Evolution of Optical Systems
To set the stage for subsequent chapters, Figure 112 shows the evolution of optical systems since the late 1980s/early 1990s to the present time [GILE99].
Figure 112 Evolution of optical systems [GILE99].
The early systems had a single channel point-to-point topology, as shown in Figure 112 (a). The short fiber spans did not use optical amplifier repeaters, so the span lengths rarely exceeded 40 km. The restriction of length was due partially to the limit of laser transmitter power of 1 mW. Optical systems at this time used a single fiber to increase the overall transmission capacity of the point-to-point link.
The discovery of erbium-doped fiber amplifiers (repeaters) was a major milestone in the ability to extend the fiber link. The optical amplifier allowed the optical transmission to extend to wide areas (see Figure 112 (b)).
Now that a link could span thousand of kilometers, it became feasible to deploy add-drop multiplexers (ADMs) to allow the connection of points along the link, shown in Figure 112 (c). (Of course, ADMs in optical networks have been around for over ten years, but wavelength ADMs (WADMs) are more recent.) As of this writing, the fixed-wavelength ADM (of selected channels) is the state-of-the-art implementation for WADMs. But WADMs that are dynamically reconfigurable are the next phase for WADMs.
As shown in Figure 112 (d), cross-connects (XCs) permit a more powerful grooming of traffic between optical networks (in this example, optical rings). This configuration has been deployed for over a decade in TDM optical systems; wavelength XCs, noted as PXCs in this book, are the next stage of the evolution.