Device for measuring a physical quantity

Abstract

The invention relates to a measurement device comprising an etalon (6), the reflection power spectrum of which comprises multiple power peaks distributed within a working range, the free spectral range of the etalon being less than or equal to 5 nm and the etalon comprising: - a waveguide (34); and - a Bragg grating (80) formed in the waveguide, the Bragg grating comprising at least three identical patterns aligned one behind the other along a longitudinal axis of the waveguide and separated from one another by a constant spacing. The spacing of the Bragg grating (80) is configured for the power spectrum of the grating to have multiple discernible harmonics with an order greater than one hundred in the working range, these harmonics thus forming the power peaks of the power spectrum of the etalon.

Description

[0001] Device for measuring a physical quantity

[0002] [1] The invention relates to a device for measuring a physical quantity and a standard for producing this device.

[0003] [2] These devices are, for example, used to measure temperature or pressure or mechanical deformation.

[0004] [3] An example of such a known measuring device is described in application CN102879022A. This known device comprises a standard and an optical transducer. The optical transducer makes it possible to transform a variation in the physical quantity to be measured into a displacement of a power peak in the power spectrum of the optical transducer. The standard makes it possible to generate a reference power spectrum which is used to correct the measurement and therefore to improve the accuracy of this measurement.

[0005] [4] For this, the power spectrum of the standard comprises a succession of very close power peaks in a predetermined working range.

[0006] [5] To date, many embodiments of such a standard have been proposed. For example, it has been proposed to produce such a standard using a Fabry Perot cavity whose diopters are mirrors connected to the ends of an optical fiber. Such mirrors have a high reflectivity, i.e. greater than 90%. Under these conditions, the peaks of the power spectrum of the standard are fine and the precision of the measuring device is high. However, the manufacture of such a Fabry Perot cavity is complex, in particular because mirrors must be connected to the ends of an optical fiber. An example of such an embodiment of a standard is described in application WO2020113147A1.

[0007] [6] In application CN102879022A, it is proposed to produce the standard by etching in the core of an optical fiber a succession of Bragg gratings located one after the other. The wavelength À B of the fundamental frequency f B resonance of each of these Bragg gratings is different from that of the other Bragg gratings. However, the widths of the power peaks of the standard of application CN102879022A are generally less fine than those obtained using a standard such as that of application W02020113147A1. In addition, such a succession of Bragg gratings is complex to produce and often leads to a fairly long and therefore bulky standard.

[0008] [7] Prior art is also known from US2019 / 178688A1 and US2020 / 271485A1.

[0009] [8] The invention aims to provide a device for measuring a physical quantity which has high precision and which is, at the same time, simple to manufacture.

[0010] [9] The invention is set forth in the attached set of claims.

[0011]

[0010] The invention will be better understood on reading the description which follows, given solely by way of non-limiting example and made with reference to the drawings in which:

[0012] - figure 1 is a schematic illustration of the architecture of a device for measuring a physical quantity,

[0013] - figure 2 is a schematic illustration of a standard of the measuring device of figure 1,

[0014] - Figure 3 is a schematic illustration, partial and in longitudinal section, of a Bragg grating used in the standard of Figure 2,

[0015] - Figure 4 is a schematic illustration, in cross-section, of a pattern of the Bragg grating of Figure 3,

[0016] - Figure 5 is a graph representing a portion of the power spectrum of the Bragg grating of Figure 3,

[0017] - Figure 6 is a flowchart of a manufacturing process for the Bragg grating of Figure 3,

[0018] - figure 7 is a flowchart of a method for measuring a physical quantity using the device of figure 1, and

[0019] - Figure 8 is a schematic illustration, partial and in longitudinal section, of another embodiment of a standard for the measuring device of Figure 1.

[0020]

[0011] 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.

[0021]

[0012] 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 III.

[0022]

[0013] Chapter I: Examples of embodiments

[0023]

[0014] Figure 1 represents a device 2 for measuring a physical quantity. For example, here, the physical quantity to be measured is a temperature of an external environment.

[0024]

[0015] The device 2 comprises an optical transducer 4 and a standard 6. The transducer 4 is exposed to variations in the physical quantity to be measured.

[0025]

[0016] The transducer 4 transforms a variation of the physical quantity to be measured into a displacement of a power peak of its power spectrum. In this text, unless otherwise indicated, the term “power spectrum” or “spectrum” designates the power spectrum in reflection. The power spectrum in reflection is the power spectrum of the optical signal reflected by an optical component. A peak in the power spectrum in reflection corresponds to an absorption line in the power spectrum in transmission of the same optical component.

[0026]

[0017] The power spectrum of the transducer 4 comprises, for example, a single power peak in a predetermined working range. This working range has a width greater than 5 nm. Typically, its width is also less than or equal to 200 nm or 120 nm. Here, the width of the working range is equal to 100 nm. The working range is located within the optical domain. The optical domain designates the range containing the wavelengths usually used in optics. More precisely, in this text, the optical domain designates the range which extends from 200 nm to 10000 nm and, frequently, from 200 nm to 5000 nm or from 400 nm to 2000 nm.

[0027]

[0018] For example, the transducer 4 is identical or similar to that described in application CN102879022A. The transducer 4 is therefore here a Bragg grating which is produced in the core of an optical fiber 14. The wavelength λ B4 of the fundamental frequency f B4of this Bragg grating is located within the predetermined working range. Preferably, the wavelength À B4 is located approximately in the middle of the working range. The wavelength of the fundamental frequency of a Bragg grating is given by the following relation (1): At B = 2*n e *A, where:

[0028] - HAS B is the wavelength of the fundamental frequency of the Bragg grating,

[0029] - n e is the effective index of the optical fiber inside which the Bragg grating is made, - A is the pitch of the Bragg grating, and

[0030] - the symbol “*” denotes the scalar multiplication operation in this text.

[0031]

[0019] The effective index n e propagation is also known as the "mode phase constant". It is defined by the following relationship: n g = n e - DNA e / dÀ, where n gis the group index and λ is the wavelength of the optical signal guided by the optical fiber. 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.

[0032]

[0020] The standard 6 has a reflection power spectrum comprising several power peaks distributed within the working range. The free spectral interval of this standard over the working range is less than or equal to 5 nm and, preferably, less than or equal to 1 nm. Subsequently, such a succession of peaks is also called “a peak comb” or simply “a comb”. Unlike the transducer 4, the standard 6 is configured so that its power spectrum is constant. In particular, the standard 6 is arranged so that its power spectrum does not shift as a function of the physical quantity measured or as a function of variations in other physical quantities of the external environment in which the standard 6 is immersed. In particular, the standard 6 is arranged so that its power spectrum does not shift as a function of the temperature of the external environment.In this text, "does not move" means that the amplitude of the displacement of the power spectrum of standard 6 is negligible compared to the amplitude AÀ of the displacement of the power spectrum of transducer 4 observed at the same time. Here, the amplitude of the displacement of the power spectrum of standard 6 is considered negligible if it is ten or one hundred times smaller than the amplitude AÀ.

[0033]

[0021] The transducer 4 is optically connected to an input / output port 10 of an optical coupler 12 via the waveguide 14. The optical coupler 12 comprises:

[0034] - an input port 16 optically connected to an output port 18 of a spectral analyzer 20 via a waveguide 22, and

[0035] - an output port 24 optically connected to an input port 26 of the spectral analyzer 20 via a waveguide 28.

[0022] The standard 6 is optically connected to an input / output port 30 of an optical coupler 32 via a waveguide 34. The optical coupler 32 comprises:

[0036] - an input port 36 optically connected to an output port 38 of the spectral analyzer 20 via a waveguide 42, and

[0037] - an output port 44 optically connected to an input port 46 of the spectral analyzer 20 via a waveguide 48.

[0038]

[0023] In this embodiment, all of the above waveguides are respective optical fibers. Thus, hereinafter, the same reference numerals are used to designate the waveguide or the optical fiber. Here, the optical fibers used are single-mode optical fibers also known by the acronym SMF (“Simple Mode Fiber”).

[0039]

[0024] The spectral analyzer 20 is capable of measuring the spectral responses of the transducer 4 and the standard 6 and then of determining the variation of the physical quantity to be measured from these measured spectral responses. For this, it comprises:

[0040] - a tunable 50 laser source,

[0041] - an optical coupler 52 which optically connects an output port 54 of the laser source 50, simultaneously, to the two output ports 18 and 38,

[0042] - two optical sensors 62 and 64 optically connected, respectively, to the input ports 26 and 46 to measure the power of the optical signal received on these input ports, and

[0043] - an electronic processing unit 70 electrically connected to the sensors 62, 64 to receive electrical signals representative of the powers of the optical signals measured by, respectively, the sensors 62 and 64.

[0044]

[0025] The laser source 50 emits, in the direction of the transducer 4 and the standard 6, a single-frequency optical signal via the port 54. The wavelength λ s of the emitted optical signal is in the optical domain. The value of the wavelength A s depends on a control signal received on a control port 66 of the laser source 50. More precisely, the wavelength λ sis related to the value of the control signal by a transfer function which, to each value of the control signal, associates a corresponding value of the wavelength À s . Typically, this transfer function is not perfectly linear. In this case, it is called "non-linear". Such a laser source 50 is also called a "scanning laser source". Indeed, by using an appropriate control signal, the wavelength λ s scans the entire working range. Here, the working range is a range of wavelengths that extends from wavelength A S min at wavelength A sm ax. The width of the working range is typically determined by the characteristics of the source 50. The width of the working range is equal to the difference λ smax “ λ smin- In this embodiment, this working range extends from 1500 nm to 1600 nm.

[0045]

[0026] The sensor 62 measures the optical signal backscattered by the transducer 4. In parallel, the sensor 64 measures the optical signal backscattered by the standard 6. Here, the sensors 62 and 64 are identical. For example, the sensors 62 and 64 are each a photodiode. Each of the sensors 62, 64 has a spectral observation range which encompasses the working range.

[0046]

[0027] The unit 70 is notably configured to:

[0047] - determine the amplitude AÀ of the displacement of the peak of transducer 4 from the spectral responses of transducer 4 and standard 6 measured separately from each other by sensors, respectively, 62 and 64, then

[0048] - establish a variation of the physical quantity to be measured from the determined amplitude AÀ.

[0049]

[0028] To carry out these operations, the unit 70 comprises a programmable microprocessor 72 and a memory 74 containing the data and instructions necessary for the operation of the spectral analyzer 20. For example, here, the memory comprises a coefficient S G sensitivity which makes it possible to establish the variation of the physical quantity from the determined amplitude AÀ. In this example, the coefficient S G is defined by the following relation AÀ / À B i4 = S G *AG, WHERE:

[0050] - HAS B i4 is the wavelength of the fundamental frequency of the Bragg grating of transducer 4 in a reference state, and

[0051] - AÀ is the amplitude of the variation in the wavelength of the fundamental frequency of the Bragg grating of transducer 4 obtained in response to a variation AG of the physical quantity to be measured.

[0052]

[0029] The wavelength ÀB>4 corresponds, here, to a reference wavelength for the power peak of the transducer 4. The amplitude AÀ is equal to the difference between the wavelength À B m4 of the fundamental frequency measured for transducer 4 and wavelength A B i4 reference. Unlike the wavelength A B i4, the wavelength A Bm 4 varies depending on the physical quantity to be measured. The value of the wavelength À B i4 is stored in memory 74. This wavelength AT B i4 can also be associated, in memory 74, with a corresponding absolute value of the physical quantity to be measured.

[0053]

[0030] Usually, the unit 70 is also connected to a human-machine interface 76 to communicate the result of the measurements carried out to a human being.

[0054]

[0031] Figure 2 shows in more detail the architecture of the standard 6. The standard 6 comprises a Bragg grating 80 made in the core of the optical fiber 34. The grating 80 is a very high order Bragg grating.

[0055]

[0032] In this text, "very high order" means that the power spectrum of the Bragg grating has discernible harmonics of order higher than N in the optical domain and, more precisely, in the working range, 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 k defined by the following relation (2): Tok = 2*n e *A / k, where:

[0056] - k is an integer equal to the order of the harmonic,

[0057] - n e is the effective index of the optical fiber in which the very high order Bragg grating is made,

[0058] - A is the pitch of the very high order Bragg grating.

[0059]

[0033] Here, this peak of order k is located inside the working range.

[0060]

[0034] The power spectrum of the network 80, within the working range, comprises a succession of peaks each corresponding to a harmonic of order higher than N. These peaks are very close together and very fine. In this text, “very close together” means that the free spectral interval is less than 5 nm and, preferably, less than or equal to 1 nm. “very fine” means that the width at half-maximum of each peak is less than the free spectral interval and, preferably, twice less than the free spectral interval. In addition, the heights of these peaks are substantially the same over the entire working range because each of these peaks corresponds to a very high order harmonic. In other words, the power spectrum of a network 80 is a comb of peaks as previously defined.An example of such a comb is shown in Figure 3 of 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”.

[0061]

[0035] 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:

[0062] - for wavelength A B of the fundamental frequency f B Bragg grating resonance either in the optical domain, or

[0063] - that only the first harmonics of order lower than twenty are in the field of optics.

[0064]

[0036] Thus, the pitch of these standard Bragg gratings is systematically less than 50 pm or 20 pm and, generally, even less than 10 pm. 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 k of these harmonics is not in the domain of optics. In other words, the wavelengths A k 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 pm or 50 pm and often greater than 100 pm. Under these conditions, the wavelength À B of the fundamental frequency f B 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.

[0065]

[0037] Standard Bragg grating patterns are commonly fabricated using ultraviolet radiation pulses or CO2 lasers, not femtosecond laser pulses. Bragg gratings fabricated without using femtosecond laser pulses exhibit only discernible harmonics of order less than twenty. This appears to be because the refractive index variations in the optical fiber obtained by implementing these other known methods are much less pronounced than those obtained using a femtosecond laser. Thus, a Bragg grating fabricated without using femtosecond laser pulses, even if it has a pitch greater than 20 pm or 50 pm, is not a very high order Bragg grating.

[0038] 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.

[0066]

[0039] 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 very high order Bragg gratings in laser sources is far from the field of measuring a physical quantity. In particular, this use does not teach that a high order Bragg grating can advantageously be used to produce a standard for a measuring device.

[0067]

[0040] The standard 6 is also arranged so that the power spectrum of the network 80 is constant despite variations in the conditions of use. For this purpose, it comprises an insulating structure 82 which isolates the network 80 from variations in the external environment inside which the standard 6 is immersed. In this embodiment, the insulating structure 82 comprises a housing 84 inside which the network 80 is fixed without any degree of freedom. The housing 84 isolates the network 80 from variations in the mechanical stresses that the external environment can exert on the network 80.

[0068]

[0041] The insulating structure 82 also comprises, housed inside the housing 84:

[0069] - a 90 temperature sensor,

[0070] - a controllable heating or cooling element 92 capable of heating or cooling the network 80, and

[0071] - a microcontroller 94 configured to control the heating element as a function of a temperature setpoint Te and the temperature measured by the sensor 90 to limit the temperature variations of the network 80 around this setpoint Te.

[0042] The element 92 is for example a Peltier module or a set of several Peltier modules.

[0072]

[0043] The microcontroller 94 comprises a programmable microprocessor 96 and a memory 98 containing the data and instructions necessary for the operation of the standard 6. Here, the memory 98 comprises the pre-recorded setpoint Te and the instructions of a control module 100. When the module 100 is executed by the microprocessor 96, the temperature of the network 80 is controlled by the setpoint Te recorded in the memory 98. For this, the microprocessor 96 controls the element 92 as a function of a difference between the setpoint Te and the temperature measured by the sensor 90 so as to reduce this difference.

[0073]

[0044] Figure 3 shows in more detail an embodiment of the network 80. The optical fiber 34 extends along a longitudinal axis 108 parallel to a Z direction of an orthogonal XYZ reference frame. Figures 3, 4 and 8 are oriented relative to this XYZ reference frame. For example, the Z direction is horizontal and the Y direction is vertical.

[0074]

[0045] To simplify Figure 3, only the portion of the optical fiber 34 that contains the grating 80 is shown. The optical fiber 34 guides the optical signal along the longitudinal axis 108.

[0075]

[0046] The optical fiber 34 comprises:

[0076] - a core 110 in which the optical signal guided by this fiber 34 propagates,

[0077] - an optical sheath 112 made of a material whose refractive index makes it possible to maintain the optical signal inside the core 110 by reflection at the interface between the core 110 and this sheath 112, and

[0078] - a mechanical sheath, typically made of polymer, which covers the sheath 112.

[0079] To simplify Figure 3, the mechanical sheath of the optical fiber 34 has not been shown.

[0080]

[0047] The network 80 is designed to obtain a comb of peaks over the working range. In addition, here, the network 80 is designed so that this comb is formed by the harmonics of the network 80 of order close to 1024.

[0081]

[0048] For this purpose, the network 80 is composed of a succession of patterns Mi arranged one behind the other along the axis 108. 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 80 is equal to 1 and the index i of the last rightmost pattern in the network 80 is equal to p. p is equal to the number of patterns Mj of the network 80. In Figure 3, only the first two and the last two patterns of the network 80 have been shown. The presence of the intermediate patterns located between the patterns M2 and M p .i is represented by small circles on the 108 axis.

[0082]

[0049] 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 greater the number p, the more the width at half-height of each peak decreases. Here, the number p is also chosen to be small enough so that the length of the network 80 remains small, i.e. less than 1 meter and, preferably, less than 10 cm. The length of the network 80 is equal to the distance between the patterns Mi and M p measured along the 108 axis. Typically, the p number is less than 200 or 100.

[0083]

[0050] Step A 80 between two patterns Mi and M i+i immediately consecutive in the Z direction is constant regardless of the index i. The step A 80 is therefore equal to the distance, along the axis 108, which separates two patterns Mi and M i +i immediately consecutive.

[0084]

[0051] Here, step A 80 is calculated so that the wavelength of a harmonic of order kc is equal to or very close to the center of the working range. Here, the order k c is chosen equal to 1024.

[0085]

[0052] For this, step A 80 is between 0.9*[k c *Àc / (2*n e )] and 1 , 1 *[k c *Àc / (2*n e )] and, preferably, between 0.98*[k c *Àc / (2*n e )] and 1.02*[k c *Àc / (2*n e )], where n e is the effective index of the optical fiber 34 and A c is the wavelength located at the center of the working range. Here, the wavelength À c is equal to 1550 nm.

[0086]

[0053] For example, the optical fiber 34 is made from an optical fiber marketed under the reference SMF-28 by the company Corning®. The index n e of this optical fiber is equal to approximately 1.4676. Under these conditions, the term k c *Xc / (2*n e ) is equal to approximately 540.8 pm. Here, the step A80 is chosen equal to 540.8 pm. With the choice of this value for step A 80 , only harmonics of order between 317 and 7936 are in the optical domain and only harmonics of order between 993 and 1058 are included in the working range. In particular, the wavelength À B so the fundamental frequency of the 80 network is not in the optical domain.

[0087]

[0054] For this step value A 80 and so that the length L 80 of the network 80 is less than 10 cm, the number p of patterns is chosen to be less than 185. Here, p is chosen equal to 120, so that the length L 80of the network 80 is approximately equal to 65 mm.

[0055] The patterns Mi are all structurally identical to each other and differ from each other only by their position along the axis 108. Thus, hereinafter, only the pattern Mj is described in detail. This pattern Mj extends mainly in a plane Pi perpendicular to the axis 108. This plane Pi is therefore parallel to the X and Y directions. In Figure 3, only the planes Pi, P2, P p -i and P p in which extend respectively, the patterns Mi, M2, M p .i and M p are represented.

[0088]

[0056] Figure 4 shows in more detail an example of embodiment of the pattern Mi. In Figure 4, only the cross section of the core 110 is shown.

[0089]

[0057] Each pattern Mi reflects a portion of the incident optical signal. Another portion of the incident optical signal passes through the pattern Mi. Finally, each pattern Mi diffuses a portion of the energy of the incident optical signal which is then neither reflected nor transmitted through this pattern Mi. This energy diffused by each pattern Mi creates insertion losses caused by the presence of the grating 80 in the core 110 of the optical fiber 34. To minimize these insertion losses, here, the surface S Mi of the cross-section of the pattern Mi occupies less than half of the surface Sno of the cross-section of the core 110. The surface S Mi is equal to the area of ​​the orthogonal projection of the pattern Mi onto the plane Pi. The area Sno is equal to the area of ​​the cross-section of the core 110. Typically, the area Sno is constant along the entire length of the optical fiber 34.

[0090]

[0058] Preferably, the surface S Miis less than O.1 *Sno or O.05*Sno or O.01 *Sno. Here, the surface Sli is less than O.05*Sno.

[0091]

[0059] To obtain sufficient reflectivity of the pattern Mi to limit the number p of patterns and therefore to limit the length L8o of the network 80, the surface S Mi is greater than 0.016 pm 2 , i.e. greater than twice the area of ​​the orthogonal projection of a spherical bubble of 100 nm diameter onto the Pi plane. In this embodiment, the area S Mi is greater than or equal to 0.032 pm 2 .

[0092]

[0060] For this purpose, the pattern Mi is made up of several bubbles B,. The index j is an identifier that makes it possible to uniquely identify the bubble B, among all the other bubbles of the same pattern Mi. The index j is here an integer between 1 and q, where q is equal to the number of bubbles B, of the pattern Mi. The number q is greater than or equal to two or four. Here, the number q is equal to six.

[0061] In this embodiment, all the bubbles B, are structurally identical to each other. Only their positions in the plane Pi make it possible to distinguish them from each other.

[0093]

[0062] Each bubble B creates a significant variation in the refractive index of the core 110 in the direction of propagation of the optical signal. For this, the difference between the index n rrefraction index of the core 110 and the refractive index n® of the bubble Bj 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 r no and n r B greater than or equal to 0.4.

[0094]

[0063] Furthermore, for the variation in refractive index to be abrupt, the diameter D, of each bubble Bj is less than 200 nm and, preferably, less than 100 nm. Generally, the diameter D, is also greater than 10 nm or 50 nm.

[0095]

[0064] Each bubble B is mainly spherical. Thus, the diameter Dj of the bubble Bj is equal to the diameter of the sphere of smallest volume which entirely contains the bubble Bj. Here, this diameter Dj is less than 100 nm.

[0096]

[0065] The center of each bubble Bj is contained in the plane Pi.

[0097]

[0066] In this embodiment, the bubbles Bj are disjoint, that is to say they do not overlap and they are not fluidically connected to each other.

[0098]

[0067] The pattern Mi is centered on the axis 108. For this, the bubbles Bj are arranged next to each other so that the barycenter of the pattern Mj is located less than 100 nm from the axis 108 and the center of at least one of the bubbles Bj is located less than 100 nm from the axis 108.

[0099]

[0068] In this first embodiment, the barycenter of the pattern Mj is located on the axis 108. In addition, the pattern Mi is symmetrical with respect to the axis 108.

[0100]

[0069] The centers of the bubbles Bj are located one behind the other on an axis Ai which intersects the axis 108 and which belongs to the plane Pi. The pattern Mi therefore comprises a line of disjoint bubbles. In this case, the arrangement of the disjoint bubbles forms what is called a “dotted line” in this text. Here, the axis Ai is parallel to the direction Y. In this embodiment, the bubbles B3 and B4 are located, respectively, above and below the axis 108. The centers of the bubbles B3 and B4 are less than 100 nm from the axis 108.

[0101]

[0070] The distance between two bubbles Bj, B j+i immediately consecutive along the axis Ai is constant. In other words, whatever the bubble pair Bj, B j+i immediately consecutive along the axis Ai, the distance separating the centers of these two bubbles is the same.

[0102]

[0071] Figure 5 represents the power spectrum of the network 80 between 1545 nm and 1555 nm. The reflectivity of the peaks of the comb obtained reaches -21 dBm.

[0103]

[0072] Figure 6 represents a method of manufacturing the optical fiber 34. This method begins with a step 120 of providing an optical fiber whose core 110 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®.

[0104]

[0073] 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 Mi patterns are to be produced.

[0105]

[0074] Then, during a step 122, the network 80 is manufactured in the core 110. For this, an operation 124 of forming the pattern Mi in the core 110 of the optical fiber provided is repeated at each location where such a pattern Mi must be formed.

[0106]

[0075] During operation 124, each bubble B is created by a single pulse from the femtosecond laser. More precisely, during operation 124, the beam of the femtosecond laser is focused on the center of the bubble B to be created, then a pulse with a duration of less than 500 fs or 250 fs is emitted and irradiates the point of the core 110 where the center of the bubble B is to be located. The bubble B is then created in the core 110. Then, 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. j+i to create, then a new pulse of the femtosecond laser is emitted.

[0107]

[0076] In this embodiment, the bubbles B are therefore created one after the other.

[0108]

[0077] The values ​​of the different 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 different parameters to create the previously characterized bubbles B is within the skills of a 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 34: - the central wavelength of the pulse of the femtosecond laser is equal to 512 nm,

[0109] - the duration of each pulse of the femtosecond laser is equal to 160 fs, and

[0110] - the power of each pulse of the femtosecond laser is equal to 45 nJ.

[0111]

[0078] The operation of the measuring device 2 will now be described with reference to the method of FIG. 7.

[0112]

[0079] During a step 130, the unit 70 controls the source 50 to vary linearly, over time, the wavelength λ s from wavelength A S min up to wavelength A sm ax. For this purpose, the unit 70 sends to the source 50 a control signal generated from an estimate of the transfer function of the source 50.

[0113]

[0080] The optical signal emitted by the source 50 is guided by the coupler 52 and the optical fibers 22, 14, 42 and 34 to the transducer 4 and the etalon 6. The transducer 4 and the etalon 6 then reflect a part of the incident optical signal. These reflected parts of the optical signal correspond to the signals backscattered, respectively, by the transducer 4 and the etalon 6.

[0114]

[0081] In parallel with step 130, during a step 132, the sensor 62 measures only the optical signal backscattered by the transducer 4 and the sensor 64 measures only the optical signal backscattered by the standard 6. More precisely, the sensors 62, 64 each generate an electrical signal whose amplitude is representative of the power of the measured optical signal. The electrical signals generated by the sensors 62, 64 are transmitted to the unit 70 which acquires them.

[0115]

[0082] Once the electrical signals have been acquired by the unit 70, during a step 134, the unit 70 determines the amplitude AÀ.

[0116]

[0083] When the wavelength A s is equal to the wavelength A Bm 4 of transducer 4, the power of the optical signal backscattered by transducer 4 passes through a maximum. Since the wavelength A s varies linearly with time, instant t m at which this maximum occurs is proportional to the current value of the wavelength λBm4. Similarly, the reference wavelength λ&4 corresponds to a reference instant t. In step 134, the unit 70 calculates the difference between instant t m measured and the reference instant t. Since the variation of the wavelength At s over time is linear, the deviation t m -ti is proportional to the amplitude AÀ. The coefficient of proportionality between the deviation t m-ti and the amplitude AÀ is equal to the target slope a c of the line representing the evolution over time of the wavelength A s . This target slope is a predetermined and known constant. Thus, during step 134, the unit 70 determines AÀ from the difference measured between the instants t m and t

[0117]

[0084] Then, during a step 136, the unit 70 establishes the variation AG of the measured physical quantity from the amplitude AÀ. For example, the variation AG is calculated using the following relationship AÀ / À Bi4 = S G *AG, WHERE SG is the sensitivity coefficient pre-stored in memory 74. If the wavelength À Bi4 is associated, in the memory 74, with a corresponding absolute value of the physical quantity to be measured, then the unit 70 also calculates this absolute value of the physical quantity measured during step 136.

[0118]

[0085] Here, during a step 140, each time the power spectrum of the standard 6 is measured by the sensor 64, the unit 70 establishes a new estimate of the transfer function of the source 50. For this, for example, the unit 70 notes for each of the peaks of order k of the comb of the standard 6, the instant t k , m at which it occurs. Then, unit 70 associates the recorded instant t k , m at wavelength A k of this peak of order k. The association of the instant t k , m and wavelength A k forms a point (To k ; t k , m ) of abscissa To k and ordinate t k , m . The estimated transfer function for source 50 then corresponds to the curve which passes through all the points (À k ; t k , m ) taken for standard 6.

[0119]

[0086] This estimated transfer function is then used during the next execution of step 130 to generate the control signal of the source 50 which makes it possible to obtain a linear variation, over time, of the wavelength λ s with the target slope a c . For example, for this, each instant t k , m is compared to a theoretical instant t k , t at which the k-order peak should have occurred if the variation in wavelength λ s was perfectly linear and with the slope a c . The theoretical instant t k , t is therefore calculated from the wavelength A k of the k-order peak and the target slope a c predetermined and known. If the amplitude of the deviation t k , m - t k , t exceeds a predetermined threshold, then the control signal is locally modified between times t k . i,t and t k ,t to limit the amplitude of this deviation. For example, if the deviation t k , m - t k , t is positive, this means that the instant t k , m is late compared to time t k , t . In this case, the control signal is modified between times t k .i, t and t k , t to make the wavelength grow faster s between these moments t k .i , t and t k , t . Conversely, if the gap t k , m - t k , t is negative, this means that the instant t k , m is ahead of time t k , t . In this case, the control signal is modified between times t k .i , t and t k , t to make the wavelength grow more slowly s between these moments t k .i ,t and t k , t .

[0087] Thus, in this embodiment, the measured spectrum of the standard 6 is used to linearize the variation, over time, of the wavelength λ s This linearization makes it possible to improve the precision of the measurement and also to compensate for drifts of the source 50. This is therefore how the measurement of the spectrum of the standard 6 intervenes in the determination of the amplitude AÀ.

[0120]

[0088] Figure 8 shows a standard 150 that can be used instead of standard 6. Standard 150 is identical to standard 6 except that a second Bragg grating 152 is made in the optical fiber 34. To simplify Figure 8, only the core 110 and the gratings 80 and 152 are shown.

[0121]

[0089] Grating 152 is a standard Bragg grating whose wavelength λ B152 of its fundamental frequency is located within the working range. The absolute value of the wavelength At B 152 is known. Here, the patterns of the 152 grating are shaped so that the amplitude of the power peak of the 152 grating at wavelength λ Bi5 2 is greater, and preferably 1.5 times or twice greater, than the amplitude of the peaks of the comb of the network 80 in the working range.

[0122]

[0090] The network 152 is, for example, produced in the core 110 at the same location as the network 80 but offset radially relative to the network 80 so that its patterns do not interfere with the patterns of the network 80. For example, the network 152 is produced in the upper part of the core 110 while the network 80 is produced in the lower part of the core 110. The upper part of the core 110 is that located above a horizontal plane containing the axis 108 and the lower part is that located below this horizontal plane.

[0123]

[0091] The power spectrum of the standard 150 is equal to the superposition of the spectrum of the grating 80 and the spectrum of the grating 152. Thus, the spectrum of the standard 150 comprises, in addition to the peak comb, an additional peak at the wavelength λ Bi5 2. This additional peak is easily identifiable because its amplitude is greater than the amplitude of the peaks of the comb of the 80 network.

[0124]

[0092] When standard 6 is replaced by standard 150, unit 70 is modified to additionally determine the absolute values ​​of the wavelengths corresponding to each of the peaks of the comb of grating 80. For this, unit 70 locates the largest power peak, i.e. that corresponding to grating 152, in the measured spectrum of standard 150. The absolute value of the wavelength at which this largest peak appears in the power spectrum of standard 150 is known and equal to wavelength A B152. The positions of the peaks of the comb of the grating 80 relative to the largest peak are determined from the measured spectrum of the standard 150. Then, for each peak of the comb of the grating 80, the unit 70 determines the absolute value of the wavelength corresponding to this peak from:

[0125] - the number of free spectral intervals which separate it from the largest peak, and

[0126] - of the absolute value of the wavelength A Bi5 2.

[0127]

[0093] For example, if a peak of the comb of the grating 80 is separated from the largest peak by 5.5 free spectral intervals, then the absolute value of the wavelength at which this peak appears is equal to λ B 152 +5.5*ISL8o, where ISLso is the known value of the free spectral interval of the 80 network.

[0128]

[0094] From the absolute values ​​of the wavelengths of the peaks of the comb, the unit 70 is then capable of estimating a transfer function for the source 50 which, to a particular value of the control signal, associates an absolute value of the wavelength À s . This simplifies the generation of a control signal that linearly varies the wavelength À s as a function of time. It is also possible, in this case, to directly establish the absolute value of the wavelength A Bm 4. Then, unit 70 establishes the absolute value of the physical quantity to be measured from a known relationship between the absolute value of the wavelength A Bm4 and the absolute value of the physical quantity.

[0129]

[0095] Chapter: Variants:

[0130]

[0096] Variants of the standard:

[0131]

[0097] The order k cof the harmonic which is in the center of the working range is here greater than 100 and, preferably, chosen greater than 500 or 1000. This order k c can also be chosen higher than 2000 or 4000 or 10000. Theoretically, there is no upper limit for this order k c . However, it follows from relation (2) that the higher the order k c is large, the larger the pitch A of the Bragg grating is and therefore the longer the very high order Bragg grating is. In practice, it is therefore the maximum desired length for the Bragg grating which imposes an upper limit for the order k c . Here, this maximum length is set at 1 m.

[0132]

[0098] Similarly, the minimum value of the pitch A is greater than 20 pm and, typically, greater than 50 pm so that very high order harmonics are included in the optical domain. Theoretically, there is no maximum value for the pitch A. Indeed, whatever the value retained for the pitch A, it is possible to find a value for the order k c which allows the wavelength to be placed at c in the center of the working range. However, the larger the pitch A, the longer the Bragg grating. In practice, it is therefore also the maximum desired length for the Bragg grating that imposes an upper limit on the value of pitch A.

[0133]

[0099] For example, by applying the teaching given in Chapter I, it is possible to obtain combs for all working ranges. This applies in particular to working ranges centered on the wavelengths commonly used in optics such as, in particular, the wavelength of 800 nm, 1000 nm, 1300 nm or 1500 nm.

[0134]

[0100] The working range may be wider than 100 nm. For example, the width of this working range is, alternatively, greater than 200 nm or 300 nm. There is no upper limit for the width of this working range other than that it must be in the optical domain and that it must be able to be scanned by the laser source of the spectral analyzer.

[0135]

[0101] 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 very high order Bragg gratings of the measuring device described here.

[0136]

[0102] The patterns of the very high order Bragg grating realized in the core of the optical fiber can 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.

[0137]

[0103] Optical fibers other than SMF-28 fiber may be used. For example, the optical fiber may be multi-mode optical fiber or MMF (Multi-Mode Fiber).

[0138]

[0104] It is not necessary for the core of the optical fiber to be made of a specific doping. Thus, the manufacturing method described can be implemented with optical fibers whose core is made of germanosilicates, pure silica, rare earth-doped aluminosilicates or sapphire.

[0139]

[0105] In a particular embodiment, the characteristics of the optical fiber 34 in which the standard 6 is made are different from the characteristics of the optical fiber 14 so that the sensitivity of the standard to variations in the physical quantity to be measured is less than the sensitivity of the optical transducer 4 to these same variations in the physical quantity.

[0140]

[0106] Variants of the insulating structure:

[0141]

[0107] Other embodiments of the insulating structure are possible. For example, the network 80 can be isolated from variations in mechanical stress by implementing the teaching of application FR3087008A1.

[0142]

[0108] The insulating structure may also be designed to isolate the network 80 from variations in hydrostatic pressure.

[0143]

[0109] In a simplified embodiment, the insulating structure is not an active insulating structure but a passive insulating structure, that is to say an insulating structure which does not consume electrical energy to insulate the optical fiber 34 from variations in the external environment. For example, a passive insulating structure comprises a material with a very low coefficient of thermal expansion on which the optical fiber is fixed without any degree of freedom. Typically this material with a very low coefficient of thermal expansion has a coefficient of thermal expansion less than 5*10'6 K' 1 . For example, this material is an alloy of iron and nickel such as the Fe-Ni alloy with 36 atomic% Nickel. This alloy is known as Invar®. In another example of a passive insulating structure, the optical fiber is buried inside a material whose thermal conductivity is less than 0.05 W / m / K.

[0144]

[0110] In another embodiment, the insulating structure comprises a material which exerts on the optical fiber a mechanical stress which compensates for the effect of the thermal expansion of this optical fiber in response to a variation in temperature.

[0145]

[0111] The housing 82 can be omitted in particular if the mechanical stress exerted by the external environment on the network 80 cannot vary or varies only negligibly.

[0146]

[0112] Optical transducer variants:

[0147]

[0113] The optical transducer does not necessarily comprise a Bragg grating. For example, as a variant, the optical transducer comprises a Fabry-Pérot interferometer instead of the Bragg grating. Like a Bragg grating, such a Fabry-Pérot interferometer has a power spectrum comprising a peak whose position varies as a function of the temperature, the elongational stress and the hydrostatic pressure exerted on this Fabry-Pérot cavity. Such a Fabry-Pérot interferometer can be produced in the core of an optical fiber. In another variant, the optical transducer is a gas cell whose transmission power spectrum has an absorption line. The position of this absorption line in the power spectrum varies, for example, as a function of the temperature.

[0148]

[0114] When the optical transducer is a Bragg grating, the power spectrum of the optical transducer is shifted in response to a temperature variation, a longitudinal deformation of the core of the optical fiber 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 of the core of the optical fiber 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.

[0149]

[0115] The measured physical quantity can also be another physical quantity than temperature, longitudinal deformation and hydrostatic pressure. For this, it is sufficient that the optical transducer is sensitive to this other physical quantity. For example, the optical transducer can be sensitive to a dose of radiation. By way of illustration, for this, the core of the optical fiber 14 is made of a photosensitive material. Here, this core is made of germanosilicate. Initially, a Bragg grating is fabricated in the core of the optical fiber 14. Then, this Bragg grating is transformed into a Bragg grating sensitive to a dose of the radiation to be measured. For this, this fabricated Bragg grating is exposed to ultraviolet radiation to create colored centers resulting from the recombination of the bonds between the germanium and the silica.When subjected to a dose of the radiation to be measured, these colored centers are modified, leading to a shift in the wavelength λ. Bm 4 of the Bragg grating of the optical transducer.

[0150]

[0116] Alternatively, the optical transducer comprises a succession of several Bragg gratings produced one after the other in the core of the same optical fiber. In this case, preferably, the wavelengths λ Bi4 of each of these Bragg gratings are different. Thanks to this, the same optical transducer can measure the physical quantity at different locations. In this embodiment, the power spectrum of the optical transducer then comprises several power peaks in the working range.

[0117] In another variant, the measuring device comprises several optical transducers optically connected in parallel to the spectral analyzer. Such a configuration of several optical transducers is for example illustrated in application CN102879022A.

[0151]

[0118] Alternatively, the optical transducer is not made in the optical fiber 14 but simply optically connected to the distal end of the optical fiber 14. For example, the optical transducer is a Fabry Pérot cavity formed between two reflecting mirrors and these mirrors are made outside the optical fiber 14.

[0152]

[0119] Variants of the spectral analyzer:

[0153]

[0120] Alternatively, the optical source is not tunable. For example, the optical source is a wide laser source, i.e. a laser source which emits an optical signal whose power spectrum simultaneously covers the entire working range. In this case, the emitted optical signal is not single-frequency. In addition, for each spectral response to be measured, the spectral analyzer then comprises a plurality of photodetectors which simultaneously measure the power of the spectral response for a large number of different wavelengths. For example, in this case, each sensor 62, 64 is a bar spectrometer. In such an embodiment, it is not necessary to vary the wavelength λ s to scan the entire working range. The estimation of the laser source transfer function can then be omitted.

[0154]

[0121] The optical source is not necessarily a laser source. For example, the optical source can also be produced using a tunable Fabry Pérot cavity. In this case, the control signal causes the displacement of at least one of the diopters of this Fabry Pérot cavity. This displacement of a diopter then causes a modification of the natural resonance frequency of the cavity and therefore a modification of the wavelength λ s .

[0155]

[0122] Other configurations of the unit 70 are possible for determining the amplitude AÀ. In particular, the amplitude AÀ can also be determined without using the time difference tm-tj. For example, alternatively, the amplitude AÀ is determined by counting the number of peaks of the spectrum of the standard located between the wavelength À Bi4 and the measured wavelength At Bm4. Such a method is for example described in the chapter “Absolute Frequency Measurement” of application W02020113147A1.

[0123] In another variant, the transfer function estimated for the source 50 is used to correct the wavelength λ Bm 4 so as to obtain a corrected wavelength closer to reality. For example, for this, the wavelength À Bm 4 corrected is calculated using the following relation: At Bm4 = [(To k+i - HAS k ) / (t k+ i, m - t k , m )]*(t m - t k ,m)+ To k , OR .

[0156] - HAS k and t k , m are, respectively, the abscissa and the ordinate of the point (À k ; t k , m ) of the transfer function estimated during step 140 for which t k , m immediately precedes time t m measured during step 134, and

[0157] - HAS k+i and t k+ i,m are, respectively, the abscissa and the ordinate of the point (À k+i ; t k+ i, m ) of the transfer function estimated during step 140 for which t k+ i, m immediately follows time t m .

[0158] In this variant, a linear interpolation of the transfer function is carried out between two successive points (À k ; t k , m ) and (To k+i ; t k+ i, m ). However, a non-linear interpolation of the transfer function between these two points is also possible.

[0159]

[0124] In the above cases, the standard 6 is not used to linearize the variation of the wavelength λ s of the optical source.

[0160]

[0125] In a simplified variant, the unit 70 only determines the variation of the measured physical quantity and not its absolute value. In this case, it is not necessary to know the value of the measured physical quantity corresponding to the wavelength λ Bi4 .

[0161]

[0126] In another embodiment, the spectral response of the standard 6 is first measured and only then is the spectral response of the optical transducer 4 measured. In this case, an optical switch is first placed in a calibration position in which it optically connects the spectral analyzer 20 only to the standard 6 to measure the spectral response of the standard. Then, this optical switch is switched to a measurement position in which it optically connects the spectral analyzer 20 only to the optical transducer 4 to measure the spectral response of the optical transducer. Generally, the spectral response of the standard 6 is then only measured intermittently and not each time the spectral response of the optical transducer 4 is measured.In this embodiment, the optical signal which illuminates the standard 6 is not necessarily strictly identical to the optical signal which illuminates the optical transducer 4 since they are emitted at two different times.

[0127] Alternatively, at least one of the optical sensors and, preferably, both optical sensors are connected to the distal end of the optical fibers 14 and 34. In this case, the spectral analyzer 20 measures the optical signals which have passed through the transducer 4 and the standard 6. Therefore, the power spectra of the measured signals are power spectra in transmission and not in reflection. However, everything which has been described in the particular case of power spectra in reflection adapts, without particular difficulty, to the case of power spectra in transmission.

[0162]

[0128] The optical couplers 12, 32 and 52 can be replaced by a single multi-channel optical coupler which alone fulfills the functions of these three optical couplers 12, 32 and 52.

[0163]

[0129] Variants of the manufacturing process:

[0164]

[0130] There are numerous variants of the method for manufacturing a very high order Bragg grating. In particular, all the manufacturing methods and their variants described in the application filed on 07 / 29 / 2022 under No. FR2207936 by the present applicant can be used to manufacture the grating 80. The manufacturing method described in article LUO2022 can also be used.

[0165]

[0131] Other variants:

[0166]

[0132] The waveguide is not necessarily an optical fiber. Everything described in this text in the particular case of optical fibers also applies to the case where the waveguides are waveguides made on a photonic chip. For example, in the latter case, the core of each waveguide is made of monocrystalline silicon or another semiconductor material and the cladding is made of a material commonly used in the field of optics on silicon such as silicon oxide.

[0167]

[0133] All that has been described previously in the particular case where the wavelength λ c of the k-order peak c is between 200 nm and 5000 nm also applies to the case where the wavelength À c is between 5000 nm and 10000 nm and, in particular, in the case where the wavelength À c is included in the infrared range. When the wavelength À cis in the infrared range, the core of the optical fiber is for example made of chalcogenide glass.

[0168]

[0134] Several of the variants described above can be combined in the same embodiment.

[0135] Chapter III: Advantages of the embodiments described:

[0169]

[0136] A very high order Bragg grating makes it possible to obtain a comb of peaks using a single Bragg grating and not a succession of several Bragg gratings as described in application CN102879022A. Thus, compared to the standard described in application CN102879022A, the standard described in the previous chapters is simpler to produce and less bulky.

[0170]

[0137] In addition, a very high order Bragg grating makes it possible to obtain a comb of peaks identical to those obtained using a Fabry Pérot cavity such as that described in application WO2020113147A1. On the other hand, with equal performance, the very high order Bragg grating is simpler to manufacture and has a smaller footprint.

[0171]

[0138] The fact that the size of the very high order Bragg grating is small reduces the size of the measuring device and also facilitates the production of the insulating structure. Indeed, it is much easier to thermally insulate a high order Bragg grating less than 10 cm long than a Fabry Pérot cavity several meters long.

[0172]

[0139] Thus, a measuring device comprising a standard manufactured using a very high order Bragg grating is, with equal performance, simpler to manufacture.

[0173]

[0140] The use of the spectral response of the standard 6 to obtain a linear variation of the wavelength emitted by the laser source 50 over the entire working range, makes it possible to simplify the structure of the sensors 62, 64 and therefore the structure of the measuring device.

[0174]

[0141] Simultaneously measuring the spectral responses of the optical transducer 4 and the standard 6 when they interact with the same optical signal ensures that the measured spectral responses are indeed spectral responses obtained in response to the same optical signal. This increases the accuracy of the measuring device.

[0175]

[0142] The fact that the standard 150 additionally comprises a Bragg grating 152 whose wavelength λ B152 is located within the working range, makes it possible to identify the absolute value of the wavelength associated with each peak of the comb of the network 80. It is therefore possible to measure the absolute value of the wavelength at which the power peak of the optical transducer 4 occurs and therefore to go back to an absolute value of the physical quantity measured.

[0143] The fact that the standard is made in an optical fiber simplifies the manufacture of the measuring device.

[0176]

[0144] Using one or more bubbles in each pattern Mi makes it possible to obtain a small pattern and therefore to substantially reduce insertion losses.

[0145] Using several disjointed bubbles makes it possible to obtain a pattern Mi that is sufficiently reflective to reduce the number p of patterns and therefore to maintain the compactness of the network 80 while limiting insertion losses. Indeed, when the bubbles overlap, the overlapping zones 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 disjointed, such overlapping zones do not exist, which limits insertion losses.

Claims

Tl Claims 1. Device for measuring a physical quantity, this device comprising: - an optical transducer (4) whose power spectrum has at least one power peak whose position varies, while remaining within a predetermined working range, depending on the physical quantity to be measured, this predetermined working range being a range of wavelengths and this working range being between 200 nm and 10000 nm, - a standard (6; 150) whose reflection power spectrum comprises several power peaks distributed within the working range, the free spectral interval of this standard being less than or equal to 5 nm, this standard comprising: - a first waveguide (34) containing a core (110) which extends along a longitudinal axis and inside which an optical signal guided by this first waveguide is able to propagate along the longitudinal axis of the first waveguide, and - a first Bragg grating (80) produced in the core of the first waveguide, this first Bragg grating comprising at least three patterns (Mi, M2, M N -I, M N ) identical aligned one behind the other along the longitudinal axis of the first waveguide and separated from each other by a first constant pitch, - an insulating structure (82) capable of isolating the first Bragg grating from temperature variations and mechanical stress variations exerted by an external environment on the standard, - a spectral analyzer (20) configured to: - separately measure the spectral response of the optical transducer in the working range and the spectral response of the standard in the same working range, then - determine the amplitude of the optical transducer peak displacement from the spectral responses of the optical transducer and the standard measured separately from each other, then - establish a variation of the physical quantity from the determined amplitude of the displacement of the peak of the optical transducer, characterized in that the first step of the first Bragg grating (80) is configured so that the power spectrum of the first Bragg grating has several discernible harmonics of order greater than one hundred in the working range, these harmonics thus forming the power peaks of the power spectrum of the standard at known wavelengths.

2. Device according to claim 1, in which: - the spectral analyzer (20) comprises a tunable optical source (50) capable of emitting a single-frequency optical signal which interacts with the optical transducer and the standard, this optical source being controllable using a control signal to vary the wavelength of the emitted optical signal, the wavelength of the emitted optical signal being linked to the control signal by a non-linear transfer function, and - the spectral analyzer (20) is also configured to: - estimate the non-linear transfer function of the controllable optical source from the measured spectral response of the standard and the known wavelengths at which the power peaks of the standard power spectrum occur over the working range, then - construct a control signal that allows to obtain a more linear variation, as a function of time, of the wavelength of the optical signal emitted over the entire working range from the estimated transfer function, and - control the optical source using this constructed control signal.

3. Device according to any one of the preceding claims, in which: - the spectral analyzer includes: - a laser source (50) equipped with an output port (54) through which an optical signal is emitted, and - an optical coupler (52) which optically connects the optical transducer and the etalon simultaneously to this output port, and - the spectral analyzer is capable of simultaneously measuring the spectral responses of the optical transducer and of the standard obtained in response to the optical signal emitted on the output port (54).

4. Device according to any one of the preceding claims, in which the waveguide (34) of the etalon is an optical fiber.

5. Device according to any one of the preceding claims, in which: - each pattern (Mi, M2, M N -I, M N ) of the first Bragg grating (80) extends mainly in a plane, called the "pattern plane", perpendicular to the longitudinal axis of the first waveguide, and - each pattern consists of one or more bubbles (Bi - B6) arranged next to each other in the plane of the pattern, and - the area of ​​the orthogonal projection of all the bubbles of the pattern onto the pattern plane is less than 50% of the area of ​​the cross-section of the core (110) of the first waveguide.

6. Device according to claim 7, in which each pattern consists of several disjoint bubbles (Bi - B6) arranged next to each other in the plane of the pattern.

7. Device according to any one of the preceding claims, in which: - the first step is greater than or equal to 20 pm, and - the difference between the refractive index of the core (110) of the first waveguide and the refractive index of each pattern of the first Bragg grating is greater than 0.

3.

8. Device according to any one of the preceding claims, in which each pattern (Mi, M2, M N -I, M N ) is performed using a pulse from a femtosecond laser.

9. Device according to any one of the preceding claims, in which the physical quantity is chosen from the group consisting of a temperature, a mechanical deformation and a hydrostatic pressure.

10. Device according to any one of the preceding claims, in which the working range is between 200 nm and 5000 nm, 11. Standard for producing a measuring device according to any one of the preceding claims, in which the reflection power spectrum of this standard comprises several power peaks distributed within a predetermined working range, the free spectral interval of this standard being less than or equal to 5 nm and this predetermined working range being a range of wavelengths and this working range being between 200 nm and 10000 nm, this standard comprising: - a first waveguide (34) containing a core (110) 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, this waveguide being able to be optically connected to a spectral analyzer, - a first Bragg grating (80) produced in the core of the first waveguide, this first Bragg grating comprising at least three identical patterns aligned one behind the other along the longitudinal axis of the first waveguide and separated from each other by a first constant pitch, - an insulating structure (82) capable of isolating the first Bragg grating from temperature variations and mechanical stress variations exerted by an external environment on the standard, this insulating structure comprising: - a temperature sensor (90), - a controllable heating or cooling element (92) capable of heating or cooling the first waveguide, and - a microcontroller (94) configured to control the heating element as a function of a temperature setpoint and the temperature measured by the sensor to limit the temperature variations of the first waveguide around this temperature setpoint, characterized in that the first step of the first Bragg grating (80) is configured so that the power spectrum of the first Bragg grating has several discernible harmonics of order greater than one hundred in the working range, these harmonics thus forming the power peaks of the power spectrum of the standard at known wavelengths.

12. Standard according to claim 10, in which the standard comprises a second Bragg grating (152) produced in the core of the first waveguide, this second Bragg grating comprising at least three identical patterns aligned one behind the other. along the longitudinal axis of the first waveguide and separated from each other by a second constant pitch, this second pitch being configured so that the wavelength of the fundamental resonant frequency of the second Bragg grating is located within the working range.

Images