Generalized multi-dimensional conservation laws for stimulated Raman and Brillouin scattering in a density gradient

TL;DR

This paper derives generalized multi-dimensional conservation laws for stimulated Raman and Brillouin scattering in density gradients, extending classical relations to include angular momentum and wave shifts.

Contribution

It introduces a Lagrangian framework and uses Noether's theorem to derive new conservation laws for SRS and SBS in complex media.

Findings

Conservation laws reduce to Manley-Rowe relations in 1D.

New symmetries lead to conservation of orbital angular momentum.

Identifies additional conserved quantities related to wave shifts.

Abstract

Generalized local and multi-dimensional conservation laws of action, energy, momentum, and angular momentum are derived for stimulated Raman (SRS) and Brillouin backscattering (SBS) in a density gradient within the paraxial ray approximation. A Lagrangian density is found that reproduces the well known envelope equations for SRS and SBS in density gradients in the absence of damping. Using Noether's theorem, the symmetries of the Lagrangian density are used to obtain local conservation laws for quantities that can easily be identified as the action, energy, and momentum. These multi-dimensional conservation laws reduce to the well known one dimensional Manley-Rowe relations, and frequency and wavenumber matching conditions. Additional symmetries of the action lead to conversation laws for new quantities that are identified as orbital angular momentum and contributions to the energy and momentum of the wave from frequency and wavenumber shifts.