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An optical fiber is a cylindrical dielectric waveguide made of low-loss materials such as silica glass. It has a central core in which the light is guided, embedded in an outer cladding of slightly lower refractive index (Fig. ). Light rays incident on the core-cladding boundary at | Fundamentals of Photonics Bahaa E. A. Saleh Malvin Carl Teich Copyright 1991 John Wiley Sons Inc. ISBNs 0-471-83965-5 Hardback 0-471-2-1374-8 Electronic CHAPTER FIBER OPTICS STEP-INDEX FIBERS A. Guided Rays B. Guided Waves C. Single-Mode Fibers GRADED-INDEX FIBERS A. Guided Waves B. Propagation Constants and Velocities ATTENUATION AND DISPERSION A. Attenuation B. Dispersion C. Pulse Propagation Dramatic improvements in the development of low-loss materials for optical fibers are responsible for the commercial viability of fiber-optic communications. Coming Incorporated pioneered the development and manufacture of ultra-low-loss glass fibers. CORNING 272 An optical fiber is a cylindrical dielectric waveguide made of low-loss materials such as silica glass. It has a central core in which the light is guided embedded in an outer cladding of slightly lower refractive index Fig. . Light rays incident on the core-cladding boundary at angles greater than the critical angle undergo total internal reflection and are guided through the core without refraction. Rays of greater inclination to the fiber axis lose part of their power into the cladding at each reflection and are not guided. As a result of recent technological advances in fabrication light can be guided through 1 km of glass fiber with a loss as low as dB . Optical fibers are replacing copper coaxial cables as the preferred transmission medium for electromagnetic waves thereby revolutionizing terrestrial communications. Applications range from long-distance telephone and data communications to computer communications in a local area network. In this chapter we introduce the principles of light transmission in optical fibers. These principles are essentially the same as those that apply in planar dielectric waveguides Chap. 7 except for the cylindrical geometry. In both types of waveguide light propagates in the form of modes. Each mode travels along the axis of the waveguide with a .