Device for measuring the sensitivity of a waveguide to a variation of a physical quantity

Abstract

Device for measuring the sensitivity of a waveguide to a variation of a physical quantity. This device comprises: - a Bragg grating (124) whose power spectrum exhibits at least ten harmonics of order greater than one hundred, each of these harmonics being located at a respective wavelength λk, - a spectral analyzer (20) configured to perform the following steps: - the measurement of a first and a second value of the wavelength λk of each of the harmonics when the value of the physical quantity is equal, respectively, to a first and a second known value, then - the determination of a sensitivity value SG from the difference between the first and second values ​​of the physical quantity and from the first and second values ​​measured for each wavelength λk. Fig. 3

Description

Description Title of the invention: Device for measuring the sensitivity of a waveguide to a variation of a physical quantity

[0001] — The invention relates to a device and a method for measuring the sensitivity Sç of a waveguide with a variation AG of a physical quantity G. The invention relates to also a method of calibrating a device for measuring physical quantity G.

[0002] — Known devices for measuring the physical quantity G comprise a fiber optics in the core of which is made at least one Bragg grating whose fundamental wavelength Ag varies depending on the physical quantity. For convert a variation of the wavelength λ3 into a current value G, of the physical quantity G, a calibration law is used, This calibration law allows to convert each variation AA, measured from the wavelength À5 into a value measured G, corresponding. This calibration law depends on the wavelength λ; and materials used to manufacture optical fiber.

[0003] — To date, to construct this calibration law, the Bragg grating is exposed to large number of G values; known from the physical quantity G and for each of these known values, the value A, ; of the wavelength A, is measured. We thus obtain a set of points with coordinates (A5, G;). Then, the coefficients of a polynomial that passes closest to these points are calculated. It is this polynomial thus calculated which is then used as the calibration law of the measuring device.

[0004] Such a calibration process is long to implement. For example, in the case where the measuring device comprises several length-division multiplexed Bragg gratings wave and made in the same optical fiber, this calibration process must be repeated for each of these Bragg gratings because they each correspond to a length fundamental wave A; different from those of other Bragg gratings.

[0005] To overcome these drawbacks, the inventors proposed to construct the law calibration from measurements of the sensitivity Sç of the optical fiber to variations AG of the physical quantity G. The sensitivity Sç is defined by the following relation AA f\p = Ss*AG, where:

[0006] — - At, is the fundamental wavelength of a Bragg grating made in the fiber optics, and

[0007] — - A, is the variation of the fundamental wavelength of the Bragg grating obtained in response to the variation AG of the physical quantity, and

[0008] — - AG is an infinitesimal variation of the value of the physical quantity G around of a given current value of the physical quantity G. It is emphasized here that this definition is given in the case of an infinitesimal variation of the quantity G around a given current value because this definition then remains true even if the value of the sensitivity Sç varies according to this current value. An infinitesimal variation is a variation ten or a hundred or a thousand times smaller than the width of the range of use within which the value of the physical quantity G varies. In the latter case, it is advantageous to have a precise method for measuring this sensitivity Sc. The invention therefore aims to propose such a method for measuring the sensitivity Sc. The invention therefore relates to a device for measuring the sensitivity Sç of a waveguide to a variation AG of a physical quantity G, the sensitivity Sç for a given value of the physical quantity G being defined by the following relation AAp / A; = Sc*AG, where: - As is the fundamental wavelength of a Bragg grating made in this waveguide, - AÀs is the variation of the fundamental wavelength of the Bragg grating obtained in response to the variation AG of the physical quantity, and - AG is an infinitesimal variation of the value of the physical quantity G around the given value of the physical quantity G, this device comprising: - a Bragg grating produced inside the waveguide, this Bragg grating comprising at least three identical patterns aligned one behind the other along a longitudinal axis of the waveguide and separated from each other by a constant pitch, - an apparatus capable of exposing the Bragg grating to a first known value of the physical quantity and then to a second known value of the physical quantity, these first and second known values ​​of the physical quantity being located on either side of a median value of the physical quantity, and - a spectral analyzer capable of detecting, in a predetermined working range between 200 nm and 5000 nm, the value of the wavelength at which a power peak appears in the reflection power spectrum of the Bragg grating in which: - the pitch of the Bragg grating is configured so that the power spectrum of this Bragg grating has at least ten harmonics of order higher than one hundred within the working range, each of these harmonics being located at a respective wavelength λ,, where k is the order number of this harmonic, - the spectral analyzer is configured to perform the following steps: - when the value of the physical quantity is the first known value, the measurement of a first value of the wavelength λ, of each of the harmonics located at inside the working range, then - when the value of the physical quantity is the second value, the measurement of a second value of the wavelength λ, of each of these harmonics located within the working range, then - the determination of the value of the sensitivity Sç for the median value of the physical quantity from the difference between the first and second values ​​of the physical quantity and from the first and second values ​​measured for each wavelength λ, Embodiments of this sensitivity measuring device may include one or more of the following features: 1) The waveguide is an optical fiber. 2) - each pattern of the Bragg grating extends mainly in a plane, called the "pattern plane", perpendicular to the longitudinal axis of the waveguide, and - each pattern consists of one or more bubbles arranged next to each other in the plane of the pattern, and - the area of ​​the orthogonal projection of all bubbles of the pattern onto the pattern plane is less than 50% of the cross-sectional area of ​​the waveguide core. 3) each pattern consists of several disjointed bubbles arranged next to each other in the plane of the pattern. 4) - the pitch of the Bragg grating is greater than or equal to 20 um, and - the difference between the refractive index of the waveguide core and the refractive index of each Bragg grating pattern is greater than 0.3. 5) each pattern is made using a pulse from a femtosecond laser. 6) the physical quantity is chosen from the group consisting of a temperature, a mechanical deformation and a hydrostatic pressure. The invention also relates to a method for measuring the sensitivity Sç of a waveguide to a variation AG of a physical quantity G, the sensitivity Sç for a given value of the physical quantity G being defined by the following relation AÀp / A; = Sc*AG, where: - A3 is the fundamental wavelength of a Bragg grating made in the waveguide, - Aks is the variation of the fundamental wavelength of the Bragg grating obtained in response to the variation AG of the physical quantity, and - AG is an infinitesimal variation of the value of the physical quantity G around the given value of the physical quantity G, this process comprising the following steps: a) the production of a Bragg grating inside the waveguide, this Bragg grating comprising at least three identical patterns aligned one behind the other along a longitudinal axis of the waveguide and separated from each other by a constant pitch, (b) exposing the Bragg grating to a first known value of the physical quantity and then to a second known value of the physical quantity, these first and second known values ​​of the physical quantity being located on either side of a median value of the physical quantity, in which: -during step a), the step of the Bragg grating produced is configured so that the power spectrum of this Bragg grating has at least ten harmonics of order greater than one hundred within a working range between 200 nm and 5000 nm, each of these harmonics being located at a respective wavelength λ,, where k is the order number of this harmonic, and - the process also includes the following steps: c) when the value of the physical quantity is the first known value, the measurement of a first value of the wavelength λ, of each of the harmonics located within the working range, then d) when the value of the physical quantity is the second value, the measurement of a second value of the wavelength λ, of each of these harmonics located within the working range, then e) determining the value of the sensitivity Sç for the median value of the physical quantity from the difference between the first and second values ​​of the physical quantity and from the first and second values ​​measured for each wavelength λ, Embodiments of this sensitivity measurement method may include one or more of the following features: 1) - repeating steps b) to €) by replacing the first and second values ​​of the physical quantity with other different values ​​of the physical quantity in order to determine the sensitivity Sa for several different median values ​​of the physical quantity, these median values ​​of the physical quantity being distributed over a predetermined range of use, then - the construction, over the range of use, of a law of evolution of the value of the sensitivity Sç as a function of the value of the physical quantity. 2) in step e), the value of the sensitivity Sç is taken equal to a value Sca which minimizes the sum of all the deviations Sc,ax— Sc,as WHERE Sc,ax Is a coarse value of the sensitivity Sç calculated from the difference between the first and second values ​​of the physical quantity and only from the first and second values ​​measured for the harmonic of order k. The invention also relates to a method for calibrating a device for measuring a physical quantity G which varies within a predetermined range of use, this device comprising: - a waveguide containing a core which extends along a longitudinal axis and inside which an optical signal guided by this waveguide is capable of propagating along the longitudinal axis of the waveguide, - a Bragg grating produced in the core of the waveguide, this Bragg grating comprising a reflection power spectrum exhibiting at least one power peak at a wavelength λ which varies according to the physical quantity to be measured so that a variation in this wavelength is representative of a variation in the value of this physical quantity, wherein the method comprises the following steps: 1) recording a law of evolution of the value of a sensitivity Sç of the waveguide as a function of the physical quantity to be measured, this law of evolution associating a value of the sensitivity Sc with each value of the physical quantity to be measured contained in the predetermined range of use and this law of evolution being constructed by implementing the above method of measuring the sensitivity Sc, 2) the acquisition of a reference value As,9 of the wavelength reflected by the Bragg grating when the value of the physical quantity is equal to a known reference value G, then 3) the calculation, for each of the values ​​G; contained in a predetermined set of several values ​​G; chosen within the range of use, of a corresponding value À; of the wavelength À using the following relation: (À; — à,) / A, = Sc; *(G,-G,), where: - Sci is the value of the sensitivity Sç of the waveguide associated with the chosen value G; by the recorded evolution law, and - À, is the value of the wavelength À when the value of the physical quantity G is equal to a value G, known, then 4) from the values ​​G;, the values ​​À; calculated for each of these values ​​G; and the values ​​Ag, and Gp of reference acquired, the construction of a calibration law which associates with each value of the wavelength À, a corresponding value of the physical quantity. Embodiments of this calibration method may include one or more of the following features: 1) the method involves recording the calibration law constructed in a memory of the device for measuring the physical quantity G as a calibration law used, by this measuring device, to establish the value of the physical quantity measured from a determined value of the wavelength λ. 2) the method comprises, after several measurements of the physical quantity by the measuring device, the repetition of steps 2) to 6) to construct a new calibration law and then the recording of this new calibration law in the memory of the device for measuring the physical quantity G as a calibration law used, by this measuring device, to convert each new measured value of the wavelength λ into a measured value of the physical quantity. 3) - for values ​​of the index i greater than zero, the calculation of the value Δ; using the relation (Δ; — Δ,) / Δ, = Sa,*(G;-G,) is carried out by taking the values ​​Δ, and G, equal, respectively, to values ​​Δ;., and G;.1, WHERE Δ;, is a previously calculated value corresponding to the value G;., closest to the value G,, and - for the index i equal to zero, the values ​​À; and G; are equal, respectively, to the acquired values ​​Àg,o €t Gp. The invention also relates to a device for measuring a physical quantity G which varies within a predetermined range of use, this device comprising: - a waveguide containing a core which extends along a longitudinal axis and inside which an optical signal guided by this waveguide is capable of propagating along the longitudinal axis of the waveguide, - a Bragg grating produced in the core of the waveguide, this Bragg grating comprising a reflection power spectrum exhibiting at least one power peak at a wavelength λ which varies according to the physical quantity to be measured so that a variation in this wavelength is representative of a variation in the value of this physical quantity, - a spectral analyzer comprising a memory in which is recorded a calibration law which associates with each value of the wavelength λ, a corresponding value of the physical quantity, this spectral analyzer being capable: - to measure the spectral response of the Bragg grating, then - to determine the value of the wavelength λ of the Bragg grating from the measured spectral response, then - to establish the value of the measured physical quantity from the determined value of the wavelength λ using the recorded calibration law, in which: - the spectra analyzer memory also contains: - a law of evolution of the value of a sensitivity Sc of the waveguide, this law of evolution associating a value of the sensitivity Sç with each value of the physical quantity to be measured contained in the predetermined range of use and this law of evolution being constructed by implementing the above method of measuring the sensitivity S., and - a reference value Às,9 of the wavelength reflected by the Bragg grating when the value of the physical quantity is equal to a known reference value Gp, and - the spectral analyzer is configured to: - calculate, for each of the values ​​G; contained in a predetermined set of several values ​​G; chosen within the predetermined range of use, a corresponding value À; of the wavelength À using the following relation: (À; — À À, = Sai*(G;-G,), where: - Sa; is the value of the sensitivity Sç of the waveguide associated with the chosen value G; by the recorded evolution law, and - À, is the value of the wavelength À when the value of the physical quantity G is equal to a value G, known, then - from the values ​​G;, the values ​​À; calculated for each of these values ​​G; and the values ​​Az, and Gp contained in its memory, construct the calibration law, then - record the constructed calibration law in the memory as the calibration law used to establish the value of the measured physical quantity from the determined value of the wavelength at. The invention will be better understood on reading the description which follows, given solely as a non-limiting example and made with reference to the drawings in which: - [Fig.1] is a schematic illustration of the architecture of a device for measuring a physical quantity G, - [Fig.2] is a flowchart of a method for calibrating the device of [Fig.1] and measuring the physical quantity G using this device of [Fig.1], - [Fig.3] is a schematic illustration of a device for measuring the sensitivity of a waveguide to a variation AG of the physical quantity G, - [Fig.4] is a schematic illustration, partial and in longitudinal section, of a Bragg grating used in the device of [Fig.3], - [Fig.5] is a schematic illustration, in cross-section, of a pattern of the Bragg grating of [Fig.4], - [Fig.6] is a graph representing a portion of the power spectrum of the Bragg grating of [Fig.4], - [Fig.7] is a flowchart of a manufacturing process for the Bragg grating of [Fig.4], - [Fig.8] is a flowchart of a method for measuring the sensitivity of a waveguide to a variation AG of the physical quantity G using the device of [Fig.3]. In these figures, the same references are used to designate the same elements. In the remainder of this description, the characteristics and functions well known to those skilled in the art are not described in detail. In this description, detailed examples of embodiments are first described in a chapter I with reference to the figures. Then, in a chapter II, variants of these embodiments are introduced. Finally, the advantages of the different embodiments are specified in a chapter II. Chapter I: Example of embodiments [Fig. 1] represents a device 2 for measuring a physical quantity G. For this purpose, the device 2 comprises a Bragg grating 4. The grating 4 is exposed to variations in the physical quantity G. In this exemplary embodiment, the device 2 is described in the particular case where the physical quantity G is measured at a single location and therefore in the particular case where the device 2 uses a single Bragg grating. Network 4 transforms a variation of the physical quantity G into a displacement of a power peak of its power spectrum. In this text, unless otherwise indicated, the term "power spectrum" or "spectrum" refers to the reflected power spectrum. The reflected power spectrum is the power spectrum of the optical signal reflected by an optical component. A peak in the reflected power spectrum corresponds to an absorption line in the transmitted power spectrum of the same optical component. The power spectrum of the network 4 comprises, for example, a single power peak in a predetermined working range. This working range has a width greater than 5 nm. Typically, its width is also less than or equal to 200 nm or 120 nm. The working range is located within the optical domain. The optical domain designates the range containing the wavelengths usually used in optics. More precisely, in this text, the optical domain designates the range which extends from 200 nm to 5000 nm and, preferably, from 400 nm to 2000 nm. Here, the power peak of the 4-array is located, in the working range, at the fundamental wavelength λ of the 4-array. The fundamental wavelength λ corresponds to the fundamental frequency fs of this 4-array. In this text, the fundamental wavelength λ of a Bragg grating is the wavelength corresponding to the fundamental frequency fg of resonance of this Bragg grating. This wavelength λ is defined by the following relation: λ = 2*n*λ, where: - n, is the effective index of the waveguide in which this Bragg grating is made, - A is the pitch of the Bragg grating, and - the symbol “*” denotes the scalar multiplication operation in this text. The effective propagation index n, is also known as the "mode phase constant". It is defined by the following relation: n, = n, - Adn, / d), where n, is the group index and λ is the wavelength of the optical signal guided by the waveguide. The effective propagation index of a waveguide depends on the dimensions of the core of this waveguide and the materials forming this core and the optical cladding of this waveguide. It can be determined experimentally or by numerical simulation. The wavelength λ varies classically according to the following physical quantities to which the grating 4 may be exposed: temperature, longitudinal deformation and hydrostatic pressure. A “longitudinal deformation” here designates a deformation of the grating 4 which stretches or shrinks it in a direction parallel to the direction of propagation of the optical signal passing through it. Thus, the grating 4 makes it possible to measure one of these physical quantities. Subsequently, the device 2 is described in the particular case where the physical quantity G measured is temperature. The network 4 is produced in a waveguide 14. The network 4 is optically connected to an input / output port 10 of an optical coupler 12 via the waveguide 14. The optical coupler 12 comprises: - an input port 16 optically connected to an output port 18 of a spectral analyzer 20 via a waveguide 22, and - an output port 24 optically connected to an input port 26 of the spectral analyzer 20 via a waveguide 28. In this embodiment, all of the above waveguides are respective optical fibers. Thus, hereinafter, the same reference numerals are used to designate the waveguide or the optical fiber. Here, the optical fibers used are single-mode optical fibers in the working range. These optical fibers are also known by the acronym SMF ("Single Mode Fiber"). The spectral analyzer 20 is capable of measuring the spectral response of the network 4 and then establishing the current value G of the physical quantity G from this measured spectral response. For this, it comprises: - an optical source 50 comprising an output port 54 optically connected to the output port 18, - an optical sensor 62 optically connected to the input port 26 to measure the power of the optical signal received on this input port, and - an electronic processing unit 70 electrically connected to the sensor 62 to receive the electrical signal representative of the power of the optical signal measured by the sensor 62. In this embodiment, the source 50 is a tunable laser source. This source 50 emits, in the direction of the grating 4, a single-frequency optical signal via the port 54. The wavelength λ of the emitted optical signal is in the optical domain. The value of the wavelength λ depends on a control signal received on a control port 66 of the source 50. More precisely, the wavelength λ is linked to the value of the control signal by a transfer function which, to each value of the control signal, associates a corresponding value of the wavelength λ. Such a source 50 is also called a “scanning laser source”. Indeed, by using an appropriate control signal, the wavelength λ scans the entire working range. This appropriate control signal is, for example, generated by the unit 70.Here, the working range is a wavelength range that extends from a wavelength λ,nin to a wavelength λ,nax. In this embodiment, the width of the working range and its limits are typically imposed by the characteristics of the source 50. The width of the working range is equal to the difference λ,mux - Asmin-. As an example, here, this working range extends from 1460 nm to 1620 nm. The sensor 62 measures the optical signal backscattered by the grating 4. For example, the sensor 62 is a photodiode. The sensor 62 has a spectral observation range that encompasses the working range. In this exemplary embodiment, the unit 70 is notably configured to: - determine a value A, of a variation AÀ of the wavelength of the peak of the network 4 with respect to a reference value As, from the spectral response of the network 4 measured by the sensor 62, then - establish a current value G, of the physical quantity G from the determined value AA. The variation AÀ is defined by the following relation: AÀ = À, — Ag 9, WHERE: - À, is the current value of the wavelength À; of the network 4 when the value of the physical quantity G is equal to the current value G,, and - the A5 values, is the value of the wavelength λ; of the network 4 when the value of the physical quantity G is equal to a known value Gp, of reference. To carry out these operations, the unit 70 comprises a programmable microprocessor 72 and a memory 74 containing the data and instructions necessary for the operation of the spectral analyzer 20. In particular, the memory 74 includes the reference values ​​Ag, and Gp. In addition, the memory 74 includes a calibration law 76 which makes it possible to establish the current value G. of the physical quantity G from the determined value Δ. of the wavelength at which the peak of the grating 4 occurs. For this, in this exemplary embodiment, the calibration law 76 makes it possible to establish the current value G, of the physical quantity G at from the value AÀ, determined from the variation AÀ. Thus, here, the law 76 is a pre-recorded function which accepts as input parameter the value AÀ, determined and which returns, in response, the current value G, corresponding to this value A), For example, in this embodiment, the law 76 comprises for this purpose a table 78 which associates with Ns possible values ​​AÀ; of the variation AÀ, a corresponding value G; of the physical quantity G. The index i is an identifier of the value G;. The number N,, is the number of points of coordinates (A), G;) contained in the table 78. The number N-6 is greater than three and, preferably, greater than ten or thirty or one hundred. Typically, the coordinate points (AÀ,, G;) are uniformly distributed over the entire operating range [Gyins Gmax] of the measuring device 2, where G,zin and Gznux are, respectively, the minimum value and the maximum value of the physical quantity G which can be measured by the device 2.These values ​​Gyin and Goax are predefined values, typically, by the manufacturer of the device 2. Here, the number N,5 is determined so that the difference between two immediately consecutive G; values ​​is less than 10°C or 1°C. Here, the G; values ​​are classified in ascending order so that, regardless of the index i greater than zero, the G; value is between the values ​​G;, and G;,1. . In this embodiment, if the value AÀ, provided as an input parameter of the law 76 is located between two values ​​AÀ; contained in the table 78, the law 76 determines the value G, to be returned by linear interpolation between the two points of the table 78 closest to the value AÀ. provided. Furthermore, in this embodiment, the memory comprises a law Sc(G) for the evolution of the sensitivity Sç of the fiber 14 as a function of the current value G, of the physical quantity G. As explained with reference to the method of [Fig.2], the law Sc(G) makes it possible to calibrate the device 2. The law Sc(G) is a function that returns the current value Se, of the sensitivity Sc corresponding to the current value G, of the physical quantity G provided as an input parameter of this law. For example, for this purpose, the law S,(G) includes a table 80 which associates with Ng possible values ​​G, of the physical quantity G, the corresponding value Sy, of the sensitivity Sc, where the index d is an identifier of the value G4. The number Ng is the number of coordinate points (G4, Sa) contained in the table 80. The number Nç is greater than two or three and, preferably, greater than four or ten. Typically, the number Ng is less than the number N75 and, preferably, two or four or ten times less than the number N75. The values ​​Gq are not necessarily the same as the values ​​G; of the physical quantity G. Preferably, the coordinate points (Ga, Sc) are uniformly distributed over the entire range of use [G,mins Gmax] of the measuring device 2. Similar to what was described in the case of law 76, here, if the current value G, provided as an input parameter of law 76 is located between two values ​​G, contained in table 80, the law Sc(G) determines the value Sc. to be returned by linear interpolation between the two points of table 80 closest to the value G, provided. The unit 70 is also connected to a human-machine interface 82 to communicate the result of the measurements carried out to a human being and, alternately, acquire new reference values ​​Ap,, Gp. Finally, preferably, the device 2 comprises an insulating structure 90 which isolates the network 4 from variations in other physical quantities which can cause its wavelength λp to vary. Thus, here, the structure 90 is arranged to maintain the longitudinal deformation of the network 4 and the hydrostatic pressure constant. For example, the structure 90 is a housing which mechanically isolates the network 4 from the mechanical stresses exerted by the external environment on the network 4. The operation of device 2 will now be described with reference to the method of [Fig.2]. The process of [Fig.2] begins with a phase 100 of calibration of the device 2. Phase 100 begins with a step 102 during which the unit 70 acquires the reference values ​​Ag and Gp. The reference values ​​Ag and Gp are, for example, obtained by placing the grating 4 in a bath at the temperature Gp and then measuring, using the unit 70, the value λ, corresponding to the wavelength λ of the grating 4. The value Gp is ​​for example acquired via the human-machine interface 82. Then, during a step 104, the unit 70 selects, within the range [Grin 3 Gmaxl] the N75 values ​​G;. Here, these values ​​G; are those already contained in the table 78. Then, for each of these values ​​G,, the unit 70 calculates the corresponding value Δ; of the wavelength Δ and the corresponding value A); of the corresponding variation Aù. The value Aà; is calculated using the following relation: A); = Δ; - Δp,0. Each pair of values ​​A), G; thus calculated forms a point with abscissa A; and ordinate G;. For this, for each value G,, the unit 70 calculates, in order of increasing index i, the corresponding value Δ; using the following relation: (Δ; - Δ,) / A, = Se.:*(G;-G,), where: - À and G, are equal, respectively, to À;, and G;;, A; being the value of the wavelength À calculated, during the previous iteration, for the value G;, of the physical quantity G, and - Sci is equal to Sc(G;). The above iterative calculation is initialized by taking Ày = Ap,0, WHERE Ap,o and Gp are the reference values ​​acquired in step 102. Since the values ​​G; are ranked in order of values, G;.; is the value closest to the value G; for which the value À; has already been calculated in this step 104. This improves the accuracy of the calculation of the value À, Sc(G;) is the value of the sensitivity of fiber 14 associated with the value G; by the law Sc (G) recorded in memory 74. Then, during a step 106, the unit 70 constructs a calibration law from the coordinate points (A); G;) and then records it in the memory 74 as a calibration law 76. For this, here, the unit 70 records, in the table 78, each point of coordinates (A};; G;) calculated during the step 104. From this moment, phase 100 of calibration of device 2 is completed and a phase 110 of measurement of the physical quantity G begins. During phase 110, network 4 is exposed to the physical quantity G to be measured. Then, during a step 112, the unit 70 controls the source 50 to vary linearly, over time, the wavelength λ, from the wavelength λ,min to the wavelength λ,pux. To this end, the unit 70 sends to the source 50 a control signal generated from an estimate of the transfer function of the source 50. The optical signal emitted by the source 50 is guided by the coupler 12 and the optical fibers 22 and 14 to the grating 4. The grating 4 then reflects a part of the incident optical signal. This reflected part of the optical signal corresponds to the signal backscattered by the grating 4. In parallel with step 112, during a step 114, the sensor 62 measures the optical signal backscattered by the network 4. More precisely, the sensor 62 generates an electrical signal whose amplitude is representative of the power of the measured optical signal. The electrical signal generated by the sensor 62 is transmitted to the unit 70 which acquires it. Once the electrical signal has been acquired by the unit 70, during a step 116, the unit 70 determines the value AÀ, of the variation AÀ. When the wavelength λ is equal to the wavelength λ of the grating 4, the power of the optical signal backscattered by the grating 4 passes through a maximum. Since the wavelength λ varies linearly as a function of time, the instant t at which this maximum occurs is proportional to the current value λ of the wavelength λ at this instant t. Similarly, the reference value λ of the wavelength λ corresponds to a reference instant ty. During step 116, the unit 70 calculates the difference between the measured instant t and the reference instant ty. Since the variation of the wavelength λ over time is linear, the difference t-t is proportional to the current value Aλ of the variation Aλ. The coefficient of proportionality between the deviation t.-t, and the current value AÀ, is equal to the slope à, of the line representing the evolution over time of the wavelength À,. This slope à is known since the generated control signal is known.Thus, during step 116, the unit 70 determines the current value AÀ, of the variation AÀ from the difference measured between the instants t, and to. Then, during a step 118, the unit 70 establishes the current value G. of the physical quantity G from the current value AÀ, determined during step 116. For this, the unit 70 uses the calibration law 76 currently stored in the memory 74. More precisely, during step 118, the unit 70 obtains, using the law 76, the value G, of the physical quantity associated with the value A, provided as an input parameter to this law 76. This value G, is then considered to be the measured value of the physical quantity G. Each measured value G, of the physical quantity G as well as the time at which it was measured are recorded in a file and / or transmitted to the interface 82 which displays the measured values ​​in response. Typically, steps 112, 114, 116 and 118 are repeated at regular intervals to measure the evolution over time of the physical quantity G. The precision of the measurement of the physical quantity G depends, in part, on the accuracy of the law Sç(G) used by the device 2 and therefore on the precision with which the sensitivity Sç of the fiber 14 is measured for different values ​​G4 of the physical quantity. [Fig. 3] shows a device 120 for measuring the sensitivity Sç of the fiber 14. The device 120 is identical to the device 2 except that the fiber 14 and the network 4 are replaced, respectively, by a fiber 122 and a network 124. In addition, unlike the device 2, the memory 74 of the device 120 is devoid of the laws 76 and Sc(G). Instead, the memory 74 contains the instructions and data necessary to execute the method of [Fig. 8]. Finally, unlike device 2, device 120 comprises an apparatus 126 which makes it possible to expose network 124 to the desired temperature. For example, for this, apparatus 126 comprises: - a liquid bath 128 inside which the portion of the fiber 122 comprising the network 124 is immersed, - a controllable heating or cooling element 130 capable of heating or cooling the bath 128, - a temperature sensor 132 arranged to measure the temperature of the bath 128, and - a microcontroller 134 configured to control the element 130 as a function of a temperature setpoint Tc and the temperature measured by the sensor 132 to limit the temperature variations of the network 124 around this setpoint Tc. The element 130 is for example a Peltier module or a set of several Peltier modules. The microcontroller 134 comprises a programmable microprocessor and a memory containing the data and instructions necessary to control the temperature of the bath 128 to the setpoint Tc. In particular, this memory comprises the setpoint Tc. The microcontroller 134 is connected to the unit 70 so that the unit 70 can modify the value of the setpoint Tec. Fiber 122 is identical to fiber 14 except that the grating made in the core of this fiber 122 is grating 124 instead of grating 4. Grating 124 is placed in the same conditions of use as grating 4. In particular, the longitudinal deformation and hydrostatic pressure to which grating 124 is exposed are the same as those to which grating 4 is exposed. Thus, the wavelength λ of grating 124 moves here only as a function of temperature. The 124 grating is a very high order Bragg grating. A very high-order Bragg grating and its fabrication process are described in the following article: Pengtao Luo et al: “Femtosecond laser plane-by-plane inscribed ultrahigh-order fiber Bragg grating and its application in multi-wavelength fiber lasers”, Optic letter, 15 / 06 / 2022. Hereinafter this article is referred to as “LUO2022”. In this text, "very high order" refers to the fact that the reflection power spectrum of the Bragg grating has discernible harmonics of order higher than N in the optical domain, where N is an integer greater than 100 and, preferably, greater than 500 or 1000. In other words, in the reflection power spectrum of a very high order Bragg grating, there are harmonics of order k, higher than N, which each correspond to a power peak distinct from the peaks corresponding to the harmonics of orders k-1 and k+1. This peak of order k is also higher than the noise. This peak of order k is located at the wavelength λ, defined by the following relation (1): λ = 2*n,* λ / k, where: - k is an integer equal to the order of the harmonic, - n, is the effective index of the optical fiber, and - A is the pitch of the Bragg grating. This k-order peak is in the optical domain. The power spectrum of a very high order Bragg grating comprises a succession of very close and very narrow peaks in a wavelength range of interest of at least 100 nm in width in the optical domain. Moreover, the heights of these peaks are approximately the same over this range of at least 100 nm in width because each of these peaks corresponds to a very high order harmonic. In other words, the power spectrum of a very high order Bragg grating is a comb of peaks. An example of such a comb is shown in [Fig.3] of the article LUO2022. It is emphasized that a very high order Bragg grating is distinguished from standard Bragg gratings commonly used in the field of optics by several characteristics. In standard Bragg gratings, the pitch of the standard Bragg grating is chosen to: - that wavelength A; either in the optical domain, or - that only the first harmonics of order lower than twenty are in the field of optics. Thus, the pitch of these standard Bragg gratings are systematically less than 50 um or 20 um and, generally, even less than 10 um. Under these conditions, the standard Bragg grating cannot be a very high order Bragg grating. Indeed, in this case, even if harmonics of order k greater than one hundred are discernible in its power spectrum, the wavelength λ of these harmonics is not in the optical domain. In other words, the wavelengths λ of harmonics of order k greater than one hundred are all less than 200 nm. Conversely, the pitch of a very high order Bragg grating is greater than 20 um or 50 um and, often greater than 100 um. Under these conditions, the wavelength λ; of the very high order Bragg grating and the wavelengths of its harmonics of order less than one hundred, are not in the domain of optics. Standard Bragg grating patterns are commonly fabricated using ultraviolet radiation pulses or CO lasers, not femtosecond laser pulses. Bragg gratings fabricated without femtosecond laser pulses exhibit only discernible harmonics of order less than twenty. This appears to be because the refractive index changes in the optical fiber obtained using these other known methods are much less pronounced than those obtained using a femtosecond laser. Thus, a Bragg grating fabricated without femtosecond laser pulses, even if it has a pitch greater than 20 μm or 50 μm, is not a very high-order Bragg grating. It is also emphasized that a Bragg grating should not be confused with a juxtaposition, along an optical fiber, of Fabry-Perot cavities. Indeed, the spectral characteristics of an optical fiber comprising such a juxtaposition of Fabry-Perot cavities depend on the lengths of each Fabry-Perot cavity as well as the reflectivity of the diopters located at each end of each Fabry-Perot cavity. Unlike a Bragg grating, the diopters are not spaced from each other at a constant pitch to form a periodic structure. Bragg gratings are also frequently used in the field of laser sources to form the end diopters of a Fabry Perot cavity of this laser source. In this case, the spectral response of this cavity is mainly determined by the length of the cavity and not by the spectral characteristics of the Bragg gratings used. More precisely, as taught in article LUO2022, the spectral characteristic of the Bragg gratings is then used to adjust the wavelength(s) of the laser source. This use of Bragg gratings is far from the field of measuring a physical quantity. In particular, this use does not teach not that a very high order Bragg grating can advantageously be used to measure the sensitivity Sç of an optical fiber. The network 124 has a reflected power spectrum comprising several power peaks distributed within a working range. Here, since the source 50 of the device 120 is the same as that of the device 2, the working range is also the same. The free spectral interval of the network 124 over the working range is such that its power spectrum has N,,, peaks within this working range, where N,24 is greater than or equal to ten and, preferably, greater than or equal to twenty, thirty or fifty. [Fig. 4] shows in more detail the architecture of the network 124. The network 124 is made in the optical fiber 122. The optical fiber 122 extends along a longitudinal axis 152 parallel to a Z direction of an orthogonal XYZ reference frame. Figures 4 and 5 are oriented relative to this XYZ reference frame. For example, the Z direction is horizontal and the Y direction is vertical. To simplify [Fig.4], only the portion of the fiber 122 that contains the grating 124 is shown. The optical fiber 122 guides the optical signal along the longitudinal axis 152. The optical fiber 122 comprises: - an optical core 154 in which the optical signal guided by this fiber 122 propagates, - an optical sheath 156 made of a material whose refractive index makes it possible to maintain the optical signal inside the core 154 by reflection at the interface between the core 154 and this sheath 156, and - a mechanical sheath, typically made of polymer, which covers the sheath 156. To simplify [Fig.4], the mechanical sheath of the optical fiber 122 has not been shown. The 124 network is designed to obtain a comb of peaks over the working range. In addition, here, the 124 network is designed so that this comb is formed by the harmonics of the 124 network of order close to 1024. For this purpose, the network 124 is composed of a succession of patterns M; arranged one behind the other along the axis 152. The index i of a pattern M; is the order number of the pattern in the Z direction. The index i of the first leftmost pattern in the network 124 is equal to 1 and the index i of the last rightmost pattern in the network 124 is equal to p. p is equal to the number of patterns M; of the network 124. In [Fig.4], only the first two and the last two patterns of the network 124 have been represented. The presence of the intermediate patterns located between the patterns Mz and M, is represented by small circles on the axis 152. The number p of patterns is greater than or equal to three and preferably greater than or equal to ten. Indeed, it has been observed that the larger the number p, the greater the width to half-height of each peak decreases. Here, the number p is also chosen small enough so that the length of the network 124 remains small, i.e., less than 1 meter and, preferably, less than 10 cm. The length of the network 124 is equal to the distance between the patterns M, and M, measured along the axis 152. Typically, the number p is less than 200 or 100. The pitch A, between two patterns M; and M,,, immediately consecutive in the Z direction is constant regardless of the index i. The pitch A2, is therefore equal to the distance, along the axis 152, which separates two patterns M, and M; ,, immediately consecutive. Here, the pitch A,.4 is calculated so that the wavelength of a harmonic of order k is equal to or very close to the center of the working range. Here, the order k, is chosen equal to 1024. For this, the pitch A;24 is between 0.9*[kce*Ac / (2*n.)] and 1.1*[Kce*Ac / (2*n2)] and, preferably, between 0.98*[kcc*A. / (2*n,)] and 1.02*[Kce*Ac / (2*n.)], where n, is the effective index of the optical fiber 122 and Δ. is the wavelength located at the center of the working range. Here, the wavelength Δ. is equal to 1550 nm. For example, optical fibers 14 and 122 are made from an optical fiber marketed under the reference SMF-28 by the company Coming®. The index n, of this optical fiber is equal to approximately 1.4676. Under these conditions, the term Kce+Ace / (2*n.) is equal to approximately 540.8 um. Here, the pitch A,24 is chosen to be equal to 540.8 Hm. With the choice of this value for the pitch A;24, only the harmonics of order between 317 and 7936 are in the optical domain and only the harmonics of order between 979 and 1087 are included in the working range. In particular, the wavelength λ5 of the fundamental frequency of the network 124 is not in the optical domain. For this value of the step A2, and so that the length L,2; of the network 124 is less than 10 cm, the number p of patterns is chosen to be less than 185. Here, p is chosen equal to 120, so that the length L;:4 of the network 124 is approximately equal to 65 mm. The motifs M; are all structurally identical to each other and differ from each other only in their position along the axis 152. Thus, in the following, only the motif M; is described in detail. This motif M; extends mainly in a plane P; perpendicular to the axis 152. This plane P; is therefore parallel to the X and Y directions. In [Fig. 4], only the planes P,, P2, P,.; and P, in which the motifs M,, M», My 1 and M, extend respectively are shown. [Fig.5] shows in more detail an example of the embodiment of the pattern M;. In [Fig.5], only the cross-section of the core 154 is shown. Each pattern M; reflects a part of the incident optical signal. Another part of the incident optical signal passes through the pattern M,. Finally, each pattern M; diffuses a part of the energy of the incident optical signal which is then neither reflected nor transmitted through this pattern M,. This energy diffused by each pattern M; creates insertion losses caused by the presence of the grating 124 in the core 154 of the optical fiber 122. To minimize these insertion losses, here, the surface Sy of the cross-section of the pattern M; occupies less than half of the surface Sys, of the cross-section of the core 154. The surface Sy; is equal to the surface of the orthogonal projection of the pattern M, on the plane P;. The surface S;s, is equal to the surface of the cross-section of the core 154. Typically, the surface Sys, is constant along the entire length of the optical fiber 122. Preferably, the area Sy; is less than 0.1*S,5, or 0.05*S,s, or 0.01*S;54. Here, the area Sm; is less than 0.05*S 54. To obtain sufficient reflectivity of the pattern M; to limit the number p of patterns and therefore to limit the length L,;; of the network 124, the surface Sy; is greater than 0.016 μm”, that is to say greater than twice the surface of the orthogonal projection of a spherical bubble of 100 nm diameter on the plane P,. In this embodiment, the surface Sy; is greater than or equal to 0.032 μm?. For this purpose, the pattern M; is made up of several bubbles B;. The index j of a bubble is an identifier that uniquely identifies the bubble B; among all other bubbles of the same pattern M,. The index j is here an integer between 1 and q, where q is equal to the number of bubbles B; of the pattern M;. The number q is greater than or equal to two or four. Here, the number q is equal to six. In this embodiment, all the bubbles B; are structurally identical to each other. Only their positions in the plane P; allow them to be distinguished from each other. Each bubble B; creates a significant variation in the refractive index of the core 154 in the direction of propagation of the optical signal. For this, the difference between the refractive index n,154 of the core 154 and the refractive index n,s of the bubble B; is greater than 0.3 or 0.4. Here, the interior of each bubble is empty or practically empty, which corresponds to a difference between the indices n,154 and N, greater than or equal to 0.4. In addition, for the variation in refractive index to be abrupt, the diameter D; of each bubble Bj is less than 200 nm and, preferably, less than 100 nm. Generally, the diameter D; is also greater than 10 nm or 50 nm. Each bubble B; is predominantly spherical. Thus, the diameter Dj; of bubble B; is equal to the diameter of the smallest volume sphere that entirely contains bubble B;. Here, this diameter Dj is less than 100 nm. The center of each bubble B; is contained in the plane P;. In this embodiment, the bubbles B; are disjoint, i.e., they do not overlap and are not fluidically connected to each other. The pattern M; is centered on the axis 152. For this, the bubbles B; are arranged next to each other so that the barycenter of the pattern M; is located less than 100 nm from the axis 152 and the center of at least one of the bubbles B; is located less than 100 nm from the axis 152. In this first embodiment, the barycenter of the pattern M; is located on the axis 152. In addition, the pattern M; is symmetrical with respect to the axis 152. The centers of the bubbles B; are located one behind the other on an axis A; which intersects the axis 152 and which belongs to the plane P,. The pattern M; therefore comprises a line of disjoint bubbles. In this case, the arrangement of the disjoint bubbles forms what is called a “dotted line” in this text. Here, the axis A; is parallel to the direction Y. In this embodiment, the bubbles B and B; are located, respectively, above and below the axis 152. The centers of the bubbles B3 and B4 are less than 100 nm from the axis 152. The distance between two bubbles B;, B;,, immediately consecutive along the axis A; is constant. In other words, whatever the pair of bubbles B;, B;.1 immediately consecutive along the axis A,, the distance which separates the centers of these two bubbles is the same. [Fig.5] represents a portion of the power spectrum of the 124 grating between 1545 nm and 1555 nm. The reflectivity of the peaks of the resulting comb reaches -20 dBm. [Fig. 7] represents a method of manufacturing the optical fiber 122. This method begins with a step 160 of providing an optical fiber whose core 154 is initially devoid of a Bragg grating. For example, the optical fiber provided is the optical fiber marketed under the reference SMF-28 by the company Corning®. Here, the mechanical sheath of this optical fiber is transparent to the pulses of a femtosecond laser so that it is not necessary to remove this mechanical sheath at the locations where the M patterns are to be made. Then, during a step 162, the network 124 is produced in the core 154. For this, an operation 164 of forming the pattern M; in the core 154 of the optical fiber provided is repeated at each location where such a pattern M; must be formed. In operation 164, each bubble B; is created by a single pulse of the femtosecond laser. More precisely, in operation 164, the beam of the femtosecond laser is focused on the center of the bubble B; to be created, then a pulse of a duration less than 500 fs or 250 fs is emitted and irradiates the point of the core 154 where the center of the bubble B, must be located. The bubble B; is then created in the core 154. Then, the optical fiber is moved relative to the focal point of the femtosecond laser so that the beam of the femtosecond laser is now focused on the center of the next bubble B;,; to be created, then a new pulse of the femtosecond laser is emitted. In this embodiment, the bubbles B; are therefore created one after the other. The values ​​of the different parameters of a femtosecond laser to create a bubble such as bubble B; depend on the characteristics of the optical fiber provided as well as the characteristics of the femtosecond laser used. The adjustment of these different parameters to create the bubbles B; previously characterized, is within the skills of the person skilled in the art. For example, by way of illustration, the reader can consult on this subject the application CN211603608U which describes in detail an example of an installation making it possible to form bubbles such as bubbles B; in the core of an optical fiber. Here, the following parameters were used to manufacture the optical fiber 122: - the central wavelength of the femtosecond laser pulse is equal to 512 nm, - the duration of each pulse of the femtosecond laser is equal to 160 fs, and - the power of each pulse of the femtosecond laser is equal to 45 nJ. The operation of the device 120 for measuring the sensitivity Sç will now be described with reference to the method of [Fig.8]. In a step 180, the unit 70 selects, within the range [Grin : Grax]» one of the Nç values ​​Ga for which the value Sc4 of the sensitivity Sç has not yet been measured. Here, the values ​​G; are the same as those contained in the table 80 of the device 2. Then, during a step 182, for the selected value Gç, the unit 70 controls the apparatus 126 to expose the network 124 to a temperature equal to Ga,. The value Ga is defined by the following relation: Ga, = Gz-€, where: - G, is the value G, selected in step 180, and - € is a predetermined constant step. Preferably, the € step is less than or equal to 10°C. Here, the € step is equal to 5°C. For example, in step 182, unit 70 sets the value of the setpoint Tc to the value Ga. Once the bath 128 has reached the Gay temperature, during a step 184, for each of the N,7, peaks of the spectrum of the network 124, the unit 70 measures the corresponding value Aa1x, where the index k is the order number of the harmonic located at the wavelength equal to Aa1x- For example, for this, the unit 70 controls the source 50 to vary the wavelength λ, from the wavelength λ,nin to the wavelength λ,max- In parallel, the unit 70 records the value of the wavelength λ, of the optical signal emitted by the source 50 each time that the electrical signal generated by the sensor 62 passes through a maximum located well above the measurement noise. Each of these recorded values ​​corresponds to a respective value λ41x. Since the spectrum of the network 124 has N'24 peaks in the range [A;mini Asmax]» during step 184, the unit 70 measures N;24 Aa1x values Then, during a step 186, the unit 70 controls the apparatus 126 to expose the network 124 at a temperature equal to Gç2. The value Ga is defined by the following relation: G, = G,4+e, where: - G, is the value G, selected in step 180, and - € is the same step as used in step 182. Step 186 is performed like step 182 except that the value G4, is replaced by the value G,…. Once the bath 128 has reached the temperature Gç2, during a step 188, for each of the N;2, peaks of the spectrum of the network 124, the unit 70 measures the corresponding value \4x. Step 188 is carried out like step 184. At the end of step 188, the unit 70 has measured N,,; values ​​Aa x. At this point, it is emphasized that the values ​​Ga; and Gas have been chosen so that the value G4 is a median value, that is, in this example, a value located equidistant from the values ​​G4, and Ge. Then, in a step 190, the unit 70 determines the value Sca of the sensitivity Sç of the fiber 122 when the temperature is equal to the value G4. It is emphasized that the sensitivity Sç of the fiber 122 is identical to the sensitivity of the fiber 14 because these two fibers are identical except for the fact that the fiber 14 comprises the network 4 while the fiber 122 comprises the network 124. Thus, measuring the sensitivity of the fiber 14 amounts to measuring the sensitivity of the fiber 122. To determine the Sca value, unit 70 uses the N124 Ag1x values ​​and the N124 Àgx values ​​measured in steps 184 and 188, respectively. Here, it is the large number of Aa1x and Agx values ​​used that increases the accuracy of the determined Sas value. This is why the number N, of peaks is chosen to be at least greater than ten. For illustration, to determine the value Sç,a, during an operation 192, for each peak k of the spectrum of the network 124 included in the range [Asmini Asmax], the unit 70 first calculates a coarse value Sç,ax of the sensitivity Sç to the temperature Ga. For this, for each peak k, the unit 70 calculates the coarse value Sçax using the following relation: Seax = (Aux — Marx Aa a*(Ga-Ga1)). Then, during an operation 194, the unit 70 calculates the arithmetic mean of the N 14 Scax values ​​calculated during operation 192. The value Sç, is taken equal to this arithmetic mean. Step 190 is then completed. Finally, during a step 196, the temperature G; and the value Sça of the associated sensitivity Sç are recorded in a table. Once step 196 is completed, the method returns to step 180 as long as there is still a value G; for which the corresponding value Sca has not yet been calculated. Once all Sc values, associated with G values, have been calculated, when from a step 198, the unit 70 constructs the law Sc(G). For this, here, the table constructed by the successive reiterations of step 196, is recorded in the memory 74 of the device 2 as table 80. Chapter II: Variants: IL! — Variants of the measurement of sensitivity S,: Variants of the 124 Bragg network: The order k of the harmonic at the center of the comb to be produced is here greater than 100 and preferably chosen to be greater than 500 or 1000. This order k can also be chosen to be greater than 2000 or 4000 or 10000. Theoretically, there is no upper limit for this order k. However, it follows from relation (1) that the greater the order k, the greater the pitch λ of the Bragg grating and therefore the longer the Bragg grating. In practice, it is therefore the maximum length desired for the grating 124 which imposes an upper limit for the order k. Here, this maximum length is set at 1 m. Similarly, the minimum value of the pitch λ is greater than 20 μm and, typically, greater than 50 μm so that very high order harmonics are included in the optical domain. Theoretically, there is no maximum value for the pitch λ. Indeed, whatever the value chosen for the pitch λ, it is possible to find a value for the order k.. which allows the wavelength λc to be placed in the optical domain. However, the larger the pitch λ, the longer the Bragg grating. In practice, it is therefore also the maximum desired length for the grating 124 which imposes an upper limit for the value of the pitch λ. For example, by applying the teaching given in chapter I, it is possible to obtain combs centered on the wavelengths commonly used in optics such as, in particular, the wavelength of 800 nm, 1000 nm, 1300 nm or 1500 nm. The working range may be wider than 100 nm. For example, the width of this working range is, alternatively, greater than 200 nm or 300 nm. There is no upper limit for the width of this working range other than that it must be within the optical domain and that it must be able to be scanned by the source 50 of the spectral analyzer. The Bragg grating patterns made in the core of the optical fiber may have different shapes. For example, as a variant, each pattern comprises a single bubble. In another embodiment, as described in article LUO2022, each pattern has the shape of an ellipse. The different variants of the very high order Bragg grating pattern described in the application filed on 07 / 29 / 2022 under No. FR2207936 by the present applicant, apply to the Bragg gratings described here. There are many variants of the method for manufacturing a Bragg grating of very high order. In particular, all the manufacturing processes and their variants described in the application filed on 07 / 29 / 2022 under No. FR2207936 by the present applicant can be used to manufacture each very high order Bragg grating. The manufacturing process described in article LUO2022 can also be used. Variants of the Sc sensitivity measurement method: The step e can be larger than 10°C. The median value is located between G4; and Ga) but not necessarily equal to (Ga,+G æ) / 2. For example, in a simplified case, the median value is taken equal to G4; or to G dz In a simplified embodiment, for the calculation of the Sc value, the unit 70 uses only a part of the peaks located within the range [A,min; Asmux]. However, the number of peaks used remains greater than ten. Other embodiments of step 190 are possible for finding the value Sca which minimizes the sum of all the deviations Sç,ax—Soa. For example, the unit 70 determines the straight line which minimizes the deviations between the points (Aax; Alax / AGa). The abscissa of the point (À ax Ala / AG;) is Aax And kax is defined by the following relation: dax = (Aazx + Aa1s) / 2. The ordinate of the point (Aax; Alax / AG4) is Adax / AG; and Alax / AGa is defined by the following relation: Akqu / AGa = (Aaax — Ac1,x) / (Gaz-Ga1). The value Soa of the sensitivity Sç when the temperature is equal to Ga is taken equal to the slope of the straight line thus determined. The method for measuring the sensitivity of a waveguide can be implemented independently of the method for calibrating the device 2. For example, the method for measuring the sensitivity of a waveguide can be implemented to measure the sensitivity of several waveguides in order to determine whether these waveguides have the same sensitivity. IL.2 - Variants of the measurement of the physical quantity G: The device 2 may also include a standard to improve the accuracy of the measurements as described, for example, in application CN102879022A. Alternatively, the fiber 14 comprises a succession of several Bragg gratings produced one after the other in the core 154. In this case, preferably, the wavelengths λ of each of these Bragg gratings are different. Thanks to this, the device 2 makes it possible to measure the physical quantity G at different locations. In this embodiment, the power spectrum of the fiber 14 then comprises several power peaks in the working range. Everything that has been described here in the particular case where the fiber 14 comprises a single Bragg grating applies to each of these additional Bragg gratings. In particular, for each additional Bragg grating, a calibration law specific to this additional grating is recorded in the memory 74 and used to convert the displacement of its wavelength λ into a current value of the physical quantity G at the location of this additional grating. This specific calibration law is constructed and used as described in the special case of calibration law 76. The sensor 62 can be connected to the distal end of the fiber 14 instead of being connected to its proximal end. In this case, the sensor 62 measures the optical signal that has passed through the network 4. Therefore, the power spectrum of the measured signal is a power spectrum in transmission and not in reflection. However, everything that has been described in the particular case where the sensor 62 is connected to the proximal end adapts, without particular difficulty, to the case where the sensor 62 is connected to the distal end. Alternatively, the source 50 is not tunable. For example, the source 50 is a wide laser source, i.e. a laser source which emits an optical signal whose power spectrum simultaneously covers the entire working range. In this case, the emitted optical signal is not single-frequency. In addition, for each spectral response to be measured, the spectral analyzer then comprises a plurality of photodetectors which simultaneously measure the power of the spectral response for a large number of different wavelengths. For example, in this case, the sensor 62 is a bar spectrometer. In such an embodiment, it is not necessary to vary the wavelength λ, to scan the entire working range. The source 50 is not necessarily a laser source. For example, the source 50 can also be made using a tunable Fabry-Pérot cavity. In this case, the control signal causes the displacement of at least one of the diopters of this Fabry-Pérot cavity. This displacement of a diopter then causes a modification of the natural resonance frequency of the cavity and therefore a modification of the wavelength λ. Other implementations of the 76 law are possible. For example, the 76 law is implemented in the form of a polynomial whose coefficient values ​​are stored in the memory 74. In this case, the table 78 is omitted. From the calculated points with coordinates (A); G;), the unit 70 of the device 2 calculates the coefficients of a polynomial which passes closest to these points. Then, it is the values ​​of the coefficients of this polynomial which are stored in the memory 74 to define the 76 law. The unit 70 can also be adapted to determine only the variation AG of the measured physical quantity and not its absolute value. In this case, the calibration law 76 is constructed in such a way as to associate with each variation A, a corresponding value of the variation AG. It is then not necessary to know the value G o of the measured physical quantity corresponding to the wavelength Ago. In another implementation variant, the reference values ​​Az, and Gp are integrated as coefficients in the calibration law. In this case, to obtain the current value G, of the physical quantity G, only the value Δ, of the wavelength Δg is provided as an input parameter of the calibration law. In this case, during step 106, preferably, the calibration law is constructed from the coordinate points (Δ;; G;) instead of the coordinate points (A\;; G;). Then, the table 78 is replaced by a table containing the coordinate points (Δ;; G;) instead of the coordinate points (A);; G;). In step 104, other choices are possible for the values ​​Δ, and G,. For example, in a simplified embodiment, in step 104, each value Δ; corresponding to a value G; is calculated using the following relationship: (Δ; — 4,) / Δ, = Sci*(G;-G,), where: - Sc; is equal to Sc(G;), and - A, and G, are systematically priced equal, respectively, to As, and Gp whatever the value of the index i. Other implementations of the Sc(G) law are possible. For example, the Sc(G) law is also implemented in the form of a polynomial whose coefficient values ​​are stored in the memory 74. In this case, the table 80 is omitted. From the recorded points (Ge, Sc.), the unit 70 of the device 120 calculates the coefficients of a polynomial which passes closest to these points. Then, during step 198, it is the values ​​of the coefficients of this polynomial which are stored in the memory 74 to define the Sc(G) law. In a simplified variant, the sensitivity Sç is considered to be constant over the entire operating range of the device 2. In this case, the calibration law 76 can be replaced by a simplified calibration law. For example, the simplified calibration law is as follows: G, = (À. - Apo) / (Aao * Sao) + Go, WHERE So, is the constant value of the sensitivity Sc. The value Sa, is measured by executing steps 180 to 196 of the method of [Fig. 8] only once. For example, during this implementation of the method of [Fig. 8], the values ​​G4, and Ga are chosen equal to, respectively, Gin and Grux When the above simplified calibration law is implemented, steps 104 and 106 of the phase 100 of calibration of the device 2, are omitted. Table 78 and the law Sc(G) are also omitted. Alternatively, grating 124 is formed in the core of fiber 14 in addition to grating 4. Preferably, grating 124 is then formed in proximity to grating 4. For example, when the patterns of gratings 4 and 124 occupy only a small fraction of the cross-section of core 154, grating 4 is formed on one side of a median plane containing axis 152 and grating 124 is formed on the other side of this median plane opposite grating 4. The height of the peak of grating 4 at wavelength λ is much greater than the height of the peaks of grating 124 in a power spectrum. Thus, unit 70 can easily discern the peak of grating 4 from the peaks of grating 124. In this case, it It is not necessary to use an optical fiber other than fiber 14 to construct the evolution law S,(G). Fiber 14 can be used instead of fiber 122. In a simplified variant, grating 4 is a very high-order Bragg grating. For example, grating 4 is identical to grating 124. In this case, it is the same very high-order grating that is used to construct the law Sc(G) and then to measure the physical quantity G. Fiber 14 can then be used instead of fiber 122. Variants of the physical quantity G: The wavelength λ of the grating 4 varies in response to a temperature variation, a longitudinal deformation or a variation in hydrostatic pressure. Thus, all the preceding embodiments can be adapted to the case where the physical quantity G is chosen from the group consisting of temperature, a longitudinal deformation and a variation in hydrostatic pressure. Using the measurement of one of these physical quantities, it is possible to deduce measurements for other physical quantities such as vibrations, acceleration or even to detect acoustic waves. The physical quantity G can also be another physical quantity than temperature, longitudinal deformation and hydrostatic pressure. For this, it is sufficient that the gratings 4 and 124 are made sensitive to this other physical quantity. For example, the gratings 4 and 124 can be made sensitive to a dose of radiation. As an illustration, for this, the core 154 of the optical fiber 14 is made of a photosensitive material. Here, this core 154 is made of germanosilicate. Initially, a Bragg grating is fabricated in the core of the optical fiber 14. Then, this Bragg grating is transformed into a Bragg grating sensitive to a dose of the radiation to be measured. For this, this fabricated Bragg grating is exposed to ultraviolet radiation to create colored centers resulting from the recombination of the bonds between the germanium and the silica.When subjected to a dose of the radiation to be measured, these color centers are modified, leading to a shift in the wavelength λ of the Bragg grating. IL.3 - Variants common to devices 2 and 120: Waveguide variants: The waveguide is not necessarily an optical fiber. Everything described in this text in the specific case of an optical fiber also applies to the case where the waveguide is a waveguide made on a photonic chip. For example, in the latter case, the core of the optical fiber is made of monocrystalline silicon or another semiconductor material and the cladding is made of a material commonly used in the field of silicon optics such as silicon oxide. Optical fibers other than SMF-28 fiber can be used. For example, the optical fiber can be multimode optical fiber or MMF (Multi-Mode Fiber). The core of the optical fiber does not need to be made of a specific doping. Thus, what is described in this text can be implemented with optical fibers whose core is made of germanosilicates, pure silica, rare earth-doped aluminosilicates or sapphire. Variants of the insulating structure: Other embodiments of the insulating structure are possible. For example, networks 4 and 124 can be isolated from variations in mechanical stress by implementing the teaching of application FR3087008A1. The insulating structure can also be designed to isolate networks 4 and 124 from variations in hydrostatic pressure. The insulating structure can be an active insulating structure, i.e. one that consumes electrical energy to operate, or a passive insulating structure that does not require electrical energy. The insulating structure may be omitted in particular if the physical quantities, other than the physical quantity G to be measured, cannot vary or vary only negligibly. Several of the variants described above can be combined in a single embodiment. The law Sc(G) recorded in the memory 74 can be constructed without using a very high order Bragg grating. For example, in another embodiment, the points (Ga Sca) are measured using a new optical fiber identical to the fiber 122 except that the grating 124 is replaced by Np, Bragg gratings each having a wavelength λ, located within the range [λ,min3 \smax] €t different from those of the other Bragg gratings made in this same fiber. The method described with reference to [Fig. 8] also works when the fiber 122 is replaced by this new fiber. Preferably, the number Ny, is greater than or equal to ten or thirty or fifty. However, in a simplified embodiment, the number N;, is equal to one. In this latter case, each value Sç, is calculated using a single power peak and not several as in the embodiments previously described. Chapter III: of the achievements described: Compared to a succession of Bragg gratings made one behind the other in a waveguide and each having their own fundamental wavelength contained within the working range, a very high order Bragg grating is much smaller and has an equal or greater number of peaks. This therefore simplifies the manufacture of the device 120 used to measure the sensitivity Sc. In addition, since the grating 124 is less bulky, this facilitates the production of the insulating structure 90 and the apparatus 126. Thus, the use of a Bragg grating of order very high also simplifies the implementation of the method for measuring the sensitivity Sc. Finally, the sensitivity S, measured is very precise because it is obtained from measurements carried out for more than ten different wavelengths, that is to say here for the wavelengths of more than ten peaks of the power spectrum of the network 124. The fact of producing the network 124 in an optical fiber simplifies the production of the device 120 for measuring the sensitivity Sc. Using one or more bubbles in each pattern M; makes it possible to obtain a small pattern and therefore to substantially reduce insertion losses. Using several disjoint bubbles makes it possible to obtain a pattern M; sufficiently reflective to reduce the number p of patterns and therefore to maintain the compactness of the network 124 while limiting insertion losses. Indeed, when the bubbles overlap, the overlapping areas between several bubbles are subjected to several successive pulses of the femtosecond laser. It has been observed that an area of ​​the core of the optical fiber which is subjected to several pulses of the femtosecond laser, degrades. This degradation increases the scattering losses. Conversely, when the bubbles are disjoint, such overlapping areas do not exist, which limits insertion losses. Constructing the evolution law Sc(G) from several Sca values ​​measured for different G values ​​of the physical quantity G, makes it possible to determine more precisely the value of the sensitivity Sç for a given value of the physical quantity G. Taking the value Sca of the sensitivity Sç equal to a value which minimizes all the deviations (Sax — Sa) makes it possible to reduce the error on the measured value of the sensitivity. Constructing the calibration law 76 using only the reference values ​​Ag, and Go and a pre-recorded law Sc(G) simplifies the calibration of the device for measuring the physical quantity G. Indeed, the law Sc(G) is independent of the wavelength λ. Thus, once the law Sc(G) has been constructed for the fiber 14, the same law Sc(G) can be used to construct the calibration law of any Bragg grating made in this fiber 14. Therefore, to calibrate a Bragg grating made in the fiber 14, it is sufficient to measure the coordinates (Az, ; Go) of a single reference point. This is simpler than known calibration methods. Indeed, unlike known methods, it is not necessary to obtain, by measurement, several points with coordinates (A);; G;). Here, these points with coordinates (AÀ;; G;) are obtained by calculation and not by measurement.Then, the law Sc(G) is the same whatever the wavelength λ, of the Bragg grating made in the fiber 14. From then on, the same law Sc(G) can be used to construct the calibration law of several Bragg gratings made. in the fiber 14 even if these Bragg gratings have different wavelengths λ. In other words, it is not necessary to repeat the measurements which made it possible to construct the law Sc(G) for each wavelength λ. In addition, the method of constructing a calibration law described here takes into account the fact that the sensitivity Sc varies according to the value of the physical quantity G. The calibration law constructed is therefore particularly precise. Repeating the calibration of the sensor after several measurements makes it possible to compensate for drifts over time in the device measuring the physical quantity G. Calculating the value Δ; using the relation (4; — 4,) / Δ, = Se,;*(G;-G,) in which the values ​​Δ, and G, are equal, respectively, to the values ​​Δ, and G;.1, WHERE Δ;1 is the previously calculated value corresponding to the value G,, closest to the value G,, improves the accuracy of the calibration law 76 and therefore the precision of the measurement of the physical quantity G.

Claims

Claims

1. Device for measuring the sensitivity Sa of a waveguide to a variation ÂG of a physical quantity G, the sensitivity Sç for a given value of the physical quantity G being defined by the relation following AAp / As = Sc*AG, where: - At is the fundamental wavelength of a Bragg grating realized in this waveguide, - AÀs is the variation of the fundamental wavelength of the grating of Bragg obtained in response to the AG variation of the physical quantity, and - AG is an infinitesimal variation of the value of the quantity physical G around the given value of the physical quantity G, this device comprising: - a Bragg grating (124) made inside the waveguide, this Bragg grating comprising at least three identical patterns aligned one behind the other along a longitudinal axis of the waveguide and separated from each other by a constant step, - an apparatus (126) capable of exposing the Bragg grating to a first known value of the physical quantity then to a second value known from the physical quantity, these first and second values known physical quantity being located on either side of a median value of the physical quantity, and - a spectral analyzer (20) capable of detecting, in a working range predetermined between 200 nm and 5000 nm, the value of the wavelength at which a power peak appears in the spectrum Bragg grating reflection power characterized in that: - the pitch of the Bragg grating (124) is configured so that the spectrum of power of this Bragg grating has at least ten harmonics of order greater than one hundred within the working range, each of these harmonics being located at a respective wavelength λ,, where k is the order number of this harmonic, - the spectral analyzer (20) is configured to perform the steps following: - when the value of the physical quantity is the first value known, the measurement of a first value of the wavelength A, of each of the harmonics located within the working range, then - when the value of the physical quantity is the second value, the measurement of a second value of the wavelength λ, of each of these harmonics located within the working range, then - the determination of the value of the sensitivity Sç for the value median of the physical quantity from the difference between the first and second values ​​of the physical quantity and from the first and second measured values ​​for each wavelength A,

2. Device according to claim 1, in which the waveguide is a optical fiber.

3. Device according to any one of the preceding claims, in which : - each pattern (M,, M, My;, Ms) of the Bragg grating extends principally- palely in a plane, called the "pattern plane", perpendicular to the axis longitudinal of the waveguide, and - each pattern is made up of one or more bubbles (B, - B«) arranged next to each other in the plane of the pattern, and - the surface of the orthogonal projection of all the bubbles of the pattern on the pattern plane is less than 50% of the section area transverse of the core (154) of the waveguide.

4. A device according to claim 3, wherein each pattern is consisting of several disjointed bubbles (B; - Bs) arranged one by one side of the others in the plane of the pattern. |Claim 5] Device according to any one of the preceding claims, in which : - the pitch of the Bragg grating is greater than or equal to 20 um, and - the difference between the refractive index of the waveguide core and the index refraction of each pattern of the Bragg grating is greater than 0.

3.

6. A device according to any preceding claim, in in which each pattern (M,, M», My.1, My) is realized using a pulse of a femtosecond laser.

7. A device according to any preceding claim, in which the physical quantity is chosen from the group consisting of a temperature, mechanical deformation and hydro- pressure static.

8. Method for measuring the sensitivity Ss of a waveguide to a variation AG of a physical quantity G, the sensitivity Sç for a given value of the physical quantity G being defined by the following relation AAg / À3 = Sc*AG, where: - Ag is the fundamental wavelength of a Bragg grating made in the waveguide, - AÀs is the variation of the fundamental wavelength of the grating of Bragg obtained in response to the AG variation of the physical quantity, and - AG is an infinitesimal variation of the value of the quantity physical G around the given value of the physical quantity G, this process comprising the following steps: a) the realization (162) of a Bragg grating inside the guide wave, this Bragg grating comprising at least three identical patterns aligned one behind the other along a longitudinal axis of the guide wave and separated from each other by a constant pitch, b) the exposure (182, 186) of the Bragg grating to a first value known physical quantity then to a second known value of the physical quantity, these first and second known values ​​of the physical quantity being located on either side of a median value of physical greatness, characterized in that: - during step a), the step of the Bragg grating produced is configured to that the power spectrum of this Bragg grating has at least ten harmonics of order greater than one hundred within a working range between 200 nm and 5000 nm, each of these harmonics being located at a respective wavelength λ,, where k is the order number of this harmonic, and - the process also includes the following steps: c) when the value of the physical quantity is the first value known, the measurement (184) of a first value of the wavelength λ, of each of the harmonics located within the working range, Then d) when the value of the physical quantity is the second value, the measurement (188) of a second value of the wavelength λ, of each of these harmonics located within the working range, then e) determining (190) the value of the sensitivity Sç for the value median of the physical quantity from the difference between the first and second values ​​of the physical quantity and from the first and second measured values ​​for each wavelength A,. [Claim A method according to claim 8, wherein the method comprises: - repeating steps b) to e) by replacing the first and second values ​​of the physical quantity by other values ​​different from the physical quantity in order to determine the sensitivity Sç for several different median values ​​of the physical quantity, these values medians of the physical quantity being distributed over a range of predetermined use, then - the construction (198), over the range of use, of an evolution law of the value of the sensitivity Sç as a function of the value of the quantity physical.

10. A method according to any one of claims 8 to 9, wherein, during step e) (190), the value of the sensitivity Sç is taken equal to a value Sca which minimizes the sum of all deviations Scax— Sca, WHERE S Gax is a rough value of the sensitivity Sç calculated from the difference between the first and second values ​​of the physical quantity and only from the first and second measured values ​​for the harmonic of order k.

11. | Method for calibrating a device for measuring a quantity physical G which varies within a predetermined range of use- completed, this device comprising: - a waveguide containing a core that extends along a long axis- gitudinal and within which an optical signal guided by this guide wave is able to propagate along the longitudinal axis of the guide wave, - a Bragg grating made in the heart of the waveguide, this grating Bragg comprising a reflection power spectrum presenting at minus a power peak at a wavelength A which varies according to of the physical quantity to be measured so that a variation of this wavelength is representative of a variation in the value of this physical size, characterized in that the method comprises the following steps: 1) the recording of a law of evolution of the value of a sensitivity S c of the waveguide as a function of the physical quantity to be measured, this law of evolution associating a value of the sensitivity Sç with each value of the physical quantity to be measured contained in the range of use predetermined and this law of evolution being constructed by putting into implements a method for measuring the sensitivity Ss in accordance with one any of claims 8 to 10, 2) the acquisition (102) of a reference value As,0 of the length wave reflected by the Bragg grating when the value of the quantity physical is equal to a known reference value Gp, then 3) the calculation (104), for each of the values ​​G; contained in a predetermined set of several G values; chosen from within the range of use, of a value corresponding to; of the length wave A using the following relation: (A; — 4,) / A, = Sc; *(G;-G,), where: - So; is the value of the sensitivity Sç of the waveguide associated with the chosen value G; by the recorded law of evolution, and - À, is the value of the wavelength À when the value of the quantity physical G is equal to a value G, known, then 4) from the values ​​G,, of the values ​​À; calculated for each of these G values; and acquired reference Ap and Gp values, the construction (106) of a calibration law which associates with each value of the length wave A, a corresponding value of the physical quantity.

12. | The method of claim 11, wherein the method comprises the recording (106) of the calibration law constructed in a memory of the device for measuring the physical quantity G as calibration law used by this measuring device to establish the value of the physical quantity measured from a determined value of wavelength A.

13. The method of claim 12, wherein the method comprises after several measurements of the physical quantity by the device of measure, the reiteration of steps 2) to 6) to construct a new law calibration and then recording this new calibration law in the memory of the device for measuring the physical quantity G in as a calibration law used, by this measuring device, for convert each new measured value of wavelength A into a measured value of the physical quantity.

14. A method according to any one of claims 11 to 13, in which : - for values ​​of the index i greater than zero, the calculation of the value À; using the relation (A; — 4,) / À, = Sc,1*(G;-G,) is achieved by taking the values ​​A, and G, equal, respectively, to values ​​A; and G;,, WHERE A;, is a previously calculated value corresponding to the value G,, the most close to the value G,, and - for the index i equal to zero, the values ​​A; and G; are equal, respec- tively, to the acquired values ​​Àp,0 and Gp.

15. Device for measuring a physical quantity G which varies within of a predetermined range of use, this device comprising: - a waveguide (14) containing a core which extends along an axis longitudinal and inside which an optical signal guided by this guide wave is able to propagate along the longitudinal axis of the waveguide, - a Bragg grating (4) produced in the core of the waveguide, this Bragg grating comprising a reflection power spectrum having at least one power peak at a wavelength λ which varies according to the physical quantity to be measured so that a variation in this wavelength is representative of a variation in the value of this physical quantity, - a spectral analyzer (20) comprising a memory (74) in which is recorded a calibration law (76) which associates with each value of the wavelength λ, a corresponding value of the physical quantity, this spectral analyzer being capable: - to measure the spectral response of the Bragg grating, then - to determine the value of the wavelength λ of the Bragg grating from the measured spectral response, then - to establish the value of the measured physical quantity from the determined value of the wavelength λ using the recorded calibration law, characterized in that: - the memory (74) of the spectral analyzer (20) also contains: - an evolution law (Sc(G)) of the value of a sensitivity Sç of the waveguide, this evolution law associating a value of the sensitivity Sç with each value of the physical quantity to be measured contained in the predetermined range of use and this evolution law being constructed by implementing a method for measuring the sensitivity S, in accordance with any one of claims 8 to 10, and - a reference value Ag of the wavelength reflected by the Bragg grating when the value of the physical quantity is equal to a known reference value G, and - the spectral analyzer is configured to: - calculate, for each of the values ​​G; contained in a predetermined set of several values ​​G; chosen within the predetermined range of use, a corresponding value Δ; of the wavelength Δ using the following relation: (Δ; — 4,) / Δ, = Sc:*(G;-G;), where: - Sa; is the value of the sensitivity Sc of the waveguide associated with the chosen value G; by the recorded evolution law, and - À, is the value of the wavelength À when the value of the physical quantity G is equal to a value G, known, then - from the values ​​G;, the values ​​À; calculated for each of these values ​​G; and the values ​​Àp.y and Gp contained in its memory, construct the calibration law, then - record the constructed calibration law in the memory as the calibration law used to establish the value of the measured physical quantity from the determined value of the wavelength λ.