 The input signal is represented in Figure 10.15a. It is quite a noisy 10 Gbit/s signal, with an optical SN of 13 dB, but the main feature is the strong ISI, corresponding to an index of 50%. The signal at the output of the regenerator/converter after filtering with an interfero- metric filter is shown in Figure 10.15b. The reduction of the ISI is evident, due to the non- linear characteristic of the converter. As a matter of fact, the ISI factor is reduced to 7%. As far as the noise is concerned, the optical SN ratio is decreased due to the fact that there is no relevant gain saturation, and the SOAs introduce their own amplified spontaneous emission (ASE) contribution. The output optical SN is about 11 dB with a degradation of 2 dB. 10.3.1.4 Wavelength Converters Based on Four-Wave Mixing The components based on cross gain modulation (XGM) and XPM are effective both as partial regenerators and WCs with a few limitations. The most important limit of such converters is that they perform "incoherent" conver- sion, generating intensity modulated signals. If a different type of signal is needed at the converter output, the converter scheme has to be complicated inserting other functionalities and getting worse and worse with the fabrication yield. The alternative is to use FWM that generates new frequency components when two or more signals at different frequencies are injected into the SOA [25,26]. While converters based on XGM or XPM can provide amplification, depending on the conditions, the FWM efficiency is generally smaller than one; thus, other means have to be found to compensate the losses. On the other hand, the conversion operation is intrinsically coherent. Moreover, a WDM comb can be amplified in a single pass through a single amplifier [27]. The layout of a monolithically integrated WC based on FWM in a SOA is reported in Figure 10.16, and it is similar in the structure to the layout of the other SOA-based WCs. The pump laser is a tunable DBR laser integrated into the WC chip, the amplifier is a very l