Frequency-modulated combs as phase solitons

TL;DR

This paper presents an exact theoretical model for frequency-modulated combs in lasers, revealing their phase dynamics as solutions to a nonlinear Schrödinger equation with a phase-dependent potential, advancing understanding of FM comb physics.

Contribution

It introduces a novel exact solution to the nonlinear Schrödinger equation describing FM combs, linking phase potentials to coherent light states and broadening the theoretical framework for nonlinear optical systems.

Findings

Exact solution for FM comb phase dynamics

Phase is piecewise quadratic in time

Applicable to large gain nonlinear optical systems

Abstract

Frequency combs are light sources with coherent evenly-spaced lines. It has been observed that in certain laser systems, combs can form whose output is frequency-modulated (FM) in time. In this state, they produce an output whose frequency sweeps linearly and periodically. These results have been replicated numerically, but a thorough understanding of their core physics remains elusive. Surprisingly, we have found that these lasers are described by a nonlinear Schrodinger equation whose potential is proportional to the phase of the electric field. This equation can be solved exactly and produces a field whose phase is piecewise quadratic in time---an FM comb. These results can be used to derive all of the salient features of FM combs, and our general theory is applicable to any nonlinear optical system with large internal gain. More generally, this result portends the development of new coherent states of light governed by phase potentials rather than amplitude potentials.