348 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.12: (a) A generic add–drop multiplexer based on optical switches (OS); (b) an add–
drop filter made with a Mach–Zehnder interferometer and two identical fiber gratings.
8.2.3 Add–Drop Multiplexers
Add–drop multiplexers are needed for wide-area and metro-area networks in which
one or more channels need to be dropped or added while preserving the integrity of
other channels. Figure 8.12(a) shows a generic add–drop multiplexer schematically; it
houses a bank of optical switches between a demultiplexer–multiplexer pair. The de-
multiplexer separates all channels, optical switches drop, add, or pass individual chan-
nels, and the multiplexer combines the entire signal back again. Any demultiplexer
design discussed in the preceding subsection can be used to make add–drop multiplex-
ers. It is even possible to amplify the WDM signal and equalize the channel powers
at the add–drop multiplexer since each channel can be individually controlled [83].
The new component in such multiplexers is the optical switch, which can be made us-
ing a variety of technologies including LiNbO 3 and InGaAsP waveguides. We discuss
optical switches later in this section.
If a single channel needs to be demultiplexed, and no active control of individual
channels is required, one can use a much simpler multiport device designed to send a
single channel to one port while all other channels are transferred to another port. Such
devices avoid the need for demultiplexing all channels and are called add–drop filters
because they filter out a specific channel without affecting the WDM signal. If only a
small portion of the channel power is filtered out, such a device acts as an “optical tap”
as it leaves the contents of the WDM signal intact.