CN114486808B - Gas detection method for enhancing spectral line absorption intensity - Google Patents
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Abstract
A method for detecting gas by enhancing spectral line absorption intensity belongs to the technical field of laser gas detection. According to the method, a pumping light source with a wavelength selected according to the wavelength of the detection light source is used, the pumping light source generates laser to be input into the air chamber to pump gas molecules to be detected from a ground state energy level to a lower energy level of characteristic absorption of the detection light, and the number of molecules on the lower energy level of the characteristic absorption is increased, so that the characteristic absorption of the gas to be detected to the detection light is improved. The invention has the remarkable advantages that the pumping light source is used for enhancing the spectral line absorption intensity of the gas to be detected on the detection light, increasing the absorption of the gas to be detected on the detection light and improving the detection limit and the sensitivity of the system.
Description
Technical Field
The invention relates to a method for detecting gas by enhancing spectral line absorption intensity, belonging to the technical field of laser gas detection.
Background
With the increasing greenhouse effect of the atmosphere in recent years, a series of problems such as global warming have been caused. The greenhouse effect is derived from greenhouse gases, and a large amount of greenhouse gases can be generated in the processes of industrial production, automobile exhaust, energy collection and transportation and the like, so that the monitoring of trace greenhouse gases in the atmosphere is very important. Meanwhile, the trace gas detection technology is also very important to be applied in the fields of medicine, mine production safety, electric power industry safety monitoring and the like.
In a paper of research on methane gas spectrum detection method based on mid-infrared DFG light source by using pump light and signal light to detect methane gas concentration through mid-infrared light source generated by Difference Frequency Generation (DFG) technology, published in 9 of 2018 by Jiulong, university of Nanjing information engineering university, chang Jianhua et al, in 39 of applied journal of optics, the detection method is not changed in principle, and a limit selected on spectral line absorption intensity exists, so that the accuracy of trace gas detection is limited.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a method for detecting gas by enhancing spectral line absorption intensity, which aims to solve the problems of low gas detection precision, too small trace gas light absorption signal and too high signal processing difficulty.
The invention is realized by the following technical scheme:
the system comprises a temperature control current source A, a temperature control current source B, a pump laser, a detection laser, an air chamber, an optical filter, a photoelectric detector, a current-to-voltage module, a data acquisition card and a computer, wherein the temperature control current source A and the temperature control current source B respectively comprise two parts of temperature control and drive current control, the temperature control current source A and the temperature control current source B are respectively connected with the pump laser and the detection laser, the output ends of the detection laser and the pump laser are coupled through optical fibers and then input into the air chamber, and the air chamber is coupled to the input end of the photoelectric detector through the optical filter taking the wavelength of the detection laser as a central wave band and the optical fibers; the output end of the photoelectric detector is connected to the current-to-voltage module, the current-to-voltage module is connected with the input end of the data acquisition card, and the output end of the data acquisition card is connected to the computer; the method comprises the following steps:
1) The system is connected, a power supply of a photoelectric detector, a current-to-voltage module, a data acquisition card and a computer is turned on, and the gas chamber is filled with the gas to be detected;
2) Firstly, power supplies of a pumping laser and a temperature control current source A are turned on, the temperature and the input current are controlled by the temperature control current source A to control the frequency and the light intensity of the output laser of the pumping laser, so that the output frequency of the pumping laser meets the frequency of transferring gas molecules to be detected from a ground state energy level to a lower energy level of the characteristic absorption of the detection light, and the output light intensity is kept constant;
3) Then, the power supplies of the detection laser and the temperature control current source B are turned on, the temperature control part is adjusted to control the temperature of the detection laser, and the driving current control part of the temperature control current source B is adjusted to enable the detection laser to output laser corresponding to the measured gas spectral line absorption peak value, namely the central frequency;
4) The output laser of the detection laser and the pump laser is input into the air chamber after being coupled by the optical fiber, and the output end of the air chamber is connected to the input end of the photoelectric detector by the optical fiber coupling through the optical filter;
5) The output current of the photoelectric detector is converted into a voltage signal through a current-to-voltage module, the voltage signal is collected by a data collection card and then is input into a computer for data processing and analysis, and the concentration result of the detected gas is obtained through the computer.
The detection principle of the method of the invention is as follows:
light emitted by the pump laser is coupled into the air chamber through the optical fiber, and the molecular number of each energy level of the gas to be detected in the air chamber accords with Boltzmann distribution:
wherein n is i And N 1 The molecular numbers of the energy level under the characteristic absorption of the gas and the energy level of the ground state are respectively, g' is the energy level E of the energy level i under the characteristic absorption of the gas during the absorption transition i Q (T) is the partitioning function of the gas absorbed at a temperature T, E i Is the energy level at the absorption transition, and k is the boltzmann constant.
Molecular number n of the gas characteristic absorption lower energy level according to Boltzmann distribution i Is the number N of molecules smaller than the ground state energy level 1 Therefore, most of the molecules of the gas to be detected are at the ground state energy level, and the lower energy level energy of the gas to be detected is higher than the ground state energy level when most of the gas is absorbed by looking up the HITRAN database, so that the Boltzmann distribution of the gas to be detected can be broken through pumping laser injection energy, the number of molecules of the lower energy level of the gas to be detected is increased, and the absorption of the gas to be detected to the detection laser can be enhanced.
The method has the advantages that:
the pump laser device is introduced, the absorption of the gas to be detected to the detection laser is enhanced by breaking the Boltzmann distribution of the gas to be detected, and the detection sensitivity of the whole system to the gas to be detected is improved.
Drawings
FIG. 1 is a schematic diagram of the whole mechanism of the detection method of the present invention.
In the figure: 1-temperature control current source A, 2-temperature control current source B, 3-pump laser, 4-detection laser, 5-air chamber, 6-photoelectric detector, 7-current-to-voltage module, 8-data acquisition card, 9-computer, 10-optical filter.
Fig. 2 is a schematic diagram showing energy level transition of the absorption of the methane molecule for the probe light feature at the time of pump light energy injection in example 1 of the present invention.
In the figure, a pump laser is used for generating 7533.49nm laser to pump partial methane gas molecules from the ground state energy level to 1327cm -1 Energy level, then using a detection laser to generate 7610.92nm laser, carrying out characteristic absorption with the excited methane molecules, pumping the methane molecules to 2641cm -1 An energy level; as is known from HITRAN, when the characteristic absorption of the laser with the wavelength of 7610.92nm and methane molecules occurs, the lower energy level energy of the absorption is 2641cm -1 。
Fig. 3 is a schematic diagram showing energy level transition of absorption of a carbon dioxide molecule for a probe light feature during pump light energy injection in embodiment 2 of the present invention.
In the figure, a pump laser is used for generating 4361.9075nm laser to pump part of carbon dioxide gas molecules from the ground state energy level to 2292cm -1 Energy level, using detection laser to generate 4396.87nm laser, characteristic absorbing with excited methane molecule, pumping carbon dioxide molecule to 4566cm -1 An energy level; according to HITRAN, when the laser with wavelength of 4396.87nm and carbon dioxide molecules are absorbed by the characteristic, the lower energy level energy of the absorption is 2292cm -1 。
Detailed Description
The invention is further illustrated, but not limited, by the following figures and examples.
Example 1:
the system comprises a temperature-controlled current source A1, a temperature-controlled current source B2, a pump laser 3, a detection laser 4, a gas chamber 5, a photoelectric detector 6, a current-to-voltage module 7, a data acquisition card 8, a computer 9 and a light filter 10, wherein the temperature-controlled current source A1 and the temperature-controlled current source B2 are respectively connected with the pump laser 3 and the detection laser 4, the temperature-controlled current source B2 of the detection laser 4 is composed of a temperature control circuit and a driving current control, the light frequency output by the detection laser 4 is ensured to be at the central frequency required by measuring the concentration of the gas to be detected by controlling the temperature and the injection current, the temperature-controlled current source A1 of the pump laser 3 is composed of a temperature control circuit and a driving current control, the light frequency output by the pump laser 3 is ensured to be stable at a frequency capable of meeting the condition that the gas to be detected is transited from a base state energy level to a lower energy level of the characteristic absorption of the detection laser, and the output light energy is kept constant; the output ends of the detection laser 4 and the pump laser 3 are connected with the air chamber 5 through optical fiber coupling, and the air chamber 5 is connected to the input end of the photoelectric detector 6 through an optical filter 10 taking the detection laser wavelength as a central wave band and the optical fiber coupling; the output end of the photoelectric detector 6 is connected to a current-to-voltage module 7, the current-to-voltage module 7 is connected with the input end of a data acquisition card 8, the data acquisition card 8 is connected to a computer 9 to read the data of the data acquisition card, and the method comprises the following steps:
1) The system is connected, a detection laser, a pumping laser, a temperature control current source A, a temperature control current source B, a computer and power supplies of all modules are turned on, and methane gas is injected into the air chamber;
2) The temperature control current source A of the pumping laser controls the central frequency of the output light of the pumping laser to be near the wavelength 7533.49nm by controlling the temperature and the injection current, and the intensity of the light is kept constant;
light emitted by the pump laser is coupled into the air chamber through the optical fiber, and the molecular number of each energy level of methane in the air chamber accords with Boltzmann distribution:
wherein the method comprises the steps ofn i And N 1 The molecular numbers of the energy level under the characteristic absorption of the gas and the energy level of the ground state are respectively, g' is the energy level E of the energy level i under the characteristic absorption of the gas during the absorption transition i Q (T) is the partitioning function of the gas absorbed at a temperature T, E i Is the energy level at the absorption transition, and k is the boltzmann constant.
Molecular number n of the gas characteristic absorption lower energy level according to Boltzmann distribution i Is the number N of molecules smaller than the ground state energy level 1 Therefore, most of the molecules of the gas to be detected are at the ground state energy level, and the lower energy level energy of the gas to be detected is higher than the ground state energy level when the HITRAN database is consulted to find that most of the gas is absorbed, so that the Boltzmann distribution of the detected gas can be broken through pumping laser injection energy, the number of molecules of the lower energy level of the absorbed gas is increased, and the absorption of the gas to be detected to the detection laser can be enhanced.
3) The temperature control current source B of the detection laser controls the output wavelength of the detection laser output light to be about 7610.92nm by controlling the temperature and the injection current;
light emitted by the detection laser is coupled and input into the air chamber through an optical fiber, and after the light is subjected to characteristic absorption with the gas to be detected excited to the lower energy level of characteristic absorption of the detection laser, the light is coupled and connected to the input end of the photoelectric detector through the optical fiber after passing through an optical filter with the central wavelength of 7610.92nm at the output end of the air chamber; the photoelectric detector is used for converting the optical signal into a current signal, the current signal is converted into a voltage signal through the current-to-voltage module and then is collected by the data acquisition card, and the computer is used for processing and analyzing the voltage signal collected by the data acquisition card to obtain the concentration content of methane gas.
As shown in FIG. 2, the energy level transition diagram of the characteristic absorption of the detection light by the methane molecule during the pumping light energy injection is shown, in this embodiment, the detection laser selects a laser with a wavelength of 7610.92nm, and referring to the HITRAN database, when the laser with the wavelength is absorbed by the methane gas characteristic, the energy difference between the lower energy level and the ground state energy level is represented by a wave number, and the value is 1327.308cm -1 So that the absorption lower energy level and ground state occurThe energy difference of the energy levels can be expressed as:
where h= 6.626 ×10 -34 (j·s) is planck constant, c=3×10 10 (cm·s -1 ) Is the speed of light, which is the speed of light,
is the wave number. While the energy of the laser can be expressed as:
where h= 6.626 ×10 -34 (j·s) is planck constant, c=3×10 10 (cm·s -1 ) Is the speed of light, which is the speed of light,
is the wave number. Therefore, in order to pump methane gas from the ground state energy level to the lower energy level of the characteristic absorption of the detection laser, the energy difference between the lower energy level of the absorption and the ground state energy level is equal to the energy of the pumping laser, and the pumping laser selects a wave number of 1327.405cm -1 I.e. a laser of wavelength 7533.49 nm. The line intensities S at the wavelengths 7610.92nm and 7533.49nm are known by looking at the HITRAN database ij 1.968×10 respectively -22 (cm -1 /(molecule·cm -2 ) And 1.16X10) -22 (cm -1 /(molecule·cm -2 ) Smaller).
The pumping laser is derived through Einstein coefficient relation to pump the gas molecules to be detected from the ground state energy level to the high-energy level particle number n i ' is:
wherein n is i ' and N 1 Respectively gas characteristic absorptionThe molecular numbers of the lower energy level and the ground state energy level, g' is the low-state i energy level energy E in absorption transition i G' is the high state j energy level energy E at the time of absorption transition j Degeneracy of (A) 21 Einstein coefficient, ρ, which is the spontaneous transition v Is the pump laser energy density.
For line intensity S ij The expression of (2) is:
wherein n is i And N 1 The particle numbers of the lower energy level and the ground state energy level of the gas generation characteristic absorption to be detected are respectively increased, the spectrum line intensity can be enhanced by increasing the particle number of the lower energy level of the methane molecule generation characteristic absorption, and the increased intensity can be n i ’/n i Expressed as:
thus, by using the method, the energy rho is fixed in the pump laser v Can inject a fixed number of particles n i The pumping to the characteristic absorption lower energy level can fixedly improve the spectral line intensity at the 7610.92nm wavelength, and the characteristic absorption of methane gas to the laser with the wavelength is enhanced.
The relative expression of the gas concentration and the absorption light intensity in the gas chamber under the condition of trace gas concentration to be measured is as follows:
I a ＝I 0 S ij PLg(v,v 0 )C (7)
wherein I is 0 Is the incident light intensity, I a Is the absorption of light intensity, S ij The spectral line intensity is C, and the concentration of the gas to be measured in the gas chamber is C. It can be seen that the gas concentration in the gas chamber and the gas absorption intensity are in linear relation, and the coefficient and spectral line intensity S ij Related, so that the line intensity S is enhanced ij After that, the absorption light intensity corresponding to a certain concentration is also enhanced, so that the detection sensitivity of the whole system to methane gas is improved.
Example 2:
the same as in example 1, except that the gas to be measured was carbon dioxide gas,
as shown in FIG. 3, the energy level transition diagram of the characteristic absorption of carbon dioxide molecules to the detection light during the pumping light energy injection is shown, the detection laser selects a laser with the wavelength of 4396.87nm, and when the HITRAN database is consulted, the energy difference between the lower energy level and the ground state energy level is represented by a wave number, and the value is 2292.6453cm -1 。
The pump laser selects a wave number of 2292.574963cm -1 I.e. a laser of wavelength 4361.9075 nm. The line intensities S at the wavelengths 4396.87nm and 4361.9075nm are known by looking at the HITRAN database ij 3.176 ×10 respectively -22 (cm -1 /(molecule·cm -2 ) And 9.018 ×10) -22 (cm -1 /(molecule·cm -2 ) By use of the present method, the pump laser energy injection can enhance the line intensity at the wavelength 4396.87 nm. The characteristic absorption of the carbon dioxide gas to the laser with the wavelength is enhanced, and the detection sensitivity of the whole system to the carbon dioxide gas is improved.
Claims (1)
1. The system comprises a temperature control current source A, a temperature control current source B, a pump laser, a detection laser, a gas chamber, an optical filter, a photoelectric detector, a current-to-voltage module, a data acquisition card and a computer, wherein the temperature control current source A and the temperature control current source B respectively comprise two parts of temperature control and drive current control, the temperature control current source A and the temperature control current source B are respectively connected with the pump laser and the detection laser, the output ends of the detection laser and the pump laser are coupled through optical fibers and then input into the gas chamber, and the gas chamber is coupled to the input end of the photoelectric detector through the optical filter taking the wavelength of the detection laser as a central wave band and the optical fibers; the output end of the photoelectric detector is connected to the current-to-voltage module, the current-to-voltage module is connected with the input end of the data acquisition card, and the output end of the data acquisition card is connected to the computer; the method comprises the following steps:
1) The system is connected, a power supply of a photoelectric detector, a current-to-voltage module, a data acquisition card and a computer is turned on, and the gas chamber is filled with the gas to be detected;
2) Firstly, power supplies of a pumping laser and a temperature control current source A are turned on, the temperature and the input current are controlled by the temperature control current source A to control the frequency and the light intensity of output laser of the pumping laser, so that the output frequency of the pumping laser meets the frequency of transition of gas molecules to be detected from a ground state energy level to a lower energy level of characteristic absorption of detection light, and the output light intensity is kept constant; the Boltzmann distribution of the gas molecules to be detected is broken through the injection energy of pumping laser, and the number of molecules of the lower energy level which are absorbed is increased;
3) Then, a power supply adjusting temperature control part of the detection laser and the temperature control current source B is turned on to control the temperature of the detection laser, and a driving current control part of the temperature control current source B is adjusted to enable the detection laser to output laser corresponding to the measured gas spectral line absorption peak value, namely the central frequency;
4) The output laser of the detection laser and the pump laser is input into the air chamber after being coupled by the optical fiber, and the output end of the air chamber is connected to the input end of the photoelectric detector by the optical fiber coupling through the optical filter;
5) The output current of the photoelectric detector is converted into a voltage signal through a current-to-voltage module, the voltage signal is collected by a data collection card and then is input into a computer for data processing and analysis, and the concentration result of the detected gas is obtained through the computer.
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