ves with different wavelength. This effect can also be represented by studying the dependence of the effective refrac- tion index n (i.e., the ratio between the group velocity of a monochromatic guided wave and the velocity of light in vacuum) on the wavelength. In particular, at short wavelengths, the effects of diffraction are smaller and the light is confined well within the core. In this condition, ne is very close to the refractive index of the core. As the wavelength increases, the effects of diffraction become more important and the light spreads slightly into the cladding. The effective refractive index decreases toward the refractive index of the cladding. Finally, at long wavelengths, the effects of diffraction dominate and the light in the fiber spreads well into the cladding, ne being very close to the refractive index of the cladding. In order to obtain an approximate expression of the waveguide dispersion parameter, we will assume a zero material dispersion, a condition that can be realized only if the refraction index does not depend on wavelength. In this condition, it is useful to substitute the angular frequency with the normalized frequency V defined in Equation 4.3 that is directly related to the guiding properties of the fiber. Thus, it is possible to write (4.17) Substituting (4.17) in the definition of the dispersion parameter it results, after expressing Dw as a function of a (4.18) The shape of the propagation constant in an ideal dielectric waveguide whose refraction index distribution does not depend on the wavelength can be varied widely by shaping the index profile being in particular either positive or negative. For example, in a step index fiber, there are two wavelength intervals: one in which the two dispersion terms are cumu- lative, the other in which they have different signs and one tends to compensate the other. Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices Material dispersion Total dispersion Waveguide dispersion Zero dispersion wavelengt