Forward Brillouin scattering in dual-mode optical fibre

Abstract

Forward stimulated Brillouin scattering (FSBS) is similar in physical origin to conventional backward SBS (BSBS) except that it occurs in a forward direction between two nondegenerate copropagating optical modes of a dual-mode optical fiber. FSBS may, however, be the only example of a nonlinear effect where the threshold falls as the mode spot size is increased. This curious behavior is a consequence of the fact that the acoustic gain depends on an electrostrictive moment and not (as in conventional SBS) a pressure. Intermodal beating between an LP mode gives rise to a moving interference pattern with regions of constructive interference that alternate across the core. In the correct matching conditions. electrostriction will feed optical energy into a flexural mode of the fiber. The results of a theoretical study of FSBS are presented. including calculations of the threshold power (only weakly dependent on the laser linewidth. owing to the copropagating nature of the process) with a discussion of the novel phenomenology caused by long phonon lifetimes. We also show how the theory may be used to design special fibers in which the FSBS threshold power is many times smaller than in BSBS.

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