al amplifiers are required to overcome loss as a result of transmission fiber, optical components, and power splitters. The main types are the erbium-doped fiber amplifier and other rare-earth variants, such as the Thulium-Doped Fiber Amplifier (TDFA), the Fiber Raman Amplifier (FRA), the Semiconductor Optical Amplifier (SOA), and the Erbium- Doped Waveguide Amplifier (EDWA) (Desurvire, 1994, 2002; Srivastava & Sun, 2002; Dutta, 2003; Kasamatsu, Yano & Ono, 2002; Zimmerman & Spiekman, 2004; Torres & Guzmán, 2007; Aozasa, 2007; Morito et al., 2003). Although their geometries, gain processes, and pumping requirements differ, optical amplifiers have important common features. They produce noise, demonstrate gain satu- ration when the signal powers become significant, have wavelength-dependent spectral profiles, show Polarization-Dependent Gain (PDG), and can exhibit transient behavior for short intervals after channels are switched on and off. Because these phenomena have consequences for optical networks within which the amplifiers reside, we address them in the following subsections. However, the issue of noise, which is SO important for network design, is covered separately in the section Amplifier Noise in Optical Systems. Further- more, the signal output powers from optical amplifiers can be high enough to cause non- linear crosstalk in the transmission fiber. This aspect is described in the final section of the chapter. Optical Amplifier Locations Amplifiers can be used in numerous network configurations, as depicted in Figure 6.5. Parts A through E illustrate discrete amplifiers, while Part F shows a distributed amplifier. A power amplifier (otherwise known as a "booster" or "post" amplifier) follows the transmit- ter and SO is often used in medium to heavy saturation. Power amplifiers are commonly EDFAs pumped at 1480 nm to maximize pump power-to-signal power conversion effi- ciency. They are beneficial to the system when the transmitter cannot provide pulses of the required power (perhaps because it include