Nonlinear Modulational Dynamics of Spectrally Stable Lugiato-Lefever Periodic Waves

TL;DR

This paper proves the nonlinear stability of spectrally stable periodic waves in the Lugiato-Lefever equation against localized perturbations, using a novel method that handles the absence of a spectral gap and incorporates phase modulation.

Contribution

It introduces a new analytical approach for establishing nonlinear stability of periodic waves in the LLE with localized perturbations, overcoming the lack of a spectral gap.

Findings

Nonlinear stability established for localized perturbations.

Decay rates match linear theory predictions.

Method handles quasilinear equations with phase modulation.

Abstract

We consider the nonlinear stability of spectrally stable periodic waves in the Lugiato-Lefever equation (LLE), a damped nonlinear Schr\"odinger equation with forcing that arises in nonlinear optics. So far, nonlinear stability of such solutions has only been established against co-periodic perturbations by exploiting the existence of a spectral gap. In this paper, we consider perturbations which are localized, i.e., integrable on the line. Such localized perturbations naturally yield the absence of a spectral gap, so we must rely on a substantially different method with origins in the stability analysis of periodic waves in reaction-diffusion systems. The relevant linear estimates have been obtained in recent work by the first three authors through a delicate decomposition of the associated linearized solution operator. Since its most critical part just decays diffusively, the nonlinear iteration can only be closed if one allows for a spatio-temporal phase modulation. However, the modulated perturbation satisfies a quasilinear equation yielding an apparent loss of regularity. To overcome this obstacle, we incorporate tame estimates on the unmodulated perturbation, which satisfies a semilinear equation in which no derivatives are lost, yet where decay is too slow to close an independent iteration scheme. We obtain nonlinear stability of periodic steady waves in the LLE against localized perturbations with precisely the same decay rates as predicted by the linear theory.