fined as corresponding to the group of frequencies around W. From a detailed analysis of propagation, group velocity is defined by (5.15) where the group index, N, of the material is defined as (5.16) Material Characteristics of Fibers-Dispersion Normal Anomalous Propagation Propagation aN(A)/ an = 0 Group Index, N Refractive Index, n Fig. 5.9 Refractive index and group index for SiO2 glass. Wavelength (um) The packet of frequencies corresponding to the pulse will arrive at the output the fiber sometime after the pulse is launched. This delay is the group delay, and it is defined (5.17) where L is the fiber length. Figure 5.9 shows the refractive index and group index of SiO2 glass fiber. For any material, the zero group dispersion is the wavelength at which the curve of the refractive index has an inflection point. For silica this wavelength is at 1300 nm, and it is known as 20. From Figure 5.9 one can distinguish two regions associated with the group index curve. The first region is the region with wavelengths less than Ao. In this region the group index decreases as the wavelength increases. This means that the spectral components of longer wavelength of the pulse travel faster than spectral components of shorter wave- lengths. This regime is identified as the normal group dispersion regime and imposes a positive chirp. For wavelengths greater than 20 the opposite behavior is observed, and the region is identified as the anomalous group dispersion regime and a negative chirp is imposed on the pulse (Buck, 2004; Agrawal, 2001). When measurements of these parameters are considered, it is useful to define a new parameter known as the material dispersion parameter, Dm(A). This parameter is defined as (5.18) where Dm(A) is expressed as picoseconds per nanometer of source bandwidth per kilometer of distance (ps/nm-km). Figure 5.10 shows the material dispersion of a pure silica glass fiber. There are two ways to write in the time domain the spread of a pulse whose spectrum is centered at As. For pulses without initia