高频率稳定度亚赫兹线宽激光系统的研制

High-frequency-stability subhertz-linewidth lasers are the key in optical atomic clock, precision spectroscopy, narrow-linewidth optical frequency combs, optical frequency synthesizer， low-phase-noise microwave generation and precision metrology. We have constructed subhertz-linewidth laser systems at 1064 nm with a frequency instability of 1.2E-15 (1 second averaging time). In this proposal, we will improve the laser frequency instability to the E-16 level and further reduce the laser linewidth. In the current laser systems, the laser frequency stability is limited by the thermal noise of their optical Fabry-Perot (FP) reference cavities. Therefore, in this proposal, we will improve the laser frequency instability by reducing the thermal noise of the reference FP cavities. After exploring the thermal noise of FP refernce cavities and their effect to the length stability of FP cavities, we will re-design and construct FP reference cavities with even lower thermal noise. Moreover, the new FP reference cavities should be designed to be extremely insensitive to environmental perturbations such as vibation, temperature fluctuation, air flow and acoustic noise. By frequency-stabilizing a laser to the resonance of such optical reference cavities with the Pound-Drever-Hall technique, the frequency instability of the laser will be improved to the E-16 level, limited by the thermal noise of the new reference cavities.

高频率稳定度亚赫兹线宽激光是研制原子光钟、精密光谱、窄线宽飞秒光梳、光学频率综合发生器、低噪声微波信号源和精密测量等课题的关键。本项目将在已实现的频率稳定度为1.2E-15（1秒平均时间）的亚赫兹线宽激光系统（1064 nm）的基础上，将激光的频率稳定度提升至E-16量级，并获得更窄的激光线宽。已实现激光系统的频率稳定度受限于其光学Fabry-Perot(FP)参考腔的热噪声，因此本项目的主要研究目标是降低参考腔的热噪声对激光频率稳定性的影响，从而提高窄线宽激光系统的频率稳定度。拟研究FP参考腔的热噪声来源、参考腔各参数与热噪声的关系，探索降低热噪声对参考腔等效腔长影响的途径，在此基础上设计加工新的具有更低热噪声的FP参考腔，并提高参考腔的抗环境噪声能力。设计精密光频锁相控制系统，将激光的频率高精度地锁定于FP参考腔上，使激光的频率稳定度提升至E-16量级，达到新参考腔的热噪声极限。

针对高频率稳定度激光在原子光钟、光频标光纤远程精密传输和高精度光学频率合成等方面的应用，本项目的目标是研制E-16频率不稳定度的激光系统。通过采用PDH技术，实现将激光的频率高精度地锁定在长度稳定的光学参考腔的谐振频率上。为此，项目系统地研究了光学参考腔的热噪声和剩余幅度调制等物理问题，并采取有效的措施减小这两种效应对激光频率稳定性的影响；研究了参考腔长度对环境振动和温度起伏的敏感性，并设计加工了对环境振动和温度起伏均不敏感的光学参考腔。在上述研究的基础上，采用10厘米长的光学参考腔研制了两套1557 nm亚赫兹线宽稳频激光系统，每套激光的线宽达到0.26 Hz，频率不稳定度达到8E-16（1-30秒平均时间），达到参考腔的热噪声限制的激光频率不稳定度，已作为高相干光源应用于光频标光纤远程精密传输系统中；采用30厘米长的光学参考腔研制了两套578 nm亚赫兹线宽稳频激光系统，通过两套激光频率比对测试和激光频率噪声评估，得到两套激光系统的频率不稳定度分别达到3E-16和8E-16(1秒平均时间)，主要受限于实验室环境的振动影响。578 nm激光系统的稳定性有望进一步改善，并将应用于高精度镱原子光钟、高精度光学分频与频率合成等研究。