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Claims  |
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What is claimed is:
1. An optical sensing device for measuring a physical parameter, to be
connected to a light source for generating a multiple frequency light
signal having predetermined spectral characteristics, said device
comprising:
a) a Fabry-Perot interferometer through which the light signal is passed,
said Fabry-Perot interferometer including two semi-reflective mirrors
substantially parallel to one another and spaced by a given distance so as
to define a Fabry-Perot cavity having transmittance or reflectance
properties which are affected by said physical parameter and which cause
said spectral properties of the light signal to vary in response to said
physical parameter, said Fabry-Perot interferometer being provided with at
least one multimode optical fiber for transmitting the light signal into
said Fabry-Perot cavity and for collecting at least a portion of the light
signal outgoing thereof;
b) optical focusing means for focusing said at least a portion of the light
signal; and
c) a Fizeau interferometer through which said focused light signal is
passed, said Fizeau interferometer including optical wedge means forming a
wedge-profiled Fizeau cavity from which exits a spatially-spread light
signal indicative of said transmittance or reflectance properties of said
Fabry-Perot interferometer;
whereby said physical parameter can be determined by means of said
spatially-spread light signal.
2. An optical sensing device according to claim 1, further comprising
photodetecting means positioned for receiving said spatially-spread light
signal, for generating a set of discrete electrical signals representing
said spatially-spread light signal.
3. An optical sensing device according to claim 2, further comprising at
least a second Fabry-Perot interferometer similar in structure to the
first Fabry-Perot interferometer, each of said Fabry-Perot interferometers
being optically coupled in parallel between said light source and said
optical focusing means, wherein said focusing means are arranged to
produce at least a second focused light signal derived from said at least
a second Fabry-Perot interferometer and distinct from the first focused
light signal, and wherein said Fizeau interferometer is arranged to
produce at least a second spatially-spread light signal derived from said
at least a second focused light signal and distinct from the first
spatially-spread light signals, said photodetecting means being a
two-dimensional photodiode array generating at least a second set of
discrete electrical signals representing said at least a second
spatially-spread light signal, whereby at least another physical parameter
can be measured by means of said at least a second Fabry-Perot
interferometer, each of said spatially-spread light signals exhibiting
respectively information on the physical parameter in respect with the
corresponding one of said Fabry-Perot interferometers.
4. An optical sensing device according to claim 1, further comprising a
microcapillary having a longitudinal bore in which said mirrors of the
Fabry-Perot interferometer are mounted, said at least one optical fiber
having a tip connected to a corresponding one of said mirrors, a portion
extending outside said bore, and a weld with the microcapillary, whereby
said microcapillary can be bonded to a body whose deformation is to be
measured, in order that the distance between said mirrors changes as a
result of an elongation of the microcapillary, thereby varying the
transmittance or reflectance properties of said Fabry-Perot cavity.
5. An optical sensing device according to claim 4, wherein said at least
one optical fiber is a pair of multimode optical fibers, each of the tips
of said optical fibers having a surface cleaved or polished at right angle
and coated with the corresponding one of said mirrors.
6. An optical sensing device according to claim 4, wherein said Fabry-Perot
interferometer is further provided with a thin wire made of a same
material as said body, said thin wire having a tip connected to the other
one of said mirrors, a portion extending therefrom in said bore, and a
weld with said microcapillary, said other one of the mirrors being made of
a light absorbing material for absorbing a portion of the light signal,
each of the tips of said at least one optical fiber and said thin wire
having a surface cleaved or polished at right angle and coated with the
corresponding one of said mirrors, whereby a thermal expansion of said
body is compensated by a similar thermal expansion of a portion of said
thin wire within said bore.
7. An optical sensing device according to claim 5, wherein one of said
optical fibers is optically coupled with said light source, and the other
of said optical fibers is optically coupled to said optical focusing
means.
8. An optical sensing device according to claim 4, further comprising an
optical coupler for coupling the light signal into said at least one
optical fiber and for coupling said at least a portion of the light signal
transmitted by said at least one optical fiber into said optical focusing
means, said optical coupler being optically coupled between said at least
one optical fiber, said optical focusing means and said light source.
9. An optical sensing device according to claim 4, in combination with said
light source, wherein said microcapillary is in quartz or stainless steel,
the mirrors of said Fabry-Perot interferometer are 30% semi-reflective
thin layers mirrors, said at least one optical fiber has a numerical
aperture below or equal to 0,2, said focusing means are a cylindrical
lens, said optical wedge means are two flat glass plates at an angle to
one another, said light source is a quartz-halogen lamp or a broadband
light emitting diode, and said photodetecting means are a linear
photodiode array.
10. An optical sensing device according to claim 5, wherein said mirrors of
the Fabry-Perot interferometer are layers of dielectric material laid all
around the tips of said optical fibers, said dielectric material having a
temperature melting point higher than a temperature melting point of said
microcapillary and said optical fibers, the welds of said optical fibers
being precisely located where said thin layers terminate over the tips of
said optical fibers, whereby a gage length defined between said welds can
be precisely determined.
11. An optical sensing device according to claim 1, further comprising at
least a second Fabry-Perot interferometer similar in structure to the
first Fabry-Perot interferometer and connected thereto in series, a first
one of said Fabry-Perot interferometers being optically coupled to said
light source, and a last one of said Fabry-Perot interferometer being
optically coupled to said optical focusing means, whereby at least another
physical parameter can be measured by means of said at least a second
Fabry-Perot interferometer, said spatially-spread light signal
simultaneously exhibiting information on the physical parameters in
respect with each of said Fabry-Perot interferometers.
12. An optical sensing device according to claim 11, wherein said wedge
means have a stepped-profiled surface for providing to said Fizeau cavity
distinct ranges of cavity length, whereby crosstalk interferences in said
spatially-spread light signal are suppressed.
13. An optical sensing device according to claim 1, further comprising at
least a second Fabry-Perot interferometer similar in structure to the
first Fabry-Perot interferometer, and an optical coupler for coupling the
light signal into each of said Fabry-Perot interferometers and for
coupling the portion of the light signal outgoing from each of said
Fabry-Perot interferometers into said optical focusing means, sad optical
coupler being optically coupled to each of said Fabry-Perot
interferometers, to said light source and to said optical focusing means,
whereby at least another physical parameter can be measured by means of
said at least a second Fabry-Perot interferometer, said spatially-spread
light signal simultaneously, exhibiting information on the physical
parameters in respect with each of said Fabry-Perot interferometers.
14. An optical sensing device according to claim 1, in combination with
said light source, further comprising at least a second light source and
at least a second Fabry-Perot interferometer respectively similar in
structure to the first light source and the first Fabry-Perot
interferometer, each of said Fabry-Perot interferometers being optically
coupled between the corresponding one of said light sources and said
optical focusing means, whereby at least another physical parameter can be
measured by means of said at least a second Fabry-Perot interferometer,
only one of said light sources operating at a time such that said
spatially-spread light signal only exhibits information on the physical
parameter in respect with the one of said Fabry-Perot interferometers
connected to said only one of the light sources.
15. An optical sensing device according to claim 1, further comprising
optical collimating means for collimating said at least a portion of the
light signal, said optical collimating means being optically coupled
between said Fabry-Perot interferometer and said optical focusing means.
16. An optical sensing device according to claim 1, wherein said optical
wedge means are formed by a thin layer of dielectric material laid down on
a flat glass plate, said thin layer having a variable width so as to form
said wedge-profiled Fizeau cavity.
17. An optical sensing device according to claim 1, wherein said
Fabry-Perot interferometer is further provided with means for filling a
space between said mirrors of the Fabry-Perot interferometer with a liquid
having a refractive index which exhibits a variation as a function of said
parameter, said distance between the mirrors being fixed.
18. An optical sensing device according to claim 1, wherein said
Fabry-Perot interferometer includes a translucent crystal having a
refractive index that exhibits a variation as a function of temperature,
and opposite flat surfaces respectively forming said mirrors of the
Fabry-Perot interferometer, said distance between the mirrors being fixed,
whereby said temperature can be measured.
19. An optical sensing device according to claim 1, wherein said
Fabry-Perot interferometer includes a translucent crystal having a
refractive index that exhibits a variation as a function of temperature,
and opposite flat surfaces on which said mirrors of the Fabry-Perot
interferometer are respectively laid down, said distance between the
mirrors being fixed, whereby said temperature can be measured.
20. An optical sensing method for measuring a physical parameter,
comprising steps of:
a) generating a multiple frequency light signal having predetermined
spectral characteristics;
b) passing the light signal in a Fabry-Perot interferometer including two
semi-reflective mirrors substantially parallel to one another and spaced
by a given distance so as to define a Fabry-Perot cavity having
transmittance or reflectance properties which are affected by said
physical parameter and which cause said spectral properties of the light
signal to vary in response to said physical parameter, the light signal
being transmitted into said Fabry-Perot cavity with at least one multimode
optical fiber, and at least a portion of the light signal outgoing from
said Fabry-Perot cavity being collected with said at least one optical
fiber;
c) focusing said at least a portion of the light signal; and
d) passing the focused light signal through a Fizeau interferometer for
converting the focused light signal into a spatially-spread light signal
indicative of said transmittance or reflectance properties of said
Fabry-Perot interferometer;
whereby said physical parameter can be determined by means of said
spatially-spread light signal.
21. An optical sensing method according to claim 20, further comprising a
step of converting said spatially-spread light signal into a set of
discrete electrical signals representing said spatially-spread light
signal.
22. An optical sensing method according to claim 20, wherein the light
signal is also passed through at least a second Fabry-Perot interferometer
in step b), whereby at least a second physical parameter can be measured
and determined by means of said spatially-spread light signal
23. An optical sensing method according to claim 20, wherein the light
signal is also passed through at least a second Fabry-Perot interferometer
in step b), the portions of the light signal outgoing each of said
Fabry-Perot interferometers are distinctly focused in step c) and
converted in step d) so as to produce at least a second spatially-spread
light signal, whereby at least a second physical parameter can be measured
and determined by means of said at least a spatially-spread light signal.
24. An optical sensing method according to claim 20, further comprising a
step of focusing at least a portion of the light signal before said step
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the field of instrumentation,
and more specifically to an optical sensing device and a method thereof,
based on Fabry-Perot type interferometry, for measuring a physical
parameter such as a pressure, a temperature, the refractive index of a
liquid, etc., and especially a strain in or a deformation of a body.
2. Description of the Prior Art
Strain sensors using optical fibers have evolved in almost all the fields
involving strain sensing measurements of mechanical micro-deformations. In
addition to traditional applications which are presently carried out by
strain gages of resistive, piezoelectric or other types, the strain
sensors using optical fibers find uses in new applications coming out from
their development. For example, this is the case for smart skin sensors
where the optical fibers, in reason to their small size, can be integrated
within the structures to monitor.
One particular type of strain sensors using optical fibers is worthy of
note. This is the Fabry-Perot type of strain sensors, which can be
compared advantageously on one hand with the conventional electrical
strain sensors, and on the other hand with the strain sensors using
optical fibers found in the literature. The electrical strain sensors
suffers however from their sensitivity to temperature and the supply
current across them causing overheating. Their response are not linear,
and they require frequent calibrations. Their output signal is moreover
low (a few milivolts) and thus highly sensitive to bad connections and
electrical or magnetic fields. Drift current may also occurs in damp
conditions.
The properties inherent to optical fibers or devices overcome these
drawbacks. For instance, sensors using optical fibers are immunized to
electromagnetic fields, provide a better precision than traditional gages
and resist to hard environment conditions.
A large number of techniques using optical fibers for strain measurements
have been already proposed. Interferometric methods are almost the only
ones providing precision, stability and dynamic ranges which satisfy most
of the applications: on-board weighing systems for road vehicles, planes
or others; systems dedicated to monitor the integrity of structures; etc.
Known in the art are the following documents: Davis U.S. Pat. No.
4,755,668, Jul. 5, 1988; MURPHY et al., "Quadrature phase-shifted,
extrinsic Fabry-Perot optical fiber sensors", Optics Letters/Feb.
1991/Vol. 16, No. 4, pp. 273-275; LESKO et al., "Embedded Fabry-Perot
fiber optic strain sensors in the macromodel composites", Optical
Engineering/Jan. 1992/Vol. 31, No 1, pp.13-22; MURPHY, "Fabry-Perot fiber
optic sensors in full-scale fatigue testing on an F-15 aircraft", Applied
Optics/Jan. 1992/Vol. 31, No 4, pp.431-433. These documents relates to
Fabry-Perot strain sensors using single mode optical fibers. Consequently,
the light sources used are laser sources requiring to be stabilized with
extreme precision. The measures, which are relative, are carried out by
scanning whether the wavelength emitted by the laser or the physical
length of an optical fiber acting as a reference, therefore increasing the
complexity of the measurement device and reducing its stability. The
measures are relative since only the interference fringes are counted with
respect to a reference number, requiring therefore further computations to
determine the sensed parameter.
Also known in the art are the following documents: HARTL et al., "Fiber
optic temperature sensor using spectral modulation", SPIE/1987/Vol. 838;
ID Systems, "Fiber-optic sensing of physical parameters",
Sensors/1987/pp.257-261; SAASKI et al., "Measurement of physical
parameters in composite materials using embedded fiberoptic sensors",
SPIE/1989/Vol. 1170, pp.143-149. These documents relates to Fabry-Perot
sensors and methods involving optical fibers and broadband light sources.
By passing a light signal of known distribution through a Fabry-Perot
interferometer subjected to the sensed physical parameter, and by
analyzing with a specific electronic processing unit the spectrum of the
light signal resulting from the interferometer, the sensed parameter can
be determined. However, the analysis carried out by the processing unit is
time consuming, without mentioning the substantial cost to manufacture
such a processing unit.
Also known in the art is the document LEFEBVRE, "White light interferometry
in optical fiber sensors", Proceedings 7th OFS conference, which reviews
the applications of white light interferometry in the domain of optical
fiber sensors. It proposes the use of a tilted Fizeau interferometer that
yields to a spatial fringe pattern that can be easily analyzed to
determine spectral characteristics of a light signal. It brings forward
the idea of connecting a Michelson interferometer to the Fizeau
interferometer for measuring a physical parameter, which in a theoretical
point of view should be possible. But the way to obtain the desired
results is far from explained in a technical point of view, without
mentioning that the application or use of such a Michelson interferometer
can be difficult depending on the situation requirements. Indeed, the
mechanical stability required for operation of the Michelson
interferometer is extremely hard to achieve, without mentioning
difficulties with piece alignments. No Fabry-Perot interferometers have
been proposed to achieve this because the author probably knew that with
such a Fabry-Perot interferometer using multimode optical fibers there
would have been, at that time, difficulties to obtain a light signal
having enough intensity to produce an analyzable fringe pattern.
Yet known in the art are the following documents: U.S. Pat. No. 4,861,136,
STONE et al., Aug. 29, 1989; European patent published under No 0,143,645,
Mallinson et al., Jun. 5, 1985; SHABUSHNIG et al., "Formulation monitoring
with a fiber optic refractive index sensor", Chemical Processing/Sept.
1988; LEE et al., "Fiber-optic Fabry-Perot temperature sensor using a
low-coherence light source", Journal of Lightwave Technology/Jan.
1991/Vol. 9, No 1, pp. 129-134; DELISLE et al., "Application de la
modulation spectrale a la transmission de l'information", Can. J.
Phys./1975/Vol. 53, pp.1047-1053. These documents relate to various
optical devices used for several purposes such as filters, communications,
etc., but are whether too complicated, not suitable or not cost-attractive
to be used for measuring physical parameters.
Therefore, an object of the present invention is to provide an optical
sensing device based on a Fabry-Perot interferometric method, for
measuring a physical parameter, which is simple, competitive to the
electrical sensors, and cost-effective with comparison to the other
sensors using optical fibers of the prior art.
It is a further object of the invention to provide such an optical sensing
device with an excellent measurement precision and stability, a linear
absolute response which do not requires additional computations before
processing to the real determination of the physical parameter, and an
adjustable high sensitivity and dynamic range to the physical parameter.
Still another object of the invention is to provide such an optical sensing
device having a gaging portion with small dimensions, which can be used to
measure several types of physical parameters, and that can be thermally
auto-compensated.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an optical sensing
device for measuring a physical parameter, to be connected to a light
source for generating a multiple frequency light signal having
predetermined spectral characteristics, said device comprising:
a) a Fabry-Perot interferometer through which the light signal is passed,
said Fabry-Perot interferometer including two semi-reflective mirrors
substantially parallel to one another and spaced by a given distance so as
to define a Fabry-Perot cavity having transmittance or reflectance
properties which are affected by said physical parameter and which cause
said spectral properties of the light signal to vary in response to said
physical parameter, said Fabry-Perot interferometer being provided with at
least one multimode optical fiber for transmitting the light signal into
said Fabry-Perot cavity and for collecting at least a portion of the light
signal outgoing thereof;
b) optical focusing means for focusing said at least a portion of the light
signal; and
c) a Fizeau interferometer through which said focused light signal is
passed, said Fizeau interferometer including optical wedge means forming a
wedge-profiled Fizeau cavity from which exits a spatially-spread light
signal indicative of said transmittance or reflectance properties of said
Fabry-Perot interferometer; whereby said physical parameter can be
determined by means of said spatially-spread light signal.
Preferably, according to the present invention, the optical sensing device
further comprises a microcapillary having a longitudinal bore in which
said mirrors of the Fabry-Perot interferometer are mounted, said at least
one optical fiber having a tip connected to a corresponding one of said
mirrors, a portion extending outside said bore, and a weld with the
microcapillary, whereby said microcapillary can be bonded to a body whose
deformation is to be measured, in order that the distance between said
mirrors changes as a result of an elongation of the microcapillary,
thereby varying the transmittance or reflectance properties of said
Fabry-Perot cavity.
According to the present invention, there is also provided an optical
sensing method for measuring a physical parameter, comprising steps of:
a) generating a multiple frequency light signal having predetermined
spectral characteristics;
b) passing the light signal in a Fabry-Perot interferometer including two
semi-reflective mirrors substantially parallel to one another and spaced
by a given distance so as to define a Fabry-Perot cavity having
transmittance or reflectance properties which are affected by said
physical parameter and which cause said spectral properties of the light
signal to vary in response to said physical parameter, the light signal
being transmitted into said Fabry-Perot cavity with at least one multimode
optical fiber, and at least a portion of the light signal outgoing from
said Fabry-Perot cavity being collected with said at least one optical
fiber;
c) focusing said at least a portion of the light signal; and
d) passing the focused light signal through a Fizeau interferometer for
converting the focused light signal into a spatially-spread light signal
indicative of said transmittance or reflectance properties of said
Fabry-Perot interferometer;
whereby said physical parameter can be determined by means of said
spatially-spread light signal.
The present invention as well as its numerous advantages will be better
understood by the following non-restrictive description of possible
embodiments made in reference to the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a Fabry-Perot interferometer;
FIGS. 2a and 2b show graphs of the transmittance as a function of
wavelength for two different Fabry-Perot cavity lengths;
FIG. 3 is an embodiment of a Fabry-Perot interferometer;
FIG. 4 is an embodiment of an optical sensing device in transmission;
FIGS. 5 and 6 show graphs of cross-correlation functions of the
transmittance (or reflectance) for two different Fabry-Perot cavity
lengths;
FIG. 7 is an embodiment of an optical sensing device in reflection;
FIG. 8 is an embodiment of a Fabry-Perot interferometer thermally
compensated;
FIG. 9 shows a graph of a cross-correlation function of the transmittance
(or reflectance) for two Fabry-Perot interferometers in series;
Figure 10 is an embodiment of an optical sensing device in which two
Fabry-Perot interferometers are multiplexed in transmission;
Figure 11 is an embodiment of an optical sensing device in which two
Fabry-Perot interferometers are multiplexed in reflection;
FIG. 12 is an embodiment of an optical sensing device in which two
Fabry-Perot interferometers are multiplexed in time;
FIG. 13 is an embodiment of an optical sensing device in which two
Fabry-Perot interferometers are multiplexed in space;
FIG. 14 is an embodiment of an optical sensing device in which five
Fabry-Perot interferometers are multiplexed in transmission, and in which
a Fizeau interferometer has a stepped-profiled cavity;
Figure 15 is an enlarged view of a masked optical fiber;
Figure 16 is an embodiment of a Fabry-Perot interferometer having a precise
gage length;
FIG. 17 shows a graph of cavity lengths of two Fabry-Perot interferometer
(of an optical sensing device as shown in FIG. 10) bonded to a deforming
body, as a function of mechanical strain measured by an electrical gage
bonded to the same body;
Figure 18 shows a graph of the cavity length as a function of load for an
optical sensing device bonded to an axle of a semi-trailer;
FIG. 19 shows a graph of relative strains as a function of time, measured
respectively by an electrical gage (upper trace) and an optical sensing
device (lower trace); and
FIG. 20 shows a graph of the refractive index as a function of cavity
length for an optical sensing device whose Fabry-Perot cavity is filled
with liquids of various refractive index.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following description and in the drawings, the same numerals refer
to same elements.
Referring to FIG. 1, the optical sensing device according to the invention
is based on a Fabry-Perot type interferometric method. A Fabry-Perot
interferometer 20 consists of two plane, parallel, reflecting surfaces 22,
24 separated by some distance d. For such a Fabry-Perot cavity 26 defined
between the reflecting surfaces 22, 24, a light signal is fully
transmitted if the cavity length d is an integer number of half
wavelength, while the other wavelengths are partly reflected. A light
plane wave 28 propagated along the normal of two mirrors 30, 32 will be
partially transmitted 34, the rest being reflected 36 (losses can be
neglected). The transmittance or reflectance function T, defined as the
ratio of the transmitted intensity to the incident intensity, of such a
Fabry-Perot cavity 26 is given by the following relation:
##EQU1##
where: d is the distance separating the mirrors 30, 32 (cavity length);
n is the refractive index of the material separating the two mirrors 30, 32
(for example for air, n=1)
.lambda. is the wavelength of the light signal 28; and
F (the finesse) is equal to [4R/(1-R)].sup.2, R being the reflectance of
the mirrors 30, 32.
For a Fabry-Perot interferometer 20 made up of two mirrors 30, 32 of a
given reflectance R, the finesse F is evidently constant. On the other
hand, the cavity length d as well as the wavelength .lambda. of the light
signal 28 propagated through the interferometer 20 can vary. Consider a
Fabry-Perot interferometer 20 with a fixed gap. As calculated with
relation (1), the transmittance or reflectance T as a function of
wavelength .lambda. takes the form of a sinus with a wavelength's
increasing period as shown in Figure 2a. Now if the cavity length d
varies, the sinus will be subjected to a phase shift accompanied by a
variation of the period as shown in FIG. 2b. For a given cavity length d,
the transmittance or reflectance T of a Fabry-Perot interferometer 20 as a
function of the wavelength .lambda. is unique. The transmittance or
reflectance function T can thus be qualified as a signature of the cavity
length d, and this is true for each value of cavity length d. Therefore,
the Fabry-Perot cavity length d can be calculated from the transmitted 34
(or reflected 36) light spectrum.
Referring to FIG. 3, the Fabry-Perot interferometer 20 (as shown in FIG. 1)
can be used for measuring a strain parameter by providing a microcapillary
42 having a longitudinal bore in which the mirrors 30, 32 of the
Fabry-Perot interferometer 20 are mounted. The optical fibers 38, 40 have
each a tip connected to a corresponding one of the mirrors 30, 32, a
portion extending outside the bore, and a weld with the microcapillary 42.
The microcapillary 42 can be bonded to a body whose deformation or strain
is to be measured, in order that the distance d between the mirrors 30, 32
changes as a result of an elongation of the microcapillary 42, thereby
varying the transmittance or reflectance properties of the Fabry-Perot
cavity 26. The two optical fibers 38, 40, having their tip polished at
right angle and coated with the 30% semi-reflective thin layer mirrors 30,
32, are inserted into the quartz microcapillary 42. The Fabry-Perot cavity
26 is made up of the mirrors 30, 32 deposited on the tip of the optical
fibers 38, 40. The optical fibers 38, 40 are then welded at the end of the
microcapillary 42 either by a CO.sub.2 laser or an electric arc. The use
of a CO.sub.2 laser allows to precisely control the gage making process,
necessary to obtain reproducible results at advantageous manufacturing
costs. If such a strain gage 44 formed by the Fabry-Perot interferometer
20 with the microcapillary 42 is bonded to the above-mentioned body, the
variation of the Fabry-Perot cavity length d due to the deformation of the
body can be translated in strain measurement. The gage length L, defined
as the distance separating the welds 46, determines the sensitivity of
this strain gage 44. Indeed, the whole elongation of the microcapillary 42
in the gage length region being completely transferred to the Fabry-Perot
cavity length d, the sensitivity of the strain gage 44 increases with an
increasing gage length L. The sensitivity, and inversely the range of
strain, can be therefore adjusted by a proper gage length L.
Referring to FIG. 4, an optical sensing device for measuring a physical
parameter is connected to a light source 48 for generating a multiple
frequency light signal having predetermined spectral characteristics. The
device comprises a Fabry-Perot interferometer 20 (such as the one shown in
FIG. 3) through which the light signal is passed, an optical focusing lens
53 (such as a cylindrical lens) for focusing at least a portion of the
light signal outgoing the Fabry-Perot interferometer 20, and a Fizeau
interferometer 50 through which the focused light signal is passed. The
Fabry-Perot interferometer 20 includes two semi-reflective mirrors 30, 32
substantially parallel to one another and spaced by a given distance so as
to define a Fabry-Perot cavity 26 having transmittance or reflectance
properties which are affected by the physical parameter and which cause
the spectral properties of the light signal to vary in response to the
physical parameter. The Fabry-Perot interferometer 20 is provided with at
least one multimode optical fiber 38 (a second multimode optical fiber 40
being also used for the present embodiment, the first optical fiber 38
being optically coupled with the light source 48, and the second optical
fiber 40 being optically coupled to the focusing lens 53) for transmitting
the light signal into the Fabry-Perot cavity 26 and for collecting the
portion of the light signal outgoing thereof. The Fizeau interferometer 50
includes an optical wedge forming a wedge-profiled Fizeau cavity 55 from
which exits a spatially-spread light signal indicative of the spectral
characteristics resulting from the Fabry-Perot interferometer 20. Thereby,
the physical parameter can be determined by means of the spatially-spread
light signal.
In operation, the luminous flux emitted by the light source 48 (formed for
example by a quartz-halogen lamp or a broadband LED) is launched into the
leading optical fiber 38. The light beam propagated inside the leading
optical fiber 38 goes through the Fabry-Perot interferometer 20 to be
partially transmitted into the collecting optical fiber 40 and partially
reflected into the leading optical fiber 38. By measuring the transmitted
light spectrum X(.lambda.) or the reflected light spectrum equal to
1-X(.lambda.), the length d of the Fabry-Perot cavity 26 can be
calculated. The calculation can be accomplished by cross-correlating the
measured spectrum X(.lambda.) with the theoretical transmittance function
T(.lambda.,d) given by relation (1). The cross-correlation coefficient is
then calculated as a function of the cavity length d with the following
relation:
##EQU2##
where the effective cavity length d is given by a maximal
cross-correlation coefficient C(d).sub.max.
However, the measurement of the transmitted (or reflected) light spectrum
needs sophisticated apparatus on one hand, and the calculation of the
cross-correlation function is very time consuming on the other hand. To
overcome those deficiencies, a method have been developed for
instantaneously providing the cross-correlation function C(d) by means of
an optical cross-correlator. This optical cross-correlator is merely the
Fizeau interferometer 50. The Fizeau interferometer 50 consists of two
flat glass plates 52, 54 each having one face with the same reflecting
properties as the mirrors 30, 32 of the Fabry-Perot interferometer 20. The
reflecting face of the two flat glass plates 52, 54 are brought closer to
form an air wedge. The distance between those reflecting faces may vary
from 0 .mu.m to 40 .mu.m, the wedge being determined by the spacer 56. To
improve the robustness of the Fizeau interferometer 50, the latter can
also be made by laying down on the plate 54 a thin layer of Al.sub.2
O.sub.3 or any other suitable dielectric material of variable width
profiled as a wedge, instead of the other plate 52. The Fizeau
interferometer 50 works like a cross-correlator with a cavity length
depending on the position on the wedge. For example, the light intensity
transmitted through the Fabry-Perot interferometer 20 having a cavity
length d of 25 .mu.m will be maximally transmitted by the Fizeau
interferometer 50 exactly at the position where the distance between the
flat glass plates 52, 54 equals 25 .mu.m. If the Fabry-Perot cavity length
d of the optical sensing device varies in response to a mechanical
deformation, the position on the Fizeau interferometer 50 of the maximally
transmitted light intensity will shift correspondingly in a fashion
similar to what is shown in FIGS. 5 and 6. Therefore, the parameter to
measure can be easily determined with respect to a shift which has
occurred in the maximally transmitted light intensity.
The cross-correlation is instantaneously produced by illuminating the whole
width of the Fizeau interferometer 50. This goal is achieved by focusing
the light signal outgoing from the optical fiber 40 on a line (limited
between the dotted lines 57) by means of the focusing lens 53, thereby
affecting the light signal (which outgoes the optical fiber 40 with a
circular geometry) only along one of its axis. Although not essential, the
optical sensing device can be further provided with a collimating lens 58
(such as a spherical lens) for collimating the light signal or reducing
its divergence. In that case, the collimating lens 58 is optically coupled
between the optical fiber 40 of the Fabry-Perot interferometer 20 and the
focusing lens 53. The light signal transmitted through the Fizeau
interferometer 50 is then detected by a photodetector 60 positioned for
receiving the spatially-spread light signal outgoing from the Fizeau
cavity 55, for generating a set of discrete electrical signals
representing the spatially-spread light signal. This photodetector 60 can
be for example a linear photodiode array or a CCD array. Therefore, the
cross-correlation function C(d) is coded on the pixels of the
photodetector 60, each pixel corresponding to a given correlated
Fabry-Perot cavity length d. The cavity length d may vary for instance
from 0 .mu.m to 40 .mu.m. The cavity length d of the Fabry-Perot
interferometer 20 is finally given by the position of the pixel reading
the maximum light intensity. The detection of the maximum can then be
translated in strain by means oz the following relation:
##EQU3##
where: .DELTA.L is the distance, on the photodetector 60, separating the
unstrained coefficient of maximum cross-correlation from the strained one;
.gamma. is the angle between the flat glass plates 52, 54 of the Fizeau
interferometer 50 (.apprxeq.0,03.degree.); and
L is the gage length of the Fabry-Perot interferometer 20.
With the Fizeau interferometer 50 can be achieved a precise, stable, fast
and inexpensive measurement.
FIG. 5 shows a typical signal reading from the photodetector 60 (shown in
FIG. 4) after being filtered with an electronic bandpass filter (not shown
in the Figures). The pixels, scanned at a rate of 1 kHz, exhibit the
cross-correlation function C(d) of the Fabry-Perot interferometer 20
cross-correlated with the Fizeau interferometer 50. The line cursor 62
indicates that the maximum intensity peak 70 is X=760,4 .mu.s apart from
the beginning of the scan indicated by the vertical line 64. For a 512
pixels photodiode array (acting as the photodetector 60 shown in FIG. 4)
scanned at a rate of 1 kHz, this corresponds to the 389th pixel. FIG. 6
illustrates the signal reading for the same Fabry-Perot interferometer 20
submitted to a given mechanical deformation. The maximum intensity peak 70
has now shifted to X'=905,6 .mu.s, corresponding to the 464th pixel.
Referring to FIGS. 4, 5 and 6, the presence of the lateral peaks 72 in the
cross-correlation function C(d) can be very useful. Indeed, the distance
between two peaks corresponds to the half of the wavelength at the center
of the wavelength range. For a wavelength range from 600 nm to 1 000 nm,
it corresponds approximately to 800 nm/2=400 nm. To each pixel of the
photodetector 60 is combined a different spacing between the plates 52, 54
of the Fizeau interferometer 60, this spacing being cross-correlated with
the Fabry-Perot interferometer 20. The first lateral peaks 72 located on
both sides of the maximal peak 70 will thus appear when the distance
between the plates 52, 54 of the Fizeau interferometer 50 will have
increased (or decreased) of 400 nm. This distance only depends of the
angle between the plates 52, 54. For a given Fizeau interferometer, the
peaks will always be equally spaced from one another whatever the cavity
length d of the Fabry-Perot interferometer 20. When the cavity length d
varies, the cross-correlation function C(d) will shift accordingly without
changing its form. By scanning the pixels of the photodetector 60 at a
constant frequency, the cross-correlation signal has always the same
frequency and it is therefore possible to filter the cross-correlation
function C(d) by means of a band-pass filter (not shown in the Figures)
for extracting only the useful signal. This allows in particular to
suppress any other signal modulation caused by the presence of dust
particles, by a light intensity variation, etc.
Actually, the only photodetectors which can be properly applied to detect
the cross-correlation function C(d) are made of silicon components. Those
which may be used in the infrared are still very expensive while having
only a few number of pixels. The use of silicon photodetectors restricts
the useful wavelength range of the optical sensing device from roughly 600
nm to 1 000 nm. On the other hand, the broadband light sources available
in this region, either quartz-halogen or LEDs, cannot be used with a
single mode optical fiber since the power launched into the optical fiber
is too low. Consequently, a multimode optical fiber as the optical fibers
38, 40 must be used. However, the visibility of the cross-correlation
function C(d) depends on the modulation depth of the light signal
transmitted (reflected) by the Fabry-Perot interferometer 20, which is
directly related to the finesse F of the Fabry-Perot cavity 26 (relation
(1)). In order to maintain a suitable finesse F, the divergence of the
light beam propagated by the optical fiber 38 must be as low as possible.
The optimal numerical aperture (NA) of the optical fiber 38, a value
representative of the divergence of the light beam, has been roughly
estimated to 0,13. Actually, the lowest numerical aperture for commercial
multimode fibers is about 0,2, a value relatively high for the purposes.
Special optical fibers have been however easily manufacture for the
current application. Their core diameter is 50 .mu.m with a NA of 0,13.
Referring to FIG. 7, in addition to the embodiment of the optical sensing
device used in transmission as shown in FIG. 4, an embodiment of the
optical sensing device in reflection is also proposed. The configuration
of such an optical sensing device in reflection is evidently more compact.
The optical sensing device further comprises an optical coupler 49
optically coupled between the optical fiber 38, the focusing lens 53 and
the light source 48, for coupling the light signal into the optical fiber
38 and for coupling the reflected portion of the light signal collected
from the Fabry-Perot cavity 26 and transmitted by the optical fiber 38
into the focusing lens 53.
Referring to FIG. 8, the reflective configuration also allows the
development of a thermally auto-compensated optical sensing device. The
optical fiber 38 is inserted in one end of the microcapillary 42 and a
thin wire 62 made of the same material as the body whose deformation is to
be measured (not shown in the Figure) is inserted in the other end of the
microcapillary 42. The tip of the wire 62 is coated with an absorbing
material 64 like Inconel (Trademark) exhibiting a reflectance of nearly
30% in order to form a mirror as the mirror 32 (as shown in FIG. 7),
absorbing the rest of the light signal. The optical fiber 38 cannot move
in the bore of the microcapillary 42 since its tip 66 is welded thereto,
while the portion 68 of the wire 62 within the bore of the microcapillary
42 can move freely. The gage length L is entirely in the region of the
portion 68 of the wire 62. A mechanical deformation will produce a
variation of the cavity length d in a similar way as described earlier. On
the other hand, a thermal expansion of the body (not shown in the Figure)
will be compensated by a similar thermal expansion of the portion 68 of
the thin wire 62 moving in the opposite direction in the bore. The optical
sensing device can be compensated for different material by changing the
material of the thin wire 62 as well.
Referring to FIGS. 9 and 10, more than one Fabry-Perot interferometer 20
can be multiplexed and simultaneously detected by the same Fizeau analyzer
50. In the embodiment shown in FIG. 10, the optical sensing device further
comprises at least a second Fabry-Perot interferometer 21 similar in
structure to the first Fabry-Perot interferometer | | |
