Abnormally low threshold gain of stimulated Brillouin scattering in long optical fiber with feedback.

We show that in the generation of stimulated Brillouin scattering (SBS) in long (> 1 km) fiber, the threshold for observing coherent Stokes emission occurs at notably lower gain than the classical lasing threshold gain for SBS. The effect is due coherent regenerative amplification of strong spontaneous Brillouin scattering, which occurs in the presence of feedback. The magnitude of the effect scales with the length of interaction.