Frequency chirp and spectra due to self-phase modulation and stimulated Raman scattering influenced by pulse walk-off in optical fibers

A numerical study is presented on the frequency chirp and spectra of light pulses affected by both self-phase modulation and stimulated Raman scattering in optical fibers. The asymmetric distortion of pump and Stokes spectra is attributed to walk-off between the pulses and found to be in good agreement with recent experimental results. The stationary-phase method is employed to explain the relation between the structure of the spectra and the form of the pulse chirp. An analytic expression describing the chirp of a depletable pump and of the first Stokes pulse is given for negligible walk-off. The dominant influence of self-phase modulation over the cross phase modulation on the buildup of the chirp in the case of large walk-off and severe pump depletion is shown.