Trace gas detection device based on high-performance spherical photoacoustic cell of photoacoustic spectroscopy

Abstract

A trace gas detection device based on high-performance spherical photoacoustic cell is proposed in this paper. Firstly, the acoustic modal optimization of spherical resonant cavity is carried out by acoustic-optic coupling modeling simulation and finite element analysis, and the resonant frequency, sound pressure distribution and the best installation site of acoustic sensor are determined. According to the above, the spherical photoacoustic cell with built-in buffer cavity is precisely processed, and the high-sensitivity acoustic sensor is embedded at the point of maximum sound pressure. The modulated light which matches the absorption peak of the gas to be measured is generated by the tunable laser, and then it is introduced into the cavity through the incident window. The spherical standing wave is excited by the absorption of light energy of the gas to be measured, and the resonance-enhanced photoacoustic signal is captured by the sensor and converted into an electrical signal. After preamplification and lock-in amplification, the gas concentration is finally inverted according to the calibration model. The device has the characteristics of compact structure and high sensitivity.

Description

Technical Field

[0001] This invention belongs to the field of gas composition detection and analysis technology, specifically relating to a photoacoustic spectroscopy gas detection device. More specifically, this invention relates to a novel device that uses a spherical photoacoustic cell as the core sensing unit and optimizes its acoustic-optical coupling structure and signal modulation and demodulation methods to achieve high sensitivity, high stability, and high selectivity in gas detection. Background Technology

[0002] In recent years, significant progress has been made in the optimization of photoacoustic spectroscopy systems. Excitation sources have evolved from near-infrared DFB-EDFA combinations to mid-infrared QCLs, significantly improving detection sensitivity. In 2013, Ma et al. used a mid-infrared quantum cascade laser as the excitation source, achieving detection limits as high as ppb and ppt for carbon monoxide (CO) and nitrous oxide (N2O), respectively. Regarding detectors, the quartz tuning fork (QTF), as a novel acoustic detector, is a core component of quartz-enhanced photoacoustic spectroscopy (QEPAS), a technology widely used in photoacoustic spectroscopy due to its high sensitivity and compactness.

[0003] The photoacoustic amplifier (PA) unit is a key component of photoacoustic absorption spectroscopy, and various types of PA units are used in this technology. Examples include cylindrical resonant cavities, differential Holmhertz resonant cavities, T-type longitudinal resonant cavities, and spherical resonant cavities. Among these, the cylindrical photoacoustic cell, with its regular axisymmetric structure, easily forms stable, high-quality-factor acoustic standing wave modes and is easy to manufacture, thus enjoying widespread use, although its Q value is relatively low. The spherical photoacoustic cell, on the other hand, has no viscous loss in radial mode and can efficiently confine acoustic wave energy within the cavity, producing extremely high acoustic quality factors (Q values), reaching the order of several hundred, thereby significantly amplifying weak photoacoustic signals. In existing research, Gong et al. developed a sensor integrating a spherical resonant cavity and an optical fiber acoustic transducer for methane detection, achieving a sensitivity of 1.23 ppm in an integration time of 10 s. Li's team utilized multiple resonance modes of the spherical resonant cavity to achieve simultaneous multi-component detection of water vapor, carbon dioxide, and methane, with sensitivities of 1.17 ppm (H2O), 83 ppm (CO2), and 1.76 ppm (CH4), respectively, and corresponding integration times of 136 seconds, 181 seconds, and 195 seconds.

[0004] In this invention, a high-quality-factor spherical photoacoustic resonator was fabricated. The acoustic characteristics of the spherical photoacoustic cell were theoretically explored, and the spherical photoacoustic cell in the first-order radial mode was simulated using finite element method (FEM) software to determine the resonant frequency and sound pressure level distribution in the first-order resonant mode. CH4 gas was selected as the test gas, and a near-infrared distributed feedback (DFB) diode laser emitting near-1648nm infrared light was used as the excitation source. Finally, its performance was evaluated. Compared with existing spherical photoacoustic cells, this photoacoustic cell has a smaller size, higher gain, and better sensitivity and robustness. Summary of the Invention

[0005] This invention aims to address the problems of poor optical mode and acoustic mode matching, mutual constraints between effective optical path and cell volume, limited improvement of acoustic quality factor (Q value), fixed operating frequency, and poor flexibility in existing photoacoustic cells, particularly cylindrical resonant cavities. The purpose of this invention is to provide a gas detection device based on a spherical photoacoustic cell, which can achieve efficient coupling of light and sound energy, and a high acoustic Q value within a small cell volume, thereby significantly improving detection sensitivity, stability, and response speed. The technical solution of this invention is as follows:

[0006] 1. A novel high-performance spherical photoacoustic cell gas detection device based on photoacoustic spectroscopy, characterized by comprising the following steps:

[0007] Step 1: Use finite element analysis software to establish an accurate three-dimensional model of the spherical resonant cavity and perform acoustic modal analysis;

[0008] Step 2: Determine its resonant frequency and the corresponding sound pressure distribution, identify the optimal location of the sound pressure maximum point, and determine the placement position of the sound wave sensor;

[0009] Step 3: Based on the optimized parameters obtained from the simulation, fabricate a high-precision spherical photoacoustic cell body and accurately install the acoustic wave sensor;

[0010] Step 4: The modulated laser light is collimated and enters the spherical cavity through the entrance window, where it interacts with the gas to be tested via a photoacoustic effect, causing the gas to generate a resonant and enhanced photoacoustic signal.

[0011] Step 5: Use an acoustic wave sensor to detect photoacoustic signals, and use a lock-in amplifier to extract effective electrical signals from background noise with the modulation frequency as a reference.

[0012] Step 6: The calculation unit calculates the precise concentration of the gas to be tested based on the extracted signal intensity and the signal-concentration curve of the calibration experiment.

[0013] Furthermore, during the design and manufacturing phases of the device, finite element software simulation analysis and optimization steps were introduced. The relationship between the resonant frequency and the quality factor and the radius was analyzed through parametric scanning. Based on practical considerations, the diameter of the spherical photoacoustic cell was finally determined. The optimal acoustic mode and its inverse node position of the spherical cavity were determined through finite element analysis, thereby accurately guiding the placement of the acoustic sensor and ensuring that the optical excitation region and the strongest acoustic mode are highly overlapped in space.

[0014] Furthermore, the optical excitation unit employs wavelength modulation spectroscopy and second harmonic detection technology, that is, while scanning the laser wavelength at the center of the gas absorption line, high-frequency intensity modulation is performed; the signal processing and inversion unit uses a lock-in amplifier to demodulate the photoacoustic signal at the second harmonic frequency, thereby effectively suppressing background noise and improving the signal-to-noise ratio and detection selectivity.

[0015] A computer program product, characterized in that, when the computer program is executed by a processor, it implements the frequency sweep processing for the function generator in the calibration experiment as described in claim 4.

[0016] The advantages and beneficial effects of this invention are as follows:

[0017] The present invention has the following beneficial effects:

[0018] 1. Extremely high sensitivity: The spherical acoustic resonant cavity has a high acoustic quality factor (Q value), which greatly amplifies the photoacoustic signal and achieves ultra-high sensitivity detection.

[0019] 2. Improved signal-to-noise ratio: Simulation results show that adding buffer cavities at both ends of the photoacoustic cell can greatly suppress airflow noise without significantly reducing the acoustic signal intensity, thereby significantly improving the signal-to-noise ratio and detection sensitivity.

[0020] 3. Fast response speed: Due to the compact structure and small internal cavity volume of the spherical photoacoustic cell, the displacement time of the gas to be measured in the cavity is significantly shortened, which enables the device to have a faster dynamic response capability.

[0021] 4. Excellent stability and anti-interference performance: Employing second harmonic detection technology, it effectively suppresses common-mode interference such as laser intensity fluctuations, floor noise, and stray light, significantly improving measurement stability and reliability. The symmetrical structure of the spherical cavity also makes it insensitive to external mechanical vibrations and other interferences. Attached Figure Description

[0022] Figure 1 This is a sound pressure level diagram based on finite element analysis of the present invention;

[0023] Figure 2 This is an overall structural diagram of the spherical photoacoustic cell of the present invention;

[0024] Figure 3 This is a complete diagram of the experimental system of this invention; Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and thoroughly described below with reference to the accompanying drawings. The described embodiments are merely some embodiments of the present invention.

[0026] The technical solution of the present invention to solve the above-mentioned technical problems is:

[0027] A novel high-performance spherical photoacoustic cell gas detection device based on photoacoustic spectroscopy is characterized by comprising the following steps:

[0028] Step 1: Use finite element analysis software to establish an accurate three-dimensional model of the spherical resonant cavity and perform acoustic modal analysis;

[0029] Step 2: Determine its resonant frequency and the corresponding sound pressure distribution, identify the optimal location of the sound pressure maximum point, and determine the placement position of the sound wave sensor;

[0030] Step 3: Based on the optimized parameters obtained from the simulation, fabricate a high-precision spherical photoacoustic cell body and accurately install the acoustic wave sensor;

[0031] Step 4: The modulated laser light is collimated and enters the spherical cavity through the entrance window, where it interacts with the gas to be tested via a photoacoustic effect, causing the gas to generate a resonant and enhanced photoacoustic signal.

[0032] Step 5: Use an acoustic wave sensor to detect photoacoustic signals, and use a lock-in amplifier to extract effective electrical signals from background noise with the modulation frequency as a reference.

[0033] Step 6: The calculation unit calculates the precise concentration of the gas to be tested based on the extracted signal intensity and the signal-concentration curve of the calibration experiment.

[0034] The sound pressure level distribution of an ideal spherical photoacoustic cell is shown in the attached figure. Figure 1 As shown, the sound pressure level gradually decreases from the center of the sphere to the wall of the sphere under the resonance of the photoacoustic pool. Therefore, we need to place the microphone as close to the center of the sphere as possible in order to obtain a larger photoacoustic signal. However, in order to block the input and output of the modulation light, we choose to place the microphone 5mm away from the sphere.

[0035] After completing the simulation optimization based on finite element analysis, a specific implementation scheme was selected according to the optimal structural parameters determined by the simulation calculation. Material selection and component processing: Considering machinability, acoustic performance, cost, and stability, aluminum alloy is preferred as the processing material for the spherical photoacoustic cell body in this embodiment. Aluminum alloy has good rigidity and internal damping characteristics, which helps to suppress unnecessary mechanical resonance, and a high-gloss inner surface can be obtained through precision machining to reduce sound wave energy loss and maintain a high acoustic quality factor (Q value). Subsequently, using precision CNC machine tools and specific spherical surface machining technology, a spherical photoacoustic cell body conforming to the design specifications was manufactured. The inner surface of its spherical resonant cavity was polished to reduce light scattering loss. Integration and assembly of core components: as shown in the attached... Figure 2 As shown, the installation of the following key components was completed: 1. Integration of acoustic sensing components: A high-sensitivity condenser microphone was installed and fixed in the simulated location using an embedded structure. This installation method ensures efficient coupling between the microphone diaphragm and the acoustic field within the cavity, while maintaining the integrity of the acoustic boundary conditions of the spherical cavity to maintain a high resonant Q value. 2. Construction of the optical path: At the optimal tangential incident position determined in the simulation, a calcium fluoride (CaF2) optical window was welded or installed via a vacuum-sealed flange. The CaF2 material has high transmittance in the infrared band (e.g., mid-infrared band) where the laser operates and is physically and chemically stable, ensuring efficient laser excitation while providing a reliable hermetically sealed environment for the entire photoacoustic cell.

[0036] At this point, a high-performance spherical photoacoustic cell core sensing component has been fabricated. This physical component strictly adheres to the simulation optimization results, providing a reliable physical basis for subsequent gas detection experiments to verify its high performance.

[0037] After successfully fabricating the core sensing component of the spherical photoacoustic cell, we followed the attached... Figure 2 The system architecture shown illustrates the construction of a complete gas detection experimental system, and the detection performance of the device was systematically tested and verified. The specific implementation process is as follows: 1. Gas path system and sample preparation are shown in the attached figure. Figure 3As shown, we constructed a mixed gas path system consisting of an air pump, a nitrogen cylinder, a methane cylinder, a gas flow meter, and a water vapor filter membrane. Nitrogen was used as the background gas, and methane as the target gas. The flow rates of each gas component were precisely controlled by the gas flow meter, and they were thoroughly mixed in the mixing chamber to prepare standard methane gas samples of different concentrations. The water vapor filter membrane in the gas path provides water to increase the relaxation rate of methane molecules. The mixed gas is then introduced into a spherical photoacoustic cell via a pipeline, and its flow direction is clearly indicated by the arrows in the figure. 2. Optical Excitation and Modulation System Optical excitation is achieved by a distributed feedback laser. The emitted laser is collimated by a collimator, then focused and guided by a CaF2 lens, and finally enters the spherical resonant cavity through the CaF2 window on the spherical photoacoustic cell. A function generator is used simultaneously to: a) inject a low-frequency scanning signal into the laser, causing its output wavelength to periodically scan through specific absorption lines of the methane gas; b) generate a high-frequency sinusoidal signal for modulating the laser intensity. 3. Signal Detection and Processing Chain: The photoacoustic signal generated by gas absorption is detected by a microphone inside the cell and converted into a weak electrical signal. This signal is first amplified by a preamplifier and then input into a lock-in amplifier. The reference channel of the lock-in amplifier is synchronized with the high-frequency modulation signal provided by the function generator, but its reference frequency is set to twice the modulation frequency, thereby accurately extracting the second harmonic signal generated by the wavelength modulation spectroscopy technique. Finally, the demodulated 2f signal is acquired, recorded, and processed by a computer. 4. Performance Testing and Results: Using the above system, we measured methane gas samples of different concentrations. The peak value of the second harmonic signal recorded by the computer showed a good linear relationship with the known gas concentration, and a standard calibration curve was plotted accordingly. Experimental results show that the detection device based on this spherical photoacoustic cell exhibits extremely high detection sensitivity and excellent signal stability for methane gas, verifying its feasibility as a high-performance gas sensing platform.

[0038] Preferably, the gas path system further includes a mass flow controller to replace or assist the gas flow meter, thereby achieving higher precision and more stable control over the flow rates of nitrogen and methane gases. Furthermore, a buffer container is added before the mixed gas enters the spherical photoacoustic cell to stabilize the airflow and reduce the interference of pressure pulsations on the acoustic resonant modes.

[0039] Preferably, the laser is a distributed feedback semiconductor laser operating near the near-infrared absorption peak of methane gas (e.g., 1653 nm). The collimator and the CaF2 lens together constitute a beam shaping system.

[0040] The device described in this invention is not limited to the detection of methane gas. By simply replacing the laser with one whose output wavelength matches the absorption spectrum of the gas to be measured (e.g., a 1576 nm DFB laser for detecting carbon dioxide, or a 1392 nm DFB laser for detecting water vapor), and accordingly adjusting the modulation frequency of the function generator to the acoustic resonance frequency of the spherical cavity in the new system, this device can be quickly applied to the high-sensitivity detection of a variety of other trace gases.

[0041] The above embodiments should be understood as illustrative only and not as limiting the scope of protection of the present invention. After reading the description of the present invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A novel high-performance spherical photoacoustic cell gas detection device based on photoacoustic spectroscopy, characterized in that... Includes the following steps: Step 1: Use finite element analysis software to establish an accurate three-dimensional model of the spherical resonant cavity and perform acoustic modal analysis; Step 2: Determine its resonant frequency and the corresponding sound pressure distribution, identify the optimal location of the sound pressure maximum point, and determine the placement position of the sound wave sensor; Step 3: Based on the optimized parameters obtained from the simulation, fabricate a high-precision spherical photoacoustic cell body and accurately install the acoustic wave sensor; Step 4: The modulated laser is collimated and enters the spherical cavity through the entrance window, where it interacts with the gas to be tested to produce a photoacoustic effect, thereby generating a resonant and enhanced photoacoustic signal in the gas to be tested. Step 5: Use an acoustic wave sensor to detect photoacoustic signals, and use a lock-in amplifier to extract effective electrical signals from background noise with the modulation frequency as a reference. Step 6: The calculation unit calculates the precise concentration of the gas to be tested based on the extracted signal intensity and the signal-concentration curve of the calibration experiment.

2. The gas detection device according to claim 1, characterized in that, Before manufacturing the spherical photoacoustic cell unit, the system further includes a simulation optimization module, which is used to model and simulate the spherical resonant cavity using the finite element analysis method to determine the resonant frequency and anti-node distribution of its acoustic modes, and optimize the size of the spherical resonant cavity and the placement position of the acoustic wave sensor based on the simulation results.

3. The gas detection device according to claim 1 or 2, characterized in that, The optical excitation unit includes: a distributed feedback laser for emitting laser light that matches the absorption peak of the gas to be measured; a function generator for modulating the laser light into a specific frequency intensity modulated light; and an optical fiber collimator for guiding the modulated laser light from the light inlet into the spherical resonant cavity.

4. The gas detection device according to claim 3, characterized in that, The signal processing and inversion unit includes: a preamplifier connected to the acoustic sensor for primary amplification of the photoacoustic signal; a lock-in amplifier whose reference input is synchronized with the modulation signal of the function generator for extracting an effective signal with the same frequency as the modulation frequency from the amplified signal; and a calculation module for inverting the concentration of the gas to be measured based on the amplitude of the effective signal and a pre-stored calibration curve.

5. The gas detection device according to claim 2, characterized in that, The acoustic sensor is a microphone or a piezoelectric ceramic sensor, which is embedded at the acoustic pressure inverse node determined by simulation optimization for the target acoustic mode.

6. A gas detection method, characterized in that, The gas detection device according to any one of claims 1 to 5 includes the following steps: a modulated laser is incident onto the resonant cavity of a spherical photoacoustic cell through an optical excitation unit to excite the gas to be tested to generate a photoacoustic signal; the photoacoustic signal generated by the gas is detected by an acoustic wave sensor located at the inverse node of the acoustic mode; and the photoacoustic signal is processed by a signal processing and inversion unit to obtain the concentration information of the gas to be tested.

7. The gas detection device according to claim 1, characterized in that, The signal processing and inversion unit is configured to: extract the signal component in the photoacoustic signal whose frequency is twice the laser intensity modulation frequency, and invert the gas concentration based on the amplitude of the second harmonic signal.

8. A computer program product, characterized in that, When the computer program is executed by the processor, it implements the frequency sweep processing for the function generator in the calibration experiment as described in claim 4.

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