Down-Looking Synthetic Aperture Imaging Ladar (SAIL) Overcomes Atmospheric Interference
The down-looking SAIL demonstrator is shown in the figure. The target is set in a building away from the demonstrator at the 4th floor of the laboratory. One target is an aluminum model of air plane, and the obtained image is given.
A synthetic aperture imaging ladar (SAIL) can provide two-dimensional active imaging of centimeter-class resolution at a long range, thus this field of research attracts much attention recently. SAILs usually work in a side-looking mode, so there are inherent defects like small optical footprint, limited receiving aperture and requirement for a matched optical delay. Moreover, the imaging will be easily affected by the phase interferences from atmospheric turbulence and mechanical trembling in the surrounding.
A down-looking synthetic aperture imaging ladar proposed by professor Jianfeng Sun's research group, from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has overcome the difficulties. This study aims to conduct the outdoor verification experiments. It is reported in Chinese Optics Letters Volume 12, No.11, 2014.
The side-looking SAIL is based on the 2D imaging with sight-distance resolving in the range direction and aperture synthesizing in the azimuth direction, and it adopts the configuration of transmitting by diffraction-limited beam and receiving by heterodyne detection with a local beam. However, the down-looking SAIL is based on the 2D imaging with lateral distance resolving on the target plane in the orthogonal direction of travel and aperture synthesizing in the travel direction, and its configuration is of projected imaging transmitting by two polarization-crossed co-axis beams and receiving by polarization-interfered self-heterodyne detection.
The down-looking SAIL demonstrator represents a completely new kind of ladar, which has promising application in spaceborne and airborne observation or inverse-SAIL radar. For the first time, the effect of phase interference caused by atmospheric turbulence under different atmospheric conditions is eliminated in outdoor experiments at a distance of over 1.2 km.
"It is a completely new concept and system to propose the down-looking SAIL, and the performance is essentially enhanced in comparison with side-looking SAIL." said Prof. Liren Liu from this group.
The following works will be focused on the development of this principle, the further increase in optical footprint size and systematic receiving sensitivity, and the improvement of the demonstrator for airborne use.
新型直视合成孔径激光成像雷达成功克服大气干扰
图为直视合成孔径激光雷达验证实验示意图。实验装置位于4楼实验室，目标放置于另一建筑内。上左图为一种漫反射铝制成的飞机模型目标，上中图为直视合成孔径激光雷达装置，上右图为目标图像。
合成孔径激光成像雷达（SAIL）能够在远距离提供厘米量级分辨率的主动二维成像,近些年成为研究热点。SAIL通常以侧视模式工作，存在许多难于克服的缺点，例如光学足址光斑尺寸很小、接收孔径有限、外差探测要求相应精确的光学延时线，而且受大气、运动平台、光雷达系统本身等相位干扰影响极大。
中国科学院上海光学精密机械研究所的孙建锋研究员课题组提出了直视合成孔径激光成像雷达的原理，克服了SAIL的上述缺点，并完成室外环境下的验证实验。相关研究成果发表Chinese Optics Letters 2014年第11期上。
侧视SAIL基于距离向视距分辨和方位向孔径合成的二维成像原理，采用衍射极限光束发射和具有本振的外差探测接收的系统结构。而直视SAIL基于交轨向目标面横向距离分辨和方位向孔径合成的二维成像原理，采用偏振正交同轴双光束成像投影发射和偏振干涉自差探测接收的系统结构。直视SAIL演示样机代表了一种全新的激光雷达，在星载和机载对地观察以及逆SAIL成像雷达领域有着重要应用价值。
该篇论文报道了与目标距离1.2 km的室外成像结果，这是国际上首次报道在各种大气条件下成功克服大气干扰的实验。课题组的刘立人研究员认为，该研究提出的直视合成孔径激光成像雷达在物理概念上和在体系结构上是全新的，与侧视SAIL相比，直视SAIL的性能获得了突破性提高。
后续工作主要是实现直视合成孔径激光成像雷达原理的拓宽和发展，进一步增大光学足趾和系统接收灵敏度，并实现雷达样机的改进并进行飞机机载对地成像观察实验。