Low-noise 750-MHz spaced Yb:fiber frequency combs

TL;DR

This paper reports the development of two high-repetition-rate, low-noise Yb:fiber frequency combs that are fully phase-locked to a CW laser, achieving record low phase noise and fractional frequency instability.

Contribution

It introduces a novel implementation of fully phase-locked 750 MHz Yb:fiber frequency combs with ultra-low phase noise and fractional frequency instability, surpassing previous repetition rate records.

Findings

Achieved <1 rad RMS phase error in locking

Fractional frequency instability of ~1.5e-18 at 1s

Highest repetition rate in fiber-based low phase noise combs

Abstract

We demonstrate two fully and tightly phase locked 750 MHz ytterbium (Yb) fiber frequency combs that are independently stabilized to a continuous wave (CW) laser with <1 rad RMS phase error. A bulk EOM and a single stack PZT are separately utilized as the fast actuators for cavity length stabilization. The carrier envelop frequencies are phase locked by single loop feedback to laser diode current, showing 1.6 MHz servo bumps. The in-loop fractional frequency instabilities are ~1.5e-18 at 1s for both combs. To the best of our knowledge, this is the highest repetition rate in fiber based low phase noise combs tightly locked to optical frequency reference.