Analysis of the 2nd Stokes wave generation through stimulated Raman scattering in hydrogen gas under four-wave mixing conditions

We present an investigation on the 2nd Stokes wave generation in molecular Raman scattering under conditions where four-wave mixing is the dominating process. We have observed that a strong focusing enhances the onset of the 2nd Stokes generation by inducing a strong annular 2nd Stokes emission. A simple analytical description of the phenomenon in the gaussian beam representation is compared to experimental observations resulting in good agreement with both the data of the angular shape and of the 2nd Stokes energy conversion in a regime where mixing is the dominant process.