t F of Figure 6.5, and two types have been demonstrated: the EDFA with very low Er3+ doping levels and the FRA. Of these only the FRA has been commercially exploited and this situation is not likely to change in the foreseeable future. Distributing the gain offers the advantage of simultaneously reducing overall optical noise and nonlinear crosstalk. A fuller discussion is to come. Amplification in Optical Systems Channel Power Equalization All optical amplifiers exhibit wavelength-dependent gain and, in the absence of corrective measures, they cause the problem for WDM systems shown in Figure 6.6. Even if the spectral gain variation is only a few decibels, the effect is cumulative and, after passage through only a small number of amplifiers, the channel powers can vary markedly. The most favored channel can be sufficiently powerful to cause nonlinear crosstalk in the transmission fiber or to saturate the detector, while the least amplified channel arrives at the receiver with such low power that it has an unacceptable bit error rate. There are three main strategies for gain equalization described here: pre-emphasis, Gain-Equalizing Filters (GEFs), and the control of pumping conditions. "Pre-emphasis" (Menif, Rusch, & Karásec, 2001) is the technique of launching unequal, but carefully selected, channel powers in order to obtain spectrally equalized channels at the end of the system, as illustrated in Figure 6.7. The required values are predicted by an optimization procedure along with a detailed model of the amplifiers (such as the Giles- Desurvire theory of EDFAs [Desurvire, 1994, 2002]) and knowledge of the transmission fiber's loss. The pre-emphasis technique is successful and is applied commercially, but the predicted solutions are valid only for one network state. Any changes due to capacity upgrades or revisions in geographical routes require the calculation to be redone. Similarly, the inclusion of reconfigurable optical add-drop multiplexers or cross-connects proves par- Fig. 6.6 Schematic of the progress