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Optical Networks: A Practical Perspective - Part 10

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Optical Networks: A Practical Perspective - Part 10. 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. | 60 Propagation of Signals in Optical Fiber in the cladding and partly in the core and thus their propagation constants satisfy kn2 fl kn . Instead of the propagation constant of a mode we can consider its effective index neff fl k. The effective index of a mode thus lies between the refractive indices of the cladding and the core. For a monochromatic wave in a single-mode fiber the effective index is analogous to the refractive index the speed at which the wave propagates is c n . We will discuss the propagation constant further in Section 2.3. The solution of 2.10 and 2.11 is discussed in Agr97 Jeu90 . We only state some important properties of the solution in the rest of this section. The core radius a the core refractive index ni and the cladding refractive index 2 must satisfy the cutoff condition V d 2 2.405 2.12 in order for a fiber to be single moded at wavelength A. The smallest wavelength A for which a given fiber is single moded is called the cutoff wavelength and denoted by -cutoff- Note that V decreases with a and A i nf n . Thus single-mode fibers tend to have small radii and small core-cladding refractive index differences. Typical values are a 4 gm and A 0.003 giving a V value close to 2 at 1.55 gm. The calculation of the cutoff wavelength Acutoff for these parameters is left as an exercise Problem 2.4 . Since the value of A is typically small the refractive indices of the core and cladding are nearly equal and the light energy is not strictly confined to the fiber core. In fact a significant portion of the light energy can propagate in the fiber cladding. For this reason the fiber modes are said to be weakly guided. For a given mode for example the fundamental mode the proportion of light energy that propagates in the core depends on the wavelength. This gives rise to spreading of pulses through a phenomenon called waveguide dispersion which we will discuss in Section 2.3. A fiber with a large value of the V parameter is called a multimode fiber and