Threshold for stimulated Brillouin scattering in optical fiber.

We show that in optical fiber the threshold exponential gain, Gth, for stimulated Brillouin scattering initiated by spontaneous Brillouin scattering is functionally and strongly dependent on the material, length and numerical aperture of the fiber and the pump wavelength. For silica fiber we show that the value of G(th) at lambda congruent with 1 microm ranges from as low as approximately 5 in long fiber ( > or =few kms) to ~10-12 in fibers of approximately 100 m length and approximately 20-23 for very short fibers (<10 cm).