CN114486808A - Gas detection method for enhancing spectral line absorption intensity - Google Patents
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Abstract
Description
技术领域technical field
本发明涉及一种增强谱线吸收强度型气体检测方法,属于激光气体检测技术领域。The invention relates to an enhanced spectral line absorption intensity type gas detection method, which belongs to the technical field of laser gas detection.
背景技术Background technique
近些年随着大气的温室效应的不断加强，已经引起了全球变暖等一系列问题。温室效应来自于温室气体，在工业生产，汽车尾气以及能源采集运输等过程中会产生大量温室气体，所以监测大气中痕量温室气体十分重要。同时，痕量气体检测技术在医学、矿井生产安全和电业安全监测等领域也有十分重要的应用。In recent years, with the continuous strengthening of the greenhouse effect of the atmosphere, it has caused a series of problems such as global warming. The greenhouse effect comes from greenhouse gases, and a large amount of greenhouse gases will be produced in the process of industrial production, automobile exhaust, energy collection and transportation, etc., so it is very important to monitor trace amounts of greenhouse gases in the atmosphere. At the same time, trace gas detection technology also has very important applications in the fields of medicine, mine production safety and electrical safety monitoring.
南京信息工程大学房久龙,常建华等人在2018年9月发表在应用光学杂志第39卷第5期的一篇文章《基于中红外DFG光源的甲烷气体光谱检测方法研究》中使用了泵浦光和信号光通过差频产生(DFG)技术产生中红外光源进行甲烷气体浓度检测，但上述检测方法在原理上并未改变待测气体的谱线吸收强度，会存在谱线吸收强度上选取的极限，从而导致痕量气体检测的精度受到限制。Nanjing University of Information Technology Fang Jiulong, Chang Jianhua and others published an article "Research on Methane Gas Spectral Detection Method Based on Mid-Infrared DFG Light Source" published in the Journal of Applied Optics, Vol. 39,
发明内容SUMMARY OF THE INVENTION
为了克服上述现有技术的不足，本发明提供了一种增强谱线吸收强度型气体检测方法，以解决气体检测的精度低、痕量气体光吸收信号过小、信号处理难度过高的问题。In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides an enhanced spectral line absorption intensity type gas detection method to solve the problems of low gas detection accuracy, too small light absorption signal of trace gas, and too high signal processing difficulty.
本发明是通过以下技术方案实现的：The present invention is achieved through the following technical solutions:
一种增强谱线吸收强度型气体检测方法，由以下系统来实现，该系统包括温控电流源A、温控电流源B、泵浦激光器、探测激光器、气室、滤光片、光电探测器、电流转电压模块、数据采集卡、计算机，温控电流源A和温控电流源B均包括温度控制和驱动电流控制两部分，温控电流源A和温控电流源B分别连接泵浦激光器和探测激光器，探测激光器和泵浦激光器的输出端通过光纤耦合后输入到气室，气室通过以探测激光波长为中心波段的滤光片和光纤耦合连接到光电探测器的输入端；光电探测器的输出端连接到电流转电压模块，电流转电压模块和数据采集卡输入端相连接，数据采集卡的输出端连接到计算机；该方法的步骤如下：An enhanced spectral line absorption intensity type gas detection method is realized by the following system, the system includes a temperature-controlled current source A, a temperature-controlled current source B, a pump laser, a detection laser, a gas chamber, an optical filter, and a photodetector , current-to-voltage module, data acquisition card, computer, temperature-controlled current source A and temperature-controlled current source B both include temperature control and drive current control, and temperature-controlled current source A and temperature-controlled current source B are respectively connected to pump lasers and the detection laser, the output ends of the detection laser and the pump laser are coupled to the gas chamber through the optical fiber, and the gas chamber is connected to the input end of the photodetector through the optical filter and the optical fiber coupling with the wavelength of the detection laser as the center band; the photoelectric detection The output end of the device is connected to the current-to-voltage module, the current-to-voltage module is connected to the input end of the data acquisition card, and the output end of the data acquisition card is connected to the computer; the steps of the method are as follows:
1)连接好上述系统，打开光电探测器、电流转电压模块、数据采集卡和计算机的电源，在气室中注入被测气体；1) Connect the above systems, turn on the power of the photodetector, the current-to-voltage module, the data acquisition card and the computer, and inject the measured gas into the gas chamber;
2)先打开泵浦激光器和温控电流源A的电源，通过温控电流源A控制温度和输入电流来控制泵浦激光器输出激光的频率和光强，使其输出频率满足将待测气体分子从基态能级跃迁到探测光发生特征吸收的下能级的频率处，且输出光强大小保持恒定；2) First turn on the power of the pump laser and the temperature-controlled current source A, and control the temperature and input current through the temperature-controlled current source A to control the frequency and light intensity of the output laser of the pump laser, so that the output frequency can satisfy the gas molecules to be measured. Transition from the ground state energy level to the frequency of the lower energy level where the characteristic absorption of the probe light occurs, and the output light intensity remains constant;
3)然后打开探测激光器和温控电流源B的电源,调整温度控制部分对探测激光器进行温控，调整温控电流源B的驱动电流控制部分使探测激光器输出对应于被测气体谱线吸收峰值即中心频率处的激光；3) Then turn on the power of the detection laser and the temperature-controlled current source B, adjust the temperature control part to control the temperature of the detection laser, and adjust the driving current control part of the temperature-controlled current source B so that the output of the detection laser corresponds to the absorption peak value of the measured gas spectral line That is, the laser at the center frequency;
4)探测激光器和泵浦激光器输出激光经过光纤耦合之后输入到气室，气室输出端通过滤光片由光纤耦合连接到光电探测器的输入端；4) The output laser of the detection laser and the pump laser is input into the gas chamber after being coupled by the optical fiber, and the output end of the gas chamber is connected to the input end of the photodetector by the optical fiber coupling through the optical filter;
5)光电探测器的输出电流经电流转电压模块转换后变成电压信号，电压信号由数据采集卡采集后输入计算机内进行数据处理和分析，经计算机得出被测气体的浓度结果。5) The output current of the photodetector is converted into a voltage signal after being converted by the current-to-voltage module. The voltage signal is collected by the data acquisition card and then input into the computer for data processing and analysis. The computer obtains the concentration result of the measured gas.
本发明方法的检测原理如下：The detection principle of the inventive method is as follows:
泵浦激光器发出的光通过光纤耦合输入到气室，气室中的待测气体各个能级分子数符合玻尔兹曼分布：The light emitted by the pump laser is coupled into the gas chamber through the optical fiber, and the number of molecules at each energy level of the gas to be tested in the gas chamber conforms to the Boltzmann distribution:
其中ni和N1分别为气体特征吸收下能级与基态能级分子数，g″是吸收跃迁时气体特征吸收下能级i能级能量Ei的简并度，Q(T)是在温度为T的时候吸收气体的配分函数，Ei是吸收跃迁时下能级的能量大小，k是玻尔兹曼常数。where n i and N 1 are the molecular numbers of the energy level and the ground state energy level under the gas characteristic absorption, respectively, g″ is the degeneracy of the energy level i energy E i under the gas characteristic absorption during the absorption transition, and Q(T) is the When the temperature is T, the partition function of the absorbing gas, E i is the energy of the energy level at the time of the absorption transition, and k is the Boltzmann constant.
根据玻尔兹曼分布，气体特征吸收下能级的分子数ni是小于基态能级的分子数N1的，所以待测气体分子大多都处于基态能级，而通过查阅HITRAN数据库，发现大部分气体发生吸收的时候，其发生吸收的下能级能量都是高于基态能级的，因此通过泵浦激光注入能量可以打破待测气体的玻尔兹曼分布，增加发生吸收的下能级的分子数，便可以增强待测气体对探测激光的吸收。According to the Boltzmann distribution, the number of molecules n i of the energy level under the gas characteristic absorption is smaller than the number of molecules of the ground state energy level N 1 , so most of the gas molecules to be tested are in the ground state energy level. When part of the gas absorbs, the lower energy level of its absorption is higher than the ground state energy level. Therefore, injecting energy through the pump laser can break the Boltzmann distribution of the gas to be tested and increase the lower energy level where absorption occurs. The number of molecules to be detected can enhance the absorption of the detection laser by the gas to be detected.
本发明方法的优点：The advantages of the method of the present invention:
引入了泵浦激光装置，通过打破待测气体的玻尔兹曼分布增强待测气体对于探测激光的吸收，提高了整个系统对于待测气体的检测灵敏度。A pump laser device is introduced, which enhances the absorption of the detection laser by the gas to be detected by breaking the Boltzmann distribution of the gas to be detected, and improves the detection sensitivity of the entire system for the gas to be detected.
附图说明Description of drawings
图1为本发明检测方法的整体机构示意图。FIG. 1 is a schematic diagram of the overall mechanism of the detection method of the present invention.
图中：1-温控电流源A、2-温控电流源B、3-泵浦激光器、4-探测激光器、5-气室、6-光电探测器、7-电流转电压模块、8-数据采集卡、9-计算机、10-滤光片。In the figure: 1- temperature-controlled current source A, 2- temperature-controlled current source B, 3- pump laser, 4- detection laser, 5- gas chamber, 6- photodetector, 7- current-to-voltage module, 8- Data acquisition card, 9-computer, 10-filter.
图2为本发明实施例1中所述的甲烷分子在泵浦光能量注入时对于探测光特征吸收的能级跃迁示意图。FIG. 2 is a schematic diagram of the energy level transition of the characteristic absorption of the probe light by the methane molecule described in
图中使用泵浦激光器产生7533.49nm激光将部分甲烷气体分子从基态能级泵浦至1327cm-1能级，接着使用探测激光器产生7610.92nm激光，与被激发的甲烷分子发生特征吸收，将甲烷分子泵浦至2641cm-1能级；根据HITRAN可知波长为7610.92nm激光与甲烷分子发生特征吸收的时候，其发生吸收的下能级能量为2641cm-1。In the figure, a pump laser is used to generate a 7533.49nm laser to pump part of the methane gas molecules from the ground state energy level to 1327cm -1 energy level, and then a probe laser is used to generate a 7610.92nm laser, which has characteristic absorption with the excited methane molecules, and the methane molecules Pumping to 2641cm -1 energy level; according to HITRAN, when the wavelength of 7610.92nm laser and methane molecules have characteristic absorption, the lower energy level of absorption is 2641cm -1 .
图3为本发明实施例2中所述的二氧化碳分子在泵浦光能量注入时对于探测光特征吸收的能级跃迁示意图。3 is a schematic diagram of the energy level transition of the characteristic absorption of the probe light by the carbon dioxide molecule described in Example 2 of the present invention when the energy of the pump light is injected.
图中使用泵浦激光器产生4361.9075nm激光将部分二氧化碳气体分子从基态能级泵浦至2292cm-1能级，使用探测激光器产生4396.87nm激光，与被激发的甲烷分子发生特征吸收，将二氧化碳分子泵浦至4566cm-1能级；根据HITRAN可知波长为4396.87nm激光与二氧化碳分子发生特征吸收的时候，其发生吸收的下能级能量为2292cm-1。In the figure, a pump laser is used to generate a 4361.9075nm laser to pump some carbon dioxide gas molecules from the ground state energy level to 2292cm -1 energy level, and a probe laser is used to generate a 4396.87nm laser, which has characteristic absorption with the excited methane molecules, and the carbon dioxide molecules are pumped. Pu to 4566cm -1 energy level; according to HITRAN, when the wavelength of 4396.87nm laser and carbon dioxide molecules have characteristic absorption, the lower energy level of the absorption is 2292cm -1 .
具体实施方式Detailed ways
下面结合附图和实施例对本发明作进一步说明，但不限于此。The present invention is further described below with reference to the accompanying drawings and embodiments, but is not limited thereto.
实施例1：Example 1:
一种增强谱线吸收强度型气体检测方法，由以下系统来实现，如图1所示，该系统包括温控电流源A1、温控电流源B2、泵浦激光器3、探测激光器4、气室5、光电探测器6、电流转电压模块7、数据采集卡8、计算机9和滤光片10，温控电流源A1和温控电流源B2分别连接泵浦激光器3和探测激光器4，其中探测激光器4的温控电流源B2由温度控制电路和驱动电流控制两部分构成，通过控制温度和注入电流保证探测激光器4输出的光频率处于测量待测气体浓度所需中心频率处，泵浦激光器3的温控电流源A1是由温度控制电路和驱动电流控制两部分构成，通过控制温度和注入电流保证泵浦激光器3输出的光频率稳定在能够满足将待测气体从基态能级跃迁到探测激光发生特征吸收的下能级的频率处，且输出的光能量保持恒定；探测激光器4和泵浦激光器3的输出端通过光纤耦合共同与气室5相连接，气室5通过以探测激光波长为中心波段的滤光片10和光纤耦合连接到光电探测器6的输入端；光电探测器6的输出端连接到电流转电压模块7，电流转电压模块7和数据采集卡8输入端相连接，数据采集卡8连接到计算机9以读取数据采集卡的数据，该方法的步骤如下：An enhanced spectral line absorption intensity type gas detection method is realized by the following system, as shown in Figure 1, the system includes a temperature-controlled current source A1, a temperature-controlled current source B2, a
1)连接好上述系统，打开探测激光器和泵浦激光器、温控电流源A、温控电流源B、计算机和各模块电源，在气室中注入甲烷气体；1) Connect the above systems, turn on the detection laser and the pump laser, the temperature-controlled current source A, the temperature-controlled current source B, the computer and the power supply of each module, and inject methane gas into the gas chamber;
2)泵浦激光器的温控电流源A通过控制温度和注入电流来控制泵浦激光输出光的中心频率在波长7533.49nm附近，且光强大小保持恒定；2) The temperature-controlled current source A of the pump laser controls the center frequency of the pump laser output light by controlling the temperature and the injection current to be near the wavelength of 7533.49nm, and the light intensity remains constant;
泵浦激光器发出的光通过光纤耦合输入到气室，气室中的甲烷各个能级分子数符合玻尔兹曼分布：The light emitted by the pump laser is coupled into the gas cell through the optical fiber, and the number of methane molecules at each energy level in the gas cell conforms to the Boltzmann distribution:
其中ni和N1分别为气体特征吸收下能级与基态能级分子数，g″是吸收跃迁时气体特征吸收下能级i能级能量Ei的简并度，Q(T)是在温度为T的时候吸收气体的配分函数，Ei是吸收跃迁时下能级的能量大小，k是玻尔兹曼常数。where n i and N 1 are the molecular numbers of the energy level and the ground state energy level under the gas characteristic absorption, respectively, g″ is the degeneracy of the energy level i energy E i under the gas characteristic absorption during the absorption transition, and Q(T) is the When the temperature is T, the partition function of the absorbing gas, E i is the energy of the energy level at the time of the absorption transition, and k is the Boltzmann constant.
根据玻尔兹曼分布，气体特征吸收下能级的分子数ni是小于基态能级的分子数N1的，所以待测气体分子大多都处于基态能级，而通过查阅HITRAN数据库，发现大部分气体发生吸收的时候，其发生吸收的下能级能量都是高于基态能级的，因此通过泵浦激光注入能量可以打破检测气体的玻尔兹曼分布，增加发生吸收的下能级的分子数，便可以增强待测气体对探测激光的吸收。According to the Boltzmann distribution, the number of molecules n i of the energy level under the gas characteristic absorption is smaller than the number of molecules of the ground state energy level N 1 , so most of the gas molecules to be tested are in the ground state energy level. When part of the gas absorbs, the energy of the lower energy level of the absorption is higher than the ground state energy level, so injecting energy through the pump laser can break the Boltzmann distribution of the detected gas and increase the absorption of the lower energy level. The number of molecules can enhance the absorption of the detection laser by the gas to be detected.
3)探测激光器的温控电流源B通过控制温度和注入电流来控制探测激光输出光的输出波长在7610.92nm附近；3) The temperature-controlled current source B of the detection laser controls the output wavelength of the detection laser output light to be around 7610.92 nm by controlling the temperature and the injection current;
探测激光器发出的光通过光纤耦合输入到气室，与被激发到探测激光特征吸收的下能级上的待测气体发生特征吸收后，通过位于气室输出端处中心波长为7610.92nm滤光片后经光纤耦合连接到光电探测器的输入端；经过光电探测器将光信号转换为电流信号，电流信号通过电流转电压模块转化为电压信号然后被数据采集卡采集，计算机对数据采集卡采集的电压信号进行处理和分析后得出甲烷气体的浓度含量。The light emitted by the detection laser is coupled into the gas chamber through the optical fiber, and after characteristic absorption occurs with the gas to be tested that is excited to the lower energy level of the detection laser characteristic absorption, it passes through a filter with a central wavelength of 7610.92 nm located at the output end of the gas chamber. Then, it is connected to the input end of the photoelectric detector through the optical fiber coupling; the optical signal is converted into a current signal through the photoelectric detector, and the current signal is converted into a voltage signal through the current-to-voltage module and then collected by the data acquisition card. After the voltage signal is processed and analyzed, the concentration of methane gas is obtained.
如图2所示为甲烷分子在泵浦光能量注入时对于探测光特征吸收的能级跃迁示意图，在本实施例中探测激光器选取了波长为7610.92nm的激光器，查阅HITRAN数据库可知，该波长的激光被甲烷气体特征吸收的时候，其下能级与基态能级的能量差用波数表示，数值为1327.308cm-1，所以发生吸收时下能级与基态能级的能量差可以表示为：Figure 2 is a schematic diagram of the energy level transition of the characteristic absorption of the probe light by the methane molecule when the pump light energy is injected. In this embodiment, the probe laser selects a laser with a wavelength of 7610.92 nm, and it can be seen from the HITRAN database that the wavelength of this wavelength is When the laser is absorbed by the characteristic of methane gas, the energy difference between the lower energy level and the ground state energy level is expressed by the wave number, and the value is 1327.308cm -1 , so the energy difference between the lower energy level and the ground state energy level when absorption occurs can be expressed as:
其中h＝6.626×10-34(J·s)是普朗克常量，c＝3×1010(cm·s-1)是光速，
其中h＝6.626×10-34(J·s)是普朗克常量，c＝3×1010(cm·s-1)是光速，
通过爱因斯坦系数关系式推导可得泵浦激光将待测气体分子从基态能级泵浦至高能级的粒子数ni’为：Through the derivation of the Einstein coefficient relationship, the number of particles ni ' that the pump laser can pump the gas molecules to be tested from the ground state energy level to the high energy level is:
其中ni’和N1分别为气体特征吸收下能级与基态能级的分子数，g”是吸收跃迁时低态i能级能量Ei的简并度,g’是吸收跃迁时高态j能级能量Ej的简并度,A21是自发跃迁的爱因斯坦系数,ρv是泵浦激光能量密度。where n i ' and N 1 are the number of molecules of the gas characteristic absorption energy level and the ground state energy level, respectively, g" is the degeneracy of the energy E i of the low state i energy level during the absorption transition, and g' is the high state of the absorption transition. The degeneracy of the j level energy E j , A 21 is the Einstein coefficient of the spontaneous transition, and ρ v is the pump laser energy density.
对于谱线强度Sij的表达式为：The expression for spectral line intensity S ij is:
其中ni和N1分别为待测气体发生特征吸收的下能级和基态能级的粒子数，增加甲烷分子发生特征吸收下能级的粒子数就能够增强谱线强度，其增加的强度可用ni’/ni表示为：where n i and N 1 are the number of particles in the lower energy level and ground state energy level of the gas to be tested, respectively. Increasing the number of particles in the lower energy level of the characteristic absorption of the methane molecule can enhance the intensity of the spectral line. The increased intensity can be used n i '/n i is expressed as:
所以通过本方法的使用，在泵浦激光固定能量ρv的注入下可以将固定数量的粒子ni’泵浦到特征吸收下能级，可以固定的提高波长7610.92nm处的谱线强度，增强了甲烷气体对于该波长激光的特征吸收。Therefore, through the use of this method, a fixed number of particles ni ' can be pumped to the energy level under the characteristic absorption under the injection of the fixed energy ρ v of the pump laser, the spectral line intensity at the wavelength of 7610.92 nm can be fixedly increased, and the The characteristic absorption of methane gas for this wavelength of laser light is obtained.
而对于痕量待测气体浓度情况下气室内气体浓度和吸收光强的相关表达式为：For the trace gas concentration to be measured, the correlation expression of the gas concentration in the gas chamber and the absorbed light intensity is:
Ia＝I0SijPLg(v,v0)C (7)I a =I 0 S ij PLg(v,v 0 )C (7)
其中I0是入射光强，Ia是吸收光强，Sij是谱线强度，C是气室待测气体浓度。可以看出气室内气体浓度和气体吸收光强成线性关系，其系数和谱线强度Sij有关，所以增强了谱线强度Sij后，某一浓度对应的吸收光强也增强了，从而提高整个系统对于甲烷气体的检测灵敏度。where I 0 is the incident light intensity, I a is the absorbed light intensity, S ij is the spectral line intensity, and C is the gas concentration in the gas chamber to be measured. It can be seen that the gas concentration in the gas chamber has a linear relationship with the gas absorption light intensity, and its coefficient is related to the spectral line intensity S ij , so after the spectral line intensity S ij is enhanced, the absorption light intensity corresponding to a certain concentration is also enhanced, thereby improving the overall The detection sensitivity of the system for methane gas.
实施例2：Example 2:
与实施例1相同，只是待测气体为二氧化碳气体,Same as Example 1, except that the gas to be measured is carbon dioxide gas,
如图3所示为二氧化碳分子在泵浦光能量注入时对于探测光特征吸收的能级跃迁示意图，所述探测激光器选取了波长为4396.87nm的激光器，查阅HITRAN数据库可知，该波长的激光被甲烷气体特征吸收的时候，其下能级与基态能级的能量差用波数表示，数值为2292.6453cm-1。Figure 3 is a schematic diagram of the energy level transition of the characteristic absorption of the probe light by carbon dioxide molecules when the energy of the pump light is injected. The probe laser selected a laser with a wavelength of 4396.87 nm. According to the HITRAN database, the laser of this wavelength is absorbed by methane. When the gas is characteristically absorbed, the energy difference between the lower energy level and the ground state energy level is represented by the wave number, and the value is 2292.6453cm -1 .
所述泵浦激光器选择了波数为2292.574963cm-1即波长4361.9075nm的激光器。通过查阅HITRAN数据库可知在波长4396.87nm和4361.9075nm处的谱线强度Sij分别为3.176×10-22(cm-1/(molecule·cm-2))和9.018×10-22(cm-1/(molecule·cm-2))，通过本方法的使用，泵浦激光能量注入能够增强波长4396.87nm处的谱线强度。增强了二氧化碳气体对于该波长激光的特征吸收，提高整个系统对于二氧化碳气体的检测灵敏度。For the pump laser, a laser with a wave number of 2292.574963 cm -1 , that is, a wavelength of 4361.9075 nm, was selected. By consulting the HITRAN database, it can be known that the spectral line intensities S ij at wavelengths 4396.87 nm and 4361.9075 nm are 3.176×10 -22 (cm -1 /(molecule·cm -2 )) and 9.018×10 -22 (cm -1 / (molecule·cm -2 )), through the use of this method, the pump laser energy injection can enhance the spectral line intensity at the wavelength of 4396.87 nm. The characteristic absorption of carbon dioxide gas for this wavelength of laser light is enhanced, and the detection sensitivity of the entire system for carbon dioxide gas is improved.
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