Multi-beam excitation enhanced photoacoustic gas sensing device and method
Through the integration of compact optical multipass cell and Helmholtz photoacoustic cell, a multi-beam excitation PAS system is constructed, which solves the problem of poor coupling of photoacoustic cell in the prior art, and realizes high sensitivity and miniaturized gas detection, which is especially suitable for trace detection of carbon dioxide.
Patent Information
- Application Number
- CN202510643543.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-19
- Publication Date
- 2025-08-19
AI Technical Summary
The prior art is difficult to achieve effective coupling between efficient multi-beam excitation and high-quality resonant photoacoustic pools in a compact space, resulting in insufficient sensitivity and structural integration of the PAS system, and it is difficult to meet the requirements of on-site deployment and high-sensitive detection.
The compact optical multi-pass cell structure is integrated with the Helmholtz photoacoustic cell to build a multi-beam excitation PAS system, and the spatial coverage of 160 laser beams is achieved in a 20mL volume through dense spot pattern design, and the resonant capillary structure in the Helmholtz photoacoustic cell is introduced into the acoustic transducer to improve the acoustic signal acquisition efficiency.
It significantly enhances the photoacoustic signal strength, improves the system detection sensitivity, achieves a minimum detection limit of 4.5ppmv, takes into account the miniaturization of the device and high sensitivity, and reduces costs.
Smart Images
Figure CN120507315A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of gas detection, and specifically relates to a multi-beam excitation enhanced photoacoustic gas sensing device and method, which is particularly suitable for high-sensitivity online monitoring of trace gases such as carbon dioxide. Background Art
[0002] Carbon dioxide (CO2), a key greenhouse gas, is closely linked to climate change and ecosystem stability. Simultaneously, real-time, online detection of CO2 concentrations in industrial process control, human metabolic monitoring, and environmental monitoring places increasing demands on sensitivity and stability. To achieve highly sensitive detection of CO2 at ppmv or even ppbv levels, laser absorption spectroscopy is widely used in trace gas detection.
[0003] In recent years, laser absorption spectroscopy based on the Lambert-Beer law has gained widespread application in trace gas detection. Photoacoustic spectroscopy (PAS), in particular, has become a key method for infrared gas detection due to its lack of reliance on high-performance photodetectors and its zero background noise. The development of tunable laser light sources has significantly improved the sensitivity and selectivity of PAS systems.
[0004] To further enhance the detection sensitivity of PAS systems, common methods include utilizing high-Q resonant photoacoustic cells to enhance the acoustic response, increasing the excitation beam power, or increasing the length of the interaction path between light and target gas molecules. Optical multipass cells (MPCs) typically consist of two or more highly reflective mirrors, enabling the incident laser beam to undergo multiple reflections within them. Examples include Herriott- or White-type MPCs, which are widely used in TDLAS systems to extend the interaction path between the laser and gas molecules and enhance detection sensitivity.
[0005] However, integrating the MPC structure with a resonant photoacoustic cell faces a series of structural and functional contradictions: on the one hand, the resonant photoacoustic cell usually requires a small geometric size (the inner diameter of the resonant photoacoustic cell is less than a few millimeters) to ensure a high quality factor (Q) and acoustic energy focusing efficiency; on the other hand, the reflected light beam trajectory in the traditional MPC has a large spatial distribution range, making it difficult to achieve high-order reflections in a compact cavity; in addition, the non-ideal contact between the laser beam and the inner wall of the resonant photoacoustic cell may also introduce background noise, affecting the system signal-to-noise ratio.
[0006] Although some attempts have been made to combine MPC with PAS systems, for example, Chen et al. proposed a structure based on the combination of a traditional multi-pass cell and a resonant photoacoustic cell, and Ma Yufei et al. enhanced the signal intensity through laser power amplification and multiple reflection beams, these systems still have problems such as large size, complex structure or low integration efficiency, making it difficult to meet the dual requirements of on-site deployment and high-sensitivity detection.
[0007] Therefore, how to achieve efficient multi-beam excitation in a compact space and effectively couple it with a high-quality resonant photoacoustic cell has become a key technical challenge to improve the sensitivity, structural integration and field applicability of the PAS system. Summary of the Invention
[0008] In response to the above problems, the present invention proposes a highly sensitive, integrated multi-beam excitation enhanced photoacoustic gas sensing device and method, which is particularly suitable for trace detection of gases such as carbon dioxide in the near-infrared band. The present invention innovatively couples and integrates a compact optical multi-pass cell structure (MPC) with a Helmholtz photoacoustic cell to construct a multi-beam excitation PAS system with a miniaturized structure, high laser energy density, and enhanced acoustic response. Among them, the MPC adopts a dense spot pattern design to achieve spatial coverage of up to 160 laser beams in a volume of less than 20mL, effectively enhancing the interaction between light and the target gas. The laser is reflected multiple times in the MPC, and the photoacoustic signal generated after interacting with the gas molecules is introduced into the acoustoelectric transducer via the resonant capillary structure-neck tube in the Helmholtz photoacoustic cell to achieve efficient acquisition of the acoustic signal. Unlike traditional photoacoustic cells designed with a single beam excitation, the sensing device of the present invention utilizes a compact multi-pass cell configuration embedded in a Helmholtz photoacoustic cell, achieving significant enhancements in photoacoustic signal generation, structural integration, and sensing robustness. At the same time, it effectively alleviates the limitation of the resonant cavity size on the number of beam reflections.
[0009] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions:
[0010] The present invention provides a multi-beam excitation enhanced photoacoustic gas sensing device, comprising a multi-beam excitation Helmholtz photoacoustic cell and a laser;
[0011] The multi-beam excitation Helmholtz photoacoustic cell comprises a first cavity and a second cavity, wherein an optical multi-pass cell is integrated in the first cavity, the first cavity and the second cavity are connected by a neck tube, a small hole is provided on the second cavity, and an acoustic-electric transducer is provided in the small hole;
[0012] The laser is used to output a laser beam, which enters the first cavity through a fiber collimator. The acoustoelectric transducer converts the generated acoustic wave signal into an electrical signal. The electrical signal passes through a preamplifier to a lock-in amplifier. The signal demodulated by the lock-in amplifier is sent to a laptop computer, and the concentration of the gas to be measured is obtained through analysis and processing. The laptop computer outputs a control signal to a laser control circuit board, and the laser control circuit board outputs a modulation signal to the laser and synchronously outputs a reference signal to the lock-in amplifier.
[0013] Furthermore, the optical multi-pass cell is formed by two identical spherical mirrors arranged opposite to each other, each spherical mirror has a diameter of 1 inch and a focal length of 25 mm, and the distance between the spherical mirrors is set to 36.4 mm.
[0014] Furthermore, the volume of the second cavity is half of that of the first cavity.
[0015] Furthermore, the acoustic electric transducer is a condenser microphone.
[0016] Furthermore, the laser uses a continuous wave fiber coupled DFB laser with a central wavelength of 1572 nm.
[0017] In a conventional Helmholtz photoacoustic cell, the photoacoustic effect occurs when target molecules absorb energy from a periodically modulated laser beam. This absorption causes changes in temperature and pressure, which generate sound waves in the first cavity. The gas in the neck tube thus acts like a piston, compressing the gas in one volume while expanding the gas in the second cavity. A sensitive acoustic-electric transducer, such as a microphone, is placed in the second cavity to convert these acoustic signals into electrical signals. The detected acoustic signal is affected by several factors: the sensitivity S0 of the microphone, the excitation light power P, the multi-beam excitation Helmholtz photoacoustic cell constant F cell , target gas concentration C, absorption coefficient per unit length α and absorption path length L0 (which is equivalent to the length of the laser when it passes through the first cavity once). Multi-beam excitation Helmholtz photoacoustic cell constant F cell The α represents the efficiency of the PA system in converting the laser power absorbed by the target gas into acoustic waves. When the resonant frequency is f0, this conversion depends solely on the geometric parameters of the multi-beam excited Helmholtz photoacoustic cell and the quality factor Q. S0, L0, P, and α are key factors in improving the signal-to-noise ratio (SNR) of PAS. However, S0 is determined by the microphone, while α is related to the absorption line intensity of the target gas. Due to the need for a compact photoacoustic unit, L0 cannot be excessively extended. However, optical folding techniques can be used within the first cavity to increase the absorption path length, thereby improving the signal.
[0018] To enhance the optical path length and signal strength in a compact structure, an MPC was integrated into a Helmholtz photoacoustic cell, known as a multi-beam excited Helmholtz photoacoustic cell. The MPC consists of two identical spherical mirrors, each with a diameter of 1 inch and a focal length of 25 mm. The spacing between the spherical mirrors is set to approximately 36.4 mm, forming an optical cavity (the first cavity) with a total volume of 20 mL. By precisely adjusting the position and angle of the incident laser, the laser beam undergoes multiple reflections between the two spherical mirrors, generating a dense pattern of light spots on their surfaces. This configuration achieves 159 reflections within the compact volume of the first cavity, significantly extending the effective optical path length and enhancing the gas-light interaction, thereby increasing the intensity of the photoacoustic signal. The MPC design was guided by a theoretical model of two spherical mirrors, which predicts 160 beam passes in this configuration. The corresponding beam spot patterns on the mirrors were calculated using this model, and the experimentally observed patterns are in good agreement with the theoretical predictions. The volume of the second cavity is set to half of the first cavity, and a high-sensitivity microphone is placed in it to capture the peak acoustic signal and convert it into an electrical signal to obtain gas concentration information.
[0019] The present invention also provides a multi-beam excitation enhanced photoacoustic gas sensing method based on the device, comprising the following steps:
[0020] Step 1: The laptop computer outputs a control signal to the laser control circuit board, which outputs a modulation signal to the laser. The laser outputs a laser beam of a corresponding wavelength. After being collimated by a fiber collimator, the laser beam enters the first cavity in the multi-beam excitation Helmholtz photoacoustic cell.
[0021] Step 2: In the first cavity, the gas molecules to be measured absorb energy from the periodically modulated laser beam, causing a photoacoustic effect, thereby generating acoustic wave signals. The acoustic-electric transducer embedded in the second cavity converts these acoustic wave signals into electrical signals.
[0022] Step 3: The electrical signal passes through a preamplifier to a lock-in amplifier, which demodulates the signal based on a reference signal from the laser control circuit board to obtain a demodulated signal.
[0023] Step 4: The demodulated signal is sent to a laptop computer, and the concentration of the gas to be measured is obtained through analysis and processing.
[0024] Furthermore, the lock-in amplifier demodulates the signal in a 2f mode synchronized with a reference signal.
[0025] Furthermore, the frequency of the modulation signal is set to half the resonance frequency of the multi-beam excited Helmholtz photoacoustic cell.
[0026] Compared with the prior art, the present invention has the following advantages:
[0027] The multi-beam Helmholtz photoacoustic cell constructed in this paper integrates a compact optical multi-pass cell structure, achieving effective reflection of up to 160 laser beams within a volume of only approximately 20 mL. Compared with traditional single-beam photoacoustic cells, the photoacoustic signal intensity is increased by 21 times, significantly enhancing the detection sensitivity of the system.
[0028] The present invention is based on the CO2 absorption line 6361.25cm -1 A near-infrared laser source is used as the excitation light source, and a minimum detection limit of 4.5ppmv can be achieved without the help of additional equipment such as erbium-doped fiber amplifiers, taking into account both sensitivity and system miniaturization.
[0029] Compared with other methods to enhance the photoacoustic effect, such as using Herriott cells or erbium-doped fiber amplifiers, multi-beam excitation Helmholtz photoacoustic cells provides an efficient and cost-effective way to enhance the photoacoustic signal. BRIEF DESCRIPTION OF THE DRAWINGS
[0030] Figure 1 Schematic diagram of the structure of the multi-beam excited Helmholtz photoacoustic cell.
[0031] Figure 2 Schematic diagram of the structure of a highly sensitive integrated photoacoustic gas sensing device.
[0032] Among them, 1 is a multi-beam excitation Helmholtz photoacoustic cell, 11 is a first cavity, 12 is a second cavity, 13 is an optical multi-pass cell, 14 is a capillary, 15 is an acoustic-electric transducer, 2 is a laser, 3 is a fiber collimator, 4 is a preamplifier, 5 is a phase-locked amplifier, 6 is a laptop computer, and 7 is a laser control circuit board.
[0033] Figure 3 This is the 2f spectrum of 2500ppmv CO2 concentration at normal pressure.
[0034] Figure 4 This is the 2f spectrum of 380ppmv CO2 concentration at normal pressure.
[0035] Figure 5 Allan deviation stability diagram for the 380 ppmv CO2 sensor.
[0036] Figure 6 This is the CO2 concentration response linearity test chart (100-1250ppmv).
[0037] Figure 7 This is the continuous monitoring data of environmental CO2 (three-day stability test). DETAILED DESCRIPTION
[0038] In order to further illustrate the technical solution of the present invention, the present invention is further described below through examples.
[0039] like Figure 1 and Figure 2 As shown, the multi-beam excitation enhanced photoacoustic gas sensing device of this embodiment includes a multi-beam excitation Helmholtz photoacoustic cell 1 and a laser 2;
[0040] The multi-beam excitation Helmholtz photoacoustic cell 1 includes a first cavity 11 and a second cavity 12, an optical multi-pass cell 13 is integrated in the first cavity 11, the first cavity 11 and the second cavity 12 are connected by a neck tube 14, a small hole is provided on the second cavity 12, and an acoustic-electric transducer 15 is provided in the small hole, and the multi-beam excitation Helmholtz photoacoustic cell 1 also includes a light inlet, an air inlet, and an air outlet (not marked in the figure);
[0041] The laser 2 is used to output a laser beam, which is emitted through a fiber collimator 3 and enters the first cavity 11 through a light inlet. The acousto-electric transducer 15 converts the generated acoustic wave signal into an electrical signal, which is transmitted through a preamplifier 4 to a lock-in amplifier 5. The demodulated signal of the lock-in amplifier 5 is sent to a laptop computer 6, and the concentration of the gas to be measured (e.g., CO2) is obtained through analysis and processing. The laptop computer 6 outputs a control signal to a laser control circuit board 7, and the laser control circuit board 7 outputs a modulation signal to the laser 2 and synchronously outputs a reference signal to the lock-in amplifier 5.
[0042] The optical multi-pass cell 13 of this embodiment is composed of two identical spherical mirrors arranged opposite to each other. The diameter of each spherical mirror is 1 inch, the focal length is 25 mm, and the distance between the spherical mirrors is set to 36.4 mm.
[0043] In this embodiment, the volume of the second cavity 12 is half of the volume of the first cavity 11 .
[0044] The acoustic electric transducer 15 of this embodiment is a condenser microphone, and may also be a low-frequency MEMS microphone, a quartz crystal oscillator, etc.
[0045] The laser 2 in this embodiment uses a continuous-wave fiber-coupled DFB laser with a central wavelength of 1572 nm. According to the HITRAN 2016 database, this wavelength corresponds to the CO2 absorption line, which is not affected by other substances in the ambient air. The operating current and temperature of the laser 2 are controlled by a custom-designed laser control circuit board 7 (CCB). By adjusting the operating current and temperature, the wave number of the emitted laser 2 can be precisely adjusted to aim at 6361.25 cm -1 At this specific wavenumber, when the operating current and temperature are set to 220 mA and 30.51°C, respectively, laser 2 can provide 40 mW of single-mode radiation.
[0046] In this embodiment, the first cavity 11 and the second cavity 12 are made of metal, such as aviation aluminum or stainless steel.
[0047] Based on the above device, a multi-beam excitation enhanced photoacoustic gas sensing method of this embodiment includes the following steps:
[0048] Step 1: The laptop computer 6 outputs a control signal to the laser control circuit board 7, which outputs a modulation signal to the laser 2. The laser 2 outputs a laser beam of a corresponding wavelength. After being collimated by the fiber collimator 3, the laser beam enters the first cavity 11 in the multi-beam excitation Helmholtz photoacoustic cell 1.
[0049] Step 2: In the first cavity 11, the gas molecules to be measured absorb energy from the periodically modulated laser beam, causing a photoacoustic effect, thereby generating acoustic wave signals. The acoustic-electric transducer 15 embedded in the second cavity 12 converts these acoustic wave signals into electrical signals.
[0050] Step 3: The electrical signal passes through the preamplifier 4 to the lock-in amplifier 5, and the lock-in amplifier 5 demodulates the signal according to the reference signal from the laser control circuit board 7 to obtain a demodulated signal;
[0051] Step 4: The demodulated signal is sent to the laptop computer 6, and the concentration of the gas to be measured is obtained through analysis and processing.
[0052] In the first cavity 11, CO2 gas of different concentrations is contained. A commercially available gas dilution system is used to dilute 2500ppmvCO2 standard gas and high-purity N2 to generate CO2 gas of different concentrations. In order to improve the detection sensitivity, a wavelength modulation technology based on second harmonic (2f) detection is adopted. The laser wavelength scanning and modulation are controlled by a laptop computer 6, and the frequency of the modulation signal is set to half the resonant frequency of the multi-beam excitation Helmholtz photoacoustic cell 1. The integration time of the sensing device is set to 1 second. The phase-locked amplifier 5 demodulates the signal in the 2f mode synchronized with the reference signal. The equivalent noise bandwidth (ENBW) of the phase-locked amplifier 5 is set to 1.25Hz.
[0053] like Figure 3 As shown, the signal amplitude of the multi-beam excitation Helmholtz photoacoustic cell 1 increased by 21 times compared with the conventional Helmholtz photoacoustic cell. -1 Laser 2 at the CO2 absorption line, such as Figure 4 As shown in Figure 2, the sensing device achieved a minimum detectable limit (MDL) of 4.5 ppmv, and the corresponding NNEA was 3.0×10 -10 cm -1 W Hz -1 / 2 .like Figure 5As shown, an optimal detection limit of 330 ppbv was established within an averaging time of 48 seconds, indicating high sensitivity. Compared to alternative methods of enhancing the photoacoustic effect (e.g., using a Herriott cell or an erbium-doped fiber amplifier), the present invention, equipped with a specially designed multi-beam excitation Helmholtz photoacoustic cell 1, provides an efficient and cost-effective method for improving the resonant photoacoustic signal. Figure 6 As shown, the linearity of the sensing device is verified. Figure 7 As shown, the long-term stability of the sensing device is verified.
[0054] The foregoing shows and describes the principal features and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the present invention is defined by the appended claims, not the foregoing description, and all variations that come within the meaning and range of equivalents of the claims are intended to be embraced therein.
[0055] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.
Claims
1. A multi-beam excitation enhanced photoacoustic gas sensing device, characterized in that: It includes a multi-beam excitation Helmholtz photoacoustic cell (1) and a laser (2); The multi-beam excitation Helmholtz photoacoustic cell (1) comprises a first cavity (11) and a second cavity (12), an optical multi-pass cell (13) is integrated in the first cavity (11), the first cavity (11) and the second cavity (12) are connected via a neck tube (14), a small hole is provided in the second cavity (12), and an acoustic-electric transducer (15) is provided in the small hole; The laser (2) is used to output a laser beam, which enters the first cavity (11) through a fiber collimator (3). The acoustic-electric transducer (15) converts the generated acoustic wave signal into an electrical signal. The electrical signal passes through a preamplifier (4) to a lock-in amplifier (5). The signal demodulated by the lock-in amplifier (5) is sent to a laptop computer (6). After analysis and processing, the concentration of the gas to be measured is obtained. The laptop computer (6) outputs a control signal to a laser control circuit board (7). The laser control circuit board (7) outputs a modulation signal to the laser (2) and synchronously outputs a reference signal to the lock-in amplifier (5).
2. The multi-beam excitation enhanced photoacoustic gas sensing device according to claim 1, characterized in that: The optical multi-pass cell (13) is formed by two identical spherical mirrors arranged opposite to each other, each spherical mirror has a diameter of 1 inch and a focal length of 25 mm, and the distance between the spherical mirrors is set to 36.4 mm.
3. The multi-beam excitation enhanced photoacoustic gas sensing device according to claim 1, characterized in that: The volume of the second cavity (12) is half of the volume of the first cavity (11).
4. The multi-beam excitation enhanced photoacoustic gas sensing device according to claim 1, characterized in that: The acoustic electric transducer (15) is a condenser microphone.
5. The multi-beam excitation enhanced photoacoustic gas sensing device according to claim 1, characterized in that: The laser (2) uses a continuous wave fiber coupled DFB laser with a central wavelength of 1572 nm.
6. A multi-beam excitation enhanced photoacoustic gas sensing method based on the device according to any one of claims 1 to 5, characterized in that: The following steps are involved: Step 1: The laptop computer (6) outputs a control signal to the laser control circuit board (7), the laser control circuit board (7) outputs a modulation signal to the laser (2), the laser (2) outputs a laser beam of a corresponding wavelength, the laser beam is collimated by the fiber collimator (3), and then enters the first cavity (11) in the multi-beam excitation Helmholtz photoacoustic cell (1); Step 2: In the first cavity (11), the gas molecules to be measured absorb energy from the periodically modulated laser beam, causing a photoacoustic effect, thereby generating acoustic wave signals, and the acoustic-electric transducer (15) embedded in the second cavity (12) converts these acoustic wave signals into electrical signals; Step 3: The electrical signal passes through the preamplifier (4) to the lock-in amplifier (5), and the lock-in amplifier (5) demodulates the signal according to the reference signal from the laser control circuit board (7) to obtain a demodulated signal; Step 4: The demodulated signal is sent to a laptop computer (6) and analyzed and processed to obtain the concentration of the gas to be measured.
7. The multi-beam excitation enhanced photoacoustic gas sensing method according to claim 6, characterized in that: The lock-in amplifier (5) demodulates the signal in a 2f mode synchronized with a reference signal.
8. The multi-beam excitation enhanced photoacoustic gas sensing method according to claim 6, characterized in that: The frequency of the modulation signal is set to half the resonance frequency of the multi-beam excited Helmholtz photoacoustic cell (1).