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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.
©2007 Optical Society of America
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 acousto-optic 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 (LiNbO3) [4–5], lithium tantalate (LiTaO3)[6], electro-optic polymeric material [7], liquid crystal [3, 8] and Lead Lanthanum Zirconate Titanate (PLZT)[9–12] 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 10 times of that of LiNbO3), availability of large volume materials, high response speed (less than 100ns), broad optical transparency range (about 0.5µm–10µm) 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.
2. Optical characteristics of PLZT electro-optic ceramic
PLZT electro-optic ceramic is demonstrated firstly in 1969, 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 9.0/65/35 (La/PbZrO3/PbTiO3), which is supposed with the largest electro-optic coefficient in the PLZT family. For the 9.0/65/35 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 10mm×2mm×1mm (length×width×thickness). Two collimators are used to collimate the incident beam and receive the transmission beam, respectively. The output beam is detected 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.
Figure 2 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 580V, the phase shift achieves 2π, 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 electro-induced 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 90% 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 600V, the electro-induced optical transmission loss is very small (less than 0.5%); however, when the applied voltage exceeds 600V, the loss will increase quickly. For example, 1000V applied voltage causes a 8% 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 4 shows the hysteresis characteristic of PLZT ceramic sample, the square dot is a voltage-raising process and the circular dot is a voltage-reducing process. The relative difference is about 15V for the same phase shift in our experiment.
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:
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, e, 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 , R 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:
Figure 6 is our experiment setup for the above analysis. A 633nm 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 20mm×4mm×0.5mm. 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=0.2rad. 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. (2). 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 500V, the deflection angles for y-polarization and x-polarization beams are 8.6mrad 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.
Fig. 6. The experiment setup for PLZT EO deflector. OA: optical attenuator, PC: polarization controller, FL: focus lens, PSD: position sensitive detector.
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 8 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 500V are measured to be 8.6, 6.3 and 4.8mrad for 30, 50 and 80°C, 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 80°C, the hysteresis characteristics become very unobvious.
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.
4. 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.10. 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 light-pass 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].
Just as Ref.[1] shows, the far-field intensity pattern of the output diffraction beam from the array phase-modulation is
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
The corresponding parameter descriptions are given in Table 1. For the PLZT ceramic with quadratic electro-optic effect, the phase difference Δφ between the closed phase-modulation units, which is induced by quadratic electro-optic effect, may be written easily as
where . 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 electro-optic effect must satisfy
From the above analysis, we can see that Δφ is not only related with the applied field, but 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 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
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 200V and 400V applied voltages, the deflection angles of the center interference fringe are about 0.4mrad and 1.2mrad, respectively. Figure 12 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 multi-interference 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.
Fig. 11. The numerical simulation far-field interference pattern of phase-modulation array varies with the applied voltage.
Fig. 12. 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 633-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 (7.2m) is received by a light screen. Figure 14 shows the photograph of our PLZT phase modulated array structure.
In our experiment, eight phase-modulation units are used, and each unit has a 300 µm period and 120 µm 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. 16 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 VN are 280V, 380V, 480V and 580V, the center fringe deflection angles are 0.59mrad, 1.14mrad, 1.57mrad and 2.35mrad, respectively. Comparing the experimental result with the numerical simulations shows in Fig.11 and 12, good consistency is obvious.
5. Summary
We have analyzed and compared systematically the characteristics of two kinds of electro-optic 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 [16]. This work is being undertaken for us.
Acknowledgment
The work is supported by the Knowledge Innovation Program of the Chinese Academy of Sciences.
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