A side-branch photoacoustic cell and a multi-component detection system comprising the same

Abstract

The application discloses a side-branch type photoacoustic cell and a multi-component detection system formed by the same. The side-branch type photoacoustic cell comprises a resonance structure, the resonance structure is communicated with a first side-branch enhancement structure and a second side-branch enhancement structure respectively, and the first side-branch enhancement structure and the second side-branch enhancement structure are located at two ends of the resonance structure respectively. The detection system comprises a signal generator, a light source controller, a laser, the side-branch type photoacoustic cell, a microphone driving circuit, a lock-in amplifier and an upper computer. The application solves the technical problems of high resonance frequency and low sensitivity of a miniaturized photoacoustic cell, realizes high-sensitivity rapid detection of multi-component gas, and has the characteristics of small size, low frequency, high sensitivity, strong multi-frequency resonance, easy processing and good universality.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, specifically to a side-branched photoacoustic cell and a multi-component detection system thereof. Background Technology

[0002] Photoacoustic spectroscopy, a novel laser spectroscopy technique, works by irradiating the gas with a light beam corresponding to its optical absorption peak. This causes the gas to periodically expand and contract, generating a sound signal. The qualitative and quantitative analysis of the gas is then achieved by controlling the wavelength of the light beam and the amplitude of the sound signal. Photoacoustic spectroscopy is renowned for its high detection sensitivity, wide dynamic range, low background noise, no need for sample pretreatment, and simple experimental system. It has enabled high-precision detection of various gases and has attracted widespread attention from researchers.

[0003] However, traditional photoacoustic spectroscopy still has some limitations. For example, the volume of air in skin respiration is extremely small, the concentration of characteristic gases is low, and the composition is complex. Traditional photoacoustic cells cannot meet the requirements of small volume, high sensitivity, and multi-frequency resonance for skin respiration detection.

[0004] As a crucial component of photoacoustic sensors, photoacoustic cells are primarily classified into resonant and non-resonant types. The photoacoustic signal of a non-resonant photoacoustic cell is independent of the absorption optical path length, but inversely proportional to the cross-sectional area of ​​the cell and the frequency of the sound wave. Therefore, non-resonant photoacoustic cells are well-suited for miniaturized applications. However, the sound waves within a non-resonant photoacoustic cell are uniformly distributed, resulting in a significantly lower detection sensitivity compared to resonant photoacoustic cells.

[0005] Resonant photoacoustic cells amplify acoustic signals through resonance, and their sensitivity is generally higher than that of non-resonant photoacoustic cells. Typically, the resonant frequency of a resonant photoacoustic cell is inversely proportional to its size; that is, a smaller resonant photoacoustic cell has a higher resonant frequency. The photoacoustic signal in a photoacoustic cell is usually captured by a condenser microphone. Since the effective response band of a condenser microphone is generally located at the human voice range (20-20 kHz), and its optimal response frequency is usually below 2 kHz, if the photoacoustic cell is too small, its resonant frequency will be too high, severely reducing its sensitivity. For example, a cylindrical photoacoustic cell with a length of 8 mm and an inner diameter of 4 mm has a first-order resonant frequency of 21.7 kHz, exceeding the effective response band of the microphone. Furthermore, an excessively high resonant frequency also limits its adaptability to different light sources and modulation methods. For example, for blackbody light sources that cannot be internally modulated, mechanical modulation methods such as choppers are required to modulate the amplitude of the incident light. The highest modulation frequency achievable by choppers is typically no more than several kilohertz, which cannot meet the 21.7 kHz modulation requirement of the aforementioned small-sized photoacoustic cell. The vibration noise generated by the chopper increases with increasing rotational speed, a phenomenon that leads to strong background noise severely interfering with photoacoustic gas detection near its highest modulation frequency. Therefore, traditional miniaturized photoacoustic cells can only be used with light sources capable of internal current modulation, limiting their widespread application in gas monitoring.

[0006] In the field of photoacoustic spectroscopy for gas detection, frequency division multiplexing (FDM) is commonly used to achieve rapid detection of multi-component gases. When the photoacoustic cell has multiple resonant modes with different frequencies, photoacoustic signals of different gases can be excited by light sources with different modulation frequencies. Each gas's photoacoustic signal corresponds to a modulation frequency, and each modulation frequency corresponds to a resonant mode. Therefore, this method can associate different resonant modes with different gas signals, enabling rapid detection of multi-component gases. There are generally two methods for achieving multi-resonant mode detection in a photoacoustic cell.

[0007] One approach involves fabricating multi-channel acoustic resonant cavities, each with a different length or inner diameter, resulting in a specific frequency resonance mode for each channel. The advantage of this method is that the intensities of multiple resonance modes are relatively close and strong. However, due to the large number of channels in this type of photoacoustic cell, the cell itself is quite large. Furthermore, this method requires high precision in the positional relationships between the multiple channels, making fabrication difficult and costly.

[0008] Secondly, higher-order resonance modes of a single-channel photoacoustic cell can be utilized. Taking the cylindrical photoacoustic cell with a buffer structure, which is most commonly used in photoacoustic spectroscopy gas detection, as an example, it has Nth-order longitudinal resonance modes. Therefore, several of these longitudinal resonance modes can be used to achieve frequency-division detection of multi-component gases. However, for a cylindrical photoacoustic cell, its first-order longitudinal resonance mode is the strongest, and the intensity of the resonance mode decays rapidly as the order increases. Therefore, this method is difficult to achieve high-sensitivity rapid detection of multi-component gases.

[0009] Chinese patent application CN116539535A discloses a photoacoustic cell, system, and method for aerosol acidity detection. The system includes a housing, optical windows, an inlet pipe, an outlet pipe, a photoacoustic resonant cavity penetrating the housing, and a piezoelectric ceramic sensor within the housing. Optical windows are installed at both ends of the junction between the photoacoustic resonant cavity and the housing. The inlet and outlet pipes are connected to both ends of the photoacoustic resonant cavity through the housing. The piezoelectric ceramic sensor is installed in the cross-sectional direction of the photoacoustic resonant cavity. Aerosol enters the photoacoustic resonant cavity through the inlet pipe and exits through the outlet pipe. The piezoelectric ceramic sensor is used to detect the photoacoustic signal of the aerosol within the photoacoustic resonant cavity. However, this photoacoustic cell has a complex structure and can only detect single-component gases. Summary of the Invention

[0010] To address the shortcomings of the prior art, the present invention aims to provide a side-supported photoacoustic cell and a multi-component detection system thereof. By setting a first side-supported reinforcement structure and a second side-supported reinforcement structure, a side-supported resonance mode is generated to solve the technical problems of excessively high resonance frequency and low sensitivity of miniaturized photoacoustic cells, thereby enabling rapid detection of multi-component gases with high sensitivity. The present invention features small size, low frequency, high sensitivity, strong multi-frequency resonance, easy processing, and good versatility.

[0011] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0012] A side-supported photoacoustic cell includes a resonant structure 1, which is connected to a first side-supported reinforcing structure 2 and a second side-supported reinforcing structure 3, respectively. The first side-supported reinforcing structure 2 and the second side-supported reinforcing structure 3 are located at opposite ends of the resonant structure 1.

[0013] The resonant structure 1 includes a cavity 101, which is cylindrical in shape. A first window 102 and a second window 105 are fixedly arranged at both ends of the cavity 101 along the axial direction. A sound guide 103 is arranged at the top of the cavity 101, and its interior communicates with the cavity 101. A sound detector 104 is arranged at the top of the sound guide 103.

[0014] The first side reinforcement structure 2 and the second side reinforcement structure 3 have the same structure. The first side reinforcement structure 2 includes a side capillary 201 with one end connected to the cavity 101, the other end of the side capillary 201 connected to one end of the side connection female port 202, the other end of the side connection female port 202 connected to the other end of the side connection male port 203, the other end of the side connection male port 203 connected to one end of the side reinforcement tube 204, and the other end of the side reinforcement tube 204 connected to the acoustic boundary unit 205.

[0015] The volume of the cavity 101 is no greater than 0.6 mL, and at this time the frequency of the first even-order resonant mode of the side-branched photoacoustic cell is no greater than 2000 Hz.

[0016] The volume of the side branch reinforcement tube 204 is greater than the volume of the cavity 101; the length of the side branch reinforcement tube 204 is at least greater than the length of the cavity 101, and not greater than 100cm.

[0017] The inner diameter of the side capillary tube 201 should simultaneously meet the following conditions:

[0018] The inner diameter of the side capillary tube 201 is no greater than 1.5 mm;

[0019] The inner diameter of the side capillary 201 is smaller than the length of the cavity 101;

[0020] The inner diameter of the side capillary 201 is smaller than the inner diameter of the cavity 101;

[0021] The inner diameter of the side capillary tube 201 is smaller than the inner diameter of the side reinforcing tube 204.

[0022] The side support reinforcement pipes of the first side support reinforcement structure 2 and the second side support reinforcement structure 3 are both detachable cylindrical pipes; the materials of the side support reinforcement pipes of the first side support reinforcement structure 2 and the second side support reinforcement structure 3 are both aluminum alloy, stainless steel, copper or Teflon; the acoustic boundary units of the first side support reinforcement structure 2 and the second side support reinforcement structure 3 are ball valves or check valves.

[0023] The first window 102 and the second window 105 are both made of calcium fluoride, zinc selenide or ultraviolet fused silica; the acoustic detector 104 is a capacitive microphone, a MEMS microphone or a PVDF piezoelectric sensor.

[0024] The first side reinforcement structure 2 and the second side reinforcement structure 3 are located on the same side or opposite side of the resonant structure 1, respectively.

[0025] A multi-component gas detection system, comprising:

[0026] Signal generator 4: The first signal output terminal of the signal generator 4 is connected to the signal input terminal of the light source controller 5, and is used to output a periodic signal of sine wave, square wave or sawtooth wave to the light source controller 5; the second signal output terminal of the signal generator 4 is connected to the first signal input terminal of the lock-in amplifier 8, and is used to output a square wave reference signal to the lock-in amplifier 8.

[0027] Light source controller 5: The signal output terminal of the light source controller 5 is connected to the signal input terminal of the laser 6, and is used to modulate the wavelength or amplitude of the laser beam emitted by the laser 6 according to the periodic signal;

[0028] Laser 6: The laser output end of the laser 6 is connected to the laser input end of the side-supported photoacoustic cell, and is used to emit a laser beam modulated by the light source controller 5 into the side-supported photoacoustic cell.

[0029] Side-supported photoacoustic cell: The signal output terminal of the acoustic detector 104 of the side-supported photoacoustic cell is connected to the signal input terminal of the microphone driving circuit 7; the gas to be tested is pre-introduced into the side-supported photoacoustic cell, and the modulated laser entering the side-supported photoacoustic cell passes through the gas to be tested to generate a photoacoustic effect; the acoustic detector 104 of the side-supported photoacoustic cell detects the sound wave signal generated by the photoacoustic effect and converts it into a voltage signal.

[0030] Microphone driver circuit 7: The signal output terminal of the microphone driver circuit 7 is connected to the second signal input terminal of the lock-in amplifier 8, and is used to transmit voltage signals to the lock-in amplifier 8;

[0031] Lock-in amplifier 8: The signal output terminal of the lock-in amplifier 8 is connected to the signal input terminal of the host computer 9, and the voltage signal is filtered, demodulated and amplified according to the square wave reference signal;

[0032] Host computer 9: Used to calculate the concentration information of multi-component gases based on voltage signals, and to complete the detection of the concentration and type of multi-component gases.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] 1. The volume of a traditional photoacoustic cell is usually inversely proportional to its resonant frequency, meaning that a small photoacoustic cell will inevitably have a high resonant frequency. However, this invention introduces a side-branch reinforcement structure, which enables a lower resonant frequency to be obtained with a very small resonant cavity. This solves the technical problem of excessively high resonant frequency in the miniaturization of traditional photoacoustic cells and improves the sensitivity of the photoacoustic cell.

[0035] 2. By setting a detachable side-branch reinforcement tube, the present invention can obtain multiple resonant modes with similar intensity by replacing side-branch reinforcement tubes of different sizes when actually detecting gases, thereby achieving effective multi-frequency resonance in a small volume. It can simultaneously detect multiple components of gas. Compared with the photoacoustic cell that is integrally formed in the prior art, the side-branch type photoacoustic cell structure of the present invention is more time-saving, has lower processing cost, higher detection efficiency, higher detection accuracy, and wider application range in actual use.

[0036] 3. Compared with the cylindrical photoacoustic cell with buffer cavity in the prior art, the gas detection linear range and gas detection dynamic range of the side-branched photoacoustic cell of the present invention are both higher than those of the cylindrical photoacoustic cell with buffer cavity, thus realizing the detection of high concentration gas.

[0037] 4. Compared with the prior art, the volume of the cavity 101 set in this invention is no more than 0.6 mL. At this time, the frequency of the first even-order resonant mode of the side-branched photoacoustic cell is no more than 2000 Hz, realizing low-frequency resonance under the small-volume photoacoustic cell structure. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of a side-supported photoacoustic cell provided by the present invention.

[0039] Figure 2 This is a simulation diagram of the side resonance mode provided by the present invention, wherein, Figure 2 (a) in the text represents the first-order mode. Figure 2 (b) in the equation represents the second-order mode. Figure 2 (c) in the equation represents the third-order mode. Figure 2 (d) in the equation represents the fourth-order mode.

[0040] Figure 3 This is the frequency response diagram of the photoacoustic cell without side-support reinforcement structure provided by the present invention.

[0041] Figure 4 This is the frequency response diagram of the side-branched photoacoustic cell with a side-branched reinforcing tube length of 10cm provided by the present invention.

[0042] Figure 5 This is the frequency response diagram of the side-branched photoacoustic cell with a side-branched reinforcing tube length of 20cm provided by the present invention.

[0043] Figure 6 This is the frequency response diagram of the side-branched photoacoustic cell when the length of the side-branched reinforcing tube is 46cm, as provided by the present invention.

[0044] Figure 7 This is the curve showing the variation of the resonant frequency with the inner diameter of the side capillary under the second-order resonant mode provided by the present invention.

[0045] Figure 8 These are the photoacoustic signal variation curves under the second-order and fourth-order resonance modes provided by this invention, as a function of the inner diameter of the side-branch reinforcing tube.

[0046] Figure 9 This is a linear response diagram of the gas absorption coefficient of the side-supported photoacoustic cell provided by the present invention.

[0047] Figure 10 This is the gas absorption coefficient response diagram of the side-supported photoacoustic cell provided by the present invention.

[0048] Figure 11 This is a schematic diagram of a cylindrical photoacoustic cell with a buffer cavity.

[0049] Figure 12 The graph shows the linear response of the gas absorption coefficient of a cylindrical photoacoustic cell with a buffer cavity.

[0050] Figure 13 The graph shows the gas absorption coefficient response of a cylindrical photoacoustic cell with a buffer cavity.

[0051] Figure 14 This is a schematic diagram of a multi-component gas detection system.

[0052] In the diagram: Resonance structure 1, cavity 101, first window 102, acoustic duct 103, acoustic detector 104, second window 105, first side branch reinforcement structure 2, side branch capillary 201, side branch female connector 202, side branch male connector 203, side branch reinforcement tube 204, acoustic boundary unit 205, acoustic boundary unit 305, second side branch reinforcement structure 3, signal generator 4, light source controller 5, laser 6, microphone driver circuit 7, lock-in amplifier 8, host computer 9. Detailed Implementation

[0053] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the implementation of the present invention is not limited thereto.

[0054] like Figure 1 As shown, a side-braced photoacoustic cell includes a resonant structure 1, which is connected to a first side-braced reinforcement structure 2 and a second side-braced reinforcement structure 3. The first side-braced reinforcement structure 2 and the second side-braced reinforcement structure 3 are located at opposite ends of the resonant structure 1. Further, the first side-braced reinforcement structure 2 and the second side-braced reinforcement structure 3 are located on the same side or opposite sides of the resonant structure 1.

[0055] The resonant structure 1 includes a cavity 101, which is a hollow cylinder with a volume not exceeding 0.6 mL. In this embodiment, the cavity 101 is a hollow aluminum alloy cylindrical pipe with an inner diameter of 4 mm and a length of 10 mm. The gas to be tested is retained in the cavity 101 and absorbs excitation light to generate an acoustic signal. A first window 102 and a second window 105 are fixedly provided at both ends of the cavity 101 along its axial direction. The first window 102 and the second window 105 allow the excitation light source to enter the cavity 101 while ensuring the airtightness of the cavity 101. The excitation light beam propagates along the axial direction of the cavity 101. In this embodiment, the first window 102 and the second window 105 are both made of calcium fluoride; a sound guide 103 is provided at the top of the cavity 101, and its interior communicates with the cavity 101. In this embodiment, the sound guide 103 is a hollow aluminum alloy cylindrical pipe with an inner diameter of 0.5 mm; a sound detector 104 is provided at the top of the sound guide 103, which is used to collect the resonant amplified sound wave signal and convert it into a voltage signal; the sound detector 104 is a capacitive microphone, a MEMS microphone, or a PVDF piezoelectric sensor. In this embodiment, the sound detector 104 is a high-sensitivity capacitive microphone with a sensitivity of 100 mV / Pa.

[0056] The first side support reinforcement structure 2 and the second side support reinforcement structure 3 have identical structures. The first side support reinforcement structure 2 includes a side support capillary 201 with one end connected to the cavity 101. The side support capillary 201 is a hollow cylinder. In this embodiment, the side support capillary 201 is a hollow aluminum alloy cylindrical pipe. The other end of the side support capillary 201 is connected to one end of the side support connecting female port 202. The side support connecting female port 202 has an internal thread structure for easy disassembly and installation. In this embodiment, the side support connecting female port 202 has an M3 internal thread. The other end of the side support connecting female port 202 is connected to one end of the side support connecting male port 203. The side support connecting male port 203 has an external thread structure. In this embodiment, the side support connecting male port 203 is a hollow aluminum alloy cylindrical pipe with M3 external threads at both ends. The other end of the side support connecting male port 203 is connected to... One end of the side branch reinforcement tube 204 is connected to the cavity 101. The side branch reinforcement tube 204 is a detachable hollow cylinder. The length of the side branch reinforcement tube 204 is at least greater than the length of the cavity 101 and not greater than 100cm. In this embodiment, the side branch reinforcement tube 204 is a hollow aluminum alloy cylindrical pipe with M3 internal threads at both ends. The volume of the side branch reinforcement tube 204 should be greater than the volume of the cavity 101. The other end of the side branch reinforcement tube 204 away from the cavity 101 is connected to the acoustic boundary unit 205. The acoustic boundary unit 205 is a ball valve or a one-way valve, used to connect the side branch reinforcement tube 204 and the external air inlet / outlet device. In this embodiment, the acoustic boundary unit 205 is a stainless steel ball valve with M3 external threads at both ends. The acoustic boundary unit 205 is connected to the outlet of the external air circuit system to allow the gas to be tested to enter the cavity 101.

[0057] The inner diameter of the side capillary tube 201 should simultaneously meet the following conditions:

[0058] The inner diameter of the side capillary tube 201 is no greater than 1.5 mm;

[0059] The inner diameter of the side capillary 201 is smaller than the height of the cavity 101;

[0060] The inner diameter of the side capillary 201 is smaller than the inner diameter of the cavity 101;

[0061] The inner diameter of the side capillary tube 201 is smaller than the inner diameter of the side reinforcing tube 204.

[0062] During air intake, the acoustic boundary unit 205 of the first side-branch reinforcement structure 2 and the acoustic boundary unit 305 of the second side-branch reinforcement structure 3 are in the open state. The gas to be tested simultaneously enters the acoustic boundary units of the first side-branch reinforcement structure 2 and the second side-branch reinforcement structure 3, the side-branch reinforcement tube, the male side-branch connection port, the female side-branch connection port, and the side-branch capillary tube from the outlet of the gas path system, and finally exits from the acoustic boundary unit 305 of the second side-branch reinforcement structure 3. When the gas to be tested completely fills the cavity 101, the acoustic boundary units 205 of the first side-branch reinforcement structure 2 and the acoustic boundary units 305 of the second side-branch reinforcement structure 3 are closed at the same time, so that the photoacoustic cell is isolated from the external gas environment, and the photoacoustic signal of the gas is detected. At this time, the acoustic boundary unit is a hard sound field boundary, and a side-branch resonance mode can be formed in the side-branch photoacoustic cell. When the gas to be tested is detected, the acoustic boundary units of the first side-branch reinforcement structure 2 and the second side-branch reinforcement structure 3 are opened, and the cavity 101 is flushed with nitrogen or clean air.

[0063] The side-branch photoacoustic cell of the present invention can form a side-branch resonance mode, and the sound pressure amplitude and phase are equal at all points in the cavity 101. Therefore, the acoustic duct 103 can be connected to any position at the top of the cavity 101.

[0064] like Figure 2 As shown, in the side resonance mode, in the first and third modes, the cavity 101 is a sound pressure node, and the acoustic boundary unit is a sound pressure antinode. Figure 2 As shown in (a), in the first mode, the sound pressure gradually increases along the direction from cavity 101 to the left acoustic boundary unit 305, and the phase at the antinode is positive; along the direction from cavity 101 to the right acoustic boundary unit 205, the sound pressure gradually increases, and the phase at the antinode is negative. There is one node and two antinodes in the entire photoacoustic cell. Figure 2 As shown in (c), in the third mode, the sound pressure along the direction from cavity 101 to the left acoustic boundary unit 305 first increases, then decreases, and then increases again, with the phase at the antinodes first positive and then negative; along the direction from cavity 101 to the right acoustic boundary unit 205, the sound pressure first increases, then decreases, and then increases again, with the phase at the antinodes first negative and then positive. There are 3 nodes and 4 antinodes in the entire photoacoustic cell. In the second and fourth modes, the cavity 101 is a sound pressure antinode, and the acoustic boundary unit is also a sound pressure antinode. Figure 2 As shown in (b), in the second mode, along the direction from cavity 101 to the left acoustic boundary unit 305, the sound pressure first decreases and then increases, and the phase at the antinodes is first positive and then negative; along the direction from cavity 101 to the right acoustic boundary unit 205, the sound pressure first decreases and then increases, and the phase at the antinodes is first positive and then negative. There are 2 nodes and 3 antinodes in the entire photoacoustic cell; Figure 2As shown in (d), in the fourth mode, along the direction from cavity 101 to the left acoustic boundary unit 305, the sound pressure first decreases, then increases, then decreases again, and then increases again, and the phase at the antinode is first positive, then negative, and then positive again; along the direction from cavity 101 to the right acoustic boundary unit 205, the sound pressure first decreases, then increases, then decreases again, and then increases again, and the phase at the antinode is first positive, then negative, and then positive again. There are 4 nodes and 5 antinodes in the entire photoacoustic cell.

[0065] Figure 2 Only the first four side resonance modes are shown; higher-order resonance modes exhibit similar patterns. Therefore, for the Nth-order mode, there are N nodes and N+1 antinodes. For odd-order modes, cavity 101 is a sound pressure node, and the acoustic boundary unit is a sound pressure antinode. Simultaneously, the amplitude variation trends of the sound pressure in the first side reinforcement structure 2 and the second side reinforcement structure 3 are the same, while their phase variation trends are opposite. For even-order modes, cavity 101 is a sound pressure antinode, and the acoustic boundary unit is also a sound pressure antinode. Furthermore, the amplitude and phase variation trends of the sound pressure in the first side reinforcement structure 2 and the second side reinforcement structure 3 are the same. Since the acoustic detector 104 is located at cavity 101, cavity 101 is a node in odd-order modes and an antinode in even-order modes. Only in even-order modes can the gas photoacoustic signal be effectively detected.

[0066] The resonance frequency of the resonance peak produced in the first-order mode is the lowest, and the resonance frequency of the resonance peak produced by the higher-order mode is.

[0067] The side-branched photoacoustic cell can generate multiple resonant modes. The concentration of the corresponding gas can be effectively detected by selecting resonant modes whose photoacoustic signal amplitude is not less than one-third of the photoacoustic signal amplitude under the strongest resonant mode.

[0068] Figure 3 The figure shows the frequency response of a photoacoustic cell without a side-supported reinforcement structure. At this time, the inner diameter of cavity 101 is 4mm and the length is 10cm. As can be seen from the figure, the resonant frequency of the photoacoustic cell is about 17000Hz, and the corresponding photoacoustic signal is about 0.025. Figure 4 The figure shows the frequency response of a side-supported photoacoustic cell. The inner diameter of the side-support reinforcing tube is 4 mm, and its length is 10 cm. As can be seen from the figure, the resonant frequency of the second-order resonant mode of the side-supported photoacoustic cell is approximately 1200 Hz, corresponding to a photoacoustic signal of approximately 1. The resonant frequency of the fourth-order resonant mode is approximately 1700 Hz, corresponding to a photoacoustic signal of approximately 0.5. Figure 3In comparison, its resonant frequency is significantly reduced, and the resonant frequency in the fourth resonant mode is 0.1 times that of the photoacoustic cell without a side-supported reinforcement structure; its photoacoustic signal is significantly enhanced, and the photoacoustic signal in the second resonant mode is 40 times that of the photoacoustic signal generated by the photoacoustic cell without a side-supported reinforcement structure, indicating that the side-supported photoacoustic cell of the present invention has high sensitivity for detecting gas concentration in a small volume.

[0069] like Figure 5 As shown, the inner diameter of the side branch reinforcement pipe is 4mm and its length is 20cm. (Compared to...) Figure 4 In comparison, the inner diameter of the side branch reinforcing pipe remains unchanged, but its length has increased; Figure 5 It can be seen that three resonance peaks were generated, and the resonance modes corresponding to their resonant frequencies from low to high are the second-order, fourth-order, and sixth-order resonance modes. The intensity of the corresponding photoacoustic signals is fourth-order > second-order > sixth-order. It is evident that the photoacoustic signal intensities generated under the second-order and fourth-order resonance modes are similar, while the photoacoustic signal intensity under the sixth-order resonance mode is less than one-third of that under the fourth-order resonance mode. Therefore, under this size of the side-branch reinforcement tube, two effective resonance modes with similar intensities were generated. Appropriately increasing the length of the side-branch reinforcement tube to 46cm yields the following... Figure 6 The schematic diagram of the frequency response of the side-supported photoacoustic cell shown is composed of... Figure 6 It is known that the photoacoustic signal is strongest in the sixth-order resonant mode, followed by the eighth-order resonant mode. The photoacoustic signal in the fourth-order resonant mode is greater than one-third of that in the sixth-order resonant mode, while the photoacoustic signals in other resonant modes are all less than one-third of those in the sixth-order resonant mode. Therefore, three effective resonant modes with similar intensities are generated within this side-branch reinforcement tube size. This indicates that as the length of the side-branch reinforcement tube increases, the present invention exhibits richer multi-frequency resonance characteristics.

[0070] Depend on Figure 4 , Figure 5 , Figure 6 It can be seen that as the length of the side-branch reinforcing tube increases, the resonant frequencies corresponding to each resonant mode decrease. This indicates that as the length of the side-branch reinforcing tube increases, the resonant frequencies of the effective resonant peaks under the same even-numbered resonant modes decrease significantly. Therefore, by setting the length of the side-branch reinforcing tube in this invention to no more than 100 cm, a lower resonant frequency can be obtained.

[0071] Figure 7 The curve showing the variation of the resonant frequency with the inner diameter of the side capillary in the second-order resonant mode is given by... Figure 7 It is known that as the inner diameter of the side capillary decreases, the second-order resonant frequency decreases significantly. Therefore, the inner diameter of the side capillary in this invention is no greater than 1.5 mm, which can achieve a lower resonant frequency.

[0072] Figure 8 The curves show the changes in photoacoustic signals under the second and fourth resonant modes as a function of the inner diameter of the side-branch reinforcing tube. Figure 8 It can be seen that as the inner diameter of the side-branch reinforcing tube increases, the photoacoustic signal in the second-order resonant mode first increases and then decreases, while the photoacoustic signal in the fourth-order resonant mode first decreases and then increases. When the inner diameter is approximately 3.5 mm, the photoacoustic signals in the second-order and fourth-order resonant modes are equal, and the intensity of the resonance peaks corresponding to these two resonant modes is relatively strong. When the intensity of the resonance peaks is relatively strong, the technical problem of inconsistent detection accuracy when simultaneously detecting multiple gases is avoided, achieving the requirement of maximizing the detection accuracy of multiple gases, and making it suitable for various application scenarios.

[0073] Figure 9 This is a linear response diagram of the gas absorption coefficient of the side-supported photoacoustic cell described in this invention. At this time, the length of cavity 101 is 10 mm. Figure 9 It can be seen that when the gas absorption coefficient is less than or equal to 0.285 cm⁻¹ -1 At this time, the photoacoustic signal and the gas absorption coefficient have a linear relationship, that is, the linear range of gas detection for the side-branched photoacoustic cell is 0–0.285 cm⁻¹. -1 . Figure 10 This is the gas absorption coefficient response diagram of the side-supported photoacoustic cell described in this invention, from... Figure 10 It can be seen that when the gas absorption coefficient is less than or equal to 5.7 cm⁻¹ -1 The photoacoustic signal is directly proportional to the gas absorption coefficient, meaning the dynamic range of gas detection for the side-branched photoacoustic cell is 0–5.7 cm⁻¹. -1 .

[0074] Figure 11 This is a schematic diagram of a cylindrical photoacoustic cell with a buffer cavity. Figure 11 The cylindrical photoacoustic cell with a buffer cavity is the most commonly used resonant photoacoustic cell structure in the field of photoacoustic spectroscopy gas detection. Figure 11 The total length of the cylindrical photoacoustic cell with buffer cavity shown is 100 mm, which is the absorption optical path. Figure 12 The linear response diagram of the gas absorption coefficient of a cylindrical photoacoustic cell with a buffer cavity is given by... Figure 12 It can be seen that when the gas absorption coefficient is less than or equal to 0.0285 cm⁻¹ -1 At this time, the photoacoustic signal and the gas absorption coefficient have a linear relationship, meaning the linear range for gas detection in a cylindrical photoacoustic cell with a buffer cavity is 0–0.0285 cm⁻¹. -1 . Figure 13 The graph shows the gas absorption coefficient response of a cylindrical photoacoustic cell with a buffer cavity. Figure 13 It can be seen that when the gas absorption coefficient is less than or equal to 0.171 cm⁻¹ -1 At this time, the photoacoustic signal is directly proportional to the gas absorption coefficient, while when the absorption coefficient is greater than 0.171 cm⁻¹... -1At this time, the photoacoustic signal decreases instead of increasing the gas absorption coefficient. Therefore, the dynamic range of gas detection in a cylindrical photoacoustic cell with a buffer cavity is 0–0.171 cm⁻¹. -1 .

[0075] Compared with the cylindrical photoacoustic cell with buffer cavity in the prior art, the side-branch photoacoustic cell of the present invention has a higher gas detection linear range and gas detection dynamic range than the cylindrical photoacoustic cell with buffer cavity, thus realizing the detection of high concentration gases.

[0076] A multi-component gas detection system, comprising:

[0077] Signal generator 4: The first signal output terminal of the signal generator 4 is connected to the signal input terminal of the light source controller 5, and is used to output a periodic signal of sine wave, square wave or sawtooth wave to the light source controller 5; the second signal output terminal of the signal generator 4 is connected to the first signal input terminal of the lock-in amplifier 8, and is used to output a square wave reference signal to the lock-in amplifier 8.

[0078] Light source controller 5: The signal output terminal of the light source controller 5 is connected to the signal input terminal of the laser 6, and is used to modulate the wavelength or amplitude of the laser beam emitted by the laser 6 according to the periodic signal;

[0079] Laser 6: The laser output end of the laser 6 is connected to the laser input end of the side-supported photoacoustic cell, and is used to emit a laser beam modulated by the light source controller 5 into the side-supported photoacoustic cell.

[0080] Side-supported photoacoustic cell: The signal output terminal of the acoustic detector 104 of the side-supported photoacoustic cell is connected to the signal input terminal of the microphone driving circuit 7; the gas to be tested is pre-introduced into the side-supported photoacoustic cell, and the modulated laser entering the side-supported photoacoustic cell passes through the gas to be tested to generate a photoacoustic effect; the acoustic detector 104 of the side-supported photoacoustic cell detects the sound wave signal generated by the photoacoustic effect and converts it into a voltage signal.

[0081] Microphone driver circuit 7: The signal output terminal of the microphone driver circuit 7 is connected to the second signal input terminal of the lock-in amplifier 8, and is used to transmit voltage signals to the lock-in amplifier 8;

[0082] Lock-in amplifier 8: The signal output terminal of the lock-in amplifier 8 is connected to the signal input terminal of the host computer 9, and the voltage signal is filtered, demodulated and amplified according to the square wave reference signal;

[0083] Host computer 9: Used to calculate the concentration information of multi-component gases based on voltage signals, and to complete the detection of the concentration and type of multi-component gases.

Claims

1. A side-branch type photoacoustic cell, characterized in that: It includes a resonant structure, which is connected to a first side-branch reinforcement structure and a second side-branch reinforcement structure, respectively, and the first side-branch reinforcement structure and the second side-branch reinforcement structure are located at both ends of the resonant structure. The resonant structure includes a cavity, which is cylindrical in shape. A first window and a second window are fixedly provided at both ends of the cavity along its axial direction. An acoustic conduit is provided at the top of the cavity, and its interior communicates with the cavity. An acoustic detector is provided at the top of the acoustic conduit. The first side-branch reinforcement structure and the second side-branch reinforcement structure are completely identical in structure; the first side-branch reinforcement structure includes a side-branch capillary with one end connected to the cavity, the other end of the side-branch capillary connected to the side-branch female port, the other end of the side-branch female port connected to the side-branch male port, the other end of the side-branch male port connected to the side-branch reinforcement tube, and the other end of the side-branch reinforcement tube connected to the acoustic boundary unit. The volume of the cavity is no greater than 0.6 mL, and at this time the frequency of the first even-order resonant mode of the side-branched photoacoustic cell is no greater than 2000 Hz; The volume of the side branch reinforcement tube is greater than the volume of the cavity; the length of the side branch reinforcement tube is at least greater than the length of the cavity, but not greater than 100cm. Both the first and second side support reinforcement structures have detachable cylindrical pipes as their side support reinforcement tubes. The inner diameter of the side capillary tube should simultaneously meet the following conditions: The inner diameter of the side capillary tube is no greater than 1.5 mm; The inner diameter of the side capillary is smaller than the length of the cavity; The inner diameter of the side capillary is smaller than the inner diameter of the cavity; The inner diameter of the side capillary is smaller than the inner diameter of the side reinforcing tube.

2. The side-supported photoacoustic cell according to claim 1, characterized in that: The side support reinforcement tubes of the first and second side support reinforcement structures are made of aluminum alloy, stainless steel, copper or Teflon; the acoustic boundary units of the first and second side support reinforcement structures are ball valves or check valves.

3. A side-supported photoacoustic cell according to claim 1, characterized in that: The first and second window sheets are made of calcium fluoride, zinc selenide, or ultraviolet fused silica; the acoustic detector is a capacitive microphone, a MEMS microphone, or a PVDF piezoelectric sensor.

4. A side-supported photoacoustic cell according to claim 1, characterized in that: The first and second side reinforcement structures are located on the same side or opposite side of the resonant structure, respectively.

5. A multi-component gas detection system comprising a side-supported photoacoustic cell as described in any one of claims 1-4, characterized in that, include: Signal generator: The first signal output terminal of the signal generator is connected to the signal input terminal of the light source controller, and is used to output a periodic signal of sine wave, square wave or sawtooth wave to the light source controller; the second signal output terminal of the signal generator is connected to the first signal input terminal of the lock-in amplifier, and is used to output a square wave reference signal to the lock-in amplifier; Light source controller: The signal output terminal of the light source controller is connected to the signal input terminal of the laser, and is used to modulate the wavelength or amplitude of the laser beam emitted by the laser according to the periodic signal; Laser: The laser output end of the laser is connected to the laser input end of the side-supported photoacoustic cell, and is used to emit a laser beam modulated by the light source controller into the side-supported photoacoustic cell; Side-supported photoacoustic cell: The signal output terminal of the acoustic detector of the side-supported photoacoustic cell is connected to the signal input terminal of the microphone driving circuit; the gas to be tested is pre-introduced into the side-supported photoacoustic cell, and the modulated laser entering the side-supported photoacoustic cell passes through the gas to be tested to generate a photoacoustic effect; the acoustic detector of the side-supported photoacoustic cell detects the acoustic wave signal generated by the photoacoustic effect and converts it into a voltage signal. Microphone driver circuit: The signal output terminal of the microphone driver circuit is connected to the second signal input terminal of the lock-in amplifier, and is used to transmit voltage signals to the lock-in amplifier; Lock-in amplifier: The signal output terminal of the lock-in amplifier is connected to the signal input terminal of the host computer, and the voltage signal is filtered, demodulated and amplified according to the square wave reference signal; Host computer: Used to calculate the concentration information of multi-component gases based on voltage signals, and to complete the detection of the concentration and type of multi-component gases.

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