Photoacoustic measurement method with wavelength tuning of a light source

Abstract

The invention relates to a method for the photoacoustic detection of at least one gas using a photoacoustic sensor (1) that comprises at least one resonant cavity (6) configured to receive the gas and at least one coherent light source (8) powered by a current source, the method comprising the steps of generating a first pulse train having a first duty cycle (DC1) and powering the light source (8) so as to emit radiation centred on a first wavelength into the cavity (6), and generating a second pulse train having a second duty cycle (DC2) and powering the light source (8) so as to emit radiation centred on a second wavelength different from the first wavelength into the cavity (6). The current is intensity-modulated at a first modulation frequency f1 for the first pulse train and at a second modulation frequency f2 for the second pulse train.

Description

[0001] Description

[0002] Title: Photoacoustic measurement method with wavelength tuning of a light source

[0003] Technical field

[0004] The present invention relates to the field of photoacoustic measurements for the detection or concentration of one or more gases in a gaseous medium.

[0005] The invention relates more specifically to a photoacoustic measurement method implementing a photoacoustic sensor comprising a light source whose wavelength is tuned in order to precisely target one or more absorption peaks of the gas(es).

[0006] Prior art

[0007] The principle of gas measurement by photoacoustic effect is based on the generation of an acoustic wave in the gas by a light source such as a continuous or pulsed laser, which can be modulated in amplitude or wavelength. The wavelength of the radiation generated by the light source, for example in the mid-infrared (MIR), near-infrared (NIR), or the visible or ultraviolet range, is chosen to interact specifically with the molecules of the gas to be detected by targeting an absorption peak of the gas. Since the emission of the light source is modulated, the energy absorbed by the gas molecules is released in the form of transient heating which generates a pressure wave, itself measured by an acoustic detector such as a microphone.

[0008] Although the photoacoustic effect has been known for a long time, its implementation for gas measurement has been made possible by the use of monochromatic light sources such as lasers, and sensitive microphones such as electret capacitive microphones.

[0009] Detection is improved by confining the gas in a cavity and modulating the laser at a frequency close to the acoustic resonance frequency of the cavity. The resulting acoustic background amplitude is directly related to the concentration of the desired gaseous compound in the gas present in the excited cavity.

[0010] The detection efficiency relies largely on the efficient coupling of the radiation from the light source with the gas contained in the resonant cavity because the measured signal is proportional to the energy absorbed, then dissipated, by the gas.

[0011] Articles [1] and [2] describe the principle of photoacoustic measurement.

[0012] Patent FR 3019653 B1 describes an example of a photoacoustic sensor.

[0013] Another example of a photoacoustic sensor is illustrated in Figure 1. The sensor 1 comprises a gas inlet 2 and an outlet 3 fluidly connected respectively to a capillary 4 and a capillary 5. The capillaries 4, 5 are each fluidly connected to two resonant cavities 6, 7. Each cavity forms an acoustic resonator. One of the cavities or both cavities can be illuminated by at least one light source 8 configured to emit coherent light whose central wavelength corresponds to an absorption peak of a gas that is to be detected. Each cavity is coupled to a microphone 9, 10 configured to measure the acoustic signals produced by the thermal dissipation of the energy absorbed by the gas.

[0014] A photoacoustic measurement by this sensor takes place as follows. A gaseous medium containing one or more gases to be detected is introduced into the sensor through the gas inlet 2. The gaseous medium spreads into the cavities 6, 7. Then, one of the cavities or both cavities are illuminated by radiation whose emission peak is centered on a wavelength corresponding to an absorption peak of a gas.

[0015] If only one cavity is illuminated, the other cavity can be used to measure the ambient noise and thus perform a differential measurement improving the signal-to-noise ratio of the measurement. Alternatively, the two cavities can be illuminated at two different wavelengths to perform two simultaneous measurements.

[0016] The excitation of the gas causes the propagation of acoustic waves in the gaseous medium which are measured by the microphones 9, 10. The concentration of the gas can be deduced from this measurement.

[0017] Preferably, the light source is powered by a current source of adjustable intensity, which makes it possible to modulate the intensity of the light source. The intensity of the light source is advantageously modulated at a frequency close to the resonance frequency of the cavity of the photoacoustic sensor, preferably in an interval of plus or minus 10% around this resonance frequency. This interval depends in particular on the quality factor Q of the resonator, which is typically between Q = 1 and Q = 10 but can reach Q = 1000. For example, for a quality factor Q = 10, the interval is preferably plus or minus 0.5% around the resonance frequency because beyond this the signal loss is rapid. For a quality factor Q = 1, the interval is preferably plus or minus 5%.

[0018] Modulating the intensity of the light source at a given frequency makes it possible to extract the gas concentration measurement by demodulating the acoustic signal measured by the microphones at this given frequency.

[0019] The demodulation of the acoustic signal is carried out at the modulation frequency of the light source, with a typical bandwidth interval between plus or minus 10 Hz and plus or minus 1 Hz, depending on the signal integration time, typically from 0.1 s to 1 s and up to 10 s.

[0020] The cavities of a photoacoustic sensor are typically configured to have a resonant frequency between 1 kHz and 20 kHz. For example, for a cavity having a resonant frequency of 5 kHz and a resonator quality factor of Q = 1, the modulation of the intensity of the light source is typically carried out at a frequency between 4700 Hz and 5300 Hz. For a resonator quality factor of Q = 5, the modulation can be carried out at a frequency between 4950 Hz and 5050 Hz.

[0021] Figure 2 is an example of the response of a photoacoustic sensor to excitation and represents the signal voltage measured by the cavity microphone as a function of the frequency of the acoustic wave generated in the cavity. The resonance peak of the sensor is at 5 kHz and the resonance range extends from approximately 4700 Hz to 5300 Hz.

[0022] The light source can be configured to emit continuous radiation or pulsed radiation. In both cases, the intensity of the radiation is modulated around an average value .

[0023] Figures 3 and 4 illustrate the modulation of the intensity of continuous radiation and pulsed radiation respectively. Patent FR 3084746 B 1 describes a photoacoustic sensor implementing a method for modulating the wavelength of the radiation from a laser source. The periodic modulation of the wavelength around a central value aims to limit the parasitic acoustic signals due to the interaction between the laser radiation and the walls of the resonant cavity. To do this, the modulation parameters of the current supplying the pulsed laser excitation of the laser are optimized. The current is modulated by two envelopes, one low to cause the wavelength modulation at the acoustic frequency, the other high, triangular in shape, to compensate for the optical power modulation linked to the variations in heating of the laser by the low modulation. The two envelopes can be phase shifted but remain at the same modulation frequency.

[0024] Gas detection using a photoacoustic sensor requires good agreement between the wavelength of the light source and the targeted absorption peaks. In the case of a laser, this can be achieved by adjusting the operating current and / or the laser temperature. Typically, the average value of the current supplied to the light source is adjusted while keeping the temperature of the latter constant.

[0025] Thus, if one wants to modify the wavelength of the light source, for example, to target different absorption peaks, it is possible to modify the current supplying the light source. However, this operation can be problematic. The time for the current to stabilize at the new setpoint is linked to the thermal equilibrium of the system to maintain a constant wavelength and can be long, typically between 2 s and 10 s, because a PID feedback loop is required to keep the temperature of the light source constant to within 0.1 °C. The current supplying the light source induces power dissipation and a local temperature change. However, to allow the measurement of different absorption peaks, the control of the light source must be able to adapt quickly to different current values.

[0026] There is therefore a need to improve existing photoacoustic measurement methods, particularly to enable rapid modification of the wavelength of the light source.

[0027] The aim of the invention is to meet at least part of this need.

[0028] Presentation of the invention To this end, the invention relates to a method for photoacoustic detection of at least one gas, implementing a photoacoustic sensor comprising at least one resonant cavity configured to receive the gas and a coherent light source powered by a current source, comprising the steps of generating by means of the current source: a / a first pulse train having a first duty cycle and powering the light source, so as to emit in the resonant cavity radiation centered on a first wavelength, b / a second pulse train having a second duty cycle different from the first duty cycle and powering the light source, so as to emit in the resonant cavity radiation centered on a second wavelength different from the first wavelength,the current supplying the light source being intensity modulated at a first modulation frequency fi for the first pulse train and at a second modulation frequency fi for the second pulse train, the modulation preferably being sinusoidal, triangular or rectangular in shape.,

[0029] Thus, the invention essentially consists of modifying the duty cycle of the power supply current of the light source in order to adjust the wavelength of the radiation generated by the latter. The invention takes advantage of the property of certain light sources according to which the wavelength of the radiation generated depends on the duty cycle of the power supply.

[0030] The light source thus generates pulsed radiation whose duty cycle is identical to that of its supply current and whose wavelength is adjustable by modifying the duty cycle.

[0031] Further pulse trains can be generated following the second pulse train.

[0032] By "duty cycle" we mean the ratio between the duration t pu ise of a pulse and the time Tcyde between two successive pulses.

[0033] According to an advantageous feature, the temperature of the light source is maintained within an interval of plus or minus 0.1 °C around an initial value and / or the average intensity of the current generated by the current source is kept constant. This makes it possible to control the emitted wavelength solely by adjusting the duty cycle. This also makes it possible to control the average optical power emitted for the photoacoustic measurement. The photoacoustic signal is therefore proportional to the optical power and the concentration of the gas to be measured.

[0034] Preferably, the light source is a laser, in particular a quantum cascade laser or an interband cascade laser. The method is not, however, limited to these light sources alone. Any light source generating radiation whose wavelength depends on the duty cycle of its supply current may be suitable. In particular, any type of light source comprising a stack of layers that contribute to the stimulated emission and confinement of the light may be suitable for modifying the emitted wavelength as a function of the duty cycle.

[0035] According to a particular embodiment, the first and second pulse trains are generated simultaneously, preferably in phase shift so that the pulses of the second train are between two pulses of the first train. This prevents the pulses of the two trains from overlapping. Preferably, the first and second pulse trains have a low duty cycle, in particular less than or equal to 5%, for example 1% and 5% respectively, and are in phase shift so as to allow the pulse trains to be well separated. The first and second pulse trains are also intensity modulated at different frequencies. Thus, the demodulation of the signal allows the two spectral signatures to be well separated.

[0036] Alternatively, the second pulse train is emitted after the first pulse train, preferably immediately after.

[0037] Preferably, the amplitude of the modulation is less than or equal to 20%, 10% or 5% of a maximum value of the current and / or greater than or equal to 0.05%, 0.1% or 1% of a maximum value of the current.

[0038] In a first variant, the first and second modulation frequencies fi and f'2 are different. Preferably, the frequency difference is greater than or equal to 1 / Tint, where Tint represents the integration time of the signal during demodulation. Tint is typically between 0.1 s and 1 s, or even 10 s. For example, if 0.1 s of signal are integrated to perform the demodulation, the frequency difference between fi and fi may be 10 Hz or more. For an integration time of 1 s, the frequency difference may be 1 Hz or more.

[0039] The use of different modulation frequencies advantageously facilitates signal processing by performing demodulations at different frequencies, which allows the simultaneous or quasi-simultaneous measurement of several absorption peaks.

[0040] In a second variant, the first and second modulation frequencies fi and f'2 are identical.

[0041] The first and second modulation frequencies fi and fi are advantageously close to the resonance frequency of the cavity of the photoacoustic sensor, preferably within an interval of plus or minus 10% around this resonance frequency, in particular within an interval of plus or minus 5% or plus or minus 1%. As explained in the preamble, the optimal interval depends in particular on the quality factor Q of the resonator. According to a preferred characteristic, the first duty cycle and the second duty cycle are between 1% and 80%, preferably greater than or equal to 2% and / or less than or equal to 50%.

[0042] The duration t pu The duration of a pulse can be between 50 ns and 1000 ns. The time T cy The delay between two pulses can be between 1 ps and 20 ps.

[0043] Preferably, the first and second pulse trains have a duration of between 30 ms and 10 s, preferably greater than or equal to 500 ms and / or less than or equal to 5 s.

[0044] Preferably, the first and second modulation frequencies are at least ten times smaller than the pulse frequency of the first and second pulse trains. For example, the modulation frequencies may be of the order of 10 kHz and the pulse frequency of the order of 100 kHz.

[0045] The duration of the pulse trains, which can be equal or different between the two trains, can be determined in particular according to the concentration of the gases to be detected. A longer pulse train in fact makes it possible to improve the signal-to-noise ratio of the measurement.

[0046] Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0047] [Fig 1] Figure 1 represents an example of a state-of-the-art photoacoustic sensor.

[0048] [Fig 2] Figure 2 illustrates the response of a photoacoustic sensor as a function of the excitation frequency.

[0049] [Fig 3] Figure 3 is a graph showing the temporal modulation of the current intensity supplying a light source emitting continuous radiation.

[0050] [Fig 4] Figure 4 is a graph showing the temporal modulation of the current intensity supplying a light source emitting pulsed radiation.

[0051] [Fig 5] Figure 5 represents a pulse train having a given duty cycle.

[0052] [Fig 6] Figure 6 represents a pulse train having a different duty cycle than that shown in Figure 5.

[0053] [Fig 7] Figure 7 represents three emission spectra of a quantum cascade laser obtained by varying the duty cycle of the generated pulse trains.

[0054] [Fig 8] Figure 8 represents the absorption spectra of several gases.

[0055] [Fig 9] Figure 9 represents pulse trains generated by a first embodiment of the method according to the invention.

[0056] [Fig 10] Figure 10 represents pulse trains generated by a second embodiment of the method according to the invention.

[0057] Detailed description

[0058] Figures 1 to 4 have already been described in the preamble and will not be commented on below.

[0059] Figures 5 and 6 show two current pulse trains feeding a light source, the two pulse trains having a different duty cycle.

[0060] In the example of Figure 5, pulses of constant intensity have a duration t pu ise and are repeated at a time interval T C yde, defining a duty cycle DC1 = t pu ise / Tcyde. In the example of Figure 6, pulses of the same duration t pu are repeated at a time interval T cy cie twice as short as in the example in Figure 1, thus defining a duty cycle DC2 = 2 * DC1.

[0061] The light source powered by these pulse trains generates pulsed radiation with a duty cycle identical to that of the supply current.

[0062] The invention takes advantage of the properties of certain light sources that generate radiation whose wavelength is in particular dependent on the duty cycle of the supply current. This is the case, for example, of quantum cascade lasers (QCL) or interband cascade lasers (ICL).

[0063] Thus, it is possible to adjust the wavelength of the generated radiation only by adjusting the duty cycle of the supply current. It is not necessary to change the intensity of the supply current or the temperature of the source. Therefore, wavelength tuning of the radiation from the light source can be done in a very short time.

[0064] This makes it possible in particular to limit energy consumption when seeking to carry out several successive measurements targeting different absorption peaks, which is particularly advantageous when the photoacoustic sensor is embedded on an energy-limited mobile support such as a drone.

[0065] Figure 7 illustrates three emission spectra generated by the same quantum cascade laser. Only the duty cycle of the laser power supply is changed between the three spectra: the laser temperature remains constant and the power supply current intensity is unchanged. In this example, the duration of a pulse is fixed at t pu ise = 50 ns and the duty cycles are 40%, 60% and 80% respectively.

[0066] Preferably, pulse durations t are used pu ise between 50 ns and 1000 ns and cycle times T C yde between 1 ps and 20 ps.

[0067] It is observed that the emission peak of the generated radiation is adjustable over a range of several cm' 1 thanks to the modification of the duty cycle.

[0068] This allows this emission peak to be precisely adjusted over a wide range of values ​​in order to target a particular absorption peak.

[0069] Figure 8 shows for reference the absorbance of different gases at a concentration typical of that found in the atmosphere: nitrous oxide at a concentration of 1 ppm, carbon dioxide at a concentration of 400 ppm, water vapor at a concentration of 10,000 ppm and carbon monoxide at a concentration of 1 ppm. Targeting a particular absorption peak requires precise control of the wavelength of the photoacoustic sensor's light source.

[0070] A photoacoustic measurement method according to the invention uses a photoacoustic sensor known as such. The light source of the sensor is used in pulsed mode and the duty cycle of its supply current is adjustable by varying the duration of a pulse and / or the time interval between two successive pulses.

[0071] Thus, a first pulse train is generated with a first duty cycle, determining a first emission wavelength targeting a first absorption peak. A second pulse train is generated, following the first pulse train or simultaneously, preferably with a phase shift, with a second duty cycle determining a second emission wavelength targeting a second absorption peak.

[0072] Of course, other pulse trains can still be generated, with other duty cycles or with duty cycles identical to those of the first and / or second pulse train.

[0073] For given values ​​of supply current, light source temperature and duty cycle, the emission of radiation having a specific wavelength is obtained.

[0074] Figure 9 illustrates a first example of implementation of the method according to the invention.

[0075] In this example, a first modulated pulse train having a first duty cycle DC1 is first generated by the current source supplying the light source.

[0076] This pulse train has a typical duration of 100 ms to 10 s.

[0077] The modulation of the current intensity is carried out at a modulation frequency fo typically close to the resonant frequency of the resonant cavity of the sensor or half of this resonant frequency.

[0078] The first pulse train is followed by a second modulated pulse train with a second duty cycle DC2. During both pulse trains, the maximum value Imax of the supply current and the temperature of the light source are maintained.

[0079] The second pulse train has a typical duration of between 100 ms and 10 s. It is modulated at the same frequency fo as the first pulse train in the illustrated example, but could be modulated at a different frequency.

[0080] Other pulse trains may optionally follow the second pulse train.

[0081] Each pulse train allows the wavelength of the excitation radiation to be adjusted, thus precisely targeting a specific absorption peak in the gaseous medium under study.

[0082] Figure 10 illustrates a second example of implementation of the method according to the invention.

[0083] In this example, a first pulse train with a typical duration of 30 ms to 10 s is generated by the current source supplying the light source with a first duty cycle DC1 and modulated at a first frequency fi.

[0084] A second pulse train is generated following the first pulse train. The second train, typically lasting 30 ms to 10 s, is generated with a second duty cycle DC2 and modulated at a second frequency fi.

[0085] The frequencies fi and fi are typically close to the resonant frequency of the photoacoustic sensor cavity or half of this frequency. For example, the values ​​of fi and fi are between 4700 Hz and 5300 Hz for a cavity resonant frequency of 5 kHz.

[0086] Advantageously, the use of different modulation frequencies for the different pulse trains facilitates the measurement and correct identification of several absorption peaks. Indeed, the processing of the signal by simultaneous demodulations for each modulation frequency used allows the quasi-simultaneous measurement of several absorption peaks, which may correspond to different peaks of a gas or to peaks of different gases. Thus, the duration of the pulse trains can be reduced to the minimum necessary to obtain a satisfactory signal-to-noise ratio.

[0087] Of course, other pulse trains with different modulation frequencies and duty cycles can still be generated after the second pulse train. Other variants and improvements can be provided without departing from the scope of the invention.

[0088] List of cited documents [1] Miklôs, A., Hess, P., & Bozôki, Z. (2001). Application of acoustic resonators in photoacoustic trace gas analysis and metrology. Review of Scientific Instruments, 72(4), 1937-1955. doi:10.1063 / 1.1353198

[0089] [2] Palzer, S. (2020). Photoacoustic-Based Gas Sensing: A Review. Sensors, 20(9), 2745. doi:10.3390 / s20092745

Claims

Claims 1. Method for photoacoustic detection of at least one gas, implementing a photoacoustic sensor comprising at least one resonant cavity configured to receive the gas and at least one coherent light source powered by a current source, comprising the steps of generating by means of the current source: a / a first pulse train having a first duty cycle and powering the light source, so as to emit in the resonant cavity radiation centered on a first wavelength, b / a second pulse train having a second duty cycle different from the first duty cycle and powering the light source, so as to emit in the resonant cavity radiation centered on a second wavelength different from the first wavelength,the current supplying the light source being intensity modulated at a first modulation frequency fi for the first pulse train and at a second modulation frequency fi for the second pulse train, the modulation preferably being sinusoidal, triangular or rectangular in shape., 2. Method according to claim 1, wherein the temperature of the light source is maintained within a range of plus or minus 0.1°C around an initial value and / or the average intensity of the current generated by the current source is kept constant.

3. Method according to one of the preceding claims, the light source being a laser, preferably a quantum cascade laser or an interband cascade laser.

4. Method according to one of the preceding claims, the first and second pulse trains being generated simultaneously, preferably in phase shift so that the pulses of the second train are between two pulses of the first train.

5. Method according to one of claims 1 to 3, the second pulse train being emitted after the first pulse train, preferably immediately after.

6. Method according to one of the preceding claims, the first and second modulation frequencies fi and fi being different.

7. Method according to one of claims 1 to 5, the first and second modulation frequencies fi and fi being identical.

8. Method according to one of the preceding claims, the first duty cycle and the second duty cycle being between 1% and 80%, preferably greater than or equal to 2% and / or less than or equal to 50%.

9. Method according to one of the preceding claims, in which the first and second pulse trains have a duration of between 30 ms and 10 s, preferably greater than or equal to 500 ms and / or less than or equal to 5 s.

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