WO2022180244A1

WO2022180244A1 - Measuring arrangement and method for monitoring a trace gas

Info

Publication number: WO2022180244A1
Application number: PCT/EP2022/054872
Authority: WO
Inventors: Stefan Palzer
Original assignee: Technische Universität Dortmund
Priority date: 2021-02-26
Filing date: 2022-02-25
Publication date: 2022-09-01
Also published as: DE102021104768A1; EP4298428A1

Abstract

The invention relates to a measuring arrangement (10) for intra-cavity absorption spectroscopy on a trace gas (12), comprising a laser (14) for generating electromagnetic radiation (16), and a measuring cell (20) for receiving the trace gas (12), the laser (14) comprising a laser medium (22) and a resonator (24), the laser medium (22) being arranged in the resonator (24), the resonator (24) and/or the measuring cell (20) being configured in such a way that the trace gas (12) is introducible into the resonator (24), the laser (14) being configured in such a way that exactly two modes (28a, 28b) are emitted, one of the two modes (28a, 28b) having a frequency in the vicinity of an absorption transition (30) of the trace gas (12), the two modes (28a, 28b) competing with each other, and a) the measuring arrangement comprising a polarization beam splitter, the two modes (28a, 28b) being two linearly polarized modes (27a, 27b) and the two linearly polarized modes (27a, 27b) being polarized orthogonally to each other, or b) the laser medium (22) providing a plurality of laser transitions, and the two modes (28a, 28b) being two modes (29a, 29b) having different laser wavelengths. Furthermore, the invention relates to a method for monitoring the trace gas (12) by means of the above measuring arrangement (10).

Description

Measuring arrangement and method for monitoring a trace gas

The invention relates to a measuring arrangement for intra-cavity absorption spectroscopy on a trace gas.

Furthermore, 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). 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, on the other hand, 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).

With cavity ring down spectroscopy (CRDS), detection limits of the trace gas in the ppb to ppq range are possible (10 ⁹ to 10 ¹⁵ ). In CRDS, a photon lifetime in a resonator is determined with high finesse, the finesse being a key figure of the resonator indicating its spectral resolution. In simplified terms, 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. During this ring down time, the light bounces back and forth between the mirrors many times, resulting in an effective decay path length of the order of a few kilometers. If the trace gas is introduced into the resonator, the mean photon lifetime is reduced because fewer round-trip reflections are required before the intensity of the light has dropped to a fraction of its original intensity. The measurement of the decay time thus enables conclusions to be drawn about the trace gas concentration.

With cavity-enhanced absorption spectroscopy (CEAS), similar detection limits as with CRDS are possible, with CEAS also coupling laser light into a resonator containing the trace gas. However, with 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.

Even though the above techniques allow for very high precision and accuracy in determining the concentration of the trace gas, their performance depends critically on the quality, especially the finesse, of the optical resonator, each of which is limited by the reflectivity of the mirrors of the resonator. Furthermore, the setup in which the laser light is coupled into the resonator makes it necessary to narrow the line width of the laser light, which is a fundamental limitation when using passive resonators for trace gas detection. At the same time, it must be ensured that the resonators are not affected by vibrations, which necessitates complex laboratory infrastructure for construction.

A method that does not require the laser light to be coupled into the optical resonator is intra-cavity absorption spectroscopy. In 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 . This leads to an increase in sensitivity compared to conventional absorption measurements, whereby use is made of the fact that the light output is greatly increased within the laser resonator. Other reasons for the high sensitivity are the large effective optical path length and mode competition within the laser resonator. If an absorption cell with trace gas is introduced into a multi-mode laser, with the trace gas absorbing certain modes of the laser but not others, very low particle concentrations of the trace gas are sufficient to suppress the absorbed modes in the laser spectrum due to mode competition. Because the mode competition leads to a redistribution of the available amplification to other modes of the laser. 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. Only after a few back and forth reflections of the laser light in the laser resonator does the laser spectrum concentrate on the spectral range of the amplification curve with the highest amplification. During this evolution, 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.

It is therefore the object of the invention to provide a measuring arrangement for intra-cavity absorption spectroscopy on a trace gas which is robust and does not require any complex laboratory setups. Furthermore, it is the object of the invention to provide a measuring arrangement and a method that enable continuous analysis of the laser spectrum.

This object is solved by the subject matter of the independent claims. Preferred developments of the invention are described in the dependent claims.

According to the invention, a measuring arrangement for intra-cavity absorption spectroscopy on a trace gas is thus provided, 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 trace gas by means of the above measurement arrangement is provided, with the trace gas being introduced into the resonator, comprising the steps

Operating the laser in two-mode operation in such a way that exactly two competing modes are emitted, with a first mode of the two modes having a frequency close to the absorption transition of the trace gas,

changing the frequency of the first mode such that its frequency matches the frequency of the absorption transition of the trace gas,

detecting an intensity of a second component of the electromagnetic radiation of the laser, which component exhibits the second mode, by means of a detector, and

monitoring the trace gas taking into account the detected intensity of the second portion of the electromagnetic radiation having the second mode.

For the purposes of the invention, 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. A key aspect of the invention is that 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. In the trace gas monitoring method, the trace gas is introduced into the resonator. If, by changing the frequency of the first modes, 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. Thus, in contrast to the prior art, due to 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. In other words, as in the prior art, 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.

With regard to reading out the absorption spectrum of the trace gas, the measuring arrangement according to the invention 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. For linearly polarized light, the direction of oscillation of the electric field vector is constant. Thus, this means that 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. In other words, 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.

In a further alternative, 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. As previously mentioned, 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. Thus, in the second alternative, it is provided that 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. For example, 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.

As already mentioned, the measuring arrangement includes the laser and the measuring cell. The laser is designed to generate electromagnetic radiation, the laser comprising the resonator and the laser medium. Preferably, the laser further includes a pump source. During operation of the laser, electromagnetic radiation is generated in the laser medium due to an optical transition of excited atoms or molecules of the laser medium into an energetically more favorable state. The central condition for a laser medium is that a population inversion can be produced. This means that the energetically higher state of the optical transition is occupied with a higher probability than the energetically more favorable state, i.e. the energetic end state. In order to bring about a population inversion, energy is pumped into the laser medium by the pump source. Depending on the laser medium, the pumping can take place optically by irradiating light or electrically, for example by electric current, or by gas discharge and in this way bring the atoms or molecules of the laser medium into excited states.

Furthermore, it is provided that the laser medium is arranged in the resonator. The resonator is preferably designed in such a way that the resonator allows feedback of the electromagnetic radiation emitted by the laser medium. The resonator is preferably a mirror resonator and more preferably a standing wave resonator. The resonator of the laser preferentially sets the direction of the induced Emission and thus the beam direction of the laser are fixed: photons emitted only along the resonator run back and forth several times in the resonator and primarily stimulate further emission running in this direction. On the other hand, a wavelength selection takes place through the resonator due to interference. 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, on the other hand, 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.

Furthermore, 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. In particular, the trace gas can be present in very low concentrations. In a first preferred alternative, it is provided that 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. As a further alternative, it can also preferably be provided that 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. In this way, the trace gas can be introduced into the resonator in a simple manner by arranging one device in the other device. Alternatively, it can preferably be provided that 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. In other words, in this configuration, the trace gas is thus received directly by the resonator. The resonator therefore provides a space for absorbing the trace gas. In this way, the trace gas can be introduced into the resonator without an additional device. In this case, 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.

According to a preferred development of the invention, it is provided that 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. In this context, it is also preferably provided that an amplification curve of the laser is such that there is room for exactly two modes in the amplification curve. the

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) and collisions between the particles in the laser medium (pressure broadening). Provision is therefore preferably made for the amplification curve and the free spectral range to be matched to one another in such a way that two successive longitudinal modes are emitted by the laser.

As an alternative to this, according to a preferred development of the invention, it is provided that 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. In this context, according to a further preferred development, it is provided that 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.

According to a preferred development of the invention, it is provided that 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.

In connection with the two linearly polarized modes, according to a further preferred development of the invention, it is provided that 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.

With regard to the arrangement of the polarization beam splitter, 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.

In connection with the resonator of the laser, it is preferably provided that 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.

In connection with the arrangement of the polarization beam splitter next to the resonator, it is further preferably provided that the polarization beam splitter is arranged next to the outcoupling mirror. In connection with the arrangement of the polarization beam splitter within the resonator, it is preferably provided that the resonator comprises at least one highly reflective mirror and comprises at least two outcoupling mirrors.

In connection with the design of the laser, in which the laser medium provides several laser transitions and the two modes are two modes with different laser wavelengths, it is further preferably provided that 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.

Provision is also preferably made for the resonator to be tunable and/or adjustable in length. This allows changing the frequency of at least one of the two modes. With regard to 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.

According to a further preferred development of the invention, 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.

In the event that the laser emits two mutually orthogonally polarized modes, it is preferably provided that 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. In other words, the detectors are arranged in such a way that the two components of the laser light can be detected individually. In the case of a polarization beam splitter that transmits one portion of the laser light and reflects the other, it is preferably provided that 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.

In the event that the laser medium provides several laser transitions and the laser emits two modes with different laser wavelengths, it is preferably provided that the measuring arrangement comprises a detector for detecting at least one intensity. In principle, 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. If, for example, a helium-neon laser is used, which emits the two competing modes of the laser transitions at 632.816 nm and at 3392.2 nm, 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.

Furthermore, it is preferably provided in this connection that 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.

In principle, 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. According to a preferred development, however, it is provided that 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. As already mentioned, it is preferably provided that 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. By changing the gas pressure, the gas temperature and/or the gas mixing ratio, the width of the gain curve and/or gain curves can be adjusted so that there is room for exactly two modes in the gain curve or that there is room for one mode in each gain curve.

In the case of narrow amplification curves in particular, it is preferably provided that the amplification curve be broadened. In this context, according to a further preferred development of the invention, it is provided that 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. Due to the Stark effect, the electrical 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 magnetic, electric and/or electromagnetic field can be a static field or an alternating field. In the case of alternating electrical fields, the AC Strak effect in particular leads to a broadening of the amplification curve. The magnetic field, the electric field and/or the electromagnetic field can be used not only to broaden the gain curve, but also to shift the gain curve and control it in such a way that the laser emits exactly two modes that compete with each other, where one of the two modes has a frequency close to the absorption transition of the tracer gas.

In this context, it can further preferably be provided that 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. Not only 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. At the field used to control the The absorption transition of the trace gas is used, it can be the same field as used to control the emission of the laser or it can be a different field.

According to a further preferred development of the invention, it is provided that 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.

With regard to the measuring cell, according to a further preferred development of the invention, it is provided that 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. In this way, 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. In particular, the absorption transitions of individual isotopes of an isotope mixture of a trace gas can also be spectrally resolved in this way.

Provision is also preferably made for the length of the resonator to be selected in such a way that exactly two competing modes can oscillate. According to a further preferred development of the invention, it is provided that 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.

It is further preferably provided that the resonator comprises an etalon, and the etalon is designed to limit a wavelength range in the resonator. In this way, 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. Furthermore, the by a long resonator Extended absorption distance, so that the measuring arrangement enables a particularly high sensitivity.

In connection with the method for monitoring the trace gas using the above measuring arrangement, it is preferably provided that 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. These measures cause the frequencies of the first mode to shift.

Furthermore, according to a preferred development, it is provided that 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.

According to a further preferred development of the method, it is provided that 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. In other words, 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.

According to a further preferred development of the method, it is provided that the method includes the step of reducing a pressure of the trace gas in the measuring cell. In other words, 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. According to a further preferred development, it is provided that 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. In particular, 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.

The invention is described in more detail below using a preferred exemplary embodiment of the invention with reference to the drawings.

In the drawings shows

1 shows a schematic representation of a measuring arrangement for the intra-cavity

Absorption spectroscopy on a trace gas, according to a preferred embodiment of the invention,

Fig. 2 is a schematic representation of a gain curve of a laser of

Measuring arrangement from Figure 1,

3 shows a schematic representation of a measuring arrangement for the intra-cavity

Absorption spectroscopy on a trace gas, according to a further preferred embodiment of the invention,

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

Measuring arrangement from Figure 4, and 6 shows a schematic representation of a measuring arrangement for the intra-cavity

Absorption spectroscopy on a trace gas, according to a further preferred embodiment of the invention.

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. In the beam direction 26 of the laser 14, 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. FIG. In the present case, the laser medium 22 is a helium-neon gas mixture. Also located in the resonator 24 is the trace gas 12, in this case methane. In this exemplary embodiment, 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 laser 14 of the present measurement arrangement 10 is designed in such a way that the laser 14 emits exactly two competing modes 28a, 28b, these being two linearly polarized modes 27a, 27b which have orthogonal polarizations to one another. Furthermore, the two modes 28a, 28b have a frequency in the vicinity of an absorption transition 30 of the trace gas 12. The helium-neon laser exhibits a lasing transition at 3.39 pm, which corresponds to the transition from the 3s ₂ energy state to the 3p ₄ energy state of the neon atom. The 3.39 pm is near the P(7) transition of the methane V3 rotational vibrational transition 30 at about 3.39 pm.

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. In the present case, 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. Furthermore, 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 || Part 32a and the second detector 34b the perpendicularly polarized 1 part 32b of the electromagnetic radiation 16.

FIG. 2 shows a schematic representation of an amplification curve 36 of the laser 14 from FIG. 1. The frequency is plotted on the x-axis 38 of the diagram in FIG. 2, and the power is plotted on the y-axis 40. Furthermore, the laser threshold 42 is shown in FIG. 2, which represents the minimum necessary pump power of the laser 14 at which gain through energy supply and loss through absorption in the laser medium 22 and through outcoupling of the generated electromagnetic radiation 16 just balance out. Laser threshold 42 thus indicates the point above which laser 14 begins to operate.

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. In the present case, the first longitudinal mode 28a, 27a, 31 is polarized in parallel || and the second longitudinal modes 28b, 27b, 31 perpendicularly polarized 1. It can also be seen in FIG. 2 that the absorption transition 30 of the methane lies spectrally between the two successive modes 28a, 28b.

The length of the resonator 24 of the laser 14 can be adjusted, so that the frequency of the two modes 28a, 28b can be changed by changing the length of the resonator, which is shown schematically by the arrow 48 in FIG. In the method for monitoring the trace gas 12, the length of the resonator 24 is changed in such a way that one of the two modes 28a, 28b - in this case the first mode 28a - of the laser 14 corresponds in its frequency to the frequency of the absorption transition 30 of the trace gas 12 and accordingly from the trace gas 30 is absorbed. The resulting change in the intensity of the second Due to the different polarizations of the modes 28a, 28b, mode 28b can be detected continuously by means of the detectors 34a and 34b. When the first mode 28a is absorbed, the available pump power of the laser 14 is used for the second mode 28b.

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. In contrast to the measuring arrangement 10 from Figure 1, the measuring arrangement of the present exemplary embodiment does not include a polarization beam splitter 18. Furthermore, 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. In the present case, the laser medium 22 is a helium-neon gas mixture. Also located in the resonator 24 is 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. Furthermore, 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.

In the present case, 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. In the present case, 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.

If the frequency of the first mode at 3392.2 nm is shifted in such a way that it matches the frequency of the absorption transition of the methane 12, this leads to an increase in the intensity of the second mode at 632.816 nm 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. In contrast to the measuring arrangement 10 in FIG. 1, in which the resonator 24 takes over the function of the measuring cell 20, it is provided in this exemplary embodiment that the resonator 24 is arranged inside the measuring cell 20. Furthermore, 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. In the present case, 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

10 measurement setup

12 trace gas

14 lasers

16 electromagnetic radiation

18 polarization beam splitter

20 measuring cell

22 laser medium

24 resonators

24a mirror

24b output mirror

26 Beam direction of the laser

27a, 27b linearly polarized modes 28a, 28b emitted modes 29a, 29b modes with different laser wavelengths

30 absorption transition of the trace gas

31 longitudinal modes 32a first portion of the electromagnetic radiation 32b second portion of the electromagnetic radiation 34 detector 36 amplification curve 38 x-axis, frequency 40 y-axis, power 42 laser threshold 44 frequency spacing 46 width of the amplification curve 48 arrow, shifting the frequency of the first mode 50 microfluidic system

II parallel polarization

1 perpendicular polarization

Claims

patent claims

1. Measuring arrangement (10) for intra-cavity absorption spectroscopy on a trace gas (12), comprising a laser (14) for generating electromagnetic radiation (16), and a measuring cell (20) for receiving the trace gas (12), the laser ( 14) comprises a laser medium (22) and a resonator (24), the laser medium (22) being arranged in the resonator (24), the resonator (24) and/or the measuring cell (20) being designed in such a way that the trace gas (12) can be introduced into the resonator (24), the laser (14) being designed such that exactly two modes (28a, 28b) are emitted, one of the two modes (28a, 28b) having a frequency near a absorption transition (30) of the trace gas (12), wherein the two modes (28a, 28b) compete with one another, and wherein a) the measuring arrangement comprises a polarization beam splitter, the two modes (28a, 28b) two linearly polarized modes (27a, 27b) and the two linearly polarized modes (27a, 27b) are orthogonal Have polarization to each other, or b) the laser medium (22) provides multiple laser transitions, and the two modes (28a, 28b) are two modes (29a, 29b) with different laser wavelengths.

2. Measuring arrangement (10) according to claim 1, wherein the laser (14) is designed such that the two modes (28a, 28b) that compete with one another are two directly consecutive longitudinal modes (31).

3. Measuring arrangement (10) according to claim 1, wherein the laser (14) is designed such that the two modes (28a, 28b) that compete with one another are not two directly consecutive longitudinal modes (31).

4. Measuring arrangement (10) according to one of the preceding claims, wherein the laser (14) is designed in such a way that a frequency of the absorption transition (30) of the trace gas (12) between the frequencies of the two modes (28a, 28b) of the laser (14 ) lies.

5. Measuring arrangement (10) according to one of the preceding claims, wherein the measuring arrangement comprises the polarization beam splitter (18), wherein the polarization beam splitter (18) is designed to convert the electromagnetic radiation (16) of the laser (14) into the first mode (27a , 28a) having the first portion (32a) and the second mode (27b, 28b) having the second portion (32b).

6. Measuring arrangement (10) according to one of the preceding claims, wherein the measuring arrangement comprises the polarization beam splitter (18), wherein the polarization beam splitter (18) is arranged in the beam direction (26) of the laser (14) next to the resonator (24) or wherein the polarization beam splitter (18) is arranged within the resonator (24) of the laser (14).

7. Measuring arrangement (10) according to any one of the preceding claims, wherein the resonator (24) comprises at least one highly reflective mirror (24a) and at least one output mirror (24b).

8. Measuring arrangement (10) according to any one of the preceding claims, wherein the laser medium (22) provides multiple laser transitions, and the two modes (28a, 28b) are two modes (29a, 29b) with different laser wavelengths, and wherein the resonator (24) has at least one mirror, the mirror reflectivity of which is different for the different laser wavelengths.

9. Measuring arrangement (10) according to one of the preceding claims, comprising at least one detector (34a, 34b) for detecting an intensity of the electromagnetic radiation (16), wherein the detector (34a, 34b) is arranged such that an intensity of the first Mode (28a) exhibiting first portion (32a) and / or an intensity of the second mode (28b) exhibiting second portion (32b) of the electromagnetic radiation (16) of the laser (14) can be detected.

10. Measuring arrangement (10) according to one of the preceding claims, wherein the laser medium (22) is gaseous and the gas pressure, gas temperature and/or gas mixing ratio of the laser medium (22) is such that exactly two modes (28a, 28b) are emitted which are mutually exclusive compete, one of the two modes (28a, 28b) having a frequency near the absorption transition (30) of the trace gas (12).

11. Measuring arrangement (10) according to any one of the preceding claims, wherein the laser medium (14) is in a magnetic field, electric field and / or electromagnetic field, and the magnetic field, electric field and / or the electromagnetic field is such that the laser (14) exactly two modes (28a, 28b) are emitted which compete with one another, one of the two modes (28a, 28b) having a frequency in the vicinity of the absorption transition (30) of the trace gas (12).

12. Measuring arrangement (10) according to any one of the preceding claims, wherein the measuring cell (20) is designed such that a pressure of the trace gas (12) in the measuring cell (20) can be changed.

13. A method for monitoring a trace gas (12) by means of a measuring arrangement (10) according to any one of the preceding claims, wherein the trace gas (12) is introduced into the resonator (24), comprising the steps

Operating the laser (14) in two-mode operation such that exactly two modes (28a, 28b) are emitted that compete with one another, with a first mode (28a) of the two modes (28a, 28b) having a frequency in near the absorption transition (30) of the trace gas (12),

Changing the frequency of the first mode (28a) such that its frequency matches the frequency of the absorption transition (30) of the trace gas (12),

detecting an intensity of a second portion (32b) of the electromagnetic radiation (16) of the laser (14) that exhibits the second mode (28b) by means of a detector (34a, 34b), and

Monitoring the trace gas (12) taking into account the detected intensity of the second portion (32b) of the electromagnetic radiation (16) having the second mode (28b).

14. The method according to the preceding claim, wherein the step of detecting the intensity of the second portion (32) of the electromagnetic radiation (16) having the second mode (28b) involves continuously detecting the intensity of the second portion ( 32b) when changing the frequency of the first mode (28a).

15. The method according to any one of the preceding method claims, wherein the monitoring of the trace gas (12) taking into account the detected intensity of the second mode (28b) having the second portion (32) of the electromagnetic radiation (16) an identification of the trace gas (12), a Determining a concentration of the trace gas (12) and/or determining a particle number density of the trace gas (12).