Optical frequency synthesis from a cryogenic microwave sapphire oscillator

We demonstrate an optical frequency comb with fractional frequency instability of </=2x10(-14) at measurement times near 1 s, when the 10th harmonic of the comb spacing is controlled by a liquid helium cooled microwave sapphire oscillator. The frequency instability of the comb is estimated by comparing it to a cavity-stabilized optical oscillator. The less conventional approach of synthesizing low-noise optical signals from a microwave source is relevant when a laboratory has microwave sources with frequency stability superior to their optical counterparts. We describe the influence of high frequency environmental noise and how it impacts the phase-stabilized frequency comb performance at integration times less than 1 s.