  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 analyze the optical
phase-noise dynamics in soliton microcombs and show that, because of the Raman
self-frequency shift, the fundamental linewidth of some of the comb lines can,
surprisingly, be narrower than the linewidth of the pump laser. This work
elucidates information about the ultimate limits in phase coherence of soliton
microcombs and illustrates a new strategy for the generation of spectrally
coherent light on chip.
