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

TL;DR

This paper presents a fiber-based femtosecond laser system that generates ultra-low noise microwave signals, demonstrating comparable stability to cryogenic sapphire oscillators and enhancing atomic fountain clock performance.

Contribution

The study introduces a fiber-based optical frequency comb system that achieves high stability microwave signals suitable for primary frequency standards, offering a practical alternative to cryogenic oscillators.

Findings

Microwave stability of 3×10^{-15} between 1-10 seconds.

Atomic fountain clock stability of 3.5×10^{-14} τ^{-1/2}.

Potential to replace cryogenic sapphire oscillators in atomic clocks.

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.