robe by the other. A counter-propagating configuration can also be devised, where the probe is inserted in parallel to the output signal and propagates in the opposite direction with respect to the input signal. However, it is demonstrated that in terms of bandwidth, the copropagating solution is quite superior if all the other parameters are kept constant [24]. In this case, gain saturation has to be avoided; thus, there is no need of amplifying the input signal unless it is really low at the device input. The idea at the base of the design is that in the absence of power incoming from the sig- nal port, the interferometer is balanced SO as to cancel the optical field at the output port. If the signal is injected, the nonlinear phase changes the balance of the interferometer creat- ing constructive interference at the output. Not only in this way the signal is not inverted, but in principle a dynamic range as low as desired can be attained due to the interferometer structure of the device. A demonstration of the possibilities of this device is reported in Figure 10.14, where the output of a regenerator/WC of interferometric type is compared with the input signal. The characteristics of the SOA integrated in the chip are reported in Table 10.2, besides other TABLE 10.2 A Few Important Parameters of an SOA Designed to Enhance XPM While Taking XGM as Low as Possible SOA length 400 um Carriers lifetime Superposition integral Bulk diffraction index Linewidth enhancement factor Local unsaturated gain (enclosing losses) 6 X 104 m-Superscript(1) Local saturation power 50 mW Next Generation Networking: Enabling Technologies, Architectures, and Performances Time (ps X 10) Time (ps X 10) FIGURE 10.15 Time domain response of a WC based on SOAs in a Mach-Zehnder configuration. The input signal is plotted in (a), while the output signal, with reduced ISI, is shown in (b). component parameters. This time, the probe is quite below the local saturation in order to avoid a strong gain saturation that would interfere with the XPM.