Spectral shift of Raman generation band in a Bessel pump beam.

A spectral shift in the first Stokes generation band has been observed at stimulated Raman scattering in a Bessel light beam. Under nanosecond pulse excitation in barium nitrate crystal, the Stokes emission propagating along the Bessel beam axis has been found to be frequency shifted toward the blue side by 3.7 GHz with respect to the Stokes emission propagating along the surface of a cone. Numerical data have shown that the observed frequency shift is directly related to the contribution of the real part of the Raman susceptibility to the refractive index. This contribution causes an increase in the Stokes gain out of exact Raman resonance owing to induced focusing, which inhibits diffraction spread of generated light and thus provides its better spatial overlap with the pump beam.