Nonlinear self-polarization of Raman amplified light in fibers

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We make a theoretical analysis of the energy transfer from an elliptically polarized pump to vectorial Stokes waves in a Raman amplification process. The analysis is done for cw waves using the general vectorial relations for the Raman susceptibility. In the circular-polarization basis, a simple equation is obtained when the phase shift between circularly polarized components is large, so that the phase-dependent components can be ignored as a result of averaging over propagation length. We show that for realistic initial conditions, most of the input energy of the elliptically polarized pump can transfer to a circularly polarized Stokes wave—an effect that was recently observed in experiments with pulse breakup and broadband spectrum formation in twisted fibers. Numerical simulations were carried out for a fixed ratio of 0.3 between the orthogonal and parallel Raman gains; this is the experimentally measured value for small Raman frequency shifts in silica fibers. The simulation shows that the polarization transfer effect is more pronounced when the input Stokes wave ellipticity coincides with the pump ellipticity.