Data Communications Networking Devices - 4th Ed.pdf

768 ___________________________________________________________ SPECIALIZED DEVICESdevices provide a mechanism for the conversion of electrically encoded informationinto light encoded information, their utilization criteria depends upon manyfactors. These factors include response times, temperature sensitivity, power levels,system life, expected failure rate, and cost.A LED is a PN junction semiconductor that emits light when biased in theforward direction. Typically a current between 25 to 100 mA is switched throughthe diode, with the wavelength of the emission a function of the material used indoping the diode.The laser diode offers users a fast response time in converting electricallyencoded information into light encoded information, thereby, they can be used forvery high data transfer rate applications. The laser diode can couple a high level ofoptical power into an optical ®ber, resulting in a greater transmission distance thanobtainable with light emitting diodes. Although they can transmit further at higherdata rates than LEDs, laser diodes are more susceptible to temperature changes andtheir complex circuitry makes such devices more costly.Optical cablesMany types of optical cables are marketed, ranging from simple one-®ber cables tocomplex 18-®ber, commonly jacketed, cables. In addition, a large variety ofspecially constructed cables can be obtained on a manufactured-to-order basis fromvendors.In its most common form an optical ®ber cable consists of a core area, cladding,and a protective coating. This is illustrated in the upper part of Figure 7.27. As alight beam travels through the core material the ratio of its speed in the core to thespeed of light in free space is de®ned as the refractive index of the core.A physical transmission property of light is that while traveling in a medium of acertain refractive index, if it should strike another material of a lower refractiveindex, the light will bend towards the material containing the higher index. SinceFigure7.27 Optical ®ber cable. Reprinted with permission from Data CommunicationsManagement, # Auerbach Publishers,New York)