FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

v
vacancy defect. In the somewhat ordered array of atoms
and molecules in optical-fiber material, a site at
which an atom or molecule is missing in the array.
The defect can serve as a scattering center, causing
diffusion, heating, absorption and resultant attenuation.
Also see interstitial defect.
valence band. In a semiconductor, the range of electron
energy, lower than that of the conduction
band, possessed by electrons that are held bound to
an atom of the material, thus reducing conductivity
for electric currents even under the influence of
an applied electric field. When electron engergies
are raised, e.g., by thermal excitation or by phonons,
electrons with the highest energy levels of
the valence band are raised to the lower energy
levels of the conduction band, thus leaving holes
in the atoms whose electrons remain in the valence
band.
velocity.
See phase velocity.
V-parameter. A parameter that can be used to calculate
or express the number of propagating modes that a
step-indexed optical fiber is capable of supporting,
expressed mathematically as:
fn =
(2na/k)(n12 - n22)1/2
where fn is the V-parameter (V-value or normalized
frequency), a is the optical fiber core radius, Ais
the source wavelength, and nl and n2 are the refractive
indices of the core and cladding of the optical
fiber. For a large number of modes, the mode volume
is given by:
N = fn2/2
where N is the number of modes, or mode volume, and
fn Is the V-parameter (V-value or normalized frequency)
above. Synonymous with normalized frequency;
V-value.
V-value. Synonym for V-parameter.
wave.
w
See electromagnetic wave; evanescent wave.
wave equation. The equation, based on Maxwell’s equations,
the constitutive relations, and the vector
algebra, that relates the electromagnetic field of
an electromagnetic wave time and space derivatives
with the transmission medium electrical permittivity
and magnetic permeability in a region without electrical
charges or currents. The solution of the wave
equation yields the electric and magnetic field
strength everywhere as a function of time and space
coordinates, field strengths, and transmission media
parameters. The wave equation is given as either:
72H- ~ca2H/at2 = o ‘r
v2E - vEa2E/at2 = o
in a current- and charge-free nonconducting medium,
where E is the electric field intensity, H is the
magnetic field intensity, c is the electric permittivity,
and p is the magnetic permeability. V is the
vector spatial derivative operator. The wave equation
applies in optical waveguides.
wavefront. A surface normal to an electromagnetic ray
as it propagates from a source, the surface of the
wavefront passing through those parts of the waves
that are in the same phase. For parallel rays, the
wavefront is a plane. For rays diverging from or
converging toward a point, the wavefront is spherical.
The wavefront is perpendicular to the direction
of propagation of the wave, and the electric
and magnetic field vectors of the wave define a
plane that is tangent to the wavefront surface at
the point that the field vectors are determined.
The front is a three-dimensional surface all the
points on which are the same optical path length
from the wave source.
waveguide. Any structure capable of confining and supporting
the energy of an electromagnetic wave to a
specific relatively narrow controllable path that is
capable of being altered, such as a rectangular
cross-section metal pipe, an optical fiber of circular
cross section, or a coaxial cable. See slab
dielectric waveguide.
waveguide delay distortion. In an optical waveguide,
the distortion in received signal caused by the differences
in propagation time for each wavelength,
(i.e., the delay versus wavelength effect for each
propagating mode), causing a spreading of a received
signal pulse at the detector. Waveguide delay distortion
contributes to group-delay distortion as
does material dispersion and multimode group-delay
spread.
waveguide dispersion. The part of the total dispersion
attributable to the dimensions of the waveguide. The
cross-section dimensions are critical. They determine
the modes that are allowed and not allowed to
propagate. Waveguide dispersion increases as the
spectral width of the source increases due to the
actual dimensions and their variation along the
length of the guide.
wavelength. The length of a wave measured from any
point on a wave to the corresponding point on the
next cycle of the wave, such as from crest to crest.
Wavelength determines the nature of the various
forms of radiant energy that comprise the electromagnetic
spectrum, e.g., it determines the color of
light. For a sinusoidal wave, the wavelength is the
distance between points of corresponding phase of
two consecutive cycles of the wave. The wavelength
k, iS related to the phase velocity v, and the frequency
f, by the relation i= vlf.
wavelength-division multiplexingg (WDM). In optical communication
systems, the multiplexing of lightwaves in
a single transmission medium or channel, such that
each of the waves are of a different wavelength and
are modulated separately before insertion into the
medium. Usually, several sources are used, such as
a laser, or several lasera, or a dispersed white
light source or aources, each having a distinctly
different center wavelength. WDM is the same as
frequency-division multiplexing (FDM) applied to
visible light frequencies of the electromagnetic
spectrum.
wave number. The value of 2n times the reciprocal of
the wavelength of a single-frequency sinusoidal wave
such as a singlefrequency uniform plane-polarized
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