CA2082686A1 - Optical fibre sensor for measuring a parameter, method of evaluating such a parameter, and application of the sensor to the measurement of a gas - Google Patents
Optical fibre sensor for measuring a parameter, method of evaluating such a parameter, and application of the sensor to the measurement of a gasInfo
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- CA2082686A1 CA2082686A1 CA002082686A CA2082686A CA2082686A1 CA 2082686 A1 CA2082686 A1 CA 2082686A1 CA 002082686 A CA002082686 A CA 002082686A CA 2082686 A CA2082686 A CA 2082686A CA 2082686 A1 CA2082686 A1 CA 2082686A1
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- Prior art keywords
- fibre
- cladding
- sensor
- parameter
- sensor according
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/75—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated
- G01N21/77—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator
- G01N21/7703—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator using reagent-clad optical fibres or optical waveguides
- G01N21/774—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator using reagent-clad optical fibres or optical waveguides the reagent being on a grating or periodic structure
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
- G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
- G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
- G02B6/021—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the core or cladding or coating, e.g. materials, radial refractive index profiles, cladding shape
- G02B6/02104—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the core or cladding or coating, e.g. materials, radial refractive index profiles, cladding shape characterised by the coating external to the cladding, e.g. coating influences grating properties
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
- G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
- G02B6/02195—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for tuning the grating
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Plasma & Fusion (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analysing Materials By Optical Means (AREA)
- Light Guides In General And Applications Therefor (AREA)
Abstract
ABSTRACT
A fibre optic sensor especially suitable for determining the gas content of a mixture comprises at least one optical fibre for measuring a physical or chemical parameter sensitive to a change in an environment (E) within which the sensor is placed. The fibre comprises a cladding (3), a core (5) surrounded by this cladding, an axis (7) with a length in the direction of this axis, and, locally along its length, parts (9) with variations in optical thickness forming an optical diffraction grating (10).
At least one part of the cladding (3) comprises in its composition an active material whose optical properties vary as a function of the change in the parameter within the environment where the sensor is placed. The active material may be a heteropolysiloxane. In order to analyse the change in the parameter, a reflectometric analysis may be provided.
A fibre optic sensor especially suitable for determining the gas content of a mixture comprises at least one optical fibre for measuring a physical or chemical parameter sensitive to a change in an environment (E) within which the sensor is placed. The fibre comprises a cladding (3), a core (5) surrounded by this cladding, an axis (7) with a length in the direction of this axis, and, locally along its length, parts (9) with variations in optical thickness forming an optical diffraction grating (10).
At least one part of the cladding (3) comprises in its composition an active material whose optical properties vary as a function of the change in the parameter within the environment where the sensor is placed. The active material may be a heteropolysiloxane. In order to analyse the change in the parameter, a reflectometric analysis may be provided.
Description
2~2~
The invention relates to a sensor, and more particularly an intrinsic fibre optic sen~or. One favoured and more particularly envisaged applicatLon of this sensor is as a gas sensor. However, the definition S of the sensor of the invention allows it equally to be used as, for example, a temperature sensor, or even as a sensor of any physical or chemical parameter which would be sensitive to the change in the environment within which the sensor will be placed.
In particular in the field of opti~al gas detectors or sensor~, it is currently known that their advantages allow reduced maintenance and operation in an oxygen-free atmosphere, their selectivity and their re~i tance to the corrosive gases having furthermore contributed to their succe~ in a number of high-technology applications.
Completely optical apparatuRes, and more par-ticularly fibre optic sensors, with ~ingle or networked optical fibres, have in addition the following advan-tages: intrinsic electrical safety (no requirement for electrical insulation), possibility of distributed measurements (combination in series, for example with continuous sensitivity), insensitivity to electro-magnetical perturbations, reduced weight and bulk.
With ~uch sensors, the optical detection in particular of a gas i8 normally performed in an open or closed cell, through which a light ray of determined wavelength interacting with the gas to be detected pas~es, in accordance with the principle of ~pectroscopic absorption. The difference in intensity between the input light signal and the output light signal of the cell gives an indication of the concentration of thi~ gas.
Other research relates finally to fibre optic chemical sensors which do not use a detection cell. Thus 35 Patent EP-A-0275275 gives an example of a fibre optic sensor for detecting a physical or chemical parameter, or alternatively a substance, using the combination of at least one selective layer disposed on a surface of the 2~&~
fibre and an optical diffraction grating with which this fibre is provided, the selective layer comprising, in its composition, an optically conducting organic material whose optical properties are sensitive to the said substance or to the said parameter, in order consequently to modify, by the intermediary of an optical diffraction grating, the optical properties of a wave propagating in the fibre.
However, this document EP-A-0275275 gives no indication a~ to the structure of the fibre; it is simply specified that it comprises an annular Bragg grating.
Difficulties in implementing this type of sensor remain, in particular when the fibre is made with a central part or core forming a waveguide, this core being surrounded by at least one optically conducting cladding.
In practice, these difficulties are in particular linked to the low proportion of light normally propagating outside the core of the fibre (the evanescent field proportion hitherto often being less than approxima~ely 1%), or to the increased fragility of the fibre when it is treated so as to have a larger evanescent field ~because of a reduction in the diameter of the fibre, and of the core in particular).
Furthermore, the selectivity or sensitivity of existing sensors is not always suitable, and it is often tricky to vary it.
Against this background, the invention provides a fibre optic sensor, of the general type of that in Patent EP-A-0275275, in which the fibre comprise~ a waveguide core surrounded by an optically conducting cladding, the selective layer occupying at least one external portion of the said cladding and surrounding, at the position of the diffraction grating, an internal optically neutral portion of this cladding which comes into contact with the core.
In this way it will be possible to obtain a sensor ensuring a compromise (which may change~ between the resilience of the fibre (mechanical strength of the 2 ~
core) and the sensitivity of the sensor (linked to the thickness of the selective layer). Alteration of this selectivity will thus be possible, by adapting it as a function of the types of detection to be performed (temperature measurement, measuring the gas content of an atmosphere, etc.).
The composition of the optical diffraction grating may be obtained by various means.
It would for example be possible to choose to make the grating with striae at the optical cladding.
This option would require deposition of a material rather than making these ctriae optically. Research into this is in progress.
However, the solution which the invention more lS particularly provide~ consists in making this grating as a Bragg grating, by creating it optically, for example by optical pumping. For this purpose the fibre (and in particular its core) will be exposed to an interference field resulting from the mutual interference of two ultraviolet radiation beams directed simultaneously towards the fibre with different acute angles of incidence with re~pect to the longitudinal axis of the fibre, so that periodic variations in the refractive index appear, at least in this core. The interference field will thus have, at the engraved grating, fringes or striae separated from each other and will be propagated transversely in the fibre with an alteration (which can be permanent if the intensity of the radiation is so adapted~ in the refractive index of the zone of creation of these striae, in correspondence with the interference field.
Given that there ha~ recently been interest in using fibre optical components which allow light to be in~ected andtor extracted, if necessary selectively tsee in particular FR-A-2,674,6Z9 or the corresponding American Application US.87/058009 of March 26 1992), an additional characteristic of the invention advantageously provides for these periodic variations in the local 2 ~
refractive index to be engraved in order to be substan-tially parallel to each other and have a normal which makes an angle ~ with the axis of the fibre, such that 0 < ~ < 90~.
In this case, in order to make the fibre operate in reflection, the cladding will then be preferentially surrounded by a mirror having a reflecting concave internal surface, directed to the fibre, in order to return to it the light wave with which it will have been illuminated.
A more detailed description of the invention will now be given by way of non-limiting example, and with reference to the attached accompanying diagrams in which:
Figure 1 is a view in longitudinal section of a first type of sensor which can be l-sed in accordance with the invention, Figure 2 is a view which is comparable with that of Figure 1 showing a first embodiment, Figure 3 is, also along a view in section identical to that in Figure 1, a representation of a second embodiment, Figure 4 is one embodiment of the cladding, solely as a detailed view corresponding to the part labelled IV in Figure 3, Figure 5 i5 an enlarged plan view showing one embodiment of construction of the mirror in particular in Figure 3 according to the detail V, and Figure 6 shows one way of optically creating the striae.
Each of the first three figures therefore illustrates the principle of an intrinsic fibre optic ~ensor, the fibre being monomode or multimode.
The following description will be lLmited, by way of example and solely for clarity of explanation, to the ca~e of a fluid sensor Quch a~ a gas sensor.
Each sensor illustrated comprises, here on the basis of only one optical fibre, an optical cladding 3, which is normally transparent, surrounding a transparent 2~?,~'$1.~
- s ~
core 5, forming a waveguide and extending longitudinally along an axis 7 in order locally to include parts 9 having variations in refractive index (or optical thick-ness) thus forming an optical diffraction grating.
It will be recalled that the optical thickness ~
of an optically homogeneous (possibly elementary) sheet of thickness e and of refractive index n is such that:
= n x e.
For such a fibre to constitute an intrinsic sensor, at least a part of the optically conducting cladding 3, or even the parts 9 with variation in refrac-tive index, CQmpriSeS in its composition an organic material which is active with respect to the substance to be detected and forms a selective layer whose optical propertie~ can thus vary as a function of the change in the parameter or in the substance P (in this case the gas content), within the environment E in which the sensor is placed.
In the examples illu~trated, the aforementioned parts with variation in refractive index are here con-stituted by zones or striae which are flat and parallel to each other and periodic. These strlae preferentially extend into the core 5 and can continue into the optically neutral part of the cladding.
Such optical diffraction gratings are known in the literature by the name Bragg Grating. The optical fibre described in the aforementioned application FR-A-2674639 is a good example of thi~, the base struc-ture of this fibre being moreover quite usable within the scope of the invention.
For any definition relating to these diffraction 5Or scattering) gratings, reference may if necessary be made to the "Dictionary of Scientific and Technical Term~", McGraw-Hill, pages 250 and 825 (see "Grating") or to the "dictionnaire de physique" [Dictionary of Physics]
by E. Levy (Presse universitaire de France), page~ 109 and 685-686.
2 0 ~ 2 ~ "? ~j In order to create the striae 9 grating, optical means are used, starting with a laser source 21 (such as a laser diode) in order to transmit to the fibre, trans-versely to its axis, an ultraviolet beam which will be separated into two waves which will be made to converge via two mirrors 25, 27, at a certain angle of incidence (~1 or o2), towards the fibre (see Figure 6), as proposed (for the grating) in Patent US-4,807,950 or alternatively in Patent US-4,867,522.
As regards the active organic material, provision may be in particular for using a heteropolysiloxane of OH
chemical formula sl_o_ with in particular R being OR
CH3 or C2H5, this material being able to be deposited in a thin film (of a few microns).
In order to ensure resilience of the assembly, the fibre will preferentially furthermore be coated with one or more mechanical claddings tfor example made of Kevlar~). So as not to complicate the figures further, no mechanical cladding has however been represented.
After this overall presentation, we shall now address ourselves to each figure independently.
It should first of all be noted that in Figure 1 the striae 9 of the Bragg grating are here made from the said active material so as to be capable of absorbing the ga~ to be detected whilst having optical characteristics, more particularly reflective properties, which are dependent on this gas concentration.
In the application adopted, the active material con8tituting these striae will advantageously have a refractive index n~ near to the refractive index nB of the cladding 3 in the absence of gas, the value of na moving away from n~ when the gas content in the active material increases.
The Bragg grating, which is almost transparent in the absence of gas, will therefore appear after the gas has been absorbed.
The optical cladding 3 may be made over part of its thickness from a base of the said active material.
This being so, it may in any case, like the external protected mechanical cladd.ing, be made of a "porous" or permeable material which allow~ the gas to be detected to penetrate laterally into the sensor, and the fibre may be made with a core and a cladding based on the same material, the difference in refractive indices existing between them being obtainable, by structural means, by doping the base material of the cladding (normally silica) with a metal oxide (such as germanium oxide GeO2).
In Figure 1 it will be noticed that the striae 9 which are parallel to each other and periodic are inclined with respect to the axis 7 of the fibre with their normal parallel to this axis. In other words, the ~triae are therefore perpendicular to the axis 7.
Thus the grating adopted will operate in reflection.
Its principle of use will be the following, irrespective of whether a single fibre or a set of sensors placed end-to-end in series is involved.
Aft~r having manufactured the striae 9 made based on the said active material embedded periodically in the fibre, so that these striae can have at least two states with different optical characteristics in the presence and absence of gas, there wlll be transmitted axially to the fibre in question, from one of its ends, an incident luminous intensity I. having at lea~t one wavelength corresponding to a resonant wavelength ~r f the optical grating in order for the latter to reflect light having the wavelength ~r~
At the same time, the luminous intensity Ir reflected by the fibre at this wavelength ~r will be measured.
The luminous intensities Ie and Ir will next be compared, the ratio between them (in general Ir/Ie) being representative of the variation in the parameter P
20(~ ?i~
analysed, and therefore in this case of the presence of the gas to be detected, and even of its concentration, inaQmuch a-q the reflected intensity Ir will increase with the gas concentration.
The example of Figure 2 differs from that of Figure 1 essentially in that here the striae 9 of the optical grating are disposed with their normal inclined by an angle ~ with respect to the axis 7 of the fibre, such that 0 < ~ ~ 90.
In this case therefore, instead of measuring the reflected luminous intensity, rather the output intensity I, will be measured, at the opposite end of the fibre, after the light has passed axially through the sensor in question over the whole of its length.
Further, in order to be able to perform these measurements, an incident luminous intensity I~ will be transmitted axially to this fibre with at least one predetermined wavelength chosen in order to make the striae 9 appear as a function of their chromatic signature, these representing, taking account of the composition of the optical grating decided upon, quaii-selective absorption.
In this case of course, the more the gas con-centration increases in the sensor, the more the intensity Ia decreases, if the incident intensity I~ is taken to be constant, the reflective properties of the striae increa~ing as does the difference between the refractive indices n, of the latter and n6 cf the cladding 3.
Let us now consider the case of Figure 3, noting that the optical fibre is here supplemented over at least a part of the periphery of the cladding by a cylindrical mirror 11 of circular cross-section whose purpose will be better understood hereinbelow, the cladding itself here being l'dividedl' into an internal part 3a, ad~acent to the core 5 and made of an "inactive" optically neutral material, that is to say one which is not sensitive to the gas to be detected (for example made of doped silica) 2~ 3 _ g _ and an external active part 3b (in particular heteropoly-siloxane) which ~urrounds it, at least at the striae 9, and whose properties of variation in transparency as a function of the gas content will essentially be u~ed. sy S way of example, for a fibre having a core with a diameter of the order of a few microns, or even a few tens of microns, the radial thickness of the layer 3a can also be a few microns, in general les~ than 10 microns. A thick-ness of a few tenths of microns may even be envisaged.
The thickness of the selective layer 3b can be a few micron.q .
As regards the mirror, its reflecting concave internal surface lla which come~ into contact with the external surface of the selective layer 3b can for example be obtained by deposition of a thin layer of a material such as gold, silver or aluminium.
As illustrated in Figure 4, the mirror 11 in its entirety will advantageously have, at least opposite the grating 10, a lacunary s~ructure with orifices 13 in order to allow the substance to be detected to pass through it. A material will thus be used which is porous to this substance and has holes 13 whose sizes are preferably markedly less than the wavelength of the signal transmitted in the fibre (for example ~/10).
The cladding 3 may also have been made from an inactive material and surrounded, as in Figure 5, with a multilayer dielectric mirror 15, which is possibly porous like the mirror 11 (cee arrows 19 in Figure 4) and the optical thickness of at least the external layer 17 of which varies as a function of the gas content of the environment (E). This being so, the result would have been comparable, the cladding 3 and the internal layers of the mirror 15 then acting as the optically neutral part 3a, and the external layer 17 replacing the selective layer 3b.
AS for the optical grating, it will have been noted that its ~triae 9 are, as in Figure 2, oriented so that their normal makes an angle ~ (0 < Q < 90) with 2a~2~,g.~
the axis 7 of the fibre, these striae here stopping at the boundary between the parts 3a and 3b of the cladding.
In order for the sensor to operate efficiently, its grating will resonate substantially around a deter-mined and known wavelength ~r~
Once this has been produced, it will be pos~ible to transmit axially into the fibre(s), still at one end, an incident luminous intensity I~ having as its wavelength or as one of its wavelengths the said resonant wavelength 10 ~r -The luminous flux will then be extracted from the fibre by the grating and reflected by the mirror 11 (or 15) in order thus to be rein~ected into the fibre in que~2tion.
When the ga3 i5 present in the active material of the selective layer, and when the waveguide used corre~-ponds to the wavelengths of absorption of the gas, the light pre~2ent in thi~2 material (and therefore in contact with the gas) will thus be absorbed, causing a drop in reflected intensity Ir.
Thus, in the presence of ga~, the absorption intensity will be a function of the length of the grating 10, of the quantity of ga-2 ~ensed, and of the thickness of the zone containing the active material.
With such a construction, it will in particular be possible to obtain a selective sensor or a series of selective sen~ors.
In fact, if the striae 9 are themselves produced with the said active material and when a polychromatic light (for example a white light) i~ transmitted to the said fibre, the grating will then itself select the resonant wavelength ~s which it will reflect, this wave-length correspondinq to the absorption line of the sensed gas, the analy~is being performed, a~ in the case in Figure 1, by picking up the incident II~) and reflected (Ir) luminou~ intensities and comparing them.
If conversely the striae of the Bragg grating are not produced based on the active material, a 2~$2~ ,3 ~j monochromatic light covering the resonant wavelength of this grating as well as the absorption line of the gas to be detected will be transmitted into this fibre, the selectivity being, in this case, performed "on input" by the operator.
In general, as means for measuring the luminous intensities a photomultiplier, a photoconducting detector or a photodiode, possibly an avalanche photodiode will be able to be used. In these four possibilities, the photo-diode is the one most commonly used. It i5 fitted to mostreflectometer~ used for measuring the reflected luminous flux (Ir)~ The incident luminous intensity (Ie) is normally supplied by the manufacturer of the apparatus.
Its value may in any case be verified by one of the aforementioned measurement means.
Another measurement method could moreover consist in using a known reflectometric method to measure the luminous intensity (Ir) reflected by the sensor or series of sensors. For all information relating to such a method, reference may be made in particular to the publication "Opto No. 63 - September/October 1991" relat-inq to optical fibre grating measurements. The following publications will also be informative to the reader:
"Principles of Optical Fiber Measurements" (Academic Press Inc., 111 Fifth Avenue, New York) and "Very High Optical Return-Loss Measurement using the OTDR Technique"
(Symposium on Optical Fiber Mea~urements, Boulder, Colorado, September 11-12 1990).
Of course, even if these methods for recording a reflected luminous flux appear attractive, the solution consisting in adopting a fibre structure comparable to those in Figures 3 to 5, with or without a mirror and with striae perpendicular to the axis of this fibre, could be selected and does not, in any case, depart in any way from the scope of the invention.
The invention relates to a sensor, and more particularly an intrinsic fibre optic sen~or. One favoured and more particularly envisaged applicatLon of this sensor is as a gas sensor. However, the definition S of the sensor of the invention allows it equally to be used as, for example, a temperature sensor, or even as a sensor of any physical or chemical parameter which would be sensitive to the change in the environment within which the sensor will be placed.
In particular in the field of opti~al gas detectors or sensor~, it is currently known that their advantages allow reduced maintenance and operation in an oxygen-free atmosphere, their selectivity and their re~i tance to the corrosive gases having furthermore contributed to their succe~ in a number of high-technology applications.
Completely optical apparatuRes, and more par-ticularly fibre optic sensors, with ~ingle or networked optical fibres, have in addition the following advan-tages: intrinsic electrical safety (no requirement for electrical insulation), possibility of distributed measurements (combination in series, for example with continuous sensitivity), insensitivity to electro-magnetical perturbations, reduced weight and bulk.
With ~uch sensors, the optical detection in particular of a gas i8 normally performed in an open or closed cell, through which a light ray of determined wavelength interacting with the gas to be detected pas~es, in accordance with the principle of ~pectroscopic absorption. The difference in intensity between the input light signal and the output light signal of the cell gives an indication of the concentration of thi~ gas.
Other research relates finally to fibre optic chemical sensors which do not use a detection cell. Thus 35 Patent EP-A-0275275 gives an example of a fibre optic sensor for detecting a physical or chemical parameter, or alternatively a substance, using the combination of at least one selective layer disposed on a surface of the 2~&~
fibre and an optical diffraction grating with which this fibre is provided, the selective layer comprising, in its composition, an optically conducting organic material whose optical properties are sensitive to the said substance or to the said parameter, in order consequently to modify, by the intermediary of an optical diffraction grating, the optical properties of a wave propagating in the fibre.
However, this document EP-A-0275275 gives no indication a~ to the structure of the fibre; it is simply specified that it comprises an annular Bragg grating.
Difficulties in implementing this type of sensor remain, in particular when the fibre is made with a central part or core forming a waveguide, this core being surrounded by at least one optically conducting cladding.
In practice, these difficulties are in particular linked to the low proportion of light normally propagating outside the core of the fibre (the evanescent field proportion hitherto often being less than approxima~ely 1%), or to the increased fragility of the fibre when it is treated so as to have a larger evanescent field ~because of a reduction in the diameter of the fibre, and of the core in particular).
Furthermore, the selectivity or sensitivity of existing sensors is not always suitable, and it is often tricky to vary it.
Against this background, the invention provides a fibre optic sensor, of the general type of that in Patent EP-A-0275275, in which the fibre comprise~ a waveguide core surrounded by an optically conducting cladding, the selective layer occupying at least one external portion of the said cladding and surrounding, at the position of the diffraction grating, an internal optically neutral portion of this cladding which comes into contact with the core.
In this way it will be possible to obtain a sensor ensuring a compromise (which may change~ between the resilience of the fibre (mechanical strength of the 2 ~
core) and the sensitivity of the sensor (linked to the thickness of the selective layer). Alteration of this selectivity will thus be possible, by adapting it as a function of the types of detection to be performed (temperature measurement, measuring the gas content of an atmosphere, etc.).
The composition of the optical diffraction grating may be obtained by various means.
It would for example be possible to choose to make the grating with striae at the optical cladding.
This option would require deposition of a material rather than making these ctriae optically. Research into this is in progress.
However, the solution which the invention more lS particularly provide~ consists in making this grating as a Bragg grating, by creating it optically, for example by optical pumping. For this purpose the fibre (and in particular its core) will be exposed to an interference field resulting from the mutual interference of two ultraviolet radiation beams directed simultaneously towards the fibre with different acute angles of incidence with re~pect to the longitudinal axis of the fibre, so that periodic variations in the refractive index appear, at least in this core. The interference field will thus have, at the engraved grating, fringes or striae separated from each other and will be propagated transversely in the fibre with an alteration (which can be permanent if the intensity of the radiation is so adapted~ in the refractive index of the zone of creation of these striae, in correspondence with the interference field.
Given that there ha~ recently been interest in using fibre optical components which allow light to be in~ected andtor extracted, if necessary selectively tsee in particular FR-A-2,674,6Z9 or the corresponding American Application US.87/058009 of March 26 1992), an additional characteristic of the invention advantageously provides for these periodic variations in the local 2 ~
refractive index to be engraved in order to be substan-tially parallel to each other and have a normal which makes an angle ~ with the axis of the fibre, such that 0 < ~ < 90~.
In this case, in order to make the fibre operate in reflection, the cladding will then be preferentially surrounded by a mirror having a reflecting concave internal surface, directed to the fibre, in order to return to it the light wave with which it will have been illuminated.
A more detailed description of the invention will now be given by way of non-limiting example, and with reference to the attached accompanying diagrams in which:
Figure 1 is a view in longitudinal section of a first type of sensor which can be l-sed in accordance with the invention, Figure 2 is a view which is comparable with that of Figure 1 showing a first embodiment, Figure 3 is, also along a view in section identical to that in Figure 1, a representation of a second embodiment, Figure 4 is one embodiment of the cladding, solely as a detailed view corresponding to the part labelled IV in Figure 3, Figure 5 i5 an enlarged plan view showing one embodiment of construction of the mirror in particular in Figure 3 according to the detail V, and Figure 6 shows one way of optically creating the striae.
Each of the first three figures therefore illustrates the principle of an intrinsic fibre optic ~ensor, the fibre being monomode or multimode.
The following description will be lLmited, by way of example and solely for clarity of explanation, to the ca~e of a fluid sensor Quch a~ a gas sensor.
Each sensor illustrated comprises, here on the basis of only one optical fibre, an optical cladding 3, which is normally transparent, surrounding a transparent 2~?,~'$1.~
- s ~
core 5, forming a waveguide and extending longitudinally along an axis 7 in order locally to include parts 9 having variations in refractive index (or optical thick-ness) thus forming an optical diffraction grating.
It will be recalled that the optical thickness ~
of an optically homogeneous (possibly elementary) sheet of thickness e and of refractive index n is such that:
= n x e.
For such a fibre to constitute an intrinsic sensor, at least a part of the optically conducting cladding 3, or even the parts 9 with variation in refrac-tive index, CQmpriSeS in its composition an organic material which is active with respect to the substance to be detected and forms a selective layer whose optical propertie~ can thus vary as a function of the change in the parameter or in the substance P (in this case the gas content), within the environment E in which the sensor is placed.
In the examples illu~trated, the aforementioned parts with variation in refractive index are here con-stituted by zones or striae which are flat and parallel to each other and periodic. These strlae preferentially extend into the core 5 and can continue into the optically neutral part of the cladding.
Such optical diffraction gratings are known in the literature by the name Bragg Grating. The optical fibre described in the aforementioned application FR-A-2674639 is a good example of thi~, the base struc-ture of this fibre being moreover quite usable within the scope of the invention.
For any definition relating to these diffraction 5Or scattering) gratings, reference may if necessary be made to the "Dictionary of Scientific and Technical Term~", McGraw-Hill, pages 250 and 825 (see "Grating") or to the "dictionnaire de physique" [Dictionary of Physics]
by E. Levy (Presse universitaire de France), page~ 109 and 685-686.
2 0 ~ 2 ~ "? ~j In order to create the striae 9 grating, optical means are used, starting with a laser source 21 (such as a laser diode) in order to transmit to the fibre, trans-versely to its axis, an ultraviolet beam which will be separated into two waves which will be made to converge via two mirrors 25, 27, at a certain angle of incidence (~1 or o2), towards the fibre (see Figure 6), as proposed (for the grating) in Patent US-4,807,950 or alternatively in Patent US-4,867,522.
As regards the active organic material, provision may be in particular for using a heteropolysiloxane of OH
chemical formula sl_o_ with in particular R being OR
CH3 or C2H5, this material being able to be deposited in a thin film (of a few microns).
In order to ensure resilience of the assembly, the fibre will preferentially furthermore be coated with one or more mechanical claddings tfor example made of Kevlar~). So as not to complicate the figures further, no mechanical cladding has however been represented.
After this overall presentation, we shall now address ourselves to each figure independently.
It should first of all be noted that in Figure 1 the striae 9 of the Bragg grating are here made from the said active material so as to be capable of absorbing the ga~ to be detected whilst having optical characteristics, more particularly reflective properties, which are dependent on this gas concentration.
In the application adopted, the active material con8tituting these striae will advantageously have a refractive index n~ near to the refractive index nB of the cladding 3 in the absence of gas, the value of na moving away from n~ when the gas content in the active material increases.
The Bragg grating, which is almost transparent in the absence of gas, will therefore appear after the gas has been absorbed.
The optical cladding 3 may be made over part of its thickness from a base of the said active material.
This being so, it may in any case, like the external protected mechanical cladd.ing, be made of a "porous" or permeable material which allow~ the gas to be detected to penetrate laterally into the sensor, and the fibre may be made with a core and a cladding based on the same material, the difference in refractive indices existing between them being obtainable, by structural means, by doping the base material of the cladding (normally silica) with a metal oxide (such as germanium oxide GeO2).
In Figure 1 it will be noticed that the striae 9 which are parallel to each other and periodic are inclined with respect to the axis 7 of the fibre with their normal parallel to this axis. In other words, the ~triae are therefore perpendicular to the axis 7.
Thus the grating adopted will operate in reflection.
Its principle of use will be the following, irrespective of whether a single fibre or a set of sensors placed end-to-end in series is involved.
Aft~r having manufactured the striae 9 made based on the said active material embedded periodically in the fibre, so that these striae can have at least two states with different optical characteristics in the presence and absence of gas, there wlll be transmitted axially to the fibre in question, from one of its ends, an incident luminous intensity I. having at lea~t one wavelength corresponding to a resonant wavelength ~r f the optical grating in order for the latter to reflect light having the wavelength ~r~
At the same time, the luminous intensity Ir reflected by the fibre at this wavelength ~r will be measured.
The luminous intensities Ie and Ir will next be compared, the ratio between them (in general Ir/Ie) being representative of the variation in the parameter P
20(~ ?i~
analysed, and therefore in this case of the presence of the gas to be detected, and even of its concentration, inaQmuch a-q the reflected intensity Ir will increase with the gas concentration.
The example of Figure 2 differs from that of Figure 1 essentially in that here the striae 9 of the optical grating are disposed with their normal inclined by an angle ~ with respect to the axis 7 of the fibre, such that 0 < ~ ~ 90.
In this case therefore, instead of measuring the reflected luminous intensity, rather the output intensity I, will be measured, at the opposite end of the fibre, after the light has passed axially through the sensor in question over the whole of its length.
Further, in order to be able to perform these measurements, an incident luminous intensity I~ will be transmitted axially to this fibre with at least one predetermined wavelength chosen in order to make the striae 9 appear as a function of their chromatic signature, these representing, taking account of the composition of the optical grating decided upon, quaii-selective absorption.
In this case of course, the more the gas con-centration increases in the sensor, the more the intensity Ia decreases, if the incident intensity I~ is taken to be constant, the reflective properties of the striae increa~ing as does the difference between the refractive indices n, of the latter and n6 cf the cladding 3.
Let us now consider the case of Figure 3, noting that the optical fibre is here supplemented over at least a part of the periphery of the cladding by a cylindrical mirror 11 of circular cross-section whose purpose will be better understood hereinbelow, the cladding itself here being l'dividedl' into an internal part 3a, ad~acent to the core 5 and made of an "inactive" optically neutral material, that is to say one which is not sensitive to the gas to be detected (for example made of doped silica) 2~ 3 _ g _ and an external active part 3b (in particular heteropoly-siloxane) which ~urrounds it, at least at the striae 9, and whose properties of variation in transparency as a function of the gas content will essentially be u~ed. sy S way of example, for a fibre having a core with a diameter of the order of a few microns, or even a few tens of microns, the radial thickness of the layer 3a can also be a few microns, in general les~ than 10 microns. A thick-ness of a few tenths of microns may even be envisaged.
The thickness of the selective layer 3b can be a few micron.q .
As regards the mirror, its reflecting concave internal surface lla which come~ into contact with the external surface of the selective layer 3b can for example be obtained by deposition of a thin layer of a material such as gold, silver or aluminium.
As illustrated in Figure 4, the mirror 11 in its entirety will advantageously have, at least opposite the grating 10, a lacunary s~ructure with orifices 13 in order to allow the substance to be detected to pass through it. A material will thus be used which is porous to this substance and has holes 13 whose sizes are preferably markedly less than the wavelength of the signal transmitted in the fibre (for example ~/10).
The cladding 3 may also have been made from an inactive material and surrounded, as in Figure 5, with a multilayer dielectric mirror 15, which is possibly porous like the mirror 11 (cee arrows 19 in Figure 4) and the optical thickness of at least the external layer 17 of which varies as a function of the gas content of the environment (E). This being so, the result would have been comparable, the cladding 3 and the internal layers of the mirror 15 then acting as the optically neutral part 3a, and the external layer 17 replacing the selective layer 3b.
AS for the optical grating, it will have been noted that its ~triae 9 are, as in Figure 2, oriented so that their normal makes an angle ~ (0 < Q < 90) with 2a~2~,g.~
the axis 7 of the fibre, these striae here stopping at the boundary between the parts 3a and 3b of the cladding.
In order for the sensor to operate efficiently, its grating will resonate substantially around a deter-mined and known wavelength ~r~
Once this has been produced, it will be pos~ible to transmit axially into the fibre(s), still at one end, an incident luminous intensity I~ having as its wavelength or as one of its wavelengths the said resonant wavelength 10 ~r -The luminous flux will then be extracted from the fibre by the grating and reflected by the mirror 11 (or 15) in order thus to be rein~ected into the fibre in que~2tion.
When the ga3 i5 present in the active material of the selective layer, and when the waveguide used corre~-ponds to the wavelengths of absorption of the gas, the light pre~2ent in thi~2 material (and therefore in contact with the gas) will thus be absorbed, causing a drop in reflected intensity Ir.
Thus, in the presence of ga~, the absorption intensity will be a function of the length of the grating 10, of the quantity of ga-2 ~ensed, and of the thickness of the zone containing the active material.
With such a construction, it will in particular be possible to obtain a selective sensor or a series of selective sen~ors.
In fact, if the striae 9 are themselves produced with the said active material and when a polychromatic light (for example a white light) i~ transmitted to the said fibre, the grating will then itself select the resonant wavelength ~s which it will reflect, this wave-length correspondinq to the absorption line of the sensed gas, the analy~is being performed, a~ in the case in Figure 1, by picking up the incident II~) and reflected (Ir) luminou~ intensities and comparing them.
If conversely the striae of the Bragg grating are not produced based on the active material, a 2~$2~ ,3 ~j monochromatic light covering the resonant wavelength of this grating as well as the absorption line of the gas to be detected will be transmitted into this fibre, the selectivity being, in this case, performed "on input" by the operator.
In general, as means for measuring the luminous intensities a photomultiplier, a photoconducting detector or a photodiode, possibly an avalanche photodiode will be able to be used. In these four possibilities, the photo-diode is the one most commonly used. It i5 fitted to mostreflectometer~ used for measuring the reflected luminous flux (Ir)~ The incident luminous intensity (Ie) is normally supplied by the manufacturer of the apparatus.
Its value may in any case be verified by one of the aforementioned measurement means.
Another measurement method could moreover consist in using a known reflectometric method to measure the luminous intensity (Ir) reflected by the sensor or series of sensors. For all information relating to such a method, reference may be made in particular to the publication "Opto No. 63 - September/October 1991" relat-inq to optical fibre grating measurements. The following publications will also be informative to the reader:
"Principles of Optical Fiber Measurements" (Academic Press Inc., 111 Fifth Avenue, New York) and "Very High Optical Return-Loss Measurement using the OTDR Technique"
(Symposium on Optical Fiber Mea~urements, Boulder, Colorado, September 11-12 1990).
Of course, even if these methods for recording a reflected luminous flux appear attractive, the solution consisting in adopting a fibre structure comparable to those in Figures 3 to 5, with or without a mirror and with striae perpendicular to the axis of this fibre, could be selected and does not, in any case, depart in any way from the scope of the invention.
Claims (10)
1. Fibre optic sensor for detecting a physical or chemical parameter or a substance, using the combination of at least one selective layer (3b, 9) disposed on a surface of the fibre and an optical diffraction grating (10) with which this fibre is provided, the said selective layer comprising, in its composition, an optically conducting organic material whose optical properties are sensitive to the said substance or the said parameter, in order consequently to modify, by the intermediary of the said optical diffraction grating, the optical properties of a wave propagating in the fibre, characterised in that the fibre comprises a waveguide core (5) surrounded by an optically conducting cladding (3), the said selective layer occupying at least one external portion (3b, 17) of the said cladding and surrounding, at the position of the said grating, an optically neutral internal portion (3a) of this cladding coming into contact with the core.
2. Sensor according to Claim 1 characterised in that the said optical diffraction grating extends into the said optically conducting cladding (3).
3. Sensor according to Claim 1 characterised in that the optical diffraction grating consists of a Bragg refraction grating created optically by exposing the fibre to an interference field resulting from the mutual interference of two ultraviolet radiation beams directed simultaneously towards the fibre, transversely to the axis of the latter, with different angles of incidence, so that periodic variations (9) in the refractive index appear in the fibre.
4. Sensor according to Claim 3 characterised in that the said periodic variations in the refractive index constitute striae (9) substantially parallel to each other and whose normal makes an angle .alpha. with the axis (7) of the fibre, such that 0° < .alpha. < 90°.
5. Sensor according to Claim 3 characterised in that the said periodic variations in the refractive index constitute striae (9) substantially parallel to each other and whose normal is parallel to the axis (7) of the fibre.
6. Sensor according to any one of the preceding claims characterised in that the said selective layer comprises an optically conducting organic material, such as a heteropolysiloxane.
7. Sensor according to Claim 6 characterised in that the said organic material is deposited on the said inter-nal portion (3a) of the optically conducting cladding (3) in order to form therein a thin film constituting the said selective layer.
8. Sensor according to Claim 6 characterised in that the said organic material is deposited in a thin film at least in the core (5) of the fibre in order to constitute therein the striae of the optical diffraction grating, the refractive index of these striae changing as a function of the variations in the said parameter or in the said substance contained in the environment within which the sensor is placed.
9. Sensor according to any one of the preceding claims characterised in that the said optically conduct-ing cladding (3) is surrounded by a mirror (11, 17) having a reflecting concave internal surface directed towards the fibre in order to return to the interior of this fibre a light wave which has been extracted therefrom.
10. Sensor according to Claim 9 characterised in that the said mirror (11, 17) which surrounds the cladding has, at least opposite the said optical diffraction grating, a lacunary structure (13) in order to allow the said substance or the said parameter to be detected to pass through it.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR9113887A FR2683628B1 (en) | 1991-11-12 | 1991-11-12 | FIBER OPTIC SENSOR FOR MEASURING A PARAMETER, PROCESS FOR EVALUATING SUCH A PARAMETER, AND APPLICATION OF THE SENSOR TO MEASURING A GAS. |
| FR9113887 | 1991-11-12 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2082686A1 true CA2082686A1 (en) | 1993-05-13 |
Family
ID=9418818
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002082686A Abandoned CA2082686A1 (en) | 1991-11-12 | 1992-11-12 | Optical fibre sensor for measuring a parameter, method of evaluating such a parameter, and application of the sensor to the measurement of a gas |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0542603A1 (en) |
| JP (1) | JPH05249039A (en) |
| CA (1) | CA2082686A1 (en) |
| FR (1) | FR2683628B1 (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4861132A (en) * | 1987-11-23 | 1989-08-29 | Hughes Aircraft Company | Self-aligning precision guide pin |
| FR2734374B1 (en) * | 1995-05-19 | 1997-06-27 | Gaz De France | OPTICAL DEVICE COMPRISING SPECTRALLY SELECTIVE SENSORS AND REFLECTORS |
| DE19630181C2 (en) * | 1996-07-26 | 2002-08-08 | Inst Physikalische Hochtech Ev | Compact optical fiber sensor for the detection of chemical or biochemical substances |
| JP5285553B2 (en) * | 2009-09-04 | 2013-09-11 | 大阪瓦斯株式会社 | Gas detection device and fire detection device |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3587641T2 (en) * | 1984-08-13 | 1994-02-17 | United Technologies Corp | DISTRIBUTED, EXPANSION METER WITH OPTICAL FIBER SET UP ACCORDING TO SPACE RESOLUTION. |
| DE3526348A1 (en) * | 1985-07-23 | 1987-02-05 | Fraunhofer Ges Forschung | SENSORS FOR THE SELECTIVE DETERMINATION OF COMPONENTS IN LIQUID OR GASEOUS PHASE |
| DE3723159A1 (en) * | 1986-07-17 | 1988-01-21 | Prosumus Ag | Chemical sensor and method which can be performed with it |
| US4996419A (en) * | 1989-12-26 | 1991-02-26 | United Technologies Corporation | Distributed multiplexed optical fiber Bragg grating sensor arrangeement |
| US5007705A (en) * | 1989-12-26 | 1991-04-16 | United Technologies Corporation | Variable optical fiber Bragg filter arrangement |
| US5042897A (en) * | 1989-12-26 | 1991-08-27 | United Technologies Corporation | Optical waveguide embedded light redirecting Bragg grating arrangement |
| CH684290A5 (en) * | 1991-02-15 | 1994-08-15 | Suisse Electronique Microtech | Methods of producing at least a diffraction grating on an optical fiber, obtained transducer device. |
| FR2674639A1 (en) * | 1991-03-29 | 1992-10-02 | Gaz De France | BRAGG NETWORK OPTIC FIBER AND ITS APPLICATIONS. |
-
1991
- 1991-11-12 FR FR9113887A patent/FR2683628B1/en not_active Expired - Fee Related
-
1992
- 1992-11-05 EP EP92402993A patent/EP0542603A1/en not_active Ceased
- 1992-11-12 CA CA002082686A patent/CA2082686A1/en not_active Abandoned
- 1992-11-12 JP JP4326294A patent/JPH05249039A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| FR2683628A1 (en) | 1993-05-14 |
| FR2683628B1 (en) | 1995-08-18 |
| JPH05249039A (en) | 1993-09-28 |
| EP0542603A1 (en) | 1993-05-19 |
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