Coupled nonlinear equations that describe the nonlinear process of stimulated Raman scattering in optical fibers are derived. These equations account in a unified manner for the Raman amplification, the Stokes generation, the induced self-frequency shift, and the interpulse stimulated Raman-scattering-induced cross-frequency shift. The equations reduce to a well-known form for relatively wide picosecond pump pulses. Using these equations, we show theoretically that the effects of cross-phase modulation, self-frequency shift, and cross-frequency shift cause two optical pulses copropagating in the anomalous dispersion regime of the fiber to shed some of their energy and evolve into a narrower soliton, which has a higher frequency shift than a single propagating soliton. It is also shown that the self-frequency shift of femtosecond pulses is detrimental to Raman generation. As the input pulse width is reduced, the spectrum of the pulse shifts by an amount comparable with the Raman shift. The shift increases continuously with propagation at such a rapid rate that the Stokes pulse has no time to build up from noise to significant energy levels.
© 1996 Optical Society of America
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