Abstract
During linear propagation through a single-mode optical fiber, a picosecond pulse is essentially affected by the fiber attenuation and the group-velocity dispersion (GVD), which respectively result in a wavelength-independent loss of power and a temporal pulse broadening. These both linear processes do not perturb the width of the pulse spectral power density. When increasing the pulse intensity nonlinear propagation will generally result in a modification of its initial spectral density. Among the large variety of nonlinear effects already observed in fibers [1], the combined actions of self-phase modulation (SPM), due to the temporal intensity dependence of the core refractive index, and stimulated Raman scattering (SRS) in silica, generating a Stokes signal at a shifted wavelength, have received much attention during the recent years because of their potential implications in fiber transmission systems, in optical pulse compression arrangements or in Raman fiber laser devices. Stolen et al. [2] succesfully described Raman gain as a response function in the time domain. This approach permits to globally treat the pump and Stokes signals propagation, but it is essentially applicable for ultrashort pulses (<100 fs) when pump bandwidth is of the order of the Raman shift. For longer pulses, simple theoretical models were proposed [3-5] to simulate the individual propagation of two interacting pump and Stokes pulses in the presence of walk-off that gradually separates the pulses because of GVD. In this simplified schemes, the Raman frequency shift is reduced to its peak value (440 cm−1) and a related effective gain is considered.
© 1993 Optical Society of America
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