                               FIBER OPTICS

     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 waveguides
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 0.2
mm (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 nonconductivity of
electricity.  In telephone systems they can provide communication
channels to customers and wideband facilities for interconnecting
switching offices.  Because they are nonconducting, 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 lightwaves.  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.
