l chirp, the change in pulse width is ITm = =-AAwidthDm(25)L - (5.19) Chapter 5 Optical Fibers Normal Propagation Anomalous Propagation Regime Regime Fig. 5.10 Material (chromatic) dispersion of pure silica. Wavelength (um) and in terms of the r.m.s. (Root Mean Square) width Om =-OrDm(As)I (5.20) where O2 is the r.m.s. width of the optical source. The value of do depends weakly on the dopants used in the fiber. For example, the use of 13.5 percent of GeO2 shifts the 20 by 0.1 nm with respect to the 20 of pure SiO2. The material dispersion is also referred to as the chro- matic dispersion. Multimode Fibers As explained in the section titled The Structure and Physics of an Optical Fiber, a multimode fiber can support the propagation of a number of modes (rays). Each of these modes carries the signal imposed on the optical wave. When the modes arrive at the receiver, they create a multi-image of the pulse launched in the waveguide. This multi-image can force the receiver to make a wrong decision regarding the transmitted bit of information. Before we discuss this feature of multimode transmission, we will first look at the loss of multimode fibers (Senior, 1992). The loss of multimode fibers should not be different from the spectral loss presented in Figure 5.8 because it is a question of material and processing. However, some manufacturers make available fiber that still has a higher loss, around 1380 nm, but far less loss than that shown in Figure 5.6. The reason for this is the high cost of completely eradicating the resid- ual loss at the OH wavelength. In Figure 5.11 the spectral loss of a multimode fiber is shown; it is important to notice that the loss at the OH wavelength is only around 0.5 dB, compared to 10 dB shown in Figure 5.6. Light propagating in a multimode fiber is subject to the material dispersion of the fiber. However, there is another form of dispersion that is specific to multimode fibers. Consider a step index multimode fiber with two propagating modes (Figure 5.12). The first is the axial mod