Note: Descriptions are shown in the official language in which they were submitted.
CA 02471401 2012-02-10
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PACKAGED OPTICAL SENSORS ON THE SIDE OF OPTICAL FIBRES
Field of the invention
The present invention generally relates to optical fibres, and more
particularly
concerns an optical fibre sensor integrated on a preferred azimuthal portion'
of the
side of an optical fibre. An exemplary application of this invention is a
temperature
measurement in vivo.
Background of the invention
Optical fibre sensors are well known and their application fields cover a
broad area
ranging from physical parameter measurement to chemical and biochemical
parameter measurement.
Optical fibres also have inherent characteristics and properties. One of these
properties, which is relevant for the purposes of the present description, is
that of an
intrinsic flexibility, which permits optical fibres to support a temporary
mechanical
deformation. The person skilled in the art will appreciate the use of this
flexibility, as
evidenced in the following articles and extracts:
A. Katzir, Optical fiber techniques (medicine), in Encyclopedia of Physical
Science
and Technology, Vol. 9, Orlando, Academic Press, pp. 630-631, (1987).
B. Selm et al., Novel flexible light diffuser and irradiation properties for
photodynamic therapy, Journal of Biomedical Optics, Vol. 12, paper 034024,
(2007).
B. Van Hoe et al., Optical fiber sensors embedded in polymer flexible foils,
Proceedings of the SPIE, Vol. 7726, paper 772603, (2010).
W. L. Lee, Optical fibers for medical sensing - A technology update,
Proceedings
of the SPIE, Vol. 1886, pp. 138-146, (1993).
J. S. Webb et al., Apparatus for optical stimulation of nerves and other
animal
tissue, U.S. Patent No. 7,736,382, June 15, 2010.
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1a
Patents and scientific papers have also been published in the field of
chemical and
biochemical measurement through Iuminescent optical fibre sensors. These cover
the biomedical field through the measurement of physiological -parameters such
as
pH, 02, glucose, and CO2 concentration in blood.
Another major area involved by the luminescent detection through optical fibre
sensors is the biomedical diagnostic domain through optical biopsy. This area
involves the evaluation of biological tissues through the measurement of a
tissue's
auto-fluorescence or through induced fluorescence by specific markers
revealing
the presence or absence of pathological tissues. These techniques are
currently
under development but some have reached the clinical level.
Of particular interest is the measurement of temperature through luminescent
optical fibre sensors since optical fibres, unlike thermistors and
thermocouples, are
not affected by microwaves used in thermal treatment of cancers.
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Luminescent optical fibre sensors usually work as follows: an excitation
wavelength is directed into the optical fibre entrance with appropriate
optical
components. The excitation light travels through the fibre up to the other end
of the
fibre, where a luminescent material has been packaged at the fibre tip. The
incoming light excites the luminescent material which in turn emits its
luminescent
light. The material is chosen such that its luminescent light properties
(intensity,
spectral content, lifetime decay) vary with the parameter to be measured. The
luminescent light follows the optical fibre path down to the fibre entrance
and is
then collected and filtered against the excitation wavelength with proper
optics and
electronics. Finally, the luminescent properties of the collected light are
analysed
to deduce the parameter value to be measured.
Most or all of these luminescent optical fibre sensors are packaged at one end
of
the fibre. Thus, few or none allow distributed measurements, either by
spatially
distributing the measurement of one parameter or through simultaneous
measurement of many parameters, through only one fibre. Furthermore, in some
cases, the fact that the sensor is placed at the end of the fibre renders its
use less
attractive.
For example, it is known that the temperature measurement of intra-arterial
walls
can be used as a diagnostic tool to detect active atheroslerotic plaque at
risk of
disrupting. These active plaques have a temperature which is higher (from 0.1
to
1.5 C) than normal arterial walls, and the temperature measurement of intra-
arterial walls can then be used to detect these plaques. If one measures the
temperature of intra-arterial walls with a luminescent optical fibre
temperature
sensor placed at the end of the fibre, one will use the small and potentially
piercing
sensing end of the fibre to make contact with the arterial wall. This is a
serious
disadvantage, since one can accidentally pierce the artery or worse, the
active
arterial plaque can be broken, which can result in a cardiac stroke.
The same configuration, i.e. the use of the sensor at one end of the fibre,
could be
used to measure the fluorescence coming from the arterial wall. In this case,
the
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optical fibre is used as a light pipe to make the excitation light reach the
arterial
wall and to gather part of the luminescent light from the wall and guide it
down to
.the fibre entrance. The luminescent light can then be analysed to identify
the type
of biological tissue and eventually diagnose the presence of plaques at risk
of
disrupting. However, to excite and collect the maximum of light level, one
needs to
put the fibre end in contact with the arterial wall, which can lead to, the
problems
described above. This is also true for any optical fibre extrinsic
spectroscopic
sensor which collect light (luminescent or not) from biological tissue or from
an
optical sensing material making contact with that tissue.
Thus the use of conventional optical fibre sensor packaged at one end of the
fibre
should be prohibited in cases where biological tissue damage can cause health
problems.
Therefore, there is a need for a sensor better adapted for a safe in vivo
spectroscopy. Moreover, it would be desirable to provide a sensor offering
more
precise measurement.
Summary of the invention
It is an object of the present invention to provide an. optical sensor on a
preferred
azimuthal portion of the side of an optical fibre.
In accordance with one aspect of the invention, this object is achieved with
an
optical sensor comprising an optical fiber for conveying a light beam, said
optical
fiber being provided with a first end for receiving said light beam and a
second end
opposed thereto, a core and a cladding surrounding said core, said optical
fiber
having a longitudinal portion extending between said first and second ends
having a
predetermined longitudinally curved permanent shape and an intrinsic
flexibility to
allow a temporary deformation thereof, said optical fiber further having at
least one
claddingless portion having a longitudinal, radial and azimuthal limited
extent, said
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azimuthal extent being less than 180 degrees, said at least one claddingless
portion
defining a shaped cavity extending on said longitudinal portion so as to
project
outwardly therefrom, said optical sensor further comprising a sensing material
extending in said cavity for forming a directional sensing area therein having
a
limited azimuthal extent less than 180 degrees in optical contact relationship
with
said core adapted to provide a directional selective contacting sensing, the
longitudinally curved permanent shape and the intrinsic flexibility of the
longitudinal
portion, in combination with the directional sensing area projecting outwardly
therefrom, enhancing contact between said directional sensing area and a
sensed
area of a solid surface, providing for discrimination between parameters of
surrounding fluid and parameters of said sensed area to be measured.
The present invention advantageously provides a sensor well adapted for
spectroscopic measurement in the biomedical and biotechnological fields.
The optical fibre has a predetermined permanent shape for projecting the
sensing
area outwardly, thereby allowing a safe use of the optical sensor for in vivo
measurement. This also allows a better contact of the sensor with the area to
be
sensed.
In a preferred embodiment, the optical sensor has a plurality of sensing areas
separated from each other by a predetermined distance. In a preferred
embodiment,
each of the sensing area extends in a longitudinal alignment relationship with
each
others. In another preferred embodiment, each of the sensing area extends in
an
azimuthal alignment relationship with each others.
Preferably, the sensing material is either a luminescent material or a
transparent
material.
Another aspect of the invention concerns the optical sensing system
comprising:
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at least one optical sensor, each comprising an optical fiber for conveying
a light beam, said optical fiber being provided with a first end for receiving
said light
beam and a second end opposed thereto, a core and a cladding surrounding said
core, said optical fiber having a longitudinal portion extending between said
first and
second ends having a predetermined longitudinally curved permanent shape and
an
intrinsic flexibility to allow a temporary deformation thereof, said optical
fiber further
having a claddingless portion having a longitudinal, a radial and an azimuthal
limited
extent, said azimuthal extent being less than 180 degrees, said claddingless
portion
defining a shaped cavity extending on said longitudinal portion so as to
project
outwardly therefrom, said optical sensor further comprising a sensing material
extending in said cavity for forming a directional sensing area therein having
a
limited azimuthal extent less than 180 degrees in optical contact relationship
with
said core adapted to provide a directional selective contacting sensing, the
longitudinally curved permanent shape and the intrinsic flexibility of the
longitudinal
portion, in combination with the directional sensing area projecting outwardly
therefrom, enhancing contact between said directional sensing area and a
sensed
area of a solid surface, providing for discrimination between parameters of
surrounding fluid and parameters of said sensed area to be measured;
a light source for injecting light into the first end of the optical fiber of
each of said at least one optical sensor;
a detector operatively connected to one of said ends of said optical fiber
of each of said at least one optical sensor for detecting light coming from
each
sensing area; and
an analyser operatively connected to said detector for analysing light
coming from each sensing area.
Brief description of drawings
The present invention and its advantages will be more easily understood after
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reading the following non-restrictive description of preferred embodiments
thereof,
made with reference to the following drawings in which:
Figure 1a is a schematic cross sectional side view of an optical sensor
according
to a preferred embodiment of the present invention.
Figure 1 b is a cross sectional front view of the optical sensor of Figure 1
a.
Figure 2 is a schematic cross sectional side view of another optical sensor
according to another preferred embodiment of the present invention;
Figure 3a to 3h are schematic cross sectional side views of different optical
sensors according to different preferred embodiments of the present invention;
each of the optical sensors having a portion of its cladding and of its core
removed
with different shapes.
Figure 4 is a side view of a permanently deformed fibre provided with an
optical
sensor according to another preferred embodiment of the present invention.
Figure 5a is a schematic representation of an optical fibre provided with a
plurality
of sensors longitudinally arranged thereon according to another preferred
embodiment of the present invention.
Figure 5b is a graph illustrating an optical technique known as Optical Time
Domain Reflectometry.
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Figure 6 is a cross-sectional front view of an optical fibre provided with a
plurality
of sensors azimuthally arranged thereon according to another preferred
embodiment of the present invention.
Figure 7a is a cross-sectional front view of an optical sensor in contact
relationship
with an arterial wall according to another preferred embodiment of the present
invention.
Figure 7b is a cross sectional side view of the fibre of Figure 7a.
Figure 8a is a schematic cross sectional side view of a fibre including a
reflection
splice proximate the sensor and extending perpendicularly to the fibre optical
axis.
Figure 8b is a schematic cross sectional side view of a fibs! including a
reflection
splice proximate the sensor and extending with an angle with respect to the
fibre
optical axis.
While the invention will be described in conjunction with an example
embodiment, it will be understood that it is not intended to limit the scope
of the
invention to such embodiment. On the contrary, it Js intended to cover all
alternatives, modifications and equivalents as may be included as defined by
the
appended claims.
DESCRIPTION OF A PREFERRED. EMBODIMENT
As mentioned previously, the present invention relates to optical fibre
sensors and
more particularly to optical fibre sensors packaged on the side of optical
fibres.
The application domain covers a large area but mainly aims the field of
luminescent and spectroscopic optical fibre sensors with application
possibilities in
the field of telecommunications.
More specifically, the packaged sensor application field aims the measurement
of
temperature and the spectroscopic measurement in the biomedical and
biotechnological domains.
AMENDED SH'EET,]
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The spectroscopic sensors cover the identification and concentration
measurement of biological, biochemical and chemical compounds aiming
applications in the biomedical, biotechnological, chemical, environmental and
industrial domains. More specifically, the concentration measurement of pH,
02,
C02, and glucose are of interest. Identification of biological pathologic
tissues
through spectroscopy is more specifically aimed.
The present invention alleviates the most glaring problem of the prior art,
viz. the
placement of the sensor at the tip of a fibre, since the material is placed
on one side of the fibre. The sensing material could, then make a gentle
contact
with the surface, to be measured through the side! of the fibre, which is not
as
piercing as its tip. Thus, this will prevent the fibre from damaging for
example, the
arterial wall in the example described above.
The present invention concerns the placement, in the side of an optical fibre
or any
other appropriate waveguide, of a sensing material. Referring to Figures 1a
and
1 b, there is shown an optical sensor 10 including an optical fibre 12 for
conveying
a light beam. The optical fibre 12 is provided with a first end 14 for
receiving the
light beam and a second end 16 opposed thereto, a core 18 and a cladding 20
surrounding the core 18. The optical fibre 12, is also provided with at least
one
claddingless portion 22 having a longitudinal, a radial and an azimuthal
extent.
Each of the at least one claddingless portion 22 defines a shaped cavity 28
extending in the optical fibre 12 between the two opposite ends 14, 16. The
optical
sensor 10 further includes a sensing material 24 extending in each of the
cavities
28 for forming a sensing area 26 therein in contact relationship with the core
18.
The claddingless portion 22 is obtained by removing a longitudinal and radial
part
of the optical fibre 12. In the preferred embodiment illustrated in Figures 1a
and
1 b, the sensing material 24 does not penetrate into the core 18 of the
optical fibre
12, but make an optical contact with it. Alternatively, according to another
preferred embodiment of the present invention illustrated in Figure 2, the
shaped
cavity 28 partially extends in the core 18 of the optical fibre 12.'Thus, the
sensing
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material 24 also extends in the core 18. Such an embodiment could
advantageously be used if a greater coupling of light is required, as further
described below. Moreover, the optical sensor 10 presented in Figures 1 and 2
may further include a thermally conductive material 30 surrounding the sensing
material 24. In other words, the sensing material 24 can be further encased in
a
thermally conductive material 30, thereby increasing the sensitivity of the
sensor
10.
The sensing material extending in the cavity 28 of the optical fibre 12 can be
a
luminescent material or can also be a transparent material. It is to be
understood
that throughout the present description, the expression "sensing material" is
intended to specifically cover such materials as well as absorbing, reflecting
or
semi-transparent materials, or even any sensing material that is able to
change
properties of light reaching it. For example, according to a particular
application,
the sensing material may be chosen to have spectral properties changing with
the
presence and/or the concentration of chemical or biochemical compounds.
One can use a transparent material in order to make a window on the side of
the
fibre. This window can then transmit an excitation light from the fibre
entrance to a
luminescent material or a biological tissue and the luminescent light from the
material or tissue back to the fibre entrance. The collected luminescent light
can
then be analysed to measure different desired parameters. In the case one uses
a
transparent material, such a transparent material preferably has an index of
refraction which is greater than or equal to that of the core of the fibre.
The
window can also be used to transmit light having a wide spectral range to
biological tissue or chemical compounds. The collected light could then be
analysed to measure its spectral content and to deduce physical or chemical
properties of the reflecting compounds or tissues.
Referring now to figures 4 and 7, a preferred application of such a sensor is
temperature measurement. In order to discriminate between ambient temperature
(i.e. the temperature around the fibre) and the actual temperature of the
target
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which can, for example, be an arterial wall 100, it is preferable that the
sensing
material be placed only on a preferred azimuthal portion of the side of the
optical
fibre, and that it covers only a part of the periphery of the fibre, as can be
better
seen on Figure 7a. This is what is meant by the expression "longitudinal,
radial
and azimuthal" used in the present description. This also increases the
sensitivity
of the sensor of the present invention. The longitudinal extent of the sensing
area
will be determined by the application requirements. If the spatial resolution
is an
important issue, then this length should be small. If it is more an.average
value
over a larger area which is of interest, then the length of the sensing area
could be
larger. In the case of the temperature measurement of arterial, walls, this
length
should preferably be between 0,5 and 5 mm. The same type of approach can be
applied to other preferred embodiments like concentration, measurements or
identification from spectral analysis.
From a theoretical point of view, it is known that the light rays going
through an
optical fibre core have an angular content comprised between rays going
parallel
along the fibre and rays reflected on the fibre core walls at a determined
angle
known as the critical total internal reflection angle (8c). This critical
angle is
determined by the following relation:
O =sin (n2 In,) Equation I
where n1 and n2 are the refractive indexes of the fibre core 18 and cladding
20
respectively. This angle is measured between a ray of light and the normal to
fibre
core walls. Rays having angle equal'to or above this value (e0) will be
reflected on
the fibre core wall and the ones having angle lower than this value will
partly go
through the wall. If one is to get light going to the sensing area without
being
totally reflected, he should either allow the light to hit the sensing area at
an angle
lower than this critical angle (9) or to choose the refractive index of the
sensing
material in order to change this critical angle, or both.
MtNDED;'SHEFIT!
19-01-2004 CA 02471401 2004-06-21 CA030023
To get fibre core rays of light hitting the sensing area at an angle lower
than the
critical angle (8.), one can shape the sensing area in order to lower the
angle of
rays hitting it, as illustrated in Figures 3a to 3h. Thus, to get the maximum
amount
of light reaching the sensing area, some preferred embodiments about the shape
5 of the sensing area will follow this assumption. The preferred embodiment
about
the making of optical fibre having a sensing area on a preferred azimuthal
portion
of their side is to remove a lateral portion of the side of the fibre and
replace it with
the appropriate sensing material. This can be used to shape the sensing area
of
the fibre by removing the material on the side of the fibre and giving it the
proper
10 shape.
The shape of the removed part of the fibre is advantageously adapted to the
application in order to get the optimum amount of light coupled from the fibre
core
to the sensing area and from the sensing area back to the fibre core. Figures
3a
to 3h illustrate some of the preferred embodiments related to the shape of the
removed part when it is made to reach the fibre core. The proper choice of the
shape and extent of the removed part will depend on several parameters such as
the particular application, the sensing parameters and material, the
sensitivity or
amount of light available from and back to the fibre core, etc, .... For
example, if
one is to sense the temperature on a very small portion of a surface, the
embodiments represented through figures 3a, 3b, and 3c are better choice than
embodiments represented by figure 3d, 3e and 3f which are more convenient for
measurement over large area or average value measurements.
If the sensing material extending in the shaped cavity. 28 'of the fibre 12,
has a
refractive index (n3) equal to or less than the refractive index of the
'cladding (n2),
then the amount of light coupled from the fibre core to the sensing area will
be the
same for embodiments illustrated by figures 3a and 3d. This comes from the
fact
that only the portion of the sensing area 28 facing the light will allow the
coupling
of the light. The flat portion parallel to the fibre wall in figure 3d will
couple very few
light since it will act identically to the fibre wall except for residual
absorption of
evanescent waves into the luminescent material. In this case, to increase the
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coupling of light to a greater extent of the sensing area 28, embodiments
illustrated on figures 3e, 3f, or 3h are preferably used.
On the other hand, if the-sensing material extending in the shaped cavity 28
of the
fibre 12, has a refractive index (n3) greater than the refractive index of the
fibre
core (ni), the light will more easily couple from the fibre core to the
sensing
material, but could suffer total internal reflection when going from the
sensing
material back to the fibre core. This will make coupling from the sensing
material
to the fibre core less efficient for embodiment illustrated by figure 3d and
again
embodiments represented by figures 3e and 3f could be useful to increase the
coupling efficiency. Embodiment illustrated in figure 3g. could also be used,
then
acting like a lens and increasing the coupling efficiency in and out the
sensing
material. If a very large extent of the sensing area is needed, many lens-like
shapes like figure 3g could be placed beside one another similarly to triangle-
like
shape of figures 3e and 3f to increase coupling efficiency. Of course, any
other
convenient shape of the cavity 28 could also be envisaged, according to a
specific
application.
To form the cavity 28, the removal of the portion of the fibre can be done by
a
chemical etching process or by laser processing, preferably a laser ablation.
The
chemical etching process could be done through masking of the fibre except for
the part to be etched. The masked fibre is then exposed to HF that dissolves
the
glass and shape the fibre in a rather linear way by removing progressively the
exposed glass. This can be used to create square-like shaped cavity 28, as
illustrated on figure 3d. However, this can hardly be used to shape the fibre
side
with irregular shapes like embodiments illustrated on figures 3a, 3b, 3c, 3e,
3f, 3g
and 3h. In these cases, the preferred technique to get irregular shapes is
laser
ablation. This technique can be used to obtain complex and precise shapes for
the cavity 28, ranging in size from few microns to many hundreds microns and
even millimetres. Furthermore, laser ablation offers' a better flexibility,
over
chemical etching of fibres and probably ensure a better mechanical and
physical
integrity of the fibre core.
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The integration of the sensing material in the shaped cavity of the fibre can
be
done in many ways. In the case where the sensing material is in the form of a
powder, it can be inserted by integrating it with an epoxy glue, preferably
having a
high thermal conductivity coefficient if the sensor is to be a temperature
sensor.
The sensing material could also be integrated into a silica powder which can
be
melted into the opening by heating it with a laser. In the case where the
sensing
material can be melted with a laser without losing its sensing properties, it
could
be directly melted into the opening. Yet alternatively, the sensing material
can be
included into a paste, which can be cured by UV or laser illumination. In the
case
1.0 where the sensing material can not be down sized to a powder or a paste,
it can
be chemically etched or laser machined to match the shape of the cavity 28 of
the
fibre. This sensing part can then be glued in place with epoxy for example or
joined to the fibre with a melting material placed between the sensing
material and
the shaped cavity of the fibre, and then heated in an oven or with a laser
beam to
complete the joining process. These techniques could be used as well for many
types of sensing materials such as, for example, but not limited to,
luminescent,
absorbing, non-linear, transparent, polarizing, porous, sol-gel or even
birefringent
materials.
As previously said and according to Equation 1, in order to obtain better
results,
the coupling of the light from the optical fibre core to the sensing area can
be
optimized through the proper choice of the sensing area shape and/or of its
refractive index with respect to the one of the fibre core. This could be
achieved
by making the refractive index of the sensing material higher than the
refractive
index of the fibre core by a proper choice of the sensing material or by
including a
material with a refractive index significantly higher than the one of the
fibre core to
the sensing material.
In the case where the shaped cavity of the fibre does not extend in the fibre
core,
as illustrated in Figure 1, one has no choice but to use a sensing material
having a
refractive index higher than the one of the fibre core. However, this can be
false if
the sensing material can absorb part of the light coming from the fibre core.
In this
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case, the absorption by the sensing material could be enough to couple a
sufficient amount of light from the fibre to the sensing area.
Advantageously, a fibre with a core occupying a greater proportion of the
section
of the fibre is used. This reduces the risk of compromising the mechanical
integrity
of the fibre, since less material needs to be removed to reach the core.
Furthermore, the manufacture of the sensor is simplified, since it is not
necessary
to penetrate deeply into the fibre. Another advantage is that the sensing
material
will be located closer to the surface of the fibre, which will promote a
better reading
of the parameter to be sensed (e.g. temperature), since the contact point will
be
less affected by the ambient environment (or average temperature) of the
fibre.
This could also lead to a faster response of the sensing element to sensed
parameter (or temperature) change. Finally, the core of the fibre being
greater, it
will be easier to couple light from the sensing material back into the fibre
core. For
example, a fibre having a total diameter of 125 pm having a core diameter of
100
pm (a standard multimode fibre) requires the removal of 13 to 25 pm. However,
in
the case of luminescent intensity time decay measuring techniques, a fibre
having
a greater core has a high modal dispersion, which can negatively impact on the
measurements if luminescent lifetime decay of the order of a nanosecond are
used. A fibre having a core of 100 pm and an index jump of 0.015 has a modal
dispersion estimated to be 0.05 ns/m. Obviously, a monomode fibre or any
convenient waveguide could also be envisaged, according to a particular
application.
According to another object of the present invention and with reference to
Figure
5, there is also provided an optical sensing system 200 including at least one
optical sensor 10 provided in an optical fibre 12, as previously described.
The
optical sensing system includes a light source for injecting light 36 into the
first end
14 of the optical fibre 12 of each of the optical sensors 10. A detector 38
operatively connected to one of the ends of the optical fibre 12 of each of
the
optical sensors 10 is also provided for detecting light coming from each of
the
sensing areas 26. Such an optical sensing system finally includes an analyser
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operatively connected to the detector for analysing light coming from each of
the
sensing areas. In a preferred application which will be described in more
details
below, such a system is used as a temperature sensing system.
Thus, in order to measure a parameter with a sensor made according to the
present invention, such as temperature, a sensing light is directed into the
optical
fibre entrance with appropriate optical components. The sensing light travels
through the fibre up to the area where the sensing material has been packaged.
The sensing light reaches the sensing material which in turn modifies its
optical
properties. The sensing material is chosen such that its optical properties
(luminescence, intensity, spectral properties, absorption, reflection,
lifetime decay,
...) vary with the parameter to be measured in a well known manner. The
returned
light is collected and filtered against the sensing light with proper optics
and
electronics. Finally, the optical properties of the collected light are
analysed to
deduce the parameter values to be measured.
In some cases, care must be taken to minimise the light reaching the area to
be
sensed through the sensing material, which could cause a parasitic signal
induced on the area to be sensed. This can be obtained by deposing an opaque
or
reflecting film 32 above the sensing material 24, as illustrated in Figure 2.
However, in the cases where this parasitic signal must be measured as the
sensing signal, such as auto-fluorescence of biological material, the sensing
material and its substrate can be a transparent material having an index of
refraction equal to or greater than the one of the fibre core. Thus, the light
reaching this transparent material will have a tendency to exit the core of
the fibre
to reach the area to be sensed. In one of the preferred applications which is
the
measurement of temperature of intra-arterial walls, it is also contemplated to
use
this technique, combined with a thermally conducting luminescent material 40
coated on the surface of the fibre 12 to increase the capability of the sensor
10 to
discriminate between the sensed area (e.g. the arterial walls) temperature and
the
ambient temperature (e.g. the blood temperature).
CA 02471401 2011-08-25
Referring now to Figures 8a and 8b, the optical sensor 10 may be further
provided
with a reflector 34 extending radially inside the optical fibre 12 between the
sensing area 26 and the second end 16 of the optical fibre 12, in the vicinity
of the
sensing area 26.
The presence of a reflector 34 near the sensing material allows to maximize
the
return of the luminescent light towards the excitation source. This can be
done by
placing on a section of the fibre a reflecting material such as, for example
TiO2.
The proximity of this reflector 34 is important in order to minimise the
temporal
shift induced by differences in optical path produced between the sensing
signal
directly reaching the sensing material and the one produced by reflection, and
also
between the signal coming from the sensing material directly towards the
source
and the one reaching it after reflection. It is even more advantageous to
place the
reflector 34 at an angle in order to collect more light, as illustrated in
figure 8b. The
use of a fibre Bragg grating reflecting only the wavelength from the sensing
material back to the entrance could be a better choice in the case where there
is a
sufficient amount of sensing light from the source. The fibre Bragg grating
could be
scribed into the fibre core by conventional UV scribing techniques or it could
be
scribed on the fibre surface by laser micro-machining of the cladding down to
the
fibre core, as well known in the art.
It should be noted that the use of the reflector 34 is optional, but
preferable in
order to increase the sensing light back to the entrance of the optical fibre.
In
some applications, it is also possible to inject light at one end of the
fibre, to detect
the parameter to be sensed at an intermediate position, and to detect the
sensing
signal at the other end of the fibre. In the case where the opposite end of
the fibre
is not used and is not provided with a reflector, it is preferable to place an
absorbent material or an index-matching material 42 (as illustrated in Figure
5a) in
CA 02471401 2011-08-25
16
order to minimise the reflection of sensing light towards the sensor, and
towards
the input of the fibre. Furthermore, in the case of the preferred embodiment
measuring temperature with a luminescent sensing material, if reflection is
permitted, it should be as close as possible to the sensor in order to
minimise a
false reading of the lifetime decay of the luminescence.
Referring again to Figure 5a, there is shown an optical sensor 10, wherein the
sensor includes a plurality of sensing areas 26 extending in line with each
other.
The sensing areas 26 are separated by a predetermined distance, and the
measurements can be taken from each of the sensing areas 26 by a technique
known as Optical Time Domain Reflectometry (OTDR), illustrated at Figure 5b.
Alternatively, referring now to Figure 6, different sensing materials with
sensing
response at different wavelengths can advantageously be used, and these can be
placed very close to each other, or even distributed on different azimuths of
the
fibre. In a preferred embodiment, each sensing area 26 extends in a radial
alignment relationship with each others around the optical fibre. The
different
sensing signals can then be distinguished through wavelength separation
techniques, which are well known in the art and won't be further exposed
therein.
Referring now to Figure 4, there is shown an optical sensor which has been
permanently deformed, for example by heating the fibre with a laser, in order
to
project the sensing area outwardly. This preferred embodiment is particularly
advantageous for inner wall temperature measurement, since it insures that the
sensing area remains in contact with the wall 100 for the duration of the
measurement ( as better shown in Figures 7a and 7b).
Although the optical sensing system according to the present invention has
been
described in details for the particular application of temperature
measurement, it
should be understood that such a sensing system could also be useful in many
other applications such as, for non-restrictive example, the measurement of a
pH
CA 02471401 2012-02-10
16a
concentration, 02, CO2 concentration or glucose concentration. Such a sensing
system may also be used as a biological tissue identifying system. Moreover,
although the present invention has been explained hereinabove by way of a
preferred embodiment thereof, it should be pointed out that any modifications
to
CA 02471401 2004-06-21
WO 03/071235 PCT/CA03/00235
17
this preferred embodiment within the scope of the appended claims is not
deemed
to alter or change the nature and scope of the present invention. More
specifically,
the present invention is not limited to temperature measurement, but can be
used
for any parameter measurement, in vivo or not, where small sensors are
required,
or where insensitivity to EM radiation is required (for example in nuclear
reactors).
Also specifically, the present invention is not limited to the use of
luminescent
sensing materials, but can use any sensing materials (such as absorbing,
reflecting, transparent, semi-transparent, non-linear, porous, sol-gel,
polarizing,
electro-optical, birefringent, ... materials) that change properties of light
(such as
wavelengths or spectral content, temporal properties, polarisation, relative
intensity or power, ...) impingent on it through absorption, reflection,
radiation (or
emission), non-linear effects, guiding properties, ...
