The stability of an optical clock laser transferred to the interrogation oscillator for a Cs fountain

TL;DR

This paper demonstrates a method to transfer optical clock laser stability to a microwave oscillator at 9.6 GHz using a fibre-based femtosecond laser frequency comb, significantly improving the stability of Cs fountain clocks.

Contribution

It introduces a novel optical-to-microwave transfer technique that enhances the stability of microwave oscillators for atomic clocks.

Findings

Achieved fractional frequency instability of 1.2E-14 at 1 s

Residual phase noise density of -97 dBc/Hz at 10 Hz offset

Potential to replace quartz oscillators in Cs fountain clocks

Abstract

We stabilise a microwave oscillator at 9.6 GHz to an optical clock laser at 344 THz by using a fibre-based femtosecond laser frequency comb as a transfer oscillator. With a second frequency comb we measure independently the instability of the microwave source with respect to another optical clock laser frequency at 456 THz. The total fractional frequency instability of this optic-to-microwave and microwave-to-optic conversion resulted in an Allan deviation sigma_y, of sigma_y=1.2E-14 at 1 s averaging time (band width 50 kHz). The residual phase noise density is -97 dBc/Hz at 10 Hz offset from the 9.6 GHz carrier. Replacing the existing quartz-based interrogation oscillator of the PTB caesium fountain CSF1 with this optically stabilised microwave source will reduce the instability contribution due to the Dick effect from the 1E-13-level at 1s averaging time to an insignificant level at the current status of CSF1. Therefore this new microwave source can be an alternative to cryogenic sapphire-loaded cavity oscillators in order to overcome the limitations of state-of-the-art quartz oscillators.