Stimulated Raman scattering of a Gaussian light beam in a beamguide system

A theoretical analysis is made of stimulated Raman scattering (STRS) of a pump beam of arbitrary width and focused in an arbitrary manner on the entry to a multimode beam waveguide (beamguide) filled with a Raman-active medium. It is shown that when a critical pump power is exceeded, a coherent STRS regime is established in which the Stokes wave quasimodes are locked and which makes it possible to increase considerably the conversion efficiency. Focusing of the pump radiation on the beamguide entry is desirable only if the number of transits between the focusing components of the beamguide is small. When this number is large, the STRS efficiency is insensitive to focusing. For a given pump power and a fixed distance between the focusing components of the beamguide the best conversion efficiency is achieved in a concentric beamguide, whereas for a constant intensity the best results are obtained at the center of a constriction of the pump beam in a planar beamguide. Stimulated Raman scattering involving the Q₀₁(1) transition in hydrogen is used as an example of conversion of the pump radiation as far as possible into the infrared range (8 μ).