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Absorption
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Absorption results from three factors: hydroxyl (OH , water) ions in the silica, impurities in the silica, and incompletely diminished residue from the manufacturing process. These impurities tend to absorb the energy of the transmitted signal and convert it to heat, resulting in an overall weakening of the optical signal. Hydroxyl absorption occurs at 1.25 and 1.39 m; at 1.7 m, the silica itself starts to absorb energy because of the natural resonance of silicon dioxide.
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Dispersion
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As mentioned earlier, dispersion is the optical term for the spreading of the transmitted light pulse as it transits the fiber. It is a bandwidthlimiting phenomenon and comes in two forms: multimode dispersion, and chromatic dispersion. Chromatic dispersion is further subdivided into material dispersion and waveguide dispersion.
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Multimode Dispersion
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To understand multimode dispersion, it is first important to understand the concept of a mode. Figure 7-19 shows a fiber with a relatively wide
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Figure 7-19 Multimode fiber: Note wide core diameter.
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Figure 7-20 Single-mode fiber: Note narrow core diameter.
core. Because of the width of the core, it allows light rays arriving from the source at a variety of angles (three in this case) to enter the fiber and be transmitted to the receiver. Because of the different paths that each ray, or mode, will take, they will arrive at the receiver at different times, resulting in a dispersed signal. Now consider the system shown in Figure 7-20. The core is much narrower and only allows a single ray, or mode, to be sent down the fiber. This results in less end-to-end energy loss and avoids the dispersion problem that occurs in multimode installations.
Chromatic Dispersion
The speed at which an optical signal travels down a fiber is absolutely dependent upon its wavelength. If the signal comprises multiple wavelengths, then the different wavelengths will travel at different speeds, resulting in an overall spreading or smearing of the signal. As discussed earlier, chromatic dispersion comprises two subcategories: material dispersion and waveguide dispersion.
Material Dispersion Simply put, material dispersion occurs because different wavelengths of light travel at different speeds through
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an optical fiber. To minimize this particular dispersion phenomenon, two factors must be managed. The first of these is the number of wavelengths that make up the transmitted signal. An LED, for example, emits a rather broad range of wavelengths between 30 and 180 nm whereas a laser emits a much narrower spectrum typically less than 5 nm. Thus, a laser s output is far less prone to be seriously affected by material dispersion than is the signal from an LED. The second factor that affects the degree of material dispersion is a characteristic called the center operating wavelength of the source signal. In the vicinity of 850 nm, red, longer wavelengths travel faster than their shorter, blue counterparts, but at 1,550 nm, the situation is the opposite: Blue wavelengths travel faster. There is, of course, a point at which the two meet and share a common minimum dispersion level; it is in the range of 1,310 nm, often referred to as the zero-dispersion wavelength. Clearly, this is an ideal place to transmit data signals, since dispersion effects are minimized here. As we will see later, however, other factors crop up that make this a less desirable transmission window than it appears. Material dispersion is a particularly vexing problem in singlemode fibers.
Waveguide Dispersion Because the core and the cladding of a
fiber have slightly different indices of refraction, the light that travels in the core moves slightly slower than the light that escapes into and travels in the cladding. This results in a dispersion effect that can be corrected by transmitting at specific wavelengths where material and waveguide dispersion actually occur at minimums.
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