Study On Improving Imaging Quality Of Laser Active Illumination

Laser active illumination imaging uses the characteristics of laser high brightness,high directivity,high monochromaticity,and good coherence to achieve target illumination in low visibility environments,overcomes the limitations of passive imaging,and improves the photoelectric imaging system at long distances,Detection and recognition capabilities in the field of dark targets.However,when the laser transmitts in the atmospheric channel,it is easily affected by atmospheric turbulence,which causes the spot drift on the receiving surface and the random fluctuation(turbulence)of the light intensity,which greatly reduces the imaging quality of the detection system.In view of the above problems,this article mainly studies the influence of atmospheric turbulence on the laser active illumination imaging system,analyzes the principle of improving the imaging quality of the system by overcoming atmospheric turbulence,and adopts two technologies of multi-beam,multi-wavelength illumination and post-image processing to reduce turbulence the impact of fluctuations on active imaging systems.The research work mainly includes the following aspects:1.First of all,we have studied the influence mechanism of atmospheric turbulence on laser beam transmission.Based on the Rytov perturbation approximation theory,the atmospheric refractive index fluctuation model is studied,and the light field distribution of the illumination light field and the echo light field under the condition of the Gaussian beam passing through the Kolmogorov spectrum weakly undulating turbulence is given,including the average intensity and intensity fluctuation of the light field Coherent length with the atmosphere,etc.2.Based on the theoretical results of the illumination light field under weak undulating turbulence,we propose the use of multiple beams and multiple wavelengths The method suppresses the optical turbulence flicker,and the post-event image processing technology corrects the turbulence distortion wavefront.According to the light field transmission model under weak turbulence and the imaging principle of the optical system,the above two methods have been theoretically verified.3.On the basis of theoretical verification,we established the active lighting imaging simulation system based on MATLAB platform.The Kolmogorov spectral model combined with power inversion method is used to simulate the weak turbulent atmosphere.Combined with free space optical transmission and step-by-step phase screen method,the numerical simulation model of the echo optical field is established,which is from the transmitted optical field to the target optical field,and then transmitted to the receiver through the atmosphere after reflection.Based on the active lighting simulation model,we carried out:(1)simulation research on turbulence suppression of multi beam emission,(2)simulation research on turbulence suppression of multi wavelength emission,(3)simulation research on turbulence suppression of multi beam and multi wavelength illumination.The simulation results are in good agreement with the theoretical analysis,which verifies the effectiveness of multi beam and multi wavelength emission in suppressing the intensity flicker of illumination field.4.The laboratory and field experiments of active lighting were carried out.Based on this experimental platform,we have completed:(1)experimental study on turbulence phenomenon of single beam illumination,(2)experimental study on turbulence suppression of beam emission,(3)experimental study on turbulence suppression of multi wavelength illumination,(4)experimental study on turbulence suppression of two beam multi wavelength illumination.The experimental results are in good agreement with the simulation and theoretical analysis,which proves the effectiveness of multi beam and multi wavelength emission in suppressing the flicker of illumination field.The field test of active lighting is carried out,and the effectiveness of active lighting technology to enhance the echo intensity of long-distance target is verified by measuring the echo data of the observed target under the condition of active lighting.5.Aiming at the problem that the target pattern collected by the receiver is disturbed by the downward turbulence and the detector photon noise in the active illumination imaging application scene,we propose an improved spatial modulation phase difference method.Firstly,the basic principle of the improved spatial modulation phase difference method is introduced,and its excellent performance in wavefront detection and image reconstruction is verified by numerical simulation and experiments.