High Precision Beam Pointing Control Technology Based On Liquid Crystal Optical Phased Array

High precision beam pointing control based on liquid crystal photoelectric device is a non-mechanical beam control technology which is widely used at home andabroad.The typical device,Liquid Crystal Optical Phased Array(LCOPA),has many advantages,such as programmable electronic control,high pixel resolution,fastresponse,small size,light weight and low power consumption.With the development of modern optoelectronic system towards integration,portability and low powerconsumption,the non-mechanical beam pointing control technology represented by Liquid Crystal on Silicon-Optical Phased Array(LCo S-OPA)has shown obvious performance improvement compared with the traditional mechanical beam steering technology with complex structure and high power consumption,which has great potential in laser radar,space laser communication,laser countermeasures and other research fields.Pointing accuracy is one of the core indexes in the field of beampointing control.To a certain extent,pointing accuracy directly determines theperformance of beam controller.However,the current beam pointing control methods still have some problems,such as insufficient pointing accuracy,limited scanningresolution and unstable pointing position.Therefore,based on phase-only LCo S,the high-precision single beam pointing control and multi-beam parallel control arestudied in this dissertation.The phase generation method commonly used in the existing single-beampointing technology is the Variable Period Grating(VPG)method.Due to the limited phase gray scale of LCo S,the pointing accuracy of this method is insufficient at some local positions and beam scanning interval is limited to micro radians or even 10micro radians.Sub-aperture Coherence method(SAC)was proposed to solve this problem,in which the LCo S panel was divided into two rectangular regions and the VPG phase with similar diffraction angle was loaded respectively.The steering angle of the output light was fine-tuned by adjusting the width of the rectangular region.Although this method can reduce the local pointing accuracy defect of VPG,it alsogenerates the problem of pointing instability when there is deviation of beam diameter and alignment position.Following the basic idea of coherent superposition of sub-beams,the Radial Sub-aperture Coherence method(RSAC)is firstly proposed,in which the double-rectangular sub-aperture structure is changed into a double-sector structure and the geometric parameter controlling the steering angle is changed from the width of the rectangular region to the central angle of the sector.This methodgreatly improves the scanning resolution and effectively alleviates the problem of unstable pointing position when the beam aperture is deviated.Then Symmetric Radial Sub-aperture Coherence(SRSAC)is proposed as an improved method of RSAC,in which the sub-aperture is further transformed into a four-sector structure and each pair of symmetrical sectors form a modulation region.SRSAC inherits the advantages of RSAC and ensures the pointing stability of the output light when the incident beam has alignment error.After the feasibility of the new phase generation method is preliminarily verified,we deeply analyze the problem of residual pointing accuracy defect in SRSAC andwork out the key parameters such as pointing accuracy defect position,width and maximum error.According to this,a pointing accuracy defect elimination method is designed to make the interpolation endpoints automatically avoid the areas withpointing accuracy defects.The residual pointing accuracy defects of SRSAC are completely eliminated by this method and the overall pointing error reduce from several microradians to sub-microradians under the same hardware conditions.On this basis,the theoretical framework of SRSAC is supplemented and improved.Two pointing error branches of SRSAC are quantitatively analyzed and their clearmathematical expressions are deduced.Finally,the correctness of the composite error formula is verified by simulation.In large field of view target detection,multi-beam parallel scanning is often used to replace single beam scanning to improve the scanning efficiency of the system.In this dissertation,a high-precision parallel scanning system with 15~°field of view and40×40 sub-beams is built by using LCo S and Liquid Crystal Polarization Grating(LCPG).This system relies on GS algorithm to calculate the beam splitting phase and separate the single collimated beam directly.Aiming at the problem of zero-orderlight leakage interfering with sub-beams and cannot be eliminated by conventional tilt method,a light leakage avoidance method based on diffraction order splicing isproposed,in which the upper half of the main diffraction order is moved up and the lower half of the upper diffraction order is moved down,so that the two parts are stitched together into a complete beam array at the cut-off frequency and the leakage spot at the zero point of spatial frequency can be avoided.At the same time,theintensity correction is carried out to ensure the overall energy uniformity of the sub-beams.Finally,the leakage avoidance without affecting the quality of the beam array is realized.In the aspect of improving pointing accuracy,an optical optimizationmethod is proposed to unify the actual spot shape by constraining the pre-set sub-spot size to the diffraction limit.On this basis,a variable step feedback correction process of pointing position is introduced to reduce the standard deviation of pointing error from the magnitude of hundred microradians to ten microradians,which meets theapplication requirements.