# Displaced phase-amplitude variables for waves on finite background

## TL;DR

This paper introduces displaced phase-amplitude variables for the nonlinear Schrödinger equation to analyze wave amplification and modulational instability, providing a new perspective on the spatial evolution of extreme waves like solitons on finite background.

## Contribution

It presents a novel transformation to displaced phase-amplitude variables, enabling autonomous oscillator equations for wave analysis and detailed study of solitons on finite background.

## Key findings

- The transformation simplifies the analysis of wave amplification.
- Autonomous oscillator equations describe spatial evolution of waves.
- Detailed characterization of soliton on finite background.

## Abstract

Wave amplification in nonlinear dispersive wave equations may be caused by nonlinear focussing of waves from a certain background. In the model of nonlinear Schr\"odinger equation we will introduce a transformation to displaced phase-amplitude variables with respect to a background of monochromatic waves. The potential energy in the Hamiltonian then depends essentially on the phase. Looking as a special case to phases that are time independent, the oscillator equation for the signal at each position becomes autonomous, with the change of phase with position as only driving force for a spatial evolution towards extreme waves. This is observed to be the governing process of wave amplification in classes of already known solutions of NLS, namely the Akhemediev-, Ma- and Peregrine-solitons. We investigate the case of the soliton on finite background in detail in this Letter as the solution that describes the complete spatial evolution of modulational instability from background to extreme waves.