A device for measuring a physical quantity using the optical Vernier effect

Abstract

A device for measuring a physical quantity using the optical Vernier effect. This device comprises: - a first Bragg grating (20) with a first step size, - an electronic unit (40) configured to calculate a measurement of the variation of the physical quantity from the envelope of a power spectrum of a signal modulated by the Vernier effect. This signal modulated by the Vernier effect corresponds to the superposition of: - a measured signal resulting from the interaction of an emitted optical signal with the first Bragg grating, and - a reference signal corresponding to the interaction of the same emitted optical signal with a second Bragg grating whose step size differs from the first. The first step size is such that the power spectrum of the first Bragg grating exhibits several discernible harmonics of order greater than one hundred within a range of wavelengths of interest located between 200 nm and 5000 nm. Fig. 1

Description

Description Title of the invention: Device for measuring a physical quantity using the optical Vernier effect

[0001] = The invention relates to a device for measuring a physical quantity using the optical Vernier effect as well as an optical fiber for the production of this device.

[0002] These devices are, for example, used to measure a temperature or a pressure or mechanical deformation.

[0003] — Such measuring devices and their operating principles are described in the following article: Yunhao Chen et al: “Advanced Fiber Sensors Based on the Vernier Effect”, Sensors 2022, 22, 2694 published on March 31, 2022. Subsequently, This article is designated by the reference "CHEN2022". Thus these explanations are not not included in this text.

[0004] — These devices comprise a standard interferometer and a measuring interferometer. For the optical Vernier effect to appear, the free spectral interval ("Free Spectral Range" in English) of the standard interferometer is different from the spectral interval free from the measuring interferometer. The measuring interferometer is made in the core of an optical fiber to a location exposed to variations in the physical quantity at measure so that its power spectrum varies according to variations in this physical quantity. Conversely, the power spectrum of the standard interferometer does not does not vary in response to a variation in the measured physical quantity.

[0005] To obtain high sensitivity, it is necessary that the power spectrum of each of the interferometers have a succession of very close and very fine peaks in a wavelength range of interest, e.g., at least 100 nm wide in the field of optics. The field of optics refers to the range of lengths waveforms usually used in optics. More precisely, in this text, the domain of optics means the range extending from 200 nm to 5000 nm and, preferably, from 600 nm to 2000 nm. In this text, "very close" means that the gap between two consecutive peaks is less than or equal to 10 nm. "very fine" means that the width at mid- height of each peak is less than 3 nm. In addition, the heights of these peaks must be substantially the same over this wavelength range of interest. By the Subsequently, such a succession of peaks is called a "comb of peaks" or simply "a Comb ".

[0006] To date, many embodiments of standard and measurement interferometers have been proposed. However, embodiments that are simple to make, especially in the heart of an optical fiber, do not allow a sens- high sensitivity. For example, it has already been proposed to use, as an interferometer, a Fabry Perot cavity made in the core of an optical fiber. The production of such a Fabry Perot cavity is quite simple. However, the sensitivity of the measuring device obtained is low, in particular because the peaks of the power spectrum are not fine enough. This is explained in particular by the fact that it is very difficult to produce diopters at each end of the Fabry Perot cavity in the core of an optical fiber, the reflectivities of which are high. To overcome this difficulty, it has been proposed to use mirrors connected to the ends of the optical fiber. Such mirrors have a high reflectivity, i.e. greater than 90%. Under these conditions, the peaks of the power spectrum are fine and the sensitivity of the measuring device is high. However, the production of Fabry Perot cavities is then complex, in particular because mirrors must be connected to the ends of an optical fiber. The invention aims to propose a device for measuring a physical quantity using the optical Vernier effect which has high sensitivity and which is, at the same time, simple to manufacture. The invention therefore relates to a device for measuring a physical quantity using the optical Vernier effect, this device comprising: - a waveguide containing a core which extends along a longitudinal axis and within which an optical signal guided by the waveguide is capable of propagating along the longitudinal axis of the waveguide, - a first Bragg grating produced in a first portion of the core of the waveguide exposed to variations in the physical quantity, this first Bragg grating comprising at least three identical patterns aligned one behind the other along the longitudinal axis of the waveguide and separated from each other by a first pitch, this first Bragg grating having a power spectrum whose peaks move in response to a variation in the physical quantity, - an optical source connected to the waveguide and capable of emitting an optical signal which propagates along the longitudinal axis of this waveguide, the wavelengths of this emitted optical signal being included within a range of wavelengths of interest located between 200 nm and 5000 nm, - a sensor connected to the waveguide and capable of measuring the optical signal emitted by the optical source after it has interacted with the first Bragg grating, - an electronic processing unit configured to: - obtain, from the signal measured by the sensor, a Vernier modulated signal, this Vernier modulated signal corresponding to the superposition of a measured signal resulting from the interaction of the optical signal emitted with the first Bragg grating and a standard signal corresponding to the interaction of the same optical signal emitted with a second Bragg grating whose pitch between the patterns is different from the first pitch and whose the peaks of the power spectrum move less in response to the same variation of the physical quantity than the peaks of the power spectrum of the first Bragg grating, and - calculate a measure of the variation of the physical quantity from the envelope of the power spectrum of the Vernier effect modulated signal obtained, characterized in that the first step is such that the power spectrum of the first Bragg grating exhibits several discernible harmonics of order greater than one hundred within the wavelength range of interest. Embodiments of this device may include one or more of the following features: 1) -the waveguide is a birefringent optical fiber having an ordinary refractive index for waves propagating in the core with a linear polarization parallel to an ordinary polarization direction and an extraordinary refractive index for waves propagating in the core with a linear polarization parallel to an extraordinary polarization direction, the extraordinary polarization direction being different from the ordinary polarization direction and the sensitivity of the ordinary refractive index to variations in the physical quantity to be measured is different from the sensitivity of the extraordinary refractive index to variations in the physical quantity to be measured, and - the optical source is capable of emitting an optical signal comprising simultaneously a wave propagating in the core with a rectilinear polarization parallel to the ordinary polarization direction and a wave propagating in the core with a rectilinear polarization parallel to the extraordinary polarization direction so that the optical signal measured by the sensor is an optical signal modulated by the Vernier effect. 2) The processing unit comprises a memory containing a digital recording of the standard signal and the processing unit is configured to digitally generate the Vernier modulated signal from the signal measured by the sensor and the digital recording of the standard signal. 3) The second Bragg grating is made in a second portion of the core of the same waveguide. 4) - the second Bragg grating is made in a second portion of the core of the same waveguide or in another waveguide, and - the sensitivity of the second Bragg grating to variations in the physical quantity is smaller than the sensitivity of the first Bragg grating to variations in the physical quantity. 5) The core of the waveguide in which the first Bragg grating is made is in germanosilicate and the first Bragg grating has color centers resulting from the recombination of bonds between germanium and silica while the second Bragg grating lacks such color centers. 6) The pitch of the second Bragg grating is between 0.95A, and 1.05A,, where A, is the first pitch of the first Bragg grating. n - each pattern of the first 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. 8) Each pattern consists of several disjointed bubbles arranged next to each other in the plane of the pattern. 9) - the number of peaks of the first and second power spectra within the wavelength range of interest is greater than a Vernier effect amplification factor M, and -the amplification factor M is defined by the following relation: M = (ISL, * ISL; M(ISL, — ISLz), where ISL, and ISL; are, respectively, the free spectral intervals of the first and second Bragg gratings. 10) For each Bragg grating made in the core of the waveguide: - the Bragg grating patterns are spaced from each other by a constant pitch 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. 11) Each pattern is made using a pulse from a femtosecond laser. 12) The physical quantity is chosen from the group consisting of a temperature at the first portion, a mechanical deformation of the first portion and a hydrostatic pressure applied to the first portion. The invention also relates to a waveguide for producing the above measuring device, in which the waveguide comprises: - a core which extends along a longitudinal axis and inside which an optical signal guided by the waveguide is able to propagate along the longitudinal axis of the waveguide, - a first Bragg grating produced in a first portion of the core of the waveguide intended to be exposed to variations in the physical quantity to be measured, this first Bragg grating comprising at least three identical patterns aligned one behind the other along the longitudinal axis of the waveguide and separated from each other by a first pitch, this first Bragg grating having a power spectrum whose peaks move in response to a variation in the physical quantity, - a second Bragg grating produced in a second portion of the core of the waveguide, this second Bragg grating comprising at least three identical patterns aligned one behind the other along the longitudinal axis of the waveguide and separated from each other by a second pitch, in which: - the first and second steps are different, and - the first step and the second step are such that the power spectra of the first and second Bragg gratings each exhibit several discernible harmonics of order greater than one hundred within a wavelength range of interest between 200 nm and 5000 nm. 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 first measuring device using the optical Vernier effect, - [Fig.2] is a schematic illustration, partial and in longitudinal section, of a first embodiment of an optical fiber of the measuring device of |Fig.1]; - [Fig.3] is a schematic illustration, in cross-section, of the optical fiber of [Fig.2]; - [Fig.4] is a graph representing a portion of the power spectrum of a Bragg grating of the optical fiber of [Fig.2]; - [Fig.5] is a flowchart of a manufacturing process for the optical fiber of [Fig.2]; - Figures 6 and 7 are graphs representing the reflection power spectra of different optical signals generated during operation of the measuring device of [Fig.1]; - figures 8 and 9 are schematic illustrations of a second and a third embodiment of a measuring device using the optical Vernier effect, - [Fig.10] is a partial schematic illustration, in longitudinal section, of a second embodiment of the optical fiber of [Fig.2]; - [Fig. 11] is a schematic illustration, in cross-section, of a third embodiment of the optical fiber of [Fig. 2], and - [Fig.12] is a partial schematic illustration, in longitudinal section, of the optical fiber of [Fig.11]. 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: Examples of embodiments [Fig. 1] represents a device 2 for measuring a physical quantity using the optical Vernier effect. This first embodiment is described in the particular case where the physical quantity to be measured is a tensile mechanical stress. By way of illustration, this mechanical stress is exerted by a carriage 4 which is pulled to the left by a force F. The device 2 comprises a waveguide 4, a distal end 6 of which is fixed, without any degree of freedom, on the carriage 4 to prevent this carriage 4 from moving under the action of the force F. The waveguide 4 also comprises a proximal end connected to a spectral analyzer 10. Waveguide 4 is an optical fiber. Therefore, the same numerical reference is used to designate this optical fiber. Optical fiber 4 is a single-mode optical fiber also known by the acronym SMF ("Simple Mode Fiber"). The spectral analyzer 10 is capable of establishing the spectral response of the optical fiber 4. For this, it comprises an optical source, such as a laser source 12, and an optical sensor 14. The laser source 12 is optically connected to the end 8 of the optical fiber 4. It emits an optical signal which propagates in the core of the optical fiber in a direction D which points towards the end 6. The wavelength of the optical signal emitted by the source 12 is in the optical domain. For example, here, the source 12 is a scanning laser source which emits a single-frequency optical signal at a wavelength λ, which varies over time to scan a predefined range of wavelengths of interest. For example, subsequently, this wavelength range of interest is the range of wavelengths between 1500 nm and 1600 nm. The sensor 14 is also optically connected to the end 8 of the optical fiber 4. The sensor 14 measures the optical signal backscattered by the optical fiber. The backscattered optical signal propagates in the optical fiber 4 in the opposite direction to the direction D. The sensor 14 has a spectral observation range located in the optical domain and which encompasses the wavelength range of interest. The optical fiber 4 is arranged so that the backscattered optical signal is representative of the stress exerted on the end 6 by the carriage 4. Furthermore, in this first embodiment, to increase the sensitivity of the device 2, the optical fiber is arranged so that the backscattered optical signal is an optical signal modulated in amplitude by the Vernier effect. For this, the optical fiber 4 comprises two Bragg gratings 20 and 22 produced one behind the other in its core. The gratings 20 and 22 are very high order Bragg gratings. 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, - A is the pitch of the Bragg grating, and -the symbol “*” denotes the scalar multiplication operation in this text. This k-order peak is in the optical domain. The reflected power spectrum is the power spectrum of the optical signal reflected by the Bragg grating. In this text, unless otherwise indicated, the term "power spectrum" refers to the reflected power spectrum. A peak in the reflection power spectrum corresponds to an absorption line in the transmission power spectrum of the same Bragg grating. The power spectrum of a very high order Bragg grating comprises a succession of very close and very fine 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 substantially 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 as defined in the introduction to this text. An example of such a comb is shown in [Fig.3] of 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 λ of the fundamental frequency fp of resonance of the Bragg grating is 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 fundamental frequency f; resonance 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-Perrot cavities. Indeed, the spectral characteristics of an optical fiber comprising such a juxtaposition of Fabry-Perrot cavities depend on the lengths of each Fabry-Perrot cavity as well as the reflectivity of the diopters located at each end of each Fabry-Perrot 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 that a Fabry Perot cavity can advantageously be replaced by a single very high order Bragg grating. The power spectrum of a Bragg grating shifts as a function of temperature, longitudinal elongation of the optical fiber, and hydrostatic pressure. To obtain a Vernier-modulated signal sensitive to force F, grating 20 is exposed to force F and, more specifically, subjected to the tensile mechanical stress exerted by this force F on end 6. Subsequently, the Bragg grating that is sensitive to the physical quantity to be measured is called the "measuring grating." Conversely, grating 22 is not or is less sensitive to the physical quantity to be measured. The Bragg grating that is less sensitive to the physical quantity to be measured is called the "standard grating." Here, a Bragg grating that is "sensitive" to a physical quantity refers to the fact that the power spectrum of this grating shifts when the physical quantity varies. In this first embodiment, to make the network 22 less sensitive to the tensile stress exerted by the force F, it is isolated, by an insulating structure 30, from the effects of the force F. For this, in this example, the structure 30 comprises a rigid arm 32 attached, without any degree of freedom, at an attachment point 34 to a segment 36 of the optical fiber. The segment 36 is located between the networks 20 and 22. The arm 32 is fixed and immobile relative to the portion 40 of the optical fiber 4 which extends from the attachment point 34 to its end 8. The arm 32 is sufficiently rigid so that the portion 40 is not subjected to the tensile force F. For a Vernier effect to appear, the free spectral interval, noted ISL22, of the network 22 is different from the free spectral interval, noted ISL.y, of the network 20. For this, the pitch Axy of the network 22 is different from the pitch As, of the network 20 in the absence of any external stress and therefore in the absence of the force F. The amplification factor M of the optical Vernier effect is defined by the following relation: M = ISL, / (ISL22-ISLa9). The larger this amplification factor, the greater the sensitivity of the device 2. To obtain a high factor M, i.e. greater than four and preferably greater than ten or twenty, the intervals ISLay and ISLn must be close and therefore the steps Asp and A» must also be close to each other. the other. Here, the deviation (ISLz,-ISL20) in absolute value is therefore less than ISLz2 / 4 and, preferably, less than ISL» / 10 or ISL, / 20. For this, typically, the step Az, is between 0.95Ax and 1.05A», or between 0.98Ax, and 1.02 A0. In order for the backscattered signal modulated by the optical Vernier effect to vary only as a function of the mechanical stress exerted by the force F, the gratings 20 and 22 operate under the same temperature and pressure conditions. For this purpose, here, the gratings 20 and 22 are located close to each other, i.e. the length of the segment 36 is small. Here, the length of the segment 36 is less than 10 cm and, preferably, less than 1 cm or 5 mm. The device 2 also comprises an electronic processing unit 40 configured to determine a variation in the measured physical quantity from the backscattered signal measured by the sensor 14. In this first embodiment, the backscattered signal is modulated by optical Vernier effect. Under these conditions, the unit 40 is programmed to extract the envelope of the backscattered signal in the wavelength range of interest and to deduce therefrom the position of a vertex of this envelope relative to a reference position. Typically, the reference position is the position of this vertex in the absence of the force F. Under these conditions, the difference between the deduced position and the reference position is representative of the variation AC of the tensile stress exerted on the end 6 compared to the case where the force F is zero. The unit 40 then determines a measured value of the tensile stress.For this, an initial value of the tensile stress in the absence of the force F is pre-recorded in the unit 40. The sensitivity S, of the network 20 to variations in this stress is also pre-recorded in the unit 40. This initial value and the sensitivity S. are typically determined during a calibration phase of the device 2. . In this text, the sensitivity Sç of a Bragg grating to variations AG of the physical quantity to be measured is defined by the following relation AAg / À5 = Sc*AG, where: - Àa is the fundamental wavelength of the 20 network, - Aks is the variation of the fundamental wavelength of the grating 20 obtained in response to the AG variation. Thus, the sensitivity S is defined by the following relation: AAp / A5 = S.*AC. To carry out these operations, the unit 40 comprises a programmable microprocessor 42 and a memory 44 containing the data and instructions necessary for the operation of the unit 40. Usually, the unit 40 also comprises a human-machine interface 46 for communicating the result of the measurements carried out to a human being. [Fig. 2] shows in more detail an embodiment of the network 20. The optical fiber 4 extends along a longitudinal axis 58 parallel to a direction Z of an orthogonal reference frame XYZ. Figures 2 and 3 are oriented relative to this reference frame XYZ. For example, the Z direction is horizontal and the Ÿ direction is vertical. To simplify [Fig.2], only the portion of the optical fiber 4 that contains the grating 20 is shown. The optical fiber 4 guides the optical signal along the longitudinal axis 58. Optical fiber 4 includes: - a core 60 in which the optical signal guided by this fiber 4 propagates, - an optical cladding 62 made of a material whose refractive index makes it possible to maintain the optical signal inside the core 60 by reflection at the interface between the core 60 and this cladding 62, and - a mechanical sheath, typically made of polymer, and which covers the sheath 62. To simplify [Fig.2], the mechanical sheath of the optical fiber 4 has not been shown. The grating 20 is designed to obtain a comb of peaks over the wavelength range of interest centered on a wavelength λ and whose width is greater than 100 nm. Here, the wavelength λ is equal to 1550 nm. Furthermore, the network 20 is designed so that this comb is formed by the harmonics of the network 20 of order close to 1024. For this purpose, the network 20 is composed of a succession of patterns M; arranged one behind the other along the axis 58. The index i is the order number of the pattern in the Z direction. The index i of the first leftmost pattern in the network 20 is equal to 1 and the index i of the last rightmost pattern in the network 20 is equal to p. p is equal to the number of patterns M; of the network 20. In [Fig.2], only the first two and the last two patterns of the network 20 have been represented. The presence of the intermediate patterns located between the patterns M and M, is represented by small circles on the axis 58. 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 more the width at half-height of each peak decreases. Here, the number p is also chosen small enough so that the length of the network 20 remains small, i.e. less than 1 meter and, preferably, less than 10 cm.The length of the network 20 is equal to the distance between the patterns M, and M, measured along the axis 58. Typically, the number p is less than 200 or 100. . The pitch Az, between two patterns M; and M,,, immediately consecutive in the Z direction is constant whatever the index i. The pitch Az, is therefore equal to the distance, along the axis 58, which separates two patterns M, and M; ,1 immediately consecutive. Here, the pitch Aa, is calculated so that the order k. of the harmonic closest to the wavelength λ is equal to 1024. For this, the step Az is between 0.9*[k.*A, / (2*ne)] and 1.1*[k.*A: / (2*n,)] and, preferably, between 0.98*[k*A / (2*ne)] and 1.02*[k.*A. / (2*ne)], where ne is the index effective fiber optic 4. The effective propagation index n, is also known as the "mode phase constant". It is defined by the following relationship: n, = n, - Adn, / d), where n, is the group index and Δ is the wavelength of the optical signal guided by the optical fiber 4. The effective propagation index of an optical fiber depends on the dimensions of the core of this optical fiber and the materials forming this core and the optical cladding of this optical fiber. It can be determined experimentally or by numerical simulation. Here, the optical fiber 4 is 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 k.«A / (2*n,) is equal to approximately 540.8 um. Here, the pitch Az is chosen to be equal to 540.8 um. With the choice of this value for the pitch Azp, only the harmonics of order between 317 and 7936 are in the optical domain. In particular, the wavelength Ås of the fundamental frequency of the network 20 is not in the optical domain. For this value of the pitch Az, and so that the length Lay of the grating 20 is less than 10 cm, the number p of patterns is chosen to be less than 185. Here, p is chosen to be equal to 120, so that the length La of the grating 20 is approximately equal to 65 mm. The patterns M; are all structurally identical to each other and differ from each other only in their position along the axis 58. Thus, in the following, only the pattern M; is described in detail. This pattern M; extends mainly in a plane P; perpendicular to the axis 58. This plane P; is therefore parallel to the X and Y directions. In [Fig. 2], only the planes P,, P, P,.; and P, in which the patterns M,, M», M, and M, extend respectively, are shown. [Fig.3] shows in more detail an example of the embodiment of the pattern M;. In [Fig.3], only the cross-section of the core 60 is shown. Each pattern M; reflects a portion of the incident optical signal. Another portion of the incident optical signal passes through the pattern M;. Finally, each pattern M; scatters a portion of the energy of the incident optical signal which is then neither reflected nor transmitted through this pattern M;. This energy scattered by each pattern M; creates insertion losses caused by the presence of the grating 20 in the core 60 of the optical fiber 4. To minimize these insertion losses, here, the surface Sy of the cross-section of the pattern M; occupies less than half of the surface Sç of the cross-section of the core 60. The surface Sy; is equal to the surface of the orthogonal projection of the pattern M, on the plane P;. The surface Se is equal to the surface of the cross-section of the core 60. Typically, the surface Se is constant along the entire length of the optical fiber 4 Preferably, the surface area Sm; is less than 0.1*Sg9 or 0.05*S« or 0.01*S4p. Here, the surface Sy;is less than 0.05#*S50. To obtain sufficient reflectivity of the pattern M; to limit the number p of patterns and therefore to limit the length Lay of the network 20, 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 is an identifier that uniquely identifies the bubble B; among all the 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 60 in the direction of propagation of the optical signal. For this, the difference between the refractive index n:o of the core 60 and the refractive index n,3 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,59 and n,s greater than or equal to 0.4. Furthermore, for the refractive index variation 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 D; of bubble B; is equal to the diameter of the smallest volume sphere that entirely contains bubble B;. Here, this diameter D 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 58 axis. 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 58 axis and the center of at least one of the bubbles B; is located less than 100 nm from the 58 axis. In this first embodiment, the barycenter of the pattern M; is located on the axis 58. In addition, the pattern M; is symmetrical with respect to the axis 58. The centers of the bubbles B; are located one behind the other on an axis A; which intersects the axis 58 and which belongs to the plane P,. The pattern M; therefore comprises a line of disjointed bubbles. In this case, the arrangement of the disjointed bubbles forms what is called a "dotted line" in this text. Here, the axis A; is parallel to the direction Y. In this mode of realization, the bubbles Bet B4 are located, respectively, above and below the axis 58. The centers of the bubbles B; and B4 are less than 100 nm from the axis 58. 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;,, immediately consecutive along the axis A, the distance separating the centers of these two bubbles is the same. [Fig.4] represents the power spectrum of optical fiber 4 between 1545 nm and 1555 nm. The reflectivity of the peaks of the obtained comb reaches -21 dBm. [Fig. 5] represents a method of manufacturing the optical fiber 4. This method begins with a step 70 of providing an optical fiber whose core 60 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 72, the network 20 is manufactured in the core 60. For this, an operation 74 of forming the pattern Mi in the core 60 of the optical fiber provided is repeated at each location where such a pattern M; must be formed. In operation 74, each bubble B; is created by a single pulse of the femtosecond laser. More precisely, in operation 74, 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 60 where the center of the bubble B; must be located. The bubble B; is then created in the core 60. Next, the optical fiber is moved relative to 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 various parameters of a femtosecond laser for creating 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 various 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 application CN211603608U which describes in detail an example of an installation for forming bubbles such as bubbles B; in the core of an optical fiber. Here, the following parameters were used to manufacture the optical fiber 4: - 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. Once the grating 20 has been manufactured, the method continues with a step 76 of manufacturing the grating 22 in the core 60 of the optical fiber 4. Step 76 is identical to step 72 except that the pitch A of the grating 22 is different from the pitch Asp of the grating 20. For example, the pitch Am is determined by applying the teaching above in the particular case where the order k, of the harmonic closest to the wavelength λ, is chosen equal to 1117. Figures 6 and 7 illustrate the operation of the device 2. In each of these figures, the top graph represents the power spectrum of the network 22 and the middle graph represents the power spectrum of the network 20. The bottom graph in Figures 6 and 7 represents the power spectrum of the optical Vernier modulated signal measured by the sensor 14. For each of these graphs, the abscissa axis is graduated in nanometers. The ordinate axis represents the power of the backscattered signal. It is graduated in an arbitrary unit ua. On each of these graphs, the dotted vertical line marks the position of the wavelength λ. The graphs in [Fig. 6] represent the power spectra in the absence of the force F. The graphs in [Fig. 7] represent the power spectra in the presence of a non-zero force F. As visible by comparing the middle graphs, the presence of force F shifts the power spectrum of network 20 to the right.This shift results in a shift of the peak of the envelope of the power spectrum of the backscattered signal. The shift of the peak of the envelope is amplified by the factor M compared to the shift observable only on the power spectrum of the network 20. Here, the factor M is equal to twelve. [Fig.8] shows a device 90 for measuring a tensile stress. The device 90 is structurally identical to the device 2 except that the optical fiber 4 is replaced by an optical fiber 94. The optical fiber 94 is identical to the optical fiber 4 except that the grating 22 is omitted. Under these conditions, the backscattered signal corresponds only to the interaction of the optical signal emitted by the source 12 with the grating 20. In this embodiment, the unit 40 is configured to obtain the Vernier modulated signal from the backscattered signal measured by the sensor 14 and from a digital recording 96 of a standard signal contained in the memory 44. The standard signal is the same as that which is backscattered by the grating 22 in the absence of the grating 20 when it interacts with the optical signal emitted by the source 12. The digital recording of this standard signal can be obtained by digital simulation or by measuring the signal backscattered by the grating 22 alone in the absence of the grating 20. Then, the signal back-scattered by the network 20 and measured by the sensor 14 and the digital recording 96 are digitally combined to obtain the optical signal modulated by Vernier effect. Thus, in this embodiment, the optical Vernier effect is simulated by calculation. The simulation of a Vernier effect by calculation is described in the following article: Chen ZHU et al: "High-sensitivity optical fiber sensing based on a computational and distributed Vernier effect," Opt. Express 30, 37566-37578 (2022). Hereinafter, this article is designated by the reference "ZHU2022". More precisely, in this article, the simulation of a Vernier effect is described in the particular case where the measurement interferometer is a Fabry-Perot cavity. However, the teaching given in this article can be directly applied to the case where the measurement interferometer is the grating 20. Indeed, the principles are the same. Thus, the various calculations implemented in unit 40 to numerically simulate the Vernier effect are not described here in more detail.The remainder of the operation of device 90 is identical to the operation of device 2. [Fig.9] shows a device 100 for multi-point measurement of tensile stress. The device 100 is structurally identical to the device 90 except that the optical fiber 94 is replaced by an optical fiber 104. The optical fiber 104 is identical to the optical fiber 94 except that it comprises N Bragg gratings 20, at 20y located one after the other along the longitudinal axis of the fiber 104. Each of the gratings 20; is located at a location where a tensile stress is to be measured, where the index i is the order number of the grating in the direction D. The number N of gratings 20; is greater than two and, for example, greater than four or ten. Each grating 20; is separated from the immediately consecutive grating 20;,, by a segment 36; of the fiber 104 devoid of Bragg grating. In this embodiment, a tensile stress is exerted directly on each segment 36; by a respective carriage 4; which is pulled by a force F, in the direction D. Each grating 20; is a very high order Bragg grating having a pitch λ; and a free spectral interval ISL; in the wavelength range of interest. The pitch λ; of grating 20; is different from the pitch of all other Bragg gratings made in optical fiber 104. For example, here, grating 20, is identical to grating 20 except that its pitch λ; is different. Under these conditions, the backscattered signal corresponds to the superposition of the signals backscattered by each of the networks 20. A digital copy 106; of a standard signal is pre-recorded for each network 20,. For each network 20, the unit 40 is configured to obtain the optical Vernier modulated signal corresponding to this network 20; from the backscattered signal measured by the sensor 14 and from the digital copy 106;. For this, the procedure is as described in the article ZHU2022. Indeed, this article describes the calculations to implement to obtain the signal modulated by optical Vernier effect by digital simulation in the case where the optical fiber has several measuring interferometers distributed along its entire length. In the preceding embodiments, the power spectrum of the measurement network shifts in response to a longitudinal deformation. It is also known that the power spectrum shifts when the measurement network is exposed to a variation in temperature and / or a variation in hydrostatic pressure. However, it is also possible to make the measurement network sensitive to a variation in another physical quantity while the standard network is less sensitive to variations in this other physical quantity. In this case, the insulating structure can be omitted. By way of illustration, [Fig. 10] represents a portion of an optical fiber 120 capable of being used to measure a dose of neutron radiation or ionizing radiation such as gamma radiation. The optical fiber 120 is identical to the optical fiber 4 except that the grating 20 is replaced by a measuring grating 122 that is more sensitive to a dose of the radiation to be measured than the grating 22. For this, the core 60 is made of a photosensitive material. Here, the core 60 is made of germanosilicate. Initially, the grating 20 is manufactured in the core 60 as described previously. Then, the grating 20 is transformed into a grating 122 that is more sensitive to a dose of the radiation to be measured. For this, only the manufactured grating 20 is exposed to ultraviolet radiation to create colored centers resulting from the recombination of the bonds between the germanium and the silica. When they are subjected to a dose of the radiation to be measured, these colored centers are modified, leading to a shift in the wavelength λ of the grating 122. Thus, the grating 122 is sensitive to a dose of the radiation to be measured while the grating 22 is not or is less so.More specifically, when both gratings 22 and 122 are exposed to the same dose of the radiation to be measured, the power spectrum of grating 122 shifts much more than the power spectrum of grating 22. The magnitude of the difference between the shift of grating 122 and the shift of grating 22 is measured by unit 40 using the optical Vernier effect. The magnitude of this measured difference is proportional to the dose of the radiation to be measured multiplied by the difference between the sensitivity Sd,», of grating 122 and the sensitivity Sd;; of grating 22. The sensitivity Sd of a Bragg grating to a radiation dose is defined by the following relationship: AÀp / À5 = Sd*D, where D is the radiation dose. Typically, the sensitivity Sd,» is less than 0.9Sd,-; or 0.5Sd,>. This difference in sensitivity is, for example, determined experimentally and then recorded in the memory 44. The unit 40 is then capable of calculating the dose of the measured radiation from the amplitude of the measured difference. In this embodiment, the insulating structure 32 is omitted. Thus, the networks 22 and 122 are exposed to the same temperature variations, the same longitudinal deformations and the same variations in hydrostatic pressure. Since the networks 22 and 122 have the same sensitivities to these variations in temperature, hydrostatic pressure and longitudinal deformation, in response to these variations, the shifts in the power spectra of the networks 22 and 122 are the same. Consequently, the shift of the peak of the envelope of the Vernier modulated signal is very small and can be neglected compared to the shift of this peak produced by a dose of the radiation to be measured. Figures 11 and 12 show an optical fiber 130 that can replace the optical fiber 4. In this embodiment, the same very high order Bragg grating is used to fulfill both the functions of measurement grating and standard grating. For this purpose, the optical fiber 130 is a birefringent optical fiber having an ordinary refractive index n for waves propagating in the core with a linear polarization parallel to an ordinary polarization direction and an extraordinary refractive index n for waves propagating in the core with a linear polarization parallel to an extraordinary polarization direction. The extraordinary polarization direction is different from the ordinary polarization direction. Here, the ordinary and extraordinary polarization directions are orthogonal. In this embodiment, the optical fiber 130 is identical to the optical fiber 4 except that: - it also includes a polarization maintenance element 132, and - network 22 is omitted. This element 132 is here located in the cladding 62. Here, the element 132 is arranged and designed to exert an asymmetric mechanical stress on the core 60. For example, the element 132 is the polarization maintaining element encountered in optical fibers known as "PANDA optical fiber" or simply "PANDA fiber". In this case, the element 132 comprises two longitudinal cylinders of boron-doped glass positioned in the cladding 62 and located on opposite sides of the core 60. Only the grating 20 is made in the core 60 of the fiber 130. In this case, when the optical signal propagating in the core 60 is linearly polarized only along the ordinary polarization direction, the power spectrum of the signal backscattered by the grating 20 has a free spectral interval ISL. When the optical signal propagating in the core 60 is linearly polarized only along the extraordinary polarization direction, the power spectrum of the signal backscattered by the grating 20 has a free spectral interval ISLe. The interval ISL is different from the interval ISLo. The sensitivity of the index n to variations in the physical quantity to be measured is different from the sensitivity of the index n, to variations in the physical quantity to be measured. Thus, the ordinary sensitivity S, of the network 20 to variations in the physical quantity to be measured when the polarization direction of the optical signal is parallel to the ordinary direction is different from the extraordinary sensitivity S, of the network 20 when the polarization direction of the optical signal is parallel to the extraordinary direction. On the other hand, the sensitivities S, and S, of the network 20 to physical quantities other than that to be measured are the same for ordinary and extraordinary polarizations. In the case of a PANDA fiber, this property is obtained when the physical quantity to be measured is hydrostatic pressure. In this case, the sensitivity S, of the network 20 to variation of the hydrostatic pressure is defined by the following relation: Adg / Àp = s,*AP, where: - AÀs is the variation of the fundamental wavelength of the Bragg grating for ordinary polarization, and - AP is the variation of hydrostatic pressure. Similarly, the sensitivity S, of the grating 20 is defined by the following relation: A)\p / \g = S.*AP, Where Ads is this time the variation of the fundamental wavelength of the Bragg grating for an extraordinary polarization. When the fiber 130 is used instead of the fiber 4, the laser source 12 emits an optical signal comprising simultaneously a wave propagating in the core with a rectilinear polarization parallel to the ordinary polarization direction and a wave propagating in the core with a rectilinear polarization parallel to the extraordinary polarization direction. Under these conditions, the backscattered signal measured by the sensor 14 is modulated by optical Vernier effect. The shift of the peak of the envelope of the backscattered signal is in this case proportional to the variation of the hydrostatic pressure. Thus, when the fiber 130 is used, the device measures the variations of the hydrostatic pressure at the level of the portion of fiber containing the network 20. Chapter II: Vari. Variants of the Bragg grating: 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 desired length for the Bragg grating 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 λ 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 Bragg grating 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 wavelength range of interest may be wider than 100 nm. For example, the width of this wavelength range of interest is, alternatively, greater than 200 nm or 300 nm. There is no upper limit to the width of this wavelength range of interest other than that it must be within the optical domain and that it must correspond to a range of wavelengths that the optical source can emit. 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 of the measuring device described here. The standard grating can be replaced by a standard interferometer that produces the same comb but using a different technology than a very high-order Bragg grating. For example, the standard grating can be replaced by a Fabry-Perot cavity obtained by attaching mirrors to the ends of an optical fiber. The Bragg grating patterns formed in the core of the optical fiber may have different shapes. For example, alternatively, each pattern comprises a single bubble. In another embodiment, as described in article LUO2022, each pattern has the shape of an ellipse. 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 a multimode optical fiber or MMF (Multi-Mode Fiber). The core of the optical fiber does not need to be specifically doped. Thus, the manufacturing process described can be implemented with optical fibers whose core is made of germanosilicates, pure silica, rare earth-doped aluminosilicates, or sapphire. Alternatively, the standard grating and the measuring grating are implemented in different optical fibers optically coupled to each other via an optical coupler. In this case, the arrangement is identical to that described with reference to [Fig. 1] of the article CHEN2022 except that the measuring interferometer and the standard interferometer are each implemented in the form of a very high order Bragg grating. When the standard network is produced in a first optical fiber different from a second optical fiber in which the measurement network is produced, then, as a variant, the characteristics of the first fiber are chosen to be different from the characteristics of the second fiber so that the sensitivity of the standard network to variations in the physical quantity to be measured is less than the sensitivity of the measurement network to these same variations in the physical quantity. Variants of the insulating structure: Other embodiments of the insulating structure are possible. For example, the measurement Bragg grating and / or the Bragg standard grating are isolated from variations in mechanical stress by implementing the teaching of application FR3087008A 1 by isolating the core of the optical fiber by micromachining. In the case where the physical quantity to be measured is temperature or hydrostatic pressure, the insulating structure is designed to isolate the standard network, respectively, from temperature variations or hydrostatic pressure variations. When the measuring network has been made sensitive to a physical quantity to which the standard network is insensitive or less sensitive, as described in the embodiment of [Fig. 10], then the insulating structure can be omitted. Variants of the manufacturing process: There are many variations of the method for manufacturing a very high order Bragg grating. In particular, all the manufacturing methods and their variations 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 method described in article LUO2022 can also be used. Other variants: The ordinary and extraordinary polarization directions may not be perpendicular to each other. In a particular embodiment, the optical fiber has more than two different refractive indices and therefore more than three possible polarization directions. In such a case, for example, only two of the po- larization are used to generate the optical signal modulated by optical Vernier effect. Other embodiments of a birefringent fiber are possible. For example, other embodiments of a birefringent fiber are known as a "bowtie optical fiber" or an "elliptical stress layer" optical fiber. Birefringence can also be achieved by using a core with an elliptical cross-section. In the latter case, the polarization-maintaining element is the shape of the core cross-section and is not located in the cladding 62. There are still other ways to achieve birefringence. One example is the use of longitudinal air holes or voids in photonic crystal fibers. In a simplified variant, unit 40 only calculates the variation of the measured physical quantity and not its absolute value. In this case, it is not necessary to know the initial value of the measured physical quantity. The sensitivity Sç of a Bragg grating to variations AG of the physical quantity to be measured can be, as a first approximation, considered to be constant over the entire operating range of the measuring device. This is particularly true if the operating range is small, i.e. the range within which the value of the physical quantity varies is small. However, as a variant, it is also possible to consider that the sensitivity Sc is not constant over the entire operating range. In the latter case, typically, during a calibration phase of the measuring device, a calibration law is constructed. This law associates with each measured variation AÀ, of the fundamental wavelength of the measuring grating, a corresponding value of the measured physical quantity.This law is then stored in memory 44 and then used by the measuring device to convert each variation A}, measured into a measured value of the physical quantity. The power spectrum of a measurement network is shifted in response to a temperature variation, a longitudinal deformation or a variation in hydrostatic pressure. Thus, all the preceding embodiments can be adapted to measure a physical quantity 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 sensor 14 can be connected to the distal end 6 of the optical fiber instead of being connected to its proximal end 8. In this case, the sensor 14 measures the optical signal that has passed through the Bragg grating. 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 14 is connected to the end 8 adapts, without particular difficulty, to the case where the sensor 14 is connected to the distal end. The source 12 is not necessarily a scanning laser source. For example, the source 12 is replaced by a wide laser source, i.e. a laser source which emits an optical signal whose power spectrum simultaneously covers the entire wavelength range of interest. 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 14 is an array spectrometer. In such an embodiment, it is not necessary to vary the wavelength λ, to scan the entire working range. In another variant, the source 12 is not a laser source. For example, the source 12 can also be produced using a tunable Fabry-Pérot cavity. Several of the variants described above can be combined in a single embodiment. Chapter II: of the achievements described: Very high order Bragg gratings have peak combs that allow high sensitivity to be achieved when used as an interferometer in a measuring device using the optical Vernier effect. In addition, very high order Bragg gratings are simple to fabricate in the core of a waveguide. In particular, there is no need for elements external to the waveguide, such as mirrors connected to the ends of the waveguide. Thus, by using very high order Bragg gratings it is possible to obtain a measuring device using the optical Vernier effect that is both simple to fabricate and exhibits high sensitivity. This improvement in sensitivity is explained by the fact that the devices described here exploit the very high order harmonics of the Bragg gratings and not the harmonics close to the wavelength λ.These very high order harmonics are finer than those obtained using known interferometers made in the core of the waveguide, which makes it possible to improve the sensitivity of the measuring device without complicating its manufacture. The use of a birefringent optical fiber makes it possible to generate the optical signal modulated by the Vernier effect using a single Bragg grating made in the core of this optical fiber. This simplifies the production of the optical fiber. The generation of the optical Vernier modulated signal from the digital recording 96 of the standard signal makes it possible to avoid the creation of the second Bragg grating in the core of the optical fiber. In addition, the digital recording of the standard signal is completely independent of variations in the physical quantity to be measured. It is therefore not necessary to provide an insulating physical structure to limit variations in this standard signal. The fact of realizing the standard network in the core of the same optical fiber as that where the measurement network is realized, simplifies the structure of the measurement device. The fact that the standard network is less sensitive to variations in the physical quantity to be measured than the measuring network makes it possible to avoid the use of an insulating structure such as structure 30. The fact that the measuring network has colored centers makes it possible to measure a radiation dose. The fact that the steps of the standard network and the measurement network are very close to each other makes it possible to obtain a significant amplification factor and therefore to increase the sensitivity of the measuring device. Using one or more bubbles in each pattern allows for a small pattern and therefore substantially reduces insertion losses. Using multiple disjoint bubbles allows for a sufficiently reflective pattern to reduce the number of patterns and thus maintain the compactness of the Bragg grating 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 that is subjected to several pulses of the femtosecond laser degrades. This degradation increases scattering losses. Conversely, when the bubbles are disjoint, such overlapping areas do not exist, which limits insertion losses.

Claims

Claims

1. Device for measuring a physical quantity using the Vernier effect optical, this device comprising: - a waveguide (4; 94; 104; 120; 130) containing a core (60) which extends along a longitudinal axis (58) and within which a optical signal guided by the waveguide is able to propagate along the longitudinal axis of the waveguide, - a first Bragg grating (20; 20, to 20, ; 122) produced in a first portion of the waveguide core exposed to variations in the physical quantity, this first Bragg grating comprising at least three identical patterns aligned one behind the other along the longitudinal axis of the waveguide and separated from each other by a first step, this first Bragg grating presenting a spectrum of power whose peaks move in response to a variation in the physical size, - an optical source (12) connected to the waveguide and capable of emitting a optical signal which propagates along the longitudinal axis of this guide wavelength, the wavelengths of this emitted optical signal being included in within a wavelength range of interest between 200 nm at 5000 nm, - a sensor (14) connected to the waveguide and capable of measuring the signal optical emitted by the optical source after it has interacted with the first Bragg grating, - an electronic processing unit (40) configured to: - obtain, from the signal measured by the sensor, a signal modulated by Vernier effect, this signal modulated by Vernier effect corresponding to the su- perposition of a measured signal resulting from the interaction of the signal optical emitted with the first Bragg grating and a cor- responding to the interaction of the same emitted optical signal with a second Bragg grating whose pitch between patterns is different from the first pitch and whose power spectrum peaks shift less in response to the same variation of the physical quantity as the peaks of the spectrum of power of the first Bragg grating, and - calculate a measure of the variation of the physical quantity from the envelope of the power spectrum of the Vernier modulated signal got, characterized in that the first step is such that the power spectrum of the first Bragg grating exhibits several discernible harmonics of order greater than one hundred within the wavelength range of interest.

2. Device according to claim 1, wherein: - the waveguide (130) is a birefringent optical fiber having a ordinary refractive index for waves propagating in the core with a linear polarization parallel to a polarization direction ordinary and an extraordinary refractive index for waves propagating in the core with a rectilinear polarization parallel to a extraordinary polarization direction, the ex-polarization direction extraordinary being different from the ordinary polarization direction and the sensitivity of the ordinary refractive index to variations in the physical quantity to be measured is different from the sensitivity of the index of extraordinary refraction to variations of the physical quantity at measure, and - the optical source (12) is capable of emitting an optical signal comprising simultaneously a wave propagating in the heart with a polarity rectilinear polarization parallel to the ordinary polarization direction and a wave propagating in the core with rectilinear polarization parallel to the direction of extraordinary polarization so that the optical signal measured by the sensor (14) is a modulated optical signal by Vernier effect.

3. A device according to claim 1, wherein the processing unit comprises a memory (44) containing a digital recording (96) of the standard signal and the processing unit is configured to generate digitally modulated signal by Vernier effect from the signal measured by the sensor (14) and digital recording of the signal standard.

4. A device according to claim 1, wherein the second network of Bragg (22) is made in a second portion of the core of the same guide wave.

5. Device according to claim 1, wherein: - the second Bragg grating (22) is made in a second portion of the core of the same waveguide or in another waveguide, and - the sensitivity of the second Bragg grating to variations in the magnitude physics is smaller than the sensitivity of the first Bragg grating to variations in physical quantity.

6. A device according to claim 5, wherein the core of the waveguide in which the first Bragg grating is made is made of germanosilicate and the first Bragg grating has colored centers from the recombination of bonds between germanium and silica while the second Bragg grating is devoid of such color centers.

7. Device according to any one of the preceding claims, in which the pitch of the second Bragg grating is between 0.95A, and 1.05A,, where Δ, is the first pitch of the first Bragg grating.

8. A device according to any preceding claim, in which : - each pattern (M,, M, My,, My) of the first Bragg grating (20; 20, at 20, ; 122) extends mainly in a plane, called the "plane of the pattern”, perpendicular to the longitudinal axis of the waveguide, and - each pattern is made up of one or more bubbles (B, - Bs) 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 (60) of the waveguide.

9. A device according to claim 8, 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.

10. A device according to any preceding claim, in which : - the number of peaks of the first and second power spectra at within the wavelength range of interest is greater than one Vernier effect amplification factor M, and - the amplification factor M is defined by the following relation: M = (ISL, * ISL:) / (ISL, — ISL:), where ISL, and ISL: are, respectively, the in- free spectral intervals of the first and second Bragg gratings.

11. | Device according to any one of the preceding claims, in which for each Bragg grating made in the core of the guide wave: - the patterns (M,, M>, Mx-1, Mn) of the Bragg grating are spaced apart from others by a constant step greater than or equal to 20 um, and - the difference between the refractive index of the core (60) of the waveguide and the refractive index of each Bragg grating pattern is greater than 0.3

12. A device according to any preceding claim, in which each pattern (M;, M», Myn.1, Mn) is made using a pulse of a femtosecond laser.

13. A device according to any preceding claim, in which the physical quantity is chosen from the group consisting of a temperature at the level of the first portion, of a deformation mechanics of the first portion and hydrostatic pressure applied to the first portion.

14. Waveguide for producing a measuring device in accordance with the claim 4, wherein the waveguide comprises: - a core (60) which extends along a longitudinal axis (58) and inside which an optical signal guided by the waveguide is able to propagate along the longitudinal axis of the waveguide, - a first Bragg grating (20; 122) produced in a first portion of the waveguide core intended to be exposed to variations of the physical quantity to be measured, this first Bragg grating comprising at least three identical patterns aligned one behind the other others along the longitudinal axis of the waveguide and separated from each other from others by a first step, this first Bragg network presenting a power spectrum whose peaks shift in response to a variation of physical quantity, - a second Bragg grating (22) produced in a second portion of the core of the waveguide, this second Bragg grating comprising at least three identical patterns aligned one behind the other along the longitudinal axis of the waveguide and separated from each other by a second step, characterized in that: - the first and second steps are different, and - the first step and the second step are such that the power spectra of the first and second Bragg gratings each present several discernible harmonics of order greater than one hundred within a wavelength range of interest between 200 nm and 5000 nm.