Research On The Method Of Fabricating Pixelated Polarization Array By Laser Micromachining

Polarization imaging can compensate for the information loss in optical intensity detection and expand the scope and depth of observation.Division of focal plane polarimeter(Do FP)polarization imaging is one of the most popular polarization imaging methods.It has the advantages of real-time,simple optical path and easy integration.Polarization filter is an important part of Do FP.Typical polarization filter fabrication methods have high fabrication difficulty,complex processing steps and strict requirements on the operating environment.Moreover,it is difficult to obtain a polarizer with large extinction ratio and uniformity by using these methods.This study overcomes the shortcomings of the existing preparation process and proposes a simple and efficient preparation method for multi-channel polarization filters.Laser micro-nano processing method has the advantages of simple process,flexible operation,no need of mask,etc.In this study,a fabrication process of multi-directional polarization filter is proposed.The process employs a picosecond laser to ablate the unidirectional sub-wavelength metal grating polarizer to form a regular polarization array,and then obtain a multi-directional polarization filter by cementation.This method can achieve large-area fabrication of polarization grating array with high extinction ratio.This paper combines simulation calculation with experimental research.The variation trend of extinction ratio and transmittance of the polarizer with the height of grid layer is simulated.By analyzing the change trend of the extinction ratio and transmittance of the polarizer,the laser ablation research is guided.The influence of laser processing parameters on the ablation morphology of subwavelength metal wire grid polarizer is studied by laser experiment,and the laser processing parameters are optimized.The transmittance and extinction ratio of the polarizer after laser ablation and the performance of the polarization filter are analyzed by building a polarization test system.The main research work and achievements of this paper are as follows:(1)Simulation of transmittance and extinction ratio of sub-wavelength metal wire grid polarizer.The variation of transmittance and extinction ratio of sub-wavelength metal wire grid polarizer with the height of grid layer is simulated by rigorous coupled wave method.The calculation results show that the transmittance of the grating increases gradually in the process of decreasing the aluminum grid layer from 120 nm to 0nm,and finally the transmittance rises to 0.96～0.97.In this process,the extinction ratio(550 nm)decreases exponentially,and when the height of the aluminum layer is less than 20 nm,the extinction ratio is close to 1.Therefore,in theory,the transmittance of the polarizer ablation area after laser ablation should be increased to 0.96 and the final grating height should be less than 20 nm.Then the extinction ratio should be close to 1.(2)Experimental study on polarization filter fabricated by laser processing.In the laser ablation experiment,the influence of four main processing parameters(laser repetition frequency,laser power density,laser pulse coincidence rate and laser scanning line spacing)is analyzed by comparing the ablation morphology.The four parameters jointly affect the total energy of the laser projected onto the polarizer and heat conduction,and determine the uniformity of the laser ablation area.The optimized laser processing parameters is determined as the laser power density 5.89×10~5 W/cm~2,pulse coincidence rate 82.17%(laser frequency 1.0 MHz,scanning rate 3333 mm/s),scanning line spacing0.008 mm.According to the comparison of SEM images,energy spectrum data of aluminum content and transmittance curve of polarizers before and after laser ablation,it is verified that the polarizability of polarizers is eliminated,and the transmittance is increased to 0.96 after laser ablation.Finally,the three-direction polarization filter is fabricated by a three-layer quasi-adhesive process.(3)Polarization performance test and imaging error analysis of polarization filter.The test system of polarization filter is built,and angle deviation and imaging calculation error of polarization filter of the polarization filter are tested.The test results of the polarization filter sample show that the polarization of the ablation area is completely eliminated and the polarization of the reserved area is retained,and the original error of the polarization filter angle is within±0.3°.The imaging test results of the 200μm polarization filter show that the filter can accurately distinguish the polarization region from the non-polarization region,and the calculated error is less than 0.5°.Two tests prove the effectiveness of the polarization filter preparation method proposed in this paper.