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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. Multimode Dispersion To understand multimode dispersion, it is first important to understand the concept of a mode. Figure 6-18 shows a fiber with a relatively wide core. Because of the width of the core, it enables 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 6-19. The core is much narrower and only enables a single ray, or mode, to be sent down the fiber.
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Transport Technologies
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Figure 6-18 Multimode fiber. Note wide core diameter.
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Figure 6-19 Single mode fiber. Note narrow core diameter.
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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.
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Material Dispersion Simply put, material dispersion occurs because dif-
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ferent wavelengths of light travel at different speeds through 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 the signal from an LED.
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Transport Technologies
Transport Technologies
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 1550 nm, the situation is the opposite: blue wavelengths travel faster. Of course, the two meet at a point and share a common minimum dispersion level; it is in the range of 1310 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 single-mode 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 a minimum.
Putting It All Together
So what does all of this have to do with the high-speed transmission of voice, video, and data A lot, as it turns out. Understanding where attenuation and dispersion problems occur helps optical design engineers determine the best wavelengths at which to transmit information, taking into account distance, the type of fiber, and other factors that can potentially affect the integrity of the transmitted signal. Consider the graph shown in Figure 6-20. It depicts the optical transmission domain as well as the areas where problems arise. Attenuation (dB/km) is shown on the Y axis; Wavelength (nm) is shown on the X axis. First of all, note that four transmission windows are in the figure. The first one is at approximately 850 nm, the second at 1,310 nm, a third at 1,550 nm, and a fourth at 1,625 nm, the last two labeled C and L band, respectively. The 850-nm band is the first to be used because of its adherence to the wavelength at which the original LED technology operated. The second window at 1,310 nm enjoys low dispersion; this is where dispersion effects are minimized. 1,550 nm, the so-called C band, has emerged as the ideal wavelength at which to operate long-haul systems and systems upon which Dense Wavelength Division Multiplexing (DWDM) has
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