er of issues. Even if in a simple experiment the signal inversion is not a big problem, in a network this feature obliges the network control plane to insert in the wavelength identifier a field that is updated every time the wavelength channel travels through a regenerator and that at the end is used to determine if the binary signal has to be inverted or not. This would be a completely new feature that has to be inserted in the OTN or in the GMPLS protocols. Besides this difficulty, the component performances also have some problems. Since the modulation of the probe is obtained via gain compression, there is a minimum dynamic range and a minimum ISI index that can be attained and that are almost always present at the device output. This is due to the fact that the SOA characteristic time is on the order of magnitude of the bit interval. In the case of Figure 10.13, the better attainable dynamic range is about 14 dB. If a sig- nal with a dynamic range of 20 dB is injected into the SOA, the output signal is degraded instead of improved, being the dynamic range of the output signal almost equal to 14 dB. Last, but not least, in order to work properly, the incoming signal probably has to be ampli- fied as in the example of Figure 10.13. Every amplification adds noise, and it is to be avoided as much as possible in a transparent network. 10.3.1.3 Wavelength Converters and Regenerators Based on Cross-Phase Modulation Instead of using the amplitude nonlinearity, the phase nonlinearity can be used in order to achieve a regenerating WC [21]. Telecommunication Networks Integrated Contact directional coupler InP chip Three-section tunable DBR Long SOA for signal processing FIGURE 10.14 Scheme of a monolithically integrated WC based on SOAs in a Mach-Zehnder configuration. A possible device scheme, with copropagating probe, is reported in Figure 10.14 [22,23]. Two SOAs are disposed along the branches of a Mach-Zehnder interferometer (MZI); the input signal is injected by one of the input ports of the interferometer, the p