CN103323115A - Method for measuring gas absorption spectrum line width and line-type coefficient based on wavelength modulation - Google Patents

Abstract

The invention discloses a method for measuring the gas absorption spectrum line width and the line-type coefficient based on wavelength modulation. According to the method for measuring the gas absorption spectrum line width and the line-type coefficient based on the wavelength modulation, the specific value between a two-time harmonic and a four-time harmonic in the wavelength modulation spectrum technology are adopted to measure the line width and the line-type coefficient of the spectral line absorbed by the gas. During measurement, the line width and the line-type of the gas absorption spectrum line to be measured are calculated by means of analysis of the specific value between the two-time harmonic and the four-time harmonic. The method for measuring the absorption gas spectrum line width and the line-type coefficient based on the wavelength modulation has the advantages of being easy to operate, high in measuring precision, and capable of eliminating influence of factors such as background signals, laser intensity fluctuation and the photoelectricity amplification coefficient, solving the problem that an existing WMS technology can not accurately measure the line-type parameters absorbed by the gas and broadening the application range of the WMS technology and the like. Due to the fact that the specific value between the two-time harmonic and the four-time harmonic are adopted to calculate the gas absorption line-type parameters, the method for measuring the gas absorption spectrum line width and the line-type coefficient based on the wavelength modulation can be called the R24 method.

Description

Based on the gas absorption spectrum line live width of wavelength-modulated and the measuring method of line style coefficient

Technical field

The present invention relates to based on the gas absorption spectrum line live width of wavelength-modulated and the measuring method of line style coefficient, the method proposes and has realized wavelength-modulated method on-line measurement gas absorption spectrum line live width and line style coefficient first.

Background technology

Tunable laser diodes absorption spectrum (Tunable diode laser absorption spectroscopy, TDLAS) technology is utilized the absorption line of the laser scanning gas of arrowband, by analyzing temperature and the concentration that is obtained gas to be measured by the laser intensity after the gas absorption.Compare with traditional sampling type gas detection technology, the TDLAS technology has advantages of non-contact type on-line measurement, strong, highly sensitive, the online fast response time of selectivity, can measure the average level of certain regional gas concentration, become important development direction and the technology main flow of current gas concentration online measuring technique.

After laser passed tested gas, laser transmittance τ (ν) can describe with the Beer-Lambert law:

In the formula, I _tAbsorption signal light intensity when gas absorption is arranged, I ₀Background signal light intensity when absorbing gas for nothing, K=PS (T) XL, P is the gas stagnation pressure, and L is the laser absorption light path, and X is gas concentration to be measured, and S (T) is the line strength of spectral line,

Be the gas absorption linear function, its expression formula is suc as formula (2), and α (υ) is spectral absorption.

In the formula, δ _VBeing line width, is half of spectral line halfwidth; f _L(ξ) and f _G(ξ) be respectively the weight coefficient of lorentzian curve and Gaussian lineshape in the Fu Yite line style, definition ξ is the line style coefficient, ξ=(δ _L-δ _G)/(δ _L+ δ _G), δ _LAnd δ _GBeing respectively Lorentz live width and Gauss's live width, is Gaussian lineshape when ξ=-1, and ξ=1 o'clock is lorentzian curve; By formula (2) as can be known, the gas absorption linear function is by live width δ _VAnd line style coefficient ξ decision; The expression formula of weight coefficient is suc as formula shown in (3):

$\left\{\begin{array}{c}{f}_L\left(\mathrm{\ξ}\right)=0.68188+0.61293\mathrm{\ξ}-0.18384{\mathrm{\ξ}}^2-0.11568{\mathrm{\ξ}}^3\\ f_G\left(\mathrm{\ξ}\right)=0.32460-0.61825\mathrm{\ξ}+{0.17681\mathrm{\ξ}}^2+0.12109{\mathrm{\ξ}}^3\end{array}---\left(3\right)\right.$

In TDLAS commercial measurement gas parameter, the gas absorption linear function is important parameter, and for example to exempt from the mark method be to obtain on the basis of known linear function to the 2f/1f that proposes based on residual amplitude modulation of R.K.Hanson seminar of U.S. Stanford university.Linear function is mainly used in describing the shape of absorption line, determines that accurately this function is to realize the online prerequisite of accurately measuring.But in fact live width and line style coefficient are decided by the parameters such as temperature, pressure and concentration of component of gas, and these parameters are unknown, that change or to be measured often in actual measurement.Owing to the uncertainty of spectrum constant in the spectroscopic datas such as HITRAN, accurate Calculation gas absorption linear function exists very large difficulty in theory simultaneously.In the actual industrial scene, especially under some rugged surroundings, in the time of greatly maybe can't measuring such as gas temperature, pressure and concentration of component fluctuation, the measuring error that molecule absorption linear function uncertainty is brought can be very large.

The measuring method of TDLAS technology mainly contains direct absorption process and wavelength-modulated method at present.

Directly absorption process (DAS) is that the direct match of the ratio of transmitted light intensity and incident intensity is obtained gas absorption rate function, the absorptivity function that match obtains has not only comprised the information such as temperature, concentration and pressure of gas to be measured, and can determine according to it spectrum constant of characteristic spectral line, therefore directly absorption process has advantages of simple to operate, directly measurement gas temperature, concentration, pressure etc.But directly absorption process is subject to the impact of the factor such as overlap of spectral lines under particle concentration, laser intensity fluctuation and the high pressure, thereby can't Accurate Curve-fitting absorptivity function, and then causes measuring error, and therefore directly absorption process is only applicable to the strong condition that absorbs.

For measuring accuracy and the sensitivity that improves TDLAS, solve the measurement difficult problem under rugged surroundings and the weak acceptance condition, professor J.Reid is incorporated into wavelength-modulated method (WMS) in the TDLAS measuring system.The advantage of wavelength-modulated absorption spectrum is effectively to have reduced the interference of background signal in the measuring system, has improved the measuring accuracy of TDLAS.But the wavelength-modulated method can not accurately be determined the gas absorption line style at present, and the measurement of its gas parameter also need to obtain on the basis of known linear function.

Consider the importance of gas absorption linear function, scientist has carried out multiple trial, for example G.Stewart seminar of Britain Strathclyde university is by analyzing first harmonic residual amplitude modulation (RAM) signal, obtain when index of modulation m less (less than＜0.5), the approximately equalised conclusion of RAM and linear function, the phasing degree by changing modulation signal is to realize recovering the purpose of line style.But when the index of modulation was very little, the RAM signal was very weak, and signal to noise ratio (S/N ratio) is lower, was difficult to obtain desirable result.

In research before, we once utilized the inspiration of the thoughts such as G.Stewart, analyzed the relation between RAM and the index of modulation and the each harmonic, derivation obtains when can eliminating the error that the index of modulation brings too greatly to odd harmonics X-axis and Y-axis signal during addition respectively, thereby simulates the gas absorption rate.But for weak acceptance condition, higher hamonic wave signal attenuation is very fast, and signal to noise ratio (S/N ratio) is low, so that there is larger error in linear fitting, therefore this method relatively is suitable for the strong environment that absorbs.

We can conclude by above research: in the Wavelength modulation spectroscopy technology, not only odd harmonics contains the information of gas absorption linear function, and even harmonics signal also necessarily contains.

Summary of the invention

For the precision that improves TDLAS commercial measurement gas concentration, widen its range of application in industry spot, the present invention utilizes the theoretical and Harmonic Theory of absorption spectrum to derive the General formulas of secondary and four-time harmonic signal ratio, analyze the relation of finding harmonic signal ratio and live width and line style coefficient, and set up on this basis based on the gas absorption spectrum line live width of wavelength-modulated and the measuring method of line style coefficient, with the precision of further raising gas absorption linear function, simplified measurement process.

Technical scheme of the present invention is as follows:

1) controls the concentration X of gas to be measured with flowmeter, with pressure transducer 5 and temperature sensor 6 respectively pressure P and the temperature T of measurement gas media;

2) according to gaseous species to be measured, from the HITRAN spectra database, choose the characteristic spectral line of gas to be measured, determine the centre frequency υ of gas characteristic spectral line ₀, line strength S (T), self widen coefficient gamma _SelfWith air broadening coefficient γ _Air

3) take semiconductor laser with tunable 3 as light source, the output frequency of controlling semiconductor laser with tunable 3 with laser controller 2 is stabilized in centre frequency υ ₀The place;

4) the high frequency sinusoidal signal that lock-in amplifier 1 is produced is input to laser controller 2, with the output frequency of modulation semiconductor laser with tunable 3;

5) laser of semiconductor laser with tunable 3 outputs is injected in the gas medium 10 behind collimation, transmitted light intensity receives and is converted to electric signal by photodetector 9, be input to again and carry out secondary and four-time harmonic detection in the lock-in amplifier (1), obtain at characteristic spectral line centre frequency υ ₀Place's secondary and four-time harmonic ratio R ₂₄:

In the formula, S _2fAnd S _4fRespectively secondary and four-time harmonic signal; A is modulation amplitude; θ ∈ [π, π];

(υ) be the gas absorption linear function, represent with the Fu Yite linear function, by live width δ _VAnd line style coefficient ξ determines live width δ _VHalf of characteristic spectral line halfwidth, line style coefficient ξ ∈ [1,1];

6) from formula (1), find out R ₂₄Only with modulation amplitude a and gas absorption line shape function Relevant; Definition index of modulation m=a/ δ _V, formula (1) is carried out simulation calculation obtain one group of permanent curve family of crossing fixed point: how pipeline type coefficient ξ does not change, along with the increase of index of modulation m, R ₂₄The value dullness reduces and the permanent fixed point O of mistake, and the coordinate that O is ordered is:

$\left\{\begin{array}{c}{m}^{*}=2.4926\\ {R_{24}}^{*}=2.1865\end{array}---\left(2\right)\right.$

7) according to the fixed point of formula (2), in experiment, regulate modulation amplitude a so that R ₂₄=R ₂₄ ^*, then the index of modulation is m ^*, namely obtain the live width δ of characteristic spectral line according to the definition of the index of modulation _V=a/m ^*=a/2.4926;

8) for same once experiment, live width is constant, therefore when reducing modulation amplitude a, obtains the R than ditty Coefficient m processed the time ₂₄, according to R ₂₄Determine line style coefficient ξ with the relation of m.

Because this method is utilized secondary and four-time harmonic ratio calculation line width and line style coefficient, therefore be also referred to as the R24 method.

The present invention is take the molecular absorption spectrum theory as the basis, derive secondary, four-time harmonic expression formula by Harmonic Theory and lock-in amplifier principle of work, then determine the live width of spectral line by the ratio of secondary and four-time harmonic, then determine the line style coefficient of spectral line by changing modulation amplitude, and then the temperature that the live width that obtains and line style coefficient can be used for measurement gas and concentration etc.Owing to parameters such as the temperature that needn't measure gas to be measured in measuring and pressure, the live width of gained and line style coefficient have reflected the relation between spectral line line style and the environmental factor in real time, have eliminated the uncertain impact that brings of environmental factor and spectral line line style.Again because secondary and four-time harmonic peak value all at core frequency place, its signal to noise ratio (S/N ratio) is high, convenient reading, the measurement burden that do not increase.The peak signal of secondary, four-time harmonic is compared, eliminated the impact of laser intensity, photoelectricity amplification coefficient, amplitude modulation coefficient and phase differential, therefore, this measuring method method is feasible in theory, has very high operability, and is applied widely.

Description of drawings

Fig. 1 is gas line width of the present invention and line style coefficient measuring system structure principle chart.

Fig. 2 is secondary and four-time harmonic ratio R in the wavelength-modulated method ₂₄Relation curve with absorption line line style coefficient and the index of modulation.

Fig. 3 adopts the R24 method to measure CO ₂The absorbance curves of absorptivity, direct absorption process and match thereof and the comparison diagram of the theoretical absorbance curves of calculating.

Among the figure: 1-lock-in amplifier; 2-laser controller; 3-semiconductor laser with tunable; 4-aspiration pump; 5-pressure transducer; 6-temperature sensor; 7a-flowmeter-1; 7b-flowmeter-2; 8a-CO ₂Gas-holder; 8b-N ₂Gas-holder; 9-photodetector; 10-gas medium; 11-oscillograph; 12-computer operating system.

Embodiment

The present invention is further illustrated below in conjunction with accompanying drawing.

The invention provides based on the gas absorption spectrum line live width of wavelength-modulated and the measuring method of line style coefficient, the method has comprised following steps:

1) controls the concentration X of gas to be measured with flowmeter, with pressure transducer 5 and temperature sensor 6 respectively pressure P and the temperature T of measurement gas media;

2) according to gaseous species to be measured, from high precision migration molecule absorption database (High-Resolution Transmission Molecular Absorption Database, HITRAN) choose the characteristic spectral line of gas to be measured in, determine the centre frequency υ of gas characteristic spectral line ₀, line strength S (T), self widen coefficient gamma _SelfWith air broadening coefficient γ _Air

3) take semiconductor laser with tunable 3 as light source, the output frequency of controlling semiconductor laser with tunable 3 with laser controller 2 is stabilized in centre frequency υ ₀The place;

4) the high frequency sinusoidal signal that lock-in amplifier 1 is produced is input to laser controller 2, and with the output frequency of modulation semiconductor laser with tunable 3, this moment, the laser instantaneous frequency was:

υ=υ ₀+ acos θ (1) wherein, a is modulation amplitude; θ ∈ [π, π]; The definition index of modulation: m=a/ δ _V, line width δ _VBe half of spectral line halfwidth.

After laser passed tested gas medium 10, laser transmittance τ (υ) can describe with the Beer-Lambert law:

Wherein, I _tAbsorption signal light intensity when gas absorption is arranged; I ₀Background signal light intensity when absorbing gas for nothing; K=PS (T) XL, P is the gas stagnation pressure, and S (T) is the line strength of spectral line, and X is gas concentration to be measured, and L is the laser absorption light path;

Be the gas absorption linear function, by line style coefficient ξ and line width δ _VDetermine, wherein the line style Coefficient Definition is ξ=(δ _L-δ _G)/(δ _L+ δ _G), ξ ∈ [1,1], δ _LAnd δ _GBeing respectively Lorentz live width and Gauss's live width, is Gaussian lineshape when ξ=-1, and ξ=1 o'clock is lorentzian curve; α (υ) is spectral absorption; Under weak absorbing state, namely when spectral absorption α (υ)＜10%, can carry out the single order Taylor series approximation to formula (1), shown in (3):

$\mathrm{\&tau;}\left(v\right)=I_t/{\mathrm{<figure-callout\; id="0"\; label="I"\; filenames="HDA00003396400800012.png"\; state="\{\{state\}\}">I</figure-callout>}}_0=\exp [-\mathrm{\&alpha;}(v_0+a\cos \mathrm{\&omega;t})]\&ap;1-\mathrm{\&alpha;}(v_0+a\cos \mathrm{\&omega;t})=1-K\underset{k=0}{\overset{\&infin;}{\mathrm{\&Sigma;}}}{H}_k(v_0,a)\cos \left(\mathrm{k\&omega;t}\right)---\left(3\right)$

In the formula, H _k(υ ₀, a) expression each harmonic amplitude, expression formula is as follows:

Wherein, δ _K0Be sign function, θ is integration variable, θ ∈ [π, π];

5) transmitted light intensity is input to carries out secondary and four-time harmonic in the lock-in amplifier and detect, obtain at core frequency υ ₀The expression formula of place's secondary and four-time harmonic is:

$\left\{\begin{array}{c}{S}_{2f}=0.5{\mathrm{KI}}_0{H}_2\\ S_{4f}=0.5{\mathrm{KI}}_0{H}_4\end{array}\right.---\left(3\right)$

In the formula, S _2fAnd S _4fBe respectively secondary and four-time harmonic signal, therefore obtain at characteristic spectral line centre frequency υ ₀Place's secondary and four-time harmonic ratio R ₂₄:

6) can find out R from formula (6) ₂₄Only with modulation amplitude a and gas absorption line shape function

Relevant.Formula (6) is carried out simulation calculation can obtain one group of permanent curve family of crossing fixed point: how pipeline type coefficient ξ does not change, along with the increase of the index of modulation, R ₂₄The value dullness reduces and the permanent fixed point O of mistake, and the coordinate that O is ordered is:

$\left\{\begin{array}{c}{m}^{*}=2.4926\\ {R_{24}}^{*}=2.1865\end{array}\right.---\left(7\right)$

7) according to the fixed point of formula (7), in experiment, regulate modulation amplitude a so that R ₂₄=R ₂₄ ^*, then the index of modulation is m ^*, obtain line width δ according to the definition of the index of modulation _V=a/m ^*=a/2.4926;

8) for same once experiment, live width is constant, therefore when reducing modulation amplitude a, obtains the R than ditty Coefficient m processed the time ₂₄, according to R ₂₄Determine line style coefficient ξ with the relation of m.

Because this method is utilized secondary and four-time harmonic ratio calculation line width and line style coefficient, therefore be also referred to as the R24 method.

Annotate: Fu Yite gas absorption linear function Definite method referring to document " Y.Liu, J.Lin, G.Huang, Y.Guo and C.Duan.Simple empirical analytical approximation to the Voigt profile.J.Opt.Soc.Am.B, 2001,18:666-672. ".

Embodiment:

1) embodiment adopts CO ₂Be example, measure CO ₂The live width of absorption line and line style coefficient.Control CO with flowmeter ₂The concentration X=13.71% of gas, with pressure P=41mbar and the temperature T=297.7K of pressure transducer 5 and temperature sensor 6 measurement gas media 9, setting laser absorbs light path L=120cm, and this moment, the absorptivity of gas was 4.65%;

2) from spectra database, choose CO ₂The centre frequency υ of absorption spectrum ₀, line strength S (T), self widen coefficient gamma _SelfWith air broadening coefficient γ _AirSuch as following table 1:

Table 1CO ₂Molecule 6982.0678cm ^-1The spectrum constant (298K) of spectral line

3) take semiconductor laser with tunable 3 as light source, the output frequency of controlling semiconductor laser with tunable 3 with laser controller 2 is stabilized in centre frequency υ ₀=6982.0678cm ^-1The place;

4) the 2.0kHz high frequency sinusoidal signal that lock-in amplifier 1 is produced is input to laser controller, with the output frequency of modulation semiconductor laser with tunable;

5) laser of laser instrument output is injected in the gas medium 10 behind collimation, transmitted light intensity receives and is converted to electric signal by photodetector 9, be input to again and carry out secondary and four-time harmonic detection in the lock-in amplifier 1, obtain the ratio at core frequency place's secondary and four-time harmonic;

6) the modulation amplitude a of adjusting lock-in amplifier makes the ratio of secondary and four-time harmonic just in time be R ₂₄ ^*=2.1865, perhaps at R ₂₄ ^*Near=2.1865 small one and large one got two values.In the experiment, work as a=2.006E-2cm ^-1The time, R ₂₄=2.3266, work as a=2.275E-2cm ^-1The time, R ₂₄=2.000, adopt interpolation algorithm to calculate at fixed point O place, work as R ₂₄ ^*=2.1865 o'clock, modulation amplitude a=2.114E-2cm ^-1, so live width δ _V=a/m ^*=8.481 * 10 ^-3Cm ^-1

7) reducing modulation amplitude is a=1.050E-2cm ^-1When (index of modulation is m=1.2381), secondary and four-time harmonic ratio are R ₂₄=6.1504, calculate the line style coefficient ξ of gas absorption spectrum line ₁=-0.3558; Reducing modulation amplitude is a=8.750E-3cm ^-1When (index of modulation is m=1.0317), secondary and four-time harmonic ratio are R ₂₄=8.2246, calculate the line style coefficient ξ of gas absorption spectrum line ₂=-0.3553; Twice experiment averaged and is ξ=-0.3556;

8) in the live width that experiment is recorded and the line style coefficient substitution Fu Yite linear function, can try to achieve the absorption line shape function of gas, and calculate absorptivity.Absorbance curves, the experiment value of direct absorption process and absorbance curves and the theoretical absorbance curves of calculating that match obtains thereof that the R24 method is tried to achieve compare, acquired results is seen accompanying drawing 3, and what show in the table 2 is R24 method, directly absorption process and theoretical comparison of calculating line width and line style coefficient.

Table 2R24 method, direct absorption process and theoretical live width and the line style coefficients comparison that calculates absorption line

Can find out from result of calculation, although direct absorption process also can comparatively accurately simulate the live width of absorption line, there is very large error in the line style coefficient that match obtains; Compare with direct absorption process, R24 method on-line measurement absorption line live width and line style coefficient have very high precision, and the linewidth error that calculates is in 1%, and the line style system errors is less than 6%.

Claims (1)

1. based on the gas absorption spectrum line live width of wavelength-modulated and the measuring method of line style coefficient, it is characterized in that the method comprises the steps:

1) controls the concentration X of gas to be measured with flowmeter, distinguish pressure P and the temperature T of measurement gas medium with pressure transducer (5) and temperature sensor (6);

2) according to gaseous species to be measured, from the HITRAN spectra database, choose the characteristic spectral line of gas to be measured, determine the centre frequency υ of gas characteristic spectral line ₀, line strength S (T), self widen coefficient gamma _SelfWith air broadening coefficient γ _Air

3) take semiconductor laser with tunable (3) as light source, the output frequency of controlling semiconductor laser with tunable (3) with laser controller (2) is stabilized in centre frequency υ ₀The place;

4) the high frequency sinusoidal signal that lock-in amplifier (1) is produced is input to laser controller (2), with the output frequency of modulation semiconductor laser with tunable (3);

5) laser of semiconductor laser with tunable (3) output is injected in the gas medium (10) behind collimation, transmitted light intensity receives and is converted to electric signal by photodetector (9), be input to again and carry out secondary and four-time harmonic detection in the lock-in amplifier (1), obtain at characteristic spectral line centre frequency υ ₀Place's secondary and four-time harmonic ratio R ₂₄:

In the formula, S _2fAnd S _4fRespectively secondary and four-time harmonic signal; A is modulation amplitude; θ ∈ [π, π];

(υ) be the gas absorption linear function, represent with the Fu Yite linear function, by live width δ _VAnd line style coefficient ξ determines live width δ _VHalf of characteristic spectral line halfwidth, line style coefficient ξ ∈ [1,1];

6) from formula (1), find out R ₂₄Only with modulation amplitude a and gas absorption line shape function

Relevant; Definition index of modulation m=a/ δ _V, formula (1) is carried out simulation calculation obtain one group of permanent curve family of crossing fixed point: how pipeline type coefficient ξ does not change, along with the increase of index of modulation m, R ₂₄The value dullness reduces and the permanent fixed point O of mistake, and the coordinate that O is ordered is:

$\left\{\begin{array}{c}{m}^{*}=2.4926\\ {R_{24}}^{*}=2.1865\end{array}\right.---\left(2\right)$

7) according to the fixed point of formula (2), in experiment, regulate modulation amplitude a, so that R ₂₄=R ₂₄ ^*, then the index of modulation is m ^*, namely obtain the live width δ of characteristic spectral line according to the definition of the index of modulation _V=a/m ^*=a/2.4926;

8) for same once experiment, live width is constant, therefore when reducing modulation amplitude a, obtains the R than ditty Coefficient m processed the time ₂₄, according to R ₂₄Determine line style coefficient ξ with the relation of m.

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