Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
  • G01N29/22—Details, e.g. general constructional or apparatus details
  • G01N29/24—Probes
  • G01N29/2418—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics
  • G01N29/2425—Probes using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics optoacoustic fluid cells therefor
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
  • G01N29/34—Generating the ultrasonic, sonic or infrasonic waves, e.g. electronic circuits specially adapted therefor
  • G01N29/341—Generating the ultrasonic, sonic or infrasonic waves, e.g. electronic circuits specially adapted therefor with time characteristics
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
  • G01N2021/1704—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in gases
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2201/00—Features of devices classified in G01N21/00
  • G01N2201/06—Illumination; Optics
  • G01N2201/069—Supply of sources
  • G01N2201/0696—Pulsed
  • G01N2201/0697—Pulsed lasers
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N2291/00—Indexing codes associated with group G01N29/00
  • G01N2291/02—Indexing codes associated with the analysed material
  • G01N2291/021—Gases

Definitions

• the invention relates to the field of photoacoustics and more specifically to the measurement of the power and the wavelength of laser radiation by photoacoustics.
• the present invention relates to laser spectroscopy methods for which it is necessary to know or precisely control the power and the wavelength of the lasers used.
• PA photoacoustic spectroscopy
• Gas analysis by PA requires a pulsed or continuous laser source modulated in intensity and/or wavelength, a cell forming an acoustic resonator containing the gas to be analyzed and a detection microphone.
• the PA effect in gas detection can be separated into 4 steps: (1) the laser radiation is absorbed by the gas thus exciting the rotational, electronic and vibrational energy levels; (2) In the case of ro-vibrational excitations, the gas will be preferentially de-excited by molecular collisions which will result in a transfer of rotational/vibrational energy and kinetic energy creating localized heating of the gas.
• the radiative emission is not predominant in the case of excitations vibrational because of the long lifetime of the ro-vibrational radiative levels compared to the non-radiative ones in the pressures usually used in PA ( ⁇ 1 bar).
• the energy absorbed by the gas is completely transferred in the form of heat by translation of kinetic energy in the gas atoms; (3) generation of an acoustic wave and a thermal wave caused by the expansion due to the heating of the gas; (4) detection by the microphone of the acoustic signal.
• the vibration amplitude of the microphone is representative of the concentration of the gas and the wavelength of laser radiation absorbed by the gas indicates its composition.
• the PA signal produced by the interaction between the laser and the gas studied is amplified before detection by the use of an acoustic cell having a resonant at a certain frequency.
• a resonant cell known from the prior art is for example a cell of the dual Helmholtz resonator type comprising two first cavities connected to the detection microphone.
• Impulse photoacoustics uses pulsed optical sources or continuous sources with an external mechanical or electro-optical modulator.
• sources operating in quasi-continuous wave regime (or QCW for Quasi Continuous Wave).
• the amplitude of the laser is modulated at a so-called repetition frequency much higher than that of the resonance of the cell, so that the laser radiation appears to be continuous from the point of view of the modulation frequency corresponding to the acoustic resonance.
• the invention aims to overcome some of the aforementioned problems of the prior art. More precisely, the invention relates to a device making it possible to determine or control the wavelength and the power of laser radiation by photoacoustics using a simple and inexpensive assembly.
• an object of the invention is a device for measuring laser radiation by photoacoustic effect comprising: a cell containing at least one gas having an absorption line with a central wavelength ⁇ c , an electro-acoustic transducer arranged within the cell and adapted to generate an electrical signal representative of the photoacoustic signal in the cell means for processing the electrical signal generated by the electro-acoustic transducer, in which an estimate of the concentration is stored of the gas or gases, at least one laser source adapted to emit laser radiation in the cell at a wavelength adapted to the excitation of at least one gas contained in the cell, said laser radiation exhibiting an oscillatorically variable wavelength around an average mean wavelength at a modulation frequency or an oscillatorily variable optical power around an average power at said frequency modulation, such that an interaction between the laser radiation and at least one gas contained in the cell induces the generation of a photoacoustic signal at a detection frequency of the electro-acoustic transducer, said cell being sealed by a membrane of so as
• the laser source or sources are further configured so that said average wavelength changes over time and so that an excursion in average wavelength comprises said wavelength central wave
• said processing means being further adapted to determine: an evolution of the phase of the photoacoustic signal over time from said electrical signal, the wavelength of the laser radiation from an evolution over time of said phase of the photoacoustic signal.
• said processing means being further adapted to determine a power P L of the laser radiation, from the electrical signal and from said estimate;
• the cell contains a plurality of distinct gases, each comprising at least one absorption line spectrally distinct from the others, said device further comprising a plurality of laser sources each located outside the cell and adapted to excite an associated gas;
• the processing means are configured to determine a phase so-called slaving of the photoacoustic signal obtained for a so-called slaving wavelength AS of the laser source or sources;
• the laser source(s) comprise(s) electrically pumped lasers, said device comprising a power supply circuit generating a pulsed electrical current called generation pumping the laser source(s), in order to cause the laser source(s) to operate in pulse mode,
• the means processor being connected to the power supply circuit, said power supply circuit being configured to further generate a so-called base current taking non-zero values between laser pulses, and having an intensity lower than the intensity of the generation current at the course of the laser pulses,
• Another object of the invention is a photoacoustic system, comprising: a measuring device according to the invention, a photoacoustic gas detection device having a laser input face, said device being adapted so that a first portion of the laser radiation emitted by at least one laser source illuminates said input face of the photoacoustic gas detection system [0013]
• said cell comprises a laser output face, the at least one laser source and said photoacoustic gas detection device being arranged so that said first portion corresponds to the laser radiation passing through said output face.
• laser output or it comprises an optical component adapted to separate said laser radiation into said first portion directed towards said laser input face of said photoacoustic system and a second portion directed into said cell.
• a cell C containing at least one gas having an absorption line with a central wavelength ⁇ c , laser radiation at a wavelength suitable for the excitation of at least one gas contained in the cell, said laser radiation having a variable wavelength in an oscillatory manner around an average average wavelength at a modulation frequency such that an interaction between the laser radiation and at least one gas contained in the cell induces the generation of a photoacoustic signal, an average wavelength excursion of the laser radiation comprising said central wavelength, said average wavelength changing over time, said cell being sealed by a membrane so as to be impermeable to the gas(es) contained in the cell;
• the measurement of the wavelength is carried out from the calculation of a maximum of a derivative of said evolution of the phase of the photoacoustic signal.
• Figure 1 a schematic view of a device for measuring the power and the wavelength of LL laser radiation by photoacoustic effect according to the invention
• Figure 2 a schematic view of a method for determining the wavelength and the power of laser radiation by photoacoustic effect according to the invention
• Figure 3 a schematic view of a device according to a first embodiment of the invention.
• Figure 4 a schematic view of a device according to a second embodiment of the invention.
• Figure 5 a schematic view of a photoacoustic detection system comprising the device D according to the invention.
• Figure 6 a schematic view of a photoacoustic detection system according to an alternative of the invention.
• Figure 7 a schematic view of a photoacoustic detection system according to one embodiment of the invention.
• FIG. 8 a schematic view of a photoacoustic detection system according to embodiment of the invention
• FIG. 1 schematically represents a device D for measuring the power and the wavelength of laser radiation LL by photoacoustic effect according to the invention.
• the device D comprises a cell C containing at least one gas G which exhibits an absorption line with a central wavelength ⁇ c .
• the device comprises at least one laser source L adapted to emit in the cell C laser radiation LL at a wavelength adapted to the excitation of the gas contained in the cell.
• the device D also comprises an electro-acoustic transducer MP arranged within the cell and adapted to generate a signal Si representative of the photoacoustic signal generated in the cell.
• the transducer makes it possible to determine the amplitude of the acoustic waves and thus to trace the power of the laser radiation, the concentration of the gaseous species studied being known (see below).
• This transducer can for example be a microphone or even a tuning fork. In another embodiment, it may be an acoustic-optical-electrical transducer.
• the laser source is configured so that the laser radiation LL has a variable wavelength in an oscillatory manner around an average average wavelength at a frequency /j of modulation, such such that an interaction between the laser radiation and the gas contained in the cell induces the generation of a photoacoustic signal (PA) at a detection frequency of the electro-acoustic transducer (first condition).
• PA photoacoustic signal
• the laser source L according to the invention is configured so that the average wavelength avg ) evolves over time over a duration T e and so that the excursion in average wavelength includes the central wavelength ⁇ c (second condition).
• This therefore means that the average wavelength varies over a duration T e and takes a plurality of distinct values such that there is a first plurality of values such that and a second plurality of values such that
• the function A avg is for example a ramp function and the function A PA is for example a triangle or square function or even a sinusoidal function.
• the joint realization of these conditions makes it possible to exactly determine the wavelength of the radiation from the phase of the signal PA.
• the laser source is configured so that the laser radiation LL has an optical power which is oscillatory variable around an average power at a modulation frequency, such that an interaction between the laser radiation and the gas contained in the cell induce the generation of a photoacoustic signal at a detection frequency of the electro-acoustic transducer.
• the cell C is sealed by a membrane MB so as to be impermeable to the gas contained in the cell.
• This membrane MB has an optical aperture transparent to the laser radiation emitted by the laser source(s) L.
• transparent it is meant here that the transmission of the membrane is sufficient for the signal-to-noise ratio of the signal Si generated by the transducer MP to be greater than 1.
• This membrane is a transparent wall that does not alter, or negligibly, the geometric properties of the beam during its propagation.
• the device comprises the membrane MB sealing the cavity in order to ensure the presence of the known gas or gases and to allow the device to function as a photoacoustic sensor.
• the concentration of the different gases in the cell is fixed and known and it is possible to determine the power of radiation thanks to the amplitude of the PA signal once the wavelength has been determined with precision (see below ).
• the device D further comprises processing means UT of the electrical signal Si generated by the transducer MP.
• the means of treatment are adapted to determine a measure of the wavelength of the LL radiation generating the PA signal.
• the processing means are configured to determine an evolution of the signal PA over time over a duration T e of evolution of the mean wavelength.
• the means UT are configured to determine an evolution of the phase of the signal PA represented by the signal Si(t).
• the phase can be calculated as follows (Equation 1 ): PA signal Si(t).
• the processing means UT of the electrical signal Si generated by the transducer MP comprise a synchronous detector adapted to simultaneously generate the signal PA and a signal representative of the evolution of the phase of the signal PA.
• the processing means are then configured to calculate the derivative of the evolution of the phase ⁇ i (t) over the duration T e and to determine the maximum of the derivative over this period of time. In a manner known per se, this maximum is reached for a wavelength equal to the central wavelength c of the absorption peak of the gas.
• a so-called servo wavelength A AS is chosen determined by the processing means corresponding to a phase called enslavement.
• This wavelength measurement is accurate to typically 0.01 cm -1 for an absorption peak with a width of 0.15 cm -1 and for a signal to noise ratio for the PA signal detected by the MP transducer of 100. These values are given by way of non-limiting example. This measurement accuracy mainly depends on the gas concentration in the sealed cavity, the laser power entering the cavity, and the performance of the cavity (quality factor and noise).
• the wavelength evolution sequence is repeated a plurality of times so that the processing means achieve an average of the evolutions of the PA signal obtained for each repetition. This makes it possible to obtain a more robust averaged PA signal and thus to improve the signal-to-noise ratio and therefore to improve the precision of the wavelength measurement.
• the concentration C(i) of the various gaseous species G(i) is known and stored in the means UT.
• the power P L of the laser radiation is calculated from the maximum Si max of the amplitude of the signal Si representative of the signal PA and of the concentration C of the gas at from which the PA signal is generated.
• the laser power P L is deduced from the Si max of the amplitude of the signal Si representative of the signal PA and a coefficient K relating to the position of the length A AS with respect to the wavelength T c and the absorption peak shape.
• the device D according to the invention is therefore a low-cost photoacoustic sensor which is an alternative to the power sensor and to the other wavelength determination techniques of the prior art.
• the concentration of the gas or gases G in the cell is greater than 1 ppm. This is necessary to obtain a sufficient signal PA to allow a sufficiently precise determination of the wavelength and of the power P L .
• the concentration of the gas or gases in the cell is greater than 100 ppm in order to be able to measure the fine variations in wavelength of the laser radiation LL.
• the cell C contains a plurality of distinct gases, each comprising at least one absorption line spectrally distinct from the others and the device comprises a plurality of monochromatic laser sources each located at the outside the cell and adapted to illuminate the cell and thus excite an associated gas.
• the device therefore makes it possible, for a plurality of laser sources, to measure the power and the wavelength.
• only one laser source and one gas in the cell will be mentioned for the sake of brevity. It is understood that this choice constitutes a simple example and that all the embodiments of the device according to the invention also apply to the case where the device comprises a plurality of laser and gas sources in the cell C.
• the source laser is adapted to emit a comb of frequencies, each of the frequencies being adapted to illuminate the cell and excite an associated gas.
• Figure 2 illustrates a method of the invention adapted to determine the wavelength and the power of laser radiation by photoacoustic effect. This method is implemented by the device of FIGS. 1 and 3-6.
• This method comprises a first step A implemented by the laser source(s) L consisting in generating, in the cell C, the laser radiation LL at a wavelength adapted to the excitation of at least one gas contained in the cell C.
• the laser radiation LL generated has a variable wavelength ⁇ (t) in an oscillatory manner around a length medium wave average at a modulation frequency fi such that an interaction between the laser radiation and at least one gas contained in the cell induces the generation of the photoacoustic signal at a detection frequency of the electro-acoustic transducer MP.
• the radiation generated has an average wavelength excursion of the laser radiation comprising the central wavelength of the gas absorption line in the cell and the average wavelength evolves over time over a period of time. T e .
• the fact that the wavelength excursion includes the central wavelength ⁇ c is necessary so that the wavelength of the radiation scans the absorption line and that one can thus correctly calibrate the length of wave by associating the maximum of the PA signal with the central wavelength of the absorption line.
• the method of FIG. 2 comprises a step B, after step A, of detection, by the electro-acoustic transducer, of the PA signal generated in the cell.
• Step B includes a step of generating the signal Si representative of the photoacoustic signal in the cell.
• the method comprises a step C for determining an evolution over time of the phase of the photoacoustic signal PA from the detected photoacoustic signal, for the duration T e .
• the determination of the phase can be carried out by a preliminary step of the calculation of the Gabor transform of the PA signal at the modulation frequency and with equation 1.
• this phase measurement is carried out by means of a synchronous detector suitable for simultaneously generating the signal PA and a signal representative of the evolution of the phase of the signal PA.
• the phase ⁇ i (t) is determined by the processing means UT from the signal generated by the transducer MP by any method known to those skilled in the art.
• the method comprises a step D of determining the wavelength of the radiation from the evolution over time of the phase of the photoacoustic signal over the duration T e .
• this determination is made by calculating the maximum of the derivative of the evolution of the phase of the photoacoustic signal over the duration T e , this maximum being obtained for a radiation wavelength equal to the central wavelength ⁇ c .
• the method of Figure 2 includes a final step E of determining the power P L of the laser radiation from the photoacoustic signal and an estimate of the gas concentration.
• Steps C, D and E are implemented by the processing means UT.
• the method of Figure 2 therefore allows the exact determination of the wavelength and then the determination of the laser power P L of the laser radiation generating the signal PA.
• FIG. 3 shows a device according to a first embodiment of the invention in which the laser source or sources are sources electrically pumped by an AC power supply circuit.
• the laser source is a quantum cascade laser or even laser diodes.
• This power supply circuit is connected to the processing means UT and generates a so-called generation pulse electric current pumping the laser source in order to cause the laser source(s) to operate in pulse mode.
• This waveform is given by way of non-limiting example, and any other waveform known to those skilled in the art and allowing the aforementioned conditions on the wavelength is able to be used ( see for example J. Saarela et al, "Wavelength modulation waveforms in laser photoacoustic spectroscopy," Appl. Opt. 48, 743-747 (2009)).
• the AC power supply circuit is configured to further generate, apart from the application of the generation current, a so-called base current taking non-zero values between laser pulses with an intensity lower than the intensity of the generation current during the laser pulses.
• This base current is modulated in intensity in order to generate the oscillatory variation of the wavelength and therefore generate the PA signal.
• the base current is the base current and not the generation current which carries the waveform repeated over the period T e .
• the power supply circuit is further controlled by the processing means and configured so that the base current is modulated in intensity so as to achieve a servo-control of the wavelength of the laser radiation LL on the central wavelength T c .
• This slaving is carried out via the evolution of the phase of the signal PA which constitutes the error signal. Indeed, the phase has a significant slope (maximum derivative) and a linear behavior around the central wavelength ⁇ c which are ideal characteristics for a servo error signal.
• the processing means controlling the power supply circuit are therefore configured so that the AC circuit injects a value of the generation current making it possible to obtain the wavelength T c .
• the servo-control is carried out by the processing means, via feedback electronics, according to conventional servo-control methods, for example, without being restrictive, with PI or PID feedback electronics for Proportional Integral Derivative, alluding to the three modes of action on the feedback electronics error signal.
• This type of feedback making it possible to make the error signal converge towards a fixed value is well known in automatic control.
• the processing means are configured to perform a servo-control on a chosen servo-control wavelength A S -
• the processing means are then configured to perform a servo-control of the AC power supply circuit or of the Temp control device (see figure 4) by maintaining the phase of the photoacoustic signal on the servo phase and therefore maintaining the wavelength of the laser source on the servo wavelength.
• the processing means are suitable for adjusting the servo phase over time when it drifts, this drift possibly being caused by numerous experimental parameters, so that the wavelength is always slaved to the wavelength ⁇ AS -
• FIG. 4 schematically shows a second embodiment of the invention identical to that of Figure 3, except that, here, the servo is performed by the processing means on a control device Temperature temp of the active zone of the laser L.
• the wavelength emitted by the laser L can be expressed as follows:
• the control of the temperature T of the active zone controlled by the processing means UT therefore makes it possible to achieve servo-control of the laser by maintaining the phase of the photoacoustic signal on the servo phase and therefore maintaining the wavelength of the laser source on the servo wavelength A AS .
• the Temp control device is a resistor attached to or placed close to the active zone so as to be able to control its temperature.
• the device Temp is a Peltier system comprising the laser L making it possible to precisely regulate the temperature T of the active zone.
• the temperature control device is the AC power supply circuit of the laser L. Indeed, a higher injection current will heat the active area and will therefore modify the emitted wavelength. It is therefore possible to achieve servo-control of the laser by controlling the temperature of the active zone by means of the power supply circuit.
• One application of the device according to the invention is to allow the control of the wavelength and of the laser power in combination with a gas detection device by photoacoustics.
• FIG. 5 schematically illustrates a photoacoustic detection system DP, comprising the device D according to the invention adapted to measure the wavelength and the power of laser radiation by photoacoustics.
• the DP system includes a conventional DPA photoacoustic gas sensing device having an EL laser entrance face.
• the DPA device is a classic photoacoustic gas detection device.
• the device D comprises an optical window called the laser output face FL traversed by a first portion of the laser radiation emitted by the source L in the cell.
• the device DPA is arranged so that this first portion crosses the face EL and makes it possible to perform gas detection. It is the fact that the cell C is sealed which makes it possible to ensure the presence of the known gas in this cell and allows the operation of the device D as a photoacoustic sensor.
• the device D is therefore a sensor that can easily be integrated into a photoacoustic assembly and which allows the measurement of the power and the wavelength of the laser radiation.
• the device D also makes it possible to achieve precise servocontrol of the laser wavelength on a target value A AS in order to avoid any drift detrimental to gas detection by the device DPA.
• FIG. 6 is an alternative to the embodiment of FIG. 5 in which a beam splitter LS separates the radiation emitted by the source L into a first portion of the laser radiation emitted by the source L towards the laser entry face EL and in the DPA device and in a second portion directed into the cell C.
• this embodiment has the advantage of not degrading the optical properties of the laser beam directed towards the device DPA and reduce constraints in component architecture.
• the DPA device comprises a laser output face and the device D is arranged so that radiation transmitted by the laser output face of the DPA device between in the cell C.
• This embodiment has the advantage of taking into account possible additional degradations of the laser power which could take place in the gas cell of the DPA device, such as an obstruction of the face laser input.
• the measurement of the wavelength and the control from the signal PA detected by the transducer MP is carried out by any method known to those skilled in the art.
• the transducer MP is a microphone, for example a MEMS microphone (for micro electromechanical system) or ECM (for electret condenser microphone).
• the microphone includes an external protective case (or cavity) which includes an acoustic entry zone adapted to let the acoustic waves enter the case in order to be detected by a DP diaphragm (for example a MEMS or ECM diaphragm).
• the box surrounding the microphone MP constitutes the cell C and is impermeable to gas.
• the membrane MB which seals the cell C so as to make it impermeable to the gas(es) contained in the cell thus forms the acoustic entry zone.
• cell C is therefore not a resonant acoustic cavity.
• the inventors have observed that for a concentration of approximately 1000ppm of H2O and an optical power of 1 mW it is not necessary to use a resonant acoustic cell in order to amplify the PA signal. It is understood that these values are given as an example and that another gas with another concentration can be used with a comparable optical power. This makes possible the use of a cell C of much smaller dimension than the resonant acoustic cavities typically used in the prior art.
• the cell C formed by the housing of the microphone MP has dimensions of less than 3 mm by 4 mm by 1.2 mm, even more preferably less than 900 ⁇ m by 300 ⁇ m by 900pm.
• the device of the invention therefore has a very high compactness.
• FIG. 7 schematically illustrates an embodiment in which the microphone MP is a MEMS microphone with a diaphragm DP.
• the device of FIG. 7 comprises an outer casing BE for protecting the diaphragm DP.
• This housing BE comprises the gas G and is sealed by the membrane MB in order to constitute the cell C.
• the device comprises a printed circuit board PCB which comprises the electrical contacts of the microphone and a connection cable DC connecting the microphone to the processing means UT.
• Means of UT processing is for example an ASIC type component (for specific printed circuit or application-specific integrated circuit in English).
• the device comprises an optional sealing layer CE, arranged above the PCB board in order to ensure the tightness of the cell C. This embodiment is very compact.
• FIG. 8 illustrates a preferred embodiment of the invention.
• the membrane MB for sealing the cavity C is formed by an assembly comprising a substrate Sub transparent to laser radiation LL deposited above the printed circuit PCB.
• the printed circuit PCB includes the electrical contacts of the microphone MP which is connected to the processing means UT by the connection cable CC.
• the PCB card has a gap H allowing the transmission of the LL radiation in the cavity C in order to allow the generation of the photoacoustic signal.
• This construction of the membrane makes it possible to obtain a sealed membrane capable of taking up both the electrical contacts of the processing means UT in the microphone and possibly the electrical contacts of the laser L. Indeed, optionally, as shown in FIG. 8, the laser L is attached to the Sub substrate in order to increase the compactness of the device.
• the device comprises welds between the substrate and the PCB board and between the PCB board and the diaphragm DP in order to seal the cell C.
• the welds between the PCB board and the diaphragm DP can be functionalized in order to produce the electrical contacts of the microphone.

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