ected at low power next to the signal (Figure 10.46a). If two signals that have almost the same power and are both RZ modulated in a synchro- nous way are injected besides the periodic probe, we obtain an NOR gate, that is, a gate generating a pulse at the output only when both the incoming signals are low (Figure 10.46b). Many other schemes can be devised using SOAs, for example, the Mach-Zendher con- figuration already used for WCs can be reproduced [65], and combining devices based on 11 Probe 12 Pump 1 FIGURE 10.45 All-optical gate based on a single SOA. 11 Filter 13 Pump 2 Depending on the input signal, it can work both as a NOT and as a NOR. Output Probe Pump 1 Pump 2 Output Probe Pump 1 Pump 2 FIGURE 10.46 NOT and NOR configurations of the all-optical gate of Figure 10.45. Next Generation Networking: Enabling Technologies, Architectures, and Performances Pump-probe detuning (nm) 1pump = 1.59 um FIGURE 10.47 Example of an FWM response of a suit- 1,000 2,000 4,000 10,000 ably designed SOA. (After Scotti, S. et al., IEEE J. Selected Areas Quantum Electron., Pump-probe detuning (GHz) 9(6), 746, 1997.) XPM and XGM, also complex logical circuits, for example, for the comparison of a given address with the address of an incoming packet can be implemented. In part, the major limitation of the use of incoherent nonlinearities in SOAs can be over- come by using FWM. As a matter of fact, FWM happens only if a pulse is present contem- porary on the pump and on the probe; thus, it works like an AND logic gate if both pumps and probe are pulse-modulated signals. We know that FWM in SOAs is a coherent effect SO that the output is proportional in phase and amplitude to the probe; moreover, due to very-high-speed nonlinearities, like two photons absorption, FWM happens also with probe and pump with very dif- ferent carrier wavelengths (results up to 80 nm have been achieved, as shown in Figure 10.47 [66]). On the other hand, the FWM in SOAs has an efficiency smaller than one, and that, generally, decreases increasing the d