helps to overcome the dispersion problems associated with step-index multimode fiber. Light that enters this type of fiber does not travel in a straight line, but rather follows a parabolic path, with all rays arriving at the receiver at more or less the same time. Graded-index fiber typically has a core diameter of 50 to 62.5 microns, with a cladding diameter of 125 microns. Some variations exist; there is at least one form of multimode graded index with a core diameter of 85 microns, somewhat larger than those described previously. Furthermore, the actual thickness of the cladding is important: If it is thinner than 20 microns, light begins to seep out, causing additional problems for signal propagation. Graded-index fiber was commonly used in telecommunications appli- cations until the late 1980s. Even though graded-index fiber is signifi- cantly better than step-index fiber, it is still multimode fiber and does not eliminate the problems inherent in being multimode. Thus was born the next generation of optical fiber: single-mode. Single-Mode Fiber There is an interesting mental conundrum that crops up with the intro- duction of single-mode fiber. The core of single-mode fiber is significantly narrower than the core of multimode fiber. Because it is narrower, 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 Chapter 7 would seem that its ability to carry information would be reduced, due to limited light-gathering ability. This, of course, is not the case. As its name implies, it allows a single mode or ray of light to propagate down the fiber core, thus eliminating the intermodal dispersion problems that plague multimode fibers. In reality, single-mode fiber is a stepped-index design, because the core's refractive index is slightly higher than that of the cladding. It has become the de facto standard for optical t