Mode structure and Stokes components of concurrent stimulated Raman scattering in optical waveguides

An experimental investigation was made of the mode structure of the Stokes components in multimode glass and capillary optical waveguides made of SiO₂:GeO₂. Preferential amplification of the dominant mode in graded-index glass fibers resulted in an anomalous reduction in the thresholds of the higher Stokes components and concentrated these components in the dominant mode of the fiber. A complex ring structure of the Stokes wave in the far-field zone observed for step-index glass fibers was due to the possibility of matching of the group velocities of the pump and scattered waves in imperfect step-index fibers.