Stimulated Raman backscattering from an ultrashort laser interacting with underdense plasmas

We present experimental results of the stimulated Raman backscattering instability (BSRS) in an ultrashort intense (45fs, 5.7×1017Wcm-2) laser pulse interacting with an optically ionized helium gas. We have studied the stimulated Raman backscattering reflectivity and the phenomenon of the transition from strongly coupled BSRS to weakly coupled BSRS. We have obtained a good agreement between our experimental results and the theory of BSRS.