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Propagation of Signals in Optical Fiber
ptical ﬁber is a remarkable communication medium compared to other media such as copper or free space. An optical ﬁber provides low-loss transmission over an enormous frequency range of at least 25 THz—even higher with special ﬁbers—which is orders of magnitude more than the bandwidth available in copper cables or any other transmission medium. For example, this bandwidth is sufﬁcient to transmit hundreds of millions of phone calls simultaneously, carry about a million high deﬁnition TV (HDTV) video streams, The low-loss property allows signals to be transmitted over long distances at high speeds before they need to be ampliﬁed or regenerated. It is because of these two properties of low loss and high bandwidth that optical ﬁber communication systems are so widely used today. Still, the ﬁber itself does impose physical limitations that must be taken into account in network design. The goal of this chapter is to provide an understanding of the three phenomena that determine ﬁber transmission limits: loss, nonlinear effects, and dispersion. Dispersion is the phenomenon whereby different components of a signal travel at different velocities. In most cases, dispersion limits the data rate of a digital signal by spreading signal pulses over time. In Chapter 5 the interaction of loss, nonlinearity, and dispersion in designing advanced systems will be discussed. We start this chapter by discussing the basics of light propagation in optical ﬁber, starting with attenuation. Then we study propagation using simple geometrical optics model as well as the more general wave theory model based on solving Maxwell’s equations. These models are used to understand dispersion, and in particular intermodal, polarization-mode, and chromatic dispersions, as well as nonlinearity. We will look at the different types of ﬁbers that have been developed to minimize the effects of ﬁber impairments. O 47 ...
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- Spring '09