Self-phase modulation and optical pulse compression influenced by stimulated Raman scattering in fibers

We have investigated experimentally the influence of stimulated Raman scattering, under conditions of pulse walkoff, on the propagation of mode-locked 1.06-,um pulses in single-mode, polarization-preserving fibers and on their subsequent pulse compression. Intense stimulated Raman scattering preferentially depletes (and steepens) the leading edge of 1.0 -,4m pump pulses and clamps the pump pulse energy. Subsequent self-phase modulation of the reshaped pump leads to nonsymmetric spectral broadening and to a nonlinear chirp. Nevertheless, high-quality compressed pulses may be obtained by using an asymmetric spectral window to select a subset of the broadened spectrum. The best compression is achieved when the downshifted spectral components, corresponding to the portion of the pump pulses most severely depleted by stimulated Raman scattering, are selected. Such pulse compression performed under conditions of strong Raman conversion is highly stabilized compared with conventional pulse compression.