WO2022180244A1 - Système de mesure et procédé de surveillance d'un gaz à l'état de traces - Google Patents

Système de mesure et procédé de surveillance d'un gaz à l'état de traces Download PDF

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Publication number
WO2022180244A1
WO2022180244A1 PCT/EP2022/054872 EP2022054872W WO2022180244A1 WO 2022180244 A1 WO2022180244 A1 WO 2022180244A1 EP 2022054872 W EP2022054872 W EP 2022054872W WO 2022180244 A1 WO2022180244 A1 WO 2022180244A1
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Prior art keywords
laser
modes
trace gas
resonator
mode
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PCT/EP2022/054872
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German (de)
English (en)
Inventor
Stefan Palzer
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Technische Universität Dortmund
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Priority to EP22708141.1A priority Critical patent/EP4298428A1/fr
Publication of WO2022180244A1 publication Critical patent/WO2022180244A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • G01N2021/391—Intracavity sample
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • G01N2021/396—Type of laser source
    • G01N2021/398—CO2 laser
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/12—Circuits of general importance; Signal processing
    • G01N2201/121—Correction signals
    • G01N2201/1218—Correction signals for pressure variations

Definitions

  • the invention relates to a measuring arrangement for intra-cavity absorption spectroscopy on a trace gas.
  • the invention relates to a method for monitoring a trace gas using the above measurement arrangement.
  • Trace gases are all gases that occur in the air in smaller proportions than the three main components nitrogen (about 78% by volume), oxygen (about 21% by volume) and argon (about 1% by volume). Trace gases can therefore be non-reactive noble gases such as neon, helium or krypton, or reactive gases that may be environmentally harmful, contribute to the greenhouse effect or - like reactive halogen compounds - contribute to ozone depletion.
  • Laser spectroscopic methods are often used for monitoring trace gases, in which lasers are used to excite the trace gas to be examined, since high sensitivities can be achieved with laser spectroscopic methods.
  • a widely used method for monitoring trace gases is direct tunable diode laser absorption spectroscopy (TDLAS).
  • TDLAS direct tunable diode laser absorption spectroscopy
  • a wavelength of the laser light is tuned to one or more characteristic absorption lines of the trace gas to be examined.
  • the particle number density of the trace gas to be examined can be determined from the measured absorption.
  • a laser diode serves as the source of the laser light.
  • the strength of the absorption per molecule is an intrinsic property of the respective molecule and only depends on the temperature.
  • the line shape of an absorption line depends on both the temperature and the pressure of the trace gas to be examined. However, the latter dependency can be eliminated by integration over the absorption line, so that the area under the absorption line is a direct measure of the particle number density of the trace gases to be examined. Accordingly is the Intensity of the signal given only by the optical path length / of the laser light through the trace gas.
  • One way to increase the sensitivity of the method is to extend the absorption distance /. This can also be done, for example, by using optical resonators, with the optical resonator multiplying the effective optical path length by reflecting the laser light back and forth.
  • Two techniques that use an optical cavity are cavity ring-down spectroscopy (CRDS) and cavity-enhanced absorption spectroscopy (CEAS).
  • CRDS cavity ring down spectroscopy
  • detection limits of the trace gas in the ppb to ppq range are possible (10 9 to 10 15 ).
  • a photon lifetime in a resonator is determined with high finesse, the finesse being a key figure of the resonator indicating its spectral resolution.
  • the finesse depends on the losses in the resonator, with high reflectivity of the mirrors of the resonator leading to high finesse.
  • a typical CRDS setup includes a laser that illuminates the optical resonator with high finesse. The laser is then turned off to allow measurement of the exponentially decaying light intensity exiting the resonator.
  • CEAS cavity-enhanced absorption spectroscopy
  • CRDS cavity-enhanced absorption spectroscopy
  • CEAS also coupling laser light into a resonator containing the trace gas.
  • CEAS it is not the decay time of the resonator that is determined, but the intensity of the laser light coupled out of the resonator.
  • Another way to monitor trace gas is photoacoustic laser spectroscopy, which combines optical and acoustic resonators. The fact is used that the sound wave amplitude is directly proportional to the light output and the number of absorbing trace gas molecules.
  • a method that does not require the laser light to be coupled into the optical resonator is intra-cavity absorption spectroscopy.
  • intra-cavity absorption spectroscopy the trace gas to be examined is introduced directly into the beam path of the laser in the laser resonator (cavity) instead of sending the emitted laser beam through the trace gas sample to be examined in an external resonator, as is the case with CRDS and CEAS .
  • Other reasons for the high sensitivity are the large effective optical path length and mode competition within the laser resonator.
  • the basic principle of intra-cavity absorption spectroscopy is as follows:
  • the trace gas sample to be examined is introduced into the laser resonator of the multi-mode laser, which laser typically comprises a laser medium with a wide gain curve and a low-loss resonator. When turning on the laser, the laser starts on many modes at the same time swing.
  • the laser spectrum concentrate on the spectral range of the amplification curve with the highest amplification.
  • weak absorption features of the trace gas can impress signatures in the laser spectrum since they can affect the laser spectrum during many round-trip reflections. Accordingly, the laser spectrum is measured a short time after switching on the laser, whereby this time should be short enough to prevent the laser spectrum from narrowing too much.
  • the disadvantage of intra-cavity absorption spectroscopy is that the laser spectrum has to be analyzed on very short time scales, which leads to complex laboratory setups.
  • a measuring arrangement for intra-cavity absorption spectroscopy on a trace gas comprising a laser for generating electromagnetic radiation and a measuring cell for receiving the trace gas, the laser comprising a laser medium and a resonator, the laser medium being arranged in the resonator, the Resonator and/or the measuring cell is designed in such a way that the trace gas can be introduced into the resonator, the laser being designed in such a way that exactly two modes are emitted, one of the two modes having a frequency in the vicinity of an absorption transition of the trace gas, wherein the two modes are in competition with each other, and wherein a) the measurement arrangement comprises a polarization beam splitter, the two modes are two linearly polarized modes and the two linearly polarized modes have orthogonal polarization to one another, or b) the laser medium provides multiple laser transitions, and the two modes n are two modes with different laser wavelengths. Furthermore, according to the invention, a method for monitoring a
  • monitoring the trace gas taking into account the detected intensity of the second portion of the electromagnetic radiation having the second mode.
  • near the absorption transition means that the frequency of one of the two modes, referred to here as the first mode, is spectrally so close to the frequency of the absorption transition of the trace gas that by changing the frequency of the first mode, the first mode can scan the absorption spectrum of the trace gas.
  • the laser is designed in such a way that exactly two competing modes are emitted, with one of the two modes, namely the first mode, having a frequency close to the absorption transition of the trace gas.
  • the trace gas is introduced into the resonator.
  • the frequency of the first mode scans the absorption spectrum of the trace gas and thus the first mode corresponds in frequency to the frequency of the absorption transition of the trace gas, this first mode is correspondingly absorbed by the trace gas.
  • An available pump power of the laser is then used for the second mode, resulting in the intensity of the second mode changing.
  • the second mode can thus be used to read out the absorption spectrum of the trace gas that is scanned with the first mode. Accordingly, the measurement arrangement enables the absorption spectrum of the trace gas to be obtained by continuously detecting the laser spectrum.
  • the measuring arrangement according to the invention and the method according to the invention it is not necessary to record and analyze the laser spectrum immediately after switching on the laser.
  • a laser in multi-mode operation in which more than two Vibration modes oscillate used, but the laser is used in two-mode operation, the two modes are in competition with each other. Competing modes means that the two modes are due to lasing transitions of the lasing medium originating from the same excited energy state of the lasing medium.
  • the measuring arrangement provides in a first alternative that the laser is designed in such a way that the two competing modes are two linearly polarized modes and the two linearly polarized modes have orthogonal polarization to one another. Accordingly, the two modes can be separated from one another by the polarization beam splitter. In this way, the detection of the intensity of the second portion of the electromagnetic radiation, which exhibits the second mode, is greatly simplified by means of the detector. According to the first alternative, it is therefore provided that the laser is designed in such a way that the two modes that compete with one another are two linearly polarized modes and the two linearly polarized modes have orthogonal polarization to one another.
  • the direction of oscillation of the electric field vector is constant.
  • the direction of oscillation of the electric field vector of the second mode is ⁇ 90° offset from the direction of oscillation of the electric field vector of the first mode.
  • the first mode is parallel polarized and the second mode is perpendicularly polarized or vice versa. Since both modes can be separated by means of the polarization beam splitter due to their mutually orthogonal polarizations, the laser spectrum and the change in the laser spectrum due to the absorption can be detected particularly easily.
  • the measurement arrangement according to the invention provides that the laser medium provides multiple laser transitions, and the two modes are two modes with different laser wavelengths. Accordingly, the two modes can be distinguished from one another simply because of their different wavelengths, which also greatly simplifies the detection of the intensity.
  • two competing modes means that the two modes are due to lasing transitions of the lasing medium originating from the same excited energy state of the lasing medium.
  • the two modes go back to laser transitions that start from the same excited energy state, but have different energetic end states and therefore modes with different laser wavelengths.
  • a helium-neon laser may have the 3s 2 - 2p 4 laser transition at 632.816 nm and the 3s 2 - 3p 4 laser transition at 3392.2 nm, both of which originate from the 3s 2 excited state.
  • a further advantage of the invention is that due to the introduction of the trace gas into the laser cavity, very high values for the finesse of the cavity can be achieved without having to use mirrors with ultra-high reflectivity for the cavity as in CEAS and CRDS. Furthermore, the measurement arrangement makes it possible to increase the photon lifetime in the resonator and consequently the finesse of the resonator in a simple manner by increasing the pump power of the laser. This can compensate for poor and/or deteriorating mirror reflectivity of the resonator.
  • the low complexity of the measuring arrangement in addition to a high level of robustness and good resistance to adverse environmental conditions, means that the measuring arrangement can be made small and light.
  • a resonant wave has the same phase or a multiple of 2p as before after one resonator round trip and can therefore interfere constructively.
  • Non-resonant waves are damped by destructive interference, since all possible phase differences can occur after several round trips.
  • the frequencies amplified by the resonator are the modes of the laser.
  • the resonator and/or the measuring cell is designed in such a way that the trace gas can be introduced into the resonator.
  • the trace gas can be a gas or a gas mixture.
  • the trace gas can be present in very low concentrations.
  • the measurement setup comprises the measurement cell, in which the trace gas can be accommodated, and the measurement cell can be and/or is arranged next to the laser medium in the resonator.
  • the resonator can be arranged and/or is arranged in the measuring cell. In other words, a device is used for the resonator and a separate device for the measuring cell.
  • the trace gas can be introduced into the resonator in a simple manner by arranging one device in the other device.
  • the measuring cell is formed by the resonator, ie a common device is present which takes over the function of the resonator and the measuring cell.
  • the trace gas is thus received directly by the resonator.
  • the resonator therefore provides a space for absorbing the trace gas.
  • the trace gas can be introduced into the resonator without an additional device.
  • the resonator is preferably not only designed in such a way that the resonator allows feedback of the electromagnetic radiation emitted by the laser medium, but is also preferably designed to absorb the trace gas.
  • the laser is designed in such a way that the two modes that are in competition with one another are two directly consecutive longitudinal modes.
  • the frequency spacing between the two directly consecutive longitudinal modes depends on the length of the resonator and is also referred to as the free spectral range.
  • This configuration is particularly advantageous when the measurement arrangement comprises the polarization beam splitter, and the two modes are the two linearly polarized modes having orthogonal polarization to each other.
  • an amplification curve of the laser is such that there is room for exactly two modes in the amplification curve.
  • Amplification curve also called gain curve, describes the frequency dependency of the radiation amplification of the laser medium.
  • the amplification curve depends, among other things, on the line profile of the laser transition.
  • a laser medium that has a lasing transition at a center frequency VL emits not only photons at VL but also (with decreasing probability) at higher and lower frequencies.
  • Reasons for this are, among other things, the natural line width of the laser transition, influences of the
  • Velocity distribution of the particles in the laser medium Doppler broadening, Doppler effect
  • collisions between the particles in the laser medium pressure broadening
  • the laser is designed in such a way that the two modes that are in competition with one another are not two directly consecutive longitudinal modes.
  • This configuration is particularly advantageous when the lasing medium provides multiple lasing transitions and the two modes are two modes with different lasing wavelengths.
  • the laser medium has two amplification curves that interact with one another, with one mode having space in each amplification curve. Provision is therefore preferably made for the amplification curves and the free spectral range to be matched to one another in such a way that the laser emits two competing modes which are not two directly consecutive longitudinal modes.
  • the laser is designed in such a way that a frequency of the absorption transition of the trace gas lies between the frequencies of the two modes of the laser.
  • the frequency of one of the two modes of the laser is therefore not only spectrally close to the absorption transition of the trace gas, but the laser is also preferably designed in such a way that one of the two modes has a lower frequency than the frequency of the absorption transition of the trace gas and the other mode has a higher frequency than the frequency of the absorption transition of the trace gas. This allows the frequency of the mode to be changed only slightly to match the frequency of the trace gas absorption transition.
  • the measuring arrangement comprises the polarization beam splitter, the polarization beam splitter being designed to divide the electromagnetic radiation of the laser into a first component having the first mode and a component the second mode share having second portion.
  • the first portion of the laser light is preferably reflected by the polarization beam splitter and the second portion of the laser light is transmitted by the polarization beam splitter, or vice versa. Since both modes are separated from one another by means of the polarization beam splitter due to their orthogonal polarizations, the laser spectrum and the change in the laser spectrum due to absorption can be easily detected.
  • a preferred development of the invention provides that the polarization beam splitter is arranged in the beam direction of the laser next to the resonator or that the polarization beam splitter is arranged inside the resonator of the laser.
  • the arrangement of the polarization beam splitter within the resonator has the advantage that the two modes with different polarization can be tuned individually.
  • the arrangement of the polarization beam splitter next to the resonator allows easy access to the polarization beam splitter.
  • the resonator comprises at least one highly reflecting mirror and at least one outcoupling mirror.
  • the output mirror preferably has a mirror reflectivity of less than 100% for at least one wavelength. In this way, the electromagnetic radiation generated by the laser emerges from the outcoupling mirror.
  • the polarization beam splitter is arranged next to the outcoupling mirror.
  • the resonator comprises at least one highly reflective mirror and comprises at least two outcoupling mirrors.
  • the resonator has at least one mirror, the mirror reflectivity of which is different for the different laser wavelengths.
  • the mirror with different mirror reflectivity is preferably the outcoupling mirror. More preferably, the mirror reflectivity for the first mode, ie that whose frequency is changed in such a way that it matches the frequency of the absorption transition of the trace gas, is higher than for the second mode.
  • the mirror reflectivity for the first mode is preferably so high that the output mirror for the first mode acts as a highly reflective mirror. In this way, only the second mode exits at the output mirror.
  • the different mirror reflectivity for the different laser wavelengths thus makes it possible to increase the sensitivity and also simplifies the detection of the intensity of the second mode.
  • the change in length of the resonator it is further preferably provided that devices for heating and/or cooling are provided, by means of which the length of the resonator can be changed.
  • Piezo-controlled mirror actuators are preferably provided.
  • the measuring arrangement comprises at least one detector for detecting an intensity of the electromagnetic radiation, the detector being arranged in such a way that an intensity of a first component exhibiting the first mode and/or an intensity of a second component exhibiting the second mode of the electromagnetic radiation of the laser can be detected.
  • the measuring arrangement comprises two detectors for detecting the intensity of the electromagnetic radiation, the detectors being arranged in such a way that the intensity of the first component having the first mode and of the the second portion of the split by the polarization beam splitter exhibiting the second mode electromagnetic radiation of the laser can be detected.
  • the detectors are arranged in such a way that the two components of the laser light can be detected individually.
  • a polarization beam splitter that transmits one portion of the laser light and reflects the other
  • one detector is arranged next to the polarization beam splitter in the transmission direction and the other detector is arranged next to the polarization beam splitter in the reflection direction.
  • the measuring arrangement comprises a detector for detecting at least one intensity.
  • a detector can be used which is designed in such a way that the intensity of both laser wavelengths can be detected.
  • a detector is particularly preferably used which is designed in such a way that only the intensity of a laser wavelength can be detected.
  • a Si photodiode is preferably used as a detector, since this detects the radiation in the middle IR - i.e. at 3392, 2 nm cannot detect.
  • the measuring arrangement comprises a spectral filter. This allows one of the two laser wavelengths to pass and the other laser wavelength to be blocked out. If the spectral filter is arranged in front of the detector, a detector that only detects the intensity of one laser wavelength can be provided in a simple manner even with laser wavelengths that are spectrally close together and whose intensities would be detected accordingly by the detector.
  • the laser medium can be solid, liquid or gaseous - i.e. the laser can basically be a solid-state laser, semiconductor laser, Raman laser, dye laser or gas laser.
  • the laser medium is gaseous and the gas pressure, gas temperature and/or gas mixing ratio of the laser medium is such that exactly two modes are emitted that compete with one another, with one of the two modes having a frequency close to the Has absorption transition of the trace gas.
  • the amplification curve of the laser is such that exactly two modes have space in the amplification curve or that the laser medium has two interacting modes Has amplification curves, one mode in each amplification curve has space.
  • the amplification curve be broadened.
  • the laser medium is located in a magnetic field, electric field and/or electromagnetic field, and the magnetic field, electric field and/or electromagnetic field is such that the laser has exactly two modes are emitted in competition with one another, with one of the two modes having a frequency close to the absorption transition of the trace gas. Due to the Zeeman effect, the magnetic field leads to a broadening of the amplification curve of the laser medium, so that the width of the amplification curve can be adapted to the free spectral range.
  • the trace gas is in a magnetic field, electric field and/or electromagnetic field and the magnetic field, electric field and/or electromagnetic field is such that the frequency of the absorption transition of the trace gas is in the vicinity of is one of the two modes of the laser.
  • the emission of the laser can be controlled via the magnetic field, electric field and/or electromagnetic field, but also the position and width of the absorption transition of the trace gas.
  • the field used to control the The absorption transition of the trace gas it can be the same field as used to control the emission of the laser or it can be a different field.
  • the laser medium is selected from the group consisting of helium-neon gas mixtures, carbon dioxide, carbon monoxide, nitrogen, noble gas, metal vapors, helium-cadmium gas mixtures and noble gas halides.
  • the measuring cell is designed in such a way that a pressure of the trace gas in the measuring cell can be changed.
  • This has the advantage that the pressure of the trace gas in the measuring cell can be reduced.
  • a spectral separation of the absorption lines of the different trace gas species can be achieved in a trace gas that comprises several trace gas species by reducing the collision broadening.
  • the absorption transitions of individual isotopes of an isotope mixture of a trace gas can also be spectrally resolved in this way.
  • the laser medium is a helium-neon gas mixture and the resonator has a length of 20 mm. Because the resonator is 20 mm long, two consecutive longitudinal modes have a frequency spacing of 7.5 GHz. Since the helium-neon laser intrinsically has a very narrow amplification curve with a half-width of approximately 1.5 GHz, it is preferably provided that the amplification curve is broadened to approximately 20 GHz by the magnetic field. In this way, the laser emits two consecutive longitudinal modes with a frequency spacing of 7.5 GHz.
  • the resonator comprises an etalon, and the etalon is designed to limit a wavelength range in the resonator.
  • the etalon in the resonator can be used to design the laser in such a way that exactly two competing modes are emitted.
  • This has the advantage that the length of the resonator can be as long as desired, so that there is sufficient space in the resonator to introduce the trace gas into the resonator.
  • the by a long resonator Extended absorption distance so that the measuring arrangement enables a particularly high sensitivity.
  • the step of changing the frequency of the first mode such that its frequency matches the frequency of the absorption transition of the trace gas, changing the length of the resonator, changing a magnetic field , electric field and/or electromagnetic field around the laser medium, changing a pressure of the laser medium and/or changing a temperature of the laser medium.
  • the method includes the step of changing a frequency of the absorption transition of the trace gas.
  • the frequency of the absorption transition of the trace gas can also be changed by applying a magnetic field, electric field and/or electromagnetic field around the trace gas. This can also be facilitated in such a way that the frequency of the first mode coincides with the frequency of the absorption transition of the trace gas.
  • the step of detecting the intensity of the second portion of the electromagnetic radiation having the second mode includes continuously detecting the intensity of the second portion having the second mode when changing the frequency of the first mode.
  • the intensity of the second mode is recorded during the scanning of the absorption transition of the trace gas with the first mode.
  • the change in the intensity of the second modes is particularly preferably detected when the frequency of the first mode changes.
  • the method includes the step of reducing a pressure of the trace gas in the measuring cell.
  • the pressure of the trace gas in the measuring cell is preferably reduced after the trace gas has been introduced into the measuring cell. This leads to the fact that the collision broadening of the absorption lines of the trace gas is reduced. In this way, with several trace gas species, a spectral separation of the absorption lines of the different trace gas species can be achieved.
  • the monitoring of the trace gas taking into account the detected intensity of the second portion of the electromagnetic radiation having the second mode, includes identifying the trace gas, determining a concentration of the trace gas and/or determining a particle number density of the trace gas.
  • the method thus enables the absorption spectrum of the trace gas to be obtained in a simple manner, namely by detecting the intensity of the second mode, by means of which the trace gas can be identified and/or the concentration and/or the particle number density of the trace gas can be determined.
  • the measuring arrangement and the method can also be used to identify individual isotopes of an isotope mixture of a trace gas and to quantify their proportions.
  • FIG. 1 shows a schematic representation of a measuring arrangement for the intra-cavity
  • Fig. 2 is a schematic representation of a gain curve of a laser of
  • FIG. 3 shows a schematic representation of a measuring arrangement for the intra-cavity
  • FIG. 4 shows a schematic representation of a measuring arrangement for the intra-cavity
  • Fig. 5 is a schematic representation of a gain curve of a laser of
  • FIG. 1 shows a schematic of a measurement arrangement 10 for intra-cavity absorption spectroscopy on a trace gas 12 according to a preferred exemplary embodiment of the invention.
  • the measuring arrangement 10 comprises a laser 14 for generating electromagnetic radiation 16.
  • a polarization beam splitter 18 is arranged next to the laser 14.
  • the laser comprises a resonator 24 and a laser medium 22, the laser medium being arranged in the resonator 24.
  • the laser medium 22 is a helium-neon gas mixture.
  • the trace gas 12 in this case methane.
  • the resonator 24 assumes the function of a measuring cell 20 in which the trace gas 12 is accommodated, in which the resonator 24 provides a space for accommodating the trace gas 12 .
  • the resonator 24 includes two mirrors 24a and 24b, due to a mirror reflectivity of a mirror 24b of less than 100%, this mirror acts as a decoupling mirror 24b for the electromagnetic radiation 16 and thus defines the beam direction 26 of the laser.
  • the polarization beam splitter 18 of the measuring arrangement 10 is designed to split the electromagnetic radiation 16 of the laser 14 into a first portion 32a having the first mode 27a, 28a and a second portion 32b having the second mode 27b, 28b.
  • the first portion 32a of the electromagnetic radiation 16 of the laser 14 is transmitted by the polarization beam splitter 18 and the second portion 32b is deflected by reflection.
  • the measurement arrangement 10 includes in this preferred Embodiment two detectors 34a, 34b which are arranged such that the first portion 32a is detected by a detector 34a and the second portion 32b by the other detector 34b.
  • the first detector 34a thus detects the parallel polarized
  • the longitudinal modes 31 are also shown in FIG. Since the resonator 24 of the laser 14 is 20 mm long in the present exemplary embodiment, two consecutive longitudinal modes 31 have a frequency spacing 44 of approximately 7.5 GHz. Since the helium-neon laser intrinsically has a narrow amplification curve 36 with a half-width of approximately 1.5 GHz, the amplification curve 36 of the laser 14 is broadened to approximately 20 GHz by means of a magnetic field (not shown), resulting in a free spectral range of 8 GHz leads. In this way, exactly two longitudinal modes 31, 28a, 28b that follow one another at a frequency spacing of 7.5 GHz are emitted by the laser 14 and have polarizations that are orthogonal to one another.
  • the first longitudinal mode 28a, 27a, 31 is polarized in parallel
  • FIG. 3 schematically shows a further measurement arrangement 10 for intra-cavity absorption spectroscopy on the trace gas 12 (not shown), according to a preferred exemplary embodiment of the invention.
  • the measuring arrangement 10 in FIG. 3 differs from the measuring arrangement 10 in FIG.
  • the resonator 24 comprises three mirrors 24a, 24b, two mirrors of which are output mirrors 24b.
  • the outcoupling mirrors 24b can be moved via piezo-controlled mirror actuators (not shown), so that the frequency of the first mode 27a, 28a can be changed independently of the second mode 27b, 28b and vice versa.
  • FIG. 4 shows another measurement arrangement 10 for intra-cavity absorption spectroscopy on the trace gas 12, according to a preferred exemplary embodiment of the invention.
  • the measuring arrangement of the present exemplary embodiment does not include a polarization beam splitter 18.
  • the laser 14 of the present exemplary embodiment is designed in such a way that the laser 14 emits exactly two modes 28a, 28b, with the first mode 28a having a frequency in near the absorption transition 30 of the trace gas 12, the two modes 28a, 28b competing with each other, and the lasing medium 22 providing multiple lasing transitions, and the two modes 28a, 28b are two modes 29a, 29b of different lasing wavelength.
  • the laser medium 22 is a helium-neon gas mixture.
  • the trace gas 12 in this case methane.
  • FIG. 5 shows a schematic representation of an amplification curve 36 of the laser 14 from FIG. 4.
  • the frequency is plotted on the x-axis 38 of the diagram in FIG. 5, and the power is plotted on the y-axis 40.
  • the laser threshold 42 is shown in FIG. 5, which represents the minimum necessary pump power of the laser 14, above which the laser 14 begins to work.
  • the laser 14 is also a helium-neon laser.
  • the laser medium 22, ie the helium-neon gas mixture, represents different laser transitions ready. It can be seen from FIG. 5 that the laser medium 22 has two mutually interacting gain curves 36, each gain curve 36 having a mode 28a, 28b. There is therefore a single-mode laser resonator for the first amplification curve 36 and a single-mode laser resonator for the second amplification curve 36. In this way, the laser 14 emits exactly two modes 28a, 28b which have different laser wavelengths.
  • the laser 14 is designed such that the laser transitions at 632.816 nm and at 3392.2 nm are supported by the laser 14, the laser transition at 632.816 nm being the second mode 28b and the laser transition at 3392.2 nm being the first mode 28a. Accordingly, the frequency of the first mode 28a is close to the absorption transition of methane 12.
  • a detector 34 here a silicon photodiode. This is not suitable for detecting the intensity at 3392.2 nm, but only detects in the spectral range from about 190 - 1100 nm. It is therefore not necessary to use a spectral filter.
  • FIG. 6 shows another measurement arrangement 10 for intra-cavity absorption spectroscopy on a trace gas 12 according to a preferred exemplary embodiment of the invention.
  • the resonator 24 takes over the function of the measuring cell 20
  • the measuring cell 20 has a microfluidic system 50 that allows the pressure of the trace gas 12 within the measuring cell 20 to be changed.
  • the measuring arrangement 10 does not include a polarization beam splitter 18 and only a detector 34.
  • the configuration of the measuring cell 20 shown can of course also be used in the embodiment with a polarization beam splitter according to FIG. Reference List

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Abstract

L'invention concerne un système de mesure (10) destiné à la spectroscopie d'absorption intracavitaire sur un gaz à l'état de traces (12), ledit système de mesure comprenant un laser (14) destiné à produire un rayonnement électromagnétique (16), et une cellule de mesure (20) destinée à recevoir le gaz à l'état de traces (12), le laser (14) comprenant un milieu laser (22) et un résonateur (24), le milieu laser (22) étant disposé dans le résonateur (24), le résonateur (24) et/ou la cellule de mesure (20) étant conçus de sorte que le gaz à l'état de traces (12) peut être introduit dans le résonateur (24), le laser (14) étant conçu de sorte qu'exactement deux modes (28a, 28b) sont émis, un des deux modes (28a, 28b) présentant une fréquence proche d'une transition d'absorption (30) du gaz à l'état de traces (12), les deux modes (28a, 28b) étant mutuellement en concurrence, et a) le système de mesure comprenant un séparateur de faisceau de polarisation, les deux modes (28a, 28b) étant deux modes polarisés de manière linéaire (27a, 27b) et les deux modes polarisés de manière linéaire (27a, 27b) présentant une polarisation orthogonale l'un par rapport à l'autre, ou b) le milieu laser (22) fournissant plusieurs transitions laser, et les deux modes (28 a, 29b) étant deux modes ayant des longueurs d'onde laser différentes. En outre, l'invention concerne un procédé de surveillance du gaz à l'état de traces (12) au moyen du système de mesure susmentionné.
PCT/EP2022/054872 2021-02-26 2022-02-25 Système de mesure et procédé de surveillance d'un gaz à l'état de traces WO2022180244A1 (fr)

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