阅读说明：本技术 一种波长调制气体浓度反演方法 (Wavelength modulation gas concentration inversion method ) 是由 杨晨光 阚瑞峰 许振宇 姚路 于 2018-06-29 设计创作，主要内容包括：本发明公开了一种波长调制气体浓度反演方法,包括下述步骤：(1)将相互正交的参考信号与探测信号相乘,并进行低通滤波后获得吸收光谱的相互正交的一组1f解调信号X<Sub>1f</Sub>、Y<Sub>1f</Sub>；(2)根据1f解调信号X<Sub>1f</Sub>、Y<Sub>1f</Sub>获得1f信号的幅角θ<Sub>1f</Sub>；(3)根据幅角θ<Sub>1f</Sub>获得被测气体浓度。本发明利用正交相位解调的1f信号X<Sub>1f</Sub>、Y<Sub>1f</Sub>,通过测量平面矢量(X<Sub>1f</Sub>,Y<Sub>1f</Sub>)幅角θ<Sub>1f</Sub>反演气体浓度,有效的屏蔽了光强抖动等噪声对气体浓度探测的影响,实现气体浓度的高灵敏度探测。(The invention discloses a wavelength modulation gas concentration inversion method, which comprises the following steps: (1) multiplying the mutually orthogonal reference signals and the detection signals, and obtaining a mutually orthogonal group of 1f demodulation signals X of the absorption spectrum after low-pass filtering 1f 、Y 1f (ii) a (2) Demodulating signal X according to 1f 1f 、Y 1f Obtaining the amplitude theta of the 1f signal 1f (ii) a (3) According to argument theta 1f And obtaining the concentration of the gas to be detected. The invention utilizes the 1f signal X of quadrature phase demodulation 1f 、Y 1f By measuring plane vectors (X) 1f ，Y 1f ) Argument theta 1f The gas concentration is inverted, the influence of noise such as light intensity jitter on the gas concentration detection is effectively shielded, and the high-sensitivity detection of the gas concentration is realized.)

1. A wavelength modulation gas concentration inversion method is characterized by comprising the following steps:

(1) multiplying the mutually orthogonal reference signals and the detection signals, and obtaining a mutually orthogonal group of 1f demodulation signals X of the absorption spectrum after low-pass filtering_1f、Y_1f；

(2) Demodulating signal X according to said 1f_1f、Y_1fObtaining the amplitude theta of the 1f signal_1f；

(3) According to the amplitude angle theta_1fAnd obtaining the concentration of the gas to be detected.

2. The wavelength-modulated gas concentration inversion method according to claim 1, wherein the frequency of the reference signal and the frequency of the detection signal are the same as the frequency of the modulation signal.

3. The wavelength modulation gas concentration inversion method according to claim 1 or 2, wherein the argument θ in step (2)_1f＝arctan(Y_1f/X_1f) And argument theta_1fIs proportional to the integrated absorbance of the gas absorption spectrum.

4. The wavelength modulation gas concentration inversion method according to any one of claims 1 to 3, wherein the measured gas concentration is obtained by standard concentration gas calibration in step (3).

5. The wavelength modulation gas concentration inversion method of claim 4, wherein the standard concentration gas calibration specifically comprises:

(1) sequentially introducing a group of gases with different concentrations, including the upper limit and the lower limit of the concentration of the gas to be measured, of known concentration into the gas absorption cell, and measuring the amplitude angle theta_1fPeak to peak value of;

(2) establishing a corresponding relation model of gas concentration and argument;

(3) measuring the amplitude angle theta of the gas to be measured_1fAnd according to the amplitude theta of the gas to be measured_1fAnd obtaining the concentration of the gas to be measured by the peak-to-peak value and the corresponding relation model.

6. The wavelength modulation gas concentration inversion method according to claim 5, wherein the gas concentration to be measured is obtained by table look-up or interpolation evaluation in a correspondence table or calculation according to a polynomial fitting relational expression.

Technical Field

The invention belongs to the field of tunable laser gas absorption spectrum detection, and particularly relates to a wavelength modulation gas concentration inversion method.

Background

The tunable laser gas absorption spectrum technology utilizes the absorption of gas molecules to laser with a certain specific wavelength, and obtains the absorption spectrum of the gas molecules at the wavelength by a wavelength tuning method to measure the concentration and the information of the gas molecules. The technology is widely applied to a plurality of fields of atmospheric pollutant monitoring, industrial control, safe production and the like.

The wavelength modulation technology is to control the wavelength and the intensity of laser by using a high-frequency modulation signal, further shift the frequency of a gas absorption spectrum signal to a high frequency, and then perform demodulation detection, thereby greatly reducing the influence of low-frequency noise on the absorption spectrum signal and improving the signal-to-noise ratio of a system.

A 1f normalized 2f demodulation method is typically used. Using a reference signal with the frequency twice the modulation frequency and the same phase as the modulation signal, multiplying the reference signal by the detected light intensity signal, and obtaining the X of the absorption spectrum by a low-pass filtering method_2rA signal. In order to mask the demodulation phase error band, two reference signals with the frequency twice the modulation frequency and the orthogonal phase are generally used, multiplied by the detected light intensity signal, and then the X of the absorption spectrum is obtained by a low-pass filtering method_2f、Y_2fA signal. In consideration of the effect of the light intensity variation on the result, the 1f signal is generally used to normalize the 1f signal. Two paths of reference signals with frequency as modulation frequency and orthogonal phase are used to be multiplied by the detected light intensity signal, and the X of the absorption spectrum is obtained by a low-pass filtering method_1f、Y_1fSignals to obtain a vector (X)_1f，Y_1f) Mode R of_1f Finally obtaining a single-channel 1f normalized 2f signal X_2f/1f(X_2f/1f＝X_2f/R_1f) Or quadrature demodulated 1f normalized 2f signal R_2f/1f

Using the obtained X_2f/1fOr R_2f/1fThere are two methods for inverting the concentration of the gas to be measured. First, X is calibrated by using gas with standard concentration_2f/1fOr R_2f/1fPeak or peak-to-peak value of the target gas absorption site, establishing a gas concentration-peak value (Or peak-to-peak) and then inverting the gas concentration by measuring the spectral peak (or peak-to-peak). Secondly, simulating a single-channel 1f normalized 2f signal with a specific concentration or an orthogonally demodulated 1f normalized 2f signal according to the Belronbo gas absorption law by using the characteristic parameters of the corresponding spectral absorption lines of the spectral database and combining the measurement environment conditions (temperature, pressure, and the like) and the measurement conditions (laser intensity, frequency characteristic parameters, and the like), and fitting the simulated single-channel 1f normalized 2f signal or the orthogonally demodulated 1f normalized 2f signal with the minimum residual error with the measurement spectrum, wherein the target gas concentration corresponding to the simulated single-channel 1f normalized 2f signal or the orthogonally demodulated 1f normalized 2f signal with the minimum residual error is the measurement target gas concentration.

In order to obtain a high-precision measurement result, various other signal demodulation spectrum inversion methods are invented on the basis of the traditional demodulation mode. Hangauer, siemens germany, for example, invented simultaneous demodulation of multiple harmonics of a modulated signal to obtain a 1f signal, a 2f signal, a 3f signal, and a 4f signal, which were simultaneously fit to invert the gas concentration. The York at Qinghua university invents methods for measuring the line width and the linear coefficient of a gas absorption spectral line based on 2 th harmonic and 4 th harmonic.

However, in the application process of various methods, gas concentration measurement errors caused by noises such as laser light intensity jitter and the like still remain to be solved.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide a wavelength modulation gas concentration inversion method, and aims to solve the problem of large gas concentration measurement error caused by laser light intensity jitter noise in the prior art.

The invention provides a wavelength modulation gas concentration inversion method, which comprises the following steps:

(1) multiplying the mutually orthogonal reference signals and the detection signals, and obtaining a mutually orthogonal group of 1f demodulation signals X of the absorption spectrum after low-pass filtering_1f、Y_1f(ii) a Wherein, X_1f、Y _1f1f demodulation signals of an X axis and a Y axis in a coordinate system respectively;

(2) demodulating signal X according to said 1f_1f、Y_1fObtaining the amplitude theta of the 1f signal_1f；

(3) According to the amplitude angle theta_1fAnd obtaining the concentration of the gas to be detected.

And the frequency of the reference signal and the frequency of the detection signal are the same as the frequency of the modulation signal.

Further, the argument θ in step (2)_1f＝arctan(Y_1f/X_1f) And argument theta_1fIs proportional to the integrated absorbance of the gas absorption spectrum.

Further, in the step (3), the measured gas concentration is obtained through standard concentration gas calibration.

Further, the standard concentration gas calibration specifically includes:

(1) sequentially introducing a group of gases with different concentrations, including the upper limit and the lower limit of the concentration of the gas to be measured, of known concentration into the gas absorption cell, and measuring the amplitude angle theta_1fPeak to peak value of;

(2) establishing a corresponding relation model of gas concentration and argument; the corresponding relation model can be a corresponding relation table or a polynomial fitting relation expression;

(3) measuring the amplitude angle theta of the gas to be measured_1fAnd according to the amplitude theta of the gas to be measured_1fAnd obtaining the concentration of the gas to be measured by the peak-to-peak value and the corresponding relation model.

Further, the concentration of the gas to be measured is obtained by looking up a table in the correspondence table or interpolating for evaluation or calculation based on a polynomial fitting relational expression.

Through the technical scheme, compared with the prior art, the amplitude of the 1f signal is far higher than that of the 2f signal, so that the wavelength modulation and demodulation method based on the amplitude of the 1f signal can still obtain an accurate demodulation result unrelated to light intensity under the severe environment with large noise interference of an original signal. The invention utilizes the 1f signal X of quadrature phase demodulation_1f、Y_1fBy measuring plane vectors (X)_1f，Y_1f) Argument theta_1fThe gas concentration is inverted, the influence of noise such as light intensity jitter on the gas concentration detection is effectively shielded, and the high-sensitivity detection of the gas concentration is realized.

Drawings

FIG. 1 is a schematic flow chart of concentration inversion according to the present invention;

FIG. 2 is a schematic representation of the raw spectral signal detected by the present invention;

FIG. 3 shows a 1f demodulated signal X of the invention_1fA schematic diagram;

FIG. 4 shows a 1f demodulated signal Y of the present invention_1fA schematic diagram;

FIG. 5 shows the amplitude θ of the 1f amplitude signal of the present invention_1fA schematic diagram;

FIG. 6 is a graph of θ for different integrated absorbances_1fA numerical calculation result;

FIG. 7 is a numerical calculation of θ_1rPeak-to-peak value corresponds to the integrated absorbance.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The invention particularly relates to a method for inverting gas concentration by using the amplitude of an orthogonal demodulation 1f signal, and high-sensitivity gas concentration detection is realized. The invention is especially suitable for gas detection environments with obvious noise such as light intensity jitter, wherein the 1f signal is a general term in the field of wavelength modulation absorption spectrum, the 1f signal (also called first harmonic) is obtained by demodulating the reference signal with the same modulation frequency as the frequency, and the 2f signal (also called second harmonic) is obtained by demodulating the reference signal with the frequency multiplied by the modulation frequency 2.

The invention aims to provide a wavelength modulation gas absorption spectrum gas concentration inversion method based on 1f signal amplitude characteristics, which shields the influence of noise such as light intensity jitter on the amplitude of a detection signal and realizes high-sensitivity gas concentration detection.

According to the invention, two paths of orthogonal reference signals with the same frequency as the modulation signal are multiplied by the detection signal, and then the X of the absorption spectrum is obtained by a low-pass filtering method_1f、Y_1fSignal, ask for Y_1fExcept for X_1fThe resulting arctangent, a vector (X) is obtained_1f，Y_1f) Amplitude theta of_1f(θ_1f＝arctan(Y_1f/X_1f))。θ_1fIs a signal similar to the first derivative of the absorption line form, theta_1rThe peak-to-peak value of the signal is proportional to the concentration of the target gas, so that the theta can be calibrated by a standard concentration gas calibration method_1fAnd obtaining the concentration of the gas to be detected.

The invention is further described with reference to the following figures and examples.

FIG. 1 is a schematic diagram of a concentration inversion process according to the present invention; the wavelength modulation absorption spectrum realizes the simultaneous modulation of light intensity and frequency through current modulation, and the light intensity I₀At a DC light intensity i₀On the basis of the amplitude of the modulation response being i₁With frequency v at reference frequency v₀On the basis of the amplitude of the modulation response being v₁The modulation frequencies are the same as ω, and a phase difference θ exists. Light intensity I₀The frequency v can be written as a function of time t; i is₀＝i₀+i₁cos(ωt+θ)，v＝v₀+v₁ cos(ωt)。

The gas absorbance τ (v) is the product of the gas concentration small pressure P, the optical path L, the line intensity S, and a linear function σ (v), where χ PLS is called the integrated absorbance and is a frequency-independent function, the linear function σ (v) has an integral of 1 over the absorption spectrum frequency range, and the gas absorbance τ (v) can be expressed in the form of a Fourier series,

τ(v₀+v₁ cos(ωt))＝χPLSσ(v₀+v₁ cos(ωt))＝∑H_k(v₀，v₁)cos(kωt)

wherein the content of the first and second substances,

the raw detection signal (as shown in fig. 2) according to belronbo's law can be expressed as:

I＝I₀ exp(-τ)

≈I₀(1-τ)＝i₀+i₁ cos(ωt+θ)-i₀τ(v₀+v₁ cos(ωt))-i₁ cos(ωt+θ)τ(v₀+v₁ cos(ωt))’

demodulating the original probe signal with the reference signal cos ω t having the same frequency and phase as the frequency modulation to obtain X_1fSignal (as shown in fig. 3):

demodulating the original detection signal with the reference signal sin ω t having the same frequency and orthogonal phase as the frequency modulation to obtain Y_1fSignal (as shown in fig. 4):

so the amplitude theta of the 1f signal_1f(as in FIG. 5) can be expressed as:

since the gas absorbance τ (v) is proportional to the integrated absorbance and the gas concentration, the Fourier series H thereof₀、H₁、H₂.... is also proportional to the integrated absorbance, gas concentration, and therefore θ_1fIs a function of the integrated absorbance, gas concentration.

Suppose i₀：i _i10, the phase difference theta between the frequency and the light intensity modulation is 150 DEG, and theta is measured under different integral absorbances_1fThe results of numerical calculations are shown in FIG. 6, and θ is calculated at different integrated absorbances_1fThe curves have distinct differences in peak-to-peak values at positions 1 and 2. 1. There was a clear monotonically increasing relationship between the peak-to-peak value at position 2 and the integrated absorbance (as shown in fig. 7). While the integrated absorbance is proportional to the gas concentration. Therefore, when the gas pressure and the measuring optical path are constant, the gas pressure and the measuring optical path can pass through theta_1fThe peak-to-peak values of the curves at positions 1 and 2 were used to measure the gas concentration.

Can measure theta of target gas spectrum to be measured by measuring different concentrations_1rSignal to obtain different concentrationsLower theta_1fThe difference value (peak-to-peak value) between the signal peak value 1 and the signal valley value 2 is used for establishing a concentration-peak-to-peak value corresponding relation, and measuring theta for target gas to be measured with unknown concentration_1fAnd the peak value of the signal is obtained by means of difference.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.