cts. The definition does not include the mode area. In order to also include this element, It can be suitably modified starting from the definition of the mode effective area that, in the hypothesis of perfectly symmetric mode, reads: (4.37) At this point, Equation 4.36 could be modified as follows: (4.38) where A is the geometrical core area. In the case of a step index fiber with a core radius of 5.2 um, A/A =0.95, SO that the results of Equations 4.36 and 4.38 practically do not differ. In other cases however, there is quite a difference. For example, in the case of a DS fiber with the same geometrical core radius, AC/Ae: =0.6 clearly indicating the greater efficiency of nonlinear effects in this kind of fiber due to the greater field confinement. However, it is the length defined by Equation 4.36 that appears in several equations regarding nonlinear effects and generally it is the quantity called characteristic length of fiber nonlinear effects. 4.2.7 Kerr Effect Under the hypotheses set in the previous section, to analyze the impact of Kerr effect, we can neglect in a first approximation the polarization evolution and imagine dealing with ideal LP modes. Several effects induced by the Kerr nonlinearity are much more efficient if the interact- ing signals polarizations are aligned SO that a scalar model results to enhance the nonlin- ear effect with respect to the real vector case. A discussion on the applicability limits of the scalar model for the analysis of transmission systems is reported in [18]. If transmission penalties have to be evaluated, this is a reasonable approach, providing a worst case system performance, if the nonlinear effect is exploited to design some fiber- based device, the correctness of the scalar assumption has to be explicitly verified due to the risk of overestimating the device performances. Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices In these conditions, it is possible to derive a propagation equation in the presence of Kerr effect by apply