Mathematical Physics Properties of Waves on Finite Background

TL;DR

This paper explores mathematical models of waves on finite backgrounds governed by the nonlinear Schrödinger equation, focusing on solutions relevant to freak wave phenomena, including their physical characteristics and spectral properties.

Contribution

It analyzes and compares two families of exact solutions (SFB1 and SFB2) of the NLS equation, highlighting their physical features and spectral differences in the context of modulational instability.

Findings

SFB1 has one pair of initial sidebands; SFB2 has two.

Both solutions exhibit wavefront dislocation and phase singularity.

The solutions model the Benjamin-Feir instability phenomenon.

Abstract

Several mathematical and physical aspects of waves on finite background are reported in this article. The evolution of the complex wave packet envelope of these type of waves is governed by the focussing-type of the nonlinear Schr\"{o}dinger (NLS) equation. The NLS equation admits a number of exact solutions; in this article, we only discuss waves on finite background type of solutions that have been proposed as theoretical models for freak wave events. Three types of waves on finite background considered in this article are known as the Soliton on Finite Background (SFB), the Ma solution and the rational solution. In particular, two families of the SFB solutions deserve our special attention. These are SFB$_{1}$ and SFB$_{2}$, where the latter one belongs to higher order waves on finite background type of solution. These families of solutions describe the Benjamin-Feir modulational instability phenomenon, which has been verified theoretically, numerically and experimentally as the phenomenon that a uniform continuous wave train is unstable under a very long modulational perturbations of its envelope. A distinct difference between the two families of solutions can be observed in the spectral domain, where SFB$_{1}$ has one pair of initial sidebands and SFB$_{2}$ has two pairs of initial sidebands within the interval of instability. The relationship between SFB$_{1}$ and SFB$_{2}$ are explained and some important physical characteristics of the two solutions are discussed. These include the amplitude amplification factor, the spatial evolution of complex-valued envelopes, their corresponding physical wave fields and the evolution of the corresponding wave signals. Interestingly, wavefront dislocation and phase singularity are observed in both families of the solution with different patterns, depending on the value of the modulation wavelength and on the choice of parameters in SFB$_{2}$.