Research On Trigger Characteristics Of Polarization Detection Of GM-APD Lidar Target Material

Polarization detection technology is an important development direction of laser imaging radar.The Geiger-mode Avalanche Photodiode(GM-APD)lidar with high sensitivity is extremely susceptible to noise,which reduces its detection effect during the day.In order to realize the high signal-to-noise ratio imaging of GM-APD lidar in the daytime and reduce the influence of noise on detection,this paper explores the material polarization detection trigger characteristics of GM-APD lidar.First,based on the six-parameter Polarized bidirectional reflectance distribution function(PBRDF),the polarization degree of the scattered light from different polarization states of the light source is studied in two cases: the transceiver is combined and the transceiver is separated.The influence of incident angle,scattering angle,material optical constant,and material surface roughness on it is analyzed.The research results show that when light sources with different polarization states are incident,the polarization states of the scattered light show obvious differences,which makes it feasible to reduce the background noise of natural light through the polarization system.When circularly polarized light is incident,the greater the refractive index of the material,the greater the absorption coefficient,the smaller the surface root mean square height,and the higher the degree of polarization of scattered light.Natural light is more complicated.The increase in surface roughness will broaden and lower the curve in each case.Secondly,in order to reduce the background noise of GM-APD detection,based on the polarized GM-APD lidar equation,the trigger characteristic law under the polarization system is studied.The influence of waveplate angle,target distance,signal-to-noise ratio,attitude angle(incident angle)on the triggering of polarization detection is analyzed.The research reveals that the polarization system can improve the target trigger probability.When the target distance is slightly farther,the lower the signal-to-noise ratio,the smaller the laser incident angle,and the more obvious the effect of the polarization system.At any distance,signal-to-noise ratio,and laser incident angle,the polarization system can dramatically reduce the false alarm probability.The optimal angle of the polarization system is the wave plate angle of 135 °.Aiming at the problem that mixed noise will cause deviations in the calculation of the echo polarization degree,the influence of noise on the echo polarization degree is studied.The research reveals that when the echo is doped with natural light noise,the calculated target polarization degree will be reduced.The lower the laser incident angle,the lower the echo signal-to-noise ratio,and the greater the root mean square height of the material,the greater the deviation.Then,the GM-APD polarization imaging lidar system was built.The influence of polarization angle and attitude angle(laser incident angle)on the trigger characteristics of each material under low noise and xenon lamp noise is studied.The research reveals that the trigger probability of each material achieves the valley and peak values at the polarization angles of 45 ° and 135 °,respectively.The greater the laser incident angle,the lower the trigger probability.The false alarm rate of each material has no obvious change with the polarization angle.The target trigger characteristics of polarized and nonpolarized systems under strong sunlight background noise are studied.The research reveals that the polarization system effectively reduces the influence of background noise on GM-APD detection,can increase the target trigger probability,and dramatically reduce the false alarm probability.The greater the incident laser energy,the smoother the surface,and the more significant the effect of the polarization system.For the metal plate material,when the polarization angle is 135 °,the trigger probability is increased by 33.8%compared with non-polarization.The false alarm probability under any polarization angle is lower than that of the non-polarized system.The false alarm probability of the metal plate is reduced by 0.08?0.10 compared with the non-polarized system,and the false alarm rate of the rough wood board is reduced by about 0.10.Finally,the change of polarization degree of different materials with the angle of laser incidence was studied,and the PBRDF model was fitted.The results show that for ceramic tile,marble,and metal materials,the polarization degree of similar materials exhibits similar laws,but there are huge differences between different materials.The degree of polarization of ceramic tile materials decreases step by step with the increase of the incident angle,marble materials change little with the incident angle,and metal materials show a similar linear decrease with the increase of the incident angle.Based on the GM-APD trigger model,a polarization correction method is proposed,and the echo polarization under the noise of the xenon lamp is corrected and compared.The results show that the polarization degree correction method can effectively remove the background noise mixed in the target echo,so as to obtain a more accurate target polarization degree.For smoother metal targets,the corrected target polarization degree can be increased by 7% compared to the uncorrected one;for slightly rough wood and cement paint surfaces,the corrected target polarization degree can also be increased by3% to 5% as a whole.