Cross-phase modulation in optical fibers

Abstract

Raman or parametric optical pulse amplification in fibers involves copropagation of a pump pulse and a weaker signal pulse. The nonlinear index change induced by the pump (δn = n 2 I p ) can phase modulate and thereby spectrally broaden the signal. The spectral features and broadening resulting from this cross-phase modulation (XPM) depend on the walk-off between the pump and signal pulses. Although these spectral features can be complicated, we show that they can be understood both qualitatively and quantitatively by concentrating on the phase change as a function of walk-off. To verify our ideas experimentally, we have used a ring cavity fiber Raman amplification soliton laser (FRASL).1 The effects of walk-off on XPM are conveniently explored in the FRASL because the pump and signal wavelengths are close to the zero dispersion wavelength λ0 in the fiber. When the fiber length is less than the walk-off length (50- and 100-m fibers), XPM dominates the spectral features. On the other hand, when there is complete walk-off between the pump and signal (400-m fiber), the effects of XPM are reduced considerably, but they are not canceled completely because the walk-off is asymmetrized by pump depletion.