V-shaped dense multi-cycle folding multi-reverse cavity and gas detection system and method

Abstract

The invention discloses a V-shaped dense multi-cycle folded multi-reflector cavity and a gas detection system and method, and belongs to the technical field of gas analys.The V-shaped dense multi-cycle folded multi-reflector cavity comprises a first spherical concave reflector, a second spherical concave reflector and a third spherical concave reflector, the other surface is a spherical concave reflecting surface; the reflecting surface of the reflecting mirror is plated with a broadband high-reflection film, and a circular hole is formed in the geometric center of the first spherical concave reflecting mirror and is used for laser entering and gas Raman scattering light emitting; the spherical concave reflecting mirror is placed in a V shape on a geometric plane, and an included angle between a connecting line of centers of a first reflecting mirror and a second reflecting mirror and a connecting line of centers of a second reflecting mirror and a third reflecting mirror is 15-30 degrees. According to the invention, multiple laser reflection times can be realized, the length of an effective action path of laser and gas is large, the intensity of an excited gas Raman signal is high, and meanwhile, the collection efficiency of gas Raman scattering light is improved.

Description

Technical Field

[0001] The present invention belongs to the field of gas analysis, and more specifically, relates to a V-shaped dense multi-cycle folded multi-reverse cavity and a gas detection system and method. Background Art

[0002] Trace gas detection plays a very important role in many fields such as electric power, medical health, food manufacturing and environmental safety, industrial production and process control. Raman spectroscopy technology has the unique advantages of non-invasiveness, calibration-free, good selectivity, high accuracy, and single wavelength laser simultaneous detection of mixed multi-component gases. However, the weak Raman effect of gas leads to a high detection limit of gas Raman spectroscopy, which cannot meet the needs of practical applications.

[0003] Inspired by absorption spectroscopy technology, researchers at home and abroad have gradually begun to use multiple reflection cavity (hereinafter referred to as "multi-reflection cavity") enhancement technology to enhance the Raman scattering signal of the gas by increasing the action path length between the laser and the gas to be measured. At present, the more commonly used multi-reflection cavities are the traditional Herriott type (the reflected light spot forms a closed-loop circular / elliptical pattern on the cavity mirror) and the dense type (there are many reflected light spots, almost filling the entire cavity mirror).

[0004] Among them, the traditional Herriott multi-reflector cavity reflects the laser less frequently, and the utilization rate of the cavity mirror reflection surface is low, so its enhancement effect on the gas Raman scattering signal is limited. The existing dense multi-reflector cavity is divided into two types: linear type (two-mirror structure) and Z type (four-mirror structure). The former is limited by the reflection theory, and its cavity mirror curvature radius is small, and the physical cavity length is generally short. The laser attenuates more with the increase of the number of reflections, which is not conducive to the excitation of Raman signals; the latter has many design parameters, complex system structure, and is difficult to adjust and implement.

[0005] Prior art document 1 (CN 108535191B) discloses a laser Raman gas detection device based on a rhombus cavity mirror, in which a reflector, a prism, and a high-pressure helium-neon tube are coaxially placed in sequence on the external light paths on the left and right sides of the gas chamber, respectively. The lasers output by the two high-pressure helium-neon tubes pass through the prisms on their respective external light paths to the reflector on the vertical light path, and pass through the prism and the high-pressure helium-neon tube from a window on one side of the gas chamber to be vertically incident on the four concave reflectors of the rhombus cavity in the gas chamber and are repeatedly refracted. However, the prior art has a limited number of reflections of the laser, resulting in a small gas Raman signal enhancement effect; and the reflection of the laser in the cavity is disordered, and the mutual interference between the light beams easily leads to a large background noise of the Raman spectrum, thereby reducing the gas Raman signal-to-noise ratio. Summary of the invention

[0006] In order to solve the deficiencies in the prior art, the present invention provides a V-shaped dense multi-cycle folded multi-reverse cavity and a gas detection system and method, thereby solving the problems existing in the prior art such as limited enhancement of the gas Raman signal by the multi-reverse cavity, low gas detection sensitivity, and difficulty in adjusting the optical path and implementing the multi-reverse cavity system.

[0007] The present invention adopts the following technical solution.

[0008] The first aspect of the present invention discloses a V-shaped dense multi-cycle folded multi-reflection cavity, comprising a first spherical concave reflector, a second spherical concave reflector and a third spherical concave reflector, wherein one side of the reflector is a frosted plane and the other side is a spherical concave reflective surface; the reflective surface of the reflector is plated with a broadband high-reflective film, wherein a circular hole is opened at the geometric center of the first spherical concave reflector for laser entry and gas Raman scattered light emission, and the diameter of the circular hole is greater than the diameter of the laser beam;

[0009] The spherical concave reflector is placed in a "V" shape on a geometric plane, and the angle between the center line of the first reflector and the second reflector lens and the center line of the second reflector and the third reflector lens is 15°.

[0010] ~30°, forming a V-shaped dense multi-circulation folded multi-reverse cavity.

[0011] Preferably, the spherical concave reflecting surfaces of the first spherical concave reflecting mirror, the second spherical concave reflecting mirror and the third spherical concave reflecting mirror are all affixed with absorption mask plates, namely the first absorption mask plate, the second absorption mask plate and the third absorption mask plate, and the mask plates are made of light-absorbing materials and are used to absorb stray lasers in the cavity and reduce interference effects in the cavity.

[0012] Preferably, the first absorption mask plate, the second absorption mask plate and the third absorption mask plate are placed close to the reflective surface of the cavity mirror after the debugging of the multi-reverse cavity is completed, and have openings at each reflection point, which does not affect the normal reflection of laser and gas Raman scattered light on the mirror surface.

[0013] The second aspect of the present invention discloses a Raman spectroscopy gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity, comprising the V-shaped dense multi-cycle folded multi-reverse cavity, a first laser, a first half-wave plate, a second laser, a second half-wave plate, a polarization beam splitter, a dichroic mirror, a focusing lens, a plane reflector, a high-pass filter group, a focusing lens, a collection optical fiber bundle, a spectrometer and a CCD;

[0014] The first laser and the second laser are both continuous single-mode single-frequency lasers, used to provide spontaneous Raman scattering signals of the light source excited gas;

[0015] The first half-wave plate and the second half-wave plate are used to adjust the polarization direction of the laser output by the laser;

[0016] The dichroic mirror is used to separate the Raman scattered light of the laser and the gas, and the mirror surface of the dichroic mirror forms an angle of 45° with the incident direction of the laser;

[0017] The focusing lens is used to focus the laser and couple it into the multi-reflector cavity, and collimate the gas Raman scattered light emitted from the multi-reflector cavity;

[0018] The plane reflector is arranged between the first spherical concave reflector and the focusing lens, and is used to couple the laser light and the gas Raman scattered light emitted from the multi-reflection cavity back into the cavity;

[0019] The high-pass filter group, focusing lens, collecting optical fiber bundle, spectrometer and CCD are arranged in sequence on the collimation line of gas Raman scattered light emitted by the multi-reverse cavity.

[0020] Preferably, the operating wavelength of the first laser and the second laser is in the visible light region, and the output power thereof is greater than 500 mW; the laser line width is less than 1 MHz; and the M 2 Factor <1.5; laser polarization ratio >100:1.

[0021] Preferably, when in use, the wavelength and line width parameters of the first laser and the second laser need to be consistent; the lasers output by the first laser and the second laser are adjusted to p-polarized light and s-polarized light respectively by the first half-wave plate and the second half-wave plate, and the two beams of light are respectively transmitted and reflected by the polarization beam splitter and then merged into one beam of light.

[0022] Preferably, the reflectivity of the plane reflector to the wavelength of the laser and the gas Raman scattered light is not less than 0.995.

[0023] Preferably, the high-pass filter group includes one or more high-pass filters, whose cut-off wavelength is slightly larger than the laser wavelength, and is used to filter out the laser and Rayleigh scattered light in the collection light path to improve the signal-to-noise ratio of the gas Raman spectrum.

[0024] Preferably, the collection fiber bundle includes several single-mode optical fibers with the same parameters, and the transmission loss of each single-mode optical fiber to the gas Raman scattered light is not higher than 50dB / km. On the collection side of the collection fiber bundle facing the focusing lens, the single-mode optical fibers are arranged in a circular shape, and their area completely covers the size of the light spot after focusing by the focusing lens; on the divergent side facing the spectrometer slit, the single-mode optical fibers are arranged in a linear shape, and their length and width completely match the height and width of the spectrometer slit.

[0025] The third aspect of the present invention discloses a method for a gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity, based on the gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity, comprising the following steps:

[0026] Step 1: The laser light emitted by the first laser is adjusted to p-polarized light by the first half-wave plate; at the same time, the laser light emitted by the second laser 3 is adjusted to s-polarized light by the second half-wave plate;

[0027] Step 2: The p-polarized light and the s-polarized light are respectively transmitted and reflected by the polarization beam splitter and then merged into a beam of light, which is then reflected by the dichroic mirror and focused by the first focusing lens in turn, and then enters the multi-reflection cavity through the opening in the center of the first spherical concave reflector;

[0028] Step 3: Adjust the placement and deflection angle of the first spherical concave reflector, the second spherical concave reflector, and the third spherical concave reflector so that the laser is reflected multiple times along different paths therebetween and forms a dense light spot pattern on each reflector; at this time, place the first absorption mask plate, the second absorption mask plate, and the third absorption mask plate close to the spherical concave reflective surfaces of the first spherical concave reflector, the second spherical concave reflector, and the third spherical concave reflector, respectively, and the absorption mask plate has openings at each reflection point, which does not affect the normal reflection of the laser and the gas Raman scattered light on the mirror surface; finally, the laser will be emitted from the opening at the center of the first spherical concave reflector along a path different from that of the incident light;

[0029] Step 4: Adjust the placement position and deflection angle of the plane reflector to recouple the emitted laser along the original optical path into the multi-reflector cavity to achieve a second circulation of the laser in the cavity;

[0030] Step 5: The laser and gas interact in the multiple reflection cavity to generate backscattered Raman light L fb After being collimated by a focusing lens, filtered out by a dichroic mirror and a high-pass filter group, the laser and Rayleigh scattered light are focused by a second focusing lens, enter the collection fiber bundle and are transmitted to the slit of the spectrometer; the gas Raman scattered light is detected by the CCD after being diffracted and split by the spectrometer;

[0031] Step 6: Multiple reflections of the forward Raman scattered light L generated in the cavity ff It is emitted from the opening in the center of the first spherical concave reflector, and then re-coupled into the multi-reflector cavity after being reflected by the plane reflector, and combined with the aforementioned back Raman scattered light L fb are collected along the same optical path;

[0032] Step 7: The laser that is reflected by the plane mirror in step 4 and then recoupled into the multi-reflector cavity interacts with the gas in the multi-reflector cavity again to generate forward gas Raman scattered light L bf The backscattered light L fb The backscattered light L is collected along the same optical path. bb After being coupled by the plane reflector again, it re-enters the multi-reflector cavity and combines with the backscattered light L fb are collected along the same optical path.

[0033] Compared with the prior art, the beneficial effects of the present invention include at least:

[0034] The present invention provides a V-shaped dense multi-cycle folded multi-reflector cavity and its Raman spectrum gas detection system, which adopts three spherical concave reflectors arranged in a V shape, and the dense reflection spot realized can almost fill the entire cavity mirror reflection surface, and the number of laser reflections that can be realized is large, that is, the effective action path length of the laser and the gas is large, and the excited gas Raman signal intensity is high; an absorption mask plate is placed close to the cavity mirror reflection surface, which effectively absorbs the stray laser and fluorescence in the cavity, and avoids the mutual interference between the laser beams, which is beneficial to improving the signal-to-noise ratio of the gas Raman spectrum; because the physical cavity length of the V-shaped folded multi-reflector cavity only needs to meet the cavity mirror curvature radius of less than 4 times the single-arm cavity length, the cavity length can be adjusted autonomously according to needs in actual use; because a retroreflector is adopted, the second circulation of the laser in the cavity is realized, and the emitted gas Raman scattered light is recoupled for collection, which improves the collection efficiency of the gas Raman scattered light, resulting in a low gas detection limit (up to the sub-ppm level); the system has a fast response (second level), a simple optical path structure and easy adjustment, a high system stability, and good robustness of the detection result. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the structure of a V-shaped dense multi-cycle folded multi-reverse cavity and gas detection system;

[0036] Figure 2 It is a schematic diagram of a V-shaped densely folded multi-reverse cavity structure;

[0037] In the figure: 1. first laser; 2. first half-wave plate; 3. second laser; 4. second half-wave plate; 5. polarization beam splitter; 6. dichroic mirror; 7. first focusing lens; 8. first spherical concave reflector; 9. second spherical concave reflector; 10. third spherical concave reflector; 11. first absorption mask; 12. second absorption mask; 13. third absorption mask; 14. plane reflector; 15. high-pass filter group; 16. second focusing lens; 17. collecting fiber bundle; 18. spectrometer; 19. CCD. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme 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 part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, other embodiments obtained by ordinary technicians in this field without creative work are all within the scope of protection of the present invention.

[0039] like Figure 2As shown, embodiment 1 of the present invention provides a V-shaped dense multi-cycle folded multi-reflection cavity, which is composed of three spherical concave reflectors, namely a first spherical concave reflector 8, a second spherical concave reflector 9 and a third spherical concave reflector 10; one side of the reflector is a frosted plane, and the other side is a spherical concave reflective surface; the reflective surface of the reflector is coated with a broadband high-reflective film, and its reflectivity at the wavelength of the laser and gas Raman scattered light is not less than 0.995; wherein a circular hole is opened at the geometric center of the first spherical concave reflector 8 for the entry of laser and the emission of gas Raman scattered light, and the diameter of the hole is slightly larger than the diameter of the laser beam.

[0040] The three spherical concave reflectors that make up the V-shaped densely folded multi-reflector cavity have the same curvature and are placed in a "V" shape on a geometric plane, and the V-shaped angle of the line connecting the centers of the three reflector lenses is controlled at 15° to 30°.

[0041] The spherical concave reflective surfaces of the V-shaped densely folded multi-reflector cavity are all affixed with absorption mask plates, namely the first absorption mask plate 11, the second absorption mask plate 12 and the third absorption mask plate 13. The mask plates are made of light-absorbing materials and are used to absorb stray lasers in the cavity and reduce the interference effect in the cavity to improve the signal-to-noise ratio of the gas Raman spectrum. After the debugging of the multi-reflector cavity is completed, each absorption mask plate is placed close to the reflective surface of the cavity mirror, and there are openings on it at each reflection point, which does not affect the normal reflection of the laser and the gas Raman scattered light on the mirror surface.

[0042] like Figure 1 As shown, embodiment 2 of the present invention provides a gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity, specifically comprising: a first laser 1, a first half-wave plate 2, a second laser 3, a second half-wave plate 4, a polarization beam splitter 5, a dichroic mirror 6, a focusing lens 7, a first spherical concave reflector 8, a second spherical concave reflector 9, a third spherical concave reflector 10, a first absorption mask plate 11, a second absorption mask plate 12, a third absorption mask plate 13, a plane reflector 14, a high-pass filter group 15, a focusing lens 16, a collecting optical fiber bundle 17, a spectrometer 18 and a CCD 19.

[0043] The first laser 1 and the second laser 3 are both continuous single-mode single-frequency lasers, used to provide a light source to efficiently excite the spontaneous Raman scattering signal of the gas. Their operating wavelengths are mainly in the visible light region, and typical values ​​include but are not limited to 532nm, 633nm, 638nm, 642nm, etc.; laser output power> 500mW; laser line width< 1MHz; laser M 2 Factor <1.5; laser polarization ratio >100:1. When in use, the wavelength, line width and other parameters of the two lasers must be consistent.

[0044] The first half-wave plate 2 and the second half-wave plate 4 are used to adjust the polarization direction of the laser output by the laser.

[0045] The dichroic mirror 6 is used to separate the Raman scattered light of the laser and the gas. Specifically, it has a very high reflectivity for the wavelength of the laser, but has a very high transmittance for the wavelength of the Raman scattered light of the gas. When in use, the mirror surface of the dichroic mirror 6 forms an angle of 45° with the incident direction of the laser.

[0046] The focusing lens 7 is an achromatic lens, which is used to focus the laser and couple it into the multi-reflector cavity, and collimate the gas Raman scattered light emitted from the multi-reflector cavity.

[0047] The first spherical concave reflector 8, the second spherical concave reflector 9, and the third spherical concave reflector 10 are all plano-concave mirrors, one side of which is a frosted plane and the other side is a spherical concave reflective surface; the reflective surface is coated with a broadband high-reflective film, and its reflectivity for the wavelength of laser and gas Raman scattered light is not less than 0.995; a circular hole is opened at the geometric center of the spherical concave reflector 8 for the entry of laser and the emission of gas Raman scattered light, and the diameter of the hole is preferably slightly larger than the diameter of the laser beam. When in use, the three reflectors are placed in a "V" shape geometrically, and the angle between the center line of the first reflector 8 and the second reflector 9 and the center line of the second reflector 9 and the third reflector 10 is 15° to 30°.

[0048] The first absorption mask plate 11, the second absorption mask plate 12, and the third absorption mask plate 13 are all made of light-absorbing materials and are used to absorb stray laser light in the cavity and reduce the interference effect in the cavity to improve the signal-to-noise ratio of the gas Raman spectrum. Each absorption mask plate is placed close to the reflection surface of the cavity mirror after the debugging of the multi-reverse cavity is completed, and there are openings on each reflection point, which does not affect the normal reflection of the laser and the gas Raman scattered light on the mirror surface.

[0049] The plane reflector 14 is used to couple the laser emitted from the multi-reflector cavity back into the cavity to achieve the second cycle of the laser in the cavity; at the same time, it couples the gas Raman scattered light generated in the cavity back into the cavity to improve the efficiency of collecting gas Raman signals. Its reflectivity at the wavelength of the laser and gas Raman scattered light is not less than 0.995.

[0050] The high-pass filter group 15 is composed of one or more high-pass filters, whose cut-off wavelength is slightly larger than the laser wavelength, and is used to filter out the laser and Rayleigh scattered light in the collection light path to improve the signal-to-noise ratio of the gas Raman spectrum. When used, it can be composed of one, two or three pieces, and the specific combination is preferably the one that obtains the highest signal-to-noise ratio of the gas Raman spectrum.

[0051] The focusing lens 16 is an achromatic lens, which is used to couple the gas Raman scattered light into the collection optical fiber bundle.

[0052] The collection fiber bundle 17 is composed of several single-mode optical fibers with the same parameters, and is used to transmit the collected gas Raman scattered light to the spectrometer slit with low loss. Among them, the transmission loss of the gas Raman scattered light by each single-mode optical fiber constituting the collection fiber bundle is extremely low, not higher than 50dB / km. On the collection side of the collection fiber bundle facing the focusing lens, the single-mode optical fibers are arranged in a circular shape, and their area just completely covers the size of the light spot after focusing by the focusing lens; on the divergent side facing the spectrometer slit, the single-mode optical fibers are arranged in a linear shape, and their length and width completely match the height and width of the spectrometer slit.

[0053] The spectrometer 18 is used to separate gas Raman scattered light of different wavelengths. When in use, the slit width has an optimal value for a specific detection, and the gas Raman spectrum obtained under this parameter has the highest signal-to-noise ratio.

[0054] The CCD 19 is used to detect and record the gas Raman scattered light and output the gas Raman spectrum.

[0055] Embodiment 3 of the present invention provides a detection method of a gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity, comprising the following steps:

[0056] Step 1: The laser light emitted by the first laser 1 is adjusted to p-polarized light by the first half-wave plate 2; at the same time, the laser light emitted by the second laser 3 is adjusted to s-polarized light by the second half-wave plate 4;

[0057] Step 2: The p-polarized light and the s-polarized light are respectively transmitted and reflected by the polarization beam splitter 5 and then merged into a beam of light, which is then reflected by the dichroic mirror 6 and focused by the first focusing lens 7, and then enters the multi-reflection cavity through the opening in the center of the first spherical concave reflector 8;

[0058] Step 3: Adjust the placement and deflection angle of the first spherical concave reflector 8, the second spherical concave reflector 9, and the third spherical concave reflector 10 so that the laser is reflected multiple times along different paths therebetween and forms a dense light spot pattern on each reflector; at this time, the first absorption mask plate 11, the second absorption mask plate 12, and the third absorption mask plate 13 are respectively placed close to the spherical concave reflective surfaces of the first spherical concave reflector 8, the second spherical concave reflector 9, and the third spherical concave reflector 10, and the absorption mask plates have openings at each reflection point, which does not affect the normal reflection of the laser and the gas Raman scattered light on the mirror surface; finally, the laser will be emitted from the opening in the center of the first spherical concave reflector 8 along a path different from that at the time of incidence;

[0059] Step 4: Adjust the placement position and deflection angle of the plane reflector 14 to recouple the emitted laser along the original optical path into the multi-reflector cavity to achieve a second circulation of the laser in the cavity;

[0060] Step 5: The laser and gas interact in the multiple reflection cavity to generate backscattered Raman light L fb After being collimated by the focusing lens 7, filtered out by the dichroic mirror 6 and the high-pass filter group 15, and focused by the second focusing lens 16, the light enters the collection fiber bundle 17 and is transmitted to the slit of the spectrometer 18; the gas Raman scattered light is diffracted and split by the spectrometer 18 and then detected by the CCD 19;

[0061] Step 6: In addition, the forward Raman scattered light L generated in the cavity is reflected multiple times ff The light is emitted from the opening in the center of the first spherical concave reflector 8, and then is reflected by the plane reflector 14 and re-coupled into the multi-reflection cavity, and the back Raman scattered light L fb are collected along the same optical path;

[0062] Step 7: Further, the laser light that is reflected by the plane reflector 14 and then recoupled into the multi-reflector cavity interacts with the gas in the multi-reflector cavity again, generating forward gas Raman scattered light L bf The backscattered light L fb The backscattered light L is collected along the same optical path. bb After being coupled by the plane reflector 14 again, it re-enters the multi-reflection cavity and combines with the backscattered light L fb are collected along the same optical path.

[0063] In order to better illustrate the effects achieved by the present invention, the proposed V-shaped dense multi-cycle folded multi-reverse cavity and gas detection system and method are further described below with reference to examples.

[0064] In the example, the central wavelength of the laser output by the first laser 1 and the second laser 3 is 532nm, the laser line width is <1MHz, and the output laser power is 5W; the laser output by the first laser 1 is p-polarized light after being adjusted by the first half-wave plate 2; the laser output by the second laser 3 is s-polarized light after being adjusted by the second half-wave plate 4; the focal length f1 of the focusing lens 7 is 300mm; the diameters of the first spherical concave reflector 8, the second spherical concave reflector 9, and the third spherical concave reflector 10 are 50.8mm, the curvature r is 400mm, and the diameter of the central opening of the first spherical concave reflector 8 is 4mm; the high-pass filter group 15, the number of high-pass filters used is 2; the focal length f2 of the focusing lens 16 is 150mm; the slit width W of the spectrometer 18 slit is 50μm.

[0065] Specifically, the laser light outputted by the first laser 1 is adjusted to p-polarized light by the first half-wave plate 2, and the laser light outputted by the second laser 3 is adjusted to s-polarized light by the second half-wave plate 4. The two beams of light are respectively transmitted and reflected by the polarization beam splitter 5 and then merged into one beam of light, which is then reflected by the dichroic mirror 6 and focused by the first focusing lens 7 in sequence, and then enters the multiple reflection cavity through the opening at the center of the first spherical concave reflector 8; after the laser light is reflected multiple times along different paths between the first spherical concave reflector 8, the second spherical concave reflector 9, and the third spherical concave reflector 10, a dense spot pattern is formed on each reflector, and finally emitted from the opening at the center of the first spherical concave reflector 8 along a path different from that at the time of incidence; the emitted laser light is reflected by the plane reflector 14 and then re-coupled into the multiple reflection cavity, realizing the second circulation of the laser light in the cavity; the laser light interacts with the gas in the multiple reflection cavity, and the back Raman scattered light L generated fb After being collimated by the focusing lens 7, filtered out by the dichroic mirror 6 and the high-pass filter group 15, and focused by the second focusing lens 16, the light enters the collection fiber bundle 17 and is transmitted to the slit of the spectrometer 18; the gas Raman scattered light is diffracted and split by the spectrometer 18 and then detected by the CCD 19; in addition, the forward Raman scattered light L generated in the cavity is reflected multiple times. ff The light is emitted from the opening in the center of the first spherical concave reflector 8, and then is reflected by the plane reflector 14 and re-coupled into the multi-reflection cavity, and the back Raman scattered light L fb are collected along the same optical path; further, the laser light re-coupled into the multi-reflector cavity after being reflected by the plane reflector 14 interacts with the gas in the multi-reflector cavity again, generating forward gas Raman scattered light L bf The backscattered light L fb The backscattered light L is collected along the same optical path. bb After being coupled by the plane reflector 14 again, it re-enters the multi-reflection cavity and combines with the backscattered light L fb are collected along the same optical path.

[0066] Table 1 Detection limits and detection repeatability of SF6 and its decomposition components

[0067] Gas composition <![CDATA[Characteristic Raman shift / cm -1 > Detection limit / ppm Detection repeatability / % <![CDATA[SF6]]> 774 0.15 99.6 <![CDATA[SO2F2]]> 849 0.49 99.5 COS 859 0.32 99.2 <![CDATA[CF4]]> 909 0.74 99.7 <![CDATA[SO2]]> 1150 0.19 99.4 <![CDATA[CO2]]> 1388 0.48 99.6 CO 2140 1.39 99.8

[0068] Compared with the prior art, the beneficial effects of the present invention include at least:

[0069] The present invention provides a V-shaped dense multi-cycle folded multi-reflector cavity and its Raman spectrum gas detection system, which adopts three spherical concave reflectors arranged in a V shape, and the dense reflection spot realized can almost fill the entire cavity mirror reflection surface, and the number of laser reflections that can be realized is large, that is, the effective action path length of the laser and the gas is large, and the excited gas Raman signal intensity is high; an absorption mask plate is placed close to the cavity mirror reflection surface, which effectively absorbs the stray laser and fluorescence in the cavity, and avoids the mutual interference between the laser beams, which is beneficial to improving the signal-to-noise ratio of the gas Raman spectrum; because the physical cavity length of the V-shaped folded multi-reflector cavity only needs to meet the cavity mirror curvature radius of less than 4 times the single-arm cavity length, the cavity length can be adjusted autonomously according to needs in actual use; because a retroreflector is adopted, the second circulation of the laser in the cavity is realized, and the emitted gas Raman scattered light is recoupled for collection, which improves the collection efficiency of the gas Raman scattered light, resulting in a low gas detection limit (up to the sub-ppm level); the system has a fast response (second level), a simple optical path structure and easy adjustment, a high system stability, and good robustness of the detection result.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A V-shaped dense multi-cycle folding multi-reverse cavity, characterized in that: The invention comprises a first spherical concave reflecting mirror (8), a second spherical concave reflecting mirror (9) and a third spherical concave reflecting mirror (10), wherein one side of the reflecting mirror is a frosted plane and the other side is a spherical concave reflecting surface; the reflecting surface of the reflecting mirror is plated with a broadband high-reflection film, wherein a circular hole is opened at the geometric center of the first spherical concave reflecting mirror (8) for the entry of laser and the emission of gas Raman scattered light, and the diameter of the circular hole is greater than the diameter of the laser beam; The spherical concave reflector is placed in a "V" shape on a geometric plane, and the angle between the center line of the first reflector (8) and the second reflector (9) and the center line of the second reflector (9) and the third reflector (10) is 15° to 30°, forming a V-shaped dense multi-circulation folded multi-reflection cavity.

2. A V-shaped dense multi-cycle folding multi-reversal cavity according to claim 1, characterized in that: The spherical concave reflection surfaces of the first spherical concave reflection mirror (8), the second spherical concave reflection mirror (9) and the third spherical concave reflection mirror (10) are all affixed with absorption mask plates, namely a first absorption mask plate (11), a second absorption mask plate (12) and a third absorption mask plate (13). The mask plates are made of light-absorbing materials and are used to absorb stray laser light in the cavity and reduce interference effects in the cavity.

3. A V-shaped dense multi-cycle folding multi-reverse cavity according to claim 2, characterized in that: The first absorption mask plate (11), the second absorption mask plate (12) and the third absorption mask plate (13) are placed close to the cavity mirror reflection surface after the multi-reflection cavity is debugged, and each of them has an opening at each reflection point, so as not to affect the normal reflection of laser and gas Raman scattered light on the mirror surface.

4. A gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity, comprising a V-shaped dense multi-cycle folded multi-reverse cavity according to any one of claims 1 to 3, characterized in that: It also includes: a first laser (1), a first half-wave plate (2), a second laser (3), a second half-wave plate (4), a polarization beam splitter (5), a dichroic mirror (6), a focusing lens (7), a plane reflector (14), a high-pass filter group (15), a focusing lens (16), a collection optical fiber bundle (17), a spectrometer (18) and a CCD (19); The first laser (1) and the second laser (3) are both continuous single-mode single-frequency lasers, used to provide spontaneous Raman scattering signals of the light source excited gas; The first half-wave plate (2) and the second half-wave plate (4) are used to adjust the polarization direction of laser light output by the laser; The dichroic mirror (6) is used to separate the laser light and the Raman scattered light of the gas, and the mirror surface of the dichroic mirror (6) forms an angle of 45° with the incident direction of the laser light; The focusing lens (7) is used to focus the laser light and couple it into the multi-reflector cavity, and at the same time collimate the gas Raman scattered light emitted from the multi-reflector cavity; The plane reflector (14) is arranged between the first spherical concave reflector (8) and the focusing lens (7) and is used to couple the laser light and gas Raman scattered light emitted from the multi-reflector cavity back into the cavity; The high-pass filter group (15), focusing lens (16), collection optical fiber bundle (17), spectrometer (18) and CCD (19) are sequentially arranged on the collimation line of gas Raman scattered light emitted by the multi-reflection cavity.

5. A gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity as claimed in claim 4, characterized in that: The operating wavelengths of the first laser (1) and the second laser (3) are in the visible light region, and their output powers are greater than 500 mW; the laser line width is less than 1 MHz; and the M 2 Factor <1.5; laser polarization ratio >100:

1.

6. A gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity as claimed in claim 4, characterized in that: When in use, the wavelength and line width parameters of the first laser (1) and the second laser (3) must be kept consistent; the lasers output by the first laser (1) and the second laser (3) are adjusted to p-polarized light and s-polarized light respectively through a first half-wave plate (2) and a second half-wave plate (4), and the two beams of light are respectively transmitted and reflected by a polarization beam splitter and then merged into one beam of light.

7. A gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity as claimed in claim 4, characterized in that: The reflectivity of the plane reflector (14) to the wavelength of the laser and the gas Raman scattered light is not less than 0.

995.

8. A gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity as claimed in claim 4, characterized in that: The high-pass filter group (15) comprises one or more high-pass filters, whose cut-off wavelength is greater than the laser wavelength, and is used to filter out the laser and Rayleigh scattered light in the collection light path, so as to improve the signal-to-noise ratio of the gas Raman spectrum.

9. A gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity as claimed in claim 4, characterized in that: The collection fiber bundle (17) includes several single-mode optical fibers with the same parameters, and the transmission loss of each single-mode optical fiber to the gas Raman scattered light is not higher than 50 dB / km. The collection fiber bundle (17) faces the collection side of the focusing lens (16), and the single-mode optical fibers are arranged in a circular shape, and their area completely covers the size of the light spot after focusing by the focusing lens; and faces the divergent side of the spectrometer (18) slit, and the single-mode optical fibers are arranged in a linear shape, and their length and width completely match the height and width of the spectrometer slit.

10. A method for a gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity, based on a gas detection system based on a V-shaped dense multi-cycle folded multi-reverse cavity according to any one of claims 4 to 9, characterized in that: The following steps are involved: Step 1: The laser light emitted by the first laser (1) is adjusted to p-polarized light by a first half-wave plate (2); at the same time, the laser light emitted by the second laser (3) is adjusted to s-polarized light by a second half-wave plate (4); Step 2: The p-polarized light and the s-polarized light in step 1 are respectively transmitted and reflected by the polarization beam splitter (5) and then merged into a beam of light, which is then reflected by the dichroic mirror (6) and focused by the first focusing lens (7) and then enters the multiple reflection cavity through the opening in the center of the first spherical concave reflector (8); Step 3: adjusting the placement positions and deflection angles of the first spherical concave reflector (8), the second spherical concave reflector (9), and the third spherical concave reflector (10) so that the laser is reflected multiple times along different paths therebetween and forms a dense light spot pattern on each reflector; at this time, a first absorption mask plate (11), a second absorption mask plate (12), and a third absorption mask plate (13) are respectively placed close to the spherical concave reflective surfaces of the first spherical concave reflector (8), the second spherical concave reflector (9), and the third spherical concave reflector (10); the absorption mask plates have openings at each reflection point, which does not affect the normal reflection of the laser and the gas Raman scattered light on the mirror surface; finally, the laser will be emitted from the opening at the center of the first spherical concave reflector (8) along a path different from that at the time of incidence; Step 4: adjusting the placement position and deflection angle of the plane reflector (14) to recouple the laser emitted in step 3 along the original optical path into the multi-reflector cavity, thereby realizing a second circulation of the laser in the cavity; Step 5: The laser and gas interact in the multi-reflector cavity to generate backscattered Raman light L fb After being collimated by a focusing lens (7), filtered out by a dichroic mirror (6) and a high-pass filter group (15), the laser and Rayleigh scattered light are focused by a second focusing lens (16) and enter a collecting optical fiber bundle (17) and transmitted to the slit of a spectrometer 18; the gas Raman scattered light is diffracted and split by the spectrometer (18) and then detected by a CCD (19); Step 6: Forward Raman scattered light L generated in the multi-reflector cavity ff The light is emitted from the opening at the center of the first spherical concave reflector (8), and after being reflected by the plane reflector (14), it is re-coupled into the multi-reflection cavity and combined with the aforementioned back Raman scattered light L fb are collected along the same optical path; Step 7: The laser light reflected by the plane reflector (14) in step 4 and then re-coupled into the multi-reflector cavity interacts with the gas in the multi-reflector cavity again to generate forward gas Raman scattered light L bf The backscattered light L fb The backscattered light L is collected along the same optical path. bb After being coupled again by the plane reflector (14), it re-enters the multi-reflection cavity and combines with the aforementioned backscattered light L fb are collected along the same optical path.

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