metals such as C-Mn steels, stainless steels and aerospace alloys. Tables 2.5.10 and 2.5.11 present process parameters for percussion drilling of different alloys using an Nd:YAG laser (model: Lumonics Inc. JK704). Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 184]

2.5 Laser separating

173

Table 2.5.10. Process parameters for a Lumonics JK704 laser percussion drilling of C2653 and INC 718 alloys [95Lum]. Material

Pulse width Energy [ms] [J]

Rep. rate Pulses [Hz] [No.]

Hole size [mm]

Power [kW]

Efficiency [10−3 mm3 /J]

2 mm C263

0.3 0.3 0.3 0.3 1 1 1 1 0.3 0.3 0.3 1 0.3

80 125 115 165 8 12.1 7 11 30 30 23 7.2 23

0.22 0.35 0.25 0.31 0.73 0.93 0.99 1.45 0.64 0.55 0.61 0.95 0.64

4 4 4 4 20 20 30 30 20 20 30 30 30

5 5.5 5.2 1.8 12.5 5.9 20.3 40 24 15.4 7.4 15.1 3.8

4 mm INC 718

1.2 1.2 1.2 1.2 20 20 30 30 6 6 9 3 9

15 21 15 51 3 6 2 2 10 10 6 6 10

Table 2.5.11. Process parameters for a Lumonics JK704 laser percussion drilling 1 mm thick Ni alloy, pulse duration 1 ms [95Lum]. Material

Spot size [mm]

Pulse power [kW]

Hole size [mm]

Total energy [J]

Power density [MW/cm2 ]

Volume drilled [mm3 ]

Ni alloy

0.45 0.66 0.92 1.14 1.36 0.45 0.68 0.92 1.14 1.36 0.45 0.68 0.92 1.14 1.36

9.1 9.1 9.1 9.1 9.1 14.7 14.7 14.7 14.7 14.7 24.3 24.3 24.3 24.3 24.3

0.58 0.72 0.94 1.2 1.43 0.62 0.78 0.93 1.25 1.49 0.73 0.85 1.06 1.29 1.6

54.6 81.9 109.2 163.8 273 52.6 88 114.4 176 220 73 73 146 146 219

5.72 2.51 1.37 0.89 0.63 9.24 4.05 2.21 1.44 1.02 15.3 6.7 3.66 2.38 1.66

0.26 0.41 0.69 1.13 1.61 0.3 0.48 0.68 1.23 1.74 0.42 0.57 0.88 1.3 2

2.5.5.3 Trepanning Trepanning is a high-accuracy drilling process that utilizes multiple-pulse drilling to create holes. The laser describes a circle as the drilling proceeds, the diameter of the circle being equal to the diameter of the hole that is required. In Tables 2.5.12 to 2.5.16 process data for trepanning holes of different sizes in stainless and mild steel are given.

Landolt-B¨ ornstein New Series VIII/1C

174

2.5.5 Drilling

[Ref. p. 184

Table 2.5.12. Process data for trepanning small holes (diameter less than the material thickness) in stainless steel [96Pen]. Plate thickness [mm] Parameter

1

2

3

4

5

6

8

Output mode Frequency [Hz] Duty cycle [%] Feed rate [mm/min.] Power [W]

Super 200 25 500 800

Super 750 50 1300 1200

Super 750 55 1000 1200

cw

cw

cw

cw

900 1500

700 1500

800 1800

500 2200

Table 2.5.13. Process data for trepanning large holes (diameter greater than the material thickness) in stainless steel [96Pen]. Plate thickness [mm] Parameter

1

2

3

4

5

6

8

Output mode Feed rate [mm/min.] Power [W]

cw 3000 1200

cw 2500 1200

cw 1800 1500

cw 1600 1500

cw 1300 2200

cw 1000 2200

cw 500 2200

Table 2.5.14. Process data for trepanning small holes (diameter less than the material thickness) in mild steel [96Pen]. Plate thickness [mm] Parameter

1

2

3

4

5

6

8

Output mode Frequency [Hz] Duty cycle [%] Feed rate [mm/min.] Power [W]

Gated 100 25 500 500

Gated 100 35 500 800

Gated 100 40 500 800

Gated 100 35 500 1000

Gated 50 30 200 1000

Gated 50 35 200 1000

Gated 30 35 125 1350

Table 2.5.15. Process data for trepanning small holes (diameter greater than 5 mm and less than 10 mm) in mild steel [96Pen]. Plate thickness [mm] Parameter

1

2

3

4

5

6

8

Output mode Frequency [Hz] Duty cycle [%] Feed rate [mm/min.] Power [W]

Gated 254 40 1000 500

Gated 300 75 1500 800

Gated 150 60 1000 800

Gated 100 60 1100 1300

Gated 80 60 900 1000

Gated 120 65 900 1300

Gated 150 65 600 1150

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Table 2.5.16. Process data for trepanning large holes (diameter greater than 10 mm) in mild steel [96Pen]. Plate thickness [mm] Parameter

1

2

3

4

5

6

8

Output mode Frequency [Hz] Duty cycle [%] Feed rate [mm/min.] Power [W]

Gated 900 70 3000 600

Gated 900 90 3000 850

Gated 1000 90 2300 1000

Gated 300 90 1900 1300

Gated 250 90 1400 1000

Gated 250 90 1500 1300

cw 1150 1250

2.5.5.4 High-speed drilling High-speed drilling results when ultra-short laser pulses are used such as those produced by Qswitched Nd:YAG or copper vapor lasers with pulse widths less than 250 ns. Power densities are in the range 108 to 1011 W/cm2 . At these power densities most materials can be drilled although the quality and precision of the resultant hole can vary considerably. In Figs. 2.5.22 to 2.5.24 the material removal rate and the drilling rate, respectively, of stainless steel and aluminum are given versus laser intensity. In Table 2.5.17 some high-speed-drilling results are listed. 6

Stainless steel

SS 316L 5

Drilling rate R [µm pulse -1 ]

Drilling rate R [µm pulse -1 ]

10

1

4

3

2

1 0.1 1

10

100 1000 Fluence I [ J cm-2 ]

10000

Fig. 2.5.22. Material removal rate of stainless steel versus laser intensity using a CVL at 4.4 kHz Pulse Repetition Frequency (PRF), pulse width 80 ns [98Cha].

Landolt-B¨ ornstein New Series VIII/1C

0 0

100

200

300 400 500 Fluence I [ J cm-2 ]

600

700

800

Fig. 2.5.23. Drilling rate of 316L stainless steel versus laser intensity using an Nd:YAG laser operating at 355 nm, pulse length 10 ns, the process is unstable when I > 400J/cm2 [99ONe].

176

2.5.6 Non-thermal ablation 60

[Ref. p. 184

Al

Drilling rate Rd [µm pulse -1 ]

50

40

30

20

10 0

0

20

40

60 80 100 120 140 160 180 Fluence I [ J cm-2 ]

Fig. 2.5.24. Drilling rate of Al versus laser intensity using an Nd:YAG laser operating at 355 nm, pulse length 10 ns [99ONe].

Table 2.5.17. High-speed-drilling results obtained with a CO2 laser, pulse width 0.05 . . . 0.2 ms [90Ols]. Material

Thickness [mm]

SS 304 SS316 Mild steel Galvanized mild steel Polycarbonate Ruby

1.0 1.0 0.5 1.0 3.0 1.0

Number of pulses

Entrance diameter [mm]

Exit diameter [mm]

Drilling time [s]

120 45 45 45 120 45

0.18 0.15 0.2 0.25 0.25 0.14

0.14 0.10 0.15 0.20 0.22 0.08

1.2 1.2 1.0 1.5 4.0 90

2.5.6 Non-thermal ablation Non-thermal-ablation processes rely on the photon quantum energy being greater than the binding energy of the molecules. For polymeric materials most can be photo-ablated at wavelengths less than 355 nm although the higher photon energy at 248 nm makes this a preferred wavelength. (248 nm is the preferred wavelength as this transition is the most efficient and provides greater power levels than the lower wavelengths of 193 and 157 nm. It should be stated that 193 and 157 nm beams are also effective in non-thermal ablation due to their high photon energies.) Sharp structures can be observed in polymeric materials with resolutions down to a few hundred nanometres. In Figs. 2.5.25 and 2.5.26 the variation of total etched depth per pulse versus fluence for PMMA and PI, respectively, is given for different wavelengths.

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 184] 6

2.5 Laser separating 1.0

PMMA

Polyimide

5

0.8

Etch depth Ed [µm pulse -1 ]

Etch depth Ed [µm pulse -1 ]

177

4

0.6

3

0.4

2

0.2

1 0 0.01

0.1

1 Fluence I [ J cm-2 ]

10

100

Fig. 2.5.25. Variation of total etched depth per pulse versus fluence for PMMA; excimer laser, pulse width approx. 60 ns, filled circles: 193 nm; open circles: 248 nm [86Sri].

0 0.01

0.1

1 Fluence I [ J cm-2 ]

10

100

Fig. 2.5.26. Variation of total etched depth per pulse versus fluence for PI; excimer laser, pulse width approx. 60 ns, filled circles: 193 nm [86Sri]; open circles: 248 nm [82Sri]; filled triangles: 193 nm [86Sri]; open triangles: 248 nm [86Sri].

2.5.7 Marking Laser marking is possibly the largest industrial laser application in terms of the total number of systems operating worldwide. The most common lasers used for marking are the Q-switched Nd:YAG and the CO2 laser. Laser marking and scribing relies on the local modification of an object such that a visible contrast can be seen by an observer. There are a number of ways to generate this contrast. These can be grouped into two distinct effects. Surface-removal methods rely on creating a local change at the surface of the object. This can be achieved by ablation, engraving through melt generation and removal, and removal of a surface layer such as paint or oxide. Surfacemodification techniques simply modify the surface using chemical decomposition through heating, thermal or photochemical activation of pigments, surface oxidation or surface melting without removal. Figure 2.5.27 provides a schematic of the most common marking processes. It is impossible to provide a full range of marking conditions for most materials since this largely depends on the degree of contrast required and the nature of the light source used. Instead, Table 2.5.18 lists the range of equipment parameters that are often used to mark industrial products.

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2.5.8 Comparison with conventional processes

[Ref. p. 184

Surface removal methods Laser irradiation

Laser irradiation

Laser irradiation Vapor

melt

Ablation

Engraving

Surface layer removal

Surface modification methods Laser irradiation

Melting

Laser irradiation

Chemical decompostion

Laser irradiation

Laser irradiation

Oxidation

Pigment activation

Fig. 2.5.27. The principles of laser marking and scribing. Table 2.5.18. Typical system parameters for industrial laser-marking machines. Process parameters

Common values

Wavelength [nm] Average power [W] Pulse duration [µs] Peak power [kW] Repetition rate [kHz] Spot size [µm] Fluence [Jcm2 ] Focal length [mm] Depth of focus [mm] Processing field [mm] Scanning speed [ms−1 ]

1064, 532, 10600 50 . . . 250 0.1 . . . 10 30 . . . 100 0 . . . 80 50 . . . 250 −100 80 . . . 250 ±1 −250 0.001 . . . 15

2.5.8 Comparison with conventional processes In this section laser-separation techniques are compared with conventional processes. In Table 2.5.19 competing marking technologies are compared. One can see that laser marking provides significant benefits compared to traditional routes, laser marking is the most widely used laser materials processing technique in industry. Table 2.5.20 presents differing cutting processes, where HAZ refers to Heat-Affected Zone, EDM refers to Electric Discharge Machining, kerf width refers to the width of the cut, and NC refers to Numerical Control. It is clear from this comparison that laser cutting provides the most efficient, flexible and reconfigurable cutting system for almost any material. Table 2.5.21 compares differing drilling processes. Lasers are more often used for particularly hard materials such as ceramics and superalloys, although they are very useful in drilling soft materials such as plastics and rubbers. If precision and the absence of a HAZ is important the laser is best replaced by mechanical drilling techniques. Table 2.5.22 compares differing machining Landolt-B¨ ornstein New Series VIII/1C

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processes. Laser machining tends to suffer from quality and accuracy issues compared to mechanical and EDM machining. This is due to the lack of a hard geometrically controlled edge that tools provide and the production of residual melt volume that clings to the machined edge. At length scales below 100 microns the laser is alone in providing effective machining capability. Table 2.5.19. Comparison of competing marking technologies [98Kap].

Laser Printing Ink jet Chemical Labels Pantograph Silk screen Metal stamp Embossing Engraving

Permanence

Throughput

Flexibility

Costs

Quality

Material Load

Maintenance

++ + + ++ – ++ – ++ ++ ++

++ – ++ – ++ – ++ ++ ++ +

++ – ++ – ++ ++ – – – –

– ++ ++ ++ ++ – – – – +

++ ++ ++ ++ ++ + ++ – ++ +

++ ++ ++ + ++ ++ ++ – – –

+ – + ++ + ++ – + + –

++ Particular attribute + Attribute – Poor attribute

Table 2.5.20. Comparison of different cutting processes [98Ste]. HAZ refers to Heat-Affected Zone, EDM refers to Electric Discharge Machining. Property

Laser

Punch Plasma Nibbling

Water jet

Wire EDM

NC Sawing Ultramilling sonic

Flame

Rate Quality Kef width Scrap & swarf Distortion Noise Metal & non metal Complex shapes Part nesting Multiple layers Equipment cost Operating cost High volume Flexibility Tool wear Automation HAZ Clamping Blind cuts Weldable edge Tool changes

+ + + + + + + + + – –

+ + + + – –

– + + + + – + + + + –

– + +

– +

– +

– –

+

+

+

+ – –

+ + + + + + + + +

+ Point of merit – Point of disadvantage

Landolt-B¨ ornstein New Series VIII/1C

– – + + – – + + – + + –

+ – – – – – + +

+ + + – + + – +

– – –

– + – – + +

+ + + + + + + +

+ + +

+

– + – + + – +

– – –

+

+ – + + – + +

–

–

+ +

–

+ – –

– +

+ –

–

180

2.5.9 How to avoid quality degradation

[Ref. p. 184

Table 2.5.21. Comparison of different drilling processes. Property

Laser

EDM

Mechanical

Rate Quality Hole diameter Scrap management Metal & non metal Angled holes Taper Equipment cost Operating cost High volume Flexibility Tool wear Automation HAZ Clamping Blind holes Tool changes

+ + + + + + – – – + + + + – + + +

– + + – – – + – – – – – + + – + –

+ + – – + – + + + + – – + + – + –

+ Point of merit – Point of disadvantage Table 2.5.22. Comparison of different machining processes. Property

Laser

EDM

Mechanical

Rate Quality Scrap management Metal & non metal Accuracy Burrs Equipment cost Operating cost High volume Flexibility Tool wear Automation HAZ Clamping Tool changes

+ – + + – – – – + + + + – + +

– + – – + + – – – – – + + – –

+ + + + + – + + + – – + + – –

+ Point of merit – Point of disadvantage

2.5.9 How to avoid quality degradation Figures 2.5.28 to 2.5.30 present typical problems associated with a particular process. In Fig. 2.5.28 laser cutting characteristics are represented where Po is laser power, Pg is process gas pressure, N is nozzle, ND is nozzle diameter, KW is kerf width, SO is plate stand-off distance, T is plate thickness, D is attached dross level, AL is axial alignment of the beam through the nozzle, DL is drag lines on the cut face, SB is side burning away from the cut line and S is the striation pattern on the cut face. Corrective actions for typical cut problems are given in Table 2.5.23. Landolt-B¨ ornstein New Series VIII/1C

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2.5 Laser separating

Laser cutting common problems

181

Power Po Pressure Pg

AL N

ND

SO

KW

S

T

DL D

D SB

S

Fig. 2.5.28. Typical problems associated with laser cutting of metals.

Table 2.5.23. Corrective action for typical problems encountered in laser cutting. Effect

Problem

Dross (D): Oxygen & N2 cutting

Insufficient melt clearance: - Processing too fast (evidence of curved drag lines DL) - Low pressure (Pg) (evidence of curved drag lines DL) - Low power (Po) - Poor focus - Nozzle (N) too narrow - Entrainment on deep sections - Boundary layer separation

Action - Reduce speed - Increase gas pressure -

Increase power Check lens Increase nozzle diameter (ND) Increase ND or pressure Pg Increase ND or pressure Pg

Side burning (SB): Oxygen cutting

Uncontrolled burning: - Oxygen Pg to high - Processing too slowly - Damaged nozzle (N)

- Reduce Pg - Increase speed - Check/replace N

Cutting unequal in x-y plane

Polarization problems: - Damaged phase retarder - Beam off center

- Check and replace - Align to nozzle

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[Ref. p. 184

In Fig. 2.5.29 laser drilling characteristics are represented where N is nozzle, HW is hole width, D is attached surface dross, T is plate thickness and CL is collateral damage of the surface adjacent to the hole. Corrective actions for typical drilling problems are given in Table 2.5.24. Laser

N HW

D

CL T

Fig. 2.5.29. Typical problems associated with laser drilling.

Table 2.5.24. Corrective action for typical problems encountered in laser drilling. Effect

Problem

Action

Dross (D): Percussion drilling

Insufficient melt clearance: - Pulse energy too low - Low gas pressure - Poor focus - Nozzle too narrow (ND)

-

Increase pulse energy Increase gas pressure Check lens Increase ND

Dross (D): High-speed drilling

Insufficient energy: - Peak power density too high - Pulse width too short - Low gas pressure - No effect with changes

-

Decrease pulse energy Increase pulse length Increase gas pressure Use trepanning

Large taper

Low energy at depth: - Focal position incorrect for depth - Hole too deep - No effect with changes

- Re-set focal point - Reduce hole depth - Use trepanning

Wide condensate

Condensate forming a ridge: - No cross jet

- Apply low-velocity cross jet

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In Fig. 2.5.30 laser machining characteristics are represented where RC is surface recast layer, RL base recast layer and S is base roughness. Corrective actions for typical machining problems are given in Table 2.5.25. Gas jet Laser beam

RC

RL

S

Fig. 2.5.30. Typical problems associated with laser machining.

Table 2.5.25. Corrective action for typical problems encountered in laser machining. Effect

Problem

Recast (RC)

Insufficient melt clearance: - Pulse energy too low (melt too viscous) - Low gas pressure - Poor focus control - Speed too fast - Peak power density too high

Action - Increase pulse energy - Increase gas pressure - Check lens - Reduce speed - Increase spot size, reduce pulse energy

Recast layer (RL)

Conduction losses high: - Speed too low - Pulse width too short - No effect with changes

- Decrease pulse energy - Increase pulse length - Use trepanning

Rough surface (S)

-

- Increase beam overlap fraction - Check for jitter - Check nozzle condition - Reduce feed

Landolt-B¨ ornstein New Series VIII/1C

Poor interaction control: Beam overlap too low Pulse-to-pulse instability Turbulent gas stream Machined layer too deep

184

References for 2.5

References for 2.5 82Rot 82Sri 84Coh

Rothe, R., Sepold, G., Teske, K.: Laserstrahlschneiden von Blechen im Dickenbereich von 3 bis 30 mm, Schweißen und Schneiden 82, DVS-Berichte 74, D¨ usseldorf, 1982. Srinivasan, V., Mayne-Banton, V.: Appl. Phys. Lett. 41 (1982) 576.

84Spe

Coherent General Everlase CO2 Laser Applications: Metal Cutting, Technical Note, 1984. Spectra Physics, Industrial Laser Division: CO2 Laser Cutting, Technical Note, 1984.

86Sri

Srinivasan, V., Smrtic, M.A., Babu, S.V.: J. Appl. Phys. 59(11) (1986) 3861.

89Sch 89Ton

Schmatjko, K.J.: Industrie-Anzeiger 89 (1989) 39. T¨onshoff, H.K., Gedrat, O.: Proc. SPIE 1132 (1989) 104.

90Lut 90Mic 90Ols

Lutz, N., Geiger, M.: Proc. ECLAT 90, Sept. 17–19, Erlangen (1990) 849. Michaelis, A., Schaefer, J.H., Uhlenbusch, J., Viol, W.: Proc. SPIE 1276 (1990) 213. Olsen, F., Heckerman, S.: Proc. ICALEO 90, Nov. 4–9, Boston (1990) 141.

92Coo 92Hue 92Pet 92Tam

Cooper, M.I., Emmony, D.C., Larson, J.H.: J. Photographic Science 40 (1992) 55. H¨ ugel, H.: Strahlwerkzeug Laser, Stuttgart: Teubner, 1992. Petring, D.: ILT-Bericht (1992) 1. Tam, A.C., Leung, W.P., Zapka, W., Ziemlich, W.: J. Appl. Phys. 71(7) (1992) 3515.

93Ebe

Eberl, G., Hildebrand, P., Kuhl, M., Sutor, U., H¨ ugel, H., Meiners, E., Widemaier, M., Zeller, T.: Laser und Optoelektronik 25(3) (1993) 80. Gabzdyl, J.: Proc. ICALEO 93, Orlando, Florida, 1993. Kulinar, M.: Materialbearbeitung durch Laserstrahl, D¨ usseldorf, 1993.

93Gab 93Kul 94Bey 94Lu

Beyer, E., Petring, D., Zefferer, H.: 26th International Seminar on Advanced Manufacturing Systems, Proc. LANE 94 1 (1994) 477. Lu, Y.F., Takai, M., Komuro, S., Shiokawa, T., Aoyagi, Y.: Appl. Phys. A 59 (1994) 281.

95Eng 95Lum 95ONe

Engelsberg, A.C.: PC Magazine, May 1995, 35. Lumonics Coporation, JK704 drilling data sheet, Rugby, UK, 1995. O’Neill, W.: Proc. I. Mech. E (B) Eng. Manufacture 215 (2001) 1.

96Bar 96Olt 96Pen 96Prc

Bartel, N., Bergman, H.W.: Proc. 4th Sheet Metal Conference 2 (1996) 385. Oltra, R., Yavas, O., Kerrec, O.: Surface and Coatings Technology 88 (1996) 157. Penz, A.: Dissertation, Technical University of Vienna, 1996. PRC Laser Corporation, Data sheet, Landing, New Jersey, 1996.

97Mar

Maravelaki, P.V., Zafiropulos, V., Kilikglou, V., Klaitzaki, M., Fotakis, C.: Spectrochimica Acta, Part B 52 (1997) 41.

98Cha 98Cin 98Geo

Chang, J.J, Warner, B.E., Dragon, E.P., Martinez, M.: J. Laser Appl. 10(6) (1998) 285. Cincinnati Laser: Cutting Capacity Charts CL-707, Cincinnati, 1999. Georgiou, S., Zafiropulous, V., Anglos, D., Balas, C., Tornari, V., Fotakis, C.: Appl. Surf. Science 51(8) (1998) 738. Kaplan, A.: Handbook of the Eurolaser Academy, London: Chapman-Hall (1998) 469.

98Kap

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References for 2.5

185

98Pow 98Ste

Powell, J.: CO2 Laser Cutting, 2nd edition, Berlin: Springer-Verlag, 1998. Steen, W.: Laser materials processing, 2nd edition, Berlin: Springer-Verlag (1998) 106.

99ONe

O’Neill, W., Sutcliffe, C.S.J., Kearns, A., Tunna, L.: Limiting factors in the production of precision deep microstructures, Proc. ICALEO 99, San Diego, FL (1999) 876.

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187

2.6 Cutting: Modeling and data W. Schulz, C. Hertzler

Laser processing covers the leading percentage (86 %) of the present West-European laser market for industrial applications, where metal sheet cutting (44.8 %) dominates metal sheet welding (27.7 %) and marking (13.5 %) as well as the remaining not yet widespread applications such as hardening, soldering, etc. Scientific research and development are related to monitoring, quality assurance and adaptive control in precision cutting. Fundamental research activities are focused on the dynamical features of the process – such as ripple formation [00Fri] and adherent dross [92Mak, 97Nem] – in contour cutting [01Pop], processing with elevated cutting speed, precision machining of microstructures [99Dau, 97Hel] and exploiting the potentials of diode-pumped solid-state lasers [00Sch]. The fundamental physical processes – also present in caving, welding and drilling – are related to the movement of free boundaries separated by the melt flow establishing the dynamical shape of the cutting front. Fundamental results are present for the physics of thin-film flow [89Jur, 90Hsi, 95Mie, 97Oro, 00Aks, 01Scl], wetting [85Gen, 98Sca] and flow separation [84Pri, 94Pri, 97Egg1, 97Egg2], as well as evaporation and condensation [97Ytr, 99Ros, 00Ros]. Actual investigations deal with the development and application of mathematical methods for the analysis of partial differential equations (asymptotic methods [88Tem, 89Con, 95Rob], integral (variational) [70Bio, 82Ell] and spectral methods [88Can]) and free-boundary problems [82Ell], which yield the formulation of approximate (asymptotically exact) models [88Tem, 89Con, 90Sir, 97Ens, 97Sch]. Recently developed numerical methods for tracking of free boundaries (level-set method [99Set, 99Ada], adaptive “sparse grids” [86Yse, 91Zen]) are applied for calculating the thermal processes [01Maz]. The comparison of approximate models with advanced numerical simulations and experimental identification [97Bee, 01Pop] by diagnostic methods with temporal and spatial resolution is used to improve the understanding and to reveal the physical limits. Historically until the 90’s, the investigations on fundamental experiments [79Ara1, 79Ara2, 80Ols, 87War] and modeling were carried out to reveal the different physical mechanisms contributing to the technical process. The physical foundations and their possible application to thermal processing are given by van Allmen [89All]. In the monograph of Steen [91Ste] the dominant physical processes and their technical relevance for laser applications are discussed. A collection of the model equations and their application to machining is compiled by Chryssolouris [91CHR] The main modeling activities until 1994 are reviewed by Steen [94Ste]. The reports of 16 German research partners are presented by Geiger [93Gei] and Sepold [98Sep]. Comprehensive surveys [93Pow, 00Pop] of the practical tasks for laser cutting are available. The physical mechanisms dominant in thermal processing – such as the global balance of the thermal energy [72Bab, 75Bun] heating up the material to be cut, the two phase transitions melting and evaporation [83Sco, 83Dec], the melt flow and its hydrodynamical stability [87Vic1] as well as the flow of the assist gas in cutting related to momentum and heat transfer [87Vic2] from the inert gas jet to the melt – are revealed by experimental evidence and discussed by the early models. The thin melt film is driven by a supersonic gas jet which is in contact with the melt at the free-moving liquid surface. Depending on the position and type of the gas nozzle as well as the shape of the cutting front the gas pressure and the shear stress to the melt film can change and the compressible flow can separate from the cut metal sheet. Performing numerical simulations of the supersonic gas flow and using the Schlieren method [87War, 88Pet2, 89Sem, 90Ber, 91Zef, 99Man] Landolt-B¨ ornstein New Series VIII/1C

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[Ref. p. 215

the characteristics of the complex gas jet like supersonic domains, shocks, expansion waves, and flow separation have become more transparent. Turbulent effects of the gas flow were discussed [92Nei]. The main interest then changed to the question how to give a closed formulation of the process and what are the additional effects introduced by coupling of the physical processes. It became essential to distinguish between the independent parameters of the process – speed of feeding, laser power, intensity distribution, etc. – and the quantities which evolve during the process – slope of the cutting front, the melt film thickness, the surface temperature, the width of the cut kerf [79Sha, 86Sch, 88Pet1, 89Sch, 94Ols, 96Kap], etc. – and have to be calculated as part of the solution. The additional effects of the reactive gas cutting process are investigated with respect to oxygen concentration [91Che, 92Nei, 92Kov, 95Nei] within the gas jet which is changed along the molten iron (low-alloyed or carbon steels) surface within the boundary layer of the gas jet due to reaction with the iron and diffusion through the metal-oxide layer. The oxygen and iron concentrations within the oxide layer are investigated [73Wag, 87Sch, 93Fra1] which yield the reaction rate and hence the energy released by the exothermal reaction. By comparison of the action of the oxygen gas jet in laser cutting and in autogenous gas cutting the high-performance laser gas cutting process – referred to as “burning-stabilized gas cutting” – is invented [93Fra2] and later applied in industries.

2.6.1 Diagnostics, monitoring and modeling With the technical aim to maintain the reachable cut quality an improved understanding of the observability and controllability is desired. In particular, precision cutting, quality surveillance and active suppression of cut-edge degradation like scouring and adherent dross in contour cutting are of interest. Experimental diagnostics of the technical process is carried out to reveal the fundamental phenomena. The interesting range of parameters is identified where the movement of a melting front (called the basis process) becomes dominant and the apparently more complex phenomena such as melt flow, evaporation, etc. (called sequel processes) contribute only small corrections. In cutting, for example, the sequel processes – such as melt flow and heat conduction in the melt film – are to be considered to describe the thermal emission from the molten surface essential for monitoring. Searching for suitable domains for the processing parameters dictates a reverse-modeling approach. Mathematical analysis of the free-boundary problem is carried out by applying asymptotic and integral methods as well as spectral methods. In dissipative infinite-dimensional dynamical systems a finite-dimensional inertial manifold exists which contains the attractor of the system. The existence of a finite-dimensional inertial manifold means, that the motion of a finite set of degrees of freedom can give a good approximation of the complete solution. Asymptotic methods are used to identify the degrees of freedom and integral methods are applied to derive models of the free-boundary problem, which reveal the structure of the solution. Applying spectral methods the dimension of the phase space can be increased and the quality of the approximate solution can be controlled. Approximate dynamical models with low dimension in phase space are available which have the important advantages of being solvable with controlled error and giving the solution to the inverse problem.

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189

2.6.2 Experiments and diagnostics In fusion cutting, the absorbed laser beam power heats the material to be cut to the melting temperature. The molten material is accelerated by the action of an inert gas jet. The main portion of the melt separates from the material and the cut kerf evolves (Fig. 2.6.1a). A characteristic pattern of the cut edge (called ripples or striations) appears. The separating liquid (Fig. 2.6.1b) shows typical effects of surface tension known from thin-film flows, where a melt thread is formed and the position of the contact line changes dynamically. Striking experimental and theoretical investigations are carried out by Pritchard [84Pri, 80Jea] and de Gennes [85Gen]. Thermal radiation emitted from the interaction zone between the laser beam and the material can be detected by a CCD-camera (Fig. 2.6.1c) and is used for monitoring the technical process [00Sch]. It may happen that the observable phenomena of the sequel processes such as a strong plasma emission in welding or a fast fluctuating thermal emission from the surface of a melt film in cutting prevent the observability of the basis process involved we are looking for. For example, the movement of the molten material in the wake of a weld seam may be apparently a fundamental subprocess. Spatially resolved experimental observations of Beersiek et al. [97Bee] revealed that the small amplitude dynamics taking place in the advance of the laser beam axis are the origin for the conspicuous and well observable response of the process in the wake. Perturbations generated in the wake have only very small feedback on the dynamic process state in the advance of the laser beam axis at the vertex line of the melting front. These observations are identified later as a consequence of the boundary layer character of heat conduction and melt flow present in the free-boundary problem [99Sch]. Apart from the emission of thermal radiation and the formation of adherent dross the striations appearing at the cut edge (Fig. 2.6.2a) are characteristic for the presence of dynamical phenomena in the ablation process. Vicanek [87Vic1] and Makashev [92Mak] argue that the melt flow is the substantial sub-process responsible for the development of striations at the cut edges. Hydrodynamical effects suggested by these authors are capillary waves propagating in axial direction and periodic droplet formation at the upper edge of the cut kerf, respectively. Abrasive water jet cutting is modelled by Friedrich and Radons [00Fri] using a Kuramoto-Sivashinsky-type equation of motion. The authors argue that ripple formation evolves through a convective instability at the interface. In comparison with these results the experimental observations in laser cutting by Zefferer [97Sch] indicate that in the upper part of the cut edge and for a speed of feeding lower than

kerf

a

b

c

Fig. 2.6.1. Laser fusion cutting (a) and methods for the observation of the dynamic phenomena involved. Cutting right at the border of the workpiece (b) is showing the striations evolving at the cut edge as well as melt flow, liquid separation and dross formation at the bottom of the sheet metal. CCD-camera images (c) are records of the thermal emission.

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2.6.2 Experiments and diagnostics

[Ref. p. 215

Fig. 2.6.2. (a) Side view of the cut edge (CrNi steel, 10 mm, power 4600 W, speed of feeding 1 m/min). The striations at the cut edge are almost perpendicular to the sheet surface, what is not obvious from the figure due to the angle of observation necessary for specular reflection of the illumination. (b) Top view of the ripple profile. Cross section at a depth of 250 µm. (c) Sketch of the cutting front which consists of the two free boundaries, namely the absorption front (liquid surface) and the melting front (CrNi steel, 2 mm, power 1200 W, speed of feeding 2 m/min).

4 m/min there is no resolidified molten material detectable at the cut edge but simultaneously the striations are present (Fig. 2.6.2b). At the upper edge the striations are U-shaped with a sharp tip between smoothly curved valleys. The cross section showing the original structure of the base material with grain boundaries (Fig. 2.6.2b) up to the cut edge gives the evidence that the cut surface at the upper cut edge is not built by resolidification of a wavy liquid, but the striations are generated directly by the moving melt front. Modeling of the process is improved by a detailed analysis of the surface morphology of the striations and their period, which increases linearly with the speed of feeding. A crucial refinement of the understanding was possible by modeling the spatial coexistence of different boundary conditions at the border line between the melting front (heating and melting) and the cut edge (only heating). As consequence of the changing boundary conditions the border line between melting front and cut edge moves (Fig. 2.6.3a) and the solution reproduces the U-shaped striations [97Sch]. From a physical point of view the melt flow influences both, the heat flux within the fluid at the melting front and the position of the liquid surface, namely the absorption front (Fig. 2.6.2c). The heat flux at the melting front enters the Stefan-type boundary condition, which governs the velocity of the melting front, while the position of the absorption front determines the absorbed intensity due to the spatial distribution of the laser beam intensity. The heat flux within the fluid is well approximated by the absorbed intensity at the absorption front if the melt film thickness dm is small compared with the penetration depth δ = κ/v0 of the temperature field: dm
δ, i.e. the temperature gradient within the liquid remains approximately constant (κ thermal diffusivity, v0 speed of feeding). Typical values for the thermal diffusivity of molten metals κ = 1.1 · 10−5 m2 s−1 and the speed of feeding v0 = 2 · 10−2 m s−1 yield the value δ = 5.5 · 10−4 m for the penetration depth, which is fairly large compared with the melt film thickness dm at the top part of the cut edge. From the theoretical description of the cutting process and experimental evidence about the surface temperature, the melt film thickness at the top part of the cutting front is of the order of 10 µm or smaller. Therefore, the influence of melt flow on the heat flux within the liquid can be negligible. This means that even the effects of the melt flow can be of minor importance for the basis process.

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Ablation in the advance of a thermal heat source takes place in caving, cutting, drilling and welding. For a wide range of the processing parameters there exists a hierarchy of physical processes, which includes a basis process, namely the motion of the melting front in the neighborhood of its vertex line. As result, the range of parameters is identified where the basis process (movement of a melting front) underlying different technical applications becomes dominant and the apparently more complex sequel processes (melt flow, evaporation, etc.) contribute only small corrections. For the revealed basis process the absorption front consists of a melting front and a not yet eroded solid part of the material. Characteristic features of the basis process are not only the non-harmonic, U-shaped formation of striations at the cut edges in cutting, but for example also the typical time scales involved in the free-boundary problem which are observable by detecting the emission of thermal radiation.

2.6.3 Mathematical formulation The experimental analysis motivates to formulate first the free-boundary problem for the movement of the melting front, namely the basis process, which is referred to as one-phase problem and the melt flow is not taken into account [97Ens]. The phase boundary can move into the material and erosion takes place or remains unchanged. Mathematically the problem is formulated as follows (Fig. 2.6.3a): The material volume Ω(t) ⊆ R2 × (0, d) is bounded by the absorption front Γ+ (t) = ∂Ω(t) ∩ {z < d} and the bottom side of the solid material Γ− (t) = ∂Ω(t) ∩ {z = d}. The laser beam intensity I = I0 (t)f ([x − v 0 t]/w0 ) is characterized by its maximum value I0 (t), the spatial distribution f (0 ≤ f ≤ 1) and the laser beam radius w0 . The laser beam is directed top to bottom and moves with the velocity v0 within positive x-direction, v 0 = v0 ex . The absorbed heat flux qa = −Ap I n · ez depends on

beam intensity

Transition from melting to heating phase

v0

absorption front

Transition from heating to melting phase

surface tempera ture θs(t) d

melt front cut edge

border line a

gy ener t en t n o c (t) Pe Q

ron A(t) f

t pos

ition

b

Fig. 2.6.3. (a) Formulation of the free-boundary problem. Movement of the border line ∂Γm (t) results in U-shaped patterns of the cut edge. (b) Representation of the solution (2.6.11) at the vertex line (x, y = 0, z = d/2) in phase space {A, Q, θs }. The temporal change of boundary conditions can take place as result of modulated laser beam power.

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2.6.4 Inertial manifolds

[Ref. p. 215

the degree of absorption Ap , where n = n(x, t) is normal with respect to the absorption front Γ+ (t). Therefore, the velocity −vb n of the free boundary depends on the polarization p of the laser beam and the angle of incidence, which also have influence on Ap . The normal component of the velocity is restricted to non-negative values, since resolidification is not allowed, i.e. vb ≥ 0. The absorption front Γ+ (t) can be subdivided in two regions: the melting front Γm (t), where the temperature equals the melting point T = Tm and erosion takes place (vb ≥ 0), and the rest Γ+ (t) \ Γm (t) (T < Tm and vb = 0). The melting front Γm (t) is free boundary. The resulting form of the cut edges – for example a ripple or striation – is a result of the motion of the border line ∂Γm (t). With these definitions the problem has the following general form: Find the solution of the heat-conduction equation ∂T = κT , T = T (x, t) , x ∈ Ω(t), ∂t

(2.6.1)

subject to the free-boundary conditions qa − λ
T · n = ρH m vb , x ∈ Γm (t) , T = Tm , x ∈ Γm (t) ,

(2.6.2)

qa − λ
T · n = 0 ,
T · n = 0 , T|x|→∞ = Ta ,

(2.6.3)

x ∈ Γ+ (t) \ Γm (t) , x ∈ Γ− (t) , x ∈ Ω(t) .

Here Ta and Tm are the ambient and the melting temperature. Hm is the melting enthalpy, λ is the heat conductivity, ρ the mass density, κ = λ/(ρc) the thermal diffusivity and c the specific heat capacity. In comparison with the well-known Stefan problem, as represented by Elliot et al. [82Ell] or Fasano et al. [77Fas1, 77Fas2, 77Fas3] there is a different and more complicated situation involved in thermal ablation by a moving heat source – the energy transfer takes place directly at the free boundary, – the heat flux absorbed at the free boundary depends on the angle of incidence and via the intensity distribution f (x, t) on the position and – the boundary conditions are changing discontinuously (Fig. 2.6.3b) at the border line ∂Γm (t) of the melting front. The free-boundary problem (2.6.1)–(2.6.3) remains too complex for a detailed mathematical analysis. One example of technical relevance is the solution of the inverse problem, which cannot be given without further analysis.

2.6.4 Inertial manifolds A typical property of dissipative infinite-dimensional dynamical systems is the existence of a finitedimensional attractor [88Tem], which is a subset of the phase space attracting all trajectories. From the theory of finite-dimensional dynamical systems it is well known that in systems with different time scales after the decay of the fast degrees of freedom the dimension in phase space is decreased. In analogy, the analysis of some examples [90Sir, 89Con, 95Rob, 97Rob] for dissipative partial differential equations yield that also the long-time dynamic of such systems can be concentrated in

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the neighborhood of a manifold having a finite dimension. Such manifolds are called inertial manifolds in the case of partial differential equations and central manifolds for the ordinary differential equations. Obviously, the attractor is a part of the inertial manifold. In principle, the existence of a finite-dimensional inertial manifold means that the motion of a finite set of degrees of freedom can give a good approximation of the complete solution. One example for such a behavior is well known from the Boltzmann equation of kinetic theory. The solutions of this differential equation are so-called distribution functions, describing the probability amplitudes for a particle in phase space. In the short-time limit, which corresponds to non-equilibrium, these solutions have a rather complex structure. In the long-time limit, which corresponds to quasi-equilibrium (equilibrium of flows), the solution is completely described by the equilibrium distribution. The equilibrium distribution is parameterized by a small number of averaged quantities like the thermodynamical temperature and the hydrodynamical drift velocity. The dynamics of these parameters are governed by the simpler hydrodynamical equations of motion. For this example the approximate dynamical system is given by the hydrodynamical approximation of the kinetic equations, where the approximating step is the assumption of the local thermodynamical equilibrium. The solution of the hydrodynamical equations of motion plays the role of the inertial manifold of the Boltzmann equation. For many partial differential equations of continuum physics the existence of an inertial manifold is proven and their dimension is estimated. By these estimates we can expect a decrease of the dimension in phase space with the dissipation in the system. For some cases the inertial manifold is explicitly constructed (for example, in [89Jol] for the Chaffee-Infante reactions-diffusion equation). Historically essential advances in the understanding of physical processes and analysis of the partial differential equations are established by the application of integral methods. Famous examples are the quantum mechanical treatment of the helium atom by Hylleraas [29Hyl, 64Hyl] applying variational methods and the formulation of the B´enard problem by Saltzmann [62Sal] and Lorenz [63Lor] using integral methods. It is shown, that the motion of the complete system is contracting on short time scales into a subspace of lower dimension. Hylleraas identified the three crucial characteristic dynamical variables for the description of the helium atom, namely the orbital radii of the two electrons and there distance. The Lorenz system consists of three ordinary differential equations of first order with respect to time. The dynamics of these three characteristic dynamical variables or degrees of freedom are already able to reproduce the important properties of the B´enard problem. Characteristic dynamical variables are encountered in the long-time dynamics near the inertial manifold. The construction of equations of motion for the characteristic dynamical variables is the crucial step for testing and finally the identification of reduced models also for free-boundary problems in thermal processing. The fundamental idea for deriving models with a reduced dimension in phase space is to find an integral representation of the partial differential equations. The variational method introduced by Biot and Daughaday [62Bio] and Biot [70Bio] allows to formulate an integral equation which is equivalent to the free-boundary problem (2.6.1), (2.6.2) and (2.6.3). The application of the variational method to the spatial one-dimensional free-boundary problem is given in [97Sch]. The approximating step is the assumption of a spatial distribution for the temperature, which is parameterized by characteristic dynamical variables.

2.6.5 Dimension in phase space To apply integral methods means to construct the reduced equations of motion for the characteristic dynamical variables which are able to reproduce the structure of the solution. Since temporal and spatial scales are closely related to each other, now the spatial scales involved in the problem

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2.6.6 Spatial one-dimensional model

[Ref. p. 215

will be identified, which cover the desired information about the structure of the solution and the characteristic dynamical variables. For this purpose the quasi-steady representation of (2.6.1) 
2 ∂T ∂2T ∂2T ∂ T (2.6.4) + + 0 = v0 +κ ∂x ∂x2 ∂y 2 ∂z 2 can be considered. Comparing the first and second derivatives with respect to the x-direction of (2.6.4) yields an internal length scale, namely the penetration depth δ = κ/v0 of the temperature within feeding direction. The penetration depth δ = κ/v0 decreases with the speed of feeding v0 and depends linearly on the thermal diffusivity κ. As a consequence of the long-tailed shape of the isophote1 of the laser beam also the absorption front has a large depth d compared with its width w0 : this holds for the capillary in welding, the shape of drilled holes as well as for the cutting front. Therefore, (2.6.4) can be considered as subject to the boundary conditions T |x2 +y2 =w02 = Tm ,

T |x2 +y2 →∞ = Ta ,

(2.6.5)

∂T ∂T |z=0 = qa (x, y, z = 0) , |z=d = 0 , (2.6.6) ∂z ∂z With the boundary conditions two external length scales are introduced, namely the lateral extent of the laser beam radius w0 and the relatively large sheet thickness d  w0 . The typical values for the temperature differences in x- and y-direction are of the same order of magnitude δTx ≈ δTy ≈ (Tm − Ta ). In the advance of the melting front and within axial direction smaller values for the temperature difference δTz ≤ (Tm − Ta ) are present, since at the top surface of the sheet and in the advance of the melting front the melting temperature is not yet reached (heating phase). Therefore, the scales x = δ ξδ , y = w0 η, z = d ζ, θ = (T − Ta )/(Tm − Ta ) can be easily motivated. Consequently, the non-dimensional diffusion equation (2.6.4)   2 2  2 2
δ ∂ θ ∂ θ ∂2θ δ ∂θ + 2 + + , (2.6.7) 0= ∂ξδ ∂ξδ w0 ∂η 2 d ∂ζ 2 −λ

together with the boundary conditions (2.6.5), (2.6.6) θ |ξδ2 +η2 =1 = 1 , −

∂θ qa (δξδ , w0 η, ζ = 0) |ζ=0 = , ∂ζ λ(Tm − Ta )/w0

θ |ξδ2 +η2 →∞ = 0 ∂θ |ζ=1 = 0 . ∂ζ

(2.6.8) (2.6.9)

demonstrates clearly the importance of the different length scales. The hierarchy δ
w0
d of the length scales induce a hierarchy of the spatial dimensions. The smallness parameter δ/w0 is called the inverse P´eclet number P e−1 (P e = w0 /δ = w0 v0 /κ). In particular, the balance equation reveals that for small values of δ/w0 , i.e. δ/w0
1, the spatial one-dimensional properties (square brackets in (2.6.7)) within feeding direction are dominant and the lateral as well as the axial contributions appear as small corrections, only.

2.6.6 Spatial one-dimensional model Using the leading order of (2.6.7) with respect to the smallness estimates δ/d
δ/w0
1 the simplest and most fundamental limiting case of the boundary value problem can be extracted. The 1

Closed surfaces defined by the intensity distribution f (x) = const. are called isophotes. Landolt-B¨ ornstein New Series VIII/1C

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spatial one-dimensional problem 0=

∂2θ ∂θ + 2 , ∂ξδ ∂ξδ

θ |ξδ2 =1 = 1 ,

θ |ξδ2 →∞ = 0

has the solution θ(ξδ ) = exp(−ξδ ) ,

ξδ =

x x = Pe = Pe ξ , δ w0

P e = w0 /δ = w0 v0 /κ .

(2.6.10)

The parameters of the quasi-steady solution for the temperature and the geometrical parameters of the melting front at their vertex line are well suited to describe the structure of the solution of the free-boundary problem. These parameters are regarded as time-dependent quantities and they are called characteristic dynamical variables. Consider at first the spatial one-dimensional part of the problem (2.6.1)–(2.6.3). The exact quasi-steady solution (2.6.10) for the temperature in the advance of the melting front motivates the ansatz 
ξ − A(τ ) , ξ ≥ A(τ ) , (2.6.11) θapp (ξ, τ ) = θs (τ ) exp − Q(τ ) which is characterized by three time-dependent parameters (degrees of freedom) . In the spatial one-dimensional problem the change of boundary conditions (Fig. 2.6.3b) can take place with respect to time only and spatial coexistence is not covered. During melting phase the surface temperature equals the melting temperature, i.e. θs (τ ) = 1. Q(τ ) is the penetration depth of the temperature. For Q(τ ) = 1/P e the quasi-steady solution (2.6.10) is reproduced by (2.6.11) and the position A(τ ) = P e τ of the melting front moves together with the beam axis at the speed of feeding P e = (w0 /κ)v0 . Of course, the spatially integrated temperature has the meaning of the energy content E ∞ θapp (ξ, τ )dξ = θs (τ )Q(τ ) ,

E(τ ) =

(2.6.12)

A(τ )

the quantity Q(τ ) equals the energy content during melting phase (θs (τ ) = 1). Starting the cutting process or while modulating the intensity the dynamical system can stay in the heating phase. The surface temperature θs (τ ) < 1 is smaller than the melting temperature and the position of the front A(τ ) = const. remains unchanged. A detailed discussion of the spatial one-dimensional solution is given in [97Sch]. Using the ansatz (2.6.11) the approximate dynamics are restricted to the time scales encountered in the motion of the slowest dynamical variables {A(τ ), θs (τ ), Q(τ )}. Similar to the treatment of the hydrodynamic equations [88Can] the heat-conduction equation can be solved by spectral methods. The exact solution then is given by including additionally an infinite-dimensional set of spectral eigenfunctions uk (ξ) with the coefficients ak (τ ) θ(ξ, τ ) = θapp (ξ, τ ) + θspectral (ξ, τ )  
∞ ξ − A(τ ) + = θs (τ ) exp − ak (τ )uk (ξ). Q(τ )

(2.6.13) (2.6.14)

k=0

It is worth to mention that the eigenvalues – inverse time scales for relaxation of the eigenfunctions – strongly depend on the spatial scales involved in the boundary problem and time scale separation can be revealed. To single out one result of the three-dimensional phase space {A, θs , Q} during melting phase θs = 1, time-scale separation of the two characteristic dynamical variables {A, Q} takes place for large P´eclet numbers. The dynamical variables are moving on a slow surface (Fig. 2.6.3b) and do not cover the whole phase space {A, Q}, namely the penetration depth is Landolt-B¨ ornstein New Series VIII/1C

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2.6.7 Spatial two-dimensional model and diffusive eikonal

[Ref. p. 215

slaved to the motion of the position of the front ((1 + hm )A˙ = γf , Q = (1 + hm )/γf ) where hm denotes the inverse Stefan number (hm = Hm /(c (Tm − Ta )), compare (2.6.2)). The properties of this solution remain of fundamental importance also for the solutions including higher spatial dimensions.

2.6.7 Spatial two-dimensional model and diffusive eikonal Consider a spatial two-dimensional region perpendicular to the laser beam axis. Additionally to the boundary problem (2.6.4), where the lateral extent w0 of the melting front is treated as a global quantity, now the radius of curvature rm at the vertex line of the melting front becomes the dynamical variable and plays the role of a parameter for the asymptotic analysis. The scales and abbreviations x = w0 ξ, y = w0 η, α = rm /w0 are introduced, where w0 is the laser beam radius. The parameter P e has to be regarded as global quantity, since it is built with the laser beam radius w0 , which is only a scale for the local shape of the boundary. The local P´eclet number P eα = P e α = rm v0 /κ depends on the radius of curvature rm of the melting front. Using these scales the boundary problem reads: 0 = Pe

∂θ ∂ 2 θ ∂2θ + 2+ 2 , ∂ξ ∂ξ ∂η

θ |ξ2 +η2 =α2 = 1 ,

θ = θ(ξ, η) ,

ρ2 = ξ 2 + η 2  α2 ,

θ |ξ2 +η2 →∞ = 0 .

(2.6.15) (2.6.16)

This formulation is frequently cited in the literature related to welding and cutting applications. Experimental evidence for this model as quasi-steady description of the most dominant properties is striking. Although this model covers as drawback a free and not directly determined parameter α = rm /w0 , namely the radius of the cutting front or the radius of the capillary in welding, the simple and plausible assumption α = O[1] for its order of magnitude is sufficient to come out with well proved time-averaged conclusions (Schulz et al. [93Sch], Zefferer [97Zef]). The solution can be given in explicit form: θ(ρ, ϕ) = exp[−P e ρ cos(ϕ)/2] S(ρ, ϕ) , S(ρ, ϕ) = θ0 K0 (P e ρ/2) + 2


In (x) θn = Kn (x)



∞ 

ρ>α,

θn Kn (P e ρ/2) cos(nϕ) ,

(2.6.17)

n=1

,

n = 0, 1, 2, . . . ,

x=P e α/2

where In , Kn are the modified Bessel functions and {ρ, ϕ} are plane polar coordinates. As before the structure of the spatial two-dimensional solution can be found by the asymptotic with respect to the smallness estimates δ/d
δ/w0
1. The properties of the exact solution from (2.6.17) suggest to introduce a new unknown function S(ξ, η) θ(ξ, η) = exp[−kξ] S(ξ, η) ,

k=

P eα , 2

(2.6.18)

which separates a spatial homogeneous part of the convective heat transport. The analogy to a ˜ having imaginary wave number k˜ = 0 + ik is already visible (pure dissipative plane wave exp(ikξ) behavior). In optics a new function for the wave amplitude is introduced, and here the new function S(ξ, η) will be called the amplitude function of the temperature. In the case of heat transport the convection is characterized by the local P´eclet number P eα = P e α, such that the wave number in optics corresponds to the P´eclet number in heat transport (P e = w0 v0 /κ , α = rm /w0 ). The Landolt-B¨ ornstein New Series VIII/1C

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transformation (2.6.18) together with (2.6.15) yields the Helmholtz equation for the amplitude function S(ξ, η) S − k 2 S = 0 ,

S = S(ξ, η) ,

S |ξ2 +η2 =α2 = exp[kξ] ,

k=

P eα , 2

(2.6.19)

S |ξ2 +η2 →∞ = 0 .

With an ansatz for a generalized plane wave S(ξ, η) = Ψ (ξ, η) exp[k L(ξ, η)] ,

k=

P eα 2

(2.6.20)

and an expression for the phase like a diffusive eikonal L(ξ, η), yields an asymptotic solution for the temperature θ(ρ, ϕ) = exp[−k ρ/α cos(ϕ)] · Ψ (ρ, ϕ) exp[k L(ρ, ϕ)] . In the limit of large P´eclet numbers P eα and as Taylor expansion with respect to the azimuth angle ϕ at the vertex line {ρ, ϕ = 0} the asymptotic expression takes the form  1/2 P eα θ(ρ, ϕ = 0) ∼  →∞. exp[−2k (ρ/α − 1)] for k = 2 ρ/α − 1/2 From this result and comparison with the asymptotic properties of the exact solution (2.6.17) the final form for an approximate ansatz θapp (ρ, ϕ, τ ) describing the time-dependent, two-dimensional movement of the melting front can be motivated:  
1 − β[Q/α] 1 + cos(ϕ) α , (2.6.21) θapp (ρ, ϕ, τ ) =  exp − (ρ/α − 1) Q 2 ρ/α − β[Q/α] β = Q/α exp[2Q/α] E1 [2Q/α] ,

Q = Q(τ ) ,

α = α(τ ) .

(2.6.22)

The spatial one-dimensional case (2.6.10) is covered within the two-dimensional ansatz (2.6.21). We introduce the distance ν from the melting front within the radial direction ν =ρ−α ,

ρ/α = 1 + ν/α.

Consider the approximate temperature θapp (ν + α, ϕ, τ ) from (2.6.21) for a fixed value of the distance ν = const. at the vertex line ϕ = 0. The transition to the temperature θ1D (ν) = exp[−ν/Q] in the advance of a spatial one-dimensional melting front is given by the asymptotic for large values of the radius α of the melting front 
exp[−ν/Q] ν lim θapp (ν + α, ϕ = 0, τ ) = lim  = exp − α→∞ α→∞ Q ν/α 1+ 1 − β[Q/α] and shows the asymptotic relation between the quantity Q(τ ) and the penetration depth δ = w0 Q of the temperature. Since the displacement β is non-negative 0  β  1/2 and remains bounded, the temperature θ1D (compare (2.6.11)) is an upper bound for the temperature in the advance of the curved melting front: θapp  θ1D . This upper bound is achieved for the limiting case of infinitely large values for the radius of the melting front. The lateral, two-dimensional effects are described by a control parameter which is the ratio Q/α = δ/rm of the penetration depth δ and the radius rm of the melting front. Although the equations of motion are derived from the limiting case of large P´eclet numbers, also down to moderate values of the P´eclet number P e = O(1) the thermal energy remains concentrated within the vicinity of the melting front (thermal boundary layer). In Fig. 2.6.4b the Landolt-B¨ ornstein New Series VIII/1C

198

2.6.7 Spatial two-dimensional model and diffusive eikonal

[Ref. p. 215

A

Front position A, radius of curvature α

1.2

α

1.0

0.8

0.6

Dynamical system A 1D A 2D α FEM-calculation A 2D,FEM α FEM FVM-calculation A 2D,FVM α FVM

0.4

0.2

0.0

0

2

4

6

8

10

12

14

16

Scaled intensity γ a

b

Fig. 2.6.4. (a) Comparison of the numerical solution using the Finite-Element Method (FEM) and the Finite-Volume Method (FVM) with the corresponding approximate solution reveals the quality of the ansatz (γ = 6.4, P e = 1.5, hm = 0.25). The shape of the free boundary can be well approximated by dynamical variables A and α. In particular, in the advance of the laser beam axis deviations of the radius of curvature from the value α remain small. (b) The squares correspond to the numerical solution of the partial differential equations (2.6.1)–(2.6.3). The results from FEM (large squares) and FVM (small squares) are shown. The solid lines correspond to the solution of the spatial two-dimensional approximate dynamical system. The dotted line is the value A1D of the position of the front owing to the spatial onedimensional dynamical system. The values for the position of the front A2D are shown as filled squares and the open squares are the values for the radius α (P e = 1.5, hm = 0.25).

approximate quasi-steady solutions of the finite-dimensional dynamical system (curves) and the numerical calculations (squares) of the partial differential equations are shown. The Finite-Element Method (FEM – large squares) is applied by Niessen [03Nie] and the Finite-Volume Method (FVM – small squares) is used by Kostrykin [97Ens]. The reduced dynamical system for the spatial two-dimensional model is explicitly given in [99Sch]. Here the appearance of the inertial manifold will be discussed in more detail. The most interesting property of the solution in dissipative systems is the reduction of dimension in phase space. With increasing P´eclet number P e the spatial two-dimensional model shows the change of phase-space dimension by decoupling and time-scale separation of the different dynamical variables. The spatial two-dimensional dynamical system for increasing P´eclet numbers becomes  
  1 + hm + b2 1 b2 b1 + , (2.6.23) γf2 − γf0 − α˙ = 1 + hm Q 1 + hm − b1 Q   b1 1 (2.6.24) γf0 − A˙ = 1 + hm − b1 Q   1 Q˙ = b1 − A˙ . (2.6.25) Q The motion of the spatial one-dimensional dynamical variables {A, Q} then is decoupled from the motion of the additional two-dimensional variable α. A Gaussian intensity distribution f (x) = exp(−2x2 ) and the maximum intensity γ at the beam axis is considered. The intensity γf enters the equations of motion by its Taylor coefficients f0 , f2

Landolt-B¨ ornstein New Series VIII/1C

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2.6 Cutting: Modeling and data ϕ2 , 2

f0 = exp(−2A¯2 ) ,

199

¯ f0 f2 = −4α(α − A)

at the vertex line, where the distance A¯ = A − RL between the position of the front ξ = A(τ ) and the moving laser beam axis ξ = RL (τ ) is introduced. In the limiting case P e → ∞ the lateral equations of motion show α˙ =

γf0 ¯ + 1] , [−4α(α − A) 1 + hm

(1 + hm )A˙ = γf0 ,

Q=

1 , A˙

(2.6.26)

in a transparent manner, how the radius α of curvature is determined. The stable (α ≥ 0) quasisteady (A˙ = P e, α˙ = 0) solution reads α=

1 ¯ 1  ¯2 A +1 , A+ 2 2

¯ = (1 + hm )P e , γf0 (A)

Q=

1 . Pe

(2.6.27)

For the quasi-steady state the radius α > A¯ is larger than the distance A¯ between the melting front and the laser beam axis. Since the lines of constant values for the intensity are circles, the radius ¯ The radius of curvature of of curvature of the isophote at the vertex line equals the distance A. the isophote is smaller than that for the melting front, and therefore the absorbed intensity has a monotone decay with respect to the azimuth angle starting at the vertex line and going along the melting front towards the cut edge. This is a direct consequence from the boundary condition (2.6.2) which states the energy density flow across the melting front is continuous. Numerical simulations [03Nie], [97Ens] are clearly indicating that moreover the radius of curvature depends only slightly on the azimuth angle. Owing to this observation as conclusion we can expect that the radius α of curvature at the vertex line already is closely connected to the width of the cut kerf.

2.6.8 Iterative refinement Approximate solutions of the variational equation describing the long-time behavior are settled. The corresponding finite-dimensional dynamical system describes the movement of time-dependent characteristic parameters (Fig. 2.6.3), like front position A, surface temperature θs , energy content Q. The melt flow – one example of a sequel process – can be treated in a similar way describing the dynamics of the melt film thickness h and mass flow m. The mass flow m is the melt velocity integrated with respect to the melt film thickness [97Sch]. The onset of evaporation at the irradiated metal surface in welding is a further example of sequel processes. Quality degradation like melt ejection in welding, scourings in oxygen cutting and unevenness in contoured fusion cutting are a direct consequence of the fast movement of the melting front and the temperature at the melt surface. The suitable dynamical variables are the melt film thickness h(τ ), the mass flow m(τ ) and the temperature θs (τ ) at the melt film surface. These variables are time-dependent parameters of the spatial distributions for the velocity and the temperature in the melt. The dynamics on shorter time scales governing the relaxation to the equilibrium distributions can be taken into account by spectral methods. Using the integral representation of the balance equations of mass, momentum, and energy the equations of motion for the dynamical variables h(τ ), m(τ ), θs (τ ) read

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2.6.8 Iterative refinement  Re

∂m 6 ∂ + ∂τl 5 ∂ζ




m2 h

 = −

∂Πg h+3 ∂ζ



Σg m − 2 2 h

∂m ∂h + = vp ∂τl ∂ζ

 
 1 ∂ 5 ∂ 1 θs θs mg2 = g3 θs hg1 + γf − 2 ∂τl 8 ∂ζ P el h m ζ=0 = h ζ=0 = θs ζ=0 = 0

[Ref. p. 215

 momentum ,

(2.6.28)

mass ,

(2.6.29)

energy ,

(2.6.30)

initial values ,

(2.6.31)

where vp = vb /v0 is the scaled velocity of the melting front (compare (2.6.2)) and gi = gi (P el h), i = 1, 2, 3 are monotone decaying functions of P el h. The dynamical system (2.6.28)–(2.6.31) for the processes in the melt is coupled to the gaseous and the solid phase by the pressure gradient ∂Πg /∂ζ and the shear stress Σg and by the velocity of the melting front vp respectively. The cutting problem is closed by the dynamical system for the motion of the melting front (2.6.23)–(2.6.25). Inserting the time-dependent characteristic parameters into the ansatz for the spatial distribution allows to reconstruct the three-dimensional representation (Fig. 2.6.5) of the movement in phase space (Fig. 2.6.3). Two examples – formation of ripples and adherent dross – will be used to demonstrate that the analysis in phase space (Fig. 2.6.3) is more transparent and reliable as the interpretation of its spatial three-dimensional representation and the latter facilitates the comparison with direct experimental observation (Fig. 2.6.1c) of the process. A characteristic dynamical feature of the cutting process is the formation of ripples. Taking into account the melt flow (2.6.28)–(2.6.31), an additional property of the cutting process – the formation of ripples of the second kind – becomes part of the solution [01Pop]. Ripples of first type are reproduced by the dynamical system as a result of direct heat absorption and diffusion (Fig. 2.6.5a). In the upper part of the sheet the influence of the melt flow on the movement of the melting front is negligible, since the heat contained in the melt film, its thickness and the mass flow of molten material remain fairly small. Heat convection in the melt film is involved when ripples of the second type are formed (Fig. 2.6.5b). Any ripple of first type causes an axial propagation of a melt wave. The subsequent heat transport – convection in axial direction – leads to an additional and delayed motion of the melting front. The ripple frequency is doubled (Fig. 2.6.5b). Introduction of quality classes by Arata [79Ara2] is applicable to fusion cutting (Fig. 2.6.6, class II, III, IV). The three main different processing domains become observable by online measurement of the thermal emission with a CCD-camera [01Pop]. These quality classes can be correlated with the dynamical process domains and can be identified with the underlying physical 2 mechanisms. The ratio of the stagnation pressure ρvmelt of the melt flow and the counteracting capillary pressure σ/dm is called Weber number W e (ρ density, σ surface tension of the melt). If the capillary forces become comparable with the inertia of the melt (W e ≈ 1) then the separation of melt flow becomes unstable. The tendency to the formation of dross is estimated from the so-

a

b

Fig. 2.6.5. (a) Ripples of first type occur at the top of the cut edge, where almost no resolidified melt is observed. Direct absorption and heat diffusion mainly determine the movement of the melting front. (b) The downward-directed heat transport by the melt flow additionally moves the melting front and ripples of second type are formed (cutting speed 1 m/min, thickness 10 mm, N2 14 bar, nozzle diameter 1.4 mm, CO2 laser power 4.6 kW, stainless steel (1.4301)). Landolt-B¨ ornstein New Series VIII/1C

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10

cutting speed v0 [m/min]

EL 8

TV IV evaporation, dross of 2. kind

6

We 4 III

2 II capillary forces, dross of 1. kind

0 0

1

2

3

4

thickness d [mm]

5

6

7

Fig. 2.6.6. As a result of the modeling quality degradation due to dross is related to the dynamics of the melt flow. Melt flow is interrupted by the dominance of capillary forces (W e) or by the onset of evaporation (TV ). Cutting speeds achievable (EL ) are only limited by high material thickness or low laser power.

lution (Fig. 2.6.6, W e) and occurs for low cutting speeds. For increased cutting speeds the melt film starts to evaporate (Fig. 2.6.6, TV ). Finally the maximum cutting speed achievable (Fig. 2.6.6, EL ) is given by energy balance. The correlations between the processing domains (W e, TV ), the characteristics of the CCD-images and the formation of the different kinds of dross is striking. For low values of the Weber number, the CCD-image shows an asymmetric position of the single melt thread (compare Fig. 2.6.1b) and there is dross at one side of the kerf. With the onset of the evaporation pressure the CCD-image shows a symmetric splitting of the melt thread and dross at both sides of the kerf appears. The capillary forces become dominant for low cutting speeds and for cw operation. The melt flow does not separate completely from the sheet metal. Low cutting speeds are inherent in contour cutting and therefore a reliable control strategy is of interest. For this task it is crucial to differentiate between the cutting speed v0 and the velocity vp (τ ) of the cutting front. From the results on the capillary forces (Fig. 2.6.6, W e) the appearance of dross is related to small values of the Weber number W e ∝ vmelt 2 dm , which has to be kept at values larger than unity. Modulation of the laser power results in correspondingly modulated velocity vp (τ ) of the melting front and hence the time-dependent mass flow vm dm = vp (τ )d  v0 d can be significantly larger than its value v0 d averaged with respect to the cycle time. The effect of power modulation on the resulting cut quality then depends on the selection of the proper pulse duration and the duty cycle. These parameters can be chosen such that only one wave of ejected melt is produced during each laser pulse. The laser pulse duration ton has to be matched to the typical time d/vm for the propagation of a melt perturbation along the whole melting front down the sheet thickness d. Also, the cycle time ton + toff has to meet the time-averaged energy balance. Using these constraints the parameters for the pulse mode operation, namely pulse duration and duty cycle, are determined. Figure 2.6.7 gives an example for a cut at a low cutting speed v0 = 1.2 m/min performed with modulated laser power which remains dross free. The CCD-images show the melt thread which remains at the center of the cutting front as for cw-cutting and parameters corresponding to quality class III in Fig. 2.6.6.

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2.6 kW ton = 1.55 ms

0 kW

toff = 0.67 ms time

Fig. 2.6.7. The cutting process can evolve periodically using modulated laser power. The recorded CCDimages in this case show the typical properties of high-quality cuts of class III known from cw operation (cutting speed 1.2 m/min, thickness 3 mm, N2 14 bar, nozzle diameter 1.4 mm, maximum CO2 laser power 2.6 kW/pulsed, duty cycle 70 %, pulse frequency 450 Hz, beam radius 150 µm, Rayleigh length 2.2 mm, focal position −2.4 mm, stainless steel (1.4301), CCD camera frame rate 2.9 kHz).

2.6.9 Cutting data Cutting data [99Tru, 00Tru, 02Tru] consist of the machining parameters which have to be set up and the criteria to evaluate the cut quality (Table 2.6.1). Table 2.6.1. Machining and quality evaluation parameters for cutting. Machining Laser

Machine

power modulation beam quality beam stability polarization

output mirrors cutting speed beam alignment gas type astigmatism gas pressure deflection mirror focal position focusing lens nozzle type nozzle diameter beam to nozzle alignment shape of nozzle exit

Processing

Quality Material type thickness surface cut shape

kerf width dross (burr) roughness perpendicularity slant tolerance groove (ripple) lag pitting heat-affected zone

In the following the items listed in Table 2.6.1 are described more extensively.

2.6.9.1 Laser power The necessary laser power depends on the type of laser used and on the type and thickness of the material to be cut. This is demonstrated in Fig. 2.6.8 which shows the maximum material thickness that can be processed for three different materials and different CO2 laser power. To achieve a high level of accuracy also during the processing of contoured cuts having fine spatial structures the laser power has to be reduced. Modern CO2 lasers can be operated at each desired time-averaged power level from 5 % to 100 % of the maximum cw-performance (cw = continuous wave).

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20

16

s [mm]

12 8

4 0 TLF 1800t TLF 2400t TLF 3000t

TLF 4000t

Fig. 2.6.8. Maximum material thicknesses s for standard cut depending on material type and CO2 laser power [02Tru]. Aluminum (ALMG3) light grey, stainless steel (1.4301) dark grey, mild steel (RSt 37-2) black.

2.6.9.2 Modulation of the laser output (gating frequency) In the cw-mode of operation, the laser beam is emitted continuously as a result of a constant energy supply for excitation. In pulsed-mode the laser output power is modulated. Laser pulses with maximum output power during the pulse duration are generated. Pulse duration and duty cycle can be controlled by the modulation frequency and the averaged laser power. For example, let us consider a cw power output of 4000 W during electrical excitation of the discharge. Running at a modulation frequency of 1 kHz and an averaged power of 2 kW the laser generates in one second 1000 pulses of 0.5 ms duration each. Two successive laser pulses are separated by 0.5 ms with no laser pumping. At modulation frequencies up to 1 kHz the laser process is fast enough to perform 100 % modulation and to follow the electrical excitation. The laser is actually in an on/off mode. An increased modulation frequency up to 10 kHz is possible, but the degree of modulation decreases and the laser output gets more and more continuous as the time between two subsequent pulses becomes shorter than the laser relaxation processes. Such high frequencies are actually used for a quasi-continuous output and not for pulsing. Whether such a quasi cw or a real pulsing behavior is desired depends strictly on the machining task. It is recommended, for example, to reduce the frequency for cutting small contours. The frequency is also reduced for piercing in ramp mode.

2.6.9.3 Beam quality and power density distribution The best cutting results are guaranteed by a power density distribution which corresponds to a loworder Transverse Electro Magnetic mode such as TEM00 and TEM01∗ . The TEM01∗ , a ring-shaped mode, has a minimum value in the center and reaches its maximum intensity on a surrounding ring. The typical cutting mode for high-power cutting lasers above 3 kW is a superposition of the fundamental mode and a ring mode. Usually, the fundamental mode is favored at low power, but with increasing amplification the higher-order modes are growing. This is not desirable, because for consistent cutting results the intensity distribution of a laser beam should be constant across the whole power range. With a proper design of the resonator, the electrodes and the gas flow within the laser, the gain profile can be controlled in such a way that the mode keeps pretty much constant from low to maximum power. There are two methods for checking the power density distribution of a laser beam: Landolt-B¨ ornstein New Series VIII/1C

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– A mode burn in acrylic glass. This simple but still very often used method has the advantage of low cost (basically an exhaust system with proper filter) and of a very high spatial resolution and intensity dynamics. – Measurement with a beam diagnostic system. The beam quality of a laser beam is indicated by the beam propagation factor K (K = 1/M 2 with M 2 = quality factor) or the beam parameter product q [98Sie]. Both quantities can be defined according to DIN EN ISO 11145 K=

λ 1 , π q

0
q=w

θ , 2

q>

λ , π

(2.6.32)

where w is the laser beam radius at the focal plane and θ is the beam spread (DIN EN ISO 11145) also referred to as full far-field divergence angle. Comparing the focusability of lasers with different wavelengths λ, the beam parameter product q is particularly suitable, since the influence of the wavelength on the focus diameter has already been taken into account. High-power cutting lasers – in the 4 and 5 kW regime – typically have K numbers of 0.5, which means, the attainable focus is twice as big as the focus of a pure Gaussian beam, which has a K number of 1. A good beam quality (a high K number) not only results in a small focus with a high intensity, but also in a small divergence of the laser beam diameter. The values used for the beam widening or deviation from the parallel propagation are the beam spread θ (beam parameter product q) and the Rayleigh length zR . A beam divergence of 1 mrad corresponds to an increment in the beam radius of 1 mm per meter of beam propagation length. With “flying optics” machines (Fig. 2.6.9), where the workpiece remains stationary while the processing optics are moved (“flown” ) over the workpiece, the divergence of the laser beam plays an important role. In order to achieve a consistent cutting quality with varying distances between the resonator and lens, the angle of divergence must be kept as narrow as possible. For this reason, a beam telescope is usually installed at the outlet of the laser. The beam telescope reduces the divergence of the laser beam by expanding the beam of the laser by 150. . . 200 %. This reduces the divergence to < 1.0 mrad. Some machines are using adaptive mirrors to control the beam diameter at the focusing lens. The greater the diameter of the laser beam, the smaller the attainable focus diameter and, consequently, the narrower the resulting kerf. The attainable focus diameter, however, is also dependent on the focal length of the lens. Focusing lenses with a focal length of 2.5 inches permit a focal diameter of < 0.12 mm; lenses with a focal length of 5 inches permit diameters of < 0.2 mm.

4 1

2

3

5

6 7

Fig. 2.6.9. Beam path of a “flying optics” cutting machine (TRUMPF TC L 3030). 1: TLF laser with beam telescope, 2: beam-bending mirror on the motion unit, 3: laser beam, 4: beam-bending mirror, 5: beam-bending mirror and phase shifter (circular polarizer), 6: beam-bending mirrors, 7: focussing lens in cutting head.

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2.6.9.4 Spatial and temporal beam stability In a cutting machine with a beam path of 10 m or more essential to have a very stable beam pointing behavior. A thermally introduced movement of the laser beam will cause a non-uniform illumination of the focussing lens or even an obstruction of the beam. This will ruin the focus and will heat up machine parts and mirror mountings, which in turn leads to further movement of the laser beam. Even damage of machine components can happen. A good laser beam source keeps the beam stable within an angle of less than 200 µrad. And a well-designed cutting machine helps to maintain this value throughout the machine. Only a constant, pre-selected laser power over the entire processing time of the workpiece can ensure consistent cutting results. After being powered up, the laser needs several minutes to attain a constant output. This is referred to as a momentary power constancy. It is also important that the guaranteed output power of the laser is maintained and not allowed to drop while cutting (long-term laser power constancy).

2.6.9.5 Polarization For cutting in two and three dimensions a circularly polarized laser beam is advantageous. Linear or elliptical polarization results in quality degradations at the cut edge as well as in a lack of the perpendicularity of the kerf. Linear polarization of the laser beam becomes noticeable, in the case of a cut-out square, in the following way: – The kerf on two opposite sides is vertical. – The kerf on the two edges set apart by 90 degrees is cut at an angle. The bevel is always formed such that the kerf becomes wider from top to bottom like a roof-shape. In extreme cases the edges set apart by 90 degrees cut direction are no longer completely separated.

2.6.9.6 Output mirrors of the laser unit A dirty output mirror absorbs laser power. This invokes a “cold/warm”response in the laser beam, causing the beam to be constricted. As a result of the constricted laser beam and the modified beam divergence, the focusing of the laser beam changes. Beam constriction increases the thermal stress on the focusing lens which causes the focus to migrate upward.

2.6.9.7 Beam alignment The laser beam must be aligned such that it is parallel and centered in the beam guideway on all axes. This is especially critical for laser cutting machines with flying optics and requires a careful adjustment of the laser beam with consistency checks. Improper alignment results in directionrelated cutting flaws which may spread uniformly across the entire working range or occur only at distinct points.

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2.6.9.8 Astigmatism If the cone of rays strikes the lens asymmetrically, it produces images at two different focal planes. The resulting line images are perpendicular to each other. Flawed cutting edges are evidence of such defects. When cutting a square, this can lead to the edges of two opposing sides to be cut with almost no deviation from perpendicularity while the other two sides are cut with slant tolerance. This is particularly noticeable when cutting thick sheets.

2.6.9.9 Deflection mirror Dirty deflection mirrors within the machine usually result in consistently poor cutting quality, regardless of the cut length. Typical effects are dross formation, deepened roughness and that mild steel begins to pit.

2.6.9.10 Focusing lens A dirty focusing lens heats up as a result of increased laser radiation absorption. This heating leads to a change in the refractive index, causing the focus position to shift upward. The effects are: – The cut starts good, but as the cut increases in length, dross forms and adherent volume becomes larger. – The kerf and roughness increase. – Mild steel begins to pit. – In extreme cases, the workpiece is no longer cut. If the focusing lens is installed upside down, the cutting quality will also be reduced because of increased optical aberrations. The power limit for ZnSe lenses has continuously increased over the years of applying laser cutting. Meanwhile there are 5 kW laser machines on the market, which are still using lenses for focusing. For applications which need even more power, like high-pressure cutting of thick stainless steel sheets, reflective optics are used. Here the cutting-gas pressure is build up by a gas flow from a double-wall conical designed nozzle [91Zef]. Such cutting heads are used with lasers operating at 8 kW and higher laser power, cutting up to 40 mm of stainless steel with good quality.

2.6.9.11 Beam to nozzle alignment The focusing lens must be set such that the focused laser beam propagates through the center of the nozzle opening. The focused beam may not vary from the center of the nozzle by more than ±0.05 mm. If a laser beam is not aligned properly in the center, the following consequences may occur: – All cut surfaces in a particular direction are of a reduced quality. – When the beam is extremely off-center, it is possible that the material will not be cut through. – The nozzle could heat up, impairing the function of the distance control system, or very high heat could destroy the nozzle itself. – Sparking on the sheet surface could result when cutting mild steel with a flame.

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2.6.9.12 Shape of nozzle exit The correct diameter and shape of the nozzle exit depends on the type of processing. For example, high-pressure cutting requires larger nozzle openings than does standard cutting. In high-speed cutting sometimes non-circular nozzles are used to reduce the gas consumption. Deformed noncircular nozzle opening (e.g. an oval shape resulting from a collision) has an effect similar to an off-center beam alignment and can lead to direction-oriented cutting flaws.

2.6.9.13 Cutting speed The cutting speed must be set based on the type and thickness of the workpiece. Speeds too high or too low can lead to unsatisfactory results such as roughness, dross formation or pitting. Generally speaking, the maximum cutting speed – decreases as sheet thickness increases, – increases almost linear with laser power and – has a maximum value depending on focal position.

2.6.9.14 Type of assist gas The type of material and the requirements for the cutting result determine which gas must be used. A flammable material like wood may not be cut with oxygen. Oxygen should also not be used with metal parts whose cutting edges should be free of oxidation. Oxygen cutting: The purity of the oxygen used in laser cutting of sheet metal is of crucial importance for the cutting results. Traces of water or nitrogen lead to dross formation. Such impurities in the cutting gas may result when gas cylinders are exchanged for contaminated cylinders. It is therefore required to use oxygen with a purity of 99.95 % (3.5). High-pressure cutting (fusion cutting): The quality of the nitrogen employed in high-pressure cutting of stainless steel is also very critical. A purity rating of 99.999 % (5.0) is recommended here. Traces of oxygen will cause formation of a fine oxide film, noticeable by a yellow discoloration of the cutting edge. In addition to this, a fine dross may form and the cut surface may be rough. The risk of gas contamination is substantially lower when using gas tanks than when using individual cylinders or cylinder bundles.

2.6.9.15 Gas pressure The gas pressure depends on the material thickness of the workpiece. When cutting with oxygen, gas pressure plays a more significant role than for cutting with nitrogen. Oxygen cutting: Low oxygen pressure, typically below 0.1 MPa, is applied. Thin plates are cut with a higher gas pressure than thick plates. Thin plates are also cut with a higher cutting speed than thick plates. The higher cutting speed creates a lack of oxygen in the gas supply which is then compensated by higher gas pressure. High-pressure cutting: Relatively high pressure ranging from 0.8 MPa to 1.5 MPa is applied in fusion cutting. A higher gas pressure is applied for cutting thicker workpieces, to expel the larger amount of viscous melt out of the kerf.

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2.6.9.16 Focal position The focal position refers to the vertical position of the focus point with respect to the sheet surface. Identifying the exact position of the focus position is an important requirement for good cutting results. Mild steel: If the focus position is too high, it results in a blister formation of adherent slag. The lower part of the cut surface is roughened. If the focus position is too low, it results in a granular accumulation of slag on the underside of the workpiece. The lower part of the cut surface is roughened. Stainless steel: If the focus position is too high, whisker-like dross forms on the underside of the workpiece. The lower part of the cut surface is roughened. If the focus position is too low, it results in a fine dross formation on the underside of the workpiece. The entire cut surface is roughened. Oxygen cutting: The following applies to cutting of mild steel (RSt 37-2): The most favorable focal position for materials up to 6 mm thick is at the workpiece surface. For sheet thicknesses of 8 mm and more, the focus point is positioned above the sheet surface. High-pressure cutting: The focus is positioned within the sheet metal. Generally speaking, the focus position has to be changed depending on sheet thickness. The focal position is established by determination of the minimum kerf width and typically is in the middle of the sheet or slightly above. To suppress adherent dross, the focal position has to be at larger depth.

2.6.9.17 Material type and composition The properties of metals (degree of absorption, heat conductivity, enthalpy for phase transitions, viscosity, etc.) are a major factor in determining their machinability with the laser. The properties of a material are determined by its components (e.g. carbon content, alloying elements).

2.6.9.18 Thickness Compared to moderate thickness and high-quality laser beams available at a medium power level high-speed cutting of thin metal foils and thick-sheet cutting at low cutting speed are qualitatively different processes. At medium and low cutting speed the melt flow takes place in front of the laser beam axis. At high cutting speed the position of melt ejection changes and takes place far behind the laser beam axis. The irradiated part of the melt surface is above the evaporation temperature and the recoil pressure dominates the assist gas pressure. Beam propagation through a laser-induced vapor and plasma takes place. As the thickness of the metal increases, so increases the roughness of the cutting edge and the laser power necessary for cutting. Complete irradiation of the extended cutting front and establishing spatially monotone acceleration of the molten material are important. Cutting thicker materials a substantially lower maximum speed is achieved when using a laser power scaled up from results with moderate thickness.

2.6.9.19 Surface condition of the sheet metal Bright material surfaces, as of pure aluminum, reflect the laser beam more strongly and result in a poor cut. A layer of scale on the sheet surface also has adverse effects on the cutting results. Landolt-B¨ ornstein New Series VIII/1C

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This is also true if the sheet has a layer of varnish or paint or is coated with plastic (e.g. stickers, lettering). Rough or matt surfaces however enable a high cutting speed. A light layer of oil, as is found on many sheets, has no negative effects on the quality of the cut. The oil film even has a positive influence when piercing at 100 % laser power as it substantially reduces slag formation on the sheet surface. A layer of rust on the workpiece leads to burnout and crater formation as the oxides induce oxygen supply, thereby increasing the amount of energy applied. Surface preparations like cold rolling, pickling or sandblasting slightly increase cut quality. High-pressure cutting allows plastic-coated stainless steel up to a thickness of 3 mm to be cut without dross formation. Galvanized steel sheets can be cut with nitrogen (high-pressure cutting) and oxygen (standard cutting) up to 4 mm and remain virtually burr- or dross free. Both, electroplate galvanized or hot-dip galvanized sheets, can be cut with good results.

2.6.9.20 Cut shape Certain cut shape or workpiece geometries, such as narrow webs, sharp angles and small holes (side length or diameter smaller 2 times sheet thickness), sometimes cause difficulties in laser cutting and the danger that excessive heat will be introduced and parts of the material will be burned off. Therefore, they are cut with more confined parameters such as reduced laser power (heat input), reduced cutting speed (machine precision) and pulse mode operation (averaged heat input). The standard DIN EN ISO 9013:2000 gives terminological definitions and describes criteria for evaluating the quality of cutting surfaces, quality classification and the dimensional tolerance. It applies e. g. in the case of laser-beam cuts for material thicknesses of 0.5. . . 40 mm.

2.6.9.21 Kerf width Laser cutting produces a kerf width which is usually narrower at the bottom of the cut than at the top. The kerf width is stated in mm. With a sheet thickness s, two values are determined with the aid of a feeler gauge: First the kerf width at a penetration depth s/3, which is referred to as upper standard value and second the kerf width at a the full penetration depth s, which is referred to as lower standard value, see Table 2.6.2. Table 2.6.2. Standard values for the kerf width. Material

Sheet thickness [mm]

Kerf width [mm]

Mild Steel (RSt 37-2)

1. . . 3 4. . . 6 8. . . 13 20

0.15 0.2. . . 0.3 0.35. . . 0.4 0.5

Stainless steel (1.4301)(N2 )

1. . . 3 4. . . 8 10. . . 12

0.15 0.2 0.5

Aluminum alloys (AlMg3,AlMgSi1)(N2 )

1. . . 3 4. . . 8

0.15 0.2. . . 0.3

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2.6.9.22 Dross formation The adherent dross may be of a highly adhesive metallic type which must be removed in finishing operations or consist of light adhesive slag, easily removed without refinishing. Laser cutting edges of metallic materials should be virtually if not completely free of dross. Dross formation on the underside of the cutting edge is not only due to certain intrinsic properties of the material but also the result of a number of laser and process parameters. Volume and shape of the adherent dross are different: – “Bead burr” is bead- or drop-like dross, with a polished metallic surface, highly adherent dross and difficult to remove (Fig. 2.6.10a). – “Crumble burr” where resolidified melt adheres in the form of fine, crumbly burr, easier to remove than “sharp burr” (Fig. 2.6.10b). – “Sharp burr” is whisker-like, rough and sharp-edged burr where parts of the dross are highly adherent. The underside of the cut surface is rough (Fig. 2.6.10c).

a

b

c

Fig. 2.6.10. (a) Mild steel, sheet thickness 15 mm, focal position +5 mm, (b) mild steel, sheet thickness 15 mm, focal position −1 mm, (c) stainless steel, sheet thickness 8 mm, focus position −4 mm.

2.6.9.23 Mean roughness In cutting, ripple or groove formation takes place at the cut edges which leads to roughness as a measure of the groove depth. The standard roughness Rz5 is the arithmetic mean calculated from the roughness (scallop height) of five consecutive, representative, individually measured sections. The roughness is stated in m. In Table 2.6.3, maximum values for the standard roughness are given. These values are based on cutting with a 4 kW CO2 laser. When working with thin materials, larger roughness can be expected when cutting small contours in mild steel (RSt 37-2) at lower values for the laser power, the gating frequency and cutting speed.

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Table 2.6.3. Maximum values for the standard roughness. Cutting with a 4 kW CO2 laser. Sheet thickness [mm]

Maximum standard roughness Rz5 [µm] for mild steel stainless steel aluminum

1 1.5 2 2.5 3 4 5 6 8 10 12 15 20

9 8 15 7 17 5 6 6 7 28 23 28 28

6 – 10 – 10 10 10 13 19 43 38 – –

18 13 17 14 22 20 19 14 46 – – – –

2.6.9.24 Perpendicularity and slant tolerance The perpendicularity of the cut edge is given by the value of the slant tolerance u. The perpendicularity encompasses the deviation from both straightness and flatness. The perpendicularity and slant tolerance are measured in mm in the case of perpendicular cuts or bevel cuts. The slant tolerance is the distance between two parallel straight lines between which the cutting surface profile must lie at the theoretically correct angle, i.e. at 90 degrees in the case of perpendicular cuts.

2.6.9.25 Drag lines In laser cutting, the edges of the workpiece have a characteristic grooved or rippled pattern. At minimum cutting speed, the grooves run almost parallel to the axis of the laser beam and the gas jet. As the cutting speed increases, the grooves bend away from the direction of cutting. The distance between top and bottom of the groove is called the drag line. The largest distance between two cut grooves in the direction of the cut is called the flute width.

2.6.9.26 Pitting Pitting refers to erosion of an irregular width, depth and form which interrupts an otherwise consistently smooth cut surface. The erosion can also take the form of notches from the bottom or top edge of the sheet or of blistering in cross-section.

2.6.9.27 Heat-affected zone In thermal machining with the laser beam, the material left and right of the cutting kerf and around the piercing point is subjected to a certain amount of heat. This influence of heat results Landolt-B¨ ornstein New Series VIII/1C

212

2.6.10 Machining data tables for cutting

[Ref. p. 215

in an alteration of the metallurgical structure whose scope can be determined most accurately by a microscopic analysis after a finishing grinding. The structural change affected by heat may be accompanied by a discoloration of the workpiece surface (annealing color = darkening). This discoloration can serve as an indirect, easily measurable criterion for determining the effect of heat. Discoloration as a measure of the heat-affected zone is stated in mm. One advantage of cutting with the laser beam is that minimal heat is introduced into the workpiece.

2.6.10 Machining data tables for cutting Machining data tables for oxygen cutting mild steel (QSt 37/2) (Table 2.6.4), high-pressure cutting (N2 ) stainless steel (1.4301) (Table 2.6.5) and aluminum (AlMg3) (Table 2.6.6) with CO2 laser having a maximum laser power of 5.0 kW; machine TC L 3050 (TC16), diameter raw beam 17 mm; straight cut or two-dimensional cut having a minimum value for the radius of curvature larger than the material thickness (large contour); gating frequency 10 kHz. Table 2.6.4. Cutting data: 5 kW CO2 laser (TLF5000), mild steel (QSt 37/2), oxygen cutting (O2 ). Thickness [mm] Focal length [inch] Nozzle diameter [mm] Kerf width [mm] Laser power [kW] Cutting speed [m/min] Nozzle distance [mm] Gas pressure [bar]

1 5 0.8 0.2 1.5 8.2 0.7 4.0

2 5 0.8 0.3 2.3 5.4 0.7 0.6

3 5 0.8 0.3 2.5 4.9 0.7 0.8

4 5 0.8 0.45 4.0 4.3 1.0 0.8

5 7.5 1.0 0.45 5.0 4.0 1.2 0.8

6 7.5 1.0 0.45 5.0 3.3 1.2 0.8

8 7.5 1.2 0.45 5.0 2.8 1.5 0.8

10 7.5 1.2 0.45 5.0 2.4 1.5 0.8

12 7.5 1.4 0.5 5.0 1.9 1.5 0.8

15∗ 7.5 1.4 0.55 5.0 1.55 1.5 0.6

20∗ 7.5 2.3 0.55 5.0 1.1 1.0 0.6

25∗ 7.5 2.3 0.7 5.0 0.8 1.0 0.6

∗

The sheet surface has to be free of scale and wetted by an oil film. At too high proximity of different cuts the sheet can be overheated and quality can be degraded.

Table 2.6.5. Cutting data: 5 kW CO2 laser (TLF5000), stainless steel (1.4301), high-pressure cutting (N2 ). Thickness [mm] Focal length [inch] Nozzle diameter [mm] Kerf width [mm] Laser power [kW] Cutting speed [m/min] Nozzle distance [mm] Gas pressure [bar] ∗

1 5 1.4 0.2 5.0 11.5 0.7 16

2 5 1.7 0.2 5.0 7.3 0.7 16

3 5 1.7 0.2 5.0 5.0 0.7 17

4 7.5 1.7 0.25 5.0 3.9 1.0 18

5 7.5 2.3 0.3 5.0 3.2 1.0 18

6 7.5 2.3 0.3 5.0 2.6 1.0 18

8∗ 7.5 2.3 0.3 5.0 1.7 1.0 19

10∗ 9 2.7 0.3 5.0 1.25 1.0 20

12∗ 9 2.7 0.3 5.0 0.9 1.0 20

15∗ 9 2.7 0.3 5.0 0.65 1.0 22

20∗ 9 2.7 0.4 5.0 0.4 1.0 22

Cut with adherent dross.

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 215]

2.6 Cutting: Modeling and data

213

Table 2.6.6. Cutting data: 5 kW CO2 laser (TLF5000), aluminum (AlMg3), high-pressure cutting (N2 ). Thickness [mm] Focal length [inch] Nozzle diameter [mm] Kerf width [mm] Laser power [kW] Cutting speed [m/min] Nozzle distance [mm] Gas pressure [bar] ∗

1 7.5 1.4 0.2 5.0 14.5 1.3 8

2 7.5 1.4 0.2 5.0 8.6 1.3 8

3 7.5 1.4 0.2 5.0 5.8 1.3 12

4 7.5 1.4 0.2 5.0 4.2 1.3 14

5 7.5 1.7 0.25 5.0 3.0 1.5 15

6 7.5 1.7 0.25 5.0 2.3 1.5 16

8∗ 9 1.7 0.25 5.0 1.2 1.5 16

10∗ 9 2.3 0.25 5.0 0.8 1.5 16

12∗ 9 2.3 0.25 5.0 0.5 2.0 18

Cut with adherent dross.

Machining data tables for oxygen cutting mild steel (QSt 37/2) (Table 2.6.7), high-pressure cutting stainless steel (1.4301) (Table 2.6.8) and aluminum (AlMg3) (Table 2.6.9) with CO2 laser having maximum laser power of 2.7 kW; machine TC L 2530, 3030, 4030, 6030 (TC8), diameter raw beam 15 mm; straight cut or two-dimensional cut having a minimum value for the radius of curvature larger than the material thickness (large contour); gating frequency 10 kHz. Table 2.6.7. Cutting data: 2.7 kW CO2 laser (TLF2700), mild steel (QSt 37/2), oxygen cutting (O2 ). Thickness [mm] Focal length [inch] Nozzle diameter [mm] Kerf width [mm] Laser power [kW] Cutting speed [m/min] Nozzle distance [mm] Gas pressure [bar]

1 5 0.8 0.15 1.5 8.2 0.7 4.5

1.5 5 0.8 0.15 1.3 6.4 0.7 4.0

2 5 0.8 0.15 1.2 5.0 0.7 4.5

2.5 5 0.8 0.15 1.0 4.1 0.7 4.0

3 5 0.8 0.15 1.0 3.6 0.7 2.5

4 5 0.8 0.15 1.2 2.9 1.0 3.0

5 7.5 1.0 0.2 2.7 3.0 1.0 0.6

6 7.5 1.0 0.2 2.7 2.6 1.0 0.6

8 7.5 1.4 0.3 2.7 1.9 1.5 0.6

10 7.5 1.4 0.3 2.7 1.7 1.5 0.6

12 7.5 1.4 0.3 2.7 1.2 1.5 0.6

15∗ 7.5 1.4 0.3 2.7 0.9 1.5 0.6

∗

The sheet surface has to be free of scale and wetted by an oil film. At too high proximity of different cuts the sheet can be overheated and quality can be degraded.

Table 2.6.8. Cutting data: 2.7 kW CO2 laser (TLF2700), stainless steel (1.4301), high-pressure cutting (N2 ). Thickness [mm] Focal length [inch] Nozzle diameter [mm] Kerf width [mm] Laser power [kW] Cutting speed [m/min] Nozzle distance [mm] Gas pressure [bar]

Landolt-B¨ ornstein New Series VIII/1C

1 5 1.4 0.15 2.7 9.2 0.7 12

1.5 5 1.4 0.15 2.7 7.2 0.7 14

2 5 1.4 0.15 2.7 5.6 0.7 15

2.5 5 1.4 0.15 2.7 4.1 0.7 15

3 5 1.4 0.15 2.7 3.0 0.7 16

4 5 1.7 0.15 2.7 2.4 1.0 18

5 7.5 1.7 0.2 2.7 1.9 1.0 18

6 7.5 1.7 0.2 2.7 1.4 1.0 18

214

2.6.10 Machining data tables for cutting

[Ref. p. 215

Table 2.6.9. Cutting data: 2.7 kW CO2 laser (TLF2700), aluminum (AlMg3), high-pressure cutting (N2 ). Thickness [mm] Focal length [inch] Nozzle diameter [mm] Kerf width [mm] Laser power [kW] Cutting speed [m/min] Nozzle distance [mm] Gas pressure [bar]

1 5 1.4 0.15 2.7 10.2 1.0 10

1.5 5 1.4 0.15 2.7 7.2 1.0 12

2 5 1.4 0.15 2.7 5.1 1.0 16

2.5 5 1.4 0.15 2.7 3.6 1.0 16

3 5 1.4 0.15 2.7 2.4 1.0 16

4 5 2.3 0.15 2.7 1.6 1.0 16

5 5 2.3 0.15 2.7 1.0 1.0 16

Machining data tables for high-pressure (N2 ) contour cutting stainless steel (1.4301) with CO2 having maximum laser power of 5 kW (Table 2.6.10); machine TC L 2530, 3030, 4030, 6030 (TC8), diameter raw beam 15 mm. The two-dimensional contour has a radius of curvature smaller than the material thickness (small contour). Table 2.6.10. Cutting data: 5 kW CO2 laser (TLF5000), small contour. Thickness [mm] Focal length [inch] Nozzle diameter [mm] Kerf width [mm] Laser power [kW] Gating frequency [kHz] Cutting speed [m/min] Nozzle distance [mm] Gas pressure [bar] ∗

1 5 1.4 0.15 0.5 2.0 1.1 0.7 13

2 5 1.7 0.2 0.75 2.0 1.1 0.7 14

3 5 1.7 0.2 0.9 2.0 1.0 0.7 16

4 7.5 1.7 0.25 1.3 2.0 1.0 1.0 16

5 7.5 2.3 0.25 2.0 10 1.0 1.0 16

6 7.5 2.3 0.3 3.0 10 0.9 1.0 18

8∗ 7.5 2.3 0.3 3.5 10 0.9 1.0 18

10∗ 9 2.7 0.3 4.0 10 0.8 1.0 18

12∗ 9 2.7 0.25 3.0 0.01 0.15 1.5 15

15∗ 9 2.7 0.25 3.5 0.01 0.15 1.5 10

Cut with adherent dross.

Machining data tables for high-pressure (N2 ) contour cutting stainless steel (1.4301) with 2.7 kW CO2 maximum laser power (Table 2.6.11); machine TC L 2530, 3030, 4030, 6030 (TC8). The two-dimensional contour has a radius of curvature smaller than the material thickness (small contour). Table 2.6.11. Cutting data: 2.7 kW CO2 laser (TLF2700), small contour. Thickness [mm] Focal length [inch] Nozzle diameter [mm] Kerf width [mm] Laser power [kW] Gating frequency [kHz] Cutting speed [m/min] Nozzle distance [mm] Gas pressure [bar]

1 5 1.4 0.15 0.5 2.0 1.0 0.7 12

2 5 1.4 0.15 0.65 2.0 1.0 0.7 14

3 5 1.4 0.15 0.8 2.0 0.7 0.7 14

4 5 1.7 0.2 1.2 2.0 0.7 1.0 14

5 7.5 1.7 0.2 2.7 10 1.4 1.0 14

6 7.5 1.7 0.2 2.7 10 1.0 1.0 12

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References for 2.6

215

References for 2.6 29Hyl

Hylleraas, E.A.: Z. Phys. 54 (1929) 347.

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Biot, M.A., Daughaday, H.: J. Aerospace Science 29 (1962) 228. Saltzmann, B.: J. Atmos. Science 19 (1962) 329.

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Jean, M., Pritchard, W.G.: Proc. R. Soc. Lond. A 370 (1980) 61. Olsen, F.O.: DVS-Berichte 63 (1980) 197.

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Elliot, C.M., Ockendon, J.R.: Weak and variational methods for moving boundary problems, Boston: Pitman, 1982.

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Decker, I., Ruge, J., Atzert, U.: Proc. SPIE 455 (1983) 81. Schu¨ ocker, D.: Proc. SPIE 455 (1983) 88. Steen, W.M., Kamalu, J.M.: Material Processing: Theory and Practice, IFS, Berlin, Germany, Vol. 3, 1983.

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Pritchard, W.G.: Lecture Notes in Physics, Berlin: Springer-Verlag 195 (1984) 305.

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Gennes, P.G. de: Rev. Mod. Phys. 57 (1985) 827.

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Schulz, W., Simon, G., Urbassek, H.M., Decker, I.: J. Phys. D: Appl. Phys. 20 (1986) 481. Schu¨ ocker, D.: J. Appl. Phys. B 40 (1986) 9. Yserentant, H.: Numer. Math. 49 (1986) 379.

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Schulz, W., Simon, G., Vicanek, M., Decker, I.: Proc. SPIE 801 (1987) 331. Vicanek, M., Simon, G., Urbassek, H.M., Decker, I.: J. Phys. D: Appl. Phys. 20 (1987) 140. Vicanek, M., Simon, G.: J. Phys. D: Appl. Phys. 20 (1987) 1191.

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87War

Fieret, J., Terry, M.J., Ward, B.A.: Proc. SPIE 801 (1987) 243.

88Can

Canuto, C., Hussaini, M.Y., Quateroni, A., Zang, T.A.: Spectral methods in fluid dynamics, Berlin New York: Springer-Verlag, 1988. Petring, D., Abels, P., Beyer, E.: Proc. ICALEO 88, Santa Clara (1988) 293. Petring, D., Abels, P., Beyer, E., Herziger, G.: Feinwerktechnik und Meßtechnik 96 (1988) 364. Temam, R.: Infinite-dimensional Dynamical Systems in Mechanics and Physics, Berlin New York: Springer-Verlag, 1988.

88Pet1 88Pet2 88Tem

89All 89Con

89Jol 89Jur 89Sch 89Sem 90Ber 90Hsi 90Sir 91Che 91Chr 91Ste 91Zef 91Zen 92Kov 92Mak 92Nei 93Fra1 93Fra2 93Gei

93Pow 93Sch

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Allmen, M. van: Laser Beam Interactions with Materials, Berlin New York: SpringerVerlag, 1989. Constantin, R., Foias, C., Nicolaenko, B., Temam, R.: Integral Manifolds and Inertial Manifolds for Dissipative Partial Differential Equations, New York: Springer-Verlag, 1989. Jolly, M.S.: J. Diff. Eq. 78 (1989) 220. Jurman, L.A., McCready, M.J.: Phys. Fluids A 1 (1989) 522. Schulz, W., Becker, D.: In: Bergmann, H.W. (ed.): Proc. European Scinetific Laser Workshop, Lisbon, Sprechsaal Pub. Group, Coburg (1989) 178. Semrau, H., T¨onshoff, H.K.: Proc. ICALEO 88, Santa Clara, USA (1989) 157. Berger, P., Herrmann, M., H¨ ugel, H.: In: Waidelich, W. (ed.): Proc. LASER 89, Berlin: Springer-Verlag (1990) 630. Hsieh, D.Y.: Phys. Fluids A 1 (1990) 1145. Sirovich, L., Knight, B.W., Rodriguez, J.D.: Quart. Appl. Math. 48 (1990) 535. Chen, S.L., Steen, W.M.: Proc. ICALEO 91, San Jose (1991) 221. Chryssolouris, G.: Laser Machining – Theory and Practice, Berlin New York: SpringerVerlag, 1991. Steen, W.M.: Laser Material Processing, Berlin New York: Springer-Verlag, 1991. Zefferer, H., Petring, D., Beyer, E.: DVS-Berichte 135 (1991) 210. Zenger, C.: Notes on Numerical Fluid Mechanics, Braunschweig: Vieweg 31 (1991) 241. Kovalenko, V., Romanenko, V., Chuck, N.: Proc. LAMP 92, Nagaoka (1992) 393. Makashev, N.K., Asmolov, N.S., Blinkov, V.V., Boris, A.Y., Buzykin, O.G., Burmistrov, A.V., Gryaznov, M.R., Makarov, V.A.: Sov. J. Quant. Electr. 22 (1992) 847. O’Neill, W.O., Steen, W.M.: Proc. ICALEO 92, Orlando (1992). Franke, J., Schulz, W., Petring, D., Beyer, E.: Proc. LASER 93 (1993) 562. Franke, J., Schulz, W., Herziger, G.: Welding and Cutting 45 (1993) 490 & 45 (1993) E161. Geiger, M., Hollmann, F. (eds.): Texte zum Berichtskolloquium der DFG im Rahmen des Schwerpunktprogramms Strahl - Stoff - Wechselwirkung bei der Laserstrahlbearbeitung 1991–1992, Bamberg: Meisenbach Verlag (1993) 123. Powell, J.: CO2 Laser Cutting, London: Springer-Verlag, 1993. Schulz, W., Becker, D., Franke, J., Kemmerling, R., Herziger, G.: J. Phys. D: Appl. Phys. 26 (1993) 1375. Olsen, F.O.: Proc. SPIE 2207 (1994) 402. Pritchard, W.G., Saavedra, P., Scott, L.R., Tavener, S.J.: In: Brown, R.A., Davies, S.H. (eds.): Free Boundaries in Viscous Flows, New York: Springer-Verlag (1994) 29.

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Steen, W.M., O’Neill, W.: Lasers in Engineering, 3(4) (1994) 281.

95Mie 95Nei 95Rob

Miesen, R., Boersma, B.J.: J. Fluid Mech. 301 (1995) 175. O’Neill, W.O., Steen, W.M.: J. Phys. D: Appl. Phys. 26 (1995) 12. Robinson, J.C.: Chaos 5 (1995) 330.

96Kap

Kaplan, A.F.H.: J. Appl. Phys. 79(5) (1996) 2198.

97Bee

Beersiek, J., Schulz, W., Poprawe, R., Mueller, R., Duley, W.W.: Proc. ICALEO 97, San Diego, USA, 1997. Eggers, J.: Phys. Bl. 53 (1997) 431. Eggers, J.: Rev. Mod. Phys. 69 (1997) 865. Enss, V., Kostrykin, V., Schulz, W., Zefferer, H., Petring, D.: In: Hoffmann, K.-H., J¨ager, W., Lohmann, Th., Schunk, H. (eds.): Mathematik – Schl¨ usseltechnologie f¨ ur die Zukunft, Berlin: Springer-Verlag (1997) 161. Hellrung, D., Gillner, A., Poprawe, R.: Proc. Laser 97, Munich, SPIE 3097 (1997) 267. Nemchinsky, V.A.: J. Phys. D: Appl. Phys. 30 (1997) 2566. Oron, A., Davies, S.H., Bankoff, S.G.: Rev. Mod. Phys. 69 (1997) 931. Robinson, J.C.: Lectures given at Depatamento de Ecuaciones Diferenciales y Analisis Numerico, Universida de Sevillia, http://www.maths.warwick.ac.uk/ jcr/lec.html (1997). Schulz, W., Kostrykin, V., Zefferer, H., Petring, D., Poprawe,R.: Int. J. Heat Mass Transfer 40(12) (1997) 2913. Ytrehus, T.: Multiphase Science and Technology 9 (1997) 205. Zefferer, H.: Die Dynamik des Schmelzschneidens mit Laserstrahlung, Thesis RWTH Aachen 1997, Aachen: Verlag Shaker, 1997, in German.

97Egg1 97Egg2 97Ens

97Hel 97Nem 97Oro 97Rob

97Sch 97Ytr 97Zef

98Sca 98Sep

98Sie 99Ada 99Dau 99Man 99Ros 99Sch 99Set 99Tru 00Aks 00Fri 00Pop 00Ros 00Sch

Sch¨affer, E., Wong, P.: Phys. Rev. Lett. 80 (1998) 3069. Sepold, G., Geiger, M. (Hrsg.): Texte zum Abschlußkolloquium der DFG im Rahmen des Schwerpunktprogramms Strahl – Stoff – Wechselwirkung bei der Laserstrahlbearbeitung, Bremen: IDEE & DRUCK, 1998. Siegman, A.E.: OSA TOPS 17 (1998) 184. Adalsteinsson, D., Sethian, J.A.: J. Comp. Phys. 148 (1999) 2. Klimentov, S.M., Garnov, S.V., Kononenko, T.V., Konov, V.I., Pivovarov, P.A., Dausinger, F.: Applied Physics A 69 (1999) 633. Man, H.C., Duan, J., Yue, T.M.: J. Phys. D: Appl. Phys. 32 (1999) 1469. Rose, J.W.: Heat and Mass Transfer 35 (1999) 479. Schulz, W., Kostrykin, V., Nießen, M., Michel, J., Petring, D., Kreutz, E.W., Poprawe, R.: J. Phys. D: Appl. Phys. 32 (1999) 1219. Sethian, J.A.: Level Set Methods and Fast Marching Methods, Cambridge: Cambridge University Press, 1999. Trumpf Lasercell 1005: Technical Documentation of Trumpf GmbH + Co. KG 12 (1999). Aksel, N.: Archive of Applied Mechanics 70 (2000) 81. Friedrich, R., Radons, G., Ditzinger, T., Henning, A.: Phys. Rev. Lett. 85(23) (2000) 4884. Poprawe, R.: Lasertechnik, CD-ROM zur Vorlesung Lasertechnik, Lehrstuhl f¨ ur Lasertechnik der RWTH Aachen, http://www.ilt.fhg.de, 2000. Rose, J.W.: Int. J. Heat and Mass Transfer 43 (2000) 3869. Schulz, W., Poprawe, R.: IEEE JSTQE 6(4) (2000) 696.

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Laser processing: Technical Documentation of Trumpf GmbH + Co. KG 10 (2000).

01Maz 01Pop 01Scl

Ki, H., Mohanty, P.S., Mazumder, T.: J. Phys. D: Appl. Phys. 34 (2001) 364. Poprawe, R., K¨ onig, W.: Annals of the CIRP 50(1) (2001) 137. Scholle, M., Aksel, N.: Z. angew. Math. Phys. 52 (2001) 749.

02Tru

Datensammlung TC L 3050: Technical Documentation of Trumpf GmbH + Co. KG 02, 2002.

03Nie

Niessen, M.: Numerical simulation of free-boundary problems related to thermal erosion, Thesis, RWTH Aachen, 2003, in preparation.

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Ref. p. 241]

2.7 Laser systems for materials processing

219

2.7 Laser systems for materials processing G. Sepold, M. Grupp

Since the first laser systems were installed for industrial applications in the 70’s as an exotic tool, the laser found a wide acceptance in many corners of today’s producing industry. After first applications in cutting, meanwhile more and more systems for welding, surface treatment and marking were installed. Laser applications have not to be regarded from the point of a single tool, but also from the point of laser-oriented design of the product, the necessary clamping technology in production and the product qualification aspects. Thus, laser system technology plays an important role for the economy of this new tool and the ability to integrate the source into a production chain [95Sep]. A system is the combination of different components with the purpose to fulfill a task. Complex tasks can be done by the combination of subsystems, where all subsystems can fulfill subtasks. Modern technical systems consist of modular components to achieve a high degree of flexibility and the option to change system components for new functions or new products. By new developments in system technology the reliability and productivity of production chains can be improved. The laser as a tool for materials processing needs to be adapted to the different applications by system components. The combination with the facilities and the adaptation to the application with regard to the necessity of the process makes the laser a multifunctional tool for various applications such as cutting, welding and surface treatment [97Wir]. In dependence of the application and the materials to be treated the system has to be designed. For a few applications such as the cutting of plane sheets there are standard systems available, but for many applications customer-designed solutions are necessary. Generally, a laser system has to fulfill various tasks [93Erh], which are – – – – – – –

handling, positioning and clamping of the workpiece, relative motion between workpiece and laser beam, guiding and forming of the laser beam, control functions between laser source and handling system, delivering of process media such as gases, cooling water and consumables, protection of the worker, measuring for process control.

The decision to replace an existing processing technology by laser technology or to establish the laser technology into the production of a new product is not only a decision of economy. The technological advantages such as a high degree of automation, process speed, flexibility and processing quality have a much higher weighting in the decision for the installation of a laser system. Due to the high investment cost the economic advantage of laser technology can only be achieved if the production chain e.g. the laser-oriented design, the material selection, the process technology with quality assurance is oriented on the product. The influences on the economy by the laser system technology are shown in Fig. 2.7.1. The fundamental set-up of a laser system is standardized in EN ISO 11145. It describes the laser unit consisting of the laser assembly, the handling and positioning system, and additional control and measurement devices for process control. The laser assembly contains the laser source with supplies and the beam guiding and shaping optics (Fig. 2.7.2).

Landolt-B¨ ornstein New Series VIII/1C

220

2.7.1 Laser macro systems

[Ref. p. 241

Planning/ simulation

Laser-oriented design Product

Clamping techniques

Beam source Laser system technology Beam guiding and forming Handling units

Efficiency

Sensor systems Controls

Fig. 2.7.1. Influences of laser system technology.

Laser unit Laser assembly Laser device Workpiece

Laser Mirrors, lenses, fibers,...

Telescope, focusing optics,...

Supply (power, cooling,...) Control and measurement Manipulation systems Robots, workpiece positioning

Fig. 2.7.2. Fundamental set-up of a laser system [EN ISO 11145].

2.7.1 Laser macro systems Laser systems for material processing can be divided in macro and micro systems. Macro systems with high-power lasers are used for cutting, welding and surface treatment whereas micro systems are used for fine cutting, drilling and ablation, and surface modification. In this section the components of laser macro systems will be described for CO2 -lasers as well as for Nd:YAG-lasers. Components for process control such as sensor systems will be described in Part 2, Chap. 8.

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 241]

2.7 Laser systems for materials processing

221

2.7.1.1 Laser sources In this section several laser sources are presented: the CO2 -laser, solid-state lasers, high-power diode lasers, fiber and disc lasers. The most important characteristic of all beam sources for materials processing is the beam quality. The beam quality is quantified by the beam parameter product (Sect. 2.7.1.3). The smaller the value of this product, the better is the ability to focus the beam. The beam quality is dependent on the resonator concept.

2.7.1.1.1 CO2 -laser The principal function of a CO2 -laser is described in LB VIII/1B. CO2 -lasers can be divided into flow and no-flow lasers of which the flow lasers achieve the higher output power. Due to these high power levels and the required cooling rates, the laser gas has to be exchanged continuously from the resonator to the heat exchanger by turbines, roots-blowers or cross-flow fans. The direction of this gas exchange is a criteria for another sub-division of the laser types. Lasers with a gas flow in direction of the resonator axis are called (fast-)axial-flow lasers (Fig. 2.7.3b), whereas lasers with a flow direction perpendicular to the resonator axis are called cross-flow systems. The geometry of the resonator of an axial-flow laser can be either straight or folded once or multiple to a U-shape, triangle or square (Fig. 2.7.3a) for longer resonators on a smaller footpoint [98Rof]. Flow lasers are commercially available from 500 W to 20 kW. For lower laser power up to 1 kW it is possible to cool the laser gas in the resonator only by heat conduction without gas exchange. These so-called sealed-off lasers have a closed resonator tube. The gas exchange is required just after 10.000 hours. Advantages of these laser types are the long maintenance intervals, the high thermal stability and the high beam quality. A new promising concept is the CO2 -slab laser (Fig. 2.7.4). This laser with an instable resonator is cooled by two copper plates with a small gap that contains the laser gas mixture. The same copper plates also serve as the electrodes for the radio-frequency gas discharge. Due to highly effective diffusion cooling and highly effective energy incoupling these lasers reach high output powers at a high beam quality. The rectangular resonator cross section causes a rectangular laser beam which is shaped outside of the resonator to a rotationally symmetric beam [99Emm].

a

b

Fig. 2.7.3. (a) Square-folded resonator concept (source: Trumpf Laser). (b) Fast-axial-flow 20 kW CO2 laser.

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Diffusion-cooled CO2 -Slab Laser (ROFIN DC Serie)

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4 7

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2 9 1

1 2 3 4 5

Laser beam Beam shaping unit Output mirror Cooling water RF excitation

6 7 8 9

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Fig. 2.7.4. Diffusion-cooled CO2 -slab laser with high beam quality (source: Rofin Sinar Laser).

2.7.1.1.2 Nd:YAG laser Due to the better energy absorption on metals and the higher flexibility of the Nd:YAG-laser regarding the beam delivery by optical fibers this kind of laser becomes more and more important in industrial applications. The laser light with a wavelength of 1.06 µm has the advantage of a much higher absorption, especially on aluminum and other high-reflective materials, compared with the wavelength of the CO2 -laser. In the early 80’s first pulsed Nd:YAG lasers with low average powers were available. Today continuos-wave lasers with an output power of 5 kW are installed for manufacturing processes. Pulsed lasers (Fig. 2.7.5) are generally pumped by krypton flash lamps and have an average power < 1 kW, but peak powers up to 15 kW. Application fields of pulsed lasers are spot welding, fine welding and fine cutting. Another possibility to achieve high peak power is the use of a Qswitch. A Q-switch is an optical chopper, which interrupts the emission of laser radiation from the resonator. During the interruption of the transmission a high energy is stored in the resonator which is set free during the transmission period of the Q-switch. In this way short pulses with a peak power of more than 1 MW are set free. Continuous-wave lasers for material processing can be either pumped by krypton arc lamps or as a new technology by high-power laser diodes. From a single Nd:YAG rod an output power of 500. . . 700 W can be achieved. For multi-kilowatt lasers the power scaling is done by connecting several rods in series. The electro-optical efficiency of a lamp-pumped laser is about 1. . . 3 % [90Iff]. The main losses are caused by the wide spectral range of the lamps and the very small absorption band of the Nd:YAG crystal. By replacing the arc lamps by high-power laser diodes with a wavelength of 808 nm which is in the absorption band of the Nd:YAG, the efficiency of the laser becomes three times higher. Beside the higher efficiency the beam quality of the laser increases due to the lower thermal stress of the rods [99Emm]. Average beam-parameter products (see Sect. 2.7.1.3) for lamp-pumped high-power systems are about 25 mm mrad whereas diode-pumped systems can achieve values less than 10 mm mrad. Another positive aspect of the diode-pumped lasers are the longer maintenance intervals due to the longer lifetime of the laser diodes. The expected lifetime of laser-diode pump modules is about 10.000 hours, whereas the lifetime of an arc lamp is about 1000 hours. Landolt-B¨ ornstein New Series VIII/1C

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Fig. 2.7.5. Principle of an industrial high-power diode-pumped Nd:YAG-laser.

2.7.1.1.3 High-power diode lasers One of the latest developments in high-power lasers for material processing are semiconductor lasers. In the telecommunication and electronic industry the semiconductor lasers with low power are common for data transmission or storage. By parallel bundling of multitude laser sources on small semiconductor bars the power can be scaled. For High-Power Diode Lasers (HPDL’s, Fig. 2.7.6) multitude bars are combined to stacks which can reach output powers up to 10 kW. With beam-forming micro-optics the spots of the different emitters are superposed to a single spot. HPDL’s are available with different wavelengths between 780 and 980 nm. In this wavelength range it is possible to transmit the laser beam through fiber optics [99Dor]. The main advantages of HPDL’s are the high efficiency of about 35 % and the small dimension and weight of the laser which enables the integration of the beam source directly into a handling system. Furthermore, the lifetime of the diode stacks is guaranteed by most producers to 10.000 hours, which avoids losses in production by maintenance periods. A disadvantage of the HPDL is the low beam quality which causes low power densities and spot sizes > 1 mm2 for multikilowatt lasers. For this reason most HPDL are installed in surface treatment or heat-conduction welding and soldering applications [97Sep, 98Sch].

Fig. 2.7.6. 1.5 kW HPDL with power supply (source: Rofin Sinar Laser).

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2.7.1.1.4 Fiber lasers and thin disc lasers Fiber lasers and thin disc lasers belong to the group of diode-pumped solid-state lasers [99Gie, 98Toe2]. These laser sources have the advantage of a higher beam quality compared to solid-state rod lasers, higher wall plug efficiency and smaller footprint. Introduced in the middle of the 1990’s with low laser power, these sources made fast development steps and have now reached output power levels for materials processing. Due to their highly efficient cooling of the laser medium, thin disc lasers achieve an excellent beam quality up to a laser power of more than 1 kW. By combining several discs, an output power of up to 4 kW with a beam quality of < 7 mm × mrad was achieved in 2003. Fiber lasers for materials processing were introduced in 2001 with an output power of less than 1 kW. Within a time period of only 2 years a scaling up of the power to 10 kW was possible and first systems were installed in the industry for cutting and welding applications [03Shi].

2.7.1.2 Laser beam guiding For the transmission of the laser beam from the beam source to the focusing head or handling system beam-guiding systems are used. In stand-alone systems the laser beam is connected between beam source and the machining system whereas a multiple-user system consists of one beam source and several handling systems. For the layout of the system various points have to be considered: – – – – –

wavelength of the laser source, kind of handling system, number of handling systems at multi-user applications, distance between laser source and focusing head, working area of the handling system.

Beam-guiding optical elements can be divided into two groups by the optical characteristics. Reflective optical elements such as Cu-mirrors are usually used for CO2 -lasers whereas transmissive and refractive elements like lenses or optical fibers are more common for Nd:YAG lasers. In the following the beam-guiding elements are divided into two groups depending on the wavelength of the source.

2.7.1.2.1 Beam-guiding systems for CO2 -lasers For simple laser processing systems with one laser source and one linear-axis handling system the beam-guiding system may consist of a small number of fixed planar Cu-mirrors for 90 ◦ -bending of the beam. The number of mirrors is depending on the number of axis and the positioning of the beam source towards the handling system. The Cu-mirrors are used for their high reflectivity at the wavelength of 10.6 µm and their high heat conduction. Even if the reflectivity of Cu-mirrors is higher than 99 %, for high-laser-power applications the mirrors have to be cooled. For minimizing the loss, the number of mirrors in a beam-guiding system should be reduced. For 3-dimensional non-linear-axis systems like robots, flexible or articulated beam-guiding arms can be applied without misadjustment of the beam. Two 90 ◦ -bending mirrors on one axis allow a rotation around the optical axis, whereby the laser beam can be deflected in 360 ◦ (Fig. 2.7.7). Complex systems for multi-user applications, where one laser source can provide more than one machining system, make the laser more cost efficient due to a higher utilization of the beam source. The beam can be sequentially switched between the handling systems by beam deflectors or split into several beams by beam splitters for parallel processing. In these beam-guiding systems with a high degree of flexibility the mirrors are manually moved or motor driven into fixed positions Landolt-B¨ ornstein New Series VIII/1C

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m r bea Lase

aa

bb

Fig. 2.7.7. CO2 -beam guiding with a flexible arm for 3D-processing (source: Rofin Sinar Laser/Arnold).

Fig. 2.7.8. x-y-scanning system for high-speed beam deflecting (source: GSI-Lumonics).

for deflection of the beam. The positioning of the deflectors can be either performed by a linear motion or by a mirror rotation of which the linear motion causes a lower expense for the beam adjustment. Very flexible beam guiding is possible by galvanometer scanners (Fig. 2.7.8). These scanners enable 1- or 2-dimensional motion of the laser beam with high speed and accuracy. The disadvantage of a scanner is the low power load of the mirrors due to insufficient cooling of the mirrors by gas streams.

2.7.1.2.2 Beam guiding for Nd:YAG-lasers Due to the excellent transmission of optical glasses like quartz glass for the Nd:YAG wavelength, most beam-guiding elements are based on refractive optics. The most common beam-guiding element is the optical fiber. The main advantages of fiber optics are the high flexibility for 3dimensional motion and the low adjustment efforts for guiding the laser beam to the workpiece. Dependent on the beam quality and the output power of the laser, fibers with a diameter of 0.1. . . 1 mm are used to transmit the laser light over wide distances (> 50 m) nearly loss-free (Fig. 2.7.9). There are two kinds of fibers: the step-index fiber and the gradient-index fiber, whereby the stepindex type is the most common in high-power laser applications. Step-index fibers consist of a quartz-glass core and a glass coating with a lower refractive index. In the interface between the core and the coating the beam is reflected and guided through the fiber. Gradient index fibers have a graded refractive index from the center to the fiber surface which prevents the emission of light through the cover.

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Collimating unit

Incoupling unit

CCD

a Beam bending cube

Fiber

Laser beam

Focusing optics f1

f2

f3

Fig. 2.7.9. Fiber coupling for Nd:YAG-lasers (source: Rofin Sinar Laser).

Fig. 2.7.10. Triple beam switch for Nd:YAGlasers.

For smaller lasers there are also beam-guiding systems with transmissive and reflective beamguiding elements. Here, the laser beam is guided directly from the beam source into the focusing head where the beam is bent by a 45 ◦ -coated mirror into the optical axis of the focusing optics. These mirrors are transparent for visible light and allow a coaxial process control and workpiece adjustment via CCD-cameras. High-power Nd:YAG lasers with fiber connections often have a beam switch to guide the beam to several fiber incoupling plugs (Fig. 2.7.10). These switches work, similar like for CO2 -lasers, by moving mirrors. For high numbers of fiber incoupling units so-called multiplexers with the rotating-mirror principle are used to switch the beam to the selected fiber. With short switching times (< 50 ms) auxiliary process times are minimized. By replacing these full-reflective mirrors for beam bending by partially reflective ones, beam splitters with defined division rates can be realized. With these beam splitters simultaneous operations can be carried out and pre- or postheating processes become possible.

2.7.1.3 Beam-forming elements Beam-forming elements for transforming the propagation parameters like the beam diameter or the divergence angle can be as well installed at the beginning of the optical path as at its end to focus the beam onto the workpiece. A very important unit for beam forming is the telescope, especially for long distances between beam source and handling system or working head. Due to the divergent propagation of the laser beam the beam diameter changes over the distance from the source. However, for constant processing results the beam diameter on the focusing optics should not change within a certain range. The beam parameter product, which is the product of the half-beam-waist diameter w and the half divergence angle Θ, is constant: Landolt-B¨ ornstein New Series VIII/1C

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Beam diameter [mm]

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8 6 4 0

working range of the laser system

2000 4000 6000 8000 10000 12000 Beam path length [mm]

Fig. 2.7.11. Shift of the beam waist in the working range of the handling system.

f1 <0

a

f2 >0

b

Fig. 2.7.12. (a) Mirror-telescope (source: BIAS). (b) Lens telescope.

(Θ · w0 ) (Θ · w) = = const. 4 4

(2.7.1)

By changing the beam diameter with optical elements, the divergence angle and the beam waist can be adapted to the necessities of the laser system. Increasing the beam diameter allows the decreasing of the divergence angle by the same factor. In laser systems with a variable beam path length, like in “flying optics” systems (see Sect. 2.7.1.4) where the beam path length changes during the motion of the machine axis, telescopes are used to shift the beam waist into the working range of the machine to keep the beam diameter changes within a minimum. An example for the propagation of a laser beam with a telescope is given in Fig. 2.7.11. Telescopes can be realized as well with lenses as with mirror optics. An example for both kinds is given in Fig. 2.7.12.

2.7.1.3.1 Focusing optics The focusing optics as the last beam-forming element before the process have a main influence on the processing result. The working head of a laser system contains beside the optical elements for beam shaping also units for process and shielding gas supply, consumable delivery, lens or mirror protection elements and sensors for process monitoring. Dependent on the process, the design of the working head has to be performed. Landolt-B¨ ornstein New Series VIII/1C

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For the focusing of high-power CO2 -lasers spherical or parabolic Cu-mirrors are used, which are produced by high-precision milling. Parabolic off-axis mirrors have the advantage of no spherical aberration, which means that also beams far from the optical axis will be bundled in the focus (see LB VIII/1A). Lens optics for CO2 -lasers are only used for low-power applications and cutting. For Nd:YAG lasers lenses of quartz glass are used for nearly all applications. Due to the much higher price coated mirrors for Nd:YAG-lasers are only used for special applications, where lenses cannot be used.

2.7.1.3.1.1 Laser cutting heads In laser cutting heads, the focusing lens has the task to focus the beam onto the workpiece surface and to separate the beam path from the pressure-filled part of the working head. In a cutting head a gas pressure up to 20 bar for melt-cutting applications is expanded coaxially to the laser beam through a nozzle which is positioned close to the surface of the material. During the expansion, the pressure is transformed to a high-velocity gas stream which is used to blow out the melt from the cutting kerf. The design of the nozzle should ensure a long laminar flow of the gas stream for a deep penetration of the melt. Due to the high gas pressure in the working head for cutting applications thick lenses or protective windows are used. The gas pressure is dependent on the process. For fusion-cutting application with inert gases higher pressures are used than for flame cutting with oxygen. Figure 2.7.13 shows two examples for laser cutting heads: for CO2 and Nd:YAG-lasers.

a

b

Fig. 2.7.13. (a) Cutting head for CO2 -lasers (source: Prectitec). (b) Cutting head for Nd:YAG-lasers (source: HIGHYAG).

2.7.1.3.1.2 Welding heads Welding heads for CO2 - and Nd:YAG-lasers differ from cutting heads by other process supplies, such as shielding gas, additional material feed and cross-jet nozzles (Fig. 2.7.14). A gas stream coaxial to the laser beam protects the melt and the seam surface from oxidation and avoids at Landolt-B¨ ornstein New Series VIII/1C

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Plan mirror

Adjustment device Cross jet

a

b

Fig. 2.7.14. (a) Welding head for CO2 -lasers (source: Precitec). (b) Welding head for Nd:YAG-lasers with wire feed and clamping tool (source: HIGHYAG).

the same time the entry of fumes into the focusing head. For a higher efficiency of the melt pool protection, additional protection gas is often supplied by nozzles which can be directed in or towards the moving direction. When using additional material, the shielding gas can also be coaxially added to the material to reduce the amount of nozzles. For many welding applications, especially for aluminum, additional material is added to alloy the substrate material or to bridge welding gaps. The supply of additional material, e.g. wire, is done by nozzles which are directed to the melt. Cross-jet nozzles with a gas stream perpendicular to the beam axis are used to avoid damages of the optics. Especially in the welding of aluminum, process-induced fumes and spatters from the melt directed to the optics can cause serious damages. Even the destruction of a protection glass can cause high expenses due to the loss of production. A new technology in laser welding is the hybrid welding process. Conventional MIG (Metal Inert-Gas), TIG (Tungsten Inert-Gas) or plasma welding processes are combined with laser welding. By this process, the laser power can be reduced dramatically to achieve the same welding depth and speed. Thus the investment costs of a welding system will be reduced. Another advantage of the process combination is the possibility to bridge larger gaps, which reduces the costs for seam preparation. Hybrid welding heads can be created either by a simple combination of a welding torch and a conventional laser welding head or by a compact all-in-one hybrid head with a positioning device between the laser focus and the electric-arc interaction zone.

2.7.1.3.1.3 Working heads for surface treatment Working heads for surface treatment contain special optics for beam forming. Most surfacetreatment applications like hardening or cladding demand a homogeneous laser power density distribution in a line or area to produce a constant heat-affected zone in a material. Lines with constant intensity can be either created by cylindrical lenses or mirrors or by scanning mirrors. Using scanning mirrors, the length of the line can be adjusted by the amplitude of oscillating movement. A homogeneous density distribution in an area can be either created by scanning mirrors, diffractive elements, beam integrators or projection lenses. Scanner optics have the advantage of a variable spot geometry but the disadvantage of high required scanning frequencies to produce a

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homogeneous line. Due to the insufficient power load of scanner mirrors the laser beam has to be expanded which restricts the scanning frequency by using large mirror with high inertia. Beam integrators which produce a homogeneous density distribution are used for CO2 -laser beams. The beam is multiply reflected at the walls of a rectangular mirror tube to integrate the density distribution. Due to the multiple reflection the absorption loss of beam integrators is very high.

2.7.1.4 Handling devices For the numerous different laser applications different system concepts for handling devices have to be regarded. The choice of a system depends on the application, the workpiece geometry, the laser source, the required path accuracy and the number of products. Only for a few applications with low demands on workpiece geometry standard systems are available, but for most applications the system has to be designed or adapted to the needs of the product and the process. Today’s laser-machining producers have a modular product mix for customer-designed solutions. Starting from a basic system the working range or the number of controlled axis can be adapted to fulfill the tasks of the process. In high-automated production chains, secondary handling systems can be added for workpiece loading and removing. Additional quality-assurance equipment and clamping devices complete the handling systems. The control of all functions of the axis is done by Numeric Control (NC). The control has to fulfill also additional laser-specific tasks like power control or closed-loop control for process-control equipment.

2.7.1.4.1 System concepts For the selection of a handling device several criteria have to be considered: – 1-, 2- or 3-dimensional processing, – linear axis or robot systems, – moved workpiece or moved optics. Dependent on the workpiece geometry and the processing direction the basic system concept has to be chosen. The combination of controlled axis for workpiece or working-head motion leads to numerous system concepts which will be described in the following.

2.7.1.4.1.1 1-dimensional systems 1-dimensional systems are introduced for special applications in laser processing and will be mentioned here just for the sake of completeness. These systems work as well with CO2 -lasers as with Nd:YAG-lasers. Examples for them are tube welding systems with a rotation axis and a fixed optical system for linear welding or cutting systems (Fig. 2.7.15). In general, the positioning is manually done or by an additional axis with a small working range. These additional axis are also used e.g. for seam tracking for process control and quality assurance.

2.7.1.4.1.2 2-dimensional systems 2-dimensional systems are the most common systems in laser materials processing. They are installed for flat-sheet-metal processing, especially for cutting and welding operations. Treated materials are all kinds of steels, non-ferrous metals, but also synthetic materials. Due to the multiple Landolt-B¨ ornstein New Series VIII/1C

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Fig. 2.7.15. Principles of 1D-processing systems.

2D - Systems

a

b

c

d

e

Fig. 2.7.16. Principles of 2D-processing systems: (a) moved workpiece, (b) moved workpiece, moved optics, (c) moved optics, (d) moved optics, moved laser, (e) moved workpiece.

combinations of the relative motion between workpiece and optics, numerous concepts have been performed (Fig. 2.7.16). Variant (a) with two linear axis for workpiece motion is common for small workpiece dimensions. The fixed-optic system ensures a high stability against maladjustment of the laser beam in the beam-guiding system for a good reproducibility of the process. For larger workpiece dimensions this concept is unsuitable due to the necessity of a wide extension arm for the optics. Variant (b) and (c) are the most common concepts for medium and large cutting systems. The optics are in motion in one or two dimensions which led to the name of the “flying optics”. This concept has the advantage that large and heavy workpieces are stationary or moved only in one dimension while the main motion is done by the optics with low inertia (Fig. 2.7.17). The principle of the flying optics demands high requirements to the optical system. Due to the changing distance between the beam source and the focusing optics the laser beam needs to be kept at a constant beam diameter on the focusing lens or mirror for a constant focus distance. This adaptation is performed by telescopic units, which expand the beam and shift the beam waist into the working range of the system (see Sect. 2.7.1.3). With fixed telescopes the beam diameter is shifted to the middle of the working range to keep the diameter changes during the motion in a tolerable range. Active or adaptive telescopes change the degree of expansion due to signals from the system control of the current axis position to keep the beam diameter on the focusing optics constant. Variant (d) can be found on very large cutting systems (Fig. 2.7.18). The beam source is mounted on one moving axis and the change of the beam path length is only dependent on the length of the second axis. This concept has lower demands on the beam-guiding systems, but higher ones on the mechanic drives. Due to the high weight of the laser source and the consequential mechanic load, the system has to be equipped with strong drives. Systems with moved beam sources are suitable for applications with poor requirements on the processing speed. High speeds

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Fig. 2.7.17. Laser cutting system with flying optics (source: BIAS).

Fig. 2.7.18. Large cutting system with flying optics and moved laser source (source: ESAB).

or accelerations would have negative influences on the optical stability of the beam source. Systems like this one can be found with working ranges in the direction of the laser movement up to 30 m. Variant (e) shows a handling unit for the processing of rotationally symmetrical workpieces. A controlled turn-table system mounted on a linear axis enables 2-dimensional processing of tubes. All listed concepts are normally equipped with an additional third controlled axis for the positioning of the working head to the surface. In cutting systems, working heads with capacitive distance sensors keep the distance between the workpiece and the cutting nozzle constant. This can be either done by a closed-loop controlled action of the machine’s z-axis or by an additional short positioning axis. In high-speed cutting systems additional axis with short positioning ranges and a low mass are common due to their low inertia and the short reaction time. Basically, all shown system concepts are also suitable for Nd:YAG-laser systems, and, due to the beam guiding via fiber optics and the low-weight working heads, Nd:YAG-laser systems even have lower requirements to the mechanical and optical set-up.

2.7.1.4.1.3 3-dimensional systems Similar to the 2-dimensional laser systems, 3-dimensional systems can be designed by numerous combinations of the relative motion between workpiece and working head. Due to this high amount

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b

axis 4

axis 3

axis 6

axis 5

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axis 1

c

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Fig. 2.7.19. (a) 5-axis gantry system (source: Prima Industrie). (b) 5-axis column system (source: Trumpf Lasertechnik). (c) Working range of a 6-axis robot system (source: KUKA Roboter). (d) Robot system with flexible beam guiding for CO2 -laser (source: KUKA Roboter).

of combination possibilities only a few important and most common ones will be mentioned in the following. With an extension of a 2-dimensional system by one additional axis or turn-table unit simple 3D-applications can be created. For complex geometries and applications like cutting and welding of deep-drawn or hydro-formed parts, especially in car body manufacturing, there are higher demands on the motion units. Also in surface treatment for rejection cladding in tool manufacturing a free motion and positioning of the working head is necessary to achieve a constant processing quality by a rectangular beam incidence onto the workpiece surface. For Cartesian systems with 3 linear axis, two axis of rotation have to be added to enable full 3-dimension motions in the working area (Fig. 2.7.19b). These rotations can either be done by the working head or the workpiece. Figure 2.7.19a shows a 5-axis gantry system with two axis of rotation for the working head with a free accessible working range for workpiece positioning. Similar to 2-dimensional cutting systems the motion in the main axis can either be done by a moving table or by the motion of the gantry. The motion of the gantry has the advantage of a free approachable working area, for example for autonomous workpiece delivering systems, but the disadvantage of the motion of the heavy gantry with the need of a synchronized actuation on both sides. A low-cost alternative to linear-axis systems with lower demands on path accuracy are nonCartesian robot systems (Fig. 2.7.19c). They can handle the workpiece as well as the working head. For handling the working head of an Nd:YAG-laser the demands on the beam guiding are low due to the flexible fiber delivery. For CO2 -lasers articulated beam-guiding arms (see Sect. 2.7.1.2.1) can Landolt-B¨ ornstein New Series VIII/1C

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bend the beam to follow the motions of the robot (Fig. 2.7.19d). But, due to inflexible tubes of the beam-guiding system the working range of the system will be restricted. A very specialized concept is a robot with internal beam-guiding system. Internal beam-bending mirrors follow the movement of the robot articulation and guide the laser beam through the robot body. The disadvantage of the system is the necessity of beam-bending mirrors in each axis, which cause a loss of laser power. Robot systems have, in comparison to portal systems, an obviously smaller working range. An increase of the working range by longer arms comes along with a decrease of the accuracy. A possibility to increase the working range in one dimension is to combine the robot with a linear axis for a transversal motion of the system. Such robot systems are mainly installed in the automotive industry. For rapid-prototyping and rapid-tooling applications compact systems for laser processing are available. For the Selective-Laser-Sintering (SLS) process combined systems with high-speed beam deflecting by scanner optics and a linear axis are common. After a thin layer of metal or polymer powder is sintered by beam deflection via scanner optics, the powder bed will be lowered to process the next layer. In this way 3D-shapes with undercut can be built up.

2.7.1.4.2 New developments New developments for handling systems enable high-dynamic processes with high accuracy. A new promising technique with meanwhile a few installed systems in the industry are the parallel or hybrid kinematic systems [98Toe1, 99Toe]. The structure of these systems with three (“tricept”) or six (“hexapods”) linear actuators enables 3-dimensional movements with high dynamics, low masses and high rigidity. Additional rotation axis for the working head complete the system for 3-dimensional laser processing. The disadvantage of this system is the reduced working range due to the fix suspension of the actuators. With additional translation axis for the robot the working range can be increased in one dimension. Figure 2.7.20a shows a tricept robot system with beam

a

b

Fig. 2.7.20. (a) Tricept robot with articulated-arm beam guiding for CO2 -lasers (source: LMTB Berlin). (b) Hybrid kinematic system with Nd:YAG working head (source: IFW Uni Hannover).

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Fig. 2.7.21. Principle of remote welding systems (source: Trumpf Lasertechnik).

guiding via articulated arm and two additional rotation axis for free 3D-motions and Fig. 2.7.20b a hybrid kinematic system for Nd:YAG-laser processing. A new welding technology, enabled by the development of high-power beam sources with high beam quality, are the so-called remote welding systems. The laser beam is focused by a long focal length of f ≥ 1000 mm onto the workpiece and is deflected by an x-y-scanning system (Fig. 2.7.21). The high beam quality enables the focusing of the laser beam into a spot which is sufficient to achieve the necessary power density for deep-penetration welding [00Goe, 01Klo, 03Gru]. This system enables highly flexible processing with short positioning times. A disadvantage of the long distance between the working head (scanner system) and the workpiece is the difficulty to feed the work area with additional material or process gas, which is normally integrated into the working head. The requirement of shielding gas can be covered by integrating the shielding-gas supply in the clamping devices.

2.7.1.4.3 Special systems The more the laser is getting established in various industries the more special systems and applications are developed. The combination of different processing technologies to increase productivity and efficiency of a process is in the foreground of the developments. One of the former-days combined laser systems was the laser cutting and punching system. While the laser is cutting large or changing geometries the punching unit can produce small standard geometries like holes with high efficiency (Fig. 2.7.22). New developments in laser technology enable the integration of the laser into machining centers to assist conventional applications. Laser-assisted turning or milling with Nd:YAG-laser or HPDL reduce the cutting forces and increase the cutting velocity. By the integration of an HPDL in a milling center the production chains can be shortened by a sequential processing of milling and laser hardening [97Bra]. Moreover, for deep-drawing processes, the laser can be used for local or selective heating to change the material’s characteristics to decrease the

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Fig. 2.7.22. Combined laser cutting and punching system (source: Trumpf).

stress and increase the drawing ratio. With increase of the drawing ratio the drawing forces can be reduced [97Sch].

2.7.1.4.4 Actuation and control of laser systems The choice of the actuation of all laser systems is dependent on the requirements of path accuracy and path feed rate. The actuation of the axis together with the path-measurement system determines the accuracy of the motion. Standard systems are equipped with linear drives with recirculating ball screws. With these systems path feed rates of v = 30 m/min and an accuracy of 0.01 mm can be achieved. The accuracy is limited by the slackness of the actuation screw and gear box. For high-dynamic systems with path feed rates up to v = 300 m/min direct linear drives with optical contactless path-measurement systems are used. Direct drives enable acceleration rates up to 20 m/s2 and a high accuracy due to the redundancy of gear boxes. The numeric system control has, beside the standard functions of machine tools, laser-specific functions. They can be for example the correction of the position by signals of a seam-tracking sensor during the processing of a program step or the contour-related laser power control. In laser cutting and welding systems, this function enables a constant line energy, that means by lowering the speed, e.g. in contour edges, the laser power will also be reduced. The programming of the path in Cartesian systems is done by standardized codes, e.g. the NCcode DIN 66025. The programming can either be manually performed or offline by CAD/CAMsystems, which generate the program code directly from the construction data. For the programming of 3D-motions, especially for non-Cartesian systems, there is also the possibility of “teaching” the system by a manual motion of the working head to certain points on the path and the storage of the position. The control system calculates then the path between the points by line, circular or spline interpolation.

2.7.1.4.5 Clamping devices The clamping technique for laser material processing, especially for welding applications, plays a very important role in the system technology. Due to the postulated small tolerances and the concentrated energy input into a small spot, gap-free workpiece positioning for butt and overlap welding have to be ensured by a special clamping technology. Beside the conventional clamping technology known from the conventional welding processes, special laser-oriented flexible technologies are developed. In car-body manufacturing with robot systems different pressure-clamping devices with moved or fixed tools are applied (Fig. 2.7.23) [97Han]. Moved roller devices enable

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Movable double clamping tool

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Fig. 2.7.23. Flexible clamping tools for 3D-processing (source: Volvo).

continuous seams by a movement of a pressure roller close to the joining position. For spot or stitch welding, fix devices which are shaped in dependence of the stitch length enable a gap-free joint. For each non-linear or 3-dimensional application the clamping tools have to be adapted to the workpiece geometry. For this reason the development of clamping techniques should be carried out simultaneously to the product design to achieve optimum functionality.

2.7.2 Laser microtechnology Laser microtechnology is a combination of laser-assisted technological processes for precise treatment, modification and synthesis of materials in the domain of micrometer sizes. This specific area of laser technology has found its most effective application in precision microprocessing of small passive and active elements and components of microelectronics, optics, optoelectronics and micromechanics. In many cases its advantage results from the relatively low cost of the technological equipment and from the high efficiency and unique capabilities of the laser-assisted technological methods. Laser microtechnology is applicable to various areas of the device-building industry, but its specific capabilities are best demonstrated when high precision, spatial resolution and reliability are required [98Met]. Typical areas of laser microtechnology are micromachining of thin films, microprocessing and modification of materials, laser micropatterning and laser-assisted synthesis of polycomponent thinfilm systems. Basically, the system technology consists of the same components as in macrotechnology – beam source, beam guiding, beam shaping and workpiece handling – but, due to the smaller working ranges, with much higher accuracy and smaller focal sizes. The role of the technical equipment in laser microtechnology is to perform a controllable action of the laser radiation on the material to be treated. It initiates a concrete technological process in areas of precisely controllable shape and size. The laser is the main unit of equipment and its characteristics determine, to great extent, the qualitative and quantitative parameters of the technological treatment.

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2.7.2.1 Beam sources For the various micro applications with multiple demands to the beam quality and wavelength different laser types are available. The laser source with a certain wavelength has to be chosen by the properties of the material to be treated. Further characteristics of the laser source with influence on the processing result are the beam quality, laser power, pulse duration and repetition rate. In Table 2.7.1 the beam sources for different micro applications are listed. The applications are divided into the three main groups of thin-film machining, micro welding/shaping and micro lithography/patterning. Table 2.7.1. Beam sources for laser micro machining. Application

Laser source

Wavelength

Thin-film machining

Nd:YAG-laser (cw/pulsed/cw-Q-switch) 1064 nm/532 nm/355 nm/266 nm 10600 nm CO2 -laser various metal vapor laser (e.g. Cu)

Micro welding/ micro shaping

Nd:YAG-laser Nd:glass-laser CO2 -laser

1064 nm/532 nm/355 nm/266 nm 1064 nm 10600 nm

Micro lithography/ micro patterning

Nd:glass Excimer Ar+ Kr+

1064 nm 150. . . 350 nm 351.1. . . 528.7 nm 337.4. . . 752.5 nm

For many applications, except the lithography, the Nd:YAG-laser is the most often used laser due to its high stability and reliability. The wavelength of 1.06 µm is sufficiently absorbed by most metals. Furthermore, the short response time of the Q-switched Nd:YAG-laser enables this type for high-speed applications. cw-Nd-YAG-lasers are common for high-speed beam-scanning applications with feed rates > 10 m/s.

2.7.2.2 Beam-guiding and -forming techniques Compared to macro systems, micro systems do not need long beam paths, but, also on short distances, the same beam-forming and deflecting elements. Basically, the optical arrangements for the different processing techniques are divided into three groups: – beam contour (focusing), – projection, – contour projection. At the beam-contour technique the image or processing contour is generated by a relative motion of a laser beam focused onto the workpiece. The motion can either be done by the laser beam or the workpiece. The workpiece motion by micro CNC-tables achieves high accuracy on small working ranges. The positioning accuracy of these tables with closed-loop-controlled drives is about 2.5 µm in a 7.5 × 7.5 cm2 working range. Due to the relatively high inertia of the tables the speed and acceleration is limited. For higher speed and acceleration, scanning mirrors are used when the mass of the optical system is lower than that of an NC-table with sample. The accuracy

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Amplifier 2 Amplifier 1 Oscillator Telescope

x-y-coordinate table Fig. 2.7.24. Laser micro system for micro drilling (source: BIAS).

of this electro-magnetical galvanometer scanner is about 62.5 µm over a 1.25 × 1.25 cm2 field. The accuracy and speed is dependent on the mirror size and the corresponding inertia and thus on the diameter of the laser beam to be deflected. For applications like micro drilling on a large area, a combination of a scanning unit and a linear drive is used. With the high-speed beam deflection of the scanner unit a small area can be processed and the linear drive positions allow the workpiece to be processed in a following area [99Koc]. Figure 2.7.24 shows the set-up of such a combined drilling system. The projection technique is used for treating a workpiece surface with a shape produced by a mask. The laser light with high power illuminates a mask and reproduces the image via a projection optics onto the sample. The advantage of this technique is to treat a complex shape, larger than the laser focus, with high resolution simultaneously without any motion devices. The size of the image can be varied by the projection system without changing the mask size. The main requirement in this process is a uniform illumination of the mask. But, this leads to the disadvantage of a high energy loss at the non-transparent areas of the mask. This disadvantage is reduced by the technique of the contour projection. In the contour-projection technique, a combination of the projection and the beam-contour technique, simple contours like squares or circles are projected onto the surface. By a motion of the sample, more complex shapes can be produced by addition of the simple contours. The advantage of this process is a uniform intensity distribution in the interaction zone, compared to the Gaussian distribution in the simple beam-contour technique.

2.7.3 Conclusions and outlook The laser system technology with all its sub-technologies is a growing industry. The world market of laser systems has increased until 2002 to about 4 billion EUR (1998: 2.5 billion EUR). The greatest part of the installed systems are standard 2-dimensional cutting and welding systems. They are installed in the sheet-fabricating industry and, to a wide part, in the automotive and electronic industries. With an increase of the degree of automation the laser processing technology will be more and more applied, due to its high potential of automation. New developments in laser system technology replace conventional methods and develop new applications by laser processes with economical and technological benefits. But only by a consequent laser-oriented design of

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products the economical benefits can be achieved notwithstanding most lower investment cost of conventional technologies. By the development of new beam sources with high beam quality new applications can be enabled. Further higher beam qualities allow longer working distances between the optical elements and the interaction zone which leads to a higher security of the optical elements and better accessibility to the process. Due to the higher power density in the focus, high-speed processes for cutting and welding can be realized even with lower laser power. With increasing beam quality of HPDL, processes with a high efficiency can be achieved. The compact size of HPDL also allows the integration of this kind of laser into existing machining centers for e.g. “laser-assisted processes”. With these new and improved beam sources the application field of laser processing will be enlarged to other industrial branches. To use all advantages of the high-precision tool laser, also the other parts of the system technology beside the beam sources have to fulfill the demands of accuracy and repeatability. First of all the handling devices which guide the laser beam over the workpiece have to fulfill the demands of speed and path accuracy. As the laser is qualified for high-speed processing in cutting and welding, the handling systems have to be able to act and react in short times with a high accuracy. The control of the working result after and during the process by online process monitoring increases production quality and supports the acceptability of laser system technology in the industry. The techniques for process monitoring will be described in Part 2, Chap. 8.

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241

References for 2.7 90Iff

Iffl¨ander, R.: Festk¨orperlaser in der Materialbearbeitung, Berlin Heidelberg New York: Springer-Verlag, 1990.

92Sep

Sepold, G., Heidenreich, B., Binroth, C.: First Laser Cutter for Shipbuilding Installed, Industrial Laser Review, Sturbridge, USA, 1992.

93Erh

Erhard, K.-M., Heine, A., Prommersberger, H.: Laser in der Materialbearbeitung, W¨ urzburg: Vogel Buchverlag, 1993. Herziger, G., Loosen, P.: In: Herziger, G., Loosen, P. (eds.): Werkstoffbearbeitung mit Laserstrahlung, M¨ unchen Wien: Carl Hanser Verlag, 1993.

93Her

95ISO 95Sep

EN ISO 11145: Laser und Laseranlagen, 1995. Sepold, G., Schubert, E., Engler, I.: Lasersystemtechnik – Quo vadis. In: Proc. of ”Mechanik und Optik” 25–27.04.1995, ISL (1995) 349.

97Arn

Arnold, T., Gnann, R.: Machine Concepts for Laser Processing Facilities. In: Sepold, G., J¨ uptner, W. (eds.): Strahltechnik Bd. 5, Bremen: BIAS Verlag, 1997. Brandner, M., Sigel, J.: Proc. Innovation durch Technik und Organisation FTK ’97. Gesellschaft f¨ ur Fertigungstechnik, Berlin: Springer-Verlag (1997) 476. Geiger, M., Otto, A., Fleckenstein, M., Hoffmann, P.: Product and Process Innovations by Intelligent System Technology. In: Sepold, G., J¨ uptner, W. (eds.): Strahltechnik Bd. 5, Bremen: BIAS Verlag, 1997. Hanicke, L., Johannson, G.: Laser Processing at Volvo. In: Sepold, G., J¨ uptner, W. (eds.): Strahltechnik Bd. 5, Bremen: BIAS Verlag, 1997. Hofmann, A., Vollertsen, F.: Proc. LANE’97 (1997) 719. Rippl, P.: Manufacturing Systems for Laser Welding Using Robots for the Body-inWhite. In: Sepold, G., J¨ uptner, W. (eds.): Strahltechnik Bd. 5, Bremen: BIAS Verlag, 1997. Ripper, G.: ILS-Quinta-N-System for 3-d-Materials Processing with Solid State Laser. In: Sepold, G., J¨ uptner, W. (eds.): Strahltechnik Bd. 5, Bremen: BIAS Verlag, 1997. Schu¨ ocker, D., Schr¨ oder, K.: Proc. LANE’97 (1997) 713. Sepold, G., Schubert, E., Franz, T., Seefeld, T.: Processing with a 1.4 kW Diode Laser, Industrial Laser Review, October 1997. St¨ umke, A., Bayerlein, H., Eckl, F.: Laser Applications at Audi. In: Sepold, G., J¨ uptner, W. (eds.): Strahltechnik Bd. 5, Bremen: BIAS Verlag, 1997. Wirth, P., Emmelmann, C.: Laserschweißen im Automobilbau – Aktueller Status und zuk¨ unftige Weiterentwicklungen. In: Strahltechnik Bd. 10, Bremen: BIAS Verlag, 1997.

97Bra 97Gei

97Han 97Hof 97Rip1

97Rip2 97Sch 97Sep 97Stu 97Wir

98Met 98Rof 98Sch 98Toe1 98Toe2

Metev, S.M., Veiko, V.P.: Laser-assisted Microtechnology, Springer Series in Materials Science, 2nd Edition, Berlin Heidelberg New York: Springer-Verlag, 1998. Information Booklet of Rofins Sinar Laser GmbH: Introduction in Industrial Laser Materials Processing, Hamburg, 1998. Schubert, E., Grupp, M., Sepold, G.: Materialbearbeitung mit Hochleistungsdiodenlasern, Lasermagazin, 2/1998. T¨onshoff, H.K., G¨ unther, G., Grendel, H.: VDI Berichte 1427, D¨ usseldorf: VDI-Verlag (1998) 249. T¨onshoff, H.K., Ostendorf, A., Sch¨ afer, K.: Fiber Laser – Compact Source for MicroWelding, ICALEO’98, Orlando, USA, 16.–19. November 1998.

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242 99Dor 99Emm

99Gie 99Koc 99NN 99Toe

References for 2.7 Dorsch, F., Daiminger, F., Hennig, O., Bl¨ umel, V.: 2kW sw Fiber-Coupled Diode Laser System, AHPLA99, SPIE Proc. Vol. 3889, Osaka 1999. Emmelmann, C., Lunding, S.: Schweißen mit Lasern hoher Strahlqualit¨ at. In: Schweißen und Schneiden ’99, DVS 204, D¨ usseldorf: Verlag f¨ ur Schweißen und verwandte Verfahren 1999. Giesen, A.: Scheibenlaser mit neuem Pumpdesign. Laseropto 31(1) (1999). Koch, J., Wolf, M., Sepold, G.: Laser Manufacturing of Micro Perforated Metal Sheets. Wt-Werkstattstechnik 89(11/12) (1999). N.N.: Zeitschrift Laser – Entwicklung und industrielle Anwendung H. 5, 12/99 (1999) 26. T¨onshoff, H.K., Grendel, H., Kaak, R.: Structure and Characteristics of the Hybrid Manipulator Georg V. In Boer, C.R. (ed.): Parallel Kinematic Machines, Proc. of the First European-American Forum on PKM, Milan, Italy, 31.08.–01.09.1998, London: Springer, 1999.

00Goe

Goebel, G., Havrilla, D., Wetzig, A., Beyer, E.: Laser welding with long focal length optics, Proc. ICALEO 2000, 2.–5. Oct. 2000, Dearborn, MI, USA, Vol. 89B, 2000.

01Klo

Klotzbach, A., Morgenthal, L., Schwarz, T., Fleischer, V., Beyer, E.: Laser welding on the fly with coupled axes systems, Proc. ICALEO 2001, Jacksonville, Florida, USA, 2001.

03Gru

Grupp, M., Seefeld, T., Vollertsen, F.: Proc. 2nd Intern. Conf. on Lasers in Manufacturing (LIM), Munich, Germany (2003) 375. Shiner, B.: Opto & Laser Europe 102(01) (2003) 24.

03Shi

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2.8 Process monitoring and closed-loop control W. Wiesemann

2.8.1 Introduction It is a well known paradigm of modern production strategy to accomplish product quality by safely mastered process technologies rather than by off-line inspection of the production lot. Mastering any sophisticated process technology commonly relies on more or less sophisticated on-line monitoring or closed-loop process control means. While monitoring systems will immediately create failure messages in case of detected not allowed process irregularities, closed-loop control systems are designed to continue the process by applying appropriate feedback actions in order to instantaneously compensate the beginning decrease of process quality. Monitoring means are also being used in scientific environments to gain a better understanding of the interaction process of the laser radiation and the material. Laser material treatment basically is a thermal process taking place in the interaction zone where the laser radiation is hitting the workpiece. The treatment process can be considered as a complexly coupled system of subprocesses like laser energy deposition, energy transformation and dissipation by temperature increase and heat conduction, material phase transition, flow dynamics of the liquid and gaseous phases, and radiation emission. By properly adjusting the strength of the interaction process and by simultaneously applying suitable solid, liquid or gaseous assist materials, directed or diffuse, pressured or not, the workpiece can be cut, drilled, welded, soldered, bent, partially ablated, sintered, or surface-treated. Up to now the majority of the standard industrial laser material treatment machines, mainly used for cutting and marking, are operating without process monitoring means with up times close to 100 %. Obviously this is due to the high level of the machines’ reliability. Standardly a big number of machine parameters like supply voltages, laser output power, cooling system and structure temperatures, stand-off distance of the cutting-head nozzle, etc. is closed-loop controlled. Within the scope of this chapter this is not regarded as process surveillance. In case of machine malfunctions most machine suppliers are capable of checking or even adjusting the relevant machine parameters remotely via telemetry. However, there exist a few more issues being relevant to the treatment performance. For example, there are some important properties of the laser beam like, e.g., focal beam diameter or focal energy density distribution which cannot be monitored directly during the process. They may vary unnoticed since they are strongly influenced by barely detectable minor deformations of the beam shaping and guiding mirrors and lenses. The deformations – though occurring quite infrequently – result from thermal expansion in case there is an increased laser power absorption caused by damages or uncleanness of the optical surfaces. In total, process-related machine parameters like beam parameters or feed speed as well as parameters characterizing the assist material and workpiece condition are called process input parameters. They determine the performance and conditions of the interaction process which in turn is characterized by a multitude of intrinsic process parameters like energy input efficiency, liquid and gaseous phase dynamics, temperature gradient in the workpiece near the interaction zone, etc.

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Furthermore, there are workpiece features like thermal conductivity, reflectivity, melting and vaporization temperature, viscosity of the molten material, chemical reactivity, etc., which define important boundary conditions for the design of laser treatment processes and for the adjustment of the machine parameters. Quality problems may arise, if one or more of the workpiece parameters vary across the processing track. Such variations can be caused by internal chemical or structural irregularities. Also, workpiece surface irregularities like uncleanness or shape distortions may occur. Finally, stochastically occurring short-term process-inherent disturbances like, e.g., plasma or particle shielding or strong laser radiation reflections from highly reflecting liquid parts of the workpiece into the laser resonator may severely deteriorate the treatment process. Fortunately, there is a certain margin for the parameters to fluctuate in without seriously affecting the quality of the treatment result. The range of permissible variations for all relevant process parameters is called process window. The width of that multi-dimensional window of course is depending on the specific kind of treatment and on the required degree of quality. Permanently meeting the process window during long production time periods is a critical issue for a growing number of innovative treatment applications. Even for standard applications there will be a growing demand for the documentation and evidence of regular treatment conditions due to the increasing importance of international quality standards. This is calling for an increased use of process-surveillance methods, too. In the following sections the basics of laser-treatment surveillance methods are outlined and some examples are given which characterize the state of the art of the scientific activities and industrial applications. Finally, an attempt is made to foresee future developments.

2.8.2 Basics of process monitoring and closed-loop control 2.8.2.1 General On-line process surveillance has to be based on monitoring some significant indicator(s) for the instantaneous condition of the laser-beam interaction zone and/or adjacent areas, since the reactions within that region are determining the quality of the laser treatment. Evidently, process monitoring means have to operate contactless since there must be no disturbing influence on the interaction zone. Fortunately, this requirement creates no problem because laser material treatment is accompanied by a variety of remotely observable treatment phenomena: depending on its temperature and shape the interaction zone is emitting more or less strong electromagnetic radiation and, during some types of treatment, acoustic radiation also. Gaseous and sometimes liquid workpiece material carrying electrical charge and emitting electromagnetic radiation may be ejected, and backscattering of laser power from the interaction zone may occur to some extend. All these process output phenomena carry some potential to be used as indicators of the process condition and, implicitly, of the treatment quality. The process output phenomena are quantitatively characterized by process output parameters which can be measured by suitable sensors. The sensor signals are carrying more or less hidden information on the condition of the process and, hence, the treatment result. As a simple example, the detection of the optical radiation emission of a workpiece being surface-treated for hardening allows a fair estimation of the temperature of the emitting part of the surface. The temperature in turn is indicating the depth of hardening if the interaction time is kept constant. Obviously, the design of process-surveillance systems has to be adapted to the particular application. This first of all includes the identification of the most suitable output parameter that has to be monitored. In the following, process output parameters which can be used for quality control are called quality indicators. Furthermore, a suitable signal assessment method has to be prepared. Landolt-B¨ ornstein New Series VIII/1C

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As part of the assessment, filed information on the influence of all possibly occurring conditions of the process on the treatment result and on the selected quality indicator has to be used in some automatic mode. This information has to be gained by thorough research, if not already available, and has to be stored in real-time accessible data formats for any specific application. In the following sections the basics of process-surveillance methods are outlined. This includes the discussion of the industrial and scientific user’s motivation for establishing process surveillance means, the description of methods for quality indicator detection, signal assessment, and the discussion of the limits for the achievable span of surveillance.

2.8.2.2 Process-surveillance objectives and strategies Process-surveillance methods are supporting routine industrial laser-treatment applications as well as scientific work aimed to a better theoretical understanding of the laser-treatment processes. The general structure of the different types of process surveillance is sketched in Fig. 2.8.1. The scope of support is briefly addressed below. Scientific research

Production monitoring Production closed loop control

Interaction process

Interaction process

Interaction process

Output phenomena

Output phenomena

Output phenomena

Output phenomena detection

Quality indicator detection

Quality indicator detection

Scientific sensor signal evaluation

Sensor signal assessment routine *)

Sensor signal assessment routine *)

Improved knowledge about process

Alarm

Process input parameter variation

*) The involvement of the machine user, needed for the adjustment of the signal assessment routine to his particular application, is not displayed in this figure.

Fig. 2.8.1. General structure of process-surveillance methods.

2.8.2.2.1 Support for scientific research by monitoring of process output parameters Today’s understanding of the interaction between the laser beam and the treated material was gained by thorough research on the influence of the process input parameters on the interaction subprocesses and on the treatment result. The research is still in progress. It is covering an increasing variety of materials which had been regarded to be untreatable or, at least, permitting just poor treatment quality like, e.g., composites, ceramics, Zn coated steel, and materials having extraordinary high reflectivity and thermal diffusivity. The work is being supported by a multitude of scientific process output parameter monitoring and evaluation methods. They are mainly based on optical and acoustic sensing methods which had to be developed for this purpose. As an example, by detecting the optical radiation emission of the plasma/vapor plume created during steel welding and measuring the intensity ratio of two suitable emission lines of Fe atoms the temperature and density of the plume can be calculated and correlated to theoretical models as well as to the treatment results. Occasionally, scientific monitoring schemes have been further adapted for successful industrial use. However, there are also purely scientific designs like, e.g., the X-ray transmission Landolt-B¨ ornstein New Series VIII/1C

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techniques for the on-line monitoring of bubble and pore formation and key hole dynamics, which cannot be transferred to industrial production sites. 2.8.2.2.2 On-line treatment fault probability assessment and documentation during serial production Most of the relevant machine-related process input parameters are standardly closed-loop controlled in today’s laser machines. The numerical parameter levels to be maintained by these control systems are determined by figures to be defined specifically for any treatment application. During the treatment the instantaneous parameter levels are being monitored by machine-integrated detectors like power meters, flow meters, etc. The detectors are not directly linked to the treatment process. Hence this kind of closed-loop control is not further discussed in the following sections. However, there are several additional input parameters like focal-plane beam features and chemical, dimensional or structural workpiece conditions which are strongly affecting the treatment result but, as a common feature, cannot be closed-loop controlled by the machine-integrated means mentioned above. Depending on the specific situation this may bring up the need for a direct online surveillance of one or more treatment quality indicators. If the treatment fault probability calculated on-line exceeds a predetermined level, failure messages may be generated. Retrospective analysis of the recorded process surveillance data may help to trace the origin of the treatment fault. As an example, it can be distinguished quite easily whether a faulty condition occurred suddenly or grew up during several production cycles. At the highest level of current system sophistication, automatic fault class determination may be accomplished to some extend. Even if no faults are detected, the recorded data may be needed to meet the requirements of internationally standardized quality assurance systems. 2.8.2.2.3 Closed-loop control during serial production By monitoring a few or just one quality indicator(s) and further assessment of the detected signal(s), a controllable process input parameter like beam power, feed speed or focal point position is being automatically adjusted in case the signal assessment is indicating a treatment deterioration. This scheme, however, is still suffering from the fact, that the input parameters which are available for closed-loop control actions as well as the quality indicators are by far outnumbered by the variety of potentially occurring irregularities. Even if there were more suitable input parameters, the system would have to “decide” which of them had to be varied to what extend. Therefore, closedloop process control systems have to be designed very specifically for the individual treatment application of interest. In particular, they have to be focused just to a few – or even only one – potentially occurring irregularities like, for example, the slightly varying thickness of the sheet during high-speed thin-sheet cutting or the excessive gap width between the sheets during overlap welding. At a higher level of sophistication, automatic fault tracing and compensating methods are under development for future use. As a prerequisite the cause of any detected relevant treatment irregularity has to be analyzed automatically and traced on-line to its origin. Only then the appropriate process input parameter(s) can be selected by the assessment circuit as control variable(s) for an effective fault-compensating feedback action. However, tracing the origin of faults is complicated by the mutual interdependencies of the numerous intrinsic process parameters. Also, it cannot be foreseen up to now that for any commonly occurring origin of faults future research activities will come up with a suitable process input parameter, which is capable to compensate the treatment irregularities sufficiently and without creating process disturbing side effects. For example, while the axial shift of the laser-beam focus position relative to the workpiece surface created by a slight thermal deformation of the focusing lens or mirror can be monitored and compensated during Landolt-B¨ ornstein New Series VIII/1C

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welding, an increase of the focal beam diameter created simultaneously for the same reason or by, e.g., the plasma plume, cannot be traced unambiguously. Even if it could be traced, it could not be compensated correctly by means of the currently applicable methods.

2.8.2.3 Treatment quality indicators, span of surveillance Laser-treatment quality is commonly being judged according to some criteria, which are nonexhaustively listed in Table 2.8.1. Concerning the ease of the on-line judgement, the criteria can be classified into two categories: directly and not directly assessable criteria like, e.g., spatter ejection and the degree of weld penetration, respectively. As already mentioned, the instantaneous condition of the interaction zone is determining the intensity of appearance of the output phenomena. An overview of relevant process output phenomena is given in Table 2.8.2. These phenomena can be characterized quantitatively by numerous process output parameters. For example, the electromagnetic radiation emission of the key hole can be characterized by parameters like, e.g., the spectral frequency pattern, the angular emission Table 2.8.1. Quality criteria for laser-treatment results. Laser treatment

Quality criteria

Cutting and drilling

- cut kerf shape (walls flat, parallel and correct angle to surface, shape of edges) - hole shape (walls cylindrical and correct angle to surface) - dross attachment - increased heat-affected zone - surface contamination (spatter, oxidation) - occurrence of cracks (ceramics cutting) - incomplete penetration - burns - surface roughness of cut kerf - accuracy of cut contour

Welding

- occurrence of disturbed seam surface shape (underfill, crater pits, top bead depression, etc.) - spatter occurrence - degree of penetration - occurrence of cracks and porosity - occurrence of incomplete fusion - seam cross section area

Hardening

-

depth of hardness lateral hardness depth profile occurrence of overheating degree of hardness lateral profile of degree of hardness

Alloying and cladding

-

constancy of layer thickness and mixing ratio occurrence of cracks, pores lateral layer homogeneity fusion with substrate

Caving

- smoothness of surface - accuracy of depth profile

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Table 2.8.2. Process output phenomena. Process

Phenomenon a )

Commonly used types of detectors d )

Cutting and drilling

e.r. emission from interaction zone gaseous workpiece material ejections fluid workpiece particle ejections l.r. scattered back from interaction zone l.r. passing through the kerf acoustic emission

photo diode (VIS, IR) photo diode, capacitive photo diode (IR), capacitive b ) b ) microphone

Welding

e.r. emission from vapor or plasma plume e.r. emission from keyhole opening wall c ) e.r. emission from keyhole finger part c )

Hardening

e.r. emission of interaction zone

photo diode (VIS, IR)

Alloying and e.r. emission of interaction zone cladding

photo diode (VIS, IR)

Caving

e.r. emission of interaction zone

photo diode (VIS)

Cleaning

acoustic emission from surface e.r. emission of plume

microphone photo diode (VIS)

photo diode (UV, VIS), electronic camera photo diode (VIS, IR), electronic camera off-axis photo diode (VIS, IR), on-axis electronic camera e.r. emission of melt pool and neighboring area photo diode (VIS, IR), electronic camera microphone acoustic emission through keyhole opening charge-collecting device electrical charge of plasma plume photo diode (IR) fluid workpiece particle ejections b l.r. scattered back from interaction zone ) e.r. emission from the backside of the workpiece photo diode (IR)

a

) e.r.: electromagnetic radiation, l.r.: laser radiation. ) Ge- or InGaS-photo diode for Nd:YAG radiation (λ = 1.06 µm); HgCdTe, pyroelectric detector, bolometer for CO2 laser radiation (λ = 10.6 µm). c ) Keyhole formation occurring during deep penetration only. d ) Spectral range: IR = infrared, VIS = visible, UV = ultraviolet. e ) Plasma = ionized vapor containing free electrons.

b

pattern, mean radiation intensity, variance of radiation intensity time histories, frequency pattern of intensity fluctuations, etc., and combinations and derivatives of these and other parameters. Basically, the instantaneous condition of the treatment process is determining the treatment quality. Due to this linkage the surveillance of relevant output phenomena is ideally suited to support the on-line assessment of the treatment quality. In fact, it has been shown in numerous applications that several relevant laser-treatment quality degradations can be concluded on-line from monitoring a few, or just one, selected process output parameters. These parameters then may serve as treatment quality indicators for monitoring systems, however not necessarily for closed-loop control systems. In order to be used for reliable process surveillance, quality indicators have to meet some basic requirements. Ideally, they comply with the following requirements: 1. There must be well-known unambiguous functional dependencies of at least one quality indicator on the most relevant treatment-quality criteria for the considered application (limited span of surveillance). 2. Full span of surveillance: There must be well-known unambiguous functional dependencies of a limited number of quality indicators on all relevant treatment-quality criteria for the considered application. Landolt-B¨ ornstein New Series VIII/1C

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3. There must be an on-line assessment method available for any treatment-quality indicator signal. 4. The quality indicator must not be susceptible to any intrinsic process parameter without relevance to the treatment quality. 5. The general noise level of the processed sensor signal has to be significantly lower than the sensor signal variations created by relevant process disturbances. Concerning the span of surveillance it should be considered that in any complex system the probability of the occurrence and the severity and impact of system faults are depending on the type of the particular fault. For laser material treatment there are numerous process-parameter irregularities which potentially may occur in general. Actually, though, occasionally just a few types of relevant irregularities are occurring in practice, if machine, process design, and workpiece preparation meet the state of the art. Consequently, surveillance effort may be focused to a small number of quality indicators, which may be different for different applications. Monitoring more than one quality indicator however usually calls for more sophisticated multiple-sensor arrangements. Depending on the probability ranking of the occurrence of process irregularities in a particular treatment application, it may even be sufficient to concentrate on the monitoring of just one quality indicator. The given laser-treatment situation and the expected results therefore determine the effort devoted to fault-preventing precautions and to the degree of sophistication of monitoring or closed-loop control systems.

2.8.2.4 Process output parameter detection 2.8.2.4.1 Theoretical introduction As can be seen from Table 2.8.2, electromagnetic radiation emission is the most frequently occurring output phenomenon. Laser radiation is a special case of electromagnetic radiation. The heated ejected materials are emitting electromagnetic radiation, too. Therefore, all phenomena listed in Table 2.8.2 except for the acoustic emissions can be detected with electromagnetic radiation sensors like photo diodes, electronic cameras, pyroelectric detectors, bolometers, etc. For an ideal black body1 having the absolute temperature T the electromagnetic radiation energy emitted per second (also called radiation power or intensity) and per surface area unit throughout the entire wavelength spectrum is determined by:
Wλ dλ = σ T 4 , (2.8.1) where σ = 5.669 × 10−8 Wm−2 K−4 , λ = wavelength and Wλ = energy emitted per second and per surface area unit within the wavelength interval dλ. The temperature of the emitter does not only determine the intensity but also the distribution of Wλ across the electromagnetic wavelength range and, thereby, the position of the radiation maximum Wλ, max . The relation between the temperature and the wavelength λmax of the maximum Wλ, max of Wλ is given by: λmax T = const. = 2898 µm K . 1

(2.8.2)

A radiating “black body” is realized approximately by an isotropically heated cavity, where the dimensions of the radiation-emitting cavity opening are small compared to the cavity’s inner dimensions. The opening of an unheated black body therefore appears black to the human eye. The emission of real objects is lower than the emission of a black body of the same temperature. The ratio of the real emission and the emission of a black body is called emissivity.

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In general, λmax decreases with increasing temperature. For example, the intensity maximum of materials having room temperature (T ≈ 293 K) is located in the 10 µm region. The radiation intensity maximum of steel being transformation-hardened (T ≈ 1100 K) is located just below the 3 µm wavelength region, and there is also a detectable intensity contribution in the visible region. For gases, the intensity of the emitted radiation is also increasing for rising temperatures. However, the electromagnetic emission spectra of gases are more complex than those of solids or liquids. In general, gaseous spectra exhibit sharp peaks (line emission) located at specific wavelengths which are typical for the emitting chemical element. Commonly occuring line emissions may range from the near ultraviolet to the far infrared spectral region depending on the type and the level of energy excitation of the emitting gas. The spacing between the lines emitted in a particular spectral region may be so small, that individual lines cannot be distinguished unless highly resolving means are used for recording the spectra. Plasma spectra may also show a comparatively weak continuum emission created by electron-ion recombination and electron Bremsstrahlung. 2.8.2.4.2 Radiation emission from the interaction zone In total, the electromagnetic radiation emitted from the interaction zone contains contributions from heated solid, fluid, and gaseous parts of the workpiece and of the assist material(s). The radiation in its entirety is called process radiation. The detectors respond to all contributing parts of the emission which are covered by their spectral and angular detection characteristics. However, depending on the type of process phenomenon to be detected, just a part of the process radiation carries relevant information. All other parts contribute to the detector signal noise level or even create interferences. Consequently, to enhance surveillance significance and reliability, reducing the broadband detector receiving characteristics frequently is advantageous. This can be done by using spectral-selective means like optical filters or spectrometers. Normally, the emission of laser-generated vapor and plasma of the workpiece material and assist gas is monitored in the ultraviolet and visible region while the radiation of the heated solid or liquid parts of the interaction zone is observed in the visible and near infrared region. As can be seen from Fig. 2.8.2, CO2 -laser-created weld-process radiation is typically exhibiting a strong UV/blue component which is missing in the Nd:YAG-laser-created process radiation. This component stems from the plasma2 originated from the steel vapor by strong absorption of the laser radiation. Furthermore, some strong argon assist gas lines are present. Due to its shorter wavelength and for intensities below 108 Wcm−2 , Nd:YAG-laser radiation is weakly absorbed by the vapor and no plasma formation occurs. Due to spectral clipping of the receiving optical components, the strong short-wavelength plasma components (300. . . 400 nm) of the CO2 -laser weld emissions are not fully developed in Fig. 2.8.2. The intensity emitted by the fluid parts of the interaction zone increases continuously in the near infrared region until reaching the maximum which is located in the 2. . . 3 µm region according to (2.8.2). 2.8.2.4.3 Radiation reflection and transmission at the interaction zone A fraction of the impinging laser radiation is reflected at the interaction zone. Since the power level and the angular distribution of the reflected radiation is dependent on the shape of that area, these parameters carry some implicit information on the status of the process. Additionally, a small part emerges from the backside of the workpiece in the cases of piercing breakthrough, cutting, and full-penetration welding. 2

Plasma = (partly) ionized gas; ionization in that case created due to strong laser radiation absorption.

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Fig. 2.8.2. Spectral distribution of process radiation created during welding with an Nd:YAG and CO2 laser. Mild steel, thickness: 5 mm, assist gas: argon [98Tsu]. The curves are modulated by the spectral characteristics of the optical instruments (see text).

2.8.2.4.4 Radiation detection and sensor arrangement Depending on the process output phenomenon to be observed in a particular application, the sensors and radiation antennas like lenses, mirrors, and front ends of fiber-optical cables may be placed inside or outside the laser-beam delivery systems, above the workpiece or, quite seldom, under the workpiece. Antennas which are placed coaxially with respect to the laser beam axis are providing on-axis insight into the interaction zone. This may be preferable for all three-dimensionally shaped interaction zones occurring during drilling, cutting and key hole welding, where the deeper parts of the interaction zone are blocked against off-axis detection directions. Electromagnetic radiation emitted from the interaction zone of surface treatment applications, from the weld pool, from vapor or plasma plumes, and from spatters can be detected by off-axis antennas as well. On-axis detection needs some beam-separating elements like pinhole mirrors, diffractive bending mirrors, and birefringent mirrors (Fig. 2.8.3). For cutting and welding there may be a speed-dependent lag of the position of the lower part of the key hole or cut front with respect to the upper part due to the relative movement between laser beam and workpiece. This results in bended key holes or cut fronts. As a consequence, in such cases the upwards guided radiation is maximum in front of the laser beam axis. In any case, all radiation emissions from lower parts of the interaction zone will be superimposed by radiation contributions from upper parts. Obviously, signals detected at different off-axis angles may contain information from different parts of the interaction zone. Moreover, off-axis signals may be used in multi-sensor schemes to subtract the influence of the upper parts of the interaction zone from the on-axis measured signals to get almost unobstructed information from the deeper interaction zone parts. If the vapor density of the emitted cloud is very low compared to the keyhole vapor density, the vapor/plasma emissions of the deeper interaction zone parts can be estimated with on-axis detector configurations only. In order to concentrate the signal evaluation on specifically selected relevant parts of interest of the interaction zone and adjacent areas, the angular field of view of the optical detection systems can be limited by optical means like apertures. For welding, the observable emission strength of most of the optical output phenomena from the interaction zone is non-isotropic with respect to the observation angle relative to the feed direction. Therefore, for fixed sensors the influence of non-isotropic emissions during varying movement Landolt-B¨ ornstein New Series VIII/1C

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Laser beam

Highspeed CCD camera

Single or multiple sensor modules

Imaging optic

Imaging optic

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Laser beam

Focusing optic Pinhole mirror

v Workpiece a

v Workpiece b

Fig. 2.8.3. Examples for on-axis sensor arrangements. Separation of the imaging beam achieved (a) by a scraper mirror for CO2 -laser applications and (b) by a birefringent mirror for Nd:YAG-laser applications. By courtesy of Fraunhofer Institut f¨ ur Lasertechnik, Aachen, Germany.

directions has to be taken into account by, e.g., experimentally taught correction functions. Alternatively, the sensors have to be turned synchronously to any turns of the feed direction. As already mentioned, optical process radiation can be analyzed with respect to its spectral properties, too. Broadband analysis, as shown in Fig. 2.8.2, is performed by compact spectrographs. Better resolution, up to the discrimination of single emission lines of gases, separated from each other by even less than 0.05 nm, can be achieved by more space-consumptive systems. If a multichannel detection design is used, the speed of the system is determined by the electro-optical components. Narrowband spectral regions of particular interest can also be selected by optical filters, placed in front of photo diodes, having a minimum bandwidth of several nanometers. As mentioned above, the spectral features of the electromagnetic emission carry information on the temperature and on the elemental consistence of the emitting material. Particular emission lines contained in the plasma radiation can be used to calculate temperature and density of the free electrons, which in turn are influenced by the condition of the interaction zone. Nd:YAG lasers operate in the near infrared region, having an emission wavelength of 1.06 µm. The power level of the laser beam reflections from the interaction zone normally is several orders of magnitude stronger than the process radiation. Therefore, process radiation detectors having a residual sensitivity in the near-infrared region have to be protected against saturating overload by efficient filtering of Nd:YAG-laser stray light, even if the process radiation to be detected stems from the visible spectral region. 2.8.2.4.5 Two-dimensionally resolved radiation emission Photo diodes integrate the detected radiation laterally across their field of view. Signal processing can therefore only be performed in the domain of time and time derivatives. Electronic camera signals additionally contain valuable information on the radiation distribution in the domain of space yielding two-dimensionally resolved information on the radiative situation of the interaction zone. If arranged in triangulation setups, cameras also may yield 3-dimensional information. Optionally, color cameras give some rough information on the radiation wavelength region. For on-line usage Landolt-B¨ ornstein New Series VIII/1C

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the relevant information content of the camera signals has to be extracted and assessed in time intervals which at least have to be equal to the typical fluctuation time periods of the phenomena to be observed. Some relevant process output parameters may vary significantly within milliseconds. This calls for extremely speedy hard- and software which have to be designed and adapted specifically to any particular application. Up to now, this has been accomplished just in a few cases of production-line applications. However, post-treatment evaluation of high-speed electronic-camera information content is also a valuable tool to support scientific work on the general understanding of laser-treatment processes. Cameras are preferably used in off-axis configurations, but a few on-axis designs have been realized, too. 2.8.2.4.6 Sound detection Acoustic signals are also created in the interaction zone. Airborne sound is created by the emission of vaporized workpiece material. The vapor is ejected with considerable high flow-speed rates which may range up to several hundred meters per second. Due to beam-workpiece dynamics and resonance effects the ejections are discontinuous. The emerging vapor requires a displacement of the surrounding atmosphere which in turn acts as a source of airborne sound. Sound intensity and frequency spectrum depend on the vapor flow rate fluctuations which of course are influenced significantly by the above mentioned dynamics. Therefore, sound analysis may be considered a tool for monitoring the instantaneous condition of the interaction zone, too. Typical vapor-created sound frequencies are ranging in the audible region. Microphones having a frequency response bandwidth of 20 Hz. . . 20 kHz are convenient for the process-sound pick-up. Workpiece-borne sound may also occur, having frequency bandwidths which range up to 200 kHz. They may also carry information on treatment quality aspects like crack formation, penetration depth, etc., but the sound pick-up has to be accomplished inconveniently by transducers coupled firmly to the workpiece. Airborne acoustic emissions during cleaning treatments are carrying information on the laser flux and the cleanliness of the surface. 2.8.2.4.7 Electrical-charge detection Due to their high temperature, the gaseous emissions from the interaction zone are electrically charged since they contain free electrons and ions. This can be used for sensing the strength of the emissions by, e.g., charge-collecting devices or by measuring the electrical conductance between the workpiece and an electrode placed near the workpiece, preferably concentrically around the laser beam. For cutting and drilling applications the nozzle of the processing head may serve as an electrode for the detection of charged particles. It should be noted, though, that the particle concentration is subject to rapid variations. Normally, it decreases with increasing distance to the interaction zone due to temperature decrease and recombination, but the particle concentration of gaseous clouds may also increase by the absorption of CO2 -laser radiation. 2.8.2.4.8 Multiple-sensor fusion It has been demonstrated that combinations of different types of sensors like photo diode and electronic camera, or photo diode and microphone, or combinations of photo diode and filter arrangements being sensitive to either the IR, VIS or UV spectral region, sometimes being aimed at different parts of the interaction zone, give an increased significance for the detection and classification of treatment faults and at the same time reduce the false alarm probability by, e.g., correlation-based signal assessment methods. When correlating acoustic signals to optical signals the time delay due to the comparatively low speed of sound has to be considered. Landolt-B¨ ornstein New Series VIII/1C

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2.8.2.5 Signal assessment methods The sensor signals representing the quality indicators selected for a particular surveillance task have to be assessed on-line according to a suitable strategy. The assessment is accomplished in real-time or stepwise in very short time periods, yielding the information needed to create some on-line reactions like setting alarm, stopping the process or perform a closed-loop control action. The user interface, containing means for setting and adjusting the assessment parameters and for the visualization of the assessment results, usually runs under non-real-time conditions. The general structure of such assessment methods is outlined in this section. While the number of suitable treatment quality indicators is limited by physics, there are less limitations for designing signal assessment strategies. Consequently, a multitude of signalprocessing and fault-evaluating methods have been created according to the specific situation of any particular treatment application. Basically, however, all assessment methods created so far for serial production surveillance are structured quite similarly. They are based on the thesis, that constant quality of the treatment results can be accomplished only if there is a good repeatability of the interaction zone condition for any repeated treatment cycle. As a consequence, fairly repeated process output parameters and quality indicator time history patterns for any treatment cycle are created, too. Therefore, as a common element of the assessment methods discussed here, the assessment results are finally achieved by judging the preprocessed sensor signal formats according to some kind of referencing information. This information has to be gained for any individual surveillance system and particular application by the system itself under the condition of accepted treatment quality prior to the start of serial production. The judging is performed according to some general system-inherent criteria which have to be fine tuned by the machine user with respect to the particular treatment situation and quality demands. In a quite simple design the preprocessed sensor signal is compared with one or more preset upper and/or lower signal level(s), see Fig. 2.8.4. In case of exceeding these thresholds, alarm or control actions may be taken. More sophisticated, prior to any action the instantaneous deviations from the thresholds are being analyzed with respect to the amount and/or the time duration and, sometimes, the frequency of occurrence during predefined time periods. Depending on the application, the preset thresholds are either constant during the entire treatment period or they

Welding error because of incorrect clamping tool Fig. 2.8.4. Signal assessment by thresholds. By courtesy of Precitec Optronik, Rodgau, Germany. Landolt-B¨ ornstein New Series VIII/1C

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o.k.process

3

No rm aliz ed sca le v

Normalized scale v 2

n.o.k.process

Criteria vector

Normalized scale v 1

Equipotential surface Fig. 2.8.5. Signal assessment by criteria vectors in a 3-dimensional space. By courtesy of LZH, Germany.

can be ramped or taught from recorded data taken under conditions that produced acceptable treatment results. The thresholds may also be allowed for slow floating to some predefined extend during the production shift by using automatically updating refreshing modes. This method allows for the adaptation to some expected, but not avoidable treatment-inherent long-term drift effects of the process, which are not significantly affecting the treatment quality. Further improvement of the reliability of process surveillance systems can be achieved by multisensor fusion systems. In this case the sensor signals are assessed isolated from each other to some extend. The outputs then are linked to each other and assessed as a set in a final stage. This, for example, can be done by creating criteria in a multi-dimensional space, see Fig. 2.8.5. Obviously, the performance of signal-assessing circuits using predefined referencing information depends on the user’s skills about the acquisition of this information, especially setting the thresholds, because there are no general guidelines to be applied for this task. This mainly results from the fact that there are no general rules for theoretically defining the range of the maximum permissible variations of the process parameters, referred to earlier as process window. The widths of the process windows depend on the respective laser-treatment process, the workpiece parameters, and on the particular level of quality required for the particular treatment result. Consequently, they have to be examined experimentally. Therefore, apart from very few exceptions, any current signal-assessing circuit needs the cooperation of a skilled laser machine user for defining the appropriate thresholds or, e.g., the limits of a multi-dimensional decision space which represent the transition from acceptable to non-acceptable treatment quality. By defining these limits the user can emphasize either maximum detection safety or minimum false-alarm probability but reduced fault-detection safety. Some significant differences between the assessment methods developed so far result from the signal-processing techniques. To give an overview, the numerous techniques are arranged below into the groups of time-domain analysis, frequency-domain analysis, and statistical and neural analysis. The latter methods are also in use for the final assessment stage. Time-domain analysis: The on-line preprocessing of the instantaneous sensor signal, which has to be sensitive to the relevant quality indicator, may be done by various types of electronic filtering or integrating. Moreover, time derivatives of the signal may be useful for further assessment. Integrating may be applied for time periods ranging from short subsequent or selected processing periods to the entire duration of one treatment cycle. The latter, of course cannot be regarded as a true on-line technique.

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Frequency-domain analysis: As mentioned earlier, the time-domain analysis may be sensitive to spurious process-inherent performance changes of the treatment system, the detection system, or the environment, which are not relevant to the treatment quality. Resulting false alarms may be avoided not only by the floating-threshold method mentioned above but also to some extend by analyzing the sensor signal in the frequency domain. In this case the strengths of the signals within selected characteristic frequency bandwidths are compared with each other and/or with preset figures or taught functions as described above. Furthermore, for some applications frequency-domain analysis may yield an increased fault-discrimination performance by, e.g., yielding higher signal-to-noise ratios and at the same time increased inclinations of the quality indicator characteristics. However, frequency-domain analysis by definition needs some time if performed by mathematical means (Fast Fourier Transformation FFT, estimate of signal power spectrum). Statistical and neural analysis: At a higher level of sophistication and more time consuming, statistical and neural algorithms and correlation computing methods are being used mainly in post-treatment modes in scientific environments. Some of the post-treatment methods developed so far carry the potential of being speeded up to real-time performance which is necessary for on-line usage in industrial production sites. In a few cases experimental real-time neural networks have demonstrated superior reliability in terms of fault-detection probability and defect class separation. Neural networks are supposed to provide self-adapting capabilities to the machines in the future. However, as a prerequisite, training sets have to be generated for teaching the network to distinguish “ordinary” data events corresponding to acceptable treatment quality from “extraordinary” data events corresponding to unacceptable treatment faults. Presently, it seems to be extremely difficult to create training sets which define one of these events in its entirety. As a consequence, in the near future neural networks most probably will not enable surveillance systems to perform completely self-adapting functions. They will, however, significantly increase the reliability of multiple-sensor fusion-based process-monitoring systems. A special format of signals to be assessed is produced by electronic cameras. In this case, digital processing techniques generate sets of characteristic numerical figures for every frame according to the application-specific data compressing and extracting algorithms. These figures then may be directly compared against predefined data sets or taught functions. Prior to this last stage of assessment they may be further processed in the frequency domain or by using statistical or neural algorithms. However, due to the high data rate and the speed requirements for the dataprocessing routines, relatively few camera-based systems were reported so far to be capable of on-line surveillance applications. The various signal evaluation techniques can be distinguished, among other criteria, by their speed of evaluation. A slow evaluation speed, of course, delays the overall response time of the surveillance system. For closed-loop control systems the response time has to be shorter than the rise time of the irregularity to be compensated. Depending on the kind of irregularity the appropriate rise time may vary by several orders of magnitude. There are stochastic short-term irregularities like keyhole collapsing or spatter ejection affecting the process just for some milliseconds or even shorter, but creating severe treatment defects. On the other hand, there are long-term drifts elongated during hours or days like focal-length variation due to an increasing dust contamination of the lens surface.

2.8.2.6 Control actions There are several kinds of control actions which may be taken if process surveillance is indicating the beginning of a faulty treatment. The most simple kind of reaction is setting alarm and stopping Landolt-B¨ ornstein New Series VIII/1C

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the process instantaneously. Depending on the kind of treatment application it may be feasible then to restart the process automatically at a workpiece position shortly in front of the fault or at the very beginning. This retry function has up to now been integrated just in some standard software configurations for cutting applications. Closed-loop control is obviously the most advanced kind of reaction since it keeps the machine producing accepted quality without loss of time. The general structure of closed-loop systems is shown in Sect. 2.8.2.2, Fig. 2.8.1. While there are several process output parameters which are suitable to serve as quality indicators, only a few process input parameters have been used so far as controlling parameters for closed-loop control actions. They are listed in Table 2.8.3. Examples of realized applications are given in Sect. 2.8.3. As mentioned earlier, for a process input parameter to be suitable for closed-loop control, it has to carry the potential to compensate the most probably occurring process disturbances. Table 2.8.3. Parameters suitable for closed-loop control. Controlled parameter

Commonly used controlling variable

Total beam power

Average laser material excitation energy

Time of beam-workpiece interaction

Feed speed Beam scanning deflection velocity Excitation energy pulse duration modulation

Axial focus position

Axial lens position Focal length variation by adaptive mirror

Nozzle stand-off distance

Axial position of the focusing head

Lateral beam power distribution

Individually controlled fiber matrix element power delivery Segmented adaptive mirror

Assist material supply

Supply rate (ejection pressure, flow-channel cross section, wire feed speed)

Up to now just a few closed-loop control systems based on quality indicator monitoring have been realized. The process input parameter preferably used for control actions is the laser beam power. This is reflecting the fact that the subprocesses involved in the energy transfer from the beam to the workpiece have a tendency to become unstable, especially during welding and drilling treatments. Moreover, laser power may be reflected back in short bursts from the workpiece into the laser resonator via the beam-guiding system stochastically and at remarkable intensity levels causing strong short-term output-power variations. The need for closed-loop power control also may stem from surface conditions influencing the absorptivity, from the heat sink capacity, which is depending on the mass volume directly surrounding the interaction zone, and on the temperature of this area immediately before the treatment. All these parameters may vary considerably and cannot entirely be predetermined along the processing track. The energy input into the workpiece can also be varied by controlling the beam-workpiece interaction time. This is accomplished by either changing the workpiece feed speed or the speed of the processing head or the deflection velocity in case a beam scanner is used or by changing the pulse width in case a pulsed laser excitation is used. By changing the axial position of the focus point, the radius of the impinging beam and consequently the beam power density are varied. The axial position can be changed by simply varying the distance between the focusing head and the workpiece, which however also changes the position of other relevant parts of the head like assist gas nozzles. This can be avoided by movement of Landolt-B¨ ornstein New Series VIII/1C

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the focusing element only or, more speedy, by variations of the focal length performed by beam forming elements (mirror, lens) having controllable radii of curvature. The stand-off distance of the nozzle of cutting and drilling heads commonly is controlled by axial movements of the entire heads. This, of course, also changes the axial focus position. The flow rate of gaseous assist materials can be controlled by valves, placed as near as possible to the ejection opening, which determine the internal pressure of the material, or by mass flow meters. The supply rate of filler wire is controlled by motors determining the feed speed.

2.8.3 State of the art of process monitoring and control technology As discussed in Sect. 2.8.2.2, process-surveillance methods are useful for both scientific and industrial use. The continuous improvement of the mastering of the laser-treatment processes was accompanied from the beginning by process-monitoring means. Pioneering work was done by acoustic and optical monitoring of welding processes and preferably optical monitoring of cutting processes. The improvement of monitoring methods gained some speed roughly during the late 1980 decade and yielded a variety of new process-surveillance schemes and patent applications, but comparatively few methods were introduced for industrial use during the next ten years [88Bec, 91Jor, 93Miy, 96Kai]. During the last years, however, a growing number of international publications demonstrated the general feasibility and improved reliability of on-line process surveillance techniques. Recent scientific effort is mainly directed to enhance the level of the detection probability of faulty parts and to automatically identify the cause of the faults. There also has been remarkable progress in the theoretical understanding and modeling of laser treatment processes, which was supported by experimental validation based on innovative process-monitoring methods. Industrial process surveillance proved to be helpful in particular for the laser welding technology to penetrate the markets of large lot-size production. This preferably applies for parts, where structural and/or functional reliability of the final products are key issues. In such cases international production quality assurance standards or at least treatment documentation regulations have to be met. While there are still no standards for the on-line surveillance of the laser treatment quality, there is a growing number of matured methods for industrial on-line process monitoring. For a specific treatment application the most suitable method can be selected and assessed according to its potential of becoming part of the individual production-line quality assurance system that is required by the applying general international standard. Industrial process monitoring also may yield advantages if no standards apply. Depending on the specific circumstances, process monitoring may avoid or reduce the effort and uncertainty of on-line visual inspection or the cost and delay effects of destructive post-process analyses. Skillfully designed signal-processing systems based on taught reference data additionally provide fault class separation capability. This, for the time being, shortens the down-time, and is one of several prerequisites for the design of future highly automated autonomous production cells. In the following sections an overview is given on the state of the art of scientific and industrial process surveillance. Due to the large number of publications and patents covering this subject just an exemplary selection can be referenced to. The bigger part of them is dealing with welding applications.

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2.8.3.1 Cutting and drilling 2.8.3.1.1 General Process input parameters influencing the treatment quality of cutting and drilling applications are: lateral laser power density profile, polarization, focus point position relative to the nozzle opening and to the workpiece surface, beam axis inclination relative to the workpiece surface inclination, shape and stand-off distance of the nozzle, cutting speed, workpiece pre-process temperature, pressure and chemical consistence of the assist gas, workpiece consistence and surface cleanliness. The tolerable maximum margins of fluctuations of these parameters determine the process window of a particular treatment. For a wide range of laser-treatment applications the optimum set of parameter figures are filed by the machines’ manufacturers. For standard cutting applications the process-window boundaries are comfortably spaced as a result of the continued improvement of the laser-cutting process technology. Also post-process inspection of cutting, drilling and marking results usually can be performed if considered to be necessary. Consequently, the use of processmonitoring means is quite rare but operators still have to be available on site to take appropriate actions in case of obvious process malfunctions. To ensure constant cutting quality the nozzle stand-off distance is being controlled contactless routinely in many cutting machines [88Top]. A constant stand-off distance between cutting-nozzle front surface and upper workpiece surface ensures a constant lens distance during the treatment as well as unchanged flow characteristics of the assist gas for every repeated production cycle. The permissible stand-off tolerance range is about ±0.05. . . ±0.25 mm, depending on the beam characteristic and on the type of application. However, the variations of shape and position of workpieces to be cut in serial production with high-power lasers frequently exceed this tolerance range, which brings up the need for a stand-off distance control. Additionally, due to thermal deformation caused by the heat dissipated from the interaction zone the z-position of the workpiece surface may change during the treatment. The process windows are getting considerably smaller if the treatments are expected to meet the utmost currently achievable limits of quality or productivity. In such cases the use of process monitoring or closed-loop control systems should be considered. Comparatively small process windows are also typical for non-standard cutting situations. These include the treatment of highly heat-conducting metals like copper, highly reflecting metals like aluminum, highly reactive materials such as titanium, or metals with increased viscosity of the liquid phase like stainless steel, if the workpiece thickness exceeds certain limits. In the past these limits have been extended continuously towards higher figures. It should be noted that for running any production-monitoring or closed-loop control system the cooperation of the machine user is needed for adjusting the threshold conditions like, e.g., alarm levels of the signal assessment routines, according to the particular situation of his application. 2.8.3.1.2 Scientific research 2.8.3.1.2.1 Cutting During the early times of commercial laser cutting considerable research has already been started on the potential of on-line monitoring of the cutting process based on the detection of spatter emerging from the lower side of the workpiece [83Ols], diode detection [84Ish, 86Dec] as well as CCD detection [88Ols] of light emitted from the interaction zone towards the cutting head and laser light passing through the cut front [88Ols]. Later, additional insight into the details of the cutting process was gained [92Miy, 92Ols, 94Hil] resulting in some significant process optimizations. Special emphasis was drawn to the detection of increased roughness, striations and burns of the cut

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surface of mild steel by photo-diode detection of the light reflected or emitted from the cut front [91Jor, 96Kap, 97Dec] or by air-borne sound detection [92Ols]. It was shown, that the dominating frequency of the diode signal frequency spectrum increases with decreasing roughness and that the diode signal variance increases with increasing roughness and dross attachment while the signal mean value increases significantly in the case of incomplete cuts. CCD-camera analysis of the heat-affected-zone area was reported to correlate to some extend with the cut quality of thin (0.5 mm) stainless steel, too [98Haf]. Camera analysis was also used for the study of the dynamics when cutting transparent material such as soda lime glass [76Ara]. Reflections of the laser radiation were used for the detection of poor cutting quality of aluminum [92Ols]. Minimizing the occurrence and dimensions of cracks during cutting alumina ceramics was accomplished by a closed-loop laser-beam-pulse-frequency control based on the detection of the radiation emission of the plasma plume [98Toe]. Closed-loop process control based on plasma-plume intensity detection was suggested to increase the stability of the cutting process close to the maximum feed speed range for cutting thick (up to 8.5 mm reported) stainless steel [01Toe]. Furthermore, it has been shown that plasma-plume intensity detection can be used for an automatic focal point search and detection routine [02Pre]. By using a CCD camera aligned coaxially to the laser beam axis the mild steel melt dynamics have been monitored [99Abe] in order to support the dross and ripple formation analysis by mathematical modeling [99Sch]. Except for the diode detection of cut-front emissions during high-speed cutting none of these layouts has been further developed and frequently used for routine applications in industrial production sites. This is due to the general high level of reliability of the cutting process mentioned above. 2.8.3.1.2.2 Drilling, piercing Relatively little research work has been done for monitoring or controlling the drilling and piercing process. In general, the same detection techniques as for cutting are applicable. By measuring the infrared radiation (900 nm) emitted from the interaction zone a laser-pulse-width control system was established which kept the temperature of the interaction zone between an upper and lower limit during oxygen-assisted piercing [89Abe]. Smaller hole diameters for oxygen-assisted piercing of mild steel and shortened piercing times for stainless steel drilling were yielded by this method. The detection of radiation emitted from the interaction zone also permits the identification of the moment of the breakthrough. Some patents are focused to this subject [91Sch, 96Spo], however, the technique is limited to relatively low aspect ratios (depth-to-diameter-ratio). By measuring the intensity of the reflected laser light or the light of an additional low-power probe laser the time of the breakthrough can be detected, too, up to an aspect ratio of at least 25. The significance of acoustic emissions for breakthrough detection seems to be comparatively limited. They may serve as an additional indicator for future multi-sensor systems, though. Drilling transparent material has been documented by the use of high-speed photographic images to investigate the instantaneous drilling velocity and melt flow dynamics [00Low]. In case of drilling glass-epoxy-printed wired boards, the end of the process, reaching the bottom copper foil surface, clearly can be recognized by the detection of an increased level of the reflected laser power. Process productivity could be increased by 30 % and the variations of the bottom diameter of the holes could be reduced significantly [98Kar]. By measuring the light emission from the bottom of the hole, layers of residual epoxy resin can be detected if the layer thickness exceeds 2 µm [99Miy2]. Formation of droplets and plasma during CO2 -laser drilling of alumina ceramic has been investigated by high-speed videography (exposure time 10 ns) [02Vil]. The drilling of small hollow workpieces like turbine blades is a non-standard application calling for process control in order to safely achieve the breakthrough and at the same time prevent damages of the inner surface adjacent to the hole. Detection of the radiation emission of the backing material was proposed to solve this problem [00Bec]. Landolt-B¨ ornstein New Series VIII/1C

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2.8.3.1.3 Industrial applications Apart from standardly used non-contact capacitive nozzle stand-off distance control systems, there are a few industrially used on-line cutting-process control systems aimed at productivity increase: During piercing, the bottom of the interaction zone produces remarkable light and plasma/vapor emissions. If the aspect ratio does not exceed a certain limit, the emissions can be detected by optical or electronic sensors commonly integrated into the cutting head or placed somewhere along the direction of the laser beam without clipping it. By comparing the instantaneous signal level with a preset minimum value, the end of the piercing process can be detected and cutting can be started immediately without the need for any preprogrammed time-consuming safety margin. Incomplete penetration and even dross formation during the cutting process can be monitored by the same method and a retry function can be started by the machine’s CNC. The emissions are also being used for closed-loop control of the piercing and cutting process. Piercing can be performed in different modes. “Soft” modes avoid material overheating. They take more time but are producing less surface-polluting spatter than “full-power” modes. For minimizing the time duration of soft piercing modes, the temperature of the molten material of the interaction zone has to be controlled within a narrow bandwidth. This again is done by measuring the radiation emission of the interaction zone, comparing the resultant signals with preset upper and lower levels, and accordingly controlling the laser input energy. The same strategy is being applied for piercing stainless steel. This may be accompanied by plasma formation which is retarding the piercing process. Feed-speed control systems allow the machine to be operated with maximum laser power while the feed speed is kept just below the limit for the occurrence of incomplete cuts. As an indication of an upcoming incomplete cut there is an increased lag of the lower part of the cut front with respect to the upper part. The cut front consists of molten material emitting electromagnetic radiation which is partly directed upwards. Due to the lagging lower part the radiation intensity to be detected above the workpiece is increased. In case the detected signal exceeds a preset level, the feed speed is automatically reduced followed by a stepwise increase as soon as the emissions fall below the preset signal level. Speed control also is an issue when cutting materials like zinc-plated steel, which are creating strong plasma preferably at high cutting speed. To avoid process disturbances caused by excessive plasma formation the cutting speed has to be reduced temporarily. Closed-loop systems have been commercially realized based on capacitive or optical plasma sensing devices.

2.8.3.2 Welding 2.8.3.2.1 General and historical Laser welding is performed either by heat-conduction welding or by penetration welding, see also Part 2, Chap. 4. The former is based on a comparatively uncomplex process of surface beam energy absorption and heat conduction. It is restricted to thin-sheet treatment due to the limited energy penetration depth accompanied by a wide Heat-Affected Zone (HAZ), which is characteristic for this technique. However, process irregularities which may call for surveillance actions cannot be excluded totally. Contrary to this, penetration welding is based on a complex beam-metal vapor/plasmaworkpiece interaction. It keeps the HAZ advantageously small while allowing considerable deep penetration depths due to a beam-generated capillary, called key hole. It is pressured and thereby prevented from collapsing by the partly vaporized wall material. As a drawback, the process windows for laser-penetration-welding applications are generally smaller than for conduction welding. This specifically applies for galvanized steel and materials having high reflectivity and heatLandolt-B¨ ornstein New Series VIII/1C

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conduction coefficients like, e.g., aluminum alloys. Therefore, the quality of penetration-welding treatment results is by far more susceptible to process-input-parameter irregularities. Additionally, keyhole-assisted weld processes are endangered by inherently existing resonances which may be stimulated by minor influences thereby creating severe disturbances of the weld process. This situation causes some considerable and enduring need for quality assuring and documenting measures to be taken. The state of the art of weld process surveillance, based on output phenomena detection, will be addressed in the next sections. Early scientific monitoring work started and partly continued with acoustic monitoring techniques, workpiece-borne [69Jol] or air-borne [78Wes, 82Shi, 83Dix, 85Jon, 86Ste, 89Ham, 92Li, 96Gu], the latter giving information on the dynamics of the plasma/vapor emerging from the key hole, which in turn is related to the shape and temperature of the key hole. Furthermore, combinations with other types of sensors (optical and charge collecting) have been investigated [88Gat, 91Bro, 92Ste]. As it seems now, air-borne sound detection may have the potential to serve as an accompanying method in combination with optical sensing [97Bee, 98Far, 98Kam, 99Far], but not as a stand-alone method. Pioneering optical-monitoring work showed that the optical emissions of the interaction zone contain the broadest scope of useful information on the instantaneous condition of the process: Infrared and visible emissions carry information on the shape and temperature of the liquid parts of the key hole, weld pool and spatter, while ultra-violet and visible emissions are dependent on the density, temperature and shape of the plasma/vapor inside and outside the key hole. The information can be extracted by spectroscopic analysis [87Roc, 89Sok] or by time history analysis of signals detected by photo diodes [83Dix, 84Ish, 84Miy, 87Bey, 88Bec, 91Che, 91Mai, 92Ste] or CCD cameras [88Ols, 90Voe, 91Hof, 92Bag, 93Miy], the latter being additionally capable of tracing the surface contour and position of the plasma plume, key hole and weld pool. The first proposal to axially resolve the keyhole plasma radiation made use of several diodes arranged at different angles with respect to the beam axis [95Miy]. While the use of single-detector arrangements gained scientific insight into specific aspects of the laser-welding process, sensor-fusion schemes have been primarily developed for industrial process monitoring aimed at false treatment detection. Probably the first experimental sensor fusion made use of the simultaneous recording of air-borne sound and UV plasma plume radiation [83Dix]. The first commercial dual-sensor photo-diode arrangement (UV/IR) aimed at on-line cw CO2 -laser welding monitoring entered the market in the late eighties [90Hat]. About 6 years later pulsed Nd:YAG-laser welding monitoring was successfully demonstrated for industrial production purposes [97Gri]. At the end of the last decade the first camera-based closed-loop control system was introduced into an automotive production site [98Die]. Innovative signal assessment based, e.g., on fuzzy logic algorithms was pioneered since the early nineties [94Gu]. The last 10 years showed the successful diffusion of reliable commercial photo-diode-based process monitors into industrial production lines. Process modeling and monitoring research were considerably intensified, mainly based on camera-type or multiple-sensor systems. Accordingly, new signal assessment algorithms and surveillance schemes have been developed and are still under development. They are aiming at autonomous self-learning production cells. This calls additionally for an advanced understanding especially of the numerous effects forming the fluid flow in the key hole and melt pool. Innovative scientific process-monitoring schemes such as X-ray melt-pool transmission and holographic techniques will support and verify future process-modeling efforts. 2.8.3.2.2 Recent scientific research The continual momentum driving scientific process-monitoring activities is resulting from the demands for an even better understanding of the conventional welding process fundamentals as well as novel process options and from the increasing need for improved on-line monitoring and control systems. Main targets of recent scientific process-monitoring activities have been: theoretical Landolt-B¨ ornstein New Series VIII/1C

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modeling of the beam-plasma-workpiece interaction; discrimination of partial penetration vs. full penetration; investigation of situations creating treatment quality faults like humping, pores, undercut and burns; giving support to the development of methods for preventing or reducing the occurrence of such faults; developing methods for monitoring the occurrence of unavoidable faults. The information given in the following sections is arranged according to the type of the detection method. 2.8.3.2.2.1 Optical emissions, photo-diode detection Currently, the most often investigated optical sensing techniques are based on photo diodes aimed at selected radiation emitting parts of the interaction zone like, e.g. upper parts or full length of the key hole or at the plasma plume above the key hole opening or at parts of the weld pool behind the key hole or at the root side of the weld. The spectral sensitivity of the sensor module has to be optimized by filters according to the spectral properties of the source of emission within the UV, VIS and IR region (see Table 2.8.2). Since the welding process is highly dynamic, the emissions and thereby the detected signal strength are strongly fluctuating. This, at a first glance, may appear stochastic and consequently of poor value for analytical purposes. However, it has been shown frequently during the last decade that not only useful information can be extracted from optical emission signals in spite of their strong fluctuations, but that also these fluctuations carry valuable information on relevant quality indicators like, e.g., the recognition of full vs. partial penetration [97Sfo, 97Mor, 98San, 99Far, 99Ike], the control of the root-side seam width [03Bag], the recognition of surface defects (holes, burns) [97Sfo], and deep-welding penetration depth [99Miy1]. Analysis of signal fluctuations also created valuable input for scientific process modeling like information on, e.g., laser-beam-plasma interaction [89Sok, 98Far, 98Gei, 99Ino], resonant stimulation frequencies [98Gei] yielding reduced penetration depth variations, keyhole-vapor-emission flow rate, and vapor plume size vs. vapor flow rate [98Far], plasma temperature vs. focus point position. Even the variance of the on-axis measured visible process-radiation fluctuations was shown to be a discriminant between a good weld and non-joining errors, cut and half join and dropping through [98Han]. The time derivative of the plasma-plume emissions turned out to be proportional to the acoustic signal strength [98Far]. Angular and optical broad-band resolution of the visible process radiation confirmed the understanding of the temperature gradients between the plasma temperatures of the plume, in the keyhole opening, and in the keyhole finger part. This in turn experimentally confirmed assumptions on the magnitude of plasma-related laser absorption inside the key hole due to inverse Bremsstrahlung. The same technique confirmed the presence of a wavelength-dependent attenuation of the keyhole plasma radiation due to scattering by particles within or surrounding the surface-borne CO2 laser plasma plume [02Tu]. A review on investigations of the plume scattering and absorption effects occurring due to clustered particles of the workpiece material in CO2 - and Nd:YAG-laser welding is presented in [02Gre]. Optical narrow-band filters placed in front of the detectors may extract some significant treatment failures such as oxidation of titanium or stainless steel occurring due to insufficient shield-gas supply [98Fox]. Increased process reliability was gained for pulsed Nd:YAG-laser spot welding of titanium by an analysis of the infrared radiation detected on-axis, separately performed for the keyhole-formation phase and the subsequent penetration phase [98Kog]. The occurrence of Laser-Shock-Cleaning-type (LSC-type) plasma shielding during CO2 -laser welding was shown to be correlated with strong radiation emission of nitrogen which was detected on-axis via a scraper mirror in the near IR region [98Sei]. Using a high-speed closed-loop control system (reaction time 50 µs) plasma shielding could be suppressed by switching off the laser power for a short time period (100 µs) [95Abe]. Axial-focus-position closed-loop control based on process-output-phenomena detection has been accomplished by different methods such as intensity and phase detection of plasma-plume intensity Landolt-B¨ ornstein New Series VIII/1C

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oscillations which are intentionally induced by small periodical axial variations of the focal point [97Neg] and by using an off-axis dual-view detection method for monitoring two different parts of the optical plasma-plume emissions [00Gu]. It has been shown, however, that the oscillation method mentioned above may be affected by varying gap widths [00Toe]. Moreover, axial auto-focus control was accomplished by making advantage of the chromatic aberration of the focusing/receiving optics of an Nd:YAG laser system while on-axis detecting the part of the radiation which is emitted by the molten pool [98Kim] or by the plasma [97Har]. Gap detection has been performed by using a twin-spot detection technique. Here one direction views the plasma plume while the other looks through the gap at the front of the radiating liquid wall of the key hole. Based on these signals, an adaptive axial-focus-position control system has been established which is capable of maintaining sufficient welds for varying gap widths [00Gu]. 2.8.3.2.2.2 Optical emissions, camera detection Increasingly frequently, electronic cameras are used for recording two-dimensionally resolved intensity patterns of the plasma plume, the keyhole area, or the whole weld-pool area. The information to be gained from these areas depends on the kind of processing algorithms and on the speed of the camera and the processing means. It has been demonstrated that simultaneous on-line surveillance of a few important parameters such as axial focus position [98Die, 98Koj] and penetration depth [98Die] or shield gas flow rate and absorbed pulsed laser energy [98Koj] can be achieved by the assessment of the camera data with respect to two criteria, like plasma plume intensity and ejection velocity [98Koj] or position of the key hole and size of the weld pool [98Die], respectively. Figure 2.8.6 shows an example of an unprocessed weld pool image of a CO2 -laser butt-welding.

Fig. 2.8.6. Camera-recorded image of the weldpool area for a non-ideal butt weld configuration: The missing white area in the front part of the keyhole areal is due to a gap between the parts to be welded.

In some cases the extraction of just one criterion turned out to be sufficient for the detection of, e.g., the occurrence of humps [98Bag]. Furthermore, high-resolution on-axis monitoring of the optical emissions of the keyhole area [97Bee, 99Kra] has been demonstrated to yield information on the spatially resolved keyhole shape, penetration depth, seam track deviation, and spatter formation. Weld-pool dynamics and resonant frequencies have been studied for Nd:YAG welding of aluminum alloys by high-speed monitoring of the surface wave movement, illuminated by short (2 ns) periodic Nd:YLF-laser pulses [98Mue]. By using the illumination of an Ar+ -laser during Nd:YAG-laser welding of 10 mm thick stainless steel the optimizing influence of side gas flow on pore reduction and bead narrowing has been investigated [02Kam]. 2.8.3.2.2.3 Reflected laser radiation Up to now this method has been used almost exclusively for Nd:YAG lasers since the near IR wavelength falls within the range of standard photo detectors. As the interaction zone is not capable of totally absorbing the impinging laser radiation, a small part of it is reflected back, even from the lower part of the key hole. Obviously, the level of the reflected radiation is dependent on the shape of the key hole. It gives additional information on the condition of the treatment Landolt-B¨ ornstein New Series VIII/1C

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process, such as the shape of the interaction zone [96Gri, 98Kog, 98Mue, 98Tsu], occurrence of pores and blow holes [98Mue], and gap formation during overlap welds [98Kog]. Deviations of the axial focus point from its optimum position can also be detected [98Mue] since the focus position influences the keyhole shape which in turn determines the level of reflected laser radiation. If the backscattered laser radiation and the emitted near-infrared thermal radiation are analyzed by an electronic camera with respect to the width of the detected lateral intensity distribution, peak amplitude, and peak position on the camera, it should be possible to identify variations of three types of process parameters (axial laser focus position, workpiece temperature, average laser power) [98Pet]. Penetration discrimination (full vs. partial) and hole detection could be achieved with 100 % reliability during laser hybrid butt welding (Nd:YAG laser from top and diode laser from the bottom) of aluminum tailored blanks by photo-diode detection of the backscattered laser radiation from the top and transmitted Nd:YAG-laser radiation from the bottom side [03Ort]. 2.8.3.2.2.4 Electrical charge collection The heated gaseous and liquid emissions of the interaction zone carry electrical charge, which can be used for monitoring purposes. It has been shown that a lot of treatment faults and process conditions like, e.g. humping, undercut and varying penetration depth, missing joint of overlap weld, and defocusing, create clearly detectable variations of the signal of the charge-collecting device [90Li, 97Hil, 99Bie]. As for the optical plasma detection, it seems to be impossible to clearly distinguish the treatment faults from the detected signal variations. The significance of statistical signal-assessment methods was shown to be superior to time-history-signal-level analyses or even to Fast-Fourier-Transformation (FFT) analyses [97Hil]. 2.8.3.2.2.5 Multiple sensor schemes Most researchers used more than one sensor in order to document the process condition as completely as possible, which is desirable if an improved understanding of the complex process has to be gained. Multiple sensor arrangements are also increasingly under development to improve the monitoring performance in terms of fault detection probability and false-alarm avoidance. The sensor techniques commonly used for combined analysis are already described in the preceding sections. Additionally, in some cases air-borne noise was recorded. This, however, did not significantly increase the reliability of features extracted from simultaneously recorded plasma-generated UV signals. Feature extraction from each sensor signal is performed according to the signal pre-assessment methods described in Sect. 2.8.2.5. For final classification or rating of the laser-treatment result various methods like, e.g., Principal Component Analysis (PCA) [02Che], statistical, neural [99Far, 03Gha], or fuzzy networks [98Ogm] have been used, isolated or in combination. As an example, four different methods have been applied in parallel to the signals of three different sensors (UV, IR and microphone) for a binary penetration depth classification during mild steel lap CO2 -laser welding in an automotive production line. PCA accompanied by Class Mean Scatter analysis turned out to be the only method in that case yielding 100 % correct classification for all three types of sensor signals. However, only the UV plasma detection signals yielded 100 % correct classification regardless of the assessment method applied [00Sun]. Applying neural methods to coaxially detected UV, VIS and IR signals in a CO2 -laser automotive transmission weld yielded 93 % porosity detection probability and 0 % false alarm [03Gha], see Sect. 2.8.3.2.3. Figure 2.8.7 demonstrates the benefits of combined sensor signal assessments in the case of hybrid laser (Nd:YAG, deep penetration-type from top, and diode, heat-conduction-type from bottom) butt welding of aluminum tailored blanks: Since partial penetration and holes as well Landolt-B¨ ornstein New Series VIII/1C

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3.2 Hole Full penetration Partial penetration

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Median YAG top [a.u.]

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0

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Fig. 2.8.7. Example for the benefits of 2dimensional feature extraction in combination with statistical methods. Hybrid laser butt welding of aluminum tailored blanks [03Ort]. Sheet thickness: 1.2 mm / 2.5 mm, weld speed: 10 m/min, laser power: 4 kW (top), 2 kW (bottom), shield gas: argon, sampling rate: 50 kHz. Median YAG top and Median YAG bottom: By statistical method pre-assessed amplitude of sensors detecting the backscattered laser radiation of the keyhole area on top and the transmitted laser radiation under the bottom of the workpiece.

as holes and full penetration, respectively, are creating approx. the same signal levels for the discrimination features, plotted in vertical and horizontal direction, respectively, the three situations can only be distinguished by using two quality indicators simultaneously [03Ort]. Alignment errors and supply-gas failures also could be identified by additionally sensing the temperature of the weld bead near the key hole. 2.8.3.2.2.6 X-ray and visible-light shadowgraphy, holography, laser beam probe, ultrasonic inspection As a quite exotic but informative tool, micro-focused X-ray on-line absorption measurement perpendicular to the key hole and feed direction has been used for the investigation of the sub-surface liquid motion and porosity formation and suppression methods during high-power CO2 laser welding of specially prepared aluminum alloy specimens [99Mat]. A similar technique was used to observe the key hole and porosity formation during high-power pulsed-Nd:YAG-laser spot welding of stainless steel [98Fuj, 02Kap] and liquid Zn [02Kap]. The results may be used for validation of process-modeling theories [02Dau, 02Fab]. X-ray analysis has already been used for the study of the electron-beam-welding processes though [70Ton]. The smoothing effect of resonant-frequency process stimulation by modulated laser power (400 Hz) on the weld-penetration-depth variations (see also Sect. 2.8.3.2.2.1) and decrease of porosity formation has been demonstrated directly by camera detection perpendicular to the CO2 -laser-induced key hole in frozen glycerin [03Cho]. Examples for further unique arrangements are: the combination of a real-time holographic interferometer and a high-speed digital camera for quantitative visualization of the removal of the laser-induced plume by the shield gas [98Bai], the application of a probe CO2 laser beam for measuring the lateral plasma plume density variations building up a few milliseconds prior to the occurrence of humping due to resonant process instabilities [98Kla]. Non-contact ultrasonic inspection may have the potential for in-process detection of pinhole defects [00Kle]. In total, numerous sophisticated signal assessment algorithms have been developed during the last decade while there was comparatively slower progress concerning the further improvement of the process-output-phenomena sensing techniques. This is leaving some space for inventions preferably in the latter area, which will be based on innovative commercial sensing hardware like advanced cameras and integrated optical devices.

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2.8.3.2.3 Industrial applications 2.8.3.2.3.1 Monitoring systems A few types of matured commercial photo-diode-based weld-monitoring systems are presently running in several hundred industrial production lines mainly in Europe and North America. Main application areas are the welding of tailored blanks, automotive power train parts, body in white parts, and a variety of small parts for the automotive, medical, and electronic industry such as fuel injection parts and capsules for, e.g., air bags and cardiac pacemakers. The driving forces for in-line installations of process monitoring systems have already been addressed in Sects. 2.8.2.2.2 and 2.8.3. In summary they are: process status documentation, fault detection with or without fault class separation, obeying quality assurance standards while reducing or avoiding effort for conventional quality assurance means like visual inspection and destructive tests. To obtain maximum surveillance reliability in terms of fault-detection safety, false-alarm prevention, and fault class separation preferably more than one process phenomenon (see Table 2.8.2) have to be monitored by multi-sensor arrangements and assessed simultaneously. The principles of the detection of the process phenomena and the assessment of the sensor signals are described in Sects. 2.8.2.4 and 2.8.2.5, respectively. Commonly used quality indicators are the signal amplitude variations of the optical emission of the key hole or the plasma/vapor plume, in some cases measured simultaneously at different wavelengths, the near infrared radiation of the weld pool or parts of it, and the reflected laser power from the key hole as described in the previous sections. Several years ago, camera-based systems were also successfully assessed at automotive production sites for the surveillance of the gear-part welding process [98Die, 01Kai]. Yet there is still considerable adaptive work needed for any specific application to gain additional on-line information on the treatment process from a machine vision system compared to the information yield of a matured photo-diode system. However, machine vision systems have already proved to be a valuable tool for post-process inspection of the seam geometry and the detection of seam faults. A comprehensive review of commercial process-monitoring systems is given in [99Sun]. Commercial on-line weld surveillance systems for industrial applications are usually based on threshold assessing methods. These methods (see Sect. 2.8.2.5) need reference data. They are created by recording and assessing some sensor signal patterns taken for production cycles which yielded proven good treatment quality. Then, by adding tolerance margins, processing thresholds have to be defined. Finally, some parameters have to be entered which influence the analyzing mode of threshold violating data traces. By these measures the surveillance systems are adapted to the individual needs of a specific application. For example, by increasing the tolerance margin and/or increasing the permitted accumulated time of threshold violations, the sensitivity for failure detection is reduced compared to common situations. This may be appropriate, e.g., for welding applications where joints are needed just at a comparatively low percentage of the complete welding track. To make full use of the versatility of commercial surveillance systems, significant skillful work for configurating all assessment parameters and margins has to be performed. If process influencing parts or parameters of the welding equipment are even slightly modified, the configuration of the surveillance system may have to be reworked. Therefore, current weld process surveillance systems are preferably suited for long-lasting single-task applications rather than for frequently changing treatment situations. Since the invention of the laser continuous effort has been undertaken to increase the span of industrial applications of laser material treatments by using novel approaches like, e.g., the twinspot technique. During the last years the high power laser welding technology has been further expanded by some new methods like, e.g., remote welding and hybrid (laser/arc) welding. For most applications based on these methods the use of process-surveillance equipment appears to be advantageous, if not mandatory. Both single element detectors and machine vision systems are being used, and in some cases it became obvious that at least a 2-dimensional feature extraction scheme (for example see Fig. 2.8.7) is needed for safe fault detection. Landolt-B¨ ornstein New Series VIII/1C

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2.8.3.2.3.2 Closed-loop control systems Closed-loop control systems are supposed to compensate for the unwanted effects of any irregularly changing process parameters on the treatment result. As a prerequisite it is necessary to trace the origin of a detected irregularity of the monitored quality indicators on-line during its occurrence with a reliability close to 100 %. This seems to be a remaining challenge for mid-term future development work. Additionally, there is the issue of focus-point positioning. Especially for butt joints a very precise lateral positioning of the focus point is mandatory so that the beam hits the center of the gap between the parts to be joined. Deviations of ±0.05 mm from the optimum position may already be crucial. The positioning is achieved by commercially available non-contact optical or inductive gapfollowing systems. The optical systems are based on high-resolution pattern-recognition methods while the inductive systems detect the strength of magnetic fields created by electrical eddy currents induced by the sensor itself within the surface layer of the two parts to be joined. The accuracy of the inductive systems is limited to about ±0.05 mm. High-end optical systems are more accurate and they are capable of calculating the gap volume of butt joints, thereby creating input data for adaptive filler-wire feed-control systems. Since the field of view of the sensors is adjusted in front of the interaction zone, some feed-speed-dependent coordinate-transformation effort is needed for the control actions. Mechanical contacting sensor modules can also be used for lateral focus-point positioning if the topography of the joint area is suitable for steering the contact pin which, e.g., is the case for overlap joints. The lateral position of the focus point is considered as a machine parameter, like the position of the workpiece. Moreover, none of the mentioned gaptracking methods is based on any process output phenomenon. Therefore, the state of the art of lateral focus-point monitoring systems is not further discussed in this section. Both inductive and optical systems are also measuring the stand-off distance. Less critical than the lateral focus-point misalignment are axial misalignment and defocusing effects, if these are small compared to the Rayleigh length of the focused beam. They may result from thermal drift effects of the beam-guiding optics, from workpiece shape and position tolerances, incorrectly taught robot motions, and from plasma lensing effects. Monitoring the axial position may be accomplished by detection of process radiation, charge-collecting devices or backreflected laser power, however, these methods yet have to mature for industrial applications. Matured capacitive and optical triangulation technologies using pilot beams have the disadvantages of measuring the distance to the surface rather than the axial focal position while the measured area is not exactly coincident with the interaction zone either. Some capacitive systems tend to be sensitive to plasma occurrence. This may restrict their application to pulsed systems (measurement during the off-periods) and to Nd:YAG systems.

2.8.3.3 Transformation hardening Laser transformation hardening of ferrous material is performed by shortly rising the temperature of the material above the austenitizing temperature but well below the melting condition, maintaining the temperature for a short time, and by a subsequent rapid temperature drop (104 K/s) occurring while the laser beam passes by, due to the heat dissipation into the bulk material. The austenitizing temperature for common materials is ranging about 1100 ± 100 K depending on the material. The achieved degree of hardening mainly depends on the content of carbon atoms being reoriented during the short high-temperature maintaining period. The resulting hardness depth mainly is a function of the peak temperature which depends on a variety of parameters like laser power density, feed speed, temperature level immediately prior to the beam exposure, workpiece shape, and surface conditions influencing the absorptivity. Therefore, by monitoring the surface temperature at the

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location of the impinging laser radiation the achieved depth of hardening can be indirectly surveyed or even controlled. Increased accuracy for reaching and maintaining the correct temperature is needed for materials having a comparatively small difference between austenitizing and melting temperature. Temperature control is also of increased importance if preheating effects occur like during largearea hardening by repeated treatments with adjacent and partly overlapping treatment tracks. The surface temperature determines the intensity and the spectrum of the electromagnetic radiation emitted from the surface according to (2.8.1). Thereby, the temperature can in principle be calculated from the measurement of the radiation level. The calculation, however, is quite complex because (2.8.1) has to be adapted to the specific conditions of the measuring device and the material’s properties. Fortunately, sticking to the basic formula is not necessary. Even for adaptive control it is sufficient to compare the on-line measured intensity levels with a preset level generated from proven good treatments and to control the laser power accordingly. Temperature fluctuations can be limited to ±5 K by that method. Up to now no commercial systems for laser-hardening process monitoring or control systems have entered the market. However, a few successful pioneering industrial applications have been reported covering the high-volume treatment of torsion springs for car doors [00Dre] and the treatment of coated crankshafts including laterally resolved control of the power distribution profile [99Dre]. The latter application has been realized by a matrix of 20 fiber cables which transmitted the power emissions of 20 independently controlled diode lasers.

2.8.3.4 Cladding, alloying Like hardening, cladding and alloying are surface treatment processes. The processing patterns mostly consist of repeated partly overlapping treatment tracks. This, like during hardening, may cause preheating situations, which cause the instantaneous temperature of the interaction zone to leave the boundaries of the process window. Poor treatment quality is the result. Also, variations of the powder flow rate and geometrical variations of the workpiece influence the temperature of the interaction zone. To achieve acceptable treatment quality, the interaction-zone temperature has to remain constant. Regularly occurring preheating can be accounted for and the effects of intentionally varied powder flow rates on the temperature may be compensated by ramping the feed speed or laser beam power according to pretested and optimized functions. However, this preparative procedure may turn out to become uncomfortably time-consumptive. Furthermore, this scheme provides no adaptation to stochastically occurring disturbances. As for hardening applications, temperature closed-loop control systems provide more safety and flexibility. Mainly depending on the materials and masses involved, the melt pool surface temperatures may range from 930 K to 2330 K. According to (2.8.2), this calls for radiation sensors having their maximum sensitivity in the near-infrared region. Therefore, thermographic CCD cameras or IR diodes are used for measuring the melt-pool dimensions and temperature distribution, and the average surface temperature, respectively. After some pioneering feasibility demonstrations [98Bac] a closed-loop control system having a set of sensors integrated into a commercial cladding head entered the market recently [03Pre].

2.8.3.5 Cleaning, caving During laser-surface-cleaning applications every pulse creates a rapidly expanding plasma which in turn emits a shock wave. By detecting these waves via microphones placed near the interaction zone [95Lu] and analyzing the data according to their frequency spectrum, amplitude distribution Landolt-B¨ ornstein New Series VIII/1C

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or chromatic properties (dominant frequency, energy level, excitation purity), for example the laser flux [98Lee1] and the surface cleanliness [98Lee2] can be monitored by using neural networks for signal assessment. Potential for serving as quality indicators has been predicted also for reflection measurements and the determination of plume radiation intensity and plume radiation dominant wavelength vs. number of laser pulses [00Lee]. Scientific research of pulsed-laser ablation has been performed for about 15 years [88Chr]. The photoacoustic sensing method has been used to determine the damage threshold and energy coupling efficiency vs. several variables such as: number of pulses, kind of material, and surface cleanliness [00Ito]. Spectroscopic methods (e.g. Laser-Induced Breakdown Spectroscopy, LIBS) proved to be a valuable tool for determining the elemental composition [03Rod] and to investigate micro machining processes [03Rus]. Process monitoring for industrial caving treatments is aimed at depth control and on-line surface roughness measurement. Conventional on-line non-contact depth monitoring systems are not applicable because of the required high resolution (< 10 µm) and the need for measuring exactly at the position of the interaction zone. Consequently, the process radiation is used directly for triangulation. As a main feature, the sensor must not be sensitive to variations of the lateral shape of the interaction zone and to variations of the angular emission characteristics [95Hae].

2.8.4 Outlook The advantages gained so far by today’s commercially available surveillance systems for laser material-treatment applications are already indicating the huge potential of future improved and integrated process-surveillance strategies. Triggered by the customers needs, there will be an increasing impetus driving the further refinement of the mastery of laser material-treatment processes. The production industry has to cope with a permanently growing demand concerning cost, quality, functionality, environmental and resource saving aspects, etc. which applies to production methods and product features as well. This situation calls for an increased and skillful future usage of innovative production methods like laser treatment even in areas where these methods are uncommon at present or are regarded as risky like, e.g., treatment of components being part of safety devices, low personnel or fully automated large-volume production of costly parts, laser treatment with hand-held processing heads, large volume laser micro-material treatment, and laser treatment of materials, combinations of materials and shapes, which presently are regarded as unreliably treatable. This development will be supported by a further increasing knowledge of the laser-treatment fundamentals, new designs of compact laser sources allowing for increased production-cell-design flexibility, and still decreasing cost per kW of laser power. The areas of innovative laser-treatment applications mentioned above represent a considerably growing production volume for which for economical reasons and such of safety there is a need to avoid as completely as possible the production of faulty devices. Therefore, process surveillance presently can be regarded to be just in an intermediate stage of need and acceptance. There will be a growing need for standardly accepted treatment surveillance and documentation methods, the latter requested by international-certification and quality-assurance standards. To keep up with the increasing need for standardization the relevant committees are – even though a little bit slowly – evaluating, which of the currently accepted surveillance principles are suitable for being part of an upcoming system of laser-treatment quality-assuring standards. To keep pace with the demands of the production industry, future systems of laser process monitoring and control have to provide a further increasing level of reliability and flexibility. This will be accomplished by innovative and highly sophisticated real-time signal assessment systems,

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which are fed by extended reference and calibration data files, which are a-priori defined and/or self-generated for specific treatment situations, and by the on-line acquired signals of integrated multi-sensor fusion setups. In spite of increased internal sophistication and complexity, standard future systems need to be more user friendly than current products. This includes features like standardized interfaces and increased autonomous functionality. Presently, the latter appears to be partly visionary for industrial applications. It includes features like, e.g., automated startup monitoring and assessment-parameter acquisition and setting, automated functional self-test routines, automated fault class identification and fault probability calculation, and suggestion of correctional measures. Spin-offs of further developments may also result in low-cost systems of reduced functionality. They may be used for monitoring situations which tolerate an increased false-alarm rate like mass production of relatively simple devices of lower value or for parts which can be easily refed into the production process for a repeated treatment cycle. But also the highest level of the technological high end of process monitoring and signal assessment, being too sophisticated for industrial use, is under need of further innovative progress to continuously support the ongoing research of the complex nonlinear laser-workpiece-interaction processes. The scientific and technological potential for the improvement of already existing monitoring and control techniques and for the creation of innovative new features and methods is demonstrated, e.g., by the continuously increasing number of so far several hundred patents covering the area of process surveillance. Peculiarly, just a very small percentage of the patent claims which focuse on closed-loop control have been realized so far for industrial use. This reflects the difficulty of identifying automatically the origin of any commonly occurring treatment fault and of addressing the correct process input parameter which is capable of compensating the fault origin without any unwanted side effect. Evidently, for these reasons closed-loop control is also an issue carrying remarkable potential for future innovative development activities which then will overcome these obstacles.

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References for 2.8

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70Ton

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76Ara

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78Wes

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82Shi

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83Dix 83Ols

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84Miy 84Ish

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85Jon

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86Dec 86Ste

Decker, I., Hansmann, M., Ruge, J.: SPIE 650 (1986) 279. Steen, W.M., Weerasinghe, V.M.: Proc. SPIE 668 (1986) 160.

87Bey 87Roc

Beyer, E., Herziger, G., Gasser, A., Sokolowski, W.: SPIE 801 (1987) 178. Rockstroh, T., Mazumder, J.: J. Appl. Phys. 61 (1987) 917.

88Bec 88Chr 88Gat 88Ols 88Top

Beck, R., Jurca, M.: DVS Berichte 113 (1988) 58. Chryssolouris, G., Bredt, J., Kordas, S., Wilson, E.: Proc. 16th North American Manufacturing Research (NAMRC) (1988) 217. Gatzweiler, W., Maischner, D., Beyer, E.: Proc. SPIE 1020 (1988) 142. Olsen, F.O.: Opto Elektronik Magazin 4 (1988) 168 (in English). Topkaya, A., Schmall, K.-H., Majoli, R.: Proc. of SPIE 1024 (1988) 103.

89Abe 89Ham 89Sok

Abels, P., Petring, D., Beyer, E.: Proc. Laser’89 (1989) (in English). Hamann, C., Rosen, H.G., LaBiger, B.: Proc. SPIE 1132 (1989) 275. Sokolowski, W., Herziger, G., Beyer, E.: Proc. SPIE 1132 (1989) 288.

90Hat 90Li 90Voe

Hatwig, A., Kutzner, R., Jurca, M.: Laser Magazin 4 (1990) 20. Li, L., Qi, N., Brookfield, D.J., Steen, W.M.: Proc. ICALEO’90 (1990) 411. Voelkel, D., Mazumder, J.: Appl. Opt. 29 (1990) 1718.

91Bie 91Bro 91Che 91Hof 91Jor 91Mai 91Sch

Biermann, S., Geiger, M.: Proc. SPIE 1415 (1991) 330. Brookfield, D.J., Li, L., Steen, W.M.: UK Patent Application GB 2 260 402 A (1991). Chen, H.B., Li, L., Brookfield, D.J., Williams, K., Steen, W.M.: ICALEO’91 (1991) 113. Hoffman, T.: Advanced Materials & Processes 9 (1991) 37. Jorgensen, H., Olsen, F.O.: SPIE 1412 (1991) 198. Maischner, D.: Proc. Laser’91 (1991) 515. Schu¨ ocker, D., Aussenegg, F.: Austrian Patent Application 7A 1071/91 (1991).

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92Bag 92Li 92Miy 92Sch 92Ste 92Ols

Bagger, C., Miyamoto, I., Olsen, F., Maruo, H.: Proc. LAMP’92 (1992) 553. Li, L., Steen, W.M.: ICALEO’92 75 (1992) 719. Miyamoto, I., Maruo, H.: Welding in the World 29 (1992) 283. Schnars, U., Sepold, G., J¨ uptner, W.: Proc. ISATA’92 (1992) 526. Steen, W.M.: Proc. LAMP’92 1 (1992) 439. Olsen, F.O., Jorgensen, H., Bagger, C.: Proc. LAMP’92 (1992) 405.

93Jur 93Miy

Jurca, M.: ISATA’93 (1993) 179. Miyamoto, I., Kamimuki, K., Maruo, H., Mori, K., Sakamoto, M.: ICALEO’93 Techn. Digest (1993) 413.

94Gu 94Hil

Gu, H., Duley, W.W.: Proc. ICALEO’93 79 (1994) 77. Hillebrand, A., Heyn, H., Decker, I., Wohlfahrt, H.: SPIE 2207 (1994) 414.

95Abe

Abels, P., Beyer, E., Drenker, A., Kaierle, S., Kirby, C., Nitsch, H.: Proc. ALAW’95, Detroit, USA (1995). H¨ausler, G., Herrmann, J.M.: Feinwerktechnik F&M (1995) 103. Lu, Y.F., Aoyagi, Y.: Jap. J. Appl. Phys. 34 (1995) 1557. Miyamoto, I., Mori, K.: Proc. ICALEO’95 (1995) 759.

95Hae 95Lu 95Miy 96Gri 96Gu 96Kap 96Kai 96Spo 97Bee 97Dec 97Gri 97Har 97Hil 97Kuh 97Mor 97Neg 97Sch 97Sfo 97Toe 98Bac 98Bag 98Bai 98Bra 98Die 98Far

Griebsch, J., Schlichtermann, L., Jurca, M., Hoving, W., Nillesen, W.: ICALEO’96 (1996) 164. Gu, H., Duley, W.W.: J. Phys. D 29 (1996) 550. Kaplan, A.F.H., Schu¨ ocker, D., Wangler, O.: Laser u. Optoelektronik 28 (1996) 68. Kaierle, S., Dahmen, M., Furst, B., Kittel, J., Kreutz, E.-W., Poprawe, R.: Proc. ICALEO’96 (1996) 154. Sp¨ orl, G.: DE Patent 19644101 (1996). Beersiek, J., Poprawe, R., Schulz, W., Gu, H., Mueller, R.E., Duley, W.W.: Proc. ICALEO’97 (1997) 30. Decker, I., Heyn, H., Martinen, D., Wohlfart, H.: SPIE 3097 (1997) 29. Griebsch, J., Schlichtermann, L., Jurca, M., Hoving, W., Nillesen, C.: Proc. ICALEO’97 (1997) B-164. Haran, F.M., Hand, D.P., Peters, C., Jones, J.D.C.: Appl. Opt. 36 (1997) 5246. Hillerich, B., Schuhmacher, J., von Alvensleben, F.: Schweißen und Schneiden 4 (1997) 226. Kuhl, M., Wrba, P.: Proc. LANE’97 Germany (1997) 857. Mori, K., Miyamoto, I.: J. of Laser Appl. 9 (1997) 155. Negendanck, M., Neubauer, N.: Proc. LANE’97 (1997) 459. Schneider, S., Bingener, F.D., Riehn, H.-D.: Laser und Optoelektronik 29 (1997) 59. Sforza, P., de Blasiis, D., Lombardo, V., Santacesaria, V., Dell’Erba, M.: SPIE 3097 (1997) 97. T¨onshoff, H.K., von Alvensleben, F., Overmeyer, L., Specker, W.: LANE’97 (1997) 485. Backes, G., Kreutz, E.W., Gasser, A., Stromeyer, R., Wissenbach, K.: Proc. ECLAT’98 (1998) 227. Bagger, C., Olsen, C.: Proc. ICALEO’98 85c (1998) 43. Baik, S.-H., Chung, C.-M., Kim, C.-J.: Proc. ICALEO’98 85c (1998) 278. Brassel, J.-O., Breitenbach, F., Schmidt, M.: Proc. MIO’98 (1998) 301. Dietz, C., Jurca, M., Schlichtermann, L., Kogel-Hollacher, M., Breitschwerdt, S., Schmid, C., Rowold, I.: Proc. ICALEO’98 85c (1998) 178. Farson, D.F., Kim, K.R.: Proc. ICALEO’98 85c (1998) 1.

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References for 2.8 Fox, M.D.T., Peters, C., Blewett, I.J., Hand, D.P., Jones, J.D.C.: Proc. ICALEO’98 85c (1998) 62. Fujinaga, S., Takenaka, H., Narikiyo, T., Katayama, S., Ohmori, A.: ICALEO’98 85c (1998) 158. Geiger, M., Geisel, M., Otto, A.: Annals of the WPG: Production Engineering V (1998) 13. Haferkamp, H., Goede, M., von Busse, A., Th¨ urk, O.: Proc. ICALEO’98 85c (1998) 11. Hand, D.P., Peters, H., Fox, M.D.T., Jones, J.D.C.: Proc. ICALEO’98 85c (1998) 86. Kamikuki, K., Nakabayashi, T., Inoue, T., Watanabe, M., Matsunawa, A.: Proc. ICALEO’98 85c (1998) 34. Karasaki, H., Isaji, K., Kinoshita, H., Kato, M.: ICALEO’98 Techn. Digest (1998) 50. Kim, C.-J., Kim, M.-S., Chung, C.-M.: Proc. ICALEO’98 85c (1998) 226. Klassen, M., Skupin, J., Schubert, E., Sepold, G.: Proc. ECLAT’98 (1998) 297. Kogel-Hollacher, M., Jurca, M., Dietz, C., Janssen, G., Lozada, E.F.: Proc. ICALEO’98 85c (1998) 168. Kojima, T., Ohmura, E., Miyamoto, I., Nagashima, T., Ishide, T.: Proc. ICALEO’98 85c (1998) 113. Lee, J.M., Watkins, K.G., Steen, W.M.: Proc. ICALEO’98 85c (1998) 18. Lee, J.M., Watkins, K.G., Steen, W.M., Ryan, J.D., Russell, P.C.: Proc. ICALEO’98 85c (1998) 236. M¨ uller, M., Dausinger, F., H¨ ugel, H.: Proc. ICALEO’98 85c (1998) 122. Ogmen, O.: Proc. ICALEO’98 85c (1998) 188. Peters, C., Fox, M.D.T., Haran, F.M., Hand, D.P., Jones, J.D.C., Steen, W.M.: Proc. ICALEO’98 85c (1998) 149. Sanders, P.G., Leong, K.H., Keske, J.S., Kornecki, G.: J. Laser Appl. 10 (1998) 205. Seidel, B.: PhD Thesis 1997, Aachen, Germany: Shaker Verlag, 1998 (in German). T¨onshoff, H.K., Ostendorf, A., Graumann, C., Kral, V., Thiessen, B.: Proc. ECLAT’98 (1998) 391. Tsukihara, H., Ichikawa, E., Kojima, E., Kimura, S.: Proc. ICALEO’98 (1998) 93. Abels, P., Kratzsch, C., Schulz, W., Kaierle, S., Poprawe, R.: Proc. ICALEO’99 87 (1999) E-99. Biermann, S., Kessler, B.: DVS Berichte 205 (1999). Drenker, A., Bosse, L., H¨ ansch, D., Sch¨ urmann, B., P¨ utz, H., Treusch, H.-G., Poprawe, R.: DVS Report 205 (1999) 124. Farson, D.F., Ali, A., Li, X.C.: J. Laser Appl. 11 (1999) 47. Ikeda, T., Kojima, T., Sano, T., Ohmura, T., Miyamoto, I., Nagashima, T., Tsubota, S., Ishide, T.: ICALEO’99 Technical Digest (1999) 66. Inoue, T., Sano, T., Miyamoto, I., Ono, K., Adachi, K., Matsumoto, Y.: ICALEO’99 Technical Digest (1999) 74. Kratzsch, C., Abels, P., Kaierle, S., Poprawe, R., Schulz, W.: SPIE 3888 (1999) 472. Matsunawa, A., Seto, N., Kim, J.-D., Mizutani, M., Katayama, S.: Proc. SPIE 3888 (1999) 1. Miyamoto, I., Inoue, T., Sano, T., Ono, K., Adachi, K., Matsumoto, Y.: ICALEO’99 Technical Digest (1999) 75. Miyamoto, I., Nakayama, T., Sano, T.: ICALEO’99 Technical Digest (1999) 51. Schulz, W., Kostrykin, V., Nießen, M., Michel, J., Petring, D., Kreutz, E.W., Poprawe, R.: J. Phys. D: Appl. Phys. 32 (1999) 1219. Sun, A., Kannatey-Asibu, E., Gartner, M.: J. Laser Appl. 11 (1999) 153.

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Beck, T.: LMTB, Berlin, Private communication. Drenker, A., Dallmeyer, S., Kaierle, S., Sch¨ urmann, B., Vitr, G., Wissenbach, K., Flemming, R., Rinsdorf, A.: Proc. ISATA 2000 (Automotive and Transportation Technology), Lasers and Joining Appl. (2000) 163. Gu, H., Duley, W.W.: Proc. ICALEO’00 89 (2000) E-11. Ito, Y., Oguro, I., Kiyoku, S.: Proc. ICALEO’00 89 (2000) E-242. Klein, M.B., Pouet, B., Kercel, S., Kisner, R.: Proc. ICALEO’00 89 (2000) E-23. Lee, J.M., Watkins, K.G., Steen, W.M.: Proc. ICALEO’00 89 (2000) E-232. Low, D.K.Y., Li, L.: Proc. ICALEO’00 89 (2000) E-105. Sun, A., Kannatey-Asibu, Jr., E., Gartner, M.: Proc. ICALEO’00 89 (2000) E-24. T¨onshoff, H.K., Ostendorf, A., Specker, W.: Proc. ICALEO’00 89 (2000) E-252. Kaierle, S., Abels, P., Kapper, G., Kratzsch, C., Michel, J., Schulz, W., Poprawe, R.: Proc. ICALEO’01 (2001) E-12. T¨onshoff, H.K., Ostendorf, A., Thiessen, B.: Proc. SPIE 4276 (2001) 80. Chen, X., Kannatey-Asibu, Jr., E.: Proc. ICALEO’02 (2002) C-21. Dausinger, F., Berger, P., H¨ ugel, H.: Proc. ICALEO’02 (2002) A-27. Fabbro, R.: Proc. ICALEO’02 (2002) A-5. Greves, J., Hilton, P.A., Barlow, C.Y., Steen, W.M.: Proc. ICALEO’02 (2002) C-27. Kamimuki, K., Inoue, T., Yasuda, K., Muro, M., Nakabayashi, T., Matsunawa, A.: J. Laser Appl. 14 (2002) 136. Kaplan, A.F.H., Mizutani, M., Katayama, S., Matsunawa, A.: Proc. ICALEO’02 (2002) A-17. Precitec KG, Gaggenau: Patent pending (2002). Tu, J.F., Miyamoto, I., Inoue, T.: J. Laser Appl. 14 (2002) 146. Villareal, F., Gulia, K., Baker, J.H., Hand, D.P., Hall, D.R.: Proc. ICALEO’02 (2002) A-13. Precitec: Laser 3 (2003) 16. Bagger, C., Olsen, F.O.: J. Laser Appl. 15 (2003) 19. Cho, M.H., Farson, D.F.: J. Laser Appl. 15 (2003) 161. Ghasempoor, A., Wild, P., Auger, M., Mueller, R.: J. Laser Appl. 15 (2003) 77. Ortmann, J., Kreutz, E.W., Maier, C., Wehner, T., Kogel-Hollacher, M., Kaierle, S., Poprawe, R.: Proc. ICALEO’03 (2003) C-9. Rodolfa, K.T., Cremers, D.A.: Proc. ICALEO’03 (2003) C-8. Russo, R.E., Mao, S.S., Zeng, X., Mao, X.: Proc. ICALEO’03 (2003) C-7.

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3.1 Lasers in biology and medicine ¨ rschel, G. Mu ¨ller O. Minet, K. Do

3.1.1 Light and heat transport in tissue If (laser) light strikes biological tissue a number of biological effects can be investigated, starting from enzyme reactions at low power densities up to an optical breakdown and plasma formation at highest power densities (see Sect. 3.1.3). However, all effects are initially based on two physical phenomena: The first one is the primary distribution of light within the treated tissue volume (fluence) resulting in the absorption process, mainly combined with a transformation from electromagnetic to thermal energy. The second one is the subsequent heat transport mechanism which determines the final extent of a tissue reaction. Both mechanisms and the underlying parameters are briefly discussed in this chapter. A comprehensive overview is given in [95Wel].

3.1.1.1 Light transport and optical parameters Most biological tissue must be considered as a highly scattering and absorbing medium. The specific distribution of laser light within the tissue depends on the reflection and refraction properties of the surface and on the scattering and absorption properties inside the tissue. Surface (Fresnel) reflection and refraction can be estimated from the refractive index of the tissue, whereas the internal distribution of laser light is a complex function of the optical parameters and given by the transport equation [78Ish]
dL (r, sˆ) = −µa L (r, sˆ) − µs L (r, sˆ) + µs p (s, sˆ) L (r, sˆ
) dω + S (r, sˆ) , (3.1.1) ds where L is the radiance and S the irradiance at position r. Absorption and scattering are described by the absorption coefficient µa and the scattering coefficient µs , respectively, both giving the average number of absorption respectively scattering events per unit length of a photon passing through the medium. The absorption of light is determined by excitation of higher molecular states in the tissue. High absorption in the visible wavelength region is due to chromophores like melanin, hemoglobin, or porphyrin. Absorption in the near-infrared and infrared wavelength region is mainly a result of water absorption while ultra-violet absorption is mostly caused by proteins. Scattering in tissues is due to structural inhomogeneities in the refractive index as caused by membranes or lipids [96Beu1]. In addition to µs , the phase function of scattering p (ˆ s, sˆ
) describes the directional properties of the scattering process, giving the relative probability for a photon to be scattered from the direction sˆ into the direction sˆ
. For most tissues the phase function is assumed to depend only on the angle θ between input and output direction, hence p (ˆ s, sˆ
) ≈ p (ˆ s · sˆ
) = p (cos θ) .

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[Ref. p. 303

A common function often applied to tissue scattering is the Henyey-Greenstein function [41Hen] which approximates tissue scattering: p (ˆ s, sˆ
) =

1 − g2 3/2

4π (1 + g 2 − 2g cos θ)

.

(3.1.3)

An important measure for scattering in biological tissue is the anisotropy factor g defined as the first momentum of the scattering phase function:
g = p (ˆ s, sˆ
) (ˆ s · sˆ
) dω . (3.1.4) The anisotropy factor ranges between −1 and 1 and is an indicator how scattering differs from isotropic conditions (g = 0). In most biological cases scattering is strongly forward-directed, i.e. g > 0. The total attenuation coefficient is defined as µt = µa + µs . However, µt only describes the attenuation of the so-called ballistic photons of the incident beam. In weakly absorbing tissues the total intensity may exceed the intensity of ballistic photons by orders of magnitude as a result of forward scattering. For most practical problems the transport equation (3.1.1) cannot be solved analytically. A common numerical approach for calculating the laser-light distribution in tissue is the MonteCarlo simulation of a large number of photon trajectories based on probability density functions for scattering and absorption [83Wil]. However, this method requires large computation times so that approximations of the transport equation were developed based on the diffusion theory [78Ish]. Their precision depends on the irradiation conditions and the optical parameters themselves. To get reliable results it is important that scattering dominates absorption and that scattering can be considered as isotropic. The latter condition can be matched by introducing the reduced scattering coefficient µs
which reduces non-isotropic scattering to isotropic conditions by a similarity transformation: µ
s = µs (1 − g) .

(3.1.5)

From the diffusion approximation the effective attenuation coefficient µeff can be introduced which characterizes the effective penetration depth δeff in scattering and absorbing media: µeff =

 3µa (µa + µ
s )

and

δeff =

1 . µeff

(3.1.6)

The determination of the optical parameters µa , µs and g is a complex task as they have no analytical relationship to measurable quantities as reflectance or transmission. Among several techniques, integrating spheres are often used for a precise determination of the diffusely remitted, the total transmitted and the collimated, unscattered part of monochromatic light incident on a thin tissue specimen. The extraction of µa , µs and g from these macroscopic measures requires the use of numerical models, e.g. Monte-Carlo simulation, in combination with a variation technique. This provides the most precise determination of optical parameters without restrictions to boundary conditions and the applied scattering phase function [93Mue, 93Pic, 97Rog]. Other models employ the diffusion theory in various approximations, including the so-called Kubelka-Munk approximation. Another well-known model is the iterative adding-doubling method [93Pra]. In Table 3.1.1 the optical penetration depths of several medical laser wavelengths in certain tissue types are given, Table 3.1.2 lists optical parameters of some human tissues in vitro, and in Fig. 3.1.1 the water and hemoglobin absorption vs. wavelength is shown.

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Table 3.1.1. Optical penetration depth of several medical laser wavelengths in certain tissue types. Laser type

CO2 Er:YAG Nd:YAG Ar-ion Excimer

Wavelength [nm]

Penetration depth [mm] Skin

Liver

Muscle

Blood

10600 2940 1064 514 193

0.01 0.001 4 2 0.001

0.01 0.001 5 0.5 0.001

0.01 0.001 4 1 0.001

0.01 0.001 0.8 0.3 0.001

Table 3.1.2. Optical parameters of some human tissues in vitro. Tissue

λ [nm]

µa [cm−1 ]

µs [cm−1 ]

µeff [cm−1 ]

g

Method

Reference

Aorta advent. Aorta advent. Aorta intima Aorta intima Aorta normal Bladder Bladder Blood (Hct 45 %, Oxy. > 98 %) Bone (skull) Bone (skull) Bone (skull) Brain (white) Brain (white) Brain (gray) Breast

633 476 633 476 1064 633 633 700 950 1100 488 514 1064 633 1064 633 450 650 450 700 400 700 400 700 633 635 1064 635 515 1064 633 635

5.8 18.1 3.6 14.8 0.5 1.4 1.4 4.0 9.5 4.2 1.4 1.3 0.5 1.58 0.4 2.63 0.15 0.07 2.6 1.2 41 1.5 14 0.9 4.0 2.3 0.3 8.1 11.2 0.4 1.9 0.35

195 267 171 237 239 88.0 29.3 73.5 65.0 63.0 200 190 120 51.0 110 60.2 262 185 320 195 260 205 295 240 182 313 150 324 530 110 264 394

27.3 68.9 17.8 51.5 6.0 4.5 4.1

0.81 0.74 0.85 0.81 0.9 0.96 0.91 0.986 0.992 0.993 0.87 0.87 0.90 0.96 0.95 0.88 0.87 0.87 0.94 0.96 0.91 0.94 0.90 0.97 0.94 0.68 0.93 0.75 – 0.96 0.91 0.69

diffuse approx. diffuse approx. diffuse approx. diffuse approx. Monte-Carlo Kubelka-Munk Kubelka-Munk Monte-Carlo

[89Kei] [89Kei] [89Kei] [89Kei] [92Ess] [87Che] [89Spl] [96Yar]

Monte-Carlo Monte-Carlo Monte-Carlo Kubelka-Munk Monte-Carlo Kubelka-Munk diffuse approx./goniometer diffuse approx./goniometer Adding Doubling Adding Doubling Adding Doubling Adding Doubling Adding Doubling Adding Doubling diffuse approx. Lambert’s law Monte-Carlo Lambert’s law Lambert’s law Monte-Carlo Monte-Carlo Lambert’s law

[95Rog] [95Rog] [95Rog] [89Spl] [95Rog] [89Spl] [01Sas] [01Sas] [94Qu] [94Qu] [94Qu] [94Qu] [94Qu] [94Qu] [93Mai] [89Mar] [95Rog] [89Mar] [89Mar] [95Rog] [93Gra] [89Mar]

Bronchial epithelium Bronchial submucosa Bronchial cartilage Gallbladder Liver Liver Lung Muscle Prostate Skin Uterus

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10.7 10.1 4.3 4.1 2.7 8.8 3.9 2.3 13.0 5.7 89.0 7.9 42.7 4.7 13.4 26.6 3.1 46.5 – 2.4 36.8 11.3

282

3.1.1 Light and heat transport in tissue

[Ref. p. 303

100000 Water

Absorption coefficient m a [cm -1]

10000 1000 100 10 1 0.1 0.01 0.001 0

2000

a

4000 6000 8000 Wavelength l [nm]

10000

12000

Absorption coefficient m a [cm -1]

1000 Hemoglobine oxygenized Hemoglobine desoxygenized Water

100

10

1

0.1 300 b

500

700 900 1100 Wavelength l [nm]

1300

1500

Fig. 3.1.1. Water [93Kou] and hemoglobin (26.7 g/l) absorption vs. wavelength.

3.1.1.2 Heat transport and thermal parameters Thermal heating by means of laser radiation is the most common effect using lasers in biology and medicine. Starting from the primary laser-light distribution, the thermal tissue parameters determine the resulting temperature distribution by various transport mechanisms. Heat transport in tissues mainly occurs by convection and conduction. The time-dependent temperature distribution is given by the bioheat transport equation, neglecting radiation losses and metabolic effects which only play a minor role during laser applications [48Pen]: ρ cp

∂T = ∇ (λ (r, T ) ∇T (r, t)) + QL (r, t) + QB (r, t) , ∂t

(3.1.7)

where ρ is the mass density, cp the specific heat, T the temperature and λ the thermal conductivity at time t and position r. QL is the heat source and represents laser energy that was converted to thermal energy by absorption. The heat source can be calculated from the fluence distribution Ψ (r): Landolt-B¨ ornstein New Series VIII/1C

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QL (r, t) = µa Ψ (r, t)

with

Ψ (r) =

L (r, sˆ) dω .

(3.1.8)

4π

Heat loss by convection in tissue is a result of the specific blood perfusion and is taken into consideration by the parameter QB : QB (r, t) = υB ρ ρB cB (TA − T (r, t)) ,

(3.1.9)

where υB is the blood perfusion rate of a specific tissue volume, ρB and cB are the mass density and the specific heat capacity of blood and TA is the temperature of the supplying arterial blood. In Table 3.1.3 the approximate thermal penetration depth of soft tissues as a function of time is given. Table 3.1.4 gives some perfusion rates of human tissues. Table 3.1.5 gives the mass density of different tissues. Table 3.1.3. Approximate thermal penetration depth of soft tissue as a function of time according to (3.1.1)–(3.1.14). Time t [s]

Thermal penetration depth dth [mm]

0.000001 0.0001 0.01 1 100

0.001 0.01 0.1 1 10

Table 3.1.4. Blood perfusion rates of some selected human organs. Data according to [85Sva, 96Nie]. Tissue

Perfusion rate υB [ml min−1 g−1 ]

Fat Muscle Skin Brain Kidney Thyroid gland

0.012 . . . 0.015 0.02 . . . 0.07 0.15 . . . 0.5 0.46 . . . 1.0 ≈ 3.4 ≈ 4.0

Table 3.1.5. Mass density of different tissues, values obtained from [90Duc]. Tissue

Mass density ρ [kg m−3 ]

Tissue

Mass density ρ [kg m−3 ]

Whole body Brain Fat Eye Kidney Liver Muscle Stomach Skin Horny layer Epidermis

1070 men, 1040 women 1030 . . . 1041 917 . . . 939 1022 . . . 1030 1050 1050 . . . 1070 1040 . . . 1060 1048 . . . 1050 1093 . . . 1190 1500 1110 . . . 1190

Bone, cortical –, trabecular Marrow Cartilage Hair Nails Teeth Enamel Dentin Blood, whole –, plasma

1990 ± 27 1920 ± 20 (bone alone) 923 . . . 1047 1092 . . . 1104 1310 1300 2090 . . . 2240 2890 . . . 3020 2030 . . . 2350 1052 . . . 1064 1025 . . . 1030

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[Ref. p. 303

Table 3.1.6. Relative water content of various human tissues. Tissue

Relative water content [weight-%]

Reference

Skeleton Fat Skin Brain white mass Pancreas Liver Intestine Muscle Spleen Heart Brain gray mass Lung Kidney

31.8 50.0 64.7 72.0 73.1 77.0 79.1 79.5 80.0 81.0 82.0 83.7 84.0

[45Mit] [45Mit] [45Mit] [92Run] [45Mit] [71Coo] [45Mit] [45Mit] [71Coo] [71Coo] [92Run] [45Mit] [71Coo]

In biological tissue the thermal parameters mainly depend on its relative water content wwater . A good approximation for the thermal parameters as a function of wwater at 37 ◦ C is given by the basic paper [77Tak]: λ37◦ C = 4.19 (0.133 + 1.36 wwater )10−3 cp,37◦ C = 4.19 (0.37 + 0.63 wwater ) ρ37◦ C = 1.3 − 0.3 wwater

[ 102 W K−1 m−1 ] ,

(3.1.10)

[ 103 J kg−1 K−1 ] , [ 10−1 kg m−1 ] .

(3.1.11) (3.1.12)

In Table 3.1.6 the water content of various human tissues is given. The density of most soft tissues is slightly higher than that of water, typically ranging between 1.00 and 1.07 g cm−3 . The presence of fat lowers the average density of tissue. From the thermal parameters the so-called thermal diffusivity α can be calculated: α=

λ . ρ cp

(3.1.13)

Values for the thermal conductivity λ and the diffusivity α of different human tissues are given in Table 3.1.7. Beside the optical penetration depth it is useful to define a thermal penetration depth dth . If a short heat pulse strikes a tissue surface, it takes the time τ before a significant temperature rise is noticeable in the depth √ dth ≈ α τ . (3.1.14) Finally, a thermal relaxation time τrel can be defined, giving the approximate time a heated volume with a characteristic length l needs to cool down to the ambient temperature: τrel ≈

l2 . α

(3.1.15)

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Table 3.1.7. Thermal conductivity λ and diffusivity α of different human tissues (for an extended overview see [95Gie]). Tissue

Temperature T [◦ C]

Conductivity λ [Wm−1 K−1 ]

Reference

Diffusivity α [10−7 m2 s−1 ]

Reference

Aorta Blood Blood hemolyzed Blood, 43% Hct Bone, rib Brain, cortex Breast Eye: vitreous Fat: subcutaneous Kidney Liver Lung Muscle: cardiac Muscle: forearm Skin: forearm Tooth: enamel Tooth: dentin Thyroid

25 . . . 80

0.444 0.570 ± 0.010 0.492 ± 0.009 0.530 0.373 . . . 0.496 0.515 0.499 ± 0.004 0.594 0.23 . . . 0.27 0.513 . . . 0.564 0.512 0.302 . . . 0.550 0.492 . . . 0.562 0.75 0.48 . . . 2.8 0.932 0.99 0.526 . . . 0.533

[86Gem] [77Bal] [77Bal] [66Pop] [81Bow] [85Val] [81Bow] [66Pop] [59Hen] [81Bow] [85Val] [81Bow] [81Bow] [91Duc] [85Cha] [70Bro] [71Dem] [90Duc]

1.22 1.21 1.19

[86Gem] [77Bal] [77Bal]

1.47

[85Val]

1.37 . . . 1.47 1.41 1.31 1.47

[85Val] [85Val] [85Val] [85Val]

0.4 . . . 1.6 4.09 16

[85Cha] [70Bro] [71Dem]

24 . . . 38 37 37 37 24 . . . 38 in-vivo 37 37 37 37 33 (in vivo) in vivo 37

3.1.2 Laser-tissue interactions 3.1.2.1 Laser diagnostics by transillumination and induced fluorescences Transillumination is increasingly used in medicine as an almost noninvasive method for various diagnostic procedures. In the weakly absorbing but highly scattering range of tissue spectra, the so-called optical window from 800 nm to 1000 nm (Fig. 3.1.1), the collimated laser light changes its properties while crossing the biological sample into a dominant contribution of scattered (i.e. diffuse) light and ballistic or so-called snake-like photons according to (3.1.1). The resulting image (2-D intensity distribution) is a mixture between anatomic and functional tissue behavior. From a historical point of view, before the invention of the laser, the sun or artificial light sources were used to transilluminate e.g. the testis or the female breast in a darkened room according to the examiner’s accommodation [93Mue, 98Min]. In recent years, selected parts of the human body like frontal sinus changes or rheumatism of small joints have been successfully transilluminated using incoherent light [93Beu] or lasers [02Beu, 03Hen]. The recent use of the photoacoustic effect with pulsed laser illumination also shows promising results in animal experiments [03Wan]. Laser-induced fluorescence originates from either endogeneous chromophores or/and by application of fluorescent markers. In both cases, fluorescence light is generated by the coherent or step-wise absorption of radiation, which may also be composed of two photons and subsequent relaxation from the excited into the ground state of the molecule which is shown in the Jablonski diagram (Fig. 3.1.2). The lifetime τ of the excited state S1 is coupled with the rate kF of fluorescence to the ground state S0 , the rate kIC of internal conversion to S0 , the rate kISC of the intersystem crossing from the singlet S1 to the triplet T1 , and the rate of non-radiative (vibration) transfer kET to the environment: kF + kIC + kISC + kET = 1/τ . Landolt-B¨ ornstein New Series VIII/1C

(3.1.16)

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3.1.2 Laser-tissue interactions

[Ref. p. 303

Fig. 3.1.2. Jablonski diagram for 1-photon (left transition S0 → S2 ) and 2-photon (right transition S0 → S2 ) excitation. Straight lines mark radiative transitions and wavy lines non-radiative transitions (further details see text).

For thin tissue layers at the excitation wavelength λ the following relation holds in a first approximation: IF (λ) ∼ = I0 ε(λ)cq ,

(3.1.17)

showing that the fluorescence intensity is proportional to the spectral extinction coefficient ε(λ), the concentration of the chromophore c and the quantum yield q [98Beu, 02Sch]. The behavior of both excitation and fluorescence light are governed by the radiation transport equation (3.1.1) for a given wavelength. The fluorescence can be detected continuously or time-resolved (time correlation or phase fluorometry). According to the polarization status of the fluorescence or Fluorescent Resonant Energy Transfer (FRET) insight into the intermolecular architecture of cells is provided [02Zan]. The basics of spectroscopy on chromophores of cell metabolism had been investigated already by Chance [54Cha, 62Cha]. In vivo investigations followed [82Ren, 90Beu, 91Loh]. The so-called optical biopsy investigates typical endogenous chromophores like pyridine nucleotides, the flavins, and the reduced nicotinamide adenine dinucleotide NADH, which play an important role in cellular energy metabolism [90Beu, 03Ric, 03Wag]. These endogenous fluorophores show fluorescence at 500 nm being excited in the UV [92Lak, 92Sho]. Metabolic activities are monitored by essential changes in spectral properties. Free NADH in folded conformation has an emission maximum around 465 nm, whereas the emission maximum of bound NADH is at 440 nm; binding results in a 2-fold increase in fluorescence intensity and a larger lifetime τ according to (3.1.16) (400 ps in comparison to 1 s) [92Lak, 03Min]. Due to the low penetration depth of the excitation radiation in the UV, endogenous chromophores are used, e.g. for the diagnosing of superficial tumors [98Beu, 02Sva]. There are various possibilities to avoid this disadvantage of optical biopsy. One of these possibilities is the 2-photon excitation of endogenous fluorophores having excitation wavelengths in the NIR range (Fig. 3.1.2). The microscopic use of this approach is well established [01Dia, 01Roe]. Another possibility is to use the enhanced sensitivity of synthetic fluorescence probes with exogenous dyes which started in 1914 by S. von Provazek in experimental cytology [96Fal, 03Lax]. Due to their synthesis such fluorescent probes designed for optical molecular imaging have a better quantum yield and photostability and show different fluorescence wavelengths in comparison to endogenous chromophores to avoid interferences. According to Weissleder [99Wei2], these chromophores should offer suitable ligands and a sensivity to metabolic deviations. Typical fluorophores are fluorescine, Alexa Fluor dyes, the Schering dye NIR96010 [02Lic, 02Min, 03Min], or the green fluorescent protein GFP [93Cod, 99Ika]. This type of diagnosis takes Landolt-B¨ ornstein New Series VIII/1C

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advantage of the fact that the fluorescence signal of an organic dye molecule may be affected by its local environment. For instance, energy transfer, fluorescence quenching or de-quenching processes can be used to report molecular or physiological conditions. Beside different organic fluorescent dyes and GFP-based biosensors, which have been routinely used for fluorescent imaging of cells, the luminescent “quantum dots” have recently entered the field of bioanalytics. Quantum dots are semiconductor nanocrystals of small geometry with special optical properties [97Wog, 03Qdo]. A typical structure consisting of some thousands of GaAs/AlGaAs atoms is as small as 4 nm in diameter, and a In/P/AlGaAs structure even 3 nm. Emission wavelengths are directly correlated with particle diameters [97Wog]. For example, a quantum dot made from Cd/Se radiates green light of 520 nm or red light of 630 nm depending on whether the diameter is 3 nm or 5.5 nm. Quantum dots can be excited using a broad spectrum, i.e. a mixture of different quantum dots can be excited by a laser source of a single wavelength (between UV and red) for multiple-color imaging [02Cha]. The potential in future biological and medical use offers a variety of possibilities, for example the analysis of thermal damage of internal cellular structures (micro-dosimetry) [04Min]. The image got by any described chromophore above is blurred both by photon migration in the excitation and fluorescence wavelength due to the radiative transport equation (3.1.1) and has to be rescaled according to the respective tissue optical parameters [02Min, 04Min].

3.1.2.2 Laser-induced photochemistry Photochemistry occurs whenever visible or ultraviolet radiation interacts with tissue biomolecules themselves or by dyes or drugs introduced into a tissue [98Mos, 99Lip, 02Way]. First of all, from a physical point of view, a rational kind of dosimetry should be established [02Jac]. There are beneficial photochemical reactions such as isomerization, dissociation and synthesis which can also be laser-induced reactions. Photosynthesis, the basis of life, is probably the most well-known photochemical reaction. The reaction is initiated by the direct conversion of light which is absorbed by chlorophyll and results in bioactive compounds which can be stored and used as a pool for further biosynthesis. The UV-induced synthesis of the biopolymer melanin leads to enhanced protection against UV-radiation. Laser-induced isomerization forms the basis of the mechanism of vision and is effected by the E/Z-isomerization of the rhodopsine chromophore. This type of reaction is also essential for the blue-light therapy which brings about a conformational change in the biomolecule bilirubin. Moreover, the biosynthesis of vitamin D3 is effected by an isomerization of intermediate compounds of the biochemical pathway. In the medical field, the principle of photoinduced dissociation is utilized in PhotoDynamic Therapy (PDT) [95Bon]. PDT is a kind of a photochemotherapy by which the interaction of a photosensitizer with laser light leads to the production of cytotoxic substances (singlet oxygen) causing the tissue damage followed by necrosis [92Hen, 01Jac]. Adverse photochemical effects in tissue, such as erythema and photokeratoconjunctivitis, are well documented. Most of the novel chemistry induced by lasers involves infrared multiphoton excitation.

3.1.2.3 Photothermal effects The thermal effects of laser radiation on tissue are based on the absorption of the radiation and its conversion into heat. Depending on the temperature the irradiated tissue may remain coagulated in situ or may be removed totally. At low and medium radiation levels and long irradiation times

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[Ref. p. 303

typical thermal tissue effects are photocoagulation and photovaporization. At very high radiation levels and short pulse duration times effects like photoablation and photodisruption take place. 3.1.2.3.1 Coagulation The coagulation process (irreversible denaturation of proteins) is a major result of heating up biological tissue to temperatures of above 43 ◦ C. The massive protein denaturation in tissue takes place above 60 ◦ C and effects its optical properties [93Cil, 91Jac, 03Chi]. However, the real amount of irreversible denatured proteins depends on the temporal evolution of the applied temperature. This behavior can be described by an approximation of the underlying chemical rate equation, first introduced by Arrhenius [47Hen, 74Joh, 84Wel, 91Jac, 95Spl, 03Dia]: C(t) = −A ln C0


t 0

 ∆E dt
≡ −Ω , exp −A RT (t
) 

(3.1.18)

where C0 is the initial concentration of intact proteins, C(t) the residual concentration of intact proteins at time t, A and ∆E are the Arrhenius parameters, R is the gas constant and T the temperature in Kelvin. Ω is called the damage integral and ranges from 0 to ∞. Table 3.1.8 shows some experimental parameters of ∆E and A for some kinds of soft tissue. Table 3.1.8. Arrhenius parameters of some tissues. Tissue Retina Skin (T < 50 ◦ C) Skin (T > 50 ◦ C) Skin Liver (rat)

A [s−1 ]

∆E [J/mol] 44

1.0 × 10 4.3 × 1064 9.3 × 10104 3.1 × 1098 2.7 × 1038

5

2.9 × 10 4.2 × 105 6.7 × 105 6.3 × 105 2.6 × 105

Reference [71Vas, 84Wei] [77Tak] [77Tak] [47Hen] [91Jac]

3.1.2.3.2 Evaporation of tissue Above 100 ◦ C the water in the tissue is boiling and the tissue becomes dehydrated. At high laser power density the temperature reaches some hundred ◦ C very rapidly and the tissue evaporates because of the strong boiling effect and pyrolysis of the tissue matrix.

3.1.2.4 Effects of short-pulsed laser radiation For a pulse width below the thermal relaxation time the absorbed energy in the irradiated tissue is conserved. In laser application the beam width at the tissue surface or the optical penetration depth, whatever is smaller, may be considered as the characteristic length l which is important for the thermal relaxation time. Using short-pulsed lasers heat-transport effects can be neglected if the pulse width is smaller than the thermal relaxation time. The removal of tissue in this time domain is named “photoablation”. The photoablation is independent of the temporal pulse profile, thus only characterized by the deposited energy per volume. This rule is violated when the power density becomes high enough or the pulse width becomes short enough that multiphoton effects, ionization, inverse bremsstrahlung or other non-linear effects occur significantly. Landolt-B¨ ornstein New Series VIII/1C

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3.1.2.4.1 Photoablation Photoablation is the process of tissue or other materials’ removal from a surface by applying pulsed laser radiation. According to the optical properties of the material the radiation is spatially distributed and absorbed leading to a deposited spatial distribution of energy and with the thermal material parameters to a spatial distribution of a temperature rise. Due to the restriction to short pulses – as defined before – there are no major energy losses during the laser pulse. With a low temperature rise resulting from low energy densities the material will cool down after a certain time leaving the tissue in place. Above a certain tissue-dependent threshold (ablation threshold) in temperature rise, energy density or absorbed energy/volume a part of the irradiated material will explosively be removed. From the energy balance it can be found that the energy necessary to remove a certain amount of material is roughly the energy necessary for evaporation [92Kar]. This holds true for different wavelengths from UltraViolet (UV) to InfraRed (IR) as well as for different pulse duration in the domain of photoablation which can range from picoseconds to some microseconds. The distribution of absorbed energy per volume is governed by the optical parameters of the tissue and the incidence of radiation onto the tissue surface leading in general to a decrease of absorbed energy per volume in depth and laterally. In the volume in which the energy is above the ablation threshold the cohesion of the material will be overcome and it will explode. The tissue volume where the energy is below the ablation threshold is heated and will depending on the temperature be coagulated or otherwise changed. By increasing the laser energy the border where the absorbed energy is above the ablation threshold is moved deeper into the tissue and the volume which is absorbed increases. Therefore, typical dependencies of ablated thickness versus energy density show a behavior with the ablation starting at a tissue-specific and wavelengthspecific energy-density threshold value and then increasing with increasing energy [98Olm]. It is observed, that the threshold energy density is nearly inversely decreasing with increasing absorption either by changing the wavelength or by adding absorbers to a tissue [92Kar]. This confirms a tissue-specific energy per volume for ablation. A further differentiation of ablation may be done by the laser pulse width [97Hib]. For short pulses (nanosecond range or shorter) the light is absorbed and after the laser pulse the evaporated material is exploding. In a simplified model with an exponentially decreasing fluence (Fz = F0 e−µz according to Lambert-Beer’s law) in the tissue and assuming an ablation threshold fluence Fth the ablation depth d of the ablated layer is given by:     1 1 F0 µ F0 = ln . (3.1.19) d = ln µ Fth µ Habl For longer laser pulses (microseconds to milliseconds) but still below the time for heat exchange with the surrounding the absorption, heating and evaporation take place at the same time in a continuous process. An energy balance neglecting losses gives for a tissue-specific evaporation energy per volume Habl the following equation for the evaporated layer d : d=

1 F0 − Fth F0 − Fth = . µ Fth Habl

(3.1.20)

In ablation with repetitive laser pulses each laser pulse leads to the removal of a tissue slice. In some cases the ablation is not constant with the number of pulses applied. This has its origin in a change of the tissue by previous laser pulses. Moreover, a dependence on the repetition rate is observed. With increasing frequency the ablation threshold is lowered but the ablation at higher energies keeps unchanged. This happens due to an accumulation of heat in the sub-threshold range of ablation where the heat is not removed by the ablated tissue and the time between two laser pulses is too short for a total heat dissipation by heat conduction [92Kar]. Several effects may occur during or after the photoablation. The ablated material is explosively moving away from the irradiated area in direction of the incident laser radiation. If the laser pulse Landolt-B¨ ornstein New Series VIII/1C

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Table 3.1.9. Photoablation data of different tissues. Tissue

Ablation depth [µm]

Pulse duration [µs]

Fth [J cm−2 ]

Habl [kJ cm−3 ]

Ref.

Skin

2.1 2.79 2.94

300 200 250

20 1.7 0.75 . . . 1.2

4.6 1.5 1.5

[97Hib] [97Hib] [97Hib]

Aorta

0.193 0.248 0.308 0.308 0.351 0.532 1.06 2.94 9.6 10.6

0.017 0.023 0.017 0.3 0.02 0.009 0.01 180 1.2 1.2

0.1 0.8 0.8 0.8 1.7 19 36 2 4 6

1.2 1 1 1 1.4 10 16 0.25 1.5 2.5

[92Kar] [92Kar] [92Kar] [92Kar] [92Kar] [92Kar] [92Kar] [92Kar] [92Kar] [92Kar]

Bone (Compakta, Femur)

0.308 0.308 2.1 2.94 9.6 10.6 10.6

0.025 0.25 250 180 0.3 0.04 0.3

1.8 4 100 20 2.5 3 5

16 29 36 6 10.5 13 70

[92Sch] [92Sch] [92Sch] [92Sch] [92Sch] [92Sch] [92Sch]

is very short (some nanoseconds or shorter) this happens after the end of the laser pulse and is not influencing the process of ablation. But if the pulse is longer (microseconds) vapor and fragments develop during the laser pulse and lead to absorption or scattering of the laser radiation. If the power density is additionally high the vapor can be partially ionized and a plasma will develop. This plasma can be a very strong absorber leading to a total shielding of the target (“plasma shielding”). In Table 3.1.9 photoablation data of different tissues are given. 3.1.2.4.2 Optical breakdown and plasma formation If the energy densities are higher than required for photoablation, another phenomenon occurs which can be used for laser therapy. This phenomenon is called optical breakdown. In the nanosecond range the threshold for the optical breakdown is approximately 1 GW/cm2 , depending on the tissue and the wavelength. This process, which takes place in both absorbing and transparent tissue, has two stages. In the first stage, single free electrons are generated by multiphoton ionization or thermionic emission. In the second stage, each of these free electrons is accelerated by inverse bremsstrahlung in the electric field of the laser radiation. The electrons transfer the kinetic energy to the electrons still bound by collisions. If the energy transferred by collision is higher than the ionization energy, another free electron is produced, which is accelerated in the electric field of the laser radiation like the pushing electron and can release other electrons by collision. The whole process causes an avalanche-type increase in the number of free electrons and is, therefore, called avalanche ionization. Consequently, the density of the free electrons is exponentially rising. The state of matter with many free electrons and ion bodies is called plasma. At a molecule density of approximately 3 × 1023 cm−3 in tissue the tissue properties are distinctly changed from a density of free electrons of approximately 1018 cm−3 .

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The absorption of tissue in plasma state is very large and may lead to a penetration depth of merely 20 µm for 1064 nm laser radiation at an electron density of 1020 cm−3 even if the medium, such as the eye lens, had been transparent before. We talk of plasma shielding if laser radiation shall be used for removing tissue by e.g. photoablation, but due to the development of an unwanted plasma the radiation is absorbed in the plasma already and does not penetrate to the tissue surface any more. Due to the ionization energy such plasma has a very large energy content, equivalently its temperature is very high ranging within some thousands degrees Kelvin, and so is its pressure that ranges within some kbar. This is why the plasma expands rapidly; it is so to say an explosion in the nanosecond range which, in turn, generates shock waves and cavitations bubbles. The formation of the plasma depends on the pulse duration, the pulse energy and the wavelength of the laser. The tissue absorption is of minor importance only (influence on the formation of the first free electrons) which is contrary to the major importance of the absorption for thermal laser effects [02Vog]. 3.1.2.4.3 Laser-induced shock waves and cavitations From a small plasma within the focus of a laser beam or at the end of an optical fiber the hot tissue volume expands at speeds ranging within several thousands of m/s pushing against the surrounding tissue or fluid. This causes a shock wave radially from the plasma. Due to the high pressure and the high temperature the expansion exceeds the acoustic velocity of the different tissues by far. The pressure wave, therefore, does not expand as sound wave, but as shock wave. This means that the pressure front is characterized by a sharp rise and, contrary to a sound wave, changes its shape while expanding. While the shock wave is expanding (in a homogenous medium as spherical wave), the surface of the wave front is increasing which, in turn, reduces the pressure jump. The shock wave becomes a sound wave which now expands at acoustic velocity. Due to the expansion, the hot plasma also cools down, the pressure is reduced and the plasma surface expands increasingly slower and falls behind the shock wave, which continues as pressure pulse. However, the plasma bubble continues expanding to some hundreds of µs. Its internal pressure falls below the ambient pressure due to the large volume and cooling off. Once the bubble wall is stagnant, the direction of motion reverses, which makes the bubble collapse. This process, namely the creation of a cavity by expansion and collapse, is called cavitations and the bubble is called cavitations bubble. Due to the collapse of the cavitations bubble, the bubble’s volume is strongly compressed and heated up, which causes the generation of another shock wave and another cavitations bubble, if any. This periodical continuation of the same processes is called cavitations bubble oscillations. As energy is always released into the ambience in the form of shock waves, heat and light, the oscillation is steadily weakening. While such cavitations are excellently visible in a fluid (e.g. when performing a laser lithotripsy), this type of bubble oscillation is, more or less, impeded in tissues that have structural strength [95Kru].

3.1.2.5 Laser biostimulation The Low-Level Laser Therapy (LLLT) belongs to the group of methods within molecular photomedicine which is called biostimulation [02Way]. Laser biostimulation is a phenomenon of a photobiological nature and is based on the action of light upon endogenous photoacceptors in the cell without any heating of the tissue. In 1973 this kind of clinical laser-therapy technique was introduced by Mester. More than one function of a cell can be influenced by low-level laser Landolt-B¨ ornstein New Series VIII/1C

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[Ref. p. 303

irradiation, but the mechanism of its action is unclear [90Han, 93Nem, 97And, 99Kar] yet. For example, the role of the amplification of signals in membrane receptors was observed [00Liu]. Due to the low intensities (10−4 . . . 10−2 W/cm2 ) of light, it is presumed that the radiation acts as a trigger for metabolic rearrangements inside the cell [87Kar1, 01Kar1, 01Kar2, 01Ona]. Various kinds of laser light have been used, in particular the He-Ne-laser (632.8 nm), Ga-As-diode lasers (830 . . . 905 nm) and the He-Cd-laser (441.6 nm) have been used for numerous investigations [86Bas, 86Gal, 77Gam, 86Kry], such as wound healing, musculo-sketal aliments caused by muscle pain after injury, surgery or arising from arthritic disorders [87Lyo, 90Kle, 90Sug, 97Yu, 97Wal]. On the cellular level the inhibitive, stimulative or neutral effect of visible light (blue or red) on the cellular growth and division for microorganism has been observed [76Kri, 66Ehr, 81Jag, 95Utz]. The problems of stimulation and inhibition using radiation of the same wavelength have been discussed in detail by [87Kar2].

3.1.3 Medical laser systems 3.1.3.1 Typical medical lasers In Table 3.1.10 typical medical cw-lasers and in Table 3.1.11 typical medical pulsed lasers are listed.

3.1.3.2 Laser light delivery An overview over possible laser light delivery systems between a laser and a medical-optical device is given in Table 3.1.12. Because direct coupling is only suitable with compact systems it won’t be discussed in the following. 3.1.3.2.1 Articulated arm For several wavelength ranges there are no light guides available to meet all requirements in medical applications, e.g. in IR about 10.6 µm (CO2 ) or 2.94 µm (Er:YAG). Here laser light can be brought to a hand piece resp. operation microscope or endoscopes with an articulated arm. For optimal handling at least five movement degrees of freedom are necessary, three for positioning and two for angle alignment. The rotation symmetry of the laser beam makes a third degree for angle alignment unnecessary. In commercial systems the beam passes through long tubes where seven rotatable joints and 45 ◦ mirrors technically realize the movement degrees of freedom. The quality of an articulated arm is essentially characterized by weight and accuracy of angle and position of the laser beam at all reflecting and escaping points. High accuracy requires very rigid bearings and compensation of weight with counterweights. 3.1.3.2.2 Light guide The use of fiber light guides is widely spread over medical applications as disordered fiber bundle for illumination, ordered fiber bundle for imaging, or as single fiber for laser light transmission. In most cases optical glass fibers are used but there are different types of fiber light guides under investigation for specific applications like optical fibers of special materials or hollow wave guides for IR light transmission. Landolt-B¨ ornstein New Series VIII/1C

Landolt-B¨ ornstein New Series VIII/1C

20 . . . 100 W 20 . . . 60 W 20 . . . 100 W 0.1 . . . 30 W 0.5 W 0.5 . . . 5 W 3W

10600 nm 1320 nm 1064 nm 650 . . . 980 580 . . . 700 488 . . . 514 531 . . . 676

CO2 Nd:YAG Nd:YAG Diode Dye Ar+ Kr+

nm nm nm nm

Power

Wavelength

cw-laser

Table 3.1.10. Typical medical cw-lasers.

articulated arm 400 . . . 600 µm IR-Q-fiber 400 . . . 600 µm Q-fiber 400 . . . 600 µm Q-fiber 400 . . . 600 µm Q-fiber 200 . . . 600 µm Q-fiber 200 . . . 600 µm Q-fiber

Beam delivery cutting, ablation coagulation coagulation photochemical, coagulation photochemical coagulation coagulation

Tissue interaction

surgery surgery surgery photodynamic therapy, surgery photodynamic therapy ophthalmology, dermatology ophthalmology, dermatology

Medical field

Ref. p. 303] 3.1 Lasers in biology and medicine 293

2J 0.5 . . . 3 J 0.5 . . . 3 J 5 . . . 10 mJ 1J

2940 nm

2100 nm

1440 nm

1064 nm

700 . . . 800 nm

800 nm

694 nm

580 . . . 700 nm

510/578 nm

628 nm

532 nm

Er:YAG

Ho:YAG

Nd:YAG

Nd:YAG

Alexandrite

Diode

Ruby

Dye

Cu-Vapor

Au-Vapor

Nd:YAG frequencydoubled

ArF-Excimer

193 nm

60 mJ

20 . . . 40 mJ

100 mJ 1J

10 mJ

10 mJ

1J

500 mJ

500 mJ

25 ns

120 ns

800 ns 1 . . . 100 ms

100 µs

100 µs

1 ms

1 . . . 40 ms

5 . . . 30 ms

1 . . . 20 ms

7 ns 1 ms

500 µs

500 µs

500 µs

500 µs 50 ms

Pulse duration

30 Hz

10 . . . 25 Hz

10 Hz 1 . . . 15 Hz

1000 Hz

1000 Hz

single pulse 1 . . . 5 Hz

5 . . . 20 Hz

5 . . . 30 Hz

5 . . . 15 Hz

10 . . . 100 Hz single pulse

Repetition rate

free beam

600 µm UV-fiber

400 . . . 600 µm fiber

400 . . . 600 µm fiber

400 . . . 600 µm fiber

400 . . . 600 µm fiber

400 . . . 600 µm fiber

400 . . . 600 µm fiber

400 . . . 600 µm fiber

400 . . . 600 µm fiber

400 . . . 600 µm IR-fiber

400 . . . 600 µm IR-Q-fiber

articulated arm (sapphire fiber)

articulated arm

Beam delivery

ablation

ablation, drilling

opt. break through coagulation

photochemical

photochemical

coagulation

coagulation

coagulation

opt. break through

ophthalmology

angioplasty, TMLR

lithotripsy dermatology

PDT

PDT

dermatology,

dermatology

dermatology

lithotripsy

ophthalmology dermatology

cartilage

cutting opt. break through

cartilage TMLR

hart tissue, dermatology

dermatology TMLR

Medical field

cutting drilling

ablation

cutting, drilling

Tissue interaction

3.1.3 Medical laser systems

XeCl-Excimer 308 nm

1J 40 J

10600 nm

CO2

1J

Pulse energy

Wavelength

Pulsed laser

Table 3.1.11. Typical medical pulsed lasers.

294 [Ref. p. 303

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Table 3.1.12. Light delivery systems and their properties [90Gre]. Advantage

Disadvantage

Wavelength

Direct coupling

good beam quality, coherence and preservation of polarization

only for compact laser systems

any

Articulated arm

good beam quality, coherence and preservation of polarization

not flexible, complex, difficult adjustment

any, mostly UV and IR

Light guide

flexible, cheap

restrictions in coherence, polarization and focus

near UV-VIS and near IR

3.1.3.2.3 Optical glass fiber Optical glass fibers mostly consist of a core with high index of refraction, a surrounding cladding with low index of refraction, and an additional coating for adequate strength. Therefore, the principle of light transport in fiber core is based on total reflection at the boundary between core and cladding beneath the critical angle of incidence. Light transmission depends essentially on the angle of light coupling into the fiber and on the bending radius of the fiber [93Kat, 03Utz]. The numerical aperture (NA) is a measurement of the ability of an optical fiber to capture light. The numerical aperture equals the sine of the vertex angle of the largest cone of meridional rays that can enter or leave the optical fiber, multiplied by the refractive index of the medium in which the vertex of the cone is located. Typical numerical values range from 0.1 to 0.4. Optical fibers are commonly made of silica alternatively known as quartz for both core and cladding or consist of a silica core and a plastic cladding. The most common coating for silica fibers is acrylate. In silica-silica fibers a different index of refraction is achieved with different concentrations of doping elements which increase resp. decrease the index of refraction such as Ge, P, Al resp. F. Silica fibers are highly transparent for near UV, VIS and near IR. The transport of IR laser light is possible with single-crystal-sapphire or polycrystalline fibers but they suffer from insufficient strength and high losses. For example about 10.6 µm (CO2 ) polycrystalline fibers are made of chalcogenide (Ge, Sb, Se, As, Te), silver halide (AgCl, AgBr) or ZnSe. For 2.9 µm (Er:YAG) light transmission is possible with ZrF or BaF fibers [90Gre, 92Har]. In Table 3.1.13 the transmission of optical glass fibers is given. Table 3.1.13. Transmission of optical glass fibers. Optical glass fiber

Wavelength

Attenuation (order of magnitude)

Reference

silica-silica sapphire chalcogenide silver halide zones ZrF

200 nm . . . 2 µm 300 nm . . . 3.5 µm 4 µm . . . 11 µm 3 µm . . . 15 µm

5 dB/km at 800 nm 1 dB/m at 2.94 µm 0.1 . . . 1 dB/m at 10.6 µm 1 dB/m at 10.6 µm > 0.3 dB/m at 10.6 µm 0.5 . . . 1 dB/m at 2.94 µm

[90Gre, 99Oxf] [92Har] [92Har] [99Oxf] [90Way] [90Gre]

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[Ref. p. 303

3.1.3.2.4 Hollow wave guide A different approach to guide light with fibers is to use a hollow glass tubing with air as core material. To achieve reflection inside the tube the index of refraction of the glass tubing has to be lower than air or an additional metal layer has to be formed inside. While the first type is theoretically possible [93Chr], the second type is in use but suffers from high losses for bending and for small diameters. Therefore, these hollow wave guides require great bending radii and tube diameter. Thus, wave guides with sufficient transmission are less flexible and difficult to handle.

3.1.3.3 Applicator systems Applicator systems deliver the laser light into the tissue and are therefore located at the end of a light guide. Applicator systems are necessary to provide the desired beam geometry or a certain light distribution at the point of application. Both, beam geometry and light distribution are important to produce a specific tissue reaction such as photochemical, thermal or non-linear tissue effects. Since the laser has been introduced into medicine, a number of different applicators have been developed for clinical use and their number has been expanding with the introduction of new fields of application. Table 3.1.14 gives an overview of applicator sytems [93Joh, 95Cas, 95Hei, 95Rog, 99Mor, 99Saj].

3.1.4 Laser applications in medicine Thanks to technical progress and improved application techniques the steadily increasing number of laser systems has rapidly become more and more versatile in application over the past years. In the medical sector lasers have meanwhile become common standard in most clinical disciplines [92McL, 92Ste, 95Whe, 97Dus, 97Sch, 97Zop, 99Buk, 99Sch, 99Lut, 02Tuc, 02Way, 03Ber]. Generally, lasers can be used in therapy either as a surgical tool or as a central therapy procedure. Monitoring metabolism and imaging procedures are the main aim for the use of lasers in diagnostics. According to their technical realization and therapy approach all laser procedures can be applied percutaneously, endoscopically or during open surgery. The explanation of laser processes in therapy, the distinguishing between the fields of application and the laser range in the treatment must be taken into account when laser position in medicine is considered. According to the parameters of laser radiation photochemical, photothermal and photomechanical effects on tissue can be observed. The effect most often employed clinically is coagulation and vaporization of tissue. At temperatures between 60 ◦ C and 100 ◦ C tissue coagulation occurs. This kind of tissue effect is used for thermal destruction of diseased structures. After several weeks the treated area heals by fibrosis. A transcutaneous coagulation can be achieved by tissue cooling with an ice cube or special cooling devices [95Phi1]. Interstitial coagulation is used for Laser-Induced ThermoTherapy (LITT) by introducing a bare fiber or a special laser applicator into the pathological tissue after puncturing it with a specially developed puncture set. LITT is mainly used for the treatment of liver metastases [95Vog, 97Vog], brain tumors [96Men] and benign prostatic hyperplasia [95Mus]. For vaporization the CO2 laser is mainly used for microsurgical preparations with a very narrow coagulation zone. For cuts having a wide coagulation edge, as required for hemostasis, an Nd:YAG laser with a focusing hand piece or a bare fiber in contact can be used [90Ber, 90Phi]. Ablation of homogenous and extended areas is best achieved by using a CO2 laser with a scanner system or Landolt-B¨ ornstein New Series VIII/1C

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Table 3.1.14. Overview of applicator systems. Applicator

Description / specification

Laser

Medical effect

Application

Comment

Handpiece

Handheld case with lenses focusing the laser beam into a focal spot (focal length e.g. 50 mm). Power density can be varied by adapting the distance between handpiece and tissue.

VIS

coagulation

NIR

coagulation, cutting

superficial, non-contact

gas-flushed to protect lenses from ablated particles

IR

cutting, ablation

Clean broken end of a fiber light guide without any additional changes or devices

UV

ablation, disruption

universal use, low cost

VIS

coagulation, disruption

NIR

coagulation, cutting, disruption

superficial / interstitial / endoscopical / intraluminal, contact / non-contact

Specially prepared bare fiber end which distributes the laser light uniformly into all directions over a specific length (e.g. 20 mm)

VIS

hyperthermia, coagulation

interstitial, contact

NIR

hyperthermia, coagulation

possibly liquid-flushed to allow large coagulation volumes

Specially designed fiber tip to deliver the laser beam in a 90 ◦ angle away from the fiber

NIR

coagulation

intraluminal, contact / non-contact

special applications only e.g. BPH treatment

VIS

coagulation

NIR

coagulation, cutting

superficial, non-contact

IR

coagulation, cutting

only in combination with surgical microscopes, colposcopes, etc.

Light guide Bare fiber

Diffusor tip

Side fiber tip

Micro Lenses and mirrors intemanipulator grated e.g. in a surgical microscope. A joystick allows precise positioning of the laser beam without restrictions to the surgeon’s visibility. Endoscopic coupler

Adapter between articular arm and rigid endoscopes containing lenses with long focal lengths

IR

coagulation, ablation

endoscopical, non-contact

special device, has to be adapted to a certain endoscope

Scanner

Case containing a mirror system which enables an automated scan in different shapes

VIS

coagulation

IR

ablation

superficial, non-contact

efficient ablation of large tissue areas

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3.1.5 Medical laser safety

[Ref. p. 303

pulsed lasers delivering high energy during very short pulse times (e.g. Er:YAG, Ho:YAG or CO2 lasers) [91Ste, 98Bak]. Photoablation by pulsed lasers can be used in ophthalmology for keratotomy [98Ben] and in angioplasty for the revascularisation of atherosclerotic occluded vessels [98Vis]. Photochemical effects (photodisruption) are mainly employed in lithotripsy [90Fra, 99Cal, 99Taw, 99Wei1] and the treatment of certain vascular disorders (e.g. FDL therapy of port wine stains). In some cases, particularly in the tumor therapy [95Phi2] and in the therapy of congenital vascular disorders [95Phi1], the best results can be achieved by using a combination of various application methods. The PhotoDynamic Therapy (PDT) is used for the treatment of superficial lesions such as dysplasia and early carcinoma in dermatology [94Cai], gastroenterology [93Ove, 99Web], urology [96Nse], gynecology [99Hil] and neurosurgery [96Pop]. It is also used in ophthalmology for the treatment of diseases caused by neovascularisation processes [96Hus, 99Oba]. The choice of a laser for the PDT depends mainly on the photosensitizer used for this procedure [87Cas, 97Bay, 97Tso, 98Kes]. The selective accumulation of photosensitizers in diseased tissue can also be used for PhotoDynamic Diagnostics (PDD). In this case, radiation power – having a value below that used for PDT – does not trigger creation of oxygen radicals, but leads to fluorescence. The difference in fluorescence intensity of healthy and diseased tissue enables the distinguishing between these regions [96Ken, 96Pan]. For this purpose it is also possible to use the autofluorescence properties of the tissue [96Beu2]. Scattered and transmitted photons can be detected and consequently utilized for optical tomography or diaphanoscopy [93Beu, 97Mue, 98Min, 99Fin, 02Beu]. Lasers emitting in the near infrared allow penetration of thicker tissue layers and transmission of laser light via flexible and thin light guides brings the light source into the body interior. Improved infrared cameras will deliver high resolution images. Thus, intraoperative differentiation between cystic and solid structures is possible during endoscopic procedures. In Table 3.1.15 indications for the laser application in medicine are given.

3.1.5 Medical laser safety 3.1.5.1 Medical laser pyrolysis products Unlike conventional surgery techniques, a laser can produce a substantial quantity of plume resulting from the tissue vaporized during incision, ablation and/or coagulation. Laser plume is diluted by the surrounding atmospheric gases, to give laser fume. As a result of the physical reaction, human tissues will be thermally destroyed, explosive vapor and gases are given off, viscous fluids made up of droplets and tissue fragments are formed during the laser/tissue interaction and are atomized in all directions. Large aggregates are formed by agglomerization. These effects are comparable to the process of pyrolysis which is characterized by the incomplete oxidative thermal disintegration of organic material with the release of volatile components, particles, gases and supersaturated vapors. Chemical reactions occur inside the fume under atmospheric conditions as well as inside the focus of the laser radiation where laser pyrolysis products act as new targets for laser-induced photochemical and photothermal reactions [96Doe]. This eventually results in the formation of secondary pyrolysis products. The main pyrolysis products of the laser fume are: bioaerosols and gases. With regard to the bioaerosol there are various mechanisms for the formation: Laser aerosols are divided into environmental laser smoke (diameter < 2 µm) and environmental laser dust (diameter > 2 µm). Particle formation and emission is a dynamic process. Size distribution and the amount of particles are influenced by the power density, the mechanical and Landolt-B¨ ornstein New Series VIII/1C

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Table 3.1.15. Indications for the laser application in medicine. Indication

Lasers

Dermatology

benign skin tumors condylomata accuminata warts mollusca contagiosa malignant tumors (palliative treatment) metastases (palliative treatment) keloids, hypertrophic scars tattoo removal skin resurfacing hair removal wound healing

CO2 , argon, Nd:YAG, diode, KTP CO2 , Nd:YAG, diode CO2 , Nd:YAG CO2 , argon CO2 , Nd:YAG CO2 , Nd:YAG CO2 , argon, FDL, (Nd:YAG) ruby, alexandrite, pulsed Nd:YAG, (CO2 ) CO2 (pulsed or scanner systems), Er:YAG ruby, alexandrite, diode CO2

Vascular system

vascular malformations (except port wine stains) port wine stains hemangiomas teleangiectasia of the face teleangiectasia of the legs Osler’s disease laser angioplasty

Nd:YAG, diode, KTP FDL, (argon) Nd:YAG, diode, KTP argon, FDL Nd:YAG, diode, KTP argon, KTP, diode, Nd:YAG excimer

Otorhinolaryngology (ENT)

laryngeal and tracheal stenoses papillomas, polyps, granulomas tumors of the tongue leucoplakia uvulopalatoplasty, velum partial resection gingival hyperplasia reduction of hyperplastic nasal conchas diaphanoscopy, sinusitis maxillaris

CO2 , Nd:YAG, diode Nd:YAG, diode CO2 , Nd:YAG, diode CO2 CO2 , Nd:YAG, diode Nd:YAG, diode Nd:YAG, diode, CO2 diode

Bronchopulmology

tracheal and bronchial fistulas tracheal and bronchial stenoses bronchial tumors (resections, destruction) wedge lung resection chest tumor resection

Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG,

diode diode diode diode diode, CO2

Gastrointestinaltract (GI)

esophago-tracheal fistulas tumors polyps esophagus stenoses LIF

Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, diode

diode diode diode diode

General surgery

transmyocardial laser revascularisation (TMLR) appendectomy adhesiolysis cholecystectomy herniorraphy lymph node resection orchidolysis vagotomy organ resection tumors – resection, destruction (LITT)

excimer, Ho:YAG, CO2

Landolt-B¨ ornstein New Series VIII/1C

Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG,

diode diode diode diode diode diode diode diode diode

300

3.1.5 Medical laser safety

[Ref. p. 303

Table 3.1.15 continued. Indication

Lasers

Proctology

anal stenosis marisque excision coagulation of anal fissures fistulas pilonidal cystectomy polyps hemorrhoids

Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG,

diode diode, CO2 diode diode diode diode diode

Gynecology

condylomata acuminata pre-cancerous lesions endometriosis adhesions fimbrioplasty ectopic pregnancies tubal sterilization hysteroscopic treatment of fibroma, polyps, synechies, myomas and endometrial ablation

Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG,

diode, diode, diode, diode, diode, diode, diode, diode

Urology

benign tumor of bladder bladder carcinomas cystitis lesions ureter tumors kidney tumors urethra strictures interstitial coagulation of benign prostatic hyperplasia (BPH) penis carcinoma condylomata and warts

Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG, Nd:YAG,

diode diode diode diode diode diode diode, (Ho:YAG)

Neurosurgery

vascular malformations excision of brain tumors interstitial coagulation of brain tumors LIF

Nd:YAG, diode CO2 Nd:YAG, diode diode

Orthopedics

percutaneous endoscopic lumbar discectomie percutaneous lumbar disc decompression plica removal synovectomie

Nd:YAG, diode Nd:YAG, diode Ho:YAG, Nd:YAG, diode Ho:YAG, Nd:YAG, diode

photorefractive keratotomy (PRK) laser- in situ-keratomileus (LASIK) laser thermokeratoplasty (LTK) laser cyclophotocoagulation laser trabeculoplasty laser sclerostomy photocoagulation of retina laser capsulotomy

excimer excimer Ho:YAG, diode Nd:YAG, diode argon Ho:YAG, Er:YAG argon, FDL Nd:YAG

Ophthalmology

CO2 CO2 CO2 CO2 CO2 CO2 CO2

Nd:YAG, diode Nd:YAG, diode

Landolt-B¨ ornstein New Series VIII/1C

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thermal nature of the tissue [95Hah, 94Mei2, 94Zie]. Over the last decades several investigations have indicated the possible chemical and biological hazards of laser plume [86Wal, 87Nez]. With a view to developing safety guidelines for medical lasers efforts are still being made to characterize laser pyrolysis product methods [95Fra, 93Lad, 94Lad, 97Web]. Qualitative and quantitative analyses of these contaminants have been documented as a function of laser parameters, type of tissue and application techniques [93Wae, 95Whe, 95Sto, 96Spl, 94Mei1]. Particular bioaerosols and gases are of major importance, with regard to hygiene, as they may have carcinogenic and toxic properties. A number of studies have dealt with the analysis of the chemical composition of fumes, vapors and gases in the vaporized target tissue [88Kok, 97Web, 97Wei]. Early investigations were also focused on to the pyrolysis products resulting from the removal and renewal of implantants [87Gan] using an Nd:YAG laser. All studies clearly show that the main components of the gaseous products are carbon dioxide and water vapor. Small amounts of toxic compounds such as hydrogen cyanide and organic compounds (nitrile, aldehydes, aromates) are also released. The interaction of toxic gaseous and volatile organic compounds has been explored with regard to the respiratory tract [82Dal, 91Cas, 78Dol]. Infected tissue is thought to represent a greater risk of airborne contaminant than aerosol emission. The risk of infection resulting from medical laser treatment has already been dealt within a number of studies. Several publications have dealt with the vaporization of warts, infected by human papilloma virus [89Saw, 88Gar, 89Lob, 91Ste, 95Glo, 94Ber]. Using biomolecular analytical methods, intact viral DNA was detected in the fume [89Saw]. The polymerase chain-reaction method has also been used for monitoring the human papilloma virus contamination [94Ber, 91Kas, 96Med]. Similar results have been obtained using an in-vitro bacteriophage system [85Mul, 87Byr, 91Mat1, 91Mat2, 95Jah]. [91Mat1] concluded that bacteria will only survive when they are added onto large particles. The risk of contamination by spreading cells is negligible, whereas the presence of the human-immuno-deficiency-virus DNA which has been found in laser smoke [91Bag] presents a much greater problem. A realistic assessment of the risk potential involved for pyrolysis products was recorded during laser treatment under OR conditions [94Alb, 95Whe]. To reduce the risk of infection for medical staff and patients appropriate measure should be taken, see [94Cai, 89Cli, 96Hus, 95Pru, 90Smi, 83Ves, 94Wei, 96Woe].

3.1.5.2 Regulatory requirements for medical laser systems The hazard potential of medical laser systems is considered as very high, since a human body is the target for the laser radiation and any malfunction may result in serious personal damage. Therefore, in most countries medical lasers have to comply with specific design requirements and have to undergo some kind of approval in advance of clinical use. Requirements for medical laser systems can be found in the international standard IEC 606012-22 which is recognized in most countries. In the US the Federal Laser Product Performance Standard 21CFR1040.11 is mandatory. In the member states of the EC compliance with the “essential requirements” has to be documented by a conformity assessment according to the Medical Device Directive 93/42/EEC. In the US an approval of the system (with respect to the intended use) by the Center for Devices and Radiological Health (CDRH) of the Food and Drug Administration (FDA) is necessary. Procedural safety measures for the safe use of lasers in health care facilities are often determined in national regulations. In Germany e.g. the instructions for the prevention of accidents BGV B 2 (formerly VBG 93) “Laser radiation” include under § 16 additional regulations for medical applications. In the US the nationally recognized standard ANSI Z136.3 is imperative.

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3.1.5 Medical laser safety

[Ref. p. 303

3.1.5.3 Specific aspects of medical laser safety In addition to ocular hazards and damages to the skin as results of unintentional irradiation some specific risks occur when using the laser as medical instrument [95Sli]. Therefore, “Guidelines for the safe use of medical laser equipment” are provided by the international technical report IEC TR 60825-8. Under disadvantageous circumstances laser beam reflections present the same danger as the direct beam. The surfaces of all surgical instruments used in connection with lasers have to have a matte finish allowing only for diffuse reflections. Convex surfaces should be preferred, because concave surfaces may result in a refocusing of the beam. There is only a German pre-standard for surfaces of surgical instruments suitable for use with lasers (DIN V 18735) which defines surface roughness parameters and degrees of reflection for laser-proofed instruments. Dry swabs, as they are often used in surgery, will ignite immediately when hit by laser radiation. This can be prevented by using swabs which have been soaked in physiological saline solution. The only known lethal incident in laser surgery is the so-called airway fire, which means the ignition of flexible endoscopes and/or tracheal tubes inside the airways of a patient. “Guidance on airway management during laser surgery of upper airway” is given by the international technical report ISO TR 11991. Laser-proofed tracheal tubes are available in two different types: Some have highly but diffuse reflecting metallic surfaces whereas another kind is provided with an absorbing porous layer of high water content using the energy of vaporization for cooling. Test methods for the determination of laser resistance of tracheal tube shafts are described in the international standard ISO 11990. The only way to prevent the plastic parts of a flexible endoscope from being ignited by a laser fiber inside of its working channel is to make absolutely sure that the distal end of the fiber is outside the working channel and that the fiber inside is not broken or even sharply bent. It is always safer to use rigid endoscopes whenever possible.

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References for 3.1 Schmolke, S., Ruhmann, O., Lazovic, D.: Orthop¨ ade 26 (1997) 267. Tsoukas, M.M., Lin, G.C., Lee, M.S., Anderson, R.R., Kollias, N.: J. Invest. Dermatol. 108 (1997) 501. Vogl, T.J., Mack, M.G., Straub, R., Roggan, A., Felix, R.: Lancet 350 (1997) 9070. Walsh, L.J.: Aust. Dent. 42 (1997) 247. Weber, L., Spleiss, M.: J. Anal. Appl. Pyrol. 39 (1997) 65. Weigmann, H.-J., Lademann, J., Meffert, H., Sterry, W.: Proc. SPIE 2923 (1997) 164. Woggon, U.: Optical Properties of Semiconductor Quantum Dots, Berlin, Heidelberg, New York: Springer, 1997. Yu, W., Naim, J.O., Lanzafame, R.J.: Lasers Surg. Med. 20 (1997) 56. Zopf, T., Riemann, J.F.: Z. Gastroenterol. 35 (1997) 987. Baker, J., Stuzin, J.M., Baker, T.M.: Facial Skin Resurfacing, St. Louis, Missouri: Quality Medical Publishing, 1998. Bende, T., Jean, B., Derse, M., Rassmann, B., Thiel, H.J.: Graefes Arch. Clin. Exp. Ophthalmol. 236 (1998) 405. Beuthan, J., Minet, O., M¨ uller, G.: Ann. N.Y. Acad. Sci. 838 (1998) 150. Kessel, D.: Science & Medicine July/August (1998) 46. Minet, O., M¨ uller, G., Beuthan, J.: Selected Papers on Optical Tomography, SPIE Milestone Series MS 147, Bellingham: SPIE Optical Engineering Press, 1998. Moser, J.G. (ed.): Photodynamic Tumor Therapy, Amsterdam: Harwood Academic Publisher, 1998. Olmes, A., Brand, M., Raible, M., Lubatschowski, H., Ertmer, W., Baensch, E., Dziuk, G.: Proc. SPIE 3195 (1998) 208. Visona, A., Perissinotto, C., Lusiani, L., Bonanome, A., Pesavento, R., Miserocchi, L., Liessie, G., Pagnan, A.: Angiology 49 (1998) 91. Bukala, B., Denstedt, J.D.: J. Endourol. 13 (1999) 215. Calvano, C.J., Moran, M.E., White, M.D., Borhan-Manesh, A., Mehlhaff, B.A.: J. Endourol. 13 (1999) 113. Dierichx, C., Alora, M.B., Dover, J.S.: Dermatol. Clin. 17 (1999) 357. Finger, P.T., Iezzi, R., Esteveo, M.L., Szechter, A., Rosen, R.B., Berson, A.: Int. Radiat. Oncol. Biol. Phys. 44 (1999) 887. Hillemanns, P., Korell, M., Schmitt-Sody, M., Baumgartner, R., Beyer, W., Kimmig, R., Untch, M., Hepp, H.: Int. J. Cancer 81 (1999) 34. Ikawa, M., Yamada, S., Nakanishi, T., Okabe, M.: Curr. Top. Dev. Biol. 44 (1999) 1. Karu, T.I.: Proc. SPIE 3829 (1999) 42. Landthaler, M., Hohenleutner, U.: Lasertherapie in der Dermatologie, Berlin, Heidelberg: Springer-Verlag, 1999. Lippert, B.M., Schmidt, S., Werner, J.A. (eds.): Fluoreszenzdiagnostik und Photodynamische Therapie, Aachen: Shaker Verlag, 2000. Lutter, G., Martin, J., Beyersdorf, F.: Zentralbl. Chir. 124 (1999) 171. Morita, T., Okamura, Y., Ookubo, K., Tanaka, K.: Journal of clinical laser medicine & surgery 17 (1999) 57. Obana, A., Gohto, Y., Kaneda, K., Nakajima, S., Takemura, T., Miki, T.: Lasers Surg. Med. 24 (1999) 209. Oxford Electronics Ltd: Optical Fibres, Product information, 1999. Sajben, F.P., Ross, E.V.: Dermatologic surgery 25 (1999) 41. Schenk, L.M., Coddington, C.C.: Obstet. Gynecol. Clin. North Am. 26 (1999) 1. Tawfiek, E.R., Bagley, D.H.: Urology 53 (1999) 25. Webber, J., Herman, M., Kessel, D., Fromm, D.: Ann. Surg. 230 (1999) 12.

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00Liu

Liu, T.C., Duan, R., Yin, P.J., Li, S.L.: Proc. SPIE 4224 (2000) 186.

01Dia

Diaspro, A. (ed): Confocal and Two-Photon Microscopy: Foundations, Applications and Advances, Berlin: Wiley, 2002. Jacques, S.L., Bargo, P.R.: Proc. SPIE 4257 (2001) 65. Karu, T.I., Afanesyeva, N.I., Kolyakov, S.F., Pyatibrat, L.V., Welser, L.: Proc. SPIE 4431 (2001) 306. Karu, T.I., Kolyakov, S.F., Pyatibrat, L.V., Mikhailov, E.L., Kompanets, O.N.: Proc. SPIE 4433 (2001) 97. Onac, I., Pop, L., Ungur, R., Giurgiu, I.: Proc. SPIE 4430 (2001) 684. Roelofs, T.A., Graschew, G., Schneider, M., Rakowsky, S., Sinn, H.J., Schlag, P.M.: Proc. SPIE 4262 (2001) 259. Sassaroli, A., Martelli, F., Zaccanti, G., Yamada, Y.: Appl. Opt. 40 (2001) 185.

01Jac 01Kar1 01Kar2 01Ona 01Roe 01Sas 02Beu 02Cha 02Jac 02Lic 02Min

02Sch 02Sva 02Tuc 02Vog 02Way 02Zan 03Ber 03Chi 03Dia 03Hen 03Lax 03Min 03Qdo 03Ric 03Utz

Beuthan, J., Netz, U., Minet, O., Klose, A.D., Hielscher, A.H., Scheel, A., Henniger, J., M¨ uller, G.: Quantum Electronics 32 (2002) 945. Chan, W.C.W., Maxwell, D.J., Gao, X., Bailey, R.E., Han, M., Nie, M.: Curr. Opin. Biotechnol. 13 (2002) 40. Jacques, S.L.: Proc. SPIE 4612 (2002) 59. Licha, K.: In: Krause, W. (ed.): Contrast Agents II (Topics in Current Chemistry, Vol. 222), Berlin, Heidelberg, New York: Springer (2002) 1. Minet, O., Beuthan, J., Licha, K., Mahnke, C.: In: Kraayenhof, R., Visser, A.J.W.G., Gerritsen, H.C. (eds.): Fluorescence Spectroscopy: Imaging and Probes (Springer Series on Fluorescence Methods and Applications, Vol. 2), Berlin, Heidelberg, New York: Springer (2002) 349. Schneckenburger, H., Stock, K., Steiner, R., Strauss, W., Sailer, R.: In: Tuchin, V. (ed.): Handbook of optical biomedical diagnostics, Bellingham: SPIE Press PM 107 (2002) 825. Svanberg, S.: In: Waynant, R.W. (ed.): Lasers in medicine, Boca Raton, London, New York, Washington: CRC Press (2002) 135. Tuchin, V.V. (ed.): Handbook of Biomedical Diagnostics, SPIE PM107, 2002. Vogel, A., Noack, J., Huettmann, G., Paltauf, G.: Proc. SPIE 4633 (2002) 23. Waynant, R.W.: Lasers in Medicine, Boca Raton, London: CRC Press, 2002. Zander, C., Enderlein, J., Keller, R.A.: Single molecule detection in solution, Berlin: Wiley-VCH, 2002. Berlien, H.P., M¨ uller, G.: Applied Laser Medicine, Heidelberg: Springer, 2003. Chin, L.C.L., Whelan, W.M., Vitkin, I.A.: Phys. Med. Biol. 48 (2003) 543. Diaz, S.H., Nelson, J.S., Wong, B.J.F.: Phys. Med. Biol. 48 (2003) 19. Henniger, J., Minet, O., Dang, H.T., Beuthan, J.: Laser Phys. 13 (2002) 796. http://laxmi.nuc.ucla.edu:8248/M248 98/synprob/part1/history.html Minet, O., Beuthan, J., Mildaˇziene, V., Baniene, R.: Annual Rev. Fluorescence 1 (2003) in press. http://www.qdots.com Richards-Kortum, R., Drezek, R., Sokolov, K., Pavlova, I., Follen, M.: In: Mycek, M.A., Pogue, B.W. (eds): Handbook of biomedical fluorescence, New York, Basel: Marcel Dekker (2003) 237. Utzinger, U., Richards-Kortum, R.R.: Jour. Biomed. Opt. 8 (2003) 121.

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References for 3.1 Wagni`eres, G., McWilliams, A., Lam, S.: In: Mycek, M.A., Pogue, B.W. (eds): Handbook of biomedical fluorescence, New York, Basel: Marcel Dekker (2003) 361. Wang, X., Pang, Y., Ku, G., Stoica, G., Wang, L.V.: Opt. Lett. 28 (2003) 1739. Minet, O., Dressler, C., Beuthan, J.: Heat stress of cancer cells: Fluorescence imaging of structural changes with Quantum DotsTM 605 and AlexaTM 488. In: Hof, M., Hutterer, R., Fidler, V. (eds.): Fluorescence Spectroscopy: Imaging and Probes (Springer Series Methods and Applications of Fluorescence Spectroscopy, Vol. 3), Berlin, Heidelberg, New York: Springer, 2004, in press.

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3.2 Laser chemical processing ¨uerle D. Ba

3.2.1 Introduction The object of Laser-induced Chemical Processing (LCP) of materials is the patterning, coating and physicochemical modification of solid surfaces by activation of real chemical reactions. An overview of the various possibilities is presented in Fig. 3.2.1. The figure includes reactions that result in material deposition, etching, ablation, synthesis, surface modification, and polymerization. Laserinduced activation or enhancement of a reaction can take place heterogeneously or homogeneously or via a combination of both. A heterogeneous reaction is induced in an adsorbate-adsorbent system, at a gas-solid or liquid-solid interface, or within the solid surface itself (Fig. 3.2.1a, b, d, e). A homogeneous reaction is activated within the ambient medium (Fig. 3.2.1c) or within the bulk of the material. Symbolically, the first step in a laser-induced reaction can be described, in many cases, by AB + M + Photons → A(↓) + B(↑) + M

(3.2.1)

including the case B ≡ A. A shall be the relevant species for surface processing. If B = A, the interaction of species B (atoms or molecules) with the substrate surface shall be weak or negligible. M can be a gas, a liquid solvent, or a solid. Both heterogeneous and homogeneous laser-induced reactions may be activated thermally (photothermally, pyrolytically), photochemically (photolytically), or photophysically (combination of thermal and non-thermal mechanisms). The characteristics of such reactions are discussed in [00Bae]. Laser-induced material removal can be based on either ablation or etching. The term laserinduced ablation, or simply laser ablation, is used if material removal can be performed, at least in principle, in vacuum or in an inert ambient medium. Thus, ablation can take place only if the laser light is either directly absorbed by the material to be ablated or if absorption is mediated via an evaporated layer, a paint, or a plasma. Laser-induced chemical etching denotes thermal or non-thermal material removal in a reactive ambient medium. Laser-induced material deposition has been demonstrated from gases and condensed phases (Fig. 3.2.1). Laser-induced Chemical Vapor Deposition (LCVD) can be employed to fabricate microstructures of different types and to grow large-area thin films. Adsorbates frequently play an important role in laser-CVD and in deposition techniques using a combination of laser and atomic (molecular) beams. Material deposition from liquids has been demonstrated with ordinary liquids and with electrolytes with and without an external ElectroMotive Force (EMF). Thin films can also be fabricated from solid targets by pulsed-laser ablation or laser-induced evaporation and subsequent material condensation onto a substrate (Pulsed-Laser Deposition (PLD) process). Laser-Induced Forward Transfer (LIFT) is mainly employed for micropatterning. Laser-induced surface modifications can be classified into physical, chemical and physicochemical transformations. Physical surface transformations can be performed in an inert atmosphere and they take place without any changes in the overall chemical composition of the material. Here, no real chemical reaction between the different constituents, if present, is activated. Chemical transLandolt-B¨ ornstein New Series VIII/1C

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Fig. 3.2.1. Laser chemical processing at or near solid surfaces. The laser beam is always shown at perpendicular incidence to the substrate (workpiece) except in case (c) where it propagates parallel to the surface. For simplicity, not all reaction products are included in the formulas. Arrows ↓ refer to deposition or condensation of products and arrows ↑ to desorption of species, surface ablation, or etching. ←→ denotes reactions that can be reversed by shifting the chemical equilibrium to the other side. In PLD (PLD = Pulsed-Laser Deposition), the material ablated from the substrate (target) is condensed on another substrate (not shown in the figure). Additional abbreviations are: nc = nanocrystalline; Me = metal; PTFE = polytetrafluoroethylene (Teflon); PI = polyimide; MMA = methyl-methacrylate; PMMA = polymethyl-methacrylate [00Bae].

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formations are characterized by an overall change in the chemical composition of the material or the activation of a real chemical reaction. This shall include material synthesis, decomposition, and some types of surface modifications. The latter can take place either in a chemically reactive ambient medium or they are performed by adding a new material to the surface, or by depleting a certain constituent of the material from the surface. For physicochemical transformations both physical and chemical processes are important. Large-area surface modifications are performed with excimer lasers, Nd:YAG or Nd:glass lasers, with high-power CO2 lasers and, more recently, with diode lasers. For localized processing by “direct writing” and “projection”, low-power cw lasers such as Ar+ or Kr+ lasers and pulsed lasers, in particular frequency-doubled/-tripled Nd:YAG, excimer, and Ti-sapphire lasers are employed.

3.2.2 Pulsed-laser ablation Material removal caused by short high-intensity laser pulses is often termed Pulsed-Laser Ablation (PLA). Throughout the literature, the terms laser-assisted evaporation and laser sputtering are also frequently used. Within the regime under consideration, material removal takes place far from equilibrium and may be based on thermal or non-thermal microscopic mechanisms. For this reason, we will prefer the term laser ablation, which is less suggestive with respect to the fundamental mechanisms involved in the process. PLA permits one to widely suppress the dissipation of the excitation energy beyond the volume that is ablated during the pulse. This is fulfilled if the thickness of the layer ablated per pulse, ∆h, is of the order of the heat penetration depth, lT ≈ 2(Dτl )1/2 with D = thermal diffusion constant and τl = laser pulse duration, or the optical penetration depth, lα = α−1 with α = absorption coefficient, depending on which is the larger, i.e., ∆h ≈ max(lT , lα ) .

(3.2.2)

This (simplified) condition is, in fact, the basic requirement for applications of the technique. Laser ablation has been demonstrated to be a powerful tool in micropatterning of hard, brittle, and heat-sensitive materials, and in the fabrication of thin films with complex stoichiometry. The latter technique is termed Pulsed-Laser Deposition (PLD, see Sect. 3.2.6). It is evident that (3.2.2) is a crude estimation. Because of the fast heating and cooling rates achieved with pulsed lasers, material damage or material segregation in multicomponent systems can often be ignored even in cases where the ablated layer thickness is considerably smaller than the value obtained from (3.2.2). For many materials, (3.2.2) can be reasonably well fulfilled with UV-laser light and nanosecond pulses. With VIS- and IR-laser radiation, this condition is often more difficult to fulfill because of the lower absorptivity observed for many materials at longer wavelength. Additionally, with increasing wavelength, laser-plasma interactions become more pronounced; these result in plasma shielding, oscillations in the energy-substrate coupling, etc. [00Bae]. With both longer wavelengths and enhanced laser-plasma interactions, the resolution achieved in micropatterning decreases. For certain materials, nanosecond laser pulses are too long for high-quality and high-resolution surface patterning. Among those are metals, many semiconductors, thin films of high-temperature superconductors, etc. Because of the high thermal diffusivity of these materials, (3.2.2) can be well fulfilled only with picosecond- or femtosecond-laser pulses. A similar problem arises for materials whose bandgap energy, Eg , exceeds the photon energy of the UV-laser sources presently available. In such cases, lα  ∆h, and (3.2.2) cannot be fulfilled. An exception are materials in which the laser radiation itself generates defects (incubation centers) that, in turn, absorb the laser light. Other wide-bandgap materials, e.g. glasses such as a-SiO2 , are quite stable even to ArF-laser radiation. However, well-defined patterning of such materials has been demonstrated with picosecond and femtosecond Ti:sapphire- and with 157 nm F2 -laser pulses. Landolt-B¨ ornstein New Series VIII/1C

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Because PLA permits one to preserve the stoichiometry during the ablation process (this is known as congruent ablation), this technique, in combination with a mass spectrometer, can be used for chemical analysis of multicomponent materials. A significant increase in the available mass range and resolution has been achieved by Matrix-Assisted Laser Desorption Ionization (MALDI). Further experimental and theoretical details can be found in [00Bae, 03Lip, 98Mil, 99Hor, 94Sri] and [03Zhi, 00Bae, 98Arn2, 99Ani, 98Luk], respectively.

3.2.2.1 Surface patterning The efficiency of material removal under the action of laser light is described by the ablation rate. This is defined by the total layer thickness ablated per laser pulse, WA ≡ ∆h [µm/pulse], by the average ablation velocity per pulse, v ≡ ∆h/τl [µm/s], or by the ablated volume per pulse, WA ≡ ∆m/ρ [µm3 /pulse], where ∆m is the mass loss. Significant material ablation is observed only if the laser fluence, φ, exceeds a certain threshold fluence, φth . The fluences employed in surface patterning are above the ablation threshold, φ > φth , and they are, typically, between 0.1 J/cm2 and several J/cm2 , depending on the particular material and laser parameters. Surface patterning by pulsed-laser ablation can be performed by direct focusing (Fig. 3.2.2a) of the laser light onto the substrate, by direct masking (Fig. 3.2.2b) [03Dye], by laser-light projection (Fig. 3.2.3a, b), laser-beam interference, by Scanning-Near-field-Optical-Microscopy-type (SNOMtype) techniques [97Dut, 99Nol1], or by means of 2D lattices of microspheres (Fig. 3.2.4) [03Den1, 02Bae, 02Pig, 02Den]. At present, the fields of applications of nanosecond PLA include: – Fabrication of microholes and grooves with variable aspect ratios for ink-jet printers, sensors, etc. – Fabrication of components for micromechanical devices, motors, sensors, etc. [95End]. – Fabrication of optoelectronic and microoptical devices such as waveguides, surface-relief gratings, graded transmission dielectric masks [99Sch1], etc. – Via formation (for vertical interconnections) mainly in polyimide (PI), for thin-film packaging of MultiChip Modules (MCM) [97Wol]. – Formation of holes with shallow wall angle in PI passivation layers on semiconductor wafers [97Wol].

Fig. 3.2.2. Scanning Electron Microscope (SEM) pictures showing different patterns produced on ceramic PbTi1−x Zrx O3 (PZT) by means of 308 nm XeCl-laser radiation (τl ≈ 15 ns). The groove in (a) was obtained with a stationary line focus (φ = 10.8 J/cm2 , w(F W HM ) = 50 µm, number of laser pulses Nl = 4 × 103 , pulse repetition rate 5 Hz). (b) Line focus scanned perpendicular to directly masked sample (φ = 15 J/cm2 , scanning velocity vs = 0.84 µm/s) [87Eye2].

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Fig. 3.2.3. Projection patterning by excimer-laser ablation. (a) Single-crystalline LiNbO3 (λ = 308 nm, φ = 2.7 J/cm2 , 2w = 175 µm, 500 pulses; vacuum) [87Eye1]. (b) YBa2 Cu3 O7 film on (100) SrTiO3 substrate (λ = 248 nm, φ ≈ 1.5 J/cm2 , τl ≈ 17 ns; film thickness h1 ≈ 0.1 µm) [90Hei].

Fig. 3.2.4. Top: White-light microscope pictures generated by a-SiO2 microspheres of diameter d = 2r = 3 ± 0.15 µm. (a) Substrate within the focal plane, i.e. z ≈ f ≈ rn/2(n − 1) where n is the refractive index of the spheres; the distance between the intensity maxima is equal to the diameter of spheres. (b) z = f + ε. The distance between interference maxima is about 800 nm [02Bae]. (c) Holes produced by local KrF-laser-induced ablation of PI (polyimide) using microspheres. Curve (d) shows the (uncorrected) depth profile of holes measured by means of an AFM. (e) Holes produced at a distance z = f + ε with ε ≈ 2.7 µm. The arrangement of holes is equal to that of the interference maxima in (b) (see rhombus).

– – – – – – –

Wirestripping [94Lam]. Marking [97Alv]. Surface cleaning from particulates and contamination layers [00Bae, 02Luk]. Different types of trimming. Link cutting, in particular in redundancy technology [89Ric]. Certain types of lithography [98Suz]. The fabrication of masters which are subsequently used for economic replication by standard techniques. Such masters are, e.g., masks or real 3D structures fabricated by laser LIGA (LIthographie, Galvanoformung, Abformung). This process combines excimer-laser patterning with LIGA. Laser LIGA provides a fast and flexible technique that is complementary to traditional LIGA, which employs X-ray lithography [95Arn]. – Medical applications. Among the most promising areas are ophthalmology, dermatology, angioplasty, and cellular microsurgery [97Deu]. Another interesting field is the fabrication of medical devices. – Laser-microdissection for the analysis of biological tissues.

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For materials with high thermal diffusivity, damage-free patterning requires picosecond or even femtosecond pulses. This has been demonstrated for metals, many types of semiconductors, and for thin films of High-Temperature Superconductors (HTS), as e.g. YBa2 Cu3 O7 [94Pro]. Another field of applications of ultrashort pulses is the patterning and micromachining of materials for which the photon energy of practical lasers is not high enough either for direct (single-photon) bandgap excitation or for efficient defect generation. Among the examples are widebandgap materials such as alkali and earth-alkali halides, various oxides, e.g., SiO2 , Al2 O3 , etc. Figure 3.2.5 shows NaCl irradiated with 16 ns and 300 fs KrF-laser pules. Well-defined patterning without detectable thermal damage is only achieved with femtosecond pulses. Similar results have been obtained for different types of glasses [99Cam, 99Len, 03Dye], and for Teflon and PMMA [89Kue]. Real and potential applications of ultrashort-pulse laser ablation include: – The patterning of thin films of materials with large thermal diffusivity that cannot be satisfactorily patterned by conventional techniques or by nanosecond-laser ablation [03Maj, 91Sch1]. – Drilling of high-aspect-ratio microholes in metals for fuel-injector nozzles [99Nol2], hydraulic or pneumatic equipment, functionalization of surfaces, etc. – Machining of memory metals. – Fabrication of medical implants, e.g., metallic stents used as cardiovascular implants. – Micromachining of metal-layer electrodes for polymer electrolyte fuel cells (PEFC) [96Kru2]. – Micromachining of materials for microoptical and optoelectronic devices, e.g., the fabrication of gratings in a-SiO2 , LiNbO3 [97Che], waveguides in Ta2 O5 [99Bei]. – Micro-perforation of membranes for biosensors [97Woe]. – Patterning of composite materials as, e.g., carbon and silicon-carbide reinforced alumo- and borosilicate glasses [96Kru1]. – Ablation of soft [99Koe] and hard biological tissues [99Kru, 00Kim, 99Per]. – Nanopatterning of material surfaces including nanolithography. This can be achieved by a combination of femtosecond-laser pulses with SNOM techniques [99Nol1]. However, the situation for the application of ns- and ultrashort laser pulses may be more complicated. Amorphous and crystalline SiO2 are most precisely ablated with the shortest pulses employed, about 5 fs. On the other hand, brittle materials like MgF2 , CaF2 and MgO are most precisely machined with picosecond pulses. Here, nanosecond- and femtosecond-laser ablation results in strong fracturing and exfoliation within and far around the interaction volume. With nanosecond pulses, this observation is mainly related to thermal stresses. With femtosecond pulses, mechanical stresses related to defect accumulation and/or the laser-induced shock may be responsible for

Fig. 3.2.5. SEM pictures of NaCl surfaces ablated by 248 nm KrF-laser radiation. (a) With nanosecond pulses (τl ≈ 16 ns, φ = 4.2 J/cm2 , Nl = 15) an undefined crater is observed with cracks reaching deep into the surrounding material. (b) With femtosecond pulses (τl ≈ 300 fs, φ = 500 mJ/cm2 , Nl = 500) well-defined patterning without any cracks within the surrounding material is possible [89Kue].

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material damage. The width of the Shock-Affected Zone (SAZ) can be modified via the shape of the laser beam. This has been demonstrated with a-SiO2 and MgF2 . Here, the damage around deep holes can be significantly reduced by using a top-hat instead of a Gaussian laser-beam profile [99Cam].

3.2.2.2 The threshold fluence With laser fluences φ < φth , changes in surface morphology and microstructure, the generation of defects, and the depletion of one or several components of the material are frequently observed [90Hei, 97Bae]. Figure 3.2.6 shows the behavior of YBa2 Cu3 O7 (YBCO) films on (100) MgO substrates irradiated by KrF-laser light. Stoichiometric ablation is only observed for φ > φth ≈ 0.75 J/cm2 . For most inorganic insulators, φth is between 0.5 and 2 J/cm2 . For organic materials this range is, 2 typically, 0.01 J/cm ≤ φth ≤ 1 J/cm2 .

Fig. 3.2.6. Surface damage/ablation of YBa2 Cu3 O7 films on (100) MgO substrates as a function of KrF-laser fluence. Different laser-beam spot sizes on the film surface, 2w, are indicated by different symbols. Film thicknesses were between 0.5 and 1.5 µm [90Hei].

For finite absorption, φth decreases with increasing absorption coefficient, irrespective of whether this is related to a decrease in laser wavelength, the addition of dopants, or to the generation of defects. Successive laser pulses may increase the number of defects and thereby the absorptivity within the irradiated volume. Thus, φth for multiple-pulse ablation is lower than for single-pulse ablation. For shorter pulses, the spatial dissipation of the excitation energy is diminished and φth is reached at lower fluences (Figs. 3.2.7, 3.2.8). Clearly, the mechanisms of energy dissipation may be quite different with different systems. With thin films, φth becomes independent of film thickness h1 only if lT < h1 (Fig. 3.2.9). If, however, lα , lT  h1 , φth becomes again constant and depends on the properties of the substrate only.

3.2.2.3 Ablation rates The ablation rate depends on the photon energy [00Erm, 99Rub, 03Lip, 93Kue, 98Arn1, 97Par, 00Bae], the laser fluence [01Bon, 01Oba, 87Eye1, 90Beu], laser pulse length [98Len, 96Him2, Landolt-B¨ ornstein New Series VIII/1C

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Fig. 3.2.7. Dependence of threshold (damage) fluence, φth , on laser pulse duration, τ
. (a) Single-shot laser irradiation of PI. Full symbols refer to 302 nm Ar+ -laser and open symbols to 308 nm XeCl-laser radiation. The scattering in XeCl-laser data could be related to differences in the laser pulse shapes. Dotted and dashed curves were calculated with Tth = 1450 K for 3D and 1D heat transport, respectively. Full curve: 3D heat transport with threshold temperature equal to temperature of stationary ablation, which depends on (constant) intensity I0 , Tth = Tst (I0 ); the dash-dotted curve shows the corresponding dependence of Tth on τ
. For τl > 10−4 s dotted and full curves can be described by φth ∝ τ
ε with ε ≈ 1 [00Bae, 98Pig]. (b) Damage threshold for a-SiO2 (filled circles) and CaF2 (filled diamonds) exposed to 1053 nm Ti:sapphire-laser radiation. 1/2 Full lines are τ
fits to the “long” pulse results [95Stu].

96Him1, 96Kau], repetition rate [00Kim], and width of focus [87Eye1, 90Beu], the heat or optical penetration depth [90Ihl], the enthalpy of vaporization, internal stresses, the type and pressure of the ambient atmosphere [97Zha], etc. With shallow structures, the ablated depth increases linearly with pulse number [02Dum, 96Kau, 87Eye2]. With deep holes or grooves, WA becomes dependent on the number of laser pulses [87Eye2]. The ablation rates in nanosecond surface micropatterning are between some 0.1 µm/pulse and several µm/pulse. The corresponding laser fluences are between 0.1 J/cm2 and several J/cm2 . Typical ablation rates achieved with metal targets and picosecond to femtosecond Ti:sapphire-laser pulses are 0.01 µm/pulse to 0.2 µm/pulse. The corresponding fluences are between 0.5 J/cm2 and 10 J/cm2 . The size of the illuminated spot on the substrate surface, 2w, determines the width of the generated pattern and the expansion of the plasma plume. With nanosecond or even longer laser pulses the ablation rates are higher for smaller laser-beam spot sizes [87Eye1]. Above a certain value, WA becomes independent of w. This effect originates from the attenuation of the incident laser radiation by the expanding plasma plume. The velocity of species ejected from the ablated surface is of the order of 1 to 100 µm/ns. Thus, even when ablation starts instantaneously, almost no plasma plume can develop during a picosecond or femtosecond pulse, and plasma shielding is Landolt-B¨ ornstein New Series VIII/1C

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Fig. 3.2.8. Threshold fluence for surface damage of fused silica as a function of the duration of Ti:sapphire-laser pulses. Data refer to different laser wavelengths, number of laser pulses, and authors. Filled downward triangles, filled circles: Nl ≈ 600 [96Stu], filled squares: Nl ≈ 50 [97Kru], filled upward triangles: Nl ≈ 50 [98Len]. The full curves have been calculated from a model which includes both MPI (MultiPhoton Ionization) and avalanche ionization [00Bae]; the straight line represents the multiphoton ionization limit calculated with the same parameters [96Stu].

Fig. 3.2.9. Damage threshold for Ni and Au films of different thicknesses on a-SiO2 for two laser pulse durations. Filled and empty squares: 14 ns 248 nm KrF-laser radiation. Filled and empty circles: 200 fs 400 nm laser radiation. The dashed curves were calculated from the heat diffusion equation. The full curves are numerical solutions of the two-temperature model [99Wel].

strongly diminished or even avoided. The ablation rate is then independent of laser-beam spot size [00Bae].

3.2.2.4 Material damage An important question for applications of pulsed-laser ablation in surface micro-patterning is the degree and extension of material damage beyond the volume ablated during the laser pulse. Among the different damages observed are defect formation, changes in morphology and chemical composition, doping, material distortions, indications for melting, cracks, exfoliation, etc. The type and degree of damage depends on the laser parameters and the specific material, including its microstructure, pureness, internal stresses, etc. In many cases, material damages can be diminished Landolt-B¨ ornstein New Series VIII/1C

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by either increasing the absorption strength via the laser wavelength or material doping [90Ihl], or/and by decreasing the laser-pulse length (Fig. 3.2.5).

3.2.2.5 Influence of an ambient atmosphere A reactive atmosphere can increase the rate of material removal. This is known as (dry) etching. Additionally, such an atmosphere can change the physical and chemical properties of the ablated surface, which is the basis of many types of laser-induced surface modifications. A reactive atmosphere can also change the chemical composition of ablated species. This is of particular importance in pulsed-laser deposition. A non-reactive atmosphere mainly influences the transport of species and thereby the ablation rate and the formation of debris. Within any type of medium, pulsed-laser ablation can generate shock waves [00Bae].

3.2.2.6 Instabilities, structure formation Laser ablation can be strongly influenced by various types of instabilities and structures which develop within certain parameter regimes. Examples are ripples, hydrodynamic instabilities, and cone-type structures [00Bae].

3.2.3 Materials etching Material removal by a gaseous, liquid, or solid etchant may be enhanced or only induced under the action of laser light. This is called laser-induced chemical etching. In dry etching, the precursor molecules most commonly used are halides, in particular Cl2 and Br2 , and halogen compounds. The etching mechanisms are often based on the interaction between halogen radicals and charge carriers at or within the solid surface. Dry etching of metals and semiconductors is often classified into spontaneous etching, diffusive etching, and passivating reactions. Wet etching is mainly performed in aqueous solutions of acids like HCl, HNO3 , H2 SO4 , and H3 PO4 , or in lyes like NaOH and KOH, or in neutral salt solutions of NaCl, NaNO3 , K2 SO4 , etc. Also mixtures of different acids or of different lyes with or without other additives have been employed. Neutral salt solutions have the advantage of being less corrosive than acids or lyes. Laser-induced etching can be considered as a new technique that not only permits direct maskless etching at high rates but also makes it possible to process a wide variety of materials that cannot be processed by standard etching techniques, or only very inefficiently. The etch rates achieved in laser-induced dry etching can compete with those in conventional techniques. Wet etching permits higher rates and a greater versatility which is related to the large variety of reactants available without restrictions on volatility, etc. Laser-induced wet etching is therefore a useful tool in micromachining (cutting, drilling, shaping, etc.). For details see [00Bae].

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3.2.3.1 Etching of metals Spontaneous etching denotes a situation where the material dissolves within the ambient medium without any external influence. Among the examples are Al in Cl2 atmosphere or Mo, Ta, Ti, W in XeF2 , etc. Small mass losses due to etching can conveniently be measured by means of a Quartz Crystal Microbalance (QCM). In most of these experiments, the metal film is directly evaporated onto the quartz. Light can significantly enhance the etch rate [85Ses]. Diffusive etching systems are characterized by strong physisorption or chemisorption of the reactant and by the diffusion of corresponding radicals into the bulk. Among the model systems investigated are Ag and Cu in Cl2 atmosphere. Laser-light irradiation removes the chlorinated layer [86Ses]. The class of passivating reaction systems investigated includes: Fe, Mo, Ni, W, and Nix Fey alloys in Cl2 atmosphere, and Ti in Br2 and CCl3 Br. Here, the etchant chemisorbs on the metal surface and forms a stable and dense metal-halide layer (10 . . . 30 ˚ A thick). Laser-light irradiation yields a local temperature rise which results in the desorption of this layer. Localized etching by means of focused cw Ar+ -laser radiation was investigated for Mo [89Mog] and W films on glass substrates immersed in Cl2 . The fabrication of holes in W films using 2D lattices of a-SiO2 microspheres, Ar+ -laser radiation and WF6 as an etchant has been demonstrated in [03Den2]. Wet etching was studied for Al, Cu, Fe, steel, and Ni. Most of the experiments have been performed by means of Ar+ -laser radiation. In the absence of an external voltage, the etch rates achieved were, typically, between one and several µm/s. Spatial resolutions of better than 2 µm have been demonstrated. Laser-enhanced Electrochemical Etching (LEE) is achieved by simply reversing the polarity of the cathode and anode. Holes in stainless steel using aqueous NiCl2 as an electrolyte and Ar+ -laser radiation have been etched with rates of up to 10 µm/s.

3.2.3.2 Etching of semiconductors and insulators Laser-induced dry etching of semiconductors has been studied mainly for Si, GaAs, and InP. Laserinduced wet etching has been studied mainly for compound semiconductors and insulators. High-resolution etching of Si in Cl2 atmosphere has been demonstrated mainly by direct writing using either focused Ar+ -laser radiation [00Bae, 95Mue, 93Ehr] or a SNOM-type setup (Fig. 3.2.10). The chlorine gas pressures employed range from about 1 mbar to several hundred mbar. Figure 3.2.11 shows the etch rate of Si in Cl2 achieved with pulsed-irradiation at three different wavelengths. The results obtained with 308 nm XeCl-laser radiation show three characteristic regimes: – For laser fluences which cause negligible surface heating (φ∗ = φ/φm < 0.2 with φ∗ = normalized laser fluence, φm = fluence required for surface melting), etching is purely photochemical and based on both chlorine radicals produced within the gas phase and electron-hole pairs generated within the Si surface [00Bae]. – At medium energy densities, thermal processes become important, but photogenerated Cl radicals are still required. – At laser fluences that cause surface melting, i.e. with φ∗ > 1, etching is mainly thermally activated. The etch rate of single-crystalline Si depends on crystal orientation. With laser powers corresponding to temperatures well below the melting point, the etch rate for (100) surfaces exceeds that for (111) surfaces by at least two orders of magnitude [00Bae]. Photochemical etch rates in n-type Si exceed those in p-type Si.

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POSITION CONTROL UNIT

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Fig. 3.2.10. SNOM-type set-up used for laser-induced chemical etching of Si in Cl2 atmosphere. Bottom: Detailed view of the fiber tip and sample, and also the cross-section of a hole in a Si wafer measured by means of an Atomic Force Microscope (AFM) [01Wys].

Light-enhanced etching of Si in halogen compounds such as XeF2 , NF3 , and SF6 atmosphere has been investigated mainly by means of Ar+ - and CO2 -laser radiation [81Chu]. Ar+ -laser-induced etching of tunnels and cavities in Si under SiO2 or Si3 N4 films using Cl2 atmospheres has been demonstrated in [96She]. Thermal etching of GaAs, InP, and InSb in CCl4 atmosphere has been demonstrated by using focused Ar+ -laser radiation [88Tak2]. The maximum resolution achieved with a 1.2 µm focal spot was about 0.6 µm. Scanning speeds of up to 60 µm/s have been employed. At medium to high laser powers, changes in the stoichiometry of the surrounding material have been found. Etching of GaAs in CF3 Br and CH3 Br and of InP in HBr and Cl2 (Fig. 3.2.12) have been demonstrated by direct masking and excimer-laser-light projection. A resolution of about 0.2 µm was achieved.

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TEMPERATURE [ K ] 400

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Fig. 3.2.11. Etch rate of (100) Si (slightly pdoped with ≈ 1014 B atoms/cm3 , resistivity ρ = 100 to 150 Ω cm) as a function of (normalized) laser fluence, and for three different wavelengths (the 423 nm and 583 nm lines were obtained from a XeCl-laser-pumped dye laser). φm (λ) is the fluence required for surface melting [87Kul].

Fig. 3.2.12. (a) InP microlens fabricated by KrF-laser-light projection in Cl2 atmosphere, (b) nominal and etched lens profiles (after [97Mat]).

Etching of Atomic Layers (EAL) also termed as digital etching is the inverse of Atomic-Layer Epitaxy (ALE). Figure 3.2.13 shows the etch rate of InP in Cl2 atmosphere as a function of ArF-laser fluence. At a (uniform) substrate temperature of Ts ≈ 140 ◦ C and a fluence of about A/pulse, which corresponds to somewhat less than 0.12 J/cm2 the etch rate saturates at about 2.3 ˚ one monolayer of InP. Single-crystalline (100) Mn-Zn ferrites have been etched crack-free in CCl4 with rates of up to 68 µm/s [88Tak1]. Similar experiments have been performed in aqueous solutions of KOH and H3 PO4 . With Fe:Al:Si (Sendust) etch rates up to 400 µm/s and aspect ratios of 40 have been achieved [94Tak]. Etching of PbTi1−x Zrx O3 in H2 atmosphere has been demonstrated in [86Eye]. For high-resolution laser-enhanced wet etching, laser-induced reactions within the bulk liquid must be avoided. Figure 3.2.14 shows the etch rate for holes in (111) Si and ceramic Al2 O3 /TiC. Landolt-B¨ ornstein New Series VIII/1C

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Fig. 3.2.13. InP etch rate as a function of ArFlaser fluence (10 Hz, p(Cl2 ) = 2.5 mbar, Ts ≈ 140 ◦ C). The dashed curve represents the calculated etch rate near threshold if etching would be limited by sublimation of InCl3 [90Don].

Fig. 3.2.14. Volume etch rate for (111) Si and ceramic Al2 O3 /TiC in KOH as a function of Ar+ -laser power (τl = 5 s). Full curves are to guide the eye [82Gut].

Average (depth) etch rates up to 15 µm/s have been achieved. Figure 3.2.15a shows a Scanning Electron Microscope (SEM) picture of a via hole in GaAs. The high aspect ratio achieved can be attributed to waveguiding of the laser light. Deep trenches have been produced by translating the substrate with respect to the laser beam. The gratings shown in Fig. 3.2.15b have been etched by using laser-beam interference. Etching of GaAs has also been investigated in HNO3 [91Rub]. Wet etching has also been demonstrated for CVD diamond films [97Sha]. Well-defined micro-patterning by rear-side illumination of a-SiO2 in contact with an absorbing solution has been demonstrated in [99Wan, 02Din, 02Zim].

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Fig. 3.2.15. Ar+ -laser-induced photochemical etching of GaAs in aqueous H2 SO4 + H2 O2 (volume ratio H2 SO4 : H2 O2 : H2 O = 1 : 1.3 : 25). (a) SEM picture of a via hole in GaAs, λ(SH Ar+ ) = 257 nm [84Pod]. (b) Gratings produced by 514.5 nm Ar+ -laser light interference. The different profiles were obtained by varying the angle of incidence, Θ [83Pod].

3.2.4 Laser-induced chemical vapor deposition (Laser-CVD) The decomposition of precursor molecules in Laser-induced Chemical Vapor Deposition (LCVD) can be activated thermally (pyrolytic LCVD) or non-thermally (photolytic or photochemical LCVD) or by a combination of both (photophysical LCVD). The type of process activation can be verified from the morphology of the deposit and from measurements of the deposition rate as a function of laser power, wavelength, substrate material, etc.; additional information is obtained from the analysis of data on the basis of theoretical models. The subsequent discussion concentrates on examples which were studied in most detail. Nevertheless, this discussion is very general in the sense that most of the trends, features, and results apply to all of the corresponding systems. For further details and the modeling of LCVD see [00Bae].

3.2.4.1 Microstructures A model system that has been investigated in great detail is the laser-induced (pyrolytic) deposition of W from WX6 (X ≡ F, Cl) with or without H2 or an inert carrier gas, M. The overall reaction can be described by k1 , k3 WX6 + 3 H2 + M ↔ W(↓) + 6 HX + M . k2

(3.2.3)

The rate constants k1 and k3 describe the decomposition of WX6 at the surface and in the adjacent gas, respectively. k2 describes etching of condensed W by HX.

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Fig. 3.2.16. SEM pictures (a)–(c) and optical transmission microscope picture (a ) of W spots deposited from WF6 + H2 onto SiO2 substrates by means of Ar+ -laser radiation (λ ≈ 515 nm, w0 (1/e) ≈ 1.1 µm). (a), (a ) P = 120 mW, τl = 0.2 s, pressure ratio Γp = 2 (10/5 ≡ 10 mbar H2 + 5 mbar WF6 ). (b) P = 110 mW, τl = 0.5 s, Γp = 5 (25/5). (c) P = 120 mW, τl = 0.5 s, Γp = 50 (250/5) [92Tot].

The morphology of spot-like deposits depends mainly on the type of precursor molecules, the partial pressures of gases, the laser-induced temperature distribution, and the illumination time (Figs. 3.2.16, 3.2.17). Lateral growth of photothermally deposited spots saturates for long laser-beam illumination times τl . With certain systems, this saturation is accompanied by an increase in axial growth and the formation of a fiber along the axis of the laser beam (Fig. 3.2.18) [83Ley, 82Bae, 81Ley]. Under quasi-stationary conditions, as characterized by a constant fiber diameter, the temperature in the tip of the fiber can be measured in situ with high precision (Fig. 3.2.19). The morphology and microstructure of fibers depend on the laser-induced temperature and on the gas pressure. Fibers have been grown in amorphous (B, SiOx , SiO2 , Si3 N4 ), polycrystalline (Ni, C, Si, SiC), and singlecrystalline form (B, Si, W) (Fig. 3.2.20). With partial pressures of precursor gases of up to 1 bar, the axial growth rates are, typically, 10 . . . 100 µm/s. With higher pressures, growth rates up to 103 µm/s have been achieved [94Wal]. Because of the high deposition rates and the possibility of in-situ temperature measurements, the growth of fibers is a unique technique for both rapid determination of apparent chemical activation energies and investigations on gas-phase processes (Fig. 3.2.21). The tensile strengths of fibers with about 15 µm diameter were about 7.6 GPa for B, 3 GPa for C, and about 2 GPa for SiC. Laser direct writing permits single-step surface patterning of planar and non-planar substrates. The process can most easily be studied by translating the substrate in one dimension perpendicular to the focused laser beam. The morphology, geometry and electrical properties of written lines strongly vary with laser parameters (Figs. 3.2.22–3.2.24). Laser-CVD permits to fabricate smooth microstructures of good morphology and well-defined height-to-diameter ratio in a single-step maskless process. Among the real and potential applications of the technique in planar and non-planar material processing are the fabrication of contacts, circuit repair, interconnects, mask repair, etc. Laser-CVD offers unique possibilities for the coating, patterning, and fabrication of non-planar three-dimensional (3-D) objects (Fig. 3.2.25) [95Leh, 97Wan].

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Fig. 3.2.17. Diameter/height of W spots as a function of laser-beam illumination time. The substrate material was SiO2 covered with a 700 ˚ A thick layer of sputtered W. (a) Deposited from 0.49 mbar WCl6 + 50 mbar H2 by using different laser wavelengths but constant absorbed laser power Pa = P (1 − R) with 2w0 ≈ 15 µm [92Kul]. (b) Deposited from 5 mbar WF6 + 500 mbar H2 at various Kr+ -laser powers (λ ≈ 647 nm, 2w0 ≈ 2.1 µm) [88Szo].

Fig. 3.2.18. Silicon fiber grown from SiH4 by means of 488 nm Ar+ -laser radiation (P = 400 mW, p ≡ p(SiH4 ) = 133 mbar) [83Bae1].

3.2.4.2 Thin-film formation Light-assisted CVD opens up new possibilities in thin-film fabrication. Laser light permits one to selectively generate high concentrations of atomic or molecular intermediate species that are present either not at all or only in small equilibrium concentrations in standard CVD using the same precursor molecules. Thus, laser-CVD enables one to study new reaction pathways and altered kinetics in thin-film growth. Lasers are often preferred over high-intensity lamps, at least in fundamental investigations, because of their high experimental versatility related to their intensity, monochromaticity, tunability, and directionability. In particular, at parallel incidence to the substrate surface, lasers permit pure gas-phase excitation. With perpendicular laser-beam irradiation, gas and surface excitations, including adsorbed layers, are important (Fig. 3.2.1). Landolt-B¨ ornstein New Series VIII/1C

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Fig. 3.2.19. Experimental setup employed for in-situ temperature measurements during laser-CVD of fibers. AMP DIV: amplifier and analog divider, BS: beamsplitter, CH: light chopper, D: power meter, FI: electronic filter, P: peak detector, PD: Si photodiode, PH: pinhole, S: switch, TR CON: translation control [86Dop].

Fig. 3.2.20. SEM pictures of the tip of lasergrown single-cystalline fibers. (a) Si grown from SiH4 [83Bae2]. (b) W grown from WF6 + H2 [00Bae].

From a practical point of view, light-assisted film growth is mainly studied with the intention of fabricating high-quality films at lower substrate temperatures (Fig. 3.2.26). Laser-CVD based on gas-phase heating (parallel incidence only; Fig. 3.2.1c) or laser-induced photolysis of gas- or adsorbed-phase precursors (Fig. 3.2.1a, b) permits one to deposit thin films without significant substrate heating. Elevated substrate temperatures are employed only for improving film morphologies. This is one of the main advantages over conventional CVD. Furthermore, in contrast to plasma-CVD, there are no problems with Vacuum UltraViolet (VUV) radiation or particle bombardment. Extended thin films have been fabricated with areas up to about 10 cm2 . Laser-CVD permits monolayer control of film thicknesses and thereby well-defined fabrication of heterostructures [88Low].

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Fig. 3.2.21. Arrhenius plot for the growth of Si from SiH4 by LCVD (Kr+ -laser, λ ≈ 531 nm) and standard CVD. The broken line separates regions of single- and polycrystalline growth; the intersection points with LCVD and CVD curves are at 1555 K and 1262 K, respectively [83Bae3].

Fig. 3.2.22. SEM pictures of W stripes deposited from WF6 + H2 with Kr+ -laser light (λ = 647 nm, w0 (1/e) ≈ 1.3 µm, vs = 100 µm/s). (a) p(WF6 ) = 5 mbar, p(H2 ) = 400 mbar, P = 146, 132, 120, 107, 97, and 83 mW (left to right). (b) p(WF6 ) = 5 mbar, p(H2 ) = 100 mbar, P = 135 mW [87Zha1].

3.2.4.3 Adsorbed layers, hybrid techniques Adsorbed precursor molecules, reaction products, or impurities may play an important role in various types of laser surface processing. They may control reaction rates, the spatial resolution in surface patterning, nucleation times in material deposition or synthesis, concentration profiles in surface doping, etc. Adsorbates are of importance also in new techniques that use a combination of laser and molecular/atomic beams (hybrid techniques) [00Bae]. Laser-assisted Molecular-Beam Epitaxy (laser-MBE) describes all cases of MBE where lasers have been incorporated in the deposition process. In MBE using laser-induced material ablation one or several standard MBE ovens (effusion cells) are replaced by solid or liquid sources (targets) from which the material is ablated under the action of laser light. In Atomic-Layer Epitaxy (ALE) and atomic-layer etching light can enhance the decomposition of strongly adsorbed species or it can desorb reaction products. Laser-assisted ALE (laser-ALE) has been demonstrated for semiconductors, and in particular for GaAs [93Iss].

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Fig. 3.2.23. Height and width of W stripes as a function of Kr+ -laser power for two substrate materials (λ = 647 nm, vs = 100 µm/s). (a) Fused quartz substrate covered with 1200 ˚ A amorphous silicon. The laser focus was w0 (1/e) ≈ 1.3 µm [87Zha1]. (b) Si-wafer substrate [87Zha2].

Fig. 3.2.24. Electrical resistance (left scale) and resistivity ratio of the deposit, ρD , and the bulk, ρB , (right scale) of W stripes as a function of laser power (647 nm Kr+ , w0 = 1.3 µm). The partial pressures of WF6 and H2 were p(WF6 ) = 5 mbar and 100 mbar ≤ p(H2 ) ≤ 800 mbar. For normalization we used ρB (W) = 5.33 × 10−6 Ω cm [87Zha1].

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Fig. 3.2.25. Examples of non-planar processing and fabrication of 3-D objects by LCVD. (a) Microsolenoid with silicon core and tungsten helix [93Wes]. (b) Al-grid structure fabricated by laser direct writing onto poly-carbonate which was dissolved after LCVD [91Leh]. (c) Free-standing boron microspring [92Joh].

Fig. 3.2.26. Arrhenius plot for the deposition of a-Si:H films on thermally oxidized (100) Si (p = 5 Torr; 40 sccm of 10 % Si2 H6 / 90% H2 and 390 sccm of He window flush). Ts is the (uniform) substrate temperature. Open symbols refer to standard CVD and full symbols to laser-CVD (ArF laser at parallel incidence; 20 Hz, 750 mW/cm2 ) [88Ere].

Organic Molecular-Beam Deposition (OMBD) permits the fabrication of ordered organic films. By combining OMBD with KrF-laser photoisomerization, well-oriented crystalline films of trans,trans-BESB (Bis(ethynylstyryl) benzene) have been grown in KBr substrates [98Fuc]. Laser-focused atomic deposition uses a standing-wave laser field together with an atomic beam for surface nanopatterning [99And].

3.2.5 Deposition from liquids Laser-enhanced/-induced liquid-phase processing is mainly applied for material deposition and etching. Both electrolytic solutions and ordinary liquids (non-electrolytes) are employed. The preLandolt-B¨ ornstein New Series VIII/1C

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cursor molecules are dissolved in the liquid (solvent) and possibly dissociate. The dipoles or ions formed in this way interact with each other, with the solvent, and with the substrate. Thus, electrochemical effects will often play an important or even decisive role. They may arise from the illumination of the liquid-solid interface (Fig. 3.2.27), or from an external ElectroMotive Force (EMF).

Fig. 3.2.27. Laser-induced thermobattery on a metal surface [00Bae]. φe is the electrochemical potential and T the temperature.

In the simplest case, electrical interactions can be ignored and the precursor molecules are just thermally decomposed at the liquid-solid interface. This situation frequently applies to laser processing using ordinary liquids. Pyrolytic laser-induced metal deposition has been demonstrated with organic solutions containing compounds with zero (metal) valency. Among those are triphenylphosphine complexes of Au and complexes of Cr, Fe, Mo, and W [91Bro]. C, Si, etc. can be deposited from liquid hydrocarbons, silanes, etc. Clearly, this process permits deposition onto both conducting and insulating substrates.

3.2.5.1 Electroless plating In electroless plating the charge balance is maintained via a reducing agent, incorporated into the solution. Thus, plating can be performed without simultaneous etching. Laser-induced/-enhanced electroless plating has been employed for fast localized deposition of metals. In general, virtually no plating takes place outside the laser-illuminated region. The technique can be applied also to electrically insulating substrates. Among the experimental examples is the electroless plating of premetallized glass substrates with Ni from solutions containing sodium hypophosphate (I) as reducing agent. With Ar+ -laser radiation local deposition rates up to 0.1 µm/s have been achieved. The background plating rates were about 5 ˚ A/s [84Gut]. Other types of electroless plating have been demonstrated for laser-pretreated surfaces. Among the examples are Au films on n-doped GaAs [92Sug], Ni and Cu films on modified [93Nii1] or ablated [93Nii2] polymer surfaces, etc.

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3.2.5.2 Electrochemical plating In laser-enhanced electrochemical plating, an external battery is applied in such a way that, in general, the substrate is negatively biased with respect to a counterelectrode. The applied voltages are, typically, 1 . . . 2 V. Detailed experiments have shown that the enhancement of the reaction rate is based on local laser-induced heating [84Gut, 81Pui]. By reversing the polarity of the electrodes, the same process can be employed to etch material surfaces. Laser-enhanced electrochemical plating (etching) has been studied most extensively with Ar+ and Kr+ -lasers. The power densities employed were between 102 and 106 W/cm2 . Both continuous and pulsed plating (etching) were demonstrated by modulating the external voltage source, the laser output power, or both synchronously. Plating has been studied in detail for Au, Cu and Ni. The substrates were glass and c-Al2 O3 , both covered with 0.1 µm thick films of Au, Cu, Ni, Mo, or W. The resolution achieved in these experiments was a few micrometers. Detailed investigations on electrochemical Au plating have revealed that dense, small-grained, crack-free, and uniform deposits of good adhesion are formed at elevated temperatures and high concentrations of gold within the electrolyte. Here, the operating potential should be below the mass-transport limit. Near this limit, Au of good morphology was deposited over areas of 500 µm in diameter with rates of up to 1 µm/s. Direct writing of Cu lines on premetallized glass substrates was possible with widths of ≥ 2 µm. In laser-enhanced jet-plating the deposition rates are significantly higher. Here, the mass transport to the substrate is increased by a jet [84Gut]. The flow velocities are, typically, 103 cm/s. Jet plating permits high-quality, rapid, localized plating. The electrochemical and hydrodynamical parameters determining the mechanical and metallurgical properties of deposits have been investigated, in particular for Au. Here, plating rates of up to 12 µm/s have been achieved. Laserenhanced plating can be applied for circuit and mask repair [90Jac], the fabrication of interconnects, in customization and ohmic contact formation, etc.

3.2.6 Pulsed-laser deposition (PLD) Lasers can be used to fabricate thin extended films by condensing on a substrate surface the material that is ablated from a target under the action of laser light. Depending on the specific laser and material parameters, ablation takes place under quasi-equilibrium conditions, as in laserinduced thermal vaporization, or far from equilibrium, as in many cases of Pulsed-Laser Ablation (PLA). Thin-film formation based on PLA is termed Pulsed-Laser Deposition (PLD). Instead of PLD, terms such as Laser-Sputter Deposition (LSD), Pulsed-Laser Evaporation (PLE), and others, are also used in the literature. PLD is of particular interest because it enables one to fabricate multicomponent stoichiometric films from a single target. Novel functional materials such as smart materials, nanocomposites etc. can be fabricated by sequential ablation of several targets, by reactive PLD, etc. From the aspect of film formation, the detailed ablation mechanisms are in many cases of minor relevance. It is only important that ablation takes place on a time scale that is short enough to suppress the dissipation of the excitation energy beyond the volume ablated during the pulse. Only with this condition, can damage of the remaining target and its segregation into different components be largely avoided. In this regime of interactions, the relative concentrations of species within the plasma plume remain almost unchanged for successive laser pulses and almost equal to those within the target material. This is the main reason why PLD has been found to be useful, in particular, for the deposition of epitaxial or large-grain oriented films with complex stoichiometry.

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PLD is a very reliable technique. It offers great experimental versatility, it is fairly simple, and fast – as long as films of up to several square-centimeters are to be fabricated. The use of corrosive and/or hazardous chemicals employed in material synthesis by standard techniques can widely be avoided. The short turn-around times enable one to efficiently study a great variety of different compounds and film dopings. For these reasons, PLD is particularly suitable in materials research and development. The short interaction times and the strong non-equilibrium conditions in PLD allow some unique applications: – The synthesis of metastable materials that cannot be produced by standard techniques. – The formation of films from species that are generated only during PLA. With certain systems, the physical properties of such films are superior to those fabricated by standard evaporation, electron-beam evaporation, etc. – The fabrication of nanocrystalline films. – The formation of composite films consisting of different materials. The major disadvantage of PLD is the relatively low throughput that can be achieved. Another problem, in particular with thin epitaxial films, can be the particulates that frequently occur on the substrate and film surface. Clearly, other thin-film techniques have their peculiarities as well. For example, Radio-Frequency (RF) sputtering enables one to produce large-area films with good thickness uniformity and small surface roughness (typically < 100 ˚ A with 1000 ˚ A thick films). Here, the control over the correct stoichiometry is, however, much more problematic. Furthermore, sputtering requires large targets and longer preparation cycles, and it affords less experimental versatility.

3.2.6.1 Overview of materials and film properties The large variety of different materials deposited as thin films by laser ablation is listed in [00Bae]. PLD permits a precise and fast control of the material composition. This capability is of particular importance in band-gap engineering and the fabrication of heterostructures. The lasers mainly employed in film fabrication are excimer lasers, Nd:YAG or Nd:glass lasers, pulsed CO2 lasers and, more recently, Ti-sapphire lasers. Some of the experiments have been performed with scanned cw-laser beams. Films deposited by laser ablation are amorphous, polycrystalline, or single crystalline. Higher substrate temperatures favor, in general, crystalline growth. With multiphase materials, the particular crystalline phase formed during deposition is determined by both the substrate temperature and the gas pressure. The film growth rates achieved range from a few ˚ A/s to some ten µm/s. 3.2.6.1.1 Inorganic materials Non-reactive PLD from metal targets results in the formation of metallic films [02Sch3, 96Sve]. Reactive deposition in oxygen atmosphere yields semiconducting or insulating metal oxides. In a similar way, metal nitrides, halides, and other metal compounds can be synthesized. Semiconducting films of pure Si and hydrogenated Si have been fabricated by ablation of Si targets in vacuum and H2 atmosphere, respectively. Deposition rates of 10 to 100 µm/s were obtained [83Han]. Ge films have been grown by using molten Ge targets. Epitaxial films of GaN have been fabricated by ablation of liquid or solid Ga targets in N2 or NH3 atmosphere [99Gro, 98Opo]. Other compound semiconductors have been synthesized by employing either multiple targets for the individual elements, or single stoichiometric targets. In the latter case, additional sources must frequently be employed to obtain stoichiometric films. Among the heterostructures fabricated by pulsed-laser deposition are InSb/CdTe, InSb/PbTe, Bi/CdTe, etc. The single layers Landolt-B¨ ornstein New Series VIII/1C

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were polycrystalline or epitaxial with thicknesses down to less than 50 ˚ A. The capability of pulsedlaser ablation in combination with MBE has been demonstrated for Hg1−x Cdx Te [92Che]. Here, pulsed-laser ablation is used to produce a modulated flux of Cd and Te, simply by changing the pulse repetition rate. In this way, the composition of the material, and thereby its band-gap, can be controlled. Diamond-Like Carbon (DLC) films were produced by excimer laser [96Kau, 98Yam] and Nd:YAG laser [96Ong] ablation of graphite. Growth rates between 3 and 10 ˚ A/s were achieved. The films were amorphous, had a hardness of up to 33 GPa, a refractive index n(633 nm) = 2.2 ± 0.2, a resistivity around 108 Ω cm, and an optical band-gap of 1.1 ± 0.2 eV. Such films could not be dissolved in solutions of HF + HNO3 . Among the dielectric materials deposited as thin films are SiO2 , ZrO2 , Al2 O3 , Si3 N4 , and many fluorides such as MgF2 , CaF2 , SrF2 , etc. The deposition rates achieved vary, typically, between 1 ˚ A/s and 50 ˚ A/s. The depletion of oxygen observed for many oxides can be compensated by reactive ablation in oxygen atmosphere at pressures between 10−4 and 1 mbar. Among the ferroelectric films fabricated by PLD are oxides of BaTiO3 , Ba1−x Srx TiO3 [03Nav], KaTa1−x Nbx O3 (KTN), PbTi1−x Zrx O3 (PZT), SrTiO3 and perovskite-like materials such as Bi4 Ti3 O12 , LiNbO3 , etc. Epitaxial films of BaTiO3 have been grown on (100) LiF substrates by XeCl-laser ablation of sintered BaTiO3 pellets [89Dav]. In a similar way, polycrystalline BaTiO3 was deposited on (100) Si substrates. The dielectric constant of these films was ε ≈ 200 and their dielectric strength 1 MV/cm. Crystalline films of PbTi1−x Zrx O3 (x ≈ 0.48) were deposited on bare and Pt or Au coated Si substrates by means of KrF-laser (ε ≈ 850, remanent polarization ≈ 22 µC/cm2 , coercive field ≈ 40 kV/cm, ρ ≈ 1013 Ω cm; [92Roy]) and Nd-YAG laser radiation [97Ver]. Oriented films of Bi4 Ti3 O12 (BTO) have been grown on SrTiO3 , LaAlO3 , MgO, etc., mainly by means of KrF-laser radiation. The fabrication of ferroelectric/superconductor multilayer structures has been demonstrated for BaTiO3 /YBCO/YSZ/Si, YBCO/PZT/YBCO/YSZ/Si and YBCO/BaTiO3 /SrTiO3 . Such structures are of interest for various device applications, such as superconducting nonvolatile memories and high-density Dynamic Random Access Memories (DRAMs). Ferromagnetic films of La2/3 Ba1/3 MnOx which show a giant negative magnetoresistance have been deposited from stoichiometric ceramic targets onto SrTiO3 substrates. Thin films of High-Temperature Superconductors (HTS) have been fabricated by pulsed-laser deposition in both reactive and non-reactive atmosphere. Non-reactive deposition was mainly performed in vacuum. It requires post-annealing for establishing the superconducting phase. Reactive deposition in O2 , O3 , N2 O, or NO2 permits in situ fabrication of high-quality superconducting films [00Bae, 91Sch2, 90Eib, 90Ven] including different types of compounds (Fig. 3.2.28). In most of the experiments, UV excimer-laser radiation and high-density ceramic targets are used. The preference for excimer lasers is related to their short wavelength, high pulse energies, and short pulse lengths, typically 10 . . . 40 ns. These properties favor strong light absorption and congruent target ablation. Stoichiometric laser ablation of (ceramic) targets requires energy densities of, typically, 1 . . . 5 J/cm2 (Fig. 3.2.6). With low-energy short pulses and high-energy long pulses, compositional changes have been observed in the targets, and thus in deposited films [88Auc, 90Hei]. In-situ epitaxial growth of heterostructures of YBa2 Cu3 O7 /Y1−x Prx Ba2 Cu3 O7 has been demonstrated over the entire range 0 ≤ x ≤ 1 [90Ven, 92Low]. Pulsed-laser deposition permits one to synthesize metastable compounds that can be prepared either not in single-phase or not at all by solid-state reactions or by standard evaporation techniques. This unique possibility is related to the lower substrate temperatures that can be employed in PLD, to the type and energy of species involved in this process, and to the short turn-around times. Synthesis can be achieved by ablation of either multiple targets or a single target consisting of non-reacted or multiphase mixtures of the individual components. Among the synthesized materials are YBax Sr1−x Cu3 O7−δ , LuBa2 Cu3 O7 , LuBaSrCu3 O7 and TmBaSrCu3 O7 . Films with step-like morphology permit one to investigate the strongly anisotropic properties of materials. This has been demonstrated for YBCO [02Mar, 97Mar], Bi-2212 [00Lan],

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Fig. 3.2.28. Dependence of the transition temperature Tc0 for thin films of REBa2 Cu3 O7 on (001) SrTiO3 (filled diamonds) and (001) MgO (filled squares) substrates on the radius of RE3+ ions. REBaSrCu3 O7 films on (001) SrTiO3 (filled triangles) and (001) MgO (filled circles). The empty squares and empty circles refer to ceramic samples [98Bae].

Fig. 3.2.29. (a) Schematic picture which shows an off c-axis oriented (001) SrTiO3 or MgO substrate (surface normal n ˆ ) and a step-like grown film. The angle between n ˆ and the (001) direction of the substrate is θs . The angle between (001)s and the c-axis of the film is 0 ≤ θf ≤ 2 ◦ . (b) TEM picture of a Bi-2212 film on a SrTiO3 substrate with θs = 10 ◦ [00Roe].

and Hg-1212 [02Ped] films deposited on off c-axis oriented (001) SrTiO3 and MgO substrates (Figs. 3.2.29, 3.2.30). The films also show a strong Seebeck effect and thermopower [95Zeu, 97Zah]. 3.2.6.1.2 Organic materials Pulsed-laser deposition of polymers was mainly investigated for PTFE (polytetrafluoroethylene) (Figs. 3.2.31, 3.2.32) [01Hub, 98Sch, 98Li, 03Hop]. The fabrication of amorphous PVDF (polyvinylidene fluoride) films [96Nor], PAN (polyacrylonitrile) films [96Nis], and the laser-induced degradation and decomposition kinetics of various polymers [88Han] have also been investigated. Deposition of polymer films using infrared radiation both resonant and nonresonant with vibrational modes in the target material was studied in [02Bub]. The effect of cone formation on the deposition rate of HPDS (hexaphenyldisilane) was studied in [99Zen].

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Fig. 3.2.30. Electrical resistivities ρab and ρc of Bi-2212 films deposited by PLD on vicinal (100) SrTiO3 substrates (λ = 248 nm, φ ≈ 3.5 J/cm2 , τl ≈ 25 ns, p(O2 ) ≈ 2.5 mbar) as a function of film thickness (Nl = 170 to 2500) [00Roe].

NORMALIZED SURFACE POTENTIAL

Fig. 3.2.31. Optical polarization micrographs of PTFE-Teflon films deposited by KrF-laser radiation on 100 nm PdAu/(100)Si. The parameters employed were φ(KrF) ≈ 3 J/cm2 , τl ≈ 20 ns, Ts ≈ 360 ◦ C, p(Ar) ≈ 0.3 mbar. The target was fabricated from pressed and sintered PTFE powder [98Li].

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Fig. 3.2.32. Thermally stimulated surface potential decay for PTFE foils and PLD films [01Hub].

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3.2.6.2 Nanocrystalline films Uniform films are synthesized primarily from a flux of atoms/ions and small molecules impinging onto the substrate surface. However, during the initial phase of growth and for certain systems, laser parameters, and substrate temperatures, islands or non-uniform films of nanocrystals may be formed on the substrate by nucleation, growth, and coalescence of clusters. The situation changes significantly when higher ambient gas pressures, typically 1 to several hundred mbar, are employed. Such conditions favor vapor-phase condensation and the formation of clusters and nanocrystals [01Sco]. The latter have diameters of about 1 to 20 nm and contain 102 to 106 atoms. By condensing the nanoparticles onto a substrate, nanocrystalline or cluster-assembled films can be fabricated [01Ayy]. Nanocrystalline (nc) films have physical properties that are quite different from those grown from a predominantly atomic/molecular flux. They may show quantum confinement effects, or may consist of entirely new composite materials formed, e.g., in a reactive atmosphere. Such films are promising materials for applications in photonics, optoelectronics requiring room-temperature PhotoLuminescence (PL), gas-sensor technology, etc. Among the materials investigated are films of nc-Si and Silicon-Rich Silicon Oxide (SRSO) [00Kab, 98Mar, 98Yos], ZnTe [98Low], ZnO [98Kaw], Cax Fey Oz [98Sas], etc. 3.2.6.2.1 Nanocomposite materials Another class of nanostructured materials consists of nanocrystals embedded in a matrix. Such films can be fabricated by employing a composite target, or by sequential or simultaneous ablation of several targets. In this technique, film synthesis can be performed also in vacuum. Here, the nanocrystals are formed on the substrate. The crystallite size can be controlled via the laser fluence and/or the number of laser pulses on each target, or via the substrate temperature. Among the composites investigated in detail are Bi nanocrystals embedded in a-Ge [98Ser], Bi and Cu in Al2 O3 [99Afo], Pt in TiO2 [99Bec], Si in SiO2 [98Mak] and CdTe in SiO2 [93Oht]. Figure 3.2.33 shows a high-resolution TEM picture of a film fabricated by sequential ablation of Cu and Al2 O3 targets by means of ArF-laser radiation. The size distributions of crystallites are quite small and narrower than those achieved with other techniques as, e.g., ion implantation or sol-gel methods. Optical properties of various nanocomposite materials fabricated by different techniques are listed in [99Afo].

Fig. 3.2.33. High-resolution TEM picture of Cu nanocrystals (d ≈ 3 ± 0.6 nm) embedded in Al2 O3 . The film was grown by sequential ArF-laser ablation of Cu and Al2 O3 targets with Nl (Cu)= 160, φ ≈ 2/cm2 , τl = 12 . . . 20 ns, νr = 5 . . . 10 Hz [99Afo].

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3.2.6.2.2 Size-selective ablation Metal nanoclusters with well-defined dimensions and very small size distributions can be fabricated by selective laser ablation [99Bos]. In this technique, the nanoclusters are irradiated with nanosecond laser pulses. Because the resonance frequency of plasmons depends on cluster size, d, laser-light absorption, and thereby heating of clusters, will depend on d. By using two laser wavelengths, it is possible to totally evaporate the smallest clusters of the distribution and to shrink the largest clusters. By this means, only clusters with a well-defined size and a very small size distribution remain on the surface. For example, Ag clusters have been fabricated with a size of 5 ± 0.5 nm.

3.2.6.3 Hybrid techniques The short response times achieved in laser ablation are the reason why it is useful for certain materials, to integrate pulsed-laser ablation into Molecular-Beam Epitaxy (MBE) [00Bae, 92Che]. The combination of PLA and plasma techniques permits low-temperature thin film growth [01Sun].

3.2.6.4 Laser-induced forward transfer Laser-Induced Forward Transfer (LIFT) employs laser radiation to transfer a film (target) initially precoated on an optically transparent support onto a substrate (Fig. 3.2.34). With certain systems, good results can also be achieved by laser irradiation of the target film via the (transparent) substrate. Patterning is achieved by direct writing, projection, or laser-beam interference. Pattern formation by LIFT has been demonstrated mainly for metals [98Zer, 94Kan, 93Tot], but also for some semiconductors, in particular In2 O3 [98Zer] and high-temperature superconductors [90Fog]. The smallest pattern sizes achieved with metal films and ns laser pulses are several µm wide. Submicrometer resolution has been demonstrated for Cr dots by femtosecond KrF-LIFT [98Zer].

Fig. 3.2.34. Schematic picture demonstrating the LIFT technique. The target film which is precoated on a transparent support is brought into close proximity to the substrate. Irradiation can take place also from the left side, if the substrate and the film are transparent at the laser wavelength so that the laser light is absorbed only in the support [00Bae].

3.2.7 Chemical surface transformations Chemical surface transformations are characterized by an overall change in the chemical composition of the material or the activation of a real chemical reaction. This shall include material synthesis, decomposition, and some types of surface modifications. The latter can take place either Landolt-B¨ ornstein New Series VIII/1C

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in a chemically reactive ambient medium or they are performed by adding a new material to the surface, or by depleting a certain constituent of the material from the surface. With physicochemical transformations both physical and chemical processes are important [00Bae].

3.2.7.1 Doping Laser-induced surface doping takes advantage of the high heating and cooling rates that can be achieved with lasers. The short temperature cycles enable one to produce very shallow heavily doped layers within solid surfaces. Dopant incorporation takes place via high-temperature diffusion or liquid-phase transport. The dopant source may be an adsorbate, a gas, a liquid, or an evaporated film. The thickness of the doped layer can be controlled via the laser-beam dwell time. Of special importance is the doping of Si; the great stability of Si allows one to study wide parameter ranges. Large-area thin-layer doping (sheet doping) of Si was mainly performed with excimer lasers using adsorbed layers of BCl3 , B2 H6 , BF3 , and PCl3 as precursors [84Deu] or gas-phase precursors such as AsH3 , BCl3 , BF3 , B2 H6 , PCl3 and PH3 (Fig. 3.2.35) [90Mat]. Liquid-phase doping with P and Sb has been demonstrated in [81Stu]. Doping with Al, B, Bi, Ga, In, P, and Sb was also performed by laser-induced heating of spun-on or evaporated dopant films [96Zha, 91Inu, 85Fog, 84Deu]. Laserinduced sheet doping of Si can be used to produce very shallow junctions [93Bol, 93Kra, 90Mat]. Local doping of Si by cw Ar+ -laser direct writing has been demonstrated for lateral dimensions down to submicrometer levels [82Ehr]. Local doping of GaAs with Si has been demonstrated by KrF-laser light projection [90Sug] and with Zn by Ar+ -laser direct writing [90Lic]. Linewidths between about 0.3 and 3 µm have been achieved.

Fig. 3.2.35. ArF-laser-induced doping of (100) Si with B. (a) SIMS profiles of the B concentration (φ ≈ 1 J/cm2 , τl ≈ 21 ns; 2 Hz; p(BCl3 ) ≈ 6.7 mbar) [90Sla]. (b) Calculated normalized concentration profile of B. Nl is the number of laser pulses and ll the estimated diffusion length of species within the liquefied layer [00Bae].

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3.2.7.2 Alloying and synthesis Laser alloying can be applied to modify the physical and chemical properties of solid surfaces and to fabricate new metastable materials. Laser synthesis describes the fabrication of stoichiometric compounds, mainly in the form of thin films and fibers [00Bae]. Among the most recent investigations is the formation of Fe3 C surface layers by laser plasma cementation [02Car], the formation of tungsten silicides [01Lub], the photo-assisted formation of ZrO2 layers [02Yu].

3.2.7.3 Oxidation, nitridation, reduction For many applications such as local hardening, chemical passivation, electrical insulation, etc., it is desirable to increase the thickness of the oxide layer, or to stimulate oxidation on material surfaces that do not spontaneously oxidize in an oxygen-containing environment. In many cases, the reoxidation of oxygen-deficient oxide layers and the transformation of oxides Mx Oy into Mx±ε Oy±δ is desirable as well. The latter includes the depletion of oxygen in a reducing ambient medium. Laser-enhanced surface oxidation was mainly performed in air and oxygen atmosphere; nitridation in gaseous and liquid NH3 and N2 . Nitridation is extremely sensitive to traces of oxygen. An overview on the various systems so far investigated is given in [00Bae]. Semiconductor oxidation takes place mainly at the semiconductor-oxide interface while metal oxidation frequently proceeds at the oxide surface. Laser-induced surface oxidation and nitridation of metals and semiconductors results in the formation of films with thicknesses of, typically, 10 ˚ A to 1 µm. The technique is complementary to laser-CVD which permits to grow films with thicknesses of, typically, 1 to 100 µm. Investigations of laser-enhanced oxidation of metals have been performed with metal plates and thin metal films. With plate-like samples, the laser-oxidized surface layer is very thin in comparison to the sample thickness. With thin metal films, having thicknesses of, typically h ≤ 0.1 µm, oxidation can succeed throughout the whole film. In this case, oxidation often results in the synthesis of a single stoichiometric oxide (Fig. 3.2.36). Laser-induced oxidation of metal films in water has been reported in [02Hae]. Very thin metal-oxide layers have been produced by Pulsed-Laser Plasma Chemistry (PLPC) [00Bae]. Nitridation of metal surfaces was studied for Al [01Sic], Fe [02Sch2], Ti and Zr. Thick TiN overlayers have been fabricated by CO2 -laser-induced surface melting in combination with an N2 /Ar-gas jet. For an overview see [02Sch1]. On silicon, oxide films have been grown in O2 atmosphere up to a thickness of about 0.2 µm and an average rate of about 3 × 10−5 µm/s [88Orl]. The average dielectric breakdown was about 6.5 × 106 V/cm [87Boy]. Among the more recent investigations is the oxidation of Si1−x Gex layers [99Cra1, 99Cra2]. Surface nitridation of Si has been investigated mainly for N2 and NH3 atmospheres and excimer-laser radiation. Laser-induced reoxidation permits direct writing of superconducting YBa2 Cu3 O7 lines into semiconducting YBa2 Cu3 O6 [94Sob, 91She, 88Lib2] films [88Lib2]. Similar experiments have been performed for oxidic perovskites [85Bae, 84Ott]. Just as laser-induced heating of certain materials in oxygen-rich atmosphere permits to incorporate oxygen into the lattice or to form a surface oxide layer, specific materials can give up their oxygen to a reducing environment under the appropriate laser-heating conditions. Laser-induced reduction and metallization of oxidic perovskites allows single-step conductive patterning of the otherwise insulating material surface [88Bae, 86Kap1]. Electrodes of areas up to 0.5 × 0.5 cm2 have been fabricated on PLZT surfaces in a similar way [86Kap2]. Laser-processed electrodes could be superior to conventional electrodes in microprocessing. The physical properties of High-Temperature

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Fig. 3.2.36. Ar+ -laser-induced oxidation of 0.05 µm thick Cu films on sapphire (λ = 514.5 nm, w0 ≈ 80 µm, p(O2 ) ≈ 200 mbar). (a) Time-resolved reflectance measured by HeNe-laser probe beam. Vertical lines indicate beginning of Ar+ -laser irradiation. Note changes in time scales with 2.2, 3.3, and 4.4 W curves. (b) Oxide-layer thickness as a function of irradiation time. (c), (d) Calculated temporal dependence of the reflectivity of HeNe- and Ar+ -laser beams, of the film thickness, and of the temperature for P = 4.4 W. Oscillations end when the film is oxidized over the whole thickness, adapted from [87Bau].

Superconductors (HTS) depend sensitively on the oxygen content. The temperature-dependent resistance of strip lines in YBa2 Cu3 O7−δ before and after laser treatment has been investigated in [88Lib1, 91She]. Similar experiments have recently been performed by means of 2D lattices of SiO2 microspheres [03Bae]. Microspheres can be employed for surface patterning, trimming, tuning of critical currents, the fabrication of weak links for SQUID’s (Superconducting Quantum Interference Devices), etc. The dehydroxylation of silica surfaces was investigated in [00Hal]. Etch frustration in LiNbO3 single crystals induced by femtosecond UV-laser irradiation has been studied in [02Mai].

3.2.7.4 Transformation of organic materials, laser lithography Laser fluences below or around the ablation threshold can modify the surface morphology, crystallinity, chemical composition, reactivity, and resistivity of surfaces. Laser-light irradiation with fluences φ ≤ φth can significantly enhance the relative amount of amorphous material within polymer surfaces. Recrystallization may result in the formation of dendrites [99Klo]. Successful material coating and bonding is closely related to the adhesion forces between the materials under consideration. Laser-enhanced adhesion may be based on different types of surface roughening, the scission of surface chemical bonds and their saturation with

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Fig. 3.2.37. AFM (Atomic Force Microscope) picture showing the surface roughening of an ultraflat PET foil after single-pulse KrF-laser irradiation (φ ≈ 41 mJ/cm2 ) [00Bae].

Fig. 3.2.38. Adhesion force of 200 nm thick Co80 Ni20 films on PET foils (thickness ≈ 50 µm) irradiated with 248 nm KrF-laser light prior to film deposition. Two different types of peel tests have been employed (filled squares: EAA (ethylene-acrylic acid) test, filled circles: modified test), adapted from [94Hag].

other atoms/radicals, etc. Systematic investigations of this type have been performed for polymers (Figs. 3.2.37, 3.2.38) [00Bae, 94Hag, 95Bah, 01Nii]. Photochemical exchange of species on polymer surfaces was studied in detail for PTFE (Teflon). Irradiation of PTFE in a reactive ambient medium may result in defluorination of the PTFE surface and substitution of F atoms by different functional groups. An example is the photochemical reaction [95Mur] F F H3 C CH3 | | | | B(CH3 )3 + [− C − C −]n + hν (193 nm) → BF3 + [ − C − C − ]n . | | | | F F F F The laser fluences employed in this processing mode are well below the ablation threshold and, typically, φ < 50 mJ/cm2 . Because PTFE has negligible absorption at 193 nm, direct breaking of Landolt-B¨ ornstein New Series VIII/1C

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[Ref. p. 345

C–F bonds by ArF-laser radiation (hν ≈ 6.4 eV) seems to be unlikely. Photochemical exchange may instead be based on the reaction of B atoms with F (bond energy ∆ED (BF) ≈ 7.9 eV; ∆ED (CF) ≈ 5.5 eV). In any case, this reaction makes the originally lipophobic PTFE surface lipophilic. In similar investigations using an aqueous solution of B(OH)3 , F atoms are substituted by OH. This makes the originally hydrophobic PTFE surface hydrophilic. The shear tensile strength for epoxy bonding of such surfaces to stainless steel is increased, typically, from 20 to 1280 N/cm2 . ArF-laser irradiation of PTFE in N2 H4 or NH3 atmosphere leads to the substitution of F by NH2 radicals [96Nii, 96Hei2, 96Hei1]. The modified PTFE surface becomes hydrophilic and permits metallization, e.g. for printed circuit boards or the adhesion and growth of biological cells [01Svo, 03Gum] etc. Photochemical exchange of species was also studied for PC, PE (polyethylene), PET, and PP (polypropylene). UV-laser irradiation with fluences φ ≈ φth may cause drastic changes in the composition of polymer surfaces. ArF- or KrF-laser irradiation of PI significantly decreases the surface concentration of oxygen and nitrogen with respect to carbon. This process results in a dramatic increase in surface conductivity [93Are, 95Bal1, 95Bal2, 95Bae, 01Qin]. Laser-induced surface modifications of polymers in combination with electroless plating permit selective area metallization. Light-induced transformations within organic materials are widely studied in connection with photochromic reactions [97Tor, 97Dvo], photopolymerization [01Miw, 02Pik, 95Kaw, 03Ser], and photodecomposition reactions [88Han]. The most important applications include: – Laser lithography. Here, the Hg-lamp (λ = 436 nm or 365 nm) employed in conventional lithography is substituted by an UV laser. At present, the only lasers that fulfill the requirements for high-resolution lithography, in particular with respect to wavelength, spectral band width, intensity, etc. are excimer lasers. While 248 nm KrF-lasers are already on line, lithography with 193 nm ArF- and in particular 157 nm F2 -laser radiation is still under development. With ArFand F2 -lasers feature sizes of 110 nm and 80 nm, respectively, have been achieved [98Rot]. For applications that do not require submicrometer resolution, one or several of the processing steps employed in standard lithography can be replaced by laser techniques as well. For example, Thin-Film Transistors (TFT) used in Liquid Crystal Displays (LCD) have lateral dimensions of several ten µm. Such patterns can be manufactured by direct laser ablation of the photoresist. Successful experiments of this type have been performed with KrF-laser radiation [97Suz]. – The fabrication of optical waveguides, 3D-patterns for optical data storage, photonic band-gap structures, etc. [99Cum, 99Kir]. – Surface patterning of materials by decomposition of precursor films. Metallopolymers or metal acetates permit the fabrication of metal patterns [85Fis, 85Fen]. Subsequent to laser direct writing, non-transformed parts of the precursor film are dissolved. The same technique has been used for a variety of other materials [00Bae].

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Chuang, T.J.: J. Chem. Phys. 74 (1981) 1453. Leyendecker, G., B¨auerle, D., Geittner, P., Lydtin, H.: Appl. Phys. Lett. 39 (1981) 921. Puippe, J.C., Acosta, R.E., Gutfeld, R.J. von: J. Electrochem. Soc. 128 (1981) 2539. Stuck, R., Fogarassy, E., Muller, J.C., Hodeau, M., Wattieux, A., Siffert, P.: Appl. Phys. Lett. 38 (1981) 715.

82Bae

B¨auerle, D., Irsigler, P., Leyendecker, G., Noll, H., Wagner, D.: Appl. Phys. Lett. 40 (1982) 819. Ehrlich, D.J., Tsao, J.Y.: Appl. Phys. Lett. 41 (1982) 297. Gutfeld, R.J. von, Hodgson, R.T.: Appl. Phys. Lett. 40 (1982) 352.

82Ehr 82Gut 83Bae1 83Bae2 83Bae3 83Han 83Ley 83Pod

84Deu 84Gut 84Ott 84Pod 85Bae 85Fen

85Fis 85Fog 85Ses 86Bae 86Dop 86Eye 86Kap1

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86Ses 87Bau 87Boy 87Eye1 87Eye2 87Kul 87Zha1 87Zha2 88Auc 88Bae 88Ere 88Han 88Lib1 88Lib2 88Low 88Orl 88Szo 88Tak1 88Tak2

89Dav 89Ehr 89Kue 89Mog 89Ric 90Beu 90Don 90Eib 90Fog 90Hei 90Ihl 90Jac 90Lic 90Mat 90Sla 90Sug

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96Nii 96Nis 96Nor 96Ong 96She 96Stu 96Sve 96Zha 97Alv 97Bae 97Che 97Das 97Deu 97Dut

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References for 3.2 Schw¨ odiauer, R., Bauer-Gogonea, S., Bauer, S., Heitz, J., Arenholz, E., B¨ auerle, D.: Appl. Phys. Lett. 73 (1998) 2941. Serna, R., Missana, T., Afonso, C.N., Ballesteros, J.M., Petford-Long, A.K., Doole, R.C.: Appl. Phys. A 66 (1998) 43. Suzuki, K., Matsuda, M., Hayashi, N.: Appl. Surf. Sci. 127–129 (1998) 905. Yamamoto, K., Koga, Y., Fujiwara, S., Kokai, F., Heimann, R.B.: Appl. Phys. A 66 (1998) 115. Yoshida, T., Yamada, Y., Orii, T.: J. Appl. Phys. 83 (1998) 5427. Zergioti, I., Mailis, S., Vainos, N.A., Papakonstantinou, P., Kalpouzos, C., Grigoropoulos, C.P., Fotakis, C.: Appl. Phys. A 66 (1998) 579. Afonso, C.N., Solis, J., Serna, R., Gonzalo, J., Ballesteros, J.M., Sande, J.C.G. de: SPIE Proc. 3618 (1999) 453. Anderson, W.R., Bradley, C.C., McClelland, J.J., Celotta, R.J.: Phys. Rev. A 59 (1999) 2476. Anisimov, S.I., Inogamov, N.A., Oparin, A.M., Rethfeld, B., Yabe, T., Ogawa, M., Fortov, V.E.: Appl. Phys. A 69 (1999) 617. Beck, K.M., Sasaki, T., Koshizaki, N.: Chem. Phys. Lett. 301 (1999) 336. Beinhorn, F., Ihlemann, J., Simon, P., Marowsky, G., Maisenh¨ older, B., Edlinger, J., Neuschl¨afer, D., Anselmetti, D.: Appl. Surf. Sci. 138–139 (1999) 107. Bosbach, J., Martin, D., Stietz, F., Wenzel, T., Tr¨ ager, F.: Appl. Phys. Lett. 74 (1999) 2605. Campbell, E.E.B., Ashkenasi, D., Rosenfeld, A.: In: Agarnala, R.P. (ed.): Lasers in Materials, Uetikon-Zurich, Switzerland: Trans. Tech. Pub. (1999) Chap. 5. Craciun, V., Boyd, I.W., Hutton, B., Williams, D.: Appl. Phys. Lett. 75 (1999) 1261. Craciun, V., Boyd, I.W., Perriere, J., Hutton, B., Nicholls, E.J.: J. Mater. Res. 14 (1999) 3525. Cumpston, B.H., Ananthavel, S.P., Barlow, S., Dyer, D.L., Ehrlich, J.E., Erskine, L.L., Heikal, A.A., Kuebler, S.M., Lee, I-Y.S., McCord-Maughon, D., Qin, J., Rockel, H., Rumi, M., Wu, X.-L., Marder, S.R., Perry, J.W.: Nature 398 (1999) 51. Gross, M., Henn, G., Ziegler, J., Allenspacher, P., Cychy, C., Schr¨ oder, H.: Materials Science and Engineering B 59 (1999) 94. Horwitz, J.S., Krebs, H.-U., Murakami, K., Stuke, M. (eds.): Appl. Phys. A, 69 (Suppl.) (1999) S1–S952. Kirkpatrick, S.M., Baur, J.W., Clark, C.M., Denny, L.R., Tomlin, D.W., Reinhart, B.R., Kannan, R., Stone, M.O.: Appl. Phys. A 69 (1999) 461. Klose, S., Arenholz, E., Heitz, J., B¨ auerle, D.: Appl. Phys. A 69 (Suppl.) (1999) S487. K¨ onig, K., Riemann, I., Fischer, P., Halbhuber, K.-J.: Cellular and Molecular Biology 45 (1999) 195. Kr¨ uger, J., Kautek, W., Newesely, H.: Appl. Phys. A 69 (Suppl.) (1999) S403. Lenzner, M., Kr¨ uger, J., Kautek, W., Krausz, F.: Appl. Phys. A 68 (1999) 369. Nolte, S., Chichkov, B.N., Welling, H., Shani, Y., Lieberman, K., Terkel, H.: Opt. Lett. 24 (1999) 914. Nolte, S., Momma, C., Kamlage, G., Ostendorf, A., Fallnich, C., Alvensleben, F. von, Welling, H.: Appl. Phys. A 68 (1999) 563. Perry, M.D., Stuart, B.C., Banks, P.S., Feit, M.D., Yanovsky, V., Rubenchik, A.M.: J. Appl. Phys. 85 (1999) 6803. Rubahn, K., Ihlemann, J., Rubahn, H.-G.: J. Appl. Phys. 86 (1999) 2847. Sch¨ afer, D., Ihlemann, J., Mann, K., Marowsky, G.: Appl. Phys. A 69 (Suppl.) (1999) S319. Schieche, H., Piglmayer, K.: Appl. Surf. Sci. 138–139 (1999) 280. Wang, J., Niino, H., Yabe, A.: Appl. Phys. A 69 (Suppl.) (1999) S271. Landolt-B¨ ornstein New Series VIII/1C

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01Ayy 01Bon 01Hub 01Lub 01Miw 01Nii 01Oba 01Qin 01Sco 01Sic 01Sun 01Svo 01Wys

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Wellershoff, S.S., Hohlfeld, J., G¨ udde, J., Matthias, E.: Appl. Phys. A 69 (Suppl.) (1999) S99. Zeng, X., Koshizaki, N., Sasaki, T., Yabe, A., Rossignol, F., Nagai, H., Nakata, Y., Okutani, T., Suzuki, M.: Appl. Surf. Sci. 140 (1999) 90. B¨auerle, D.: Laser Processing and Chemistry, 3rd Ed., Berlin, Heidelberg: Springer, 2000. Ermer, D.R., Papantonakis, M.R., Baltz-Knorr, M., Nakazawa, D., Haglund, R.F.: Appl. Phys. A 70 (2000) 633. Halfpenny, D.R, Kane, D.M., Lamb, R.N., Gong, B.: Appl. Phys. A 71 (2000) 147. Kabashin, A.V., Charbonneau-Lefort, M., Meunier, M., Leonelli, R.: Proc. SPIE 3933 (2000) 192. Kim, B., Feit, M.D., Rubenchik, A.M., Joslin, E.J., Eichler, J., Stoller, P.C., Da Silva L.B.: Appl. Phys. Lett. 76 (2000) 4001. Lang, W., Bittner, R., Gehringer, P., Pedarnig, J.D., B¨ auerle, D.: Physica B 284 (2000) 993. Praschak, D., Bahners, T., Schollmeyer, E.: Appl. Phys. A 71 (2000) 577. R¨ossler, R., Pedarnig, J.D., B¨ auerle, D., Connolly, E.J., Zandbergen, H.W.: Appl. Phys. A 71 (2000) 245. Ayyub, P., Chandra, R., Taneja, P., Sharma, A.K., Pinto, R.: Appl. Phys. A 73 (2001) 67. Bonse, J., Wrobel, J.M., Kr¨ uger, J., Kautek, W.: Appl. Phys. A 72 (2001) 89. Huber, N., Heitz, J., B¨ auerle, D., Schw¨ odiauer, R., Bauer, S., Niino, H., Yabe, A.: Appl. Phys. A 72 (2001) 581. Luby, S., Majkova, E., Jergel, M., Leo, G., Tundo, S., Vasanelli, L., D’Anna, E., Luches, A., Martino, M.: Materials Science and Engineering C 15 (2001) 187. Miwa, M., Juodkazis, S., Kawakami, T., Matsuo, S., Misawa, H.: Appl. Phys. A 73 (2001) 561. Niino, H., Kr¨ uger, J., Kautek, W.: Appl. Phys. A 72 (2001) 53. Obata, K., Sugioka, K., Akane, T., Aoki, N., Toyoda, K., Midorikawa, K.: Appl. Phys. A 73 (2001) 755. Qin, Z.Y., Du, B.Y., Zhang, J., He, T.B., Qin, L., Zhang, Y.S.: Appl. Phys. A 72 (2001) 711. Scott, C.D, Arepalli, S., Nikolaev, P., Smalley, R.E.: Appl. Phys. A 72 (2001) 573. Sicard, E., Boulmer-Leborgne, C., Andreazza-Vignolle, C., Frainais, M.: Appl. Phys. A 73 (2001) 55. Sun, J., Wu, J.D., Ying, Z.F., Shi, W., Zhou, Z.Y., Wang, K.L., Ding, X.M., Li, F.M.: Appl. Phys. A 73 (2001) 91. Svorcik, V., Walachov´ a, K., Heitz, J., Gumpenberger, T., Bac´ akov´ a, L.: J. Mater. Sci. Lett. 20 (2001) 1941. Wysocki, G., Dai. S.T., Brandstetter, T., Heitz, J., B¨ auerle, D.: Appl. Phys. Lett. 79 (2001) 159. An, S.-J., Park, W.I., Yi, G.-C., Cho, S.: Appl. Phys. A 74 (2002) 509. B¨auerle, D., Piglmayer, R., Denk, N., Arnold, N.: Lambda Physik Highlights 60 (2002) 1. Bubb, D.M., Papantonakis, M.R., Toftmann, B., Horwitz, J.S., McGill, R.A., Chrisey, D.B., Haglund, R.F.: J. Appl. Phys. 91 (2002) 9809. Carpene, E., Schaaf, P.: Appl. Phys. Lett. 80 (2002) 891. Denk, R., Piglmayer, K., B¨ auerle, D.: Appl. Phys. A 74 (2002) 825. Ding, X., Kawaguchi, Y., Niino, H., Yabe, A.: Appl. Phys. A 75 (2002) 641.

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03Bae 03Den1 03Den2 03Dye 03Gum 03Hop 03Lip 03Maj 03Nav 03Ser 03Zhi

References for 3.2 Dumitru, G., Romano, V., Weber, H.P., Sentis, M., Marine, W.: Appl. Phys. A 74 (2002) 729. Haefliger, D., Stemmer, A.: Appl. Phys. A 74 (2002) 115. Luk’yanchuk, B. (ed.): Laser Cleaning, New Jersey, London, Singapore, Hong Kong: World Scientific, 2002. Mailis, S., Brown, P.T., Sones, C.L., Zergioti, I., Eason, R.W.: Appl. Phys. A 74 (2002) 135. Markowitsch, W., Stockinger, C., Lang, W., R¨ ossler, R., Pedarnig, J.D., B¨ auerle, D.: Physica C 366 (2002) 277. Pedarnig, J.D., R¨ ossler, R., Delamare, M.P., Lang, W., B¨ auerle, D., K¨ ohler, A., Zandbergen, H.W.: Appl. Phys. Lett. 81 (2002) 2587. Piglmayer, K., Denk, R., B¨ auerle, D.: Appl. Phys. Lett. 80 (2002) 4693. Pikas, D.J., Kirkpatrick, S.M., Tomlin, D.W., Natarajan, L., Tondiglia, V., Bunning, T.J.: Appl. Phys. A 74 (2002) 767. Schaaf, P.: In: Ashby, M.F., Cantor, B., Massalski, T.B. (eds.): Progress in Materials Science, Vol. 47, Oxford: Pergamon Elsevier Science Ltd. (2002) 1. Schaaf, P., Han, M., Lieb, K.-P., Carpene, E.: Appl. Phys. Lett. 80 (2002) 1091. Scharf, T., Krebs, H.U.: Appl. Phys. A 75 (2002) 551. Yu, J.J., Boyd, I.W.: Appl. Phys. A 74 (2002) 143. Zimmer, K., B¨ohme, R., Braun, A., Rauschenbach, B., Bigl, F.: Appl. Phys. A 74 (2002) 453. B¨auerle, D., Denk, R., Pedarnig, J.D., Piglmayer, K., Heitz, J., Schrems, G.: Appl. Phys. A 77 (2003) 203. Denk, R., Piglmayer, K., B¨ auerle, D.: Appl. Phys. A 76 (2003) 1. Denk, R., Piglmayer, K., B¨ auerle, D.: Appl. Phys. A 76 (2003) 549. Dyer, P.E., Maswadi, S.M., Walton, C.D.: Appl. Phys. A 76 (2003) 817. Gumpenberger, T., Heitz, J., B¨ auerle, D., Kahr, H., Graz, I., Romanin, C., Svorcik, V., Leisch, F.: Biomaterials 24 (2003) 5139. Hopp, B., Smausz, T., Kresz, N., Nagy, P.M., Juhasz, A., Ignacz, F., Marton, Z.: Appl. Phys. A 76 (2003) 731. Lippert, T., Dickinson, J.T.: Chem. Rev. 103 (2003) 453. Majkova, E., Luby, S., Senderak, R., Chushkin, Y., Jergel, M., Zergioti, I., Papazoglou, D., Manousaki, A., Fotakis, C.: Appl. Phys. A 76 (2003) 763. Navi, N., Kim, H., Horwitz, J.S., Wu, H.D., Qadri, S.B.: Appl. Phys. A 76 (2003) 841. Serbin, J., Egbert, A., Ostendorf, A., Chichkov, B.N., Houbertz, R., Domann, G., Schulz, J., Cronauer, C., Fr¨ ohlich, L., Popall, M.: Optics Letters 28 (2003) 301. Zhigilei, L.V., Leveugle, E., Garrison, B.J., Yingling, Y.G., Zeifman, M.I.: Chem. Rev. 103 (2003) 321.

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4.1 Communication ¨ hrle, H. Venghaus M. Mo

4.1.1 Introduction Lasers in optical communication systems are needed with very different specifications depending on the specific purpose a laser under consideration is expected to serve. In submarine intercontinental links e.g. bandwidth is an extremely valuable commodity. Hence transmitters (lasers) for such systems should be available for many different channels, should be compatible with narrow channel spacing, and they should enable high modulation rates (10 Gbit/s or even more). Lasers for the access area, on the other hand, should primarily be low cost and enable operation without the need for temperature stabilization, while demands with respect to wavelength accuracy, modulation rate, or output power are much more relaxed. In contrast, particularly high output power and strong linearity is required for lasers used in CATV (Community Antenna Television System) distribution systems, and lasers to be used in METRO-rings or in data communication links are designed with still other specifications. With respect to this situation it is immediately obvious that there is no “standard” laser for optical communication, but a certain number of classes of lasers have been developed for the different application areas. Lasers with emission wavelengths in the 1.3 µm and 1.5 µm range are rather similar with respect to material system and device design. Nevertheless, there are distinct differences between the vast majority of 1.3 µm and 1.5 µm lasers as a consequence of system-design aspects. Actually the fiber most widely deployed in Europe and the US has the dispersion minimum at 1.3 µm and lowest attenuation in the 1.5 µm range. As a consequence the 1.5 µm range is predominantly used for long distance, high-bit-rate trunk lines and (D)WDM ((Dense) Wavelength Division Multiplexing) systems, while data links and moderate-bit-rate links in the access area are commonly associated with the 1.3 µm region. The fiber predominating in Japan is dispersion-shifted with its dispersion minimum around 1.55 µm, which causes particular difficulties with WDM systems due to non-linear interactions, in particular with even, narrow channel spacing. We will focus in the following sections on the one hand on the properties of such lasers, which are commercially available, and in addition we will cover lasers and important R&D results, which have so far been reported in the literature only. This will also include a brief section on laser-based devices for future all-optical networks. GaAs-based Vertical Cavity Surface Emitting Lasers (VCSELs) with emission wavelengths around 850 nm primarily and at 980 nm to a lower extent have found widespread use in shortreach data links using multimode fibers (Gigabit Ethernet), with an initial focus on single-fiber or multi-fiber parallel links and potential for future reconfigurable multi-point networks. However, there are strong efforts to develop GaAs- or InP-based long wavelength VCSELs for transmission in the 1.3 or 1.5 µm wavelength regions using single-mode fibers (9/125 µm core/cladding diameter). Expected spans are 10 or even 40 km (1.5 µm), which would enable the VCSELs to migrate into metro networks considered so far to be a clear telecom domain. This will be just another example of softening the previous distinction between datacom and telecommunication, and as a consequence VCSELs will also be covered in this chapter.

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4.1.2 Heterostructures Most semiconductor lasers emitting in the wavelength range around 1.3 µm and 1.5 µm consist of an active layer surrounded by an optical WaveGuide (WG) and embedded between n- and p-doped InP-material. This heterostructure layout allows to optimize independently both the optical confinement of the optical mode and the electrical confinement of electrons and holes in the active layer. Two different types of heterostructures are commonly distinguished: the Separate-Confinement (SC) structure and the GRaded-INdex Separate-Confinement Heterostructure (GRINSCH). In the SC structure the optical waveguide consists of a material with fixed composition (Fig. 4.1.1a), whereas in the GRINSCH structure the composition of the waveguide is epitaxially continuously changed from the InP-cladding towards the active layer (Fig. 4.1.1b). The latter structure is often used in lasers with thin active layers in order to achieve optimum carrier injection into the active layer. The most widely used active-layer material is Gax In1−x Asy P1−y , grown lattice-matched on InP-substrates. By varying the Ga- and As-fractions the emission wavelength of this material can be adjusted between 0.92 µm and 1.65 µm. Alternatively lattice-matched Gax In1−x−y Aly As covering the wavelength range between 0.83 µm and 1.65 µm can also be used as active layer. A combination of both material systems is possible as well. The schematic band structure of these materials is shown in Fig. 4.1.2. SC - layer E

GRINSCH Ec

active layer

GRINSCH Ec

active layer

hν

Ec active layer

hν

hν

Ev

Ev

a

Ev

b

Fig. 4.1.1. Schematic band diagram of different heterostructure types: (a) Separate-Confinement (SC) heterostructure, (b) Graded-Refractive-INdex Separate-Confinement Heterostructure (GRINSCH).

Energy E

Energy E CB

E FC Eg E FV LH k x,y

HH

kz

dn /dE

ρ

Fig. 4.1.2. Schematic energy-band structure E(k) of a bulk active layer together with the corresponding spectral carrier density dn/dE at transparency condition. Note the parabolic density of states ρ in both conduction and valence band, respectively. Due to the high hole mass the conduction-band Fermi level EFC is shifted to high energies within the conduction band at threshold. HH, LH: heavy hole, light hole valence band.

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Due to the fact that the Heavy Holes (HH) and the Light Holes (LH) are degenerate at k = 0, the optical gain of these bulk materials is polarization-independent, i.e. the gain for TE- and the TM-polarized optical waves is equal. Thus TE- or TM polarization of the laser emission can be obtained depending on the waveguide design chosen. Typical active-layer thicknesses of such bulk lasers range from 100 nm to 250 nm. The respective Density Of carrier States (DOS) ρ in the Conduction and Valence Band (CB, VB) is parabolic (Fig. 4.1.2). Generally, transparency in laser-active material is obtained if the difference of the quasi Fermi levels in the conduction and valence band EFC − EFV is larger than the bandgap energy Eg of the material itself. It is worthwhile to note, that in this case – due to the large hole mass – transparency is only obtained, if the injected carrier density is high enough to shift EFC into the conduction band (Fig. 4.1.2) [99Kap1]. Modern epitaxy techniques such as Metal Organic Vapor Phase Epitaxy (MOVPE) or Molecular Beam Epitaxy (MBE) allow the reproducible growth of epitaxial layers with thicknesses of several atomic layers only. The availability of these new techniques led to a breakthrough in the performance of semiconductor lasers, because they facilitated the realization of so-called Quantum Well (QW) heterostructures. Quantum wells are obtained, if thin layers (wells) are embedded between slightly thicker layers (barriers) of larger bandgap than that of the wells. The quantization of electron- and hole states in the QWs occurs, if their thickness is smaller than the De Broglie wavelength of the electrons and holes in the material due to thermal excitation [93Ebe]. One big advantage of QW structures is the fact, that the emission wavelength can be epitaxially adjusted within a wide range simply by varying the thickness of these QWs. Typical thicknesses of the QWs and the barrier layers range from 4 nm to 10 nm and from 5 nm to 20 nm, respectively. In contrast to bulk material the quantization of the carriers in QW structures leads to a step-like DOS ρ, which is no longer energy-dependent (Fig. 4.1.3, cf. also [86Agr]). In a first approximation, the gain is proportional to the product of the spectral carrier densities dn/dE in the CB and the VB close to the respective band edges, where the optical transition takes place. Consequently, QW structures show higher gain at equal injected carrier densities compared to bulk structures (Figs. 4.1.2 and 4.1.3). The optimized gain together with the much smaller active volume lead to a clear reduction of the threshold currents and also to higher optical output powers [91Kaz]. Energy E

Energy E

CB E FC Eg E FV HH

dn /dE

LH k x,y

kz

ρ

Fig. 4.1.3. Schematic energy-band structure E(k) of unstrained quantum wells together with the corresponding spectral carrier density dn/dE at transparency condition. Due to the quantization effect heavy and light holes are no longer degenerate at k = 0. Note the step-like density of states ρ in both conduction and valence band, respectively. HH, LH: heavy hole, light hole valence band.

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Due to the lower mass of the light holes the quantized level is shifted to lower energies compared to the heavy-hole level (Fig. 4.1.3). Therefore, mainly electron-heavy-hole transitions take place and the laser emission is predominantly TE-polarized. However, it is more advantageous to use strained QWs instead of lattice-matched ones. Strained QWs are realized in such a way that GaInAsP or AlGaInAs layers are grown with a composition corresponding to a lattice constant different from that of InP. If the lattice constant of the epitaxially grown material is larger than the lattice constant of InP compressively strained QWs are obtained, in the opposite case the QWs are tensilely strained relative to the InP crystal. A kind of natural limit to the applicable amount of strain in a QW structure is the critical strain-thickness product which amounts to about 25 nm% on InP substrates. Strained layer stacks with strainthickness products exceeding this limit tend to a relaxation of the built-in strain and a correlated formation of severe crystal defects. To compensate the high accumulated strain in strained MultiQuantum-Well (MQW) structures the barrier layers are often grown with opposite strain compared to that of the QWs. Typical strain values of QWs and barriers range from ±0.3 % to ±2.0 % lattice mismatch. The major physical effect associated with the strain in QWs is a drastic change of the VB structure. In the case of compressive strain the heavy-hole mass in the plane of the QW layer (x, ydirection) becomes lighter and the light-hole mass becomes heavier (Fig. 4.1.4). In addition to that the energy difference between the light- and the heavy-hole band increases. Thus electron-light-hole transitions do not take part in optical transitions and the light emitted by compressively strained QWs is therefore purely TE-polarized [91Thi]. As a consequence of the reduced in-plane heavy-hole mass the corresponding DOS ρ in the VB is also reduced (Fig. 4.1.4). This causes a stronger shift of the quasi Fermi level EFV towards the valence band with increasing carrier density compared to unstrained QWs. Thus (a) the transparency condition EFC − EFV > Eg and at the same time the laser threshold is achieved at lower carrier densities and (b) the differential gain is increased. Beyond that the changed VB structure also decreases – especially in 1.5 µm lasers – the inherent loss processes as there are intervalence band absorption and Auger recombination losses in the QWs [94Sue]. Due to higher gain and reduced internal losses lasers with strained QWs show much higher output power than lasers with Energy E

Energy E

CB E FC Eg E FV HH

dn /dE

LH k x,y

kz

ρ

Fig. 4.1.4. Simplified schematic energy-band structure E(k) of compressively strained quantum wells together with the corresponding spectral carrier density dn/dE at transparency condition. Note the steplike density of states ρ in both conduction and valence band, respectively. Compared to the unstrained case (Fig. 4.1.3) the conduction-band Fermi level EFC is less shifted to high energies because of the smaller hole mass. The compressive strain also causes an increase of the energy difference between the quantized states of heavy and light holes (cf. also Chap. 2 in [99Kap1]).

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unstrained QWs. For these reasons most of the actual commercial lasers have active layers with strained QW structures. In tensilely strained QWs the VB changes in the opposite direction. The in-plane light holes become heavier, the heavy holes become lighter. With increasing tensile strain the light-hole band shifts towards higher energies. From a certain amount of tensile strain, depending on the actual QW-barrier-strain configuration, the light-hole band surpasses the heavy-hole band. Thus in these cases only electron-light-hole transitions take place and the emitted light is then mainly TMpolarized [91Thi]. The principal advantages offered by strained QW structures, as discussed above, hold in the case of lasers with tensile-strained QWs as well. Actual research is not restricted to 1-dimensional quantization of carriers (QWs), but focuses on 2- and 3-dimensional quantization as well. Two-dimensional quantization is achieved, if for instance QWs are structured into stripes with a width smaller than the De Broglie wavelength of the carriers. Such structures are called quantum wires. The highest degree of quantization is obtained if quantum wires are divided into quantum boxes, where the carriers are quantized in 3 dimensions. These 2- and 3-dimensional quantized structures promise improved laser performance compared to QW structures, e.g. lower threshold currents, lower temperature-dependent shift of emission wavelength, smaller linewidth, or particularly wide tuning range of tunable lasers [86Asa, 99New, 00Var, 02Sch, 03Zha].

4.1.3 Material systems Lasers for 1.3 µm and 1.5 µm operation in telecommunication systems are primarily fabricated in the GaInAsP/InP material system. QWs are typically made from GaInAs(P), embedded between quaternary barrier layers. However, GaInAsP/InP-based lasers exhibit moderate high-temperature performance only, and in order to improve the temperature characteristics alternative material systems are developed. For 1.5 µm operation these include AlGaInAs/InP and InAsP/GaInP/InP (for more details cf. Sect. 4.1.6.1). Improved temperature characteristics are also expected from lasers on GaAs substrates using GaInAsN strained QW active layers. This material system has been used for the fabrication of vertical cavity lasers (cf. Sect. 4.1.13), but 1.3 µm edge emitting lasers have also been reported [96Kon, 99Bor, 00Kag, 00Ill, 00Mer]. The actual threshold current densities of corresponding broad-area lasers reveal a material quality significantly inferior so far to GaInAsP/InP [02Rie]. GaInAsN/GaAs-based 1.5 µm edge-emitting lasers have also been fabricated recently [00Fis], but these are even more remote from commercial device quality.

4.1.4 Diode laser structures Commercially available laser structures rely on index guiding and can be divided into the following two groups: Ridge-Waveguide (RW) and Buried-Heterostructure (BH) lasers.

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4.1.4.1 Ridge-waveguide (RW) lasers In Ridge-Waveguide (RW) lasers structured during laser processing. To provide the guiding of the optical mode the p-InP-cladding layers are processed into a ridge (Fig. 4.1.5). Both lateral and transversal single-mode behavior of RW-lasers is obtained by a suitable adjustment of the waveguide composition and layer thicknesses and the width of the ridge.

Fig. 4.1.5. Cross section of Ridge-Waveguide (RW)-type lasers: (a) Reversed Mesa (RM) RW-laser, (b) Vertical Mesa (VM) RW-laser.

Typical widths of the ridges lie between 2 µm and 2.5 µm. The weak lateral optical guiding leads to a highly asymmetric far field of RW-lasers. The lateral and transversal 3dB far-field angles are typically about 25 degrees and 40 degrees, respectively. Lateral electrical confinement is obtained, because only directly below the p-InP-ridge holes are injected into the active layer. Depending on the etching process used for ridge fabrication either Reversed Mesa (RM) (Fig. 4.1.5a) or Vertical Mesa (VM) (Fig. 4.1.5b) RW-laser structures are obtained. In principle due to the wider contact region RM-RW-structures show lower series resistance and improved heat dissipation, if mounted p-side-down on heatsinks [97Aok]. However, if the lasers are mounted p-side-up the differences between the laser performance of the two structures are small. Basically, RW-lasers are low-cost lasers, because they need a single epitaxial step only (FabryPerot (FP) lasers). High reliability of the devices is achieved relatively easily, because the active layer is not affected during the processing of the lasers, i.e. processing-induced damage of the active layer which reduces the lifetime of the devices is avoided. The main disadvantage of RW-lasers is the inherent effect of lateral diffusion of injected carriers in the waveguide and active layer beneath the ridge, which leads to a certain amount of leakage current in these lasers and in that way to increased threshold current values. Typical values for 0.4 mm long RW-FP lasers with cleaved facets are about 10 . . . 30 mA, depending on the active-layer composition.

4.1.4.2 Buried-heterostructure (BH) lasers In Buried-Heterostructure (BH) lasers the waveguide and the active layer are structured into stripes. Then in subsequent epitaxy steps these stripes are embedded in InP material. In contrast to RW-lasers here the buried stripe represents a strong-guiding waveguide. Lateral and transversal single-mode behavior is obtained by suitable adjustment of the width of the buried stripe. Typical widths of laser stripes lie between 1 µm and 2 µm. The 3 dB far-field angles are typically about 30 degrees in both, the lateral and transverse direction. To achieve electrical confinement of the Landolt-B¨ ornstein New Series VIII/1C

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injected carriers in BH-lasers three different approaches are commonly used. They are presented in Sects. 4.1.4.2.1–4.1.4.2.3. Typical threshold currents of 0.4 mm long BH-FP-lasers with cleaved facets are about 5 . . . 10 mA. Compared to RW-lasers BH-lasers show lower threshold currents, lower power consumption and improved temperature behavior. However, due to their more sophisticated processing technology and the structuring of the active-layer region it is more difficult to assure high reliability of these devices. 4.1.4.2.1 Conventional BH-lasers In a first selective epitaxy step both side walls of the stripe are first covered with a p-doped and then an n-doped InP blocking layer (growth of layers directly on the laser stripe is prevented). After this, p-cladding layers are grown (Fig. 4.1.6a). In this configuration the blocking layers represent an inverted pn-junction compared with the laser diode (buried stripe), which inhibits leakage currents. BH-lasers with pn-blocking layers are the most frequently used BH laser type. Beside the described BH-laser Fig. 4.1.6b, c also shows two alternative approaches to realize BH-lasers with pn-blocking layers: the V-grooved BH-laser and the double-channel planar BH-laser [82Mit, 99Fuk].

Fig. 4.1.6. Cross section of Buried-Heterostructure (BH)-type lasers I: (a) standard buried heterostructure, (b) V-grooved buried heterostructure [99Fuk], (c) double-channel planar buried heterostructure [82Mit, 99Fuk].

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4.1.4.2.2 Semi-insulating BH-lasers Using a selective epitaxial step both side walls of the stripe are covered with a semi-insulating InP layer (Fig. 4.1.7a). Since Fe-doped InP exhibits a high sheet resistance for electrons, possible electron leakage currents are usually suppressed by a Fe-doped InP-layer, and an additional ndoped InP layer on top of this inhibits hole leakage currents. In a third and final epitaxial step the p-InP cladding and the contact layer are grown in a non-selective epitaxy process [91Thi]. Actual research activities focus on replacing the Fe-doped InP by differently doped InP layers which block both electrons and holes simultaneously [98Dad].

Fig. 4.1.7. Cross section of Buried-Heterostructure (BH)-type lasers II: (a) semi-insulating buried heterostructure, (b) buried ridge-stripe structure.

Because of their low built-in capacitance BH-lasers with semi-insulating blocking layers are predominantly used for lasers to be directly modulated. 4.1.4.2.3 Buried-Ridge-Stripe (BRS) lasers The stripe is completely embedded in p-InP material (Fig. 4.1.7b). The electrical confinement is obtained by implantation of protons into the p-InP material on both sides of the buried stripe. The implantation causes a strong increase of the respective sheet resistance, which reduces the leakage current. The main advantage of this type of BH-lasers is the fact that only two non-selective epitaxy steps are needed for fabrication [91Kaz].

4.1.5 Laser types 4.1.5.1 Multimode devices (Fabry-Perot (FP) lasers) In Fabry-Perot (FP) lasers cleaved facets act as laser mirrors. The reflectivities of the cleaved facets amount to about 30 %, providing sufficient optical feedback to achieve lasing. Typical laser lengths range from 0.25 mm to 1.5 mm. Landolt-B¨ ornstein New Series VIII/1C

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The longitudinal mode separation ∆λ in FP-lasers with emission wavelength λ, length L, effective refractive index neff and dispersion dneff /dλ of the laser structure, is given by [93Ebe]: ∆λ =

2 · L · neff

λ2  .
· 1 − λ · dneff dλ

(4.1.1)

Typical values for 0.3 mm long 1.3 µm and 1.55 µm RW-FP-lasers are about 0.7 . . . 1.0 nm. Depending on the used active-layer type the width of the gain spectra at threshold lies between 10 nm and 30 nm. Thus no mode selection occurs and FP-lasers always show multimode emission. A typical FP-spectrum above threshold is shown in Fig. 4.1.11, see below. If temperature effects can be neglected (pulsed operation), the optical output power of FP-lasers at the front facet Pfront can approximately be described by: Pfront (I) =

hν ηin · ln(Rfront ) (I − Ith ) , e ln(Rfront Rrear ) − 2αopt L

(4.1.2)

where Ith is the threshold current, hν the photon energy, e the electron charge, αopt the optical loss, R the facet reflectivity and ηin the internal quantum efficiency. In FP-lasers with cleaved facets (Rfront = Rrear ) the same optical power is emitted on both sides: Pfront = Prear . Equation (4.1.2) shows that for a given length L the optical output power is mainly dependent on the threshold current Ith , the internal quantum efficiency ηin and the inherent optical losses αopt . However, in most practical applications only the output power of one of the laser facets is used. For this reason a High-Reflective (HR) coating is frequently applied to the rear facet of FP-lasers: Rrear > 90 %. According to (4.1.2) this leads to a drastic increase of the usable optical power from the front facet Pfront , i.e. the external differential quantum efficiency ηex =

d(Pfront /hν) d(I/e)

increases. In cw-operation a certain amount of self-heating of the laser structure inevitably occurs due to the inherent ohmic series resistance and absorptive loss processes. To reduce this heating effect FP-lasers are commonly mounted on suitable metal heatsinks in order to guarantee sufficient heat dissipation into the heat sink during operation of the lasers. Even so, at high injection currents a so-called thermal roll-off of the P (I)-characteristic occurs, which means that the linear relation Pfront ∝ I does not hold anymore. A similar behavior is found for DFB-lasers (cf. Sect. 4.1.5.2.1). Figure 4.1.13 (see below) illustrates these characteristics. If the environmental temperature T is varied during operation of the lasers, the threshold current change can in many cases and over a reasonable temperature range be approximated by the following relation Ith ∝ exp(T /T0 ) ,

(4.1.3)

where T0 denotes the characteristic temperature of the laser structure. The threshold current increases at higher temperatures due to a decrease of the optical gain. The lower gain necessitates a higher threshold-current density and this in turn raises the internal optical losses, e.g. leakage current, Auger recombination and intervalence band absorption [99Pip]. In addition, the internal optical losses themselves get larger with increasing temperature. The decrease of the optical gain results from the higher thermal activation of carriers in the CB and VB at higher temperatures. The carriers are distributed over a wider energy range within the respective bands (Figs. 4.1.2–4.1.4), and this leads to a reduction of the Fermi level separation EFC − EFV , which in turn reduces the optical gain. Landolt-B¨ ornstein New Series VIII/1C

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The amount of leakage current in an MQW laser structure depends strongly on the built-in CBand VB-discontinuity between the quantized levels in the QWs and the respective barrier energy bands. Low discontinuity values lead to a thermally activated carrier overflow out of the QWs into the waveguide and cladding layers, which raises the threshold current. Because of the much smaller mass of the electrons compared to the holes the T0 -value depends mainly on the value of the CB discontinuity. Auger recombination and intervalence-band absorption losses decrease with decreasing hole mass in the active layer [99Kap1]. Therefore by using active layers with strained QWs a strong decrease of these losses is obtained (see Sect. 4.1.2). Altogether the T0 -value is a complicated function of the following laser parameters: laser type, material system of the active layer, number of QWs, length, facet coating and method of mounting (p-side-down or p-side-up). In order to assure comparability it is recommended that T0 be derived from the increase of Ith if the operation temperature is raised from 20 to 70 ◦ C [90Bel]. Typical T0 -values of 1.3 µm-GaInAsP-MQW-lasers are between 60 K and 90 K. With 1.3 µmAlGaInAs-MQW-lasers and 1.3 µm-InAsP/AlGaInAs-MQW-lasers higher T0 -values between 90 K and 140 K (or even 270 K, see Sect. 4.1.6.1) are achieved, due to the higher CB discontinuity in these structures [99Yam]. The T0 -values of 1.5 µm lasers are somewhat lower due to higher inherent optical loss processes such as Auger recombination and intervalence-band recombination resulting from the less favorable band structure in these lasers [99Kap1].

4.1.5.2 Single-mode devices 4.1.5.2.1 Distributed-feedback (DFB) lasers In contrast to FP-lasers in Distributed-FeedBack (DFB) lasers the optical feedback is accomplished by a built-in refractive-index grating in longitudinal direction and not by the facets of the lasers. In most cases the refractive-index grating is realized by etching periodic trenches into the respective pside waveguide layer. After grating definition the structure is overgrown with the p-cladding layers in an additional epitaxy process. The period of the grating Λ is related to the Bragg wavelength λB of the grating via λB = 2 neff Λ .

(4.1.4)

Due to the implemented grating a wavelength mode selection occurs: The threshold gain of the possible longitudinal optical modes with emission wavelength λ increases with increasing absolute wavelength deviation from the Bragg wavelength |λB − λ|. However, these index-coupled DFB lasers with both facets Anti-Reflection (AR) coated can principally not lase directly at the Bragg wavelength, but show a spectrum consisting of two degenerate modes, which are located symmetrically around the Bragg wavelength. The physical background of this effect is that the periodic refractive-index change in the waveguide direction represents a kind of one-dimensional photonic crystal with a corresponding photonic band structure. The normal photon dispersion relation ν = c/neff λ is modified in the vicinity of the Bragg wavelength λB and the dispersion relation shows a forbidden frequency gap, where the propagation of optical modes is impossible [72Kog]. This forbidden spectral band is called stopband. The width of the stopband increases with increasing coupling strength of the index grating. To achieve single-mode emission two different approaches are possible: 1. λ/4-phase-shifted DFB-lasers with both facets AR-coated: If in the middle of the laser cavity an additional λ/4-phase shift is implemented, i.e. if the two parts of the gratings are shifted relative to each other by Λ/2, single-mode emission exactly at the Bragg wavelength is achieved (Fig. 4.1.8a) [99Kap2]. Landolt-B¨ ornstein New Series VIII/1C

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2. DFB-lasers with AR/HR-coated facets: If one of the facets is AR-coated and the other facet is HR-coated the symmetry in the optical feedback is broken and single-mode emission on one of the two modes on both sides of the stopband can be achieved (Fig. 4.1.8b). The latter approach is mostly used for commercial index-coupled DFB-lasers. The achievable single-mode yield is higher than 50 %. The asymmetric facet reflection raises at the same time the optical output power from the front facet (see Sect. 4.1.5.1). The Side-Mode-Suppression-Ratio (SMSR) values of these lasers are typically beyond 35 dB within the whole usable current range. An alternative way to obtain single-mode emission on one of the two stopband modes is the implementation of an amount of loss or gain coupling in addition to the index coupling. The loss or gain coupling in these so-called complex-coupled DFB lasers is achieved when the grating is etched in an absorptive waveguide layer or into the active layer of the laser, respectively (Fig. 4.1.8c, d). Both effects break the degeneracy of the two stopband modes, which in turn leads to single-mode emission. If the gain or loss periodicity is in phase with the index periodicity, always the stopband mode on the long wavelength side of the Bragg wavelength emits [93Li]. If the gain or loss periodicity is anti-phase compared to the index periodicity, the stopband mode on the short-wavelength side emits. Compared to purely index-coupled DFB lasers complex-coupled DFB lasers are less sensitive to external reflexions of the emitted laser light. Even if the facets are not AR-coated a high yield of single-mode lasers is achieved [93Li]. This feature makes them particularly attractive for system applications where a small amount of back reflexion of the laser light is always present. A typical spectrum of a single-mode DFB-laser is shown in Fig. 4.1.12, see below. It should be noted that because of the different thermal shift of the gain peak λgp (about +0.5 nm/K) and the DFB-wavelength λDFB (about +0.1 nm/K) the threshold-current and outputpower behavior of DFB lasers is a strong function of the spectral position of the DFB wavelength λDFB relative to the gain peak wavelength λgp . Thus the simple temperature dependence of the threshold current given in (4.1.3) does not hold here. In fact there is always a trade-off between low

Fig. 4.1.8. Cross section of Distributed-FeedBack (DFB) lasers: (a) index-coupled DFB-laser with implemented λ/4 phase shift, (b) index-coupled DFB-laser with asymmetric facet coating, (c) complex-(loss)coupled DFB-laser, (d) complex-(gain)-coupled DFB-laser. Landolt-B¨ ornstein New Series VIII/1C

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threshold current and high optical output power on the one side and good temperature behavior on the other side. Within the specified temperature range the emission wavelength of DFB-lasers λDFB can be thermally tuned without any mode jumps (SMSR is not affected) by varying the heatsink temperature. This means that in case of possible operation temperatures between 20 ◦ C and 80 ◦ C the tuning range amounts to about 6 nm. Electrical tuning is only possible in multi-section DFB laser devices by electrical detuning of the different DFB sections relative to each other (see Sect. 4.1.8). The linewidth δν of single-mode DFB-lasers is approximately given by [99Fuk] δν =

vg2 · hν · nsp (1 + α2 )(αem + αopt )αem , P 4π

(4.1.5)

where vg is the group velocity, α the linewidth enhancement factor (see below (4.1.6)), nsp the spontaneous emission factor, αem are the equivalent mirror losses and P is the total optical output power. The spontaneous emission factor nsp is typically between 1 and 2 for DFB lasers. The value of the equivalent mirror losses αem depends mainly on the coupling coefficient κ and length L of the DFB lasers. Increasing κ L-values lead in principle to reduced linewidth values [99Kap2]. The spectral linewidth also decreases inversely with increasing optical output power P . However, if the κ L-values or the optical output power are too high, spatial hole burning occurs which deteriorates the linewidth. Another important parameter determining the linewidth is the linewidth enhancement factor α which is defined as α=−

4π dneff /dn . λ dg/dn

(4.1.6)

Due to their higher differential gain dg/dn and smaller internal optical losses αopt DFB lasers with strained QW active layers show smaller linewidths compared to DFB lasers with bulk active layers (see Sect. 4.1.2). Typical α-values for DFB lasers with strained QW layers lie between 1.5 and 3. However, the linewidth enhancement factor in DFB lasers is a strong function of the spectral position of the DFB wavelength λDFB relative to the peak of the gain spectrum λgp . Due to the higher differential gain on the short-wavelength side of the gain spectrum (smaller α-value) a negative detuning of the DFB wavelength λDFB − λgp leads to the smallest linewidth values [87Ogi]. The emission of DFB lasers can be directly modulated easily by varying the operating current. The modulated optical power P (νm ) depends strongly on the modulation frequency νm and shows a pronounced resonance behavior (Fig. 4.1.14, see below). The resonance frequency νr can be described by [93Ebe]      I c 1 1 1 · · Γ no dg /d n + −1 , (4.1.7) νr = 2π τs neff τ Ith where no is the transparency carrier density, τs the carrier lifetime at threshold, τ the photon lifetime in the laser cavity and Γ the optical confinement factor. With increasing injection current I the resonance νr shifts to higher frequencies. Measures to achieve high νr -values are the use of lasers with high differential gain dg/dn, short cavities and high κ-values. However, the maximum usable modulation frequency is often limited by capacity effects of the laser structures themselves. Thus lasers for high-speed direct modulation have to be specially designed towards small capacity of the laser structure and the contact pads (see Sect. 4.1.4). Because of α > 0 (4.1.6) any intensity modulation of the laser light is accompanied by a modulation of the optical frequency of the emitted radiation. This so-called chirp of the laser frequency is given by [91Pet] Landolt-B¨ ornstein New Series VIII/1C

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367 (4.1.8)

where κp is a measure of the gain saturation at high optical power according to g = go (1 − κp P ). The first term in (4.1.8) describes the transient part of the chirp, which shows a phase shift compared to the optical power modulation P (t) and dominates at high frequencies. The second term results from the gain saturation in laser diodes and describes the adiabatic part of the chirp. 4.1.5.2.2 Distributed-Bragg-reflector (DBR) lasers In contrast to DFB lasers Distributed-Bragg-Reflector (DBR) lasers consist of three or four sections which are operated independently: a gain section comprising the active layer, a phase section and one or two passive grating sections (Fig. 4.1.9). The optical feedback is accomplished by one cleaved facet together with a grating section in case of three-section DBR lasers or by two grating sections in case of four-section DBR lasers. The main advantage of DBR lasers is the possibility of electrical tuning of the lasing wavelength. To this end the three or four different operation currents have to be adjusted in a way that the phase roundtrip and the mode-selection condition for single-mode emission are fulfilled simultaneously (see Sect. 4.1.9). Thus the single-mode operation of DBR lasers is much more complicated than the operation of one-section DFB lasers. Moreover, the long-term stability of the parameters required for electrical wavelength tuning is difficult to obtain. Therefore most commercial single-mode lasers are of DFB type.

Fig. 4.1.9. Cross section of Distributed-Bragg-Reflector (DBR) laser comprising gain section, phase tuning section and one or two grating sections.

4.1.6 Characteristics of 1.3 µm and 1.5 µm lasers The complete characterization of a semiconductor laser requires several 10 different parameters, ranging from epitaxial layer sequences, composition, strain and doping levels over geometrical dimensions to performance data such as threshold current, output power-current-characteristics, slope efficiency or high-frequency behavior, to name only a selection of the more important parameters. Additional data are needed to describe module characteristics or systems performance of a laser, for example. In reality such a comprehensive set of data is never available for any laser (module). Instead, the information provided is normally one out of two frequently found subsets of data: In the case of Landolt-B¨ ornstein New Series VIII/1C

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commercially available lasers device behavior is rather well documented with very little and rather general information on the laser chip itself. On the other hand, in the case of research papers, the technological details of the laser chip are normally well documented supplemented by the characteristics of specific interest in the respective paper, for example particularly low threshold current or chirp, record values for the high-frequency cut-off etc., while parameters of minor interest and/or general systems performance are typically not given. This situation will also be reflected in the following section, where the current status of commercially available lasers and the actual R&D status will be documented. It may be worthwhile to note that many parameters are strongly interdependent and as a consequence they cannot be chosen or optimized independently. Instead, fixing any parameter to a certain value puts restrictions (more or less severe) on many others. In other words, optimizing one specific property normally implies that some other property (-ies) is (are) impaired simultaneously. An illustrative example referring to laser cavity optimization is given by Coldren and Corzine for example [95Col1].

4.1.6.1 Fabry-Perot (FP) lasers Fabry-Perot (FP) lasers are commercially available for digital and analogue operation in the 1.3 µm and 1.5 µm wavelength range as well. Devices with 1.3 µm emission are predominantly fabricated in the GaInAsP/InP or AlGaInAs/InP material systems with strained MQW active layer, and commercial devices may be specified for uncooled or cooled operation. The maximum modulation rate is in some cases limited to 622 Mbit/s but may also be as high as 10 Gbit/s. Typical applications include Telecom access, supervisory channel (cf. also Sect. 4.1.6.4), Telecom trunk (cooled), narrowband video, downstream telephony, and data for example. Typical performance data of low-power ridge RW FP lasers for operation in the 1.3 µm wavelength range are given in Table 4.1.1, which also reflects the parameters which are normally chosen in order to characterize commercially available lasers. Corresponding current-output-power characteristics for different temperatures are shown in Fig. 4.1.10, which also illustrates the increase of the threshold current (cf. (4.1.3)) and a decrease of the slope efficiency. The thermal roll-off of the P -I-curves mentioned in Sect. 4.1.5.1 occurs beyond the current range shown in Fig. 4.1.10. Figure 4.1.11 illustrates the temperature-induced wavelength shift of the emission spectrum. FP lasers are available with output power up to 10 mW, slightly shorter or longer rise/fall times and lower beam divergence than given in Table 4.1.1. Lasers are frequently offered as complete modules with fiber pigtail. Due to unavoidable chipfiber coupling losses the fiber output power and – as a consequence the quantum efficiency – are significantly lower from a pigtailed module compared to the bare laser. This has to be kept in mind, if the performance of different lasers is compared! FP laser modules exhibit typical output powers in the range from 0.1 to 2.5 mW and slope efficiencies ranging from 6 to 250 µW/mA. Lasers operating in local networks should enable high-temperature operation with low threshold current and high output power without the need for thermoelectric coolers. For the conventional GaInAsP/InP material system T0 is essentially restricted to the range 50 . . . 80 K [99Phi], and better high-temperature performance requires alternative material systems, where the conduction-band offset between the wells and the barriers ∆Ec considerably exceeds the value for GaInAsP/InP (∆Ec = 0.40 ∆Eg ; see also Sect. 4.1.5.1). Various improvements have been reported in the past: 1. One approach is a GRINSCH structure with AlInAs-AlGaInAs waveguides, strained AlGaInAs quantum wells and AlGaInAs barriers corresponding to ∆Ec = 0.72 ∆Eg [94Zah]. A charac-

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Table 4.1.1. Characteristics of typical low-power 1.3 µm RW FP lasers (all data refer to the operation temperature Top = 25 ◦ C if not otherwise stated). Symbol

Parameter

Typical value

Unit

Ith

threshold current

8

mA

Iop

operating current

20

mA

Pout

output power

5

mW

η

slope efficiency

0.4 0.25

mW/mA

λ

lasing wavelength

1310 ± 20

nm

Top

operating temperature

−40 to +85

◦

θparallel

beam divergence (parallel)

30

deg.

θperp

beam divergence (perpendicular)

40

deg.

trise , tfall

rise, fall time

0.5

ns

∆λ

spectral bandwidth

1

nm

∆λ(T )

temperature coefficient

0.4 (. . . 0.5)

nm/K

Remarks

at Top = 25 ◦ C at Top = 85 ◦ C

C

10 %/90 % or 90 %/10 %

Fig. 4.1.10. Optical output power vs. forward current for a 1.3 µm low-power MQW FP laser diode.

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Fig. 4.1.11. Temperature dependence of the lasing spectrum of a low-power MQW FP laser diode.

2.

3. 4.

5.

teristic temperature T0 = 105 K has been found up to 85 ◦ C [94Zah, 95Wan] as well as cw operation beyond 170 ◦ C [95Wan]. ∆Ec > 300 meV has been derived for compressively strained InAsP QWs separated by tensilestrained AlGaInAs barriers and InP spacer layers in between [97Ana, 99Yam]. Corresponding MQW ridge waveguide lasers exhibited a characteristic temperature as high as T0 = 143 K in the range between 25 ◦ C and 85 ◦ C. Another reported approach enabling high T0 is the use of compressive InAsP wells embedded between InGaP tensile barriers (on InP substrate), which provided T0 = 117 K [95Oug] for as-cleaved lasers. InAsP/GaInAsP strained MQW lasers are another variety enabling particularly good hightemperature performance [95Ooh]. With compressively strained InAsP wells and straincompensating tensile strain GaInAsP barrier layers cw operation up to 160 ◦ C and a slope efficiency exceeding 0.55 W/A have been observed at 90 ◦ C on BH laser structures. GaInAsN/GaAs QW lasers with 1.2 µm emission wavelength have been operated up to 170 ◦ C (300 µm device length) and exhibited a maximum characteristic temperature of T0 = 270 K (750 µm device length) [00Kag]. However, this particularly high T0 is valid up to 50 ◦ C operation temperature only, and in addition, the actual threshold current is close to 400 mA, i.e. rather high. As a consequence a considerable amount of additional work is still required before commercial applications based on this material system may become attractive.

Many lasers exceeding the performance data compiled above have been reported in the literature during the last decade. Due to the high relevance of high output power and/or high operation temperature of FP lasers in optical communication systems, a corresponding large number of R&D papers focuses on these properties. Record output powers at 1.3 µm are 245 mW from a 0.5 mm long FP laser with 5 %/95 % reflectance [91Bou] and very recently even 825 mW (630 mW) chip (fiber-coupled) output power from dual-channel ridge waveguide MQW GaInAsP/InP Fabry-Perot lasers (while the output power of corresponding DFB-lasers with otherwise identical structure is 450 mW (315 mW)) [01Men]. FP lasers for operation in the 1.5 µm wavelength range are fabricated mostly in the GaInAsP/InP material system and are suited for applications such as low-cost high-speed short-range links or local networks, but also for moderate bit rate (≤ 622 Mbit/s) long haul transmission. Typical Landolt-B¨ ornstein New Series VIII/1C

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output powers are about 6 . . . 8 mW from lasers or 0.25 . . . 1.5 mW from modules with fiber pigtail. Maximum direct modulation rates are specified from 622 Mbit/s up to 10 Gbit/s, and slope efficiencies for lasers are 0.2 . . . 0.25 mW/mA. As the basic design of 1.3 µm and 1.5 µm FP lasers is the same, similar characteristics are observed as well. The highest output power reported for 1.5 µm FP lasers are 325 mW from one facet of a 1 mm long GaInAs/InP MQW laser with asymmetric facet coating (4 %/98 %) [91Thi] and > 300 mW output power at 1 A driving current from a reverse-mesa RW laser (cf. Sect. 4.1.4 and Fig. 4.1.6a) with 1.2 mm long cavity and 3 %/90 % reflective ends [95Aok].

4.1.6.2 Distributed-feedback (DFB) lasers 4.1.6.2.1 General 1.3 µm DFB lasers are commercially available mostly as modules for various applications in telecommunications, data, video and telephony (downstream) applications, but for measuring instruments such as Optical Time Domain Reflectometry (OTDR) as well. They can be directly modulated up to 2.5 Gbit/s or 10 Gbit/s, or, alternatively, they are specified as a DC source which requires external modulation. Fiber-launched output powers are offered in a wide range from about 100 µW to about 13 mW. 2 to 6 mW are typical output powers for telecom applications, while the high-power lasers are designed for long-haul AM CATV systems. The maximum chip output power reported so far for 1310 nm DFB lasers is 450 mW with 315 mW fiber-coupled power (cf. Sect. 4.1.6.1) [01Men]. Typical slope efficiencies are 0.2 . . . 0.35 mW/mA for lasers and about 0.05 . . . 1 mW/mA for modules. Several other parameters are compiled in Table 4.1.2. According to systems considerations (and the characteristics of widely deployed fibers) the development of 1.5 µm DFB lasers has strongly been determined by the expected – and currently strongly rising – demand for (D)WDM sources. The initial demand for 1.5 µm lasers for telecom applications was an operation wavelength fitting into the EDFA (Erbium-Doped Fiber Amplifier) window (1530 . . . 1565 nm, “C-band”). Subsequently, lasers became available for the various ITU (International Telecommunication Union) channels including the extended EDFA-(L-)band, so that now any wavelength between 1525 and 1615 nm (roughly corresponding to 196.6 and 185.6 THz, respectively) is available in 100 GHz (0.8 nm) intervals. Such high wavelength precision requires wavelength locking, i.e. thermal tuning (0.1 nm/K) plus a wavelength reference. The latter may be a dielectric multilayer filter, a fiber Bragg grating or an Arrayed Waveguide Grating (AWG). The wavelength locker may be a unit separate from the laser module, it may be incorporated into the transmitter module micro-optically or by Planar Lightwave Circuit (PLC) technology, and it might eventually be monolithically integrated with the laser. Wavelength locking is also the key issue for 50 GHz or 25 GHz channel spacing, while the laser performance itself is of minor relevance in this respect. Table 4.1.2. Typical data of commercially available 1.3 µm DFB lasers. Symbol

Parameter

Typical value

Unit

λ SMSR trise , tfall RIN ∆λ(T )

lasing wavelength side mode suppression ratio rise, fall time relative intensity noise temperature coefficient

1285 . . . 1330 40 0.15 −155 0.1

nm dB ns dB/Hz nm/K

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A typical output spectrum of a DFB laser is shown in Fig. 4.1.12 (compressively strained GaInAsP MQW BH-laser [93Mor]), and P -I-curves of a GaInAsP-RW-DFB laser with compressively strained QWs and barriers with tensile strain in the temperature range 20 ◦ C ≤ T ≤ 90 ◦ C are shown in Fig. 4.1.13. Thermal roll-off at 90 ◦ C is clearly visible.

Fig. 4.1.12. Typical emission spectrum of a DFB laser (compressive-strain MQW laser, 380 µm device length, 70 %/2 % HR/AR coating) [93Mor].

Fig. 4.1.13. P -I-curves of a GaInAsP-RW-DFB laser (compressive-strain QWs, tensile-strain barriers, 0.4 mm length, both facets AR-coated) for 20 ◦ C ≤ T ≤ 90 ◦ C.

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Output powers of commercially available 1.5 µm DFB lasers are in the range from several mW to about 60 mW. Maximum output powers from the fiber end of modules may be as high as 20 . . . 50 mW. Lasers with up to about 20 mW can be directly modulated up to 2.5 Gbit/s, the higher output devices are intended for DC operation and external modulation. Applications include digital telecom and data transmission, RF (Radio Frequency) on fiber, and hybrid-fibercoax/CATV systems used for supertrunking and distribution. 4.1.6.2.2 Linewidth The laser linewidth depends on design and operation parameters as given in (4.1.5). Its experimental determination can be accomplished by many different techniques (heterodyne using a local oscillator, delayed self-heterodyne, self-homodyne, Michelson-interferometer-based techniques) [99Pao, 98Ban], but it is by no means a straightforward matter. In a round-robin study within the European collaboration project COST 240 a number of lasers have been characterized in different laboratories and in particular the linewidth values obtained exhibited pronounced differences. Normally – although not always –, general trends could be observed, but significant differences of the absolute values of the linewidth were found, ranging from a factor of about three up to more than two orders of magnitude. These differences can be well explained by physical phenomena in combination with the specific experimental technique chosen [99Pao], but it is obvious that linewidth measurements should be made with great care and results given in the literature have to be viewed with appropriate caution. A particularly narrow linewidth is of minor concern for standard digital data transmission, but it is important for analogue and digital video signal distribution via fiber-to-the-home systems using optical coherent detection, or for opto-mm wave systems using heterodyne signals as RF source, for example. Linewidths as narrow as 50 . . . 60 kHz for 25 mW output power [99Ooh] or 80 kHz at 40 mW [94Kan] have been reported, while the linewidth of standard DFB lasers is typically around one or several MHz. 4.1.6.2.3 HF characteristics According to (4.1.7) the high-frequency behavior of a laser depends on design parameters, but on the driving current as well, and exhibits a pronounced resonance behavior. Lasers suited for 10 Gbit/s direct modulation are commercially available, and 3 dB cut-off frequencies up to 28 GHz have been reported [93Lu]. A set of spectra of a laser with 22.5 GHz cut-off frequency is shown in Fig. 4.1.14 [93Mor]. The maximum transmission distance of high-bit-rate data streams along standard single-mode fibers is limited by chromatic dispersion in combination with chirp (cf. (4.1.8)). Calculated maximum transmission lengths for Gaussian pulses propagating along a standard fiber (17 ps/nm/km dispersion) in the 1.5 µm wavelength range are shown in Fig. 4.1.15 [85Koy]. It is obvious that high-bit-rate transmission requires particular measures in order to enable reasonable spans. Beyond 10 Gbit/s dispersion (or even dispersion slope) compensation is mandatory, up to 10 Gbit/s assuring a sufficiently small (or negative) chirp parameter is most rewarding. For directly modulated lasers α is essentially limited to ≥ 1.5, and as a consequence 2.5 Gbit/s direct laser modulation enables maximum transmission distances between 70 and 170 km typically (1200 . . . 3000 ps/nm dispersion). Much better performance is obtained, if cw sources are externally modulated, either by an Electro-Absorption (EA) or a Mach-Zehnder-modulator. These devices will be covered in more detail in Sect. 4.1.8.

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Fig. 4.1.14. High-frequency characteristics of a 1.5 µm laser for different driving currents (same device as Fig. 4.1.12 [93Mor]).

Fig. 4.1.15. Calculated maximum transmission length (1 dB penalty) of Gaussian-shaped pulses along standard dispersion fiber (17 ps/nm/km) as a function of the chirp parameter α for different bit rates [85Koy]. Operation wavelength: 1.5 µm. Ranges of α for Electro-Absorption- (EA), MachZehnder- (MZ) and direct modulation indicated, bit rates in Gbit/s.

4.1.6.3 Lasers for uncooled operation The need to reduce cost, size, and power consumption of transmitter modules has spurred a great interest to develop lasers which are suited for uncooled operation up to about +85 ◦ C and which do not need thermoelectric cooling. One specific focus of such lasers is 10 Gigabit Ethernet for Local Area Network (LAN) and Wide Area Network (WAN) applications, and as a consequence FP- and DFB-lasers enabling uncooled operation in the 1.3 µm region (dispersion minimum of Landolt-B¨ ornstein New Series VIII/1C

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optical fiber) are developed for 10 Gbit/s direct modulation, although devices suited for 2.5 Gbit/s direct modulation are of high interest as well. High-speed, high-temperature direct modulation requires high T0 and high differential gain even at elevated temperatures, and it is frequently argued that strained AlGaInAs MQWs (cf. Sec. 4.1.3) combined with semi-insulating blocking layers are the most promising approach to reach these goals. However, good characteristics have also been achieved using strained InAsP/GaInAsP, InAsP/GaInP, InAsP/AlGaInAs or “standard” GaInAsP MQW structures [02Mel]. Beside InP:Fe blocking layers, pnpn layers adjacent to the active area have been shown to be a good choice to suppress unwanted leakage currents, in particular at higher operation temperature [02Mel]. For short-distance applications uncooled 1.55 µm FP- and DFB-lasers have also been developed. Uncooled directly modulated lasers have reached commercial commodity status in a rather short time due to a strong pull from the particularly promising Gigabit Ethernet transponder market. Lasers with quantum-dot active layers are also candidates for uncooled high-speed direct modulation, however, they are still in a research status so far (cf. for example [03Zha]).

4.1.6.4 Lasers for supervisory channels Supervision of optical systems is normally performed using out-of-band channels, and corresponding wavelengths are 1480 or mostly 1510 nm. The corresponding lasers and laser modules are essentially comparable to ITU channel transmitters. Supervision does not need particularly high bit rates nor high power, and consequently lasers for supervision are typically specified up to 622 Mbit/s and with a few mW output power only.

4.1.7 Fiber-based lasers 4.1.7.1 Fiber lasers DBR and DFB fiber lasers can be fabricated by inducing a Bragg grating by UV-irradiation into a rare-earth-doped fiber [92Zys]. Sufficiently high stable dynamic output power is achieved with moderate Er-doping and about one order of magnitude higher Yb-codoping [97Din]. Fiber lasers are normally pumped with 980 nm radiation, but are also commercially available with 1480 nm pump, their emission wavelengths range from 1520 to 1620 nm. Typical output powers of Erbium-doped fiber lasers are 2 mW (at 80 mW pump power), but considerably higher values are also available. Fiber lasers exhibit rather narrow linewidths (several 10 kHz), high signal-to-noise ratio (60 . . . 70 dB) and low thermal shift of the emission wavelength (0.01 . . . 0.03 nm/K). Fiber laser arrays with 100, 50 or even 25 GHz channel separation and a pump redundancy scheme have been shown recently [99Ibs]. In order to enable wavelength tuning fiber lasers are mounted on a thermoelectric cooler/heater and 100 K temperature variation yields about 3 nm emission wavelength change (0.03 nm/K). In principle, fiber lasers constitute an alternative to semiconductor lasers for (D)WDM systems, but they are not yet deployed to a significant extent. For applications in component testing fiber lasers are available with output powers as large as several W under cw operating conditions and as pulse sources with ns pulse duration, kHz repetition frequency and kW peak output power. High-power cw lasers are also used for Raman amplification, see Sect. 4.1.12.2.

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4.1.7.2 Hybrid Fabry-Perot fiber Bragg grating lasers Hybrid lasers consisting of a Fabry-Perot laser with one facet AR coated and combined with a fiber Bragg grating have been reported recently [98Fan, 98Pao, 01Ler]. The technical challenge is to eliminate reflections within the cavity and to realize an efficient fiber-chip coupling. On the other hand, such hybrid lasers exhibit low temperature dependence (e.g. 11 pm/K [01Has]) and particularly low chirp. 10 . . . 20 MHz/mA chirp for sinusoidal drive have been observed [98Pao, 98Fan], which has to be compared to 250 MHz/mA for a comparable DFB laser having the same structure and the same active layer. Thus these directly modulated hybrid DBR lasers might become competitors to integrated EA-LD structures (cf. Sect. 4.1.8), the more so, since more than 15 GHz small-signal modulation bandwidth and 10 Gbit/s modulation capability have already been shown [98Pao]. Another variety is a hybrid laser with particular emphasis on wavelength agility, where the amplifying medium and the fiber grating are packaged separately in two parts of a low-cost MT connector. By proper choice of the grating emission wavelengths between 1530 and 1560 nm using the same active section have been demonstrated [01Ler], and direct modulation at 2.5 Gbit/s should be possible. Finally, hybrid-fiber Bragg-grating external-cavity lasers have been demonstrated to act as a short-pulse source (see Sect. 4.1.11.1).

4.1.8 Integrated laser-modulator Operating a laser in the cw mode and modulating the light by an external modulator is the preferred choice for ≥ 10 Gbit/s data rate transmission. In addition to stand-alone modulators exhibiting bandwidths beyond 50 GHz based on LiNbO3 and GaAs, GaInAsP/InP modulator chips have been developed recently. They can be copackaged with the laser into the same module. Alternatively, lasers can be monolithically integrated with InP-based modulators. Both solutions are commercially available, but only the latter will be treated in somewhat more detail in the following. The main advantage of the integrated version is the much smaller insertion loss into the modulator due to the integration resulting in higher optical power at the modulator output. Integrable modulators could either be of Mach-Zehnder [99Bis2, 99Bis1] or Electro-Absorption (EA) type. Due to its smaller geometrical dimensions, which can directly be converted into lower cost, the LD (Laser Diode)-EA-modulator integration is the widely preferred solution, although the Mach-Zehnder modulator offers significantly larger spectral bandwidth (several 10 nm compared to  10 nm for EA modulators). High-speed modulators (40 Gbit/s) have initially been developed exclusively for the 1.55 µm region, where transmitters with low (or dedicated) chirp are a prerequisite for taking advantage of the low fiber attenuation. Only very recently EA modulators with 1.3 µm operation wavelength have been presented for very short-reach applications [03Kaw]. The approach is considered favorable, since fiber attenuation is no matter of concern as long as the transmission distance is small, and fiber dispersion is negligible at 1.3 µm anyway. The LD-EA-modulator integration can be accomplished in different ways as outlined in detail by Ramdane et al. [96Ram]. For commercial applications two different approaches are favored: the identical-layer concept [96Ram] and the selective-area-growth concept [98Kud]. In the identical-layer concept the same active layer is used for both the DFB laser and the modulator, which makes fabrication technology simple. However, in this approach the DFB-laser is inevitably positively detuned relative to the gain-peak wavelength, which leads to inferior laser performance such as higher threshold current, lower optical output power and high α-values in Landolt-B¨ ornstein New Series VIII/1C

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the DFB-laser, see (4.1.2). These high α-values do not only raise the linewidths but also increase the sensitivity of the DFB-laser to external reflexions. The selective-area concept allows to obtain active layers with different bandgaps within one epitaxial step. Thus both, the detuning of the DFB-wavelength relative to the gain peak and the detuning relative to the modulator bandgap, can be optimized separately [93Aok]. However, there always remains a trade-off between good laser and good modulator performance. The most versatile integration concept is butt coupling [97Tak], because it allows to optimize independently the integrated lasers and modulators. But it is at the same time the most challenging (and consequently the most expensive) concept. Analogue to the case of lasers the linewidth enhancement factor in EA-modulators can be defined as αEA =

4π dneff /dV · , λ dαabs /dV

(4.1.9)

where dαabs /dV describes the change of the absorption with applied voltage. The resulting chirp is given by (4.1.8) with κp ≡ 0, i.e. EA-modulators show transient chirp behavior only. Beyond that the particularly good chirping properties of integrated LD-EA modulators rely on the possibility to realize very small αEA -values. Depending on the applied bias voltage and the wavelength difference between the integrated DFB-laser and the modulator bandgap λDFB − λmod αEA -values ranging from +1.5 to −1 can be obtained [96Bin, 99Ram]. Spectra of the optical output power and the wavelength chirp from a 10 Gbit/s DFB-EA transmitter with negative αEA are shown in Fig. 4.1.16 [99Sah].

Fig. 4.1.16. Optical power (left scale) and wavelength chirp (right scale) of a 10 Gbit/s DFB-electroabsorption modulator with negative chirp parameter (electrical drive signal is about 1.9 V p-p) [99Sah].

Integrated DFB-EA modulators can be tailored within wide limits according to the customer’s needs, however, there are several trade-offs. One of these is output power versus extinction ratio and chirp, another one is modulation bandwidth versus extinction ratio and output power. Commercially available high-speed LD-EA modulators exhibit the following typical characteristics: peak-to-peak modulation voltages of 2 . . . 2.7 V, about 10 dB dynamic extinction ratio and output power in the 1 mW range. About 1 . . . 2 dB dispersion penalty is observed for 2.5 Gbit/s modules after the data have traveled up to several 100 km over standard fiber (with an average dispersion of 18 ps/nm/km). For 10 Gbit/s modules the same dispersion penalty is observed after Landolt-B¨ ornstein New Series VIII/1C

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distances of 20 . . . 90 km, depending on the specific device design. For a given device dispersion penalties are generally smaller for lower output power. Integrated laser-modulator modules offer higher system benefits, if the emission wavelength can be varied. Thus wavelength-tunable (three-section DBR) lasers are also integrated with EA modulators. So far 10 nm quasi-continuous tuning (enabling up to 16 channels with 50 GHz spacing), 1 mW output power and 2.5 Gbit/s modulation rate are offered commercially, while 10 Gbit/s modules are announced for the near future. A similar device, comprising a two-section wavelength tunable DBR laser (20 channels with 50 GHz spacing), an SOA (Semiconductor Optical Amplifier) and an EA-modulator has already been used for 10 Gbit/s long-distance transmission [01Joh]. Integrated laser-EA modulators with more than 40 GHz modulation bandwidth and 10 dB dynamic extinction ratio have already been demonstrated and commercial products are ready for delivery, however, there is no significant market demand for 40 Gbit/s devices at present. One approach to obtain high-speed modulators is the reduction of device capacitance by limiting the modulator length (to about 100 µm or less) [01Kaw, 01Tak], an alternative concept relies on traveling-wave electrodes enabling more than 50 GHz bandwidth) [01Tak, 01Aka].

4.1.9 Tunable lasers 4.1.9.1 Standard devices Tunable lasers are particularly useful for WDM communication systems. Depending on the tuning range a single (or small number of) tunable laser(s) can serve as back-up for a much larger number of single-channel transmitters, which reduces inventory cost. Tunable lasers can also support wavelength routing in the optical domain and thus represent a key element for future all-optical networks (“IP (Internet Protocol) over WDM”). Other applications might be in network protection. Tunable lasers should be addressable – with as little control effort as possible – to any desired ITU-channel, at present primarily in the EDFA-band(s) (C-band: 1530 . . . 1565 nm, L-band: 1565 . . . 1625 nm), in the long run to any channel from about 1.2 µm to about 1.7 µm. Due to their importance tunable lasers have found strong interest for many years, and a comprehensive treatment of the topic is given by Amann and Buus [98Ama]. Tunability may either be strictly continuous, quasicontinuous (i.e. exhibiting mode hops, but each wavelength within the tuning range is accessable) or discontinuous. The “classical” tunable lasers are 3- or 4-section DBR or DFB lasers. In the case of DBR lasers the refractive index in the Bragg section is normally tuned by carrier injection resulting in an overall tuning range of ∆λ = 5 . . . 10 nm, typically with a certain number of mode hops. A discontinuous tuning range of 17 nm has been shown by Delorme et al. [97Del], and thermal tuning of the Bragg section (while the temperature of the gain section is kept constant) combined with ambipolar biasing has even resulted in > 20 nm discontinuous tuning [91Oeb]. A maximum of about 6 nm continuous wavelength variation is typically achieved with thermal tuning of DFB lasers (e.g. 0.1 nm/K wavelength shift and heat-sink temperatures Ths in the range −10 ◦ C ≤ Ths ≤ +50 ◦ C).

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4.1.9.2 Semiconductor lasers with enhanced tuning range Tuning ranges in considerable excess of 10 nm require more sophisticated designs, and many different varieties have been developed in the last decade. Transverse integration schemes relying on codirectional coupling enable continuous tuning with a single tuning section (in contrast to moresection DBR lasers for example). The Tunable-Twin-Guide (TTG) laser falls into this category, and 13 nm continuous tuning has been shown for BH TTG DFB lasers [98Ama]. A shortcoming of the TTG concept is the inherent impossibility to independently optimize the threshold current and the tuning range. About 50 nm tuning range has been demonstrated for the Amplifier-Coupler-Absorber (ACA) laser, the design of which is based on the TTG concept [91Ill]. Another codirectional-type tunable laser is the Vertical Coupler Filter (VCF) (∆λ > 70 nm, [94Kim]), which needs a single current for wavelength control, but suffers from limited SMSR. Tunable lasers based on interferometric effects represent another class of widely tunable lasers, comprising the Y-laser [93Hil], the Y3-laser as a closely related concept [93Kuz] and the VerticalMach-Zehnder-type (VMZ-type) structure [94Bor]. Particularly large tuning ranges can be achieved by the use of Super-Structure Gratings (SSG), including Sampled-Grating (SG) lasers as a special case [93Jay]. One corresponding example is the Grating-Coupler-Sampled-Reflector (GCSR) laser shown in Fig. 4.1.17, which is a combination of a codirectional (vertical) coupler and an SG or SSG. The codirectional coupler provides coarse tuning, the Bragg section assures intermediate-accuracy wavelength selection, and fine tuning – assuring high SMSR – is accomplished by the phase section current. In essence, a GCSR laser can be considered being equivalent to several interleaving tunable DBR lasers. 67 nm continuous [96Rig] and 114 nm discontinuous tuning [95Rig] have been reported so far. The practical deployment of GCSR lasers requires a microprocessor for wavelength control. As the full characterization in 4-dimensional current space is impossible (too expensive), a combination of measurements and smart algorithms is the preferred choice. Another variety of a widely tunable laser is the four-section super-structure grating DBR laser [96Ish]. In addition to a gain and a phase section the SSG-DBR laser has a front and a rear SSG section with different sampling periods. The two reflectors exhibit reflection peaks with different wavelength separation and the lasing mode is the one where two reflection peaks coincide. Wavelength tuning is accomplished by a kind of Vernier effect. A rather similar device is a widely-tunable Sampled-Grating Distributed Bragg Reflector (SGDBR) laser comprising a sampled-grating front mirror, a gain and a phase section, and a back mirror with a sampling period different from that of the front mirror. An additional monolithicallyintegrated amplifier section enhances the output power and can also assure beam blanking during wavelength switching and can be operated as a variable optical attenuator as well [03Col]. Another concept to achieve wide wavelength tuning (1531 . . . 1564 nm, 10 mW output power) has been based on a multi-wavelength DFB laser array (comprising 12 lasers with 10 µm pitch) and a Micro-ElectroMechanical (MEM) mirror that selects one element of the array at a time.

Fig. 4.1.17. Schematic cross section of a GratingCoupler-Sampled-Reflector (GCSR) laser [96Rig].

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Selection of the appropriate laser provides coarse wavelength tuning, while fine tuning is achieved by adjusting the chip temperature [02Pez].

4.1.9.3 Commercial tunable single-chip (SC) lasers Commercially available are/have been the following tunable Single-Chip (SC) lasers: – Tunable DBR lasers, tuning range > 6 nm, output power about 1 mW. – Tunable DFB lasers, thermal tuning range about 5 nm. – Tunable SSG-DBR or GCSR lasers, tuning range around 30 nm (1530 . . . 1560 nm), about 2 mW chip output and 0.2 . . . 0.5 mW module output power. – Widely tunable DFB-lasers, three grating sections on each of two waveguides and their outputs combined on-chip by a 3-dB coupler. Only one DFB-section is operated and thermally tuned at a time (thus rather simple tuning), Bragg wavelengths are chosen such that laser emission is not absorbed in unexcited sections traversed. 33.6 nm total tuning range and 1 mW fiber launched power have been specified [99Cla]. – Four section SGDBR laser plus monolithically-integrated amplifier section, ≥ 10 mW output power, ≤ 5 MHz linewidth, full C-band (1528 . . . 1563 nm) or full L-band (1568 . . . 1608 nm) tuning range [03Col]. Commercial availability of specific widely-tunable lasers has been discontinued repeatedly in the past due to insufficient market demand, and this may occur in the future as well.

4.1.9.4 Linewidth of widely tunable lasers Tuning by current injection impairs the linewidth of all tunable lasers [90Ama, 98Ama]. In the case of a single tuning current the induced linewidth broadening is given by c2 ∆νt = 4π e 4 λ



dλ dIt

2 It ,

(4.1.10)

with It being the current in the tuning section, while all other symbols have their usual meaning. A similar expression holds for the case of more than one tuning current [98Ama], for example for DBR lasers with tuning currents in the phase and in the Bragg section. Due to current-induced tuning linewidths of a few MHz at zero tuning may increase up to several 10 MHz under large tuning-current injection. The linewidth degradation can be circumvented by thermal tuning using separate heaters on each electrode and by this approach 400 kHz linewidth over 40 nm tuning range have been achieved [95Ish].

4.1.9.5 Wavelength tunable/selectable fiber laser As already mentioned in Sect. 4.1.7 fiber lasers are alternative choices to semiconductor lasers. A wavelength selectable fiber laser with large tuning range has recently been presented [99Hab]. It is an Erbium-Doped Fiber (EDF) ring laser with a Fiber Fabry-Perot Interferometer (FFPI) and a Fiber Fabry-Perot Tunable Filter (FFP-TF). The high-finesse FFPI defines a set of uniformly spaced resonances with narrow linewidth assuring single-mode lasing, and the FFP-TF selects any of these resonances, i.e. lasing wavelengths. 50 GHz channel separation, 7 mW output power, Landolt-B¨ ornstein New Series VIII/1C

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±0.3 GHz accuracy and ±0.05 GHz stability over 47 nm tuning range, and 45 dB SMSR have been demonstrated.

4.1.9.6 External-cavity laser External-Cavity Lasers (ECL) are widely used in the field of optical communication. They are offered essentially for measurement and characterization purposes, but include (D)WDM telecom applications as well [02Ant]. The gain medium of an ECL is a conventional laser diode, in which the laser resonator is disabled by antireflection coating of one of the laser facets. The resonator is rebuilt by adding a diffracting grating as a wavelength-selective external reflector. The resonator can accommodate a large number of lasing wavelengths, and by proper setting of the grating angle combined with a fine-tuning of the cavity length one mode can be selected [95Dua]. ECLs operating in the 1.3 µm region cover the wavelength range from 1.25 µm to 1.36 µm. Maximum output powers are > 3 dBm in the central wavelength region and at least −3 dBm all over the specified wavelength range. Absolute wavelength accuracy is typically ±0.1 nm, laser linewidth is 100 kHz typically (holds for 1.3 and 1.5 µm as well). ECLs operating in the 1.5 µm region are offered with individual tuning ranges from 50 nm to 140 nm and in total they actually cover the wavelength range from 1460 to 1640 nm. Maximum output powers are 4 . . . 8 dBm, wavelength accuracy is typically about ±0.01 nm.

4.1.10 Monolithic integrations 4.1.10.1 Integrated spot size converter The assembly and packaging of standard lasers account for the largest fraction of the total module cost due to a strong mismatch of the laser and optical-fiber-guided modes and due to required submicron alignment tolerances. The coupling efficiency can be significantly improved and the alignment tolerances can be relaxed to about 2 µm simultaneously by integrating a spot size converter (frequently designated as “taper”) to the laser diode. The underlying principle is an adiabatic (i.e. lossless) mode transformation, which is achieved essentially by continuously reducing the size of the core WG (WaveGuide). Various technological concepts have been demonstrated in the past few years, both, for BH and for RW lasers. In the case of BH lasers the following approaches are the most relevant: The active stripe may be narrowed in the horizontal direction only, which induces a horizontal and vertical beam expansion already. Current state of the art is a minimum WG width of 0.5 µm, and passive alignment of such tapered 1.5 µm lasers with more than 50 % coupling efficiency has been reported [94Lea, 95Col2]. The narrowing of the laser stripe may occur over a limited length section only, or alternatively the laser stripe may be designed with a narrowing width along the total cavity length [02Moe], which makes the fabrication particularly simple, as the integrated taper does not require any additional processing effort. Alternatively, one can also realize a vertical reduction of the active WG thickness. This additional vertical tapering can most simply be accomplished by narrow-stripe selective MOVPE. In the case of simultaneously grown active and tapered regions residual coupling losses of −2.8 dB have been achieved for a 1.3 µm laser [98Fur], while coupling losses of −1.06 dB only have been obtained for the case of a separately optimized taper butt coupled to a 1.3 µm laser [95Toh]. Alignment tolerances (1 dB additional loss limit) are typically about ±2 µm for these tapers. Landolt-B¨ ornstein New Series VIII/1C

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For RW lasers the narrowing of the active layer is not sufficient in order to obtain a good spotsize converter, thus one has to introduce so-called guiding layers as well. These layers are grown about 1 µm below the active layer of the LD and assure guiding of the light wave in the taper region, after the beam has been strongly expanded. The narrowing of the active layer is either made in the horizontal direction only or horizontally and vertically (as for BH lasers). Vertical WG tapering has been made using shadow masks [94Wen] or semitransparent masks [96Wen] and yielded coupling losses of about 2 dB. On the other hand, 3 dB coupling loss has been reported for a horizontally tapered ridge waveguide [98Bis]. The latter concept is considered most promising for practical applications as it constitutes a good compromise between process simplicity and coupling efficiency plus alignment tolerance.

4.1.10.2 Integrated multi-wavelength sources The monolithic integration of several lasers with different emission wavelength and well defined channel separation can be accomplished in different ways. In the most simple fashion lasers with well defined channel spacing are fabricated next to each other and the emitted radiation of each laser is fed into a separate fiber. In more advanced schemes the emitted radiation is combined on chip so that a single fiber has to be coupled to the multi-channel emitter. The combination of the laser-emitted power can be accomplished in basically two different ways: using 3 dB- or star couplers, or alternatively, using wavelength-selective combiners, which may significantly reduce combiner losses, in particular for high numbers of channels. An example of the former is a multi-wavelength DFB laser array with 20 lasers, an integrated star coupler and integrated optical amplifiers for compensating the high combiner losses [97Zah]. Electrical crosstalk from neighboring channels at 2.5 Gbit/s has been found negligible, however, crosstalk in the optical amplifiers due to four-wave mixing and cross-gain modulation is a matter of concern. A total wavelength coverage of 46.9 nm has been reported for a monolithic structure comprising 16 DFB lasers, an MMI coupler plus an SOA [01Ooh]. The emission wavelength difference for adjacent lasers is 3 nm and 30 ◦ C temperature tuning assures the selection of any wavelength within the 46.9 nm interval. In a similar concept four DFB-lasers, an MMI combiner and an SOA are integrated monolithically on a single chip, and five different chip varieties with contiguous wavelengths are fabricated on a single wafer [01Kud]. 25 ◦ C temperature variation provides 8 nm emission-wavelength tuning for a single chip, and the full set of five chips provides essentially full coverage of the C-band (1530 . . . 1562 nm). Along the same lines full coverage of the S-, C-, and L-bands (1477 . . . 1508 nm, 1525 . . . 1559 nm, and 1566 . . . 1606 nm, respectively) has been obtained using six wavelengthselectable laser chips fabricated from two different wafers [02Hat]. Substantially reduced combiner losses are observed in multi-channel emitters with an Arrayed Waveguide Grating (AWG) in their cavity and gain-providing sections at the end of each AWG arm [98Mon, 99Joy] as shown in Fig. 4.1.18. The approach is particularly appealing as AWGs (also designated as PHASARs) determine channel spacings very accurately [96Smi]. Characteristics such as 100, 200 and 400 GHz channel separation, > 50 dB SMSR, < 1 MHz linewidth, and > 1 mW single-channel output have already been achieved. Direct modulation of this multi-channel laser is currently limited to < 1 Gbit/s due to its long cavity [98Mon]. In the case of 16 channels modulated simultaneously at 622 Mbit/s electrical crosstalk induced a 3 dB penalty compared to the case of single-channel modulation [97Mon]. Another design combines AWGs and Semiconductor Optical Amplifiers (SOAs) to form an integrated multi-wavelength ring laser [02Bes].

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Fig. 4.1.18. Schematic drawing of an integrated multi-frequency laser [97Mon].

4.1.10.3 Integrated mm-wave source The integration of lasers is not only a particularly rewarding means to fabricate multi-channel sources, but integration can also provide other functionalities. A photodiode can detect the beat frequency, if two lasers are monolithically integrated, the output from these two lasers with slightly different emission wavelengths is combined on-chip, and the combined signal is fed into a photodiode. The beat frequency can be adjusted to the mm-wave regime, and thus a chip as shown in Fig. 4.1.19 represents an optical mm-wave source [95Tro]. Adjustment of the mm-wave frequency can easily be accomplished by thermal tuning of the frequency difference. Applications are the optical generation of mm-waves and their distribution to remote base stations along optical fibers in mobile communication systems.

Fig. 4.1.19. Integrated optical mm-wave source [95Tro].

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[Ref. p. 395

4.1.10.4 Transceiver The deployment of optical-fiber links to residential customers (fiber-to-the-home) or data communication links require cheap transmitters-receivers (“transceivers”). At present such transceivers are in most cases assembled as Planar Lightwave Circuits (PLCs) and comprise single laser chips specified according to the particular application. However, the demand for such transceivers is expected to rise steeply in the years to come, and as a consequence monolithic-integrated transceivers are being developed (based on the assumption that monolithic OptoElectronic ICs (OEICs) will in the end provide the most cost-efficient solution). Monolithic transceivers have already been fabricated, the standard configuration for telecom application is a 1.3 (1.55) µm laser and a 1.55 (1.3) µm detector for upstream (downstream) communication [94Wil, 98Ham, 98Nak]. Most challenging is to assure a sufficiently high degree of crosstalk isolation, which is a prerequisite to achieve −28 . . . −32 dBm receiver sensitivity as required for different access network architectures and simultaneous bidirectional communication, while the demands for datacom applications are somewhat more relaxed.

4.1.11 Lasers for advanced optical systems 4.1.11.1 Short-pulse sources Very high-speed optical data transmission (single-channel rate 40 Gbit/s or even higher) requires very short transform-limited, chirpless picosecond pulses of < 5 ps duration and repetition rates in the range from 5 to 20 GHz. In addition, the repetition rate must be controllable in order to enable signal synchronization and, moreover, the signal wavelength should be tunable within reasonable ranges. Optical pulses meeting these demands can be generated by different techniques. The most important are: gain switching of DFB laser diodes, mode locking of LDs, harmonic mode locking of EDF lasers, and supercontinuum generation. More details to each of these techniques, their potential and limitations, are given by Saruwatari [01Sar2]. Examples of active mode-locking include: synchronous mode-locking of a monolithic DBR laser delivering a 33 GHz optical output pulse train from a 1.65 GHz (20th subharmonic) optical input [97Nir] and a monolithic active/passive mode-locked DBR laser for 4 × 10 Gbit/s OTDM applications at 1.55 µm, which delivered nearly time-bandwidth-limited (< 0.4) and wavelength-tunable (∆λ = 4.3 nm) optical RZ pulses of down to 8 ps pulse width [97Zie]. Short pulses have also been created by active mode locking of a hybrid fiber Bragg grating external cavity laser. 9.63 mm cavity length assures 9.953 Gbit/s (STM-64 or OC-192) bit rate, pulse length was 3.8 ps [01Due].

4.1.11.2 Self-pulsating lasers Multi-section DFB lasers can be designed in such a way that they exhibit self-pulsations although the lasers are dc-driven. Self-pulsations in two-section DFB lasers due to dispersive self-Q-switching have been reported for the first time by M¨ ohrle et al. [92Moe]. These lasers have been improved and investigated in system experiments since then in great detail [01Sar1]. The most relevant application of such self-pulsating lasers is for all-optical high-speed clock recovery. This application relies on the property of the lasers to lock to an external optical signal. The locking range is about 50 . . . 100 MHz once the self-pulsation frequency has been adjusted roughly to the clock frequency, Landolt-B¨ ornstein New Series VIII/1C

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Fig. 4.1.20. Frequency spectrum of a selfpulsating laser under free-running conditions and in the locked mode [01Moe].

and locking can be obtained with polarization and wavelength independence over the complete wavelength range of the laser gain spectrum. Figure 4.1.20 shows the frequency spectrum of a self-pulsating laser under free-running conditions and in the locked state [01Moe]. Self-pulsations on a chip level up to 80 GHz have been observed, and 10 Gbit/s optical 3R-regeneration [99Lav] and error-free demultiplexing from 40 to 10 Gbit/s have also been reported [98Bux].

4.1.12 Pump lasers for optical amplification 4.1.12.1 Pump lasers for erbium-doped fiber amplifiers (EDFAs) Optical communication systems have been revolutionized by the invention and deployment of the Erbium-Doped Fiber Amplifier (EDFA) about a decade ago. Since then signal attenuation ceased being a matter of concern, initially in the 1530 . . . 1565 nm range, today from about 1525 nm beyond 1600 nm. EDFAs are pumped by lasers with 980 or 1480 nm emission wavelength, and one parameter of prime importance is output power, which should be as high as possible. Initially only 1480 nm pump lasers were available, fabricated in the GaInAsP/InP material system with strained layer MQW active layers. Actual maximum fiber-launched output powers of commercially available 1480 nm-laser modules are about 200 . . . 300 mW, although chip output powers close to 1 W and about 30 % less fiber-coupled power have already been reported [01Gar]. In addition to pumping EDFAs, 1480 nm modules are also used for Raman amplification (see Sect. 4.1.12.2). The first 980 nm pump lasers were introduced in a fiber link in the US in 1993, and since then the importance of 1480 nm pump lasers has dropped considerably, which is mainly due to the fact that 980 nm pumping provides about 1 dB lower amplification noise. 980 nm pump lasers are based on GaAs/AlGaAs and GaInAs/AlGaAs heterostructures. Output powers from different manufacturers are in the range from 200 mW to 300 mW. As pump lasers operate at extremely high output powers reliability is a key issue, since the sudden failure of a pump laser would have extremely detrimental consequences for network operators. Due to corresponding efforts reliability of pump lasers has made tremendous progress in recent years. Landolt-B¨ ornstein New Series VIII/1C

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[Ref. p. 395

4.1.12.2 Pump lasers for Raman amplification Raman amplification in optical fibers occurs when higher-energy pump photons scatter off the vibrational modes of the glass matrix (optical phonons) and coherently add to lower-energy signal photons. In silica fibers the Raman gain has a peak at about 100 nm longer wavelengths than the pump. The bandwidth is about 50 nm (full-width at half-maximum) and can be increased to > 100 nm by multi-wavelength pumping. As Raman amplification is a non-resonant process and thus Raman gain can be obtained at any desired wavelength by using appropriate pump wavelengths, Raman amplification is of particular interest for wavelength regions outside the EDFA C- and L-bands (i.e. outside 1530 . . . 1625 nm). The Raman gain efficiency is orders of magnitude lower than that of EDFAs, and thus Raman amplification occurs over many km of fiber and requires pump powers of several 100 mW or even in the Watt range. A common configuration of fiber Raman lasers comprises semiconductor lasers with 915 nm or 950 . . . 980 nm emission wavelength, which pump a Yb3+ double-clad fiber laser (emission wavelength around 1.1 µm), which in turn pumps the Raman resonator delivering the longwavelength optical output. Commercially available fiber Raman lasers have emission wavelengths in the 1.3 . . . 1.5 µm range and output powers of 0.5 . . . 2 W. Raman lasers emitting around 1.2 µm have also been demonstrated. Raman scattering is highly polarization sensitive and thus Raman pumps have to be unpolarized in order to avoid polarization-dependent amplification. High-power semiconductor laser modules, commercially available with up to 300 mW output power and emission wavelengths in the 1420 . . . 1500 nm range, constitute an alternative to fiber Raman lasers and can be directly used for distributed Raman amplification. Highest reported output powers of such lasers are beyond 1 W chip output (about 30 % less fiber-coupled power) at about 1400 nm emission wavelength [01Gar], while the maximum reported output power at 1550 nm is 440/340 mW (chip/module) [01Men].

4.1.13 Vertical-cavity surface-emitting lasers (VCSELs) Vertical-Cavity Surface-Emitting Lasers (VCSELs, also designated as VC lasers) represent an alternative to standard edge-emitting lasers for many applications. The concept of VCSELs had been proposed and demonstrated already in the late 1970’s [79Sod], however, a major breakthrough did not occur for ten more years until the first monolithic device was finally presented [89Jew]. Since their emergence as commercial products in 1996 VCSELs have made tremendous progress and are widely used, primarily in short-reach datacom links. In contrast to edge-emitting lasers VC lasers have resonator mirrors above and below the junction layer and thus light oscillates in a cavity vertical to the pn junction (hence their designation as “vertical-cavity lasers”). For a recent review of the topic see e.g. [03Li]. A generic view of a VC laser is given in Fig. 4.1.21. The mirrors above and below the junction are most favorably formed as Distributed Bragg Reflectors (DBRs) by depositing a series of quarterwavelength thick layers of semiconductor material with two alternating compositions that differ in refractive index. Due to their geometry VC lasers have low gain per single pass through the cavity and thus the reflectivity of the mirrors has to be high. Normally, one mirror is almost totally reflecting (> 99.5 . . . 99.9 %), while the other typically transmits about 0.5 . . . 1 % of the light striking the mirror. An additional element of VC lasers are spacers (typically on either side of the junction), which control the cavity length and thus determine the output wavelength, coinciding with the wavelength of highest DBR reflectivity. As a consequence of the short-cavity-length VCSEL light emission is longitudinally single-mode, whereas transverse single-mode emission can be assured by a sufficiently small active area. ShortLandolt-B¨ ornstein New Series VIII/1C

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light emission

}

top DBR

}

active bottom DBR

substrate Fig. 4.1.21. Generic view of a VC laser.

wavelength VCSELs used in multimode fiber-based datacom applications usually emit in several transverse modes. On the other hand, single-mode behavior is a prerequisite for long-wavelength VCSELs to be employed in single-mode optical fiber networks. The fabrication process of VC lasers is more complex than that of edge-emitting lasers. However, the latter are incomplete until the wafer is cleaved into bars, and hence device testing is tedious and expensive. On the other hand, VCSELs can be completed and tested on wafer, which compensates the higher fabrication effort. In addition, VCSELs easily lend themselves to the fabrication of one- and two-dimensional arrays being attractive for short-distance space division multiplexing transmission schemes. Besides, DBR mirror reflectance and active-layer gain, electrical and thermal conductance are other critical parameters, as they determine how efficiently carriers can be injected into the pnjunction region and how efficiently heat can be dissipated. The first monolithic VCSELs have been made in the GaAs/AlGaAs material system, since GaAs and AlGaAs are lattice-matched and moreover have a high refractive-index contrast, which enables low-loss DBR mirrors with a moderate number of layers. The operation wavelengths of such VCSELs are in the 780 . . . 860 nm range and around 980 nm as well. These short-wavelength VCSELs are commercially available abundantly at present, and they have widely and successfully been used in high-speed multimode fiber Local-Area Networks (LANs), e.g. supporting GigabitEthernet or Fiber-Channel protocols. However, the desire to penetrate into the application areas of Fabry-Perot- and DFB-type lasers, serving longer transmission distances in LANs or MetropolitanArea Networks (MANs), as well as the general increase of bit rates (as in the current transition from Gigabit Ethernet to 10-Gigabit Ethernet) drive the development of long-wavelength 1.3 µm or even 1.55 µm VCSELs, and commercial devices have already become available. Beyond applications in Gigabit Ethernet, Fiber Channel and (uncooled) CWDM transceiver modules for example longwavelength VCSELs are also offered at specific wavelengths in the 1400 . . . 2050 nm range for gas sensing applications.

4.1.13.1 Short-wavelength VCSELs Short-wavelength VCSELs are commercially available for 1.25, 2.5, 3.125 and 10 Gbit/s modulation rates, and they come as single devices, in 1D arrays from 1 × 2 to 1 × 12 and in 2D arrays with, e.g., Landolt-B¨ ornstein New Series VIII/1C

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[Ref. p. 395

8 × 8 elements. In accordance with the definition of the 770 . . . 860 nm wavelength regime in the Gigabit Ethernet standard, the most common output wavelength is around 850 nm. Red-emitting as well as 980 nm wavelength VCSELs are available as custom devices. In addition, VCSELs with emission wavelength differences of about 10 . . . 25 nm have successfully been operated in coarse WDM transmission experiments, where the rather small temperature coefficient (0.06 nm/K) is favorable for systems not requiring temperature control. Characteristic operation parameters of 850 nm VCSELs are compiled in Table 4.1.3. The geometrical size of the light-emitting area is typically of the order of 10 µm in diameter. Table 4.1.3. Typical characteristics of commercially available short-wavelength VCSELs. Parameter

Single-mode operation

Multimode operation

threshold current [mA]

0.5 . . . 1.5

1...3

operating current [mA]

1...5

2 . . . 10

operating voltage [V]

1.7 . . . 2.5

1.7 . . . 2.3

series resistance [Ω]

70 . . . 300

30 . . . 70

peak emission wavelength [nm]

780 . . . 860, 940 . . . 980

780 . . . 860, 940 . . . 980

spectral width [nm]

0.5 . . . 2

output power [mW]

0.5 . . . 2

0.5 . . . 3

slope efficiency [mW/mA]

0.2 . . . 0.6

0.2 . . . 0.6

beam divergence [FWHM, deg.]

10 . . . 12

12 . . . 16

temperature tuning coefficient [nm/K]

0.06

0.06

current tuning coefficient [nm/mA]

0.15 . . . 0.3

0.15 . . . 0.3

rise/fall time [ps] modulation speed [Gbit/s]

≤ 10

25 . . . 150 1.25, 2.5, 3.125, 10

relative intensity noise [dB/Hz]

−140 . . . −120

−140 . . . −120

4.1.13.2 Long-wavelength (1.3 µm, 1.55 µm) VCSELs Similar to the case of edge-emitting lasers, ternary and quaternary InP-based compounds can be considered the most obvious materials choice for 1.3 µm and 1.55 µm VCSELs. However, the development of corresponding VCSELs has proven much more difficult than that of shortwavelength ones, primarily for the following reasons: Ternary and quaternary InP-based compounds exhibit 1. considerably smaller refractive index contrast than GaAs/AlAs, 2. more than one order of magnitude lower thermal conductivity and 3. a stronger temperature dependence of the optical gain. Landolt-B¨ ornstein New Series VIII/1C

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The low refractive-index contrast leads to thick DBR mirror stacks (usually more than 40 layer pairs). Such a large total mirror thickness makes the inadequate heat dissipation even more of a problem, and this in turn aggravates the gain reduction with increasing temperature. Various approaches have been tried to overcome these limiting conditions. The concepts include: wafer fusion, metamorphic layer growth, optical pumping, and current injection by tunnel junctions. Another important technological process beside the fabrication of the vertical layer stack is the lateral definition of the VCSEL. In the short-wavelength GaAs/AlGaAs material system a well established approach to confining the pump current to the desired cross section is the selective conversion (steam oxidation) of AlGaAs into Al2 O3 (or, more general, Alx Oy ) [00Dep, 00Cho]. This technique is not applicable to InP-based compounds and it provides inadequate results for AlAsSb lattice-matched to InP [97Leg]. Consequently, alternative solutions have to be developed for those long-wavelength VCSELs, which do not use GaAs/Al(Ga)As mirrors (i.e. most of the 1.55 µm VCSELs). The majority of the long-wavelength VCSELs are top-emitting devices, but emission via the substrate is encountered for 1.3 µm and 1.55 µm VCSELs as well. 4.1.13.2.1 1.3 µm VCSELs 1.3 µm VCSELs are generally based on GaAs substrates and lattice-matched bottom GaAs/ Al(Ga)As DBR mirrors. The optical cavities, however, including the active layers, and top DBR mirrors have been fabricated in a considerable number of different material systems. The various approaches chosen represent different attempts to combat the various impediments such as low electric and thermal conductivity, unwanted free-carrier absorption, thick DBR layer stacks, low optical gain, and undue gain reduction at elevated temperatures. Table 4.1.4 compiles the most relevant solutions. Wafer fusion offers the highest design flexibility, however, it is the most complex technological approach, and as a consequence interest in this approach has declined most recently. Wafer-fused devices reported so far comprise electrically pumped devices [98Bla, 97Qia1, 97Qia2], as well as a 1300 nm VCSEL, which is optically pumped by an adjacent 850 nm VCSEL [00Jay]. Advantages of this approach are an undoped 1300 nm cavity with reduced free-carrier losses and resistive heating being primarily confined to the 850 nm cavity, which enabled cw operation up to 115 ◦ C. Optical pumping of VCSELs, as mentioned above, is an exception, and the standard design relies on electrical pumping. The active layers reported so far include AlGaInAs-, GaAsSb-, or GaInAsN quantum-well structures with 2 to 9 QWs, where the GaInAsN approach appears to be most mature. The lateral definition of the active area is generally achieved by oxidation of an AlAs layer into Alx Oy , similar to the technology established for short-wavelength VCSELs. An alternative active-layer concept relies on self-organized InAs quantum dots with several dot layers vertically stacked on top of each other [00Lot]. At present only pulsed lasing has been achieved and the ultimate potential of this approach is difficult to predict. 4.1.13.2.2 1.55 µm VCSELs 1.55 µm VCSELs are either based on GaAs substrates in combination with wafer fusion [98Ohi, 01Kar] or alternatively on InP substrates with either metamorphic GaAs/Al(Ga)As DBR mirrors or lattice-matched layer stacks of different composition. The current state of the art is reflected by the data compiled in Table 4.1.5. Continuous-wave operation temperatures, not included in Table 4.1.5, have already been raised to 105 ◦ C [01Kar]. In order to alleviate the problem of low current (and heat) conductivity of p-type In(GaAs)P and that of excessive free-carrier absorption in highly p-doped DBR mirrors it has become a widely

Landolt-B¨ ornstein New Series VIII/1C

GaAs/AlGaAs

AlGaAs/AlAs

GaAs/AlGaAs

GaAs/AlAs

AlGaAs/GaAs

AlGaAs/GaAs

AlAs/GaAs

GAs/AlAs

GaAs/AlO

GaAs

GaAs

p-GaAs

n-GaAs

n-GaAs

GaAs

GaAs

GaAs

GaAs

GaAs/AlO

GaAs/AlAs

AlGaAs/GaAs

AlGaAs/GaAs

AlGaAs/GaAs

GaAs/AlAs

2

1.3

1.28

–

≤ 3.5 mW

0.7 (9 mA)

–

SM

SM

pulsed operation only

optically pumped (980 nm) 3 )

intracavity-contacted configuration

[00Lot]

[03Jou]

[01Ste]

[01Kag]

25 dB SMSR SM MM

0.43 (3.5 mA) > 1 (8 mA)

1.185 1.28

[01Jac] including tunnel junction

SM MM

[01Ana]

[00Yam]

[97Qia2]

[00Quo]

[00Jay] [00Ges]

0.75 (5.2 mA) 1.43 (–)

bottom-emitting laser

wafer fused

optically pumped (810 nm) 3 ), pulsed operation only

wafer fused, optically pumped (850 nm) 2 )

Reference

1.27 1.26

–

MM

–

SM SM

Transverse Comment(s) single-mode (SM)/ multimode (MM)

–

0.1 (4 mA)

0.035 (2 mA)

–

1.65 (20 mA) 1 (13 mA) fibercoupled

RT cw output power 1 ) [mW]

0.06

1.295

1.23

1.3

1.29

SiO2 /TiO2

ZnSe/MgF

1.3

Emission wavelength [µm]

GaAs/AlGaAs

Top DBR

) Data not directly comparable due to different active diameters. ) Monolithically integrated 850 nm pump laser. 3 ) External optical pump.

GaInAs quantum dots

GaInAsN MQW

GaInAsN MQW

GaInAsN MQW

GaAsSb MQW

AlGaInAs MQW

GaAsSb MQW

InP-based

Active layer

4.1.13 Vertical-cavity surface-emitting lasers (VCSELs)

1

Bottom DBR

Substrate

Table 4.1.4. Characteristics of VCSELs emitting in the 1.3 µm wavelength range.

390 [Ref. p. 395

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metamorphic p-AlGaAs/GaAs

n-InP/GaInAsP GaInAsP MQW

AlAsSb/ AlGaAsSb

lattice-matched AlGaInAs MQW AlGaInAs/ AlInAs

lattice-matched GaInAs MQW n-InGaAlAs/ InAlAs

1.5 period GaInAlAs MQW MgF2 /a-Si plus Au-coating

n-InP

undoped InP

n-InP

InP

InP

1

AlGaInAs/ AlInAs

GaAs/AlAs

lattice-matched AlGaInAs/ AlInAs

AlAsSb/ AlGaAsSb

metamorphic GaAs/AlAs

GaAs/AlGaAs

) Data not directly comparable due to different active diameters.

AlGaInAs MQW

GaInAsP/InP MQW

n-InP/GaInAsP + SiO2 /TiO2

GaAs

GaInAsP MQW

GaAs/AlAs

Top DBR

n-GaAs

Active layer

Bottom DBR

Substrate

wafer fused

1.55

1.6

1.55

1.55

0.4 (4.5 mA) 2 (25 mA)

0.45 (9 mA)

–

0.9 (9 mA) 2 (20 mA)

1 (55 mA)

SM MM

SM

–

SM MM

MM

–

0.65 (8 mA)

1.53 1.55

–

0.025 . . . 0.030 (9 . . . 15 mA);

1.54

including tunnel junction

multiple-active region, diff. quant. eff. > 1, no RT cw operation

including tunnel junction

including tunnel junction

wafer fused

wafer fused, SiO2 /TiO2 by e-beam evaporation

–

1.55

0.009 (12 mA)

Transv. Comment single-mode (SM)/ multimode (MM)

Emission RT cw output wavelength Power 1 ) [µm] [mW]

Table 4.1.5. Characteristics of VCSELs emitting in the 1.55 µm wavelength range.

[00Ort2] [01Ama]

[00Yue]

[00Kim]

[01Nak] [99Hal]

[99Bou1], [99Bou2]

[01Kar]

[95Bab]

[98Ohi]

Reference

Ref. p. 395] 4.1 Communication 391

392

4.1.14 Reliability

[Ref. p. 395

adopted practice to introduce a tunnel junction into the 1.55 µm VCSEL layer sequence [00Ort1] and simultaneously replace the highly p-doped mirrors by a lower-doped n-type mirror. The problem of inefficient current flow through thick DBR layer stacks has also been circumvented by different measures including a double-intracavity contacted structure, which provides efficient current injection as well as lateral heat spreading [01Nak] or a lower DBR consisting of a 1.5 period MgF2 /a-Si with Au-coating only [00Ort2, 01Ama]. If the device design does not include Al(Ga)As layers suited for steam oxidation, the lateral definition of the cavity is typically done by etching 3-dimensional pillars [99Hal] or by proton implantation [99Bou1, 99Bou2].

4.1.13.3 Tunable VCSELs Tunable VCSELs with several 10 nm tuning range have been reported for the short-wavelength [96Lar, 98Li] and the long-wavelength region as well, for more details see e.g. [00Cha]. Tunability of long-wavelength VCSELs has been achieved based on two related principles: using a cantilever-type top DBR [00Cha] or a curved, translatable top DBR mirror [99Vak]. In the former case the upper DBR mirror is freely suspended about 1.2 µm above the bottom DBR by a cantilever, which is moved by electrostatic force, enabled by doping the mirrors to form a pn junction. In agreement with the general design criterion to have the active layers in the center of the cavity in order to assure maximum overlap of gain and optical field, the air gap has to be located off the center (which enables efficient tuning at the same time). Tuning ranges of such VCSELs, which have been announced to be commercially available soon, are 4, 8, 16, or 32 channels with 100 GHz separation (about 4 to 28 nm wavelength variation) in the 1530 . . . 1610 nm wavelength range. Maximum tuning ranges obtained so far amount to 40 nm, and the highest modulated output power is 250 µW. 1 to 2 dB dispersion penalty is accumulated after 100 km propagation of 2.5 Gbit/s signals along standard dispersion fiber. The second type of tunable VCSELs relies on a top DBR mirror realized as a membrane, which is moved by electrostatic force due to a voltage between the membrane and the InP substrate [99Vak]. More than 6 mW optical output power has been achieved for 70 mW optical pump power at 980 nm, and mode-hop free tuning of > 43 nm, covering the full C-band (1535 . . . 1565 nm) with an SMSR exceeding 50 dB has been demonstrated [00Vak]. With only 2 mW output even 50 nm tuning has been observed [99Vak].

4.1.14 Reliability Reliability of lasers is an important issue in optical communication, and it may be of extreme commercial relevance in high-speed trunk lines. As a consequence suppliers have to guarantee a certain median life or, alternatively, Mean Time To Failure (MTTF). Lasers, similar to other optoelectronic devices, exhibit a certain initial failure rate due to production, mounting, bonding, etc. defects. Once these defective devices have been sorted out the failure rate is fairly stable for a long period at a value significantly lower than the initial one, until the failure rate eventually increases in the wear-out period due to accumulated deterioration and fatigue. Failure of a device tends to be a complete loss of function, if it occurs in the initial phase, while the end of its useful life is normally reached, when the device no longer meets the required specifications. Laser diodes experience a slow degradation in threshold current and slope efficiency. As a consequence, an increasing current is needed to assure a constant light output P0 . This behavior is illustrated in Fig. 4.1.22. The end of (useful) life is frequently defined as the time, when Landolt-B¨ ornstein New Series VIII/1C

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Fig. 4.1.22. Change of the P -I-characteristics of a laser diode as a function of operation time ti (schematic).

the operating current Ii required to assure the initial output power P0 at time ti has increased to (1.3 . . . )1.5 times its initial value I0 . An alternatively recommended end-of-life criterion is a 50 % increase in the initial threshold current Ith (t = 0) [90Bel]. In (D)WDM systems the required emission-wavelength accuracy may be another failure criterion. The maximum accepted wavelength deviation is ±5 % of the system channel separation, e.g. < ±0.05 nm for 100 GHz channel spacing in the 1.55 µm wavelength range, and this has to be assured for a 20 year life of the laser. The emission wavelength may exhibit a long-term drift due to various reasons, but the increased operation current It needed to keep the output power constant normally induces a shift of the emission wavelength ∆λ as well. In the case of DFB lasers ∆λ is given by ∆λ = (It − I0 )dλDFB /dI ,

(4.1.11)

where dλDFB /dI is the normal variation of emission wavelength as a function of driving current (typical value: 0.01 nm/mA) and It is the current needed at ti to keep the emission intensity at P0 . A long-term wavelength change as the decisive failure criterion has been recently reported for GCSR lasers [99Bro] (10 GHz change over 13.9 . . . 22.8 years extrapolated life time, corresponding activation energy 0.62 . . . 0.71 eV, see below). The MTTF of LDs under normal operation conditions is in the order of 105 hours. Such times cannot be determined experimentally. Instead – in good agreement with observation – it is assumed that the MTTF depends exponentially on the inverse junction temperature, and room-temperature lifetimes are regularly derived by extrapolating high-temperature data to room temperature. Empirical formulas frequently used are [90Bel]   −Ea ∆Ith (t) = AIth (t = 0) tm exp (4.1.12) kT or

 ∆Ith (t) = BIth (t = 0) (α + βt) exp

−Ea kT

 ,

(4.1.13)

where A, B, α, and β are empirical constants, m is an (empirically found) exponent, Ea is the activation energy, k is Boltzmann’s constant, and T is the temperature in K. Even in the same Landolt-B¨ ornstein New Series VIII/1C

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4.1.14 Reliability

[Ref. p. 395

material system Ea may be different for different laser designs, and Ea is definitely different for a laser diode and the corresponding module. Preferably Ea should be derived from life tests performed at a minimum of three different temperatures, however, if such data cannot be provided, a recommended activation energy is Ea = 0.4 eV [90Bel]. Device lifetime depends also on the light output power. There is no quantitative formula, but to a reasonable approximation for output powers P1 , P2 with P1 < P2 MTTF(P1 )/MTTF(P2 ) = (P2 /P1 )n

(4.1.14)

holds, where n has to be determined empirically and values between 2 and 1/2 are reported. A similar expression holds for the increase of threshold current: ∆Ith (P1 )/∆Ith (P2 ) = (P1 /P2 )n ,

(4.1.15)

where ∆Ith (Pi ) is the increase in threshold current for a laser providing output power Pi during long-term operation (ageing).

Landolt-B¨ ornstein New Series VIII/1C

References for 4.1

395

References for 4.1 72Kog

Kogelnik, H., Shank, C.V.: J. Appl. Phys. 43 (1972) 2327.

79Sod

Soda, H., Iga, K., Kitahara, C., Suematsu, Y.: Jpn. J. Appl. Phys. 18 (1979) 2329.

82Mit

Mito, I., Kitamura, M., Kobayashi, Ke., Kobayashi, Ko.: Electron. Lett. 18 (1982) 953.

85Koy

Koyama, F., Suematsu, Y.: IEEE J. Quantum Electron. QE-21 (1985) 292.

86Agr 86Asa

Agrawal, G.P., Dutta, N.K.: Long Wavelength Semiconductor Lasers, New York: Van Nostrand, 1986. Asada, M., Miyamoto, Y., Suematsu, Y.: IEEE J. Quantum Electron. 22 (1986) 1915.

87Ogi

Ogita, S., Yano, M., Ishikawa, H., Imai, H.: Electron. Lett. 23 (1987) 393.

89Jew

Jewell, J.L., Scherer, A., McCall, S.L., Lee, Y.H., Walker, S., Harbison, J.P., Florez, L.T.: Electron. Lett. 25 (1989) 1123.

90Ama 90Bel

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Besten, J.H. den, Broeke, R.G., Vries, T. de, Leitjens, X.J.M., Smit, M.K.: Opt. Fiber Commun. Conf. (OFC 2002), Anaheim, CA, USA, Techn. Digest (2002) 203. Hatakeyama, H., Naniwae, K., Kudo, K., Suzuki, N., Sudo, S., Ae, S., Muroya, Y., Yashiki, K., Satoh, K., Morimoto, T., Mori, K., Sasaki, T.: Opt. Fiber Commun. Conf. (OFC 2002), Anaheim, CA, USA, Techn. Digest (2002) 205. Meliga, M.: Proc. 28th Europ. Conf. Opt. Commun. (ECOC 2002), Copenhagen, Denmark, Vol. 2 (2002) paper 5.3.1. M¨ ohrle, M., Roehle, H., Sigmund, A., Suna, A., Reier, F.: Proc. 14th Int. Conf. on Indium Phosphide and Related Materials (IPRM), Stockholm, Sweden (2002) 27. Pezeshki, B., Vail, E., Kubicky, J., Yoffe, G., Zou, S., Heanue, J., Epp, P., Rishton, S., Ton, D., Faraji, B., Emanuel, M., Hong, X., Sherback, M., Agrawal, V., Chipman, C., Razazan, T.: IEEE Photon. Technol. Lett. 14 (2002) 1457. Riechert, H.: Proc. 14th Int. Conf. on Indium Phosphide and Related Materials (IPRM), Stockholm, Sweden (2002) 19. Schwertberger, R., Gold, D., Reithmaier, J.P., Forchel, A.: IEEE Photon. Technol. Lett. 14 (2002) 735. Coldren, C., Strand, T., Hegblom, E., Akulova, Y., Fish, G., Larson, M., Coldren, L.: Opt. Fiber Commun. Conf. (OFC 2003), Georgia, USA, Techn. Digest Vol. 1 (2003) 73. Jouhti, T., Okhotnikov, O., Konttinen, J., Gomes, L.A., Peng, C.S., Karirinne, S., Pavelescu, E.-M., Pessa, M.: New J. Phys. 5 (2003) 84.1. Kawanishi, H., Suzuki, T., Nakamura, K., Mineo, N., Shibuya, Y., Sasaki, K., Yamada, K., Wada, H.: Opt. Fiber Commun. Conf. (OFC 2003), Georgia, USA, Techn. Digest Vol. 1 (2003) 270. Li, H., Iga, K. (eds.): Vertical-Cavity Surface-Emitting Laser Devices, Heidelberg, Germany: Springer, 2003. Zhang, L., Wang, R., Zou, Z., Gra, A., Olona, L., Newell, T., Webb, D., Varangis, P., Lester, L.: Opt. Fiber Commun. Conf. (OFC 2003), Georgia, USA, Techn. Digest Vol. 2 (2003) 678.

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5.1 High-precision optical metrology for surfaces H.J. Tiziani, M. Totzeck

5.1.1 Introduction Optical metrology for surfaces provides a height map of surfaces on various length scales. In particular, we distinguish the macrostructure and the microstructure of the surface. While the former denotes the shape of the surface, the latter describes its structure in the µm region and below. The surface metrology has been playing a key role for many years. The function of optical components depends critically on both the macro- and the microstructure of the surfaces. A good example is the fabrication and testing of telescope mirrors. While the first metrology tool was a function test for the correct overall form as well as visual inspection for scratches and other surface irregularities, it became apparent in the late 19th century that these methods are neither accurate nor reliable enough for the fabrication of high-quality mirrors. More elaborate methods like the Foucault knife-edge test [1859Fou] and the Hartmann test with an array of holes [04Har] were developed in 1859 and 1904, respectively, followed by interferometrical methods since 1918 [18Mic]. In microscopy it was the pioneering work of H. Helmholtz [1874Hel], Lord Rayleigh [1896Ray], and E. Abbe [10Lum] that formulated the physical limits for the optical resolution. In particular, Abbe’s theory of image formation has put, together with C. Zeiss for the mechanics and O. Schott for the optical glasses, the hitherto purely empirical fabrication of optical microscopes on a sound scientific basis. The current applications of surface inspection in the µm region cover optical components, wafer inspection, lithography, and magnetic storage media. In particular, lithography is one of the main driving forces for high-resolution methods. In accordance with the roadmap of lithography (Table 5.1.1), the attainable minimum lateral structure size is continuously decreasing to currently < 100 nm. The measurable height deviations are < 1 nm. Compared to methods with an inherent better spatial resolution, like scanning-tunneling and scanning-force microscopy, optical methods have the advantage of being fast, non-destructive, and they work in a wide range of environments. This chapter intends to give an overview of optical methods for surface metrology with focus on high precision, in lateral direction and for large fields in vertical direction. The overview can’t be complete, but we hope it is representative for the state of the art and emerging developments. The presented optical methods and their attainable resolution are depicted in Fig. 5.1.1 together with electron and scanning-probe microscopy. Optical metrology of engineering surfaces, where a large unambiguity range of the measured height is very important, is discussed in depth in [96Ti2]. A note of caution: We did not try to give full credit to the literature for the methods presented here. This would have gone far beyond the scope of a short review as the present one. Instead, we focus on the current research situation. What we did try to do, however, was to cite exemplary articles (reviews, if available) from which the reader could obtain, or at least derive, the historical development.

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Table 5.1.1. The road map of chip production ([98Rei] modified). Wavelength for photo lithography [nm]

Light source

[Gbit]

Minimum structure size [nm]

1997

0.256

250

248

excimer laser (KrF)

2001

4

130

193

excimer laser (ArF)

2003

16

90

193

excimer laser (ArF)

2007

64

65

193 immersion? 157?

excimer laser (ArF, F2 )?

2010

256

45

193 immersion? 157? 13 (EUV)? electrons (EPL)?

excimer laser (ArF, F2 )? plasma source?

Year

DRAM

10 µm VIS eld p y r-fi o s c o a F icr m

100 nm 10 nm 1 nm

ric et m s ro od rfe th te me

AFM STM

In

Vertical resolution

1 µm

al

1Å 0.1 Å 0.1 Å 1 Å

UHV STM

1 nm 10 nm 100 nm 1 µm 10 µm Lateral resolution

Fig. 5.1.1. Resolution of surface-imaging techniques: Ultra-High-Vacuum (UHV) ScanningTunneling Microscopy (STM), Atomic-Force Microscopy (AFM), interferometrical methods, and far-field VISible (VIS) microscopy.

In Sect. 5.1.2 the physical foundations of microstructure metrology are presented: We discuss the optical resolution limit and technical attempts for resolution improvement. The utilization of a-priori information is presented firstly in rather general terms and then focused on threshold and extreme-value criteria. Section 5.1.3 presents the state-of-the-art instrumentation and methods for microstructure metrology: various kinds of field-measuring and confocal microscopy, as well as near-field methods. Large-field metrology is the subject of Sect. 5.1.4 where we focus on interferometrical methods for spherical and aspherical surfaces, heterodyne interferometry, and the Shack-Hartmann sensor. We conclude this chapter by a look into the future in Sect. 5.1.5.

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5.1.2 Microstructure metrology In microstructure metrology, the quality of the measurements is described by the terms accuracy, precision, and resolution. The accuracy is the agreement between the (conventional) true value and the measured quantity. For a high accuracy, systematical errors have to be eliminated. They may be caused by calibration errors and physical-optics effects. The precision is determined usually by the repeatability and reproducibility of the measurements. In this respect, a low precision is caused by noise and drift of the set-up. Because precision is a less well defined term (frequently precision is used in a broader context covering also accuracy) it should be avoided in the quantification of data [84DIN]. We use it here in a loose sense as a generic term for accuracy and reproducibility. The resolution denotes the minimum lateral distance and depth of spatial features of the object that can be distinguished in the image plane. Lateral and depth resolution are determined by aberrations of the optical system and the diffraction caused by the finite-sized pupils. Because of large improvements in optical design even high-magnification optical systems are nearly diffraction-limited. For the applicability of optical methods for microstructure inspection and also for microstructure generation by optical lithography, the diffraction-limited resolution is now the main delimiter.

5.1.2.1 Resolution in optical imaging In this section we will give a brief introduction into the optical resolution limit and its various formulations. For detailed derivations we must refer to the literature. Because the resolution limit is a consequence of the imaging process we start with a brief summary of optical imaging theory. According to H.H. Hopkins [53Hop] an optical system satisfying the sine condition can mathematically be represented by an entrance and an exit pupil centered on the Gaussian reference sphere (Fig. 5.1.2). A Fourier-transform relation exists between the object plane and the entrance pupil and between the exit pupil and the image plane. The pupil function describes the transfer of a plane-wave component incident under an angle ϑ and leaving the system under an angle ϑ
. The relation between ϑ and ϑ
is obtained from the sine condition using the magnification M n sin ϑ x
y
= = =M .

n sin ϑ x y

fx

x

fx'

J y

Object

(5.1.1)

x' J'

fy

Entrance pupil

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Exit pupil

y'

Image

z

Fig. 5.1.2. Optical system satisfying the sine condition.

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n and n
are the refractive indices in object- and image-space, respectively. To facilitate the notation, we assume for the following considerations M = 1. The maximum acceptance angle α and the refractive index n determine the numerical aperture NA = n sin α of the system. The pupil function H(fx , fy ) is in the most general case a complex function of the pupil coordinates of which the phase term describes the aberrations. For a perfect optical system the pupil function is constant within the numerical aperture. The most convenient mathematical formulation of optical image formation is within the framework of linear system theory [96God]. There, the system is characterized either by its Point-Spread Function (PSF) or, alternatively, by the Fourier transform of the point-spread function, the Optical Transfer Function (OTF). Provided the imaging process is linear, the relation between the object-field distribution B(x, y) and its image B
(x
, y
) is given within the isoplanatic region by the convolution with the PSF G(x
, y
)






∞
∞




B (x , y ) =

B(x, y) G(x
− x, y
− y) dx dy .

(5.1.2)

−∞ −∞

According to the convolution theorem the spatial frequency spectrum of the image, i.e. its Fourier transform, is obtained by multiplying the object spectrum with the OTF, i.e. b
(fx , fy ) = b(fx , fy ) g(fx , fy ) ,

(5.1.3)

where b
, b and g are the Fourier transforms of B
, B and G, respectively. The space and the spatial-frequency formulation of the imaging process indicate that also the optical resolution can be formulated in either space. The resolution limit derived from the OTF is called Abbe resolution limit. It is obtained from the highest transmitted spatial frequency fmax according to ∆xA =

1 fmax

=

λ . NA

(5.1.4)

Oblique illumination allows a shift of the object spectrum up to λ−1 , resulting in a minimum Abbe resolution limit of ∆xM =

λ . 2 NA

(5.1.5)

The resolution limit in space is equivalent to the minimum resolvable distance of two point objects. It is called Rayleigh resolution limit and is usually written as ∆xR = κ

λ NA

(5.1.6)

with a system-dependent parameter κ that depends on experimental parameters like illumination, signal-to-noise ratio of the detectors, a-priori information, and, the dominating factor, the transfer function of the optical system. According to Rayleigh, the limit is chosen such that, for the imaging of two neighbored point objects, the maximum of one image coincides with the minimum of the other. This provides a relative minimum of 20 % for coherent and of 26 % for incoherent illumination. For the κ-values see Table 5.1.2. For optical systems, the linear transmission is valid in the coherent and incoherent limit. The coherent PSF is the Fourier transform of the pupil and the incoherent PSF is its squared magnitude. The corresponding transfer functions are the pupil function itself and its autocorrelation, respectively. These functions and the resulting resolution limits are given in Table 5.1.2 for an   −1 2 aberration-free, circular pupil with k0 = 2πλ , ρ = fx + fy2 and r = x2 + y 2 . OTF and PSF are sketched in Fig. 5.1.3. Landolt-B¨ ornstein New Series VIII/1C

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Table 5.1.2. Point-spread function, transfer function and resolution limits for a circular pupil. Coherence

OTF

PSF

Coherent


 circ NA−1 λρ

2




circ NA

Incoherent

−1






−1

λρ ⊗ circ NA

λρ



∆xAbbe

∆xRayleigh

J1 (k0 r NA) k0 r NA

λ NA

λ 0.83 NA

   J1 (k0 r NA) 2  2  k0 r NA 

λ 2NA

λ 0.61 NA

OTF amplitude

1.0 coherent

0.5 incoherent

0 0

0.5

1.0 r l / NA

1.5

2.0

PSF amplitude

1.0 coherent 0.5

0.61 0.83

incoherent 0 -2

-1

0 r NA / l

1

2

Fig. 5.1.3. Amplitude of the optical transfer function (top) and of the point-spread function (bottom) in normalized coordinates.

The form of the transfer function is also of considerable importance. It should be constant in the transferred region of spatial frequencies because otherwise the image may be changed in a manner that is not describable as a simple resolution limitation. For partially coherent illumination a linear transfer function does not exist. The transfer becomes bilinear, but it possesses still an upper limit for the transmitted spatial frequency and therefore a resolution limit. The degree of coherence of the illumination depends on the numerical aperture of the illumination and by these means on the numerical aperture of the condenser. The sum of condenser NA and imaging NA determines the maximum transmitted spatial frequency of the object. Therefore, we could write the lower resolution limit as ∆xL =

λ . NAImg + NACond

(5.1.7)

The vertical resolution in image formation is defined in a strict sense for pure 3D-microscopy only, for instance in confocal microscopy. There it is the minimum detectable distance of two vertically separated point objects. For field-measuring microscopy, like conventional bright-field microscopy, it is usually identified with the depth of field. This is provided by the Rayleigh λ/4 Landolt-B¨ ornstein New Series VIII/1C

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criterion [87Bor]: If we consider a wavefront deviation of a quarter wavelength at the rim of the pupil as tolerable, we get a depth of field of ∆z =

nλ . 2NA2

(5.1.8)

In interferometry, the term axial resolution is used frequently for the minimum detectable height difference. This is limited only by the noise level of the interferometer and might be as low as 10−3 λ. According to the investigation of the optical uncertainty relation [53Ing, 90Hau] the detectable height difference is related with the lateral resolution limit according to an expression like  2 λ ∆x ∆z ≥ m , (5.1.9) 2 where m is the order of the interference.

5.1.2.2 Improving the optical resolution The optical resolution, cf. (5.1.6), can be improved by either decreasing the wavelength λ, increasing the numerical aperture NA, or decreasing the prefactor κ. In this section we’ll discuss briefly the theoretical foundations and limitations of the different approaches. Practical realizations are deferred to the next section. Generally, techniques subsumed under the term “superresolution” also fall in this category. For a review on this subject see [97Dek].

5.1.2.2.1 Decreasing the wavelength Decreasing the wavelength is possible by either embedding the object into a medium of refractive index n what results in a shorter wavelength λn = λ/n, or by using radiation of higher frequency ν with the correspondingly reduced wavelength λ = c0 /ν (c0 = velocity of light in vacuum). Embedding the object yields immersion-type microscopy which is applied mainly in biology where the object is already situated within a medium. For optimum resolution, the refractive index of the immersion fluid should be at least as high as the index of the embedding medium. Otherwise high-angle components of the diffraction far field of the object may be totally reflected at the interface to the lower-index medium. In the utilization of higher frequencies, optical inspection is very likely to follow the road paved by optical lithography (see Table 5.1.1 in Sect. 5.1.1) that proceeded from the Hg i-line at 365 nm to KrF excimer laser radiation at 248 nm, ArF excimer laser radiation at 193 nm, and the F2 excimer laser wavelength at 157 nm in the near future. A leap to 13 nm Extreme-UltraViolet (EUV) optics is very likely to follow. The main technical problems of the utilization of higher frequencies are the proportionally increased photon energy E = hν (= 1240 eV/λ [nm]) and the λ−4 increase of the cross section for Rayleigh scattering. The comparatively high photon energy (of, for instance, 7.9 eV at λ = 157 nm) may damage the surface of reflecting materials and induce color centers into transparent substances. The effects are reduced lifetimes of both the object and the optics. The increased scattering cross section imposes severe limitations onto the allowed surface roughness. Furthermore, the number of materials that can be used for optical components is strongly reduced: glass varieties are transparent down to approximately 300 nm and quartz is transparent for λ > 190 nm. For optical components to be used at 157 nm, CaF2 and MgF2 are the almost sole materials. Reflection optics becomes of increasing importance. In the EUV, reflection optics has to be used. Landolt-B¨ ornstein New Series VIII/1C

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5.1.2.2.2 Increasing the numerical aperture (NA) Increasing NA beyond NA = n implies a transition to the near field because of the low-pass characteristics of free-space propagation: Like optical imaging, free space propagation can be described in frequency space by multiplication with a transfer function, cf. (5.1.3). This follows from the convolution-type diffraction integrals. Derived from the first Rayleigh-Sommerfeld diffraction integral the transfer function for propagation along a distance z becomes   1/2 P (fx , fy , z) = exp +i2π n2 λ−2 − fx2 − fy2 z , (5.1.10)

, z)

2 1/2

10 0 10-10

2

P (f =(fx +fy )

Amplitude of

which results for spatial frequencies fx2 + fy2 > n2 λ−2 in an exponential damping. The amplitude of P (f = (fx2 + fy2 )1/2 , z) is depicted in Fig. 5.1.4.

10-20 10-30 0

fx l

1 / NA 2

3 3

2

1

z/l

0

Fig. 5.1.4. Amplitude (logarithmic scale) of the free-space transfer function depending on distance z and spatial frequency f .

5.1.2.2.3 Decreasing the prefactor κ A decreased constant κ in the optical resolution limit can be achieved by several means: – Non-linear imaging processes like two-photon and multiphoton confocal microscopy [98Str]. Then, linear system theory and the derived resolution limits are no longer valid. Because of the non-linearity, though, the image interpretation should be performed with care. The need for a preparation of the objects makes it difficult to apply these methods to technical surfaces. – Optimized illumination techniques as in off-axis interference lithography [99Bru], by use of phase-conjugating mirrors [99Uhl], or by use of particular pupil filters [97Sal]. – Model-based imaging and extrapolation of the spatial frequency spectrum: See Sect. 5.1.2.3.

5.1.2.3 Usage of a-priori information: From model-based imaging to threshold criteria With a-priori information we denote knowledge about the structure that is known literally before the actual measurement, i.e. in particular it is independent from the measurement. A-priori information plays an important role in microstructure metrology, where some information about the structure is usually available (for instance material or etching depth due to the fabrication process). Furthermore, the task of metrology on these structures is not to reveal the complete shape but to determine certain structure parameters like the linewidth of a groove. As a consequence, the

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Table 5.1.3. Surface measurements applying a-priori information. Method

A-priori information

Obtained accuracy (∆xR = Rayleigh resolution)

fit of PSF [00San]

instrument’s PSF

∆xR /3

fit of spatial frequency spectrum [93Kru]

object type; here: double lines of known phaseshift

λ/20

thresholds [87Nyy]

object = edge

λ/10

obtainable measurement precision becomes more important than the resolution and the signal-tonoise ratio gains great practical importance [86Cox]. Table 5.1.3 lists some measurement methods that use an increasing amount of a-priori information together with the obtained accuracy. In model-based imaging a mathematical model of the image is formulated and fitted to the image with the quantities of interest as free parameters. This was shown for object- [00San] and frequency-space [93Kru]. The incorporation of multiple scattering in the object has also been shown [98Che]. Techniques for extrapolation of the spatial frequency spectrum utilize the analyticity of the spectrum. An analytic function is completely determined by a contiguous section. Therefore, it is possible to reconstruct the complete spectrum from a band-limited transfer by analytic continuation [96God]. The a-priori information comprises merely the spatial restriction of the object. However, because of practical difficulties these methods are currently of limited practical use. Of considerable importance from an application-oriented point of view in this respect is the concept of the ambiguous image [93Sem]: A band-limited image might be caused by several possible objects of which the incorrect ones have to be excluded by application of a-priori information – a fact that provides an upper bound for the extrapolation, dependent on the signal-to-noise ratio. If additional a-priori information about the object is available (e.g. pure amplitude or phase object), this can also be incorporated in the reconstruction. For instance, the reconstruction of the spatial frequency spectrum using projection methods has been shown [97Fri]. Currently, the most important use of a-priori information is in the form of threshold- and extreme-value criteria which will be discussed in Sect. 5.1.3.2.1. Their application transforms the image interpretation into a localization problem. A threshold value, for instance, is used to determine the position of an opaque edge. It is a certain relative intensity in the image that coincides with the position of the structure edge. Extreme-value criteria make use of relative maxima or minima in the image. The main advantage of both is the possible simplicity of set-up and evaluation method: A conventional bright-field or confocal microscope together with a search for prescribed relative intensity values is sufficient. The strong dependence on the a-priori information, however, makes the methods susceptible to errors. To become more independent in this respect, generalized threshold criteria are applied that make use of additional values of the image (for instance dual-threshold criteria [87Nyy]).

5.1.3 Methods and instrumentation In this section, we give an overview of state-of-the-art set-ups for high-precision metrology of surface microstructures. The methods are roughly divided into 3 categories:

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1. field-measuring microscopy, where the whole object field is imaged in parallel, 2. confocal microscopy where the image is obtained by scanning of a focus, and 3. near-field microscopy, where the distance between the imaging device and the structure is less than a wavelength. The term microscopy without prefix generally denotes far-field microscopy. If near-field microscopy is meant, it is explicitly stated. Pupil-measuring methods like scatterometry and conoscopy are not considered here. Well-known monographs on light microscopy are provided by M. Pluta [88Plu, 89Plu, 93Plu] for general instrumentation and aspects, T. Wilson [90Wil] for confocal microscopy., T.R. Corle and G.S. Kino [97Cor] for scanning microscopy in a broader context including interference- and near-field methods. As a starting point for work on scanning-probe microscopy the book edited by R. Wiesendanger and H.-J. G¨ utherodt [95Wie] is useful. A fundamental prerequisite for accurate microstructure metrology is the availability of diffraction-limited, high-NA microscope lenses in a wide wavelength region. Therefore, we start this section with a brief review of the state of the art of high-NA lens design.

5.1.3.1 High-NA lenses Microscope lenses are characterized by high numerical apertures and small fields of view [94Vol]. Virtually all modern microscope lenses are corrected at the image side to infinity, i.e. a tube lens is necessary to produce a real image. Consequently, the declared magnification refers to the focal length of the tube lens. This enables the manufactures to provide different magnifications by exchangeable tube lenses. Furthermore, additional optical components can be introduced between both lenses without affecting the image quality; at the same time the distance between both lenses can be varied. The user has to be careful, though, because different manufacturers use different standard focal lengths for their tube lenses (Zeiss: 164 mm, Olympus: 180 mm, Leica: 200 mm [94Vol]). Furthermore, part of the correction (in particular chromatic aberrations) might be left to the tube lens, so that, to be on the safe side, one should use matched microscope and tube lenses. A lens cut of a modern microscope/tube lens pair is shown in Fig. 5.1.5. For microstructure metrology, in particular high-aperture, dry (i.e. usable without immersion) objectives are of interest. They should 1. be corrected to the diffraction limit at highest NA’s, 2. be usable for a spectral region as large as possible, and 3. impose no depolarization to the transmitted field (apart from a geometrical depolarization due to different propagation angles in object and image space). Numerical apertures in object space of NAObj = 0.9 are common and high-end lenses provide NAObj = 0.95. In object space (n = 1), this corresponds to a solid angle of ±64 ◦ and ±72 ◦ , respectively. Because of compliance with the sine condition, the NAImg in image space is obtained from the magnification M by NAImg = M −1 NAObj .

(5.1.11)

State-of-the-art objectives have – for the correction wavelength – spherical aberration corrected to < λ/100 over the full pupil yielding Strehl ratios of 0.97 and more [94Vol].

Fig. 5.1.5. Ray path of a Leica 100/0.9 dry objective [00Lei]. Landolt-B¨ ornstein New Series VIII/1C

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Microscope lenses for the VISible (VIS) wavelength region are roughly divided into two classes, depending on their correction state for longitudinal chromatic aberration. In the achromat, the C(656.3 nm) and F-line (486.1 nm) are corrected against the standard d-line (587.6 nm) and the secondary spectrum is partly corrected [94Shi]. In the apochromat the color correction is made down to the g-line (422.7 nm) and the secondary spectrum is almost removed. But this is not the only difference. Generally, an apochromat is in a higher performance class, considering NA and field of view, than an achromat [94Vol]. Recently, microscope lenses that are broad-band corrected down to the i-line (365 nm) have been introduced to the market. For still shorter wavelengths, single-line corrected lenses (for instance in the Deep UltraViolet (DUV) at 248 nm) are available. As dielectric interfaces are hit at oblique angles within a lens, some polarization losses of the transmitted field must be taken [88Han]. The designers try to minimize them together with reflection losses by anti-reflection coatings. Measurements at a manufacturer [99Wes] yielded maximum polarization losses (between parallel polarizers) at the rim of the pupil of a few % for high-NA dry, and up to 15 % for high-NA oil objectives.

5.1.3.2 Field-measuring microscopy The term “field-measuring methods” summarizes microscopical imaging techniques that capture the image as a whole, i.e. perform no scanning. Different field quantities are measured: intensity, phase, polarization, or a combination of them. Additionally, a spatial frequency filter may be introduced into the exit pupil of the microscope lens allowing for a spatial-frequency-dependent preprocessing of the image. A necessary prerequisite for all quantitative, high-precision imaging techniques is a well-defined illumination. It is provided by a set-up developed in 1893 by A. K¨ ohler [1893Koe, 1899Koe] resulting in the optical path sketched in Fig. 5.1.6. According to the illumination optical path, the source, the aperture stop – located in the entrance pupil of the condenser –, and the exit pupil are conjugate planes. For the imaging optical path these are the field stop, the object plane, and the image plane. Because every point in the aperture stop yields an incident plane wave in the object plane, unavoidable inhomogenities of the illumination are equalized.

Collector

Condenser

Objective lens

Tube lens

Source Field Aperture Object Exit pupil stop stop plane

Image

Fig. 5.1.6. Illumination and imaging optical path for K¨ ohler illumination.

Another often used illumination scheme is the critical illumination where the light source is imaged directly into the object plane. Because the degree of spatial coherence of the light incident on the object depends solely on the condenser numerical aperture, it is the same as in K¨ohler illumination [87Bor].

5.1.3.2.1 Intensity microscopy Intensity microscopy is probably the most simple and robust metrology tool: A bright-field or dark-field image is captured and a threshold or extreme-value criterion is applied to determine the Landolt-B¨ ornstein New Series VIII/1C

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1.25

Intensity

1.00 0.75

incoherent

0.50 coherent

0.25 0 -2

-1

a

Intensity

1.5

0 x NA / l

1

2

1

2

j = p/ 2

1.0

0.5 j=p 0 -2

-1

b

0 x NA / l

Fig. 5.1.7. Intensity images of an opaque halfplane (top) and a phase-shifting (ϕ0 ) half-plane (bottom).

position of the edges. To avoid non-axial aberrations, the sample is often put onto a scanning stage and only the central region of the image is used for the measurements [77Nyy]. For the intensity image of an opaque edge we get in the coherent and incoherent limit the well-known intensity distributions coherent limit: incoherent limit :



2 Icoh (x) = π−1 Si(NA k0 x) + 0.5 , −1 Iinc (x) = π−1 Si(2 NA k0 x) + [cos(2NA k0 x) − 1] (2π NA k0 x) + 0.5 (5.1.12)

with the sine integral Si(z) as defined in [70Abr]
z sin t dt Si(z) = t 0

(5.1.13)

and k0 = 2πλ−1 . The results are shown in Fig. 5.1.7. The coherent threshold is 0.25 (because for the amplitude it must be 0.5 due to symmetry) and in the incoherent limit it becomes 0.5 (again due to symmetry). An extreme-value criterion indicates the edge position by a relative maximum or minimum. The coherent image of a phase edge of phase shift ϕ0 , for instance, is a well-known example:

 2 Iphase edge (x) = 1 + (exp(iϕ0 ) − 1) π−1 Si(NAk0 x) + 0.5 .

(5.1.14)

The results for two different phase shifts are shown in Fig. 5.1.7. The incoherent image shows, of course, no modulation because the phase is lost. In the coherent image, the minimum drops to zero for a π phase shift (zero crossing of the amplitude). For phase shifts deviating from π it is shallower and vanishes for ϕ0 = m 2π. Of course, threshold criteria can also be applied to other types of microstructure images like interferograms or polarization microscopy images. Landolt-B¨ ornstein New Series VIII/1C

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5.1.3 Methods and instrumentation

[Ref. p. 439

0.34

Threshold

0.32 0.30 0.28 0.26 0.24 0

0.2 0.4 0.6 0.8 Coherency parameter s

1.0

p-Pol.

0.8

.

1.0

Fig. 5.1.8. Threshold for an opaque edge dependent on the coherency parameter σ. Filled circles: values from [65Wat], line: interpolated.

l / l0

0.6 s-Pol.

0.4

1074 nm 1161 nm

0.2 1017 nm

0 -1500 -1000 -500

s-Pol. p-Pol. AFM-width

0 500 1000 1500 x [nm]

Fig. 5.1.9. Polarization effect for a bar on silicon (s-pol: E parallel edge, p-pol: E perpendicular edge) [97PTB].

The simplicity of the evaluation, however, is opposed by a strong dependence on experimental conditions – like coherence and polarization – and on structural parameters like edge shape and reflectivity. The dependence of the threshold for the image of an opaque edge on the coherency parameter σ=

NA of condenser NA of objective

(5.1.15)

is shown in Fig. 5.1.8 taken from [65Wat]. The threshold values up to σ = 1 could be described by a simple interpolation according to Ithreshold (σ) ≈

4

aj exp(2jσ)

(5.1.16)

j=0

with the empirically obtained coefficients: a0 = 2.5335 e − 1, a1 = −0.8549 e − 3, a2 = 6.3557 e − 3, a3 = −1.2634 e−3, a4 = 1.0259 e−4. Please note that this simple interpolation becomes increasingly doubtful for σ > 0.8. The dependence of the threshold values on inclination angle and phase shift for phase edges is shown in [94Kru] together with the development of a generalized line of thresholds to extract the edge properties. The threshold values are polarization-dependent as shown in [95Sro]. Figure 5.1.9 shows an example. The width of a bar of 1017 nm width and 376 nm height on silicon was measured by optical means with an imaging NA of 0.9 and an illumination NA of 0.71 at a wavelength of 525 nm. A polarization of the incident field parallel to the structure provided a width that differed by almost 90 nm to the width obtained for perpendicular polarization. Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]

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417

5.1.3.2.2 Microscopy with pupil filters Several methods were suggested and tried out for enhancing measurement accuracy by modifying the spatial frequency spectrum of the object. This is done experimentally by inserting an appropriate filter into the pupil plane. In dark-field microscopy, where the 0th order is removed by a corresponding stop, the edge position is indicated by a dark line surrounded by a bright halo [88You]. Using a non-linear crystal in combination with a polarizer as pupil filter frees the experimentalist from the need to know form and size of the 0th order in advance [98Reh]. A π/2 phase-shifting pupil filter for the 0th order realizes Zernike phase contrast. The phase distribution of week phase objects, i.e. for instance shallow topographies, is transformed into an intensity distribution that can be evaluated by threshold criteria [93Boz]. Other modifications of the spatial frequency spectrum include single-band transmission and performing the derivative by optical means using a pupil filter with linear amplitude gradient. However, the user should keep in mind that for all these methods the spatial frequency spectrum is altered, making a very careful image interpretation necessary.

5.1.3.2.3 Interference microscopy In interference microscopy, the phase of the image field is measured by superposition with a reference wave or by superposing two laterally sheared wave fields. For homogeneous materials the actual topography of the structure can be measured within the diffraction limit. Three types of set-ups are mainly in use for microstructure measurements in reflection (Fig. 5.1.10): Nomarsky interferometry [88Plu, 89Plu, 93Plu] as a lateral shearing method and Mirau and Linnik interferometry for measuring the phase-modulated object wave against an undisturbed reference wave. The major advantage of these methods is their suitability for high-numerical-aperture lenses (in particular Linnik interferometry (Fig. 5.1.10c)). Measurement and evaluation of the interferograms is usually performed according to the algorithms of phase-shifting interferometry because then no additional resolution limitation, as in the carrier frequency techniques, is imposed. The interferogram in the image plane stems from the field E O due to the object and accordingly a phase-shifted reference field E R (to simplify the notation, the spatial dependence is not written). Because interference microscopy can be combined with polarization techniques, we consider vector fields: 2

I∆ϕ = |E O + E R exp(i∆ϕ)| ,

(5.1.17)

which becomes in the usual form I∆ϕ = Ig (1 + γ cos(φ + ∆ϕ))

(5.1.18)

with phase φ, intensity Ig , and contrast γ a

b

c

Image

Nom. prism

Reference mirror

Obj. lens

Beam splitter Structure

Landolt-B¨ ornstein New Series VIII/1C

Fig. 5.1.10. Schematic representation of methods for high-aperture interference microscopy: (a) Nomarski shearing interferometry, (b) Mirau interferometry, (c) Linnik interferometry.

418

5.1.3 Methods and instrumentation 2

2

Ig = |E O | + |E R | ,

γ=

2 |E O · E R | , Ig

[Ref. p. 439

φ = arg (E O · E ∗R ) .

(5.1.19)

As there are three unknown quantities, at least 3 different measurements – different phase shifts – are necessary to obtain the phase. For compensation of phase-shift errors the number of measurements is usually larger; a detailed description of the various techniques (and means to introduce the phase shift) can be found in [88Cre] and [90Swi]. With regard to stability and measurement time, the 8-phase-step method [96Smi] is useful. There, 8 images Ij (j = 1 . . . 8) are captured where the j’th component is phase-shifted by βj = jπ/2. Phase φ, intensity I0 = Ig , and contrast γ are obtained by √ Z2 + N 2 Z (5.1.20) tan φ(x, y) = and γ(x, y) = N 32 I0 with Z = 5 I2 − 15 I4 + 11 I6 − I8 ,

N = I1 − 11 I3 + 15 I5 − 5 I7 ,

and I0 =

1 8

8  j=1

Ij .

(5.1.21)

Many interferometrical techniques require a precise mechanical movement with nm accuracy. This can be achieved by piezo-driven flexure stages with capacitive sensors. Under suitable conditions, phase images, as obtained with interference microscopy, show phase singularities. These are localized points in a wave field, where the intensity is zero and the phase is not defined. Because of the continuity of the free-space wave field the phase isolines form to a starshaped pattern around a singularity. Phase singularities, also called phase dislocations, and optical vortices occur also in the reflected near field of small diffracting structures, down to sub-wavelength width. Figure 5.1.11 shows simultaneously the amplitude and the phase isolines of the reflected field caused by a plane wave of λ = 550 nm incident perpendicularly on a rectangular elevation (width: 300 nm, height: 160 nm) of a Si-substrate. The field was computed rigorously using an integralequation approach [97To1]. Where the amplitude drops down to zero, a singularity has formed. If the lens of an interference microscope is focused to a plane containing singularities, supersteep edges are observed. They stem from the π phase jump which is due to the change of sign of the complex field when crossing the singularity. Figure 5.1.12 shows a phase image of a rectangular bar of ≈ 200 nm width and 160 nm thickness on a silicon surface measured with a Linnik interference microscope with NA = 0.9, a magnification of 400 and λ = 549 nm [97To1]. Note the supersteep edges.

1.0 0.8 0.6 0.4

Amplitude

Phase

0.2 0 800 600 z [n 400200 m] 0 -500

0 x [nm]

500

Fig. 5.1.11. Phase singularity in the reflected near field of a sub-λ structure.

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]

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419

h [nm]

1

x[µ 0 m] -1

-2

0

2 ] y [ µm

1

4

3

Fig. 5.1.12. Supersteep edges in an interferometrical image.

-1000

-1000 b = 300 nm

z [nm]

-800

d = 120 nm

d = 90 nm b = 240 nm b = 210 nm

-600

-800

d= 150 nm

d= 180 nm

-400

z [nm]

250 200 150 100 50 0 2

-600 d = 90 nm

-400 d = 60 nm

d= 210 nm

-200 0 -500

-300

a

-100 0 100 x [nm]

300

d = 120 nm

d = 150 nm

-200 0 -500

500

b

-300

-100 0 100 x [nm]

300

500

Fig. 5.1.13. Calculated phase singularity position for bars on a Si-substrate of width 300 nm (*), 240 nm (x) and 210 nm (o) and increasing height for a perpendicularly incident plane wave with λ = 549 nm. (a) s-polarization, (b) p-polarization [97To2].

Phase singularities can be located in the image with high precision, therefore they might be used as indicators for the correct edge position, i.e. a kind of a self-adapting threshold criterion. The migration of the phase singularities in the near field for different structure widths is shown for both polarizations in Fig. 5.1.13: In both cases the phase singularities follow with increasing structure height generally an arc-shaped path through the near field above the structure (solid line, the dashed line corresponds to structures of identical height). The distance of the phase singularities agrees with the true structure width only for a particular structure thickness. Comparing Fig. 5.1.13a and b, we note that the phase singularities are generated in a larger distance above the structure for s- than for p-polarization. This is due to the higher scattering cross-section for s-polarization. Because of the doubled phase shift due to the reflection configuration, interference microscopy has an unambiguity range of λ/4. This limit can be extended by using a second wavelength [99Ped], oblique incidence [91Boe] or a combination of both [98Fra]. Both, multiple-wavelength and obliqueincidence approaches generate an enlarged effective wavelength Λ in z-direction, the unambiguity range is given now by Λ/4. For a plane wave incident at an angle ϑ the effective wavelength is determined by the component of the wavevector perpendicular to the surface Landolt-B¨ ornstein New Series VIII/1C

420

5.1.3 Methods and instrumentation

[Ref. p. 439

7

Focus position [µm ]

6 5 4 3 2 1 0 1

2 3 4 Lateral position [ µm]

5

Fig. 5.1.14. Intensity of a camera line depending on the focus position above a groove in silicon [00ITO].

Λ = λ cos ϑ .

(5.1.22)

For dual-wavelength interferometry the effective wavelength Λ is the beat wavelength Λ=

λ1 λ2 . |λ1 − λ2 |

(5.1.23)

In white-light interference microscopy (also called correlation microscopy [97Cor] or coherence radar) the limit is overcome entirely by using a continuous broad-band source, scanning the object vertically through the focus and evaluating the interferogram contrast [95Win]. A measurement example is shown in Fig. 5.1.14. Here, a 570 nm deep groove of 2 µm width was measured at 780 nm with ∆λ = 50 nm (FWHM). With a Mirau objective lens with a silicon-nitride beam-splitter membrane, the method was applied to the UV-wavelength of 325 nm [98Cha].

5.1.3.2.4 Polarization interferometry In particular, for linear microstructures the near and far fields depend strongly on the polarization. Consider for instance a small groove (width  wavelength) in a good conductor: If the field is polarized parallel to the groove edges (TE-polarization) it can’t excite a propagating mode and therefore it may penetrate the groove only exponentially damped as an evanescent wave. For perpendicular polarization (TM-polarization) it penetrates as a propagating mode. Accordingly, the reflected field is scarcely modulated in TE, but definitely stronger in TM polarization. For silicon, as a strong dielectric, similar effects occur. This polarization dependence can be used to get images with a high edge sensitivity [00Tot]. To this end, the parallel and perpendicular polarized components are measured with respect to each other by means of polarization interferometry (Fig. 5.1.15): The structure is illuminated with circularly polarized light and imaged with a high-NA lens (250 × 0.95). An analyzer under an angle of 45 ◦ to the structure in front of the detector interferes both components. Prior to the interference they are phase-shifted by an adjustable amount using a Liquid-Crystal Phase Shifter (LCPS). To get accurate edge images, a two-step magnification system is used. The interferograms are captured by a CCD-camera. A single 9.2 × 8.4 µm2 pixel on the CCD corresponds to a 9.9 × 9.0 nm2 pixel in the object space. Image evaluation is performed according to the algorithms of phaseshift interferometry. Three images are obtained: phase, contrast and intensity, whereby phase and contrast are particularly sensitive to structure defects. Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]

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421

CCD Polarizer LCPS Lens

3.2 x

l /4 retarder

Tube lens BS

Laser diode Polarizer

{

Köhler illum.

Rotating ground glass

250/0.95

Lens Structure .

z piezo

Fig. 5.1.15. Set-up for polarization interferometry.

Frequently, polarization-optical methods allow a better imaging of sub-wavelength structures than intensity microscopy. This is demonstrated by two measurement examples obtained with λ = 635 nm. For instance, a 330 nm wide groove in silicon (depth: 160 nm) allows a clear edge detection in contrast and phase, but not in intensity (Fig. 5.1.16a–c). The measured width of 200 nm, though, is smaller than the actual width of 330 nm. As can be shown by rigorous numerical simulations, these deviations are due to systematical errors depending on structure depth. They may be calibrated if the depth is known. A second example (Fig. 5.1.16d–f) shows five neighbored trenches with 800 nm pitch etched 120 nm deep into silicon. Again, the contrast and phase image show an improved edge-detection capability compared with the intensity image.

5.1.3.3 Confocal microscopy In confocal microscopy, a point light source is imaged by a lens as a focus spot into the object plane. The spot is imaged by a second lens onto a pinhole detector. The pinhole acts as a kind of focus plane filter, because only light from the proper focus plane is transmitted without attenuation. Light from object parts in front or behind the focus plane is imaged defocused onto the pinhole yielding a strongly reduced detector signal, see Fig. 5.1.17. The name confocal denotes the fact that the foci of the illumination and imaging lens agree. In reflection mode, the focusing and imaging lens are the same. Optical sectioning is obtained by scanning the focus through the object. The depth response, and therefore, in a sense, the vertical resolution, depends on the kind of the investigated object. Two kinds of objects are of particular importance: point objects in biology, and surface patches in engineering. The resulting lateral and axial response and the corresponding Full Width Half Maxima (FWHM) are listed in Table 5.1.4, u and v are normalized lateral and axial coordinates according to v = k0 NA r

and

u = 2nk0 (1 − cos α) z

(5.1.24)

(α is the maximum acceptance angle, see Sect. 5.1.2.1). With decreasing NA, u in (5.1.24) approaches the small-angle formula u = nk0 z sin2 α. The lateral intensity response equals the square of the point spread function obtained for incoherent field microscopy. Therefore a 27 % smaller FWHM of the spot is obtained what results in an improved resolution of about 20 % for incoherent scatterers (as encountered in fluorescence Landolt-B¨ ornstein New Series VIII/1C

422

5.1.3 Methods and instrumentation

a

Intensity

4

Intensity

4

3

[Ref. p. 439

d

3

y [µ m ]

1 .0 0 .8

2

0. 8

2

0 .6 0 .4

1

0. 6 0. 4

1

0. 2 0

0 0 4

1

2

3

Contrast X [µ m ]

0

4

b

1

2

3

Contrast

4

3

y [µm ]

1. 0

4

e

3 0.8

0 .8 0 .6

2

0.6

2

0 .4 0 .2

1

1

0.2

0

0 0

1

2

3

Phase

4

0

4

c 4

1

2

3

Phase

4

3

y [ µm ]

0.4

4

f

3 1.5

2 2

1.0

2 0 -2

1

0

0.5 0

1

0 0

1

2

3

4

0

1

2

3

4

x [µm ]

x [µm ]

Fig. 5.1.16. Intensity (a), contrast (b), and phase image (c) of a small trench of 330 nm width and 160 nm depth in Si between two bars. Intensity (d), contrast (e), and phase image (f ) of 5 trenches of 120 nm depth and 800 nm pitch in Si.

Object

Pinhole

Det.

Fig. 5.1.17. Effect of pinhole detector.

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Ref. p. 439]

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423

Table 5.1.4. Lateral and axial response, and focus size in confocal microscopy in reflection. Objects

point

Lateral response intensity I(r)  4 J (v) 2 1v

Axial response intensity I(z)  4 sin (u/4) u/4 

surface

–

sin (u/2) u/2

Lateral FWHM ∆r

Axial FWHM ∆z

λ 0.37 NA

0.64 λ n (1 − cos α)

–

0.44 λ n (1 − cos α)

2

Collector

Light source Condenser Imaging lens

CCD-camera

Beam splitter

Rotating Nipkow-disk

Tube lens

Microscope lens Object

Fig. 5.1.18. Confocal microscopy with Nipkow disk.

microscopy). For coherent scatterers, the point spread function is not given by the intensity but by the amplitude and a resolution equivalent to incoherent field microscopy is obtained. Yet another remarkable fact is that the halfwidth of the axial response is ≈ 50 % larger for isolated point objects than for surface objects. The reason for this is the reflection geometry. The measurement volume is elongated in axial direction. Confocal variants with further decreased measurement volume are multiphoton confocal microscopy [98Str] and 4π confocal microscopy [92Hel]. In the former the strong nonlinearity caused by the multiphoton excitation reduces the effective volume and in the latter the axial spot size is reduced by simultaneous illumination from back and front. Currently, both procedures seem difficult to apply for technical surfaces. For this field of application, the need for scanning either sample or optical system and the resulting slow measurement rate is a major drawback of confocal microscopy. A variant that allows fast, confocal measurements on technical surfaces uses a Nipkow disk which consists of pinholes arranged in a spiral shape (typical values are for instance 20 µm pinholes at 200 µm distance [98Jor]). The rotating disk is illuminated by a plane wave and acts as a scanning multiple-point light source that is imaged into the focal plane of the objective lens (Fig. 5.1.18); i.e., here not only one but many focal spots are imaged onto the object. After reflection or scattering, each Nipkow pinhole acts as its own detector pinhole. The depth-discriminated xy-information is imaged onto a CCD camera. During one rotation of the disk an xy-section of the sample is obtained in video real-time. By an additional z-scan of the specimen a stack of n camera frames is acquired each corresponding to a certain axial object Landolt-B¨ ornstein New Series VIII/1C

424

5.1.3 Methods and instrumentation b

zn z n -1 • • • z2 z1

Intensity

a

[Ref. p. 439

FWHM

z1

z

zn

yl

Fig. 5.1.19. Reconstruction of topography from frames: (a) frame stack, (b) single-pixel intensity.

xk

20 0

y [ µm]

15 0

1 00

50

0 0

50

100 15 0 x [µ m]

200

Fig. 5.1.20. Measurement example for confocal microscopy: wafer topography.

position (Fig. 5.1.19). The 3D-topography can be reconstructed with a resolution of about 1 % of the FWHM (Fig. 5.1.20). A reliable method to obtain the topographical height is to determine the center of gravity of intensity values above the FWHM. To increase the light yield, microlenses might be aligned above the holes with the focal spots precisely in the pinholes. A very stable setup with high luminous intensity can be obtained by just replacing the Nipkow pinholes by microlenses aligned in spiral shape [00Tiz]. However, lateral and vertical resolution may be slightly reduced. Even the vertical scanning becomes dispensable if chromatic length aberrations are utilized to get an image where the height information is coded in the color. The basic idea is that for an optical system with chromatic length aberrations, and spherical aberration corrected for each wavelength, each color has its own focal plane. For a multicolor light source, the confocal image of a topography becomes colored and every color indicates a certain height range [94Tiz]. The measurement range is given by the chromatic length aberration as large as 10 µm. Furthermore, a chromatic principle works with diffractive elements by sequential selection of the different wavelengths of the light source [00Tiz]. In an experimental set-up (Fig. 5.1.21) four stabilized semiconductor diode lasers were used as light sources. The diode lasers were focused by graded-index lenses into single-mode optical fibers and coupled together to form a single point source. In the microlens arrangement, 100 × 100 microlenses were used. They are a kind of complementary to the Nipkow-disk arrangement. While in the Nipkow-disk arrangement one objective and many pinholes are used, the microlens arrangement applies many objectives (i.e. the microlenses) and one pinhole. A tube lens images the microlens pupils onto a CCD-camera. In particular, diffractive microlenses have a large chromatic aberration together with an inverse proportionality between focal length and wavelength when operating in the first order. It was found by computer simulations that the optimal distance between two wavelength focal lengths matches the FWHM, hence the wavelengths chosen were 750 nm, 780 nm, 810 nm, and 840 nm.

Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]

5.1 High-precision optical metrology for surfaces

4 Laserdiodes (l1... l4 )

Tube lens

425

Pinhole CCDcamera

Beam splitter 1 2 3 4

Diffractive microlens-array

I

Df(chro.) l

Fig. 5.1.21. Confocal chromatic microscopy with microlenses.

5.1.3.4 Near-field microscopy Diffraction, scattering and even reflection of radiation of frequency ν0 = c0 λ−1 at material structures have the potential to produce spatial frequencies that can’t propagate in the surrounding medium of refractive index n. These are bounded to the structure and form an exponentially damped field: the near field. It applies to all spatial frequencies that exceed the inverse free-space wavelength of the surrounding, i.e. |f | >

n . λ

(5.1.25)

The spatial-frequency-dependent damping constant is obtained from (5.1.10). If either source or detector are situated in the near field of the structure, spatial frequencies exceeding the limit (5.1.25) can also contribute to the image. By this means the numerical aperture is exceeded beyond the free-space limit of 1. The best known and manageable near field is the evanescent field stemming from total reflection. It is generated by a plane wave incident at angle α – beyond the critical angle of total reflection – from a high-index medium of refractive index n onto a plane interface to a low-index medium n0 . The evanescent field at the interface declines exponentially into the lower-index medium according to (5.1.10)   1/2 U (x, z) = exp −k0 n2 sin2 α − 1 z exp (i nk0 sin α x) . (5.1.26) A few degrees above the critical angle the penetration depth (1/e amplitude) z1/e =



λ

1/2

2π n2 sin2 α − 1

(5.1.27)

becomes already small compared to the wavelength. However, if a second interface is brought within this distance to the first, energy couples into the second medium. The evanescent wave at the interface excites a propagating wave in the second medium with an amplitude of approximately U (x, d). Figure 5.1.22 shows a rigorous numerical simulation of this process – the frustrated total reflection. Note the evanescent field in the air gap and the frustrated total-reflected field in the lower part. It is considerably stronger below the smaller air gap. Conversely, the interference contrast in the upper part is stronger for the larger gap, because there the reflected amplitude is almost unaffected. Because of the close analogy to electron tunneling, frustrated total reflection is called photon tunneling. Of course, the evanescent wave may also be absorbed or scattered at the interface. Landolt-B¨ ornstein New Series VIII/1C

426

5.1.3 Methods and instrumentation

[Ref. p. 439

incident wave

z[l]

glass

air glass

Fig. 5.1.22. Frustrated total reflection at an air (n = 1)–glass (n = 1.5) step in s-polarization according to a rigorous simulation. The gray scale presents the amplitude of the real part of the electric field.

x [l]

a

b Oil immersion lens

c

n n Transducer

Low-NA dry lens

High-NA dry lens

n0

Hemispherical lens

a a'

n

d

Aplanatic lens

Fig. 5.1.23. The three basic configurations for PTM [90Gue, 92Kin, 97Cor].

The near-field effects are deployed in Photon-Tunneling Microscopy (PTM) [90Gue, 92Kin] (also called frustrated total reflection microscopy or solid-immersion-lens microscopy [92Kin]). A thorough review can be found in [97Cor]. The various forms of PTM are depicted in Fig. 5.1.23. In Fig. 5.1.23a [90Gue] an immersion-type microscope lens is used to illuminate conically a glass-air interface (the “transducer”) under angles beyond the critical angle. An evanescent field forms at the bottom side of the transducer. Figure 5.1.23b [92Kin] uses a high-NA dry objective and a hemispherical lens. The need for a high-NA objective is relaxed in Fig. 5.1.23c [97Cor], where an aplanatic lens is used instead. The aplanatic lens is free of spherical aberration and also satisfies the sine condition exactly, i.e. it is free of coma. Furthermore, it provides a magnification of 2.3. The basic formulas are [94Shi] d=

n+1 r n

and β =

n sin α = 2.3 , sin α


(5.1.28)

where r is the radius of the sphere forming the aplanatic lens. Provided a calibration is performed that relates reflected intensity and gap width according to (5.1.27), three-dimensional images can be obtained. Using such a calibration, PTM was applied, for instance, for monitoring and measurement of latent resist images (i.e. photoresist patterns after exposure, but before development) [98Lid]. The lateral resolution of PTM is only moderately improved. It is determined by the xdependence in (5.1.26) and therefore increased by a factor of n compared to far-field microscopy. In Fig. 5.1.23a and b, the lateral resolution limit does not exceed the resolution obtained with an immersion fluid of refractive index n. In Fig. 5.1.23c the refractive power of the aplanatic lens Landolt-B¨ ornstein New Series VIII/1C

Ref. p. 439]

a

5.1 High-precision optical metrology for surfaces

<< l

427

c

fiber metal (Al) detect. Dz << l

b

d

Fig. 5.1.24. SNOM modes: (a) collection mode, (b) illumination mode, (c) reflection mode, (d) STOM.

yields a further resolution improvement in relation to the applied low-NA lens. However, if we include the aplanatic lens into the effective NA, the improvement by just a factor of n is restored. A further resolution improvement is possible by a lateral localization of the light fields in the near field of the investigated structure. This is done by letting the diffraction near field of the object interact with a small probe of dimensions  λ (Fig. 5.1.24). The resolution of the resulting Scanning Near-field Optical Microscopy (SNOM, for review articles see [91Poh, 93Cou, 95Poh]) is not restricted by the wavelength but rather by scanning distance and probe size. With the time, a multitude of different SNOM set-ups has been developed: In Scanning Tunneling Optical Microscopy (STOM, Fig. 5.1.24d) an evanescent wave field is scanned by a small receiving tip (for instance tipped glass fiber [89Cou] or the Si3 N4 tip of atomic force microscopy [95Bor]). In illumination-mode SNOM (Fig. 5.1.24b) the object is illuminated by a sub-wavelength light source – usually a small aperture in a waveguiding device – and the transmitted field is collected behind the object without spatial resolution. In collection-mode SNOM (Fig. 5.1.24a) the set-up is reversed: The object is illuminated by an incident free space wave in transmission and the resulting diffraction near field is scanned by a small probe. In reflection-mode SNOM (Fig. 5.1.24c) the object is illuminated by a small probe and the near field is collected by the same probe, or, alternatively, the probe measures the free-space-reflected near field of the sample. Although different kinds of probes are under consideration (among them lasing spheres, coaxial tips, tetraeder tips, optoelectronic probes, fiber lasers) the probe type applied most often is the coated-fiber probe: a monomode fiber is drawn to a sharp tip. The formed taper is coated with metal in a way that a sub-wavelength aperture forms at the tip [91Bet]. Aperture sizes between 50 nm and 100 nm are typical. The overall taper length is typically 0.5. . . 1 mm. The metal coating is usually made of aluminum with a skin depth of ≈ 6 nm. For distance regulation it is usually combined with another kind of scanning probe microscopy, as shear-force microscopy [92Bet] or atomic-force microscopy [95Dan]. The key element of the fiber probe is the taper with the nano-aperture at the tip: It determines the achievable spatial resolution and the transmitted energy. We may approximate the resolution as given by the effective diameter of the aperture (which is increased by the penetration of the field into the coating due to the skin effect). The transmission efficiency, i.e. the quotient of the transmitted and the incident energy, is for a circular aperture of radius R in a perfectly conducting screen of vanishing thickness ∼ R6 according to Bethe [44Bet]. For realistic probes with a cladding of finite conductivity and realistic tapers, transmission efficiencies between 10−7 and 10−4 are feasible. Experimental investigations on this subject are reported in [95Val] and computations in [00Moa].

Landolt-B¨ ornstein New Series VIII/1C

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5.1.4 Large-field metrology

[Ref. p. 439

5.1.4 Large-field metrology The subject of this section is the precise measurement of the general form of a surface for a large (i.e. macroscopic) field. Applications of these ultra-precise-shaped surfaces range from semiconductor industry and data storage to optical fabrication and telecommunication to outer-space technology. Depending on the precise application, shape accuracies of ˚ A and below are required. To ensure this accuracy for the required large fields, optical interferometry is the method of choice. In addition, methods like the Shack-Hartmann test, combined with short wavelengths, and heterodyne interferometry also have the potential to satisfy the needs.

5.1.4.1 Interferometry for spherical surfaces The basic problem of interferometrical testing is the limited spatial resolution of the available detectors. The fringe density should not exceed the spatial resolution of the imaging device, usually a CCD chip. But the fringe density increases with the slope deviations of the interfering wavefronts. Therefore a compensation of the general form of the surface under test is required, so that only the deviations from this form are measured. The device to accomplish this task is the “null lens”. Interferometry for spherical surfaces is well known and a number of textbooks discuss the various methods [92Mal]. The prototype of a two-beam interferometer for testing spherical surfaces is the Twyman-Green interferometer (Fig. 5.1.25), which is a modified Michelson interferometer with an objective as spherical null lens. Its focus and the center of the sphere must coincide. Then the incident spherical wavefront, formed by the objective, matches the spherical surface and the resulting interferogram shows constant intensity or straight fringes for an inclined reference, provided the sphere is perfect. Deviations of the perfect sphere yield distortions of the interferogram. In terms of ray optics we say that every ray hits the (perfect) sphere at the right angle. Reference

Objective F

Sphere under test

Center of sphere

Spatial filter

CCD Fig. 5.1.25. Twyman-Green interferometer.

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5.1.4.2 Aspherics and their testing methods Aspherical components are used in an increased manner in complex optical systems. Applications of aspheres are mirror telescopes, photolenses, lithographic systems, ophthalmic lens systems, etc. For an optical designer, they provide additional design freedom compared with conventional spherical optics. The most obvious advantage of using aspheres is that an improved image quality with an equal or decreasing number of optical surfaces can be achieved. Due to the additional freedom of a variable surface curvature, aberrations can be separated from paraxial image properties. Further advantages of aspheres are reduced weigth and at the end lower costs of the whole optical system, provided production and test of the aspheric components can be improved further. Today, highly precise aspheric surfaces can be manufactured by new processes, like Ion Beam Figuring (IBF), Computer-Controlled Polishing (CCP), and MagnetoRheological Finishing (MRF). The manufacturing of aspherics must be combined with a careful optical testing, see Table 5.1.5.

5.1.4.3 Interferometry for aspherical surfaces Simple null tests can only be performed in the case of aspheres based on pure conics, which have at least one focus (paraboloid, hyperboloid, ellipsoid). However, in the case of general aspheres, the simple transformation of a spherical wave to a plane wave by means of an objective is not possible. When the aspheric deformation is weak, the asphere can still be measured in a non-null configuration interferometrically. When the deviation from the sphere rises, the fringe density in the interferogram increases rapidly. The main problem in testing aspheres interferometrically is how to compensate the aspheric test wave in such a way, that an evaluable fringe pattern results at the superposition of the reference and test wavefront. Conventional methods for interferometric measurement of aspheres are null tests using compensating systems. These compensators are designed such that, combined with the asphere, they form a stigmatic image of a point source. Or, in other words, the test wavefront is transformed in such a way, that it fits the asphere under test. The compensator can be of refractive, reflective or diffractive type. Several methods for aspheric surface testing using null compensators have been developed, cf. [92Mal].

5.1.4.3.1 The computer-generated hologram (CGH) null A Computer-Generated or synthetic Hologram (CGH) is a binary representation of an interferogram that would be obtained by superposition of the aspheric test wavefront and any reference wavefront. Firstly it is calculated in the computer and secondly it is generated in reality, e.g. by a laser or e-beam writer. To create a suitable optical test setup for a specific measuring task, the following interacting criteria concerning the CGH design have to be evaluated: – – – – –

interferometer type, position of the CGH within the interferometer, amplitude or phase CGH, inline or off-axis CGH, possibility for manufacture of the CGH.

Often, the opportunities are restricted by the interferometer type and by the measuring task. Customary practice is to place the CGH in the test arm of a commercial Fizeau- or Twyman-Green

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Table 5.1.5. Optical testing methods for aspherical elements. Method

Principle

References

Interferometry with CGH null

see Sect. 5.1.4.3

Heterodyne interferometry

see Sect. 5.1.4.4

Shack-Hartmann sensor

see Sect. 5.1.4.5

Multi-λinterferometry

see Sect. 5.1.3.2.3

Interferometry with adaptive mirror reference

An adaptive membrane mirror is used for partial compensation of the [00Daf] aspheric wavefront in a conventional interferometer. The advantage is the flexibility of the membrane, which can be adapted to several shapes. Up to now, the dynamic range of membrane deflection is limited to about 15 µm.

Subaperture testing

Interferometric method which evaluates wavefront subapertures consec- [88Yin, utively in such a way, that the deviation is within the measurement 93Mel] range of the interferometer, e.g. by means of defocusing. After all separate measurements, the subapertures are stitched together. The general problem is to recover the full-aperture aberration from the separate subaperture data.

Sub-Nyquist interferometry

This extended-range interferometry uses the a-priori information that [87Gre, the aspheric wavefront under test has continuous derivations. 96Gre]

Shearing interferometry

The surface-slope distribution is measured by superposition of two lat- [96Ser, erally displaced surface images. Height obtained by integration. For two- 99Wei] dimensional measurements at least 2 linear independent displacements are necessary.

Interferometry with phaseconjugate mirrors

A distorted wavefront and its inverse are superposed. No reference flat [91Tiz, is necessary and the sensitivity is doubled. 99Kow]

Point-diffraction interferometry

The distorted wavefront is superposed with a low-passed version of itself [96Mer, provided by pinhole diffraction. This common-path interferometry is 99Nau] particularly suited for short wavelengths (EUV) because of the minimum number of components.

interferometer, because this position is easily accessible and no critical data of interferometer optic are necessary. To achieve an optimal fringe contrast in an interferogram, the intensities of reference and measuring wavefront should be matched. Due to their higher efficiency, two-level binary-phase CGHs are well-suited for double pass, whereas binary-amplitude CGHs are well-suited for single pass or single reflection. The concept of inline holograms was developed by Gabor. Real and virtual image are located on the optical axis of the reconstructed wave, whereby disturbances caused by twin images and the

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431

c +1. +1. 0. -1.

b

0.

d -1.

Fig. 5.1.26. Structure and principle of inline CGH’s (left), (a) binary fringepattern, (b) diffraction orders, and off-axis CGH’s (right), (c) binary fringepattern, (d) diffraction orders.

Phase [p]

5

0

-5

0

continuous phase continuous phase mod 2 p two-level binary phase 0.4 0.6 0.2 Normalized radius

0.8

1.0

Fig. 5.1.27. Binary quantization of a continuous phase function.

undiffracted zeroth order occur. Leith and Upatnieks overcome this disadvantage by introducing off-axis or carrier-frequency holograms. Here, a spatial separation of the images occurs, because the reconstructed waves and the zeroth order propagate in different directions. In Fig. 5.1.26 the binary diffraction structure and the optical principle are shown. In spite of the insufficient spatial filtering, inline CGH’s have some advantages: the whole bandwidth of the CGH can be used for aspheric compensation and, as a result of its rotational symmetry, the adjustment of the interferometer is much simpler, because it is not sensitive to rotation. Furthermore, when using a circular laser writer which operates in polar coordinates, the amount of data required for the fabrication is reduced considerably.

5.1.4.3.2 Computation and fabrication of CGH’s During the CGH design process it must be noted that two conditions for successful aspheric testing with CGH null have to be met: no caustic and no vignetting of the test wavefront in the CGH plane. Furthermore, it is important that the maximal spatial frequency in the CGH can still be achieved by means of the customary fabrication process In most cases, computation of the aspheric CGH nulls is made by ray-tracing methods. This requires that all optical components in the interferometer are known. Commercial ray-tracing programs represent the CGH as a thin phase element. The design method is based on geometrical optics and describes the CGH as a grating with a local period (Fig. 5.1.27). This approach is permitted only if the grating period p is significantly larger than the design wavelength λ, i.e. d/λ  1.

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Reference Aspheric system under test

CGH

Spatial filter

CCD

Fig. 5.1.28. Test setup for aspheric testing in transmission using single-pass reflection CGH.

Laser beam writers operate either in polar or Cartesian coordinates. For inline-CGH’s with circular symmetry it is appropriate to use a polar-coordinate writing machine, because its symmetry is adapted to the hologram symmetry. This reduces the necessary amount of data storage during fabrication. For pattern generation, the thermochemical writing process in chromium or direct writing process in photoresist are used. The exposition is carried out by scanning the substrate under an intensity-modulated, focused laser beam. Spatial frequencies of about 500 line pairs per millimeter (lp/mm) can be achieved. Electron beam writers use a focused or variable-shaped electron beam for exposition of the hologram pattern into a photoresist layer. Here, a combination of beam deflection and substrate translation is performed. The whole pattern is stitched of small pattern areas of about 500 µm up to 3 mm. With older machines the stitching error was a problem, modern machines achieve an alignment accuracy which is better than 5 nm. The resolution of an e-beam writer is better than those of laser beam writers, because the e-beam spot size can be reduced down to 100 nm or less. Due to the complicated data handling for e-beam pattern generation, several encoding algorithms have been developed [85Arn]. Fabrication errors of CGH’s: The main source of error when using a rotational plotter is due to plotter distortion. The wavefront error introduced by an incorrect hologram pattern is proportional to the slope of the recorded aspheric test wavefront, i.e. to the design phase function of the CGH. To minimize the effect, the CGH should be arranged such that the aspheric test wavefront has a minimum departure from its reference wavefront.

5.1.4.3.3 CGH application and error reduction As mentioned above, CGH nulls can be of transmittive or reflective type. The application of phase or amplitude CGH’s results from its diffraction efficiency: either as CGH null mirror or as CGH null lens. In Figs. 5.1.28 and 5.1.29 typical interferometrical setups with amplitude and phase CGH’s within the test arm of a Twyman-Green interferometer are shown respectively. A similar arrangement of the CGH’s can be used for Fizeau interferometry. Wavefront aberrations introduced due to errors in the hologram pattern influence the result of interferometrical measurement and therefore they must be calibrated. The wavefront error ∆W caused by a lateral zone displacement ζ for a CGH used in single pass, can be calculated by [76Fer]

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Reference

Objective

CGH

Asphere under test

Spatial filter

Fig. 5.1.29. Test setup for aspheric testing in reflection using double-pass transmission CGH.

CCD

∆W = mλ

ζ , p

(5.1.29)

where λ is the wavelength of light, m the fringe order, and p the local grating period resulting from design data of the CGH. If the lateral zone displacement error which is the same as the writing error is well known or monitored during the writing process, the wavefront error ∆W can be predicted and can be used for calibration of the measuring system. The adjustment of CGH and asphere is much more complicated than of spherical components, because a rotationally symmetric asphere has a predefined optical axis and thus a decentering cannot be completely compensated by tilt and vice versa. In the rotationally symmetric case, five degrees of freedom for CGH as well as for asphere have to be adjusted: x−, y−, z-position, x−, y−tilt. Errors in adjustment result in wavefront errors, which must not be interpreted as surface errors of the asphere. For small adjustment errors a compensation of these errors has been derived [85Dor]. There are further critical aspects for aspheric surface testing which affect the wavefront: lateral distortion, field curvature and interference imaging [99Sil, 00Mur]. Measurement examples are shown in Figs. 5.1.30 and 5.1.31.

5.1.4.4 Heterodyne interferometry In heterodyne interferometry the phase difference of two waves is transformed into the phase difference of two oscillations at a frequency f that can be measured by electronical means (usually f < 1 GHz). This is achieved by superposition of two phase- and frequency-shifted light waves and phase detection at the beat frequency. Set-ups using external references and differential beamsplitting have been realized.

5.1.4.4.1 Principle of external reference Consider two fields at frequency ν1 and ν2 reflected at two surfaces at z and z + ∆z according to U1 = A cos [i (ω1 t − 2k1 z)] U2 = A cos [i (ω2 t − 2k2 (z + ∆z))]

(5.1.30)

with ω = 2πν, k = ω/c, and the time-dependent interference signal 2

I = |U1 + U2 | = I0 (1 + V cos [(ω1 − ω2 ) t + (k2 − k1 ) z + k2 ∆z] ) , Landolt-B¨ ornstein New Series VIII/1C

(5.1.31)

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20 10 40 0 -20

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10

0

5 0 -5 -10 -15

-10

-40 -20

-20 -20

a

[nm]

20

y [mm]

[Ref. p. 439

-10

0 x [mm]

10

20

-20

-10

b

0 x [mm]

10

20

20

40 20

0

0

[nm]

y [mm]

10

-20

-10

-40 -20 -20

c

-10

0 x [mm]

10

20

Fig. 5.1.30. Elimination of the spokes error (phase Fresnel zone plate f /5 used in double pass): (a) measured wavefront, (b) reconstructed systematical error with a PV-value of 32.12 nm, and (c) residual wavefront aberrations after the calibration procedure. The rms-value is reduced from 13.1 nm to 11.8 nm.

i.e. I(t, ∆z) is a beat signal oscillating with the frequency ω1 −ω2 . The phase of this beat frequency is a constant offset of (k2 − k1 )z and the phase from the optical path difference. The detection is usually performed relative to an unshifted reference signal. Figure 5.1.32 illustrates the principle, where it is assumed that both frequency-shifted beams are polarized orthogonally with respect to each other, so that a polarizing beam splitter can provide the spatial separation. There are different techniques to introduce the frequency shift itself and the necessary spatial separation of both beams into signal- and object-beam: cavity methods as Zeeman splitting in the laser cavity, producing two orthogonally polarized, frequency-shifted beams, or two laser diodes with correlated, slightly different wavelengths. Alternatively, a monochromatic beam can be frequency-shifted by transmission through an Acousto-Optical Modulator (AOM), a rotating grating or rotating retarders. A high-frequency AOM splits the incident beam into an undiffracted and diffracted beam with the original and the shifted frequency, respectively.

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50

0

0

-200

[nm]

y [mm]

200

-400 -600

-50

-50

0 x [mm]

Frequency shifter Laser

50

z

n1

Fig. 5.1.31. Example for residual aberrations of an aspheric surface with an aspheric departure of 38 µm. Piston, tilt, and defocus are removed from measured data, the rms value is 88 nm. The test was performed by using a CGH null in double pass.

z+Dz

BS n2 PBS

n 1, n 2 Pol.

Pol.

Reference

Signal Phase detection electronics

Fig. 5.1.32. Schematic of heterodyne interferometry; BS = beam splitter, PBS = polarizing beam splitter.

The sensitivity of heterodyne interferometry can be driven to the shot-noise limit. The unambiguity range of heterodyne interferometry is equal to the one of homodyne interferometry. As there, it can be increased by using two wavelengths (double heterodyne interferometry) [91Sod, 88Dan]. While the main application of heterodyne interferometry are distance measurements (without lateral resolution) it can also be applied to topometry by focussing the beam onto a small object area and scanning the object.

5.1.4.4.2 Scanning differential heterodyne interferometry Differential interferometry is a very robust method that is not very sensitive to disturbances like mechanical vibrations. In a differential interferometer reference and object beams are reflected from the surface under test (cf. Sect. 5.1.3.2.3). The local slope angle of the surface can be calculated from the beam separation of the object and the phase value. For point measurements as in heterodyning, scanning the object provides the spatial distribution of the surface slope. The topography is obtained by integration. Double heterodyne interferometry can be applied to increase the unambiguity range.

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For separating two beams, frequently a Wollaston prism is used [81Som] where the two separated beams are polarized perpendicularly to each other. For heterodyne interferometry one of the two beams is shifted relative to the other by a frequency f . Both beams are combined and interference occurs on traversing a polarizer. Alternatives for beam separation are based on holographic optical elements. In the profilometer shown in Fig. 5.1.33 the lateral scanning occurs by means of a variable deflection angle due to acousto-optic deflectors. The linearly polarized light of the laser diode LD1 traverses a polarizing beam splitter PBS and a polarizer POL. The first acousto-optic deflector AOD1 splits the incident beam into two orthogonally polarized beams. The angle of deviation depends on the grating period in the crystal due to the frequency f of the ultrasonic wave that propagates perpendicular to the optical axis in the crystal of the deflector [96Ti1]. The frequency of the first diffraction order (upper ray) is Doppler-shifted by f . The two beams pass a half-wave plate HWP in order to rotate the direction of polarization. A microscope objective MO focuses the reference and measuring beams onto the surface under test with variable separation. The beams returning from the surface under test follow almost the same path back. The beam splitter BS directs the beams to the second acousto-optic deflector AOD2. The beams superimpose by pairs and interfere on the detector. From the diffraction of the zeroth order of AOD1 (ν1 ) results a plus first order (ν1 + f2 ) that is up-shifted in its frequency by f2 . From the plus first order of AOD1 (ν1 + f1 ) results a minus first order (ν1 + f1 − f2 ) that is down-shifted by f2 . Furthermore, there are the unaffected beams of AOD1 with the frequencies ν1 and ν1 + f1 . On the detector DET1 two light components interfere: the zeroth order of AOD2, which arises from the zeroth order of AOD1, and the minus first order of AOD2, which arises from the plus first order of AOD1. Both beams are stationary, because the deflection by means of AOD1 LD 2

830 nm PBS

DET 1

Pol AOD 2

DET 2 LD 1

measuring beam

L1

AOD 1 HWP Pol

L2 QWP

BS

810 nm PBS

specimen

reference

a

MO

Ampl. [nm]

110

0

-110 0

b

275 x [ µm]

550

Fig. 5.1.33. Setup of a scanning differential heterodyne interferometer with acousto-optic deflectors AOD 1 and AOD 2. Inset: Measurement of a rectangular grating, nominal depth 200 nm.

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is compensated by a reverse deflection caused by AOD2. For double heterodyne interferometry, the dashed indicated second laser-diode LD2 and detector DET2 are used.

5.1.4.5 Shack-Hartmann sensors Shack-Hartmann sensors gain an increasing popularity for high-precision wavefront measurements because of their simple set-up and the robustness [94Lia]. They may be applied as a low-price alternative to interferometry for the quality control of optical components from the VIS to the DUV [03Roc]. A Shack-Hartmann device is a static sensor to measure the local slope of a wave front. This can be achieved by the use of a microlens array that focuses the wavefront onto a CCD chip (Fig. 5.1.34). The local obliqueness of the wavefront yields a well-defined lateral focus shift in the CCD plane behind a single microlens. Considering the lateral focus shifts for the entire microlens array, a “shift field” is obtained that represents the lateral wavefront deviations:   1 dx {∇W (xp , yp )} ij = . (5.1.32) dy f ij Numerical integration of the vector field of the focus shifts yields the wavefront W (x, y) itself. To measure DUV wavefronts, the microlenses must be transparent in this regions. A convenient microlens array is produced by etching diffractive microlenses in a checkerboard pattern into a quartz substrate. Figure 5.1.35 shows a set-up for DUV measurements (here for measurement of aberrations introduced by optical components in transmission). The microlenses of the array can be refractive or diffractive, in our case they were diffractive 4-level phase holograms with a quadratic aperture of 200 µm diameter and a focal length of 25 mm. A 30 × 30 lens array equals the size of a CCD chip. The theoretical diffraction efficiency of 81 % is sufficient for secure focus detection. For a highly accurate Shack-Hartmann sensor the focused spots should be diffraction-limited and the stray-light background should be low [94Kra]. For a precise detection of the focus spots, fabrication errors, like misalignment of the different levels to each other, phase errors caused by wrong etching depth and pattern distortions, should be small compared to the size of the diffractive structure. A critical problem for DUV applications is the stray light caused by scattering at the rough surfaces of the microlenses. The surface roughness should be kept low by special fabrication techniques [03Roc]. Measured focus-spot arrays are shown in Fig. 5.1.34. The distance of the spot positions contains the derivatives of the wavefront. Figure 5.1.36 shows the wavefront of a defocused DUV optics. The Peak-Valley (PV) value equals 20 nm corresponding to a wavefront error of λ/30 for the HeNe wavelength. Wave front

0 80

Microlens array

80 60 40 20

100

el]

dx,dy

200

pix

f

60 40 y [p ix e l]

x[

a

Intensity

CCD

W (xp,yp )

20

0 0

Fig. 5.1.34. Measurement principle of the Shack-Hartmann sensor and obtained array of focus spots.

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Test object 30×30 microlenses

Adjustable

Frequency-doubled Argon laser

6 mm

16 mm

Pinhole

Collimator

Analog CCD camera (Pulnix TM 765)

Relay optic Exit pupil

Entrance pupil

Frame grabber computer

10

2

0 -10

0

x[ mm ]

Wave front [nm]

Fig. 5.1.35. Scheme of set-up for a Shack-Hartmann sensor. Adjustable relay optic to minimize defocusing [03Roc].

3

2

1

0 y [mm]

-2 -1

-2

Fig. 5.1.36. Spherical wavefront measured with a Shack-Hartmann sensor.

5.1.5 A look into the future The current trend to shorter wavelengths is highly visible. However, the accompanying high photon energy, desirable for material processing and lithography, might emerge as a delimiter for the applications for non-destructive surface inspection. New methods that make use of several information channels and derive a multi-parameter image might be competitive; in particular, if they are combined with optical pre-processing and numerical post-processing. Improving solid-state cameras and increasing numerical facilities make measurement procedures with a large evaluation overhead feasible. This is not only true for versatile stand-alone set-ups, but also for highly specialized devices incorporated into a production line for fabrication measurements and quality control. History of lithography has proven that optical methods have a tendency to be more flexible and better adaptable than previously supposed. We expect it to continue this way.

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References for 5.1

439

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Foucault, L.: Ann. Paris (M´em) 5 (1859) 197.

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Courjon, D., Sarayeddine, K., Spajer, M.N.: Opt. Commun. 71 (1989) 23. Pluta, M.: Advanced light microscopy, 3 Volumes, Vol. II, Amsterdam: Elsevier, 1989.

90Gue 90Hau 90Swi 90Wil

Guerra, J.M.: Appl. Opt. 29 (1990) 3741. H¨ausler, G.: Proc. SPIE 665 (1986) 182. Schwider, J.: Progr. Opt. 28 (1990) 272. Wilson T.: Confocal Microscopy, San Diego: Academic Press, 1990.

91Bet 91Boe 91Poh 91Sod 91Tiz

Betzig, E., Trautman, J.K., Harris, T.D., Weiner, J.S., Kostelak, R.L.: Science 251 (1991) 1468. Boebel, D., Packross, B., Tiziani H.J.: Opt. Eng. 30 (1991) 1910. Pohl, D.W.: Adv. Opt. Electron Micr. 12 (1991) 243. Sodnik, Z., Fischer, E., Ittner, T., Tiziani, H.J.: Appl. Opt. 30 (1991) 3139. Tiziani, H.J., Schmidt, J.: Internat. J. Opt. Comput. 2 (1991) 293.

92Bet 92Hel 92Kin 92Mal

Betzig, E., Finn, P.L., Weiner, J.S.: Appl. Phys. Lett. 60 (1992) 2484. Hell, S., Stelzer, E.H.K.: J. Opt. Soc. Am. A 9 (1992) 2159. Kino, G.S., Mansfield, S.M.: SPIE Proc. 1556 (1992) 2. Malacara, D. (Ed.): Optical Shop Testing, New York: Wiley, 1992.

93Boz 93Cou 93Kru 93Mel 93Plu 93Sem

Bozyk, M.: Opt. Eng. 32 (1993) 3229. Courjon, D., Pohl, D.W. (Eds.): Near Field Optics, NATO ASI Series E 242, Dordrecht: Kluwer, 1993. Krumb¨ ugel, M.A., Totzeck, M.: Opt. Commun. 98 (1993) 47. Melozzi, M., Pezzati, L., Mazzoni, A.: SPIE-Proc. 1781 (1993) 232. Pluta, M.: Advanced light microscopy, 3 Volumes, Vol. III, Amsterdam: Elsevier, 1993. Sementilli, P.J., Hunt, B.R., Nadar, M.S.: J. Opt. Soc. Am. A 11 (1993) 2265.

94Kra 94Kru 94Lia 94Shi 94Tiz 94Vol

Krackhardt, U., Streibl, N., Schwider, J.: Optik 95 (1994) 137. Krumb¨ ugel, M.A., Totzeck, M.: Appl. Opt. 33 (1994) 7864. Liang, J., Grimm, B., Goelz, S., Bille, J.: J. Opt. Soc. Am. A 11 (1994) 1949. Shimizu, Y., Takenaka, H.: Adv. Opt. Electron Micr. 14 (1994) 249. Tiziani, H.J., Uhde, H.-M.: Appl. Opt. 33 (1994) 1838. Vollrath, W.: OSA Proc. 22 (1994) 214.

95Bor

95Win

Borgonjen, E.G., Moers, M.H.P., Ruiter, A.G.T., van Hulst, N.F.: SPIE-Proc. 2535 (1995) 125. Danzebrink, H.U., Ohlsson, O., Wilkening, G.: Ultramicr. 61 (1995) 131. Pohl, D.W.: In: Wiesendanger, R., G¨ untherodt, H.-J. (Eds.): Scanning Tunneling Microscopy II, Springer Series in Surface Sciences Vol. 28, 2nd ed., Berlin: Springer (1995) 235. Schr¨ oder, K.-P., Mirand´e, W., Geuther, H., Herrmann, C.: Opt. Commun. 115 (1995) 568. Valaskovic, G.A., Holton, M., Morrison, G.H.: Appl. Opt. 34 (1995) 1215. Wiesendanger, R., G¨ untherodt, H.-J. (Eds.): Scanning Tunneling Microscopy II, Springer Series in Surface Sciences Vol. 28, 2nd ed., Berlin: Springer, 1995. Windecker, R., Haible, P., Tiziani, H.J.: J. Mod. Opt. 42 (1995) 2059.

96God 96Gre 96Mer 96Ser 96Smi

Goodman, J.W.: Introduction to Fourier Optics, 2nd ed., New York: McGraw-Hill, 1996. Greivenkamp, J.E., Lowman, A.E., Palum, R.J.: Opt. Eng. 35 (1996) 2962. Mercer, C.R., Creath, K.: Appl. Opt. 35 (1996) 1633. Servin, M., Malacara, D., Marroquin, L.: Appl. Opt. 35 (1996) 4343. Schmit, J., Creath, K.: Appl. Opt. 35 (1996) 5642.

95Dan 95Poh

95Sro 95Val 95Wie

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96Ti1 96Ti2

Tiziani, H.J., Maier, N., Rothe, A.: Opt. Commun. 123 (1996) 34. Tiziani, H.J.: Optical Metrology of Engineering Surfaces – Scope and Trends, In: Rastogi, P.K. (Ed.): Optical Measurement Techniques and Applications (1996) 15.

97Cor

Corle, T.R., Kino, G.S.: Confocal Scanning Optical Microscopy and Related Imaging Systems, San Diego: Academic Press, 1997. den Dekker, A.J., den Bos, A.: J. Opt. Soc. Am. A 14 (1997) 547. Friedmann, M., Shamir, J.: Appl. Opt. 36 (1997) 1747. Measurement performed at the laboratory for optics at the Physikalisch Technische Bundesanstalt Braunschweig. Sales, T.R.M., Morris, G.M.: J. Opt. Soc. Am. A 14 (1997) 1637. Totzeck, M., Tiziani, H.J.: Opt. Commun. 136 (1997) 61. Totzeck, M., Tiziani, H.J.: Opt. Commun. 138 (1997) 365.

97Dek 97Fri 97PTB 97Sal 97To1 97To2 98Cha 98Che 98Fra 98Jor 98Lid 98Reh 98Rei 98Str 99Bru 99Kow 99Nau 99Ped 99Sil 99Uhl 99Wei 99Wes

00Daf 00ITO 00Lei 00Moa 00Mur 00San 00Tiz 00Tot 00Vol 03Roc

Chang, F., Kino, G.S.: Appl. Opt. 37 (1998) 3471. Chen, F.-C., Chew, W.C.: Appl. Phys. Lett. 72 (1998) 3080. Franze, B., Tiziani, H.J.: J. Mod. Opt. 45 (1998) 861. Jordan, H.-J., Wegener, M., Tiziani, H.: Meas. Sci. Technol. 9 (1998) 1142. Liddle, J.A., Johnson, J.A., Cirelli, R., Mkrtchyan, M., Novembre, A.E., Peabody, M.L.: J. Vac. Sci. Technol. B 16 (1998) 3651. Rehn, H., Kowarschik, R.: Opt. Laser Technol. 30 (1998) 39. Reiss, S.M.: Optics and Photonics News, June 1998 (1998) 31. Straub, M., Hell, S.W.: Bioimaging 6 (1998) 177. Brueck, S.R.J., Chebn, X.: J. Vac. Sci. Technol. B 17 (1999) 908. Kowarschik, R., Wenke, L., Baade, T., Esselbach, M., Kiessling, A., Notni, G., Uhlendorf, K.: Appl. Phys. B 69 (1999) 435. Naulleau, P.P., Goldberg, K.A., Lee, S.H., Chang, C., Attwood, D., Bokor, J.: Appl. Opt. 38 (1999) 7252. Pedrini, G., Fr¨ oning, P., Tiziani, H.J., Gusev, M.E.: Appl. Opt. 38 (1999) 3460. Schillke, F.: SPIE-Proc. 3739 (1999) 317. Uhlendorf, K., Notni, G., Kowarschik, R.: Appl. Opt. 38 (1999) 869. Weing¨artner, I., Schulz, M., Elster, C.: SPIE-Proc. 3782 (1999) 306. Wesner, J., Heil, J., Sure, T.: Apodisation and pupil-function of microscope lenses of high aperture, talk at the 100th meeting of the German Society for Applied Optics, Berlin, 1999. Daffner, M., Reichelt, S., Tiziani, H.: Proceedings of the 2nd International Workshop on Adaptive Optics for Industry and Medicine, Singapore: World Scientific (2000) 141. Measurement performed at the Institut f¨ ur Technische Optik, Stuttgart, August 2000. Design by Leica Microsystems GmbH, provided by W. Vollrath, 2000. Moar, P., Ladouceur, F., Cahill, L.: Appl. Opt. 39 (2000) 1966. Murphy, P.E., Brown, T.G., Moore, D.T.: Appl. Opt. 39 (2000) 2122. Santos, A., Young, I.T.: Appl. Opt. 39 (2000) 2948. Tiziani, H.J., Wegner, M., Steudle, D.: Opt. Eng. 39 (2000) 32. Totzeck, M., Jacobsen, H., Tiziani, H.J.: Appl. Opt. 39 (2000) 6295. Vollrath, W.: SPIE Proc. 4099 (2000) 23. Rockt¨aschel, M.: Wellenfrontanalyse mittels diffraktiver optischer Elemente, Dissertation, Universit¨ at Stuttgart, 2003.

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5.2 Environmental control M. Ulbricht

5.2.1 Introduction Environmental analytics deals with the detection of small concentrations of molecules. For many applications, laser analytical techniques are the method of choice. Beside their high sensitivity, they reduce or even avoid direct contact with the measured object, allowing for detecting the substances in their original environment. Furthermore, some optical techniques offer the capability of open-path measurements and remote detection, e.g. of air pollutants. Open-path measurements are important, if the concentration shows a strong spatial variation and integral values over a certain range are needed (e.g. for pollutant flux measurements of a plume). Remote sensing with a range of up to several tens of kilometers enables measurements of parameters which are difficult to access (e.g. plume tracking, continuous stratospheric measurements, etc.). Many analytical methods have been developed since laser sources became available. In this chapter, some methods successfully applied in gas detection and atmospheric analysis are briefly introduced. For a comprehensive overview see [94Sig, 94Fri, 84Mea, 88Mea].

5.2.2 Tunable diode laser spectroscopy (TDLAS) Diode lasers are widely used for the detection of atmospheric trace gases by molecular absorption techniques [92Gri]. They are available over a wide range of the optical spectrum (Fig. 5.2.1). For quantitative gas detection, the laser radiation is send through a volume containing the gas of interest. The number density N of this gas in the cell is given by Beer-Lambert’s law: 
1 I0 (λ) , (5.2.1) N= ln S(T, λ)g(λ)L I(λ) where S is the temperature-dependent line strength, g is the line-shape function, L is the absorption length, I0 is the reference intensity and I is the detected intensity behind the gas volume. Diode lasers allow fast wavelength tuning. To enhance the sensitivity, the laser wavelength is tuned over the selected absorption line and the second derivative of the intensity is measured. Two different measurement setups are applied. The first uses an absorption cell, usually a multipass cell in Herriott- or White-configuration, to extend the absorption length L. This method enables measurements with low pressure to reduce the width of the absorption lines caused by pressure broadening and hence a better spectral separation between different gases. Secondly, instead of using an absorption cell, open-path measurements are made. In this case, the laser is directed to a retroreflector, which redirects the beam back to the detector. Path lengths of several hundred meters have been demonstrated.

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5.2.3 Cavity ring-down spectroscopy (CRDS)

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Fig. 5.2.1. Top: Wavelength coverage of different diode lasers. Bottom: Atmospheric transmission caused by absorption of CO2 and H2 O.

An analysis of the detection limits for different gases in environmental and industrial environment is compiled in [98Wer]. Applying an adequate detection scheme for molecules like O2 [94Ngu, 98Mih1], CO2 [98Son, 01Web], CO [98Son, 00Wan], H2 O [99Ups], OH [99Ups, 99Aiz], NO [97Son], CH4 [99Wer, 96Nag], NH3 [01Web, 98Mih2], N2 O [98Mih2], HCHO [99Wer, 97Fri], NO2 [96Son], C2 H2 [95Oh], HCl [99Sco] and other gases, with detection limits down to 100 ppt applying InGaAsP/InP, GaAs/AlGaAs, or PbCdS, PbSSe, or PbSnSe diode lasers are reported. Whereas the first group of laser diodes are operated at room temperature, the lead salt diodes need cryogenic cooling. This disadvantage is partially compensated by the higher absorption coefficients of the lines under study in the lead salt spectral range. New developments of quantum cascade diode lasers [97Kas2, 02Kos] emitting in the Mid-InfraRed (MIR) region with thermoelectric cooling or at room temperature and the application of a difference-frequency diode-laser spectrometer with Near-InfraRed (NIR) laser diodes as pump sources for the generation of MIR radiation [99Kel, 98Lan] could overcome the disadvantage of cryogenic cooling.

5.2.3 Cavity ring-down spectroscopy (CRDS) Cavity Ring-Down Spectroscopy (CRDS) was originally developed to measure the reflectivity of mirrors [84And] and has been modified for sensitive absorption measurements of gases [88OKe]. The system consists of a pulsed tunable laser, the Ring-Down Cavity (RDC) consisting of two high-reflecting (R > 99.9 %) dielectric mirrors for the wavelength range of interest, a sensitive detector and a data-acquisition system. A laser pulse is coupled on the cavity axis. A small fraction of the laser pulse penetrates through the entrance mirror and is trapped in the cavity. In an ideal cavity of length l and mirror reflectivity R (no absorption inside the cavity) the trapped pulse looses 1−R of energy at every reflection. The light which is transmitted through the mirror is detected with a fast detector. The light intensity decays exponentially:

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445 (5.2.2)

with a decay time τ0 = − cl ln R. During this time τ0 the light makes n=−

1 2 ln R

(5.2.3)

roundtrips. The high reflectivity causes a much longer optical path of the probe light through a sample in the cavity than achievable with Herriott- or White-cells. A stable resonator configuration makes the cell insensitive against misalignments. If the cavity is filled with gas and the laser is tuned on an absorption line of this gas, additional light attenuation according to the Beer-Lambert law occurs. Thus the decay rate is given by 1 1 = + cN σ , τ τ0

(5.2.4)

where σ is the molecular absorption coefficient and N is the number density of the gas molecules of interest. By measuring the decay time, the gas concentration can be determined. Special care has to be taken that the cavity mirrors remain stable and clean, because even small changes in R have a strong influence on τ0 . The advantage of CRDS compared to conventional laser absorption spectroscopy is that a decay time instead of the ratio of intensities is measured. This method is insensitive against laser power variations, and very long light paths of the light through the probe are obtained: with a cavity length of 1 m and R = 0.9999 after 5000 roundtrips (10 km light path) the light intensity in the cavity has decayed to 1/e. Therefore, very low concentrations of many gases like CHCl3 [98He], NO [98He], NO2 [97Rom], N2 O [97Rom], NH3 [95Jon] and different alcohols [00Tot] have been detected. For an overview see [98Whe] and [97Sch]. Different techniques have been developed to use narrowband cw-lasers instead of pulsed lasers [96Leh]. They are based either on the measurement of the decay time after switching off the laser or on the detection of a phase shift [80Her] caused by the losses in the RDC.

5.2.4 Photoacoustic spectroscopy (PAS) The methods described before are absorption techniques, where the light is detected, which is not absorbed. PhotoAcoustic Spectroscopy (PAS) is also an absorption method, but here the absorbed energy is detected: When a molecule absorbs a photon of an appropriate wavelength, the excited state of the molecule subsequently can release its energy via a nonradiative transfer. This produces a temperature variation in the environment and hence a pressure change. The pressure change can be detected with a sensitive microphone. A photoacoustic spectrometer consists of a pulsed or modulated tunable laser, a PAS-cell with build-in microphone and a data-acquisition system. Often CO2 -lasers, Optical Parametric Oscillators (OPOs) or dye-lasers are used. Compared to TDLAS or CRDS, where gas-mixing ratios could be directly determined, if the molecular absorption cross section is known, photoacoustic spectroscopy requires a calibration, which is performed with reference gas mixtures. Many gases have been detected, for an overview see [95Rep] and references therein.

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5.2.5 Lidar

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5.2.5 Lidar Lidar (LIght Detection And Ranging) techniques are powerful optical methods to remotely measure atmospheric parameters [84Mea] within a radius of several kilometers. They are based on a laser, which emits a short light pulse into the atmosphere. The light interacts with the atmosphere or a target and scatters a small fraction of the light back to the receiver. The backscattered light is detected with high temporal resolution. The propagation time of the light from the light source to the object and back to the system is used to determine the distance of the object. The backscattersignal intensities are used to derive the properties of the atmosphere or the target. A distinction between different lidar variants is made depending on the interaction process of light and matter.

5.2.5.1 Backscatter lidar A simple backscatter lidar system consists of a pulsed laser, a receiver telescope, a fast detector and a data-acquisition system (Fig. 5.2.2). When a laser pulse is transmitted into the atmosphere, the light is partially scattered by molecules (Rayleigh scattering) and particles (Mie scattering1 ). Furthermore, the light undergoes attenuation by molecular and particle absorption. The backscattered light is collected by a telescope and focused onto a fast detector. The temporal resolution of the detected backscatter signal corresponds to a spatial resolution, thus range-resolved properties of the atmosphere can be derived.

Fig. 5.2.2. Schematic of a biaxial backscatter lidar system. A backscatter signal is detected beginning at the distance x0 , where the laser beam enters the receiver-telescope field of view.

The lidar signal is given by I(x, λ) = I0 (λ)

C β(x, λ)τ 2 (x, λ) , x2

(5.2.5)

where I(x, λ) is the intensity received from distance x at wavelength λ, I0 (λ) is the emitted laser power, C is an instrument constant, β(x, λ) is the backscatter coefficient, which describes the fraction of the incident light scattered back to the system, and τ (x,λ) is the atmospheric extinction 1

In this chapter the term “Mie scattering” is used for light scattering by all particles, although the Mie theorie is only valid for spherical particles. Landolt-B¨ ornstein New Series VIII/1C

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due to absorption and scattering given by Beer-Lambert’s law (where α(ξ, λ) is the atmospheric extinction coefficient): τ (x, λ) = e−


x 0

α(ξ,λ)dξ

.

(5.2.6)

With α and β (5.2.5) and (5.2.6) contain two atmospheric parameters, but only one measured quantity. As long as there is a well-known relation between α and β, the parameters can be derived from the measurement. If no gaseous absorption occurs and particles are absent, this relation for pure Rayleigh scattering is known. If particles are present, it becomes more difficult. The optical properties of particles depend on various parameters like size, shape and chemical composition. Due to the large variety of different suspended particles in the air and its complex optical properties, assumptions about the relation between α and β have to be made depending on the type of particles [81Kle, 85Kle]. Even if it is difficult to evaluate absolute backscatter- or extinction coefficients, relative changes of the particle distribution can be detected. These changes display e.g. aspects of the stratification of the atmosphere. In atmospheric regions, where Mie scattering can be excluded (e.g. in higher altitudes), the scattering intensity can be used to determine the atmospheric density. In practice, many backscatter lidar systems are based on Nd:YAG-lasers with Second Harmonic Generation (SHG), 532 nm, and/or Third Harmonic Generation (THG), 355 nm. If daytime measurements are performed, narrowband daylight filtering is essential. To determine further properties of particles like shape or size distribution, more advanced methods like depolarization or multi-wavelength measurements are applied.

5.2.5.2 Differential absorption lidar (DIAL) An important application of the lidar technique is the remote measurement of gas concentrations. For this, the DIfferential-Absorption-Lidar (DIAL) method is applied [84Mea, 99VDI]. This technique is based on the specific light absorption of pollutant molecules. Two laser pulses with different wavelengths are emitted into the atmosphere. One wavelength (λon ) is tuned to a specific absorption line of the molecule of interest. The second wavelength (λoff ) is a reference wavelength with no specific absorption of the pollutant molecule. Usually the wavelength separation should be small to reduce the influence of other wavelength-dependent effects like Mie or Rayleigh scattering. The ratio of both backscatter signals is given by
x

I(x, λon ) I0 (λon )β(x, λon )e−2 0 (αRayleigh (ξ,λon )+αMie (ξ,λon )+αGas (ξ,λon ))dξ
x . = I(x, λoff ) I0 (λoff )β(x, λoff )e−2 0 (αRayleigh (ξ,λoff )+αMie (ξ,λoff )+αGas (ξ,λon ))dξ

(5.2.7)

If λon ≈ λoff , the backscattering as well as the Rayleigh and Mie extinction coefficients for both wavelengths are approximately equal, because they vary slowly with the wavelength: αRayleigh (λon ) = αRayleigh (λoff ) , αMie (λon ) = αMie (λoff ) ,

(5.2.9)

β(λon ) = β(λoff ) .

(5.2.10)

(5.2.8)

The absorption of the molecule of interest is described by: αGas (x, λ) = NGas (x)σGas (λ) ,

(5.2.11)

where N (x) is the number density and σ(λ) the molecular absorption cross section of the molecule of interest. If the absorption spectrum of the relevant gas is known, the gas concentration can be directly calculated from the lidar signals using Landolt-B¨ ornstein New Series VIII/1C

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[Ref. p. 453

d 1 I(x, λoff ) ln 2∆σGas dx I(x, λon )

(5.2.12)

with ∆σGas = σGas (λon ) − σGas (λoff ) .

(5.2.13)

Many pollutant gases like SO2 [79Fre, 01Fuj, 94Edn], NO [92Koe, 82Ald], NO2 [89Koe, 88Gal], CH4 [87Men], NH3 [85For], Cl2 [87Edn], HCl [87Men], Hg(g) [89Edn], and different Volatile Organic Compounds (VOCs) [82Men, 92Mil] have been detected with DIAL. Different applications like remote emission detection, flux measurements, the determination of diffuse emissions, vertical profiling of the atmosphere, impact studies, model evaluation [90Ben] etc. have been demonstrated. For some applications scanning lidar systems are required, which can steer the beam in different directions to obtain a 2D- or 3D- picture of the pollutant concentration. An example is shown in Fig. 5.2.3. 0 −200 µg/m3 1200 1000 750 500 250 0

Height [m]

− 400 − 600 −800 −1000

Mean 58.69 µg/m3 Std.dev. 162.13 µg/m 3 Area 1.1427 km2

−1200 −1400 −1600

0

200 400 600 800 1000 1200 1400 1600 Distance [m]

Fig. 5.2.3. Measurement of the horizontal SO2 -distribution over a refinery area. The scan comprises 12 beams. Two stacks with high SO2 -emissions can be pinpointed. The measurement was done with a mobile scanning UV-DIAL system manufactured by Elight Laser Systems, Germany.

Two gases are of special interest. Water vapor is the most important trace gas in the atmosphere. Its spatial and temporal distribution plays a major role in meteorology and climatology. DIAL is an excellent method for monitoring the vertical distribution of water vapor. A comprehensive description is given in [98Wul1, 98Wul2, 98Boe]. Tropospheric ozone plays a major role in summer smog [98Wei]. To understand this phenomenon, ozone transport is of major interest. Ozone stratification and transport phenomena can be observed using DIAL systems. In the stratosphere, ozone is relevant for the absorption of cosmic UV radiation which would cause damage of vegetation and human skin. Here, DIAL is an essential tool for the investigation of the stratosphere [90Pap]. Due to their high sensitivity, DIAL systems for ambient air monitoring usually work in the UV spectral range. For stratospheric ozone measurements excimer lasers (XeCl, λon = 308 nm), Nd:YAG-lasers with THG (λoff = 355 nm), for tropospheric multi-pollutant UV-DIAL systems tunable sources like Ti:Sapphire- (with SGH and THG), Ce:LiCAF-, dye-lasers (with SHG) or OPOs are used.

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The number of pollutants which can be detected in the UV is limited to a few important gases like O3 , SO2 , NO, NO2 , and aromatic hydrocarbons. To detect industrial emissions of NH3 , HCl, and other VOCs, IR-DIAL systems can be applied in two different atmospheric windows (3–5 µm and 9–11 µm). Due to weak Rayleigh scattering in the IR, these DIAL systems require the presence of aerosols or topographic targets to increase the backscatter signal. Beside measuring the concentration of atmospheric trace gases, DIAL is also used to retrieve the temperature profile of the atmosphere. The basis of this method is the well-known mixing ratio of oxygen in the air and the temperature dependence of its absorption cross section [93The]. DIAL requires lasers with a bandwidth more narrow than the selected absorption line. An alternative is the Fourier-transform lidar [95Dou]. A broadband laser is transmitted, and the received light is detected with an optical multi-channel analyzer. This method has the potential for multi-pollutant detection, if several pollutants have absorption lines within the bandwidth of the laser. For some applications, the scattering of a solid target instead of the atmospheric scattering is investigated. The advantage is the much stronger intensity of the scattered light signal. Compared to atmospheric scattering, concentrations of gases between system and target can be derived from a single pulse pair instead of averaging over hundreds of pair. This is of special importance, if small gas concentrations with limited spatial extent are measured from a fast moving platform (e.g. a helicopter, car). The disadvantage of this method is the loss of spatial information along the laser beam. Furthermore, special care must be taken to reduce the influence of target albedo variations and speckles. This technique has been successfully applied to the survey of underground natural gas pipelines. An infrared (∼ 3.3 µm) DIAL system based on a seeded OPO with fast alternation between λon and λoff has been implemented in a container mounted on the cargo hook of a small helicopter to detect even smallest emissions of methane, the main constituent of natural gas. A scanner allows for the survey of an 18 m wide sector centered on the pipeline. The system is equipped with navigation and an inertial measurement system to pinpoint methane locations with a precision of a few meters. With a survey speed of 100 km/h simulated emission rates of less than 50 l/h have been detected (Fig. 5.2.4).

Fig. 5.2.4. Helicopter-based gas pipeline surveillance system developed by Ruhrgas AG, DLR, and ADLARES GmbH.

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5.2.5.3 Raman lidar If a laser pulse propagates through the air, it undergoes not only elastic scattering processes like Rayleigh or Mie scattering, but also inelastic Raman scattering (Fig. 5.2.5). The molecular Raman scattering cross section is typically three orders of magnitude smaller than the Rayleigh scattering cross section. Therefore, the Raman backscatter signal is much weaker than the signal from elastic scattering processes. Practically, Raman scattering can be observed only from major constituents of the atmosphere like N2 , O2 , and H2 O.

Fig. 5.2.5. Different Raman-scattering processes: (a) rotational Stokes, (b) rotational anti-Stokes, (c) vibrational Stokes, (d) vibrational anti-Stokes.

Since Raman scattering is a non-resonant process, the wavelength of the laser is less critical. For selecting the appropriate laser source the focus is high power and good beam quality. Usually excimer or Nd:YAG lasers are used. The lidar return signal always contains both, the strong Rayleigh and the weak wavelengthshifted Raman fraction. The detection of the Raman signal requires an excellent suppression of the Rayleigh fraction, which is typically realized with narrowband filters or gratings. Due to the weakness of the signal, mostly photon-counting techniques are applied. Compared to elastic backscatter lidar, the Raman technique has the advantage that there is no backscatter from particles, i.e. β = βRayleigh . Therefore, Raman lidar enables to determine α and β separately without further assumptions [92Ans, 95Gat, 95Gro]. As mentioned above, the determination of the vertical distribution of water vapor in the atmosphere is an important meteorological application of lidar. Instead of using DIAL, where sophisticated lasers are required, it is possible to measure the water-vapor Raman-scattering signal [98Gol, 69Mel]. The water-vapor mixing ratio w(z) in air is proportional to the ratio of the number density of water vapor to nitrogen: w(z) ∝

nH2 O (z) , nN2 (z)

(5.2.14)

which can be derived from the signal intensities from both detection channels (H2 O-Raman and N2 -Raman). Another application of Raman lidar is the remote measurement of the atmospheric temperature. The relative population of molecules in different rotational levels is given by the Boltzmann distribution. By observation of the anti-Stokes rotational Raman scattering, the temperature can be derived from the relative intensities of the backscatter signals of different Raman lines [93Vau, 93Ned]. The main difficulty for rotational Raman lidar is the suppression of the Rayleigh line. Alternatively, combined vibrational-rotational Raman spectroscopy can be applied to overcome this problem [97Hea]. Due to the use of lasers with industrial quality and the availability of excellent filter techniques, Raman lidar can be routinely applied. A major limitation is the reduced performance during Landolt-B¨ ornstein New Series VIII/1C

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daytime caused by background light. Good optical design and better daylight filtering can overcome this limitation [98Gol].

5.2.5.4 Fluorescence lidar Fluorescence techniques are widely used in analytical chemistry, biosciences, and medicine. Fluorescence lidar extends this wide field of applications to the remote sensing. The majority of applications does not investigate the atmosphere, but the properties of a solid or liquid target. Only few atmospheric applications like the determination of the temperature of the mesosphere were demonstrated [87Jen]. One field of application is the remote sensing of the chemical composition of surfaces. Here, fluorescence lidar is used for the detection of organic components, e.g. mineral oil spilled on the sea surface [80ONe]. For these applications only the scattering signal from the target is evaluated, hence the only spatial information of interest is the distance of the target. Solid targets like the surface of historical buildings can also be investigated with fluorescence lidar [01Wei, 01Bal]. Two different aspects play a role here. The first is the early detection of biodeteriogens before their presence can be observed with other techniques. Secondly, different stone types and treatments can be identified, which open new perspectives for the reconstruction of the building’s history. The observation of the biosphere is another topic for fluorescence lidar. Stress (water- or nutrient deficiency, diseases) changes the fluorescence spectrum of plants. This change can be observed with a fluorescence lidar before plant damages become visible [95Sva] to better pinpoint countermeasures at an early stage. Other applications are e.g. the detection of phytoplankton in ocean water masses [98Hog].

5.2.5.5 Doppler lidar Lidar is also used to remotely detect wind velocity [80Bil, 01Wer, 96Huf, 89Cha, 01VDI]. The detection principle is based on the optical Doppler effect. A laser pulse with a frequency f0 is transmitted into the atmosphere. Due to the wind the molecules or particles are moving relative to the light source with the wind velocity component in direction of the laser beam (line of sight) vLOS . The movement causes a shift ∆f of the frequency f of the laser light received by the detection system: ∆f = f  − f0 = 2

vLOS f0 . c

(5.2.15)

For detecting the small frequency shift in the order of 10−6 usually a heterodyne detection is applied. Here, light from a second laser working as Local Oscillator (LO) is mixed with the light received from the atmosphere. This yields a detector current which can be split into three parts: 

iSignal = iDC + ρ

2Ix,λ ILO {cos [2π (fLO − (f0 ± ∆f )) t] + cos [2π (fLO + (f0 ± ∆f )) t]} . (5.2.16)

The detector cannot follow the high frequencies fLO + (f0 ± ∆f ), hence only the beat frequency fLO − (f0 ± ∆f ) is detected. Due to the small frequency shift, a very narrow laser bandwidth combined with high frequency stability of the laser is required. Usually CO2 lasers or Tm:YAG lasers are used. In case of a MOPO Landolt-B¨ ornstein New Series VIII/1C

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design, the master oscillator can be used as seeder for the power oscillator and as local oscillator, hence the beat frequency is the frequency shift to be detected. Using the above method, only the wind velocity in beam direction is measured. To obtain the wind vector, the measurement must be done in several non-coplanar directions. This can be either done by sequential scanning of a few beam directions (Doppler Beam Swinging (DBS)) or by applying the Velocity Azimuth Display (VAD) scan technique. In the case of the VAD scan technique, a conical scan of the laser beam is performed. The result is a sinusoidal curve of vLOS , from which all components of the wind vector can be derived. Instead of using a pulsed laser, as well a cw-laser can be applied. To obtain spatial resolution, the laser beam is focused at a certain distance. By variation of the focal distance, the wind vector is determined as a function of range. This method is called Laser Doppler Anemometry (LDA) [95Wer]. Direct detection, as an alternative to heterodyne detection, has also been demonstrated [92Abr, 02Irr]. Here a spectroscopic element must be implemented to resolve the spectroscopic shift. It must be either scanned in wavelength (etalon) [97McG] or have an extremely well-known spectral response (narrowband filter) [92Kor].

5.2.5.6 Lidar using intense femtosecond laser pulses With the development of the Chirped-Pulse-Amplification (CPA) technique [85Str], femtosecond light pulses with peak power in the TW range became available. They show a highly nonlinear interaction with matter, which may open new paths for environmental sensing. When a high-power laser pulse is weakly focused into the atmosphere, it shows self-focusing due to the non-linear Kerr-effect (Kerr-lensing), which further increases the power density. Above a certain power-density level, multiphoton ionization of the molecules of the air occurs. This ionization creates a plasma, which causes a defocusing of the laser beam. The balance of both effects (Kerr-lens, plasma-defocusing) leads to a phenomenon called self channeling [95Bra]. The laser beam is guided through small plasma filaments with a diameter of about 100 µm over distances of several hundred meters [99Fon]. Inside the plasma channels the laser light of a limited bandwidth is partially converted into white light [00Kas]. This white light can be used for the detection of atmospheric constituents. In combination with a multispectral detection system, a femtosecond lidar could detect different components with a single pulse [00Rai]. Special interest is in the investigation of particles suspended in the air. While “classical” lidar methods do only yield limited information about particles, nonlinear interaction between light and matter like the generation of high harmonics [93Hil, 93Lea, 97Kas1], multiphoton fluorescence [00Hil] or laser-induced breakdown [96Ham] may generate further information about these particles, which in turn may help to better understand their role in atmospheric chemistry.

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References for 5.2 69Mel

Melfi, S.H., Lawrence, J.D., jr., McCormick, M.P.: Appl. Phys. Lett. 15 (1969) 295.

79Fre

Frederiksson, K., Galle, B., Nystrom, K., Svanberg, S.: Appl. Opt. 18 (1979) 2998.

80Bil 80Her 80ONe

Bilbro, J.W.: Opt. Eng. 19 (1980) 533. Herbelin, J.M., McKay, J.A., Kwok, M.A., Ueunten, R.H., Urevig, D.S., Spencer, D.J., Benard, D.J.: Appl. Opt. 19 (1980) 144. O’Neill, R.A., Buja-Bijunas, L., Rayner, D.M.: Appl. Opt. 19 (1980) 863.

81Kle

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82Ald 82Men

Ald´en, M., Edner, H., Svanberg, S.: Opt. Lett. 7 (1982) 543. Menyuk, N., Killinger, D.K., DeFeo, W.E.: Appl. Opt. 21 (1982) 2275.

84And 84Mea

Anderson, D.Z., Frisch, J.C., Masser, C.S.: Appl. Opt. 23 (1984) 1238. Measures, R.M.: Laser Remote Sensing, New York: Wiley, 1984.

85For 85Kle 85Str

Force, A.P., Killinger, D.K., DeFeo, W.E., Menyuk, N.: Appl. Opt. 24 (1985) 2837. Klett, J.D.: Appl. Opt. 24 (1985) 1638. Strickland, D., Mourou, G.: Opt. Comm. 56 (1985) 219.

87Edn 87Jen 87Men

Edner, H., Fredriksson, K., Sunesson, A., Wendt, W.: Appl. Opt. 26 (1987) 3183. Jenkins, D.B., Wareing, D.P., Thomas, L., Vaughan, G.: J. Atmos. Terr. Phys. 49 (1987) 287. Menyuk, N., Killinger, D.K.: Appl. Opt. 26 (1987) 3061.

88Gal 88Mea 88OKe

Galle, B., Sunesson, A., Wendt, W.: Atmosph. Environ. 22 (1988) 569. Measures, R.M. (ed.): Laser Remote Chemical Analysis, New York: Wiley, 1988. O’Keefe, A., Deacon, D.A.G.: Rev. Sci. Instrum. 59 (1988) 2544.

89Cha

Chanin, M.L., Garnier, A., Hauchecorne, A., Porteneuve, J.: Geophys. Res. Lett. 16 (1989) 1273. Edner, H., Faris, G.W., Sunesson, A., Svanberg, S.: Appl. Opt. 28 (1989) 921. K¨ olsch, H.J., Rairoux, P., Wolf, J.P., W¨ oste, L.: Appl. Opt. 28 (1989) 2052.

89Edn 89Koe 90Ben 90Pap 92Abr 92Ans 92Gri 92Koe 92Kor 92Mil

Beniston, M., Wolf, J.P., Beniston-Rebetez, M., K¨ olsch, H.J., Rairoux, P., W¨ oste, L.: J. Geophys. Res. 95(D7) (1990) 9879. Papayannis, A., Ancellet, G., Pelon, J., M´egie, G.: Appl. Opt. 29 (1990) 467. Abreu, V.J., Barnes, J.E., Hays, P.B.: Appl. Opt. 31 (1992) 4509. Ansmann, A., Wandinger, U., Riebesell, M., Weitkamp, C., Michaelis, W.: Appl. Opt. 31 (1992) 7113. Grisar, R., B¨ ottner, H., Tacke, M., Restelli, G. (eds.): Monitoring of Gaseous Pollutants by Tunable Diode Lasers, Dordrecht: Kluwer, 1992. K¨ olsch, H.J., Rairoux, P., Wolf, J.P., W¨ oste, L.: Appl. Phys. B 54 (1992) 89. Korb, C.L., Gentry, B.M., Weng, C.Y.: Appl. Opt. 31 (1992) 4202. Milton, M.J.T., Woods, P.T., Jolliffe, B.W., Swann, N.R.W., McIlveen, T.J.: Appl. Phys. B 55 (1992) 41.

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References for 5.2 Hill, S.C., Leach, D.H., Chang, R.K.: J. Opt. Soc. Am. B 10 (1993) 16. Leach, D.H., Chang, R.K., Acker, W.P., Hill, S.C.: J. Opt. Soc. Am. B 10 (1993) 34. Nedeljkovic, D., Hauchecorne, A., Chanin, M.L.: IEEE Trans. Geosci. Remote Sens. 31 (1993) 90. Theopold, F.A., B¨ osenberg, J.: J. Atmos. Ocean. Tech. 10 (1993) 165. Vaughan, G., Wareing, D.P., Pepler, S.J., Thomas, L., Mitev, V.: Appl. Opt. 32 (1993) 2758. Edner, H., Ragnarson, P., Svanberg, S., Wallinder, E., Ferrara, R., Cioni, R., Raco, B., Taddeucci, G.: J. Geophys. Res. 99 (1994) 18827. Fried, A., Killinger, D.K., Schiff, H.I. (eds.): Proc. SPIE 2112, 1994. Nguyen, Q.V., Dibble, R.W., Day, T.: Opt. Lett. 19 (1994) 2134. Sigrist, W.M. (ed.): Air Monitoring by Spectroscopic Techniques, Chemical Analysis Series, Vol. 127, New York: Wiley, 1994. Braun, A., Korn, G., Liu, X., Du, D., Squier, J., Mourou, G.: Opt. Lett. 20 (1995) 73. Douard, M., Bacis, R., Rambaldi, P., Boss, A., Wolf, J.P.: Opt. Lett. 20 (1995) 2140. Gathen, P. von der: Appl. Opt. 34 (1995) 463. Gross, M.R., McGee, T.J., Singh, U.N., Kimvilakani, P.: Appl. Opt. 34 (1995) 6915. Jongma, R.T., Boogaarts, M.G.H., Holleman, I., Meijer, G.: Rev. Sci. Instrum. 66 (1995) 2821. Oh, D.B., Hove, D.C.: Appl. Opt. 34 (1995) 7002. Repond, P., Sigrist, M.W.: Appl. Opt. 35 (1995) 4065. Svanberg, S.: Phys. Scr. T58 (1995) 79. Werner, C., K¨ opp, F., Schwiesow, R.L.: J. Clim. and Appl. Meteor. 34 (1995) 2055. Hammer, D.X., Thomas, R.J., Noojin, G.D., Rockwell, B.A., Kennedy, P.K., Roach, W.P.: IEEE J. Quantum Electron. 32 (1996) 670. Huffaker, R.M., Hardesty, R.M.: Proc. IEEE 84 (1996) 181. Lehmann, K.K.: US patent No. 5528040, 1996. Nagali, V., Chou, S.I., Baer, D.S., Hanson, R.K., Segall, J.: Appl. Opt. 35 (1996) 4026. Sonnenfroh, D.M., Allen, M.G.: Appl. Opt. 35 (1996) 4053. Fried, A., Sewell, S., Henry, B., Wert, B.P., Gilpin, T., Drummond, J.: J. Geophys. Res. 102 (1997) 6253. Heaps, W.S., Burris, J., French, J.A.: Appl. Opt. 36 (1997) 9402. Kasparian, J., Kr¨ amer, B., Dewitz, J.P., Vajda, S., Rairoux, P., Vezin, B., Boutou, V., Leisner, T., H¨ ubner, W., Wolf, J.P., W¨ oste, L., Bennemann, K.H.: Phys. Rev. Lett. 78 (1997) 2952. Kastner, J.F., Sassenscheid, K., Halford, B., Lambrecht, A., Tacke, M.: Proc. SPIE 3106 (1997) 103. McGill, M.J., Skinner, W.R., Irrgang, T.D.: Appl. Opt. 36 (1997) 1253. Romanini, D., Kachanov, A.A., Stoeckel, F.: Chem. Phys. Lett. 270 (1997) 538. Scherer, J.J., Paul, J.B., O’Keefe, A., Saykally, R.J.: Chem. Rev. 97 (1997) 25. Sonnenfroh, D.M., Allen, M.G.: Appl. Opt. 36 (1997) 7970. B¨osenberg, J.: Appl. Opt. 37 (1998) 3845. Goldsmith, J.E.M., Blair, F.H., Bisson, S.E., Turner, D.D.: Appl. Opt. 37 (1998) 4979. He, Y., Hippler, M., Quack, M.: Chem. Phys. Lett. 289 (1998) 527. Hoge, F.E., Wright, C.W., Kana, T.M., Swift, R.N., Yungel, J.: Appl. Opt. 37 (1998) 4744. Lancaster, D.G., Richter, D., Curl, R.F., Tittel, F.K.: Appl. Phys. B 67 (1998) 339. Landolt-B¨ ornstein New Series VIII/1C

References for 5.2 98Mih1 98Mih2 98Son 98Wei 98Wer 98Whe 98Wul1 98Wul2 99Aiz 99Fon 99Kel 99Sco 99Ups 99VDI 99Wer 00Hil 00Kas 00Rai 00Tot 00Wan 01Bal 01Fuj 01VDI 01Web 01Wei 01Wer

02Irr 02Kos

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Mihalcea, R.M., Baer, D.S., Hanson, R.K.: Meas. Sci. Technol. 9 (1998) 327. Mihalcea, R.M., Webber, M.E., Baer, D.S., Hanson, R.K., Feller, G.S., Chapman, W.B.: Appl. Phys. B 67 (1998) 283. Sonnenfroh, D.M., Allen, M.G.: Appl. Opt. 36 (1998) 3298. Weidauer, D., Kambezidis, H.D., Rairoux, P., Melas, D., Ulbricht, M.: Atmosph. Environ. 32 (1998) 2173. Werle, P.: VDI Berichte 1366 (1998) 1. Wheeler, M.D., Newman, S.M., Orr-Ewing, A.J., Ashfold, M.N.R.: J. Chem. Soc., Faraday Trans., 94 (1998) 337. Wulfmeyer, V.: Appl. Opt. 37 (1998) 3804. Wulfmeyer, V., B¨ osenberg, J.: Appl. Opt. 37 (1998) 3825. Aizawa, T., Kamimoto, T., Tamaru, T.: Appl. Opt. 38 (1999) 1733. Fontaine, B. la, Vidal, F., Jiang, Z., Chien, C.Y., Comtois, D., Desparois, A., Johnson, T.W., Kieffer, J.C., Peppin, H.: Phys. Plasmas 6 (1999) 1615. Kelz, T., Schumacher, A., N¨ agele, M., Sumpf, B., Kronfeldt, H.-D.: J. Quant. Spectrosc. Radiat. Transfer 61 (1999) 591. Scott, D.C., Herman, R.L., Webster, C.R., May, R.D., Flesh, G.J., Moyer, E.J.: Appl. Opt. 38 (1999) 4609. Upschulte, B.L., Sonnenfroh, D.M., Allen, M.G.: Appl. Opt. 38 (1999) 1506. VDI Guideline 4210, Part 1, Berlin: Beuth, 1999. Werle, P., Popov, A.: Appl. Opt. 38 (1999) 1494. Hill, S.C., Boutou, V., Yu, J., Ramstein, S., Wolf, J.P., Pan, Y., Holler, S., Chang, R.K.: Phys. Rev. Lett. 85 (2000) 54. Kasparian, J., Sauerbrey, R., Mondelain, D., Niedermeier, S., Yu, J., Wolf, J.P., Andr´e, Y.B., Franco, M., Prade, B., Mysyrowicz, A., Tzortzakis, S., Rodriguez, M., Wille, H., W¨ oste, L.: Opt. Lett. 25 (2000) 1397. Rairoux, P., Schillinger, H., Niedermeier, S., Rodriguez, M., Ronneberger, F., Sauerbrey, R., Stein, B., Waite, D., Wedekind, C., Wille, H., W¨ oste, L.: Appl. Phys. B 71 (2000) 573. Totschnig, G., Baer, D.S., Wang, J., Winter, F., Hofbauer, H., Hanson, R.K.: Appl. Opt. 39 (2000) 2009. Wang, J., Maiorov, M., Baer, D.S., Garbuzov, D.Z., Connolly, J.C., Hanson, R.K.: Appl. Opt. 39 (2000) 5579. Ballerini, G., Bracci, S., Pantani, L., Tiano, P.: Opt. Eng. 40 (2001) 1579. Fuji, T., Fukuchi, T., Goto, N., Nemoto, K., Takeuchi, N.: Appl. Opt. 40 (2001) 949. VDI Guideline 3786, Part 14, Berlin: Beuth, 2001. Webber, M.E., Claps, R., Englich, F.V., Tittel, F.K., Jeffries, J.B., Hanson, R.K.: Appl. Opt. 40 (2001) 4395. Weibring, P., Johansson, T., Edner, H., Svanberg, S., Sundn´er, B., Raimondi, V., Cecchi, G., Pantani, L.: Appl. Opt. 40 (2001) 6111. Werner, C., Flamant, P.H., Reitebuch, O., K¨ opp, F., Streicher, J., Rahm, S., Nagel, E., Klier, M., Herrmann, H., Loth, C., Delville, P., Drobinski, P., Romand, B., Boitel, C., Oh, D., Lopez, M., Meissonnier, M., Bruneau, D., Dabas, A.: Opt. Eng. 40 (2001) 115. Irrgang, T.D., Hays, P.B., Skinner, W.R.: Appl. Opt. 41 (2002) 1145. Kosterev, A.A., Tittel, F.K., K¨ ohler, R., Gmachl, C., Capasso, F., Sivco, D.L., Cho, A.Y., Wehe, S., Allen, A.G.: Appl. Opt. 41 (2002) 1169.

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6.1 Laser safety H. Welling

Lasers are commonly used in industrial, medical and telecommunication applications and in many other fields. Their unique flexibility allows changing the processing or process parameters or the material within a very short time. Lasers are thus a universal tool, but on the other hand, can also have a high complexity of possible hazards related with their use. A systematical approach to laser safety is not limited to the hazards of and the protection against laser radiation. It includes all parts of the laser installation – electric devices, laser gases, optics, handling devices, screens etc. Furthermore, emissions – such as fumes, gases or UV radiation generated by the interaction of the laser beam with materials or the atmosphere – must be taken care of. Beyond this, the evaluation of administrativa like training and education of the staff, provision and use of personal protection measures is essential. A proper safety concept ensuring a safe laser workplace and an environmentally compatible use of lasers therefore requires all technical, administrative and personal protection measures. Depending on the special laser application, hazards and safety measures are sometimes comparable to those for conventional techniques (e.g. fume emissions for welding), but often high demands are put on safety measures, due to the special hazards (e.g. in medical applications, filters for fumes containing bacteria, viral DNA). Though laser technology is relatively new and is developing dynamically, a high level of knowledge concerning hazards and safety measures could be achieved. An evaluation of statistical data of laser-related accidents for the last 30 years showed that the number of accidents per laser installation is very low, but still severe injuries were reported [97Roc2]. These resulted from technical failures, but also either from ignorance or carelessness of the laser operator. With the rapid development of laser technology in industrial and medical applications (wavelengths, output power, pulse length), new hazards will arise, which will have to be taken care of. Especially the development of ultrashort-pulsed lasers requires the validation of known safety measures (e.g. eye-protection) [97Roc2, 98Rob]. In this chapter, a systematical approach to laser-related hazards and safety measures is given.

6.1.1 Hazard potentials Hazards related to the use of lasers can generally be divided into primary and secondary hazards (Fig. 6.1.1). The primary hazard is the laser beam, which can affect humans or objects – as a raw beam, focused beam, directly reflected beam or as scattered radiation. In this context, the primary hazards are often designated as laser-related hazards. Secondary hazards are further subdivided into direct or indirect hazards: – Direct hazards are caused by technical components of the laser installation. – Indirect secondary hazards are generated by the interaction of the laser with materials or the atmosphere.

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Primary hazard potential Laser radiation Secondary hazard potential

Direct

Indirect

Device related

Application related

Electrical components

Emitted hazardous substances

Pump radiation

Ignition of explosives

Laser gases

Fire hazard

Optical components

Secondary radiation

Fig. 6.1.1. Hazard potentials for laser-material interaction [98Toe].

6.1.2 Norms and standards for laser safety To ensure a safe use of machinery and to protect operators against hazards by chemical, physical or biological substances, the legislator has defined a number of fundamental directives, which are supplemented by a multitude of technical norms, standards and guidelines. A systematical survey on laser-related norms and standards would include an enormous number of directives and cannot be given here. Laser-related norms and standards either refer directly to lasers or to the use of lasers in special applications or they describe the demands on laser safety devices or generally define requirements for a safe workplace. In the following text, an extract of the most important European and American standards is given. European regulations which generally refer to machinery safety or workplace health and safety are the Machinery Directives (e.g. 89/392/EEC ) [89EEC2], based on Article 100a/EEC, and the Directives related to Health and Safety (e.g. 88/642/EEC) [88EEC], based on Article 118a/EEC. The European technical standard EN 292 “Safety of Machinery” generally defines requirements on the design and the function of machinery [95EN]. Regulations which are more related to lasers can be found in the field of primary and direct secondary laser hazards. A very important international standard is the IEC 825 (EN 60825) “Safety of Laser Products”, which describes the effects of laser radiation on the eye and the skin and defines threshold limits and safety measures [96IEC]. Further important European standards are e.g.: – EN 31553: “Laser and laser-related equipment: Safety of machines using laser radiation to process materials” [98EN1], – EN 60601-2-22: “Medical electrical equipment: Particular requirements for the safety of diagnostic and therapeutic laser equipment” [96EN]. For the USA, laser-related regulations are given by: – Food and Drug Administration/Center for Devices and Radiological Health (FDA/CDRH), e.g. “CFR 50 (161) 1085 Performance standard for Laser Products” [85CFR], – Occupational Safety and Health Administration (OSHA), e.g. “PUB 8.-1.7 Guidelines for Laser Safety and Hazard assessment” [91OSH], – American National Standards Institute (ANSI): – ANSI Z-136.1 “Safe use of lasers” [93ANS], – ANSI Z-136.2 “Safe Use of Optical Fiber Communications Systems Utilizing Laser Diode and LED sources” [88ANS], – ANSI Z-136.3 “Safe Use of Lasers in the Health Care Environment” [96ANS]. Landolt-B¨ ornstein New Series VIII/1C

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In the field of secondary indirect hazards and safety measures (e.g. hazardous substances generated during laser processing), so far no laser-specific norms and standards exist. In these cases, norms and standards for industrial safety, workplace safety and environmental compatibility are used. Typically, the regulations only deal with national concerns. In the following, some major guidelines for Germany and the USA are given: – German law: Chemikaliengesetz, Gefahrstoffverordnung, Technische Regeln f¨ ur Gefahrstoffe [80Che, 98Gef, 03TRG], – German law: Bundesimmissionsschutzgesetz, Technische Anleitung zur Reinhaltung der Luft [90BIM, 86TAL], – OSHA: Respiratory Protection 29CFR1910.134, Toxic and hazardous substances 29CFR Subp. Z [89OSH1, 89OSH2], – ACGIH: American Conference of Governmental Industrial Hygienists: Threshold limit values for chemical substances and physical agents and biological exposure indices [98ACG]. Norms and standards related to lasers are permanently under further development. A number of technical committees and working groups (e.g. IEC TC, CEN, CENELEC) are responsible for carrying out these developments.

6.1.3 Effects of laser radiation and safety measures The outstanding properties of the laser radiation – high power, monochromasy (and coherence) – offer high spectral and temporal radiation intensities [72Wel, 75Wel]. Especially the pulsed operating mode offers extremely high peak powers [93Wel1, 93Wel2, 96Wel]. This advantageous properties on one side cause special laser-related hazards on the other side: – Depending on the wavelength, the monochromatic radiation leads to very specific effects on human tissues. In particular, hazards arise from the fact that the radiation can be completely invisible, which can pretend a false safety. – The low divergence of a few mrad – and as a consequence the excellent focusability of laser radiation – causes high intensities, even in large distances. – Pulsed laser systems can emit – for short times (ns) – peak powers of several terrawatts. Since the human eye only recognizes the time-average power, the extremely high peak power of short pulses is noticed by the eye as being low power. This means a considerable hazard potential. In cases of malfunction, if the laser radiation meets objects within the laser-controlled area or the human body, these high intensities can cause hazards or severe damage of the eye and the skin. But even the intensity of an unfocused beam can be sufficient to injure the human body.

6.1.3.1 Effects of laser radiation on biological tissue The interaction of laser radiation and tissue mainly depends on the wavelength, the irradiance and the time of interaction. Depending on irradiance and time of interaction, five different interaction processes can be identified (Fig. 6.1.2). The biological effects on the skin and the eye are illustrated in Fig. 6.1.3. For typical levels of irradiance (less than 1 W/cm2 ) and interaction times (greater than 10 s) photochemical reactions are induced by laser radiation in distinct wavelength ranges. Irradiances

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Intensity [W/cm²]

10

photodisruption

12

10

9

10

6

10

3

10

0

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10

photoablation 10

0 10 J/cm ² J 1 J /cm ² / 0.1 cm J/c ² m²

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vaporization

coagulation

photochemistry 10

-3

10 10

-12

10

-9

10

-6

10

-3

10

0

10

-3

3

Time of interaction [s] Fig. 6.1.2. Classification of laser-radiation-tissue interactions [98Toe]. Type of laser

Wavelength 5

10,060 nm

CO 2 -laser

10

infrared

C

Wavelength [nm]

10

Biological effects

4

Thermal effects

B

3.000

1,064 nm

1.400 10

3

A

Nd:YAG-laser

Thermal effects Photochemical effects

780 visible

10

A

Cataract, pigmentation ultra-violet

nm nm nm nm

B

351 308 246 193

C

Excimer-laser

380 315 280

Erythema Photoconjunctivitis, photoceratitis Skin cancer, aging of skin

2

Fig. 6.1.3. Biological effects of radiation on skin and eye [96IEC].

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between 10 W/cm2 and 1 MW/cm2 and interaction times between 10 ms and 1 s result in thermal interactions. The tissue is heated by the absorbed laser radiation and will be coagulated or vaporized. The dimension of the necrosis zone depends on the laser process parameters (e.g. wavelength, pulse duration). Interaction times of nano- to microseconds and irradiances of several MW/cm2 cause the so-called “photoablation” which means spontaneous removal of tissue. Extremely high irradiances (greater than 1 GW/cm2 ) are able to ionize the tissue by non-linear absorption such as multiphoton processes. Due to the extremely high temperatures and pressures the tissue is disrupted [96Nie]. The wavelengths of laser radiation comprise nearly the whole region of radiation, air allows beam propagation (ca. 180 nm. . . 1000 µm). The absorption of the laser radiation and the properties of the tissue determine the penetration depth of the radiation, the damage mechanism and accordingly the extent of the injury. Damage can be assigned to thermal, opto-acoustic and photochemical effects. Thermal and opto-acoustic effects are predominately present in the infrared region, whereas photochemical mechanisms dominate in the ultraviolet region [96Nie, 97Hen]. Thermal effects of laser radiation on biological tissue are the most common damage mechanisms. Effects reach from denaturation of proteins to vaporization of the tissue. Here, the thermal effects depend on the intensity, the duration of exposure and the absorption of the tissue. Due to the fact that biological tissues consist mainly of water (70. . . 80 %), the absorption of water decisively influences the penetration depth. Opto-acoustic effects are closely linked to thermal ones. For extremely short-duration high intensities (pulsed lasers), the heat conduction of the tissue is not fast enough to ensure a thermal diffusion process. Therefore, the tissue is spontaneously heated, so that the water is vaporized and rapid expansion occurs. Since this phase transition happens explosively, surrounding tissue is mechanically damaged (popcorn effect). Photochemical effects are based on the fact that monochromatic laser radiation can stimulate electronical pathways in molecules of the tissue. Due to this, chemical properties of the material exposed can be altered, and biological functions can be disturbed. Photochemical effects are highly dependent on the wavelength of the radiation. The damage of the skin in the infrared range is mainly caused by thermal effects and leads to localized burns. Continued exposure to low irradiances in the infrared range – insufficient to cause burns – can dilate blood capillaries and overload the natural cooling mechanism of the human body, resulting in inflammation. For wavelengths less than 500 nm, the photochemical effects become dominant. Here, repeated or long-time exposure even at low irradiances has to be considered, due to accumulation effects. Skin exposure to radiation in the UV-A range leads to pigmentation. UV-B radiation causes erythema and enhances the hazard for cancer, whereas radiation in the spectral UV-C band accelerates the aging of the skin and promotes skin cancer [80Sli, 89Sut, 97Hen]. Damage of the eye depends decisively on the wavelength of the radiation. The optical properties of the eye are illustrated in Fig. 6.1.4. Due to the fact that the cornea, the lens and the vitreous are transparent for radiation between 400. . . 1400 nm, the light is focused on the retina. For a pupil aperture of 7 mm, the light is focused to a diameter of about 10 µm on the retina, which is an amplification by a factor of 5 × 105 . These extremely high intensities cause severe damage of the retina up to a total loss of vision. Therefore, the spectral band between 400. . . 1400 nm has very low threshold limit values. Radiation of wavelengths less than 400 nm is absorbed by the cornea, and photochemical effects lead to cataracts, photoceratitis or photoconjunctivitis. Radiation above 1400 nm is mainly absorbed by water inside the eye, which reduces the penetration depth and leads mainly to burns of the cornea [80Sli, 89Sut, 97Hen].

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Retina

Cornea Microwave gamma radiation 400 nm - 1400 nm 300 nm - 400 nm < 300 nm > 2500 nm

Vitreous Optic nerve Iris Lens

Fig. 6.1.4. Cross-section of the human eye and optical properties [97Hen].

6.1.3.2 Threshold limit values and laser classification To avoid hazards related to laser radiation, threshold limits for exposure have been internationally established, below which it is believed to be safe. These limits, termed “Maximum Permissible Exposure” (MPE), are tabulated as a function of the wavelength and the exposure time. Due to the transmission properties of the eye and the severity of the retinal damages for radiation from 400 nm to 1400 nm, there are separate MPE’s for the eye and the skin (Fig. 6.1.5). In addition, source-size-dependent (collimated or extended source) and wavelength-dependent correction factors are included. MPE’s are given in the standard IEC 825 [96IEC] and standards derived from it. To simplify the hazard assessment related to a distinct laser system, the lasers are classified in the order in which the maximum intensity emitted by the laser can affect the human body. The threshold limit values are termed “Accessible Emission Limit” (AEL), see Fig. 6.1.5. Lasers are classified in 4 distinct laser classes, whereby Class 3 is subdivided into Class 3A and 3B. The AEL’s for each laser class are given in tabular form (IEC 825, CDRH) [96IEC], presented as a function of emission time and wavelength. Wavelength- and time-dependent correction factors are also included.

MPE

AEL

Maximum permissible exposure

Accessible emission limit

Skin

400nm-1400nm

Eye

Influencing variables: - wavelength - operational mode - irradiation geometry - time of irradiation

Laser classification

Fig. 6.1.5. Threshold limit values MPE and AEL [98Toe].

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Inherently safe under any condition. Very low emission of the laser device itself or enclosure of lasers with higher outputs. Covers only visible radiation. Possible eye hazard but safe for accidental momentary viewing. Eye protection normally by natural blink reflex (0.25 s). Eye hazard if magnifying viewing instruments are used to view the beam. For visible radiation, safe for momentary viewing (0.25 s). Hazard to the unaided eye. Viewing diffuse reflection may be safe. Exposure to concentrated beams can exceed the safety threshold limit for skin. Eye and skin hazard. Diffuse reflections may be hazardous. Possible fire hazard and danger of explosions.

6.1.3.3 Safety measures In dealing with the class of the laser system, particular technical as well as administrative safety measures have to be provided: – marking of lasers according to the laser classes (e.g. warning signs), – design of the laser (e.g. protective enclosures, safety interlocks, key control, warning light, beam attenuator), – definition of laser-controlled areas (for Class 3b and Class 4 lasers), – protective wear (e.g. protective eye wear), – education (e.g. laser safety officer, operator). Requirements for safety measures or measuring equipment for laser radiation are given in the standard IEC 825 and further standards more related to certain measures or devices (e.g. EN 31553, “Laser processing machines” [98EN1], DIN 5335 “Guards for laser workplace” [93DIN], EN 61040 “Power and energy measuring detectors” [92EN1] etc.). Besides, the qualification of certain safety measures has been subject of many investigations (e.g. screens for laser radiation [96Alu]). To prevent eye hazards in cases where there is the risk of being exposed to levels exceeding the MPE, protective filters have to be used. These filters reduce the irradiance to MPE levels or below by ensuring sufficient transmission in the visible range. These filters are used both within enclosures (Class 1) for viewing the laser material processing zone or for protective eye wear. Generally, two types of filters can be distinguished: absorbing filters and coated filters reflecting the radiation. The determination and calculation of necessary optical densities of the filters and further requirements on the properties of both for filters in enclosures and protective eyewear are addressed in e.g. IEC 825, EN 207, EN 208 [96IEC, 93EN1, 93EN2].

6.1.3.4 Hazard distances The assessment of hazards within a certain area exposed to laser radiation can be realized by determination of the distance over which a propagating laser beam is hazardous [80Sli, 89Sut, 97Hen]. The distance to the laser source, where the maximal irradiance of the laser beam is equal to the MPE, is termed “Nominal Ocular Hazard Distance” (NOHD). This concept includes different beam geometries (collimated beam/divergent beam, beam propagation beyond focal spot of focused beams, optical fibers and point or extended sources by direct or diffuse reflection). The area around a laser for which an eye hazard exists is called the “Nominal Ocular Hazard Area” (NOHA). The range of the NOHA can be narrowed down by suitable guards or screens. Laser hazards may not only arise from the direct beam, but also as a result of reflection of the beam from irradiated surfaces. The amount of reflected radiation depends on the reflectance Landolt-B¨ ornstein New Series VIII/1C

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level of the surface. The geometry of the reflected beam is also influenced by the surface. Specular reflecting plane surfaces redirect the beam, whereas curved surfaces may increase or decrease the reflected beam divergence. Diffuse reflections, caused by mat surfaces, redistribute the reflected radiation in all directions. Due to the complete change of beam geometry, the irradiated area on the surface must be considered as a secondary source of radiation (according to area size as point or extended source).

6.1.3.4.1 Specular-reflected beam In cases where an accidentally misdirected beam of a > Class 3 laser is specularly reflected, exposures can easily exceed the MPE’s. Due to the accidental situation, exposure times will usually be only a few seconds (respond of beam control etc.). Care must be taken to have proper beam alignment and a beam control [93Hie].

6.1.3.4.2 Diffuse-reflected beam The level of diffuse scattered CO2 -laser radiation in a typical environment of laser processing with a 2.5 kW laser is found to be low, under normal conditions [93Hie]. Due to the fact that diffuse scattered radiation can last over a long time if protective measures are not taken, longtime exposure (8 h) has to be considered. Nevertheless, in cases of misalignment and a highly reflective workpiece surface, high levels of radiation can occur, so protection is recommended. For Nd:YAG lasers, protection against scattered light is essential, despite the lower reflectance of most materials for 1.06 µm, because of the hazardous effects on the eye and the accordingly low MPE for 1.06 µm. When using excimer lasers, the diffuse scattered radiation can easily exceed the MPE. Total enclosures with UV-absorbing windows are therefore recommended [93Hie].

6.1.4 Secondary hazard potentials and safety measures 6.1.4.1 Laser system and components 6.1.4.1.1 Electrical safety Lasers are generally electrically powered. Depending on the laser system, respectively the electrical components, there are different hazards. Especially pulsed laser systems using high voltage for excitation give rise to serious hazards. These lasers (CO2 ) use large capacitor banks to store the electric charge. Precautions must be taken, since high voltage remains even after the system has been switched off and disconnected from the main power supply. During maintenance, the safe discharge of capacitors (short-circuited, grounding) and the prevention of a direct contact between charged parts and the human body (e.g. interlocks) have to be ensured [80Sli, 89Sut, 97Hen]. Furthermore, the generation of electromagnetic fields can lead to potential hazards. Many laser systems use High electromagnetic Frequency fields (HF) for excitation, some also generate radiation in the Low Frequency (LF) and microwave region. In cases of insufficient shielding, electromagnetic radiation is emitted and can lead to health risks for operators. To prevent harmful radiation, parts emitting Radio Frequency (RF) radiation must be shielded by a safely grounded electrically conducting grid or net (Faraday cage). Exposure limits for LF/HF radiation are given Landolt-B¨ ornstein New Series VIII/1C

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in norms and standards (e.g. German DIN VDE 0848, Part2). Maximum permissible interference levels for technical components are given in the European standard EN 55011 [98EN2]. Requirements on the compatibility with other devices concerning electromagnetic radiation come from the Electromagnetic Compatibility Directive (89/336 EEC) and norms and standards (e.g. IEC 801) [89EEC3]. General requirements for the safety of electrical components of machines are addressed within EN 60204 “Safety of Machinery – Electrical Equipment for Machines” [93EN3].

6.1.4.1.2 Optical components Under normal operation conditions, hazards from materials used for laser optics do not arise. However, in cases of misalignment or pollution of the optics, absorption of the laser radiation can rapidly increase, resulting in damage of the optics [95Kre]. If optical components made of gallium-arsenide or zinc-selenide are used, absorption can lead to evaporation and bursting of the optics. Due to chemical reactions (> 200◦ C) with the ambient air, arseniuretted hydrogen (arsine) respectively hydrogen selenide can be generated. Both gases are toxic and have to be immediately extracted from the workplace to avoid health risks. The remaining particles (fumes, dust) contain toxic arsenic or arsenic oxides, respectively selenium or selenium oxides, and must also be handled with care (using personal protective equipment). Furthermore, a minor hazard results from the handling of arc and flash lamps used for optical pumping of solid-state lasers. During maintenance (replacement of lamps), exploding lamps can cause injuries, and protective wear is recommended.

6.1.4.1.3 Laser gases Laser systems can contain hazardous gases or liquids. In particular, excimer lasers shall be concentrated on, which use inert-halogen gas mixtures. In cases of leakage, the fluorine or chlorine can contaminate the workplace (as molecular halogen or hydrogen halogenide). Hazards can be easily assessed by calculating conditions for a spontaneous leakage in the halogen gas supply equipment. Assuming a laser cabin of 10 × 10 × 5 m3 (500 m3 ) the German threshold limit value for fluorine “Maximale Arbeitsplatz-Konzentration” (MAK) of 0.1 ml/m3 (also peak limit 0.1 ml/m3 according to TRGS 900) [03TRG] means a maximum of 50 ml fluorine in the workplace air. Typically the halogen gas mixture contains 5 % fluorine (rest He), which correlates with a maximum permissible leakage gas volume of 1 l. Therefore, the room or cabin where the gas supply and the laser installation are located should be equipped with gas sensors activating warning lights, emergency extraction systems and automatically controlling safety valves, which stop the gas supply. Leakage of the resonator (e.g. mirror sealings) generally causes a creeping pressure loss (about 1000 Pa/h). Since the halogen concentration is considerably lower than in the halogen gas supply and usually extraction systems are integrated in the laser system, this failure is of secondary meaning to a leakage in the gas supply. In comparison, gas leakage for CO2 lasers is less critical than for excimer lasers, due to the threshold limit value for CO2 in the workplace air, which is magnitudes higher (CO2 : 5000 ml/m3 ). Dye lasers can emit volatile substances in cases of leakage. The solvent fumes are highly inflammable, and ignition by heat, sparks or open flames must be avoided. Precautions must be taken, due to the respirability of the substances and their toxic effects on persons.

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6.1.4.1.4 Handling devices The development of lasers with increasing output powers and excellent beam properties will result in constantly increasing processing velocities. Parallel to this, the technical progress of handling devices (2-D, 3-D) leads to higher positioning velocities. Hazards related to handling devices mainly result from these high feed rates. At present, typical feed rates of handling devices industrially used are several 10’s of meters per minute, and devices exceeding 100 m/min. are already entering the market [92Dau, 94Pre]. In cases of malfunctions or striking objects within the path of the devices, high deceleration values cause high mechanical stresses. This leads to high damage rates or – if people are affected – to severe injuries or the death of person. Moreover, insufficient protection of even slow moving components can cause severe bruises in cases of accidents. Due to the severity of possible accidents, special care must be taken to control both the function and position of the device as well as the area of movement, to avoid uncontrolled access of persons. Depending on the distinct application, sufficient covering of moving parts and mechanical barriers, light barriers, camera systems etc. which provide rapid emergency stops are adequate protective measures. Norms and standards related to handling components of machinery are arranged under the basic Machinery Directive and the EN 292 “Safety of Machinery”: – EN 418: “Safety of Machinery – Emergency Stop Equipment” [92EN2], – EN 349: “Safety of Machinery – Minimum Gaps to avoid Crushing or Parts of the Human Body” [93EN4], – EN 294: “Safety of Machinery – Safety Distances to Prevent Danger Zones being Reached by Lower Limbs” [93EN3] etc.

6.1.4.2 Secondary radiation The main emphasis shall be placed upon the non-coherent UV and blue-light radiation, due to their capability of causing eye hazards, and upon UV-radiation below 242 nm which generates the toxic gas ozone or other harmful substances. This secondary radiation is formed during laser processing by ionized substances (e.g. metal vapor or shielding gases), due to the very high temperatures in the interaction zone. Assessing the hazards related to secondary radiation, the bio-effects are more complex than for monochromatic radiation due to the broad frequency band from the UV to the IR. Focussing on the UV to blue-light radiation, the bio-effects are mostly photochemical in nature (photoceratitis, photoconjunctivitis, cataracts). Due to the low threshold limits in this range (TLV by ACGIH [98ACG]) and cumulative effects, the radiation (UV-C, -B, -A to blue light) may reach critical values within certain periods (Table 6.1.1) [95Hur]. Measurements for laser welding at a typical operating distance (0.5 m) show, in comparison to conventional arc welding, low total levels of radiation, but they can exceed levels typical for Tungsten Inert Gas (TIG) and Manual Metal Arc (MMA) welding. The secondary radiation increases with the laser power, and the choice of assist gas remarkably effects the radiation levels. As shown in Table 6.1.1, the maximum permissible exposure times for laser welding can be reached within seconds. Therefore, especially for high-power laser processing (welding, punching operations), UV-A and blue-light absorbing screens and protective eyewear are recommended [95Hur]. Ultraviolet radiation below 242 nm also generates ozone by dissociating oxygen in the interaction zone. Ozone is a toxic gas which has to be extracted out of the working area to avoid health risks (see also Sect. 6.1.4.4). Landolt-B¨ ornstein New Series VIII/1C

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Table 6.1.1. Secondary radiation from laser welding [95Hur]; UVeff : effective irradiance of all UVradiation, tUV : time to reach threshold limit for UV-radiation, blueeff : effective irradiance of blue light, tblue : time to reach threshold limit for blue light, butt: butt joint, BOP: bead on plate, T: T-seam, ms: mild steel, ss: stainless steel. Laserpower [kW] 9 9 9 8 2.5 2.5 2.5 2.5 9 7 6 10

Weld type butt BOP BOP butt butt butt butt butt T butt butt butt

Material

Shielding gas ms He ss He ss He ss Ar ms Ar ms He ss Ar ss He ms He AlMg5 He AlMg4.5Mn Ar/He Titanium He

UVeff [W/m2 ] 3.0 6.6 3.8 19.3 2.5 2.2 2.4 1.3 6.7 0.3 3.6 7.0

tUV [s] 10.0 4.5 8.0 1.6 12.0 13.6 12.6 22.7 4.5 100.0 8.3 4.3

blueeff [W/m2 ] 1.0 5.0

tblue [s] 100 20

5.6 0.6 0.4 0.5 0.7 1.2 0.1 0.8 2.0

18 180 220 220 400 83 1000 1300 50

6.1.4.3 Explosive atmospheres and fire hazards The use of high-power lasers always implies latent fire hazards. Demands on suitable materials for guards, screens, medical instruments, covering materials etc. have already been mentioned in Sect. 6.1.3. Care must be taken that no inflammable materials (except a minimum for use) are inside the area the laser beam can affect, under normal and faulty conditions. Special caution is required for laser applications in oxygen-enriched atmospheres, due to the higher ignition threshold of materials in this atmosphere. In this context, it shall also be pointed out that fire hazards can as well result from the filter systems for gases and particulates. Melt spatter or igniting organic gases can cause a smouldering fire of the filter material. To avoid hazards, deflector plates for melt spatter, a sufficient distance between process and filter and frequent cleaning of deposits from the extraction pipe are suitable measures. If explosive mixtures are inside the laser areas of Class 3B and Class 4 lasers, explosion protection must be ensured. Requirements are given in the explosion protection guidelines (e.g. EX-RL) [86ZH1].

6.1.4.4 Emission of gases and fumes 6.1.4.4.1 Characteristics of laser-generated air contaminants The physical basis of “Laser-Generated Air Contaminants” (LGAC) is the absorption of laser radiation in the interaction zone. Depending on the wavelength and the material properties, the energy is transformed into thermal energy or can cause direct disbonding. As a result, the material is partially melted, vaporized or sublimated and leaves the interaction zone at high velocities. Due to the thermodynamic conditions, the by-products can immediately condense into fumes or remain in the gas phase as Volatile Organic Compounds (VOC’s). If vaporization takes place in layers below the surface, the gas pressure mechanically “explodes” the material or tissue in the upper layers, and the fragments accelerate to reach high velocities.

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The quantity, the kind (gaseous, particulate) and the chemical composition of the emissions generated depend mainly on the material processed, the process parameters and the process atmosphere [01Haf, 02Bar]. While VOC’s are generally airborne, the size and weight of aerosol particles determines their behavior. For usual air speeds at ambient air temperature at the workplace from 0.1 m/s to 3 m/s, particles smaller than 10 µm can be regarded as airborne [93VDI]. Molten pearls or larger fragments sediment out, due to their size and weight near the processing zone. Apart from these material-specific by-products, process-specific emissions like ozone and nitrogen oxides can be generated, independent of the material processed in certain temperature and radiation ranges. The toxic gas ozone is generated mainly by secondary radiation in the UV-range (120. . . 242 nm). Since it is thermodynamically unstable, ozone is reduced to oxygen at high temperatures. Moreover, aerosol particles generated by the laser process intensify the reduction as a catalyser. The toxic nitrogen oxides are generated at temperatures above 1000 K by oxidation of nitrogen. For industrial applications, three groups of applications can be identified, based on typical emission rates (Fig. 6.1.6) [98Haf2]. Cutting applications cause the highest emission rates. Applications with moderate emission rates are, e.g. welding, cladding and material removal. Emission rates below 1 mg/s are characteristic for laser marking and micro-machining. With the progressive use of laser systems with higher kW powers (greater than 10 kW) it can be expected that the emission rates will increase with increasing cutting speeds.

Aerosols Hydrocarbon Nitrogen oxides Ozone

100 10

0.1

10

-6

10

-7

Micromaching

-5

Marking (metals)

10

Marking (plastics)

-4

Cladding

10

Material removal

-3

Welding

10

Cutting (metals)

0.01

Cutting (plastics)

Rate of emissions [mg/s]

1

Fig. 6.1.6. Emission rates for different laser processes [98Haf2].

When laser processing metals, aerosols are the main emission products. Nitrogen oxides and ozone are two orders of magnitude smaller [98VDI]. Investigations of the fume particle morphology show that for all materials and tissues processed, the fume particles are of a spherical shape. Most of the particles, about 90 %, are smaller than 1 µm aerodynamic diameter (Figs. 6.1.7 and 6.1.8) [97VDI, 98VDI]. They exist as single particles or as agglomerates. The smallest single particles have geometric diameters of about 5. . . 10 nm. The properties of the particles are due to the formation process by vaporization and condensation.

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AlCuMg2 Ti99.4 RST37-2 X5CrNi189 St02Z275NA ST14.03

50

Relative mass frequency [%]

471

40

30

20

10

0 0

0.03

0.06

0.13

0.25

0.50

1.0

2.0

4.0

8.0

16

Aerodynamic particle diameter [µm]

Fig. 6.1.7. Size distributions of aerosols for cutting metals [98Haf2].

Pa6 PE,N PE PC PP,N PP PVC PS,N PMMA

Relative mass frequency [%]

60

40

1 nm

20

0 0

0.03

0.06

0.13

0.25

0.50

1.0

2.0

4.0

8.0

16

Aerodynamic particle diameter [µm]

Fig. 6.1.8. Size distributions of aerosols for cutting plastics [97Haf].

Because of their size, the fume particles are highly respirable (more than 80 % according ACGIH [98ACG, 93Rei]). When processing metals, fumes mainly consist of metal oxides and elements with low evaporation temperatures (e.g. Zinc, Manganese) are overrepresented compared to the chemical composition of the material processed [98VDI]. Landolt-B¨ ornstein New Series VIII/1C

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300 250 200 150

Aerosole Hydrogenchlorid

100

80 PE, N PE PP, N PP PS, N PS/SB PA6 PC PVC PMMA

60

40

100 50 0

20

laser beam power

350

Total Hydrocarbons

thickness

120

cutting speed : v f = 2 m/min processing gas : synthetic air processing : 0.05 MPa gas pressure

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material

400

Emission rate [mg/s]

Concentration in the exhaust air [mg/m 3 N ]

472

[mm] 3 3 6 3 4 3 3 3 3 3

[W] 750 750 750 750 630 630 775 630 450 500

0 PA 6

PE,N

PE

PC

PP

PP,N

PVC PS/SB

PS,N PMMA

Fig. 6.1.9. Emission rates for laser cutting of plastics [98Haf2]. The concentration in the exhaust air refers to normal conditions, i.e. 1013 hPa and 273.15 K.

During laser processing of organic materials (polymers, wood, tissue), apart from aerosol emissions, a large number of different volatile hydrocarbon compounds is also emitted [98VDI, 00Haf, 00Haf2]. As shown in Fig. 6.1.9, plastics can be roughly divided into two groups [02Bar]: – materials which mainly emit aerosols, – materials which are mainly pyrolyzed into gases. Hydrocarbon compounds are generated by chemical fragmentation of the organic molecules and the following chemical reactions. Fragments (e.g. belonging to the groups of olefines, acrylates, aldehydes, aromates etc.) usually refer to the chemical structure of the material processed [03Bar]. Polystyrene (PS) and polymethylmethacrylate (PMMA) typically represent polymers, which are broken down (retropolymerized) into their monomers styrene resp. methylmethacrylate. In most cases, stable compounds like aromatic substances (benzene, toluene, xylene) and polycyclic aromatic hydrocarbons (PAH) are generated [98VDI, 00Haf2, 01Haf]. Investigations of the particle morphology show that again most of the fume particles, about 90 %, are smaller than 1 µm (aerodynamic diameter). For some plastics, like PA 6 or woods, the particles are viscous and can easily stick together [95Fri, 97Haf, 98VDI, 01Haf]. To assess the hazards resulting from emission products and to plan safety measures, the user of a laser installation is obliged by laws to carry out a hazard analysis. According to this, all emissions have to be characterized and compared with threshold limit values for the contamination of the workplace. Especially, when processing organic materials, characterization can comprise more than 50 single compounds. Concerning the aerosols, the hazard potential of the LGAC’s arises from the respirability of the fume particles (particles in the range of 0.05. . . 1 µm remain from 40. . . 60 % in the alveoli and are taken in by the so-called cleaning cells “alveolar macrophages”) and their morphological as well as their chemical characteristics [95Mal]. Many substances emitted during laser processing are known as toxic (Mn, Zn) or as carcinogenic (Ni, Co, particle-bounded PAH) in industrial medicine. Especially when cutting stainless steel, the toxic soluble chromium (VI) and the carcinogenic insoluble chromium (VI) have to be considered [95Leu, 98VDI]. For medical applications, the possibility of virus-DNA-fragments, transported with particles, is a significant hazard potential, if they infect tissues [97Wol2, 97VDI]. Gases and volatile organic components (VOC) are inhaled and

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affect the respiratory tract, respectively the mucous membranes. Due to their interactions with the organism, most of the substances emitted are allergic, toxic or carcinogenic. To avoid health risks for the laser operators, contamination of the workplace atmosphere must be reduced well below threshold limit values (TLV) given by occupational medicine. It is not possible to conclude the contamination of the workplace atmosphere from the emission characterizations. A value which allows the comparison of different processes and which gives answers to extraction systems to be installed is the “Nominal Hygienic air requirement Limit value” (N HL) [73IIW]. The N HL describes the amount of fresh air to be supplied to keep the threshold limit values for air contaminants: 
 A B 3 N HL = m ˙ G · 3600 · + + ... [m /h] (6.1.1) T LVA T LVB with mG A, B, . . . T LVA , T LVB , . . .

emission rate [mg/s], emission rate of each component related to mG , threshold limit value of each component [mg/m3 ].

Assuming a homogeneous distribution of the LGAC’s in a typical workplace area with a volume of 1000 m3 , the air exchange rate for, e.g. laser cutting mild steel, is larger than 10, for stainless steel larger than 20 and for plastics more than 50 [00Haf3]. The air exchange based on natural ventilation (windows, doors) is about 0.5 [98Haf2]. Forced ventilation systems grant an exchange rate from 5 to 10. However, it has to be taken into account that a high exchange rate significantly increases the costs for heating in winter. From this data it can be concluded that direct capturing of the emissions near the point of generation is most efficient to minimize the work-air contamination [00Haf].

6.1.4.4.2 Extraction systems To avoid critical contamination of the workplace atmosphere, efficient extraction and additional room ventilation systems are necessary. In general capturing devices can be divided according to VDI 2262 into 3 types [93VDI]: – open design (e.g. capture hood, extractor tube), – semi-open design (e.g. extraction stand, extraction table), – enclosed design (e.g. encapsulation, working cabin). In all cases, the capturing device has to be positioned as near as possible to the point of the emission generation, to ensure entire removal of the emissions. Dependent on the laser application, different types of capture systems can be used [98Haf2]: – capture hoods, extractor tube, working-head-integrated system, – table extraction system, – total enclosures/cabins. Open systems like capture hoods can only be satisfactorily used for applications where there is a constant convectional flow (e.g. welding, ablating). Working-head-integrated capture systems are favorable for 3-D applications (e.g. welding, surface treatment) but the stand-off distance of the head to the workpiece has to be well-controlled to ensure both effective shielding and efficient extraction [98VDI]. Extractor tubes are typically used for medical applications. The design (funnel, half-funnel, tube) of the capture inlet depends on the procedure (endoscopic or open surgery) [97VDI]. Semi-open systems are used for applications where a directional emission flow is present. During laser cutting, most (greater than 90 %) of the emissions are released below the workpiece, due to Landolt-B¨ ornstein New Series VIII/1C

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the processing gas stream [98Haf2]. Therefore, the table-integrated capture system allows a high capture efficiency for adequate extraction air volumes adjusted. Advantageous – to reduce volume flow and heat losses in winter – are small moving systems integrated in the table, which can be synchronously positioned to the movement of the laser head [00Haf]. Alternatively, if the building design permits, an enclosed working area with an emission extraction system provides the best solution, where the enclosure also isolates the operator from laser and secondary radiation hazards. It must be taken into account that the capturing efficiency also depends on the pressure loss of the filtering device. If the filter system is clogged, efficiency will drop rapidly, and contamination of the working atmosphere increases.

6.1.4.4.3 Filtration When passing the extracted air to the environment, high demands are put upon filtration systems, due to the complex mixtures of very small aerosols, gases and volatile organic components and their properties (toxicity, carcinogenity, infection). When processing organic materials, generally filtration must be divided into a pre-filtration system for aerosols and a filtration system for volatile organic components. An overview of qualified types of filtration techniques depending on industrial applications and materials processed is given in Table 6.1.2 [98Haf2]. Table 6.1.2. Proven techniques for different laser applications and materials processed [98Haf2]. Application cutting welding material removal cladding marking micromaching ∗

Material metals org. materials metals metals org. materials metals metals org. materials metals org. materials

Filtration of aerosols self-cleaning filter self cleaning/fabric filter, washer∗ self-cleaning filter self-cleaning filter self cleaning/fabric filter, washer∗ self-cleaning filter fabric filter fabric filter fabric filter fabric filter

Filtration of gases not necessary catalytic, biological, adsorptive∗ not necessary not necessary catalytic, biological, adsorptive∗ not necessary not necessary adsorption not necessary adsorption

Specific filtration technics necessary, depending on application and material processed.

For airborne non-adhesive aerosol emissions (fumes) resulting from the processing of metals, inorganic materials (ceramics, glasses) and some types of organic materials, self-cleaning surface filters provide high separation efficiencies (greater than 99.995 %) and stable pressure losses. For applications with high aerosol rates (e.g. cutting, welding) storage filters will be clogged within a short operation time, and filters have to be changed frequently. Storage filters are therefore only suitable for applications with low aerosol emission rates such as marking or micro-machining. If adhesive particles which can be generated during the cutting of plastics and woods are present, then self-cleaning surface filters will also become clogged. For these applications, special solutions where the filter is continuously pre-coated with lime powder have to be considered [98Haf2, 98VDI]. For medical applications, mostly fabric pre-filters with separation efficiencies greater than 99.995 (High-Efficiency Particulate Air (HEPA) filters) and secondary Ultra-Low-Penetrating Air (ULPA) filters are suitable for separating aerosols [97Wol1]. In all cases, the contaminated filters, respectively the separated material must be disposed of. Especially for medical applications when filters are contaminated with carcinogenic or infectious substances, special treatment of the filters (e.g. disinfection, handling) is needed [97Wol1, 97VDI]. Landolt-B¨ ornstein New Series VIII/1C

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Laser processing of organic materials (plastics, wood, paper, human tissue) requires gas filtration to keep at or below permissible threshold limit values and to avoid bad odor. For these applications, no standard solutions can be used. In general, either – adsorption technique, – catalytic combustion or – biological filtration can be applied. The choice depends on the laser application, the properties of the filter system, the space available and the costs [00Haf2]. Adsorbing charcoal filters are mostly used, but there are effects (e.g. desorption during laser off-times, displacement of high-volatile compounds) which impair performance and which have to be controlled during operating and non-operating times [03Bar]. Systems based on catalytic combustion are useful for complex mixtures of VOC’s, but due to the energy costs, such systems are not suitable for a stand-alone laser installation. Biological air-cleaning systems based on the bio-trickling method (liquid nutrient solution) show good results for reducing LGAC’s and bad odor. The advantage of the bio-filtration systems is the low pressure loss in comparison with conventional bio-filters working with shredded bark, cleaning adaptability and the ease of control during non-operation [98Haf1, 00Haf2].

6.1.5 Risk assessment The assessment of (non-avoidable) safety and health risks at the workplace is a requirement on the employers which is defined in the framework of the European Directives (e.g. 89/391 EEC, 88/642 EEC) [89EEC1, 88EEC]. There are many different approaches to risk assessment (“WhatIf-Method”, “Failure Mode and Effect Analysis – FMEA”, “Hazard and Operability Study – HAZOP”, “Fault Tree Analysis – FTA” etc.) [95Gre, 97Hen]. Since the discussion of method details would go too far, some fundamentals shall be focused on. The risk factors to be considered in any assessment might include: – – – – – –

likelihood of incidents occurring which could cause harm, likelihood of harm being caused, should an incident occur, harm, extent of harm (number of people who could be affected), exposure period of hazard, exposure frequency.

Risk assessment should identify which factors have to be addressed to reduce the risk to an acceptable level. Records should be kept to document the factors for any particular hazard being assessed. They are the basis for structured judgement and for comparisons with future re-assessment. Risk assessment of a laser installation should both fulfill the regulatory duty and provide a useful basis for product or process improvements. It must include three major areas according to IEC 825-1: – capability of the laser/laser system to injure personnel, – environment in which the laser is used, – level of training of personnel operating the laser or those who may be exposed to this radiation.

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6.1.6 Training and education

[Ref. p. 477

The laser risk assessment process should at least cover the following distinct stages: – establishing background information (laser, environment, norm-standards etc.), – identify hazards in distinct areas (laser process, laser installation, beam delivery), – perform risk assessment. In Europe, some methods have been tested for the suitability of laser-processing risk assessment [95Gre, 97Hen]. Moreover, software programs have been developed to facilitate risk management using computers [95Tyr].

6.1.6 Training and education Training and education in laser safety are of decisive importance to ensure safe utilization of laser technology. The international safety standard IEC 825 [96IEC] requires that all persons controlling laser systems of Class 3A, Class 3B and Class 4 (German VBG 93 [95VBG] additional: “operate Class 2 to Class 4 laser systems or have access to Class 3B or Class 4 laser safety areas (“Nominal Ocular Hazard Areas” (NOHAs))” have to be trained to an appropriate level. The training should include at least the following items: – – – – –

familiarization with systems operating procedures, proper use of hazard control procedures, warning signs etc., need for personal protection, accident reporting procedures, bioeffects of the laser upon the eye and the skin.

The standard also establishes the need for a Laser Safety Officer (LSO) within an organization where Class 3B and Class 4 laser systems are in use. According to the IEC 825, the LSO is the person, “who is knowledgeable in the evaluation and control of laser hazards and has responsibility for oversight of the control of laser hazards”. According to the VBG 93, the duties of the LSO are partially reduced (responsibilities are firstly focused on the employer) and defined as follows: – supervision of the operation of laser installations, – assistance of the employer with regard to safe operation and necessary safety measures, – co-operation with specialists for workplace safety and training on important affairs of laser protection. More detailed guidance on the duties of the LSO and safety management than in the IEC or EN standards is given by the American standard ANSI Z136.1. Also, the ANSI Z136.1 provides more guidance to the training of LSO and laser users. However, the ANSI Z136.1 tends towards describing the role of the LSO as a more executive one [96IEC, 93ANS]. In addition to the “laser-specific duties” of training and education, the framework of the European Directives 89/391/EEC (transferred to national norms) also obliges the employer to provide to employees adequate health and safety training. Beneath general instructions for workplace safety, training and education for dealing with hazardous substances (which can be generated during laser processing) should be an integral part [89EEC1]. Worldwide, many institutes are involved in the development and optimization of training and education, e.g. summarizing minimum criteria for LSO courses, optimization and harmonization of syllabuses for the education of laser staff, e.g. [93Bel, 97Ray, 97Smi].

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References for 6.1

477

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Welling, H., Gerhard, H., G¨ uttner, A.: Z. Phys. 253 (1972) 113.

73IIW

Swedish Delegation: Draft standard for the classification of electrodes for manual welding according to the kind and composition of their fumes, IIW-Document: VIII-543-73.

75Wel

Welling, H., Schr¨ oder, H.W., Wellegehausen, B.: Spectrosc. Lett. 8(9) (1975) 685.

80Che 80Sli

Chemikaliengesetz (ChemG) vom 16.09.1980, BGBl.I (1980) 1718. Sliney, D., Wolbarsht, M.: Safety with lasers and other optical sources, Plenum Press, 1980.

85CFR

CFR 50 (161) 1085: Food and Drug Administration/Center for Devices and Radiological Health: Performance standard for laser products, 1985.

86TAL

Technische Anleitung zur Reinhaltung der Luft (TA-Luft) vom 04.04.1986, GMBl (1986) 202. ZH 1/10: Explosionsschutz-Richtlinien (Ex-RL): Richtlinien f¨ ur die Vermeidung der Gefahren durch explosionsf¨ ahige Atmosph¨aren. Carl Heymanns, 1986.

86ZH1

88ANS 88EEC

89EEC1 89EEC2 89EEC3 89OSH1 89OSH2 89Sut

ANSI Z-136.2: Safe use of optical fiber communications systems utilizing laser diode and LED sources, 1988. 88/642/EEC: Directive of the European Economic Community: Protection of employees against hazards by chemical, physical and biological substances at work, 1988. 89/391/EEC: Directive of the European Economic Community: Introduction of measures to encourage improvements in the safety and health of workers at work, 1989. 89/392/EEC: Directive of the European Economic Community: Machinery Directive, 1989. 89/336/EEC: Directive of the European Economic Community: Electromagnetic Compatibility Directive, 1989. Occupational Safety and Health Administration (OSHA): Respiratory protection, 29CFR 1910.134, 1989. Occupational Safety and Health Administration (OSHA): Toxic and hazardous substances, 29CFR, subpart Z, 1989. Sutter, E., Schreiber, P., Ott, G.: Handbuch Laser-Strahlenschutz, Berlin: Springer, 1989.

90BIM

Bundesimmissionsschutzgesetz (BImSchG) vom 14.05.1990, BGBl.I (1990) 880.

91OSH

Occupational Safety and Health Administration (OSHA): Guidelines for laser safety and hazard assessment, Pub 8.-1.7, 1991.

92Dau

Dausinger, F., H¨ ugel, H.: Nutzungspotentiale von Lasern in der Blechbearbeitung, B¨ander/Bleche/Rohre, 7/1992. EN 61040: European Standard: Power and energy measuring detectors – Instruments and equipment for laser radiation, 1992. EN 418: European Standard: Safety of machinery – Emergency stop equipment, functional aspects, 1992.

92EN1 92EN2

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92EN3

EN 294: European Standard: Safety of machinery – Safety distances to prevent danger zones being reached by upper limbs, 1992.

93ANS 93Bel

ANSI Z-136.1: American National Standard: Safe use of lasers, 1993. Bellido, F., Montejo, J.F., Botts, M., Engstrom, H., Garcia, J., Green, M., Laitinen, H., Woolnough, R.: Proc. Industrial Laser Safety Forum ’93, United Kingdom (1993) 245. DIN 5335: German Standard: Screens for laser work places, 1993. EN 207: European Standard: Personal eye-protection, filters and eye-protection against laser radiation, 1993. EN 208: European Standard. Personal eye-protection, eye-protectors for adjustment work on lasers and laser systems, 1993. EN 60204-1: European Standard: Safety of machinery, Electrical equipment of machines, 1993. EN 349: European Standard: Safety of machinery – Minimum gasps to avoid crushing of parts of the human body, 1993. Hietanen, M., Schr¨ oder, K., Meijer, J.: Proc. Industrial Laser Safety Forum ’93, United Kingdom (1993) 115. Reist, P.C.: Aerosol science and technology, McGraw-Hill Inc., 1993. VDI 2262: VDI-Guideline: Workplace air, reduction of exposure to air pollutants, 1993. Welling, H., Mitschke, F., Steinmeyer, G.: In: Walter, H., Koroteev, N., Scully, M. (eds.): Frontiers in nonlinear optics – The Sergei Akhmanov Memorial Volume, IOP Publishing (1993) 240. Welling, H., Mitschke, F., Steinmeyer, G., Ostermeyer, M., Fallnich, C.: Appl. Phys. B 56 (1993) 124.

93DIN 93EN1 93EN2 93EN3 93EN4 93Hie 93Rei 93VDI 93Wel1

93Wel2

94Pre

Preißig, K.-U., Petring, D., Herziger, G.: Proc. SPIE 2207 (1994) 96.

95EN 95Fri

EN 292: European Standard: Safety of machinery, 1995. Friedrich, G., St¨ ahle, H.J.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 257. Green, M., Tozer, B.A.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 23. Hurup, K., Glandholm, A., Hietanen, M., Nandelstadh, P. von, Schr¨ oder, K.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 91. Kreutz, E.W., Dahmen, M., Hass, C.R., Wesner, D.A.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 83. Leummens, M., Herber, R.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 105. Malkusch, W., Rehn, B., Bruch, J., Hechler, B.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 115. Tyrer, J., Vassie, L.: Proc. Industrial Laser Safety Forum ’95, Denmark (1995) 39. VBG 93: Unfallverh¨ utungsvorschrift der Berufsgenossenschaften: Laserstrahlung, Fassung von 10/95, 1995.

95Gre 95Hur 95Kre 95Leu 95Mal 95Tyr 95VBG

96Alu 96ANS 96EN 96IEC 96Nie 96Wel

Alunovic, M., Kreutz, E.W.: Abschirmungen an Laserarbeitspl¨ atzen, Wirtschaftsverlag NW, 1996. ANSI Z-136.3: Safe use of lasers in the health care environment, 1996. EN 60601-2-22: European Standard: Medical electrical equipment. Particular requirements for the safety of diagnostic and therapeutic laser equipment, 1996. IEC 845: International Electrotechnical Commission: Safety of laser products, 1996. Niemz, M.: Laser-Tissue Interactions, Berlin: Springer, 1996. Wellegehausen, B., Welling, H., Momma, C., Feuerhake, M., Mossavi, K., Eichmann, H.: Opt. Quantum Electron. 28 (1996) 267.

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00Haf 00Haf2 00Haf3

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Haferkamp, H., Alvensleben, F. von, Seebaum, D., Goede, M., P¨ uster, T.: Proc. International Laser Safety Conference ’97, USA (1997) 209. Henderson, R.: A guide to laser safety, Chapman&Hall, 1997. Raymond, E.A., Tyrer, J.R.: Proc. International Laser Safety Conference ’97, USA (1997) 338. Rockwell, B.A., et al.: Proc. International Laser Safety Conference ’97, USA (1997) 159. Rockwell, R.J.: Proc. International Laser Safety Conference ’97, USA (1997) 564; Laser Accident Database, LaserNet Homepage, http://www.rli.com. Smith, J.F., Jones, J.E.: Proc. International Laser Safety Conference ’97, USA (1997) 354. VDI-TZ Physikalische Technologien: Bewertung von Abbrandprodukten bei der medizinischen Laseranwendung, 1997. W¨ ollmer, W.: Proc. International Laser Safety Conference ’97, USA (1997) 373. W¨ ollmer, W.: Proc. International Laser Safety Conference ’97, USA (1997) 383. American Conference of Governmental Industrial Hygienists (ACGIH), Threshold limit values for chemical substances and physical agents and biological exposure indices, 1998. EN 31553: European Standard: Laser and laser-related equipment, 1998. EN 55011: European Standard: Industrial, scientific and medical radio-frequency equipment, radio disturbance characteristics, 1998. Gefahrstoffverordnung (GefStoffV), Fassung vom 12.06.1998, BGBl.I (1998) 1286. Haferkamp, H., Alvensleben, F. von, Goede, M., Barcikowski, S., Daniel, S.: Z. Wasser, Boden, Luft (WLB), Februar 1998. Haferkamp, H., Bach, Fr.-W., Goede, M., P¨ uster, T., Seebaum, D.: Emissions generated during laser cutting and safety precautions, IIW-Document I-1040-96 41(3) 1998. Robinson, K.: Photonics Spectra, October 1998 (1998) 92. T¨onshoff, H.K., Lubatschowski, H., Goede, M.: Vortragsband 25, Sicherheitsfachtagung Krankenhaus, Medizinische Hochschule Hannover (1998) 143. VDI-TZ Physikalische Technologien: Sicherheitstechnische und medizinische Aspekte bei der Laserstrahlmaterialbearbeitung, 1998. Haferkamp, H., Bunte, J., Barcikowski, S.: Furnier Magazin, Dezember 2000 (2000) 18. Haferkamp, H., Goede, M., P¨ uster, T., Barcikowski, S.: Proc. USC-TRG Conf. on Biofiltration 2000, Los Angeles, CA, USA, 19.–20. Oct. 2000 (2000) 115. Haferkamp, H., Goede, M., Barcikowski, S., Feld, A.: Joint International Congress and Exhibition “Electronics Goes Green 2000+”, Berlin, 11.–13. Sept. 2000, Proc. Vol. 1, Technical Lectures, (2000) 613.

01Haf

Haferkamp, H., Goede, M., Barcikowski, S.: LaserOpto 04/01, August 2001 (2001) 68.

02Bar

Barcikowski, S., Goede, M., Ostendorf, A.: Proc. HPL 2002, 1st International Symposium at LAMP 2002, 27.–31. May 2002, Osaka, Japan (2002) 459.

03Bar

Barcikowski, S., Bunte, J., Haferkamp, H., P¨ uster, T., Sattari, R.: Proc. Expo Laser 2003, March 2003, Ancona, Italy (2003) 124. Technische Regeln f¨ ur Gefahrstoffe (TRGS), Kissing: WEKA Fachverlag, Februar 2003.

03TRG

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