Optical frequency synthesis from a cryogenic sapphire oscillator using a fiber-based frequency comb

We have demonstrated optical frequency synthesis using a fiber-based frequency comb system, referenced to an ultrastable microwave oscillator. The oscillator is based on a high Q-factor cryogenic sapphire resonator cooled with liquid helium. The 100-MHz signal synthesized from the 10.8-GHz oscillation frequency of the cryogenic sapphire oscillator (CSO) was used to stabilize the repetition frequency of the mode-locked fiber laser. When the synthesized optical frequency was compared with a rubidium two-photon stabilized laser at 778 mu, the measured fractional frequency stability was similar to 6 x 10(-14) tau(-1/2) for the averaging times tau between 1 and 100 s, and the best frequency stability was 3.0 x 10(-15) for an averaging time of 1280 s.