A gas cell and gas sensing device based on Faraday magneto-optical rotation spectroscopy

Abstract

This invention relates to the field of gas detection technology, specifically to a gas cell and gas sensing device based on Faraday magneto-optical rotation spectroscopy. It includes a gas absorption cell and permanent magnet rings coaxial with and sleeved on the gas absorption cell. Ten to twenty permanent magnet rings are arranged sequentially from the inlet to the outlet of the gas absorption cell. Each permanent magnet ring is of the same size and shape, and the distance between any two adjacent permanent magnet rings is 0 to 10 mm. The length of the permanent magnet rings is adapted to the length of the gas absorption cell, and they are symmetrically arranged from the center of the gas absorption cell towards the inlet and outlet. This invention utilizes a non-equidistant array of permanent magnet rings to generate a static magnetic field, thereby effectively changing the magnetic circular birefringence by modulating the laser wavelength under static Zeeman splitting, ultimately generating a Faraday rotation spectral signal, thus improving the applicability of magneto-optical rotation spectroscopy technology in the field of gas detection.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, specifically to a gas cell and gas sensing device based on Faraday magneto-optical rotation spectrum of rare-earth ring permanent magnet array. Background Art

[0002] Faraday rotation spectroscopy, also known as Faraday modulation spectroscopy, utilizes the magnetic circular birefringence caused by Zeeman splitting of the absorption lines of paramagnetic samples (NO, NO2, O2, OH, HO2) under an external magnetic field to detect molecular concentration. Its basic principle is as follows: First, a polarizer defines the polarization axis of the linearly polarized light. Under the influence of an external axial magnetic field, due to the increased degeneracy of molecular energy levels, the rotational-vibrational transition state of the paramagnetic molecule is split into several substates. When the linearly polarized light propagates in the absorption cell containing the paramagnetic sample, the polarization plane of the linearly polarized light rotates because left-handed and right-handed circularly polarized light have different refractive indices. By placing an analyzer, nearly orthogonal to the polarizer, after the absorption cell, the rotation of the polarization plane is converted into a change in laser intensity. Finally, by detecting this change, the concentration of the paramagnetic sample can be determined. The rotation angle of the linearly polarized light is proportional to the concentration of the paramagnetic sample.

[0003] By employing a pair of nearly orthogonal polarizers to effectively suppress laser noise, Faraday rotation spectroscopy can achieve higher detection sensitivity within a shorter optical path compared to traditional direct absorption spectroscopy or wavelength modulation spectroscopy. A longitudinal magnetic field is a necessary condition for generating the Faraday rotation spectral signal. Currently, this signal is mainly generated by modulating the Zeeman splitting of the absorption line using an alternating magnetic field induced by an AC solenoid coil. However, this method suffers from drawbacks such as high power consumption, difficulty in external field applications, excessive Joule heating, and excessive electromagnetic interference. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a gas cell based on Faraday magneto-optical rotation spectrum of rare earth ring permanent magnet array.

[0005] The present invention adopts the following technical solution:

[0006] A gas cell based on Faraday magneto-optical rotation spectroscopy includes a gas absorption cell and permanent magnet rings coaxial with and sleeved on the gas absorption cell. The gas absorption cell has an inner cavity for containing the gas to be measured and an opening connecting the inner cavity to the external environment. The opening includes an inlet at the inlet end of the gas absorption cell and an outlet at the outlet end of the gas absorption cell. 10 to 20 permanent magnet rings are arranged sequentially from the inlet end to the outlet end of the gas absorption cell. Each permanent magnet ring has the same size and shape. The distance between any two adjacent permanent magnet rings is 0 to 10 mm. The length of the permanent magnet rings is adapted to the length of the gas absorption cell and they are arranged symmetrically from the center of the gas absorption cell towards the inlet end and the outlet end, respectively.

[0007] Preferably, the permanent magnet ring is made of neodymium iron boron, and the cross-section of the permanent magnet ring and the cross-section of the ring arm are rotationally symmetrical. The longitudinal static magnetic field strength formed by multiple permanent magnet rings is 200 to 400 Gauss.

[0008] Preferably, the permanent magnet ring has a circular cross-section, an outer diameter of 100mm, an inner diameter of 60mm, a rectangular cross-section for the ring arm, and a thickness of 25mm for the ring arm.

[0009] Preferably, a total of 14 permanent magnet rings are provided. The spacing between the permanent magnet rings from the center of the gas absorption pool toward the inlet or outlet is 0, 0, 1, 3, 4, and 10 mm respectively. The distance between the two permanent magnet rings closest to the center of the gas absorption pool is 0 mm. The longitudinal static magnetic field formed by the 14 permanent magnet rings has a magnetic field length of 386 mm and a magnetic field strength of 300 Gauss.

[0010] Preferably, the main material of the gas absorption cell is a non-ferromagnetic material such as hard plastic to avoid damaging the magnetic field.

[0011] The present invention also provides a sensing device using the above-mentioned Faraday magneto-optical rotational spectral gas cell, comprising a signal source, a laser source, and a Faraday magneto-optical rotational spectral gas cell disposed on the laser optical path. The laser source consists of a laser generator and a laser controller electrically connected thereto. The laser controller is electrically connected to the signal source. The outlet end of the spectral gas cell is also sequentially provided with a photodetector, a lock-in amplifier, and a data acquisition card for detecting and acquiring optical rotation signals passing through the spectral gas cell. The lock-in amplifier is also electrically connected to the signal source to obtain a reference signal.

[0012] Preferably, along the laser light path, a polarizer and an aperture are sequentially arranged between the laser source and the absorption cell, and an analyzer and an aperture are sequentially arranged between the absorption cell and the photodetector. The relative deflection angle θ between the polarizer and the analyzer satisfies 80°<θ<100°, and θ≠90°.

[0013] Preferably, the data acquisition card is also electrically connected to an embedded industrial control computer for recording the acquired information.

[0014] Preferably, a focusing lens is also provided between the second aperture and the photodetector.

[0015] Preferably, the laser generator is a quantum cascade laser, a semiconductor laser, or a vertical-cavity surface-emitting laser.

[0016] Preferably, the photodetector is a thermoelectrically cooled mercury cadmium telluride photodetector or a gallium arsenide photodetector.

[0017] The beneficial effects of the present invention are:

[0018] This invention utilizes a non-equidistant array of permanent magnet rings to generate a static magnetic field. This magnetic field, under static Zeeman splitting, effectively alters the magnetic circular birefringence by modulating the laser wavelength, ultimately producing a Faraday rotation spectral signal. The application of the ring-shaped permanent magnet array significantly reduces the high power consumption and heat generation drawbacks of existing technologies that commonly use AC solenoid coil induction, facilitating the further development of this technology into a field-applicable, unattended optical measurement platform.

[0019] By applying a magneto-optical gas absorption cell containing non-equidistantly spaced permanent magnet rings to gas detection, a Faraday magneto-optical rotational spectroscopy gas sensing device is obtained. By using a pair of nearly orthogonal polarizers to suppress laser intensity noise, a higher detection sensitivity than direct absorption spectroscopy or wavelength modulation spectroscopy with the same optical path can be obtained.

[0020] Faraday magneto-optical rotation spectroscopy signals are only related to paramagnetic molecules or free radicals (such as NO2, OH, NO, O2, HO2, etc.) that have magnetic dipole moments in the ground state or on-electronic state, and are not affected by diamagnetic molecules (CO2, H2O, etc.), thus significantly improving the accuracy of measurement results. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the Faraday magneto-optical rotation spectroscopy gas cell of the present invention;

[0022] Figure 2 for Figure 1 Side view;

[0023] Figure 3 This is a schematic diagram of the sensing device using a Faraday magneto-optical rotational spectroscopy gas cell according to the present invention;

[0024] Figure 4 This is the Faraday rotation spectral signal of 100 ppb NO2 obtained by the sensing device of the present invention.

[0025] The meanings of the symbols marked in the figure are as follows:

[0026] 10-Spectroscopic gas cell; 11-Gas absorption cell; 111-Inlet; 112-Outlet; 12-Permanent magnet ring

[0027] 20-Signal Source

[0028] 31-Laser generator 32-Laser controller

[0029] 40 - Photodetector; 50 - Lock-in amplifier; 60 - Data acquisition card; 61 - Embedded industrial computer

[0030] 71-Polarizer 72-Analyzer 81-First Aperture 82-Second Aperture

[0031] 90-Focusing Lens Detailed Implementation

[0032] The technical solution of the present invention will be described in more detail below with reference to the accompanying drawings:

[0033] like Figure 1-2 As shown, a Faraday magneto-optical rotational spectroscopy gas cell includes a gas absorption cell 11 and a permanent magnet ring 12 coaxial with and sleeved on the gas absorption cell 11. The gas absorption cell 11 has an inner cavity for containing the gas to be measured and an outlet connecting the inner cavity to the external environment. The outlet includes an inlet 111 at the inlet end of the gas absorption cell 11 and an outlet 112 at the outlet end of the gas absorption cell 11. The inlet 111 and the outlet 112 are used to realize the real-time exchange of paramagnetic molecules such as NO, NO2, O2, OH, HO2, etc.

[0034] The permanent magnet rings 12 are arranged in a line from the inlet end to the outlet end of the gas absorption tank 11, with a total of 10 to 20 rings. Each permanent magnet ring 12 is the same size and shape. The distance between any two adjacent permanent magnet rings 12 is in the range of 0 to 10 mm. The length of the permanent magnet rings 12 is equal to the length of the gas absorption tank 11. Furthermore, they are arranged symmetrically from the center of the gas absorption tank 11 towards the inlet end and the outlet end, respectively.

[0035] In this invention, the gas absorption cell 11 can be prepared using existing gas absorption cells, but the main material of the gas absorption cell should be a non-ferromagnetic material such as hard plastic to avoid damaging the magnetic field generated by the permanent magnet 12.

[0036] In one specific embodiment, the Faraday magneto-optical rotational spectroscopy gas cell provided by this invention has a length L1 of 386 mm and is equipped with 14 NdFeB permanent magnet rings 12. Each permanent magnet ring 12 is a circular ring with a rectangular cross-section of its ring arms. The outer diameter L2 of a single permanent magnet ring 12 is 100 mm, the inner diameter L3 is 60 mm, the thickness L4 is 25 mm, and the magnetic field strength is 1.4 Tesla. Since the magnetic field strength induced by the ring magnets is affected by their longitudinal spacing, the magnetic field strength and uniformity at different ring magnet spacings (spacing adjustment range 0–10 mm) were measured using a gaussmeter. The final determination of the ring magnet spacing satisfies the following: the spacing from the center of the gas absorption cell 11 towards the inlet or outlet is 0, 0, 1, 3, 4, and 10 mm respectively, with the distance between the two permanent magnet rings closest to the center of the gas absorption cell 11 being 0 mm. The resulting static magnetic field has a length of 386 mm and a magnetic field strength of 300 Gauss. The gas absorption cell 11 and the annular permanent magnet array 12 are coaxially aligned and have basically the same length, which allows the static magnetic field to be effectively applied to the Zeeman splitting of paramagnetic molecules.

[0037] In practical applications, the number, shape, and size of the permanent magnet rings 12 can be adjusted according to the required static magnetic field strength and the detection sensitivity of the Faraday rotation spectrum. For example, the shape can be a circular ring, a square, or a hexagon with rotational symmetry. This invention does not impose any specific limitations.

[0038] Trace gas sensing devices using the aforementioned Faraday magneto-optical rotational spectroscopy gas cell, such as... Figure 3 As shown, it includes a signal source 20, a laser source, and a Faraday magneto-optical rotational spectral gas cell 10 disposed on the laser optical path.

[0039] The laser source consists of a laser generator 31 and a laser controller 32 electrically connected to it. The laser generator 31 is a quantum cascade laser, a semiconductor laser, or a vertical cavity surface-emitting laser. The laser controller 32 is electrically connected to a signal source 20. The signal source 20 simultaneously outputs triangular wave and sine wave signals to the laser controller 32 for wavelength scanning and modulation of the laser generator 31.

[0040] The outlet of the spectral gas cell 10 is also sequentially equipped with a photodetector 40, a lock-in amplifier 50, a data acquisition card 60, and an embedded industrial control computer 61 for detecting and acquiring the optical rotation signal passing through the spectral gas cell 10. The photodetector 40 is a thermoelectrically cooled mercury cadmium telluride photodetector or gallium arsenide photodetector. It converts the intensity change of linearly polarized light caused by the Faraday rotation effect into an electrical signal, which is transmitted to the phase-sensitive lock-in amplifier 50. The signal source 20 is electrically connected to the phase-sensitive lock-in amplifier 50. The signal source 20 provides a reference signal to demodulate the modulated laser intensity caused by the Faraday rotation effect, ultimately acquiring the optical rotation signal... Figure 4The image shows a second-harmonic Faraday rotation spectral signal containing molecular concentration information. The data acquisition card 60 is electrically connected to the phase-sensitive lock-in amplifier 50 and the embedded industrial computer 61, respectively, to realize real-time acquisition and recording of the Faraday rotation spectral signal.

[0041] A polarizer 71 and a first aperture 81 are sequentially arranged between the laser source and the spectral gas cell 10, and an analyzer 72 and a second aperture 82 are sequentially arranged between the spectral gas cell 10 and the photodetector 40.

[0042] The polarizer 71 is used to define the polarization axis of linearly polarized light, the analyzer 72 is used to convert the rotation of the polarization plane into the modulated light intensity reaching the photodetector, and the phase-sensitive lock-in amplifier 50, which is electrically connected to the photodetector 40, is used for demodulating the light intensity signal.

[0043] By adjusting the opening size of the first aperture 81, interference between the small portion of the laser beam reflected back from the polarizer 71 and the emitted laser beam can be prevented, reducing the etalon effect. By adjusting the opening size of the second aperture 82, ordinary and unusual light can be effectively isolated, thereby achieving effective detection of ordinary light.

[0044] In addition, the Faraday rotation spectral signal disappears when the polarizer 71 and the analyzer 72 are completely orthogonal. Therefore, in this invention, the relative deflection angle θ between the polarizer 71 and the analyzer 72 satisfies 80° < θ < 100° and θ ≠ 90°. If the analyzer 72 deviates from the polarizer 71 by more than 10°, the laser noise will usually increase significantly.

[0045] In this invention, the specific deflection angle of the polarizer 72 can be determined by detecting the effect of different angles on signal enhancement and noise suppression.

[0046] A focusing lens 90 is also provided between the second aperture 82 and the photodetector 40. The focusing lens 90 is used to converge ordinary light. The photodetector 40 is placed at the focal point of the focusing lens 90 to better detect ordinary light containing information on the concentration of paramagnetic molecules.

[0047] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A gas cell based on Faraday magneto-optical rotation spectroscopy, characterized in that, The system includes a gas absorption pool (11) and a permanent magnet ring (12) coaxial with the gas absorption pool (11) and sleeved on the gas absorption pool (11). The gas absorption pool (11) has an inner cavity for accommodating the gas to be tested and an opening for connecting the inner cavity with the external environment. The opening includes an air inlet (111) at the air inlet end of the gas absorption pool (11) and an air outlet (112) at the air outlet end of the gas absorption pool (11). 10 to 20 permanent magnet rings (12) are arranged sequentially from the air inlet end to the air outlet end of the gas absorption pool (11). Each permanent magnet ring (12) has the same size and shape. The distance between any two adjacent permanent magnet rings (12) is 0 to 10 mm. The length of the permanent magnet rings (12) is equal to the length of the gas absorption pool (11) and they are arranged symmetrically from the center of the gas absorption pool (11) towards the air inlet end and the air outlet end, respectively. There are 14 permanent magnet rings (12). The spacing between the permanent magnet rings (12) from the center of the gas absorption pool (11) toward the gas inlet or gas outlet is 0, 0, 1, 3, 4, 10 mm respectively. The distance between the two permanent magnet rings that are close to the center of the gas absorption pool (11) is 0 mm.

2. The gas cell based on Faraday magneto-optical rotation spectrum as described in claim 1, characterized in that, The permanent magnet ring (12) is made of neodymium iron boron. The cross-section of the permanent magnet ring (12) and the cross-section of the ring arm are both rotationally symmetrical. The magnetic field strength of the longitudinal static magnetic field formed by multiple permanent magnet rings (12) is 200~400 Gauss.

3. A gas cell based on Faraday magneto-optical rotation spectrum as described in claim 2, characterized in that, The permanent magnet ring (12) has an outer diameter of 100 mm, an inner diameter of 60 mm, a rectangular cross-section of the ring arm, and a thickness of 25 mm.

4. A gas cell based on Faraday magneto-optical rotation spectrum as described in claim 3, characterized in that, The longitudinal static magnetic field composed of 14 permanent magnet rings (12) has a magnetic field length of 386 mm and a magnetic field strength of 300 Gauss.

5. A gas sensing device using a gas cell based on Faraday magneto-optical rotation spectrum as described in any one of claims 1-4, characterized in that, The system includes a signal source (20), a laser source, and a gas pool (10) disposed on the laser path. The laser source consists of a laser generator (31) and a laser controller (32) electrically connected to it. The laser controller (32) is electrically connected to the signal source (20). The gas pool (10) is also provided with a photodetector (40), a lock-in amplifier (50), and a data acquisition card (60) for detecting and acquiring the laser rotation signal passing through the gas pool (10). The lock-in amplifier (50) is also electrically connected to the signal source (20) to obtain a reference signal.

6. The gas sensing device as described in claim 5, characterized in that, Along the laser beam path, a first aperture (81) and a polarizer (71) are sequentially arranged between the laser source and the gas cell (10), and an analyzer (72) and a second aperture (82) are sequentially arranged between the gas cell (10) and the photodetector (40). The relative deflection angle θ between the polarizer (71) and the analyzer (72) satisfies 80°<θ<100° and θ≠90°.

7. The gas sensing device as described in claim 6, characterized in that, A focusing lens (90) is also provided between the second aperture (82) and the photodetector (40).

8. The gas sensing device as described in claim 5, characterized in that, The laser generator (31) is a quantum cascade laser, a semiconductor laser, or a vertical cavity surface-emitting laser.

9. The gas sensing device as described in claim 5, characterized in that, The photodetector (40) is a thermoelectrically cooled mercury cadmium telluride photodetector or a gallium arsenide photodetector.

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