Self-adaptive high-sensitivity long-optical-path laser absorption spectrum system

Abstract

The invention discloses a self-adaptive high-sensitivity long-optical-path laser absorption spectrum system, which relates to the technical field of trace gas detection, and mainly comprises a light source generation module for generating signal light, an aspherical mirror multi-reflection pool for reflecting the signal light for multiple times, and a light source detection module for detecting the signal light, the signal processing module is used for monitoring output light of the aspherical mirror multi-reflection pool; and the reference pool module is self-adaptive. According to the invention, the signal light is emitted into the aspherical mirror multi-reflection cell, so that the portability of the system is ensured, and the effective absorption optical path of the gas optical path cell is improved. In addition, self-adaption of the laser wavelength is achieved through a pure methane reference gas pool in a light path in combination with a software program, and no extra circuit needs to be added. The system is simplified, the cost is controlled, and synchronous high-sensitivity detection of various trace gases is realized.

Description

Technical Field

[0001] The invention relates to the technical field of trace gas detection, in particular to an adaptive high-sensitivity long-light-path laser absorption spectroscopy system. Background Art

[0002] With the rapid development of the economy and the acceleration of urbanization, air pollution has become increasingly prominent. Although the concentration of air pollutants has decreased in recent years, formaldehyde (HCHO) and ammonia (NH3) are important air pollutants, and their monitoring and control are of special significance.

[0003] Tunable semiconductor laser absorption spectroscopy (TDLAS) technology has excellent selectivity and is not affected by background gas interference. It has developed into an extremely sensitive atmospheric trace gas monitoring technology and plays an important role in many fields such as industrial processing and pollution monitoring. In addition, TDLAS technology also shows its unique advantages in multi-component gas measurement. By selecting the appropriate laser center wavelength, the absorption lines of multiple gases can be separated under low-pressure conditions so that they do not interfere with each other. Therefore, by using a single laser and detector, it is possible to achieve simultaneous measurement of multiple gases under low-pressure conditions. This method not only reduces costs but also simplifies the system structure.

[0004] For the same concentration of the gas being measured, the longer the absorption path, the larger the absorption signal and the higher the detection sensitivity of the system. One way to increase the effective absorption path length of a gas is to increase the length of the multi-reflector cell, thereby increasing the absorption path length while maintaining the same number of reflections. However, when the multi-reflector cell is too long, the instrument's portability is greatly reduced, making it inconvenient for real-time field measurements. Another approach is to increase the number of light spots distributed on the mirror surface, thereby increasing the number of reflections of the signal beam within the multi-reflector cell. The traditional Herriott cell light spot is only uniformly distributed in a single circle along the edge of the mirror, resulting in relatively low mirror utilization. Using aspheric mirrors with different structural parameters as reflectors at both ends of the multi-reflector cell creates a light spot distribution different from that of the traditional Herriot cell, which can improve mirror utilization. Furthermore, compared with direct absorption technology, tunable semiconductor laser absorption spectroscopy combined with wavelength modulation spectroscopy (2f-WMS) effectively suppresses system background noise, especially 1 / f noise, thereby significantly improving detection sensitivity. Summary of the Invention

[0005] In order to overcome the above-mentioned defects in the prior art, the present invention provides an adaptive high-sensitivity long-path laser absorption spectroscopy system, which increases the effective absorption path of the gas light flux cell while ensuring the portability of the system, and realizes the synchronous high-sensitivity detection of multiple trace gases while simplifying the system and controlling costs.

[0006] To achieve the above object, the present invention adopts the following technical solutions, including:

[0007] An adaptive, highly sensitive, long-path laser absorption spectroscopy system includes a light source generating module for generating signal light, an aspheric mirror multi-reflector cell for multiple reflections of the signal light, and a signal processing module for collecting and analyzing the output light of the aspheric mirror multi-reflector cell;

[0008] The aspheric mirror multi-reflection pool includes a first high-reflection mirror, a second high-reflection mirror and a rotating platform; the first high-reflection mirror and the second high-reflection mirror are plano-concave high-reflection mirrors with the same parameters; the concave surface is a reflecting surface, and the curvature radii in the x-axis direction and the y-axis direction of the mirror surface are different, and the concave surfaces are both facing the inner side of the aspheric mirror multi-reflection pool; the first high-reflection mirror is provided with holes for the incident and emitted signal light; the second high-reflection mirror is installed on the rotating platform, and the second high-reflection mirror is rotated around the central axis by the rotating platform; the central axis is the connecting axis between the center of the first high-reflection mirror and the center of the second high-reflection mirror.

[0009] Preferably, the system also includes a reference cell containing pure CH4. The signal light generated by the light source generating module enters the aspherical mirror multi-reflection cell after passing through the reference cell to achieve signal calibration. When analyzing the measurement signal and background signal, the CH4 absorption signal is regarded as a reference signal.

[0010] Preferably, the signal light enters the aspherical mirror multi-reflection pool through the hole of the first high-reflection mirror at a certain incident angle, and after multiple reflections between the reflective surface of the second high-reflection mirror and the reflective surface of the first high-reflection mirror, forms a periodic stable light field distribution in the aspherical mirror multi-reflection pool, and then exits from the hole of the first high-reflection mirror;

[0011] The light transmission trajectory between the first high-reflective mirror and the second high-reflective mirror is determined by the following preset calculation formula:

[0012]

[0013]

[0014] Where, ABCD matrix is the light transmission matrix; d is the distance between the first high reflective mirror and the second high reflective mirror; R x is the curvature radius of the high reflective mirror along the x-axis; R y is the curvature radius of the high-reflectivity mirror on the y axis; R(R x 、R y ) is the mirror reflection matrix; D(d) is the free space propagation matrix; T(θ) is the rotation matrix of the second high-reflection mirror around the central axis; θ is the rotation angle of the second high-reflection mirror around the central axis; Z nis a matrix consisting of the coordinates and angles of the nth reflection of the light, n = 1, 2, ..., N, N is the number of light spots formed on the surface of the second high-reflection mirror, that is, the number of light reflections in the multi-reflection pool of the aspherical mirror; x n ,y n is the position coordinate of the nth light spot, x n ',y n ' is the angle between the light and the x and y axes during the nth reflection.

[0015] Preferably, the parameters of the aspherical mirror multi-reflector pool are determined by the following preset calculation formula:

[0016] Nθ x =M x π

[0017] Nθ y =M y π

[0018]

[0019] N max =(R pout π / R pin ) 2

[0020] R pin ≥(R beam +R hole )

[0021]

[0022] Where N is the number of light reflections in the multi-reflection pool of the aspheric mirror, that is, the number of light spots formed on the surface of the second high-reflection mirror; M x M is the number of circles of the light spot along the x-axis of the second high-reflection mirror. y is the number of circles of the light spot along the y-axis of the second high-reflection mirror; θ x and θ y is the advancing angle of the light spot with adjacent reflection times in the x-axis and y-axis directions of the second high-reflection mirror; x 、M y}, N must be an even number and there must be no other common factors between the three numbers except 2; N max is the theoretical maximum number of reflections; R x is the curvature radius of the high reflective mirror along the x-axis, R y is the curvature radius of the high-reflectivity mirror on the y axis; R beam is the laser radius; R hole is the hole radius in the first high reflective mirror; R pout R is the maximum circle radius of the light spot on the mirror surface; pin It is the minimum circle radius of the light spot distribution on the mirror surface.

[0023] Preferably, the position deviation caused by the wavelength drift is corrected by the CH4 absorption signal of the reference cell:

[0024] The signal of the signal light passing through the reference cell is regarded as the reference signal;

[0025] In the background mode, the measured signals include the reference signal and the background noise signal, and the second harmonic peak position in the background mode is recorded as the background mode signal;

[0026] In the measurement mode, the measured signals include the gas signal to be measured, the reference signal and the background noise signal, and the second harmonic peak position in the measurement mode is recorded;

[0027] Based on the background mode signal, the second harmonic position in the measurement mode is adjusted in real time, and then the background mode signal is deducted to correct the position deviation caused by wavelength drift.

[0028] Preferably, the hole of the aspherical mirror multi-reflection cell is located at the center of the first high-reflection mirror.

[0029] Preferably, in the aspheric mirror multi-reflective cell, the first high-reflective mirror is mounted on an integrated high-reflective mirror mounting plate, the second high-reflective mirror is mounted on a high-reflective mirror mounting slot, the high-reflective mirror mounting slot is connected to a rotating platform, the rotating platform is connected to a multi-reflective cell sealing plate, the first high-reflective mirror and the second high-reflective mirror are both fixed using high-reflective mirror pressure rings; the bases on both sides of the aspheric mirror multi-reflective cell are fixed using multi-reflective cell fixing rods; the aspheric mirror multi-reflective cell is provided with an air inlet and an air outlet for the circulation of the gas to be measured.

[0030] Preferably, the light source generating module includes a laser control module, a quantum cascade laser, a focusing lens, a first plane reflecting mirror and a second plane reflecting mirror which are sequentially arranged along the signal transmission direction.

[0031] Preferably, the signal processing module includes a photodetector, a lock-in amplifier and a processor which are sequentially arranged along the signal transmission direction.

[0032] Preferably, the laser control module performs current modulation and temperature control on the quantum cascade laser respectively, so that the absorption of HCHO and NH3 gases is included in the output range of the quantum cascade laser; by controlling the vacuum pump at the outlet end of the aspherical mirror multi-reflector cell and the mass flow meter at the inlet end of the aspherical mirror multi-reflector cell, the internal pressure of the aspherical mirror multi-reflector cell is controlled, so that the absorption lines of the two gases are separated, and the second harmonic signal is extracted through a lock-in amplifier.

[0033] The advantages of the present invention are:

[0034] (1) The present invention proposes an adaptive, highly sensitive, long-path laser absorption spectroscopy system. By injecting signal light into a long-path aspheric mirror multi-reflector cell, the effective absorption path of the gas light channel cell is increased while ensuring system portability. Furthermore, a pure methane reference gas cell in the optical path is combined with a software program to achieve adaptive adjustment of the measurement signal and background signal without the need for additional circuitry. This invention achieves simultaneous, highly sensitive detection of multiple trace gases while simplifying the system and controlling costs.

[0035] (2) The present invention is a tunable semiconductor laser absorption spectroscopy system based on a long optical path aspheric mirror multi-reflector cell. While ensuring that the spatial length of the equipment remains unchanged, the effective absorption optical path of the system gas far exceeds that of the traditional Herriott cell.

[0036] (3) The present invention provides a design method for a long optical path aspheric mirror multi-reflector pool, which can adapt to the requirements of different spatial sizes and effective absorption optical paths.

[0037] (4) The long optical path aspheric mirror multi-reflector proposed in the present invention can adjust the angle between the mirror axes through the electrically controlled displacement rotation platform, thereby achieving adjustment of the absorption optical path within a certain range.

[0038] (5) The present invention achieves the simultaneous measurement of HCHO and NH3 using one laser by controlling the low pressure of the multi-reflector cell and the wavelength output range of the laser, thereby reducing the complexity of the system while controlling the cost of the system.

[0039] (6) The present invention realizes adaptive adjustment of the measurement signal and the background signal through a high-purity CH4 reference cell without adding additional circuits, and combines wavelength modulation spectroscopy technology and fast background subtraction to realize synchronous high-sensitivity detection of multiple trace gases. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a structural block diagram of a tunable semiconductor laser absorption spectroscopy system for synchronously measuring multi-component gases with an aspheric mirror multi-reflector provided in an embodiment of the present invention.

[0041] Figure 2 This is a Tracepro optical path simulation diagram of the aspheric mirror multi-reflector provided in an embodiment of the present invention.

[0042] Figure 3 This is the simulated distribution diagram of the mirror light spot at the output end of the aspherical mirror multi-reflector.

[0043] Figure 4 A diagram showing the mechanical structure of an aspheric mirror multi-reflector cell provided in an embodiment of the present invention.

[0044] Figure 5A flowchart of software-based signal adaptive adjustment provided by an embodiment of the present invention.

[0045] Figure 6 The peak signals of HCHO alone and mixed gases under different concentration conditions are shown.

[0046] Figure 7 These are the peak signals of NH3 alone and mixed gases under different concentration conditions.

[0047] Description of reference numerals:

[0048] Laser control module 1, quantum cascade laser 2, focusing lens 3, first plane reflector 4, reference cell 5, second plane reflector 6, aspherical mirror multi-reflector 7, photodetector 8, lock-in amplifier 9 and processor 10, first high-reflector 71, second high-reflector 72, rotating platform 73, air outlet 74, air inlet 75, high-reflector pressure ring 76, multi-reflector sealing plate 77, high-reflector mounting groove 78, high-reflector mounting plate 79, multi-reflector fixing rod 710, quartz glass tube 711, glass tube pressure ring 712. DETAILED DESCRIPTION

[0049] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0050] Figure 1 A block diagram of a tunable semiconductor laser absorption spectroscopy system for synchronously measuring multi-component gases using a long-path aspheric mirror multi-reflector cell, provided in an embodiment of the present invention. The present invention provides an adaptive, highly sensitive long-path laser absorption spectroscopy system comprising: a light source generating module for generating signal light; an aspheric mirror multi-reflector cell 7 for multiple reflections of the signal light; an adaptive reference cell 5; and a signal processing module for monitoring the output signal of the aspheric mirror multi-reflector cell 7.

[0051] The light source generating module includes a laser control module 1 , a quantum cascade laser 2 , a focusing lens 3 , a first plane reflecting mirror 4 and a second plane reflecting mirror 6 .

[0052] The signal processing module includes a photodetector 8 , a lock-in amplifier 9 and a processor 10 .

[0053] The aspherical mirror multi-reflection cell 7 includes a first high-reflection mirror 71 , a second high-reflection mirror 72 and a rotating platform 73 .

[0054] To simplify the design, the first high-reflection mirror 71 and the second high-reflection mirror 72 are plano-concave high-reflection mirrors with the same parameters. The curvature radii of the plano-concave high-reflection mirrors in the x-axis and y-axis directions of the mirror surface are different. The concave surface, i.e., the reflective surface, faces the inner side of the aspherical mirror multi-reflection cell 7 and is coated with a high-reflectivity film layer in the absorption line band of the gas to be measured. Figure 2 As shown, the z-axis direction is perpendicular to the plano-concave high-reflection mirror, that is, the central axis direction.

[0055] The second highly reflective mirror 72 is mounted on a rotating platform 73. The rotating platform 73 can be used to rotate the second highly reflective mirror 72 about its central axis (z-axis) to adjust the light spot distribution on the mirror surface, thereby adjusting the effective optical path of the long optical path aspherical mirror multi-reflector within a certain range. In this embodiment, an electrically controlled displacement rotating platform 73 is used.

[0056] The first high-reflection mirror 71 is provided with a hole for the signal light to enter and exit. In this embodiment, the hole is located at the center of the first high-reflection mirror 71. The signal light beam enters the aspherical mirror multi-reflection pool 7 through the hole (central small hole) of the first high-reflection mirror 71 at a certain incident angle, and after multiple reflections between the reflecting surface (concave surface) of the second high-reflection mirror 72 and the reflecting surface (concave surface) of the first high-reflection mirror 71, a periodic stable light field distribution is formed in the aspherical mirror multi-reflection pool 7, and then it is emitted from the hole (central small hole) of the first high-reflection mirror 71.

[0057] The light transmission trajectory between the first high-reflective mirror 71 and the second high-reflective mirror 72 is determined by the following preset calculation formula:

[0058]

[0059] Where, d is the distance between the first high reflective mirror 71 and the second high reflective mirror 72, R x is the curvature radius of the high reflective mirror along the x-axis, R y is the curvature radius of the high-reflectivity mirror on the y-axis, R(R x 、R y ) is the mirror reflection matrix, D(d) is the free space propagation matrix, T(θ) is the rotation matrix of the second high reflective mirror 72 around the central axis, θ is the rotation angle of the second high reflective mirror 72 around the central axis, ABCD matrix is the light transmission matrix, Z n is a matrix consisting of the coordinates and angles of the nth reflection of the light, n = 1, 2, ..., N, N is the number of light spots formed on the surface of the second high-reflection mirror 72, that is, the number of light reflections in the aspherical mirror multi-reflection pool 7, x n ,y n is the position coordinate of the nth light spot, x n ',y n ' is the angle between the light and the x and y axes during the nth reflection.

[0060] The aspheric mirror (plano-concave high-reflection mirror) used in the present invention makes the light spot of the multi-reflection cell distributed in the shape of a Lissajous figure. Its light transmission can be described by an ABCD matrix, but the influence of the rotation of the second high-reflection mirror 72 needs to be taken into account. The parameters of the aspheric mirror multi-reflection cell 7 used in the present invention can be determined by the following preset calculation formula:

[0061] Nθ x =M x π

[0062] Nθ y =M y π

[0063]

[0064] N max =(R pout π / R pin ) 2

[0065] R pin ≥(R beam +R hole )

[0066]

[0067] Where, N is the number of light reflections in the aspherical mirror multi-reflection pool 7, that is, the number of light spots formed on the surface of the second high-reflection mirror 72; M x M is the number of circles of the light spot along the x-axis of the second high-reflection mirror 72, y θ is the number of circles of the light spot along the y-axis of the second high-reflection mirror 72; x and θ y is the advancing angle of the light spot with adjacent reflection times in the x-axis direction and the y-axis direction of the mirror surface of the second high-reflection mirror 72. x 、M y}, N must be an even number and there must be no other common factors between the three numbers except 2. max is the theoretical maximum number of reflections. x is the curvature radius of the high reflective mirror along the x-axis, R y R is the curvature radius of the high reflective mirror on the y axis. beam is the laser radius, R hole is the radius of the central aperture of the first high-reflection mirror 71, R pout R is the maximum circle radius that the light spot can be distributed on the mirror surface, pin It is the minimum circle radius that the light spot can be distributed on the mirror surface.

[0068] Assuming that the working goal is to design an aspherical mirror multi-reflector with a base length of 65 cm and a mirror diameter of 8 cm, the N can be calculated according to the above formula. max =980, according to the design process and N and Mx 、M y Finally, we find {650, 324, 314} meets the conditions. At this time, the high reflective mirror parameter R x =657.9mm; R y =728.5mm; light incident angle x' = -2°; y' = 2°; the second high-reflection mirror rotates around the central axis by an angle θ = 2°; the final optical path is approximately 425m. Under the conditions of the same size spherical reflector and base length, the number of spots distributed on the traditional Herriott cell mirror surface is m = π*a / D size , D size is the diameter of the light spot on the surface of the plano-concave high-reflection mirror (spherical mirror) (1 / e of the peak light intensity 2 The calculated m = 54 indicates the optical path length of a conventional Herriott cell, with a being the spot size corresponding to the location of the beam, and a being the radius of the spot pattern. This ensures full utilization of the mirror surface without causing overlapping interference between the spots. The calculated m = 54 indicates that the optical path length of a conventional Herriott cell is approximately 35.5 m. The aspherical mirror multi-reflector cell of the present invention achieves an optical path length far exceeding that of a conventional Herriott cell, given the same mirror surface size and base length.

[0069] Figure 2 This is the Tracepro simulated optical path diagram of the aspheric mirror multi-reflector under this condition. Figure 3 This is the simulated distribution diagram of the mirror light spot at the output end of the aspherical mirror multi-reflector. Figure 3 The a in the figure is the simulated distribution diagram of the mirror spot at the output end of the aspherical mirror multi-reflector cell. As the curvature of the aspherical mirror is not rotationally symmetrical, if the mirror axes of the two mirrors are not aligned with the designed angle during installation, the mirror spot distribution at the output end of the multi-reflector cell will deviate from the designed one. Figure 3 Figure b is a simulated distribution diagram of the mirror light spot at the output end of the aspherical mirror multi-reflector when the second high-reflection mirror is rotated 2° around the central axis. At this time, the distribution and number of light spots change.

[0070] The mechanical structure of the aspherical mirror multi-reflection cell 7 is as follows Figure 4As shown, the first high reflective mirror 71 is installed on the integrated high reflective mirror mounting plate 79, and the second high reflective mirror 72 is installed on the high reflective mirror mounting groove 78, and the high reflective mirror mounting groove 78 is connected to the rotating platform 73, and the rotating platform 73 is connected to the multi-reflection pool sealing plate 77. The first high reflective mirror 71 and the second high reflective mirror 72 are all fixed with high reflective mirror pressure ring 76. In order to prevent that when the multi-reflection pool base on both sides will move or deform because of the pressure difference and thus affect the final optical path of the multi-reflection pool when low pressure is pumped out, the multi-reflection pool fixing rod 710 is used, which is respectively connected to the base on both sides of the multi-reflection pool. Through this design, the angle of the two aspherical mirrors relative to the mirror axis in the aspherical mirror multi-reflection pool 7 can be fine-tuned. The aspherical mirror multi-reflection pool 7 is also provided with an air inlet 75 and an air outlet 74, for passing the gas to be measured to the aspherical mirror multi-reflection pool 7. The aspherical mirror multi-reflection pool 7 body is a quartz glass tube 711, which is fixed by a glass tube pressure ring 712.

[0071] The output wavelength of the quantum cascade laser 2 may drift due to changes in external environmental factors such as temperature and drive circuits. Long-term wavelength drift will increase the error between the measured signal and the background signal. To calibrate the signals in both modes of the system, a reference cell 5 containing pure CH4 is placed in the optical path of the system. The internal pressure of the reference cell 5 is 0.04atm. The position deviation caused by wavelength drift is corrected by the detection signal of pure CH4 in the reference cell 5. Figure 5 It is a software-based adaptive flow chart. The signal of the laser passing through the reference cell 5 is regarded as the reference signal. In the background mode, the measured signal includes the reference gas signal and the background noise signal, and the second harmonic peak position of the reference signal in this mode is recorded. Subsequently, the measurement mode is switched to. At this time, the measured signal includes the gas signal to be measured, the reference signal and the background noise signal, and the second harmonic peak position of the reference signal in this mode is also recorded. With the background mode signal as the benchmark, the second harmonic position in the measurement mode is adjusted in real time in the software processing, and then the background mode signal is deducted, thereby suppressing the signal and background measurement errors caused by wavelength drift, completing the calibration of the laser output wavelength, and improving the detection sensitivity of the system. This method does not require additional circuits, simplifies the system and is conducive to cost control.

[0072] During operation, the laser control module 1 performs current modulation and temperature control on the quantum cascade laser 2, respectively, so that the absorption of HCHO and NH3 is included in the output range of the quantum cascade laser 2; by controlling the vacuum pump at the outlet end 74 of the aspherical mirror multi-reflector cell 7 and the mass flow meter at the inlet end 75 of the aspherical mirror multi-reflector cell 7, the low pressure inside the aspherical mirror multi-reflector cell 7 is controlled, so that the absorption lines of the two gases are separated, and the second harmonic signal is extracted through the lock-in amplifier 8.

[0073] In this embodiment, the central wavelength of the quantum cascade laser 2 is 5.682 μm, and the laser output wavelength range can be tuned by the laser driving module 1 so that the laser output range includes HCHO (1759.72 cm -1 )、NH3(1759.82cm -1 ) and CH4(1759.64cm -1 )Absorption of three gases.

[0074] Figure 6 The peak signals of HCHO alone and mixed gas under different HCHO concentration conditions are: Figure 7 The peak signals of a single NH3 gas and a mixed gas at different NH3 concentrations are shown in Figure 1. The difference between the peak signals of a single gas and a mixed gas at different concentrations is less than one thousandth, which is negligible. This indicates that the present invention eliminates interference when simultaneously measuring two gases. Figure 6 and Figure 7 The horizontal axis represents the concentration (volume fraction) of formaldehyde and ammonia, and the vertical axis represents the peak signal of the second harmonic.

[0075] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An adaptive high-sensitivity long-path laser absorption spectroscopy system, characterized in that: The system comprises a light source generating module for generating signal light, an aspherical mirror multi-reflecting pool (7) for multiple reflections of the signal light, and a signal processing module for collecting and analyzing the output light of the aspherical mirror multi-reflecting pool (7); The aspheric mirror multi-reflection pool (7) comprises a first high-reflection mirror (71), a second high-reflection mirror (72) and a rotating platform (73); the first high-reflection mirror (71) and the second high-reflection mirror (72) are plano-concave high-reflection mirrors with the same parameters; the concave surface is a reflecting surface, and the curvature radius in the x-axis direction and the y-axis direction of the mirror surface is different, and the concave surface is both facing the inner side of the aspheric mirror multi-reflection pool (7); the first high-reflection mirror (71) is provided with a hole for the incident and emitted signal light; the second high-reflection mirror (72) is installed on the rotating platform (73), and the rotating platform (73) is used to rotate the second high-reflection mirror (72) around a central axis; the central axis is a connecting axis between the center of the first high-reflection mirror (71) and the center of the second high-reflection mirror (72).

2. The adaptive high-sensitivity long-path laser absorption spectroscopy system according to claim 1, characterized in that: The system also includes a reference cell (5) containing pure CH4. The signal light generated by the light source generating module enters the aspherical mirror multi-reflection cell (7) after passing through the reference cell (5) for realizing signal calibration. When analyzing the measurement signal and the background signal, the CH4 absorption signal is regarded as the reference signal.

3. The adaptive high-sensitivity long-path laser absorption spectroscopy system according to claim 1, characterized in that: The signal light enters the aspherical mirror multi-reflection pool (7) through the hole of the first high-reflection mirror (71) at a certain incident angle, undergoes multiple reflections between the reflection surface of the second high-reflection mirror (72) and the reflection surface of the first high-reflection mirror (71), forms a periodic stable light field distribution in the aspherical mirror multi-reflection pool (7), and then exits from the hole of the first high-reflection mirror (71); The light transmission trajectory between the first high-reflection mirror (71) and the second high-reflection mirror (72) is determined by the following preset calculation formula: Wherein, ABCD matrix is the light transmission matrix; d is the distance between the first high reflective mirror (71) and the second high reflective mirror (72); R x is the curvature radius of the high reflective mirror along the x-axis; R y is the curvature radius of the high-reflectivity mirror on the y axis; R(R x 、R y ) is the mirror reflection matrix; D(d) is the free space propagation matrix; T(θ) is the rotation matrix of the second high reflective mirror (72) around the central axis; θ is the rotation angle of the second high reflective mirror (72) around the central axis; Z n is a matrix consisting of the coordinates and angles of the nth reflection of the light, n=1, 2, ..., N, N is the number of light spots formed on the surface of the second high-reflection mirror (72), that is, the number of light reflections in the aspherical mirror multi-reflection pool (7); x n ,y n is the position coordinate of the nth light spot, x n ',y n ' is the angle between the light and the x and y axes during the nth reflection.

4. The adaptive high-sensitivity long-path laser absorption spectroscopy system according to claim 3, characterized in that: The parameters of the aspherical mirror multi-reflector pool (7) are determined by the following preset calculation formula: Nth x =M x p Nth y =M y p N max =(R pout p / R pin ) 2 R pin ≥(R beam +R hole ) Where, N is the number of light reflections in the aspherical mirror multi-reflection pool (7), that is, the number of light spots formed on the surface of the second high-reflection mirror (72); M x M is the number of circles of the light spot along the x-axis of the second high-reflection mirror (72), y is the number of circles of the light spot along the y-axis of the mirror surface of the second high-reflection mirror (72); θ x and θ y is the advancing angle of the light spot with adjacent reflection times in the x-axis direction and the y-axis direction of the mirror surface of the second high-reflection mirror (72); x 、M y }, N must be an even number and there must be no other common factors between the three numbers except 2; N max is the theoretical maximum number of reflections; R x is the curvature radius of the high reflective mirror along the x-axis, R y is the curvature radius of the high-reflectivity mirror on the y axis; R beam is the laser radius; R hole is the hole radius in the first high reflective mirror (71); R pout R is the maximum circle radius of the light spot on the mirror surface; pin It is the minimum circle radius of the light spot distribution on the mirror surface.

5. The adaptive high-sensitivity long-path laser absorption spectroscopy system according to claim 2, characterized in that: The position deviation caused by wavelength drift is corrected by the CH4 absorption signal of the reference cell (5): The signal of the signal light passing through the reference cell (5) is regarded as a reference signal; In the background mode, the measured signals include the reference signal and the background noise signal, and the second harmonic peak position in the background mode is recorded as the background mode signal; In the measurement mode, the measured signals include the gas signal to be measured, the reference signal and the background noise signal, and the second harmonic peak position in the measurement mode is recorded; Based on the background mode signal, the second harmonic position in the measurement mode is adjusted in real time, and then the background mode signal is deducted to correct the position deviation caused by wavelength drift.

6. The adaptive high-sensitivity long-path laser absorption spectroscopy system according to claim 1, characterized in that: The hole of the aspherical mirror multi-reflection pool (7) is located at the center of the first high-reflection mirror (71).

7. The adaptive high-sensitivity long-path laser absorption spectroscopy system according to claim 1, characterized in that: In the aspheric mirror multi-reflection pool (7), a first high-reflection mirror (71) is mounted on an integrated high-reflection mirror mounting plate (79), a second high-reflection mirror (72) is mounted on a high-reflection mirror mounting groove (78), the high-reflection mirror mounting groove (78) is connected to a rotating platform (73), the rotating platform (73) is connected to a multi-reflection pool sealing plate (77), the first high-reflection mirror (71) and the second high-reflection mirror (72) are both fixed using a high-reflection mirror pressure ring (76); bases on both sides of the aspheric mirror multi-reflection pool (7) are fixed using a multi-reflection pool fixing rod (710); and an air inlet (75) and an air outlet (74) for the circulation of a gas to be measured are provided on the aspheric mirror multi-reflection pool (7).

8. The adaptive high-sensitivity long-path laser absorption spectroscopy system according to claim 1, characterized in that: The light source generation module comprises a laser control module (1), a quantum cascade laser (2), a focusing lens (3), a first plane reflector (4), and a second plane reflector (6), which are sequentially arranged along a signal transmission direction.

9. The adaptive high-sensitivity long-path laser absorption spectroscopy system according to claim 1, characterized in that: The signal processing module comprises a photodetector (8), a lock-in amplifier (9) and a processor (10) which are sequentially arranged along the signal transmission direction.

10. The adaptive high-sensitivity long-path laser absorption spectroscopy system according to claim 1, characterized in that: The laser control module (1) performs current modulation and temperature control on the quantum cascade laser (2) so that the absorption of the two gases HCHO and NH3 is included in the output range of the quantum cascade laser (2); the internal pressure of the aspherical mirror multi-reflector cell (7) is controlled by controlling the vacuum pump at the outlet end (74) of the aspherical mirror multi-reflector cell (7) and the mass flow meter at the inlet end (75) of the aspherical mirror multi-reflector cell (7), so that the absorption lines of the two gases are separated, and the second harmonic signal is extracted through the lock-in amplifier (8).

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