Time fluctuations of the radiation of the Stokes component of induced Raman scattering

The fluctuations in the field of the Stokes component of induced Raman scattering (IRS) with a characteristic time determined by the spectral width of the phonon wave excited in the medium were found. The fluctuations are observed in IRS in compressed hydrogen and liquid nitrogen using both narrow-band and broad-band pumping. A system consisting of a single-mode ruby laser and an amplifier is used to excite the IRS. Depending on the method of Q-factor modulation of the laser, the spectral width of pumping may be about 0.01 or 0.3-0.5 $cm^{-1}$. The characteristic time $\tau$ of the fluctuations in hydrogen at pressures of 40 and 20 atm is found to be 2.3 and 4.3 nsec, respectively; the value in liquid nitrogen is 4.5 nsec. Theoretical consideration of the effect leads to the conclusion that, in conditions of reproduction and manipulation of the pumping wavefront, the fluctuations occur synchronously in the beam cross section, while the IRS field with spatially coherent pumping is spatially coherent. Good agreement of the theoretical and experimental values of $\tau$ is observed.