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).
An optical frequency synthesizer referenced to an optical clock
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.
