Experimental research of circular incoherently synthetic aperture imaging ladar using chirped-laser and heterodyne detection

Abstract

Synthetic aperture imaging ladar (SAIL) technique belongs fully coherent processing in both the time domain and space domain and has a rather high implement difficulty. To solve this problem, the concept of circular incoherently SAIL is introduced. A speckle version image of a two-dimensional (2D) letter 'E' target is reconstructed from E-field projection data detected by a circular incoherently SAIL system. The experimental system is constructed by three subsystems using chirped-pulse laser as the light source and heterodyne detection to get the range information of the target. The reconstruction of the image and the noise effect are also discussed in detail.OCIS The technique of high resolution imaging from a long distance has been pursued by researchers for many years, and many different imaging methods have been developed, in which the most promising method is the synthetic aperture imaging ladar (SAIL) technique [1] . SAIL technique integrates reflective information of the target from different angles in coherent ways and can provide the centimeter-class resolution with an aperture size no larger than a few meters on an observation range of thousand kilometers or more. But the SAIL technique belongs fully coherent processing in both the time domain and space domain, and has a rather high implement difficulty. To solve this problem, the concept of circular incoherently SAIL technique is introduced in this letter.The circular operating mode is one of the spotlightmodes in synthetic aperture radar (SAR), which is based on the relative rotation between the radar and target. It is first proposed by M. Soumekh [2,3] . After that, TszKing Chan et al. proposed a confocal algorithm for circular SAR [4−6] . Experiment of radar division of Georgia Tech Research Institute [7−9] and Technische Universitat Miinchen [10−13] has proved the feasibility of circular trajectory in signal collecting. The circular incoherently SAIL collects 360• data of projections around the target, and a two-dimensional (2D) image of the object is reconstructed by tomography algorithm from incoherent processing. Compared with other spotlight-modes, circular scanning trajectory breaks up the restriction on the extent of spatial spectrum, and as a result, high resolution image can be acquired from the circular incoherently SAIL.In this letter, we propose a circular incoherently SAIL experimental system which can get the image of the whole target region covered by the laser beam. In the experiment, a 2D target of letter "E" is placed on a plane with a tilt angle to the horizontal plane and rotated about the axis perpendicular to the tilt plane. During the experiment, the target is illuminated by a linear FM laser, and the range-dependent return signal is collected by a heterodyne detective optical system. Filtered backprojection tomography algorithm is used to reconstruct the target, and a speckle version image result which has distinct feature description of the target is obtained.For an incoherently SAIL, the range-resolved are first collect...