FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

APPENDIX
FIBEROPTIC SENSORS GLOSSARY
This Glossary on fiberoptic aensors is intended to
provide definitions of the terms used in this Handbook
and to provide supplementary information directly related
to the topics discussed. Many topics that were
introduced in the various chapters are developed in
further detail in thia Glossary. l%is approach was used
to avoid burdening the reader with details and explanations
of terms aa topics were covered. For example,
Maxwell’s equations, solid state electronics, electrooptic
effects, electromagnetic theory, multiplexing
and modulation methods, and various coefficient for
transmission, reflection and attenuation are covered
in this Gloasary.
The definitions in this Glossary are consistent
with international, national, Federal, military, and
technical society atandards. Many were taken from the
more comprehensive Fiberoptic and Lightwave Communications
Standard Dictionary, Illustrated, 284 pages;
and from the Communications Standard Dictionary,
Illustrated. 1045 Dazes. .-. by . Martin H. Weik, Van
Nostrand Reinhold Company, 135 W. 50th Street, New
York, New York, 10020.
A
absorption. The transference of some or all of the
energy contained in an electromagnetic wave to the
substance or medium in which it ia propagating or
upon which is is incident. Abaorbed energy from
incident or transmitted lightwaves is converted into
energy of other forms, usually heat, within the
transmission medium, with the resultant attenuation
of the intensity. See intrinsic absorption.
acceptance angle. The maximum angle, measured from the
longitudinal axis or centerline of an optical fiber
to an incident ray, within which the incident ray
will be accepted for transmission along the fiber,
that is, total internal reflection of the incident
ray occurs. If the acceptance angle for the fiber
is exceeded, total internal reflection will not occur
and the incident ray will be lost by leakage,
scattering, diffuaion, or absorption in the cladding.
The acceptance angle is dependent upon the
refractive indicea of the two media that determine
the critical angle. For a cladded fiber in air,
the sine of the acceptance angle is given by the
square root of the difference of the squares of the
indices of refraction of the fiber core and ~he cla -
ding. In mathematical notation, sine= (n -n
7
where 0 ia the acceptance angle, n, is tiie r~~l~c~
tive index of the core, and n2 is the refractive
index of the cladding. Synonymous with acceptance
one-half angle.
acceptance cone. A solid angle whose included apex
angle is equal to twice the acceptance angle. Rays
of light within the acceptance cone can be coupled
into the end of an optical fiber and still maintain
total internal reflection for all the rays in the
cone. Typically, an acceptance cone is 40°.
acceptance one-half angle.
angle.
Synonym for acceptance
acceptor. In an intrinaic semiconducting material (such
as galium arsenide), a dopant (such as germanium
that has nearly the same electronic bonding structure
as the intrinsic material, but with one less
electron among its valence electrons than that required
to complete the intrinsic bonding structural
pattern. This pattern leaves a “space” or “hole”
for one electron for each dopant atom in the structure.
The dopant atoms are relatively few and are
far apart and hence to not interfere with the electrical
conductivity of the intrinsic material. An
electron from a neighboring intrinsic material atom
can fill the hole at the dopant site, leaving a hole
from whence it came; thus, the hole can appear to
move or wander about, although with less mobility
than the electrons that are free and exceas to donor
atoms. Also see donor; electron; hole.
acoustooptic effect. The changes in diffraction gratings
or phase patterns produced in a transm.lssion
medium conducting a lightwave when the medium is
subjected to a sound (acoustic) wave, due to the
photoelaatic changes that occur. The acoustic waves
might be created by a force developed by an impinging
sound wave, the piezoelectric effect, or magnetostriction.
The effect can be used to modulate a
light beam in a material since many properties,
such as lightconducting velocities, reflection and
transmission coefficients at interfaces, acceptance
angles, critical angles, and transmission modes,
are dependent upon the diffractive changes that
occur. The effect includea the phase transduction
mechanism used in fiberoptic sensors, i.e., t h e
change in phase that occurs due to the change in
length and refractive index caused by the acoustic
presaure. Also see electrooptic effect.
acoustooptics. The study and application of the interrelation
of acoustics and optics. Synonymous with
optoacoustics.
amplification by stimulated emission of radiation
(laser). See light amplification by stimulated emission
of radiation (laser).
amplitude modulation (AM). The modulation of the amplitude
of a wave serving as a carrier, by another wave
serving as the modulating signal. The amplitude excursions
of the carrier are made proportional to a
parameter of the modulating signal that bears the
information to be transmitted.
A-1