Passive mode-locking under higher order effects

TL;DR

This paper investigates the robustness of passive mode-locking mechanisms under higher order effects like third order dispersion, self-steepening, and Raman gain, showing that mode-locking persists and behaves predictably in different regimes.

Contribution

It demonstrates that passive mode-locking remains stable under higher order effects and applies soliton perturbation and asymptotic theories to verify numerical results in different regimes.

Findings

Mode-locking persists with higher order effects in both regimes.

Anomalous regime pulses behave like classical solitons.

Bi-solitons act as dark solitons on a stable background.

Abstract

The response of a passive mode-locking mechanism, where gain and spectral filtering are saturated with the energy and loss saturated with the power, is examined under the presence of higher order effects. These include third order dispersion, self-steepening and Raman gain. The locking mechanism is maintained even with these terms; mode-locking occurs for both the anomalous and normal regimes. In the anomalous regime, these perturbations are found to affect the speed but not the structure of the (locked) pulses. In fact, these pulses behave like solitons of a classical nonlinear Schrodinger equation and as such a soliton perturbation theory is used to verify the numerical observations. In the normal regime, the effect of the perturbations is small, in line with recent experimental observations. The results in the normal regime are verified mathematically using a WKB type asymptotic theory. Finally, bi-solitons are found to behave as dark solitons on top of a stable background and are significantly affected by these perturbations.