Fiber optics is a branch of optics concerning the transmission of light by meansof optical fibers, which are thin strands of glass or other opticallytransparent materials. Optical fibers can be used to guide light–which iselectromagnetic radiation in a certain frequency range–in much the same waythat metal wave guides or coaxial cables can be used to guide lower-frequencyelectromagnetic radiation. Optical Fiber An optical fiber is usually circular incross section and consists of a core and cladding. An optical fiber forcommunication applications is typically between about 0.
1 and 0. 2mm (0. 004 and0. 008 in) in diameter. In order that the light waves be guided by the fiber, thecore must have a higher index of refraction than the cladding. One such fiber iscalled a step-index fiber because the index changes abruptly at the interfacebetween the core and the cladding.
An important variation of this structure isthe graded-index fiber, so called because the index of refraction decreasessmoothly outward from the center with no abrupt step. Transmission of Light Inthe step-index fiber, the light wave is guided by a process called totalinternal reflection. Only rays that have an angle of incidence at thecore-cladding interface greater than the critical angle will be reflected backinto the core and thus guided by the fiber. Some rays follow a longer paththrough the fiber than do others. Thus a pulse of energy entering the fiberundergoes dispersion. This effect limits the bandwidth of the fiber and reducesthe amount of information it can transmit.
This undesirable feature can bepartly overcome by the use of graded-index fibers of proper design. ApplicationsFiber optics is used in several areas of telecommunications. Advantages ofoptical fibers include their wide bandwidth, low attenuation, lightness, smallcross section, and non-conductivity of electricity. In telephone systems theycan provide communication channels to customers and wideband facilities forinterconnecting switching offices.
Because they are non-conducting, they can beused to provide telecommunications services to locations in electrically hostileenvironments, such as electric power stations. Because they are completelyimmune to induced currents from external electromagnetic fields, optical fibersare also useful in environments where electrical noise exists, such as hospitalsand factories. Finally, their lightness makes them attractive for use inaircraft and spacecraft as well as in portable communications systems requiredfor tactical military applications. All these properties make them desirable forinterconnecting computers and other sophisticated electronic equipment. Incommunication-system applications, individual fibers usually are used to guidelight waves. Other applications employ bundles of fibers.
One such applicationis the transmission of light for illumination. Fibers used for this purpose neednot have the cladding or the index gradient of single-fiber light guides becausethe index step at the glass-air interface serves to guide the light. Anotherapplication of fiber bundles is the transmission of images. For this applicationthe fibers must be arranged in the bundle in a coherent fashion. By arrangingthe locations of the fibers at one end (the output) of the bundle in certainways with respect to their location at the other end (the input), such functionsas magnification, inversion, rotation, distortion, and scrambling of the imagecan be performed. Bundles of this type can be used for viewing otherwiseinaccessible areas, an example being the physician’s endoscope.
In order toachieve high resolution, fibers with diameters as small as 0. 02 mm (0. 0008 in)are used in these applications. Fiber bundles are also used in photography,spectroscopy, and image processing.