 been found not to impose significant additional con- tributions to the PMD. However, because L-band amplifiers require longer spans, PMD can become an issue in some circumstances. Fortunately, as reported in Sachiano (2005), the effect can be alleviated by winding the doped fiber on a small-radius bobbin. As described in the earlier discussion of chromatic dispersion mitigation, chirped fiber Bragg gratings can be used for dispersion compensation of one or a small number of chan- nels by achieving large but negative group velocity dispersions over centimeter lengths. Unfortunately, the combination of possible asymmetries in the writing process, the high chromatic dispersion, and the presence of group delay spectral ripples can lead to high localized PMDs. It is therefore necessary to impose strict standards on the fabrication process and to control the curvature of the gratings within their packages. PMD Compensation PMD can be compensated for optically or electronically; in a WDM system there is one compensator per channel (Bülow & Lanne, 2005; Kudou et al., 2000). Whether optical or electronic, PMD compensation is preferably performed at the receiver end of the system. The objective is always to ensure that the accumulated DGD resulting from passage through the system plus the DGD exerted by the compensator sum to a value that is as close as pos- sible to zero bit periods. Unfortunately, the tracking of high-speed variations (especially those caused by mechanical vibrations due to weather or human activities) is very demand- ing. Therefore, the best that any compensation technique can achieve is to reduce the outage probability to a tolerable value. Optical compensation is performed prior to the detection process. A first-order optical PMD compensator consists of a polarization controller, such as a liquid crystal device, fol- lowed by a highly birefringent element, such as a span of high-birefringence fiber. The polarization controller responds to a feedback signal that originates from a measurement of signal