CN115950872B - A Raman spectroscopic gas analyzer and analysis method based on hollow-core optical fiber - Google Patents
A Raman spectroscopic gas analyzer and analysis method based on hollow-core optical fiberInfo
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- CN115950872B CN115950872B CN202211362998.0A CN202211362998A CN115950872B CN 115950872 B CN115950872 B CN 115950872B CN 202211362998 A CN202211362998 A CN 202211362998A CN 115950872 B CN115950872 B CN 115950872B
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Abstract
A Raman spectrum gas analyzer based on hollow fiber comprises a laser, a concave spherical reflecting mirror, an internal standard gas chamber, a first hollow fiber adapter, a hollow fiber, a second hollow fiber adapter, a first focusing lens, a dichroic mirror, a high-pass filter, a second focusing lens, a spectrometer and a CCD which are sequentially arranged on the same optical axis, wherein a first adjustable diaphragm is arranged between the second hollow fiber adapter and the dichroic mirror, a second adjustable diaphragm is arranged between the dichroic mirror and the second focusing lens, and the second hollow fiber adapter is also connected with a medium-pressure pump system through a gas pipeline and a gas filter. The invention can reduce the gas detection limit, shorten the response time of the system, reduce the difficulty of light path adjustment and the fluctuation of detection results, and improve the stability of the system.
Description
Technical Field
The invention belongs to the technical field of gas analysis, and particularly relates to a Raman spectrum gas analyzer based on hollow fiber and an analysis method.
Background
The Raman spectrum is an emerging molecular fingerprint spectrum technology, has the advantages of high selectivity, no calibration, no contact/damage/no consumption of samples, no component separation and the like, can simultaneously detect almost all gases except rare gases by utilizing single-wavelength laser, and has wide application prospect in the field of multi-component gas analysis. However, the gas raman effect is extremely weak, and the raman signal intensity needs to be improved by a corresponding technical means so as to reach the detection limit of practical application requirements.
The microstructure hollow fiber such as hollow photon band gap fiber, hollow anti-resonance fiber and the like has the characteristics of low transmission loss, wide transmission band, low dispersion and the like, can be used as a miniature air chamber while guiding laser efficiently, greatly prolongs the effective action path length of laser-gas, improves the collection efficiency of space Raman scattered photons, has extremely small gas demand, and is a hotspot for research of a domestic and foreign gas Raman spectrum enhancement method.
The detection limit of the traditional fiber reinforced Raman spectrum technology on gas is in ppm level, and although the detection limit can be lower by extending the length of the hollow fiber, the response time of the system, namely the time of gas inlet/outlet of the hollow fiber, is increased in square times along with the increase of the length, which limits the real-time detection and analysis of trace gas in specific applications. In addition, the detection result of the traditional optical fiber reinforced Raman spectrum technology is greatly influenced by factors such as light path tiny fluctuation, instrument performance fluctuation, hollow fiber performance degradation and the like.
Disclosure of Invention
In order to solve the defects in the prior art, the invention provides a Raman spectrum gas analyzer and an analysis method based on hollow fiber, which can reduce the gas detection limit, shorten the system response time, reduce the light path adjustment difficulty and the detection result fluctuation, and improve the system stability.
The invention adopts the following technical scheme.
A Raman spectrum gas analyzer based on hollow fiber comprises a laser, a concave spherical reflecting mirror, an internal standard gas chamber, a first hollow fiber adapter, a hollow fiber, a second hollow fiber adapter, a first focusing lens, a dichroic mirror, a high-pass filter, a second focusing lens, a spectrometer and a CCD which are sequentially arranged on the same optical axis;
a first adjustable diaphragm is arranged between the second hollow fiber adapter and the dichroic mirror;
a second adjustable diaphragm is arranged between the dichroic mirror and the second focusing lens;
The second hollow fiber adapter is also connected with the medium pressure pump system through a gas pipeline and a gas filter.
The invention further comprises the following preferable schemes:
Preferably, the laser is a single longitudinal mode, narrow linewidth and continuous wave laser, is used for exciting spontaneous Raman signals of gas, the working wavelength of the laser can be any wavelength of visible light-near infrared wave bands, the linewidth of the laser is less than 0.00001nm, and the M 2 factor of the laser is less than 1.5.
Preferably, the dichroic mirror is a 45 ° high-pass filter for separating laser light from gas raman scattered light while filtering the gas rayleigh scattered light, which has a reflectance of not less than 98% for the rayleigh scattered light and a transmittance of not less than 93% for the raman scattered light, and the mirror surface of the dichroic mirror is 45 ° to the laser light incident direction.
Preferably, the first focusing lens is an achromatic lens for coupling laser light into the core of the hollow core optical fiber while collimating back-gas raman scattered light emitted from the core of the hollow core optical fiber;
The first focusing lens is arranged on the triaxial adjustable micro-displacement platform, the laser light spot at the focus is coupled into the fiber core of the hollow fiber by finely adjusting the spatial position of the lens, and the coupling condition is judged by monitoring the laser power output by the tail end of the hollow fiber and the far-field light spot shape in real time;
the second focusing lens is an acromatic lens for coupling the gas raman scattered light into the slit of the spectrometer.
Preferably, the focal length f of the first focusing lens is:
Lambda and D, D MFD are the laser wavelength, the spot size of the laser at the focusing lens and the mode field diameter of the hollow fiber, respectively.
Preferably, the first adjustable diaphragm and the second adjustable diaphragm are used for adjusting the clear aperture size of the diaphragm to perform space optimal filtering of the gas Raman scattered light;
The first adjustable diaphragm and the second adjustable diaphragm determine the diaphragm aperture of optimal filtering through position adjustment, and the constraint conditions of the position adjustment are as follows:
A first tunable aperture is positioned between the dichroic mirror and the second hollow fiber adapter and along the optical axis;
a second adjustable aperture is positioned between the dichroic mirror and the second focusing lens and along the optical axis.
Preferably, the second hollow fiber adapter and the first hollow fiber adapter comprise an adapter body, an optical window, an optical flange plate, a screw, a fiber clamp, a sealing ring, a pressure gauge and an air inlet/outlet, and are used for fixing and installing the hollow fiber and guaranteeing the integral air tightness of the adapter, and simultaneously realizing the coupling of laser and the hollow fiber, the collection of gas Raman scattered light and the gas inlet/outlet;
The air inlet/outlet of the second hollow fiber adapter is connected with the medium pressure pump system through the gas pipeline, the gas to be detected is continuously pumped into the hollow fiber in a pressure driving mode, the air inlet/outlet of the second hollow fiber adapter and the air inlet/outlet of the first hollow fiber adapter are closed when a stable pressure gradient is built in the fiber core, and the gas balance time in the fiber core of the hollow fiber is monitored in real time through the pressure gauge.
Preferably, the hollow fiber comprises a hollow photonic bandgap fiber and a hollow antiresonant fiber, and is used for guiding laser and providing a place for the interaction of laser and gas, and the generation of gas Raman scattered light has a transmission bandwidth covering visible light-near infrared band;
The air holes around the hollow fiber cores are all sealed so as to ensure that the gas pressure of the cores is always greater than the gas pressure in the surrounding air holes in the experimental process.
Preferably, the internal standard gas chamber comprises a gas chamber body, 2 optical windows along two ends of an optical axis, 2 optical flanges, screws, a sealing ring, a pressure gauge and a gas inlet/outlet, and is used for quantitative analysis of gas Raman spectrum;
The plane where the 2 optical windows are located is perpendicular to the laser transmission direction, and the interior of the internal standard gas chamber is filled with high-purity single standard gas, namely internal standard gas, and the type of the internal standard gas is different from that of the gas to be measured.
Preferably, the concave spherical reflecting mirror is used for re-coupling the laser and the gas Raman scattered light which are output from the tail end of the hollow fiber and pass through the first hollow fiber adapter and the internal standard gas chamber into the fiber core of the hollow fiber, and finally detected by the CCD;
The curvature r of the concave spherical reflecting mirror is equal to the distance d from the center of the mirror surface to the tail end of the hollow optical fiber;
the reflectivity of the mirror surface of the concave spherical reflecting mirror to laser and gas Raman scattered light is not lower than 99.5%.
Preferably, the high-pass filter is used for filtering the scattered laser light in the space and the gas Rayleigh scattered light transmitted back along the optical axis, the reflectivity of the gas Rayleigh scattered light is not lower than 98%, and the transmittance of the gas Rayleigh scattered light is not lower than 93%;
the mirror surface of the high-pass filter mirror forms 0 degree with the laser transmission direction.
Preferably, the spectrometer is used for separating the gas raman scattered light with different wavelengths so that the gas raman scattered light can be recorded by different detection units of the CCD;
The slit width W slit of the spectrometer is:
Wslit＝f2dfiber/f1。
Wherein f 1 is the focal length of the first focusing lens;
f 2 is the focal length of the second focusing lens;
d fiber is the core diameter of the hollow core fiber.
Preferably, the CCD is used for detecting and recording gas Raman scattered light and converting the gas Raman scattered light into an electric signal to be output;
The medium pressure pump system is used for pumping gas into the hollow optical fiber;
The gas transmission pipeline is used for inputting gas from the medium pressure pump system into the second hollow fiber adapter;
the gas filter is used for adsorbing moisture and solid particles in the gas to be detected;
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.
A raman spectroscopy gas analysis method based on hollow fiber, the method comprising:
Step one, pumping calibration gas with known concentration into a hollow fiber, carrying out Raman spectrum detection by an analyzer after gas balance, and simultaneously obtaining a Raman spectrum S c of the calibration gas, a Raman spectrum S cs of the internal standard gas and a pressure reading P cs of an internal standard gas chamber;
step two, fully flushing the hollow optical fiber by using high-purity argon;
Pumping gas to be detected with unknown concentration into the hollow fiber, carrying out Raman spectrum detection by an analyzer after gas balance, and simultaneously obtaining a Raman spectrum S m of the gas to be detected, a Raman spectrum S ms of the internal standard gas and a pressure reading P ms of the internal standard gas chamber;
Step four, calculating a gas Raman peak area A c for calibration, an internal standard gas Raman peak area A cs for calibration, a gas Raman peak area A m to be measured and an internal standard gas Raman peak area A ms for measurement respectively through the acquired S c、Scs、Sm、Sms;
Meanwhile, the gas concentration c c for calibration, the pressure reading P cs of the internal standard gas chamber during calibration and the pressure reading P ms of the internal standard gas chamber during measurement are known;
the concentration of the gas to be measured is calculated according to the following formula:
preferably, the process of raman spectrum detection by the analyzer comprises:
Step 1, a laser emits laser, and the laser enters a hollow fiber through a second hollow fiber adapter after being reflected by a dichroic mirror and focused by a first focusing lens;
step 2, the laser interacts with the gas in the fiber core of the hollow fiber to generate forward Raman scattered light L ff and backward Raman scattered light L fb;
Step 3, back Raman scattered light L fb passes through a second hollow fiber adapter, is collimated by a first focusing lens, is filtered by a dichroic mirror and a high-pass filter, and enters a slit of a spectrometer after being focused by the second focusing lens;
The gas Raman scattered light is detected by a CCD after being diffracted and split by a spectrometer;
step 4, forward Raman scattered light L ff passes through the first hollow fiber adapter and the internal standard gas chamber, and then re-enters the hollow fiber after being coupled by the concave spherical reflector, and is detected and collected by the CCD along the same light path with the backward Raman scattered light L fb in the step 3;
And 5, enabling laser output by the tail end of the hollow fiber to pass through the first hollow fiber adapter and the internal standard gas chamber, coupling by the concave spherical reflector, re-entering the hollow fiber, enabling the laser to interact with gas in the hollow fiber again, enabling the generated forward gas Raman scattered light L bf and the generated back scattered light L fb of the step 3 to be collected along the same light path, enabling the generated back scattered light L bb to pass through the first hollow fiber adapter and the internal standard gas chamber, re-entering the hollow fiber after coupling by the concave spherical reflector again, and collecting the back scattered light L fb of the step 3 along the same light path.
Preferably, when the analyzer detects Raman spectrum, the aperture sizes of the first adjustable diaphragm and the second adjustable diaphragm are slowly adjusted, simultaneously, the Raman spectrum of gas detected by the CCD is recorded, and the optimal diaphragm aperture is determined by calculating a plurality of groups of spectrum signal to noise ratios so as to realize optimal filtering.
Preferably, when the analyzer detects raman spectrum, the air inlet/outlet of the second hollow fiber adapter is connected with the medium pressure pump system through a gas pipeline, and the air inlet/outlet of the first hollow fiber adapter is kept open;
the medium pressure pump system provides constant pressure to continuously pump gas into the second hollow fiber adapter and enter the hollow fiber, the second hollow fiber adapter and the gas inlet/outlet of the first hollow fiber adapter are closed when a stable pressure gradient is built in the hollow fiber, and the gas balance time in the hollow fiber is monitored in real time through the pressure gauge.
Compared with the prior art, the invention has the advantages that the gas detection limit can reach ppb level, the system realizes second level response, the light path structure is simple, the adjustment is easy, the system stability is high, the robustness of the detection result is good (the influence by factors such as light path tiny fluctuation, instrument performance fluctuation, hollow fiber performance degradation is less), and the like, and the invention has the following characteristics:
1. According to the invention, the air holes around the hollow fiber core are sealed, so that the air pressure of the fiber core is always higher than the air pressure in the surrounding air holes in the experimental process, namely the medium refractive index of the fiber core is higher than the equivalent medium refractive index of the surrounding air holes, the transmission efficiency of the gas Raman scattered light in the hollow fiber core can be improved, and the strength of the detected gas Raman signal can be further improved.
2. The invention re-couples the laser and gas Raman scattered light which are output by the tail end of the hollow fiber and pass through the internal standard gas chamber into the fiber core of the hollow fiber by arranging the concave spherical reflecting mirror at a certain distance along the optical axis at the tail end of the hollow fiber, and finally detects the laser and the gas Raman scattered light by the CCD, thereby improving the utilization efficiency of the laser and the collection efficiency of the gas Raman scattered light (under the condition of not considering optical loss) and improving the gas Raman signal to 4 times at the same time;
3. According to the invention, two adjustable diaphragms are inserted into the optical axis, and the size of the clear aperture of each diaphragm is adjusted to perform space optimal filtering of gas Raman scattered light, so that compared with the traditional space filtering system, the structure is simple, the light path alignment requirement is low, the adjustment difficulty is low, and the application cost is low;
4. According to the invention, the maximum gas Raman spectrum signal-to-noise ratio, namely the minimum gas detection limit, is obtained by setting the width W slit of the slit of the optimal spectrometer.
Wslit＝f2dfiber/f1
Wherein f 1 is the focal length of the first focusing lens;
f 2 is the focal length of the second focusing lens;
d fiber is the core diameter of the hollow core fiber.
5. According to the invention, an internal standard air chamber is arranged at the tail end of the hollow optical fiber at a certain distance along the optical axis, so that the influence of factors such as light path tiny variation (laser-hollow optical fiber coupling, gas Raman signal-spectrometer slit coupling and the like), instrument performance fluctuation, hollow optical fiber performance degradation and the like on the detection result of the gas analyzer is reduced;
6. the invention pumps the gas to be measured into the hollow fiber and passes through the gas filter so as to adsorb solid particles such as moisture, dust and the like in the gas to be measured, and can prevent the solid particles from being adsorbed on the inner wall of the fiber to cause pollution, thereby causing the performance degradation of the hollow fiber.
Drawings
FIG. 1 is a schematic diagram of a Raman spectrum gas analyzer based on hollow fiber according to the present invention;
FIG. 2 is a schematic diagram of two exemplary microstructured hollow-core optical fibers in accordance with an embodiment of the present invention;
The reference numerals in FIG. 1 are 1-laser, 2-dichroic mirror, 3-first focusing lens, 4-first adjustable diaphragm, 5-second hollow fiber adapter, 6-hollow fiber, 7-first hollow fiber adapter, 8-internal standard gas cell, 9-concave spherical mirror, 10-high pass filter, 11-second adjustable diaphragm, 12-second focusing lens, 13-spectrometer, 14-CCD, 15-medium pressure pump system, 16-gas pipeline, 17-gas filter, 18-cage system.
Detailed Description
In order to make the objects, technical solutions and advantages of the present application more apparent, the technical solutions of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. The described embodiments of the application are only some, but not all, embodiments of the application. All other embodiments, which can be made by those skilled in the art without making any inventive effort, are within the scope of the present application.
As shown in fig. 1, embodiment 1 of the present invention provides a raman spectrum gas analyzer based on hollow fiber, which in a preferred but non-limiting embodiment of the present invention comprises a laser 1 and a concave spherical reflecting mirror 9, an internal standard gas cell 8, a first hollow fiber adapter 7, a hollow fiber 6, a second hollow fiber adapter 5, a first tunable diaphragm 4, a first focusing lens 3, a dichroic mirror 2, a high-pass filter 10, a second tunable diaphragm 11, a second focusing lens 12, a spectrometer 13 and a CCD14, which are sequentially arranged on the same optical axis;
and the second hollow fiber adapter 5 is also connected with the medium pressure pump system 15 through a gas pipeline 16 and a gas filter 17.
Further preferably, the laser 1 is a single longitudinal mode, narrow linewidth, continuous wave laser for exciting spontaneous raman signals of the gas. The working wavelength can be any wavelength of visible light-near infrared wave band, and typical values include but are not limited to 532nm, 633nm, 642nm and 785nm, laser linewidth <0.00001nm and M 2 factor <1.5 of the laser.
The dichroic mirror 2 is a 45 DEG high-pass filter and is mainly used for separating laser light and gas Raman scattered light, and filtering most of gas Rayleigh scattered light. The reflectivity of the high-pass filter is not lower than 98% to laser (Rayleigh scattered light) and the transmissivity of the high-pass filter to the Raman scattered light with longer wavelength is not lower than 93%, and when the high-pass filter is used, the mirror surface of the high-pass filter is placed at 45 degrees with the incidence direction of the laser.
The first focusing lens 3 is an achromatic lens, and is used for coupling laser into the fiber core of the hollow fiber 6, and simultaneously collimating the back gas raman scattered light emitted by the fiber core of the hollow fiber, and the focal length f 1 of the first focusing lens needs to comprehensively consider parameters such as the laser wavelength lambda, the spot size D of the laser at the focusing lens, the mode field diameter D MFD of the hollow fiber, and the like, and the specific calculation formula is as follows: When the three-axis micro-displacement adjustable optical fiber is used, the focusing lens is arranged on the three-axis micro-displacement adjustable platform, the laser light spots at the focus are coupled into the fiber cores of the hollow optical fibers by finely adjusting the spatial positions of the lens, and the coupling condition is judged by monitoring the laser power and the far-field light spot shape output by the tail ends of the hollow optical fibers in real time.
The second focusing lens 12 is an acromatic lens for coupling gas raman scattered light into the slit of the spectrometer. The focal length is f 2.
A first adjustable aperture 4 for spatial filtering of the gas raman scattered light. Specifically, the laser interacts with the end face of the hollow fiber, the inner wall of the hollow fiber, and the like to generate 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), and further limits the detection limit of the gas analyzer, so that the gas analyzer must be "cleaned" (filtered) by adopting a spatial filtering mode. The traditional space filtering system consists of a focusing lens, a pinhole (the aperture is in the micron order), a collimating lens and the like, and has the advantages of complex structure, difficult light path alignment and higher cost (ten thousand yuan).
According to the invention, the adjustable diaphragm is inserted into the light path, and the size (millimeter magnitude) of the clear aperture of the diaphragm is adjusted to carry out spatial filtering, so that the device has a simple structure and is easy to operate, and meanwhile, the application cost is greatly reduced.
When the CCD-based gas Raman spectrum measuring device is used, the aperture size of the diaphragm is slowly adjusted, the Raman spectrum of gas detected by the CCD is recorded, and the optimal diaphragm aperture is determined by calculating the signal-to-noise ratio of a plurality of groups of spectra. It is noted that the position of the first adjustable aperture 4 in the optical path is not particularly required, but must be located between the dichroic mirror 2 and the second hollow fiber adapter 5 and must be placed along the optical axis. According to different placement positions, the optimal diaphragm aperture is different.
The second adjustable diaphragm 11 is matched with the first adjustable diaphragm 4 and is used for spatial filtering of the gas Raman scattered light. When the CCD-based gas Raman spectrum measuring device is used, the aperture size of the diaphragm is slowly adjusted, the Raman spectrum of gas detected by the CCD is recorded, and the optimal diaphragm aperture is determined by calculating the signal-to-noise ratio of a plurality of groups of spectra. It is noted that the position of the second adjustable stop 11 in the light path is likewise not particularly required, but must be located between the dichroic mirror 2 and the second focusing lens 12 and must be placed along the optical axis. The optimal diaphragm aperture is also different according to the placement positions.
The second hollow fiber adapter 5 is composed of an adapter body, an optical window, an optical flange plate, a screw, an optical fiber clamp, a sealing ring, a pressure gauge, an air inlet/outlet and the like, and is used for fixing and installing the hollow fiber and guaranteeing the integral air tightness of the adapter, and can realize the coupling (collection of gas Raman scattered light) and gas inlet/outlet of the laser-hollow fiber. When the pressure sensor is used, the air inlet/outlet of the pressure sensor is connected with the medium pressure pump system 15 through the air transmission pipeline 16, the air to be measured is continuously pumped into the hollow fiber in a pressure driving mode, the air inlet/outlet is closed when a stable pressure gradient is built in the fiber core, and the air balance time in the fiber core of the hollow fiber is monitored in real time through the pressure gauge.
The first hollow fiber adapter 7 is composed of an adapter body, an optical window, an optical flange plate, a screw, an optical fiber clamp, a sealing ring, a pressure gauge, an air inlet/outlet and the like, and is used for fixing and installing the hollow fiber and guaranteeing the integral air tightness of the adapter, and can realize the coupling (collection of gas Raman scattered light) and gas inlet/outlet of the laser-hollow fiber. When the pressure meter is used, the air inlet/outlet is kept open in advance, the air inlet/outlet is closed when a stable pressure gradient is established in the fiber core, and the air balance time in the hollow fiber core is monitored in real time through the pressure meter.
As shown in fig. 2, the hollow core optical fiber 6 specifically includes a hollow core photonic bandgap fiber (HC-PBGF) and a hollow core antiresonant fiber (HC-ARF) for guiding laser light and providing a desired place for laser-gas interaction (generation of gas raman scattered light). The transmission bandwidth covers the visible light-near infrared band and mainly comprises 500-1200 nm, and the diameter d fiber of the fiber core is in the micrometer level, and the typical value is 20-50 mu m. When the device is used, the air holes around the hollow fiber cores are all sealed (arc discharge causes collapse of the air holes), so that the gas pressure (usually a few atmospheres) of the cores in the experimental process is always higher than the gas pressure (1 atmosphere) in the surrounding air holes, and the effective refractive index of the cores is higher than that of the surrounding air holes. The method can effectively improve the transmission efficiency of the gas Raman scattered light in the hollow fiber core, thereby improving the gas Raman signal intensity.
The internal standard gas chamber 8 consists of a gas chamber body, 2 optical windows along two ends of an optical axis, 2 optical flanges, screws, a sealing ring, a pressure gauge, a gas inlet/outlet and the like, and is used for quantitative analysis of a gas Raman spectrum so as to reduce the influence of factors such as light path tiny fluctuation (laser-hollow fiber coupling, gas Raman signal-spectrometer slit coupling and the like), instrument performance fluctuation, hollow fiber performance degradation and the like on a detection result of a 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 of the 2 optical windows of the gas chamber), and the inside is filled with 20bar of high-purity single standard gas (the type of the internal standard gas is different from that of the gas to be measured). Specifically, firstly, a calibration gas with known concentration is pumped into a hollow fiber, raman spectrum detection is carried out after gas balance, meanwhile, the Raman spectrum S c of the calibration gas, the Raman spectrum S cs of an internal standard gas and the pressure reading P cs of an internal standard gas chamber are obtained, the process is a calibration process, the hollow fiber is fully flushed by high-purity argon, a gas to be detected with unknown concentration is pumped into the hollow fiber, raman spectrum detection is carried out after gas balance, and meanwhile, the Raman spectrum S m of the gas to be detected is obtained, Raman spectrum S ms of the internal standard gas, pressure reading P ms of the internal standard gas cell, this process is the measurement process. The obtained S c、Scs、Sm、Sms can calculate the gas Raman peak intensity (peak area, peak height) I c for calibration, the internal standard gas Raman peak intensity I cs for calibration, the gas Raman peak intensity I m to be measured, The Raman peak intensity I ms of the internal standard gas during measurement is known, and the gas concentration c c for calibration, the internal standard gas concentration c cs for calibration and the internal standard gas concentration c ms during measurement are known, so that the gas concentration to be measuredWherein c ms/ccs can be represented by the ratio P ms/Pcs of the pressures recorded by the pressure gauge connected to the internal standard gas cell at calibration. At this time, the concentration of the gas to be measured
The concave spherical reflecting mirror 9 is used for re-coupling the laser and the gas Raman scattered light which are output from the tail end of the hollow fiber and pass through the internal standard gas chamber into the fiber core of the hollow fiber, and finally, the laser and the gas Raman scattered light are detected by the CCD. Ideally (irrespective of the loss of the optical elements of each part) the method increases the detected gas raman signal by a factor of 4. The curvature r of the concave spherical reflecting mirror is equal to the distance d from the center of the mirror surface to the tail end of the hollow optical fiber, and the reflectivity of the mirror surface of the concave spherical reflecting mirror to laser and gas Raman scattered light is not lower than 99.5%.
A high pass filter 10 for filtering stray laser light in space and gas rayleigh scattered light transmitted back along the optical axis. The reflectivity of the laser light (gas Rayleigh scattered light) is not lower than 98%, and the transmittance of the laser light (gas Rayleigh scattered light) with longer wavelength is not lower than 93%. When in use, the mirror surface of the high-pass filter is placed at an angle of 0 DEG with the laser transmission direction.
A spectrometer 13 for separating the gas raman scattered light of different wavelengths so that it can be recorded by different detection units of the CCD. For a particular hollow fiber raman spectroscopy gas analyzer system, there is an optimum value for the slit width W slit of the spectrometer, typically W slit＝f2dfiber/f1.
Wherein f 1 is the focal length of the first focusing lens 3;
f 2 is the focal length of the second focusing lens 12;
d fiber is the core diameter of the hollow core fiber.
And a CCD14 for detecting and recording the Raman scattered light of the gas and converting it into an electric signal for output.
A medium pressure pump system 15 for pumping gas into the hollow core fiber. Which can provide a pressure of not less than 5 bar.
A gas line 16 for inputting gas from the intermediate pumping system into the second hollow fiber adapter 5. The pipe of the gas transmission pipeline is synthetic metal, and has high temperature resistance, corrosion resistance, high mechanical strength and strong plasticity.
And the gas filter 17 is used for adsorbing solid particles such as moisture, dust and the like in the gas to be detected, and preventing the solid particles from being pumped into the hollow fiber and being adsorbed on the inner wall of the fiber to cause pollution, thereby causing performance degradation of the hollow fiber.
The cage system 18 is characterized in that a first adjustable diaphragm 4, a first focusing lens 3, a dichroic mirror 2, a high-pass filter lens 10, a second adjustable diaphragm 11 and a second focusing lens 12 are fixedly connected in the cage system 18, so that light path tiny changes caused by factors such as mechanical looseness, external disturbance and the like are reduced, and the accuracy of a detection result of the gas analyzer is affected.
The embodiment 2 of the invention provides a Raman spectrum gas analysis method based on an air-core optical fiber, which is realized based on the analyzer, and comprises the following steps:
Step one, pumping calibration gas with known concentration into a hollow fiber, carrying out Raman spectrum detection by an analyzer after gas balance, and simultaneously obtaining a Raman spectrum S c of the calibration gas, a Raman spectrum S cs of the internal standard gas and a pressure reading P cs of an internal standard gas chamber;
step two, fully flushing the hollow optical fiber by using high-purity argon;
Pumping gas to be detected with unknown concentration into the hollow fiber, carrying out Raman spectrum detection by an analyzer after gas balance, and simultaneously obtaining a Raman spectrum S m of the gas to be detected, a Raman spectrum S ms of the internal standard gas and a pressure reading P ms of the internal standard gas chamber;
Step four, calculating a gas Raman peak area A c for calibration, an internal standard gas Raman peak area A cs for calibration, a gas Raman peak area A m to be measured and an internal standard gas Raman peak area A ms for measurement respectively through the acquired S c、Scs、Sm、Sms;
Meanwhile, the gas concentration c c for calibration, the pressure reading P cs of the internal standard gas chamber during calibration and the pressure reading P ms of the internal standard gas chamber during measurement are known;
the concentration of the gas to be measured is calculated according to the following formula:
in the first to fourth steps, the inside of the internal standard gas chamber is always filled with high-purity single standard gas.
Further preferably, the process of raman spectrum detection by the analyzer comprises:
Step 1, a laser 1 emits laser, and the laser is reflected by a dichroic mirror 2, focused by a first focusing lens 3 and enters a hollow fiber 6 through a second hollow fiber adapter 5;
step 2, the laser interacts with the gas in the fiber core of the hollow fiber to generate forward Raman scattered light L ff and backward Raman scattered light L fb;
Step 3, back raman scattered light L fb passes through a second hollow fiber adapter 5, is collimated by a first focusing lens 3, is filtered by a dichroic mirror 2 and a high-pass filter 10, and enters a slit of a spectrometer 13 after being focused by a focusing lens 12;
the gas Raman scattered light is detected by CCD14 after being diffracted and split by a spectrometer 13;
Step 4, forward Raman scattered light L ff passes through the first hollow fiber adapter 7 and the internal standard gas chamber 8, and is coupled by the concave spherical reflector 9, then enters the hollow fiber 6 again, and is detected and collected by the CCD14 along the same light path with the backward Raman scattered light L fb in step 3;
Step 5, the laser output from the tail end of the hollow fiber passes through the internal standard gas chamber 8, and is coupled by the concave spherical reflector 9, and then reenters the hollow fiber 6, and interacts with the gas in the hollow fiber 6 again, the generated forward gas raman scattered light L bf and the generated back scattered light L fb in step 3 are collected along the same optical path, and the generated back scattered light L bb is coupled by the concave spherical reflector 9 again, and then reenters the hollow fiber 6, and is collected along the same optical path with the back scattered light L fb in step 3.
When the analyzer detects Raman spectrum, the aperture sizes of the first adjustable diaphragm 4 and the second adjustable diaphragm 11 are slowly adjusted, the Raman spectrum of gas detected by the CCD is recorded, and the optimal diaphragm aperture is determined by calculating a plurality of groups of spectrum signal to noise ratios so as to realize optimal filtering.
The first adjustable diaphragm 4 and the second adjustable diaphragm 11 are used in combination for filtering a silica background raman signal distributed along the radial direction of the optical axis.
When the analyzer detects Raman spectrum, the air inlet/outlet of the second hollow fiber adapter 5 is connected with the medium pressure pump system 15 through the gas pipeline 16, and the air inlet/outlet of the first hollow fiber adapter 7 is kept open;
The medium pressure pump system provides constant pressure of 5bar to continuously pump gas into the hollow fiber adapter 5 and enter the hollow fiber, the gas inlet/outlet of the second hollow fiber adapter 5 and the gas outlet of the first hollow fiber adapter 7 are closed when a stable pressure gradient is built in the hollow fiber, the gas balance time in the hollow fiber is monitored in real time through the pressure gauge, and experimental records show that the gas balance time is within 20 s.
In example 2, the wavelength of the laser 1 was 532nm, the output laser power was 1.5W, the core of the hollow fiber 6 was 26 μm, the use length was 50cm, the focal length f 1 of the first focusing lens 3 was 60mm, the curvature r of the concave spherical mirror 9 was 60mm, the focal length f 2 of the second focusing lens 12 was 150mm, the slit width W slit of the spectrometer 13 was 65 μm, the optimal aperture of the first tunable diaphragm 4 was 3mm, the optimal aperture of the second tunable diaphragm 11 was 0.8mm, and the internal standard gas was SF 6.
The obtained transformer fault characteristic gas detection limit and quantitative accuracy are shown in table 1.
Table 1 transformer fault signature gas detection limits and quantitative accuracy
| Gas composition | Characteristic Raman frequency shift/cm -1 | Detection limit/ppm | Quantitative accuracy/% |
| H2 | 588 | 0.9 | 98.6 |
| CO | 2140 | 2.5 | 98.5 |
| CO2 | 1388 | 0.6 | 98.2 |
| CH4 | 2917 | 0.2 | 98.7 |
| C2H6 | 2955 | 0.7 | 98.4 |
| C2H4 | 1342 | 0.3 | 98.6 |
| C2H2 | 1972 | 0.3 | 98.8 |
Compared with the prior art, the invention has the advantages that the gas detection limit can reach ppb level, the system realizes second level response, the light path structure is simple, the adjustment is easy, the system stability is high, the robustness of the detection result is good (the influence by factors such as light path tiny fluctuation, instrument performance fluctuation, hollow fiber performance degradation is less), and the like.
The present disclosure may be a system, method, and/or computer program product. The computer program product may include a computer readable storage medium having computer readable program instructions embodied thereon for causing a processor to implement aspects of the present disclosure.
The computer readable storage medium may be a tangible device that can hold and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer-readable storage medium include a portable computer diskette, 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 Disc (DVD), a memory stick, a floppy disk, a mechanical encoding device, punch cards or intra-groove protrusion structures such as those having instructions stored thereon, and any suitable combination of the foregoing. Computer-readable storage media, as used herein, are not to be construed as transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., optical pulses through fiber optic cables), or electrical signals transmitted through wires.
The computer readable program instructions described herein may be downloaded from a computer readable storage medium to a respective computing/processing device or to an external computer or external storage device over a network, such as the internet, a local area network, a wide area network, and/or a wireless network. The network may include copper transmission cables, fiber optic transmissions, wireless transmissions, routers, firewalls, switches, gateway computers and/or edge servers. The network interface card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium in the respective computing/processing device.
The computer program instructions for performing the operations of the present disclosure may be assembly instructions, instruction Set Architecture (ISA) instructions, machine-related instructions, microcode, firmware instructions, state setting data, or source or object code written in any combination of one or more programming languages, including an object oriented programming language such as SMALLTALK, C ++ or the like and conventional procedural programming languages, such as the "C" programming language or similar programming languages. The computer readable program instructions may be executed entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer through any kind of network, including a Local Area Network (LAN) or a Wide Area Network (WAN), or may be connected to an external computer (for example, through the Internet using an Internet service provider). In some embodiments, aspects of the present disclosure are implemented by personalizing electronic circuitry, such as programmable logic circuitry, field Programmable Gate Arrays (FPGAs), or Programmable Logic Arrays (PLAs), with state information of computer readable program instructions, which can execute the computer readable program instructions.
Finally, it should be noted that the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made to the specific embodiments of the present invention without departing from the spirit and scope of the present invention, and any modifications and equivalents are intended to be included in the scope of the claims of the present invention.
Claims (10)
1. A Raman spectrum gas analyzer based on hollow optical fibers is characterized by comprising a laser, a concave spherical reflecting mirror, an internal standard gas chamber, a first hollow optical fiber adapter, a hollow optical fiber, a second hollow optical fiber adapter, a first focusing lens, a dichroic mirror, a high-pass filter, a second focusing lens, a spectrometer and a CCD which are sequentially arranged on the same optical axis, wherein a first adjustable diaphragm is arranged between the second hollow optical fiber adapter and the dichroic mirror;
The air holes around the hollow fiber cores are all sealed, so that the air pressure of the cores is always higher than the air pressure in the surrounding air holes in the experimental process;
the internal standard gas chamber comprises a gas chamber body, 2 optical windows along two ends of an optical axis, 2 optical flanges, screws, a sealing ring, a pressure gauge and a gas inlet/outlet port, and is used for quantitative analysis of gas Raman spectrum, wherein the plane of the 2 optical windows is perpendicular to the laser transmission direction, and the internal standard gas chamber is filled with high-purity single standard gas, namely internal standard gas, and the type of the internal standard gas is different from that of the gas to be measured;
The concave spherical reflector is used for re-coupling the laser and the gas Raman scattered light which are output from the tail end of the hollow optical fiber and pass through the internal standard gas chamber into the fiber core of the hollow optical fiber and finally detecting the laser and the gas Raman scattered light by the CCD, wherein the curvature r of the concave spherical reflector is equal to the distance d from the center of the mirror surface to the tail end of the hollow optical fiber, and the reflectivity of the mirror surface of the concave spherical reflector to the laser and the gas Raman scattered light is not lower than 99.5 percent;
The high-pass filter is used for filtering stray laser in a space and gas Rayleigh scattered light transmitted back along an optical axis, the reflectivity of the high-pass filter to the gas Rayleigh scattered light is not lower than 98 percent, and the transmissivity of the high-pass filter to the gas Rayleigh scattered light is not lower than 93 percent, and the mirror surface of the high-pass filter forms 0 degree with the laser transmission direction;
the first adjustable diaphragm, the first focusing lens, the dichroic mirror, the high-pass filter mirror, the second adjustable diaphragm and the second focusing lens are fixedly connected in a cage system;
the Raman spectrum gas analysis method based on the hollow fiber, which is realized based on the analyzer, comprises the following steps:
Step one, pumping calibration gas with known concentration into a hollow fiber, carrying out Raman spectrum detection by an analyzer after gas balance, and simultaneously obtaining a Raman spectrum S c of the calibration gas, a Raman spectrum S cs of the internal standard gas and a pressure reading P cs of an internal standard gas chamber;
step two, fully flushing the hollow optical fiber by using high-purity argon;
Pumping gas to be detected with unknown concentration into the hollow fiber, carrying out Raman spectrum detection by an analyzer after gas balance, and simultaneously obtaining a Raman spectrum S m of the gas to be detected, a Raman spectrum S ms of the internal standard gas and a pressure reading P ms of the internal standard gas chamber;
Step four, respectively calculating a gas Raman peak area A c for calibration, an internal standard gas Raman peak area A cs for calibration, a gas Raman peak area A m for measurement and an internal standard gas Raman peak area A ms for measurement through the acquired S c、Scs、Sm、Sms, simultaneously knowing the concentration c c of the gas for calibration, the pressure reading P cs of the internal standard gas chamber for calibration and the pressure reading P ms of the internal standard gas chamber for measurement, and calculating the concentration of the gas for measurement according to the following formula:
2. a hollow fiber based raman spectroscopy gas analyzer according to claim 1, wherein:
The laser is a single longitudinal mode, narrow linewidth and continuous wave laser, is used for exciting spontaneous Raman signals of gas, the working wavelength of the laser can be any wavelength of visible light-near infrared wave bands, the linewidth of the laser is less than 0.00001nm, and the M 2 factor of the laser is less than 1.5.
3. A hollow fiber based raman spectroscopy gas analyzer according to claim 1, wherein:
The dichroic mirror is a 45-degree high-pass filter mirror and is used for separating laser light and gas Raman scattered light, filtering the gas Rayleigh scattered light, wherein the reflectivity of the gas Rayleigh scattered light to the Rayleigh scattered light is not lower than 98 percent, the transmissivity of the gas Rayleigh scattered light to the Raman scattered light is not lower than 93 percent, and the mirror surface of the dichroic mirror is 45 degrees with the incident direction of the laser light.
4. A hollow fiber based raman spectroscopy gas analyzer according to claim 1, wherein:
The first focusing lens is an achromatic lens and is used for coupling laser into the fiber core of the hollow fiber, and simultaneously collimating the back gas Raman scattered light emitted by the fiber core of the hollow fiber;
The first focusing lens is arranged on the triaxial adjustable micro-displacement platform, the laser light spot at the focus is coupled into the fiber core of the hollow fiber by finely adjusting the spatial position of the lens, and the coupling condition is judged by monitoring the laser power output by the tail end of the hollow fiber and the far-field light spot shape in real time;
the second focusing lens is an achromatic lens for coupling the gas raman scattered light into a slit of a spectrometer;
the focal length f of the first focusing lens is:
Lambda and D, D MFD are the laser wavelength, the spot size of the laser at the focusing lens and the mode field diameter of the hollow fiber, respectively.
5. A hollow fiber based raman spectroscopy gas analyzer according to claim 1, wherein:
The first adjustable diaphragm and the second adjustable diaphragm are used for adjusting the size of the clear aperture of the diaphragm to perform space optimal filtering of the gas Raman scattered light;
The first adjustable diaphragm and the second adjustable diaphragm determine the diaphragm aperture of optimal filtering through position adjustment, and the constraint conditions of the position adjustment are as follows:
A first tunable aperture is positioned between the dichroic mirror and the second hollow fiber adapter and along the optical axis;
a second adjustable aperture is positioned between the dichroic mirror and the second focusing lens and along the optical axis.
6. A hollow fiber based raman spectroscopy gas analyzer according to claim 1, wherein:
The second hollow fiber adapter and the first hollow fiber adapter comprise an adapter body, an optical window, an optical flange, screws, a fiber clamp, a sealing ring, a pressure gauge and an air inlet/outlet, and are used for fixing and installing the hollow fiber and guaranteeing the integral air tightness of the adapter, and simultaneously realizing the coupling of laser and the hollow fiber, the collection of gas Raman scattered light and the gas inlet/outlet;
The air inlet/outlet of the second hollow fiber adapter is connected with the medium pressure pump system through the gas pipeline, the gas to be detected is continuously pumped into the hollow fiber in a pressure driving mode, the air inlet/outlet of the second hollow fiber adapter and the air inlet/outlet of the first hollow fiber adapter are closed when a stable pressure gradient is built in the fiber core, and the gas balance time in the fiber core of the hollow fiber is monitored in real time through the pressure gauge.
7. A hollow fiber based raman spectroscopy gas analyzer according to claim 1, wherein:
The hollow fiber comprises a hollow photon band gap fiber and a hollow anti-resonance fiber, is used for guiding laser and providing a place for the interaction of laser and gas, and the generation of gas Raman scattered light, wherein the transmission bandwidth covers visible light-near infrared band, and the diameter of the fiber core is in the micron order.
8. A hollow fiber based raman spectroscopy gas analyzer according to claim 1, wherein:
the CCD is used for detecting and recording the Raman scattered light of the gas and converting the Raman scattered light into an electric signal to be output;
The medium pressure pump system is used for pumping gas into the hollow optical fiber;
The gas transmission pipeline is used for inputting gas from the medium pressure pump system into the second hollow fiber adapter;
the gas filter is used for adsorbing moisture and solid particles in the gas to be detected;
The spectrometer is used for separating the gas Raman scattered light with different wavelengths so that the gas Raman scattered light can be recorded by different detection units of the CCD;
The slit width W sli t of the spectrometer is:
Wslit＝f2dfiber/f1
Wherein f 1 is the focal length of the first focusing lens;
f 2 is the focal length of the second focusing lens;
d fiber is the core diameter of the hollow core fiber.
9. The hollow fiber based raman spectroscopy gas analyzer of claim 1, wherein:
The process of carrying out Raman spectrum detection by the analyzer comprises the following steps:
Step 1, a laser emits laser, and the laser enters a hollow fiber through a second hollow fiber adapter after being reflected by a dichroic mirror and focused by a first focusing lens;
step 2, the laser interacts with the gas in the fiber core of the hollow fiber to generate forward Raman scattered light L ff and backward Raman scattered light L fb;
Step 3, back Raman scattered light L fb passes through a second hollow fiber adapter, is collimated by a first focusing lens, is filtered by a dichroic mirror and a high-pass filter, and enters a slit of a spectrometer after being focused by the second focusing lens;
The gas Raman scattered light is detected by a CCD after being diffracted and split by a spectrometer;
step 4, forward Raman scattered light L ff passes through the first hollow fiber adapter and the internal standard gas chamber, and then re-enters the hollow fiber after being coupled by the concave spherical reflector, and is detected and collected by the CCD along the same light path with the backward Raman scattered light L fb in the step 3;
And 5, enabling laser output by the tail end of the hollow fiber to pass through the first hollow fiber adapter and the internal standard gas chamber, coupling by the concave spherical reflector, re-entering the hollow fiber, enabling the laser to interact with gas in the hollow fiber again, enabling the generated forward gas Raman scattered light L bf and the generated back scattered light L fb of the step 3 to be collected along the same light path, enabling the generated back scattered light L bb to pass through the first hollow fiber adapter and the internal standard gas chamber, re-entering the hollow fiber after coupling by the concave spherical reflector again, and collecting the back scattered light L fb of the step 3 along the same light path.
10. The hollow fiber based raman spectroscopy gas analyzer of claim 9, wherein:
when the analyzer detects Raman spectrum, the aperture sizes of the first adjustable diaphragm and the second adjustable diaphragm are slowly adjusted, and simultaneously, the Raman spectrum of gas detected by the CCD is recorded;
when the analyzer detects Raman spectrum, the air inlet/outlet of the second hollow fiber adapter is connected with the medium pressure pump system through a gas pipeline, and the air inlet/outlet of the first hollow fiber adapter is kept open;
the medium pressure pump system provides constant pressure to continuously pump gas into the second hollow fiber adapter and enter the hollow fiber, the second hollow fiber adapter and the gas inlet/outlet of the first hollow fiber adapter are closed when a stable pressure gradient is built in the hollow fiber, and the gas balance time in the hollow fiber is monitored in real time through the pressure gauge.
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