Science 351, 357–360 (2016)

On-chip microresonator-based frequency combs have been demonstrated before, but the challenge remains in achieving broadband coherent combs. Such a device would be attractive for frequency metrology and spectroscopy. Now, by exploiting soliton Cherenkov radiation in the presence of higher-order dispersion, Victor Brasch and colleagues from EPFL, Switzerland have generated a fully coherent optical frequency comb spanning two-thirds of an octave and whose phase can be stabilized to the sub-hertz level. Their experimental platform is based on an integrated silicon nitride optical microresonator with a thickness of 800 nm and a diameter of 238 μm, embedded in SiO2. It is continuous wave-pumped at 1,560 nm and dispersion engineered with an anomalous group velocity dispersion for wavelengths around 1,500 nm. When pump power is at 2 W, soliton Cherenkov radiation is generated, appearing as a sharp feature around 1,930 nm in the optical spectrum.