photoacoustic detection device

Abstract

Photoacoustic detection device (1) comprising: - ° an illumination module (2) emitting a plurality of light beams of different wavelengths, - a photonic chip comprising: ° a substrate (40) comprising a face with an etching (100), ° a plurality of waveguides (41) extending parallel to the etched face of the substrate (40), each waveguide (41) guides at least one light beam emitted by the illumination module (2), ° a plurality of diffraction gratings (42) extracts the light beam from the waveguide (41), towards the etching (100) of the substrate (40), in which said etching (100) extracts the light beams towards the medium to be analyzed, - a photoacoustic cell (5) formed by the photonic chip comprising: ° an open contact surface formed by the etched face of the substrate (40) ° a photoacoustic cavity (51) formed by the etching (100) of the substrate (40), - a transducer (3) connected to the cavity (51).Figure for abstract: Figure 2.

Description

Title of the invention: photoacoustic detection device FIELD OF THE INVENTION

[0001] The present invention relates to a photoacoustic detection device for measuring a parameter of interest in a medium to be analyzed. More specifically, the photoacoustic detection device comprises a photonic chip for multispectral infrared spectroscopy, the photonic chip forming a photoacoustic cell. TECHNOLOGICAL BACKGROUND

[0002] Photoacoustic detection can be used in the field of detection devices, in particular for detecting parameters of interest such as chemical components in a medium. The medium can be an organic tissue, such as the skin of a human being.

[0003] Photoacoustic detection is based on the irradiation of a medium M to be analyzed by a light beam emitted by a light source. The light beam is a light beam of a chosen wavelength. The wavelength is chosen according to the type of parameter of interest to be measured.

[0004] Light sources of different types can be used, in particular depending on the desired application. A first application can be an indirect photoacoustic application, in which the photoacoustic detection is based on the detection of a pressure wave caused by a thermal wave. The thermal wave is generated under the effect of the absorption of the light beam by the medium. This absorption creates a local heating of the chemical components of the medium, where the light beam has been absorbed. The thermal wave propagates in the medium M before propagating outside the medium. More precisely, when the thermal wave leaves the medium, after its diffusion, a pressure variation is generated, which can be detected.The transition from a thermal wave to an acoustic wave takes place in a photoacoustic cavity acting as a thermoacoustic transducer, then a microphone-type sensor picks up the acoustic wave, acting as an acoustoelectric transducer.

[0005] Photoacoustic detection can be made specific to particular chemical components by adjusting the wavelength of the light beam. More specifically, the wavelength can be adjusted to match an absorption peak of the component to be analyzed. Indirect photoacoustic detection then provides a non-invasive means of analyzing a medium of interest. Many devices for Photoacoustic detection has been developed. In particular, such devices have been developed to measure glucose levels in the epidermis for a person.

[0006] Direct photoacoustic detection is performed on a solid or liquid object in an indirect measurement configuration. In this case, it is necessary to detect the acoustic wave produced by the mechanical expansion of the sample under the effect of its heating. This is a modality that is notably used for imaging biological tissues. Direct photoacoustic detection is notably used for the detection or analysis of gases.

[0007] The article Sensors (Basel). 2020 May; 20(9): 2745. Published online 2020 May 11. Photoacoustic-Based Gas Sensing: A Review, by Stefan Palzer deals with direct and indirect photoacoustic detection.

[0008] These photoacoustic detection devices generally comprise a photoacoustic cell, a light source emitting a light beam, a guide element guiding the light beam towards the photoacoustic cell and at least one transducer capable of detecting a signal generated in the photoacoustic cell by the photothermal effect in the medium in response to the irradiation of the medium by the light beam.

[0009] A first requirement related to these detection devices is that they may be intended to be worn continuously, that is to say constantly, by a person. They must therefore be sufficiently compact so as not to disturb the person wearing them. In other cases, the devices may be embedded in other devices and must therefore also be sufficiently compact to allow their integration.

[0010] A second requirement of these detection devices is that, in order to obtain precise and correct results, the medium to be analyzed must be illuminated equally at all wavelengths.

[0011] The present invention aims to propose a multispectral photoacoustic detection device meeting the requirements presented above. Summary of the invention

[0012] Thus, the invention relates to a photoacoustic detection device for measuring a parameter of interest in a medium to be analyzed, the device comprising: - ° at least one illumination module configured to emit a plurality of light beams, in which at least two light beams among the plurality of light beams have different wavelengths, - a photonic chip comprising: ° a substrate comprising a face with an engraving, ° a plurality of waveguides extending parallel to a plane formed by the etched face of the substrate, each waveguide being configured to guide at least one light beam emitted by the illumination module, ° a plurality of diffraction gratings, each diffraction grating being respectively formed in a waveguide of the plurality of waveguides and each diffraction grating being configured to extract from the waveguide in which it is formed, towards the etching of the substrate, the light beam propagating in said associated waveguide, wherein said etching of the substrate is configured to extract the light beams from the substrate and towards the medium to be analyzed, - a photoacoustic cell formed by the photonic chip, such that the photoacoustic cell comprises: ° an open contact surface formed by the etched face of the substrate, the open contact surface being intended to be in contact with the medium to be analyzed, ° a photoacoustic cavity formed by the etching of the substrate, ° said light beams extracted from the substrate being configured to propagate in the photoacoustic cavity then pass through the open contact surface to reach the medium, - a transducer connected to the cavity, the transducer being configured to detect a signal generated in the cavity of the photoacoustic cell by a photothermal effect in the medium in response to the irradiation of the medium by the light beams extracted from the substrate.

[0013] Thus, the photoacoustic detection device is particularly compact since by combining a photonic chip allowing the guidance and collimation of the light beams towards the medium to be analyzed with a photoacoustic cell, the device does away with a component taking up space. Indeed, in order to be able to direct light beams towards the scene to be illuminated, to carry out the analyses of the medium, a light guiding element is necessary. Similarly, a photoacoustic cell is also necessary for the conversion of the photothermal signal into an acoustic signal in order to analyze the medium from the generated signal. The device therefore proposes to combine these two elements into a single one, which considerably reduces the final size of the device and facilitates its manufacture.

[0014] In addition, the distribution of the sources and diffraction gratings around the engraving and in the same plane allows for homogeneous illumination at the same angle of incidence of the medium for all the sources.

[0015] According to different aspects, it is possible to provide one and / or the other of the characteristics below taken alone or in combination.

[0016] The diffraction gratings are distributed around the etching so that a circle with a center corresponding to the center of the etching of the substrate passes through the barycenters of all the diffraction gratings.

[0017] This particular arrangement is suitable in the case where a single source is associated with a network but also in the case where several sources are associated with a single network. In the case where several sources are associated with the same network, the sources are multiplexed upstream of the entry of the light beams into the waveguides.

[0018] Advantageously, the networks are distributed regularly around the engraving according to radial symmetry.

[0019] According to a variant, the symmetry is not radial and the networks can be distributed around the etching on the circle described above irregularly. This is particularly the case when the etching of the substrate is obtained by isotropic etching of the substrate. This can be advantageous for reasons of space requirements and integration of other components into the device.

[0020] the etching is configured to be obtained by an anisotropic etching of the substrate, such that said etching forms, according to a top view of the etched face of the substrate, a polygonal shape.

[0021] Advantageously, the polygonal shape is square, rectangular or octagonal. The engraving can therefore be made from four prints (square shape) or eight prints (octagonal shape). Several types of anisotropic engravings can therefore be used.

[0022] The device comprises as many diffraction gratings as there are sides of the polygonal shape.

[0023] Since at least one light source is associated with a diffraction grating, this therefore makes it possible to use at least four sources for the square shape, and at least eight sources for the octagonal shape. Furthermore, each diffraction grating is arranged so as to be associated with only one imprint so as to obtain a symmetrical arrangement of the sources and / or diffraction gratings around the engraving. This has the advantage of easily obtaining homogeneous illumination at the same angle of incidence for all the light beams.

[0024] The etching is configured to be obtained by isotropic etching of the substrate, such that said etching forms, according to a top view of the etched face of the substrate, a circle shape.

[0025] The number of sources is then theoretically infinite, the only limit being the space available to put the sources.

[0026] The device further comprises an input waveguide into which all the light beams are intended to be injected, the photonic chip comprising in in addition to at least one demultiplexer configured to demultiplex the light beams at the output of the input waveguide so as to inject at least one light beam into at least two waveguides of the plurality of waveguides.

[0027] The device further comprises a plurality of input waveguides into each of which a portion of the light beams is intended to be injected, the photonic chip further comprising as many demultiplexers as there are input waveguides, each demultiplexer being associated with an input waveguide, the demultiplexers being configured to demultiplex the light beams at the output of the input waveguide so as to inject at least one light beam into waveguides of the plurality of waveguides.

[0028] the etched face of the substrate, i.e. the open contact surface of the photoacoustic cell, is closed by a window.

[0029] The window makes it possible to protect the photoacoustic cavity from the external environment, while allowing the passage of light beams towards the medium, and the passage of the generated signal towards the photoacoustic cavity.

[0030] the etching of the substrate is an etching opening onto the waveguides so that the photoacoustic cavity is open on the side of the etched face of the substrate and on the side of the waveguides, the transducer being positioned against the entrance opening onto the waveguides, so as to seal this said entrance of the photoacoustic cavity.

[0031] The cavity is closed and therefore protected on this side. The other opening of the cavity is closed by the middle or the window. The transducer is therefore easily connected to the photoacoustic cavity to measure the generated signal.

[0032] The substrate comprises etchings between the cavity and the external environment, said etchings forming a channel connecting the photoacoustic cavity to a transducer.

[0033] There is therefore the possibility of using several transducers capable of measuring different types of generated signals. This embodiment can also be applied to a single transducer, for example if the etching is closed at the level of the waveguides. The etchings can be right-angle etchings, at 45° or 54.74°, or etchings obtained by isotropic or anisotropic etching of the substrate.

[0034] The transducer is configured to be fabricated on the same silicon wafer as the substrate.

[0035] This therefore has a high advantage in that everything is done at the same time during manufacturing since the cavity and the optical coupler are also directly integrated on silicon on the photonic chip.

[0036] The sources are configured to emit light beams in the mid-infrared range. Brief description of the drawings

[0037] Embodiments of the invention will be described below with reference to the drawings, briefly described below:

[0038] [Fig-1] is a block diagram of an acoustic detection device according to a realization,

[0039] [Fig.2] is a cross-sectional view of a photoacoustic detection device according to one embodiment.

[0040] [Fig.3] is a cross-sectional view of a photoacoustic detection device according to another embodiment.

[0041] [Fig.4] illustrates the extraction of a light beam according to one embodiment.

[0042] [Fig.5] is a cross-sectional view of a photoacoustic detection device according to another embodiment.

[0043] [Fig.6] is a top view of an example of placement of the waveguides relative to the etching of the substrate.

[0044] [Fig.7] is a top view of an example of placement of the waveguides relative to the etching of the substrate

[0045] [Fig.8] is a top view of an example of placement of the waveguides relative to the etching of the substrate,

[0046] [Fig.9] is a side view of an example of placement of the waveguides relative to the etching of the substrate,

[0047] [Fig. 10] is a side view of an example of placement of the waveguides relative to the etching of the substrate,

[0048] [Fig. 11] is another example of waveguide placement using a demultiplexer.

[0049] In the drawings, like references designate identical or similar objects. DETAILED DESCRIPTION

[0050] In a particular application of indirect photoacoustic detection, the photoacoustic detection device 1 is particularly suitable for measuring a parameter of interest in a medium M to be analyzed. The medium can be solid or liquid.

[0051] In a non-limiting example, the photoacoustic detection device 1 (or “detection device 1” or “device 1” in the following description) is intended to be worn by a person. The medium M may be an organic tissue such as the skin of the person wearing the detection device 1.

[0052] A parameter of interest may be a chemical component present in the person's skin, such as molecules. The parameter of interest may include glucose, cholesterol, triglyceride, urea, albumin, and / or alcohol. This list is not exhaustive, and several other parameters of interest may be measured.

[0053] The measured parameters can then be analyzed to determine the blood concentration of glucose, cholesterol, etc.

[0054] The detection device 1 can be worn by a person at any location allowing a parameter of interest to be measured in the person's epidermis.

[0055] According to one example, the device 1 is worn on the arm or wrist of a person. The detection device 1 may be attached to the person by means of a bracelet.

[0056] The detection device 1 can allow continuous monitoring of the person P, by repeatedly measuring the parameters of interest of the person, while it is worn.

[0057] In another particular application in which the device is used for direct photoacoustic detection, the detection device 1 is used to detect one or more gases in a medium. The device can then be placed in any medium allowing access to the targeted gas. The medium M can then be air, for example. Such a device can find applications in biomedical, for the analysis of a person's exhaled air for monitoring certain diseases (for example detection of NO for asthma), or even environmental for monitoring atmospheric concentrations of greenhouse gases, in defense for the detection of explosive(s) (TNT) or even in security for the detection of fire outbreaks.

[0058] [Fig.l] is a block diagram of the detection device 1 according to one embodiment.

[0059] The detection device 1 comprises:

[0060] an illumination module comprising at least two light sources 2, configured to each emit at least one light beam, advantageously at least two light beams are emitted at different wavelengths.

[0061] a transducer 3 acquiring a signal coming from the medium M,

[0062] a signal processing module 4 for analyzing the signal detected by the transducer.

[0063] In one embodiment, the transducer 3 may be an acoustic transducer detecting an acoustic signal generated in response to the irradiation of the medium M by the light beam.

[0064] The transducer 3 can be connected to the signal processing module 4 so that the signal processing module 4 is adapted to receive a signal from the transducer 3.

[0065] In a non-limiting embodiment, the signal processing module 4 may comprise an analog-to-digital converter, converting the signal acquired by the transducer 3 into a digital signal.

[0066] In another embodiment, the transducer 3 can directly transmit a digital signal to the signal processing module 4.

[0067] The signal processing module 4 can be implemented by a processor (not shown) which may or may not be remote from the detection device 1.

[0068] The detection device 1 may comprise other components. For example, the detection device 1 may also comprise a module 5 for adapting the irradiation parameters of the light source 2 and a memory 6, for example. These components will not be described in further detail.

[0069] In one embodiment, the light sources 2 emit a light beam at a chosen wavelength, in the direction of the medium M to be analyzed. The chosen wavelength can be chosen according to the parameters of interest to be measured.

[0070] More specifically, the wavelength may correspond to the absorption peak of the parameter of interest to be measured. For example, to detect glucose, the wavelength may be 1034 cm-1, which corresponds to the absorption peak of glucose.

[0071] The light sources can be lasers, and more particularly quantum cascade lasers (QCL), or interband cascade lasers (ICL), with external or internal cavity.

[0072] The light sources 2 emit light beams in the mid-infrared, that is to say at wavelengths between 2 and 12 pm.

[0073] Each light source 2 emits at least one light beam whose wavelength is always the same. In one configuration, the wavelengths of the light beams emitted by the sources may be different for at least two sources, so as to perform multispectral photoacoustic detection.

[0074] In another variant, a light source 2 can emit several light beams if the light beams are at close wavelengths, typically if the difference in wavelengths between the light beams is equal to or less than 0.3 pm.

[0075] In the particular application of indirect photoacoustic detection, the light beams emitted by the light sources 2 propagate towards the medium M and through it. This phenomenon is represented by the dotted arrows in [Fig.2]. The light beam is absorbed by the constituents of the medium M at a characteristic depth z depending on the chosen wavelength of the light beam, and on the composition of the medium M.

[0076] The absorption of the energy of the light beam causes local heating of the medium M. Consequently, a thermal signal propagates in the medium M (phenomenon illustrated by the solid arrows in [Fig.l]), in particular towards the surface of the medium M. In addition, the thermal signal can create, outside the medium M and in the photoacoustic cavity of the photoacoustic cell, a pressure wave which propagates outside the medium M and in the photoacoustic cavity. Such a pressure wave can be detected by the acoustic transducer 3.

[0077] In the case where the device is adapted for gas detection, the light beams emitted by the light sources 2 propagate towards the gas contained in a photoacoustic cavity and inside it. The light beam is absorbed by the constituents of the gas.

[0078] The absorption of the energy of the light beam causes heating of the gas. Consequently, a pressure wave is created and propagates in the photoacoustic cavity. Such a pressure wave can be detected by the acoustic transducer 3.

[0079] Figures 2 and 3 are cross-sectional views of a detection device 1 according to two embodiments.

[0080] Generally, the detection device 1 comprises an illumination module comprising the light sources 2, at least one transducer 3, a photonic chip 4 and a photoacoustic cell 5 formed by the photonic chip 4.

[0081] The photonic chip 4 is a lighting photonic chip, intended to combine light beams belonging to the infrared domain for lighting a scene.

[0082] The photonic chip 4 comprises a substrate 40. The substrate 40 is a crystalline silicon substrate.

[0083] The substrate 40 has a face comprising an etching 400. Advantageously, in the case of indirect detection, the medium M is located at the level of the etched face of the substrate 40. As visible in FIGS. 2 or 9, for example, the etching 400 can be obtained by an anisotropic etching of the substrate 40. Advantageously, the etching has a square or octagonal shape according to a top view of the etching 400 of the substrate 40.

[0084] The principle of anisotropic etching forming an octagon seen from above is described in the scientific article by Rola, Krzysztof P., Konrad Ptasinski, Adrian Zakrzewski, and Irena Zubel. 2014. “Silicon 45° Micromirrors Fabricated by Etching in Alkaline Solutions with Organic Additives”. Microsystem Technologies 20(2): 221-26.

[0085] In Figures 3, and 6 to 8, in particular, the etching 400 is obtained by an isotropic etching of the substrate 40, so that, seen from above, the etching 400 has a circular shape.

[0086] The photonic chip also comprises at least one waveguide 41, and preferably a plurality of waveguides 4L. The waveguides 41 are linear or planar waveguides, capable of guiding a light beam F propagating in said waveguides 41 by successive reflections of the light beam F on mutually parallel plane faces. The waveguides 41 are formed by a core 410 in which the light beam F circulates and a cladding 411, ensuring a desired optical index difference between the core and the medium surrounding the core. With regard to the waveguides illustrated, each guide comprises a core 410 interposed between two sheaths 411. The light beam F is guided in the core 410 by successive reflections at the interfaces between the core 410 and each of the sheaths 411 between which the core 410 is interposed.

[0087] Advantageously, the cores 410 of the waveguides 41 are made of germanium and the claddings 411 are made of a silicon-germanium alloy, for example with 60% silicon and 40% germanium. The waveguides 41 are inscribed on the substrate 40. In a first configuration, the waveguides 41 are configured to guide a single light beam of a particular wavelength. Advantageously, at least two waveguides 41 among the plurality of waveguides are configured to respectively guide light beams of different wavelengths, so that the photonic chip 4 is suitable for multispectral lighting in the infrared.

[0088] According to a second configuration, which may come as an alternative or in addition to the first configuration described above, at least one waveguide 41 or a majority of waveguides 41 or all the waveguides 41, are capable of guiding several light beams F. In this configuration, a waveguide 41 may be capable of guiding two light beams of identical wavelength or of different wavelengths, if the wavelengths of the light beams propagating in the guide are close. In one example, two wavelengths are considered to be close if their difference is less than or equal to 0.3 pm.

[0089] Advantageously, the waveguides 41 are single-mode for all the wavelengths of the light beams F considered. That is to say that the waveguides 41 are single-mode at the smallest wavelength considered.

[0090] The waveguides 41 each have an input zone 412, at which the light beams emitted by the light sources 2 are injected into the waveguides, and an output zone 413, at which the light beams injected into the waveguides are extracted from the waveguides. Typically, a light beam is injected from the light sources 2.

[0091] All the waveguides 41 of the photonic chip 4 may have the same thickness, for ease of manufacturing. The thickness of the waveguides 41 may be between 500nm and 3pm.

[0092] Alternatively, the input and / or output zones 112, 113 of the waveguides 41 may have, in top view, a rectangular, prismic or adiabatic parallelepiped shape in order to facilitate the injection of light into the waveguide or its extraction. The width of a waveguide 41 may therefore be variable at its input and / or output zones 112, 113. In this configuration and in order to ensure the monomodality of the waveguide 41, the width of the waveguide 41 in its narrowest part is between 500nm and 3pm. In the widest part of the waveguide 41, the width of the waveguide 41 is between the value of the narrowest width (for example 500nm) and the size of the area to be illuminated.

[0093] Each waveguide 41 is associated with a diffraction grating 42 allowing the extraction of light from the associated waveguide 11. Each diffraction grating 42 is optimized for a particular wavelength. The diffraction gratings 42 have pitches 420 which may be constant or variable. The order of magnitude of the pitches 420 of the diffraction gratings 42 is between one micron and a few microns. The pitches 420 are chosen to allow the extraction of the light beam F from the waveguide 11 associated with the diffraction grating 42 in question.

[0094] As a result, the output zones 413 of the waveguides 41 correspond to the location of the diffraction gratings 42 in the waveguide.

[0095] As visible in the figures, the waveguides 11 extend linearly in the same plane, intended to be parallel to a plane (P) formed by the etched face of the substrate.

[0096] The term "plane (P) formed by the etched face" means the flat face of the substrate in which the etching has been carried out. The etching itself is not taken into account in the definition of the plane (P) formed by the etched face of the substrate.

[0097] According to one embodiment, the diffraction gratings 42 of each waveguide 41 are positioned symmetrically with respect to the etching 400 of the substrate 40, along a plane of symmetry of said etching 400 of the substrate 40, said plane of symmetry being perpendicular to the plane (P) formed by the etched face of the substrate.

[0098] Advantageously, the diffraction gratings 42 of the waveguides are arranged according to a radial symmetry of the photonic chip seen from above. In other words, the diffraction gratings 42 of the waveguides are distributed around the etching so that a circle with a center corresponding to the center of the etching 400 of the substrate 40 passes through the barycenters of all the diffraction gratings 42, whether the etching 400 is obtained by an isotropic or anisotropic etching of the substrate.

[0099] This makes it possible to obtain the same angle of incidence on the surface to be illuminated of the medium for all the light sources 2. This also ensures the same depth of penetration of the light beams into the medium, which makes it easier to calibrate and characterize the device. This also makes it possible to obtain equivalent illuminated surfaces for all the light sources. Finally, the placement of the diffraction gratings around the etching 400 and in the same plane makes it possible to optimize the space for adding additional elements (for example thermistors next to the light sources).

[0100] Figures 6 to 8 illustrate different examples of placement of the sources and the waveguides relative to an etching 400 obtained by an isotropic etching of the substrate 40. In these figures, the waveguides are arranged regularly around the etching and according to a radial symmetry.

[0101] [Fig.9] illustrates another example in which the waveguides are arranged irregularly around the etching and according to radial symmetry.

[0102] [Fig. 10] illustrates an example of placement of the sources and waveguides relative to an etching 400 forming a square in top view. These examples are not limiting and other arrangements respecting the radial symmetry of the etchings 400 are possible. The number of waveguides and sources is also given as an example.

[0103] Regarding Figures 6 to 9, the position of the waveguides is optimized to limit bending losses. The round-shaped etching, seen from above, is particularly advantageous insofar as the number of light sources is restricted only by the desired compactness of the device.

[0104] [Fig. 10] illustrates a variant with an etching 400 obtained by anisotropic etching of the substrate 40 and four light sources. In this example, the etching 400 comprises four prints and therefore forms a square when viewed from above. In another example, the etching 400 may comprise eight prints and form an octagon when viewed from above. In this case, eight light sources may be provided.

[0105] The light beams extracted by the diffraction gratings 42 from the waveguides 41 are directed towards the etching 400 of the substrate 40 so as to reach the medium.

[0106] According to the Snell-Descartes law for refraction, the critical angle beyond which the light beams resulting from the refraction of the refraction gratings 42 will undergo the phenomenon of total internal reflection and will actually be able to exit the substrate 40 and illuminate the medium is approximately 17° in the wavelength range considered, silicon being very slightly dispersive in the infrared. Thus, the light beams resulting from the diffraction of the gratings must be included in a cone with an angle of ± 17° relative to the normal to the silicon / air diopter.

[0107] The light beams extracted by the diffraction gratings 42 propagate equally in two opposite directions, towards the substrate 40 and towards the upper cladding 411. Consequently, advantageously, if the extraction angle is greater than the critical angle of 17° or if the etched face of the substrate 40 is metallized, the thickness of the upper cladding 411 at the exit zone 413 of the core 410 is determined so as to obtain, at the wavelength of use, constructive interference between the light beam extracted from the upper cladding 411 and the light beam resulting from the reflection on the etched face of the substrate 40.

[0108] To allow the light beams to be effectively extracted from the substrate 40 towards the scene to be illuminated, the etching angle 400 of the substrate in the case where the etching 400 is obtained by anisotropic etching of the substrate 40 ([Fig.2]) is determined by the crystal planes of the silicon forming the substrate 40.

[0109] For example, etching can be done on the crystal planes using a substrate -000) and the etching angle is then equal to 54.74°.

[0110] According to another example, the etching angle may be equal to 45° using a (100) substrate.

[0111] The pitch 420 of the diffraction gratings 42 can then advantageously be constant and chosen so that at the wavelength of use the extraction angle after refraction is equal to the desired angle of incidence on the scene to be illuminated which is intended to be at the level of the etching of the substrate, see [Fig.4].

[0112] The pitch 420 of the diffraction gratings 42 is then chosen as follows:

[0113] To p = ----------- rn . —n siniQ ) cff àiOÆ à.'Otr

[0114] Where X is the operating wavelength or the median operating wavelength, neff is the effective index of the mode at wavelength X, nSiGe is the index of the cladding 111 in Silicon Germanium and ÔSiGe is the extraction angle considered in Silicon Germanium. To obtain the extraction angle in air Oair, it is sufficient to use the Snell-Descartes law.

[0115] In this variant, since the light beams extracted by the diffraction gratings 42 are collimated, the length of the diffraction gratings 42 depends on the characteristic length of the surface to be illuminated and the extraction angle Oair.

[0116] In a particular case, if the extraction angle of the diffraction gratings 42 is chosen such that the light beam arrives perpendicularly on the sides of the etching 400 obtained by anisotropic etching of the substrate 40, then the length of the diffraction gratings 42 is equal to the characteristic length of the surface to be illuminated of the medium.

[0117] [Fig.4] illustrates this achievement.

[0118] Optionally, the sides of the etching 400 may be anti-reflectively treated to limit Fresnel losses at the Silicon / air interface. The anti-reflective treatment applied is, for example, a deposition of a thin layer of ZnS. The anti-reflective coating is optimized for a particular wavelength in the range of wavelengths of use, for example the median wavelength of use. The thickness of the anti-reflective layer is, for example, equal to a quarter of the median wavelength of use, typically between 1 and 3 pm.

[0119] Considering the case where the etching is obtained by an isotropic etching of the substrate 40 ([Fig.3]), the profile of the etching 400 visible in [Fig.2] is curved, concave and divergent. The profile is not completely circular because the etching 400 is more effective in the vertical direction than in the horizontal direction.

[0120] Advantageously, in this embodiment the pitch 420 of the diffraction gratings 42 is not constant and the extraction angle OSiGe is also not constant, in order to obtain a collimated light beam outside the substrate 40 taking into account the concave diopter of the etched face, acting as a diverging lens.

[0121] The pitch 420 of the gratings 42 is chosen such that at the wavelength of use and at each pattern of the diffraction gratings 42, the extraction angle after refraction is equal to the desired angle of incidence on the scene. The variable pitch 420 is then chosen using the first-order grating law.

[0122] In particular, the order of magnitude of the variable pitch 120 of the gratings 12 for the wavelengths considered is the micron, or even a few microns.

[0123] In this variant, the length of the diffraction gratings 42 depends on the characteristic length of the surface to be illuminated of the medium M, the extraction angle Oair and the profile of the etching 400 obtained by an isotropic etching of the substrate.

[0124] For each etching variant (obtained by anisotropic or isotropic etching of the substrate), the filling factor is determined so as to homogenize the illumination on the scene by compensating for the exponential law of the Beer-Lambert principle. The filling factor is chosen to increase as a function of the position of the pattern of the gratings 12, in the direction of propagation of the light.

[0125] Optionally, the sides of the etching 100 may be antireflectively treated to limit Fresnel losses at the interface of the silicon substrate and the air. The antireflective treatment applied is, for example, a deposition of a thin layer of ZnS. The antireflection is optimized for a particular wavelength of the range of wavelengths of use, for example the median wavelength of use. The thickness of the antireflective layer is, for example, equal to a quarter of the median wavelength of use, typically between 1 and 3 pm.

[0126] In the following description, the device is described with respect to indirect photoacoustic detection. The person skilled in the art will know how to make the necessary changes for the application of the device to indirect photoacoustic detection.

[0127] Advantageously, the surface to be illuminated of the medium M is of the order of a millimeter or a few millimeters.

[0128] Once the medium M has been illuminated, or irradiated, by the light beams by means of the photonic chip 4, the absorption, by the medium, of the light beams results in the creation of a thermal wave propagating in the medium M. When the thermal wave exits the medium M and into the photoacoustic cavity, an acoustic wave is generated. This acoustic wave can be detected by the transducer 3.

[0129] To do this, a photoacoustic cell 5 is formed from the photonic chip. More specifically, the photoacoustic cell 5 comprises a surface open contact 50 corresponding to the etched face of the substrate 40 and a cavity 51 formed by the etching 400 of the substrate 40.

[0130] The person skilled in the art, a specialist in particular in photoacoustics, will be able to use his general knowledge in order to size the photoacoustic cavity in order to obtain the desired acoustic response. In particular, the person skilled in the art will know that the cavity must have the smallest possible dimensions in order to optimize the impulse response of the cavity at any modulation frequency in the case of a closed photoacoustic cell. In the case of an open photoacoustic cell, the dimensions can be chosen to obtain the resonance frequency at the desired frequency.

[0131] The transducer 3 is necessarily connected to the cavity 51. The transducer 3 is configured to detect a generated signal, the generated signal being generated in the cavity 51 of the photoacoustic cell 5 by the photothermal effect in the medium M in response to the irradiation of the medium by the light beams extracted from the substrate 40.

[0132] As visible in Figures 2 and 3, the etching 100 can be open at the level of the waveguides 41, that is to say, pass through them entirely. The transducer 3 can then be placed at the level of the input opening onto the waveguides 41 so as to close the cavity 51 on this side. The cavity 51 is advantageously closed at the level of the etched face of the substrate 40 by the medium M.

[0133] According to another embodiment, the etching 400 does not open out at the level of the waveguides 4L. According to this embodiment, an additional etching 401 can be made in the substrate 40 in order to connect the transducer 3 to the cavity 51.

[0134] An example is illustrated in [Fig. 5]. [Fig. 5] illustrates the case where the etching 400 is obtained by an anisotropic etching of the substrate, although this embodiment is applicable to the case where the etching 400 is obtained by an isotropic etching of the substrate 40. In the example of [Fig. 5], the etching 300 connecting the transducer to the cavity 51 can be obtained by right-angle etchings or by isotropic or anisotropic etchings of the substrate 40.

[0135] For a better understanding of [Fig.5], it is specified that the transducer 3 is in the foreground, while the photonic chip is in the background.

[0136] Other engravings may also be added in order to add other transducers such as thermal or humidity transducers, for example.

[0137] According to another embodiment, the etching 400 is open but the transducer 3 is connected to the cavity 51 by an etching 401 as described above. A window 6 can then be placed at the level of the entrance of the etching 400 opening onto the waveguides 41 in order to close the cavity 51. The window 6 can thus also make it possible to protect the cavity. The window 6 can be manufactured in silicon to facilitate its manufacture or its transfer to the device.

[0138] According to another embodiment, the transducer 3 can be directly integrated on the silicon wafer used for the substrate. This embodiment has an advantage in terms of ease of manufacturing since the photoacoustic cell, the photonic chip and the transducer can be integrated at the same time on silicon.

[0139] For example, the transducer 3 is built on a silicon wafer which is then directly bonded to the silicon wafer forming the substrate 40.

[0140] Optionally, as illustrated in Figures 2 and 3, a window 6 can be interposed between the etched face of the substrate 40 and the medium M.

[0141] The window 6 protects the cavity 51 from dust or humidity. It allows the light beams to pass towards the medium and the generated signal towards the cavity.

[0142] Window 6 may be made of a material transparent in the wavelength range considered. Window 6 is for example made of silicon.

[0143] Alternatively, the window 6 may be made of a non-transparent material and comprise an opening allowing the light beams to pass towards the medium and the generated signal towards the cavity. The size of the window opening is determined by the standard, i.e. the characteristic size less than a millimeter of the surface to be illuminated of the medium and / or by the characteristic size of the surface to be illuminated of the medium.

[0144] Advantageously, the window 6 is made of silicon and manufactured or transferred onto the same silicon wafer used for the substrate. This makes it possible to reduce manufacturing costs.

[0145] An anti-reflection treatment may be applied to the window to limit Fresnel losses at the window / air interface. If the window is made of silicon, the anti-reflection treatment applied is, for example, a deposition of a thin layer of ZnS. The anti-reflection is optimized for a particular wavelength of the wavelength range considered, for example the median wavelength. The thickness of the anti-reflection layer is, for example, equal to a quarter of the median wavelength. Typically, the thickness of the anti-reflection layer is between 1 and 3 μm.

[0146] [Fig. 11] illustrates an alternative embodiment, in which the device 1 is illustrated in a top view. The device 1 comprises an etching 400 obtained by anisotropic etching of the substrate, although the alternative embodiment of [Fig. 11] is also applicable to the case where the etching 400 is obtained by isotropic etching.

[0147] In this variant, the device 1 comprises, in addition to the plurality of waveguides 41, an input waveguide 43. This input waveguide 43 is intended to receive the plurality of light beams F. A demultiplexer 44 is provided, which is associated with the input waveguide 43. The demultiplexer 44 is configured to inject at least one light beam into at least two of the waveguides 41 of the plurality of waveguides 41 associated with it. All the waveguides 41 extend in the same plane, intended to be parallel to the plane defined by the etched face of the substrate. The output zones 413 of each waveguide 41 are positioned symmetrically with respect to the etching 400 of the substrate 40, according to a plane of symmetry of said etching 400 of the substrate 40, said plane of symmetry being perpendicular to the plane defined by the etched face of the substrate 400.

[0148] Advantageously, the number of waveguides 41 is equal to the number of wavelengths in the wavelength range considered. Alternatively, at least one waveguide 41 may be associated with two different wavelengths if they are close, for example if the difference in wavelengths is less than or equal to 0.3 pm.

[0149] In this embodiment, the input waveguide 43 is single-mode for the smallest wavelength considered and, therefore, for the entire range of wavelengths considered.

[0150] Advantageously, the thickness of the input waveguide 43 and of the plurality of waveguides 41 is the same, in order to facilitate the manufacture of the device 1. The input waveguide 43 may have, in top view, a rectangular parallelepiped or prism shape or an adiabatic shape to facilitate the injection of the light beams into the input waveguide 3.

[0151] The demultiplexer 44 may be of the AWG type (Arrayed Waveguide Grating), of the PCG type (Planar Concave Grating), of the MZI type (Mach-Zehnder Interferometer) or of the MMI type (MultiModal Interference Coupler).

[0152] In another variant, more than one input waveguide may be used. In this variant, the number of demultiplexers used is equal to the number of input waveguides.

Claims

Claims

1. Photoacoustic detection device (1) for measuring a parameter of interest in a medium to be analyzed, the device comprising: - ° at least one illumination module (2) configured to emit a plurality of light beams, in which at least two light beams among the plurality of light beams have different wavelengths, - a photonic chip comprising: ° a substrate (40) comprising a face with an etching (100), ° a plurality of waveguides (41) extending parallel to a plane formed by the etched face of the substrate (40), each waveguide (41) being configured to guide at least one light beam emitted by the illumination module (2), ° a plurality of diffraction gratings (42),each diffraction grating (42) being respectively formed in a waveguide (41) of the plurality of waveguides (41) and each diffraction grating (42) being configured to extract from the waveguide (41) in which it is formed, towards the etching (100) of the substrate (40), the light beam propagating in said associated waveguide (41), wherein said etching (100) of the substrate (40) is configured to extract the light beams from the substrate (40) and towards the medium to be analyzed, - a photoacoustic cell (5) formed by the photonic chip, such that the photoacoustic cell (5) comprises: ° an open contact surface formed by the etched face of the substrate (40), the open contact surface being intended to be in contact with the medium to be analyzed, ° a photoacoustic cavity (51) formed by the etching (100) of the substrate (40),° said light beams extracted from the substrate (40) being configured to propagate in the photoacoustic cavity (51) and then pass through the open contact surface to reach the medium, - a transducer (3) connected to the cavity (51), the transducer (3) being configured to detect a signal generated in the cavity (51) of the photoacoustic cell (5) by a photothermal effect in the, medium in response to irradiation of the medium by said light beams extracted from the substrate (40) and passed through the open contact surface.

2. Device (1) according to claim 1, wherein the diffraction gratings (42) are distributed around the etching (100) so that a circle with a center corresponding to the center of the etching (100) of the substrate (40) passes through the barycenters of all the diffraction gratings (42).

3. Device (1) according to claim 1, wherein the etching (100) is configured to be obtained by an anisotropic etching (100) of the substrate (40), so that said etching (100) forms, according to a top view of the etched face of the substrate (40), a polygonal shape.

4. Device (1) according to the preceding claim, comprising as many diffraction gratings (42) as there are sides of the polygonal shape.

5. Device (1) according to one of the preceding claims, in which the etching (100) is configured to be obtained by an isotropic etching (100) of the substrate (40), so that said etching (100) forms, according to a top view of the etched face of the substrate (40), a circle shape.

6. Device (1) according to one of the preceding claims, further comprising an input waveguide (41) into which all the light beams are intended to be injected, the photonic chip further comprising at least one demultiplexer (44) configured to demultiplex the light beams at the output of the input waveguide (41) so as to inject at least one light beam into at least two waveguides (41) of the plurality of waveguides (41).

7. Device (1) according to one of claims 1 to 5, further comprising a plurality of input waveguides (41) into each of which a portion of the light beams is intended to be injected, the photonic chip further comprising as many demultiplexers (44) as there are input waveguides (41), each demultiplexer (44) being associated with an input waveguide (41), the demultiplexers (44) being configured to demultiplex the light beams at the output of the input waveguide (41) so as to inject at least one light beam into waveguides (41) of the plurality of waveguides (41).

8. Device (1) according to one of the preceding claims in which the etched face of the substrate (40), i.e. the open contact surface of the photoacoustic cell (5), is closed by a window.

9. Device (1) according to one of the preceding claims, in which the etching (100) of the substrate (40) is an etching (100) opening onto the waveguides (41) so that the photoacoustic cavity (51) is open on the side of the etched face of the substrate (40) and on the side of the waveguides (41), the transducer (3) being positioned against the entrance opening onto the waveguides (41), so as to seal this said entrance of the photoacoustic cavity (51).

10. Device (1) according to one of the preceding claims, in which the substrate (40) comprises etchings between the cavity (51) and the external environment, said etchings forming a channel connecting the photoacoustic cavity (51) to a transducer (3).

11. Device (1) according to one of the preceding claims, wherein the transducer (3) is configured to be manufactured on the same silicon wafer as the substrate (40).

12. Device (1) according to one of the preceding claims, wherein the illumination module (2) (2) is configured to emit light beams in the mid-infrared range.

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