Optical device for detecting acoustic waves

Abstract

The present invention provides an apparatus and method for detecting acoustic waves propagating toward a membrane carrying a waveguide including an optical cavity defining a resonant frequency. Under the influence of vibrations of the membrane, the resonant frequency of the optical resonator changes. The apparatus includes a light source for directing an optical wave into the optical cavity and a servo circuit for servo-controlling the wavelength of the optical wave to the resonant wavelength of the optical cavity. By monitoring the change in wavelength of the optical wave, it is possible to estimate the amplitude of the acoustic wave.

Description

[Technical field]

[0001] The technical field of the invention is the detection of acoustic waves using optical resonant cavities. [Background technology]

[0002] Sound waves are usually detected by devices based on MEMS or NEMS electromechanical resonators (MEMS is an acronym for micro-electro mechanical systems and NEMS is an acronym for nano-electro mechanical systems). This type of transducer can be manufactured using wafer-level microfabrication processes. As a result, moderate manufacturing costs can be achieved, making such transducers particularly suitable for use in everyday components. These resonators are based on a membrane that vibrates under the effect of exposure to sound waves, and the vibrations are detected via electrical transduction. The transduction can be achieved via piezoelectric materials or via the capacitive effect.

[0003] However, in certain applications it is advantageous to detect the vibrations of the membrane via optical transduction. In particular, optical transduction can allow remote measurement without contact. This is particularly appropriate in certain environments (subject to stresses of a thermal or electromagnetic nature) where electrical transduction is not possible. Furthermore, optical transduction is suitable for environments where there is a risk of explosion.

[0004] Devices based on photoconversion are described in the following documents: [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2014 / 195372 [Non-patent literature]

[0006] [Non-Patent Document 1] Gallego D. “High-sensitivity ultrasound interferometric single-mode polymer optical fiber sensors for biomedical applications”, Opt Letter, Vol.34, n°12, p. 1807, 2009.06 [Non-Patent Document 2] Chen K. “Fiber-optic Fabry Perot Interferometer based high-sensitive cantilever microphone”, Sense. Actuators Phys., vol.279, p. 107-112, 2018.08 [Non-Patent Document 3] SMLeinders “A sensitive optical micromachined ultrasound sensor (OMUS) based on a silicon photonic ring resonator on a acoustical membrane”, Sci. Rep., vol.5, N°1, 2015.11 Summary of the Invention [Problem to be solved by the invention]

[0007] The inventors have designed a device that is particularly compact, easy to manufacture, and has high detection sensitivity. [Means for solving the problem]

[0008] A first subject of the invention is a device for detecting sound waves, comprising: a membrane configured to vibrate at an acoustic frequency and carrying a waveguide; a first reflector and a second reflector, each reflector reflecting light in a reflection spectral band, the waveguide configured to vibrate with the membrane; the first reflector and the second reflector are spaced apart from one another to form an optical resonant cavity, the optical resonant cavity defining a resonant wavelength in the reflection spectral band; the waveguide transmits light at the resonant wavelength and reflects light in the reflection spectral band but not at the resonant wavelength; The device also a laser light source configured to emit light waves at an emission wavelength into the waveguide; a photodetector positioned to detect light waves transmitted by the waveguide at the resonant wavelength; a servo circuit coupled to the light source and the photodetector and configured to servo the wavelength of the light wave to the resonant wavelength of the optical resonant cavity at various times; a processing unit connected to the servo circuit and configured as follows: determining a periodic time-dependent change in the emission wavelength under the effect of vibration of the membrane, the periodic time-dependent change in the emission wavelength corresponding to a periodic change in the resonant wavelength; An amplitude of an acoustic wave is estimated based on the periodic time-dependent change in the emission wavelength.

[0009] According to one embodiment, each reflector is a Bragg mirror formed by a periodic modulation of the refractive index along the waveguide.

[0010] Preferably, the servo circuit comprises a servo loop connected to the light source and configured to servo the emission wavelength of the light waves emitted by the light source to a resonant wavelength of the optical resonant cavity, the servo circuit being in particular capable of implementing a top-of-fringe locking servo technique.

[0011] Preferably, the laser source emits light waves with an emission spectral band less than 10 pm wide, or less than 1 pm wide, more typically the emission spectral band is narrower than the width of the resonance peak of the optical resonant cavity.

[0012] According to one embodiment, the first reflector is a first Bragg mirror and the second reflector is a second Bragg mirror, the first Bragg mirror and the second Bragg mirror form one and the same Bragg mirror, the latter having a defect, the first Bragg mirror and the second Bragg mirror respectively corresponding to portions of the Bragg mirror located on either side of the defect.

[0013] The membrane, under the effect of vibration, exhibits at least one vibration antinode, the amplitude of said vibration being maximum at each vibration antinode. The waveguide is preferably at the same height as at least one vibration antinode.

[0014] According to one embodiment, the waveguide is formed directly on the membrane, in particular by photo-sculpting with a femtosecond laser.

[0015] According to one embodiment, said waveguide is a microstructured optical fiber deposited on a membrane.

[0016] A second subject of the invention is a method for detecting the amplitude of a sound wave using a device according to the first subject of the invention, comprising: a) vibration of the membrane, which under the effect of the acoustic waves vibrates at a vibration frequency corresponding to the frequency of the acoustic waves, the vibration of the membrane causing a vibration of the waveguide under the effect of the vibration such that the resonant wavelength of the waveguide is periodically modulated; b) activating a laser light source such that the light source emits light waves into the waveguide at the emission wavelength; c) servoing the emission wavelength to the resonant wavelength using a servo circuit such that the emission wavelength varies periodically, the time-dependent oscillations of the emission wavelength corresponding to said periodic modulation of the resonant wavelength at the frequency of the acoustic wave; d) estimating the amplitude of the acoustic wave by a processing unit based on the time-dependent change in the emission wavelength obtained via the servo circuit.

[0017] A third subject of the invention is a method for manufacturing a device according to the first subject of the invention, in which the waveguide is formed directly on the membrane, the method comprising the steps of: depositing a thin layer of a first material onto the film to form a waveguide; The waveguide is photo-sculpted with a femtosecond laser beam to obtain a periodic modulation of the refractive index of the waveguide.

[0018] The invention will be better understood by referring to the figures listed below and reading the description of the exemplary embodiments set forth in the remainder of the description. [Brief description of the drawings]

[0019] [Figure 1A] 1 shows a first example of a device according to the invention. [Figure 1B] 1 shows a first example of a device according to the invention. [Figure 1C] 1 shows a first example of a device according to the invention. [Figure 2A] 1 shows a microstructured optical fiber forming a Bragg mirror. [Figure 2B] 2B shows the reflectance spectral band of the optical fiber shown diagrammatically in FIG. 2A. [Figure 2C] A microstructured optical fiber is shown forming an optical resonant cavity based on two spaced apart Bragg mirrors. [Figure 2D] 2D shows the reflectance spectral band of the optical fiber shown diagrammatically in FIG. 2C. [Figure 3A] 1 shows an unmodified waveguide containing an optical resonant cavity. [Figure 3B] 1 shows a waveguide including a modified optical resonant cavity. [Figure 3C] 1 shows a waveguide with non-uniform deformation. [Figure 3D] 3D shows the change in the reflection spectrum band of a waveguide as shown in FIG. 3C under the effect of deformation. [Figure 3E] The amplitude of radial deformation along the diameter of the membrane is shown. [Figure 3F]Optimally positioned waveguides are shown for the deformation shown in FIG. 3D. [Figure 4A] 1 shows a schematic diagram of a circuit for servo-controlling the wavelength of a light source using the top-of-fringe locking method. [Figure 4B] The change in the error function obtained using the top-of-fringe locking method is shown. [Figure 5A] 1 shows the shift in the resonant wavelength of an optical resonant cavity under the effect of a membrane vibration. [Figure 5B] 1 shows the time-dependent modulation of the optical resonance wavelength. [Figure 5C] 13A-B show schematic diagrams illustrating an estimation of the vibration amplitude of a membrane based on the time-dependent modulation of the resonant wavelength. [Figure 6] The transfer function of the membrane in the operating frequency range is shown. [Figure 7A] 1 illustrates the main steps of the fabrication process that allows contacting the membrane to form a microstructured waveguide. [Figure 7B] 1 illustrates the main steps of the fabrication process that allows contacting the membrane to form a microstructured waveguide. [Figure 7C] 1 illustrates the main steps of the fabrication process that allows contacting the membrane to form a microstructured waveguide. [Figure 7D] 1 illustrates the main steps of the fabrication process that allows contacting the membrane to form a microstructured waveguide. [Figure 8A] 1 shows diagrammatically two possible configurations of a device according to the invention; [Figure 8B] 1 shows diagrammatically two possible configurations of a device according to the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] 1A-1C show an example of a device 1 according to the invention. The device comprises a flexible membrane 2 configured to vibrate when exposed to incident sound waves 5. The frequency of the incident sound waves is included in the range of audible or ultrasonic sound waves. Thus, the membrane 2 can vibrate between 20 Hz and 20 kHz (audible range) and / or in a frequency band above 20 kHz, for example between 20 kHz and several tens of MHz.

[0021] In the illustrated example, the membrane 2 is aligned in a radial plane P perpendicular to the transverse axis Z. XY It's inside.

[0022] The membrane 2 is connected via its periphery to a substrate 3 forming the body of the device. The diameter Φ of the membrane, or its longest diagonal, may be between 1 mm and 10 mm. The thickness of the membrane along the horizontal axis is between 10 μm and a few mm, depending on the radius r or diameter Φ of the membrane. The thickness is a few thousandths of the radius (10 -3 ) and 1 / 10(10 -1 ), for example, it can be equal to 1 / 100 of the radius.

[0023] The membrane 2 preferably comprises openings 6 which extend parallel to the transverse axis Z. Such openings make it possible to balance the pressures on both sides of the membrane. They prevent the pressure difference on both sides of the membrane from appearing slowly, which would cause the membrane to deform. By slowly appearing we mean a pressure difference which occurs at a frequency lower than the operating frequency range of the membrane. The diameter of the openings 6 is, for example, less than 1 / 10 of the diameter of the membrane. For example, it is of the order of 10 μm, 20 μm.

[0024] The waveguide 20 contacts the membrane 2 and extends parallel to the membrane 2. The waveguide has an inlet 20 i and Exit 20 o The waveguide has a first refractive index n1 The first material 21 has a first thickness.

[0025] The waveguide 20 may be an optical fiber, in which case the first material is the material of the core of the optical fiber. It may also be a matter of a waveguide formed from the deposition of a thin layer of a first material 21, for example SiON (silicon oxynitride), which corresponds to the example shown in Figures 1A to 1C. The waveguide is surrounded by a confining material 23 whose refractive index is lower than the refractive index n3 of the first material n1. If the waveguide 20 is an optical fiber, the confining material 23 is the cladding of the optical fiber.

[0026] The waveguide 20 is advantageously formed from a thin layer of a first material 21. The confining material 23 may simply be air surrounding the first material. The thickness along the transverse axis Z of the waveguide is preferably less than 10 μm or 5 μm. A process for forming such a waveguide is described with reference to Figures 7A to 7D. By forming the waveguide directly on the membrane, a step of bonding an optical fiber to the membrane can be avoided. Another advantage of the use of optical fibers is that a less rigid waveguide is obtained.

[0027] Regardless of the configuration chosen, the refractive index n3 of the encapsulation material 23 is lower than the refractive index n1 of the first material 21. If the first material 21 is deposited directly on the membrane, then the refractive index n1 of the first material 21 is higher than the refractive index of the material from which the membrane 2 is formed.

[0028] An example of a waveguide 20 is shown in FIG. 1B. In this example, a first material 21 is deposited on a film 2. The waveguide comprises segments of a second material 22 of a second refractive index n2, which are periodically distributed along the waveguide 20. The second refractive index n2 is different from the first refractive index n1. The relative change between the first and second refractive indices is less than 0.01% (10 -4 ) to 0.1% (10 -3 )

[0029] Along the axis along which the waveguide extends, the refractive index is periodically modulated between n1 and n2, resulting in a reflection spectral band Δλ 20The structure of the Bragg mirror is known to those skilled in the art and is a structure in which the refractive index changes periodically, where two segments of different refractive indexes are formed alternately along the axis along which light propagates, and the optical thickness of each segment is λ B / 4n i and the optical thickness of each segment is λ B is the reflection spectrum band Δλ 20 is the central wavelength of i is the refractive index of the material (n i = n1 or n i = n2). The lower the refractive index contrast, the higher the number of periods.

[0030] Reflectance spectrum band Δλ 20 is the Bragg wavelength λ B Also called the resonant wavelength λ r It is centered on the following: TIFF0007657228000001.tif651, where n eff is the effective index of the lattice, which is given by: TIFF0007657228000002.tif1042Λ is the spatial period of the grating, i.e. the length of two successive segments 21 and 22 along the axis of the waveguide.

[0031] The waveguide 20 is such that the Bragg mirror, formed of alternating segments 21 and 22, contains a defect. A defect means a local break in the periodicity of the refractive index modulation. The defect is for example a continuous defect 25 of a given material, for example the first material 21, extending the length of one period Λ or the length of several consecutive periods. See FIG. 1B. At the level with the defect, the waveguide contains a single material and extends a distance along the axis of the waveguide 20. The distance d is such that d=kλ B / n eff (1') (k is a positive natural integer), the resonant wavelength λ r If the defect 25 spans a single period Λ, then λ r = λ B It is.

[0032] Also, d>kλ B / n eff If the reflectance spectrum band Δλ 20 is the Bragg wavelength λ B Other resonant wavelengths λ r In this case, it is preferable to keep the resonance wavelength at which the resonance peak is narrowest.

[0033] This allows separation of the first Bragg mirror 241 and the second Bragg mirror 242 in the waveguide 20. The assembly formed by the first Bragg mirror 241, the second Bragg mirror 242 and the defect 25 between the Bragg mirrors forms a resonant cavity 26.

[0034] The waveguide is then configured as follows: The reflection spectral band Δλ of the Bragg mirrors 241, 242 20 Reflects light with a resonant wavelength of λ r The resonant cavity 26 does not reflect light having a resonant wavelength λ r It allows light to pass through.

[0035] The device 1 also includes an inlet 20 of a waveguide 20 i The light source 10, in particular a laser diode, is arranged to emit an incident light wave 12 towards the 12 Emission spectrum band Δλ centered at 12 The emission spectrum band Δλ 12 is the reflection spectrum band Δλ 20 Preferably, the resonant wavelength λ r The emission spectrum band Δλ is narrower than the width of the resonance peak associated with 12 Since the width of is narrower than the width of the resonance peak, the emission wavelength λ 12 can be accurately adjusted to the resonant wavelength of the waveguide. This adjustment is performed by a servo circuit 41, which will be described later.

[0036] Emission spectrum band Δλ 12The width of the emission spectrum band Δλ is preferably less than 10 pm or 1 pm. 12 The width of refers to the full width at half maximum of the emission spectral band. The light source 10 is preferably a continuous wave laser. It may for example be a DFB laser diode (DFB is the acronym for distributed feedback) with a power of 1 mW, emitting at a wavelength of 1.55 μm and having a spectral width of the order of 1 pm. This type of laser diode is commonly used in the field of telecommunications.

[0037] The device 1 includes a photodetector 16, preferably a high-speed photodetector, here a photodiode. The photodetector detects a reflection spectral band Δλ 20 Detection spectral band Δλ including 16 has.

[0038] The device 1 is configured to have a resonant wavelength λ of the resonant cavity 26 under the effect of the vibration of the membrane. r The time-dependent change in λ r (t) of the light wave 12 emitted by the light source 10. Such a circuit is described below with reference to Figures 4A and 4B. The servo circuit 41 is arranged to servo-control the light source 10 to follow the emission wavelength λ of the light wave 12 emitted by the light source. 12 is the resonant wavelength λ of the resonant cavity 26 r Make it correspond to.

[0039] The device includes a processing unit 42 configured to estimate the amplitude of the sound wave 5 under the effect of the vibration of the membrane 2. The emission wavelength λ determined by the servo circuit 41 12 The time-dependent change in λ 12 The amplitude of the sound wave is estimated based on (t). The operation of the processing unit 42 is described in further detail with reference to FIG.

[0040] The device comprises a cover 8 defining a rear volume, the rear volume corresponding to the volume extending between the membrane 2 and the cover 8 .

[0041] In general, the waveguide 20 includes a resonant cavity 26 formed from a first reflector 241 and a second reflector 242, which are obtained by microstructuring the waveguide 20 to induce a periodic variation in the refractive index.

[0042] An important aspect of the present invention relates to the fact that the light source 10 is activated and its emission wavelength λ 12 is the resonant wavelength of the waveguide 20 (more precisely, the resonant wavelength of the resonant cavity 26), λ r , the waveguide 20 will reflect a reflected wave 12′. However, when the light source 10 is activated and the resonant wavelength λ of the resonant cavity 26 is r The emission wavelength λ 12 When the lightwave 12 having an emission wavelength λ is emitted, the waveguide 20 transmits the transmitted wave 14 to the photodetector 16. 12 is the resonant wavelength λ r The closer it is to , the higher the intensity of the transmitted wave 14 becomes.

[0043] The present invention provides an acoustic amplitude A a When exposed to sound wave 5 of frequency f a As a result, the waveguide 20 is periodically deformed, and under the influence of the deformation, the resonant wavelength λ r is the periodic time-dependent change λ r (t) shows the amplitude of the time-dependent modulation A λr depends on the amplitude of the membrane vibration, which is the acoustic amplitude A a For example, the acoustic amplitude A a Thus, the amplitude A λr By estimating the acoustic amplitude A a can be estimated.

[0044] FIG. 1C shows the radial plane P XY In this example, the membrane 2 takes the form of a thin disk with a thickness of 1 / 100th of the radius.

[0045] 2A shows an embodiment in which the waveguide 20 is a microstructured optical fiber in which a Bragg grating is formed. This type of microstructure in an optical fiber is usually called a fiber Bragg grating (FBG). The optical fiber comprises a first material 21 forming the core and an encapsulation material 23 forming the cladding. In the core of the optical fiber, an inclusion or cavity of a second material 22, the refractive index of which is different from that of the first material, is formed.

[0046] FIG. 2B shows the reflection spectrum of such a finely divided optical fiber. The reflection spectrum corresponds to the reflection intensity normalized by the illumination intensity (y-axis) as a function of wavelength (x-axis - in nm). The reflection spectrum band Δλ 20 Such a waveguide, when illuminated with a light wave 12, has a reflection spectral band Δλ 20 So, the reflection spectrum band Δλ 20 Overall, it reflects light wave 12'.

[0047] 2C shows a similar optical fiber in which two Bragg mirrors 241 and 242 are separated by a defect 25 filled with a first material 21, as described with reference to FIG. 1B. The length of the defect 25 is such that the resonant wavelength k / n falls within the reflection spectral band. eff , the optical fiber includes a resonant cavity 26.

[0048] FIG. 2D shows the reflection spectrum of the optical fiber thus miniaturized. r Except for the reflectance spectrum band Δλ 20 When illuminated with a light wave 12, such a waveguide has a reflection spectral band Δλ 20 At wavelength λ 12 is different from the resonant wavelength, the light wave 12' is reflected and the wavelength λ 12 is located at a resonance peak, it transmits a light wave 14, called the transmitted light wave.

[0049] 2C and 2D were obtained using a model written in Matlab® (Mathworks), in which the structure is considered to extend along a length L of 3 mm, with a refractive index contrast between the first and second materials 21, 22 of 10 -3 and the period of each Bragg mirror was taken to be about 0.5 μm. Each Bragg mirror therefore has a number of periods equal to 3000.

[0050] 3A to 3C show the resonant wavelength λ resulting from the deformation of the waveguide 20 as shown in FIG. 1B or FIG. 2C. r 3A and 3B show the waveguide 20 in its undeformed and deformed states, respectively. Under the effect of the deformation, the spatial period of the refractive index modulation changes from Λ to Λ'=Λ+dΛ. Applying equation (1), the Bragg wavelength λ B Shift in dλB which broadens the reflection spectral band Δλ. λB The shift is as follows: TIFF0007657228000003.tif1377Here, ε is 10 -4 μ equivalent to % ε (microstrain: strain on the order of micrometers), and the deformation ε is the normalized variation in length, as follows: TIFF0007657228000004.tif1243Note that × is the multiplication operator.

[0051] Equation (3) is for the case where the film is SiO2 and the exponential jump between the first and second materials is 10 -3 This is based on the assumption that each Bragg mirror is uniformly deformed, as shown in FIG. 3B. According to equation (3), 1μ ε For the deformation of B Shift dλ in B is 1.2 pm.

[0052] In FIG. 3C, non-uniform deformation of the waveguide 20 is shown, with some segments of the Bragg mirror being less deformed than other segments.

[0053] FIG. 3D is a model showing the change in the reflection spectrum of the Bragg mirror in a configuration as described with reference to FIG. 3C. Curves a, b, c correspond, respectively, to no deformation, a linear deformation comprised between 0 and 10 microstrain along the axis of the waveguide, and a linear deformation comprised between 4 and 6 microstrain along the axis of the waveguide. The spectral shift is small, less than 10 pm. Curves b and c correspond to the same average deformation of the waveguide, equal to 5 microstrain. The shift of the resonant wavelength between these two configurations is due to the variation of the strain along the axis of the waveguide and ranges from 0 to 10 microstrain and from 4 to 6 microstrain, respectively. The more uniform the deformation, the greater the spectral shift of the resonant wavelength under the effect of the deformation.

[0054] Preferably, the waveguide 20 extends over the part of the membrane 2 that undergoes the greatest deformation. The membrane 2 exhibits one or more vibration antinodes where the amplitude of the vibration is maximum. Each antinode can be determined by modeling and / or experiment. Preferably, the waveguide 20 extends beyond at least one vibration antinode of the membrane's vibration. This maximizes the deformation of the waveguide 20, further increasing the spectral shift due to the deformation. In this way, better sensitivity is obtained.

[0055] We have modelled the deformation of a membrane 2 as shown diagrammatically in Figures 1A and 1C. The modelled membrane is made of SiO2 with a radius of 1 mm, a thickness of 10 μm and is subjected to a pressure of 1 Pa. The deformation along one of the membrane's diameters is shown in Figure 3E. The x-axis corresponds to the distance (mm) from the centre of the membrane and the y-axis corresponds to the deformation in microstrain. The resonant cavity 26 is preferably placed at the level with the largest amplitude of deformation, i.e. at the centre of the membrane 2. The simulation shown in Figure 3E shows that for this membrane, a pressure of 1 Pa causes a deformation of about 10 -2 This shows that microstrain deformation is induced.

[0056] Moreover, in order to achieve a deformation that is as uniform as possible, the resonant cavity 26 is preferably located in a portion of the membrane whose curvature is as uniform as possible under the effect of the deformation, in other words, it is a matter of a portion of the membrane whose derivative of the curvature is low.

[0057] In Figure 3E, the deformation of the membrane is c Negative, peripheral part 2 p The resonant cavity 26 is, as is merit, arranged on a segment of the membrane 2 which, under the effect of the vibrations of the membrane, undergoes deformations of the same sign, whether it is a matter of compression or expansion.

[0058] FIG. 3F shows that the resonant cavity 26 is located in the center 2 of the membrane 2. c 3E. Under the effect of the vibrations of the membrane, the deformation becomes alternately negative (ε<0) and then positive as shown in FIG. 3E. When the deformation is negative, the optical cavity is compressed and the segments of the second material 22 move closer to each other. When the deformation is positive, the optical cavity is expanded and the segments of the second material 22 move away from each other.

[0059] 4A and 4B show the operation of the servo circuit 41 in a simplified manner, the function of which is to regulate the emission wavelength λ 12 is the resonant wavelength λ of the resonant cavity 26 formed in the waveguide 20. r The servo circuit 41 controls the wavelength λ 12 is the resonant wavelength λ rThis is done using a top-of-fringe lock. For example, this is a matter of circuits using the Pound-Dever-Hall servo technique, such circuits being described in the publications of Chow JH “Phase-sensitive interrogation of fiber Bragg grating resonators for sensing applications”, J.Light Technol., vol.23, No.5, pp.1881-1889, May 2005, or Black E. “An introduction to Pound-Dever-Hall laser frequency stabilization”, Am.J.Phys. 69(1), January 2001.

[0060] The servo circuit 41 detects the wavelength λ of the light wave 12 emitted by the light source 10. 12 The modulator 411 includes a tunable filter 412 for modulating the emission wavelength λ at a high modulation frequency that can vary from 10 kHz to several hundred MHz. 12 The modulation frequency of is much higher than the maximum acoustic frequency addressed by the device, for example, it may be at least ten times higher than the maximum acoustic frequency addressed by the device. The intensity of light wave 14 transmitted through waveguide 20 and detected by photodetector 16 is sent to circuitry 41, which measures a function h that represents the change in intensity detected by photodetector 16 as a function of wavelength modulation.

[0061] Depending on the sign of the function h, an error signal is sent to the light source, which determines the emission wavelength λ 12 increases or decreases. For example, if the change in detected intensity with increasing wavelength is negative, the emission wavelength gradually decreases. If the change in detected intensity with increasing wavelength is positive, the emission wavelength increases. If the change in detected intensity due to modulation is close to zero, the emission wavelength corresponds to the resonant wavelength of the waveguide. The servo circuit 41 uses the following fact: λ 12 <λ r In the case of 12 As the wavelength λ increases, the intensity of the transmitted wave 14 increases. 12A decrease in the intensity of the transmitted wave 14 results. λ 12 >λ r In the case of 12 As the wavelength λ increases, the intensity of the transmitted wave 14 decreases. 12 A decrease in results in an increase in the intensity of the transmitted wave 14.

[0062] Therefore, the wavelength λ of the emitted light wave 12 12 and observing the effect of the modulation on the intensity of the transmitted light wave 14, the light source 10 can be servo-controlled to change the emitted wavelength λ 12 The resonant wavelength λ of the waveguide 20 r can be made to follow.

[0063] By tracking the resonance wavelength by the top-of-fringe lock, when the acoustic frequency of the sound wave 5 is higher than 10 kHz, -6 pm, and 10 for acoustic frequencies below 1 kHz. -3 It is possible to track the resonant wavelength with a wavelength sensitivity on the order of pm. Considering the formula (3) applied to SiO2 films, it is believed that such sensitivity can estimate film deformations on the order of several pico strains, or several mPa. Therefore, the Pound-Dever-Hall method is an appropriate method considering that the spectral shift of the resonant cavity 26 is small, probably on the order of several pm.

[0064] Furthermore, tracking of the resonance wavelength by top-of-fringe locking can make tracking of the resonance wavelength of the cavity 26 less sensitive to variations in environmental parameters such as temperature and humidity.

[0065] 5A to 5C show the emission wavelength λ when the membrane 2 vibrates under the influence of the sound wave 5. 12 Periodic time-dependent change in λ 12 Due to the servo control performed by the servo circuit 41, the emission wavelength λ 12The periodic time-dependent fluctuations in (t) are due to the resonant wavelength λ caused by the membrane vibration. r 5A shows the spectrum of the transmitted light wave 14 under the effect of the deformation of the waveguide 20 and the spectral shift dλ. r FIG. 5B shows the resonant wavelength λ resulting from the deformation of the waveguide 20. r shows a time-dependent modulation of the acoustic frequency f a The frequency fλ corresponds to r The servo circuit 41 modulates the emission wavelength λ 12 is the resonant wavelength λ r The processing unit 42 determines such modulation by analysing the periodic variation of the emission wavelength and determining the frequency f a The amplitude Aλ of the modulation of the resonance wavelength (or emission wavelength) at r The processing unit 42 is configured to estimate the amplitude Aλ. r Based on this, the amplitude A of sound wave 5 a Estimate.

[0066] Amplitude Aλ of the modulation of the resonance wavelength r Sound amplitude A based on a An estimate of can be determined via simulation and / or experimental calibration.

[0067] Sound Amplitude A a The determination of does not necessarily require the determination of the value of the resonant wavelength, but the amplitude Aλ of the modulation. r Note that we only need to determine accurately

[0068] FIG. 6 corresponds to the selection of the operating frequency range of the device 1. It shows the amplitude of the deformation of the membrane (y-axis, in dBV / Pa) as a function of the acoustic frequency (x-axis, in Hz). The operating frequency range of the device corresponds to the flattest part of the curve. The operating frequency range is determined by the lower cut-off frequency f low and the mechanical resonance frequency f res In this example, the frequency range is chosen to cover audible sound waves between 100 Hz and 20 kHz.

[0069] The lower cutoff frequency is: TIFF0007657228000005.tif1345 where R quantifies the viscous loss through the opening 6 in the membrane 2 and C 8 corresponds to the compressibility of the rear volume of the device as defined with reference to FIG. 1A. R 6 is expressed as follows: TIFF0007657228000006.tif1252 where N corresponds to the number of openings 6 in the membrane, η is the viscosity of air (Pa·s), e is the thickness of the membrane, and s6 is the cross-sectional area of ​​the opening 6, the cross-section of which is taken along the radial plane P XY is cut parallel to the Furthermore, C8 is expressed as follows: TIFF0007657228000007.tif1336Where, V8 is the rear volume (cm 3 ), and c0 is the speed of sound in air (ms -1 ) and ρ0 is the density of air (g cm -3 ). The resonant frequency of the membrane can be calculated by the following formula: TIFF0007657228000008.tif1881 where r is the radius of the membrane 2 (cm), E is the Young's modulus of the material on which the membrane is formed, and ρ is the density of the material on which the membrane is formed (g cm -3 ) and ν is the Poisson's ratio of the material forming the membrane.

[0070] It is therefore possible to select the operating frequency range as a function of the parameters defined above so as to cover all or part of the audible or ultrasonic range. The greater the thickness of the membrane and the smaller the diameter, the lower the resonant frequency f res Conversely, the thinner the membrane and the larger the diameter, the higher the resonant frequency f res will be lower.

[0071] 7A to 7D show the main steps making it possible to form a non-optical fiber waveguide 20 on the membrane 2. FIG.

[0072] The substrate 3 is, for example, a Si substrate, on which a first layer 31 made of, for example, 4 μm-thick SiO2 (refractive index: 1.44) and a second layer 32 made of, for example, 1 μm-thick SiON (silicon oxynitride, refractive index: 1.60) are deposited (see FIG. 7A).

[0073] The method includes the following: Photolithography is used to etch the second layer 32 to form the waveguide 20 (see FIG. 7B). In this example, SiON corresponds to the first material 21 of the waveguide. The rear surface of the substrate 3 is etched so as to peel off a portion of the first layer 31, forming a floating film 2 in that portion (see FIG. 7C). It is exposed to femtosecond laser pulses in order to form cavities in the second material 22 (see FIG. 7D). Under the effect of the laser irradiation, the index of the SiON changes locally. The exposed SiON corresponds to the second material 22 and its refractive index n2 differs from that of the unexposed SiON. In particular, the exposure generates microbubbles, which cause a change in the refractive index. This exposure is performed so as to obtain a regular and periodic distribution of cavities 22. As a result, the refractive index of the waveguide 20 is modulated in the waveguide along the axis of propagation of the light. The duration of each pulse is, for example, equal to 100 fs at a wavelength of 800 nm and the energy of each pulse is 30 nJ. The pulse frequency can be configured between a few Hz and 200 kHz. Another exposure technique is UV photoengraving, which is described in the publication of Chow JH “Phase-sensitive interrogation of fiber Bragg grating resonators for sensing applications”, J.Light.Technol., vol.23, No.5, pp.1881-1889, May 2005. For example, UV photoengraving makes it possible to microstructure optical fibers.

[0074] The refractive index modulation due to exposure is 10 -3However, photo-sculpting with femtosecond lasers makes it possible to produce Bragg mirrors with lengths as short as 1 mm. This type of exposure makes it possible to obtain highly precise resonant cavities 26, with widths of the resonant peaks less than a few tens of pm, or even less than 10 pm, and in some cases less than 5 pm.

[0075] It is also possible to increase the length that each Bragg mirror extends, which further reduces the width of the resonance peak.

[0076] Figures 8A and 8B show an example of the device. In Figure 8A, the device 1 is placed on a carrier 7 closed by a cover 8. A laser source 10 is arranged on the emitting surface P XY 1, which is reflected by a reflector 11 towards the membrane 2. An input coupling grating 13 couples the light wave 12 to the entrance 20 of the waveguide 20. i The output coupling grating 15 allows directing all or part of the transmitted light wave 14 to a photodetector 16.

[0077] In Fig. 8B the device is shown in which the active optical components (light source 10, photodetector 16, servo circuit 41 and processing unit 42) are remotely located in a peripheral part 9 attached to a cover 8. The deformation sensitive elements, i.e. the waveguide 20 and the membrane 2, are enclosed in a casing formed from a carrier 7 closed by a cover. The cover 8 comprises a transparent part for transmitting the outgoing light 12 and the transmitted light 14.

[0078] The invention may be used to create miniature detection devices, the main application being the detection of sound waves in the audible and ultrasonic ranges.

Claims

1. A device (1) for detecting sound waves (5), comprising: The frequency of the sound wave (f ａ a membrane (2) configured to vibrate at a wavelength of 100 nm and carrying a waveguide (20); The first reflector (24 １ ) and a second reflector (24 ２ ), each of the reflectors having a reflectance spectral band (Δλ ２０ and the waveguide configured to reflect light of the first order (a) and vibrate with the membrane; The first reflector and the second reflector are spaced apart from one another to form an optical resonant cavity (26), the optical resonant cavity having a reflectance spectral band (Δλ ２０ ) at the resonant wavelength (λ ｒ ) The waveguide has a resonant wavelength (λ ｒ ) and transmits light in the reflection spectrum band (Δλ ２０ ) reflects light other than the resonant wavelength (λ r ); The apparatus further comprises: The light emitting wavelength (λ １２ a laser light source (10) configured to emit a light wave (12) of The resonant wavelength (λ ｒ a photodetector (16) configured to detect light waves (14) transmitted by the waveguide at A laser light source (10) and a photodetector (16) are connected to the laser light source (10) and the photodetector (16), and the wavelength (λ １２ ) to the resonant wavelength (λ) of the optical resonant cavity (26). ｒ a servo circuit (41) configured to servo-control the a processing unit (42) connected to the servo circuit and configured as follows: The emission wavelength (λ １２ ) a periodic time-dependent change in the emission wavelength corresponding to a periodic change in the resonant wavelength; estimating an amplitude of an acoustic wave based on the periodic time-dependent change in the emission wavelength; Where: The waveguide (20) is formed directly on the membrane (2), the membrane exhibits at least one vibration antinode under the effect of vibration, the amplitude of the vibration being maximum at each vibration antinode; The waveguide (20) is at the same height as at least one vibration antinode. Device.

2. Each of the reflectors (24 １ , 24 ２ ) is the refractive index along the waveguide (n １ , n ２ ) is a Bragg mirror formed by periodic modulation of 2. The apparatus of claim 1.

3. The servo circuit (41) is connected to the laser light source (10) and controls the wavelength (λ) of the light wave emitted by the laser light source. １２ ) to the resonant wavelength (λ ｒ ) a servo loop configured to servo 2. The apparatus of claim 1.

4. The servo circuit (41) implements a top-of-fringe lock servo technique.

4. The apparatus of claim 3.

5. The laser light source has an emission spectral band (Δλ １２ emitting said light waves at 2. The apparatus of claim 1.

6. The first reflector (24 １ ) is the first Bragg mirror, The second reflector (24 ２ ) is the second Bragg mirror, the first Bragg mirror and the second Bragg mirror form the optical resonant cavity, the optical resonant cavity having a defect (25), the first Bragg mirror and the second Bragg mirror each corresponding to a portion of the optical resonant cavity located on either side of the defect; 2. The apparatus of claim 1.

7. The waveguide is formed by etching a thin layer; said thin layer having been previously deposited on a first layer forming said membrane, An apparatus according to any one of claims 1 to 6.

8. The amplitude (A) of the sound wave (5) can be measured using the device (1) according to claim 1. ａ ), comprising the steps of: a) the vibration of the membrane (2), which, under the effect of the sound waves, vibrates at the frequency of the sound waves (f ａ ) at a vibration frequency corresponding to the resonant wavelength of the waveguide (20), the vibration of the membrane causing the waveguide to vibrate under the effect of the vibration such that the resonant wavelength of the waveguide is periodically modulated; b) the laser light source emits light having the emission wavelength (λ １２ activating the laser light source (10) to launch the light wave into the waveguide at c) Using the servo circuit (41), the emission wavelength (λ １２ The emission wavelength (λ) is changed periodically. １２ ) to the resonant wavelength (λ ｒ ) to control the time-dependent oscillation of the emission wavelength (λ １２ (t)) is the frequency of the sound wave (f ａ ) corresponds to the periodic modulation of the resonant wavelength (λr(t)) at d) estimating the amplitude of the sound wave by a processing unit (42) based on the time-dependent change in the emission wavelength obtained via the servo circuit (41); method.

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