Raman-induced dynamics of ultrafast microresonator solitons

TL;DR

This paper develops a new theory for Raman-induced soliton self-frequency shift in microresonators, validated by experiments, offering guidelines to improve broadband soliton microcombs for applications like spectroscopy.

Contribution

A novel theory of Raman-induced SSFS in microresonators valid for ultrashort pulses, supported by simulations and experiments, enhancing understanding and control of microcomb spectra.

Findings

Reduced SSFS dependence on pulse duration for ultrashort pulses

Expanded soliton existence range due to the new theory

Validated predictions through experiments on Si3N4 microresonators

Abstract

Soliton microcombs are evolving towards octave-spanning for $f$-$2f$ self-referencing and expanding applications in spectroscopy and timekeeping. As spectra broaden and pulses shorten, the Raman-induced soliton self-frequency shift (SSFS) becomes a principal limitation: it reduces pump-to-comb conversion efficiency, constrains achievable span, and can, in extremes, preclude stationary operation. We develop a complementary theory of SSFS in microresonators that remains valid when the soliton duration $\tau_s$ is shorter than the Raman response timescale. The theory predicts a reduced dependence of the SSFS on $\tau_s$ which also expands the soliton existence range. Such predictions are validated by numerical simulations and by experiments on Si$_3$N$_4$ microresonators. Our results provide practical guidelines for engineering efficient and broadband soliton microcombs.