FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

angstrom. A unit of length equal to 10-10 meter, 10-1
nanometer, and 10-4 micron.
aperture. See numerical aperture (N. A. ).
array.
See sensor array.
attenuation. The decrease in power of a signal, light
beam, or lightwave, either absolutely or as a fraction
of a reference value. The decrease usually
occurs as a result of absorption, reflection, diffusion,
scattering, deflection, or dispersion from
an original level and usually not as a result of
geometric spreading, i.e., the inverse sqwre of
the distance. In an optical fiber, attenuation Is
undesirable for transmission purposes but desirable
for prevention of leakage or clandestine detection.
Optical fibers have been classified as high-loss
(over 100 dB/km), medium loss (20 to 100 db/km),
and 10W1OSS (less than 20 dB/km).
band.
B
See conduction band; energy band; valence band.
bandwidth. 1. A range of frequencies, usually specifying
the number of hertz of the band or the upper
and lower limiting frequencies. 2. The range of
. frequencies that a device is capable of generating,
handling, passing, or allowing, usually the range
of frequencies in which the response is not reduced
greater than 3 dB from the maximum response.
baseband. The band of frequencies associated with or
comprising an original signal from a modulated
source. In the process of modulation, the baseband
is occupied by the aggregate of the transmitted signals
used to modulate a carrier. In demodulation,
it is the recovered aggregate of the transmitted
signals. The termis commonly applied to cases where
the ratio of the upper to the lower limit of the
frequency band is large compared to unity.
beam splitter. h optical device for dividing a light
beam into two separated beams. One simple beam
splitter consists of a plane parallel plate, with
one surface coated with a dielectric or metallic
coating that reflects a portion and transmits a portion
of the incident beam; i.e., part of the light
is deviated through an angle of 90°, and part iS
unchanged in direction. A beam splitter may also
be made by coating the hypotenuse face of a 45°-90”
prism and cementing it to the hypotenuse face of
another. The thickness of the metallic beam splitting
interface will determine the proportions of the
light reflected and transmitted. In metallic beam
splitters, an appreciable amount of light is lost by
absorption in the metal. It may also be necessary
to match the reflected and transmitted beam for
brightness and for color. In these cases, it is
necessary to use a material at the interface that
gives the same color of light by transmission and
reflection. Nhere color matching at the surface or
interface cannot be accomplished, a correcting color
filter may be placed in one of the beams. In a
fiber-to-fiber beam splitter, evanescent coupling
can be used to transfer optical energy from one
fiber to another.
birefringence. The splitting of a light beam into two
divergent components upon passage through a doublyrefracting
transmission medium, with the two components
propagating at different velocities in the
medium. In an optical fiber, birefringence is related
to the strain in the fiber which causes the
fiber to be a single polarization transmission
medium.
Bragg cell. An acoustooptlc device that accepts fixed
frequency monochromatic light and that has a baseband
vibrating element capable of modulating the input
lightwaves producing an output lightwave with a
frequency equal to the frequency of the input lightwave
plus the frequency of the baseband input signal.
The Bragg cell has application as part of an interferometer
in which heterodyne detection is used.
Brewster angle. The angle, measured with respect to
the normal, at which an electromagnetic wave incident
upon an interface surface between two dielectric
media of different refractive indices is totally
transmitted into the second medium. The magnetic
component of the incident wave must be parallel to
the interface surfa~~l The Brewster angle is given
by: tan B = (~2/E1)
, where B is the Brewster angle,
c1 is the electric permittivity of the incident
medium, and e2 is the electric permittivity of the
transmitted medium. The Brewster angle is a convenient
angle to transmit all the energy in an optical
fiber to an outside detector. There is no Brewster
angle, for which there is total transmission and
therefore zero reflection, when the electric field
component is parallel to the interface, except when
the permittivities are equal, in which case there
is no interface. Mso, for entry into a more dense
medium, such as from air into an optical fiber: tan
B = (n2/nl), and from a more dense medium into a
less dense medium, such as fiber to air: tan B =
(nl/n2), where nl and n2 are the refractive indices
of the air and fiber, respectively.
brightfield sensor. In fiberoptic, a sensor in which
the optical power modulated by the sensor is all or
a large fraction of the total optical power fed to
or available to the sensor. Synonymous with lightfield
sensor. Contrast with darkfield sensor.
budget. See optical power budget; power budget; risetime
budget.
bulk coupler. In fiberoptic, a coupler that has one
input and many outputs.
bundle jacket. The outer protective covering applied
over a bundle of optical fibers.
bus. 1. One or more conductors that serve as a common
connection for a related group of devices. 2. One
or more conductors used for transmitting optical or
electrical power or signals.
bend.
See ordinary bend.
bend loss.
See microbend loss.
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