efractive index slightly. The refractive index of both pure and lightly doped SiO2 decreases slowly towards longer wavelengths. This variation of refractive index with wavelength is called "material dispersion," or, more fully, "the material component of chromatic dispersion." Chromatic Dispersion: System Implications In a dielectric fiber the optical power travels in both the core and the cladding. Continu- ity of electromagnetic field is required at the core-cladding interface, which means that the overall waveguide mode must propagate at a velocity jointly influenced by the two refrac- tive indices. Light travels faster in a glass with the refractive index of the cladding than in one with the refractive index of the core. Optical waveguide theory accounts for this with the concept of an effective refractive index, Neff, to determine the speed of light of the whole guided mode. neff is governed by the ratio of power in the core to power in the cladding. For a mode to be guided, neff must be greater than the power of the cladding but less than that of the core. In a step index fiber two factors dictate the core-cladding ratio of guided power. The first is the difference between the two refractive indices. When the indices are similar, light spreads into the cladding to a greater extent, but when they are different the guided wave is more tightly confined to the core. The second factor is the ratio of core diameter to wave- length of light. The mode spreads further into the cladding when this ratio is relatively small. We now have a simple chain of logic. The fiber's refractive index profile, its core diam- eter, and the wavelength of light determine the extent to which the guided mode spreads into the cladding. The ratio of power in the core to that in the cladding dictates the effective index of the guided mode. However, the effective mode index governs the speed of light of the whole guided wave. Thus, for a fixed waveguide structure with a given core diameter and refractive indices, Neff varies with the wavel