超低噪声光纤干涉仪稳频激光器研究

Ultra-stable laser is not only a key element for optical frequency standards, but also has a variety of applications in the field of generation of low-phase-noise microwave signals, improvement of atomic fountain clock, transfer of stable optical frequencies, tests of relativity and so on. Currently, most Hz linewidth lasers are realized by stabilizing laser frequency onto an ultra stable high-finesse Fabry-Perot cavity with PDH method. The cavity-stabilized laser exhibits excellent performance and the effects of the cavity's material, shape and mounting were sufficiently studied. As a result, sub-Hz linewidth laser sources have been developed. However, this approach is expensive, bulky, fragile and requires stable and fine alignment of free space optical elements and complicated optics. Moreover, the PDH method is not convenient to tune the laser frequency. In this project, we will develop an ultralow noise laser frequency stabilization by using an optical fiber interferometer as a frequency reference. The fiber-interferometer stabilized laser is realized by all-fiber optics and simple, robust, frequency-tunable and in this way we can demonstrate a compact, transportable ultra-stable laser.

超稳激光器不仅仅是原子光频标的重要组成部分，还广泛应用于超低噪声微波信号的产生、喷泉钟的升级、光频传输网络、引力波的测量以及相对论的验证等。目前世界上所使用的赫兹量级的稳频激光器最多的是采用基于Pound-Drever- Hall（PDH）方法锁频的超稳腔稳频激光器。这种稳频激光器具有优良的性能，经过多年在超稳腔材料、形状和安装方式上的研究，目前稳频激光器的线宽可以达到亚赫兹量级。尽管超稳腔稳频激光器性能十分优良，但是它也存在着一些缺点：光学结构复杂，需要光路和腔的空间模式精确匹配；系统昂贵，体积巨大，对环境参数敏感；不容易对激光器的频率进行调节。本项目拟实现的光纤干涉仪稳频激光器频率噪声低，结构简单，环境敏感度较低，体积小，频率可调谐，并为超稳激光器的小型化、可移动化打下基础。

利用光纤干涉仪实现对激光频率的锁定，首次实现了亚赫兹线宽的全光纤结构超稳激光器。通过比对测量两个相同的激光系统，测得激光的线宽为0.34Hz，其短期的频率稳定度为5x10-15（0.1-1s），频率噪声为0.05Hz2/Hz@10Hz和0.008Hz2/Hz（200Hz-3kHz）。设计加工了低振动灵敏度的光纤干涉仪，振动灵敏度为6×10-12/ms-2，并在此基础上设计了轴向振动灵敏度与径向振动灵敏度均小于1×10-12/ ms-2的超低振动灵敏度的光纤干涉仪。国际上首次在实验中观测到次声波频段的光纤热噪声并确定其1/f的频谱特性，检验了热噪声幅度与光纤长度的正比关系，并且验证了通过改变光纤的涂覆层可实现降低热噪声，这对进一步提高光纤干涉仪稳频激光器的性能非常重要。圆满完成了项目提出的各项指标，为实现小型化的超稳激光以及应用于高精度原子钟打下坚实的基础。