Optical heat flow sensor

Abstract

The invention relates to a sensor (10) for measuring heat flux by means of a distributed temperature measurement, comprising: an optical guide (OG) extending along a Z-axis and having an end (Ex), a tube (TU) in which is inserted a portion of the optical guide defining a sensitive section of the optical guide, a centering device (DC) configured to position the sensitive section at the center of the tube, without any contact between the sensitive section of the optical guide and an internal wall of the tube, the sensitive section of the optical guide comprising a plurality of at least two Bragg gratings (RBi), arranged along the sensitive section, each center (Ci) of a Bragg grating being disposed at a position along Z (zi) distinct from the positions of the other centers, each Bragg grating having a distinct Bragg wavelength (λBi). Figure 1

Description

Title of the invention: Optical heat flux sensor FIELD OF INVENTION

[0001] The present invention relates to the field of optical sensors for measuring heat flux by means of a distributed temperature measurement, i.e. by carrying out several multiplexed temperature measurements at different points of an area to be thermally characterized. STATE OF THE TECHNOLOGY

[0002] Sensors using thermocouples are known to perform a distributed temperature measurement, allowing the characterization of a heat flow between a fluid and the wall of a structure.

[0003] The sensor is typically inserted into a wall (which is therefore perforated to position the sensor), for example, a pipe or the wall of a tank, the wall being in contact, on its "hot surface," with a fluid. The temperature measurement is performed at several points distributed along an axis perpendicular to the local plane tangent to the wall on its "hot surface." The devices performing the local temperature measurement are typically: i) thermocouples, as described in US 8583397, often connected in series to form a thermopile, or ii) resistive probes (impedance measurement), as described in US 2023 / 0400365, or iii) Bragg gratings, as described in FR 2959309.In this latter document, however, the resulting temperature measurement may be biased by friction resulting from differential expansions in the event of contact between the sensitive part carrying the Bragg gratings and the internal part of its housing.

[0004] From the various temperature measurements taken within the wall in which the one-dimensional heat flux sensor is inserted, the heat flux passing perpendicularly through this wall is characterized using an inverse algorithm. This algorithm requires at least two temperature measurements, preferably at least three to minimize errors, with all measurements being distinct in position within the wall's thickness. An inverse conduction algorithm, which calculates a heat flux and the temperature at the wall / fluid interface, typically within the wall in which the sensor is inserted, is described, for example, in the first two documents cited above.

[0005] These sensors have several drawbacks. They are not suitable for all operating conditions (nature, shape and wall thickness, space and access). available to install the sensor and its connectors) and operating conditions (high temperatures, exposure to ionizing radiation).

[0006] They are complex to manufacture, and therefore expensive. The size of the thermocouple-based sensor cannot be easily reduced to a diameter of less than 6 mm because it requires, for three temperature measurements, the very precise positioning and welding of 6 thermocouple microwires, whose orientations must remain perpendicular to the temperature gradient to be measured, these microwires then having to be routed to the output of the sensor located on the side of the "cold surface".

[0007] One object of the present invention is to remedy the aforementioned drawbacks by proposing a sensor for measuring a heat flux based on a distributed temperature measurement which is compact, simple to make and low manufacturing cost, which allows measurement at several points (at least three), which where appropriate is resistant to ionizing radiation, and which is compatible with a remote measurement electronics and associated computing means in a remote and secure area. DESCRIPTION OF THE INVENTION

[0008] The present invention relates to a sensor for measuring heat flux by means of distributed temperature measurement, comprising: • an optical guide extending along a Z-axis and having one end, • a tube into which is inserted a portion of the optical guide defining a sensitive section of the optical guide, said end of the optical guide opening onto the same plane as one end of the tube and being in direct contact with an external medium, • a centering device configured to position the sensitive section at the center of the tube, without any contact between the sensitive section of the optical guide and an internal wall of the tube, • the sensitive section of the optical guide comprising a plurality of at least two Bragg gratings, arranged along the sensitive section, each center of a Bragg grating being arranged at a position along Z distinct from the positions of the other centers, each Bragg grating having a distinct Bragg wavelength.

[0009] According to a first variant the optical guide comprises a bare optical cladding of a single-mode optical fiber and at least one optical core.

[0010] According to one embodiment, the single-mode optical fiber also includes, beyond the sensitive section, a mechanical sheath.

[0011] According to one embodiment, the end of the optical cladding is cleaved at an angle.

[0012] According to another variant the optical guide is a rigid bar.

[0013] According to one embodiment, the material of the bar is sapphire.

[0014] According to one embodiment, each Bragg grating of said plurality is a photo-inscribed Bragg grating of type I or type II.

[0015] According to one embodiment, the Bragg network of said plurality is a point-to-point photo-inscribed Bragg network.

[0016] According to one embodiment, each Bragg grating consists of periodically spaced micromotifs.

[0017] According to one embodiment, Bragg gratings are radially offset.

[0018] According to one embodiment, a part of a first and a second network of Bragg are respectively located at the same position along the Z axis.

[0019] According to one embodiment, Bragg networks are configured to be queried at an order greater than or equal to 2.

[0020] According to one embodiment, each Bragg grating has a length of less than 1 mm.

[0021] According to one embodiment the sensitive section has a length less than or equal to 15 mm.

[0022] According to one embodiment, the sensor according to the invention further comprises an external mechanical housing.

[0023] According to one embodiment, the Bragg gratings are configured so that the Bragg wavelengths have an increasing value towards said end.

[0024] According to one embodiment, the Bragg networks are apodized.

[0025] According to one embodiment, an intermediate zone between the optical guide and the wall The inside of the tube is filled with a lubricating material.

[0026] According to one embodiment, a material of the optical guide is resistant to ionizing radiation.

[0027] The following description presents several embodiments of the device of the invention: these examples are not limiting to the scope of the invention. These embodiments present both the essential features of the invention and additional features related to the embodiments considered.

[0028] The invention will be better understood and other features, objectives and advantages thereof will become apparent from the following detailed description and with reference to the accompanying drawings given by way of non-limiting examples and in which:

[0029] Fig. 1 illustrates a sensor for measuring heat flux by distributed temperature profile measurement according to the invention.

[0030] Figure 2 illustrates the sensor according to the invention in an example of operation, implemented in a wall in contact with a fluid on the side of its "hot surface".

[0031] Figure 3 illustrates an example of sensor implementation according to a first embodiment of the invention in which the optical guide GO comprises a bare single-mode optical fiber, without its protective mechanical sheath in the sensitive area. measurement, and an optical core Co arranged on its axis at its center (case of a single-core fiber).

[0032] Figure 4 illustrates an embodiment of an optical fiber for a sensor according to the first variant in which each Bragg grating extends over the entire radial dimension of the optical core.

[0033] Figure 5 illustrates another embodiment of an optical fiber for a sensor according to the first variant of the invention in which the Bragg gratings located in the optical core are of type III (formed of a succession of micro-vacuums).

[0034] Figure 6 illustrates another embodiment of an optical fiber for a sensor according to the first variant of the invention in which the Bragg gratings located in the optical core are of type III and radially offset.

[0035] Figure 7 illustrates an embodiment of an optical fiber for a sensor according to the first variant of the invention in which the Bragg gratings are of type III, some of these Bragg gratings being radially offset within the optical core, their positions along the Z axis overlapping longitudinally.

[0036] Figure 8 illustrates an embodiment of the bar acting as an optical guide for a sensor according to the second variant of the invention, in which the Bragg gratings are of type III and are radially offset.

[0037] Fig. 9 illustrates an embodiment of the bar for a sensor according to the second variant of the invention, in which the Bragg gratings are of type III and are radially offset, and their positions along the Z axis overlap longitudinally.

[0038] Fig. 10 illustrates an example of the implementation of six type III Bragg gratings for a sensor according to the first variant of the invention.

[0039] Figure 11 illustrates the reflected spectral intensity I of six Bragg gratings, as illustrated in Figure 10, as a function of wavelength.

[0040] Figure 12 illustrates the temperature behavior of one of the sensor's Bragg gratings, as illustrated in Figure 10, and more particularly its temperature measurement hysteresis. DETAILED DESCRIPTION OF THE INVENTION

[0041] The invention relates to a sensor 10 for measuring a heat flux, typically passing through a wall, by means of a distributed temperature measurement along an axis typically normal to the plane tangent to the measurement surface of this wall, as illustrated in [Fig. 1]. [Fig. 2] illustrates the sensor 10 according to the invention in an example of operation, implemented in a wall Pa in contact with a fluid F. The surface of the wall Pa in contact with the fluid F defines a "hot surface", and its opposite surface a "cold surface".

[0042] The sensor 10 includes an optical guide GO extending along a Z axis and having an end Ex. An optical guide is understood to be a structure adapted to guiding light in a given orientation, here along the orientation of the Z axis. In operation the Z axis corresponds to the measurement axis, that is to say it is oriented according to the thermal gradient that we seek to characterize.

[0043] According to a first embodiment, the optical guide comprises the optical cladding (bare, without additional mechanical cladding) of a single-mode optical fiber (FOM) and at least one optical core (Co) of the fiber. In one embodiment, the fiber comprises a single optical core disposed at its center, preferably on its axis. In another embodiment, the optical fiber comprises a plurality of optical cores within the optical cladding.

[0044] The light then propagates in the optical core(s) of the optical fiber.

[0045] By way of example, for a single-mode silica-doped optical fiber, in an application around the wavelength 1.55 pm, the diameter of the optical cladding is typically in the range [40 pm - 150 pm] with an optical core of a diameter of a few micrometers up to about ten micrometers.

[0046] According to a second embodiment, the optical guide GO is a rigid bar, the material of which is typically sapphire (single-crystal Al12O3). The typical diameter of such a bar is on the order of a hundred micrometers (for example, between 40 pm and 1 mm).

[0047] The sensor 10 also includes a protective tube TU into which a portion of the optical guide GO is inserted, the portion inserted in the tube defining a sensitive section of the guide. The end Ex of the GO guide opens onto the same plane P as the end of the tube and is in direct contact with the external environment.

[0048] This external environment, when the sensor is in place in the wall, is that of a fluid F (liquid or gas). The surface of the wall Pa in contact with the fluid F defines a "hot surface," and its opposite surface a "cold surface"; the temperature difference between these two surfaces generates a temperature gradient in the wall, through which the sensor 10, in which it is inserted, measures the heat flux passing. In operation, that is, when the sensor is inserted in the wall, the end Ex of the optical waveguide GO is flush with the hot surface.

[0049] The sensor according to the invention is also applicable to heat exchangers. In general, the sensor according to the invention is applicable to all industrial systems where knowledge of the heat flux is an essential parameter for studying: - The sizing of a device; - The characterization of thermal stress on a component; - The measurement of heat fluxes under specific conditions (mixtures, boiling, condensation); - Thermal fatigue (through knowledge of thermal loading).

[0050] The sensor 10 also includes a DC centering device, for example a centering ring, configured to position the sensitive section at the center of the tube, without any contact between the sensitive section of the guide and the inner wall of said tube. There is thus an intermediate zone ZI between the sensitive section of the optical guide and the inner wall of the tube.

[0051] The internal diameter of the TU tube is adapted to the type of optical guide, which can be of different dimensions (rigid bar, bare optical fiber, ...).

[0052] Optionally, the sensor 10 includes an external mechanical enclosure (EME). The external mechanical enclosure protects the sensor and makes it handleable without unreasonable risk of damage. This external enclosure, or sensor body, can be made of any material suitable for the application (depending on the intended temperature range) and the type of wall in which the sensor is intended to be inserted, referred to as the host wall. Preferably, the external enclosure is made of a material whose thermal properties are identical to, or as close as possible to, those of the host wall in the temperature ranges considered. For a high-temperature application, the material is chosen, for example, from zirconia, alumina, yttrium, magnesia, titanium oxide, or any other temperature-resistant material. For lower temperature ranges, in one embodiment, the enclosure is made of ceramic, glass, or a composite material.

[0053] Furthermore, the sensitive section of the waveguide comprises a plurality of at least two RBi Bragg gratings, indexed i ranging from 1 to n, arranged along the sensitive section. A Bragg grating exhibits a periodic index modulation of the refractive index of the waveguide in which it is located. Each Bragg grating exhibits index modulation along the Z-axis and is configured to reflect an incident beam along the Z-axis into a reflected beam propagating in the opposite direction along the same Z-axis.

[0054] Each medium Ci of a Bragg grating, called a center, is disposed at a position zi along Z, distinct from the positions (zj)j of the other centers (Cj)j. Moreover, each Bragg grating RBi presents, for the same temperature, a different Bragg wavelength XB i in the measurement range of the associated instrumentation.

[0055] In the architecture of the sensor 10 according to the invention, the Bragg gratings are located along the thermal gradient to be measured.

[0056] Bragg gratings are typically photo-inscribed, that is, the refractive index modulation results from a light / matter interaction via illumination with a beam luminous, typically a laser (phase masking process, point-to-point inscription, ...).

[0057] The use of Bragg gratings photo-inscribed in an optical fiber as a temperature sensor is known to those skilled in the art, see for illustration documents FR2959309, EP3864378 and CN117451215, but according to different arrangements of the sensor according to the invention.

[0058] The Bragg grating photo-inscribed in a medium reflects light according to a chosen diffraction order m, so as to correspond to the measurement range of the associated instrumentation.

[0059] The Bragg wavelength XB reflected by the grating is given by the formula:

[0060] , AB ~ m

[0061] where A is the grating pitch, m is the grating order, and neff is the effective index of the optical guide, this relationship differentiating to obtain the Bragg relation used in instrumentation, expressed below in the absence of any hydrostatic pressure effect, according to:

[0062] AT + bx A,_(2)

[0063] with a = 12 pmi 0 C and b = 0.78,

[0064] where AT is the temperature variation of the transducer relative to an arbitrary reference state, and Aemec is its variation of longitudinal mechanical deformation relative to the same reference state.

[0065] The determination of the variation of the Bragg wavelength AXB=XBr-XB ref between this reference value (XB ref) at the reference temperature Tref, and its current value XBr(T) at the temperature T, then allows us to deduce the temperature variation: AT=T-Tref.

[0066] However, as illustrated by relation (2), any variation in mechanical deformation Aemec undergone by the optical fiber introduces a bias on the temperature measurement, which is a difficulty for the accurate temperature measurement, since without additional information it is impossible to separate the two origins, mechanical or thermal, of the variation in the Bragg wavelength, since the transducers are sensitive to these two effects.

[0067] When the sensor is in operation, as illustrated [Fig.2], preferably the optical guide GO is coupled with a forward fiber FOD which allows: • Sending an interrogation beam comprising a plurality of wavelengths or a continuous spectrum in the spectral band of interest [Xmin Xmax], with spectral extent AXint=Xmax-Xmin, • The recovery of light beams reflected by the RBi gratings at different XBli Bragg wavelengths, and propagating in the optical guide, • Sending the beams to a DD detection device to identify the value of these XBri Bragg wavelengths (typically by spectrometry).

[0068] A processing unit UT then performs the processing. First, temperature values ​​Ti(zi), where zi is known precisely, are deduced from the variations in Bragg wavelengths AXi.

[0069] The position zi of each grating is, for example, measured with a microscope and the simultaneous injection of laser light in the visible range, for example using a defect detector. The Bragg grating then acts as a diffracting element, making its position visible from outside the optical fiber.

[0070] For a photo-inscribed Bragg grating, this position can also be known from the photo-inscription process itself, generally employing a displacement stage to position the optical fiber under the photo-inscription beam at the location where each Bragg grating should be.

[0071] Then a heat flux is determined from the Ti(zi). If the temperature TF of the fluid in the vicinity of the hot surface is also measured (by a dedicated sensor not shown in [Fig.2]), a heat transfer coefficient can be determined.

[0072] According to a first option, adapted to n=2, the determination of the heat flux is carried out from the known formula of the gradient.

[0073] According to a second option, the heat flux is determined by inverse calculation using an algorithm known to those skilled in the art, for example, that described in the aforementioned US patents 8583397 and 2023 / 0400365. To minimize error and improve the reliability of the calculation, the value of n, the number of Bragg gratings and therefore of distinct measurement points, is at least 3, and typically a value of n between 3 and 10 is preferred. Thus, according to one embodiment of the invention, n is greater than or equal to 3.

[0074] The sensor structure according to the invention has many advantages compared to thermocouple or resistive probe temperature sensors, and also compared to Bragg grating temperature sensors according to the prior art.

[0075] The sensor according to the invention allows the implementation of several Bragg gratings in the same guide, aligned along the direction of the guide, allowing the multiplexing of a plurality of temperature measurements along the Z axis, which can be read simultaneously and reliably over time.

[0076] The Bragg grating closest to the end (numbered i=l) can be placed very close to the Ex end to perform a temperature measurement very close to the hot surface. All the Bragg gratings, each typically less than a millimeter long (see below), can be concentrated over a short length, for example, less than 1 cm. This increases the density of temperature measurement points over the same length compared to a conventional flux sensor, and consequently allows for a greater number of measurement points for the same wall thickness, even for walls less than 1 cm thick. Thus, in one embodiment, the sensitive cross-section of the optical guide has a length of 15 mm or less.

[0077] Its single-guide structure housed in a tube allows for the creation of a compact, very small, and minimally intrusive sensor, for example, with a diameter, including the outer casing, on the order of a millimeter for the first fiber variant. The compact sensor 10 according to the invention can also be positioned in hard-to-reach areas (e.g., fillets) and / or in small spaces.

[0078] Due to the original structure of the sensor according to the invention, the Bragg gratings have no mechanical interaction with their immediate environment since the sensitive section of the optical guide in which they are photo-inscribed is attached to the sensor body in a region located beyond the last Bragg grating (i=n: the furthest from the end of the optical fiber) via the DC centering device. It should be noted that simple contact can induce friction in the presence of differential expansion, and therefore a longitudinal mechanical force (and consequently a longitudinal mechanical deformation), and subsequently a temperature measurement bias as explained by equation (2).The presence of the centering device, which allows the sensitive part of the optical guide to remain perfectly centered in its tube, ensures that the Bragg gratings are only sensitive to the thermal effects of their environment; any mechanical force exerted on the sensitive part of the optical guide is not transmitted to them. A clearance exists between the tube and the sensitive part of the optical guide, allowing the latter to move relative to the tube without mechanical friction.

[0079] Along its sensitive area, the optical guide is bare, without any coating: there is therefore no additional thermal barrier (other than the very thin film of air surrounding it), nor any mechanical influence from a possible mechanical coating resulting from differential expansions with it.

[0080] The sensor according to the invention has a low thermal inertia, at best equal to that of the wall in which it is housed in the temperature range considered.

[0081] The simplicity of implementation of the sensor ensures a low manufacturing cost.

[0082] Transmission in the optical guide of reflected Bragg wavelengths This process is carried out with minimal loss, as is the transmission through the remote optical fiber where applicable. This allows the optoelectronic measurement unit (detection device DD) and the processing unit (UT) to be relocated to a remote and secure area. This relocation of the DD and UT enables safe access to measurements for personnel and prevents these devices from sustaining irreversible damage related to the sensor's immediate environment, such as ionizing radiation (e.g., measurements in a nuclear environment).

[0083] Depending on the type of guide chosen, the sensor according to the invention can address a wide range of temperatures exceeding 900°C: typically at least up to 900°C for a conventional doped silica optical fiber (first variant) and beyond for a sapphire (A12O3 single crystal) guide (second variant).

[0084] Finally, when calculating the heat flux, the principle of which is based on the pairwise differences of the measurements made by each of the Bragg networks, any measurement biases, since they have a similar effect on each of them, are therefore minimized, or even perfectly compensated, thus strengthening the reliability of the measurement.

[0085] According to one embodiment, the RBi Bragg gratings are configured such that the Bragg wavelengths / .H of said plurality have an increasing value towards the Ex end. For example, i=l is chosen for the Bragg grating closest to the end: XB i > XB i+i. This distribution of Bragg wavelengths makes it possible to have a reflected spectrum with the best signal-to-noise ratio.

[0086] According to one embodiment, the Bragg gratings are apodized, for example by photo-inscribing type III Bragg gratings (see below) at an angle in the optical guide. This avoids disturbing the algorithms used to determine the Bragg wavelengths reflected from the different gratings.

[0087] According to one embodiment, the intermediate zone ZI, between the sensitive section of the optical guide GO and the inner wall of the tube TU, is filled with a lubricating material, which further limits any potential friction between the sensitive section and the tube in case of a risk of misalignment. For example, the zone ZI is lubricated with alumina micro-powder A12O3, the temperature resistance of which allows its use up to more than 1500°C.

[0088] According to one embodiment, the optical guide material is resistant to ionizing radiation. For example, the optical guide is a silica matrix optical fiber resistant to ionizing radiation, known as "RadHard".

[0089] Figure 3 illustrates an example of the implementation of a sensor 10 according to the first embodiment of the invention. The optical guide GO comprises the bare optical cladding GON of a single-mode optical fiber FOM and an optical core Co disposed at its center. The single-mode optical fiber FOM also comprises, beyond the sensitive section, a mechanical cladding GM. Thus, the optical guide consists of the terminal portion of a single-mode optical fiber from which its mechanical cladding has been removed. The five Bragg gratings RB1, ... RB5 are photo-inscribed in the optical core Co of the FOM fiber.

[0090] Single-mode optical fiber is, for example, made of doped silica. The optical fiber is single-mode in order to have only one spectral profile per Bragg grating to be used.

[0091] The tube is for example a commercially available needle tube, the diameter of which is adjusted to the diameter of the optical cladding of the bare fiber, with sufficient clearance so that there is no contact.

[0092] According to one embodiment, the Ex end of the optical cladding is cleaved at an angle. This prevents unwanted reflections and avoids the formation of a Fabry-Pérot cavity, which would result in significant noise in the measurement spectrum. This is particularly important when the first Bragg grating RB 1 is positioned very close to the Ex end of the optical fiber.

[0093] According to one embodiment, the zone 31 between the optical fiber with its mechanical sheath and the external casing of the sensor is embedded in an adhesive or cement, in order to ensure the absorption of forces that may be exerted by the remote cable making the connection to the remote measuring instrumentation.

[0094] The optical fiber with its mechanical sheath can be used to carry out the transmission to the remote measurement system (detection and processing) itself, or can be coupled to a single-mode optical transmission fiber.

[0095] The sensor according to the invention is compatible with any operating wavelength range, the condition being the propagation of the operating wavelength in the guide and the adaptation of the pitch A of the Bragg grating.

[0096] An example is a wavelength range around 1.55 pm. A standard telecom bare cladding diameter is 125 pm, but optical fibers of smaller diameters, such as 80 pm or 40 pm (non-limiting values), can be used to reduce the thermal inertia of the sensor. The optical core of these fibers is typically a few micrometers, for example, 9 pm.

[0097] One advantage is the existence at these wavelengths of commercially available broadband sources, coupled with an associated detection device, which allow, via a The spectrometer identifies the position of the reflection peaks of Bragg gratings in real time. This creates a low-cost sensor.

[0098] Another advantage is that the optical telecom window at 1.55 pm exhibits low spectral attenuation, around 0.2 dB / km, and that at 1.31 pm, an attenuation around 0.34 dB / km. This allows for the remote location of the optoelectronic measurement unit over long distances (multi-kilometer), enabling it to be positioned in areas where it is unlikely to suffer any damage (for example, located outside an area exposed to ionizing radiation, whereas the sensor according to the invention is configured to be able to withstand such radiation).

[0099] For use in a radiative or space environment, according to one embodiment, wavelengths around 1064 nm are chosen. The telecom windows around 1310 nm and 1550 nm are suitable for standard use, as is the window around 850 nm.

[0100] A sensor based on doped silica optical fiber makes it possible to address a temperature range of at least up to 900°C, the maximum temperature being, moreover, also dependent on the type of Bragg grating implemented.

[0101] The operation, characteristics, and advantages of the single-core fiber sensor according to the invention described above are applicable to a multi-core fiber, in which at least two cores comprise Bragg gratings. The advantage of such a fiber is to increase the density of Bragg gratings along the optical fiber, in proportion to the number of cores compared to a single-core fiber.

[0102] According to the second variant, the optical guide of the sensor according to the invention is a rigid bar, typically a sapphire bar, typically with a diameter of about one hundred micrometers.

[0103] In this case, to ensure the sensor's operation (signal interrogation, retrieval, and processing), it is necessary to couple this bar with a remote optical fiber. According to one embodiment, when the bar diameter is sufficiently small, the coupling is achieved via a fusion splice. According to another embodiment, for a bar diameter significantly different from that of the remote optical fiber, the coupling is achieved using a coupling module.

[0104] The advantage of such a sensor is the durability of the sapphire, as well as the photo-inscribed Bragg gratings (see below), which are resistant to very high temperatures, above 900°C, up to 1500°C or even 1800°C. The optical fiber for the remote sensor, not subjected to the high temperatures, can be conventional (for example, germanosilicate, such as that used in optical telecommunications). The sensor housing must be adapted to withstand these temperatures. A body made of tungsten or titanium, or even alumina, can be used.

[0105] The fact that the sensitive part of the optical guide carrying the Bragg gratings is made of sapphire gives it an overall multimode behavior as long as there is no optical core giving it a single-mode behavior, with a measurement error on the order of ten degrees Celsius: in the absence of any other solution beyond 1000°C, and in expectation of strong thermal gradients within the wall whose heat flux we are trying to measure, this error can nevertheless be considered acceptable.

[0106] According to one embodiment, the bar is split at an angle at its end on the side of the "hot surface" to avoid unwanted reflections.

[0107] Different types of photo-inscribed Bragg gratings are compatible with the sensor according to the invention.

[0108] According to an embodiment adapted to the first variant illustrated in [Fig.4], each Bragg grating extends over the entire radial dimension of the Co core. The Bragg gratings are located along the Z axis without overlapping.

[0109] These Bragg gratings are, for example, photo-inscribed using an interferometric method that involves placing the optical fiber in an interference pattern between two light waves (so-called "Type I" gratings). Examples include the so-called "holographic" interferometric method and the interferometric method using a phase mask, both known to those skilled in the art. In another embodiment, Type I gratings extend along a radial dimension smaller than the radial dimension of the core. These Type I gratings exhibit continuous index modulation, for example, sinusoidal in shape.

[0110] According to one embodiment, type II Bragg gratings, photo-inscribed by femtosecond laser, can also be used. They are characterized by continuous lines throughout the core of the optical fiber.

[0111] According to another embodiment, compatible with both variants, each Bragg grating is a point-to-point photo-inscribed grating, typically with a femtosecond laser.

[0112] Typically, such a grating consists of periodically spaced micro-patterns or MM defects, the micropatterns having a refractive index distinct from the rest of the optical guide. Such a grating is called a "Type III" grating, and the micropatterns are usually referred to as "voids." The chain of micropatterns constituting the Bragg grating is created point-by-point by damaging the medium to be inscribed with a focused pulsed femtosecond laser beam. The very intense energy density modifies the refractive index of the medium at the exact location of each micropattern. The micropatterns are typically pseudo-spherical in shape, with a characteristic diameter between 200 nm and 600 nm.

[0113] Fig. 5 illustrates an example of optical fiber for a sensor 10 according to the first variant with type III Bragg gratings made up of such micropatterns.

[0114] One advantage of type III Bragg gratings is their inscription stability at very high temperatures; therefore, they are the preferred type of Bragg grating for applications above 900°C. Another advantage is their resistance to ionizing radiation.

[0115] According to an embodiment made possible by the small size of the micro-motifs constituting the type III Bragg gratings, these gratings are radially offset as illustrated [Fig.6] for a sensor according to the first variant and [Fig.8] for a sensor according to the second variant.

[0116] According to an embodiment illustrated in [Fig. 7] for the first variant and [Fig. 9] for the second variant, a portion of a first and a second Bragg grating are respectively located at the same position along the Z-axis. The gratings are both radially offset and longitudinally overlapped. This allows the centers of the different Bragg gratings to be brought closer together so as to increase the density of measurement points along the sensitive area, in particular to evaluate the temperature profile as reliably as possible as close as possible to the "hot surface".

[0117] Preferably, type III Bragg gratings are configured to operate at an order m greater than or equal to 2, typically 4 or 5. This allows the distance between two neighboring micromotifs to be greater than the typical size of the micromotif itself, so as to prevent these micromotifs from forming a continuous defect within the optical guide, which would then have the effect of breaking the periodicity of photoinscription, with the consequence of the impossibility of obtaining a resonant structure in the spectral domain, of the Bragg grating type.

[0118] Preferably, each type III Bragg grating has a length of less than 1 mm, preferably less than 600 pm (compared to lengths of 4 mm to 12 mm for standard Bragg gratings used in instrumentation), which is possible with this type of Bragg grating, while obtaining a high reflection coefficient.

[0119] Short Bragg gratings also offer the advantage of greater reliability in determining their Bragg wavelength. Indeed, when a long Bragg grating is subjected to a steep temperature gradient, the deformation (broadening) of its spectral profile, under the effect of the thermal gradient, can lead to a splitting of its Bragg peak into two distinct Bragg peaks, each of which can be approximated by an average Bragg wavelength associated with each end of the grating. One way to reduce this bias is therefore to use the shortest possible Bragg gratings, particularly if the temperature gradients are large (for example, greater than 100°C / cm in the wall thickness).

[0120] The short length of the Bragg gratings, combined with the possibility of offsetting them radially in the optical core of the fiber or bar, makes it possible to generate Bragg gratings whose respective centers are, from one point to the next, at most one millimeter apart.

[0121] A sensor can thus be made with about ten measurement points per optical core along a sensitive section of length less than or equal to 15 mm, preferably less than or equal to 10 mm.

[0122] The wavelength sensitivity of this type of Bragg grating is, for example, close to 12 pm / °C at 1550 nm. With a Bragg grating interrogator whose spectral measurement range is 160 nm, more than 10 Bragg gratings can be interrogated simultaneously in a medium whose temperature varies by more than 1000°C.

[0123] Thus, a dozen transducers with an individual length of 600 pm each (photo-inscribed in order 4 or 5 so that the periodic micro-patterns together form a resonant device at a Bragg wavelength located in the measurement range of the interrogator) can be arranged in the same optical core of an optical fiber, over a length of 7 mm to 8 mm, which can thus correspond to the typical thickness of a wall whose heat flux through which one seeks to measure is being measured.

[0124] An example of a sensor according to the invention having six type III Bragg gratings of length 600 pm, made up of “void” type micropatterns and configured to reflect wavelengths in the spectral window extending from 1500 nm to 1600 nm, has thus been made.

[0125] Type III Bragg gratings were photo-inscribed in a single-mode silica-doped optical fiber having an optical cladding diameter of 125 pm and an optical core diameter of 9 pm. The TU tube is a stainless steel needle tube, having an inner diameter of 150 pm and an outer diameter of 650 pm, resulting, in association with the 125 pm diameter GON cladding, in a clearance of 12.5 pm between the cladding and the inner wall of the tube.

[0126] The distribution of type III Bragg gratings in the optical fiber is illustrated [Fig. 10], with the distances D and y expressed in millimeters. Di denotes the distance between the end Ex and the center of the first grating RB1, and Di+i the distance between the center of grating i and that of grating i+1. The first Bragg grating is located very close to the end Ex, allowing a measurement point very close to the hot surface. The Bragg gratings are spaced approximately 1 mm apart in pairs.

[0127] Fig. 11 illustrates the reflected spectral intensity I (in dB) as a function of wavelength X, showing 6 peaks corresponding to the 6 Bragg wavelengths, distributed between 1516 nm (X6B) and 1565 nm (XiB).

[0128] Figure 12 illustrates the temperature behavior of the sensor comprising the Bragg gratings shown in Figure 10. The sensor is placed in an oven from which... The temperature is varied from 0°C to 50°C over several cycles to characterize its hysteresis. For each temperature, the shift AX (in pm) is measured here for the RB2 Bragg grating between the Bragg wavelength reflected at Tref = 17°C and the Bragg wavelength reflected at temperature T, XBr2(T). A very small difference is observed around 15°C-20°C between a first curve 90 and a second curve 91 from a different cycle (the upper curve).

[0129] This difference between two measured values ​​of AX induces a temperature error of AT, shown on curves 92 below, relative to a reference temperature measurement corresponding to the temperature inside the oven. A maximum value of AT of approximately 1°C is measured around 17°C. This value is an upper bound that does not increase with temperature, with a temperature difference of less than 1°C at 30°C. The waveform of the measurement error AT, reproducible from cycle to cycle (mainly due to the Bragg wavelength calculation algorithm, but also to spectral subsampling of the measurement), can also be taken into account during the calibration of the sensor, so as to linearize its behavior within the measurement range considered; the measurement error can thus be reduced to a value of less than 0.5°C throughout the entire measurement range considered.

Claims

1.

2.

3.

4.

5.

6.

7. Demands Sensor (10) for measuring heat flux by means of distributed temperature measurement, comprising: • an optical guide (OG) extending along a Z-axis and having one end (Ex), • a tube (TU) in which is inserted a portion of the optical guide defining a sensitive section of the optical guide, said end of the optical guide opening onto the same plane (P) as an end of the tube and being in direct contact with an external medium, • a centering device (CD) configured to position the sensitive section at the center of the tube, without any contact between the sensitive section of the optical guide and an internal wall of the tube, • the sensitive section of the optical guide comprising a plurality of at least two Bragg gratings (RBi), arranged along the sensitive section, each center (Ci) of a Bragg grating being arranged at a position along Z (zi) distinct from the positions of the other centers, each Bragg grating having a distinct Bragg wavelength (XB i). Sensor according to the preceding claim in which the optical guide comprises a bare optical cladding (GON) of a single-mode optical fiber (FOM) and at least one optical core (Co). Sensor according to the preceding claim in which the single-mode optical fiber also comprises, beyond the sensitive section, a mechanical sheath (GM). Sensor according to one of claims 2 or 3 wherein the end (Ex) of the optical cladding is cleaved at an angle. Sensor according to claim 1 wherein the optical guide is a rigid bar. Sensor according to the preceding claim in which a material of the bar is sapphire. Sensor according to any one of the preceding claims wherein each Bragg grating of said plurality is a photo-inscribed Bragg grating of type I or type II.

8. Sensor according to any one of claims 1 to 6 wherein each Bragg grating of said plurality is a point-to-point photo-inscribed Bragg grating.

9. Sensor according to the preceding claim in which each Bragg grating consists of periodically spaced micropatterns.

10. Sensor according to any one of claims 8 or 9 in which Bragg gratings are radially offset.

11. Sensor according to the preceding claim in which a part of a first and a second Bragg grating are respectively located at the same position along the Z axis.

12. Sensor according to any one of claims 8 to 11 wherein the Bragg gratings are configured to be polled at an order greater than or equal to 2.

13. Sensor according to any one of claims 8 to 12 wherein each Bragg grating has a length of less than 1 mm.

14. Sensor according to any one of claims 8 to 13 wherein the sensitive section has a length less than or equal to 15 mm.

15. Sensor according to any one of the preceding claims further comprising an external mechanical enclosure (EME).

16. Sensor according to any one of the preceding claims wherein the Bragg gratings are configured such that the Bragg wavelengths exhibit an increasing value towards said end (Ex).

17. Sensor according to any one of the preceding claims in which the Bragg gratings are apodized.

18. Sensor according to any one of the preceding claims wherein an intermediate zone between the optical guide and the inner wall of the tube is filled with a lubricating material.

19. Sensor according to any one of the preceding claims wherein a material of the optical guide is resistant to ionizing radiation.

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