Reducing optical interference signals by means of oscillatory movement of optically active element of spectrometer

Abstract

The invention relates to reduction of optical interference signals by means of oscillatory movement of an optically active element of a spectrometer. The spectrometer includes: a laser light source having coherent radiation; a photodetector arranged to receive coherent radiation after passing through the path length; at least one optically active surface arranged along a path length between the laser output and the photodetector, and an evaluation unit, characterized in that the laser light source and / or the first optical element is mounted on a movable carrier, in which the movement of the carrier is an oscillating movement with an amplitude and / or frequency and changes the path length of the radiation, in this way, interference signals generated as optical interference on the at least one optical element due to the superposition of light rays of different optical path lengths caused by at least partial reflections on the at least one optical surface are suppressed, wherein the suppression occurs due to interference between radiation emitted from the light source and a change in path length of the beam path due to movement on the movable carrier.

Description

Technical Field

[0001] The present invention generally relates to improved detection of gases, for example for use in monitoring the ambient atmosphere, gases formed in wastewater treatment plants, in process monitoring, inspection and control, in quality assurance in gas production and in industrial processes where gases are formed, and in the transport of gases. In particular, the present invention relates to improvements in the detection and measurement of gas concentrations and gas emissions based on tunable diode lasers. Background Art

[0002] It is known that the detection sensitivity of TDL-WMS spectroscopy (tunable diode laser wavelength modulation spectroscopy) is limited by interference fringes rather than by the theoretical limit given by the detector noise. The interference fringes are caused by the reflecting or scattering surfaces of optical elements, the end faces of glass fibers and the Fabry-Perot etalon between the components of the multipass cell. The first quantitative analysis of the effect of interference fringes on detection sensitivity was performed by Reid et al. (Reid et al., Optical and quantum electronics, Vol. 17, 1985). In their study, the authors estimated that if they could eliminate the interference fringes, they could increase the detection sensitivity by at least a factor of 5.

[0003] The oscillating movement with amplitude and / or frequency changes the path length of the radiation so that an interference signal generated as optical interference on at least one optical element due to superposition of light rays of different optical path lengths caused by partial reflections on at least one optical surface is suppressed, wherein the suppression occurs due to interference between the radiation emitted from the light source and the change in the path length of the beam path due to the movement of the optical surface on the movable carrier. The change in the path length occurs at a frequency that is much higher than the bandwidth of the measurement signal.

[0004] In addition to the optical fringes of the optical cavity, any reflections returning to the laser change the laser characteristics. If we can vary the back reflections at a much higher frequency than the bandwidth of the measured signal, the effects caused by the back reflections can also be filtered out. As a result, the filtered or conditioned signal corresponds more closely to the actual signal than if no suppression was applied. Summary of the invention

[0005] Therefore, the object of the device according to the invention and the method according to the invention is to provide a device and a corresponding method that are more robust and simplified than the prior art in order to suppress interference signals caused by optical interference due to the superposition of light rays of different optical path lengths caused by at least partial reflection on at least one optical surface.

[0006] This object is achieved by a spectrometer, the spectrometer comprising:

[0007] (i) a laser light source having coherent radiation, comprising a tunable diode laser having a coherent laser output, wherein the coherent laser light source is configured to modulate the frequency of the coherent laser output;

[0008] (ii) a photodetector arranged to receive coherent radiation from the laser output after traversing the path length;

[0009] (iii) at least one optical surface, preferably a first optical element, more preferably a first refractive optical element such as a first lens or a first reflective optical element such as a mirror, arranged along the path length between the laser output and the photodetector,

[0010] as well as

[0011] (iv) an evaluation unit electrically connected to the photodetector in order to receive signals from the photodetector, analyze them and send instructions to a control unit electrically connected to the evaluation unit,

[0012] Features

[0013] Laser light source and / or

[0014] The first optical element is mounted on a movable carrier, wherein the carrier is moved by a drive driven by a control unit,

[0015] wherein the movement of the carrier is an oscillatory movement with amplitude and / or frequency and changes the path length of the radiation such that an interference signal arising as optical interference on at least one optical element due to a superposition of light rays of different optical path lengths caused by at least partial reflection on at least one optical surface is suppressed, wherein the suppression occurs due to interference between the radiation emitted from the light source and the change in the path length of the beam path due to the movement on the movable carrier.

[0016] The process proposed here for suppressing interference signals uses a method and a device for mechanical oscillatory movement of at least one optically active surface of at least one optically active element, preferably of a spectrometer, which changes the optical path length of a light beam.

[0017] The advantage of the device according to the invention and the method according to the invention is that the optically active surface provided for the oscillating movement, preferably the optical element, is a mechanical component and not a radiation source whose power depends on the feed current and is therefore susceptible to interference. This means that the electrical connection to the laser does not have to be designed so that it remains unaffected by mechanical movements. This is beneficial both for the measurement accuracy and the long-term stability of the system.

[0018] In one embodiment, the movable carrier has a rotator and / or a translator, wherein the rotator can rotate continuously or stepwise by less than 10°, preferably less than 2° around the x, y or z axis or a combination thereof, and wherein the translator can move continuously or stepwise along the x, y or z axis or any combination thereof.

[0019] In one embodiment, the amplitude is a multiple of the laser wavelength, preferably 2 to 10 times, wherein the amplitude is preferably 0.5 μm to 50 μm, preferably 1 μm to 10 μm.

[0020] In one embodiment, the movement of the carrier is sinusoidal, sawtooth, triangular or rectangular in time and / or suppresses interfering signals at frequencies between 1 and 200 kHz.

[0021] In one embodiment, the frequency of the movement of the carrier is greater than 1 kHz, preferably 1 to 200 kHz, more preferably between 2 kHz and 100 kHz, even more preferably between 2 kHz and 50 kHz.

[0022] In one embodiment, the wavelength of the laser light source is between the mid-infrared range and the visible light range, preferably between 380 nm and less than 6000 nm, and more preferably between 380 nm and 3000 nm.

[0023] In one embodiment, the spectrometer has one, two, three, four or five optical elements, preferably four or five optical elements.

[0024] In one embodiment, the optical element is or represents the first optical element arranged immediately after the laser light source along a path length from the laser output to the photodetector.

[0025] In a preferred embodiment, the optical element is a first refractive element. In another embodiment, the optical element is a second or third refractive element.

[0026] In one embodiment, the spectrometer has two optical elements, wherein preferably one optical element is designed with at least one reflector element or a cross-stacked back-reflector arrangement in the path between the laser and the photodetector, wherein the back-reflector arrangement is fixedly mounted and therefore not arranged on a movable carrier.

[0027] In one embodiment, the spectrometer is an absorption spectrometer, preferably a laser spectrometer, more preferably a diode laser absorption spectrometer.

[0028] In one embodiment, the evaluation unit comprises a measuring circuit and evaluation electronics, which are designed as

[0029] - modulate the frequency of the coherent laser output, and

[0030] - Convert it into an electrical signal on a photodetector and record the electrical signal, which is used to determine

[0031] a) the concentration of at least one gas to be analyzed; and

[0032] b) Optionally, pressure and / or

[0033] c) Optionally, temperature.

[0034] In one embodiment, the evaluation unit has a measuring circuit and evaluation electronics which are additionally designed to determine translational and / or rotational movement instructions which are sent to a controller which controls a carrier on which the laser is mounted and / or a carrier on which the first optically active surface, preferably the first optical element, is mounted in order to suppress interference signals arising as optical interference on at least one optical element due to superposition of light rays of different optical path lengths due to at least partial reflection on at least one optical surface.

[0035] In an alternative embodiment, the pressure and / or temperature is measured by means of a pressure sensor or a temperature sensor specifically built into the spectrometer.

[0036] In one embodiment, the control signal for moving one or more carriers is converted into a rotational and / or translational movement by an actuator, wherein the actuator is selected from a piezoelectric actuator, an electromechanical drive, a hydraulic drive and / or a pneumatic drive, wherein the actuator is preferably a piezoelectric actuator.

[0037] In one embodiment, at least one optical element has an anti-reflective coating.

[0038] The invention further relates to an analysis device for measuring the concentration of at least one gas, preferably one, two, three or four gases, wherein the analysis device comprises a spectrometer according to the invention or an embodiment thereof.

[0039] The invention also relates to a method for suppressing interference signals of a spectrometer, preferably an absorption spectrometer, by means of a tunable laser with a spectrometer according to the invention or an embodiment thereof, the tunable laser preferably being a diode laser, the method comprising:

[0040] - directing a coherent laser beam from a coherent laser source comprising a tunable diode laser along a path having a path length onto a photodetector,

[0041] - modulate the frequency of the coherent laser output,

[0042] - measures the light signal on the photodetector, converts the light signal into an electrical signal and sends the electrical signal of the photodetector to an evaluation unit,

[0043] - sending one or more instructions to a control unit electrically connected to the evaluation unit,

[0044] - varying the path length of the laser beam by means of an oscillating movement via a translational and / or rotational movement of the carrier controlled by a control unit,

[0045] The laser is mounted on a carrier, and / or

[0046] The first optical element is mounted on a carrier,

[0047] The movement is thereby selected such that interference signals generated on at least one optical surface, preferably on at least one optical element, due to scattering and / or reflection are suppressed; and

[0048] - Determine the electronic signal of the photodetector.

[0049] The invention further relates to the use of a spectrometer according to the invention or an embodiment thereof for analyzing atmospheric gases and / or gases of a sewage treatment plant. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In the figure:

[0051] Figure 1 It is a schematic flow chart of the working principle of an embodiment of the present invention. DETAILED DESCRIPTION

[0052] Figure 1 Schematic representation of the beam path of a TDL-WMS (tunable diode laser wavelength modulation spectroscopy) beam splitter including the correction of the interference pattern caused at least in part by back reflections. The correction is performed using a first refractive element. The effect is a change in all path length sections including the section between the radiation source and the first refractive element.

[0053] (a) shows the first portion between the radiation source (2) and the first refractive / reflective element (4).

[0054] (a b ) represents at least partial reflection of radiation between the first refractive / reflective element (4) and the radiation source (2).

[0055] (b i ) one or more portions of a radiation path.

[0056] (b i,b ) represents at least partial reflection of radiation between at least one further refractive / reflective element (5) and the radiation source (2).

[0057] (c) The last section between the last optical element (5) and the detector.

[0058] (c b ) denotes at least partial reflection of radiation between at least one further refractive element, preferably the last refractive element (5), and the radiation source (2).

[0059] The dashed lines show at least partial back reflections in portions a, b and c.

[0060] In each case, all embodiments of the inline sensor and the method described above can be combined with one another as far as this is technically possible.

[0061] List of reference numerals

[0062] (1) Control electronics for the light source and for performing the mechanical oscillation

[0063] (2) Radiation source

[0064] (3) Mechanical oscillation element

[0065] (4) First refractive / reflective element

[0066] (5) Refractive / reflective elements

[0067] (6) Detector

[0068] (7) Data collection and processing with filtering

Claims

1. A spectrometer, comprising: (i) a laser light source having coherent radiation, the laser light source having coherent radiation comprising a tunable diode laser having a coherent laser output, wherein the coherent laser light source is configured to modulate the frequency of the coherent laser output; (ii) a photodetector arranged to receive the coherent radiation from the laser output after passing through a path length; (iii) at least one optically active surface, preferably a first optical element, more preferably a first refractive optical element such as a first lens, or a first reflective optical element such as a mirror, the at least one optically active surface being arranged along the path length between the laser output and the photodetector, as well as (iv) an evaluation unit electrically connected to the photodetector in order to receive signals from the photodetector, analyze them and send instructions to a control unit electrically connected to the evaluation unit, Features The laser light source, and / or The first optical element is mounted on a movable carrier, wherein the carrier is moved by a drive driven by the control unit, wherein said movement of said carrier is an oscillatory movement with amplitude and / or frequency and changes the path length of said radiation so as to suppress an interference signal as optical interference generated on at least one optical element due to the superposition of light rays of different optical path lengths caused by at least partial reflection on at least one optical surface, wherein said suppression occurs due to interference between said radiation emitted from said light source and the change in the path length of the beam path caused by said movement on said movable carrier.

2. The spectrometer according to claim 1, wherein the movable carrier having a rotator, wherein the rotator can rotate continuously or stepwise about the x, y or z axis or a combination thereof by less than 10°, preferably less than 2°, and / or There is a translator, wherein the translator can move continuously or stepwise along the x, y or z axis or any combination thereof.

3. The spectrometer according to any of the preceding claims, wherein the amplitude is a multiple of the laser wavelength, preferably 2 to 10 times, wherein preferably the amplitude is 0.5 to 50 μm, preferably 1 to 10 μm.

4. Spectrometer according to any of the preceding claims, wherein the movement of the carrier is sinusoidal, sawtooth, triangular or rectangular in time and / or suppresses interfering signals at frequencies between 1 and 200 kHz. 5 . The spectrometer according to claim 4 , wherein the frequency of the movement of the carrier is greater than 1 kHz, preferably 1 to 200 kHz, more preferably between 2 kHz and 100 kHz, even more preferably between 2 kHz and 50 kHz.

6. The spectrometer according to any of the preceding claims, wherein the wavelength of the laser light source is between the mid-infrared range and the visible light range, preferably between 380 nm and less than 6000 nm, more preferably between 380 nm and 3000 nm.

7. The spectrometer according to any of the preceding claims, wherein the spectrometer has one, two, three, four or five optical elements, more preferably four or five optical elements.

8. A spectrometer according to claim 7, wherein the optical element is or represents the first optical element arranged immediately after the laser source along a path length from the laser output to the photodetector.

9. The spectrometer according to claim 1 , wherein the spectrometer has two optical elements, wherein preferably one optical element is designed with at least one reflector element or a cross-stacked back-reflector arrangement in the path between the laser and the photodetector, wherein the back-reflector arrangement is fixedly mounted.

10. The spectrometer according to any of the preceding claims, wherein the spectrometer is an absorption spectrometer, preferably a laser spectrometer, more preferably a diode laser absorption spectrometer.

11. The spectrometer according to any one of claims 1 to 10, wherein the evaluation unit comprises a measuring circuit and evaluation electronics, which are designed: - modulating the frequency of said coherent laser output, and - converting it into an electrical signal on the photodetector and recording the electrical signal, wherein the electrical signal is used to determine a) the concentration of at least one gas to be analyzed; and b) Optionally, pressure and / or c) Optionally, temperature.

12. A spectrometer according to any of the preceding claims, wherein the control signal for moving one or more carriers is converted into a rotational and / or translational movement by an actuator, wherein the actuator is selected from a piezoelectric actuator, an electromechanical drive, a hydraulic drive and / or a pneumatic drive, wherein the actuator is preferably a piezoelectric actuator.

13. The spectrometer according to any of the preceding claims, wherein the at least one optical element has an anti-reflection coating.

14. An analytical device for measuring the concentration of at least one gas, preferably one, two, three or four gases, wherein the analytical device comprises a spectrometer according to any one of claims 1 to 13.

15. A method for suppressing interference signals of a spectrometer by means of a tunable laser having a spectrometer according to any one of claims 1 to 13, the spectrometer being preferably an absorption spectrometer, the tunable laser being preferably a diode laser, the method comprising: - directing a coherent laser beam from a coherent laser light source comprising a tunable diode laser onto a photodetector along a path having a path length, - modulating the frequency of said coherent laser output, - measuring the optical signal on the photodetector, converting the optical signal into an electrical signal and sending the electrical signal of the photodetector to an evaluation unit, - sending one or more instructions to a control unit electrically connected to said evaluation unit, - varying said path length of said laser beam by means of an oscillating movement via a translational and / or rotational movement of a carrier controlled by a control unit, The laser is mounted on the carrier, and / or The first optical element is mounted on the carrier, The movement is thus selected such that interference signals generated on the at least one optically active surface, preferably on the at least one optical element, due to scattering and / or reflection are suppressed; as well as - determining the electronic signal of said photodetector.

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