FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

diffraction. 1. The procesa by which the propagation
of radiant waves or lightwaves are modified as the
waves interact with objects or obstacles. Some of
the rays are deviated from their path by diffraction
at the objects, whereaa other rays remain undeviated
by diffraction at the objects. As the objects become
small in comparison with the wavelength, the
concepts of reflection and refraction become useless,
and diffraction plays the dominant role in determining
the redistribution of the rays following incidence
upon the objects. Diffraction results from
the deviation of light from the paths and foci prescribed
by the rectilinear propagation laws of geometrical
optics. Thus, even with a very small, distant
source, some light, in the form of bright and
dark bands, is found within a geometrical shadow
because of the diffraction of the light at the edge
of the object forming the shadow. Diffraction gratings,
with spacings of the order of the wavelength
of the incident light also cause diffraction that
results in the formation of light and dark areas
called “diffraction patterns.” Such gratings can
be ruled grids, spaced spots, or crYstal lattice
structures. 2. The bending of radio, sound, or
lightwaves around an object, barrier, or aperture
edges.
diode. See light-emitting diode (LED).
dispersion. 1. The process by which rays of light of
different wavelength are deviated angularly by different
amounts; e.g. , as with prisms and diffraction
gratings. 2. Phenomena that cause the refractive
index and other optical properties of a transmission
medium to vary with wavelength, also refers to the
frequency dependence of any of several parameters,
for example, in the process by which an electromagnetic
signal is distorted because the various frequency
components of that signal have different propagation
characteristics and paths. Thus, the components
of a complex radiation are dispersed or
separated on the basis of some characteristic. A
prism disperses the components of white light by
deviating each wavelength a different amount. For
example, 2.5 nsec/km might be a maximum allowable
dispersion for an 18.7 Mbit/see pulse repetition
rate with 10 km repeater spacing. 3. The allocation
of circuits between two points over more than
one geographic or physical route. See intermodal
dispersion; intramodal dispersion; material dispersion;
modal dispersion; waveguide dispersion.
distortion.
See waveguide delay distortion.
diversity. See polarization diversity.
donor. In an intrinsic semiconducting material, a dopant
that has nearly the same electronic bonding
structure as the intrinsic material, but with one
more electron among its valence electrons than that
required to complete the intrinsic bonding pattern,
thus leaving one “extra” or “excess” electron for
each impurity (dopant) atom in the structure. The
dopant (i.e., the donor) atoms are relatively few
and far apart and hence to not interfere with the
electrical conductivity of the intrinsic material.
Tin or tellurium can serve as a dopant for galium
arsenide. The extra electron moves or wanders from
atom to atom more freely than the bound electrons
that ar required to complete the bonding structure,
although interchanges actually occur with the bound
electrons. The extra electrons move about more
freely than the holes created by acceptors. Hence,
the electrons are more mobile than the holes. Under
A-6
the influence of electric fields, the electrons and
holes move in the direction of the field according
to their sign, thus constituting an electric current.
Also see acceptor; electron; hole.
E
electromagnetic interference. Interference caused or
generated in a circuit by electromagnetic radiation
energy coupling. The radiation may be lightwaves,
radio waves, gamma rays, high-energy neutrons, x-
rays, or microwaves. Sources include artifical
transmissions and emissions as well as natural
sources, such as cosmic and solar sources. The
phenomenon of interference is considered to occur
when electromagnetic energy causes an unacceptable
or undesirable response, malfunction, degradation,
or interruption of the intended operation or performance
of electronic equipment.
electromagnetic pulse (EMP). A broadband, high-intensity,
short-duration burst of electromagnetic energy,
such as might occur from a nuclear detonation.
In the case of a nuclear detonation, the electromagnetic
pulse (signal) consists of a continuous spectrum
with most of its energy distributed throughout
the lower frequencies between 3 Hz and 30 kHz.
electromagnetic wave (EMW). The effect obtained when a
time-varying electric field and a time-varying magnetic
field interact, causing electrical and msgnetic
energy to be propagated in a direction that is
dependent upon the spatial relationship of the two
interacting fields that are interchanging their energies.
The most common EMU consists of time-varying
electric and magnetic fields that are directed at
right angles to each other, thus defining a plane in
which they both lie, i.e., polarization plane. The
direction of energy propagation is perpendicular to
this plane, and the wave is called plane polarized.
A plane-polarized wave may be linearly, circularly,
or elliptically polarized depending on the phase
relationship between the varying electric and msgnetic
fields. When launched initially, the interacting
and interrelated time-varying electric and magnetic
fields are produced by an electric current,
consisting of moving electric charges that oscillate
in time and space, such as might oscillate In a wire,
called an antenna. If an electric field is made to
vary in time in a conductive medium in order to produce
an oscillating current, an electromagnetic wave
will be launched that can propagate energy through
material media and a vacuum. If the time and spatial
distributions of currents are given, the electromagnetic
field intensities, power flow rates, and energy
densities can be determined everywhere in space, provided
also that the parameters of the material in
the space are known. Lightwaves are electromagnetic
waves that can travel in optical fibers where they
can be trapped and guided, and can be made to energize
photodetectors.
electron. A basic negatively charged particle with a
char e of 1.6021 x 10-19 C and a mass of 9.1091 x
10-3 f kg. It is outside the nucleus of the chemical
elements, exists with different discrete energy
levels in a given chemical element, differentiates
the elements by its population outside the nucleus,
and is the moving matter that contributes the most
to the formation of electric currents and voltages.
Also see acceptor; donor; hole.