The structure of the Stokes´ fields reflected in SBS in a light guide

The calculation of gain increments and the spatial structure of the scattered Stokes´ radiation is carried out on the basis of a proposed theoretical method. The so-called rotated modes are shown to be trapped into a spatial resonance with the central reversed mode. These spatial configurations are discovered experimentally with the help of an interferometer with wavefront-reversing mirrors based on stimulated Brillouin scattering. The rotated modes are concentrated in a so-called `background pedestal´ for which the divergence θ_b increases with the gain increment at the longitudinal correlation length Γ_c. For a fixed Γ_c the relationship between the energy of the Stokes´ signal reproducing exactly the spatial structure of the pump radiation and the energy of the background pedestal varies randomly from one laser pulse to another, resulting in a strong dispersion of the energy of the Stokes´ waves measured in both interferometer arms