Experimental investigation of optical beam deflection based on PLZT electro-optic ceramic

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1 Experimental investigation of optical beam deflection based on PLZT electro-optic ceramic Qing Ye, Zuoren Dong, Ronghui Qu, and Zujie Fang Lab of Information Optics, Shanghai Institute of Optics and Fine Mechanics, CAS, P.O. Box -11, Shanghai 1, P.R. China Abstract: Based on the optical characteristics of PLZT electro-optic ceramic, two kinds of electro-optic deflectors, triangular electrode structure and optical phased array technology, are studied in detail by using transverse electro-optic effect. Theoretically, the electro-optic deflection characteristics and mechanisms of the deflectors are analyzed. Experimentally, the optical characteristics of ceramic wafer, such as the phase modulation, the hysteresis and the electro-induced loss characteristics, are measured firstly, and then the beam deflection experiments are designed to verify the theoretical results. Moreover, the effect of temperature on the performance of triangular electrode deflector is investigated. The characteristics of both deflectors are also compared and illuminated. 7 Optical Society of America OCIS codes: (3.9) Electro-optical devices; (1.1) Electro-optical materials; (.5) Phase modulation. References and links 1. J.A.Thomas, and Y.Fainman, Optimal cascade operation of optical phased array beam deflectors, Appl. Opt. 37, 19-1 (199).. D.P.Resler, D.S.Hobbs, and R.C.Sharp. High efficiency Liquid-crystal optical phased array beam steering, Optics L ett. 1, 9-91 (199). 3. P.F.Mcmanamon, T.A.Dorschner, and D.L.Corkum, Optical phased array technology, Proceedings of the IEEE, -9 (199).. S.J.Barrington, A.J.Boyland, and R.W.Eason, Resolution considerations in electro-optic, single interface deflectors, Appl. Opt. 3, (). 5. D.Djukic, R.Roth, J.Yardley, R.Osgood, S.Bakhru, and H.Bakhru, Low-voltage planar- waveguide electrooptic prism scanner in Crystal-Ion-Sliced thin-film LiNbO3, Opt. Express 1, ().. R.A.Meyer, Optical beam steering using a multichannel lithium tantalate crystal, Appl. Opt. 11, 13 1 (197). 7. Lin Sun, Jin-ha Kim, Chiou-hung Jang, Dechang An, and Xuejun Lu, Polymeric waveguide prism-based electro-optic beam deflector, Opt. Eng., (1).. P.F.Mcmananmon, E.A.Watson, and T.A.Dorshner, Applications look at the use of liquid crystal writable gratings for steering passive radiation, Opt. Eng. 3, 57- (1993). 9. Qiwang Song, Xuming wang, R.Bussjager, and J.Osman, Electro-optic beam-steering device based on a lanthanum-modified lead zirconate titanate ceramic wafer, Appl. Opt, 35, (199). 1. Tsuyoshi Tatebayashi, Takashi Yamamoto, and Heihachi Sato, Electro-optic variable focal-length lens using PLZT ceramic, Appl. Opt. 3, (1991). 11. T.Utsunomiya, K.Nagata, and K.Okazaki, Optical deflector using PLZT ceramics, Jpn. J. Appl. Phys., Supplement -, 1-3 (195). 1. T.Utsunomiya, Optical deflector with tandem electrodes using PLZT ceramics, Jpn. J. Appl. Phys., Supplement -, 1-1 (199). 13. D.Goldring, Z.Zalevsky, E.Goldenberg, A. Shemer, and D. Mendlovic, Optical characteristics of the compound PLZT, Appl. Opt., (3). 1. Feng Liu, Qing Ye, Fufei Pang, Ronghui Qu, and Zujie Fang, Polarization analysis and experimental implementation of PLZT electro-optical switch using fiber Sagnac interferometers, J. Opt. Soc. Am. B 3, (). 15. Heihachi Sato, Tsuyoshi Tatebayashi, Takashi Yamanioto and Kunihiko Hayashi, Electro-optic lens composed of transparent electrodes on PLZT ceramic towards optoelectronic devices, Proc. SPIE 1319, 93-9 (199). #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 1933

2 1. A.L.Glebov, M.G. Lee, Lidu Huang, Shigenori Aoki, Kishio Yokouchi, Masatoshi Ishii, and Masayuki Kato, Electrooptic planar deflector switches with thin-film PLZT active elements, IEEE J. of Selected Topics in Quantum electronics, 11, -3 (5). 1. Introduction High speed spatial beam scanning devices [1-3] have important applications in varieties of areas, such as optical communication, optical memory and optical interconnections. Mechanical rotator type beam deflector has been used widely, while a deflector without moving component is more attractive for its higher speed and higher reliability, among which the acousto-optic and electro-optic deflectors have had a development largely. The acoustooptic deflector has a relative high cost, and its response speed and angle resolution also limit its development to a certain extent. The electro-optic deflector may provide a high-speed beam scanning due to motionless component, and the corresponding structure design may help to realize high angle scanning resolution, so it becomes a good candidate for practical applications. With the development of material science and technology, the electro-optic deflectors based on different kinds of materials, such as lithium niobate (LiNbO 3 ) [-5], lithium tantalate (LiTaO 3 )[], electro-optic polymeric material [7], liquid crystal [3, ] and Lead Lanthanum Zirconate Titanate (PLZT)[9-1] ceramic, have obtained a high-speeding development. However, compared with other electro-optic materials, PLZT electro-optical ceramic has some distinct advantages [13-15], such as the high electro-optic coefficient (about 1 times of that of LiNbO 3 ), availability of large volume materials, high response speed (less than 1ns), broad optical transparency range (about.5µm-1µm) and low cost, so it has very extensive applications in the area of opt-electro component. At present, the electro-optic deflector based on PLZT ceramic generally includes two different structures. One is electro-optic prism-type PLZT deflector, which possesses a triangular up-and-down electrode structure. The electro-optic effect controls the refractive index variation in the electrode area and makes the transmission beam deflect on the interface. The other is the optical phased array technology. The output beams passing through different phase modulated unit can obtain a linear phase distribution, and the multiple diffraction beams will interfere in the far-field and form interference fringe. Different voltages will make the interference fringe deflect. Usually, the electro-optic prism-type PLZT deflector has a relative larger deflection angle, while the optical phased array technology has a higher angle scanning resolution. In this paper, we analyze some optical characteristics of PLZT electro-optic ceramic firstly, such as the phase modulation, the hysteresis, the electro-induced loss and the temperature effect. Two different structures PLZT electro-optic deflectors are investigated systematically for in theory and experiment. Finally some related problems are discussed and the corresponding suggestions are also provided.. Optical characteristics of PLZT electro-optic ceramic PLZT electro-optic ceramic is demonstrated firstly in 199, and it represents a class of high optical transparency and large electro-optic coefficient ceramics. The characteristics of this ceramic have been studied extensively [13]. In order to illuminate the following performances of deflectors clearly and provide some evidences for the future practical applications, some other electro-optic characteristics, such as the phase modulation, the electro-induced loss and the hysteresis, are analyzed theoretically and measured experimentally. The PLZT ceramic used here is a quadratic electro-optic material and its composition is 9./5/35 (La/PbZrO 3 /PbTiO 3 ), which is supposed with the largest electro-optic coefficient in the PLZT family. For the 9./5/35 PLZT electro-optic ceramic, only quadratic terms are considered since the un-poled quadratic PLZT ceramic exhibits very weak linear electro-optic effect. Figure 1 shows our experimental setup. The size of PLZT ceramic sample is 1mm mm 1mm (length width thickness). Two collimators are used to collimate the incident beam and receive the transmission beam, respectively. The output beam is detected #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 193

3 by an optical power meter and phase detector, respectively. In our scheme, the applied voltage will generate a transverse electro-optic effect for the transmission beam. +V Ti/Pt/Au electrode Coupler Phase detector Laser Collimator PLZT ceramic Collimator Power meter Fig. 1. The experimental setup for the optical characteristics measurement of PLZT ceramic. Figure shows the variation of the phase shift with the applied voltage for PLZT ceramic sample. A quadratic relation is clearly to be seen and the experimental and theoretical results show good agreement. When the applied voltage is 5V, the phase shift achieves π, however, this periodic phase shift voltage will reduce as the applied voltage increases due to the quadratic relation. Besides the electro-optic phase-modulated characteristic, the electroinduced optical loss is also very important. It will influence the energy-efficiency of component. Usually, the PLZT ceramic has a very good optical transparency (more than 9% without considering the interface reflection). However, an intense applied electrical field will induce a strong optical transmission loss, and this is now shown in Fig.3. For our experiment, when the applied voltage is less than V, the electro-induced optical transmission loss is very small (less than.5%); however, when the applied voltage exceeds V, the loss will increase quickly. For example, 1V applied voltage causes a % transmission loss, which is also approved in Ref.[13]. Therefore, investigating the phase-modulated characteristic, not only decreases the systematical applied voltage, but also reduces the transmission loss of optical field. On the other hand, the hysteresis characteristic of PLZT ceramic is also a noticeable problem for the electro-optic deflector applications. It will influence the deflection angle scanning precision and the adding-means of the applied voltage, which will be illuminated in the following part. Figure shows the hysteresis characteristic of PLZT ceramic sample, the square dot is a voltage-raising process and the circular dot is a voltagereducing process. The relative difference is about 15V for the same phase shift in our experiment. Phase shift δ /π 1 1 Experimental result Theoritical result 1 1 Applied voltage (V) Fig.. The phase shift of PLZT electro-optic ceramic sample versus the applied voltage. #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 1935

4 1 Loss (%) 1 1 Applied voltage(v) Fig. 3. The measured electro-induced optical loss of PLZT electro-optic ceramic sample. Phase shiftδ /π Voltage-increasing Voltage-reducing Applied voltage (V) Fig.. The hysteresis characteristic of PLZT electro-optic ceramic sample 3. Electro-optic deflector with triangular electrode Based on the optical characteristics of PLZT ceramic, an electro-optic deflector with triangular electrode is studied firstly. This deflector is a very simple high-speed deflector by using the electro-optic prism effect. It usually possesses a pair of parallel triangular electrodes on the up-and-down surfaces of rectangle PLZT ceramic substrate, just as shown in Fig.5. When an electrical field is applied to the PLZT ceramic, the refractive index of the material under the triangular electrode area will change due to the quadratic electro-optic effect, and form an electro-optic prism. This will cause the transmission beam deflection at the interface. Incident lights with different polarizations will show different deflection angles and directions because the electro-optic coefficient depends on light polarization strictly (shown in Fig.5). Take a y-polarization beam for sample, the deflection angle can be deduced easily from the refractive law in geometrical optics: π n sin( α) = nsinγ, (1) π nsin( α γ ) = sin β #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 193

5 where α is the acute angle of the triangular electrode, β is the beam deflection angle as Fig.5 denoted and γ is the refractive angle, n ' and n are the refractive indices of the PLZT ceramic sample with and without the applied voltage. Since the electro-optic induced index change ' 1 3 Δ n( = n n = n R33E, E = V t, R33 is electro-optic coefficient, V and t are the applied voltage and the thickness of PLZT wafer) is quite small compared with the refractive index n of PLZT ceramic, the beam deflection angle β can be obtained approximately from Eq.(1) as follows: β arcsin cos [ α ( n sin α nδncos α ( n + Δn) sinα )] = α α + 1 α 3 arcsin cos sin n R33E cos n n R33E sinα n, () y-polarization Triangular electrode γ β x, n π α Triangular electrode z α n x-polarization Fig. 5. The principle of the electro-optic prism based on PLZT ceramic for different polarization. Figure is our experiment setup for the above analysis. A 33nm He-Ne laser beam incidents on the polished front-face of PLZT ceramic perpendicularly, and an optical attenuator (OA) and a polarization controller (PC) are used to adjust the incident intensity and polarization. The size of PLZT sample is mm mm.5mm. A Ti:Pt:Au triangular electrodes are deposited on the up-and-down surfaces by photo-lithograph and sputtering method, and the corresponding acute angle α =.rad. The deflection angle is measured by spot displacement on the position sensitive detector (PSD), which is placed 3.5 meters from the sample. The relation between the displacement and deflection angle should be calibrated. Moreover, a controllable heating device is also fixed on the down surface of PLZT sample in order to investigate the effect of temperature on the performance of the electro-optical deflector. Figure 7(a) and (b) show the deflection angle variations with the applied voltage for the y-polarization and x-polarization beams respectively. The point is the experimental result and the solid line is the theoretical result from Eq. (). It is worth noticing that the deflections of x-polarization and y-polarization beams are not only different in amplitude, but also different in deflection direction, which is coincident with the above analysis as the solid line shows. Under the condition of 5V, the deflection angles for y-polarization and x- polarization beams are.mrad and -1.5mrad, respectively. Furthermore, the acute angle α has also a large influence on the deflection angle, smaller acute angle α will produce larger deflection angle β with the same applied voltage. #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 1937

6 Ti:Pt:Au surface electrode 33nm He-Ne laser OA y E y x z x PC heating device + z x y β E z FL PSD Computer Fig.. The experiment setup for PLZT EO deflector. OA: optical attenuator, PC: polarization controller, FL: focus lens, PSD: position sensitive detector. Deflection angle (mrad) 1 (a) Experimental result Theoretical result Deflection angle (mrad) (b) Experimental result Theoretical result Applied voltage (V) Applied voltage (V) Fig. 7. The deflection angle versus the applied voltage for (a) y-polarization and (b) x- polarization. The temperature effect on the electro-optic deflector is also studied in detail. Figure shows the beam deflection under different temperature conditions. It can be seen that the deflection angle decreases with the increase of the temperature. For instance, the deflection angles for y-polarization beam at 5V are measured to be.,.3 and.mrad for 3, 5 and, respectively. This reflects that the PLZT ceramics electro-optic coefficient reduces with the increase of the temperature. The main cause is that the temperature dependence can be attributed to more vigorous molecular movement at higher temperature, which will weaken the electro-optic polarization. So this is a very important characteristic for PLZT ceramics in practical applications. Additionally, the effect of temperature on the hysteresis is also investigated as shown in Fig. 9. From the three-different experiment results, we can see that the hysteresis becomes weaker and weaker with the increase of the temperature. At, the hysteresis characteristics become very unobvious. Deflection angle (mrad) 1 3 o C 5 o C o C Applied Voltage(V) #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 193

7 Fig.. The effect of temperature on PLZT electro-optic deflector for y-polarization. Deflection angle (mrad) 1 (a) 3 o C Voltage-increasing Voltage-reducing 1 (b) 5 o C Voltage-increasing Voltage-reducing (c) o C Voltage-increasing Voltage-reducing Applied voltage(v) Applied voltage(v) Applied voltage(v) Fig. 9. The effect of temperature on the hysteresis of PLZT ceramics From the above experimental results, one can see clearly that the material hysteresis and temperature have distinct effect on the performance of the PLZT electro-optic deflector. Therefore, some methods should be adopted to eliminate the corresponding negative effects in practical applications. For example, some temperature compensation should be considered for temperature effect. And in order to eliminate the scanning error caused by the hysteresis, unidirectional applied voltage scanning is recommended, such as sawtooth-wave voltage scanning scheme. Moreover, because the beam scanning of electro-optic prism is a kind of continuous scanning, and the corresponding scanning resolution is related with the size of optical beam, large optical beam diameter will decrease scanning resolution, therefore it is very difficult to locate the precise position of far-field scanning target in practical military and communication applications.. Electro-optic deflector with optical phased array technology In order to obtain the spatial beam precisely-scanning, optical phased array technology is one of the very effective schemes. Generally, the optical beam deflector based on optical phased array is composed of multi-independent phase-modulation units as shown in Fig.1. Multiple diffraction beams, whose phases are modulated by the applied voltages, will interfere in the far-field and form multi-order interference fringes, and different applied voltage will induce the interference fringe deflection. In our scheme, a strip electrode array is deposited on the top surface of PLZT ceramic by photo-lithograph and sputtering method, and its period and lightpass aperture can be adjusted easily through the design of the mask. The down electrode is a plane electrode by sputtering method. A plane beam incidents on the front face of the modulator, and the output diffraction beams will form a quantization linear phase distribution in near-field by the action of each phase-modulated unit for different applied voltages. Then the far-field interference pattern will show deflection. Compared with the above deflector with triangular electrode, this optical phased array technology can obtain a relative high beam scanning resolution due to small beam diameter of interference fringes. Moreover, it can obtain multiple precise deflection positions within the whole scanning range which is decided by the array number N and the period of each modulated unit [1]. #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 1939

8 Fig. 1. The basic scheme of optical phased-array beam deflector Just as Ref.[1] shows, the far-field intensity pattern of the output diffraction beam from the array phase-modulation is I( θ ) = C EF( θ ) AF( θ ) (3) where C is a constant of proportionality. EF(θ ) and AF (θ ) are the element factor and the array factor, and the related expressions are w wθ EF ( θ ) = sin c, (a) d λ ( ) d sin ( ) d AF θ = sin ( ) Nπ θ θ f N π θ θ f, (b) λ λ Δϕ λ θ f = π d. (c) The corresponding parameter descriptions are given in Table 1. For the PLZT ceramic with quadratic electro-optic effect, the phase difference Δ ϕ between the closed phase-modulation units, which is induced by quadratic electro-optic effect, may be written easily as 3 ϕ = ϕ i + 1 ϕi = k L ( Δni 1 Δni ) = πln R33( E i + 1 E i ), (5) λ Δ + Vi t where Δni = n R33Ei = n R33. In order to realize the quantization linear phase distribution of output beams (i.e. the phase induced by the electrical field is proportional to the electrical field square, and the phase difference between the closed phase-modulation units is also uniform), the applied voltage for each phase-modulation unit through transverse electrooptic effect must satisfy V i = i V 1, i = 1... N. () Δ ϕ is not only related with the applied field, but From the above analysis, we can see that also proportional to the length of the phase modulator. Therefore, increasing the length L and decreasing the thickness t of the modulator (i.e. increasing the electrical field) may reduce the applied voltage, and at the same time, the relative electro-induce optical loss will also be #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 19

9 reduced. Accordingly, we optimize the structure design of phased array modulators relative to Refs. [1] and [9] and obtain a relative large angle deflection under the same conditions. Table 1. The descriptions of related parameters Parameter Description Parameter value d w t L N, i D, f Incident wavelength The period of phase modulated unit The light-pass aperture of phase modulated unit The thickness of phase modulator The length of phase modulator The number of phase modulate units and the ith unit The width of whole phase modulated array The angle variable and related beam deflection angle 33nm 3µm 1µm.mm 1mm ϕ The phase difference between closed phase-modulation units n k R 33 n i The refractive index of PLZT ceramic The wave vector of incident field The electro-optic coefficient of PLZT ceramic The refractive index variation of the ith phase modulated unit m /V V i The applied voltage of the ith phase modulated unit ϕ The phase shift of the ith phase modulated unit i Figure 11 shows the corresponding numerical simulation results based on the above analysis and parameters in Table 1. It displays that the interference fringe deflects when the applied voltage increases. In the case of V and V applied voltages, the deflection angles of the center interference fringe are about.mrad and 1.mrad, respectively. Figure 1 shows the deflection angle variation with different applied voltage, and the curve satisfies a good quadratic relation. Usually, the far-field interference fringe is determined by the interaction of the element factor and the array factor. The element factor decides the envelope of multiinterference fringes, and the array decides the interference order and period. Moreover, it should be pointed out that the beam scanning has an angle scanning range because of multiple interference fringes. The range for each order interference fringe is decided by the period of interference fringes. Intensity (a.u.) V V V Angle (rad) Fig. 11. The numerical simulation far-field interference pattern of phase-modulation array varies with the applied voltage. #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 191

10 . Deflection angle (rad) Applied voltage (V) Fig. 1. Numerical simulated deflection angle of phase modulated array varies with the applied voltage. Based on the above theoretical analysis, we design a corresponding experimental setup to realize optical beam deflection, which is shown in Fig.13. In our scheme, the incident beam from a 33-nm He-Ne laser is polarized by a polarization controller, expanded by the combination of a collimating lens and a cylindrical lens, then selected by a diaphragm and an amplitude mask, and finally injects into a PLZT electro-optical ceramic phased-array modulator. An optical attenuator is also used to control the intensity of the incident optical field. Multiple output diffraction beams from different phase modulators will interfere in the far-field and form interference fringes. The parameters of phase-modulation array are listed in Table 1. The far-field interference pattern (7.m) is received by a light screen. Figure 1 shows the photograph of our PLZT phase modulated array structure. 33nm He-Ne laser Collimating lens Amplitude mask Voltage controller OA PC Diaphragm Cylindrical lens PLZT phased array Light screen Fig. 13. The experimental setup. #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 19

11 Electrodes(-) Electrodes(+) Electrodes(+) PLZT phased array Fig. 1. The photograph of PLZT phase modulated array structure In our experiment, eight phase-modulation units are used, and each unit has a 3 m period and 1 μ m light-pass aperture (i.e. electrode area). In order to keep the beam transmitting in light-passing aperture, an amplitude mask is applied, which possesses the same period and light-passing aperture with phase-array modulation unit. Figure 15 shows the phase distribution of each modulated-phase unit with the different applied voltage, and Fig. 1 shows the corresponding beam deflection. Evidently that the far-field interference fringes obtain an obvious deflection as the applied voltages increase. When the applied voltages V N are V, 3V, V and 5V, the center fringe deflection angles are.59mrad, 1.1mrad, 1.57mrad and.35mrad, respectively. Comparing the experimental result with the numerical simulations shows in Fig.11 and 1, good consistency is obvious. μ Phase-shift (rad) V 3V V 5V different phase-modulation unit n Fig. 15. The phase distribution of each phase modulation unit with different applied voltages. #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 193

12 (a) V N =V mrad (b) V N =V.59 mrad (c) V N =3V 1.1 mrad (d) V N =V 1.57 mrad (e) V N =5V.35 mrad Fig. 1. The beam deflection of phase modulation array with different applied voltage. 5. Summary We have analyzed and compared systematically the characteristics of two kinds of electrooptic deflectors based on PLZT ceramic transverse electro-optic effect in experiment and theory. The optical characteristics of PLZT ceramic, such as the phase modulation, the hysteresis and the electro-induced loss, are measured experimentally, and their effect on the deflector is illuminated. For the deflector with electro-optic prism, the effect of temperature on its performances is also investigated. Considering the electro-optic deflector based on PLZT ceramic, one shortcoming is the high working voltage due to the thickness of PLZT wafer used in the experiments. This will limit its applications in some areas, such as optical communication. In order to solve this problem, a thinner PLZT ceramic, especially using thin film, will be effective which has a huge attraction for the forthcoming compact optic-electro components [1]. This work is being undertaken for us. Acknowledgment The work is supported by the Knowledge Innovation Program of the Chinese Academy of Sciences. #357 - $15. USD Received Oct 7; revised 11 Nov 7; accepted 15 Nov 7; published Dec 7 (C) 7 OSA 1 December 7 / Vol. 15, No. 5 / OPTICS EXPRESS 19