Gain and index guiding in a Raman generator with diffraction-free pump beams

Gain spectrum for a diffraction-free solution of the steady-state paraxial wave equation for the Stokes amplitude is shown to be frequency-asymmetric relative to an exact Raman resonance. The asymmetry is directly related to a light guiding originated due to Raman-induced change of the refractive index and imprinted in the Stokes far field. The index guiding with a zero-order Bessel beam pump is experimentally observed at the Stokes generation in high-pressure hydrogen and barium nitrate, and reasonable agreement with numerical data is demonstrated. (paper)