Device for measuring a radius of curvature

Abstract

Device for measuring a radius of curvature. This device comprises: - an array (16) of Bragg gratings, each Bragg grating being fabricated in the core of a multimode optical fiber (14) and having at least three identical motifs aligned one behind the other, - a spectral analyzer (42) configured to: - emit, into the multimode optical fiber, an optical excitation signal the majority of whose power lies within a wavelength range [λmin; λmax], then - establish a value for the radius of curvature from the measured amplitude of a power peak in the measured spectral response of a Bragg grating from the array (16), in which, for each Bragg grating from the array (16), the largest transverse dimension of the motifs of that Bragg grating is less than the wavelength λmin. Fig. 3

Description

Title of the invention: Device for measuring a radius of curvature

[0001] The invention relates to a device and a method for measuring a radius of curvature.

[0002] Many known devices for measuring a radius of curvature use a transducer comprising an optical fiber and a Bragg grating made in this optical fiber. These known devices offer many advantages, such as, for example, the insensitivity of the transducer to electromagnetic interference.

[0003] The following article discloses such a measuring device: Zhao, Y. et al.: “Simultaneous directional curvature and temperature sensor based on a tilted few-mode fiber Bragg grating” Applied Optic, AO 57(7), pages 1671-1678, 01 / 03 / 2018. Hereafter, this article is referred to by the reference “Zhao2018”.

[0004] The measurement device disclosed in Zhao2018 establishes the radius of curvature from the amplitude of a power peak in a Bragg grating spectral response in a group of spatial modes queried outside the fundamental spatial mode group. This is interesting because the amplitude of this power peak varies with the radius of curvature of the optical fiber but does not vary, or varies very little, with temperature. Thus, the measurement of the radius of curvature using this measurement device is relatively insensitive to temperature variations.

[0005] However, to implement the measurement device disclosed in Zhao2018, a particular multimode optical fiber known by the acronym FMF (Few Mode Fiber) must be used. Furthermore, a particular type of Bragg grating must be used, namely a tilted Bragg grating known by the acronym TFBG (Tilted Fiber Bragg Grating). As taught in the following article, a tilted Bragg grating modifies the intensity of the light transmitted through the optical cladding as a function of the radius of curvature: Jang, M., Kim et al.: “Ultra-high curvature sensors for multi-bend structures using fiber Bragg gratings,” Optics Express, Vol. 27, No. 3, pages 2074-2084, 4 / 02 / 2019.Thus, when an inclined Bragg grating is used to measure a radius of curvature, it is necessary to excite this inclined Bragg grating by emitting an optical excitation signal in one end of the optical fiber and to measure the optical signal which varies as a function of the radius of curvature at the opposite end of this optical fiber.

[0006] Due to the various constraints outlined in the preceding paragraph, the implementation of the measurement device described in Zhao2018 is quite complex, particularly for the following reasons:

[0007] 1) The transmission spectrum of the Bragg grating must be measured, which requires to emit the excitation signal from one end of the optical fiber and to perform the measurement from the other end of the fiber.

[0008] 2) It is assumed that the first four absorbed bands (“core modes”) on the [Fig. 7] from Zhao (2018) corresponds to the couplings of the fundamental mode LP01 to the four guided LP modes (LP01, LP11, LP21, LP02). In practice, to obtain such a result, only the LP01 mode needs to be injected, which requires a specific and more complex setup without a modal demultiplexer. Other modes injected into the fiber could produce other absorbed bands corresponding to the couplings from these modes to all the guided modes of the fiber, and these could overlap with the absorption bands of interest, making the measurement more complex.

[0009] 3) The observed absorption bands will be more numerous for a fiber containing more mode groups. It will therefore be more difficult to separate the peaks for a fiber with a higher mode number.

[0010] The invention aims to remedy this drawback by proposing a device for measuring a radius of curvature that is simpler to implement.

[0011] The invention therefore relates to a device for measuring a radius of curvature, this device comprising:

[0012] - an optical transducer comprising:

[0013] - a multimode optical fiber containing a core that extends along a central axis and within which an optical signal guided by this multimode optical fiber is capable of propagating according to several different spatial mode groups, and

[0014] - a set of one or more Bragg gratings, each Bragg grating of this an assembly being made in the core of the optical fiber and comprising at least three identical patterns aligned one behind the other along a longitudinal axis parallel to or coinciding with the central axis of the optical fiber,

[0015] - a spectral analyzer configured for:

[0016] - to emit, in the multimode optical fiber, an optical excitation signal whose the majority of the power is contained within a wavelength range [Xmin; Xmax], and

[0017] - measure a spectral response of the set of one or more Bragg gratings in response to the emitted optical excitation signal, then extract from the measured spectral response, for each Bragg grating in the assembly, the amplitude of a power peak corresponding to the power peak of a spectral response of that Bragg grating in a group of spatial modes queried different from the group of the fundamental spatial mode, the amplitude of this power peak varying as a function of the radius of curvature of the multimode optical fiber at the location of that Bragg grating, then

[0018] - establish a value of the radius of curvature from the measured amplitude of the peak power,

[0019] wherein, for each Bragg grating in the set, the largest transverse dimension of the motifs of that Bragg grating is less than the wavelength

[0020] Embodiments of this device may include one or more of the following features:

[0021] 1) The assembly comprises a central Bragg grating whose longitudinal axis is confused with the central axis of the optical fiber and whose patterns are centered on the central axis.

[0022] 2) The assembly comprises only the central Bragg network.

[0023] 3)

[0024] - the assembly comprises at least two eccentric Bragg gratings, each of these Eccentric Bragg gratings exhibiting the following characteristics:

[0025] - its longitudinal axis is eccentric with respect to the central axis of the optical fiber multimode, and

[0026] - the pitch of this Bragg grating is different from the pitch of other Bragg gratings of this set, the spacing of a Bragg grating being the distance separating the motifs of this Bragg grating,

[0027] - the eccentric Bragg networks of this set are contained in planes secants, the plane containing an eccentric Bragg grating being the plane containing both the central axis of the multimode optical fiber and the longitudinal axis of this eccentric Bragg grating,

[0028] - all the Bragg gratings of the set extend on both sides of the same median plane perpendicular to the central axis of the multimode optical fiber, and

[0029] - the spectral analyzer is configured to establish, in addition, an orientation value of the plane of curvature containing the osculating circle for which the value of the radius of curvature is established.

[0030] 4) The angle, expressed in degrees, between two consecutive intersecting planes around the axis central is between 0.9*360 / (2*Nps) and 1.1*360 / (2*1^), where Nps is the number of intersecting planes.

[0031] 5) The longitudinal axes of all eccentric Bragg gratings are uniformly distributed on the periphery of one or more cylinders of revolution centered on the central axis.

[0032] 6) The assembly comprises at least three eccentric Bragg gratings whose axes longitudinal are uniformly distributed on the periphery of the same cylinder of revolution.

[0033] 7) The spectral analyzer comprises:

[0034] - an optical source capable of emitting, in one end of the optical fiber multimode, an optical excitation signal that propagates in at least one group of spatial modes of the multimode optical fiber,

[0035] - an acquisition apparatus capable of measuring the spectral response of the assembly of one or more Bragg gratings in response to the emitted optical excitation signal, and

[0036] - an electronic processing unit electrically connected to the equipment acquisition, this electronic processing unit being configured:

[0037] - to extract from the measured spectral response, for each Bragg grating, the amplitude of the peak power of the spectral response of this Bragg grating in the group of spatial modes being queried, and

[0038] - to establish the value of the radius of curvature from the measured amplitude of the peak of power.

[0039] 8)

[0040] - the spectral analyzer includes a modal demultiplexer optically connected to a at the end of the multimode optical fiber, this modal demultiplexer is capable of directing only the optical signals propagating in the group of spatial modes being queried to an input port of the acquisition equipment, and

[0041] - the acquisition apparatus is capable of measuring, for each Bragg grating of the whole, the amplitude of the peak power of the spectral response of this Bragg grating in the group of spatial modes queried only from optical signals directed to its input port by the modal demultiplexer.

[0042] 9) The acquisition apparatus is optically connected to the same end of the multimode optical fiber than the one to which the optical source is connected.

[0043] The invention also relates to a method for measuring a radius of curvature using the above measuring device, this method comprising the following steps:

[0044] - the emission in the multimode optical fiber of an optical excitation signal whose the majority of the power is contained within a wavelength range [Xmin; Xmax], and

[0045] - the measurement of a spectral response of the entire set of one or more networks of Bragg in response to the emitted optical excitation signal, then

[0046] - the extraction, from the measured spectral response, for each Bragg grating of the whole, the amplitude of a power peak corresponding to the power peak of a spectral response of this Bragg grating in a group of spatial modes queried different from the group of the fundamental spatial mode, the amplitude of this power peak varying as a function of the radius of curvature of the multimode optical fiber at the location of this Bragg grating, then

[0047] - establishing a value for the radius of curvature from the measured amplitude of peak power,

[0048] wherein, during the emission step, the wavelength is greater than the largest transverse dimension of the patterns of each Bragg grating in the assembly.

[0049] 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:

[0050] - Fig. 1 is a schematic illustration of a cross-section of a optical transducer

[0051] - [Fig. 2] is a schematic illustration of a Bragg grating of the transducer the [Fig.l],

[0052] - [Fig. 3] is a schematic illustration of a first embodiment of a measuring device using the transducer of [Fig.1],

[0053] - Figures 4 to 7 are graphs illustrating different spectral responses of the transducer of the [Fig.l],

[0054] - [Fig. 8] is a flowchart of a measurement method using the device of the [Fig.3],

[0055] - Figures 9 and 10 are schematic illustrations, respectively, of a second and a third embodiment of a measuring device using the transducer of [Fig.1],

[0056] - [Fig. 1 1] is a graph illustrating spectral responses measured using the device of the [Fig. 10],

[0057] - Figures 12 and 13 are schematic cross-sectional illustrations of two other embodiments of the transducer of [Fig.l].

[0058] 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. In Chapter III, variants of these embodiments are presented. Finally, the advantages of the different embodiments are specified in Chapter IV.

[0059] Chapter I: Definitions, terminology and conventions:

[0060] In the figures, the same references are used to designate the same elements.

[0061] In the remainder of this description, the well-known characteristics and functions of a person skilled in the art are not described in detail.

[0062] The symbol “*” denotes the scalar multiplication operation.

[0063] The optical domain refers to the range containing the wavelengths commonly used in optics. More precisely, 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.

[0064] A spatial mode group of a multimode optical fiber is a spatial mode group that contains all the spatial modes that have propagation constants identical or nearly identical. Thus, a mode in this group can couple very strongly with 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." A spatial mode group contains all the 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 central 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 (graded index fiber).

[0065] Hereafter, the expression "mode group" refers to a group of spatial modes.

[0066] The word "mode" designates 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.

[0067] 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".

[0068] 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.

[0069] A polled mode group is a group of modes in which the spectral response of a Bragg grating is measured. A mode group is polled from the moment at least one mode of that group is polled, that is, from the moment the spectral response of that group is measured from an optical signal propagating in at least one mode of that group.

[0070] 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 its core. Optical fiber and the materials forming its core and cladding. It can be determined experimentally or by numerical simulation.

[0071] 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:

[0072] - only from the optical signal propagating in only one of the modes of this group of modes, or

[0073] - from optical signals that propagate in several of the modes of this group of modes.

[0074] 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.

[0075] 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.

[0076] Typically, the spectral response of a particular group of a Bragg grating has, within a predetermined working range, a single peak whose power can be at its maximum. This peak of maximum power is subsequently called the "main peak".

[0077] 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.

[0078] A Bragg grating is sensitive to a physical quantity when a variation in that physical quantity results in a corresponding change in a spectral response of that Bragg grating. This change may be a change in the amplitude of a power peak in that spectral response and / or a change in the position of that power peak in the spectral response.

[0079] A sensitivity coefficient Sij of a Bragg grating to a physical quantity is a constant defined by the following relation AAij = Sij*AG, where:

[0080] - AAij is the variation in the amplitude of the power peak in a spectral response of a group j of the Bragg lattice i obtained in response to a variation AG of the physical quantity to be measured,

[0081] - i is an index that identifies the Bragg grating used to measure the variation AA^ , And

[0082] - j is an index that identifies the group of modes for which the spectral response of the group was measured.

[0083] The variation AA^ is equal to the difference Ajj - A^f, where:

[0084] - Aij is the amplitude of the power peak measured in the spectral response of the group j of fashions, and

[0085] - Aij ref is a predetermined and constant reference amplitude.

[0086] Unlike the amplitude A^f, the amplitude A^ varies according to the physical quantity to be measured.

[0087] The term "plane of curvature" refers to the plane that contains the osculating circle whose radius of curvature is measured. This plane therefore contains the portion of the central axis of the optical fiber that is curved and whose radius of curvature must be measured.

[0088] The largest transverse dimension of a motif in a Bragg grating is equal to the length of the longest side of the smallest rectangle that entirely contains the orthogonal projection of the motif onto a plane passing through the motif and perpendicular to the longitudinal axis of the Bragg grating. Thus, in the case where the motif is a substantially ellipsoidal bubble, the largest transverse dimension of this motif is close to or equal to the diameter of the smallest sphere that entirely contains this substantially spherical bubble.

[0089] Chapter II: Examples of embodiments

[0090] Figure 1 shows a cross-section of an optical transducer 4 in a median plane Pm. The transducer 4 is capable of transforming a variation in its radius of curvature to be measured into variations in the amplitudes of power peaks in spectral responses measured over a working range. This working range has a width greater than 5 nm or 10 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.

[0091] The transducer 4 comprises for this purpose a multimode optical fiber 14 and a set 16 of Bragg gratings.

[0092] The fiber 14 contains a core 18 extending along a central axis 20. Optical signals guided by this fiber 14 propagate inside the core 18 along the axis 20. The core 18 is centered on the axis 20. Here, the cross-section of the core 18 visible in [Fig. 1] is a disk of diameter D[8]. The diameter D[8 is generally greater at 25 pm or 50 pm and less than 200 pm or 100 pm. This core 18 is surrounded by a cladding 22 of a different refractive index to guide the optical signals along the axis 20. The outer diameter D22 of the cladding 22 is often greater than 100 pm. For example, the diameter D22 is equal to 125 pm. The cladding 22 may itself be surrounded by one or more protective envelopes for mechanical, thermal, and chemical protection.

[0093] The fiber 14 is shaped to allow the propagation of optical signals along the axis 20 according to the fundamental mode group and according to at least one other different mode group. Typically, the number of different mode groups may be greater than three, five, or ten.

[0094] In this embodiment, assembly 16 comprises a central Bragg grating Bi and three eccentric Bragg gratings B2 to B4. Each of these Bragg gratings is made in the core 18 and comprises at least three identical motifs aligned one behind the other along a longitudinal axis parallel to the axis 20. Here, the Bragg gratings of assembly 16 are structurally identical to each other except that their pitch A; are different. Thus, only the structure of grating B; is shown in more detail in [Fig. 2], where the index i is an index identifying grating B; among the different Bragg gratings of assembly 16. Here, the index i is an integer between 1 and 4.

[0095] The lattice B; is composed of a succession of motifs Mp arranged one behind the other along a longitudinal axis A;. The axis A; is parallel to or coincides with the axis 20. Each motif Mp is centered on the axis A;. The index p is the sequence number of the motif along the axis A;. The index p of the first leftmost motif in the lattice B; is equal to 1 and the index p of the last rightmost motif in the lattice B; is equal to n. n is equal to the number of motifs Mp in the lattice B;. In [Fig. 2], only the first two motifs, one motif Mp, and the last two motifs of the lattice B; are shown. The presence of intermediate motifs located between motifs M2 and Mp and between motifs Mp and Mn is represented by black dots on the axis A;.

[0096] The number n of motifs is greater than or equal to three and, preferably, greater than or equal to ten. Here, the number n is also chosen to be sufficiently small so that the length of the grid B; remains small, that is, less than 1 meter and, preferably, less than 10 cm or 1 cm. The length of the grid B; is equal to the distance between the motifs Mi and Mn measured along the axis A;. Typically, the number n is such that the number of motifs per millimeter remains less than 500 or 1000.

[0097] Here, the step A; between two immediately consecutive patterns Mp and Mp+i along the axis A; is constant regardless of the index p. The step A; is therefore equal to the distance, along the axis Ai, which separates two immediately consecutive patterns Mp and Mp+i.

[0098] Here, the step A; is chosen so that the wavelength Xij of the array B;, in the group j of queried modes, is included in the working range, where the index j is a number The integer that identifies the group of modes being queried. The step A; is also chosen so that the wavelength Xij of the grating B; is sufficiently different from the wavelengths Xkjj, in the same group of modes being queried j, of the other Bragg gratings in set 16, where the index k is different from the index i. Thus, the spectral responses of the different gratings B; can be isolated from each other.

[0099] In the case of a multimode optical fiber, the array B; has a wavelength Xjj for each group of modes j queried in the fiber 14. The wavelength Xjj of the array B; in the group j of modes queried is given by the following relation (1): Xij = 2*neij*Ai / mi, where:

[0100] - Xij is the wavelength of the grating B; for the group j of modes queried,

[0101] - neij is the effective index of the group j of modes, and

[0102] - A; is the step of network B;

[0103] - m; is the order of the Bragg lattice.

[0104] In the embodiments described here, the order m; is less than ten and, for example, equal to one.

[0105] Since the effective index neij varies according to the mode group, there are as many wavelengths Xîj as there are possible mode groups in the fiber 14 for a given interrogation setup.

[0106] The motifs Mp of the lattice B are all identical to each other. Here, the motif Mp consists of a single, predominantly spherical bubble. Thus, the diameter of this bubble is equal to the diameter of the smallest sphere that entirely contains this bubble.

[0107] Each bubble creates a significant variation in the refractive index of the core 18 in the direction of propagation of the optical signal. For this reason, the difference between the refractive index nr8 of the core 18 and the refractive index nrB of the bubble 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 nr8 and nrB greater than or equal to 0.4.

[0108] Here, each bubble is produced using a pulse from a femtosecond laser. In this embodiment, the characteristics of the femtosecond laser pulse are adjusted so that the diameter of each bubble is less than 1000 nm, 500 nm, or 100 nm. Generally, the diameter of each bubble is also greater than 10 nm or 50 nm. Thus, in this embodiment, the largest transverse dimension of each Mp pattern is less than 1000 nm, 500 nm, or 100 nm.

[0109] The B networks are located at the same point along fiber 14. For this purpose, for example, all the B networks extend on either side of the same median plane Pm perpendicular to axis 20. Preferably, the B networks are centered on this plane median Pm, that is to say that the plane Pm divides each network B into two portions of the same length.

[0110] Network Bi is said to be "central" because its axis Ai coincides with axis 20. Conversely, networks B2 to B4 are said to be "eccentric" because their axes A2 to A4 are eccentric with respect to axis 20.

[0111] The eccentric Bragg gratings of set 16 are contained in Nps intersecting planes, where Nps is an integer greater than or equal to two. All the intersecting planes intersect each other along axis 20. In this embodiment, there are three intersecting planes P2, P3, and P4 which contain, respectively, the gratings B2, B3, and B4.

[0112] Preferably, the angle, expressed in degrees, between two consecutive intersecting planes about axis 20 is between 0.9*360 / (2*Nps)° and 1.1*360 / (2*Nps)° or between 0.95*360 / (2*Nps)° and 1.05*360 / (2*Nps)° and advantageously equal to 360 / (2*Nps)°. Thus, in this example, the angle between two consecutive intersecting planes is equal to 60°.

[0113] Here, the axes A2 to A4 of the lattices B2 to B4 are uniformly distributed around the periphery of a cylinder 26 of revolution centered on the axis 20. Thus, the axes A2 to A4 are located at the same distance from the axis 20. The distance between each of the axes A2 to A4 and the axis 20 is sufficiently large so that the motifs Mp of the lattices B; of the set 16 are disjoint. To this end, the distance between each of the axes A2 to A4 and the axis 20 is greater than the diameter of the bubbles that form each of the motifs Mp.

[0114] Figure 3 represents a device 40 for measuring a radius of curvature and The orientation of the plane of curvature with respect to a reference plane Pref. Subsequently, the plane Pref is the plane coinciding with the plane P2 when the axis 20 is straight and therefore in the absence of curvature of the fiber 14. The orientation of the plane of curvature therefore corresponds to the angle between the plane of curvature and the plane Pref.

[0115] To this end, the device 2 comprises the transducer 4 and a spectral analyzer 42. The spectral analyzer 42 is capable of measuring spectral responses of the transducer 4 and then determining the radius of curvature and the orientation of the plane of curvature of the fiber 14 at the location of the assembly 16 from these measured spectral responses. For this purpose, the spectral analyzer 42 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.

[0116] Subsequently, the analyzer 42 is described in the particular case where only group j of modes of fiber 14 is interrogated. Group j is different from the fundamental mode group. Group j is chosen from among the groups of modes of fiber 14 in which the spectral response of the arrays B; exhibits a power peak whose amplitude varies according to the radius of curvature to be measured. Among several peaks in the spectral response of a group of an array B;, the peak whose amplitude varies the The peak in response to a change in the radius of curvature is called the "main peak." It is noted that the spectral response of a group of B-networks exhibits a power peak whose amplitude varies with the radius of curvature only in certain groups of fiber modes, referred to as "even-numbered" groups. Conversely, groups of fiber modes in which the spectral response of the B-networks does not include a power peak whose amplitude varies with the radius of curvature are called "odd-numbered" groups. The fundamental mode group is one of these odd-numbered mode groups.

[0117] This is illustrated in the graphs of Figures 4 to 7. The graphs in Figures 4 to 7 represent the spectral responses of grating Bi measured from the optical signals reflected by this grating Bi in the mode groups, respectively, 2, 4, 1 and 3. Group 1 corresponds to the fundamental mode group. Groups 2 and 4 are the first two even groups. Group 3 is the second odd group. In these graphs, the x-axis corresponds to the wavelength and the y-axis represents the power of the optical signal reflected by grating Bb. In each of these graphs, the solid line represents the spectral response of the group measured in the absence of curvature of fiber 14 and the dashed line represents the same spectral response but measured in the presence of a curvature radius of 1 cm.As can be seen in the graphs of Figures 4 and 5, the amplitude of the main peak in the spectral responses of groups 2 and 4 varies significantly in response to a bend in fiber 14. Conversely, as can be seen in the graphs of Figures 6 and 7, the amplitude of the main peak in the spectral responses measured in groups 1 and 3 does not vary in response to this same bend in fiber 14.

[0118] This phenomenon is explained by the fact that the transverse distribution, i.e., in a cross-section of the core 18, of the power of the optical signals propagating in a mode group is symmetrical with respect to a center of symmetry. In the case of even-numbered mode groups, the power of the optical signals is zero at this center of symmetry. When the axis 20 is straight, the center of symmetry of these transverse power distributions is located on the axis 20. Thus, in the absence of curvature of the fiber 14, the optical signals propagating in the even-numbered mode groups do not interact with the grating Bi, whose axis Ai coincides with the axis 20. This is why, in the graphs of Figures 4 and 5, in the absence of curvature of the fiber 14, the amplitude of the main peak is minimal.However, when fiber 14 is bent, the center of symmetry of the transverse power distributions of the even-mode groups shifts with respect to axis 20. The power of the optical signals propagating in these even-mode groups is then no longer zero at axis 20. They therefore interact with the Bb network, which explains why the amplitude of the main peak in the graphs of figures 4 and 5 increases when fiber 14 is bent.

[0119] Conversely, arrays B2 to B4 are located in areas of the transverse power distribution of the even-mode groups where the optical signal power is significant when axis 20 is straight. When fiber 14 is bent, the center of symmetry of the transverse power distributions of the even-mode groups moves closer to one of arrays B2 to B4. In response, the amplitude of the main peak in the spectral response of the even-mode group of that array decreases. Thus, it has been observed that, in the case of arrays B2 to B4, the behavior is the inverse of that observed for array Bb; that is, a bend in fiber 14 causes a decrease in the amplitude of the main peak of at least one of arrays B2 to B4.

[0120] Here, the group of modes being questioned is the even group of modes for which, in response to a given variation in the radius of curvature, the variation in the amplitude of the main peak is the greatest.

[0121] The group of modes being examined contains several modes. To simplify the figures and explanations, the following description assumes that only two modes of the group of modes being examined are used. However, what is described in this simplified case also applies to the case where all or virtually all the modes of the group of modes being examined are used to perform measurements of the radius of curvature and the orientation of the plane of curvature.

[0122] In this embodiment, the excited mode group is the fundamental mode group.

[0123] For this purpose, the analyzer 42 comprises:

[0124] - an optical source 50,

[0125] - a modal demultiplexer 52 optically connected, via a optical connector, at the proximal end of fiber 14,

[0126] - an acquisition apparatus 54 optically connected to the modal demultiplexer 52,

[0127] - 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

[0128] - a human-machine interface 58, connected to the unit 56, for communicating the result of measurements taken on a human being.

[0129] 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 It also allows the optical signals propagating simultaneously in several modes of fiber 14 to be directed, each to the respective input / output port associated with that mode. Thus, the modal demultiplexer 52 also allows the demultiplexing, that is, the separation, of the different optical signals propagating simultaneously in different modes of fiber 14.

[0130] To simplify [Fig. 3], 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 same even group of queried modes.

[0131] The optical source 50 emits an optical signal that is used to excite the fundamental mode of the fiber 14. For this purpose, the optical source 50 is optically connected directly to port 61 of the demultiplexer 52. Here, the optical source 50 is a broadband 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. The majority of the power of the optical excitation signal, and typically more than 90% or 95% of the power of the optical excitation signal, lies, within the power spectrum of this optical excitation signal, in a continuous range of wavelengths [Xmin; Xmax]. The wavelength is greater than the largest lateral dimension of the patterns Mp of the arrays Bi to B4.In this embodiment, the wavelength is two or three times greater than the largest lateral dimension of the Mp patterns of the Bi to B4 gratings. Here, the wavelength Xmin is greater than 1550 nm.

[0132] In this regard, it is pointed out that tests carried out with Bragg gratings whose largest transverse dimension was greater than the wavelength showed that the effect illustrated in Figures 4 and 5 does not occur under these conditions.

[0133] The acquisition apparatus 54 measures the spectral responses in the group of modes being examined. In this first embodiment, the apparatus 54 measures, one after the other, the spectral responses in each mode of the group of modes being examined. For this purpose, the apparatus 54 comprises an optical switch 80 and a single-channel spectrometer 82.

[0134] The switch 80 has two input ports 86a and 86b and one output port 88. Ports 86a and 86b are permanently optically connected to ports 62a and 62b, respectively, of the demultiplexer 52. The switch 80 also has a control port 90 electrically connected to the unit 56. Depending on the command received on this port 90, the switch 80 toggles between the following states:

[0135] - a first state where it optically connects only port 86a to port 88, and

[0136] - a second state where it optically connects only port 86b to port 88.

[0137] The spectrometer 82 has a single measurement port 92 optically permanently connected to port 88 of the switch 80. The spectrometer 82 also has an output port 94 electrically connected to the unit 56. The spectrometer 82 measures the spectral response of the optical signal received at its measurement port 92 and outputs the measured spectral response to 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 the switch 80, the spectrometer 82 outputs to port 94 either the measured spectral response of the optical signal received at port 62a or the spectral response of the optical signal received at port 62b of the demultiplexer 52.

[0138] Unit 56 is specifically configured to implement the process of [Fig. 8]. For this purpose, in particular, Unit 56 is programmed to:

[0139] - determine, for each of the networks B;, the variation AALJ of the peak amplitude main in the spectral response of this network B; in the j group of modes queried, then

[0140] - establish the value of the radius of curvature and the value of the orientation of the plane of curvature from the determined amplitudes AAjj.

[0141] 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. 8]. In particular, the memory includes all the sensitivity coefficients Sk, i, j that relate a variation of the k-th physical quantity to a variation of the amplitude AAjj, where:

[0142] - k is an index that identifies the physical quantity,

[0143] - i is an index that identifies the Bragg grating used to measure the variation AAjj , And

[0144] - j is an index that identifies the group of modes being queried.

[0145] In this example, k is equal to 1 to identify the radius of curvature and equal to 2 to identify the orientation of the plane of curvature.

[0146] In the case of the Bb grating, the variation AAij is independent or practically independent of the orientation of the plane of curvature because the longitudinal axis of the Bi grating coincides with the central axis 20 of the fiber 14. Consequently, the variation AAij is related to the value AR of the variation in the radius of curvature by the following predetermined relation: AAij = fi(AR), where fi is a predetermined function. For example, the function fi is a linear function that can be written in the form AAij = Si.ij * AR, where Sijj is the sensitivity coefficient of the Bi grating to the variation in the radius of curvature.

[0147] Conversely, in the case of the B2 to B4 lattices, the variation AA^j depends on both the variation of the radius of curvature and the orientation of the plane of curvature. Thus, in the case of the B2 to B4 lattices, the variation AA^j is related to the variation of the radius of curvature and the orientation of the plane of curvature by the following predetermined relation: AA^j = f; (AR, AO), where:

[0148] - fi is a predetermined function associated with the network B;,

[0149] - AR is the variation of the radius of curvature, and

[0150] - AO is the orientation of the plane of curvature with respect to the Pref plane.

[0151] By way of example, the function f; can be chosen to be linear. In this case, the relation AAij = fi(AR, AO) can be written in the form AAij = Si,ij*AR + S2jij*AO.

[0152] The relation AAij = fi(AR) and the three relations AA^ = f;(AR, AO) form a system (1) of equations that can be solved to establish the value AR of the variation of the radius of curvature and the value AO of the orientation of the plane of curvature from the determined amplitudes AA^. 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 algorithm that minimizes the error, such as the least squares method.

[0153] Typically, the Skjij coefficients are determined experimentally by implementing the method of [Fig.8] for known variations of the radius of curvature and for different known orientations of the plane of curvature.

[0154] Memory 102 also contains, for each network B;, the reference amplitude Aijref of the main power peak of its spectral response in the group of modes polled. Typically, the amplitudes A^f are measured when the radius of curvature of fiber 14 is zero.

[0155] Fig. 8 represents a method for measuring the radius of curvature and the orientation of the plane of curvature using device 2.

[0156] During a calibration phase 110, the coefficients Sk,ij and the amplitudes Aijref are determined experimentally and then recorded in memory 102.

[0157] Next, it is possible to proceed to a phase 112 of measuring the radius of curvature and the orientation of the plane of curvature. 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 each grating B;, a spectral response of the grating B; can be measured in other groups of modes of the fiber 14 than the one excited.

[0158] Here, during a step 120, the optical source 50 excites a group of modes different from the group or groups of modes being queried. Here, the excited group of modes is the fundamental group. For this purpose, the optical source 50 emits an optical excitation signal into the fiber 14. For example, this optical excitation signal is emitted continuously. This optical excitation signal is transmitted to port 61 of the demultiplexer 52. The demultiplexer 52 transmits this optical excitation signal only in the fundamental mode of fiber 14. The optical excitation signal then propagates to the B gratings, and the B gratings reflect 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 62a and 62b deliver the optical signals reflected by the B gratings, respectively, in two distinct modes of mode group j. It is the power spectrum of the optical signal reflected in mode group j that forms the spectral response of the B gratings in this mode group j.

[0159] The optical signals delivered on ports 62a and 62b are transmitted continuously on ports 86a and 86b of switch 80, respectively.

[0160] Then, in parallel with step 120, during a step 122, the switch 80 is commanded by the unit 56 to measure the spectral response of the B networks in a particular mode of the group j of modes. For example, during the first iteration of step 122, the unit 56 commands the switch 80 to place it in its first state where it optically connects port 86a to port 88.

[0161] Then, in step 122, the spectrometer 82 measures the spectral response of the optical signal emitted on the port 88 of the switch 80. Thus, in the first iteration of step 122, the apparatus 54 measures the spectral response of the networks B; in a first mode of group j and then transmits this first measured spectral response to the unit 56 which acquires it.

[0162] Step 122 is repeated several times. During the second iteration of step 122, unit 56 commands switch 80 to place it in the second state. 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 a first spectral response in a first mode and a second spectral response in a second mode from group j of modes.

[0163] Once unit 56 has acquired each of the spectral responses to be measured, in step 130, unit 56 determines the AAjj variations. To do this, in this embodiment, unit 56 processes each of the acquired spectral responses to extract an amplitude Aij>m of the main peak for each of the grids B, where the index m is an identifier of the mode of group j in which the processed spectral response was measured. Subsequently, m is equal to 1 to denote the first mode of group j and m is equal to 2 to denote the second mode of group j. In a measured spectral response, unit 56 distinguishes the main peak of a grid B from the main peaks of the other grids in set 16 by using the fact that the grids in set 16 have different wavelengths, sufficiently far apart to allow separation of the main peaks of the different B-networks present in the same measured spectral response. Thus, in this embodiment, during step 130, unit 56 extracts four amplitudes and four amplitudes Aij>2 from the first and second measured spectral responses.

[0164] The amplitudes Aij>m for all modes in group j are theoretically identical. Thus, in this embodiment, to improve measurement accuracy, the amplitudes Aij>m extracted from the spectral responses measured for the same lattice B; are averaged. Consequently, in this example, the amplitude ALJ of the main peak of the spectral response of lattice B; in group j of modes is taken to be equal to the arithmetic mean of the amplitudes A^^ and Aij>m.

[0165] Then, still during step 130, for each network B;, unit 56 calculates the difference between the amplitudes Ajj and A^f to obtain the variation AAjj.

[0166] Once the variations AA^ have been determined, in step 132, unit 56 establishes the AR and AO values ​​of the variations of the radius of curvature and the orientation of the plane of curvature from the determined variations AA^. To do this, in step 132, unit 56 solves the system (1) of equations.

[0167] In a step 134, the unit 56 commands the interface 58 to communicate the established values ​​to a human being. The measured value of the radius of curvature can be communicated in the form of the relative value AR or in the form of an absolute value Rref + AR, where Rref is the value of the radius of curvature of the fiber 14 when the amplitudes Aij are equal, respectively, to the amplitudes Aij ref.

[0168] Fig. 9 represents a measuring device 160 identical to the device 40 except that the spectral analyzer 42 is replaced by a spectral analyzer 162. The spectral analyzer 162 is identical to the spectral analyzer 42 except that the acquisition apparatus 54 is replaced by an acquisition apparatus 164. Device 164 is identical to device 54 except that switch 80 is replaced by an optical coupler 166. The optical coupler 166 has two inputs optically connected to ports 62a and 62b of the demultiplexer 52, respectively. The optical coupler 166 also has an output optically connected to port 92 of the spectrometer 82. The coupler 166 allows ports 62a and 62b to be connected simultaneously to port 92 of the spectrometer 82. Thus, the optical coupler 166 delivers an optical signal on port 92 corresponding to the combination of the optical signals delivered on 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.

[0169] The operation of device 160 is identical to the operation of device 40 except that:

[0170] - during step 122 no switch, such as switch 80, is ordered, and

[0171] - during step 130, for each network B;, the amplitude A^ is taken equal to the amplitude of the main peak of grating B; in the spectral response of the combined optical signal received on port 92 of spectrometer 82. Indeed, in this embodiment, only one spectral response is measured for the group of modes. It is therefore not necessary to repeat step 122 for several modes of the group.

[0172] Fig. 10 represents a measuring device 170 identical to the device 40 except that the transducer 4 is replaced by a transducer 4a and the spectral analyzer 42 is replaced by a spectral analyzer 172.

[0173] Transducer 4a is, for example, identical to transducer 4 except that the step A; of each grating B; is chosen so that all wavelengths Xij at which a detectable power peak occurs are within a wavelength range distinct from that of the other gratings in the set 16. Thus, the power spectra, such as that shown in [Fig. 11], of the different gratings B; do not overlap.

[0174] Spectral analyzer 172 is identical to spectral analyzer 42 except that:

[0175] - the demultiplexer 52 is replaced by an optical circulator 174, and

[0176] - the acquisition apparatus 54 is replaced by an acquisition apparatus 176.

[0177] The optical circulator 174 comprises:

[0178] - a port 175 optically connected to the optical source 50 to receive the signal excitation optics,

[0179] - a port 176 optically connected directly to the proximal end of the fiber 14 of transducer 4, and

[0180] - a port 177 optically connected to the acquisition apparatus 176.

[0181] Circulator 174 therefore allows:

[0182] - to transmit, only on port 176, the optical excitation signal emitted by the source 50, and

[0183] - to transmit, only on port 177, the optical signal reflected by the networks B; in all mode groups of fiber 14.

[0184] Device 176 is identical to device 54 except that switch 80 is omitted. Thus, port 177 of circulator 174 is optically connected directly to the measuring port 92 of spectrometer 82.

[0185] The operation of device 170 is identical to the operation of device 40 except that:

[0186] - during step 122, the command for switch 80 is omitted, and

[0187] - during step 130 the treatments carried out to determine the AA^j variations are different because, for each network B;, the processed spectral response includes all the principal peaks of all the mode groups.

[0188] The graph in [Fig. 11] represents two spectral responses of the Bi grating measured by the spectrometer 82 of the device 170. The solid line represents the spectral response of the Bi grating in the absence of curvature of the fiber 14. The dashed line represents the spectral response of the Bi grating when the fiber 14 is curved. On this graph, the x-axis corresponds to the wavelength and the y-axis represents the power of the optical signal reflected by the Bb grating. As can be seen on this graph, the amplitude of the principal peaks located at the wavelengths Xij of the even-numbered mode groups varies significantly in response to a curvature of the fiber 14. These peaks correspond to the power peaks of the spectral responses of this Bragg grating in the interrogated even-numbered groups.Conversely, the amplitude of the main peaks located at the wavelength locations Xij of the odd mode groups does not vary in response to this same curvature of fiber 14. These peaks correspond to the power peaks of the spectral responses of this Bragg grating in the interrogated odd groups.

[0189] Therefore, in step 130, for each network B;, a variation AA^ is determined for at least one even group of modes from the spectral response measured by the spectrometer 82. Then, steps 132 and 134 are identical.

[0190] Fig. 12 represents a cross-section of a transducer 190 that can be used in place of the transducer 4. The transducer 190 is identical to the transducer 4 except that the assembly 16 includes only the central array Bh. When the transducer 190 is used in place of the transducer 4, only the AR value of the radius of curvature is established during step 132 because the array Bi is not or practically not sensitive to the orientation of the plane of curvature.

[0191] Figure 13 shows a cross-section of a transducer 200 that can be used in place of the transducer 4. The transducer 200 is identical to the transducer 4 except that the assembly 16 comprises eight eccentric arrays B2 to B9. These eccentric arrays are constructed and positioned by applying the principles established for the specific case of the transducer 4. In particular, in this embodiment, the axes A2 to A9 of the arrays B2 to B9 are uniformly distributed around the periphery of the cylinder 26 of revolution centered on the axis 20. In this embodiment, the number Nps is equal to four.

[0192] When transducer 200 is used instead of transducer 4, the operation of the measuring device follows from the teaching given in the particular case of the transducer 4. In particular, the number of AA^j variations determined is greater since there are more eccentric networks in transducer 200 than in transducer 4.

[0193] Chapter III: Variants:

[0194] Variants of the Bragg lattice set:

[0195] The motif Mp is not necessarily composed of a single bubble. For example, alternatively, each motif Mp is formed of several bubbles arranged side by side in a plane perpendicular to axis A. When the motif Mp is formed of several bubbles, these bubbles may partially overlap, for example, to form channels. It is also possible for these bubbles to be disjoint and not overlap. In another embodiment, the bubble that forms the motif Mp is not spherical but rather flattened to extend mainly in a plane, for example, perpendicular to axis A. In fact, there is no particular constraint on the shape of a motif Mp except that its largest dimension must be less than

[0196] Alternatively, the B networks; of set 16 may differ from each other by additional characteristics other than their pitch A;. For example, the B networks do not all have the same number n of patterns Mp and / or the patterns Mp are not identical in each of the B networks;.

[0197] In another embodiment, the central Bragg grating is omitted, and the assembly 16 comprises only one or more eccentric Bragg gratings. If the assembly comprises only one eccentric Bragg grating, then the measuring device can establish, in the same way as described for the central Bragg grating, the value of the radius of curvature from this single eccentric Bragg grating. However, it is not possible to establish a value for the orientation of the plane of curvature. Moreover, in this highly simplified embodiment, the sensitivity of the transducer depends on the orientation of the plane of curvature. Thus, this highly simplified embodiment is primarily usable in contexts where the orientation of the plane of curvature is constant.

[0198] In most practical cases, when the central Bragg grating is omitted, the assembly 16 comprises at least two eccentric Bragg gratings and, preferably, at least three eccentric Bragg gratings, contained in separate planes. In this case, it is always possible to simultaneously establish a value for the radius of curvature and a value for the orientation of the plane of curvature. However, in a simplified version, only the value of the orientation of the plane of curvature or only the value of the radius of curvature is established.

[0199] Set 16 may also contain a greater number of eccentric Bragg gratings than in the examples previously described.

[0200] In variants, the eccentric networks of set 16 are contained in only two intersecting planes or, on the contrary, in more than three or four intersecting planes.

[0201] The longitudinal axes of the eccentric Bragg gratings can be distributed over the periphery of several concentric cylinders of revolution. In this case, the gap between the radii of two consecutive cylinders of revolution is greater than the largest transverse dimension of the Mp motifs.

[0202] In variants the longitudinal axes of the eccentric Bragg gratings arranged on the same cylinder of revolution are not uniformly distributed around the periphery of this cylinder of revolution.

[0203] Other variants of the transducer:

[0204] 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 to implement 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 variation of the refractive index in a transverse direction can be by index step or, conversely, by a continuous variation following a linear or parabolic function.

[0205] 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 six or three different mode groups.

[0206] Alternatively, the fiber 14 comprises several sets 16 of Bragg gratings arranged one after the other in its core. In this case, the wavelengths Xij of the Bragg gratings in each of these sets are different. Under these conditions, the same optical transducer makes it possible to measure the radius of curvature and / or the orientation of the plane of curvature at the location of each of these sets of Bragg gratings by implementing for each of these sets the same measurement procedure as that described with reference to [Fig. 8].

[0207] Variants of the spectral analyzer:

[0208] 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 means of Using a suitable control signal, the Xs wavelength 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 B gratings in the mode group(s) being examined. In this case, the spectrometer 82 can be simplified because it does not need to be able to measure the power of the received optical signal simultaneously for different wavelengths.

[0209] 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.

[0210] 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 must be able to be scanned by the optical source of the spectral analyzer.

[0211] 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.

[0212] The interferometer 82 can be replaced by a multi-channel interferometer which is capable of simultaneously measuring the spectral responses of the optical signals delivered on ports 62a and 62b. In this case, it is not necessary to repeat step 122 for each mode interrogated.

[0213] Variants of the measurement method:

[0214] The excitation signal can also be emitted in an odd group of modes other than the fundamental group

[0215] When the excited odd group comprises several modes, the optical excitation signal can be transmitted to one or more modes of this excited odd group. It is also possible to simultaneously excite only several different mode groups without exciting all the mode groups. To simultaneously excite several modes, an optical coupler is used to simultaneously transmit the optical excitation signal to the different input / output ports of the demultiplexer 52 associated with these modes.

[0216] In a simplified variant, even for groups of modes that are surveyed, only one spectral response in a single particular mode of that group of modes is measured. In this case, during determination step 130, the variation AA^j is equal to the variation AAij>m determined from the spectral response measured for that single particular mode m.

[0217] Alternatively, several groups of even modes are queried. In this case, in step 130, AALJ variations are determined for different values ​​of the index j. Then, in step 132, the AR and AO values ​​of the variations of the radius of curvature and the orientation of the plane of curvature are established from these AALJ variations determined in several distinct groups of modes.

[0218] The determination of the amplitude Aij in the group j comprising several modes can be carried out differently. For example, alternatively, instead of averaging the different amplitudes Aij determined, it is possible to first average the different spectral responses measured in several modes of the group j in order to obtain an average spectral response for the group j. Then, the amplitude Aij is obtained from the average spectral response thus obtained.

[0219] To increase the accuracy of the measurements, it is possible to repeat the measurement phase 112 several times and then average several AR and AO values ​​to obtain the final values, respectively, of the variation of the radius of curvature and the orientation of the plane of curvature.

[0220] In a simplified variant, unit 56 determines only the variation of the radius of curvature and not its absolute value. In this case, it is not necessary to know the reference value Rref of the radius of curvature.

[0221] Several of the variants described above can be combined in the same embodiment.

[0222] Chapter IV: Advantages of the embodiments described:

[0223] Using a Bragg grating with a transverse dimension of motifs smaller than allows the amplitude of the main peak in the interrogated mode group to be measured from both a reflected and transmitted power spectrum. Furthermore, because of this characteristic, the measurement device works with all types of multimode optical fibers and not just FMF fibers. Therefore, the measurement device described herein is simpler to implement than the one described in the Zhao2018 article.

[0224] The fact that the Bi network is centered on the central axis 20 of the optical fiber allows this Bi network to have the same sensitivity to variations in the radius of curvature in all possible planes of curvature and therefore including between orthogonal planes of curvature.

[0225] The use of a set of eccentric Bragg gratings contained in distinct secant planes also allows the orientation of the plane of curvature to be measured.

[0226] The fact that the angle between two successive intersecting planes is between 0.9*360 / (2*N)° and 1.1*360 / (2*N)°, allows the accuracy of the measuring device to be improved.

[0227] The fact that the longitudinal axes A; of all the eccentric networks B; are uniformly distributed on the periphery of one or more cylinders of revolution makes it possible to obtain a sensitivity of the transducer to the orientation of the plane of curvature uniform in all transverse directions.

[0228] Using at least three eccentric Bragg gratings whose axes A; are uniformly distributed on the periphery of the same cylinder of revolution makes it possible to improve the accuracy of the measurement of the orientation of the plane of curvature.

[0229] Connecting the acquisition equipment and the optical source to the same end of the optical fiber simplifies the implementation of the measurement device.

Claims

Demands

1. A device for measuring a radius of curvature, this device comprising: - an optical transducer (4; 190; 200) comprising: - a multimode optical fiber (14) containing a core (18) extending along a central axis (20) and within which an optical signal guided by this multimode optical fiber is capable of propagating according to several different spatial mode groups, and - an assembly (16) of one or more Bragg gratings (B;), each Bragg grating of this assembly being made in the core of the optical fiber and comprising at least three identical patterns (Mp) aligned one behind the other along a longitudinal axis parallel to or coinciding with the central axis of the optical fiber, - a spectral analyzer (42; 162; 172) configured to: - emit, in the multimode optical fiber, an optical excitation signal the majority of whose power is within a wavelength range [Xmin;Xmax], and - measure a spectral response of the set (16) of one or more Bragg gratings in response to the emitted optical excitation signal, then extract from the measured spectral response, for each Bragg grating in the set (16), the amplitude of a power peak corresponding to the power peak of a spectral response of that Bragg grating in a group of spatial modes queried different from the group of the fundamental spatial mode, the amplitude of that power peak varying as a function of the radius of curvature of the multimode optical fiber at the location of that Bragg grating, then - establish a value of the radius of curvature from the measured amplitude of the power peak, characterized in that, for each Bragg grating in the set (16), the largest transverse dimension of the patterns of that Bragg grating is less than the wavelength;

2. Device according to claim 1, wherein the assembly comprises a central Bragg grating (Bi) whose longitudinal axis coincides with the central axis of the optical fiber and whose patterns are centered on the central axis.

3. Device according to claim 2, wherein the assembly (16) comprises only the central Bragg grating.

4. A device according to any one of the preceding claims, wherein: - the assembly comprises at least two eccentric Bragg gratings (B2 to B4; B2 to B9), each of these eccentric Bragg gratings having the following characteristics: - its longitudinal axis is eccentric with respect to the central axis of the multimode optical fiber, and - the pitch of this Bragg grating is different from the pitch of the other Bragg gratings of this assembly, the pitch of a Bragg grating being the distance that separates the motifs of this Bragg grating, - the eccentric Bragg gratings of this assembly are contained in intersecting planes (P2 to P4;P2 to P5), the plane which contains an eccentric Bragg grating being the plane which contains both the central axis (20) of the multimode optical fiber and the longitudinal axis (Ai) of this eccentric Bragg grating, - all the Bragg gratings of the set extend on either side of the same median plane (Pm) perpendicular to the central axis of the multimode optical fiber, and - the spectral analyzer (42; 162; 172) is configured to establish, in addition, a value of the orientation of the plane of curvature containing the osculating circle for which the value of the radius of curvature is established.;

5. Measuring device according to claim 4, wherein the angle, expressed in degrees, between two consecutive intersecting planes around the central axis (20) is between 0.9*360 / (2*Nps) and 1.1*360 / (2*Nps), where Nps is the number of intersecting planes.

6. Measuring device according to claim 5, wherein the longitudinal axes of all the eccentric Bragg gratings are uniformly distributed over the periphery of one or more cylinders of revolution (26) centered on the central axis (20).

7. Measuring device according to claim 6, wherein the assembly (16) comprises at least three eccentric Bragg gratings whose longitudinal axes are uniformly distributed over the periphery of the same cylinder of revolution (26).

8. A device according to any one of the preceding claims, wherein the spectral analyzer (42; 162; 172) comprises: - an optical source (50) capable of emitting, in one end of the multimode optical fiber, an optical excitation signal which propagates in at least one group of spatial modes of the multimode optical fiber, - an acquisition apparatus (54; 164; 176) capable of measuring the spectral response of the assembly (16) of one or more Bragg gratings in response to the emitted optical excitation signal, and - an electronic processing unit (56) electrically connected to the acquisition apparatus, this electronic processing unit being configured: - to extract from the measured spectral response, for each Bragg grating, the amplitude of the power peak of the spectral response of this Bragg grating in a group of spatial modes queried, and - to establish the value of the radius of curvature from the measured amplitude of the power peak.

9. A measuring device according to claim 8, wherein: - the spectral analyzer (40; 160) comprises a modal demultiplexer (52) optically connected to one end of the multimode optical fiber, this modal demultiplexer being capable of directing only the optical signals propagating in the group of spatial modes being interrogated to an input port of the acquisition apparatus, and - the acquisition apparatus (54; 164) is capable of measuring, for each Bragg grating of the assembly, the amplitude of the peak power of the spectral response of this Bragg grating in the group of spatial modes being interrogated only from the optical signals directed to its input port by the modal demultiplexer.

10. Measurement device according to claim 8 or 9, wherein the acquisition apparatus (54; 164; 176) is optically connected to the same end of the multimode optical fiber as that to which the optical source (50) is connected.

11. A method for measuring a radius of curvature using a measuring device according to any one of the preceding claims, said method comprising the following steps: - the emission (120) into the multimode optical fiber of an optical excitation signal the majority of whose power is within a wavelength range [Xmin; Xmax], and - the measurement of a spectral response of one or more Bragg gratings in response to the emitted optical excitation signal, then - the extraction (122), from the measured spectral response, for each Bragg grating in the set, of the amplitude of a power peak corresponding to the power peak of a spectral response of this Bragg grating in a group of spatial modes queried different from the group of the fundamental spatial mode, the amplitude of this power peak varying as a function of the radius of curvature of the multimode optical fiber at the location of this Bragg grating, then - the establishment (132) of a value of the radius of curvature from the measured amplitude of the power peak, characterized in that, during the emission step (120), the wavelength is greater than the largest transverse dimension of the patterns of each Bragg grating in the set.

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