Ring-shaped backward stimulated Raman scattering driven by stimulated Brillouin scattering

Backward stimulated Raman scattering is generated in water, pumped by pre-compressed pulses from a single-cell stimulated Brillouin scattering pulse compressor. The maximum energy efficiency of 9% is achieved by employing a circularly-polarized pump pulse at its energy of 50 mJ, around which point the backward stimulated Raman scattering also exhibits a ring-shaped profile. The correlations between spatial and temporal profiles as well as the intensities of the backward stimulated Raman and the stimulated Brillouin scattering generated from Raman cell indicate that the ring-shaped backward stimulated Raman is driven by intense stimulated Brillouin scattering. We demonstrate the latter process to be much more efficient for the backward Raman generation than the conventional process in which the laser itself pumps a backward stimulated Raman beam. It is shown that a further increase in pump energy leads to a drop in efficiency, combined with a break-up of the ring pattern of backward stimulated Raman. These effects are associated with filament generation above a certain threshold.