Excitation of stimulated Raman scattering in graded-index fiber waveguides by an arbitrary Gaussian beam

A theoretical investigation is made of stimulated Raman scattering in graded-index fiber waveguides with a parabolic distribution of the refractive index in the transverse direction. It is assumed that a fiber is pumped by a Gaussian light beam of arbitrary width and with arbitrary focusing at the entry to the fiber waveguide. The process of stimulated Raman scattering is described by expanding the interacting waves in terms of the waveguide quasimodes which are in the form of Gauss–Hermite functions of width variable along the waveguide. Equations for the amplitudes of these functions make it possible to allow for the parametric coupling between the Stokes modes. The conditions are found for the generation of multimode and singlemode Stokes components. It is shown that the efficiency of stimulated Raman scattering is independent of the focusing of a pump beam at the entry of a fiber waveguide and that it increases on increase in the refractive index gradient.