Low-noise 750 MHz spaced ytterbium fiber frequency combs

We demonstrate two low-noise 750 MHz ytterbium fiber frequency combs that are independently stabilized to a continuous-wave laser. A bulk electro-optic modulator and a single-stack piezo-electric transducer are employed as fast actuators for stabilizing the respective cavity length to heterodyne beat notes. Both combs exhibit in-loop fractional frequency instabilities of ∼10−18 at 1 s. To the best of our knowledge, this is the first demonstration of tightly phase-locked (<1  rad root mean square phase noise integrated from 0.1 Hz to 10 MHz) fiber frequency combs with 750 MHz fundamental repetition rate.