Origin of backward to forward wave dominance in broadband Raman scattering in hydrogen

We have observed that in a multilens H2 cell it is possible to obtain a total dominance of the first Stokes backscattered wave with respect to the forward wave [1]. This dominance of the backward wave is produced by a combination of a strong focusing and a matched backward mode stimulated by the forward generation. A quantitative analysis of the backward gain and of the forward gain suppression due to the interference with a matched antiStokes generation is performed. Both the gain and bandwidth analysis are in agreement with a theory of multimode broadband scattering in quasi-stationary regime. A matched backward mode is observed for the pump and its mixing with the first Stokes backward emission induces weak high order backward Raman generations that should be in principle not observable.