CN115343233B - Real-time measurement method and device for trace gas concentration on open path - Google Patents
Real-time measurement method and device for trace gas concentration on open path Download PDFInfo
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Abstract
The invention discloses a real-time measurement method and device for trace gas concentration on an open path, wherein the used device comprises a signal generator, two tunable diode lasers, two optical isolators, two optical power amplifiers, a wavelength division multiplexer, two optical fiber beam splitters, two collimating mirrors, a parabolic reflector, a pyramid prism, a dichroic mirror, a beam splitter, four photoelectric detectors, three narrow-band optical filters, a high-pass filter, a low-pass filter, a data acquisition card and the like; the beat frequency signals of the measuring light and the reference light are used for path distance measurement, two easily acquired absorption spectrum lines are used for path average temperature measurement, and a single absorption spectrum line of the gas to be measured is used for absolute concentration measurement, so that dependence on the absorption spectrum line of the gas to be measured is reduced; the invention can simultaneously measure the gas concentration, the average temperature of the path and the path distance on the open path, and can be applied to long-term monitoring of the greenhouse gas content in farmlands, animal farms, cities and other areas.
Description
Field of the art
The invention provides a real-time measurement method and device for trace gas concentration on an open path, and belongs to the technical fields of tunable diode laser absorption spectrum, temperature measurement and absolute distance measurement.
(II) background art
The global temperature rise not only has a great influence on the ecological environment, but also has disturbed the normal operation of human society. With the effectiveness of Paris agreement, the world is highly concerned about the emission of greenhouse gases, and corresponding emission reduction measures are formulated according to the national conditions of the world. At present, greenhouse gases emitted by human activities mainly comprise carbon dioxide, methane, nitrous oxide, hydrofluorocarbon, sulfur hexafluoride, chlorofluorocarbon compounds, ozone and the like; wherein, the contribution rate of carbon dioxide and methane to the greenhouse effect exceeds 90 percent. The greenhouse gas real-time monitoring technology not only can identify the source of greenhouse gas emission, but also can provide accurate data support for formulating energy-saving and emission-reducing policies.
The gas sensor based on the spectrum technology has the advantages of non-invasiveness, high selectivity, high precision, high sensitivity, simple structure, small volume, low cost and the like, and is widely researched and applied in the fields of environmental monitoring, industrial process control, medical diagnosis and the like. The most common spectroscopic techniques in these optical sensors are: ring down cavity spectra (CAVITY RING-down Spectroscopy, CRDS), quartz-enhanced photoacoustic spectra (Quartz-enhanced Photoacoustic Spectroscopy, QEPAS), chirped laser dispersion spectra (CHIRPED LASER Dispersion Spectroscopy, CLaDS), and tunable diode laser absorption spectra (Tunable Diode Laser Absorption Spectroscopy, TDLAS). The open ring-down cavity methane sensor "(Open-path cavity ring-down methane sensor for mobile monitoring of natural gas emissions,Optics Express) for mobile monitoring of natural gas emission is described in detail in Laura E.Mchale et al, volume 27 of optical Rapid Common, 14 th stage 20084-20097, wherein a typical ring-down cavity measuring device adopts a pair of concave spherical reflectors with high reflectivity to form a high-precision optical cavity, a gas to be measured is packaged in the optical cavity, light is reflected back and forth between two mirror surfaces, the attenuation rate of the light in the optical cavity is recorded, and then the concentration of the gas to be measured is determined according to the Bill law. Kaiyuan Zheng et al, in volume 29 of optical Rapid Communicator, 4, pages 5121-5127, describe in detail a Quartz enhanced photoacoustic spectroscopy technique (Quartz-enhanced photoacoustic-photothermal spectroscopy for TRACE GAS SENSING, optics Express) in a trace gas sensor based on the Quartz enhanced photoacoustic photothermal spectroscopy technique, which uses the photoacoustic effect to perform spectroscopic detection, typically using tuning fork Quartz crystal as a high Q factor resonator to detect weak acoustic waves in trace gas detection, the tuning fork Quartz crystal being an acoustic quadrupole, and thus having a strong environmental noise immunity. The technique has high quality requirements on the light beam, and the laser beam cannot touch the surface of the tuning fork when passing through the vibrating arm gap of the tuning fork. Because of the high requirements of ring-down cavity spectroscopy and quartz-enhanced photoacoustic spectroscopy techniques on the measurement apparatus, both techniques are not suitable for gas concentration measurement on an open path. In measuring the gas concentration, the gas absorption signal is proportional to the product of the gas concentration and the path length. For a more accurate measurement of gas concentration, a more accurate path length needs to be obtained. The conventional trace gas concentration measuring method adopts a measuring ruler or an additional laser range finder to measure the length of an absorption path in advance and then measure an absorption signal. However, during long measurement times, the path length may change due to structural deformation or mechanical vibration. Therefore, it is necessary to develop a device and method that can measure both path length and gas absorption.
The chirped laser dispersion spectrum realizes spectrum detection by utilizing a molecular dispersion effect, and measures the concentration of gas by detecting refractive index fluctuation near the absorption peak of the gas. The method does not depend on absorption effect and does not need normalization treatment on received light power, so that the method is suitable for high-concentration gas detection and long-optical-path gas remote sensing. Nart S. Dsghaestani et al, in 2014, optical Rapid Communications, volume 22, 7, pages 1731-1743, paper, analysis and demonstration "(Analysis and demonstration of atmospheric methane monitoring by mid-infrared open-path chirped laser dispersion spectroscopy,Optics Express), based on atmospheric methane monitoring of the chirped laser dispersion spectrum of the mid-infrared open path, uses chirped laser dispersion spectroscopy to measure the atmospheric methane concentration on the open path, and uses mid-infrared laser with a center wavelength of 7.7942 μm on the 90-meter path to continuously measure the ambient gas for 2 hours, thus realizing a methane concentration detection lower limit of 100 ppb. The scheme uses a mid-infrared quantum cascade laser, the output power of the laser is smaller and is only 5.8mW, and a detector with high sensitivity is needed when measuring the gas concentration under an open path. In addition, in order to obtain the best measurement accuracy, the modulation frequency of the laser light intensity needs to reach the order of 3.4GHz, which has higher requirements on the bandwidth of the detector, and the current middle infrared detector cannot realize the bandwidth above GHz. Michal Nikodem et al, in volume 119 of applied Physics B, paper 3-9, measured the atmospheric methane concentration on an open path of 35 meters by using a chirped laser dispersion spectroscopy technique in an open path sensor "(Open-path sensor for atmospheric methane based on chirped laser dispersion spectroscopy,Applied Physics B) of atmospheric methane based on chirped laser dispersion spectroscopy, and continuously measured for 2.7 hours by using a methane spectral line near 1.653 μm, and the result shows that the system can reach a methane detection lower limit of 1.3ppmv, and basically meets the atmospheric methane concentration measurement requirement. The scheme needs a high-sensitivity detector with bandwidth reaching GHz level, and the current photoelectric detector cannot simultaneously realize high bandwidth and high sensitivity, so that the detection sensitivity of a measuring system is inevitably reduced. In addition, the system needs to introduce a reference gas pool to calibrate the concentration of the gas to be measured in the atmosphere in the measurement process, and the gas concentration value in the reference gas pool directly influences the accuracy of the concentration of the gas to be measured. Wuwen Ding et al, in volume 55 of applied optics, page 31, pages 8698-8704, describe in detail the principle of measuring gas concentration by chirped laser dispersion spectroscopy technology in double sideband heterodyne dispersion Spectrum "(Dual-sideband heterodyne of dispersion spectroscopy based on phase-sensitive detection,Applied Optics) based on phase sensitive detection, and measure methane gas at 940.5ppm.m under normal temperature and pressure, and the lower detection limit of methane concentration can reach 0.2ppb at integration time of 30 s. Although this method can achieve high-precision gas concentration measurement, the relationship between the peak-to-peak value of the dispersion spectrum and the gas concentration needs to be calibrated before measurement. In order to obtain a double-sideband dispersion spectrum, the modulation frequency of the laser light intensity needs to reach the GHz level, which has higher bandwidth requirements on a detection system and an acquisition system. YIFENG CHEN et al, in volume 46 of optical communication, 13, 3005-3008, in open path isolated chirped laser dispersion spectrometry-based transient methane detection "(Fugitive methane detection using open-path stand-off chirped laser dispersion spectroscopy,Optics Letters), combine a phase-sensitive chirped laser dispersion spectrometry technique with a frequency modulated continuous wave ranging technique to measure the concentration of methane in the atmosphere, and experimentally measure the concentration of methane in the atmosphere to be 2.9ppm. The scheme can not measure the gas concentration and the path length at the same time, the measuring light path of the two parameters needs to be manually switched, the measuring system is relatively complex, and a reference gas pool is also needed to be introduced for real-time calibration in the measuring process. Andreas Hangauer et al, volume 46, optical communication, phase 2, pages 198-201, used chirped laser dispersion spectroscopy in spectrochemical sensing and distance detection "(Chirped laser dispersion spectroscopy for spectroscopic chemical sensing with simultaneous range detection,Optics Letters) to simultaneously measure gas concentration and path length using chirped laser dispersion spectroscopy. The gas concentration is related to the second harmonic peak value of the modulation signal, and the relation between the two needs to be calibrated in advance; the path length is related to the baseline frequency offset of the modulated signal, and the path length is obtained by extracting the baseline frequency offset of the modulated signal in a linear relationship. In order to obtain the best gas concentration measurement accuracy, the modulation frequency of the laser intensity needs to be controlled in the GHz level, and a detector with higher bandwidth is needed. In addition, the method can only realize the ranging accuracy of the order of 0.1 meter at present under the influence of the modulation frequency and the change rate of the modulation frequency. The simultaneous measurement of gas concentration and path length is achieved by using a phase-sensitive chirped laser scattering spectroscopy technique in the paper of Mingli Zou et al, volume 29, optical Rapid Communicator, phase 8, pages 11683-11692, simultaneous measurement of gas absorption and path length based on dispersive spectroscopy double sideband heterodyne phase-sensitive detection "(Simultaneous measurement of gas absorption and path length based on the dual-sideband heterodyne phase-sensitive detection of dispersion spectroscopy,Optics Express). The ranging range of this technique is inversely proportional to the modulation frequency, which is on the order of MHz, and can only measure distances in the range of 10 meters. However, the gas concentration measurement requires modulation frequencies above GHz, and in order to balance the measurement requirements of two parameters, the method can only realize simultaneous measurement of the two parameters in the range of 1 meter. The above schemes all adopt the chirped laser dispersion spectrum technology to realize gas concentration measurement, and the technology has potential application value in open path gas concentration measurement. But in practical use this technique requires an electro-optic modulator with a modulation frequency above GHz and a detector of corresponding bandwidth. Because of the limitations of devices such as electro-optic modulators, detectors and the like, the method is mainly used in a near infrared band of 1-2 mu m at present, and the selectable spectral lines in the band range are fewer and the spectral line intensity is weaker. In addition, the gas concentration cannot be directly obtained from the dispersion spectrum, the relationship between the peak value and the concentration of the dispersion spectrum needs to be calibrated, then the measured gas concentration is deduced through the calibrated relationship between the peak value and the concentration of the dispersion spectrum, and the accuracy of gas concentration measurement depends on the calibrated result.
There are currently two main implementations of tunable diode laser absorption spectroscopy (Tunable Diode Laser Absorption Spectroscopy, TDLAS) technology: direct absorption spectroscopy (direct absorption spectroscopy, DAS) and wavelength modulation spectroscopy (WAVELENGTH MODULATION SPECTROSCOPY, WMS). The wavelength modulation spectrum technology loads a high-frequency modulation signal to a laser, modulates the wavelength, and measures gas parameters by detecting harmonic signals. The technology can effectively inhibit the background noise of the system, thereby improving the detection sensitivity and being widely applied to the measurement of trace gas concentration. The calibration-free wavelength modulation spectroscopy technique, which requires two or more absorption lines of the gas to be measured, is described in detail in the paper "calibration-free wavelength modulation spectroscopy "(Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments,Applied Optics) for gas temperature and concentration measurement in harsh environments" by Gregory B.Rieker et al, application optics, volume 48, 29, pages 5546-5560. In the case of gas concentration measurement, the path length needs to be determined in advance, and the path length may change during the measurement, so that the path length measured in advance cannot truly characterize the path taken by the laser. Liang Mei et al, in 2011, optical Rapid communication, volume 36, 16, pages 3036-3038, in a paper, based on wavelength modulation continuous wave diode laser scattering medium gas spectrum and path length measurement "(Gas spectroscopy and optical path-length assessment in scattering media using a frequency-modulated continuous-wave diode laser,Optics Express), combine a wavelength modulation spectrum technology and a frequency modulation continuous wave ranging technology to realize measurement of gas concentration in a porous medium. Jinbao Xia et al, volume 117 of optical and laser engineering in 2019, pages 4, 21-28, used wavelength modulation spectroscopy to measure the concentration of methane and carbon dioxide in turbulent atmosphere in the detection "(Probing greenhouse gases in turbulent atmosphere by long-range open-path wavelength modulation spectroscopy,Optics and Laser in Engineering) of greenhouse gases in turbulent atmosphere based on long Cheng Kaifang path wavelength modulation spectroscopy, and continuously measured on a 2.6 km path for 10 hours, wherein the lower detection limits of methane and carbon dioxide can reach 2ppb and 20ppm, respectively. According to the method, a reference gas pool is required to be introduced in the measurement process, the measurement accuracy of the concentration of the gas to be measured is related to the concentration of the reference gas, the length of a reference light path and the distance between a light source and a reflector, and the measurement accuracy of the three parameters directly influences the accuracy of the concentration of the gas to be measured. Xinqian Guo et al in 2019, optical and laser engineering, volume 115, 23, pages 243-248, a portable ammonia in-situ measurement sensor "(A portable sensor for in-situ measurement of ammonia based on near-infrared laser absorption spectroscopy,Optics and Laser in Engineering) based on near infrared laser absorption spectroscopy, built a set of portable ammonia in-situ measurement sensor by using a multi-reflection gas cell, and used for continuously measuring 30ppm ammonia for 1 hour, wherein the lower limit of ammonia concentration detection can reach 0.16ppm. According to the scheme, before measuring the ammonia concentration, ammonia is required to be filled into the gas tank, so that the actual state of the gas to be measured is not reflected. When the temperature is lower than 415K, ammonia gas is adsorbed on glass on the inner wall of the gas cell, resulting in inaccurate measured gas concentration. Guishi Wang et al, in volume 27 of optical Rapid Communicator, 4, pages 4878-4885, in wavelength modulation Spectroscopy for sensing sensitive gases, used a wavelength modulation spectroscopy technique to measure 50ppm methane at ambient temperature and pressure in laser frequency locking and intensity normalization "(Laser frequency locking and intensity normalization in wavelength modulation spectroscopy for sensitive gas sensing,Optics Express), the effective optical path of the used multi-reflection gas cell was 26.4 meters, and the lower limit of detection of the methane concentration by the sensor was 2.5ppbv. In practical use, the solution requires that the ambient gas is sampled and then filled into the gas cell, which will change the actual state of the gas to be measured. Mingli Zou et al, in the paper of optical short message of 2020, volume 28, 8, 11573-11582, and on the basis of wavelength modulation spectrum, built an acetylene sensor of three rows of circular multiple reflection gas tanks in the acetylene sensor "(Acetylene sensing system based on wavelength modulation spectroscopy using a triple-row circular multi-pass cell,Optics Express), the effective optical path of the acetylene sensor can reach 21.9 meters, and the detection sensitivity test of 100.6ppm of acetylene gas is performed by using the sensor, so that the detection lower limit of the acetylene sensor under the integration time of 340s is 76.75ppb. When measuring the ambient gas, the method also needs to sample the gas to be measured, and changes the real state of the gas to be measured. Chenguang Yang et al, volume 28, optical Rapid Communicator, 3, pages 3289-3297, measured "(Simultaneous measurement of gas absorption and path length by employing the first harmonic phase angle method in wavelength modulation spectroscopy,Optics Express) simultaneously for trace gas concentrations and path lengths using the first harmonic phase angle of wavelength modulation spectroscopy, based on the first harmonic phase angle method of wavelength modulation spectroscopy. The maximum measurement distance of the method is inversely proportional to the modulation frequency of a sine wave in a wavelength modulation spectrum technology, and the modulation frequency of a laser needs to be reduced during long-distance measurement. When the modulation frequency of the sine wave is reduced to the kHz magnitude, the change of the first harmonic phase angle with distance is not obvious, resulting in poor spatial resolution. Hongbin Lu et al, in 2021, sensor, volume 21, 7, 2448-2462, TDLAS technology-based ammonia leakage monitoring remote sensing system (A Remote Sensor System Based on TDLAS Technique for Ammonia Leakage Monitoring, sensors), built a set of open path ammonia monitoring Sensors based on wavelength modulation spectroscopy, using the Sensors to measure a simulated ammonia leakage source, system stability test results showed that: the sensor can realize the lower limit of ammonia concentration detection of 16.6ppm, and can be used for monitoring an ammonia leakage source. Although this scheme can measure ammonia concentration on the open path, the lower limit of detection of ammonia concentration is higher, and ammonia concentration in the atmosphere cannot be detected yet. In the scheme, the wavelength modulation spectrum technology is adopted to measure the concentration of trace gas, and in actual use, the technology can only extract the demodulated harmonic component and can not directly obtain the concentration of gas from the harmonic component. In order to link the gas concentration with the harmonic component, not only the amplitude of the linear and nonlinear light intensity modulation of the laser and the phase difference between the frequency modulation of the linear and nonlinear light intensity modulation of the laser are required to be determined, but also the harmonic signal is required to be calibrated by utilizing the reference gas, the measured gas concentration is deduced through the relation between the harmonic signal and the gas concentration, and the accuracy of the gas concentration measurement depends on the accuracy of the calibration result. In addition, the trace gas detection scheme based on the wavelength modulation spectrum technology does not have high-precision ranging capability, and the real path length of the laser cannot be accurately obtained, which affects the accuracy of the gas concentration measurement result.
Compared with the wavelength modulation spectrum technology, the direct absorption spectrum technology can directly extract the absorption spectrum from the transmitted light intensity, and absolute measurement of gas parameters can be realized without standard gas calibration, so that the direct absorption spectrum technology occupies an irreplaceable position in gas concentration measurement. Chuantao Zheng et al, volume 28 of IEEE photon technology communication, 21, pages 3036-3038, used direct absorption spectroscopy to measure methane concentration in air at room temperature at 700Torr in an infrared dual gas CH4/C2H6 sensor "(Infrared Dual-Gas CH4/C2H6 Sensor Using Two Continuous-Wave Interband Cascade Lasers,IEEE Photonics Technology Letters) using two continuous band cascade lasers, the effective optical path of the multi-reflection gas cell was 54.6 meters, and the average concentration of methane in air was measured to be 2.7ppm. The lower limit of methane concentration detection was analyzed by Allen variance, and at an average time of 1s, a lower limit of methane concentration detection of 2.7ppbv was achieved. Although the method can realize higher measurement precision, the gas to be measured needs to be sampled, and the air pressure of the sampled gas is slightly less than 1 standard atmosphere, which affects the accuracy of measuring the gas parameters. The Lei Dong et al, volume 108 of the application physical fast report, pages 1-1106, designed a compact, low power trace gas concentration measurement device in a compact CH4 sensor system "(Compact CH4 sensor system based on a continuous-wave,low power consumption,room temperature interband cascade laser,Applied Physics Letters) based on continuous wave, low power consumption, inter-temperature band cascade laser in 2016, which uses a multiple reflection gas cell as a core element, and the system size is limited within a volume of 32 x 20 x 17cm 3, which is convenient for use on mobile equipment. The effective optical path of the device is 54.6 meters, and the lower limit of methane concentration detection of 1.4ppbv can be realized at normal temperature and normal pressure. The method also needs to sample the gas to be measured, the output power of the laser is only 1.5mW, and the signal-to-noise ratio of the measurement signal is low. Jingsong Li et al in 2016, "sensor and actuator B: the paper of chemistry, volume 231, pages 723-732, discloses that a single quantum cascade laser sensor based on a double-spectrum technology can simultaneously detect CO, N 2 O and H2O"(Simultaneous atmospheric CO,N2O and H2O detection using a single quantum cascade laser sensor based on dual-spectroscopy techniques,Sensors and Actuators B:Chemical) in the atmosphere, the direct absorption spectrum technology is adopted to measure the ambient gas with the pressure of 100mbar and the temperature of 300K, the effective optical path of a multi-reflection gas tank is 76 meters, the central wavelength of the laser is 4566nm, and the result shows that the sensor can realize the lower limit of CO concentration detection of 1.64ppb and the lower limit of N 2 O concentration detection of 1.15ppb within the integration time of 1 s. The method not only needs to sample the gas to be detected, but also the air pressure of the sampled gas is less than 1 standard atmosphere. Fang Song et al, in 2017, optical Rapid communication, 25 th, 31876-31888, in a medium infrared methane sensor "(Interband cascade laser based mid-infrared methane sensor system using a novel electrical-domain self-adaptive direct laser absorption spectroscopy(SA-DLAS),Optics Express) based on an interband cascade laser and an adaptive filtering direct absorption spectroscopy technology, measure the methane concentration in the air at normal temperature and pressure by using a multi-reflection gas cell with an effective optical path of 16 meters, inhibit the noise of the sensor by combining the adaptive filtering technology, calculate the filtered direct absorption spectroscopy data, and measure the methane concentration in the air to be 1.876ppm. At an average time of 6s, the method can realize a methane concentration detection lower limit of 43.9ppbv, but the system cannot stabilize the temperature in the gas tank, and the temperature fluctuation directly influences the measurement accuracy of the methane concentration. Nicolas Sobanski et al, volume 11 of application science, 3 pages 1222-1243, have studied in high precision nitrogen dioxide measurement "(Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy,applied sciences) by using direct absorption spectroscopy to measure nitrogen dioxide with pressure of 80mbar, temperature of 300K and concentration of 1ppbv, and the effective optical path of the multi-reflection gas cell is 110 meters, which shows that the sensor can realize a concentration detection lower limit of 0.8ppbv within 150s of integration time. The scheme also needs to sample the gas and destroy the original state of the gas. The technology adopts the direct absorption spectrum technology to measure the concentration of trace gas, and has the advantages of wide applicability, simple data interpretation, capability of extracting absorption spectrum, no need of standard gas calibration and the like, but the detection sensitivity is slightly lower than that of the wavelength modulation spectrum technology due to the interference of system background noise and laser power fluctuation. For the direct absorption spectrum technology, the signal to noise ratio of the absorption spectrum signal can be improved by increasing the path length, so that the detection sensitivity is improved. Furthermore, the above method uses a multi-reflection gas cell as a core element of a measurement system for trace gas concentration detection, and no document has yet reported using direct absorption spectroscopy technology for ambient gas concentration measurement on an open path.
Temperature is one of the core parameters in gas concentration measurement. The current gas concentration measurement is to obtain a temperature value near the sensor through a temperature measuring device such as a thermometer or a thermocouple, and the temperature only reflects the temperature of a certain fixed point and cannot reflect temperature information on an open path. To provide a more accurate temperature value, replacing the temperature of the fixed point with the path average temperature may better reflect the change in temperature over the open path. According to the beer lambert law, the ratio of the absorption intensities of two spectral lines is a single-valued function of temperature, so that the average temperature on the absorption path can be solved by using the ratio of the absorption intensities of two different spectral lines of the same substance, and the temperature measurement method is called colorimetry. When colorimetric method is used for measuring temperature, two isolated absorption lines of a certain gas without interference of other gases are firstly selected, and then whether the temperature measurement sensitivity of the two absorption lines meets the requirement is judged. In general, it is difficult to select two absorption lines suitable for temperature measurement. Considering that the response wave band of the common photoelectric detector is 1100nm-1700nm, taking the absorption spectrum line of methane as an example, the detection range of the sensor is assumed to be 100-200 m, and under normal temperature and normal pressure, the spectrum line intensity is proper and the absorption spectrum line is isolated: 1.6481 μm, 1.651 μm and 1.653 μm. In addition, the cross interference of water molecules and the sensitivity of colorimetric temperature measurement are taken into consideration, and double spectral lines meeting the temperature measurement requirement are not available. According to the beer lambert law and the colorimetric temperature measurement principle, a single spectral line cannot measure the average temperature of a path, and the precondition of gas concentration measurement is that the gas pressure, the average temperature of the path and the path length are obtained, so that absolute concentration measurement cannot be realized by only relying on the absorption spectral line of methane. In contrast, the water molecules have wide absorption spectrum line distribution in the range of 1100nm-1700nm, and more isolated spectrum lines, so that two absorption spectrum lines suitable for open path average temperature measurement are easily selected. In order to solve the problem that temperature measurement cannot be achieved by a single spectral line, two absorption spectral lines with proper spectral line intensity can be selected from the absorption spectral lines of water molecules, and the average temperature of the same path as that of the gas to be measured is obtained in a common-path mode so as to finish the concentration measurement of the gas to be measured.
Based on the background, the invention provides a real-time measurement method and device for the concentration of trace gas on an open path, which can measure the concentration of trace gas on the open path, the average temperature of the path and the path length at the same time. The measuring device comprises two measuring arms, wherein one arm is used for detecting gas to be measured, the other arm is used for providing a reference signal, and after laser beam combination of the two arms, beat signals containing information such as trace gas concentration, path average temperature, path length and the like are generated. The invention combines the direct absorption spectrum technology, the frequency modulation continuous wave distance measurement technology, the colorimetric method temperature measurement technology and the like together, and develops a device and a method for measuring the concentration of trace gas on an open path. The method uses a frequency modulation continuous wave ranging technology to obtain high-precision distance information, uses a colorimetric method temperature measurement technology to obtain path average temperature information of the gas to be detected, and uses a direct absorption spectrum technology to obtain absolute concentration information of the gas to be detected. In order to reduce the dependence on the absorption spectrum line of the gas to be detected, the invention adopts two easily-obtained gas molecular absorption spectrum lines to carry out path average temperature measurement, and uses a single absorption spectrum line to carry out trace gas concentration measurement; the common light path mode is adopted to make the average temperature of the paths of the two gas measurements identical. The device is simple, convenient to carry and fast in data processing, and is not only suitable for monitoring the greenhouse gas emission of processing enterprises, but also suitable for long-term monitoring of the greenhouse gas content in farmlands, animal farms, cities and other areas.
(III) summary of the invention
The invention aims to solve the defects in the existing trace gas concentration detection technology on an open path, provides a real-time measurement device and method for trace gas concentration, path average temperature and path length on the open path, and belongs to the three technical fields of tunable diode laser absorption spectrum, temperature measurement and absolute distance measurement. The components used include a signal generator, two tunable diode lasers, two optical isolators, two optical power amplifiers, a wavelength division multiplexer, two fiber optic splitters, two collimating mirrors, a parabolic mirror, a pyramid prism, a dichroic mirror, a beam splitter, four photodetectors, three narrowband filters, a high pass filter, a low pass filter, a data acquisition card, and the like.
The technical scheme adopted by the invention is as follows: the signal generator is connected with the two tunable diode lasers, the laser frequencies of the two lasers are linearly changed by adjusting the injection current of the lasers, the wavelength output by one of the lasers covers one absorption spectrum line of the gas to be detected, and the wavelength output by the other laser covers two absorption spectrum lines of the easily acquired gas molecules. The two lasers are respectively connected with an optical isolator, the two lasers emitted by the optical isolator amplify the laser power respectively by using an optical power amplifier, and then a wavelength division multiplexer is used for coupling the lasers of the two wave bands together so as to realize the common optical path of the two lasers. The laser beam passing through the common light path of the wavelength division multiplexer is incident into two optical fiber beam splitters connected in series for beam splitting, so as to obtain three beams of light, wherein one beam is used as measuring light (90%), the other beam is used as reference light (5%) for frequency modulation continuous wave ranging, and the other beam is used as monitoring light (5%) for measuring the laser power of the system. After being collimated by the collimating lens, the measuring light sequentially passes through the parabolic reflector with a small hole in the center and the gas to be measured, is reflected by the primary path of the pyramid prism, and is received by the parabolic reflector. The method comprises the steps of separating measuring light according to wavelength by a dichroic mirror, detecting sweep laser containing a temperature measuring absorption spectrum line by a photoelectric detector, combining the sweep laser containing the gas absorption spectrum line to be measured with reference light by a beam splitter, and detecting the combined laser by the two photoelectric detectors. A narrow band filter is arranged in front of each photodetector to filter interference of other light. And after the light and the reference light are combined, beat frequency occurs on the photoelectric detectors, the two photoelectric detectors are respectively connected with a high-pass filter and a low-pass filter, the beat frequency signals after the high-pass filtering are used for measuring the path length, and the direct current signals after the low-pass filtering are used for measuring the concentration of the gas to be measured. The signals of all the photodetectors are collected by the collection card for subsequent signal processing. The specific implementation process is as follows:
Step one: the signal generator generates sawtooth wave signals to modulate the wavelength output by the laser, and the laser controller adjusts the laser frequency output by the tunable diode laser by changing injection current; in order to linearly change the laser frequency, the sawtooth waveform is continuously modified according to the relation between the injection current and the laser frequency until the laser frequency output by the lasers is linearly changed, and the laser frequency generated by each laser meets the following conditions:
v (T) =v 0 +at (1), where v 0 represents the starting frequency of laser frequency modulation, a=Ω/T represents the modulation rate of the laser frequency, T represents the period of laser modulation, Ω represents the bandwidth of the laser frequency modulation.
Step two: adjusting the working temperature of the lasers, so that the laser wavelength generated by one of the lasers covers one absorption spectrum line of the gas to be measured, and the laser frequency generated by the other laser covers two absorption spectrum lines for measuring the temperature, and driving the two lasers in a time division multiplexing mode; an optical isolator is arranged between the laser and the wavelength division multiplexer to protect the laser; two beams of laser emitted by the optical isolator amplify laser power respectively by using an optical power amplifier; the wavelength division multiplexer couples the laser generated by the two lasers together and realizes a common optical path of the two lasers; two fiber optic splitters are connected in series according to 90:5:5, dividing the laser into three paths of output, wherein one path with the proportion of 90% is taken as measuring light, the light intensity is marked as I m, and the two paths with the proportion of 5% are respectively taken as reference light for frequency modulation continuous wave ranging and monitoring light for measuring the laser power of the system, and the light intensities are respectively marked as I r and I monitor.
Step three: after being collimated by the collimating lens, the measuring light sequentially passes through the parabolic reflector with a small hole in the center and the gas to be measured, is reflected by the primary path of the pyramid prism and is received by the parabolic reflector; the dichroic mirror separates the measuring light according to the wavelength, the sweep frequency laser containing the temperature measuring absorption spectrum is reflected by the dichroic mirror, and is detected by a photoelectric detector after passing through the narrow-band filter; the sweep frequency laser for measuring the absorption spectrum of the gas to be measured is transmitted by the dichroic mirror, the beam splitter combines the transmitted light with the reference light which does not pass through the gas to be measured, the combined light is detected by the two photoelectric detectors after passing through the narrow-band filter, and detection signals of the two photoelectric detectors are respectively uploaded to the acquisition card after passing through a high-pass filter and a low-pass filter; the change of the laser power of the measuring system is recorded by a fourth photoelectric detector; the signals uploaded to the acquisition card by the four photoelectric detectors are respectively expressed as:
I1(t)＝Imexp(-αx(v(t))) (2)
I3(t)＝Ir+Imexp(-αtarget(v(t))) (4)
I 4(t)＝Imonitor (5), wherein I 1(t)、I2(t)、I3(t)、I4 (t) is the signals of the 4 photoelectric detectors acquired by the acquisition card respectively; τ 1、τ2 is the time delay introduced by the laser when passing through the measuring path and the reference path; τ m is the time difference between the measurement light and the reference light reaching the photodetector; α x(v(t))、αtarget (v (t)) is the absorbance at the thermometry absorption line and the absorbance at the gas absorption line to be measured, respectively, α i (v), i=x, target can be expressed as:
Wherein P is the measurement area gas pressure, L is the path length, X (L) is the mole fraction of the gas to be measured, T (L) is the temperature of the gas to be measured, S [ T (L) ] is the temperature-dependent line intensity, Is a linear function of the absorption spectrum, satisfies the normalization condition, and l is a position on the expression path.
Step four: extracting the beat frequency from the beat signal I 2 (t), the relationship between the path length and the beat frequency can be expressed as:
wherein c represents the speed of light in the air, f IF＝aτm is the frequency of the beat signal, and the signal I 2 (t) collected by the photodetector is subjected to Fourier transform to obtain a frequency spectrum, and the frequency corresponding to the maximum amplitude point in the frequency spectrum is the required frequency f IF.
Step five: as can be seen from formulas (2) and (4) in the third step, the absorption rate at the absorption line of the measured temperature and the absorption rate α i (v) at the absorption line of the gas to be measured, i=x, tar get are respectively expressed as:
The reference light I r is obtained according to a certain light splitting ratio, and can be obtained by calculating the reference light I r from the monitoring light according to a certain proportion; measuring light I m is obtained through baseline fitting, selecting sections [ t 1,t2 ] and [ t 3,t4 ] with extremely small absorption at two sides of an absorption section in I mexp(-αi (v)), and enabling the amplitude of I m of the two sections to approach the amplitude of I mexp(-αi (v)) through a data fitting mode to obtain I m which does not contain the term exp (-alpha i (v)); the absorption spectrum can be calculated from the absorption curve α i (v).
Step six: the integrated absorption area A i of the two absorption lines is extracted from the absorption curve alpha x (v (t)) of the temperature-measuring absorption line, i=1 or 2, the path average temperature is calculated by a colorimetry method, and the integrated absorption area A i meets the following conditions:
From equation (10), the ratio of the integrated absorption areas at the two absorption lines is only dependent on the gas temperature:
obtaining a path average temperature T through the calculated integral absorption area ratio; then, according to the beer lambert law, the concentration of the gas to be measured can be calculated by the following formula:
The invention has the advantages that: the easily obtained temperature measurement absorption spectrum is combined with the absorption spectrum of the gas to be measured, so that the requirement on the absorption spectrum of the gas to be measured is reduced, and the problem that the concentration of the gas to be measured cannot be measured because two proper absorption spectrum of the gas to be measured cannot be found is avoided. The scheme can simultaneously realize the measurement of the concentration of the gas to be measured on the open path, the average temperature of the path and the path length, is not only suitable for monitoring the greenhouse gas emission of processing enterprises, but also suitable for long-term monitoring of the greenhouse gas content in farmland, animal farm, city and other areas.
(IV) description of the drawings
Fig. 1 is a structural diagram of a measuring apparatus.
Fig. 2 is a modulated signal of the laser injection current.
(Fifth) detailed description of the invention
Taking the measurement of the average temperature of a path by using double spectral lines of water molecules and the measurement of the concentration by using single spectral lines of methane as an example, and further describing the technical scheme of the invention with reference to the accompanying drawings, the invention provides a real-time measurement method and a device for the concentration of trace gas on an open path, wherein the measurement device is shown in fig. 1, and comprises a signal generator 1, a tunable diode laser 2, a tunable diode laser 3, an optical isolator 4, an optical isolator 5, an optical power amplifier 6, an optical power amplifier 7, a wavelength division multiplexer 8, an optical fiber beam splitter 9, an optical fiber beam splitter 10, a collimating mirror 11, a collimating mirror 12, a parabolic mirror 13, a dichroic mirror 14, a beam splitter 15, a narrow-band filter 16, a narrow-band filter 17, a narrow-band filter 18, a photoelectric detector 19, a photoelectric detector 20, a photoelectric detector 21, a photoelectric detector 22, a pyramid prism 23 and a data acquisition card 24, wherein:
The signal generator 1 is connected with the tunable diode laser 2 and the tunable diode laser 3, adjusts injection current by using a sawtooth waveform, linearly changes laser frequencies generated by the tunable diode laser 2 and the tunable diode laser 3 by correcting the shape of the sawtooth waveform, enables the wavelength output by the tunable diode laser 2 to cover one absorption spectrum line of methane, and enables the wavelength output by the tunable diode laser 3 to cover two absorption spectrum lines of water molecules, as shown in fig. 2.
The tunable diode laser 2 is sequentially connected with the optical isolator 4 and the optical power amplifier 6, the tunable diode laser 3 is sequentially connected with the optical isolator 5 and the optical power amplifier 7, the sweep laser amplified by the optical power amplifier 6 and the optical power amplifier 7 is coupled together by the wavelength division multiplexer 8, and a two-path laser common path is realized, the laser of the common path is incident into the optical fiber beam splitter 9 with the split ratio of 90:10 for splitting, one beam with the split ratio of 90% is used as measuring light, one beam with the split ratio of 10% is split again by the optical fiber beam splitter 10 with the split ratio of 50:50, one beam (5%) is used as reference light for frequency modulation continuous wave ranging, and the other beam (5%) is used for monitoring the laser power of the measuring system.
After being collimated by the collimating mirror 11, the measuring light sequentially passes through the parabolic mirror 13 with a small hole in the center and the gas to be measured, and is reflected by the pyramid prism 23 in the original path and then received by the parabolic mirror 13.
The dichroic mirror 14 separates sweep frequency laser output by the tunable diode laser 2 and the tunable diode laser 3 according to wavelength, the sweep frequency laser for measuring the absorption spectrum of water molecules is reflected by the dichroic mirror 14, detected by the photoelectric detector 20 after passing through the narrow-band filter 16, and the detection signal is directly collected by the data collecting card 24; the sweep frequency laser for measuring the methane absorption spectrum is transmitted by the dichroic mirror 14, the beam splitter 15 combines the transmitted light with the reference light which does not pass through the gas to be measured, the combined light passes through the narrow-band filter 17 and the narrow-band filter 18 respectively and then is detected by the photoelectric detector 19 and the photoelectric detector 21, the signal detected by the photoelectric detector 19 is collected by the data collection card 24 after being filtered by the low-pass filter, and the signal detected by the photoelectric detector 21 is collected by the data collection card 24 after being filtered by the high-pass filter; the signals measured by the photodetector 22 are collected directly by the data acquisition card 24 without processing.
The tunable diode laser 2 selects two suitable easily available molecular absorption spectral lines according to the path length, and the tunable diode laser 3 depends on the kind of gas molecules to be detected.
A method for real-time measurement of trace gas concentration on an open path, comprising the steps of:
Step one: modulating the wavelength of the tunable diode laser 2 and the tunable diode laser 3 by adopting a sawtooth waveform, and continuously correcting the waveform of the injection current by utilizing the relation between the injection current and the laser frequency until the laser frequency linearly changes, wherein the corrected sawtooth waveform is shown in fig. 2; the laser frequencies generated by the tunable diode laser 2 and the tunable diode laser 3 satisfy the following conditions:
v(t)＝ν0+at (1)
Where v 0 denotes the starting frequency of laser frequency modulation, a=Ω/T denotes the modulation rate of the laser frequency, T denotes the period of laser modulation, Ω denotes the bandwidth of the laser frequency modulation.
Step two: the wavelength ranges output by the tunable diode laser 2 and the tunable diode laser 3 are adjusted by changing the working temperature of the lasers, so that the wavelength output by the tunable diode laser 2 covers one absorption spectrum line of methane, and the wavelength output by the tunable diode laser 3 covers two absorption spectrum lines of water molecules, and the two lasers are driven in a time division multiplexing mode; an optical isolator 4 and an optical power amplifier 6 are connected in sequence after the tunable diode laser 2; an optical isolator 5 and an optical power amplifier 7 are sequentially connected after the tunable diode laser 3; the optical isolator is used for protecting the safety of the laser, and the optical power amplifier amplifies the power of the laser, so that the remote open path measurement is facilitated; the wavelength division multiplexer 8 couples the sweep laser of the two paths of amplified laser power together and realizes a common optical path of the two paths of laser; the laser of the common light path is incident into the optical fiber beam splitter 9 with the beam splitting ratio of 90:10 for beam splitting, one beam with the beam splitting ratio of 90% is used as measuring light, the light intensity is marked as I m, one beam with the beam splitting ratio of 10% is split by the optical fiber beam splitter 10 with the beam splitting ratio of 50:50, one beam (5%) is used as reference light for frequency modulation continuous wave ranging, the other beam (5%) is used as monitoring light for measuring the laser power of the system, and the light intensities are respectively marked as I r and I monitor.
Step three: after being collimated by the collimating mirror 11, the measuring light sequentially passes through the parabolic mirror 13 with a small hole in the center and the gas to be measured, and is reflected by the pyramid prism 23 in a primary way and then received by the parabolic mirror 13; the dichroic mirror 14 separates the measurement light according to the wavelength, and the sweep laser for measuring the absorption spectrum of the water molecules is reflected by the dichroic mirror 14, passes through the narrow-band filter 16 and is detected by the photoelectric detector 20; the sweep frequency laser for measuring the methane absorption spectrum is transmitted by the dichroic mirror 14, the beam splitter 15 combines the transmitted light with the reference light which does not pass through the gas to be measured, and the combined light passes through the narrow-band filter 17 and the narrow-band filter 18 respectively and is detected by the photoelectric detector 19 and the photoelectric detector 21; the change in the laser power of the measurement system is recorded by the photodetector 22; the detection signals uploaded to the data acquisition card by the four photoelectric detectors are respectively expressed as follows:
I4(t)＝Imonitor (5)
Wherein I 1(t)、I2(t)、I3(t)、I4 (t) is the laser light intensity measured by the photodetector 20, the photodetector 21, the photodetector 19, and the photodetector 22, respectively; τ 1、τ2 is the time delay introduced by the laser when passing through the measuring path and the reference path; τ m is the time difference between the measurement light and the reference light reaching the photodetector; The absorption rates of water molecules and methane at the absorption lines, respectively, α i(v),i＝H2O,CH4 can be expressed as:
Wherein P is the measurement area gas pressure, L is the path length, X (L) is the mole fraction of the gas to be measured, T (L) is the temperature of the gas to be measured, S [ T (L) ] is the temperature-dependent line intensity, Is a linear function of the absorption spectrum, satisfies the normalization condition, and l is a position on the expression path.
Step four: extracting the beat frequency from the beat signal I 2 (t), the relationship between the path length and the beat frequency can be expressed as:
wherein c represents the speed of light in the air, f IF＝aτm is the frequency of the beat signal, and the signal I 2 (t) collected by the photodetector is subjected to Fourier transform to obtain a frequency spectrum, and the frequency corresponding to the maximum amplitude point in the frequency spectrum is the required frequency f IF.
Step five: as can be seen from formulas (2) and (4) in step three, the absorption curves α i(v),i＝h2o,CH4 of water molecules and methane are expressed as:
The reference light I r is obtained according to a certain light splitting ratio, and can be obtained by calculating the reference light I r from the monitoring light according to a certain proportion; measuring light I m is obtained through baseline fitting, selecting sections [ t 1,t2 ] and [ t 3,t4 ] with extremely small absorption at two sides of an absorption section in I mexp(-αi (v)), and enabling the amplitude of I m of the two sections to approach the amplitude of I mexp(-αi (v)) through a data fitting mode to obtain I m which does not contain the term exp (-alpha i (v)); the absorption spectrum can be calculated from the absorption curve α i (v).
Step six: from the absorption curve of water moleculesThe integrated absorption area a i of two absorption lines is extracted, i=1 or 2, the path average temperature is calculated by using a colorimetry method, and the integrated absorption area a i meets the following conditions:
From equation (10), the ratio of the integrated absorption areas of water molecules at the two absorption lines is only dependent on the gas temperature:
obtaining a path average temperature T through the calculated integral absorption area ratio; the methane concentration can then be calculated according to the beer's law from the following equation:
The above description of the invention and its embodiments is not limited thereto, but is shown in the drawings as only one of its embodiments. Without departing from the spirit of the invention, a similar structure or embodiment to the technical proposal is not creatively designed, and the invention belongs to the protection scope of the invention.
Claims (2)
1. The trace gas monitoring device based on the open path comprises a signal generator, two tunable diode lasers, two optical isolators, two optical power amplifiers, a wavelength division multiplexer, two optical fiber beam splitters, two collimating mirrors, a parabolic mirror with a small hole in the center, a pyramid prism, a dichroic mirror, a beam splitter, four photodetectors, three narrow-band optical filters, a high-pass filter, a low-pass filter and a data acquisition card; wherein: the signal generator is connected with the two tunable diode lasers, the laser frequency of the two tunable diode lasers is linearly changed by adjusting the injection current of the lasers, the wavelength output by one tunable diode laser covers one absorption spectrum line of the gas to be detected, and the wavelength output by the other tunable diode laser covers two absorption spectrum lines of the easily acquired gas molecules; the two tunable diode lasers are respectively connected with two optical isolators, two beams of laser emitted by the two optical isolators are respectively amplified by an optical power amplifier, and then a wavelength division multiplexer is used for coupling the laser of two wave bands together to realize a common optical path of the two laser beams; laser passing through a common light path of the wavelength division multiplexer is incident into two optical fiber beam splitters connected in series for beam splitting, so that three beams of light are obtained, wherein one beam of light with the power ratio of 90% is used as measuring light, the other beam of light with the power ratio of 5% is used as reference light for frequency modulation continuous wave ranging, and the other beam of light with the power ratio of 5% is used as monitoring light for monitoring the laser power of the device; after being collimated by the collimating mirror, the measuring light sequentially passes through the parabolic mirror with a small hole in the center and the gas to be measured, and is reflected by the primary path of the pyramid prism and then received by the parabolic mirror with the small hole in the center; the dichroic mirror separates the measuring light according to the wavelength, the sweep frequency laser containing the temperature measuring absorption spectrum line is reflected by the dichroic mirror, and is detected by a photoelectric detector after passing through the narrow-band filter; the sweep frequency laser containing the absorption spectrum line of the gas to be detected is transmitted by a dichroic mirror, the beam splitter combines the transmitted light with the reference light which does not pass through the gas to be detected, the combined light is detected by two photoelectric detectors after passing through a narrow-band filter, and the signals of the two photoelectric detectors are respectively filtered by a high-pass filter and a low-pass filter before being uploaded to the acquisition card; the change of the laser power of the monitoring device is recorded by a photoelectric detector; the detection signals of the four photoelectric detectors are uploaded to a computer through a collection card for real-time processing, and the average temperature, the path length and the absolute concentration of the gas to be detected are inverted, and the method is characterized by comprising the following steps:
Step one: the signal generator generates sawtooth wave signals to modulate the output wavelength of the two tunable diode lasers, and the laser controller adjusts the output laser frequency of the tunable diode lasers by changing injection current; in order to linearly change the laser frequency, the sawtooth waveform is continuously modified according to the relation between the injection current and the laser frequency until the laser frequency output by the tunable diode lasers linearly changes, and the laser frequency v (t) generated by each tunable diode laser meets the following conditions:
ν(t)＝v0+at (1)
Wherein v 0 represents the starting frequency of laser frequency modulation, a=Ω/T represents the modulation rate of the laser frequency, T represents the period of laser modulation, Ω represents the bandwidth of the laser frequency modulation;
Step two: adjusting the working temperature of the tunable diode lasers, so that the laser frequency generated by one tunable diode laser covers one absorption spectrum line of the gas to be measured, and the laser frequency generated by the other tunable diode laser covers two absorption spectrum lines of the temperature measuring gas, and driving the two tunable diode lasers in a time division multiplexing mode; an optical isolator is arranged between the tunable diode laser and the wavelength division multiplexer to protect the tunable diode laser; two beams of laser emitted by the optical isolator amplify laser power respectively by using an optical power amplifier; the wavelength division multiplexer couples the laser generated by the two tunable diode lasers together to realize a common optical path of the two laser beams; two fiber optic splitters are connected in series according to 90:5:5, dividing the laser into three paths of output, wherein one path of laser with the proportion of 90% is used as measuring light, the light intensity is marked as I m, and two paths of laser with the proportion of 5% are respectively used as reference light for frequency modulation continuous wave ranging and monitoring light for monitoring the laser power of the device, and the light intensities are respectively marked as I r and I monitor;
Step three: after being collimated by the collimating mirror, the measuring light sequentially passes through the parabolic mirror with a small hole in the center and the gas to be measured, is reflected by the primary path of the pyramid prism, and is received by the parabolic mirror with the small hole in the center; the dichroic mirror separates the measuring light according to the wavelength, the sweep frequency laser containing the temperature measuring absorption spectrum is reflected by the dichroic mirror, and is detected by a photoelectric detector after passing through the narrow-band filter; the sweep frequency laser comprising the measurement of the absorption spectrum of the gas to be measured is transmitted by the dichroic mirror, the beam splitter combines the transmitted light with the reference light which does not pass through the gas to be measured, the combined light is detected by the two photoelectric detectors after passing through the narrow-band filter, and the detection signals of the two photoelectric detectors are respectively uploaded to the acquisition card after passing through a high-pass filter and a low-pass filter; the change of the laser power of the monitoring device is recorded by a fourth photoelectric detector; the signals uploaded to the acquisition card by the four photoelectric detectors are respectively expressed as:
I1(t)＝Imexp(-αx(v(t))) (2)
I3(t)＝Ir+Imexp(-αtarget(v(t))) (4)
I4(t)＝Imonitor (5)
Wherein, I 1 (t) is a measurement signal of an absorption spectrum which is easy to obtain, I 2 (t) is a measured beat signal, I 3 (t) is a measurement signal of an absorption spectrum of the gas to be detected, and I 4 (t) is a monitoring signal of laser power of a monitoring device; τ 1、τ2 is the time delay introduced by the laser when passing through the measuring path and the reference path; τ m is the time difference between the measurement light and the reference light reaching the photodetector; α x(v(t))、αtarget (v (t)) is the absorbance at the thermometry absorption line and the absorbance at the gas absorption line to be measured, respectively, α i (v), i=x, target is expressed as:
Wherein P is the measurement area gas pressure, L is the path length, X (L) is the mole fraction of the gas to be measured, T (L) is the temperature of the gas to be measured, S [ T (L) ] is the temperature-dependent line intensity, Is a linear function of the absorption spectrum, satisfies the normalization condition, and l is a position on the representation path;
step four: the beat frequency is extracted from the beat signal I 2 (t), and the relationship between the path length L and the beat frequency f IF is expressed as:
Wherein c represents the speed of light in the air, f IF＝aτm is the frequency of the beat frequency signal, the signal I 2 (t) collected by the photoelectric detector is subjected to Fourier transform to obtain a frequency spectrum, and the frequency corresponding to the maximum amplitude point in the frequency spectrum is the required frequency f IF;
Step five: as can be seen from formulas (2) and (4) in the third step, the absorption rate at the absorption line of the measured temperature and the absorption rate α i (v) at the absorption line of the gas to be measured, i=x, and target are expressed as:
The reference light I r is obtained according to a certain light splitting ratio and is obtained by calculating the reference light I r from the monitoring light according to a certain proportion; measuring light I m is obtained through baseline fitting, selecting sections [ t 1,t2 ] and [ t 3,t4 ] with extremely small absorption at two sides of an absorption section in I mexp(-αi (v)), and enabling the amplitude of I m of the two sections to approach the amplitude of I mexp(-αi (v)) through a data fitting mode to obtain I m which does not contain the term exp (-alpha i (v)); the absorption spectrum is calculated from an absorption curve alpha i (v);
Step six: the integrated absorption area a i of the two absorption lines, i=1 or 2, is extracted from the absorption curve α x (v (t)) of the thermometric absorption line, the path average temperature is calculated by colorimetry, and the integrated absorption area a i satisfies:
from equation (10), the ratio R of the integrated absorption areas at the two absorption lines is only dependent on the gas temperature:
obtaining the path average temperature by calculating the ratio of the integral absorption areas Then, according to the beer lambert law, the concentration of the gas to be measured is calculated by the following formula: /(I)。
2. The method for detecting the concentration of the trace gas monitoring device in real time on the basis of the open path according to claim 1, wherein the trace gas monitoring device on the open path uses a wavelength division multiplexer to couple the sweep lasers of two wave bands together so as to realize the common optical path of the lasers of two different wavelengths; separating the sweep frequency laser containing the temperature measurement absorption spectrum from the sweep frequency laser containing the gas absorption spectrum to be measured by using a dichroic mirror, wherein the signal containing the temperature measurement absorption spectrum is used for calculating the path average temperature; the sweep laser and the reference light containing the absorption spectrum line of the gas to be detected are combined by a beam splitter, the beat frequency signal generated by the combined light is used for calculating the path length, and the direct current component of the combined light is used for calculating the absolute concentration of the gas to be detected.
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| CN108760681A (en) * | 2018-04-26 | 2018-11-06 | 北京航空航天大学 | A kind of path averaged temperature measuring system and method decomposed based on waveform |
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| CN111077109A (en) * | 2020-01-16 | 2020-04-28 | 北京航空航天大学 | Temperature and concentration measuring system and method based on double-optical comb spectrum technology |
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| 基于TDLAS的长光程环境大气痕量CO监测方法研究;姚路;刘文清;刘建国;阚瑞峰;许振宇;阮俊;戴云海;;中国激光;20150228(第02期);全文 * |
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