be only slightly higher than that of the PDF. Noise due to the interaction of RBS with the amplifiers is less of an issue. The PDF-NDF splice losses can be acceptably small. The fiber's effective area is larger than the DCF in modules, which leads to lower nonlinear crosstalk. Optical Dispersion Compensation: Chirped Fiber Bragg Gratings Several component technologies have been developed to provide discrete optical dispersion compensation (Othonos et al., 2006). Figure 6.24 illustrates the one that is the most favored: the chirped Fiber Bragg Grating (FBG) (Kashyap, 1999; Ennser, Laming, & Zervas, 1998). Chapter 6 Optical Fiber Transmission Three-port Circulator Channel 1 Fig. 6.24 Discrete dispersion compensators based on chirped Channel 2 FBGs and an optical circulator. Where necessary, more than two gratings can be used. The FBG is a segment of optical fiber, typically a few centimeters long, with a low- magnitude, longitudinal perturbation to its core refractive index. If the perturbation is periodic, such as a sinusoid with respect to the length, there will be a very narrow band- width reflection of any incident light with a peak wavelength in the glass that is twice the periodicity of the perturbations. However, the structure used in a dispersion compensator is a chirped grating, which is one that has a superposed variation SO that the period of the refractive index changes slowly over the length of the grating. Therefore, as illustrated in Figure 6.24, the refractive index perturbations are more closely spaced at one end of the grating than the other. In this way, the grating reflects a greater range of wavelengths, as illustrated by the arrows in the enlarged insert in the figure. When a pulse enters the chirped grating, its long- and short-wavelength components propagate into the grating to different extents and therefore experience different phase delays. It is thus possible to a good approximation to use the chirped grating to cancel the group delay that the pulse has accumulated during its progress ov