Fiber optics is a branch of optics concerning the transmission of light by means
of optical fibers, which are thin strands of glass or other optically
transparent materials. Optical fibers can be used to guide light--which is
electromagnetic radiation in a certain frequency range--in much the same way
that metal wave guides or coaxial cables can be used to guide lower-frequency
electromagnetic radiation. Optical Fiber An optical fiber is usually circular in
cross section and consists of a core and cladding. An optical fiber for
communication applications is typically between about 0.1 and 0.2mm (0.004 and
0.008 in) in diameter. In order that the light waves be guided by the fiber, the
core must have a higher index of refraction than the cladding. One such fiber is
called a step-index fiber because the index changes abruptly at the interface
between the core and the cladding. An important variation of this structure is
the graded-index fiber, so called because the index of refraction decreases
smoothly outward from the center with no abrupt step. Transmission of Light In
the step-index fiber, the light wave is guided by a process called total
internal reflection. Only rays that have an angle of incidence at the
core-cladding interface greater than the critical angle will be reflected back
into the core and thus guided by the fiber. Some rays follow a longer path
through the fiber than do others. Thus a pulse of energy entering the fiber
undergoes dispersion. This effect limits the bandwidth of the fiber and reduces
the amount of information it can transmit. This undesirable feature can be
partly overcome by the use of graded-index fibers of proper design. Applications
Fiber optics is used in several areas of telecommunications. Advantages of
optical fibers include their wide bandwidth, low attenuation, lightness, small
cross section, and non-conductivity of electricity. In telephone systems they
can provide communication channels to customers and wideband facilities for
interconnecting switching offices. Because they are non-conducting, they can be
used to provide telecommunications services to locations in electrically hostile
environments, such as electric power stations. Because they are completely
immune to induced currents from external electromagnetic fields, optical fibers
are also useful in environments where electrical noise exists, such as hospitals
and factories. Finally, their lightness makes them attractive for use in
aircraft and spacecraft as well as in portable communications systems required
for tactical military applications. All these properties make them desirable for
interconnecting computers and other sophisticated electronic equipment. In
communication-system applications, individual fibers usually are used to guide
light waves. Other applications employ bundles of fibers. One such application
is the transmission of light for illumination. Fibers used for this purpose need
not have the cladding or the index gradient of single-fiber light guides because
the index step at the glass-air interface serves to guide the light. Another
application of fiber bundles is the transmission of images. For this application
the fibers must be arranged in the bundle in a coherent fashion. By arranging
the locations of the fibers at one end (the output) of the bundle in certain
ways with respect to their location at the other end (the input), such functions
as magnification, inversion, rotation, distortion, and scrambling of the image
can be performed. Bundles of this type can be used for viewing otherwise
inaccessible areas, an example being the physician's endoscope. In order to
achieve 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.