Research On Liquid Crystal Spatial Light Modulator Beam Pointing Control Method

Liquid crystal spatial light modulator(LCSLM)is a programmable phase modulation optical device that modulates the wavefront phase of the incident beam by applying a voltage to achieve beam deflection.It has the characteristics of fast scanning speed,beam agility,and high resolution.It is widely used in the fields of lidar systems,laser shaping and space optical communications.Due to the limitation of the manufacturing process level of the liquid crystal spatial light modulator and the influence of atmospheric interference,the error of the beam deflection angle increases and the pointing accuracy decreases.Therefore,the optimization method based on the particle swarm algorithm and the BP neural network PID control strategy are used to carry out the research on the beam pointing accuracy optimization of the liquid crystal spatial light modulator and the LCSLM beam deflection closed-loop control system respectively.The details are as follows:The beam modulation mechanism of the modulator of the liquid crystal spatial light modulator is studied,the electro-optical characteristics of the liquid crystal molecules are analyzed,the basic structure of the reflective liquid crystal spatial light modulator is introduced,and the total Gibbs free energy in the liquid crystal cell is calculated.The difference iteration method is used to solve the free energy density,the director distribution of the liquid crystal molecules is derived,and the corresponding relationship between the voltage and the phase retardation of the liquid crystal is obtained from the refractive index anisotropy of the uniaxial crystal.Based on Kirchhoff's diffraction theory,the far-field light intensity distribution of the beam deflection of the liquid crystal spatial light modulator is explored,which provides theoretical support for the optimization of the pointing accuracy of the liquid crystal spatial light modulator.In the process of beam modulation,the pointing accuracy is reduced due to disturbance factors.Based on the basic principle of beam deflection,a beam deflection model of liquid crystal spatial light modulator is established.The aberrations caused by edge effect,duty cycle of electrodes,fluctuation of LCD box surface,voltage quantization and external electric field frequency are analyzed.An optimization method for directional accuracy based on particle swarm optimization is proposed,which takes the deflection angle of far field as the optimization target.The simulation results show that the algorithm can reduce the beam deflection angle error very well,and the optimized beam deflection angle is close to the ideal angle.Aiming at the problems of slow beam deflection response speed,poor stability,and susceptibility to interference,the system modeling and control system design research of liquid crystal spatial light modulators were carried out,the relaxation characteristics of liquid crystals were analyzed,and the control model of liquid crystal spatial light modulators was established through system identification.Because the traditional PID control system has shortcomings such as large overshoot,long adjustment time,and susceptibility to interference,the BP neural network control strategy is adopted.The simulation results show that the PID control system based on BP neural network can learn and adjust the control parameters online,which has a good suppression effect on overshoot,and the system's anti-interference ability is also improved.The BP neural network PID control strategy has better stability and robustness for the liquid crystal spatial light modulator beam deflection closed-loop system.The simulation results show that the PID control system based on BP neural network can learn and adjust the control parameters online,which has a good suppression effect on overshoot,and the anti-interference ability of the system is also improved.The BP neural network PID control strategy is effective for the beam deflection of the liquid crystal spatial light modulator The closed-loop system has better stability and robustness.It provides theoretical basis and guidance for the demand of high-precision beam pointing of liquid crystal phased array.