Optical device for detecting acoustic waves

Abstract

The invention is a device and a method for detecting an acoustic wave propagating towards a membrane, the membrane carrying a waveguide comprising an optical cavity, the optical cavity defining a resonance frequency. The resonance frequency of the optical cavity is changed under the effect of the membrane vibration. The device comprises a light source for directing a light wave into the optical cavity, and a servo circuit for servo-controlling the wavelength of the light wave to the resonance wavelength of the optical cavity. Monitoring the change in wavelength of the light wave allows to estimate the amplitude of the acoustic wave.

Description

Technical Field

[0001] The technical field of this invention is the use of optical resonant cavities to detect sound waves. Background Technology

[0002] Sound waves are typically detected using devices based on MEMS or NEMS electromechanical resonators (MEMS stands for Microelectromechanical Systems, and NEMS stands for Nanoelectromechanical Systems). This type of transducer can be fabricated using wafer-level microfabrication processes. This results in modest manufacturing costs, making these transducers particularly suitable for everyday components. These resonators are based on a diaphragm that vibrates when exposed to sound waves, and this vibration is detected by electroconversion. This conversion can be achieved using piezoelectric materials or via the capacitive effect.

[0003] However, in certain applications, detecting membrane vibrations via optical conversion is advantageous. Specifically, optical conversion allows for remote measurement without contact. This is particularly suitable in environments where electrical conversion is not feasible (e.g., under thermal or electromagnetic stress). Furthermore, optical conversion is suitable for environments with explosion risks.

[0004] Devices based on optical conversion have been described in publications:

[0005] -Gallego D. "High-sensitivity ultrasound interferometric single-mode polymer optical fiber sensors for biomedical applications", Opt Letter, Vol. 34, No. 12, p. 1807, June 2009;

[0006] -Chen K. "Fiber-optic Fabry Perot interferometer based high sensitive cantilever microphone", Sens. Actuators Phys., Vol. 279, pp. 107-112, August 2018;

[0007] -WO 2014 / 195372;

[0008] -SMLeinders "A sensitive optical micro-machined ultrasound sensor (OMUS) based on a silicon photonic ring resonator on an acoustical membrane", Sci.Rep, Volume 5, Issue 1, November 2015.

[0009] The inventors designed a particularly compact, simple to manufacture and highly sensitive detection device. Summary of the Invention

[0010] The first subject of this invention is a device for detecting sound waves, comprising:

[0011] - A membrane carrying a waveguide, which is configured to vibrate at the frequency of sound waves;

[0012] - The waveguide includes a first reflector and a second reflector, each of which reflects light in the reflected spectral band, and the waveguide is configured to vibrate together with the membrane;

[0013] - The first reflector and the second reflector are spaced apart from each other to form an optical resonant cavity that defines the resonant wavelength in the reflection spectral band;

[0014] -This makes the waveguide:

[0015] • Transmits light at its resonant wavelength;

[0016] • The reflection spectrum includes light other than the resonant wavelength;

[0017] The device also includes:

[0018] - A laser source configured to emit light waves into a waveguide at an emission wavelength;

[0019] - A photodetector, which is configured to detect light waves transmitted by a waveguide at a resonant wavelength;

[0020] - A servo circuit, which is connected to the light source and the photodetector, and is configured to servo control the wavelength of the emitted wave to the resonant wavelength of the optical resonant cavity at different times.

[0021] - A processing unit, which is connected to the servo circuit and is configured as follows:

[0022] • Determine the periodic time-dependent change in the emission wavelength under the action of membrane vibration, which corresponds to the periodic change in the resonant wavelength;

[0023] • The amplitude of the sound wave is estimated based on the periodic time-dependent variation of the emitted wavelength.

[0024] According to one embodiment, each reflector is a Bragg mirror, which is formed by periodic modulation of the refractive index along the waveguide.

[0025] Preferably, the servo circuit includes a servo loop connected to the light source and configured to servo-control the emission wavelength of the light wave emitted by the light source to the resonant wavelength of the optical resonant cavity. The servo circuit can particularly implement edge-top locking servo technology.

[0026] Preferably, the laser source emits light waves with a spectral width of less than 10 pm or even less than 1 pm. More generally, the emission spectral width is narrower than the resonant peak of the optical resonant cavity.

[0027] According to one embodiment:

[0028] -The first reflector is the first Bragg mirror;

[0029] -The second reflector is a second Bragg mirror;

[0030] - The first and second Bragg mirrors form the same Bragg mirror including the defect, and the first and second Bragg mirrors respectively correspond to the portions of the Bragg mirror located on either side of the defect.

[0031] Under vibration, the membrane exhibits at least one antinode, with the vibration amplitude being maximum at each antinode. The waveguide is preferably flush with at least one antinode.

[0032] According to one embodiment, the waveguide is formed directly on the film. The waveguide can be formed, in particular, by femtosecond laser writing.

[0033] According to one embodiment, the waveguide is a microstructured optical fiber deposited on a film.

[0034] A second aspect of the present invention is a method for detecting sound wave amplitude using an apparatus according to the first aspect of the present invention, the method comprising:

[0035] a) The membrane vibrates at a frequency corresponding to the frequency of the sound wave under the action of the sound wave. The vibration of the membrane causes the waveguide to vibrate. Under the action of this vibration, the resonant wavelength of the waveguide is periodically modulated.

[0036] b) Activate the laser source so that the source emits light waves into the waveguide at the emission wavelength;

[0037] c) Using a servo circuit, the transmission wavelength is servo controlled to the resonant wavelength, so that the transmission wavelength changes periodically, and the time-dependent change of the transmission wavelength corresponds to the periodic modulation of the resonant wavelength at the sound wave frequency.

[0038] d) The amplitude of the sound wave is estimated by the processing unit based on the time-related variation of the transmitted wavelength obtained via the servo circuit.

[0039] A third aspect of the present invention is a method for producing an apparatus according to a first aspect of the present invention, the apparatus such that a waveguide is formed directly on a film, the method comprising:

[0040] A thin layer of the first material is deposited on the film to form a waveguide;

[0041] Waveguides are etched using femtosecond laser beams to obtain periodic modulation of the waveguide's refractive index.

[0042] The invention will be better understood by reading the description of exemplary embodiments, which will be described in the remainder of the description with reference to the accompanying drawings listed below. Attached Figure Description

[0043] Figures 1A to 1C A first example of a device according to the invention is shown.

[0044] Figure 2A The microstructure of the optical fiber forming the Bragg mirror is shown.

[0045] Figure 2B It shows Figure 2A The reflection spectral bands of the optical fiber are schematically shown in the diagram.

[0046] Figure 2C The microstructured optical fiber is shown, which is based on two spaced-apart Bragg mirrors forming an optical resonant cavity.

[0047] Figure 2D It shows Figure 2C The reflection spectral bands of the optical fiber are schematically shown in the diagram.

[0048] Figure 3A A waveguide including an optical resonant cavity is shown, which is not deformed.

[0049] Figure 3B A waveguide including a deformable optical resonant cavity is shown.

[0050] Figure 3C The waveguide exhibits non-uniform deformation.

[0051] Figure 3D This shows that under the action of deformation, such as Figure 3C The waveguide's reflection spectral bands are shown.

[0052] Figure 3E The radial deformation amplitude of the membrane along its diameter direction is shown.

[0053] Figure 3F It shows that for Figure 3DThe waveguide with the best-positioned deformation is shown.

[0054] Figure 4A The circuit schematically illustrates a method for servo-controlling the wavelength of a light source using an edge-top locking approach.

[0055] Figure 4B The variation of the error function obtained using the edge-top locking method is shown.

[0056] Figure 5A This shows the shift in the resonant wavelength of the optical resonant cavity under the influence of membrane vibration.

[0057] Figure 5B The time-correlated modulation of the optical resonant wavelength is shown.

[0058] Figure 5C The estimation of the vibration amplitude of the membrane based on time-correlated modulation of the resonant wavelength is schematically shown.

[0059] Figure 6 The transfer function of the membrane in the operating frequency band is shown.

[0060] Figures 7A to 7D The main steps of the manufacturing process are shown, which allows the microstructured waveguide to be formed in contact with the film.

[0061] Figure 8A and Figure 8B Two possible configurations of the device according to the invention are illustrated schematically. Detailed Implementation

[0062] Figures 1A to 1C An example of a device 1 according to the invention is shown. The device includes a flexible diaphragm 2 configured to vibrate when exposed to an incident sound wave 5. The frequency of the incident sound wave can include the range of audible or ultrasonic waves. Thus, the diaphragm 2 can vibrate in the frequency band of 20 Hz to 20 kHz (audible range) and / or beyond 20 kHz, for example, from 20 kHz to tens of MHz.

[0063] In the example shown, membrane 2 is located in a radial plane P perpendicular to the transverse axis Z. XY middle.

[0064] Membrane 2 is connected to substrate 3 via its periphery, and substrate 3 forms the body of the device. The diameter φ of the membrane or its longest diagonal can be from 1 mm to 10 mm. The thickness of the membrane along its transverse axis can be from 10 μm to several mm, depending on the radius r or diameter φ of the membrane. The thickness can be a few thousandths of the radius (10 μm). -3 ) to one-tenth (10 -1 For example, it can be equal to one percent of the radius.

[0065] Membrane 2 preferably includes through-holes 6 extending parallel to the transverse axis Z. These holes allow the pressure on either side of the membrane to remain balanced. They prevent the membrane from deforming due to a slowly developing pressure difference across the membrane. A slowly developing pressure difference refers to a pressure difference that appears at a frequency below the operating frequency range of the membrane. The diameter of the holes 6 is, for example, less than one-tenth of the membrane diameter. For example, it is on the order of 10 μm or 20 μm.

[0066] Waveguide 20 extends above membrane 2, contacts membrane 2, and is parallel to membrane 2. The waveguide is located at inlet 20. i Export 20 o The waveguide extends between the two sides. The waveguide is made of a first material 21 having a first refractive index n1.

[0067] Waveguide 20 can be an optical fiber, in which case the first material is the material of the fiber core. Alternatively, it could be a waveguide formed from a deposition of a thin layer of the first material 21, such as SiON (silicon oxynitride), which corresponds to... Figures 1A to 1C The example shown is a waveguide confined by a confining material 23, the refractive index of which is lower than that of the first material n1 (n3). When the waveguide 20 is an optical fiber, the confining material 23 is the cladding of the optical fiber.

[0068] Waveguide 20 is advantageously formed from a thin layer of the first material 21. The confining material 23 can simply be the air surrounding the first material. The thickness of the waveguide along the transverse axis Z is preferably less than 10 μm or 5 μm. (Reference) Figures 7A to 7D The process used to form this waveguide is described. Forming the waveguide directly on the film allows the step of bonding the optical fiber to the film to be avoided. Another advantage of using optical fiber is that this allows for waveguides with lower rigidity.

[0069] Regardless of the configuration chosen, the refractive index n3 of the limiting material 23 is lower than the refractive index n1 of the first material 21. When the first material 21 is directly deposited on the film, the refractive index n1 of the first material 21 is higher than the refractive index of the material forming the film 2.

[0070] Figure 1B An example of waveguide 20 is shown. In this example, a first material 21 is deposited on film 2. The waveguide includes portions of a second material 22 having a second refractive index n2, which are periodically distributed along 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 can be from 0.01% (10) -4 ) changed to 0.1% (10 -3 ).

[0071] Along the axis extending from the waveguide, the refractive index is periodically modulated between n1 and n2, thus affecting the reflection spectral band Δλ. 20A Bragg mirror is formed in this process. The structure of a Bragg mirror is known to those skilled in the art. It is a structure with a periodically changing refractive index, such that along the axis of light propagation, the mirror is formed by periodically alternating sections with two different refractive indices, each section having an optical thickness of λ. B / 4n i , where λ B It is the reflectance spectral band Δλ 20 The center wavelength, and n i The refractive index (n) of the material under discussion i =n1 or n i =n2). The lower the refractive index contrast, the higher the number of periods.

[0072] Reflectance spectral band Δλ 20 With resonant wavelength λ r (also known as the Bragg wavelength λ) B Centered on ), and such that:

[0073] λ r =λ B =2n eff Λ (1)

[0074] Where, r

[0075] -n eff It is the effective refractive index of the grating that makes

[0076]

[0077] -Λ is the spatial period of the grating, which is the length of the two consecutive parts 21 and 22 along the waveguide axis.

[0078] Waveguide 20 causes the Bragg mirror, formed by alternating sections 21 and 22, to include defects. A defect is defined as a periodic, localized interruption in refractive index modulation. A defect, for example, corresponds to a continuous space 25 of length Λ or the length of multiple consecutive periods, made of a given material, such as first material 21, extending over one period. See also... Figure 1B Flush with the defect, the waveguide comprises a single material extending a distance d along the axis of waveguide 20. When the distance d makes Where k is a positive natural number, forming a resonant Fabry-Perot optical cavity 26, and the resonant wavelength λ is defined. r When defect 25 extends beyond a single cycle Λ, λ r =λ B .

[0079] When d>kλ B / n eff At that time, in the reflection spectral band Δλ 20 Other resonant wavelengths λ may appear in the middle. rThe resonant wavelength is different from the Bragg wavelength λ. B In this case, it is preferable to maintain the resonant wavelength with the narrowest resonant peak.

[0080] Therefore, in waveguide 20, this defect allows the first Bragg mirror 241 and the second Bragg mirror 242 to separate. The assembly formed by the first Bragg mirror 241, the second Bragg mirror 242, and the space 25 between the Bragg mirrors forms a resonant cavity 26.

[0081] Then, the waveguide is constructed as follows:

[0082] -Reflecting Bragg mirrors 241 and 242 in the reflection spectral band Δλ 20 In addition to the resonant wavelength λ r The light;

[0083] - The resonant wavelength λ of the transmission resonant cavity 26 r The light.

[0084] Device 1 also includes a light source 10, specifically a laser diode, which is configured to face the entrance 20 of waveguide 20. i An incident light wave 12 is emitted. The light wave 12 travels at an emission wavelength λ. 12 The emission spectral band Δλ centered on 12 Emission in the middle. Emission spectral band Δλ 12 Included in the reflectance spectral band Δλ 20 In the middle. It is preferably narrower than the width of the resonant peak associated with the resonant wavelength. The width of the emission spectral band Δλ 12 The fact that the width of the peak is narrower than the resonance peak allows for the emission wavelength λ 12 Relative to the resonant wavelength λ of the waveguide r It is precisely adjusted. This adjustment is performed by the servo circuit 41 described below.

[0085] The width Δλ of the emission spectral band 12 Preferably less than 10 pm or 1 pm. The width of the emission spectral band Δλ 12 It refers to the full width at half maximum (FWHM) of the emission spectral band.

[0086] The light source 10 is preferably a continuous-wave laser. For example, it can be a DFB laser diode (DFB is an abbreviation for Distributed Feedback), with a power of 1mW, an emission wavelength of 1.55μm, and a spectral width of approximately 1pm. This type of laser diode is commonly used in the telecommunications field.

[0087] Device 1 includes a photodetector 16, preferably a fast photodetector, here a photodiode. The photodetector has a reflectance spectral band Δλ. 20 The detection spectral band Δλ 16 .

[0088] Device 1 includes a servo circuit 41 configured to follow the resonant wavelength λ of resonant cavity 26 under the action of diaphragm vibration. r Time-related changes λ r (t). See below for reference. Figure 4A and 4B This circuit is described. The servo circuit 41 allows the light source 10 to be servo-controlled, for example, causing the emission wavelength λ of the light wave 12 emitted by the light source to... 12 The resonant wavelength λ corresponding to resonant cavity 26 r .

[0089] The device includes a processing unit 42 configured to estimate the amplitude of an acoustic wave 5, under which the diaphragm 2 vibrates. This is based on the emission wavelength λ determined by the servo circuit 41. 12 Time modulation λ 12 (t) is used to estimate the amplitude of the sound wave. Reference will be made to... Figures 5A to 5C The operation of processing unit 42 is described in more detail.

[0090] The device includes a cover 8 that defines a rear volume corresponding to the volume extending between the membrane 2 and the cover 8.

[0091] Typically, waveguide 20 includes a resonant cavity 26 formed by a first reflector 241 and a second reflector 242, which are obtained by microstructuring waveguide 20 in a manner that causes periodic changes in refractive index.

[0092] An important aspect of this invention (which will be explained below) relates to the following fact:

[0093] - When the light source 10 is activated and emits light wave 12, the emission wavelength λ of the light wave 12 The resonant wavelength λ that does not correspond to waveguide 20 (or more precisely, resonant cavity 26) r Waveguide 20 reflects wave 12'; however

[0094] - When light source 10 is activated and emits light wave 12, its emission wavelength λ 12 The resonant wavelength λ corresponding to resonant cavity 26 r Waveguide 20 transmits the transmitted wave 14 to photodetector 16. The emitted wavelength λ 12 The closer to the resonant wavelength λ r The higher the intensity of the transmitted wave 14.

[0095] This invention is based on the fact that when exposed to sound amplitude A a When the sound wave 5 is emitted, the membrane 2 emits sound waves at frequency f. a The amplitude of the vibration fluctuates. This causes periodic deformation of waveguide 20, and under the action of this deformation, the resonant wavelength λ... rThe time-dependent modulation λ exhibits periodicity r (t). Amplitude of time-dependent modulation Depending on the vibration amplitude of the membrane, the amplitude is related to the acoustic amplitude A. a Related, for example, proportional to the sound amplitude. Therefore, by estimating the amplitude This device allows for estimation of sound amplitude λ. a .

[0096] Figure 1C References are shown Figure 1A The described elements are in the radial plane P XY The view in the image. In this example, membrane 2 takes the form of a thin disk with a thickness of one-hundredth of its radius.

[0097] Figure 2A An embodiment is shown where waveguide 20 is a microstructured optical fiber with a Bragg grating formed within it. In optical fibers, this type of microstructure is commonly referred to as a fiber Bragg grating (FBG). The optical fiber includes a first material 21 forming the core and a confinement material 23 forming the cladding. An inclusion or cavity of a second material 22, with a refractive index different from that of the first material, is formed within the core of the optical fiber. Figure 2B The reflection spectrum of this microstructured optical fiber is shown. The reflection spectrum corresponds to the reflection intensity normalized to the illumination intensity (y-axis) as a function of wavelength (x-axis - units in nm). The reflection is expressed in the reflection spectral band Δλ. 20 The maximum value is in the middle. Therefore, when this waveguide is reflected by the spectral band Δλ... 20 When light wave 12 is irradiated, it reflects the entire reflection spectrum Δλ. 20 The light wave 12' in the middle.

[0098] Figure 2C A similar optical fiber is shown, wherein two Bragg mirrors 241 and 242 are separated by a space 25 filled with a first material 21, as shown in the reference. Figure 1B The length of space 25 corresponds to the resonant wavelength contained in the reflection spectral band. When the optical fiber is doubled, it includes a resonant cavity 26.

[0099] Figure 2D The reflection spectrum of the optical fiber with such a microstructure is shown. Except at the resonant wavelength λ... r At that point, the reflection occurs in the reflection spectral band Δλ. 20 The maximum value is found at this point. Therefore, when this waveguide is reflected by the spectral band Δλ... 20 When light wave 12 is irradiated, if wavelength λ 12 Unlike the resonant wavelength, it reflects light wave 12', and when the wavelength λ... 12 When it is located at the resonance peak, it transmits light waves called transmitted light waves 14.

[0100] Figure 2C and Figure 2D It was obtained using a model written in Matlab (registered trademark Mathworks), where the structure was considered to extend along a length L of 3 mm, and the refractive index contrast between the first material 21 and the second material 22 was considered to be 10. -3 The period of each Bragg mirror is considered to be approximately 0.5 m. Therefore, the number of periods of each Bragg mirror is equal to 3000.

[0101] Figures 3A to 3C It shows the result of, as Figure 1B or Figure 2C The resonant wavelength λ caused by the deformation of waveguide 20 shown r The changes. Figure 3A and Figure 3B Waveguide 20, both undeformed and deformed, are shown respectively. Under the influence of deformation, the spatial period of refractive index modulation changes from Λ to Λ'=Λ+dΛ. The application of expression (1) results in the Bragg wavelength λ. B offset dλ B This wavelength extends the reflection spectral band Δλ. B The offset makes:

[0102]

[0103] in:

[0104] -ε corresponds to deformation, denoted by με (micro-strain), corresponding to 10. -4 %. The deformation ε is a normalized change in length, such that:

[0105]

[0106] -× is the multiplication operator.

[0107] Considering that the film is made of SiO2 and the refractive index jumps to 10 between the first and second materials. -3 In this case, we obtain expression (3). It is based on the uniform deformation of each Bragg mirror, such as Figure 3B As shown. According to expression (3), for a deformation of 1με, the Bragg wavelength λ B offset dλ B It's 1.2 pm.

[0108] exist Figure 3C The non-uniform deformation of waveguide 20 has been shown, with some parts of the Bragg mirror deforming less than others.

[0109] Figure 3D It is shown in, for example, reference Figure 3CThe described configuration is a model of the variation in the reflection spectrum of the Bragg mirror. Curves a, b, and c correspond to no deformation, linear deformation with 0 to 10 microstrain along the waveguide axis, and linear deformation with 4 to 6 microstrain along the waveguide axis, respectively. 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 in resonant wavelength between these two configurations is due to the variation in strain along the waveguide axis, in the ranges of 0–10 microstrain and 4–6 microstrain, respectively. Under the influence of deformation, the more uniform the deformation, the greater the spectral shift in resonant wavelength.

[0110] Preferably, waveguide 20 extends above the portion of membrane 2 that experiences maximum deformation. Membrane 2 exhibits one or more antinodes, at which the vibration amplitude is maximized. Each antinode can be determined through modeling and / or experimentation. Preferably, waveguide 20 extends above at least one antinode of the membrane's vibration. This maximizes the deformation of waveguide 20, which further increases the spectral shift caused by deformation. Therefore, better sensitivity is achieved.

[0111] The inventors have simulated the deformation of membrane 2, such as Figure 1A and Figure 1C The diagram is schematically shown. The model membrane is made of SiO2, has a radius of 1 mm, a thickness of 10 μm, and is subjected to a pressure of 1 Pa. The deformation of the membrane along one of its diameters is shown below. Figure 3E As shown, the x-axis corresponds to the distance from the center of the membrane (in mm), and the y-axis corresponds to the deformation (in microstrain). The resonant cavity 26 is preferably placed flush with the maximum deformation amplitude, i.e., at the center of the membrane 20. Figure 3E The simulations shown indicate that for this type of membrane, applying a pressure of 1 Pa will cause approximately 10 -2 Deformation due to micro-strain.

[0112] Furthermore, to achieve the most uniform possible deformation, it is preferable to place the resonant cavity 26 in a region where the curvature of the membrane is as uniform as possible under deformation. In other words, this addresses the region where the derivative of the membrane's curvature is low.

[0113] exist Figure 3E In the middle, the deformation of the membrane is in the central part 2 c It is negative, in the outer part 2 p It is positive. The resonant cavity 26 is advantageously placed on a portion of the diaphragm 2, in which the deformation under the vibration of the diaphragm has the same sign, regardless of whether it is a matter of compression or expansion.

[0114] Figure 3F Waveguide 20 is schematically shown, with its resonant cavity 26 located in the central portion 2 of membrane 2. cAbove, the waveguide extends 0.5 mm from the center on either side of the membrane center. Under the vibration of the membrane, the deformation alternately becomes negative (ε < 0), such as... Figure 3E As shown, the deformation is positive. When the deformation is negative, the optical cavity is compressed: the portions of the second material 22 become closer to each other. When the deformation is positive, the optical cavity expands: the portions of the second material 22 move away from each other.

[0115] Figure 4A and 4B The operation of the servo circuit 41 is schematically illustrated; its function is to transmit the wavelength λ. 12 Servo control is applied to the resonant wavelength λ of the resonant cavity 26 formed in the waveguide 20. r The servo circuit 41 will control the wavelength λ. 12 Locked to the resonant wavelength λ r It uses edge-top locking to do this. For example, this is a circuit problem using Pound-Drever-Hall servo technology, which is described in the publication Chow JH “Phase-sensitive interrogation of fiber Bragggrating resonators for sensing applications”, J. Light. Technol., Vol. 23, No. 5, pp. 1881-1889, May 2005, or even in the publication Black E. “An introduction to Pound-Drever-Hall laser frequency stabilization”, Am. J. Phys. 69(1), January 2001.

[0116] Servo circuit 41 includes modulator 411 for modulating the wavelength λ of light wave 12 emitted by light source 10 at a high modulation frequency that can vary from 10 kHz to several hundred MHz. 12 Emission wavelength λ 12 The modulation frequency is much higher than the maximum sound frequency that the device can handle. For example, it can be at least 10 times higher than the maximum sound frequency that the device can handle. The intensity of the light wave 14 transmitted by the waveguide 20 and detected by the photodetector 16 is transmitted to the circuit 41, which measures the change in intensity detected by the photodetector 16 as a function of wavelength modulation, h.

[0117] Depending on the sign of the function h, an error signal is sent to the light source to increase or decrease the emission wavelength λ. 12For example, when the detected intensity change with increasing wavelength is negative, the transmission wavelength gradually decreases. When the detected intensity change with increasing wavelength is positive, the transmission wavelength increases. When the detected intensity change caused by modulation is close to zero, the transmission wavelength corresponds to the resonant wavelength of the waveguide. The servo circuit 41 utilizes the following facts:

[0118] -when λ 12 <λ r At that time, the emission wavelength λ 12 The increase in wavelength λ leads to an increase in the intensity of transmitted wave 14. Conversely, the wavelength λ... 12 The reduction in ...

[0119] -when λ 12 >λ r At that time, the emission wavelength λ 12 The increase in wavelength λ leads to a decrease in the intensity of transmitted wave 14. Conversely, the wavelength λ... 12 The decrease in ...

[0120] Therefore, by adjusting the wavelength λ of the emitted light wave 12 12 By applying a small modulation and observing the effect of the modulation on the intensity of the transmitted wave 14, the light source 10 can be servo-controlled to make the emission wavelength λ... 12 Tracking waveguide 20 resonant wavelength λ r .

[0121] When the sound frequency of sound wave 5 is higher than 10kHz, the tracking resonant wavelength is allowed to be 10kHz by locking the top edge. -6 Wavelength sensitivity on the order of pm tracks the resonant wavelength, or when the sound frequency is below 1kHz, at 10 -3 The wavelength sensitivity is on the order of pm to track the resonant wavelength. According to the expression (3) applicable to SiO2 films, estimating this sensitivity will allow for the estimation of film deformation on the order of several picograms of strain, or equivalent to several mPa. Therefore, given the small spectral shift of the resonant cavity 26, which may be on the order of several pm, the Pound-Drever-Hall method is suitable.

[0122] Locking the resonant wavelength at the top edge also allows the tracking to remain insensitive to fluctuations in the resonant wavelength of cavity 26 in the face of any changes in environmental parameters such as temperature and humidity.

[0123] Figures 5A to 5C This shows the emission wavelength λ when the membrane 2 vibrates under the action of sound wave 5. 12 Periodic time-related changes λ 12 The relationship between (t) and the acoustic wave amplitude. Due to the servo control performed by the servo circuit 41, the periodic time-dependent change λ of the emitted wavelength. 12(t) is considered to correspond to the resonant wavelength λ caused by membrane vibration. r Time-dependent modulation of (t). Figure 5A The spectrum of transmitted light wave 14 is shown, as well as the spectral shift dλ under the deformation of waveguide 20. r . Figure 5B The resonant wavelength λ caused by the deformation of waveguide 20 is shown. r The time-dependent modulation, which is periodic, and its frequency... Corresponding to sound frequency f a By transmitting the wavelength λ 12 Servo control to resonant wavelength λ r The servo circuit 41 allows this modulation to be determined. The processing unit 42 is configured to analyze the periodic changes in the transmitted wavelength and estimate the frequency f. a Modulation amplitude of the lower resonant wavelength (or emission wavelength) Based on amplitude Processing unit 42 estimates the amplitude A of sound wave 5. a .

[0124] Amplitude of modulation based on resonant wavelength Acoustic amplitude A a The estimate can be determined through simulation and / or experimental calibration.

[0125] It should be noted that the acoustic amplitude A a Determining the value of the resonant wavelength is not necessarily required; only the modulation amplitude needs to be precisely determined.

[0126] Figure 6 The selection corresponds to the operating frequency range of device 1. Figure 6 This corresponds to the membrane deformation amplitude (y-axis - dBV / Pa) as a function of sound frequency (x-axis - unit Hz). The operating frequency range of the device corresponds to the flattest part of the curve. The operating frequency range extends to the low cutoff frequency f. low With mechanical resonant frequency f res The frequency range extends between these ranges. In this example, the frequency range is chosen to cover audible sound waves from 100Hz to 20kHz.

[0127] A low cutoff frequency makes:

[0128]

[0129] in,

[0130] R6 quantifies the viscosity loss caused by pores 6 in membrane 2;

[0131] C8 corresponds to the compressibility of the rear volume of the device, which is referenced to... Figure 1A definition.

[0132] This can be shown as:

[0133]

[0134] in:

[0135] -N corresponds to the number of pores 6 in the membrane;

[0136] -η is the viscosity of air, measured in Pa·s.

[0137] -e represents the thickness of the membrane;

[0138] -s6 is the cross-sectional area of ​​hole 6, which is parallel to the radial plane P. XY Cutting.

[0139] also:

[0140]

[0141] in,

[0142] -V8 represents the volume of the rear section, in cm³. 3

[0143] -c0 is the speed of sound in air, measured in milliseconds (ms). -1 ;

[0144] -ρ0 is the density of air, in g·cm³. -3 .

[0145] The resonant frequency of the membrane can be calculated using the following expression:

[0146]

[0147] in:

[0148] -r is the radius of membrane 2, expressed in cm;

[0149] -E is the Young's modulus of the material forming the film;

[0150] -ρ is the density (g·cm³) of the material forming the film. -3 );

[0151] -ν is the Poisson ratio of the materials that make up the membrane.

[0152] Therefore, it is possible to select an operating frequency range that varies with the parameters defined above in order to cover all or part of the audible or ultrasonic domain. The greater the membrane thickness and the smaller the diameter, the higher the resonant frequency f. res The higher the resonant frequency f, the higher the resonant frequency f. Conversely, the thinner the film and the larger the diameter, the higher the resonant frequency f. res The lower.

[0153] Figures 7A to 7D The main steps that allow the formation of a non-fiber waveguide 20 on membrane 2 are shown.

[0154] A substrate 3, such as a Si substrate, is provided on which a first layer 31, for example, 4 μm thick SiO2 (refractive index 1.44) and a second layer 32, for example, 1 μm thick SiON (silicon oxynitride - refractive index 1.60), have been deposited. See also Figure 7A .

[0155] The method includes:

[0156] - The second layer 32 is etched using photolithography to form the waveguide 20. See also Figure 7B In this example, SiON corresponds to the first material 21 of the waveguide.

[0157] - The back side of substrate 3 is etched to release a portion of the first layer 31, which forms the suspended film 2. See also Figure 7C .

[0158] Exposure to femtosecond laser pulses to form a cavity in the second material 22. See also Figure 7D When exposed to laser light, the refractive index of SiON changes locally. The exposed SiON then corresponds to a second material 22, whose refractive index n2 differs from that of the unexposed SiON. Specifically, exposure generates microbubbles, which cause changes in the refractive index. Exposure is performed to obtain a regular and periodic distribution in cavity 22. This results in the refractive index of waveguide 20 being modulated along the propagation axis of light within the waveguide. The duration of each pulse is, for example, equal to 100 fs, the wavelength is 800 nm, and the energy of each pulse is 30 nJ. The pulse frequency can range from a few Hz to 200 kHz.

[0159] Another exposure technique is UV lithography, described in the publication 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 lithography allows optical fibers to be microstructured.

[0160] The modulation of the refractive index caused by exposure is relatively small, approximately 10. -3 On the order of magnitude. However, writing with femtosecond lasers allows for the creation of Bragg mirrors on short lengths on the order of 1 mm. This type of exposure allows for the acquisition of highly precise resonant cavities26, with resonance peak widths less than tens of pm, or even less than 10 pm, and possibly on the order of less than 5 pm.

[0161] The length of each Bragg mirror extension can be increased. This further reduces the width of the resonance peak.

[0162] Figure 8A and Figure 8B An example of the device is shown. Figure 8A In this configuration, device 1 is placed on carrier 7, which is enclosed by cover 8. Laser source 10 emits light parallel to radial plane P. XY The light wave 12 is reflected by reflector 11 along the direction of membrane 2. Input coupling grating 13 allows the light wave 12 to enter the waveguide 20. i Coupling. The output coupling grating 15 allows all or some of the transmitted light waves 14 to be directed to the photodetector 16.

[0163] exist Figure 8B The diagram illustrates an apparatus in which active optical components (light source 10, photodetector 16, and servo circuitry 41 and processing unit 42) are remotely located in a peripheral component 9 attached to a cover 8. Deformation-sensitive elements (i.e., waveguide 20 and membrane 2) are confined within a shell formed by a carrier 7 enclosed by the cover. The cover 8 includes transparent portions to allow the transmission of emitted light waves 12 and transmitted light waves 14.

[0164] This invention could be used to form a compact detection device, with the primary application being the detection of sound waves in the audible and ultrasonic domains.

Claims

1. A device (1) for detecting acoustic waves (5), comprising: - a membrane (2) carrying a waveguide (20), the membrane being configured to vibrate at the frequency of the acoustic wave; ) - the waveguide comprises a first reflector (241) and a second reflector (242), each reflector reflecting light in a reflection spectral band ((Δλ 20 ) and the waveguide is configured to vibrate with the membrane; - said first and second reflectors are spaced apart from each other so as to form an optical resonant cavity (26) defining a resonant wavelength (λ r ) in a reflection spectral band ((Δλ 20 ) - such that the waveguide: • transmitting light of said resonant wavelength (λ r ) • reflect light in the reflection reflection band other than at the resonance wavelength (Δλ 20 ); The device further comprises: - a laser light source (10) configured to emit light waves (12) into the waveguide (20) at an emission wavelength (l 12 ); - a photodetector (16) arranged to detect light waves (14) transmitted by the waveguide at a resonant wavelength (λ r ); - a servo circuit (41) connected to the light source (10) and to the photodetector (16) and configured to servo control the wavelength (λ 12 ) of the emitted wave to the resonance wavelength (λ r ) of the optical resonator (26) at different times; - a processing unit (42) connected to the servo circuit and configured to: • determining a periodic time-dependent variation of an emission wavelength (λ 12 ) under the influence of the vibration of the membrane, the periodic time-dependent variation of the emission wavelength corresponding to a periodic variation of the resonance wavelength; • estimate the amplitude of the acoustic waves based on the periodic time-dependent variation of the emission wavelength; wherein: - the waveguide (20) is directly formed on the membrane (2); - under the action of the vibrations, the membrane presents at least one vibration antinode at which the vibration amplitude is maximum; - the waveguide (20) is flush with the at least one vibration antinode, wherein the first and second reflectors are configured to deform under the action of the vibrations of the membrane.

2. The apparatus of claim 1, wherein, Each reflector (241, 242) is a Bragg mirror formed by a periodic modulation of the waveguide refractive index (n1, n2).

3. The apparatus of claim 1, wherein, The servo circuit (41) comprises a servo loop connected to the light source (10) and configured to servo control the emission wavelength (λ 12 ) of the light wave emitted by the light source to the resonance wavelength (λ r ) of the optical resonator cavity.

4. The apparatus of claim 3, wherein, The servo circuit (41) implements an edge top-locked servo technique.

5. The apparatus of claim 1, wherein, The laser light source emits light waves having a width of less than 10 pm in an emission spectral band (Δλ 12 ) 6. The device according to claim 1, wherein: - the first reflector (241) is a first Bragg mirror; - the second reflector (242) is a second Bragg mirror; - the first and second Bragg mirrors form the same Bragg mirror comprising a defect (25), the first and second Bragg mirrors respectively corresponding to portions of the Bragg mirror located on either side of the defect.

7. A method for detecting the amplitude of a sound wave (5) using the apparatus (1) according to claim 1. The method includes: a) the vibration of the membrane (2) which, under the action of the acoustic wave, vibrates with a frequency of vibration corresponding to the frequency (f ) of the acoustic wave, the vibration of the membrane causing the vibration of the waveguide (20) which, under the action of the waveguide vibration, is periodically modulated in its resonant wavelength (λ r ). b) activating the laser light source (10) such that the light source emits light waves into the waveguide at an emission wavelength (λ 12 ); c) using a servo circuit (41), servo-controlling the emission wavelength (λ 12 ) to the resonance wavelength (λ r ) such that the emission wavelength (λ 12 (t)) varies periodically, the time-dependent variation of the emission wavelength (λ (t)) corresponding to a periodic modulation of the resonance wavelength (λ r (t)) at the frequency (f) of the acoustic wave; d) estimating, by the processing unit (42), the amplitude of the acoustic waves based on the time-dependent variation of the emission wavelength obtained via the servo circuit (41).

8. A method for manufacturing the device according to claim 1, the method comprising: - depositing a thin layer of a first material on the membrane so as to form the waveguide; - inscribing the waveguide with a femtosecond laser beam so as to obtain a periodic modulation of the refractive index of the waveguide.

Images