Electro-optic Tunable Phase Array Beam Splitter Based On PPLN"

Array beam splitter is one kind of microstructure devices widely used in optical information fields. The tunable phase array beam splitter based on Periodically Poled LiNbO3 (PPLN) crystal is a new style array device with Talbot diffraction employing the special construction of PPLN. As described below is the principle of this device:Applied electric field along the polarization direction of PPLN, the crystal’s refractive index and thickness changed with the polarization period by reason of electro-optic effect and anti-piezoelectric effect. So, phase grating controlled by electric filed is formed and is used to split the light wave by Talbot diffraction.The splitter is suitable for used in the whole transmission spectra of LN crystal. It overcomes the weak point that multi-step phase grating is only used for the certain wavelength. And it can compensate the effect to array device from temperature and photorefractive in the high power applications. It has an important value to develop the tunable phase array beam splitter as to act as the research content of this paper. The main work is as follows.Theoretical simulation:Theory models of Talbot effect diffraction about electro-optical tunable phase array beam splitter based on PPLN has been built. Simulation on diffraction field intensity distribution of two hexagonal array construction (Compact style and No-compact style) by the Matlab software has been completed, and the effects to light intensity from electric field, position and duty ration of array cell had been studied. Finally, the optimal parameters for array construction design had been obtained. Simulation showed that:The duty cycle was key to beam splitter design, the highest compression ratio had been found with the same area of positive polarization as negative polarization and the minimum required electric field. For compact hexagonal array, optimal duty ratio was 71%, the maximum compression ratio appeared at position of fraction Talbot coefficient β= 0.20, in which peak relative intensity gradually increased with phase different in 0-0.75π up to 20 units.PPLN preparation:the temperature characteristic of polarization reverse about the LN crystal and MgLN crystal applied external electric field had been studied. The PPLN and PPMgLN with optimal design had been prepared according to the determined polarization reverse parameters (for example:temperature, pulse wave and repeat times) for the size of samples. It was found that:a polarization inversion voltage decreases with increasing temperature, LN poled best temperature is 200℃, pulse wave bias voltage 2kV, peak voltage 6.4kV; MgLN poled optimum temperature is 100℃, pulse wave bias voltage lkV, peak voltage 1.8kV.Transparent electrode preparation:Magnetron sputtering ITO film on PPLN in order to tune the phase by electro-optic effect through electrode with low resistivity and high transmittance. The optical properties of ITO film had been tested and the optimal sputtering conditions had been obtained by changing substrate temperature, sputtering time and other parameters. When the conditions of sputtering ITO film were best, substrate temperature 320℃ and sputtering time of 50 min, the prepared film have resistivity of 3.41×10-4Ω.cm and average visible light transmittance of 74.38%.Optical experiment:Phase array beam splitter based on PPLN and PPMgLN were tested. The affect from electric field intensity, diffraction position and array cell duty on the intensity distribution of diffraction field had been studied. The experimental results correspond to the results of simulation. The experiment results show that:the distribution of diffraction field intensity variations with the electric field intensity, the diffraction spot size and intensity could be adjusted by changing diffraction position or applied electric field; The MgLN crystal should be the candidate material for array device because of its lower reverse electric field, higher ability of resisting to photorefractive, neater appearance in periodically poled samples and better beam splitter than LN crystal.Finally, we summarized the research results, the improvement of phase array beam splitters in the design and optimization was proposed according to the situation of array in real application.