Research On Influencing Factors And Improvement Methods Of Imaging Quality Of Sheared-beam Imaging

Sheared-beam imaging(SBI)is a computational imaging technique which utilized three sheared coherent laser beams for illumination,and a detector array to receive the intensity of speckle pattern reflected from the target.SBI can be used to image remote objects through atmosphere without the need of any adaptive optics.In this dissertation the key technology of improving SBI image quality is the main research content.The imaging technology was systematically studied and discussed from the aspects of imaging principle,system composition,imaging quality influencing factors and laboratory experiment verification.The main work and achievements of this dissertation include:Based on the study of the basic principle of SBI,the specific relationship between the resolution of imaging system and the baseline length of detector array was deduced.The effect of target spectrum amplitude information in image reconstruction was studied.The results showed that the imaging quality was mainly affected by the phase,which pointed out the direction for further research on the influencing factors of imaging quality.Finally,a reference-free image quality evaluation method suitable for evaluating the image quality of SBI was proposed.In the study of the factors affecting the imaging quality in SBI system,according to the theoretical analysis and simulation results,laser energy allocation method for SBI high-quality imaging was proposed.According to the effects of laser frequency error on the spectrum of the object,solutions were proposed.In addition,a shear error influence model of SBI transmitter system was established.The mechanism of shear error affecting image quality was analyzed,and the allowable range of shear error was proposed.The formula for calculating the field of view of SBI was deduced,and a detector array arrangement scheme for high resolution imaging of the local areas of target was proposed.Finally,the influence of detector array spacing error on image quality was analyzed.The image quality degradation effects caused by various atmospheric turbulence effects were analyzed.The influence of 25 km atmosphere on SBI beams was simulated by using multi-layer phase screen model,and the effects of phase fluctuation on laser wavefront quality on target surface under different laser emission apertures and imaging distances were studied.Finally,an imaging method by laser common aperture transmission was proposed to suppress various atmospheric turbulence effects.The influence of target shape and surface characteristics on speckle field were analyzed.The mechanism of laser echo signal degradation caused by target depth was studied.The combined effects of target depth and laser frequency difference on SBI imaging quality was simulated by computer.Finally,the influence of target translation and rotation on SBI imaging were analyzed.A method of estimating target velocity by calculating the frequency change of echo signal was proposed.The source and characteristics of speckle noise in SBI images were studied,and a registration method based on morphological filtering was proposed.By acquiring the transformation parameters between images after morphological filtering,the target areas of original SBI images were aligned.Finally,the speckle noise in the image was eliminated by multiple-image averaging.Image registration accuracy and image quality were improved.The experimental results showed that the contrast-to-noise ratio was increased by an average of about 8%.SBI laboratory verification system was built.SBI principle and imaging quality influence factors were studied experimentally.The parameters of SBI system,models of the influence of turbulence and target on laser echo signal and the method of speckle noise elimination,which were studied in this dissertation,would provide theoretical basis for improving the imaging performance of SBI.The researches would effectively propel the progress of SBI technology.