FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

vacuum, and t= is the overall risetime to be calculated
later in this chapter. This value of ta can be reduced
to the extent that the pulse width can be reduced.
If there are only a few sensors in the array the
second term in bracketa in Eq. (6.6) is significant.
As the number of sensors is increased, the second term
in Eq. (6.6) becomes less significant. Eq. (6.6) does
not apply to one senaor because then L = O, the second
term in brackets becomes indeterminate, and in fact
there is no array. In this case, only the risetime
requirement will place a limit on the sampling period.
The maximum permissible sampling rate PR~s is:
requirement for any specific spatial relationship among
the sensors. The minimum sampling period for each sensor
tssen in the array will be:
‘asen
= mtad (6.9)
where m is the number of sensors in the array and tad
is the analog-to-digital conversion time. This presumes
a photodetector for each sensor output, a multiplexer
(for polling each photodetector output), and a
aingle A-D converter with its sample-and-hold and other
associated circuitry aa shown in Fig. 6.8.
Pluqs.= l/ts (6.7)
where ts is defined in Eq. (6.6). The sampling rate
can be arbitrarily reduced, but should not be less than
twice the highest significant frequency component in
the baseband signal in order to obtain reproduction of
the baseband signal without significant distortion
(Nyquist criterion).
Another method for energizing an array of sensors
is to connect the array to a pulsed electrical bus
as shown in Fig. 6.7, rather than the pulsed optical
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Fig. 6.8
A fiberoptic star-coupler-fed sensor random
array telemetry system with multiple and
single cable returns.
Fig. 6.7
A fiberoptic lightfield pulsed
bus-fed sensor array telemetry
single optical return bus.
electricalsystem
with
The random array of fiberoptic sensors may be
energized instead by a direct-current electrical bus
as shown in Fig. 6.9. In this case, there is a contin-
bus just described. A light source, such as an LED, is
optically connected to each sensor. Thia is the brightfield
form of senaor energization. The return bus can
be the same, as was shown in Fig. 6.6, and the analysis
above still applies except that the propagation time
in the electrical bus will be different. In this case,
the electrical sampling pulae period will be:
CONTFWOUS (DC) ELECTRICAL FEED BUS
telec=l/2 [ts+[2+l.8/(m-l)]L/v+trc] (6.8)
where the variables are as in Equation (6.6), v is the
velocity of propagation in the electrical bus, and trc
is the combined electrical and optical risetimes that
will be discussed later in this chapter.
Another configuration for energizing a randomly-distributed
(non-linear) array of fiberoptic aensors
is shown in Fig. 6.8. A continuous source of
light (laser) energizes a star coupler that may be
placed at the center of the array to reduce fiber requirements.
The output of the coupler is fed to each
fiberoptic sensor where it is modulated by the baseband
signal. The senaor outputs are individually fed to an
arrayof photodetector (PDs). In the arrangement
shown in Fig. 6.8, the inputs to the photodetector (PD)
array (one PD for each sensor) are time-division multiplexed,
sampled, digitized, and telemetered as a digital
data bit stream via a fiberoptic cable to another
location for exploitation. In this case, there ia no
6-4
Fig. 6.9
A fiberoptic electrical direct-current busfed
sensor array telemetry aystem with
multiple and single cable return.
uous-output light source (LED) for each sensor. The
baseband-modulated optical output of each sensor is
separately cabled to a photodetector array and processed
there in a manner similar to the preceding arrangement
that had the continuous optical feed as was shown
in Fig. 6.8.
J