Research on Working Mode of Down-looking Synthetic Aperture Imaging Ladar

合成孔径激光成像雷达发展自微波合成孔径雷达，使用光波波段代替微波对目标主动探测，具有全天候、远距离、高分辨目的。在军事、航天遥感、机载遥感、海洋陆地监测等领域具有非常广的应用价值。合成孔径激光成像雷达的发展经历了从简单到复杂、从实验室内到外场环境的过程，由于机载、星载远距离高分辨的需求日益强烈，传统的微波合成孔径雷达要达到高分辨需要很长的合成孔径长度，不仅成像时间长而且处理复杂。因此，通过减小雷达工作波长来提高分辨率的方法提上日程，由此，最初的合成孔径激光成像雷达原理来自于微波合成孔径雷达原理，这种侧视的体制要求严格的相位控制，外差拍新信号初始相位必须严格同步以及抑制非线性啁啾效应，这些技术在实验室内可以通过复杂的相位控制技术完成，但是在外场环境下，复杂多变的湍流对侧视合成孔径激光成像雷达的工程化推进造成了很大的难题，如果不能很好的克服湍流对侧视体制的影响，那么机载与星载的使用就会成为一个很大的问题。 直视合成孔径激光成像雷达正是为解决侧视工程化难题而应运而生的一种新的合成孔径激光成像雷达体制。本人所在的课题组通过采用自由空间波前变换技术，有效的发挥了光学优势，降低了系统复杂度，根据直视合成孔径激光成像雷达原理实现的合成孔径激光成像雷达条带模式通过实验室内以及1.2km的外场实验得到了很好的验证，直视条带模式最大优点在于大大降低湍流对系统相位的影响，系统分辨率与光学足趾可以自由设计，并且系统在探测距离尺度上具有设计上的可扩展性。但由于实际机载、星载条件下，雷达相对目标运动时间较快，加之激光波段的光学足趾相对微波波束覆盖区域而言非常小，因此直视条带模式在有限时间内接收到的回波信号能量非常有限，这就需要提高激光器的发射功率，但是这会影响其它光学元件的性能甚至发生烧毁的情况。因此，为了解决直视条带模式在将来实际使用过程中的功率问题，本课题组开展了相关探索工作，并取得一系列成果。 本论文在直视条带模式的基础之上，对合成孔径激光成像雷达的工作模式进行了探索，主要包括以下四个方面： （1）基于波动光学衍射理论，系统建立了侧视、直视合成孔径激光成像雷达的数学模型。理论分析了侧视合成孔径激光成像雷达的光外差接收的基本过程，推导了一般情况下的合成孔径激光成像雷达的数据接收方程，分析了不同雷达系统口径对于分辨率的影响。 （2）提出使用直视聚束模式的方案，降低直视条带模式中系统对激光器功率的要求，通过延长雷达对目标的凝视时间，增加了回波相干累积时间，提高了图像对比度，通过10m焦距的平行光管在实验室内验证了直视聚束模式的可行性，对下一步机载实验有一定的参考意义。 （3）提出使用直视滑动聚束模式的方案，扩大直视聚束模式的成像范围，同时相对直视条带模式而言，延长了雷达对目标的凝视时间，在提高图像对比度的同时，对下一步机载实验具有重要的参考价值。 （4）提出使用自适应光学系统实现侧视合成孔径激光成像雷达距离向相位同步的方案，搭建实验进行了相关方面的探索。使用C++实现了一套自适应光学的闭环控制程序，其能否应用于侧视合成孔径激光成像雷达系统未来仍待未来实验进一步论证。Synthetic aperture imaging ladar is derived from synthetic aperture radar. In order to accomplish the purpose of detecting targets under all-weather, long-range, high-resolution, using the optical wavelength to replace the micowave. Synthetic aperture imaging ladar has widely application in military, space remote sensing, aviation remote sensing, the ocean and terrain monitoring. The development of synthetic aperture imaging ladar has gone through from simple to complex, from in-door simulation to out-field test. Because of the increasing demand on high-resolution in long range, traditional synthetic aperture radar will need extremely long synthetic aperture length to achieve the estimated resolution. and it will lead to long synthetic aperture time as well as complex processing method. A good method to solve this problem is that using shorter wavelength to increase the resolution ability. The straightforward thought would be copy the principle of synthetic aperture radar directly to synthetic aperture imaging ladar, which is called side-looking synthetic aperture imaging ladar. It needs sophisticated even complicated phase error control, synchronizes the initial phase of heterodyne beat signal in each chirp pulse, suppresses the nonlinear chirp effect, etc. In-door experiment can avoid these problems, but outfield experiment cannot, because the turbulence is always changing, the vibration of working platform is unavoidable. This imposes much difficulties for the synthetic aperture imaging ladar engineering, if it did not address well, the application in airborne and spaceborne will be impossible. Down-looking synthetic aperture imaging ladar is borned for overcoming the engineering problems in the development of side-looking synthetic aperture imaging ladar. My research group uses the free space optical techniques, takes the advantages of optical properties, which leads to ,reducing system complexity. Based on the principle of down-looking synthetic aperture imaging ladar, we conducted outfield experiment in the distance of 1.2km, which turns out to be a good chance to verify the correctness of down-looking synthetic aperture imaging ladar. Under the principle of down-looking synthetic aperture ladar, the stripmap mode can reduce the influences of turbulence, the size of optical footprint and resolution can be designed freely as well as the system’s scalability. Due to the fast relative speed in the real conditions, the flight time within a single optical footprint is very short, which causes that there is very limited energy can be collected. In order to get the imaging result, increasing the power of laser is unavoidable, which may cause the damage of other optical elements. In order to solve the power issue in the stripmap mode, my research group conduct several research work to address this problems. This thesis’s experiments is developed based on the stripmap mode, and explores several ways to improve the performance of down-looking synthetic aperture ladar. My work can be concluded in four aspects as follow: (1) Build a side-looking and down-looking synthetic aperture imaging ladar mathematical model based on the theory of wave optics diffraction theory. Conduct theoretical analysis of the basic process of optical heterodyne balance receiving process. Deduce the general formation of synthetic aperture imaging ladar data collection equation. Analysis the influences on resolution of system stop. (2) Put forward the spotlight mode down-looking synthetic aperture imaging ladar scheme, it reduces the requirement of laser power, extends the illumination time, which increasing the echo coherent integration time and improve image contrast. Using collimator of 10m focal length in the laboratory to simulate the far field optical field distribution. The indoor experiment verifies the feasibility of spotlight mode synthetic aperture imaging ladar and is useful for the airborne application in next experiment phase. (3) Put forward the slide spotlight mode down-looking synthetic aperture imaging ladar scheme, which can expand the imaging range as well as increasing illumination time compared to stripmap mode. It also improves image contrast. Slide spotlight mode down-looking synthetic aperture imaging ladar has significant value for the airborne and spaceborne applications. (4) Put forward the experiment scheme to synchronize the initial phase of range direction in side-looking synthetic aperture imaging ladar, using adaptive optical system. Develop a real-time close loop adaptive optical system software with C++. It needs further discussions and explorations to achieve a final conclusion in the future