Polarization properties of noise-initiated stimulated Brillouin scattering in linear birefringent fibers

The transient theory of stimulated Brillouin scattering (SBS) is developed for optical fibers with constant modal birefringence along its length and arbitrary polarization of the pump and Stokes fields. SBS is initiated by thermally excited acoustic waves distributed along the length of the optical fiber, and the generated Stokes field fluctuates in time. It is shown that when a single-frequency pump field is launched such that the fiber is nearly equally excited along the slow and fast axes, the generated Stokes field is partially polarized for fiber lengths longer than its polarization beat length, due to temporal fluctuations in power and polarization. The Stokes field degree of polarization is reduced as the birefringence of the fiber is increased. For launched pump polarizations resulting in nonequal excitation along the slow and fast axes of the fiber, the generated Stokes field is preferentially linearly polarized along the principal axis that exhibits the highest Brillouin gain for fibers with birefringence such that its length is greater than twice its polarization beat length. As the fiber birefringence is reduced, the generated Stokes field state of polarization moves towards the launched pump polarization state, with the opposite sense of rotation. Numerical results are presented to quantify these effects as a function of single-pass Brillouin gain, fiber birefringence, and launched pump polarization state. (C) 2020 Optical Society of America