In the early days of optical fiber communications, optical link design engineers only needed to be concerned with the operation of a few component types. Mainly these included light sources, optical fibers, photodetectors, connectors, splices, and couplers. The links carried a single wavelength, and the data rates were low enough that the design did not require a great deal of special signal processing to compensate for distortion effects. However, the push to increase the data rate, provide longer transmission distances, and send many wavelengths simultaneously over the same fiber has resulted in the development of numerous, highly sophisticated passive and active optical components to meet the new and ever-increasing link performance demands. The design, installation, and operation of WDM links now have become more complex with the use of these new components.

This chapter describes the major modules of a typical WDM link and explains their functions. The first part outlines performance requirements for wideband long-distance networks (known as long-haul networks), for metro networks, and for local-area networks (LANs). Section 13.2 gives a top-level view of how various optical devices and modular components fit into a WDM system and what their functions and impacts are on system operation. Next, Sec. 13.3 gives some examples of coarse WDM (CWDM) and dense WDM (DWDM) networks. Finally, Sec. 13.4 introduces the concepts of monitoring the performance and health of WDM links. Chapter 18 presents further details on this last topic.