GAS CELL

Abstract

A gas cell (1) for the spectroscopic, in particular absorption spectroscopic, analysis of a gas, in which the gas is exposed to an incident beam of rays (S) of electromagnetic radiation and a beam of rays (S.sub.A) of electromagnetic radiation exiting the gas is detected to form a measurement signal, wherein the gas cell (1) comprises a body (10) formed by a porous, electromagnetic radiation-scattering material, an in-coupling device (20) for coupling the incident beam of rays (S) into the gas cell (1) and an out-coupling device (30) for coupling the exiting beam of rays (S.sub.A) out of the gas cell (1), wherein, according to the invention, the gas cell is further developed according to the invention by forming a material-free cavity (12) in the body (10), which is surrounded by an inner surface (14) running within the material and is both diffusely reflecting and transmitting the electromagnetic radiation.

Claims

1. A gas cell for the spectroscopic, in particular absorption spectroscopic, analysis of a gas, in which the gas is exposed to an incident beam of rays of electromagnetic radiation and a beam of rays of electromagnetic radiation exiting from the gas to form a measurement signal is recorded, wherein the gas cell comprises: a body consisting of a porous material that scatters electromagnetic radiation, with a cavity formed in the porous material, the cavity having an inner surface running inside the porous material, such that electromagnetic radiation is diffusely reflected and transmitted; an in-coupling device for coupling the incident beam of rays into the gas cell; and an out-coupling device for out-coupling the exiting beam of rays from the gas cell.

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

3. The gas cell according to claim 1, wherein the porous material has a porosity that is between 25% and 70%.

4. The gas cell according to claim 1, wherein the cavity has the form of a spherical shell with radius R.sub.K.

5. The gas cell according to claim 4, wherein: the radius R.sub.K of the spherical shell is selected considering a signal-to-noise ratio; and the radius R.sub.K of the spherical shell is a minimum of 2 mm.

6. The gas cell according to claim 1, wherein the body has an outer surface which surrounds the inner surface of the cavity at a distance such that an amount of a leakage radiation exiting at the outer surface is less than a predetermined loss limit of the intensity of the incident beam of rays.

7. The gas cell according to claim 1, wherein a relationship Lo/V of an optical path length Lo traveled in the gas cell by the beam of rays to a volume V enclosed by the external surface of the body of the gas cell is greater than or equal to 1×10.sup.5 m.sup.−2.

8. The gas cell according to claim 1, wherein the in-coupling device is designed for divergent coupling of the incident beam of rays into the cavity.

9. The gas cell according to claim 1, further comprising a light guide in at least one of: the in-coupling device and the out-coupling device.

10. The gas cell according to claim 1, wherein at least one of the following applies: the in-coupling device is designed for coupling the electromagnetic radiation of the incident beam of rays into the cavity; and the out-coupling device is designed for out-coupling the electromagnetic radiation forming the exiting beam of rays from the cavity.

11. The gas cell according to claim 1, wherein an amount of the area occupied in the body by at least one of the in-coupling device and the out-coupling device is selected considering at least one of: an absorbance and a signal-to-noise ratio.

12. The gas cell according to claim 1, wherein at least one of the following applies: the in-coupling device is configured for coupling electromagnetic radiation of the incident beam of rays into the area of the body containing the porous material; and the out-coupling device is configured for out-coupling the electromagnetic radiation forming the exiting beam of rays from the area of the body containing the porous material.

13. An arrangement for the absorption spectroscopic analysis of a gas, comprising: a gas cell according to claim 1; a source of electromagnetic radiation for generating the incident beam of rays; and a detector arrangement for detecting the exiting beam of rays.

14. The arrangement according to claim 13, wherein the source of electromagnetic radiation has a tuneable laser diode.

15. The gas cell according to claim 1, wherein the porous material has a porosity that is between 25% and 70%.

16. The gas cell according to claim 6, wherein the predetermined loss limit is less than or equal to 99% of the intensity of the incident beam of rays.

Description

[0049] In the following description, the invention will be explained, referring to the drawings as examples. In the drawings, the following are shown:

[0050] FIG. 1 a sectional view of a first embodiment of a gas cell according to the invention, wherein part of the beam path inside the gas cell is shown by way of example,

[0051] FIG. 2a a sectional view of the gas cell shown in FIG. 1,

[0052] FIG. 2b a sectional view of a first modification of the gas cell shown in FIG. 2a,

[0053] FIG. 2c a sectional view of a second modification of the gas cell shown in FIG. 2a,

[0054] FIG. 3a a sectional view of a second embodiment of a gas cell according to the invention,

[0055] FIG. 3b a sectional view of a modification of the gas cell shown in FIG. 3a,

[0056] FIG. 4a a sectional view of a third embodiment of a gas cell according to the invention,

[0057] FIG. 4b a sectional view of a modification of the gas cell shown in FIG. 4a,

[0058] FIG. 5 a sectional view of components of a fourth embodiment of a gas cell according to the invention,

[0059] FIG. 6 a sectional view of a fifth embodiment of a gas cell according to the invention.

[0060] FIG. 1 is a sectional view of a first embodiment of a gas cell according to the invention 1, wherein part of the beam path within the gas cell 1 is shown as an example. The gas cell 1 has a body 10. The body 10 can, for example, be block-shaped. The body 10 is made of a porous material that scatters electromagnetic radiation. In the body 10 a material-free cavity 12 is formed. In the present embodiment the cavity 12 has the form of an spherical shell. The cavity 12 is delimited by an inner surface 14. The body 10 has an outer surface 16 which surrounds the inner surface 14 of the cavity 12 at a distance.

[0061] The gas cell 1 also has an in-coupling device 20 for coupling an incident beam of rays S into the gas cell 1. In the present embodiment, the in-coupling device 20 has a light guide. For this purpose, a material-free area is provided in the body 10, through which the light guide can be introduced from the outside of the gas cell 1 into the cavity 12. In the present embodiment, the in-coupling device 20 couples all of the electromagnetic radiation of the incident beam of rays into the cavity 12. In addition, the gas cell 1 has an out-coupling device 30. In the present embodiment the out-coupling device 30 out-couples all of the electromagnetic radiation forming the exiting beam of rays S.sub.A out of the cavity. Furthermore, in the present embodiment the out-coupling device 30 has a light guide. An additional material-free area in the body 10 is provided, through which the light guide can pass from the cavity 12 to the outside of the body 10.

[0062] The beam of rays is coupled through the light guide of the in-coupling device 20 from the outside of the body 10 into the cavity 12. The in-coupling device 20 can be designed in such a way that the incident beam of rays S is coupled into the cavity 12. For this purpose, for example, a light guide with a high numerical aperture can be used. Alternatively, a lens can be provided at the end of the light guide opening into the cavity 12.

[0063] In the following, the beam path within the gas cell 1 will be explained. To facilitate visualization, only the track of the principal ray of the beam of rays (also abbreviated as beam S) will be shown.

[0064] The beam S coupled into the cavity 12 passes through an area of the cavity 12 and then strikes a point P1 on the inner surface 14 of the cavity 12. According to the invention, the inner surface 14 is both diffusely reflective and transmitting for electromagnetic radiation. Thus, part of the beam S will be diffusely reflected at point P1 on the inner surface 14 of the cavity 12, and part of the radiation will be transmitted into the porous material at point P1 on the inner surface 14. By way of example, three reflected beams S (R1), S.sub.1 (R1) and S.sub.2 (R1) are shown in FIG. 1. However, to facilitate visualization, only the further path of the reflecting beam S (R1) is shown. The path of beams S.sub.1 (R1) and S.sub.2 (R1) is not further shown, and therefore the beams are represented as dashed arrows. The transmitted portion of the beam S enters the porous material in various directions. As in the case of the reflected radiation, to simplify visualization, only the path of a single beam S (T1) will be shown. For clarification, three additional beams entering the material at point P1 are drawn in. Since their path will not be further described hereinafter, these are shown as dashed arrows. The beam S (T1) traveling in the porous material will be scattered multiple times in the porous material before again entering the cavity 12.

[0065] The path of the reflected beam S (R1) and the transmitted beam S (T1) will now be described. The reflected beam S (R1) strikes the inner surface 14 of the cavity 12 at a second point P2 and at that point part of it is again reflected diffusely while another part is transmitted into the porous material. Here also, as in the case of point P1, there are several reflected and transmitted beams, wherein to facilitate visualization only one reflected beam and one transmitted beam are shown in each case. The reflected part S (R1, R2) travels further through the cavity 12 until it strikes a point P5 on the inner surface 14 of the cavity 12. There again part of it is reflected and another part is transmitted.

[0066] The beam S (T1) transmitted at point P1, after re-entering the cavity 12, passes through this until it strikes the inner surface 14 of cavity 12 at a point P3, where it is partially reflected and partially transmitted. The reflected part S (T1, R2) enters the light guide of the out-coupling device 30 and is decoupled from the gas cell 1 as a contribution to the exiting beam of rays S.sub.A (for simplification, only one beam S.sub.A is shown in the drawing). Similarly, the other beams continue to travel through the cavity 12 and partially through the porous material, finally entering the light guide of the out-coupling device 30 and being out-coupled from the gas cell 1.

[0067] Thus, the exiting beam of rays S.sub.A is composed of a plurality of exiting beams, which have covered different optical path lengths within the gas cell 1. The exiting beam of rays S.sub.A has covered an effective path length L.sub.eff=L+Z, where L represents the effective path length inside of the cavity 12 and Z represents the effective path length within the porous material.

[0068] Hereinafter the die optical path length extension of the gas cell 1 shown in FIG. 1 will be compared with that of an Ulbricht sphere. In the latter, the inner surface is almost completely reflecting, and the cavity is not surrounded by a porous material, so that the path length extension is only produced by the path within the spherical shell. For an 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 case of the embodiment shown in FIG. 1, the effective path length is more than 3 m. In the case of an spherical shell with a radius of 15 mm and a reflectivity of 0.985 the effective path length for the Ulbricht sphere is 1.33 m, and the effective path length for the gas cell shown in FIG. 1 is more than 8 m. In other words, using the gas cell 1 according to the invention, the effective path length can be increased considerably compared with an Ulbricht sphere.

[0069] FIGS. 2a to 2c show three possibilities for setting up the out-coupling device 30 for a gas cell 1 with a cavity 12 in the form of an spherical shell. These possibilities, however, are not limited to the cavity in the form of an spherical shell and can be used for any arbitrary form of a cavity.

[0070] FIG. 2a shows the first embodiment of the invention, presented in FIG. 1. As was explained in connection with FIG. 1, both the in-coupling device 20 and the out-coupling device 30 (for example in the form of light guides, in particular optical fibers) extend from the outside of the gas cell 1 out through the porous material into the cavity 12. The following problems can occur with this arrangement of the in-coupling and out-coupling devices 20, 30:

[0071] There is a part of the exiting beam of rays S.sub.A which was reflected only at the reflecting surface after entry into the cavity. Therefore, this part of the beam of rays has not traveled through the porous material. This results in a decrease of the attainable optical path length.

[0072] In addition, the in-coupling and out-coupling devices 20, 30 reduce both the area of the inner surface 14 at which the electromagnetic radiation can be reflected and the volume of the porous material in which the electromagnetic radiation can be reflected.

[0073] One solution to these problems is shown in FIG. 2b. Here, the out-coupling device 30 does not reach the cavity 12 but ends in the porous material. To be decoupled by the out-coupling device 30, the electromagnetic radiation therefore must travel at least the area between the inner surface 14 of the cavity 12 and the entry end 32 of the out-coupling device 30 in order to be decoupled from the gas cell 1 (see the path of the beam S′, which is transmitted into the porous material at point P′ and enters the light guide as the transmitted beam S.sub.T). Here, the radiation passes through the porous material and thus undergoes an additional path length extension. The out-coupling device 30 thus out-couples the electromagnetic radiation forming the exiting beam of rays S.sub.A from the area 18 of the body 10 containing the porous material.

[0074] As a result of the modification shown in FIG. 2c, a further enlargement of the effect shown in FIG. 2b can be achieved. Here, the out-coupling device 30 is located completely outside of the body 10. In this case, however, the distance of the entry end 32 of the out-coupling device 30 from the cavity 12 must be small enough to receive an adequate signal. In the case of a light guide, an improvement of the signal-to-noise ratio can be achieved by increasing the diameter of the light guide.

[0075] FIGS. 3a and 3b show a sectional view of a second embodiment of a gas cell 1 according to the invention. Here the cavity 12 has a convex shape, but not a spherical one. In the embodiment shown in FIG. 3a the out-coupling device 30 is arranged at a distance from the cavity 12 in the area 18 of the body 10 containing the porous material. Thus, in each case a path length extension resulting from the scattering in the porous material is achieved.

[0076] FIG. 3b shows a modification of the embodiment shown in FIG. 3a. The out-coupling device 30, for example a light guide, is arranged completely outside of the gas cell 1. With this, an especially large fraction of the radiation reflected in the porous material is obtained (see beam S.sub.T).

[0077] FIG. 4a shows a sectional view of a third embodiment of a gas cell 1 according to the invention. Here, the cavity 12 has the form of an ellipsoid. The in-coupling device 20 is connected with the cavity 12. The out-coupling device 30 is located at a distance from the cavity 12 in the area 18 of the body 10 containing the porous material. Thus, in any case, the radiation decoupled from the gas cell 1 by the out-coupling device 30 has traveled through the porous material (see beam S.sub.T).

[0078] FIG. 4b shows a modification of the third embodiment shown in FIG. 4a. Here, the out-coupling device 30 is arranged such that the entry end 32 is arrange at the focal point of the ellipsoid.

[0079] FIG. 5a shows a sectional view of components of a fourth embodiment of a gas cell 1 according to the invention. The body 10 of this gas cell 1 consists of three parts, a left-hand part 10a, a center part 10b and a right-hand part 10c. In parts 10a and 10c, in each case a spherical shell is hollowed out. In the center part 10b a cylindrical section is hollowed out. These recesses can be created by milling in a ceramic body. Parts 10a, 10b and 10c are then assembled and together form the body 10, in which the cavity 12 is formed.

[0080] FIG. 6 shows a sectional view of a fifth embodiment of a gas cell according to the invention 1. In the body 10, two separate cavities 12a and 12b are provided, which in the view presented in FIG. 6 have the form of a spherical shell. The in-coupling device 20 is connected to the cavity 12a, so that electromagnetic radiation can be coupled in through the in-coupling device 20 into the cavity 12a. The out-coupling device 30 is connected with the cavity 12b. In order for the die radiation to be decoupled by the out-coupling device 30, it must pass from the first cavity 12a through the porous material into the second cavity 12b (beam S.sub.T). Thus, in this embodiment as well, a contribution is obtained as a path length extension through the path of the radiation within the porous material.

REFERENCE SIGNS LIST

[0081] 1 Gas cell [0082] 10 Body [0083] 12 Cavity [0084] 14 Inner surface [0085] 16 Outer surface [0086] 18 Area containing the porous material [0087] 20 In-coupling device [0088] 30 Out-coupling device [0089] 32 Entry end [0090] S Incident beam of rays [0091] S.sub.A Exiting beam of rays [0092] Lo Optical path length [0093] V Gas cell volume