Method for setting up a photoacoustic detector

Abstract

Method for configuring a photoacoustic detector, the photoacoustic detector comprising: a measuring chamber (10), intended to be occupied by a gas; a laser source (15), configured to illuminate the gas; an acoustic transducer; a processing unit (20), configured to perform a demodulation of each detection signal, according to a demodulation parameter, and estimate the concentration of a target gaseous species; the method being characterized in that: each illumination parameter and / or each demodulation parameter form acquisition parameters respectively associated with each detection signal; the method comprises a parameterization phase, implemented by the processing unit, making it possible to define at least one acquisition parameter so as to minimize a measurement error.

Description

Title of the invention: Method for setting the parameters of a photoacoustic detector Technical field

[0001] The technical field of the invention is photoacoustic detection. PREVIOUS ART

[0002] Photoacoustic detection allows detection of a low concentration of a gaseous species, present in a gas or mixture of gases, in trace amounts. The operating principle is based on periodic illumination of the gas at a wavelength corresponding to an absorption spectral band of the gaseous species sought. The illumination can be pulsed or, more generally, modulated in amplitude and / or wavelength, according to a predetermined modulation frequency. The illumination leads to periodic heating of the gas, the latter generating a pressure wave. The pressure wave is detected by an acoustic transducer. Thus, the detection of the acoustic wave makes it possible to quantify a concentration of the gaseous species in the analyzed gas. Photoacoustic detection allows the design of compact gas sensors for uses in the industrial or medical fields.

[0003] Generally, the illumination is carried out in a wavelength in the near or mid-infrared range, typically between 0.8 pm and 12 pm. These wavelengths correspond to vibrational absorptions (pure or compound) of specific chemical bonds contained in the gas molecule(s) (for example: CH, CO, NH, SO, CC, CF...). Visible or ultraviolet wavelengths are also used for gases which do not have an effective spectral response in the infrared range (for example, ozone will be used, the strongest absorption of which is in the UV-C band around 0.25 pm).

[0004] The illumination is modulated in amplitude and / or wavelength according to a modulation frequency generally between 100 Hz and 50 kHz. The modulation frequency depends in particular on the geometry of a measuring chamber, occupied by the gas analyzed. The illumination can be subject to wavelength scanning, which makes it possible to successively address different gaseous species.

[0005] The determination of the concentration of the gaseous species assumes a demodulation phase of the detection signal resulting from the photoacoustic transducer. The demodulation is carried out according to a harmonic, corresponding to the modulation frequency or to an integer multiple of the modulation frequency.

[0006] Generally, the parameters for modulating the illumination and demodulating the signal resulting from the acoustic transducer are defined empirically. The inventors propose determining modulation or demodulation parameters, so as to optimize the detection performance. The invention particularly addresses a configuration according to which the analyzed gas comprises several gaseous species, absorbing light in the same spectral band. Statement of the invention

[0007] A first object of the invention is a method for parameterizing a photoacoustic detector, the photoacoustic detector comprising: - a measuring chamber, intended to be occupied by a gas, the gas comprising at least one target gaseous species for which a concentration is to be determined; - a laser source, configured to illuminate the gas, the laser source being modulated in emission power and / or in wavelength, the emission power and its temporal modulation being defined by at least one illumination parameter; - an acoustic transducer, configured to form a detection signal or several successive detection signals, each detection signal being representative of a modulation of a pressure in the measuring chamber, under the effect of the modulation of the laser source; - a processing unit, configured to perform a demodulation of each detection signal, according to a demodulation parameter, and estimate the concentration of the target gaseous species;

[0008] the method being characterized in that: - each illumination parameter and / or each demodulation parameter forms acquisition parameters respectively associated with each detection signal; - the method includes a parameterization phase, implemented by the processing unit, comprising the following steps:

[0009] (a) definition of a measurement error as a function of at least one parameter acquisition;

[0010] (b) determining at least one acquisition parameter minimizing the error of measure ;

[0011] (c) for each detection signal, selection of each acquisition parameter determined in step (b).

[0012] By determining an acquisition parameter, we mean determining a value of the acquisition parameter or a type of acquisition parameter: It may in particular be a value of an illumination parameter or a type of demodulation parameter.

[0013] According to one possibility: - the processing unit is configured to estimate the concentration of the target gaseous species using several detection signals; - at least one acquisition parameter of a detection signal is different from the acquisition parameter of another detection signal.

[0014] Thus, each detection signal is obtained differently from at least one other detection signal, or even from the other detection signals: the illumination is different and / or the demodulation is different. Preferably,

[0015] According to one possibility: - in step a), the measurement error depends on the concentration of the target gaseous species; - in step b), the minimization of the error is carried out for several concentration ranges of the target gaseous species; - during step c), at least one acquisition parameter is different for two concentration ranges of the target gaseous species.

[0016] According to one possibility, for each detection signal, the acquisition parameters comprise at least: - an emission power of the laser source; - and / or a modulation amplitude of the emission power of the laser source; - and / or a demodulation harmonic of the detection signal.

[0017] The demodulation harmonic can be chosen from a first harmonic, at the modulation frequency, or a second harmonic, at twice the modulation frequency.

[0018] According to one possibility: - the processing unit takes into account two detection signals to estimate the concentration of the target gaseous species; - each detection signal is demodulated according to the first harmonic; - the illumination parameters for each detection signal are different. The transmission power and / or the modulation amplitude of the transmission power may be different for each detection signal.

[0019] According to one possibility, steps a) to c) are implemented by considering at least two detection signals demodulated by the processing unit to estimate the concentration of the target gaseous species.

[0020] Step c) can be implemented using response functions established for each gaseous species, as a function of at least one acquisition parameter.

[0021] According to one embodiment, the gas comprises the target gaseous species and another gaseous species, called interfering, the target gaseous species and the interfering gaseous species absorbing light in the same absorption spectral band. The measurement error may depend on the concentration of the target gaseous species and the concentration of the interfering gaseous species.

[0022] According to one embodiment, the measurement error depends on the concentration of the target gaseous species and the concentration of the interfering gaseous species.

[0023] According to one embodiment, step c) is implemented using response functions established for the target gaseous species and for the interfering gaseous species, as a function of at least one acquisition parameter.

[0024] A second object of the invention is a photoacoustic detector, comprising: - a measuring chamber, intended to be occupied by a gas, the gas comprising at least one target gaseous species whose concentration is to be determined; - a laser source, configured to illuminate the gas, the laser source being modulated in emission power and / or in wavelength, the emission power of the laser source and its temporal modulation being defined by at least one illumination parameter; - an acoustic transducer, configured to form a detection signal or several successive detection signals, each detection signal being representative of a modulation of a pressure in the measuring chamber, under the effect of the modulation of the laser source; - a processing unit, configured to perform a demodulation of each detection signal, so as to estimate the concentration of the target gaseous species, the demodulation of the signal being defined by a demodulation parameter; - the detector being characterized in that an acquisition parameter, chosen from an illumination parameter and / or a demodulation parameter, is defined by implementing steps a) to c) of a method according to the first subject of the invention.

[0025] A third object of the invention is a photoacoustic detector, comprising: - a measuring chamber, intended to be occupied by a gas, the gas comprising at least one target gaseous species whose concentration is to be determined; - a laser source, configured to illuminate the gas, the laser source being modulated in emission power and / or in wavelength, the emission power of the laser source and its temporal modulation being defined by at least one illumination parameter; - an acoustic transducer, configured to form several successive detection signals, each detection signal being representative of a modulation of a pressure in the measuring chamber, under the effect of the modulation of the laser source; - a processing unit, configured to perform a demodulation of each detection signal, to estimate the concentration of the target gaseous species, the demodulation of the signal being defined by a demodulation parameter; - the detector being characterized in that an acquisition parameter, chosen from an illumination parameter and / or a demodulation parameter, is different for two successive detection signals.

[0026] A fourth object of the invention is a method for estimating a concentration of a target gaseous species, using a photoacoustic detector according to the second or third object of the invention, the method comprising: - (i) illumination of the gas occupying the measuring chamber using the source laser, depending on an illumination parameter; - (ii) during step (i), formation of a detection signal by the acoustic transducer; - (iii) demodulation of one or more detection signals, each detection signal being demodulated according to a demodulation parameter, so as to estimate a concentration of the target gaseous species;

[0027] the method being such that: - each illumination parameter and each demodulation parameter form acquisition parameters of each detection signal; - the acquisition parameters of each detection signal are defined for different concentration ranges of the target gaseous species;

[0028] the method comprises a reiteration of steps (i) to (iii), so that - during a first iteration, the acquisition parameters are initialized arbitrarily or according to an a priori relative to the concentration of the target gaseous species; - during a second iteration, the acquisition parameters are selected according to the concentration of the target gaseous species resulting from the previous iteration.

[0029] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0030] [Fig. 1] describes an example of a photoacoustic detector.

[0031] [Fig.2A] represents electro-optical characteristics of a laser source. This is the optical power of a laser beam emitted by the laser source (left ordinate axis - unit mW) as well as the wave number of the laser beam (right ordinate axis: unit cm1) as a function of the intensity of a supply current of the laser source.

[0032] [Fig.2B] shows the absorption spectral bands of two gaseous species as a function of the wave number (x-axis - cm1). The y-axis corresponds to a linear absorption coefficient (y-axis - cm1).

[0033] Figure 3 shows an evolution of a relative error (y-axis) in Cdà function of the concentration cdet of a target gaseous species (abscissa axis - ppm), as well as the evolution of different components of the relative error.

[0034] Figure 4A shows an evolution of a relative error (y-axis) in cdet function of the concentration cdet of a target gaseous species (x-axis - cm1), and this for different acquisition configurations.

[0035] Figure 4B represents an evolution of the continuous component (left ordinate axis - unit A) and the amplitude of the modulation component (right ordinate axis - unit A) of the supply current of a laser as a function of the concentration cdet of a target gaseous species (abscissa axis - ppm).

[0036] [Fig.4C] represents an evolution of a response coefficient (gray level) of a photoacoustic detector for a gaseous species, in this case GB, as a function of the intensity of the continuous component (ordinate axis - unit A) and the amplitude of the modulation component (abscissa axis - unit A) of the supply current of a laser.

[0037] [Fig.4D] represents an evolution of a response coefficient (grey level) of a photoacoustic detector for another gaseous species, in this case CO2, as a function of the intensity of the continuous component (ordinate axis - unit A) and the amplitude of the modulation component (abscissa axis - unit A) of the supply current of a laser.

[0038] [Fig.5A] shows a ratio of a measurement error (y-axis) as a function of the concentration of a target species (x-axis - ppm) taking into account different acquisition parameters respectively.

[0039] [Fig.5B] shows an evolution of the relative measurement error (y-axis) as a function of the concentration of a target species (x-axis - ppm), for two measurement configurations.

[0040] [Fig. 6] shows schematically the main steps of a method according to the invention. PRESENTATION OF PARTICULAR EMBODIMENTS

[0041] Figure 1 represents a photoacoustic detector 1. The device comprises a measuring chamber 10 configured to be occupied by a gas to be analyzed. The measuring chamber 10 is configured to be exposed to a laser beam L modulated in amplitude and / or in frequency according to a modulation frequency f. The laser beam L is emitted at a wavelength corresponding to an absorption wavelength of a gaseous species likely to be present in the gas. Under the effect of the absorption, the illuminated gas heats up, heating being modulated according to the modulation frequency of the laser beam. The modulated heating causes a succession of compressions - expansions of the gaseous species. This causes a pressure variation in the measuring chamber. The laser beam is emitted by one or more laser sources 15.

[0042] The device comprises an acoustic measuring transducer 11 allowing measurement of a pressure variation AP(f) in the measuring chamber under the effect of the modulation of the laser beam. The pressure variation AP(Z) is modulated according to the modulation frequency f.

[0043] The laser is for example of the QCL type (Quantum Cascade Laser), this type of laser being well suited to integration into compact measuring devices. The emission wavelength is infrared, typically between 3 and 12 pm. In this spectral range, many gaseous species have absorption lines.

[0044] The device comprises a processing unit 20, configured to process the signal coming from the detector or from each detector, according to processing parameters, and to estimate a concentration of the gaseous species. The processing unit may in particular comprise a microprocessor.

[0045] The device may comprise a control unit 25, configured to control emission parameters of the laser, for example the optical power or the amplitude of the modulation.

[0046] The power and the emission wavelength of the laser 15 are controlled by the temperature and the supply current I. The latter comprises a continuous component A) (offset current) and a modulation component îj, modulated at the frequency w. Thus, I [°° 47 1 xo= / o+^cosWœ

[0048] The processing unit is connected to the measuring transducer and to the possible reference transducer. The processing unit is configured to form and process a detection signal S(t) representative of the pressure variation AP(Z) in the measuring chamber.

[0049] The detection signal can be expressed in the form:

[0050] S(t) =kP(t)p(t) (2)

[0051] where: - & (mV.cm.W ') is a detector response coefficient for the species gaseous considered; - (W) is the optical power of the laser beam; - fi (J) (cm1) is the absorption of the gas at the laser emission wavelength.

[0052] The detection signal S(t) comprises a 5° offset signal to which harmonics of different orders are added:

[0053] 5(0=^ + ^cos (wt) + S2cos(2w) (3).

[0054] Where $7 denotes the harmonic of rank j. Each harmonic corresponds to j times the modulation frequency, where j is a positive integer. In expression (3), we are limited to the first two harmonics.

[0055] The first harmonic S1 can be considered as representative of the first derivative of the absorption of the gaseous species with respect to the wavelength. The second harmonic S2 can be representative of the second derivative of the absorption of the gaseous species with respect to the wavelength, assuming a linear variation of the absorption coefficient of the molecule in the applied modulation.

[0056] The processing unit is configured to demodulate the detection signal, so as to extract the first harmonic or the second harmonic, or a higher harmonic.

[0057] Figure 2A represents the evolution of the laser emission power of an example of a QCL laser source (curve a: left-hand ordinate axis - unit mW) as a function of the power supply current intensity / ( / ) (abscissa axis - unit A) as well as the evolution of the wave number of the emitted light beam (curve b: right-hand ordinate axis - unit cm1) as a function of / ( / ). It is observed that the modulation of the power supply current causes both a modulation of the emission power and of the wave number. The laser power supply threshold is 0.4 A.

[0058] [Fig.2B] shows two absorption spectra of two gaseous species likely to be present in a gas to be detected, the latter typically being air. The ordinate axis corresponds to an absorption coefficient (cm1) and the abscissa axis is the wave number (cm1). The two gaseous species considered exhibit significant absorption in the same absorption spectral band.

[0059] Among these two gaseous species there is a gaseous species to be detected, called the target gaseous species, of concentration cdet in the analyzed gas, and a gaseous species called the interfering species, of concentration cint in the analyzed gas. In the example shown, the gaseous species to be detected is Sarin gas (international name Sarine gas - GB), with a concentration of 1 ppm, and the interfering gaseous species is CO2, with a concentration of 500 ppm. The dotted curve represents the absorption spectrum of CO2, which can be modeled, in a narrow spectral band of the order of 1 cm4, by a Lorentzian function (solid curve). The double arrow shows an example of variation of the wave number during modulation of the laser supply current.

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[0068] In this example, we have two gaseous species: a "target" species, to be detected, mixed with an interfering species. In order to lower the detection limit, A' successive detection signals S, can be acquired, where A is greater than or equal to 2. Lowering the detection limit by accumulating detection signals is a known approach. Each detection signal is parameterized by acquisition parameters, which affect the illumination of the gas by the laser and the processing of the detection signal, more precisely the demodulation. The acquisition parameters include: - the intensity of the offset current ^oj; - the intensity of the modulation current / u; - the harmonic h{ taken into account during demodulation: first harmonic or second harmonic, or higher harmonic. The value of h is j, where j denotes the rank of the harmonic previously defined in relation to (3). An important aspect of the invention is that at least two detection signals can be acquired taking into account at least one different acquisition parameter. Each detection signal S{ can be explained according to expression (2), taking into account a hypothesis of linearity with respect to the concentrations cdet and cmt. ~ ^detj ( ^0 / ' ) Ciet Where Ædet j and are the detector response coefficients for the parameters acquisition ( Iqj, ly, h- ), respectively with respect to the target species and the interfering species. Taking into account A detection signals, we obtain a direct model: s / Av. ^det,N Cdet Glct ^int (5) G is a detector response matrix for the set of N parameters respectively associated with each detection signal, of dimension (N, 2). The response matrix depends on the acquisition parameters. Obtaining the concentrations [cdet L ^int. is performed by inversion of the direct model, for example by a method of type

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[0078] least squares. The inversion of the forward model is performed by processing unit 20. The following developments aim to determine a variable whose minimization makes it possible to define the optimum acquisition parameters for the target gaseous species. The variable adet (unit ppm) is an estimation error of the concentration cdet. ^det has three components, described below: - ae is the variance of the noise of the measurement chain, which includes the acoustic transducer 15, the demodulation implemented by the processing unit, the analog-digital conversion. The noise of the measurement chain is considered to follow a Gaussian distribution, with variance The variance is independent of the concentration of the gaseous species. The noise of the measurement chain is an additive term in the detection signal. can for example be taken equal to 10 pV. - (unit mV), corresponds to the measurement noise, resulting from the power supply electrical noise of the laser light source. The noise ai has two components, which correspond respectively to the continuous component and the modulation component By deriving (2) with respect to 70 and I\, and taking into account a maximum value of the interfering gaseous species, we obtain expressions (6) and (7). Cj^x is previously determined. It is a value of a maximum concentration of Cm. ( I dk.^ II j \ (6 ) "L | dlu pdet+ j dlü pto / / I dkApl II dk^. I \ H) ( | ” pdet + | I And 'AL and are respectively the variances of the noises associated with the C and A components, these noises are considered to follow a centered Gaussian distribution. We can take for example = 2mA and ~ 1 mA. In the case of CO2 in ambient air, c1^ = 10000 ppm. This concentration should be compared with the concentrations usually found in unpolluted air, outdoors (500 ppm) or in a closed living room (2000 ppm). - ^int is an additive noise resulting from the presence of the interfering species. This is not a statistical noise, but an additive component, considered as a bias. (9) The three components ae and are combined to form adet according to:

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[0093] ^det ^and In Figure 3, we have represented (curve a) a ratio of the relative error (axis of the cdet ordinates) as a function of ^.(axc of the abscissas - ppm unit), taking into account = 1, h = 1 (first harmonic), - 0.44A, Ix- 0.048A . Figure 3 also represents different contributions to 2*t: cdet - curve b: 2*, as a function of cda ^det - curve c:, as a function of cdet * det - curve d: , as a function of '"sky In Figure 3, a 100% curve is also shown, for which the relative error 2*t _ |, forming a limit of use. It is considered that it is not possible to carry out measurements for 2*1 > i cdet — 1 We can define two ranges on the curve representing the relative error 2*1 as a function of cdet; on a first range, corresponding to low concentrations cda, tends towards 2*1, which means that cdet tends towards a constant C, such that cdet C +£i>n c = K" 'Met On a second range, which corresponds to high concentrations cdei, the relative error 2*1 tends towards another constant D such that: cda At high concentrations, the predominant source of error is 42.. A concentration limit can be defined between low concentrations and high concentrations, such as: r* ~ L t cdet ~ Ds Taking into account N detection signals respectively parameterized by at minus a different acquisition parameter, we can write ^deC & detains^ , > < xl (15), with . . & detains ^ini G is the detector response matrix, of dimension (N, 2), as defined in relation to (5); ^int is the equivalent of adet for the species of interest. It is calculated by implementing expressions (6) to (10) taking into account a maximum concentration Cdet* for the species to be detected; ^detained are correlation terms'

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[0105] S is a diagonal measurement covariance matrix of dimensions (N, N), such that that 0 (16), with Sy = (Te2 + (7 / p (17). The unit of sü is mV2. c / r is the term A for the measure of rank i, with 1< 1< N In the matrix E, the cross terms S'J with i * j, are null because the measures are independent. From (15), taking into account the fact that the matrix (g?E4G) is invertible for N > 2, we obtain, for / V = 2: W (18) -,__»1.1 >22_____________________________ 4^ Aôii.2] ^^«.2 V [—+— +“ .M.-K7-+-5T— J ^deti ct ^intj are the detector responses for the parameters ( Içy, [y, ) respectively with respect to the target species and the interfering species, described in connection with expression (5). In expression (18), ^det depends on cdet by the term taken into account in SU, cf. expressions (17), (10), and (6) to (8). Expression (18) defines a measurement configuration that minimizes adef By measurement configuration, we mean the measurement parameters ( ly, h; ) for each of the N measurements. In this example, N = 1 or N = 2. In other words, =argmin( Au'AjA \, the number N being previously fixed. The optimal acquisition parameters are those minimizing adet. They are obtained by implementing a minimization algorithm. The minimization algorithm uses stored values of Ant,i and k^We recall that when extracting the first harmonic, we consider, as a first approximation, that Ant and Aiet depend on the derivative of absorption as a function of wavelength. When extracting the second harmonic, Ant and kda depend on the second derivative of absorption as a function of wavelength. Thus, for each harmonic, we have memorized values of kmt and Ata, stored in memory 21. The values 3^ can be 3 / 0 ' a ' din ' also stored. The values of Ant and Ata, or their derivatives with respect to A or A) are obtained by knowing the emission parameters of the laser source as a function of Ii and Aj: wave number and light power (see curve 2A).

[0106] The input data of the algorithm are (or a variation range of cint, a variation range of cdet, and these values being fixed by the user according to the device used.

[0107] The input data may also include a variation range of / ], of 4, of N as well as the harmonics potentially usable for demodulating the detection signal: harmonic of rank 1, of rank 2 or possibly of higher rank. Example of application

[0108] Expression (18) was implemented to define optimal measurement configurations considering GB (sarin gas) and CO2 as target gas species and interfering gas species. Different configurations were tested. Each configuration included a single measurement (N = 1) or two successive measurements (N = 2).

[0109] When A = 1, ^det was calculated according to (10). When N = 2, adet was calculated according to (18), under the assumption that at least one parameter of each of the N measurements is different.

[0110] Five configurations were tested: - configuration 1:^=1 - measurement taking into account the first harmonic 2 harmonic configuration ^2; 3 harmonic hx configuration; 4 harmonic h2 configuration; : N = 1 - measurement taking into account the second : N = 2 - measurements taking into account the first AT = 2 - measures taking into account the second configuration 5: N = 2 - measurements taking into account the first and the second harmonic h} and h2-, Laser wave number: 1049.665 cm 1; Optical power of the laser depending on the intensity of the supply current according to a slope of 0.3 W / A (see curve a of [Fig.2A]); this corresponds to the characteristics of a commercial QCL laser. Wavenumber of the laser beam depending on the intensity of the supply current according to a slope of -10 cm ' / A. (see curve b of [Fig.2A]). This is also a characteristic considered standard for a commercial QCL laser; Lj varying between 0.4 and 0.6 A, according to 500 discretization steps regularly spaced; - I[ varying between 0.001 and 0.05A according to 250 regularly spaced discretization steps; «x=104ppm; = 10 pV ; = 2mA ; ^=lmA.

[0111] Figure 4A shows the relative error as a function of cdet for different cdels different configurations tested: curve a: configuration 1; curve b: configuration 2; curve c: configuration 3; curve d: configuration 4; curve e: configuration 5. Curves a and d, corresponding respectively to configurations 1 and 4, are superimposed.

[0112] In Figure 4A, a 100% curve is also shown, for which - |, forming a usage limit. It is considered that it is not possible to carry out measurements for > i. cdet —

[0113] From the results of Figure 4A, we observe that the best configuration is the third configuration (curve c), for which the ratio corresponding to cdet the relative measurement error is minimal for all cdet concentrations. The optimal configuration corresponds to two successive measurements, each being carried out by demodulating the detection signal according to the first harmonic.

[0114] For high concentrations, the fifth configuration (curve e) also shows good performance in terms of relative error.

[0115] Figure 4B shows the optimal intensities / 0,i, A12, ^1,2 for the third configuration, as a function of the cda concentration. We observe that the optimal intensities, i.e. the intensities minimizing 2^, vary as a function of the cdn concentration.

[0116] A remarkable piece of information is that the continuous component h of the laser supply current varies between two optimal values / 0.1, ^0.2 between the two measurement configurations: a first value / 0.1 depends on the concentration cdet^ while the second value / 0.2 can be considered, at least to the first order, as independent of the concentration cdet.

[0117] Thus: - for Cdet - 0-02 ppm, ^0.1= ^0.1.1 0.425 A; - for 0.02 ppm cJel < 200 ppm, Iqa = ^0.1.2 = 0.5 A; - for Cdet - 0-02 ppm, ^o,i = ^0,1,3 = 0.6 A. - A),2= 0.6 A and this whatever cdet, which corresponds to the optical power maximum. - h,2 = h,2 = 0.05 A: Whatever cdet, which corresponds to the amplitude of maximum modulation taken into account.

[0118] Thus, during a measurement, which here corresponds, arbitrarily, to the first measurement, the optimal intensity Axi takes three different values ^0.13 in function of the concentration cdet. It will be understood that it is equivalent that the intensity ^0.1 is constant and equal to 0.6 A and that the intensity ^0.2 depends on the concentration cdet.

[0119] In Figures 4C and 4D, the different values of the response coefficients are represented as a function of the intensity Aj (ordinate axis) and 1} (abscissa axis). The different values ^o, Lb ^0.12 and A), 1.3 and / 0.2 described in connection with Figure 4B have also been positioned, knowing that the optimal value of / 1 is 0.05 mA for each measurement configuration.

[0120] It is observed that the optimal value / 0.2 corresponds to a maximum response coefficient £det for the target gaseous species (GB) and a minimum response coefficient Æint for CO2.

[0121] When cdet < 0.02 ppm, the values fo,j = and / 0,2 correspond to currents in which the values &int are respectively opposite. The detection signal being formed from the first harmonic, &int corresponds to the derivative of the absorption of the interfering species (CO2) with respect to the wavelength. It is assumed that the value of ^0,1,1 was retained because it corresponds to a range of values of in which the derivatives | I and I ^and I are small, which contributes to minimizing (see expressions I di0 | ja / ] I (6) to (8)).

[0122] When 0.02 ppm cdet < 200 ppm, the value hi - h.1,2 corresponds to a value of ^int close to 0, and to a coefficient &det higher than when cdet < 0.02 ppm.

[0123] When - 200 ppm, ~ ^0.2.

[0124] The results show that the use of different configurations is more suitable for low cda concentrations (i.e. cdet < 200 ppm) than for high concentrations. At high concentrations, i.e. when c^et - 200 ppm, two measurements are carried out with the same parameters, which amounts to obtaining a gain, in terms of signal-to-noise ratio, solely linked to the measurement statistics.

[0125] Figure 5A represents, for different concentrations c <iet (axe des abscisses) un ratio entre : determined for two measurements based on the first harmonic, taking into account the acquisition parameters described in connection with Figures 4A to 4D; determined for a single measurement based on the first harmonic, taking into account the optimal acquisition parameters for each cdet concentration.

[0126] We observe that for high concentrations, the ratio 111) tends towards -L, which which corresponds to a value due to the measurement statistics: it is expected that when the number of measurements is doubled, according to the same acquisition parameters, the theoretical gain in terms of signal to noise ratio is

[0127] It is interesting to observe that for low concentrations, the ratio ''WAA) moves away, by decreasing, from the limit value of -L, in particular when cdet < 10 ppm, and even more when cdet < 1 ppm. This shows that performing two acquisitions taking into account two different parameters provides an additional gain compared to the purely statistical gain.

[0128] Figure 5B shows the evolution, as a function of cdet (x-axis), of the ratio (y-axis) for the configurations (z, h Y taking into account eda v*l' '*V fixed acquisition parameters for all cda values. Curve a corresponds to taking into account the acquisition parameters defined for edM = 103 ppm. Curve b corresponds to taking into account the acquisition parameters defined for ^det = 103 ppm. Here we see the interest of adapting the acquisition parameters according to the cd concentration: the optimal parameters at the concentration 103 ppm (curve a) lead to an increase in the relative error for low cdet concentrations. ^det Conversely, the optimal parameters at the 103 ppm concentration (curve a) lead to an increase in the relative error for high cdet concentrations. ^det

[0129] [Fig.6] summarizes the main steps of a method implementing the invention.

[0130] During a step 100, a gas mixture, comprising at least two species gaseous is introduced into a photoacoustic detector.

[0131] During a step 110, the gas mixture is subjected to a number A7 of measurements, each measurement being parameterized by power supply parameters of the laser source. In this example, the parameters are the continuous component / 0; and the amplitude of the modulation component lu-

[0132] During a step 120: the detection signal resulting from the detector is demodulated, according to a previously defined harmonic. The demodulation makes it possible to obtain an estimate of the concentration of each gaseous species, or of at least one gaseous species present in the mixture.

[0133] Steps 110 and 120 are implemented from previously defined acquisition parameters, for example for different expected concentration ranges of each gaseous species. The acquisition parameters are established during calibration phases 80 and 90.

[0134] During phase 80, values or ranges of values of measurement parameters are defined: laser illumination parameters, harmonics used for demodulation of the detection signal, maximum number of measurements, concentration ranges of each gaseous species (or maximum value of the concentration of a gaseous species). This makes it possible to obtain an analytical expression of the measurement error adet, such as (18).

[0135] Step 90 is a step of minimizing the measurement error adet, so as to identify the optimum acquisition parameters, i.e. the laser power supply parameters and / or the demodulation parameters, in particular the choice of the harmonic. Cf. (19).

[0136] It has been observed that the optimum parameters can vary depending on the concentration of at least one species to be detected, in this case cdet in the example previously described. Either we have an a priori on the value of cdet to be measured, or at least on a range of values, in which case the acquisition parameters are defined according to this a priori.

[0137] When no a priori is available, steps 110 to 120 can be carried out iteratively by adjusting, between two successive iterations, the acquisition parameters as a function of the concentration cdet obtained. During a first iteration, the acquisition parameters are selected arbitrarily or randomly, or on the basis of an a priori. A first estimate of cdet is obtained. Steps 110 to 120 are then repeated, so that the concentration cdet of an iteration of rank q-1 is used to select the acquisition parameters of the following iteration of rank q. The iterations follow one another until a predetermined number of iterations or when the value cda is stabilized.

[0138] Although described in connection with a "target" species to be detected mixed with an interfering species, the method can be applied to an estimation of the concentrations of two different target species.

[0139] In this case, the parameters are established to optimize the detection errors of the two gaseous species, taking into account a compromise between the detection errors. The cost function to be minimized can, for example, combine the measurement errors of each gaseous species.< / iet>

Claims

Claims

1. Method for setting a photoacoustic detector (1), the photoacoustic detector comprising: - a measuring chamber (10), intended to be occupied by a gas, the gas comprising at least one target gaseous species of which it is desired to determine a concentration; - a laser source (15), configured to illuminate the gas, the laser source being modulated in emission power and / or in wavelength, the emission power and its temporal modulation being defined by at least one illumination parameter; - an acoustic transducer, configured to form a detection signal or several successive detection signals, each detection signal being representative of a modulation of a pressure in the measuring chamber, under the effect of the modulation of the laser source; - a processing unit (20), configured to perform a demodulation of each detection signal, according to a demodulation parameter, and estimate the concentration of the target gaseous species; the method being characterized in that: - each illumination parameter and / or each demodulation parameter forms acquisition parameters respectively associated with each detection signal; - the method includes a parameterization phase, implemented by the processing unit, comprising the following steps: (a) definition of a measurement error as a function of at least one acquisition parameter; (b) determination of at least one acquisition parameter minimizing the measurement error; (c) for each detection signal, selection of the acquisition parameter determined during step (b).

2. The method of claim 1, wherein: - the processing unit is configured to estimate the concentration of the target gaseous species using several detection signals; - at least one acquisition parameter of a detection signal is different from said acquisition parameter of another detection signal.

3. Method according to any one of claims 1 or 2, wherein - in step a), the measurement error depends on the concentration of the target gaseous species; - in step b), the minimization of the error is carried out for several concentration ranges of the target gaseous species; - in step c), at least one acquisition parameter is different for two concentration ranges of the target gaseous species.

4. Method according to any one of the preceding claims, in which for each detection signal, the acquisition parameters comprise at least: - an emission power of the laser source; - and / or a modulation amplitude of the emission power of the laser source; - and / or a demodulation harmonic of the detection signal.

5. The method of claim 4, wherein the demodulation harmonic is selected from a first harmonic, at the modulation frequency, or a second harmonic, at twice the modulation frequency.

6. Method according to claim 5, in which - the processing unit takes into account two detection signals to estimate the concentration of the target gaseous species; - each detection signal is demodulated according to the first harmonic; - the illumination parameters for each detection signal are different.

7. A method according to any preceding claim, wherein steps a) to c) are implemented by considering at least two of the detection signals demodulated by the processing unit to estimate the concentration of the target gaseous species.

8. A method according to any preceding claim, wherein step c) is implemented using response functions established for the target gaseous species, as a function of at least one acquisition parameter.

9. Method according to any one of the preceding claims, in which the gas comprises the target gaseous species and another gaseous species, called interfering, the target gaseous species and the interfering gaseous species absorbing light in the same spectral absorption band.

10. The method of claim 9, wherein the measurement error depends on the concentration of the target gaseous species and the concentration of the interfering gaseous species.

11. A method according to any one of claims 9 or 10, wherein step c) is implemented using response functions established for the target gaseous species and for the interfering gaseous species, as a function of at least one acquisition parameter.

12. Photoacoustic detector, comprising: - a measuring chamber, intended to be occupied by a gas, the gas comprising at least one target gaseous species whose concentration is to be determined; - a laser source, configured to illuminate the gas, the laser source being modulated in emission power and / or in wavelength, the emission power of the laser source and its temporal modulation being defined by at least one illumination parameter; - an acoustic transducer, configured to form a detection signal or several successive detection signals, each detection signal being representative of a modulation of a pressure in the measuring chamber, under the effect of the modulation of the laser source; - a processing unit, configured to perform a demodulation of each detection signal, so as to estimate the concentration of the target gaseous species, the demodulation of the signal being defined by a demodulation parameter; - the detector being characterized in that an acquisition parameter, chosen from an illumination parameter and / or a demodulation parameter, is defined by implementing steps a) to c) of a method according to any one of the preceding claims.

13. Photoacoustic detector, comprising: - a measuring chamber, intended to be occupied by a gas, the gas comprising at least one target gaseous species whose concentration is to be determined; - a laser source, configured to illuminate the gas, the laser source being modulated in emission power and / or in wavelength, the emission power of the laser source and its temporal modulation being defined by at least one illumination parameter; - an acoustic transducer, configured to form several successive detection signals, each detection signal being representative of a modulation of a pressure in the measuring chamber, under the effect of the modulation of the laser source; - a processing unit, configured to perform a demodulation of each detection signal, to estimate the concentration of the target gaseous species, the demodulation of the signal being defined by a demodulation parameter; - the detector being characterized in that an acquisition parameter, chosen from an illumination parameter and / or a demodulation parameter, is different for two successive detection signals.

14. A method of estimating a concentration of a target gaseous species, using a photoacoustic detector according to any one of claims 12 or 13, the method comprising: - (i) illumination of the gas occupying the chamber of measurement using the laser source, according to an illumination parameter; - (ii) during step (i), formation of a detection signal by the acoustic transducer; - (iii) demodulation of one or more detection signals, each detection signal being demodulated according to a demodulation parameter, so as to estimate a concentration of the target gaseous species; the process being such that: - each illumination parameter and each demodulation parameter form acquisition parameters of each detection signal; - the acquisition parameters of each detection signal are defined for different concentration ranges of the target gaseous species; the method comprises a repetition of steps (i) to (iii), so that - during a first iteration, the acquisition parameters are initialized arbitrarily or according to an a priori relative to the concentration of the target gaseous species; - during a second iteration, the acquisition parameters are selected according to the concentration of the target gaseous species resulting from the previous iteration.

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