fficiently transparent SO that the signal does not have to be re- freshed, amplified and repeated for short distances. The photode- tectors must be inexpensive, sensitive to the proper frequencies, and exhibit low noise levels. 1.3.1 Fiber Selection A choice of fiber type determines the amount of power the emitter or light source can place in the fiber and the attenuation of the light IRED/ILD Information flow PIN/APD Electrical Electrical signal signal Fiber cable Source emitter Detector Fig. 1-6. Major elements of a fiber optics systems. THE MEDIUM AND THE MARKET 11 power as it travels down the fiber. A basic understanding of this can be obtained by comparing various fiber types. A step index fiber has an abrupt change in the index of refraction. In a graded index fiber, the index of refraction changes gradually from a high to a lower value as position varies radially from the center. This causes the rays to gradually bend back towards the center, instead of reflecting, and keeps the rays traveling along the fiber at the same velocity. This velocity matching property in a multimode fiber generally gives graded index fibers higher bandwidth-distance characteristics. The highest bandwidth-distance fibers are designed to operate like mi- crowave waveguides. (see Fig. 1-7). The velocity of light in the fiber, as in any medium, varies with its wavelength-the longer the wavelength, the greater the velocity. Thus, if the source emits light over a range of wavelengths, the shorter wavelengths of light will trail behind the longer, broadening the signal. However, if the diameter of a fiber is less than about two wavelengths, only one mode will be propagated, which is called a single mode fiber. At this diameter, fiber optics begins to resemble the transmission of microwaves. Single mode fibers provide a greater bandwidth than do multi- mode fibers because the signal is not subjected to modal dispersion. However, less power can be coupled because of the severe constraint imposed by the single characteristic angle. The de