Research On Overdrive Technology Of Liquid Crystal Spatial Light Modulator In Optical Communication

Space laser communication has been widely studied and applied due to its advantages of large information capacity,high confidentiality,small size and light weight.In the spaceborne environment,the traditional ATP system has certain limitations in terms of the weight,the size,and the power consumption,therefore it is urgently needed to achieve more dexterous and convenient micro-mechanical or non-mechanical beam deflection technology.As a non-mechanical beam deflecting device,liquid crystal has become a research hotspot due to its advantages of low power consumption,high beam deflection accuracy,and large beam deflection.The slow response of the liquid crystal in the liquid crystal spatial light modulation system is a key problem to be solved urgently.Overdrive method accelerates the response of the liquid crystal by increasing the drive voltage.The method has a simple control system,a small system size,and low power consumption.This paper uses the overdrive method to improve the response speed of the liquid crystal spatial light modulator.First,analyzing the physical basis of overdrive technology.According to the theory of liquid crystal elastomer,the distribution of deflection angles of liquid crystal molecules with position is obtained by differential iterative method.Based on the effect of electronically controlled birefringence,the relationship between the modulation phase of the nematic liquid crystal and the driving voltage is simulated,and the feasibility of the overdrive method is proved.Based on the above analysis,the key issues that need to be solved in this project are: measuring the relationship between the modulation phase of liquid crystal and the driving voltage,selecting the appropriate modulation phase interval and accurately applying the response time of the overvoltage driving,and aiming at solving these problems on the liquid crystal overdrive method research.In the process of establishing the liquid crystal overdrive system,this paper proposes a measurement method of the liquid crystal phase modulation characteristics,establishes a liquid crystal diffraction grating model,and derives the correspondingrelationship between the light intensity of the far field diffraction and the modulation phase,through the liquid crystal drive voltage and by modulating the phase mapping relationship,the phase modulation characteristics of the liquid crystal are finally obtained.Then,according to the characteristics of the phase modulation of the liquid crystal,the measurement method of the phase response of the liquid crystal is discussed,and the basic measure of the measurement of the response time of the liquid crystal is obtained.According to the definition of liquid crystal response time,the calculation and selection methods of modulation phase interval and optimal driving voltage are proposed.An over-drive system based on FPGA(Field-Programmable Gate Array)of liquid crystal spatial light modulator was established.In this system,the quantification of the liquid crystal modulation phase,the overdrive table lookup,and the generation of the PWM(Pulse Width Modulation)control wave are all done in the FPGA.This method does not occupy CPU resources and can respond to CPU instructions more quickly.Finally,set up the experimental light path platform,and the phase modulation characteristics of the liquid crystal are measured.Changing the driving circuit of the liquid crystal,measuring the response speeds of the liquid crystal under normal driving voltage and different over driving voltage conditions,and selecting the phase modulation interval of different over driving voltages,comparing the response speed of the liquid crystal after the selected segment,and finally obtaining the best drive voltage of the liquid crystal device.The experimental results show that the application of FPGA-based over-driving method eventually achieved rapid deflection of the liquid crystal molecules,and improved the response speed of the liquid crystal spatial light modulator system by nearly one order of magnitude,which has important research significance.