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Optical Networks - _2_2 Intermodal Dispersion_24

Optical Networks - _2_2 Intermodal Dispersion_24 - 2.2...

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2.2 Intermodal Dispersion 51 (ZiFr 4 ) in order to realize the low loss that is potentially possible by operating at these wavelengths [KK97, p. 69]. 2.1.1 Bending Loss Optical fibers need to be bent for various reasons both when deployed in the field and particularly within equipment. Bending leads to “leakage” of power out of the fiber core into the cladding, resulting in additional loss. A bend is characterized by the bend radius —the radius of curvature of the bend (radius of the circle whose arc approximates the bend). The “tighter” the bend, the smaller the bend radius and the larger the loss. The bend radius must be of the order of a few centimeters in order to keep the bending loss low. Also, the bending loss at 1550 nm is higher than at 1310 nm. The ITU-T standards specify that the additional loss at 1550 nm due to bending must be in the range 0.5–1 dB, depending on the fiber type, for 100 turns of fiber wound with a radius of 37.5 mm. Thus a bend with a radius of 4 cm results in a bending loss of < 0.01 dB. However, the loss increases rapidly as the bend radius is reduced, so that care must be taken to avoid sharp bends, especially within equipment. 2.2 Intermodal Dispersion An optical fiber consists of a cylindrical core surrounded by a cladding. The cross section of an optical fiber is shown in Figure 2.3. Both the core and the cladding are made primarily of silica (SiO 2 ), which has a refractive index of approximately 1.45. The refractive index of a material is the ratio of the speed of light in a vacuum to the speed of light in that material. During the manufacturing of the fiber, certain impurities (or dopants) are introduced in the core and/or the cladding so that the refractive index is slightly higher in the core than in the cladding. Materials such as germanium and phosphorus increase the refractive index of silica and are used as dopants for the core, whereas materials such as boron and fluorine that decrease the refractive index of silica are used as dopants for the cladding. As we will see, the resulting higher refractive index of the core enables light to be guided by the core, and thus propagate through the fiber. Multimode and Single-Mode Fiber Just as there are different grades of copper cables, there are many grades of optical fiber. The most fundamental divide is between single-mode and multimode fiber. The difference between the two is so profound it is often better to think of them
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52 Propagation of Signals in Optical Fiber Figure 2.3 Cross section and longitudinal section of an optical fiber showing the core and cladding regions. a denotes the radius of the fiber core. as completely different media types, almost as different as copper and fiber. The majority of this book is concerned with single-mode fiber because that is the medium for networks of any length above a few hundred meters. However, multimode fiber will be discussed in this section.
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Optical Networks - _2_2 Intermodal Dispersion_24 - 2.2...

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