A device that measures laser radiation via photoacoustic effects.

Abstract

The present invention relates to a device (D) for measuring the power and wavelength of laser radiation via the photoacoustic effect, which comprises: -center wavelength λ c a cell (C) containing at least one gas (G) having an absorption line having an electro-acoustic transducer (MP) arranged within the cell and suitable for generating a signal (Si) representative of the photoacoustic signal within the cell; - means for processing the signal generated by the electro-acoustic transducer (UT), in which estimates of the concentration of one or more gases are stored; - at least one laser source (L) suitable for emitting laser radiation (LL) into the cell at a wavelength suitable for exciting at least one gas contained in the cell, said laser radiation having a mean wavelength λ at a modulation frequency (f1) such that the interaction between the laser radiation and the at least one gas contained in the cell induces the generation of an optoacoustic signal at the detection frequency of the electro-acoustic transducer; moy a laser source having a wavelength that is tunable in an oscillation manner around and the cell is sealed by a membrane (MB) so as to be impermeable to one or more gases contained in the cell and has an optical aperture that is transparent to laser radiation; the processing means is configured to determine a variation in a phase Φ(t) of the photoacoustic signal based on the photoacoustic signal; The processing means is adapted to measure the wavelength of the radiation based on the photoacoustic signal.

Description

Technical Field

[0001] The present invention relates to the field of photoacoustic methods, and more particularly, to the measurement of the power and wavelength of laser radiation via photoacoustic methods.

Background Art

[0002] The present invention relates to a method of laser spectroscopy in which it is necessary to know or accurately control the power and wavelength of the laser being used.

[0003] Of these techniques, photoacoustic spectroscopy (PA) is well known to those skilled in the art. This qualitative and quantitative analytical technique allows for the determination of the composition of various solids, liquids, and gases. The technique is based on the interaction of laser radiation with a material, which generates sound waves that are then analyzed to characterize the material under consideration. This is particularly well suited to detecting gases using a monochromatic source due to the natural selectivity of the linear absorption spectra of gas atoms. The rapid development of compact infrared laser sources (e.g., laser diodes) in the past decade has transformed gas detection via PA into a stable, compact, and simple solution. Gas analysis based on PA requires an amplitude and / or wavelength-modulated pulsed or continuous-wave laser source, a cell forming an acoustic resonator to house the gas to be analyzed, and a detection microphone. In gas detection, the PA effect can be separated into four stages: (1) absorption of laser radiation by the gas, thereby exciting rotational, electronic, and vibrational energy levels; (2) in the case of Ro-vibrational excitation, the gas is preferably de-excited via molecular collisions, which results in the transfer of rotational / vibrational and kinetic energy, thereby generating localized heating of the gas. Radiation emission is not dominant in the case of Ro-vibrational excitation at the pressures typically used in PA (approximately 1 bar), compared to non-radiative emissions, due to the long lifetime of the Ro-vibrational radiation level. In fact, the energy absorbed by the gas is completely converted into heat through the transfer of kinetic energy to the gas atoms; (3) generation of sound waves and heat waves resulting from expansion due to the heating of the gas; and (4) detection of the acoustic signal by a microphone. The amplitude of the microphone's vibration represents the gas concentration, and the wavelength of the laser radiation absorbed by the gas indicates its composition.

[0004] In most cases, during the analysis of the gas, the PA signal generated by the interaction between the laser and the gas under consideration is amplified before detection using an acoustic cell that exhibits resonance at a specific frequency. Naturally, this requires that the PA signal be generated at the same frequency as the resonance frequency of the cell. For example, one resonant cell known in the prior art, such as a dual Helmholtz resonator type cell, has two first cavities linked to a detection microphone.

[0005] It is known to modulate the amplitude of the laser emission at the resonance frequency of the cell until the end of amplification, in which case this results in the modulation of the PA signal at the same frequency. The amplitude modulation of the laser emission has many forms, and the related techniques can be divided into two categories: the continuous wave photoacoustic method and the pulsed photoacoustic method.

[0006] The pulsed photoacoustic method uses a pulsed light source or a continuous wave source with an external mechanical or electro - optical modulator. To generate a PA signal at the resonance frequency of the cell, it is known to use a source operating in a quasi - continuous wave (QCW) mode. In this case, the amplitude of the laser is modulated at a frequency called the repetition rate, which is much higher than that of the resonance of the cell, and thus the laser emission appears as continuous from the perspective of the modulation frequency corresponding to the acoustic resonance.

[0007] Furthermore, in photoacoustic spectroscopy, it is known that the wavelength of the laser emission can be modulated, as shown in J. Saarela et al, “Wavelength modulation waveforms in laser photoacoustic spectroscopy”, Appl. Opt. 48, 743-747 (2009). In this case, the laser wavelength is modulated around the absorption peak of the gas under study. Theoretically, modulating the laser wavelength prevents noise caused by the interaction between the laser emission and the cell wall from interfering with the determination of the gas concentration under study.

[0008] One drawback of laser spectroscopy techniques is that obtaining accurate gas measurements requires complete knowledge of the laser wavelength and its power fluctuations over time. Specifically, wavelength errors can lead to errors in estimates of the concentration of the gas being detected, for example. Often, such instruments incorporate power detectors, resulting in complex assembly and increased equipment costs. Scanning methods can be used to determine the wavelength, but these have been found to be relatively inefficient when measuring gas concentrations close to the instrument's sensitivity limit. In any case, the time-dependent fluctuations in the laser characteristics can be very small, but are sufficient to necessitate adjustment of the gas detection instrument. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] J.Saarela et al, “Wavelength modulation waveforms in laser photoacoustic spectroscopy”, Appl.Opt.48, 743-747(2009) [Overview of the project] [Problems that the invention aims to solve]

[0010] The present invention aims to alleviate some of the aforementioned problems of the prior art. More precisely, the present invention relates to a device that allows the wavelength and power of laser radiation to be determined or controlled via photoacoustics using a simple and inexpensive assembly. [Means for solving the problem]

[0011] To this end, one subject of the present invention is a device for measuring laser radiation via photoacoustic effects, which is, -Center wavelength λ c A cell containing at least one gas having an absorption line having, - An electro-acoustic transducer positioned within a cell and suitable for generating electrical signals representing photoacoustic signals within the cell, -Means for processing electrical signals generated by an electro-acoustic transducer, wherein estimated values ​​of the concentrations of one or more gases are stored. - A cell comprising at least one laser source suitable for emitting laser radiation at a wavelength suitable for exciting at least one gas contained within the cell, wherein the laser radiation is modulated at an average wavelength λ such that the interaction between the laser radiation and the at least one gas contained within the cell induces the generation of a photoacoustic signal at the detection frequency of an electro-acoustic transducer. moy A laser source having an optical power that is variable in the oscillation method, with the average power being variable in the oscillation method, with the wavelength being variable in the modulation frequency, centered around the frequency, It has, - The cell is sealed by a membrane so as to be impermeable to one or more gases contained within the cell, and has an optical aperture that is transparent to laser radiation. - The processing means is suitable for determining the wavelength of laser radiation from a photoacoustic signal.

[0012] According to a particular embodiment of the present invention, -One or more laser sources are further configured such that the average wavelength changes over time and the deviation in the average wavelength includes the center wavelength, thereby the processing means - The change in the phase Φ(t) of the photoacoustic signal over time based on the aforementioned electrical signal, -The wavelength of the laser radiation based on the time variation of the phase of the photoacoustic signal, It is more suitable for making that determination. - The processing means determines the power P of the laser radiation based on the electrical signal and the estimated value. L It is more suitable for making that determination. - The cell contains several distinct gases, each having at least one absorption line spectrally distinct from the others, and the apparatus further comprises several laser sources, each located outside the cell and suitable for exciting one associated gas. - The processing means uses a wavelength λ, which is referred to as the servo wavelength of one or more laser sources. AS The phase Φ, referred to as the servo phase, is used to servo-control the photoacoustic signal acquired for this purpose. AS It is configured to determine, -One or more laser sources have electronically pumped lasers, and the apparatus has a power supply circuit that generates a pulsed current called a generated current for pumping one or more laser sources so that one or more laser sources operate in pulsed mode, and a processing means is connected to the power supply circuit, and the power supply circuit is configured to further generate a current called a base current which has a non-zero value between laser pulses and an amplitude which is less than the amplitude of the generated current at the time of the laser pulse, and the base current is amplitude modulated to generate the oscillation fluctuations of the wavelength, - The power supply circuit is configured such that the base current is amplitude-modulated to servo-control the phase of the photoacoustic signal with respect to the servo phase. - The apparatus has a device for controlling the temperature of the active regions of one or more laser sources, the device for controlling the temperature is connected to a processing means and is configured to adjust the temperature of the active regions of one or more laser sources so as to servo-control the phase of the photoacoustic signal with respect to the servo phase. -The device for controlling the temperature is a resistor, a thermoelectric system, or the power supply circuit. - The concentration of one or more gases is greater than 1 ppm, and preferably greater than 100 ppm. - The transducer is a microphone having a package that is impermeable to one or more gases, and inside it contains a diaphragm suitable for detecting photoacoustic signals, in which case the cell is formed by the package.

[0013] Another subject of this invention is a photoacoustic system, which is, - Measuring device according to the present invention, - A photoacoustic gas detection device having a laser inlet surface, wherein a first portion of laser radiation emitted by at least one laser source is configured to illuminate the inlet surface of the photoacoustic gas detection system, It holds.

[0014] According to a particular embodiment of this system, the cell has a laser exit surface, and at least one laser source and the photoacoustic gas detection device are configured such that the first portion corresponds to the laser radiation passing through the laser exit surface, or it has an optical component suitable for splitting the laser radiation into a first portion directed toward the laser inlet surface of the photoacoustic system and a second portion directed into the cell.

[0015] Another means of the present invention is a method for determining the wavelength and power of laser radiation via the photoacoustic effect, which is, -Center wavelength λ cA step of generating laser radiation at a wavelength suitable for exciting at least one gas contained in a cell C containing at least one gas having an absorption line having λ, wherein the laser radiation has an average wavelength λ at a modulation frequency such that the interaction between the laser radiation and at least one gas contained in the cell induces the generation of a photoacoustic signal. moy The laser has a wavelength that changes in an oscillation manner centered on a certain point, the average wavelength deviation of the laser radiation includes the center wavelength, the average wavelength changes with time, and the cell is sealed by a membrane so as to be impermeable to one or more gases contained within the cell, step, - A step of detecting the photoacoustic signal and generating an electrical signal (Si) representing the photoacoustic signal in the cell, - A step of determining the time-dependent variation of the phase Φ(t) of the photoacoustic signal based on the aforementioned photoacoustic signal, - A step of measuring the wavelength of radiation based on the time-dependent variation of the phase of the photoacoustic signal, - The power P of the laser radiation is determined based on the electrical signal and the estimated concentration of one or more gases. L The steps to measure and It holds.

[0016] According to a particular embodiment of the method of the present invention, the wavelength is measured based on the calculation of the maximum value of the derivative of the phase variation of the photoacoustic signal.

[0017] Other features, details, and advantages of the present invention will become apparent when referring to the description provided with reference to the accompanying drawings, each of which is shown below. [Brief explanation of the drawing]

[0018] [Figure 1] This is a schematic diagram of an apparatus for measuring the power and wavelength of laser radiation LL via the photoacoustic effect according to the present invention. [Figure 2]Schematic diagram of a method for determining the wavelength and power of laser radiation via the photoacoustic effect according to the present invention. [Figure 3] Schematic diagram of an apparatus according to a first embodiment of the present invention. [Figure 4] Schematic diagram of an apparatus according to a second embodiment of the present invention. [Figure 5] Schematic diagram of a photoacoustic detection system having an apparatus D according to the present invention. [Figure 6] Schematic diagram of a photoacoustic detection system according to an alternative embodiment of the present invention. [Figure 7] Schematic diagram of a photoacoustic detection system according to an embodiment of the present invention. [Figure 8] Schematic diagram of a photoacoustic detection system according to another embodiment of the present invention.

Modes for Carrying Out the Invention

[0019] Reference numerals in the figures correspond to the same elements when they are the same.

[0020] In the figures, elements are not shown to scale unless otherwise indicated.

[0021] FIG. 1 schematically shows an apparatus D for measuring the power and wavelength of a laser radiation LL via the photoacoustic effect according to the present invention. The apparatus D has a cell C containing at least one gas G having an absorption line with a central wavelength λ c In addition to this, the apparatus has at least one laser source L suitable for emitting a laser radiation LL at a wavelength suitable for exciting the gas contained within the cell, within the cell C.

[0022] Furthermore, apparatus D has an electro-acoustic transducer MP suitable for generating a signal Si representing a photoacoustic signal located within and generated within the cell. The transducer allows determination of the amplitude of the sound wave and, therefore, the power of the laser radiation, in which case the concentration of the gas species under consideration is known (see below). This transducer may be, for example, a microphone, or, in some cases, a tuning fork. In another embodiment, this may be an acoustic-optical-electric transducer.

[0023] In a known manner, the laser source is configured such that the interaction between the laser radiation and the gas contained within the cell induces the generation of a photoacoustic (PA) signal at the detection frequency of the electroacoustic transducer (condition 1), with the laser radiation LL having an average wavelength λ at the modulation frequency f1. moy It is configured to have a wavelength that changes depending on the oscillation method, centered around [a specific point].

[0024] In addition, the laser source L according to the present invention has an average wavelength λ moy (t) is period T e The average wavelength deviation changes over time, and the center wavelength λ c It is constructed in a manner that includes (second condition). Therefore, this is λ moy (t) < λ c There exist a first set of multiple values ​​such that λ moy (t>λ c The average wavelength is such that there are a second set of multiple values, and the period T e This means that it changes and obtains multiple distinct values.

[0025] Specifically, under the two conditions described above, the wavelength emitted by the laser source L is λ(t) = λ moy (t)+λ PA It may also be expressed in the expression (t), in which case, λ moy (t) is a function corresponding to the average value of the wavelength of the radiation, and λ PAλ is a function that represents the oscillation portion of the wavelength at frequency f1, and its average value is equal to 0, which allows the PA signal to be generated within the cell. moy For example, it is a ramp function and also a function λ PA These are, for example, triangular, square, or sinusoidal functions. As will be discussed later, satisfying both of these conditions allows for accurate determination of the wavelength of radiation based on the phase of the PA signal. These waveforms are given as non-limiting examples, and any other waveform known to those skilled in the art that allows for the satisfaction of the above conditions in relation to wavelength may be used.

[0026] In another embodiment, the laser source is configured such that the laser radiation LL has an optical power that changes in an oscillating manner around the average power at a modulation frequency f1, such that the interaction between the laser radiation and the gas contained within the cell induces the generation of a photoacoustic signal at the detection frequency of the electroacoustic transducer. In this embodiment, the laser source is then configured such that the emitted wavelength is simply λ(t)=λ PA It is configured to acquire the form (t).

[0027] In particular, in all embodiments of the present invention, cell C is sealed by a membrane MB so as to be impermeable to the gas contained within the cell. This membrane MB has an optical aperture that is transparent to laser radiation emitted by one or more laser sources L. Transparent, as used herein, means that the transmittance of the membrane is sufficient so that the signal-to-noise ratio of the signal Si generated by the transducer MP is greater than 1. This membrane is a transparent wall that either does not change or negligibly changes the geometric properties of the beam as the beam propagates. In contrast to conventional photoacoustic gas detection devices, the device has a membrane MB that seals a cavity to ensure the presence of one or more known gases and to allow the device to operate as a photoacoustic sensor. Thus, the concentrations of various gases in the cell are set and known, and the power of the radiation can be determined by the amplitude of the PA signal, once the wavelength is determined with accuracy (see below).

[0028] Apparatus D further comprises means UT for processing the electrical signal Si generated by transducer MP. The processing means determines the measured wavelength of the radiation LL, thereby making it suitable for generating a PA signal. To accomplish this, the processing means determines one period T of the variation in the average wavelength. e It is configured to determine the fluctuation of the PA signal over time. Based on this fluctuation, means UT is configured to determine the fluctuation of the phase Φ(t) of the PA signal represented by the signal Si(t). For example, the phase Φ(t) may be calculated in the following manner (Equation 1),

number

number

[0029] Alternatively, the means UT for processing the electrical signal Si generated by the transducer MP has a synchronous detector suitable for simultaneously generating the PA signal and a signal representing the phase variation of the PA signal.

[0030] According to one embodiment, the processing means then processes period T e The system is configured to calculate the derivative of the phase variation Φ(t) over a certain period and to determine the maximum value of the derivative over this period. As is known, this maximum value corresponds to the central wavelength λ of the gas absorption peak. c It reaches a wavelength equal to λ. Therefore, the determination of the maximum value of the derivative of the phase variation is based on the value of the wavelength of the laser emission, λ = λ. c It allows for the precise determination of the point in time when this occurs. In the case of an electrically pumped laser source, the laser emission then occurs at this wavelength λ=λ c The value of the supply current that is permissible to be generated can be determined. In the following, this supply current value will be used to servo-control the laser for this wavelength (see below).

[0031] In addition to this, λ = λ c After the determination that this is true, the phase of the PA signal is associated with the wavelength λ(t) of the laser emission at time t by comparing the known variation in wavelength λ(t) over time with the measured variation in the phase Φ(t) of the PA signal. Therefore, subsequent measurements of the phase Φ(t) of the PA signal allow for the instantaneous determination of the corresponding wavelength of the laser emission.

[0032] In the following, the phase Φ will be referred to as the servo phase. AS The corresponding wavelength is called the servo wavelength λ AS This is selected by the processing means.

[0033] This wavelength measurement is typically performed with a width of 0.15 cm. -1In the case of an absorption peak and a signal-to-noise ratio of the PA signal detected by transducer MP of 100, 0.01 cm -1 It is accurate within the cavity. These values ​​are given as non-limiting examples. This measurement accuracy mainly depends on the gas concentration within the sealed cavity, the laser power input into the cavity, and the performance of the cavity (quality factor and noise).

[0034] According to one embodiment, the sequence of wavelength variations is repeated multiple times so that the processing means can obtain the average of the variations in the PA signal obtained in each iteration. This allows for a relatively stable average PA signal to be obtained, and therefore allows for an improvement in the signal-to-noise ratio, and therefore allows for an improvement in the accuracy of wavelength measurement.

[0035] Importantly, in the apparatus D of the present invention, the concentrations C(i) of various gas species G(i) are known and stored within means UT. AS Laser length λ AS =λ c In this case, the power P of the laser radiation L This is the maximum amplitude Si of the signal Si representing the PA signal. max The PA signal is calculated based on the gas concentration C from which it is generated.

[0036] The following equation is obtained: P L =A × C × Si max In this case, A is the conversion coefficient determined by the prior calibration of the instrument.

[0037] Laser length λ AS ≠λ c In that case, laser power P L This represents the amplitude of the signal Si representing the PA signal. max , wavelength λ c Length λ in relation to AS It is estimated based on a coefficient K related to a portion of the function and the shape of the absorption peak.

[0038] The following equation is obtained. P L =A × K × C × Si max

[0039] Therefore, this is due to the fact that cell C is sealed to have a known gas concentration, which allows for the accurate determination of the wavelength and the determination of the power of the corresponding laser radiation. Thus, the apparatus D according to the present invention is a low-cost photoacoustic sensor that is an alternative to power sensors and to other wavelength determination techniques of the prior art.

[0040] Preferably, the concentration of one or more gases G in the cell is greater than 1 ppm. This is due to the wavelength and power P L This is required to obtain a PA signal sufficient to allow for a sufficiently accurate determination. Preferably, the concentration of one or more gases in the cell is greater than 100 ppm so that slight variations in the wavelength of the laser emission LL can be measured.

[0041] According to one embodiment of the present invention, cell C contains a plurality of distinct gases, each having at least one absorption line spectrally distinct from the others, and the apparatus has a plurality of monochromatic laser sources, each positioned outside the cell and suitable for irradiating the cell and thus for exciting one associated gas. Thus, in this embodiment, the apparatus is capable of measuring the power and wavelength of the plurality of laser sources. For the remainder of the description, for the sake of brevity, only one laser source and a single gas in the cell will be mentioned. It should be understood that this is only one example, and all embodiments of the apparatus according to the present invention also apply to cases in which the apparatus has multiple laser sources and multiple gases in cell C. Alternatively, the laser sources may also be suitable for emitting frequency combs, in which case each of the frequencies of the comb is suitable for irradiating the cell and for exciting one associated gas.

[0042] Figure 2 illustrates the present invention's method, which is suitable for determining the wavelength and power of laser radiation via photoacoustic effects. This method is implemented by the apparatus shown in Figures 1 and 3-6.

[0043] The method has a first step A, which is implemented by one or more laser sources L and generates laser radiation LL at a wavelength suitable for exciting at least one gas contained within cell C. As described above, the generated laser radiation LL has an average wavelength λ at a modulation frequency f1 such that the interaction between the laser radiation and at least one gas contained within the cell induces the generation of a photoacoustic signal at the detection frequency of the electro-acoustic transducer MP. moy It has a wavelength λ(t) that is variable by the oscillation method, centered around [a specific point]. In addition, the generated radiation has an average laser emission wavelength deviation that includes the center wavelength of the absorption line of the gas in the cell, and the average wavelength has a period T e It changes over time. The wavelength shift is the central wavelength λ of the radiation used to scan the absorption lines. c It is necessary that this be included, and therefore, that the wavelength can be accurately calibrated by relating the maximum value of the PA signal to the center wavelength of the absorption line.

[0044] The method in Figure 2 includes step B, after step A, in which the PA signal generated within the cell is detected using an electro-acoustic transducer. Step B includes the step of generating a signal Si representing the photoacoustic signal within the cell.

[0045] After step B, the method is period T eThroughout the process, there is a step C in which the time-dependent variation of the phase Φ(t) of the photoacoustic signal is determined based on the detected photoacoustic signal. As understood above, the phase can be determined by a preliminary step of calculating the Gabor transform of the PA signal at the modulation frequency f1, and by Equation 1. Alternatively, this phase measurement is performed using a synchronous detector suitable for simultaneously generating the PA signal and a signal representing the phase variation of the PA signal. According to another alternative, the phase Φ(t) is determined by the processing unit UT based on the signal generated by the transducer MP by using any method known to those skilled in the art.

[0046] After step C, the method is period T e The process includes step D, which determines the wavelength of the radiation based on the time-dependent variation in the phase of the photoacoustic signal over period T. As described above, this is based on period T e This is done by calculating the maximum value of the derivative of the phase variation of the photoacoustic signal over the center wavelength λ, in which case this maximum value is the center wavelength λ c This has been obtained for radiation wavelengths equal to [the specified value].

[0047] Finally, the method in Figure 2 calculates the power P of the laser radiation based on the photoacoustic signal and the estimated gas concentration. L It has a final step E to determine the following. Specifically, since the concentration of the gas in the cell is known, the processing means is based on formula P L =A×C×S max By using the center wavelength λ c Power P of laser radiation obtained for wavelengths equal to L It is configured to perform calculations.

[0048] Steps C, D, and E are implemented by the processing means UT.

[0049] Therefore, the method in Figure 2 involves determining the wavelength of the laser radiation that generates the PA signal, and then the laser power P. L This allows for accurate determination of the facts.

[0050] Figure 3 shows an apparatus according to a first embodiment of the present invention, in which one or more laser sources are sources that are electrically pumped by a power supply circuit CA. For example, the laser sources are quantum cascade lasers or, in some cases, laser diodes. This power supply circuit is connected to a processing means UT and generates a pulsed current, called a generated current, which pumps the laser sources so that one or more laser sources operate in pulsed mode.

[0051] The laser source generates a current of the form I(t) = I0 + g.t + h.sin(2πf1t) with a period T e The laser source L is powered by a power supply circuit CA that applies power to the laser source L in a repeating state, where 0 ≤ t ≤ T e And I0 is the current offset, g is the current slope, and h is the amplitude of the modulated current. This waveform is given as a non-limiting example, and any other waveform known to those skilled in the art and that allows the above conditions in relation to wavelength to be satisfied may be used (see, for example, J. Saarela et al, “Wavelength modulation waveforms in laser photoacoustic spectroscopy”, Appl. Opt. 48, 743-747 (2009)).

[0052] Furthermore, in a manner known in itself, the power supply circuit CA is configured to generate a current called a base current, which, in addition to applying the generated current, acquires a non-zero value between laser pulses and has an amplitude less than the amplitude of the generated current during the laser pulse. This base current is amplitude-modulated to generate wavelength oscillation variations and, therefore, to generate the PA signal.

[0053] Alternatively, according to another embodiment, period T eThe waveform that is repeated by this is carried not by the generated current, but by the base current. Therefore, for example, the base current carries the above waveform g.t+h.sin(2πf1t), and in this case, 0≦t≦T e Furthermore, the generated current simply takes the form I(t)=I0.

[0054] In the apparatus shown in Figure 3, which is the first modification, the power supply circuit is further controlled by the processing means, and the center wavelength λ c The base current is amplitude-modulated to servo-control the wavelength of the laser emission LL. This servo control is achieved through a phase variation of the PA signal, which forms the error signal. Specifically, the phase is determined by the large slope (maximum derivative) and the center wavelength λ c It exhibits linear behavior centered on λ, i.e., ideal characteristics for a servo error signal, and . Therefore, the processing means for controlling the power supply circuit is wavelength λ c The circuit CA is configured to inject a value of generated current that allows the acquisition of a certain value.

[0055] Servo control is performed by a processing unit using conventional servo control methods, via feedback electronics, and not limited to, using, for example, PI or PID feedback electronics (PID is an abbreviation for Proportional Integral Derivative, and these terms suggest three operating modes for the error signal of the feedback electronics). This type of feedback, which allows the error signal to converge to a setpoint, is well known in the field of automatic control.

[0056] According to the second modification, the processing means selects the servo control wavelength λ AS ≠λ cThe system is configured to servo-control the power supply circuit CA or the control device Temp (see Figure 4) so ​​as to maintain the phase of the photoacoustic signal in the servo phase and, therefore, the wavelength of the laser source in the servo wavelength.

[0057] Advantageously, the processing means is such that the wavelength is always wavelength λ AS Servo control is performed in relation to the servo phase Φ AS When it drifts (this drift is probably caused by many experimental parameters), the servo phase Φ changes over time. AS It is suitable for adjusting.

[0058] Figure 4 schematically shows a second embodiment of the present invention, which is identical to the embodiment in Figure 3, with the exception that servo control is performed by a processing means using the device Temp to control the temperature of the active region of the laser L. Specifically, in a laser that is electrically pumped in a manner known in itself, when the current applied to the laser exceeds the injection threshold, the wavelength emitted by the laser L may be expressed as follows: λ(I,T) = λ0 + aT + bI Here, λ0 is the theoretical length of the laser at 0K and with a current of 0mA, a is the temperature coefficient of the wavelength, b is the current coefficient of the wavelength, T is the temperature of the laser, and I is the current applied to the laser.

[0059] Therefore, controlling the temperature T of the active region by the processing means UT is equivalent to controlling the servo phase Φ AS By maintaining the phase of the photoacoustic signal, and therefore the wavelength of the laser source is servo-controlled to the wavelength λ AS By maintaining this, the laser is allowed to be servo-controlled.

[0060] According to one embodiment, the control device Temp is a resistor mounted on or positioned in close proximity to the active region so as to be able to control its temperature. Alternatively, according to another embodiment, the device Temp is a thermoelectric system having a laser L and allowing the temperature T of the active region to be precisely controlled.

[0061] Alternatively, according to the third embodiment, the device controlling the temperature is the power supply circuit CA of the laser L. Specifically, a relatively large injection current will heat the active region and, consequently, change the emitted wavelength. Therefore, the laser can be servo-controlled by controlling the temperature of the active region through the power supply circuit.

[0062] One application example of the apparatus according to the present invention is to allow control of the laser wavelength and power in combination with a photoacoustic gas detection device. Accordingly, Figure 5 shows a photoacoustic detection system having apparatus D according to the present invention, which is suitable for measuring the wavelength and power of laser radiation via the photoacoustic method. S This provides a schematic overview of DP. In addition to this, the system S DP has a conventional photoacoustic gas detector DPA having a laser inlet surface EL. Device DPA is a conventional photoacoustic gas detector. In the embodiment shown in Figure 5, device D has an optical window designated as the laser exit surface FL, which is passed through by a first portion of the laser radiation emitted by source L into the cell. Device DPA is configured such that this first portion passes through surface EL and allows gas detection to occur. The fact that cell C is sealed allows for the assurance of the presence of a known gas in the cell and allows device D to function as a photoacoustic sensor. Thus, device D is a sensor that can be easily integrated into a photoacoustic assembly and allows the power and wavelength of the laser radiation to be measured. In addition, device D is configured such that the laser wavelength is set to a target value λ to avoid any drift that would be detrimental to gas detection by device DPA. AS It allows for precise servo control.

[0063] Figure 6 shows an alternative to the embodiment of Figure 5, in which the beam splitter LS divides the radiation emitted by the source L into a first laser radiation portion directed toward the laser entrance surface EL and into the DPA apparatus, and a second portion directed into the cell C. In relation to the embodiment of Figure 5, this embodiment has the advantage of not degrading the optical properties of the laser beam directed toward the apparatus DPA and reducing constraints on the component architecture.

[0064] In another alternative to the embodiment of Figure 5, the apparatus DPA has a laser exit surface, and apparatus D is configured such that radiation transmitted by the laser exit surface of apparatus DPA enters cell C. This embodiment has the advantage of taking into account possible further degradation of the laser power that may occur within the gas cell of apparatus DPA, such as interference with the laser entry surface. According to another embodiment, the wavelength is measured, and servo control based on the PA signal detected by transducer MP is performed using any method known to those skilled in the art.

[0065] In this invention, the transducer MP is a microphone, such as a MEMS microphone (MEMS is an abbreviation for Micro-ElectroMechanical System) or an ECM microphone (ECM is an abbreviation for Electret Condenser Microphone). The microphone has an external protective package (or cavity) having an acoustic entry zone suitable for allowing sound waves to penetrate into the package so as to be detected by a diaphragm DP (e.g., a MEMS or ECM diaphragm). In this invention, the package surrounding the microphone MP forms a cell C and is impermeable to gases. Therefore, the membrane MB sealing the cell C to be impermeable to one or more gases contained within the cell forms the acoustic entry zone. This feature allows for improved compactness of the device without causing excessive degradation of performance. Importantly, therefore, in this invention, the cell C is not a resonant acoustic cavity as a result. Specifically, the inventors observed that, at an H2O concentration of approximately 1000 ppm and an optical power of 1 mW, it is not necessary to use a resonant acoustic cell to amplify the PA signal. These values ​​are given as examples, and it should be understood that other gases with different concentrations can be used with comparable optical power. This allows for the use of a cell C that is significantly smaller in size than the resonant acoustic cavities typically used in the prior art. As a non-limiting example, in a preferred embodiment of the present invention, the cell C formed by the microphone MP package has dimensions of less than 3 mm × 4 mm × 1.2 mm, and in some cases more preferably less than 900 μm × 300 μm × 900 μm. Thus, the apparatus of the present invention is extremely compact.

[0066] Figure 7 schematically shows one embodiment in which the microphone MP is a MEMS microphone having a diaphragm DP. The device in Figure 7 has an external package BE for protecting the diaphragm DP. This package BE contains gas G and is sealed by a membrane MB to form a cell C. Furthermore, the device has a printed circuit board PCB having the electrical contacts of the microphone and a connecting cable CC connecting the microphone to a processing means UT. The processing means UT is, for example, an application-specific integrated circuit (ASIC). In addition, the device has an optional sealing layer CE disposed on the substrate PCB to ensure the sealing of cell C. This embodiment is very compact.

[0067] Figure 8 shows a preferred embodiment of the present invention. In this embodiment, the membrane MB sealing the cavity C is formed by an assembly having a substrate Sub that is transparent to laser radiation LL deposited on a printed circuit board PCB. The printed circuit board PCB has electrical contacts for a microphone MP, which is connected to a processing unit UT by a connecting cable CC. In addition, the substrate PCB has a gap H that allows radiation LL to be delivered into the cavity C to allow a photoacoustic signal to be generated. The structure of this membrane makes it possible to obtain a membrane that is sealing and capable of redistributing both the electrical contacts of the processing unit UT to the electrical contacts of the microphone and, optionally, the laser L. Specifically, optionally, as shown in Figure 8, the laser L is mounted on the substrate Sub to improve the compactness of the device.

[0068] Preferably, in the embodiment shown in Figure 8, the device has solder joints between the substrate and the substrate PCB and between the substrate PCB and the diaphragm DP to ensure the sealing of cell C. Advantageously, the solder joint between the substrate PCB and the diaphragm DP can be configured to generate electrical contacts for the microphone.

Claims

1. A device (D) for measuring laser radiation via photoacoustic effects, -center wavelength λ ｃ A cell (C) containing at least one gas (G) having an absorption line having, - An electro-acoustic transducer (MP) which is a microphone having a diaphragm (DP) that is impermeable to one or more of the aforementioned gases, wherein the diaphragm is located within the cell and is suitable for generating an electrical signal (Si) representing a photoacoustic signal within the cell, and the cell (C) forms the package of the microphone, - Processing means (UT) configured to process the electrical signal generated by the electro-acoustic transducer, wherein the processing means stores estimated values ​​of the concentrations of one or more gases, - The cell comprises at least one laser source (L) suitable for emitting laser radiation (LL) at a wavelength suitable for exciting at least one gas contained within the cell, wherein the laser radiation is modulated at a frequency (f) such that the interaction between the laser radiation and the at least one gas contained within the cell induces the generation of a photoacoustic signal at the detection frequency of the electro-acoustic transducer. １ ) at the average wavelength λ ｍｏｙ The wavelength or the modulation frequency (f) changes in the oscillation method centered around this point. １ A laser source having optical power that varies depending on the oscillation method, centered around the average power, It has, The cell is sealed with a membrane (MB) so as to be impermeable to the one or more gases contained within the cell and has an optical aperture that is transparent to the laser radiation, and the cell has dimensions of less than 3 mm × 4 mm × 1.2 mm. The processing means is an apparatus suitable for determining the wavelength of the laser radiation from the photoacoustic signal.

2. The one or more laser sources are further configured such that the average wavelength changes with time and the deviation in the average wavelength includes the center wavelength, and the processing means is - The time-dependent phase Φ(t) of the photoacoustic signal based on the electrical signal (Si), - The wavelength of the laser radiation based on the time-dependent variation of the phase of the photoacoustic signal, The apparatus according to claim 1, which is further suitable for determining.

3. The processing means determines the power P of the laser radiation based on the electrical signal (Si) and the estimated value. Ｌ The apparatus according to claim 2, which is further suitable for determining.

4. The apparatus according to any one of claims 1 to 3, wherein the cell contains a plurality of distinct gases, each having at least one absorption line spectrally distinct from the others, and the apparatus further comprises a plurality of laser sources, each located outside the cell and suitable for exciting one associated gas.

5. The processing means uses the wavelength λ of the one or more laser sources, which is referred to as the servo wavelength. ＡＳ In order to servo-control the obtained photoacoustic signal, a phase Φ, which is called the servo phase, is used. ＡＳ The apparatus according to any one of claims 1 to 4, configured to determine

6. The one or more laser sources have electrically pumped lasers, and the apparatus has a power supply circuit (CA) that generates a pulsed current (CG), called a generated current, for pumping the one or more laser sources so that they operate in pulsed mode. The apparatus according to claim 5, wherein the processing means is connected to the power supply circuit, and the power supply circuit is configured to further generate a current called a base current (CB) which has a non-zero value between laser pulses and has an amplitude less than the amplitude of the generated current during the laser pulse, and the base current is amplitude modulated to generate oscillation fluctuations of the wavelength.

7. The apparatus according to claim 6, wherein the power supply circuit is configured such that the base current is amplitude modulated to servo control the phase of the photoacoustic signal with respect to the servo phase.

8. The apparatus according to claim 6, comprising a device (Temp) for controlling the temperature of the active regions of one or more laser sources, wherein the device for controlling the temperature is connected to the processing means and is configured to adjust the temperature of the active regions of the one or more laser sources so as to servo control the phase of the photoacoustic signal with respect to the servo phase.

9. The apparatus according to claim 8, wherein the device for controlling the temperature is a resistor, a thermoelectric system, or the power supply circuit.

10. The apparatus according to any one of claims 1 to 9, wherein the concentration of the one or more gases is greater than 1 ppm or greater than 100 ppm.

11. A photoacoustic gas detection system (SDP), - A measuring device according to any one of claims 1 to 10, - A photoacoustic gas detection device (DPA) having a laser entrance surface (EL), wherein a first portion of the laser radiation emitted by at least one laser source is configured to illuminate the entrance surface of the photoacoustic gas detection system, A system that has

12. The system according to claim 11, wherein the cell has a laser exit surface (SL), and the at least one laser source and the photoacoustic gas detection device are configured such that the first portion corresponds to the laser radiation that has passed through the laser exit surface.

13. The system according to claim 11, further comprising an optical component (LS) suitable for splitting the laser radiation into a first portion guided toward the laser inlet surface of the photoacoustic gas detection system and a second portion guided into the cell.

14. A method for determining the wavelength and power of laser radiation via photoacoustic effects, A. Central wavelength λ ｃ In a cell C containing at least one gas (G) having an absorption line with a central wavelength λ, generating a laser emission (LL) at a wavelength suitable for exciting at least one gas contained in the cell, wherein the laser emission has a wavelength that varies in an oscillatory manner around an average wavelength λ ｍｏｙ at a modulation frequency (f １ ), the average wavelength shift of the laser emission includes the central wavelength, the average wavelength varies with time, and the cell is sealed by a membrane (MB) so as to be impermeable to the one or more gases contained in the cell, a step;​​ B. A step of detecting the photoacoustic signal using a microphone having a diaphragm (DP) and a package (BE) that is impermeable to one or more gases, wherein the diaphragm is located within the cell, the cell (C) is formed by the package, and the diaphragm is suitable for generating an electrical signal (Si) representing the photoacoustic signal within the cell. C. A step of determining the time-dependent variation of the phase Φ(t) of the photoacoustic signal based on the photoacoustic signal, D. A step of measuring the wavelength of the radiation based on the time variation of the phase of the photoacoustic signal, E. The power P of the laser radiation based on the electrical signal and the estimated concentration of one or more gases. Ｌ The steps to measure and A method of having.

15. The method according to claim 14, wherein the wavelength is measured based on the calculation of the maximum value of the derivative of the phase variation of the photoacoustic signal.

Images