Gas Cell

Abstract

A gas is exposed to an incident beam of electromagnetic radiation (S) and an outgoing beam of electromagnetic radiation (S A A gas cell (1) for spectroscopic analysis of gases, in particular absorption spectroscopic analysis, in which a measurement signal is generated by detecting a light beam (S) emitted from the gas cell (1), the gas cell (1) comprising a body (10) formed by a porous electromagnetic radiation scattering material, an incoupling device (20) for coupling an incident light beam (S) into the gas cell (1), and an outgoing light beam (S). A ) from the gas cell (1), and in accordance with the present invention the gas cell is further developed in accordance with the present invention by forming a material-free cavity (12) in the body portion (10), the cavity (12) extending into the material and surrounded by an inner surface (14) that is both diffusely reflective and transparent to electromagnetic radiation.

Description

[Technical Field]

[0001]

[0001] The present invention relates to a gas cell for spectroscopic analysis of a gas, particularly for absorption spectroscopy, in which a gas is exposed to an incident beam of electromagnetic radiation and the beam of electromagnetic radiation emitted from the gas is detected to form a measurement signal, the gas cell comprising a body made of a material that scatters electromagnetic radiation, and an incoupling device for coupling the incident beam of electromagnetic radiation into the gas cell and an outcoupling device for outcoupling the emitted beam of electromagnetic radiation from the gas cell. [Background technology]

[0002]

[0002] Spectroscopic analysis of gases, such as absorption spectroscopy or Raman spectroscopy, allows for the determination of characteristic properties, particularly gas concentration. This has a wide range of applications, for example, in industrial processes or in observing environmental changes.

[0003]

[0003] Absorption spectroscopy is based on measuring the absorption of an incident beam of electromagnetic radiation as it passes through a gas. When the frequency of the electromagnetic radiation matches the resonance frequency of the gas, absorption of the electromagnetic radiation occurs, which can be detected by a decrease in the intensity of the emitted beam. Suitable radiation sources include, in particular, lasers that are tuned within a specified frequency range. In this case, absorption can be detected as a decrease in the intensity of the emitted beam at the corresponding resonance frequency of the gas.

[0004]

[0004] The basis of absorption spectroscopy is the Lambert-Beer law. In the case of a light beam incident on a gas of known concentration with intensity I0, the intensity I of the radiation transmitted by the gas can be obtained by the following equation. I = I0·e -α·z

[0005] Here, z is the optical path length, i.e., the path length through which the radiation travels in the gas. α is the absorption coefficient, α = c·ε, where ε is the extinction coefficient and c is the gas concentration. Therefore, if the extinction coefficient and optical path length are known, the gas concentration c can be determined.

[0005]

[0006] A special technique within absorption spectroscopy is so-called absorption spectroscopy using a tunable diode, also known as TDLAS (Tunable Diode Laser Absorption Spectroscopy). Here, the laser radiation source is a laser diode. The laser radiation is tuned using the typical absorption lines of the gas, and the decrease in the intensity of the emitted light beam is measured using a detector. TDLAS enables the measurement of very low gas concentrations.

[0006]

[0007] The presented equation clearly shows that the detection sensitivity of the absorption spectroscopy method used to determine gas concentration depends on the optical path length. Therefore, the goal is to extend the optical path length in the absorption spectroscopy method to achieve improved detection sensitivity.

[0007]

[0008] One known method for extending the optical path length involves using so-called multi-gas cells. These cells repeatedly induce electromagnetic radiation through the gas being examined. Known examples of multi-path gas cells include the White gas cell and the Heliot gas cell. In these, the achievable optical path length is proportional to the gas cell volume (defined as the volume surrounded by the outer surface of the main body), and in the case of the White gas cell, the optical path length L is proportional to the gas cell volume V. O Ratio L O / V=7.5·10 2 m -2 , and in the case of a heliot gas cell, L O / V=3·10 4 m -2 This can be achieved. Therefore, relatively large gas cell volumes are required to extend the optical path length over a range of several meters. However, gas cells with large gas cell volumes are not feasible in certain applications. The response time of the gas cell, which increases with the gas cell volume, is also unfavorable for use.

[0008]

[0009] The detection sensitivity of multipath gas cells is also limited by interference effects caused by the formation of Fabry-Perot etalons between the reflective or dispersion surfaces of the multipath gas cell, such as mirrors and lenses. Further optical components are added to the gas cell to reduce interference effects; however, these result in diffuse reflection of electromagnetic radiation that causes speckle formation.

[0009]

[0010] To solve these problems, the use of a so-called Ulbricht sphere for absorption spectroscopy has been proposed (see, e.g., J. Hodgkinson et al., Applied Optics Vol. 48, No. 30, 2009). Here, a spherical depression for introducing the gas to be measured is formed in the main body. The surface of the depression is reflective to the radiation used. In this case, the optical path length within the spherical shell is given by equation g

[0010]

number

[0011] It can be approximated by the equation, where R is the radius of the spherical shell and ρ is the reflectance on the inner surface of the spherical shell. Since the optical path length is a linear function of the radius of the spherical shell, the use of a spherical shell with a large radius is required, which may be disadvantageous for certain applications. A further problem associated with Ulbricht spheres is gas exchange. The gas must be introduced into the recess, and the provision of inlet and outlet devices reduces the achievable optical path length.

[0012]

[0011] Another possibility for extending the optical path length is provided by the so-called GASMAS method (gas in scattering media absorption spectroscopy) (M. Sjoholm et al., Optics Letters Vol. 26, 16 - 18, 2001). This method is based on the use of a porous material for extending the path length. The porous material is penetrated by the gas to be measured. The material is selected to have high dispersion characteristics for the absorption wavelength of the gas and low absorbency. The electromagnetic radiation incident on the material is then scattered multiple times (multiple scattering) in the pores, thus enabling the achievement of an optical path length corresponding to a multiple of the thickness of the porous material. Further possibilities for the use of the GASMAS method are disclosed, for example, in L. Mei et al., "Pathlength Determination for gas in Scattering Media absorption Spectroscopy", Sensors 2014;14(3):3871 - 3890, and include the use of a macroscopically homogeneous porous medium and a heterogeneously porous medium in which one or more larger cavities are formed within a pore-free dispersive matrix material. WO2018 / 210583 A1 discloses the use of a porous ceramic for a gas cell.

[0013]

[0012] In the past, GASMAS has been combined with TDLAS for performing absorption spectroscopy of gases in porous materials, thus achieving an improvement in detection sensitivity. However, in such cases, the detection sensitivity is limited by optical interference noise. To solve this problem, so-called laser beam dithering has been proposed, but this requires the optical components to be moved, which is difficult to achieve in a commercially available cell. WO2018 / 210583 A1 proposes, as an alternative, the use of a broad unfocused laser beam impinging on a gas cell in a porous material. This results in the removal of the interference effect.

[0014]

[0013] To date, the use of porous materials for absorption spectroscopy has provided an optical path length that increases with the thickness of the porous layer used. However, the intensity of the transmitted radiation increases with the layer thickness, and thus the layer thickness can only be increased until the intensity of the outgoing beam has decreased to the minimum value necessary to detect it. Therefore, this effect and the dimensions of the gas cell limit the maximum achievable optical path length.

[0015]

[0014] EP2 520 924 A1 and US Patent No. 4,709,150 A also disclose gas measurement systems.

Summary of the Invention

Problems to be Solved by the Invention

[0016]

[0015] From the perspective of the problems in the described prior art, the object of the present invention is to further develop a gas cell of the type first described such that high detection sensitivity is achieved with a simultaneous small design.

Means for Solving the Problems

[0017]

[0016] This object is achieved according to the invention in that a cavity without material is formed in the body part, surrounded by a surface that diffusely emits and also transmits electromagnetic radiation.

[0017] The present invention is based on the understanding that a gas cell of this type provides a considerably higher ratio of optical path length to gas cell volume compared to conventional gas cells. In this case, the gas cell volume is defined as the volume enclosed by the outer surface of the body part of the gas cell.

[0018]

[0018] Here, the high ratio of optical path length to gas cell volume is brought about by both diffuse reflection of radiation on the inner surface of the cavity and transmission of radiation into a radiation-scattering porous material. The interaction of the two effects can be understood as follows.

[0019]

[0019] A beam of electromagnetic radiation, which can be coupled into a gas cell using an incoupling device, enters the cavity. The incident beam of radiation may be focused, divergent, or cylindrical. The beam of radiation strikes a region on the inner surface of the cavity, is partially diffusely reflected on the surface, and partially transmitted into the porous material. The diffusely reflected portion of the radiation strikes further areas on the inner surface of the cavity, where again, diffuse reflection or transmission occurs.

[0020]

[0020] The portion of the radiation that has been transmitted enters the porous material. As described above, multiple scattering of radiation occurs within the porous material. Preferably, the material exhibits little to no absorption of radiation. Following multiple scattering, the radiation transmitted into the material re-enters the cavity. At this point, the transmitted radiation travels an additional path length compared to the radiation reflected at the surface. Once the radiation re-enters the cavity, it collides with further regions on the inner surface of the cavity. There, as just described above, the radiation is partially reflected and also partially transmitted into the porous material. This process is repeated until the radiation is decoupled from the gas cell by the decoupling device.

[0021]

[0021] As a result of diffuse reflection on the inner surface and scattering within the porous material, the emitted beam of electromagnetic radiation consists of multiple rays covering different path lengths in the main body and within the cavity. On average, the emitted beam travels a path length L + Z, where L is the portion of the average path length inside the cavity and Z is the portion of the average path length within the porous material. In this case, the average path length Z within the porous material is much larger than the average path length L within the cavity.

[0022]

[0022] Therefore, the gas cell described above is very well suited for use in absorption spectroscopy. For this purpose, the gas cell is in contact with the gas being measured. The porous material is permeable to the gas being measured. The gas penetrates into the porous material and into the cavities. When the incident light beam is then coupled into the gas cell, the exit light beam travels, on average, through the sample gas over a path length L + Z. Since Z is much larger than L, the majority of the absorbance is attributable to the interaction of electromagnetic radiation with the gas in the porous material.

[0023]

[0023] Electromagnetic radiation enters the porous material surrounding the cavity multiple times, where it interacts with the gas. Therefore, an ultra-high ratio of optical path length to gas cell volume can be achieved by a gas cell according to the present invention. This enables a compact design with high detection sensitivity and high speed. 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 gas exchange across the entire surface of the gas cell. At the same time, the porous structure enables gas exchange within seconds and therefore high-speed detection of concentration changes. Gas input or output by a complex system of tubing and pumps is not required for this. As a result of the compact design, gas cells according to the present invention have numerous potential applications and can be produced at reasonable cost.

[0024]

[0024] As described above, some of the electromagnetic radiation reflected from the inner surface of the cavity enters the partially porous material as it strikes the inner surface of the cavity again. As a result, the loss of intensity of the exiting light beam, which is caused by reflection on the entrance side of the light beam and is seen in conventional gas MA processes, is avoided.

[0025]

[0025] Furthermore, interference effects are avoided due to the diffuse scattering of radiation on the cavity surface and within the material. This results in a further improvement in the detection sensitivity of the gas cell.

[0026] A further advantage of the gas cell according to the present invention is that the porous material acts as a barrier, preventing larger molecules from entering the cavities or the porous material itself. Only molecules smaller than the size of the pores can penetrate the nanoporous material or pass through it to reach the cavities. Thus, the porous material acts as a filter or membrane. Consequently, the inside of the cavities is protected from contamination and remains clean.

[0026]

[0027] Furthermore, the material surrounding the cavity acts as a flame arrestor. On the one hand, the porous material itself is not easily flammable. On the other hand, due to multiple reflections within the gas cell, measurements can be performed with generally lower power output so that heating of the gas cell by the incident light beam can be prevented, or at least significantly reduced. Nevertheless, if ignition occurs within the cavity, the flame will not leak out of the gas cell because the gas has already cooled extremely. Thus, higher optical performance can be enabled, and as a result, this gas cell can simultaneously meet the requirements of ATEX and / or FM explosion prevention guidelines.

[0027]

[0028] According to one embodiment of the present invention, the material includes a porous ceramic material in which cavities are formed. The optimal size of the pores and their distribution are determined experimentally and depend on the wavelength used for the gas being evaluated and the ceramic material used. Pore sizes of 20 nm to 10 μm are favorable for radiation in the mid-infrared region. For example, pore sizes of 100 to 300 nm may be used for a wavelength of 760 nm.

[0028]

[0029] Porous ceramic materials may contain, for example, zirconium oxide, aluminum oxide, titanium oxide, silicon oxide, magnesium oxide, yttrium oxide, gallium phosphide, porous silicon, or mixtures thereof.

[0029]

[0030] The porosity can be at least 25%, preferably at least 30%, more preferably at least 35%, and also at most 70%, preferably at most 65%, and more preferably at most 60%. When the porosity is less than 25%, the material is very dense and the effect of extending the path length due to scattering in the pores is slight. On the other hand, when the porosity is greater than 70%, the porous material is unstable and not suitable for use as the main body of the gas cell.

[0030]

[0031] The cavities can have any arbitrary shape. In one potential embodiment, the cavity has a spherical shell with a radius R K However, the cavities can also have other shapes, such as an ellipsoidal shape.

[0031]

[0032] Here, the volume and / or inner surface of the cavity can be selected considering the signal-to-noise ratio. In fact, it has been experimentally shown that the signal-to-noise ratio is a function of the cavity volume and the internal surface area, and thus an improvement in the signal-to-noise ratio can be achieved by suitably sizing the cavity volume and / or the internal surface area. When the cavity has the form of a spherical shell, the radius R K of the spherical shell can be selected considering the signal-to-noise ratio, and the radius R K of the spherical shell can preferably be at least 2 mm, more preferably at least 5 mm, even more preferably at least 10 mm, and still more preferably at least 15 mm. If a spherical shell radius that is too small is selected, the absorbance is too low, resulting in a poor signal-to-noise ratio.

[0032]

[0033] According to one embodiment of the present invention, the main body has an outer surface surrounding the inner surface of the cavity, at a distance such that the leak radiation emanating from the outer surface is below a specified loss limit. The loss limit is preferably 99% of the intensity of the incident light beam, more preferably 95%, and even more preferably 90%. As previously described, the inner surface of the cavity is both reflective and penetrating to electromagnetic radiation. Transmitted radiation incident on the material reaches the outer surface of the main body, where it exits the main body as leak radiation. However, if the distance between the inner surface of the cavity and the outer surface of the main body is selected to be sufficiently large, only a small portion of the radiation that penetrates into the porous material reaches the outer surface. The remainder is redirected into the cavity by scattering through the pores of the material. For strongly scattering materials, a distance of 3-4 mm between the inner surface of the cavity and the outer surface of the main body may be sufficient to limit the leakage of leak radiation to a reasonable amount. This improves the intensity of the emitted light beam and, therefore, the signal-to-noise ratio.

[0033]

[0034] In a gas cell according to the present invention, the optical path length L is the optical path length L that is traveled by the light beam within the gas cell relative to the surrounding gas cell volume V. O Ratio L O / V is 10 5 m -2 The above, preferably 2.10 5 m -2 The above is about comfort level 3.10. 5 m -2 The above is possible. For example, V = 4 · 10 -5 m 3 In a gas cell with a gas cell volume of , the optical path length L exceeds 8 m. O It can reach V = 4.5·10 -6 m 3 In a further exemplary embodiment of the present invention using the gas cell volume, the optical path length L is 1.4 m. O This can be achieved. This represents a significant improvement over conventional gas cells such as White gas cells and Heliot gas cells.

[0034]

[0035] In one embodiment of the present invention, an incoupling device for divergent incoupling of an incident light beam may be formed in a cavity. When a divergent light beam enters the cavity, the development of destructive interference effects is further suppressed. The incoupling device and / or outcoupling device may include an optical guide. Divergent incoupling can then be achieved by using an optical guide having the highest possible numerical aperture. For example, a fiber with a numerical aperture of 0.22 to 0.50 may be used. In addition, or alternatively, a lens that broadens the light beam may be disposed at the end of the optical guide that couples into the gas cell.

[0035]

[0036] An incoupling device can be used to couple the electromagnetic radiation of an incident light beam into a cavity. An incoupling device may be designed to couple all of the electromagnetic radiation of the incident light beam into the cavity. Alternatively, an incoupling device may be designed to couple only a portion of the electromagnetic radiation of the incident light beam into the cavity. Additionally or alternatively, an outcoupling device may also be designed to couple out the electromagnetic radiation forming an outgoing light beam from the cavity. An outcoupling device may be designed to couple all of the electromagnetic radiation of the outgoing light beam out of the cavity. Alternatively, an outcoupling device may be designed to outcoup only a portion of the electromagnetic radiation of the outgoing light beam from the cavity. If the incoupling device and / or outcoupling device have an optical guide, this may guide the radiation from the outer surface of the body, through the body, into the cavity. In other words, the optical guide extends to or into the cavity. This allows for particularly simple incoupling and / or outcoupling of electromagnetic radiation into / out of the cavity.

[0036]

[0037] In the embodiments described above, the optical guide intersects the inner surface of the cavity. As a result, the inner surface area from which electromagnetic radiation can be reflected can be reduced. In addition, because the optical guide extends through the porous material, in this area, incident electromagnetic radiation cannot be scattered within the porous material. Both effects result in a reduction in the optical path length, which reduces absorbance. On the other hand, the use of an optical guide with a large cross-section allows for an improvement in the signal-to-noise ratio. Therefore, in one embodiment of the present invention, the size of the incoupling and / or outcoupling devices within the body is selected considering the absorbance and / or signal-to-noise ratio. Radius R K In a cavity with a spherical shell morphology having a radius of 7 mm, particularly good absorbance can be achieved with a fiber having a radius of 300 μm. On the other hand, the signal-to-noise ratio can be optimized using a fiber having a radius of 750 μm. K For cavities with a radius of 7 mm, optical fibers with radii of 300 μm to 750 μm are advantageous.

[0037]

[0038] An incoupling device may be designed to couple the electromagnetic radiation of an incident light beam into a porous material-containing area of ​​the main body. The incoupling device may be designed so that all of the electromagnetic radiation of the incident light beam is coupled into the porous material-containing area of ​​the main body. Alternatively, the incoupling device may be designed to couple only a portion of the electromagnetic radiation of the incident light beam into the porous material-containing area of ​​the main body. Additionally or alternatively, an outcoupling device may also be designed to outcouple the electromagnetic radiation forming an outgoing light beam from the porous material-containing area of ​​the main body. The outcoupling device may be designed to outcouple all of the electromagnetic radiation of the outgoing light beam from the porous material-containing area of ​​the main body. Alternatively, the outcoupling device may be designed to outcouple only a portion of the electromagnetic radiation forming an outgoing light beam from the porous material-containing area of ​​the main body.

[0038]

[0039] For example, if the outcoupling device has an optical guide, the radiation from the end of the optical guide that outcouples the radiation from the main body may be positioned at a distance a from the inner surface of the cavity in the porous material. In this way, it is possible to prevent the light beam incident on the cavity from being immediately decoupled again from the cavity after reflection without passing through the porous material. This is due to the distance between the inner surface of the cavity and the outcoupling end of the fiber, so that the electromagnetic radiation always travels at least a distance a before entering the outcoupling device. For example, a may be 1 mm.

[0039]

[0040] The outcoupling device may also be positioned entirely outside the main body. However, in this case, the distance between the inner surface of the cavity and the outer surface of the main body may be selected so that electromagnetic radiation can reach the outcoupling device with sufficient intensity. This arrangement allows for maximum absorbance.

[0040]

[0041] In one embodiment of the present invention, the incoupling device can couple all of the electromagnetic radiation of the incident light beam into the cavity. The outcoupling device may then be designed to outcouple all of the electromagnetic radiation forming the outgoing light beam from the cavity. Alternatively, the outcoupling device may be designed to outcouple all of the electromagnetic radiation forming the outgoing light beam from the region of the body containing a porous material.

[0041]

[0042] A gas cell according to the present invention can be used for absorption spectroscopy analysis of a gas. For this purpose, the gas cell is brought into contact with the gas to be measured. The porous material is permeable to the gas being measured. The gas penetrates into the porous material and into the cavities. When electromagnetic radiation of a suitable wavelength is coupled into the gas cell through an incoupling device, the gas concentration can be determined by measuring the intensity of the radiation decoupled through an outcoupling device according to the Lambert-Beer law (see above). The optical path length in the gas cell is determined in advance using measurements with gas of known concentration.

[0042]

[0043] In addition, the present invention relates to an apparatus for gas absorption spectroscopy analysis, and this apparatus is

[0044] A gas cell according to any one of the above embodiments,

[0045] An electromagnetic radiation source for generating an incident light beam, and

[0046] It has a detector device for recording the emitted light beam.

[0043]

[0047] The electromagnetic radiation source may have a laser light source. In particular, it may have a tunable laser diode. The detector device may have a photodetector.

[0048] Gas cells and apparatus according to the present invention for gas absorption spectroscopy analysis may 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 sulfide, hydrogen halides such as HCl or HF, water, or mixtures thereof.

[0044]

[0049] In the following description, the present invention will be described with reference to the drawings as an example. [Brief explanation of the drawing]

[0045] [Figure 1]

[0050] This is a cross-sectional view of a first embodiment of a gas cell according to the present invention, showing an example of a portion of the beam path inside the gas cell. [Figure 2a] Figure 1 is a cross-sectional view of the gas cell. [Figure 2b] Figure 2a is a cross-sectional view of the first modified form of the gas cell. [Figure 2c] Figure 2a is a cross-sectional view of the second modified form of the gas cell. [Figure 3a] This is a cross-sectional view of a second embodiment of a gas cell according to the present invention. [Figure 3b] Figure 3a is a cross-sectional view of a modified gas cell. [Figure 4a] This is a cross-sectional view of a third embodiment of a gas cell according to the present invention. [Figure 4b] Figure 4a is a cross-sectional view of a modified gas cell. [Figure 5] This is a cross-sectional view of a component of a fourth embodiment of a gas cell according to the present invention. [Figure 6] This is a cross-sectional view of a fifth embodiment of a gas cell according to the present invention.

[0046]

[0051] Figure 1 is a cross-sectional view of a first embodiment of a gas cell 1 according to Invention 1, where a portion of the beam path within the gas cell 1 is shown as an example. The gas cell 1 has a main body 10. The main body 10 may be, for example, block-shaped. The main body 10 is made of a porous material that scatters electromagnetic radiation. A cavity 12 without material is formed within the main body 10. In this embodiment, the cavity 12 has the form of a spherical shell. The cavity 12 is demarcated by an inner surface 14. The main body 10 has an outer surface 16 that surrounds the inner surface 14 of the cavity 12 at a certain distance.

[0047]

[0052] The gas cell 1 also has an incoupling device 20 for coupling the incident light beam S into the gas cell 1. In this embodiment, the incoupling device 20 has an optical guide. For this purpose, a materialless area is provided in the main body 10 through which the optical guide can be introduced from the outside of the gas cell 1 into the cavity 12. In this embodiment, the incoupling device 20 couples all of the electromagnetic radiation of the incident light beam into the cavity 12. In addition, the gas cell 1 has an outcoupling device 30. In this embodiment, the outcoupling device 30 couples the outgoing light beam S A All of the electromagnetic radiation forming is outcoupled from the cavity. Furthermore, in this embodiment, the outcoupling device 30 has an optical guide. An area without further material is provided within the main body 10 through which the optical guide can travel from the cavity 12 to the outside of the main body 10.

[0048]

[0053] The light beam is coupled from the outside of the main body 10 into the cavity 12 through the optical guide of the incoupling device 20. The incoupling device 20 may be designed so that the incident light beam S is coupled into the cavity 12. For this purpose, for example, an optical guide with a high numerical aperture may be used. Alternatively, a lens may be provided at the end of the optical guide that opens into the cavity 12.

[0049]

[0054] The beam path within gas cell 1 is described below. To facilitate visualization, only the path of the principal ray of the light beam (also abbreviated as beam S) is shown.

[0055] The beam S coupled into the cavity 12 passes through the area of ​​the cavity 12 and then collides with 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 transparent to electromagnetic radiation. Therefore, a portion of the beam S is diffusely reflected at point P1 on the inner surface 14 of the cavity 12, and a portion of the radiation is transmitted into the porous material at point P1 on the inner surface 14. As an example, three reflected beams S(R1), S1(R1), and S2(R1) are shown in Figure 1. However, for the sake of visualization, only the path of the reflected beam S(R1) is shown. The paths of beams S1(R1) and S2(R1) are not shown further, so the beams are represented by dashed arrows. The transmitted portion of beam S enters the porous material in various directions. As in the case of reflected radiation, only the path of a single beam S(T1) is shown for the sake of simplification of visualization. For clarity, three additional beams entering the material at point P1 are depicted. Their paths are not further explained and are therefore shown as dashed arrows. The beam S(T1) traveling through the porous material is scattered multiple times within the porous material before re-entering cavity 12.

[0050]

[0056] The paths of the reflected beam S(R1) and the transmitted beam S(T1) are now described. The reflected beam S(R1) collides with the inner surface 14 of the cavity 12 at a second point P2, where a portion of the reflected beam S(R1) is diffusely reflected again, while another portion is transmitted into the porous material. Here again, as in the case of point P1, there are several reflected and transmitted beams, but to facilitate visualization, only one reflected beam and one transmitted beam are shown in each case. The reflected portion S(R1,R2) continues to travel through the cavity 12 until it collides with point P5 on the inner surface 14 of the cavity 12. Here again, a portion is reflected and another portion is transmitted.

[0051]

[0057] The beam S(T1) transmitted at point P1 re-enters the cavity 12 and continues through the cavity 12 until it collides with the inner surface 14 of the cavity 12 at point P3, where it is partially reflected and partially transmitted. The reflected portion S(T1,R2) enters the optical guide of the outcoupling device 30, and the emitted beam S A As a contribution, it is decoupled from gas cell 1 (for simplicity, one beam S A (Only the beam shown in the drawing is shown). Similarly, the other beam continues to travel through the cavity 12 and partially through the porous material, eventually entering the optical guide of the outcoupling device 30 and being outcoupled from the gas cell 1.

[0052]

[0058] Therefore, the emitted beam S A It consists of multiple output beams covering different optical path lengths within the gas cell 1. Output beam S A The effective path length L eff The equation =L+Z is covered, where L represents the effective path length inside the cavity 12, and Z represents the effective path length within the porous material.

[0053]

[0059] Hereafter, the extension of the die optical path length of the gas cell 1 shown in Figure 1 will be compared with that of the Ulbricht sphere. In the latter, the inner surface is almost completely reflective, and the cavity is not surrounded by a porous material, so the extension of the path length is brought about solely by the path within the spherical shell. For a spherical shell with a radius of 5 mm and a reflectivity of 0.985, the effective path length in the case of the Ulbricht sphere is 0.44 m. In the embodiment shown in Figure 1, the effective path length is more than 3 m. For a spherical shell with a radius of 15 mm and a reflectivity of 0.985, the effective path length of the Ulbricht sphere is 1.33 m, and the effective path length of the gas cell shown in Figure 1 is more than 8 m. In other words, using the gas cell 1 according to the present invention, the effective path length can be significantly increased compared to the Ulbricht sphere.

[0054]

[0060] Figures 2a and 2c show three possibilities for setting up an outcoupling device 30 for a gas cell 1 having a cavity 12 in the form of a spherical shell. These possibilities, however, are not limited to a cavity in the form of a spherical shell and can be used for any arbitrary form of cavity.

[0055]

[0061] Figure 2a shows a first embodiment of the present invention presented in Figure 1. As described in relation to Figure 1, both the incoupling device 20 and the outcoupling device 30 (e.g., in the form of an optical guide, particularly an optical fiber) extend from the outside of the gas cell 1 through a porous material into the cavity 12. The following problems may arise from this arrangement of the incoupling and outcoupling devices 20, 30.

[0056]

[0062] After entering the cavity, the emitted beam S is reflected only from the reflective surface. A A portion of the material is porous. Therefore, this portion of the light beam does not travel through the porous material. This results in a reduction in the achievable optical path length.

[0057]

[0063] In addition, the incoupling and outcoupling devices 20 and 30 reduce both the area of ​​the inner surface 14 from which electromagnetic radiation can be reflected, and the volume of the porous material from which electromagnetic radiation can be reflected.

[0058]

[0064] One solution to these problems is shown in Figure 2b. Here, the outcoupling device 30 does not reach the cavity 12 but terminates within the porous material. In order to be decoupled by the outcoupling device 30, the electromagnetic radiation must therefore travel at least through the area between the inner surface 14 of the cavity 12 and the inlet end 32 of the outcoupling device 30 (at point P', it is transmitted into the porous material, and the transmitted beam S T(Refer to the path of beam S' entering the optical guide). Here, the radiation passes through the porous material and therefore undergoes further path length extension. Therefore, the outcoupling device 30 connects to the emitted light beam S A The electromagnetic radiation forming the structure is outcoupled from area 18 of the main body 10 containing the porous material.

[0059]

[0065] As a result of the modified configuration shown in Figure 2c, a further expansion of the effect shown in Figure 2b can be achieved. Here, the outcoupling device 30 is located completely outside the main body 10. However, in this case, the distance from the cavity 12 to the inlet end 32 of the outcoupling device 30 must be small enough to receive a suitable signal. In the case of an optical guide, an improvement in the signal-to-noise ratio can be achieved by increasing the diameter of the optical guide.

[0060]

[0066] Figures 3a and 3b show cross-sectional views of a second embodiment of the gas cell 1 according to the present invention. Here, the cavity 12 has a convex shape and is not spherical. In the embodiment shown in Figure 3a, the outcoupling device 30 is positioned at a distance from the cavity 12 within an area 18 of the main body 10 containing a porous material. Thus, in either case, path length extension resulting from scattering within the porous material is achieved.

[0061]

[0067] Figure 3b shows a modified form of the embodiment shown in Figure 3a. The outcoupling device 30, for example, the optical guide, is positioned completely outside the gas cell 1. This allows a particularly large portion of the radiation reflected within the porous material (beam S) to be captured. T (See reference).

[0062]

[0068] Figure 4a shows a cross-sectional view of a third embodiment of the gas cell 1 according to the present invention. Here, the cavity 12 has an ellipsoidal shape. The incoupling device 20 is connected to the cavity 12. The outcoupling device 30 is located at a distance from the cavity 12 within area 18 of the main body 10 containing a porous material. Therefore, in all cases, the radiation decoupled from the gas cell 1 by the outcoupling device 30 travels through the porous material (beam S T (See reference).

[0063]

[0069] Figure 4b shows a modified form of the third embodiment shown in Figure 4a. Here, the outcoupling device 30 is positioned such that the inlet end 32 is located at the foci of the ellipsoid.

[0064]

[0070] Figure 5a shows a cross-sectional view of the components of a fourth embodiment of the gas cell 1 according to the present invention. The main body 10 of this gas cell 1 consists of three parts: a left portion 10a, a central portion 10b, and a right portion 10c. In portions 10a and 10c, the spherical shell is hollowed out in both cases. In the central portion 10b, the cylindrical area is hollowed out. These recesses can be created by milling the ceramic main body. The portions 10a, 10b, and 10c are then assembled together to form the main body 10, in which a cavity 12 is formed.

[0065]

[0071] Figure 6 shows a cross-sectional view of a fifth embodiment of the gas cell 1 according to Invention 1. The main body 10 is provided with two separate cavities 12a and 12b, which, as presented in Figure 6, have the form of a spherical shell. An incoupling device 20 is connected to cavity 12a so that electromagnetic radiation can be coupled into cavity 12a through the incoupling device 20. An outcoupling device 30 is connected to cavity 12b. For die radiation to be decoupled by the outcoupling device 30, it must travel from the first cavity 12a through the porous material into the second cavity 12b (beam S TTherefore, in this embodiment as well, a contribution was obtained in the form of extending the path length of radiation through the path within the porous material. [Explanation of symbols]

[0066]

[0072] 1 gas cell 10 Main body 12 Cavity 14. Inner self 16 Exterior 18 Area containing porous material 20 Incoupling Devices 30 Out-coupling devices 32 Inlet end S Incident light beam S A Emitting beam L O optical path length V Gas cell volume

Claims

1. A gas is exposed to an incident light beam (S) of electromagnetic radiation and a light beam (S) of electromagnetic radiation emerging from said gas is detected to form a measurement signal. Ａ 1. A gas cell (1) for the spectroscopic analysis of gases, in which a photoelectron spectroscopy (PES) is recorded, the gas cell (1) comprising: A body (10) made of a porous material that scatters electromagnetic radiation; and an in-coupling device (20) for coupling the incoming light beam (S) into the gas cell (1), and an outgoing light beam (S Ａ an outcoupling device (30) for outcoupling a gas from the gas cell (1); the in-coupling device (20) and the out-coupling device (30) are arranged on the same side of the gas cell (1); A gas cell (1), characterized in that a cavity (12) is formed in said body (10), extends inside said porous material and is surrounded by an inner surface (14) that is diffusely reflective and transparent to said electromagnetic radiation.

2. 2. The gas cell (1) according to claim 1, wherein the material comprises a porous ceramic material in which the cavity (12) is formed.

3. 3. The gas cell (1) according to claim 1 or 2, wherein the porosity is a minimum of 25% and a maximum of 70%.

4. A gas cell (1) according to any one of claims 1 to 3, wherein the cavity (12) has a radius R Ｋ The gas cell (1) has the form of a spherical shell having

5. 5. A gas cell (1) according to claim 4, wherein the radius R of the spherical shell Ｋ is selected with consideration of the signal-to-noise ratio, and the radius R of the spherical shell Ｋ is between 2 mm and 15 mm, gas cell (1).

6. 6. The gas cell (1) according to any one of claims 1 to 5, wherein the optical path length (L) travelled by the light beam (S) within the gas cell (1) relative to the gas cell volume (V) enclosed by the outer surface (16) of the body (10) is Ｏ ) is 10 ５ m －２ This is the gas cell (1).

7. 7. The gas cell (1) according to any one of claims 1 to 6, wherein the in-coupling device (20) is designed for divergent coupling of the incident light beam (S) into the cavity (12).

8. The gas cell (1) according to any one of the preceding claims, wherein the in-coupling device (20) and / or the out-coupling device (30) comprises a light guide.

9. 9. The gas cell (1) according to any one of claims 1 to 8, wherein the in-coupling device (20) is designed for coupling electromagnetic radiation of the incident light beam (S) into the cavity (12) and / or the out-coupling device (30) is designed for coupling electromagnetic radiation of the exit light beam (S Ａ ) from said cavity (12).

10. A gas cell (1) according to any one of claims 1 to 9, wherein the size of the area occupied by the incoupling and / or outcoupling devices (20, 30) in the body (10) is selected taking into account absorbance and / or signal-to-noise ratio.

11. The gas cell (1) according to any one of claims 1 to 10, wherein the in-coupling device (20) is configured for coupling electromagnetic radiation of the incident light beam (S) into an area (18) of the body (10) containing the porous material and / or the out-coupling device (30) is configured for coupling electromagnetic radiation of the exiting light beam (S) into an area (18) of the body (10) containing the porous material. Ａ 1. A gas cell (1) configured for outcoupling electromagnetic radiation forming a gas having a porous material from the area (18) of the body (10) containing the porous material.

12. A gas cell (1) according to any one of claims 1 to 11, an electromagnetic radiation source for generating an input light beam (S) and an output light beam (S Ａ 2. An apparatus for the absorption spectroscopy of gases, comprising a detector arrangement for detecting a component (a) of the gas.

13. 13. The apparatus of claim 12, wherein the electromagnetic radiation source is a tunable laser diode.

14. The apparatus of claim 12, wherein the body portion (10) has an outer surface (16) surrounding the inner surface (14) of the cavity (12) at a distance at which leakage radiation emitted at the outer surface (16) is less than a predetermined loss limit, the predetermined loss limit being less than 99% and greater than 0% of the intensity of the incident light beam (S).

Images