632.8-nm visible region waveguide polarizer fabricated by proton exchange

SCIENCE IN CHINA (Series A )

VOI. 40 NO. 7

JUIY1997

632.8-nm visible region waveguide polarizer fabricated by proton exchange FENG Kecheng (?3~$%),LI Ling ($

?&)* , JIAO Wentao

and WANG Zhaomin ( 3%

(%%%)

)

(Department of Optics Physics, Institute of Optics and Fine Mechanics, Changchun 130022, China) Received January 10. 1997 Abstract Optical waveguide polarizer at 632. 8 nm was fabricated for th'e first time using proton exchange Ti: LiNb03. The setup measuring the characteristic parameters of the polarizer was given. The extinction ratio of the polarizer was theoretically analyzed and calculated by means of physical optics and dispersion theory for waveguide. Various factors affecting the device performance are analyzed. Theoretical calculations are in good agreement with the experimental results. Keywords: waveguide.

proton exchange, optics waveguide polarizer, polarization extinction ratio, dispersion theory for

With the development of science and technology in optical communication, optical fiber gyro, optical fiber sensor and the core of information processing equipment, easily modulated polarizing optical source are widely employed. The appearance of waveguide polarizer undoubtedly offers us a new choice. Especially when jointly used with semiconductor lasers, it has the advantages of compact, high output stability, high extinction ratio, good mode selection and being easy to integrate with other waveguide devices. Great interest has been aroused at home and abroad. However, the polarizing devices reported are mostly used in infrared or near infrared region['-31. Reports of fabrication, measurement and theoretical analyses for 6 3 2 . 8 nm or shorter wavelengths have not been seen. We have designed and fabricated for the first time the 632.8-nm device with the extinction ratio larger than 40 dB. We also set up the measuring system to measure the characteristics of the devices. The extinction ratio of the waveguide polarizer is analyzed and calculated using physical optics and dispersion theory for waveguide. The theoretical calculation is in good agreement with the experimental results.

1 Theoretical analyses 1.1 Physical interpretation for waveguide polarizer The LiNb03 crystal itself is a kind of ferroelectric crystal. The variation of its spontaneous polarization could greatly affect the index of refraction. Under paraelectric phase, LiC is located in the oxygen triangle plane of octahedron, N b C 5is located at the center of oxygen octahedron. Under ferroelectric phase, Lit and N b + 5have a small shift along C axis. T h e shift is .in the same direction as the spontaneous polarization. Therefore the total spontaneous polarization of LiNb03 crystal is contributed by PsLiC and P S N ~ ' ~ .

* Department

of Physics, Siping Education College, Siping 136000, China.

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The infrared measurement shows that for proton exchange LiNb03 waveguide, after the proton exchange, H + is not exchanged to the Li+ position but shifts along C axis. Under the action of internal electric field, it enters the oxygen triangle plane near Lif . It is evident that for each replacement of Lif by H + there would be a reduction of PsLiC contribution to the total polarization. On the waveguide layer if the lack concentration of Lif is A N ( x ) (here, the concentration refers to atoms per unit volume, x is the depth of the waveguide layer), the total spontaneous polarization is expressed as AP, = P A N ( x ) , (1) where P is the polarization decrease due to the lack of one Li' . Therefore An,, the variation of the extraordinary index of refraction (e-light) due to the linear electrooptical effect can be expressed as An, = - Y33 n: AN(rc) P / 3 ~ ~ ( ~ - 31 3) . (2)

.

.

T o examine the correctness of the above expression, we consider a crystal where the distance of Li to the nearest oxygen plane is 0 . 0 7 1 4 nm and concentration of Li atom is 1 . 8 9 x loz8 ~ m - At ~ . room temperature, for A = 632.8 nm, taking Y33 = 3 2 . 2 X 1 0 - l 2 m - V - ' , €33 = 28, € 0 = 8.854 X 1 0 - ~ ~ ~ . m -n ', =, 2 . 2 0 1 9 . Substituting the above result into eqs. ( 1 ) and ( 2 ) , the calculation gives An, = 0 . 1 1 , while the experimental result is 0 . 1 2 , which are in good agreement. Jolivares et a1 . [41 gave an empirical formula for the variation of Ano as a function of An,: Ano = 0.007 - 0 . 4 An,. (3) From eq. ( 4 ) , Ano = - 0 . 0 4 can be calculated. It is obvious that extraordinary index of refraction is evidently increased, while the ordinary index of refraction is decreased. This effect forms a special waveguide. For ordinary light it has total reflection only on the upper surface, whereas on the lower surface (the boundary between waveguide and the substract) there is a wave leakage which attenuates the transmitted light. As a result, the emerging light is polarized. 1.2

Calculation of the extinction ratio Extinction ratio is an important physical parameter of a polarizer. It is defined as

.

E = - 10 log( I:(t/ I & ) ,

where

(4)

IZ, represents the output intensity of the light whose polarization direction is parallel to the

incident plane ( i . e. the T M wave or 0-light) ; I:, represents the output intensity of the light whose polarization direction is perpendicular to the incident plane ( i . e. TE wave or e-light). Since the T E wave formed a guided wave in the waveguide when the transmission loss is neglected, I&,* I A . The calculation of extinction ratio is mainly the calculation of the output intensity of the T M wave. In waveguided optics the dispersion equation for plane waveguide isr5I (1,2) 2nlkdcos0, - 6//

where 6;;'

')

and 6;;'

3,

(1,3)

- 2 m x ( m = 0,1,2,

- a * ) ,

(5)

represent the reflection phase difference in ;he two boundaries respective-

ly, n l is the index of refraction of the waveguide layer. For our waveguide polarizer, because of the wave leakage of T M wave at the boundary between the waveguide and the substrate, the phase difference takes 6):'2) = x , while for the upper surface,

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where n2 and n 3 are the index of refraction of the upper and lower layer respectively, 8; is the reflection angle at the boundary between the upper and lower layer (figure 1 ) . Substituting eq. ( 6 ) into the dispersion equation, we have

( m = 0,1,2,...). (7) According to eq. (7) different mode order m corresponds to different 0;. For T M mode, the amplitude ratio between each reflected wave at the lower surface and the incident wave is

As shown in fig. 1, if the thickness and length of the waveguide are d , L respectively, thus traveling length of light along the axis for each back and forth reflection at the lower boundary is 2 s , and s = d tgOi ; the reflection times are n = ( L / 2 d . tgOi). After n times reflection, the ratio between the left energy and the incident energy, i. e . the total reflectivity for T M mode is R// = r" , while the output energy is ~,/d, = R // I{ . Thus, for the same TE and T M incident intensi- Fig. 1 . The transmission of light in the plane waveguide. ty, the extinction ratio can be calculated: E = - 10 l ~ ~ ( l L ( ~ / l) l=; ;- 10 log(l;[,/1[) = - 10 logR//

.

It can be seen from eq. ( 9 ) that the smaller the transmission angle is, the bigger the extinction ratio is. Fig. 2 shows the behavior of extinction ratio plotted as a function of transmission angle. In fact, as 8,decreases, all the mode would be attenuated rapidly except the fundamental mode. Therefore the proton exchanged waveguide polarizer can only transmit with fundamental mode. Fig. 3 shows the calculated and experimental results of extinction ratio of a waveguide polarizer for A = 6 3 2 . 8 nm. The difference between n l , the index of refraction of waveguide layer, and 722, the index of refraction of the substrate, An is one of the major factors affecting the performance of the polarizer. It is the key to improve the performance of the device. Our design parameters are: A = 632.8 nm, d = 3 pm, L = 2 000 pm. Fig. 4 shows the relation between An = ( n l - n 2 ) and the extinction index. It is obvious that the bigger the A n , the smaller the extinction ratio. T o improve the performance, it is necessary to improve the exchange condition in order that An is not over 0.05. So the 0-light can be well extinguished. On the other hand, it can be seen from eq. ( 9 ) that the length of the waveguide is also an important parameter. According to eq. ( 9 ) , the E-L relation is linear. But L should not be too long; otherwise the insert loss will be increased. The third factor affecting the performance of the polarizer is its thickness d . The E-d relation is more complicated. T o obtain the E-d dependence the d-O,, relation must be found in advance. The re-

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Propagation angle/rad

L/pm

Fig. 2 . Extinction ratio of po!arizer versus propaga-

'Fig. 3 . Length of proton exchange region versus

tion angle ( rad) .

extinction ratio for I = 632.8 nm.

sults are shown in figure 5. 160

Difference index of refraction A n

I

I

I

20

30

40

'Thickness of waveguide d / p m

Fig. 4 . Difference between index of refraction of the

Fig. 5. Thickness d of waveguide versus extinction ratio

waveguide region of the substrate An and extinction ratio.

E.

In brief, the extinction ratio of a waveguide polarizer is proportional to its length. But the selection of L must take the loss into account. Since the bigger the difference A n of index of refraction the smaller the extinction ratio. The proton exchange time and annealing time must be controlled carefully. The thickness of the waveguide can limit the mode and greatly affect the extinction ratio as well. In the design of polarizer, it is unwise to consider the extinction ratio as the only factor, especially when the polarizer is to combine with a semiconductor laser. 2

Fabrication and measurement of the device

2.1

Fabrication of the device The polarizer device employs X-cut and Y-transmission strip waveguide structure as shown in fig. 6. A combination structure is formed by cutting off a segment waveguide and introducing a length of proton exchange waveguide region. T o assure the fabrication, the width of Ti diffusion is selected as 9 pm; the width of optical waveguide is selected as 10 pm. Interrupted strip Ti film is fabricated on a fine polished and cleaned LiNb03 crystal by high frequency spatter and peeling off technique. After 9-h diffusion at 1050°C in flowing wet Ar gas, the strip is cooled to 600°C in oxygen atmosphere. The T i : LiNbOs single mode waveguide is prepared. Then a layer of 0 . 2 pm A1 film is coated on the surface of the substrate. On the A1 film the proton exchange waveguide pattern is formed by block titanium diffusion in the interrupted re-

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gion. The A1 mask on proton exchange region is removed by etching. The proton exchange technique is a key step in the fabrication. The device is put in a 30 cm long glass tube in which benzoxy powder is filled to 10 cm high. The glass tube is sealed and put into the exchange furnace at 210°C for 25 min. Then the sample is taken out and annealed in air at room temperature for 2 . 5 h . Thus the optical waveguide polarizer chip is fabricated.

1

2

A /,'/

3

Fig. 6. Structure of Ti d~ffusionproton exchange LiNbOs optical waveguide polarizer. 1 , LiNb03 substrate; 2 , Ti diffusion region; 3, proton exchange region. d , thickness of exchange region; L , length of exchange region.

The purpose of annealing is to eliminate the sudden change of index of refraction on the surface of the device. It can be seen from fig. 7 that the curve of index of refraction is much smoother than that before annealing. It is approximately linear. Finally, the two ends of the device are cleaned and polished and glued to a 632.8-nm optical fiber. The device is mounted in a box of 2 . 3 c m x 1 cm. 2 . 2 Measurements of the devices The setup measuring the polarizer extinction ratio is shown in fig. 8. Random polarized A = 632.8 nm light beam from a He-Ne laser is changed into circularity polarized by passing through a h / 4 place polarizer. The energizing light is changed into linearly polarized light by a Glan prism. The 2.2 0.5 emerging light is coupled to the input end of optical fiber by Dlpm beam expander and lens of short focal length. The light from Fig. 7 . Curves of the index of refraction the output end of optical fiber is coupled to the detector, and versus depth before and after annealing. 1. measured by a P-W power meter. The curve for extinction Before annealing; 2, after annealing. ratio versus direction of polarization of the incident light can be obtained by changing the orientation of the Glan prism. The parameters of our device are L = 2mm, d = 3 pm. Power of He1 2 3 4 Ne laser is 3 mW, temperature of measurement is 21. 6°C . The results are shown in table 1. The advantages of this measuring method are that the results are apparent. The requirement Fig. 8. Experimental setup for measurement of extinction ratio. 1 , He-Ne for the laser power is not high, and laser; 2, polarizer; 3, h/4 plate; 4 , Glan prism; 5 , lens of short focal the external effect is small because the length; 6, single-mode optical fiber; 7, waveguide polarizer; 8 , detector;

a

10

m

0

C Y I

9, power meter.

D

emerging light from the polarizer is directly coupled into the detector to reduce the background light. However, this method requires a strict coaxiality of the measuring devices. Otherwise, the measurement accuracy would be badly affected.

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Table 1 Measured results of extinction ratio of the waveguide polarizer -

-

-

Output power

V TE TM Extinction ratio

24.8 nW

16.64 nW

1 9 . 5 1 nW

19.42 nW

Mean value

23.80 nW

2 . 2 5 pW

1.56 pW

1 . 6 8 pW

1 . 3 4 pW

2 . 3 7 pW

40.29 dB

40.28 dB

40.43 dB

40.50 dB

40.54 dB

40.54 dB

Conclusions

3

Proton exchange waveguide polarizer can be theoretically analyzed with guided-wave theory and numerical simulative calculation for various factors affecting the extinction ratio to conduct the optimum design to improve the performance of the device. Waveguide polarizers fabricated by proton exchange have the advantages of low transmission loss and easy fabrication. By annealing the light loss can be further decreased and the proton diffusion depth can be increased to meet the mode matching in the waveguide region. The selection of the design parameters is extremely important in the fabrication. Those factors such as proton exchange temperature, exchange time, annealing time and temperature, the length and thickness of waveguide region, the difference of index of refraction, would greatly affect the transmission efficiency loss and extinction. As for the measurements, the stability of the light source, the background light and the coaxiality of various device would directly affect the measurement accuracy. T o improve the device performance and propose more accurate measuring technique is an important topic in the research. On the other hand, experiences extracted from the fabrication of proton exchanged waveguide polarizer are instructive to the fabrication of other type of polarizers, such as metal cladding optical waveguide polarizers. In the future, with the increasing applications of the polarizers, there are prospective developments in this field.

References 1

Suchoski, P.G, Findakly, T . K . , Leonberger. F. J . , Low-loss high extinction polarizers fabricated in LiNb03 by proton exchange, O p t L e t t . , 1988, 1 3 ( 2 ) :170.

2

Veselka, J . J . , Bogerro, G. A. , Low-loss TM-pass polarizer fabricated by proton exchange for Z-cut Ti: LiNbOs waveguides, Election. L e t t . , 1987, 23(1) :29. Gao Fubin, Jin Feng. Xing Rubing et a1 . , 1.5 pm proton exchanged waveguide polarizer for TEo mode, Optics Journal (in Chinese), 1995, 15(8):1102. Jolivares Diaz-Gabrera, M . A . , Direct measurement of ordinary refractive index of proton exchanged LiNb03 waveguides, O p t . C o r n m a n . , 1992, 92(1-3) :40. Xu Senlu, Ling Shide, Opticul Waveguides and Applications (in Chinese 1, Hangzhou: Zhejiang University Publishing House, 1990, 46-53.

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