An optical sound excited raman gas detection system and method based on hollow core fiber self-seeding injection enhancement

Abstract

This invention discloses a photoacoustic stimulated Raman gas detection system and method based on hollow-core optical fiber seed injection enhancement, belonging to the field of laser spectroscopy trace gas detection technology. The system includes a light source and beam splitting module, a hollow-core optical fiber seed generation module, an optical delay module, and a short-path power amplification module. The main laser beam is incident on a beam splitter and splits into a first beam and a strong pump beam. In the hollow-core optical fiber seed generation module, the first beam is reflected by a fifth mirror, a hollow-core optical fiber seed generation unit, a sixth mirror, and a filter, outputting a weak seed beam. The strong pump beam enters the optical delay module and synchronizes with the weak seed beam output from the filter. In the short-path power amplification module, the weak seed beam and the strong pump beam output from the optical delay module are combined and strictly coaxial at a dichroic mirror, then focused by a convex lens and enter the short-path Raman frequency shifter. This invention achieves a significant reduction in the oscillation threshold of the Raman frequency shifter by utilizing seed light-induced stimulated emission.

Description

Technical Field

[0001] This invention belongs to the field of laser spectroscopy trace gas detection technology, specifically relating to a photoacoustic stimulated Raman gas detection system and method based on hollow fiber self-seed injection enhancement. Background Technology

[0002] Photoacoustic stimulated Raman spectroscopy (PARS) is a spectroscopic sensing technique that combines nonlinear stimulated Raman scattering with high-sensitivity photoacoustic detection. Compared to traditional spontaneous Raman spectroscopy, PARS utilizes stimulated emission to significantly enhance signal intensity, enabling precise quantitative analysis of trace gases. Since photoacoustic detection only detects acoustic signals generated by nonradiative relaxation processes, it avoids interference from scattered light and background fluorescence, resulting in an extremely high signal-to-noise ratio. The key to achieving PARS detection lies in obtaining a high-power, narrow-linewidth dual-wavelength (pump light and Stokes light) coherent radiation source. In early implementations, stimulated Raman conversion typically relied on high-power lasers to achieve frequency conversion through a long-path Raman cell in free space. To address the issues of large volume and high oscillation threshold of traditional cells, recent research has introduced air-filled hollow fiber (HCF) as the Raman gain medium. While the strong confinement of the optical field by the fiber and its extremely long effective operating distance have reduced the oscillation threshold and system size to some extent, this improvement has also introduced new problems such as fiber end-face damage and limited energy load. In summary, existing frequency conversion solutions have the following significant problems:

[0003] Traditional stimulated Raman scattering (SRS) is large in size and typically requires extremely high pump power or extremely long interaction distance (usually requiring a Raman frequency shifter of 1-2 meters) to overcome the threshold and obtain sufficient Stokes light conversion efficiency, resulting in a large system size that is difficult to integrate.

[0004] Optical fibers are vulnerable. Although the interaction distance can be extended to confine the light intensity within a very small core and reduce the oscillation threshold, in photoacoustic stimulated Raman detection, a peak power in the megawatt (MW) range is typically required to obtain a stronger acoustic signal. Ordinary hollow-core optical fibers have a low laser damage threshold and cannot withstand pulsed laser energy of tens or even hundreds of millijoules (mJ), thus limiting the increase in pump power.

[0005] Unstable oscillation: The randomness of oscillation based on spontaneous Raman scattering is relatively large. The randomness of the noise-based oscillation process leads to instability in the energy and spectral quality (linewidth) of the output Stokes light pulse, which affects the signal-to-noise ratio of the photoacoustic signal. Summary of the Invention

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

[0007] A photoacoustic stimulated Raman gas detection system based on hollow fiber seed injection enhancement includes: a light source and beam splitting module, a hollow fiber seed generation module, an optical delay module, a short optical path power amplification module, a photoacoustic detection module, and a signal acquisition and processing module.

[0008] The light source and beam splitting module includes a pulsed laser and a beam splitter. The main laser beam output by the pulsed laser is incident on the beam splitter, which splits the main laser beam into a reflected first beam and a transmitted strong pump beam.

[0009] The hollow fiber seed generation module includes a fifth reflector, a hollow fiber seed generation unit, a sixth reflector, and a filter. The first beam is reflected by the fifth reflector and coupled into the hollow fiber seed generation unit. The beam output by the hollow fiber seed generation unit is reflected by the sixth reflector and passes through the filter to output pure weak seed light.

[0010] The optical delay module includes a mirror group consisting of several mirrors arranged in sequence. Strong pump light enters the optical delay module and is synchronized with the weak seed light output from the filter after being output.

[0011] The short optical path power amplifier module includes a dichroic mirror, a convex lens, and a short optical path Raman frequency shifter; the weak seed light and the strong pump light output from the optical delay module are combined and strictly coaxial at the dichroic mirror, then focused by the convex lens and enter the short optical path Raman frequency shifter.

[0012] The short-path Raman frequency shifter, photoacoustic detection module, and signal acquisition and processing module are connected in sequence.

[0013] A photoacoustic stimulated Raman gas detection method based on hollow-core optical fiber self-seed injection enhancement, used in the aforementioned photoacoustic stimulated Raman gas detection system based on hollow-core optical fiber self-seed injection enhancement, comprising:

[0014] Step 1: Turn on the laser, adjust the multi-dimensional precision adjustment frame of the beam splitter, the fifth reflecting mirror and the hollow fiber seed generator unit to make the first beam efficiently coupled into the hollow fiber seed generator unit, and observe and confirm that a stable weak seed light has been generated behind the filter.

[0015] Step 2: Monitor the time waveforms of the strong pump light and the weak seed light at the position of the dichroic mirror, and adjust the multi-dimensional precision adjustment frame of the reflector group contained in the optical delay module until the two pulses of the strong pump light and the weak seed light coincide on the oscilloscope. Then, finely adjust the angle of each reflector in the reflector group to ensure that the strong pump light and the weak seed light coincide in the far field spot to form a combined beam.

[0016] Step 3: After the combined beam is passed into the short-path Raman frequency shifter, the output is residual pump light and amplified first-order Stokes light; the pulse energy of the residual pump light and the amplified first-order Stokes light are measured in the photoacoustic cell respectively; the air pressure in the short-path Raman frequency shifter is changed to find the optimal air pressure value that maximizes the product of the pulse energy of the residual pump light and the amplified first-order Stokes light and then fixes it.

[0017] Step 4: Introduce known standard gases of different concentrations into the photoacoustic cell. When each concentration of gas is introduced, collect the acoustic wave signal excited in the photoacoustic cell, extract the photoacoustic signal amplitude at the resonant frequency of the photoacoustic cell, and perform linear fitting on the gas concentration-photoacoustic signal amplitude to obtain the calibration curve.

[0018] Step 5: Introduce the gas sample to be tested, collect its photoacoustic signal amplitude, and calculate the concentration of the gas sample based on the calibration curve in Step 4.

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

[0020] (1) This invention utilizes low-threshold Stokes light generated by hollow optical fiber for external seed injection, thereby significantly reducing the oscillation threshold of the Raman frequency shifter by using seed light-induced stimulated emission. This allows the traditional Raman gas cell, which originally required a length of 1-2 meters, to be reduced to a short optical path gas cell of tens of centimeters to achieve extremely high conversion efficiency.

[0021] (2) By adopting a system architecture of split seeding-spatiotemporal synchronization-stimulated amplification, the present invention uses the easily damaged hollow fiber only to generate low-energy seed light, and places the high-energy power amplification process in a short optical path Raman frequency shifter in free space. This achieves high beam quality seeding while perfectly avoiding the damage problem of the hollow fiber end face being unable to withstand high-power pulsed laser (MW level) coupling, thereby allowing the system to inject higher energy pump light to improve detection sensitivity.

[0022] (3) By introducing seed light from hollow fiber with high beam quality and narrow linewidth characteristics for injection locking, the present invention realizes the transformation of the stimulated Raman scattering process from random spontaneous emission noise oscillation to a deterministic stimulated amplification process, which significantly improves the pulse energy stability and spectral quality of the output Stokes light, thereby greatly improving the signal-to-noise ratio of the photoacoustic signal. Attached Figure Description

[0023] Figure 1This is a schematic diagram of the photoacoustic stimulated Raman gas detection system based on hollow fiber self-seed injection enhancement of the present invention, wherein: 1-beam splitter, 2-first reflector, 3-second reflector, 4-third reflector, 5-fourth reflector, 6-fifth reflector, 7-sixth reflector, 8-dichroic mirror, 9-convex lens, 10-short optical path Raman frequency shifter, 11-hollow fiber seed generation unit, 12-pressure valve, 13-achromatic lens, 14-first optical window, 15-second optical window, 16-third optical window, 17-fourth optical window, 18-photoacoustic cell, 19-microphone, 20-optical trap, 21-pulsed laser, 22-preamplifier, 23-filter, 24-data acquisition card, 25-host computer, 26-filter. Detailed Implementation

[0024] 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.

[0025] To address the problems existing in the prior art, this invention discloses a photoacoustic stimulated Raman gas detection system and method based on hollow fiber self-seed injection enhancement, involving a novel architecture of beam splitting seeding-spatiotemporal synchronization-stimulated amplification.

[0026] The overall structure of the photoacoustic stimulated Raman gas detection system (hereinafter referred to as the system) based on hollow optical fiber self-seed injection enhancement of the present invention is as follows: Figure 1 As shown, the system mainly consists of six parts: a light source and beam splitting module, a hollow fiber seed generation module, an optical delay module, a short optical path power amplification module, a photoacoustic detection module, and a signal acquisition and processing module.

[0027] Light source and beam splitting module: including pulsed laser 21 and beam splitter 1. Pulsed laser 21 (e.g., Nd:YAG solid-state laser) serves as the main pump source, outputting high-energy (e.g., 100mJ), narrow-pulse-width (e.g., 7ns) linearly polarized pulsed laser (i.e., the main laser beam). The main laser beam is incident on beam splitter 1, which splits the main laser beam into two beams according to an energy ratio (e.g., 1:99): a small portion of the reflected energy (e.g., 1%) serves as the first beam (weak light) used to generate seed light; the majority of the transmitted energy (e.g., 99%) serves as the second beam (strong pump light) used for subsequent power amplification.

[0028] The hollow-core fiber seed generation module (i.e., the seed production optical path) includes a fifth reflecting mirror 6, a hollow-core fiber seed generation unit 11, a sixth reflecting mirror 7, and a filter 26. The first beam, after being reflected by the fifth reflecting mirror 6, is coupled into the hollow-core fiber seed generation unit 11, which is filled with pure gas (such as hydrogen). Utilizing the extremely high confinement capability and long interaction distance of the hollow-core fiber seed generation unit 11, even a 1% low-energy pulse can efficiently excite stimulated Raman scattering within the fiber, generating high-quality, narrow-linewidth weak seed light (first-order Stokes seed light). The beam output from the hollow-core fiber seed generation unit 11 (containing first-order Stokes seed light and residual pump light) is reflected by the sixth reflecting mirror 7 and then filtered by the filter 26 to remove the residual pump light, outputting pure weak seed light (first-order Stokes seed light) on the hollow-core fiber seed generation module.

[0029] The optical delay module (i.e., the pump light delay branch) consists of a first reflector 2, a second reflector 3, a third reflector 4, and a fourth reflector 5 (the first reflector 2, the second reflector 3, the third reflector 4, and the fourth reflector 5 form a reflector group; the number of reflectors in the reflector group depends on the required delay time and spatial constraints; if the delay time is short, two reflectors are sufficient; if a long delay is required, more reflectors are needed for folding the optical path). The strong pump light enters the optical delay module. Because the weak seed light experiences a time delay during transmission in the optical fiber, the second beam (strong pump light) must be compensated for by the optical delay module. By adjusting the delay distance of the reflector group, the time difference in transmission of the weak seed light in the optical fiber is compensated, ensuring precise synchronization between the strong pump light output from the optical delay module and the weak seed light output from filter 26 in both the time and spatial domains.

[0030] The short-path Raman power amplifier module includes a dichroic mirror 8, a convex lens 9, and a short-path Raman frequency shifter 10. The weak seed light and the delayed strong pump light output from the optical delay module are combined and strictly coaxial at the dichroic mirror 8, then focused by the convex lens 9 and enter the short-path Raman frequency shifter 10 (e.g., optical path only 20-30cm). Since a high-quality weak seed light already exists, the strong pump light entering the gas chamber directly amplifies the weak seed light through stimulated emission. This method eliminates the need for long-distance accumulation as in traditional techniques, achieving extremely high conversion efficiency over short distances. The short-path Raman frequency shifter 10 is equipped with a pressure valve 12. The pressure valve 12 adjusts the internal gas pressure of the short-path Raman frequency shifter 10, controlling the conversion efficiency of the strong pump light to Stokes light, ensuring that the energy product of the output residual pump light and the amplified first-order Stokes light is optimized. Reaching the maximum value, where, The residual pump pulse energy output after passing through the short-path Raman frequency shifter 10 The energy of a first-order Stokes light pulse after being stimulated and amplified.

[0031] The photoacoustic detection module includes an achromatic lens 13, a photoacoustic cell 18, and an optical trap 20. The amplified dual-wavelength beam (including residual pump light and amplified first-order Stokes light) output from the short-path Raman frequency shifter 10 is focused by the achromatic lens 13 into the photoacoustic cell 18. At this point, the frequency difference between the two laser beams is exactly equal to the Raman transition frequency of the corresponding gas molecules, thus efficiently exciting the photoacoustic signal of the gas under test within the photoacoustic cell 18. The transmitted beam passing through the photoacoustic cell 18 is ultimately absorbed by the optical trap 20 to prevent stray light interference.

[0032] The signal acquisition and processing module includes a microphone 19, a preamplifier 22, a filter 23, a data acquisition card 24, and a host computer 25. The microphone 19, positioned on the wall of the photoacoustic cell 18, acquires the photoacoustic signal generated after the gas molecules absorb laser energy. This signal is amplified by the preamplifier 22 and undergoes preliminary noise reduction processing by the filter 23 (low-pass filter circuit). The data acquisition card 24 then acquires the time-domain photoacoustic signal, which is finally transmitted to the host computer 25. The host computer 25 performs a Fast Fourier Transform (FFT) on the time-domain photoacoustic signal to convert it into a frequency-domain signal, extracts the photoacoustic signal amplitude at the resonant frequency of the photoacoustic cell 18, and uses a preset calibration model to deduce the concentration of the gas under test.

[0033] The following embodiments are provided to specifically illustrate the system, which includes:

[0034] The light source and beam-splitting module includes a pulsed laser 21 and a beam splitter 1. A high-average-power, high-repetition-rate acousto-optic modulated Nd:YAG solid-state laser is selected as the main pump source. The laser wavelength of the main pump source is set to 532nm, the pulse width to approximately 7ns, the repetition rate to 10Hz, the single-pulse energy to 100mJ, the spot diameter to approximately 6mm, and the output peak power to the MW level. The linearly polarized pulsed light output from the pulsed laser 21 is first incident on the beam splitter 1. The beam splitting ratio of the beam splitter 1 is set to 1:99, splitting the incident main laser beam into two beams. The first beam, with an energy percentage of approximately 1% (approximately 1mJ), enters the hollow-core fiber seed generation module to generate seed light; the second beam (strong pump light), with an energy percentage of approximately 99% (approximately 99mJ), enters the optical delay module for subsequent power amplification.

[0035] In the hollow-core fiber seed generation module, the first beam, after reflection or transmission, is coupled into the hollow-core fiber seed generation unit 11. The hollow-core fiber seed generation unit 11 is a section of gas-filled anti-resonant hollow-core fiber (HCF), with a core diameter of approximately 30-50 μm and a length of approximately 1-2 meters. Both ends of the hollow-core fiber seed generation unit 11 are sealed and filled with a high-purity active gas (such as hydrogen, methane, etc.; the gas type must be consistent with the gas to be measured or the gas in the subsequent short-path Raman frequency shifter 10). Due to the extremely strong confinement of the optical field by the hollow-core fiber seed generation unit 11, even a low-energy pulse of 1 mJ can achieve extremely high power density within the fiber, thereby efficiently exciting stimulated Raman scattering (SRS). The beam output from the hollow-core fiber seed generation unit 11 includes residual pump light (wavelength 532 nm) and newly generated first-order Stokes seed light (e.g., the wavelength of first-order Stokes light from hydrogen is 683 nm). A filter 26 (such as a long-pass filter with high reflectivity of 532nm and high transmittance of 683nm) is set at the output end of the hollow fiber seed generation unit 11 to filter out residual pump light and retain only high-quality, narrow-linewidth first-order Stokes seed light.

[0036] The second beam (strong pump light) enters the optical delay module. Because the first beam experiences a time delay during transmission within the hollow fiber seed generation unit 11 (due to fiber refractive index and optical path length), the second beam (strong pump light) must travel through a sufficiently long free-space optical path (such as a spatially folded optical path, a specific implementation of a free-space optical path where a straight optical path is folded to save space) to compensate for this delay. By adjusting the physical spacing of the mirror assembly (e.g., mounting the third mirror 4 and the fourth mirror 5 on a precision translation stage), the optical path length of the second beam (strong pump light) is precisely controlled, ensuring that the time difference between its arrival at the beam combining point and the weak seed light is within nanoseconds, thus achieving precise synchronization of the strong pump light and the weak seed light in both the temporal and spatial domains.

[0037] The short-path power amplifier module includes a dichroic mirror 8, a convex lens 9, and a short-path Raman frequency shifter 10. The delayed second beam (strong pump light) and the weak seed light converge at the dichroic mirror 8. The dichroic mirror 8 is coated with a film that is highly transparent to 532nm (strong pump light) and highly reflective to 683nm (weak seed light). By adjusting the angles of the fourth reflecting mirror 5 and the dichroic mirror 8, the two beams (weak seed light and strong pump light) are made strictly coaxial in space. The coaxial dichroic beams (strong pump light and weak seed light) are focused by the convex lens 9 and enter the short-path Raman frequency shifter 10. The short-path Raman frequency shifter 10 is configured as a stainless steel gas chamber with a length of 20-30cm. A first optical window 14 and a second optical window 15 are respectively sealed at both ends of the stainless steel gas chamber. A pressure valve 12 is installed in the stainless steel gas chamber for real-time monitoring and precise control of the internal air pressure. The stainless steel gas chamber is filled with the same high-purity gas (such as high-purity hydrogen) as the gas filled in the hollow fiber seed generation unit 11. Inside the stainless steel gas chamber, since there is already a weak seed light (first-order Stokes seed light), the second beam (strong pump light) (99mJ) rapidly transfers energy to the first-order Stokes seed light through stimulated emission, thus achieving subordinate amplification.

[0038] In the photoacoustic detection module, a high-intensity dual-color beam (containing residual pump light and amplified first-order Stokes light with wavelengths of 532 nm and 683 nm, respectively) output from the short-path Raman frequency shifter 10 is focused by an achromatic lens 13 and coupled into the center of the photoacoustic cell 18. The achromatic lens 13 is coated with a VIS (visible light band) antireflective film to effectively correct the axial chromatic aberration of the dual-color beam, ensuring that the two-color beams coincide at the center of the photoacoustic cell 18, maximizing the nonlinear interaction volume. A third optical window 16 and a fourth optical window 17 are sealed at both ends of the photoacoustic cell 18, and the cell is filled with a sample containing trace amounts of the gas to be measured (such as trace amounts of hydrogen in ambient air). Because the frequency difference of the incident dual-color beams is precisely locked to the Raman transition frequency of the analyte molecule (such as 4155 cm⁻¹ for hydrogen), the photoacoustic cell is effectively controlled. -1 The molecules undergo resonant stimulated Raman transitions, and the excited-state molecules release thermal energy and generate sound waves through nonradiative relaxation. The light beam passing through the photoacoustic cell 18 eventually enters the optical trap 20 and is absorbed to prevent stray light interference.

[0039] In the signal acquisition and processing module, the weak acoustic signal generated within the photoacoustic cell 18 (the weak acoustic signal originates from the photoacoustic effect and is proportional to the concentration of the gas being measured. The gas absorbs light energy and undergoes periodic thermal expansion; the higher the concentration, the stronger the sound wave) is picked up by a high-sensitivity microphone 19 mounted on the wall of the photoacoustic cell 18. The analog electrical signal output by the microphone 19 is first amplified by a low-noise preamplifier 22. It then enters a filter 23 (a low-pass or band-pass filter) to filter out high-frequency electronic noise and low-frequency environmental interference. The signal is then converted from analog to digital by the data acquisition card 24 and finally transmitted to the host computer 25. The host computer software performs a Fast Fourier Transform (FFT) on the time-domain signal to extract the signal amplitude at the resonant frequency of the photoacoustic cell 18. Combined with a pre-calibrated gas concentration-photoacoustic signal amplitude (usually the signal amplitude at the resonant frequency) system response curve (i.e., the calibration curve), the concentration value of the gas being measured is inverted and displayed in real time.

[0040] In addition, to meet the hardware requirements of strict coaxial optical path, high-efficiency fiber coupling and nanosecond-level spatiotemporal synchronization, the system's beam splitter 1, each reflector (first reflector 2 to sixth reflector 7), dichroic mirror 8, and the coupling input end of hollow fiber seed generation unit 11 are all equipped with multi-dimensional precision adjustment frames (including fine-tuning frames for angle yaw and pitch adjustment, and optical displacement stages for spatial translation and positioning).

[0041] This invention further provides a photoacoustic stimulated Raman gas detection method based on hollow-core optical fiber self-seed injection enhancement, comprising:

[0042] Step 1: Optical Path Collimation and Seed Generation. Turn on the pulsed laser 21, and adjust the multi-dimensional precision adjustment frame configured on the beam splitter 1, the fifth reflecting mirror 6, and the hollow fiber seed generation unit 11 to efficiently couple the first beam into the hollow fiber seed generation unit 11. Observe and confirm the generated stable weak seed light (first-order Stokes seed light) behind the filter 26; the wavelength can be monitored using a spectrometer.

[0043] Step 2, Spatiotemporal Synchronization Adjustment. Using a high-speed photodetector and an oscilloscope, monitor the temporal waveforms of the second beam (strong pump light) and the weak seed light (first-order Stokes seed light) at the position of the dichroic mirror 8. Adjust the multi-dimensional precision adjustment frame of the reflector group contained in the optical delay module until the two pulses of the dichroic beam (strong pump light and weak seed light) coincide on the oscilloscope. Then, finely adjust the angles of each reflector in the reflector group to ensure that the dichroic beam (strong pump light and weak seed light) coincides in the far-field spot (spatial coaxial), forming a combined beam.

[0044] Step 3, Amplifier Parameter Optimization. After the combined beam is passed into the short-path Raman frequency shifter 10, it outputs residual pump light and amplified first-order Stokes light. An energy meter is set at the photoacoustic cell 18 to measure the pulse energy of the two beams (residual pump light and amplified first-order Stokes light) respectively. The pressure valve 12 is adjusted to change the air pressure inside the short-path Raman frequency shifter 10 to find the optimal balance. The optimal air pressure value that maximizes the product is fixed. Among them, The residual pump pulse energy output after passing through the short-path Raman frequency shifter 10 The energy of a first-order Stokes light pulse after being stimulated and amplified.

[0045] Step 4, Signal Detection and Calibration. Various known standard gases of different concentrations are introduced into the photoacoustic cell 18. When each gas concentration is introduced, a microphone 19 collects the acoustic wave signal emitted within the photoacoustic cell 18. After processing by a preamplifier 22 and a filter 23, the host computer 25 extracts the photoacoustic signal amplitude at the resonant frequency of the photoacoustic cell 18. The least squares method is used to linearly fit the gas concentration-photoacoustic signal amplitude data to obtain the system response curve (i.e., the calibration curve), thus completing the system calibration.

[0046] Step 5, Measurement of the gas to be tested. The gas sample to be tested is introduced, and the system automatically collects its photoacoustic signal amplitude and calculates the concentration of the gas to be tested based on the calibration curve in Step 4.

[0047] The above description is merely an embodiment of the present invention and does not limit the scope of the invention. Any equivalent structural or procedural transformations made based on the description and drawings of this invention, or direct or indirect applications in other related system fields, are similarly included within the protection scope of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A photoacoustic stimulated Raman gas detection system based on hollow-core optical fiber self-seed injection enhancement, characterized in that, include: The system includes a light source and beam splitter module, a hollow fiber seed generator module, an optical delay module, a short optical path power amplifier module, a photoacoustic detection module, and a signal acquisition and processing module. The light source and beam splitting module includes a pulsed laser and a beam splitter. The main laser beam output by the pulsed laser is incident on the beam splitter, which splits the main laser beam into a first reflected beam and a second transmitted beam: a strong pump light. The hollow fiber seed generation module includes a fifth reflector, a hollow fiber seed generation unit, a sixth reflector, and a filter. The first beam is reflected by the fifth reflector and coupled into the hollow fiber seed generation unit. The beam output by the hollow fiber seed generation unit is reflected by the sixth reflector and passes through the filter to output pure weak seed light. The optical delay module includes a mirror group consisting of several mirrors arranged in sequence. Strong pump light enters the optical delay module and is synchronized with the weak seed light output from the filter after being output. The short optical path power amplifier module includes a dichroic mirror, a convex lens, and a short optical path Raman frequency shifter; the weak seed light and the strong pump light output from the optical delay module are combined and strictly coaxial at the dichroic mirror, then focused by the convex lens and enter the short optical path Raman frequency shifter. The short-path Raman frequency shifter, photoacoustic detection module, and signal acquisition and processing module are connected in sequence.

2. The photoacoustic stimulated Raman gas detection system based on hollow optical fiber self-seed injection enhancement according to claim 1, characterized in that, In the light source and beam splitting module, the pulsed laser outputs linearly polarized pulsed laser light as the main laser beam; the main laser beam is incident on the beam splitter, which splits the main laser beam into two beams according to the energy ratio: a small portion of the energy reflected is used as the first beam; and most of the energy transmitted is used as the second beam.

3. The photoacoustic stimulated Raman gas detection system based on hollow optical fiber self-seed injection enhancement according to claim 1, characterized in that, The optical delay module includes a spatially folded optical path composed of a first mirror, a second mirror, a third mirror, and a fourth mirror arranged in sequence. The first mirror, the second mirror, the third mirror, and the fourth mirror constitute a mirror group. By adjusting the delay distance of the mirror group, the time difference of weak seed light in the optical fiber is compensated, ensuring that the strong pump light output from the optical delay module and the weak seed light output from the filter are synchronized in the time and spatial domains.

4. The photoacoustic stimulated Raman gas detection system based on hollow optical fiber self-seed injection enhancement according to claim 1, characterized in that, The short-path Raman frequency shifter is equipped with a pressure valve. By adjusting the internal air pressure of the short-path Raman frequency shifter through the pressure valve, the conversion efficiency of the strong pump light to Stokes light is controlled, so that the energy product of the output residual pump light and the amplified first-order Stokes light is multiplied. Reaching the maximum value; among which, This refers to the residual pump pulse energy output after passing through the short-path Raman frequency shifter. The energy of a first-order Stokes light pulse after being stimulated and amplified.

5. The photoacoustic stimulated Raman gas detection system based on hollow-core optical fiber self-seed injection enhancement according to claim 1, characterized in that, The photoacoustic detection module includes an achromatic lens, a photoacoustic cell, and an optical trap. The residual pump light and the amplified first-order Stokes light output from the short-path Raman frequency shifter are focused by the achromatic lens into the center of the photoacoustic cell. At this time, the frequency difference between the residual pump light and the amplified first-order Stokes light is equal to the Raman transition frequency of the corresponding gas molecules, which efficiently excites the photoacoustic signal of the gas to be tested in the photoacoustic cell. The transmitted light beam passing through the photoacoustic cell is finally absorbed by the optical trap.

6. The photoacoustic stimulated Raman gas detection system based on hollow optical fiber self-seed injection enhancement according to claim 5, characterized in that, The surface of the achromatic lens is coated with a visible light band antireflection film to effectively correct the axial chromatic aberration of the residual pump light and the amplified first-order Stokes light, ensuring that the residual pump light and the amplified first-order Stokes light coincide at the center of the photoacoustic cell.

7. The photoacoustic stimulated Raman gas detection system based on hollow optical fiber self-seed injection enhancement according to claim 1, characterized in that, The signal acquisition and processing module includes a microphone, a preamplifier, a filter, a data acquisition card, and a host computer connected in sequence. The microphone is set on the wall of the photoacoustic cell and is used to acquire the photoacoustic signal generated by the gas molecules under test after absorbing laser energy.

8. The photoacoustic stimulated Raman gas detection system based on hollow-core optical fiber self-seed injection enhancement according to claim 1, characterized in that, The short-path Raman frequency shifter is configured as a stainless steel gas chamber, with a first optical window and a second optical window respectively sealed at both ends of the stainless steel gas chamber; the inside of the stainless steel gas chamber is filled with the same high-purity gas as that filled in the hollow fiber seed generation unit; a third optical window and a fourth optical window are respectively sealed at both ends of the photoacoustic cell.

9. The photoacoustic stimulated Raman gas detection system based on hollow optical fiber self-seed injection enhancement according to claim 1, characterized in that, The beam splitter, the fifth reflector, the sixth reflector, several reflectors in the optical delay module, the dichroic mirror, and the coupling input end of the hollow fiber seed generator are all equipped with multi-dimensional precision adjustment frames.

10. A photoacoustic stimulated Raman gas detection method based on hollow-core optical fiber self-seed injection enhancement, used in the photoacoustic stimulated Raman gas detection system based on hollow-core optical fiber self-seed injection enhancement as described in any one of claims 1 to 9, characterized in that, include: Step 1: Turn on the pulsed laser, adjust the multi-dimensional precision adjustment frame of the beam splitter, the fifth reflecting mirror and the hollow fiber seed generation unit to efficiently couple the first beam into the hollow fiber seed generation unit, and observe and confirm that a stable weak seed light has been generated behind the filter. Step 2: Monitor the time waveforms of the strong pump light and the weak seed light at the position of the dichroic mirror, and adjust the multi-dimensional precision adjustment frame of the reflector group contained in the optical delay module until the two pulses of the strong pump light and the weak seed light coincide on the oscilloscope. Then, finely adjust the angle of each reflector in the reflector group to ensure that the strong pump light and the weak seed light coincide in the far field spot to form a combined beam. Step 3: After the combined beam is passed into the short-path Raman frequency shifter, the output is residual pump light and amplified first-order Stokes light; the pulse energy of the residual pump light and the amplified first-order Stokes light are measured in the photoacoustic cell respectively; the air pressure in the short-path Raman frequency shifter is changed to find the optimal air pressure value that maximizes the product of the pulse energy of the residual pump light and the amplified first-order Stokes light and then fixes it. Step 4: Introduce known standard gases of different concentrations into the photoacoustic cell. When each concentration of gas is introduced, collect the acoustic wave signal excited in the photoacoustic cell, extract the photoacoustic signal amplitude at the resonant frequency of the photoacoustic cell, and perform linear fitting on the gas concentration-photoacoustic signal amplitude to obtain the calibration curve. Step 5: Introduce the gas sample to be tested, collect its photoacoustic signal amplitude, and calculate the concentration of the gas sample based on the calibration curve in Step 4.

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