Opto-mechanical transduction system for photoacoustic spectrometry

Abstract

Title: Opto-mechanical transduction system for photoacoustic spectrometry. The invention relates to an opto-mechanical system for transducing optically phase-shifted displacement, comprising a mechanical sensor including a sensing element. The sensing element has an upper face extending primarily in a plane called the longitudinal plane when the system is at rest. The mechanical sensor is designed to receive an acoustic wave to cause the sensing element to vibrate at a vibration frequency. The system further comprises an optical detector capable of guiding light radiation substantially parallel to the longitudinal plane. The optical detector has an evanescent field. The vibration of the sensing element modifies the evanescent field of the optical detector, and the sensing element moves in a direction called the transverse direction, substantially perpendicular to the longitudinal plane when it is vibrating. Figure for the abstract: Fig. 1.

Description

Title of the invention: Opto-mechanical transduction system for photo-acoustic spectrometry Technical field

[0001] The present invention relates to optomechanical detection using resonant micromechanical and / or nanomechanical structures. It finds a particularly advantageous application in photoacoustic detection, in particular for gas detection. It can in particular be used for photoacoustic spectrometry. STATE OF THE ART

[0002] Photoacoustic spectrometry is an analysis technique that makes it possible, in particular, to determine the concentration of a gas within a medium. It is based on the following photoacoustic effect: when modulated light radiation, and in particular laser radiation, is emitted into a medium containing the gas to be analyzed, the species constituting the gas absorbs at least part of this radiation if it has a wavelength in the absorption range of the species, causing excitation of the molecules of the species. The relaxation of these excited molecules can take place in several different ways, including: a. By radiative relaxation causing the emission of a photon, b. By collision with another molecule, causing heat emission, or GOOD c. By a chemical event such as a chemical bond rearrangement.

[0003] The emission of heat due to a collision with another molecule causes, among other things, a phenomenon of expansion and contraction of the gas, which generates an acoustic wave. The amplitude of this wave is proportional to the concentration of the gas within the medium.

[0004] The amplitude of this wave can be measured using different means, and in particular by a mechanical sensor coupled to capacitive detection means, such a system being commonly referred to as a resonant microelectromechanical system, or in English "resonant capacitive microelectromechanical system". However, the results obtained in the prior art are not satisfactory. Indeed, to maximize the mechanical detection of the acoustic wave it is necessary to increase the reception surface of the wave at the level of the mechanical sensor. In doing so, the size of the system is increased, which is unfavorable in a logic of optimization of the integration. Furthermore, the air located between the mechanical sensor and the electrode located opposite to allow capacitive detection is alternately sucked in and evacuated by the mechanical sensor during its vibration. This air constitutes a damper viscous for the mechanical sensor. This phenomenon, commonly referred to as the "squeeze film effect" cannot be avoided and is very disadvantageous for detection.

[0005] Thus, current integrated photoacoustic spectrometry systems do not allow results to be obtained as precise as desired.

[0006] An objective of the present invention is thus to propose an alternative to existing integrated systems that can be used for photoacoustic spectrometry. Preferably, this alternative guarantees more precise detection than existing integrated systems. SUMMARY

[0007] To achieve this objective, a first aspect of the invention relates to an opto-mechanical system for transducing a displacement into optical phase shift comprising: a. a mechanical sensor comprising a sensor element, the sensor element having an upper face extending mainly in a plane called the longitudinal plane when the system is at rest, the mechanical sensor being intended to receive an acoustic wave to set the sensor element into vibration at a vibration frequency, b. an optical detector capable of guiding light radiation substantially parallel to the longitudinal plane, the optical detector having an evanescent field.

[0008] The system is characterized in that the vibration of the sensor element modifies the evanescent field of the optical detector and in that the sensor element moves in a direction called the transverse direction substantially perpendicular to the longitudinal plane when it is vibrating.

[0009] Thus, the detection of the amplitude of the acoustic wave resulting from the excitation of a gas is carried out using a mechano-optical transduction system. The system according to the invention thus makes it possible to overcome the constraints specific to capacitive detection mentioned in the introduction. In particular, it is easy to limit or even eliminate the squeeze film effect, which could not be avoided in the case of a capacitive detection system. The system thus allows precise detection of the concentration of a gas. The system also has, thanks to this, improved sensitivity compared to existing detection devices. It makes it possible to detect very low gas concentrations, which is particularly advantageous in certain fields such as the detection of toxic gases.

[0010] The present invention thus provides a precise and sensitive detection system, while remaining compact.

[0011] A second object of the invention relates to a photoacoustic spectrometer comprising a cavity configured to accommodate at least one gas, the cavity including: a. an opto-mechanical system according to the first aspect of the invention, b. a first light source configured to inject detection radiation into an input of the optical detector of the opto-mechanical system, c. detection means capable of detecting the power of the detection radiation at a level of the optical detector, d. a second light source for injecting into the cavity an excitation radiation capable of being at least partly absorbed by said at least one gas.

[0012] The advantages of the system according to the first aspect of the invention apply mutatis mutandis to the spectrometer according to the second aspect of the invention.

[0013] Furthermore, the invention makes it possible to produce a spectrometer enabling several gases to be detected simultaneously, using a single inlet (an inlet enabling the gas to enter the cavity) and a single outlet (at the detection means). BRIEF DESCRIPTION OF THE FIGURES

[0014] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

[0015] [Fig.l] [Fig.l] represents the system according to an embodiment of the invention in which the optical detector comprises a waveguide and an optical resonator and in which the acoustic wave is received by a receiving element driving the sensor element into vibration.

[0016] [Fig.2] [Fig.2] is a top view of a system according to the same embodiment as in [Fig.l], this time comprising two mechanical sensors and two optical resonators.

[0017] [Fig.3A] Figures 3A and 3B illustrate different variants for the position of the sensor element relative to the optical detector.

[0018] [Fig.3B]

[0019] [Fig.4A] Figures 4A and 4B represent the system according to an embodiment of the invention in which the optical detector comprises a waveguide and an optical resonator and in which the acoustic wave is received by the sensor element.

[0020] [Fig.4B]

[0021] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily to the scale of practical applications. In particular, the dimensions are not representative of reality. DETAILED DESCRIPTION

[0022] Before beginning a detailed review of embodiments of the invention, optional features which may possibly be used in combination or alternatively are set out below:

[0023] According to an advantageous embodiment, the mechanical sensor is connected to a substrate, the substrate having an upper face extending mainly in a plane parallel to the longitudinal plane and located opposite the mechanical sensor, the substrate having an opening passing entirely through it in the transverse direction, the opening being located at least partially opposite the mechanical sensor in the transverse direction.

[0024] According to one example, the sensor element and the optical detector are arranged side by side in projection in the longitudinal plane.

[0025] According to one example, the sensor element and the optical detector are arranged opposite each other in the transverse direction.

[0026] According to an advantageous embodiment, the mechanical sensor comprises a receiving element intended to receive the acoustic wave and to be set into vibration by the acoustic wave at the vibration frequency, the mechanical sensor being configured so that the receiving element sets the sensor element into vibration at the vibration frequency when it is set into vibration by the acoustic wave at the vibration frequency.

[0027] According to one example, the receiving element and the sensor element are connected by a junction, the junction being connected to a substrate.

[0028] According to an advantageous example, the sensor element comprises a plurality of cantilever beams extending from the junction.

[0029] According to an advantageous embodiment, the receiving element, the junction and the sensor element are aligned in a first direction, and when the system is at rest, each of the beams has a first dimension Ln5>x in the first direction and a second dimension Ln5 Y in a second direction perpendicular to the first direction and parallel to the longitudinal plane, with Ln5>x > 10*Lii5,Y, preferably Ln5>x > 20*Ln5>Y. Typically, Ln5 x is between 20 pm and 100 pm. Typically, Ln5 Y is between 100 nm and 500 nm.

[0030] According to a preferred example, the receiving element, the junction and the sensor element are aligned in a first direction, and, when the system is at rest, the sensor element has a first dimension Ln0jX in the first direction and a second dimension Ln0jY in a second direction perpendicular to the first direction and parallel to the longitudinal plane, with Ln0jX > 10*Ln0jY, preferably Ln0jX > 100:|'Liio,y-

[0031] According to a preferred example, the receiving element, the junction and the sensor element are aligned in a first direction, and, when the system is at rest, the element sensor has a first dimension LnojX in the first direction and the receiving element has a first dimension Li2o,x in the first direction, with Ln0>x > O,42*Li2o,x-

[0032] We can also define a second dimension L^o^ of the receiving element according to the second direction. We then preferably have LhOjy= Li2o,y.

[0033] According to an advantageous example, the sensor comprises a second sensor element separated from the sensor element by the receiver element, the second sensor element and the receiver element being connected by a second junction, the second junction being connected to the substrate via anchor points, the second sensor element, the second junction, the receiver element, the junction and the sensor element being aligned in the first direction, the system having a plane of symmetry perpendicular to the first direction, and, when the system is at rest, the sensor element and the second sensor element each have a first dimension LnojX in the first direction and the sensor has a first dimension Lioo,x in the first direction, with: 2Liio,x / Lioo,x>0.46.In particular, we have Lioo^=2Lno^+Li2o,x- We can note fanch=2Lno>x / Lioo,x- This fanch factor is representative of the positioning of the junction and the second junction, which separate the receiving element from the sensor element and the second sensor element. It was observed that when fanch >0.46, a significant displacement was obtained at the ends of the sensor elements. This makes it possible to maximize the effect of the vibration of the sensor elements on the evanescent field and therefore to improve the detection sensitivity.

[0034] According to one embodiment, the sensor element is intended to receive the acoustic wave directly.

[0035] According to one embodiment, the sensor element is connected to a substrate by at least one junction. This substrate advantageously does not vibrate when the system receives the acoustic wave. This substrate can be described as fixed.

[0036] According to an advantageous example, the optical detector comprises an optical resonator and the optical resonator is at least partly housed in the opening.

[0037] According to an advantageous embodiment of the spectrometer, the second light source is a laser source and the excitation radiation is laser radiation having a main wavelength greater than 700 nm.

[0038] According to an advantageous embodiment of the spectrometer, the cavity further comprises a conduit configured to bring the acoustic wave towards the mechanical sensor. Advantageously, the conduit is also configured to amplify the acoustic wave. It thus has an acoustic resonator function.

[0039] The terms “substantially”, “approximately”, “of the order of” mean, when they relate to a value, “within 10%” of this value or, when they relate to an angular orientation, “within 10°” of this orientation. Thus, a direction sen ably normal to a plane means a direction having an angle of 90+10° with respect to the plane.

[0040] In the detailed description which follows, an XYZ reference frame shown in the figures will be used.

[0041] The system 1 according to different embodiments of the invention will now be described with reference to FIGS. 1 to 5.

[0042] The elements making up the system 1 may be formed in the same substrate 10 or, for some, may have been deposited on this substrate 10. Certain elements may be formed during the same manufacturing steps and be made of the same material, for example silicon or polysilicon. The substrate 10 may for example comprise a support substrate 11, typically based on silicon. A buried oxide layer 12 and an active layer 13 may cover the support substrate 11, the buried oxide layer 12 being located between the support substrate 11 and the active layer 13. The various elements of the system 1 described above are advantageously formed in the active layer 13 by conventional microelectronic methods.

[0043] The substrate 10 has an upper face 10a extending mainly along a plane called the longitudinal plane XY. The longitudinal plane XY is defined by a first direction X and a second direction Y perpendicular to each other.

[0044] The system 1 comprises a mechanical sensor 100 and an optical detector 200.

[0045] The mechanical sensor 100 is intended to receive an acoustic wave 5 so as to be set into vibration by this acoustic wave 5 at a vibration frequency.

[0046] The structural characteristics (dimensioning, materials, etc.) of the mechanical sensor 100 are such that the latter is capable of being set into vibration in a frequency range. Typically, the mechanical sensor 100 is capable of vibrating in a frequency range including a resonance frequency corresponding to a given gas. Those skilled in the art have perfect mastery of how to produce such a sensor so that it can vibrate at a precise frequency, or at least in a frequency range including this frequency, and preferably centered around this frequency. Thus, it is possible, for example, to design the mechanical sensor 100 so that it is capable of vibrating in a frequency range including a resonance frequency fCO2 or fNH3, and preferably capable of vibrating at a frequency substantially equal to fCO2 or fNH3, corresponding respectively to the frequencies of the acoustic waves emitted following the excitation of CO2 and NH3.The vibration amplitude of the mechanical sensor 100 will then correspond to the concentration of CO2 or NH3, respectively. It should be noted that the frequencies fCO2 or fNH3 are dependent on the thermal relaxation time of CO2 and NH3, respectively, when these gases are excited by the excitation radiation. These gases are given as examples, but it is understood that these explanations are valid for any gas. It is understood that, although the sensor . mechanical sensor 100 is sized to be able to vibrate at the resonance frequency corresponding to a given molecule, the effective vibration frequency of the mechanical sensor 100 may not be exactly this resonance frequency. The system 1 will still be functional.

[0047] The mechanical sensor 100 comprises a sensor element 110 which, when the mechanical sensor 100 receives the acoustic wave 5, is set into vibration at the resonance frequency. The sensor element 110 can directly receive the acoustic wave 5 or be set into vibration via a separate region having received this acoustic wave 5 and forming part of the mechanical sensor 100. These two variants will be considered in more detail in the context of embodiments described further on.

[0048] The sensor element 110 has an upper face 110a which, when the system 1 is at rest, extends mainly in the longitudinal plane XY. It is understood that the system 1 is at rest when it is not vibrating due to the reception of an acoustic wave 5. It is then immobile relative to the substrate 10.

[0049] The following paragraphs describe the optical detector 200.

[0050] The optical detector 200 typically comprises a waveguide 210, conventionally linear, and an optical resonator 220, for example in the form of a ring - we then speak of an optical ring -, a disk or more generally a “race track” (which can be translated into French by the term “circuit”). The waveguide 210 and the optical resonator 220 are coupled by an evanescent coupling.

[0051] The waveguide 210 comprises an input 211 and an output 212 between which, when the system is in operation, light radiation, typically laser radiation, called detection radiation, is diffused. The coupling between the waveguide 210 and the optical resonator 220 is such that at least a portion of the detection radiation is injected into the optical resonator 220 and then collected again by the waveguide 210. The vibration of the sensor element 110 in the vicinity of the optical resonator 220 causes a modification of the effective optical index of the latter and therefore disturbs the detection radiation passing through the optical resonator 220.

[0052] In order to allow the detection of the vibration of the sensor element 110 by the optical detector 200, the optical resonator 220 and the sensor element 110 are positioned so that at least a portion of the sensor element 110 is in the evanescent field of the optical resonator 220. The distance between the sensor element 110 and the optical resonator 220 may for example be of the order of 100 nm. Furthermore, the relative arrangement of the optical resonator 220 and the sensor element 110 is such that when the sensor element 110 is vibrating, the distance between these two elements varies and the sensor element 110 remains in the evanescent field of the resonator 220 optics.

[0053] The optical detector 200 further comprises means for detecting the power of the light radiation at the output 212 of the waveguide 210. This power is proportional to the displacement of the sensor element 110. The analysis of the evolution of this power thus makes it possible to determine the amplitude of the vibration of the sensor element 110 and its evolutions, therefore the amplitude of the acoustic wave and therefore to determine the concentration of the gaseous species studied in the medium.

[0054] The means for detecting the power of the light radiation may for example comprise a spectrometer, a photodetector such as a photodiode, or an external laser and interferometric detection means.

[0055] A first embodiment of the system 1 will now be described with reference to figures 1 and 2. It is understood that in [Fig.2] two mechanical sensors 100 appear, but that the system 1 can perfectly well comprise a single mechanical sensor 100, as in [Fig.l], or more than two mechanical sensors 100.

[0056] The system 1 comprises an optical detector 200 as described above.

[0057] In this first embodiment, the mechanical sensor 100 comprises a receiving element 120 intended to receive acoustic wave 5 and to be set into vibration by this acoustic wave 5 at the resonance frequency. This receiving element 120 is configured to, when it is vibrating, set the sensor element 110 into vibration.

[0058] Thus, the receiving element 120 and the sensor element 110 are mechanically coupled. For example, they may be connected by a junction 130 itself connected to the substrate 10. The position of the junction 130 defines the coupling between the receiving element 120 and the sensor element 110. The junction 130 may be a beam connected at each of its two ends to anchoring points 140. The beam preferably extends mainly along an axis parallel to the second direction Y, called the coupling axis. The beam has the capacity to deform in torsion around the coupling axis.

[0059] According to an advantageous example, the characteristics of the sensor element 110, such as for example its length LnojX in the first direction X, those of the receiver element 120 and those of the junction 130 are such that these three elements 110, 120, 130 together form a mechanism for amplifying the mechanical vibration such that the amplitude of movement at the flank 110c of the sensor element 110 is greater than the amplitude of movement at the main region 122 of the receiver element 120. Such amplification is notably enabled by a lever arm phenomenon.

[0060] According to a variant, in order to allow good mechanical coupling between the receiving element 120 and the sensor element 110, the receiving element 120 preferably has an openwork region 121 adjacent to the junction 130 in projection in the plane Ion- horizontal XY. More precisely, it is at the level of the openwork region 121 that the receiving element 120 is connected to the junction 130. The openwork region 121 preferably comprises a plurality of beams, each being fixed at one end to the junction 130.

[0061] The receiving element 120 preferably comprises a main region 122 which is solid, i.e. not openwork. In projection in the longitudinal plane XY when the system 1 is at rest, the receiving element 120 has a closed contour defining a surface Stot and the main region 122 has a surface area S122. Preferably, Si22>0.5*Stot and preferably Si22>0.9*Stot. The presence of a solid region makes it possible to promote the vibration of the receiving element 120 by the acoustic wave 5.

[0062] According to a preferred variant, the receiving element 120 does not comprise an openwork region 121 and is formed solely from the solid main region 122. This makes it possible to maximize the mechanical-acoustic interaction between the receiving element and the acoustic wave 5.

[0063] Preferably, when the system 1 is at rest, the receiving element 120 and the sensor element 110 are in the extension of one another along the longitudinal plane XY.

[0064] In order to allow good mechanical coupling between the receiving element 120 and the sensor element 110 and good mobility of the sensor element 110, the sensor element 110 is preferably perforated and thus has through openings in the transverse direction Z. Typically, the sensor element 110 comprises a plurality of beams 115, for example three, extending from the junction 130. These beams preferably extend mainly in the first direction X, perpendicular to the main direction of the junction 130. The beams 115 can also be connected to each other by transverse beams, as illustrated in [Fig. 2]. Such transverse beams make it possible to increase the rigidity of the sensor element 110 and to limit the number of undesirable vibration modes or even eliminate them.

[0065] According to another variant, the sensor element 110 consists of a single beam preferably extending mainly in the first direction X.

[0066] According to one embodiment, the mechanical sensor 100 comprises a single sensor element 110.

[0067] According to another embodiment, as illustrated in [Fig.l], the mechanical sensor 100 may have a plane of symmetry perpendicular to the longitudinal plane XY and comprising the second direction Y. This plane of symmetry intersects in particular the receiving element 120. Thus, typically, the mechanical sensor 100 comprises a second junction 130' and a second sensor element 110'. The junction 130 and the sensor element 110 on the one hand and the second junction 130' and the second element sensor 110' on the other hand are located on either side of the receiving element 120. These elements are preferably in continuity with each other along the first direction X when the system 1 is at rest.

[0068] It is understood that although the second sensor element 110 has all the characteristics of the sensor element 110, it is not necessary to associate it with an optical detector. A reading of the vibration of the mechanical sensor 100 at the sensor element 110 is sufficient in itself to trace the vibration amplitude and the concentration of the analyzed gas. However, the possibility is provided of including in the system 1 a second optical detector associated with the second sensor element 110. The presence of two optical detectors can indeed be advantageous.

[0069] As also illustrated in [Fig. 1], the mechanical sensor 100 advantageously has a plane of symmetry perpendicular to the longitudinal plane XY and comprising the first direction X. This allows good balance of the mechanical sensor 100 and thus better detection of the vibration amplitude.

[0070] The sensor element 110 may be positioned in different ways relative to the optical resonator 220, as illustrated in FIGS. 3A and 3B.

[0071] According to a first example illustrated in [Fig.3A], in projection in the longitudinal plane XY, the sensor element 110 and the optical resonator 220 are located side by side. The sensor element 110 then has a flank 110c facing an interaction face 220c of the optical resonator 220. Advantageously, and as illustrated in [Fig.2], the flank 110c of the sensor element 110 located facing the optical resonator 220 has a shape complementary to the latter. This makes it possible to maximize the mechano-optical interaction between the moving mass 110 and the optical detector 200. For example, when the optical resonator 220 is an optical ring and therefore has a crown shape in projection in the longitudinal plane XY, it is advantageous to provide that the sensor element 110 has a concave shape towards the optical resonator 220, that is to say at the level of its flank 110c.

[0072] According to a second example illustrated in [Fig.3B], the sensor element 110 and the optical resonator 220 are located opposite each other in the transverse direction Z. Thus, in projection in the longitudinal plane XY, the image of one is projected onto the image of the other. This embodiment makes it possible to maximize the amplitude of the distance between the sensor element 110 and the optical resonator 220 during the vibration of the sensor element 110. This makes it possible to maximize the effect of the vibration on the evanescent field and consequently the optomechanical effect. The output signal is therefore greater, and the detection sensitivity is therefore improved.

[0073] In this embodiment illustrated in [Fig.3B], the optical resonator is advantageously a photonic crystal. A resonator of this type has a field evanescent highly concentrated out-of-plane (i.e. here in the transverse direction Z). Furthermore, the size of the optical resonance of a photonic crystal is comparable to the typical values ​​that can be given to the width of the beam forming the sensor element 100 (typically between 200 nm and 1 pm). System 1 is therefore more sensitive than in the case of an optical resonator in the form of a disk or ring.

[0074] A second embodiment of the system 1 will now be described with reference to FIGS. 4A and 4B.

[0075] The system 1 comprises an optical detector 200 as described above.

[0076] In this second embodiment, the acoustic wave 5 is directly received by the sensor element 110. The latter is therefore directly set into vibration at the resonance frequency.

[0077] The sensor element 110 is advantageously connected to the substrate 10 via anchoring points 140. In the example illustrated, the sensor element 110 is connected to the anchoring points 140 via junctions 150, typically beams. Each junction 150 is connected at each of its ends to an anchoring point 140, along a coupling axis specific to said junction 150. Each junction 150 has the capacity to deform in torsion around its coupling axis.

[0078] In order to allow good mechanical deformation of the sensor element 110 when it receives the acoustic wave 5, the latter is preferably perforated and thus has through openings in the transverse direction Z. Typically, the sensor element 110 comprises a plurality of beams 116 extending between the junctions 150 and a central region 117 of the sensor element 110.

[0079] In the advantageous example illustrated in Figures 4A and 4B, the central region 117 has a rectangular shape, preferably square, in projection in the longitudinal plane XY when the system 1 is at rest. In projection in the longitudinal plane XY and when the system 1 is at rest, the central region 117 has dimensions Ln7jX and Ln7jY taken respectively in the second direction X and in the excitation direction Y. Ln7X and Ln7 Y are each typically between 1 pm and 1000 pm.

[0080] The system 1 comprises four anchoring points 140 and four junctions 150 each extending between two anchoring points 140. Each coupling axis of the junctions 150 is preferably substantially parallel to one side of the central region 117. The sensor element 110 comprises eight beams 116 connecting it to the junctions 150. Two beams 116 connect each side of the central region 117 to the junction 150 located opposite when the system 1 is at rest. The beams 116 may each have a length of between 50 nm and 20 pm, this length being measured in the first direction X or the second direction Y depending on the orientation of the beam 116.

[0081] In this embodiment, the sensor 100 may have a width of between 3 pm and 5000 pm.

[0082] As illustrated in Figures 4A and 4B, the mechanical sensor 100 advantageously has a plane of symmetry perpendicular to the longitudinal plane XY and comprising the second direction Y. As is also illustrated in Figures 4A and 4B, the mechanical sensor 100 advantageously has a plane of symmetry perpendicular to the longitudinal plane XY and comprising the first direction X. These symmetries are advantageous for setting the mechanical sensor 100 into resonance. They are also advantageous for attenuating unwanted vibration modes.

[0083] Figures 4A and 4B illustrate a particular example of embodiment of the sensor element 110, but it is understood that its structure may be different, in particular the sensor element 110 could comprise another number of beams 116.

[0084] In the embodiment illustrated in Figures 4A and 4B, the optical resonator is advantageously a photonic crystal. A photonic crystal can indeed have small dimensions, which contributes to the reduction of viscous losses.

[0085] In each of the embodiments of the system 1 described previously, it is possible to provide the presence of an opening 15 in the substrate 10 under the mechanical sensor 100. Thus, in projection in the longitudinal plane XY, the image of one is projected onto the image of the other.

[0086] This opening 15 is preferably located, in the transverse direction Z, at least partly under the zone of the mechanical sensor 100 receiving the acoustic wave. Thus, in the embodiment described with reference to [Fig. 2], the opening is preferably located at least partly under the receiving element 120. Preferably, the opening 15 underlies the entirety of the receiving element 120. Advantageously, still in this embodiment illustrated in [Fig. 2], the opening 15 also extends under at least a portion and preferably the entirety of the sensor element 110. In the embodiment described with reference to FIGS. 4A and 4B, the opening is preferably located at least partly under the sensor element 110. Preferably, the opening 15 underlies the entirety of the sensor element 110.

[0087] The presence of such an opening makes it possible to reduce the attenuation of the vibration of the mechanical sensor 100, and thus to maximize the detection of the acoustic wave by the system. In particular, it makes it possible to prevent an air film from forming between the substrate and the mechanical sensor 100, such an air film causing undesirable viscous damping.

[0088] In the case of the embodiment described with reference to FIGS. 4A and 4B, this opening 15 advantageously accommodates the optical resonator 220 of the optical detector 200. This makes it possible to make the system both more compact and more robust.

[0089] According to an example which can be applied to all the embodiments described above Alternatively, the mechanical sensor 100 may be formed from a non-resonant membrane, for example graphene, coupled to an acoustic resonator.

[0090] Multiple moving mass detection system

[0091] According to an advantageous embodiment, the system according to the invention may comprise a plurality of mechanical sensors 100, each mechanical sensor 100 being placed so as to modify an evanescent field of the optical detector 200.

[0092] The system 1 then advantageously comprises as many optical resonators 220, all coupled with the waveguide 210, as mechanical sensors 100. Each mechanical sensor 100 can then be associated with an optical resonator 220 for the detection of a different vibration frequency (see [Fig.2]), or all the mechanical sensors 100 can be associated with the same optical resonator 220. The mechanical sensors 100 have different dimensions from each other so that each is capable of being set into vibration at a distinct vibration frequency.

[0093] Whatever the embodiment chosen, a system having several mechanical sensors makes it possible to simultaneously detect several types of gas.

[0094] Photoacoustic spectrometer comprising a system according to the invention

[0095] Another object of the invention relates to a photoacoustic spectrometer comprising a detection system as described previously.

[0096] The system 1 can in fact be arranged within a cavity 2 which can accommodate one or more gases to be analyzed (see [Fig.l]).

[0097] The cavity 2 then further comprises a first light source 21 enabling so-called detection radiation to be injected into an input 211 of the optical detector 200, as well as means 22 for detecting the radiation at the output 212 of the optical detector 200.

[0098] The cavity 2 also comprises a second light source 30 intended to inject into the cavity 2 so-called excitation radiation making it possible to excite the gas(es) contained in the cavity 2. The second light source 30 is typically a modulated laser source. The modulated nature of the source in fact makes it possible to scan the gas to be analyzed in a range included in its absorption range, typically in an absorption peak.

[0099] The excitation radiation, typically the excitation laser, advantageously has a spectral width that is narrower than the absorption range of the gas to be analyzed. Furthermore, the higher the laser intensity, the more relaxations of molecules composing this gas will be detected and therefore the greater the output signal will be.

[0100] The excitation radiation or laser generally has a main wavelength in the infrared, typically between 1 pm and 20 pm. In this range, in fact, most molecules have a single absorption line, which is advantageous for output signal analysis.

[0101] [Fig.l] illustrates an embodiment of the spectrometer in which the first light source 21 and the second light source 30 are distinct. However, it is conceivable, according to another variant, that these two sources 21, 30 are in reality one and the same source. In this variant, the radiation would be parallel to the second direction Y. Whatever the variant chosen, the radiation must be focused vertically, in the direction Z, of the receiving element 120, and preferably vertically of the center of the main region 122.

[0102] According to one example, it is provided that the acoustic waves are conducted to the mechanical sensor 100 (to the receiving element 120 or to the sensor element 110, depending on the embodiment) by a conduit. In the case of a system comprising several mechanical sensors 100, the presence of several conduits or a conduit having an opening opposite each mechanical sensor 100 is provided. The different conduits - or the single conduit - will then guide and amplify acoustic waves generated by lasers having distinct main wavelengths.

[0103] The use of a conduit to conduct the acoustic waves towards the receiving element 120 or directly towards the sensor element 110 is particularly advantageous when the latter has small dimensions. The conduit then makes it possible to focus the wave towards the receiving element and maximize the mechanical-acoustic interaction. The choice of whether or not to use a conduit is therefore made mainly according to the dimensions of the receiving element 120 or the sensor element 110.

[0104] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention.

Claims

Claims

1. Opto-mechanical system (1) for transducing a displacement into optical phase shift comprising: • a mechanical sensor (100) comprising a sensor element (110), the sensor element (110) having an upper face (110a) extending mainly in a plane called the longitudinal plane (XY) when the system (1) is at rest, the mechanical sensor (100) being intended to receive an acoustic wave to vibrate the sensor element (110) at a vibration frequency, • an optical detector (200) capable of guiding light radiation substantially parallel to the longitudinal plane (XY), the optical detector (200) having an evanescent field, characterized in that the vibration of the sensor element (110) modifies the evanescent field of the optical detector (200) and in that the sensor element (110) moves in a direction called the transverse direction (Z) substantially perpendicular to the longitudinal plane (XY) when it is in vibration.

2. System (1) according to the preceding claim in which the mechanical sensor (100) is connected to a substrate (10), the substrate (10) having an upper face (10a) extending mainly in a plane parallel to the longitudinal plane (XY) and located opposite the mechanical sensor (100), the substrate (10) having an opening (15) passing entirely through it in the transverse direction (Z), the opening (15) being located at least partially opposite the mechanical sensor (100) in the transverse direction (Z).

3. System (1) according to any one of the preceding claims in which the sensor element (110) and the optical detector (200) are arranged side by side in projection in the longitudinal plane (XY).

4. System (1) according to any one of claims 1 and 2 wherein the sensor element (110) and the optical detector (200) are arranged opposite each other in the transverse direction (Z).

5. System (1) according to any one of the preceding claims in which the mechanical sensor (100) comprises a receiving element (120) intended to receive acoustic sound and to be vibrated by the acoustic wave at the vibration frequency, the mechanical sensor (100) being configured so that the receiving element (120) vibrates the sensor element (110) at the vibration frequency when it is vibrated by the acoustic wave at the vibration frequency.

6. System (1) according to the preceding claim in which the receiving element (120) and the sensor element (110) are connected by a junction (130), the junction (130) being connected to a substrate (10) via anchoring points (140).

7. System (1) according to the preceding claim in which the receiving element (120), the junction (130) and the sensor element (110) are aligned in a first direction (X), and in which, when the system (1) is at rest, the sensor element (110) has a first dimension L iio,x in the first direction (X) and the receiving element (120) has a first dimension Li2o,x in the first direction (X), with Ln0,x > 0.42*L120.x.

8. System (1) according to either of the two preceding claims wherein the sensor (10) comprises a second sensor element (110') separated from the sensor element (110) by the receiver element (120), the second sensor element (110') and the receiver element (120) being connected by a second junction (130'), the second junction (130') being connected to the substrate (10) via anchor points (140), the second sensor element (110'), the second junction (130'), the receiver element (120), the junction (130) and the sensor element (110) being aligned along the first direction (X), the system (1) having a plane of symmetry perpendicular to the first direction (X), and, when the system (1) is at rest, the sensor element (110) and the second sensor element (110') each have a first dimension LnojX in the first direction (X) and the sensor (100) has a first dimension Li0o,x in the first direction (X),with: 2L11Ojx / Lioo,x^O,46.,

9. System (1) according to any one of claims 1 to 4 in which the sensor element (110) is intended to directly receive the acoustic wave.

10. System (1) according to the preceding claim in which the sensor element (110) is connected to a fixed substrate (10) by at least one junction (130).

11. System (1) according to any one of the two preceding claims- preceding claims in combination with claim 4 wherein the optical detector comprises an optical resonator (220) and wherein the optical resonator (220) is at least partly housed in the aperture (15).

12. Photoacoustic spectrometer comprising a cavity configured to accommodate at least one gas, the cavity comprising: • an opto-mechanical system (1) according to any one of the preceding claims, • a first light source (21) configured to inject detection radiation into an inlet (211) of the optical detector (200) of the opto-mechanical system (1), • detection means (22) capable of detecting the power of the detection radiation at an outlet (212) of the optical detector (200), • a second light source (30) for injecting into the cavity excitation radiation capable of being at least partly absorbed by said at least one gas.

13. Photoacoustic spectrometer according to the preceding claim wherein the second light source is a laser source and wherein the excitation radiation is laser radiation having a main wavelength greater than 700 nm.

14. A photoacoustic spectrometer according to any one of claims 12 and 13 wherein the cavity further comprises a conduit configured to bring the acoustic wave towards the mechanical sensor (110).

Images