Maxwell-Bloch equations without spectral broadening: the long-time asymptotics of an input pulse in a long two-level laser amplifier

TL;DR

This paper analyzes the long-time behavior of electromagnetic pulses in a two-level laser amplifier using advanced inverse scattering techniques, revealing new soliton phenomena generated by boundary conditions rather than poles.

Contribution

It introduces a modern Riemann-Hilbert approach to the Maxwell-Bloch equations, uncovering novel soliton dynamics influenced by boundary conditions in a long laser amplifier.

Findings

New solitons with velocities below the speed of light are found.

Solitons are generated by zeros of the reflection coefficient, not poles of the transmission coefficient.

Asymptotic results differ significantly from previous studies near the light cone and in the tail region.

Abstract

We study the problem of propagation of an input electromagnetic pulse through a long two-level laser amplifier under trivial initial conditions. In this paper, we consider an unstable model described by the Maxwell-Bloch equations without spectral broadening. Previously, this model was studied by S.V. Manakov in 1982 and together with V.Yu. Novokshenov in 1986. We consider this model in a more natural formulation as an initial-boundary (mixed) problem using a modern version of the inverse scattering transform method in the form of a suitable Riemann-Hilbert (RH) problem. The RH problem arises as a result of applying the Fokas-Its method of simultaneous analysis of the corresponding spectral problems for the Ablowitz-Kaup-Newell-Segur (AKNS) equations. This approach makes it possible to obtain rigorous asymptotic results at large times, which differ significantly from the previous ones. Differences take place both near the light cone and in the tail region, where a new type of solitons is found against an oscillating background. These solitons are physically relevant, their velocities are smaller than the speed of light. The number of such solitons can be either finite or infinite (in the latter case, the set of zeros has a condensation point at infinity). Such solitons can not be reflectionless, they are generated by zeros of the reflection coefficient of the input pulse (and not by poles of the transmission coefficient). Thus our approach shows the presence of a new phenomenon in soliton theory, namely, the boundary condition (input pulse) of a mixed problem under trivial initial conditions can generate solitons due to the zeros of the reflection coefficient, while the poles of the transmission coefficient do not contribute to the asymptotics of the solution.