Dynamics of the Modulational Instability in Microresonator Frequency Combs

TL;DR

This paper analyzes the formation and stability of microresonator frequency combs through a nonlinear Schrödinger equation framework, revealing the role of modulational instability and fixed points in their dynamics.

Contribution

It introduces a linear stability analysis considering cavity boundary conditions and a three-wave model connecting coupled mode theory with modulational instability, providing new insights into comb dynamics.

Findings

Stable comb states as attractive fixed points

Bistability and excitation states in normal and anomalous regimes

Agreement between analytical predictions and numerical simulations

Abstract

A study is made of frequency comb generation described by the driven and damped nonlinear Schr\"odinger equation on a finite interval. It is shown that frequency comb generation can be interpreted as a modulational instability of the continuous wave pump mode, and a linear stability analysis, taking into account the cavity boundary conditions, is performed. Further, a truncated three-wave model is derived, which allows one to gain additional insight into the dynamical behaviour of the comb generation. This formalism describes the pump mode and the most unstable sideband and is found to connect the coupled mode theory with the conventional theory of modulational instability. An in-depth analysis is done of the nonlinear three-wave model. It is demonstrated that stable frequency comb states can be interpreted as attractive fixed points of a dynamical system. The possibility of soft and hard excitation states in both the normal and the anomalous dispersion regime is discussed. Investigations are made of bistable comb states, and the dependence of the final state on the way the comb has been generated. The analytical predictions are verified by means of direct comparison with numerical simulations of the full equation and the agreement is discussed.