Transient refractive-index changes in stimulated Raman scattering

We present a theory of two sources of phase aberrations during amplification by stimulated Raman scattering. Both are caused by intensity nonuniformity in the incident pump laser beam. First, nonuniform population transfer of molecules undergoing the Raman transition results in a cumulative refractivity nonuniformity. For the Q(1) vibrational transition in H2 this effect becomes large at a fluence nonuniformity level ≳ 3 J/cm2. Second, collisional relaxation of the excited population during the pulse causes heating and a nonuniform density distribution. The equations coupling the pump and Stokes fields to the refractivity and density changes are solved in the ray-optics limit of collimated beams, and the effect on beam quality is determined.