High-gain single-pass stimulated Raman scattering and four-wave mixing in a focused beam geometry: A numerical study

Abstract

The coupled Maxwell nonlinear equations for focused beam propagation are solved numerically to describe the cascade single-pass Stokes conversion (stimulated Raman scattering and four-wave mixing) in a high Raman gain molecular medium. A steady-state paraxial description is assumed, with a gain reduction parameter which approximates transient effects. The importance of the four-wave mixing averaging effect on the Stokes energy spectrum is shown. A parametric study is conducted in which the focusing lens focal length, the type and partial pressure of the Raman-active gas and of added rare gas, and the quality of the pump beam, are varied. The conversion efficiency and beam quality of the Stokes and anti-Stokes spectrum are computed. Four-wave mixing is shown to degrade the beam profile, the effect of which is most pronounced for the higher Stokes. Beam cleanup (i.e. the decrease of the M2 parameter of the Stokes beam compared to the pump beam) occurs at low gain (i.e. before the intensity averaging regime appears), whereas it is the opposite case at high gain. The main qualitative features of the modelling match previously published experimental data well, and help to explain the commonalities and discrepancies between the results of different experimental set-ups. © 1997 IOP Publishing Ltd.