High-frequency mechanical wave measuring device

Abstract

Apparatus for measuring high-frequency mechanical waves This apparatus comprises: - a demultiplexer (52) capable of extracting optical signals reflected by measurement Bragg gratings, this demultiplexer comprising for this purpose a series of bandpass filters (Fpi) in which each bandpass filter (Fpi) is associated with a respective measurement Bragg grating (Bmi), each of these bandpass filters comprising a bandwidth centered on a central wavelength and the width of this bandwidth being greater than the reflection range of the measurement Bragg grating with which it is associated, and - a servo-control device (130) capable of varying the central wavelength of each bandpass filter (Fpi) to cancel variations in the continuous component of the optical signal reflected by the measurement Bragg grating with which it is associated. Fig. 3

Description

Title of the invention: Device for measuring high-frequency mechanical waves

[0001] The invention relates to a device for measuring high-frequency mechanical waves and to an instrumented structure comprising this measuring device.

[0002] The invention applies in particular, but not exclusively, to the field of non-destructive testing and structural integrity monitoring.

[0003] For example, a high-frequency acoustic wave measuring apparatus is disclosed in the following article: ZHAO Yang et al: “A laser-based fiber Bragg grating ultrasonic sensing System for structural health monitoring”, IEEE Photonics Technology Letters, 2016, vol. 28, no. 22, pp. 2573-2576. Hereinafter, this article is referred to as “article Zhao2016”. This known measuring apparatus comprises:

[0004] - an optical fiber to be fixed on a structure to be monitored in which are made several Bragg gratings,

[0005] - a generator of an excitation signal formed by several laser sources which each emit an excitation beam reflected by a respective Bragg grating,

[0006] - a demultiplexer capable of extracting the optical signals reflected by each of the Bragg gratings, this demultiplexer comprising for this purpose a series of bandpass filters, and

[0007] - a set of photodiodes capable of measuring each of the optical signals extracted by the demultiplexer.

[0008] Each of the bandpass filters of the demultiplexer has a -3 dB bandwidth centered on a central wavelength. The width of this bandwidth is greater than the reflection range of a respective measurement peak of one of the Bragg gratings. The width of this bandwidth is also sufficiently narrow so as not to encroach on a measurement peak of another Bragg grating.

[0009] An acoustic wave propagating in the monitored structure deforms the Bragg grating it encounters. This deformation of the Bragg grating slightly changes the fundamental wavelength XB of this Bragg grating. This change in the wavelength XB changes the power of the optical signal reflected by the Bragg grating at a given wavelength. Thus, when environmental conditions are constant, a change in the amplitude of the reflected optical signal, measured by a photodiode, is a measure of the acoustic wave at the location of this Bragg grating.

[0010] The fundamental wavelength XB can also vary depending on the conditions in environmental conditions such as temperature or mechanical deformation of the structure. Because of this, the bandwidth of each bandpass filter must be large enough to always include the XB wavelength of the corresponding Bragg grating. Thus, in each measuring device such as the one described in Zhao2016, the bandwidth of each bandpass filter is sized so that, even when extreme environmental conditions are encountered, the XB wavelength always lies within this bandwidth. Because of this, the bandwidths of the demultiplexer's bandpass filters are much wider than the measurement peaks. Furthermore, since each bandwidth must contain at most one measurement peak, the wider the bandwidths of the bandpass filters, the further the XB wavelengths of the Bragg gratings must be separated from each other.Thus, the number of Bragg gratings that can be interrogated by the same generator decreases as the bandwidth of the bandpass filters increases. In other words, the density of Bragg gratings decreases as the bandwidth of the bandpass filters increases.

[0011] The invention aims to provide a high-frequency acoustic wave measuring apparatus in which, even when environmental conditions vary, a higher Bragg grating density can be achieved.

[0012] The invention therefore relates to an apparatus for measuring high-frequency mechanical waves, i.e. mechanical waves with frequencies greater than 1 kHz, this apparatus comprising:

[0013] - an optical fiber in which several Bragg measuring gratings are made intended to be exposed to high-frequency mechanical waves, each of these measurement Bragg gratings having a reflection spectrum which includes a measurement peak centered on a measurement wavelength and the width of which defines a reflection range which contains all the wavelengths reflected by this measurement peak, the measurement wavelengths of each of these Bragg gratings being different from each other and spaced from each other in such a way that the different reflection ranges of these measurement Bragg gratings are distinct and do not overlap,

[0014] - a generator capable of emitting an optical excitation signal in the optical fiber, the emission spectrum of this optical excitation signal comprising as many emission peaks as there are measurement Bragg gratings, each of these emission peaks being located, at least in part, within the reflection range of a respective measurement peak,

[0015] - a demultiplexer capable of extracting the optical signals reflected by each of the Bragg measurement gratings, this demultiplexer comprising for this purpose a series of bandpass filters in which each bandpass filter is associated with a respective measurement Bragg grating, each of these bandpass filters having a -3 dB bandwidth centered on a central wavelength and the width of this bandwidth being greater than the reflection range of the measurement peak of the measurement Bragg grating with which it is associated and sufficiently narrow so as not to encroach on a measurement peak of another measurement Bragg grating, and

[0016] - a set of photodiodes capable of measuring each of the optical signals extracted by the demultiplexer,

[0017] in which the apparatus comprises a first servo-control device capable of varying the central wavelength of each band-pass filter to cancel the variations of the continuous component of the optical signal reflected by the measuring Bragg grating with which it is associated.

[0018] Embodiments of this apparatus may include one or more of the following features:

[0019] 1) Each bandpass filter comprises an optically filtering Bragg grating connected to the optical fiber to receive the optical signal to be demultiplexed and optically connected to one of the photodiodes to deliver to this photodiode the optical signal extracted by this bandpass filter, the reflection spectrum of this filtering Bragg grating comprising a filtering peak centered on the central wavelength of this bandpass filter and the width of this filtering peak being greater than the width of the reflection range of the measurement peak of the measurement Bragg grating associated with this bandpass filter.

[0020] 2) The first servo device comprises, for each Bragg grating of filtering, a transducer capable of mechanically deforming this filtering Bragg grating to modify the central wavelength of this bandpass filter.

[0021] 3) The generator comprises:

[0022] - an optical source capable of emitting on an output an incoherent light whose power spectrum has an emission band that extends over all reflection ranges of the measurement peaks, and

[0023] - a filtering device comprising an input optically connected to the output of the optical source and an output optically connected to the optical fiber, the power spectrum of this filtering device comprising as many -3 dB bandwidths as there are measurement Bragg gratings, each of these bandwidths being centered on a respective emission peak to form this emission peak in the excitation optical signal.

[0024] 4) The filtering device comprises as many source Bragg gratings as there are measurement Bragg gratings, each source Bragg grating being associated with a respective measurement Bragg grating, the reflection spectrum of each source Bragg grating comprising the emission peak which lies, at least in part, within the reflection range of the measurement peak of the measurement Bragg grating associated with it.

[0025] 5) The width of the emission peak of each source Bragg grating is equal to or greater than smaller than the width of the measurement peak of the associated measurement Bragg grating.

[0026] 6) Each emission peak is centered on a central wavelength and the device comprises a second servo device capable of varying the central wavelength of each source Bragg grating to cancel out variations in the continuous component of the optical signal reflected by the measurement Bragg grating with which it is associated.

[0027] 7) The second servo device comprises, for each Bragg grating source, a mechanical transducer capable of mechanically deforming this source Bragg grating to modify the central wavelength of this source Bragg grating.

[0028] 8) The first and second servo devices are combined and each transducer is capable of mechanically deforming both the filtering Bragg grating of the bandpass filter and the source Bragg grating which are associated with the same measurement Bragg grating.

[0029] 9) Each transducer is a piezoelectric material on which the Bragg grating filtering and the source Bragg grating are fixed without any degree of freedom.

[0030] The invention also relates to an instrumented structure comprising:

[0031] - a structure in which a defect, capable of modifying the propagation of the waves high frequency canines in this structure or to generate a high frequency mechanical wave, may appear,

[0032] - a system for detecting a defect in this structure, this system comprising

[0033] - the above apparatus for measuring high-frequency mechanical waves is propagating in the structure, this device comprising an optical fiber in which several measuring Bragg gratings are made, these measuring Bragg gratings being fixed without any degree of freedom to the structure, and

[0034] - a unit for monitoring the appearance of a defect in the structure from the high-frequency mechanical waves measured by this device.

[0035] The invention will be better understood on reading the description which follows, given solely by way of non-limiting example and made with reference to the drawings in which:

[0036] - [Fig.l] is a schematic illustration of the architecture of an ins structure instrumented comprising a device for measuring high-frequency acoustic waves,

[0037] - [Fig.2] is a schematic illustration of a measurement peak and an emission peak used in the measuring device of [Fig.l],

[0038] - Figures 3 and 4 are schematic illustrations of a reader of the apparatus of measurement of [Fig.l],

[0039] - [Fig.5] is a flowchart of a method of operation of the ins structure trumented with [Fig.l].

[0040] In this description, the terminology, conventions and definitions of the terms used in this text are introduced in a chapter I. Then, detailed examples of embodiments are described in a chapter II with reference to the figures. In a chapter III, variants of these embodiments are presented. Finally, the advantages of the different embodiments are specified in a chapter IV.

[0041] Chapter I: Definitions, terminologies and conventions:

[0042] In the figures, the same references are used to designate the same elements.

[0043] In the remainder of this description, the characteristics and functions well known to those skilled in the art are not described in detail.

[0044] In this text, the symbol “*” designates the scalar multiplication operation.

[0045] The term “high frequency mechanical wave” or simply “mechanical wave” means a mechanical wave whose fundamental frequency is greater than 1 kHz and preferably greater than 10 kHz. Typically, it is an acoustic wave.

[0046] An ultrasonic wave is an acoustic wave whose frequency is greater than 20 kHz.

[0047] A "reflection power spectrum" or simply "reflection spectrum" is the power spectrum of the optical signal reflected by a Bragg grating.

[0048] An "emission spectrum" is the power spectrum of an optical source that emits an optical signal.

[0049] A peak in the reflection spectrum corresponds to an absorption line in the transmission power spectrum of the same Bragg grating.

[0050] The "width" of a peak in a power spectrum refers to its full width at half maximum. Full width at half maximum is also known by the acronym FWHM ("Full Width at Half Maximum").

[0051] The fundamental wavelength XB of a Bragg grating is defined by the following relation: XB = 2*ne*A, where:

[0052] - ne is the effective index of the optical fiber in which the network is made of Bragg, and

[0053] - A is the pitch of the Bragg grating.

[0054] The effective propagation index ne is also known as the "mode phase constant". It is defined by the following relationship: ng = neff - Xdneff / dk, where ng is the group index and X is the wavelength of the optical signal guided by the optical fiber. The effective propagation index of an optical fiber depends on the dimensions of the core of this optical fiber and the materials forming this core and the optical cladding of this optical fiber. It can be determined experimentally or by numerical simulation.

[0055] Chapter II: Examples of embodiments

[0056] [Fig.l] represents an instrumented structure 2 comprising:

[0057] - a structure 6, and

[0058] - a system 8 for detecting a defect in the structure 6.

[0059] The structure 6 is a structure in which a defect, capable of modifying the propagation of ultrasonic waves in this structure, may appear. The defect detectable using the system described here is, for example, a crack or a microcrack. It may also be a defect such as a trace of corrosion or a local modification of the porosity of the structure.

[0060] The structure 6 is a mechanical part. By way of illustration, the structure 6 is a thin structure. For example, here, the thin structure 6 has an external face and an internal face separated from each other by the thickness e6 of the thin structure 6. The thickness e6 is sufficiently small so that the external and internal faces guide the propagation of an elastic wave or a Lamb wave in the thin structure in directions parallel to these external and internal faces. For this purpose, typically, the thickness e6 is ten or one hundred times smaller than a length and / or a width of the thin structure 6. Here, the thin structure 6 is a composite panel constituting the fuselage of an airplane.

[0061] For example, the thin structure 6 is made of laminated composite materials, that is to say by a stack, in a direction perpendicular to the external face, of a succession of layers each made of a respective material. As explained in chapter III on the variants, the teaching given in this particular case can be transposed without particular difficulty to many other possible structures.

[0062] To simplify [Fig.l], the thin structure 6 is represented in the form of a simple rectangle. However, in reality, the shape of the thin structure 6 is more complex. In particular, in the particular case of a composite panel of the fuselage of an aircraft, the thin structure 6 typically has rounded curves.

[0063] The system 8 makes it possible in particular to detect the appearance of a defect by measuring the ultrasonic waves which propagate in the structure 6. For this purpose, the system 8 comprises:

[0064] - a device 10 for measuring ultrasonic waves propagating in the structure 6,

[0065] - an emitter 12 capable of emitting a predefined ultrasonic wave which propagates in structure 6, and

[0066] - a monitoring unit 14 which monitors the occurrence of a defect in the structure 6 from device measurements 10.

[0067] The apparatus 10 comprises:

[0068] - an optical fiber 20,

[0069] - Bragg measuring gratings Bmj to BmN made one behind the other on along the axis of the optical fiber 20, in its core, and

[0070] - a reader 22 capable of measuring the fundamental wavelengths of the Bm networks, BmN.

[0071] The fiber 20 comprises a sensitive portion 24 in which the gratings Bmj to BmN are made. This sensitive portion 24 is fixed, without any degree of freedom, to the structure 6 so that the ultrasonic waves which propagate in the structure 6 are transmitted to the fiber 20 and modify the fundamental wavelength of the gratings Bm, to BmN when these ultrasonic waves encounter these gratings Bmj to BmN. For example, the portion 24 is glued to one face of the structure 6. The wavy lines which separate the sensitive portion 24 into two segments indicate that only a part of this sensitive portion is shown in [Fig.l].

[0072] A proximal end of the fiber 20 is optically connected to the reader 22.

[0073] Subsequently, the symbol “Bm;” denotes the i-th Bragg grating produced in the optical fiber 20, where the index i equals one, corresponds to the Bragg grating closest to the reader 22. Here, N is equal to the total number of Bm gratings; made in the fiber 20. Each Bm grating; has a fundamental wavelength / .H at which a reflection peak appears in the reflection spectrum of this Bm grating;. Subsequently, this reflection peak of the Bm grating; is called "measurement peak" and noted "Pm;" to distinguish it from other reflection peaks of other Bragg gratings described later. The portion of the reflection spectrum of the Bm grating; which contains its Pm peak; is represented in [Fig.2]. In [Fig.2], the abscissa axis is graduated in nanometers and the ordinate axis is graduated in arbitrary units (au). The Pm peak; of the Bm grating; is represented by a Gaussian-shaped curve. The peak of the Pm curve; is centered on the wavelength XBi.On either side of this peak, the curve Pm; comprises, going from left to right, a rising edge Fm; then a falling edge Fd;. The edges Fm; and Fd; are substantially linear at mid-height of the peak Pm;. The peak Pm; is higher than the background noise within a range [XBi- ôBi ; XBi+ ôBi] of wavelengths. This range [XBi- ôBi ; XBi+ ôBi] is subsequently called the “reflection range” because an incident optical signal on the grating Bm; whose wavelength is included within the range [XBi- ôBi ; XBi+ ôBi] is, at least in part, reflected by the grating Bm;. Conversely, if the incident optical signal is completely outside this range [XBi- ôBi ; XBi+ ôBi], this optical signal is not reflected by the grating Bm;. This range [XBi- ôBi ; XBi+ ôBi] moves as the XBi wavelength changes.

[0074] The wavelengths / .H are all different from each other and spaced apart from each other so that the reflection ranges of each of the gratings Bm; do not overlap. do not overlap.

[0075] The reader 22 interrogates, for example simultaneously, each network Bm; to measure the optical signal S;(t) reflected by each of the networks Bm;. A detailed embodiment of this reader 22 is described with reference to FIGS. 3 and 4.

[0076] The emitter 12 is controlled by the unit 14 to emit, typically at regular intervals, a predefined ultrasonic wave which propagates in the structure 6 until it reaches each of the networks Bm;. The ultrasonic wave generated by the emitter 12 is typically an elastic wave. In the case of a thin structure, this elastic wave is, for example, a Lamb wave. For example, the emitter 12 is a piezoelectric actuator fixed, without any degree of freedom, on the face of the structure 6.

[0077] The monitoring unit 14 acquires the measurements from the device 10 and, based on the acquired measurements, detects the occurrence of a fault if such a fault appears in the structure 6. Thus, the unit 14 makes it possible to monitor the health of the structure 6 and to inform a maintenance operator thereof. For this purpose, the unit 14 comprises an electronic computer 30 and a human / machine interface 32 connected to the computer 30.

[0078] The computer 30 comprises a programmable microprocessor 34 and a memory 36. The memory 36 comprises the instructions and data necessary for the execution of the method of [Fig.5], when these instructions are executed by the microprocessor 34. For example, here, for each network Bm;, the memory 36 comprises a pre-recorded reference signal Sref;(t). This signal Sref;(t) is the ultrasonic signal emitted by the transmitter 12 and measured using the network Bm; in the absence of a defect in the structure 6.

[0079] The human / machine interface 32 is capable of communicating, in a manner directly intelligible to a human being, the results of the implementation of the detection method of [Fig.5]. For example, the interface 32 comprises a screen.

[0080] To simplify the description of the reader 22, in [Fig. 3], only the optical components of the reader 22 associated with a single network Bm; are shown. The optical components associated with this single network Bm; are those used to measure the ultrasonic wave at the location of this network Bm;. In [Fig. 4], the optical components associated with several networks Bm; of the reader 22 are simultaneously shown. More precisely, [Fig. 4] represents the networks Bm; of index one to three and only the optical components associated with these first three networks Bm;. In [Fig. 4] the optical components already described with reference to [Fig. 3] in the case of the network Bm;, bear the same numerical references except that the index i is replaced by the indices one to three. Thus, in [Fig.4], the optical components which bear the indices “1”, “2” and “3” are those associated with the networks, respectively Bmb Bm2 and Bm3.Similarly in the remainder of this description, all components or peaks which have the same index "i" are specifically associated with the Bm network; and therefore used for . acquire the ultrasonic wave which deforms this Bm network;.

[0081] The reader 22 comprises:

[0082] - a generator 50 of an excitation signal of the Bmi5 networks

[0083] - a demultiplexer 52 which extracts the optical signals reflected by each of the Bm networks;,

[0084] - a set 54 of photodiodes which measures each of the optical signals extracted by the die multiplexer 52, and

[0085] - a device 56 for acquiring each of the signals measured by the set 54 of pho todiodes.

[0086] The generator 50 emits, on an output 60, an optical excitation signal whose emission spectrum comprises as many emission peaks Pg; as there are gratings Bm;. A peak Pgi is represented in [Fig.2]. Each peak Pg; is located, at least in part, within the reflection range of the peak Pm;. This peak Pg; is centered on a wavelength XGi. The width of the peak Pg; is such that when the wavelength / .H varies, the power of the optical signal reflected by the grating Bm; also varies. Preferably, the width of this peak Pg; is equal to or less than the width of the peak Pm;. For example, here, the width of each peak Pg; is twice less than the width of the peak Pm;. The width of the peak Pgi is however, preferably, greater than 0.1 nm to obtain an incoherent light emission peak.

[0087] The wavelength XGi is located in the middle of one of the fronts Fm; and Fd;. For example, in [Fig.2], the wavelength XGi is located in the middle of the rising front Fm;, that is to say at a location where the slope of the rising front is maximum and substantially linear. Under these conditions, a slight displacement of the wavelength XBi of the grating Bm; caused by an ultrasonic wave, causes a variation in the optical signal reflected by the grating Bm; large enough to be detected by the photodiodes Pd; (Figures 3 and 4). This is illustrated in [Fig.2], where the symbol Si; denotes a slight ripple in the wavelength / .H and the symbol Sp denotes the corresponding ripple in the optical signal reflected by the grating Bm;.

[0088] To produce this excitation signal, the generator 50 comprises:

[0089] - an optical source 62 which emits incoherent light on an output 64, and

[0090] - a tunable filtering device 66 comprising an optically connected to output 64 and an output merged with output 60 of the generator.

[0091] The source 62 emits incoherent light whose power spectrum comprises a continuous emission band which extends over all the reflection ranges of the Pm; peaks. Thus, for all the wavelengths included within this emission band, the power of the incoherent light is greater than the background noise and therefore capable of producing a reflected optical signal when it encounters one of the Bm; gratings.

[0092] The filtering device 66 filters the incoherent light emitted by the source 62 to form each of the peaks Pg;. To this end, for each of the peaks Pg,, the filtering device 66 comprises a bandwidth at - 3 dB centered on the wavelength XGi and whose width is equal to the width of this peak Pg;.

[0093] For this, the filtering device 66 comprises an optical circulator 72 and, for each network Bm;, a source Bragg network Bs;.

[0094] The circulator 72 comprises a first input optically connected to the input 68, a second input 74 optically connected to one end of an optical fiber 80 in which each of the networks Bs are made, and an output optically connected to the output 60. This circulator 72:

[0095] - directs the incoherent light received on input 68 only to its second input 74, and

[0096] - directs the optical signal received on its second input 74 only to the output 60.

[0097] Thus, the optical excitation signal emitted on the output 60 is only formed by the optical signals reflected by the networks Bs; produced in the fiber 80.

[0098] The gratings Bs; are made one behind the other along the axis of the fiber 80. Each grating Bs; is shaped so that its reflection spectrum includes the peak Pg;, that is to say the peak centered on the wavelength XGi. Here, the peak Pg; of the grating Bs; is centered on the fundamental wavelength of the grating Bs;. For this, typically, the pitch and the number of patterns of the grating Bs; are adjusted to obtain this peak Pg; in its reflection spectrum.

[0099] The output 60 of the generator 50 is optically connected to a first input 90 of an optical circulator 92. The circulator 92 comprises a second input 94 optically connected to the end of the fiber 20 and an output 96 optically connected to an input 98 of the demultiplexer 52. The circulator 92:

[0100] - directs the excitation signal received on its input 90 only to its input 94, and

[0101] - directs the optical signal received on its input 94 only to output 96.

[0102] Thus, only the optical signals reflected by the Bm; networks are received by the demultiplexer 52.

[0103] The demultiplexer 52 extracts the optical signals reflected by each of the gratings Bm; and transmits each of the extracted optical signals to a respective photodiode Pd; of the set 54. The demultiplexer 52 comprises for this purpose a series of tunable bandpass filters Fp;. Each filter Fp; has a bandwidth at - 3 dB [Xfi- ôfi ; Xfi+ ôfi] centered on a central wavelength Xfi and whose width 2*ôfi is greater than the width of the reflection range [XBi- ôBi ; XBi+ ôBi] of the peak Pm;. The wavelength and the range [Xfi- ôfi ; Xfi+ ôfi] are visible in [Fig.2]. The width 2*ôfi is also sufficiently narrow so as not to encroach on another measurement peak. For example, here the width 2*ôfi is greater than or equal to 4*ôBi or 6*ôBi and in- less than 10*ôBi or 20*ôBi.

[0104] Here, each filter Fp; comprises an optical circulator Co; and a filtering Bragg grating Bf;.

[0105] The gratings Bf; are made in an optical fiber 110. More precisely, these gratings Bf; are made, for example, in the increasing order of index i, one after the other along the axis of this fiber 110. Each grating Bf; is shaped so that its reflection spectrum includes a filtering peak Pf; centered on the central wavelength Xfl. The width of this peak Pf; is equal to 2*ôfl. Here, the peak Pf; of the grating Bf; is centered on the fundamental wavelength of the grating Bf;. For this, typically, the pitch and the number of patterns of the grating Bf; are adjusted to obtain this peak Pf; in its reflection spectrum.

[0106] The circulator Co; of each filter Fp; is located just upstream of the network Bf;, that is to say on the side of the input 98 with respect to the network Bf;. Thus, with the exception of the circulator Coi which is interposed between the input 98 and the network Bfb, each circulator Co; is interposed between the networks Bf; i and Bf;.

[0107] The circulator Co; comprises a first input Eh directly optically connected to the input 98 for the circulator Coi and optically directly connected to the network Bf i for the other circulators Co;, a second input E2; optically connected to the network Bf and an output Si optically connected to the photodiode Pd;. This circulator Co;:

[0108] - directs the optical signal received on its input El; only towards its input E2;, and

[0109] - directs the optical signal received on its input E2; only towards its output Si.

[0110] When the input El; is connected to the previous network Bf H this input receives the optical signal that has passed through the networks Bfi to Bf b This input El; therefore receives the portion of the optical signal reflected by the networks Bm; and which is outside the reflection bands [Xfi- ôfi ; Xfi+ ôpf] of the previous networks Bfi to Bf The input El; therefore receives in particular the portion of the reflected optical signal which is in the band [Xfl- ôfi ; Xfl+ ôfi] of the network Bf. This portion of the reflected optical signal is transmitted to the network Bf via the input E2;. The network Bf reflects only the portion of the reflected optical signal which is inside this band [Xfl- ôfi ; Xfl+ ôfi]. The other portion of the reflected optical signal is transmitted to the input E2i+i of the circulator Coi+i. The portion of the optical signal which is inside the band [Xfl- ôfi ; Xfl+ ôfi] is emitted by the output Si to the photodiode Pd;.Thus, the photodiode Pd; measures only the reflected optical signal which is inside the band [Xfi- ôfi ; Xfi+ ôfi] and therefore only the power of the peak Pm;. .

[0111] The acquisition device 56 acquires the electrical signals generated by each of the photodiodes Pd; and delivers to the computer 30 the digitized signals corresponding to each of these electrical signals. For this purpose, typically, for each photodiode Pd,, the acquisition device 56 comprises an analog-digital converter CANi connected to the output of the photodiode Pd;.

[0112] To reduce the width of each band [Xfl- ôfi ; Xfl+ ôfi] while retaining the ability to use the apparatus 10 in environmental conditions that may vary, the reader 22 further comprises a servo device 130. This device 130 modifies the wavelength Xfl of each grating Bf; to cancel out the variations in the DC component of the optical signal reflected by the grating Bm; associated with this grating Bf;. The DC component is generated by slow variations in the wavelength XBi, that is to say variations much slower than those caused by an ultrasonic wave. These slow variations in the wavelength XBi are typically those caused by variations in the environmental conditions. Indeed, the environmental conditions vary at a frequency of less than 500 Hz and, typically, less than 100 Hz or 10 Hz or 1 Hz.

[0113] For this, the device 130 comprises a microcontroller 132 and a mechanical transducer Tm; for each network Bf;.

[0114] The transducer Tm; is capable of mechanically deforming the grating Bf; without deforming the other filtering Bragg gratings. The transducer Tm; directly deforms the grating Bf; without resorting to local heating. For this, the transducer Tm; is a piezoelectric material on which the grating Bf is fixed, without any degree of freedom. For example, the grating Bf; is glued to one face of this piezoelectric material.

[0115] The microcontroller 132 receives the electrical signals generated by each of the photodiodes Pdi and estimates, from each electrical signal received, the value of the DC component of the optical signal reflected by each network Bm;. For example, here, it is the cumulative power of the components of the electrical signal located, in the power spectrum, at frequencies lower than 100 Hz which is used as a representative value of the DC component. Then, depending on the estimated value of the DC component, the microcontroller 132 generates a command of the transducer Tm; which aims to compensate for the variations of this DC component and thus maintain the wavelength Xfl centered on the wavelength XBi. In other words, the command generated by the microcontroller 132 makes it possible to cancel the variations of the measured DC component.To do this, the microcontroller 132 executes a control algorithm that controls the value of the estimated DC component to a predetermined setpoint. Typically, the predetermined setpoint is the value of the estimated DC component in the absence of an ultrasonic wave and when the wavelength is correctly centered on the wavelength XBi. For example, this setpoint is measured during a preliminary calibration phase. For example, the control algorithm is a PID (Proportional, Integral, Derivative) regulator. Thus, the central wavelength Xflest automatically shifted to compensate for variations in the DC component of the optical signal reflected by the grating Bm;. Under these conditions, the wavelength Xfise shifts like the wavelength XBi when this shift in the wavelength / .H is caused by a variation in environmental conditions.

[0116] Furthermore, here, the same servo-control device 130 is also used to servo-control the wavelength XGi of each grating Bs; on the continuous component of the optical signal reflected by the grating Bm; associated with this grating Bs;. This makes it possible to keep the relative position of the wavelength XGi with respect to the wavelength XBi substantially constant when the wavelength XBi varies in response to a change in the environmental conditions. For this purpose, here, the grating Bs; is also fixed without any degree of freedom on a face of the piezoelectric material of the transducer Tm;.

[0117] In [Fig.4], to simplify the representation of the reader 22, the device acquisition 56 and the microcontroller 132 have not been shown. Furthermore, in [Fig.4] only part of the digital references of the inputs / outputs of the optical components have been reported.

[0118] The operation of the system 8 will now be described with reference to the method of [Fig.5].

[0119] During an instrumentation step 200, the networks Bm; are each fixed to a respective location on the structure 6.

[0120] Then, during a step 202, the reference signal Sref;(t) is recorded for each of the networks Bm;. For example, for this, in the absence of a defect in the structure 6, an ultrasonic wave is measured, using the device 10, by each of the networks Bm;. It is this ultrasonic wave measured in the absence of a defect, using the network Bm;, which is then recorded in the memory 36 as the reference signal Sref;(t) associated with this network Bm;. During this step 202, each signal Sref;(t) is measured in the same way as during the operating phase of the system 8 for detecting a defect.

[0121] Then, a phase 210 of operation of the system 8 for detecting a fault is executed. During this phase 210, the following steps are repeated at regular intervals.

[0122] During a step 212, the computer 30 controls the transmitter 12 to emit a predefined ultrasonic wave into the structure 6.

[0123] In parallel, during a step 214, the generator 50 continuously emits, in the fiber 20, the excitation signal. Each network Bm; then reflects the part of the excitation signal which corresponds to the peak Pg;.

[0124] Also in parallel with step 214, during a step 216, the demultiplexer 52 extracts each optical signal reflected by a particular network Bm; and directs it to the corresponding photodiode Pd;. Thus, during step 216, each photodiode Pd; transforms the extracted optical signal into a corresponding electrical signal.

[0125] As the photodiodes Pd; generate the electrical signals, during a step 218, the acquisition device 56 transforms them into digital signals Si (t) which are transmitted to the computer 30.

[0126] Here, the electrical signals S;(t) are also transmitted to the servo-control device 130 which uses, during a step 220, the continuous component of each of these electrical signals to control a displacement of the wavelengths XGi and Xfide so that these wavelengths XGi and Xfl follow the displacement of the wavelength XB; which is caused by a variation in the environmental conditions. This displacement of the wavelengths XGi and / .(.thus compensates for the variation in the environmental conditions.

[0127] Once the complete signals S;(t) have been acquired by the computer 30, during a step 230, the computer 30 compares each signal S;(t) to the signal Sref;(t) to deduce the existence of a fault in the event of a significant difference between these two signals. If a fault is detected, the computer 30 controls the interface 32 to transmit this information to an operator.

[0128] Chapter III: Variants:

[0129] Variants of the structure:

[0130] The measuring apparatus described here applies to thin structures other than an aircraft fuselage panel. For example, the thin structure may also be a plate, a rail, a tube, a bar or any other part whose thickness is small compared to its length or its width. In particular, for example in the case of a bar, the thin structure does not necessarily have both an external face and an internal face.

[0131] The structure is not necessarily a thin structure. For example, the structure may be a civil engineering structure such as a bridge or a road on which a vehicle travels. In this case, the emitted signal is adapted to propagate, without being attenuated too much in the structure and, preferably, parallel to a face of this structure. For example, for this, the Lamb wave is replaced by a Rayleigh wave which propagates parallel to a face of the structure.

[0132] The structure may be made of materials other than a laminated composite material. For example, the structure may be made of a non-laminated or non-composite material. In this case, for example, the structure is a blade of a turbine or a propeller. Thus, the measuring apparatus described herein may also be used with structures made of metal or concrete.

[0133] Generator variants:

[0134] The emission peak Pg; may be wider. For example, the width of the peak Pg; is equal to the width of the peak Pm;. In another embodiment, the width of the peak Pg; is su smaller than the width of the peak Pm,. In all cases, the width of the peak Pg; is small enough that a variation in the wavelength XBi results in a variation in the power of the optical signal reflected by the grating Bm;.

[0135] Alternatively, the generator 50 is additionally equipped with a sensor which measures the source signal reflected by each of the gratings Bs;. This source signal measured for each of the gratings Bs; is acquired by the computer 30. When processing the optical signal reflected by one of the gratings Bm;, the computer normalizes the power of the reflected optical signal. For this, for example, the power of the reflected optical signal is normalized by dividing it by the power of the source signal measured for the grating Bs;. This makes it possible to compensate for part of the noise of the excitation signal.

[0136] The generator 50 is not necessarily an incoherent light generator. For example, if the distance separating each of the Bm; gratings from the generator 50 is short, the generator 50 can be replaced by a generator which comprises as many laser sources as there are Bm; gratings. Each of these laser sources emits a coherent excitation beam at a respective wavelength / .g. Each of these laser sources emits a monochromatic beam, i.e. a beam corresponding to a Pg; peak whose width is less than 1 nm and, preferably, less than 0.5 nm. In this case, the noise of the excitation signal is lower than that obtained by using an incoherent light source. The noise attenuation device of the generator can therefore be omitted.

[0137] In another variant, for each grating Bm;, the generator comprises a source produced using a phase-shifted Bragg Grating and an amplifier which amplifies the signal reflected by the phase-shifted Bragg Grating before reinjecting it. Such a source is for example described in detail in the following article: Laffont G. et Al: “Wavelength tunable fiber ring laser for high-speed interrogation of fiber Bragg grating sensors”, Proc. SPIE 5855, 17th International Conference on Optical Fibre Sensors, 23 May 2005.

[0138] In a simplified embodiment, the filtering device 66 is not tunable. In this case, in response to a variation in the environmental conditions, the power of the reflected optical signal varies. However, even in such a case, it remains possible to separate the high-frequency component, which corresponds to the measured ultrasonic signal, from the low-frequency component which corresponds to the variations in the environmental conditions. In such an embodiment, the width of the peak Pg; is chosen to be sufficiently large so that, in response to a variation in the environmental conditions, at least a portion of this peak Pg; remains within the range [XBi- ô; ; W ÔJ.

[0139] Demultiplexer variants:

[0140] Other embodiments of the filters Fp; are possible. In particular, in alternatively, Fp filters are made without using Bragg gratings. For example, the Fpi filter can also be made from an absorbing or dichroic optical filter.

[0141] Alternatively, the filters Fp; are produced in several optical fibers connected to the same input 98 via an optical coupler.

[0142] Variants of the servo device:

[0143] Other embodiments of the transducers Tm; are possible. For example, the transducer Tm; comprises an electrical resistance which heats a material on which the networks Bs; and Bf; are fixed, without any degree of freedom. In this case, it is the thermal expansion of this material which mechanically deforms the networks Bs; and Bf;.

[0144] The servo device 130 may implement other servo algorithms than a PID regulator. For example, as a variant, the microcontroller 132 implements PI (Proportional, Integral) regulators.

[0145] The preceding embodiments have been described in the case where the servo device 130 is common to the networks Bf; and Bs;, that is to say that the same mechanical transducer and the same servo algorithm are used to deform at the same time and in the same manner the networks Bf; and Bs;. However, as a variant, this common servo device can be replaced by a first and a second servo devices. The first servo device uses first transducers to mechanically deform each of the networks Bf independently of each other. The second servo device uses second transducers, different from the first transducers, to deform each of the networks Bs; independently of each other. Each first and each second transducer are typically piezoelectric materials mechanically distinct from each other.In this case, the servo algorithm executed by the first servo device may be different from the servo algorithm executed by the second servo device. To do this, the first and second servo devices each receive, as input, the electrical signal delivered by each Pd photodiode. In this case where there are two servo devices independent of each other, one of them may be a master servo device while the other is a slave servo device. Thus, in the latter case, the first transducers are mechanically independent of the second transducers but the servo algorithms of the first and second servo devices are not necessarily so.

[0146] Other variants of the measuring device:

[0147] Other methods of fixing the sensitive portion 24 of the fiber 20 to the structure 6 are possible. For example, if the thickness of the structure 6 is sufficient, the sensitive portion 24 is integrated, without any degree of freedom, into the thickness of this structure during its manufacture.

[0148] As a variant, the apparatus 10 does not comprise a single optical fiber 20 but several optical fibers 20, each of these optical fibers 20 comprising several Bm networks; . In this case, what has been described here in the case of the optical fiber 20 applies to each of these optical fibers. This variant can be transposed to the case of a multi-core fiber. In the latter case, several measurement Bragg networks are produced in each of the cores of this multi-core fiber.

[0149] Each optical circulator can be replaced by a set of optical couplers and optical isolators which perform the same function.

[0150] Variants of the method for detecting a defect:

[0151] Other processing operations than those consisting of comparing the ultrasonic signal measured by a network Bm; with the reference signal Sref;(t) are possible. For example, as a variant, the processing carried out to detect a defect from the measurements of a network Bm; uses vibro-acoustic modulation. Such processing is described in detail in application EP4155724. In this case, it is not necessary to use a pre-recorded reference signal.

[0152] In other variants, the transmitter 12 is omitted. In this case, the Bm; networks are used to measure the acoustic background generated by the defect itself when it appears in the structure. In this case, the transmitter 12 may be omitted. Such processing for identifying a defect from the acoustic waves generated by the defect itself is for example described in application RU2737235C1.

[0153] Alternatively, the measurements of the apparatus 10 can also be used to determine the time intervals that have elapsed for an acoustic wave to propagate from the location of the defect to the locations of the Bm; networks. Then, knowing the positions of each of the Bm; networks, these determined time intervals are used, for example, to estimate the position of the defect in the structure.

[0154] Other variants:

[0155] The apparatus 10 can also be adapted to measure mechanical waves of frequencies between 1 kHz and 20 kHz. In particular, these acoustic waves of frequencies between 1 kHz and 20 kHz can also be used to detect defects in structures.

[0156] The microprocessor 34 may be a generic processor, a specific processor, an application-specific integrated circuit (also known as ASIC for “Application-Specific Integrated Circuit”) or an in situ programmable gate array (also known as FPGA for “Field-Programmable Gate Array”).

[0157] Several of the variants described above can be combined in the same embodiment.

[0158] Chapter IV: Advantages of the described embodiments:

[0159] Varying the central wavelength Xfl of each bandpass filter Fp; to cancel the continuous component of the optical signal reflected by each grating Bm; makes it possible to reduce the width of each passband [Xfl- ôfi ; Xfl+ ôfl], compared to the case where the wavelengths Xfl are constant. Indeed, when the wavelengths Xfl are constant, the width of each passband [Xfl- ôfi ; Xfl+ ôfl] must be large enough to always include the peak Pm; even if the wavelength / .H varies according to the environmental conditions. It is therefore necessary to widen the passband [Xfl- ôfi ; Xfl+ ôfl] so that even when extreme environmental conditions are encountered, the peak Pm; is always located within this passband.Conversely, when the wavelength Xfl is modified according to the DC component of the optical signal reflected by the grating Bm;, the wavelength X fi moves like the wavelength XBi in response to variations in environmental conditions. Thus, the bandwidth [Xfi- ôfl ; Xfi+ ôfi] moves according to the environmental conditions encountered to always maintain the peak Pm; within this bandwidth. Therefore, the width of the bandwidth [Xfi- ôfi ; Xfi + ôfi] can be much smaller since it does not depend on the extreme environmental conditions likely to be encountered. However, being able to reduce the width of the bandwidth [Xfi- ôfi ; Xfi+ ôfi] of each filter Fp; makes it possible to reduce the gap between successive peaks Pm;. This therefore makes it possible to increase the density of Bm networks;, that is to say that within the same spectral band, it is possible to use a greater number of them.

[0160] The use of a filtering Bragg grating Bf; to produce the filter Fp; simplifies the production of this bandpass filter.

[0161] The use of a transducer Tm; which directly mechanically deforms the grating Bf; to vary the wavelength Xfl makes it possible to vary this wavelength Xfl very quickly. This improves the accuracy of the measurement.

[0162] Using incoherent light and the filtering device 66 to form the excitation signal, instead of using laser sources, makes it possible to obtain an excitation signal that generates less noise when it propagates in the fiber 20. Thus, this optical excitation signal makes it possible to interrogate Bm; networks more than one kilometer away from the generator 50. The generator therefore makes it possible to interrogate Bm; networks over greater distances than when a coherent laser beam is used to do this.

[0163] The use of the network Bs; to generate the source peak Pg; makes it possible to simply generate this source peak.

[0164] The fact that the width of the source peak Pg; is equal to or smaller than the width of the measurement peak Pm; increases the sensitivity of the device and avoids ranges of loss of sensitivity.

[0165] Varying the central wavelength XGi of each grating Bs; to cancel out variations in the continuous component of the optical signal reflected by the grating Bm;, allows the apparatus 10 to operate correctly even if the grating Bm; is subjected to environmental conditions which cause its wavelength XBi to vary slowly.

[0166] The use of a mechanical transducer Tm; to vary the wavelength / . Gi allows this central wavelength to be varied very quickly. This improves the accuracy of the measurement.

[0167] Using the same transducer Tm; to mechanically deform both the network Bs; and the network Bf; simplifies the production of the apparatus 10.

[0168] Using a piezoelectric material as a transducer simplifies the production of the servo device.

Claims

Claims

1. Apparatus for measuring high-frequency mechanical waves, i.e. mechanical waves with frequencies greater than 1 kHz, this apparatus comprising: - an optical fiber (20) in which several measuring Bragg gratings (Bm;) are made, intended to be exposed to high-frequency mechanical waves, each of these measuring Bragg gratings having a reflection spectrum which comprises a measurement peak (Pm;) centered on a measurement wavelength (XBi) and the width of which defines a reflection range which contains all the wavelengths reflected by this measurement peak, the measurement wavelengths (XBi) of each of these Bragg gratings being different from each other and spaced from each other in such a way that the different reflection ranges of these measuring Bragg gratings are distinct and do not overlap, - a generator (50) capable of emitting an optical excitation signal in the optical fiber, the emission spectrum of this optical excitation signal comprising as many emission peaks (PgJ) as there are measurement Bragg gratings, each of these emission peaks being located, at least in part, within the reflection range of a respective measurement peak, - a demultiplexer (52) capable of extracting the optical signals reflected by each of the measurement Bragg gratings, this demultiplexer comprising for this purpose a series of bandpass filters (Fp;) in which each bandpass filter (FpO is associated with a respective measurement Bragg grating (Bm;), each of these bandpass filters comprising a bandwidth at - 3 dB centered on a central wavelength (X fi) and the width of this bandwidth being greater than the reflection range of the measurement peak of the measurement Bragg grating with which it is associated and sufficiently narrow so as not to encroach on a measurement peak of another measurement Bragg grating, and - a set (54) of photodiodes (Pd;) capable of measuring each of the optical signals extracted by the demultiplexer, characterized in that the apparatus comprises a first servo-control device (130) capable of varying the central wavelength (Xfi) of each band-pass filter (Fp;) to cancel the variations of the continuous component of the optical signal reflected by the Bragg grating measurement to which it is associated.

2. Apparatus according to claim 1, wherein each bandpass filter (Fpi) comprises a filtering Bragg grating (Bf;) optically connected to the optical fiber (20) to receive the optical signal to be demultiplexed and optically connected to one of the photodiodes to deliver to this photodiode the optical signal extracted by this bandpass filter, the reflection spectrum of this filtering Bragg grating comprising a filtering peak (Pf;) centered on the central wavelength (Xfl) of this bandpass filter and the width of this filtering peak being greater than the width of the reflection range of the measurement peak (Pm;) of the measurement Bragg grating associated with this bandpass filter.

3. Apparatus according to claim 2, in which the first servo-control device (130) comprises, for each filtering Bragg grating (Bf; ), a transducer (¾) capable of mechanically deforming this filtering Bragg grating to modify the central wavelength (Xfi) of this bandpass filter.

4. Apparatus according to any one of the preceding claims, in which the generator comprises: - an optical source (62) capable of emitting on an output (64) an incoherent light whose power spectrum comprises an emission band which extends over all the reflection ranges of the measurement peaks (Pm;), and - a filtering device (66) comprising an input (68) optically connected to the output of the optical source and an output optically connected to the optical fiber (20), the power spectrum of this filtering device comprising as many passbands at - 3 dB as there are measurement Bragg gratings, each of these passbands being centered on a respective emission peak (Pg;) to form this emission peak in the optical excitation signal.

5. Apparatus according to claim 4, in which the filtering device (66) comprises as many source Bragg gratings (Bs;) as measurement Bragg gratings (Bm;), each source Bragg grating being associated with a respective measurement Bragg grating, the reflection spectrum of each source Bragg grating comprising the emission peak (Pg;) which is located, at least in part, within the reflection range of the measurement peak of the measurement Bragg grating associated with it.

6. Apparatus according to claim 5, wherein the width of the emission peak (Pg;) of each source Bragg grating (Bs;) is equal to or smaller than the width of the measurement peak of the associated measurement Bragg grating.

7. Apparatus according to claim 5 or 6, wherein each emission peak (Pg;) is centered on a central wavelength (XGi) and the apparatus comprises a second servo device capable of varying the central wavelength (XGi) of each source Bragg grating to cancel out variations in the continuous component of the optical signal reflected by the measurement Bragg grating with which it is associated.

8. Apparatus according to claim 7, in which the second servo device comprises, for each source Bragg grating, a mechanical transducer capable of mechanically deforming this source Bragg grating to modify the central wavelength (XGi) of this source Bragg grating (Bs;).

9. Apparatus according to claims 3 and 8 taken together, in which the first and second servo devices (130) are merged and each transducer (Tm;) is capable of mechanically deforming both the filtering Bragg grating (Bf) of the bandpass filter (Fp;) and the source Bragg grating (Bs;) which are associated with the same measurement Bragg grating (Bm;).

10. Apparatus according to claim 9, wherein each transducer (TmO is a piezoelectric material on which the filter Bragg grating (Bf) and the source Bragg grating (Bs;) are fixed without any degree of freedom.

11. Instrumented structure comprising: - a structure (6) in which a defect, capable of modifying the propagation of high-frequency mechanical waves in this structure or of generating a high-frequency mechanical wave, can appear, - a system (8) for detecting a defect in this structure, this system comprising - an apparatus (10) for measuring high-frequency mechanical waves propagating in the structure, this apparatus comprising an optical fiber (20) in which several measuring Bragg gratings (Bm;) are made, these measuring Bragg gratings being fixed without any degree of freedom to the structure, and - a unit (14) for monitoring the appearance of a defect in the structure from the high-frequency mechanical waves measured by this apparatus, characterized in that the apparatus (10) for measuring the mechanical waves high frequencies is in accordance with any one of the preceding claims.

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