CN111103260A - System and method for detecting intermediate infrared all-fiber cavity ring-down trace gas - Google Patents
System and method for detecting intermediate infrared all-fiber cavity ring-down trace gas Download PDFInfo
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Abstract
The invention discloses a system and a method for detecting trace gas decayed in a mid-infrared all-fiber optical cavity. Measuring the ring-down time difference of the optical cavity when the air chamber is vacuumized and filled with the measured gas, and calculating the single-wavelength gas absorption coefficient of the characteristic absorption wavelength of the measured gas; measuring the relation between the absorption spectrum coefficient and the wavelength of the gas to be measured in the waveband range by tuning the wavelength of the laser; the composition and concentration of the measured trace gas are determined qualitatively and quantitatively by processing the data. The invention has high sensitivity and strong anti-interference capability, makes the coupling of complex light paths simple and easy, and has compact system structure, small volume and easy portability.
Description
Technical Field
The invention relates to a trace gas detection system and method, in particular to a full-fiber structure cavity ring-down trace gas detection system and method based on a mid-infrared quantum cascade laser.
Background
The mid-infrared 3-14 micron wave band has important practical value, and life-related fingerprint fundamental frequency vibration absorption spectral lines of gas molecules containing carbon, hydrogen, oxygen and nitrogen and volatile organic compounds fall in the wave band, so that the mid-infrared spectrum technology can perform synchronous online, rapid and accurate qualitative and quantitative analysis on multi-component trace gas, and has important requirements in a plurality of application scenes such as environment monitoring, industrial production process monitoring, toxic and explosive gas detection, disease diagnosis and the like.
The cavity ring-down spectroscopy technology is a special absorption spectroscopy detection technology based on a resonant cavity, has extremely high sensitivity and resolution, is not influenced by the fluctuation of light source power, and is suitable for testing an extremely low-concentration sample; therefore, the method is widely used for high-precision absorption spectrum tests of various atoms, molecules, radicals and the like, and has good effects in the fields of atmospheric environment monitoring, biomedicine and the like.
The cavity ring-down spectroscopy technology is based on an optical resonant cavity formed by two high reflectors, when the cavity is filled with substances (gas or liquid) which have certain absorption on laser wavelength entering the resonant cavity through the reflectors, laser signals are reflected for multiple times in the cavity, so that an equivalent result of a long optical path can be realized in a short cavity, loss information in the cavity is obtained through light intensity attenuation time in the resonant cavity, and the absorption coefficient of substances with extremely low concentration is calculated.
In contrast to direct absorption spectroscopy, which is generally based on the Lambert-Beer law, cavity ring-down spectroscopy decays exponentially with time t (I ═ I) by the intensity I in the cavity0Exp (- (cA/nL) · (t/τ))), where n is the refractive index of the intra-cavity medium, for gases, n is 1, L is the resonator length, a is the total transmission loss in the cavity, and c is the speed of light in vacuum; the output light intensity I of the ring-down cavity is attenuated to the initial light intensity I0The time τ of 1/e (36.8%) is a ring-down time (τ ═ nL)/(cA)); calculating the absorption coefficient of the test gas according to the ring-down time difference of the tested gas and the background of the tested gas without the tested gas; it must be noted that since the cavity ring-down spectroscopy technique measures an amount of time on the order of microseconds to nanoseconds, the intensity fluctuations of the laser source during this short time interval can be neglected; therefore, when the technology is used for testing substances with extremely low concentration, the influence on the testing precision due to the fluctuation of the laser intensity can be avoided.
In cavity ring-down spectroscopy test, high reflectivity and low reflection loss of a resonant cavity need to be ensured, reflectors at two ends of the cavity need to be plated with high-reflectivity materials, and meanwhile, two cavity mirrors need to be accurately aligned, so that the whole optical system is constructed with high complexity. The near infrared (0.8-2.4 microns) wave band can be covered by a low-loss single-mode quartz optical fiber, so that the full-fiber cavity ring-down spectroscopy based on the quartz optical fiber can detect trace gas with ppm (parts per million) level concentration in the near infrared wave band.
The all-fiber cavity ring-down spectroscopy technology needs to use a laser light source with fiber output, a fiber Bragg grating, a fiber gas chamber and a plurality of fiber devices with photoelectric probes with fiber input; in the near infrared band, various low-loss quartz optical fiber devices can meet the requirements; however, in the near-infrared band, each gas only has higher harmonic absorption peaks such as overtone and mixing, the absorption cross section of the gas is 2-3 orders of magnitude lower than the fundamental frequency absorption intensity of the gas in the middle-infrared band, and meanwhile, because the overtone and mixing absorption peaks of a plurality of gases in the near-infrared band are easy to coincide on the wavelength, the sensitivity on the intensity and the resolution on the spectrum are far inferior to those of the middle-infrared spectrum analysis technology when the near-infrared spectrum analysis technology carries out qualitative and quantitative analysis on the trace gas.
The key to realizing the mid-infrared all-fiber cavity ring-down spectroscopy is the all-fiber. The low-loss fusion between the intermediate infrared laser light source with single-mode intermediate infrared fiber output, the intermediate infrared fiber Bragg grating, the intermediate infrared fiber air chamber and the intermediate infrared fiber device is a core link. Taking a mid-infrared quantum cascade laser as an example, the laser is already in practical stage at present, the wavelength can cover the mid-infrared wavelength of more than 4 microns and even the terahertz waveband of more than 100 microns, the size is small, the weight is light, the laser output linewidth is about megahertz (megahertz), the power level is between hundred milliwatts and watt level, and the laser is an ideal mid-infrared laser light source which meets the requirements of high-precision mid-infrared trace gas spectrum analysis technology. However, the existing intermediate infrared quantum cascade laser only has reports and products of output of intermediate infrared fiber pigtails with multimode large core diameter and intermediate infrared hollow fiber pigtails, and can not be butted with intermediate infrared single-mode fiber Bragg gratings; at present, reports or products which can solve the problem of low-loss fusion among the intermediate infrared single-mode pigtail, the intermediate infrared fiber grating, the intermediate infrared fiber bragg grating, the intermediate infrared hollow fiber air chamber and the intermediate infrared fiber device do not exist, so that the realization of the intermediate infrared all-fiber cavity ring-down spectrum testing device with compact structure and high portability is hindered, and theoretically, the intermediate infrared all-fiber cavity ring-down spectrum technology realizes a high-precision and high-resolution gas spectrum detection technology with ultralow concentration from sub-ppm (one million) to ppt (one billion).
Disclosure of Invention
The purpose of the invention is as follows: aiming at the technical problem of realizing high-precision mid-infrared all-fiber cavity ring-down spectrum detection, the invention provides a high-precision mid-infrared all-fiber cavity ring-down spectrum detection system and method.
The technical scheme is as follows: in order to realize the purpose of the invention, the technical scheme adopted by the invention is as follows:
a mid-infrared all-fiber cavity ring-down trace gas detection system, comprising: the device comprises a wavelength-tunable intermediate infrared quantum cascade laser, a first intermediate infrared single-mode fiber, a first intermediate infrared fiber Bragg grating, an intermediate infrared hollow fiber, a first three-way valve, a first gas pipeline, a first micro-tank, a second gas pipeline, a second three-way valve, a vacuum pump, a pressure pump, a second intermediate infrared fiber Bragg grating, a second intermediate infrared single-mode fiber, an intermediate infrared photoelectric probe, a lead, an oscilloscope and a computer;
the intermediate infrared quantum cascade laser is welded with the incident end of a first intermediate infrared fiber Bragg grating through a first intermediate infrared single-mode fiber, the emergent end of the first intermediate infrared fiber Bragg grating is connected with a first micro box, the two ends of a hollow intermediate infrared fiber are respectively connected with the first micro box and a second micro box, the second micro box is connected with the incident end of a second intermediate infrared fiber Bragg grating, the emergent end of the second intermediate infrared fiber Bragg grating is welded with an intermediate infrared photoelectric probe through a second intermediate infrared single-mode fiber, and the intermediate infrared photoelectric probe is connected with an oscilloscope through a conducting wire; the first intermediate infrared fiber Bragg grating and the second intermediate infrared fiber Bragg grating form an all-fiber ring-down cavity at two ends of the hollow-core fiber gas chamber; the computer is used for processing all test data and controlling the switches of the intermediate infrared quantum cascade laser, the first three-way valve, the second three-way valve, the vacuum pump and the pressure pump;
the first micro-tank is connected with one end of a first three-way valve through a first gas pipeline, and the other two ends of the first three-way valve are respectively connected with a gas inlet end to be detected and a pressure pump; the second micro-tank is connected with one end of a second three-way valve through a second gas pipeline, and the other end of the second three-way valve is connected with a vacuum pump;
the detected gas reaches a first micro-box which is hermetically sealed through a first three-way valve and a first gas pipeline;
the input end of the intermediate infrared hollow optical fiber is drilled with a micropore with the diameter of d micrometers, the micropore extends from the surface of the hollow optical fiber to the hollow core of the optical fiber, and the surface roughness of the inner wall of the micropore is lower than a micrometers; the gas to be detected enters the hollow optical fiber through the micropores and fills the whole core of the hollow optical fiber; preferably, the diameter d ranges from 20 to 500 microns, and a is 1;
the output end of the intermediate infrared hollow optical fiber is drilled with a micropore with the diameter of d micrometers, the micropore extends from the surface of the hollow optical fiber to the hollow core of the optical fiber, and the surface roughness of the inner wall of the micropore is lower than a micrometers; the gas to be detected leaves the hollow-core optical fiber through the micropores and enters a second airtight closed micro-box;
the detected gas passes through the second micro-tank and is discharged out of the detection system through a second gas pipeline and a second three-way valve.
Further, the first mid-infrared fiber bragg grating and the second mid-infrared fiber bragg grating are a pair of mid-infrared fiber bragg gratings with the same central wavelength and the same high reflectivity.
The working principle of the system is as follows:
the wavelength-tunable intermediate infrared quantum cascade laser is used as a light source, and a section of intermediate infrared single-mode fiber is used as a laser output tail fiber; a section of fiber Bragg grating based on the intermediate infrared single-mode fiber is used as a reflecting cavity mirror at the incident end of the laser; a section of intermediate infrared hollow fiber is used as a gas chamber for loading gas to be detected;
the method comprises the following steps that an optical fiber fusion splicer is used, so that low-loss fusion splicing is achieved between the intermediate infrared single-mode fiber Bragg grating and a single-mode tail fiber output by a quantum cascade laser, two ends of the intermediate infrared hollow fiber and the intermediate infrared single-mode fiber Bragg grating are subjected to low-loss fusion splicing, and the outgoing end of the intermediate infrared single-mode fiber Bragg grating and the input tail fiber of the intermediate infrared photoelectric probe are subjected to low-loss fusion splicing;
the gas to be tested firstly enters the testing system through the inlet and reaches the first micro-box which is hermetically sealed through the first three-way valve and the first gas pipeline; the gas to be detected enters the hollow-core optical fiber through the micropores at the input end of the intermediate infrared hollow-core optical fiber and fills the whole core of the hollow-core optical fiber; the detected gas leaves the hollow-core optical fiber through the micropores at the output end of the intermediate infrared hollow-core optical fiber, enters a second airtight and closed micro-box and is discharged through a second gas pipeline and a second three-way valve; when gas is exhausted, negative pressure is generated in the hollow optical fiber gas chamber through the vacuum pump, and the whole hollow optical fiber gas chamber is filled with accelerated gas;
a pair of mid-infrared fiber Bragg gratings with the same central wavelength and high reflectivity form an all-fiber ring-down cavity at two ends of a hollow fiber gas chamber, a laser signal ring-down through the cavity enters a mid-infrared photoelectric probe through a mid-infrared single-mode tail fiber, is converted into an electric signal, is connected to an oscilloscope through a lead, and is tested to obtain a ring-down time constant of the cavity; before each test, a vacuum is formed in the hollow-core optical fiber air chamber by closing the three-way valve and opening the vacuum pump, and the real spectral absorption coefficient of the gas to be tested at the wavelength to be tested can be calculated by subtracting the background ring-down time constant of the vacuum state from the ring-down time constant of the gas to be tested in the ring-down test process of the optical cavity; measuring the relation between the absorption spectrum coefficient and the wavelength of the gas to be measured in the waveband range by tuning the wavelength of the laser; and finally, processing all data by a computer, and comparing the data with an international common HITRAN gas database so as to qualitatively and quantitatively determine the components and the concentration of the detected trace gas.
In the gap of testing different gas samples, high-purity dry nitrogen is blown into the hollow optical fiber air chamber through the pressure pump, residual gas is removed, and interference to next test is avoided.
The computer processes the test data and controls the switches of the intermediate infrared quantum cascade laser, the first three-way valve, the second three-way valve, the vacuum pump and the pressure valve.
The invention also provides a method for detecting the intermediate infrared all-fiber cavity ring-down trace gas based on the system, which comprises the following steps:
s1, closing the first three-way valve, closing an idle valve of the second three-way valve, opening a vacuum pump, and vacuumizing the whole air chamber channel until the vacuum degree reaches p atmospheric pressures; closing the vacuum pump, opening the first three-way valve to the pressure pump, and introducing the cleaning nitrogen into the gas chamber; closing the first three-way valve, closing an idle valve of the second three-way valve, opening a vacuum pump, vacuumizing the whole air chamber channel until the vacuum degree reaches p atmospheric pressures, and closing the vacuum pump; preferably, p is 0.01;
s2, turning on the intermediate infrared quantum cascade laser, and setting the working wavelength as lambda1The laser signal enters the hollow-core optical fiber air chamber through the first intermediate infrared single-mode optical fiber and the first intermediate infrared optical fiber Bragg grating; the laser signal is oscillated in a ring-down cavity formed by a first fiber Bragg grating and a second fiber Bragg grating and enters a second intermediate infrared single-mode fiber, the signal is received by an intermediate infrared photoelectric probe and is connected to an oscilloscope by a lead, and thus the wavelength lambda of the full-fiber ring-down cavity in a vacuum state (namely, no gas) is measured1Background ring-down time constant τ (λ)1,bkg)；
S3, tuning output wavelength lambda in the working waveband range of the intermediate infrared quantum cascade laseriRepeating the step S2, and measuring the relation tau (lambda) between the background ring-down time constant and the wavelength of the mid-infrared quantum cascade laser in the whole working wavelength range in the vacuum state of the all-fiber ring-down cavityi,bkg)；
S4, closing the second three-way valve, and keeping the air chamber at vacuum negative pressure; opening a first three-way valve to a detected gas inlet, and injecting detected gas; the whole air chamber channel is quickly filled with the detected gas under the action of negative pressure in the air chamber, the first three-way valve is closed, and the air chamber channel is kept in a fully closed state;
s5, turning on the intermediate infrared quantum cascade laser, and setting the working wavelength as lambda1The laser signal enters the hollow-core optical fiber air chamber through the first intermediate infrared single-mode optical fiber and the first intermediate infrared optical fiber Bragg grating; the laser signal is oscillated in a ring-down cavity formed by the first and second fiber Bragg gratings and enters the second intermediate infrared single-mode fiber, the signal is received by the intermediate infrared photoelectric probe and is connected to an oscilloscope by a lead, thereby measuring the wavelength lambda of the gas to be measured1Ring down time constant of (d)1) (ii) a Computer according to T (lambda)1) And τ (λ)1,bkg) The measured gas is obtained by calculating the difference value of the measured gas and the measured gas at the wavelength of lambda1Absorption coefficient α (λ)1)；
S6, tuning output wavelength lambda in the working waveband range of the intermediate infrared quantum cascade laseriRepeating the step S5, and measuring the relation tau (lambda) of the ring-down time constant and the wavelength of the measured gas in the whole working wavelength rangei) (ii) a Computer according to T (lambda)i) And τ (λ)i,bkg) The wavelength lambda of the measured gas in the whole working waveband range is obtained through calculation of the difference valueiAbsorption coefficient α (λ)i)；
S7, the computer compares the absorption coefficient α of the detected gas in the whole working waveband range with the absorption coefficient spectrum of the gas in the international HITRAN gas database, thereby qualitatively and quantitatively determining the components and the concentration of the detected trace gas.
Has the advantages that: compared with the prior art, the technical scheme of the invention has the following beneficial technical effects:
a) the whole device has compact structure, small volume and light weight due to the full optical fiber ring-down cavity structure, is easy to carry, and is beneficial to being used in the fields, vehicles, aircrafts and other scenes needing a light portable high-precision gas detection system;
b) the all-fiber resonant cavity structure avoids the complex collimation adjustment of the original ring-down cavity light path coupled in the free space;
c) the sample volume of the gas to be detected of the all-fiber gas chamber structure realized by the femtosecond microstructure drilling technology and the low-loss fiber fusion welding technology is small, and the volume of the fiber core of the meter-level length hollow fiber is only in the magnitude of submillimeter, so that the method is particularly favorable under the condition that the volume of the gas to be detected is limited (such as early disease detection through the gas exhaled by a human body);
d) the sample volume of the gas to be detected of the all-fiber gas chamber structure realized by the femtosecond microstructure drilling technology and the low-loss fiber fusion welding technology is small, and the volume of the fiber core of the meter-level length hollow fiber is only in the sub-milliliter order, so that the online rapid sampling is facilitated;
e) the full optical fiber structure ensures that the whole device structure has high mechanical stability, and is beneficial to stable use in a complex use environment;
drawings
FIG. 1 is a schematic diagram of a trace gas detection system according to the present invention;
FIG. 2 is an enlarged partial view of the input end of a hollow core fiber in a first micro-chamber;
FIG. 3 is an enlarged view of a portion of the output end of a hollow core fiber in a second microchamber;
FIG. 4 is a graph of a pair of intermediate infrared InF single-mode fiber Bragg transmission spectra;
FIG. 5 is a schematic cross-sectional view of a mid-infrared hollow-core optical fiber used;
FIG. 6 is a graph of transmission loss of the employed mid-infrared hollow-core fiber at mid-infrared 3-11 μm;
FIG. 7 is a drawing of a drilling object through ultrafast femtosecond micromachining technology on the side surface of a mid-infrared hollow fiber;
FIG. 8 is a graph of absorption spectra at 4.3-4.5 micron wavelength bands for carbon 12 carbon dioxide gas and carbon 13 isotope carbon dioxide extracted from the HITRAN gas database;
wherein, 1-intermediate infrared quantum cascade laser; 2-a first mid-infrared single mode fiber; 3-a first mid-infrared fiber bragg grating; 4-mid-infrared hollow-core optical fiber; 5-inlet end of gas to be detected; 6-a first three-way valve; 7-a first gas conduit; 8-a first micro-chamber; 9-a second micro-chamber; 10-a second gas conduit; 11-a second three-way valve; 12-a vacuum pump; 13-a pressure pump; 14-a second mid-infrared fiber bragg grating; 15-a second mid-infrared single-mode fiber; 16-mid-infrared photoelectric probe; 17-a wire; 18-an oscilloscope; 19-computer.
Detailed Description
The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
The present invention is illustrated by the example of detecting gastric helicobacter pylori by carbon-13 isotope exhalation.
The invention relates to a mid-infrared all-fiber optical cavity ring-down trace gas detection system, which comprises a pulse type wavelength-tunable narrow-linewidth mid-infrared distributed feedback quantum cascade laser 1 as a light source, wherein the center wavelength of the laser is 4.401 micrometers, the linewidth is 100 MHz, and the tunable wavelength range is 4.401-4.405 micrometers; a section of first intermediate infrared single-mode fiber 2 is used as a laser output tail fiber;
a section of intermediate infrared hollow optical fiber 4 which is plated with metal silver and dielectric material silver iodide film on the inner wall of the quartz glass capillary tube is used as a gas chamber for loading gas to be detected;
the first intermediate infrared fiber Bragg grating 3 and the second intermediate infrared fiber Bragg grating 14 with the same central wavelength and the same high reflectivity form an all-fiber ring-down cavity at two ends of a gas chamber of a hollow-core fiber 4;
the low-loss fusion joint is realized by using a commercial VYRON optical fiber fusion splicer at the fusion joint 3a between the incident end of the first intermediate infrared fiber Bragg grating 3 and the laser output tail fiber, namely the first intermediate infrared single-mode fiber 2, at the fusion joint 3b between the emergent end of the first intermediate infrared fiber Bragg grating 3 and the intermediate infrared hollow-core fiber 4, at the fusion joint 14a between the intermediate infrared hollow-core fiber 4 and the incident end of the second intermediate infrared fiber Bragg grating 14, and at the fusion joint 14b between the emergent end of the second intermediate infrared fiber Bragg grating 14 and the second intermediate infrared single-mode fiber 15.
In this embodiment, the first intermediate infrared single-mode fiber 2 and the second intermediate infrared single-mode fiber 15 are intermediate infrared indium fluoride glass single-mode fibers.
The detected exhaled gas enters the detection system through a detected gas inlet end 5 and reaches a first micro-box 8 which is hermetically sealed through a first three-way valve 6 and a first gas pipeline 7; the intermediate infrared hollow-core optical fiber 4 is drilled with a micropore 4a with the diameter of 100 microns by an ultrafast femtosecond drilling technology, as shown in figure 2, the micropore 4a extends from the surface of the hollow-core optical fiber to the hollow core of the optical fiber; the gas to be detected enters the hollow optical fiber through the micropores 4a and fills the whole core of the hollow optical fiber; the output end of the intermediate infrared hollow-core optical fiber 4 is drilled with a micropore 4b with the diameter of 100 microns by an ultrafast femtosecond drilling technology, as shown in figure 3, the micropore 4b extends from the surface of the hollow-core optical fiber to the hollow core of the optical fiber; the gas to be detected leaves the hollow optical fiber through the micropores 4b, enters the second airtight closed micro-box 9, and is discharged through the second gas pipeline 10 and the second three-way valve 11; when the gas is exhausted, negative pressure is generated in the gas chamber of the hollow-core optical fiber 4 at the gas output end through the vacuum pump 12, and the whole gas chamber of the hollow-core optical fiber 4 is filled with accelerated gas.
In the present embodiment, a pair of mid-infrared indium fluoride glass single mode fiber bragg transmission spectra are actually measured as shown in fig. 4, wherein the center wavelengths of the mid-infrared indium fluoride glass single mode fiber bragg transmission spectra are 4.401 micrometers, and the 3dB bandwidths of the mid-infrared indium fluoride glass single mode fiber bragg transmission spectra are 4 nanometers; the transmission at the peak wavelength dropped to-20 dB compared to the baseline level, and its reflectivity at a peak wavelength of 4.401 microns was calculated to be about 99%.
The schematic cross-sectional structure of the mid-infrared hollow-core optical fiber 4 used in this embodiment is shown in fig. 5; the transmission loss graph of the adopted mid-infrared hollow-core optical fiber 4 in the mid-infrared range of 3-11 microns is shown in figure 6; the loss around the operating wavelength of 4.4 microns in the example was 0.45 dB/m; fig. 7 shows a real figure of the side surface of the intermediate infrared hollow-core optical fiber 4 drilled at the positions 4a and 4b by the ultrafast femtosecond micro-processing technology, wherein the outer diameter of the optical fiber is 1 mm, and the diameter of the micro-hole is 100 micrometers.
The laser signal subjected to cavity ring-down enters a mid-infrared photoelectric probe 16 through a second mid-infrared single-mode fiber 15, is converted into an electric signal, is connected to an oscilloscope 18 through a lead 17, and is tested to obtain a cavity ring-down time constant; before each test, vacuum is formed in the air chamber of the hollow-core optical fiber 4 by closing the first three-way valve 6 and opening the vacuum pump 12, and the real spectral absorption coefficient of the detected gas at the detected wavelength can be calculated by subtracting the ring-down time constant of the vacuum state from the ring-down time constant of the detected gas in the cavity ring-down test process; measuring the relation between the absorption spectrum coefficient and the wavelength of the gas to be measured in a wide band range by tuning the wavelength of the laser;
finally, all data are processed by the computer 19 and compared with the international common HITRAN gas database, so as to qualitatively and quantitatively determine the components and the concentration of the detected trace gas.
In this embodiment, the mid-infrared photoelectric probe 16 is a non-refrigeration high-speed infrared InAsSb detector, and detects the corresponding wavelength range of 3-11 μm.
FIG. 8 is absorption lines of carbon 12 carbon dioxide gas and carbon 13 isotope carbon dioxide gas extracted from the HITRAN gas database at 4.3-4.5 micron wavelength bands; since the carbon 12 isotope carbon dioxide gas is overlapped with the absorption spectrum line of the carbon 13 isotope carbon dioxide below 4.38 microns, and the carbon 13 isotope carbon dioxide has the maximum absorption coefficient at 4.401-4.403 microns, 4.401-4.403 microns are the optimal absorption spectrum wavelength of the carbon 13 isotope carbon dioxide to be tested.
The invention also provides a method for detecting the intermediate infrared all-fiber cavity ring-down trace gas based on the system, which comprises the following steps:
s1, closing the first three-way valve 6, closing an idle valve of the second three-way valve 11, opening the vacuum pump 12, and vacuumizing the whole air chamber channel until the vacuum degree reaches 0.01 atmosphere; the vacuum pump 12 is closed, the first three-way valve 6 is opened to the pressure pump 13, and the cleaning nitrogen enters the gas chamber; closing the first three-way valve 6, closing an idle valve of the second three-way valve 11, opening the vacuum pump 12, vacuumizing the whole air chamber channel until the vacuum degree reaches 0.01 atmosphere, and closing the vacuum pump 12;
s2, turning on the intermediate infrared quantum cascade laser 1, and setting the working wavelength as lambda1The laser signal enters a hollow-core optical fiber 4 air chamber through a first intermediate infrared single-mode optical fiber 2 and a first intermediate infrared fiber Bragg grating 3; the laser signal is oscillated in a ring-down cavity formed by a first fiber Bragg grating and a second fiber Bragg grating and enters a second intermediate infrared single-mode fiber 15, the signal is received by an intermediate infrared photoelectric probe 16 and is connected to an oscilloscope 18 through a lead 17, and therefore the wavelength lambda of the full-fiber ring-down cavity in a vacuum state (namely, without any gas) is measured1Background ring-down time constant τ (λ)1,bkg)；
S3 atTunable output wavelength lambda within the working band range of the mid-infrared quantum cascade laser 1iRepeating the step S2, and measuring the relation tau (lambda) between the background ring-down time constant and the wavelength of the mid-infrared quantum cascade laser 1 in the whole working wavelength range in the vacuum state of the all-fiber ring-down cavityi,bkg)；
S4, closing the second three-way valve 11, and keeping the air chamber at vacuum negative pressure; opening a first three-way valve 6 to the inlet of the gas to be detected, and injecting the gas to be detected; due to the action of negative pressure in the air chamber, the whole air chamber channel is quickly filled with the detected air, the first three-way valve 6 is closed, and the air chamber channel is kept in a fully closed state;
s5, turning on the intermediate infrared quantum cascade laser 1, and setting the working wavelength as lambda1The laser signal enters a hollow-core optical fiber 4 air chamber through a first intermediate infrared single-mode optical fiber 2 and a first intermediate infrared fiber Bragg grating 3; the laser signal is oscillated in a ring-down cavity formed by a first fiber Bragg grating and a second fiber Bragg grating and enters a second intermediate infrared single-mode fiber 15, the signal is received by an intermediate infrared photoelectric probe 16 and is connected to an oscilloscope 18 by a lead 17, and thus the measured gas with the wavelength of lambda is measured1Ring down time constant of (d)1) (ii) a The computer 19 is based on τ (λ)1) And τ (λ)1,bkg) The measured gas is obtained by calculating the difference value of the measured gas and the measured gas at the wavelength of lambda1Absorption coefficient α (λ)1)；
S6, tuning output wavelength lambda in the working waveband range of the intermediate infrared quantum cascade laser 1iRepeating the step S5, and measuring the relation tau (lambda) of the ring-down time constant and the wavelength of the measured gas in the whole working wavelength rangei) (ii) a The computer 19 is based on τ (λ)i) And τ (λ)i,bkg) The wavelength lambda of the measured gas in the whole working waveband range is obtained through calculation of the difference valueiAbsorption coefficient α (λ)i)；
S7, the computer 19 compares the absorption coefficient α of the measured gas in the whole working wave band range with the absorption coefficient spectrum of the gas in the international HITRAN gas database, thereby qualitatively and quantitatively determining the components and the concentration of the measured trace gas.
The foregoing is a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.
Claims (6)
1. The utility model provides a mid-infrared all-fiber optical cavity ring down trace gas detecting system which characterized in that: the detection system comprises: the device comprises a wavelength-tunable intermediate infrared quantum cascade laser, a first intermediate infrared single-mode fiber, a first intermediate infrared fiber Bragg grating, an intermediate infrared hollow fiber, a first three-way valve, a first gas pipeline, a first micro-tank, a second gas pipeline, a second three-way valve, a vacuum pump, a pressure pump, a second intermediate infrared fiber Bragg grating, a second intermediate infrared single-mode fiber, an intermediate infrared photoelectric probe, a lead, an oscilloscope and a computer;
the intermediate infrared quantum cascade laser is welded with the incident end of a first intermediate infrared fiber Bragg grating through a first intermediate infrared single-mode fiber, the emergent end of the first intermediate infrared fiber Bragg grating is connected with a first micro box, the two ends of a hollow intermediate infrared fiber are respectively connected with the first micro box and a second micro box, the second micro box is connected with the incident end of a second intermediate infrared fiber Bragg grating, the emergent end of the second intermediate infrared fiber Bragg grating is welded with an intermediate infrared photoelectric probe through a second intermediate infrared single-mode fiber, and the intermediate infrared photoelectric probe is connected with an oscilloscope through a conducting wire; the first intermediate infrared fiber Bragg grating and the second intermediate infrared fiber Bragg grating form an all-fiber ring-down cavity at two ends of the hollow-core fiber gas chamber; the computer is used for processing all test data and controlling the switches of the intermediate infrared quantum cascade laser, the first three-way valve, the second three-way valve, the vacuum pump and the pressure pump;
the first micro-tank is connected with one end of a first three-way valve through a first gas pipeline, and the other two ends of the first three-way valve are respectively connected with a gas inlet end to be detected and a pressure pump; the second micro-tank is connected with one end of a second three-way valve through a second gas pipeline, and the other end of the second three-way valve is connected with a vacuum pump;
the detected gas reaches a first micro-box which is hermetically sealed through a first three-way valve and a first gas pipeline;
the input end of the intermediate infrared hollow optical fiber is drilled with a micropore with the diameter of d micrometers, the micropore extends from the surface of the hollow optical fiber to the hollow core of the optical fiber, and the surface roughness of the inner wall of the micropore is lower than a micrometers; the gas to be detected enters the hollow optical fiber through the micropores and fills the whole core of the hollow optical fiber;
the output end of the intermediate infrared hollow optical fiber is drilled with a micropore with the diameter of d micrometers, the micropore extends from the surface of the hollow optical fiber to the hollow core of the optical fiber, and the surface roughness of the inner wall of the micropore is lower than a micrometers; the gas to be detected leaves the hollow-core optical fiber through the micropores and enters a second airtight closed micro-box;
the detected gas passes through the second micro-tank and is discharged out of the detection system through a second gas pipeline and a second three-way valve.
2. The mid-infrared all-fiber cavity ring-down trace gas detection system of claim 1, wherein: the first intermediate infrared fiber Bragg grating and the second intermediate infrared fiber Bragg grating are a pair of intermediate infrared fiber Bragg gratings with the same central wavelength and the same high reflectivity.
3. The mid-infrared all-fiber cavity ring-down trace gas detection system of claim 1 or 2, wherein: the diameter d of the micropores ranges from 20 to 500 microns.
4. The mid-infrared all-fiber cavity ring-down trace gas detection system of claim 1 or 2, wherein: the roughness of the inner wall surface of the micropore is lower than 1 micron.
5. The system of claim 1 is used for realizing a method for detecting the ring-down trace gas in the mid-infrared all-fiber optical cavity, which is characterized in that: the method comprises the following steps:
s1, closing the first three-way valve, closing an idle valve of the second three-way valve, opening a vacuum pump, and vacuumizing the whole air chamber channel until the vacuum degree reaches p atmospheric pressures; closing the vacuum pump, opening the first three-way valve to the pressure pump, and introducing the cleaning nitrogen into the gas chamber; closing the first three-way valve, closing an idle valve of the second three-way valve, opening a vacuum pump, vacuumizing the whole air chamber channel until the vacuum degree reaches p atmospheric pressures, and closing the vacuum pump;
s2, turning on the intermediate infrared quantum cascade laser, and setting the working wavelength as lambda1The laser signal enters the hollow-core optical fiber air chamber through the first intermediate infrared single-mode optical fiber and the first intermediate infrared optical fiber Bragg grating; the laser signal is oscillated in a ring-down cavity formed by the first and the second fiber Bragg gratings and enters the second intermediate infrared single-mode fiber, the signal is received by the intermediate infrared photoelectric probe and is connected to an oscilloscope by a lead, and thus, the wavelength lambda of the full-fiber ring-down cavity in a vacuum state is measured1Background ring-down time constant τ (λ)1,bkg)；
S3, tuning output wavelength lambda in the working waveband range of the intermediate infrared quantum cascade laseriRepeating the step S2, and measuring the relation tau (lambda) between the background ring-down time constant and the wavelength of the mid-infrared quantum cascade laser in the whole working wavelength range in the vacuum state of the all-fiber ring-down cavityi,bkg)；
S4, closing the second three-way valve, and keeping the air chamber at vacuum negative pressure; opening a first three-way valve to a detected gas inlet, and injecting detected gas; the whole air chamber channel is filled with the detected gas, the first three-way valve is closed, and the air chamber channel is kept in a fully closed state;
s5, turning on the intermediate infrared quantum cascade laser, and setting the working wavelength as lambda1The laser signal enters the hollow-core optical fiber air chamber through the first intermediate infrared single-mode optical fiber and the first intermediate infrared optical fiber Bragg grating; the laser signal is oscillated in a ring-down cavity formed by the first and second fiber Bragg gratings and enters the second intermediate infrared single-mode fiber, the signal is received by the intermediate infrared photoelectric probe and is connected to an oscilloscope by a lead, thereby measuring the wavelength lambda of the gas to be measured1Ring down time constant of (d)1) (ii) a Computer according to T (lambda)1) And τ (λ)1,bkg) The measured gas is obtained by calculating the difference value of the measured gas and the measured gas at the wavelength of lambda1Absorption coefficient α (λ)1)；
S6, tuning output wavelength lambda in the working waveband range of the intermediate infrared quantum cascade laseriRepeating the step S5, and measuring the relation tau (lambda) of the ring-down time constant and the wavelength of the measured gas in the whole working wavelength rangei) (ii) a Computer according to T (lambda)i) And τ (λ)i,bkg) The wavelength lambda of the measured gas in the whole working waveband range is obtained through calculation of the difference valueiAbsorption coefficient α (λ)i)；
S7, the computer compares the absorption coefficient α of the detected gas in the whole working waveband range with the absorption coefficient spectrum of the gas in the international HITRAN gas database, thereby qualitatively and quantitatively determining the components and the concentration of the detected trace gas.
6. The method of claim 5, wherein the method comprises the steps of: when the whole air chamber channel is vacuumized, the vacuum degree reaches 0.01 atmosphere.
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