Temperature sensor with increased sensitivity

Abstract

The invention relates to a temperature sensor comprising: an optical fibre (3) having a portion comprising a Bragg grating (4); a capillary (1) surrounding the portion; and a filling material (2) between the optical fibre and the capillary. The filling material (2) has a coefficient of thermal expansion strictly greater than the optical fibre and it adheres to the fibre but not to the capillary, such that, when it expands under the effect of temperature, the filling material exerts an axial pull on the Bragg grating. Alternatively, the capillary has a coefficient of thermal expansion strictly greater than the optical fibre and the filling material adheres both to the capillary and to the optical fibre, such that, when it expands under the effect of temperature, the capillary exerts an axial pull on the Bragg grating by way of the filling material.

Description

[0001] DESCRIPTION

[0002] TITLE: Increased Sensitivity Temperature Sensor

[0003] This application relates to the field of temperature measurement requiring a very high level of accuracy and / or resolution, for example for the purpose of structural health monitoring, particularly in constrained environments such as aircraft. More specifically, the invention relates to fiber optic temperature sensors.

[0004] Previous art

[0005] We are familiar with optical fiber sensors equipped with a Bragg grating. Such a sensor comprises an optical fiber, a section of which, referred to here as the measurement section, incorporates a Bragg grating. The Bragg grating is a series of periodic refractive modifications on a short segment of the fiber. The Bragg grating acts as an optical filter, reflecting a specific wavelength of the light passing through the fiber, while allowing other wavelengths to pass through. A reflection peak, called the Bragg peak, therefore appears on the spectrum of wavelengths reflected by the sensor. The Bragg peak depends on the period (i.e., the registration pitch) of the grating and the refractive index of the fiber.

[0006] Bragg grating sensors are widely used for temperature measurement. Indeed, a variation in the temperature to which the fiber and its Bragg grating are subjected induces a variation in the refractive index and therefore a shift in the Bragg peak.

[0007] The accuracy of fiber optic Bragg grating sensors depends directly on two factors: the temperature sensitivity of the Bragg grating; the variation of the Bragg grating's refractive index with temperature depends on the operating wavelength and the material. Once these two parameters are fixed, the curves showing the variation of the refractive index, and therefore of the Bragg peak, are fixed; the temperature is directly deduced from the shift of the Bragg peak relative to the initial operating wavelength (which is the Bragg length at the initial temperature, for example, room temperature); and the sensitivity and resolution of the interrogator.

[0008] Furthermore, knowing the sensitivity of the Bragg grating, the element that determines the resolution of the sensor (i.e., the resolution of the temperature measurement) is the interrogator.

[0009] As an example, considering an interrogator with 1pm resolution and 20pm precision and silica optical fibers, we obtain the following table.

[0010] In the preceding table, the sensitivity of the Bragg grating (expressed in pm / °C) is interpreted as follows. A sensitivity of 10 to 15 pm / °C means that if a Bragg grating inscribed in a silica optical fiber is used in the 1550 nm range, the generated peak shifts by approximately 10 to 15 pm for every one-degree change in temperature. Similarly, the range of 6 to 8 pm / °C obtained for a silica optical fiber and an operating wavelength of 850 nm means that the generated peak shifts by approximately 6 to 8 pm for each degree of temperature change.

[0011] Given the above, it appears that two levers can theoretically be used to improve the sensitivity of the temperature sensor:

[0012] - improve the sensitivity of the Bragg grating by adjusting the working wavelength of the grating and / or the optical fiber material,

[0013] - to improve the interrogator's performance.

[0014] In practice, regarding the first lever, the choice of wavelength is limited to traditional ranges (1550nm, 1300nm and 850nm) and the fibers are generally made of silica with slight differences related to the type of doping used.

[0015] In practice, regarding the second lever, one quickly encounters technological limitations that are difficult or impractical to overcome because unfavorable compromises result. State-of-the-art interrogators generally have a resolution on the order of 1 µm and an accuracy on the order of 20 µm.

[0016] A well-known solution for increasing the sensitivity of a physical phenomenon is the Vernier effect, notably used to facilitate the reading of a length on a ruler.

[0017] The same principle applies to Bragg gratings. It is possible to increase the measurement sensitivity of a Bragg grating by coupling it with a Vernier system.

[0018] The drawback of this method is that it requires adding extra reference components to the interrogator. Furthermore, the interrogator must include ways to maintain these reference components at a stable temperature or to correct for the effect of temperature variations on these components. In addition, adding components and modifying the architecture can limit the interface between the interrogator and the Bragg gratings, in terms of the number of gratings, spectral positions, and so on. Incorporating such a Vernier effect can also limit the type and number of Bragg gratings that can be used.

[0019] We also know about the temperature sensor disclosed by

[0020] US 2001 / 022804, wherein: conventionally, the optical fiber comprises a glass (silica) waveguide core surrounded by a protective outer sheath; the Bragg grating is inscribed in a bare portion, i.e., a portion without a protective sheath, of the fiber; this bare portion is inserted into a glass capillary, and the volume separating the bare fiber from the capillary is filled with an adhesive material having a coefficient of thermal expansion different from that of the capillary. The adhesive material is preferably an epoxy resin having a coefficient of thermal expansion of 90 x 10⁻¹⁰⁸⁻¹. 6 K' 1 , while optical fiber (or more precisely its waveguide core) and the capillary, both made of silica, have a coefficient of thermal expansion of 0.5x10' 6 K' 1As the temperature increases, the epoxy resin expands much more than the optical fiber and the capillary. The Bragg grating is then subjected to radial pressure imposed by the resin under the pressure of the capillary. This radial pressure is temperature-dependent and leads to a corresponding change in the grating geometry, resulting in a further shift in the Bragg peak. This shift is in addition to the shift caused by the effect of temperature on the fiber's refractive index.

[0021] This known temperature sensor is advantageous in that it exhibits improved sensitivity through the combination of the effects of temperature on the refractive index of the fiber and the effects of temperature on the mechanical stresses imposed on the Bragg grating by the differential expansion of the epoxy resin relative to the capillary.

[0022] However, this sensor is not without its drawbacks. The lack of a protective sheath on a portion of the fiber significantly weakens it. Furthermore, the stripping step prior to embedding the Bragg grating within the optical fiber complicates and increases the time and cost of sensor manufacturing. Most importantly, the sensor's sensitivity remains insufficient for certain applications. The need for sensors with improved sensitivity and resolution remains undiminished.

[0023] We are also familiar with the sensor disclosed by EP 1 144 696, whose optical fiber with its Bragg grating is inserted into a tube made of natural or synthetic quartz or Pyrex® or Vycor® type glass. The tube and the fiber are fused, meaning that a molecular bond is established between the inner wall of the tube and the outer wall of the optical fiber, such that it is no longer possible to distinguish an interface between the inner wall of the tube and the outer wall of the optical fiber. This fusion can be carried out along the entire length of the measurement segment or only on a portion of it, either directly above the Bragg grating or on either side of it. The tube is thus designed to absorb the axial forces exerted on the fiber so that the Bragg grating is affected only by temperature variations.

[0024] This sensor is particularly complex to manufacture and its sensitivity remains insufficient for certain applications.

[0025] The sensor disclosed by US2017 / 0049341 is now known. It comprises an optical fiber equipped with one or more fiber Bragg gratings (FBGs) and a sensing body whose ends are connected to the fiber on either side of the FBG, either directly by soldering or gluing, or via terminal plates. The sensing body is made of a rigid, temperature-sensitive material—that is, a material that expands or contracts, particularly along the axial direction, depending on the temperature. The change in the size of the sensing body alters the length of the fiber at the FBG location. The fiber may be associated with a capillary filled with adhesive, which provides radial support for the fiber and prevents it from buckling under axial pressure.The deformations of the sensing body are then transmitted to the fiber via the capillary which is rigidly fixed to the ends of the sensing body and which is glued to the fiber along the entire length of the measurement section except possibly at the FBG where the capillary can be omitted.

[0026] Here again, the proposed sensor is particularly complex and expensive to manufacture. While the presence of the sensing body improves the sensor's temperature sensitivity, it increases the sensor's weight and size, thus precluding its use for monitoring aircraft structures. Description of the invention

[0027] The invention aims to provide a temperature sensor having improved sensitivity and / or resolution(s), while remaining lightweight and compact and being able to be manufactured simply and cheaply from a standard silica optical fiber, and whose Bragg grating is configured to be able to work in a common wavelength range (i.e. around 1550nm or 1300nm or 850nm), and this without adding complex and expensive software and optoelectronic components to the interrogator.

[0028] To achieve this, the invention proposes a temperature sensor comprising:

[0029] - an external detection face intended to be brought into contact with a structure to be monitored (which structure is not part of the claimed sensor),

[0030] - an optical fiber comprising, on the one hand, a waveguide core having a section, called the measurement section, in which a Bragg grating is inscribed, and on the other hand, a protective sheath which encloses said waveguide core,

[0031] - a capillary into which the measurement section of the optical fiber is inserted, the capillary having an internal diameter strictly greater than the external diameter of the optical fiber so that an annular space exists between an external face of the optical fiber and an internal face of the capillary along the entire length of the measurement section,

[0032] - a filling material in the annular space between the optical fiber and the capillary; characterized in that:

[0033] - the outer face of the optical fiber and the inner face of the capillary are not fused,

[0034] - an external face of the capillary constitutes the external sensing face of the temperature sensor at the level of the measuring section; in other words, the temperature sensor according to the invention is devoid of an additional sensing body such as that disclosed by US2017 / 0049341.

[0035] The temperature sensor according to the invention is further characterized in that:

[0036] - either the filling material and the capillary constituent material are such that the filling material has a coefficient of thermal expansion strictly greater than that of the optical fiber, and the filling material adheres to the optical fiber but not to the capillary,

[0037] - either the filling material and the constituent material of the capillary are such that the capillary has a coefficient of thermal expansion strictly greater than that of the optical fiber and the filling material adheres to both the capillary and the optical fiber.

[0038] Note that the expression "thermal expansion coefficient" more precisely refers to the linear thermal expansion coefficient of the materials or elements in question, in the axial direction of the optical fiber.

[0039] In the first case defined above, as the temperature increases, the filler material expands more than the optical fiber. Since it does not adhere to the capillary, it is not constrained by it and can move axially within the capillary due to this expansion. Because it adheres to the fiber, it then exerts axial forces on the fiber.

[0040] Note that in the earlier sensor disclosed by US 2001 / 022804, the forces generated on the optical fiber by the expansion of the adhesive material (filler material) are essentially radial since this material adheres to the capillary and is therefore axially constrained by the silica capillary which has the same coefficient of thermal expansion as the optical fiber; these forces therefore have a limited effect on the geometry of the Bragg grating, which results in a limited increase in the sensitivity of the sensor.

[0041] In the second case defined above concerning the invention, when the temperature increases, the capillary expands more than the fiber in the axial direction. Since the filler material adheres to both the capillary and the optical fiber, the axial expansion of the capillary generates axial forces on the filler material, forces which are transmitted to the optical fiber.

[0042] Thus, in both cases, an increase in temperature leads to an axial pull on the optical fiber, either by the filling material (first case), or by the capillary (second case) or by both (second case also, if the filling material also has a coefficient of thermal expansion strictly greater than that of the optical fiber).

[0043] This axial tension causes a significant change in the Bragg grating's registration pitch, resulting in a significant shift in the Bragg peak. This shift is compounded by the temperature-induced change in the fiber's refractive index. The sensor's sensitivity is thus greatly improved, even when the optical fiber is a conventional silica fiber and is used with a basic, commercially available interrogator.

[0044] According to particular embodiments of the invention, the temperature sensor further meets the following characteristics, implemented individually or in any technically possible and operational combination.

[0045] In certain embodiments, the protective sheath encloses the waveguide core, including along the measurement section. Indeed, the sensor design according to the invention does not require prior stripping of the optical fiber at the Bragg grating, and the previously stated result is obtained even if the protective sheath is retained along the measurement section. The only requirement is that the filler material adheres to the protective sheath so that it can pull axially on the Bragg grating when the filler material or capillary expands. The protective sheath can thus advantageously be retained along the entire length of the optical fiber.

[0046] In some embodiments, the filler material is chosen from polymers that adhere to the optical fiber in the polymerized state and are sufficiently fluid in the unpolymerized state to allow the optical fiber to slide within the filler material. As will be understood later, this property simplifies the manufacturing of the sensor.

[0047] In some embodiments, the optical fiber measurement section extends from one end of the optical fiber, called the measurement end, which measurement end is located inside the capillary near a first, closed end of said capillary, the optical fiber exiting the capillary at a second end thereof.

[0048] The first end of the capillary can be closed using a dot of glue or cement or by LASER sealing.

[0049] Depending on the filling material used, the second end of the capillary can be left open; preferably, this second end is closed with a drop of glue or cement, to ensure that the filling material remains confined inside the capillary regardless of the sensor's operating conditions.

[0050] The invention extends to a method for manufacturing a temperature sensor as previously defined. In particular, the invention relates to a method for manufacturing a temperature sensor comprising an external sensing face intended to be brought into contact with a structure to be monitored, the manufacturing method using:

[0051] - an optical fiber comprising, on the one hand, a waveguide core having a section, called the measurement section, in which a Bragg grating is inscribed, and on the other hand, a protective sheath which encloses said waveguide core,

[0052] - a capillary into which the measurement section of the optical fiber is inserted, the capillary having an internal diameter strictly greater than the external diameter of the fiber so that an annular space exists between an external face of the optical fiber and an internal face of the capillary over the entire length of the measurement section, - and a filling material to fill the annular space between the optical fiber and the capillary.

[0053] The manufacturing process according to the invention is characterized in that:

[0054] - an external face of the capillary constitutes the external sensing face of the temperature sensor at the level of the measuring section (no additional sensing body, such as that disclosed by US2017 / 0049341, is therefore attached to the capillary at the ends of the measuring section),

[0055] - no fusion operation is performed between an outer face of the optical fiber and an inner face of the capillary (contrary to what is stipulated in EP 1 144 696), and in that:

[0056] - either the filler material and the capillary constituent material are chosen such that the filler material has a coefficient of thermal expansion strictly greater than that of the optical fiber and the filler material adheres to the fiber but not to the capillary,

[0057] - either the filling material and the constituent material of the capillary are chosen such that the capillary has a coefficient of thermal expansion strictly greater than that of the optical fiber and the filling material adheres to both the capillary and the optical fiber.

[0058] In some embodiments, the optical fiber measuring section is inserted into the capillary without first stripping the optical fiber.

[0059] In some embodiments:

[0060] - the measurement section of the optical fiber extends from one end of the optical fiber, called the measurement end,

[0061] - the filling material is chosen from polymers that adhere to the optical fiber in the polymerized state and are sufficiently fluid in the unpolymerized state to allow the optical fiber to slide within the filling material,

[0062] - the capillary tube is filled with filling material in a fluid state;

[0063] - One end of the capillary tube is sealed using a dab of glue or cement, or by laser sealing.

[0064] - The measuring section of the optical fiber is inserted into the capillary through its second end.

[0065] - then the filling material is polymerized or allowed to polymerize, - optionally, the second end of the capillary is closed with a drop of glue or cement, before or after polymerization of the filling material.

[0066] Brief description of the drawings

[0067] The invention, according to an exemplary embodiment, will be well understood and its advantages will be more apparent upon reading the detailed description that follows, given by way of example and not limiting in any way, with reference to the attached drawings in which: o Figure 1 illustrates an example of a temperature sensor according to the invention at different stages of its manufacture, o Figure 2 represents the sensor of Figure 1 at the final stage.

[0068] Identical elements represented in the aforementioned figures are identified by identical numerical references.

[0069] Figure 1 illustrates the different assembly steps of an example of a temperature sensor according to the invention, this sensor can be seen fully assembled in Figure 2.

[0070] The sensor shown includes:

[0071] - a capillary 1, which can be made of plastic or metal, for example,

[0072] - an optical fiber 3, comprising on the one hand a waveguide core provided with a Bragg grating 4 inscribed in said core near an end 30, called the measurement end, of said fiber, the optical fiber comprising on the other hand a protective sheath (not shown) which envelops the waveguide core; the other end of the optical fiber (not visible in the drawings) is intended to be connected to an interrogator; for this purpose, it may be equipped with an optical connector;

[0073] - and a filling material 2, filling the volume between the optical fiber 3 and the capillary 1.

[0074] The first step in the manufacturing process consists of supplying the capillary 1 and the optical fiber 3. The filler material 2 is then injected in a fluid state into the capillary 2, for example, using a syringe (not shown). A delayed-curing gel, which is almost liquid upon insertion into the capillary and then hardens after a few minutes or tens of minutes inside the capillary, can be used as the filler material. One end 10 of the capillary is sealed using a laser process or by means of a dot of cement or adhesive 12. This step can be carried out after (as illustrated) or before filling the capillary 1 with the filler material 2.It seems preferable to proceed as illustrated to expel the air present in the capillary as the filler material is injected, thus ensuring that the capillary is completely filled. The filler material can be injected under pressure into the capillary. Alternatively, the filler material can be introduced into the capillary by immersing one end of the capillary in a volume of filler material and then aspirating it from the other end, for example, using a syringe securely attached to the other end of the capillary.

[0075] The measuring end 30 of the optical fiber 3 is then inserted into the capillary through its second end 11, at least until the Bragg grating 4 is entirely located inside the capillary. Once the fiber is thus inserted into the capillary, the Bragg grating 4 is, for example, located approximately in the central part of the capillary. Throughout this description, by convention, the optical fiber segment that incorporates the Bragg grating 4 and is delimited by the capillary 1 is called the measuring segment.

[0076] The sealing of the capillary end 10 can be performed after the capillary has been filled, before the optical fiber is inserted into the capillary (as illustrated and described previously). Alternatively, it can be performed after the optical fiber has been inserted, and even after the filler material has cured. The order in which the various manufacturing steps described here are carried out may be dictated by the time available before the filler material hardens.

[0077] The second end 11 of the capillary can be sealed with a dab of glue or cement 13 to prevent the filler material 2 from escaping the capillary 1 when it expands due to heat, particularly if the filler material is a gel that does not adhere to the capillary and has a coefficient of thermal expansion greater than that of the capillary. As previously explained, according to the invention, the material constituting the capillary 1 and the filler material 2 are chosen such that the capillary and / or the filler material exert axial tension on the optical fiber when the temperature increases.

[0078] To this end, the polymerized filler material 2 must adhere to the optical fiber 3. The filler material is preferably chosen from among those that adhere to the protective cladding (not shown in detail) of the optical fiber, thus allowing the cladding to be retained along the entire length of the fiber, including its measurement section, opposite the Bragg grating. Alternatively, although not desirable, it is possible to choose a filler material that adheres to the waveguide core but not to the protective cladding; in this case, a preliminary step of at least partial stripping of the optical fiber on the measurement section will be necessary.

[0079] Furthermore, in order for the capillary and / or the filling material to exert axial traction on the optical fiber when the temperature increases, two embodiments are proposed.

[0080] Either the filler material also adheres to the capillary, and the capillary, through the filler material, provides most of the tension in the optical fiber. In this case, the capillary is chosen to have a coefficient of thermal expansion strictly and significantly higher than that of the optical fiber, although the filler material can have any coefficient of thermal expansion. Or the filler material does not adhere to the capillary, and it alone provides the tension in the optical fiber. In this case, the filler material is chosen to have a coefficient of thermal expansion strictly and significantly higher than that of the optical fiber, although the capillary can have any coefficient of thermal expansion.

[0081] In all cases, the filling material, the capillary and the optical fiber are also chosen according to the following constraints, which a person skilled in the art is able to take into account without demonstrating inventive activity: - the choice not only of the filling material and the capillary, but also of the optical fiber itself, depends on the expected operating temperatures according to the intended application, the materials not having to degrade at the temperatures to which the sensor will be subjected; thus for example, an optical fiber with a protective sheath of polyimide or metal will be preferred to optical fibers sheathed in acrylate for operating temperatures above 300°C;

[0082] - the viscosity of the filling material is preferably sufficiently low, before polymerization, to allow its injection into the capillary and then the placement of the optical fiber in the capillary filled with filling material, or to allow the injection of filling material into the capillary around the optical fiber previously inserted into said capillary;

[0083] - the crosslinking (polymerization) time is preferably as short as possible in order to limit manufacturing time and costs, and the crosslinking method (in particular the curing temperature if crosslinking cannot be considered at room temperature in a reasonable time) must be compatible with the materials used for all the constituent elements of the sensor;

[0084] - given the target operating temperatures, the filling material must be able to undergo thermal expansion without cracking or being destroyed, in order to remain attached to the optical fiber;

[0085] - The axial tension exerted on the optical fiber by the filler material and / or capillary at target temperatures must not be such as to cause breakage of said fiber. Typically, a fiber with a silica waveguide core and a protective cladding of acrylate or polyimide can withstand a maximum elongation of 3%.

[0086] Two series of tests were conducted to evaluate the performance of the sensors according to the invention. Table 1 below lists the components of the sensors in the first series of tests (sensors No. 1 to No. 4), which aimed to assess the suitability of a gel as a filling material, in this case the two-component "Magic Fluid" marketed as an embedding and insulating fluid by Raytech® in a double pouch under reference 100210-mag, combined with two types of conventional fibers and two types of capillaries. Table 2 below lists the components of the sensors in the second series of tests (sensors No. 5 to No. 8), which aimed to assess the suitability of an adhesive as a filling material, in this case a cyanoacrylate adhesive, combined with the fibers and capillaries from the first series of tests.

[0087] As previously mentioned, two types of optical fibers were used. One objective of the invention being to provide sensors from commercially available optical fibers operating at common wavelengths, compatible with commercial interrogators, the fibers used are conventional optical fibers with a silica waveguide core 125pm in diameter.

[0088] The first type of fiber (sensors No. 1, No. 2, No. 5, and No. 6) has a protective polyimide sheath 10 µm thick, giving the optical fiber a total external diameter of 145 µm. It is generally accepted that optical fibers of this type have a coefficient of thermal expansion equal to that of silica, i.e., 0.5 × 10⁻¹⁰⁸⁻¹. 6 / K (the protective polyimide sheath, which is very thin, has little impact on the expansion of the assembly).

[0089] The optical fibers of the second type (sensors No. 3, No. 4, No. 7, and No. 8) have a protective acrylate sheath and a total diameter of 245 µm (i.e., a sheath thickness of 62.5 µm). Again, although thicker (but less heat-resistant), the protective sheath has little impact on the overall coefficient of thermal expansion, and it is generally accepted that the coefficient of thermal expansion for this type of fiber is that of silica, i.e., 0.5 x 10⁻¹⁰⁸⁻¹. 6 / K.

[0090] Note that a silica optical fiber sheathed in metal (aluminum, copper, gold for example) has, on the other hand, a coefficient of thermal expansion equal to (or close to) that of the metal in question.

[0091] Two types of capillaries were used: Inconel® 600 capillaries and AISI 304L stainless steel capillaries, with internal diameters of 600 µm and 700 µm respectively, and both with an external diameter of 1 mm. The coefficient of thermal expansion of Inconel® 600 is 13.3 x 10⁻³ 6 / K between 20°C and 100°C, while that of AISI304L stainless steel is 16.10' 6 / K between 20°C and 100°C and ranges from 17.10 6 / K at 18.5.10 6 / K between 200°C and 700°C.

[0092] Table 1

[0093] Table 2

[0094] It was observed that the Magic Fluid® used in sensors No. 1 to No. 4 adheres correctly to the optical fiber, whether it has an acrylate or polyimide sheath, but does not adhere to the metallic capillary (Inconel® or stainless steel), even escaping from the capillary when it expands if the second end 11 of the sensor is left open. The extent of the elongation experienced by the optical fiber and the geometric changes in the Bragg grating confirmed the axial tensile effect of the gel on the fiber.

[0095] For sensors No. 5 to No. 8 in the second series of tests, the filler material (cyanoacrylate adhesive) adheres to both the optical fiber and the capillary. It was observed that this adhesion is not affected by temperature (sensors with polyimide-coated fibers were tested up to 80°C). In these sensors, it is therefore the capillary that pulls axially on the fiber. Temperature measurements were taken with each of the aforementioned sensors No. 1 to No. 8, using a control sensor made from the same optical fiber incorporating an identical Bragg grating and from the same capillary, but without filler material, the measurement section of the optical fiber being left free inside the capillary.

[0096] Each sensor according to the invention and its control sensor were exposed to the same temperatures, and the spectra obtained with the two sensors were compared. A significantly larger shift in the Bragg peak was observed with the sensors according to the invention compared to their respective control sensors.

[0097] As demonstrated by the tests conducted, the temperature sensors according to the invention exhibit improved sensitivity and resolution, while remaining transparent to state-of-the-art interrogators. In other words, the interrogator sees a conventional Bragg grating but with increased sensitivity. The sensors according to the invention are directly compatible with commercially available interrogators, without the need to add any additional optoelectronic components to these interrogators.

[0098] The invention is not limited to the materials of the tests reported above. By way of example, the capillary can be made of plastic, the filling material can be an epoxy adhesive, etc., provided they exhibit the claimed properties.

[0099] The temperature sensors according to the invention can be used in structural health monitoring applications and, more generally, in any application requiring precise temperature measurement.

Claims

DEMANDS 1. Temperature sensor including - an external detection face intended to be brought into contact with a structure to be monitored, - an optical fiber (3) comprising on the one hand a waveguide core having a section, called the measurement section, in which a Bragg grating (4) is inscribed, and on the other hand a protective sheath which envelops said waveguide core, - a capillary (1) into which the measurement section of the optical fiber (3) is inserted, the capillary having an internal diameter strictly greater than the external diameter of the optical fiber so that an annular space exists between an external face of the optical fiber and an internal face of the capillary over the entire length of the measurement section, - a filling material (2) in the annular space between the optical fiber and the capillary, characterized in that: - the outer face of the optical fiber and the inner face of the capillary are not fused, - the external face of the capillary constitutes the external detection face of the temperature sensor at the measuring section, and in that: - either the filling material (2) and the capillary constituent material (1) are such that the filling material has a coefficient of thermal expansion strictly greater than that of the optical fiber and the filling material adheres to the optical fiber but not to the capillary, - either the filling material (2) and the constituent material of the capillary (1) are such that the capillary has a coefficient of thermal expansion strictly greater than that of the optical fiber and the filling material adheres to both the capillary and the optical fiber.

2. Temperature sensor according to claim 1, characterized in that the protective sheath encloses the waveguide core including on the measuring section.

3. Temperature sensor according to any one of claims 1 or 2, characterized in that the filling material (2) is chosen from polymers which adhere to the optical fiber (3) in the polymerized state and which are sufficiently fluid in the unpolymerized state to allow the optical fiber to slide in the filling material.

4. Temperature sensor according to any one of claims 1 to 3, characterized in that the measuring section of the optical fiber (3) extends from one end (30) of the optical fiber, said measuring end, which measuring end of the optical fiber is located inside the capillary (1) near a first closed end (10) of said capillary, the optical fiber exiting the capillary at a second end (11) thereof.

5. Temperature sensor according to claim 4, characterized in that the first end (10) of the capillary is closed using a dot of glue or cement or by LASER sealing (12).

6. Temperature sensor according to any one of claims 4 or 5, characterized in that the second end (11) of the capillary is closed with a dot of glue or cement (13).

7. A method for manufacturing a temperature sensor comprising an external sensing face intended to be in contact with a structure to be monitored, the manufacturing method using: - an optical fiber (3) comprising on the one hand a waveguide core having a section, called the measurement section, in which a Bragg grating (4) is inscribed, and on the other hand a protective sheath which envelops said waveguide core, - a capillary (1) into which the measurement section of the optical fiber is inserted, the capillary having an internal diameter strictly greater than the external diameter of the fiber so that an annular space exists between an external face of the optical fiber and an internal face of the capillary over the entire length of the measurement section, - a filling material (2) for filling the annular space between the optical fiber and the capillary, characterized in that: - one external face of the capillary constitutes the external detection face of the temperature sensor at the level of the measuring section, - no fusion operation is performed between an outer face of the optical fiber and an inner face of the capillary, and in that: - either the filling material (2) and the capillary constituent material (1) are chosen such that the filling material has a coefficient of thermal expansion strictly greater than that of the optical fiber and the filling material adheres to the fiber but not to the capillary, - either the filling material (2) and the constituent material of the capillary (1) are chosen such that the capillary has a coefficient of thermal expansion strictly greater than that of the optical fiber and the filling material adheres to both the capillary and the optical fiber.

8. Method for manufacturing a temperature sensor according to claim 7, characterized in that the measuring section of the optical fiber (3) is inserted into the capillary (1) without stripping it beforehand.

9. A method for manufacturing a temperature sensor according to claim 7 or 8, characterized in that: - the measurement section of the optical fiber extends from one end (30) of the optical fiber, called the measurement end, - the filling material is chosen from polymers that adhere to the optical fiber in the polymerized state and are sufficiently fluid in the unpolymerized state to allow the optical fiber to slide within the filling material, - the capillary tube is filled with filling material (2) in a fluid state, - one end (10) of the capillary is closed using a dot of glue or cement or by LASER sealing (12), - the measuring section of the optical fiber (3) is inserted into the capillary through its second end (11), - then the filler material is either polymerized or allowed to cure, - Optionally, the second end (11) of the capillary is closed with a dot of glue or cement (13), before or after polymerization of the filling material (2).

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