  Soliton microcombs provide a versatile platform for realizing fundamental
studies and technological applications. To be utilized as frequency rulers for
precision metrology, soliton microcombs must display broadband phase coherence,
a parameter characterized by the optical phase or frequency noise of the comb
lines and their corresponding optical linewidths. Here, we analyse the optical
phase-noise dynamics in soliton microcombs generated in silicon nitride high-Q
microresonators and show that, because of the Raman self-frequency shift or
dispersive-wave recoil, the Lorentzian linewidth of some of the comb lines can,
surprisingly, be narrower than that of the pump laser. This work elucidates
information about the physical limits in phase coherence of soliton microcombs
and illustrates a new strategy for the generation of spectrally coherent light
on chip.
