Polarization Imaging Based On Twisted Liquid Crystal Micropolarizer Array

The division of focal-plane polarimeter(Do FP)has unique advantages of compact structure and real-time dynamic image acquisition by introducing micropolarizer array on the image sensor.Do FP has important potential applications,such as camouflage target reconnaissance,biological medicine,defect detection and other fields of military and civilian,and becomes the research hotspot in the field of polarization imaging.At present,metal-wire-grid polarizer(MWGP)array is the core element in the mainstream Do FP.However,its uniformity and optical properties are limited by the nanoscale-accuracy processing technology,thus,it is difficult to meet the requirements of high precision polarization measurement and imaging applications.While liquid crystal micropolarizer(LCMP)array based on the photoalignment technology has unique advantages of flexible design,simple micron-scale fabrication process and superior performance,which is expected to achieve cost-effective and high-precision polarization measurement and imaging.Therefore,the design,fabrication and polarization imaging of the twisted liquid crystal micropolarizer array are studied in this thesis.Firstly,based on the polarization rotation mechanism of the twisted liquid crystal(LC),the LCMP adopts the bidirectional-twisted structure of ± 22.5° and ± 67.5°,the LC material with large birefringence LC-4k and 6.6 ?m in the LC layer thickness.Their average polarization efficiency can reach 0.9996 and 0.9966 in the 400-800 nm visible band,respectively.Based on the theory of LC elastomer,the distribution deviation of LC director between different twisted sub-pixels are discussed.The optimal structure of LCMP and the size limitation of the sub-pixel is given.Then,rubbing-photo mixed alignment technology is adopted for fabrication and the digital micromirror device(DMD)projection lithography system is used to carry out the pixelated photoalignment.Additionally,the parallelism error of the rotatable polarizer causes the sub-pixel overlap dislocation and the gap of DMD micromirrors causes the grid defect.These two problems are well settled by introducing piezoelectric platform in DMD system for real-time dislocation correction and superposition exposure.A series of uniformed twisted LCMP samples without defects are successfully fabricated.The extinction ratios of 22.5° and 67.5° single twisted sample are about 6000 and 1500(@ 632.8 nm),respectively,and the maximum polarization transmittance is consistent with that of the back-end polarizer(about75%),which can meet the application requirements.Furthermore,a Do FP imaging system is designed and built based on the principle of secondary imaging using the twisted LCMP array.The alignment between twisted LCMP sub-pixels and CMOS sensor pixels is studied.The rotation error is controlled within 0.03° by Moiré fringe method.The coordinates of each sub-pixel in twisted LCMP on the sensor are determined by using correlation algorithm,and the measurement matrix of each super-pixel is calibrated by using polarization data reduction matrix method.After calibration,the polarization imaging system has a high precision of linear polarization degree(Do LP)and angle of polarization(Ao P)which are less than 1.10% and 0.30°,respectively.And the natural objects whose Do LP change of about 0.1 can be clearly distinguished.Then,a novel polarization-intensity dual-mode imaging based on switching the back-end polarizer is proposed.These results verified the good performance and diversified application potential of twisted liquid crystal micropolarizer array.The research content and related results in this thesis provide theoretical basis and practical experience for the development of polarization imaging based on twisted liquid crystal micropolarizer array.It is expected to realize practical application and has important research significance.