tached to a mouthpiece, as shown in Figure 7-14. At the receiving end (see Figure 7-15), the light was concentrated by a parabolic mirror onto a selenium resistor, which was connected to a battery and speaker. The diaphragm vibrated when struck by the human voice, which in turn caused the intensity of the light striking the resistor to vary. The selenium resistor, then caused the cur- rent flow to vary in concert with the varying sunlight, which caused the received sound to come out of the speaker with remarkable fidelity. This represented the birth of optical transmission. Optical transmission in its early days was limited in terms of what it was capable of doing. Consider the following analogy: If you look through a 2-foot-square pane of window glass, it appears clear- - if the glass is clean, it is virtually invisible. However, if you turn the pane on edge and look through it from edge to edge, the glass appears to be dark green. 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 Transport Technologies Figure 7-15 photophone receiver Very little light passes from one edge to the other. In this example, you are looking through 2 feet of glass. Imagine trying to pass a high-band- width optical signal through 40 or more kilometers of that glass! In 1966, Charles Kao and Charles Hockham at the UK's Standard Telecommunication Laboratory (now part of Nortel Networks) published their seminal work, demonstrating that optical fiber could be used to carry information provided its end-to-end signal loss could be kept below 20 dB per kilometer. Keeping in mind that the decibel scale is logarith- mic, 20 dB of loss means that 99 percent of the light would be lost over each kilometer of distance. Only 1 percent would actually reach the receiver-and that's a 1-kilometer run. Imagine the loss over today's fiber cables that are hundred