igher refractive index) Figure 9.23 Structure of optical fiber consisting of a central core and a peripheral transparent cladding surrounded by protective packaging. 9.4 FIBER-OPTIC COMMUNICATION LINKS Figure 9.24 Ray paths for several angles of incidence (n1 > n2). just grazes the surface. Figure 9.24c is an example of total internal reflection. This occurs when the angle of incidence exceeds the critical angle. A glass fiber, for the effective transmission of light, requires total internal reflection. Figure 9.25 illustrates a model of a fiber-optic link. Besides the supporting electrical circuitry, it shows the three basic elements in an optical fiber transmission system: (1) the optical source, (2) the fiber link, and (3) optical detector. Regarding the fiber-optic link itself, there are two basic impairments that limit the length of such a link without resorting to repeaters or that can limit the distance between repeaters. These impairments are loss (attenuation), usually expressed in decibels per kilometer, and dispersion, usually expressed as bandwidth per unit length, such as megahertz per kilometer. A particular fiber-optic link may be power-limited or dispersion-limited. Dispersion, manifesting itself in intersymbol interference at the receive end, can be brought about by several factors. There is material dispersion, modal dispersion, and chromatic dispersion. Material dispersion can manifest itself when the emission spectral line is very broad, such as with a light-emitting diode (LED) optical source. Certain frequencies inside the emission line travel faster than others, causing some transmitted energy from a pulse to arrive later than other energy. This causes intersymbol interference. Modal dispersion occurs when several different modes are launched. Some have more reflections inside the fiber than other modes, thus, again, causing some energy from the higher-order modes to be delayed, compared with lower-order modes. Let us examine the effect of dispersion on a train of pulses arriving at a light