Pulse generation in birefringent fibers by four-wave mixing in the presence of Raman scattering is theoretically modeled by a set of coupled nonlinear Schrödinger equations that are solved numerically. We discuss phase matching in the positive group-velocity dispersion regime for the split-pump configuration, which places the parametric frequency shift within the Raman band, and derive the combined initial gain. It is found that for shorter fiber lengths the symmetry-breaking roles of Raman–Stokes gain and Raman–anti-Stokes loss is balanced by four-wave mixing, resulting in a common effective power gain for both components. The importance of the relative phase of the four participating pulses as a switching parameter for the direction of the energy flow is demonstrated. It is further shown that, as a result of pulse walk-off, Raman scattering becomes the dominant process for longer fiber lengths. Theoretical results are compared with experimental cross-correlation pulse shapes.
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