Research of Wavefront Transformation Based on Liquid Crystal Spatial Light Modulator
|School||Harbin Institute of Technology|
|Course||Instrument Science and Technology|
|Keywords||liquid crystal spatial light modulator phase-only modulation wavefront transformation phase retrieval|
LCSLM(Liquid crystal spatial light modulator) , due to its advantages of low-cost, micro-power and programmable characteristic, can be used for one-dimensional or two-dimensional real-time spatial modulation towards information like polarization state and the phase of beam etc. Therefore, LCSLM is more and more widely applied in modern optics, optoelectronic hybrid information processing system for research and application. The phase-only modulation technology obtaining a transmission rate of 100% and the wave-front transform technology able to change the wave-front properties distribution of incident light according to the specific needs are thriving recently; they are both very important for researches and applications in the areas like agile beam control, laser scanning, target search, tracking and so on.From the concept of coherent wave-front transform, we analyze the wave-front transform principle of LCSLM in this paper. Firstly, an experimental system to measure the phase of spatial light modulator can be established based on birefringence effect of light transforming in liquid crystal and Twyman-Green interference principle, measuring the phase-shifting characteristics against a 256×256 pixel liquid crystal spatial light modulator from BNS(Boulder Nonlinear Systems) of the United States, plotting its phase-shift modulation curve simultaneously. The results showed that when the driver gray value of the device is between 60 and 200, the linearity of phase modulation is better, while the deteriorating degree intensified of a sudden when it is greater than 200. Therefore, the gray value should be driven less than 200 in practical applications. Meanwhile, according to the theory of the Fraunhofer diffraction and the principle of phase-only modulation, algorithm of phase retrieval optimization can be improved by the use of pseudo-random coding technology, namely solving the issue of the sensitization of initial value by GS(Gerchberg-Saxton) improved algorithm, which can also improved the convergence rate. Finally, we did the experiment of transforming the wavefront of a coherent plane wave, using 256×256 pixel liquid crystal spatial light modulator of the BNS as phase-only components. The result obtained looks fine, vivid, and uniformly good in strength of bright spot. Error was less than 8%, and graphics occurred within 150 ms. The experimental results demonstrate the rapid convergence of improved GS algorithm and the feasibility of realizing wave-front transformation by the phase-only liquid crystal spatial light modulator.In this paper, the conclusions provide us a method and basis for wave-front transform of the coherent plane wave, and can be used as a reference for research of beam transformation and control, target search, tracking and so on. Meanwhile, the testing methods of spatial light modulator also provide value for the performance test of other optical modulation devices.