channel in Washington, DC, and a third channel in Richmond, Virginia. In addition, if traffic war- ranted it, channels could also be added to the fiber at those or other locations along the route from New York to Miami. Now that we have Fiber in the WAN an appreciation for the role of fiber Bragg gratings, let's discuss a third area of technology that enables WDM and DWDM to become a reality. That area of technology is erbium amplification. ERBIUM AMPLIFICATION Optical transmission is similar to elec- trical transmission in that both types of signaling become weaker and distorted as transmission distance increases. In an optical environment, which in many ways is similar to a copper cable environment, the ability to extend transmission requires signals to be periodically amplified. Light pulses can typically travel between 40 and 80 km before becoming too attenuated and dispersed. Until the late 1980s the only method available to amplify a light signal was through the use of an electronic repeater. In this method, the repeater performed an optical-to-electrical and electrical-to-optical con- version as illustrated in Figure 6.2. In effect, this action regenerated a new optical signal, and the device was referred to as an optical regenerator. The use of an optical regenerator considerably adds to the cost of a long-distance fiber. Thus, the realization that erbium has an electron shell or band whose energy-level difference from the base configura- tion of erbium ions, referred to as a metastable band, is close to the ener- gy of a photon in the 1550-nm range, permitted this rare-earth ele- ment to serve as an optical amplifier. The rationale for the use of erbium is the fact that the 1550-nm region represents a midpoint posi- tion in an optical "window," where signal loss values are lowest in that optical region. The optical region between 1530 and 1565 nm is referred Optical Repeater Figure 6.2 The optical repeater represents an optical- to-electrical and elec- Optical Fiber Optical Fiber trical-to-optical con- v