用于铝光钟直接激光冷却的 234 nm 紫外光梳光源研究

The Al+ ion optical clock is a very promising optical frequency standard. Based the program of direct laser cooling, it will be greatly improved over the current sympathetic-cooling type to achieve the highest uncertainty ion optical clock system. The ultra-violet optical frequency comb operating at 234 nm is essential as a rare light source for direct laser cooling and needs to be studied and realized. In this research project, we propose a feasible scheme of using the femtosecond optical parametric oscillator combined with cascade cavity-enhanced optical frequency doubler, making full use of the advantages of intra-cavity laser resonance and broadly tunable property to improve the overall nonlinear frequency conversion efficiency and promote the flexibility of widely tunable pulse wavelength generation. With a high-power, Yb-doped fiber laser as the pump light, and by designing and optimizing the structure of resonant cavities as well as different suitable crystal elements, the high-efficiency frequency conversion process from near-infrared pump source to generate an 234 nm ultra-violet frequency coherent pulsed laser with a power level of up to 100 mW is realized. Based on the above, we investigate both the behaviors and mechanism of main accumulated noise in the system, thus explore the carrier-envelope offset (CEO) frequency characteristics of optical parametric oscillator to find an effective way to improve the signal-to-noise ratio of beat signals. Finally, design phase lock-loop electronic circuits to achieve the complete control of ultra-violet frequency comb, with the frequency stability of comb teeth reaching the level of 10-12 in an integration time of 1 s, which meets requirements of direct laser cooling Al+ ion optical clocks, moreover, the proposal will also lay a theoretical and technical foundation for the development of new tunable deep ultra-violet frequency combs in the future.

铝光钟是很有前途的一种光频标。基于激光直接冷却的铝离子光频标，有望超过目前的协同冷却方式，实现最高不确定度的离子光钟系统。234 nm紫外光梳作为直接激光冷却的稀缺光源，至关重要，亟待研究和实现。本项目研究，我们提出采用飞秒光学参量振荡器结合级联倍频腔方案，充分利用激光腔内谐振以及腔体可调谐波长的优势，提升系统整体非线性频率变换效率，促进脉冲光谱调谐灵活性，以高功率掺镱光纤激光脉冲作为泵浦光，经优化设计腔体结构和非线性晶体，实现近红外泵浦激光高效频率变换产生高达 100 mW 水平的 234 nm 紫外频率相干脉冲光源；在此基础，通过研究链路噪声累积过程与演变规律，探明飞秒光学参量振荡器光源 CEO 特性，寻求提升信噪比的有效途径；最终，设计反馈锁定环路，实现紫外频率梳严格锁定，稳定度达到 10-12 @ 1s 量级，以满足铝光钟直接激光冷却需求，为发展波长可调谐的深紫外光梳光源奠定理论与技术基础。

光学原子钟（简述光钟）基于拍赫兹跃迁，相对微波原子钟，具有更高的Q值，能够提供更好的稳定性和准确性，有望成为下一代的时间和频率标准。27Al+离子1S0-3P0钟跃迁自然线宽只有8mHz，并且是目前所有光钟体系中对黑体辐射最不敏感的离子，因此，27Al+光钟是很有前途的一种光频标。最近报道的利用234nm深紫外光梳对27Al+光钟进行直接激光冷却的方案，有望超过目前的协同冷却方式，实现最高不确定度的离子光钟系统。针对该紫外光梳光源短缺问题，本项目着重于234nm紫外光梳光源产生、光梳非线性变频的关键技术，从理论和实验方面开展研究。.自主设计和研制了一套波长可灵活非线性转换且能够产生250MHz 234nm深紫外光梳光源系统。该系统基于一台双焦点倍频光学参量振荡器以及级联频率转换过程。通过采用χ(2)非线性包络方程仿真技术，指导设计非线性晶体和优化光参量振荡器腔体结构，充分利用腔内激光谐振以及腔体可调谐波长的优势，实现了1041nm泵浦脉冲高效频率转换至850nm倍频信号光，功率高达1.16W；通过设计脉冲合波与同步控制方案，高效和频产生了229mW的468nm蓝光脉冲激光；进一步通过构建单通倍频单元，优化Boyd-Kleinman聚焦因子，获得了平均功率为3mW的234nm紫外脉冲激光，20分钟采集时间内测得激光功率的RMS抖动量小于1%。以此为基础，采用“共模抑制”技术，成功探测到OPO光梳的载波包络偏移频率，信噪比达到30dB；最后通过采用经典的微波锁定方案，建立锁相环电子线路，初步实现了234nm紫外光梳锁定至氢钟参考。在20h测量时间内，重复频率时域抖动标准差为70μHz，秒稳的稳定度达到3×10-13；锁定后的载波包络偏移频率抖动量由自由运转下的MHz量级降至kHz量级。.上述研究的成果可望用于直接激光冷却铝离子光钟系统，为解决原子光谱分析等领域面临的光源瓶颈问题开辟一条新途径，并将推动飞秒激光非线性变频技术的进步，具有极其重要的科学意义和实用价值。