Research On Optimization Method Of Diffraction Efficiency Of Liquid Crystal Spatial Light Modulator

The liquid crystal spatial light modulator（LCSLM）is an optical device that enables nonmechanical deflection of the light beam.With the advantages of small size,light weight,low power consumption,good stability and random pointing,it has a wide range of applications in the fields of laser x radar,spatial optical communication,autonomous driving and laser guidance.The LCSLM rotates the liquid crystal molecules by applying different driving voltages to each array element,changing the effective refractive index of the liquid crystal,which in turn changes the phase distribution of the emitted light to achieve different angular beam deflection.In laser phased-array radar,the diffraction efficiency of the beam is a very important system performance index,and a low diffraction efficiency will cause a decrease in the radar detection distance and target detection probability.Therefore,this thesis carries out relevant research of the optimization methods of diffraction efficiency of LCSLM,which is mainly as follows.The electro-optic modulation and diffraction properties of LCSLM were studied.Firstly,the electro-optic effect of liquid crystal molecules was explored,and the director distribution of liquid crystal was solved.Then,the principle of beam deflection of LCSLM is explored and electrooptical modulation methods based on periodic and non-periodic flashing gratings are clarified.Secondly,the study of Kirchhoff diffraction principle and Fraunhofer diffraction theory was carried out to explore the far-field diffraction characteristics of LCSLM.Finally,the influencing factors of the diffraction efficiency of the device are analyzed in depth,and the influence of factors such as the flatness of the liquid crystal cell,the phase depression and the return region on the diffraction efficiency of the LCSLM was investigated.And it provides theoretical support for the optimization of beam diffraction efficiency.A diffraction efficiency optimization system for LCSLM was designed to address the problem of low diffraction efficiency of LCSLM caused by the above factors.And diffraction efficiency optimization method based on Stochastic Parallel Gradient Descent（SPGD）and Alopex algorithm are proposed.Firstly,the optimization effect of the two optimization algorithms on the diffraction efficiency under different array element periods are simulated by MATLAB to verify its feasibility.Secondly,a test system for the diffraction efficiency of the LCSLM was built.Finally,the effectiveness of the two optimization algorithms are verified by experiments.The experimental results show that after the optimization of the Alopex algorithm,the diffraction efficiency of the specified order beam is increased by 28.70%-40.37% compared with the unoptimized one,and by 0.74%-3.20% compared with the SPGD algorithm.Aiming at the problem that SPGD algorithm and Alopex algorithm have many iterations and are prone to fall into local optimum,a method for optimizing diffraction efficiency of LCSLM based on spline interpolation was proposed.Firstly,a phase modulation measurement system is built according to the Tyman-Green interference principle.Load the grayscale image of step change on the LCSLM,and draw the relationship curve between the input grayscale and the phase difference of the LCSLM by calculating the movement amount of the interference fringes.Secondly,the phase modulation curve was corrected by the cubic spline inverse interpolation method,so that the input grayscale of the LCSLM and the corresponding phase are close to the ideal linear relationship,and a new voltage grayscale look-up table（Look Up Table,LUT）is gneraed,by loading the newly generated linear LUT table,the compensation for the phase modulation amount is realized.Finally,the effectiveness of the method is verified by experiments.The experimental results show that the cubic spline inverse interpolation method does not need iterative search and does not fall into local optimum,which can effectively improve the beam diffraction efficiency.After applying this method to optimize the diffraction efficiency of the LCSLM,the beam diffraction efficiency is increased by 34.91%-44.65%.Compared with the SPGD algorithm,the diffraction efficiency is increased by 5.62%-7.99%.Compared with the Alopex,the diffraction efficiency is improved.3.0%-6.21%.