Trace gas detection device based on micro differential column-cone complex photoacoustic cell

Abstract

The invention discloses a trace gas detection device based on a micro differential column-cone complex photoacoustic cell, and relates to the technical field of photoacoustic spectrometry and trace gas detection. The device comprises a photoacoustic signal detection part and a data processing display part, wherein the photoacoustic signal detection part comprises a laser modulation unit, a distributed feedback semiconductor laser, an optical fiber collimator, a micro differential cylindrical cone complex photoacoustic cell and an MEMS microphone which are connected in sequence; the data processing and displaying part comprises a pre-voltage amplifier, a phase-locked amplification module, an analog-to-digital converter and an upper computer which are connected in sequence; to-be-detected gas is communicated with the micro differential column-cone complex photoacoustic cell; and the MEMS microphone is communicated with the pre-voltage amplifier. According to the invention, the column-cone complex structure of the photoacoustic cell enables resonance amplification of photoacoustic signals and suppresses common-mode noise at the same time, so that the signal-to-noise ratio is improved; the system is compact in overall structure, and a solution is provided for trace gas detection in a miniaturized application scene.

Description

Technical Field

[0001] The present invention relates to the technical field of photoacoustic spectroscopy and trace gas detection, in particular to a trace gas detection device based on a micro differential rod-cone composite photoacoustic cell. Background Art

[0002] In recent years, photoacoustic spectroscopy has demonstrated significant advantages in trace gas detection and has been widely used in environmental pollutant monitoring, industrial process control, and medical breath analysis. Its non-contact, interference-resistant, and real-time online monitoring capabilities offer innovative solutions for gas analysis under complex working conditions. When gas molecules absorb modulated light energy of a specific wavelength, the excited molecules convert the light energy into heat through a nonradiative relaxation process, causing cyclical local temperature fluctuations and generating an acoustic pressure signal within the photoacoustic cell. This acoustic pressure signal is captured using a highly sensitive acoustic sensor, and its intensity is positively correlated with the target gas concentration. Compared to traditional gas detection techniques, photoacoustic spectroscopy requires no complex sample pretreatment and offers inherently high selectivity. By matching a tunable laser with the characteristic absorption lines of a gas, it can accurately identify multi-component gases. With the continuous development and advancement of science and technology, the application of gas detection is expanding from the laboratory to complex industrial sites, multi-point environmental monitoring networks, and personal health management. Therefore, the integration of various modules in a photoacoustic spectroscopy detection device is not only an inevitable trend in technological development but also a key path to breaking through barriers to industry application.

[0003] In the photoacoustic spectroscopy gas detection system, the photoacoustic cell serves as the core sensing unit, and its design and performance directly affect the detection sensitivity, signal-to-noise ratio, and system stability. The interior of the photoacoustic cell can provide a closed physical space for the light-heat-acoustic energy conversion process, and achieve efficient coupling and amplification of the signal by optimizing the structural design, optical and acoustic properties. Structurally, the photoacoustic cell is usually designed as a cylindrical or rectangular cavity, and the acoustic resonant cavity enhances the sound pressure signal. For example, the resonant photoacoustic cell can significantly increase the amplitude of the sound pressure signal and reduce the detection limit by matching the modulated laser frequency with the inherent acoustic resonant frequency of the cavity.

[0004] Currently, the most common types of resonant photoacoustic cell structures are H-type and T-type. The H-type resonant photoacoustic cell consists of two symmetrical buffer chambers and a resonant cavity. This design makes the manufacturing process relatively simple. However, due to its structural characteristics, the H-type photoacoustic cell is larger in size, resulting in a longer time for gas diffusion, which affects the gas response speed. This delay may directly affect the timeliness of gas detection, especially in scenarios requiring real-time monitoring or rapid dynamic analysis. The T-type resonant photoacoustic cell consists of only one buffer chamber and a resonant cavity. Although its volume is smaller than the H-type, its resonant frequency is lower, which may lead to insufficient response sensitivity to certain specific gases. Therefore, designing a photoacoustic cell with a compact structure, high signal-to-noise ratio, and sensitive response is a key step in achieving the integration of the entire detection device.

[0005] In addition to the photoacoustic cell, other modules of a gas detection device play an irreplaceable role in the complete system, and their performance significantly impacts the overall detection performance. While traditional split-type designs can independently optimize the performance of each module, they also suffer from bulky size, high power consumption, and poor environmental adaptability, making them difficult to meet the detection needs of complex scenarios.

[0006] In summary, miniaturization has become an inevitable trend in the development of photoacoustic spectroscopy gas detection applications. Therefore, the development of compact photoacoustic cells with a high signal-to-noise ratio, sensitive response, and high-performance compact gas detection devices is of great practical significance. Summary of the Invention

[0007] In order to overcome the problem of difficult balance in volume, signal-to-noise ratio, sensitivity and response speed of a resonant photoacoustic cell, the purpose of the present invention is to provide a trace gas detection device based on a micro differential rod-cone composite photoacoustic cell.

[0008] The internal volume of the micro differential rod-cone composite photoacoustic cell of the present invention is only about 6200mm 3 The cone tips at both ends amplify the photoacoustic signal, and the differential characteristics of the sound pressure distribution can be further enhanced through subsequent differential subtraction, while suppressing common-mode noise and improving the signal-to-noise ratio. In addition to the design of the photoacoustic cell, the various components and modules used in the entire detection device have been made more compact without sacrificing performance, which is more conducive to the integration of the entire detection system.

[0009] The present invention provides a new idea for the design and production of resonant photoacoustic sensors and the integration of the entire gas detection system, and has great application prospects in the field of photoacoustic spectroscopy trace gas detection.

[0010] To achieve the above object, the technical solution of the present invention is:

[0011] 1. Trace Gas Detection Device Using a Micro-Differential Rod-Cone Composite Photoacoustic Cell

[0012] The device includes a photoacoustic signal detection component and a data processing and display component, wherein:

[0013] The photoacoustic signal detection component includes a laser modulation unit, a distributed feedback semiconductor laser, a fiber collimator, a micro differential rod-cone complex photoacoustic cell and a MEMS microphone connected in sequence;

[0014] The data processing and display component includes a pre-voltage amplifier, a phase-locked amplifier module, an analog-to-digital converter and a host computer connected in sequence;

[0015] The gas to be measured is connected to the micro differential rod-cone complex photoacoustic cell;

[0016] The MEMS microphone is connected to the pre-voltage amplifier.

[0017] 2. A micro differential rod-cone complex photoacoustic cell

[0018] The photoacoustic cell comprises a laser incident port, a positive cavity, an air inlet valve, a connecting pipe, an air outlet valve and a negative cavity;

[0019] Its location and connection relationship are:

[0020] The positive cavity and the negative cavity are symmetrically arranged on both sides, connected by a connecting pipe in the middle. An air inlet valve is arranged on the upper part of the positive cavity, an air outlet valve is arranged on the upper part of the negative cavity, and a laser incident port is arranged at the bottom of the positive cavity.

[0021] The main body of the photoacoustic cell consists of three parts: a positive electrode cavity, a negative electrode cavity and a connecting tube. The positive and negative electrode cavities are exactly the same in shape and size, and the two cavities are connected in the middle by a connecting tube; an air inlet valve and a laser incident hole are provided on the side wall of the positive electrode cavity column, and an air outlet valve is provided on the side wall of the negative electrode cavity; the positive and negative electrode cavities are composed of a cylindrical cavity and a conical cavity. The gas 0 to be measured enters from the air inlet valve provided on the side wall of the cylindrical cavity of the positive electrode cavity, and the modulated laser light source enters from the laser incident port provided on the side wall of the positive electrode cavity and is diffusely reflected on the cylindrical side wall of the positive electrode cavity. The laser interacts with the target gas to be measured, and after the gas 0 to be measured absorbs the laser energy, it triggers the resonance of the column-cone composite photoacoustic cell to generate a photoacoustic signal, and the distribution of the resonant sound pressure signal intensity presents a differential characteristic and is respectively concentrated at the cone tips of the positive and negative electrode cavities. This resonance amplification ability of the sound pressure signal and the distribution characteristic of the sound pressure signal difference enable the photoacoustic cell to have the advantages of fast response speed, high signal-to-noise ratio and strong stability even when the volume is extremely small.

[0022] 3. Trace Gas Detection Method Using a Micro-Differential Rod-Cone Composite Photoacoustic Cell

[0023] This method comprises the following steps:

[0024] ① The laser modulation unit in the photoacoustic signal detection component controls the distributed feedback semiconductor laser to output laser light of a specific wavelength and power, and uses the control circuit inside the unit to provide low-frequency sawtooth waves and high-frequency sine waves to modulate the output laser light. The modulated laser light enters the positive cavity of the micro differential rod-cone complex photoacoustic cell through the fiber collimator;

[0025] ② Open the inlet valve at the positive cavity of the photoacoustic cell, close the outlet valve at the negative cavity, and slowly introduce the target gas to be measured into the photoacoustic cell. The laser and gas interact in the photoacoustic cell to stimulate the photoacoustic effect. Due to the unique design of the present invention, the maximum value of the generated photoacoustic signal is concentrated at the two cone tips of the positive and negative cavities, respectively, and exhibits a differential characteristic with the same amplitude and opposite polarity. Two MEMS microphones are used to collect the sound pressure signals at the two cone tips.

[0026] ③ The two sound pressure signals collected by the MEMS electrical microphone are sequentially passed through the pre-voltage amplifier in the data processing and display unit and a small phase-locked amplifier module with differential input characteristics to extract the second harmonic signal. The extracted harmonic signal enters the analog-to-digital converter and is converted into a digital signal that can be processed by the host computer;

[0027] ④ The host computer of the data processing and display component collects the digital signal, inverts the concentration of the gas to be tested and displays the result of the gas detection concentration; finally, the outlet valve is opened to exhaust the gas in the photoacoustic cell to facilitate the next detection.

[0028] Compared with the prior art, the present invention has the following advantages and positive effects:

[0029] ① The present invention abandons the light source coupling method used in traditional resonant photoacoustic cell designs and confines the laser to diffuse reflection within the positive cavity, thereby making the sound pressure signal distribution of the entire photoacoustic cell exhibit differential characteristics. The subsequent differential subtraction process can effectively suppress common-mode noise and improve the signal-to-noise ratio of the entire detection device.

[0030] ② The positive and negative cavities are composed of a cylinder and a cone. The presence of the cone can concentrate and amplify the photoacoustic signal, resulting in a high quality factor. Subsequently, two MEMS microphones are used to extract the sound pressure signals at the tips of the cones at both ends of the positive and negative cavities, solving the problem of low detection limit of traditional photoacoustic cells.

[0031] ③The volume of the entire photoacoustic pool is about 6200mm 3 , greatly reducing the gas consumption during gas detection and improving the response speed of the system. Its compact size makes it more suitable for deployment in a small space, facilitating the integration of the entire detection device.

[0032] ④ In addition to the design of the micro-photoacoustic cell, the selection of MEMS microphones, analog phase-locked amplifier modules, distributed feedback semiconductor lasers, preamplifiers and other modules used in the entire device ensures high performance while taking into account the module volume, making the entire device more compact and able to achieve real-time detection of gas concentration.

[0033] In summary, the cylindrical-cone composite structure of the photoacoustic cell in this invention resonates and amplifies the photoacoustic signal while suppressing common-mode noise, improving the signal-to-noise ratio. Furthermore, the overall system is compact, providing a solution for trace gas detection in miniaturized applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a structural diagram of the device, in which:

[0035] A—photoacoustic signal detection component; B—data processing and display component;

[0036] 0—gas to be tested;

[0037] 1—Laser modulation unit;

[0038] 2—Distributed feedback semiconductor laser (referred to as laser);

[0039] 3—Fiber collimator,

[0040] 4—Micro differential rod-cone complex photoacoustic cell (photoacoustic cell for short),

[0041] 5—MEMS microphone,

[0042] 6—Pre-voltage amplifier,

[0043] 7—Phase-locked amplifier module,

[0044] 8—Analog-to-digital converter,

[0045] 9—Upper computer.

[0046] Figure 2 Schematic diagram of the structure of a micro differential rod-cone complex photoacoustic cell 4, in which:

[0047] 41—laser incident port;

[0048] 42—positive cavity;

[0049] 43—intake valve;

[0050] 44—connecting pipe;

[0051] 45—exhaust valve;

[0052] 46—Negative electrode cavity. DETAILED DESCRIPTION

[0053] The following is a detailed description with reference to the accompanying drawings and embodiments.

[0054] 1. Trace gas detection device based on a micro differential rod-cone composite photoacoustic cell

[0055] 1. Overall

[0056] like Figure 1 , the device includes a photoacoustic signal detection component A and a data processing and display component B;

[0057] The photoacoustic signal detection component A comprises a laser modulation unit 1, a distributed feedback semiconductor laser 2, a fiber collimator 3, a micro differential rod-cone complex photoacoustic cell 4 and a MEMS microphone 5 which are connected in sequence;

[0058] The data processing and display component B includes a pre-voltage amplifier 6, a phase-locked amplifier module 7, an analog-to-digital converter 8 and a host computer 9 connected in sequence;

[0059] The gas to be measured 0 is connected to the micro differential rod-cone complex photoacoustic cell 4;

[0060] The MEMS microphone 5 is in communication with the pre-voltage amplifier 6 .

[0061] 2. Photoacoustic signal detection component A

[0062] 1) Laser modulation unit 1

[0063] The laser modulation unit 1 is composed of a precise laser driving circuit and a modulation control module. The unit can provide a stable and adjustable electrical signal input for the distributed feedback semiconductor laser 2 and realize high-precision modulation of its output light wavelength; the laser driving circuit monitors the temperature and operating current of the distributed feedback semiconductor laser 2 in real time through a closed-loop feedback mechanism to ensure that the distributed feedback semiconductor laser 2 operates stably at the preset center wavelength while suppressing mode jumping and power fluctuations; the modulation module is based on direct modulation technology and dynamically controls the output light intensity or frequency of the distributed feedback semiconductor laser 2 by applying a current or voltage signal of a specific frequency, thereby generating modulated light that matches the absorption characteristics of the target gas.

[0064] 2) Distributed feedback semiconductor laser 2

[0065] A distributed feedback semiconductor laser2 is a light source device that implements a distributed feedback mechanism of light by integrating a periodic Bragg grating inside the laser resonator. This design enables it to have a narrower linewidth (typically <1 MHz), better wavelength stability, and more precise wavelength tunability (capable of covering specific gas absorption peaks, such as acetylene 1532nm or methane 1653nm, through current / temperature adjustment).

[0066] Driven and modulated by the laser modulation unit 1, the distributed feedback semiconductor laser 2 can output continuous laser light ranging from several milliwatts to tens of milliwatts, the wavelength of which is highly matched with the narrowband absorption spectrum of the gas molecule vibration-rotation energy level transition, so that the concentration of the target gas in the gas to be measured 0 can be accurately detected subsequently; in addition, its compact butterfly package also makes it possible to integrate the entire gas detection device.

[0067] 3) Fiber collimator 3

[0068] The fiber collimator 3 is a passive optical device, which is an integrated cylindrical structure of optical fiber and lens. It converts the divergent light in the optical fiber into collimated light through a self-focusing lens.

[0069] The modulated laser is emitted from the distributed feedback semiconductor laser 2 and enters the fiber collimator 3. The end of the fiber collimator 3 is connected to the fiber entrance port in the micro differential rod-cone complex photoacoustic cell 4, realizing the process of coupling the light source into the photoacoustic cell.

[0070] 4) Micro differential rod-cone complex photoacoustic cell

[0071] As the core component of the photoacoustic signal detection component A, the micro differential cylindrical-cone complex photoacoustic cell 4 not only provides an exclusive enclosed space for the interaction between laser and gas, but the photoacoustic signals excited by light and gas can also resonate with the photoacoustic cell, and realize secondary amplification through the cone tips in the positive and negative pole cavities at the positive and negative ends of the photoacoustic cell; in addition, the sound pressure signal distribution at the cone tip in the resonant state presents a differential characteristic, and the subsequent differential subtraction processing of the signals collected at the two cone tips can effectively suppress the common-mode noise in the photoacoustic cell; the structure and process of this photoacoustic cell will be further explained later.

[0072] 5) MEMS microphone 5

[0073] The MEMS microphone 5 serves as a highly sensitive acoustic sensor, used to detect the acoustic wave signals generated by the photoacoustic effect. When the modulated infrared light source irradiates the gas sample 0 to be measured, specific gas molecules absorb the light energy and convert it into heat energy, causing periodic changes in the gas temperature, which in turn causes periodic fluctuations in the pressure in the photoacoustic cell, forming acoustic waves. These acoustic wave signals are directly related to the concentration of the gas. Two MEMS microphones 5 are placed at the two cone tips of the photoacoustic cell. Through their high sensitivity and wide bandwidth characteristics, they can accurately capture these weak acoustic wave signals and convert them into electrical signals, thereby achieving accurate measurement of the gas concentration.

[0074] At the same time, the outstanding advantages of MEMS microphone 5 lie in its miniaturized design, high signal-to-noise ratio and excellent wide-band response range, making it very suitable for the detection needs of weak acoustic signals in photoacoustic spectroscopy gas detection; in addition, this microphone also has low cost and high reliability, which can meet the needs of real-time gas monitoring and trace detection.

[0075] 3. Data processing and display component B

[0076] 1) Pre-voltage amplifier 6

[0077] The function of the pre-voltage amplifier 6 is to amplify the weak electrical signal collected by the MEMS microphone 5, thereby improving the signal-to-noise ratio of the signal and ensuring the accuracy of subsequent signal processing and gas concentration inversion. When the sound pressure signal generated by the photoacoustic effect is converted into an electrical signal by the MEMS microphone 5, it is usually very weak and easily interfered by environmental noise. The pre-voltage amplifier 6 can significantly enhance the amplitude of the signal through its high gain and low noise characteristics, while suppressing background noise, providing high-quality input signals for the subsequent phase-locked amplifier module 7 or other signal processing modules.

[0078] 2) Lock-in amplifier module 7

[0079] The phase-locked amplifier module 7 is the most critical signal processing module in the data processing and display component B. Its function is to extract specific frequency signals related to the photoacoustic signal from the noise background, thereby significantly improving the signal-to-noise ratio of the detection; after the current voltage amplifier 6 amplifies the weak electrical signals collected by the MEMS microphone 5, these signals usually still contain a lot of noise, especially low-frequency noise and environmental interference; the phase-locked amplifier module 7 uses its "phase-locked" characteristics, that is, using the phase relationship between the reference signal and the input signal, to accurately lock and extract the components consistent with the frequency of the photoacoustic signal, while suppressing noise of other frequencies.

[0080] The phase-locked amplifier module 7 in this detection device supports differential signal input. The two sound pressure signals collected by the MEMS microphone 5 are amplified by the pre-voltage amplifier 6 and then enter the two input terminals of the phase-locked amplifier module 7 for differential processing. Compared with the single-channel signal, the signal amplitude after differential processing is significantly enhanced and the common-mode noise is effectively suppressed. In addition, this module also supports the extraction and output of harmonics. The second harmonic of the extracted sound pressure signal can be used to invert the concentration of the gas to be measured.

[0081] 3) Analog-to-digital converter 8

[0082] The function of the analog-to-digital converter 8 is to convert the analog signal processed by the phase-locked amplifier 7 into a digital signal to facilitate the visualization of subsequent signal processing and data processing results. After the photoacoustic signal is extracted by the phase-locked amplifier 8, the output is a second harmonic signal related to the gas concentration, which is an analog voltage signal. The analog-to-digital converter 8 can convert these analog signals into digital signals in real time through its high precision and high sampling rate, thereby facilitating further analysis and calculation by the subsequent host computer 9.

[0083] 4) Host computer 9

[0084] The host computer 9 in the present invention uses a single-chip microcomputer to complete the comprehensive control and data analysis of the entire detection system; the host computer 9 receives data converted by the analog-to-digital converter 8 in real time, including gas type, gas concentration and environmental parameters, and processes and stores these data, and displays the detection results on the display interface; in addition, the host computer 9 can also complete the driving of the A laser modulation unit 1 in the photoacoustic signal acquisition component and the opening and closing state control of the inlet and outlet valves of the photoacoustic cell 4, and is the control center of the entire detection device.

[0085] 2. Specific Structure of the Micro-Differential Rod-Cone Complex Photoacoustic Cell 4

[0086] The above article has given a preliminary explanation of the general functions and basic features of the photoacoustic cell 4 in the device. Next, the specific structure and process of the micro differential rod-cone complex photoacoustic cell 4 will be described in more detail.

[0087] 1. Overall structure

[0088] like Figure 2 The photoacoustic cell 4 includes a laser incident port 41, a positive cavity 42, an air inlet valve 43, a connecting pipe 44, an air outlet valve 45 and a negative cavity 46;

[0089] Its location and connection relationship are:

[0090] The positive cavity 42 and the negative cavity 46 are symmetrically arranged on both sides, connected by a connecting pipe 44 in the middle. An air inlet valve 43 is provided on the upper part of the positive cavity 42, an air outlet valve 45 is provided on the upper part of the negative cavity 46, and a laser incident port 41 is provided at the bottom of the positive cavity 42.

[0091] 1) Laser entrance port 41

[0092] The laser incident port 41 is located on the cylindrical side wall of the positive cavity 42 and is constructed in the form of a side wall opening. The periphery of the entrance is connected with a sealing ring made of fluororubber, the size of which is equal to the size of the fiber collimator 3, ensuring that the fiber collimator 3 can be smoothly coupled with the photoacoustic cell 4 and has good airtightness; the modulated laser is vertically injected from the laser incident port 41 into the cylindrical side wall of the positive cavity 42 of the micro differential column-cone complex photoacoustic cell 4, and is diffusely reflected, which facilitates the generation of the photoacoustic effect and the differential characteristic distribution of the sound pressure signal.

[0093] 2) Positive cavity 42

[0094] The positive cavity 42 is the site where light and gas interact in the photoacoustic cell. It is composed of a composite of a cylinder and a cone. After the laser enters the positive cavity 42 from the laser incident port 41, it is diffusely reflected by the cylindrical side wall of the positive cavity 42. The plane of the entire reflected light path is parallel to the circular surface of the cylinder, so that the light source is confined within the positive cavity 42 and does not enter the negative cavity 46. This design can make the photoacoustic signal of the photoacoustic cell 4 in the resonant state present a differential distribution. In addition, the presence of the cone causes the photoacoustic signal to be concentrated at the tip of the cone and resonantly amplified. This design facilitates the extraction of the photoacoustic signal by the MEMS microphone 5, improving the signal-to-noise ratio of the photoacoustic cell 4 and the sensitivity of the detection device.

[0095] 3) Intake valve 43

[0096] The inlet valve 43 is used to control the entry of the gas to be measured O. It is also located on the cylindrical side wall of the positive electrode cavity 42, but the apertures of the two are not collinear. The purpose of this design is, on the one hand, to provide a reflective surface for diffuse reflection of the incident laser, and on the other hand, to reduce the airflow noise caused by the influx of gas inside the entire photoacoustic cell.

[0097] 4) Connecting pipe 44

[0098] The connecting tube 44 is composed of a small section of a column, which connects the positive cavity 42 and the negative cavity 46 to form the cavity inside the photoacoustic cell 4. The existence of the connecting tube 44 allows the gas entering the photoacoustic cell 4 from the air inlet valve 43 to fill the entire internal cavity of the photoacoustic cell 4. In addition, the radius of the main body circular surface is much smaller than the positive and negative cavities 42 and 46. The purpose of this size difference is to more effectively confine the light source to the positive and negative cavities 42 and 46, so that the sound pressure signal distribution in the resonant state can present a differential characteristic.

[0099] 5) Exhaust valve 45

[0100] The outlet valve 45 is used to control the outflow of the gas to be measured O. It is located on the cylindrical side wall of the negative electrode cavity 46. Its physical position is symmetrical with the inlet valve 43 along the direction perpendicular to the central axis of the cavity.

[0101] 6) Negative electrode cavity 46

[0102] The structure of the negative electrode cavity 46 is exactly the same as that of the positive electrode cavity 2, and is also composed of a composite of a cylinder and a cone; during the photoacoustic gas detection process, only gas exists in the negative electrode cavity 46 but no laser; in the resonant state, the photoacoustic signal generated by the cone tip of the negative electrode cavity 46 is equal in size to that of the cone tip of the positive electrode cavity 42, but has opposite polarity.

[0103] 3. Process

[0104] 1) The radius and sidewall length of the cylinder constituting the positive electrode chamber 42 and the negative electrode chamber 46 are 8 mm and 11 mm, respectively;

[0105] 2) The radii of the upper and lower circular surfaces of the cones constituting the positive electrode chamber 42 and the negative electrode chamber 46 are 1 mm and 2 mm, respectively, and the height is 3 mm;

[0106] 3) The connecting tube 44 is cylindrical, and the cylindrical radius and side wall length are 6 mm and 34 mm respectively. The resonant frequency of the photoacoustic cell with such dimensions is 18848 Hz, and the quality factor is 282.1.

[0107] 3. Trace gas detection method based on micro differential rod-cone complex photoacoustic cell 4

[0108] The above device can realize real-time dynamic detection of the concentration of a certain trace gas in the mixed gas. The specific detection method is as follows (taking acetylene as an example):

[0109] ① In the photoacoustic signal detection component A, the laser modulation unit 1 controls the distributed feedback semiconductor laser 2 to output a laser with a wavelength of 1532nm and a power of 200mW. The control circuit inside the unit provides a low-frequency sawtooth wave and a high-frequency sine wave to modulate the output laser. The modulated laser enters the positive cavity 42 of the micro differential rod-cone complex photoacoustic cell 4 through a fiber collimator.

[0110] ② Open the air inlet valve 43 at the positive cavity 42 of the photoacoustic cell, close the air outlet valve 45 at the negative cavity 46, and slowly introduce the target gas 0 to be measured into the photoacoustic cell 4. The laser and the gas interact in the photoacoustic cell 4 to stimulate the photoacoustic effect. Due to the unique design of the present invention, the maximum values of the generated photoacoustic signals are respectively concentrated at the two cone tips of the positive and negative cavities and exhibit differential characteristics with the same amplitude and opposite polarity. The sound pressure signals at the two cone tips are collected using two MEMS electrical microphones 5;

[0111] ③ The two sound pressure signals collected by the MEMS microphone 5 are sequentially passed through the pre-voltage amplifier 6 and the small phase-locked amplifier module 7 with differential input characteristics in the data processing and display component B to extract the second harmonic signal. The extracted second harmonic signal enters the analog-to-digital converter 8 and is converted into a digital signal that can be processed by the host computer 9;

[0112] ④ The host computer 9 of the data processing and display unit B collects the digital signal, inverts the acetylene concentration in the test gas O, and displays the result and response time. Finally, the outlet valve 45 is opened to exhaust the gas in the micro differential rod-cone complex photoacoustic cell 4, facilitating the next test.

[0113] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A trace gas detection device based on a micro differential rod-cone composite photoacoustic cell, characterized by: It includes a photoacoustic signal detection component (A) and a data processing and display component (B), wherein: The photoacoustic signal detection component (A) comprises a laser modulation unit (1), a distributed feedback semiconductor laser (2), a fiber collimator (3), a micro differential rod-cone complex photoacoustic cell (4) and a MEMS microphone (5) connected in sequence; The data processing and display component (B) includes a pre-voltage amplifier (6), a phase-locked amplifier module (7), an analog-to-digital converter (8) and a host computer (9) connected in sequence; The gas to be measured (0) is connected to the micro differential rod-cone complex photoacoustic cell (4); The MEMS microphone (5) is connected to the pre-voltage amplifier (6).

2. The trace gas detection device according to claim 1, characterized in that: The structure of the micro differential column-cone complex photoacoustic cell (4) is as follows: It includes a laser incident port (41), a positive electrode cavity (42), an air inlet valve (43), a connecting pipe (44), an air outlet valve (45) and a negative electrode cavity (46); The position and connection relationship are as follows: the positive electrode cavity (42) and the negative electrode cavity (46) are symmetrically arranged on both sides, and are connected by a connecting pipe (44) in the middle. An air inlet valve (43) is provided on the upper part of the positive electrode cavity (42), an air outlet valve (45) is provided on the upper part of the negative electrode cavity (46), and a laser incident port (41) is provided at the bottom of the positive electrode cavity (42).

3. The trace gas detection device according to claim 1, characterized in that: The process of the micro differential column-cone complex photoacoustic cell (4) 1) The radius and side wall length of the cylinder constituting the positive electrode cavity (42) and the negative electrode cavity (46) are 8 mm and 11 mm, respectively; 2) The radii of the upper and lower circular surfaces of the cone constituting the positive electrode cavity (42) and the negative electrode cavity (46) are 1 mm and 2 mm respectively, and the height is 3 mm; 3) The connecting tube (44) is cylindrical, and the cylindrical radius and side wall length are 6 mm and 34 mm respectively. The resonant frequency of the photoacoustic cell with this size is 18848 Hz, and the quality factor is 282.

1.

4. The detection method of the trace gas detection device according to claim 1, 2 or 3, characterized in that The following steps are included: ① The laser modulation unit (1) in the photoacoustic signal detection component controls the distributed feedback semiconductor laser (2) to output laser light of a specific wavelength and power and uses the control circuit inside the unit to provide a low-frequency sawtooth wave and a high-frequency sine wave to modulate the output laser light. The modulated laser light enters the positive cavity (42) of the micro differential rod-cone complex photoacoustic cell (4) through the optical fiber collimator (3); ② Open the air inlet valve (43) at the positive cavity (42) of the photoacoustic cell, close the air outlet valve (45) at the negative cavity (46), and slowly introduce the target gas to be measured into the photoacoustic cell. The laser and the gas interact in the photoacoustic cell to stimulate the photoacoustic effect. Due to the unique design of the present invention, the maximum value of the generated photoacoustic signal is concentrated at the two cone tips of the positive and negative cavities respectively and exhibits a differential characteristic with the same amplitude and opposite polarity. The sound pressure signals at the two cone tips are collected using two MEMS microphones (5); ③ The two sound pressure signals collected by the MEMS microphone (5) are sequentially passed through the pre-voltage amplifier (6) in the data processing and display component and the phase-locked amplifier module (7) with differential input characteristics to extract the second harmonic signal. The extracted harmonic signal enters the analog-to-digital converter (8) and is converted into a digital signal that can be processed by the host computer; ④ The host computer (9) of the data processing and display component collects the digital signal, inverts the concentration of the gas to be tested (0) and displays the result of the gas detection concentration; finally, the outlet valve (45) is opened to exhaust the gas in the photoacoustic pool to facilitate the next detection.

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