PHOTO-ACOUSTIC DETECTOR

Abstract

A photoacoustic detector (100) is adapted to reveal the presence of a target compound in a gas to be analyzed (G). At least one radiation beam (F) is used during detector operation, entering a chamber (30) containing the gas to be analyzed and / or exiting said chamber at one or more locations, each adjacent to or superimposed on a pressure node of a standing acoustic wave (AC). The detector is compatible with configurations where the radiation beam passes through the chamber multiple times, and alignment of detector components can be simplified. The detector's sensitivity to external disturbances is also reduced. (Abstract figure: Figure 1)

Description

Title of the invention: PHOTO-ACOUSTIC EFFECT DETECTOR Technical field

[0001] The present description relates to a photoacoustic effect detector. Previous technique

[0002] The use of the photoacoustic effect to detect the presence of a compound that absorbs radiation at a wavelength identified for that compound is known, for example, from US 7,245,380 B2. It consists of sending a beam of radiation, modulated at a frequency such that the compound, if present in the gaseous medium, produces intermittent local heating of that gaseous medium by absorbing the radiation. This intermittent local heating then generates an acoustic wave that is detected, and its amplitude can be related to the concentration of the compound in the gaseous medium. More precisely, the acoustic wave is detected by means of a mechanical or electromechanical resonator that is set into vibration by the acoustic wave, or by means of an electromechanical oscillator whose frequency is modified by the acoustic wave, as described in FR 3 057 078 Bl.

[0003] Several improvements to this detection technique have been proposed to increase its sensitivity, including the following: - the resonator can be a quartz tuning fork; - The resonator can be placed inside an acoustic cavity whose resonance band contains the vibration frequency of that resonator, and the modulation frequency of the radiation beam is chosen to match the vibration frequency of the resonator. WO 2018 / 020096 A1 indicates a preferred placement for a tuning fork inside the acoustic cavity, in order to optimize the coupling between the vibration of the tuning fork and the standing acoustic wave that is generated in the cavity; and - The radiation beam can follow a multi-pass path inside the cavity to increase radiation absorption by the compound, which is effective in generating the standing acoustic wave. Such a configuration is described, for example, in the article entitled "Intracavity Quartz-Enhanced Photoacoustic sensor" by S. Borri et al., Applied Physics Letters, 104, 01114, 2014, DOI: 10.1063 / 1.4867268.

[0004] However, the detector of S. Borri et al., as described in the aforementioned article, requires that its optical components be aligned very precisely, as does the The radiation beam is affected relative to the tuning fork, particularly when the radiation is in the infrared range due to beam divergence. To maximize the sensitivity of this detector by S. Borri et al., a large number of passes of the radiation beam are used, necessitating the formation of a resonant optical cavity. However, such a resonant optical cavity requires the detector components to be precisely aligned to an additional degree. For this reason, the detector by S. Borri et al. is time-consuming to adjust geometrically and is very sensitive to external disturbances such as variations in ambient temperature or external vibrations. Technical problem

[0005] From this situation, one aim of the present invention is to propose a new photoacoustic detector which does not require precise alignment of its components and which does not exhibit high sensitivity to external disturbances.

[0006] A complementary object of the invention is to propose a photoacoustic detector configuration that can be easily adapted to implement several passes of modulated radiation beam through the chamber containing the resonator and the gas to be analyzed. Summary of the invention

[0007] To achieve this or another objective, a first aspect of the invention proposes a novel photoacoustic detector adapted to reveal the presence of a target compound in a gas to be analyzed, for which the detector is suitable. This detector of the invention comprises: - a mechanical or electromechanical resonator, adapted to vibrate during detector operation; - at least one optical source, adapted to produce during the operation of the detector, at least one beam of radiation which has at least one wavelength value contained within a spectral absorption range of the target compound, with a modulation of the radiation such that an absorption of this radiation by the target compound produces in a chamber a standing acoustic wave which generates or modifies a vibration of the resonator; - a closed enclosure, forming the chamber within the enclosure, with at least a portion of the resonator intended to vibrate during detector operation, located inside the chamber, and the enclosure being equipped with means for introducing the gas to be analyzed into the chamber, and further equipped with at least one optical passage arranged so that the radiation beam enters the chamber or emerge through at least one optical passage during detector operation; and - detection methods, adapted to reveal the vibration of the resonator or a modification thereof which is caused by the standing acoustic wave.

[0008] According to a first feature of the invention, at least one radiation beam is partially superimposed on a pressure antinode of the standing acoustic wave. The detector's sensitivity to the presence of the target compound is thus increased. Indeed, this first feature constitutes an optimization of the optical-thermal-acoustic coupling sequence implemented in the detector's operation.

[0009] According to a second feature of the invention, the detector is arranged so that each optical passage is crossed by at least one radiation beam at one or more locations, each of which is adjacent to or superimposed on a pressure node of the standing acoustic wave. This avoids or reduces disturbances in the detector's operation that could be caused by an interaction between the radiation beam and each optical passage through the chamber wall. Indeed, stray light absorption of the radiation by each optical passage can cause, via a layer of gas in contact with the wall on the chamber side, parasitic acoustic emission, independent of the presence of the target compound. Positioning each optical passage at a pressure node of the standing acoustic wave reduces the coupling between the parasitic acoustic emission and the resonator.The potential parasitic acoustic emission therefore does not cause vibration of the resonator or any significant modification of its vibration, other than that caused by the target compound. The sensitivity and reliability of the detector are thus improved. The respective locations of the optical passage(s), when there are several such locations, can be superimposed on the same pressure node of the stationary acoustic Fonde, or on different pressure nodes.

[0010] Thanks to the superposition of each optical passage traversal by the radiation beam with a pressure node of the standing acoustic wave, a reduced number of passages through the chamber by the radiation beam(s) is sufficient, so that it is not necessary to implement a resonant optical cavity. Therefore, no precise optical alignment is required, and the detector is relatively insensitive to external disturbances, such as thermal variations or external vibrations. The use of a tuned optical cavity remains possible, however.

[0011] Finally, the detector of the invention is compatible with a mechanical or electromechanical resonator that is not miniaturized, for example, whose dimensions may The size of the resonator can range from one to several millimeters, or even several centimeters. This makes it easy to adapt the detector configuration to produce multiple passes of at least one radiation beam through the chamber. Furthermore, these non-miniaturized dimensions of the resonator also facilitate the alignment of its optical components with respect to the resonator. However, the use of a miniaturized mechanical or electromechanical resonator remains a viable option.

[0012] For the purposes of the invention, a place which is adjacent or superimposed on a pressure node, or pressure antinode, of the standing acoustic wave means a distance between that place and the pressure node, or pressure antinode, which is less than one quarter, preferably less than one tenth, of a distance between two adjacent pressure nodes of the standing acoustic wave.

[0013] For the purposes of this invention, a detector suitable for detecting the presence of a target compound is defined as having at least one wavelength value of the radiation produced by the optical source within a spectral absorption range of the target compound. This spectral absorption range may be a line or an absorption band, depending on its width, for the target compound. The target compound(s) for which the detector is suitable may be indicated on the detector or in its instructions. Alternatively, the wavelength value(s) of the radiation used in the detector may be indicated on the detector or in its instructions. A user of the detector can thus determine the target compound(s) that can be detected using this detector, based on the tabulated values ​​of the absorption lines and bands of known compounds.The detector's instruction manual can be attached to the detector, or accessed online using a link provided with the detector.

[0014] Preferably, the means for introducing the gas to be analyzed into the chamber can also be adjacent to or superimposed on one or more pressure nodes of the standing acoustic wave. These pressure nodes for the means for introducing the gas to be analyzed can be the same as, or different from, those of each optical passage. Disturbances in the detection of the target compound that these gas introduction means could cause are thus reduced, thereby further increasing the reliability and sensitivity of the detector. In the context of the invention, the means for introducing the gas to be analyzed into the chamber can be means for circulating this gas through the chamber. They then also include means for removing the gas to be analyzed from the chamber, such removal means preferably also being adjacent to or superimposed on one or more pressure nodes of the standing acoustic wave.

[0015] In various possible embodiments, the detector may further comprise: - at least one mirror located outside the chamber or forming part of at least one wall of the enclosure, arranged so that at least one radiation beam passes through the chamber several times along successive optical path segments separated at each point by a reflection of the radiation beam on the mirror or one of the mirrors, each optical path segment being partially superimposed on a pressure antinode of the standing acoustic wave. Furthermore, each mirror may be arranged so that each optical path segment passes through at least one optical passage at a respective location adjacent to or superimposed on a pressure node of the standing acoustic wave.Alternatively, when the mirror forms part of the chamber wall, it is arranged so that the radiation beam is reflected at a point on the mirror that is adjacent to or superimposed on a pressure node of the standing acoustic wave. By placing each mirror outside the chamber, or using it to form one of the chamber walls, the chamber can be smaller, and therefore the time required to renew the chamber's gas content can be reduced. Furthermore, since at least one mirror is not inside the chamber, it does not disturb the standing acoustic wave, and consequently does not cause a reduction in detector sensitivity.Finally, the multiple crossings of the chamber by the radiation beam provide superior sensitivity to the detector of the invention, thanks to the resulting multiplication of the proportion of radiation absorbed by the target compound, for an identical quantity of this target compound present in the chamber.

[0016] Preferably, the detector of the invention may comprise at least two mirrors arranged so that the radiation beam traverses the chamber along at least three successive optical path segments, each separated by a reflection of the radiation beam on one of the mirrors, and preferably along fewer than twenty successive optical path segments, each separated by a reflection of the radiation beam on one of the mirrors. Such embodiments of the invention exhibit high detection sensitivity while remaining simple, in particular without the need for a resonant optical cavity.

[0017] Generally for the invention, the detector may optionally further include means for adjusting a dimension of the chamber, or means for adjusting a temperature of the gas to be analyzed, which are adapted so that with the gas to be analyzed present in the chamber, a stationary acoustic background frequency is between 0.95 and 1.05 times a frequency of an acoustic natural mode of the chamber equipped with the resonator.

[0018] In preferred embodiments of the invention, the resonator may comprise a tuning fork, with two prongs of the tuning fork intended to vibrate during the The detector's operation involves extending its rays within the chamber parallel to a central longitudinal axis of the tuning fork, this central longitudinal axis being an axis of symmetry of the tuning fork, and a median plane of the tuning fork, which contains the central longitudinal axis and is superimposed on the two arms, being parallel to any displacements of the arms caused by the vibration of the tuning fork. For such preferred embodiments, the chamber can be adapted so that components of the standing acoustic wave propagate parallel to a transverse direction contained in the median plane of the tuning fork and perpendicular to the central longitudinal axis of the tuning fork.

[0019] In first preferred tuning fork embodiments, the detector can be arranged so that the central longitudinal axis of the tuning fork is superimposed on a stationary acoustic Fonde pressure antinode, in particular the pressure antinode to which the radiation beam is partially superimposed.

[0020] Preferably, the detector can then be arranged so that the inner faces of the two prongs of the tuning fork, which are facing each other, are each adjacent to or superimposed on a pressure antinode of the standing acoustic wave during the operation of the detector. Optionally, the pressure antinode can be the same for these two inner faces of the prongs of the tuning fork. Furthermore, it can be the same pressure antinode as that of the central longitudinal axis of the tuning fork, if applicable. In addition, the detector can be arranged so that the outer faces of the two prongs of the tuning fork, which are facing in opposite directions, are each adjacent to or superimposed on other pressure antinodes of the standing acoustic wave during the operation of the detector, these other pressure antinodes being in opposite phase with respect to those on which the inner faces of the prongs of the tuning fork are superimposed.

[0021] Such first preferred embodiments may benefit from the following additional features: - when several crossings of the chamber by radiation are implemented with at least one mirror, each segment of optical path can intersect the central longitudinal axis of the tuning fork, and these intersections can preferably be located at levels of this central longitudinal axis which are all offset with different respective offset lengths from a projection of free ends of the arms of the tuning fork onto the central longitudinal axis; - The optical source can be oriented so that a first segment of the optical path, according to the direction of radiation propagation, makes, in a meridian plane containing this first segment of the optical path and the central longitudinal axis of the tuning fork, a first angle between 45° and 90° with the central longitudinal axis of the tuning fork. In other words, the radiation beam can be substantially transverse with respect to the central longitudinal axis, being inclined with respect to this central longitudinal axis until it can be perpendicular to it; - the optical source can further be oriented so that the beam of radiation makes, when projected into a section plane which is perpendicular to the central longitudinal axis of the tuning fork, a second angle which is between 1° and 87°, with a line of intersection between this section plane and the median plane of the tuning fork; - the optical source can be positioned so that the first segment of the optical path of the radiation beam intersects the central longitudinal axis of the tuning fork with an initial offset length between -30 mm and 50 mm, relative to the projection of the free ends of the tuning fork's arms onto the central longitudinal axis, this initial offset being counted as positive when moving away from the base of the tuning fork; and - each mirror can be positioned so that the two successive segments of optical path which are separated by a reflection on this mirror, intersect the central longitudinal axis at points which are separated by a gap of offset between 0 mm and 40 mm.

[0022] In second preferred tuning fork embodiments, the detector can be arranged so that the radiation beam is located on only one side of the median plane of the tuning fork.

[0023] When several radiation passes through the chamber with at least one mirror in such preferred second embodiments, the detector can be arranged so that the optical path segments of the radiation beam are all located on the same common side of the tuning fork's median plane, offset by different spacing distances from the median plane, each optical path segment forming a fourth angle of less than 45° with a perpendicular projection of that segment onto the median plane. In other words, each optical path segment is only slightly inclined with respect to the tuning fork's median plane in these preferred second embodiments of the invention. Such preferred second embodiments may have the following additional features: - the optical source and each mirror can be arranged so that the same stationary acoustic Fonde pressure antinode that is superimposed on the central longitudinal axis of the tuning fork is also superimposed on a part of each optical path segment of the radiation beam; - each mirror can be oriented so that successive points of crossing each optical passage by the radiation beam are aligned at the level of this optical passage perpendicular to the median plane of the tuning fork; - the optical source and each mirror can be arranged so that the first segment of the optical path, which is closest to the tuning fork, is separated from the median plane by a first separation distance between -50 mm and +50 mm, measured perpendicular to this median plane, with negative values ​​signifying a change of side relative to the median plane; and - the optical source and each mirror can also be arranged so that, for each optical passage, successive points of entry of the radiation beam into the chamber through said optical passage, according to the direction of propagation of the radiation, are separated by a subsequent separation distance which is between 0 mm and 40 mm.

[0024] In other embodiments of the invention, the tuning fork in the resonator can be replaced by a straight beam intended to vibrate during the operation of the detector. In this case, the beam has two opposite ends, one fixed end rigidly embedded in a base portion of the resonator and the other free end located in the chamber. Displacements of the beam caused by the vibration of the resonator are perpendicular to the longitudinal axis of the beam. The chamber is then adapted so that components of the standing acoustic wave propagate parallel to the beam's displacements. The preferred characteristics mentioned above for the first and second preferred embodiments can be applied to the single-beam embodiments, provided they are compatible.

[0025] Generally, in a detector according to the invention: - The faces of the resonator that move during its vibration can preferably be superimposed on pressure antinodes of the standing acoustic wave, and when dealing with two opposite faces of the same tuning fork or beam arm, their respective pressure antinodes are in opposite phase in the standing acoustic wave. Such an arrangement of the resonator with respect to the standing acoustic wave produces a stronger coupling between the latter and the vibration of the resonator; - each mirror can be further oriented so that the two successive segments of optical path which are separated by a reflection on this mirror, form between them a third angle which is between 0° and 50°; - Each optical passage can be a porthole or window that is transparent to the radiation from the optical source, or a hole in a wall of the enclosure. Possibly, such a transparent window can form all or part of a flat wall of the enclosure; - where appropriate, each porthole or transparent window may be parallel to the median plane. But advantageously, one of the portholes or transparent windows may make a non-zero angle with another, in particular to avoid a resonant optical cavity effect of the Fabry-Pérot type for the radiation beam; - The detector may include several optical sources that produce radiation with identical spectral distributions, and which are activated simultaneously during detector operation. In this case, the radiation beam produced by each optical source separately from the others constitutes a respective segment of the optical path; The detector may include several optical sources, each adapted to produce, simultaneously or during its respective operation, a beam of radiation with at least one wavelength different from that of each other optical source. The detector is then adapted to activate at least one of these optical sources with each operation. Each optical source can then correspond to a target compound different from those of the other optical sources, so the detector is thus suitable for detecting the presence of several target compounds selectively.

[0026] A second aspect of the invention proposes a method for revealing the presence of a target compound in a gas to be analyzed, this method comprising the following steps: / 1 / provide a detector which conforms to the first aspect of the invention, and of which at least one wavelength value of the optical source is contained within a spectral absorption range of the target compound; / 2 / inject the gas to be analyzed into the chamber; Optionally: adjust at least one dimension of the chamber or the temperature of the gas to be analyzed so that, with the gas to be analyzed present in the chamber, the frequency of the standing acoustic wave is between 0.95 and 1.05 times the frequency of an acoustic eigenmode of the chamber equipped with the resonator; and / 3 / simultaneously activate at least one optical source and the detection means.

[0027] Such a method can be used for methane (CH4), carbon dioxide (CO2) ), ammonia (NH3), a nitrogen oxide (NOx), including nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O), sulfur fluoride (SF6), oxygen (O2), hydrogen sulfide (H2S), sulfur(IV) oxide (SO2), hydrogen fluoride (HF), hydrogen chloride (HCl), benzene (C6H6), toluene (C7H8), xylene (C8H0), ozone (O3), water (H2O), etc. as a target compound. Brief description of the figures

[0028] The features and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments, with reference to the accompanying figures, among which:

[0029] [Fig-1] is a perspective diagram of a first photo-acoustic effect detector according to the invention;

[0030] [Fig.2] is a cross-section of the detector of [Fig.1], showing antinodes and nodal pressure planes of a standing acoustic wave occurring during detector operation;

[0031] [Fig.3] is a perspective diagram of a second photo-acoustic effect detector according to the invention;

[0032] [Fig.4] is a cross-section of the detector of [Fig.3], also showing the antinodes and nodal pressure planes of the standing acoustic wave;

[0033] [Fig.5] corresponds to [Fig.1] for a first variant of the embodiment;

[0034] [Fig.6] also corresponds to [Fig.1] but for a second embodiment; and

[0035] [Fig.7] illustrates an improvement of the invention with several target gases. Detailed description of the invention

[0036] For clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or actual dimension ratios. Furthermore, some of these elements are represented only symbolically, and identical reference numerals shown in different figures designate identical elements or elements with identical functions.

[0037] In various possible embodiments of the invention, at least some of the detector components may be of varying models and designs. In particular, the following variants are possible:

[0038] - for the resonator: it can have the shape of a tuning fork or another shape, by For example, the shape of a vibrating beam, one end of which is free inside the chamber and the other end is rigidly connected to a base part of the resonator. The resonator can be made of quartz or any other piezoelectric material such as langatate, or of a non-piezoelectric material such as silicon, a metal or stainless steel;

[0039] - for the vibration of the resonator: this can be a continuous vibration when the The resonator is associated with a loss compensation system, and the presence of the target compound in the chamber generates a modification of this vibration, for example a modification of its oscillation frequency. Alternatively, when the resonator is not associated with a loss compensation system, its vibration is generated by radiation in the presence of the target compound by optical-thermal-acoustic-mechanical coupling;

[0040] - for the detection means: their design is adapted to the resonator model which is used. When this resonator is made of quartz, the detection means may include excitation and detection electrodes carried by the resonator, which connect it to an electronic oscillator circuit. In such a case, the resonator is said to be electromechanical. The detection means may then further include a system for measuring the amplitude or frequency of oscillation of the oscillator, and the result of each amplitude or frequency measurement indicates the absence of the target compound or reveals its presence. When the resonator is not made of a piezoelectric material, the detection means may be optical, in particular based on optical interference, in order to measure the amplitude of the resonator's vibrations. The resonator may then be purely mechanical, notably without a loss compensation system;

[0041] - for each mirror: it can be external to the enclosure that limits the chamber or may constitute at least partially one of the walls of this enclosure. Each mirror may be dedicated to a single reflection of the radiation beam or be common to several reflections of this beam. Each mirror may be flat or curved, in particular to focus the radiation beam at desired locations in the chamber;

[0042] - for each optical passage _ allowing _ the radiation beam of to pass through a wall of the enclosure: this optical passage can be a porthole or window that is transparent to the radiation from the optical source and that constitutes one of the walls of the enclosure or part thereof. Alternatively, the optical passage can be a small hole through the wall to allow the entry or exit of the radiation beam while causing only minimal gas leakage. Alternatively still, when a wall of the enclosure is formed at least partially by a mirror, the mirror can be made in the form of a reflective coating that is supported by a transparent substrate, and the optical passage can be made by locally removing the reflective coating; and

[0043] - for the optical source: it can be of any type whose spectrum The emission wavelength is known. Preferably, the optical source can be of the laser type, in particular a quantum cascade laser or an interband cascade laser, or of the light-emitting diode type, or even of the optical parametric oscillator type. For the same optical path of the radiation in the chamber, the optical source can be located at one end of this optical path or at the other end in equivalent embodiments, for which the radiation propagates along the same optical path in one direction or the opposite direction. The optical source has at least one emission wavelength, which can be between 0.2 pm (micrometers) and 300 pm.

[0044] With reference to [Fig.1]-[Fig.6], a photoacoustic effect detector 100 comprises a resonator 1, an optical source 2, a closed and rigid enclosure 3, detection means 4, and two mirrors 51 and 52 for [Fig.1]-[Fig.4] and [Fig.6].

[0045] In the two embodiments now described in detail by way of non-limiting examples, the resonator 1 is a tuning fork, with a fixed base portion 10 and two straight and parallel prongs 11 and 12 extending with the same branch length from the base portion 10. Each prong 11, 12 thus extends longitudinally parallel to an axis L, called the central longitudinal axis. Each of the prongs 11, 12 has a respective free end 11L, 12L, and a respective fixed end 11F, 12F which is continuous with the base portion 10. The two prongs 11 and 12 have respective internal lateral faces which are opposite each other, parallel to the central longitudinal axis L, and designated by the reference numerals Fin for prong 11 and FI for prong 12.They also possess respective external lateral faces FEn and FE^ which are also parallel to the central longitudinal axis L and each rotated in the opposite direction to the other arm. This tuning fork can be formed by engraving a quartz plate. The lateral faces Fin, Fin, FEn, and FE^ are then formed by the engraving of the plate. Such a tuning fork resonator, referred to as tuning fork 1 hereafter, is designed to vibrate in a mode where the free ends 1IL and 12L move towards and away from each other parallel to the plane of the plate and perpendicular to the central longitudinal axis. The following geometric elements are defined based on tuning fork 1: median plane PM: a plane that is parallel to the tuning fork plate 1 and passes through it at mid-thickness, central longitudinal axis L: axis which is contained in the median plane PM and which is equidistant from the two branches 11 and 12, section plane PS: plane that is perpendicular to the central longitudinal axis L, transverse direction T: direction that is contained in the median plane PM and that is perpendicular to the central longitudinal axis L, line of intersection LI: line of intersection between the section plane PS and the median plane PM, meridian plane PMR: a plane that contains the central longitudinal axis L, while being distinct from the median plane PM, and projection PR: projection point of the free ends 1 IL and 12L of the branches 11 and 12 of the tuning fork 1 on the central longitudinal axis L.

[0046] For example, the two arms 11 and 12 of the tuning fork 1 may have a common length of 13.6 mm measured from the base part 10 parallel to the central longitudinal axis L, a width of 8 mm measured parallel to the direction transverse T, and a thickness of 2 mm measured perpendicular to the median plane PM.

[0047] When made of quartz, the tuning fork 1 is equipped with electrodes (not shown) that connect it to the detection means 4 (not shown). These means may include an electronic circuit to form an oscillator including the tuning fork 1, and a frequency detector. The frequency detector is adapted to measure the oscillation frequency of the oscillator, which is equal to the vibration frequency of the prongs 11 and 12 of the tuning fork 1 during operation of the detector 100. The result of the frequency measurement then constitutes the measurement result of the detector 100. In particular, a non-zero frequency shift measured in this way reveals the presence of a target compound in a gas that is in contact with the prongs 11 and 12 of the tuning fork 1. A possible structure for such detection means 4 is described in FR 3 057 078 B1.

[0048] The closed and rigid enclosure 3 defines an internal chamber 30 which contains the arms 11 and 12 of the tuning fork 1. In the embodiments described hereafter, again by way of non-limiting example, the chamber 30 has a parallelepiped shape with two principal faces that are parallel to the median plane PM, two end faces that are perpendicular to the central longitudinal axis L, and two lateral faces that are perpendicular to the transverse direction T. In the embodiment of Figures 1 and 2, the two principal faces are formed by respective windows 33 and 34, which are transparent to radiation used in the operation of the detector 100. In the embodiment of Figures 3 and 4, the windows 33 and 34 constitute the two end faces, respectively.The two lateral faces can be covered, inside the chamber 30, by plates 61 and 62 which allow adjustment of a width l30 of the chamber 30 measured parallel to the transverse direction T, as described later. Typically, the width l30 of the chamber 30 is between 5 mm and 40 mm.

[0049] The chamber 3 is sealed against a gas G that is likely to contain the target compound, except for an inlet port 31 for the gas G into the chamber 30 and an outlet port 32 for the gas G. Thus, the gas G can be introduced into the chamber 30 and then trapped therein for a static detection measurement. Alternatively, a continuous scan of the chamber 30 can be performed with the gas G, and successive detection measurements of the target compound can be carried out in real time during the gas scan, the concentration of the target compound in the gas G possibly varying over time.

[0050] The optical source 2 produces a beam of radiation F which is introduced into the enclosure 30 through one of the two windows, for example window 33 in the case shown in figures 1 and 2, and which can exit through either of the two windows.

[0051] In the described embodiments, mirrors 51 and 52 are external to chamber 30 and are arranged and oriented to reflect the radiation beam F back through chamber 30 via windows 33 and 34, such that this beam F passes through chamber 30 at least three times between the two windows 33 and 34. In the embodiments shown, the beam F is reflected out of chamber 30 several times on the side of each window 33 and 34 by a mirror dedicated to each side. Thus, mirror 51, which may optionally be flat, is located on the side of window 34 to reflect the beam F back through it. Similarly, the other mirror 52, which may also be flat, is located on the side of window 33 to reflect the beam F back through window 33.In this way, the radiation beam F travels through several successive optical path segments, each passing through chamber 30 between the two windows 33 and 34. Two consecutive optical path segments are separated by a reflection of the beam F either on mirror 51 or on mirror 52, as shown in [Fig. 1], [Fig. 3], and [Fig. 6]. F1, F2, F3, F4... then denote the successive optical path segments of the beam F that are thus created through chamber 30. The number of these optical path segments can be arbitrary, but it can advantageously be less than twenty. Thanks to this multiplicity of optical path segments in chamber 30, the interaction between the radiation beam F and the gas G is increased, thus improving the sensitivity of detector 100. Orientations of the optical source 2 and of mirrors 51 and 52 will be given later for two embodiments of the invention.

[0052] The chamber 30 filled with gas G has an acoustic natural mode that is coupled to the vibration of the tuning fork 1. The resulting vibration mode, called acousto-mechanical, comprises a standing acoustic wave AC consisting of the superposition of two standing wave components propagating in chamber 30 in opposite directions parallel to the transverse direction T and with equal amplitudes. In the jargon of those skilled in the art, chamber 30 equipped with the tuning fork 1 constitutes an acoustic cavity. The standing acoustic wave AC has pressure nodal surfaces that can be substantially planar and perpendicular to the transverse direction T, as well as pressure antinodes that are intermediate between pairs of neighboring pressure nodal surfaces.As is known, pressure nodal surfaces alternate with pressure antinodes along the transverse direction T, and the two lateral faces of chamber 30 that are perpendicular to the transverse direction T constitute two pressure antinodes. In [Fig. 1]-[Fig. 6], the indications "0" show the respective positions of the pressure nodal surfaces, with pressure denoted p, and the indications "+" and "-" show the positions of the pressure antinodes. Two pressure antinodes that are separated by an odd number of . Nodal pressure surfaces vibrate in opposite phase, symbolized by the associated signs which are opposite. The diagrams at the top of [Fig.1] and [Fig.3] symbolically illustrate nodes and antinodes of the pressure p of the standing acoustic wave AC, which appear successively following the transverse direction T.

[0053] To achieve efficient coupling between the absorption of radiation from beam F by the target compound and the tuning fork 1, the standing acoustic wave AC preferably corresponds to an acoustic eigenmode of the chamber 30, through adjustment of the width l30, or to adjustment of the temperature of the gas G. In the jargon of those skilled in the art, the chamber 30 constitutes a resonant acoustic cavity for the modulation frequency of the radiation beam F. Furthermore, in order for the standing acoustic wave AC to be efficiently coupled to the vibration of the tuning fork 1, the dimensions of the chamber 30 can be adjusted so that the frequency of the resonant standing acoustic wave AC is close to or equal to the vibration frequency of the tuning fork 1. The frequency of the resonant standing acoustic wave AC depends on the temperature and pressure of the gas G, as well as on the width l30 of the chamber 30.Plates 61 and 62, by choosing an appropriate thickness for each of them, make it possible to achieve frequency matching between the acoustic resonance of chamber 30 and a natural mode of vibration of the tuning fork 1. In addition, the coupling between the standing acoustic wave AC and the vibration of the tuning fork 1 is maximal when the two internal lateral faces Fin and Fin are each superimposed on a pressure antinode of the standing acoustic wave AC, corresponding to synchronous pressure variations due to the standing acoustic wave AC, and when simultaneously the two external lateral faces FEn and FEn are also each superimposed on another pressure antinode of the standing acoustic wave AC, corresponding to pressure variations due to the standing acoustic wave AC which are in opposite phase with those at the level of the internal lateral faces Fin and FI12.By "superimposed" is further understood that the distance between the relevant lateral face of the tuning fork 1 and the corresponding pressure antinode is less than 25% of the distance between two nodal pressure surfaces measured parallel to the transverse direction T. For the embodiments of detector 100 shown in [Fig. 1]-[Fig. 4], the two internal lateral faces Fin and FI^ are superimposed on the same pressure antinode, which is also superimposed on the central longitudinal axis L, and the two external lateral faces FEn and FEn are each superimposed on the next pressure antinode, respectively in one direction and in the other towards plate 61 or 62. In the example shown, the external lateral face FEn (respectively FEn) is superimposed on the same pressure antinode as that of the adjustment plate of width 61 (resp. 62). The phase opposition of the variations of . The pressure between the pressure antinode that is superimposed on the internal lateral faces Fin and FI12, and the two pressure antinodes that are superimposed respectively on the external lateral faces FEn and FE^, is shown by the signs “+” and “-” in the figures.

[0054] During operation of the detector 100, the radiation emitted by the source 2 is modulated so that an acoustic excitation caused by the absorption of the radiation has a frequency equal to that of the acousto-mechanical resonant vibration mode just described. To achieve this, the radiation of the beam F as it passes through the chamber 30 can be modulated either in intensity or in wavelength. In the first case, the intensity modulation frequency of the beam F is chosen to be approximately equal to that of the acousto-mechanical resonant vibration mode. In the second case, the wavelength modulation frequency of the beam F is chosen to be approximately equal to half that of the acousto-mechanical resonant vibration mode.

[0055] The detector 100 has the following additional characteristics: / i / the optical source 2 is oriented such that successive optical path segments Fl, F2, F3, F4... all pass through a pressure antinode of the standing acoustic wave AC; and / ii / the optical source 1 and the mirrors 51 and 52 are further arranged and oriented so that each optical path segment Fl, F2, F3, F4... intersects the face of each porthole 33 and 34 which is turned towards the chamber 30 at the level of a nodal pressure surface of the standing acoustic wave AC.

[0056] The / i / characteristic ensures maximum efficiency for the excitation of the acousto-mechanical vibration mode resulting from the absorption of the radiation modulated by the target compound. For this, the wavelength of the radiation emitted by the optical source 2 belongs to a spectral absorption range of the target compound, outside of its possible wavelength modulation.

[0057] The feature / ii / ensures that local heating of the gas G in contact with the windows 33 and 34 which may present an undesired residual absorption for the radiation of the optical source 2, are not or almost not coupled to the acousto-mechanical vibration mode of the chamber 30 coupled to the tuning fork 1.

[0058] [Fig.1] and [Fig.2] on the one hand, and [Fig.3] and [Fig.4] on the other hand, correspond to two different ways of realizing these characteristics / i / and / ii / .

[0059] In the embodiment of [Fig. 1] and [Fig. 2], the beam F and the optical path segments Fl, F2, F3, F4... are highly inclined with respect to the central longitudinal axis L. The windows 33 and 34 then constitute the principal faces of the chamber 30. The beam F and the optical path segments Fl, F2, F3, F4... are contained in a meridian plane PMR, and the first optical path segment Fl makes a first angle ai with the central longitudinal axis L inside this meridian plane PMR, passing at a distance di from the projection PR of the free ends of the tuning fork 1, in the opposite direction to that of the base portion 10 of the tuning fork 1. For example, the first angle ai can be equal to 80°, and the distance dh, which has been called the first offset length in the general part of this description and is preferably non-zero, can be equal to 5 mm. The meridian plane PMR makes a second angle a2 with the line of intersection LI inside the section plane PS. The value of the second angle a2 is chosen such that the meridian plane PMR intersects the faces of the windows 33 and 34 that are facing chamber 30 at the intersection of these main faces with pressure nodal surfaces, as shown in [Fig. 2], which is a section of detector 100 in the section plane PS.The nodal pressure surface containing the points of penetration of the porthole 33 by beam F can be intermediate between the pressure antinode of the internal lateral faces Fin and Fin on the one hand, and that of the external lateral face FEn on the other. Symmetrically, the nodal pressure surface containing the points of penetration of the porthole 34 by beam F can be intermediate between the pressure antinode of the lateral faces Fin and Fin on the one hand, and that of the external lateral face FEi 2 on the other. As an example, the angle a2 can be equal to 40°, when the chamber 30 has a depth p30 equal to 10 mm, measured perpendicular to the median plane PM.

[0060] Simultaneously, the mirrors 51 and 52 can be arranged and oriented so that any two of the optical path segments Fl, F2, F3, F4... which are separated by a single reflection form a third angle a3, non-zero and less than 50°, for example equal to 15°. Furthermore, the mirrors 51 and 52 can be arranged and oriented so that any two of the optical path segments Fl, F2, F3, F4... which are separated by a single reflection intersect the central longitudinal axis L at respective points A1, A2, A3, A4... which are separated by a non-zero distance Ad, less than 40 mm and for example equal to 6 mm. This distance Ad has been called the offset from the projection PR of the free ends 1IL and 12L of the tuning fork arms 1 in the general part of this description.

[0061] In the embodiment of [Fig. 3] and [Fig. 4], the beam F and the optical path segments Fl, F2, F3, F4... are only slightly inclined with respect to the median plane PM. The windows 33 and 34 then constitute the end faces of the chamber 30. The optical source 2 and the mirrors 51 and 52 are such that the optical path segments Fl, F3... enter the chamber 30 by intersecting the end face formed by the window 33 at respective points Bl, B3... which are aligned perpendicularly to the median plane PM, and exit the chamber 30 by intersecting the end face formed by the window 34 at respective points Bl', B3'... which are also aligned. perpendicular to the median plane PM. Similarly, the optical path segments F2, F4... enter the chamber 30 by intersecting the end face formed by the port 34 at respective points B2, B4... which are aligned perpendicular to the median plane PM, and exit the chamber 30 by intersecting the end face formed by the port 33 at respective points B2', B4'... which are again aligned perpendicular to the median plane PM. When the chamber 30 has a height H30 of 55 mm, measured parallel to the central longitudinal axis L, a fourth angle α4 between the first optical path segment Fl and its projection onto the median plane PM, perpendicular to the latter, can be less than 45°, for example, equal to 10°.The third angle α3 between two optical path segments separated by a single reflection on mirror 51, for example between segments F1 and F2, or on mirror 52, for example between segments F2 and F3, can again be less than 50°, for example 15°. The separation distance αpi between the median plane PM and point B1 can be 1 mm, for example, and the separation distance αp between points B1 and B3 can be less than 30 mm, for example 5 mm, the same separation αp being possible between points B2' and B4'. Mirror 51 can advantageously be placed closer to the port 34 so that points B1' and B2 are themselves very close to each other, and similarly for mirror 52 relative to the port 33 so that points B2' and B3 are also very close to each other.For example, the angle a3 between the optical path segments Fl and F2 at the level of the mirror 51, can be equal to 50°, being identical to the angle between the optical path segments F2 and F3 at the level of the mirror 52. [Fig.4], which is a superposition of the respective sections of the detector 100 in the two end faces of the windows 33 and 34 for the embodiment of [Fig.3], shows the alignments of the points Bl, B2', B3, B4'... and Bl', B2, B3', B4... perpendicular to the median plane PM, and in superposition with the nodal pressure surfaces of the stationary acoustic background AC.

[0062] The optical detector 100 of [Fig. 5] is obtained from that of [Fig. 1] and [Fig. 2] by replacing the use of mirrors 51 and 52 with the use of several optical sources 2b 23, 25... to create the multiplicity of optical path segments. The optical sources 2b 23, 25... then have identical spectral emission characteristics and emit respective radiation beams along the optical path segments Fl, F3, F5... These optical path segments Fl, F3, F5... are not necessarily parallel, but they are each contained in the meridian plane PMR so that each window 33, 34 is crossed by all the optical path segments at the level of an acoustic wave pressure node. For such an embodiment with several optical sources 2b 23, 25... with identical spectral emission characteristics, all these optical sources 2b 23, 25... are modulated synchronously, preferably with identical modulations, so that these effective modulations for radiation absorption by the target compound combine constructively to generate the AC standing acoustic wave with maximum amplitude. All the geometric and optimized operating characteristics described in relation to [Fig. 1]-[Fig. 4] can be applied identically to the embodiment of [Fig. 5], insofar as these characteristics do not concern mirrors 51 and 52 or optical path segments F2, F4...

[0063] The optical detector 100 of [Fig. 6] is obtained from that of [Fig. 1] and [Fig. 2] by replacing the tuning fork with a beam having one fixed end and one free end to form the resonator 1. Typically, the beam can be formed by the arm 11 without changing its position in the chamber 30, by removing the arm 12. The reference numerals 11F and 1IL then designate the fixed end and the free end of the beam, the fixed end 11F being rigidly connected to a base not shown, and the free end 1IL still vibrating parallel to the transverse direction T. In this way, the faces Fin and FEn remain optimally positioned with respect to the pressure antinodes of the standing acoustic wave AC. All the geometric and optimized operating characteristics described in relation to [Fig. 1]-[Fig. 4] can then be applied identically to the embodiment of [Fig. 6].

[0064] [Fig. 7] illustrates an improvement of detector 100 of [Fig. 1], [Fig. 3], or [Fig. 6], in which this detector again comprises several optical sources, for example, seven optical sources referenced 2i to 27, but these now have different emission wavelength values. These optical sources 2i to 27 can be activated simultaneously or sequentially, each replacing the others, to emit the radiation beam F, which is sent into the chamber 30. They can be optically connected to the path of the beam F by means of a multi-input optical connection component 20. The set of optical sources 2i to 27 with the optical connection component 20 then replaces the optical source 2 in the embodiments of [Fig. 1], [Fig. 3], and [Fig. 6].The optical connection component 20 can be, for example, a lens that directs each elementary radiation beam from one of the optical sources 2r27 towards a common focal point in the chamber 30, this focal point being on the central longitudinal axis L. Because of their different emission wavelengths, each of the optical sources 2i to 27 can be selected to detect a specific target compound that differs from that of the other optical sources. The selection criterion is that the optical source used has an emission wavelength within a spectral range. absorption of the target compound being sought in gas G. When several of the optical sources 2i to 27 are activated simultaneously, they can be modulated at different frequencies, while remaining within the resonance band of the acoustic cavity, to allow their respective contributions to the detection signal, which is collected by the detection means 4, to be separated. It is thus possible to simultaneously reveal several target compounds in the same gas to be analyzed G. In preferred embodiments, the optical sources 2i to 27 are of the quantum cascade type and implemented as an array of elementary optical sources on a common bar-shaped substrate.

[0065] The detector 100 of the invention can be particularly useful in many technical and industrial fields, in particular for monitoring industrial processes such as refining or purifying gases, in the field of security, for environmental monitoring, including for characterizing air pollution and greenhouse gases, as well as in the field of health, in particular for characterizing the air exhaled by a patient.

[0066] It is understood that the invention can be reproduced by modifying secondary aspects of the embodiments described in detail above, while retaining at least some of the advantages mentioned. In particular, the following modifications can be applied without inventive step on the part of a person skilled in the art, in addition to the alternatives already mentioned: - chamber 30 can have a shape other than that of a rectangular parallelepiped, for example a cylindrical shape; - the pressure antinodes of the standing acoustic wave AC which are superimposed on the lateral faces of the tuning fork may be different from those indicated, in particular when the standing acoustic wave corresponds to a natural mode of the chamber which has a greater number of pressure antinodes; - The alteration in the resonator's vibration produced by the presence of the target compound, and which is measured, may be other than an alteration in the resonator's vibration amplitude. It may, for example, be an alteration in the resonator's vibration frequency; and - the number, arrangement and shape of the mirrors used to increase the number of passes of the F radiation beam in chamber 30 can be changed.

[0067] Finally, all the numerical values ​​that have been cited have been cited only as examples, and may be changed.

Claims

Demands

1. A photoacoustic detector (100) adapted to reveal in a gas to be analyzed (G) the presence of a target compound, the detector comprising: - a mechanical or electromechanical resonator (1) adapted to vibrate during operation of the detector (100); - at least one optical source (2) adapted to produce during operation of the detector (100), at least one beam (F) of radiation which has at least one wavelength value contained within a spectral absorption range of the target compound, with a modulation of the radiation such that an absorption of said radiation by said target compound produces in a chamber (30) a standing acoustic wave (AC) which generates or modifies a vibration of the resonator (1);- a closed enclosure (3), forming the chamber (30) inside the enclosure, with at least a part of the resonator (1) intended to vibrate during the operation of the detector (100), which is located inside the chamber, and the enclosure being provided with means (31, 32) for introducing the gas to be analyzed (G) into the chamber, and further provided with at least one optical passage (33, 34) arranged so that the beam (F) of radiation enters the chamber or exits from said chamber through said at least one optical passage during the operation of the detector;and - detection means (4), adapted to reveal the vibration of the resonator (1) or a modification of said vibration of the resonator which is caused by the standing acoustic wave (AC), the detector (100) being characterized in that it is arranged so that, during the operation of said detector, at least one beam of radiation is partially superimposed on a pressure antinode of the standing acoustic wave (AC), and that each optical passage (33, 34) is crossed by at least one beam (F) of radiation at one or more places which are each adjacent to or superimposed on a pressure node of the standing acoustic wave.;

2. Detector (100) according to claim 1, wherein the means (31, 32) for introducing the gas to be analyzed (G) into the chamber (30) are adjacent to or superimposed on one or more pressure node(s) of the standing acoustic wave (AC), identical or different from the pressure node to which each optical passage is superimposed (33, 34).

3. Detector (100) according to claim 1 or 2, further comprising at least one mirror (51, 52) located outside the chamber (30) or forming at least part of a wall of the enclosure (3), and arranged such that at least one beam (F) of radiation passes through the chamber (30) several times along successive optical path segments (F1, F2, F3, F4...) which are separated at each time by a reflection of the radiation beam on the mirror or one of the mirrors, each optical path segment being partially superimposed on a pressure antinode of the standing acoustic wave (AC), in which each mirror (51, 52) is arranged such that each segment (F1, F2, F3, F4...) optical path crosses at least one optical passage (33, 34) at a respective location which is adjacent to or superimposed on a pressure node of the standing acoustic wave (AC), or when the mirror constitutes a part of the wall of the enclosure (3), said mirror is arranged so that the beam (F) of radiation is reflected at a location of said mirror which is adjacent to or superimposed on a pressure node of the standing acoustic wave.

4. Detector (100) according to claim 3, comprising at least two mirrors (51, 52) which are arranged so that the beam (F) of radiation passes through the chamber (30) along at least three successive segments (F1, F2, F3, F4...) of optical path separated each time by a reflection of the radiation beam on one of the mirrors, preferably along less than twenty successive segments of optical path separated each time by a reflection of the radiation beam on one of the mirrors.

5. Detector (100) according to any one of claims 1 to 4, wherein the resonator (1) comprises a tuning fork, with two prongs (11, 12) of the tuning fork, intended to vibrate during the operation of the detector, extending into the chamber (30) parallel to a central longitudinal axis (L) of the tuning fork, said central longitudinal axis being an axis of symmetry of said tuning fork, and a median plane (PM) of the tuning fork, which contains the central longitudinal axis and is superimposed to the two branches, being parallel to displacements of said branches caused by the vibration of the tuning fork, and in which the chamber (30) is adapted so that components of the standing acoustic wave (AC) propagate parallel to a transverse direction (T) which is contained in the median plane (PM) of the tuning fork and which is perpendicular to the central longitudinal axis (L) of said tuning fork.

6. Detector (100) according to claim 5, arranged so that the central longitudinal axis (L) of the tuning fork (1) is superimposed on a pressure antinode of the standing acoustic wave (AC), in particular the pressure antinode to which the beam (F) of radiation is partially superimposed.

7. Detector (100) according to claim 6 and claim 3 or 4, arranged so that the optical path segments (F1, F2, F3, F4...) each intersect the central longitudinal axis (L) of the tuning fork, preferably at levels of said central longitudinal axis which are all offset with different respective offset lengths from a projection (PR) of free ends (1 IL, 12L) of the arms (11, 12) of the tuning fork onto said central longitudinal axis.

8. Detector (100) according to claim 5 or 6, arranged so that the beam (F) of radiation is located on only one side of the median plane (PM) of the tuning fork (1).

9. Detector (100) according to claim 8 and claim 3 or 4, arranged such that the optical path segments (F1, F2, F3, F4...) of the radiation beam (F) are all located on the same common side of the median plane (PM) of the tuning fork (1), being offset by different spacing distances from said median plane, each optical path segment of the beam forming with a perpendicular projection of said optical path segment onto said median plane an angle (a4) which is less than

10. HJ . Detector (100) according to claim 9, wherein the optical source (2) and each mirror (51, 52) are arranged so that the same pressure antinode of the standing acoustic wave (AC) which is superimposed on the central longitudinal axis (L) of the tuning fork (1) is also superimposed on a part of each segment (F1, F2, F3, F4...) of optical path of the beam (F) of radiation.

11. Detector (100) according to any one of claims 1 to 4, wherein the resonator (1) comprises a straight beam (11) intended to vibrate during the operation of the detector, said beam having two opposite ends comprising a fixed end (11F) rigidly embedded in a base portion (10) of said resonator and another free end (1IL) located in the chamber (30), displacements of the beam caused by the vibration of the resonator being perpendicular to said beam, and wherein the chamber (30) is adapted so that components of the standing acoustic wave (AC) propagate parallel to the displacements of the beam (11).

12. Method for revealing the presence of a target compound in a gas to be analyzed (G), comprising the following steps: / 1 / providing a detector (100) which conforms to any one of claims 1 to 11, of which at least one wavelength value of the optical source (2) is contained within a spectral absorption range of the target compound; / 2 / inject the gas to be analyzed (G) into the chamber (30); and / 3 / simultaneously activate at least one optical source (2) and the detection means (4).

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