Research On Design And Mid-Frequency Information Improving For Optical Sparse Aperture System

Optical sparse aperture(OSA)imaging system uses the combination of multiple small apertures system to equivalent the imaging effect of large aperture optical system,so as to improve the high frequency performance of optical system.OSA imaging system can effectively improve the resolution of optical system while maintaining low emission volume and weight.However,due to the discretization and sparsity of the subaperture distribution of the OSA system,the modulation transfer function(MTF)of the system is reduced significantly or even missing in the mid-frequency band,which will inevitably decrease the image contrast and quality.The main purpose of this paper is to improve the imaging performance of OSA system.In order to improve and compensate the mid-frequency information of OSA system,the relationship between filling factor and MTF of OSA system is studied.The research of mid-frequency information enhancement and compensation for two kinds of systems with large filling factor and small filling factor is carried out.The system design and aberration analysis of OSA system are the basis of engineering application of OSA system.The structure of OSA system is different from that of traditional single aperture system,which brings new problem s to the design of OSA system.Firstly,the position error and aberration tolerance of sub-aperture are analyzed based on wave aberration theory.Then,based on the given tolerance of sub-aperture system,the design methods of two types of OSA system are analyzed,and the basic design process of two types OSA system is given.Finally,two kinds of systems are designed based on Zemax.The simulation results of Zemax show that the two systems have good imaging quality,which proves the feasibility of the design process given in this paper.At present,image restoration method is often used to improve the frequency information in OSA system.However,there are few studies on the influence of space-variant point spread function(PSF)of optical system on frequen cy information improving in image restoration.In order to solve this problem,this paper firstly explains the phenomenon of edge ringing and false texture in image restoration process caused by the space-variant point spread function.Then this paper points out that the use of space-variant image restoration method will better reduce the phenomenon of edge ringing and false texture.Then,in order to select the restoration methods types,the paper defines the Degree of Variance(DOV)as a index of space-variant characteristics of optical systems.On this basis,a method of image restoration types selection is proposed.And the reference value of DOV is 0.01 by a lot of simulation experiments.That is,when the DOV of the system is less than 0.01,the space-invariant image restoration method is adopted,and when the DOV is greater than 0.01,the space-variant restoration method is used.Finally,the simulation model of space-variant OSA system is presented,which can be used to simulate the images of OSA syst em and test the image restoration algorithms.A method of improving mid-frequency MTF of OSA system with large filling factor based on a direct space-variant image restoration method is proposed.Firstly,the basic problems of space-variant image restoration are analyzed.Aiming at the acquisition of space-variant PSF of all field of view(FOV),principal component analysis(PCA)and point interpolation method(PIM)is proposed.On this basis,the framework of OSA image restoration method based on space-variant image restoration is given.Laplace regularization image restoration method is modified to space-variant image restoration method.Finally,the digital simulation results show that space-variant restoration method can improve the image quality better than spatial invariant restoration algorithm.For three typical images,the improved signal-to-noise ratio(ISNR)of restored images can be increased by 10% at 20 dB,12% at 30 dB and 17% at 50 dB,respectively.A method of compensating mid-frequency MTF of OSA system with small filling factor based on image fusion is proposed.Firstly,the correctness of the principle of mid-frequency loss is proved by experiments.Then the models of mid-frequency compensation schemes for OSA systems are established based on Fourier optics principle.Then,the images obtained by the single-aperture system and the OSA system are fused by a pixel-level image fusion method to obtain the full spatial frequency images.Finally,in the digital simulation experiment,the target images obtained by the two systems are fused.The experimental results show that the spatial frequency of the image is well compensated.