CN117030638A - Gas temperature and concentration measuring method based on open optical path adjustable ring-down cavity - Google Patents

Gas temperature and concentration measuring method based on open optical path adjustable ring-down cavity Download PDF

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CN117030638A
CN117030638A CN202311006512.4A CN202311006512A CN117030638A CN 117030638 A CN117030638 A CN 117030638A CN 202311006512 A CN202311006512 A CN 202311006512A CN 117030638 A CN117030638 A CN 117030638A
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cavity
signal
ring
laser
gas
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曹章
李心语
徐立军
周文彬
李雪娇
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Beihang University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
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    • G01K11/003—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using absorption or generation of gas, e.g. hydrogen
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    • G01—MEASURING; TESTING
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    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
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Abstract

The invention provides a gas temperature and concentration measuring method based on an open optical path adjustable ring-down cavity, which mainly comprises a laser generating module, an optical modulation module, an open optical ring-down cavity module, a signal acquisition and processing module, a feedback circuit module and the like; the tunable laser is coupled into the cavity of the open ring-down cavity, a periodical light intensity attenuation signal is formed through pattern matching, the influence of abnormal extinction caused by large particle scattering on frequency domain fitting is reduced according to the window selection of time domain analysis, and then the frequency domain signal is converted into a frequency domain signal to change the attenuation coefficient tolerance band for correction, so that the measurement of absorption spectrum, path average temperature and component concentration in the atmosphere open environment is realized. The invention has simple structure and high system stability, reduces the influence of aerosol extinction in an open environment on measurement precision by a time domain analysis and frequency domain iterative filtering method, is suitable for outdoor portable measurement, and has higher use value and wide application prospect in the field of gas detection.

Description

Gas temperature and concentration measuring method based on open optical path adjustable ring-down cavity
Field of the art
The invention provides a gas temperature and concentration measuring method based on an open optical path adjustable ring-down cavity, and belongs to the technical field of ring-down absorption spectrum of optical cavities and gas temperature and concentration measurement.
(II) background art
With the rapid development of socioeconomic performance, the industrial sector has put higher demands on the precise measurement of the gas concentration and the lower limit of the measurement. Trace gas detection becomes a significant subject of scientific research, and is widely applied to various fields such as industrial production, atmospheric detection, biomedical treatment, scientific research and the like. The gas concentration detection technique is classified into a chemical detection method and an optical detection method according to the detection method. The traditional chemical detection method is generally contact measurement, and not only can interfere with a gas flow field to be detected, but also is easily influenced by environmental change and air disturbance. In addition, the method has high requirements on stability of measured parameters, and generally cannot realize online real-time monitoring. The optical detection technology is mainly an absorption spectrum technology, utilizes absorption spectrum information of the detected gas to establish a relation between the spectral absorptivity and the gas concentration, and avoids the influence of direct contact gas on the measurement precision. The optical detection technology has the advantages of long service life, high stability, high detection speed, high sensitivity, low power consumption, low cost and the like, and is widely used in the field of gas concentration measurement.
Ring-down Cavity spectroscopy (CRDS) is divided into Pulsed laser ring-down Cavity absorption spectroscopy (Pulsed-CRDS), continuous laser ring-down Cavity absorption spectroscopy (cw-CRDS) and Cavity enhanced absorption spectroscopy (Cavity-enhanced absorption spectroscopy, CEAS). In 1988O' Keefa and Anthony published in the paper of science instruments overview 59, pages 2544-2551 (Review ofScientific Instruments) of the absorption rate measurement based on Pulsed laser (Cavity ring-down spectrometer-down optical spectrometer for absorption measurements usingpulsed laser sources), high-power Pulsed laser is used as light source to measure the vibration absorption spectrum of ions and free radicals at the Pulsed discharge nozzle, which proves that the gas measurement sensitivity of the Pulsed-CRDS method can reach 10 -6 Magnitude. Because the linewidth of the pulse laser is large, single-mode laser is difficult to generate, and multi-mode laser can increase noise, and the measuring precision of the system is improvedThe influence is caused; in the paper of Karpf et al, applied Optics, 55 volume 16, page 4497 (Applied Optics), ultrasensitive real-time tracking gas detection technology (Ultrasensitive, real-time trace gas detection using a high-power, multimode diode laser and cavity ringdown spectroscopy) using a high-power multimode diode laser and ring-down cavity, the influence of multimode laser on measurement accuracy is eliminated by multiple measurement and averaging, and the characteristic of resisting mechanical vibration is realized. Compared with a pulse laser, the continuous laser has a narrower linewidth (< 50 MHz), can realize wavelength tuning in a smaller spectral range, and has higher spectral resolution and better wavelength repeatability. In the continuous ring-down cavity spectrum technology (Chemical Physics Letters) published in the fast report of Physics and chemistry, volume 264, pages 316-322 (Chemical Physics Letters) of 1997, continuous light is used as a light source for the first time, and piezoelectric ceramics are utilized to continuously modulate the cavity length of the ring-down cavity in a plurality of free spectral ranges, so that the measurement sensitivity of target gas reaches 10 -9 cm -1 The method comprises the steps of carrying out a first treatment on the surface of the He and Orr in 2000 were published in "ring-down cavity enhanced absorption spectroscopy technique using a continuous wave tunable diode laser and a fast swept optical cavity" (Ringdown and cavity-enhanced absorption spectroscopy using a continuous-wave tunablediode laser and a rapidly swept optical cavity) at pages 131-137 (Chemical Physics Letters) of "physical chemistry fast report" 319 volume 1, and CRDS and CEAS absorption spectroscopy techniques were simultaneously employed to rapidly change the resonant frequency of the cavity, rapidly accumulate energy in the cavity, rapidly break away from the resonant state, and replace the effect of optical shutdown, but the measurement accuracy was reduced; in the continuous wave cavity ring-down technology principle derivation and experimental study of pages 452-456 of the application laser, 6 of 2006, tan Zhongji et al, the loss in the cavity is analyzed according to the energy conservation principle, the continuous wave cavity ring-down technology measurement principle is studied, a cavity loss measurement system is built, and the measurement result shows that the measurement error of the cavity loss of the system is about 0.15ppm; high precision of greenhouse gases (CO 2and CH 4) using cavity ring-down spectroscopy (CRDS) technology, published in atmospheric measurement technology, 3 volume, 2 nd phase 375-386 page (Atmospheric MeasurementTechniques) by Chen et al 2009In continuous air measurement (High-accuracy continuous airborne measurements ofgreenhouse gases (CO 2and CH 4) using the cavity ring-down spectroscopy (CRDS) technology), a set of integrated airborne measuring instrument is developed, the oscillation starting of a ring-down cavity is realized by adopting a laser wavelength scanning mode, the content of carbon dioxide and methane in the atmosphere is measured, and the air in the cavity is dried by using drying equipment, so that the interference of water vapor on measurement in the measuring process is avoided, and the measuring standard deviation which is better than 0.3ppm is realized; in 2012 Zhao Hui et al published in "measuring carbon dioxide in the atmosphere by using an off-axis cavity enhanced absorption spectroscopy technique" on pages 41-45 of "spectroscopy and spectral analysis" 32 volumes 1 ", a wavelength modulation technique is used to modulate the laser wavelength, and high-frequency background noise can be effectively suppressed by harmonic detection, and an effective absorption optical path of a resonant cavity is obtained by referring to an absorption cell with a known optical path length. The method needs to introduce a reference absorption pool, so that the complexity of the system is increased; in 2018 Li et al (general environmental science) 613, p.131-139 (Science ofThe Total Environment) in-situ CRDS and CEAS (Intercomparison ofin situ CRDS and CEAS for measurements ofatmospheric N O5in Beijing, china) for atmospheric N2O5, the measured gas component in the cavity is exhausted by using a vacuum mechanical cavity, the optical loss in the cavity is calibrated, and the atmospheric N is compared with CRDS and CEAS instruments 2 O 5 The measurement is carried out, and the measurement result shows good consistency; in an optical switch dual-wavelength cavity ring-down spectrometer (Optically Switched Dual-Wavelength Cavity Ring-Down Spectrometer for High-Precision Isotope Ratio Measurements of Methane delta D in the Near Infrared) for near infrared methane delta D high-precision isotope ratio measurement, published by Chen et al, analytical chemistry, 93, 16 th stage 6375-6384 (Analytical Chemistry), a waveguide optical switch is used to achieve rapid alternation between the two wavelengths, and in methane isotope ratio measurement, common mode noise caused by temperature and frequency fluctuations is reduced, but the measurement system cannot achieve cavity temperature and laser frequency stabilization. In summary, the existing method has the common problem that complicated calibration equipment or standard gas is required to be introduced for cavity calibration so as to obtain accurate cavity length and accurate cavity lengthMirror reflectivity, which increases the complexity of the measurement system and introduces a new source of error to the gas concentration measurement.
The open ring-down cavity calibration-free measurement method becomes a hot spot of current research, the open cavity can uniformly distribute atmospheric gas to be measured between two cavity mirrors, loss introduced by calibration sampling and adsorption effect of the cavity walls on the gas are reduced, and the time response rate of the system is improved. In 2005 Bitter et al published in "Wide-Spectrum cavity Ring-down spectrometer for in-situ measurement of atmospheric trace gas" (Abroadband cavity ringdown spectrometer for in-situ measurements ofatmospheric trace gases) volume 5, 9 th edition 2547-2560 (Atmospheric Chemistry andPhysics), the gas absorptivity obtained by fitting the differential structure in the broad-Spectrum cavity Ring-down spectrum by means of differential absorption Spectrum method, the measurement accuracy of gas concentration mainly depends on the influence of vapor absorption and aerosol on the target spectrum, and it is assumed that the aerosol introduces linear offset when data is processed at present, so that the influence of processing aerosol can introduce larger measurement error; in the method of measuring IO free radical concentration in a marine boundary layer (Measurement ofIO radical concentrations in the marine boundary layer using a cavity ring-down spectrometer) by using a cavity ring-down spectrometer, wada et al published in journal of Dai chemistry, 58, pages 1-87 (Journal ofAtmospheric Chemistry), an open cavity ring-down spectrometer for measuring iodine monoxide free radicals in the atmosphere is provided, viscous marine gas is measured by using the marine boundary layer, an accurate IO absorption section is obtained under the condition that the influence of aerosol is lower than the noise level, and the measurement lower limit of 10pptv is realized; in Gordon et al, no. 2015, no. 9, no. 717-726 (Aerosol Science and Technology), designs of novel open-diameter aerosol extinction cavity ring-down spectrometer (Design ofaNovel Open-PathAerosol Extinction Cavity Ringdown Spectrometer), an open-circuit CRDS system is provided, wherein a mechanical shell is closed periodically to clear the cavity at fixed time, solve the cavity loss, and the measurement system measures the lower limit of 0.05cm in 1s of average data -1 The measurement accuracy is comparable to a sealed ring down cavity system, but requires a mechanical systemThe dynamic calibration is carried out, so that the complexity of the system is increased; in 2019 Mcha et al published in optical expression 27 volume 14 page 20084 (Optics Express) Open cavity ring-down methane sensor for mobile monitoring of natural gas emission (Open-path cavity ring-down methane sensor for mobile monitoring ofnatural gas Emissions), the influence of atmospheric aerosol on measurement results is eliminated by an iterative filtering mode, and the proposed box-division filtering and global iterative filtering algorithm are sensitive to the accuracy of a fitting mode; in 2020, song Zhenyuan et al, published in optical fast report 45, pp.3, 746-749 (Optics Letters), continuous wave CRDS System without measurement of concentration of calibration gas (A random vibration-driven continuous wave CRDS system for calibration-free gas concentration measurement), cavity length variation is driven by random vibration, calibration of cavity loss is avoided, and due to randomness of mechanical vibration, the occurrence of effective signals is also random, so that the acquisition efficiency of effective signals is low, and the variation value of cavity length with vibration cannot be given.
Based on the background, the invention provides a gas temperature and concentration measurement method based on an open optical path adjustable ring-down cavity, which is used for realizing atmospheric environment temperature and component concentration measurement in an open environment, reducing the influence of abnormal extinction of open path large particle aerosol on frequency domain fitting through time domain fitting, converting the influence into a frequency domain signal to change an attenuation coefficient tolerance band for correction, and realizing measurement of absorption spectrum, path average temperature and component concentration in the atmospheric open environment.
(III) summary of the invention
The invention provides a gas temperature and concentration measuring method based on an open optical path adjustable ring-down cavity, belonging to the technical field of ring-down absorption spectrum of an optical cavity and gas temperature and concentration measurement; the tunable laser is coupled into the cavity of the open ring-down cavity, a periodical light intensity attenuation signal is formed through pattern matching, the influence of abnormal extinction caused by large particle scattering on frequency domain fitting is reduced according to the window selection of time domain analysis, and then the frequency domain signal is converted into a frequency domain signal to change the attenuation coefficient tolerance band for correction, so that the measurement of absorption spectrum, path average temperature and component concentration in the atmosphere open environment is realized.
The implementation device comprises: the device comprises a laser generation module, an optical modulation module, an optical ring-down cavity, a signal acquisition and processing module and a feedback circuit module.
The technical scheme of the invention is as follows: the laser generating module transmits a periodically tuned laser signal to enable the periodically tuned laser signal to be matched with the ring-down cavity mode, and a ring-down signal is generated; the feedback circuit module generates a control signal of the light modulation module by comparing the acquired signal with a threshold value, so as to realize on-off switching of incident laser; the tunable laser is coupled with the indicating light by a two-in-two coupler, one path is used for detecting the wavelength of the tunable laser by a wavemeter, the other path is collimated by a collimating mirror and then enters an open optical ring-down cavity, the tunable laser is reflected for multiple times in high reflectors at two sides of the ring-down cavity, a light intensity attenuation signal is generated at the emergent end of the ring-down cavity, the optical signal is converted into an electric signal by a photoelectric detector, and the electric signal is led into an acquisition and processing module. The method is characterized in that according to the relation between the exponential decay coefficient and the absorption rate, the influence of atmospheric aerosol extinction on the measurement accuracy is removed through time domain analysis and frequency domain iteration.
First, when the tuning laser and the optical ring-down cavity meet the resonance condition, the laser with the frequency v is coupled into the open ring-down cavity composed of the high-reflectivity reflector:
wherein q is the ordinal number of the longitudinal mode of the laser, and L is the cavity length of the optical ring-down cavity;
the signal generator generates a stepped modulation signal, and superimposes a sine wave with small amplitude on the modulation signal, wherein the amplitude of the sine wave is larger than one free spectral range and smaller than the adjusting voltages of two free spectral ranges, and the superimposed signal is used for modulating the wavelength of the tunable diode laser, so that the ring-down cavity system generates ring-down signals with oscillation periods at each step length; the initial wavelength of the tunable laser is read by a wavemeter, and the frequency of each point is corrected according to the free spectral range:
v q ＝v 0 +FSR·q (2)
the tunable laser is emitted from the collimating mirror and converts the optical fiber light into space light, the laser which meets the requirement of ring-down cavity mode matching is reflected in the optical ring-down cavity for multiple times, the light intensity signal transmitted by the rear reflector is exponentially attenuated along with time, the photoelectric detector converts the light signal into an electric signal and transmits the electric signal to the acquisition system, and the acquired light intensity signal I (t) can be expressed as:
wherein I is 0 For initial light intensity of ring down signal, t is time, c is light speed, R is reflectivity of high reflector, α (v) is coefficient of spectral absorptivity, and under the condition of multicomponent gas, total absorptivity of gas can be expressed as:
wherein n is the number of gas absorption lines contained in a measurement wave band, P is the gas pressure in a measurement area, xabs is the mole fraction of the gas to be measured, S (T, v 0 ) Phi is the absorption line intensity function voigt (v,X abs ) As a linear function of the absorption spectrum Voigt;
absorption line intensity function S (T, v 0 ) Can be expressed as:
wherein T is the measured temperature, v 0 For spectral line center frequency, T 0 Q (T) is a distribution function of gas molecules to be detected at the temperature T, h is a Planck constant, c is the speed of light, E' is the low-energy state energy of energy level transition, and k is a Boltzmann constant;
the expression of the absorbance profile Voigt line is:
wherein Deltav D For Doppler half-width, a is the parameter relation between Doppler broadening and collisional broadening, w is the dimensionless number of the spectrum length of the center of the distance absorption spectrum, and y is the integral variable;
according to the formula (3), carrying out exponential fitting on the acquired ring-down cavity light intensity attenuation signals to obtain an exponential attenuation coefficient beta (v):
the relationship between β (v) and α (v) is obtained from formula (7):
wherein (1-R)/L is the cavity loss of the ring down cavity; considering the influence of the aerosol present at the time of open optical path measurement, α (v) can be corrected to α' (v):
α'(v)＝α(v)+α ext (9)
wherein,extinction ratio for uniformly distributed aerosol small particles, +.>The extinction ratio of aerosol large particles which are unevenly distributed is achieved; acquiring each step position of the step wave for multiple times to obtain an exponential decay coefficient beta (v) of a corresponding wavelength, wherein the distribution of the exponential decay coefficient is composed of Gaussian distribution and high absorption tail generated by aerosol with uneven distribution;
as can be seen from the formula (8), the absorption rate and the cavity loss of the gas to be detected and the extinction coefficient of the aerosol are in a linear superposition relation; converting the measured frequency domain exponential decay coefficient into a time domain for analysis, namely performing Fourier inverse transformation on two sides of the formula (8) to obtain a decay signal A (t) of the time domain:
the cavity loss, the small particle absorption and the gas absorptivity change slowly along with the optical frequency, and the component attenuation speed of the signals is faster than that of signals such as high-frequency noise in the time domain, so that a proper area can be selected for carrying out absorption spectrum fitting, the influence of the high-frequency noise in the frequency domain on spectrum fitting is suppressed, and a weighting function is used for selecting a fitting area:
wherein t is 1 To start molecular absorption time, t 2 A moment at which the molecular response decays to a signal noise level;
and carrying out iterative correction on the attenuation coefficient, firstly setting the temperature to be measured, the gas mole fraction, the cavity loss and the small particle absorption loss as free variables, and carrying out nonlinear least square fitting on the measured absorption rate and the simulated absorption rate in the database according to a formula (11).The light intensity loss caused by the method can not be completely separated from gas absorption on a time scale, the fitted attenuation coefficient distribution is subjected to Fourier transformation and converted into a frequency domain signal, frequency points with larger correction errors of the attenuation coefficient distribution tolerance band are iteratively changed according to a formula (8), and the concentration and the temperature of the open path gas components are solved.
The invention has the advantages that: 1. according to the invention, the laser generation module is used for transmitting the periodically tuned laser signal, so that the periodically tuned laser signal is matched with the ring-down cavity mode to generate the ring-down signal, the precision requirement of discrete points on a fitting mode is reduced through time domain analysis, and then the frequency points with larger correction errors are changed through iteration of a frequency domain to change the attenuation coefficient distribution tolerance band, so that the calibration-free measurement of the gas temperature and the component concentration under the influence of aerosol is realized. 2. The open type measuring system provided by the invention has the advantages of simple structure, uniform distribution of measured gas, gas sampling loss avoidance, system energy consumption reduction, simple operation of a filtering algorithm, no need of introducing other devices, and improvement of the measuring precision of average temperature and component concentration in an open optical path.
(IV) description of the drawings
Reference is made to the accompanying drawings
FIG. 1 is a schematic diagram of the overall system structure of the present patent
101. Laser generating module 102, optical modulating module 103, and open optical ring-down cavity
104. Signal low sampling rate acquisition and processing module 105 and feedback circuit module
FIG. 2 is a schematic diagram of a spatial light path structure
Fig. 3 is a flow chart of the method of the present patent.
Fig. 4 is a comparison of the method of the present patent with a conventional frequency domain fit solution.
(fifth) detailed description of the invention
The invention will be further described with reference to specific examples.
The experiment is carried out under the following conditions, and the experiment is carried out under the open environment of standard atmospheric pressure, as shown in the space light structure diagram of fig. 2, the space light is adjusted to be coaxial through a cage structure, and the relative distance between the focal length of the collimating lens and the matching lens is adjusted according to calibration data, so that the optimal cavity length of the ring-down cavity is selected; the laser generating module (101) tunes the emergent laser with the central wave number of 5937.7cm -1 Step-by-step scanning is started at the position, the scanning step length is slightly larger than the free spectrum range, and the coverage of methane and water vapor is 5937.7cm -1 To 5938.7cm -1 The absorption spectrum line at the position has a period of 1Hz, sine waves with a frequency of 1kHz are applied to carry out pattern matching while step scanning, the periodic turn-off is controlled by a feedback circuit module (105) through an optical modulation module (102), and then the incident light is formed by a group of high-reflectivity reflectors and a designed open mirror frameIn the ring-down cavity (103), the light intensity exponential decay signal of the emergent end is led into a signal acquisition and processing module (104) for acquisition, and the sampling frequency is 14MHz. The method specifically comprises the following steps:
step one, tuning the outgoing center wave number of the laser generating module (101) to 5937.7cm -1 The step length and the sine wave amplitude are set to be 10mV; the red pen adjusts the space light coaxial; by adjusting the relative distance between the focal length of the collimating lens and the matching lens shown in fig. 2, the cavity length is adjusted according to the base waist position and the spot size of the outgoing beam calibrated by the mass beam analyzer, so that the cavity length is as long as possible, the outgoing signal intensity is less influenced by path aerosol, the optimal cavity length parameter L is selected to be 16.5cm, and the free spectrum range is 0.0302cm -1 ；
Step two, a signal generator generates a stepped modulation signal, and a sine wave with small amplitude is superimposed on the modulation signal, wherein the amplitude of the sine wave is larger than one free spectral range and smaller than the adjusting voltage of two free spectral ranges, and the superimposed signal is used for modulating the wavelength of the tunable diode laser, so that a ring-down cavity system generates ring-down signals with oscillation periods at each step length; the initial wavelength of the tunable laser is read by a wavemeter, and the frequency of each point is corrected according to the free spectral range:
v q ＝v 0 +FSR·q (2)
step two, tunable laser is emitted from a collimating mirror and converted into space light, laser which meets the requirement of ring-down cavity mode matching is reflected for multiple times in an optical ring-down cavity, a light intensity signal transmitted by a rear reflector is exponentially attenuated along with time, a photoelectric detector converts the light signal into an electric signal and transmits the electric signal to an acquisition system, and an acquired light intensity signal I (t) can be expressed as:
wherein I is 0 For the initial light intensity of ring-down signal, t is time, c is light speed, R is reflectivity of high reflector, alpha (v) is coefficient of spectral absorptivity, under the condition of multicomponent gas, the gasThe total absorption rate can be expressed as:
wherein n is the number of gas absorption lines contained in a measurement wave band, P is the gas pressure in a measurement area, and X abs Is the mole fraction of the gas to be measured, S (T, v) 0 ) Phi is the absorption line intensity function voigt (v,X abs ) As a linear function of the absorption spectrum Voigt;
absorption line intensity function S (T, v) 0 ) Can be expressed as:
wherein T is the measured temperature, v 0 For spectral line center frequency, T 0 Q (T) is a distribution function of gas molecules to be detected at the temperature T, h is a Planck constant, c is the speed of light, E' is the low-energy state energy of energy level transition, and k is a Boltzmann constant;
the expression of the absorbance profile Voigt line is:
wherein Deltav D For Doppler half-width, a is the parameter relation between Doppler broadening and collisional broadening, w is the dimensionless number of the spectrum length of the center of the distance absorption spectrum, and y is the integral variable;
step three, according to the formula (3), carrying out exponential fitting on the acquired ring-down cavity light intensity attenuation signals to obtain an exponential attenuation coefficient beta (v):
the relationship between β (v) and α (v) is obtained from formula (7):
wherein (1-R)/L is the cavity loss of the ring down cavity; considering the influence of the aerosol present at the time of open optical path measurement, α (v) may be corrected to α' (v):
α'(v)＝α(v)+α ext (9)
wherein,extinction ratio for uniformly distributed aerosol small particles, +.>The extinction ratio of aerosol large particles which are unevenly distributed is achieved; acquiring each step position of the step wave for multiple times to obtain an exponential decay coefficient beta (v) of a corresponding wavelength, wherein the distribution of the exponential decay coefficient is composed of Gaussian distribution and high absorption tail generated by aerosol with uneven distribution;
as can be seen from the formula (8), the absorption rate and the cavity loss of the gas to be detected and the extinction coefficient of the aerosol are in a linear superposition relation; converting the measured frequency domain exponential decay coefficient into a time domain for analysis, namely performing Fourier inverse transformation on two sides of the formula (8) to obtain a decay signal A (t) of the time domain:
the cavity loss, the small particle absorption and the gas absorptivity change slowly along with the optical frequency, and the component attenuation speed of the signals is faster than that of signals such as high-frequency noise in the time domain, so that a proper area can be selected for carrying out absorption spectrum fitting, the influence of the high-frequency noise in the frequency domain on spectrum fitting is suppressed, and a weighting function is used for selecting a fitting area:
wherein t is 1 To start molecular absorption time, t 2 A moment at which the molecular response decays to a signal noise level;
and fourthly, carrying out iterative correction on the attenuation coefficient, firstly setting the temperature to be measured, the gas mole fraction, the cavity loss and the small particle absorption loss as free variables, and carrying out nonlinear least square fitting on the measured absorption rate and the simulated absorption rate in the database according to a formula (11).The light intensity loss caused by the method can not be completely separated from gas absorption on a time scale, the fitted attenuation coefficient distribution is subjected to Fourier transform and converted into a frequency domain signal, the frequency points with larger correction errors of the attenuation coefficient distribution tolerance band are iteratively changed according to a formula (8), the tolerance band is set by using the histogram distribution frequency in the example, and the concentration and the temperature of the open path gas components are solved.
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 structure or an embodiment similar to the technical scheme is not creatively designed, and the structure or the embodiment belongs to the protection scope of the invention.

Claims (2)

1. The system comprises a laser generation module, an optical modulation module, an open optical ring-down cavity module, a signal acquisition and processing module, a feedback circuit module and the like; the laser generating module transmits a periodically tuned laser signal to enable the periodically tuned laser signal to be matched with the ring-down cavity mode, and a ring-down signal is generated; the feedback circuit module generates a control signal of the light modulation module by comparing the acquired signal with a threshold value, so as to realize on-off switching of incident laser; the tunable laser is coupled with the indicating light by a two-in-two coupler, one path is used for detecting the wavelength of the tunable laser by a wavemeter, the other path is collimated by a collimating mirror and then enters an open optical ring-down cavity, the tunable laser is reflected for multiple times in high reflectors at two sides of the ring-down cavity, a light intensity attenuation signal is generated at the emergent end of the ring-down cavity, the optical signal is converted into an electric signal by a photoelectric detector, and the electric signal is led into an acquisition and processing module. The method is characterized in that according to the relation between the exponential decay coefficient and the absorption rate, the influence of atmospheric aerosol extinction on the measurement precision is removed through time domain analysis and frequency domain iteration, and the method specifically comprises the following steps:
step one, when the tuning laser and the optical ring-down cavity meet the resonance condition, the laser with the frequency v is coupled into an open ring-down cavity formed by a high-reflectivity reflecting mirror:
wherein q is the ordinal number of the longitudinal mode of the laser, and L is the cavity length of the optical ring-down cavity;
the signal generator generates a stepped modulation signal, and superimposes a sine wave with small amplitude on the modulation signal, wherein the amplitude of the sine wave is larger than one free spectral range and smaller than the adjusting voltages of two free spectral ranges, and the superimposed signal is used for modulating the wavelength of the tunable diode laser, so that the ring-down cavity system generates ring-down signals with oscillation periods at each step length; the initial wavelength of the tunable laser is read by a wavemeter, and the frequency of each point is corrected according to the free spectral range:
v q ＝v 0 +FSR·q (2)
step two, tunable laser is emitted from a collimating mirror and converted into space light, laser which meets the requirement of ring-down cavity mode matching is reflected for multiple times in an optical ring-down cavity, a light intensity signal transmitted by a rear reflector is exponentially attenuated along with time, a photoelectric detector converts the light signal into an electric signal and transmits the electric signal to an acquisition system, and an acquired light intensity signal I (t) can be expressed as:
wherein I is 0 For initial light intensity of ring down signal, t is time, c is light speed, R is reflectivity of high reflector, α (v) is coefficient of spectral absorptivity, and under the condition of multicomponent gas, total absorptivity of gas can be expressed as:
wherein n is the number of gas absorption lines contained in a measurement wave band, P is the gas pressure in a measurement area, and X abs S (T, v) 0 ) Phi is the absorption line intensity function voigt (v,X abs ) As a linear function of the absorption spectrum Voigt;
absorption line intensity function S (T, v) 0 ) Can be expressed as:
wherein T is the measured temperature, v 0 For spectral line center frequency, T 0 Q (T) is a distribution function of gas molecules to be detected at the temperature T, h is a Planck constant, c is the speed of light, E' is the low-energy state energy of energy level transition, and k is a Boltzmann constant;
the expression of the absorbance profile Voigt line is:
wherein Deltav D For Doppler half-width, a is the parameter relation between Doppler broadening and collisional broadening, w is the dimensionless number of the spectrum length of the center of the distance absorption spectrum, and y is the integral variable;
step three, according to the formula (3), carrying out exponential fitting on the acquired ring-down cavity light intensity attenuation signals to obtain an exponential attenuation coefficient beta (v):
the relationship between β (v) and α (v) is obtained from formula (7):
wherein (1-R)/L is the cavity loss of the ring down cavity; considering the influence of the aerosol present at the time of open optical path measurement, α (v) may be corrected to α' (v):
α'(v)＝α(v)+α ext (9)
wherein,extinction ratio for uniformly distributed aerosol small particles, +.>The extinction ratio of aerosol large particles which are unevenly distributed is achieved; acquiring each step position of the step wave for multiple times to obtain an exponential decay coefficient beta (v) of a corresponding wavelength, wherein the distribution of the exponential decay coefficient is composed of Gaussian distribution and high absorption tail generated by aerosol with uneven distribution;
as can be seen from the formula (8), the absorption rate and the cavity loss of the gas to be detected and the extinction coefficient of the aerosol are in a linear superposition relation; converting the measured frequency domain exponential decay coefficient into a time domain for analysis, namely performing Fourier inverse transformation on two sides of the formula (8) to obtain a decay signal A (t) of the time domain:
the cavity loss, the small particle absorption and the gas absorptivity change slowly along with the optical frequency, and the component attenuation speed of the signals is faster than that of signals such as high-frequency noise in the time domain, so that a proper area can be selected for carrying out absorption spectrum fitting, the influence of the high-frequency noise in the frequency domain on spectrum fitting is suppressed, and a weighting function is used for selecting a fitting area:
wherein t is 1 To start molecular absorption time, t 2 A moment at which the molecular response decays to a signal noise level;
and fourthly, carrying out iterative correction on the attenuation coefficient, firstly setting the temperature to be measured, the gas mole fraction, the cavity loss and the small particle absorption loss as free variables, and carrying out nonlinear least square fitting on the measured absorption rate and the simulated absorption rate in the database according to a formula (11).The light intensity loss caused by the method can not be completely separated from gas absorption on a time scale, the fitted attenuation coefficient distribution is subjected to Fourier transformation and converted into a frequency domain signal, frequency points with larger correction errors of the attenuation coefficient distribution tolerance band are iteratively changed according to a formula (8), and the concentration and the temperature of the open path gas components are solved.
2. The method for measuring the gas temperature and the concentration of the ring-down cavity based on the open optical path according to claim 1, wherein the optimal cavity length parameter in the open environment is selected by the adjustable coaxial ring-down cavity, comprising the following steps:
the method comprises the steps that firstly, a space light path device is coaxially arranged through a cage structure, a collimating lens and a matching lens sleeve are installed through a two-dimensional lens holder, an open optical ring-down cavity comprises an optical high-reflectivity reflecting lens which is symmetrically installed and an open installation lens holder, an inner shell of the lens holder is connected through an O ring, and the outer shell of the lens holder is of the cage structure; the micro pump and the air high-efficiency filter are connected with the air guide hole of the lens holder, form vortex in front of the lens, and purge and protect the lens;
step two, calibrating the positions and the sizes of the Gaussian light base waists corresponding to different relative positions of the matched lenses in the sleeve;
and thirdly, adjusting the lens frame to change the cavity length on the premise of ensuring the stability of the device and the small change of the central axis offset by utilizing a cage structure in an experiment, adjusting the distance between the matched lens and the pitching of the lens frame according to a calibration range, observing the emergent signal strength until the transmission peak value is maximum, enabling the cavity length to be as long as possible and enabling the emergent signal strength to be less influenced by path aerosol, so that the optimal cavity length parameter is rapidly selected.
CN202311006512.4A 2023-08-10 2023-08-10 Gas temperature and concentration measuring method based on open optical path adjustable ring-down cavity Pending CN117030638A (en)

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