Differential photoacoustic spectrum detection system and method based on common-mode noise suppression

Abstract

The invention relates to a differential photoacoustic spectrum detection system and method based on common-mode noise suppression. The system comprises an adjustable light source, a differential photoacoustic cell and a detection unit which are connected in sequence, the differential photoacoustic cell comprises a measuring cavity and a reference cavity which are asymmetrically arranged, and the measuring cavity is a high-Q-value resonant cavity and is used for enhancing a target gas signal; the reference cavity is a cavity with a low Q value, the reference cavity is consistent with the measuring cavity in resonant frequency and different in acoustic impedance through active tuning, the common-mode noise transfer function is consistent, and the reference cavity and the measuring cavity are communicated through a channel; a microphone array is arranged on the measuring cavity to collect a central sound pressure signal; a piezoelectric film sensor is attached to the outer wall of the reference cavity to detect a vibration signal; and the detection unit performs adaptive difference subtraction on the measurement cavity signal and the reference cavity signal to obtain a target signal. Compared with the prior art, the method solves the problem of signal-to-noise ratio degradation caused by common-mode noise interference and target signal frequency band overlapping, and improves the quality and reliability of signal detection.

Description

Technical Field

[0001] The present invention relates to the technical field of trace gas detection, and in particular to a differential photoacoustic spectroscopy detection system and method based on common-mode noise suppression. Background Art

[0002] As a highly sensitive method for trace gas detection, photoacoustic spectroscopy offers unique advantages, including non-contact, high selectivity, and a wide dynamic range. It directly inverts gas concentrations by exploiting the principle that gas molecules absorb laser light of a specific wavelength, generating thermal expansion acoustic waves. This avoids the reliance of traditional spectroscopy techniques on complex optical path calibration. Combined with the narrow linewidth characteristics of tunable lasers, it can accurately identify the fingerprint absorption spectra of gas molecules, achieving ultra-low detection limits ranging from ppb (parts per billion) to ppt (parts per trillion). This technology has been widely used in environmental monitoring (tracking greenhouse gases CO2 / CH4), industrial safety (early warnings of flammable and explosive gas leaks), medical diagnostics (analysis of VOCs markers in exhaled breath), and scientific research (studying the mechanisms of atmospheric chemical reactions), becoming a core tool in the field of gas sensing.

[0003] However, the existing photoacoustic spectroscopy technology has the technical problem of common-mode noise interference (such as environmental vibration and light source fluctuation) overlapping with the target signal band in trace gas detection, which is difficult to distinguish with traditional single-channel detection, resulting in a serious degradation of the signal-to-noise ratio.

[0004] For example, the low-noise differential Helmholtz photoacoustic spectroscopy detection device disclosed in Chinese patent application CN115201116A uses a symmetrically structured differential Helmholtz photoacoustic cell to suppress environmental co-frequency noise by differentially suppressing the acoustic wave signals of the excitation cavity and the compensation cavity. However, its symmetrical dual cavities respond uniformly to the target signal and common-mode noise, resulting in an inability to effectively separate signals and noise with overlapping frequency bands, a single noise suppression mechanism, and insufficient signal detection accuracy. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned prior art, such as the inability to effectively separate signals and noise with overlapping frequency bands, the single noise suppression mechanism, and the insufficient signal detection accuracy, and to provide a differential photoacoustic spectroscopy detection system and method based on common-mode noise suppression.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] As a first aspect of the present invention, a differential photoacoustic spectroscopy detection system based on common-mode noise suppression is provided, characterized in that it includes an adjustable light source, a differential photoacoustic cell, and a detection unit connected in sequence;

[0008] The differential photoacoustic cell is an asymmetric dual-cavity cell, comprising a measurement cavity and a reference cavity. The measurement cavity is a high-Q resonant cavity for enhancing the target gas signal; the reference cavity is a low-Q cavity, which is actively tuned so that its resonant frequency is consistent with that of the measurement cavity and its acoustic impedance is lower than that of the measurement cavity. The common-mode noise transfer function of the reference cavity and the measurement cavity are consistent; the measurement cavity and the reference cavity are connected by a micro-acoustic waveguide.

[0009] A microphone array is deployed on the measurement cavity to collect the central sound pressure signal of the measurement cavity; a piezoelectric film sensor is attached to the outer wall of the reference cavity to detect the vibration signal of the reference cavity;

[0010] The detection unit is connected to the microphone array and the piezoelectric film sensor signal, and is used to perform adaptive differential subtraction and second harmonic demodulation processing on the central sound pressure signal of the measurement cavity and the vibration signal of the reference cavity to obtain the target signal.

[0011] As a preferred technical solution, the adjustable light source includes a light source and a light source modulator;

[0012] The light source adopts a distributed feedback laser, and the wavelength is locked to the absorption peak of the target gas;

[0013] The light source modulator adopts dual-frequency modulation technology, wherein the main modulation frequency matches the resonance frequency of the measurement cavity, and the auxiliary modulation frequency is used to actively tune and compensate the reference cavity, so that the acoustic impedance of the reference cavity is close to the acoustic impedance of the measurement cavity;

[0014] The output of the light source is evenly divided and input into the measurement cavity and the reference cavity through an optical fiber beam splitter.

[0015] As an optimal technical solution, the measurement cavity is a cylindrical structure. By adjusting the diameter / length ratio of the cavity, the Q value of the measurement cavity at the laser modulation frequency corresponding to the target gas absorption spectrum is greater than the first threshold.

[0016] As a preferred technical solution, the inner wall of the measuring cavity is plated with a high-reflectivity gold film and is integrated with a silicon-based microstructured anti-vibration diaphragm.

[0017] As a preferred technical solution, the reference cavity is a tapered gradually changing acoustic impedance cavity, and the resonance bandwidth is broadened by adjusting the geometric shape of the gradually changing cavity cross-sectional area, so that the Q value of the reference cavity is less than the second threshold;

[0018] The reference cavity is equipped with an actively tuned piezoelectric ceramic piece, which dynamically adjusts the acoustic impedance of the reference cavity according to the resonance frequency of the measurement cavity, so that the transfer functions of the reference cavity and the measurement cavity to common mode noise are consistent.

[0019] As a preferred technical solution, the measuring cavity and the reference cavity are connected through a Helmholtz channel, and the channel is filled with a porous sound-absorbing material of ceramic fibers.

[0020] As an optimal technical solution, the microphone array is a MEMS microphone array deployed at multiple symmetrical points, and the central sound pressure signal of the measurement cavity is extracted through a beamforming algorithm.

[0021] As a preferred technical solution, the piezoelectric film sensor is arranged at a position far away from the light inlet and the acoustic waveguide channel of the reference cavity.

[0022] As a second aspect of the present invention, a differential photoacoustic spectroscopy detection method based on common-mode noise suppression is provided. The method is based on the differential photoacoustic spectroscopy detection system based on common-mode noise suppression as described above, and the steps include:

[0023] The central sound pressure signal of the measurement cavity is extracted through the microphone array, and the vibration signal of the reference cavity is collected through the piezoelectric film sensor;

[0024] Adopting an adaptive differential algorithm, the noise coupling characteristics of the two-way signal of the measurement cavity and the reference cavity are dynamically evaluated. The differential weight is adjusted in real time based on the coherence function in the frequency domain. The differential subtraction is performed on the collected sound pressure signal at the center of the measurement cavity and the vibration signal of the reference cavity based on the differential weight.

[0025] Perform second harmonic demodulation, perform phase-locked amplification on the adaptive differential output signal, extract the second harmonic component, and obtain the target signal.

[0026] As a preferred technical solution, the adaptive difference algorithm is specifically as follows:

[0027] Synchronous sampling of the measurement cavity and reference cavity signals in the time domain;

[0028] For the time-domain synchronous sampling photoacoustic signal, the effective frequency band within the set bandwidth of the measurement cavity resonance frequency is retained;

[0029] Divide the signal into frames and calculate the auto-power spectrum density G of the measurement cavity signal and the reference cavity signal in each frame xx (f), G yy (f) and the cross power spectrum density G xy (f), and calculate the coherence function of the two:

[0030]

[0031] Based on the coherence function, for each frequency point f k Calculate the frequency domain weight w(f k ), and obtain the frequency domain weight matrix:

[0032]

[0033] Perform frequency domain weighted difference on each frame signal and then perform inverse FFT to convert it back to time domain signal:

[0034] V out (f k )=w(f k )·V measure (f k )-[1-w(f k )]·V refercence (f k )

[0035] Where V measure (f k ) indicates that the measurement cavity is at the frequency f k The signal at V refercence (f k ) represents the reference cavity at frequency f k The signal at.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1) The differential photoacoustic spectroscopy detection system based on common-mode noise suppression proposed in this invention, by designing an asymmetric dual-cavity structure, with the measurement cavity as a high-Q resonant cavity and the reference cavity as a low-Q cavity and actively tuned, ensures that the common-mode noise transfer function is consistent. The two cavities are connected by a micro-acoustic waveguide, which can balance air pressure fluctuations and suppress crosstalk between the cavities, achieving a common-mode noise suppression ratio of more than 20dB, solving the problem of common-mode noise interference and target signal frequency band overlap leading to signal-to-noise ratio degradation. When the resonant frequency of the reference cavity is consistent with that of the measurement cavity and the acoustic impedance is adjusted to a suitable state, the response characteristics of the two cavities to common-mode noise will be similar, that is, the common-mode noise transfer function is consistent. When faced with environmental vibrations, the two cavities will convert the vibrations into changes in the acoustic waves in the cavities in a similar manner. Since the acoustic structures of the two cavities are consistent after active tuning, the propagation and reflection processes of these acoustic wave changes in the two cavities are also similar, making the transfer characteristics of common-mode noise in the two cavities identical. In this way, when the two cavity signals are subsequently differentially processed, common-mode noise can be effectively suppressed.

[0038] 2) The present invention utilizes a high-Q (narrow bandwidth) resonant cavity. By optimizing the cavity's diameter / length ratio, the cavity achieves a Q greater than 100 at a laser modulation frequency of 1 to 10 kHz, corresponding to the target gas absorption line. A high-reflectivity gold film is applied to the cavity's inner wall to minimize optical energy loss. Furthermore, a silicon-based microstructured anti-vibration diaphragm is integrated to reduce mechanical noise coupling. This enhances the target gas signal and improves detection sensitivity, enabling detection of even lower gas concentrations and facilitating ultra-low detection limits in the ppb (parts per billion) to ppt (parts per trillion) range.

[0039] 3) The structural design of the reference cavity of the present invention is a conical gradient acoustic impedance cavity, which broadens the resonance bandwidth Q value <20 by adjusting the geometric shape of the gradual change of the cavity cross-sectional area; a built-in active tuning piezoelectric ceramic piece dynamically adjusts the acoustic impedance of the reference cavity according to the resonant frequency of the measuring cavity, that is, the feedback control bandwidth is ≥1kHz. Since the reference cavity is designed as a conical gradient acoustic impedance cavity, this structure makes its resonance bandwidth wider (Q value <20), and it is more convenient to achieve flexible adjustment of the acoustic impedance through slight deformation of the piezoelectric ceramic piece. By monitoring the resonant frequency of the measuring cavity in real time and feedback controlling the electrical signal of the piezoelectric ceramic piece, the resonant frequency of the reference cavity can be kept consistent with that of the measuring cavity, while ensuring that the transfer functions of the two cavities to common mode noise are consistent.

[0040] 3) The present invention uses an adaptive differential algorithm and second harmonic demodulation to process the detection signal. The adaptive differential algorithm dynamically evaluates the noise coupling characteristics of the two signals and adjusts the differential weight in real time to maximize the cancellation of common-mode noise and minimize the loss of the target signal. Second harmonic demodulation performs phase-locked amplification on the differential output signal, extracting the second harmonic component and further suppressing low-frequency 1 / f noise, thereby improving signal quality and detection reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A schematic flow chart of a method for designing a differential photoacoustic spectroscopy detection system based on common-mode noise suppression provided by an embodiment of the present invention;

[0042] Figure 2 Schematic diagram of step 2 of the design method of a differential photoacoustic spectroscopy detection system based on common-mode noise suppression provided by an embodiment of the present invention

[0043] Figure 3 A schematic structural diagram of a differential photoacoustic spectroscopy detection system based on common-mode noise suppression provided by an embodiment of the present invention.

[0044] The numbers in the figure are as follows: 1. Light source; 2. Light source modulator; 3. Differential photoacoustic cell; 4. Detection unit; 5. Measurement cavity; 6. Reference cavity; 7. Helmholtz channel; 8. Microphone array; 9. Piezoelectric film sensor; 10. Fiber optic beam splitter. DETAILED DESCRIPTION

[0045] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0046] Example 1

[0047] The differential photoacoustic spectroscopy detection system based on common-mode noise suppression provided by the present invention accurately separates signals and noise through an asymmetric cavity structure, enhances system stability through micro-channels, optimizes light sources through dual-frequency modulation and fiber optic splitting, and deeply reduces noise through array detection and intelligent algorithms. This systematically solves the problems of existing patents in improving signal-to-noise ratio in complex noise environments, limited detection sensitivity, and insufficient environmental adaptability. It is particularly suitable for high-precision detection scenarios of trace gases at the ppb to ppt level. Figure 1 As shown, the system includes an adjustable light source, a differential photoacoustic cell 3 and a detection unit 4 connected in sequence.

[0048] The differential photoacoustic cell 3 is an asymmetric dual-cavity structure, comprising a measurement cavity 5 and a reference cavity 6. The measurement cavity 5 selects a high-Q value (narrow bandwidth) resonant cavity to enhance the target gas signal; the reference cavity 6 selects a low-Q value (wide bandwidth) cavity, and through active tuning, its resonant frequency is consistent with that of the measurement cavity 5, but the acoustic impedance is slightly lower, ensuring that the common-mode noise transfer function is consistent, while the target signal response is significantly different. The two cavities are connected by a micro-Helmholtz channel 7, and the connecting channel is filled with porous sound-absorbing material to balance air pressure fluctuations while suppressing high-frequency crosstalk. A microphone array 8 is deployed on the measurement cavity 5 to collect the central sound pressure signal of the measurement cavity 5; a piezoelectric film sensor 9 is attached to the outer wall of the reference cavity 6 to detect the vibration signal of the reference cavity 6.

[0049] The detection unit 4 is connected to the microphone array 8 and the piezoelectric film sensor 9, and uses an adaptive differential algorithm and second harmonic demodulation to process the detection signal of the detection unit, and performs adaptive differential subtraction on the measurement cavity signal and the reference cavity signal to obtain the target signal.

[0050] In this embodiment, the tunable optical element includes a light source 1 and a light source modulator 2 for adjusting the output of light source 1. Light source 1 uses a distributed feedback (DFB) laser, with its wavelength locked to the absorption peak of the target gas. Light source modulator 2 employs dual-frequency modulation technology, where the primary modulation frequency matches the resonant frequency of measurement cavity 5, and the secondary modulation frequency is used for active tuning compensation of reference cavity 6. The light source is evenly divided into two cavities via a fiber optic beam splitter 10.

[0051] Furthermore, the structure of the measurement cavity 5 is designed to be a cylindrical structure. The diameter / length ratio of the cavity is optimized through COMSOL finite element simulation so that it reaches Q>100 at the laser modulation frequency of 1 to 10 kHz corresponding to the absorption spectrum of the target gas; the inner wall of the cavity is coated with a high-reflectivity gold film to reduce light energy loss; at the same time, a silicon-based microstructured anti-vibration diaphragm is integrated to reduce mechanical noise coupling.

[0052] Furthermore, the structure of the reference cavity is designed as a conical gradient acoustic impedance cavity, which broadens the resonance bandwidth Q value <20 by adjusting the geometric shape of the gradual change of the cavity cross-sectional area; a built-in active tuning piezoelectric ceramic piece dynamically adjusts the acoustic impedance of the reference cavity according to the resonance frequency of the measurement cavity, that is, the feedback control bandwidth is ≥1kHz, ensuring that the transfer functions of the two cavities to common-mode noise are consistent.

[0053] The reference cavity is set up in this application to form an asymmetric dual cavity with the measurement cavity, so that the resonant frequencies of the two are consistent, but the reference cavity does not respond to the target gas, and only transmits common-mode noise. Through differential processing, the noise is offset, the signal is retained, and the anti-interference ability is improved.

[0054] Auxiliary frequency modulation works by altering the laser's frequency in the reference cavity, where the laser interacts with the gas within it. Lasers of varying frequencies alter the energy state of the gas molecules, thereby affecting the gas's acoustic properties. By varying the laser's frequency, auxiliary frequency modulation changes the gas's response to acoustic waves within the reference cavity 6, thereby fine-tuning the acoustic impedance of the reference cavity 6, bringing it closer to that of the measurement cavity 5. This creates the conditions for aligning the resonant frequencies and common-mode noise transfer functions of the two cavities.

[0055] Active tuning mechanism of piezoelectric ceramics: During the active tuning process, based on the resonant frequency of the measurement cavity 5, a corresponding electrical signal is applied to the piezoelectric ceramic, causing it to undergo mechanical deformation, changing the cavity structure of the reference cavity 6, and thus changing the acoustic impedance of the reference cavity 6. Since the reference cavity 6 is designed as a conical gradient acoustic impedance cavity, this structure gives it a wide resonance bandwidth (Q value <20), making it easier to achieve flexible adjustment of the acoustic impedance through slight deformation of the piezoelectric ceramic. By monitoring the resonant frequency of the measurement cavity 5 in real time and feedback-controlling the electrical signal of the piezoelectric ceramic, the resonant frequency of the reference cavity 6 can be kept consistent with that of the measurement cavity 5, while ensuring that the transfer functions of the two cavities to common-mode noise are consistent.

[0056] The principle of achieving consistency in the common-mode noise transfer function: Common-mode noise refers to noise that affects both the measurement cavity 5 and the reference cavity 6, such as environmental vibrations, light source fluctuations, etc. When the resonant frequency of the reference cavity 6 is consistent with that of the measurement cavity 5 and the acoustic impedance is adjusted to an appropriate state, the response characteristics of the two cavities to common-mode noise will be similar, that is, the common-mode noise transfer functions are consistent. When faced with environmental vibrations, the two cavities will convert the vibrations into changes in the sound waves within the cavities in a similar manner, and because the acoustic structures of the two cavities are consistent after active tuning, the propagation and reflection processes of these sound wave changes in the two cavities are also similar, making the transfer characteristics of the common-mode noise in the two cavities the same. In this way, when the two cavity signals are subsequently differentially processed, the common-mode noise can be effectively suppressed.

[0057] The micro Helmholtz channel 7 has a diameter of 0.5 to 1 mm and a length of ≤5 mm. The channel is filled with a porous sound-absorbing material of ceramic fiber, which balances the air pressure fluctuations while suppressing the high-frequency crosstalk attenuation rate of ≥30 dB.

[0058] A MEMS microphone array 8 is deployed at four symmetrical points, and a beamforming algorithm is used to extract the central sound pressure signal of the measurement cavity; a piezoelectric film sensor 9 is used to detect the vibration signal of the reference cavity.

[0059] The piezoelectric film sensor 9 is attached to the outer wall of the reference cavity 6, away from the light inlet and the acoustic waveguide channel, to avoid direct interference of the photoacoustic signal or the acoustic wave in the cavity with the sensor measurement.

[0060] Furthermore, the piezoelectric film sensor 9 detects the mechanical vibration signal of the reference cavity 6. Since the wavelength of the light source in the reference cavity 6 is in the band that gas does not absorb, it does not generate photoacoustic signals. Therefore, its vibration signal is almost entirely caused by common mode noise, such as environmental vibration (laboratory bench vibration, fan noise), mechanical jitter of the light source module, and cavity deformation caused by gas path pressure fluctuations.

[0061] Correlation with the sound pressure signal at the center of measurement cavity 5:

[0062] Commonality: Both contain common-mode noise components (e.g., synchronous vibrations of the measurement cavity and the reference cavity caused by the same environmental vibration source).

[0063] Difference: Measurement cavity signal = target photoacoustic signal (acoustic wave generated by gas absorbing light energy) + common mode noise.

[0064] Reference cavity signal = common mode noise (no target photoacoustic signal because the light source wavelength is not absorbed by the gas).

[0065] Performing differential subtraction of the signals from measurement cavity 5 and reference cavity 6 in the frequency domain based on the coherence function suppresses common-mode noise. This is fundamentally due to the difference in correlation between common-mode noise and the target signal in the two cavity signals. Common-mode noise (such as ambient vibrations or light source fluctuations) affects both cavities simultaneously, causing the noise components in the two cavity signals to exhibit strong correlation at the same frequency. However, the target photoacoustic signal exists only in measurement cavity 5 and has no corresponding component in reference cavity 6, resulting in a very weak correlation between the two at the target signal frequency.

[0066] The photoacoustic cell of the present application adopts an asymmetric dual-cavity design. The measurement cavity is a high-Q value narrow-bandwidth resonant cavity, and the reference cavity is a low-Q value wide-bandwidth cavity, which solves the problem that the symmetrical cavity cannot distinguish between signals and overlapping frequency band common-mode noise; the signal detection unit deploys a MEMS microphone array at four symmetrical points in the measurement cavity, extracts the central sound pressure signal through the beamforming algorithm, and the reference cavity uses a piezoelectric film sensor to detect the vibration signal, which improves the spatial resolution and can capture signals in multiple dimensions; the signal processing adopts an adaptive differential algorithm combined with second harmonic demodulation, which solves the problem that the existing fixed differential weights cannot dynamically adapt to noise changes and the low-frequency noise suppression is insufficient.

[0067] Example 2

[0068] As another embodiment of the present invention, this embodiment provides a detection method using the differential photoacoustic spectroscopy detection system of the above-mentioned embodiment 1, comprising the following steps:

[0069] The central sound pressure signal of the measurement cavity 5 is extracted through the microphone array 8, and the vibration signal of the reference cavity 6 is collected through the piezoelectric film sensor 9;

[0070] Adopting an adaptive differential algorithm, the noise coupling characteristics of the two-way signal of measurement cavity 5 and reference cavity 6 are dynamically evaluated. The differential weight is adjusted in real time based on the coherence function in the frequency domain. Based on the differential weight, the collected sound pressure signal at the center of measurement cavity 5 and the vibration signal of reference cavity 6 are differentially subtracted to achieve maximum cancellation of common-mode noise and minimize loss of target signal.

[0071] Perform second harmonic demodulation, perform phase-locked amplification on the adaptive differential output signal, extract the second harmonic component, further suppress the low-frequency 1 / f noise, and obtain the target signal.

[0072] The specific steps of the adaptive difference algorithm are as follows:

[0073] In step 2.1, the two signals are sampled synchronously in the time domain using a 24-bit ADC (sampling rate ≥ 100 kHz) to eliminate timing deviation.

[0074] Step 2.2, retaining the effective frequency band of ±10% of the resonance frequency of the photoacoustic signal measurement cavity 5;

[0075] Step 2.3: Divide the signal into frames (frame length 1024 points, overlap rate 50%) and calculate the autopower spectral density (PSD) of each frame:

[0076] G xx (f)=|X(f)| 2

[0077] G yy (f)=|Y(f)| 2

[0078] Among them, G xx (f): represents the measured cavity self-power spectrum; G yy (f) represents the reference cavity self-power spectrum.

[0079] Cross Power Spectral Density (CPSD):

[0080] G xy (f) = X * (f)Y(f)

[0081] Among them, G xy (f) represents the cross power spectrum of the measurement cavity and the reference cavity.

[0082] Coherence function:

[0083]

[0084] Where γ(f) represents the coherence function, 0≤γ(f)≤1.

[0085] Step 2.4, for each frequency point f k Calculate the frequency domain weight w(f k ), and obtain the frequency domain weight matrix:

[0086]

[0087] In step 2.5, a sliding window with a length of 10 frames is used to complete the time domain weight averaging to avoid weight jumps.

[0088] Step 2.6: Perform frequency domain weighted difference on each frame signal and then perform inverse FFT to convert it back to time domain signal:

[0089] V out (f k )=w(f k )·V measure (f k )-[1-w(f k )]·V refercence (f k )

[0090] Where V measure (f k ) indicates that the measurement cavity is at the frequency f k The signal at V refercence (f k ) represents the reference cavity at frequency f k The signal at.

[0091] Those skilled in the art will clearly understand that, for the convenience and brevity of description, the specific working processes of the systems, devices and units described above can refer to the corresponding processes in the aforementioned method embodiments and will not be repeated here.

[0092] In the several embodiments provided in this application, it should be understood that the disclosed devices and systems can be implemented in other ways. For example, the device embodiments described above are merely schematic. For example, the division of units is only a logical function division. In actual implementation, there may be other division methods, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0093] Units described as separate components may or may not be physically separate, and components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of these units may be selected to achieve the purpose of this embodiment according to actual needs.

[0094] In addition, the functional units in the various embodiments of the present invention may be integrated into a single processing unit, each unit may exist physically separately, or two or more units may be integrated into a single unit. The aforementioned integrated units may be implemented in the form of hardware or software functional units.

[0095] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for enabling a computer device (which can be a personal computer, server, or network device, etc.) to execute all or part of the steps of the various embodiments of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, and other media that can store program code.

[0096] The above describes in detail the preferred embodiments of the present invention. It should be understood that those skilled in the art can make numerous modifications and variations based on the concepts of the present invention without inventive effort. Therefore, any technical solutions that can be derived by those skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. A differential photoacoustic spectroscopy detection system based on common mode noise suppression, characterized in that: It comprises an adjustable light source, a differential photoacoustic cell (3) and a detection unit (4) connected in sequence; The differential photoacoustic cell (3) is in an asymmetric dual-cavity form, comprising a measuring cavity (5) and a reference cavity (6), wherein the measuring cavity (5) is a high-Q value resonant cavity for enhancing the target gas signal; and the reference cavity (6) is a low-Q value cavity. The reference cavity (6) is actively tuned so that the resonant frequency is consistent with the resonant frequency of the measuring cavity and the acoustic impedance is lower than the acoustic impedance of the measuring cavity. The common-mode noise transfer function of the reference cavity and the measuring cavity are consistent. The measuring cavity and the reference cavity are connected through a micro acoustic waveguide; A microphone array (8) is disposed on the measuring cavity (5) for collecting a central sound pressure signal of the measuring cavity (5); a piezoelectric film sensor (9) is attached to the outer wall of the reference cavity (6) for detecting a vibration signal of the reference cavity (6); The detection unit (4) is connected to the microphone array (8) and the piezoelectric film sensor (9) for signal connection, and is used to perform adaptive differential subtraction and second harmonic demodulation processing on the central sound pressure signal of the measurement cavity and the vibration signal of the reference cavity to obtain a target signal.

2. The differential photoacoustic spectroscopy detection system based on common mode noise suppression according to claim 1, characterized in that: The adjustable light source comprises a light source (1) and a light source modulator (2); The light source (1) adopts a distributed feedback laser, and its wavelength is locked to the absorption peak of the target gas; The light source modulator (2) adopts a dual-frequency modulation technology, wherein the main modulation frequency matches the resonance frequency of the measurement cavity (5), and the auxiliary modulation frequency is used for active tuning compensation of the reference cavity (6), so that the acoustic impedance of the reference cavity (6) is close to the acoustic impedance of the measurement cavity (5); The output of the light source (1) is evenly divided and input into the measurement cavity (5) and the reference cavity (6) through an optical fiber beam splitter (10).

3. The differential photoacoustic spectroscopy detection system based on common mode noise suppression according to claim 1, characterized in that: The measuring cavity (5) is a cylindrical structure, and by adjusting the diameter / length ratio of the cavity, the Q value of the measuring cavity (5) at the laser modulation frequency corresponding to the target gas absorption spectrum line is greater than a first threshold.

4. The differential photoacoustic spectroscopy detection system based on common mode noise suppression according to claim 3, characterized in that: The inner wall of the measuring cavity (5) is plated with a high-reflectivity gold film and is integrated with a silicon-based microstructured anti-vibration diaphragm.

5. The differential photoacoustic spectroscopy detection system based on common mode noise suppression according to claim 1, characterized in that: The reference cavity (6) is a tapered gradually changing acoustic impedance cavity, and the resonance bandwidth is broadened by adjusting the geometric shape of the gradually changing cavity cross-sectional area, so that the Q value of the reference cavity (6) is less than a second threshold value; The reference cavity (6) has an internal actively tuned piezoelectric ceramic piece, which dynamically adjusts the acoustic impedance of the reference cavity according to the resonance frequency of the measurement cavity (5), so that the transfer functions of the reference cavity (6) and the measurement cavity (5) to common mode noise are consistent.

6. The differential photoacoustic spectroscopy detection system based on common mode noise suppression according to claim 1, characterized in that: The measuring cavity (5) and the reference cavity (6) are connected via a Helmholtz channel (7), and the channel is filled with a porous sound-absorbing material of ceramic fibers.

7. The differential photoacoustic spectroscopy detection system based on common mode noise suppression according to claim 1, characterized in that: The microphone array (8) is a MEMS microphone array deployed at multiple symmetrical points, and extracts the central sound pressure signal of the measurement cavity through a beamforming algorithm.

8. The differential photoacoustic spectroscopy detection system based on common mode noise suppression according to claim 1, characterized in that: The piezoelectric film sensor (9) is arranged at a position away from the light inlet and the acoustic waveguide channel of the reference cavity (6).

9. A differential photoacoustic spectroscopy detection method based on common mode noise suppression, characterized in that: The method is based on a differential photoacoustic spectroscopy detection system based on common mode noise suppression as described in any one of claims 1 to 8, and the steps include: The central sound pressure signal of the measurement cavity (5) is extracted by a microphone array (8), and the vibration signal of the reference cavity (6) is collected by a piezoelectric film sensor (9); Adopting an adaptive differential algorithm, the noise coupling characteristics of the two-way signals of the measurement cavity (5) and the reference cavity (6) are dynamically evaluated, the differential weight is adjusted in real time based on the coherence function in the frequency domain, and the differential subtraction is performed on the collected central sound pressure signal of the measurement cavity (5) and the vibration signal of the reference cavity (6) based on the differential weight; Perform second harmonic demodulation, perform phase-locked amplification on the adaptive differential output signal, extract the second harmonic component, and obtain the target signal.

10. The differential photoacoustic spectroscopy detection method based on common mode noise suppression according to claim 9, characterized in that: The adaptive difference algorithm is specifically as follows: Synchronous sampling of the two signals of the measurement cavity (5) and the reference cavity (6) in the time domain; For the time-domain synchronous sampling photoacoustic signal, an effective frequency band within the set bandwidth of the resonance frequency of the measurement cavity (5) is retained; Divide the signal into frames and calculate the auto-power spectrum density G of the measurement cavity signal and the reference cavity signal in each frame xx (f), G yy (f) and the cross power spectrum density G xy (f), and calculate the coherence function of the two: Based on the coherence function, for each frequency point f k Calculate the frequency domain weight w(f k ), and obtain the frequency domain weight matrix: Perform frequency domain weighted difference on each frame signal and then perform inverse FFT to convert it back to time domain signal: V out (f k )=w(f k )·V measure (f k )-[1-w(f k )]·V refercence (f k ) Where V measure (f k ) indicates that the measurement cavity is at the frequency f k The signal at V refercence (f k ) represents the reference cavity at frequency f k The signal at.

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