Method for measuring variations of a first and a second independent physical quantity using an optical transducer

Abstract

Method for measuring variations of a first and a second independent physical quantity using an optical transducer. This method comprises: - the measurement (122) of a spectral response of a Bragg grating in, respectively, a first and a second group of spatial modes, then - the determination (130) of a first displacement amplitude of a first power peak in the spectral response of the Bragg grating in the first group of spatial modes and the determination (130) of a second displacement amplitude of a second power peak in the spectral response of the Bragg grating in the second group of spatial modes, then - the establishment (132) of the variations of the first and second physical quantities from the first and second displacement amplitudes determined. Fig. 2

Description

Title of the invention: Method for measuring variations of a first and a second independent physical quantity using an optical transducer

[0001] The invention relates to a method and a device for measuring variations of a first and a second independent physical quantity using an optical transducer.

[0002] To measure physical quantities such as temperature and mechanical deformation, it is advantageous to use an optical fiber and a Bragg grating fabricated in the core of this optical fiber as a transducer. In particular, such a transducer is insensitive to electromagnetic interference.

[0003] The power spectrum of the Bragg grating varies both with temperature and with mechanical deformation. Thus, to measure both a temperature variation and a mechanical deformation variation using the same Bragg grating, it is necessary to be able to isolate the changes in the Bragg grating's power spectrum caused by the temperature variation from those caused by the mechanical deformation.

[0004] To this end, it has already been proposed to use Bragg gratings with specific structures. For example, the following article proposes using a Superstructure Fiber Grating (SFG) Bragg grating inscribed in a single-mode optical fiber: Chi, H. et al.: “Simultaneous measurement of axial strain, temperature, and transverse load by a superstructure fiber grating,” Optical Letter, OL 26(24), 1949-1951 (2001). Another example of the state of the art is represented by the following article, which uses a tilted Fiber Bragg Grating inscribed in a Polarization Maintaining Few-Mode Fiber (PM-FMF) optical fiber: Chongxi Wang et al.: “Simultaneous Temperature and Strain Measurements Using Polarization-Maintaining Few-Mode Bragg Gratings,” Sensor 2019, 19, 5221.

[0005] Thus, to isolate a temperature variation from a mechanical deformation variation using a single Bragg grating, it has already been proposed to use a Bragg grating with a particular structure and / or optical fibers with a particular structure, such as PM-FMF optical fibers. The use of a Bragg grating with a particular structure or an optical fiber with a particular structure complicates the fabrication of the optical transducer and therefore the implementation of such measurement methods.

[0006] The invention aims to provide a method for measuring the variations of two independent physical quantities using a single Bragg grating which is simpler to implement.

[0007] The invention therefore relates to a method for measuring variations of a first and a second independent physical quantity using an optical transducer comprising:

[0008] - a multimode optical fiber containing a core that extends along an axis longitudinal and within which an optical signal guided by this multimode optical fiber is capable of propagating along the longitudinal axis of the multimode optical fiber according to at least a first and a second group of different spatial modes, and

[0009] - a Bragg grating implemented in the core of the multimode optical fiber, the responses spectral of this Bragg grating in, respectively, the first and second groups of spatial modes comprising, respectively, a first and a second power peak whose positions vary according to the variations of both the first and second physical quantities at the location of this Bragg grating, the sensitivity coefficient which relates the variation of the position of the first peak to a variation of the first physical quantity being different from the sensitivity coefficient which relates the variation of the position of the second peak to a variation of the first physical quantity,

[0010] in which the process comprises the following steps:

[0011] - in response to the excitation of a single group of spatial modes of the fiber multimode optics, the measurement of a spectral response of the Bragg grating in, respectively, the first and second groups of spatial modes, then

[0012] - the determination of a first and a second displacement amplitudes at starting from the spectral responses measured only in, respectively, the first and second groups of spatial modes, the first displacement amplitude being representative of the displacement amplitude, relative to a first predetermined reference position, of the first power peak in the spectral response of the Bragg grating in the first group of spatial modes and the second displacement amplitude being representative of the displacement amplitude, relative to a second reference position, of the second power peak in the spectral response of the Bragg grating in the second group of spatial modes, the first and second reference positions being equal to the positions, respectively, of the first and second power peaks when the first and second physical quantities have reference values, then

[0013] - the establishment of the variations of the first and second physical quantities compared to their respective reference values ​​from the first and second determined displacement amplitudes.

[0014] Embodiments of this process may include one or more of the following features:

[0015] 1)

[0016] - the measurement step comprises, for the second group of spatial modes which comprises several spatial modes, the measurement of several spectral responses, each of these spectral responses being measured in a spatial mode distinct from this second group of spatial modes, then

[0017] - during the determination of the first and second amplitudes of the displacement, the second displacement amplitude is determined from spectral responses measured in several distinct spatial modes of the second group of spatial modes.

[0018] 2) The measurement step includes, for the second group of spatial modes which It includes several spatial modes:

[0019] - the combination, using an optical coupler, of reflected optical signals or transmitted by the Bragg grating and propagating in several distinct spatial modes of the second group of spatial modes, to obtain a combined optical signal, then

[0020] - the measurement of the spectral response in the second group of spatial modes from of this combined optical signal.

[0021] 3) During the measurement step, each of the spectral responses is measured from of an optical signal reflected by the Bragg grating.

[0022] 4) The measurement step comprises:

[0023] - the excitation of a single group of spatial modes of the multimode optical fiber, and

[0024] - in response to the excitation of this single group of spatial modes, the measurement of minus a spectral response of the Bragg network in, respectively, the first and second groups of spatial modes.

[0025] 5) Exciting a single group of spatial modes consists of exciting the group of spatial modes which includes the fundamental spatial mode.

[0026] 6) The first group of spatial modes is the one that includes the spatial mode fundamental and the second group of spatial modes is that which includes the spatial modes whose effective index is the lowest.

[0027] 7) The first and second physical quantities are chosen from the group composed of temperature, mechanical deformation, hydrostatic pressure and radiation dose.

[0028] The invention also relates to a device for measuring variations of a first and a second independent physical quantity, this device comprising:

[0029] - an optical transducer comprising:

[0030] - a multimode optical fiber containing a core that extends along an axis longitudinal and within which an optical signal guided by this multimode optical fiber is capable of propagating along the longitudinal axis of the multimode optical fiber according to a first and a second group of different spatial modes, and

[0031] - a Bragg grating implemented in the core of the multimode optical fiber, the responses spectral of this Bragg grating in, respectively, the first and second groups of spatial modes comprising, respectively, a first and a second power peak whose positions vary according to the variations of both the first and second physical quantities at the location of this Bragg grating, the sensitivity coefficient which relates the variation of the position of the first peak to a variation of the first physical quantity being different from the sensitivity coefficient which relates the variation of the position of the second peak to a variation of the first physical quantity,

[0032] - a spectral analyzer configured to establish the variations of the first and second physical quantities derived from the spectral responses of the Bragg grating,

[0033] wherein the spectral analyzer is configured to perform the following steps:

[0034] - in response to the excitation of a single group of spatial modes of the optical fiber multimode, the measurement of spectral responses of the Bragg grating in, respectively, the first and second groups of spatial modes, then

[0035] - the determination of a first and a second displacement amplitudes at starting from the spectral responses measured only in, respectively, the first group and the second group of spatial modes, the first displacement amplitude being representative of the displacement amplitude, relative to a first predetermined reference position, of the first power peak in the spectral response of the Bragg grating in the first group of spatial modes and the second displacement amplitude being representative of the displacement amplitude, relative to a second reference position, of the second power peak in the spectral response of the Bragg grating in the second group of spatial modes, the first and second reference positions being equal to the positions, respectively, of the first and second power peaks when the first and second physical quantities have reference values, then

[0036] - the establishment of the variations of the first and second physical quantities compared to their respective reference values ​​from the first and second determined displacement amplitudes.

[0037] Embodiments of this device may include one or more of the following features:

[0038] 1) The spectral analyzer comprises:

[0039] - an optical source capable of generating an optical excitation signal,

[0040] - a modal demultiplexer optically connected to the optical source and to a at the end of the multimode optical fiber, this modal demultiplexer being capable of transmitting the optical signal generated by the optical source in a single group of excited spatial modes and of directing the optical signals reflected by the Bragg grating in a first spatial mode of the first group of spatial modes and in a second spatial mode of the second group of spatial modes, to, respectively, a first and a second output ports,

[0041] - an acquisition apparatus capable of measuring spectral responses in each from the first and second groups of spatial modes from the optical signals directed, respectively, to the first and second output ports, and

[0042] - an electronic processing unit electrically connected to the equipment acquisition, this electronic processing unit being configured:

[0043] - to determine the first and second displacement amplitudes from the spectral responses measured by the acquisition equipment in, respectively, the first and second groups of spatial modes, and

[0044] - to establish the variations of the first and second physical quantities at starting from the first and second determined displacement amplitudes.

[0045] 2)

[0046] - the modal demultiplexer includes a third output port on which is only directed the optical signal reflected or transmitted by the Bragg grating into a third spatial mode of the second group of spatial modes different from the second spatial mode,

[0047] - the acquisition apparatus comprises:

[0048] - an optical coupler that combines the optical signals delivered on the second and third output ports to obtain a combined optical signal, and

[0049] - a spectrometer equipped with a measurement port on which the optical signal is received combined to measure the spectral response of the Bragg grating in the second group of spatial modes from this combined optical signal.

[0050] 3)

[0051] - the modal demultiplexer includes a third output port on which is only directed the optical signal reflected or transmitted by the Bragg grating into a third spatial mode of the second group of spatial modes different from the second spatial mode,

[0052] - the acquisition apparatus comprises:

[0053] - a spectrometer equipped with a first and a second optically measuring ports connected, respectively, to the second and third output ports to measure, in parallel, spectral responses of the Bragg grating, respectively, in the second and third spatial modes of the second group of spatial modes, and

[0054] - the electronic processing unit is configured to determine the second displacement amplitude from spectral responses measured in the second and third spatial modes of the second group of spatial modes.

[0055] 4) The acquisition apparatus comprises:

[0056] - a spectrometer equipped with a measuring port, and

[0057] - an optical switch capable of optical connection, in response to a command switching emitted by the electronic processing unit, the first output port to the measurement port of the spectrometer and, alternately, the second output port to the measurement port of the spectrometer to measure, one after the other, the spectral responses of the Bragg grating in the first and second groups of modes.

[0058] 5) The spectral analyzer includes an optical coupler capable of optically connecting the optical source simultaneously to several input ports of the modal demultiplexer to simultaneously excite several spatial modes of the same spatial mode group of the multimode optical fiber.

[0059] The invention will be better understood upon reading the following description, given solely by way of non-limiting example and made with reference to the drawings in which:

[0060] - Figures 1, 3, 5 to 8 are schematic illustrations of four architectures possible for a measuring device,

[0061] - [Fig. 2] is a flowchart of a measurement method using the device measurement of [Fig. 1], and

[0062] - [Fig. 4] is a flowchart of a measurement method using the device measurement of [Fig.3].

[0063] In this description, the terminology, conventions, and definitions of the terms used in this text are introduced in Chapter I. Detailed examples of embodiments are then described in Chapter II with reference to the figures. Variants of these embodiments are presented in Chapter III. Finally, the advantages of the different embodiments are specified in Chapter IV.

[0064] Chapter I: Definitions, terminology and conventions:

[0065] In the figures, the same references are used to designate the same elements.

[0066] In the remainder of this description, the well-known characteristics and functions of a person skilled in the art are not described in detail.

[0067] The symbol “*” denotes the scalar multiplication operation.

[0068] The optical domain refers to the range containing the wavelengths commonly used in optics. More specifically, in this text, the optical domain refers to the range extending from 200 nm to 10000 nm and, frequently, from 200 nm to 5000 nm or from 400 nm to 2000 nm.

[0069] A spatial mode group of a multimode optical fiber is a group of spatial modes that contains all spatial modes that have identical or nearly identical propagation constants. Thus, a mode in this group can couple very strongly to another mode in the same group as soon as there is a very small imperfection in the optical path. For example, a very small imperfection in the optical path could be an asymmetry in the core of the optical fiber. These spatial modes that can couple very strongly together are also known as "degenerate modes." Thus, a spatial mode group contains all modes that have the same or nearly the same effective propagation index. Spatial mode groups vary according to the characteristics of the multimode optical fiber, such as its geometry and the variation of its refractive index in a transverse direction.The transverse direction is a direction that is perpendicular to the longitudinal axis of the optical fiber. For example, the spatial mode groups are not the same for a step-index multimodal optical fiber and for a multimode optical fiber whose refractive index varies continuously along a transverse direction.

[0070] Hereafter, the expression "mode group" refers to a group of spatial modes.

[0071] The word "mode" refers to a "spatial mode". Each mode can be decomposed into two orthogonal polarization modes. Hereafter, unless otherwise specified, the word "mode" is used to refer to the two orthogonal polarizations of that mode. Thus, for example, when it is stated that a mode is excited, in the absence of any contrary information, this means that the two orthogonal polarization modes are simultaneously excited.

[0072] The mode corresponding to the highest effective index is called the fundamental mode. The mode group that contains the fundamental mode generally includes only the two fundamental modes of orthogonal polarizations. This mode group that includes the fundamental mode is called the "fundamental mode group".

[0073] An excited mode group is a group of modes in which an optical excitation signal is transmitted. A mode group is excited from the moment at least one mode of that group is excited.

[0074] A polled mode group is a mode group in which the spectral response of a Bragg grating is measured. A mode group is polled from from the moment when at least one mode of this group is interrogated, that is to say from the moment when the spectral response of this group is measured from an optical signal which propagates in at least one mode of this group.

[0075] The effective propagation index ne is also known as the "mode phase constant." It is defined by the following relation: [3] = ne*2*Jt / X, where [3] is the phase constant and X is the wavelength of the optical signal in a vacuum. The effective propagation index of an optical fiber depends on the dimensions of the fiber core and the materials forming the core and the cladding of the optical fiber. It can be determined experimentally or by numerical simulation.

[0076] In this text, a spectral response of a group is a power spectrum of a Bragg grating measured solely from optical signals propagating in that particular group of modes of a multimode optical fiber in which that Bragg grating is implemented. Thus, for a given spectral analyzer, the same Bragg grating exhibits as many spectral responses of a group as there are possible mode groups in the multimode optical fiber. When a mode group comprises several modes, the spectral response of that mode group can be obtained:

[0077] - only from an optical signal propagating in only one of the modes of this group of modes, or

[0078] - from optical signals that propagate in several of the modes of this group of modes.

[0079] A "spectral response of a mode" or "spectral response in a mode" is a Bragg grating power spectrum measured using only optical signals propagating in that particular mode. This spectral response of a mode can be considered, on its own, as representative of the spectral response of the group containing that particular mode. In this case, the spectral response in that group can be considered as identical to the spectral response of that particular mode within that group.

[0080] In this text, the expression "a spectral response in a group" refers both to the spectral response of that group and to the spectral response of a mode of that group.

[0081] In this text, unless otherwise indicated, the term "power spectrum" or "spectrum" refers to the reflected power spectrum. The reflected power spectrum is the power spectrum of the optical signal reflected by an optical component. A peak in the reflected power spectrum corresponds to an absorption line in the transmitted power spectrum of the same optical component.

[0082] A Bragg grating is sensitive to a physical quantity when a variation of this physical quantity causes a corresponding shift in the power spectrum of this Bragg grating.

[0083] A sensitivity coefficient Skji of a Bragg grating to a physical quantity is a constant defined by the following relation AX; = Sk.i*AGk, where:

[0084] - AXi is the amplitude of the variation in wavelength at which the peak of Bragg grating reflection obtained in response to a variation AGk of the physical quantity to be measured,

[0085] - i is an index that identifies a particular group of modes, and

[0086] - k is an index that identifies the measured physical quantity.

[0087] The amplitude AX; is equal to the difference between the measured wavelength X; at which the reflection peak of the Bragg grating is located in mode group i and a predetermined reference wavelength Xi>ref. Unlike the wavelength Xi>ref, the wavelength X; varies depending on the physical quantity being measured.

[0088] The term "mechanical deformation" refers to:

[0089] - a force exerted on the optical fiber that longitudinally stretches the fiber core optics, and

[0090] - a force exerted on the optical fiber that causes a bend in the optical fiber at the location of the Bragg network.

[0091] Chapter II: Examples of embodiments

[0092] Figure 1 represents a device 2 for measuring two independent physical quantities. Thus, these two physical quantities can vary independently of each other. For example, here, the two physical quantities to be measured are the temperature of an external environment and a mechanical deformation.

[0093] The device 2 includes an optical transducer 4. The transducer 4 is exposed to variations in the two physical quantities to be measured. More precisely, the transducer 4 transforms a variation in the two physical quantities to be measured into shifts in power peaks within their respective spectral responses. For example, each spectral response has a single maximum power peak within 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 100 nm. The working range lies within the optical domain.

[0094] The transducer 4 comprises a multimode optical fiber 14 and a Bragg grating 16.

[0095] The fiber 14 contains a core extending along a longitudinal axis 18 and within which the optical signals guided by this fiber propagate. This core is surrounded by a cladding of a different refractive index to guide the optical signals along the axis 18. The fiber 14 is shaped to allow the propagation of optical signals along the axis 18 according to at least two different groups of modes. Typically, the number of different mode groups can be greater than three, five, or ten.

[0096] The Bragg grating 16 is inscribed in the core of the fiber 14. This grating 16 is formed by a succession of identical motifs arranged one after the other in a direction parallel to the axis 18. These motifs are spaced from each other by a regular pitch A. Each motif corresponds to a sharp variation in the refractive index of the core of the fiber 14. Typically, the difference between the refractive index of the motif and the refractive index of the core of the fiber 14 is greater than 0.2 or 0.4.

[0097] The grating 16 has a wavelength Xi for each mode group, where the index i is an identifier of a mode group of the fiber 14. The spectral response of group i has a reflection peak centered on this wavelength Xi. For each mode group used to measure the two physical quantities, the wavelength Xi lies within the predetermined working range. Preferably, each wavelength Xi is located substantially in the middle of the working range. The wavelength Xi of the Bragg grating 16 is given by the following relation (1): Xi = 2*nei*A / m, where:

[0098] - Xi is the wavelength for the group of modes identified by the index i,

[0099] - nei is the effective index of the mode group identified by the index i,

[0100] - m is the order of the Bragg lattice, and

[0101] -A is the step of network 16.

[0102] In the embodiments described here, the order m is less than ten and, for example, equal to one.

[0103] Since the effective index does not vary according to the mode group, there are as many X wavelengths as there are possible mode groups in fiber 14 for a given spectral analyzer.

[0104] The device 2 also includes a spectral analyzer 20. The spectral analyzer 20 is capable of measuring spectral responses of the transducer 4 and then determining the variations of the two physical quantities at the location of the grating 16 based on these measured spectral responses. For this purpose, the spectral analyzer 20 is optically connected to a proximal end of the fiber 14. The other end of the fiber 14, referred to as the "distal" end, is free.

[0105] Subsequently, the analyzer 20 is described in the particular case where only a first and a second group of modes of fiber 14 are queried. Here, the first group of modes queried is the fundamental mode group, which contains only the fundamental mode. The second group of modes queried is the group of modes of fiber 14 that corresponds to the lowest effective index, that is, the effective index furthest from the effective index of the fundamental mode group. The second group It contains several modes. To simplify the figures and explanations, the description of the different embodiments of the measuring device assumes that only two modes from the second group of modes are used. However, what is described in this simplified case also applies to the case where all or virtually all the modes of the second group are used to measure the two physical quantities. Furthermore, in the case of analyzer 20, the excited mode group is also the fundamental mode group. Thus, in this particular embodiment, the first group of modes is both excited and interrogated.

[0106] To this end, the analyzer 20 comprises:

[0107] - an optical source 50,

[0108] - a modal demultiplexer 52 optically connected, via a optical connector, at the distal end of fiber 14,

[0109] - an acquisition apparatus 54 optically connected to the modal demultiplexer 52,

[0110] - an electronic processing unit 56 electrically connected to the equipment 54 to receive electrical signals representative of the spectral responses acquired by apparatus 54, and

[0111] -a human-machine interface 58, connected to the unit 56, to communicate the result of the measurements carried out to a human being.

[0112] The modal demultiplexer 52 has several input / output ports, each of these input / output ports being associated with a single respective mode of the fiber 14. When an optical signal is received on one of these input / output ports, the modal demultiplexer 52 transmits this optical signal only in the mode of the fiber 14 associated with that input / output port. Thus, the demultiplexer 52 makes it possible to excite a single mode of the fiber 14 chosen from among several possible modes. Conversely, the demultiplexer 52 also makes it possible to direct optical signals that propagate simultaneously in several modes of the fiber 14, each to the respective input / output port associated with that mode. Thus, the modal demultiplexer 52 also makes it possible to demultiplex, that is to say, to separate, the different optical signals that propagate simultaneously in different modes of the fiber 14.

[0113] To simplify [Fig. 1], only three input / output ports 61, 62a, and 62b are shown. Port 61 is associated with the fundamental mode. Ports 62a and 62b are associated with two different modes from the second group of modes.

[0114] The optical source 50 emits an optical signal that is used to excite the first group of modes of the fiber 14. Here, the optical source 50 is a broad-spectrum source, that is, a source that emits an optical signal whose power spectrum simultaneously covers the entire operating range. The emitted optical signal is therefore not single-frequency.

[0115] In this embodiment, since the first group of modes is both excited and queried, the optical source 50 is connected to port 61 via an optical circulator 70. The optical circulator comprises:

[0116] - a port 71 optically connected to the optical source 50 to receive the signal excitation optics,

[0117] - a port 72 optically connected to port 61 of the demultiplexer 52 to transmit the optical excitation signal on this port 61, and

[0118] - a port 73 optically connected to the acquisition apparatus 54.

[0119] Circulator 70 therefore allows:

[0120] - to transmit, only on port 61, the optical excitation signal received on the Gate 71, and

[0121] - to transmit, only on port 73, the optical signal reflected by the network of Bragg 16 in the first group of modes.

[0122] The acquisition apparatus 54 measures spectral responses in the groups of modes being examined. In this first embodiment, the apparatus 54 measures spectral responses in several modes of the groups being examined. These spectral responses in the different modes are measured one after the other. For this purpose, it includes an optical switch 80 and a single-channel spectrometer 82.

[0123] The switch 80 has three input ports 84, 86a, and 86b and one output port 88. Ports 84, 86a, and 86b are permanently optically connected to ports 73, 62a, and 62b, respectively. The switch 80 also has a control port 90 electrically connected to unit 56. Depending on the command received on this port 90, the switch 80 toggles between the following states:

[0124] - a first state where it optically connects only port 84 to port 88,

[0125] - a second state where it optically connects only port 86a to port 88, and

[0126] - a third state where it optically connects only port 86b to port 88.

[0127] The spectrometer 82 has a single measurement port 92 optically permanently connected to the output port 88 of the switch 80. The spectrometer 82 also has an output port 94 electrically connected to the unit 56. The spectrometer 80 measures the spectral response of the optical signal received at its measurement port 92 and outputs the measured spectral response at its port 94. Here, the spectrometer 82 has a plurality of photodetectors that simultaneously measure the power of the received optical signal for a large number of different wavelengths. For example, the spectrometer 82 is a strip spectrometer. Thus, depending on the state of switch 80, spectrometer 82 delivers on port 94 either the spectral response measured in the first mode, or the spectral response measured in one of the two different modes of the second group, that is to say the spectral response of the optical signal received on port 62a or on port 62b of demultiplexer 52.

[0128] Unit 56 is specifically configured to implement the process of [Fig. 2] or 4. For this purpose, in particular, Unit 56 is programmed to:

[0129] - determine the amplitudes AXi and AX2 of the displacement of the power peaks in the spectral responses of the first and second groups of modes questioned, then

[0130] - establish the variations of the two physical quantities to be measured from the AXi and AX2 amplitudes determined.

[0131] To perform these operations, the unit 56 includes a programmable microprocessor 100 and a memory 102 containing the data and instructions necessary to execute the process of [Fig. 2] or 4. In particular, the memory includes all the sensitivity coefficients Skji that relate a variation of the k-th physical quantity to a variation of the amplitude AX;, where:

[0132] - the index k is an identifier of the physical quantity, and

[0133] - the index i is an identifier of the group of modes queried.

[0134] In this example, k is equal to one to identify the first physical quantity and equal to two to identify the second physical quantity. The index i is equal to one to identify the first group of modes queried and equal to two to identify the second group of modes queried.

[0135] Since the amplitudes AXi and AX2 vary with respect to both the first and second physical quantities, these amplitudes are related to the variations of these two physical quantities by the following system (1) of equations: AXi = Si,i*AGi + S2,i*AG2 and AX2 = Si,2*AGi + S2>2*AG2, where AGi and AG2 are the variations, respectively, of the first and second physical quantities. This system of equations can be solved as long as the determinant Si i*S2>2 - S2,i*Si,2 is not zero. However, for the same physical quantity to be measured, the Skji coefficients are not the same depending on the group of modes examined. Moreover, the Skji coefficients are not the same for the first and second physical quantities measured. Thus, in practice, using the system (1) of equations, it is always possible to establish the values ​​of the variations AGi and AG2 from the measured amplitudes AXi and AX2.

[0136] Typically, the Skji coefficients are determined experimentally by implementing the procedure of [Fig.2] for known variations of the first and second physical quantities.

[0137] Memory 102 also contains the reference positions Xi>ref and X2>ref of the power peaks in the spectral responses of the first and second groups of modes, respectively. Typically, the reference positions Xi>ref and X2>ref are determined experimentally by measuring the positions of the power peaks of the spectral responses of the first and second groups of modes, respectively, when the values ​​of the first and second physical quantities are equal to their Reference values ​​are denoted, respectively, Gi>ref and G2jref. For example, the reference value of the first physical quantity is an ambient temperature of 25°C and the reference value of the second physical quantity corresponds to the absence of mechanical deformation of the network 16.

[0138] Fig. 2 represents a method for measuring the first and second physical quantities using device 2.

[0139] During a calibration phase 110, the coefficients Sk,i and the positions Xi>ref and X2>ref are determined experimentally and then recorded in memory 102.

[0140] Next, it is possible to proceed to a phase 112 of measuring the variations of the first and second physical quantities. For this, the measurement method exploits the fact that even when only one group of modes of the fiber 14 is excited, after reflection by the grating 16, a spectral response of the grating 16 can be measured in other mode groups of the fiber 14 than the one excited. It has also been observed that when only one group of modes is excited, the observable power peak in the spectral response of a particular group of modes is not the result of the superposition of spectral responses from several different groups of modes. Thus, it is only under these conditions that it is possible to accurately measure the amplitude A / ., of the displacement of a peak in the spectral response of a particular group of modes.

[0141] Here, during a step 120, the optical source 50 emits an optical excitation signal. For example, this optical excitation signal is emitted continuously. This optical excitation signal is transmitted, by the circulator 70, to the port 61 of the demultiplexer 52. The demultiplexer 52 transmits this optical excitation signal only in the first group of modes, i.e., here in the fundamental mode of the fiber 14. The optical excitation signal then propagates to the array 16, and the array 16 reflects only a few wavelengths of this optical excitation signal in each of the mode groups. The demultiplexer 52 then directs the optical signals reflected in each mode to the input / output port associated with that mode. Thus, ports 61, 62a and 62b deliver the optical signals reflected by the network 16, respectively, in the fundamental mode and in two distinct modes of the second group of modes.The power spectrum of the optical signal(s) reflected in a particular group of modes forms the spectral response of that group of modes.

[0142] The optical signal delivered by port 61 is then directed continuously, by the circulator 70, to port 84 of the switch 80. In parallel, the optical signals delivered on ports 62a and 62b are transmitted continuously on ports 86a and 86b respectively of the switch 80.

[0143] Then, in a step 122, the switch 80 is controlled by the unit 56 to measure the spectral response of a particular mode. For example, in the first iteration of step 122, unit 56 commands switch 80 to place it in its first state where it optically connects port 84 to port 88.

[0144] In parallel, during step 122, the spectrometer 82 measures the spectral response of the optical signal emitted on the port 88 of the switch 80. Thus, during the first iteration of step 122, the apparatus 54 measures the spectral response of the fundamental mode and then transmits this measured spectral response to the unit 56 which acquires it.

[0145] Step 122 is repeated several times. During the second and third iterations of step 122, unit 56 commands switch 80 to place it in the second and third states, respectively. Thus, after repeating step 122 for each mode for which a spectral response is to be acquired, unit 56 has acquired each of these spectral responses. More precisely, here, unit 56 has acquired one spectral response in the first group of modes and two spectral responses in the second group of modes.

[0146] Once unit 56 has acquired each of the spectral responses to be measured, in step 130, unit 56 determines the amplitudes AX1 and AX2. To do this, in this embodiment, the unit processes each of the acquired spectral responses to extract the position Xi,j of the power peak, where the index j is an identifier of a particular mode in the group i of modes. Thus, in this example embodiment, in step 130, the unit extracts the positions Xi1, X2>i, and X2>2 of the power peak from the spectral responses of the optical signals delivered on ports 61, 62a, and 62b, respectively.

[0147] The positions Xi,j for all modes of the same mode group are theoretically identical. Thus, in this embodiment, to improve measurement accuracy, the positions extracted from the spectral responses measured in different modes of the same mode group are averaged. Consequently, in this example, the position X2 of the power peak of the spectral response of the second mode group is taken to be equal to the arithmetic mean of the positions X2>i and X2>2. Since the first group contains only the fundamental mode, the position Xi of the power peak of the spectral response of the first group is taken to be equal to Xij.

[0148] Then, still during step 130, unit 56 calculates the gap between positions Xi and Xi>ref to obtain the amplitude AXi and the gap between positions X2 and X2>ref to obtain the amplitude AX2.

[0149] Once the amplitudes AXi and AX2 have been determined, in step 132, unit 56 establishes the values ​​of the variations AGi and AG2 of the first and second physical quantities from the determined amplitudes AXi and AX2. To do this, in step 132, unit 56 solves the system (1) of equations.

[0150] In step 134, unit 56 commands interface 58 to communicate the established variations to a human being. The measured variations may be communicated in the form of relative values ​​AGi and AG2 or in the form of absolute values ​​Gijref + AGi and G2>ref+ AG2.

[0151] Figure 3 shows a measuring device 140 identical to device 2 except that the spectral analyzer 20 is replaced by a spectral analyzer 142. The spectral analyzer 142 is identical to the spectral analyzer 20 except that the acquisition apparatus 54 is replaced by an acquisition apparatus 144. The apparatus 144 includes a multi-channel interferometer 146 which has input ports 84, 86a, and 86b. The interferometer 146 is capable of simultaneously measuring the spectral responses of the optical signals received at its inputs 84, 86a, and 86b. For example, for this purpose, the interferometer 146 includes three copies of the interferometer 82, the three measuring ports 92 of these three copies forming the measuring ports 84, 86a, and 86b. Thus, the interferometer 146 is capable of simultaneously measuring the spectral responses of optical signals reflected by the grating 16 in the fundamental mode and in the two distinct modes of the second group of modes.Under these conditions, the use of an optical switch such as switch 80 is omitted.

[0152] Figure 4 illustrates the measurement method using the measuring device 140. This method is identical to the method in Figure 2, except that step 122 is replaced by step 150. In step 150, the spectrometer 146 simultaneously measures the spectral responses of the optical signals received on its three ports 84, 86a, and 86b. Thus, unlike the method described in Figure 2, here the three spectral responses used to determine the amplitudes AX1 and A / 2 are simultaneously measured and then transmitted to the unit 56.

[0153] Fig. 5 represents a measuring device 160 identical to device 2 except that the spectral analyzer 20 is replaced by a spectral analyzer 162. The spectral analyzer 162 is identical to the spectral analyzer 20 except that the acquisition apparatus 54 is replaced by an acquisition apparatus 164. Device 164 is identical to device 54 except that it also includes an optical coupler 166. Optical coupler 166 has two inputs optically connected to ports 62a and 62b of demultiplexer 52, respectively. Optical coupler 166 also has an output optically connected to port 86a of optical switch 80. Coupler 166 allows ports 62a and 62b to be connected simultaneously to the same port 86a of optical switch 80. Thus, optical coupler 166 delivers an optical signal to port 86a corresponding to the combination of the optical signals delivered to ports 62a and 62b.The spectral response of this combined optical signal exhibits an improved signal-to-noise ratio compared to the spectral response of the signal delivered only on port 62a or 62b.

[0154] The operation of device 160 is identical to the operation of device 2 except that:

[0155] - during step 122 the switch 80 is commanded to switch only between the first and second states since port 86b is not used, and

[0156] - during step 130, the position X2 is taken to be equal to the position of the power peak of the spectral response of the combined optical signal received on port 86a of switch 80. Indeed, in this embodiment, only one spectral response is measured for the second group of modes.

[0157] Figure 6 shows a measuring device 180 identical to device 160 except that the spectral analyzer 162 is replaced by a spectral analyzer 182. The spectral analyzer 182 is identical to the spectral analyzer 162 except that the acquisition apparatus 164 is replaced by an acquisition apparatus 184. The apparatus 184 allows the acquisition of:

[0158] - the spectral response of the first group of modes in response to the single excitation of this first group of modes, then

[0159] - the spectral response of the second group of modes in response to the sole excitation of this second group of modes.

[0160] For this purpose, the apparatus 184 comprises the same components as the apparatus 164, except that they are optically connected to each other differently. Here, the difference lies in the fact that:

[0161] - the output port 88 of switch 80 is optically connected directly to port 72 of circulator 70, and

[0162] - port 73 of the circulator is optically connected directly to measurement port 92 of spectrometer 82.

[0163] The operation of device 180 is identical to that of device 160 except that, because port 72 of the circulator 70 is connected to port 88 of the switch 80, when the switch 80 is in its second state, the optical excitation signal is transmitted in the second mode group. Under these conditions, when the first mode group is excited, only a spectral response of this first mode group is measured. No spectral response of the second mode group is measured in response to the excitation of the first mode group. Conversely, when only the second mode group is excited, only a spectral response of this second mode group is measured. No spectral response of the first mode group is measured in response to the excitation of the second mode group.Thus, in this embodiment, the spectral responses of the first and second groups are measured in response to different excitations and at different times.

[0164] Figure 7 represents a measuring device 190 identical to the measuring device 160 except that spectral analyzer 162 is replaced by spectral analyzer 192. Spectral analyzer 192 is identical to spectral analyzer 162 except that the mode group The excited mode group comprises several modes, and the demultiplexer 52 has two input / output ports 61a and 61b, each associated with a respective mode from the excited mode group. In this example, the spectral analyzer 192 includes an optical coupler 194 that simultaneously connects port 72 of the circulator 70 to the two ports 61a and 61b of the demultiplexer 52. Under these conditions, two different modes from the same group are simultaneously excited. The optical signal received at port 84 of the switch 80 corresponds to the combination of the optical signals delivered at ports 61a and 61b.

[0165] The operation of device 190 is identical to that of device 160 except that, because of the presence of coupler 194, two modes are simultaneously excited and the signal received on port 84 results from the combination of optical signals delivered on ports 61a and 61b.

[0166] Figure 8 shows a measuring device 200 identical to device 180 except that the spectral analyzer 182 is replaced by a spectral analyzer 202. The spectral analyzer 202 is identical to the spectral analyzer 182 except that the excited mode group comprises several modes and the demultiplexer 52 has two input / output ports 61a and 61b, each associated with a respective mode of the excited mode group. In this example, the spectral analyzer 202 includes an optical coupler 204 that simultaneously connects port 84 of switch 80 to the two ports 61a and 61b of demultiplexer 52. Under these conditions, as in the previous embodiment, two different modes of the same group are simultaneously excited. The optical signal received on port 84 of switch 80 corresponds to the combination of the optical signals delivered on ports 61a and 61b.

[0167] The operation of device 200 is identical to that of device 180 except that, because of the presence of coupler 204, two modes are simultaneously excited and the signal received on port 84 results from the combination of optical signals delivered on ports 61a and 61b.

[0168] Chapter III: Variants:

[0169] Transducer variants:

[0170] Many different embodiments are possible for the fiber 14. In fact, as long as the optical fiber allows the propagation of at least two different spatial mode groups, then it can be used as a multimode optical fiber for the transducer 4. In particular, there are many different profiles of variation of the refractive index of a multimode optical fiber in a transverse direction. For example, this profile can be a step-index profile or, conversely, a graded-index profile. In a step-index profile, the refractive index has only two different values, one inside the core and the other outside the core. In the graded-index profile, the index varies continuously, following, for example, a linear or parabolic function.

[0171] Multimode optical fiber can be an optical fiber known by the acronym FMF (“Few Mode Fiber”), that is, an optical fiber allowing the propagation of the optical signal according to fewer than three, or six, different mode groups.

[0172] Alternatively, the fiber 14 comprises several Bragg gratings arranged one after the other in its core. In this case, for each mode group, the wavelengths Xi of each of these Bragg gratings are different. For example, the wavelengths Xi of these different Bragg gratings are separated from each other by at least 5 nm or 10 nm. Thanks to this, the spectral responses of the different Bragg gratings in the optical fiber can be isolated from each other. Under these conditions, the same optical transducer allows the two physical quantities to be measured at the location of each of these Bragg gratings by implementing for each of these Bragg gratings the same measurement procedure as that described with reference to [Fig. 2] or 4.

[0173] Variants of the spectral analyzer:

[0174] The optical source is not necessarily a broadband source. For example, the optical source 50 can be replaced by a monochromatic tunable laser source that emits a single-frequency optical signal at a wavelength Xs in the optical range. The value of the wavelength Xs depends on a control signal received at a control port of the laser source. Such a laser source is also called a "scanning laser source." Indeed, by using a suitable control signal, the wavelength Xs sweeps across the entire working range. By sweeping the working range with such an optical source, it is also possible to measure the spectral responses of the Bragg grating 16 in the first and second mode groups. In this case, the spectrometer 82 can be simplified because it is not necessary for it to be able to measure the power of the received optical signal simultaneously for different wavelengths.

[0175] The optical source is not necessarily a laser source. For example, the optical source can also be implemented using a tunable Fabry-Pérot cavity. In this case, the control signal causes the displacement of at least one of the interfaces of this Fabry-Pérot cavity. This displacement of an interface then causes a change in the cavity's natural resonant frequency and therefore a change in the wavelength Xs.

[0176] The working range can 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 except that it must be within the optical domain and that it must be able to be scanned by the optical source of the spectral analyzer.

[0177] In a simplified variant, unit 56 determines only the variations of the measured physical quantities and not their absolute values. In this case, it is not necessary to know the reference values ​​Gi>ref and G2>ref corresponding to the reference positions Xijref and X2>ref.

[0178] Alternatively, the acquisition apparatus is optically connected to the distal end of the fiber 14. In this case, the acquisition apparatus measures the optical signals that have passed through the network 16. Consequently, the spectral responses measured are transmission power spectra and not reflection power spectra. However, everything described for the case where reflection power spectra are used can be transposed, without particular difficulty, to the case where transmission power spectra are used.

[0179] Variants of the measurement method:

[0180] Alternatively, the excited mode group is not the mode group containing the fundamental mode. For example, the second mode group is excited instead of the first mode group. For this purpose, for example, in the embodiments shown in Figures 1 and 3, ports 72 and 73 of the circulator 70 are optically connected to port 62a and port 86a, respectively. Port 61 of the demultiplexer 52 is directly connected to port 84 of the switch 80 without passing through the circulator 70.

[0181] The excited mode group may be different from the polled mode groups. For example, alternatively, a third mode group, different from the first and second polled mode groups, is excited to measure, in response, the spectral responses of the network 16 in these first and second mode groups. According to another alternative, the spectral response of the first group is measured by exciting only the second group and vice versa.

[0182] In all the embodiments described, when the excited mode group comprises several modes, it is possible to excite this mode group according to one of the following alternatives:

[0183] - to excite a single mode from this group of modes,

[0184] - to simultaneously excite several modes of this mode group but not all, or

[0185] - simultaneously excite all modes of this mode group.

[0186] To simultaneously excite several modes of the same mode group, an optical coupler is used to simultaneously transmit the optical excitation signal on the different input / output ports of the modal multiplexer associated with these modes.

[0187] Alternatively, the first group of modes queried does not contain the fundamental mode. In this case, the first group of modes contains the modes with a lower effective index than that of the fundamental mode.

[0188] Similarly, the second group of modes is not necessarily the group of modes containing the spatial modes with the lowest effective index. The effective index of the modes in the second group of modes may therefore be higher.

[0189] In another embodiment, the number of different mode groups polled is equal to Nm, where Nm is greater than two. In this case, for example, additional input / output ports of the demultiplexer 52 are optically connected to corresponding additional input ports of the switch 80. Then, in step 130, the unit 56 determines a respective amplitude AX; for each of the polled mode groups. Finally, in step 132 of determining the variations AG1 and AG2, the system (1) of equations comprises as many equations AX; = Sij*AG1 + S2ji*AG2 as there are mode groups polled. Thus, in this case, the system (1) of equations to be solved comprises Nm equations and only two unknowns. Such a system of equations can be solved using an error-minimizing algorithm such as the least squares method.

[0190] In a simplified variant, even for groups of modes that are examined, only one spectral response of a single particular mode from that group of modes is measured. In this case, during the determination step, the amplitude AXi is equal to the amplitude AXij determined from the measured spectral response of that single particular mode. Here, the index j corresponds to the index identifying that particular mode.

[0191] The determination of the amplitude of the power peak shift of the network 16 in a particular group comprising several modes can be carried out differently. For example, alternatively, instead of averaging the different amplitudes AXij determined from each of the spectral responses of the different modes in this group, it is possible to first average the different spectral responses measured for the different modes in this group in order to obtain an average spectral response. Then, the amplitude AXij is obtained from the average spectral response thus obtained. Typically, in this case, the amplitude AXij is taken to be equal to the difference between the position Xij of the peak measured in the average spectral response and the position Xi>ref

[0192] To increase the accuracy of the measurements, it is possible to repeat the measurement phase 112 several times and then average several measurements of the variation of the same physical quantity.

[0193] Other variants:

[0194] The first and second physical quantities simultaneously measurable using a single Bragg grating are not necessarily, respectively, temperature and mechanical strain. In practice, one of the temperature and Mechanical deformation can be replaced by another physical quantity, different from temperature and mechanical deformation. For this to happen, the Bragg grating simply needs to be sensitive to this other physical quantity, and the determinant Si,i*Sk>2 - Sk,i*Si,2 must be non-zero, where the coefficients Skji are the Bragg grating's sensitivity coefficients to this other physical quantity. For example, mechanical deformation can be replaced by the hydrostatic pressure exerted on the optical fiber at the Bragg grating's location. The other physical quantity can also be a radiation dose. Indeed, a Bragg grating can be made sensitive to a radiation dose, for example, by making the optical fiber's core from a photosensitive material. For example, the core is made of germanosilicate, and then the Bragg grating is inscribed within it.Next, this Bragg grating is transformed into a dose-sensitive Bragg grating by exposing it to ultraviolet radiation, which creates colored centers resulting from the recombination of bonds between germanium and silica. When subjected to a dose of the radiation to be measured, these colored centers are modified, leading to a shift in the wavelengths Xi of the Bragg grating.

[0195] The spectral analyzer can also be adapted to simultaneously measure the variations of Ng in different physical quantities, where Ng is greater than two. This is illustrated in the particular case of three physical quantities being measured simultaneously, such as temperature, mechanical strain, and a radiation dose. Since there are three physical quantities to be measured using the same Bragg grating, at least three distinct mode groups must be interrogated in order to determine three amplitudes AX1. In this case, the system (1) of equations comprises three equations of the following form: AX1 = S1,i*AG1 + S2,i*AG2 + S3,i*AG3, where AG3 is the variation in radiation dose and S31 is the sensitivity coefficient to the variation in AG3. Therefore, from the measurements of the amplitudes AX1 in three different mode groups, it is possible to establish the corresponding variations in temperature, mechanical strain, and radiation dose.What has just been described in the case where Ng is equal to three can be generalized to the case where Ng is greater than three, provided that the system (1) of equations is solvable.

[0196] Several of the variants described above can be combined in the same embodiment.

[0197] Chapter IV: Advantages of the embodiments described:

[0198] Exciting only one group of modes at a time to measure a spectral response in a given mode group avoids the superposition of spectral responses from different mode groups that is observed when several different mode groups are simultaneously excited. This simplifies and increases the Accuracy in determining the amplitude AX;. Furthermore, the sensitivity coefficient of the Bragg grating to the same physical quantity differs depending on the mode group used to determine the amplitude AXi. Therefore, by exploiting these two characteristics, the same Bragg grating can be used to measure the variations of two independent physical quantities by determining only amplitudes AX;. Now, the amplitude AX; is measurable with greater accuracy than, for example, the amplitude of a power peak. Moreover, the method described here imposes no limitations on the type of multimode optical fiber used or the type of Bragg grating employed. In particular, the measurement method works equally well with FMF optical fibers and with multimode optical fibers, which allow the propagation of the optical signal in a much larger number of mode groups.Thus, the measurement method described here is both simple and precise.

[0199] Using spectral responses measured in several modes of the same mode group to determine the amplitude AX; of the power peak shift in this mode group, improves the accuracy of the process.

[0200] Combining, using an optical coupler, the optical signals that propagate in different modes of the same mode group and then measuring the spectral response of this group from this combined optical signal, makes it possible to improve the accuracy of the process without having to measure and process individually each of the optical signals that propagate in the different modes of this mode group.

[0201] The fact that the measured spectral responses are reflection power spectra simplifies the implementation of the method because excitation and measurement are carried out from the same end of the optical fiber 14.

[0202] Exciting only one group of modes and measuring, in response, the spectral responses in two distinct groups of modes simplifies the measurement process since only one group of modes is excited to measure spectral responses in several distinct groups of modes.

[0203] Exciting the fundamental mode group improves the coupling between the excited group and the polled groups when the Bragg grating patterns are located at the center of the multimode optical fiber core.

[0204] Using groups of modes whose effective indices are as far apart as possible to measure spectral responses increases the differences between the sensitivity coefficients. This ultimately improves the accuracy of the measurement method.

[0205] Using a spectral analyzer capable of simultaneously acquiring several spectral responses in several modes of the same mode group makes it possible, with equal precision, to make the measurement faster.

[0206] Using an optical switch to alternately connect several output ports of the modal multiplexer to the same measurement port of the spectrometer 82 allows a single-channel spectrometer to be used to measure several spectral responses. This therefore simplifies the design of the acquisition equipment.

[0207] Exciting several modes of the same mode group simultaneously improves the signal-to-noise ratio of the measured spectral responses and thus improves the accuracy of the measurement.

Claims

1. Demands Method for measuring variations of a first and a second independent physical quantity using an optical transducer comprising: - a multimode optical fiber containing a core extending along a longitudinal axis and within which an optical signal guided by this multimode optical fiber is capable of propagating along the longitudinal axis of the multimode optical fiber according to at least a first and a second group of different spatial modes, and - a Bragg grating made in the core of the multimode optical fiber, the spectral responses of this Bragg grating in, respectively, the first and second groups of spatial modes comprising, respectively, a first and a second power peak whose positions vary according to the variations of both the first and second physical quantities at the location of this Bragg grating,the sensitivity coefficient that relates the variation in the position of the first peak to a variation in the first physical quantity being different from the sensitivity coefficient that relates the variation in the position of the second peak to a variation in the first physical quantity, characterized in that the process comprises the following steps: - in response to the excitation of a single group of spatial modes of the multimode optical fiber, the measurement (122; 150) of a spectral response of the Bragg grating in, respectively, the first and second groups of spatial modes, then, - the determination (130) of a first and a second displacement amplitudes from the spectral responses measured only in, respectively, the first and second groups of spatial modes, the first displacement amplitude being representative of the displacement amplitude, relative to a first predetermined reference position, of the first power peak in the spectral response of the Bragg grating in the first group of spatial modes and the second displacement amplitude being representative of the displacement amplitude, relative to a second reference position, of the second power peak in the spectral response of the Bragg grating in the second group of spatial modes, the first and second reference positions being equal to the positions, respectively, of the first and second power peaks when the first and second physical quantities have reference values, then - the establishment (132) of the variations of the first and second physical quantities with respect to their respective reference values ​​from the first and second determined displacement amplitudes.

2. A method according to claim 1, wherein: - the measurement step (122; 150) comprises, for the second group of spatial modes which comprises several spatial modes, the measurement of several spectral responses, each of these spectral responses being measured in a spatial mode distinct from this second group of spatial modes, then - during the determination (130) of the first and second displacement amplitudes, the second displacement amplitude is determined from the spectral responses measured in several spatial modes distinct from the second group of spatial modes.

3. A method according to claim 1, wherein the measurement step (122; 150) comprises, for the second group of spatial modes which comprises several spatial modes: - the combination, using an optical coupler, of the optical signals reflected or transmitted by the Bragg grating and which propagate in several distinct spatial modes of the second group of spatial modes, to obtain a combined optical signal, and then - the measurement of the spectral response in the second group of spatial modes from this combined optical signal.

4. A method according to any one of the preceding claims, wherein, during the measurement step (122; 150), each of the spectral responses is measured from an optical signal reflected by the Bragg grating.

5. A method according to any one of the preceding claims, wherein the measurement step (122; 150) comprises: - the excitation (120) of a single group of spatial modes of the multimode optical fiber, and - in response to the excitation of this single group of spatial modes, the measurement (122; 150) of at least one spectral response of the Bragg grating in, respectively, the first and second groups of spatial modes.

6. A method according to claim 5, wherein the excitation (120) of a single group of spatial modes consists of exciting the group of spatial modes which includes the fundamental spatial mode.

7. A method according to any one of the preceding claims, wherein the first group of spatial modes is that which includes the fundamental spatial mode and the second group of spatial modes is that which includes the spatial modes with the lowest effective index.

8. A method according to any one of the preceding claims, wherein the first and second physical quantities are chosen from the group consisting of a temperature, a mechanical deformation, a hydrostatic pressure and a radiation dose.

9. Device for measuring variations of a first and a second independent physical quantity, this device comprising: - an optical transducer (4) comprising: - a multimode optical fiber (14) containing a core extending along a longitudinal axis (18) and within which an optical signal guided by this multimode optical fiber is capable of propagating along the longitudinal axis of the multimode optical fiber according to a first and a second group of different spatial modes, and - a Bragg grating (16) made in the core of the multimode optical fiber, the spectral responses of this Bragg grating in, respectively, the first and second groups of spatial modes comprising, respectively, a first and a second power peak whose positions vary according to the variations of both the first and second physical quantities at the location of this Bragg grating,the sensitivity coefficient that relates the variation in the position of the first peak to a variation in the first physical quantity being different from the sensitivity coefficient that relates the variation in the position of the second peak to a variation in the first physical quantity, - a spectral analyzer (20; 142; 162; 182) configured to establish the variations of the first and second physical quantities from the spectral responses of the Bragg grating, characterized in that the spectral analyzer (20; 142; 162; 182) is configured to perform the following steps:, - in response to the excitation of a single group of spatial modes of the multimode optical fiber, the measurement of spectral responses of the Bragg grating in, respectively, the first and second groups of spatial modes, then - the determination of a first and a second displacement amplitudes from the spectral responses measured only in, respectively, the first and second groups of spatial modes, the first displacement amplitude being representative of the displacement amplitude, relative to a first predetermined reference position, of the first power peak in the spectral response of the Bragg grating in the first group of spatial modes and the second displacement amplitude being representative of the displacement amplitude, relative to a second reference position, of the second power peak in the spectral response of the Bragg grating in the second group of spatial modes,the first and second reference positions being equal to the positions, respectively, of the first and second power peaks when the first and second physical quantities have reference values, then - the establishment of the variations of the first and second physical quantities with respect to their respective reference values ​​from the first and second determined displacement amplitudes.

10. A device according to claim 9, wherein the spectral analyzer comprises: - an optical source (50) capable of generating an optical excitation signal, - a modal demultiplexer (52) optically connected to the optical source and to one end of the multimode optical fiber, this modal demultiplexer being capable of transmitting the optical signal generated by the optical source in a single group of excited spatial modes and of directing the optical signals reflected by the Bragg grating in a first spatial mode of the first group of spatial modes and in a second spatial mode of the second group of spatial modes, respectively, to a first and a second output port (61; 62a), - an acquisition apparatus (54; 144; 164; 184) capable of measuring spectral responses in each of the first and second spatial mode groups from optical signals directed, respectively, to the first and second output ports, and - an electronic processing unit (56) electrically connected to the acquisition equipment, this electronic processing unit being configured: - to determine the first and second displacement amplitudes from the spectral responses measured by the acquisition equipment in, respectively, the first and second groups of spatial modes, and - to establish the variations of the first and second physical quantities from the first and second determined displacement amplitudes.

11. Device according to claim 10, wherein: - the modal demultiplexer (52) has a third output port (62b) to which only the optical signal reflected or transmitted by the Bragg grating is directed in a third spatial mode of the second group of spatial modes different from the second spatial mode, - the acquisition equipment (164; 184) comprises: - an optical coupler (166) which combines the optical signals delivered on the second and third output ports to obtain a combined optical signal, and - a spectrometer (82) equipped with a measurement port (92) on which the combined optical signal is received to measure the spectral response of the Bragg grating in the second group of spatial modes from this combined optical signal.

12. Device according to claim 10, wherein: - the modal demultiplexer (52) has a third output port (62b) to which only the optical signal reflected or transmitted by the Bragg grating is directed in a third spatial mode of the second group of spatial modes different from the second spatial mode, - The acquisition equipment includes: - a spectrometer (146) equipped with a first and second measurement ports (86a, 86b) optically connected, respectively, to the second and third output ports (62a, 62b) to measure, in parallel, spectral responses of the Bragg grating, respectively, in the second and third spatial modes of the second group of spatial modes, and - the electronic processing unit (56) is configured to determine the second displacement amplitude from the spectral responses measured in the second and third spatial modes of the second group of spatial modes.

13. Device according to claim 10, wherein the acquisition apparatus comprises: - a spectrometer (82) equipped with a measurement port (92), and - an optical switch (80) capable of optically connecting, in response to a switching command issued by the electronic processing unit, the first output port (61) to the measurement port (92) of the spectrometer and, alternately, the second output port (62a) to the measurement port of the spectrometer to measure, one after the other, the spectral responses of the Bragg grating in the first and second groups of modes.

14. Device according to claim 10, wherein the spectral analyzer (182) comprises an optical coupler (166) capable of optically connecting the optical source (50) simultaneously to several input ports (62a, 62b) of the modal demultiplexer to simultaneously excite several spatial modes of the same group of spatial modes of the multimode optical fiber.

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