Albedo and laser threshold of a diffusive Raman gain medium

TL;DR

This paper models the diffuse reflectance and transmittance of a Raman gain medium, identifying the laser threshold and effects of scattering parameters and external mirrors through semi-analytic equations.

Contribution

It introduces a semi-analytic two-stream model for Raman gain media that accurately predicts albedo, transmittance, and laser threshold behavior considering scattering effects.

Findings

Raman albedo and transmittance diverge at a critical gain, indicating laser threshold.

The ratio of transmittance to albedo approaches a finite limit depending on scattering properties.

Adding external mirrors reduces the laser threshold gain significantly.

Abstract

The diffuse reflectance (albedo) and transmittance of a Raman random gain medium are calculated via semi-analytic two-stream equations with power-dependent coefficients. The results show good agreement with the experimental data for barium nitrate powder. Both the Raman albedo AR and Raman transmittance TR diverge at a critical gain gc, interpreted as the threshold for diffusive Raman laser generation. However, the ratio TR/AR approaches a finite limiting value dependent on particle scattering albedo v and scattering asymmetry g. The dependence of the generation threshold on the scattering parameters is analysed and the feedback effect of Fresnel reflection at the gain boundaries evaluated. The addition of external mirrors, particularly at the pumped surface, significantly reduces the threshold gain.