Raman spectrum gas detection system and method for cooperative surface enhancement of optical fiber resonant cavity

Abstract

The invention discloses an optical fiber resonant cavity cooperative surface enhanced Raman spectrum gas detection system and method, and the method comprises the following steps: laser emitted by a laser is subjected to beam diameter increase through a beam expander, is transmitted through an optical isolator, is output to a first optical coupler, is focused through the first optical coupler, and enters an optical fiber resonant cavity unit; the output end of the optical fiber resonant cavity unit outputs gas Raman scattering light to the second optical coupler, the gas Raman scattering light is collimated by the second optical coupler and then input into the high-pass filter, the Raman scattering light is transmitted by the high-pass filter, silicon Raman signals are filtered out by the spatial filter, then the gas Raman scattering light is input into the third optical coupler, and the gas Raman scattering light is focused by the third optical coupler and then input into the signal collection optical fiber; gas Raman scattering light is transmitted to a slit of the spectrometer through the signal collection optical fiber and is input to the CCD after being diffracted and split by the spectrometer, and the CCD is used for detecting the gas Raman scattering light and converting an optical signal into an electric signal to be output. The multi-component mixed gas can be detected at the same time, the detection sensitivity is improved, and the response speed is high.

Description

Technical Field

[0001] The present invention belongs to the field of gas analysis, and more specifically, relates to a Raman spectroscopy gas detection system and method based on the cooperation of an optical fiber resonator and surface enhancement. Background Art

[0002] The gas composition information (such as H2, O2, H2O, CO, CO2, C x H y 、N x O y etc.) in the narrow and enclosed space of complex equipment (such as energy storage batteries, aerospace and military equipment, etc.) is an effective characteristic quantity for characterizing the function maintenance of the structural materials of the equipment and the physical mechanism of service damage in a special environment. Therefore, it is of great significance to carry out research on the long-term reliable, real-time and stable detection technology of multi-component gases in narrow and enclosed spaces.

[0003] At present, the detection methods of multi-component characteristic gases mainly include chromatography, electrochemistry, infrared absorption spectroscopy, photoacoustic spectroscopy, etc., but they generally have problems such as cross-interference of detected gases, long detection cycle, and poor long-term reliability.

[0004] Raman spectroscopy is a spectroscopic analysis method based on the Raman scattering effect. By directly measuring the Raman scattered light generated by a substance due to laser irradiation, the properties and content of the substance can be inferred. Compared with traditional detection methods, it has many advantages such as non-contact, non-destructive, non-consumption of sample molecules, and the ability to simultaneously detect almost all gas components (except single-atom molecular gases) and multi-component mixed gases with a single laser source, and good detection repeatability and long-term stability.

[0005] However, the gas Raman effect is extremely weak, and corresponding technical means are required to increase the Raman signal intensity to meet the detection limit requirements of practical applications.

[0006] CN118090698A discloses a multi-gas detection method and device for cavity-enhanced Raman spectroscopy collection. The method includes forming a reflection cavity through a first and a second concave mirror; the laser emitted by the laser light source is collimated by a first collimating mirror and then enters the reflection cavity for multiple reflections, and trace gas Raman scattered light is excited at the center of the reflection cavity; the Raman scattered light is divided into x, y, and z directions; the Raman scattered light in the x and z directions is collected by two light collection devices respectively; the collected Raman scattered light is collimated by a second collimating mirror, and then sequentially passes through a narrowband filter and a converging mirror and then enters a Raman spectroscopy detector. The device includes a laser light source module, a multiple reflection cavity module, a multi-angle Raman scattered light collection module, and a Raman spectroscopy detection module. However, the enhanced cavity used has a large volume and a large gas demand, and is not suitable for the application scenarios of narrow and enclosed spaces;

[0007] CN115326777A discloses a device for enhancing gas Raman signals in a hollow-core optical fiber, which includes a hollow-core optical fiber and a capillary. The hollow-core optical fiber is axially divided into an outer-fiber and an inner-fiber. The left end of the outer-fiber is the input end, the right end of the outer-fiber is connected to the left end of the inner-fiber, and the right end of the inner-fiber is the output end. The right end of the capillary is flush with the right end of the inner-fiber, and the left end of the capillary is flush with the left end of the inner-fiber. The inner-fiber is concentrically arranged inside the capillary, and an annular cavity is formed between the fiber cladding and the reflective layer of the inner-fiber. In the system, the hollow-core optical fiber and the capillary are located between a laser and a multi-mode fiber bundle. The multi-mode fiber bundle sends the received signal to a data processing device via a spectrometer. However, its detection sensitivity is low, and the gas exchange time between the inside of the optical fiber and the external environment is long, which cannot meet the requirements of real-time online monitoring. Summary of the Invention

[0008] To solve the deficiencies in the prior art, the present invention provides a Raman spectroscopy gas detection system and method based on the cooperation of an optical fiber resonator and surface enhancement.

[0009] The present invention adopts the following technical solutions.

[0010] A first aspect of the present invention proposes a Raman spectroscopy gas detection system based on the cooperation of an optical fiber resonator and surface enhancement, which includes: a laser 1, a beam expander 2, an optical isolator 3, a first optical coupler 4, an optical fiber resonator unit 6, a second optical coupler 8, a high-pass filter 9, a spatial filter 10, a third optical coupler 11, a signal collection optical fiber 12, a spectrometer 13, a CCD 14, and an optical trap 17, characterized in that:

[0011] The laser beam emitted by the laser 1 passes through the beam expander 2 to increase the beam diameter, and then passes through the optical isolator 3 and is transmitted to the first optical coupler 4. After being focused by the first optical coupler 4, it enters the optical fiber resonator unit 6. The optical fiber resonator unit 6 is placed in a sealed space where the gas to be detected is located. The output end of the optical fiber resonator unit 6 outputs gas Raman scattered light to the second optical coupler 8. After being collimated by the second optical coupler 8, it is input to the high-pass filter 9. The first output end of the high-pass filter 9 reflects the laser and Rayleigh scattered light to the optical trap 17. The second output end of the high-pass filter 9 transmits the Raman scattered light. Then, after passing through the spatial filter 10 to filter out the silicon Raman signal, it is input to the third optical coupler 11. After being focused by the third optical coupler 11, it is input to the signal collection optical fiber 12, and is transmitted through the signal collection optical fiber to the slit of the spectrometer 13. After being diffracted and spectroscopically analyzed by the spectrometer 13, it is input to the CCD 14. The CCD 14 is used to detect the gas Raman scattered light and convert the optical signal into an electrical signal for output.

[0012] Preferably, the detection system further includes a photodetector 15 and a frequency locking system 16;

[0013] Part of the light in the fiber optic resonator unit 6 is output from the input end of the fiber optic resonator unit 6, and is output to the photodetector 15 after passing through the first optical coupler 4 and the optical isolator 3 in sequence. The photodetector 15 is used to provide an error signal for the frequency stabilization of the fiber optic resonator unit 6; the error signal is transmitted to the frequency locking system 16, which is connected to the control end of the laser 1 and is used to lock the laser frequency emitted by the laser 1 at the resonance frequency of the fiber optic resonator unit 6.

[0014] Preferably, both ends of the fiber optic resonator unit 6 are fixed to the sealed space through the first fiber optic adapter 5 and the second fiber optic adapter 7 respectively. The first fiber optic adapter 5 and the second fiber optic adapter 7 are both airtight fiber optic adapters, with a fluororubber sealing column inside. The center of the sealing column is penetrated, and the diameter of the through hole is adapted to the outer diameter of the fiber optic resonator; there are threads engraved on the outside, and it is fastened to the outer wall of the sealed space by rotation, and the gas-tight pressure resistance values of both of them are not less than 1 MPa.

[0015] Preferably, the fiber optic resonator unit 6 includes a first fiber grating 18, a single-mode fiber 19, a second fiber grating 20, and surface-enhanced metal nanoparticles with various morphological characteristics.

[0016] The first fiber grating 18 and the second fiber grating 20 are two reflectors of the fiber optic resonator unit 6; the single-mode fiber 19 is connected between the first fiber grating 18 and the second fiber grating 20 and serves as the cavity of the fiber optic resonator unit 6; the signal input into the fiber optic resonator unit 6 is reflected back and forth between the first fiber grating 18 and the second fiber grating 20 through the single-mode fiber 19 for multiple times to form a stable standing wave; surface-enhanced metal nanoparticles with various morphological characteristics are deposited on the outer wall of the cladding of the single-mode fiber 19, and the surface-enhanced metal nanoparticles are used to further excite the Raman scattering of nearby gas molecules. The different morphological characteristics include spheres, tetrahedrons, and cubes, and the sizes are all in the nanometer range.

[0017] Preferably, the first fiber grating 18 and the second fiber grating 20 are fixed to both ends of the single-mode fiber 19 by fusion splicing, and their reflectivity to laser is not less than 0.9995, and the single-end fusion splicing loss is not higher than 0.002 dB.

[0018] Preferably, the coating layer of the single-mode fiber 19 is stripped off, the core diameter is 5 μm, the core diameter ratio is 0.04, and the core mode field is adapted to the mode fields of the first fiber grating 18 and the second fiber grating 20; the transmission loss of the single-mode fiber 19 to the wavelengths of laser and gas Raman scattering light is not higher than 10 dB / km.

[0019] Preferably, the laser 1 is a single longitudinal mode, narrow linewidth, continuous wave laser. The output wavelength of the laser is in the visible or near-infrared band, including 532 nm, 638 nm, 642 nm, 785 nm, 1064 nm, etc.; its linewidth is not greater than 0.00001 nm; the M 2 factor is not higher than 1.2.

[0020] Preferably, the magnification of the beam expander 2 is continuously adjustable between 1 and 5.

[0021] Preferably, the isolation degree of the optical isolator 3 is above 30 dB; and the reflectivity to the laser is not less than 0.995.

[0022] Preferably, the reflectivity of the high-pass filter to the laser and Rayleigh scattered light is not less than 0.98, and the transmittance to the gas Raman scattered light is not less than 0.95.

[0023] Preferably, the transmission loss of the signal collection optical fiber 12 to the wavelength of the gas Raman scattered light is not higher than 30 dB / km.

[0024] The second aspect of the present invention proposes a fiber optic resonator collaborative surface enhanced Raman spectroscopy gas detection method using the system described in the first aspect of the present invention, which is characterized in that:

[0025] Place the fiber optic resonator unit 6 in the sealed space where the gas to be detected is located;

[0026] The laser 1 emits laser light, and the laser light enters the fiber optic resonator unit 6 after being focused; part of the laser light leaks from the cladding of the single-mode optical fiber 19 of the fiber optic resonator unit 6 to form a surface evanescent wave; the fiber evanescent wave undergoes Raman scattering with the gas molecules of the gas to be detected on the surface of the cladding of the single-mode optical fiber 19 of the fiber optic resonator unit 6 to generate gas Raman scattered light;

[0027] And the laser light is reflected back and forth multiple times in the fiber optic resonator unit to form a stable standing wave, increasing the laser power, thereby increasing the intensity of the evanescent wave; at the same time, by collecting part of the reflected light output from the fiber optic resonator unit 6, detecting the resonance frequency of the fiber optic resonator unit, and controlling the frequency of the emitted laser to be locked at the resonance frequency of the fiber optic resonator unit;

[0028] The surface enhanced nanoparticles with different morphological characteristics deposited on the cladding of the single-mode optical fiber 19 in the fiber optic resonator unit 6 adsorb different gas components by means of chemical bonding, and under the action of the fiber evanescent wave, a surface plasmon resonance effect is generated, forming a local electromagnetic field enhancement, thereby further exciting the Raman scattering of the gas molecules near the fiber optic resonator unit;

[0029] After the generated gas Raman scattered light is collimated, the laser light, Rayleigh scattered light, and silicon Raman signal are filtered out;

[0030] The filtered gas Raman scattered light is focused and diffraction spectroscopied and then input into the CCD14. The CCD14 records the Raman spectrum of the gas Raman scattered light, and the gas species and concentration are identified through spectral analysis.

[0031] The beneficial effects of the present invention are as follows. Compared with the prior art, the present invention uses a fiber optic resonator unit, which is small in size, bendable, and has a long service life. The present invention uses surface-enhanced metal nanoparticles with different morphological characteristics to adsorb different gas components, and forms a local electromagnetic field enhancement under the action of the evanescent wave on the surface of the single-mode optical fiber, further exciting the Raman scattering signal of the gas, and can perform simultaneous detection of multi-component mixed gases, improving the detection sensitivity. And an optoelectronic detector and a frequency locking system are used to lock the laser frequency at the resonance frequency of the fiber optic resonator, providing a stable laser power enhancement inside the fiber optic resonator, further improving the detection sensitivity. The present invention is based on the principle of fiber optic evanescent wave detection. The interaction between the laser and the gas occurs in the free space on the outer wall of the cladding of the fiber optic resonator unit, and the gas does not need to enter the narrow fiber core, thus saving the time for the gas to enter and exit the fiber core and having a fast response speed. The present invention uses an optical trap to collect the laser and Rayleigh scattered light output from the fiber optic resonator to maintain experimental safety. Description of the Drawings

[0032] Figure 1 It is a schematic structural diagram of a Raman spectroscopy gas detection system with a fiber optic resonator cooperating with surface enhancement;

[0033] Figure 2 It is a schematic structural diagram of a fiber optic resonator unit. Detailed Embodiments

[0034] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0035] The present invention adopts the following technical solutions.

[0036] As Figure 1 shown, Embodiment 1 of the present invention discloses a Raman spectroscopy gas detection system with a fiber optic resonator cooperating with surface enhancement, including: a laser 1, a beam expander 2, an optical isolator 3, a first optical coupler 4, a fiber optic resonator unit 6, a second optical coupler 8, a high-pass filter 9, a spatial filter 10, a third optical coupler 11, a signal collection optical fiber 12, a spectrometer 13, a CCD14, an optoelectronic detector 15, a frequency locking system 16, and an optical trap 17, characterized in that:

[0037] After the laser emitted by the laser 1 passes through the beam expander 2 to increase the beam diameter, it passes through the optical isolator 3 and is transmitted to the first optical coupler 4, and after being focused by the first optical coupler 4, it enters the fiber optic resonator unit 6; the fiber optic resonator unit 6 is placed in the closed space where the gas to be detected is located, and the output end of the fiber optic resonator unit 6 outputs the gas Raman scattered light to the second optical coupler 8, and after being collimated by the second optical coupler 8, it is input to the high-pass filter 9. The first output end of the high-pass filter 9 reflects the laser and Rayleigh scattered light to the optical trap 17, and the second output end of the high-pass filter 9 transmits the Raman scattered light, and then after passing through the spatial filter 10 to filter out the silicon Raman signal, it is input to the third optical coupler 11, and after being focused by the third optical coupler 11, it is input to the signal collection optical fiber 12, and is transmitted through the signal collection optical fiber to the slit of the spectrometer 13, and after being diffracted and dispersed by the spectrometer 13, it is input to the CCD 14. The CCD 14 is used to detect the gas Raman scattered light and convert the optical signal into an electrical signal for output;

[0038] Part of the light in the fiber optic resonator unit 6 is output from the input end of the fiber optic resonator unit 6, and passes through the first optical coupler 4 and the optical isolator 3 in sequence and then is output to the photodetector 15. The photodetector 15 is used to provide an error signal for the frequency stabilization of the fiber optic resonator unit 6; the error signal is transmitted to the frequency locking system 16, and the frequency locking system is connected to the control end of the laser 1 and is used to lock the laser frequency emitted by the laser 1 to the resonance frequency of the fiber optic resonator unit 6.

[0039] Both ends of the fiber optic resonator unit 6 are fixed to the closed space through the first fiber optic adapter 5 and the second fiber optic adapter 7 respectively. The first fiber optic adapter 5 and the second fiber optic adapter 7 are both airtight fiber optic adapters. The inside thereof is a fluororubber sealing column. The center of the sealing column is through, and the diameter of the through hole is adapted to the outer diameter of the fiber optic resonator; the outside thereof is engraved with threads and is rotationally fastened to the outer wall of the closed space, and their gas sealing pressure resistance values are not less than 1 MPa.

[0040] Specifically, the focal length of the first optical coupler 4 in this embodiment is 10 mm; the slit width of the spectrometer 13 is 50 μm.

[0041] As Figure 2 shown, the fiber optic resonator unit 6 includes a first fiber grating 18, a single-mode fiber 19, a second fiber grating 20, and surface-enhanced metal nanoparticles with various different morphological characteristics;

[0042] The first fiber grating 18 and the second fiber grating 20 are two reflectors of the fiber resonator unit 6; the single-mode fiber 19 is connected between the first fiber grating 18 and the second fiber grating 20 and serves as the cavity of the fiber resonator unit 6; the signal input into the fiber resonator unit 6 is reflected back and forth multiple times between the first fiber grating 18 and the second fiber grating 20 through the single-mode fiber 19 to form a stable standing wave; surface-enhanced metal nanoparticles with various morphological features are deposited on the outer wall of the cladding of the single-mode fiber 19, and the surface-enhanced metal nanoparticles are used to further excite the Raman scattering of nearby gas molecules. The different morphological features include spheres, tetrahedrons, and cubes, and the sizes are all in the nanometer range.

[0043] Specifically, this embodiment uses five surface-enhanced metal nanoparticles 21, 22, 23, 24, and 25 with different morphological features.

[0044] The first fiber grating 18 and the second fiber grating 20 are fixed at both ends of the single-mode fiber 19 by fusion splicing. Their reflectivity to laser is not less than 0.9995, and the single-end fusion splicing loss is not higher than 0.002 dB.

[0045] Specifically, the reflectivity of the first fiber grating 18 and the second fiber grating 20 in this embodiment to laser is 0.9999, and the single-end fusion splicing loss is 0.0015 dB.

[0046] The coating layer of the single-mode fiber 19 is stripped off. The core diameter is 5 μm, the core diameter ratio is 0.04, and the core mode field adapts to the mode fields of the first fiber grating 18 and the second fiber grating 20; the transmission loss of the single-mode fiber 19 to the wavelengths of laser and gas Raman scattered light is not higher than 10 dB / km.

[0047] Specifically, the core of the single-mode fiber 19 in this embodiment is 5 μm, the cladding diameter is 125 μm, and the length is 30 cm.

[0048] The laser 1 is a single-longitudinal-mode, narrow-linewidth, continuous-wave laser. The output wavelength of the laser is in the visible or near-infrared band, including 532 nm, 638 nm, 642 nm, 785 nm, 1064 nm, etc.; its linewidth is not greater than 0.00001 nm; the M 2 factor is not higher than 1.2.

[0049] Specifically, the wavelength of the laser 1 in this embodiment is 532 nm, and the output laser power is 1.5 W.

[0050] The magnification of the beam expander 2 is continuously adjustable between 1 and 5.

[0051] Specifically, the magnification of the beam expander 2 in this embodiment is 2.

[0052] The isolation degree of the optical isolator 3 is above 30 dB; and the reflectivity to the laser is not less than 0.995.

[0053] The reflectivity of the high-pass filter to the laser and Rayleigh scattered light is not less than 0.98, and the transmittance to the gas Raman scattered light is not less than 0.95.

[0054] The transmission loss of the signal collection optical fiber 12 to the wavelength of the gas Raman scattered light is not higher than 30 dB / km.

[0055] Embodiment 2 of the present invention proposes a fiber optic resonator collaborative surface enhanced Raman spectroscopy gas detection method using the system described in Embodiment 1 of the present invention, characterized in that:

[0056] Place the fiber optic resonator unit 6 in the closed space where the gas to be detected is located;

[0057] The laser 1 emits laser light, and the laser light enters the fiber optic resonator unit 6 after focusing; part of the laser light leaks from the cladding of the single-mode optical fiber 19 of the fiber optic resonator unit 6 to form a surface evanescent wave; the fiber evanescent wave undergoes Raman scattering with the gas molecules of the gas to be detected on the surface of the cladding of the single-mode optical fiber 19 of the fiber optic resonator unit 6 to generate gas Raman scattered light;

[0058] And the laser light is reflected back and forth multiple times in the fiber optic resonator unit to form a stable standing wave, increasing the laser power, thereby increasing the intensity of the evanescent wave; at the same time, by collecting part of the reflected light output from the fiber optic resonator unit 6, detecting the resonance frequency of the fiber optic resonator unit, and controlling the frequency of the emitted laser to be locked at the resonance frequency of the fiber optic resonator unit;

[0059] The surface enhanced nanoparticles with different morphological characteristics deposited on the cladding of the single-mode optical fiber 19 in the fiber optic resonator unit 6 adsorb different gas components by means of chemical bonding, and under the action of the fiber evanescent wave, a surface plasmon resonance effect is generated, forming a local electromagnetic field enhancement, thereby further exciting the Raman scattering of gas molecules near the fiber optic resonator unit;

[0060] After the generated gas Raman scattered light is collimated, the laser light, Rayleigh scattered light, and silicon Raman signal are filtered out;

[0061] The filtered gas Raman scattered light is input into the CCD 14 after being focused and diffraction spectroscopically analyzed. The CCD (14) records the Raman spectrum of the gas Raman scattered light, and the gas type and concentration are identified through spectral analysis.

[0062] The characteristic gas components and their detection limits in the narrow and closed space of complex equipment that can be detected in this embodiment are shown in Table 1.

[0063] Table 1 Characteristic gas components and detection limits in the narrow and closed space of complex equipment

[0064]

[0065] As shown in Table 1, in this embodiment, the detection limits of gases such as H2, O2, H2O, CO, CO2, and C x H y , N x O y are better than 2 ppm, the response time is better than 10 s, the detection accuracy is better than 2% FS, and the maximum range is in the range of 1000 - 10000 ppm.

[0066] Moreover, the core fiber optic gas detection unit is small in volume (diameter 5 μm, length 0.3 - 0.5 m, bendable), weighs no more than 200 g, and has a service life of no less than 6 years.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: it is still possible to modify the specific implementation manners of the present invention or make equivalent replacements, and any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A Raman spectroscopy gas detection system with fiber optic resonator collaborative surface enhancement, comprising: Laser (1), beam expander (2), optical isolator (3), first optical coupler (4), fiber optic resonator unit (6), second optical coupler (8), high-pass filter (9), spatial filter (10), third optical coupler (11), signal collection optical fiber (12), spectrometer (13), CCD (14) and optical trap (17), characterized in that: The laser emitted by the laser (1) passes through the beam expander (2) to increase the beam diameter, then passes through the optical isolator (3) and is transmitted to the first optical coupler (4), and after being focused by the first optical coupler (4), it enters the fiber optic resonator unit (6); the fiber optic resonator unit (6) is placed in the closed space where the gas to be detected is located, and the output end of the fiber optic resonator unit (6) outputs the gas Raman scattered light to the second optical coupler (8), and after being collimated by the second optical coupler (8), it is input to the high-pass filter (9). The first output end of the high-pass filter (9) reflects the laser and Rayleigh scattered light to the optical trap (17), and the second output end of the high-pass filter (9) transmits the Raman scattered light. Then, after passing through the spatial filter (10) to filter out the silicon Raman signal, it is input to the third optical coupler (11), and after being focused by the third optical coupler (11), it is input to the signal collection optical fiber (12), and is transmitted through the signal collection optical fiber to the slit of the spectrometer (13), and after being diffracted and dispersed by the spectrometer (13), it is input to the CCD (14). The CCD (14) is used to detect the gas Raman scattered light and convert the optical signal into an electrical signal for output.

2. A Raman spectroscopic gas detection system with fiber optic resonator collaborative surface enhancement according to claim 1, characterized in that: The detection system further includes a photodetector (15) and a frequency locking system (16); Part of the light in the fiber optic resonator unit (6) is output from the input end of the fiber optic resonator unit (6), passes through the first optical coupler (4) and the optical isolator (3) in sequence, and then is output to the photodetector (15). The photodetector (15) is used to provide an error signal for the frequency stability of the fiber optic resonator unit (6); The error signal is transmitted to the frequency locking system (16). The frequency locking system is connected to the control end of the laser (1) and is used to lock the laser frequency emitted by the laser (1) at the resonance frequency of the fiber optic resonator unit (6).

3. A Raman spectroscopic gas detection system with fiber optic resonator collaborative surface enhancement according to claim 2, characterized in that: Both ends of the fiber optic resonator unit (6) are fixed to the closed space through the first fiber optic adapter (5) and the second fiber optic adapter (7) respectively. The first fiber optic adapter (5) and the second fiber optic adapter (7) are both airtight fiber optic adapters. The inside thereof is a fluororubber sealing column. The center of the sealing column is through, and the diameter of the through hole is adapted to the outer diameter of the fiber optic resonator; the outside is engraved with threads and is fastened to the outer wall of the closed space by rotation. The gas sealing pressure resistance values of both of them are not less than 1 MPa.

4. A Raman spectroscopic gas detection system with fiber optic resonator collaborative surface enhancement according to claim 3, characterized in that: The fiber optic resonator unit (6) includes a first fiber grating (18), a single-mode fiber (19), a second fiber grating (20), and surface-enhanced metal nanoparticles with various morphological features. The first fiber grating (18) and the second fiber grating (20) are two reflectors of the fiber optic resonator unit (6); the single-mode fiber (19) is connected between the first fiber grating (18) and the second fiber grating (20) and serves as the cavity of the fiber optic resonator unit (6); the signal input into the fiber optic resonator unit (6) is reflected back and forth multiple times between the first fiber grating (18) and the second fiber grating (20) through the single-mode fiber (19) to form a stable standing wave; surface-enhanced metal nanoparticles with various morphological features are deposited on the outer wall of the cladding of the single-mode fiber (19), and the surface-enhanced metal nanoparticles are used to further stimulate the Raman scattering of nearby gas molecules. The different morphological features include spheres, tetrahedrons, and cubes, and the sizes are all in the nanometer range.

5. The Raman spectroscopy gas detection system with fiber optic resonator synergistic surface enhancement according to claim 4, characterized in that: The first fiber grating (18) and the second fiber grating (20) are fixed at both ends of the single-mode fiber (19) by fusion splicing. Their reflectivity to laser is not less than 0.9995, and the single-end fusion splicing loss is not higher than 0.002 dB.

6. The Raman spectroscopy gas detection system with fiber optic resonator synergistic surface enhancement according to claim 5, characterized in that: The coating layer of the single-mode fiber (19) is stripped off. The core diameter is 5 μm, the core diameter ratio is 0.04, and the core mode field is adapted to the mode fields of the first fiber grating (18) and the second fiber grating (20); the transmission loss of the single-mode fiber (19) to the wavelengths of laser and gas Raman scattered light is not higher than 10 dB / km.

7. The Raman spectroscopy gas detection system with fiber optic resonator synergistic surface enhancement according to claim 1, characterized in that: The laser (1) is a single longitudinal mode, narrow linewidth, continuous wave laser. The output wavelength of the laser is in the visible or near-infrared band, including 532 nm, 638 nm, 642 nm, 785 nm, 1064 nm, etc. Its line width is not greater than 0.00001 nm; M 2 The factor is not higher than 1.

2.

8. The Raman spectroscopy gas detection system with fiber optic resonator synergistic surface enhancement according to claim 1, characterized in that: The magnification of the beam expander (2) is continuously adjustable between 1 and 5.

9. The Raman spectroscopy gas detection system with fiber optic resonator synergistic surface enhancement according to claim 1, characterized in that: The isolation degree of the optical isolator (3) is above 30 dB; and its reflectivity to laser is not less than 0.

995.

10. The Raman spectroscopy gas detection system with fiber optic resonator synergistic surface enhancement according to claim 1, characterized in that: The reflectivity of the high-pass filter to laser and Rayleigh scattered light is not less than 0.98, and the transmittance to gas Raman scattered light is not less than 0.

95.

11. The Raman spectroscopy gas detection system with fiber optic resonator synergistic surface enhancement according to claim 1, characterized in that: The transmission loss of the signal collection optical fiber (12) for the gas Raman scattered light wavelength is not higher than 30 dB / km.

12. A fiber optic resonator collaborative surface enhanced Raman spectroscopy gas detection method using the system according to any one of claims 4-11, characterized in that: The fiber optic resonator unit (6) is placed in a closed space where the gas to be detected is located; The laser (1) emits laser light, and the laser light enters the fiber optic resonator unit (6) after being focused; part of the laser light leaks from the cladding of the single-mode fiber (19) of the fiber optic resonator unit (6) to form a surface evanescent wave; the fiber evanescent wave undergoes Raman scattering with gas molecules of the gas to be detected on the surface of the cladding of the single-mode fiber (19) of the fiber optic resonator unit (6) to generate gas Raman scattered light; And the laser light reflects back and forth multiple times in the fiber optic resonator unit to form a stable standing wave, increasing the laser power, thereby increasing the intensity of the evanescent wave; at the same time, by collecting part of the reflected light output from the input end of the fiber optic resonator unit (6), the resonance frequency of the fiber optic resonator unit is detected, and the frequency of the emitted laser light is controlled to be locked at the resonance frequency of the fiber optic resonator unit; The surface enhanced nanoparticles with different morphological characteristics deposited on the cladding of the single-mode fiber (19) in the fiber optic resonator unit (6) adsorb different gas components by means of chemical bonding, and under the action of the fiber evanescent wave, a surface plasmon resonance effect is generated to form a local electromagnetic field enhancement, thereby further exciting the Raman scattering of gas molecules near the fiber optic resonator unit; The generated gas Raman scattered light, after being collimated, filters out the laser light, Rayleigh scattered light, and silicon Raman signal; The filtered gas Raman scattered light is input into the CCD (14) after being focused and diffraction spectroscopied, and the CCD (14) records the Raman spectrum of the gas Raman scattered light, and the gas type and concentration are identified through spectral analysis.

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