Apparatus for measuring high-frequency mechanical waves

Abstract

The invention relates to an apparatus for measuring high-frequency mechanical waves, which includes: - a demultiplexer (52) able to extract optical signals reflected by measurement Bragg gratings, the demultiplexer including a series of band-pass filters (Fpi) for this purpose, wherein each band-pass filter (Fpi) is associated with a respective measurement Bragg grating (Bmi), each of the band-pass filters having a passband centred on a central wavelength, the width of the passband being greater than the reflection range of the measurement Bragg grating with which it is associated, and - a servo-control device (130) able to vary the central wavelength of each band-pass filter (Fpi) in order to cancel out variations of the continuous component of the optical signal reflected by the measurement Bragg grating with which it is associated.

Description

High-frequency mechanical wave measuring apparatus[1] The invention relates to a high-frequency mechanical wave measuring apparatus and an instrumented structure comprising this measuring apparatus.[2] The invention applies in particular, but not exclusively, to the field of non-destructive testing and structural health monitoring.[3] For example, a high-frequency acoustic wave measuring apparatus is disclosed in the following article: ZHAO Yang et al: “A laser-based fiber Bragggrating 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 “Zhao2016 article”.This known measuring apparatus comprises: - an optical fiber to be fixed on a structure to be monitored in which several Bragg gratings are made, - a generator of an excitation signal formed by several laser sources which each emit an excitation beam reflected by a respective Bragg grating, - 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 - a set of photodiodes capable of measuring each of the optical signals extracted by the demultiplexer.[4] Each of the bandpass filters of the demultiplexer has a bandwidth at – 3 dB 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.[5] An acoustic wave propagating through the structure being monitored distorts the Bragg grating it encounters. This distortion of the Bragg grating slightly changes the fundamental wavelength λB of that Bragg grating. This change in wavelength λB 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 that Bragg grating.[6] The fundamental wavelength λB can also vary depending on environmental conditions such as temperature or mechanical deformation of the structure. Because of this, the bandwidth of each of the bandpass filters must be large enough to always include the wavelength λ. Bof the corresponding Bragg grating. Thus, in each measuring device such as the one described in Zhao2016, the bandwidth of each bandpass filter is dimensioned so that, even when extreme environmental conditions are encountered, the wavelength λB always lies within this bandwidth. Because of this, the bandwidths of the bandpass filters of the demultiplexer are much wider than the measurement peaks. Moreover, since each bandwidth must contain at most one measurement peak, the wider the bandwidths of the bandpass filters, the further the wavelengths λB 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 bandwidths of the bandpass filters increase. In other words,the density of Bragg gratings decreases when the bandwidth of the bandpass filters increases.[7] The invention aims to provide a high-frequency acoustic wave measuring apparatus operating like that described in Zhao2016 but in which, even when environmental conditions vary, a higher Bragg grating density can be achieved.[8] The invention is set forth in the attached set of claims. Embodiments of this invention include, for example:1) An apparatus for measuring high-frequency mechanical waves, i.e. mechanical waves with frequencies above 1 kHz, this apparatus comprising:- an optical fiber in which several measuring Bragg 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 whose width 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, - 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, - a demultiplexer 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 in which each bandpass filter is associated with a respective measurement Bragg grating, each of these bandpass filters comprising a bandwidth at – 3 dB 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 - a set of photodiodes capable of measuring each of the optical signals extracted by the demultiplexer, in which the apparatus also comprises a first servo device capable: - for each measurement Bragg grating,to estimate a value of the continuous component in the optical signal reflected by this Bragg grating from the signal measured by the photodiode which receives the optical signal extracted by the bandpass filter associated with this measuring Bragg grating, and in response - for each bandpass filter, to vary the central wavelength of this bandpass filter, as a function of the estimated value of the continuous component in the optical signal reflected by the measuring Bragg grating associated with this bandpass filter, to cancel the variations of the continuous component of the optical signal reflected by the measuring Bragg grating associated with this bandpass filter.[9] 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:- Figure 1 is a schematic illustration of the architecture of an instrumented structure comprising a high-frequency acoustic wave measuring device, - Figure 2 is a schematic illustration of a measurement peak and an emission peak used in the measuring device of Figure 1,- Figures 3 and 4 are schematic illustrations of a reader of the measuring device of Figure 1,- Figure 5 is a flowchart of a method of operation of the instrumented structure of Figure 1.,

[0010] 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.

[0011] Chapter I: Definitions, terminologies and conventions:

[0012] In the figures, the same references are used to designate the same elements.

[0013] In the remainder of this description, the features and functions well known to those skilled in the art are not described in detail.

[0014] In this text, the symbol "*" denotes the scalar multiplication operation.

[0015] The term "high-frequency mechanical wave" or simply "mechanical wave" refers to a mechanical wave whose fundamental frequency is greater than 1 kHz and, preferably, greater than 10 kHz. Typically, this is an acoustic wave.

[0016] An ultrasonic wave is an acoustic wave with a frequency greater than 20 kHz.

[0017] A "reflected power spectrum" or simply "reflected spectrum" is the power spectrum of the optical signal reflected by a Bragg grating.

[0018] An "emission spectrum" is the power spectrum of an optical source that emits an optical signal.

[0019] A peak in the reflection spectrum corresponds to an absorption line in the transmission power spectrum of the same Bragg grating.

[0020] 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).

[0021] The fundamental wavelength λB of a Bragg grating is defined by the following relation: λB = 2*ne*Λ, where: - n e is the effective index of the optical fiber in which the Bragg grating is made, and - Λ is the pitch of the Bragg grating.

[0022] The effective propagation index ne is also known as the "mode phase constant". It is defined by the following relation: n g = n eff - λdn eff / dλ, where n g is the group index and λ is the wavelength of the optical signal guided by the optical fiber. The effective propagation index of an optical fiber depends on the dimensions of the core of this optical fiber and the materials forming this core and the optical cladding of this optical fiber. It can be determined experimentally or by numerical simulation.

[0023] Chapter: Examples of embodiment

[0024] The figure represents an instrumented structure 2 comprising: - a structure 6, and - a system 8 for detecting a fault in the structure 6.

[0025] 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.

[0026] The structure 6 is a mechanical part. For illustration, the structure 6 is a thin structure. For example, here, the thin structure 6 has an outer face and an inner face separated from each other by the thickness e6 of the thin structure 6. The thickness e6 is small enough for the outer and inner faces to guide the propagation of an elastic wave or a Lamb wave in the thin structure in directions parallel to these outer and inner 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 aircraft.

[0027] 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.

[0028] To simplify Figure 1, 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.

[0029] 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: - a device 10 for measuring the ultrasonic waves propagating in the structure 6, - a transmitter 12 capable of emitting a predefined ultrasonic wave which propagates in the structure 6, and - a monitoring unit 14 which monitors the appearance of a defect in the structure 6 based on the measurements of the device 10.

[0030] The device 10 comprises: - an optical fiber 20, - Bragg measuring gratings Bm1 to Bm N made one behind the other along the axis of the optical fiber 20, in its core, and - a reader 22 capable of measuring the fundamental wavelengths of the networks Bm 1 to Bm N .

[0031] The fiber 20 comprises a sensitive portion 24 in which the gratings Bm1 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 Bm1 to BmN when these ultrasonic waves encounter these gratings Bm1 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. 1.

[0032] A proximal end of the fiber 20 is optically connected to the reader 22.

[0033] Subsequently, the symbol “Bmi” designates the i-th Bragg grating made in the optical fiber 20, where the index i equal to one, corresponds to the Bragg grating closest to the reader 22. Here, N is equal to the total number of Bm i gratings made in the fiber 20. Each Bmi grating has a fundamental wavelength λBi at which a reflection peak appears in the reflection spectrum of this Bm i grating. Subsequently, this reflection peak of the Bmi grating is called “measurement peak” and noted “Pmi” to distinguish it from other reflection peaks of other Bragg gratings described later. The portion of the reflection spectrum of the Bmi grating which contains its Pmi peak is represented in Figure 2. In Figure 2, the abscissa axis is graduated in nanometers and the ordinate axis is graduated in arbitrary units (au). The Pm peak i of the Bm network iis represented by a Gaussian-shaped curve. The peak of the curve Pm i is centered on the wavelength λBi. On either side of this peak, the curve Pm i has, going from left to right, a rising edge Fm i then a falling edge Fdi. The edges Fmi and Fdi are approximately linear at mid-height of the peak Pm i. The peak Pmi is higher than the background noise within a range [λBi - δBi ; λBi + δBi] of wavelengths. This range [λBi - δBi ; λBi + δBi] is subsequently called the "reflection range" because an optical signal incident on the grating Bm i whose wavelength is within the range [λBi - δBi ; λBi + δBi] is, at least in part, reflected by the grating Bm i. Conversely, if the incident optical signal is completely outside this range [λBi - δBi ; λBi + δBi], this optical signal is not reflected by the grating Bm i. This range [λBi - δBi ; λBi + δBi] moves at the same time as the wavelength λBi changes.

[0034] The wavelengths λBi are all different from each other and spaced apart from each other so that the reflection ranges of each of the Bm gratings i do not overlap.

[0035] The reader 22 interrogates, for example simultaneously, each network Bm i to measure the optical signal reflected Si(t) by each of the networks Bmi. A detailed embodiment of this reader 22 is described with reference to figures 3 and 4.

[0036] 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 gratings Bm i. 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.

[0037] The monitoring unit 14 acquires the measurements from the device 10 and, from 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.

[0038] 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 i, the memory 36 comprises a pre-recorded reference signal Srefi(t). This signal Srefi(t) is the ultrasonic wave emitted by the transmitter 12 and measured using the network Bm i in the absence of a defect in the structure 6.

[0039] 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.

[0040] To simplify the description of the reader 22, in Figure 3, only the optical components of the reader 22 associated with a single grating Bm i are shown. The optical components associated with this single grating Bm i are those used to measure the ultrasonic wave at the location of this grating Bm i. In Figure 4, the optical components associated with several gratings Bm i of the reader 22 are simultaneously shown. More precisely, Figure 4 represents the gratings Bm i of index one to three and only the optical components associated with these first three gratings Bm i. In Figure 4, the optical components already described with reference to Figure 3 in the case of the grating Bm i, bear the same numerical references except that the index i is replaced by the indices one to three. Thus, in Figure 4, the optical components which bear the indices “1”, “2” and “3” are those associated with the gratings, respectively Bm1, Bm2 and Bm3.Similarly, in the remainder of this description, all the oupic components which bear the same index “i” are specifically associated with the network Bm i and therefore used to acquire the ultrasonic wave which deforms this network Bm i.

[0041] The reader 22 comprises: - a generator 50 of an excitation signal of the Bm i networks, - a demultiplexer 52 which extracts the optical signals reflected by each of the Bm networks i , - a set 54 of photodiodes which measures each of the optical signals extracted by the demultiplexer 52, and - a device 56 for acquiring each of the signals measured by the set 54 of photodiodes.

[0042] The generator 50 emits, on an output 60, an optical excitation signal whose emission spectrum comprises as many emission peaks Pg i as there are gratings Bmi. A peak Pgi is represented in figure 2. Each peak Pg i is located, at least in part, within the reflection range of the peak Pm i. This peak Pgi is centered on a wavelength λGi. The width of the peak Pgi is such that when the wavelength λBi varies, the power of the optical signal reflected by the grating Bm i also varies. Preferably, the width of this peak Pgi is equal to or less than the width of the peak Pm i. For example, here, the width of each peak Pgi is twice less than the width of the peak Pm i. The width of the peak Pgi is however, preferably, greater than 0.1 nm to obtain an emission peak of incoherent light.

[0043] The wavelength λGi is located in the middle of one of the fronts Fm i and Fdi. For example, in Figure 2, the wavelength λGi is located in the middle of the rising front Fmi, i.e. at a location where the slope of the rising front is maximum and substantially linear. Under these conditions, a slight displacement of the wavelength λBi of the grating Bmi caused by an ultrasonic wave causes a variation of the optical signal reflected by the grating Bm i large enough to be detected by the Pdi photodiodes (Figures 3 and 4). This is illustrated in Figure 2, where the symbol Si denotes a slight ripple in the wavelength λBi and the symbol Sri denotes the corresponding ripple in the optical signal reflected by the Bm i grating.

[0044] To produce this excitation signal, the generator 50 comprises: - an optical source 62 which emits incoherent light on an output 64, and - a tunable filtering device 66 comprising an input 68 optically connected to the output 64 and an output coinciding with the output 60 of the generator.

[0045] Source 62 emits incoherent light whose power spectrum includes a continuous emission band that extends over all the reflection ranges of the Pm peaks i . Thus, for all 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 i .

[0046] The filtering device 66 filters the incoherent light emitted by the source 62 to form each of the peaks Pgi. For this purpose, for each of the peaks Pg i, the filtering device 66 comprises a bandwidth at – 3 dB centered on the wavelength λGi and whose width is equal to the width of this peak Pg i .

[0047] For this, the filtering device 66 comprises an optical circulator 72 and, for each network Bmi, a source Bragg network Bsi.

[0048] 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 i are made, and an output optically connected to the output 60. This circulator 72: - directs the incoherent light received on the input 68 only towards its second input 74, and - directs the optical signal received on its second input 74 only towards the output 60.

[0049] Thus, the optical excitation signal emitted on the output 60 is only formed by the optical signals reflected by the Bsi networks produced in the fiber 80.

[0050] The Bsi networks are made one behind the other along the axis of the 80 fiber. Each Bs network i is shaped so that its reflection spectrum includes the Pgi peak, that is, the peak centered on the wavelength λGi. Here, the Pgi peak of the Bsi grating is centered on the fundamental wavelength of the Bs i grating. For this, typically, the pitch and the number of patterns of the Bs i grating are adjusted to obtain this Pgi peak in its reflection spectrum.

[0051] The output 60 of the generator 50 is optically connected to a first input 90 of an optical circulator 92. The circulator 92 has 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: - directs the excitation signal received on its input 90 only to its input 94, and - directs the optical signal received on its input 94 only to the output 96.

[0052] Thus, only the optical signals reflected by the Bm i networks are received by the demultiplexer 52.

[0053] The demultiplexer 52 extracts the optical signals reflected by each of the gratings Bmi 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 Fpi. Each filter Fpi has a bandwidth at – 3 dB [λfi - δfi ; λfi + δfi] centered on a central wavelength λfi and whose width 2*δfi is greater than the width of the reflection range [λBi - δBi ; λBi + δBi] of the peak Pmi. The wavelength λfi and the range [λfi - δfi ; λfi + δfi] are visible in Figure 2. The width 2*δ fi is also sufficiently narrow so as not to encroach on another measurement peak. For example, here the width 2*δfi is greater than or equal to 4*δBi or 6*δBi and less than 10*δBi or 20*δ Bi .

[0054] Here, each filter Fpi comprises an optical circulator Co i and a filtering Bragg grating Bfi.

[0055] The Bfi networks are made in an optical fiber 110. More precisely, these Bfi networks are made, for example, in the increasing order of index i, one after the other along the axis of this fiber 110. Each Bf i network is shaped so that its reflection spectrum includes a filtering peak Pf i centered on the central wavelength λ fi . The width of this peak Pf i is equal to 2*δ fi . Here, the Pf peak i of the Bf network i is centered on the fundamental wavelength of the Bf i grating. For this, typically, the pitch and the number of patterns of the Bf i grating are adjusted to obtain this Pf i peak in its reflection spectrum.

[0056] The circulator Coi of each filter Fpi is located just upstream of the network Bf i, that is to say on the side of the input 98 with respect to the network Bf i. Thus, with the exception of the circulator Co1 which is interposed between input 98 and network Bf1, each circulator Coi is interposed between networks Bfi-1 and Bfi.

[0057] The circulator Coi comprises a first input E1i directly optically connected to input 98 for the circulator Co1 and optically directly connected to the network Bfi-1 for the other circulators Co i, a second input E2i optically connected to the network Bfi and an output Si optically connected to the photodiode Pd i. This circulator Co i : - directs the optical signal received on its input E1i only to its input E2i, and - directs the optical signal received on its input E2i only to its output Si.

[0058] When the input E1i is connected to the previous network Bfi-1, this input receives the optical signal which has passed through the networks Bf1 to Bfi-1. This input E1i therefore receives the portion of the optical signal reflected by the networks Bm i and which is outside the reflection bands [λ fi - δ fi; λ fi + δ pf ] from previous networks Bf1 to Bf i-1 . Entrance E1 i therefore receives in particular the portion of the reflected optical signal which is in the band [λfi - δfi ; λfi + δfi] of the network Bfi. This portion of the reflected optical signal is transmitted to the network Bfi via the input E2 i. The network Bfi reflects only the portion of the reflected optical signal which is inside this band [λfi - δfi ; λfi + δfi]. The other portion of the reflected optical signal is transmitted to the input E2i+1 of the circulator Coi+1. The portion of the optical signal which is inside the band [λfi - δfi ; λfi + δfi] is emitted by the output Si to the photodiode Pdi. Thus, the photodiode Pdi measures only the reflected optical signal which is inside the band [λfi - δfi ; λfi + δfi] and therefore only the power of the peak Pmi.

[0059] The acquisition device 56 acquires the electrical signals generated by each of the photodiodes Pdi and delivers to the computer 30 the digitized signals corresponding to each of these electrical signals. For this purpose, typically, for each photodiode Pdi, the acquisition device 56 comprises an analog-to-digital converter CAN i connected to the output of the Pd photodiode i .

[0060] To reduce the width of each band [λ fi - δfi ; λfi + δfi] 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 λfi of each grating Bfi to cancel out variations in the continuous component of the optical signal reflected by the grating Bm i associated with this grating Bfi. The continuous component is generated by slow variations in the wavelength λBi wavelength, that is, variations much slower than those caused by an ultrasonic wave. These slow variations in the λBi wavelength are typically those caused by variations in environmental conditions. Indeed, environmental conditions vary at a frequency lower than 500 Hz and, typically, lower than 100 Hz or 10 Hz or 1 Hz.

[0061] For this, the device 130 comprises a microcontroller 132 and a mechanical transducer Tm i for each Bf network i .

[0062] The Tmi transducer is capable of mechanically deforming the Bf grating without deforming the other filtering Bragg gratings. The Tm i transducer directly deforms the Bf grating iwithout resorting to local heating. For this, the Tmi transducer is a piezoelectric material on which the Bfi network is fixed, without any degree of freedom. For example, the Bfi network is glued to one side of this piezoelectric material.

[0063] 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 grating Bm i. 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 Tmi which aims to compensate for the variations of this DC component and thus maintain the wavelength λfi centered on the wavelength λBi. 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 λfi is correctly centered on the wavelength λBi. For example, this setpoint is measured during a prior calibration phase. For example, the control algorithm is a PID (Proportional, Integral, Derivative) regulator. Thus, the central wavelength λfi is automatically shifted to compensate for variations in the DC component of the optical signal reflected by the grating Bm i. Under these conditions, the wavelength λ. fi moves as the wavelength λ Bi when This shift in the wavelength λBi is caused by a variation in environmental conditions.

[0064] Furthermore, here, the same servo device 130 is also used to servo the wavelength λGi of each grating Bsi on the continuous component of the optical signal reflected by the grating Bmi associated with this grating Bsi. For this purpose, the servo device 130 is capable, for each source Bragg grating, of varying the central wavelength λ Giof this source Bragg grating, as a function of the estimated value of the DC component in the optical signal reflected by the measurement Bragg grating associated with the bandpass filter which is associated with the same measurement Bragg grating as this source Bragg grating, to cancel the variations of the DC component of the optical signal reflected by the measurement Bragg grating with which it is associated. This makes it possible to keep the relative position of the wavelength λGi with respect to the wavelength λBi substantially constant when the wavelength λBi varies in response to a change in the environmental conditions. For this purpose, here, the grating Bs i is also fixed without any degree of freedom on a face of the piezoelectric material of the transducer Tm i.

[0065] In Figure 4, to simplify the representation of the reader 22, the acquisition device 56 and the microcontroller 132 have not been shown. In addition, in Figure 4 only a part of the digital references of the inputs / outputs of the optical components have been reported.

[0066] The operation of system 8 will now be described with reference to the method of Figure 5.

[0067] During an instrumentation step 200, the Bm i networks are each fixed to a respective location on the structure 6.

[0068] Then, during a step 202, the reference signal Sref i(t) is recorded for each of the Bmi networks. For example, for this, in the absence of a fault in the structure 6, an ultrasonic wave is measured, using the device 10, by each of the Bmi networks. It is this ultrasonic wave measured in the absence of a fault, using the Bmi network, which is then recorded in the memory 36 as the reference signal Srefi(t) associated with this Bmi network. During this step 202, each signal Srefi(t) is measured in the same way as during the operating phase of the system 8 for detecting a fault.

[0069] 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.

[0070] During a step 212, the computer 30 controls the transmitter 12 to emit a predefined ultrasonic wave into the structure 6.

[0071] In parallel, during a step 214, the generator 50 continuously emits, in the fiber 20, the excitation signal. Each network Bmi then reflects the part of the excitation signal which corresponds to the peak Pgi.

[0072] Also in parallel with step 214, in a step 216, the demultiplexer 52 extracts each optical signal reflected by a particular Bm i grating and directs it to the corresponding photodiode Pdi. Thus, in step 216, each photodiode Pd i transforms the extracted optical signal into a corresponding electrical signal.

[0073] As the photodiodes Pd i 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.

[0074] Here, the electrical signals S i(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 λGi and λfi so that these wavelengths λGi and λfi follow the displacement of the wavelength λBi which is caused by a variation in the environmental conditions. This displacement of the wavelengths λGi and λfi thus compensates for the variation in the environmental conditions.

[0075] Once the complete signals Si(t) have been acquired by the computer 30, during a step 230, the computer 30 compares each signal S i(t) to the signal S refi(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.

[0076] Chapter III: Variants:

[0077] Structure variations:

[0078] The measuring apparatus described herein 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 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.

[0079] 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 is traveling. In this case, the emitted signal is adapted to propagate, without being too attenuated 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.

[0080] The structure can be made of materials other than a laminated composite material. For example, the structure can 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 device described here can also be used with structures made of metal or concrete.

[0081] Generator Variants:

[0082] The emission peak Pgi may be broader. For example, the width of the peak Pg i is equal to the width of the peak Pmi. In another embodiment, the width of the peak Pg i is greater than the width of the peak Pmi. In any case, the width of the peak Pg i is small enough that a variation in the wavelength λBi results in a variation in the power of the optical signal reflected by the grating Bm i.

[0083] Alternatively, the generator 50 is additionally equipped with a sensor which measures the source signal reflected by each of the networks Bs i. This source signal measured for each of the networks Bs i is acquired by the computer 30. When processing the optical signal reflected by one of the gratings Bm i, 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 i This compensates for some of the noise in the excitation signal.

[0084] The generator 50 is not necessarily an incoherent light generator. For example, if the distance separating each of the gratings Bm i from the generator 50 is short, the generator 50 can be replaced by a generator which comprises as many laser sources as gratings Bm i. Each of these laser sources emits a coherent excitation beam at a respective wavelength λGi. Each of these laser sources emits a monochromatic beam, i.e. a beam corresponding to a peak Pgi 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.

[0085] In another variant, for each grating Bm i, the generator comprises a source made 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.

[0086] 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 Pgi is chosen to be sufficiently large so that, in response to a variation in the environmental conditions, at least a part of this peak Pg i remains within the range [λ Bi - δ i ; λ Bi + δ i ].

[0087] Demultiplexer variants:

[0088] Other embodiments of Fpi filters are possible. In particular, as a variant, Fp filters iare made without using Bragg gratings. For example, the Fpi filter can also be made from an absorbing or dichroic optical filter.

[0089] Alternatively, the Fpi filters are made in several optical fibers connected to the same input 98 via an optical coupler.

[0090] Variants of the servo device:

[0091] Other embodiments of the Tm i transducers are possible. For example, the Tmi transducer comprises an electrical resistance which heats a material on which the Bs i and Bfi networks are fixed, without any degree of freedom. In this case, it is the thermal expansion of this material which mechanically deforms the Bsi and Bfi networks.

[0092] The servo device 130 may implement other servo algorithms than a PID regulator. For example, alternatively, the microcontroller 132 implements PI (Proportional, Integral) regulators.

[0093] The preceding embodiments have been described in the case where the servo device 130 is common to the networks Bf i and Bsi, 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 way the networks Bf i and Bsi. 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 i independently of each other. The second servo device uses second transducers, different from the first transducers, to deform each of the networks Bsi 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 photodiode Pdi. 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.

[0094] Other variations of the measuring device:

[0095] 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.

[0096] Alternatively, the device 10 does not comprise a single optical fiber 20 but several optical fibers 20, each of these optical fibers 20 comprising several Bmi 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 gratings are made in each of the cores of this multi-core fiber.

[0097] Each optical circulator can be replaced by a set of optical couplers and optical isolators that perform the same function.

[0098] Variants of the fault detection process:

[0099] Other treatments than those consisting of comparing the ultrasonic wave measured by a Bm network i to the reference signal Sref i (t) are possible. For example, as a variant, the processing performed to detect a fault from the measurements of a Bmi network 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.

[0100] In other variants, the transmitter 12 is omitted. In this case, the Bm arrays are used to measure the acoustic wave 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.

[0101] 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 networks Bm i. Then, knowing the positions of each of the networks Bm i, these determined time intervals are used, for example, to estimate the position of the defect in the structure.

[0102] Other variants:

[0103] The apparatus 10 may 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 may also be used to detect defects in structures.

[0104] The microprocessor 34 may be a generic processor, a specific processor, an application-specific integrated circuit (also known as an ASIC for “Application-Specific Integrated Circuit”) or an in situ programmable gate array (also known as an FPGA for “Field-Programmable Gate Array”).

[0105] Several of the variants described above can be combined in a single embodiment.

[0106] Chapter IV: Advantages of the embodiments described:

[0107] Varying the central wavelength λ fi of each bandpass filter Fpi to cancel the DC component of the optical signal reflected by each grating Bmi makes it possible to decrease the width of each passband [λ fi - δfi ; λfi + δfi], compared to the case where the wavelengths λfi are constant. Indeed, when the wavelengths λfi are constant, the width of each passband [λ fi - δfi ; λfi + δfi] must be large enough to always include the peak Pm i even if the wavelength λBi varies depending on the environmental conditions. It is therefore necessary to widen the passband [λfi - δfi ; + δfi] so that even when extreme environmental conditions are encountered, the peak Pm i is always located within this bandwidth. Conversely, when the wavelength is modified according to the continuous component of the optical signal reflected by the grating Bm i, the wavelength moves as the wavelength λBi in response to variations in environmental conditions. Thus, the bandwidth [λ fi - δfi ; λfi + δfi] moves according to the environmental conditions encountered to always maintain the peak Pmi within this bandwidth. Therefore, the width of the bandwidth [λ fi -δ fi ; λ fi + δ fi ] can be much smaller since it does not depend on the extreme environmental conditions that may be encountered. However, being able to reduce the width of the bandwidth [λ fi - δfi ; λfi + δfi] of each Fpi filter makes it possible to reduce the gap between successive Pmi peaks. This therefore makes it possible to increase the density of Bmi networks, i.e. within the same spectral band, it is possible to use a greater number of them.

[0108] The use of a filtering Bragg grating Bf i to produce the filter Fpis simplifies the production of this bandpass filter.

[0109] The use of a transducer Tmi which directly mechanically deforms the network Bfi to vary the wavelength λ fi makes it possible to vary this wavelength λ fi very quickly. This improves the accuracy of the measurement.

[0110] 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 i networks that are more than one kilometer away from the generator 50. The generator therefore makes it possible to interrogate Bm i networks over greater distances than when a coherent laser beam is used to do this.

[0111] Using the Bsi network to generate the source peak Pgi makes it possible to simply generate this source peak.

[0112] The fact that the width of the source peak Pg i is equal to or smaller than the width of the measurement peak Pmi increases the sensitivity of the device and avoids ranges of loss of sensitivity.

[0113] Varying the central wavelength λGi of each grating Bsi to cancel out variations in the DC component of the optical signal reflected by the grating Bmi, allows the apparatus 10 to operate correctly even if the grating Bm i is subjected to environmental conditions which cause its wavelength λBi to vary slowly.

[0114] Using a mechanical transducer Tm i to vary the wavelength λGi allows this central wavelength to be varied very quickly. This improves the measurement accuracy.

[0115] Using the same transducer Tm i to mechanically deform both the Bsi network and the Bfi network simplifies the construction of the device 10.

[0116] Using a piezoelectric material as a transducer simplifies the construction of the servo device.

Claims

Claims 1. Appareil de mesure d’ondes mécaniques hautes fréquences, c’est-à-dire d’ondes mechanical devices with frequencies greater than 1 kHz, this device comprising: - une fibre optique (20) dans laquelle sont réalisés plusieurs réseaux de Bragg de mesure (Bmi) destinés à être exposés aux ondes mécaniques hautes fréquences, each of these measuring Bragg gratings presenting a reflection spectrum which comporte un pic de mesure (Pm i) centré sur une longueur d’onde de mesure (λBi) et whose width defines a reflection range that contains all the wavelengths reflected by this measurement peak, the measurement wavelengths (λBi) 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, - un générateur (50) apte à émettre un signal optique d’excitation dans la fibre optique, le spectre d’émission de ce signal optique d’excitation comprenant autant de pics d’émission (Pgi) qu’il y a de réseaux de Bragg de mesure, chacun de ces pics d’émission se situant, au moins en partie, à l’intérieur de la plage de réflexion d’un pic respective measurement, - 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 filtres passe-bande (Fpi) dans laquelle chaque filtre passe-bande (Fp i) est associé à un réseau de Bragg de mesure (Bmi) respectif, chacun de ces filtres passe-bande comportant une bande passante à – 3 dB centrée sur une longueur d’onde centrale (λ 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 (Pdi) capable of measuring each of the optical signals extracted by the demultiplexer, characterized in that the apparatus comprises a first servo device (130) capable to vary the central wavelength (λfi) of each bandpass filter (Fpi) to annuler les variations de la composante continue du signal optique réfléchi par le Bragg measuring grating with which it is associated.

2. Appareil selon la revendication 1, dans lequel chaque filtre passe-bande (Fp i) comporte un réseau de Bragg de filtrage (Bf i) optiquement raccordé à la fibre optique (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 peak of filtrage (Pfi) centré sur la longueur d’onde centrale (λ fi) de ce filtre passe-bande et la largeur de ce pic de filtrage étant supérieure à la largeur de la plage de réflexion du picmeasurement (Pmi) of the measurement Bragg grating associated with this bandpass filter.

3. Appareil selon la revendication 2, dans lequel le premier dispositif (130) d’asservissement comporte, pour chaque réseau de Bragg de filtrage (Bf i), un transducer (Tmi) capable of mechanically deforming this filtering Bragg grating to modify the central wavelength (λ fi ) of this bandpass filter.

4. Appareil selon l’une quelconque des revendications précédentes, dans lequel le 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 i ), And - un dispositif (66) de filtrage comportant une entrée (68) optiquement raccordée à la output of the optical source and an output optically connected to the optical fiber (20), le spectre de puissance de ce dispositif de filtrage comportant autant de bandes pass-through at – 3 dB than measurement Bragg gratings, each of these bands passantes étant centrée sur un pic d’émission (Pgi) respectif pour former ce pic emission in the optical excitation signal.

5. Appareil selon la revendication 4, dans lequel le dispositif (66) de filtrage comporte autant de réseaux de Bragg source (Bs i) que de réseaux de Bragg de mesure (Bm i ), each source Bragg grating being associated with a measurement Bragg grating respective, the reflection spectrum of each source Bragg grating comprising the emission peak (Pgi) which lies, at least in part, within the reflection range of the measurement peak of the measurement Bragg grating associated with it.

6. Appareil selon la revendication 5, dans lequel la largeur du pic d’émission (Pg i) of each source Bragg grating (Bsi) is equal to or smaller than the measurement peak width of the measurement Bragg grating associated with it.

7. Appareil selon la revendication 5 ou 6, dans lequel chaque pic d’émission (Pg i) est centré sur une longueur d’onde centrale (λGi) et l’appareil comporte un second dispositif d’asservissement apte à faire varier la longueur d’onde centrale (λGi) de each source Bragg grating to cancel variations in the continuous component of the optical signal reflected by the measurement Bragg grating with which it is associated.

8. Appareil selon la revendication 7, dans lequel le second dispositif d’asservissement comporte, pour chaque réseau de Bragg source, un transducteur mechanics capable of mechanically deforming this source Bragg grating to modify the central wavelength (λGi) of this source Bragg grating (Bsi).

9. Appareil selon les revendications 3 et 8 prises ensemble, dans lequel les premier and second servo devices (130) are merged and each transducer (Tmi) est apte à déformer mécaniquement à la fois le réseau de Bragg de filtrage (Bf i) du filtre passe-bande (Fpi) et le réseau de Bragg source (Bsi) qui sont associés au same Bragg measuring grating (Bmi).

10. Appareil selon la revendication 9, dans lequel chaque transducteur (Tm i) est un matériau piézoélectrique sur lequel le réseau de Bragg de filtrage (Bf i) et le réseau deBragg sources (Bsi) are fixed without any degree of freedom.

11. Structure instrumentée comportant : - 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, may appear, - a system (8) for detecting a defect in this structure, this system comprising - un appareil (10) de mesure des ondes mécaniques hautes fréquences se propagating in the structure, this device comprising an optical fiber (20) in the aquelle sont réalisés plusieurs réseaux de Bragg de mesure (Bm i), ces réseaux Bragg measuring beams 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 device, caractérisée en ce que l’appareil (10) de mesure des ondes mécaniques hautes frequencies is in accordance with any one of the preceding claims.

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