FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

Michelson interferometer. b interferometer in which
an electromagnetic wave is split. One half is then
reflected from a fixed mirror and back through the
splitter to a photodetector, the other half is passed
directly through the splitter to a movable mirror
(transducer) that reflects it back to the splitter
where it is reflected to the same photodetector.
The two waves can phase enhance or cancel and thus
modulate the intensity at the photodetector in accordance
with a baseband input signal to the movable
mirror. If an optical fiber is used, the ends of
the fiber form the reflecting surfaces. Moving one
end with respect to the other produces the same effect
as moving the mirror.
microbend. A small bend or induration in the outer
surface of an optical fiber core that causes lightwaves
in the core to penetrate into the cladding
and thus leak from the fiber. Contrast with ordinary
bend.
microbend loss. In an optical fiber, the loss or attenuation
in signal power caused by small bends, kinks,
or abrupt discontinuities in the direction of the
fibers, usually caused by fiber cabling or by wrapping
fibers on drums. Microbending losses usually
result from a coupling or guided modes among themselves
among the radiation modes when light rays
enter the cladding at the microbends or get reflected
at larger than critical angles and hence also enter
the cladding. Use can be made of microbend loss by
creating microbends in number and amplitude in accordance
with a baseband signal and thus modulating
leakage. Contrast with ordinary bend.
microbend sensor. A transducer capable of converting
mechanical mvement, such as displacement actuated
by applied forces or pressures, into a modulated
lightwave in an optical fiber. The microbender
introduces microbends in the fiber thus modulating
the lightwave intensity by causing leakage in accordance
with the information-bearing baseband signal.
micrometer. See micron.
micron (lhn or P). A unit of length in the metric system
equal to one-millionth of a meter, i.e., 10-6
meter. Synonymous with micrometer.
mismatch 10ss. See refractive-index-profile-mismatch
loss.
mixing box.
See optical mixing box.
mixing rod. See optical mixing rod.
modal dispersion. 1. The difference in propagation
time for each of the modes propagating in an optical
fiber, resulting in a broadening of a light pulse.
2. In the propagation of an electromagnetic wave or
pulse in a waveguide, the changes introduced in the
relative magnitudes of the frequency components of
the wave or pulse. The guide is capable of supporting
or introducing only a fixed number of frequencies
depending on its geometry and material parameters,
such aa permeability, permittivity, and conductivity.
where a is the fiber radius, nl and n2 are the refractive
indexes of core and cladding, and 1 is the
wavelength. There are additional degenerate modes
that can be supported, such as polarization and
evanescent modes. 2. In communication systems, a
form or medium for transmission of voice, image,
digital data, or other signala.
mode stripper.
See cladding mode stripper.
mode volume. For a large number of modes, N = fn2/2,
where fn is the V-parameter (normalized frequency or
V-value).
modulation. 1. The variation of a characteristic or
parameter of a wave in accordance with a characteristic
or parameter of another wave. For example, a
variation of the amplitude, frequency, or phase of a
carrier wave in accordance with the wave form that
represents information by means of superposition,
mixing, or transduction. The carrier may be a continuous
direct-current (DC) signal or a continuous
alternating signal such as a sinusoidal wave. The
carrier is used as a means of propagation. The
superimposed or mixed aignal is used as the intelligence-bearing
signal. The variation of the modulated
carrier is detected at the receiver. The information
or intelligence frequencies are normally called
the baseband. 2. In fiberoptic, the variation
of a characteristic or parameter of a lightwave in
order to superimpose an information-bearing signal
on a carrier wave. For example, a variat~on of the
amplitude, frequency, or phase of a lightwave by an
analog or digital baseband signal in a fiberoptic
sensor, transmitted in an optical fiber, and recovered
by a photodetector. The carrier may be a continuous
lightwave when it is not modulated by the sensor.
Contrast with demodulation. See amplitude
modulation; phase modulation; polarization modulation;
frequency modulation.
modulator. A device that accomplishes modulation, that
is, has the capability of varying one signal in accordance
with the variations of another aignal. Thus,
it converts baseband signal into a modulated carrier.
moving grating sensor. A sensor consisting of a fixed
and moveable grating of transparent and opaque areas
such that the intensity of light passing through is
modulated according to the amount that the transparent
areas of both gratings coincide (overlap) aa
the movable grating is moved according to the applied
baseband aignal.
multimode fiber. An optical fiber waveguide that will
allow (support) more than one mode to propagate.
Multimode optical fibers have a much larger core (25
to 75 microns) than aingle-mode fibers (2 to 12
microns diameter) and thus permit nonaxial rays or
modes to propagate through the core.
multiplexing. See frequency-division multiplexing; polarization
multiplexing; space-division multiplexing;
time-division multiplexing (TDM); wavelengthdivision
multiplexing (WDM).
mode. 1. A specific condition or arrangement of electromagnetic
waves in a transmission medium, particularly
in a waveguide. The total number of dimensional
modes that a step-index optical fiber can support,
couple to, or radiate into is given by:
1= ~2a2 (n12-n22)/A2
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