ABSTRACT Microresonator‐based Kerr frequency combs (microcombs) provide compact femtosecond pulse sources with repetition rates exceeding 10 GHz, combining a small footprint with intrinsically low timing jitter. However, free‐running microcombs suffer from long‐term timing drift, and direct stabilization to optical references is hindered by the difficulty of carrier‐envelope offset frequency detection at such high repetition rates. Here we demonstrate optical‐to‐microcomb stability transfer via a mode‐locked laser (MLL), which enables femtosecond synchronization and directly imprints optical‐reference stability onto the microcomb repetition rate. Using an electro‐optic‐sampling‐based timing detector, we synchronize a 22‐GHz silica microcomb to a 250‐MHz fiber MLL, reaching 1.3 as/Hz 0.5 timing resolution and fractional frequency stability of 10 −18 level at 1000‐s averaging. When the MLL is stabilized to a compact and alignment‐free optical fiber‐delay reference, its stability is transferred to the microcomb, yielding a single‐sideband phase noise of −125 dBc/Hz at 100‐Hz offset from a 22‐GHz carrier—the lowest phase noise reported to date for a microcomb‐based microwave source at offset frequencies below ∼1 kHz. This approach enables direct transfer of optical‐reference stability to microcombs, providing a practical route toward high‐repetition‐rate and ultralow‐noise photonic oscillators for ultrastable timing, precision metrology, coherent communication, and next‐generation sensing systems.
Ahn et al. (Thu,) studied this question.