Temperature sensor with increased sensitivity

Abstract

A temperature sensor comprising: an optical fiber (3) having a segment containing a Bragg grating (4); a capillary (1) surrounding said segment; and a filler material (2) between the optical fiber and the capillary. The filler material (2) has a coefficient of thermal expansion strictly greater than that of the optical fiber and adheres to the fiber but not to the capillary, such that, when it expands due to temperature, the filler material exerts an axial pull on the Bragg grating. Alternatively, the capillary has a coefficient of thermal expansion strictly greater than that of the optical fiber, and the filler material adheres to both the capillary and the optical fiber, such that, when it expands due to temperature, the capillary exerts an axial pull on the Bragg grating through the filler material. Figure for the abstract: Fig. 2

Description

Title of the invention: Temperature sensor with increased sensitivity

[0001] 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 optical fiber-based temperature sensors. Previous art

[0002] Optical fiber sensors incorporating a Bragg grating are known. Such a sensor integrates an optical fiber equipped with a Bragg grating, the Bragg grating being 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.

[0003] 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.

[0004] The accuracy of Bragg grating fiber optic sensors depends directly on two factors:

[0005] - the sensitivity of the Bragg grating to temperature; the variation of the index of The refraction of the Bragg grating with temperature depends on the working wavelength and the material. Once these two parameters are fixed, the curves of variation of the refractive index and therefore of the Bragg peak are fixed; the temperature is deduced directly from the shift of the Bragg peak with respect to the initial working wavelength (which is the Bragg length at the initial temperature, for example room temperature);

[0006] - the sensitivity and resolution of the interrogator.

[0007] Moreover, knowing the sensitivity of the Bragg grating, the element which determines the resolution of the sensor (i.e. the resolution of the temperature measurement) is the interrogator.

[0008] By way of example, considering an interrogator with a resolution of Ipm and a precision of 20pm and silica optical fibers, we obtain the following table. Spectral range Temperature sensitivity of the Bragg grating (silica fiber) Sensor resolution Sensor accuracy 1550 nm 10-15 pm / °C 0.06-0.1 °C 1.33-2 °C 850 nm 6-8 pm / °C 0.125-0.16 °C 2.5-3.33 °C

[0009] 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 each time the temperature changes by one degree. Similarly, the range of 6 to 8 pm / °C obtained for a silica optical fiber and a working wavelength of 850 nm means that the generated peak shifts by approximately 6 to 8 pm for each degree of temperature variation.

[0010] In view of the above, it can be seen that two levers can theoretically be used to improve the sensitivity of the temperature sensor:

[0011] - improve the sensitivity of the Bragg grating by adjusting the wavelength of network work and / or work on the optical fiber material,

[0012] - improve the performance of the interrogator.

[0013] In practice concerning 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 put in place.

[0014] In practice, with regard to the second lever, one quickly encounters technological limits which are difficult or not very relevant to overcome because unfavorable compromises result.

[0015] In the state of the art, interrogators generally have a resolution on the order of Ipm and an accuracy on the order of 20pm.

[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 equivalent also exists for Bragg gratings. It is possible to increase the measurement sensitivity of a Bragg grating by coupling it to 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 means 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 number of networks, spectral positions... The incorporation of such a Vernier effect can also limit the type and number of Bragg gratings that can be used.

[0019] A temperature sensor with an optical fiber and Bragg grating is also known, in which: conventionally, the fiber comprises a glass (silica) core forming a waveguide, surrounded by a peripheral protective 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 10⁶ K⁻¹, while the optical fiber (or more precisely its waveguide core) and the capillary, both made of silica, have a coefficient of thermal expansion of 0.5 x 10⁶ K⁻¹.As the temperature increases, the epoxy resin expands much more than the optical fiber and the capillary. The Bragg grating is then subjected to pressure exerted by the resin under the pressure of the capillary, which is temperature-dependent and leads to a corresponding change in the grating geometry and therefore an additional shift in the Bragg peak, which is added to the shift generated by the effect of temperature on the refractive index of the fiber.

[0020] 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.

[0021] However, this sensor is not without its drawbacks. The absence of a sheath on a portion of the fiber greatly weakens it. Furthermore, the stripping step prior to inscribing the Bragg grating inside the optical fiber complicates and increases the time and cost of manufacturing the sensor.

[0022] Moreover, and most importantly, the sensor's sensitivity remains insufficient for certain applications. The need for sensors with better sensitivity, as well as better resolution, remains entirely unmet. Description of the invention

[0023] The invention aims to provide a temperature sensor having improved sensitivity and / or resolution(s), which can be made 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), without adding complex and expensive software and optoelectronic components to the interrogator.

[0024] To this end, the invention proposes a temperature sensor comprising: - an optical fiber having, 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 that encloses said waveguide core, - a capillary in which the measurement section of the optical fiber is inserted, - a filling material between the optical fiber and the capillary, characterized in that: - 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, - 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.

[0025] Note that the expression "coefficient of thermal expansion" more precisely designates the coefficient of linear thermal expansion of the materials or elements concerned, in the axial direction of the optical fiber.

[0026] In the first case defined above, when the temperature increases, the filling 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.

[0027] It should be noted that in the earlier sensor described in paragraph

[0019] above, the forces generated on the optical fiber by the expansion of the adhesive material (filling material) are essentially radial since this material adheres to the capillary and is therefore constrained axially 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.

[0028] 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 filling material adheres to both the capillary and the optical fiber, the axial expansion of the capillary generates axial forces on the filling material, forces which are transmitted to the optical fiber.

[0029] Thus, in both cases, an increase in temperature results in axial traction of the optical fiber, either by the filling material (first case), or by the capillary (second case), or by both (second case also, if the material the filling material also has a coefficient of thermal expansion strictly greater than that of optical fiber).

[0030] This axial tension causes a significant change in the Bragg grating's registration pitch, which in turn leads to a significant shift in the Bragg peak. This shift is added to the shift resulting from the change in the fiber's refractive index due to temperature. The sensor's sensitivity is thus greatly improved, even if the optical fiber is a conventional silica fiber and is used with a basic, commercially available interrogator.

[0031] According to particular embodiments of the invention, the temperature sensor further meets the following characteristics, implemented individually or according to any technically possible and operational combination.

[0032] 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 constraint is that the filler material adheres to said 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.

[0033] In certain 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 facilitates the manufacture of the sensor.

[0034] In some embodiments, the measurement section of the optical fiber 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.

[0035] The first end of the capillary can be closed using a dot of glue or cement or by LASER sealing.

[0036] Depending on the filling material used, the second end of the capillary can be left open; preferably, this second end is closed with a dot of glue or cement, in order to ensure that the filling material remains confined inside the capillary regardless of the sensor's operating conditions.

[0037] 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 using: - 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 envelops said waveguide core, - a capillary into which the measurement section of the optical fiber is inserted, - and a filling material to fill the space between the optical fiber and the capillary.

[0038] The manufacturing process according to the invention is characterized in that: - 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, - 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.

[0039] In some embodiments, the optical fiber measuring section is inserted into the capillary without first stripping the optical fiber.

[0040] In certain embodiments: - the measurement section of the optical fiber extends from one end of the optical fiber, called the measurement end, - the filling material is chosen from polymers which adhere to the optical fiber in the polymerized state and which are sufficiently fluid in the unpolymerized state to allow the optical fiber to slide in the filling material, - the capillary is filled with filling material in the fluid state; - One end of the capillary tube is sealed using a dab of glue or cement, or by laser sealing. - The measuring section of the optical fiber is inserted into the capillary through its second end. - then the filler material is either polymerized or allowed to cure, - Optionally, the second end of the capillary is closed with a drop of glue or cement, before or after polymerization of the filling material. Brief description of the drawings

[0041] The invention, according to an exemplary embodiment, will be better understood and its advantages will become more apparent upon reading the following detailed description, given by way of example and in no way limiting, with reference to the accompanying drawings in which: • Figure [1] illustrates an example of a temperature sensor according to the invention at different stages of its manufacture, • [Fig.2] represents the sensor of [Fig.1] at the final stage.

[0042] Identical elements represented in the aforementioned figures are identified by identical numerical references.

[0043] Fig. 1 illustrates the different assembly steps of an example of a temperature sensor according to the invention, this sensor being able to be observed fully assembled in Fig. 2.

[0044] The illustrated sensor comprises: - a capillary 1, which may be made of plastic or metal, for example, - 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; - and a filling material 2, filling the volume between the optical fiber 3 and the capillary 1.

[0045] 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.

[0046] A first end 10 of the capillary is sealed using a LASER process or by means of a drop 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 in order to expel the air present in the capillary as the filler material is injected into the capillary, and thus ensure that the capillary is completely filled with filler material. 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 said capillary first in a volume of filler material, and being drawn into the capillary from the other end. capillary, for example using a syringe coupled in a hermetic fashion to the second end of the capillary.

[0047] 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 referred to as the measuring segment.

[0048] Sealing the end 10 of the capillary can be carried out 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 performed can be dictated by the time available before the filler material hardens.

[0049] The second end 11 of the capillary can be closed with a dot of glue or cement 13 in order to prevent the filling material 2 from coming out of the capillary 1 when it expands under the effect of heat, particularly in the case where said filling material is a gel which does not adhere to the capillary and which has a coefficient of thermal expansion greater than that of the capillary.

[0050] As previously explained, according to the invention, the constituent material of the capillary 1 and the filling material 2 are chosen so that the capillary and / or the filling material exert(s) an axial pull on the optical fiber when the temperature increases.

[0051] To this end, the filler material 2 in its polymerized state 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 on 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.

[0052] Furthermore, in order for the capillary and / or the filling material to exert an axial pull on the optical fiber when the temperature increases, two embodiments are proposed.

[0053] Either the filler material also adheres to the capillary, and it is the capillary that provides most of the traction for the optical fiber, via the filler material. In this case, the capillary is chosen so as to have a coefficient thermal expansion strictly and significantly greater than that of the optical fiber, the filling material can have any coefficient of thermal expansion.

[0054] Either the filler material does not adhere to the capillary and it is the filler material alone that provides the traction of the optical fiber. In this case, the filler material is chosen so as to have a coefficient of thermal expansion strictly and significantly greater than that of the optical fiber, the capillary being able to have any coefficient of thermal expansion.

[0055] 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:

[0056] - 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 must not 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;

[0057] - 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;

[0058] - 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 within a reasonable time) must be compatible with the materials used for all the constituent elements of the sensor;

[0059] - taking into account the target operating temperatures, the filling material must be able to withstand thermal expansion without cracking or being destroyed, in order to remain attached to the optical fiber;

[0060] - the axial tension that the filling material and / or the capillary exert(s) on the Optical fiber 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 acrylate or polyimide cladding can withstand a maximum elongation of 3%.

[0061] 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 evaluate the suitability of a gel as The filler 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, was 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 evaluate the suitability of an adhesive as a filler material, specifically a cyanoacrylate adhesive, combined with the fibers and capillaries from the first series of tests.

[0062] As previously stated, two types of optical fibers were used. An 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 having a silica waveguide core with a diameter of 125pm.

[0063] The fibers of the first type (sensors No. 1, No. 2, No. 5 and No. 6) have a protective polyimide sheath 1 Opm thick, the optical fiber consequently having 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. equal to 0.5 x 10⁶ / K (the protective polyimide sheath, whose thickness is very small, having little impact on the overall expansion).

[0064] 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). Here again, although thicker (but not very heat-resistant), the protective sheath has little impact on the coefficient of thermal expansion of the assembly, and it is accepted that the coefficient of thermal expansion of this type of fiber is that of silica, i.e., 0.5 x 10⁶ K.

[0065] Note that a silica optical fiber sheathed in metal (aluminium, 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.

[0066] Two types of capillaries were used, namely Inconel® 600 capillaries and AISI 304L stainless steel capillaries, having an internal diameter of 600pm and 700pm respectively and both having an external diameter of 1mm.

[0067] The coefficient of thermal expansion of Inconel® 600 is 13.3.10 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 to 18.5.10 6 / K between 200°C and 700°C.

[0068] [Tables 1] sensor No. 1 sensor No. 2 sensor No. 3 sensor No. 4 Optical fiber Silica core, polyimide sheath Silica core, polyimide sheath Silica core, acrylate sheath Silica core, acrylate sheath Magic Fluid bic filling material Magic Fluid bic Magic Fluid bic Magic Fluid bic filling component component component component Capillary Inconel® 600 stainless steel AISI 304L Inconel® 600 stainless steel AISI 304L

[0069] [Tables2] Sensor No. 5 Sensor No. 6 Sensor No. 7 Sensor No. 8 Optical fiber Silica core, polyimide sheath Silica core, polyimide sheath Silica core, acrylate sheath Silica core, acrylate sheath Filling material Cyanoacrylate adhesive Cyanoacrylate adhesive Cyanoacrylate adhesive Cyanoacrylate adhesive Capillary Inconel® 600 stainless steel AISI 304L Inconel® 600 stainless steel AISI 304L

[0070] It was observed that the Magic Fluid used in sensors No. 1 to No. 4 adheres correctly to the optical fiber, whether the fiber has an acrylate or polyimide sheath, but that it 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 elongations undergone by the optical fiber and the geometric changes in the Bragg grating confirmed the axial tensile effect of the gel on the fiber.

[0071] 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 a polyimide-sheathed fiber were tested up to 80°C). In these sensors, it is therefore the capillary that pulls axially on the fiber.

[0072] Temperature measurements were carried out with, for each of the previous sensors No. 1 to No. 8, a control sensor made from the same optical fiber incorporating an identical Bragg grating and from the same capillary, but without filling material, the measurement section of the optical fiber being left free inside the capillary.

[0073] 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.

[0074] As the tests carried out have proven, 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 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.

[0075] 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 that they exhibit the claimed properties.

[0076] 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. A temperature sensor comprising: - an optical fiber (3) having 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 encloses said waveguide core, - a capillary (1) in which the measurement section of the optical fiber (3) is inserted, - a filler material (2) between the optical fiber and the capillary, characterized in that: - either the filler material (2) and the material constituting the capillary (1) are 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 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 selected 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 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 that encloses said waveguide core, - a capillary (1) into which the measurement section of the optical fiber is inserted, - a filler material (2) to fill the space between the optical fiber and the capillary, characterized in that: - either the filler material (2) and the material constituting the capillary (1) 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,- either the filling material (2) and the capillary constituent material (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 of 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 any one of claims 7 or 8, characterized in that: - the measuring section of the optical fiber extends from one end (30) of the optical fiber, referred to as the measuring end, - the filling material is chosen from polymers that adhere to the optical fiber in the polymerized state and that are sufficiently fluid in the unpolymerized state to allow the optical fiber to slide in 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 drop 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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