Optical-frequency combs are versatile tools for measuring time, identifying chemicals, and generating quantum states. A new direction is to produce frequency combs through intriguing nonlinear behaviors of light in Kerr microresonators. Experiments with whispering-gallery-mode and waveguide ring configurations have been highly productive, exploring the formation, properties, and uses of soliton pulses that are the nonlinear eigenstate of the resonator. The soliton’s spectrum is a comb with a repetition frequency given by the free-spectral range of the resonator. Fabry-Perot (FP) cavities with sufficient Kerr nonlinearity also support soliton pulses. I will discuss experiments that probe soliton frequency combs and ultraprecise measurements with FP cavities based on bulk fused silica, optical fiber, and nanofabricated photonic crystal reflectors.
Microresonator frequency combs, or microcombs, provide a new opportunity for ultra-compact and chip-integrated optical and microwave frequency synthesis. A high-Q dielectric microresonator pumped by a continuous wave laser generates a frequency comb (typically 10-1000 GHz mode spacing) via parametric four‑wave mixing. Due to the tight waveguide confinement and high nonlinearity of the integrated-photonics platform, comb generation occurs with only milliwatts of input power. Additionally, precise geometric control of the waveguide dispersion has led to ultra‑broad bandwidth microcombs suitable for f-2f self-referencing. We have leveraged these advances to demonstrate a dual microcomb optical frequency synthesizer that employs inter-locked silicon nitride and silica microcombs. A silicon nitride resonator with 1 THz mode spacing provides an octave-spanning spectra for self-referencing and connection of the 200 THz pump laser to the silicon nitride repetition rate. The mode-spacing of this comb is subsequently phase-locked with a 22 GHz silica comb that provides the countable repetition rate and final link between microwave and optical domains. This dual microcomb system thus forms a backbone in the telecom C-band against which a tunable chip-integrated laser is phase-locked with user-defined frequency at 1 Hz resolution and absolute accuracy. The core elements of this system are fully reversible, and can also be used to generate low-noise microwaves on both short and long timescales.