istance between the probe and pump. The exam- ple of a very wide FWM efficiency curve is reported in Figure 10.48 [66] where the efficiency is represented with the ratio between the output FWM power and the input probe power. Moreover, observing Figure 10.48, it is possible to notice that not only the FWM effi- ciency varies sensibly as a function of the pump wavelengths, but the shape of variation depends strongly on the detuning. Pump-probe detuning 1000 GHz 2000 GHz FIGURE 10.48 Two values of a very wide FWM effi- ciency curve versus the pump wavelength. (After Scotti, S. et al., IEEE J. Selected Areas Pump wavelength (um) Quantum Electron., 9(6), 746, 1997.) Telecommunication Networks Probe A.A 0 Pump (Ap) Filter High Kerr fiber Probe (10) Probe spectrum FIGURE 10.49 Possible scheme of an all-optical gate realized exploiting a high nonlinear coefficient fiber. In order to achieve higher efficiency at very high speeds, nonlinear elements adopting the fiber need to be used, instead of the SOA. Fiber nonlinear dynamics contain Kerr effect that is practically instantaneous as far as telecommunications are concerned. Several possible gate structures have been proposed based on special and on standard fibers. One of the simpler architectures is shown in Figure 10.49. It is based on a single high nonlinear coefficient fiber that was optimized SO to minimize the required length and the spurious absorption does not require other special components [67]. Let us call A = {ak} and B={bk = the bit streams that have to be processed. Let us also image that A and B are coded into two synchronized RZ IM signals modulated at 10 Gb/s by means of picosecond pulses. The two signals are launched in a suitable length of small core area fiber that presents an enhanced Kerr effect with respect to SSMF. The spectrum of the probe, that is assumed to be of sufficiently low power SO as not to influence the phenomenon, experiences an XPM-induced frequency shift that is propor- tional to the slope of the pump signal power profile. The prob