Abstract

The nonlinear regime of stimulated Raman scattering (SRS) may be reached in two ways. The best known is that of depletion of the pump laser as a large number of photons are scattered into the Stokes field. There is, however, another way in which gain saturation can occur, that is, by depletion of the atomic population while the laser remains undepleted. The equations which describe SRS, including damping and population inversion,¹ are remarkably similar to those of well-known theories of two-level superfluorescence, and it is possible by analogy to define a delay time ^τD for cooperative Raman scattering (CRS). This delay time is inversely proportional to the spontaneous scattering rate, which depends on the pump laser intensity. A sufficient condition for CRS is that the spontaneous Stokes linewidth Γ is very much greater than ${\tau }_D^{-1}$. The only other requirement is that there be more laser photons than scatterers, so that ^τD is independent of the number of scatterers. The S(1) transition in molecular hydrogen, when excited by a short (≅ 100-ps) laser pulse, provides a possible system for observing pure CRS. Similar studies have been carried out previously² for $\Gamma \cong {\tau }_D^{-1}$.

© 1985 Optical Society of America