Study On Beam Deflection With Liquid Crystal Polarization Grating At Oblique Incidence

Liquid crystal polarization grating(LCPG)is a geometrically phase-based diffractive optical element,light weight and programmable control,which has a wide range of applications in the fields of LIDAR,space optical communication and autonomous driving.The LCPG adjusts the phase and polarization of incident light by electronically changing the orientation of the liquid crystal,and overcomeing the disadvantages of traditional mechanical beam deflection systems,such as large size,high power consumption and complex control systems.However,a single LCPG can only deflect three angles and to achieve multi-angle and large field of view beam deflection,multiple LCPGs need to be cascaded.However,the cascade technique can cause the beam to be tilted to the back of the LCPGs,in addition,in practical engineering applications,the mismatch in system mounting can cause the incident light to be tilted to the cascade system,both of which can result in a change in the operating voltage of the LCPGs and a reduction in diffraction efficiency.Therefore,this paper carries out a study of beam deflection techniques under oblique incidence of LCPGs.The main work of the paper is as follows:The beam deflection mechanism and diffraction characteristics of LCPG are investigated.Firstly,the anisotropy of liquid crystal and the theory of continuous elastomer deformation are investigated to clarify the light polarization and its representation;then,the basic structure of LCPG is studied from the preparation principle,and the beam deflection mechanism of LCPG is explored;finally,the electro-optical properties of the LCPG are investigated,and the relationship between the phase delay and diffraction efficiency of the LCPG at vertical incidence is derived by combining the Jones matrix,and the diffraction characteristics of the liquid crystal polarized grating at vertical incidence are analyzed to provide theoretical support for the study of beam deflection technology of LCPG at oblique incidence.The beam deflection characteristics of LCPG under oblique incidence are studied.The effects of liquid crystal box thickness,birefringence and Fresnel loss on the diffraction efficiency of l LCPG are analyzed,and a diffraction efficiency analysis method for LCPG under oblique incidence is proposed.Based on the theory of elastic deformation of liquid crystal continuum,the method solves the relationship between the driving voltage and the tilt angle of liquid crystal molecules by using the difference iteration method,derives the phase delay of LCPG at different oblique incidence angles,characterizes the transmittance of liquid crystal polarized grating at different incidence angles by using the extended Jones matrix,and obtains the mathematical expression of oblique incidence angle-driving voltage-diffraction efficiency by combining with the vector diffraction theory.The mathematical expressions of oblique incidence angle-drive voltage-diffraction efficiency is obtained.The proposed method is validated by constructing an experimental platform for testing the diffraction efficiency of LCPG under oblique incidence.The results show that the optimal diffraction efficiency and the optimal operating voltage can be quantitatively solved for different oblique incidence angles.The design of cascaded LCPGs and the performance of the system under oblique incidence were investigated,binary,binary sum and binary-like cascading schemes were explored,the number of liquid crystal devices,system transmittance and maximum deflection angle of the three schemes were analyzed,a binary-like beam deflection system was designed,and a beam deflection with a maximum deflection angle of 10.05° and an angular resolution of 0.67° was achieved;a multi-channel programmable driver and control software for the cascaded system are developed,and a method for analyzing the beam deflection characteristics under oblique incidence of cascaded LCPGs is proposed,using which the effects of the incident beam tilt and oblique incidence within the cascaded system on the diffraction efficiency and operating Voltage of the system are analyzed.Experimental verification is carried out for both oblique incidence cases.The results show that when the beam is incident vertically,the cascade internal structure causes the cascade system to have different diffraction efficiency of the outgoing beam at different deflection angles,and the operating voltage of the LCPGs gradually increases as the beam deflection angle increases.At the beam deflection of 10.05°,the system oblique incidence angle is 4.69° and the operating voltage increases by 0.1V,by correcting the operating voltage diffraction efficiency increases by 3%;when the beam is tilted by 10° to be incident on the cascade system,the operating Voltage changes by 0.2V and the diffraction efficiency of the system is reduced by 17.4%,by correcting the operating voltage the diffraction efficiency is increased by 5%.