Detector employing a photo-acoustic effect

Abstract

The invention relates to a detector (100) employing a photo-acoustic effect, the detector being configured to reveal a presence of a target compound in a gas (G) to be analysed. At least one beam (F) of radiation is used during operation of the detector, the beam entering into a chamber (30) containing the gas to be analysed and / or exiting from the chamber in one or more locations each adjacent to or superposed on a pressure node of a stationary acoustic wave (AC). The detector is compatible with configurations in which the beam of radiation passes through the chamber multiple times, and components of the detector can be aligned more simply. A sensitivity of the detector to external disturbances is also decreased.

Description

[0001] PHOTO-ACOUSTIC DETECTOR

[0002] Technical field of the invention

[0003] This description relates to a photoacoustic detector.

[0004] Previous technique

[0005] 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 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 via a mechanical or electromechanical resonator that is set into vibration by the acoustic wave, or via

[0006] of an electromechanical oscillator whose frequency is modified by the acoustic wave as described in FR 3 057 078 B1.

[0007] Several improvements to this detection technique have been proposed to increase its sensitivity, including the following:

[0008] - the resonator can be a quartz tuning fork;

[0009] - 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 A 1 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

[0010] - 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.

[0011] However, the detector of S. Borri et al., as described in the aforementioned article, requires that its optical components be aligned very precisely, as well as the radiation beam relative to the tuning fork, particularly when the radiation is in the infrared range due to beam divergence in this case. To maximize the sensitivity of this detector, a large number of passes of the radiation beam are used, necessitating the formation of a resonant optical cavity. Such a resonant optical cavity requires the detector components to be aligned with an additional degree of precision. For this reason, the detector of 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.

[0012] Technical problem

[0013] 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.

[0014] A further objective of the invention is to provide a photoacoustic detector configuration that can be easily adapted to implement multiple passes of modulated radiation beams through the chamber containing the resonator and the gas to be analyzed. Summary of the invention

[0015] 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;

[0016] - 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;

[0017] - 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 or exits the chamber through this at least one optical passage during detector operation; and

[0018] - detection methods adapted to reveal the vibration of the resonator or a modification thereof which is caused by the standing acoustic wave.

[0019] 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.

[0020] 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 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.Any unwanted acoustic emission therefore does not cause vibration of the resonator or any significant change in 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, can be superimposed on the same pressure node of the standing acoustic wave, or on different pressure nodes.

[0021] Thanks to the superposition of each optical passage of 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, thus eliminating the need for a resonant optical cavity. Therefore, no precise optical alignment is required, and the detector is relatively insensitive to external disturbances such as temperature variations or external vibrations. However, the use of a tuned optical cavity remains possible.

[0022] Finally, the detector of the invention is compatible with a non-miniaturized mechanical or electromechanical resonator, for example, one with dimensions ranging from one to several millimeters, or even several centimeters. It is then easy to adapt the detector's configuration to produce multiple passages of the chamber by at least one radiation beam. 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 possibility.

[0023] 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.

[0024] For the purposes of this invention, a detector suitable for detecting the presence of a target compound is defined as one in which at least one wavelength of the radiation produced by the optical source falls within the spectral absorption range of the target compound. This spectral absorption range may be a line or a 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 documentation. Alternatively, the wavelength(s) of the radiation used in the detector may be indicated on the detector or in its documentation. A user of the detector can thus determine the target compound(s) that can be detected using this detector, based on tabulated values ​​of the absorption lines and bands of known compounds.The detector's instruction manual can be attached to the detector, or accessed via the internet using a link provided with the detector.

[0025] 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 gas introduction means 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.

[0026] In various possible embodiments, the detector may further include:

[0027] - at least one mirror which is located outside the chamber or constitutes at least part of a wall of the enclosure, and which is arranged so that at least one beam of radiation passes through the chamber several times along successive segments of optical path which are separated each time by a reflection of the beam of radiation on the mirror or one of the mirrors, each segment of optical path being partially superimposed on a pressure antinode of the standing acoustic wave.

[0028] Furthermore, each mirror can be arranged so that each optical path segment passes through at least one optical passage at a 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 location on the mirror 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 gaseous content renewal time can be shorter. Moreover, 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 for the proportion of radiation that is absorbed by the target compound, for an identical quantity of this target compound that is present in the chamber.

[0029] Preferably, the detector of the invention may comprise at least two mirrors arranged such 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, notably without the need for a resonant optical cavity.

[0030] In general 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 frequency of the standing acoustic wave is between 0.95 and 1.05 times a frequency of an acoustic eigenmode of the chamber equipped with the resonator.

[0031] 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 operation of the detector, extending 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 prongs, being parallel to displacements of the prongs caused by the vibration of the tuning fork. For such preferred embodiments, the chamber may 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.

[0032] In first preferred embodiments of tuning fork detectors, the detector can be arranged so that the central longitudinal axis of the tuning fork is superimposed on a pressure antinode of the standing acoustic wave, in particular the pressure antinode to which the radiation beam is partially superimposed.

[0033] Preferably, the detector can then be arranged so that the inner faces of the two opposite prongs of the tuning fork are adjacent to or superimposed on each of 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 tuning fork prongs. 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 opposite prongs of the tuning fork are adjacent to or superimposed on each of 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 tuning fork prongs are superimposed.

[0034] Such first preferred embodiments may benefit from the following additional features:

[0035] - 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;

[0036] - the optical source can be oriented so that a first segment of the optical path, according to the direction of propagation of the radiation, makes, in a meridian plane which contains this first segment of the optical path and the central longitudinal axis of the tuning fork, a first angle which is between 45° (degree) and 90° with the central longitudinal axis of the tuning fork. In other words, the beam of radiation can be substantially transverse with respect to the central longitudinal axis, by being inclined with respect to this central longitudinal axis until it can be perpendicular to it;

[0037] - 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;

[0038] - 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

[0039] - 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.

[0040] In second preferred embodiments of the tuning fork, the detector can be arranged so that the radiation beam is located on only one side of the median plane of the tuning fork.

[0041] When multiple 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 forms 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:

[0042] - the optical source and each mirror can be arranged so that the same pressure antinode of the standing acoustic wave which 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;

[0043] - 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;

[0044] - 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

[0045] - 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.

[0046] In other embodiments of the invention, the tuning fork in the resonator can be replaced by a straight beam designed to vibrate during detector operation. 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 within the chamber. Displacements of the beam caused by the resonator's vibration are perpendicular to its longitudinal axis. 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.

[0047] In general, in a detector according to the invention:

[0048] - 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;

[0049] - 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°;

[0050] - 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;

[0051] - 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;

[0052] - 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;

[0053] 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.

[0054] 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:

[0055] / 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;

[0056] 121 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 this gas to be analyzed present in this chamber, the frequency of the standing acoustic wave is between 0.95 and 1.05 times a frequency of an acoustic eigenmode of the chamber equipped with the resonator; and

[0057] / 3 / activate simultaneously at least one optical source and the detection means.

[0058] Such a process can be used for methane (CH4), carbon dioxide (CO2), ammonia (NH3), and nitrogen oxides (NOx). X), including nitrogen monoxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O), sulfur fluoride (SFe), oxygen (O2), hydrogen sulfide (H2S), sulfur(IV) oxide (SO2), hydrogen fluoride (HF), hydrogen chloride (HCl), benzene (CeHe), toluene (CyHs), xylene (CsH), ozone (O3), water (H2O), etc. as target compounds.

[0059] Brief description of the figures

[0060] The features and advantages of the present invention will become clearer in the following detailed description of non-limiting embodiments, with reference to the accompanying figures, among which:

[0061] Figure 1 is a perspective diagram of a first photoacoustic detector according to the invention; Figure 2 is a cross-section of the detector of Figure 1 showing antinodes and nodal pressure planes of a standing acoustic wave occurring during operation of the detector;

[0062] Figure 3 is a perspective diagram of a second photoacoustic detector according to the invention;

[0063] Figure 4 is a cross-section of the detector in Figure 3, also showing the antinodes and nodal pressure planes of the standing acoustic wave;

[0064] Figure 5 corresponds to Figure 1 for a first variant of the implementation;

[0065] Figure 6 also corresponds to Figure 1 but for a second embodiment; and

[0066] Figure 7 illustrates an improvement of the invention with several target gases.

[0067] Detailed description of the invention

[0068] For clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or ratios of actual dimensions. Furthermore, some of these elements are represented only symbolically, and identical references shown in different figures designate identical elements or elements with identical functions.

[0069] 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:

[0070] - for the resonator: it can be in the shape of a tuning fork or another shape, 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;

[0071] - for the vibration of the resonator: this can be a continuous vibration when 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;

[0072] - For the detection means: their design is adapted to the type of resonator 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;

[0073] - for each mirror: it can be external to the enclosure that limits the chamber or it can constitute at least partially one of the walls of this enclosure. Each mirror can be dedicated to a single reflection of the radiation beam or be common to several reflections of this beam. Each mirror can be flat or curved, in particular to focus the radiation beam at desired locations in the chamber;

[0074] - for each optical passage allowing the radiation beam to pass through a wall of the enclosure, this optical passage may 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 a part thereof. Alternatively, the optical passage may 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 may be in the form of a reflective coating that is supported by a transparent substrate, and the optical passage may be made by locally removing the reflective coating; and

[0075] - For the optical source: it can be of any type whose emission spectrum is known. Preferably, the optical source can be a laser, in particular a quantum cascade laser or an interband cascade laser, or a light-emitting diode (LED), or an optical parametric oscillator. For the same optical path of 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.With reference to Figures 1 to 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 Figures 1, 4 and 6.

[0076] In the two embodiments described in detail now by way of non-limiting examples, the resonator 1 is a tuning fork, with a fixed base portion 10 and two straight, parallel prongs 11 and 12 extending with the same 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 Fl 12 for prong 12.They also possess respective external lateral faces FEn and FE12, 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 etching a quartz plate, as is known. The lateral faces Fin, FI12, FEn, and FE12 are then formed by the etching 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 11L 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 from the tuning fork 1: median plane PM: plane which is parallel to the plate of the tuning fork 1 and which 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 arms 11 and 12, section plane PS: plane which is perpendicular to the central longitudinal axis L, transverse direction T: direction which is contained in the median plane PM and which is perpendicular to the central longitudinal axis L, line of intersection L1: line of intersection between the section plane PS and the median plane PM, meridian plane PMR: plane which contains the central longitudinal axis L, while being distinct from the median plane PM, and projection PR: point of projection of the free ends 11L and 12L of the arms 11 and 12 of the tuning fork 1 onto the central longitudinal axis L.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 transverse direction T, and a thickness of 2 mm measured perpendicular to the median plane PM.

[0077] 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 incorporating 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.

[0078] 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 parallel to the median plane PM, two end faces perpendicular to the central longitudinal axis L, and two lateral faces perpendicular to the transverse direction T. In the embodiments 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 embodiments of Figures 3 and 4, the windows 33 and 34 form the two end faces, respectively.The two lateral faces can be covered, inside chamber 30, by plates 61 and 62 which allow adjustment of a width ho of chamber 30 measured parallel to the transverse direction T, as described later. Typically, the width ho of chamber 30 is between 5 mm and 40 mm.

[0079] The chamber 3 is sealed against a gas G that may 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 contained within it for static detection measurements. 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, as the concentration of the target compound in the gas G may vary over time. The optical source 2 produces a beam of radiation F that is introduced into the chamber 30 through one of two windows, for example, window 33 in the case shown in Figures 1 and 2, and can exit through either of the two windows.

[0080] Mirrors 51 and 52 are external to chamber 30 in the described embodiments 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 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 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 Figures 1, 3, and 6. F1, F2, F3, F4... then designate the successive optical path segments of the beam F that are thus formed 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.

[0081] The chamber 30, filled with gas G, possesses an acoustic natural mode coupled to the vibration of the tuning fork 1. The resulting vibration mode, called acousto-mechanical, comprises a standing acoustic wave AC, which is formed by 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 terminology 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 adjacent 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 Figures 1 and 6, the markings "0" indicate the respective positions of the pressure nodal surfaces, with pressure denoted p, and the markings "+" and "-" indicate the positions of the pressure antinodes. Two pressure antinodes separated by an odd number of pressure nodal surfaces vibrate in opposite phase, symbolized by the opposite signs associated with them. The diagrams at the top of Figures 1 and 3 symbolically illustrate the nodes and pressure antinodes p of the standing acoustic wave AC, which appear successively along the transverse direction T.

[0082] 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 chamber 30, through adjustment of the width ho, or to adjustment of the temperature of the gas G. In the jargon of those skilled in the art, chamber 30 constitutes a resonant acoustic cavity for the modulation frequency of the radiation beam F. Furthermore, for the standing acoustic wave AC to be efficiently coupled to the vibration of the tuning fork 1, the dimensions of 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 ho of chamber 30.Plates 61 and 62, by choosing an appropriate thickness for each of them, make it possible to achieve the frequency tuning 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 Fl 12 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 FE11 and FE12 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 Fl 12.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 the detector 100 which are shown in Figures 1 and 4, the two internal lateral faces Fin and Fl 12 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 FE12 are each superimposed on the next pressure antinode, respectively in one direction and in the other towards the plate 61 or 62. In the example shown, the external lateral face FEn (respectively FE12) is superimposed on the same pressure antinode as that of the adjustment plate of width 61 (respectively 62).The phase opposition of the pressure variations between the pressure antinode that is superimposed on the internal lateral faces Fin and Fl 12, and the two pressure antinodes that are superimposed respectively on the external lateral faces FEn and FE12, is shown by the signs “+” and “-” in the figures.

[0083] During operation of detector 100, the radiation emitted by 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 beam F as it passes through chamber 30 can be modulated either in intensity or in wavelength. In the first case, the intensity modulation frequency of 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 beam F is chosen to be approximately equal to half that of the acousto-mechanical resonant vibration mode.

[0084] Detector 100 has the following additional features:

[0085] / i / The optical source 2 is oriented such that successive optical path segments F1, F2, F3, F4... all pass through a pressure antinode of the standing acoustic wave AC; and

[0086] / ii / the optical source 1 and the mirrors 51 and 52 are further arranged and oriented so that each segment of optical path F1, 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.

[0087] 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 to occur, the wavelength of the radiation emitted by the optical source 2 belongs to a spectral absorption range of the target compound, independent of any wavelength modulation.

[0088] The characteristic / ii / ensures that local heating of the gas G in contact with the windows 33 and 34, which may present an undesirable residual absorption for the radiation from the optical source 2, is not or almost not coupled to the acousto-mechanical vibration mode of the chamber 30 coupled to the tuning fork 1. Figures 1 and 2 on the one hand, and Figures 3 and 4 on the other hand, correspond to two different ways of realizing these characteristics / i / and / ii / .

[0089] In the embodiment of Figures 1 and 2, the beam F and the optical path segments F1, 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 F1, F2, F3, F4... are contained in a meridian plane PMR, and the first optical path segment F1 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 part 10 of the tuning fork 1. For example, the first angle ai can be equal to 80°, and the distance di, 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 02 with the line of intersection L1 inside the section plane PS. The value of this second angle 02 is chosen such that the meridian plane PMR intersects the faces of the windows 33 and 34, which are oriented towards 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 pressure nodal surface containing the points where beam F passes through window 33 can be intermediate between the pressure antinode of the internal lateral faces Fin and Fl12 on the one hand, and that of the external lateral face FEn on the other. Symmetrically, the nodal pressure surface which contains the crossing points of the porthole 34 by the beam F can be intermediate between the pressure antinode of the lateral faces Fin and Fl 12 on the one hand, and that of the external lateral face FE12 on the other hand.As an example, angle 02 can be equal to 40°, when chamber 30 has a pao depth equal to 10 mm, measured perpendicular to the median plane PM.

[0090] Simultaneously, mirrors 51 and 52 can be arranged and oriented so that any two optical path segments F1, F2, F3, F4... separated by a single reflection form a third angle O3, non-zero and less than 50°, for example, equal to 15°. Furthermore, mirrors 51 and 52 can be arranged and oriented so that any two optical path segments F1, F2, F3, F4... separated by a single reflection intersect the central longitudinal axis L at respective points A1, A2, A3, A4... separated by a non-zero distance Ad, less than 40 mm and, for example, equal to 6 mm. This distance Ad has been referred to as the offset from the projection PR of the free ends 11L and 12L of the tuning fork arms 1 in the general part of this description. In the embodiment of Figures 3 and 4, the beam F and the optical path segments F1, F2, F3, F4... are slightly inclined with respect to the median plane PM.The portholes 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 F1, F3... enter the chamber 30 by cutting the end face which is constituted by the porthole 33 at respective points B1, B3... which are aligned perpendicularly to the median plane PM, and exit the chamber 30 by cutting the end face which is constituted by the porthole 34 at respective points BT, B3'... which are also aligned perpendicularly to the median plane PM. Similarly, the optical path segments F2, F4... enter the chamber 30 by cutting the end face which is constituted by the porthole 34 at respective points B2, B4... which are aligned perpendicularly to the median plane PM, and exit the chamber 30 by cutting the end face which is constituted by the porthole 33 at respective points B2', B4'... which are again aligned perpendicularly 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 04 between the first optical path segment F1 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 03 between two optical path segments that are separated by a single reflection on the mirror 51, for example, between segments F1 and F2, or on the mirror 52, for example, between segments F2 and F3, can again be less than 50°, for example, equal to 15°. The separation distance Api between the median plane PM and point B1 can be equal to 1 mm, for example, and the separation distance Ap between points B1 and B3 can be less than 30 mm and, for example, equal to 5 mm, the same separation Ap being possible between points B2' and B4'.Mirror 51 can advantageously be brought closer to window 34 so that points BT and B2 are themselves very close to each other, and similarly for mirror 52 in relation to window 33 so that points B2' and B3 are also very close to each other. For example, the angle 03 between optical path segments F1 and F2 at mirror 51 can be equal to 50°, being identical to the angle between optical path segments F2 and F3 at mirror 52. [Fig. 4], which is a superposition of the respective sections of detector 100 in the two end faces of windows 33 and 34 for the embodiment of [Fig. 3], shows the alignments of points B1, B2', B3, B4'... and BT, B2, B3', B4... perpendicular to the median plane PM, and in superposition with the nodal pressure surfaces of the standing acoustic wave AC.

[0091] The optical detector 100 in Figure 5 is derived from that of Figures 1 and 2 by replacing the use of mirrors 51 and 52 with the use of several optical sources 2i, 23, 2s... to create the multiplicity of optical path segments. The optical sources 2i, 23, 2s... then have identical spectral emission characteristics and emit respective radiation beams along the optical path segments F1, F3, F5... These optical path segments F1, F3, F5... are not necessarily parallel, but they are each contained within 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 2i, 23, 2s... with identical spectral emission characteristics, all these optical sources 2i, 23, 2s...are modulated synchronously, preferably with identical modulations, so that these modulations, effective 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 Figures 1 to 4 can be applied identically to the embodiment of Figure 5, insofar as these characteristics do not concern mirrors 51 and 52 or optical path segments F2, F4.

[0092] The optical detector 100 of Figure 6 is derived from that of Figures 1 and 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 altering its position in the chamber 30, by removing the arm 12. The references 11F and 11L then designate the fixed and free ends of the beam, the fixed end 11F being rigidly connected to a base not shown, and the free end 11L 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 Figures 1 to 4 can then be applied identically to the embodiment of Figure 6.

[0093] Figure 7 illustrates an improvement to detector 100 of Figures 1, 3, and 6, in which this detector again comprises several optical sources, for example, seven optical sources referenced 2i to 2ε, but these now have different emission wavelengths. These optical sources 2i to 2ε can be activated simultaneously or sequentially, each replacing the others, to emit the radiation beam F that is sent into chamber 30. They can be optically connected to the path of beam F by a multi-input optical connection component 20. The set of optical sources 2i-2ε with the optical connection component 20 then replaces optical source 2 in the embodiments of Figures 1, 3, and 6.The optical connection component 20 can be, for example, a lens that directs each elementary radiation beam from one of the optical sources 2i-2? to a common focal point in the chamber 30, this focal point being located on the central longitudinal axis L. Because their respective emission wavelengths differ, each of the optical sources 2i to 2? can be selected to detect a specific target compound that differs from that detected by the other optical sources. The selection criterion is that the optical source used has an emission wavelength within the spectral absorption range of the target compound being sought in the gas G.When several of the optical sources 2i to 2? 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 2? are of the quantum cascade type and implemented as an array of elementary optical sources on a common substrate in the form of a bar.

[0094] The detector 100 of the invention can be particularly useful in many technical and industrial fields, especially for monitoring industrial processes such as refining or gas purification, in the field of security, for environmental monitoring, including for characterizing air pollution and greenhouse gases, as well as in the field of health, especially for characterizing the air exhaled by a patient.

[0095] It is understood that the invention can be reproduced by modifying minor 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:

[0096] - chamber 30 can have a shape other than that of a rectangular parallelepiped, for example a cylindrical shape;

[0097] - 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;

[0098] - 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

[0099] - 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.

[0100] Finally, all the numerical values ​​that have been cited have been for illustrative purposes only, and may be changed.

Claims

DEMANDS 1. A photoacoustic detector (100) adapted to reveal the presence of a target compound in a gas to be analyzed (G), 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 the 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) within the enclosure, with at least a portion 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 radiation beam (F) 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 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 (33, 34) is superimposed.

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 segments (F1, F2, F3, F4...) of optical path, each segment separated by a reflection of the radiation beam on the mirror or one of the mirrors, each segment of optical path 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, which extend in 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 which is superimposed on the two prongs, being parallel to displacements of said prongs caused by the vibration of the tuning fork, and wherein 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 such that the central longitudinal axis (L) of the tuning fork (1) is superimposed on a pressure antinode of the acoustic wave stationary (AC), in particular the pressure antinode to which the radiation beam (F) is partially superimposed.

7. Detector (100) according to claim 6 and claim 3 or 4, arranged so that the segments (F1, F2, F3, F4...) of optical path 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 (11L, 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 so that the segments (F1, F2, F3, F4... ) of optical path of the beam (F) of radiation are all located on the same common side of the median plane (PM) of the tuning fork (1), being offset with respective spacing distances which are different with respect to said median plane, each segment of optical path of the beam forming with a perpendicular projection of said segment of optical path on said median plane an angle (04) which is less than 45°.

10. Detector (100) according to claim 9, wherein the optical source (2) and each mirror (51, 52) are arranged such that the same pressure antinode of the standing acoustic wave (AC) is superimposed on the central longitudinal axis (L) of the tuning fork (I) is also superimposed on a part of each segment (F1, F2, F3, F4...) of the 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) designed 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 (11L) 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 (II).

12. A method for detecting the presence of a target compound in a gas to be analyzed (G), comprising the following steps: / 1 / provide 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; 121 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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