FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

CONTINUOUS
LIGHT
MODE
STRIPPER
. .
: : . .
MICROBEND SENSOR ARRAY
MODE STRIPPER
STRIPPER
,
energized and either the return optical cables are each
returned to a photodetector in an array, or the return
bus may be a single optical cable with time-division
multiplexed signals from equally-spaced sensors fed to
a single photodetector as discussed earlier.
The sensor array can also consist of an array
of optical grating sensors that modulate the output of
individually-coupled light sources (LEDs) powered by
light pulses on a common electrical bus as shown in
Fig. 6.5. The grating outputs are individually coupl-
Fig. 6.2
A fiberoptic darkfield microbend sensor array
telemetry system with multiple cable
return.
ELECTRICAL POWER BUS
>
I
Brightfield sensing can also be accomplished
in a fiberoptic sensor array as shown in Fig. 6.3. A
STAR COUPLER
I
.
CONTINUOUS \ .)
:
fl “ ‘
PHOTODETECTOR
ARRAY
Fig. 6.5
))
A fiberoptic optical grating electricalbua-fed
senaor array telemetry system.
Fig. 6.3
DETECTOR
ARRAY
)
: MICROBEND BRIGHTFIELD
SENSOR ARRAY
A fiberoptic microbend brightfield starcoupler-fed
sensor array telemetry system.
star coupler is fed by a continuous light source, e.g.,
a laser or an LED. Each output of the star-coupler is
fed to a baseband-modulated microbend fiberoptic brightfield
sensor. The output signal of each fiberoptic
sensor in the array is separately fed to an array of
photodetectors for further processing and transmission.
~so, an electrical power bus to a light source (LED)
at each sensor can be used to energize the basebandmodulated
microbend fiberoptic brightfield sensor as
shown in Fig. 6.4. The electrical bus is continuously
LIGHT SOURCE
ELECTRICAL POWER BUS
)
‘“Ḟ .
I
(LED) (LED) (LED)
. ● MICROBEND SENSOR
.
. ARRAY
.
.
.
.
/
Fig.
DETECTOR ARRAY
6.4 A fiberoptic microbend brightfield electrical-bus-fed
sensor array telemetry system.
6-2
ed to photodetectors in an array via fiberoptic cables.
Some of the performance features of these and other
fiberoptic sensor array configurations will now be
discussed in some detail.
An array of sensors may be used to beam-form
or to signal average. In the former case it is necesaary
to distinguish, i.e. maintain separation of, the
output signals from the individual sensors. In the
latter case the signals from a number of sensors are
summed (OR-gated). For example, beam forming is used
in echo ranging, while averaging can be used to discriminate
between signals that arrive normal (perpendicular,
transverse) to a linear array and those that
arrive parallel (longitudinal) to the array. Thus,
very often the spatial distribution of an array of
fiberoptic sensors can be used to accomplish the multiplexing,
mixing, or summing, of signals from the array.
Although the principle of operation of only a linear
array of sensors will be discussed, the same principles
can be applied to multidimensional arrays.
A linear array of fiberoptic sensors may be
energized by means of a common bus. The bus may be an
electrical conductor, fed by a direct-current power
source or an alternating-current power source, or the
bus may be an optical fiber fed by a relatively highpowered
optical continuous-output source, such as a
laser. Alternatively, in each of these situations, the
power source output can be pulsed rather than be continuous,
giving rise to four posaible arrangements,
namely continuous electrical, continuous optical,
pulsed electrical, and pulsed optical power. In any
caae, the fiberoptic sensors (transducer, modulators)
in the linear array may be either directly connected
to the optical bus by means of a fiberoptic coupler,
or a light source at each sensor may be connected to
the electrical bus. The selection of the appropriate