Research On The Distributed Holographic Aperture Imaging Technology

Resolution is one of the most important parameters for all imaging systems.In order to improve resolution,it is often necessary to increase the aperture of the system.Accordingly,the large aperture technology and the distributed aperture technology are developed,of which,the development of a monolithic aperture imaging system is limited by optical fabrication technology,support of large aperture primary mirror,and carrying capacity.The distributed aperture imaging is an alternative scheme to monolithic aperture imaging.The wavefront of target can be sampled by an array of sub-apertures,and a high-resolution image can be synthesized by co-phasing these wavefronts of sub-apertures.It is able to reduce the weight,volume,and process difficulty.However,the detection and correction for atmospheric turbulence aberrations and the co-phasing error among sub-apertures make the system quite complex,so that it is also a limitation and challenge for the applications with relatively strict weight and volume requirements.With the development of electronic information technology,the digital distributed aperture imaging technology has been developed gradually.It occupies the digital holography technology to digitally detect the back-propagation wave-fronts scatterred by a distant target.Then the high-resolution image can be synthesized digitally in the computer.Therefore,the complex Adaptive Optics systems and Active Optics systems can be discarded so that the weight and volume can be greatly reduced.However,some new challenges have been brought,such as accurate recording of complex amplitude,real-time detection and correction of common phase error.In this paper,the digial holography technology is occupied to digitally detect the sub-aperture complex amplitude,and the distributed holographic aperture imaging system is designed.Moreover,the equivalent resolution of the system,complex amplitude recording and extraction conditions of off-axis digital holography,digital co-phasing and speckle suppression technology are analyzed and studied.Because the CTF(coherent transfer function)of the distributed aperture imaging system under coherent illumination is discontinuous,the equivalent aperture of the system can not be accurately analyzed by the cut-off frequency of CTF.It proposes a new method to estimate the equivalent aperture.Because the image of an object for an coherent imaging system is the coherent superposition of all image points on the image surface,this paper calculates the coherent superposition of ASF(amplify spread function)curves of the corresponding target image point and its adjacent image points by scanning the distance and relative phase of them,so as to judge whether the image point can be distinguished.According to Rayleigh criterion,when the target image point can be distinguished from the nearest image points for any phase relationship,the angle between the target points and the nearest image point is considered as the angular resolution of the coherent distributed aperture imaging system.The ratio of unit aperture resolution to this resolution under coherent illumination is regareded as the equivalent aperture.Theory and experiment demonstrate that this method is useful,and the resolution can be estimated accurately.For the problem of co-phase between sub-apertures,this paper designs the algorithm of high-order Zernike aberration correction,complex amplitude shape and position errors' correction,and the co-phase between sub-apertures.Firstly,the shape and position errors of the wavefront of each sub-aperture are analyzed,and the limitations of the existing methods for estimating the wavefront shape and position errors between sub-apertures based on the similarity transformation model are analyzed.It is proved that the results of the synthetic co-phase method based on the mosaic of pupil field have large errors.A new digital co-phase algorithm for image plane interference is proposed.Based on the principle of interference imaging,the shape and position errors of complex amplitudes of each sub-aperture are corrected on the image plane,and then the co-phasing errors between sub-apertures are corrected by optimization algorithm.The high-resolution imaging is realized gradually,avoiding solving the pupil field transformation matrix by the similarity transformation model according to the image plane transformation matrix.Therefore,it has a better effect and wider application.For the speckle noise problem in the single-aperture reconstruction image and the 4-aperture synthetic image,it proposes an average algorithm by various tip-tilt phase modulation to suppress the speckle of the reconstruction image in single-shot digital hologram or the synthesized image for the 4-aperture distributed imaging system.The method is demonstrated to be able to attenuate the speckled noise almost without decreasing the resolution.And it also proposes to apply this average method as the preprocessing method of BM3 D in order to denoise with minimum resolution loss and maximum denoising effect as much as possible.Lastly,a four-aperture distributed aperture imaging is designed by moving the holographic aperture for three times.And a high-resolution image for four-aperture distributed holographic aperture imaging is finally realized.Compared with single aperture imaging,the imaging resolution and signal-to-noise ratio are greatly improved.Theoretical and experimental results demonstrate that distributed holographic aperture imaging can improve the resolution compared to the single aperture imaging,so it is a very potential imaging technology.