erials than through others. The index of refraction of either the core or the cladding (or both) is changed from the values of pure silica by the incorporation of small amounts of dopant mate- rials, such as germanium and phosphorus. The index of refraction is defined in reference to two media (see Figure 2-3) and represents the ratio of the speeds of light propagation in the two media. In physics tables, the value is expressed in reference to a vacuum. The index of refraction is also depen- dent on the wavelength, as shown in Figure 2-4. Optical fiber is constructed with a cylindrical core of a higher index of refraction than its cladding material. Figure 2-5 illustrates the fiber core composition of transparent glass with a relatively high index of refraction. Optics Primer Figure 2-3 Definition of index or refraction. Medium 1 Fiber Core Reflected Incident Refracted Medium 2 Water Fiber Cladding Notes A1 = A1 sin A1 A1 = n21 = Index of refraction of medium 2 with respect to medium 1 sin A2 or, by further derivation which is the ratio of the speeds of light in the two media In turn, the cladding is enveloped by a protective outer cable material, which plays no role in light propagation. The generic specifications for opti- cal fiber and fiber cables for metro and long-haul applications cover the spe- cific requirements for optical, geometric, mechanical, cable-jacket, environmental, and electrical-protection features. Fiber acts as a waveguide for the signal. Figure 2-6 shows how light trav- els in a fiber. Because there is a difference of speed in the two media (core and cladding), the light is reflected and refracted at the boundary; the goal is to achieve total internal reflection, SO that the light beam is projected for- ward. This occurs when the angle of refraction is 90 degrees. Because, by Chapter 2 Index of refraction of one medium with respect to another medium Figure 2-4 generally varies with the wavelength of the incident beam/ray Index of refraction's dependence on wavelength. 3000 A 8000 A Some in