Photoacoustic or photothermal detectors comprising optical transducers

Abstract

The present invention relates to an apparatus and method for detecting an analyte in a medium. An excitation light source generates an excitation light wave that propagates to the medium and heats the medium. The apparatus includes a transducer for detecting the heating of the medium. According to one embodiment, the transducer is a thermal transducer configured to detect changes in the temperature of the medium. According to another embodiment, the transducer is an acoustic transducer configured to detect photoacoustic waves propagating from the medium. In either embodiment, the transducer comprises a membrane having a waveguide disposed thereon. The waveguide comprises a resonant optical cavity. Energy conversion is achieved by analyzing changes in the resonant wavelength of the optical cavity.

Description

Technical Field

[0001] The technical field of the present invention is the detection of analytes in a medium based on the principles of photoacoustic detection or photothermal detection. Background Art

[0002] Photoacoustic detection is based on the detection of acoustic waves generated by the absorption of pulsed or amplitude-modulated electromagnetic excitation waves (called excitation waves) by the medium being analyzed. Upon heating of absorbing molecules present in the medium being analyzed, the absorption of the excitation waves generates acoustic waves. Heating causes a modulated thermal expansion of the medium, which results in acoustic waves.

[0003] By setting the wavelength of the excitation wave to the absorption wavelength of the analyte, photoacoustic detection can be targeted to a specific analyte. Consequently, photoacoustic detection has been applied to detecting gaseous species in gases or the presence of specific molecules in biological tissue. The wavelength of the incident wave is often in the infrared.

[0004] Photoacoustic detection is a non-invasive analytical technique that can be applied to diffuse or opaque media.

[0005] The application of photoacoustic detection to biological tissue is described, for example, in the following publications:

[0006] -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);

[0007] - "Windowless ultrasound photoacoustic cell for in-vivo mid-IRspectroscopy of human epidermis: Low interference by changes of air pressure, temperature, and humidity caused by skin contact opens the possibility for anon-invasive monitoring of glucose in the interstitial fluid", Rev.Sci.Instrum.84,084901(2013).

[0008] These publications use a pulsed laser source activated at a frequency of tens of kilohertz. The goal is to estimate the glucose concentration in the interstitial fluid at a depth of 10 to 50 μm below the user's skin. For this purpose, a photoacoustic detection device is used, positioned in close proximity to the user's skin.

[0009] Photothermal detection techniques are also known, which are based on detecting the temperature change of the analyte medium due to the absorption of pulsed or amplitude-modulated excitation electromagnetic waves by the medium. The temperature change is caused by the heating of absorbing molecules present in the analyte medium due to the absorption of the excitation wave.

[0010] By evaluating the change in the refractive index of the medium under the influence of temperature changes, it is possible, for example, to detect periodic temperature modulations, as described, for example, in EP 3 359 949 B1.

[0011] Whether photothermal or photoacoustic detection, the sample can be a gas sample, the purpose of which is to detect certain gaseous substances or certain particles that can be considered as pollutants. It can also involve liquid or solid samples, perhaps in industrial fields (such as the food industry) or, as mentioned above, in the health field.

[0012] The inventors have designed a transducer that can be used for applications employing photoacoustic detection or for applications employing photothermal detection. This allows obtaining a device specific to each application, with high detection sensitivity and a simple design. Summary of the Invention

[0013] A first subject of the invention is a detection device intended to be applied via a contact surface to a medium to be analyzed, which medium may contain an analyte which absorbs light at at least one absorption wavelength, said device comprising:

[0014] - an opening formed in the contact surface;

[0015] an excitation light source configured to emit an excitation light wave in an excitation spectral band including the absorption wavelength, said excitation light wave being pulsed or amplitude modulated at an excitation frequency, said device being arranged so that the excitation light wave propagates through the opening towards the medium to be analyzed;

[0016] - a transducer intended to induce the response of said medium after the analyte absorbs a portion of the excitation light wave;

[0017] The device is characterized in that the transducer comprises:

[0018] - a membrane carrying a waveguide;

[0019] - a waveguide comprising a first reflector and a second reflector, each reflector reflecting light in a reflection spectral band;

[0020] - a first reflector and a second reflector, the first reflector and the second reflector being spaced apart from each other to form a resonant optical cavity, the resonant optical cavity defining a resonant wavelength in a reflection spectral band;

[0021] So that the waveguide:

[0022] Transmit light at a resonant wavelength;

[0023] Reflection: Light outside the resonant wavelength in the reflection spectral band;

[0024] The transducer further comprises:

[0025] - an auxiliary laser source configured to launch an auxiliary light wave in a reflection spectral band into the waveguide;

[0026] - a photodetector arranged to detect light waves transmitted by the waveguide at the resonant wavelength;

[0027] A servo circuit is connected to the photodetector and is configured to determine a periodic time-dependent modulation of the resonant wavelength of the resonant optical cavity.

[0028] The apparatus may include a processing unit connected to the servo circuit and configured to:

[0029] - estimating the amplitude of the time-dependent modulation of the resonant wavelength;

[0030] - Based on the estimated amplitude, detecting the presence of the analyte in the medium.

[0031] The processing unit may be configured to estimate the concentration of the analyte in the medium based on the estimated amplitude.

[0032] According to one embodiment, referred to as a photoacoustic embodiment, the apparatus includes a cavity opening into an opening, a transducer coupled to the cavity. The transducer is an acoustic transducer configured to detect the amplitude of a photoacoustic wave propagating from the opening through the cavity, such that, upon illumination of the medium by the excitation light wave, the membrane vibrates at an excitation frequency, resulting in a time-dependent modulation of the resonant wavelength at a modulation frequency equal to the excitation frequency.

[0033] The membrane may be positioned parallel to the contact surface.The membrane may define a portion of the cavity.

[0034] According to one embodiment, referred to as the photothermal embodiment, the apparatus is such that:

[0035] - the membrane of the transducer forms a contact surface of the device, said contact surface being intended to be applied in contact with the medium;

[0036] - an opening extending through the membrane;

[0037] The transducer is a thermal transducer, so that under the action of the medium being irradiated with an excitation light wave, the temperature of the membrane exhibits a periodic temporal variation, resulting in a periodic time-dependent modulation of the resonant wavelength.

[0038] Whatever the embodiment, at least one reflector or each reflector is a Bragg mirror formed by a periodic modulation of the refractive index along the waveguide.

[0039] Regardless of which embodiment, the servo circuit comprises a servo loop connected to the auxiliary light source and configured to servo-control the wavelength of the light wave emitted by the auxiliary light source to the resonant wavelength of the resonant optical cavity. The servo circuit may particularly implement a Pound-Drever-Hall type servo technique.

[0040] Regardless of the embodiment, the device can enable:

[0041] - the first reflector is a first Bragg mirror;

[0042] - the second reflector is a second Bragg mirror;

[0043] - the first Bragg mirror and the second Bragg mirror form the same Bragg mirror comprising a defect, the first Bragg mirror and the second Bragg mirror respectively corresponding to the portions of the Bragg mirror situated on either side of the defect.

[0044] In a photoacoustic embodiment, the apparatus may be such that:

[0045] - the membrane exhibits, under vibration, at least one vibration antinode, the vibration amplitude being maximum at each antinode;

[0046] The waveguide is flush with at least one antinode of the vibration.

[0047] Regardless of which embodiment, the waveguide may be formed directly on the film.The first and second reflectors may be obtained by engraving the waveguide with a laser beam to obtain a periodic modulation of the refractive index in the waveguide.

[0048] Regardless of the embodiment, the waveguides may be microstructured optical fibers deposited on a film.

[0049] A second subject matter of the present invention is a method for detecting an analyte in a medium, said analyte absorbing light at at least one absorption wavelength, said method comprising the following steps:

[0050] a) applying the device according to the first subject matter of the invention to a medium such that the contact surface of the device remains on the medium;

[0051] b) activating an excitation light source that emits an excitation light wave in a wavelength corresponding to the absorption wavelength of the analyte, the excitation light wave being pulsed or amplitude modulated at an excitation frequency;

[0052] c) determining, by a servo circuit, a periodic modulation of a resonant wavelength of a waveguide of the transducer at a modulation frequency corresponding to the excitation frequency, the resonant wavelength corresponding to a transmission peak of the waveguide;

[0053] d) detecting the presence of an analyte in the medium based on the periodic modulation determined by the servo circuit.

[0054] The device may be the device described in relation to the photoacoustic embodiment. The method may include:

[0055] - after step b), periodically heating the medium at an excitation frequency to induce emission of photoacoustic waves propagating through the cavity and under the effect of which the membrane of the transducer vibrates at the excitation frequency so as to modulate the resonant frequency of the waveguide of the transducer at a modulation frequency equal to the excitation frequency;

[0056] - In step d):

[0057] Estimating the amplitude of the periodic modulation of the resonant wavelength at the modulation frequency;

[0058] Based on the estimated magnitude, detecting the presence of the analyte.

[0059] The device may be the device described in the photothermal embodiment. The method may include:

[0060] - after step b), periodically heating the medium at the excitation frequency to cause periodic heating of the membrane at the excitation frequency so as to modulate the resonant frequency of the waveguide of the transducer at a modulation frequency corresponding to the excitation frequency;

[0061] - In step d):

[0062] Estimating the amplitude of the periodic modulation of the resonant wavelength at the modulation frequency;

[0063] Based on the estimated magnitude, detecting the presence of the analyte.

[0064] Regardless of the embodiment, step d) may also comprise estimating the concentration of the analyte in the medium being analyzed.

[0065] A third subject of the invention consists in a method for manufacturing a device according to the first subject of the invention, the waveguide being formed directly on the film, said method comprising:

[0066] - depositing a thin layer of a first material on the membrane to form a waveguide;

[0067] - Engraving the waveguide with a femtosecond laser beam to obtain a periodic modulation of the waveguide's refractive index.

[0068] The present invention will be better understood by reading the description of the embodiments described in the remainder of this specification with reference to the drawings listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figures 1A to 1E The main components of a device according to one embodiment, referred to as the photoacoustic embodiment, are schematically shown.

[0070] Figure 2A Shown is a microstructured optical fiber forming a Bragg mirror.

[0071] Figure 2B Show Figure 2A The reflection spectrum band of the optical fiber is schematically shown in FIG.

[0072] Figure 2C A microstructured optical fiber is shown forming a resonant optical cavity based on two spaced-apart Bragg mirrors.

[0073] Figure 2D Show Figure 2C The reflection spectrum band of the optical fiber is schematically shown in FIG.

[0074] Figure 3A A waveguide comprising a resonant optical cavity is shown, the waveguide being undeformed.

[0075] Figure 3B A waveguide comprising a resonant optical cavity is shown, the waveguide being deformed.

[0076] Figure 3C A waveguide is shown, the deformation of which is not uniform.

[0077] Figure 3D Shown as Figure 3C Shown is the change in the reflectance spectral band of the waveguide under deformation.

[0078] Figure 3E The deformation amplitude of the membrane along its diameter is shown.

[0079] Figure 3F The optimally positioned waveguide is shown, and the deformation of the waveguide is shown as Figure 3D shown.

[0080] Figure 4A The diagram schematically shows a circuit for servo-controlling the wavelength of the auxiliary light source using a wavelength locking method.

[0081] Figure 4B Shown is the variation of the error function obtained using the wavelength locking method.

[0082] Figure 5A The shift of the resonant wavelength of the resonant optical cavity due to the vibration of the membrane is shown.

[0083] Figure 5BShows the time-dependent modulation of the resonant light wavelength in the fiber.

[0084] Figure 5C Schematic illustration of the estimation of the vibration amplitude of the membrane based on the time-dependent modulation of the resonant wavelength.

[0085] Figures 6A to 6D The main steps of the manufacturing method allowing the formation of a microstructured waveguide in contact with a membrane are shown.

[0086] Figure 7A The main steps of a detection method using an apparatus according to a photoacoustic embodiment are shown.

[0087] Figure 7B The main steps of a detection method using a device according to a photothermal embodiment are shown.

[0088] Figures 8A to 8C The main components of a device according to one embodiment, referred to as the photothermal embodiment, are schematically shown. DETAILED DESCRIPTION

[0089] Figures 1A to 1E A first embodiment, referred to as the photoacoustic embodiment, of the device 1 according to the invention is shown. The device 1 is configured to be applied to a medium 2 to be analyzed.

[0090] The device comprises an excitation light source 10 configured to emit an excitation light wave 11 that propagates to the medium 2 to be analyzed. 11 , the light source 10 is pulsed or amplitude modulated. The light wave 11 is in the excitation spectrum band Δλ including the absorption wavelength λ4 of the analyte 4 present in the medium. 11 The purpose of the device 1 is to detect the presence of an analyte 4 and possibly estimate its concentration.

[0091] The excitation spectral band is preferably located in the visible or infrared, for example extending between a wavelength of 3 μm and 15 μm. 11 Narrow enough to make the device 11 specific for a single analyte. For example, the width of the emission spectrum band is about 1 cm -1 When the analyte is glucose, the emission band is centered at the absorption wavelength of glucose, for example, 1034 cm -1 The excitation light source 10 may be a pulsed laser source, for example a wavelength-tunable laser of the QCL (Quantum Cascade Laser) type, with an emission spectrum band Δλ located in the infrared.

[0092] Analytes 4 can be molecules present in the medium being analyzed. When the medium is biological tissue, this can involve glucose present in the body fluid of the biological tissue. As described in conjunction with the prior art, the analytes can be gas molecules, and the medium is a gas. These can be, for example, gas molecules considered contaminants. Alternatively, the medium can be a liquid, and the analytes can be molecules potentially present in the liquid.

[0093] The device 1 comprises a contact surface 3 intended to be applied to the medium to be analyzed 2, in order to come into contact with the medium to be analyzed. The contact surface 3 is designed to conform to the medium 2 on which it is intended to be applied. It is, for example, flat.

[0094] The device 1 comprises a housing 17 extending from the contact surface 3 and defining a cavity 16. The cavity 16 comprises an opening 13 formed in the contact surface 3 to provide access to the medium 2. The excitation light source 10 is configured such that an excitation light wave 11 propagates through the cavity 16 and through the opening 13 to the medium 2.

[0095] Under the action of the analyte 4 in the medium 2, a photoacoustic wave 6 is formed. The photoacoustic wave 6 is caused by the medium being excited at the excitation frequency f 11 The acoustic wave 6 is formed by periodically heating the incident light wave 11 with the amplitude down-modulated. A portion of the photoacoustic wave 6 propagates through the cavity 16 to be detected by the transducer 15.

[0096] In the photoacoustic embodiment, the transducer 15 is an acoustic transducer. Its function is to measure the amplitude and / or frequency of the photoacoustic wave 6. More specifically, in the target application, the transducer 15 allows estimating the excitation frequency f of the excitation light wave. 11 The amplitude of the photoacoustic wave 6.

[0097] The transducer 15 comprises a flexible membrane 18 configured to vibrate when it is exposed to the photoacoustic waves 6. The membrane 18 is preferably parallel to the radial plane P XY The diameter of the membrane, or its largest diagonal, is 1 mm to 10 mm. The thickness of the membrane, parallel to the transverse axis Z perpendicular to the radial plane, is preferably 10 μm to 500 μm, preferably 10 μm to 100 μm. The thickness of the membrane is preferably 1 / 10 to 1 / 200 of the radius of the membrane (or its largest half-diagonal).

[0098] The excitation light source 10 is configured to allow the excitation light wave 11 to pass through the cavity 16 and propagate to the medium 2. Figure 1A In the example shown, the excitation light wave 11 propagates through a second opening 19 which is formed by the membrane 18 .

[0099] Second opening 19 formed in the membrane also allows the pressure on either side of membrane 18 to equalize at low frequencies. This prevents possible deformation of membrane 18 due to low-frequency variations in pressure on either side of membrane 18. Low-frequency variations in pressure refer to pressure differences occurring at frequencies below the membrane's operating frequency range. The diameter of second opening 19 is, for example, less than one-tenth the diameter of the membrane. It can be, for example, approximately 10 μm or 20 μm.

[0100] The transducer 15 comprises a waveguide 20 extending over, in contact with and parallel to the membrane 18. The waveguide is at the input 20 i The waveguide is made of a first material 21 having a first refractive index n1.

[0101] The waveguide 20 may be an optical fiber, in which case the first material is the material of the core of the optical fiber. It may also involve a waveguide formed by deposition of a thin layer of a first material 21, such as SiON (silicon oxynitride), which corresponds to Figures 1A to 1E The waveguide is defined by a confining material 23 having a refractive index less than the refractive index n3 of the first material n1. When the waveguide 20 is an optical fiber, the confining material 23 is the sheath of the optical fiber.

[0102] The light guide 20 is advantageously formed on the basis of a thin layer of a first material 21, the confinement material 23 being simply air surrounding the first material. The thickness of the waveguide along the transverse axis Z is preferably less than 10 μm or 5 μm. Figures 6A to 6D A method for forming such a waveguide is described. Forming the waveguide directly on the membrane allows avoiding the step of gluing the optical fiber to the membrane. Another advantage over using optical fibers is that this allows obtaining a less rigid waveguide.

[0103] Regardless of which arrangement is used, the refractive index n3 of the confinement material 23 is less than the refractive index n1 of the first material 21. When the first material 21 is deposited directly on the film 18, it is preferred that the refractive index n1 of the first material 21 is greater than the refractive index of the material forming the film 18.

[0104] exist Figure 1B An example of a waveguide 20 is shown in detail in FIG. In this example, a first material 21 is deposited on a membrane 18. The waveguide includes portions of a second material 22 having a second refractive index n2 that are periodically distributed along the waveguide 20. The second refractive index n2 is different from the first refractive index n1. It may be greater than the first refractive index n1. The relative change between the first and second refractive indices may be 0.01% (10 -4 ) to 0.1% (10 -3 ).

[0105] Along the axis defined by the waveguide, the refractive index is periodically modulated between n1 and n2 to reflect the spectrum in the spectral band Δλ.20 A Bragg mirror is formed in the quartz crystal. The structure of a Bragg mirror is known to those skilled in the art. This involves a structure in which the refractive index varies periodically so that along the axis along which the light propagates, the mirror is formed by alternating sections of two different refractive indices, each section having an optical thickness of λ. B / 4n i , where λ B is the reflection spectral band Δλ 20 The central wavelength, n i is the refractive index of the material under consideration (n i =n1 or n i =n2). The lower the refractive index contrast, the greater the number of periods.

[0106] Reflection spectrum band Δλ 20 Centered on the resonant wavelength λ r The resonant wavelength is such that:

[0107] λ r =λ B =2n eff Λ(1)

[0108] in:

[0109] -n eff is the effective refractive index of the grating, such that:

[0110]

[0111] -Λ is the spatial period of the grating, ie the length of two consecutive portions 21 and 22 along the axis of the waveguide.

[0112] The waveguide 20 is such that the Bragg mirror formed by the alternation of the portions 21 and 22 includes a defect. A "defect" is a local interruption in the period of the refractive index modulation. A defect corresponds, for example, to a continuous space 25 made of a given material (for example, the first material 21) extending for the length of one period Λ or for the length of a plurality of consecutive periods. At the defect, the waveguide comprises a single material extending for a distance d along the axis of the waveguide 20. When the distance d is such that When k is a positive natural number, the resonant wavelength λ is limited. r The resonant Fabry-Perot cavity 26. When the defect extends the length of a single period Λ, λ r =λ B .

[0113] When d>kλ B / n eff When the reflection spectrum band Δλ 20 Other resonant wavelengths λ may appear in r, the resonant wavelength is different from the Bragg wavelength λ B In such a case, it is preferable to retain the resonance wavelength at which the resonance peak is narrowest.

[0114] Thus, the defect allows separation of the first Bragg mirror 241 and the second Bragg mirror 242 in the waveguide 20. The assembly formed by the first Bragg mirror 241, the second Bragg mirror 242, and the space 25 between the Bragg mirrors forms a resonant cavity 26.

[0115] The waveguide 20 is structured as follows:

[0116] -Reflection at the resonant wavelength λ r The reflection spectrum band Δλ of the Bragg mirrors 241 and 242 is other than 20 the light in

[0117] - The resonant wavelength λ emitted in the resonant cavity 26 r The light of the place.

[0118] The transducer 15 also includes an input 20 arranged to feed a waveguide 20. i The auxiliary light source 30, in particular a laser diode, emits an auxiliary light wave 32. The auxiliary light wave 32 is centered at the emission wavelength λ 32 The emission band Δλ 32 Medium emission. Emission spectrum band Δλ 32 Preferably included in the reflection spectral band Δλ 20 middle.

[0119] Preferably, the emission spectrum band Δλ 32 The width of the reflection spectrum band Δλ 20 The width of the emission spectrum is narrower. For example, the emission spectrum band Δλ 32 The width of the emission spectrum can be 1nm, or even less than 500pm or 100pm. 32 The width refers to the full width at half maximum of the emission spectrum band.

[0120] The auxiliary light source 30 is preferably a continuous wave laser. It can be, for example, a DFB (DFB is the abbreviation of Distributed Feedback) type laser diode with a power of 1 mW, an emission wavelength of 1.55 μm, and a spectral width of about 1 pm. This type of laser diode is commonly used in the telecommunications field.

[0121] The transducer 15 comprises a photodetector 36, preferably a fast photodetector of the photodiode type. The photodetector has a reflective spectral band Δλ 20 Detection spectral band Δλ 36 .

[0122] The transducer 15 includes a configuration to follow the resonant wavelength λ of the resonant cavity 26.r Time-dependent modulation λ r (t) servo circuit 41. Such a circuit is combined with Figure 4A and 4B The servo circuit 41 allows the auxiliary light source 30 to be servoed so that the emission wavelength λ of the auxiliary light wave 32 emitted by the auxiliary light source is 32 Corresponding to the resonant wavelength λ of the resonant cavity 26 r .

[0123] The device comprises or is connected to a circuit configured to calculate the time-dependent modulation λ determined by the servo circuit 41 r Frequency (t) or amplitude The processing unit 42 is designed or programmed to process the excitation light wave 11 at an excitation frequency f. 11 The corresponding frequency The operation of the processing unit 42 will be combined with Figures 5A to 5C Explain in more detail.

[0124] The device comprises a cover 48 defining a back volume corresponding to the volume extending between the membrane 18 and the cover 48. In general, the waveguide 20 comprises a resonant cavity 26 formed by a first reflector 241 and a second reflector 242 obtained by microstructuring the waveguide 20. In the examples provided in this specification, the first reflector 241 and the second reflector 242 are Bragg mirrors, but other types of microstructures are contemplated.

[0125] An important aspect of the present invention, which will be explained below, is related to the following facts:

[0126] - When the auxiliary light source 30 is activated and emits the emission wavelength λ 32 does not correspond to the resonant wavelength λ of the waveguide 20 (or more specifically, the resonant cavity 26). r When the corresponding light wave 32 is reflected, the waveguide 20 reflects the reflected wave 32'; but

[0127] - When the auxiliary light source 30 is activated and emits the emission wavelength λ 32 Corresponding to the resonant wavelength λ of the resonant cavity 26 r When the light wave 32 is emitted, the waveguide 20 transmits the transmitted wave 34 to the photodetector 36. 32 The closer to the resonant wavelength λ r , the greater the intensity of the transmitted wave 34.

[0128] The present invention is based on the fact that when exposed to sound of frequency f a When the acoustic wave 6 is generated, the membrane 18 is at the frequency f of the photoacoustic wave 6. a Next, according to the vibration amplitude Aa This results in periodic deformation of the waveguide 20. Under the action of this periodic deformation, the resonant wavelength λ r Presents periodic time-dependent modulation λ r (t). Amplitude of the time-dependent modulation of the resonant wavelength Depends on the membrane and the sound amplitude A a Proportional to the vibration amplitude. Time-dependent modulation of the frequency Corresponding to the audio frequency f a , which also corresponds to the excitation frequency f 11 By calculating the frequency The modulation amplitude under The device allows the detection of the presence of an analyte in a medium and possibly the estimation of the concentration, e.g. Figures 5A to 5C As stated.

[0129] Figure 1C Show combination Figure 1A Some of the elements are in the radial plane P XY In this example, the membrane 18 takes the form of a thin disk with a thickness of one hundredth of the radius.

[0130] Figure 1D is a 3D view showing the position of the membrane 18 and the waveguide 20 relative to the medium 2 being analyzed.

[0131] Figure 1E An example of a device is shown, wherein an excitation light source 10 is associated with a reflector 14. The excitation light wave 11 is parallel to the radial plane P XY is emitted and then reflected by the reflector 14 toward the opening 13 to propagate toward the medium 2. The auxiliary light source 30 and the photodetector 36 are aligned relative to the waveguide 20. Alternatively, the auxiliary light source 30 and / or the photodetector 36 may be coupled to the waveguide 20 through a photonic crystal.

[0132] According to a variant, the membrane 18 of the transducer 15 is connected to the cavity 16 by an acoustic channel which transmits a portion of the photoacoustic wave 6 towards the membrane 18 .

[0133] Figure 2A An embodiment is shown in which the waveguide 20 is a microstructured optical fiber having a Bragg grating formed therein. This type of microstructuring in optical fiber is often referred to as a fiber Bragg grating (FBG). The optical fiber comprises a first material 21 forming the core and a confining material 23 forming the sheath. A cavity or cavity containing a second material 22 having a refractive index different from that of the first material is formed in the core of the optical fiber. Figure 2BThe reflection spectrum of the optical fiber thus microstructured is shown. The reflection spectrum corresponds to the reflected intensity (y axis) normalized by the irradiation intensity according to the wavelength (x axis - in nm). 20 Thus, when the waveguide is reflected by the spectral band Δλ 20 When the light wave 32 is irradiated, it reflects the whole spectrum of Δλ 20 The reflected light wave 32'.

[0134] 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 conjunction with Figure 1B When the length of the space 25 corresponds to the resonant wavelength included in the reflection spectrum band When the optical fiber comprises a resonant cavity 26 .

[0135] Figure 2D The reflection spectrum of the thus microstructured optical fiber is shown. r In addition, in the reflection spectrum band Δλ 20 Thus, when such a waveguide is supplemented by a light source 30 in the reflection spectrum band Δλ 20 When the light wave 32 emitted from the 32 Different from the resonant wavelength, it reflects the light wave 32' and when the wavelength λ 32 When in the resonance peak, it transmits a light wave 34 called the transmitted light wave.

[0136] Figure 2C and 2D was obtained using a model written in Matlab (registered trademark - Mathworks), where the structure is considered to extend along a length L of 3 mm and the refractive index contrast between the first and second materials is 10 -3 , and the period of each Bragg mirror is considered to be about 0.5 μm. Thus, each Bragg mirror has a period number equal to 3000.

[0137] Figures 3A to 3C Shown as Figure 1B Or the resonant wavelength λ caused by the deformation of the waveguide 20 described in 2C r changes. Figure 3A and 3B The waveguide 20 is shown without deformation and with deformation, respectively. Under the effect of the deformation, the spatial period of the refractive index modulation changes from Λ to Λ'=Λ+dΛ. Applying expression (1) results in the reflection spectral band Δλ extending around the Bragg wavelength λ B The offset dλ B .Offset dλ B is as follows:

[0138]

[0139] in:

[0140] -ε corresponds to the deformation, which corresponds to 10 -4 The deformation ε is the normalized change in length such that:

[0141]

[0142] -× is the multiplication operator.

[0143] By considering that the film 18 is composed of SiO2 and the refractive index jump between the first material and the second material is 10 -3 When , expression (3) is obtained. It is based on the uniform deformation of each Bragg mirror, as Figure 3B According to expression (3), for a deformation of 1 microstrain, the Bragg wavelength λ B The offset dλ B It is 1.2pm.

[0144] exist Figure 3C In FIG, a non-uniform deformation of the waveguide 20 is shown, with some parts of the Bragg mirror being less deformed than others.

[0145] Figure 3D is shown in conjunction with Figure 3C Model of the variation in the Bragg mirror's reflection spectrum for the described configuration. Curves a, b, and c correspond to no deformation, deformations between 0 and 10 microstrain, and deformations between 4 and 6 microstrain, respectively. The spectral shift is small, less than 10 pm. Curves b and c correspond to the same average waveguide deformation of 5 microstrain. The shift in the resonant wavelength between these two settings is due to the variation in deformation along the waveguide axis in the ranges of 0 to 10 microstrain and 4 to 6 microstrain, respectively. The more uniform the deformation, the greater the spectral shift in the resonant wavelength due to the deformation.

[0146] Preferably, the waveguide 20 extends over the portion of the membrane 18 that experiences the greatest deformation. The membrane 18 exhibits one or more vibration antinodes, where the vibration amplitude is greatest. 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 also increases the spectral shift caused by the deformation. This results in better sensitivity.

[0147] like Figure 1A and 1D As shown schematically, the inventors have modeled the deformation of a membrane 18. The modeled 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. Figure 3E The deformation of the membrane along one of its diameters is shown in , with the x-axis corresponding to the distance in mm relative to the center of the membrane and the y-axis corresponding to the deformation in microstrain. The resonant cavity 26 is preferably arranged flush with the maximum deformation amplitude, i.e. at the center of the membrane 18. Figure 3E The simulations shown show that for this membrane, applying a pressure of 1 Pa results in a tens of -2 Microstrain deformation.

[0148] exist Figure 3E The deformation of the membrane is in the central part 2 c The center is negative, while the peripheral part 2 p The resonance cavity 26 is advantageously arranged in the portion of the membrane 2 in which, under the effect of the vibrations of the membrane, the deformations have the same sign, whether compression or expansion.

[0149] Figure 3F The waveguide 20 is schematically shown, the resonant cavity 26 of which is arranged in the central part 2 of the membrane 2 c The waveguide extends on either side of the center of the membrane by a distance of ±0.5 mm relative to the center. Under the effect of the vibration of the membrane, the deformation is alternately negative (ε<0), as Figure 3E When the deformation is negative, the optical cavity is compressed: the parts of the second material 22 move closer to each other. When the deformation is positive, the optical cavity is expanded: the parts of the second material 22 move away from each other.

[0150] Figure 4A and 4B The operation of the servo circuit 41 is schematically shown, and its function is to convert the wavelength λ 32 Servo-controlled to the resonant wavelength λ of the resonant cavity 26 formed in the waveguide 20 r The servo circuit 41 sets the wavelength λ 32 Locked to the resonant wavelength λ rWavelength locking is often referred to as "top of fringe locking". This involves circuits using a Pound-Drever-Hall type servo technique, as described, for example, 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 even in the publication Black E. "An introduction to Pound-Drever-Hall laser frequency stabilization", Am. J. Phys. 69 (1), January 2001.

[0151] The servo circuit 41 includes a circuit for modulating the wavelength λ of the auxiliary light wave 32 emitted by the auxiliary light source 30 at a modulation frequency varying from 10 kHz to several hundred MHz. 32 Modulator 411. Emission wavelength λ 32 The modulation frequency is significantly higher than the maximum sound frequency handled by the device. It can be, for example, 10 times the maximum sound frequency handled by the device. The intensity of the light wave 34 emitted from the waveguide 20 and detected by the photodetector 36 is transmitted to the servo circuit 41, which measures a function h representing the variation of the intensity detected by the photodetector 36 as a function of the wavelength modulation.

[0152] 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 32 For example, when the detected intensity change relative to the wavelength increase is negative, the emission wavelength is gradually reduced. When the detected intensity change relative to the wavelength increase is positive, the emission wavelength is increased. When the detected intensity change caused by the modulation is close to zero, the emission wavelength corresponds to the resonant wavelength of the waveguide. The servo circuit 41 utilizes the following facts:

[0153] -When λ 32 <λ r When the wavelength λ 32 The increase of λ results in an increase in the intensity of the transmitted wave 34. Conversely, decreasing the wavelength λ 32 resulting in a reduction in the intensity of the transmitted wave 34;

[0154] -When λ 32 >λ r When the wavelength λ 32 The increase of λ results in a decrease in the intensity of the transmitted wave 34. Conversely, decreasing the wavelength λ 32This results in an increase in the intensity of the transmitted wave 34 .

[0155] Thus, by applying the wavelength λ of the auxiliary light wave 32 32 By observing the effect of the modulation on the intensity of the transmitted light wave 34, the auxiliary light source 30 can be servo-controlled so that the wavelength λ of the auxiliary light wave 32 is 32 Following the resonant wavelength λ of the waveguide 20 r .

[0156] Tracking the resonant wavelength by edge locking allows for a gain of approximately 10 at frequencies above 10 kHz. -6 The wavelength sensitivity of pm, or about 10 when the sound frequency is below 1kHz -3 The wavelength sensitivity of the resonant wavelength is pm, tracking the resonant wavelength. According to expression (3) applicable to SiO2 films, it is estimated that this sensitivity will allow the estimation of film deformations of the order of a few pico-strains, or equivalent to a few mPa. Therefore, the Pound-Drever-Hall method is suitable considering the small spectral shifts of the resonant cavity 26, which may be of the order of a few pm.

[0157] Tracking the resonant wavelength by edge-locking the wavelength also allows for insensitivity to fluctuations in the resonant wavelength of the cavity 26 due to changes in environmental parameters such as temperature or humidity.

[0158] Figures 5A to 5C It shows that when the film 18 is under the action of the photoacoustic wave 6, the resonant wavelength λ r The periodicity of λ changes with time r The relationship between (t) and the amplitude of the acoustic wave. Due to the servo control performed by the servo circuit 41, the periodicity of the emission wavelength changes with time λ 32 (t) is considered to correspond to the time-dependent modulation λ of the resonant wavelength due to the vibration of the membrane r (t). Figure 5A The spectrum of the transmitted light wave 34 and the spectral shift dλ due to the deformation of the waveguide 20 are shown. r . Figure 5B shows the resonant wavelength λ due to the deformation of the waveguide 20 r The time-dependent modulation of Corresponding to the audio frequency f a By changing the wavelength λ 32 Servo-controlled to the resonant wavelength, the servo circuit 41 allows determining such a modulation. This modulation is transmitted to the processing unit 42 which calculates the modulation amplitude of the resonant wavelength Based on the amplitude, the processing unit 42 estimates the vibration amplitude of the membrane, which corresponds to the acoustic frequency f of the photoacoustic wave 6 a Based on the audio frequency f aThe vibration amplitude of the membrane can be used to determine the presence of the analyte 4 in the medium or to estimate the concentration of the analyte 4 in the medium.

[0159] Estimation of the concentration of analyte 4 in the medium may require prior calibration to establish:

[0160] - Relationship between the concentration of the analyte and the vibration amplitude of the membrane;

[0161] - or the relationship between the concentration of the analyte and the modulation amplitude of the resonant wavelength.

[0162] It should be pointed out that the sound amplitude A a The determination of the resonant wavelength does not necessarily require the determination of the value, but only the precise determination of the modulation amplitude

[0163] Figures 6A to 6D The main steps that allow the formation of the non-fiber optic waveguide 20 on the film 18 are shown.

[0164] A substrate 100, for example made of Si, is provided on which a first layer 101, for example made of SiO2 (refractive index 1.44) with a thickness of 4 μm and a second layer 102, for example made of SiON (silicon oxynitride - refractive index 1.60) with a thickness of 1 μm have been deposited. Figure 6A .

[0165] The method comprises:

[0166] - Etching the second layer 102 by photolithography to form the waveguide 20. Figure 6B In this example, SiON corresponds to the first material 21 of the waveguide.

[0167] - Etching on the back side of the substrate 100 to release a portion of the first layer 101 which forms the suspended membrane 18. This step also allows the formation of the housing 17 which defines the cavity 16. Figure 6C .

[0168] - Exposing the waveguide 20 point by point to femtosecond laser pulses to form cavities in the second material 22, an operation often referred to as engraving. Figure 6D Upon exposure to the laser, the refractive index of the 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, the exposure generates microbubbles, which induce a change in the refractive index. This results in a modulation of the refractive index of the waveguide 20 along the axis of light propagation within the waveguide.

[0169] The duration of each pulse is, for example, 100 fs, and at a wavelength of 800 nm, the energy of each pulse is 30 nJ. The pulse frequency can be from several Hz to 200 kHz.

[0170] Another exposure technique is UV light engraving as described in the following 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. UV light engraving allows, for example, optical fibers to be microstructured.

[0171] The modulation of the refractive index caused by exposure is relatively small, about 10 -3 However, engraving with the aid of a femtosecond laser allows the realization of Bragg mirrors extending along a short length (e.g., about 1 mm). This type of exposure allows obtaining resonant cavities 26 with high finesse, with the width of the resonance peak being less than a few tens of pm, or even less than 10 pm, and possibly of the order of or less than 5 pm.

[0172] The extension length of each Bragg mirror can be increased. This reduces the width of the resonance peak.

[0173] Thus, when the first photoacoustic embodiment is adopted, by implementing Figure 7A The following steps are shown, making it possible to detect the presence of an analyte in an analyte medium and even to estimate its concentration.

[0174] - Step 110 : applying the device 1 to the medium so that the contact surface remains on the medium;

[0175] - Step 120: Activate the excitation light source 10, which emits an excitation light wave 11 at a wavelength corresponding to the absorption wavelength λ4 of the analyte, said excitation light wave at an excitation frequency f 11 Pulse or amplitude modulation;

[0176] - Step 130: Under the action of the medium being irradiated by the excitation light wave, a photoacoustic wave 6 is emitted which propagates through the cavity 16, after which the membrane of the transducer is heated at the excitation frequency f 11 Vibrate so that it is equal to the excitation frequency f 11 Modulation frequency Periodically modulate the resonant frequency λ of the transducer 20 r .

[0177] - Step 140: Determine the resonant wavelength λ of the waveguide 20 through the servo circuit 41 r At the excitation frequency f 11 The time-dependent modulation under the condition of φ is the same as that under the condition of φ, the resonant wavelength corresponds to the transmission peak of the waveguide;

[0178] - Step 150: Calculation of the modulation amplitude of the resonant wavelength at a frequency dependent on the excitation frequency, in particular at a frequency twice the excitation frequency, from the time-dependent modulation determined by the servo circuit;

[0179] - Step 160: Detecting the presence of an analyte in the medium and / or estimating the analyte concentration based on the modulation amplitude.

[0180] Detecting the presence of an analyte or estimating the concentration can be performed by taking into account a calibration performed by performing a calibration sample representing the medium 2 comprising a known amount of the analyte.

[0181] Figures 8A to 8C A second embodiment of the device 1 ′ according to the invention, referred to as photothermal, is shown. The device 1 ′ is designed to be applied to a medium 2 to be analyzed.

[0182] The device 1 ' comprises the components as described in connection with the first embodiment. One difference is that the membrane 18 forms the support wall 3 through which the opening 13 is formed.

[0183] The device 1 ′ comprises an excitation light source 10 that emits an excitation light wave 11. This is a pulsed light source, with a pulse frequency of, for example, 10 Hz to 500 Hz, for example 100 Hz. The excitation light wave 11 propagates into the medium 2. According to this embodiment, the excitation light source is configured such that the excitation light wave 11 propagates into the medium 2 through an opening 13 formed by a membrane 18.

[0184] When analytes 4 are present in the medium, a portion of the excitation wave is absorbed. This results in heating 5 of the medium 4. When the analytes are present in the surface portion of the medium, the heating 5 of the medium propagates by thermal diffusion to the membrane 18 forming the contact surface. The surface portion of the medium refers to the portion between the contact surface and a depth of two or three times the thermal penetration depth of the material forming the medium being analyzed.

[0185] The membrane 18 preferably has a thermal conductivity such that the temperature of the membrane can be regarded as following (possibly after a time lag) the temperature variations of the medium 2. The membrane is thin enough to have such a thermal conductivity.

[0186] According to this embodiment, the transducer 15 is a thermal transducer: its function is to detect, and preferably quantify, the 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 .

[0187] The transducer 15 includes an auxiliary light source 30, a waveguide 20, a photodetector 36, and a servo circuit 41 as described in conjunction with the photoacoustic embodiment.

[0188] As a result of the heating of the membrane 18, the temperature of the waveguide 20 changes. This is due in particular to a change in the refractive index of the material 21, 22 forming the waveguide, which results in a change in the resonance wavelength λr.

[0189] The following expression can be used to explain the change of resonant wavelength under temperature:

[0190]

[0191] T corresponds to temperature.

[0192] When the wavelength λ of the auxiliary light source 32 32 When the sensitivity is equal to 1.55 μm, the sensitivity of the transducer can be estimated to be 11 pm / °C.

[0193] When using the photothermal embodiment, by implementing Figure 7B The following steps are shown, making it possible to detect the presence of an analyte in an analyte medium and even to estimate its concentration.

[0194] - Step 110 : applying the device 1 to the medium so that the contact surface (here the membrane) remains on the medium;

[0195] - Step 120: Activate the excitation light source 10 at an excitation frequency f 11 emitting an excitation light wave 11 at a wavelength corresponding to the absorption wavelength λ4 of the analyte;

[0196] - Step 135: Under the action of the excitation light wave irradiating the medium, a periodic heating of the medium 2 is obtained, and the generated heat is propagated to the film 18 by thermal diffusion, so that the medium 2 is heated at a frequency equal to the excitation frequency f. 11 Modulation frequency The resonant frequency λ of the waveguide 20 of the transducer is periodically modulated r ;

[0197] - Step 140: Determine the excitation frequency f by the servo circuit 41 11 The resonant wavelength λ of the waveguide 20 is r The time-dependent modulation of,the resonance wavelength corresponds to the transmission peak of the fluctuation,modulation;

[0198] - Step 155: Calculating the amplitude of the change in the resonant wavelength based on the time change determined by the servo circuit;

[0199] - Step 165: Based on the change, detecting the presence of the analyte in the medium and / or estimating the analyte concentration.

[0200] The presence of an analyte can be detected or its concentration estimated by calibration using representative medium 2 and a calibration sample containing a known amount of the analyte.

[0201] The present invention may be implemented on gaseous, liquid or solid samples in analyte detection applications in the environmental field, the industrial field (for example the food industry field) or the biomedical field.

Claims

1. A detection device configured to be applied to a medium to be analyzed via a contact surface, the medium to be analyzed being likely to contain an analyte that absorbs light in at least one absorption wavelength, the device comprising: - an opening formed in said contact surface; - an excitation light source configured to emit an excitation light wave in an excitation spectral band including the absorption wavelength, the excitation light wave being pulsed or amplitude modulated at an excitation frequency, the device being arranged so that the excitation light wave propagates through the opening towards the medium to be analyzed; - a transducer intended to measure the response of said medium following a periodic heating of said medium caused by the absorption of a portion of the excitation light wave by said analyte; Wherein, the transducer comprises: - a membrane, which carries the waveguide; - the waveguide comprising a first reflector and a second reflector, each reflector reflecting light in a reflection spectral band; - the first reflector and the second reflector are spaced apart from each other to form a resonant optical cavity, the resonant optical cavity defining a resonant wavelength in the reflection spectral band; so that the waveguide: transmitting light at the resonant wavelength; reflecting light other than the resonant wavelength in the reflection spectral band; The transducer further comprises: - an auxiliary laser source configured to emit auxiliary light waves in said reflection spectral band into said waveguide; - a photodetector arranged to detect light waves transmitted by said waveguide at said resonant wavelength; - a servo circuit connected to the photodetector and configured to determine a periodic time-dependent modulation of the resonant wavelength of the resonant optical cavity; in: - the waveguide is formed directly on the membrane; - The membrane is configured to deform under the effect of periodic heating of the medium.

2. The device according to claim 1 , comprising a processing unit connected to the servo circuit and configured to: - estimating the amplitude of the time-dependent modulation of the resonant wavelength; - Based on the estimated amplitude, detecting the presence of said analyte in the medium.

3. The device according to claim 2, wherein The processing unit is configured to estimate the concentration of the analyte in the medium based on the estimated amplitude.

4. The device of claim 1 , comprising a cavity leading to an opening, the transducer being connected to the cavity, and wherein The transducer is an acoustic transducer configured to detect the amplitude of a photoacoustic wave propagating from the opening through the cavity, so that under the action of the medium being illuminated by the excitation light wave, the membrane vibrates at an excitation frequency, resulting in a time-dependent modulation of the resonant wavelength at a modulation frequency equal to the excitation frequency.

5. The device according to claim 4, wherein The membrane extends parallel to the contact surface.

6. The apparatus according to claim 4, wherein The membrane defines a portion of the cavity.

7. The apparatus of claim 1 , wherein: - the membrane of the transducer forms the contact surface of the device, said contact surface being intended to be applied in contact with the medium; - said opening extends through said membrane; - the transducer is a thermal transducer, so that under the action of the medium being irradiated with an excitation light wave, the temperature of the membrane follows a periodic temporal variation, resulting in a periodic time-dependent modulation of the resonant wavelength.

8. The apparatus according to claim 1, wherein At least one reflector or each reflector is a Bragg mirror formed by a periodic modulation of the refractive index along the waveguide.

9. The apparatus according to claim 1, wherein The servo circuit includes a servo loop connected to the auxiliary light source and configured to servo-control the wavelength of the light wave emitted by the auxiliary light source to a resonant wavelength of the resonant optical cavity.

10. The apparatus according to claim 9, wherein The servo circuit implements a Pound-Drever-Hall type servo technique.

11. The apparatus of claim 1 , wherein: - 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 the same Bragg mirror comprising a defect, the first Bragg mirror and the second Bragg mirror respectively corresponding to portions of the Bragg mirror located on either side of the defect.

12. The apparatus of claim 1, wherein: - the membrane presents, when it is deformed, at least one vibration antinode, the vibration amplitude being maximum at each antinode; - The waveguide is flush with at least one antinode.

13. The apparatus according to claim 1, wherein The first and second reflectors are obtained by engraving a waveguide with a laser beam to obtain a periodic modulation of the refractive index in the waveguide.

14. A method for detecting an analyte in a medium, wherein the analyte absorbs light at at least one absorption wavelength, the method comprising: a) applying the device according to any one of claims 1 to 13 to the medium such that the contact surface of the device remains on the medium; b) activating the excitation light source, which emits an excitation light wave at a wavelength corresponding to the absorption wavelength of the analyte, the excitation light wave being pulsed or amplitude modulated at an excitation frequency; c) determining, by the servo circuit, a periodic modulation of a resonant wavelength of a waveguide of the transducer at a modulation frequency corresponding to the excitation frequency, the resonant wavelength corresponding to a transmission peak of the waveguide; d) detecting the presence of the analyte in the medium based on the periodic modulation determined by the servo circuit.

15. The method according to claim 14, wherein The device is a device according to claim 4, and the method comprises: - after said step b), periodically heating said medium at said excitation frequency so as to cause the emission of photoacoustic waves propagating through said cavity and under the effect of which the membrane of said transducer vibrates at said excitation frequency so as to modulate the resonant frequency of the waveguide of said transducer at a modulation frequency equal to said excitation frequency; - In step d): estimating an amplitude of the periodic modulation of the resonant wavelength at the modulation frequency; Based on the estimated magnitude, the presence of the analyte is detected.

16. The method according to claim 14, wherein The device is a device according to claim 7, and the method comprises: - after step b), periodically heating the medium at the excitation frequency to cause periodic heating of the membrane at the excitation frequency so as to modulate the resonant frequency of the waveguide of the transducer at a modulation frequency corresponding to the excitation frequency; - In step d): estimating an amplitude of the periodic modulation of the resonant wavelength at the modulation frequency; Based on the estimated magnitude, the presence of the analyte is detected.

17. The method according to claim 14, wherein: The step d) further comprises estimating the concentration of the analyte in the analyzed medium.

18. A method for manufacturing an apparatus according to any one of claims 1 to 13, the method comprising: - depositing a thin layer of a first material on the membrane to form a waveguide; - Engraving the waveguide with a femtosecond laser beam to obtain a periodic modulation of the refractive index of the waveguide.

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