CN111122496B - Calibration-free gas concentration measuring device and method - Google Patents
Calibration-free gas concentration measuring device and method Download PDFInfo
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- CN111122496B CN111122496B CN201911328202.8A CN201911328202A CN111122496B CN 111122496 B CN111122496 B CN 111122496B CN 201911328202 A CN201911328202 A CN 201911328202A CN 111122496 B CN111122496 B CN 111122496B
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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
- 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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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
- G01N2021/391—Intracavity sample
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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
- 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
- G01N2021/396—Type of laser source
- G01N2021/399—Diode laser
Abstract
A calibration-free gas concentration measuring device and method comprises a direct absorption spectrum device and a ring-down spectrum device, wherein gas chambers of the direct absorption spectrum device and the ring-down spectrum device are communicated with each other and share a laser. Emergent light of the laser is divided into two beams, one beam enters the direct absorption spectrum device, the other beam enters the ring-down spectrum measuring device, and time-sharing measurement of the two spectrum technologies is achieved through program control. The method obtains the gas concentration by directly fitting the transmitted light intensity, and realizes direct absorption spectrum calibration-free measurement. And calculating the cavity ring-down time by utilizing the gas absorption rate measured by the direct absorption spectrum and the ring-down time of gas absorption in the ring-down spectrum measurement, thereby realizing calibration-free measurement of the ring-down spectrum. The invention intelligently selects the measurement result according to the measurement precision and the measuring range of the two spectra, the measurement precision is about 10-20ppm at high concentration and less than 1ppm at low concentration, and the measurement process does not need calibration, thereby having the advantages of high measurement speed, simple operation and the like.
Description
Technical Field
The invention relates to a gas concentration measuring method based on a direct absorption spectrum technology and a cavity ring-down technology, and belongs to the technical field of measurement.
Background
Tunable laser diode absorption spectroscopy (TDLAS for short) has the advantages of non-contact, fast response, wavelength selectivity, and the like. The Direct Absorption Spectroscopy (DAS) in TDLAS has a clear physical concept and simple operation, is commonly used for measuring parameters such as concentration and temperature of gas, and is widely applied to a high-concentration and strong-absorption environment. For trace gas monitoring and weak absorption, more complex techniques such as Cavity ring-down spectroscopy (CRDS) are often used.
The minimum detectable absorbance alpha of DAS when the length of the gas absorption cell is 1mminAbout 10-4～10-5The corresponding absorption coefficient kappa is about 10-6～10-7cm-1(ii) a For CRDS, the maximum and minimum values of κ are about 10, respectively-6～10-7cm-1And 10-11～10-13cm-1Therefore, the intersection of the measurement ranges of the two technologies is small, and the two technologies are suitable for measurement of gas with higher concentration and gas with lower concentration respectively. However, in practical measurements, the concentration of some gases may be in a wide rangeAnd sometimes even fluctuates dramatically. Monitoring H in the atmosphere of a high-rise building, for example on a moving aircraft2O, its concentration may vary from 1ppm to 1% and fluctuates sharply as the aircraft passes through clouds. Non-premixed CH in atmospheric environment4In laminar air flow flames, the CO concentration can vary from 1ppm to 4% at different heights in the flame. Engine exhaust gases (e.g., NO and CO) may range from 1ppm to 0.3% (NO) and 1ppm to 4% (CO) in a single engine cycle. HO of plasma effluents in plasma jet at atmospheric pressure2The radial distribution of the concentration also varies greatly. In these cases, it is difficult for a single method to respond quickly in such a wide concentration measurement range and to maintain high measurement accuracy over a wide range, and therefore it is necessary to consider a gas sensor having a wider range, high accuracy, and quick response. Some researchers consider a combination of the two methods, such as a combination of DAS and Wavelength Modulation Spectroscopy (WMS). WMS is calibrated in real time by using the measurement result of DAS, and the water concentration on-line measurement with wide range and high precision can be realized. The combination of CRDS and laser induced fluorescence can realize real-time, wide-range and high-precision measurement of NOx in the atmosphere. Complementation of cavity enhanced absorption spectrum and CRDS (cyclic redundancy check) can also realize the N in the atmosphere2O5Real-time high-precision detection.
Based on the research, the invention combines DAS and CRDS to establish a gas concentration measuring method with wide range, high precision and no calibration. The method combines the advantages of DAS (rapid, wide-range and measurable absorption rate function) and CRDS (high precision), and the accurate cavity ring-down time tau can be directly calculated by the absorption rate measured by DAS and the ring-down time of the gas absorption0The calibration-free cavity ring-down time is realized, the measurement mode is simplified, and the measurement speed is improved.
Disclosure of Invention
The invention aims to realize calibration-free measurement of gas concentration, and provides a measuring device and a method based on a direct absorption spectrum technology and a ring-down spectrum technology, which are characterized in that: 1) directly calculating cavity ring-down time by using the absorption rate measured by the direct absorption spectrum and the ring-down time measured by the ring-down spectrum, so as to realize calibration-free of the cavity ring-down time; 2) the device combines a direct absorption spectrum and ring-down spectrum measuring device, can adopt a measuring result of a spectrum technology with higher precision when different concentrations are used, and realizes calibration-free, wide-range and high-precision gas concentration measurement.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a calibration-free gas concentration measuring device comprises a tunable semiconductor laser, a laser controller, a signal generator, a computer, an interferometer, a first photoelectric detector and a long-optical-path gas absorption cell; the method is characterized in that: the gas concentration measuring device also comprises a ring-down spectrum measuring device, wherein the device comprises a ring-down cavity, a second photoelectric detector, a pulse delayer and an acousto-optic modulator, the ring-down cavity is communicated with the long-optical-path gas absorption pool through a connecting pipe (24), and gas to be measured is filled in the ring-down cavity; a laser beam emitted by the tunable semiconductor laser is divided into two beams after passing through an optical isolator and a beam splitter, and one beam passes through a first optical collimator, a long-optical-path gas absorption cell, a second focusing lens and a first photoelectric detector and is collected by a collection card;
the other beam passes through the acousto-optic modulator, the second optical collimator, the ring-down cavity, the second focusing lens and the second photoelectric detector and then is divided into two paths which respectively enter the acquisition card and the pulse delayer; two paths of signals collected by the acquisition card enter a computer for analysis and processing and are fed back to the laser controller.
The invention provides a calibration-free gas concentration measuring method, which is characterized by comprising the following steps:
1) selecting corresponding absorption spectrum lines from a spectrum database according to the type of the gas to be detected; having a central wavelength v0The linear intensity is S, and the linear intensity is obtained by inquiring the American spectral database; the pressure of the gas to be detected is P, and the temperature is T;
2) measurement of gas absorption rate and gas concentration: as shown in FIG. 1, a tunable semiconductor laser is used as a light source, and the current and temperature of the laser are set so that the wavelength of the laser is fixed at a center wavelength v0At least one of (1) and (b); pass meterThe computer turns on the external modulation of the laser controller so that it can load the triangular wave modulation signal generated by the signal generator. The signal generator generates a triangular wave signal and inputs the triangular wave signal into the laser controller so as to modulate the current of the laser controller and further modulate the wavelength of the laser output by the tunable semiconductor laser; laser beams output by the tunable semiconductor laser sequentially pass through the optical isolator and the beam splitter, one laser beam enters the long-optical-path gas absorption cell after passing through the first optical collimator, emergent light is converged to the first photoelectric detector through the first condensing lens, and a signal I is obtainedt(t) storing and processing the signals by adopting a computer; obtaining the relation between the relative wavelength v of the laser and the scanning time t by using an interferometer, and calculating the ratio of the relative wavelength v of the laser to the scanning time tt(t) conversion to It(v) In that respect The transmitted light intensity through the gas cell is, according to Beer-Lambert's law:
in the formula I0(v) The transmitted light intensity without absorption is described by a polynomial; n is a polynomial order, typically a 1 or 2 degree polynomial; a isiIs the coefficient of the ith polynomial, L is the optical path of the gas absorption cell,
representing the linear function of the spectral line to be measured by a Rautian function; using formula (1) for measured It(v) Fitting to obtain the gas absorptivity alpha (v) and the central wavelength v0Absorption rate of
3) Measuring ring-down time τ of gas absorption: the computer turns off the external modulation of the laser controller. The laser beam output by the tunable semiconductor laser passes through an optical isolator, a beam splitter, an acousto-optic modulator and a second optical collimator in sequence, and all the devices are connected by adopting optical fibers. The light beam enters the ring-down cavity after being collimated, and then is focused on the second photoelectric detector through the first focusing lens. Received by the detectorThe signal is divided into two paths, one path is collected by a collection card and input into a computer, and the other path generates a pulse signal through a pulse delayer and inputs the pulse signal into an acousto-optic modulator; the piezoelectric ceramic is tightly attached to the second cavity mirror in the ring-down cavity, the cavity length of the ring-down cavity is scanned through the piezoelectric ceramic, and the scanning length is slightly larger than the laser wavelength, so that the mode matching of the incident laser wavelength and the eigenmode of the ring-down cavity is realized; at the moment, the incident laser can be effectively coupled into the ring-down cavity, and the light intensity received by the second photoelectric detector is the maximum; when the signal of the photoelectric detector reaches a threshold level (such as 3.5V), the pulse delayer generates a pulse signal, the rising edge of the pulse signal is in the nanosecond level, and the acousto-optic modulator can be instantly turned off, so that the laser entering the ring-down cavity is turned off; because the light source of the ring-down cavity is cut off, the light intensity signal reaching the second photoelectric detector is attenuated in a single exponential manner, the attenuation signal is collected by a collection card, and the attenuation signal is fitted by a computer to calculate the wavelength v0The following corresponding ring down times τ, τ are related to the gas concentration X as follows:
in the formula, τ0Cavity ring-down time, c is the speed of light; obtained according to step 2)
And equation (2) yields:
the measured ring-down times τ and α (v)0) Substituting the formula (3) to obtain tau0(ii) a When measuring the gas concentration, P, S, T is known,
Using calculated τ0And tau containing gas absorption information obtained by measurement is substituted into the formula (2) to obtain the gas concentration;
4) the switch of the external modulation of the laser controller is controlled by the computer, so that the time-sharing measurement of the ring-down spectroscopy technology and the direct absorption spectroscopy technology is realized, the measurement precision of the two technologies on the gas concentration is compared, and the result with higher measurement precision is intelligently selected; when the gas concentration is higher, the gas concentration is measured by adopting a direct absorption spectrum technology, and when the gas concentration is lower, the gas concentration is measured by adopting a ring-down spectrum technology.
Compared with the prior art, the method has the following advantages and prominent technical effects: the device combines a direct absorption spectrum and ring-down spectrum measuring device, and can realize time-sharing measurement of the same light source; the cavity ring-down time is directly calculated by the absorption rate measured by the direct absorption spectrum and the ring-down time measured by the ring-down spectrum, so that the calibration-free cavity ring-down time is realized, the measurement steps are simplified, the maintenance amount is reduced, and the measurement speed is increased; the device can intelligently select the measurement result of the spectrum technology with higher precision when different concentrations exist, for example, the measurement result of the direct absorption spectrum is adopted when the concentration is high, and the measurement result of the ring-down spectrum is adopted when the concentration is low, so that the gas concentration measurement with the measuring range exceeding 5 orders of magnitude and high precision is realized.
Drawings
Fig. 1 is a schematic diagram of the system architecture of the present invention.
Figure 2 is the absorbance of CO measured with DAS at a concentration of 1.09% and its best fit results in the present invention.
Figure 3 is the absorbance of a DAS measurement of CO at a concentration of 101ppm and its best fit in the present invention.
Fig. 4 is a CRDS measurement principle in the present invention.
FIG. 5 is a graph showing the measurement results of ring-down time of CRDS for different concentrations of CO in the present invention
FIG. 6 is a graph showing the results of long-term measurements of different concentrations of CO by CRDS and DAS in the present invention
FIG. 7 is a histogram of typical CRDS and DAS measurements of the present invention for three CO concentrations of 101ppm, 3650ppm, 1.09%
FIG. 8 is a plot of the absorption rate measured by DAS and ring-down time of trapped gas absorption measured by CRDS, and the cavity ring-down time obtained from a linear fit of the absorption rate and ring-down time of trapped gas absorption in accordance with the present invention
FIG. 9 is the Allan standard deviation of the two methods of the present invention
In the figure: 1-a computer; 2-a laser controller; 3-a tunable semiconductor laser; 4-an optical isolator; 5-a beam splitter; 6-acousto-optic modulator; 7-a second light collimator; 8-ring down cavity; 9-a first cavity mirror; 10-a second cavity mirror; 11-piezoelectric ceramics; 12-a second focusing lens; 13-a second photodetector; 14-acquisition card; 15-a pulse delay; 16-a first light collimator; 17-a long optical path gas absorption cell; 18-a first mirror with an aperture; 19-a second mirror; 20-a first focusing lens; 21-a first photodetector; 22-a signal generator; 23-an interferometer; 24-connecting the pipes.
Detailed Description
The invention will be further described with reference to the accompanying drawings.
Fig. 1 is a schematic structural principle diagram of a calibration-free gas concentration measuring device provided by the present invention, the measuring device includes a tunable semiconductor laser 3, a laser controller 2, a signal generator 22, a computer 1, an interferometer 23, a first photodetector 21, a long-range gas absorption pool 17, and a ring-down spectrum measuring device, the ring-down spectrum measuring device includes a ring-down cavity 8, a second photodetector 13, a pulse delay 15, and an acousto-optic modulator 6, the ring-down cavity 8 is connected with the long-range gas absorption pool 17 through a pipeline; a laser beam emitted by the tunable semiconductor laser 3 is divided into two beams after passing through an optical isolator 4 and a beam splitter 5, and one beam is collected by a collection card 14 after passing through a first optical collimator 16, a long-optical-path gas absorption cell 17, a first focusing lens 20 and a first photoelectric detector 21; the other beam passes through an acousto-optic modulator 6, a second optical collimator 7, a ring-down cavity 8, a second focusing lens 12 and a second photoelectric detector 13 and then is divided into two paths which respectively enter an acquisition card 14 and a pulse delayer 15; two paths of signals collected by the acquisition card 14 enter the computer 1 for analysis and processing and are fed back to the laser controller 2.
The invention provides a calibration-free gas concentration measuring method, which specifically comprises the following steps:
1) according to the waitingMeasuring the gas type, and selecting corresponding absorption spectrum lines from a spectrum database; having a central wavelength v0The linear intensity is S, and the linear intensity is obtained by inquiring a U.S. HITRAN spectral database; the pressure of the gas to be detected is P, and the temperature is T;
2) measurement of gas absorption rate and gas concentration: as shown in FIG. 1, a tunable semiconductor laser 3 is used as a light source, and the laser controller 2 sets the current and temperature of the tunable semiconductor laser so that the wavelength of the laser light is fixed at a center wavelength v0To (3). The external modulation of the laser controller 2 is switched on by the computer 1 so that it can be loaded with the triangular wave modulation signal generated by the signal generator 22. The signal generator 22 generates a triangular wave signal which is input to the laser controller 2 to modulate the current of the laser controller and thus the laser wavelength output by the tunable semiconductor laser 3. The laser beam output by the tunable semiconductor laser 3 sequentially passes through an optical isolator 4 and a beam splitter 5, wherein the laser beam enters a long-optical-path gas absorption cell 17 after passing through a first optical collimator 16, the emergent light is converged to a first photoelectric detector 21 through a first focusing lens 20, and a signal I is obtainedt(t) and storing and processing the signals by using the computer 1. Obtaining the relation between the relative wavelength v of the laser and the scanning time t by using the interferometer 23, and calculating It(t) conversion to It(v) In that respect The transmitted light intensity through the gas cell is, according to Beer-Lambert's law:
in the formula, base line I0(v) Described using polynomials, n being the order of the polynomial, typically a polynomial of degree 1 or 2, aiIs the coefficient of the ith polynomial, L is the optical path of the gas absorption cell,
the linear function of the spectral line to be measured is expressed by a Rautian function. Using formula (1) for measured It(v) Fitting to obtain the gas absorptivity alpha (v) and the central wavelength v0Absorption rate of
3) Measuring ring-down time τ of gas absorption: the computer 1 closes the external modulation of the laser controller 2, the laser beam output by the tunable semiconductor laser 3 passes through the optical isolator 4, the beam splitter 5, the acousto-optic modulator 6 and the second optical collimator 7 in sequence, and all the devices are connected by adopting optical fibers. The light beam is collimated and passes through the ring-down cavity 8, and then is focused on the second photodetector 13 through the second focusing lens 12. The received signal is divided into two paths, one path is collected by a collection card 14 and input into the computer 1, and the other path generates a pulse signal through a pulse delay device 15 and inputs the pulse signal into the acousto-optic modulator 6. The piezoelectric ceramic 11 is tightly attached to the second cavity mirror 10 in the ring-down cavity, the cavity length of the ring-down cavity 8 is scanned through the piezoelectric ceramic, the scanning length is slightly longer than the laser wavelength, so that the mode matching of the incident laser wavelength and the eigenmode of the cavity is realized, at the moment, the incident laser can be effectively coupled into the cavity, and the light intensity received by the second photoelectric detector 13 is the maximum; when the signal reaches a threshold level (such as 3.5V), the pulse delayer 15 generates a pulse signal, the rising edge of the pulse signal is in the nanosecond level, and the acousto-optic modulator 6 can be turned off instantly, so that the laser entering the ring-down cavity 8 is turned off; since the light source is cut off, the light intensity signal reaching the second photodetector 13 is attenuated in a single exponential manner, the attenuated signal is collected by the collection card 14, and the wavelength v can be calculated by fitting the attenuated signal by the computer 10The lower corresponding ring down time τ. τ is related to the gas concentration X as follows:
in the formula, τ0Cavity ring-down time, c is the speed of light. Obtained according to step 2)
And equation (2) yields:
ring down times τ and α (v)0) Substituting into formula (3) to obtain tau0. When measuring the gas concentration, P, S, T is known,
Using calculated τ0And tau containing the gas absorption information obtained by measurement is substituted into the formula (2) to obtain the gas concentration.
4) The computer (1) is used for controlling the switch of external modulation of the laser controller (2), so that time-sharing measurement of the ring-down spectroscopy technology and the direct absorption spectroscopy technology is realized, the measurement precision of the two technologies on gas concentration is compared, and a result with higher measurement precision is intelligently selected. When the gas concentration is higher, the gas concentration is measured by adopting a direct absorption spectrum technology, and when the gas concentration is lower, the gas concentration is measured by adopting a ring-down spectrum technology.
Example (b):
1) with CO and N2Taking the mixed gas as an example, the absorption spectrum line is selected from a HITRAN spectrum database, and the center frequency v of the absorption spectrum line is0Is 6374.406cm-1Line intensity S of 2.13X 10-23cm-1/(molec·cm-2) (ii) a The temperature T of the mixed gas is about 25 ℃, the pressure P is about 100.6kPa, and the effective optical length L of the long-optical-path gas absorption cell is 120 m. Configured CO and N2The range of the concentration X of the mixed gas is 4ppm to 1.09 percent;
2) measuring gas absorption rate and gas concentration by using a direct absorption spectroscopy technology: as shown in FIG. 1, a tunable semiconductor laser 3 is used as a light source, and the laser controller 2 sets the current and temperature of the tunable semiconductor laser so that the wavelength of the laser light is fixed at a center wavelength v0To (3). The external modulation of the laser controller 2 is switched on by the computer 1 so that it can be loaded with the triangular wave modulation signal generated by the signal generator 22. The signal generator 22 generates a triangular wave signal with a frequency of 1kHz, and the triangular wave signal is input into the laser controller 2 to modulate the current of the laser controller, so as to modulate the output of the tunable semiconductor laser 3Laser wavelength of (1). Laser beams output by the tunable semiconductor laser 3 sequentially pass through the optical isolator 4, the beam splitter 5 and the first optical collimator 16 and then enter the long-optical-path gas absorption cell 17, emergent light is converged to the first photoelectric detector 21 through the first focusing lens 20, and a signal I is obtainedt(t) and storing and processing the signals by using the computer 1. Obtaining the relation between the relative wavelength v of the laser and the scanning time t by using the interferometer 23, and calculating It(t) conversion to It(v) In that respect The transmitted light intensity through the gas cell is, according to Beer-Lambert's law:
in the formula, base line I0(v) Described by an nth degree polynomial, which is a polynomial order, where n is 2, aiIs the coefficient of the ith-order polynomial,
the linear function of the spectral line to be measured is expressed by Rautian, and is defined as:
in the formula (I), the compound is shown in the specification,
from the measured spectral line Gauss line width gammaDLorentz line width gammaLAnd Dicke convergence coefficient beta, k is an integral form variable; as shown in FIGS. 2 and 3, formula (1) is used to measure It(v) Fitting to obtain gas absorption rate alpha (v) and gas concentration X and central wavelength v0Absorption rate of
At a concentration of 1.09%, the peak absorptionThe rate can reach 0.352, the signal-to-noise ratio can reach 1760, and the transmitted light intensity at the peak can still reach 0.6V, which means that the concentration range exceeding 1.09 percent can still be measured with high precision; at 101ppm, the absorption was only 3.27X 10-3The signal-to-noise ratio can still reach 42.
3) Measuring the gas concentration by adopting a ring-down spectroscopy: the computer 1 closes the external modulation of the laser controller 2, and the laser beam output by the tunable semiconductor laser 3 sequentially passes through the optical isolator 4, the beam splitter 5, the acousto-optic modulator 6, the second optical collimator 7 and the ring-down cavity 8, and then is focused on the second photoelectric detector 13 through the second focusing lens 12. The detector receives signals which are divided into two paths, one path is collected by a collecting card 14 and input into the computer 1, and the other path generates pulse signals through a pulse delayer 15 and inputs the pulse signals into the acousto-optic modulator 6. The piezoelectric ceramic 11 is tightly attached to the second cavity mirror 10 in the ring-down cavity, as shown in fig. 4, the cavity length of the ring-down cavity 8 is scanned by the piezoelectric ceramic, the scanning length is slightly longer than the laser wavelength, so that the matching of the incident laser wavelength and the eigenmode of the cavity is realized, at the moment, the incident laser can be effectively coupled into the cavity, and the light intensity received by the first photoelectric detector 13 is the maximum; when the signal reaches a threshold level (3.5V), the pulse delayer 15 generates a pulse signal, the rising edge of the pulse signal is in the nanosecond level, the acousto-optic modulator 6 is instantly turned off, and the laser entering the ring-down cavity 8 is cut off; as shown in FIG. 5, the light intensity signal reaching the second photodetector 13 is attenuated by a single exponential, the attenuated signal is collected by the collection card 14, and the wavelength v can be calculated by fitting the attenuated signal by the computer 10The following corresponding ring down time τ:
in the formula, τ0Cavity ring-down time, c is the speed of light. Obtained according to step 2)
And equation (2) yields:
ring down times τ and α (v)0) Substituting into formula (3) to obtain tau064.74 μ s, the standard deviation from the cavity ring-down time measured several times in the experiment was 0.007 μ s. The reflectivity of the first and second cavity mirrors of the ring down cavity is 0.999975, the cavity length is 0.5 m, and the theoretical cavity ring down time is 66.7 mus. The calculated and measured cavity ring-down time is different from the theory, because certain impurities exist in the cavity, and the loss of the cavity is increased due to water absorption, lens pollution and the like, so that the cavity ring-down time is smaller than the theoretical value. By using the calculated cavity ring-down time (64.74 μ s) and the measured ring-down time for the absorption of the trapped gas, the relative error between the obtained gas concentration and the gas concentration obtained in step 2) is not more than 0.05%.
4) By comparing the measurement accuracy of the gas concentration in the ring-down spectroscopy and the direct absorption spectroscopy, the computer 1 can intelligently select a result with higher measurement accuracy. As shown in fig. 6 and 7, in the case of long-time measurement, when the gas concentration is high (3650ppm to 1.09%), the accuracy of the direct absorption spectroscopy measurement is higher to 20 ppm; when the gas concentration is in the range crossing region (101 ppm-3650 ppm) of two spectral techniques, the precision of the two methods is the same to 10ppm, the measurement result of any method can be adopted, and the cavity ring-down time tau can be calculated in the region through the step 3)0As shown in fig. 8; when the gas concentration is low (4 ppm-101 ppm), the measurement precision of the ring-down spectroscopy is higher to 1ppm, and tau calculated in the range intersection region of the direct absorption spectroscopy and the ring-down spectroscopy is utilized0And the measured tau containing the gas absorption information is substituted into the formula (3) to obtain the gas concentration X. The measuring speed of the device can reach 0.1ms, the range can reach 4 ppm-1.09%, and the measuring precision is 1-20 ppm. As shown in fig. 9, the lower detection limit may be 35 ppb when the integration time is 25 s.
Claims (2)
1. A calibration-free gas concentration measuring device comprises a tunable semiconductor laser (3), a laser controller (2), a signal generator (22), a computer (1), an interferometer (23), a first photoelectric detector (21) and a long-optical-path gas absorption cell (17); the method is characterized in that: the gas concentration measuring device also comprises a ring-down spectrum measuring device, wherein the device comprises a ring-down cavity (8), a second photoelectric detector (13), a pulse delayer (15) and an acousto-optic modulator (6); the ring-down cavity (8) is connected with the long-optical-path gas absorption pool (17) through a pipeline; a laser beam emitted by the tunable semiconductor laser (3) is divided into two beams after passing through an optical isolator (4) and a beam splitter (5), and one beam is collected by a collection card (14) after passing through a first optical collimator (16), a long-optical-path gas absorption cell (17), a first focusing lens (20) and a first photoelectric detector (21);
the other beam passes through an acousto-optic modulator (6), a second optical collimator (7), a ring-down cavity (8), a second focusing lens (12) and a second photoelectric detector (13) and then is divided into two paths which respectively enter an acquisition card (14) and a pulse delayer (15); two paths of signals acquired by the acquisition card (14) enter the computer (1) for analysis and processing and are fed back to the laser controller (2).
2. A calibration-free gas concentration measurement method using the apparatus of claim 1, characterized in that the method comprises the steps of:
1) selecting corresponding absorption spectrum lines from the spectrum database according to the type of the gas to be detected, wherein the center frequency of the absorption spectrum lines is v0The spectral line intensity is S; the long-optical-path gas absorption pool is communicated with the ring-down cavity through a connecting pipe (24), and gas to be detected is filled in the long-optical-path gas absorption pool, the temperature of the gas to be detected is T, and the pressure of the gas to be detected is P;
2) the temperature and current of the laser are controlled by a computer to ensure that the laser wavelength is at the center frequency v of the spectral line0Scanning nearby in the form of triangular waves; the instantaneous incident light intensity signal before being incident on the long-optical-path gas absorption cell is I0(ii) a After passing through the long optical path absorption cell, the instantaneous transmitted light intensity signal is It(ii) a According to the beer-Lambert law, ItCan be expressed as:
in the formula: l isIs the effective optical path of the gas absorption cell, X is the concentration of the gas to be measured,
calibrating the relative laser wavelength v for the linear function of the gas spectral line to be measured by an interferometer, and measuring I by adopting a formula (1)tFitting to obtain an absorption rate function alpha (v);
3) the temperature and current of the laser are controlled by a computer, so that the laser wavelength is fixed at the center frequency v of the spectral line0Scanning the cavity length of the ring-down cavity by utilizing piezoelectric ceramics; when the received light intensity of the second photoelectric detector is maximum, the pulse delayer sends out a pulse signal, the signal enables the acousto-optic modulator to instantly turn off laser, so that a single-exponential attenuated light intensity signal is obtained, and the ring-down time tau is obtained by fitting the signal:
in the formula: c is the speed of light and c is the speed of light,
is a central frequency v0Is determined by the linear function, tau, of the gas spectral line to be measured0Cavity ring down time; according to step 2) to obtain
The center frequency v0Therein is provided with
Substituting it into equation (2) yields:
the measured ring-down time τ and center frequency v0Alpha (v) of (a)0) Substituting the formula (3) to obtain the cavity ring-down time tau0Then the calculated tau is calculated0And substituting the formula (2) to realize the measurement of the gas concentration X.
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| CN112525841B (en) * | 2020-12-01 | 2021-11-19 | 北京航空航天大学 | Vibration and gas temperature concentration measuring method based on ring-down cavity |
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