MULTI-PASSAGE PHOTOACOUSTIC DEVICE FOR DETECTING GAS

Abstract

A multi-passage photoacoustic device for detecting gas includes a first portion having a stable optical cavity function and having a diameter D and having a concavity with a bend radius R, the diameter D and the bend radius R being such that

[00001]$0{\left(1-\frac{D}{R}\right)}^{2}1;$

a second portion having an acoustic resonator function and having a first end having a first diameter D1 and a second end having a second diameter D2 less than the first diameter D1, the diameter of the second portion decreasing between its first and second ends; an opening for the introduction of a light beam; a gas supply system for introducing a gas to detect; an acoustic detector coupled with the second end of the second portion.

Claims

1. A multi-passage photoacoustic device for detecting gas comprising: a first portion having a stable optical cavity function, the first portion having a first closed end, a second open end and a side wall extending between the first and second ends, the first portion having a diameter D and having a concavity with a bend radius R, the diameter D and the bend radius R being such that: $0{\left(1-\frac{D}{R}\right)}^{2}1$ a second portion having an acoustic resonator function, the second portion having a first open end arranged in the extension of the second open end of the first portion and a second open end, the first end having a first diameter D1 and the second end having a second diameter D2 less than the first diameter D1, the diameter of the second portion decreasing between its first and second ends; an opening for the introduction of a light beam; a gas supply system for introducing a gas to detect; an acoustic detector coupled with the second end of the second portion.

2. The multi-passage photoacoustic device according to claim 1, wherein the second portion has a length L between its first and second ends and wherein the first diameter D1 of the first end is less than the length L.

3. The multi-passage photoacoustic device according to claim 1, wherein the second portion has a diameter that decreases continuously between the first diameter D1 of its first end and the second diameter D2 of its second end.

4. The multi-passage photoacoustic device according to claim 1, wherein the first end of the first portion is a reflective optic in such a way that the first portion has a folded stable optical cavity function.

5. The multi-passage photoacoustic device according to claim 1, wherein the first portion and the second portion are of circular section.

6. The multi-passage photoacoustic device according to claim 16, wherein the second portion is of flattened cone shape.

7. The multi-passage photoacoustic device according to claim 1, wherein the gas supply system for introducing a gas to detect is at least one vent, each vent being arranged in the second portion in line with a pressure node of an acoustic mode of the acoustic resonator to favour.

8. The multi-passage photoacoustic device according to claim 1, wherein the first portion having a stable optical cavity function has a reflection coefficient strictly greater than 95%.

9. The multi-passage photoacoustic device according to claim 1, wherein the opening for the introduction of the light beam is arranged in the side wall of the first portion or in the first end of the first portion or at the junction between the side wall and the first end of the first portion or at the junction between the second end of the first portion and the first end of the second portion.

10. The multi-passage photoacoustic device according to claim 1, further comprising a second acoustic detector arranged in line with the pressure node of the fundamental acoustic mode of the acoustic resonator.

11. A method for detecting gas with a multi-passage photoacoustic device according to claim 1, comprising: introducing the light beam into the device via the opening; introducing the gas to detect into the device via the gas supply system; carrying out a measurement with the multi-passage photoacoustic detector.

12. The method for detecting gas according to claim 11, wherein the light beam is introduced into the device via the opening with a first angle 1 measured, in a straight section of the first portion passing through the opening, with respect to a diameter of the first portion passing through the opening and/or with a second angle 2 measured, in a plane perpendicular to said straight section and passing through the opening, with respect to the diameter of the first portion passing through the opening, the first and second angles 1, 2 being such that: $\frac{d.Math..Math.40}{D}.Math..Math.1,.Math..Math.210\frac{d.Math..Math.40}{D}$ with d40 the diameter of the opening.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0035] The figures are presented for indicative purposes and in no way limit the invention.

[0036] FIG. 1a shows a schematic representation of a multi-passage photoacoustic device for detecting gas according to a first embodiment of the invention.

[0037] FIG. 1b shows a second schematic representation of a multi-passage photoacoustic device for detecting gas according to the first embodiment of the invention.

[0038] FIG. 1c shows a third schematic representation of a multi-passage photoacoustic device for detecting gas according to the first embodiment of the invention.

[0039] FIG. 2a shows a schematic representation of a multi-passage photoacoustic device for detecting gas according to a second embodiment of the invention.

[0040] FIG. 2b shows a second schematic representation of a multi-passage photoacoustic device for detecting gas according to the second embodiment of the invention.

[0041] FIG. 3a is a schematic representation in section of a first portion, having a first geometry, of a multi-passage photoacoustic device for detecting gas according to any of the embodiments of the invention.

[0042] FIG. 3b is a schematic representation in section of a first portion, having a second geometry, of a multi-passage photoacoustic device for detecting gas according to any of the embodiments of the invention.

[0043] FIG. 3c is a schematic representation in section of a first portion, having a third geometry, of a multi-passage photoacoustic device for detecting gas according to any of the embodiments of the invention.

[0044] FIG. 4a schematically shows a first arrangement of an opening for the introduction of a light beam, in a multi-passage photoacoustic device for detecting gas according to any of the embodiments of the invention.

[0045] FIG. 4b schematically shows a second arrangement of an opening for the introduction of a light beam, in a multi-passage acoustic device for detecting gas according to any of the embodiments of the invention.

[0046] FIG. 4c schematically shows a third arrangement of an opening for the introduction of a light beam, in a multi-passage acoustic device for detecting gas according to any of the embodiments of the invention.

[0047] FIG. 4d schematically shows a fourth arrangement of an opening for the introduction of a light beam, in a multi-passage acoustic device for detecting gas according to any of the embodiments of the invention.

[0048] FIG. 4e schematically shows a fifth arrangement of an opening for the introduction of a light beam, in a multi-passage acoustic device for detecting gas according to any of the embodiments of the invention.

[0049] FIG. 5 shows calculated acoustic resonance modes in a multi-passage photoacoustic device for detecting gas according to an embodiment of the invention.

[0050] FIG. 6a shows a digital simulation of the pressure field of the fundamental mode of the photoacoustic device having the acoustic resonance modes of FIG. 5.

[0051] FIG. 6b shows a digital simulation of the pressure field of the second harmonic of the photoacoustic device having the acoustic resonance modes of FIG. 5.

[0052] FIG. 7a schematically shows a first angle of introduction of the light beam, used in the first, second and third arrangements of FIGS. 4a, 4b and 4c according to an embodiment of the invention.

[0053] FIG. 7b schematically shows a second angle of introduction of the light beam, used in the first, second and third arrangements of FIGS. 4a, 4b and 4c according to an embodiment of the invention.

DETAILED DESCRIPTION

[0054] Unless stated otherwise, a same element appearing in different figures has a single reference.

[0055] FIG. 1a shows a schematic representation of a multi-passage photoacoustic device 1 for detecting gas, according to a first embodiment of the invention. To be more concise, the multi-passage photoacoustic device 1 for detecting gas according to the first embodiment of the invention will be simply designated device 1 in the remainder of the description. FIGS. 1b and 1c respectively show second and third schematic representations of the device 1 according to the first embodiment of the invention. FIGS. 1a, 1 b and 1c are described jointly.

[0056] The device 1 according to the first embodiment comprises: [0057] a first portion 10 having a stable optical cavity function, [0058] a second portion 20 having an acoustic resonator function, [0059] an opening 40 for the introduction of a light beam, [0060] an acoustic detector 30 for the detection of a pressure wave generated by photoacoustic effect, and [0061] a gas supply system for introducing a gas to detect.

[0062] The first portion 10 has a first closed end 11, a second open end 12 and a side wall 13 extending between the first and second ends 11, 12. In order to ensure the stable optical cavity function, the first portion 10 has a diameter D and a concavity with a bend radius R, the diameter D and the bend radius R being such that:

[00004]$0{\left(1-\frac{D}{R}\right)}^{2}1$

[0063] The stable optical cavity has an axis of symmetry A and a plane of symmetry P. The diameter D of the first portion 10 is measured in the plane of symmetry P, perpendicular to the axis of symmetry A. The stable optical cavity has, in an embodiment, a reflection coefficient as high as possible, and at least 95%, in order to contribute to improving the sensitivity of the device 1.

[0064] Any section of the first portion 10 through a plane perpendicular to its axis of symmetry A is, in an embodiment, circular, but may also be elliptical or regular polygonal. FIG. 3a schematically shows a section of the first portion 10 through a plane perpendicular to its axis of symmetry A, in the configuration where any section of the first portion 10 through a plane perpendicular to the axis of symmetry A is circular. FIG. 3b schematically shows a section of the first portion 10 through a plane perpendicular to its axis of symmetry A, in an alternative configuration where any section of the first portion 10 through a plane perpendicular to the axis of symmetry A is elliptical. FIG. 3c schematically shows a section of the first portion 10 through a plane perpendicular to its axis of symmetry A, in a second alternative configuration where any section of the first portion 10 through a plane perpendicular to the axis of symmetry A is regular polygonal.

[0065] The second portion 20 has a first open end 21, which is arranged in the extension and in the continuity of the second end 12 of the first portion 10, and a second open end 22. The first end 21 of the second portion 20 has a first diameter D1, and the second end 22 of the second portion 20 has a second diameter D2 which is less than the first diameter D1. The first diameter D1 is typically of the order of several centimetres whereas the second diameter D2, linked to the size of the acoustic detector 30, is typically of the order of a fraction of a millimetre. The second portion 20 has a diameter that decreases between its first and second ends 21, 22. The second portion 20 may be defined by extrusion of a substantially circular shape, of which the diameter decreases between the first diameter D1 and the second diameter D2, along a straight or curved generating line, for example a line folded into a U or wound in a spiral. The diameter of the second portion 20 is the maximum dimension in each section perpendicular to the generating line. The diameter of the second portion 20, in an embodiment, decreases continually between its first and second ends 21, 22. According to an alternative, the diameter of the second portion 20 may decrease by stages between its first and second ends 21, 22. In this alternative, the stages are chosen as a function of the acoustic wavelength: the smaller the difference in diameter between two consecutive stages compared to the acoustic wavelength, the less the propagation of the acoustic wave is perturbed. The difference in diameter between two consecutive stages is, in an embodiment, chosen less than or equal to of the acoustic wavelength, and, in another embodiment, chosen less than or equal to 1/10 of the acoustic wavelength.

[0066] The second portion 20 is, in an embodiment, of flattened cone shape. Alternatively, the second portion 20 may have a folded shape, for example U shaped, or a wound shape, for example a spiral.

[0067] The acoustic detector 30, or microphone, may for example be a moveable membrane type detector with a capacitive sensor or piezoelectric strain gauge, or a detector using a diapason technique in which the variation in resonance frequency is detected, or instead a detector using a surface movement detection technique (lever) by optical means.

[0068] The opening 40 for the introduction of a light beam may be arranged in different manners in the first portion 10 or in the second portion 20. Different arrangements of the opening 40 are described later, in relation with FIGS. 4a to 4e.

[0069] By their dimensioning described previously, the first and second portions 10, 20 form an enclosure that resonates at certain frequencies, also called harmonic modes. The harmonic mode having the lowest frequency is called first harmonic or fundamental mode. It is this fundamental mode that it is wished to favour, to the detriment of other harmonics of higher rank. Indeed, benefit is thus made of an excitation frequency of the gas to detect that is as low as possible, better suited to the relaxation time required by the gas after each excitation. The first end 11 of the first portion 10 being closed and the second end 22 of the second portion 20 being able to be considered as closed by the acoustic detector 30, the enclosure formed by the first and second portions 10, 20 behaves like a closed enclosure and the fundamental mode has a stationary pressure field antinode at its two ends and a single stationary pressure field node between its two ends.

[0070] The device 1 comprises a gas supply system for introducing a gas to detect into the enclosure formed by the first and second portions 10, 20. The gas supply system may be a single vent 50 or a plurality of vents 50, for example two vents 50. Each vent 50 is arranged in the second portion 20, for example in line with the stationary pressure field node of the fundamental mode, which contributes to favouring the fundamental mode by not dampening it and by on the contrary dampening potential other harmonic modes. FIG. 5 shows for example acoustic resonance modes, notably the fundamental mode H1, the second harmonic H2 and the third harmonic H3, calculated in an enclosure formed of the first and second portions 10, 20 having the following characteristics: first portion 10 of 10 mm height; second portion 20 of flattened cone shape and 30 mm height. For this enclosure, with the same characteristics, FIG. 6a shows a digital simulation of the pressure field of the fundamental mode H1, and FIG. 6b shows a digital simulation of the pressure field of the second harmonic H2. The fundamental mode H1 of FIG. 6a has a first pressure antinode H1_V1 at the first end of the second portion 20, a second pressure antinode H1_V2 at the second end of the second portion 20 and a pressure node H1_N between the first and second pressure antinodes H1_V1, H1_V2. The second harmonic H2 of FIG. 6b has a first pressure node H2_N1, a second pressure node H2_N2, a first pressure antinode H2_V1 between the first and second pressure nodes H2_N1, H2_N2 and a second pressure antinode H2_V2 at the second end of the second portion 20. The enclosure of FIGS. 6a and 6b comprises first and second vents 50 arranged in line with the pressure node H1_N of the fundamental mode H1. Alternatively, the gas supply system may be a porous wall, for example to better sense emanations from a surface in contact with the porous wall. In this case, it is desirably the first end 11 of the first portion 10 according to the first embodiment of the invention that is a porous wall, in order that the introduction of the gas via the pores of said wall perturbs as little as possible the operation of the acoustic cavity. Within the scope of the present invention, porous wall is taken to mean a wall provided with pores of small dimension compared to the acoustic wavelength, that is to say of dimension less than 1/10 of the acoustic wavelength and in an embodiment less than 1/100 of the acoustic wavelength, each pore being distant from the other pores by a distance 3 to 30 times greater than the dimension of the pores. The acoustic wavelength being of the order of a cm, typically comprised between 3 cm and 10 cm, the pores may be of dimensions comprised between 0.3 mm and 1 cm. In comparison, each vent 50 is in an embodiment of millimetric size, notably in order to protect the enclosure from potential external sound pollution. It will thus be understood that according to the given definition, a pore is not necessarily of smaller dimension than a vent.

[0071] FIG. 2a shows a schematic representation of a multi-passage photoacoustic device for detecting gas 1, according to a second embodiment of the invention. To be more concise, the multi-passage photoacoustic device 1 for detecting gas according to the second embodiment of the invention will be simply designated device 1 in the remainder of the description. FIG. 2b shows a second schematic representation of the device 1 according to the second embodiment of the invention.

[0072] FIGS. 2a and 2b are described jointly.

[0073] The device 1 according to the second embodiment of the invention comprises: [0074] a first portion 10 having a folded stable optical cavity function, [0075] the second portion 20 having the acoustic resonator function, [0076] the opening 40 for the introduction of a light beam, and [0077] the acoustic detector 30 for the detection of a pressure wave generated by photoacoustic effect.

[0078] The second portion 20, the acoustic detector 30 and the opening 40 of the device 1 according to the second embodiment of the invention have been described previously for the device 1 according to the first embodiment of the invention.

[0079] The first portion 10 having a folded stable optical cavity function has a first closed end 11, the second open end 12 and a side wall 13 extending between the first and second ends 11, 12. Just like the stable optical cavity of the first portion 10 according to the first embodiment, the folded stable optical cavity of the first portion 10 according to the second embodiment has the axis of symmetry A and the plane of symmetry P. In order to ensure the folded stable optical cavity function, the first end 11 is a reflective optic arranged in the plane of symmetry P. The device 1 according to the second embodiment of the invention, of which the first portion 10 fulfils the folded stable optical cavity function, may beneficially be manufactured by moulding without draw taper difficulty.

[0080] Different arrangements of the opening 40 will now be described, in relation with FIGS. 4a to 4e.

[0081] FIG. 4a schematically shows a first example according to which the opening 40 is arranged in the side wall 13 of the first portion 10 according to the first embodiment. The opening 40 may likewise be arranged in the side wall 13 of the first portion 10 according to the second embodiment.

[0082] FIG. 4b schematically shows a second example according to which the opening 40 is arranged at the junction between the first end 11 and the side wall 13 of the first portion 10 according to the first embodiment. The opening 40 may likewise be arranged at the junction between the first end 11 and the side wall 13 of the first portion 10 according to the second embodiment.

[0083] FIG. 4c schematically shows a third example according to which the opening 40 is arranged at the junction between the side wall 13 and the second end 12 of the first portion 10 according to the first embodiment. The opening 40 may likewise be arranged at the junction between the side wall 13 and the second end 12 of the first portion 10 according to the second embodiment.

[0084] FIG. 4d schematically shows a fourth example according to which the opening 40 is arranged in the first end 11 of the first portion 10 according to the first embodiment of the invention. The opening 40 may likewise be arranged in the first end 11 of the first portion 10 according to the second embodiment of the invention.

[0085] FIG. 4e schematically shows a fifth example according to which the opening 40 is arranged in a zone of the second portion 20 of the device 1 according to the first embodiment, the zone extending over a length H from the first end 21 of the second portion 20, the length H being equal to the height of the stable optical cavity, that is to say to the height of the first portion 10, measured along the axis of symmetry A. The opening 40 may likewise be arranged in a zone of the second portion 20 of the device 1 according to the second embodiment, the zone extending over the length H from the first end 21 of the second portion 20, the length H being equal to the height of the folded stable optical cavity, that is to say double the height of the first portion 10, measured along the axis of symmetry A.

[0086] In order to avoid that the light beam does not come out via the opening 40 and in order to maximise the optical path covered by the light beam within the optical cavity, notably in the first, second and third examples of FIGS. 4a, 4b and 4c, the light beam is in an embodiment introduced via the opening 40 with a first angle 1 or with a second angle 2 or both with the first and second angles 1 and 2. The first angle 1, represented in FIG. 7a, is measured, in a straight section of the first portion 10 passing through the opening 40, with respect to a diameter of the first portion 10 passing through the opening 40. The second angle 2, represented in FIG. 7b, is measured, in a plane perpendicular to the straight section defined previously and passing through the opening 40, with respect to the diameter of the first portion 10 passing through the opening 40. The first angle 1 and the second angle 2 are in an embodiment small, that is to say such that:

[00005]$\frac{d.Math..Math.40}{D}.Math..Math.1,.Math..Math.210\frac{d.Math..Math.40}{D}$

[0087] FIGS. 4a, 4b and 4c show that only the first portion 10 has a reflective wall and plays the role of an optical cavity. However, in the first, second and third examples of FIGS. 4a, 4b and 4c, the second portion 20 could alternatively also have a reflective wall, for example at least on a zone extending over the length H from the first end 21 of the second portion 20. FIGS. 4d and 4e show that, in addition to the first portion 10, the entire second portion 20 has a reflective wall and plays the role of an optical cavity. However, in the fourth and fifth examples of FIGS. 4d and 4e, the second portion 20 could alternatively have a reflective wall occupying only a zone extending over the length H from the first end 21 of the second portion 20.