e explanation for this is pro- vided in a later subsection on the Friis cascade formula.) The favored technologies are long-period fiber gratings, thin-film filters (with an isolator), and Arrayed Waveguide Grat- ings (AWGs) (Othonos et al., 2006). A problem with all of these components is that they are normally static filters. Amplifier gain profiles can change significantly with the launched pump power and the number and power of the channels. Therefore, system upgrades and reconfigurations alter the gain spectrum, which means that static GEFs are often no longer able to achieve the necessary equalized powers. Dynamic GEFs, based on liquid crystals or other means, have been successfully demonstrated (Afonso et al., 2006; Barge, Battarel, & De Bougrenet de la Tocnaye, 2005). Their operation requires additional power-monitoring and control software, which increases the amplifier's cost and complexity but they are nec- essary to enable wavelength-routed networks. Amplification in Optical Systems In fiber Raman amplifiers, gain equalization is usually achieved by multi-wavelength pumping (Ritwitt, 2005; Islam, DeWilde, & Kuditcher, 2004). Each pump creates its own spectrally dependent gain profile that is shifted to lower frequencies with a peak at ~13.2 THz and a full width at half-maximum of ~7 THz in silicate glass. As will be discussed later, an important category is the dispersion-compensating fiber amplifier module, in which up to about a dozen pumps can provide very wide and flat spectral profiles in discrete modules. Distributed Raman gain is also commonly used, with the aim of overcoming loss in the transmission fiber, but then three to six pumps are more common. In either case, as the pumps propagate in the fiber, each one contributes to the amplification of the signals. More- over, the shorter-wavelength (i.e., higher-frequency) pumps provide gain for pumps with longer wavelengths. Indeed, the conceptual difficulties are increased by the saturating influence of the bidirectional amplified spontaneous sca