ssion engineers who design optical networks. Optical fiber has a number of advantages over copper: It is light- weight, has enormous bandwidth potential, has significantly higher ten- sile strength, can support many simultaneous channels, and is immune to electromagnetic interference. It does, however, suffer from several dis- ruptive problems that cannot be discounted. The first of these is loss or attenuation, the inevitable weakening of the transmitted signal over dis- tance that has a direct analog in the copper world. Attenuation is typi- cally the result of two subproperties, scattering and absorption, both of which have cumulative effects. The second is dispersion, which is the spreading of the transmitted signal and is analogous to noise. 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 Scattering Scattering occurs because of impurities or irregularities in the physical makeup of the fiber itself. The best known form of scattering is called Rayleigh Scattering; it is caused by metal ions in the silica matrix and results in light rays being scattered in various directions. Rayleigh Scattering occurs most commonly around wavelengths of 1,000 nm, and is responsible for as much as 90 percent of the total atten- uation that occurs in modern optical systems. It occurs when the wave- lengths of the light being transmitted are roughly the same size as the physical molecular structures within the silica matrix; thus, short wave- lengths are affected by Rayleigh Scattering effects far more than long wavelengths. In fact, it is because of Rayleigh Scattering that the sky appears to be blue: The shorter (blue) wavelengths of light are scattered more than the longer wavelengths of light. Absorption Absorption results from three factors: hydroxyl (OH-, water) ions in the silica, impurities in the silica, and incompl