Spatially filtered differential resonant photoacoustic stimulated raman detection system and method

Abstract

The application discloses a kind of spatial filtering type differential resonance photoacoustic stimulated raman detection systems and methods, belong to gas detection technical field, the system includes laser source module, raman frequency shifter module, spatial filtering type differential resonance photoacoustic cell and signal acquisition and processing unit;High-energy pulsed laser generated by laser source module is shot into raman frequency shifter module after collimation;The double-color coherent light beams output by raman frequency shifter module are focused after achromatic lens, and are coaxially shot into the center of spatial filtering type differential resonance photoacoustic cell, and the photoacoustic signals excited are collected by two microphones of spatial filtering type differential resonance photoacoustic cell;Signal acquisition and processing unit is electrically connected each microphone;Two acoustic tubes of spatial filtering type differential resonance photoacoustic cell are symmetrically arranged in the inside of large tube buffer cavity, and two Brewster windows are respectively sealed and installed at the two ends of large tube buffer cavity.The application can effectively inhibit window optical damage, improve system stability.

Description

Technical Field

[0001] This invention belongs to the field of gas detection technology, specifically relating to a spatial filtering differential resonance photoacoustic stimulated Raman detection system and method. Background Technology

[0002] Photoacoustic spectroscopy, as an indirect absorption spectroscopy technique, boasts advantages such as high sensitivity, good selectivity, rapid response, and zero background detection, making it widely used in trace gas detection. However, gas analysis techniques based on infrared absorption principles, relying on changes in molecular dipole moments, have a fundamental limitation: they cannot detect homonuclear diatomic molecules such as hydrogen. While Raman spectroscopy can achieve fingerprint recognition of nonpolar molecules, its spontaneous Raman scattering signal is extremely weak. Traditional stimulated Raman scattering (PARS), although capable of identifying hydrogen, essentially extracts weak light intensity changes under strong background laser light, significantly limiting its sensitivity and stability for low-concentration gases. Existing photoacoustic Raman Spectroscopy (PARS) combines the molecular selectivity of stimulated Raman transitions with the "zero background" detection advantage of photoacoustic spectroscopy, achieving high signal-to-noise ratio measurements by detecting the acoustic signal generated by the release of heat energy during non-radiative relaxation of molecules. In the detection of homonuclear diatomic gases (such as H2, N2, and O2), photoacoustic stimulated Raman spectroscopy typically requires extremely high-power pulsed lasers to induce nonlinear optical effects. However, high-power pulsed lasers encounter two core technical problems when passing through the photoacoustic cell window:

[0003] Window slat damage risk: Traditional vertically mounted or non-Brewster angle mounted windows are highly susceptible to optical damage due to energy absorption or surface reflection when subjected to high-energy pulsed lasers of Nd:YAG (neodymium-doped yttrium aluminum garnet) at the tens of millijoules level.

[0004] Background thermal noise: The unavoidable weak absorption of the window leads to local temperature rise and pressure waves. This window noise will directly diffuse into the resonant cavity, forming background noise that cannot be completely eliminated by electrical means. It is very easy to drown out the effective signal generated by trace gases, which seriously limits the system's breakthrough to the ppb (parts per billion) level detection limit. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A spatially filtered differential resonant photoacoustic stimulated Raman spectroscopy (SRS) detection system includes: a laser source module, a Raman frequency shifter module, a spatially filtered differential resonant photoacoustic cell, and a signal acquisition and processing unit. The high-energy pulsed laser generated by the laser source module is collimated and then incident on the Raman frequency shifter module. The two-color coherent beam output from the Raman frequency shifter module is focused by an achromatic lens and coaxially incident on the radial center of the spatially filtered differential resonant photoacoustic cell. The excited photoacoustic signal is acquired by two microphones in the spatially filtered differential resonant photoacoustic cell. The signal acquisition and processing unit is electrically connected to each microphone.

[0007] The spatial filtering differential resonant photoacoustic cell includes a first acoustic tube, a second acoustic tube, a large tube buffer cavity, a first Brewster window, a second Brewster window, a first microphone, a second microphone, and an optical trap. The first and second acoustic tubes are symmetrically arranged inside the large tube buffer cavity. The first and second Brewster windows are respectively sealed and installed at both ends of the large tube buffer cavity. The optical trap is located at the end of the optical path on one side of the light outlet of the spatial filtering differential resonant photoacoustic cell. The first and second microphones are respectively sealed and installed at the middle of the first and second acoustic tubes along their axial direction.

[0008] A spatially filtered differential resonant photoacoustic stimulated Raman spectroscopy method includes:

[0009] S1, preheat the laser; set the laser's operating parameters via the host computer;

[0010] S2, monitor and adjust the pressure of the high-purity Raman active gas in the Raman frequency shifter; the pump light output by the laser undergoes stimulated Raman scattering in the cavity to generate first-order Stokes light; scan different pressures, measure the energy product of the residual pump light and the first-order Stokes light, take the pressure corresponding to the maximum value as the optimal experimental pressure and fix it to obtain the optimal two-color coherent light source.

[0011] S3, determine the longitudinal resonant mode frequency of the spatial filter-type differential resonant photoacoustic cell;

[0012] S4, exhaust the air in the large tube buffer chamber and the first and second acoustic tubes, and then press the gas sample to be tested into the air inlet.

[0013] S5, the two-color coherent beam output by the Raman frequency shifter is focused by an achromatic lens and converges at the geometric center of the first acoustic tube; photoacoustic signals are excited in the first and second acoustic tubes;

[0014] S6, the first microphone and the second microphone respectively collect the photoacoustic signals in the first acoustic tube and the second acoustic tube. The two photoacoustic signals enter the differential circuit at the same time for inversion and summation, and output a pure differential mode photoacoustic signal.

[0015] S7, the differential mode photoacoustic signal passes through the preamplifier and bandpass filter in sequence, and is acquired by the data acquisition card to obtain the time-domain photoacoustic signal;

[0016] S8, the host computer receives the time-domain photoacoustic signal, converts the time-domain photoacoustic signal into a frequency-domain signal, and extracts the signal amplitude at the resonant frequency as the effective photoacoustic signal value corresponding to the target gas to be measured.

[0017] S9. Prepare and measure multiple sets of gas samples with different standard concentrations in sequence according to S4 to S8. Perform linear fitting between the concentration of the target gas and the effective photoacoustic signal amplitude to obtain the system calibration curve.

[0018] S10: For a gas sample of unknown concentration, substitute the time-domain photoacoustic signal into the system calibration curve in S9 to retrieve the concentration of the gas sample.

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

[0020] The differential resonance photoacoustic Raman detection system and method with spatial filtering effect provided by this invention, combined with stimulated Raman scattering to endow photoacoustics with fingerprint recognition capability, especially provides an ideal and reliable detection method for gases such as hydrogen that do not have infrared reactivity. The specific beneficial effects are as follows:

[0021] (1) By adopting Brewster window combined with P-polarized light transmission design and oblique installation process, this invention utilizes the characteristic that the theoretical value of Brewster angle for P-polarized light reflection loss is 0, which eliminates the multiple reflections and energy accumulation of high-energy pulsed laser on the surface of the window (first Brewster window 6 and second Brewster window 7), and reduces the power density per unit area by increasing the spot projection area, effectively solving the problem that the window is susceptible to optical damage under high-power pulsed laser excitation, and at the same time weakening the background thermal noise generated by window absorption from the source, thus improving the stability of the system.

[0022] (2) By constructing a nested spatial filtering structure of a large tube enclosing a small tube, the present invention utilizes the acoustic impedance mismatch between the outer large tube buffer cavity and the inner acoustic tube to form a physical isolation area, which dilutes and blocks the divergent thermoacoustic waves generated by the window, thereby achieving effective spatial filtering of the background noise of the window and preventing it from directly coupling into the core detection area to interfere with the target signal, thus greatly optimizing the detection lower limit of the system.

[0023] (3) By designing a parallel symmetrical differential resonant dual-tube structure, the present invention utilizes the characteristic that the signal amplitudes in the two acoustic resonant tubes are the same but the phases are opposite to perform differential operations, thereby achieving efficient cancellation of non-coherent common-mode noise such as external environmental vibration, temperature fluctuations and electromagnetic interference. At the same time, the effective photoacoustic signal amplitude is theoretically doubled, significantly improving the detection signal-to-noise ratio of the system.

[0024] (4) The present invention forms a leak-free acoustic path by embedding and sealing the microphone, and uses an achromatic lens to correct the axial chromatic aberration of the two wavelengths (pump light and Stokes light), which solves the problem of low acquisition efficiency caused by the leakage of sound pressure signal into the buffer cavity, and the problem of low excitation efficiency caused by the non-overlapping of the focal space of the two beams, thus ensuring the efficient excitation and complete acquisition of weak photoacoustic signals and improving the stability of detection. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall structure of the spatial filtering differential resonant photoacoustic stimulated Raman detection system of the present invention, wherein 1-first acoustic tube, 2-second acoustic tube, 3-large tube buffer cavity, 4-first microphone, 5-second microphone, 6-first Brewster window, 7-second Brewster window, 8-optical trap, 9-achromatic lens, 10-differential circuit, 11-preamplifier, 12-bandpass filter, 13-data acquisition card, 14-pressure valve, 15-Raman frequency shifter, 16-long focal length plano-convex lens, 17-optical window, 18-laser, 19-host computer, 20-first acoustic signal output tube, 21-second acoustic signal output tube. Detailed Implementation

[0026] 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 merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0027] This invention provides a spatially filtered differential resonant photoacoustic stimulated Raman detection system (hereinafter referred to as the system), which aims to achieve high signal-to-noise ratio, high damage threshold, and low thermal noise photoacoustic Raman detection through Brewster window, large tube-within-small tube structure, differential dual tube design and closed microphone installation method.

[0028] like Figure 1 As shown, the spatially filtered differential resonant photoacoustic stimulated Raman detection system of the present invention includes: a laser source module, a Raman frequency shifter module, a spatially filtered differential resonant photoacoustic cell, and a signal acquisition and processing unit; wherein, the high-energy pulsed laser generated by the laser source module is collimated and then injected into the Raman frequency shifter module; the two-color coherent beam output by the Raman frequency shifter module is focused by an achromatic lens 9 and coaxially injected into the radial center of the spatially filtered differential resonant photoacoustic cell, and the excited photoacoustic signal is acquired by two microphones of the spatially filtered differential resonant photoacoustic cell; the signal acquisition and processing unit is electrically connected to each microphone and is used to process and invert the concentration of the photoacoustic signal acquired by each microphone.

[0029] The laser source module includes a laser 18, and the laser beam emitted by the laser 18 passes through the optical window 17 and enters the Raman frequency shifter module.

[0030] The Raman frequency shifter module includes a Raman frequency shifter 15, a pressure valve 14 disposed on the Raman frequency shifter 15, an optical window 17 sealed and installed at the light inlet of the Raman frequency shifter 15, and a long focal length plano-convex lens 16 sealed and installed at the light outlet of the Raman frequency shifter 15. The pressure valve 14 is used to monitor and adjust the internal gas pressure of the Raman frequency shifter 15, the optical window 17 is used to transmit the pump laser and seal the gas, and the long focal length plano-convex lens 16 is used to collimate the output beam of the Raman frequency shifter 15.

[0031] The spatially filtered differential resonant photoacoustic cell includes a first acoustic tube 1, a second acoustic tube 2, a large-tube buffer cavity 3, a first Brewster window 6, a second Brewster window 7, a first microphone 4, a second microphone 5, and an optical trap 8. The first acoustic tube 1 and the second acoustic tube 2 are symmetrically arranged inside the large-tube buffer cavity 3. The first Brewster window 6 and the second Brewster window 7 are respectively sealed and installed at both ends of the large-tube buffer cavity 3. The optical trap 8 is located at the end of the optical path on the light outlet side of the spatially filtered differential resonant photoacoustic cell. An achromatic lens 9 is positioned between the Raman frequency shifter module and the spatially filtered differential resonant photoacoustic cell. The first microphone 4 and the second microphone 5 are respectively sealed and installed in the middle of the axial direction of the first and second acoustic tubes, forming a leak-free acoustic acquisition path to ensure that only the photoacoustic signal inside the tube is picked up and external noise is isolated.

[0032] The signal acquisition and processing unit includes a differential circuit 10, a preamplifier 11, a bandpass filter 12, a data acquisition card 13, and a host computer 19. The input terminals of the differential circuit 10 are connected to the first microphone 4 and the second microphone 5, respectively, and the output terminals are connected to the preamplifier 11, the bandpass filter 12, and the data acquisition card 13 in sequence. The data acquisition card 13 is communicatively connected to the host computer 19.

[0033] Main pump source: Laser 18 adopts a mature 532nm Nd:YAG (neodymium-doped yttrium aluminum garnet) solid-state laser, with pulse energy set to a maximum of 100mJ (adjustable from 1 to 100mJ), operating frequency of 10Hz (adjustable from 1 to 20Hz), pulse width of 7ns, beam diameter of approximately 6mm, and peak power of up to 14.3MW, which is sufficient to excite stimulated Raman effect.

[0034] The Raman frequency shifter 15 is made of stainless steel and is approximately 1 meter in length. It is filled with a high-purity Raman-active gas, the same as the target gas; in this embodiment, hydrogen is used as an example. The input end of the Raman frequency shifter 15, the optical window 17, is sealed with a fused silica window.

[0035] The output end uses a long focal length plano-convex lens 16 to seal and collimate the diverging beam. The surface of the Raman frequency shifter 15 is provided with a vent. Through the vent, the inside of the Raman frequency shifter 15 is first evacuated, and then pure hydrogen gas is forced in. The surface of the Raman frequency shifter 15 is provided with a pressure valve 14 to monitor the internal pressure in real time.

[0036] The achromatic lens 9, used as a beam-combining focusing lens, has a focal length of 15cm and is coated with a visible light anti-reflection film. The dichromatic light output from the Raman frequency shifter 15 has a natural collinearity characteristic. After being focused by the achromatic lens 9, it can effectively suppress the focus drift problem caused by different wavelengths and fully excite the photoacoustic Raman signal in the spatially filtered differential resonant photoacoustic cell.

[0037] The spatial filtering type differential resonant photoacoustic cell is configured as a differential resonant photoacoustic cell with a double-layer spatial filtering effect. The inner layer consists of parallel and symmetrical acoustic resonant tubes: the first acoustic tube 1 and the second acoustic tube 2, and the outer layer is a large-volume isolation buffer cavity: the large tube buffer cavity 3. The large spatial volume is used to physically attenuate the sound waves, thereby achieving a spatial filtering effect.

[0038] Furthermore, to reduce background noise at the source, a first Brewster window 6 and a second Brewster window 7 are sealed at both ends of the spatially filtered differential resonant photoacoustic cell. Utilizing the characteristic that the Brewster angle has zero reflection of P-polarized light, the energy deposition and heat absorption of the laser on the surface of the windows (first Brewster window 6 and second Brewster window 7) are greatly reduced, thereby significantly reducing the initial intensity of the window thermoacoustic wave (window noise).

[0039] Through the above design, this invention achieves dual noise reduction. First, the first Brewster window 6 and the second Brewster window 7 suppress the noise generation amplitude at the source. Second, the large tube buffer cavity 3 spatially isolates and attenuates residual weak noise along the propagation path, thereby ensuring that the microphone located in the middle of the inner acoustic resonator (first acoustic tube 1 and second acoustic tube 2) only picks up pure gas photoacoustic signals. The inner parallel symmetrical acoustic resonator: The center of the first acoustic tube 1 and the second acoustic tube 2 is respectively opened with a small hole with an inner diameter of about 2 mm. The two holes are respectively set with cylindrical lead-out tubes with a height of about 1 mm: the first acoustic signal lead-out tube 20 and the second acoustic signal lead-out tube 21. The first acoustic signal lead-out tube 20 and the second acoustic signal lead-out tube 21 are respectively sealed and connected to the second microphone 5 and the first microphone 4, forming a leak-free photoacoustic signal acquisition path. In addition, in order to withstand high-power pulsed laser and prevent optical damage, the two ends of the spatial filter type differential resonant photoacoustic cell are respectively sealed and installed with the first Brewster window 6 and the second Brewster window 7.

[0040] Both ends of the large-tube buffer cavity 3 are precision-machined into Brewster angle bevels conforming to the refractive index relationship. The first Brewster window 6 and the second Brewster window 7 are respectively embedded in the annular positioning countersunk holes on the bevels at both ends of the large-tube buffer cavity 3. A six-hole fastening flange covers the top of the first Brewster window 6 and the second Brewster window 7, and the first Brewster window 6 and the second Brewster window 7 are securely fixed using stainless steel bolts. A high-elasticity vacuum sealing ring is used at the sealing interface, achieving adhesive-free airtight sealing through axial pressure. Furthermore, air inlets and outlets are respectively provided at both ends of the large-tube buffer cavity 3 (e.g., located at the lower parts of both ends of the large-tube buffer cavity 3) for the introduction and discharge of the gas sample to be tested, thus forming a through-flow gas path to ensure that the gas inside the photoacoustic cell can be quickly and fully replaced.

[0041] The first microphone 4 and the second microphone 5 are embedded in the side wall of the large tube buffer cavity 3 through stepped holes and sealed around the periphery with sealing rings. This installation structure forms a leak-free, sealed acoustic path between the probe of the first microphone 4 and the second acoustic signal output tube 21, and between the probe of the second microphone 5 and the first acoustic signal output tube 20, respectively, forming a sealed microphone coupling cavity to fully acquire the generated photoacoustic signal. In the signal acquisition and processing unit, the differential circuit 10 differentially processes the photoacoustic signals of the first acoustic tube 1 and the second acoustic tube 2, effectively suppressing incoherent noise in the environment. The preamplifier 11 amplifies the differentially processed photoacoustic signal, and the bandpass filter 12 is set to a bandwidth of 1000-2000Hz to initially filter out low-frequency noise in the environment. The data acquisition card 13 acquires the time-domain photoacoustic signal, and the host computer 19 performs spectral analysis on the time-domain photoacoustic signal and extracts the characteristic spectrum through Fourier transform, and then inverts to obtain the concentration of the target gas (hydrogen) to be measured.

[0042] This invention further provides a spatially filtered differential resonance photoacoustic stimulated Raman detection method, comprising:

[0043] S1, Laser 18 preheating and operating parameter setting. Preheat laser 18 to ensure stable output energy. Set the operating parameters of laser 18 through the visual operation interface of the host computer 19: set the laser pulse energy to an energy value that can effectively excite stimulated Raman scattering (preferably 100mJ in this embodiment, but can be adjusted within the range of 50-100mJ according to actual needs), and set the operating frequency to a frequency value that matches the sampling rate (preferably 10Hz in this embodiment, but can also be adjusted within the range of 1-20Hz).

[0044] S2, Raman frequency shifter 15 output optimization. The internal pressure of the Raman frequency shifter 15 is monitored in real time, and the pressure of the high-purity Raman-active gas (such as high-purity hydrogen) injected internally is precisely controlled through pressure valve 14. High-energy pump light of a preset wavelength (preferably 532nm in this embodiment) output from laser 18 enters the Raman frequency shifter 15, inducing stimulated Raman scattering of the high-purity Raman-active gas (hydrogen), causing part of the pump light to be converted into the corresponding first-order Stokes light (683nm in this embodiment; note: if the pump light wavelength or Raman medium changes, the Stokes light wavelength will also change). During this process, the light intensity of the 532nm residual pump light and the 683nm first-order Stokes light under different pressures (within the range of 1-24 atm) is measured sequentially using an energy meter, and the energy of the two is multiplied (…). , The pulse energy of the residual pump light, The pressure corresponding to the maximum value of the pulse energy of the first-order Stokes light is calibrated as the optimal experimental pressure, and this optimal experimental pressure is fixed in order to obtain the optimal two-color coherent light source.

[0045] S3, Determination of the resonant frequency of the spatially filtered differential resonant photoacoustic cell. The longitudinal resonant mode frequency of the spatially filtered differential resonant photoacoustic cell is determined through finite element simulation or frequency sweep experiment. In this embodiment, the resonant frequency of the first acoustic tube 1 and the second acoustic tube 2 is determined to be 1693Hz.

[0046] S4, Target gas replacement. Use a vacuum device to purge the air from the large tube buffer chamber 3 and the internal acoustic tubes: first acoustic tube 1 and second acoustic tube 2, and then pressurize the target gas sample: ppm (parts per million) nitrogen / hydrogen mixture through the air inlet.

[0047] S5, Two-color light focusing and signal excitation. The Raman frequency shifter 15 outputs a two-color coherent beam of 532nm and 683nm, which, after being focused by the achromatic lens 9, converges at the geometric center of the first acoustic tube 1. At this point, the frequency difference between the 532nm and 683nm two-color coherent beams exactly matches the frequency of hydrogen molecules at 4155cm. -1 The energy level transition frequency. The molecules of the gas sample (hydrogen) are excited to transition from the ground state to a higher energy state (undergoing stimulated Raman transition), and then undergo a non-radiative relaxation process accompanied by heat release, resulting in periodic thermal expansion and contraction in the first acoustic tube 1 and the second acoustic tube 2 (the laser only enters the first acoustic tube 1, but sound waves are generated inside both the first acoustic tube 1 and the second acoustic tube 2, with the same amplitude and opposite phase), thereby exciting a photoacoustic signal.

[0048] S6, Differential acquisition of photoacoustic signals. The first microphone 4 and the second microphone 5 acquire the photoacoustic signals from the first acoustic tube 1 and the second acoustic tube 2, respectively. At this time, the photoacoustic signals in both acoustic tubes are superimposed from the target photoacoustic signal and background noise (according to the differential resonance principle, the target photoacoustic signals in the first acoustic tube 1 and the second acoustic tube 2 have the same amplitude but opposite phase, while the background noise in the first acoustic tube 1 and the second acoustic tube 2 is usually in phase). The two acquired photoacoustic signals simultaneously enter the differential circuit 10 for inverse summation (differential) operation, thereby effectively suppressing spatially incoherent environmental common-mode noise (in-phase subtraction) and multiplying the effective signal (out-of-phase subtraction), ultimately outputting a pure differential-mode photoacoustic signal.

[0049] S7, the differential mode photoacoustic signal output by the differential circuit 10 is amplified by the preamplifier 11 with low noise gain, filtered by the bandpass filter 12 (bandwidth covering 1693Hz) to remove flow noise and stray noise other than the resonant frequency, and then acquired by the data acquisition card 13 at high speed to obtain the time domain photoacoustic signal.

[0050] S8, Frequency Domain Conversion and Feature Extraction. The host computer 19 receives the time-domain photoacoustic signal and converts it into a frequency-domain signal using a Fast Fourier Transform (FFT). The signal amplitude at the resonant frequency (1693Hz) is extracted, and this amplitude is the effective photoacoustic signal value corresponding to the target gas (hydrogen).

[0051] S9, System Calibration. Following steps S4 to S8, prepare and measure four sets of test gas samples (hydrogen samples) with different standard concentrations. Use the least squares method to perform linear fitting between the concentration of the target gas (hydrogen) and the effective photoacoustic signal amplitude to obtain the system calibration curve (sensitivity coefficient).

[0052] S10, Concentration Inversion. For a gas sample with an unknown concentration, the time-domain photoacoustic signal is substituted into the system calibration curve obtained in S9 to invert the concentration of the gas sample.

[0053] In this embodiment, the target gas is hydrogen, and the Raman frequency shifter 15 is filled with hydrogen. The system's laser frequency difference is locked at the hydrogen transition frequency. By changing the frequency shifter medium, the system can also detect other gases. This system has fingerprint recognition capabilities and can detect specific target components in a gas mixture (such as measuring hydrogen concentration from a nitrogen / air mixture), unaffected by background gas interference.

[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 spatially filtered differential resonant photoacoustic stimulated Raman detection system, characterized in that, include: The system comprises a laser source module, a Raman frequency shifter module, a spatially filtered differential resonant photoacoustic cell, and a signal acquisition and processing unit. The high-energy pulsed laser generated by the laser source module is collimated and then incident on the Raman frequency shifter module. The two-color coherent beam output from the Raman frequency shifter module is focused by an achromatic lens and coaxially incident on the radial center of the spatially filtered differential resonant photoacoustic cell. The resulting photoacoustic signal is acquired by two microphones within the spatially filtered differential resonant photoacoustic cell. The signal acquisition and processing unit is electrically connected to each microphone. The Raman frequency shifter is filled with a high-purity Raman-active gas identical to the target gas. The spatial filtering differential resonant photoacoustic cell includes a first acoustic tube, a second acoustic tube, a large tube buffer cavity, a first Brewster window, a second Brewster window, a first microphone, a second microphone, and an optical trap. The first and second acoustic tubes are symmetrically arranged inside the large tube buffer cavity. The first and second Brewster windows are respectively sealed and installed at both ends of the large tube buffer cavity. The optical trap is located at the end of the optical path on one side of the light outlet of the spatial filtering differential resonant photoacoustic cell. The first and second microphones are respectively sealed and installed at the middle of the first and second acoustic tubes along their axial direction.

2. The spatial filtering type differential resonance photoacoustic stimulated Raman detection system according to claim 1, characterized in that, The laser source module includes a laser, and the laser beam emitted by the laser passes through an optical window and enters the Raman frequency shifter module; the laser is a neodymium-doped yttrium aluminum garnet solid-state laser.

3. The spatial filtering differential resonance photoacoustic stimulated Raman detection system according to claim 2, characterized in that, The Raman frequency shifter module includes a Raman frequency shifter, a pressure valve disposed on the Raman frequency shifter, an optical window sealed and installed at the light inlet of the Raman frequency shifter, and a long focal length plano-convex lens sealed and installed at the light outlet of the Raman frequency shifter.

4. The spatial filtering type differential resonance photoacoustic stimulated Raman detection system according to claim 1, characterized in that, The signal acquisition and processing unit includes a differential circuit, a preamplifier, a bandpass filter, a data acquisition card, and a host computer. The input terminals of the differential circuit are connected to the first and second microphones, respectively, and the output terminals are connected to the preamplifier, the bandpass filter, and the data acquisition card in sequence. The data acquisition card communicates with the host computer. The differential circuit performs differential processing on the photoacoustic signals of the first and second acoustic tubes. The preamplifier amplifies the differentially processed photoacoustic signals, and the bandpass filter initially filters out low-frequency noise in the environment. The data acquisition card acquires the time-domain photoacoustic signals, and the host computer performs spectral analysis on the time-domain photoacoustic signals, extracts the characteristic spectrum, and then inverts to obtain the concentration of the target gas to be measured.

5. The spatial filtering differential resonance photoacoustic stimulated Raman detection system according to claim 3, characterized in that, The optical window is sealed with a fused silica window, and the output end is sealed with a long focal length plano-convex lens to collimate and diverge the beam. The surface of the Raman frequency shifter is provided with a vent. The Raman frequency shifter is first evacuated through the vent, and then high-purity Raman active gas is forced in.

6. The spatial filtering type differential resonance photoacoustic stimulated Raman detection system according to claim 1, characterized in that, A small hole is opened in the center of the first acoustic tube and the second acoustic tube respectively. The first acoustic signal lead-out tube and the second acoustic signal lead-out tube are respectively sealed and connected between the small hole and the microphone to lead the photoacoustic signal to the first microphone and the second microphone.

7. The spatial filtering differential resonance photoacoustic stimulated Raman detection system according to claim 1, characterized in that, Achromatic lenses, used as beam-combining and focusing lenses, have a visible light anti-reflection coating on their surface.

8. The spatial filtering type differential resonance photoacoustic stimulated Raman detection system according to claim 1, characterized in that, The two ends of the large tube buffer cavity are machined into Brewster angle bevels that conform to the refractive index relationship using a precision beveling process; the first Brewster window and the second Brewster window are respectively embedded in the annular positioning countersunk holes on the bevels at both ends of the large tube buffer cavity; an air inlet and an air outlet are respectively provided at both ends of the large tube buffer cavity.

9. The spatial filtering differential resonance photoacoustic stimulated Raman detection system according to claim 3, characterized in that, Raman frequency shifters are made of stainless steel.

10. A spatially filtered differential resonant photoacoustic stimulated Raman detection method, used in the spatially filtered differential resonant photoacoustic stimulated Raman detection system as described in any one of claims 1 to 9, characterized in that, include: S1, preheat the laser; The laser's operating parameters are set via a host computer. S2, monitor and adjust the pressure of the high-purity Raman active gas in the Raman frequency shifter; the pump light output by the laser undergoes stimulated Raman scattering in the cavity to generate first-order Stokes light; scan different pressures, measure the energy product of the residual pump light and the first-order Stokes light, take the pressure corresponding to the maximum value as the optimal experimental pressure and fix it to obtain the optimal two-color coherent light source. S3, determine the longitudinal resonant mode frequency of the spatial filter-type differential resonant photoacoustic cell; S4, exhaust the air in the large tube buffer chamber and the first and second acoustic tubes, and then press the gas sample to be tested into the air inlet. S5, the two-color coherent beam output from the Raman frequency shifter is focused by an achromatic lens and converges at the geometric center of the first acoustic tube; photoacoustic signals are excited in the first and second acoustic tubes; S6, the first microphone and the second microphone respectively collect the photoacoustic signals in the first acoustic tube and the second acoustic tube. The two photoacoustic signals enter the differential circuit at the same time for inversion and summation, and output a pure differential mode photoacoustic signal. S7, the differential mode photoacoustic signal passes through the preamplifier and bandpass filter in sequence, and is acquired by the data acquisition card to obtain the time-domain photoacoustic signal; S8, the host computer receives the time-domain photoacoustic signal, converts the time-domain photoacoustic signal into a frequency-domain signal, and extracts the signal amplitude at the resonant frequency as the effective photoacoustic signal value corresponding to the target gas to be measured; S9. Prepare and measure multiple sets of gas samples with different standard concentrations in sequence according to S4 to S8. Perform linear fitting between the concentration of the target gas and the effective photoacoustic signal amplitude to obtain the system calibration curve. S10: For a gas sample of unknown concentration, substitute the time-domain photoacoustic signal into the system calibration curve in S9 to retrieve the concentration of the gas sample.

Images