FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

electron-hole recombination. The combining of an electron
and a hole resulting in a decrease in electron
energy and the production of a photon.
electronically-controllable coupler. An optical element
that enables other optical elements to be coupled
to, or uncoupled from, each other, in accordance
with an applied electrical signal. For example, two
parallel slab dielectric waveguides with an optical
material between them whose refractive index can be
altered by application of an electronic signal, thus
turning the coupling of the waveguides on or off according
to the signal.
electrooptic coefficient. A measure of the extent to
which the refractive index changes with applied high
electric field, auch as several parts per 10 thousand
for applied fields of the order of 20 V/cm.
Since the phase shift of a lightwave is a function
of the refractive index of the transmission medium
in which it is propagating, the change in index can
be used to phase-modulate the lightwave by shifting
its phase at a particular point along the guide, by
changing propagation time to the point.
electrooptic device. 1. An electronic device that uses
electromagnetic radiation in the visible, infrared,
or ultraviolet regions of the frequency spectrum;
emits or modifies noncoherent or coherent electromagnetic
radiation in these same regions; or uaes
such electromagnetic radiation for its internal operation.
The wavelengths handled by theae devices
range from approximately 0.3 to 30 microns. 2. Electronic
devices associated with light, aerving aa
sources, conductors, or detectors. Synonymous with
optoelectronic device.
electrooptic effect. The change in the refractive index
of a material when subjected to an electric field.
It can be used to modulate a light beam in a mater–
ial since many light propagation properties are dependent
upon the refractive indices of the transmission
medium in which the light travels.
electrostrictive effects. The change in physical dimensions
that occurs to certain materials when they are
placed in an electric field.
EMI.
EMP.
emission.
See electromagnetic interference.
See electromagnetic pulse.
See spontaneous emission.
emission of radiation. See light amplification by stimulated
emiasion of radiation (laser).
energy band. A specified range of energy levels that a
constituent particle or component of a substance may
have. The particles are uaually electrons, protons,
ions, neutrons, atoms, or molecules. Some energy
bands are allowable and some are unallowable for
specific particles. For example, electrons of a
given element at a specific temperature occupy only
certain energy bands. Examples of energy bands are
the higher and lower level ranges of the conduction
and valence bands.
energy gap. The difference in energy level between the
lower limit of the conduction band and the upper
limit of the valence band.
energy level. The discrete precise amount of kinetic
and potential energy possessed by a body, such as an
orbiting electron. A quantum of energy is absorbed
or radiated depending on whether an electron moves
from a lower to a higher energy level or vice versa.
evanescent-field coupling. Coupling between two waveguides,
auch as an optical fiber or an integrated
optical-circuit (IOC), in which the waveguides are
held parallel to each other in the coupling region,
with the evanescent waves on the outside of one of
the waveguides entering the coupled waveguide, bringing
some of the light energy with it into the coupled
waveguide. In optical fibers and planar dielectric
waveguides, close-to-core proximity or fusion
is required. The evanescent field of the core modes
can be made available by etching away the fiber
cladding or locally modifying the refractive index.
evanescent wave. In a waveguide conducting a transverse
electromagnetic wave, the wave on the outside of the
guide. It will radiate away at sharp bends in the
guide if the radiua of the bend is less than the
critical radius. It uaually has a frequency smaller
than the cutoff frequency above which true propagation
occurs and below which the waves decay exponentially
with distance from the guide. Evanescent
wavefronts of constant phase may be perpendicular
or at an angle less than 90” to the surface of the
guide.
extrinsic fiber loss. Optical power loss in an optical
fiber aplice, connector, or coupling caused by end
separation, axial displacement, axial misalignment,
reflection, or other external condition involved in
implementation or use and subject to the control of
the uaer.
F
Fabry-Perot interferometer. A high-resolution multiple-beam
interferometer consisting of two optically
flat and parallel glasa or quartz plates held a short
fixed diatance apart, the adjacent aurfaces of the
platea or interferometer flata being made almoat
totally reflecting by a thin silver film or multilayer
dielectric coating. If one plate is moved with
respect to the other, interference patterns are produced.
If the ends of an optical fiber are made reflective,
moving one end with reapect to the other
will also result in an output signal when monochromatic
light is inserted into the fiber.
Faraday effect.
FDM.
Synonym for magnetooptic effect.
See frequency-division multiplexing.
fiber. See graded-index fiber; low-loss fiber; multimode
fiber; optical fiber; SELFOCc fiber; self-focusing
optical fiber; single-mode fiber; step-index
fiber; optical-fiber coating.
fiber length-bandwidth product. The product of the
length of an optical fiber and the spectral width of
lightwavea propagating within it, usually expressed
in micron-kilometers.
fiber loss. See extrinsic fiber loss; intrinsic fiber
loss; optical fiber loss.
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