logy have caused the use of multimode fiber to fall out of favor; single-mode fiber is far more commonly used today. Multimode fiber is manufactured in two forms: step-index fiber and graded-index fiber. We will examine each in turn. Multimode Step-Index Fiber In step-index fiber, the index of refraction of the core is slightly higher than the index of refraction of the cladding. Remember that the higher the refractive index, the slower the signal travels through the medium. Thus, in step-index fiber, any light that escapes into the cladding because it enters the core at too oblique an angle will actually travel slightly faster in the cladding (assuming it does not escape altogether) than it 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 would if it traveled in the core. Of course, any rays that are reflected repeatedly as they traverse the core also take longer to reach the receiver, resulting in a dispersed signal that causes problems for the receiver at the other end. Clearly, this phenomenon is undesirable; for that reason, graded-index fiber was developed. Multimode Graded-Index Fiber Because of the dispersion that is inherent in the use of step-index fiber, optical engineers created graded index fiber as a way to overcome the signal degradation that occurred. In graded-index fiber, the refractive index of the core actually decreases from the center of the fiber outward. In other words, the refrac- tive index at the center of the core is higher than the refractive index at the edge of the core. The result of this rather clever design is that, as light enters the core at multiple angles and travels from the center of the core outward, it is actually accelerated at the edge and slowed down near the center, causing most of the light to arrive at roughly the same time. Thus, graded-index fiber 