pposite occurs in the event of an increase in the input signal power. Thus, when used in intermedi- ate saturation, total output power varies slowly with total input power. Input signal powers can change for a number of reasons. One possibility is a "soft failure" (i.e., gradual power reduction) of an amplifier pump in a previous part of an ampli- fier chain. Another is extra loss after performing a repair in a subsea system, where it is necessary to add a cable span that is at least twice the depth of the sea at the point of cable damage (Horne, 1989). The process shown in Figure 6.9 can be regarded as a form of passive automatic gain control. When there is a chain of amplifiers in intermediate saturation, the signal output powers tend rapidly to the original value as they progress along the chain. In a WDM system, self-regulation refers to the total input power of all of the channels (as in Figure 6.9). Sometimes the power variation affects only one channel-for example, owing to the reconfiguration of an OADM somewhere in the network. The power of the affected channel then differs from the others throughout the remainder of the amplifier chain, even if each amplifier has a well-flattened gain profile. In such circumstances, other measures, such as dynamic GEFs (Afonso et al., 2006; Barge, Battarel, & De Bougrenet de la Tocnaye, 2005) are required to reestablish power equalization. Amplifier Noise in Optical Systems All of the amplifiers in an optical system produce noise, which interacts in often subtle ways as it is passed through successive amplifiers toward the receiver. The noise, superposed on the signals at the detector, influences the assignment of the detected 1s and Os to their correct Amplifier Noise in Optical Systems values. This section explains how we account for noise accumulation in an amplified fiber system. When an amplifier's gain does not depend on the phase of the incident signals, it is said to be "phase insensitive." The majority of amplifiers-such as rare-earth-doped fiber and waveguide 