Method for disturbance-robust photothermal spectroscopy, and disturbance-robust photothermal spectrometer

Abstract

The invention relates to a method for the disturbance-robust determination and / or monitoring of a measurement variable of a medium by means of photothermal spectroscopy. The method comprises disturbance-robust control of a probe laser (2) and / or of an interferometer (6) by means of a disturbance-robust control method, wherein in the disturbance-robust control method, the actual value is controlled to a target value (SW) by adjustment of a manipulated variable (SG) of the probe laser (2), in particular a laser current of the probe laser (2), such that a frequency dependent disturbance response (SA) - has a first frequency range (Δf1), the first frequency range extending from a first frequency (f1) of less than the detection frequency (fD) to a second frequency (f2) of greater than the detection frequency (fD), and the disturbance response (SA) being greater than or equal to a minimum amplification (Min) for all frequencies from the first frequency range (Δf1), - has a second frequency range (Δf2) which is different from the first frequency range (Δf1), the second frequency range (Δf2) lying above the first frequency range (Δf1), in particular directly adjacent thereto, and the disturbance response (SA) being less than a maximum amplification (Max) for all frequencies from the second frequency range (Δf2), and - wherein the minimum amplification of the first frequency range is at least as large as the maximum amplification of the second frequency range. The invention also relates to a photothermal spectrometer (100) which is designed to carry out the method according to the invention.

Description

[0001] Methods for interference-resistant photothermal spectroscopy and interference-resistant photothermal spectrometer

[0002] The invention relates to a method for the disturbance-resistant determination and / or monitoring of a measured quantity of a medium by means of photothermal spectroscopy, as well as a photothermal spectrometer.

[0003] Photothermal (molecular) spectroscopy is based on the excitation of a medium (or a sample thereof) using an excitation light source. The absorption of photons leads to the excitation of molecular energy levels, such as molecular vibrational modes like rotational vibrations and / or transverse vibrations. This excitation of molecular energy levels, in turn, leads to a change in the temperature, pressure, and density of the medium or sample. The temperature change occurs when an excited molecule relaxes from the excited energy level back to its original energy level via a non-radiative transition.

[0004] A temperature increase caused by photoabsorption generates a pressure change within the sample, which in turn propagates in the form of an acoustic wave. The detection of this acoustic wave for molecular spectroscopy is called photoacoustic spectroscopy and is based on similar principles to the photothermal spectroscopy of the present application.

[0005] Once the pressure has decreased to the equilibrium pressure, a density change proportional to the temperature remains. In combination, temperature and density changes affect other properties of the sample, such as the refractive index. Photothermal spectroscopy (PTS) is based on the detection of these photoinduced changes in the sample temperature, which is typically monitored via the sample's refractive index. Unlike classical transmission spectroscopy based on the Lambert-Beer law, PTS is an indirect method for optical absorption analysis, measuring a photoinduced change in the thermal state of the sample. The heat output is proportional to the sample's absorption coefficient and the incident light intensity.

[0006] Photothermal spectroscopy (PTS) systems for detecting refractive index changes typically use the aforementioned excitation light source, such as an excitation diode or an excitation laser, which serves to photo-induced heating of the sample, and a probe laser, which monitors the change resulting from the heating, e.g., the refractive index. The change in the refractive index causes a phase shift of the light passing through the heated sample and can be measured with high sensitivity using an interferometer. It is partly due to this high sensitivity that photothermal spectroscopy is used to measure trace gases. For example, a photothermal spectrometer for the detection of trace gases, which uses Fabry-Perot interferometers (FPIs), is disclosed in European patent EP 3 485 254 B1. The excitation laser described therein is modulated in its wavelength by a modulation frequency.The photothermal spectrometer described in EP 3 485 254 B1 has a high sensitivity and is suitable for laboratory operation, in which, as a rule, controllable and, in particular, constant environmental conditions prevail.

[0007] There is a growing desire to use photothermal spectroscopy for the detection of trace gases in process automation technology, for example in industrial plants, particularly as so-called field devices. Field devices are defined as, in principle, all measuring instruments for determining and / or monitoring process variables that are used close to the process and provide or process process-relevant information. The applicant manufactures and distributes such field devices in a wide variety of configurations.

[0008] In process automation technology, changing, and sometimes even abrupt, environmental conditions often occur, including changes in physical environmental parameters such as temperature, air pressure, humidity, etc., which can disrupt the operation of the photothermal spectrometer. This is especially true if the aforementioned changes include frequency components that exceed the modulation frequency of the excitation light source. In this case, compensating for these changing environmental conditions, which are perceived as disturbances, becomes a challenging task.

[0009] Particularly in Fabry-Perot interferometers, if there is a sufficiently large disturbance, the interrogation light source may drift out of the typically monotonically rising or falling range of an interferometer characteristic curve specified by the interferometer, for which the photothermal spectrometer is designed in terms of control technology.

[0010] In this case, a special algorithm must be executed to adjust the photothermal spectrometer to a control range valid under the changed environmental conditions. During this time, no measurements can be taken. Such measurement pauses are a major disadvantage in process automation technology and are potentially undesirable, especially in the relevant case of trace gas detection, for example, for safety reasons.

[0011] The invention is therefore based on the objective of providing a method for interference-resistant photothermal spectroscopy and an interference-resistant photothermal spectrometer. With regard to the method, this objective is achieved by a method for the interference-resistant determination and / or monitoring of a measured quantity of a medium by means of photothermal spectroscopy, the method comprising:

[0012] Illumination of excitation light into the medium or into a sample of the medium, by means of at least one, in particular narrowband, excitation light source, wherein the frequency and / or intensity of the excitation light source is modulated with an excitation modulation frequency;

[0013] Irradiation of probe light by means of a probe laser into the medium or sample excited by the excitation light,

[0014] Guiding the excitation light from the excitation light source and the probe light from the probe laser into an interferometer such that the excitation light and the probe light overlap in the interferometer.

[0015] Specification of a detection frequency for detecting a photothermal measurement signal, which detection frequency is in particular a natural multiple, preferably an even natural multiple, of the excitation modulation frequency,

[0016] Capturing the light transmitted and / or reflected by the interferometer from the probe laser as measuring light, using an optical detector unit,

[0017] Determining an actual value for a measuring light measurement signal, wherein the measuring light measurement signal is determined based on the detected measuring light,

[0018] Disturbance-robust control of the probe laser and / or the interferometer, by means of a disturbance-robust control method, wherein in the disturbance-robust control method the actual value is regulated to a setpoint value, by adjusting a manipulated variable of the probe laser, in particular a laser current of the probe laser, such that a frequency-dependent disturbance response

[0019] - has a first frequency range, wherein the first frequency range extends from a first frequency smaller than the detection frequency fd to a second frequency larger than the detection frequency fd, wherein for all frequencies from the first frequency range the disturbance response is greater than or equal to a minimum gain, - has a second frequency range different from the first frequency range, which second frequency range lies above the first frequency range, in particular directly adjacent to it, wherein for all frequencies from the second frequency range the disturbance response is less than a maximum gain, and

[0020] -where the minimum gain of the first frequency range is at least as large as the maximum gain of the second frequency range,

[0021] Detecting a photothermal measurement signal using the optical detector unit, using the detection frequency, wherein the probe laser and / or the interferometer are controlled by means of the disturbance-robust control method, and

[0022] Determining and / or monitoring the measured quantity based on the detected photothermal measurement signal.

[0023] In particular, for all frequencies smaller than the first frequency, the interference response is also smaller than the maximum gain.

[0024] The determination and / or monitoring of the measured quantity involves, for example, the determination of the presence and / or the determination of the concentration of a target gas, such as the aforementioned trace gas, which preferably has a substance concentration of no more than 10%, in particular 5%, in the medium, e.g. a gas mixture.

[0025] The determined actual value for the measurement light signal represents, for example, the light intensity of the transmitted and / or reflected measurement light. This actual value is compared with a target value and adjusted to the target value by setting the manipulated variable. This serves precisely to regulate to a control point, which, for example, is related to a resonance to be detected in a frequency spectrum of the light intensity of the transmitted and / or reflected measurement light.

[0026] The term "natural multiple" naturally also includes the single multiple.

[0027] The interference-robust control of the invention makes it possible to suppress the aforementioned interferences with frequencies above the photothermal signal—which lies precisely at the detection frequency used for photothermal spectroscopy—and thus compensate for them through control engineering. At the same time, the photothermal measurement signal itself is not suppressed around the detection frequency. This property is achieved by ensuring that the interference response, known from control engineering, is always greater in the first frequency range around the detection frequency than in the second frequency range above the first. This is because the maximum gain, which serves as the upper limit for the second frequency range, is always smaller than the minimum gain, which serves as the lower limit for the first frequency range.

[0028] In one embodiment of the method, the minimum gain of the first frequency range is not less than -20 dB, in particular not less than -5 dB.

[0029] In one embodiment of the method, the frequency-dependent disturbance response assumes a disturbance response maximum in the first frequency range, in particular at least a local one, wherein for all frequencies from the first frequency range the disturbance response is not greater than the disturbance response maximum, and wherein in particular the disturbance response maximum is greater than -10 dB, preferably greater than -5 dB.

[0030] In one embodiment of the method, the frequency-dependent disturbance response in the first frequency range exhibits no extrema other than the disturbance response maximum. Preferably, the disturbance response maximum is the only extremum of the first frequency range.

[0031] In one embodiment of the method, the first frequency and the second frequency are chosen such that for all frequencies from the first frequency range, the interference response is at least as large as to be at most 5dB smaller, in particular at most 3dB smaller, than the interference response maximum.

[0032] In one embodiment of the method, the first frequency is not less than 0.75 times, in particular not less than 0.9 times, the detection frequency, and the second frequency is not more than 1.25 times, in particular not more than 1.1 times, the detection frequency.

[0033] In one embodiment of the method, the second frequency range borders directly on the first frequency range and extends from the second frequency to a cutoff frequency, wherein, in the event that the disturbance response for all frequencies from the first frequency range is at least so large that it is at most 3dB smaller than the disturbance response maximum, the cutoff frequency is greater than 6 times the detection frequency.

[0034] In one embodiment of the method, the disturbance-robust control procedure uses a first control loop with one or more filter elements to filter out the first frequency range, namely a bandstop filter whose filter bandwidth corresponds to the first frequency range and / or a high-pass filter with the second frequency as its high-pass cutoff frequency combined with a low-pass filter with the first frequency as its low-pass cutoff frequency. Alternatively or additionally, the manipulated variable of the probe laser set in the disturbance-robust control procedure, in particular...the laser current of the probe laser, and / or an interferometer control variable adjustable in the disturbance-robust control method, having a high-pass control variable with a high-pass behavior with the second frequency as the high-pass cutoff frequency, wherein the setting of the high-pass control variable is combined with the setting of a low-pass control variable of the probe laser and / or the interferometer, having a low-pass behavior with the first frequency as the low-pass cutoff frequency.

[0035] In one iteration of the procedure, this includes:

[0036] Using the laser current of the probe laser as a control variable in the disturbance-robust control method;

[0037] Specification of a target laser current for the probe laser;

[0038] Implementing a second control procedure, which is coupled to the disturbance-robust control procedure in such a way that the laser current of the probe laser, which serves as a manipulated variable in the disturbance-robust control procedure and is set to an actual laser current, is adjusted to the target laser current by means of the second control procedure, using at least a second manipulated variable of the second control procedure, in particular a laser temperature of the probe laser, a mirror spacing of the interferometer and / or an interferometer temperature.

[0039] With regard to the photothermal spectrometer, the task is solved by a photothermal spectrometer for the interference-resistant determination and / or monitoring of a measured quantity of a medium, in particular the determination of a concentration and / or the monitoring of the presence of a trace gas in the medium.

[0040] The photothermal spectrometer comprises: an excitation light source, in particular a narrowband one, a probe laser, an optical detector unit, an interferometer, a first control circuit for robust control of the probe laser and / or the interferometer, and an evaluation unit for determining and / or monitoring the measured quantity on the basis of a photothermal measurement signal controlled by the first control circuit and detected by the optical detector unit, wherein the photothermal spectrometer is configured to carry out the method according to the invention.

[0041] In one embodiment of the spectrometer, the interferometer is at least one of the following:

[0042] Fabry-Perot Interferometer

[0043] Mach-Zehnder interferometer.

[0044] In one embodiment of the spectrometer, the first control circuit has one or more filter elements for filtering out the first frequency range, namely a bandstop filter whose filter width corresponds to the first frequency range, in particular an analog bandstop filter and / or a high-pass filter with the second frequency as its high-pass cutoff frequency combined with a low-pass filter with the first frequency as its low-pass cutoff frequency and / or wherein the manipulated variable of the probe laser set in the disturbance-robust control method, in particularthe laser current of the probe laser, or an adjustable interferometer control variable in the disturbance-robust control method, exhibiting a high-pass control variable with a high-pass behavior with the second frequency as the high-pass cutoff frequency, and the interferometer and / or the probe laser exhibiting an adjustable low-pass control variable with a low-pass behavior with the first frequency as the low-pass cutoff frequency.

[0045] In one embodiment of the spectrometer, the bandstop filter in the first control circuit is arranged such that it is positioned in front of a current driver for the probe laser.

[0046] In one embodiment of the spectrometer, the probe laser is arranged in relation to the interferometer such that the light introduced into the interferometer by the probe laser is directed into the interferometer along a probe axis, wherein the probe axis does not deviate from an interferometer axis by more than 10°, and wherein, in particular, the excitation light source is arranged in relation to the interferometer such that the light introduced into the interferometer by the excitation light source is directed into the interferometer along an excitation axis, and wherein either an enclosed angle between the excitation axis and the probe axis is between 80 degrees and 100 degrees, or wherein the excitation axis and the probe axis substantially coincide.

[0047] In one embodiment of the spectrometer, the evaluation unit includes a lock-in amplifier for detecting the photothermal measurement signal at the detection frequency. In another embodiment, the spectrometer comprises a second control loop for implementing the second control procedure.

[0048] In one embodiment of the spectrometer, the first control loop and / or the second control loop features a PID controller.

[0049] The invention is explained in more detail with reference to the following figures, which are not to scale, where identical reference numerals denote identical features. Where clarity requires it or it otherwise appears appropriate, previously mentioned reference numerals are omitted in subsequent figures.

[0050] All variations of the method are also encompassed, mutatis mutandis, by the photothermal spectrometer and vice versa.

[0051] They show:

[0052] Fig. 1 : A schematic design of a photothermal spectrometer 100;

[0053] Fig. 2: A disturbance response SA according to the disturbance-robust control method according to the invention;

[0054] Fig. 3a: A first control circuit 3 designed to carry out the method according to the invention, in a first embodiment of the invention

[0055] Fig. 3b: A further first control circuit 3 designed for carrying out the method according to the invention, in a further embodiment of the invention, and

[0056] Fig. 4: A second control circuit 31 designed for carrying out the method according to the invention, which is coupled to the first control circuit 3, in a further embodiment of the invention.

[0057] Fig. 1 shows an embodiment of a photothermal spectrometer 100 in which the disturbance-robust control method according to the invention is used. An excitation light source 1, in this case an excitation laser, shines excitation light into a sample of the medium arranged in an interferometer s. The excitation laser 1 is optionally also controlled – however, this is independent of the disturbance-robust control and is not explained in detail here for the sake of clarity. In this embodiment, the intensity of the excitation light source 1 is modulated with an excitation modulation frequency fA. The narrowband excitation light source 1 can, mutatis mutandis, also be, for example, a diode. The excitation light is shone into an interferometer 6, which here is designed as a Fabiya-Perot interferometer. The interferometer 6 typically comprises two mirrors parallel to each other, arranged along an interferometer axis IA perpendicular to the mirror plane.The two mirrors of interferometer 6 have a spacer between them; a line parallel to the mirror planes points into the image plane. A probe laser 2 emits probe light into interferometer 6 along a probe axis AbA. The excitation light from the excitation light source 1 and the probe light from the probe laser 2 are directed into interferometer 6 such that the excitation light and the probe light superimpose within the interferometer 6.

[0058] The probe axis AbA coincides with the interferometer axis IA. In contrast, the excitation light from the excitation light source 1 is directed into the interferometer s along an excitation axis AnA, with the excitation axis AnA forming an angle of approximately 90° to the probe axis AbA. Alternatively, the probe axis AbA and the excitation axis AnA can also coincide, in which case the interferometer s, for example, for a Fabry-Perot interferometer, is configured such that it is a high-finesse interferometer s (e.g., >10) for the wavelengths of the probe light, but a low-finesse interferometer s (e.g., <6, especially <2) for the wavelengths of the probe light. This can be achieved, for example, by a suitably designed coating on the mirrors of the interferometer 6. For a Mach-Zehnder interferometer s, on the other hand, the excitation light source 1 is only present in one measurement path of the interferometer 6.

[0059] The light transmitted and / or reflected by the interferometer 6 from the probe laser 2 is detected as measuring light by means of an optical detector unit 5. The optical detector unit 5 is, for example, a photodiode. Based on the detected measuring light, an actual value IW for a measuring light measurement signal is determined and transmitted to a control circuit 3 for the implementation of the disturbance-robust control method according to the invention, in order to subsequently control the actual value IW to a setpoint SW, cf. Figs. 3a, b and 4.

[0060] Subsequently, a photothermal measurement signal is detected in the optical system, comprising the probe laser 2 and the interferometer s, which is controlled by the disturbance-robust control method, using a detection frequency fD. This is preferably twice the excitation modulation frequency fA.

[0061] The photothermal measurement signal is transmitted to a lock-in amplifier s adapted to the detection frequency fD. The photothermal spectrometer 100 includes an evaluation unit 7, which contains the lock-in amplifier s. The evaluation unit 7 is designed to determine the measured quantity based on the detected photothermal measurement signal. The measured quantity is the concentration of a target gas, which is present as a trace gas in the sample. The excitation light source 1 is designed to excite molecular vibrations of the target gas. The evaluation unit 7 is, for example, a processing unit such as a microcontroller. The microcontroller may include programmable logic, such as an FPGA, for real-time data processing. The evaluation unit 7 is also equipped with, in particular, analog electronic components for data acquisition.

[0062] In the embodiment shown in Fig. 1, only the probe laser 2 is controlled by means of the disturbance-robust control method, whereby the invention also includes, mutatis mutandis, the additional and / or alternative control of the interferometer 6. This is because the detected measuring light is always adjusted by the entire optical system, comprising the interferometer s and the probe laser 2. The interferometer s can be controlled, for example, by means of a piezoelectric element via its mirror spacing.

[0063] Fig. 2 shows the disturbance response SA of the disturbance-robust control method according to the invention, which is commonly used in control engineering. For the frequency-dependent disturbance response SA, the gain is usually plotted against the frequency (e.g., in Hz) in decibels, with the frequency axis in Fig. 2 having a logarithmic scale. For conventional control loops 3 according to the prior art, the frequency-dependent disturbance response SA shown here is1 According to the current state of the art (dashed line). However, in this case, the photothermal measurement signal is also suppressed at the detection frequency fD.

[0064] The noise response SA of the method according to the invention is shown as the solid line. According to the invention, in a first frequency range Af1 around the detection frequency fD, the noise response SA is always greater than or equal to a minimum gain Min, while in a second frequency range Af2 above the first frequency range Af1, the noise response SA is always less than or equal to a maximum gain Max. The minimum gain Min thus serves as a lower limit, whereas the maximum gain Max represents an upper limit. The maximum gain Max of the second frequency range Af2 coincides here with the minimum gain Min of the first frequency range Af1, whereby it is only essential for the invention that the minimum gain Min is always greater than or equal to the maximum gain Max.

[0065] Only above a cutoff frequency fG, which limits the second frequency range Af2, does the gain again become as high as the smallest gain from the first frequency range Af1, which, however, is always greater than the minimum gain Min of the first frequency range Af1. Furthermore, the interference response SA assumes an interference response maximum SAM – at least locally – in the first frequency range Af1, predominantly at the detection frequency fD.

[0066] The first and second frequencies f1, f2, which limit the first frequency range Af1, are preferably equidistant around the detection frequency fD. The first and second frequencies f1, f2 are preferably chosen such that the first frequency range Af1 encompasses the -3 dB range commonly used in interference analysis. This refers to the range in which the interference response has dropped to at least -3 dB of the interference response maximum SAM.

[0067] For the photothermal spectrometer 100 shown in Fig. 1, the detection frequency fD is, for example, 520 Hz, and for the minimum gain Min shown here greater than -20 dB, the first frequency range Af1 is limited by the first frequency f1 = 420 Hz and by the second frequency f2 = 620 Hz.

[0068] The interference response SA increases again at frequencies far above the cutoff frequency fG, e.g., frequencies above 40 GHz. At these frequencies, even modern controllers can no longer suppress the aforementioned external interference.

[0069] To achieve the inventive behavior of the disturbance response SA, a first control circuit 3 with a bandstop filter 4 is preferably used, wherein other means known to the skilled person for achieving the disturbance response SA shown in Fig. 2 are mutatis mutandis included in the invention.

[0070] This is shown in more detail in Fig. 3a. The bandstop filter 4 (first box) has a filter width that is essentially the first frequency range Af1. In the first control loop 3, the bandstop filter 4 is connected upstream of a PID controller 9 (second box) and the optical system (third box), the optical system comprising the interferometer s and a driver 21 of the probe laser 2, as well as the probe laser 2. The position of the bandstop filter 4 in the first control loop 3 is not essential for the invention. Furthermore, the bandstop filter 4 is preferably an analog bandstop filter 4. Instead of the bandstop filter 4, other filter elements can also be used, including, for example, a high-pass filter with the second frequency f2 as its high-pass cutoff frequency, which is combined with a low-pass filter with the first frequency f1 as its low-pass cutoff frequency. In Fig.Figure 3a shows the bandstop filter 4 as the filter element and the PID controller 9 as two different elements of the control loop 3.

[0071] Fig. 3b further illustrates another control engineering alternative in which the control is directly combined with the filtering to achieve the bandpass behavior for filtering out the first frequency range Af1. Instead of separate filter elements such as the bandstop filter 4 shown in Fig. 3a, the filtering out of the first frequency range Af1 is achieved by, for example, using the manipulated variable SG of the probe laser, such as the laser current of the probe laser 2, as a high-pass manipulated variable HPS, with a high-pass behavior and a second frequency f2 as the high-pass cutoff frequency. A separate controller 9a is used to set the high-pass manipulated variable HPS. This controller is, for example, combined with a high-pass filter before or after the controller 9a, such as a PID controller (not shown).

[0072] Setting the high-pass control variable HPS of, for example, the probe laser 2, is combined with setting a low-pass control variable TPS of the probe laser 2 and / or the interferometer 6, exhibiting low-pass behavior with the first frequency f1 as the low-pass cutoff frequency. Controlling the laser temperature of the probe laser 2, and thus setting the wavelength of the probe laser 2, exhibits, for example, intrinsic low-pass behavior.

[0073] Alternatively, the mirror spacing of the interferometer 6, and thus its resonant frequency, can be adjusted via temperature control. This temperature control also exhibits low-pass characteristics. A separate controller 9b, e.g., a PID controller, is also used for setting the low-pass control variable TPS or the high-pass control variable HPS.

[0074] The intrinsic low-pass behavior of the temperature control may not correspond to the desired low-pass behavior with f1 as the low-pass cutoff frequency and a specified accuracy. In this case, the temperature control can be combined with an electronic low-pass filter (not shown).

[0075] By adjusting the laser current as the quickly adjustable (first) manipulated variable SG of the first control loop 3, the actual value IW is adjusted to the setpoint SW. Before being passed to the bandstop filter 4, the control deviation (i.e., the difference between the setpoint SW and the actual value IW) still contains the photothermal measurement signal, while afterwards the first frequency range Af 1 around the detection frequency fD is just excluded from the control deviation. Only then is the control deviation passed to the PID controller 9. This allows disturbances caused by changing environmental conditions to be quickly compensated for by the PID controller 9 before the manipulated variable SG is adjusted and passed to the scanning laser 2 or its driver 21, without suppressing the photothermal measurement signal itself.

[0076] The previously mentioned rapid tuning of the probe laser 2 is achieved via the laser current as the control variable SG. However, this tuning is limited to a rather small wavelength range, namely a wavelength range of approximately 1 nm for the photothermal spectrometer 100 shown in Fig. 1, which corresponds roughly to the distance between two resonance peaks, and for a Fabry-Perot interferometer with a mirror spacing of 1 mm, as the interferometer s

[0077] It is also possible to tune the probe laser 2 via its temperature. While this tuning is slower than the previously discussed tuning via the laser current as the (first) manipulated variable SG, it is possible over a larger range (2 to 3 nm). Therefore, in the embodiment shown in Fig. 4, the previously described first control loop 3 is coupled with a second control loop 31. The second control loop 31 uses a predetermined target laser current SL as a setpoint, at which the probe laser 2 exhibits the smallest possible line width. This is usually specified by the manufacturer of the probe laser 2.

[0078] A second controller 91, also a PID controller 91, is used, which, via the setting of the laser temperature as the second manipulated variable SG2 of the second control loop 31, returns the actual laser current IL (which in turn represents the manipulated variable SG of the first control loop 3) to the aforementioned target laser current SL, at which a measurement with the best performance of the probe laser 2 can be carried out.

[0079] Any disturbance that occurs is therefore initially compensated for via the laser current as a rapidly adjustable (first) control variable SG of the probe laser 2, in order to comprehensively track the probe laser 2 and / or the interferometer 6. The second control, via the second control loop 31, ensures that the probe laser 2 subsequently operates again at its target laser current SL, albeit with a slight time delay.

[0080] By means of the two interconnected control loops 3 and 31, disturbances that occur can first be quickly compensated for by adjusting the laser current to the actual laser current SL. Subsequently, the probe laser 2 is operated at its optimal current again by adjusting the second manipulated variable SG2, e.g., the laser temperature of the probe laser 2.

[0081] The first controller 9, which uses the laser current as the (first) manipulated variable SG, is significantly faster than the second controller 91, which uses the second manipulated variable SG2. For example, the laser current can be adjusted much faster than the laser temperature, a typical second manipulated variable SG2. For example, the first controller 9 has a 3 dB bandwidth with a first upper cutoff frequency fOG1, where the first upper cutoff frequency fOG1 is greater than 1 Hz, in particular greater than 100 Hz, preferably greater than 1 kHz. The second controller 91, on the other hand, has a 3 dB bandwidth with a second upper cutoff frequency fOG2, where the second upper cutoff frequency is lower than the first upper cutoff frequency.

[0082] The first controller 9 and / or the second controller 91 can also be implemented in software. The two controllers 9 and 91 may also be combined in a common control unit, which then has one input and two outputs. Of course, one or more control units can also be implemented in software. For example, the first controller 9 and / or the second controller 91 can be implemented, at least partially, using programmable logic (e.g., an FPGA).

[0083] As an alternative to using the laser temperature of the probe laser 2 as the second control variable SG2, the mirror spacing of the interferometer 6 – adjustable, for example, by means of a piezoelectric element – ​​or the temperature of the interferometer 6 can also be set. The temperature of the probe laser 2 and / or the interferometer 6 can be adjusted, for example, by means of at least one actively heating and / or cooling temperature control element, in particular by means of a Peltier element.

[0084] Reference signs and symbols

[0085] 1. A light source

[0086] 2 Query lasers

[0087] 21 drivers for 2

[0088] 3 first regulatory circuit

[0089] 31 second regulatory circle

[0090] 4 bandstop filters

[0091] 5 optical detector unit

[0092] 6 Interferometer

[0093] 7 Evaluation unit

[0094] 8 Lock-in amplifiers

[0095] 9, 91, 9a, 9b PID controller

[0096] 100 Photothermal spectrometer

[0097] SA disturbance response with disturbance-robust control

[0098] SA' disturbance response without robust control fA excitation modulation frequency fD detection frequency

[0099] SW target value

[0100] IW Actual Value

[0101] Af1 first frequency range

[0102] Af2 second frequency range f1 first frequency f2 second frequency fG cutoff frequency

[0103] Minimum gain

[0104] Max Maximai reinforcement

[0105] SAM noise response maximum

[0106] SG Control variable

[0107] HPS high-pass filter control variable

[0108] TPS Low-pass control variable

[0109] SG2 second control variable

[0110] AnA excitation axis

[0111] AbA query axis

[0112] IA Interferometer axis

[0113] SL Target laser current

[0114] IL Actual laser current fOG1 , fOG2 Cutoff frequencies of 9, 91

Claims

Patent claims 1. Method for the disturbance-resistant determination and / or monitoring of a measured quantity of a medium using photothermal spectroscopy, comprising: Illumination of excitation light into the medium or into a sample of the medium, by means of at least one, in particular narrowband, excitation light source (1), wherein the frequency and / or intensity of the excitation light source (1) is modulated with an excitation modulation frequency (fA); Irradiation of probe light by means of a probe laser (2) into the medium or sample excited by the excitation light, Guiding the excitation light from the excitation light source (1) and the probe light from the probe laser (2) into an interferometer (6) such that the excitation light and the probe light overlap in the interferometer (6), Specification of a detection frequency (fD) for detecting a photothermal measurement signal, which detection frequency (fD) is in particular a natural multiple, preferably an even natural multiple, of the excitation modulation frequency (fA), Capturing the light transmitted and / or reflected by the interferometer (6) from the probe laser (2) as measuring light, using an optical detector unit (5), Determining an actual value (IW) for a measuring light measurement signal, wherein the measuring light measurement signal is determined based on the detected measuring light, Disturbance-robust control of the probe laser (2) and / or the interferometer (6) by means of a disturbance-robust control method, wherein in the disturbance-robust control method the actual value (IW) is controlled to a setpoint (SW), by adjusting a manipulated variable (SG) of the probe laser (2), in particular a laser current of the probe laser (2), such that a frequency-dependent disturbance response (SA) has a first frequency range (Af1), wherein the first frequency range extends from a first frequency (f1) smaller than the detection frequency (fD) to a second frequency (f2) larger than the detection frequency (fD), wherein for all frequencies from the first frequency range (Af1) the disturbance response (SA) is greater than or equal to a minimum gain (Min). a second frequency range (Af2) that differs from the first frequency range (Af1), with the second frequency range (Af2) being above the first frequency range (Af1), in particular directly adjacent to it, wherein for all frequencies from the second frequency range (Af2) the disturbance response (SA) is less than a maximum gain (Max), and - where the minimum gain of the first frequency range is at least as large as the maximum gain of the second frequency range, Detecting a photothermal measurement signal using the optical detector unit (5), using the detection frequency (fD), wherein the probe laser (2) and / or the interferometer (6) are controlled by the disturbance-robust control method, and Determining and / or monitoring the measured quantity based on the detected photothermal measurement signal.

2. Method according to claim 1, wherein the minimum gain of the first frequency range (Af1) is not less than -20 dB, in particular not less than -5 dB.

3. Method according to at least one of the preceding claims, wherein the frequency-dependent disturbance response (SA) in the first frequency range (Af1) assumes a disturbance response maximum (SAM), in particular at least a local one, and for all frequencies from the first frequency range (Af1) the disturbance response (SA) is not greater than the disturbance response maximum (SAM), and wherein in particular the disturbance response maximum (SAM) is greater than -10 dB, preferably greater than -5 dB.

4. Method according to claim 3, wherein the frequency-dependent disturbance response (SA) in the first frequency range (Af1) has no further extrema besides the disturbance response maximum (SAM).

5. Method according to claim 3 or 4, wherein the first frequency (f1) and the second frequency (f2) are selected such that for all frequencies from the first frequency range (Af 1 ) the disturbance response (SA) is at least as large as to be at most 5dB smaller, in particular at most 3dB smaller, than the disturbance response maximum (SAM).

6. Method according to at least one of the preceding claims, wherein the first frequency (f1) is not less than 0.75 times, in particular not less than 0.9 times, the detection frequency (fD) and the second frequency (f2) is not more than 1.25 times, in particular not more than 1.1 times, the detection frequency (fD).

7. Method according to at least one of the preceding claims 3 to 6, wherein the second frequency range (Af2) is directly adjacent to the first frequency range (Af1) and extends from the second frequency (f2) to a cutoff frequency (fG), and wherein, in the event that for all frequencies from the first frequency range (Af1) the noise response (SA) is at least so large that it is at most 3dB smaller than the noise response maximum (SAM), the cutoff frequency (fG) is greater than 6 times the detection frequency (fD).

8. Method according to at least one of the preceding claims, wherein in the disturbance-robust control method a first control loop (3) with one or more filter elements for filtering out the first frequency range (Af 1 ) is used, namely with a bandstop filter (4) whose filter width corresponds to the first frequency range (Af1) and / or with a high-pass filter with the second frequency (f2) as its high-pass cutoff frequency combined with a low-pass filter with the first frequency (f1) as its low-pass cutoff frequency and / or wherein the manipulated variable (SG) of the probe laser (2) set in the disturbance-robust control method, in particularthe laser current of the probe laser (2), and / or an interferometer control variable adjustable in the disturbance-robust control method having a high-pass control variable (HPS) having a high-pass behavior with the second frequency (f2) as the high-pass cutoff frequency, and wherein the setting of the high-pass control variable (HPS) is combined with the setting of a low-pass control variable (TPS) of the probe laser (2) and / or the interferometer (6), having a low-pass behavior with the first frequency (f1) as the low-pass cutoff frequency.

9. Method according to at least one of the preceding claims, comprising: Using the laser current of the probe laser (2) as a manipulated variable (SG) in the disturbance-robust control method; Specification of a target laser current (SL) of the probe laser (2); Implementing a second control procedure, which is coupled to the disturbance-robust control procedure in such a way that the laser current (IL) of the probe laser (2), which serves as the manipulated variable (SG) in the disturbance-robust control procedure and is set to an actual laser current, is readjusted to the target laser current (SL) by means of the second control procedure, using at least a second manipulated variable (SG2) of the second control procedure, in particular a laser temperature of the probe laser (2), a mirror spacing of the interferometer (6) and / or an interferometer temperature 10. Photothermal spectrometer (100) for the disturbance-robust determination and / or monitoring of a measured quantity of a medium, in particular the determination of a concentration and / or the monitoring of the presence of a trace gas in the medium, comprising: an excitation light source (1), in particular a narrowband one, a probe laser (2), an optical detector unit (5), an interferometer (6), a first control circuit (3) for the disturbance-robust control of the probe laser (2) and / or the interferometer (6), and an evaluation unit (7) for the determination and / or monitoring of the measured quantity on the basis of a photothermal measurement signal controlled by at least the first control circuit (3) and detected by the optical detector unit (5), wherein the photothermal spectrometer (100) is configured to carry out the method according to at least one of the preceding claims.

11. Photothermal spectrometer (100) according to claim 10, wherein the interferometer (6) is at least one of the following: Fabry-Perot interferometer or Mach-Zehnder interferometer.

12. Photothermal spectrometer (100) according to claim 10 or 11, wherein the first control circuit (3) has one or more filter elements for filtering out the first frequency range (Af1), namely a bandstop filter (4) whose filter width corresponds to the first frequency range (Af1), in particular an analog bandstop filter (4) and / or a high-pass filter with the second frequency (f2) as its high-pass cutoff frequency combined with a low-pass filter with the first frequency (f1) as its low-pass cutoff frequency and / or wherein the manipulated variable (SG) of the probe laser (2) set in the disturbance-robust control method, in particularthe laser current of the probe laser (2), and / or an interferometer control variable adjustable in the disturbance-robust control method having a high-pass control variable (HPS) with a high-pass behavior with the second frequency (f2) as the high-pass cutoff frequency, and the interferometer (6) and / or the probe laser (2) having an adjustable low-pass control variable (TPS) with a low-pass behavior with the first frequency (f1) as the low-pass cutoff frequency. 13 Photothermal spectrometer (100) according to claim 12, wherein the bandstop filter (4) is arranged in the first control circuit (3) such that it is arranged in front of a driver (21) for the probe laser (2).

14. Photothermal spectrometer (100) according to at least one of claims 10 to 13, wherein the probe laser (2) is arranged in relation to the interferometer (6) such that the light introduced into the interferometer (6) by the probe laser (2) is directed into the interferometer (6) along a probe axis (AbA), wherein the probe axis (AbA) does not deviate by more than 10° from an interferometer axis (IA), and wherein, in particular, the excitation light source (1) is arranged in relation to the interferometer (6) such that the light introduced into the interferometer (6) by the excitation light source (1) is directed into the interferometer (6) along an excitation axis (AnA), and wherein either an included angle between the excitation axis (AnA) and the probe axis (AbA) is between 80 degrees and 100 degrees. is or where the stimulus axis (AnA) and the query axis (AbA) essentially coincide.

15. Photothermal spectrometer (100) according to at least one of claims 10 to 14, wherein the evaluation unit (7) has a lock-in amplifier (8) for detecting the photothermal measurement signal at the detection frequency (fD).

16. Photothermal spectrometer (100) according to at least one of claims 10 to 15, comprising a second control circuit (31) for carrying out the second control method.

17. Photothermal spectrometer (100) according to at least one of claims 10 to 16, wherein the first control circuit (3) and / or the second control circuit (31) comprises a PID controller (9;91).

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