Interferometric element, Device for detecting a compound comprising an interferometric element, and Method for detecting a compound

Abstract

Interferometric element (IE) intended for a device for detecting at least one compound (C) exhibiting resonant absorption over a predetermined spectral region centered on a resonance wavelength, said interferometric element comprising a detection subset (DS) optimized for said resonance wavelength and comprising: a plurality of resonant Fabry-Pérot type optical cavities at said resonance wavelength, each cavity comprising a reflective layer (RS) at said resonance wavelength and a partially transparent layer (TS) at said resonance wavelength, the partially transparent layer of the sensitive cavities being permeable to the compound(s) to be detected (C); an encapsulation layer impermeable to the compound(s) to be detected (C) and encapsulating a first subset of optical cavity(ies), called reference cavities,so that each reference cavity is devoid of said compound to be detected between the reflective layer (CR) and the partially transparent layer, the encapsulating layer not encapsulating a second subset of optical cavity(ies), called sensitive cavities, so that each sensitive cavity can include the compound(s) to be detected (C) between the reflective layer (CR) and the partially transparent layer. [Fig. 1]

Description

Title of the invention: Interferometric element, Device for detecting a compound comprising an interferometric element, and Method for detecting a compound. Technical field

[0001] The present invention relates to the field of particle detection and more particularly to particle detection by optical interferometry. Prior art

[0002] In many fields of application, such as food processing, defense, and chemicals, the detection and identification of particles are necessary in order to provide early warning of a potential attack or contamination. Numerous techniques known to those skilled in the art allow for the determination of the chemical composition of a sample.

[0003] For example, chemical sensors and biosensors enable rapid detection and real-time control of the interaction between the sample or compounds to be detected and the sensor. Such sensors use a chemical or biomolecular detection layer to recognize a compound to be detected by binding to it. This layer may, for example, include molecules such as antibodies, enzymes, hormones, DNA, neurotransmitter receptors, etc.

[0004] This type of sensor is not reusable because the bonding step between the compound to be detected and the detection layer is generally not reversible. These sensors are therefore used once and then discarded.

[0005] Integrated optical sensors provide an attractive alternative to these chemical sensors. Indeed, the techniques for manufacturing waveguides integrated onto optical chips by photolithography and microfabrication allow for mass, low-cost, and repeatable production of integrated optical sensors. Most of these integrated optical sensors are Mach-Zender interferometers (MZIs).

[0006] Fourier transform infrared (FTIR) spectroscopy is an extremely widespread analytical technique in which the molecules of the sample absorb the incident radiation, thereby modifying their vibrational energies. Depending on the bonds and chemical functions present in the sample, a characteristic infrared (IR) spectrum is obtained.

[0007] Besides FTIR spectroscopy, there are a multitude of techniques for identifying particles within a sample, such as Raman spectroscopy, laser-induced plasma spectroscopy (LIBS), or fluorescence-induced photofragmentation (PF-LIF), which is specific to NO2 bonds.

[0008] Techniques involving infrared absorption can be implemented in several ways.

[0009] A first method is "passive" IR imaging in transmission or backscattering, by direct absorption of light by the particle. In this case, the collected optical power decreases in the presence of the sample. For example, it is known to analyze a gas generated by the decomposition of a sample subjected to an intense pulsed laser in mid-infrared (MIR) backscattering. The detected radiation is the thermal radiation (blackbody type) of the objects constituting the observed scene. This scene is observed by an infrared imager whose detection spectral band is either broadband (covering the entire IR spectrum) or restricted to a portion of the spectrum. Image processing uses the contrasts, both in the spectral and spatial domains, between the two types of images and deduces an absorption level and therefore a gas concentration.This type of system is well-suited to high gas concentrations and large scenes, for example, for monitoring emissions from an industrial site. This imaging technique makes it possible to detect and identify gas clouds. It is not possible to use this technique for small objects such as particles because the optical path traveled within the object is too short.

[0010] A second method consists of performing "active" thermal imaging. Indeed, there are active sensors that integrate an infrared source, filters, and detectors into a single module, enabling the detection of signal variations based on the presence of the gas being sought. This type of system can achieve a very high level of sensitivity. However, it can be bulky, expensive, and requires a power source. These drawbacks can be prohibitive if the goal is to detect a gas in a confined or difficult-to-access environment, or if low-cost technologies are required.

[0011] Finally, a last technique is photoacoustics, which can be separated into 4 stages:

[0012] (1) absorption of laser radiation by the gas, thus exciting the energy levels rotational, electronic and vibrational;

[0013] (2) in the case of ro-vibrational excitations, de-excitation of the gas preferentially through molecular collisions which will result in a transfer of rotational / vibrational energy and kinetic energy creating a localized heating of the gas;

[0014] (3) generation of an acoustic wave and a thermal wave caused by expansion due to gas heating;

[0015] (4) detection by the microphone of the acoustic signal. The vibration amplitude of the The microphone is representative of the gas concentration and the wavelength of the laser radiation absorbed by the gas indicates its composition.

[0016] This latter method is interesting, but it does not allow for imaging an area. It would require scanning the laser onto the sample, which necessitates instrumentation and measurement time. Furthermore, this technique requires a laser source with adjustable intensity or wavelength, resulting in a significant cost for the detection device.

[0017] The invention aims to overcome certain problems of the prior art by providing a passive interferometric element enabling the remote detection of a predetermined compound, possibly through a material wall transparent to the wavelength range used. The interferometric element of the invention is particularly advantageous for low-cost applications (e.g., the inspection of consumer products), or in industrial applications in harsh environments (high or low temperatures, explosive or corrosive atmospheres). Summary of the invention

[0018] To this end, an object of the invention is an interferometric element intended for a detection device for at least one compound exhibiting resonant absorption over a predetermined spectral region centered on a resonance wavelength 2r, said interferometric element comprising at least one detection subset optimized for said resonance wavelength År and comprising: - at least two Fabry-Pérot type optical cavities exhibiting resonance at said resonance wavelength Âr, each cavity comprising a reflective layer (CR) at said resonance wavelength Âr and a partially transparent layer at said resonance wavelength Âr, - an encapsulation layer impermeable to the compound(s) to be detected and encapsulating at least one optical cavity, called the reference cavity, so that said reference cavity is devoid of said compound to be detected between the reflective layer and the partially transparent layer, the encapsulation layer not encapsulating the second optical cavity, called the sensitive cavity, and the partially transparent layer of the sensitive cavity being permeable to the compound(s) to be detected so that the sensitive cavity can include the compound(s) to be detected between the reflective layer and the partially transparent layer.

[0019] Preferably, the interferometric element comprises a plurality of sensitive cavities and a plurality of reference cavities, and the sensitive cavities and the reference cavities are arranged in a predetermined configuration such that the position of the sensitive cavities and the reference cavities can be determined by processing an image of said interferometric element. Even more preferably, said predetermined configuration is such that the sensitive cavities and the reference cavities are arranged alternately along a line or a plurality of preferably parallel lines.

[0020] According to one embodiment, the interferometric element includes an optical target adapted so as to be able to determine an orientation and a position of said interferometric element by processing an image of said interferometric element.

[0021] According to one embodiment, the sensitive cavity or cavities are adapted to present a reflection coefficient R^Ar) at the resonance wavelength Ar and the reference cavity or cavities are adapted to present a reflection coefficient R^Àr) at the resonance wavelength Àr such that R^A^ - Rs(Aj > 1%, and preferably R^A^ - RjAr) > 2%, for a concentration of 1% of the compound or compounds to be detected between the reflective layer and the partially transparent layer of the sensitive cavities.

[0022] According to one embodiment, the interferometric element comprises a plurality of detector subsets, each optimized for a respective resonance wavelength and different from the other resonance wavelength(s). Preferably, the resonance wavelengths are separated from each other by less than 50%. Alternatively, the resonance wavelengths are separated from each other by at least 5%.According to a variant of this embodiment, the optical cavities of each detection subset comprise, between said partially transparent layer and said reflective layer, an identical dielectric layer respectively associated with said detection subset, a refractive index and a thickness of said dielectric layer respectively associated with said detection subset being different from a refractive index and a thickness of the dielectric layer(s) respectively associated with the other detection subset(s) and being adapted so that the optical cavities have the same thickness.

[0023] According to one embodiment, the partially transparent layer is separated by a distance px Arj 2, from the reflective layer with p € N* > 2, preferably p>4.

[0024] Another object of the invention is a device for detecting at least one compound exhibiting resonant absorption over a spectral region centered on a resonance wavelength 2r, said device comprising: - a suitable light source to generate a first incident beam having at least said resonance wavelength - an interferometric element according to the invention - arranged so that the first beam illuminates the partially transparent layer of at least one sensitive cavity and the partially transparent layer of at least one reference cavity - a sensor comprising a plurality of pixels and adapted to acquire an image of the first incident beam reflected by the interferometric element, called the first image, - a processing unit connected to the sensor and configured to detect a possible presence of the compound(s) to be detected from a comparison of an intensity between, on the one hand, at least a first region of pixels where the first incident beam reflected by said at least one sensitive cavity is detected and on the other hand at least a second region of pixels where the first incident beam reflected by said at least one reference cavity is detected.

[0025] Another object of the invention is a device for detecting at least one compound exhibiting resonant absorption over a spectral region centered on a resonance wavelength 2r, said device comprising - a light source adapted to generate a first incident beam having at least said resonance wavelength Ar and to generate a second incident beam having no wavelength included in said resonant absorption and having at least one wavelength called non-resonance wavelength Âur, - an interferometric element according to the invention arranged such that the first and second beams illuminate the partially transparent layer of at least one sensitive cavity and the transparent layer of at least one reference cavity - a sensor comprising a plurality of pixels and adapted to acquire an image of the first incident beam reflected by the interferometric element, called the first image, and an image of the second incident beam reflected by the interferometric element, called the second image - a processing unit connected to the sensor and configured to detect a possible presence of the compound(s) to be detected from a comparison of the intensity of the first image and the second image.

[0026] Another object of the invention is a method for detecting at least one compound exhibiting resonant absorption over a spectral region centered on a resonance wavelength 2r, said method comprising the following steps: - generate a first incident beam exhibiting at least the said resonance wavelength År and a second incident beam not exhibiting any wavelength included in said resonant absorption and exhibiting at least one wavelength called the non-resonance wavelength Ånr, - illuminate an interferometric element according to the invention with the first and second beams so that they illuminate the partially transparent layer of at least one sensitive cavity and the transparent layer of at least one reference cavity - to acquire an image of the first incident beam reflected by the interferometric element, called the first image, and an image of the second incident beam reflected by the interferometric element, called the second image - detect a possible presence of the compound(s) to be detected from a comparison of the intensity of the first image and the second image.

[0027] According to one embodiment, said detection comprises the following steps:

[0028] A- calculate a third image by the difference between the second image and the first image

[0029] B- in the third image, compare the intensity between, on the one hand, at least a first region of pixels where the first incident beam reflected by said at least one sensitive cavity is detected, and on the other hand, at least a second region of pixels where the first incident beam reflected by said at least one reference cavity is detected.

[0030] Preferably, the detection includes a step A0, implemented before said step A, consisting of realigning the first image and the second image.

[0031] According to one embodiment, said detection is carried out when an average IlïlOy^ of the intensity of said at least a first region and an average Imoy^ of the intensity of said at least a second region are such that: -Imoy^ with S between 0.5% and 5%.

[0032] According to one embodiment, the first image and the second image are acquired simultaneously or within a time interval of less than 5 seconds, preferably less than 1 second. Brief description of the drawings

[0033] Other features, details and advantages of the invention will become apparent from the description given with reference to the accompanying drawings provided by way of example, which represent, respectively:

[0034] [Fig. 1], a schematic view of a device 1 for detecting at least one compound according to the invention,

[0035] [Fig. 2A], a cross-sectional view along a yz plane of an interferometric element along an embodiment of the invention,

[0036] [Fig.2B], the evolution of the reflection coefficient of the sensitive cavities (curve Cl) and reference cavities (C2 curve) as a function of wavelength,

[0037] [Fig.2C], the evolution of the reflection coefficient of the optical cavities as a function of the concentration of the compound in the ambient environment of the interferometric element,

[0038] [Fig.3A], an example of the images acquired for an interferometric element according to an embodiment of the invention,

[0039] [Fig.3B], an interferometric element according to an embodiment of the invention

[0040] [Fig.4], a graphical representation of the evolution of the reflection coefficient of a cavity according to an embodiment as a function of wavelength, for five different values ​​of H2S concentration in the cavity,

[0041] [Fig. 5A], [Fig. 5B], an example of an image off-resonance and at resonance respectively, and histograms of the pixels of these two images acquired for an interferometric element according to an embodiment of the invention.

[0042] [Fig.6], the evolution of the value of the reflection coefficient as a function of the loss tangent at resonance, for three different distances L.

[0043] [Fig.7], an embodiment in which the interferometric element comprises two detection subsets, each optimized for a respective resonance wavelength different from the other,

[0044] [Fig.8], an object of the invention which is a packaging for example food comprising an AL food and comprising the interferometric element of the invention.

[0045] In the figures, unless otherwise indicated, the elements are not to scale and identical references designate identical elements. Description of the implementation methods

[0046] Fig. 1 illustrates in a simplified manner a device 1 according to the invention for detecting at least one compound C. The device 1 essentially comprises a light source SL, an interferometric element El according to the invention, a sensor Det and a processing unit UT.

[0047] For the sake of simplicity, it is assumed that device 1 is optimized to detect a single compound C unless explicitly stated otherwise. The compound to be detected, C, is known and predetermined and has resonant absorption. on a spectral region centered on a resonance wavelength Àr. As a non-limiting example, the compound to be detected is hydrogen sulfide (H2S). The absorption spectrum of H2S gas has several bands of strong absorption in the terahertz range, in particular a line centered at 612 GHz. Studies have shown the appearance of this gas and the increase in its concentration inside food packaging within a few days, making it an indicator of product spoilage (see L. Kuuliala et al., "Spoilage evaluation of raw Atlantic salmon (Salmo salar) stored under modified atmospheres by multivariate statistics and augmented ordinal regression". International Journal of Food Microbiology, vol. 303, p. 46-57, August 2019, doi: 10.1016 / j.ijfoodmicro.2019.04.011).

[0048] The light source SL is adapted to generate at a minimum a first incident beam Fil having at least the resonance wavelength Ar. The light source SL is not specific to the invention and is, for example, a laser source.

[0049] According to a principal embodiment illustrated in Figure 1, the light source SL is further adapted to generate a second incident beam FI2 having no wavelength within the resonant absorption of compound C and having at least one wavelength referred to as the non-resonant wavelength Ånr. By "a second incident beam FI2 having no wavelength within the resonant absorption of compound C," it is meant here that the second beam has no wavelength within a spectral range dependent on the spectral width of the resonant absorption, which is, for example, such as [0.98 × 10⁻¹ År; 1.02 × 10⁻¹ År], and preferably no wavelength within a spectral range such as [0.96 × 10⁻¹ Ų År; 1.04 × 10⁻¹ Ų Ų].

[0050] According to a first alternative of the main embodiment, the light source SL is a source adapted to simultaneously generate the first and second beams. For example, the light source SL comprises a monochromatic laser source emitting at the resonant wavelength Ar and includes an optical architecture for generating at least the off-resonant wavelength from the resonant wavelength Ar, typically by taking a portion of the first beam Fil and doubling its frequency. Alternatively, the laser source SL comprises two laser sources, one monochromatic at the off-resonant wavelength Ånr and the other at the resonant wavelength 2r. Alternatively, the source SL is a broadband source covering a spectral band sufficiently wide to encompass the off-resonant wavelength Ånr and the resonant wavelength 2r.

[0051] According to a second alternative, the light source SL is capable of generating the first and second beams successively in time. For example, the source SL includes a wavelength-tunable laser source or two alternating laser sources, respectively monochromatic at the off-resonance wavelength and at the resonance wavelength Ar.

[0052] Alternatively, according to a secondary embodiment, the light source SL is adapted to generate only the first incident beam FIL. In the following, the operation of the device according to the main embodiment will be described first, then the operation of the device according to the secondary embodiment.

[0053] The interferometric element El comprises at least one detection subset SE (not visible in Figure 1 but visible in Figure 2A) optimized for the resonance wavelength to enable the detection of compound C. According to some embodiments, the interferometric element El comprises several detection subsets, each optimized to detect one or more respective compounds (see [Fig. 7] described below). It is important to emphasize that the interferometric element El of the invention is a passive element requiring no power supply to enable the detection of compound C.

[0054] Figure 2A is a cross-sectional view along a yz plane of an interferometric element El according to an embodiment of the invention.

[0055] The SE subassembly optimized to enable the detection of compound C comprises a plurality of Fabry-Pérot type FP optical cavities exhibiting resonance at the resonance wavelength Ar. It is further required that the FP optical cavities not exhibit resonant absorption at the off-resonance wavelength . For this, we will choose for example Àar such that, for each resonance band of index i of the cavities FP, centered on a wavelength 2- and of full width at half height ÔÀjj on ai F n 6 A t . 1 ^nr ¢- [Aj - ~ “

[0056] By way of non-limiting example, the SE subset of the interferometric element El of Figure 2A comprises two cavities FP aligned along the y direction.

[0057] Each of the FP cavities comprises a reflective layer CR at the resonance wavelength and a partially transparent layer CT at the resonance wavelength Ar. Typically, each reflective layer CR has a reflection coefficient greater than 99%, preferably greater than 99.5% at the resonance wavelength Ar and each partially transparent layer CT has a reflection coefficient between 90% and 99%, preferably between 97% and 99% at the resonance wavelength Àr.

[0058] For example, the CT and CR layers are metallic layers, the CT layer being able to include a spatial structure in order to increase its transmission.

[0059] Alternatively, the CT layer and / or the CR layer is / are a Bragg mirror made up of a stack of dielectric materials of different optical indices and thicknesses that are multiples of a quarter of the resonance wavelength.

[0060] The CT and CR layers of each cavity are stacked along a stacking direction (z in the illustration of Figure 2A) so as to be separated by a distance L. This distance is adapted according to the medium(s) separating the CT and CR layers so that each FP cavity exhibits a resonance at the resonance wavelength 2r. As will be explained in more detail in Figure 6, the greater this distance L, the greater the sensitivity of the sensor.

[0061] As is known, a Fabry-Pérot interferometer exhibits a path difference between each ray transmitted by the interferometer equal to 2nLcos(θ), where θ is the ray propagation angle between the CR and CR layers of the interferometer and a medium of refractive index n separates the CT and CR layers. It can be shown that the transmission of a Fabry-Pérot interferometer is maximum when this path difference is equal to a multiple of the wavelength of the rays propagating in the interferometer. At normal incidence, the transmission of each cavity FP is therefore maximum for a distance T_Pr, with n ≤ N*. Also, assuming 2n In a medium of refractive index n separating the CT and CR layers of the FP cavities of the interferometric element El of the invention, the separation distance L between the CT and CR layers is such that t _ , with n ≤ N* 2n

[0062] The SE subset of the interferometric element El further comprises an encapsulation layer CE that is impermeable to the compound to be detected C and transparent to wavelengths Ar and Ànr. By "transparent," it is understood here that the encapsulation layer CE has a transmission greater than 80%, preferably greater than 90%, for wavelengths 2r and

[0063] This encapsulation layer CE is arranged to encapsulate a first subset of optical cavity(ies), the reference cavity(ies) REF-, such that each reference cavity is devoid of the compound to be detected between the CT and CR layers. Thus, according to one embodiment, the CT and CR layers of the reference cavity(ies) are separated by a vacuum or by a neutral medium (typically a non-resonant gas at wavelengths λ and λ, for example nitrogen).

[0064] Furthermore, the encapsulation layer CE does not encapsulate a second subset of optical cavity(ies): the sensitive cavity(ies) SNS. Thus, the sensitive cavity(ies) SNS of the interferometric element can contain the compound to be detected C between its reflective layer CR and its partially transparent layer CT when the surrounding medium contains compound C. For example, as illustrated in [Fig. 2A], the encapsulation layer CE includes an aperture OV such that the The SNS sensitive cavity must be exposed to the ambient environment. For this to happen, the partially transparent CT layer of each SNS sensitive cavity must be permeable to the compound to be detected, C.

[0065] Thus, the sensitive cavity(ies) and the reference cavity(ies) exhibit a very high (typically greater than 99%) identical (or almost identical) reflection coefficient at the non-resonance wavelength Anr and a lower and different reflection coefficient at the resonance wavelength Ar. This result is illustrated in [Fig. 2B] which represents the evolution of the reflection coefficient of the sensitive cavities (curve Cl) and the reference cavities (curve C2) as a function of wavelength.

[0066] On curve C2, it can be observed that the reference cavities exhibit a reflection coefficient close to 100% over most of the spectrum and slightly lower than the resonance wavelength Ar due to the higher losses generated by the electromagnetic resonance phenomenon.

[0067] On the Cl curve, it can be observed that the sensitive cavities also exhibit a reflection coefficient close to 100% over most of the spectrum because compound C has no significant effect on transmission in the absence of resonance. On the other hand, the reflection coefficient is significantly reduced compared to the reference cavity at the resonance wavelength 2r due to the absorption of compound C.

[0068] We denote by AR(Ar) = Rr(Ar) - Rs(Ar) the difference between the value R^) of the reflection coefficient at the resonance wavelength of the reference cavities Ar and the value R^A^ of the reflection coefficient at the resonance wavelength of the sensitive cavities. It can be shown that this difference R^A^ is approximately proportional to the concentration of compound C. This result can be observed in [Fig. 2C], which illustrates the evolution of the reflection coefficient of the optical cavities as a function of the concentration of compound C in the ambient medium of the interferometric element E1 (and therefore between the CR and CT layers of the sensitive cavities).

[0069] Curves C3 and C5 of [Fig.2C] illustrate respectively the evolution of the reflection coefficient of the sensitive cavities as a function of the concentration of compound C at the resonance wavelength and at the non-resonance wavelength.

[0070] Curves C4 and C6 of [Fig.2C] illustrate the evolution of the reflection coefficient of the reference cavities as a function of the concentration of compound C at the resonance wavelength and at the non-resonance wavelength respectively.

[0071] The reference cavities exhibit a constant or quasi-constant reflection coefficient. The value of this reflection coefficient is close to 100% and is significantly higher for the non-resonant wavelength.

[0072] Sensitive cavities exhibit a high and slightly decreasing reflection coefficient with gas concentration outside of resonance (C5) and a strongly decreasing reflection at resonance (C3).

[0073] Preferably, the sensitive and reference cavities are adapted—by means of the reflection coefficients of the CR and CT layers—to have a reflection coefficient Rs[Xr] and a reflection coefficient R^X^ respectively such that AR(Xr) = R^X^ - R^Xj > 1%, and preferably AR(Xr) > 2%, for a concentration of 1% of the compound to be detected between the reflective layer and the partially transparent layer of the sensitive cavities. This value facilitates the detection of compound C.

[0074] As illustrated in [Fig. 1], the interferometric element El is arranged so as to be illuminated by the first and second beams Fil, FI2. More specifically, the beams Fil, FI2 illuminate the CT layer of at least one sensitive cavity SNS and the CT layer of at least one reference cavity REF.

[0075] The sensor Det comprises a plurality of pixels and is adapted to acquire an image of the first incident beam reflected FRI by the interferometric element, called first image II or resonance image IL. In addition, the sensor is capable of acquiring an image of the second incident beam reflected FR2 by the interferometric element, called second image 12 or non-resonance image 12.

[0076] The Det sensor is not specific to the invention and will be adapted by a person skilled in the art according to the light source SL without going out of the scope of the invention.

[0077] For example, according to the embodiment in which the light source SL is a broadband source, the sensor Det comprises a pixel array associated with a spectral filter system capable of capturing images at Xr and X^. Typically, the spectral filter system is a spectral filter wheel optically positioned upstream of the pixel array. Alternatively, the sensor Det is a multispectral pixel array and comprises, for example, several subsets of pixel arrays, each having a distinct spectral filter.

[0078] According to another embodiment in which the source emits a plurality of monochromatic beams successively or simultaneously, the Det sensor is for example a wideband pixel matrix such as a CCD or CMOS camera.

[0079] Finally, the processing unit UT is connected to the sensor Det and is configured to detect a possible presence of the compound to be detected C from a comparison of the intensity of the first image II and the second image 12.

[0080] Thus, the passive interferometric element El of the invention allows for the remote detection of compound C. Depending on the resonance wavelength Xr used, this detection can be carried out through an optically transparent element, by For example, a food-type packaging (see [Fig. 8] described below). The interferometric element of the invention is particularly interesting for low-cost applications (e.g., the inspection of consumer products), or in industrial applications in harsh environments (high or low temperatures, explosive or corrosive atmospheres).

[0081] Indeed, as mentioned above, the structure of the interferometric element implies that the pixel regions where the reflection of the FRI, FR2 beams by the sensitive cavities is detected, called first regions Rs, have a different intensity between the first and second image II, 12. Conversely, the pixel regions where the reflection of the FRI, FR2 beams by the reference cavities is detected, called second regions Rr, have an equal or substantially equal intensity between the first and second image II, 12. By comparing the intensity of these Rs and Rr regions, the processing unit UT allows the detection of compound C within the sensitive cavities.

[0082] This result is illustrated in Figures 3A and 3B. More specifically, [Fig.3A] is an example of the images II and 12 acquired for an interferometric element El in the presence of the compound to be detected C in which the sensitive cavities SNS and references REF are arranged according to the illustration in [Fig.3B].

[0083] In the embodiment of [Fig. 3B], by way of non-limiting example, the sensitive cavities SNS and reference cavities REF are arranged in a "checkerboard" pattern, i.e., the cavities are arranged alternately in a plurality of parallel lines. This arrangement is advantageous because it allows intensity measurements in the images of sensitive and reference cavities that are close together and therefore associated with nearly identical optical paths.

[0084] In [Fig.3A], it can be observed that the Rs and Rr regions associated respectively with the sensitive and reference cavities have an almost identical intensity in the non-resonance image 12 and have a different intensity in the resonance II image. Indeed, given the presence of compound C between the CT and CR layers of the sensitive SNS cavities which absorb the Fil beam but not the FI2 beam, the reflection coefficient of the sensitive SNS cavities is lower than that of the reference REF cavities for the Fil beam (see [Fig.2B]).

[0085] This is why, as illustrated in Figure II of [Fig.3A], the intensity of the Rs regions is lower than that of the Rr regions.

[0086] According to one embodiment of the main embodiment, the processing unit is adapted so that the detection step includes a first step consisting of calculating a third image by the difference between the second image 12 and the first image II.

[0087] Thus, provided that images II and 12 are acquired simultaneously or within a sufficiently close time interval, the first step makes it possible to overcome the characteristics of the optical path from light source SL to interferometric element El to sensor Det (propagation losses, optical losses, sensor orientation, reflection / diffraction phenomena in the system environment, etc.). Calculating the third image also makes it easier to locate the Rs and Rr regions than in images II and 12.

[0088] By "a sufficiently close time interval", it is meant here that the first image and the second image are acquired in a time interval of less than 5 seconds, preferably less than 1 second.

[0089] In a second step, the processing unit UT is configured to compare the intensity between at least one first region Rs and at least one second region of pixels R2 in the third image. When the processing unit identifies a significant difference in intensity between the region(s) Rs and the region(s) Rr, the processing unit UT considers that compound C has been detected.

[0090] Preferably, the processing unit UT considers that compound C is detected when an average Imcy^ of the intensity of the first regions Rs and an average Imoy^ of the intensity of the second regions Rr are such that:

[0091] a t _ _ c Imoy^ >

[0092] with S a predetermined threshold depending on: - the minimum concentration of compound C between the CT and CR layers that we wish to detect, - the reflection coefficient of cavities sensitive to Ar and Ânr - the reflection coefficient of the reference cavities at Àr and Ànr.

[0093] In a manner known per se, the reflection coefficients are determined by the coefficients of the CT and CR layers.

[0094] Typically, this threshold S is between 0.5% and 5% for a minimum concentration of 1%.

[0095] Preferably, the first step of calculating the third image is preferably preceded by a preliminary registration step between the first and second images. This registration step is known per se and can be implemented by any method known to those skilled in the art. In image processing, registration is a technique that consists of "matching images" for the purpose of comparing or combining them.

[0096] According to one embodiment, this registration step is carried out by image processing methods based on the 2D correlation of images II and 12.

[0097] According to one embodiment, compatible with all embodiments of the invention, the sensitive cavities and the reference cavities are arranged in a predetermined configuration so as to allow the position of the sensitive and reference cavities to be determined by image processing. Thus, the registration step is carried out by the image processing unit, thanks to this predetermined configuration previously stored in the processing unit.

[0098] According to another embodiment, compatible with all embodiments of the invention, the interferometric element comprises an optical target. This optical target is adapted so as to be able to determine an orientation and position of the interferometric element from an image. The optical target thus facilitates the registration of images II and 12.

[0099] Preferably, the interferometric element El comprises a plurality of sensitive cavities and reference cavities (for example, more than five sensitive cavities and more than five reference cavities). This allows the Rs and Rr regions to be averaged in the II and 12 (or directly in the third image), thus enabling the intensity comparison step of the Rs and Rr regions implemented by the processing unit UT to be performed on the average of the Rs regions and the average of the Rr regions. This eliminates dispersions from the detectors, resonant cavities, or the environment (for example, an object obscuring part of the interferometric element).

[0100] According to one embodiment, the processing unit UT is further configured to determine the concentration of compound C from the value of A / . Indeed, as mentioned previously, the difference A_R(2r) between the value of the reflection coefficient between the sensitive cavities SNS and that of the reference cavities REF, at the resonance wavelength, is substantially proportional to the concentration of compound C. Naturally, this difference A_R(Ar) is proportional to the value of AI. It is therefore possible to perform calibration by prior measurements with predetermined concentrations of compound C and thus associate a value of AI with a concentration of compound C.

[0101] As mentioned previously, in the secondary embodiment, the SL source is adapted to emit only the Fil beam exhibiting the resonance wavelength. Also, in the secondary embodiment, the Det sensor acquires a single image: image II. The processing unit UT is then configured to detect the possible presence of the compound to be detected, C, directly from the first image II, by comparing the intensity between at least one region Rs and at least one region Rr. Alternatively, as in the main embodiment, in the secondary embodiment this detection can be implemented by comparing the intensity between an average of the Rs regions and an average of the Rr regions.

[0102] Compared to the main embodiment, the secondary embodiment has the advantage of being easier to implement because the interferometric element E1 is interrogated using a single beam Fil. It is therefore possible to use a monochromatic laser source, for example. Indeed, in theory, it is possible to detect compound C by analyzing only the image E1 under very good observation conditions. This assumes that the difference in reflectivity between sensitive cavities and the resonance reference cavities is sufficient to locate the Rs and Rr regions in image E1, or that their position in image E1 is known. However, in practice, the signal-to-noise ratio in image E1 will be low due to uncontrolled and / or random spatial variations in the scene imaged by the detector.Thus, the detection of compound C is more difficult in the secondary embodiment and the false negative detection rate of compound C will be higher than in the primary embodiment.

[0103] Furthermore, in the main embodiment, image 12 is advantageous because it greatly facilitates the identification of the Rs and Rr regions in image II.

[0104] Example of implementation _ of the interferometric element:

[0105] According to one embodiment of the invention, denoted MRI, the interferometric element El comprises cavities FP optimized to exhibit a resonance at the resonance wavelength 2r= 612 GHz which is the central wavelength of an absorption line of the H2S gas.

[0106] In this embodiment, the CR reflective layers are formed by a metallic layer. The partially transparent CT layers each comprise a stack consisting of a lower silicon layer 5 µm thick, an intermediate layer comprising a matrix of 47x47 µm metallic tiles with a pitch of 50 µm, and an upper silicon oxide layer 2 µm thick. The CT and CR layers are separated by a distance L = 250 nm. This distance L is close to 2π / 2 (245 µm) but not exactly equal due to the phase transmission of the layers in the CT layer stack.

[0107] Preferably, the metallic elements are made of a good conductive metal, such as copper, gold or failing that aluminum, in order to minimize the associated losses.

[0108] Figure 4 is a graphical representation of the evolution of the reflection coefficient of a cavity according to the MRI embodiment as a function of wavelength, for five different values ​​of H2S concentration in the cavity. More specifically, curve C7 is obtained for a dielectric loss tangent (tanô) of 0.01, curve C8 is obtained for a dielectric loss tangent (tanô) of 0.05, the curve C9 is obtained for a dielectric loss tangent (tanô) of 0.001, curve CIO is obtained for a dielectric loss tangent (tanô) of 0.0001 and curve Cil is obtained for a dielectric loss tangent (tanô) of zero.

[0109] As a reminder, the dielectric loss tangent is directly related to the absorbance Q of compound C by the following relation: tQn& where μ' is the permittivity of the compound C. Thus, the loss tangent is proportional to the concentration of species C.

[0110] In Figure 4, we observe that the reflection coefficient has a minimum at År = 612 GHz, with a depth increasing with the loss tangent, i.e., increasing with the gas concentration. The non-resonance reflection coefficient, below 590 GHz or above 630 GHz, is close to 100% for low or moderate gas concentrations. On curve C7, we observe that the reflection coefficient at resonance is zero. It will therefore potentially be easy to identify the Rs regions in image II for this concentration.

[0111] By simulations calculating the evolution of the reflection coefficients off and at resonance as a function of the gas concentration, the reflection values ​​at resonance and off resonance for a gas concentration of zero and 1% are determined and presented in the following table (hereinafter "Table 1"): tanô 0 7.2e-5 Concentration (%) 0 1% Reflection at 580 GHz (%) 99.35% 99.32% Reflection at resonance (%) 73.29% 71.51%

[0112] From these reflection values, we can simulate the images obtained by a detector matrix having a noise level of 250 pVrms (typical value) and a signal-to-noise ratio (SNR) of 10. By way of non-limiting example, the element El of the MRI embodiment comprises a 5x5 FR cavity matrix with among them 13 reference cavities and 12 sensitive cavities arranged in a checkerboard pattern.

[0113] Figures 5A and 5B present the results of this simulation. More specifically, [Fig. 5A] shows image 12 on the left and displays the histogram of the pixel values ​​of regions Rs and Rr in image 12. [Fig. 5B] shows image II on the left and displays the histogram of the pixel values ​​of regions Rs and Rr in image II. As an example, regions Rs and Rr each consist of approximately 3000 pixels.

[0114] First, it is observed that in images II and 2, it is not possible to distinguish the two types of cavities with the naked eye and therefore to detect the Rs and Rr regions. On the other hand, it is possible to plot their histogram and calculate their average values.

[0115] In the histogram of [Fig. 5A], it can be observed that the average value of the pixels in the Rs region is 2.485 mV and is approximately equal to the average value of the pixels in the Rr region, which is 2.479 mV. Indeed, outside of resonance, the reflection coefficient of the sensitive cavities and that of the reference cavities are almost equal (see Table 1).

[0116] In the histogram of [Fig. 5B], it can be observed that the average value of the pixels in the Rs region is 1.786 mV and is significantly lower than the average value of the pixels in the Rr region, which is 1.826 mV. Indeed, at resonance, the reflection coefficient of the sensitive cavities is lower than that of the reference cavities due to the absorption of compound C, which has a concentration of 1% (see Table 1).

[0117] Here, the detection of compound C can therefore be carried out from image II only by comparing the average intensity of the Rs and Rr regions or by calculating the third image from the difference between image 12 and II, then comparing the average intensity of the Rs and Rr regions in this third image.

[0118] Figure 6 illustrates the evolution of the reflection coefficient value as a function of the resonance loss tangent for three different distances L in an optical cavity according to the MRI embodiment. More precisely, curve C12 is obtained for L = 2πr, curve C13 is obtained for L = πr, and curve C12 is obtained for π = 2πr / Z

[0119] Figure 6 shows that choosing a relatively larger distance L improves the sensitivity of the interferometric element El because a significant change in reflection will be obtained for a lower loss tangent (i.e., gas concentration). This is explained by the fact that a larger distance L allows for a greater quantity of gas in the cavity.

[0120] Also, preferably, the separation distance L between the partially transparent layer and the reflective layer in the FP cavities of the interferometer of the invention is such L = p X / 2 with p ≤ N* > 2, preferably p > 4. This makes it possible to improve the sensitivity of the interferometric element El.

[0121] Figure 7 illustrates an embodiment in which the interferometric element El comprises two detection subsets SE and SE', each optimized for a respective and different resonance wavelength. Thus, the interferometric element El of Figure 7 allows the detection of two different compounds C or allows the detection of two different resonances of the same gas.

[0122] More generally, according to one embodiment, the interferometric element El comprises a plurality of detection subsets, each optimized for a respective resonance wavelength and different from the other or other resonance wavelengths.

[0123] Preferably, the resonance wavelengths are less than 50% apart from each other. Thus, manufacturing uncertainties can be eliminated.

[0124] Alternatively, the resonance wavelengths are separated from each other by at least 5%. Thus, several different compounds C or several different resonances of the same compound can be detected.

[0125] As illustrated in [Fig. 7], the optical cavities FP of a subassembly SE may include an identical dielectric layer DA disposed between the CT and CR layers. This dielectric layer DA is preferably formed from a low-loss material in order to reduce the thickness between the two surfaces by increasing the average refractive index of the medium located between the CT and CR layers.

[0126] Preferably, the refractive index and the thickness of the dielectric layer DA are adapted so that the optical cavities have the same thickness (i.e., the same dimensions along the stacking direction). This simplifies the fabrication of the interferometric element.

[0127] Figure 8 illustrates an object of the invention, which is a packaging EA, for example, a food packaging containing a food product AL. This packaging EA includes the interferometric element El of the invention within the packaging in the volume where the food product AL is stored and arranged. By way of non-limiting example, the structure of each of the cavities FP of the interferometric element El is optimized to exhibit resonance at a resonant absorption of the H2S gas, such as 612 GHz, since the detection of hydrogen sulfide in perishable food products such as fresh meat or fish is an indicator of product spoilage. Thus, when the interferometric element El is interrogated by a light source SL emitting at least at a wavelength 2r = 612 GHz, it will be possible to detect any potential spoilage of the food product AL due to the presence of H2S gas.

[0128] It is preferable that the SL light source be adapted according to the food packaging so that the latter has sufficient transmission (for example greater than 50%) at the wavelength 2r. For example, in the terahertz range, it will be possible to pass through a wall made of plastic, paper, cardboard, or even fabric.

Claims

Demands

1. Interferometric element (El) for a detection device of at least one compound (C) exhibiting resonant absorption over a predetermined spectral region centered on a resonance wavelength Ar, said interferometric element comprising at least one detection subset (SE) optimized for said resonance wavelength År and comprising: - at least two Fabry-Pérot type optical cavities (FP) exhibiting resonance at said resonance wavelength Ar.each cavity comprising a reflective layer (CR) at said resonance wavelength Ar and a partially transparent layer (CT) at said resonance wavelength Ar, - an encapsulation layer (CE) impermeable to the compound(s) to be detected (C) and encapsulating at least one optical cavity, called the reference cavity (REF), such that said reference cavity is devoid of said compound to be detected between the reflective layer (CR) and the partially transparent layer, the encapsulation layer not encapsulating the second optical cavity, called the sensitive cavity (SNS), and the partially transparent layer of the sensitive cavity being permeable to the compound(s) to be detected (C) so that the sensitive cavity can include the compound(s) to be detected (C) between the reflective layer (CR) and the partially transparent layer.

2. Interferometric element according to claim 1, comprising a plurality of sensitive cavities and a plurality of reference cavities, wherein the sensitive cavities and the reference cavities are arranged in a predetermined arrangement so as to be able to determine a position of the sensitive cavities and the reference cavities by processing an image of said interferometric element.

3. Interferometric element according to the preceding claim, wherein said predetermined arrangement is such that the sensitive cavities and the reference cavities are arranged alternately along a line or a plurality of lines preferably parallel.

4. Interferometric element according to any one of the preceding claims, comprising an optical target adapted so as to be able to determine an orientation and a position of said interferometric element by processing an image of said interferometric element.

5. An interferometric element according to any one of the preceding claims, wherein the sensitive cavity or cavities are adapted to exhibit a reflection coefficient R^A^ at the resonance wavelength and the reference cavity or cavities are adapted to exhibit a reflection coefficient R^A^ at the resonance wavelength Xr such that R^X^ - Rs(Àr) > 1%, and preferably R^A^ " > 2%, for a concentration of 1% of the compound(s) to be detected (C) between the reflective layer (CR) and the partially transparent layer of the sensitive cavities.

6. Interferometric element according to any one of the preceding claims, comprising a plurality of detection subsets (SE, SE') each optimized for a respective resonance wavelength and different from the other or other resonance wavelengths.

7. Interferometric element according to the preceding claim, wherein the resonance wavelengths are separated from each other by less than 50%.

8. Interferometric element according to claim 6, wherein the resonance wavelengths are separated from each other by at least 5%.

9. An interferometric element according to any one of claims 6 to 8, wherein the optical cavities of each detection subset comprise, between said partially transparent layer and said reflective layer, an identical dielectric layer (DA) respectively associated with said detection subset, a refractive index and a thickness of said dielectric layer respectively associated with said detection subset being different from a refractive index and a thickness of the dielectric layer(s) respectively associated with the other detection subset(s) and being adapted so that the optical cavities have the same thickness.

10. Interferometric element according to any one of the preceding claims, wherein the partially transparent layer is separated by a distance px Àr[ 2, from the reflective layer with p GN* > 2, preferably p> 4.

11. Use of said interferometric element according to any one of the preceding claims, disposed in a package including a food, to detect an alteration of said food.

12. Device (1) for detecting at least one compound (C) exhibiting resonant absorption over a spectral region centered on a resonance wavelength 2r, said device comprising: - a light source (SL) adapted to generate a first incident beam (Wire) having at least said resonance wavelength 2r - an interferometric element (El) according to any one of claims 1 to 10 arranged such that the first beam illuminates the partially transparent layer of at least one sensitive cavity and the partially transparent layer of at least one reference cavity - a sensor (Det) comprising a plurality of pixels and adapted to acquire an image of the first incident beam reflected by the interferometric element, said first image,- a processing unit (PU) connected to the sensor and configured to detect the possible presence of the compound(s) to be detected (C) by comparing the intensity between, on the one hand, at least a first region of pixels where the first incident beam reflected by said at least one sensitive cavity is detected, and on the other hand, at least a second region of pixels where the first incident beam reflected by said at least one reference cavity is detected.

13. A device for detecting at least one compound (C) exhibiting resonant absorption over a spectral region centered on a resonance wavelength 2r, said device comprising

14. - a light source (SL) adapted to generate a first incident beam (Fil) having at least said resonance wavelength Ar and to generate a second incident beam (FI2) having no wavelength included in said resonant absorption and having at least one wavelength called non-resonance wavelength Ânr, - an interferometric element (El) according to any one of claims 1 to 10 arranged such that the first and second beams illuminate the partially transparent layer of at least one sensitive cavity and the transparent layer of at least one reference cavity - a sensor (Det) comprising a plurality of pixels and adapted to acquire an image of the first incident beam reflected by the interferometric element (FRI), called the first image (II), and an image of the second incident beam reflected by the interferometric element (FR2), called the second image (12) - a processing unit (PU) connected to the sensor and configured to detect a possible presence of the compound(s) to be detected (C) from a comparison of the intensity of the first image and the second image. Method for detecting at least one compound (C) exhibiting resonant absorption over a spectral region centered on a resonance wavelength 2r, said method comprising the following steps: - generate a first incident beam (Fil) having at least said resonance wavelength hr and a second incident beam (FI2) having no wavelength included in said resonant absorption and having at least one wavelength said non-resonance wavelength - illuminate an interferometric element (El) according to any one of claims 1 to 10 with the first and second beams so that they illuminate the partially transparent layer of at least one sensitive cavity and the transparent layer of at least one reference cavity - acquire an image of the first incident beam reflected by the interferometric element, said first image, and an image of the second incident beam reflected by the interferometric element, said second image - detect a possible presence of the compound(s) to be detected (C) from a comparison of the intensity of the first image and the second image.

15. Method according to the preceding claim, wherein said detection comprises the following steps: A. calculate a third image by the difference between the second image and the first image B. in the third image, compare the intensity between, on the one hand, at least a first region of pixels where the first incident beam reflected by said at least one sensitive cavity is detected, and on the other hand, at least a second region of pixels where the first incident beam reflected by said at least one reference cavity is detected.

16. Method according to the preceding claim, wherein said detection includes an AO step, implemented before said A step, consisting of re-registering the first image and the second image.

17. A method according to claim 15 or 16, wherein said detection is performed when an average Imoy of the intensity of said at least a first region and an average Imoy of 25 the intensity of said at least a second region are such that: llmoy^fmoyj with S between 0.5% and 5%.

18. A method according to any one of claims 15 to 17, wherein the first image and the second image are acquired simultaneously or within a time interval of less than 5 seconds, preferably less than 1 second.

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