Research On Liquid Crystal Beam Deflection For Space Optical Communication

Space optical communication has many advantages such as high bandwidth,high security,high anti electromagnetic interference and so on,and it has important position in the national economy,national defense science and technology.Europe,Japan and the United States and other developed countries have begun to study the key technologies of space optical communication,and a lot of research results have been obtained.Acquisition,Tracking and Pointing is one of the key technologies in space laser communication,The traditional mechanical beam steering technologies such as gimbal mount and the fast steering mirrors,it cannot meet the acquisition of weight,volume and power requirements in space laser communication,Therefore,it is of great strategic significance to study the non-mechanical beam deflection technique with small size,light weight and low power consumption.Liquid crystal optical phased array beam deflection technology is the most promising technology in non mechanical beam deflection technology researching.Space optical communication requires the ATP subsystem to control the deflection of the laser beam accurately and quickly,Therefore,the research work of the liquid crystal optical phased array is based on the precision and fast,which lays the foundation for the engineering application of the liquid crystal optical phased array in the space laser communication.In this paper,the physical basis of the liquid crystal phase modulation is first studied.Based on the liquid crystal curvature elasticity theory,the relationship between the change of liquid crystal molecular directional vector and the molecular position is studied,and the numerical simulation is used to solve the distribution of liquid crystal molecular directional vector under a certain voltage,then the modulation characteristic curve of liquid crystal phase modulation with the voltage is simulated.The phase modulation characteristics of liquid crystal optical phased array are related to the deflection angle accuracy and diffraction efficiency,In order to obtain the accurate phase modulation characteristic curve,a new method for measuring the phase modulation characteristics based on the liquid crystal phase modulation grating is proposed.The method establishes the relationship between the phase modulation and the maximum intensity of the zero order diffraction,and has the advantages of large dynamic range,high resolution and high measurement accuracy.In order to meet the requirement of high precision beam deflection in space optical communication,The blazed grating phased array model is analyzed,In this model,the beam deflection angle is discrete and discontinuous,and most of the beam deflection can only be distributed in the small angle region,which can not realize the high precision beam deflection.A model based on microwave phased array radar is proposed,The liquid crystal as a whole in this model,The phase difference between the liquid crystal electrodes is fixed,This model can realize beam high precision continuous deflection.and the deflection angle is determined by the phase difference between the liquid crystal electrodes.The continuous and high precision deflection of liquid crystal optical phased array can be realized.In order to meet the requirement of fast and high bandwidth adjustment in space optical communication,The relaxation characteristics of liquid crystal are analyzed.Liquid crystal molecular steering relaxation is the greatest impact on response time.A new method of liquid crystal over driving based on FPGA look-up table is proposed to solve this problem,The response time of liquid crystal is improved by increasing the driving voltage and finding the appropriate phase modulation interval,The rise time of the liquid crystal device is shortened from 600 ms to 25 ms,and the down time is shortened from 350 ms to 25 ms.In order to verify the beam deflection accuracy and the closed-loop bandwidth,In The closed-loop control system is built,the closed-loop control precision and the closed-loop bandwidth based on the phased array radar model and over driving are tested,It is proved that the liquid crystal optical phased array can realize the tracking of moving targets function of the traditional photoelectric tracking system,Liquid crystal optical phased array near a pixel tracking accuracy and 2Hz closed-loop bandwidth of simulated target perturbations in the fast steering mirror.Although the current loop bandwidth can not meet the requirement of space optical communication now,but with the further development of liquid crystal device,liquid crystal material response speedup,using liquid crystal overdrive method and phased array radar model,The performance of liquid crystal optical phased array will be more and more suitable for space laser communication system.