Dynamics of the Modulational Instability in Microresonator Frequency Combs

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.