Fiber Optic Communications: Fundamentals and Applications

Chapter 10Nonlinear Effects in Fibers

10.1 Introduction

So far, we have treated the fiber optic system as a linear system, but it is actually a nonlinear system because the refractive index of the fiber changes with the intensity of signal due to the Kerr and Raman effects. In Section 10.2, the origin of linear and nonlinear refractive indices and the Kerr effect are discussed. Since the change in refractive index due to the Kerr effect translates into a phase shift, the signal phase is modulated by its power distribution, which is known as self-phase modulation (SPM). SPM leads to spectral broadening and the exact balance between dispersion and SPM leads to soliton formation. A soliton is a pulse that propagates without any change in shape over long distances. Sections 10.3, 10.4, 10.5, 10.6 present the effects of dispersion, SPM, and soliton formation. In WDM systems, several channels co-propagate down the fiber. The phase of a signal in a channel is modulated not only by its channel power, but also by other channels, which is known as cross-phase modulation (XPM). In addition, nonlinear interaction among two or more channels leads to four-wave mixing (FWM), which acts as noise on channels. The impact of XPM and FWM on the system performance of a WDM system is discussed in Section 10.7. In a high-bit-rate highly dispersive single-channel system, signal pulses overlap strongly in the time domain, leading to intra-channel four-wave mixing (IFWM) and intra-channel cross-phase ...