FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

method of sensor energization will depend on the power
budget, risetime budget, distances to and within the
array, and other matters related to the specific application.
For the pulsed-bus method of energization of
an equally-spaced linear array of sensors, spatial distribution
of the sensors will cause the baaeband-modulated
output signal pulse from each sensor of the array
to occur at a different time according to the time it
takes for a pulse to propagate from one sensor to an -
other. If these signals are all fed into a single
common return bus, they will be automatically timedivision
multiplexed on that bus. This arrangement is
shown in Fig. 6.6. Assume a pulse of light is dis-
‘Warray = l/t = c(m - 1)/2nL (6.2) length of the array, c is the velocity of light in a
For 50 sensors in a linear array, an optical
fiber bus core refractive index of 1.5, a linear array
500 meters long, Eq. (6.1) indicates that the time between
the leading edges of array output pulses is:
to=(2)( l.5)(500)/3(108) (49)=102 ns (6.3)
The corresponding pulse repetition rate, PRR, is:
PRR = l/t. = 1/102(10-9) = 9.8 kfPPS. (6.4)
Thus, the input feed bus pulses cannot be
wider than to. This is the rate at which the basebandsignal
modulated pulses will emerge from the input-output
end of the array. Also, some time should be allowed
between pulses for random variations of sensor spacing,
delays through sensor leads from and to the busses, and
pulse risetimes. Therefore, the pulse length should
not exceed 0.9to or about 90 ns for the above example.
If they are wider, they are liable to overlap in the
return bus. They can be narrower. The pulses will ar -
rive as a train of pulses at the photodetector. The
pulses in the train will come from and be in the same
sequence as the sensors are positioned in the linear
array. These events occur each time the feed bus is
pulsed and as many pulses will be in each array output
pulse train as there are sensors in the array. The
photodetector must be capable of responding to about
10 Mpps for the above example. If any analog-to-digital
(A-D) conversion is to take place in a single A-D
PHOTO– PULSED
OETECTOR SOURCE
converter fed by the return bus, the length of the
pulses must be long enough to allow for analog to digital
conversion. The repetition rate of the pulses in
I r PULSED SOURCE OPTICAL FEEO BUSwlTH M. l
LENGTHL~
the train can be reduced by placing additional fiber
)J LINEAR ARRAY between the sensors or by using a fiber with a higher
refractive index, as indicated by Eq. (6.2).
SPACEDINTENSITY-
. . .
MODULATION
{r
)J
Various methods can be used to telemeter the
sensor-modulated signals from the sensor array to a
EQUALLY-SPACEO PULSES IN AN OPTICAL RETURN SUS
photodetector. For example, the detector could be an
A fiberoptic darkfield optical-bus-fed sensor
array telemetry system with single op-
is required, or the output of the photodetector could
optical repeater if long distance optical transmission
tical return bus.
be transmitted electrically, via a wire line, coaxial
cable, or radio-link. The radio frequency carrier
could be modulated by the detected analog pulses or by
A coupler at each sensor an analog-to-digital converter output. The above discussion
applies each time a single light pulse is sent
along the fiber bus. The maximum safe pulse duration
was shown to be, from Eq. (6.1) and allowing a 10%
The modulated pulse travels via the return bus safety margin:
‘msx = 1.8nL/c(m - 1) (6.5)
The maximum rate at which the pulses can be
dispatched down the optical fiber feed bus is limited
The distance between sensors is by the overall length of the array. Each pulse must
travel twice the full length of the array and clear the
The wave has to travel first sensor before the next pulse can be applied to
the array. This maximum pulse rate is the maximum rate
at which the baseband signal inputs to the fiberoptic
aensors can be sampled. The minimum time between leading
edges of feed bus pulses is twice the time length
of the array, plus the pulse length (maximum possible
The time pulse width is assumed) plus a safety margin for risetime
and settling time. Therefore, these considerations
the time between leading edges of output pulses
will yield a minimum sampling period ts of:
to=2[L/(m-1)] /(c/n)=2nL/c(m-1) (6.1)
t+ = 2nL/c + 1.8nL/c(m - 1) + tr
(6.6)
ts = [2 + 1.8/(m - l)]nL/c + tr
where m is the number of aenaors in the linear array, n
is the refractive index of the fiber busses, L is the
The physical parameter variation (baseband)
to be sensed, such as a sound wave, a magnetic field
variation, a pressure wave, or a force variation, will
modulate the optical input to the fiberoptic sensor,
thus producing an optical baseband-modulated signal output.
This sensor output signal can be telemetered to a
distant location (for detection and processing) in any
of a number of different ways depending on spatial,
timing, compositional, and other factors.
Fig. 6.6
patched along the feed bus.
location taps a fraction of the light from the bus.
The pulse of light enters each sensor in turn where it
is modulated by the baseband signal imposed by the sensor.
bsck to the photodetector for further processing. The
minimum time that can be allowed for the spacing between
the leading edges of pulses in the return bus is
the propagation time between a given sensor location
and the next sensor in the array and return to the given
sensor location.
L/(m - 1), where L is the length of the linear array
and m is the number of sensors.
the distance between aensors in the feed bus and in the
return bus, therefore the travel distance is 2L/(m - 1).
The speed of propagation of a lightwave in the bus is
c/n, where c is the speed of light in a vacuum and n
is the refractive index of the core. It iS assumed the
refractive index is the same for both busses.
of propagation is the distance divided by the speed,
thus,
is:
The pulse repetition rate (PRR) of the pulses emanating
from the linear senaor array is given by:
6-3