ong SOA since in this case it can be shown that the nonlinear effects generating FWM are enhanced sensibly by increasing the interaction length [28]. A typical configuration of the spectra of signal, pump, and the main FWM product is also shown in Figure 10.16. In this plot, it can be noted that the FWM operation creates a Telecommunication Networks Integrated Spectrum directional coupler InP chip Signal Pump laser Long SOA for three-section maximum FWM tunable DBR Wavelength efficiency Ps = -3 dBm Pump power (dBm) FIGURE 10.16 Scheme of a monolithically integrated WC based on FWM in an SOA. The schematic component layout and the spectrum of the output field before filtering are shown in (a) while the conversion efficiency versus the pump power or a shift of 420 GHz in an SOA amplifier, 1 mm long, is shown in (b). Continuous and dashed lines rep- resent efficiency for a signal frequency on the right and on the left of the pump frequency (After D'Ottavi, A. et al., IEEE Photon. Technol. Lett., 7(4), 357,1995.). mirrored replica of the signal spectrum, thus creating a phase conjugated signal at the new frequency. The analysis of the FWM efficiency in an SOA is quite complex and cannot be carried out in any case on the ground of the simplified model of the amplifier we have used up to now. As a matter of fact, different causes generate FWM [28]. At low detunings between the signal and the pump (10-15GHz), the main mechanism is the carrier density pulsation induced by the pump-signal beating inside the active region. At higher values of detun- ing, FWM is mainly caused by nonlinear gain and index saturation due to intraband car- rier dynamics. Two intraband mechanisms are prevalent in causing FWM-spectral hole burning (SHB) and carrier heating (CH). The first is dominant if the pump frequency is below the amplifier gain peak, the second is dominant above the gain peak. An example of conversion efficiency for a shift of 420 GHz in an SOA amplifier, 1 mm long, whose parameters are reported in [28], is shown in Figure 1