Page 207 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
12

Wavelength Division Multiplexing

A powerful aspect of an optical communication link is that many different wave-
lengths selected from the spectral regions ranging from the O-band through the
L-band can be sent along a single fiber simultaneously. The technology of com-
bining a number of wavelengths onto the same fiber is known as wavelength
division multiplexing, or WDM. Conceptually, the WDM scheme is the same as
frequency division multiplexing (FDM) used in microwave radio and satellite
systems. Just as in FDM, the wavelengths (or optical frequencies) in WDM
must be spaced properly to avoid interference between adjacent channels. The
key system features of WDM are the following:

■ Capacity upgrade. The classical application of WDM has been to upgrade the
capacity of existing point-to-point fiber optic transmission links. If each wave-
length supports an independent network channel of a few gigabits per second,
then WDM can increase the capacity of a fiber system dramatically with each
additional wavelength channel.
■ Transparency. An important aspect of WDM is that each optical channel can
carry any transmission format. Thus, by using different wavelengths, fast or
slow asynchronous and synchronous digital data and analog information can
be sent simultaneously, and independently, over the same fiber without the
need for a common signal structure.
■ Wavelength routing. Instead of using electronic means to switch optical signals
at a node, a wavelength-routing network can provide a pure optical end-to-end
connection between users. This is done by means of lightpaths that are routed
and switched at intermediate nodes in the network. In some cases, lightpaths
may be converted from one wavelength to another wavelength along their route.
Chapter 17 gives a detailed treatment of wavelength routing and switching.

This chapter begins by addressing the generic operating principles of WDM
and the international standards that have evolved for its implementation. The

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