ing the slowly varying approximation and considering the Kerr effect response time about zero SO to deal with an instantaneous effect (being the Kerr character- istic time of the order of 100 fs this, approximation is always valid in telecommunications). This equation, called nonlinear Schrödinger equation, writes [6,19] (4.39) where T is the time in the pulse reference frame, and the nonlinear coefficient given by with rm representing the modal radius. Also within this approximation, the evolution in the presence of a wideband signal complex and, in order to understand from a qualitative point of view the signal distortions introduced by Kerr effect, signal propagation is generally studied assuming a few simple expressions for the fiber input signal. These particular signals are conceived to put in evidence particular aspects of the Kerr effect, aspects that are called, for historical reasons "effects." Thus, we speak about self phase modulation (SPM) effect, cross phase modulation (XPM) effect, four wave mixing (FWM) effect, and SO on. However, all these phenomena are Kerr effect-induced evolu- tions of particular input signals. If the input signal is complex, the evolution can be qualitatively understood by considering the input as a superposition of a certain number of simple signals and using the fact that the nonlinear effect is small to try to model the final result as the superimposition of the effects on each simple signal (compare in Chapter 6 the rule of penalties addition and Appendix C). This approach is quite useful to understand complex phenomena also, but has clear limits either when the power is very high and the superposition of the individual effects cannot be assumed or when the bandwidth is very large, SO that parameters dependence on wavelength cannot be neglected. These conditions are not SO far from the operational conditions of long-haul high-capacity WDM systems; thus, the validity of all the approxi- mations has always to be verified when in front of a real system. To access more quanti