PHOTOACOUSTIC OR PHOTOTHERMAL DETECTOR WITH AN OPTICAL TRANSDUCER

Description

DOMAINE TECHNIQUE

[0001] The technical field of the invention is the detection of an analyte in a medium according to the principle of photoacoustic detection or photothermal detection. ART ANTERIEUR

[0002] Photoacoustic detection is based on the detection of an acoustic wave generated by the absorption, by an analyzed medium, of an electromagnetic excitation wave, called an excitation wave, which is pulsed or amplitude modulated. The acoustic wave is formed following heating of absorbing molecules, present in the analyzed medium, under the effect of absorption of the excitation wave. The heating causes a modulated thermal expansion of the medium, the latter being the origin of the acoustic wave.

[0003] Photoacoustic detection can be specific to a particular analyte, by adjusting the wavelength of the excitation wave to an absorption wavelength of the analyte. Photoacoustic detection has thus been applied to detect gaseous species in a gas, or to detect the presence of particular molecules in biological tissues. The wavelength of the incident wave is frequently in the infrared.

[0004] Photoacoustic detection then constitutes a non-invasive analysis technique, which can be implemented in diffusing or opaque environments.

[0005] Applications of underwater photoacoustic sensing have been described in the Rosenthal publication “Embedded ultrasound sensor in a silicon-on-insulator photonic platform” and in the Guan publication “Acoustic and Ultrasonic Detection With Radio-Frequency Encoded Fiber Laser Sensors”.

[0006] The paper Yang et al "Time-resolved photoacoustic spectroscopy using fiber Bragg grating acoustic transducers" describes the use of an optical fiber, including a Bragg grating, as a detector of a photoacoustic wave.

[0007] Other photoacoustic detection devices are described in Guan B. “Acoustic and ultrasonic detection with radio-frequency”, as well as in Juntao W. “Fiber-optic photoacoustic spectroscopy sensor for harsh environment gas detection”.

[0008] The publication WEI H. "Direct laser writing of a phase-shifted Bragg grating waveguide for ultrasound detection" describes the fabrication of a waveguide by femtosecond laser writing.

[0009] Optical resonators are also described in WO2019170884 or in Leinders, S “A sensitive optical micro-machined ultrasound sensor (OMUS) based on a silicon photonic ring resonator on an acoustical membrane”.

[0010] Des applications de détection photoacoustique à des tissus biologiques sont par exemple décrits dans les publications suivantes : Bauer AJ. "IR-spectroscopy for skin in vivo : Optimal skin sites and properties for non-invasive glucose measurement by photoacoustic and photothermal spectroscopy" ; Journal of biopohtonics 11 (2018) ; "Windowless ultrasound photoacoustic cell for in-vivo mid-IR spectroscopy of human epidermis : Low interference by changes of air pressure, temperature, and humidity caused by skin contact opens the possibility for a non-invasive monitoring of glucose in the interstitial fluid", Rev. Sci. Instrum. 84, 084901 (2013).

[0011] In these publications, a pulsed laser light source is used, activated at a frequency of several tens of kHz. The objective is to estimate a glucose concentration in the body's interstitial fluid, at a depth of between 10 µm and 50 µm under the skin of a user. This is done using a photoacoustic detection device placed against the user's skin.

[0012] The principles of photothermal detection are also known, based on the detection of a variation in the temperature of an analyzed medium, under the effect of the absorption, by the medium, of a pulsed or amplitude-modulated electromagnetic excitation wave. The temperature variation results from the heating of absorbing molecules, present in the analyzed medium, under the effect of the absorption of the excitation wave.

[0013] Periodic temperature modulation can for example be detected by estimating a variation in the optical index of the medium, under the effect of the temperature variation. This is for example described in EP3359949B1.

[0014] Whether it is photothermal detection or photoacoustic detection, the sample can be a gaseous sample, the objective being the detection of certain gaseous species or certain particles, not considered as pollutants. It can also be liquid or solid samples, the applications being able to concern the field of industry, for example the food industry, or the field of health, as previously mentioned,

[0015] The inventors have designed a transducer that can be used either for photoacoustic detection applications or for photothermal detection applications. It allows for devices dedicated to each application, with high detection sensitivity and a simple design. EXPOSE DE L'INVENTION

[0016] A first object of the invention is a detection device according to claim 1.

[0017] The device may comprise a processing unit, connected to the control circuit, and configured to: estimate an amplitude of the temporal modulation of the resonance wavelength; detect the presence of the analyte in the medium based on the estimated amplitude.

[0018] The processing unit can be configured to estimate a concentration of the analyte in the medium based on the estimated amplitude.

[0019] According to one embodiment, called photoacoustic, the device comprises a hollow cavity, opening onto the opening, the transducer being connected to the hollow cavity. The transducer is an acoustic transducer, configured to detect an amplitude of a photoacoustic wave propagating from the opening, through the hollow cavity, such that under the effect of the illumination of the medium by the excitation light wave, the membrane vibrates according to the excitation frequency, causing the temporal modulation of the resonance wavelength, at a modulation frequency equal to the excitation frequency.

[0020] The membrane may extend parallel to the contact face. The membrane may delimit a portion of the hollow cavity.

[0021] According to one embodiment, called photothermal, the device is such that: the membrane of the transducer forms the contact face of the device, intended to be applied in contact with the medium; the opening extends through the membrane; the transducer is a thermal transducer, such that under the effect of the illumination of the medium by the excitation light wave, the temperature of the membrane follows a periodic temporal variation, resulting in the periodic temporal modulation of the resonance wavelength.

[0022] Regardless of the embodiment, at least one reflector, or each reflector, is a Bragg mirror, formed by a periodic modulation of a refractive index along the waveguide.

[0023] The servo circuit can notably implement a Pound-Drever-Hall type servo.

[0024] Whatever the embodiment, the device may be such that: the first reflector is a first Bragg mirror; the second reflector is a second Bragg mirror; the first Bragg mirror and the second Bragg mirror form a single Bragg mirror comprising a defect, the first Bragg mirror and the second Bragg mirror corresponding to the parts of the Bragg mirror extending respectively on either side of the defect.

[0025] In the photoacoustic embodiment, the device may be such that: the membrane has, under the effect of the vibration, at least one vibration antinode, the vibration amplitude being maximum at the level of each vibration antinode; the waveguide extends at the level of at least one vibration antinode.

[0026] The waveguide is formed directly on the membrane. The first reflector and the second reflector can be obtained by writing the waveguide with a laser beam, so as to obtain a periodic modulation of the refractive index in the waveguide.

[0027] A second subject of the invention is a method, according to claim 12, for detecting an analyte in a medium, the analyte absorbing light according to at least one absorption wavelength, the method comprising the following steps: a) applying a device according to the first subject of the invention against the medium, such that the contact face of the device is held against the medium; b) activating the excitation light source, the excitation light source emitting an excitation light wave, pulsed or amplitude modulated, according to an excitation frequency, in a wavelength corresponding to an absorption wavelength of the analyte; c) determining, by the control circuit, a periodic modulation of a resonance wavelength of the transducer waveguide, at a modulation frequency corresponding to the excitation frequency, the resonance wavelength corresponding to a transmission peak of the waveguide; d) depending on the periodic modulation determined by the control circuit, detecting the presence of the analyte in the medium.

[0028] The device may be as described in connection with the photoacoustic embodiment. The method may then comprise: following step b), periodic heating of the medium, according to the excitation frequency, resulting in an emission of a photoacoustic wave, propagating through the hollow cavity, under the effect of which the membrane of the transducer vibrates at the excitation frequency, such that the resonance frequency of the waveguide of the transducer is modulated according to a modulation frequency equal to the excitation frequency; during step d): an estimation of an amplitude of the periodic modulation of the resonance wavelength, at the modulation frequency; a detection of the presence of analyte according to the estimated amplitude.

[0029] The device may be as described in connection with the photothermal embodiment. The method may then comprise: following step b), periodic heating of the medium, according to the excitation frequency, resulting in periodic heating of the membrane at the excitation frequency, such that the resonance frequency of the transducer waveguide is modulated according to a modulation frequency corresponding to the excitation frequency; during step d): an estimation of an amplitude of the periodic modulation of the resonance wavelength at the modulation frequency; a detection of the presence of analyte according to the estimated amplitude.

[0030] Regardless of the embodiment, step d) may also include an estimation of a concentration of the analyte in the medium analyzed.

[0031] The invention will be better understood by reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0032] THE figures 1A à 1E schematize the main components of a device according to an embodiment, called photoacoustic embodiment. The figure 2A shows a microstructured optical fiber, forming a Bragg mirror. The figure 2B represents a spectral band of reflection of the optical fiber shown schematically on the figure 2A . There figure 2C shows a microstructured optical fiber, forming a resonant optical cavity based on two spaced Bragg mirrors. The figure 2D represents a spectral band of reflection of the optical fiber shown schematically on the figure 2C . There figure 3A shows a waveguide, comprising a resonant optical cavity, undistorted. The figure 3B shows a waveguide, comprising a resonant optical cavity, deformed. The figure 3C shows a waveguide whose deformation is not homogeneous. The figure 3D shows an evolution of the reflection spectral band of a waveguide, as represented in the figure 3C , under the effect of deformations. The figure 3E represents an amplitude of deformation of a membrane, along a diameter of the latter. The figure 3F shows an optimal positioning of a waveguide, taking into account the deformation represented on the figure 3D . There figure 4A schematizes a circuit implementing a servocontrol of the wavelength of the auxiliary light source according to the wavelength locking method. The figure 4B shows the evolution of an error function obtained by a wavelength locking method. The figure 5A illustrates a shift in the resonance wavelength of a resonant optical cavity, under the effect of a vibration of the membrane. The figure 5B shows a temporal modulation of the optical resonance wavelength. The figure 5C schematizes an estimate of the vibration amplitude of a membrane from the temporal modulation of the resonance wavelength. figures 6A à 6D show the main steps of a manufacturing process for forming a microstructured waveguide in contact with a membrane. The figure 7A shows the main steps of a detection method implementing the device according to the photoacoustic embodiment. The figure 7B shows the main steps of a detection method implementing the device according to the photothermal embodiment. The figures 8A à 8C schematize the main components of a device according to an embodiment, called photothermal embodiment. EXPOSE DE MODES DE REALISATION PARTICULIERS

[0033] THE figures 1A à 1E illustrate a first embodiment, called photoacoustic, of a device 1 according to the invention. The device 1 is configured to be applied against a medium 2 to be analyzed.

[0034] The device comprises an excitation light source 10, configured to emit an excitation light wave 11 propagating to the medium 2 to be analyzed. The light source 10 is pulsed or amplitude modulated, according to an excitation frequency f 11. The light wave 11 is emitted in an excitation spectral band Δλ 11 comprising an absorption wavelength λ 4 of an analyte 4 present in the medium. An objective of the device 1 is to detect the presence of the analyte 4 and possibly to estimate a concentration thereof.

[0035] The excitation spectral band preferably extends into the visible or infrared range, for example between wavelengths of 3 µm and 15 µm. Preferably, the excitation spectral band Δλ 11 is sufficiently narrow so that the device 1 is specific to a single analyte. For example, the width of the emission spectral band is of the order of 1 cm -1< . When the analyte is glucose, the emission spectral band is centered on an absorption wavelength of glucose, for example 1034 cm -1< . The excitation light source 10 may in particular be a pulsed laser source, for example a wavelength-tunable laser of the QCL (Quantum Cascade Laser) type. The emission spectral band Δλ is then located in the infrared.

[0036] The analyte 4 may be a molecule present in the medium being analyzed. When the medium is a biological tissue, it may be glucose present in a bodily fluid of the biological tissue. As mentioned in connection with the prior art, the analyte may be a gaseous molecule, the medium being a gas. For example, it may be a gas molecule considered to be a pollutant. The medium may also be a liquid, the analyte being a molecule potentially present in the liquid.

[0037] The device 1 comprises a contact face 3, intended to be applied against the medium 2 to be analyzed, in contact with the latter. The contact face 3 is designed to conform to the medium 2 against which it is intended to be applied. It is for example flat.

[0038] The device 1 comprises an enclosure 17, extending from the contact face 3, and delimiting a hollow cavity 16. The hollow cavity 16 comprises an opening 13, arranged in the contact face 3, so as to open onto the medium 2. The excitation light source 10 is configured such that the excitation light wave 11 propagates to the medium 2 through the hollow cavity 16, as well as through the opening 13.

[0039] Under the effect of the presence of an analyte 4 in the medium 2, a photoacoustic wave 6 is formed. The photoacoustic wave 6 is an acoustic wave formed from a periodic heating of the medium by the incident light wave 11, the latter being modulated in amplitude at the excitation frequency f 11. A portion of the photoacoustic wave 6 propagates through the hollow cavity 16 so as to be detected by a transducer 15.

[0040] In the photoacoustic embodiment, the transducer 15 is an acoustic transducer. Its function is to measure an amplitude and / or frequency of the photoacoustic wave 6. More precisely, in the intended application, the transducer 15 allows an estimation of an amplitude of the photoacoustic wave 6 at the excitation frequency. f 11 of the excitation light wave.

[0041] The transducer 15 comprises a flexible membrane 18, configured to vibrate when exposed to the photoacoustic wave 6. The membrane 18 preferably extends parallel to a radial plane P XY . The diameter of the membrane, or its largest diagonal, is between 1 mm and 10 mm. The thickness of the membrane, parallel to a transverse axis Z, perpendicular to the radial plane, is preferably between 10 µm and 500 µm, and preferably between 10 µm and 100 µm. The thickness of the membrane is preferably between 1 / 10 and 1 / 200 of the radius of the membrane (or its largest half-diagonal).

[0042] The excitation light source 10 is configured such that the excitation light wave 11 propagates to the medium 2 through the hollow cavity 16. In the example shown in the figure 1A , the excitation light wave 11 propagates through a secondary opening 19 made through the membrane 18.

[0043] The secondary opening 19 made in the membrane can also make it possible to establish a pressure balance on either side of the membrane 18, at low frequency. This makes it possible to avoid possible deformation of the membrane 18 under the effect of a variation in the low-frequency pressure on either side of the membrane 18. By low-frequency pressure variation, we mean a pressure difference occurring at a frequency lower than a frequency operating range of the membrane. The diameter of the secondary opening 19 is for example less than one tenth of the diameter of the membrane. It is for example of the order of 10 µm or 20 µm.

[0044] The transducer 15 comprises a waveguide 20 which extends over the membrane 18, in contact with the latter and parallel to the latter. The waveguide extends between an input 20 i and an output 20 o . The waveguide is made from a first material 21 with a first refractive index n 1 .

[0045] Outside the scope of the present invention, the waveguide 20 may be an optical fiber, in which case the first material is the core of the optical fiber. According to the invention, it is a waveguide formed from the deposition of a thin layer of the first material 21, for example SiON (Silicon oxynitride), which corresponds to the example shown in the figures 1A à 1E The waveguide is delimited by a confinement material 23, the refractive index of which is lower than the refractive index n 3 of the first material n 1 . When the waveguide 20 is an optical fiber, the confinement material 23 is the cladding of the optical fiber.

[0046] The waveguide 20 is formed from a thin layer of the first material 21, the confining material 23 may 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. The method of forming such a waveguide is described in connection with the figures6A à 6D . Forming a waveguide directly on the membrane makes it possible to avoid a step of bonding the optical fiber to the membrane. Another advantage, compared to the use of an optical fiber, is that this makes it possible to obtain a less rigid waveguide. Whatever the configuration chosen, the refractive index n 3 of the confinement material 23 is lower than the refractive index n 1 of the first material 21. When the first material 21 is directly deposited on the membrane 18, it is preferable that the refractive index n 1 of the first material 21 is higher than the optical index of the material forming the membrane 18. An example of a waveguide 20 is detailed on the figure 1B . In this example, the first material 21 is deposited on the membrane 18. The waveguide comprises portions of a second material 22, of a second refractive index n 2 , periodically distributed along the waveguide 20. The second refractive index n 2 is different from the first refractive index n 1 . It may be greater than the first refractive index n 1 . The relative variation between the first refractive index and the second refractive index may vary between 0.01% (10 -4 < ) to 0.1% (10 -3 < ).

[0047] Along the axis defined by the waveguide, the refractive index is periodically modulated, between n 1 and n 2 , so as to form a Bragg mirror in a reflection spectral band Δλ 20 . The structure of a Bragg mirror is known to those skilled in the art. It is a structure according to which the refractive index varies periodically, such that along the axis along which the light propagates, the mirror is formed by alternating portions of two different indices, the optical thickness of each portion being λ B / 4n i , where λ B is a central wavelength of the reflection spectral band Δλ 20 and ni is the refractive index of the material considered (ni = n 1 or ni = n 2 ). The lower the refractive index contrast, the higher the number of periods.

[0048] The reflection spectral band Δλ 20 is centered on a resonance wavelength λ r . The latter is such that: λ r = λ B = 2 n eff Λ Or n eff is an effective index of the network, such that n eff = n 1 + n 2 2 Λ is the spatial period of the grating, that is to say the length of two successive portions 21 and 22 along the axis of the waveguide.

[0049] The waveguide 20 is such that the Bragg mirror, formed by the alternation of the portions 21 and 22, comprises a defect. By defect is meant a localized break in the periodicity of the refractive index modulation. The defect corresponds for example to a continuous space 25, formed by the same material, for example the first material 21, along a period Λ or along several successive periods. At the defect, the waveguide comprises the same material, extending along a distance d along the axis of the waveguide 20. When the distance d is such that d = kλ B n eff where k is a positive natural integer, a Fabry-Perot type resonant optical cavity 26 is formed, defining a resonance wavelength λ r . When the fault extends over a single period Λ, λ r = λ B

[0050] When d > kλ B / n eff , other resonance wavelengths λ r may appear, in the reflection spectral band Δλ 20 , the resonance wavelengths being different from the Bragg wavelength λ B In such a case, the resonance wavelength at which the resonance peak is the finest is preferably retained.

[0051] Thus, the defect makes it possible to separate, in the waveguide 20, a first Bragg mirror 24 1 and a second Bragg mirror 24 2 . The assembly formed by the first Bragg mirror 24 1 , the second Bragg mirror 24 2 , and the space 25 between the Bragg mirrors forms the resonant cavity 26.

[0052] The waveguide 20 is then structured to: reflect light in the reflection spectral band Δλ 20 of the Bragg mirrors 24 1 , 24 2 , outside the resonance wavelength λ r ; transmit light in the resonance wavelength λ r of the resonant cavity 26.

[0053] The transducer 15 also comprises an auxiliary light source 30, in particular a laser diode, arranged to emit an auxiliary light wave 32 towards the input 20 i of the waveguide 20. The auxiliary light wave 32 is emitted in an emission spectral band Δλ 32 , centered on an emission wavelength λ 32 . The emission spectral band Δλ 32 is preferably included in the reflection spectral band Δλ 20 .

[0054] Preferably, the width of the emission spectral band Δλ 32 is narrower than that of the reflection spectral band Δλ 20 . For example, the width of the emission spectral band Δλ 32 may be 1 nm, or even less than 500 pm or 100 pm. By width of the emission spectral band Δλ 32 is meant a width at half-maximum of the emission spectral band.

[0055] The auxiliary light source 30 is preferably a continuous laser. It may, for example, be a DFB (Distributed Feedback) type laser diode, with a power of 1 mW, emitting at a wavelength of 1.55 µm, with a spectral width of the order of 1 pm. This type of laser diode is commonly used in the field of telecommunications.

[0056] The transducer 15 comprises a photodetector 36, preferably a fast photodetector, of the photodiode type. The photodetector has a detection spectral band Δλ 36 which includes the reflection spectral band Δλ 20 .

[0057] The transducer 15 comprises a servo circuit 41, configured to follow a time modulation λ r ( t ) of the resonance wavelength λ r of the resonant cavity 26. Such a circuit is described later, in connection with the figures 4A et 4B . The control circuit 41 allows control of the auxiliary light source 30, such that the emission wavelength λ 32 of the auxiliary light wave 32 emitted by the auxiliary light source corresponds to the resonance wavelength λ r of the resonant cavity 26.

[0058] The device comprises or is connected to a processing unit 42, configured to calculate a frequency f λr or an amplitude A λr of temporal modulation λ r ( t ) determined by the servo circuit 41. The processing unit 42 is for example designed or programmed to estimate a modulation amplitude of the resonance wavelength, at a frequency f λr , corresponding to the excitation frequency f 11 of the excitation light wave 11. The operation of the processing unit 42 is described in more detail in connection with the figures 5A à 5C .

[0059] The device comprises a cover 48, delimiting a rear volume, the rear volume corresponding to the volume extending between the membrane 18 and the cover 48.

[0060] Generally speaking, the waveguide 20 comprises a resonant cavity 26, formed from a first reflector 24 1 and a second reflector 24 2 , the latter being obtained by microstructuring the waveguide 20. In the examples given in this description, the first reflector 24 1 and the second reflector 24 2 are Bragg mirrors, but other types of microstructures are conceivable.

[0061] An important point of the invention, explained later, is based on the fact that: when the auxiliary light source 30 is activated, and emits a light wave 32 whose emission wavelength λ 32 is not tuned with the resonance wavelength λ r of the waveguide 20 (or more precisely of the resonant cavity 26), the waveguide 20 reflects a reflected wave 32'; when the auxiliary light source 30 is activated, and emits a light wave 32 whose emission wavelength λ 32 corresponds to the resonance wavelength λ r of the resonant cavity 26, the waveguide 20 transmits a transmitted wave 34 to the photodetector 36. The closer the emission wavelength λ 32 is to the resonance wavelength λ r , the greater the intensity of the transmitted wave 34.

[0062] The invention is based on the fact that when exposed to an acoustic wave 6, of acoustic frequency f a , the membrane 18 vibrates according to a vibration amplitude A a at the frequency f a of the photoacoustic wave 6. This results in a periodic deformation of the waveguide 20, under the effect of which the resonance wavelength λ r follows a periodic temporal modulation λ r ( t ). The amplitude A λr the temporal modulation of the resonance wavelength depends on the vibration amplitude of the membrane, the latter being proportional to the acoustic amplitude A a . The frequency f λr of the temporal modulation corresponds to the acoustic frequency f a , which also corresponds to the excitation frequency f 11. By calculating the amplitude of the modulation A λr at the frequency f λr , the device makes it possible to detect the presence of the analyte in the medium, and possibly to estimate a concentration, as described in connection with the figures 5A à 5C .

[0063] There figure 1C shows a view of some of the elements described in connection with the figure 1A , in the radial plane P XY . In this example, the membrane 18 takes the form of a thin disc, the thickness being one hundredth of the radius.

[0064] There figure 1D is a 3D view representing the position of the membrane 18 and the waveguide 20 relative to the analyzed medium 2.

[0065] There figure 1E shows an example of a device in which the excitation light source 10 is associated with a reflector 14. The excitation light wave 11 is emitted parallel to the radial plane P XY , then is reflected, by the reflector 14, towards the opening 13, so as to propagate towards the medium 2. The auxiliary light source 30 and the photodetector 36 are aligned with respect to the waveguide 20. Alternatively, the auxiliary light source 30 and / or the photodetector 36 can be coupled to the waveguide 20 by a photonic crystal.

[0066] According to a variant, the membrane 18 of the transducer 15 is connected to the cavity 16 by an acoustic channel, the latter transmitting a part of the photoacoustic wave 6 towards the membrane 18.

[0067] Outside the scope of the present invention, the figure 2A illustrates an embodiment in which waveguide 20 is a microstructured optical fiber, inside which a Bragg grating is formed. This type of microstructuring, in an optical fiber, is usually referred to as "Fiber Bragg Grating". The optical fiber comprises a first material 21 forming the core and a confinement material 23 forming the cladding. Inclusions or cavities of a second material 22 are formed in the core of the optical fiber, the refractive index of which is different from that of the first material. figure 2B shows a reflection spectrum of the optical fiber thus microstructured. The reflection spectrum corresponds to a reflected intensity normalized by the illumination intensity (y-axis) as a function of the wavelength (x-axis - unit nm). The reflection is maximum in the reflection spectral band Δλ 20 . Thus, when such a waveguide is illuminated by a light wave 32, in the reflection spectral band Δλ 20 , it reflects a light wave 32', in the entire reflection spectral band Δλ 20 .

[0068] There figure 2C illustrates a similar optical fiber, in which two Bragg mirrors 24 1 and 24 2 are separated by a space 25 filled with the first material 21, as described in connection with the figure 1B . When the length of space 25 corresponds to k n eff times a resonance wavelength, included in the reflection spectral band, the optical fiber comprises a resonant cavity 26.

[0069] There figure 2D shows a reflection spectrum of the optical fiber thus microstructured. The reflection is maximum according to the reflection spectral band Δλ 20 , with the exception of the resonance wavelength λ r . Thus, when such a waveguide is illuminated by a light wave 32, emitted by the auxiliary light source 30, in the reflection spectral band Δλ 20 , it reflects a light wave 32', if the wavelength λ 32 is different from the resonance wavelength, and transmits a light wave 34, called the transmitted light wave, when the wavelength λ 32 is located in the resonance peak.

[0070] THE figures 2C et 2D are derived from models, calculated by Matlab type calculation software (registered trademark - Mathworks), considering a structure extending along a length L of 3 mm, the index contrast between the first and second materials being 10 -3< , the period of each Bragg mirror being approximately 0.5 µm. Thus, each Bragg mirror has a number of periods equal to 3000.

[0071] THE figures 3A à 3C illustrate the variation of the resonance wavelength λ r resulting from a deformation of a waveguide 20 as described in the figures 1B Or 2C . THE figures 3A et 3B show the undeformed and deformed waveguide 20 respectively. Under the effect of the deformation, the spatial period of the index modulation varies from A to A' = A + dA. The application of expression (1) results in a shift dλ B of the Bragg wavelength λ B , around which the reflection spectral band Δλ extends. The shift dλ B is such that: 1 λ B × ∂ λ B ∂ ε = 0.78 × 10 − 6 μ ε − 1 Or : ε corresponds to the deformation, expressed in µε (microstrain), corresponding to 10 -4< %. The deformation ε is a normalized variation of the length, such that: ε = d Λ Λ × 10 − 2 × is the multiplication operator.

[0072] Expression (3) was obtained by considering that when the membrane 18 is made of SiO 2 , and the index jump between the first material and the second material is 10 -3< . It is based on a homogeneous deformation of each Bragg mirror, as shown in the figure 3B . According to expression (3), for a deformation of 1 microstrain, the shift dλ B of the Bragg wavelength λ B is 1.2 pm.

[0073] On the figure 3C , a non-uniform deformation of the waveguide 20 has been shown, certain portions of the Bragg mirrors being less deformed than others.

[0074] There figure 3D is a model showing the evolution of the reflection spectrum of a Bragg mirror, in a configuration as described in connection with the figure 3C . Curves a, b and c correspond respectively to no deformation, to a deformation of 0 to 10 microstrain as well as a deformation between 4 and 6 microstrain. 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 resonance wavelengths between these two configurations is due to the variation of deformation along the axis of the waveguide, respectively in the range 0-10 microstrain and 4-6 microstrain. The more homogeneous the deformation, the greater the spectral shift of the resonance wavelength, under the effect of the deformation, is important.

[0075] Preferably, the waveguide 20 extends over the parts of the membrane 18 subjected to the greatest deformation. The membrane 18 comprises one or more vibration antinodes, at which the vibration amplitude is maximum. Each antinode can be determined by modeling and / or experimentally. Preferably, the waveguide 20 extends over at least one vibration antinode of the membrane. This maximizes the deformation of the waveguide 20, which further increases the spectral shift resulting from the deformation. This results in better sensitivity.

[0076] The inventors modeled a deformation of a membrane 18, as shown schematically in the figures 1A And 1D The modeled membrane was made of SiO 2 , with a radius of 1 mm and a thickness of 10 µm, subjected to a pressure of 1 Pa. The deformation of the membrane, along one of its diameters, is represented on the figure 3E , the abscissa axis corresponding to the distance from the center of the membrane (in mm) and the ordinate axis corresponding to the deformation, in microstrain. The resonant cavity 26 is preferably arranged at the level of the maximum deformation amplitude, that is to say at the center of the membrane 18. The simulation illustrated on the figure 3E shows that with this membrane, the application of a pressure of 1 Pa induces a deformation of some 10 -2< microstrain.

[0077] On the figure 3E , the deformation of the membrane is negative in the central part 2 c and positive in the peripheral part 2 p . The resonant cavity 26 is advantageously placed on a portion of the membrane 2 in which under the effect of the vibration of the membrane, the deformation is of the same sign, whether it is a compression or an expansion.

[0078] On the figure 3F , a waveguide 20 is shown diagrammatically, the resonant cavity 26 of which is positioned on the central part 2 c of the membrane 2, on either side of the center of the membrane, at a distance of ±0.5 mm from the center. Under the effect of the vibration of the membrane, the deformation is alternately negative (ε < 0), as shown in the figure 3E , then positive. When the deformation is negative, the optical cavity is compressed: the portions of the second material 22 move closer to each other. When the deformation is positive, the optical cavity is expanded: the portions of the second material 22 move away from each other.

[0079] THE figures 4A et 4B schematize the operation of the control circuit 41, the function of which is to control the wavelength λ 32 to the resonance wavelength λ r of the resonant cavity 26 formed in the guide 20. The control circuit 41 performs a locking of the wavelength λ 32 with respect to the resonance wavelength λ r . Wavelength locking is usually designated by the Anglo-Saxon term "top of fringe locking". This is a Pound-Drever-Hall type circuit, such a circuit being for example described in the publication Chow JH "Phase-sensitive interrogation of fiber Bragg grating resonators for sensing applications", J. Light. Technol., vol. 23, n°5, p. 1881-1889, May 2005, or in the publication Black E. "An introduction to Pound-Drever-Hall laser frequency stabilization", Am. J. Phys. 69 (1), January 2001.

[0080] The control circuit 41 comprises a modulator 41 1 , for modulating the wavelength λ 32 of the auxiliary light wave 32 emitted by the auxiliary light source 30, according to a modulation frequency which can vary from 10 kHz to several hundred MHz. The modulation frequency of the emission wavelength λ 32 is much higher than the maximum acoustic frequency addressed by the device. It can for example be higher than 10 times the maximum acoustic frequency addressed by the device. The intensity of the light wave 34, emerging from the waveguide 20 and detected by the photodetector 36, is transmitted to the control circuit 41, the latter measuring a function h translating a variation in the intensity detected by the photodetector 36 with respect to the wavelength modulation.

[0081] Depending on the sign of the function h, an error signal is sent to the light source, so as to increase or decrease the emission wavelength λ 32 . For example, when the change in the detected intensity with respect to an increase in wavelength is negative, the emission wavelength is gradually decreased. When the change in the detected intensity with respect to an increase in wavelength is positive, the emission wavelength is increased. When the change in the detected intensity with respect to the modulation is close to zero, the emission wavelength corresponds to the resonance wavelength of the waveguide. The control circuit 41 uses the fact that: when λ 32 < λ r , an increase in wavelength λ 32 results in an increase in the intensity of the transmitted wave 34. Conversely, a decrease in wavelength λ 32 results in a decrease in the intensity of the transmitted wave 34; when λ 32 > λ r , an increase in wavelength λ 32 results in a decrease in the intensity of the transmitted wave 34. Conversely, a decrease in wavelength λ 32 results in an increase in the intensity of the transmitted wave 34.

[0082] Thus, by performing a weak modulation of the wavelength λ 32 of the auxiliary light wave 32, and by observing the effect of the modulation on the intensity of the transmitted light wave 34, the auxiliary light source 30 can be controlled, so that the wavelength λ 32 of the auxiliary light wave 32 follows the resonance wavelength λ r of the waveguide 20.

[0083] Resonance wavelength tracking by wavelength locking allows for resonance wavelength tracking with a wavelength sensitivity of the order of 10 -6< pm when the acoustic frequency is greater than 10 kHz, or of the order of 10 -3< pm when the acoustic frequency is less than 1 kHz. Given expression (3), it is estimated that this allows for an estimation of the membrane deformation of the order of a few picostrain, i.e. the equivalent of a few mPa based on a membrane formed of SiO 2 . The Pound-Drever-Hall method is therefore suitable, given the small spectral shifts of the resonant cavity 26, which can be of the order of a few pm.

[0084] Tracking the resonance wavelength by wavelength locking also makes it possible to be insensitive to fluctuations in the resonance wavelength of the cavity 26 under the effect of variations in environmental parameters, such as variations in temperature or humidity.

[0085] THE figures 5A à 5C illustrate the link between the periodic temporal variation λ r (t) of the resonance wavelength λ r , when the membrane 18 vibrates under the effect of a photoacoustic wave 6, and the amplitude of the acoustic wave. Due to the control carried out by the control circuit 41, the periodic temporal variation of the emission wavelength λ 32 (t) is considered to correspond to the temporal modulation of the resonance wavelength λ r (t) induced by the vibration of the membrane. On the figure 5A , we have schematized a spectrum of the transmitted light wave 34, and a spectral shift dλ r under the effect of the deformation of the waveguide 20. The figure 5B shows the temporal modulation of the resonance wavelength λ r resulting from the deformation of the waveguide 20, the modulation being periodic and of frequency f λr corresponding to the acoustic frequency f a . The control circuit 41, by controlling the wavelength λ 32 to the resonance wavelength, makes it possible to determine such a modulation. This modulation is transmitted to the processing unit 42, the latter calculating an amplitude A λr modulation of the resonance wavelength. From the latter, the processing unit 42 estimates the vibration amplitude of the membrane, which corresponds to the acoustic frequency f a of the photoacoustic wave 6. From the vibration amplitude of the membrane, at the acoustic frequency f a , we can determine the presence of analyte 4 in the medium, or estimate a concentration of analyte 4 in the medium.

[0086] Estimating the concentration of analyte 4 in the medium may require prior calibration, in order to establish a link: between the concentration of the analyte and the vibration amplitude of the membrane; or between the concentration of the analyte and the modulation amplitude of the resonance wavelength.

[0087] It is observed that the determination of the acoustic amplitude A a does not necessarily imply a determination of the value of the resonance wavelength, but a precise determination of the modulation amplitude A λr .

[0088] THE figures 6A à 6D illustrate the main steps for forming a non-fibered waveguide 20 on a membrane 18.

[0089] A substrate 100 is provided, for example made of Si, on which a first layer 101 has been deposited, for example made of SiO 2 (index 1.44), with a thickness of 4 µm, and a second layer 102, for example made of SiON (silicon oxynitride - index 1.60), with a thickness of 1 µm. Cf. figure 6A .

[0090] The process includes: an etching of the second layer 102, by photolithography, so as to form the waveguide 20. Cf. figure 6B . In this example, SiON corresponds to the first material 21 of the waveguide. an etching on the rear face of the substrate 100, so as to release a part of the first layer 101, the latter forming the suspended membrane 18. This step also makes it possible to form the enclosure 17, delimiting the hollow cavity 16. Cf. figure 6C . an insolation, usually designated by the term inscription, point by point, of the waveguide 20 by femtosecond laser pulses, so as to form cavities of a second material 22. Cf. figure 6D . Under the effect of laser insolation, the SiON undergoes a local variation in index. The insolated SiON then corresponds to a second material 22, whose refractive index n 2 is different from that of the non-insolated SiON. Indeed, the insolation generates microbubbles, which induces a variation in the refractive index. This results in a modulation of the refractive index of waveguide 20, along the axis of propagation of the light, inside the waveguide.

[0091] The duration of each pulse is for example equal to 100 fs, at the wavelength of 800 nm, the energy of each pulse being 30 nJ. The pulse frequency can be between a few Hz and 200 kHz.

[0092] Another exposure technique is UV photoinscription, described in the publication Chow JH "Phase-sensitive interrogation of fiberBragg grating resonators for sensing applications", J. Light. Technol., vol. 23, n°5, pp. 1881-1889, May 2005. UV photoinscription allows, for example, the microstructuring of optical fibers.

[0093] During insolation, the modulation of the refractive index is relatively low, of the order of 10 -3 < . However, femtosecond laser inscription allows the production of Bragg mirrors extending over short lengths, for example of the order of mm. This type of insolation makes it possible to obtain a very fine resonant cavity 26, the width of the resonance peak being less than a few tens of pm, or even less than 10 pm, and possibly being of the order of or less than 5 pm.

[0094] It is possible to increase the length by which each Bragg mirror extends. This results in an even sharper resonance peak.

[0095] Thus, when using the first photoacoustic embodiment, it is possible to detect the presence of an analyte, or even estimate its concentration, in an analyzed medium, by implementing the following steps, represented on the figure 7A : Step 110: application of the device 1 to the medium, such that the contact face is held against the medium; Step 120: activation of the excitation light source 10, the excitation light source emitting an excitation light wave 11, pulsed or amplitude modulated, according to an excitation frequency f11, in a wavelength corresponding to an absorption wavelength λ 4 of the analyte; Step 130: under the effect of the illumination of the medium by the excitation light wave, emission of a photoacoustic wave 6, propagating through the hollow cavity 16, following which the membrane of the transducer vibrates at the excitation frequency f 11, such that the resonant frequency λ r of the waveguide 20 of the transducer is periodically modulated according to a modulation frequency f λr equal to the excitation frequency f 11 . Step 140: determination, by the control circuit 41, of a temporal modulation of a resonance wavelength λ r of the waveguide 20, at the excitation frequency f11, the resonance wavelength corresponding to a transmission peak of the waveguide; Step 150: as a function of the temporal modulation determined by the control circuit, calculation of a modulation amplitude of the resonance wavelength at a frequency dependent on the excitation frequency, and in particular at a frequency twice the excitation frequency; Step 160: detection of the presence of the analyte in the medium and / or estimation of an analyte concentration as a function of the modulation amplitude.

[0096] The detection of the presence of analyte or the estimation of the concentration can be carried out by taking into account a calibration, carried out by implementing a calibration sample, representative of medium 2, comprising a known quantity of analyte.

[0097] THE figures 8A à 8C illustrate a second embodiment, called photothermal, of a device 1' according to the invention. The device 1' is configured to be applied against a medium 2 to be analyzed.

[0098] The device 1' comprises components as described in connection with the first embodiment. A difference is that the membrane 18 forms the support wall 3, through which an opening 13 is provided.

[0099] The device 1' comprises an excitation light source 10, emitting an excitation light wave 11. This is a pulsed laser source, the pulse frequency being for example between 10 Hz and 500 Hz, for example 100 Hz. The excitation light wave 11 propagates to the medium 2. According to this embodiment, the excitation light source is configured such that the excitation light wave 11 propagates to the medium 2 through the opening 13 made through the membrane 18.

[0100] Under the effect of the presence of an analyte 4 in the medium, a part of the excitation wave is absorbed. This results in a heating 5 of the medium 4. When the analyte is present in a superficial part of the medium, the heating of the medium 5 propagates, by thermal diffusion, to the membrane 18 forming the contact face. By superficial part of the medium, we mean a part between the contact face and a depth of up to 2 or 3 times the thermal penetration depth of the material forming the medium analyzed.

[0101] The membrane 18 preferably has thermal conductivity, such that the temperature of the membrane can be considered as following the changes in the temperature of the medium 2, possibly with a time lag. The membrane is sufficiently thin to have such thermal conductivity.

[0102] According to this embodiment, the transducer 15 is a thermal transducer: its function is to detect, and preferably to quantify, a periodic modulation of the temperature of the membrane under the effect of the periodic excitation of the medium 2 by the excitation light wave 11.

[0103] The transducer 15 comprises an auxiliary light source 30, a waveguide 20, a photodetector 36 and a servo circuit 41 as described in connection with the photoacoustic embodiment.

[0104] Under the effect of heating of the membrane 18, the temperature of the waveguide 20 varies. This results in a variation of the resonance wavelength λr, in particular due to the variation of the refractive indices of the materials 21, 22 making up the waveguide.

[0105] The variation of the resonance wavelength under the effect of temperature can be explained by the expression: 1 λ B × ∂ λ B ∂ T = 6.67 × 10 − 6 ° C − 1

[0106] T corresponds to the temperature.

[0107] When the wavelength λ 32 of the auxiliary source 32 is equal to 1.55 µm, the sensitivity of the transducer can be estimated at 11 pm / °C.

[0108] When implementing the photothermal embodiment, the presence of an analyte can be detected, or even its concentration estimated, in an analyzed medium, by implementing the following steps, shown in the figure 7B . Step 110: application of the device 1 to the medium, such that the contact face, in this case the membrane, is held against the medium; Step 120 activation of the excitation light source 10, the excitation light source emitting an excitation light wave 11, at an excitation frequency f11 in a wavelength corresponding to an absorption wavelength λ 4 of the analyte; Step 135: under the effect of the illumination of the medium by the excitation light wave, periodic heating of the medium 2, propagating, by thermal diffusion, to the membrane 18, such that the resonance frequency λ r of the waveguide 20 of the transducer is periodically modulated according to a modulation frequency f λr equal to the excitation frequency f 11. Step 140: determination, by the control circuit 41, of a temporal modulation of a resonance wavelength λ r of the waveguide 20, at the excitation frequency f11, the resonance wavelength corresponding to a transmission peak of the waveguide, the modulation; Step 155: as a function of the temporal variation determined by the control circuit, calculation of an amplitude of variation of the resonance wavelength Step 165: detection of the presence of the analyte in the medium and / or estimation of an analyte concentration as a function of the variation.

[0109] The detection of the presence of analyte or the estimation of the concentration can be carried out by taking into account a calibration, carried out by implementing a calibration sample, representative of medium 2, comprising a known quantity of analyte.

[0110] The invention may be implemented on gaseous, liquid or solid samples, in analyte detection applications in the fields of the environment, industry, for example the agri-food industry, or the biomedical field.

Claims

1. A detecting device (1, 1'), intended to be applied, via a contact face (3), against a medium to be analyzed (2), the analyzed medium being liable to contain an analyte (4), which absorbs light at at least one absorption wavelength, the device comprising: - an aperture (13), formed in the contact face (3); - an exciting light source (10), configured to emit an exciting light wave (11), which is pulsed or amplitude-modulated at an excitation frequency (f11), in an excitation spectral band (Δλ11) comprising the absorption wavelength, the device being arranged such that the exciting light wave (11) propagates through the aperture (13), toward the analyzed medium; - a transducer (15), intended to measure a response of the medium following periodic heating of the medium resulting from absorption, by the analyte, of some of the exciting light wave (11); the device being such that the transducer (15) comprises: - a membrane (18), carrying a waveguide (20); - the waveguide comprising a first reflector (241), and a second reflector (242), each reflector reflecting light in a reflection spectral band (Δλ20), - the first reflector and the second reflector being spaced apart from each other, so as to form a resonant optical cavity (26), the resonant optical cavity defining a resonant wavelength (λr), in the reflection spectral band; - such that the waveguide: • transmits light at the resonant wavelength (λr); • reflects light, in the reflection spectral band (Δλ20), not of the resonant wavelength; the transducer also comprising: - a source (30) of auxiliary laser light, configured to emit an auxiliary light wave (32), in the reflection spectral band (Δλ20), into the waveguide; - a photodetector (36), arranged to detect a light wave (34) transmitted by the waveguide at the resonant wavelength (λr); - a servo circuit (41), connected to the photodetector (36), and configured to determine a periodic time-dependent modulation (λr(t)) of the resonant wavelength of the resonant optical cavity (26); the device being such that: - the waveguide (20) is formed directly on the membrane; - the membrane (18) is configured to deform under the effect of the periodic heating of the medium; - the servo circuit (41) comprises a servo loop, connected to the auxiliary light source (30), and configured to servo-control the wavelength (λ32) of the light wave (32) emitted by the auxiliary light source (30) to the resonant wavelength (λr) of the resonant optical cavity (26).

2. The device as claimed in claim 1, comprising a processing unit (42), connected to the servo circuit, and configured to: - estimate an amplitude (Aλr) of the time-dependent modulation of the resonant wavelength (λr); - detect the presence of the analyte in the medium (2) depending on the estimated amplitude.

3. The device as claimed in claim 2, wherein the processing unit is configured to estimate a concentration of the analyte in the medium (2) depending on the estimated amplitude (Aλr).

4. The device (1) as claimed in any one of the preceding claims, comprising a hollow cavity (16) that opens onto the aperture (13), the transducer being connected to the hollow cavity, and wherein the transducer (15) is an acoustic transducer configured to detect an amplitude of a photoacoustic wave (6) that propagates from the aperture through the hollow cavity, such that, under the effect of the illumination of the medium by the exciting light wave (11), the membrane (18) vibrates at the excitation frequency (f11), resulting in the time-dependent modulation of the resonant wavelength (λr), at a modulation frequency (fλr) equal to the excitation frequency.

5. The device as claimed in claim 4, wherein the membrane (18) lies parallel to the contact face (3).

6. The device (1') as claimed in any one of claims 1 to 3, wherein: - the membrane (18) of the transducer (15) forms the contact face (3) of the device, the contact face being intended to be applied so as to make contact with the medium (2); - the aperture (13) extends through the membrane (20); - the transducer (15) is a thermal transducer, such that, under the effect of the illumination of the medium by the exciting light wave (11), the temperature of the membrane exhibits a periodic time-dependent variation, resulting in the periodic time-dependent modulation of the resonant wavelength (λr).

7. The device as claimed in any one of the preceding claims, wherein at least one reflector, or each reflector, is a Bragg mirror, formed via a periodic modulation of a refractive index along the waveguide.

8. The device as claimed in claim 8, wherein the servo circuit (41) implements a Pound-Drever-Hall servo technique.

9. The device as claimed in any one of the preceding claims, wherein: - the first reflector (241) is a first Bragg mirror; - the second reflector (242) is a second Bragg mirror; - the first Bragg mirror and the second Bragg mirror form the same Bragg mirror, the latter comprising a defect (25), the first Bragg mirror and the second Bragg mirror corresponding to the portions of the Bragg mirror lying on either side of the defect, respectively.

10. The device as claimed in any one of claims 1 to 5, wherein: - the membrane (18) exhibits, when it deforms, at least one vibration antinode, the amplitude of vibration being maximum at each antinode; - the waveguide (20) lies level with at least one antinode.

11. The device as claimed in any one of the preceding claims, wherein the first reflector and the second reflector are obtained by inscribing the waveguide with a laser beam, so as to obtain a periodic modulation of the refractive index in the waveguide.

12. A method for detecting an analyte (4) in a medium (2), the analyte (4) absorbing light at at least one absorption wavelength, the method comprising the following steps: b) applying a device (1, 1') according to any one of the preceding claims against the medium (2), such that the contact face (3) of the device is held against the medium; c) activating the exciting light source (10), the exciting light source emitting an exciting light wave (11), which is pulsed or amplitude-modulated at an excitation frequency (f11), with a wavelength corresponding to an absorption wavelength of the analyte; d) determining, by means of the servo circuit (41), a periodic modulation of a resonant wavelength (λr) of the waveguide (20) of the transducer (15), at a modulation frequency corresponding to the excitation frequency, the resonant wavelength corresponding to a transmission peak of the waveguide; e) depending on the periodic modulation determined by the servo circuit, detecting the presence of the analyte in the medium.

13. The method as claimed in claim 12, wherein the device is a device (1) as claimed in any one of claims 4 to 5, the method comprising: - following step b), heating the medium (2) periodically, at the excitation frequency (f11), so as to cause an emission of a photoacoustic wave (6), which propagates through the hollow cavity (16), and under the effect of which the membrane (18) of the transducer (15) vibrates at the excitation frequency (f11), such that the resonant frequency (λr) of the waveguide (20) of the transducer is modulated at a modulation frequency (fλr) equal to the excitation frequency (f11); - in step d): • estimating an amplitude (Aλr) of the periodic modulation of the resonant wavelength, at the modulation frequency (fλr); • detecting the presence of analyte depending on the estimated amplitude.

14. The method as claimed in claim 12, wherein the device is a device (1') as claimed in claim 6, the method comprising: - following step b), heating the medium (2) periodically, at the excitation frequency, so as to cause periodic heating (5) of the membrane (18) at the excitation frequency (f11), such that the resonant frequency (λr) of the waveguide (20) of the transducer is modulated at a modulation frequency (fλr) corresponding to the excitation frequency; - in step d): • estimating an amplitude of the periodic modulation of the resonant wavelength at the modulation frequency; • detecting the presence of analyte depending on the estimated amplitude.

15. The method as claimed in any one of claims 12 to 14, wherein step d) also comprises estimating a concentration of the analyte in the analyzed medium.