Spinel or aluminium oxynitride as reflection element in measuring device for absorption spectroscopy of gas and method of absorption spectroscopy

Description

[0001] The invention relates to a measuring device for absorption spectroscopic gas measurement, the use of spinel or aluminium oxynitride, and a method for absorption spectroscopic gas measurement.

[0002] Absorption spectroscopic gas measurements are typically performed using laser absorption spectroscopy, and in particular diode laser absorption spectroscopy (TDLAS) with a tunable laser. This spectroscopy technique is especially well-suited for gas measurements, particularly for determining the chemical and / or physical parameters of a gas. Laser absorption spectroscopy is especially advantageous for determining the concentration of a gas in a gas mixture. In addition to concentration, other parameters such as temperature, pressure, velocity, and mass flow rate can also be determined.

[0003] A measuring device for absorption spectroscopic gas measurement using diode laser absorption spectroscopy with a tunable laser (TDLAS) comprises, in particular, a radiation device by which a laser beam can be generated, a reflection element by which the laser beam can be reflected, and at least one detection element by which a laser beam can be detected. The radiation device can, in particular, be a laser diode by which a tunable laser beam can be generated. The laser beam generated by the radiation device is guided along a first optical axis to the reflection element, and the laser beam reflected by the reflection element is subsequently guided along a second optical axis from the reflection element to the detection element.

[0004] For absorption spectroscopic gas measurement using such a TDLAS, a gas mixture to be measured is irradiated with the laser beam as it travels along the first and second optical axes. During this irradiation of the gas, the emission wavelength of the tunable diode laser is tuned to the characteristic absorption bands of the gas being measured within the gas mixture. This leads to a reduction in the radiation intensity of the laser beam due to absorption. This reduction in radiation intensity results in a decrease in the signal intensity measured by the detection element, which can then be used for gas measurement, in particular for determining the gas concentration within the gas mixture. The gas concentration can, as is well known, be determined, for example, using the Beer-Lambert law.

[0005] The diode laser to be used can be selected depending on the gas being measured and the desired tuning range. For example, DFB lasers (Distributed Feedback Lasers) can cover wavelength ranges between 700 nm and 3 µm, VCSEL lasers (Vertical Cavity Surface Emitting Lasers) wavelength ranges down to about 2.1 µm, ICL lasers (Interband Cascade Lasers) wavelength ranges between 2.8 µm and 5.8 µm, and QCL lasers (Quantum Cascade Lasers) wavelength ranges above 3.5 µm.

[0006] TDLAS can be used for gas measurement of gases which have at least one characteristic absorption band or absorption line in these wavelength ranges, for example oxygen (O 2 ), carbon dioxide (CO 2 ), carbon monoxide (CO), nitrogen oxides (NOx), ammonia (NH 3 ), hydrogen sulfide (H 2 S), sulfur oxides (SOx), hydrogen halide compounds (HCl, HF), formaldehyde (CH 2 O ), water vapor (H 2 O ) or mixtures thereof.

[0007] A particular challenge in using TDLAS is selecting a suitable reflective element, as it must possess numerous, highly specific properties. Specifically, the reflective element must be transparent and reflective in the wavelength range of the laser beam generated by the radiation device and directed to it. Furthermore, the reflective element must be resistant to aggressive gases with which it may come into contact during gas measurement and must also be able to withstand high temperatures. Finally, the reflective element should be easy to clean in case of contamination.

[0008] In current technology, reflective elements made of sapphire are regularly used. However, these have numerous disadvantages. For example, sapphire is birefringent, so the reflected laser beam is split into multiple beams. Furthermore, the technically complex processing of sapphire is a disadvantage. Finally, due to its high price, sapphire is also disadvantageous from an economic perspective.

[0009] In addition, reflective elements made of YAG (yttrium aluminum garnet) are also regularly used in the prior art. However, these are disadvantageous due to the high raw material costs.

[0010] Due to these disadvantages, quartz is sometimes used as a reflective element. However, quartz can only be used for wavelength ranges below approximately 3 µm.

[0011] EP 2 065 738 A1 describes a TDLAS system with a gas cell with a cube-shaped retroreflector made of sapphire.

[0012] WO 2020 / 142138 A1 describes a gas detection probe with a cube-shaped retroreflector, optionally made of an aluminum oxide material, e.g. sapphire, and mentions an anti-reflection coating for optical elements.

[0013] WO 2021 / 156731 A1 describes silicon, sapphire, spinel, aluminium oxynitride or silicon dioxide as possible materials for a retroreflector for optical temperature measurements in high-temperature environments.

[0014] Further optical apparatus for gas measurement are disclosed, for example, in US 6,118,520 A and DE 20 2008 013557 U1.

[0015] The invention is based on the objective of providing a measuring device for absorption spectroscopic gas measurement, particularly in the form of a TDLAS, with an alternative reflective element. In particular, the reflective element should also be usable for wavelengths above 3 µm and especially in the mid-infrared wavelength range. According to a further objective, the reflective element should not be birefringent. According to a further objective, the reflective element should be chemically and thermally resistant, in particular resistant to aggressive gases and at high temperatures. According to a further objective, the reflective element should be easy to clean. Finally, according to a further objective, the reflective element should be economically advantageous, i.e., in particular easy to process and available at the lowest possible cost.

[0016] To solve these problems, a measuring device for absorption spectroscopic gas measurement according to claim 1, which defines the present invention, is provided.

[0017] The invention is based on the surprising finding that spinel or aluminium oxynitride can be used as a reflective element for a measuring device for absorption spectroscopic gas measurement, in particular in the form of a TDLAS.

[0018] Spinel, more precisely magnesia spinel, is a mineral of the spinel group with the chemical composition MgAl₂O₄, which crystallizes in the cubic crystal system. According to the invention, it has surprisingly been found that spinel is particularly advantageous as a reflecting element in a measuring device for absorption spectroscopic gas measurement, especially in a TDLAS. In this respect, spinel is transparent to a laser beam over a broad wavelength range and a laser beam can be reflected by the spinel. Specifically, spinel is transparent in a wavelength range from approximately 0.3 to 5.6 µm, and thus also in the mid-infrared wavelength range, and particularly above 3 µm.

[0019] Polycrystalline aluminum oxide, also known as "PCA" (polycrystalline alumina), is a high-density ceramic made of aluminum oxide. It is typically produced by pressing and subsequent sintering of powdered aluminum oxide. Preferably, the polycrystalline aluminum oxide is produced from very fine aluminum oxide powder, particularly preferably with a particle size of less than 1 µm. Most preferably, the polycrystalline aluminum oxide is in the form of transparent polycrystalline aluminum oxide, i.e., so-called TPCA (transparent polycrystalline alumina). It has been found that polycrystalline aluminum oxide is advantageously used as a reflective element in a measuring device for absorption spectroscopic gas measurement, especially in a TDLAS (transmission diffusion laser oscillator).In this respect, polycrystalline aluminum oxide is transparent to a laser beam across a broad wavelength range, and a laser beam can be reflected by the polycrystalline aluminum oxide. Specifically, polycrystalline aluminum oxide is transparent in a wavelength range from approximately 0.3 µm to approximately 5.6 µm, and thus also in the mid-infrared wavelength range, particularly above 3 µm.

[0020] Aluminum oxynitride is a ceramic material composed of the elements aluminum, oxygen, and nitrogen, also known as "ALON." According to the invention, it has been surprisingly found that aluminum oxynitride is advantageously usable as a reflective element in a measuring device for absorption spectroscopic gas measurement, particularly in a TDLAS. In this respect, aluminum oxynitride is transparent to a laser beam over a broad wavelength range, and a laser beam can be reflected by the aluminum oxynitride. Specifically, aluminum oxynitride is transparent in a wavelength range from approximately 0.3 µm to approximately 5.2 µm, and thus also, in particular, in the mid-infrared wavelength range, and especially above 3 µm.

[0021] In particular, spinel, polycrystalline aluminum oxide, and aluminum oxynitride are not birefringent, making it especially advantageous for the laser beam to be reflected through these materials, as the laser beam is not split into multiple beams upon reflection through any of them. Furthermore, spinel, polycrystalline aluminum oxide, and aluminum oxynitride exhibit exceptional resistance to aggressive gases and high temperatures. Finally, spinel, polycrystalline aluminum oxide, and aluminum oxynitride are easy to purify and can be supplied economically, as they are readily machinable, particularly more easily than sapphire, and can be provided at a low cost, especially lower than sapphire.However, the advantage of using these substances as a reflective element in a measuring device for absorption spectroscopic gas measurement, especially in a TDLAS, is that these substances can be coated very well, especially with an antireflection coating (AR coating), as explained in more detail below.

[0022] Although spinel, polycrystalline aluminum oxide, and aluminum oxynitride, as described above, are equally advantageous as reflective elements in a measuring instrument for absorption spectroscopic gas measurement, particularly in a TDLAS, according to the invention spinel is particularly preferred as such a reflective element, since it exhibits the best properties with regard to transmission, reflection, durability, cleanability, coatability, and cost.

[0023] In a preferred embodiment, the reflective element has a coating. Particularly preferably, the reflective element has an anti-reflective coating (AR coating). Alternatively or cumulatively, the reflective element can have a scratch-resistant coating or a temperature-resistant coating.

[0024] In a preferred embodiment, the reflective element has a coating selected from the following group: Al₂O₃ / SiO₂, TiO₂ / Al₂O₃, TiO₂ / SiO₂, Ta₂O₅, or MgF₂. According to the invention, it has been found that each of the aforementioned substances or mixtures can provide an AR coating that also offers scratch and temperature protection for the reflective element. Particularly preferably, according to the invention, the reflective element has a coating of Ta₂O₅ (tantalum pentoxide). This is because it was surprisingly found according to the invention that coating the reflective element with Ta₂O₅ reduced the reflection in the wavelength range of approximately 3,425 to 3,650 nm to below 0.05%.

[0025] For coating the reflective element with a coating, in particular a coating made of one of the aforementioned substances or mixtures of substances, technologies known from the prior art for applying a coating can be used, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD).

[0026] In a particularly preferred embodiment, the reflection element is a retroreflector. As is known, a retroreflector (also called a triple prism) is a device by which incident electromagnetic waves, including in particular light or laser beams, are reflected largely back in the direction from which they originated, regardless of the direction of incidence with respect to the orientation of the device. Designing the reflection element as a retroreflector also has the particular advantage that the laser beam reflected by the reflection element can be reflected onto the detection element particularly easily and effectively.

[0027] In a preferred embodiment, the reflecting element has an incidence surface through which a laser beam generated by the radiation device can be introduced into the reflecting element. The incidence surface is designed as a flat surface oriented at an angle other than 90° to the first optical axis. Such an inclination of the incidence surface to the first optical axis has the particular advantage that any radiation components of the laser beam reflected at the incidence surface are not reflected back in the direction of the incident laser beam, thus preventing interference with the incident laser beam. In a preferred embodiment, the normal to the incidence surface is oriented at an angle in the range of > 0 to 5°, more preferably at an angle in the range of 1 to 5°, and most preferably at an angle of 3° to the first optical axis.

[0028] The radiation device of the measuring device according to the invention is preferably a laser diode, particularly preferably a tunable laser diode. According to a preferred embodiment, a laser diode, in particular a tunable laser diode, is used, which is selected from the following group: interband cascade lasers, quantum cascade lasers.

[0029] An interband cascade laser, also known as an ICL (Interband Cascade Laser), is a laser diode for the emission of wavelengths in the range of approximately 2.8 µm to 5.8 µm.

[0030] A quantum cascade laser, also known as QKL or QCL (Quantum Cascade Laser), is a laser diode for the emission of wavelengths in the range of about 3.5 µm to at least about 14 µm.

[0031] Although interband cascade lasers and quantum cascade lasers are equally advantageous as lasers in the measuring device according to the invention, an interband cascade laser is particularly preferably used as the laser according to the invention.

[0032] Preferably, the radiation device, in particular the tunable laser diode, is a laser diode tunable from 2.0 µm to 5.0 µm.

[0033] According to the invention, it has been found that a laser diode tunable over this wavelength range interacts particularly advantageously with the reflection element in the form of spinel, polycrystalline aluminum oxide or aluminum oxynitride, since these substances are optically transparent to the laser beam in this wavelength range and reflect the laser beam particularly advantageously in this wavelength range.

[0034] The detection element can, in principle, be in the form of any detection element known from the prior art for detecting a laser beam. According to a preferred embodiment, the at least one detection element comprises at least one photodetector. As is known, a photodetector is an electronic component that converts light into an electrical signal using the photoelectric effect or exhibits an electrical resistance that depends on the incident radiation. According to a particularly preferred embodiment, the at least one detection element comprises at least one photodetector in the form of a photodiode.

[0035] The detection element converts the laser beam detected by the element into an electrical signal, which is then evaluated according to known methods for gas measurement. For example, to measure the concentration of the gas to be measured, the laser beam detected by the element can be evaluated using the Beer-Lambert law to determine the gas concentration. Preferably, the evaluation is performed using evaluation electronics. The evaluation electronics can be part of the measuring device according to the invention or can be provided as separate evaluation electronics. Such evaluation electronics can be based on those known from TDLAS.

[0036] In a preferred embodiment, a laser beam generated by the radiation device is guided through a gas mixture along a section of the first optical axis. Along this section of the first optical axis, the laser beam can interact with the gas to be measured in the gas mixture, or the radiation intensity of the laser beam can be reduced due to absorption by the gas in this section. According to a further development of this inventive concept, a laser beam generated by the radiation device can also be guided through the gas mixture along a section of the second optical axis, or the laser beam reflected by the reflection element can be guided through the gas mixture along a section of the second optical axis.Due to absorption by the gas being measured, the radiation intensity of the laser beam is further reduced during the second pass through the gas mixture along a section of the second optical axis. This greater reduction in radiation intensity can be detected more clearly by at least one detection device, thus making the gas measurement and evaluation more reliable.

[0037] In a preferred embodiment, the measuring device according to the invention comprises a probe, wherein the portion of the first optical axis extends within the probe. In a further development of this invention, the portion of the second optical axis also extends within the probe. The probe can, in particular, be designed such that it can be inserted into a gas mixture. By inserting the probe into a gas mixture, a laser beam generated by the radiation device can be guided through the gas mixture along a portion of the first optical axis and the second optical axis. In a preferred embodiment, the measuring device according to the invention further comprises a device for dispensing a purge gas, through which the reflecting element, the spinel, or the aluminum oxynitride can be rinsed with a purge gas. This protects the reflecting element from aggressive gases in the gas mixture.

[0038] Furthermore, the measuring device according to the invention can comprise any further components known from the prior art for TDLAS. In particular, the measuring device according to the invention can, for example, have transmitting optics, i.e., for example, one or more lenses and / or mirrors, by which a laser beam generated by the radiation device can be optically shaped. Furthermore, the measuring device can have receiving optics, i.e., for example, one or more lenses and / or mirrors, by which a laser beam reflected by the reflection element can be shaped before it strikes the at least one detection element.

[0039] Preferably, one or more mirrors are used as the transmitting and receiving optics, and particularly preferably no lenses are used, especially when an interband cascade laser or a quantum cascade laser is used. This is because a lens always exhibits a certain degree of reflection, to which such a laser is very sensitive. Such reflection can be reduced or even practically completely suppressed by using mirrors as the receiving optics.

[0040] The invention also relates to the use of a measuring device according to claim 1 for absorption spectroscopic gas measurement, in particular in a diode laser absorption spectrometer using a tunable laser (TDLAS). This use is particularly preferred, as described above, with the proviso that spinel is used.

[0041] In practical application, the measuring device according to the invention can be operated as follows.

[0042] A laser beam is generated by the radiation device, in particular a tunable laser diode, especially in a wavelength range of 2.0 µm to 5.0 µm. The generated laser beam can be guided along the first optical axis to the reflecting element, optionally after beam shaping by a transmitting optic. In this way, the laser beam is preferably guided along a section of the first optical axis through a gas mixture, which includes a gas to be determined, in particular a gas whose concentration in the gas mixture is to be determined. The laser beam is reflected by the reflecting element. The reflecting element, which is made of spinel or aluminum oxynitride, is preferably a retroreflector, wherein the reflecting element further preferably has an antireflection coating and the incident surface of the reflecting element is preferably oriented at an angle to the first optical axis.The laser beam reflected by the reflecting element is reflected along the second optical axis to the at least one detection element, optionally shaped by receiving optics, and then strikes the at least one detection element, particularly preferably a photodiode of the at least one detection element. Along the second optical axis, the laser beam is preferably guided again through the gas mixture along a section, whereby the laser beam interacts again with the gas to be measured in the gas mixture and thereby further loses radiation intensity.The laser beam striking the at least one detection element, in particular the photodiode, is subsequently detected by the at least one detection element, in particular converted into an electrical signal by the at least one detection element, which is then preferably evaluated by means of evaluation electronics and the gas to be measured is measured, in particular its concentration in the gas mixture is determined.

[0043] According to the invention, it has been found that the measuring device according to the invention can be advantageously used for determining the gas concentration, in particular for determining the gas concentration of at least one of the following gases, especially in a gas mixture: C₂H₂, C₂H₄, C₂H₆, CH₂O, CH₃Cl, CH₄, CO, CO₂, CS, CS₂, H₂, H₂O, H₂S, HBr, HCl, HCN, HF, Hl, HOCl, N₂, N₂O, NH₃, NO, NO₂, O₃, OCS, PH₃, SO₂, SO₃. Preferably, the measuring device according to the invention is used for determining the gas concentration of at least one of the following gases, especially in a gas mixture: C₂H₄, C₂H₆, CH₂O, CO, NO, NO₂, SO₂, SO₃. The measuring device according to the invention is particularly preferably used for determining the gas concentration of formaldehyde (CH2O), especially in a gas mixture.

[0044] The use of the measuring device according to the invention for determining the concentration of gaseous formaldehyde in a gas mixture is also described.

[0045] The invention also relates to a method for absorption spectroscopic gas measurement, comprising the following steps: Provision of a measuring device according to the invention; generation of a laser beam by the radiation device; detection of the laser beam by the detection element; performance of a gas measurement based on the detection.

[0046] The generation of the laser beam, the detection of the laser beam, and the performance of the gas measurement based on the detection can be carried out as described herein.

[0047] Further features of the invention can be found in the claims, the figures and the associated figure description.

[0048] An embodiment of the invention is explained in more detail with reference to the following description of the figures.

[0049] The figures show Figure 1 is a schematic representation of an embodiment of a measuring device according to the invention; Figure 2 is a perspective view of an embodiment of a spinel used as a reflective element according to the invention; and Figure 3 is a further embodiment of a measuring device according to the invention.

[0050] In Figure 1Figure 1 shows an embodiment of a measuring device according to the invention for absorption spectroscopic gas measurement. The measuring device, designated in its entirety by reference numeral 1, is configured for performing tunable diode laser absorption spectroscopy (TDLAS). The measuring device 1 comprises a radiation device 2 in the form of a laser diode, by which a laser beam can be generated. The measuring device 1 further comprises a reflection element 3, in the form of a spinel, by which a laser beam generated by the radiation device 2 can be reflected. Finally, the measuring device 1 comprises a detection element 4, in the form of a photodiode, by which a laser beam can be detected. A laser beam generated by the radiation device 2 can be guided along a first optical axis 5 to the reflection element 3. Along a second optical axis 6, a laser beam reflected by the reflection element 3 can be guided to the detection element 4.

[0051] The spinel or reflection element 3 is designed as a retroreflector or triple prism and has an antireflection coating made of Ta 2 O 5 applied by PVD.

[0052] The spinel or the reflection element 3 has an incidence surface 7 through which a laser beam generated by the radiation device 2 can be introduced into the spinel 3, wherein the incidence surface 7 is designed as a flat surface whose normal is aligned at an angle of 3° to the first optical axis 5.

[0053] Laser diode 2 is an interband cascade laser that can be tuned over a wavelength from 3,627 nm to 3,633 nm (for the gas being measured here, formaldehyde).

[0054] The detection element 4 consists of a photodiode, which converts a laser beam detected by the detection element 4 into an electrical signal. The detection element 4 is coupled to an evaluation unit 9 via an electronic data line 8, which allows the electrical signals generated by the detection element 4 to be evaluated. In the exemplary embodiment, the evaluation unit is in the form of an electronic data processing device.

[0055] Measuring device 1 is designed to determine the concentration of gaseous formaldehyde (CH2O) in a gas mixture.

[0056] In order to determine the concentration of formaldehyde in a gas mixture using the measuring device 1, it expediently has a probe which, in a practical embodiment of the measuring device 1 as described in Figure 3As shown, it is realized. A laser beam generated by the radiation device 2 is guided through the gas mixture along a section of the first optical axis 5 and along a section of the second optical axis 6, each of which runs in the probe.

[0057] In practical application, the measuring device 1 is used as follows to determine the concentration of formaldehyde in a gas mixture.

[0058] The measuring device 1 is initially arranged such that a section of the first optical axis 5 and the second optical axis 6 passes through a gas mixture in which the concentration of formaldehyde is to be determined. A laser beam is generated by the radiation device 2 and is directed along the first optical axis 5 to the reflecting element or spinel 3. It is then introduced into the reflecting element via the incidence surface 7 and reflected by the reflecting element 3. The path of the laser beam in the reflecting element 3 is indicated by dashed lines. The reflected laser beam is then directed along the second optical axis 6 to the detection element 4, where it is detected, converted into an electrical signal, and this electrical signal is transmitted via the electronic data line 8 to the evaluation electronics 9.The laser beam generated by the radiation device 2 is periodically modulated within a predetermined wavelength range, which includes at least one absorption band of formaldehyde. This reduces the radiation intensity of the laser beam as it passes through the gas mixture. This reduction in the radiation intensity of the laser beam is detected by the detection element 4, and the concentration of formaldehyde in the gas mixture is determined by the evaluation electronics 9 based on this detection.

[0059] By using a spinel as a reflecting element 3, this determination of the formaldehyde concentration in the gas mixture can be carried out with particular reliability. This is because the spinel does not refract the laser beam and proves to be resistant even to a hot and aggressive gas mixture. Furthermore, the spinel is transparent to the required wavelengths. Since the spinel is also designed as a retroreflector, the incident laser beam is reflected in the direction from which the laser beam entered the spinel along the first optical axis 5, i.e., parallel to the direction from which the laser beam entered the spinel.

[0060] Furthermore, by inclining the incident surface 7 to the first optical axis 5, interference between the laser beam introduced into the spinel and any radiation component of the laser beam reflected at the incident surface 7 can be prevented. In this respect, any radiation component of the laser beam reflected at the incident surface 7 is not reflected in the direction of the incident laser beam, but rather at an angle to it, as indicated by arrow 10.

[0061] The in Figure 1 The spinel of reflection element 3, shown only schematically, is in Figure 2 shown in more detail in a perspective view. It is clearly visible in Figure 2 that the reflection element 3 is designed as a retroreflector or triple prism. The beam path in the reflection element 3 is in Figure 2indicated by arrows, the laser beam is first introduced into the reflection element 3 along the first optical axis 5, is reflected three times there and then leaves the reflection element 3 along the second optical axis 6.

[0062] In Figure 3 is a measuring device according to the Figure 1 and 2 illustrated in an exemplary embodiment of a practical design.

[0063] Identical or equivalently acting elements are used in the exemplary embodiment according to Figure 3 using the same reference symbols as in the Figure 1 and 2 provided.

[0064] In the embodiment according to Figure 3The measuring device 1 further comprises a transmitting optic 11 in the form of a parabolic mirror, through which a laser beam generated by the radiation device 2 can be shaped. The measuring device 1 also comprises a receiving optic 12 in the form of a parabolic mirror, through which a laser beam reflected by the reflection element 3 can be shaped before it reaches the detection element 4.

[0065] The measuring device 1 according to Figure 3 The measuring device 1 has a steel housing 13 in which the radiation device 2, the transmitting optics 11, the reflecting element 3, the receiving optics 12, the detection element 4, and the evaluation electronics 9 are arranged. Furthermore, the measuring device 1 includes a flange 14 by which the measuring device 1 can be attached to a device (not shown). This device may, in particular, be a device comprising a gas mixture with a gas to be measured by the measuring device 1.

[0066] The measuring device 1 according to Figure 3The device further comprises a probe 15 that can be inserted into the gas mixture to be analyzed. The probe 15 is advantageously designed such that it can be inserted into the gas mixture when the measuring instrument 1 is attached to the device via the flange 14. The probe 15 has elongated openings 16 in the form of process windows.

[0067] A section of the first optical axis 5 and the second optical axis 6 pass through the probe 15, wherein a laser beam running along these sections can be guided through a gas mixture by the gas mixture penetrating the probe 15 through the openings 16.

[0068] The radiation device 2, the transmitting optics 11, the receiving optics 12, the detection element 4, and the evaluation electronics 9 are arranged in a specially protected housing 17 of the measuring device. The housing 17 is shielded from the probe 15 by glass windows 18, 19 which are transparent to the laser beam generated by the radiation device 2.

[0069] The measuring device 1 according to Figure 3 furthermore, it has a (not shown) device for the release of a purge gas, through which the reflective element 3 can be rinsed with a purge gas.

[0070] In practical application, the measuring device 1 is used according to Figure 3The device is used as follows. A laser beam generated by the radiation device 2 is shaped and deflected by the transmitting optics 11 and sent along the first optical axis 5 through the two glass windows 18, 19 to the reflecting element 3. The laser beam is reflected by the reflecting element 3 and sent along the second optical axis 6 through the two windows 18, 19 to the detection element 4. Before reaching the detection element 4, the laser beam is shaped and deflected by the receiving optics 12. The laser beam is detected by the detection element 4, and an electrical signal generated in the process is transmitted to the evaluation electronics 9. The evaluation electronics perform a gas measurement based on the detection by the detection element 4.

[0071] On the section of the first optical axis 5 and the second optical axis 6, which passes through the probe 15, the laser beam is guided through the gas mixture to be analyzed, whereby the signal strength of the laser beam decreases, as described above, due to the interaction with formaldehyde. Based on this decrease in the signal strength of the laser beam, the concentration of formaldehyde in the gas mixture is determined, as described above.

Claims

1. A measuring device (1) for absorption-spectroscopic gas measurement, including: 1.1 a radiation device (2) capable of generating a laser beam; 1.2 a reflection element (3) capable of reflecting a laser beam; 1.3 at least one detection element (4) capable of detecting a laser beam; 1.4 a first optical axis (5) along which a laser beam generated by the radiation device (2) can be guided to the reflection element (3); and 1.5 a second optical axis (6) along which a laser beam reflected by the reflection element (3) can be guided to the detection element (4); wherein 1.6 the reflection element (3) takes the form of spinel or aluminium oxynitride.

2. A measuring device (1) according to claim 1, wherein the reflection element (3) is a retroreflector.

3. A measuring device (1) according to at least one of the preceding claims, wherein the reflection element (3) has a coating.

4. A measuring device (1) according to at least one of the preceding claims, wherein the reflection element (3) has an incident surface (7) via which a laser beam generated by the radiation device (2) can be guided into the reflection element (3), wherein the incident surface (7) is configured as a flat surface that is oriented at an angle in relation to the first optical axis (5) that is not equal to 90°.

5. A measuring device (1) according to at least one of the preceding claims, wherein the radiation device (2) is a laser diode.

6. A measuring device (1) according to at least one of the preceding claims, wherein the radiation device (2) is a tunable laser diode.

7. A measuring device (1) according to at least one of the preceding claims, wherein the radiation device (2) is a laser diode that is tunable over a range from 2.0 µm to 5.0 µm.

8. A measuring device (1) according to at least one of the preceding claims, wherein the at least one detection element (4) includes at least one photodetector.

9. A measuring device (1) according to at least one of the preceding claims, wherein a laser beam generated by the radiation device (2) can be guided along a section of the first optical axis (5) through a gas mixture.

10. A measuring device (1) according to claim 9, wherein a laser beam generated by the radiation device (2) can also be guided along a section of the second optical axis (6) through the gas mixture.

11. A measuring device (1) according to claim 9, further including a probe (15), wherein the section of the first optical axis (5) extends in the probe (15).

12. A measuring device (1) according to claim 11, wherein the section of the second optical axis (6) also extends in the probe (15).

13. A measuring device (1) according to at least one of the preceding claims, further including a device for discharging a flushing gas by means of which the reflection element (3) can be flushed with a flushing gas.

14. A use of a measuring device (1) according to at least one of claims 1 to 13 for absorption-spectroscopic gas measurement.

15. A method for absorption-spectroscopic gas measurement, including the following steps: A. providing a measuring device (1) according to at least one of claims 1 to 13; B. generating a laser beam by means of the radiation device (2); C. detection of the laser beam by the detection element (4); D. carrying out a gas measurement based on the detection.