s of kilometers long, if 20 dB was the mod- ern performance criterion! Kao and Hockham proved that metallic impurities in the glass-su as chromium, vanadium, iron, and copper-were the primary cause for such high levels of loss. In response, glass manufacturers rose to the chal- lenge and began to research the creation of ultrapure products. In 1970, Peter Schultz, Robert Maurer, and Donald Keck of Corning Glass Works (now Corning Corporation) announced the development of a glass fiber that offered better attenuation than the recognized 20 dB threshold. Today, fiber manufacturers offer fiber SO incredibly pure that 10 percent of the light arrives at a receiver placed 50 kilometers away. Put another way, a fiber with 0.2 dB of measured loss delivers more than 60 percent of the transmitted light over a distance of 10 kilometers. Remember the windowpane example? Imagine glass so pure that you could see clearly through a window 10 kilometers thick. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Chapter 7 Regenerator Figure 7-16 Components of Source a typical optical network Optical Transmitter Optical Amplifiers Receiver Amplifiers O-E-O Fundamentals of Optical Networking At their most basic level, optical networks require three fundamental components, as shown in Figure 7-16: a source of light, a medium over which to transport it, and a receiver for the light. Additionally, there may be regenerators, optical amplifiers, and other pieces of equipment in the circuit. We will examine each of these generic components in turn. Optical Sources Today the most common sources of light for optical systems are either light-emitting diodes or laser diodes. Both are commonly used, although laser diodes have become more common for high-speed data applications because of their coherent signal. While lasers have gone through several iter