Observation of coherent beats in backward stimulated Raman scattering

Abstract Damped oscillations have been observed in pulses of backward stimulated Raman scattering, at the first Stokes frequency, by using a two-cell device and a weakly multimode Nd:YAG laser. Liquid benzene and hydrogen gas, compressed at moderate pressure, have been studied at room temperature. The frequency of these oscillations has been found to be constant and equal to half the frequency difference for two adjacent longitudinal modes of the exciting laser beam. However, the values of the damping time are dependent on the molecular species studied. These results could be interpreted on the assumption of a process of coherent beating between components of the backscattered Stokes wave having close frequencies, during the amplification of this wave by the exciting laser beam, combined with a cooperative effect, similar to that observed in superradiance.