Numerical simulation of stimulated-Raman-scattering conversion of femtosecond UV pulses

A numerical simulation is reported of transient forward stimulated Raman scattering (SRS) of femtosecond pulses in dispersive molecular gases. The one-dimensional two-wave approximation is used and the self-phase modulation as well as the cross-phase modulation of the pulses are taken into account. The kinetics of formation of a Stokes pulse and the transformation of the spectrum and temporal profile in the course of amplification to saturation are tackled for the first time. It is shown that a fundamental role is played by the influence of the difference between the group velocities, which governs the rate of passage of a Stokes pulse along the pump pulse, on the SRS conversion parameters. Depending on this velocity difference, which is a characteristic of the active medium and proportional to its pressure, the SRS falls into three regimes: (1) a Stokes pulse repeats the profile of a pump pulse; (2) pulse compression with optimal gain and strong peaking of the leading edge; (3) multispiking oscillation.