Stimulated Raman Scattering in H2 Pumped by a Single Longitudinal Mode Nd:YAG

Strong backward first Stokes (BS) in H2 is observed when a single longitudinal mode fundamental frequency Nd:YAG laser (1.06 μm， pulsed width about 9 ns， linewidth 0.003 cm-1) is used as the pump source. Using a pump energy of 120 mJ, photon conversion efficiency of FS and BS was determmed to be 66% and 15% respectively in 1.5 MPa H2, while in 4.0 MPa H2 the respective values are 46% and 39%. Due to their propagation in opposite directions, there is tension between FS and BS, which leads to a relaxation of the oscillation that splits both FS and BS pulses into two peaks, with the BS pulses being narrowed to about 1 ns. Surprisingly, the BS peak power reaches twice that of the pump, which can never happen in the FS case. Furthermore, the beam quality of BS is much better than that of both FS and pump. At 4 MPa of H2 pressure and a 10 Hz cycle rate, the thermal release of the Raman process deteriorates the FS beam quality, without noticeably affecting that of BS. According to this calculations, within the present experimental conditions, the stimulated Raman scattering process does not reach the steady state. Because of this, all of the experimental results can be explained explained by a related transient state, theory of stimulated Raman scattering.