高精度光钟频率比对系统研究

In the last decade, optical atomic clocks have been developed tremendously. The frequency instability and uncertainty of optical atomic clocks have been reduced to the 10^(-18) level, which will trigger a revolution in fundamental research and high technology. All the applications of optical atomic clocks rely on accurate frequency ratio measurement between optical clocks or accurate frequency transfer from optical clocks to other frequencies in the optical or microwave region. In order to accurately measure the frequency ratios between optical atomic clocks, we need both accurate optical clocks as well as frequency ratio measurement systems with accuracy beyond optical clocks. Aiming at develop accurate optical clocks and frequency comparison system, this proposal plans to develop ultra-stable laser systems with frequency instability of 10^(-16)－10^(-17) at 1 s averaging time to improve the stability and accuracy of optical clocks, and to develop a high precision optical frequency divider with division uncertainty at the 10^(-20) level to meet the requirement of frequency measurement between optical clocks. Based on both of them, a high precision frequency ratio system for optical clocks can be constructed. The system can not only generate high quality clock laser for different optical clocks, but also accurately control the frequency ratios between those clock lasers. Besides, a portable frequency ratio measurement system will be realized for many important applications of optical clocks in our country.

近年来光钟研究迅猛发展，它的频率不稳定度和不确定度都能达到10^(-18)，将为基础研究和高新技术的发展带来革命性的突破。它的这些重要应用都依赖于光钟之间高精度的频率比值测量和如何将光钟的频率精度传递到其它光学或者微波波段。高精度的光钟频率比值测量必须有高精度的光钟和优于光钟精度的频率比对系统。本项目针对高精度光钟的关键技术（超稳钟激光）和高精度光钟频率比对关键技术（光学频率分频器）开展深入的研究，拟研制频率不稳定度为10^（-16）-10^(-17)的光学频率源(1秒积分时间）和分频精度为10^(-20)的多通道、低噪声光学频率分频器。然后将两者有机地结合起来，建立高精度光钟频率比对系统。该系统不仅能根据不同的光钟产生相应的钟激光，还能精确地控制这些钟激光之间的频率比值。项目拟在上述研究的基础上争取建立可搬运的高精度光钟频率比对系统，为提升我国光钟研究和光钟应用水平做出贡献。

原子光钟的频率不稳定度和不确定度已提高到E-18，正在迈向E-19，它将为精密测量物理和高新技术的发展带来革命性的突破。光钟的应用都依赖于光钟之间的频率比值测量和如何将光钟的频率精度传递到其它光学或者微波波段。通常采用光钟控制的光梳和基于微波频率基准源的频率计数器测量光钟之间的频率比值，该方法的测量精度受限于光梳和微波频率基准源的频率噪声。本项目采用传递振荡器、光束空间共模传输和光频下转换微波频率自参考基准等技术使得光学频率比值测量对光梳和微波频率基准源的频率噪声有6个数量级的免疫效果。我们采用商用铷钟控制的钛宝石飞秒光梳实现了不同光钟工作波段之间的高精度光频传递。通过与光学倍频（不同工作原理的光学分频器）的比对测量证明：由于受限于铷钟的频率噪声，光梳的频率稳定度为2E-11（1秒积分时间），但是光学分频过程中所引入的频率噪声可低至6E-18（1秒积分时间），分频精度可达到5E-21（1万秒积分时间），它可满足当今最好光钟的频率比值测量需要。采用该系统我们按预置的频率比值将1064nm稳频激光的频率稳定性传递到578nm激光，用于探测镱原子光钟跃迁谱线，并分别在400ms和200ms探测时间获得了线宽为2Hz及4.1Hz的光谱信号。采用谱线宽度为4.1Hz的光谱信号将光学分频器输入端的激光频率稳定在镱原子光钟跃迁谱线上，因而光学分频器的输出激光将与镱原子光钟有相同的频率精度。由于我们采用的是钛宝石飞秒光梳，其光谱范围可覆盖大部分光钟波段，因此可以同时以高精度的频率比值连接多个光钟和开展不同光钟之间的频率比对和频率比值测量。由于铷原子钟具有价格低、体积小、易搬运和使用环境要求低的优点，基于商用铷钟的光学分频技术将具有更大的应用范围。同时该技术对光梳频率噪声又有很强的免疫效果，为不宜实现高精度频率控制的芯片光梳、高频相位调制型光梳、光纤光梳满足空间应用提供了新的技术路线。

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