Note: Descriptions are shown in the official language in which they were submitted.
CA 02408043 2002-10-11
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Optical sensor and readout apparatus
s FIELD OF THE INVENTION
The present invention generally relates to a method and an apparatus for
measuring and monitoring various physical parameters, such as temperature,
strain or pressure, with an optical sensor.
io
BACKGROUND OF THE INVENTION
Optical sensors in general, and fiber optic sensors in particular, have
is proven to be useful in a number of situations where access to the measurand
is
problematic with electric wiring, or where electromagnetic interference
prevents
the use of electric or electronic sensors. Optical sensors use the correlation
between the change of the optical properties of a material and the parameter
to be
measured. The material in question is interrogated by a beam of light in free
2o space, or by light guided by a waveguide such as an optical fiber, and the
transmitted, reflected, re-emitted or scattered light is measured by a remote
photodetector or other type of photosensor.
Many optical sensors use a material with bandpass properties, that is a
2s material where light is either transmitted, reflected or absorbed over a
narrow band
of wavelength. The shift of the wavelength corresponding to maximum or minimum
transmission or reflection is then correlated with the parameter to be
measured. To
measure that wavelength, one can disperse the transmitted or reflected
spectrum
of a broadband source with a spectrometer, and use, for example, a
photodetector
3o array to detect the peak wavelength. Spectrometers, however, are quite
bulky and
expensive, especially if you need a good wavelength resolution. Alternatively,
one
CA 02408043 2002-10-11
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can use a tunable monochromatic source, and scan the spectrum band of
interest.
Tunable sources, however, are typically quite expensive, and often include
fine
mechanical parts that make them not very suitable for use in the field.
s One example of a bandpass-type sensor is a so-called Fiber Bragg Grating
(FBG). This is a narrowband filter imprinted into the core of a single mode
optical
fiber, using the interference pattern created by two intersecting beams of UV
light.
Due to the photosensitive nature of some optical fibers, this results in the
creation
of a permanent periodic refractive index pattern along the fiber, which
reflects light
io over a very narrow range of wavelengths centered on ~,m~= 2 n P, where n is
the
effective index of the mode guided by the fiber, and P is the period of the
grating.
Since the wavelength of maximum reflection is dependent on the period of the
grating as well as the refractive index of the fiber, FBGs can be used to
sense
temperature, strain, pressure, and many other parameters. Their reflective
nature
is imply that the source and detector can easily be collocated. The fact that
they are
imprinted inside the fiber itself lends to very small sensor gauges, since the
fiber
diameter is typically only 125 microns. Furthermore, multiple FBGs can be
imprinted at different locations along the same optical fiber, and
interrogated with
the same instrument. However, the cost of the readout instrumentation has
2o remained so far quite high due to the complexity and cost of the components
used
(tunable lasers or spectrum analyzers).
Methods that involve measuring the peak wavelength in fact discard a large
part of the information available from the sensor, that is the shape of the
bandpass
2s spectrum itself. Given that that shape is very often a constant, its
knowledge
provides a means to track the position of the peak wavelength, just by
measuring
the amount of light that is transmitted or reflected by the sensor. The reason
this is
not done is that the transmissivity through the rest of the optical system
that
carries the light from the light source through the sensor and to the
photosensor, is
3o rarely known with good accuracy, and furthermore can vary with
environmental
conditions or over time.
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Accordingly, it would be desirable to provide a method and an apparatus for
measuring and monitoring various physical parameters (such as temperature,
strain, pressure, etc...) that alleviated some of the above-mentioned
drawbacks of
s the prior art. Furthermore, it would be desirable to provide such a method
and
apparatus requiring very few optical components, all of which being robust and
compact enough in such a way that the apparatus can be made to be portable and
is able to operate in various harsh environments.
to
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention, there
is provided a method and an apparatus for measuring and monitoring various
is physical parameters, like temperature, strain, pressure, etc... The
apparatus
comprises an optical sensor, whose spectral reflection or transmission
properties
vary with the parameter to be measured (the measurand) in a well known and
defined manner, and a readout apparatus that translates this variation into an
electronic signal.
More specifically, the present invention provides such a method and
apparatus that is able to track the difference between the wavelength of a
fixed
optical source and the wavelength of maximum reflection or transmission of a
bandpass-type optical sensor, using a knowledge of the spectral shape of the
2s sensor. It is particularly suitable to the readout of FBG sensors, since
the spectral
shape of an FBG can be tailored almost at will at fabrication time.
The present invention and its advantages will be better understood upon
reading of preferred embodiments thereof with reference to the appended
3o drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
A detailed non-restrictive description of preferred embodiments will be given
herein below with reference to the following drawings, in which like numbers
refer
s to like elements:
Figure 1 is a schematised representation of an optical sensor and readout
apparatus used in transmission according to a preferred embodiment of the
invention.
Figure 2 is a schematised representation of an optical sensor and readout
~o apparatus used in reflection according to a preferred embodiment of the
invention.
Figure 3 is a schematised representation of an arrangement of a plurality of
optical
sensors for using the same optical source to probe the plurality of sensors.
Figure 4 is a schematised representation of an arrangement of a plurality of
optical
sensors for using a plurality of wavelength multiplexed optical sources to
probe
is several sensors along the same optical fiber according to a preferred
embodiment
of the invention.
Figure 5 is a graph of a wavelength (nanometers) versus the function S'/S for
a 0.4
mm long uniform Fiber Bragg Grating.
DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The present invention provide a method and an apparatus for measuring
and monitoring various physical parameters, like temperature, strain,
pressure,
2s etc... As illustrated on Figures 1 and 2, the apparatus comprises an
optical sensor
S, whose spectral reflection or transmission properties vary with the
parameter to
be measured (the measurand) in a well known and defined manner, and a readout
apparatus that translates this variation into an electronic signal.
3o The optical sensor S is a device having spectral reflection or transmission
properties such that reflection or transmission is maximum at a given
wavelength
CA 02408043 2002-10-11
of light, denoted by ~.maX, and decreases monotonically for wavelengths
greater
and smaller than a,",ax, at least over a range of wavelengths 0~,. The shape
of the
transmission or reflection curve over that wavelength range is called the
spectral
signature of the sensor, and can be described by a function of wavelength ~,,
s which we call S(~,), normalized so that S(~.maX) = 1. The transmission or
reflection
of the sensor itself is given by Tm~ *S(~.), or RmaX *S(~.), where TmaX and
RmaX are
the value of transmission or reflection respectively at ~,max. Furthermore,
the optical
sensor is made such that when the parameter to be measured varies, ~,~,~"~
varies
in a well defined manner, but the spectral signature S(~,) remains unchanged.
to Furthermore, the function S(~,) must be such that its slope, or derivative
S'(~,),
when divided by S(~,), varies monotonically over the range 0~, around ~,m~.
For
example, if S(~,) is a gaussian function exp(-(~,/8)Z), then S'(~,)/S(~,) is a
linear
function of ~,. The value of the function S'(~,)/S(~,) is then unambiguously
attributed
to the difference between ~, and ~,meX, when ~.m~"~ varies over a range ~~..
The range
is of wavelengths 0~, corresponds to the range of variation of ~,m~ that can
be
unambiguously measured by the readout apparatus described below, and
therefore corresponds to the measurement range of the sensor. A sensor having
a
Gaussian signature is advantageous since S'(~,)/S(~,) is a linear function of
~, and,
then, there is only a single easily determined value of ~, corresponding to
2o S'(~,)/S(~,). However, the spectral signature S(~,) of the optical sensor
may have
another shape than a Gaussian function as long as it is well known and well
defined and that S(~,) decreases monotonically for wavelengths greater and
smaller than ~,m~.
Referring again to Figures 1 and 2, there is shown a schematised
2s representation of an optical sensor S and a readout apparatus according to
the
present invention. As a preferred embodiment, the optical sensor S is a fiber
Bragg grating (FBG). Advantageously, the FBG can be designed to have a
spectral signature approaching closely a gaussian function. This is achieved
by
giving the FBG a so-called "apodization profile" which is a gaussian function,
and
3o keeping the FBG reflectivity low enough (typically less than 10%). If the
FBG
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reflectivity is too strong, its spectral shape is almost flat around ~,m~,
making its
slope very small, so that the signal S'(~,)IS(7~) is also always very small.
However,
even a uniform FBG of low enough reflectivity will have a usable spectral
signature. The FBG can be designed to have a smaller or larger value of 0~..
If the
s FBG has a uniform period, then o~, is inversely proportional to its length.
On the
other hand, if the period of the FBG is linearly chirped (i.e. Increases or
decreases
in a linear fashion along the length of the FBG), then e~, is proportional to
its
length. The use of an FBG gives a lot of flexibility in the choice of 0~,. For
example,
a l0mm long FBG with a gaussian apodization profile having a full-width at
half-
io maximum (FWHM) of 3mm, and a uniform period, will have a usable e~, of
about
0.5 nm, while the same FBG with a chirped period of 15 nm/cm will have a ~~,
of
7 0 nm. Sensors of the same length can therefore have measurement ranges
varying by a factor of more than 20. Of course, any other optical sensor
presenting a well known spectral signature having a reflection peak or a
is transmission sag such as a long period fiber grating for example could be
conveniently used.
The readout apparatus, which is schematized in Figures 1 and 2 for sensors
used in transmission or reflection respectively, comprises a wavelength-locked
20 optical source (WLOS), itself comprised of an optical source OS emitting
over a
very narrow range of wavelengths, much smaller than 0~,. Preferably OS is a
laser.
Furthermore, the central wavelength of the optical source ~,o is modulated at
a
frequency f, over a range 8~,, which is also much smaller than 0~,. Therefore
the
wavelength is a function of time described by: ~,(t) _ ~.o + (8a,/2)
sin(2~ft).
For example, and as a preferred embodiment, the source can be a DFB
(Distributed FeedBack) laser diode with a current driver (D), whose central
emission wavelength ~,o can be modulated by modulating the driving current, or
the
operating temperature. The emission wavelength of a DFB laser is normally
quite
3o stable in time if the driving current and temperature are kept constant.
However, to
CA 02408043 2002-10-11
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counter possible long-term drifts of the laser wavelength, the readout
apparatus
can also comprise a means for keeping ~,o constant (locking the wavelength).
This
can be achieved by separating the light emitted by the diode with a partially
transmitting optical element (a beam splitter BS1 for example), and sending
one
s part onto a filter (F) with bandpass-like reflection or transmission
properties, and a
wavelength of maximum reflection or transmission ~,f, but with a bandwidth
smaller
than o~., the bandwidth being nevertheless larger than 8~,. The light
reflected or
transmitted by the filter F is measured by a photodetector PD1. The current
generated by the photodetector PD1 is treated by the electronic circuit EC1:
it is
1o amplified and converted to a voltage, and filtered to keep only the part
that is
modulated at the frequency f. This filtered signal is used as an error signal
in an
electronic feedback loop PID to act on the diode driving current and maintain
the
diode at the central wavelength of the filter F. The filter F can be, for
example, an
optical etalon, a thin film bandpass filter, or a gas absorption line. This
is arrangement for "locking" the laser diode wavelength is well known in the
art, and
commercial devices are available to perform that task.
The other part of the light from the optical source OS is directed onto the
optical sensor S either through free space with appropriate redirecting
optics, or
2o through an optical waveguide. If the sensor is used in transmission, the
light
transmitted by the sensor is collected by a second photodiode PD2 as can be
seen on Figure 1. If used in reflection, another beam splitting element BS2 is
used
to redirect the reflected light onto the photodiode PD2 as can be seen on
Figure 2.
If the sensor S is an FBG, then the light from the laser diode can be launched
info
2s an optical fiber into which the FBG is provided, and optical fiber couplers
can be
used as beam splitting elements.
The photocurrent from the photodiode is then treated by the electronic
circuit EC2 in the following way. The current is first amplified and converted
to a
3o voltage V proportional to the photodiode current. One part of this signal
is filtered
with a low pass filter so as to remove all modulation frequencies above a
value fm;n
CA 02408043 2002-10-11
g
< f, which gives a voltage Vd~, proportional to rd~*Po*T*S(~,), where Po is
the power
emitted by the light source, T is the transmission of the optical system from
the
source to the photodiode, and rd~ is the responsivity of the diode,
amplification and
filtering circuit in Volts per Watt of incident power. The other part is
filtered to keep
s only the signal modulated at the frequency f, which gives a voltage Vas. If
8~, is
much smaller than ~~,, then that part is equal to ray*Po*T*S'(~.)*8~,, where
rep is the
responsivity of the photodiode, amplification and ac filtering circuit. Vas is
then
divided by Vd~, either numerically after digitizing both signals with an
analog-to-
digital converter, or by controlling the gain of the amplifier to maintain Vd~
at a
to preset value, with an automatic gain control circuit, in which case ray is
also
proportionally affected, so that Vas can be directly related to S'(~,)/S(~,).
The value
of Vas thus extracted can be correlated to the difference between ~,m~ and
~.o, and
to the value of the measurand through calibration curves.
is For increased accuracy, the signal obtained from the light collected
through
the locking filter F can be treated in the same way by EC1, so that the error
signal
is made to be proportional to the difference between ~,o and ~,f. The error
signal is
always small, as it is kept close to zero by the feedback loop, however it is
a more
precise estimation of ~,o than just using ~,f. Furthermore, if ~,f is the
central
2o wavelength of a gas absorption line, then its value can be known with
extreme
accuracy, and is not prone to variations in time, as opposed to an optical
etalon or
thin film filter, whose central wavelength depends on temperature, angle of
incidence, and refractive index, all quantities which can vary with
environmental
conditions.
The accuracy and resolution of the readout system is then limited by the
noise from the photodiode and electronic amplification and filtering circuit,
by the
precision and accuracy of the calibration curves, and by the calibration
factor
proportional to s~,. One must also ensure that the wavelength modulation
3o amplitude S~, does not vary over time, or with environmental conditions.
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The main advantage of this readout system is that the signal is independent
of both Po and T, and only depends on the value of the measurand. Therefore,
the
readout system is unaffected by variations in the emitted power of the
photodiode,
s or in the transmission of the optical system that carries the light from the
source to
the sensor, and from the sensor to the photodiode. Another advantage is that
the
wavelength of the source does not need to be tunable over the range ~~, of the
sensor. It only needs to be tunable over a small range within 0~,, which makes
a
standard, commonly available, and relatively low cost DFB laser diode suitable
as
io the optical source. Other readout systems for FBG sensors typically use
expensive
tunable lasers, or tunable filters, to scan the spectrum, and locate the
wavelength
of maximum reflection.
Referring now to Figure 3, there is shown an arrangement where the same
is wavelength-locked optical source is used to probe multiple sensors, by
splitting it
into multiple beams with optical splitter SP. For each sensor, one photodiode
and
an electronic treatment circuit is required.
Figure 4 shows how the optical source and photodiode arrangement can be
2o combined with other similarly locked sources at wavelengths ~,~ outside of
the
range ~,o+/-(d~,/2), by using wavelength division multiplexing devices (WDM),
such
that multiple sensors with corresponding central wavelengths ~~ can be placed
along the path of the light. As an example, multiple FBGs can be used at
different
locations along the same piece of optical fiber, or can even be overwritten at
the
2s same location on the fiber, and measured simultaneously with the multiple
sources. Since each sensor can be designed to operate over a different range
0~,,
this scheme gives a lot of flexibility in the design of a sensing system. For
example, one could use one FBG to measure temperature over a given range
corresponding to one value of ~~., and another one to measure strain over a
range
~o corresponding to another value of 0~,, without being constrained by a
readout
CA 02408043 2002-10-11
system that is better suited to the range of one parameter only. One could
also
use two or more FBG's with slightly different central reflection wavelengths,
and
measure the same parameter over different ranges. In this arrangement, a
tunable
laser could be used for probing each FBG in a sequential manner, by being
locked
s to a different, sequentially increasing or decreasing wavelength, using a
locking
filter with periodic transmission properties, although this would not give any
additional advantage over the method described above and it would be much
more expensive to achieve. A typical DFB laser diode, however, can be tuned
over
a limited range of wavelengths (2-3 nm). If the filter used for locking the
to wavelength of the laser has a periodic response function, the DFB laser
could be
locked to a different peak of the locking filter, and as a result the reading
apparatus would operate over a different measurement range. This can be used
to
extend the measurement range. For example, if the measurement range is 2 nm
around the central wavelength of the laser, and if the locking filter has
peaks
is separated by 1.5 nm, the DFB laser could be set at either one of three
peaks of
the locking filter, given that it can be tuned over three nanorneters. The
total
measurement range would then be 5 nm. The instrument could be programmed to
automatically tune the laser to a different peak of the locking filter when
the
measurand approaches certain values near the end of the measurement range.
Another useful feature is that multiple FBG's can be overwritten at the same
location in the fiber, or written very near to one another so that they are
affected in
the same way by the measurand. The central wavelengths of the different FBG's
can be slightly different so that their measurement ranges nearly overlaps. In
this
2s configuration, the wavelength of the DFB laser only needs to be tuned to
the
center of one of the multiple measurement ranges. This has the practical
advantage that the central wavelength of the DFB laser does not need to be
specified with great accuracy, which means that a cheaper, non-wavelength-
selected laser can be used.
CA 02408043 2002-10-11
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In the preferred embodiment, the optical sensors are fiber Bragg gratings
(FBGs), but any other component having a welt defined spectral signature
including a known transmission or reflection peak could equally be used. Long
period fiber grating or an appropriately selected gas cell could for example
be
s considered. The optical source is preferably a DFB laser diode, and the
locking
filter is an etalon filter, or a gas cell such as acetylene which has many
absorption
bands in the range 1500-1550nm. The beam splitters are fused fiber couplers in
the case of an all-fiber embodiment, but could alternatively be embodied by
any
appropriate optical element or arrangement.
io
As an example, consider an FBG used for temperature sensing. The
dependence of the central wavelength of the FBG on temperature is
approximately
0.01nm1°C. To make a sensor with a measurement range of 200°C,
one needs the
range 0~, to be 2nm. This can achieved with a uniform FBG of length 0.4 mm,
and
is reflectivity of 10%. Figure 5 shows the function S'!S over the 2 nm range,
which is
nearly linear, and can be described by a calibration function derived from a
3~d or
4t" order polynomial fit, so that only 4 or 5 parameters are required for
calibrating
the sensor. A 16 bit digitization of the output voltage would give more than
10,000
points, for a resolution better than .02°C.
While embodiments of this invention have been illustrated in the
accompanying drawings and described above, it will be evident to those skilled
in
the art that changes and modifications may be made therein without departing
from the essence of this invention. All such modifications or variations are
believed
2s to be within the scope of the invention.
