COMPACT GASS SENSOR WITH SIMPLE DESIGN

Description

DOMAINE TECHNIQUE

[0001] The technical field of the invention is an optical gas sensor, and more particularly a non-dispersive infrared sensor. ART ANTERIEUR

[0002] The use of optical methods for gas analysis is quite common. Devices exist that allow the composition of a gas to be determined based on the fact that the species composing a gas exhibit different spectral absorption properties. Thus, knowing the spectral absorption band of a gaseous species, its concentration can be determined by estimating the absorption of light passing through the gas, using Beer-Lambert's law. This principle allows for an estimation of the concentration of a gaseous species present in the medium.

[0003] The light source is usually an infrared source, and the method used is commonly referred to by the English term "NDIR detection," the acronym NDIR standing for Non-Dispersive Infrared. This principle has been frequently implemented and is described in numerous documents, such as US5026992 and WO2007064370.

[0004] In most common methods, the gas being analyzed is placed between a light source and a photodetector, called a measurement photodetector. This photodetector measures a light wave emitted by the source, transmitted through the gas being analyzed, and partially absorbed by it. The methods generally include measuring a light wave, called a reference light wave, emitted by the source but not absorbed by the gas being analyzed.

[0005] Comparing the light wave in the presence of a gas with the light wave without gas allows for the characterization of the gas. This can be done, for example, to determine the quantity of a gaseous species within the gas, using a technology known as "NDIR absorption." It can also be used to estimate the quantity of particles in the gas by detecting light scattered by it within a predetermined angular scattering range.

[0006] The reference light wave is measured by a reference photodetector. This reference photodetector may be different from the measurement photodetector and positioned to face the light source. The reference photodetector is associated with a reference optical filter. The reference optical filter defines a reference spectral band in which the gas being analyzed does not exhibit significant absorption.

[0007] Documents EP2711687 and EP2891876 describe gas sensors comprising enclosures in which one or more mirrors are arranged. The mirrors maximize the path length of the light within the enclosure and focus the light rays that have passed through the gas onto the photodetector(s). This increases detection sensitivity while using compact devices.

[0008] FR 3 022 999 A1 discloses a gas sensor comprising an enclosure with a first series of mirrors and a second series of mirrors arranged relative to each other such that the radiation emitted by the light source is reflected alternately by a mirror of the second series and by a mirror of the first series, defining an optical path from the light source to the photodetector

[0009] US2022 / 0214267 describes a compact configuration of a gas sensor, compatible with a so-called "4R" format, which is a common standard in the field of gas detection.

[0010] The applicant has designed a particularly compact gas sensor, compatible with the 4R format, which is simpler and less expensive to manufacture than the sensor described in US2022 / 0214267. The result is a gas sensor with optimized performance, prioritizing compactness and cost. EXPOSE DE L'INVENTION

[0011] A first object of the invention is a gas sensor comprising a chamber configured to receive a gas, the sensor also comprising: a light source, configured to emit a light wave propagating through the enclosure, forming a first cone of light from the light source; a measurement photodetector configured to detect a light wave emitted by the light source and propagating through the enclosure; the light source and the measurement photodetector being supported by a printed circuit board; - a first-order reflective surface, forming part of an ellipsoid of revolution, having a first focus and a second focus, the first-order reflective surface being positioned opposite the light source, so as to reflect the first cone of light towards the second focus, the first reflective surface forming part of an ellipsoid of revolution;- the device being characterized in that: the printed circuit board includes a plane mirror, forming a reflective surface of rank 2, the second focus of the reflective surface of rank 1 being located on the plane mirror or at a distance of less than 1 mm from the latter; the device includes a final reflective surface, of rank; N, N being greater than or equal to 3, comprising a first focus, located at the level of a plane mirror, arranged on the printed circuit board, forming a reflective surface of rank N -1 , the last reflective surface having a second focus; such that the light wave emitted by the light source is successively reflected by the Nreflective surfaces, according to their respective ranks, before converging towards the second focus of the last reflective surface, forming a final cone of light, the measurement photodetector being arranged so as to extend into the last cone of light.

[0012] According to one possibility, the light source is positioned at the first focus of the first-order reflective surface, or at a distance of less than 1 mm from it.

[0013] In one possibility, the measurement photodetector is placed at the second focus of the last reflective surface, or at a distance of less than 1 mm from it.

[0014] According to one possibility: the device comprises several reflective surfaces of even rank, each reflective surface of even rank being a plane mirror arranged on the printed circuit; the device comprises several reflective surfaces of odd rank, each reflective surface of even rank forming part of an ellipsoid of revolution and extending opposite the printed circuit.

[0015] According to one possibility: each reflective surface of rank n is configured to reflect the light wave it receives, forming a cone of light of rank n +1, the latter extending around a propagation axis of rank n +1; when n<N, the propagation axis of rank n +1 extends between the reflective surface of rank n up to the reflective surface of rank n +1; when n is odd, and n ≤ N -2 ,Each reflective surface extends along a portion of an ellipsoid of revolution, the propagation axis of rank n +1 passing through the second focus of the surface of rank n, the latter being confused with the first focus of the surface of rank n +2; when n = N, the propagation axis of rank N +1 extends between the surface of rank N up to the measurement photodetector; such that the light wave emitted by the light source undergoes a number of reflections corresponding to the number of reflective surfaces before reaching the measuring photodetector.

[0016] According to one possibility, in which when n < N And n is odd, each cone of light of rank n +1 converges towards the second focus of the surface of rank nfrom which it propagates. Odd-numbered reflective surfaces can be arranged on an inner face of the same wall, forming a sensor cover. The enclosure can be formed by mounting the sensor cover on top of a base, with the printed circuit board placed on the base.

[0017] The plane mirror(s) can be formed by a thin layer of reflective material deposited on the printed circuit board.

[0018] The number of reflective surfaces extending between the light source and the measuring photodetector is preferably odd. The number of reflective surfaces extending between the light source and the measuring photodetector can be either 3 or 5. In one option, the sensor includes a reference photodetector configured to detect a light wave considered unattenuated by the gas.

[0019] The sensor may include a reflective reference surface, forming an ellipsoid of revolution, the reference surface comprising: a first focus, located at the level of a flat reflective surface, of even rank, or at a distance of less than 1 mm from the latter; a second focus, located at the level of the reference photodetector, or at a distance of less than 1 mm from the latter.

[0020] A second object of the invention is a method for analyzing a gas using a sensor according to the first object of the invention, the method comprising the following steps: a) introduction of the gas into the sensor enclosure; b) illumination of the gas using the light source; c) measurement of the light intensity of a light wave, emitted by the light source, and propagated through the enclosure by the measuring photodetector; d) determination of the quantity of a gaseous species in the gas, from the measured light intensity.

[0021] Step d) can be implemented by a processing unit from a signal generated by the measurement photodetector.

[0022] The process may include the detection, by a reference photodetector, of a reference light wave considered to be unabsorbed, or negligibly absorbed, by the gas. Step d) is then implemented by the processing unit using a reference signal generated by the reference photodetector.

[0023] The invention will be better understood by reading the explanation of the examples of embodiment presented, in the continuation of the description, in connection with the figures listed below. FIGURES

[0024] THE figures 1A et 1B are cross-sections of the gas sensor, respectively in a transverse plane XZ and in a principal plane XY, the principal plane being perpendicular to the transverse plane. figure 2A The diagram shows the base and cover of the sensor and indicates the location of the main sensor components: a light source, a measurement photodetector, and three reflective surfaces. In this figure, axis D1 corresponds to a circular generatrix shown on the figure 1B This representation is equivalent to unrolling the sensor enclosure along this generator. figure 2B represents a variant of the embodiment shown schematically on the figure 2A , employing a reference photodetector. The figure 3A This shows an embodiment using five successive reflective surfaces. figure 3B represents a variant of the embodiment shown schematically on the figure 3A , employing a reference photodetector. The figure 4 shows an experimental test bench designed to determine the reflectivity of different thin films. EXPOSE DE MODES DE REALISATION PARTICULIERS

[0025] THE figures 1A et 1B The diagrams represent a gas sensor 1 according to the invention. The gas sensor 1 comprises a housing 2 delimited by a base 4, a cover 5, and a transverse wall 6. The transverse wall 6 extends parallel to a transverse axis Z. The transverse wall 6 is an annular wall around the transverse axis Z. It extends between the base 4 and the cover 5. The transverse axis Z extends perpendicularly to a principal plane XY. The housing 2 forms a cylindrical casing, obtained by placing the cover 5 on the base 4.

[0026] The base 4 has an external face 4e and an internal face 4i. The cover 5 has an external face 5e and an internal face 5i. The external faces 4e and 5e of the base 4 and cover 5, respectively, are flat and parallel to the principal plane XY. They are perpendicular to the transverse axis Z around which the annular wall 6 extends. The internal faces 4i and 5i of the base 4 and cover 5, respectively, are oriented towards the interior of the enclosure 2. The enclosure 2 is connected to connecting pins 3, parallel to the transverse axis Z.

[0027] The shape and dimensions of sensor 1 conform to a standard format in the field of gas detection, known as the 4R format. The diameter Φ of the enclosure 2, in the XY plane, is 20 mm, and its height h (along the Z-axis) is 16.6 mm. Considering the thickness of the transverse wall 6, the internal diameter Φ' of the enclosure 2 is 17 mm. A positive direction of rotation around the transverse axis Z is defined, which is arbitrarily considered to be similar to the direction of clockwise rotation.

[0028] One advantage of the 4R format is its widespread use in electronics. A gas sensor conforming to this format can therefore be easily connected to existing electronic circuits. Adherence to this format also facilitates the maintenance of measurement systems by simplifying sensor replacement.

[0029] Enclosure 2 is intended to be occupied by a gas Gto be analyzed. The gas to be analyzed contains a gaseous species G x of which we wish to determine a concentration C x in the gas. The gaseous species G x absorbs light, and in particular infrared light, within a spectral absorption band Δλx. Sensor 1 also includes: ▪ A light source 10, configured to emit a light wave 11 in an emission spectral band Δλ. The emission spectral band can extend from the near-ultraviolet to the mid-infrared, for example between 200 nm and 20 µm, and most often in the infrared, the emission spectral band Δλ being, for example, between 1 µm and 20 µm. ▪ A measurement photodetector 20, configured to detect a light wave 14, called the transmitted light wave, in the absorption spectral band Δλx. The transmitted light wave is a light wave emitted by the light source 10, and having propagated through the enclosure, gradually attenuating during its passage through the gas. The measurement photodetector 20 can be associated with a bandpass filter, defining a detection spectral band Δλd included in the absorption spectral band Δλx. ▪ a reference photodetector 20ref, configured to detect a reference light wave.The reference light wave detected by the reference photodetector 20 ref is considered to be unattenuated by the gas. Gpresent in the chamber. In this example, the reference photodetector detects the reference light wave 20 ref in a reference spectral band Δλ ref, in which the attenuation of the light wave emitted by the source is considered negligible, or due to a variation in environmental conditions, for example, temperature or humidity. The reference photodetector can be associated with a reference bandpass filter, defining the reference spectral band Δλ ref. For example, the reference spectral band Δλ ref is centered around the wavelength 3.91 µm. The use of a reference photodetector is optional but advantageous. Alternatively, the reference photodetector is not placed in the chamber 2 containing the gas, in which case the reference spectral band can be similar to the measurement spectral band.

[0030] Preferably, the light source 10, the measurement photodetector 20, and the reference photodetector 20 ref are connected to the same flat support. This could, in particular, be an electronic board forming the first wall 4 of the enclosure 2. Such an arrangement simplifies the manufacture of the sensor.

[0031] The light source 10 can be pulsed, emitting light pulses with a duration generally between 50 ms and 1 s. It can, in particular, be an infrared light source of the filament type formed on a suspended membrane, the filament carrying an electric current. In this way, the suspended membrane is heated to a temperature between 400°C and 800°C, so as to emit infrared light.

[0032] The measuring photodetector 20 and the reference photodetector 20 ref can be thermopiles, pyrodetectors, or photodiodes. Each photodetector is configured to generate a detection signal, the amplitude of which A corresponds to an intensity I of the detected light wave.

[0033] The intensity I the light wave detected by the measuring photodetector 20 depends on the concentration C x of the gaseous species of interest G x according to the Beer-Lambert equation: att = I I 0 = e − μ C x l Or : µ(C x ) is an attenuation coefficient, dependent on the concentration C x wanted; l is the thickness of gas through which the light wave passes in the enclosure; I 0 corresponds to the intensity of the wave which is or would be detected by the measuring photodetector 20 in the absence of absorbing gas in the enclosure.

[0034] The comparison between I And I 0, taking the form of a ratio I / I 0 , corresponds to an attenuation att caused by the gaseous species G x considered. During each pulse of the light source 10, it is thus possible to determine µ(C x ), which allows us to estimate the quantity C x knowing that the relationship between C x And µ(C x ) is known.

[0035] The intensity I 0 can be predetermined, for example during calibration, or measured in the absence of gas in chamber 20. When the sensor includes a reference photodetector 20 ref, the reference intensity I ref measured by the reference photodetector allows for the measurement of variations in I 0.

[0036] The sensor includes a processing unit 30, which receives the signals measured respectively by the measurement photodetector 20 and the optional reference photodetector 20 ref. The processing unit 30 performs calculations to estimate the concentration of the gaseous species being sought, based on these signals. The processing unit 30 includes, for example, a microprocessor.

[0037] The dimensions of the enclosure 2 in which the gas is contained are particularly small. However, it is preferable that the light wave detected by the measuring photodetector 20 be sufficiently attenuated so that the attenuation att so that it is more easily quantifiable, taking into account the various noises in the measurement chain. It is therefore preferable that the length l The distance traveled by the rays reaching the measurement photodetector is sufficiently high. The applicant considers that the attenuation att is preferably greater than 0.2.

[0038] US patent application US2022 / 0214267 describes a particularly compact configuration of a gas sensor in which the rays are reflected by ellipsoidal reflective surfaces. An ellipsoidal surface is defined as a surface that follows a portion of an ellipsoid of revolution. The reflective surfaces allow the wave emitted by the light source to undergo multiple reflections before reaching the measuring photodetector. This results in a sufficiently short wavelength. l within chamber 2, the rays reach the measuring photodetector 20. However, such a configuration requires the creation of a large number of ellipsoidal surfaces, which must be precisely arranged relative to one another. Furthermore, it requires the creation of two different parts with complex shapes.

[0039] There figure 2A diagram of the interior of enclosure 2. On the figure 2A The interior of enclosure 2 was represented by following a circular generatrix D1 shown in dotted lines on the figure 1B This representation amounts to unrolling the enclosure along the circular generatrix D1, along which reflective surfaces extend. S n rank n. The index n is a natural number such that 1 ≤ n ≤ N, N being the number of reflective surfaces located on the optical path of the light wave emitted by the light source 10 and propagating towards the measurement photodetector 20.

[0040] In this system, depending on whether n whether even or odd, the reflective surface S n rank n is respectively planar or ellipsoidal. Thus, the sensor is formed of an alternation of ellipsoidal reflective surfaces (rank n odd) or plane ( npair). The use of flat reflective surfaces simplifies the manufacture of the sensor.

[0041] The term ellipsoidal surface S n denotes an ellipsoidal surface of revolution, obtained by rotating an ellipse in space E n around one of its axes. In the example described, each ellipsoidal surface S n , n being odd, it is associated with an ellipse E n presenting a major axis and a minor axis. By ellipsoidal surface associated with an ellipse, we mean an ellipsoidal surface formed by a rotation of the ellipse around one of its axes. In the example shown, each ellipsoidal surface S n is obtained by an ellipse rotation E n , to which it is associated, around its major axis. Such an ellipsoidal surface is said to be elongated, which corresponds to the Anglo-Saxon designation "prolate spheroid" or "prolate ellipsoid".

[0042] A reflective surface is defined as a surface whose reflection coefficient, in all or part of the emission spectral band Δλ, is greater than 40%, or even 50%, and preferably greater than 80%. In the infrared spectral range, a reflective wall can be formed using a reflective material such as a metal, for example gold.

[0043] In the configurations shown on the figures 2A et 2B , N = 3. In the configurations shown on the figures 3A et 3B , N = 5. N can be between 3 and 10, but the applicant considers that N =5 corresponds to an optimum.

[0044] Regardless of the configuration, every reflective surface S n , n being odd, it is ellipsoidal. It has a first focus P 1- n and a second focus P 2- n , the two foci being distinct from each other. When 3 ≤ n < N, the first focus P 1-n is confused with the second focus P 2- (n-2) of an ellipsoidal reflective surface S n-2 of rank n-2. When n = 1 (first reflective surface) S 1 ), the second focus P 2-1 coincides with the first focus P 1-3 of the second reflective surface S 3 . When n = N (last surface), the first focus P 1- N is confused with the second focus P 2- (N-2) of the ellipsoidal surface S N-2 rank N -2. The first reflective surface S 1 allows reflection of the incident light wave 11 emitted by the light source 10, while the last reflective surface S N allows reflection of the light wave, having passed through enclosure 10, towards the measuring photodetector 20.

[0045] Each reflective surface S n , n Since it is even, it is a plane mirror. Each reflective surface S n , n being even, is located at the level: of the second focus P 2,n-1 of the ellipsoidal reflecting surface S n-1 of the immediately lower rank; of the first focus P 1,n+1 of the ellipsoidal reflecting surface of the immediately higher rank S n+1 .

[0046] The light source 10 and the measuring photodetector 20 are mounted on a printed circuit board 7. Preferably, the light source and the measuring photodetector are mounted on the same printed circuit board. Preferably, each plane mirror, forming a reflective surface S n Each mirror of even rank is formed by a thin layer of a reflective material, such as gold or nickel, deposited directly onto the PCB. The thickness of the thin layer can be a few hundred nanometers, for example, between 50 nm and 1 µm, or between 300 nm and 1 µm. Advantageously, each mirror results from the deposition of the reflective material directly onto the PCB. This simplifies the detector manufacturing process. The flat thin layer is positioned so as to lie along the Δn-1 axis of the light cone Ωn-1 emitted by the ellipsoidal reflective surface. S n-1 of the previous rank. As previously stated, each reflective surface S n , n being even, extends at the level of the second focus P 2,n-1 of the ellipsoidal reflecting surface S n-1 of previous rank, as well as at the level of the first focus P 1,n+1 of the ellipsoidal reflecting surface S n+1 of the next rank.

[0047] One advantage of the invention is that only the odd-numbered reflective surfaces are ellipsoidal. These surfaces are formed on the cover 5, and more precisely, form the inner face 5i of the cover. Unlike the configuration described in US2022 / 0214267, the ellipsoidal reflective surfaces are formed on a single piece, intended to be positioned facing the printed circuit board 7. This limits the number of complex parts required. Furthermore, forming plane mirrors on the printed circuit board is particularly simple and inexpensive, thus reducing the cost of the sensor. Either a thin reflective layer can be deposited, or a prefabricated plane mirror can be used.

[0048] The light wave 11 emitted by the light source is successively reflected by the different reflective surfaces S n , according to their respective ranks, taken in ascending order.

[0049] In other words, reflective surfaces S 1 ... S N extend successively between the light source 10 and the measuring photodetector 20. The term extend successively is to be interpreted by considering the propagation of light between the light source 10 and the measuring photodetector 20. The path of the light wave, projected onto the base 4 or the hood 5, describes an arc of a circle.

[0050] The light source 10 is configured to emit the emission light wave 11 according to a light cone Ω 1, called the first cone, extending around a first axis Δ 1, up to the first ellipsoidal reflective surface S 1 . The light source 10 is positioned at a vertex of the first light cone Ω1. The light source 10 preferably coincides with the first focus P1-1 of the first reflective surface. S 1 .

[0051] The terms "confused" or "at the level of" should be interpreted with some leeway and can mean "to be located in the vicinity of," the vicinity being an area extending a short distance from a point. By short distance, we mean a distance of less than 1 mm, or even less than 0.5 mm.

[0052] Based on the configuration shown on the figure 2A , the first ellipsoidal reflective surface S 1 is configured to reflect the first light cone Ω1 to form a second light cone Ω2, extending around a second axis Δ2. The second light cone converges towards the second focus P2-1 of the first reflecting surface. S 1 , the latter being located at the level of the plane mirror S 2 , which forms the second reflective surface.

[0053] The sensor includes a third ellipsoidal reflective surface S 3 ,whose first focus P 1.3 extends to the level of the second reflective surface S 2 The second reflective surface S 2 is configured to reflect the second light cone Ω2 to form a third light cone Ω3, extending around a third axis Δ3. The third light cone Ω3 diverges to the third ellipsoidal reflecting surface S 3 .

[0054] The third ellipsoidal reflective surface S 3 is configured to reflect the third light cone Ω 3 to form a fourth light cone Ω 4, extending around a fourth axis Δ 4. The fourth light cone Ω 4 converges towards the measuring photodetector 20, the latter being located at the second focus P 2-3 of the third reflector S 3 .

[0055] Thus, the light wave 11 emitted by the light source 10 propagates between the different reflective surfaces S 1 ... S N , alternately ellipsoidal and planar, before reaching the measurement photodetector. In this example, N = 3. N can be equal to 5, or to another odd integer greater than or equal to 3. Between the light source 10 and the measuring photodetector 20, the light wave emitted by the light source undergoes N successive reflections, that is, as many reflections as there are reflective surfaces extending between the light source 10 and the measuring photodetector 20. This minimizes signal loss during each reflection. Between two reflective surfaces of successive ranks n , n +1, the light wave 11 does not undergo reflection by another reflective surface. The odd-rank reflective surfaces are ellipsoidal. The second-rank reflective surface is a plane mirror arranged on the printed circuit board 7.

[0056] There figure 2B shows a configuration in which the sensor 1 includes a reference photodetector 20 ref, the latter being arranged on the same printed circuit board 7 as the light source 10 and the measurement photodetector 20. The sensor includes a reflective ellipsoidal reference surface S ref , opposite the reference photodetector 20 ref. The reference reflective surface S ref includes a first zone P 1-ref and a second zone P 2-ref , the foci being distinct from one another. The first focus P 1-ref of the reference reflective surface S ref is located on the reflective surface S 2 The second focus P 2-ref of the reference reflective surface S ref is located at the level of the reference photodetector 20 ref. Like the odd-rank reflective surfaces, the reference reflective surface S ref is preferably mounted on the wall opposite to that carrying the light source, the measuring photodetector 20 and the reference photodetector 20 ref. On the figure 2B The reference cones Ω ref,1 and Ω ref,2 respectively incident on the reference reflecting surface have been delimited by dashed lines. S ref and reflected upon by the latter.

[0057] On the figure 3A We have represented a configuration in which the number N of reflective surfaces is equal to 5. On the figure 3B We have represented a configuration with 5 reflective surfaces, and a reference reflective surface that directs a reference cone of light towards the reference photodetector 20 ref. On the figure 3B The reference cones Ω ref,1 and Ω ref,2 respectively incident on the reference reflecting surface have been delimited by dashed lines. S ref and reflected upon by the latter.

[0058] The ability of a simple thin film, deposited on a printed circuit board, to reflect a light wave has been experimentally verified. figure 4 represents the experimental setup implemented.

[0059] A light source L emits filamentary light, collimated by a first lens L1, onto a flat reflective surface S. The light reflected by the thin film is focused by a second lens L2 onto a thermopile-type photodetector D. The reflective surface was either a mirror or a gold layer deposited on a PCB over a 5 µm nickel substrate. Two different PCBs were used: on the first PCB1, the gold layer was 66 nm thick, and on the second substrate, it was 744 nm thick. The light collected by the detector allowed the reflectivity of the reflective surface to be estimated. Reflectivity corresponds to the percentage of light emitted by the light source and collected by the photodetector D. The angle θ shown on the figure 4 was 30°. The distances d1 and d2 were respectively 33.5 mm and 52.9 mm.

[0060] Two PCBs were implemented, each with a different surface finish. PCB1 had a rough surface, treated using a standard electrochemical finish. The second PCB had a smoother surface, achieved through an electrolytic finish.

[0061] On PCB1, four identical flat mirrors, referenced M1 to M4, were formed. The same is true of PCB2, on which four identical flat mirrors, M6 to M10, were formed.

[0062] The table below shows the reflectivities respectively determined for each plane mirror. Table 1 Réference Type de miroir Réflectivité PCB1 M1 41 M2 42 M3 39 M4 38 PCB2 M6 62 M7 50 M8 64 M9 49 M10 46 Miroir de référence 97

[0063] The last line of the table corresponds to a reference mirror, considered to be perfect.

[0064] It is observed that improving the surface condition of the PCB significantly improves reflectivity.

[0065] The invention can be applied for gas detection in environmental monitoring, or in industrial gas control applications, particularly in the food industry, the oil industry, or gas distribution.

Claims

1. Gas sensor (1) comprising an enclosure (2) configured to receive a gas (G), the sensor also comprising: - a light source (10) configured to emit a light wave (11) propagating in the enclosure, forming a first light cone (Ω1) from the light source; - a measuring photodetector (20) configured to detect a light wave (14) emitted by the light source (10) and propagated through the enclosure; - the light source and the measuring photodetector being supported by a printed circuit board (7); - a first-order reflective surface forming part of a revolving ellipsoid, having a first focal point (P1-1) and a second focal point (P1-2), the first-order reflective surface being disposed opposite the light source (10) so as to reflect the first light cone toward the second focal point, the first reflective surface forming part of a revolving ellipsoid; - wherein: • the printed circuit board comprises a flat mirror, forming a reflective surface of rank 2, the second focal point of the reflective surface of rank 1 being located on the flat mirror or at a distance of less than 1 mm therefrom; • the device comprises a final reflective surface (SN) of rank N, N being greater than or equal to 3, the final reflective surface comprising a first focal point (P1-N), located at the level of a flat mirror, arranged on the printed circuit board, the flat mirror forming a reflective surface of rank N -1, the final reflective surface comprising a second focal point (P2-N); such that the light wave (11) emitted by the light source is successively reflected by the N reflective surfaces, according to their respective ranks, before converging towards the second focal point (P2-N) of the last reflective surface (SN), forming a last light cone (ΩN ), the measuring photodetector being arranged so as to extend into the last light cone.

2. Sensor according to claim 1, wherein the light source (10) is arranged at the first focal point (P1-1) of the reflective surface of rank 1 (S1), or at a distance of less than 1 mm therefrom.

3. Sensor according to any of the preceding claims, wherein the measuring photodetector (20) is arranged at the second focal point (P2-N) of the last reflective surface (SN ), or at a distance of less than 1 mm therefrom.

4. Sensor according to any of the preceding claims, wherein: - the device comprises a plurality of reflective surfaces of even rank, each reflective surface of even rank being a plane mirror arranged on the printed circuit board; - the device comprises a plurality of odd-ranked reflective surfaces, each odd-ranked reflective surface forming part of a revolution ellipsoid and extending facing the printed circuit board.

5. Sensor according to claim 4, wherein: - each reflective surface of rank n is configured to reflect the light wave it receives by forming a light cone (Ωn+1) of rank n+1, the latter extending around a propagation axis (Δn+1) of rank n+1; - when n<;N, the propagation axis (Δn+1) of rank n+1 extends between the reflective surface (Sn) of rank n and the surface (Sn(+1) of rank n+1; - whenn is odd, and n ≤ N-2, each reflective surface (Sn ) extends along part of a revolving ellipsoid, with the propagation axis of rank n +1 passing through the second focal point (P2- n) of the surface (Sn ) of rank n, the latter being confused with the first focal point (P1- n+ 2) of the surface (Sn+2 ) of rank n + 2; - when n = N, the propagation axis (ΔN+1) of rank N+1 extends between the surface (SN) of rank N and the photodetector (20); such that the light wave emitted by the light source (10) undergoes a number of reflections corresponding to the number (N) of reflective surfaces before reaching the measuring photodetector (20).

6. Sensor according to claim 5, wherein when n <N and n is odd, each light cone (Ωn+1) of rank n+1 converges towards the second focal point (P2-n) of the surface (Sn) of rank n from which it propagates.

7. Sensor according to any of the preceding claims, wherein the odd-ranked reflective surfaces are provided on an inner face (5i) of a single wall (5), forming a cover for the sensor.

8. Sensor according to claim 7, wherein the enclosure (2) is formed by assembling the sensor cover (5) on top of a base (4), the printed circuit board (7) being arranged on the base.

9. Sensor according to any of the preceding claims, wherein the or each plane mirror is formed by a thin layer of a reflective material deposited on the printed circuit board.

10. Sensor according to any of the preceding claims, wherein the number (N) of reflective surfaces extending between the light source (10) and the measuring photodetector (20) is odd.

11. Sensor according to claim 10, wherein the number of reflective surfaces extending between the light source and the measuring photodetector is equal to 3 or 5.

12. Sensor according to any of the preceding claims, comprising a reference photodetector (20ref), configured to detect a light wave considered to be unattenuated by the gas (G).

13. Sensor according to claim 12, comprising a reference reflective surface (Sref) forming a revolution ellipsoid, the reference surface comprising: - a first focal point (P1-ref ) located at a plane reflective surface of even rank, or at a distance of less than 1 mm from the latter; - a second focal point (P2 - ref) located at the reference photodetector or at a distance of less than 1 mm from the latter.

14. Method for analysing a gas (G) using a sensor (1) according to any of the preceding claims, the method comprising the following steps: a) introducing the gas into the chamber (2) of the sensor (1); b) illuminating the gas using the light source (10); c) measuring a light intensity of a light wave emitted by the light source and propagated through the enclosure (2) by the measuring photodetector (20); d) determining a quantity of a gaseous species in the gas (Gx) from the measured light intensity.

15. Method according to claim 14, wherein step d) is carried out by a processing unit (30) from a signal generated by the measuring photodetector (20).