t, nondegenerate modes propagate along the fiber with different group veloci- ties due to the dependence of the function appearing in Equation 4.2 from the mode numbers. The dependence of on the propagation mode (called mode dispersion [2]) generates signal distortion when transmitting a plurality of modes and greatly limits the possible span of a transmission system. Even if a single LP mode is excited at the fiber input, the presence of material imperfec- tions excites all the modes that can propagate through the fiber, thus causing multimodal propagation. In order to solve this problem, a single-mode fiber has to be designed, that is, a fiber hav- ing only one mode that propagates at the minimum attenuation wavelength. As in any kind of waveguide, modes propagating in an optical fiber are characterized by the so-called cutoff frequency, that is, the minimum frequency at which the mode can propagate. At lower frequencies, the mode propagation is impossible since the field rapidly vanishes advancing along the fiber, irradiating the greater part of the optical power out of the waveguide. The mode cutoff is generally expressed in terms of normalized frequency V, defined as (4.3) where a is the core diameter C is the light speed in vacuum a is the field wavelength W is the field angular frequency It is assumed that the diffraction index n(p) is a decreasing function of the radial coordi- nate P SO that n(0) is the index at the core center and n(oo) is the index of the cladding far from the core. At each fiber mode, a cutoff normalized frequency V can be associated, but to LP1 modes whose cutoff is zero. That means that these two polarization degenerate modes propagate at every frequency. The cutoff frequency of the lower cutoff mode depends on the index profile; for a step index fiber with the indexes given in Figure 4.1, the mode with the lower cutoff is the LP11 and V = 2.405, corresponding to a wavelength of 1262nm for n = 1.4600, ne = 1.4592, and the core radius equal to 10 um. This means that, at 1300 and 1500