Gas chamber

Abstract

The present invention relates to a gas cell (1) for spectroscopically analyzing a gas, in particular by absorption spectroscopy, in which the gas is exposed to an incident beam (S) of electromagnetic radiation and the emerging beam (SA) of the electromagnetic radiation from the gas forms a measurement signal, wherein the gas cell (1) has a body (10) made of a porous material that scatters electromagnetic radiation, an input device (20) for coupling the incident beam (S) into the gas cell (1), and an output device (30) for coupling the emerging beam (S A ) out of the gas cell (1), wherein, according to the invention, a cavity (12) without material is extended and constructed in the body (10), the cavity being surrounded by an inner surface (14) extending inside the material, which inner surface not only diffusely reflects but also transmits the electromagnetic radiation.

Description

Technical Field

[0001] The invention relates to a gas cell for spectroscopically analyzing a gas, in particular by absorption spectroscopy, in which the gas is exposed to an incident beam of electromagnetic radiation and the outgoing beam of electromagnetic radiation from the gas forms a measurement signal, wherein the gas cell has a body formed of a material that scatters electromagnetic radiation, an input device for coupling the incident beam into the gas cell, and an output device for coupling the outgoing beam out of the gas cell. Background Art

[0002] By spectroscopically analyzing a gas, such as absorption spectroscopy or Raman spectroscopy, characteristic features, in particular the gas concentration, can be determined. This is widely used, for example, in industrial processes or in the observation of environmental changes.

[0003] Absorption spectroscopy is based on the measurement of the absorption experienced by an incident beam of electromagnetic radiation as it passes through a gas. If the frequency of the electromagnetic radiation coincides with the resonance frequency of the gas, absorption of the electromagnetic radiation occurs; this can be detected by a decrease in the intensity of the outgoing beam. Suitable radiation sources in particular have lasers that are tunable in a specific frequency range. In this way, absorption can be detected as a decrease in the intensity of the outgoing beam at the corresponding gas resonance frequency.

[0004] The basis of absorption spectroscopy is the Beer-Lambert law. For a beam incident on a gas at a known temperature, the intensity I of the radiation transmitted by the gas is given by the following formula: 0 The intensity I of the radiation transmitted by the gas is given by the following formula:

[0005] I = I 0 ·e -α·z .

[0006] In this formula, z is the optical path length, that is, the path length that the radiation travels in the gas. α represents the absorption coefficient, α = c·ε, where ε is the extinction coefficient and c is the gas concentration. Therefore, when the extinction coefficient and the optical path length are known, the gas concentration c can be determined.

[0007] A special technique in absorption spectroscopy is the so-called absorption spectroscopy using a tunable laser diode, also known as TDLAS (Tunable Diode Laser Absorption Spectroscopy). Here, the laser radiation source is a laser diode. The laser radiation is tuned to the typical absorption lines of the gas, and a detector is used to measure the decrease in the intensity of the outgoing beam. TDLAS can measure very low gas concentrations.

[0008] As can be seen from the given formula, the detection sensitivity of the absorption spectroscopy method for determining the gas concentration depends on the optical path length. Therefore, the goal is to extend the optical path length in absorption spectroscopy to improve the detection sensitivity.

[0009] A known method of extending the optical path length uses so-called multi-channel gas cells. Through these gas cells, electromagnetic radiation is guided to pass repeatedly through the gas to be examined. Known examples of multi-channel gas cells are the White cell and the Herriott cell. In them, the achievable optical path length is proportional to the gas cell volume (defined as the volume enclosed by the outer surface of the body). The optical path length L of the White cell O The ratio to the gas cell volume V can reach L O / V = 7.5·10 2 m -2 , and this ratio for the Herriott cell can reach L O / V = 3·10 4 m -2 . Therefore, extending the optical path length in the range of several meters requires a relatively large gas cell volume. However, gas cells with a large gas cell volume are not feasible in some applications. The response time of the gas cell increases with the increase in the gas cell volume, which is also not conducive to applications.

[0010] The detection sensitivity of multi-channel gas cells is also limited by interference effects caused by Fabry-Perot etalons arranged between the reflective or dispersive surfaces of the multi-channel gas cells, such as mirrors, lenses, etc. To reduce the interference effects, other optical components are added to the gas cell. However, this results in diffuse reflection of electromagnetic radiation, thus causing the formation of speckles.

[0011] To solve these problems, it is proposed to use a so-called integrating sphere (for example, see J. Hodgkinson et al., Applied Optics, Vol. 48, No. 30, 2009) in absorption spectroscopy. A spherical hollow part for introducing the gas to be measured is formed in the body. The surface of the hollow part is reflective to the radiation used. The optical path length in the hollow sphere can be approximated by the expression where R is the radius of the hollow sphere and ρ is the reflectivity of the inner surface of the hollow sphere. Since the optical path length is a linear function of the radius of the hollow sphere, a hollow sphere with a large radius must be used, but this may be disadvantageous for some applications. Another problem with the integrating sphere is gas exchange. The gas must be introduced into the hollow part, and setting up inflow and outflow devices weakens the achievable optical path length.

[0012] Another possibility of extending the optical path length comes from the so-called GASMAS method (gas absorption spectroscopy in scattering media) (M. etc., Optics Letters, Vol. 26, pages 16 - 18, 2001). This method is based on using porous materials to extend the path length. The gas to be measured penetrates the porous material. Materials with high dispersibility for the absorption wavelength of the gas and low absorption need to be selected. Then the electromagnetic radiation incident on the material is scattered multiple times (multiple scattering) at the pores, enabling an optical path length corresponding to a multiple of the thickness of the porous material. For example, "Pathlength Determination for gas in Scattering Media absorption Spectroscopy" by L. Mei et al., Sensors 2014; 14(3): 3871 - 3890 discloses more possibilities of using the GASMAS method and includes the use of macroscopic homogeneous porous media and heterogeneous porous media in which one or more larger cavities are formed in a non - porous dispersive matrix material. WO 2018 / 210583 A1 discloses the use of porous ceramics for gas cells.

[0013] In the past, GASMAS and TDLAS have been combined for gas absorption spectroscopy in porous materials to improve detection sensitivity. However, in this case, the detection sensitivity is still limited by optical interference clutter. To solve this problem, so - called stimulated emission jitter has been proposed, but this requires moving optical components, which is difficult to achieve in commercial gas cells. WO 2018 / 210583 A1 proposes using amplified, unfocused stimulated emission irradiated on a gas cell with porous material as an alternative. This eliminates the interference effect.

[0014] In absorption spectroscopy using porous materials to date, an optical path length increasing with the thickness of the porous layer used is obtained. However, the intensity of the transmitted radiation decreases with the layer thickness, so when the intensity of the outgoing beam drops to the minimum required to detect it, the layer thickness can no longer be increased. This effect and the size of the gas cell thus limit the maximum achievable optical path length.

[0015] EP 2 520 924 A1 and US 4,709,150 A also disclose gas measurement systems. Summary of the Invention

[0016] In view of the problems existing in the above - mentioned prior art, the object of the present invention is to further develop a gas cell of the initially proposed type, so as to achieve high detection sensitivity while having a compact gas cell structure.

[0017] This task is solved according to the invention in the following way: A cavity without material is constructed in the main body, and the cavity is surrounded by an inner surface that not only diffusely reflects but also transmits electromagnetic radiation.

[0018] The present invention is based on the following recognition: Compared with traditional gas cells, this type of gas cell provides a significantly larger ratio of optical path length to gas cell volume. In this case, the gas cell volume is defined as the volume enclosed by the outer surface of the main body of the gas cell.

[0019] Here, the large ratio of optical path length to gas cell volume is generated by the diffuse reflection of radiation on the inner cavity surface and the transmission of this radiation into a porous material that scatters the radiation. The interaction of these two effects can be understood as follows:

[0020] An electromagnetic radiation beam that can be coupled into the gas cell by means of a coupling device enters the cavity. The incident beam can be convergent, divergent, or cylindrical. The beam irradiates an area on the inner cavity surface and is partially diffusely reflected on this surface and partially transmitted into the porous material. The diffusely reflected part of this radiation irradiates other areas on the inner cavity surface, where diffuse reflection or transmission occurs again.

[0021] The transmitted part of this radiation enters the porous material. As described above, multiple scattering of this radiation occurs in the porous material. Preferably, this material does not absorb or hardly absorbs this radiation. The radiation transmitted into this material returns to the cavity after multiple scattering. Compared with the radiation reflected on the surface, the transmitted radiation travels an additional path length. If this radiation enters the cavity again, it irradiates other areas on the inner cavity surface. As described above, there, the radiation is partially reflected and partially transmitted into the porous material. This process is repeated until the radiation is coupled out of the gas cell by the coupling-out device.

[0022] Due to the diffuse reflection on the inner surface and the scattering in the porous material, the outgoing beam of electromagnetic radiation consists of multiple radiations that travel different path lengths in the main body and the cavity. On average, the outgoing beam travels a path length L + Z, where L is the average path length part in the cavity and Z is the average path length part in the porous material. In this case, the average path length Z in the porous material is much larger than the average path length L in the cavity.

[0023] Therefore, the above gas cell is very suitable for absorption spectrometry. For this purpose, the gas cell is brought into contact with the gas to be measured. The porous material is penetrable to the gas to be measured. The gas penetrates into the porous material and enters the cavity. If an incident beam is then coupled into the gas cell, the outgoing beam passes through the measuring gas and travels an average path length L + Z. Since Z is much larger than L, most of the absorbance is determined by the interaction of the electromagnetic radiation with the gas in the porous material.

[0024] The electromagnetic radiation enters the porous material surrounding the cavity multiple times, where it interacts with the gas. Thus, a very large ratio of the optical path length to the gas cell volume can be achieved with the gas cell according to the invention. This enables a compact construction, high detection sensitivity, and a fast response time to be achieved simultaneously. The high detection sensitivity is based on the large achievable optical path length. The fast response time is due to the relatively small gas cell volume and the gas exchange over the entire surface of the gas cell. At the same time, the porous structure allows gas exchange within seconds, enabling rapid detection of concentration changes. Here, the gas input or gas output does not require a complex hose and pump system. Due to the compact construction, the gas cell according to the invention has a large number of possible applications and can be produced at a reasonable cost.

[0025] As described above, the portion of the electromagnetic radiation reflected from the inner surface of the cavity partially enters the porous material when irradiating the inner surface of the cavity again. This avoids the loss of the intensity of the outgoing beam caused by the reflection of the beam on the incident side in the conventional GASMAS process.

[0026] In addition, due to the diffuse scattering of the radiation on the cavity surface and in the material, the occurrence of interference effects is also avoided. This further improves the detection sensitivity of the gas cell.

[0027] Another advantage of the gas cell according to the invention is that the porous material acts as an obstacle that hinders larger molecules from entering the cavity or the porous material itself. Only molecules smaller than the pore size can penetrate or pass through the nanoporous material and reach the cavity. Thus, the porous material acts as a filter or membrane. The interior of the cavity is thus protected from contamination and can remain clean.

[0028] In addition, the material surrounding the cavity acts as a flame arrester. On the one hand, the porous material itself is not flammable. On the other hand, due to the multiple reflections in the gas cell, the measurement can be carried out with a generally smaller output power, thereby preventing or at least significantly reducing the heating of the gas cell by the incident beam. If ignition still occurs inside the cavity, the flame does not escape from the gas cell because the gas has been sufficiently cooled. Thus, higher optical performance can be achieved, enabling the gas cell to meet the requirements of ATEX and / or FM explosion protection guidelines simultaneously.

[0029] According to an embodiment of the invention, the material has a porous ceramic material in which a cavity is formed. The optimal size and its distribution of the pores are determined experimentally and depend on the wavelength used for the gas to be studied and the ceramic material used. A pore diameter of 20 nm to 10 μm is favorable for radiation in the mid-infrared range. For example, a pore diameter of 100 to 300 nm can be used for a wavelength of 760 nm.

[0030] The porous ceramic material can, for example, have zirconia, alumina, titanium oxide, silicon oxide, magnesium oxide, yttrium oxide, gallium phosphide, porous silicon, or a mixture thereof.

[0031] The porosity of the material can reach at least 25%, preferably at least 30%, more preferably at least 35%, while the maximum is 70%, preferably the maximum is 65%, more preferably the maximum is 60%. If the porosity is less than 25%, the material is very dense and the effect of extending the path length achieved by scattering on the pores is too small. On the other hand, if the porosity is greater than 70%, the porous material is unstable and not suitable for use as the main body of the gas chamber.

[0032] The cavity can have any shape. In one possible embodiment, the cavity has the shape of a hollow sphere with a radius R K . However, the cavity can also have other shapes, such as an ellipsoid.

[0033] Here, the signal-to-noise ratio can be considered to select the volume and / or the inner surface area of the cavity. In fact, experiments show that the signal-to-noise ratio is a function of the cavity volume size and the inner surface area, so the signal-to-noise ratio can be improved by appropriately determining the volume and / or the inner surface area of the cavity. If the cavity has the shape of a hollow sphere, the radius R of the hollow sphere K can be selected according to the signal-to-noise ratio, and the radius R of the hollow sphere K is preferably at least 2 mm, more preferably at least 5 mm, still more preferably at least 10 mm, and even more preferably at least 15 mm. If the selected radius of the hollow sphere is too small, the absorbance is too low, resulting in an unsatisfactory signal-to-noise ratio.

[0034] According to an embodiment of the present invention, the main body has an outer surface that surrounds the inner surface of the cavity at a spaced distance, in which case the loss radiation exiting on the outer surface is less than a pre-given loss limit. The loss limit is preferably less than or equal to 99% of the intensity of the incident beam, more preferably less than or equal to 95% of the intensity of the incident beam, and still more preferably less than or equal to 90% of the intensity of the incident beam. As described above, the inner surface of the cavity not only diffusely reflects but also transmits electromagnetic radiation. The transmitted radiation incident on the material can reach the outer surface of the main body and thus exit the main body as loss radiation. However, if the distance between the inner surface of the cavity and the outer surface of the main body is selected to be large enough, only a small part of the radiation entering the porous material reaches the outer surface. The rest will be redirected back into the cavity by scattering on the pores of the material. For a strongly scattering material, a distance of 3 mm to 4 mm between the inner surface of the cavity and the outer surface of the main body may be sufficient to limit the escape of the loss radiation to a reasonable amount. This will increase the intensity of the exiting beam, thereby improving the signal-to-noise ratio.

[0035] In the gas cell according to the invention, the optical path length L that the beam travels in the gas cell O and the ratio to the volume V of the gas cell enclosed by the outer surface of the body (i.e., L O / V) can be greater than or equal to 10 5 m -2 , preferably greater than or equal to 2·10 5 m -2 , more preferably greater than or equal to 3·10 5 m -2 . For example, for a gas cell with a gas cell volume of V = 4·10 -5 m 3 , an optical path length L of more than 8 m can be achieved O . In another exemplary embodiment of the invention with a gas cell volume of V = 4.5·10 -6 m 3 , an optical path length L of 1.4 m can be achieved O . This is a significant improvement over conventional gas cells (such as White cells and Herriott cells).

[0036] In one embodiment of the invention, the coupling-in device can be configured to couple an incident beam into the cavity in a divergent manner. When the divergent beam enters the cavity, the occurrence of destructive interference effects is further suppressed. The coupling-in device and / or the coupling-out device can have an optical conductor. This divergent coupling-in can then be achieved by using an optical conductor with the highest possible numerical aperture. For example, an optical fiber with a numerical aperture of 0.22 to 0.50 can be used. Additionally or alternatively, a lens for broadening the beam can be arranged at the end of the optical conductor that is coupled into the gas cell.

[0037] The coupling-in device can be used to couple the electromagnetic radiation of an incident beam into the cavity. The coupling-in device can be configured such that the coupling-in device couples all of the electromagnetic radiation of the incident beam into the cavity. Alternatively, the coupling-in device can be configured such that the coupling-in device only couples a part of the electromagnetic radiation of the incident beam into the cavity. Additionally or alternatively, the coupling-out device can also be configured such that the coupling-out device couples the electromagnetic radiation forming the output beam out of the cavity. The coupling-out device can be configured such that the coupling-out device couples all of the electromagnetic radiation of the output beam out of the cavity. Alternatively, the coupling-out device can be configured such that the coupling-out device only couples a part of the electromagnetic radiation of the output beam out of the cavity. If the coupling-in device and / or the coupling-out device has an optical conductor, the optical conductor can be guided from the outer surface of the body through the body to the cavity. In other words, the optical conductor extends to or into the cavity. This enables the electromagnetic radiation to be coupled into and / or out of the cavity particularly simply.

[0038] In the above-described embodiment, the light conductor intersects the inner surface of the cavity. The area of the inner surface that can reflect electromagnetic radiation is thereby reduced. Since the light conductor also extends through the porous material, the electromagnetic radiation incident on the porous material in this area cannot be scattered in the porous material. These two effects result in a reduction in the extension of the optical path length. This reduces the absorbance. On the other hand, using a light conductor with a large cross-section can improve the signal-to-noise ratio. Therefore, in one embodiment of the present invention, the size of the area occupied by the coupling-in and / or coupling-out device in the body is selected considering the absorbance and / or the signal-to-noise ratio. For a cavity having the shape of a hollow sphere with a radius R K = 7 mm, excellent absorbance can be achieved with an optical fiber having a radius of 300 μm. On the other hand, an optical fiber with a radius of 750 μm can be used to optimize the signal-to-noise ratio. For a cavity with a radius R K = 7 mm, an optical fiber with a radius between 300 μm and 750 μm is advantageous.

[0039] The coupling-in device can be configured such that the coupling-in device couples the electromagnetic radiation of the incident beam into the area of the body having the porous material. The coupling-in device can be configured such that the coupling-in device couples all of the electromagnetic radiation of the incident beam into the area of the body having the porous material. Alternatively, the coupling-in device can be configured such that the coupling-in device only couples a part of the electromagnetic radiation of the incident beam into the area of the body having the porous material. Additionally or alternatively, the coupling-out device can be configured such that the coupling-out device couples the electromagnetic radiation forming the outgoing beam out of the area of the body having the porous material. The coupling-out device can be configured such that the coupling-out device couples all of the electromagnetic radiation forming the outgoing beam out of the area of the body having the porous material. Alternatively, the coupling-out device can be configured such that the coupling-out device only couples a part of the electromagnetic radiation forming the outgoing beam out of the area of the body having the porous material.

[0040] For example, if the coupling-out device has a light conductor, the end of the light conductor that couples out the radiation from the body can be arranged in the porous material at a distance a from the inner surface of the cavity. This can prevent the beam incident on the cavity from being immediately coupled out of the cavity again without passing through the porous material after reflection. This is because, due to the distance between the inner surface of the cavity and the coupling-out end of the optical fiber, the electromagnetic radiation always passes through at least the distance a before entering the coupling-out device. a can be, for example, 1 mm.

[0041] The coupling-out device can also be arranged entirely outside the body. However, the distance between the inner surface of the cavity and the outer surface of the body can be selected such that the electromagnetic radiation can reach the coupling-out device with sufficient intensity. This arrangement can maximize the absorbance.

[0042] In one embodiment of the present invention, the coupling-in device can couple all the electromagnetic radiation of the incident beam into the cavity. The coupling-out device can be configured such that the coupling-out device couples all the electromagnetic radiation forming the outgoing beam out of the cavity. Alternatively, the coupling-out device can be configured such that the coupling-out device couples all the electromagnetic radiation forming the outgoing beam out of the region of the body having the porous material.

[0043] The gas cell according to the present invention can be used for absorption spectroscopy of gases. For this purpose, the gas cell is brought into contact with the gas to be measured. The porous material is permeable to the gas to be measured. The gas penetrates into the porous material and enters the cavity. If electromagnetic radiation of an appropriate wavelength is coupled into the gas cell by the coupling-in device, the gas concentration can be determined by measuring the intensity of the radiation coupled out by the coupling-out device, using the Beer-Lambert law (see above). The optical path length in the gas cell is predetermined by measurement with a gas of known concentration.

[0044] Furthermore, the present invention relates to a component for absorption spectroscopy of gases, the component having:

[0045] - a gas cell according to any one of the foregoing aspects;

[0046] - an electromagnetic radiation source for generating an incident beam;

[0047] - a detector device for detecting the outgoing beam.

[0048] The electromagnetic radiation source can have a laser light source. In particular, the electromagnetic radiation source can have a tunable laser diode. The detector assembly can have a photodetector.

[0049] The gas cell according to the present invention and the component for absorption spectroscopy of gases according to the present invention can be used to determine the concentration of one or more of the following gases: oxygen, carbon dioxide, carbon monoxide, nitric oxide, methane, amines, ammonia, hydrogen sulfide, sulfur oxides, hydrogen halides such as HCl or HF, water, or mixtures thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In the following description, the present invention will be explained by way of example with reference to the drawings. The content shown in the drawings is as follows:

[0051] Figure 1 : Cross-sectional view of a first embodiment of a gas cell according to the present invention; wherein a partial radiation path in the gas cell is shown by way of example;

[0052] Figure 2a: Figure 1 Cross-sectional view of the gas cell shown in ;

[0053] Figure 2b: Cross-sectional view of a first variant of the gas cell shown in Figure 2a;

[0054] Figure 2c: Cross-sectional view of a second variant of the gas chamber shown in Figure 2a;

[0055] Figure 3a: Cross-sectional view of a second embodiment of the gas chamber according to the present invention;

[0056] Figure 3b: Cross-sectional view of a variant of the gas chamber shown in Figure 3a;

[0057] Figure 4a: Cross-sectional view of a third embodiment of the gas chamber according to the present invention;

[0058] Figure 4b: Cross-sectional view of a variant of the gas chamber shown in Figure 4a;

[0059] Figure 5 : Cross-sectional view of components of a fourth embodiment of the gas chamber according to the present invention;

[0060] Figure 6 : Cross-sectional view of a fifth embodiment of the gas chamber according to the present invention. Detailed Description

[0061] Figure 1 is a cross-sectional view of a first embodiment of the gas chamber 1 according to the present invention, in which a partial radiation path within the gas chamber 1 is shown by way of example. The gas chamber 1 has a body 10. The body 10 can be, for example, a block. The body 10 is made of a porous material that scatters electromagnetic radiation. In the body 10, a cavity 12 without material is constructed. In this embodiment, the cavity 12 has the shape of a hollow sphere. The cavity 12 is bounded by an inner surface 14. The body 10 has an outer surface 16 that surrounds the inner surface 14 of the cavity 12 at a distance.

[0062] The gas chamber 1 also has an input device 20 for coupling an incident beam S into the gas chamber 1. In this embodiment, the input device 20 has an optical conductor. For this purpose, a region without material is provided in the body 10 through which the optical conductor can be introduced from the outside of the gas chamber 1 into the cavity 12. In this embodiment, the input device 20 couples all the electromagnetic radiation of the incident beam into the cavity 12. In addition, the gas chamber 1 has an output device 30. In this embodiment, the output device 30 couples out all the electromagnetic radiation that forms the output beam S A from the cavity. In addition, in this embodiment, the output device 30 has an optical conductor. Another region without material is provided in the body 10 through which the optical conductor can reach the outside of the body 10 from the cavity 12.

[0063] The beam is coupled into the cavity 12 from the outside of the body 10 through the light conductor of the coupling-in device 20. The coupling-in device 20 can be configured such that the incident beam S is coupled into the cavity 12. For this purpose, for example, a light conductor with a high numerical aperture can be used. Alternatively, a lens can be provided at the end of the light conductor leading into the cavity 12.

[0064] The radiation path within the gas chamber 1 is described below. For the sake of simplicity in description, only the path of the main radiation of the beam (also simply referred to as radiation S) is shown in all the figures.

[0065] The radiation S coupled into the cavity 12 passes through the region of the cavity 12 and irradiates a point P1 on the inner surface 14 of the cavity 12. According to the present invention, the inner surface 14 is both diffusely reflective and transmissive to this electromagnetic radiation. Thus, a part of the radiation S is diffusely reflected at the point P1 on the inner surface 14 of the cavity 12, while a part of this radiation is transmitted into the porous material at the point P1 on the inner surface 14. Figure 1 Three reflected radiations S(R1), S 1 (R1) and S 2 (R1) are exemplified. However, for the sake of simplicity in description, only the further path of the reflected radiation S(R1) is shown. The radiations S 1 (R1) and S 2 (R1) are not shown further, so these radiations are represented by dashed arrows. The transmitted part of the radiation S enters the porous material from different directions. As in the case of the reflected radiation, for the sake of simplicity in description, only the path of a single radiation S(T1) is shown. For clarity, three other radiations entering the material at the point P1 are also drawn. Since their paths will not be further described below, they are shown by dashed arrows. The radiation S(T1) extending in the porous material is scattered multiple times in the porous material before re-entering the cavity 12.

[0066] Now the paths of the reflected radiation S(R1) and the transmitted radiation S(T1) are described. The reflected radiation S(R1) irradiates the inner surface 14 of the cavity 12 at a second point P2 and there a part is diffusely reflected again while another part is transmitted into the porous material. Here, as in the case of the point P1, there are also several reflected and transmitted radiations, where, for the sake of simplicity in description, only one reflected radiation and one transmitted radiation are shown respectively. The reflected part S(R1, R2) continues to extend through the cavity 12 until it irradiates a point P5 on the inner surface 14 of the cavity 12. This reflected part is again partly reflected and partly transmitted there.

[0067] The radiation S(T1) transmitted at point P1 passes through the cavity 12 after re-entering it until it irradiates point P3 on the inner surface 14 of the cavity 12, where a part of the radiation is reflected and another part is transmitted. The reflected part S(T1, R2) enters the light conductor of the coupling-out device 30 and is coupled out of the gas chamber 1, contributing to the formation of the outgoing beam S A (For simplicity, only one radiation S is shown in the figure A ). The other radiations continue to extend through the cavity 12 in a similar manner and partially pass through the porous material so as to finally enter the light conductor of the coupling-out device 30 and be coupled out of the gas chamber 1.

[0068] Therefore, the outgoing beam S A consists of multiple outgoing radiations that have traveled different optical path lengths in the gas chamber 1. The outgoing beam S A has traveled an effective path length L eff = L + Z, where L represents the effective path length within the cavity 12 and Z represents the effective path length within the porous material.

[0069] The following compares the optical path length extension of the gas chamber 1 shown in Figure 1 with that of an Ulbricht sphere. In the latter, the inner surface is almost perfectly reflecting and the cavity is not surrounded by porous material, such that the path length extension is only determined by the path within the hollow sphere. For a hollow sphere with a radius of 5 mm and a reflectivity of 0.985, the effective path length in the case of an Ulbricht sphere is 0.44 m. In the case of the embodiment shown in Figure 1 , the effective path length is greater than 3 m. In the case of a hollow sphere with a radius of 15 mm and a reflectivity of 0.985, the effective path length of the Ulbricht sphere is 1.33 m, while Figure 1 the effective path length of the gas chamber shown exceeds 8 m. In other words, compared with the Ulbricht sphere, the effective path length can be significantly increased by the gas chamber 1 according to the present invention.

[0070] Figures 2a to 2c show three possibilities for arranging the coupling-out device 30 for the gas chamber 1 having a cavity 12 in the shape of a hollow sphere. However, these possibilities are not limited to a cavity in the shape of a hollow sphere but can be used for cavities of any shape.

[0071] Figure 2a shows Figure 1 the first embodiment of the present invention presented in Figure 1 . As explained in the description of

[0072] A part of the outgoing beam S A is only reflected on the reflective surface after entering the cavity. Thus, this part of the beam does not pass through the porous material. This results in a reduction of the achievable optical path length.

[0073] In addition, the coupling-in and coupling-out devices 20, 30 not only reduce the area of the inner surface 14 that can reflect electromagnetic radiation, but also reduce the volume of the porous material that can reflect electromagnetic radiation.

[0074] A solution to this problem is shown in Fig. 2b. Here, the coupling-out device 30 does not reach the cavity 12, but ends in the porous material. In order to be coupled out by the coupling-out device 30, the electromagnetic radiation must at least pass through the region between the inner surface 14 of the cavity 12 and the inlet end 32 of the coupling-out device 30 in order to be coupled out of the gas chamber 1 (see the path of the radiation S', which is transmitted into the porous material at point P' and emerges as the transmitted radiation S T entering the light guide). Here, the radiation extends through the porous material and thus obtains an additional path length extension. The coupling-out device 30 thus couples out the electromagnetic radiation forming the outgoing beam S A from the region 18 of the body 10 having the porous material.

[0075] The effect shown in Fig. 2b can be further amplified by the variant shown in Fig. 2c. Here, the coupling-out device 30 is completely arranged outside the body 10. However, in this case, the distance between the inlet end 32 of the coupling-out device 30 and the cavity 12 must be small enough to receive a sufficient signal. In the case of a light guide, the signal-to-noise ratio can be improved by increasing the diameter of the light guide.

[0076] Figs. 3a and 3b show cross-sectional views of a second embodiment of the gas chamber 1 according to the invention. The cavity 12 here has a convex shape, but is not spherical. In the embodiment shown in Fig. 3a, the coupling-out device 30 is arranged at a distance from the cavity 12 in the region 18 of the body 10 having the porous material. Thus, in any case, a path length extension due to scattering in the porous material is obtained.

[0077] Fig. 3b shows a variant of the embodiment in Fig. 3a. The coupling-out device 30 is, for example, completely arranged outside the gas chamber 1 as a light guide. Thereby, a particularly large part of the radiation reflected in the porous material is obtained (see the radiation S T ).

[0078] Figure 4a shows a sectional view of a third embodiment of the gas chamber 1 according to the present invention. Here, the cavity 12 has an ellipsoidal shape. The coupling-in device 20 is connected to the cavity 12. The coupling-out device 30 is arranged at a distance from the cavity 12 in the region 18 of the main body 10 having a porous material. Thus, in any case, the radiation coupled out from the gas chamber 1 by the coupling-out device 30 extends through the porous material (see radiation S T ).

[0079] Figure 4b shows a variant of the third embodiment shown in Figure 4a. Here, the coupling-out device 30 is arranged such that the inlet end 32 is located at the focus of the ellipsoid.

[0080] Figure 5 a shows a sectional view of the components of a fourth embodiment of the gas chamber 1 according to the present invention. The main body 10 of the gas chamber 1 consists of three parts: a left part 10a, a middle part 10b, and a right part 10c. In these parts 10a and 10c, a hollow sphere notch is respectively provided. A cylindrical section notch is provided in the middle part 10b. These notches can be produced by milling in a ceramic body. Then, these parts 10a, 10b, and 10c are combined to jointly form the main body 10, in which the cavity 12 is constructed.

[0081] Figure 6 shows a sectional view of a fifth embodiment of the gas chamber 1 according to the present invention. In the main body 10, two separate cavities 12a and 12b are provided, and the cavities have the shape of a hollow sphere in the Figure 6 view shown. The coupling-in device 20 is connected to the cavity 12a, so that electromagnetic radiation can be coupled into the cavity 12a through the coupling-in device 20. The coupling-out device 30 is connected to the cavity 12b. In order to be able to couple out the radiation through the coupling-out device 30, the radiation must pass from the first cavity 12a through the porous material into the second cavity 12b (radiation S T ). Thus, in any case, in this embodiment, a contribution to the extension of the path length due to the path of the radiation in the porous material is also obtained.

[0082] List of reference numerals

[0083] 1 Gas chamber

[0084] 10 Main body

[0085] 12 Cavity

[0086] 14 Inner surface

[0087] 16 Outer surface

[0088] 18 Region with porous material

[0089] 20 Coupling-in device

[0090] 30 Coupling-out device

[0091] 32 Inlet end

[0092] S Incident beam

[0093] S A Emergent beam

[0094] L O Optical path length

[0095] V Gas cell volume

Claims

1. A gas cell (1) for absorption spectroscopic analysis of a gas, in which the gas is exposed to an incident beam (S) of electromagnetic radiation and the electromagnetic radiation's emerging beam (S A ) is detected to form a measurement signal, and the absorption is detected as a reduction in the intensity of the emerging beam at the resonance frequency of the gas, Among them, the gas chamber (1) has: a main body (10) composed of a porous material that scatters the electromagnetic radiation, and An input device (20) for coupling the incident beam (S) into the gas chamber (1) and an output device (30) for coupling the output beam (S A ) out of the gas chamber (1). characterized in that a cavity (12) without material is formed in the main body (10), and the cavity is surrounded by an inner surface (14) extending inside the material. The inner surface not only diffusely reflects but also transmits the electromagnetic radiation. Among them, the porous material is penetrable by the gas, so that when the gas chamber (1) is in contact with the gas, the gas invades the porous material and enters the cavity (12).

2. The gas chamber (1) according to claim 1, wherein, the material has a porous ceramic material, and the cavity (12) is formed in the porous ceramic material.

3. The gas chamber (1) according to claim 1 or 2, wherein, the porosity is at least 25% and at most 70%.

4. The gas chamber (1) according to claim 1 or 2, wherein, The cavity (12) has the shape of a hollow sphere having a radius R K .

5. The gas chamber (1) according to claim 4, wherein, The radius R of the hollow sphere K is selected considering the signal-to-noise ratio.

6. The gas chamber (1) according to any one of claims 1, 2, and 5, wherein, the main body (10) has an outer surface (16) that surrounds the inner surface (14) of the cavity (12) at a distance. In this case, the loss radiation emitted on the outer surface (16) is less than a pre-given loss limit.

7. The gas chamber (1) according to any one of claims 1, 2, and 5, wherein, The optical path length (L O ) traveled by the beam (S) in the gas cell (1) and the volume (V) of the gas cell enclosed by the outer surface (16) of the body (10) have a ratio greater than or equal to 10 5 m -2 .

8. The gas chamber (1) according to any one of claims 1, 2, and 5, wherein, the coupling-in device (20) is configured to couple the incident beam (S) into the cavity (12) in a divergent manner.

9. The gas chamber (1) according to any one of claims 1, 2, and 5, wherein, the coupling-in device (20) and / or the coupling-out device (30) has an optical conductor.

10. The gas chamber (1) according to any one of claims 1, 2, and 5, wherein, The coupling-in device (20) is configured to couple the electromagnetic radiation of the incident beam (S) into the cavity (12), and / or the coupling-out device (30) is configured to couple out the electromagnetic radiation forming the output beam (S A ) from the cavity (12).

11. The gas chamber (1) according to any one of claims 1, 2, and 5, wherein, the size of the region occupied by the coupling-in and / or coupling-out device (20, 30) in the main body (10) is selected in consideration of absorbance and / or signal-to-noise ratio.

12. The gas chamber (1) according to any one of claims 1, 2, and 5, wherein, The coupling-in device (20) is configured to couple the electromagnetic radiation of the incident beam (S) into the region (18) of the body (10) having the porous material, and / or the coupling-out device (30) is configured to couple out the electromagnetic radiation forming the outgoing beam (S A ) from the region (18) of the body (10) having the porous material.

13. The gas chamber (1) according to claim 5, wherein, The radius R of the hollow sphere K is at least 2 mm.

14. The gas chamber (1) according to claim 6, wherein, the loss limit is less than or equal to 99% of the intensity of the incident beam (S).

15. A detector assembly for absorption spectroscopic analysis of a gas, the detector assembly having: the gas chamber (1) according to any one of the foregoing claims; an electromagnetic radiation source for generating the incident beam (S); and for detecting the outgoing beam (S A ).

16. The detector assembly according to claim 15, wherein, the electromagnetic radiation source has a tunable laser diode.

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