A dual-wavelength synchronous detection system and method for photoacoustic-scattering aerosol based on dual-mode resonance

Abstract

The application discloses a kind of based on bimodal resonance's photoacoustic-scattering aerosol dual-wavelength synchronous detection system and method, belong to photoelectric detection technical field, in the system, dual-wavelength light source modulation module generates two different frequency modulated laser, respectively from two ends into double-difference bulb pipe coupling photoacoustic-scattering combined gas cell;The gas cell includes the two ends of two acoustical tubes arranged in parallel and each is connected with an integrating sphere, form H type coupling structure;Scattering signal collection module, photoacoustic signal acquisition and processing module respectively acquire two-channel scattering photon counting data in two integrating spheres, sound wave signal in two acoustical tubes and two integrating spheres;Digital lock-in signal processing module based on FPGA outputs modulation signal to light source, and receives sound wave signal in two acoustical tubes and two integrating spheres to carry out full-digital demodulation and processing.The application effectively solves the problem of time-space asynchronization and sampling cumulative error caused by time-sharing multiplexing or multiple machine parallel connection.

Description

Technical Field

[0001] This invention belongs to the field of photoelectric detection technology, specifically relating to a photoacoustic-scattering aerosol dual-wavelength synchronous detection system and method based on dual-modal resonance. Background Technology

[0002] The optical properties of aerosols (mainly absorption and scattering) are key parameters for evaluating their radiative forcing effects. To obtain the wavelength dependence of aerosols (such as the absorption angstrom index AAE and the scattering angstrom index SAE), it is necessary to detect optical parameters at two or more wavelengths.

[0003] The existing technology has the following problems:

[0004] (1) Spatiotemporal asynchrony leads to cumulative error: Existing multi-wavelength detection mostly adopts the "time-division multiplexing" method, that is, lasers of different wavelengths work in turn, or multiple single-wavelength instruments are connected in parallel. The former has a time difference and cannot capture transient aerosol characteristics; the latter has spatial sampling differences (microscopic sample inconsistency), which leads to cumulative error when calculating wavelength-dependent parameters.

[0005] (2) Structural limitations: Chinese patent application CN116519604A (A system and method for synchronous detection of aerosol absorption coefficient and scattering coefficient) proposes a Π-type photoacoustic cell structure. This structure realizes the synchronous measurement of absorption and scattering, but its resonance mode is singular, mainly utilizing the longitudinal resonance of the resonant tube. It cannot physically separate and amplify the absorption signals of two different wavelengths at the same time, that is, it cannot simultaneously amplify the signal of wavelength A and the signal of wavelength B through resonance. In addition, some existing technologies adopt a scheme of single-end detection combined with buzzer calibration, which lacks a physical differential mechanism based on acoustic symmetry, and has limited ability to suppress environmental noise (especially flow noise and external vibration).

[0006] (3) Insufficient weak signal extraction capability: Traditional analog lock-in amplifiers are large in size, expensive, and have non-adjustable parameters. In multi-wavelength synchronous detection, the signal components are complex, requiring multiple lock-in amplifiers to work simultaneously, which increases the system complexity and integration difficulty. Moreover, analog circuits are difficult to achieve high-order filtering and extremely narrow bandwidth signal extraction, resulting in a limited detection lower limit. Summary of the Invention

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A photoacoustic-scattering aerosol dual-wavelength synchronous detection system based on dual-modal resonance includes: a dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell, a dual-wavelength light source modulation module, a photoacoustic signal acquisition and processing module, a scattering signal collection module, and an FPGA-based digital phase-locked signal processing module.

[0009] Among them, the dual-wavelength light source modulation module generates two laser beams modulated at different frequencies, which are injected into the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell from both ends.

[0010] The dual-difference X-ray tube coupled photoacoustic-scattering combined gas cell includes two parallel acoustic tubes and two integrating spheres; the two ends of the two acoustic tubes are respectively connected to the two integrating spheres, forming an H-type coupling structure;

[0011] The scattering signal collection module acquires dual-channel scattered photon count data from two integrating spheres; the photoacoustic signal acquisition and processing module acquires acoustic signals from two acoustic tubes and two integrating spheres; the FPGA-based digital phase-locked signal processing module outputs modulation signals to the light source through the dual-wavelength light source modulation module, and receives acoustic signals from the two acoustic tubes and two integrating spheres acquired by the photoacoustic signal acquisition and processing module for fully digital demodulation and processing.

[0012] A photoacoustic-scattering aerosol dual-wavelength synchronous detection method based on dual-modal resonance includes:

[0013] Step 1: Use a precision-machined H-shaped X-ray tube coupling structure to create a dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell; the inner walls of the two integrating spheres are coated with a high-reflectivity PTFE coating or silver-plated.

[0014] Step 2: Generate the spherical resonant frequency using the DDS inside the FPGA. Harmony tube resonant frequency The modulation signals drive the first light source and the second light source to modulate, respectively;

[0015] Step 3: The laser beams emitted by the first light source and the second light source are respectively injected into the second near-concentric cavity and the first near-concentric cavity. After multiple round-trip reflections, high-energy-density converging light spots are formed at the geometric centers of the second integrating sphere and the first integrating sphere, respectively.

[0016] Step 4: The aerosol under test absorbs light energy to generate heat waves and excites sound waves; the first and second microphones are installed at the midpoints of the first and second acoustic tubes, respectively, and the third and fourth microphones are symmetrically installed on the first and second integrating spheres, respectively; the first differential module and the second differential module differentially amplify the sound wave signals collected by the two sets of microphones to obtain the acoustic tube modal photoacoustic signal voltage. Sphere modal photoacoustic signal voltage ;

[0017] Step 5, , After being digitized by the data acquisition card, the data is sent to the FPGA. After digital phase-locked demodulation, multi-stage downsampling, and narrowband low-pass filtering, the amplitude is calculated to obtain the photoacoustic signal intensities at 532nm and 450nm, respectively. The pulse signals output by the first and second photomultiplier tubes are counted by a photon counter to obtain the first scattering intensity. Second scattering intensity ;

[0018] Step 6: A standard absorber of known concentration is introduced into the forward-biased dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell. The photoacoustic signal amplitude and scattering count values ​​of the dual-wavelength synchronous detection system based on dual-modal resonance are recorded at two wavelengths. The cell constant and scattering correction factor of the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell are calculated. The FPGA then demodulates the first absorption amplitude. Second absorption amplitude First scattering intensity Second scattering intensity The data is uploaded to a host computer, which identifies the aerosol type in real time.

[0019] The present invention has the following beneficial effects:

[0020] (1) This invention adopts a dual differential X-ray tube coupling structure, using the spherical resonance mode of the integrating sphere (corresponding to wavelength A) and the tube resonance mode of the acoustic tube (corresponding to wavelength B) as two independent acoustic detection channels. Combined with frequency division multiplexing (FDM) technology, it realizes the synchronous physical separation and detection of dual wavelength signals under a single sample injection. It effectively solves the problems of time and space asynchrony and sampling accumulation error caused by time division multiplexing or multi-machine parallel connection in the prior art.

[0021] (2) This invention establishes a physical differential mechanism based on acoustic symmetry by constructing an H-shaped dual differential acoustic structure and cooperating with a symmetrically arranged microphone array. At the same time, it integrates near-concentric cavity multiple reflection excitation technology to achieve efficient suppression of common-mode interference such as airflow noise and external vibration, as well as multiplication of photoacoustic / scattering signal source strength. It solves the problems in the prior art where the Π-shaped structure has a single resonance mode that cannot simultaneously amplify dual-wavelength signals, and the lack of a physical differential mechanism in single-end detection leads to weak noise resistance.

[0022] (3) This invention uses FPGA-based all-digital lock-in amplification technology and a specific digital filtering algorithm that cascades CIC (cascaded integrator comb filter) and FIR (finite impulse response filter) to achieve precise extraction of weak signals with extremely narrow bandwidth under complex background noise, and supports dynamic software configuration of filter bandwidth and integration time; it solves the problems of large size, high cost, non-adjustable parameters, and difficulty in achieving high-order filtering in traditional analog lock-in amplifiers, which leads to limited detection lower limit.

[0023] (4) By reusing the integrating sphere as the acoustic buffer / resonance cavity of the scattering signal collector and the photoacoustic cell, the present invention realizes the integrated deep coupling of the photoacoustic detection module and the scattering detection module. While ensuring the dual-mode resonance characteristics, it significantly reduces the system volume and improves the system integration. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the dual-wavelength synchronous detection system for photoacoustic-scattering aerosols based on dual-modal resonance of the present invention. In the diagram, 1-first acoustic tube, 2-second acoustic tube, 3-first integrating sphere, 4-second integrating sphere, 5-first microphone, 6-second microphone, 7-third microphone, 8-fourth microphone, 9-first cut-off tube, 10-second cut-off tube, 11-third cut-off tube, 12-fourth cut-off tube, 13-air inlet, 14-air outlet, 15-first photomultiplier tube, 16-... 17-Second photomultiplier tube, 18-First concave spherical mirror, 19-Third concave spherical mirror, 20-Fourth concave spherical mirror, 21-First optical window, 22-Second optical window, 23-Third optical window, 24-Fourth optical window, 25-First differential module, 26-Second differential module, 27-Photon counter, 28-Data acquisition card, 29-FPGA, 30-First light source, 31-Second light source, 32-Host computer. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of the invention described below can be combined with each other as long as they do not conflict with each other. This invention aims to solve the problems of existing technologies being unable to accurately separate aerosol dual-wavelength signals at the same time and space point, having weak noise immunity, and large signal crosstalk, and provides a photoacoustic-scattering aerosol dual-wavelength synchronous detection system based on dual-modal resonance (hereinafter referred to as the detection system), comprising: a dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell, a dual-wavelength light source modulation module, a photoacoustic signal acquisition and processing module, a scattering signal collection module, and a digital phase-locked signal processing module based on FPGA (Field-Programmable Gate Array). The system comprises two laser beams modulated at different frequencies, which are injected from both ends into a dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell. The dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell includes two integrating spheres and two acoustic tubes. The scattering signal collection module collects the scattered light signals from the two integrating spheres. The photoacoustic signal acquisition and processing module collects the acoustic wave signals from the two acoustic tubes and the two integrating spheres. The FPGA-based digital phase-locked signal processing module serves as the control core of the system. On the one hand, it outputs modulation signals to the light source through the dual-wavelength light source modulation module. On the other hand, it receives the acoustic wave signals from the two acoustic tubes and the two integrating spheres collected by the photoacoustic signal acquisition and processing module and performs fully digital demodulation and processing.

[0026] Its core principle is: utilizing the unique spherical resonance mode (corresponding to the first resonance frequency (spherical resonance frequency)) of the photoacoustic-scattering combined gas cell coupled by a dual-differential X-ray tube. ) and acoustic tube resonant mode (corresponding to the second resonant frequency (acoustic tube resonant frequency) These frequencies are used as modulation frequencies for two different wavelength lasers to achieve frequency division multiplexing in the acoustic field. At the same time, the full digital algorithm chain of DDS (Direct Digital Frequency Synthesis) + CIC (Cascaded Integral Comb Filter) + FIR (Finite Impulse Response Filter) + CORDIC (Coordinate Rotation Digital Calculation Method) inside the FPGA29 is used to accurately separate and demodulate the amplitude of the two different wavelength photoacoustic signals.

[0027] The present invention adopts the following technical solution:

[0028] like Figure 1As shown, the dual-difference X-ray tube coupled photoacoustic-scattering combined gas cell includes: a first acoustic tube 1, a second acoustic tube 2, a first integrating sphere 3, and a second integrating sphere 4. Unlike existing technologies (Π-type structure), the dual-difference X-ray tube coupled photoacoustic-scattering combined gas cell of this invention employs a dual integrating sphere + dual acoustic tube configuration to form an H-type coupling structure with Helmholtz interaction. The main body consists of two parallel acoustic resonant tubes: the first acoustic tube 1 and the second acoustic tube 2, and two integrating spheres (also serving as buffer cavities / spherical resonant cavities): the first integrating sphere 3 and the second integrating sphere 4. The two ends of the two acoustic tubes are respectively connected to the two integrating spheres, forming an H-type coupling structure.

[0029] The above symmetrical structure uses the double differential principle to suppress external environmental noise (common-mode noise). External environmental noise (such as airflow noise) is usually in phase (common-mode signal) in the first acoustic tube 1 and the second acoustic tube 2. The photoacoustic signal is out of phase (differential-mode signal) through a special phase modulation design. By performing differential (subtraction) operation on the two microphone signals, the background noise is greatly canceled and the signal-to-noise ratio is significantly improved.

[0030] High-reflectivity spherical mirrors are installed on both sides of the first integrating sphere 3 and the second integrating sphere 4: a third concave spherical mirror 19, a fourth concave spherical mirror 20, a first concave spherical mirror 17, and a second concave spherical mirror 18. The third concave spherical mirror 19 and the fourth concave spherical mirror 20 form a first near-concentric cavity, and the first concave spherical mirror 17 and the second concave spherical mirror 18 form a second near-concentric cavity. The two laser beams undergo multiple regular back-and-forth reflections (not single passes) within the first and second near-concentric cavities, respectively, and form high-energy-density convergence regions at the centers of the first integrating sphere 3 and the second integrating sphere 4. This not only significantly enhances the photoacoustic signal (increased excitation source strength) but also greatly improves the excitation efficiency of the scattered signal. Furthermore, in conjunction with cutoff tubes: a first cutoff tube 9, a second cutoff tube 10, a third cutoff tube 11, and a fourth cutoff tube 12, the scattered light collection angle is optimized, reducing stray light interference. The first cut-off tube 9 and the second cut-off tube 10 are coaxially arranged inside the laser inlet and laser outlet of the first integrating sphere 3, respectively; the third cut-off tube 11 and the fourth cut-off tube 12 are coaxially arranged inside the laser inlet and laser outlet of the second integrating sphere 4, respectively, to block surface scattering stray light generated by the optical window.

[0031] The dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell further includes: an inlet 13, an outlet 14, a first optical window 21, a second optical window 22, a third optical window 23, and a fourth optical window 24. The inlet 13 is located on the side wall of the second integrating sphere 4, allowing the aerosol to be tested to enter the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell; the outlet 14 is located on the side wall of the first integrating sphere 3, allowing the aerosol to flow out, thus forming a continuous photoacoustic-scattering combined gas path.

[0032] The first optical window 21 and the second optical window 22 are respectively sealed and installed at the outer ends of the second cut-off tube 10 and the first cut-off tube 9, and are coaxially arranged with the third concave spherical mirror 19 and the fourth concave spherical mirror 20, so that the laser passes through the first integrating sphere 3 when it reciprocates between the third concave spherical mirror 19 and the fourth concave spherical mirror 20, thereby transmitting the laser and sealing the gas pool; the third optical window 23 and the fourth optical window 24 are respectively sealed and installed at the outer ends of the fourth cut-off tube 12 and the third cut-off tube 11, and are coaxially arranged with the first concave spherical mirror 17 and the second concave spherical mirror 18, so that the laser passes through the second integrating sphere 4 when it reciprocates between the first concave spherical mirror 17 and the second concave spherical mirror 18, thereby transmitting the laser and sealing the gas pool.

[0033] The dual-wavelength light source modulation module includes a first light source 30, a second light source 31, and an FPGA 29 for controlling their modulation frequencies. The DAC (digital-to-analog converter) interface of the FPGA 29 is connected to the drive input terminals of the first light source 30 and the second light source 31, respectively. The FPGA 29 is used to drive the first light source 30 and the second light source 31 to output frequencies corresponding to the acoustic tube resonance. Resonant frequency of a sphere The modulated signal.

[0034] The photoacoustic signal acquisition and processing module includes: a first microphone 5 and a second microphone 6 symmetrically arranged in the middle of the first acoustic tube 1 and the second acoustic tube 2, respectively used to acquire sound wave signals within the first acoustic tube 1 and the second acoustic tube 2; a third microphone 7 and a fourth microphone 8 arranged symmetrically on the first integrating sphere 3 and the second integrating sphere 4, respectively used to acquire sound wave signals within the first integrating sphere 3 and the second integrating sphere 4. The first microphone 5 and the second microphone 6 are connected to the first differential module 25, and the third microphone 7 and the fourth microphone 8 are connected to the second differential module 26. The output terminals of both the first differential module 25 and the second differential module 26 are connected to the analog input channel of the data acquisition card 28.

[0035] The scattering signal collection module includes a first photomultiplier tube 15 disposed on the side wall of the first integrating sphere 3 and a second photomultiplier tube 16 disposed on the side wall of the second integrating sphere 4. The output terminals of both the first photomultiplier tube 15 and the second photomultiplier tube 16 are connected to a photon counter 27. The photon counter 27 is directly connected to the host computer 32 through a communication interface, and is used to transmit the collected dual-channel scattering photon counting data to the host computer 32 in real time for subsequent scattering coefficient calculation and parameter inversion.

[0036] The FPGA-based digital phase-locked signal processing module includes a data acquisition card 28, an FPGA 29, and a host computer 32. The data acquisition card 28 digitizes the analog signal and transmits it to the FPGA 29. After the internal logic of the FPGA 29 completes the digital phase-locked demodulation, the result is transmitted to the host computer 32 through the communication interface.

[0037] This invention employs a frequency division multiplexing method based on dual-mode resonance, specifically: utilizing the multiple acoustic eigenmodes inherent in the photoacoustic-scattering combined gas cell coupled with a dual-differential X-ray tube to achieve synchronous demodulation of two wavelengths without interference.

[0038] Mode A (spherical resonance mode, low frequency, e.g., ~270Hz) is the expansion and contraction of gas within the first integrating sphere 3 and the second integrating sphere 4, which affects the wavelength. The laser modulation frequency (e.g., 532nm) is set as the first resonant frequency (spherical resonant frequency). Mode B (acoustic tube resonant mode, higher frequency, e.g., ~1521Hz) is characterized by the reciprocating oscillation of gas within the first acoustic tube 1 and the second acoustic tube 2, resulting in a wavelength... The laser modulation frequency (e.g., 450nm) is set as the second resonant frequency (acoustic tube resonant frequency). .

[0039] Two laser beams emitted from the first light source 30 and the second light source 31 are simultaneously incident on the second integrating sphere 4 and the first integrating sphere 3 of the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell, respectively. Photoacoustic signals of different wavelengths excite corresponding acoustic modes, thereby achieving preliminary signal separation and enhancement at the physical sound field level. The microphone array (including the first microphone 5, the second microphone 6, the third microphone 7, and the fourth microphone 8) picks up frequencies of... and The superimposed signal of the modulated signals. The FPGA-based digital phase-locked signal processing module utilizes digital phase-locked technology to process signals at the first resonant frequency (spherical resonant frequency). The second resonant frequency (acoustic tube resonant frequency) Demodulation is performed at the point to obtain the absorption coefficients at both 532nm and 450nm.

[0040] This invention replaces the traditional analog lock-in amplifier with an FPGA-based digital phase-locked signal processing module, achieving high-precision and miniaturized detection. This FPGA-based digital phase-locked signal processing module is implemented with all-digital logic, constructing a dual-channel parallel quadrature demodulation architecture within the FPGA29. It integrates a DDS (Direct Digital Frequency Synthesis) module, generating a frequency strictly locked to... and High-precision sine / cosine reference signal with no frequency drift.

[0041] In signal demodulation, dual-channel (I / Q) quadrature demodulation technology is employed to eliminate the influence of signal phase difference, eliminating the need for manual phase adjustment. It is an in-phase component. It is an orthogonal component (Quadrature).

[0042] In terms of the filtering link, a heterogeneous cascaded structure of CIC (Cascaded Integral Comb Filter) and FIR (Finite Impulse Response Filter) is adopted: the CIC filter, as the first stage, is used for high-rate downsampling (decimation), efficiently processing the high sampling rate data of the front-end ADC (Analog-to-Digital Converter) and reducing the data rate to save hardware resources; the FIR filter, as the second stage, performs precise narrowband filtering (bandwidth <0.1Hz) at low sampling rates, ensuring a flat passband, linear phase, and extremely steep cutoff characteristics, thoroughly filtering out harmonic noise and the first resonant frequency (spherical resonant frequency). The second resonant frequency (acoustic tube resonant frequency) Crosstalk between them.

[0043] In signal processing, the CORDIC algorithm is used to perform coordinate rotation on the demodulated I / Q components at the hardware level to quickly calculate the amplitude. Phase and high precision (residual error can reach) (magnitude).

[0044] Furthermore, this FPGA-based digital phase-locked signal processing module features dynamic configuration capabilities as a "software-defined instrument" to adapt to detection environments with varying concentrations. Dynamic adjustment of filter bandwidth: The coefficients of the FIR filter are stored in the RAM (Random Access Memory) of the FPGA29 and can be reloaded online via host computer software. When the system detects high-concentration aerosols (e.g., standard high-concentration black carbon particles generated by an aerosol generator or high-concentration atmospheric particulate matter during heavily polluted weather) introduced through inlet 13 and filling a dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell (containing two integrating spheres and two acoustic tubes), the software issues a command to switch to fast response mode, loading coefficients with fewer taps to broaden the passband bandwidth (e.g., adjusting to 1Hz) for millisecond-level rapid tracking detection. When detecting extremely low-concentration aerosols (weak signal, low signal-to-noise ratio), it automatically switches to high-sensitivity mode, loading coefficients with multiple taps to compress the bandwidth to an extremely narrow level (e.g., 0.01Hz) to drastically narrow the noise bandwidth and improve the detection limit. Dynamic adjustment of integration time: The amplitude data output from the CORDIC circuit enters a programmable accumulator. The host computer software dynamically modifies the integration period of the accumulator via instructions based on the current signal-to-noise ratio level. At low concentrations, increasing the integration time (e.g., from 1s to 10s) utilizes the statistical properties of white noise (noise increases with the square root of time, while the signal increases linearly with time) to improve the signal-to-noise ratio; at high concentrations, decreasing the integration time increases the data refresh rate.

[0045] This invention further provides a method for simultaneous detection of photoacoustic-scattering aerosols using dual-mode resonance, comprising:

[0046] Step 1, Gas Cell Manufacturing: A dual-difference X-ray tube coupled photoacoustic-scattering combined gas cell with an H-shaped X-ray tube coupling structure is precision-machined from aluminum alloy or stainless steel. The inner walls of the first integrating sphere 3 and the second integrating sphere 4 of this dual-difference X-ray tube coupled photoacoustic-scattering combined gas cell are coated with a high-reflectivity PTFE (polytetrafluoroethylene) coating or silver-plated to prevent aerosol adsorption and ensure a diffuse reflectance >98% in the visible light band. The length and diameter of the first acoustic tube 1 and the second acoustic tube 2, as well as the volumes of the first integrating sphere 3 and the second integrating sphere 4, are rigorously designed based on finite element method (FEM) simulation results to match the predetermined first resonant frequency (sphere resonant frequency). (wavelength (e.g., 532nm laser modulation frequency, e.g., 270Hz) and second resonant frequency (acoustic tube resonant frequency). (wavelength The laser modulation frequency is (e.g., 450nm, for example, 1521Hz). The aerosol to be tested enters through the inlet 13 of the second integrating sphere 4, flows through the first acoustic tube 1 and the second acoustic tube 2, and finally exits through the outlet 14 of the first integrating sphere 3. This through-flow gas path design ensures that the aerosol sample composition in the integrating sphere region and the acoustic tube region is consistent in real time, guaranteeing the spatial synchronization of dual-wavelength detection.

[0047] Step 2, Light Source Modulation: The first resonant frequency (spherical resonant frequency) is generated by the DDS inside the FPGA29. The second resonant frequency (acoustic tube resonant frequency) The modulated signal. First resonant frequency (spherical resonant frequency). After simulation optimization, 270Hz was selected as the second resonant frequency (acoustic tube resonant frequency). The frequency was selected as 1521Hz after simulation optimization. The first light source 30 (such as a 450nm blue laser) is driven at the second resonant frequency (acoustic tube resonant frequency). Modulation drives the second light source 31 (such as a 532nm green laser) to the first resonant frequency (spherical resonant frequency). modulation.

[0048] Step 3, optical path adjustment: The laser beam emitted by the first light source 30 enters the second near-concentric cavity (located in the second integrating sphere 4) formed by the first concave spherical mirror 17 and the second concave spherical mirror 18; the laser beam emitted by the second light source 31 enters the first near-concentric cavity (located in the first integrating sphere 3) formed by the third concave spherical mirror 19 and the fourth concave spherical mirror 20.

[0049] The near-concentric cavity is constructed as follows: two concave spherical mirrors (first concave spherical mirror 17, second concave spherical mirror 18, third concave spherical mirror 19 or fourth concave spherical mirror 20) are placed opposite each other, with a radius of curvature of twice 2. (The radii of curvature of the four concave spherical mirrors are the same, all R) slightly larger than the distance between the first concave spherical mirror 17 and the second concave spherical mirror 18, or between the third concave spherical mirror 19 and the fourth concave spherical mirror 20. (i.e., satisfy) The near-concentric stability condition, and near In this configuration, the distance between the two pairs of concave spherical mirrors is the same, L. The incident angles of the two laser beams are finely adjusted so that the laser beams undergo multiple regular back-and-forth reflections (e.g., >20 times) within the first and second near-concentric cavities, thereby forming high-energy-density converging light spots at the geometric centers of the second integrating sphere 4 and the first integrating sphere 3, respectively.

[0050] Step 4, Four-microphone pickup and analog front-end processing: The aerosol under test in the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell absorbs light energy to generate heat waves and excites sound waves. Due to the use of dual-frequency modulation, the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell mixes the first resonant frequency (spherical resonant frequency). The second resonant frequency (acoustic tube resonant frequency) The acoustic wave component of the modulated signal. In this embodiment, a spatial mode matching pickup strategy is adopted: the first microphone 5 and the second microphone 6 are respectively installed at the midpoint of the first acoustic tube 1 and the second acoustic tube 2, which is the second resonant frequency (acoustic tube resonant frequency). The maximum sound pressure level is at this point. The third microphone 7 and the fourth microphone 8 are symmetrically mounted on the first integrating sphere 3 and the second integrating sphere 4, respectively. This location corresponds to the first resonant frequency (the sphere's resonant frequency). The sound pressure sensitive point. Based on the acoustic field simulation characteristics of the H-type X-ray tube coupled photoacoustic-scattering combined gas cell, under the resonance state, the photoacoustic signals at symmetrical positions are out of phase (180° phase difference), while external environmental noise (such as airflow and mechanical vibration) is usually in phase. The first differential module 25 and the second differential module 26 differentially amplify the acoustic signals collected by the two sets of microphones, respectively. (Subtracting the two signals doubles the amplitude of the photoacoustic signal in the acoustic tube mode. The common-mode noise in phase cancels each other out, resulting in a high signal-to-noise ratio output of the 450nm absorption correlation signal.) ); (Subtracting the two signals doubles the amplitude of the photoacoustic signal in the spherical mode, cancels out the in-phase noise, and outputs a high signal-to-noise ratio 532nm absorption correlation signal.) ).

[0051] Through the aforementioned differential connection, the useful photoacoustic signal is multiplied while incoherent common-mode noise is significantly canceled. Among these, and The original voltage signals collected by the first microphone 5 and the second microphone 6 are respectively. and The original voltage signals collected by the third microphone 7 and the fourth microphone 8 are respectively. The voltage of the acoustic tube modal photoacoustic signal is output after differential amplification by the first microphone 5 and the second microphone 6 through the first differential module 25. The voltage of the spherical mode photoacoustic signal is output after differential amplification by the third microphone 7 and the fourth microphone 8 through the second differential module 26.

[0052] Step 5, FPGA29 back-end all-digital signal processing: and After being digitized by the high-precision data acquisition card 28, the data is sent to the FPGA 29. The FPGA 29 has two parallel digital phase-locked demodulation channels. The first channel (processing the 532nm absorption signal): The digital signal and the first resonant frequency (sphere resonant frequency) generated by DDS. The mixing (multiplication) process is performed, and the output result is sequentially downsampled through a CIC (cascaded integrator comb) filter, and then enters an FIR (finite impulse response) filter for narrowband low-pass filtering (bandwidth < 0.1 Hz) to completely filter out the second resonant frequency (acoustic tube resonant frequency). Crosstalk and harmonic noise were eliminated. Finally, the amplitude was calculated using CORDIC to obtain the 532nm photoacoustic signal intensity. The second channel (processing the 450nm absorption signal): [The text abruptly ends here, likely due to an incomplete sentence or missing information.] The digital signal and the second resonant frequency (acoustic tube resonant frequency) generated by DDS. Frequency mixing is performed, and the signal undergoes the same CIC+FIR+CORDIC processing flow to obtain a 450nm photoacoustic signal intensity. The pulse signals output by the first photomultiplier tube 15 and the second photomultiplier tube 16 are counted by a photon counter inside the FPGA 29 to obtain the first scattering intensity. Second scattering intensity .

[0053] Step 6, Parameter Inversion and Output: Before formally detecting aerosol samples, a standard absorber of known concentration (such as nitrogen dioxide gas or standard black carbon aerosol) is introduced into the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell. The photoacoustic signal amplitude and scattering count values ​​of the dual-wavelength synchronous detection system for photoacoustic-scattering aerosols based on dual-modal resonance are recorded at two wavelengths. The cell constant and scattering correction factor of the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell are calculated as the benchmark for subsequent parameter inversion. The FPGA29 transmits the demodulated four sets of data (first absorption amplitude) via USB or serial port. Second absorption amplitude First scattering intensity Second scattering intensity The data is uploaded to the host computer. The host computer software, combined with the pre-calibrated cell constant, calculates the absolute absorption coefficient of the aerosol. and scattering coefficient Further derive wavelength-related indices (absorption angstrom index AAE, scattering angstrom index SAE) to identify aerosol types in real time (such as distinguishing between black carbon, brown carbon, dust, etc.).

[0054] The above description is merely an embodiment of the present invention and does not limit the scope of the invention. Any equivalent structural or procedural transformations made based on the description and drawings of this invention, or direct or indirect applications in other related system fields, are similarly included within the protection scope of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A photoacoustic-scattering aerosol dual-wavelength synchronous detection system based on dual-modal resonance, characterized in that, include: The system includes a dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell, a dual-wavelength light source modulation module, a photoacoustic signal acquisition and processing module, a scattering signal collection module, and an FPGA-based digital phase-locked signal processing module. Among them, the dual-wavelength light source modulation module generates two laser beams modulated at different frequencies, which are injected into the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell from both ends. The dual-difference X-ray tube coupled photoacoustic-scattering combined gas cell includes two parallel acoustic tubes and two integrating spheres; the two ends of the two acoustic tubes are respectively connected to the two integrating spheres, forming an H-type coupling structure; The scattering signal collection module acquires dual-channel scattered photon count data from two integrating spheres; the photoacoustic signal acquisition and processing module acquires acoustic signals from two acoustic tubes and two integrating spheres; the FPGA-based digital phase-locked signal processing module outputs modulation signals to the light source through the dual-wavelength light source modulation module, and receives acoustic signals from the two acoustic tubes and two integrating spheres acquired by the photoacoustic signal acquisition and processing module for fully digital demodulation and processing.

2. The photoacoustic-scattering aerosol dual-wavelength synchronous detection system based on dual-modal resonance according to claim 1, characterized in that, The dual-difference X-ray tube coupled photoacoustic-scattering combined gas cell also includes: a third concave spherical mirror and a fourth concave spherical mirror arranged on both sides of the first integrating sphere, and a first concave spherical mirror and a second concave spherical mirror arranged on both sides of the second integrating sphere. The third and fourth concave spherical mirrors form the first near-concentric cavity, and the first and second concave spherical mirrors form the second near-concentric cavity. The two laser beams undergo multiple regular back-and-forth reflections in the first and second near-concentric cavities, respectively, and form high-energy-density convergence regions at the centers of the first and second integrating spheres, respectively.

3. The photoacoustic-scattering aerosol dual-wavelength synchronous detection system based on dual-modal resonance according to claim 2, characterized in that, The dual-difference X-ray tube coupled photoacoustic-scattering combined gas cell further includes: an inlet, an outlet, a first optical window, a second optical window, a third optical window, and a fourth optical window; and a second cut-off tube and a first cut-off tube respectively disposed at the laser inlet and outlet of the first integrating sphere, and a fourth cut-off tube and a third cut-off tube respectively disposed at the laser inlet and outlet of the second integrating sphere; the first, second, third, and fourth cut-off tubes are used to block surface scattering stray light generated by the optical windows; the inlet is disposed on the side wall of the second integrating sphere for the aerosol to be tested to enter the gas cell; the outlet is disposed on the side wall of the first integrating sphere for the aerosol to be tested to enter the gas cell. The laser flows out, thus forming a continuous photoacoustic-scattering combined gas path; the first optical window and the second optical window are respectively sealed and installed at the outer ends of the second cut-off tube and the first cut-off tube, and are coaxially arranged with the third concave spherical mirror and the fourth concave spherical mirror, so that the laser passes through the first integrating sphere when it reciprocates between the third concave spherical mirror and the fourth concave spherical mirror; the third optical window and the fourth optical window are respectively sealed and installed at the outer ends of the fourth cut-off tube and the third cut-off tube, and are coaxially arranged with the first concave spherical mirror and the second concave spherical mirror, so that the laser passes through the second integrating sphere when it reciprocates between the first concave spherical mirror and the second concave spherical mirror.

4. The photoacoustic-scattering aerosol dual-wavelength synchronous detection system based on dual-modal resonance according to claim 3, characterized in that, The dual-wavelength light source modulation module includes: a first light source, a second light source, and an FPGA for controlling the modulation frequencies of the first and second light sources; the digital-to-analog converter interface of the FPGA is connected to the drive input terminals of the first and second light sources respectively, and the FPGA is used to drive the first and second light sources to output frequencies equal to the resonant frequencies of the acoustic tubes. Resonant frequency of a sphere The modulated signal.

5. The photoacoustic-scattering aerosol dual-wavelength synchronous detection system based on dual-modal resonance according to claim 4, characterized in that, The photoacoustic signal acquisition and processing module includes: a first microphone and a second microphone symmetrically arranged in the middle of the first acoustic tube and the second acoustic tube, respectively used to acquire acoustic wave signals in the first acoustic tube and the second acoustic tube; a third microphone and a fourth microphone arranged symmetrically on the first integrating sphere and the second integrating sphere, respectively used to acquire acoustic wave signals in the first integrating sphere and the second integrating sphere; The first and second microphones are connected to the first differential module, and the third and fourth microphones are connected to the second differential module; the outputs of both the first and second differential modules are connected to the data acquisition card.

6. The photoacoustic-scattering aerosol dual-wavelength synchronous detection system based on dual-modal resonance according to claim 5, characterized in that, The scattering signal collection module includes: a first photomultiplier tube disposed on the side wall of the first integrating sphere and a second photomultiplier tube disposed on the side wall of the second integrating sphere; the output terminals of the first photomultiplier tube and the second photomultiplier tube are both connected to a photon counter; the photon counter is connected to a host computer and transmits the collected dual-channel scattering photon counting data to the host computer in real time for scattering coefficient calculation and parameter inversion.

7. The photoacoustic-scattering aerosol dual-wavelength synchronous detection system based on dual-modal resonance according to claim 6, characterized in that, The FPGA-based digital phase-locked signal processing module includes: a data acquisition card, an FPGA, and a host computer. The data acquisition card digitizes the analog signal and transmits it to the FPGA. After the FPGA completes the digital phase-locked demodulation, it transmits the result to the host computer through the communication interface.

8. A method for simultaneous detection of photoacoustic-scattering aerosols based on dual-modal resonance at two wavelengths, used in the simultaneous detection system for photoacoustic-scattering aerosols based on dual-modal resonance as described in any one of claims 1 to 7, characterized in that, include: Step 1: Use a double-differential X-ray tube coupled photoacoustic-scattering combined gas cell with an H-shaped X-ray tube coupling structure precision machined from aluminum alloy or stainless steel. The inner walls of the two integrating spheres are coated with a high-reflectivity PTFE coating or silver plating; Step 2: Generate the spherical resonant frequency using the DDS inside the FPGA. Harmony tube resonant frequency The modulation signals drive the first light source and the second light source to modulate, respectively; Step 3: The laser beams emitted by the first light source and the second light source are respectively injected into the second near-concentric cavity and the first near-concentric cavity. After multiple round-trip reflections, high-energy-density converging light spots are formed at the geometric centers of the second integrating sphere and the first integrating sphere, respectively. Step 4: The aerosol under test absorbs light energy to generate heat waves and excites sound waves; the first and second microphones are installed at the midpoints of the first and second acoustic tubes, respectively, and the third and fourth microphones are symmetrically installed on the first and second integrating spheres, respectively; the first differential module and the second differential module differentially amplify the sound wave signals collected by the two sets of microphones to obtain the acoustic tube modal photoacoustic signal voltage. Sphere modal photoacoustic signal voltage ; Step 5, , After being digitized by the data acquisition card, the data is sent to the FPGA. After digital phase-locked demodulation, multi-stage downsampling, and narrowband low-pass filtering, the amplitude is calculated to obtain the photoacoustic signal intensities at 532nm and 450nm, respectively. The pulse signals output by the first and second photomultiplier tubes are counted by a photon counter to obtain the first scattering intensity. Second scattering intensity ; Step 6: A standard absorber of known concentration is introduced into the forward-biased dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell. The photoacoustic signal amplitude and scattering count values ​​of the dual-wavelength synchronous detection system based on dual-modal resonance are recorded at two wavelengths. The cell constant and scattering correction factor of the dual-differential X-ray tube coupled photoacoustic-scattering combined gas cell are calculated. The FPGA then demodulates the first absorption amplitude. Second absorption amplitude First scattering intensity Second scattering intensity The data is uploaded to a host computer, which identifies the aerosol type in real time.

9. The photoacoustic-scattering aerosol dual-wavelength synchronous detection method based on dual-modal resonance according to claim 8, characterized in that, In step 3, the first and second near-concentric cavities are constructed in such a way that the cavity shapes satisfy the following conditions: ,and near The near-concentric condition; among which, Let be the radius of curvature of each concave spherical mirror. The distance between the first and second concave spherical mirrors or between the third and fourth concave spherical mirrors.

10. The photoacoustic-scattering aerosol dual-wavelength synchronous detection method based on dual-modal resonance according to claim 8, characterized in that, In step 5, , After being digitized by the data acquisition card, the data is sent to the FPGA, and then processed by the digital phase-locked demodulation channel. , The digital signal and the spherical resonance frequency generated by DDS Acoustic tube resonant frequency The frequency is mixed, and the output is passed through a cascaded integrator comb filter for multi-stage downsampling, then enters a finite impulse response filter for narrowband low-pass filtering, and finally the amplitude is calculated by a coordinate rotation digital calculation method to obtain the photoacoustic signal intensity at 532nm and 450nm respectively.

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