The threshold for second-order stimulated Brillouin scattering (SBS) in a fiber has been investigated; the study was motivated, in part, by the need to determine the operational dynamic range of SBS fiber beam combiners. Theoretical analysis showed that the second-order Stokes threshold is approximately 130 times the first-order threshold. Experimentally, however, the threshold was found to be only 15 times greater. This dramatic reduction in threshold was determined to be due to the generation of second-order Stokes photons through four-wave mixing, which in turn seeds the second-order SBS process. Suppression of internal Fresnel reflection at the front fiber facet can help to restore the threshold to the higher value.
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