CN111982813B - All-fiber open gas monitoring system and detection method - Google Patents
All-fiber open gas monitoring system and detection method Download PDFInfo
- Publication number
- CN111982813B CN111982813B CN202010848024.8A CN202010848024A CN111982813B CN 111982813 B CN111982813 B CN 111982813B CN 202010848024 A CN202010848024 A CN 202010848024A CN 111982813 B CN111982813 B CN 111982813B
- Authority
- CN
- China
- Prior art keywords
- optical fiber
- laser
- mirror
- signal
- wavelength
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Active
Links
Images
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/01—Arrangements or apparatus for facilitating the optical investigation
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
- Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
Abstract
The invention discloses an all-fiber open gas monitoring system and a detection method, which comprise a wavelength tunable laser, an optical fiber coupler, an optical fiber beam splitter, an optical fiber collimator, a reference pool based on a hollow optical fiber, an optical fiber annular cavity interferometer, a three-mirror cavity system, a focusing lens, a photoelectric detector, a laser control module, an analog-to-digital conversion module, a digital-to-analog conversion module and a computer control unit. The invention uses hollow optical fiber and optical fiber ring cavity interferometer with small volume and high flexibility to respectively replace the traditional fragile glass absorption cell and F-P type etalon sensitive to the optical path to realize laser center frequency locking, spectral system correction and tuning frequency range correction; a three-mirror cavity system is used for replacing a traditional two-mirror linear reflection type to realize large-range area open type gas monitoring.
Description
Technical Field
The invention relates to the technical field of laser spectrum and optical fiber sensing, in particular to an all-optical fiber type open gas monitoring system and a detection method.
Background
Many toxic, harmful, flammable and explosive gases exist in the production and living environment of modern society, and potential harm is brought to the health and safety of people. In some open places, such as industrial parks, chemical plants, oil and gas storage stations, etc., fire-forbidden and fire-forbidden areas, certain harmful gases and hazardous chemicals need to be monitored and pre-warned. The traditional analysis means of sampling off-line gas back to a laboratory is far from meeting the requirement of real-time monitoring, so that real-time on-line remote sensing gas monitoring becomes the only effective means.
Under the prior art, the open monitoring of trace gases is mainly realized by a fourier infrared spectrometer or an infrared gas sensor based on TDLAS technology. Among them, although the fourier infrared spectrometer has an advantage of measuring a plurality of components at the same time, it has the following problems: first, in order to obtain higher sensitivity, the detector part needs to be cooled by liquid nitrogen, so that the detector has the problems of high cost, large volume, heavy weight and inconvenience in carrying. Secondly, it takes a long time to perform the spectrum scanning and the data calculation, resulting in a problem of slow data processing speed. The open type infrared gas sensor based on the TDLAS technology mainly uses two cavity mirrors, can only realize linear distance measurement, and generally can realize inversion of gas concentration after a glass reference cell and an air gap F-P type etalon are integrated to be used for locking the central frequency and correcting the frequency of a laser, so that the problems of complicated system structure and limited monitoring range are caused. Therefore, a brand-new trace gas monitoring system is developed to solve the problems of high cost, large volume, heavy weight, inconvenience in carrying, low data processing speed, complex structure, short monitoring distance and the like of the conventional trace gas monitoring system, and has important scientific significance and application value.
Disclosure of Invention
The invention provides an all-fiber open gas monitoring system and a detection method. Aiming at the defects in the prior art and the important significance of measuring toxic, harmful, flammable and explosive gases and atmosphere trace gases in special places, the invention provides an all-fiber open gas monitoring system and a detection method.
In order to realize the technical purpose, the invention adopts the technical scheme that:
an all-fiber open gas monitoring system, comprising: the device comprises a wavelength tunable laser, a 1X 3 optical fiber beam splitter, a three-mirror cavity system, a reference pool based on a hollow optical fiber, an optical fiber annular cavity interferometer, three photoelectric detectors, a laser control module, an analog-digital and digital-analog conversion module and a computer control unit;
the signal output end of the computer control unit is connected with a digital-to-analog conversion module, the signal output end of the digital-to-analog conversion module is connected with a laser control module, and the signal output end of the laser control module is in control connection with a wavelength tunable laser;
the wavelength tunable laser is output through an optical fiber and connected with a 1 × 3 optical fiber beam splitter, the 1 × 3 optical fiber beam splitter divides the laser into three paths according to a certain proportion and outputs the three paths, a first path of incident light with the highest proportion is collimated through an optical fiber collimator and then enters a three-mirror-cavity system, the three-mirror-cavity system comprises a first mirror, a second mirror and a third mirror, an incident small hole and an emergent small hole are respectively formed in the centers of the first mirror and the third mirror of the three-mirror-cavity system, the incident light enters the three-mirror-cavity system through the incident small hole and sequentially passes through the third mirror and the second mirror, the incident light forms multiple reflection in the three-mirror-cavity by adjusting the incident angle and finally exits from the emergent small hole, the emergent light of the three-mirror-cavity system is focused through a focusing lens and then enters a first photoelectric detector, and the first photoelectric detector is arranged at the focus of the focusing lens;
a second path of incident light generated by the 1 x 3 optical fiber beam splitter is input into a reference pool based on a hollow optical fiber through a first optical fiber coupler, gas molecules to be detected with certain concentration are filled in the reference pool based on the hollow optical fiber, and the incident light is directly coupled to a second photoelectric detector after being emitted from the hollow optical fiber;
a third path of incident light generated by the 1 × 3 optical fiber beam splitter is input to the optical fiber ring cavity interferometer through the second optical fiber coupler, interference is generated inside the optical fiber ring cavity interferometer, and the incident light is directly coupled to a third photoelectric detector after being emitted from the optical fiber ring cavity interferometer;
the signal output ends of the three photoelectric detectors are connected to a multi-channel analog-to-digital conversion module, and the analog-to-digital conversion module inputs three photoelectric signals to a computer control unit for online real-time analysis and processing.
Further, the all-fiber type open gas monitoring system is characterized in that the central wavelength of the wavelength tunable laser is determined according to the type of the gas molecules to be detected. Such as corresponding to C 2 H 2 The center wavelength of the molecule is 1520nm and CH 4 1653nm, H of the molecule 2 1576nm, and NH of the S molecule 3 1531nm for molecules, etc.
Furthermore, the all-fiber open gas monitoring system is characterized in that the first cavity mirror, the second cavity mirror and the third cavity mirror of the three-mirror-cavity system are all reflectors, and the physical distances among the three reflectors can meet the gas monitoring requirements in different area ranges according to practical application requirements.
Further, the all-fiber open gas monitoring system is characterized in that the beam splitting ratio of the 1 × 3 fiber beam splitter is 8.
The all-fiber open gas detection method is characterized in that an all-fiber open gas monitoring system is adopted to realize the detection of gas leakage in the range of an open area;
the computer control unit comprises a laser driving signal output module, a frequency signal demodulation and analysis algorithm module and an absorption spectrum fitting and inversion concentration algorithm module which are compiled by Labview software; the detection method specifically comprises the following steps:
[01] a laser wavelength tuning or modulation driving voltage signal output by a laser driving signal output module compiled by computer control unit Labview software is converted into an analog signal through a digital-to-analog conversion module and then is input into a wavelength tunable laser through a laser control module, so that the emitted wavelength tuning and modulation output of the laser are realized;
[02] the laser emitted by the wavelength tunable laser is output through the optical fiber and is directly coupled to the 1 x 3 optical fiber beam splitter, so that three paths of laser are synchronously output;
[03] the first path of laser is collimated by the optical fiber collimator and then enters the three-mirror cavity system, the incident light is emitted after being reflected for multiple times in the three-mirror cavity, the emergent light is focused by the focusing lens and enters the first photoelectric detector, and the position of the first photoelectric detector is positioned at the focus position of the focusing lens;
[04] the second path of laser directly enters a reference pool based on hollow-core optical fiber through a first optical fiber coupler, and emergent light is directly coupled to a second photoelectric detector;
[05] the third path of laser directly enters the optical fiber ring cavity interferometer through the second optical fiber coupler, interference fringes are generated in incident light inside the interferometer, and the incident light is directly coupled to the second photoelectric detector after being emitted from the hollow optical fiber;
[06] and finally, voltage signals output by the three photoelectric detectors are simultaneously input into a digital-to-analog conversion module, the digital-to-analog conversion module inputs the signals into a frequency signal demodulation analysis algorithm module and an absorption spectrum fitting inversion concentration algorithm module which are compiled by Labview software in a computer control unit, and the absorption spectrum fitting inversion concentration algorithm module carries out related processing analysis according to a preselected spectrum detection method.
Further, the all-fiber open gas detection method is characterized in that the frequency signal demodulation and analysis algorithm module is used for locking the center frequency of the laser by using a high signal-to-noise ratio reference spectrum signal generated by the hollow-core fiber-based reference cell according to a signal collected by the second photoelectric detector, and the variation of the center position of the reference spectrum is calculated and converted into a bias voltage offset, and the offset is synchronously fed back to the control module of the wavelength tunable laser, so that the problem of laser wavelength drift caused by environmental factors in the application of an external field environment is solved. Under a wavelength modulation spectrum detection mode, the hollow fiber reference absorption spectrum with known sample concentration and a multi-dimensional linear fitting algorithm can be directly used for calculating the concentration information of the molecules to be analyzed corresponding to the spectrum signals generated by the three-mirror cavity system in an inversion mode.
Furthermore, the all-fiber open gas detection method is characterized in that the absorption spectrum fitting inversion concentration algorithm module can calculate the corresponding relative tunable wave number of the wavelength tunable laser in the working current or voltage range according to the interference fringes and the photoelectric signal detected by the third photoelectric detector, and finally realize the absolute wavelength or frequency correction of the laser tuning range by combining the position of the molecular center absorption line given by the spectrum database; and correcting the obtained absolute wavelength of the laser to be used as the abscissa of the absorption spectrum, and fitting and inverting the gas concentration information by a preselected direct absorption spectrum detection method.
Further, the all-fiber type open gas detection method is characterized in that the preselected spectrum detection method comprises correction-free direct absorption spectroscopy and high-sensitivity wavelength modulation spectroscopy.
Further, the all-fiber open gas detection method is characterized in that the signal calculation processing process of the correction-free direct absorption spectrometry is as follows:
from lambert-beer's law, the change in intensity I (v) of an electromagnetic wave at a frequency v when the laser passes through an absorptive gas filled region having an optical path length L can be described by:
I(v)＝I 0 (v)e -α(v)L
in the above formula, I 0 (v) Is the initial incident intensity, I (v) is the final received light intensity after passing through the gas medium, α (v) is the absorption coefficient of the molecule, and the integral area of the molecular absorption can be calculated from the above equation:
wherein the expression of the number density of molecules
Absorption line type phi (v-v) 0 ) The normalization condition is satisfied; p is total pressure, T is actual temperature, reference temperature T 0 =296K, reference pressure P 0 =1atm; in the open type measurement, the integral area A can be calculated by fitting the absorption spectrum by selecting a Lorentz line type and a minimum quadratic fitting algorithm, and the gas concentration information can be inverted by combining the Lambert-beer law satisfied by the absorption spectrum according to the absorption spectrum line parameters (line intensity S and the like) and the known optical path L of the known gas molecules and other related physical quantity information (environmental temperature, pressure and the like), so that the concentration of the gas to be detected can be directly inverted without correcting through a standard gas sample.
Further, the all-fiber open gas detection method is characterized in that the signal calculation and processing process of the wavelength modulation spectroscopy is as follows:
the wavelength modulation spectroscopy is based on a correction-free direct absorption spectroscopy technology, and realizes effective suppression of 1/f noise limiting the sensitivity of a system by superposing a high-frequency modulation signal in a laser wavelength tuning current signal and shifting the signal frequency to a high-frequency range, thereby realizing high-sensitivity measurement; when a sinusoidal modulation signal of angular frequency ω is superimposed on the laser drive current, the laser output wavelength or frequency varies with time, as given by the following expression
I 0 (t)＝I 0 +i 0 cos(ωt+ψ)
The transmission coefficient can be derived from lambert-beer's law:
wherein:
transmission coefficient for modulated light: />
Using a fourier cosine series expansion we can obtain:
under conditions of weak absorption, the transmission can be reduced to
Widely adopted in view of higher signal-to-noise ratio of the second harmonic, the Fourier coefficient thereof can be expressed as
In practical application, the relationship between the second harmonic term and the concentration of the molecule to be detected can be obtained by using a digital phase-locked demodulation algorithm
I 2f ∝I 0 α(v)CL
From this equation, it can be seen that the actual measured secondary signal amplitude I 2f The concentration C of the gas to be analyzed satisfies a certain linear relation; in the detection method, the second harmonic signals in the signal channel of the three mirror cavities and the reference second harmonic signals measured under the known concentration in the hollow optical fiber reference pool are subjected to multi-dimensional linear fitting, and three signals can be calculatedThe average concentration is integrated within the scope of the monitoring area of the mirror cavity.
The invention has the advantages that:
the invention utilizes a three-mirror cavity system, a reference pool based on a hollow optical fiber and an optical fiber ring cavity interferometer to form an open type gas monitoring system, and can select a direct absorption spectrum technology and a wavelength modulation spectrum technology to respectively realize spectrum measurement under different gas concentration conditions according to actual requirements;
compared with the traditional Fourier infrared spectrometer based on a liquid nitrogen cooling detector and a TDLAS infrared gas sensor system based on a glass reference pool and an F-P etalon, the liquid nitrogen cooling detector has the advantages that a three-mirror cavity structure with adjustable physical length is adopted, gas leakage monitoring in a wider area can be realized, and the detector part can obtain higher sensitivity without being cooled by liquid nitrogen;
in addition, the invention uses the hollow fiber as a reference pool, and the fiber ring cavity interferometer replaces the traditional air gap F-P etalon to realize the necessary laser center frequency drift locking and frequency real-time correction process under the high-precision open measurement requirement condition, thereby leading the system structure to be simpler, the cost to be lower and the measurement result to be more reliable.
Drawings
Fig. 1 is a schematic structural diagram of an all-fiber open gas monitoring system and a detection method according to an embodiment of the present invention.
In the figure: the device comprises a wavelength tunable laser 1, an optical fiber beam splitter 2, an optical fiber collimator 3-1, an optical fiber coupler 3-2, an optical fiber coupler 3-3, a reflector 4-1, a reflector 4-2, a reflector 4-3, a hollow optical fiber 5, an optical fiber annular cavity interferometer 6, a focusing lens 7, a photoelectric detector 8-1, a photoelectric detector 8-2, a photoelectric detector 8-3, an analog-to-digital conversion module 9-1, a digital-to-analog conversion module 9-2, a computer control unit 10 and a laser control module 11.
FIG. 2 is a graph of gas concentration information obtained by the apparatus of the present invention using direct absorption spectroscopy.
Fig. 3 shows the laser modulation signal (up), the signal detected by the photodetector (middle) and the second harmonic signal after demodulation (down) used in the device of the present invention using wavelength modulation spectroscopy.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.
Example 1.
As shown in fig. 1, an all-fiber type open gas monitoring system includes: the device comprises a wavelength tunable laser 1, an optical fiber beam splitter 2, an optical fiber collimator 3-1, an optical fiber coupler 3-2, an optical fiber coupler 3-3, a reflector 4-1, a reflector 4-2, a reflector 4-3, a hollow optical fiber 5, an optical fiber annular cavity interferometer 6, a focusing lens 7, a photoelectric detector 8-1, a photoelectric detector 8-2, a photoelectric detector 8-3, an analog-to-digital conversion module 9-1, a digital-to-analog conversion module 9-2, a laser control module 11 and a computer control unit 10;
the signal output end of the computer control unit 10 is connected with the digital-to-analog conversion module 9-2, the signal output end of the digital-to-analog conversion module 9-2 is connected with the laser control module 11, and the signal output end of the laser control module 11 is in control connection with the wavelength tunable laser 1;
the wavelength tunable laser 1 is output through an optical fiber and connected with a 1 × 3 optical fiber beam splitter 2, the 1 × 3 optical fiber beam splitter 2 divides the laser into three paths according to the beam splitting ratio of 8.
The reflector 4-1, the reflector 4-2 and the reflector 4-3 form a three-mirror-cavity system, and gas molecules to be detected are in the area of the three-mirror-cavity system; the physical distance between the three reflectors can meet the gas monitoring requirement in different area ranges according to the practical application requirement.
An optical fiber beam splitter 2 and an optical fiber collimator 3-1 are sequentially arranged on a light path between a light outlet of a wavelength tunable laser 1 and a reflector 4-1, a focusing lens 7 and a photoelectric detector 8-1 are sequentially arranged on a light path at the rear end of the reflector 4-3, an incident aperture and an emergent aperture are respectively formed in the centers of the reflector 4-1 and the reflector 4-3, incident light enters a three-mirror-cavity system through the incident aperture, sequentially passes through the reflector 4-2 and the reflector 4-3, the incident light is reflected for multiple times in the three-mirror-cavity system by adjusting an incident angle and finally is emitted from the emergent aperture, emergent light of the three-mirror-cavity system is focused through the focusing lens 7 and then enters the photoelectric detector 8-1, and the photoelectric detector 8-1 is arranged at the focus of the focusing lens 7.
The second path of incident light generated by the 1 x 3 fiber splitter 2 is input to the reference cell based on hollow-core fiber through the fiber coupler 3-2.
The reference pool based on the hollow optical fiber comprises a hollow optical fiber 5 and a reference pool, wherein gas molecules to be detected with certain concentration are filled in the reference pool, an optical fiber beam splitter 2 and an optical fiber coupler 3-2 are sequentially arranged on a light path between a light outlet of a wavelength tunable laser 1 and the hollow optical fiber 5, a condensing electric detector 8-2 is sequentially arranged on a light path at the rear end of the reference pool, and incident light is directly coupled to the photoelectric detector 8-2 after being emitted from the hollow optical fiber 5.
The third path of incident light generated by the 1 x 3 optical fiber beam splitter 2 is input to the optical fiber ring cavity interferometer 6 through the optical fiber coupler 3-3, interference is generated inside the optical fiber ring cavity interferometer 6, and the incident light is directly coupled to the photodetector 8-3 after being emitted from the optical fiber ring cavity interferometer 6.
The signal output ends of the photoelectric detector 8-1, the photoelectric detector 8-2 and the photoelectric detector 8-3 are connected with an analog-to-digital conversion module 9-1, and the output end of the analog-to-digital conversion module 9-1 is connected with a computer control unit 10.
The center wavelength of the wavelength tunable laser 1 depends on the kind of the gas molecule to be detected. Such as ethanol @1.39 microns, CH 4 @1.65 micron, C 2 H 2 @1.52 μm, NH 3 @1.53 μm, CO and CO 2 @1.58 μm, etc. The modulated signal waveform may be a sine wave or other periodic signal, as shown in fig. 2.
An all-fiber type open gas detection method, the method adopts the above-mentioned all-fiber type open gas detection system to detect; the computer control unit 10 comprises a laser driving signal output module, a frequency signal demodulation and analysis algorithm module and an absorption spectrum fitting inversion concentration algorithm module which are compiled by Labview software; the detection method specifically comprises the following steps:
[01] a laser wavelength tuning or modulation driving voltage signal output by a laser driving signal output module compiled by Labview software in the computer control unit 10 is changed into an analog signal through a digital-to-analog conversion module 9-2 and then is input into the wavelength tunable laser 1 through a laser control module 11 to realize laser wavelength tuning and modulation output; the computer control unit 10 comprises a signal output module and an input module which are compiled based on Labview software, the signal output module provides a low-frequency triangular wave or sawtooth wave voltage driving signal required by laser wavelength tuning and a high-frequency sine modulation signal, and the output end of the signal output module is connected with the digital-to-analog conversion module 9-2;
[02] the signal output end of the digital-to-analog conversion module 9-2 is connected with the input end of the laser control module 11, the laser control module 11 is used for converting a driving voltage signal into a current signal to drive the laser to emit laser, and simultaneously, the working temperature of the laser is controlled to enable the laser to stably emit light at a fixed temperature, the signal output end of the laser control module 11 is connected with the wavelength tunable laser 1, the wavelength tunable laser 1 is output through an optical fiber and is directly connected to a 1 × 3 optical fiber beam splitter 2, and the 1 × 3 optical fiber beam splitter 2 divides the beam splitting proportion of the incident laser 8 1 into three paths;
[03] the first path of laser output by the optical fiber beam splitter 2 is collimated by an optical fiber collimator 3-1 and then enters a three-mirror-cavity system consisting of three high-reflection mirrors, an incident small hole is formed in the center of a reflector 4-1 of the three mirror cavity to supply incident light to enter the three-mirror-cavity system, the incident light is reflected for multiple times in the three-mirror-cavity system and finally exits through an exit small hole in the center of the reflector 4-3, the physical distance between the three reflectors of the three mirror cavity can be flexibly adjusted according to actual requirements to meet monitoring requirements in different area ranges, the emergent light passing through the first reflector is focused by a focusing lens 7 and enters a first photoelectric detector 8-1, and the position of the first photoelectric detector 8-1 is at the focal position of the focusing lens 7;
[04] referring to fig. 2, in the direct absorption spectrum detection, the second laser output by the fiber beam splitter 2 directly enters the reference cell 5 based on the hollow-core fiber through the fiber coupler 3-2, the reference cell is filled with gas components to be monitored with known concentration, and the emergent light is directly coupled to the second photodetector 8-2. The high signal-to-noise ratio reference spectrum signal generated by the hollow-core optical fiber reference pool 5 is used for locking the central frequency of the laser, and the variation of the central position of the reference spectrum is calculated and converted into the offset of the bias voltage, and the offset is synchronously fed back to the laser control module 11, so that the problem of laser wavelength drift caused by environmental factors in the application of the external field environment is solved. The third path of laser output by the optical fiber beam splitter 2 directly enters an optical fiber ring cavity interferometer 6 through an optical fiber coupler 3-3, incident light generates interference fringes inside the interferometer, the corresponding relative tuning wave number of the laser in a working current or voltage range can be calculated according to the interference fringes, and the absolute wavelength or frequency correction of the tuning range of the laser is finally realized by combining the position of a molecular center absorption line given by a spectrum database; the obtained absolute wavelength of the laser was corrected as the abscissa of the absorption spectrum.
[05] Referring to fig. 3, in the wavelength modulation spectrum detection, the second path of laser output by the optical fiber beam splitter 2 directly enters the reference cell 5 based on the hollow-core optical fiber through the optical fiber coupler 3-2, the inside of the reference cell is filled with gas components to be monitored with known concentration, and the emergent light is directly coupled to the second photodetector 8-2. A frequency signal demodulation and analysis algorithm module compiled by Labview software in the computer control unit 10 demodulates a second harmonic signal, and the integral average concentration in the monitoring area range of the three mirror cavities can be calculated by performing multi-dimensional linear fitting on the second harmonic signal in the signal channel of the three mirror cavities and a reference second harmonic signal measured under the known concentration in the hollow fiber reference pool according to the linear relation between the second harmonic signal and the concentration of the gas to be measured in the hollow fiber reference pool 5.
[06] Finally, voltage signals output by the three photoelectric detectors are simultaneously input into a digital-to-analog conversion module, the digital-to-analog conversion module inputs the signals into an absorption spectrum fitting inversion concentration algorithm module written by Labview software in a computer control unit, and the absorption spectrum fitting inversion concentration algorithm module detects the concentration according to a preselected spectrum detection method: and (4) performing relevant processing analysis by correction-free direct absorption spectrometry and wavelength modulation spectrometry.
The signal calculation processing process of the correction-free direct absorption spectrometry is as follows:
from lambert-beer's law, the change in intensity I (v) of an electromagnetic wave at frequency v when the laser passes through an absorptive gas filled region having an optical path L can be described by:
I(v)＝I 0 (v)e -α(v)L
in the above formula, I 0 (v) Is the initial incident intensity, I (v) is the final received light intensity after passing through the gas medium, α (v) is the absorption coefficient of the molecule, and the integral area of the molecular absorption can be calculated from the above equation:
wherein the expression of the number density of molecules
Absorption line type phi (v-v) 0 ) The normalization condition is satisfied; p is total pressure, T is actual temperature, reference temperature T 0 =296K, reference pressure P 0 =1atm; in the open type measurement, the integrated area A can be calculated by fitting the absorption spectrum by selecting a Lorentz line type and a minimum quadratic fitting algorithm, and the gas concentration information can be inverted by combining the Lambert-beer law satisfied by the absorption spectrum according to the absorption spectrum line parameters and the known optical path L of the known gas molecules and other related physical quantity information, so that the concentration of the gas to be detected can be directly inverted without correcting a standard gas sample.
The signal calculation processing procedure of the wavelength modulation spectroscopy is as follows:
the wavelength modulation spectroscopy is based on a correction-free direct absorption method technology, and realizes effective suppression of 1/f noise limiting the system sensitivity by superposing a high-frequency modulation signal in a laser wavelength tuning current signal and shifting the signal frequency to a high-frequency range, thereby realizing high-sensitivity measurement; when a sinusoidal modulation signal of angular frequency omega is superimposed on the laser drive current, the laser output wavelength or frequency is a function ofTime variation, given by the following expression
I 0 (t)＝I 0 +i 0 cos(ωt+ψ)
The transmission coefficient was derived from lambert-beer law:
wherein:
transmission coefficient for modulated light: />
Using a fourier-cosine series expansion we can obtain:
under conditions of weak absorption, the transmission can be reduced to
The second harmonic is widely adopted due to its higher signal-to-noise ratio, and its Fourier coefficient can be expressed as
In practical application, the relationship between the second harmonic term and the concentration of the molecule to be detected can be obtained by using a digital phase-locked demodulation algorithm
I 2f ∝I 0 α(v)CL
From this equation, the actually measured secondary signal amplitude I 2f The concentration C of the gas to be analyzed satisfies a certain linear relation; in the detection method, the second harmonic in the signal channel of the three mirror cavities is generatedAnd carrying out multi-dimensional linear fitting on the wave signal and a reference second harmonic signal measured under the known concentration in the hollow optical fiber reference pool, and calculating the integral average concentration in the monitoring area range of the three mirror cavities.
Fig. 1 is a schematic structural diagram of an all-fiber open gas monitoring system and method according to an embodiment.
Fig. 2 shows the laser scanning signal (top), the signal detected by the photodetector (middle) and the absorption signal after wavelength correction (bottom) for the device of the present invention.
Fig. 3 shows the laser modulation signal (up), the signal detected by the photodetector (middle) and the demodulated second harmonic signal (down) used in the apparatus of the present invention.
Although the embodiment of the invention takes two spectrum methods of direct absorption spectrum and wavelength modulation spectrum as examples, the embodiment of the invention can be popularized to other spectrum technology applications.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered to be within the technical scope of the present invention, and the technical solutions and the inventive concepts thereof according to the present invention should be equivalent or changed within the scope of the present invention.
Claims (10)
1. An all-fiber open gas monitoring system, comprising: the device comprises a wavelength tunable laser, a 1 x 3 optical fiber beam splitter, a three-mirror cavity system, a reference pool based on a hollow optical fiber, an optical fiber annular cavity interferometer, three photoelectric detectors, a laser control module, an analog-digital and digital-analog conversion module and a computer control unit;
the signal output end of the computer control unit is connected with a digital-to-analog conversion module, the signal output end of the digital-to-analog conversion module is connected with a laser control module, and the signal output end of the laser control module is in control connection with a wavelength tunable laser;
the wavelength tunable laser is output through an optical fiber and connected with a 1 x 3 optical fiber beam splitter, the 1 x 3 optical fiber beam splitter divides the laser into three paths according to a certain proportion and outputs the three paths, a first path of incident light with the highest proportion is collimated by an optical fiber collimator and then enters a three-mirror-cavity system, the three-mirror-cavity system comprises a first mirror, a second mirror and a third mirror, an incident aperture and an emergent aperture are respectively formed in the centers of the first mirror and the third mirror of the three-mirror-cavity system, the incident light enters the three-mirror-cavity system through the incident aperture and sequentially passes through the third mirror and the second mirror, the incident light forms multiple reflections in the three-mirror-cavity system by adjusting the incident angle and finally exits from the emergent aperture, the emergent light of the three-mirror-cavity system is focused by a focusing lens and then enters a first photoelectric detector, and the first photoelectric detector is arranged at the focus of the focusing lens;
a second path of incident light generated by the 1 x 3 optical fiber beam splitter is input into a reference pool based on a hollow-core optical fiber through a first optical fiber coupler, the reference pool based on the hollow-core optical fiber is filled with gas molecules to be detected with a certain concentration, the length and the concentration of the filled gas of the hollow-core optical fiber are selected according to actual requirements, and the incident light is directly coupled to a second photoelectric detector after being emitted from the hollow-core optical fiber;
a third path of incident light generated by the 1 × 3 optical fiber beam splitter is input to the optical fiber ring cavity interferometer through the second optical fiber coupler, interference is generated inside the optical fiber ring cavity interferometer, and the incident light is directly coupled to a third photoelectric detector after being emitted from the optical fiber ring cavity interferometer;
the signal output ends of the three photoelectric detectors are connected to a multi-channel analog-to-digital conversion module, and the analog-to-digital conversion module inputs three photoelectric signals to a computer control unit for online real-time analysis and processing.
2. The all-fiber open gas monitoring system according to claim 1, wherein the center wavelength of the wavelength tunable laser is determined according to the kind of gas molecules to be detected.
3. The all-fiber open gas monitoring system as claimed in claim 1, wherein the first cavity mirror, the second cavity mirror and the third cavity mirror of the three-cavity system are all mirrors, and the physical distances between the three mirrors can be adjusted according to practical application requirements to achieve gas monitoring requirements in different area ranges.
4. The all-fiber open gas monitoring system according to claim 1, wherein the 1 x 3 fiber splitter has a splitting ratio of 8.
5. An all-fiber open gas detection method, which is characterized in that the method adopts the all-fiber open gas monitoring system of any one of claims 1 to 4 to realize the detection of gas leakage in the open area;
the computer control unit comprises a laser driving signal output module, a frequency signal demodulation and analysis algorithm module and an absorption spectrum fitting and inversion concentration algorithm module which are compiled by Labview software; the detection method specifically comprises the following steps:
[01] a laser wavelength tuning or modulation driving voltage signal output by a laser driving signal output module compiled by computer control unit Labview software is converted into an analog signal through a digital-to-analog conversion module and then is input into a wavelength tunable laser through a laser control module to realize laser wavelength tuning and modulation output;
[02] the laser emitted by the wavelength tunable laser is output through the optical fiber and is directly coupled to the 1 x 3 optical fiber beam splitter, so that three paths of laser are synchronously output;
[03] the first path of laser is collimated by the optical fiber collimator and then enters the three-mirror cavity system, the incident light is emitted after being reflected for multiple times in the three-mirror cavity, the emergent light is focused by the focusing lens and enters the first photoelectric detector, and the position of the first photoelectric detector is positioned at the focus position of the focusing lens;
[04] the second path of laser directly enters a reference pool based on the hollow-core optical fiber through a first optical fiber coupler, and emergent light is directly coupled to a second photoelectric detector;
[05] the third path of laser directly enters the optical fiber ring cavity interferometer through the second optical fiber coupler, interference fringes are generated in incident light inside the interferometer, and the incident light is directly coupled to a third photoelectric detector after being emitted from the hollow optical fiber;
[06] and finally, voltage signals output by the three photoelectric detectors are simultaneously input into a digital-to-analog conversion module, the digital-to-analog conversion module inputs the signals into a frequency signal demodulation analysis algorithm module and an absorption spectrum fitting inversion concentration algorithm module which are compiled by Labview software in a computer control unit, and the absorption spectrum fitting inversion concentration algorithm module carries out related processing analysis according to a preselected spectrum detection method.
6. The all-fiber open gas detection method according to claim 5, wherein the frequency signal demodulation and analysis algorithm module is configured to use a high signal-to-noise ratio reference spectrum signal generated by the hollow-core fiber-based reference cell for locking a center frequency of the laser according to a signal collected by the second photodetector, calculate a variation of a center position of the reference spectrum, convert the variation into a bias voltage offset, and synchronously feed the offset back to the control module of the wavelength tunable laser, and in the wavelength modulation spectrum detection mode, the multi-dimensional linear fitting algorithm is combined with an absorption spectrum signal output by the hollow fiber reference cell with a known sample concentration, so that the multi-dimensional linear fitting algorithm can be directly used to calculate the concentration of the sample to be analyzed corresponding to the spectrum signal output by the three-mirror cavity system through inversion.
7. The all-fiber open gas detection method according to claim 5 or 6, wherein the absorption spectrum fitting inversion concentration algorithm module calculates the corresponding relative tuning wave number of the wavelength tunable laser in the working current or voltage range according to the interference fringes according to the signal collected by the third photodetector, and finally realizes the absolute wavelength or frequency correction of the laser tuning range by combining the molecular center absorption line position given by the spectral database; and correcting the obtained absolute wavelength of the laser to be used as the abscissa of the absorption spectrum, and fitting and inverting the absolute wavelength to calculate the concentration information of the gas to be detected in the preselected spectrum detection method.
8. The all-fiber open gas detection method according to claim 7, wherein said preselected spectral detection methods include calibration-free direct absorption spectroscopy and high-sensitivity wavelength modulation spectroscopy.
9. The all-fiber open gas detection method according to claim 8, wherein the signal calculation process of the correction-free direct absorption spectroscopy method comprises the following steps:
from lambert-beer's law, the change in intensity I (v) of an electromagnetic wave at a frequency v when the laser passes through an absorptive gas filled region having an optical path length L can be described by:
I(v)＝I 0 (v)e -α(v)L
in the above formula, I 0 (v) Is the initial incident intensity, I (v) is the final received light intensity after passing through the gas medium, α (v) is the absorption coefficient of the molecule, and the integral area of the molecular absorption can be calculated from the above equation:
wherein the expression of the number density of molecules
Absorption line type phi (v-v) 0 ) The normalization condition is satisfied; p is total pressure, T is actual temperature, reference temperature T 0 =296K, reference pressure P 0 =1atm; in the open type measurement, the integral area A can be calculated by fitting the direct absorption spectrum by selecting a Lorentz line type and a minimum quadratic fitting algorithm, and the concentration information of the gas to be detected can be inverted by combining the Lambert-beer law satisfied by the absorption spectrum according to the absorption spectrum line parameters of the known gas molecules and the known physical quantity temperature T and the optical path L, so that the concentration of the gas to be detected can be directly inverted without correcting a standard gas sample.
10. The all-fiber open gas detection method according to claim 9, wherein the signal calculation process of the wavelength modulation spectroscopy is as follows:
the wavelength modulation spectroscopy is characterized in that on the basis of correction-free direct absorption spectroscopy, a high-frequency modulation signal is superposed in a wavelength tuning current signal of a laser to shift the signal frequency to a high-frequency range, so that 1/f noise limiting the sensitivity of a system is effectively suppressed, and high-sensitivity measurement is realized; when a sinusoidal modulation signal of angular frequency ω is superimposed on the laser drive current, the laser output wavelength or frequency varies with time, as given by the following expression
I 0 (t)＝I 0 +i 0 cos(ωt+ψ)
The transmission coefficient can be derived from lambert-beer's law:
wherein:
transmission coefficient for modulated light: />
Using a fourier cosine series expansion we can obtain:
under conditions of weak absorption, the transmission can be simplified to
The second harmonic is widely adopted due to its higher signal-to-noise ratio, and its Fourier coefficient can be expressed as
In practical application, the relation I between the second harmonic term and the concentration of the molecule to be detected can be obtained by using a digital phase-locked demodulation algorithm 2f ∝I 0 α(v)CL
From this equation, the actually measured secondary signal amplitude I 2f The concentration C of the gas to be analyzed satisfies a linear relation; in the detection method, the second harmonic signal in the signal channel of the three mirror cavities and the reference second harmonic signal measured under the known concentration in the hollow optical fiber reference pool are subjected to multi-dimensional linear fitting, so that the integral average concentration in the monitoring area range of the three mirror cavities can be calculated.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202010848024.8A CN111982813B (en) | 2020-08-21 | 2020-08-21 | All-fiber open gas monitoring system and detection method |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202010848024.8A CN111982813B (en) | 2020-08-21 | 2020-08-21 | All-fiber open gas monitoring system and detection method |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CN111982813A CN111982813A (en) | 2020-11-24 |
| CN111982813B true CN111982813B (en) | 2023-04-07 |
Family
ID=73443627
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN202010848024.8A Active CN111982813B (en) | 2020-08-21 | 2020-08-21 | All-fiber open gas monitoring system and detection method |
Country Status (1)
| Country | Link |
|---|---|
| CN (1) | CN111982813B (en) |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113612859B (en) * | 2021-09-18 | 2023-04-18 | 中国科学院长春光学精密机械与物理研究所 | Mine environment monitoring system and monitoring method thereof |
| CN113970525B (en) * | 2021-10-26 | 2023-07-25 | 安徽大学 | All-fiber type deep sea multi-parameter in-situ detection device and method |
| CN114280003A (en) * | 2021-12-10 | 2022-04-05 | 山东大学 | Gas molecule number density detection method based on first harmonic integration processing |
| CN115112603A (en) * | 2022-06-24 | 2022-09-27 | 浙江师范大学 | Novel quasi-distributed optical fiber gas detection method |
| CN114964323B (en) * | 2022-07-12 | 2022-10-21 | 中国航空工业集团公司北京长城计量测试技术研究所 | Multi-wavelength optical signal generating device and method |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002004903A1 (en) * | 2000-07-12 | 2002-01-17 | Macquarie Research Ltd | Optical heterodyne detection in optical cavity ringdown spectroscopy |
| CN105277503A (en) * | 2015-08-20 | 2016-01-27 | 安徽大学 | Two quantum cascade laser spectrum-based multicomponent gas simultaneous detection device and method |
| WO2016127146A1 (en) * | 2015-02-06 | 2016-08-11 | Block Engineering, Llc | Quantum cascade laser (qcl) based gas sensing system and method |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2948761B1 (en) * | 2013-01-23 | 2023-06-28 | California Institute of Technology | Miniature tunable laser spectrometer for detection of a trace gas |
| US9846118B2 (en) * | 2016-04-13 | 2017-12-19 | The Hong Kong Polytechnic University Shenzhen Research Institute | Photothermal spectroscopy with hollow-core optical fiber |
-
2020
- 2020-08-21 CN CN202010848024.8A patent/CN111982813B/en active Active
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002004903A1 (en) * | 2000-07-12 | 2002-01-17 | Macquarie Research Ltd | Optical heterodyne detection in optical cavity ringdown spectroscopy |
| WO2016127146A1 (en) * | 2015-02-06 | 2016-08-11 | Block Engineering, Llc | Quantum cascade laser (qcl) based gas sensing system and method |
| CN105277503A (en) * | 2015-08-20 | 2016-01-27 | 安徽大学 | Two quantum cascade laser spectrum-based multicomponent gas simultaneous detection device and method |
Non-Patent Citations (2)
| Title |
|---|
| An open-path sensor for simultaneous atmospheric pressure detection of CO and CH4 around 2.33 μm;Jingmin Dang等;《Optics and Lasers in Engineering》;第123卷;第1-7页 * |
| 基于光纤环腔激光技术的气体传感研究;张文超等;《激光杂志》;第36卷(第10期);第102-104页 * |
Also Published As
| Publication number | Publication date |
|---|---|
| CN111982813A (en) | 2020-11-24 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CN111982813B (en) | All-fiber open gas monitoring system and detection method | |
| Ma et al. | Trace gas sensing based on multi-quartz-enhanced photothermal spectroscopy | |
| US5767976A (en) | Laser diode gas sensor | |
| US5173749A (en) | Method and apparatus for spectroscopic measurement of the concentration of a gas | |
| Xia et al. | A ppb level sensitive sensor for atmospheric methane detection | |
| Waclawek et al. | Balanced-detection interferometric cavity-assisted photothermal spectroscopy | |
| CN109991189B (en) | Fixed point wavelength modulation gas concentration measuring device based on wave number drift correction and measuring method thereof | |
| CN104280362A (en) | Online high-temperature water vapor laser spectrum detection system | |
| CN111829982A (en) | Multi-component gas concentration detection and temperature error correction method | |
| EP2342544A2 (en) | Wavelength-modulation spectroscopy method | |
| Shen et al. | Methane near-infrared laser remote detection under non-cooperative target condition based on harmonic waveform recognition | |
| CN114384045B (en) | Trace gas concentration and path length real-time detection system and method | |
| Yang et al. | A methane telemetry sensor based on near-infrared laser absorption spectroscopy | |
| Vargas-Rodríguez et al. | Design of CO, CO2 and CH4 gas sensors based on correlation spectroscopy using a Fabry–Perot interferometer | |
| CN114993987B (en) | Temperature and gas concentration measuring method and system based on absorption spectrum amplitude modulation | |
| Liang et al. | Multiplex-gas detection based on non-dispersive infrared technique: a review | |
| Meshcherinov et al. | Lidar-based remote infrared gas sensor for monitoring anthropogenic pollution: a proof of concept | |
| Wang et al. | Gas concentration sensing based on fiber loop ring-down spectroscopy: a review | |
| CN218865767U (en) | Multi-component wide-range laser gas analysis system | |
| Wang et al. | Multicomponent gas detection based on concise CW-cavity ring-down spectroscopy with a bow-tie design | |
| Liang | Standoff detection of backscattered light based on tunable diode laser absorption spectroscopy | |
| Larnimaa et al. | Photoacoustic phase-controlled Fourier-transform infrared spectroscopy | |
| CN212207093U (en) | Double-frequency comb generating device and gas detection system with same | |
| CN110361359B (en) | Absorption rate function reproduction method based on even harmonic | |
| Okajima et al. | Measurement of methane gas concentration using an infrared LED |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PB01 | Publication | ||
| PB01 | Publication | ||
| SE01 | Entry into force of request for substantive examination | ||
| SE01 | Entry into force of request for substantive examination | ||
| GR01 | Patent grant | ||
| GR01 | Patent grant |