The Stability of an Optical Clock Laser Transferred to the Interrogation Oscillator for a Cs Fountain

We stabilize a microwave oscillator at 9.6 GHz to an optical clock laser at 344 THz by using a fiber-based femtosecond laser frequency comb as a transfer oscillator. With a second frequency comb, we independently measure 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 1.2 times 10^-14 at 1 s averaging time (bandwidth of 50 kHz). The residual phase noise density is -97 dBc/Hz at a 10 Hz offset from the 9.6 GHz carrier. Replacing the existing quartz-based interrogation oscillator of the Physikalisch-Technische Bundesanstalt (PTB) caesium fountain CSF1 with this optically stabilized microwave source will reduce the instability contribution due to the Dick effect from the 10^-13 level at 1 s 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 to overcome the limitations of state-of-the-art quartz oscillators.