s a high-loss external modulator) or can only do SO with unacceptable chirp. Then a low-chirp, but low-power, transmitter can be followed by a power amplifier. Line amplifiers are often spaced periodically in a system, especially in subsea operation. However, in terrestrial use geographical features or the logistics of electrical power supplies commonly impose locations at irregular intervals. Line amplifiers normally operate in inter- mediate saturation, and they need well-equalized gain profiles, as explained in the next two subsections. Chapter 6 Optical Fiber Transmission A: Power Amplifier B: Line Amplifier C: Preamplifier D: Component Loss Compensator E: Splitter Loss Compensator F: Distributed Amplifier Fig. 6.5 Main categories of optical amplifiers in networks. Preamplifiers are located before the optical receiver and ensure sufficient incident optical power prior to photon-to-electron conversion to enable good bit error rate (BER) performance, as described later in the chapter. They should be very low noise and normally operate in rather light saturation. The required noise performance is usually achieved by an EDFA pumped at 980 nm (because it provides lower noise than 1480-nm pumping). Sometimes a preamplifier is incorporated within the receiver module. Parts D and E in Figure 6.5 represent a number of possibilities in which the aim of the amplifier is to overcome losses that are due to splitters, passive components, or even complex subsystems, rather than the result of lengthy spans of transmission fiber. There are many examples, but three important ones are At optical cross-connects within metropolitan-area and wide-area networks, where the large number of constituent components can create node losses exceeding 20 dB At dispersion-compensating fiber modules (the details of which are described in a later section) In long-reach PONs, which have target splitting ratios of 1 X 1024 or more (plus fiber spans of up to ~100 km) (Davey et al., 2006) A distributed fiber amplifier is shown schematically in Par