基于光纤延迟线稳频的飞秒激光频率梳

A passively mode-locked femtosecond laser emits millions of equally spaced coherent comb lines in optical frequency domain. The resulting optical frequency comb (OFC) has reformed optical frequency metrology and a number of research areas that rely on precise optical electrical field manipulation. The advances of OFC technique have been enabled by the innovations in laser frequency stabilization. By locking to a radio frequency standard, the long term stability of OFC can be significantly improved. However, this method is not capable for narrowing comb linewidth which is essential for a lot of high precision applications. In order to solve this problem, a narrow linewidth continuous wave (CW) laser stabilized by a high-finesse optical cavity can be used. However, this configuration is so complex that can hardly be implemented outside the best national metrology laboratories. Here, we propose a novel OFC stabilization technique that utilizes a fiber delay line as reference. The inherent thermal stability of the fiber delay line can be faithfully transferred to the frequency stability of optical comb lines, resulting in broadband frequency noise elimination and significant comb linewidth narrowing. This setup does not require additional CW lasers and the design of phase locked loop is simple. Moreover, it characterizes an all-fiber configuration. By exploring this technique for passively mode-locked Erbium fiber laser stabilization, a compact all-fiber optical frequency comb can be reached. This novel OFC stabilization technique enables a number of applications that demand on short term stability of OFC, such as molecular spectroscopy, enhanced cavity based higher harmonic generation, ultra-low phase noise microwave signal generation and coherent lidar.

被动锁模的飞秒激光器在光谱域输出成百万计等间隔排列的相干谱线，构成光学频率梳，成为光频计量以及诸多依赖精密光电场调控的科学技术领域不可或缺的工具。激光稳频技术的创新是推进光频梳的性能提升与应用拓展的核心动力。通过锁定至射频频率基准，可以有效地改善频率梳的长期稳定性，但是梳齿的谱线宽度仍然很宽，使得应用受限。为了压缩梳齿线宽，目前只能将光频梳锁定至超稳光学谐振腔稳频的窄线宽单频激光器，技术门槛与成本过高。本项目提出一种新的稳频技术，利用一段单模光纤延迟线作为参考基准，将其固有的热噪声极限的长度稳定性传递至频率梳的谱线频率稳定性，实现宽带的梳齿频率噪声消除，并压缩梳齿线宽。这种方案不需要额外的单频激光器，锁相环路简单，且具有全光纤的结构，结合掺铒光纤飞秒激光技术，能够实现紧凑型、实用化的稳频飞秒激光频率梳，在分子光谱学、基于增强腔产生高次谐波、超稳射频源、相干激光雷达等领域具有广阔的应用前景。

本项目提出了一种基于光纤延迟线的光学频率梳稳频技术，以一段公里级的经色散补偿、隔振、温控的单模光纤作为参考基准，实现光学频率梳的2个自由度的频率锁相及频率梳齿的有效窄化。基于同一套稳频系统，可以同时稳定多台独立的光学频率梳，实现双光梳、三光梳等稳频装置，满足多维光谱、绝对测距等应用。.研制了一台全光纤化的光纤延迟线稳频的掺铒光纤频率梳实验系统，通过反馈控制激光器的泵浦功率、腔外的声光频移器实现了光频梳的完全锁相。与超窄线宽的单频激光器的拍频实验表明，光学频率梳的梳齿线宽压缩至580 Hz，比自由运转时的~100 kHz下降了170倍。.全面表征了这种新型稳频光频梳的性能指标。针对稳频光学频率梳的三个关键参数：梳齿线宽、载波-包络偏移频率、脉冲序列定时抖动开展了系统的研究。尤其是，基于光纤延迟线稳频技术发展了一项高灵敏度、大动态范围的光频梳噪声测量技术，系统表征、全面解析了光频梳的各个自由度的锁相环路之间串扰的作用机制。此外，发明了一种含增益的f-2f干涉仪，实现了高精度的载波-包络相位锁相，获得了6 as的残余定时抖动记录，将光频梳的载波-包络相位的测量与控制推进至原子时间单位。.开展了窄线宽光频梳在绝对测距、分子光谱等前沿领域的应用探索。基于双光梳相干多外差光谱探测原理开展了精密光谱测量研究，在2微米中红外波段实现了单个梳齿分辨的水吸收精密光谱测量，探测到最窄40pm的水吸收峰，分析了双光梳光谱技术的测量分辨率极限。.本项目发表学术论文23篇（第一标注10篇），授权中国发明专利1项，成果涉及光纤稳频的光频梳的原理与实验、新型光频梳稳频技术、光频梳前沿应用等。其中，第一、通讯作者论文17篇，含10篇发表在Opt. Lett., Opt. Express 等国际光学期刊上的高水平学术论文，一个研究成果被“美国光学学会”（OSA）的光学聚焦专栏“Spotlight on Optics”做高亮报道。由于在低噪声光频梳领域做出了成体系的研究成果，受邀在《红外与激光工程》、英国物理学会出版社（IOP）旗下的Journal of Physics: Photonics杂志发表长篇综述文章各一篇。在国内外重要学术会议上作特邀报告4次。项目培养博士研究生3名，硕士研究生2名。