A Raman spectroscopic gas analyzer and analysis method based on hollow-core optical fiber

Abstract

A hollow-core fiber-based Raman spectroscopy gas analyzer and analysis method comprises a laser, a concave spherical reflector, an internal standard gas chamber, a first hollow-core fiber adapter, a hollow-core fiber, a second hollow-core fiber adapter, a first focusing lens, a dichroic mirror, a high-pass filter, a second focusing lens, a spectrometer, and a CCD. A first adjustable aperture is provided between the second hollow-core fiber adapter and the dichroic mirror; a second adjustable aperture is provided between the dichroic mirror and the second focusing lens. The second hollow-core fiber adapter is also connected to a medium-pressure pump system via a gas pipeline and a gas filter. The present invention can lower gas detection limits, shorten system response time, reduce the difficulty of optical path adjustment and the volatility of detection results, and improve system stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas analysis, and in particular relates to a Raman spectrum gas analyzer and an analysis method based on hollow-core optical fiber. Background Art

[0002] Raman spectroscopy is an emerging "molecular fingerprint" technology with advantages such as high selectivity, no calibration required, no contact, no destruction, and no sample consumption, and no need for component separation. It can simultaneously detect nearly all gases, except rare gases, using a single wavelength laser, and has broad application prospects in multi-component gas analysis. However, the Raman effect of gases is extremely weak, requiring appropriate technical means to enhance the Raman signal intensity to achieve the detection limits required for practical applications.

[0003] Microstructured hollow-core optical fibers, such as hollow-core photonic bandgap fibers and hollow-core antiresonant fibers, have the characteristics of low transmission loss, wide transmission band, and low dispersion. They can act as micro gas chambers while efficiently guiding lasers, greatly extending the effective action path length of the laser-gas and improving the collection efficiency of spatial Raman scattered photons. Moreover, the amount of gas required is extremely small, generally only a few microliters, and is a hot topic in the research of gas Raman spectroscopy enhancement methods at home and abroad.

[0004] Traditional fiber-enhanced Raman spectroscopy (FER) has a gas detection limit in the ppm range. While extending the hollow-core fiber length can achieve lower detection limits, the system response time—the time it takes for gas to enter and exit the hollow-core fiber—increases quadratically with the fiber length, limiting its application in the immediate detection and analysis of trace gases. Furthermore, the spatial filtering system employed in traditional FER is complex, requiring high optical path alignment and adjustment skills from the experimenter, and is costly. Furthermore, the detection results of traditional FER are significantly affected by factors such as slight variations in the optical path, fluctuations in instrument performance, and degradation of the hollow-core fiber. Summary of the Invention

[0005] To address the deficiencies in the prior art, the present invention provides a Raman spectroscopic gas analyzer and analysis method based on hollow-core optical fiber, which can reduce the gas detection limit, shorten the system response time, reduce the difficulty of optical path adjustment and the volatility of detection results, and improve system stability.

[0006] The present invention adopts the following technical solutions.

[0007] A hollow-core fiber-based Raman spectroscopic gas analyzer includes a laser and a concave spherical reflector, an internal standard gas chamber, a first hollow-core fiber adapter, a hollow-core fiber, a second hollow-core fiber adapter, a first focusing lens, a dichroic mirror, a high-pass filter, a second focusing lens, a spectrometer, and a CCD.

[0008] and a first adjustable diaphragm is provided between the second hollow-core fiber adapter and the dichroic mirror;

[0009] A second adjustable aperture is provided between the dichroic mirror and the second focusing lens;

[0010] The second hollow-core optical fiber adapter is also connected to the medium-pressure pump system through a gas pipeline and a gas filter.

[0011] The present invention further includes the following preferred embodiments:

[0012] Preferably, the laser is a single longitudinal mode, narrow linewidth, continuous wave laser used to excite the spontaneous Raman signal of the gas, and its operating wavelength can be any wavelength in the visible light-near infrared band; the laser linewidth is <0.00001nm; the laser's M 2 Factor <1.5.

[0013] Preferably, the dichroic mirror is a 45° high-pass filter, which is used to separate the laser and the gas Raman scattered light, and filter the gas Rayleigh scattered light at the same time. Its reflectivity for Rayleigh scattered light is not less than 98%, and its transmittance for Raman scattered light is not less than 93%; and the mirror surface of the dichroic mirror is at 45° to the incident direction of the laser.

[0014] Preferably, the first focusing lens is an achromatic lens, which is used to couple the laser light into the core of the hollow core optical fiber and collimate the backscattered gas Raman light emitted from the core of the hollow core optical fiber;

[0015] The first focusing lens is mounted on a three-axis adjustable micro-displacement platform. By finely adjusting the spatial position of the lens, the laser spot at the focal point is coupled into the core of the hollow-core fiber. The coupling condition is determined by real-time monitoring of the laser power output at the end of the hollow-core fiber and the shape of the far-field spot.

[0016] The second focusing lens is an achromatic lens, which is used to couple the gas Raman scattered light into the slit of the spectrometer.

[0017] Preferably, the focal length f of the first focusing lens is:

[0018]

[0019] λ, D, D MFD They are respectively the laser wavelength, the laser spot size at the focusing lens, and the mode field diameter of the hollow-core optical fiber.

[0020] Preferably, the first adjustable iris and the second adjustable iris are used to adjust the aperture size of the iris to perform spatially optimal filtering of the gas Raman scattered light;

[0021] The first adjustable iris and the second adjustable iris are adjusted to determine the aperture of the optimal filter, and the position adjustment constraint condition is:

[0022] The first adjustable aperture is located between the dichroic mirror and the second hollow core fiber adapter and is positioned along the optical axis;

[0023] The second adjustable aperture is located between the dichroic mirror and the second focusing lens and is positioned along the optical axis.

[0024] Preferably, the second hollow-core fiber adapter and the first hollow-core fiber adapter include an adapter body, an optical window, an optical flange, screws, a fiber clamp, a sealing ring, a pressure gauge, and an air inlet / outlet, which are used to fix and install the hollow-core fiber and ensure the airtightness of the adapter as a whole, while achieving laser-hollow-core fiber coupling, gas Raman scattered light collection, and gas inlet / outlet;

[0025] The gas inlet / outlet of the second hollow-core optical fiber adapter is connected to the medium-pressure pump system through a gas pipeline. The gas to be tested is continuously pumped into the hollow-core optical fiber in a pressure-driven manner. When a stable pressure gradient is established inside the fiber core, the gas inlet / outlet of the second hollow-core optical fiber adapter and the first hollow-core optical fiber adapter are closed, and the gas equilibrium time in the hollow-core optical fiber core is monitored in real time by a pressure gauge.

[0026] Preferably, the hollow-core optical fiber includes a hollow-core photonic bandgap fiber and a hollow-core antiresonant fiber, which is used to guide the laser and provide a place for the interaction between laser and gas and the generation of gas Raman scattering light. Its transmission bandwidth covers the visible light to near-infrared band; the core diameter is in the micron range;

[0027] The air holes around the core of the hollow-core optical fiber are all sealed to ensure that the gas pressure in the core is always greater than the gas pressure in the surrounding air holes during the experiment.

[0028] Preferably, the internal standard gas chamber includes a gas chamber body, two optical windows along the two ends of the optical axis, two optical flanges, screws, a sealing ring, a pressure gauge, and an air inlet / outlet, and is used for quantitative analysis of gas Raman spectroscopy;

[0029] The planes where the two optical windows are located are perpendicular to the laser transmission direction, and the interior of the internal standard gas chamber is filled with a high-purity single standard gas, namely the internal standard gas, which is different from the gas to be measured.

[0030] Preferably, the concave spherical reflector is used to recouple the laser light and gas Raman scattered light output from the end of the hollow-core optical fiber and passing through the first hollow-core optical fiber adapter and the internal standard gas chamber into the core of the hollow-core optical fiber, and finally be detected by the CCD;

[0031] The curvature r of the concave spherical reflector is equal to the distance d from the center of the mirror to the end of the hollow-core optical fiber;

[0032] The reflectivity of the concave spherical reflector to laser and gas Raman scattered light is not less than 99.5%.

[0033] Preferably, the high-pass filter is used to filter stray laser light in the space and Rayleigh scattered light of the gas transmitted back along the optical axis, and its reflectivity for Rayleigh scattered light of the gas is not less than 98%, and its transmittance for Raman scattered light of the gas is not less than 93%;

[0034] The mirror surface of the high-pass filter is 0° to the laser transmission direction.

[0035] Preferably, a spectrometer is used to separate gas Raman scattered light of different wavelengths so that it can be recorded by different detection units of the CCD;

[0036] Spectrometer slit width W slit for:

[0037] W slit =f2d fiber / f1.

[0038] Wherein, f1 is the focal length of the first focusing lens;

[0039] f2 is the focal length of the second focusing lens;

[0040] d fiber is the core diameter of the hollow-core fiber.

[0041] Preferably, a CCD is used to detect and record the gas Raman scattered light and convert it into an electrical signal for output;

[0042] a medium-pressure pump system for pumping gas into the hollow-core optical fiber;

[0043] a gas transmission pipeline, used for inputting gas from the medium-pressure pump system into the second hollow-core optical fiber adapter;

[0044] Gas filter, used to absorb moisture and solid particles in the gas to be tested;

[0045] The first adjustable diaphragm, the first focusing lens, the dichroic mirror, the high-pass filter, the second adjustable diaphragm and the second focusing lens are fixedly connected in a cage system.

[0046] A Raman spectroscopy gas analysis method based on hollow-core optical fiber, the method comprising:

[0047] Step 1: Pump the calibration gas of known concentration into the hollow-core optical fiber. After the gas is balanced, the analyzer performs Raman spectroscopy detection and obtains the Raman spectrum S of the calibration gas. c , Raman spectrum of internal standard gas S cs , the pressure reading of the internal standard gas chamber P cs ;

[0048] Step 2: Fully flush the hollow-core optical fiber with high-purity argon gas;

[0049] Step 3: Pump the unknown concentration of the gas to be tested into the hollow-core optical fiber. After the gas is balanced, the analyzer performs Raman spectrum detection and obtains the Raman spectrum S of the gas to be tested. m , Raman spectrum of internal standard gas S ms , the pressure reading of the internal standard gas chamber P ms ;

[0050] Step 4: Obtain S c 、S cs 、S m 、S ms , calculate the calibration gas Raman peak area A c , the Raman peak area of ​​the internal standard gas during calibration A cs , Raman peak area of ​​the gas to be measured A m , the internal standard gas Raman peak area A during measurement ms ;

[0051] At the same time, the concentration of the calibration gas c is known c , the pressure reading of the internal standard gas chamber during calibration P cs , the pressure reading of the internal standard gas chamber during measurement P ms ;

[0052] The concentration of the gas to be measured is calculated according to the following formula:

[0053]

[0054] Preferably, the process of performing Raman spectroscopy detection by the analyzer includes:

[0055] Step 1: The laser emits laser light, which is reflected by a dichroic mirror, focused by a first focusing lens, and then enters a hollow-core optical fiber through a second hollow-core optical fiber adapter;

[0056] Step 2: The laser and gas interact in the core of the hollow-core fiber, generating forward Raman scattered light L ff and backscattered Raman light L fb ;

[0057] Step 3: Backscattered Raman light L fb It passes through the second hollow-core fiber adapter, is collimated by the first focusing lens, and filtered out by the dichroic mirror and high-pass filter to remove the laser and Rayleigh scattered light. It is then focused by the second focusing lens and enters the slit of the spectrometer.

[0058] The gas Raman scattered light is diffracted and split by the spectrometer and then detected by the CCD;

[0059] Step 4: Forward Raman scattered light L ff Passing through the first hollow-core fiber adapter and the internal standard gas chamber, it re-enters the hollow-core fiber after being coupled by the concave spherical reflector and combines with the backscattered Raman light L in step 3.fb The light is collected by CCD along the same optical path;

[0060] Step 5: The laser output from the end of the hollow-core fiber passes through the first hollow-core fiber adapter and the internal standard gas chamber, and re-enters the hollow-core fiber after being coupled by the concave spherical reflector, and interacts with the gas in the hollow-core fiber again, generating forward gas Raman scattered light L bf The backscattered light L from step 3 fb The backscattered light L is collected along the same optical path. bb After passing through the first hollow-core fiber adapter and the internal standard gas chamber, it is coupled again by the concave spherical reflector and re-enters the hollow-core fiber to combine with the backscattered light L in step 3. fb are collected along the same optical path.

[0061] Preferably, when the analyzer performs Raman spectroscopy detection, the aperture sizes of the first adjustable iris and the second adjustable iris are slowly adjusted, while the Raman spectrum of the gas detected by the CCD is recorded. By calculating multiple sets of spectral signal-to-noise ratios, the optimal iris aperture is determined to achieve optimal filtering.

[0062] Preferably, when the analyzer performs Raman spectroscopy detection, the gas inlet / outlet of the second hollow-core optical fiber adapter is connected to the medium-pressure pump system through the gas pipeline, and the gas inlet / outlet of the first hollow-core optical fiber adapter remains open;

[0063] The medium-pressure pump system provides constant pressure to continuously pump gas into the second hollow-core optical fiber adapter and into the hollow-core optical fiber. When a stable pressure gradient is established inside the hollow-core optical fiber core, the inlet / outlet ports of the second hollow-core optical fiber adapter and the first hollow-core optical fiber adapter are closed, and the gas equilibrium time in the hollow-core optical fiber core is monitored in real time using a pressure gauge.

[0064] The beneficial effects of the present invention are that, compared with the prior art, the gas detection limit of the present invention can reach the ppb level, the system achieves a response time of seconds, the optical path structure is simple and easy to adjust, the system stability is high, and the detection results are robust (less affected by factors such as slight changes in the optical path, instrument performance fluctuations, and hollow-core optical fiber performance degradation). Furthermore:

[0065] 1. The present invention seals the air holes around the core of the hollow-core optical fiber to ensure that the gas pressure in the core is always greater than the gas pressure in the surrounding air holes during the experiment. That is, the refractive index of the medium in the core is greater than the equivalent refractive index of the medium in the surrounding air holes. This can improve the transmission efficiency of gas Raman scattered light in the core of the hollow-core optical fiber, thereby increasing the intensity of the detected gas Raman signal.

[0066] 2. The present invention places a concave spherical reflector at a certain distance along the optical axis at the end of the hollow-core fiber. This recouples the laser light and gas Raman scattered light output from the end of the hollow-core fiber and passing through the internal standard gas chamber into the core of the hollow-core fiber, where they are ultimately detected by the CCD. This can simultaneously improve the efficiency of laser utilization and the efficiency of collecting gas Raman scattered light, increasing the gas Raman signal by four times (without considering optical loss). Furthermore, the concave spherical reflector is placed outside the first hollow-core fiber adapter to actively adjust the coupling between the laser light (gas Raman scattered light) reflected by the lens and the core of the hollow-core fiber.

[0067] 3. The present invention inserts two adjustable irises into the optical axis and adjusts the aperture size of the irises to perform spatial optimal filtering of gas Raman scattered light. Compared with traditional spatial filtering systems, the present invention has a simpler structure, lower optical path alignment requirements, lower adjustment difficulty, and lower application cost.

[0068] 4. The present invention sets the optimal spectrometer slit width W slit , obtaining the highest gas Raman spectroscopy signal-to-noise ratio, that is, the lowest gas detection limit.

[0069] W slit =f2d fiber / f1

[0070] Wherein, f1 is the focal length of the first focusing lens;

[0071] f2 is the focal length of the second focusing lens;

[0072] d fiber is the core diameter of the hollow-core fiber.

[0073] 5. The present invention places an internal standard gas chamber at a certain distance along the optical axis at the end of the hollow-core fiber to reduce the impact of factors such as slight changes in the optical path (laser-hollow-core fiber coupling, gas Raman signal-spectrometer slit coupling, etc.), instrument performance fluctuations, and hollow-core fiber performance degradation on the gas analyzer detection results;

[0074] 6. The present invention passes the gas to be tested through a gas filter before being pumped into the hollow-core optical fiber to absorb moisture, dust and other solid particles in the gas to be tested, thereby preventing them from being adsorbed on the inner wall of the optical fiber to cause pollution, thereby causing performance degradation of the hollow-core optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Figure 1 This is a schematic structural diagram of a hollow-core optical fiber-based Raman spectroscopy gas analyzer according to the present invention;

[0076] Figure 2 Schematic diagrams of two typical microstructured hollow-core optical fibers according to the embodiments of the present invention;

[0077] Figure 1The figures are marked as follows: 1-laser, 2-dichroic mirror, 3-first focusing lens, 4-first adjustable aperture, 5-second hollow-core fiber adapter, 6-hollow-core fiber, 7-first hollow-core fiber adapter, 8-internal standard gas chamber, 9-concave spherical reflector, 10-high-pass filter, 11-second adjustable aperture, 12-second focusing lens, 13-spectrometer, 14-CCD, 15-medium-pressure pump system, 16-gas pipeline, 17-gas filter, 18-cage system. DETAILED DESCRIPTION

[0078] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying 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 making creative efforts are all within the scope of protection of the present invention.

[0079] like Figure 1 As shown, Example 1 of the present invention provides a Raman spectroscopic gas analyzer based on a hollow-core fiber. In a preferred but non-limiting embodiment of the present invention, the analyzer includes a laser 1 and a concave spherical reflector 9, an internal standard gas chamber 8, a first hollow-core fiber adapter 7, a hollow-core fiber 6, a second hollow-core fiber adapter 5, a first adjustable aperture 4, a first focusing lens 3, a dichroic mirror 2, a high-pass filter 10, a second adjustable aperture 11, a second focusing lens 12, a spectrometer 13, and a CCD 14.

[0080] The second hollow optical fiber adapter 5 is also connected to the medium-pressure pump system 15 via a gas pipeline 16 and a gas filter 17 .

[0081] Further preferably, the laser 1 is a single longitudinal mode, narrow linewidth, continuous wave laser used to excite the spontaneous Raman signal of the gas. Its operating wavelength can be any wavelength in the visible-near infrared range, with typical values ​​including but not limited to 532nm, 633nm, 642nm, and 785nm; the laser linewidth is less than 0.00001nm; the laser's M 2 Factor <1.5.

[0082] Dichroic mirror 2 is a 45° high-pass filter, primarily used to separate laser light from gas Raman scattered light, while simultaneously filtering out the majority of gas Rayleigh scattered light. Its reflectivity for laser light (Rayleigh scattered light) is no less than 98%, and its transmittance for longer-wavelength Raman scattered light is no less than 93%. When in use, the high-pass filter is positioned at a 45° angle to the incident laser direction.

[0083] The first focusing lens 3 is an achromatic lens, which is used to couple the laser into the core of the hollow-core fiber 6 and collimate the backscattered Raman light emitted by the hollow-core fiber core. The selection of its focal length f1 needs to comprehensively consider the laser wavelength λ, the spot size D of the laser at the focusing lens, and the mode field diameter D of the hollow-core fiber. MFD The specific calculation formula is: When in use, the focusing lens is installed on a three-axis adjustable micro-displacement platform. The spatial position of the lens is finely adjusted to couple the laser spot at the focus into the core of the hollow-core optical fiber. The coupling condition is judged by real-time monitoring of the laser power and far-field spot shape output at the end of the hollow-core optical fiber.

[0084] The second focusing lens 12 is an achromatic lens, which is used to couple the gas Raman scattered light into the slit of the spectrometer. Its focal length is f2.

[0085] The first adjustable aperture 4 is used for spatial filtering of gas Raman scattered light. Specifically, the laser will interact with the end face of the hollow-core fiber, the inner wall of the hollow-core fiber, etc. to produce spatially distributed silica Raman scattered light, which increases the background noise intensity of the gas Raman spectrum (reduces the signal-to-noise ratio of the gas Raman spectrum), thereby limiting the detection limit of the gas analyzer. Therefore, it is necessary to use spatial filtering to "purify" (filter out) it. The traditional spatial filtering system consists of a focusing lens, a pinhole (aperture in the micrometer range), a collimating lens, etc., with a complex structure, difficult optical path alignment, and a high cost (10,000 yuan).

[0086] The present invention inserts an adjustable diaphragm into the optical path and adjusts the light aperture size (millimeter level) of the diaphragm to perform spatial filtering. The invention has a simple structure and is easy to operate, while greatly reducing the application cost.

[0087] During use, the aperture is slowly adjusted while simultaneously recording the gas Raman spectrum detected by the CCD. The optimal aperture is determined by calculating the signal-to-noise ratio of multiple spectral groups. It is important to note that the position of the first adjustable aperture 4 in the optical path is not specified; however, it must be located between the dichroic mirror 2 and the second hollow-core fiber adapter 5 and along the optical axis. Depending on the placement, the optimal aperture will vary.

[0088] The second adjustable aperture 11, in conjunction with the first adjustable aperture 4, provides spatial filtering of gas Raman scattered light. During operation, the aperture is slowly adjusted while simultaneously recording the gas Raman spectrum detected by the CCD. The optimal aperture is determined by calculating the signal-to-noise ratio of multiple spectral groups. It is important to note that the position of the second adjustable aperture 11 in the optical path is not specifically required; however, it must be located between the dichroic mirror 2 and the second focusing lens 12 and along the optical axis. Depending on its placement, the optimal aperture will vary.

[0089] The second hollow-core fiber adapter 5, consisting of an adapter body, an optical window, an optical flange, screws, a fiber clamp, a sealing ring, a pressure gauge, and an air inlet / outlet, is used to secure and install the hollow-core fiber and ensure the overall airtightness of the adapter. It also enables laser-hollow-core fiber coupling (collection of gas Raman scattered light) and gas inlet / outlet. During use, its air inlet / outlet is connected to a medium-pressure pump system 15 via a gas pipeline 16. The gas to be tested is continuously pumped into the hollow-core fiber via a pressure-driven method. The air inlet / outlet is closed once a stable pressure gradient is established within the core. The pressure gauge then monitors the gas equilibrium time within the hollow-core fiber core in real time.

[0090] The first hollow-core fiber adapter 7, consisting of an adapter body, an optical window, an optical flange, screws, a fiber clamp, a sealing ring, a pressure gauge, and air inlet and outlet ports, is used to secure and install the hollow-core fiber and ensure the adapter's overall airtightness. It also facilitates laser-to-hollow-core fiber coupling (collection of gas Raman scattered light) and gas inlet and outlet. During use, the air inlet and outlet ports are initially open and closed once a stable pressure gradient is established within the core. The pressure gauge monitors the gas equilibration time within the hollow-core fiber in real time.

[0091] like Figure 2 As shown, the hollow-core fiber 6, specifically including hollow-core photonic bandgap fiber (HC-PBGF) and hollow-core antiresonant fiber (HC-ARF), is used to guide the laser and provide an ideal location for laser-gas interaction (generation of gas Raman scattering light). Its transmission bandwidth covers the visible-near infrared band, mainly including 500-1200nm; the core diameter d fiber At the micron level, typical values ​​are 20 to 50 μm. During use, the air holes surrounding the hollow-core fiber core are sealed (arc discharge causes the air holes to collapse), ensuring that the gas pressure in the core (typically several atmospheres) is always greater than the gas pressure in the surrounding air holes (1 atmosphere) during the experiment. This results in a higher effective refractive index in the core than in the surrounding air holes. This method can effectively improve the transmission efficiency of gas Raman scattered light within the hollow-core fiber core, thereby increasing the intensity of the gas Raman signal.

[0092] The internal standard gas chamber 8 is composed of a gas chamber body, two optical windows along the two ends of the optical axis, two optical flanges, screws, sealing rings, a pressure gauge, an air inlet / outlet, etc., and is used for quantitative analysis of gas Raman spectra to reduce the impact of factors such as slight changes in the optical path (laser-hollow-core fiber coupling, gas Raman signal-spectrometer slit coupling, etc.), instrument performance fluctuations, and hollow-core fiber performance degradation on the detection results of the gas analyzer. When in use, the internal standard gas chamber is placed along the optical axis (the laser transmission direction is perpendicular to the plane where the two optical windows of the gas chamber are located), and the interior is filled with 20 bar of high-purity single standard gas (the type of internal standard gas is different from the gas to be measured). Specifically, a calibration gas of known concentration is first pumped into the hollow-core fiber, and Raman spectrum detection is performed after the gas is balanced, and the Raman spectrum S of the calibration gas is obtained at the same time. c , Raman spectrum of internal standard gas S cs , the pressure reading of the internal standard gas chamber P cs This process is the calibration process; the hollow core fiber is fully flushed with high-purity argon gas; the gas to be tested at an unknown concentration is pumped into the hollow core fiber, and Raman spectrum detection is performed after the gas is balanced, and the Raman spectrum S of the gas to be tested is obtained at the same time. m , Raman spectrum of internal standard gas S ms , the pressure reading of the internal standard gas chamber P ms This process is the measurement process. c 、S cs 、S m 、S ms , the Raman peak intensity (peak area, peak height) of the calibration gas can be calculated respectively c , Raman peak intensity of internal standard gas during calibration I cs , Raman peak intensity of the gas to be measured I m , the Raman peak intensity of the internal standard gas during measurement I ms , and the calibration gas concentration c is known c , internal standard gas concentration during calibration c cs , internal standard gas concentration c during measurement ms , then the concentration of the gas to be measured is where c ms / c cs The ratio of the pressure recorded by the pressure gauge connected to the internal standard gas chamber during measurement and calibration can be used. ms / P cs At this time, the concentration of the gas to be measured is

[0093] The concave spherical reflector 9 is used to couple the laser light and gas Raman scattered light emitted from the hollow-core fiber and passing through the internal standard gas chamber back into the core of the hollow-core fiber for detection by the CCD. Ideally (ignoring losses in the various optical components), this method can increase the detected gas Raman signal by a factor of four. The curvature r of the concave spherical reflector is equal to the distance d from the center of the mirror to the end of the hollow-core fiber; the reflectivity of the concave spherical reflector for the laser light and gas Raman scattered light is no less than 99.5%.

[0094] The high-pass filter 10 is used to filter stray laser light in space and Rayleigh scattered light from the gas traveling in the opposite direction of the optical axis. Its reflectivity for laser light (Rayleigh scattered light from the gas) is no less than 98%, and its transmittance for the longer-wavelength Raman scattered light from the gas is no less than 93%. When in use, the high-pass filter's mirror surface is positioned at a 0° angle to the direction of laser transmission.

[0095] The spectrometer 13 is used to separate the gas Raman scattered light of different wavelengths so that it can be recorded by different detection units of the CCD. For a specific hollow-core fiber Raman spectroscopy gas analyzer system, the slit width W of the spectrometer is slit There is an optimal value, usually W slit =f2d fiber / f1.

[0096] Wherein, f1 is the focal length of the first focusing lens 3;

[0097] f2 is the focal length of the second focusing lens 12;

[0098] d fiber is the core diameter of the hollow-core fiber.

[0099] CCD14 is used to detect and record the Raman scattered light of the gas and convert it into an electrical signal for output.

[0100] The medium-pressure pump system 15 is used to pump gas into the hollow-core optical fiber and can provide a pressure of no less than 5 bar.

[0101] The gas pipeline 16 is used to input gas from the medium pressure pump system into the second hollow fiber adapter 5. The gas pipeline is made of synthetic metal, which is resistant to high temperature and corrosion, has high mechanical strength and strong plasticity.

[0102] The gas filter 17 is used to absorb solid particles such as moisture and dust in the gas to be measured to prevent them from being pumped into the hollow-core optical fiber and being adsorbed on the inner wall of the optical fiber to cause pollution, thereby causing performance degradation of the hollow-core optical fiber.

[0103] Cage system 18: The first adjustable aperture 4, the first focusing lens 3, the dichroic mirror 2, the high-pass filter 10, the second adjustable aperture 11, and the second focusing lens 12 are fixedly connected in a cage system 18 to reduce minor changes in the optical path caused by factors such as mechanical looseness and external disturbances, which may affect the accuracy of the gas analyzer's detection results.

[0104] Embodiment 2 of the present invention provides a Raman spectroscopy gas analysis method based on hollow-core optical fiber, which is implemented based on the above-mentioned analyzer, and the method includes:

[0105] Step 1: Pump the calibration gas of known concentration into the hollow-core optical fiber. After the gas is balanced, the analyzer performs Raman spectroscopy detection and obtains the Raman spectrum S of the calibration gas. c , Raman spectrum of internal standard gas S cs , the pressure reading of the internal standard gas chamber P cs ;

[0106] Step 2: Fully flush the hollow-core optical fiber with high-purity argon gas;

[0107] Step 3: Pump the unknown concentration of the gas to be tested into the hollow-core optical fiber. After the gas is balanced, the analyzer performs Raman spectrum detection and obtains the Raman spectrum S of the gas to be tested. m , Raman spectrum of internal standard gas S ms , the pressure reading of the internal standard gas chamber P ms ;

[0108] Step 4: Obtain S c 、S cs 、S m 、S ms , calculate the calibration gas Raman peak area A c , the Raman peak area of ​​the internal standard gas during calibration A cs , Raman peak area of ​​the gas to be measured A m , the internal standard gas Raman peak area A during measurement ms ;

[0109] At the same time, the concentration of the calibration gas c is known c , the pressure reading of the internal standard gas chamber during calibration P cs , the pressure reading of the internal standard gas chamber during measurement P ms ;

[0110] The concentration of the gas to be measured is calculated according to the following formula:

[0111]

[0112] During steps 1 to 4, the interior of the internal standard gas chamber is always filled with a high-purity single standard gas.

[0113] Further preferably, the process of the analyzer performing Raman spectroscopy detection includes:

[0114] Step 1: Laser 1 emits laser light, which is reflected by dichroic mirror 2, focused by first focusing lens 3, and then passes through second hollow-core fiber adapter 5 into hollow-core fiber 6;

[0115] Step 2: The laser and gas interact in the core of the hollow-core fiber, generating forward Raman scattered light L ff and backscattered Raman light L fb ;

[0116] Step 3: Backscattered Raman light L fb The laser beam passes through the second hollow-core fiber adapter 5, is collimated by the first focusing lens 3, filtered out by the dichroic mirror 2 and the high-pass filter 10, and then focuses by the focusing lens 12 before entering the slit of the spectrometer 13.

[0117] The gas Raman scattered light is diffracted and split by the spectrometer 13 and then detected by the CCD 14;

[0118] Step 4: Forward Raman scattered light L ff Passing through the first hollow-core fiber adapter 7 and the internal standard air chamber 8, it is coupled by the concave spherical reflector 9 and then re-enters the hollow-core fiber 6, and the back Raman scattered light L in step 3 fb It is detected and collected by CCD14 along the same optical path;

[0119] Step 5: The laser output from the end of the hollow-core fiber passes through the internal standard gas chamber 8, is coupled by the concave spherical reflector 9, and then re-enters the hollow-core fiber 6, where it interacts with the gas in the hollow-core fiber 6 again, generating forward gas Raman scattered light L bf The backscattered light L from step 3 fb The backscattered light L is collected along the same optical path. bb The first hollow-core fiber adapter 7 and the internal standard gas chamber 8 are coupled again through the concave spherical reflector 9 and then re-enter the hollow-core fiber 6 to combine with the backscattered light L in step 3. fb are collected along the same optical path.

[0120] When the analyzer performs Raman spectrum detection, the aperture sizes of the first adjustable iris 4 and the second adjustable iris 11 are slowly adjusted, and the gas Raman spectrum detected by the CCD is recorded at the same time. By calculating the signal-to-noise ratio of multiple groups of spectra, the optimal iris aperture is determined to achieve optimal filtering.

[0121] The first adjustable aperture 4 and the second adjustable aperture 11 are used in combination to filter the silicon dioxide background Raman signal distributed radially along the optical axis.

[0122] When the analyzer performs Raman spectroscopy detection, the air inlet / outlet of the second hollow-core optical fiber adapter 5 is connected to the medium-pressure pump system 15 through the air pipeline 16, and the air inlet / outlet of the first hollow-core optical fiber adapter 7 remains open;

[0123] The medium-pressure pump system provides a constant pressure of 5 bar to continuously pump gas into the hollow-core fiber adapter 5 and into the hollow-core fiber. When a stable pressure gradient is established inside the hollow-core fiber core, the inlet / outlet ports of the second hollow-core fiber adapter 5 and the first hollow-core fiber adapter 7 are closed, and the gas equilibrium time in the hollow-core fiber core is monitored in real time using a pressure gauge. The experimental records show that the gas equilibrium time is within 20 seconds.

[0124] In Example 2, the wavelength of the laser 1 is 532 nm, and the output laser power is 1.5 W; the core of the hollow-core optical fiber 6 is 26 μm, and the length used is 50 cm; the focal length f1 of the first focusing lens 3 is 60 mm; the curvature r of the concave spherical reflector 9 is 60 mm; the focal length f2 of the second focusing lens 12 is 150 mm; the slit width W of the spectrometer 13 is 150 mm; slit is 65μm; the optimal aperture of the first adjustable iris 4 is 3mm; the optimal aperture of the second adjustable iris 11 is 0.8mm; and the internal standard gas is SF6.

[0125] The obtained detection limits and quantitative accuracy of transformer fault characteristic gases are shown in Table 1.

[0126] Table 1 Detection limits and quantitative accuracy of transformer fault characteristic gases

[0127] Gas composition <![CDATA[Characteristic Raman shift / cm -1 > Detection limit / ppm Quantitative accuracy / % <![CDATA[H2]]> 588 0.9 98.6 CO 2140 2.5 98.5 <![CDATA[CO2]]> 1388 0.6 98.2 <![CDATA[CH4]]> 2917 0.2 98.7 <![CDATA[C2H6]]> 2955 0.7 98.4 <![CDATA[C2H4]]> 1342 0.3 98.6 <![CDATA[C2H2]]> 1972 0.3 98.8

[0128] The beneficial effects of the present invention are that, compared with the prior art, the gas detection limit of the present invention can reach the ppb level, the system achieves a response in seconds, the optical path structure is simple and easy to adjust, the system stability is high, and the detection results are robust (less affected by factors such as slight changes in the optical path, fluctuations in instrument performance, and degradation of hollow-core optical fiber performance).

[0129] The present disclosure may be a system, method and / or computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0130] A computer-readable storage medium can be a tangible device that can hold and store instructions for use by an instruction execution device. A computer-readable storage medium can be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination thereof. More specific examples (a non-exhaustive list) of computer-readable storage media include: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanical encoding device, such as a punch card or a raised structure in a groove on which instructions are stored, and any suitable combination thereof. As used herein, a computer-readable storage medium is not to be construed as a transient signal per se, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagating through a waveguide or other transmission medium (e.g., a light pulse through a fiber optic cable), or an electrical signal transmitted through an electrical wire.

[0131] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or downloaded to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network can include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. The network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions to be stored in the computer-readable storage medium in each computing / processing device.

[0132] The computer program instructions for performing the operations of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, and conventional procedural programming languages ​​such as "C" language or similar programming languages. Computer-readable program instructions may be executed entirely on a user's computer, partially on a user's computer, as an independent software package, partially on a user's computer, partially on a remote computer, or entirely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., utilizing an Internet service provider to connect via the Internet). In some embodiments, an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), may be personalized by utilizing the state information of the computer-readable program instructions. The electronic circuit may execute the computer-readable program instructions, thereby realizing various aspects of the present disclosure.

[0133] 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 field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A Raman spectroscopic gas analyzer based on hollow-core optical fiber, characterized in that: The system comprises a laser and a concave spherical reflector, an internal standard gas chamber, a first hollow-core fiber adapter, a hollow-core fiber, a second hollow-core fiber adapter, a first focusing lens, a dichroic mirror, a high-pass filter, a second focusing lens, a spectrometer, and a CCD, all arranged in sequence on the same optical axis. A first adjustable aperture is provided between the second hollow-core fiber adapter and the dichroic mirror; a second adjustable aperture is provided between the dichroic mirror and the second focusing lens; the second hollow-core fiber adapter is further connected to a medium-pressure pump system via a gas pipeline and a gas filter; and the optimal aperture is determined by calculating the signal-to-noise ratio of multiple spectral groups to achieve optimal filtering. The air holes around the core of the hollow-core optical fiber are all sealed to ensure that the gas pressure in the core is always greater than the gas pressure in the surrounding air holes during the experiment; The internal standard gas chamber includes a gas chamber body, two optical windows along the two ends of the optical axis, two optical flanges, screws, a sealing ring, a pressure gauge, and an air inlet / outlet, and is used for quantitative analysis of gas Raman spectroscopy; the planes where the two optical windows are located are perpendicular to the laser transmission direction, and the interior of the internal standard gas chamber is filled with a high-purity single standard gas, i.e., the internal standard gas, which is different from the gas to be measured; The concave spherical reflector is used to recouple the laser light and gas Raman scattered light output from the end of the hollow-core optical fiber and passing through the internal standard gas chamber into the core of the hollow-core optical fiber, and finally be detected by the CCD. The curvature r of the concave spherical reflector is equal to the distance d from the center of the mirror to the end of the hollow-core optical fiber. The reflectivity of the concave spherical reflector to the laser light and gas Raman scattered light is not less than 99.5%. The high-pass filter is used to filter stray laser light in the space and Rayleigh scattered light of gas transmitted in the back direction along the optical axis. Its reflectivity for Rayleigh scattered light of gas is not less than 98%, and its transmittance for Raman scattered light of gas is not less than 93%. The mirror surface of the high-pass filter is at 0° with the transmission direction of the laser light. The first adjustable iris, the first focusing lens, the dichroic mirror, the high-pass filter, the second adjustable iris, and the second focusing lens are fixedly connected in a cage system; The hollow-core optical fiber Raman spectroscopy gas analysis method implemented by the analyzer includes: Step 1: Pump the calibration gas of known concentration into the hollow-core optical fiber. After the gas is balanced, the analyzer performs Raman spectroscopy detection and obtains the Raman spectrum S of the calibration gas. c , Raman spectrum of internal standard gas S cs , the pressure reading of the internal standard gas chamber P cs ; Step 2: Fully flush the hollow-core optical fiber with high-purity argon gas; Step 3: Pump the unknown concentration of the gas to be tested into the hollow-core optical fiber. After the gas is balanced, the analyzer performs Raman spectrum detection and obtains the Raman spectrum S of the gas to be tested. m , Raman spectrum of internal standard gas S ms , the pressure reading of the internal standard gas chamber P ms ; Step 4: Obtain S c 、S cs 、S m 、S ms , calculate the calibration gas Raman peak area A c , the Raman peak area of ​​the internal standard gas during calibration A cs , Raman peak area of ​​the gas to be measured A m , the internal standard gas Raman peak area A during measurement ms ; At the same time, the calibration gas concentration c is known c , the pressure reading of the internal standard gas chamber during calibration P cs , the pressure reading of the internal standard gas chamber during measurement P ms ; The concentration of the gas to be measured is calculated according to the following formula:

2. The hollow-core fiber Raman spectroscopic gas analyzer according to claim 1, characterized in that: The laser is a single longitudinal mode, narrow linewidth, continuous wave laser used to excite the spontaneous Raman signal of the gas. Its operating wavelength can be any wavelength in the visible light-near infrared band; the laser linewidth is <0.00001nm; the laser's M 2 Factor <1.

5.

3. The hollow-core fiber Raman spectroscopic gas analyzer according to claim 1, characterized in that: The dichroic mirror is a 45° high-pass filter used to separate laser light and gas Raman scattered light, while filtering gas Rayleigh scattered light. Its reflectivity for Rayleigh scattered light is not less than 98%, and its transmittance for Raman scattered light is not less than 93%. The mirror surface of the dichroic mirror is at 45° to the incident direction of the laser.

4. The hollow-core fiber Raman spectroscopic gas analyzer according to claim 1, characterized in that: The first focusing lens is an achromatic lens, which is used to couple the laser into the core of the hollow-core fiber and collimate the backscattered gas Raman light emitted from the core of the hollow-core fiber; The first focusing lens is mounted on a three-axis adjustable micro-displacement platform. By finely adjusting the spatial position of the lens, the laser spot at the focal point is coupled into the core of the hollow-core fiber. The coupling condition is determined by real-time monitoring of the laser power output at the end of the hollow-core fiber and the shape of the far-field spot. The second focusing lens is an achromatic lens, which is used to couple the gas Raman scattered light into the slit of the spectrometer; The focal length f of the first focusing lens is: λ, D, D MFD They are respectively the laser wavelength, the laser spot size at the focusing lens, and the mode field diameter of the hollow-core optical fiber.

5. The hollow-core fiber-based Raman spectroscopic gas analyzer according to claim 1, characterized in that: The first adjustable iris and the second adjustable iris are used to adjust the aperture size of the iris to perform spatially optimal filtering of the gas Raman scattered light; The first adjustable iris and the second adjustable iris are adjusted to determine the aperture of the optimal filter, and the position adjustment constraint condition is: The first adjustable aperture is located between the dichroic mirror and the second hollow core fiber adapter and is positioned along the optical axis; The second adjustable aperture is located between the dichroic mirror and the second focusing lens and is positioned along the optical axis.

6. The hollow-core fiber-based Raman spectroscopic gas analyzer according to claim 1, characterized in that: The second hollow-core fiber adapter and the first hollow-core fiber adapter include an adapter body, an optical window, an optical flange, screws, a fiber clamp, a sealing ring, a pressure gauge, and an air inlet / outlet. They are used to fix and install the hollow-core fiber and ensure the airtightness of the adapter as a whole. They also achieve laser-hollow-core fiber coupling, gas Raman scattered light collection, and gas inlet / outlet. The gas inlet / outlet of the second hollow-core optical fiber adapter is connected to the medium-pressure pump system through a gas pipeline. The gas to be tested is continuously pumped into the hollow-core optical fiber in a pressure-driven manner. When a stable pressure gradient is established inside the core, the gas inlet / outlet of the second hollow-core optical fiber adapter and the first hollow-core optical fiber adapter are closed, and the gas equilibrium time in the hollow-core optical fiber core is monitored in real time by a pressure gauge.

7. The hollow-core fiber-based Raman spectroscopic gas analyzer according to claim 1, characterized in that: Hollow-core optical fibers include hollow-core photonic bandgap fibers and hollow-core antiresonant fibers, which are used to guide lasers and provide a place for laser-gas interaction and the generation of gas Raman scattering light. Their transmission bandwidth covers the visible light-near-infrared band; the core diameter is in the micron range.

8. The hollow-core fiber-based Raman spectroscopic gas analyzer according to claim 1, characterized in that: CCD, used to detect and record gas Raman scattered light and convert it into electrical signal output; a medium-pressure pump system for pumping gas into the hollow-core optical fiber; a gas transmission pipeline, used for inputting gas from the medium-pressure pump system into the second hollow-core optical fiber adapter; Gas filter, used to absorb moisture and solid particles in the gas to be tested; Spectrometer, used to separate gas Raman scattered light of different wavelengths so that it can be recorded by different detection units of the CCD; Spectrometer slit width W sli t is: W slit =f2d fiber / f1 Wherein, f1 is the focal length of the first focusing lens; f2 is the focal length of the second focusing lens; d fiber is the core diameter of the hollow-core fiber.

9. The hollow-core fiber-based Raman spectroscopic gas analyzer according to claim 1, characterized in that: The process of Raman spectroscopy detection by the analyzer includes: Step 1: The laser emits laser light, which is reflected by a dichroic mirror, focused by a first focusing lens, and then enters a hollow-core optical fiber through a second hollow-core optical fiber adapter; Step 2: The laser and gas interact in the core of the hollow-core fiber, generating forward Raman scattered light L ff and backscattered Raman light L fb ; Step 3: Backscattered Raman light L fb It passes through the second hollow-core fiber adapter, is collimated by the first focusing lens, and filtered out by the dichroic mirror and high-pass filter to remove the laser and Rayleigh scattered light. It is then focused by the second focusing lens and enters the slit of the spectrometer. The gas Raman scattered light is diffracted and split by the spectrometer and then detected by the CCD; Step 4: Forward Raman scattered light L ff Passing through the first hollow-core fiber adapter and the internal standard gas chamber, it re-enters the hollow-core fiber after being coupled by the concave spherical reflector and combines with the backscattered Raman light L in step 3. fb The light is collected by CCD along the same optical path; Step 5: The laser output from the end of the hollow-core fiber passes through the first hollow-core fiber adapter and the internal standard gas chamber, and re-enters the hollow-core fiber after being coupled by the concave spherical reflector, and interacts with the gas in the hollow-core fiber again, generating forward gas Raman scattered light L bf The backscattered light L from step 3 fb The backscattered light L is collected along the same optical path. bb After passing through the first hollow-core fiber adapter and the internal standard gas chamber, it is coupled again by the concave spherical reflector and then re-enters the hollow-core fiber to combine with the backscattered light L in step 3. fb are collected along the same optical path.

10. The hollow-core optical fiber-based Raman spectroscopic gas analyzer according to claim 9, characterized in that: When the analyzer performs Raman spectrum detection, the aperture sizes of the first adjustable iris and the second adjustable iris are slowly adjusted, and the Raman spectrum of the gas detected by the CCD is recorded at the same time; When the analyzer performs Raman spectroscopy detection, the gas inlet / outlet of the second hollow-core optical fiber adapter is connected to the medium-pressure pump system through the gas pipeline, and the gas inlet / outlet of the first hollow-core optical fiber adapter remains open; The medium-pressure pump system provides constant pressure to continuously pump gas into the second hollow-core optical fiber adapter and into the hollow-core optical fiber. When a stable pressure gradient is established inside the hollow-core optical fiber core, the inlet / outlet ports of the second hollow-core optical fiber adapter and the first hollow-core optical fiber adapter are closed, and the gas equilibrium time in the hollow-core optical fiber core is monitored in real time using a pressure gauge.

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