Suppression of the soliton frequency shifts by nonlinear pairing of pulses

The combined effects of interpulse Raman scattering and cross phase modulation from a pump pulse on the propagation of picosecond signal pulses in an optical fiber is studied. It is shown that although interpulse Raman scattering can suppress the frequency downshifts, it is XPM that has a more significant effect. A new expression for XPM in the presence of Raman scattering is derived. The copropagation of a bright and dark pulse in the so-called reverse or inverted case, in the presence of Raman scattering is also studied. A dark pulse is seen to stabilize the bright pulse against the frequency downshifts.