宽带线性调频光电振荡器的研究

Synthetic Aperture Radar (SAR), as the best imaging radar, is highly demanded by scientific research, military, security, space exploration, Earth observation, and many other applications. Chirped radio frequency (RF) pulse, where the carrier frequency changes (almost) linearly along time, is mostly utilized by SAR. Future advanced SAR requires chirped RF pulse with large (> several GHz) bandwidth, long duration, as well as low phase noise. Compared with current electronic approaches, optics has significant advantages for chirped pulse generation, especially on pulse bandwidth broadening to tens of GHz. Scientists name the optical way as “microwave photonics technologies”, which have been globally studied. However, the current photonic generation of chirped RF pulse has very limited “duration times bandwidth over phase noise”, which cannot satisfy the future SAR waveform. In this project, we will study optoelectronic oscillator (OEO) after our extensive investigation of current microwave photonics technologies, since we believe the target chirped RF pulse can benefit greatly from the ultra-low phase noise nature of OEO. Two problems have to be solved before its final application, i.e., the ultra-fast tuning and the mode-hopping-free tuning. We here introduce two brand new concepts for this purpose. An “optical-radio-intermediate frequencies” mixed oscillation structure is expected to fast tune the OEO, and an operation called “Fourier-domain mode-locking” is expected for direct chirped-pulse oscillation inside OEO. After an OEO is built based on the above new structure, its output waveform, which is expected with large “duration times bandwidth over phase noise”, will be precisely measured. Stable oscillation and exact frequency modulation inside the target chirping will also be studied, by feedback control. During the above research, three unsolved theories will be studied. Firstly, how will the broadband RF pulse be manipulated by a time-variant filter with high quality factor? Secondly, how will a chirped RF pulse oscillate in a multi-mode, time-variant optoelectronic cavity, and what are its characteristics? Thirdly, how to map the errors of generated broadband chirped pulse from the target linear frequency modulation to an easily-obtained baseband signal? We hope the above studies can show a solution for chirped RF pulse with broad bandwidth, long duration, as well as low phase noise, which can support the demanded imaging quality for future SAR. Also we hope the theory of new OEO shows significance in the field of microwave photonics.

合成孔径雷达（SAR）是军事、科技等众多领域重要的成像雷达，大带宽、大时宽、低噪声的线性调频波形是高端SAR的长久追求。微波光子技术在打开SAR带宽孔径方面有巨大潜力，其研究广受国内外重视。然而，当前光子技术仍被有限的“时间×带宽÷噪声”所制约。项目拟从具有天然低相噪的光电振荡器（OEO）入手，引入并研究两个全新概念解决OEO的“快调谐”、“无跳模”难题：“光—射频—中频”的混合振荡结构、和“频域脉冲锁模”的振荡机理；在此基础上，研究大时间带宽积雷达波形的精确质量评估、及实现稳定准确线性调频的反馈控制技术。项目拟解决三个关键科学问题：（1）时变高品质因子滤波器对信号的作用机理，（2）多模光电振荡腔内的啁啾振荡机理，和（3）线性调频误差映射机制。通过研究，项目拟提出线性调频信号产生方案以满足未来SAR高质量成像所需的大带宽、大时宽、低噪声，并提出OEO新模型以填补微波光子学当前的理论空白。

合成孔径雷达（SAR）是军事、科技等众多领域重要的成像雷达，大带宽、大时宽、低噪声的线性调频波形是高端SAR的长久追求。微波光子技术在打开SAR带宽孔径方面有巨大潜力，其研究广受国内外重视。然而，目前的光子技术仍被有限的“时间×带宽÷噪声”所制约。本项目提出了一种基于光电振荡器技术的频域锁模光电振荡技术，发挥光电振荡器在低相噪方面的优势，实现宽带线性调频信号的直接、低相位噪声产生。项目完成了如下研究内容：快速时变的高Q值微波光子滤波器、频域锁模光电振荡器理论模型与实验评估、基于光子欠采样技术的信号采集与优化、基于参量过程的光电振荡器相位稳定、以及宽带射频信号的时频分析，完整的覆盖了宽带线性调频光电振荡器的原理、结构、关键器件、性能分析等从理论模型到实验验证的各个方面。项目提出的“宽带振荡”理论，填补了光电振荡器理论的空白；在实验上实现了覆盖X波段的连续调频信号的产生，时宽达到22.22 us，带宽大于10 GHz，10 kHz频偏处相噪为-134.5 dBc/Hz。项目执行期间发表SCI论文11篇、均已标注。项目负责人在国内外会议上做邀请报告4次。申请国家发明专利2项，其中一项已授权。除此之外，研究团队与中科院半导体研究所研究团队于2018年合作、在著名期刊、Nature Communications上发表了题为《Breaking the limitation of mode building time in an optoelectronic oscillatior》的文章，详细报道了频域锁模光电振荡器和宽带振荡的理论和实验结果。