| Being real-time and flexible, spatial light modulator is widely used in many important fields such as pattern recognition, image processing, digital holography, spatial filter and binary optics. It plays an important role in modern optics and optic-electronic information processing. Liquid crystal spatial light modulator based on twisted-nematic LC is introduced in this paper. The modulation of LCD includes phase variation and intensity variation, however in some fields phase-only modulation is required or intensity varies in small range. We emphasize phase-only modulation in the paper.After analyzing and summarizing the research of phase-only modulation home and abroad, we introduce the principle of phase-only modulation based on twisted-nematic LC. Balancing several factors, an approach based on improving the driving voltage of commercial LCD is proposed. We change the original driving circuit and enlarge the range of voltage on LCD to improve the phase depth.A LCD which fits for SLM is chosen and common principle of driving is given. Companied with the consideration of voltage control, we design and make the driving circuit which includes analog signal module, digital signal module, control module and power module. The LCD plays steadily.Then the modulation character of LCD is measured in optical system. Generally it needs polarizers and quarter-wave plates. The phase modulation character changes with the rotation of two polarizers beside the LCD. The relationship between them is given through the experiment. Companied with measurement of intensity modulation, the phase-only modulation mode and the amplitude-only modulation mode are found. It can be deduced from the modulation figure that the phase modulation depth is improved for the new driving circuit as predicted.Finally, a comparison of modulation character between our system and LC2002 spatial light modulator is demonstrated. In the application of image decoding, the result is satisfactory. |
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