Bragg-grid abrasion sensor for harsh environments

Description

TECHNICAL FIELD

[0001] The invention relates to the field of fiber optic sensors and, more specifically, to the field of Bragg grating fiber optic sensors. It relates to an erosion sensor comprising a variable-pitch Bragg grating inscribed on a measurement section intended to undergo erosion.

[0002] The invention is particularly applicable in the field of predictive maintenance, for monitoring the wear of parts by means of integrated sensors. It is of particular interest when the fiber optic sensors are subjected to difficult environments, for example high temperatures and / or ionizing radiation irreversibly modifying the optical properties of the optical fiber. The invention is also particularly useful when the fiber optic sensors are subjected to significant thermomechanical variations, notably varying the sensitivity of the sensor. ÉTAT DE LA TECHNIQUE ANTÉRIEURE

[0003] Chirped fiber Bragg gratings are optical components that create a direct, potentially linear, relationship between their physical dimension and the spectral width of the radiation reflected by these Bragg gratings. Bragg gratings can thus be used as erosion sensors, the physical length of a grating being determined from the measurement of the spectral width of the reflected radiation. In harsh environments, the optical fiber and the Bragg grating inscribed in this fiber undergo accelerated aging leading to a change in their optical properties. Aging can be accelerated in particular by exposure to high temperatures and ionizing radiation.Furthermore, the spectral response of variable pitch Bragg gratings is also influenced by the temperature, pressure and strain conditions to which the Bragg gratings are subjected. As long as these conditions remain relatively stable and close to the calibration conditions, the reflected radiation remains mainly influenced by the decrease in the length of the variable pitch Bragg grating.

[0004] XU OU's publication: "High-Speed ​​Interrogation of a Fiber Grating Sensor based on Wavelength-to-Time Mapping and a Fabry-Perot Configuration", Asia Communication and Photonics Conf., IEEE, October 26, 2018, pages 1-3 describes a strain sensing system using fiber optic Bragg gratings (FBGs) with variable pitch; a measurement FBG and a reference FBG, both with variable pitch.

[0005] On the other hand, when a fiber Bragg grating with variable pitch is subjected to significant variations in temperature, pressure and / or strain, the spectrum of the radiation reflected by the Bragg grating undergoes both a frequency shift and a change in its width. As a result, the aging of the fiber and the variations in the physical parameters to which the fiber Bragg grating is subjected distort the measurement of the physical length of the fiber Bragg grating.

[0006] Consequently, although some optical fibers incorporating Bragg gratings are suitable for use as sensors in extreme environments, particularly due to their mechanical strength, their measurement accuracy is impaired in these environments.

[0007] An aim of the invention is therefore to propose a solution for reliably measuring the physical length of a variable-pitch fiber Bragg grating despite its aging and variations in its environmental parameters. The invention also aims to provide an erosion sensor whose design, manufacturing and maintenance costs are compatible with use on an industrial scale. DISCLOSURE OF THE INVENTION

[0008] To this end, the invention is based on taking into account the environment of the variable-pitch Bragg grating, called the measurement Bragg grating, by measuring the radiation reflected by another Bragg grating, called the reference Bragg grating, inscribed in the same optical fiber or in a neighboring optical fiber, and therefore subject to physical parameters identical or similar to those to which the variable-pitch Bragg grating is subjected.

[0009] More specifically, the invention relates to a Bragg grating erosion sensor according to claim 1.

[0010] For given temperature, pressure and strain conditions, and for a given physical length of the measurement Bragg grating, said measurement Bragg grating is arranged to reflect radiation in a wavelength range called the "measurement range". When the measurement Bragg grating has not undergone any erosion, the measurement range is called the "initial measurement range". For the same temperature, pressure and strain conditions, the width of the measurement range decreases with the erosion of the measurement section. Furthermore, each reference Bragg grating is arranged to reflect radiation in a wavelength range called the "reference range". In the presence of several reference Bragg gratings, the different reference ranges may be identical, disjoint, or partially overlap.In the present description, any comparison between two wavelength ranges implies that the Bragg gratings considered are subjected to identical or similar temperature, pressure and deformation conditions.

[0011] When the erosion sensor comprises several optical fibers, the optical fibers are preferably arranged relative to each other so as to be subjected to the same temperature and pressure conditions. The optical fibers may in particular be mechanically linked to each other. In particular, they may be mechanically linked over a section encompassing both the measurement Bragg grating and the one or more reference Bragg gratings. Thus, the reference Bragg grating(s) are subjected to the same deformations as the measurement Bragg grating.

[0012] According to the invention at least one of the one or more reference Bragg gratings is a constant pitch Bragg grating. Such a Bragg grating is formed of patterns spaced from each other by the same distance along the longitudinal axis of the optical fiber. It reflects incident radiation at a wavelength called the "Bragg wavelength" or "reference wavelength". In practice, the radiation can be reflected over a wavelength range of a few nanometers.

[0013] The measurement Bragg grating and at least one of the one or more reference Bragg gratings may be inscribed in the same core of an optical fiber. Each reference Bragg grating may be inscribed in the measurement section or outside this measurement section. Preferably, each reference Bragg grating is arranged to reflect radiation in a reference range separate from the measurement range. Thus, the spectra of the radiation reflected by the different Bragg gratings can be distinguished from each other.

[0014] Still according to a particular embodiment, at least one Bragg grating is formed by a set of bubbles. The measurement Bragg grating and / or one or more of the reference Bragg gratings may be Bragg gratings each formed by a set of bubbles. Such a Bragg grating corresponds to a type III point-to-point grating inscribed by femtosecond laser pulses. Each bubble has a shape approaching a sphere whose diameter may be less than or equal to 1 µm. In any event, each bubble has a maximum dimension (diameter) less than or equal to the diameter of the core of the optical fiber in which the patterns are inscribed. A single core may thus accommodate a plurality of Bragg gratings on a single section. In this case, the core of an optical fiber may accommodate the measurement Bragg grating and one or more reference Bragg gratings on a single section.

[0015] According to yet another particular embodiment, one of the one or more optical fibers comprises a measurement core and a reference core. The measurement Bragg grating can then be inscribed in the measurement core and at least one of the one or more reference Bragg gratings can be inscribed in the reference core.

[0016] According to this latter particular embodiment, each reference Bragg grating inscribed in the reference core can be arranged to reflect radiation in a range of wavelengths (reference range) included in a range of wavelengths reflected by the measurement Bragg grating (measurement range).

[0017] The measurement section may extend between a proximal end, arranged to receive incident radiation, and a distal end, arranged to undergo erosion. The one or more reference Bragg gratings may then comprise a first reference Bragg grating inscribed on a first reference section located in the vicinity of the proximal end. The first reference Bragg grating is then located at a distance from the end subjected to erosion and may provide a reference measurement throughout the erosion of the measurement Bragg grating, or substantially throughout this erosion.

[0018] According to a first embodiment, the first reference section is located upstream of the measurement section, i.e. on a section of optical fiber normally not subject to erosion.

[0019] According to a second embodiment, the first reference section is included in the measurement section. The first reference Bragg grating is then likely to undergo erosion with the measurement Bragg grating. However, the first reference Bragg grating generally has a relatively short physical length compared to the physical length of the measurement Bragg grating. This length may be less than the minimum safety length of the measurement section, for which the erosion sensor must be replaced. For example, the first reference Bragg grating may have a physical length of 1 mm and the measurement Bragg grating may have a physical length of between a few millimeters and a few centimeters or decimeters.

[0020] According to a particular embodiment, the one or more reference Bragg gratings comprise a plurality of second reference Bragg gratings distributed along the measurement section. These second reference Bragg gratings make it possible to measure a variation of a physical parameter along the measurement section.

[0021] Preferably, the second Bragg gratings are arranged to reflect radiation in wavelength ranges (reference ranges) that are distinct from each other. More preferably, the second Bragg gratings are arranged to reflect radiation in wavelength ranges (reference ranges) that are disjoint from each other.

[0022] According to this latter particular embodiment, one of the one or more optical fibers may comprise a measurement core and a reference core. The measurement Bragg grating may then be inscribed in the measurement core and the second reference Bragg gratings may be inscribed in the reference core. Advantageously, each second reference Bragg grating has a pitch equal to a local pitch of the measurement Bragg grating.

[0023] The erosion sensor according to the invention may further comprise a processing unit arranged to measure a spectral width of radiation reflected by the measuring Bragg grating, measure a wavelength of radiation reflected by at least one reference Bragg grating, and determine a physical length of the measuring section as a function of the spectral width of the radiation reflected by the measuring Bragg grating and the wavelength of the radiation reflected by the at least one reference Bragg grating. Thus, the determination of the physical length of the measuring section from the spectral width of the radiation reflected by the measuring Bragg grating is corrected for the effects of other parameters influencing the spectral response of the measuring Bragg grating.

[0024] The processing unit may be arranged to determine the physical length of the measurement section as a function of the spectral width of the radiation reflected by the measurement Bragg grating and a wavelength difference between a wavelength of the radiation reflected by the measurement Bragg grating and the wavelength of the radiation reflected by the at least one reference Bragg grating. In particular, the wavelength considered for the radiation reflected by the measurement Bragg grating may be the lower limit, the upper limit, or an average value of the wavelength range in which the measurement Bragg grating reflects the radiation (measurement range). This is, for example, the limit closest to the wavelength of the radiation reflected by the at least one reference Bragg grating.

[0025] Furthermore, the processing unit may be arranged to measure a wavelength of the radiation reflected by each of the second reference Bragg gratings, determine a variation of a physical parameter along the measurement section as a function of the wavelengths of the radiation reflected by the second reference Bragg gratings, and determine the physical length of the measurement section as a function of the spectral width of the radiation reflected by the measurement Bragg grating and the variation of the physical parameter along the measurement section. The physical parameter likely to vary along the measurement section may be a temperature, a pressure or a deformation within an optical fiber.

[0026] The invention also relates to a method implementing the erosion sensor as described above. More specifically, the invention relates to a method for determining the physical length of the measurement section of the erosion sensor as described above, according to claim 14. BRÈVE DESCRIPTION DES DESSINS

[0027] Other characteristics, details and advantages of the invention will emerge from reading the description which follows, given solely by way of example and referring to the appended drawings in which: there figure 1A schematically represents an example of a variable-pitch Bragg grating inscribed in the core of an optical fiber; figure 1B represents, by a graph, the variation of the pitch of the Bragg grating of the figure 1A along the longitudinal axis of the optical fiber; figure 1C represents, by a graph, the spectral response in reflection of the Bragg grating with variable pitch of the figure 1A ; there figure 2A schematically represents the variable pitch Bragg grating of the figure 1A after erosion; the figure 2B represents, by a graph similar to that of the figure 1C , the spectral response in reflection of the variable pitch Bragg grating of the figure 2A ; there figure 3A schematically represents a variable-pitch Bragg grating as a set of constant-pitch Bragg gratings; figure 3B represents the sensitivity of each Bragg grating at constant pitch of the figure 3A , depending on the variation of a physical parameter; the figure 3C represents, by two graphs, the spectral response in reflection of each Bragg grating at constant pitch of the figure 3A , for two different physical states; the figure 4represents, by means of a graph, an example of the evolution of the width of the wavelength range of radiation reflected by a variable-pitch Bragg grating during a temperature ramp varying between 750°C and 20°C; figure 5A schematically represents a first example of embodiment of a Bragg grating erosion sensor according to the invention, comprising a single reference Bragg grating; the figure 5B represents, by a graph, the variation of the pitch of the measurement Bragg grating and the reference Bragg grating of the erosion sensor of the figure 5A , along the longitudinal axis of the optical fiber; the figure 5C represents, by a graph, the spectral response in reflection of the erosion sensor of the figure 5A ; there figure 6 represents, by a graph, the spectral response in reflection of the erosion sensor of the figure 5A , for two different physical states; the figure 7represents, by means of a graph, the spectral response in reflection of an example of an erosion sensor produced in accordance with the invention; the figure 8 represents, by a graph, the error in determining the physical length of the Bragg grating measuring the erosion sensor of the figure 7 , with and without compensation for the variation in sensitivity of the measuring Bragg grating; figure 9A schematically represents a second exemplary embodiment of a Bragg grating erosion sensor according to the invention, comprising a plurality of reference Bragg gratings inscribed in the same core as the measurement Bragg grating; the figure 9B represents, by a graph, the spectral response in reflection of the erosion sensor of the figure 9A ; there figure 10Aschematically represents a third exemplary embodiment of a Bragg grating erosion sensor according to the invention, comprising a measurement Bragg grating inscribed in a first core of a multi-core optical fiber and a plurality of reference Bragg gratings inscribed in a second core of the optical fiber; the figure 10B represents, by a graph, the spectral response in reflection of the erosion sensor of the figure 10A ; there figure 11A schematically represents a fourth example of an embodiment of a Bragg grating erosion sensor according to the invention, comprising a first optical fiber in which a measuring Bragg grating is inscribed and a second fiber in which a plurality of reference Bragg gratings are inscribed; the figure 11B represents, by a graph, the spectral response in reflection of the erosion sensor of the figure 11A ; there figure 12represents an example of a method for determining the physical length of a measuring Bragg grating in an erosion sensor according to the invention. EXPOSÉ DÉTAILÉ DE MODES DE RÉALISATION PARTICULIERS

[0028] A Bragg grating is a periodic or pseudo-periodic modulation of the refractive index in a waveguide, for example in the core of an optical fiber. For a period of pitch A and an effective refractive index no, a Bragg grating reflects radiation at the Bragg wavelength: λ Bragg = 2.L. no. Bragg gratings are sensitive to physical parameters such as temperature, mechanical deformations and hydrostatic pressure. This sensitivity results in a shift in the Bragg wavelength Δ λ Bragg defined by the equation: Δ λ Bragg λ Bragg = 1 + p e Δ ε + α + ζ Δ T + c . Δ P Or yes is the photoelastic constant, Δ e mechanical deformation, α the coefficient of thermal expansion, gthe thermo-optical coefficient, Δ T temperature variation, c pressure sensitivity and Δ P pressure variation. The sensitivity of a Bragg grating to the physical parameters imposed on the optical fiber and the Bragg grating allows its use as a sensor.

[0029] A chirped fiber Bragg grating is a Bragg grating whose pitch is not constant but varies along the longitudinal axis of the optical fiber. The variation of the pitch can be linear. Typically, the spectral response of a chirped fiber Bragg grating is characterized by a reflection spectral band whose width depends on the variation of the grating pitch, between the smallest pitch and the largest pitch. Thus, it is possible to determine the physical length of a chirped fiber Bragg grating by measuring the width of the reflection spectral band. In the case of a grating whose pitch varies linearly, any decrease in its physical length leads to a proportional decrease in the width of its reflection spectral band. Chirped fiber Bragg gratings can thus be used as an erosion sensor, for example to measure the level of wear on a part.

[0030] There figure 1A schematically represents an example of a variable-pitch Bragg grating inscribed in the core of an optical fiber. The optical fiber 10 comprises a core 11 surrounded by an optical cladding 12. A variable-pitch Bragg grating 20 is inscribed in the core 11 of the optical fiber 10 on a section called the “measuring section”. The patterns, or fringes, of the Bragg grating 20 are spaced from each other with a pitch A(z) varying linearly along the longitudinal axis of the optical fiber 10. The variation of the pitch A(z) along the longitudinal axis of the optical fiber 10 is also represented on the figure 1B by a graph. The abscissa axis corresponds to the longitudinal position z of the patterns and the ordinate axis corresponds to the step A(z) between two successive patterns at the longitudinal position considered. The figure 1Crepresents, by a second graph, the spectral response in reflection of the Bragg grating with variable pitch 20 before erosion. The abscissa axis corresponds to the wavelength l of the radiation reflected by the Bragg grating 20 and the y-axis corresponds to the amplitude R of the reflected radiation. The variable-pitch Bragg grating 20 is arranged to reflect radiation over a wavelength range called the "initial measurement range". This initial measurement range has an initial half-maximum width FWHM i between a minimum wavelength l minutes , i and a maximum wavelength l max , i .

[0031] There figure 2A schematically represents the variable pitch Bragg grating 20 of the figure 1A after erosion. The figure 2B represents, by a graph similar to that of the figure 1C, the reflection spectral response of the Bragg grating 20. The optical fiber 10 and the Bragg grating 20 have undergone erosion over part of the measurement section. This erosion results in a reduction in the physical length of the measurement section and causes a shift in the maximum wavelength λ max reflected by the Bragg grating 20. The full width at half maximum FWHM of the measuring range is then reduced compared to the initial half-height width FWHM i .

[0032] Determining the width of the measuring range of a variable-pitch Bragg grating therefore allows its physical length to be deduced. However, like any other Bragg grating, a variable-pitch Bragg grating is also sensitive to temperature, deformation and hydrostatic pressure. Due to chromatic dispersion, the variation of a physical parameter does not generate an identical wavelength shift for all wavelengths in the measuring range. Thus, any variation of at least one parameter introduces not only a shift in the reflection spectral band of the variable-pitch Bragg grating, but also a change in its width. The determination of the length of the variable-pitch Bragg grating, and therefore of the erosion of the optical fiber, is therefore distorted.

[0033] THE figures 3A, 3B and 3C illustrate the influence of the variation of the sensitivity of a variable-pitch Bragg grating as a function of the wavelength. The figure 3A schematically represents a variable-pitch Bragg grating as a set of constant-pitch Bragg gratings 31, 32, 33, 34, 35, 36, 37, the different Bragg gratings having different pitches from each other. figure 3B represents the sensitivity of each constant-pitch Bragg grating 31-37 to a physical parameter. The x-axis represents a temperature variation Δ T , a deformation Δ e or a pressure variation ΔP. The y-axis represents the Bragg wavelength shift Δ λ Bragg of each constant-pitch Bragg grating as a function of the variation of the physical parameter. It can be observed that, for the same variation of a given physical parameter, the Bragg wavelength shift Δ λ Bragg is different for different Bragg gratings. The figure 3Crepresents, by two graphs, the spectral response in reflection of each Bragg grating 31-37 for two different physical states. On each graph, the abscissa axis represents the wavelength l of the radiation reflected by the Bragg gratings 31-37, and the y-axis represents the amplitude R of this radiation. It can also be observed, between the two physical states, a shift in the Bragg wavelength Δ λ Bragg distinct for the different Bragg gratings, leading to a first width FWHM 1 of the wavelength range reflected by all 31-37 Bragg gratings in the first state, and a second width FWHM 2 of the wavelength range reflected by all Bragg gratings 31-37 in the second state, FWHM 2 being distinct from FWHM 1 .

[0034] There figure 4represents an example of the evolution of the width of the wavelength range of radiation reflected by a variable-pitch Bragg grating during a temperature ramp evolving between 750°C and 20°C. The variable-pitch Bragg grating has a physical length of 50 mm (millimeters) and a pitch between 1.052 µm (micrometers) and 1.086 µm. On the graph of the figure 4 , the x-axis represents the time, in hours, and the y-axis represents the width of the reflection spectrum FWHM, in nanometers. The width FWHM evolves between 50.16 nm at the temperature of 750°C and 49.85 nm at the temperature of 20°C.

[0035] There figure 5Arepresents a first exemplary embodiment of a Bragg grating erosion sensor according to the invention. The erosion sensor 100 comprises an optical fiber 110, a variable-pitch Bragg grating 120 and a constant-pitch Bragg grating 130. The optical fiber 110 comprises a core 111 and an optical cladding 112 surrounding the core 111. The variable-pitch Bragg grating 120 and the constant-pitch Bragg grating 130 are inscribed in the core 111 of the optical fiber 110. The variable-pitch Bragg grating 120 is intended to undergo erosion by a distal end of the optical fiber 110, i.e. the end opposite the proximal end connected to a measuring instrument. The variable-pitch Bragg grating 120 is therefore also called a “measuring Bragg grating” and is inscribed on a measuring section.The constant-pitch Bragg grating 130 is intended to provide reference information to correct the measurement error due to the variation in the sensitivity of the measuring Bragg grating 120 as a function of the wavelength. It is therefore also called a "reference Bragg grating" and is inscribed on a reference section. The reference Bragg grating 130 is inscribed in the vicinity of the measuring Bragg grating 120. It is therefore subjected to the same environment, i.e. the same variations in temperature and hydrostatic pressure, the same deformations, and the same effects of aging. In the example of the . figure 5A , the reference section is located upstream of the measurement section, that is to say it is located closer than the measurement section to the proximal end of the optical fiber 110.

[0036] There figure 5Brepresents the variation of the pitch A(z) of the Bragg gratings 120 and 130 along the longitudinal axis of the optical fiber 110. The abscissa axis represents the longitudinal position z of the patterns of each grating and the y-axis represents the pitch A(z) between two successive patterns at the longitudinal position considered. The pitch of the measurement Bragg grating 120 evolves linearly along the longitudinal axis. The pitch of the reference Bragg grating 130 is less than the minimum pitch of the measurement Bragg grating. Thus, the radiation reflected by the reference Bragg grating 130 has a wavelength outside the spectral range of the radiation reflected by the measurement Bragg grating 120 and can be identified.

[0037] There figure 5C represents the spectral response in reflection of the Bragg gratings 120 and 130. The abscissa axis corresponds to the wavelength lof the radiation reflected by one of the Bragg gratings 120 and 130, and the y-axis corresponds to the amplitude R of the reflected radiation. The measuring Bragg grating 120 is arranged to reflect radiation over a wavelength range of width FWHM and the reference Bragg grating is arranged to reflect radiation at a Bragg wavelength λ Bragg_ref . Due to the difference in pitch between the 120 and 130 Bragg gratings, the Bragg wavelength λ Bragg_ref is located outside the wavelength range of the radiation reflected by the measuring Bragg grating 120. The wavelength deviation between the Bragg wavelength λ Bragg_ref and the lower limit of the spectral range of the radiation reflected by the measuring Bragg grating 120 is denoted Δ l This wavelength deviation is used to correct the measurement error, as shown below.

[0038] There figure 6represents, in a similar way to the figure 5C , the reflection spectral response of the Bragg gratings 120 and 130 for two different physical states. In a first state, the measuring Bragg grating 120 reflects radiation over a wavelength range of width FWHM 1 and the reference Bragg grating 130 reflects radiation at a Bragg wavelength l Bragg_ref 1. The wavelength difference between the Bragg wavelength l Bragg_ref 1 and the lower limit of the spectral range of the radiation reflected by the measuring Bragg grating 120 is then noted Δ l 1. In a second state, the measuring Bragg grating 120 reflects radiation over a range of wavelengths of width FWHM 2 and the reference Bragg grating 130 reflects radiation at a Bragg wavelength l Bragg_ref 2. The wavelength difference between the Bragg wavelength λ Bragg_ref2and the lower limit of the spectral range of the radiation reflected by the measuring Bragg grating 120 is then noted Δ l 2. As a first approximation, it is possible to consider that the sensitivity of a Bragg grating to a physical parameter varies linearly as a function of the Bragg wavelength at which this grating was inscribed. With such an approximation, the ratio between the width FWHM of the spectrum reflected by the Bragg grating of measurement 120 and the wavelength difference Δ l is constant for a given physical length of the Bragg grating of measurement 120. Thus, for two physical states, the following relation is verified: FWHM 1 Δ λ 1 ≈ FWHM 2 Δ λ 2

[0039] As an example, let's take the case of temperature. Consider a temperature variation ΔT between temperatures T 1 and T 2, for a reference grating whose Bragg wavelength is λ r, and a measuring Bragg grating whose extreme wavelengths are λ m And λ M . We then define the following quantities Δ l = λ m - λ r And FWHM = λ M - λ m . We note the quantities defined above respectively l r 1 , l m 1 , Δ l 1 , FWHM 1 à the temperature T 1 and l r 2 , , l m 2 , Δ l 2 , FWHM 2 at temperature T 2. We have seen previously that: Δ λ Bragg λ Bragg = α + ζ Δ T

[0040] Or again: Δ λ Bragg λ Bragg = β Δ T

[0041] With β = α + ζ the thermal sensitivity. To give an order of magnitude, in the case of a standard germanosilicate fiber, β ≈ 6.32 E -6< / °C.

[0042] So we have: dλ Bragg dT − β . λ Bragg = 0 λ Bragg T = a . e βT with a a real constant pour T = T 1 : λ Bragg T 1 = λ 1 soit a = λ 1 e − βT 1 λ Bragg T = λ 1 e β T − T 1 soit λ 2 = λ 1 e β Δ T

[0043] In real applications we always have β Δ T "1, we can therefore make the following development limited to order 1: λ 2 = λ 1 1 + β Δ T

[0044] We can therefore write: Δ λ 2 = λ m 2 − λ r 2 Δ λ 2 = λ m 1 1 + β Δ T − λ r 1 1 + β Δ T Δ λ 2 = λ m 1 − λ r 1 1 + β Δ T Δ λ 2 = Δ λ 1 1 + β Δ T Δ λ 2 Δ λ 1 = 1 + β Δ T

[0045] Likewise : FWHM 2 = λ M 2 − λ m 2 FWHM 2 = λ M 1 1 + β Δ T − λ m 1 1 + β Δ T FWHM 2 = λ M 1 − λ m 1 1 + β Δ T FWHM 2 = FWHM 1 1 + β Δ T FWHM 2 FWHM 1 = 1 + β Δ T

[0046] So in the end: Δ λ 2 Δ λ 1 = FWHM 2 FWHM 1

[0047] Or again: ∀ Δ T , FWHM 1 Δ λ 1 = FWHM 2 Δ λ 2 = C , C ∈ ℝ

[0048] This constant ratio can then be used as a correction coefficient for the measurement of the physical length of the measuring Bragg grating. Let us now consider two erosion states 3 and 4, at constant temperature, with a Bragg grating of length L. We note respectively the quantities defined previously L 3 , Δ l 3 and FWHM 3 à state 3 and L 4, Δ l 4 and FWHM 4à state 4. Considering a variable-pitch Bragg grating inscribed according to a proportionality relationship between its length and its width at mid-height, we can write: L = a FWHM , a ∈ ℝ Δ λ 3 = Δ λ 4

[0049] Either : L 4 L 3 = FWHM 4 / Δ λ 4 FWHM 3 / Δ λ 3

[0050] Or again: L 4 = FWHM 4 Δ λ 4 L 3 FWHM 3 / Δ λ 3

[0051] Considering state 3 as the initial state of the grating at the time of its registration, we have a direct proportionality relationship between, on the one hand, the length of the measuring Bragg grating and, on the other hand, the ratio between its width at half-height and its distance from the reference grating whatever the temperature. ∀ T , L ∝ FWHM Δ λ

[0052] Obviously, it is possible to correct more finely the measurement of the physical length of the Bragg grating of measurement 120 by taking into account the non-linearity of the sensitivity of a Bragg grating as a function of the wavelength.

[0053] The inventors have produced a Bragg grating erosion sensor according to the invention. A variable-pitch Bragg grating and a constant-pitch Bragg grating were inscribed in the core of an optical fiber by femtosecond laser. The variable-pitch Bragg grating has a physical length of 50 mm and a pitch varying between 1 µm and 1.1 µm, giving a spectral width of 44 nm at room temperature. The constant-pitch Bragg grating has a length of 2 mm and is positioned 1 mm upstream of the variable-pitch Bragg grating. figure 7 represents, by means of a graph, the spectral response in reflection of this erosion sensor. On this graph, the abscissa axis represents the wavelength of the radiation reflected by the Bragg gratings, in nanometers, and the ordinate axis represents the power of the reflected radiation, in dBm.

[0054] In order to test the effect of sensitivity variation compensation on the measurement of the physical length of the measuring Bragg grating, the optical fiber was subjected to a temperature ramp varying from 750°C to room temperature. The width at half maximum FWHM of the spectrum reflected by the measuring Bragg grating and the wavelength difference Δ l were measured during the temperature reduction of the optical fiber. The physical length of the measuring Bragg grating was then determined, on the one hand, from the width FWHM and, on the other hand, according to the same width FWHM and the wavelength difference Δ l . There figure 8represents, by a graph, the error in determining the physical length of the measuring Bragg grating. The abscissa axis represents the time, in hours, and the ordinate axis represents the length error, in millimeters. A first curve 81 corresponds to the length error determined without compensation for the sensitivity variation and a second curve 82 corresponds to the length error determined with compensation for the sensitivity variation. It can be seen that, without compensation, the maximum length error is 330 µm at room temperature. With compensation, the maximum length error is 30 µm. The error in determining the physical length of the Bragg grating is thus reduced by a factor of 11.

[0055] There figure 9Aschematically represents a second exemplary embodiment of a Bragg grating erosion sensor according to the invention. The erosion sensor 200 comprises an optical fiber 210, a variable-pitch Bragg grating 220, a first constant-pitch Bragg grating 230 and four second constant-pitch Bragg gratings 241, 242, 243 and 244. The optical fiber 210 comprises a core 211 and an optical cladding 212 surrounding the core 211. The variable-pitch Bragg grating 220, also called the “measurement Bragg grating”, is inscribed in the core 211 of the optical fiber 210 on a measurement section. The first constant-pitch Bragg grating 230, also called the “first reference Bragg grating”, is inscribed in the core 211 on a first reference section, upstream of the measurement section.The second constant-pitch Bragg gratings 241, 242, 243, 244, also called "second reference Bragg gratings", are inscribed in the core 211 on second reference sections distributed along the measurement section. The measurement Bragg grating 220 and the second reference Bragg gratings 241, 242, 243, 244 are point-to-point gratings. The reference Bragg gratings 230, 241, 242, 243, 244 have pitches distinct from each other so as to reflect radiation at Bragg wavelengths that can be differentiated. Since the reference Bragg gratings 230, 241, 242, 243, 244 are inscribed in the same core as the measurement Bragg grating 220, they must also have a pitch different from the pitch range of the measurement Bragg grating 220.

[0056] There figure 9Brepresents the spectral response in reflection of Bragg gratings 220, 230, 241, 242, 243 and 244. The abscissa axis corresponds to the wavelength l of the radiation reflected by one of the Bragg gratings, and the y-axis corresponds to the amplitude R of the reflected radiation. The Bragg wavelengths of the different constant-pitch Bragg gratings are distinct from each other and located outside the wavelength range of the radiation reflected by the measurement Bragg grating 220. The reference Bragg gratings and, in particular, the second reference Bragg gratings 241, 242, 243, 244, make it possible to measure a variation of a physical parameter along the measurement section and thus to refine the estimation of the physical length of the measurement Bragg grating 220.

[0057] There figure 10Aschematically represents a third exemplary embodiment of a Bragg grating erosion sensor according to the invention using a multi-core optical fiber. The erosion sensor 300 comprises an optical fiber 310, a variable-pitch Bragg grating 320, a first constant-pitch Bragg grating 330 and four second constant-pitch Bragg gratings 341, 342, 343 and 344. The optical fiber 310 comprises a first core 311, called the “measurement core”, a second core 312 called the “reference core”, and an optical cladding 313 surrounding the two cores 311, 312. The measurement Bragg grating 320 is inscribed in the measurement core 311 on a measurement section. The first reference Bragg grating 330 is inscribed in the reference core 312 on a first reference section, upstream of the measurement section.The second reference Bragg gratings 341, 342, 343, 344 are inscribed in the reference core 312 on second reference sections distributed along the measurement section. The reference Bragg gratings 330, 341, 342, 343, 344 have pitches that are distinct from each other so as to reflect radiation at Bragg wavelengths that can be differentiated. On the other hand, the second reference Bragg gratings 341, 342, 343, 344 each have a pitch equal to or close to the local pitch of the measurement Bragg grating 320. In other words, each second reference Bragg grating has a pitch equal to or close to the average pitch of the measurement Bragg grating 320 on the second reference section considered.

[0058] There figure 10Brepresents the spectral response in reflection of the Bragg gratings 320, 330, 341, 342, 343 and 344. The Bragg wavelengths of the different constant-pitch Bragg gratings are distinct from each other. The Bragg wavelengths of the second reference Bragg gratings 341, 342, 343 and 344 are, however, located in the wavelength range of the radiation reflected by the measurement Bragg grating 320. Thus, the variation of sensitivity of the Bragg grating with the wavelength can be estimated finely.

[0059] There figure 11A schematically represents a fourth exemplary embodiment of a Bragg grating erosion sensor according to the invention using two single-core optical fibers. The erosion sensor 400 comprises a first optical fiber 401, a second optical fiber 402, a variable-pitch Bragg grating 420, a first constant-pitch Bragg grating 430 and four second constant-pitch Bragg gratings 441, 442, 443 and 444. The optical fiber 401 comprises a core 411 and an optical cladding 413 surrounding the core 411. The optical fiber 402 comprises a core 412 and an optical cladding 414 surrounding the core 412. The measurement Bragg grating 420 is inscribed in the core 411 of the optical fiber 401 on a measurement section. The first reference Bragg grating 430 is inscribed in the core 412 of the second optical fiber 402 on a first reference section, upstream of the measurement section.The second reference Bragg gratings 441, 442, 443, 444 are inscribed in the core 412 of the second optical fiber 402 on second reference sections distributed along the measurement section. In a manner identical to the third exemplary embodiment, the reference Bragg gratings 430, 441, 442, 443, 444 have pitches distinct from one another so as to reflect radiation at Bragg wavelengths that can be differentiated. The second reference Bragg gratings 441, 442, 443, 444 each have a pitch equal to or close to the local pitch of the measurement Bragg grating 420. Preferably, the two optical fibers 401, 402 are mechanically linked to each other, so that the reference Bragg gratings 430, 441, 442, 443, 444 undergo the same deformations as the measurement Bragg grating 420.

[0060] There figure 11B represents the spectral response in reflection of the Bragg gratings 420, 430, 441, 442, 443 and 444. This spectral response is identical to that of the third embodiment.

[0061] Other exemplary embodiments may be envisaged within the scope of the invention. In particular, the erosion sensor may only comprise reference Bragg gratings inscribed on the measurement section. In other words, the erosion sensor may not comprise a first reference Bragg grating inscribed on a section upstream of the measurement section. Furthermore, the erosion sensor may comprise any number of reference Bragg gratings. The number of these gratings may in particular vary depending on the physical length of the measurement Bragg grating and / or the desired measurement accuracy.

[0062] There figure 12 represents an example of a method for determining the physical length of a measurement Bragg grating in an erosion sensor according to the invention. The method 500 comprises a first step 501 in which incident radiation is injected into the core of each optical fiber in which a Bragg grating is inscribed. The incident radiation must have a wavelength range covering the wavelength range of the measurement Bragg grating and the wavelength range of each of the reference Bragg gratings. In a second step 502, the radiation reflected by the different Bragg gratings is received by a processing unit. In a step 503, the spectral width of the radiation reflected by the measurement Bragg grating is measured. The spectral width is for example the width at half-maximum of the peak maximum ( FWHM). In a step 504, the Bragg wavelength of each reference Bragg grating is measured. It should be noted that steps 503 and 504 can be performed simultaneously or consecutively, in any order. In a fifth step 505, the physical length of the measurement section is determined as a function of the spectral width of the radiation reflected by the measurement Bragg grating and the Bragg wavelength of each reference Bragg grating. In particular, when the erosion sensor comprises a single reference Bragg grating, the physical length of the measurement section can be determined as a function of the spectral width of the radiation reflected by the measurement Bragg grating and the wavelength deviation between the Bragg wavelength of the reference Bragg grating and a lower or upper limit of the wavelength range of the radiation reflected by the measurement Bragg grating.

Claims

1. Bragg-grating erosion sensor (100, 200, 300, 400) comprising: - one or more optical fibres (110, 210, 310, 401, 402), each optical fibre comprising one or more cores (111, 211, 311, 312, 411, 412) and an optical sheath (112, 212, 313, 413, 414) surrounding the core(s), - a variable-pitch measurement Bragg grating (120, 220, 320, 420) inscribed in one of the cores (111, 211, 311, 411) of one of the optical fibres in a measurement section intended to undergo an erosion, and - one or more reference Bragg gratings (130, 230, 241, 242, 243, 244, 330, 341, 342, 343, 344, 430, 441, 442, 443, 444), each reference Bragg grating being inscribed in one of the cores (111, 211, 312, 412) of one of the optical fibres, characterised in that at least one of the one or more reference Bragg gratings (130, 230, 241, 242, 243, 244, 330, 341, 342, 343, 344, 430, 441, 442, 443, 444) is a constant-pitch Bragg grating.

2. Erosion sensor according to claim 1, wherein the measurement Bragg grating (120, 220) and at least one of the one or more reference Bragg gratings (130, 230) are inscribed in the same core (111, 211) of an optical fibre.

3. Erosion sensor according to claim 1 or 2, wherein at least one Bragg grating is formed by a set of bubbles.

4. Erosion sensor according to one of the preceding claims, wherein one of the one or more optical fibres (310) comprises a measurement core (311) and a reference core (312), the measurement Bragg grating (320) being inscribed in the measurement core (311) and at least one of the one or more reference Bragg gratings (330, 341, 342, 343, 344) being inscribed in the reference core (312).

5. Erosion sensor according to claim 4, wherein each reference Bragg grating (341, 342, 343, 344) inscribed in the reference core (312) is arranged to reflect a radiation in a range of wavelengths comprised in a range of wavelengths reflected by the measurement Bragg grating (320).

6. Erosion sensor according to one of the preceding claims, wherein the measurement section extends between a proximal end, arranged to receive an incident radiation, and a distal end, arranged to undergo an erosion, the one or more reference Bragg gratings comprising a first reference Bragg grating (130, 230, 330, 430) inscribed in a first reference section located near the proximal end.

7. Erosion sensor according to claim 6, wherein the first reference section is located upstream of the measurement section.

8. Erosion sensor according to claim 6, wherein the first reference section is included in the measurement section.

9. Erosion sensor according to one of the preceding claims, wherein the one or more reference Bragg gratings comprise a plurality of second reference Bragg gratings (241, 242, 243, 244, 341, 342, 343, 344, 441, 442, 443, 444) distributed along the measurement section.

10. Erosion sensor according to claim 9, wherein one of the one or more optical fibres (310) comprises a measurement core (311) and a reference core (312), the measurement Bragg grating (320) being inscribed in the measurement core (311) and the second reference Bragg gratings (341, 342, 343, 344) being inscribed in the reference core (312), each second reference Bragg grating having a pitch equal to a local pitch of the measurement Bragg grating.

11. Erosion sensor according to one of the preceding claims, further comprising a processing unit arranged to measure a spectral width of a radiation reflected by the measurement Bragg grating (120, 220, 320, 420), measure a wavelength of a radiation reflected by at least one reference Bragg grating (130, 230, 241, 242, 243, 244, 330, 341, 342, 343, 344, 430, 441, 442, 443, 444), and determine a physical length of the measurement section according to the spectral width of the radiation reflected by the measurement Bragg grating and the wavelength of the radiation reflected by the at least one reference Bragg grating.

12. Erosion sensor according to claim 11, wherein the processing unit is arranged to determine the physical length of the measurement section according to the spectral width of the radiation reflected by the measurement Bragg grating (120, 220, 320, 420) and a wavelength difference between a wavelength of the radiation reflected by the measurement Bragg grating and the wavelength of the radiation reflected by the at least one reference Bragg grating (130, 230, 241, 242, 243, 244, 330, 341, 342, 343, 344, 430, 441, 442, 443, 444).

13. Erosion sensor according to one of claims 11 and 12, taken with one of claims 10 and 11, wherein the processing unit is arranged to measure a wavelength of the radiation reflected by each of the second reference Bragg gratings (241, 242, 243, 244, 341, 342, 343, 344, 441, 442, 443, 444), determine a variation of a physical parameter along the measurement section according to the wavelengths of the radiation reflected by the second reference Bragg gratings, and determine the physical length of the measurement section according to the spectral width of the radiation reflected by the measurement Bragg grating (120, 220, 320, 430) and the variation of the physical parameter along the measurement section.

14. Method for determining the physical length of the measurement section of the erosion sensor (100, 200, 300, 400) according to one of the preceding claims, comprising the steps of: - measuring (503) a spectral width of a radiation reflected by the measurement Bragg grating (120, 220, 320, 420), - measuring (504) a wavelength of a radiation reflected by at least one reference Bragg grating (130, 230, 241, 242, 243, 244, 330, 341, 342, 343, 344, 430, 441, 442, 443, 444), and - determining (505) a physical length of the measurement section according to the spectral width of the radiation reflected by the measurement Bragg grating and the wavelength of the radiation reflected by the at least one reference Bragg grating.