Effect on beam quality of transient refractive-index changes in stimulated Raman scattering

Two sources of phase aberrations during amplification by stimulated Raman scattering have been studied. 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 associated with the difference between the polarizabilities of the two molecular states. The fractional polarizability difference for the Q(1) vibrational transition of H/sub 2/ is ≅0.15 - 0.20. As a result, this effect becomes large at a fluence nonuniformity level >3J/cm/sup 2/. The dependence on fluence, rather than intensity, arises from the cumulative population transfer that takes place when the relaxation of the upper level during the laser pulse only weakly affects the population difference. Second, collisional relaxation of the excited population during the pulse causes heating and a nonuniform density distribution; this effect becomes more important as the pulselength is increased or the spatial scale size of the intensity gradients is decreased