Diffraction controlled backscattering threshold and application to Raman gap

TL;DR

This paper investigates how diffraction influences the stimulated Raman scattering (SRS) threshold, revealing that diffraction effects can explain the observed Raman gap near half the laser frequency, especially in wide speckled laser beams.

Contribution

It introduces a diffraction-based mechanism that can account for the Raman gap, extending previous models beyond one-dimensional constraints.

Findings

Diffraction increases SRS threshold near the Raman gap.

Diffraction effects depend on interaction length and speckle size.

Diffraction may explain the disruption of Langmuir wave scattering near critical densities.

Abstract

The range of stimulated Raman scattering (SRS) frequencies covers a domain which at the low end abuts half the laser frequency, omega_0 / 2, according to the simplest SRS theories, corresponding to scatter from electron densities near 1/4 critical. Experiments, on the other hand, clearly point to a frequency gap: SRS is not observed at frequencies close to and above omega_0 / 2, indicating a drastic disruption of scatter from Langmuir waves as electron densities approaches 1/4 critical from below. Several one-dimensional mechanisms, linear and nonlinear, have been proposed to explain this "Raman gap". In this paper we release the one-dimensional constraint by allowing diffraction of the scattered light. In the linear convective regime we find that diffractive effects on SRS from a wide speckled laser beam tend to increase the SRS threshold with increase of density, so long as the interaction length is comparable to or larger than a speckle length. This may lead to a new, diffraction controlled, contribution to the Raman gap.