Open type multi-light-path absorption enhanced trace gas detection device and method

Abstract

The invention discloses an open type multi-light-path absorption enhanced trace gas detection device and method. The detection device comprises a signal generator, a tunable laser light source, an open type multi-light-path photoacoustic cell, a lock-in amplifier and a processor, wherein the open type multi-light-path photoacoustic cell consists of a gas absorption cavity and a detection cavity. Accurate matching of excitation light output by the tunable laser light source and a gas absorption line is realized through wavelength modulation, a gas absorption light path is prolonged by utilizing the reflecting mirror, a photoacoustic signal is enhanced, and the detection limit is further reduced. According to the invention, open excitation of sample gas is realized by using a separated structure, and absorbed laser excites carbon powder to generate a solid photoacoustic signal; and the phase-locked amplifier is used for demodulating, filtering and amplifying the solid photoacoustic signal by taking the frequency of the signal generator as a reference and transmitting the solid photoacoustic signal to the processor, so that the inversion of gas concentration information is realized.

Description

Technical Field

[0001] The present invention belongs to the field of substance detection technology, and further relates to an open multi-path absorption-enhanced trace gas detection device and method in the field of trace gas detection technology. The present invention can be used for high-sensitivity trace gas detection in environmental pollution, industrial process control, and hazardous environment alarms. Background Art

[0002] High-sensitivity trace gas detection can monitor leaks of flammable and toxic gases (such as methane and carbon monoxide) in industrial production in real time, preventing explosions and poisoning accidents. Detecting hypoxia or harmful gases during operations in confined spaces can prevent suffocation. In the environmental protection sector, analyzing atmospheric pollutants (such as sulfur dioxide and PM2.5) can aid pollution control. Furthermore, gas leak alarms and laboratory safety monitoring applications also rely on accurate gas detection. Existing gas detection technologies based on photoacoustic spectroscopy typically require modulating the excitation light source frequency to match the resonant frequency of the optical resonator to amplify the photoacoustic signal, thereby reducing the detectable gas concentration limit. In this method, the excitation light source directly passes through the resonator to achieve a single excitation of the gas. The microphone is built into the optical resonator. Long-term direct contact between some highly oxidizing and corrosive gas samples and the microphone can cause corrosion, affecting detection accuracy or even preventing gas detection. Furthermore, in some high-flow gas environments, the high flow noise can reduce the signal-to-noise ratio (SNR), ultimately affecting the detection limit.

[0003] Jiangsu Normal University has disclosed a device and method for calibrating effective optical path and gas concentration measurements in its patent application, "An Effective Optical Path and Gas Concentration Measurement Calibration Device and Application Method" (Application No. 202311157955.3, Publication No. CN 117147494 A). The device comprises a sealed gas detector with a dual-beam hybrid laser at its left end and a single-beam laser at its right end. The sealed gas detector is connected to an information analysis system. The sealed gas detector comprises a first gas absorption cell and a second gas absorption cell. The integrated absorption areas of the two absorption cells are compared to obtain the effective absorption optical path of the integrating cavity system, thereby performing real-time correction of gas concentration inversion. The process is simple; the invention patent performs real-time correction of gas concentration inversion by comparing the integrated absorption areas of the two absorption cells. However, this invention still has drawbacks: the use of a photodetector as the receiving end for signal acquisition results in a slow response speed. Furthermore, due to the selective absorption of gases by lasers, photodetectors with different wavelengths are required for light intensity detection, resulting in relatively high detection costs.

[0004] Zhejiang University's patent application, "Gas Detection Method and System Based on a Nonlinear Resonant Photoacoustic Cell and Optical Multipass," (Application No. 202410909592.2, Publication No. CN 118624534 A), discloses a gas detection method and system based on a nonlinear resonant photoacoustic cell and an optical multipass. This photoacoustic gas detection method utilizes a nonlinear resonant photoacoustic cell and a matching optical multipass structure. The nonlinear resonant photoacoustic cell contains a nonlinear resonant tube, and its shape is designed to closely match the beam shape within the optical multipass structure. The photoacoustic sensing system includes a laser controller, a laser source, a collimator, a nonlinear resonant photoacoustic cell, an optical multipass structure, a microphone, a custom-made pre-transimpedance amplifier circuit board, a lock-in amplifier, an acquisition card, and a host computer. The high-quality, shape-optimized nonlinear resonant photoacoustic cell amplifies the photoacoustic signal, and the optical multipass structure enhances the effective absorption path, further enhancing the photoacoustic signal. This system offers advantages such as low detection limit and high sensitivity. The system utilizes acoustic resonance to amplify photoacoustic signals by designing a hyperbolic cosine-shaped nonlinear resonant photoacoustic cell. However, a drawback remains: the system extends the optical path by adding an optical multi-pass structure at both ends of the resonant photoacoustic cell. During this process, the laser passes through the end windows of the resonant photoacoustic cell multiple times at specific angles, causing reflections on the window surfaces. This results in a loss of excitation light intensity, which in turn weakens the expected photoacoustic signal strength within the resonant photoacoustic cell.

[0005] South-Central University for Nationalities disclosed a resonant photoacoustic gas sensor and process based on a multi-optical path ellipsoidal cavity and secondary amplification in its patent application “An effective optical path and gas concentration measurement calibration device and application method” (application number 202411365439.4 application publication number CN 119290773A). This invention utilizes the geometric advantages of the ellipsoidal focal characteristics to achieve multiple reflections of the light path inside the ellipsoid and amplify the photoacoustic signal. However, the invention still has shortcomings in that the device uses two semi-ellipsoidal cavities to synthesize a complete cavity, which is difficult to process and difficult to accurately control the consistency between the two. The asymmetry of the ellipsoidal cavity will affect the device's resonant amplification of the photoacoustic signal, thereby reducing the minimum detection limit of the gas. In addition, the device extends the light path by coating the inner wall of the cavity, and the coating process also directly affects the light path reflection efficiency and light intensity. Summary of the Invention

[0006] The purpose of the present invention is to address the shortcomings of the above-mentioned existing technologies and propose an open multi-path absorption-enhanced trace gas detection device and method, aiming to solve the problems of possible damage to the microphone when measuring corrosive gases in existing photoacoustic spectroscopy gas detection devices in actual applications, large noise at high gas flow rates, and limitations in the selection of photoelectric detector bands in traditional methods.

[0007] The technical idea for achieving the purpose of the present invention is to design an open multi-path absorption-enhanced trace gas detection device, which is mainly composed of a signal generator, a tunable laser light source, an open multi-path photoacoustic cell, a phase-locked amplifier and a processor. Among them, the open multi-path photoacoustic cell is the primary component for realizing the function of this device. The open multi-path photoacoustic cell is used for trace gas detection. The open multi-path photoacoustic cell consists of a gas absorption chamber and a detection chamber. The gas absorption chamber is realized by a first mirror frame, a second mirror frame, a third mirror frame, a first reflector, a second reflector and four support rods and cap nuts at the four corners of the mirror frame at both ends. Buffer pads are provided between the first mirror frame and the second mirror frame, and between the third mirror frame and the detection chamber shell to prevent the reflector from breaking. The detection chamber shell is the main component of the detection chamber. A window made of single crystal infrared calcium fluoride glass is horizontally embedded in the left side of the detection chamber shell to form a sealing structure with the carbon powder chamber. The absorption light path reflected by the reflector one and the reflector two finally enters the carbon powder chamber through the window through the off-axis light hole two. A microphone is vertically embedded in the top of the detection chamber shell. The microphone is connected to the carbon powder chamber through the sound hole. A rubber sealing ring is provided on the contact surface between the carbon powder chamber and the detection chamber shell to ensure sealing. The carbon powder chamber and the detection chamber shell are fixed by fixing bolts. The detection method of the present invention utilizes a signal generator to achieve precise matching of the excitation light output by a tunable laser light source with the gas absorption line through wavelength modulation. The signal generator also provides a modulation reference frequency for a lock-in amplifier. The detection method of the present invention utilizes a lock-in amplifier to process the solid-state photoacoustic signals received by a microphone within the detection chamber of an open multi-path photoacoustic cell and transmit them to a processor, thereby inverting information about the gas type and concentration.

[0008] The technical solution for achieving the purpose of the present invention is:

[0009] The trace gas detection device of the present invention includes a signal generator, a tunable laser light source, an open multi-path photoacoustic cell, a lock-in amplifier and a processor; the first output end of the signal generator is connected to the input end of the tunable laser light source; the tunable laser light source injects excitation light into the open multi-path photoacoustic cell to generate a solid photoacoustic signal, and the solid photoacoustic signal is output to the first input end of the lock-in amplifier; the second input end of the lock-in amplifier is connected to the second output end of the signal generator, and the output end of the lock-in amplifier is connected to the processor; the open multi-path photoacoustic cell is composed of a gas absorption cavity and a detection cavity connected together.

[0010] Furthermore, the gas absorption chamber is composed of a first mirror frame, a first reflector, a second mirror frame, a support rod, a third mirror frame and a second reflector; the first mirror frame, the first reflector, the second mirror frame, the third mirror frame and the second reflector are coaxially arranged; the first reflector is located between the first mirror frame and the second mirror frame.

[0011] Furthermore, the support rod is located between the second mirror frame and the third mirror frame, and is formed into a cage structure by four support rods; each support rod is a cylinder that is thick in the middle and thin at both ends, and is threaded; the left end of each support rod passes through the first mirror frame and is fixed with a cap nut at the end, and its right end passes through the third mirror frame and is fixed to the detection chamber housing by threads.

[0012] Furthermore, a first buffer pad and a second buffer pad are respectively tightly attached between the first reflector, the first mirror frame and the second mirror frame to prevent the pressure at the contact surface from crushing the reflector; the first buffer pad is attached to the first mirror frame, and the second buffer pad is embedded in the second mirror frame.

[0013] Furthermore, a third buffer pad and a fourth buffer pad are respectively tightly attached between the contact surfaces of the second reflector, the third mirror frame and the detection chamber shell to prevent the pressure at the contact surface from crushing the reflector; the third buffer pad is embedded in the third mirror frame, and the fourth buffer pad is embedded in the detection chamber shell.

[0014] Furthermore, the first reflector and the second reflector are respectively provided with a first off-axis light-through hole and a second off-axis light-through hole; the edges of the first reflector and the second reflector are both flat, and the centers are both concave; the first reflector and the second reflector are both made of BK7 glass, and the concave surfaces of the first reflector and the second reflector are both coated with a high-reflectivity silver film.

[0015] Furthermore, the first off-axis light-through hole and the second off-axis light-through hole are coaxially arranged with the window and the carbon powder cavity. Carbon powder with a particle diameter of micrometer level is punched into the carbon powder cavity. The contact surface between the carbon powder cavity and the detection cavity shell is provided with a rubber sealing ring to ensure sealing. The window is single crystal calcium fluoride glass in the infrared band.

[0016] Furthermore, the detection chamber is composed of a detection chamber shell, a fixing bolt, a carbon powder chamber and a window; the microphone is vertically embedded in the detection chamber shell and connected to the carbon powder chamber through a sound hole; the window is parallelly embedded in the detection chamber shell and forms a sealing structure with the carbon powder chamber, and the fixing bolt is connected to the detection chamber shell through a thread and is close to the back of the carbon powder chamber.

[0017] The steps of an open multi-path absorption-enhanced trace gas detection method of the present invention are as follows:

[0018] Step 1: Excitation light is incident on the gas absorption cavity of the open multi-path photoacoustic cell at a specific angle, and the light path of the gas absorption is extended by light reflection between the reflectors in the gas absorption cavity;

[0019] Step 2: The gas to be tested enters the gas absorption chamber in a free diffusion manner;

[0020] Step 3: The excitation light fully absorbed by the gas to be measured is incident on the carbon powder in the detection cavity through the second off-axis light hole on the second reflector, thereby exciting the carbon powder to generate a solid photoacoustic signal;

[0021] Step 4: The microphone converts the collected solid-state photoacoustic signal into an electrical signal and transmits it to a lock-in amplifier. The lock-in amplifier filters, demodulates, and amplifies the solid-state photoacoustic signal based on the reference frequency output by the signal generator.

[0022] In step 5, the processor performs gas concentration inversion.

[0023] Furthermore, the tunable laser light source sinusoidally modulated by the signal generator has an output wavelength that matches the absorption line of the gas to be measured.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] First, the present invention designs an open multi-path absorption-enhanced trace gas detection device, primarily composed of a signal generator, a tunable laser light source, an open multi-path photoacoustic cell, a lock-in amplifier, and a processor. This overcomes the issues of existing gas detection devices and methods, such as cumbersome detection procedures, susceptibility to airflow velocity interference, and corrosion. This results in a longer service life and greater detection stability for the device. It enables highly sensitive detection of corrosive trace gases in open air environments.

[0026] Second, the detection device of the present invention uses a reflector to extend the gas absorption light path, thereby enhancing the photoacoustic signal and reducing the detection limit; the laser light after absorbing the gas enters the carbon powder cavity through the second off-axis light hole and the window to excite the carbon powder to generate a solid photoacoustic signal, indirectly inverting the gas concentration; since the carbon powder has full-spectrum absorption characteristics, it can absorb laser light of any wavelength band, which increases the selectivity of the detection device, has low economic cost, and broadens the application range of the detection device of the present invention.

[0027] Third, the method of the present invention designs an open multi-path photoacoustic cell, and the gas sample enters the gas absorption cavity by free diffusion to absorb the excitation light source, eliminating the sampling step of using an air pump to extract gas in the traditional method, so that the present invention shortens the time required for detection.

[0028] Fourth, in the method of the present invention, the microphone does not need to directly contact the sample, which reduces the risk of microphone corrosion and reduces the impact of airflow fluctuations on detection accuracy; allowing the method of the present invention to achieve high-sensitivity detection of corrosive trace gases in an open gas environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the open multi-light path absorption-enhanced trace gas detection device of the present invention;

[0030] Figure 2 Schematic diagram of the structure of the open multi-path photoacoustic cell in the device of the present invention;

[0031] Figure 3 is a cross-sectional view of an open multi-path photoacoustic cell in the device of the present invention;

[0032] Figure 4 Schematic diagram of the structure of the carbon powder cavity in the open multi-path photoacoustic cell in the device of the present invention;

[0033] Figure 5 This is a diagram of the light path simulation model of the open multi-path photoacoustic cell in the device of the present invention;

[0034] Figure 6 This is a light spot distribution diagram of the reflector in the light path simulation model of the open multi-path photoacoustic cell of the device of the present invention;

[0035] Figure 7 The photoacoustic signal fluctuation diagram under different gas flow rates measured by the method of the present invention;

[0036] Figure 8 This is a diagram showing the calibration results of acetylene concentration detected by the trace gas detection device with open multi-light path absorption enhancement in the method of the present invention. DETAILED DESCRIPTION

[0037] The technical solutions of the present invention are further described in detail below with reference to the accompanying drawings and specific embodiments. The described specific embodiments are only used to explain the present invention, but the present invention is not limited to the following technical solutions.

[0038] like Figure 1-Figure 3 As shown, an open multi-path absorption-enhanced trace gas detection device includes a signal generator 1, a tunable laser light source 2, an open multi-path photoacoustic cell 3, a lock-in amplifier 4, and a processor 5. The first output end of the signal generator 1 is connected to the input end of the tunable laser light source 2.

[0039] The tunable laser light source 2 injects excitation light into the open multi-path photoacoustic cell 3 to generate a solid photoacoustic signal, and the solid photoacoustic signal is output to the first input end of the lock-in amplifier 4 .

[0040] The second input end of the lock-in amplifier 4 is connected to the second output end of the signal generator 1 , and the output end of the lock-in amplifier 4 is connected to the processor 5 ; the open multi-path photoacoustic cell 3 is composed of a gas absorption chamber 31 and a detection chamber 32 connected together.

[0041] The gas absorption chamber 31 is composed of a first mirror frame 310, a first reflector 314, a second mirror frame 311, a support rod 312, a third mirror frame 316 and a second reflector 328; the first mirror frame 310, the first reflector 314, the second mirror frame 311, the third mirror frame 316 and the second reflector 328 are coaxially arranged; the first reflector 314 is located between the first mirror frame 310 and the second mirror frame 311.

[0042] The support rod 312 is located between the second mirror frame 311 and the third mirror frame 316, and forms a cage structure with four support rods; each support rod is a cylinder with a thick middle and thin ends and is threaded; the left end of each support rod passes through the first mirror frame 310 and is fixed with a cap nut 315 at the end, and its right end passes through the third mirror frame 316 and is fixed to the detection chamber housing 320 by threads.

[0043] A first buffer pad 310a and a second buffer pad 311a are respectively tightly attached between the first reflector 314, the first mirror frame 310 and the second mirror frame 311 to prevent the pressure at the contact surface from crushing the reflector; the first buffer pad 310a is attached to the first mirror frame 310, and the second buffer pad 311a is embedded in the second mirror frame 311.

[0044] A third cushion 316a and a fourth cushion 320a are respectively positioned between the contact surfaces of the second reflector 328, the third mirror holder 316, and the detection chamber housing 320 to prevent the reflector from being crushed by pressure at the contact surfaces. The third cushion 316a is embedded in the third mirror holder 316, and the fourth cushion 320a is embedded in the detection chamber housing 320.

[0045] The first reflector 314 and the second reflector 328 are respectively provided with a first off-axis light-through hole 313 and a second off-axis light-through hole 327. The edges of the first reflector 314 and the second reflector 328 are both flat and the centers are both concave. Both the first reflector 314 and the second reflector 328 are made of BK7 glass, and the concave surfaces of both the first reflector 314 and the second reflector 328 are coated with a high-reflectivity silver film.

[0046] The first off-axis light-through hole 313 and the second off-axis light-through hole 327 are coaxially arranged with the window 326 and the carbon powder cavity 324 . The window 326 is made of single-crystal calcium fluoride glass in the infrared band.

[0047] The detection chamber 32 is composed of a detection chamber shell 320, a microphone 321, a fixing bolt 323, a carbon powder chamber 324 and a window 325; the microphone 321 is vertically embedded in the detection chamber shell 320 and is connected to the carbon powder chamber 324 through the sound hole 322; the window 325 is parallelly embedded in the detection chamber shell 320 and forms a sealing structure with the carbon powder chamber 324; the fixing bolt 323 is connected to the detection chamber shell 320 through a thread and is close to the back of the carbon powder chamber 324.

[0048] like Figure 3 、 Figure 4 As shown, carbon powder 325 with a particle diameter of micrometer level is punched into the carbon powder cavity 324, and a rubber sealing ring 324 (a) is provided on the contact surface between the carbon powder cavity 324 and the detection cavity housing 320 to ensure sealing.

[0049] The working principle of the present invention is as follows: Figures 1 to 4 Figure 2 shows the basic structure of an open-type trace gas detection device with multi-path absorption enhancement according to the present invention, comprising a signal generator 1, a tunable laser light source 2, an open-type multi-path photoacoustic cell 3, a lock-in amplifier 4, and a processor 5. The signal generator 1 transmits a modulated signal to the tunable laser light source 2 via a BNC cable, so that the central wavelength of the output modulated excitation light matches the absorption peak of the gas to be detected. The excitation light is then incident on the open-type multi-path photoacoustic cell 3 to achieve gas absorption and solid-state photoacoustic signal excitation and detection.

[0050] Reference Figure 3 、 Figure 4 、 Figure 5 , the working principle of the open multi-path photoacoustic cell in the device of the present invention is further described.

[0051] The excitation light enters the gas absorption cavity 31 through the first off-axis light hole 313 of the first reflector 314, and the excitation light is reflected back and forth between the coaxially arranged second reflector 328 and the first reflector 314, and finally exits from the second off-axis light hole 327. Figure 3 As shown. The emitted excitation light is fully absorbed by the sample gas and carries the concentration information of the sample gas. The emitted excitation light enters the carbon powder cavity 324 through the window 326 to excite the carbon powder 325 to generate a solid photoacoustic signal, as shown Figure 4 As shown. The solid photoacoustic signal is detected by the microphone 321 through the sound hole 322 and transmitted to the lock-in amplifier 4 in the form of a voltage signal. The lock-in amplifier 4 filters, demodulates, and amplifies the voltage signal based on the reference signal input by the signal generator 1. The processor 5 inverts the filtered, demodulated, and amplified voltage signal to detect the gas, as shown in FIG. Figure 5 shown.

[0052] Reference Figure 6, a further description is given of the process of detecting spot distribution of the reflector in the optical path simulation model of the open multi-path photoacoustic cell in the device of the present invention.

[0053] Using TracePro 73 optical simulation software, Figure 5 The irradiance analysis is performed by simulating the gas absorption optical path of the open multi-path photoacoustic cell established in the embodiment, and the spot distribution of the concave surface of the reflector 1 314 and the reflector 2 328 can be calculated, as shown in FIG. Figure 6 (a) and Figure 6 As shown in (b), the first reflector 314 and the second reflector 328 each have 48 irradiance sampling points, that is, the open multi-path photoacoustic cell 3 in the device of the present invention can achieve 48×2 absorption light path reflections, which greatly increases the absorption light path length of the gas to the excitation light source.

[0054] Reference Figure 7 , further describes the method of the present invention used in Example 1 of the present invention to detect photoacoustic signal fluctuations at different gas flow rates.

[0055] Figure 7 The horizontal axis represents different gas flow rates in sccm (ml / min), and the vertical axis represents the corresponding photoacoustic signal.

[0056] Using 100 ppm (parts per million) of acetylene as the sample gas, the photoacoustic signal amplitude was measured at gas flow rates ranging from 0 to 1000 sccm, with an average value of 1.637 mV. The photoacoustic signal amplitude fluctuation caused by changes in gas flow rate measured in the detection chamber 32 was calculated using the following formula:

[0057]

[0058] Where N is the number of samples, x i is the sample point, and μ is the sample mean.

[0059] According to equation (1), the fluctuation in the amplitude of the photoacoustic signal caused by the change in gas flow rate measured by the detection chamber 32 is only 0.71‰, thanks to the design of the separate gas absorption chamber 31 and detection chamber 32. This proves that the present invention can resist the interference of airflow noise, exhibits good robustness under high gas flow rates, and can be applied to detection in dynamic airflow environments.

[0060] Reference Figure 8 , further describes the calibration results of acetylene concentration detected by the trace gas detection device with open multi-light path absorption enhancement in Example 2 of the present invention.

[0061] Figure 8The horizontal axis is the sample gas concentration, and the vertical axis is the corresponding photoacoustic signal amplitude. Through the open multi-path absorption enhanced trace gas detection device of the present invention, the photoacoustic signals of acetylene samples at concentrations of 5ppm, 20ppm, 60ppm, 100ppm, 300ppm, 600ppm, and 1000ppm were tested and linear fitting calibration was performed. The fitting function is shown in formula (2). From the fitting function in formula (2), it can be seen that the detection sensitivity of the present invention is 0.013mV / ppm, and the linear fitting goodness R 2 The linearity is 0.99949, which is excellent and can achieve accurate detection of gases with different concentrations.

[0062] y=0.013x+0.2171 (2)

[0063] Where x represents the gas concentration and y represents the photoacoustic signal amplitude.

[0064] The above embodiments are merely detailed descriptions of the present invention, but the present invention is not limited to the above embodiments. Any modifications, replacements and changes made to the present invention within the spirit of the present invention and the scope of protection of the claims are within the scope of protection of the present invention.

Claims

1. An open multi-path absorption-enhanced trace gas detection device, characterized in that: The detection device comprises a signal generator (1), a tunable laser light source (2), an open multi-path photoacoustic cell (3), a lock-in amplifier (4) and a processor (5); the first output end of the signal generator (1) is connected to the input end of the tunable laser light source (2); the tunable laser light source (2) injects excitation light into the open multi-path photoacoustic cell (3) to generate a solid photoacoustic signal, and the solid photoacoustic signal is output to the first input end of the lock-in amplifier (4); the second input end of the lock-in amplifier (4) is connected to the second output end of the signal generator (1), and the output end of the lock-in amplifier (4) is connected to the processor (5); the open multi-path photoacoustic cell (3) is composed of a gas absorption cavity (31) and a detection cavity (32) connected together.

2. The detection device according to claim 1, characterized in that The gas absorption chamber (31) is composed of a first mirror frame (310), a first reflector (314), a second mirror frame (311), a support rod (312), a third mirror frame (316) and a second reflector (328); the first mirror frame (310), the first reflector (314), the second mirror frame (311), the third mirror frame (316) and the second reflector (328) are coaxially arranged; and the first reflector (314) is located between the first mirror frame (310) and the second mirror frame (311).

3. The detection device according to claim 2, characterized in that The support rod (312) is located between the second mirror frame (311) and the third mirror frame (316), and a cage structure is formed by four support rods; each support rod is a cylindrical body with a thick middle and thin ends and tapped with threads; the left end of each support rod passes through the first mirror frame (310) and is fixed with a cap nut (315) at the end, and the right end passes through the third mirror frame (316) and is fixed to the detection chamber housing (320) by means of threads.

4. The detection device according to claim 2, characterized in that A first buffer pad (310a) and a second buffer pad (311a) are respectively closely attached between the first reflector (314), the first mirror frame (310) and the second mirror frame (311), for preventing the reflector from being crushed by pressure at the contact surface; the first buffer pad (310a) is attached to the first mirror frame (310), and the second buffer pad (311a) is embedded in the second mirror frame (311).

5. The detection device according to claim 2, characterized in that A third buffer pad (316a) and a fourth buffer pad (320a) are respectively closely attached between the contact surfaces of the second reflector (328), the third mirror frame (316) and the detection chamber housing (320), so as to prevent the pressure at the contact surfaces from crushing the reflector; the third buffer pad (316a) is embedded in the third mirror frame (316), and the fourth buffer pad (320a) is embedded in the detection chamber housing (320).

6. The detection device according to claim 2, characterized in that The first reflector (314) and the second reflector (328) are respectively provided with a first off-axis light-through hole (313) and a second off-axis light-through hole (327); the edges of the first reflector (314) and the second reflector (328) are both plane, and the centers are both concave; the first reflector (314) and the second reflector (328) are both made of BK7 glass, and the concave surfaces of the first reflector (314) and the second reflector (328) are both coated with a high-reflectivity silver film.

7. The detection device according to claim 6, characterized in that The first off-axis light-through hole (313), the second off-axis light-through hole (327), the window (326), and the carbon powder cavity (324) are coaxially arranged. Carbon powder (325) with a particle diameter of micrometer level is punched into the carbon powder cavity (324). A rubber sealing ring (324a) is provided on the contact surface between the carbon powder cavity (324) and the detection cavity housing (320) to ensure sealing. The window (326) is single crystal calcium fluoride glass in the infrared band.

8. The detection device according to claim 1, characterized in that The detection chamber (32) is composed of a detection chamber housing (320), a microphone (321), a fixing bolt (323), a carbon powder chamber (324) and a window (325); the microphone (321) is vertically embedded in the detection chamber housing (320) and communicates with the carbon powder chamber (324) through a sound hole (322); the window (325) is parallely embedded in the detection chamber housing (320) and forms a sealing structure with the carbon powder chamber (324); the fixing bolt (323) is connected to the detection chamber housing (320) through a thread and is in close contact with the back of the carbon powder chamber (324).

9. An open multi-path absorption-enhanced trace gas detection method according to the detection device of claims 1-8, characterized in that: The trace gas detection method is as follows: Step 1: Excitation light is incident on the gas absorption cavity of the open multi-path photoacoustic cell at a specific angle, and the light path of the gas absorption is extended by light reflection between the reflectors in the gas absorption cavity; Step 2: The gas to be tested enters the gas absorption chamber in a free diffusion manner; Step 3: The excitation light fully absorbed by the gas to be measured is incident on the carbon powder in the detection cavity through the second off-axis light hole on the second reflector, thereby exciting the carbon powder to generate a solid photoacoustic signal; Step 4: The microphone converts the collected solid-state photoacoustic signal into an electrical signal and transmits it to a lock-in amplifier. The lock-in amplifier filters, demodulates, and amplifies the solid-state photoacoustic signal based on the reference frequency output by the signal generator. In step 5, the processor performs gas concentration inversion.

10. The detection method according to claim 9, characterized in that: The tunable laser light source (2) sinusoidally modulated by a signal generator (1) has an output wavelength that matches the absorption line of the gas to be measured.

Images