Method for configuring a photoacoustic detector

Abstract

A method for setting up a photoacoustic detector, the photoacoustic detector comprising: a measurement 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 to 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 includes a setting phase, implemented by the processing unit, allowing at least one acquisition parameter to be defined so as to minimize a measurement error.

Description

Title of the invention: Method for parameterizing a photoacoustic detector. Technical field.

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

[0002] Photoacoustic detection enables the detection of low concentrations of a gaseous species present in trace amounts in a gas or gas mixture. The operating principle is based on the periodic illumination of the gas at a wavelength corresponding to an absorption spectral band of the gaseous species being 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, which generates a pressure wave. The pressure wave is detected by an acoustic transducer. Thus, the detection of the acoustic wave makes it possible to quantify the concentration of the gaseous species in the analyzed gas. Photoacoustic detection allows the design of compact gas sensors for industrial or medical applications.

[0003] Generally, illumination is carried out at 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 that do not have an effective spectral response in the infrared range (for example, ozone, whose strongest absorption 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 measurement chamber, occupied by the analyzed gas. The illumination can be swept in wavelength, which makes it possible to successively address different gaseous species.

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

[0006] Generally, the illumination modulation and demodulation parameters of the signal resulting from the acoustic transducer are defined empirically. The inventors propose a method for determining modulation or demodulation parameters in order to optimize detection performance. The invention particularly addresses a configuration in which the analyzed gas comprises several gaseous species that absorb light in the same spectral band. Description 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 whose concentration is to be determined; - a laser source, configured to illuminate the gas, the laser source being modulated in emission power and / or wavelength, the emission power and its temporal modulation being defined by at least one illumination parameter; - an acoustic transducer, configured to form one or more 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 demodulation of each detection signal, according to a demodulation parameter, and estimate the concentration of the target gaseous species;

[0008] The process being characterized in that: - each illumination parameter and / or each demodulation parameter form acquisition parameters respectively associated with each detection signal; - The process 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) determination of at least one acquisition parameter minimizing the error of measure ;

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

[0012] By determining an acquisition parameter, we mean determining a value of the acquisition parameter or a type of acquisition parameter: It This could include, in particular, 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: - during step a), the measurement error depends on the concentration of the target gaseous species; - during 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 include at least: - the 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 of the 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 the interfering gaseous species, the target gaseous species and the interfering gaseous species absorbing light in the same spectral absorption 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 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 one or more 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, in order 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 process according to the first object 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 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 measurement chamber, under the effect of the modulation of the laser source; - a processing unit, configured to perform 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 the 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, 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;

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

[0028] The process involves a repetition of steps (i) to (iii), such that - during a first iteration, the acquisition parameters are initialized arbitrarily or according to 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 later in this description, in connection with the figures listed below. FIGURES

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

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

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

[0033] Figure 3 shows the evolution of a relative error (ordinate axis) in cdet function of the concentration C 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 ILm (ordinate axis) as a function of the concentration ^det of a target gaseous species (abscissa axis -cm1), and this for different acquisition configurations.

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

[0036] Fig. 4C represents an evolution of a response coefficient (grey 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 measurement error (ordinate axis) as a function of the concentration of a target species (abscissa axis - ppm) taking into account different acquisition parameters respectively.

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

[0040] Figure 6 schematically illustrates the main steps of a process according to the invention. DESCRIPTION OF SPECIFIC EMBODIMENTS

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

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

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

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

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

[0046] The power and emission wavelength of the laser 15 are controlled by the temperature and the supply current I. The latter has a DC component Iq (offset current) and a modulation component 1p modulated at the frequency w. Thus, I

[0047] l(t) = +

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

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

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

[0051] where: - k (mV.cm.W') is a detector response coefficient for the gaseous species considered; - P(t) (W) is the optical power of the laser beam; - t) (cm') is the absorption of the gas at the emission wavelength of the laser.

[0052] The detection signal S(t) comprises an offset signal g0 to which harmonics of different orders are added:

[0053] S(t)= S°+ 5^08(wt) + S2CQ^2wt) (3).

[0054] Where gJ denotes the jth harmonic. Each harmonic corresponds to j times the modulation frequency, where j is a positive integer. In expression (3), we have limited ourselves to the first two harmonics.

[0055] The first harmonic cjl can be considered representative of the first derivative of the absorption of the gaseous species with respect to the wavelength. The second harmonic çp 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-order harmonic.

[0057] Figure 2A shows the evolution of the laser emission power of an example QCL laser source (curve a: left y-axis - unit mW) as a function of the supply current intensity (x-axis - unit A) and the evolution of the wavenumber of the emitted light beam (curve b: right y-axis - unit cm1) as a function of θ(t). It can be observed that modulating the supply current j(f) results in both a modulation of the emission power and the wavenumber. The laser supply threshold is 0.4 A.

[0058] Figure 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 to the wavenumber (cm¹). The two gaseous species considered exhibit significant absorption in the same spectral absorption band.

[0059] Among these two gaseous species, there is a gaseous species to be detected, called the target gaseous species, of concentration ^det in the analyzed gas, and a gaseous species called interfering gas, with a concentration Cmt in the analyzed gas. In the example shown, the gas to be detected is Sarin gas (international designation Sarin gas - GB), with a concentration of 1 ppm, and the interfering gas is CO2, with a concentration of 500 ppm. The dashed curve represents the absorption spectrum of CO2, which can be modeled, in a narrow spectral band on the order of 1 cm⁴, by a Lorentzian function (solid line). The double arrow shows an example of wavenumber variation during modulation of the laser's supply current.

[0060] 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, N successive detection signals S can be acquired, N being greater than or equal to 2. Lowering the detection limit by accumulating detection signals is a known approach.

[0061] Each detection signal S is parameterized by acquisition parameters, which affect the illumination of the gas by the laser and the processing of the detection signal, more specifically the demodulation. The acquisition parameters include: - the intensity of the offset current Zq7; - the intensity of the modulation current I; - the h harmonic; taken into account during demodulation: first harmonic or second harmonic, or higher-order harmonic. The value of 11; is j, where j denotes the rank of the harmonic previously defined in relation to (3).

[0062] An important aspect of the invention is that at least two detection signals can be acquired by taking into account at least one different acquisition parameter.

[0063] Each detection signal S; can be explained according to expression (2), taking into account an assumption of linearity with respect to the concentrations Cdet and ^'int.

[0064] -^deti ( h; ) (4)

[0065] Where and j are the detector response coefficients for the acquisition parameters ( ly, h; ), respectively with respect to the target species and the interfering species.

[0066]

[0067] By taking into account N detection signals, we obtain a direct model: , ^det,N ■^int,l [ Gdet , L ^int, ^int,N, -G ^det ^mt.

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[0074] (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. The concentrations [^detl] are obtained by inverting the direct model, for example by a method £ int J of the least squares type. The inversion of the direct model is performed by processing unit 20. The following developments aim to determine a variable G^det, the minimization of which allows us to define the optimum acquisition parameters for the target gaseous species. The variable G^det (unit ppm) is an error in estimating the concentration C^det and has three components, described below: - is the variance of the noise in the measurement chain, which includes the Acoustic transducer 15, demodulation implemented by the processing unit, analog-to-digital conversion. The noise in the measurement chain is considered to follow a Gaussian distribution, with variance °e. The variance is independent of the concentration of the gaseous species. The noise in the measurement chain is an additive term in the detection signal. ae can, for example, be taken as equal to 10 pV. - (mV unit), corresponds to the measurement noise resulting from the power supply electrical of the laser light source. The noise has two components, which correspond respectively to the continuous component Iq and the modulation component 1^. By differentiating (2) with respect to Iq and I and taking into account a maximum value of the interfering gaseous species, we obtain expressions (6) and (7). is determined beforehand. This is a value of a maximum concentration of ^int. ZTr — / y T (I ir _i_ |^121 | rmax dIQ |^det ' ] â / 0 / (6) CTr = cr r (I —| r _i_ | I max] (7) And = (8)

[0075] and are respectively the variances of the noises associated with the components Iq and 1^, these noises are considered to follow a centered Gaussian distribution. We can take, for example, = 2mA and = 1 mA.

[0076] In the case of CO2 in ambient air, = 10000 ppm. This concentration is to be compared with the concentrations usually found in unpolluted air, outdoors (500 ppm) or in a closed inhabited room (2000 ppm). - is an additive noise resulting from the presence of the interfering species. This is not statistical noise, but an additive component, considered as a bias.

[0077]

[0078] The three components % and are combined to form & det according to:

[0079] (1°) adet~ kdet

[0080] Figure 3 shows (curve a) a ratio of the relative error °det (axis of Cdet ordinates) as a function of CrfetXaxis of abscissas - unit ppm), taking into account N= l,h= 1 (first harmonic), IQ = 0.44A, I1= 0.048A.

[0081] Figure 3 also represents different contributions to: Cdet - curve b: , as a function of ^det Cdet - curve c: -Ss- , as a function of ^det ^det - curve d: üi , as a function of ^det %et

[0082] Figure 3 also shows a 100% curve, for which the relative error adet _ -i, forming a limit of use. It is considered that it is not Cdet 1 it is conceivable to take measures for adet > d Cdet — 1

[0083] Two ranges can be defined on the curve representing the relative error £*2 in cdet as a function of ^det: on a first range, corresponding to low concentrations ^det, ^£1 tends towards , which means that ^det tends towards a constant G, such that Cdet C that

[0084] (11) C = £ Aiet

[0085] On a second range, which corresponds to high concentrations ^det, the error relative °^t tends towards another constant D, such that: Cdet I0O86J (12) D=----

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[0103] At high concentrations, the predominant source of error is . Cdet We can define a limiting concentration C^et, between low and high concentrations, such that: r* — c ^det Taking into account N detection signals respectively parameterized by at minus one different acquisition parameter, we can write / , ii (15), with: = (6^6) G is the detector response matrix, of dimension (N, 2), as defined in link with (5); int is the equivalent of & det for the species of interest. It is calculated by implementing expressions (6) to (10) taking into account a maximum concentration ^et For the species to be detected; hold are correlation terms ' S is a diagonal covariance matrix of measures, of dimensions (N, N), such that that SN.N, (16), with = cre2 + (Jj j2 (17). The unit of si,i is mV2. (j^ is the term for the measure of rank J, with 1 < 1 < N In the matrix S, the crossed terms si,j with ij, are nuisance because the measures are independent. From (15), taking into account the fact that the matrix is ​​invertible for N > 2, we obtain, for N = 2; (18) ^detj ct are the detector responses for the parameters ( 1qly, 11^) respectively with respect to the target species and the interfering species, described in relation to expression (5). In expression (18), G det depends on Pæ" the term taken into account in si,i, cf. expressions (17), (10), and (6) to (8). Expression (18) allows us to define a measurement configuration that minimizes & det- By measurement configuration, we mean the measurement parameters (Iqj, In, for each of the N measurements. In this example, N = 1 or N = 2. In other words, (¼ hi) = argmin(o-det) Ay / u A , the number N being fixed beforehand.

[0104] The optimal acquisition parameters are those minimizing θdet. These parameters are obtained by implementing a minimization algorithm. The minimization algorithm uses stored values ​​of θ4 and θdet. It is recalled that when extracting the first harmonic, it is considered, as a first approximation, that θdet and θdet depend on the derivative of absorption with respect to wavelength. When extracting the second harmonic, θdet and θdet depend on the second derivative of absorption with respect to wavelength.

[0105] Thus, for each harmonic, we have stored values ​​of and stored in memory 21. The values ​​^det dkdet gkirit akillt can be dIQ ' dly ' dl() > dly also stored. The values ​​of and k^^, or their derivatives with respect to 1^ or Iq are obtained by knowing the emission parameters of the laser source as a function of 1^ and Iq: wave number and light power (see curve 2A).

[0106] The input data of the algorithm are C^x (or a range of variation of ^int, a range of variation of ^det, ae, and ^s,!, these values ​​being fixed by the user according to the device used.

[0107] The input data may also include a range of variation of 1^, of Iq, of N as well as the harmonics potentially usable to demodulate the detection signal: harmonic of rank 1, of rank 2 or possibly of higher rank. Application example

[0108] Equation (18) was implemented to define optimal measurement configurations taking into account GB (sarin gas) and CO2 as the target gaseous species and interfering gaseous species. Different configurations were tested. Each configuration involved a single measurement (N = 1) or two successive measurements (N = 2).

[0109] When N = 1, & det was calculated as a function of (10). When N = 2, ^det was calculated as a function of (18), under the assumption that at least one parameter of each of the N measurements is different.

[0110] Five configurations were tested: - configuration 1: N = 1 - measurement taking into account the first harmonic ^1; configuration 2 harmonic; configuration 3 harmonic h1; 4th harmonic configuration; : N = 1 - measure taking into account the second : N = 2 - measures taking into account the first : N = 2 - measures taking into account the second Configuration 5: N = 2 - measures taking into account the first and the second harmonic hi and h^, - Laser wave number: 1049.665 cm 1; - Optical power of the laser depends 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⁻¹ / A. (see curve b of [Fig. 2A]). This is also a characteristic considered standard for a commercial QCL laser; - Iq varying between 0.4 and 0.6 A, according to 500 regularly spaced discretization steps; - J j varying between 0.001 and 0.05A according to 250 regularly spaced discretization steps; C™* = 104 PPm Oe = 10 p V ; - = 2mA; - = 1 mA.

[0111] Figure 4A shows the relative error as a function of ^det for different Cdet values 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] Figure 4A also shows a 100% curve, for which — 1, forming a usage limit. It is considered that it is not feasible Cdet 1 to take measures to > i.

[0113] According to the results in Figure 4A, the best configuration is the third configuration (curve c), for which the ratio , corresponding to % and The relative measurement error is minimal for all concentrations ^det- La 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 exhibits correct performance in terms of relative error.

[0115] Figure 4B shows the optimal intensities Iq j, Iq^, lu, 2 For the third configuration, as a function of the concentration ^det- We observe that the optimal intensities, i.e. the intensities minimizing , vary as a function of the Cdet concentration ^det-

[0116] A noteworthy piece of information is that the DC component Io of the laser supply current varies between two optimal values ​​Iq j, Iq^ between the two measurement configurations: a first value Iq j depends on the concentration ^det, while the second value Iq 2 can be considered, at least to the first order, as independent of the concentration Cdet-

[0117] Thus: for Cdet < 0.02 ppm, IQ1= Iq^i 0.425 A; for 0.02 ppm < C det <200ppm, I 01 = I o l2 = O.5A; for Cdet > 0.02 ppm, I01 = I013 = Q.6A. - Iq,2 = 0 - 6 A and this regardless of ^det, which corresponds to the power Maximum optics. = 2 = 0.05 A: Whatever ^det, which corresponds to the amplitude of maximum modulation taken into account.

[0118] Thus, during a measurement, which here arbitrarily corresponds to the first measurement, the optimal intensity Iq takes three different values ​​Iq 11, Iq 12 and Iq 13 depending on the concentration ^det- It will be understood that it is equivalent for the intensity Iq j to be constant and equal to 0.6 A and for the intensity Iq 2 to depend on the concentration Cdet-

[0119] Figures 4C and 4D show the different values ​​of the response coefficients as a function of the intensity Iq (ordinate axis) and 1^ (abscissa axis). The different values ​​of Iq 1, Iq 12, Iq 13, and Iq 2 described in relation to Figure 4B are also shown, given that the optimal value of 1^ is 0.05 mA for each measurement configuration.

[0120] It is observed that the optimal Iq 2 value corresponds to a maximum response coefficient kde^ for the target gaseous species (GB) and a minimum response coefficient for CO2.

[0121] When cdet < 0.02 ppm, the values ​​Jq 1 = A) 11ct A,2 correspond to currents in which the values ​​are respectively opposite. The detection signal, being formed from the first harmonic, corresponds to the derivative of the absorption of the interfering species (CO2) with respect to the wavelength. The value of Iq ja is presumed to have been retained because it corresponds to a range of values ​​of jfjnt in which the derivatives I a^det I and l^det I are small, which contributes to IIII minimize (cf. expressions (6) to (8)).

[0122] When 0.02 ppm < Cdet < 200 ppm, the value Iq 1= -A 12 corresponds to a value close to 0, and to a higher coefficient than when cdet< 0.02 ppm.

[0123] When Cdet > 200 ppm, Io Io 2.

[0124] The results show that using different configurations is more suitable for low Cdet concentrations (i.e., Cdet < 200 ppm) than for high concentrations. At high concentrations, i.e., when Cdei > 200 ppm, two measurements are taken with the same parameters, which amounts to obtaining a gain, in terms of signal-to-noise ratio, solely related to the measurement statistics.

[0125] Figure 5A represents, for different concentrations Cdet (abscissa axis) a ratio between: determined for two measures based on the first harmonic, taking into account the acquisition parameters described in connection with figures 4A to 4D; - J determined for a single measurement based on the first harmonic, taking into account the optimal acquisition parameters for each concentration Cdet,

[0126] It is observed that for high concentrations, the ratio ^i) tends towards -L, this 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 note that for low concentrations, the ratio hi) moves away, by decreasing, from the limiting value of -L, particularly when Cdet — 10 ^2 ppm, and even more so 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 ^det (x-axis), of the ratio (y-axis) for the J configurations, taking into account fixed acquisition parameters for all ^det values. Curve a corresponds to taking into account the acquisition parameters defined for ^det = 103 ppm. Curve b corresponds to taking into account the acquisition parameters defined for ^det = 103 ppm. Here we see the advantage of adapting the acquisition parameters according to the concentration ^det: the optimal parameters at the concentration 103 ppm (curve a) lead to an increase in the relative error for low concentrations C^et. Conversely, the parameters cdet Optimal values ​​at a concentration of 103 ppm (curve a) lead to an increase in the relative error for high concentrations ^det- Cdet

[0129] Figure 6 summarizes the main steps of a process implementing the invention.

[0130] During a step 100, a gaseous 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 N of measurements, each measurement being parameterized by laser source feed parameters. In this example, the parameters are the DC component law and the amplitude of the modulation component Iy.

[0132] During a step 120: the detection signal resulting from the detector is demodulated according to a predefined 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 using predefined 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 demodulating the detection signal, maximum number of measurements, concentration ranges of each gaseous species (or maximum value of the concentration of a gaseous species). This allows obtaining an analytical expression of the measurement error ^det, such as (18).

[0135] Step 90 is a measurement error minimization step C” in order to identify the optimum acquisition parameters, i.e. the laser 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 ^det in the example described above. Either we have a priori knowledge of the value of ^det to be measured, or at least of a range of values, in which case the acquisition parameters are defined according to this prior knowledge.

[0137] When no prior knowledge is available, steps 110 to 120 can be performed iteratively by adjusting the acquisition parameters between successive iterations according to the obtained concentration C-rfet. During a first iteration, the acquisition parameters are selected arbitrarily or randomly, or based on a prior knowledge. A first estimate of ^det- is obtained. Steps 110 to 120 are then repeated, so that the concentration ^det of an iteration of rank q-1 is used to select the acquisition parameters of the next iteration of rank q. The iterations continue until a predetermined number of iterations or when the value G det 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 trade-off between the detection errors. The cost function to be minimized can, for example, combine the measurement errors of each gaseous species.

Claims

Demands

1. A method for parameterizing 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 whose concentration is to be determined; - a laser source (15), configured to illuminate the gas, the laser source being modulated in emission power and / or wavelength, the emission power and its temporal modulation being defined by at least one illumination parameter; - an acoustic transducer, configured to form one or more 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 process being characterized in that: - each illumination parameter and / or each demodulation parameter form acquisition parameters respectively associated with each detection signal; - The process includes a parameterization phase, implemented by a minimization algorithm, 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 measurement error; (c) for each detection signal, selection of the acquisition parameter determined in step (b).

2. The method according to 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. A 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. A method according to any one of the preceding claims, wherein for each detection signal, the acquisition parameters include 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. Method according to claim 4, wherein the demodulation harmonic is chosen from a first harmonic, at the modulation frequency, or a second harmonic, at twice the modulation frequency.

6. A method according to claim 5, wherein - 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 one of the preceding claims, 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 one of the preceding claims, wherein step c) is implemented using established response functions for the target gaseous species, as a function of at least one acquisition parameter.

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

10. A method according to 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 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 one or more 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 it is configured to implement a minimization algorithm, programmed to execute steps a) to c) of a method according to any one of the preceding claims, to define at least one acquisition parameter, chosen from an illumination parameter and / or a demodulation parameter.

13. Photoacoustic detector, comprising:

14. - a measuring chamber, intended to be occupied by a gas, the gas comprising at least one target gaseous species whose concentration we wish to determine; - a laser source, configured to illuminate the gas, the laser source being modulated in emission power and / or 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 measurement chamber, under the effect of the modulation of the laser source; - a processing unit, configured to perform 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. A method for estimating the 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 for each detection signal; - the acquisition parameters of each detection signal are defined for different concentration ranges of the target gaseous species; the process involves a repetition of steps (i) to (iii), such that - during a first iteration, the acquisition parameters are initialized arbitrarily or according to 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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