FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

N
nanometer. One thousandth of a micron, i.e. , 10-9
met er.
near total internal reflection sensor. In fiberoptic,
a fiberoptic sensor that operates on the principle
of varying the property of an optical fiber in such
a manner that the amount of light that leaks into
the cladding ie altered by varying the critical angle
in accordance with a baseband signal, auch as by
altering the refractive index or the ordinary bend
radius, thus altering the internal reflection.
noise. In a fiberoptic system, the sum of unwanted or
disturbing energy introduced into the system from
natural or man-made sources, such as unwanted lightwaves
coupled into an optical fiber or unwanted modulation
of lightwaves in an optical fiber due to environmental
conditions that alter the propagation or
modulation characteristics of an optical fiber or
fiberoptic sensor.
noise power. The power that is developed by unwanted
electromagnetic waves from all sources in the output
of a device, such as a transmission channel or amplifier.
Noise power is usually the total noise power
of waves with frequencies within the passband of the
system or device. Croastalk, distortion, and lntermodulation
products are usually classed as noise.
normalized frequency.
See V-parameter.
numerical aperture (N.A.). A measure of the light-accepting
property of an optical fiber. For example,
glass; given by:
N.A. = (n12-n22)1/2
i.e., the square root of the difference of the
squares of the refractive indices of the core, nl,
and the cladding, n2. If nl is 1.414 (glass) and n2
is 1.0 (air), the numerical aperture is 1.0 and all
incident rays will be trapped. The numerical aperture
is a measure of the characteristic of an optical
waveguide in terms of its acceptance of impinging
light. The degree of openness, light-gathering
ability, angular acceptance, and acceptance cone are
all terms describing the characteristic. It may be
necesaary to specify that the refractive indices are
for step index fibera and for graded index fibers;
nl is the maximum index in the core and n2 is the
minimum Indexin the cladding. As a number, theN.A.
expresses the Mghtgathering power of a fiber. It
is mathematically equal to the sine of the acceptance
angle. A method of meaauring the N.A. is to excite
the fiber in the visible region and display the light
emerging from the end perpendicularly on a screen
about 10 to 30 cm away. The measured diameter of
the projected circle of light divided by twice the
distance from the fiber end to the screen is the
numerical aperture. The numerical aperture is also
equal to the sine of the half-angle of the widest
bundle of rays capable of entering a lens, multiplied
by the refractive index of the medium containing
that bundle of rays, i.e., the incident medium.
Typical numerical apertures for plastic-clad fused
silica optical fibers range from 0.25 to 0.45.
optic.
optical cable.
See fiberoptic.
optical circuit.
optical data link.
0
See fiberoptic cable.
See integrated optical circuit.
See fiberoptic data link.
optical fiber. A single discrete optical transmission
element or waveguide usually consisting of a fiber
core and a fiber cladding that can guide a lightwave
and is usually cylindrical in shape. It consists
either of a cylinder of transparent dielectric material
of a given refractive index whose walls are in
contact with a second dielectric material of a lower
refractive index; or of a cylinder whose core has a
refractive index that gets progressively lower away
from the center. The length of a fiber is usually
much greater than its diameter. The fiber relies
upon internal reflection to transmit light along its
axial length. Light enters one end of the fiber and
emerges from the opposite end with losses dependent
upon length, absorption, scattering, and other factors.
A lightwave in an optical fiber can be modulated
by changing the light propagation parameters
of the fiber. A bundle of fibers has the ability
to transmit a picture from one of its surfaces to
another, around curves, and into otherwise inaccessible
places with an extremely low losa of definition
and light, by the proceas of total internal reflection.
One optical fiber classification scheme is
to divide them into plastic, glaas, or plastic-clad
fuaed silica fibers; then into step-index multimode,
graded-index multimode, or step-index single mode
fibers. Plastic is less brittle than glass but has
increased attenuation compared to glass. Synonymous
with light pipe. See self-focusing optical fiber.
optical fiber coating. A protective material bonded to
an optical fiber over the cladding to preserve fiber
atrength and inhibit cabling losses by providing protection
againat mechanical damage, protection against
moisture and debilitating environments, compatibility
with fiber and cable manufacture, and compatibility
with the jacketing proceas. Coatings include
fluorpolymers, Teflonc, Kynarc, polyurethane,
and many others. Application methods include dipcoating
(for those in solution), extrusion, spray
coating, and electrostatic coating.
optical fiber jacket. A material used to cover an optical
fiber, whether or not it is cladded or coated.
optical fiber loss. The optical power loss in an optical
fiber, usually expressed in dB/km.
optical fiber preform. Specially-shaped material from
which an optical fiber is made, usually by drawing
or rolling. For example, a solid glass rod made with
a higher refractive index than the tube into which
it is alipped,
into a cladded optical fiber; or
tive-index rods surrounding a
index rod heated and drawn into a
drawing procesa results in fiber
than the preforma.
to be heated and drawn or rolled
four lower-refrachigher-refractivecladded
fiber. The
many times longer
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