Frequency combs and coherent dissipative structures in nonlinear optical microresonators

Laser-driven high-Q Kerr-nonlinear optical microresonators enable parametric oscillation with low-power continuous-wave lasers and host a variety of coherent dissipative structures, including dissipative Kerr solitons and switching waves. These time-periodic structures constitute coherent optical frequency combs, and photonic-chip integration has miniaturized them to the chip scale. Such photonic-integrated, microresonator-based frequency combs - often termed 'microcombs' or 'Kerr combs' - have been demonstrated in various system-level and scientific applications. They complement femtosecond-laser-based frequency combs when high repetition rates, broad bandwidths, or high power per comb line are needed. This review introduces the field of microcombs and outlines the fundamental physical principles governing the generation of coherent frequency combs in microresonators.