An acoustic pressure focusing type photoacoustic gas sensor for suppressing interference of airflow noise and a detection method thereof

Abstract

The application discloses a sound pressure focusing type photoacoustic gas sensor for inhibiting airflow noise interference and a detection method thereof. The sensor comprises a ball-beam waist coupling T type photoacoustic cell, a laser generating module and a signal processing module. The ball-beam waist coupling T type photoacoustic cell comprises a cylindrical buffer cavity, a beam waist curved surface resonant cavity and a ball coupling structure. The beam waist curved surface resonant cavity is vertically coupled with the cylindrical buffer cavity to form a T type structure. The ball coupling structure is arranged at the top of the beam waist curved surface resonant cavity. The beam waist curved surface resonant cavity is used for realizing sound field focusing and concentrating sound pressure on the ball coupling structure. The ball coupling structure is used for inhibiting airflow noise and mounting an acoustic detection element. The laser generating module is used for generating modulated laser and introducing the modulated laser into the photoacoustic cell, so that the modulated laser interacts with a to-be-detected gas in the photoacoustic cell to generate a photoacoustic signal. The signal processing module is used for receiving and demodulating the photoacoustic signal to obtain concentration information of the to-be-detected gas. The application can realize airflow noise interference inhibition and sound pressure focusing improvement.

Description

Technical Field

[0001] This invention belongs to the field of gas detection technology, and particularly relates to an acoustic pressure focusing photoacoustic gas sensor and its detection method for suppressing airflow noise interference. Background Technology

[0002] With the development of laser technology, various spectroscopic gas detection methods have emerged, including direct absorption spectroscopy (DAS), tunable diode laser absorption spectroscopy (TDLAS), cavity-enhanced absorption spectroscopy (CEAS), and photoacoustic spectroscopy (PAS). Among them, PAS technology stands out due to its advantages of high sensitivity, fast response, and non-destructive detection, becoming one of the core technologies for trace gas detection. The core principle of PAS technology is that the interaction between modulated laser and gas molecules induces periodic thermal expansion of the gas. The resulting acoustic signal is detected by a detector, and the gas concentration is then deduced. The performance of the photoacoustic sensor is highly dependent on the structural design of the photoacoustic cell; therefore, the development of a high-performance photoacoustic cell is crucial for achieving accurate detection of trace gases.

[0003] Currently, photoacoustic cells are mainly divided into resonant and non-resonant types. Non-resonant photoacoustic cells are small in size but are easily affected by noise, while resonant photoacoustic cells have strong noise immunity but are usually larger in size. Common structures of resonant photoacoustic cells mainly include H-type, T-type and Helmholtz type. Among them, the T-type resonant photoacoustic cell has achieved miniaturization due to the vertical coupling between the buffer cavity and the resonant cavity, and has attracted much attention in the field of trace gas detection.

[0004] Traditional T-type photoacoustic cell resonators are cylindrical structures with uneven sound pressure distribution and no effective sound field focusing design. The photoacoustic signal amplitude is low, and the one-dimensional resonance mode formed by a single cavity limits energy superposition. The acoustic amplification efficiency is much lower than that of H-type and Helmholtz-type dual-cavity symmetrical resonant structures. At the same time, they lack airflow disturbance suppression design, resulting in high airflow noise. During dynamic detection, airflow-induced fluid noise easily interferes with the detection of acoustic signals, resulting in a low signal-to-noise ratio (SNR). It is difficult to further improve performance through existing methods such as optimizing the buffer cavity and increasing optical power.

[0005] Among existing T-shaped photoacoustic cell improvement schemes, the multi-resonator T-shaped photoacoustic cell fails to solve the sound pressure dispersion problem of the cylindrical resonator and lacks airflow disturbance suppression design; the spherical-cylindrical coupled T-shaped photoacoustic cell achieves a certain degree of sound pressure concentration, but cannot effectively suppress environmental noise and airflow noise, with the equivalent airflow noise masking the weak photoacoustic signal; the geometric discontinuity of the stepped T-shaped photoacoustic cell exacerbates airflow noise and lacks effective sound pressure focusing capability, resulting in poor signal amplification. Furthermore, some high-sensitivity photoacoustic cell improvement schemes require increasing the cavity volume and laser power, sacrificing the miniaturization advantages of the T-shaped photoacoustic cell and limiting its industrial applications.

[0006] The method of increasing optical power to improve detection sensitivity requires the use of equipment such as fiber optic amplifiers, but its applicable wavelength range is only a small part of the near-infrared wavelength, which has limitations in application. The method of increasing the effective optical path by in-cell contact reflection will introduce additional noise and reduce the system's SNR due to the solid photoacoustic effect caused by the material's light absorption, resulting in poor performance.

[0007] In other words, the existing technology cannot accurately achieve the synergy of sound pressure concentration, noise suppression and small volume at the same time, making it difficult to achieve high-sensitivity C2H2 trace detection while maintaining the miniaturization advantage of the T-shaped photoacoustic cell. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention proposes a sound pressure focusing photoacoustic gas sensor and its detection method that suppresses airflow noise interference. This invention can achieve a synergistic improvement in sound field focusing, sound pressure concentration, and airflow noise suppression, while maintaining the advantage of small size and significantly improving detection sensitivity and signal-to-noise ratio.

[0009] To achieve the above objectives, the present invention provides an acoustic pressure focusing photoacoustic gas sensor for suppressing airflow noise interference, comprising: a sphere-beam waist coupled T-shaped photoacoustic cell, a laser generation module, and a signal processing module; The spherical-waist-coupled T-shaped photoacoustic cell includes a cylindrical buffer cavity, a waist-shaped curved resonant cavity, and a spherical coupling structure. The waist-shaped curved resonant cavity is perpendicularly coupled to the cylindrical buffer cavity to form a T-shaped structure. The spherical coupling structure is disposed at the top of the waist-shaped curved resonant cavity. The waist-shaped curved resonant cavity is used to focus the sound field and concentrate the sound pressure on the spherical coupling structure. The spherical coupling structure is used to suppress airflow noise and install acoustic detection elements. The laser generating module is used to generate a modulated laser and guide the modulated laser into the photoacoustic cell, so that the modulated laser interacts with the gas to be tested in the photoacoustic cell to generate a photoacoustic signal. The signal processing module is used to receive and demodulate the photoacoustic signal to obtain the concentration information of the gas to be measured.

[0010] Optionally, the cylindrical buffer cavity is provided with an optical window at the bottom, an air outlet at the top, and an air inlet at the top; The optical window is used to transmit the modulated laser; The air inlet and the air outlet are positioned opposite each other on both sides of the optical window to form a symmetrical airflow distribution within the waisted curved resonant cavity.

[0011] Optionally, the laser generating module includes: a signal generator, a current driver, a temperature controller, and a distributed feedback laser; The signal generator is used to generate a composite modulated signal; The current driver is used to drive the distributed feedback laser according to the composite modulation signal; The temperature controller is used to stabilize the operating temperature of the distributed feedback laser, so that the output wavelength of the distributed feedback laser corresponds to the absorption peak of the gas to be tested.

[0012] Optionally, the signal processing module includes: a lock-in amplifier and a data acquisition unit; The lock-in amplifier is used to receive photoacoustic signals and reference signals, and extract the second harmonic signal through lock-in demodulation; The data acquisition unit is used to acquire the second harmonic signal and transmit it to the host computer for concentration inversion to obtain the concentration information of the gas to be measured.

[0013] This invention also provides a detection method for an acoustic pressure focusing photoacoustic gas sensor that suppresses airflow noise interference, comprising: Acquire modulated laser; The modulated laser is introduced into the photoacoustic cell through the optical window of the spherical-waist-coupled T-shaped photoacoustic cell, so that the modulated laser interacts with the gas molecules to be tested in the photoacoustic cell, exciting periodic thermal expansion to obtain photoacoustic signals. The spherical-waist-coupled T-shaped photoacoustic cell includes: a cylindrical buffer cavity, a waist curved resonant cavity, and a spherical coupling structure. The photoacoustic signal is acquired by an acoustic detection element installed at the apex of the spherical coupling structure, and the photoacoustic signal is subjected to phase-locked demodulation processing to extract the second harmonic signal related to the concentration of the gas to be measured. The concentration of the gas to be measured is calculated by inverting the amplitude of the second harmonic signal.

[0014] Optionally, before introducing the modulated laser into the photoacoustic cell, the following steps are included: A composite modulation signal, which is a superposition of a triangular wave scanning signal and a sine wave modulation signal, is generated by a signal generator. The composite modulation signal drives a distributed feedback laser, so that the center wavelength of the laser output by the distributed feedback laser is aligned with the characteristic absorption peak of the gas under test. The frequency of the sine wave modulation signal is set to half of the resonant frequency of the photoacoustic cell, thereby achieving synchronous modulation of wavelength scanning and photoacoustic excitation.

[0015] Optionally, performing phase-locked demodulation processing on the photoacoustic signal includes: The photoacoustic signal output by the acoustic detection element and the reference signal are input into a lock-in amplifier. The reference signal is in phase and frequency with the sinusoidal modulation signal used to excite the photoacoustic signal. The second harmonic component of the photoacoustic signal is extracted using the lock-in amplifier, and environmental noise and airflow-induced noise are filtered out to obtain the amplitude of the second harmonic signal that is proportional to the gas concentration.

[0016] Optionally, before acquiring the photoacoustic signal via an acoustic detection element mounted at the apex of the spherical coupling structure, the following steps are included: The surface parameters of the waisted resonant cavity and the radius of curvature of the spherical coupling structure were optimized using COMSOL Multiphysics simulation software to maximize the absolute sound pressure of the photoacoustic cell at the target resonant frequency and minimize the gas flow velocity at the apex of the spherical coupling structure, thereby determining the final structural dimensions of the photoacoustic cell.

[0017] Compared with the prior art, the present invention has the following advantages and technical effects: The STR photoacoustic cell of this invention achieves sound field focusing and sound pressure concentration through a composite design of a waisted curved surface and a spherical structure, increasing the absolute sound pressure by an order of magnitude and the 2f signal amplitude by 4.6 times. The spherical coupling structure effectively reduces gas flow velocity, significantly suppressing airflow noise, increasing the SNR from 503 to 7000, reducing the detection limit from 4970 ppb to 357 ppb, and improving sensitivity by 14 times. The optical path-to-volume ratio increases from 8.57 mm / mL to 10.52 mm / mL, and the NNEA coefficient decreases by an order of magnitude, significantly improving optical energy utilization efficiency and noise suppression capability. At the same time, it maintains the advantage of a small volume of 3.1 cm³, balancing miniaturization and high sensitivity, making it more suitable for practical applications in industrial settings. Attached Figure Description

[0018] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a detailed structural diagram of an acoustic pressure focusing photoacoustic gas sensor for suppressing airflow noise interference according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the photoacoustic cell structure according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the simulation results of an embodiment of the present invention, wherein (a) is a simulated cloud map of the photoacoustic field distribution of the traditional T-shaped photoacoustic cell and the STR photoacoustic cell, (b) is the simulated frequency response curve at detection points A and B, (c) is the simulated gas flow rate at detection points A1 and B1, and (d) is the curve of gas flow rate at detection points A1 and B1 changing with time. Figure 4 This is a schematic diagram of the experimental results of an embodiment of the present invention, wherein (a) is the 2f signal of 2500ppm acetylene measured using the STR photoacoustic cell and the conventional T-type photoacoustic cell, and (b) is the noise signal of 2500ppm acetylene measured using the STR photoacoustic cell and the conventional T-type photoacoustic cell. Detailed Implementation

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0020] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0021] Terminology Explanation: Spherical-waist-coupled T-type photoacoustic cell (STR): A novel T-type photoacoustic cell.

[0022] Photoacoustic (PA): When a pulsed laser irradiates a gas sample, the gas pressure will cause periodic expansion and contraction, thereby generating an acoustic signal. We call this acoustic signal generated by light excitation a photoacoustic signal.

[0023] Photoacoustic spectroscopy (PAS): A novel spectral analysis and detection technology based on the photoacoustic effect.

[0024] Signal-to-noise ratio (SNR): This is a metric that measures the ratio of signal strength to background noise strength.

[0025] This embodiment proposes a sound pressure focusing photoacoustic gas sensor that suppresses airflow noise interference, specifically including: a sphere-beam waist coupled T-shaped photoacoustic cell, a laser generation module, and a signal processing module; The spherical-waist-coupled T-shaped photoacoustic cell includes a cylindrical buffer cavity, a waist-shaped curved resonant cavity, and a spherical coupling structure. The waist-shaped curved resonant cavity is perpendicularly coupled to the cylindrical buffer cavity to form a T-shaped structure. The spherical coupling structure is disposed at the top of the waist-shaped curved resonant cavity. The waist-shaped curved resonant cavity is used to focus the sound field and concentrate the sound pressure on the spherical coupling structure. The spherical coupling structure is used to suppress airflow noise and install acoustic detection elements. The laser generating module is used to generate a modulated laser and guide the modulated laser into the photoacoustic cell, so that the modulated laser interacts with the gas to be tested in the photoacoustic cell to generate a photoacoustic signal. The signal processing module is used to receive and demodulate the photoacoustic signal to obtain the concentration information of the gas to be measured.

[0026] Specifically, this embodiment includes a laser, a signal generator, a photoacoustic cell, a subtraction circuit, a lock-in amplifier, a data acquisition card, a current driver, a temperature controller, and a LabVIEW host computer. A schematic diagram of the gas sensor detection system is shown below. Figure 1 As shown.

[0027] The core of the technical solution in this embodiment lies in designing a sphere-waist coupled T-type photoacoustic cell (STR) and building a complete high-sensitivity acetylene gas sensor system based on this photoacoustic cell. Through structural optimization and module collaboration, trace acetylene detection under low light power is achieved.

[0028] In this embodiment, the sensor detection system first generates a composite modulation signal, which is a superposition of a triangular wave scanning signal and a sine wave modulation signal, through a signal generator. This signal is then used to drive a C2H2 laser via a current driver and a temperature controller. The laser driver stabilizes the temperature of the DFB laser and supplies the driving current, enabling the dedicated C2H2 laser with a center wavelength of 1532.68 nm to operate. The laser output power is approximately 17 mW. After being collimated by an optical fiber collimator, the laser beam passes vertically through an optical window into the sealed STR photoacoustic cell. It interacts with the C2H2 gas molecules within the cell, causing them to absorb laser energy and undergo periodic thermal expansion, generating a photoacoustic signal. A microphone mounted at the apex of the spherical structure of the STR photoacoustic cell collects the photoacoustic signal and transmits it to a lock-in amplifier. The signal generator has one sine wave and one square wave for modulation, and another sine wave for demodulating the signal in the lock-in amplifier. The lock-in amplifier demodulates and extracts the second harmonic signal related to the gas concentration. This 2f signal is then sent to a LabVIEW monitoring platform embedded in a personal computer (PC) via a data acquisition card, enabling real-time display, concentration inversion, and data storage of the signal. Standard C2H2 gas and pure nitrogen (N2) are mixed in a specific ratio using a gas dilution system to prepare C2H2 gas of different concentrations. These gases are then introduced into a STR photoacoustic cell for dynamic and continuous measurement. After measurement, the residual gas in the cell is pumped to an exhaust gas treatment unit for further processing.

[0029] Furthermore, the cylindrical buffer cavity is provided with an optical window at the bottom, an air outlet at the top, and an air inlet at the top; The optical window is used to transmit the modulated laser; The air inlet and the air outlet are positioned opposite each other on both sides of the optical window to form a symmetrical airflow distribution within the waisted curved resonant cavity.

[0030] Furthermore, the laser generating module includes: a signal generator, a current driver, a temperature controller, and a distributed feedback laser; The signal generator is used to generate a composite modulated signal; The current driver is used to drive the distributed feedback laser according to the composite modulation signal; The temperature controller is used to stabilize the operating temperature of the distributed feedback laser, so that the output wavelength of the distributed feedback laser corresponds to the absorption peak of the gas to be tested.

[0031] Furthermore, the signal processing module includes: a lock-in amplifier and a data acquisition unit; The lock-in amplifier is used to receive photoacoustic signals and reference signals, and extract the second harmonic signal through lock-in demodulation; The data acquisition unit is used to acquire the second harmonic signal and transmit it to the host computer for concentration inversion to obtain the concentration information of the gas to be measured.

[0032] In this embodiment, the STR photoacoustic cell differs from the traditional T-shaped PA absorption cell. It abandons the traditional cylindrical resonant cavity and adopts a composite design of a waisted curved resonant cavity and a top spherical coupling structure. Combined with the vertical coupling of the cylindrical buffer cavity and the resonant cavity, it forms an improved T-shaped structure. A schematic diagram of the STR photoacoustic cell is shown below. Figure 2 As shown, the STR photoacoustic cell mainly consists of a waisted curved resonant cavity, a spherical coupling structure, a cylindrical buffer cavity, an optical window, an air inlet, an air outlet, and a microphone mounting position. The optical window is located at the bottom of the buffer cavity, providing high light transmittance and reducing laser transmission loss. Both the air inlet and outlet are located at the bottom of the buffer cavity, and parameter optimization ensures uniform airflow distribution, maximizing the suppression of airflow noise. The microphone is mounted at the apex of the spherical coupling structure, where the sound pressure is at its maximum, enabling efficient acquisition of photoacoustic signals. The waisted curved resonant cavity utilizes the curved surface characteristics to focus the sound field, concentrating the sound pressure at the top of the cavity. The spherical coupling structure further enhances the sound pressure concentration effect while effectively reducing gas velocity, minimizing airflow disturbance, and suppressing airflow noise. The laser interacts fully with gas molecules within the photoacoustic cell, achieving efficient conversion of photoacoustic signals. It can be used to detect C2H2 gas with concentrations ranging from 0 to 2500 ppm.

[0033] The STR photoacoustic cell was modeled and simulated using COMSOL Multiphysics software. The sound pressure distribution and airflow velocity within the cell were simulated using both the pressure acoustic frequency domain module and the turbulence module to determine the photoacoustic signal amplitude and airflow noise level. In the pressure acoustic frequency domain simulation, the temperature was set to 20℃ and standard atmospheric pressure, with air used as the model medium for the photoacoustic cell. Hard acoustic walls were used as boundary conditions, and a line source was used to simulate the laser beam with a step size of 1Hz. The simulation results are shown below. Figure 3 As shown. By Figure 3 From (a)-(b), we can see that the resonant frequency of the traditional T-shaped photoacoustic cell is 5038Hz, and the sound pressure distribution is dispersed. In contrast, the resonant frequency of the STR photoacoustic cell is 5204Hz, and the sound pressure is highly concentrated at the apex of the spherical coupling structure. The absolute sound pressure is an order of magnitude higher than that of the traditional T-shaped photoacoustic cell, and the photoacoustic signal amplitude is significantly enhanced. In the turbulence module simulation, the normal inflow velocity at the inlet is set to 2 m / s, the wall is a no-slip boundary condition, and the outlet is a fully flowing boundary condition. The simulation results are as follows: Figure 3 As shown in (c)-(d), the gas flow velocity at the detection point of the traditional T-shaped photoacoustic cell is stable at 5.85 × 10⁻⁶. -5The gas flow rate at the STR photoacoustic cell detection point in this embodiment is significantly reduced, stabilizing at 4.72 × 10 m / s. -14 m / s, thus verifying that the designed STR photoacoustic cell has the characteristic of lower airflow noise.

[0034] The performance of the STR photoacoustic cell was verified experimentally. An STR photoacoustic cell (manufacturing precision 0.1 mm) and a traditional T-type photoacoustic cell were fabricated using photosensitive resin via 3D printing technology, and their performance was compared under the same experimental conditions. Gas samples were prepared using a dynamic gas distribution method with a gas dilution system. C2H2 concentrations ranging from 0 to 2500 ppm were introduced into the STR photoacoustic cell, and multiple measurements were taken for each C2H2 concentration. Experimental results showed that the resonant frequency of the STR photoacoustic cell was 5204 Hz, corresponding to a sinusoidal modulation frequency of 2602 Hz, at which the 2f signal amplitude was maximized. Further optimization of the modulation amplitude resulted in a modulation amplitude of 170 mV (corresponding to a modulation depth of 0.68 cm). -1 At this point, the 2f signal amplitude reaches its peak, achieving optimal interaction efficiency between the laser and gas molecules. In quantitative detection, gradient concentration C2H2 samples were prepared using a gas dilution system. The 2f signal characteristics at different concentrations were clearly distinguishable, and the signal amplitude increased stepwise with increasing concentration. The STR photoacoustic cell showed a good linear relationship between the 2f signal amplitude and gas concentration for C2H2 detection, with a goodness of fit of 0.999 and a detection sensitivity of 0.314 μV / ppm. The STR photoacoustic cell exhibits excellent response speed and repeatability. After being purged with 2500ppm C2H2, the sensor stabilizes within 3.1 seconds and recovers within 3.9 seconds after being purged with pure N2. When alternating between 2500ppm C2H2 and pure N2, the signal amplitude fluctuation is small, and the repeatability is good. Figure 4 As shown in (a)-(b), when 2500ppm C2H2 is introduced, the 2f signal amplitude of the STR photoacoustic cell reaches 7.98V, while the 2f signal amplitude of the traditional T-type photoacoustic cell is only 1.72V. The photoacoustic signal amplitude of the STR photoacoustic cell is 4.6 times that of the traditional T-type photoacoustic cell. Furthermore, when detecting system noise under pure nitrogen gas, the standard deviation (1σ) of the noise signal of the traditional T-type photoacoustic cell is 3.42mV, resulting in a calculated SNR of 503 and a corresponding minimum detection limit (LoD) of 4970ppb. The standard deviation (1σ) of the noise signal of the STR photoacoustic cell of this invention is 1.14mV, resulting in a calculated SNR of 7000 and a corresponding LoD of 357ppb. Compared to the traditional T-type photoacoustic cell, the SNR of the STR photoacoustic cell is significantly improved, and the detection limit is reduced by an order of magnitude, demonstrating the superiority of the STR photoacoustic cell used in this invention.

[0035] The noise of the system was detected under the condition of pure nitrogen gas. The standard deviation of the noise signal of the conventional T-type photoacoustic cell is (1σ). The optical energy utilization efficiency of the STR photoacoustic cell of the present invention can be further improved by optimizing the structural parameters. The optical energy utilization efficiency of the photoacoustic cell is evaluated by calculating the optical path-to-volume ratio (OPVR). The optical path of the STR photoacoustic cell is about 32.6 mm and the volume is 3.1 mL, as shown in formula (1): (1); The OPVR reaches 10.52 mm / mL, while the OPVR of the traditional T-type photoacoustic cell is only 8.57 mm / mL with an optical path of 30 mm and a volume of 3.5 mL. The optical energy utilization efficiency of the STR photoacoustic cell is significantly improved. At the same time, the normalized noise equivalent absorption (NNEA) coefficient can be calculated by formula (1).

[0036] (2); in, α min The noise equivalent absorption (NEA) is 1.98 × 10⁻⁶. -7 cm -1 Laser power W in It is 17mW, with a noise equivalent bandwidth. BW The frequency is 0.25 Hz. According to equation (2), the NNEA of the STR photoacoustic cell is 6.73 × 10⁻⁶. -9 cm -1 •W•Hz -1 / 2 Compared to the traditional T-shaped photoacoustic cell with a diameter of 9.38×10, -8 cm -1 •W•Hz -1 / 2 Reducing noise by an order of magnitude significantly improves noise suppression capability and overall performance.

[0037] This embodiment also provides a detection method for an acoustic pressure focusing photoacoustic gas sensor that suppresses airflow noise interference, including: Acquire modulated laser; The modulated laser is introduced into the photoacoustic cell through the optical window of the spherical-waist-coupled T-shaped photoacoustic cell, so that the modulated laser interacts with the gas molecules to be tested in the photoacoustic cell, exciting periodic thermal expansion to obtain photoacoustic signals. The spherical-waist-coupled T-shaped photoacoustic cell includes: a cylindrical buffer cavity, a waist curved resonant cavity, and a spherical coupling structure. The photoacoustic signal is acquired by an acoustic detection element installed at the apex of the spherical coupling structure, and the photoacoustic signal is subjected to phase-locked demodulation processing to extract the second harmonic signal related to the concentration of the gas to be measured. The concentration of the gas to be measured is calculated by inverting the amplitude of the second harmonic signal.

[0038] Further, before introducing the modulated laser into the photoacoustic cell, the following steps are included: A composite modulation signal, which is a superposition of a triangular wave scanning signal and a sine wave modulation signal, is generated by a signal generator. The composite modulation signal drives a distributed feedback laser, so that the center wavelength of the laser output by the distributed feedback laser is aligned with the characteristic absorption peak of the gas under test. The frequency of the sine wave modulation signal is set to half of the resonant frequency of the photoacoustic cell, thereby achieving synchronous modulation of wavelength scanning and photoacoustic excitation.

[0039] Further, the phase-locked demodulation processing of the photoacoustic signal includes: The photoacoustic signal output by the acoustic detection element and the reference signal are input into a lock-in amplifier. The reference signal is in phase and frequency with the sinusoidal modulation signal used to excite the photoacoustic signal. The second harmonic component of the photoacoustic signal is extracted using the lock-in amplifier, and environmental noise and airflow-induced noise are filtered out to obtain the amplitude of the second harmonic signal that is proportional to the gas concentration.

[0040] Furthermore, before acquiring the photoacoustic signal through the acoustic detection element installed at the apex of the spherical coupling structure, the following steps are included: The surface parameters of the waisted resonant cavity and the radius of curvature of the spherical coupling structure were optimized using COMSOL Multiphysics simulation software to maximize the absolute sound pressure of the photoacoustic cell at the target resonant frequency and minimize the gas flow velocity at the apex of the spherical coupling structure, thereby determining the final structural dimensions of the photoacoustic cell.

[0041] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A sound pressure focusing photoacoustic gas sensor for suppressing airflow noise interference, characterized in that, include: Spherical-waist-coupled T-shaped photoacoustic cell, laser generation module, and signal processing module; The spherical-waist-coupled T-shaped photoacoustic cell includes a cylindrical buffer cavity, a waist-shaped curved resonant cavity, and a spherical coupling structure. The waist-shaped curved resonant cavity is perpendicularly coupled to the cylindrical buffer cavity to form a T-shaped structure. The spherical coupling structure is disposed at the top of the waist-shaped curved resonant cavity. The waist-shaped curved resonant cavity is used to focus the sound field and concentrate the sound pressure on the spherical coupling structure. The spherical coupling structure is used to suppress airflow noise and install acoustic detection elements. The laser generating module is used to generate a modulated laser and guide the modulated laser into the photoacoustic cell, so that the modulated laser interacts with the gas to be tested in the photoacoustic cell to generate a photoacoustic signal. The signal processing module is used to receive and demodulate the photoacoustic signal to obtain the concentration information of the gas to be measured.

2. The acoustic pressure focusing photoacoustic gas sensor for suppressing airflow noise interference according to claim 1, characterized in that, The cylindrical buffer cavity is provided with an optical window and an air outlet at the bottom and an air inlet at the top; The optical window is used to transmit the modulated laser; The air inlet and the air outlet are positioned opposite each other on both sides of the optical window to form a symmetrical airflow distribution within the waisted curved resonant cavity.

3. The acoustic pressure focusing photoacoustic gas sensor for suppressing airflow noise interference according to claim 1, characterized in that, The laser generating module includes: a signal generator, a current driver, a temperature controller, and a distributed feedback laser; The signal generator is used to generate a composite modulated signal; The current driver is used to drive the distributed feedback laser according to the composite modulation signal; The temperature controller is used to stabilize the operating temperature of the distributed feedback laser, so that the output wavelength of the distributed feedback laser corresponds to the absorption peak of the gas to be tested.

4. The acoustic pressure focusing photoacoustic gas sensor for suppressing airflow noise interference according to claim 1, characterized in that, The signal processing module includes: a lock-in amplifier and a data acquisition unit; The lock-in amplifier is used to receive photoacoustic signals and reference signals, and extract the second harmonic signal through lock-in demodulation; The data acquisition unit is used to acquire the second harmonic signal and transmit it to the host computer for concentration inversion to obtain the concentration information of the gas to be measured.

5. A detection method for a sound pressure focusing photoacoustic gas sensor that suppresses airflow noise interference, used to implement the sensor as described in any one of claims 1-4, characterized in that, include: Acquire modulated laser; The modulated laser is introduced into the photoacoustic cell through the optical window of the spherical-waist-coupled T-shaped photoacoustic cell, so that the modulated laser interacts with the gas molecules to be tested in the photoacoustic cell, exciting periodic thermal expansion to obtain photoacoustic signals. The spherical-waist-coupled T-shaped photoacoustic cell includes: a cylindrical buffer cavity, a waist curved resonant cavity, and a spherical coupling structure. The photoacoustic signal is acquired by an acoustic detection element installed at the apex of the spherical coupling structure, and the photoacoustic signal is subjected to phase-locked demodulation processing to extract the second harmonic signal related to the concentration of the gas to be measured. The concentration of the gas to be measured is calculated by inverting the amplitude of the second harmonic signal.

6. The detection method of a sound pressure focusing photoacoustic gas sensor for suppressing airflow noise interference according to claim 5, characterized in that, Before introducing the modulated laser into the photoacoustic cell, the following steps are included: A composite modulation signal, which is a superposition of a triangular wave scanning signal and a sine wave modulation signal, is generated by a signal generator. The composite modulation signal drives a distributed feedback laser, so that the center wavelength of the laser output by the distributed feedback laser is aligned with the characteristic absorption peak of the gas under test. The frequency of the sine wave modulation signal is set to half of the resonant frequency of the photoacoustic cell, thereby achieving synchronous modulation of wavelength scanning and photoacoustic excitation.

7. The detection method of a sound pressure focusing photoacoustic gas sensor for suppressing airflow noise interference according to claim 5, characterized in that, Phase-locked demodulation processing of the photoacoustic signal includes: The photoacoustic signal output by the acoustic detection element and the reference signal are input into a lock-in amplifier. The reference signal is in phase and frequency with the sinusoidal modulation signal used to excite the photoacoustic signal. The second harmonic component of the photoacoustic signal is extracted using the lock-in amplifier, and environmental noise and airflow-induced noise are filtered out to obtain the amplitude of the second harmonic signal that is proportional to the gas concentration.

8. The detection method of a sound pressure focusing photoacoustic gas sensor for suppressing airflow noise interference according to claim 5, characterized in that, Before acquiring the photoacoustic signal via an acoustic detection element installed at the apex of the spherical coupling structure, the following steps are included: The surface parameters of the waisted resonant cavity and the radius of curvature of the spherical coupling structure were optimized using COMSOL Multiphysics simulation software to maximize the absolute sound pressure of the photoacoustic cell at the target resonant frequency and minimize the gas flow velocity at the apex of the spherical coupling structure, thereby determining the final structural dimensions of the photoacoustic cell.

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