barcode generator in vb.net 2010 Figure 4-30 Cross-section of multimode fiber. in Software

Create Code 3/9 in Software Figure 4-30 Cross-section of multimode fiber.

Figure 4-30 Cross-section of multimode fiber.
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Core index of refraction: 1.5 Cladding index of refraction: 1.3
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Figure 4-31 End view showing relative diameter of core in multimode fiber.
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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.
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shown in Figure 4-30. 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 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 gradedindex 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, as shown in Figure 4-31. In other words, the refractive 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 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 (Figure 4-32), 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; at least one form of multimode graded-index is available 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.
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Figure 4-32 Light propagation in multimode graded index fiber.
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Graded-index fiber was commonly used in telecommunications applications until the late 1980s. Even though graded-index fiber is significantly better than step-index fiber, it is still multimode fiber and does not eliminate the problems inherent in being multimode. Thus, the next generation of optical fiber was born: single-mode fiber.
Modes: An Analogy
The concept of modes is sometimes difficult to understand, so let me pass along an analogy that will help. Imagine a shopping mall that has a wide, open central area that all the shops open onto. An announcement comes over the PA system informing people that The mall is now closed; please make your way to the exit. Shoppers begin to make their way to the doors on the left, as shown in Figure 4-33, but some wander from store to store, window-shopping along the way, while others take a relatively straight route to the exit. The result is that some shoppers take longer than others to exit the mall because different modes of exiting are available. Now consider a mall that has a single, very narrow corridor that is only as wide as a person s shoulders. Now when the announcement comes, everyone heads for the exit, but they must form a single file line and head out in an orderly fashion, as shown in Figure 4-34. If you understand the difference between these two examples, you understand single versus multimode fiber. The first example represents multimode; the second represents single mode.
Figure 4-33 Multimode shoppers in a shopping mall. Note the various paths that shoppers take as they exit the building.
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