10 - Interaction of four-wave mixing and stimulated Raman scattering in optical fibers

Summary

Introduction

Parametric four-wave mixing (FWM) and stimulated Raman scattering (SRS) are two fundamental processes of third-order nonlinear optical materials, and silica optical fibers in particular (Agrawal, 2006). As will be clear from many of the preceding chapters in this book, they also play a central role in supercontinuum generation. On the one hand, FWM involves the elastic interaction between two pump photons as well as a Stokes and an anti-Stokes photon symmetrically located around the pump frequency (Stolen, 1975; Stolen and Bjorkholm, 1982). It is of tremendous interest for large bandwidth optical amplification (Hansryd et al., 2002), including ultra-low-noise phase-sensitive amplification (McKinstrie and Radic, 2004; Oo-Kaw et al., 2008), but also for parametric oscillators (Coen and Haelterman, 2001; Sharping, 2008), wavelength converters (Islam and Boyraz, 2002) including noise-free schemes (Gnauck et al., 2006; Méchin et al., 2006) or optical signal processing in general (Salem et al., 2008). Its efficiency critically depends on a phase-matching relation, which is related to momentum conservation. On the other hand, SRS is characterized by the stimulated inelastic conversion of photons from an intense optical pump wave into lower frequency Stokes photons through the resonant excitation of a vibration in the transmission medium (i.e., optical phonons) (Bloembergen, 1967; Stolen et al., 1972). Here phase-matching is automatically satisfied. The SRS gain, in contrast to the FWM gain, exhibits an asymmetric profile (Stolen and Ippen, 1973): only the down-shifted Stokes waves are amplified while the up-shifted anti-Stokes waves are exponentially absorbed.