An optical frequency synthesizer referenced to an optical clock
An optical frequency synthesizer referenced to an optical clock
Electromagnetic waves with higher spectral purity, frequency stability, and accuracy are consistently pursued in communication, radar systems and global position systems for the goal of large communication capacity and high positioning accuracy. Microwave frequency synthesizers are frequently employed for their generation at any desired frequencies in the microwave region with the same excellent frequency stability and accuracy as those of microwave oscillators or clocks, supporting the variety of applications listed above.
In the past decade, the frequency stabilities and accuracies provided by electromagnetic waves in the optical domain outperform their microwave counterparts by more than two orders of magnitude: the frequency instabilities of the most stable lasers have reached 10−17, and the frequency accuracies of the most accurate optical clocks have achieved 10−18. Such extraordinary electromagnetic waves emitted from optical oscillators or optical clocks will be the new generation of time and frequency standards, and they constitute extremely sensitive tools for basic research, such as searching for possible time variations of fundamental constants, measurement of changes in gravitational potentials on the centimeter scale, and tests of relativity.
However, optical oscillators or clocks only work at a few specific wavelengths. To make use of their full power, we need an optical frequency synthesizer, which can generate optical waves or microwaves at desired frequencies with the same performance as optical oscillators or optical clocks. The key to realize an optical frequency synthesizer is to maintain the coherence and frequency accuracy during frequency synthesis.
The team led by Prof. Yanyi Jiang from State Key Laboratory of Precision Spectroscopy, East China Normal University realized low noise, accurate optical frequency synthesis with the help of comb-frequency noise reduction techniques, which provides possibilities to use chip-scale combs.
This work was published in Photonics Research, Vol. 9, No. 2, 2021 (Yuan Yao, Bo Li, Guang Yang, Xiaotong Chen, Yaqin Hao, Hongfu Yu, Yanyi Jiang, Longsheng Ma. Optical frequency synthesizer referenced to an ytterbium optical clock[J]. Photonics Research, 2021, 9(2): 02000098).
To simulate the large frequency noise of chip-scale combs, they phase-locked a comb to a rubidium (Rb) clock at 10 MHz, resulting in frequency instability of the comb teeth to be 2 × 10−11 at 1 s averaging time. Benefitting from the noise-reduction techniques adopted in optical frequency synthesis, e.g., the transfer oscillator scheme and the optically self-referenced time base, the synthesis noise is largely immune to comb frequency noise. They achieved a synthesis noise of 6 × 10−18 at 1 s averaging time and a synthesis uncertainty of 5 × 10−21, supporting optical frequency synthesis from the state-of-the-art optical clocks.
Using the comb phase-locked to the Rb clock, they transferred the coherence from an ultra-stable laser at 1064 nm to 578 nm, and resolved hertz-level-linewidth Rabi spectrum of the ytterbium clock transition. Any frequency deviation from the transition is corrected by adjusting the frequency of the 1064 nm laser, the internal oscillator of the optical frequency synthesizer. Thereby the optical frequency synthesizer acquires frequency stability in the long term as well as high frequency accuracy.
Prof. Ma from this research team recalled that about 40 years ago when he used microwave frequency synthesizer for the first time he dreamed of an optical frequency synthesizer for precision spectroscopy. His dream did not come true until optical frequency combs were invited in 2000. Since then, he and his team developed narrow-linewidth ultra-stable lasers, low noise optical frequency synthesis based on optical frequency combs together with comb-frequency-reduction techniques, and cold ytterbium atoms trapped in optical lattices. They combined all these parts together to build an accurate optical frequency synthesizer referenced to the ytterbium optical clock. Such an optical frequency synthesizer enables possibilities to output optical waves with excellent coherence, frequency stability, and accuracy in the region of 530–1100 nm for precision spectroscopy and measurement, supporting many cutting-edge applications of most optical clocks. In the near future, as the SI second is redefined based on the optical clocks, optical frequency synthesizer will become an increasingly indispensable tool to meet the varied applications.
圆梦：实现以光钟为频率基准的高精度光学频率合成系统
以光钟为频率基准的光学频率合成器
在通讯、雷达、全球定位系统等应用中，人们不断追求噪声更低、精度更高的电磁波，从而达到通讯容量增大、定位精度提高的目的。其中，微波频率合成器发挥了重要作用，它能在所需要的频率处输出与微波钟性能相当的电磁波信号。
近年来，光波波段的电磁波（频率比微波高四个数量级）的性能已超越了微波：低噪声光学振荡器的频率稳定度已达到10−17量级（1s积分时间），光钟的精度已达到10−18量级，比铯喷泉钟还好两个数量级。有了性能如此优异的电磁波，不仅能建立新一代时间频率标准，在高新技术应用中发挥不可替代的作用，还能以前所未有的精度和灵敏度去进行基本物理常数是否随时间变化的探索、厘米量级引力势测量、相对论验证等研究。
由于光波振荡器或光钟只工作在特定光频处，故需要利用光学频率合成器，将低噪声和高精度的光钟频率特性传递到所需要的频率处。光学频率合成器的研究关键在于：光频合成过程不能破坏频率精度和相干性。
近日，华东师范大学精密光谱科学与技术国家重点实验室的蒋燕义研究员团队在Photonics Research 2021年第2期（Yuan Yao, Bo Li, Guang Yang, Xiaotong Chen, Yaqin Hao, Hongfu Yu, Yanyi Jiang, Longsheng Ma. Optical frequency synthesizer referenced to an ytterbium optical clock[J]. Photonics Research, 2021, 9(2): 02000098）发表的文章中报道了有望采用芯片光梳实现高精度的光学频率合成。
该研究团队将光梳锁定于铷钟，光梳梳齿的频率稳定度为2×10−11（1s平均时间）。使用频率噪声如此大的光梳，借助光梳频率噪声免疫技术可实现低噪声高精度的光学频率合成：光频合成噪声为6 × 10−18（1s平均时间），光频合成精度为5 × 10−21，表明它能对世界上最低频率噪声或最高频率精度的光波进行频率合成而不影响其性能。
利用该技术，研究团队将1064 nm稳频激光的性能传递到578 nm，获得了线宽为Hz量级的镱原子光钟跃迁谱线，并将偏离跃迁中心频率的误差信号反馈给1064 nm稳频激光，最终将镱原子光钟的频率精度高保真地传递到了530~1100 nm范围中。
华东师范大学的马龙生教授对此深有感触：40年前当他第一次使用无线电波频率合成器时，他就梦想拥有一台光学频率合成器，用于开展精密光谱研究；2000年光梳的诞生为实现这个梦想铺平了道路；经过20年的奋斗，他们相继研制了窄线宽稳频激光、基于光梳频率噪声免疫技术的光学频率合成器、冷镱原子光钟系统，并将它们集成为一个具有光钟精度的频率合成系统，为光钟应用于精密光谱和精密测量迈出了必要的一步。