System and method for determining a measure for an absorption of a sample

Abstract

The present invention relates to a system for determining a measure for an absorption of a sample. Such a system has an excitation radiation source, a sample holder, a detection device, and an evaluation device. During operation of the system, the excitation radiation source generates and emits electromagnetic excitation radiation having an excitation wavelength. In addition, it periodically modulates the excitation wavelength within a spectral tuning range during operation of the system. A sample can be received on the sample holder and, during operation of the system, the excitation radiation illuminates the sample received on the sample holder. The detection device detects periodic heating and cooling generated by the absorption of the excitation radiation in the sample, generates a detection signal which represents the periodic heating and cooling, and outputs said detection signal. The evaluation device is operatively connected to the detection device in such a way that the evaluation device receives the detection signal from the detection device during operation of the system, wherein the evaluation device is designed in such a way that it determines the measure for the absorption of the sample from the detection signal during operation of the system. According to the invention, the system has a filter gas cell in addition to the sample holder. The filter gas cell is arranged in a beam path of the excitation radiation between the excitation radiation source and the sample holder. A filter gas having a spectral absorption range is received in the filter gas cell, wherein the spectral absorption range and the spectral tuning range of the excitation radiation source overlap in such a way that the modulation of the excitation wavelength leads to an intensity modulation of the excitation radiation behind the filter gas cell in a beam direction.

Description

[0001] System and method for determining a measure of the absorption of a sample

[0002] The present invention relates to a system for determining a measure of the absorption of a sample. The system comprises an excitation radiation source, a sample holder, a detection device, and an evaluation device. The excitation radiation source is configured such that, during operation of the system, it generates and emits electromagnetic excitation radiation with an excitation wavelength and periodically modulates the excitation wavelength within a spectral tuning range. The sample holder is configured and arranged such that a sample can be received on the sample holder and that, during operation of the system, the excitation radiation illuminates the sample received on the sample holder.The detection device is designed and arranged such that, during system operation, it detects periodic heating and cooling caused by the absorption of excitation radiation in the sample and generates and outputs a detection signal representing this periodic heating and cooling. The evaluation device is effectively connected to the detection device such that, during system operation, the evaluation device receives the detection signal from the detection device and is configured to determine the measure of sample absorption from the detection signal during system operation.

[0003] The present invention further relates to a method for determining a measure of absorption of a sample, wherein the method comprises the steps:

[0004] Generation and emission of electromagnetic excitation radiation with an excitation wavelength, periodic modulation of the excitation wavelength within a spectral tuning range,

[0005] Illuminating the sample with the excitation radiation,

[0006] Detecting periodic heating and cooling generated by the absorption of excitation radiation in the sample and

[0007] Determining the measure of absorption from periodic heating and cooling.

[0008] Systems and methods for determining the absorption of a sample are known from the prior art, which utilize photoacoustic and photothermal methods. These are also referred to as photoacoustic spectroscopy (PAS) and photothermal spectroscopy (PTS).

[0009] Both photoacoustic and photothermal spectroscopy are typically used to characterize gaseous samples. However, it is also possible to characterize liquids or solids, especially non-linear optical crystals.

[0010] In both methods, the sample, particularly the molecules in a gaseous sample, is excited with periodically modulated electromagnetic excitation radiation to generate a photoacoustic or photothermal signal that depends on the sample's absorption. This approach exploits the fact that the absorption of the excitation radiation in the sample is always accompanied by local heating of the sample. If the intensity of the applied excitation radiation is changed periodically, the temperature of the sample also changes locally and periodically. Such a periodic change in temperature, in turn, is accompanied by a local periodic change in the sample's density and / or pressure. The periodic pressure change can be measured as an acoustic signal. The periodic density change can be measured as a periodic change in the sample's refractive index.

[0011] In photoacoustic spectroscopy, the sound wave propagating through the sample is detected using an acoustic detector. In photothermal spectroscopy, the change in the refractive index is measured. This involves observing, for example, how the transmission of an electromagnetic probe beam, which also illuminates the area of ​​the sample where the excitation radiation is absorbed, changes due to the refractive index change.

[0012] The signal generated by both methods changes with the concentration of the sample absorbing the excitation radiation. Therefore, the intensity of the detected sound wave or the amplitude of the modulated refractive index change is a measure of the target gas concentration.

[0013] Since the sample is frequently gaseous, the following text will mostly use such a gaseous sample or target gas as an example. However, the considerations apply analogously to liquid or solid samples.

[0014] Both methods are based primarily on the periodic modulation of the excitation radiation. For both methods, it is possible to subject the excitation radiation to either intensity modulation or wavelength modulation.

[0015] It is known from the prior art to modulate the intensity of the excitation radiation source. Mechanical choppers, acousto-optic modulators, electro-optic modulators, and also directly modulated radiation sources, such as light-emitting diodes or semiconductor lasers, are available for this purpose in the prior art.

[0016] Several state-of-the-art approaches are also available for wavelength modulation, i.e., shifting the excitation wavelength relative to the absorption line of the sample. For example, wavelength modulation can be imposed on a tunable semiconductor laser by current modulation or temperature modulation.

[0017] Lasers as excitation radiation sources have proven advantageous because, due to the spectral narrowband nature of their emission, they exhibit high selectivity, so that only the target substance itself is excited and not other components in the sample or sample holder.

[0018] Intensity modulation of lasers, especially semiconductor lasers, remains complex. In particular, intensity modulation by modulating the driver current in semiconductor lasers is often accompanied by unusable background signals. Furthermore, a wavelength reference via purely optical detection is usually required for measurement robustness.

[0019] In contrast, it is an object of the present invention to at least partially avoid the disadvantages of the prior art. A further object of the present invention is to provide a system and a method for photoacoustic or photothermal spectroscopy that also provide sufficient selectivity (distinguishability of the origin of the signal change) and sensitivity for broadband absorbing samples.

[0020] At least one of the aforementioned problems is solved by a system according to independent claim 1. For this purpose, the system of the type mentioned at the outset comprises, in addition to the sample holder, a filter gas cell. The filter gas cell is arranged in a beam path of the excitation radiation between the excitation radiation source and the sample holder. The filter gas cell contains a filter gas with a spectral absorption range, wherein the spectral absorption range and the spectral tuning range of the excitation radiation source overlap such that modulating the excitation wavelength leads to an intensity modulation of the excitation radiation in a beam direction downstream of the filter gas cell.

[0021] The underlying idea of ​​the invention is to make the advantages technically associated with generating excitation radiation by modulating the excitation wavelength applicable to samples with spectrally broad or shallow absorption bands, for which conventional wavelength modulation does not yield good measurement results. The wavelength modulation of the excitation radiation source, in particular a laser or semiconductor laser, becomes usable through the filtering with the filter gas according to the invention.

[0022] According to the invention, the filter gas in the filter gas cell serves to convert the wavelength modulation of the excitation radiation into an intensity modulation. Modulating the excitation wavelength leads to a change in the absorption of the excitation radiation in the filter gas cell and thus to an intensity modulation of the excitation radiation in the direction of radiation behind the filter gas cell. Therefore, the excitation radiation is also intensity-modulated when it enters a sample.

[0023] In principle, all radiation sources that allow the excitation radiation to be modulated with a specific excitation wavelength within a spectral tuning range are suitable as excitation radiation sources. In one embodiment of the invention, the excitation radiation source is a tunable laser, in particular a semiconductor laser or a solid-state laser.

[0024] The spectral tuning range of the excitation radiation source is the wavelength range over which the excitation wavelength can be tuned.

[0025] The excitation radiation itself has a linewidth; that is, the excitation wavelength is typically not exactly one wavelength, but a range of wavelengths around the excitation wavelength. This range of wavelengths is then periodically shifted within the spectral tuning range during the periodic modulation of the excitation wavelength. The linewidth of the excitation radiation around the excitation wavelength is smaller than the spectral tuning range.

[0026] In one embodiment, the wavelength modulation of the excitation radiation is sinusoidal or cosinusoidal between an upper wavelength limit and a lower wavelength limit of the spectral tuning range. In such an embodiment, for example, a semiconductor laser is driven as the excitation radiation source with a sinusoidally modulated current.

[0027] In an alternative embodiment, the wavelength modulation of the excitation radiation is essentially linear between an upper and a lower wavelength limit of the spectral tuning range. In such an embodiment, for example, a semiconductor laser is driven as the excitation radiation source with a periodic sawtooth signal. Independent claim 1 claims the system without the actual sample and only with the sample holder for receiving the sample. It is understood that in an embodiment of the invention, the system also includes the sample itself.

[0028] The sample holder must be selected to suit the sample. In one embodiment, the sample holder is a gas measuring cell for a gaseous sample or a measuring cell for a liquid sample, for example, a cuvette. In another embodiment of the invention, the measuring cell is a hollow fiber whose fiber core can be filled with the liquid or gaseous sample. If the sample is a solid, for example, a crystal, the sample holder can be formed by a support surface onto which the sample is clamped or glued.

[0029] In one embodiment of the invention, the sample comprises a target gas.

[0030] The system according to the invention is particularly suitable for target gases with broadband absorption. Broad spectral absorption ranges are exhibited, for example, by target gases with large molecules and target gases under high pressure.

[0031] The design of the detection device depends crucially on whether the system according to the invention is used for a photoacoustic or a photothermal detection method. The resulting variants for each method are explained in detail below.

[0032] In one embodiment, the evaluation device is a processor with software running on it that implements the corresponding evaluation steps.

[0033] The filter gas cell is a gas cell, preferably enclosed, that allows excitation radiation to be passed through one or more filter gases contained within the filter gas cell. In one embodiment of the invention, the filter gas cell is a transmission cell with two opposing windows for the entry and exit of the excitation radiation.

[0034] In one embodiment, the filter gas cell is a hollow fiber whose core is filled with the filter gas. Such an embodiment allows for a space-saving extension of the interaction length between the excitation radiation and the filter gas.

[0035] The spectral absorption range of the filter gas is a range of wavelengths in which the filter gas absorbs electromagnetic radiation, particularly the excitation radiation. Crucial for converting the wavelength modulation into an intensity modulation of the excitation radiation is the way in which the spectral absorption range of the filter gas and the spectral tuning range of the excitation radiation overlap.

[0036] In principle, any wavelength modulation leads to an intensity modulation if the absorption of the filter gas changes across the spectral tuning range of the excitation radiation source. However, it is advantageous if this change in absorption is as large as possible for the smallest possible change in wavelength. In other words, the ideal absorption change of the filter gas is a steep slope in its absorption characteristics.

[0037] In one embodiment of the invention, the spectral absorption range of the filter gas within the tuning range is designed such that a change in the excitation wavelength (expressed in wavenumbers) of 0.1 cm -1 or less leads to a change in the absorption of the filter gas of 10% or more.

[0038] In one embodiment, over a wavelength range that is equal to or smaller than the spectral tuning range, the absorption of the filter gas changes from maximum absorption to no absorption.

[0039] In one embodiment of the invention, the spectral absorption range of the filter gas lies entirely within the spectral tuning range of the excitation radiation source and is spectrally narrower than the spectral tuning range. In this way, when the excitation wavelength sweeps across the entire spectral tuning range, the excitation radiation is switched off and then on again.

[0040] In one embodiment of the invention, the filter gas exhibits zero absorption section by section (i.e., only section by section) within the spectral tuning range.

[0041] In one embodiment, the spectral absorption range in which the filter gas exhibits non-zero absorption for the excitation radiation is spectrally wider than the linewidth of the excitation radiation.

[0042] In one embodiment of the invention, the spectral tuning range is larger than the spectral absorption range of the filter gas in which the filter gas exhibits non-zero absorption for the excitation radiation. In another embodiment of the invention, the spectral absorption range is smaller than the spectral width of the target gas absorption.

[0043] In one embodiment of the present invention, the filter gas contains methane, ethane or CO2.

[0044] In one embodiment of the invention, the system is a system for photoacoustic spectroscopy.

[0045] Photoacoustic measurement methods are based on exciting a sample, usually a gas, with electromagnetic excitation radiation. Due to the absorption of this radiation within the sample, local heating and expansion occur, which in turn leads to a pressure increase. If the intensity of the electromagnetic excitation radiation is periodically modulated, the sample undergoes periodic excitation, resulting in alternating heating and cooling. This periodic heating and cooling, in turn, leads to periodic expansion and contraction, generating a sound wave within the sample. This sound wave can be detected using a conventional acoustic measuring device.

[0046] The amplitude of the sound wave propagating through the sample is proportional to the radiation absorption of the excitation radiation in the sample. Radiation absorption, in turn, is proportional to the concentration of the absorbing sample. Therefore, the concentration of the sample can be determined from the amplitude of the detected sound wave.

[0047] Such photoacoustic measurement methods are used in trace gas analysis, for example to detect air pollutants. They can detect concentrations of methane in nitrogen as low as ten parts per billion.

[0048] Therefore, in one embodiment of the invention, the detection device is a sound measuring device, wherein the sound measuring device is configured and arranged on the sample holder, in particular on a gas measuring cell, such that during operation of the system, a sound wave generated by the periodic heating and cooling produced in the sample can be detected by the sound measuring device. In one embodiment of the invention, the sound measuring device is a microphone.

[0049] In an alternative embodiment, the system is a photothermal spectroscopy system. In such an embodiment, the detection device comprises a probe radiation source and a radiation detector. The probe radiation source is configured such that, during system operation, it generates and emits electromagnetic probe radiation with a probe wavelength that differs from the excitation wavelength. Furthermore, an excitation beam path and a probe beam path are configured and arranged such that, during system operation, the sample is illuminated by both the excitation radiation and the probe radiation in such a way that the excitation beam path and the probe beam path overlap within the sample.Furthermore, the radiation detector is designed and arranged such that, during system operation, it detects the probe radiation in the probe beam path behind the sample and generates a detection signal representing the periodic heating and cooling generated in the sample. In one embodiment, the excitation beam path and the probe beam path intersect in the sample at an angle other than 0°.

[0050] An example of a photothermal method for determining a measure of sample absorption is photothermal common-path interferometry (PCI). This method exhibits high sensitivity for the sample, particularly for a target gas, and enables highly accurate concentration determination. PCI is also known as thermal lens spectroscopy. This form of absorption sensing is characterized by a linear range spanning many orders of magnitude and by low achievable detection limits for absorption, and thus low detectable substance concentrations.

[0051] The basic principle of PCI (Periodic Electron Radiation Detection) is based on the periodically modulated generation of a thermal lens within the sample itself. Excitation radiation is injected into the sample with an excitation intensity that varies across the cross-section of the excitation beam. The absorption of this radiation within the sample heats the sample within this cross-section. Due to the varying excitation intensity across the beam cross-section, the power input into the sample, and thus the temperature of the sample, also varies across this cross-section. This creates a thermal lens. Changes in the thermal lens, occurring when the absorption of the excitation radiation within the sample changes, result in more or less probe radiation reaching the radiation detector.

[0052] In one embodiment of the invention, the excitation beam path and the interrogation beam path intersect in the sample at an angle other than 0°.

[0053] At least one of the aforementioned problems is also solved by the inventive method according to the independent claim directed thereto. For this purpose, the method of the type mentioned at the outset additionally comprises the step of illuminating a filter gas in a beam direction of the excitation radiation in front of the sample, wherein the filter gas has a spectral absorption range and wherein the spectral absorption range and the spectral tuning range overlap in such a way that modulating the excitation wavelength leads to an intensity modulation of the excitation radiation in the beam direction after the filter gas.

[0054] In one embodiment of the invention, an absorption signal not generated by the sample is used as a reference signal to determine the wavelength.

[0055] Further advantages, features, and applications of the present invention will become clear with reference to the following description of embodiments thereof and the accompanying figures. In the figures, identical elements are designated by identical reference numerals.

[0056] Figure 1 is a block diagram illustrating the basic principle of the present invention.

[0057] Figure 2 is a schematic plot of the absorption of the target gas and the transmission of the filter gas cell.

[0058] Figure 3 is a schematic side view of a first embodiment of the system according to the invention for photoacoustic spectroscopy.

[0059] Figure 4 is a schematic side view of a second embodiment of the system according to the invention for photothermal spectroscopy.

[0060] Figures 1 and 2, taken together, illustrate the principle underlying the present invention. The system 1 according to the invention comprises the following elements: a laser 2 as an excitation radiation source, a filter gas cell 3, a gas measuring cell 4 as a sample holder for receiving a target gas 8 as a sample, a detection device 5, and an evaluation device 6. The evaluation device is connected to the detection device 5 in such a way that, during operation of the system 1, it receives and evaluates a detection signal from the detection device.

[0061] For the examples considered here, the concentration of methane as target gas 8 in the gas measuring cell 4 is to be determined. The absorption of electromagnetic excitation radiation 7 by the methane in the gas measuring cell 4 depends on the concentration of the target gas in the gas measuring cell 4. The system 1 according to the invention detects a measure of the absorption of the target gas 8 in the gas measuring cell 4. The excitation radiation 7 has an excitation wavelength that extends over a spectral tuning range of 0.1 cm'. 1The excitation radiation 7 generated and emitted by the laser 2 can be tuned and periodically modulated within the spectral tuning range. For this purpose, the semiconductor laser 2 is driven with a sinusoidally periodically modulated current. This drive current is shown schematically in the inset diagram labeled 9 in Figure 1. Consequently, the excitation radiation 7 generated and emitted by the laser 2 also undergoes a sinusoidally modulated wavelength change over time. This is shown in the inset diagram labeled 10 in Figure 1, where wavelength is plotted on the x-axis and time on the y-axis. The excitation wavelength undergoes a sinusoidal variation between the lower wavelength limit of the spectral tuning range and the upper wavelength limit of the spectral tuning range of the laser 2.

[0062] The photothermal and photoacoustic measurement methods considered here are based on the periodic heating and cooling of the target gas 8 due to the modulated absorption of the excitation radiation 7 in the target gas 8. Since hydrocarbons tend to exhibit a spectrally broad absorption characteristic, the linear change in the excitation wavelength 10 applied to the laser 2 would result in only very small changes in the local temperature of the target gas 8 in the gas measuring cell 4. Neither the photoacoustic nor the photothermal detection would generate a quantitatively reliable signal.

[0063] In order to still be able to take advantage of the benefits of generating the wavelength-modulated excitation radiation 7 using the semiconductor laser 2, a filter gas cell 3 with a filter gas 11 is arranged in the direction of the excitation radiation 7 in front of the gas measuring cell 4.

[0064] In the embodiments discussed here, the filter gas 11 is also methane. The filter gas 11 is selected such that it has a spectral absorption range that lies entirely within the spectral tuning range of the laser 2. The filter gas 11 is at a significantly lower pressure than the target gas 8. In this way, the spectral absorption range of the filter gas 11 is narrower than that of the target gas 11. Within the spectral tuning range 14 of the laser 2, the absorption 15 of the filter gas 11 rises from zero to a local maximum and then falls back to zero. In the specific example chosen, therefore, when the excitation wavelength is periodically modulated across the spectral tuning range 14, the excitation wavelength sweeps out an absorption peak of the filter gas 11.Due to the periodic modulation, in the example shown, the absorption maximum is swept twice during one period of wavelength modulation; the excitation radiation is "switched off" twice. In alternative embodiments, the target gas 8 and the filter gas 11 are different gases. For example, the target gas 8 could be a large hydrocarbon molecule (C ≥ 2) and the filter gas methane at normal or reduced pressure.

[0065] Ideally, the situation arises as shown schematically in the diagram in Figure 2. The absorption 15 of the target gas 8 is shown as a solid line in Figure 2. The transmission of the filter gas cell 3 is shown as a dashed line 13. In the illustrated embodiment, the filter gas cell 3 with the filter gas 11 serves as a notch filter for the excitation radiation 7. If the excitation wavelength is varied across the spectral tuning range 14, the intensity of the excitation radiation 7 drops once in the direction of travel behind the filter gas cell 3, and then increases again. In other words, the excitation radiation is switched off and then on again as it sweeps across the spectral tuning range 14.

[0066] Since the absorption 15 of the target gas 8 varies only slightly over the spectral tuning range 14, this modulation of the excitation radiation 7 in the target gas 8 manifests itself as an alternation of heating, cooling, and heating. According to the invention, this periodic alternation of heating and cooling is detected by the detection device 5.

[0067] The local heating of the target gas in the area of ​​the excitation radiation beam 7 in the gas measuring cell 4 leads to a pressure change that can be measured as a sound wave, or to a density change that can be measured as a change in the refractive index. The two embodiments of system 1 shown schematically in Figures 3 and 4 are used for detection at this point.

[0068] Figure 3 shows a photoacoustic system 1. To detect the sound wave generated by the periodic heating and cooling in the target gas 8, a microphone 16 is arranged in the gas measuring cell 4 as a sound measuring device. This microphone 16 is connected to the evaluation unit 6.

[0069] In this embodiment, the filter gas cell 3 is arranged directly adjacent to the gas measuring cell 4 for the target gas 8, with the filter gas cell 3 and the gas measuring cell 4 being separated from each other by a transparent partition 17. Crucially for the system 1 according to the invention, the evaluation unit 6 uses the detection signal 18 of the microphone 16 as a measure of the concentration of the target gas 8 in the measuring gas cell 4, while the filter gas 11 serves only to convert the wavelength modulation of the excitation radiation 7 into an intensity modulation, as previously described. In contrast, the system 1 from Figure 4 serves for the photothermal detection of the absorption of the excitation radiation 7.

[0070] System 1 is shown in a schematic top view in Figure 1. System 1 comprises a laser 2 as a radiation source for the excitation radiation 7, a laser 20 as a radiation source for the probe radiation 21, a detector 22 for detecting an interference pattern, a detector 23 for detecting the total power of the excitation radiation 7, a processing unit in the form of a lock-in amplifier 24, and a computer 25.

[0071] In the example described here, the target gas 8 is analyzed as a sample, the task of which of the photothermal absorption sensor 1 is to determine the content of methane as target gas 8 in a gas mixture in the measuring gas cell 4.

[0072] In the embodiment of Figure 4, the filter gas cell 3 with the filter gas 11 also converts the wavelength modulation of the excitation radiation 7 specified to the laser 2 by the lock-in amplifier 24, as previously explained for the system 1 from Figure 3, into an intensity modulation of the excitation radiation 7.

[0073] The excitation radiation 7 generates thermal heating in the target gas 8 that varies across the excitation beam cross-section and essentially follows the intensity distribution of the excitation radiation 7 within the excitation beam cross-section. The lasers 2 and 20 generate both the excitation radiation 7 and the probe radiation 21 with Gaussian intensity distributions across their respective beam cross-sections.

[0074] The target gas 8 absorbs at the excitation wavelength of the excitation radiation 7 of 3057.7 cm. -1Therefore, the excitation radiation in the center of the excitation beam cross-section generates a stronger heating of the sample gas than symmetrically around the center.

[0075] The described local distribution of the heating of the target gas 8 across the excitation jet cross-section leads to a refractive index variation across the excitation jet cross-section, which essentially follows the heating profile across the excitation jet cross-section 2.

[0076] To enable later normalization to the total power of the excitation radiation 7 in the lock-in amplifier 24 or in the computer 25, the power of the excitation radiation 7 is measured by the detector 23 downstream of the gas measuring cell 4. The probe radiation 21 has a probe wavelength that differs from the excitation wavelength and from the absorption wavelength of the target gas 8. The probe radiation 21 therefore passes through the target gas 8 essentially without significant absorption. However, the refractive index within the target gas 8, which varies across the beam cross-section of the excitation radiation 7, leads to self-interference of the probe radiation 21, as can also be observed, for example, in the Fresnel biprism experiment.

[0077] The probe beam 21 intersects the excitation beam 7 such that the excitation beam path 26 of the excitation beam 7 and the probe beam path 25 of the probe beam 21 intersect at an angle 31 of approximately ten degrees. With ideal alignment of the system 1, the center axes of the excitation beam path 26 and the probe beam path 27 intersect.

[0078] In the near field behind the intersection point of the excitation radiation 7 and the probe radiation 21 in the gas measuring cell 4, an interference pattern is formed which is characteristic of the beam cross-section of the excitation radiation 7, of the modulation of the excitation radiation 7 and of the absorption of the excitation radiation 7 by the methane contained in the gas measuring cell 4.

[0079] If the gas measuring cell 4 does not contain the methane to be analyzed, the excitation radiation 7 is not absorbed in the target gas 8 and no interference pattern is generated. The probe radiation 21 also exhibits an essentially undisturbed Gaussian intensity distribution even beyond the intersection point in the target gas 8. As soon as the sample gas contains methane, an interference pattern characteristic of the methane concentration is formed. This pattern is detected by a detector 22 in the detector plane just beyond the intersection point in the target gas 8.

[0080] To detect the interference pattern, a detector 22 is provided, which has a first sensor section 28 at a first location, a second sensor section 29 at a second location, and a third sensor section 30 at a third location in the detector plane. Unlike the schematic representation in Figure 1, the detector plane is arranged perpendicular to the direction of the probe beam 21. Thus, the detector 22 does not extend in the plane of the illustration (sheet plane) as shown, but perpendicular to it.

[0081] The absorption, and thus the thermal heating, of the target gas 8 is modulated by the modulation frequency specified by the lock-in amplifier 24. This modulation is also reflected in the interference pattern. The change in the interference pattern is then the measure of the absorption by the target gas 8 in the gas measuring cell 4. For the purposes of the original disclosure, it is pointed out that all features, as they can be deduced from the present description, the drawings, and the claims, even if they have been specifically described only in connection with certain other features, can be combined individually or in any combination with other features or groups of features disclosed herein, unless this has been expressly excluded or technical circumstances render such combinations impossible or pointless.A comprehensive, explicit description of all conceivable combinations of features is omitted here solely for the sake of brevity and readability.

[0082] While the invention has been illustrated and described in detail in the drawings and the preceding description, this illustration and description are merely exemplary and are not intended to limit the scope of protection as defined by the claims. The invention is not limited to the disclosed embodiments.

[0083] Variations of the disclosed embodiments are obvious to a person skilled in the art from the drawings, the description, and the accompanying claims. In the claims, the word "have" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude multiple features. The mere fact that certain features are claimed in different claims does not preclude their combination. Reference numerals in the claims are not intended to limit the scope of protection.

[0084] List of reference signs

[0085] 1 system

[0086] 2 lasers for the excitation radiation

[0087] 3 Filter gas cell

[0088] 4 gas measuring cell

[0089] 5 Detection device

[0090] 6 Evaluation unit

[0091] 7 Excitation radiation

[0092] 8 Target gas

[0093] 9 Graphics inset (modulated driver current)

[0094] 10 Excitation wavelength

[0095] 11 Filter gas

[0096] 13 Transmission of the gas measuring cell

[0097] 14 spectral tuning range

[0098] 15 Absorption of the target gas

[0099] 16 microphones

[0100] 17 transparent partition walls

[0101] 18 Detection signal

[0102] 20 lasers for the interrogation beam

[0103] 21 Query radiation

[0104] 22 Radiation detector for the interference pattern

[0105] 23 Detector for the excitation radiation

[0106] 24 Lock-in Amplifiers

[0107] 25 computers

[0108] 26 Excitation beam path

[0109] 27 Interrogation beam path

[0110] 28 first sensor section

[0111] 29 second sensor section

[0112] 30 third sensor section

[0113] 31 angles

Claims

P a t e n t a n s p r ü c h e 1. System (1) for determining a measure of absorption of a sample (8), wherein the system (1) comprises an excitation radiation source (2), wherein the excitation radiation source (2) is configured such that, during operation of the system (1), it generates and emits electromagnetic excitation radiation (7) with an excitation wavelength, and, during operation of the system (1), it periodically modulates the excitation wavelength within a spectral tuning range (14); a sample holder (4), wherein the sample holder (4) is configured and arranged such that a sample (8) can be received on the sample holder (4) and that, during operation of the system (1), the excitation radiation (7) illuminates the sample (8) received on the sample holder (4); and a detection device (5), wherein the detection device (5) is configured and arranged such thatthat in the operation of the system (1) it detects a periodic heating and cooling generated by the absorption of the excitation radiation (7) in the sample (8) and generates and outputs a detection signal (18) representing the periodic heating and cooling, and has an evaluation device (6) wherein the evaluation device (6) is effectively connected to the detection device (5) such that in the operation of the system (1) the evaluation device (6) receives the detection signal (18) from the detection device (5), and wherein the evaluation device (6) is configured such that in the operation of the system (1) it determines the measure of the absorption of the sample (8) from the detection signal (18), characterized in that the system (1) has a filter gas cell (3) in addition to the sample holder (4),wherein the filter gas cell (3) is arranged in an excitation beam path (26) of the excitation radiation (7) between the excitation radiation source (2) and the sample holder (4), wherein a filter gas (11) with a spectral absorption range is contained in the filter gas cell (3) and wherein the spectral absorption range and the spectral tuning range overlap such that the modulation of the excitation wavelength, leading to an intensity modulation of the excitation radiation (7) in a beam direction behind the filter gas cell (3).

2. System (1) according to the preceding claim, wherein the spectral absorption range lies entirely within the spectral tuning range.

3. System (1 ) according to one of the preceding claims, wherein the filter gas (11 ) has a section-wise modulation of zero within the spectral tuning range.

4. System (1) according to one of the preceding claims, wherein the spectral absorption range of the filter gas (11) within the tuning range is configured such that a change in the excitation wavelength of 0.1 cm -1 or less leads to a change in the absorption of the filter gas of 10 percent or more.

5. System (1) according to any one of the preceding claims, wherein the filter gas contains methane, ethane or CO2.

6. System (1) according to one of the preceding claims, wherein the sample holder (4) is a gas measuring cell.

7. System (1) according to one of the preceding claims, wherein the detection device (5) comprises a sound measuring device (16), wherein the sound measuring device (16) is configured and arranged on the sample holder (4) such that, during operation of the system (1), a sound wave generated by the periodic heating and cooling produced in the sample (8) can be detected by the sound measuring device (16).

8. System (1) according to any one of claims 1 to 6, wherein the detection device (5) is a probe radiation source (20), wherein the probe radiation source (20) is configured such that, during operation of the system (1), the probe radiation source (20) generates and emits electromagnetic probe radiation (21) with a probe wavelength, wherein the probe wavelength is different from the excitation wavelength, and wherein an excitation beam path (26) of the excitation radiation (7) and a probe beam path (27) of the probe radiation (21) are configured and arranged such that, during operation of the system (1), the sample (8) is illuminated with the excitation radiation (7) and with the probe radiation (21). the excitation beam path (26) and the probe beam path (27) overlap in a volume of the sample (8), and comprises a radiation detector (22), wherein the radiation detector (22) is designed and arranged such that, in the operation of the system (1), the radiation detector (22) detects the probe radiation (21) in the probe beam path (27) behind the sample (3) and generates a detection signal (18) representing the periodic heating and cooling.

9. Method for determining a measure of absorption of a sample (8), wherein the method comprises the steps: Generation and emission of electromagnetic excitation radiation (7) with an excitation wavelength, periodic modulation of the excitation wavelength within a spectral tuning range, Illuminating the sample (8) with the excitation radiation (7), Detecting periodic heating and cooling generated by the absorption of excitation radiation (7) in the sample (8) and Determining the measure of absorption from periodic heating and cooling, characterized in that the method further comprises the steps Illumination of a filter gas in a beam direction of the excitation radiation (7) in front of the sample (8), wherein the filter gas (11) has a spectral absorption range and wherein the spectral absorption range and the spectral tuning range overlap such that modulating the excitation wavelength leads to an intensity modulation of the excitation radiation (7) in the beam direction after the filter gas (11).

10. Method according to the preceding claim, wherein an absorption signal not generated by the sample is used as a reference signal to determine the wavelength.

11. Method according to claim 9 or 10, wherein the sample (8) is a target gas.

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