Study On Optimization Method Of Diffraction Efficiency Of Cascade Liquid Crystal Polarization Gratings

Liquid crystal polarization grating is a new type of liquid crystal device that can realize nonmechanical beam deflection.It has the characteristics of lightweight,miniaturization and low power consumption.It is widely used in laser radar,space optical communication,scanning imaging and other fields.However,the scanning field of view of the monolithic liquid crystal polarization grating is limited.In order to realize beam scanning with high resolution and large field of view,it is necessary to alternately cascade multiple liquid crystal polarization gratings and liquid crystal variable retarders to form a cascaded liquid crystal polarization gratings.However,the oblique incidence phenomenon of the beam generated during the propagation of the cascaded liquid crystal polarization gratings will depolarize the circularly polarized light emitted by the liquid crystal variable retarder,resulting in a decrease in the energy utilization rate of the liquid crystal polarization grating,thereby reducing the diffraction efficiency.Therefore,in order to achieve beam deflection with high diffraction efficiency,it is of great theoretical and application value to study the diffraction efficiency optimization method of cascaded liquid crystal polarization gratings.The main research contents are as follows:Starting from the electro-optic effect of liquid crystal,the diffraction characteristics of single-chip liquid crystal polarization grating and the phase modulation characteristics of liquid crystal variable retarder are analyzed.Using the binary cascade scheme,a cascade system consisting of four sets of liquid crystal polarization gratings and liquid crystal variable retarders is designed,which can realize beam deflection with a scanning field of view of ± 5° and an angular resolution of 0.33°.The factors affecting the diffraction efficiency of the cascaded liquid crystal polarization gratings are analyzed.Combined with the extended Jones matrix,Stokes parameters and vector diffraction theory,a three-dimensional model of the oblique incident angledepolarization-diffraction efficiency of the cascaded liquid crystal polarization gratings is constructed.The mathematical formulas of the depolarization and the retardation of the liquid crystal variable retarder are derived,which provides theoretical support for the optimization of diffraction efficiency.Aiming at the problem of low diffraction efficiency of cascaded liquid crystal polarization gratings,the optimization method of diffraction efficiency based on immune algorithm and Alopex algorithm is analyzed.Under different deflection angles of cascaded system,the optimization effect of diffraction efficiency is simulated by two optimization algorithms.The results show that the two optimization algorithms can improve the diffraction efficiency,but it is easy to fall into local optimum.In order to further improve the diffraction efficiency of cascaded liquid crystal polarization gratings,a retardation model of liquid crystal variable retarder under oblique incidence is constructed.Based on the phase modulation characteristics of liquid crystal variable retarder,combined with the mathematical formula of depolarization and retardation of liquid crystal variable retarder,a diffraction efficiency optimization method based on polarization compensation is proposed.This method can calculate the working voltage of the liquid crystal variable retarder corresponding to the optimal diffraction efficiency of the output light of the cascade system,and optimize the diffraction efficiency.The results show that when the deflection angle of the cascaded liquid crystal polarization gratings is 1°,the diffraction efficiency of the cascaded system is optimized by the polarization compensation method,and the diffraction efficiency is increased from 84.21% to 91.75%,which is increased by 7%.Compared with immune algorithm and Alopex,the diffraction efficiency is improved by 6% and 4% respectively.In order to realize the beam scanning with high diffraction efficiency of cascaded liquid crystal polarization gratings,an overall scheme of control software for cascaded liquid crystal polarization gratings based on polarization compensation method is designed.Input the working voltage of the liquid crystal variable retarder through the human-computer interaction interface,and use the serial port communication module to send voltage commands to the lower computer,and then modulate the retardation of the beam passing through the liquid crystal variable retarder,and optimize the diffraction efficiency of the cascaded liquid crystal polarization gratings.Using the scanning control module to control the cascaded liquid crystal polarization gratings,the beam scanning with a scanning field of view of ± 5° and an angular resolution of 0.33° is realized.Compared with before voltage optimization,the diffraction efficiency is improved by more than4%,which verifies the effectiveness of the proposed method in practical application scenarios.