Optical linewidth of soliton microcombs

TL;DR

This paper investigates the phase noise and linewidth of soliton microcombs in silicon nitride resonators, revealing that certain comb lines can have narrower linewidths than the pump laser, advancing understanding of their coherence limits.

Contribution

It demonstrates that Raman effects and dispersive waves can lead to narrower linewidths in soliton microcombs, providing new insights into their phase coherence properties.

Findings

Some comb lines have narrower linewidths than the pump laser.

Raman self-frequency shift influences the phase noise dynamics.

Dispersive-wave recoil affects the optical linewidths.

Abstract

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.