8.2. WDM COMPONENTS 339
Figure 8.7: Channel selection through a tunable optical filter.8.2 WDM Components
The implementation of WDM technology for fiber-optic communication systems re-
quires several new optical components. Among them are multiplexers, which combine
the output of several transmitters and launch it into an optical fiber (see Fig. 8.2);
demultiplexers which split the received multichannel signal into individual channels
destined to different receivers; star couplers which mix the output of several transmit-
ters and broadcast the mixed signal to multiple receivers (see Fig. 8.5); tunable optical
filters which filter out one channel at a specific wavelength that can be changed by
tuning the passband of the optical filter; multiwavelength optical transmitters whose
wavelength can be tuned over a few nanometers; add–drop multiplexers and WGRs
which can distribute the WDM signal to different ports; and wavelength shifters which
switch the channel wavelength. This section focuses on all such WDM components.
8.2.1 Tunable Optical Filters.....................
It is instructive to consider optical filters first since they are often the building blocks
of more complex WDM components. The role of a tunable optical filter in a WDM
system is to select a desired channel at the receiver. Figure 8.7 shows the selection
mechanism schematically. The filter bandwidth must be large enough to transmit the
desired channel but, at the same time, small enough to block the neighboring channels.
All optical filters require a wavelength-selective mechanism and can be classified
into two broad categories depending on whether optical interference or diffraction is
the underlying physical mechanism. Each category can be further subdivided accord-
ing to the scheme adopted. In this section we consider four kinds of optical filters;
Fig. 8.8 shows an example of each kind. The desirable properties of a tunable opti-
cal filter include: (1) wide tuning range to maximize the number of channels that can
be selected, (2) negligible crosstalk to avoid interference from adjacent channels, (3)
fast tuning speed to minimize the access time, (4) small insertion loss, (5) polariza-
tion insensitivity, (6) stability against environmental changes (humidity, temperature,
vibrations, etc.), and (7) last but not the least, low cost.