here is an averaging effect.) Where there are two or more WDM channels in an SOA, each one changes the state of saturation and as a result imposes a modulation on all of the others. This phenomenon, called "cross-gain modulation," is an important limitation, especially in systems at channel datarates of 10 Gbit/s or higher. The effect can be reduced somewhat by using one strongly saturating wavelength that does not convey any data, together with the M data channels, or by encouraging laser oscillation outside the range of channel wavelengths (Zimmerman & Spiekman, 2004; Gutiérrez-Castrejón & Filios, 2006; Michie et al., 2007). Cross-gain modulation remains a constraint, however. For this reason SOAs are most likely to be applied where the number and datarate of the channels Chapter 6 Optical Fiber Transmission Small-Signal Region (seldom used) Operating Point Fig. 6.9 Self-regulating behavior of Initial a C-band EDFA in intermediate Operating Point saturation: 40 channels with 100-GHz Increases spacing. The vertical axis is defined as G(dB) = 10log1ol{Pout(2i)}/MPin], where M = 40, fiber length is 6 m, launched pump power is 900 mW, Input Power and pump wavelength is 1480 nm. Reduces The amplifier is heavily pumped and has a suboptimal length to increase saturation output power. (Simulation by García López, 2008.) Total Launched Signal Power (dBm) are low, such as in optical access networks, lower-capacity metropolitan networks, and fiber- based LANs. (This issue is discussed in detail in Volume 3.) In EDFAs and FRAs, operation with several decibels of gain compression below the small signal limit can be beneficial for the whole system. This situation is illustrated in Figure 6.9, which is a graph of the gain variation of an EDFA with launched signal power summed over all channels (García López, 2008). It shows a self-regulating mechanism. If the input channel power decreases by a few decibels during the system's operating life, the gain increases and SO the output power remains close to its original value. The o