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Photonic chip–based optical frequency comb using soliton Cherenkov radiation

Timing on a chip Laser-induced optical frequency combs allow precision measurements and affect a broad range of technologies. Brasch et al. generated optical frequency combs on an optical chip (see the Perspective by Akhmediev and Devine). They induced an optical soliton, or optical bullet, and propagated it in an engineered microcavity waveguide. The emitted output light formed a coherent comb of frequencies spanning two-thirds of an octave. Such an on-chip demonstration bodes well for miniaturization of metrological technology and its adaption for widespread application. Science, this issue p. 357; see also p. 340 A broadband and coherent frequency comb is generated with an optical microresonator. [Also see Perspective by Akhmediev and Devine] Optical solitons are propagating pulses of light that retain their shape because nonlinearity and dispersion balance each other. In the presence of higher-order dispersion, optical solitons can emit dispersive waves via the process of soliton Cherenkov radiation. This process underlies supercontinuum generation and is of critical importance in frequency metrology. Using a continuous wave–pumped, dispersion-engineered, integrated silicon nitride microresonator, we generated continuously circulating temporal dissipative Kerr solitons. The presence of higher-order dispersion led to the emission of red-shifted soliton Cherenkov radiation. The output corresponds to a fully coherent optical frequency comb that spans two-thirds of an octave and whose phase we were able to stabilize to the sub-Hertz level. By preserving coherence over a broad spectral bandwidth, our device offers the opportunity to develop compact on-chip frequency combs for frequency metrology or spectroscopy.