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.
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