354 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.17: Schematic of an optical cross-connect based on optical switches.8.2.6 Optical Cross-Connects
The development of wide-area WDM networks requires a dynamic wavelength routing
scheme that can reconfigure the network while maintaining its nonblocking (transpar-
ent) nature. This functionality is provided by anoptical cross-connect(OXC) which
performs the same function as that provided by electronic digital switches in telephone
networks. The use of dynamic routing also solves the problem of a limited number of
available wavelengths through the wavelength-reuse technique. The design and fab-
rication of OXCs has remained a major topic of research since the advent of WDM
systems [103]–[118].
Figure 8.17 shows the generic design of an OXC schematically. The device hasN
input ports, each port receiving a WDM signal consisting ofMwavelengths. Demul-
tiplexers split the signal into individual wavelengths and distribute each wavelength to
the bank ofMswitching units, each unit receivingNinput signals at the same wave-
length. An extra input and output port is added to the switch to allow dropping or
adding of a specific channel. Each switching unit containsNoptical switches that can
be configured to route the signals in any desirable fashion. The output of all switch-
ing units is sent toNmultiplexers, which combine theirMinputs to form the WDM
signal. Such an OXC needsNmultiplexers,Ndemultiplexers, andM(N+ 1 )^2 optical
switches. Switches used by an OXC are 2×2 space-division switches which switch
an input signal to spatially separated output ports using a mechanical, thermo-optic,
electro-optic, or all-optical technique. Many schemes have been developed for per-
forming the switching operation. We discuss some of them next.
Mechanical switching is perhaps the simplest to understand. A simple mirror can
act as a switch if the output direction can be changed by tilting the mirror. The use of
“bulk” mirrors is impractical because of a large number of switches needed for mak-