Raman spectroscopy device for measuring the concentration of a gas by means of raman spectroscopy

Abstract

The invention relates to a Raman spectroscopy device (30) for measuring the concentration of a gas. The crux of the invention is that the Raman spectroscopy device (30) has an open-loop and closed-loop control device (72), wherein: a first closed-loop control circuit (73) and a second closed-loop control circuit (74) are implemented in the Raman spectroscopy device (30); the first closed-loop control circuit (73) is designed to control the temperature of the laser diode (16) to a first target value; and the second closed-loop control circuit (74) is designed to control the temperature of the detector (70) to a second target value.

Description

[0001] R.416707

[0002] - 1 -

[0003] Raman spectroscopy device for measuring the concentration of a gas using Raman spectroscopy

[0004] Technical field

[0005] The invention relates to a Raman spectroscopy device for concentration measurement and / or quantitative concentration evaluation of a gas or a gas mixture by means of Raman spectroscopy, with at least one gas measurement chamber for the gas or the gas mixture.

[0006] State of the art

[0007] From the prior art (A. Stratmann and G. Schweiger, Fluid Phase Equilibria of Ethanol and Carbon Dioxide Mixtures with Concentration Measurements by Raman Spectroscopy, Appl. Spectrosc. 56 (6) 2002, 783-788), it is known that the particle concentration (N / V) and thus the gas density p can be determined locally at the Raman measurement volume V using Raman spectroscopy. A prerequisite for measurement using Raman spectroscopy is a powerful, compact light source that illuminates the sample gas and thus excites it. For this purpose, a high-power light source, e.g., with a wavelength of 440 nm in the form of a diode laser, can be used.

[0008] From DE 10 2021 107 229 A1, DE 10 2009 026 744 A1 and EP 3 748 339 A2 measuring devices for a concentration measurement of a gas are known, in which powerful diode lasers are used to excite the gas to be measured.

[0009] Description of the invention

[0010] The inventors realized that stable measurement with a Raman spectroscopy device is only possible if the temperature of the sample is as R.416707.

[0011] - 2 -

[0012] The light source used, the laser diode, is kept as constant as possible to prevent wavelength drift and changes in the optical power of the emitted laser diode light. Both changing wavelength and changing optical power would distort the measurement result, as the gas being measured would be illuminated with these altered values ​​during the measurement, and the wavelength and optical power of the Raman scattered light would change accordingly. The inventors also recognized that it is equally necessary to monitor and keep as constant as possible the temperature of the detector, which detects the Raman scattered light or the Stokes response, otherwise the detector signal would not be fully reproducible. For example, high detector temperatures would lead to so-called detector noise and thus to a distortion of the measurement result.

[0013] The proposed solution ensures stable operation of the laser diode and the detector, thereby enabling the Raman spectroscopy device to produce the most accurate measurement results possible.

[0014] The invention relates to a Raman spectroscopy device for measuring the concentration of a gas, comprising a gas measurement chamber for the gas, which has one or more optical inlets and one or more optical outlets, and a gas supply line for supplying the gas to the gas measurement chamber during the concentration measurement, wherein the Raman spectroscopy device includes a laser diode configured for focused illumination of the gas in the gas measurement chamber, wherein the Raman spectroscopy device comprises a collecting optical system with at least one filter and at least one aperture and a spectral analysis unit, and wherein the Raman spectroscopy device supplies Raman scattered light through the collecting optical system with at least one filter and at least one aperture to the spectral analysis unit, wherein the spectral analysis unit comprises a detector for spectrally resolved detection of the Raman scattered light.

[0015] The core of the invention is that the Raman spectroscopy device has a control and regulating device, wherein the Raman spectroscopy device has a first control loop and a second control loop R.416707

[0016] - 3 - are realized, wherein the first control loop is designed to control the temperature of the laser diode to a first setpoint, and the second control loop is designed to control the temperature of the detector to a second setpoint.

[0017] The first control loop integrates the components for temperature management of the laser diode, such as a temperature sensor, a setpoint generator, and at least one active temperature control element. The second control loop integrates the components for temperature management of the detector, such as a temperature sensor, a setpoint generator, and at least one active temperature control element.

[0018] The first control loop is configured, in particular, to measure the actual temperature of the laser diode, and furthermore, to set a target temperature of the laser diode based on the measured actual temperature. The second control loop is configured, in particular, to measure the actual temperature of the detector, and furthermore, to set a target temperature of the detector based on the measured actual temperature. The control device according to the invention described above preferably includes a control amplifier that converts the difference signal between the target value and the actual value into a control signal that actuates the respective active temperature control element.

[0019] According to the invention, the setpoint temperature of the laser diode can be between +10° and +50°, particularly between +24° and +26°, and most preferably +25°. Furthermore, the setpoint temperature of the detector can be between -35° and +7°, particularly between -9° and -11°, and most preferably -10°. The first setpoint is preferably higher than the second. In principle, it is therefore preferred that the detector is operated at a cooler temperature than the laser diode, or that the first setpoint is higher than the second setpoint.

[0020] Furthermore, it can be advantageous to thermally connect the laser diode and the detector. Cooling is possible (R.416707).

[0021] - 4 - the laser diode results from a temperature gradient between the laser diode and the detector. One can then speak of a so-called cascade cooling between the laser diode and the detector.

[0022] In particular, it can be advantageously provided that the laser diode and the detector are thermally connected to each other by means of passive temperature control elements, in particular by means of rod conductors, wherein the rod conductors consist of a material with high thermal conductivity, for example a metal, for example aluminum, copper, gold or silver; or for example a non-metal, for example diamond, boron nitride, silicon carbide, graphite or aluminum oxide.

[0023] Rod conductors are, in particular, solid materials extending in a longitudinal direction with a cross-sectional area that is, for example, round or polygonal, e.g. rectangular, and extends perpendicular to the longitudinal direction.

[0024] Preferably, the number of rod(s) is 1 to 100, the length of the rod(s) is between 1 and 200 mm and / or the cross-section of the rod(s) is between 0.1 and 1000 mm. 2 amounts.

[0025] It can be provided that the first control loop includes at least one temperature sensor for measuring the temperature of the laser diode and at least one temperature control element for maintaining the temperature of the laser diode, and that the second control loop includes at least one temperature sensor for measuring the temperature of the detector and at least one temperature control element for maintaining the temperature of the detector. Furthermore, it is preferred that the at least one temperature sensor and the at least one temperature control element of the first control loop are configured separately from the at least one temperature sensor and the at least one temperature control element of the second control loop. This is advantageous because both the sensors for detecting the temperature, i.e., the control variable of the control loop, and the active temperature control elements can each be optimally positioned on the laser diode and the detector, respectively.

[0026] Generally, "tempering" is understood to mean cooling, the term R.416707

[0027] - 5 -

[0028] However, in exceptional cases, temperature control can also include at least temporary heating, e.g., when operating in very cold environments.

[0029] It is possible that the temperature control elements are designed as Peltier elements, wherein in particular one to four Peltier elements may be provided per temperature control element, wherein the Peltier elements may have a square or rectangular shape, wherein the cross-sectional area of ​​the individual rectangular Peltier elements may vary from 15mm x 15mm to 50mm x 50mm, wherein in particular the power per Peltier element may be between 20W and 70W, and wherein the Peltier elements may be formed from a single layer or from several layers of thermoelectric material.

[0030] A Peltier element is a thermoelectric device used for cooling or heating. A distinction is made between "single-stage" and "multi-stage" Peltier elements. These terms refer to the number of thermoelectric layers used in a Peltier element. A single-stage Peltier element consists of a single layer of thermoelectric material. It can generate a limited temperature difference between the two sides of the element and is typically used for applications requiring lower cooling capacity. A multi-stage Peltier element consists of several layers of thermoelectric material connected in series. This allows for a larger temperature difference between the two sides of the element and higher cooling capacity. Multi-stage Peltier elements are used in applications requiring greater cooling capacity.

[0031] It is also highly advantageous to have a thermal connection between the laser diode and the detector via passive heat conduction structures. This allows the laser diode to be kept at a higher target temperature (e.g., set point: 25°C) and the detector at a lower operating temperature (e.g., set point: -10°C) during spectrometer operation, while reducing energy consumption and optimizing the dynamics of both control loops.

[0032] In a further development of the invention, it can be provided that the passive heat conduction structures are formed from heat conduction tubes, which transport the heat from the laser diode to the detector. Heat conduction tubes or R.416707

[0033] - 6 - Heat pipes are defined as rod-shaped conductors with an interior that is either closed or open. It is also conceivable that the interior is filled with a medium. The heat pipes can be made of solid material, coated material, or, for example, as hollow tubes, i.e., capillaries with closed ends and filled with a fluid refrigerant.

[0034] In this process, the refrigerant may evaporate at the warm end and condense in a cyclical process at the cooler end of the heat pipe. The heat pipe or heat conduction tubes' mounting points, i.e., the base points for attaching the passive conductive elements to the laser diode or detector, can also be made of metallic materials or non-metallic materials with high thermal conductivity coefficients.

[0035] It may be advantageous to provide that the heat conduction tubes are made of a metallic material, in particular of Al [235 W / mK], Au [310 W / mK] Cu [400 W / mK], Ag [430 W / mK] or alloys of these metals.

[0036] Alternatively, the heat conduction tubes may be made of a non-metallic material, in particular tool diamond [2000 W / mK], boron nitride [anisotropic up to 500 W / mK], silicon carbide [270 W / mK], graphite [anisotropic up to 200 W / mK] or aluminum oxide or corundum [30 W / mK].

[0037] Furthermore, it is possible to thermally insulate the surface of the heat pipes, creating an improved temperature gradient along their length from heat input to heat output while simultaneously reducing the influence of the ambient temperature. This insulation can be achieved through a coating or a casing, for example, made of plastic.

[0038] Using the materials described above in the manner described above is advantageous because it ensures high thermal conductivity, good workability, high ductility, good sealing properties, and high corrosion resistance. R.416707

[0039] - 7 -

[0040] It is possible that the number, length, and cross-sectional area of ​​the heat pipes depend on the amount of heat to be dissipated and the desired temperature gradient between the laser diode and the detector, with the number of heat pipes varying from 1 to 100, the length of the heat pipes varying from 1 to 200 mm, and the cross-sectional area of ​​the heat pipes varying from 0.1 to 1000 mm². 2 may vary.

[0041] In a further development of the invention, it can be provided that the detector is designed as a CCD sensor or as a CMOS sensor or as a SPAD or Si photodiode array or as Si photodiodes or as InGaAs photodiodes or as a multi-pixel photon counter or as an N MOS sensor or as avalanche photodiodes or as a phototube detector.

[0042] The Peltier elements mentioned above can be controlled, for example, by means of pulse width modulation.

[0043] Brief description of the drawing

[0044] Embodiments of the invention are explained in more detail with reference to the drawings and the following description.

[0045] They show:

[0046] Figure 1 shows a Raman spectroscopy apparatus,

[0047] Figure 2 shows a three-dimensional representation of a Raman spectroscopy device according to the invention.

[0048] Figure 3 shows a schematic representation of a Raman spectroscopy device according to the invention.

[0049] Embodiments of the invention R.416707

[0050] - 8 -

[0051] Fig. 1 schematically shows the setup of a measuring device 10 or Raman spectroscopy device 30. This device comprises a high-power laser 14, here a laser diode 16, as the radiation source 12. This laser is operated within the visible spectral range, particularly within the blue spectral range. A focusing optic 18, shown only schematically here, is connected downstream of the radiation source 12 as depicted in Fig. 1. This optic focuses the laser radiation generated by the at least one laser diode 16 onto a portion of a gas measurement chamber 20. The gas measurement chamber 20 contains a gas 22 or a gas mixture 24. The gas measurement chamber 20 can be part of a conduit or bypass line 26 through which a gas flow 66 passes.

[0052] The gas measurement room 20 includes at least one optical access 28 and at least one optical output for the laser radiation generated by the at least one laser diode 16.

[0053] The illustration in Fig. 1 further shows that the laser radiation exiting from the at least one optical output enters a radiation absorber 32 in order to avoid scattered light influences.

[0054] From the gas measurement chamber 20, Raman scattered light 34 enters a Raman scatter intensity-enhancing optic 36, which is part of a spectral analysis unit 38. This unit comprises a spectrograph that includes at least one dispersing element, which can be configured, for example, as a grating and at least one prism. Multiple gratings and multiple prisms, as well as combinations of gratings and prisms, can also be provided. The spectral analysis unit 38 also includes a light detector 48, for example, in the form of a CCD camera 40 or a CMOS device and / or a number of receiver diodes. Furthermore, receiver diodes can be arranged within the light detector 48 at the Raman wavelengths of the gas(es) to be analyzed.By appropriately designing the bypass 26 or using optical windows in the gas measurement chamber 20, gases 22 or gas mixtures 24 can also be measured in different pressure and temperature ranges. R.416707.

[0055] - 9 -

[0056] In the Raman spectroscopy device 30 shown in Fig. 1, the gas 22 to be measured is illuminated by the laser diode 16, preferably in the visible blue spectral range, through a focusing optic 18. The Raman scattering light 34 is captured by the Raman scattering intensity-enhancing optic 36 and supplied to the spectral analysis unit 38.

[0057] Fig. 2 shows an example of a Raman spectroscopy device 30 according to the invention, in which the spectral analysis unit 38 has a detector 70 for spectrally resolved detection of the Raman scattered light.

[0058] Furthermore, Fig. 2 shows that both the laser diode 16 and the detector 70 are thermally connected to each other by means of passive temperature control elements 76 and are also both temperature controlled by means of individually assigned active temperature control elements 71.

[0059] Figure 3 shows that the Raman spectroscopy device 30 has a control and regulation device 72 and that the Raman spectroscopy device 30 has a first control loop 73 and a second control loop.

[0060] 74 is realized, wherein the first control loop 73 is designed to control the temperature of the laser diode 16 to a first setpoint, and the second control loop 74 is designed to control the temperature of the detector 70 to a second setpoint.

[0061] Figure 3 further shows that the first control loop 73 has a temperature measuring element 75 for measuring the temperature of the laser diode 16 and a temperature control element 71 for temperature control of the laser diode 16. Figure 3 also shows that the second control loop 74 has a temperature measuring element 75 for measuring the temperature of the detector 70 and a temperature control element 71 for temperature control of the detector 70, and that the temperature measuring element 75 and the temperature control element 71 of the first control loop 73 are independent, i.e., separate, from the temperature measuring element 75 of the first control loop 73.

[0062] 75 and the temperature control element 71 of the second control loop 74 are formed.

[0063] As shown in both Fig. 2 and Fig. 3, the thermal connection of the laser diode 16 to the detector 70 is provided by means of passive R.416707

[0064] - 10 -

[0065] Heat conduction structures 76 are manufactured. In the example shown, the passive heat conduction structures 76 are formed by heat pipes, which generally transport heat from the laser diode 16 to the detector 70. The four heat pipes shown in Fig. 2 are merely an example. For instance, only a single heat pipe, perhaps with a rectangular cross-section, may be used. The cross-sectional area of ​​this heat pipe can be relatively large, for example, 10–400 mm². 2 or even 10 - 1000 mm 2 .

Claims

R.416707 - 11 - Claims 1. Raman spectroscopy device (30) for measuring the concentration of a gas (22) comprising a gas measurement chamber (20) for the gas (22), which has one or more optical inlets (28) and one or more optical outlets, with a gas supply line for supplying the gas (22) into the gas measurement chamber (20) during the concentration measurement, wherein the Raman spectroscopy device (30) comprises a laser diode (16) configured for focused illumination of the gas (22) in the gas measurement chamber (20), wherein the Raman spectroscopy device (30) comprises a collecting optical system (36) with at least one filter and at least one aperture and a spectral analysis unit (38), and wherein the Raman spectroscopy device (30) directs Raman scattered light (34) through the collecting optical system (36) with at least one filter and at least one aperture of the spectral analysis unit. (38) bringswherein the spectral analysis unit (38) comprises a detector (70) for spectrally resolved detection of the Raman scattered light (34), characterized in that the Raman spectroscopy device (30) comprises a control and regulation device (72), wherein a first control loop (73) and a second control loop (74) are implemented in the Raman spectroscopy device (30), wherein the first control loop (73) is configured to control the temperature of the laser diode (16) to a first setpoint, and wherein the second control loop (74) is configured to control the temperature of the detector (70) to a second setpoint.

2. Raman spectroscopy device according to claim 1, characterized in that the first setpoint is between +10° and +50°, in particular between +24° and +26°, particularly preferably +25°.

3. Raman spectroscopy device according to claim 1 or 2, characterized in that the second setpoint is between -35° and +7°, R.416707 - 12 - in particular between -9° and -11°, especially preferably -10°.

4. Raman spectroscopy device according to claim 1, 2 or 3, characterized in that the first control loop (73) has at least one temperature measuring element (75) for measuring the temperature of the laser diode (16) and at least one temperature control element (71) for temperature control of the laser diode (16), and that the second control loop (74) has at least one temperature measuring element (75) for measuring the temperature of the detector (70) and has at least one temperature control element (71) for temperature control of the detector (70) and that the at least one temperature control element (75) and the at least one temperature control element (71) of the first control loop (73) are formed separately from the at least one temperature measuring element (75) and the at least one temperature control element (71) of the second control loop (74).

5. Raman spectroscopy device according to claim 4, characterized in that the temperature control elements (71) are designed as Peltier elements, wherein one to four Peltier elements are provided per temperature control element (71), wherein the Peltier elements have a square or rectangular shape, wherein the cross-sectional area of ​​the individual rectangular Peltier elements is between 15mm x 15mm and 50mm x 50mm, wherein in particular the power per Peltier element is between 20W and 70W and wherein the Peltier elements are formed from a single layer or from several layers of thermoelectric material.

6. Raman spectroscopy apparatus according to one of the preceding claims, characterized in that the laser diode (16) and the detector (70) are thermally connected to each other.

7. Raman spectroscopy device according to claim 6, characterized in that the laser diode (16) and the detector (70) are thermally connected by means of at least one passive thermal conductivity structure (76). R.416707 - 13 - 8. Raman spectroscopy device according to claim 7, characterized in that the passive thermal conductivity structure (76) is formed from one or more rod conductors, wherein the rod conductor(s) preferably consists of a material with high thermal conductivity, for example aluminum, copper, gold or silver; diamond, boron nitride, silicon carbide, graphite or aluminum oxide.

9. Raman spectroscopy device according to claim 8, characterized in that the number of rod lines is 1 to 100, that the length of the rod lines is between 1 and 200 mm and that the cross-section of the rod lines is between 0.1 and 1000 mm² 2 amounts.

10. Raman spectroscopy device according to claim 7, characterized in that the passive heat conduction structure (76) is formed from one or more heat conduction tubes.

11. Raman spectroscopy apparatus according to claim 10, characterized in that the heat conduction tubes consist of a metallic material, in particular of aluminium, gold, copper or silver, or of an alloy which in particular has a proportion of aluminium, gold, copper or silver.

12. Raman spectroscopy apparatus according to claim 10, characterized in that the heat conduction tubes consist of a non-metallic material, in particular tool diamond, boron nitride, silicon carbide, graphite or aluminum oxide.

13. Raman spectroscopy device according to claim 11 or 12, characterized in that the number of heat conduction tubes is 1 to 100, that the length of the heat conduction tubes is between 1 and 200 mm and that the cross-sectional area of ​​the heat conduction tubes is between 0.1 and 1000 mm 2 amounts.

14. Raman spectroscopy apparatus according to one of the preceding claims, characterized in that the detector (70) is a CCD- R.416707 - 14 - sensor or as a CMOS sensor or as a SPAD or Si photodiode array or as Si photodiodes or as InGaAs photodiodes or as a multi-pixel photon counter or as an N MOS sensor or as avalanche photodiodes or as a phototube detector (70).

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