Optical Frequency Synthesis From a Cryogenic Sapphire Oscillator Using a Fiber-Based Frequency Comb

We have demonstrated optical frequency synthesisusing a fiber-based frequency comb system, referenced to anultrastable microwave oscillator. The oscillator is based on a highQ-factor cryogenic sapphire resonator cooled with liquid helium.The 100-MHz signal synthesized from the 10.8-GHz oscillationfrequency of the cryogenic sapphire oscillator (CSO) was used tostabilize the repetition frequency of the mode-locked fiber laser.When the synthesized optical frequency was compared with arubidium two-photon stabilized laser at 778 nm, the measuredfractional frequency stability was ~6 × 10−14 τ −1/2 for theaveraging times τ between 1 and 100 s, and the best frequencystability was 3.0 × 10−15 for an averaging time of 1280 s.