Abstract

Optical nonlinearities of the third order in single-mode fibers have been the subject of many studies and much attention has been devoted to the generation of pulses either through the process of stimulated Raman scattering (SRS), or parametric four-wave mixing (VWM).¹ For a complete description both processes require the inclusion of self- and crossphase modulation (SPM/XPM) as well as linear effects such as pulse walk-off and group velocity dispersion (GVD) in the interaction scheme. While the simulation of Raman pulse generation is facilitated through the fact that SRS is a two-pulse, self-phase-matched interaction, the process of VWM requires certain measures to satisfy the phase-matching condition between the four pulses. The 'relative phase' ϕ_r, defined by the difference between the phases of the pumps and the parametric components, gains crucial importance and is responsible for magnitude and sign of the amplification.² In birefringent fibers operated in the positive dispersion regime VWM is possible with the so-called 'split-pump-configurations',³ which requires the pump-pulse at fiber input to exite both polarization axes. Under this condition the Stokes pulse appears on the 'slow axis' and the anti-Stokes pulse on the 'fast axis' with a typical frequency shift between 3 and 12 THz, where SRS also occurs. The new pulses are thus commonly generated by both VWM and SRS. In addition all nonlinear processes mentioned, together with the linear effects of pulse walk-off and GVD—directly or indirectly—contribute to ϕ_r.

© 1994 IEEE