Note: Descriptions are shown in the official language in which they were submitted.
CA 02285708 1999-10-08
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for optical
demultiplexing multiple Bragg grating sensors in a serial array in optical
fibers.
BACKGROUND OF THE INVENTION
Fiber optic Bragg gratings may be used as sensors to monitor
perturbations in their environment. A Bragg grating is formed in a single mode
optical fiber by creating a periodic refractive index perturbation in the
fiber core
as described by Kawaski, Hill, Johnson and Fuhjii in Optics Letters, Vol. 3,
pp.
66-68, 1978. The diffraction grating in the fiber core will reflect optical
frequencies within a narrow bandwidth around the Bragg wavelength of the
optical grating. The Bragg wavelength of the diffraction grating can be
altered by
changing the grating pitch. If an external influence alters the grating pitch
then
the reflection spectrum of the grating can be monitored to determine the
magnitude of the external influence. If the grating is subject to varying
strain or
temperature, the pitch of the grating is altered as described by Morey, Meltz
and
Glenn in the Proceeding of the IEEE, vol. 1169, pp. 98-107, 1989. By coupling
the grating to an appropriate transducer, the grating can be used to monitor a
wide variety of parameters including but not limited to strain, temperature,
vibration, pressure, and acceleration.
Fiber optic Bragg grating sensors offer many advantages over traditional
electrical sensors for monitoring the various parameters. They provide
inherent
immunity to electromagnetic interference and provide a reliable signal with
very
little noise. They can also withstand large variations in temperature and
pressure
and are compact in size allowing them to be used in locations where
conventional sensors are impractical. Bragg grating fiber sensors have the
additional advantage that the signal is encoded directly into an absolute
wavelength shift of the optical signal, so the signal is insensitive to
optical power
fluctuations and other signal perturbations.
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Unfortunately, the design of Bragg grating sensor systems is often more
costly than the conventional electrical sensor alternatives and this has
prevented
their widespread adoption in many applications. To increase the utility of
Bragg
grating sensors, it would be advantageous to be able to multiplex many grating
sensors in the same optical fiber in order share expensive resources such as
the
optical source and the sensor measurement unit among the many sensors
thereby dramatically reducing the cost per sensor. The placement of many
sensors in the same fiber often simplifies the installation of the sensors in
structures or systems by reducing bulk and complexity. It is also desirable
that
the functionality and performance of the system not be degraded by the
multiplexing technique.
These potential advantages have motivated significant efforts into
developing methods of multiplexing Bragg grating sensors. It would be very
beneficial to be able to multiplex a hundred sensors or more in a single
optical
fiber using only one light source and spectral measurement system. Current
systems have fallen short of this goal with about ten sensors per fiber in
demonstrated systems that do not severely restrict the sensor's application.
As
the number of sensors grows there is an increased demand on the optical
source power and the complexity of the multiplexing and/or demultiplexing. For
a
very large number of sensors the cross talk between the sensors can become a
significant problem.
Many different multiplexing techniques have been developed for Bragg
grating sensors. The most successful techniques for use with a large number of
sensors have been wavelength division and time division multiplexing. Examples
of these systems are described in the paper by Kersey et al. in the Journal of
Lightwave Technology vol. 15, pp.1442-1462, 1997.
In wavelength division multiplexing, the Bragg wavelength of each sensor
is set at a separate and unique wavelength. The separations of the Bragg
wavelengths are made to be far enough apart so that any reasonable external
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influence to the grating sensors will not be sufficient to cause the Bragg
wavelengths of any two sensors to overlap. Thus each sensor is given a unique
wavelength band or slot for its Bragg wavelength. In many situations, the size
of
each wavelength slot may need to be very large. This requirement can result
from the necessity to be able to detect a large range of the parameter being
sensed or due to the fact there may be uncertainty in the nominal Bragg
wavelength of the sensors. Uncertainty may arise from variations in the
fabrication process of the gratings, by static strains or uncertain operating
temperatures when the sensor is used. The variability can necessitate a
wavelength slot for each sensor in excess of 15 nm for Bragg wavelengths near
1550 nm. When the number of multiplexed sensors is large, the bandwidth
requirement on the optical source can become intractable thus limiting
wavelength division multiplexing to well controlled sensors that are subject
to
small external influences.
To overcome the aforementioned problems associated with limited optical
bandwidth, the Bragg wavelengths of the sensors may be fabricated with nearly
identical Bragg wavelengths and multiplexed with time division multiplexing.
In
this method a short optical pulse is sent along the fiber containing the Bragg
sensors. The pulse will partially reflect off of each sensor and return the
sensor
information from each grating. The signals from each sensor can be
distinguished by their time of arrival. Previous demonstrations of time
division
multiplexing have determined the time of arrival of the signal by converting
the
optical pulses into an electrical signal and then gating the electrical signal
with a
known time delay. Only the pulse that is passing through the electronic
detector
at the time of the gate is measured. By varying the time delay of the gate,
the
signals from each of the sensors can be read out.
A previous method used in the art to identify the sensor signals is to
electrically gate the sensor signals as disclosed in U.S. Patent No.
5,680,489.
Since the sensors are now identified by time discrimination instead of
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wavelength, bandwidth requirements of the source will not limit the number of
sensors. However, different problems can be encountered in time division
multiplexing that can limit the performance of the system. Time division
multiplexed systems generally experience more noise than wavelength division
multiplexed systems. A significant contribution of the noise is from multiple
reflection between the different grating sensors that cause a pulse to arrive
back
from the sensor array at a time later than expected. Noise is also be
contributed
by the optical source which may not be pulsed in an ideal manner so that there
is a finite level of optical power between successive pulses.
Bragg grating sensor systems often require a very high dynamic range of
eighty to a hundred and twenty decibels. Therefore any small sources of noise
can be significant. To optimize the performance of the system it is necessary
to
perform the signal gating in as short a time period as possible. This allows
the
system to reject a large portion of the noise that does not return at the same
time
as a sensor pulse. With the method of gating used previously in the art, the
performance of the system is limited. An electronic circuit performs the
gating
action after an optical detector has detected the optical signal. Therefore
the
electronic circuit must be operated at the speed of arrival of the optical
pulses. It
is difficult to operate electronic circuits at very high speed and still
maintain very
high signal fidelity due to noise and distortion. Since the gating is done
after the
optical signal is detected, the wavelength measurement on the signals must be
done before the gating. Therefore any noise or distortions in the gating
process
will create errors in the sensor signal. Furthermore, the limited operation of
this
gating method will reduce the spatial resolution of the sensor system since
the
pulses from the sensor array must be spaced far apart in time.
It would therefore be very advantageous to provide a method and
apparatus for time division optical multiplexing multiple serial Bragg
gratings in
optical fibers which reduces noise associated with the gating process and
allows
for very fast gating times.
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SUMMARY OF INVENTION
It is an object of the present invention to provide a method and apparatus
to facilitate multiplexing many Bragg grating sensors along an optical fiber
that
can all share the same optical source and sensor processing unit.
The present invention provides a pulse read-out system to implement time
division multiplexing of a fiber optic Bragg grating sensor array. The pulse
read-
out system allows for a reduction in system noise and an increase in sensor
resolution and flexibility. The essential idea of the invention is that the
optical
signal from the grating sensors is gated by an electronically controlled
optical
modulator before any wavelength measurement is performed to determine the
sensor information. This offers significant advantages since the sensor
information is encoded into the wavelength of the optical signal and not its
intensity. Therefore the sensor signal information is not distorted by the
gating.
Since the gating is performed on the optical signal, the speed of the
electronic
processing needs only to be performed at the speed of variation of the sensor
information and the choice of methods of wavelength measurement is not
influenced by the gating action.
The gating or switching action of the optical modulator will modify the
optical power transmitted to the sensor information-processing portion of the
system, but will not modify the spectral content of the optical signal.
Therefore
distortion and noise in the gating signal will not alter the sensor reading
thus
providing a more robust read-out system. This allows the system to operate at
very short gating times and provides a measure of immunity from unwanted
signals returning from the sensor array and provides superior sensor spatial
resolution.
The present invention provides a means for evaluating the sensor
configuration of the network to high degree of precision if it is not known
beforehand. A means is also provided to implement synchronous detection of
the sensor signal in combination with the gating action of the optical signal.
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An additional advantage of the present method is its flexibility with sensor
signal decoding techniques. Depending on the application of the sensors,
different demands may be required of the system. For example, one may want to
measure rapidly varying signals or quasi-static signals. One may require a
large
dynamic range or a large sensing range. Many different techniques of decoding
the sensor information of Bragg gratings have been developed but all of them
must measure the wavelength of the returned signal. Therefore the present
sensor read-out technique can be easily integrated with a wide variety of
sensor
measurement methods since the optical gating does not alter the wavelength
information of the optical signal. This is in contrast to previous techniques
where
the electronic gating is performed after the wavelength detection making it
more
difficult to integrate the demultiplexing with the sensor decoding technique.
The present invention provides an optical fiber serial Bragg grating
sensor device, comprising:
a) a light source adapted to produce optical pulses;
b) an optical fiber network including an optical fiber optically coupled to
said light source, the optical fiber including a Bragg sensor array having at
least
two spaced apart Bragg gratings; and
c) an optical transmission element connected to a section of said optical
fiber network adapted to receive optical pulses reflected from said at least
two
Bragg gratings, a wavelength detection means optically coupled to said optical
transmission element, switch means connected to said optical transmission
element for switching said optical transmission element between an attenuating
state in which said optical transmission element attenuates light and a
transmission state in which light is transmitted through said optical
transmission
element to said wavelength detection means, said switch means being activated
at selectively adjustable times after production of said optical pulses.
The present invention also provides a device for time domain
demultiplexing serial optical fiber Bragg grating sensor networks, the network
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including a light source adapted to produce optical pulses connected to an
optical fiber network with the optical fiber network including a sensor array
having at plurality of spaced Bragg gratings. The device comprises
an optical transmission element connected to a section of said optical
fiber network adapted to receive optical pulses reflected from said at least
two
Bragg gratings, switch means connected to said optical transmission element
for
switching said optical transmission element between a transmission state in
which said optical transmission element transmits light therethrough and an
attenuating state in which said optical transmission element attenuates light,
said switch means being activated at selectively adjustable times after
production of said optical pulses; and
wavelength detection means connected to said optical transmission
element.
The present invention also provides a method for time domain
demultiplexing a serial fiber Bragg grating sensor network, the sensor network
including an optical fiber having at least two spaced Bragg gratings and a
light
source for producing light pulses that propagate along the sensor network and
are incident on said at least two Bragg gratings. The method comprises:
directing optical pulses reflected by said at least two Bragg gratings to an
optical transmission element;
spectrally analyzing optical pulses reflected from a selected Bragg grating
by switching said optical transmission element to a state of transmission at
effective periods of time after preselected optical pulses are produced, said
periods of time being equal to a transit time of said optical pulses from a
light
source to said selected Bragg grating and from said selected Bragg grating to
said optical transmission element; and
maintaining said optical transmission element in the state of transmission
for an effective period of time to permit the reflected light pulses from said
selected Bragg grating to be transmitted through said optical transmission
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element to a wavelength detection means and thereafter switching said optical
transmission element to a state of attenuation to block optical pulses
reflected
from all other Bragg gratings.
BRIEF DESCRIPTION OF THE DRAWINGS
The method and apparatus for time division optical demultiplexing serial
Bragg gratings in optical fibers will now be described, by way of example
only,
reference being had to the accompanying drawings, in which:
Figure 1 is a block diagram of a system for time division optical
demultiplexing of multiple Bragg gratings in an optical fiber;
Figure 2 is a block diagram of a pulsed read-out system forming part of a
time-division multiplexed fiber optic Bragg grating sensor array;
Figure 3 is a more detailed block diagram of the pulsed read-out unit of
Figure 2;
Figure 4A shows the optical spectrum of a sensor array having two
multiplexed Bragg gratings without use of time division demultiplexing;
Figure 4B shows the optical spectrum of the sensor array of Figure 4A
using a pulsed read-out system using a delay of a gating pulse so that only
the
optical spectrum from the first Bragg grating sensor in the sensor array is
detected;
Figure 4C is similar to Figure 4B but using a differently delayed gating
pulse so that only the optical spectrum from the second Bragg grating sensor
in
the sensor array is detected;
Figure 5 illustrates a method of determining the configuration of the
sensors; and
Figure 6 is a second embodiment of the invention to implement
synchronous detection.
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DETAILED DESCRIPTION OF THE INVENTION
Referring first to Figure 1, an apparatus used for time division optical
demultiplexing multiple Bragg gratings in optical fibers is shown generally at
10.
A light source 12 launches optical pulses 13 into a optical fiber 14
containing a
fiber splitter 16 and a serial array of Bragg grating sensors 18 located on
the
other side of splitter 16 from source 12. The optical fiber used is preferably
a
single mode silica optical fiber however any other optical fiber or waveguide
in
which a Bragg grating can be written may be used. Each sensor in sensor array
18 will return an optical pulse with wavelength encoded information, producing
a
train of pulses that are directed towards an optical demultiplexing system 20.
The sensors in array 18 are coupled to one or more external parameters that
they are to monitor so that changes in these parameters will modify the Bragg
wavelength of the sensors. The coupling may be achieved by embedding or
bonding the fiber sensors 18 to the structure or apparatus to be monitored so
that changes in temperature or strain are also experienced by the sensors. The
sensors may also be coupled to an appropriate transducer known in the art to
convert other parameters into a shift in the sensor's Bragg wavelength. The
optical fiber near the sensors has the protective buffer removed to permit the
sensors to be directly coupled to the appropriate structure, apparatus or
transducer.
The optical demultiplexing system 20 is essentially an optical
transmission device that can be rapidly switched between a transmission state
in
which light is transmitted through it and an attenuation state in which light
is
attenuated. The optical transmission device includes an optical modulator 22,
a
preferred optical modulator is a commercial lithium niobate opto-electronic
modulator that is gated (switched) using a switching mechanism comprising an
electrical signal from a short pulse generator 24 so that light is only
allowed to
pass through the modulator 22 to a wavelength detection system 40 when the
gating voltage signal is applied. The switch also includes a variable
electrical
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delay generator 24 connected to the short pulse generator 24. By varying the
time delay of the gating signal using the variable electrical delay generator
24,
the individual reflected optical pulses transmitted through the modulator to
the
wavelength detection system are selected. The optical demultiplexing system 20
may include a polarization control 28. The polarization control is useful for
adjusting the polarization of the sensor signals to a preferred polarization
state if
the optical modulator 22 is sensitive to the polarization of the optical
signal. The
polarization control may be performed by inducing birefringence into the
optical
fiber after the fiber splitter 16 or by other methods known in the art.
Referring to Figure 2, Bragg grating sensor array 18 includes several
Bragg gratings 30A, 30B ... 30N are written at separate locations in the
single
mode optical fiber 14. Optical pulse 13 from light source 12 (containing
sufficient
optical bandwidth to cover the expected range of Bragg wavelengths of any
given Bragg grating sensor in array 18) is launched into the serial sensor
array
18 through the optical coupler 16. The Bragg grating sensors 30 are each
fabricated to be reflective within the bandwidth of the optical source and
wavelength measurement capability of the system for any reasonable
perturbations of the sensors.
The reflectivity of each Bragg grating sensor in array 18 at each of their
respective Bragg wavelengths is designed to be a few percent or less so that
only a small portion of the pulse 13 launched into the array is back-reflected
at
each sensor. The rest of the optical pulse is allowed to propagate to sensors
further down the array 18 and be likewise reflected. The arrows 32 indicate
the
possible paths of the optical signal. Thus, from the single optical pulse 13
launched into the sensor array 18, a train of pulses 36 are returned from the
sensor array through the fiber path 38 after passing through coupler 16. Each
returned pulse has a spectral content corresponding to the spectral
reflectivity of
the Bragg grating sensor that it originated from. In general the duration of
the
pulses must be shorter than the duration of the optical gate and the
repetition
CA 02285708 1999-10-08
rate must be lower than the time for the pulse to traverse the fiber and
return to
the pulse read-out system.
The minimum physical spacing of the Bragg sensors in the array 18 is
given by the temporal duration of the optical gate. The time for the optical
pulse
to travel twice the distance between the two nearest sensors must be longer
than the gating time. The maximum number of sensors is limited to the ratio of
the total physical length of the sensor array, from the first sensor to the
last, to
the minimum physical spacing between sensors. The maximum number can also
be expressed as the ratio of twice the time for an optical pulse to travel
from the
first sensor to last, to the temporal duration of the optical gate.
In a preferred embodiment the pulses from the source are made to be
shorter in duration than the time for a pulse to travel twice the distance
between
the two spatially closest sensors on the sensor array. In this preferred
embodiment a mode-locked fiber laser producing sub-picosecond pulses with a
bandwidth > 10 nm may be used. However those skilled in the art will
understand that other light sources may be used as long as they meet the
requirements described above. Each of the individual pulses making up pulse
train 36 from the sensor array 18 will return from the sensor array at unique
times. The pulses containing the sensor information in the optical fiber
branch
38 are directed towards the pulse read-out system 20. The optical source 10
launches a series of pulses at a fixed repetition rate into the sensor array
to
repeat the process described above. The period between pulses is greater than
the time for a pulse to travel twice the distance from the first sensor to the
last
sensor in the array.
The sensor information contained within each pulse of pulse train 36 may
be identified as coming from the appropriate Bragg grating sensor by the time
of
arrival of the pulse at the pulse read-out unit 20. The pulse read-out unit 20
allows the optical signal to propagate to the wavelength detection unit 40 for
a
short period of time and acts as an optical gate on the returned optical
signal.
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The duration of the optical gate is chosen to be longer than the temporal
duration of the pulse response from any one Bragg grating and shorter than the
time between two pulses arriving from spatially adjacent Bragg grating sensors
of array 18.
The timing of the optical modulator is determined by a timing signal
derived from the pulses from the optical source 12. The timing signal may be
generated by the optical detector 44 and passed to the pulse read out unit 20
through path 46. The signal may also be generated directly at the optical
source
12. For example, if the optical source 12 is pulsed directly using an
electrical
control signal, then this signal may be used for timing by the pulse read-out
unit
20.
The timing signal is delayed in the pulse read-out unit 20 and used to
trigger the optical gate. The delay is chosen so that only one pulse is
allowed to
pass through the optical gate for each pulse of pulse train 36 returning from
the
sensor array 18. Thus, only the signal from one Bragg grating sensor will
reach
the wavelength detection unit 40, and the wavelength detection can be
performed as if only one Bragg grating sensor was being monitored. The
wavelength detection unit 40 may be of any standard design that is suitable
for
measuring the sensor signal and interrogation of the optical pulse may be
performed using techniques known in the art.
The operation of the pulse read-out unit 20 is more closely detailed in
Figure 3. The pulse read-out unit 20 includes electronic delay generator 26
connected to short electrical pulse generator 24 which is connected to electro-
optical modulator 22 that modifies the transmission of light in accordance
with
the electrical signal applied to it.
The train of pulses 36 along path 38 of the fiber is shown at the input to
the optical modulator 22 in Figure 3. Each individual pulse has a central
wavelength, denoted by AB,,'XB2 ... XBI corresponding to the Bragg grating
wavelength of the sensor from which the pulse originates. By choosing a
suitable
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CA 02285708 1999-10-08
delay of the trigger pulse with the electrical delay generator 26, the short
pulse
generator can be triggered to produce an electrical pulse to the electro-
optical
modulator 22 when one of the pulses, for example the pulse containing XB2, is
passing through the modulator. The gating of the optical pulses is
demonstrated
graphically by 23. The top set of pulses in 23 shows the progression in time
of
the set of pulses 36. The gating action of the modulator is shown below these
pulses. The gating is synchronized with the pulses containing AB2. Below the
gating pulses, the selected optical pulses are shown containing only AB2. The
short pulse generator 24 produces a very short electrical pulse that is wider
than
the temporal width of the pulse to be gated. It is found that if the pulse
from the
optical source 12 is several picoseconds or less in temporal duration, then
the
reflected pulses typically have a temporal width of fifty to a hundred
picoseconds. The temporal gate width of the optical modulator 22 should be
slightly larger than the width of the pulse, however the lower limit may be
restricted by the dynamic response of the modulator or the speed of the
electrical pulse generator 24 that produces the gating signal 50. Typical
gating
times may be from five hundred to a thousand picoseconds. The optical
modulator 22 can be implemented, among other methods known in the art, by a
Mach-Zehnder integrated optic modulator that is controlled through the electro-
optic effect or by a semiconductor electro-absorption modulator.
The process described above is repeated at the repetition rate of the
optical source 12 so that only the pulse from one Bragg grating sensor is
allowed to pass through the modulator 22 for each pulse launched into the
system. This is shown in Figure 3 by the single pulse 54 that exits from the
modulator 22 for the train of pulses incident on the modulator 22. A train of
pulses will then arrive at the wavelength detection unit 40 at the repetition
rate of
the optical source 12. This repetition rate is made to be greater than the
electrical bandwidth of the wavelength detection unit 40. The lower bandwidth
of
the detection electronics will make the train of pulses appear as a continuous
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signal that varies at the rate of perturbations to the Bragg grating sensors.
The
average level of detected signal is given by the average optical power from
the
pulse read-out unit. In this way, the wavelength detection unit effectively is
decoding a sensor signal as if there was only one sensor in the system. Thus,
any one of the numerous methods known in the art for signal decoding a single
Bragg grating sensor may be used.
Different sensors may be monitored by altering the pulse 54 that is
selected by the pulse read-out unit from the train of pulses 36 corresponding
to
each Bragg grating sensor in array 18. This selection is achieved by altering
the
delay in the electrical delay generator 26 so the gating pulse 50 is applied
to the
optical modulator 22 when the desired pulse passes through the modulator.
The gating pulse 50 is made to be slightly longer than the optical pulses
returning from the sensors. The time between pulses from the optical source
will
typically be much longer then the gating time. For example if the length of
the
sensor array 18 is made to be a hundred meters and the gating time was 1
nanosecond, then the optical gate would be open 0.1 % of the time. This
enables
the sensor system to reject a large portion of unwanted signals from sensor
array 18. Such unwanted signals include multiple reflections between grating
sensors, reflections from fiber splices and other components and noise from
the
optical source that may be caused by a small continuous light output in
addition
to the pulsed output. In this way, the pulse read-out system 20 helps to
reject
erroneous signals from the sensor array 18.
It is to be noted that electrical noise in the gating pulse 50 does not affect
the sensor reading. Variations in the gating pulse amplitude will cause
variations
in the optical signal at the output of the pulse read-out unit 20, but will
not affect
the spectral content of the optical signal. Therefore the sensor information
can
still be recovered despite imperfections in the high speed gating pulse.
Figure 4 shows the result of the operation of the pulsed read-out unit with
a time multiplexed sensor array using two Bragg grating sensors. These figures
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show the optical spectrum from the sensor array as obtained on a standard
optical spectrum analyzer. The optical spectrum from the sensor array without
the pulse read-out system is shown in Figure 4A. In Figure 4A there are
clearly
two peaks corresponding to the reflection from the two sensors and some
background optical signal. With the use of the pulse read-out system only the
optical spectrum from the first Bragg grating sensor in the sensor array is
seen
at the spectrum analyzer as shown in Figure 4B. In Figure 4C the delay of the
gating pulse is set so that the spectrum analyzer only measures the spectrum
from the second Bragg grating sensor. The pulse read-out unit allows one to
identify and isolate the sensor information from each of the Bragg grating
sensors.
Figure 5 illustrates a method of using the pulse read-out unit to identify
each of the sensor gratings to determine their location in the sensor array
and to
choose the correct delay to read-out each sensor. An arbitrary starting delay
is
chosen for the delay generator 26 of Figure 3. The value of the delay, denoted
by the i axis of Figure 5 is swept from the starting point given by i equal to
zero
to the time for one repetition of the optical source. The optical power at the
output of the optical modulator 22 in Figure 3 versus the delay ti reveals the
pulse response of the sensor array. By calibrating the distance along the
sensing fiber that the optical signal will travel for a given delay ti, the
physical
location of each sensor may be determined. Therefore the gratings may be
placed in the sensor without detailed knowledge of their positions. By
determining the positions of each sensor, and by calculating their Bragg
wavelengths, the effects of cross talk due to multiple reflections may also be
reduced since the occurrences of multiple reflections can be predicted if the
configuration and state of the sensor array is known.
An alternative embodiment of the invention is shown at 80 in Figure 6.
The operation of the pulse read-out system 80 in Figure 6 is similar to the
system 20 of Figure 3 except a low frequency modulating signal 82 is
multiplied
CA 02285708 1999-10-08
with the timing signal to the modulator at junction 84. This junction 84 may
be
placed before the delay generator 26 as shown or between the delay generator
26 and the pulse generator 24 (not shown). The modulating signal 82
alternately
turns the timing signal on and off at a rate of a few kilohertz. This allows
the
output from the pulse read-out unit 80 to be modulated at the same rate. The
modulation signal 82 is also passed to the wavelength detection unit for
reference. The modulation allows for synchronous detection to be used in
measuring the sensor signal. Synchronous detection permits the system to
obtain higher sensitivity by rejecting noise such as the dark current from
optical
detectors and noise in electrical amplifiers.
The foregoing description of the preferred embodiments of the invention
has been presented to illustrate the principles of the invention and not to
limit
the invention to the particular embodiment illustrated. It is intended that
the
scope of the invention be defined by all of the embodiments encompassed within
the following claims and their equivalents.
16
