应用于SAL中的超宽带线性啁啾光波发射源的研究

Synthetic Aperture Ladar (SAL) is a technique which uses signal processing to improve the resolution beyond the limitation of physical antenna aperture. It has been shown that the diffraction limit is no longer a limitation for 1D-imaging alone the direction of the flight track in a synthesis aperature ladar system, however, for 2D-imaging the range resolution cannot be improved by the technique of SAL. The range resolution is determined by the swept frequency range of the linearly chirped continuous light waves in a frequency modulation continuous wave (FM/CW) SAL system. For example, for the imaging with 1mm resolution, it needs the swept frequency range of more than 150 GHz. In this project a system which can generate a linearly chirped CW light waves with a super broadwide band of 200 GHz is going to be built up as a light source in a high definition SAL system with 2D imaging resolutions of 1mm × 1mm. The weight and volume of this kind of SAL systems can be reduced significantly comparing to traditional optical imaging systems with the same 2D resolutions, and also it can be replaced by no one at all. A new method of generation of linearly chirped continuous light waves is proposed here in which an optical modulator is used to generate a set of multi-wavelenght CW lights at first. Then each wavelength wave is seperated from the set by optical switches and an interleaver in order to let each of them go through another optical modulator one by one which is driven by a linearly chirped electrical signals. The outputs from the later optical modulator are combined together without any gap in time domain to form a super broadband linearly chirped lightwave in 200 GHz as the final output of the whole system. Only can the SAL system with the light soure like the system built in this project be recognized as an unreplacable 2D high definition imaging system.

综合孔径激光雷达(SAL)在航道维度上的分辨率可以突破衍射极限，但遗憾的是在垂直维度的距离分辨率无法靠综合孔径来提高。当使用宽带线性啁啾连续光波成像时，距离分辨率只与线性啁啾光波的扫频宽度有关，1mm的距离分辨率需要扫频宽度不能低于150GHz。如果本项目研制的超宽带（200GHz）线性啁啾光波发生器能作为SAL的光源，就可以充分发挥SAL本身在航道维度上的分辨率优势，构成一个分辨率为1mm×1mm的远距离二维高清晰度成像系统。该系统的二维成像分辨率不受孔径的限制，可以大大减小设备的体积和重量，是传统光学成像系统不能取代的。本项目拟通过对单一波长激光器的连续输出光进行腔外单边带调制，产生多个波长，并分别对每个波长再进行连续线性啁啾调制，最后再将各个线性啁啾的光波汇接成一个在200GHz范围内线性啁啾的光波。只有这样的光源，才能充分发挥SAL的优势，构成一个不可替代的二维高清晰度成像系统。

研制激光雷达光源的动力来自于长距离高分辨率成像技术的应用，其中最主要的应用是实现无人机的机载雷达对地面的人脸识别功能。为了识别人脸，要求成像系统的分辨能力必须达到1 mm以下，如果用传统的CCD透镜成像方法对距离大于1 km之外人脸成像，需要的透镜直径至少超过1.5 m，隐蔽式无人机无法承载这样大的透镜，因此是不可行的。由此将激光综合孔径雷达技术（SAL）与连续波频率调制（FMCW）激光雷达技术结合起来，实现1 mm距离分辨能力，是本项目立项的初衷和背景。在本项目后期，无人驾驶的车载激光雷达开始涌现商机，但主流技术仍然是传统的脉冲光激光雷达（TOF），TOF激光雷达受到分辨率低和精度低的限制，性能差强人意。我们的激光雷达用的是连续光波，虽然技术复杂、昂贵，但它的高精度高分辨率是其他技术不能实现的，因此有望用于航空航天领域中的卫星编队飞行的感知雷达中。.本项目研究内容是通过宽带光调制器，将激光外调制技术与先进的射频信号产生技术有机地结合起来，实现在超宽频域内，光强恒定的连续光的频率随时间严格线性变化，变化速率和线性度均超过其他技术产生的指标。.我们获得国家专利1项，发表二区SCI文章3篇，发表EI检索SPIE国际会议论文13篇。培养博士研究生4名，其中2名已经顺利毕业。.已实现的关键数据是扫频范围200 GHz，扫频速率36 GHz/μs, 线性度R平方大于0.9998，估算瞬时线宽小于50kHZ。.光强恒定的连续光的频率随时间严格线性快速变化的光是区别于单频连续光和超短脉冲光的另一种光波形式，对这种新形式的的光波的研究还处于空白，主要是由于它的产生技术还不普遍。目前对这类光波进行测量的设备也十分匮乏，根本就没有带宽200 GHz的实时示波器，来完成记录下一个周期的波长。而且我们的扫频光波的周期非常长，如果用取样示波器，它一般没有那么大的存储深度，来存下每个周期的200 GHz带宽的信号。因此如何标定和评估这类光波就成了摆在我们面前的最大的科学问题了。这也是我们下一步要重点研究的课题。