ilter or a dynamic control of the channels power at the amplifier input. Transmission Systems Architectures and Performances Gain of the signal at 11 (dB) Input power of the signal at 12 (mW) FIGURE 6.36 Decrease of the gain of an amplifier at a certain wavelength due to the presence of a strong signal at a different wavelength. Gain spectrum (dB) P1 = 0.5 P2=0.5 mW P1=0.5 P2=3mW P1 = 3 P2 = 3 mW Wavelength (um) FIGURE 6.37 Dependence of the gain spectrum of an ideal amplifier on the power of a strong signal transiting in the active medium. The first solution is always possible and relies, essentially, on a set of filters, one for each spectral slice, that can be automatically adjusted by the element manager when a new channel is inserted in the ring. Naturally, these filters introduce a greater loss with respect to fixed filters. The other solution is viable only if the channels are demultiplexed at each node, as happens when the OADM is realized with a couple of back to back AWG with optical switches in between (as shown in Figure 6.38). Here a set of variable attenuators equalize the channels before entering in the second amplification stage SO to achieve a globally flat amplification. A second issue arising in DWDM optical rings is related to the fact that, in the absence of a node where the continuity of the optical signal is interrupted, both spurious signal contributions and the ASE out of the signal bandwidth can circulate all around the ring and, if the overall ring gain is positive, could create laser action. Telecommunication Networks FIGURE 6.38 Block scheme of an OADM realized with AWG multiplexer/demultiplexer. As far as ASE is considered, if no node interrupts the ring fibers continuity, it is possible to set up in one location along the ring a periodic filter that passes all the channels but stops the ASE out of the channel bandwidths. The situation is completely different for the spurious signal components that arise from imperfect filtering of the drop OADM and continue circulation along the rin