Magnetic Rotation Spectroscopy Multipass Absorption Cell and Gas Sensing Device Based on Steady-State Static Magnetic Field

Abstract

This invention relates to the field of gas detection technology, specifically to a magnetic rotation spectroscopy multi-pass absorption cell and gas sensing device based on a steady-state static magnetic field. The absorption cell includes a dual-wavelength multi-pass cell and permanent magnet rings disposed on the dual-wavelength multi-pass cell. The permanent magnet rings are sleeved on the dual-wavelength multi-pass cell and coaxially arranged with it. Ten to twenty permanent magnet rings are arranged sequentially from the inlet to the outlet of the dual-wavelength multi-pass 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 ring arrangement matches the length of the dual-wavelength multi-pass cell, and they are symmetrically arranged from the center of the dual-wavelength multi-pass cell towards the inlet and outlet. This invention uses permanent magnets to replace the electromagnetic induction steady-state magnetic field, effectively avoiding the limitations of high energy consumption and excessive heat generation associated with AC solenoid coil induction of electromagnetic fields. It can promote the development of magnetic rotation spectroscopy into a high-sensitivity spectroscopic detection device that meets the requirements of external field applications.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, specifically to a dual-wavelength magnetic rotation spectroscopy multi-pass absorption cell and a paramagnetic sample gas sensing device based on a steady-state static magnetic field. Background Technology

[0002] Nitrogen oxides (NO) X The sources of NOx (NO + NO2) mainly include human activities such as fossil fuel combustion and biomass combustion, and natural processes such as microbial processes in the soil, wildfires, and lightning. It is an important trace gas in the Earth's troposphere, playing a crucial role in regulating the chemical properties and life cycles of many trace gases and free radicals in the atmosphere. Therefore, high-sensitivity, real-time, and synchronous detection of NOx concentration is of great significance. Currently, NOx concentration is mainly measured using chemiluminescence immunoassay, but this technique exhibits cross-sensitivity to other nitrogen compounds such as peroxyacetyl nitrate and nitric acid. Furthermore, this method has a slow detection rate for both NO and NO2, with time resolution often reaching the minute level or higher.

[0003] Magnetic rotation spectroscopy utilizes magnetic circular birefringence observed near the Zeeman splitting absorption line to detect paramagnetic molecules. Under the influence of an external longitudinal magnetic field, the rotational-vibrational transition of a paramagnetic molecule causes Zeeman splitting into ΔM... J =+1 and ΔM J = -1 two components. When linearly polarized light (which can be considered as the superposition of left-handed and right-handed circularly polarized light) passes through paramagnetic molecules immersed in an external magnetic field, its polarization plane rotates due to magnetic circular birefringence. By measuring this optical rotation signal, the concentration of paramagnetic molecules can be detected. Magnetic rotation spectroscopy significantly reduces laser intensity noise by using a pair of nearly orthogonal polarizers, thereby significantly improving the detection sensitivity of the system. In addition, the magnetic rotation spectral signal is generally not affected by diamagnetic molecules (such as water vapor and CO2), thus enabling highly selective measurement of paramagnetic molecules such as NO and NO2.

[0004] Currently, magnetic rotation spectroscopy mainly uses an AC solenoid coil to induce an electromagnetic field, and then detects a single component in a single cavity immersed in a longitudinal magnetic field. However, this AC magnetic field has limitations such as high power consumption and high heat generation. In addition, each cavity can only detect a single component, which cannot meet the requirement of simultaneous measurement of multiple components in a single cavity. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a dual-wavelength magnetic rotation spectroscopy paramagnetic sample sensing device based on a steady-state static magnetic field.

[0006] The present invention adopts the following technical solution:

[0007] A magnetic rotation spectral multipass absorption cell based on a steady-state static magnetic field includes a dual-wavelength multipass cell and permanent magnet rings disposed on the dual-wavelength multipass cell. The dual-wavelength multipass cell includes a gas cavity for forming a gas chamber to accommodate the gas to be measured, and a first concave mirror fixed at the gas inlet end and a second concave mirror fixed at the gas outlet end of the gas cavity. The permanent magnet rings are sleeved on the dual-wavelength multipass cell and are coaxially arranged with the dual-wavelength multipass cell. 14 permanent magnet rings are arranged sequentially from the gas inlet end to the gas outlet end of the dual-wavelength multipass 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 dual-wavelength multipass cell and is symmetrically arranged from the center of the dual-wavelength multipass cell towards the gas inlet end and the gas outlet end, respectively.

[0008] Preferably, the first concave mirror and the second concave mirror are each provided with an air hole at the center for gas exchange between the air chamber and the outside world. The first concave mirror and the second concave mirror are also provided with two light-transmitting holes that avoid the positions of the air holes.

[0009] Preferably, the diameter of the light-transmitting hole is 1 to 3 mm, and the two light-transmitting holes on the same concave mirror are positioned to avoid each other and are not on the same radius line.

[0010] Preferably, the first concave mirror and the second concave mirror are the same size, with a diameter of 25-60 mm and a radius of curvature of 500-1000 mm. The concave surfaces of the first concave mirror and the second concave mirror are arranged opposite each other, with a spacing of 100-500 mm.

[0011] 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 magnetic field strength of the longitudinal static magnetic field formed by multiple permanent magnet rings is 100 to 500 Gauss.

[0012] 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.

[0013] Preferably, a total of 14 permanent magnet rings are provided. The spacing between the permanent magnet rings from the center of the dual-wavelength multi-pass cell toward the air 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 dual-wavelength multi-pass cell 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.

[0014] The present invention also provides a sensing device using the above-mentioned magnetic rotation spectral multi-pass absorption cell. The device includes a signal source, a laser source, and a polarizer, a magnetic rotation spectral multi-pass absorption cell, and an analyzer arranged sequentially on the laser optical path. The signal source is connected to a first laser source and a second laser source respectively through an adder. The emitted light from the first laser source and the second laser source is polarized by the polarizer, enters the magnetic rotation spectral multi-pass absorption cell through a first concave mirror, and after secondary reflection in the gas cavity, exits through a second concave mirror and enters the analyzer. The output end of the analyzer is sequentially provided with a photodetector for detecting and acquiring laser optical rotation signals and a lock-in amplifier. The lock-in amplifier is also electrically connected to the signal source to obtain a reference signal.

[0015] Preferably, the polarizer and analyzer are arranged in pairs, including a first polarizer and a first analyzer corresponding to the first laser source, and a second polarizer and a second analyzer corresponding to the second laser source. The relative deflection angle between the polarization planes of the first polarizer and the first analyzer is θ1, and the relative deflection angle between the polarization planes of the second polarizer and the second analyzer is θ2. Both θ1 and θ2 satisfy 80° < θ < 100°, and θ ≠ 90°.

[0016] Preferably, the signal source outputs two sinusoidal signals, which are superimposed on the triangular wave signals output by the adder and then input to the first laser source and the second laser source, respectively. The frequency difference between the two sinusoidal signals is 1 to 10 kHz.

[0017] Preferably, the laser source further includes a laser control module, wherein the adder is electrically connected to the first laser control module to control the first laser source to generate outgoing light, and is electrically connected to the second laser control module to control the second laser source to generate outgoing light, and the modulation frequency of the laser source is greater than 10KHz.

[0018] The beneficial effects of this invention are as follows:

[0019] This invention utilizes a non-equidistant array of permanent magnet rings to induce a steady-state static magnetic field. This allows for effective alteration of the magnetic circular birefringence by modulating the laser wavelength under static Zeeman splitting, ultimately generating a Faraday rotation spectral signal. By replacing the electromagnetically induced steady-state static magnetic field with permanent magnets, the limitations of high energy consumption and excessive heat generation associated with AC solenoid coil-induced electromagnetic fields are effectively avoided. This promotes the development of magnetic rotation spectroscopy into a highly sensitive trace gas detection device with power consumption comparable to traditional direct absorption spectroscopy and wavelength modulation spectroscopy.

[0020] The polarization rotation angle Θ can be expressed as Θ = ΔnLπ / λ, where λ is the laser wavelength, L is the effective optical path length of the linearly polarized light interacting with the paramagnetic sample in the magnetic field, and Δn = n R -n LThis represents the difference in refractive index between right-handed and left-handed circularly polarized light. The invention introduces a dual-wavelength multi-pass cell, which effectively increases the interaction between two beams of linearly polarized light of different wavelengths and two different paramagnetic samples, thereby amplifying the Faraday rotation angle and improving the detection sensitivity of magnetic rotation spectroscopy.

[0021] The magnetic rotation spectroscopy multi-pass absorption cell based on a steady-state static magnetic field provided by this invention has the characteristics of a single cavity and dual optical path, which can satisfy the simultaneous entry of two laser beams of different wavelengths into a single cavity, realize the synchronous detection of two paramagnetic samples, and overcome the defect of current magnetic rotation spectroscopy that can only detect a single paramagnetic sample in a single cavity.

[0022] Furthermore, according to experimental requirements, the wavelength and power of the laser source can be adjusted accordingly based on the sample being tested and the required detection sensitivity. The optical path of the dual-wavelength multi-pass cell can be optimized by using a White cell or a high-precision cavity-enhanced absorption cell to improve the interaction between linearly polarized light and paramagnetic samples, thereby improving the detection limit. The optimized design of the concave reflector in the multi-pass cell can extend the dual-wavelength magnetic rotation spectrum to a three-wavelength (or more) magnetic rotation spectrum for the simultaneous detection of multiple paramagnetic molecules. The length and intensity of the uniform magnetic field induced by the permanent magnet can be adjusted according to the actual detection sensitivity requirements by adjusting the number and size of the permanent magnet rings, thereby meeting various detection needs. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of a magnetic rotation spectral multipass absorption cell based on a steady-state static magnetic field;

[0024] Figure 2 for Figure 1 Side view;

[0025] Figure 3 This is a schematic diagram of the sensing device using a magnetic rotation spectral multi-pass absorption cell according to the present invention.

[0026] Figure 4 This is a schematic diagram showing the distribution of concentric ring-shaped light spots of different radii formed on the concave mirror of a magnetic rotation spectroscopy multi-pass absorption cell.

[0027] Figure 5 The magnetic rotation spectral signals of 300 ppb NO2 and NO were obtained using this device.

[0028] The meanings of the symbols marked in the figure are as follows:

[0029] 10-Magnetic rotation spectroscopy multi-pass absorption cell; 11-Dual wavelength multi-pass cell; 111-First concave mirror; 112-Second concave mirror; 12-Permanent magnet ring; 13-Pore.

[0030] 20-Signal Source

[0031] 31-First laser source; 32-Second laser source; 33-First laser control module; 34-Second laser control module

[0032] 40 - Photodetector; 50 - Lock-in amplifier

[0033] 71-First polarizer 72-First analyzer 73-Second polarizer 74-Second analyzer

[0034] 80-Focusing Lens Detailed Implementation

[0035] The technical solution of the present invention will be described in more detail below with reference to the accompanying drawings:

[0036] like Figure 1-2 As shown, a magnetic rotation spectral multi-pass absorption cell based on a steady-state static magnetic field includes a dual-wavelength multi-pass cell 11 and a permanent magnet ring 12 disposed on the dual-wavelength multi-pass cell 11. The dual-wavelength multi-pass cell 11 includes a cylinder for forming a gas cavity to accommodate the gas to be measured, and a first concave mirror 111 fixed at the gas inlet end and a second concave mirror 112 fixed at the gas outlet end of the cylinder. The concave surfaces of the first concave mirror 111 and the second concave mirror 112 are arranged opposite to each other. The center of the first concave mirror 111 and the second concave mirror 112 is provided with a pore 13 for connecting the gas cavity and the outside world to exchange gases. The pore is used to realize the real-time exchange of paramagnetic molecules such as NO and NO2.

[0037] The first concave mirror 111 and the second concave mirror 112 are also provided with two through holes that avoid the position of the vent 13 for the entry and exit of the light beam. The diameter of the through holes is 1 to 3 mm and should be larger than the diameter of the laser beam. The two through holes on the same concave mirror are positioned to avoid each other and are not on the same radius line. In actual use, the distance between the through holes is set to ensure that the light spots formed on the reflecting mirror surface do not overlap.

[0038] The permanent magnet ring 12 is sleeved on the dual-wavelength multi-pass cell 11 and coaxially arranged with it. The permanent magnet rings 12 are arranged in a line from the air inlet to the air outlet of the dual-wavelength multi-pass cell 11, with a total of 10 to 20 rings. Each permanent magnet ring 12 is the same size and shape, and 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 dual-wavelength multi-pass cell 11, and they are symmetrically arranged from the center of the dual-wavelength multi-pass cell 11 towards the air inlet and outlet. In this invention, the length of the dual-wavelength multi-pass cell 11 is between 100 and 500 mm, the permanent magnet rings 12 are made of neodymium iron boron, and the length of the magnetic field formed is basically the same as the length of the dual-wavelength multi-pass cell 11, with a longitudinal static magnetic field strength in the range of 100 to 500 Gauss.

[0039] In this invention, the first concave mirror 111 and the second concave mirror 112 of the dual-wavelength multi-pass cell 11 are the same size, with a diameter of 25-60 mm and a radius of curvature of 500-1000 mm. The concave surfaces of the first concave mirror 111 and the second concave mirror 112 are arranged opposite each other, with a spacing of 100-500 mm, which is equal to the length of the dual-wavelength multi-pass cell 11. The dual-wavelength multi-pass cell 11 can be an existing gas absorption cell such as a White cell or a high-precision cavity-enhanced absorption cell, 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.

[0040] In one specific embodiment, the magnetic rotation spectral multi-pass absorption cell based on a steady-state static magnetic field provided by the present invention is 386 mm long and consists of 14 NdFeB permanent magnet rings 12. Each permanent magnet ring 12 is circular with a rectangular cross-section of its ring arm. The outer diameter L2 of a single permanent magnet ring 12 is 100 mm, the inner diameter L3 is 60 mm, and the thickness L4 of the ring arm is 25 mm. Since the magnetic field strength induced by the ring magnets is affected by their axial spacing, the magnetic field strength and uniformity at different ring magnet spacings (spacing adjustment range 0–10 mm) are measured using a gaussmeter. The final ring magnet spacing is determined to satisfy 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 is approximately 386 mm long and has a longitudinal magnetic field strength of 300 gauss. The dual-wavelength multi-pass cell 11 and the ring permanent magnet 12 array are coaxially matched and have basically the same length, which can effectively apply the induced static magnetic field to the Zeeman splitting of paramagnetic molecules.

[0041] In practical applications, the number, shape, and size of the permanent magnet rings 12 can be adjusted and determined according to the required static magnetic field strength and the length and diameter of the dual-wavelength multi-pass cell 11 used. The shape can be a circular ring, a square, or a hexagon, etc., with rotational symmetry. This invention does not impose any specific limitations.

[0042] Sensing devices employing the aforementioned magnetic rotation spectroscopy multi-pass absorption cell, such as... Figure 3 As shown, it includes a signal source 20, a laser source, and a polarizer, a magnetic rotation spectral multi-pass absorption cell 10, and an analyzer arranged sequentially on the laser optical path.

[0043] In this device, two laser sources are provided: a first laser source 31 and a second laser source 32. The signal source 20 is electrically connected to the first laser source 31 and the second laser source 32 via an adder 21. The signal source 20 outputs a triangular wave signal and two sine wave signals with a frequency difference of 1-10 kHz. The adder 21 superimposes the triangular wave signal and the two sine wave signals and outputs them to the first laser source 31 and the second laser source 32, respectively. The laser source in this invention also includes a laser control module. The adder 21 is electrically connected to the first laser control module 33 to control laser I generated by the first laser source 31, and electrically connected to the second laser control module 34 to control laser II generated by the second laser source 32. Because the target molecules being detected are different, laser I and laser II have different wavelengths.

[0044] The emitted light from the first laser source 31 and the second laser source 32 is polarized by a polarizer, enters the magnetic rotation spectral multi-pass absorption cell 10 through the first concave mirror 111, and after multiple reflections within the gas cavity, exits through the second concave mirror 112 and enters the analyzer. The polarizer is used to establish linearly polarized light, and the analyzer is used to convert the optical rotation signal resulting from the interaction of linearly polarized light with paramagnetic molecules into a modulated change in light intensity.

[0045] In this device, two sets of polarizers and analyzers are set according to the number of laser sources, including a first polarizer 71 and a first analyzer 72 corresponding to the first laser source 31, and a second polarizer 73 and a second analyzer 74 corresponding to the second laser source 32.

[0046] The polarizer and analyzer surfaces are each coated with an antireflection film corresponding to the laser wavelength, and their deflection angles are close to 90°. The Faraday rotation spectral signal disappears when the polarizer and analyzer are completely orthogonal. Specifically, the relative deflection angle between the polarization planes of the first polarizer 71 and the first analyzer 72 is θ1, which can be expressed as ψ1 = 90° - θ1. The relative deflection angle between the polarization planes of the second polarizer 73 and the second analyzer 74 is θ2, which can be expressed as ψ1 = 90° - θ2. The selection of ψ1 or ψ2 should satisfy the optimal signal-to-noise ratio of the magnetic rotation spectrum signal. The signal-to-noise ratio of the magnetic rotation spectrum is defined as the ratio of the peak value of the magnetic separation spectrum signal to the standard deviation of the data points at the non-absorption positions. By measuring the relationship between this signal-to-noise ratio and the deflection angle ψ1 or ψ2 of the analyzer, the angle ψ1 or ψ2 corresponding to the maximum signal-to-noise ratio can be determined. It is generally within the range of 10°. Therefore, the values ​​of θ1 and θ2 are 80° < θ < 100°, and θ ≠ 90°.

[0047] The first analyzer 72 and the second analyzer 74 are respectively equipped with a photodetector 40 for detecting and acquiring optical rotation signals from the magnetic rotation spectral multi-pass absorption cell 10, and a lock-in amplifier 50. The lock-in amplifier 50 is also electrically connected to the signal source 20 to obtain a reference signal. A focusing lens 80 is also provided between the analyzer and the photodetector 40. The focusing lens 80 is used to converge ordinary light, and the photodetector 40 is positioned at the focal point of the focusing lens 90 to better detect ordinary light containing information on the concentration of paramagnetic molecules.

[0048] In one embodiment, the quantitative detection of NO and NO2 using this device is performed as follows:

[0049] First, based on the desired detection sensitivity for paramagnetic molecules, the base length of the dual-wavelength multipass cell 11 was determined to be 386 mm, and the inner diameter to be 50 mm. A pair of concave mirrors with a radius of curvature of 1000° were installed on both sides of the cell's gas cavity, with each mirror having two small holes with a diameter of 2 mm, thus forming a dual-wavelength multipass cell with an optical path of 24 m. The concave mirrors allow for multiple back-and-forth reflections of the two beams within a single gas cavity, enabling simultaneous detection of two-component paramagnetic molecules. Figure 4 The image shows the distribution of concentric ring-shaped light spots of different radii formed on the reflecting mirror surface after multiple reflections.

[0050] Based on the length of the dual-wavelength multi-pass cell 11, the length and spacing of the permanent magnet ring array 12 are set to obtain a uniform magnetic field with an axial magnetic field strength of about 300 Gauss, thereby establishing a magnetic rotation spectral multi-pass absorption cell 10.

[0051] Signal source 20 outputs one triangular wave signal and two sine wave signals respectively. Adder 21 adds the triangular wave signal output by signal source 20 to one of the sine wave signals respectively, and then sends them to the first laser control module 33 and the second laser control module 34 for electrical modulation of the first laser source 31 and the second laser source 32 respectively. To obtain the Faraday rotation spectral signals of NO and NO2, the output wavelengths of the laser sources are 1875.8 cm⁻¹. -1 (5.331μm) and 1613.25cm -1 (6.199μm), the frequencies of the two sinusoidal signals differ by 10KHz, preferably 20KHz and 30KHz.

[0052] The emitted light from the first laser source 31 and the second laser source 32 passes through the first polarizer 71 and the second polarizer 73 respectively to establish two linearly polarized beams. Then, they enter the magnetic rotation spectral multi-pass absorption cell 10. The linearly polarized light is amplified by the Faraday rotation effect of its interaction with paramagnetic molecules through multiple reflections inside the magnetic rotation spectral multi-pass absorption cell 10. After passing through the first analyzer 72 and the second analyzer 74, the optical rotation signal is converted into a modulated light intensity change.

[0053] Two CaF2 focusing lenses 80, coated with antireflective films, focus ordinary light, after passing through the first analyzer 72 and the second analyzer 74, onto two photodetectors 40, respectively. The photodetectors 40 convert the detected modulated light intensity changes into electrical signals, which are then input to two lock-in amplifiers 50 electrically connected to them. The lock-in amplifiers 50 also receive a reference signal from the signal source 20, which has the same frequency as the two sinusoidal signals, for demodulating the magnetic rotation spectral signals of the two paramagnetic samples. The magnetic rotation spectral signals are as follows: Figure 5 As shown, it contains concentration information of NO and NO2 molecules, thus enabling the quantitative detection of NO and NO2.

[0054] 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 magnetic rotation spectroscopy multi-pass cell based on a steady-state static magnetic field, characterized in that, The application relates to a double-wavelength multi-pass cell (11) and a permanent magnet ring (12) arranged on the double-wavelength multi-pass cell (11), wherein the double-wavelength multi-pass cell (11) comprises a gas cavity for containing a gas to be measured, a first concave mirror (111) fixed at the gas inlet end of the gas cavity, and a second concave mirror (112) fixed at the gas outlet end of the gas cavity; the permanent magnet ring (12) is coaxially arranged on the double-wavelength multi-pass cell (11) and is arranged from the gas inlet end to the gas outlet end of the double-wavelength multi-pass cell (11) in sequence; the size and shape of each permanent magnet ring (12) are consistent; the interval between any two adjacent permanent magnet rings (12) is 0-10 mm; the arrangement length of the permanent magnet rings (12) is matched with the length of the double-wavelength multi-pass cell (11); and the permanent magnet rings (12) are symmetrically arranged from the center of the double-wavelength multi-pass cell (11) to the gas inlet end and the gas outlet end. The permanent magnet ring (12) is arranged in a total of 14 numbers, and the interval of the permanent magnet ring (12) arranged from the center of the double-wavelength multi-pass cell (11) to the gas inlet end or the gas outlet end is 0, 0, 1, 3, 4, and 10 mm in sequence.

2. A magnetic rotation spectroscopy multi-pass cell based on a steady-state static magnetic field as claimed in claim 1, wherein, The center of the first concave mirror (111) and the second concave mirror (112) is provided with a gas hole (13) for communicating the gas cavity with the outside for gas exchange; and the first concave mirror (111) and the second concave mirror (112) are further provided with two light transmission holes which avoid the position of the gas hole (13).

3. A multi-pass cell for magnetic rotation spectroscopy based on a steady-state static magnetic field as claimed in claim 2, characterized in that, The diameter of the light transmission hole is 1-3 mm, the two light transmission holes are consistent in size, the two light transmission holes on the same concave mirror are arranged to avoid each other, and the two light transmission holes are not on the same radius line.

4. A multi-pass cell for magnetic rotation spectroscopy based on a steady-state static magnetic field as defined in claim 2, wherein, The first concave mirror (111) and the second concave mirror (112) are consistent in size, the diameter is 25-60 mm, the curvature radius is 500-1000 mm, the concave surfaces of the first concave mirror (111) and the second concave mirror (112) are oppositely arranged, and the interval is 100-500 mm.

5. A magnetic rotation spectroscopy multi-pass cell based on a steady-state static magnetic field as defined in claim 1, wherein, The material of the permanent magnet ring (12) is neodymium iron boron, the cross section of the permanent magnet ring (12) and the cross section of the ring arm are rotationally symmetrical, the magnetic field strength of the longitudinal static magnetic field formed by the plurality of permanent magnet rings (12) is 100-500 Gauss.

6. A magnetic rotation spectroscopy multi-pass cell based on a steady-state static magnetic field as claimed in claim 5, characterized in that, The outer diameter of the permanent magnet ring (12) is 100 mm, the inner diameter is 60 mm, the cross section of the ring arm is rectangular, and the thickness of the ring arm is 25 mm.

7. A magnetic rotation spectroscopy multi-pass cell based on a steady-state static magnetic field as claimed in claim 6, characterized in that, The magnetic field length of the longitudinal static magnetic field formed by the 14 permanent magnet rings (12) is 386 mm, and the magnetic field strength is 300 Gauss.

8. A gas sensing device using the multi-pass cell for magnetic rotation spectroscopy based on a steady-state static magnetic field according to any one of claims 1 to 7, characterized in that, The application relates to a magnetic rotation spectrum multi-pass absorption cell, which comprises a signal source (20), a laser source and a polarizer, a magnetic rotation spectrum multi-pass absorption cell (10) and a detection polarizer arranged in sequence on a laser light path, the signal source (20) is connected with a first laser source (31) and a second laser source (32) through an adder (21), the emergent light of the first laser source (31) and the second laser source (32) is polarized by the polarizer, enters the magnetic rotation spectrum multi-pass absorption cell (10) from a first concave mirror (111), passes through the magnetic rotation spectrum multi-pass absorption cell (10) for multiple times of reflection in a gas cavity, and finally enters the detection polarizer, the emergent light end of the detection polarizer is provided with a photoelectric detector (40) and a lock-in amplifier (50) in sequence for detecting and collecting a rotation signal, the lock-in amplifier (50) is also electrically connected with the signal source (20) to obtain a reference signal.

9. The gas sensing device of claim 8, wherein, The polarizer and the detection polarizer are arranged in pairs and comprise a first polarizer (71) corresponding to the first laser source (31), a first detection polarizer (72), a second polarizer (73) corresponding to the second laser source (32) and a second detection polarizer (74), the relative rotation angle between the polarizing plane of the first polarizer (71) and the first detection polarizer (72) is theta1, the relative rotation angle between the polarizing plane of the second polarizer (73) and the second detection polarizer (74) is theta2, theta1 and theta2 both satisfy 80 DEG < theta < 100 DEG, and theta is not equal to 90 DEG.

10. The gas sensing device of claim 8, wherein The signal source (20) outputs two sinusoidal signals and one triangular wave signal, the two sinusoidal signals are superposed with the triangular wave signal through the adder (21) and then input into the first laser source (31) and the second laser source (32) respectively, and the frequency difference between the two sinusoidal signals is 1-10 KHz.

11. The gas sensing device of claim 10, wherein, The laser source further comprises a laser control module, the adder (21) is electrically connected with a first laser control module (33) to control the first laser source (31) to generate emergent light and is electrically connected with a second laser control module (34) to control the second laser source (32) to generate emergent light, and the modulation frequency of the laser source is greater than 10 KHz.

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