e spectrum at the fiber output is composed of two lobes. One lobe is identical to the probe spectrum before XPM and a second lobe is its spectrally shifted replica. If a band pass filter removes the spectral components that are shifted by XPM and keeps the original frequency components, we get a signal corresponding to the operation (10.33) where and represent the sum and multiplication between integer numbers repre- sented in base two. On the other hand, if the filter removes the original frequency components and keeps the XPM shifted spectral components, the output signal corresponds to the operation (10.34) Using such gates, either based on SOAs or on Kerr effect in fibers, complex elaboration systems have been constructed in laboratory experiments. Next Generation Networking: Enabling Technologies, Architectures, and Performances Payload header Payload header All-optical deserializer separation FIGURE 10.50 Block scheme of a header recognition and packet routing processor. (After Kurumida, J. et al., All-optical header recognition sub-system based on SOA-MZI switches, Pacific Rim Conference on Lasers and Electro-Optics, 2005. CLEO/Pacific Rim 2005, Tokyo, Japan, IEEE, s.l., 2005, pp. 1790-1791.) A good example is constituted by the header recognition and packet routing processor presented in [68], whose functional scheme is reported in Figure 10.50. The base hypothesis is that the switch fabric of the packet switch is a Banyan network (see Chapter 7), SO that routing is performed stage by stage looking at a single bit of the packet address. In the case depicted in the figure, we see a 4 X 4 processor, but it is simply an example that can be scaled to every power of two. In principle, the working of the system is easy-the header is separated by the payload and parallelized, SO that every bit can drive the corresponding stage of the switch fabric that is assumed to be built with all-optical switches. Much less trivial is the implementation of the whole system using all-optical components. The block scheme of th