A multiple-reflection enhanced differential-resonance photoacoustic stimulated raman detection system and method

Abstract

The application discloses a kind of multiple reflection enhanced differential resonance photoacoustic stimulated raman detection systems and methods, belong to gas detection technical field, in the system, laser beam output by laser outputs after entering first stage multiple reflection enhancement module after convex lens;First stage multiple reflection enhancement module includes being arranged in the Raman frequency shifter of light source emission module downstream;First stage multiple reflection enhancement module, optical coupling module and second stage multiple reflection enhancement module, signal acquisition and processing module for inverting the concentration of gas to be measured are sequentially connected;Raman frequency shifter outputs two-color coherent light beam containing pump light and first-order stokes light mixture;Two-color coherent light beam passes through optical coupling module and eliminates axial chromatic aberration;Second stage multiple reflection enhancement module receives two-color coherent light beam, and the light beam that passes through second stage multiple reflection enhancement module is finally shot into optical trap and is absorbed.The application realizes the extension of light and gas molecule interaction distance and the promotion of photoacoustic signal excitation source intensity.

Description

Technical Field

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

[0002] Photoacoustic stimulated Raman spectroscopy (PARS) combines the molecular fingerprinting capabilities of stimulated Raman scattering with the high sensitivity and "zero background" detection advantages of photoacoustic spectroscopy, making it a cutting-edge technology in the field of trace gas detection. However, in the pursuit of higher detection sensitivity and practical system application, existing PARS technologies still face significant challenges in both light source acquisition and signal detection.

[0003] First, there is an inherent contradiction between conversion efficiency and device size / safety in the acquisition of stimulated Raman light sources. Traditional Raman frequency shifters mostly adopt a single-pass structure. According to stimulated Raman gain theory, the conversion efficiency of Stokes light depends exponentially on the pump light intensity, the gain coefficient of the Raman medium, and the interaction length. In order to overcome the oscillation threshold of stimulated Raman scattering and obtain sufficient conversion efficiency in a single pass, existing technologies usually adopt two extreme strategies: one is to greatly increase the length of the gas chamber (often exceeding 1 meter), resulting in a huge instrument size that is difficult to meet the requirements of portability or integration; the other is to drastically increase the internal pressure of the gas chamber (usually as high as 20-50 atm) to increase the molecular density of the medium. However, the high-pressure environment not only places stringent requirements on the sealing process and mechanical strength of the gas chamber, but also significantly increases the risk of high-pressure gas leakage, making the system a serious safety hazard in industrial or field applications.

[0004] Secondly, in the detection of photoacoustic signals, there is a dual bottleneck: "window thermal noise limitation" and "low excitation efficiency." Stimulated Raman spectroscopy outputs a high-energy dual-color pulsed laser (typically in the millijoules or even hundreds of millijoules range) containing residual pump light and strong Stokes light. When such a high-energy beam directly enters a conventional photoacoustic cell (such as a cylindrical or H-shaped photoacoustic cell), the windows at both ends of the cell undergo periodic thermal expansion and contraction due to unavoidable medium absorption, resulting in strong window thermal noise. Because this noise is in phase and frequency with the photoacoustic signal, traditional electronic filtering or lock-in amplification techniques are insufficient to filter it out, directly limiting the system's detection limit. Furthermore, conventional photoacoustic cells often employ a single-pass design, where the interaction path between the beam and the target gas is limited to the physical length of the photoacoustic cell. Most of the laser energy is transmitted without being absorbed by the gas, resulting in extremely low excitation efficiency and a significant waste of valuable laser energy.

[0005] In summary, existing technologies urgently require a comprehensive solution that can simultaneously address the issues of low light source conversion efficiency, large device size, high window background noise, and weak signal excitation. On one hand, a multi-reflection optical path system is introduced into the Raman frequency shifter and photoacoustic cell. Utilizing the principle of optical folding, the interaction distance between light and the medium is extended by tens of times within a limited physical space. This allows the Raman frequency shifter to achieve efficient and high-quality conversion of Stokes light under relatively low safety pressure, while significantly reducing the device size; it also greatly increases the effective excitation optical path within the photoacoustic cell, thereby multiplying the photoacoustic signal amplitude without increasing laser power. On the other hand, a low-loss optical path with a Brewster window and a differential Helmholtz detection structure are constructed. Brewster windows are configured at both the inlet and outlet ports of the Raman frequency shifter and the photoacoustic cell. By utilizing the lossless transmission characteristics of the Brewster window for P-polarized light, the single-reflection loss of the Raman frequency shifter's multiple-reflection cavity is first minimized to maintain a high energy density within the cavity and ensure the stimulation threshold. Secondly, the energy deposition of high-energy laser light on the surface of the photoacoustic cell window is reduced at its physical source, suppressing thermal noise generation. Combining Helmholtz acoustic resonance enhancement with a differential detection structure, environmental noise is further offset through common-mode suppression, thereby achieving a net gain in the photoacoustic signal and significantly improving the system's detection signal-to-noise ratio and sensitivity. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A differential resonant photoacoustic stimulated Raman detection system with multiple reflection enhancement includes: a light source emission module, a first-stage multiple reflection enhancement module, an optical path coupling module, a second-stage multiple reflection enhancement module, and a signal acquisition and processing module.

[0008] The light source emitting module includes a laser and a convex lens; the laser beam output by the laser enters the first-stage multiple reflection enhancement module after passing through the convex lens.

[0009] The first-stage multiple reflection enhancement module includes a Raman frequency shifter located downstream of the light source emission module for generating Stokes light;

[0010] An optical path coupling module is located between the first-stage multiple reflection enhancement module and the second-stage multiple reflection enhancement module; the beam output from the Raman frequency shifter is a two-color coherent beam containing a mixture of pump light and first-order Stokes light; the two-color coherent beam passes through the optical path coupling module to eliminate axial chromatic aberration;

[0011] The second-stage multiple reflection enhancement module receives the two-color coherent beam output from the optical path coupling module. The beam passing through the second-stage multiple reflection enhancement module is finally absorbed in the optical trap.

[0012] The signal acquisition and processing module includes several microphones installed in the second-stage multiple reflection enhancement module. These microphones are used to extract the amplitude of the photoacoustic signal synchronized with the laser pulse from the photoacoustic signal detected by the microphones, and to invert the concentration of the gas to be measured.

[0013] A differential resonant photoacoustic stimulated Raman detection method with multiple reflection enhancement, used in the aforementioned differential resonant photoacoustic stimulated Raman detection system, comprising:

[0014] S1, turn on the laser, rotate the half-wave plate inside the laser, monitor the transmission power after the first Brewster window module of the Raman frequency shifter through the power meter, adjust to the maximum transmission power, and ensure the output of pure P-polarized light.

[0015] S2, P-polarized light is injected into the Raman frequency shifter. The P-polarized light is reflected back and forth in the multi-channel optical path formed by the first concave spherical mirror and the second concave spherical mirror. Under low pressure conditions, it fully interacts with the Raman active gas, efficiently exciting stimulated Raman scattering, and outputting a two-color coherent beam that is a mixture of pump light and first-order Stokes light.

[0016] S3, the two-color coherent beam maintains the P-polarization characteristic and passes through the exit window of the Raman frequency shifter without reflection loss: the second Brewster window module and the entrance window of the photoacoustic cell: the third Brewster window module;

[0017] S4, the two-color coherent beam enters the photoacoustic cell and undergoes multiple round-trip reflections under the action of the third and fourth concave spherical mirrors. The gas molecules to be tested in the signal cavity are excited to generate a strong resonant photoacoustic signal.

[0018] S5 uses the first and second microphones installed in the reference cavity and signal cavity of the photoacoustic cell respectively to pick up the resonant photoacoustic signal. The two photoacoustic signals are subtracted and then amplified and processed by lock-in amplification. Finally, the amplitude of the photoacoustic signal proportional to the gas concentration is demodulated, and the concentration of the gas to be measured is inverted.

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

[0020] (1) This invention innovatively introduces a “two-stage multiple reflection” optical path mechanism in the Raman frequency shifter and photoacoustic cell, thereby achieving a several-fold increase in the interaction distance between light and gas molecules and a several-fold increase in the intensity of the photoacoustic signal excitation source. It effectively solves the technical problems of traditional single-pass through-type systems, such as the large size of the device due to low light source conversion efficiency, serious gas leakage safety hazards caused by reliance on ultra-high pressure environment (20-50 atm), and extremely low laser energy excitation efficiency of conventional photoacoustic cells.

[0021] (2) By constructing a low-loss optical path with a Brewster window in the entire link and strictly cooperating with the transmission of P-polarized light, the present invention achieves the maintenance of high energy density under low reflection loss in the Raman frequency shifter and blocks the energy deposition of high-energy laser on the surface of the window in the photoacoustic cell from the physical source. It effectively solves the technical bottleneck of strong "window thermal noise" caused by high-energy dual-color pulse laser directly entering the conventional photoacoustic cell, and the fact that this noise is difficult to be filtered out by traditional electronic technology because it is in phase with the photoacoustic signal, thus seriously limiting the detection limit of the system.

[0022] (3) By combining differential Helmholtz acoustic resonance structure for signal detection, this invention achieves physical-level suppression of environmental common-mode noise such as external vibration and airflow noise and amplification of photoacoustic signal by using a completely symmetrical cavity; effectively solves the problem that conventional photoacoustic cells are easily interfered with by complex environmental noise in actual industrial sites or field applications, resulting in limited system detection signal-to-noise ratio and poor environmental adaptability. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of a differential resonant photoacoustic stimulated Raman detection system enhanced by multiple reflections. 1-Laser, 2-Convex lens, 3-Raman frequency shifter, 4-Photoacoustic cell, 5-First concave spherical mirror, 6-Second concave spherical mirror, 7-Third concave spherical mirror, 8-Fourth concave spherical mirror, 9-First mirror, 10-Second mirror, 11-Achromatic lens, 12-Optical trap, 13-Differential module, 14-Filtering module, 15-Amplification module, 16-Data acquisition module, 17-Host computer;

[0024] Figure 2 This is a schematic diagram of the Raman frequency shifter, where 3.1 - first Brewster window module, 3.2 - second Brewster window module, 3.3 - first air inlet, 3.4 - first air outlet, and 3.5 - Raman frequency shifter body;

[0025] Figure 3 This is a schematic diagram of the photoacoustic cell structure, where 4.1-third Brewster window module, 4.2-fourth Brewster window module, 4.3-fifth Brewster window module, 4.4-sixth Brewster window module, 4.5-second air inlet, 4.6-second air outlet, 4.7-first microphone, 4.8-second microphone, 4.9-connecting pipe, 4.10-reference cavity, 4.11-signal cavity. 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] like Figure 1 As shown, the present invention provides a differential resonant photoacoustic stimulated Raman detection system with multiple reflection enhancement (hereinafter referred to as the system), including a light source emission module, a first-stage multiple reflection enhancement module (including a Raman frequency shifter), an optical path coupling module, a second-stage multiple reflection enhancement module (photoacoustic cell), and a signal acquisition and processing module.

[0028] The light source emission module includes a laser 1 and a convex lens 2. Laser 1 uses a linearly polarized pulsed laser as the pump source. In this embodiment, laser 1 is an Nd:YAG (neodymium-doped yttrium aluminum garnet) solid-state pulsed laser with an output wavelength of 532 nm, a pulse width of approximately 7 ns, a repetition frequency of 10 Hz, and a single pulse energy of approximately 100 mJ. To accommodate the design of the Brewster window module in the subsequent optical path, the laser output from laser 1 needs to be adjusted to P-polarized light (i.e., linearly polarized light with the electric field vector parallel to the incident plane). This can be achieved by rotating the half-wave plate inside the laser or by adding a polarization adjustment device in the optical path between laser 1 and convex lens 2, ensuring that the laser beam is adjusted to P-polarized light before entering convex lens 2. After output, the laser beam is collimated or shaped by convex lens 2 to match the mode matching requirements of the subsequent multiple reflection cavity, and then enters the first-stage multiple reflection enhancement module.

[0029] The first-stage multiple reflection enhancement module includes a Raman frequency shifter 3. The Raman frequency shifter 3 is located downstream of the light source emission module and is used to generate Stokes light. The structure of the Raman frequency shifter 3 is as follows... Figure 2As shown, the device includes a high-pressure resistant Raman frequency shifter body 3.5, which has a first inlet 3.3 and a first outlet 3.4 for filling with Raman active gas (high-purity hydrogen in this embodiment). A pair of first concave spherical mirrors, 5 and 6, are installed at both ends of the Raman frequency shifter 3. The surfaces of the first and second concave spherical mirrors 5 and 6 are coated with a high-reflectivity film (R>99.9%), and they are placed opposite each other to form a multi-channel reflection cavity, using a Herriott-type optical path or a near-concentric optical path arrangement. The incident pump light enters the Raman frequency shifter body 3.5 through a coupling hole on the edge of the first concave spherical mirror 5, undergoing dozens of round-trip reflections between the first and second concave spherical mirrors 5 and 6, thereby significantly extending the interaction distance between the light and hydrogen molecules within a limited physical length (e.g., tens of centimeters). This allows the stimulated Raman threshold to be reached at relatively low pressures (e.g., 1-5 atm), enabling efficient generation of 683nm first-order Stokes light.

[0030] The two ends of the Raman frequency shifter 3 are sealed by the first Brewster window module 3.1 and the second Brewster window module 3.2, respectively. The installation angles of the first Brewster window module 3.1 and the second Brewster window module 3.2 satisfy the condition that the P-polarized light is incident at the Brewster angle, thereby achieving zero-reflection loss transmission of the pump light and maintaining a high energy density in the cavity.

[0031] The optical path coupling module includes a first reflector 9, a second reflector 10, and an achromatic lens 11. Spatially, this module is positioned in the optical path between the first-stage multiple reflection enhancement module and the second-stage multiple reflection enhancement module. The beam output from the Raman frequency shifter 3 is a two-color coherent beam containing a mixture of pump light (532nm) and first-order Stokes light (683nm). This two-color coherent beam undergoes optical path folding and guidance via the first reflector 9 and the second reflector 10, and then passes through the achromatic lens 11 towards the photoacoustic cell 4. The achromatic lens 11 is used to focus or collimate the two-color coherent beam, eliminating axial chromatic aberration and ensuring that the two beams spatially coincide within the subsequent photoacoustic cell 4.

[0032] The second-stage multiple reflection enhancement module includes a photoacoustic cell 4, and a third concave spherical mirror 7 and a fourth concave spherical mirror 8 respectively disposed on both sides of the signal cavity of the photoacoustic cell 4. In terms of spatial position and optical path connection, this module is positioned downstream of the optical path coupling module to receive the bichromatic coherent beam output via the achromatic lens 11. The structure of the photoacoustic cell 4 is as follows... Figure 3 As shown, a differential Helmholtz resonance structure is adopted.

[0033] The main body of the photoacoustic cell 4 comprises two symmetrically arranged resonant chambers: a reference chamber 4.10 and a signal chamber 4.11. The two chambers are connected by a fine connecting pipe 4.9, which is used to balance static pressure and suppress low-frequency flow noise. The photoacoustic cell 4 is provided with a second air inlet 4.5 located in the reference chamber 4.10 and a second air outlet 4.6 located in the signal chamber 4.11 for introducing the gas to be measured.

[0034] A pair of second concave spherical mirrors (a third concave spherical mirror 7 and a fourth concave spherical mirror 8) are installed on the outer sides of the signal cavity 4.11. This mirror group is coated with a highly reflective film. After the bicolor coherent beam enters the signal cavity 4.11, it undergoes multiple round-trip reflections (e.g., 20-40 times) between the third concave spherical mirror 7 and the fourth concave spherical mirror 8, which multiplies the effective excitation path of the beam in the gas under test, significantly enhancing the excitation intensity of the photoacoustic signal. No beam passes through the reference cavity 4.10; it serves only as an acoustic reference. A third Brewster window module 4.1 and a fourth Brewster window module 4.2 are installed at both ends of the signal cavity 4.11 for optical path sealing; a fifth Brewster window module 4.3 and a sixth Brewster window module 4.4 are installed at both ends of the reference cavity 4.10 for symmetrical balance sealing. The key is that the installation orientation of the third Brewster window module 4.1 and the fourth Brewster window module 4.2 is consistent with that of the windows of the Raman frequency shifter 3: the first Brewster window module 3.1 and the second Brewster window module 3.2. This ensures that the high-energy two-color beam with P-polarization characteristics can pass through without loss at the Brewster angle, avoiding the deposition of laser energy on the surface of the window (i.e., the Brewster window module) and the generation of thermal noise. The beam passing through the photoacoustic cell 4 is finally absorbed in the optical trap 12 to prevent stray light interference.

[0035] The signal acquisition and processing module includes a first microphone 4.7 and a second microphone 4.8 installed at the midpoint antinodes of the reference cavity 4.10 and signal cavity 4.11, respectively, and sequentially connected to a differential module 13, a filtering module 14, an amplification module 15, a data acquisition module 16, and a host computer 17. In terms of specific connection and working principle, the gas molecules to be tested in the signal cavity 4.11 are excited to generate a strong resonant photoacoustic signal. The first microphone 4.7 and the second microphone 4.8 act as high-sensitivity acoustic sensors. The detected resonant photoacoustic signal is simultaneously input to the differential module 13 (which includes a differential amplification circuit). After subtraction, a pure differential-mode signal with ambient common-mode noise removed is output. This signal is then sequentially filtered by the filtering module 14 (bandpass filtering) and amplified by the amplification module 15 (low-noise amplification) before being sent to the data acquisition module 16 (high-speed acquisition card) for analog-to-digital conversion to obtain a digital signal. The host computer 17 receives digital signals, uses digital phase-locked loop algorithm or Fourier transform to extract the amplitude of the resonant photoacoustic signal synchronized with the laser pulse, and inversely calculates the concentration of the gas to be measured.

[0036] This invention further provides a differential resonance photoacoustic stimulated Raman detection method for gas detection using the above system with enhanced multiple reflections, comprising the following steps:

[0037] S1, Light source adjustment: Turn on laser 1, rotate the half-wave plate inside the laser, monitor the transmission power after passing through the first Brewster window module 3.1 of the Raman frequency shifter 3 using a power meter, and adjust it to the maximum transmission power to ensure that the output to the subsequent optical path is pure P-polarized light.

[0038] S2, First-order multiple reflection seeding and amplification: P-polarized light is injected into Raman frequency shifter 3. The P-polarized light is reflected back and forth in the multi-channel optical path formed by the first concave spherical mirror 5 and the second concave spherical mirror 6. Under low pressure conditions, it fully interacts with the Raman active gas to efficiently excite stimulated Raman scattering and output a two-color coherent beam of pump light mixed with first-order Stokes light with high beam quality.

[0039] S3, end-to-end lossless coupling: the two-color coherent beam maintains the P-polarization characteristic and passes through the exit window of the Raman frequency shifter 3, the second Brewster window module 3.2, and the entrance window of the photoacoustic cell 4, the third Brewster window module 4.1, without reflection loss, thus avoiding thermal shock and thermal noise caused by the absorption of high-energy laser on the window surface from the physical source.

[0040] S4, Second-stage multiple reflection excitation: The two-color coherent beam enters the photoacoustic cell 4 and undergoes multiple round-trip reflections under the action of the second concave spherical mirror pair: the third concave spherical mirror 7 and the fourth concave spherical mirror 8, which greatly extends the effective excitation optical path. The gas molecules to be tested in the signal cavity 4.11 are excited to generate a strong resonant photoacoustic signal.

[0041] S5, Differential Noise Reduction Detection: The photoacoustic signal is picked up by the photoacoustic cell 4 with the differential Helmholtz resonant structure. Combined with the physical suppression of thermal noise by the third Brewster window module 4.1, the fourth Brewster window module 4.2, the fifth Brewster window module 4.3 and the sixth Brewster window module 4.4, the two photoacoustic signals are subtracted and then amplified and processed by lock-in. Finally, the amplitude of the photoacoustic signal proportional to the gas concentration is demodulated, and the concentration of the gas to be measured is inverted.

[0042] 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 differential resonant photoacoustic stimulated Raman detection system with enhanced multiple reflections, characterized in that, include: The system includes a light source emission module, a first-stage multiple reflection enhancement module, an optical path coupling module, a second-stage multiple reflection enhancement module, and a signal acquisition and processing module. The light source emitting module includes a laser and a convex lens; the laser beam output by the laser enters the first-stage multiple reflection enhancement module after passing through the convex lens. The first-stage multiple reflection enhancement module includes a Raman frequency shifter located downstream of the light source emission module for generating Stokes light; An optical path coupling module is located between the first-stage multiple reflection enhancement module and the second-stage multiple reflection enhancement module; the beam output from the Raman frequency shifter is a two-color coherent beam containing a mixture of pump light and first-order Stokes light; the two-color coherent beam passes through the optical path coupling module to eliminate axial chromatic aberration; The second-stage multiple reflection enhancement module receives the two-color coherent beam output from the optical path coupling module. The beam passing through the second-stage multiple reflection enhancement module is finally absorbed in the optical trap. The signal acquisition and processing module includes several microphones installed in the second-stage multiple reflection enhancement module. These microphones are used to extract the amplitude of the photoacoustic signal synchronized with the laser pulse from the photoacoustic signal detected by the microphones, and to invert the concentration of the gas to be measured.

2. The differential resonant photoacoustic stimulated Raman detection system with enhanced multiple reflections according to claim 1, characterized in that, Also includes: A half-wave plate placed inside the laser or a polarization adjustment device placed between the laser and a convex lens is used to adjust the laser output from the laser to P-polarized light.

3. The differential resonant photoacoustic stimulated Raman detection system with enhanced multiple reflections according to claim 1, characterized in that, The laser selected is an Nd:YAG solid-state pulsed laser.

4. The differential resonant photoacoustic stimulated Raman detection system with enhanced multiple reflections according to claim 1, characterized in that, The Raman frequency shifter includes a Raman frequency shifter body, which is provided with a first air inlet and a first air outlet for filling with Raman active gas. A pair of first concave spherical mirrors is installed at both ends of the Raman frequency shifter: a first concave spherical mirror and a second concave spherical mirror; the surfaces of the first concave spherical mirror and the second concave spherical mirror are coated with a high reflectivity film, and they are placed opposite each other to form a multi-pass reflection cavity, and are arranged in a Heriot-type optical path or a near-concentric optical path.

5. The differential resonant photoacoustic stimulated Raman detection system with enhanced multiple reflections according to claim 1, characterized in that, The two ends of the Raman frequency shifter are sealed by a first Brewster window module and a second Brewster window module, respectively; the installation angles of the first Brewster window module and the second Brewster window module satisfy that the P-polarized light is incident at the Brewster angle.

6. The differential resonant photoacoustic stimulated Raman detection system with enhanced multiple reflections according to claim 1, characterized in that, The optical path coupling module includes a first reflector, a second reflector, and an achromatic lens. A two-color coherent beam, consisting of a mixture of pump light and first-order Stokes light, is folded and guided by the first and second reflectors, and then directed to the second-stage multiple reflection enhancement module through the achromatic lens. The achromatic lens is used to focus or collimate the two-color coherent beam to eliminate axial chromatic aberration.

7. The differential resonant photoacoustic stimulated Raman detection system with enhanced multiple reflections according to claim 1, characterized in that, The second-stage multiple reflection enhancement module includes a photoacoustic cell, and a third and a fourth concave spherical reflector respectively disposed on both sides of the signal cavity of the photoacoustic cell.

8. The differential resonance photoacoustic stimulated Raman detection system with enhanced multiple reflections according to claim 1, characterized in that, The main body of the photoacoustic cell includes two symmetrically arranged resonant chambers: a reference chamber and a signal chamber; the two chambers are connected by a fine connecting tube, which is used to balance static pressure and suppress low-frequency flow noise; the photoacoustic cell is provided with a second air inlet in the reference chamber and a second air outlet in the signal chamber for introducing the gas to be measured. A second pair of concave spherical mirrors is installed on both sides of the signal cavity: a third concave spherical mirror and a fourth concave spherical mirror. After the two-color coherent beam enters the signal cavity, it undergoes two or more round-trip reflections between the third concave spherical mirror and the fourth concave spherical mirror.

9. The differential resonant photoacoustic stimulated Raman detection system with enhanced multiple reflections according to claim 1, characterized in that, The signal acquisition and processing module includes a first microphone and a second microphone installed at the antinodes in the middle of the reference cavity and the signal cavity, respectively, as well as a differential module, a filtering module, an amplification module, a data acquisition module and a host computer connected in sequence. The resonant photoacoustic signals detected by the first and second microphones are simultaneously input into the differential module. After subtraction, the differential mode signal with ambient common-mode noise removed is output. The differential mode signal passes through the filtering module and the amplification module in sequence, and is sent to the data acquisition module for analog-to-digital conversion to obtain a digital signal. The host computer receives the digital signal, extracts the amplitude of the resonant photoacoustic signal synchronized with the laser pulse, and inversely calculates the concentration of the gas to be measured.

10. A differential resonance photoacoustic stimulated Raman detection method with enhanced multiple reflections, used in the differential resonance photoacoustic stimulated Raman detection system with enhanced multiple reflections as described in any one of claims 1 to 9, characterized in that, include: S1, turn on the laser, rotate the half-wave plate inside the laser, monitor the transmission power after the first Brewster window module of the Raman frequency shifter through the power meter, adjust to the maximum transmission power, and ensure the output of pure P-polarized light. S2, P-polarized light is injected into the Raman frequency shifter. The P-polarized light is reflected back and forth in the multi-channel optical path formed by the first concave spherical mirror and the second concave spherical mirror. Under low pressure conditions, it fully interacts with the Raman active gas, efficiently exciting stimulated Raman scattering, and outputting a two-color coherent beam that is a mixture of pump light and first-order Stokes light. S3, the two-color coherent beam maintains the P-polarization characteristic and passes through the exit window of the Raman frequency shifter without reflection loss: the second Brewster window module and the entrance window of the photoacoustic cell: the third Brewster window module; S4, the two-color coherent beam enters the photoacoustic cell and undergoes multiple round-trip reflections under the action of the third and fourth concave spherical mirrors. The gas molecules to be tested in the signal cavity are excited to generate a strong resonant photoacoustic signal. S5 uses the first and second microphones installed in the reference cavity and signal cavity of the photoacoustic cell respectively to pick up the resonant photoacoustic signal. The two photoacoustic signals are subtracted and then amplified and processed by lock-in amplification. Finally, the amplitude of the photoacoustic signal proportional to the gas concentration is demodulated, and the concentration of the gas to be measured is inverted.

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