ase, the constants describing the material characteristics in the wave equation depend on the field, and nonlinear effects appear. Nonlinear propagation effects can be divided in two categories: scattering effect and the Kerr effect. Scattering effect includes the phenomena in which a propagating field photon is scat- tered by some material alteration caused by the field itself. Kerr effect includes all the phenomena due to the dependence of the real part of dielec- tric susceptibility on the field. Both the classes of effects are important in telecommunication systems, because they are either exploited to improve the system performance or avoided to avoid a too high system penalty. The starting point for the propagation problem is always the wave equation with the boundary conditions corresponding to the fiber structure. In particular, indicating with P(E) the polarization vector of the glass composing the fiber, the wave equation writes [16] (4.32) where P(E) is the polarization vector. In a nonlinear propagation regime, the polarization is not proportional to the field, but it depends on the field in a more complex way. The consequence of this more complex dependence is the presence of wavelengths in the output signal that were not present in the input signal. If the more powerful of these nonlinear products is much smaller than the linear com- ponent (i.e., the signal power remaining along propagation in the input bandwidth) the propagation regime is told to be weakly nonlinear. In the weak nonlinear propagation regime a few approximations can be used that simplify the problem. In normal propagation conditions, the impact of nonlinear propagation can be classified into three main effects: 1. Brillouin effect: It is caused by photons scattering from coherent phonons. This is a backscattering effect. 2. Raman effect: It is caused by photon scattering from glass molecules in an excited state. 3. Kerr effect: It is caused by incoherent interaction between molecules' external orbit- als and the traveling field. In