Ultra-low noise microwave generation with fiber-based optical frequency comb and application to atomic fountain clock (0901.3654v1)

Abstract: We demonstrate the use of a fiber-based femtosecond laser locked onto an ultra-stable optical cavity to generate a low-noise microwave reference signal. Comparison with both a liquid Helium cryogenic sapphire oscillator (CSO) and a Ti:Sapphire-based optical frequency comb system exhibit a stability about $3\times10^{-15}$ between 1 s and 10 s. The microwave signal from the fiber system is used to perform Ramsey spectroscopy in a state-of-the-art Cesium fountain clock. The resulting clock system is compared to the CSO and exhibits a stability of $3.5\times10^{-14}\tau^{-1/2}$. Our continuously operated fiber-based system therefore demonstrates its potential to replace the CSO for atomic clocks with high stability in both the optical and microwave domain, most particularly for operational primary frequency standards.

• The paper demonstrates an ultra-low noise microwave generation technique using a fiber-based optical frequency comb, achieving 3×10^-15 fractional stability at 1 s.
• The paper details how locking a femtosecond laser to an ultra-stable cavity mitigates phase noise and overcomes limitations from quantum projection noise and the Dick effect.
• The paper validates the approach with Allan deviation and phase noise spectral density measurements, indicating strong promise for enhancing atomic fountain clock performance.

Ultra-low Noise Microwave Generation with Fiber-based Optical Frequency Comb

The paper presented demonstrates a significant advancement in microwave reference signal generation by utilizing a fiber-based femtosecond laser locked onto an ultra-stable optical cavity. This technique exhibits a remarkable stability of approximately $3\times10^{-15}$ between 1 and 10 seconds, positioning it as a robust alternative to existing cryogenic sapphire oscillators (CSO) for high precision atomic fountain clocks. Such atomic fountain clocks are instrumental in the definition of the SI second and precise tests of fundamental physics.

The authors explore the inadequacies in current microwave atomic fountain clock technologies which are inherently limited by quantum projection noise (QPN) and the Dick effect, predominantly due to phase noise in the microwave signals used. Herein lies the importance of ultra-low noise microwave oscillators that can mitigate these effects and improve frequency standards' stability.

The current method involves optical frequency combs with optical reference transfer to the microwave domain. A noteworthy development in this paper is the application of a fiber-based optical frequency comb (FOFC) demonstrating superior reliability and maintenance compared to Ti:Sapphire-based optical frequency combs (TSOFC). The FOFC system shows an instable microwave generation at levels of $3\times10^{-15}$ at 1\,s, and further experiments with transmission setup to distant laboratories ensure robustness for various remote operations.

Numerical results provided highlight the phase noise power spectral density at 9.2 GHz and the fractional frequency instability compared against CSO at 11.932 GHz. The Allan deviation measurements substantiate the low noise characteristics, aligning with theoretical predictions and demonstrating potential improvements in microwave generation systems for advanced atomic clock applications.

The implications of this research are broad, offering practical advancements in atomic fountain clocks for tasks such as metrology and deep space navigation. Additionally, the discussion on VEGA laser design and implementation highlights promising directions for further reductions in vibration sensitivity—critical for enhancing optical cavity performance.

In conclusion, the paper signals a shift towards non-cryogenic alternatives for high precision atomic standards and opens avenues for further research in extreme low-noise systems for applications such as radar and VLBI. As the transition from CSO to fiber-based systems continues, it will likely catalyze improvements in both microwave and optical domains. Future work could investigate optimizing these systems for even lower noise levels, thus further pushing the boundaries of precision navigation and metrology technologies.