Optical Networks: A Practical Perspective - Part 9. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 50 Propagation of Signals in Optical Fiber Figure Cross section and longitudinal section of an optical fiber showing the core and cladding regions a denotes the radius of the fiber core. theory model based on solving Maxwell s equations. We then devote the rest of the chapter to understanding the basics of chromatic dispersion and fiber nonlinearities. Designing advanced systems optimized with respect to these parameters is treated in Chapter 5. Light Propagation in Optical Fiber An optical fiber consists of a cylindrical core surrounded by a cladding. The cross section of an optical fiber is shown in Figure . Both the core and the cladding are made primarily of silica S1O2 which has a refractive index of approximately . 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 phosphorous 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. Geometrical Optics Approach We can obtain a simplified understanding of light propagation in optical fiber using the so-called ray theory or geometrical optics approach. This approach is valid when the fiber that is used has a core radius a that is much larger than the operating wavelength X. Such fibers are termed multimode and first-generation optical communication links were built using such fibers with a in the range of 25-100 am and A around gm. Light Propagation in Optical Fiber 51 Figure Reflection and .
