CN114942235A - Method for extracting effective absorption information in complex background environment - Google Patents
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Abstract
The invention provides a method for extracting effective absorption information in a complex background environment, which is characterized by obtaining a light intensity signal containing absorption information according to a beer-Lambert law combined with a wavelength modulation spectrum technology, obtaining a background harmonic signal through a background gas characteristic absorption line combined with an absorption model accurate construction method, obtaining target molecular characteristic absorption through a double-channel background deduction mode, inhibiting the influence generated by an initial phase of a reference signal, and obtaining integral absorbance, absorption line central frequency, Gaussian broadening and pressure broadening through a nonlinear least square fitting method for accurately calculating the concentration of background gas. The method solves the problem of accurate measurement of trace gas in a high-concentration and fast-change complex background environment, applies the weak absorption signal extraction method to the related engineering field, solves the key technical problem, is suitable for severe environments with complex components, serious absorption spectrum line superposition, fast change of background gas concentration and the like, and further expands the application range of the wavelength modulation spectrum technology.
Description
Technical Field
The invention relates to the technical field of measurement and testing, in particular to a method for extracting effective absorption information in a complex background environment.
Background
Accurate measurement of molecular concentration and state parameters (such as temperature, pressure, flow rate, etc.) in a gas medium is of great significance in various fields such as environmental monitoring, industrial process control, combustion flow field diagnosis, respiratory detection, etc. The tunable semiconductor absorption spectrum (TDLAS) technology adopts a tunable semiconductor laser, the output wavelength of the laser is tuned by changing the injection current or temperature of the laser, the laser scans a single or multiple absorption lines of gas molecules to obtain a high-resolution gas absorption spectrum, and the spectrum is analyzed to obtain gas parameter information.
In practical engineering application, complex background absorption influence often exists in a measurement environment, even if a large number of absorption lines are selected and analyzed, isolated target absorption is difficult to find, especially, the low-concentration gas content is measured under a high-concentration background, the absorption of target molecules is submerged by overall absorption, and the background gas concentration is in a continuous change state, so that greater difficulty is increased for accurate extraction of weak absorption signals.
Disclosure of Invention
The invention aims to solve the problem that weak absorption signals cannot be accurately extracted, and provides a method for extracting effective absorption information in a complex background environment, which comprises the steps of firstly determining background absorption concentration information by adding a reference light path, and inputting background concentration serving as an environment parameter into an accurate absorption model; and establishing a background absorption signal through an absorption model, performing sub-channel background subtraction on the measurement signal containing background absorption, and performing normalization processing, thereby solving the problem of accurate measurement of the concentration of the gas in the complex background absorption environment.
The invention provides a method for extracting effective absorption information in a complex background environment, which comprises the following steps:
s1, obtaining a light intensity signal containing absorption information: after the first laser and the second laser are continuously scanned, the light intensity I of the absorbed background medium is obtained 1 (t) and the intensity of the absorbed light I containing the target medium 2 (t) obtaining background gas volume fraction absorbance A by polynomial fitting and frequency domain integration 1 Then calculating to obtain the background gas concentration x 1 ；
S2, obtaining a background absorption light intensity signal: obtaining a background absorption light intensity signal I through a molecular absorption model according to an environment input parameter and a background gas dynamic concentration value b2 (t)；
S3, reducing the target absorption harmonic signal: the absorbed light intensity I of the target medium 2 (t) and background absorbed light intensity signal I b2 (t) simultaneously performing phase-locked demodulation and low-pass filtering by using the same phase-locked demodulation and low-pass filtering, and reducing by using a sub-channel background subtraction method to obtain a target absorption harmonic signal S 2f/1f-bgsub ；
S4, extracting the harmonic signal peak value of the target molecule: extracting a target absorption harmonic signal S 2f/1f-bgsub The peak value of (a) is obtained,obtaining target molecular concentration chi according to the monotonic curve of the peak value and the concentration 2 And the effective absorption information extraction is completed.
In the method for extracting effective absorption information in a complex background environment, as a preferred mode, in step S1, the scanning method of the first laser and the second laser includes: the first laser and the second laser work simultaneously, the first laser is used for carrying out direct scanning to obtain background information, the second laser is used for carrying out modulation scanning penetrating through a measuring medium, output light of the first laser and output light of the second laser are coupled and then divided into three paths of output light in equal proportion to be emitted, the first path of output light obtains light-emitting frequency time response upsilon of the first laser through an etalon 1 (t) and the second laser's optical frequency time response upsilon 2 (t); the second path of output light has an optical path of l 1 Obtaining the light intensity I of the background medium after absorption 1 (t); the third path of output light has an optical path l 2 To obtain the absorbed light intensity I containing the target medium 2 (t)。
The method for extracting effective absorption information in complex background environment, provided by the invention, is used as an optimal mode, wherein the background gas concentration x 1 The calculating method comprises the following steps: a. the 1 ＝Pχ 1 l 1 S 1 (T)；
Where P is the pressure of the absorption medium and l 1 To absorb the optical path of the medium, S 1 (T) is the absorption line intensity, background gas volume partial absorbance A 1 By converting the light intensity transmittance into absorbance alpha 1 And then integrated in the frequency domain.
As a preferred mode, the light intensity transmittance tau (v) is obtained according to the following formula:
wherein tau is light intensity transmittance, upsilon is laser output frequency, upsilon and wavelength form reciprocal relation, I 0 For input of light intensity, I t To output light intensity, S j The intensity of the absorption line of j transition of a single gas molecule, T is the temperature of the absorption medium, χ is the concentration of the absorption medium, l is the optical path of the absorption medium, φ j Is a linear function of the j transition of a single gas molecule.
The method for extracting effective absorption information in complex background environment is used as an optimal mode, and the linear function phi is used as the optimal mode j For the Voigt profile, the Voigt profile is a convolution of the Lorentzian and Gaussian profiles:
wherein phi D Is of Gaussian linear type, phi C Of the Lorentzian type, the function of the profile phi for a single transition of a gas molecule j The full frequency domain integral of (1);
the absorption line strength S (T) is:
wherein upsilon is 0 For transition of the center frequency, T 0 The reference temperature is 296K, c is the speed of light, h is the planckian constant, K is the boltzmann constant, E "is the low-state energy level of the molecular absorption line, Q is the partition function, Q is only related to temperature, and the partition function Q is obtained by discrete point interpolation in a standard database.
The method for extracting the effective absorption information in the complex background environment is used as an optimal mode, and the background gas volume is divided into absorbance A 1 The obtaining method comprises the following steps: carrying out multimodal nonlinear least square fitting on the background gas absorbance by using a Voigt line type, automatically obtaining the number of absorption peaks by a peak searching algorithm during fitting, obtaining optimal input parameters by setting absorption peak threshold values and intervals, wherein each absorption comprises four fitting parameters, the fitting parameters are integral absorption, absorption line central frequency, Gaussian broadening and pressure broadening, and the number of the absorption peaks is increased when each absorption line is increasedFour fitting parameters.
In the method for extracting effective absorption information in a complex background environment, step S1 preferably includes the light intensity I of the target medium after absorption 2 (t) is:
I 2 (t)＝I 02 (t)·τ(υ 2 (t))；
wherein, I 02 (t) is the light intensity signal without absorption, I 02 (t) is obtained by using segmented L-M fitting, tau is the light intensity transmittance,
is the light-emitting center frequency of the laser, a represents the modulation depth, f is the sine modulation frequency, upsilon 2 (t) and τ (v) 2 (t)) are even functions of time t, including the absorbed light intensity I of the target medium 2 (t) includes background absorption accumulation and target molecule absorption accumulation.
In the method for extracting effective absorption information in a complex background environment, as a preferred mode, in step S2, the parameters of the molecular absorption model include: spectral line parameters, flow field parameters of a measuring medium and laser parameters;
obtaining spectral line parameters from a HITRAN database, wherein when the spectral line parameters are spectral line parameters of a single absorption line, the spectral line parameters comprise absorption center frequency, line intensity at a reference temperature, an air broadening coefficient, a self broadening coefficient, an air broadening temperature dependence coefficient and a pressure frequency shift coefficient;
the flow field parameters of the measuring medium comprise the temperature, the background gas dynamic concentration value and the pressure value in the measuring environment, and the background gas dynamic concentration value is obtained in real time according to the step S1;
the laser parameters comprise light-emitting intensity and light-emitting frequency, the light-emitting intensity takes a zero-absorption background intensity signal as an input parameter, and the light-emitting frequency is obtained by combining an etalon with an intelligent processing algorithm.
The method is used for extracting the complex background environmentThe method for effectively absorbing information preferably includes step S3 of determining the absorbed light intensity I of the target medium 2 (t) and background absorbed light intensity signal I b2 And (t) transferring the absorption information to a high-frequency part, realizing by superposing high-frequency sinusoidal modulation on a scanning light source of the measuring light path, and obtaining harmonic signals of each order by a double-channel demodulation mode.
In the method for extracting effective absorption information for complex background environment according to the present invention, as a preferred mode, in step S3, the absorbed light intensity I of the medium containing the target is measured 2 (t) and background absorbed light intensity signal I b2 (t) simultaneously obtaining the background harmonic signals of two channels through the same phase-locked demodulation and low-pass filtering
And
double-channel measurement harmonic signal X 1f (t)、Y 1f (t) and X 2f (t)、Y 2f (t) wherein
For one measurement of the background signal for the X channel,
for one measurement of the background signal for the Y channel,
the background signal is measured for a second time for the X channel,
for secondary measurement of the background signal of the Y channel, X 1f (t) first measurement harmonic signal of X channel, Y 1f (t) first harmonic signal of measurement for Y channel, X 2f (t) is the second measurement harmonic signal of the X channel, Y 2f (t) is the second measurement harmonic signal of the Y channel;
normalizing the second harmonicTo obtain a target absorption harmonic signal S 2f/1f-bgsub ：
Wherein R is 1f For the first measurement harmonic signal of the two channels,
background signal for two channels;
wherein n is 1 or 2.
A method for extracting effective absorption information in a complex background environment is used for obtaining a light intensity signal containing absorption information according to a beer-Lambert law combined with a wavelength modulation spectrum technology, wherein absorption of target molecules in the absorption information is covered by complex background absorption, weak absorption signals of the target molecules need to be accurately extracted according to a physical transmission process, a sub-channel background subtraction method is used for reducing target absorption harmonic signals, then a harmonic signal peak value is extracted, and the method is used for calculating the gas concentration in an absorption medium according to a monotonic curve of the peak value and the concentration:
the light intensity follows beer-lambert law through the medium as follows:
wherein tau is light intensity transmittance, and upsilon is laser output frequency (unit: cm) -1 Inverse relation to wavelength), I 0 And I t Respectively input and output light intensity, S j 、φ j The absorption line intensity and the linear function of the j transition of a single gas molecule are respectively shown, and T, P, chi and l are respectively the temperature, the pressure, the mole fraction and the optical path of an absorption medium. Absorption linetype and linewidth of molecules, mainly due to emission or absorption spectrum of molecules themselvesThe line is not a single frequency distribution and includes two widening mechanisms, uniform widening, which is generally represented by the lorentzian line type, and non-uniform widening, which is represented by the gaussian line type. In actual gas measurement, it is more accurate to describe using the Voigt line type, which is a convolution form of Lorentzian line type and Gaussian line type, and the expression is as follows:
wherein phi D And phi C Gaussian and lorentzian, respectively, the full-frequency-domain integral of the linear function is 1 for a single transition of the gas molecule. The linear intensity of molecular absorption is a function of temperature, and the linear intensity at temperature T can be expressed as:
wherein upsilon is 0 Indicating the transition center frequency, T 0 The reference temperature is generally 296K, c is the speed of light, h is the planckian constant, K is the boltzmann constant, E "is the low state energy level of the molecular absorption line, Q is the partition function, is only related to temperature, and can be interpolated from discrete points in a standard database.
The complex absorption background requires two steps to acquire:
1) increasing a reference light path to obtain complex background concentration information;
2) and inputting the background concentration information into an absorption model to obtain a background absorption light intensity signal.
And a reference light path obtains an absorbed light intensity signal in a continuous scanning mode, the scanning range only contains background absorption and does not contain a target molecule absorption line which is finally measured, then the absorbance is fitted by utilizing a Voigt line type to obtain integral absorbance, the concentration of background gas is further calculated, and a Levenberg-Marquardt (L-M) method with higher efficiency is selected for fitting, so that multi-parameter nonlinear fitting is realized. During fitting, the number of absorption peaks is automatically obtained through a peak searching algorithmObtaining the optimal input parameters by setting the threshold value and interval of absorption peaks, wherein each absorption comprises integral absorbance A and central frequency upsilon of an absorption line 0 Gaussian broadening Δ ν D And a pressure spread Δ ν C Four fitting parameters, four for each additional absorption line.
The absorption model is a molecular absorption model which can be directly compared with an actually measured signal, and the model comprises spectral line parameters, flow field parameters of a measured medium and laser parameters. The spectral line parameters are obtained from a HITRAN database updated and maintained by Harvard university, and for a single absorption line, the spectral line parameters comprise absorption center frequency, line intensity under reference temperature (296K), air broadening coefficient, self broadening coefficient, air broadening temperature dependence coefficient and pressure frequency shift coefficient; measuring medium parameters including temperature, background gas concentration and pressure values in the measurement environment, the temperature and pressure being monitored in real time by the sensors, the background gas concentration being obtained in real time by claim 3; the laser parameters consist of two parts: firstly, the change of the light-emitting intensity along with the time, in order to ensure the accuracy of the model, a zero absorption background intensity signal is taken as an input parameter, and secondly, the change of the light-emitting frequency along with the time is obtained by combining an etalon with an intelligent processing algorithm.
In order to realize the accurate extraction of the weak absorption signal of the target molecule, the comprehensive absorption information of the background and the target molecule is transferred to a high-frequency part, the high-frequency sine modulation is superposed on a scanning light source of a measuring light path, and harmonic signals of each order are obtained in a dual-channel demodulation mode.
And carrying out background subtraction and normalization processing on the obtained measured harmonic signals, wherein normalization is carried out by using first harmonic, and finally the expression of the obtained harmonic signals is as follows:
wherein X 2f 、
Representing the second harmonic signal of the X channel and the background signal, Y, respectively 2f 、
Representing the second harmonic signal of the Y channel and the background signal, R, respectively 1f 、
Representing the primary measured harmonic signal and the background signal of the two channels, respectively.
The technical scheme adopted by the invention is as follows: the two lasers work simultaneously by driving current in a time-sharing mode, the first laser is used for obtaining background information in a direct scanning mode, the second laser is used for modulating and scanning to penetrate through a measuring medium, and two beams of light are coupled and then divided into three paths in equal proportion to be emitted out: one path passes through an etalon to obtain the light-emitting frequency time response upsilon of the two lasers 1 (t) and upsilon 2 (t); one-way optical path length is l 1 Obtaining the light intensity I after absorption 1 (t); the last path has an optical path length of l 2 To obtain a light intensity I 2 (t)；
The passing optical path is l 1 The light beam transmission of the absorption medium follows Beer-Lambert law, and the absorbance can be directly obtained through transformation, wherein the expression is as follows:
in which I 01 (t) is the background of no absorption by the pair I 1 (t) the non-absorption part polynomial is obtained by piecewise fitting, and the other linear function phi 1 The method has the normalization characteristic, and the absorbance is integrated on a frequency domain, so that the integrated absorbance irrelevant to a linear function can be obtained, and the expression is as follows:
A 1 ＝Pχ 1 l 1 S 1 (T) (2)
the absorbance fitting is realized through an L-M nonlinear fitting algorithm to obtain integral absorbance, and thenCalculating background absorption gas concentration x 1 The fitting is carried out in the frequency domain, the frequency domain signal upsilon 1 (t) is obtained from the optical path through the etalon.
The passing optical path is l 2 The light beam of the absorption medium is modulated and absorbed, and comprises background and absorption accumulation of target molecules, and the light intensity after absorption is expressed as follows:
I 2 (t)＝I 02 (t)·τ(υ 2 (t)) (3)
wherein
Is the light-emitting center frequency of the laser, a represents the modulation depth, f is the sine modulation frequency, upsilon 2 And (t) is an even function of time t, the transmittance of the light transmitted by the light source is also the even function of time t, and corresponding harmonic signals are obtained according to different frequency values of the reference signals by multiplying the reference signals by the absorbed light intensity and then passing through a low-pass filter.
The background subtraction process is shown in fig. 1 and is divided into four steps:
the method comprises the following steps: creating a background absorption model to obtain the light intensity I b2 (t) four inputs are involved, including environmental parameters, background absorption line parameters, dynamic concentration values of the background, and laser parameters.
The environmental parameters in the step one are temperature values and pressure intensities of the environment, and accurate measurement values are obtained through corresponding sensors;
referring to the value in the HITRAN database for background absorption spectrum line parameters in the step I, wherein each absorption line correspondingly comprises an absorption center frequency, a line intensity at a reference temperature (296K), an air broadening coefficient, a self broadening coefficient, an air broadening temperature dependence coefficient and a pressure frequency shift coefficient, and in order to ensure that the finally obtained I b2 (t) accuracy, considering the effect of all absorption lines in the laser scanning range;
the background concentration value in the first step refers to a gas concentration value which is beyond a target component in a measurement environment and has an influence on a measurement result, multimodal nonlinear least square fitting is performed by using a Voigt linear type with the measured absorbance as a target, the influence of light jitter is considered during fitting, namely, third-order polynomial bias is added into a fitting model, and the expression is as follows:
wherein the offset represents the combined effect of optical jitter and calculation error, and is a non-linear function of frequency;
the laser parameters in step one include background of non-absorption light intensity 02 (t) and frequency-time response upsilon of the modulation mode of operation 2 (t), the non-absorption light intensity signal is directly obtained by utilizing segmented L-M fitting, and the frequency time response is accurately obtained by an automatic peak searching algorithm.
Step two: the signal demodulation is realized in a software demodulation mode, the operation is easier compared with hardware demodulation, and when the parameters of the filter are fixed, the background absorbs the light intensity I b2 (t) and measuring the intensity of the absorbed light I 2 (t) obtaining two groups of double-channel background harmonic signals through the same processing
And
two-group double-channel harmonic signal X 1f (t)、Y 1f (t) and X 2f (t)、Y 2f (t)。
Step three: obtaining a background-subtracted normalized 2-order harmonic signal S 2f/1f-bgsub The subtraction method is described in the formula (4) in the claims, wherein
Step four: obtaining the target molecular concentration χ 2 By step III of the harmonic signal S 2f/1f-bgsub The monotonic relationship with concentration is directly obtained.
The invention has the following advantages:
(1) the invention discloses a method for extracting effective absorption information in a complex background environment, which obtains dynamically-changed background gas concentration information by adding a reference light path, and solves the problem that the weak absorption information of dynamically-absorbed background target molecules cannot be accurately extracted; the method for accurately constructing the absorption model based on the modulation spectrum technology is adopted to create an accurate dynamic absorption background, the modulation technology transfers target absorption to a high-frequency part, low-frequency noise is inhibited, the signal-to-noise ratio is improved, the concentration value is inverted by utilizing the harmonic signal peak value absorbed by target molecules, and the problem of accurately measuring weak absorption components in the strong absorption background environment is effectively solved.
(2) The invention discloses a method for extracting effective absorption information in a complex background environment, which inhibits the influence of reference signal phase value by a double-channel demodulation mode for modulating absorption light intensity; the 1-order harmonic is adopted to carry out normalization processing on the 2-order harmonic, the influence of gain and light-emitting jitter is eliminated, common-mode noise and non-resonant transmission loss of a light source and a detector are inhibited, and the measurement precision is further improved.
(3) The method for extracting the effective absorption information in the complex background environment can be used in the fields of complex industrial process control and special measurement, and can widen the application range of spectral measurement.
Drawings
FIG. 1 is a flow chart of a method for extracting effective absorption information for complex background environments;
FIG. 2 shows an embodiment of a method for extracting effective absorption information in a complex background environment, which is 26349.5cm -1 ～6352.5cm -1 Absorbance of the range;
FIG. 3 is a diagram of a method for extracting effective absorption information in a complex background environment embodiment 2 direct absorption light intensity signal for background concentration measurement;
FIG. 4 is a method for extracting effective absorption information in a complex background environment example 2 methane absorption fitting integral absorbance
FIG. 5 is a diagram of a method for extracting effective absorption information in a complex background environment, embodiment 2 includes a modulated absorption light intensity signal of an absorption background;
FIG. 6 is a method for extracting effective absorption information in complex background environment embodiment 2 background-subtracted normalized 2-th harmonic signal;
fig. 7 is a method for extracting effective absorption information in a complex background environment, in which 2, the harmonic signal peak value is in linear relation with the concentration.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.
Example 1
As shown in fig. 1, a method for extracting effective absorption information in a complex background environment includes the following steps:
1. a method for extracting effective absorption information in a complex background environment is characterized by comprising the following steps: the method comprises the following steps:
s1, obtaining a light intensity signal containing absorption information: after the first laser and the second laser are continuously scanned, the light intensity I of the absorbed background medium is obtained 1 (t) and the intensity of the absorbed light I containing the target medium 2 (t) obtaining background gas volume fraction absorbance A by polynomial fitting and frequency domain integration 1 Then calculating to obtain the background gas concentration x 1 ；
The scanning method of the first laser and the second laser comprises the following steps: the method comprises the steps that a first laser and a second laser work simultaneously, the first laser is used for conducting direct scanning to obtain background information, the second laser is used for conducting modulation scanning penetrating through a measuring medium, output light of the first laser and output light of the second laser are coupled and then divided into three paths of output light in equal proportion to be emitted, the first path of output light obtains the first laser light emitting frequency time response upsilon through an etalon 1 (t) and the second laser's optical frequency time response upsilon 2 (t); the second path of output light has an optical path of l 1 Obtaining the light intensity of the background medium after absorptionI 1 (t); the third path of output light has an optical path l 2 To obtain the absorbed light intensity I containing the target medium 2 (t)；
Background gas concentration χ 1 The calculation method comprises the following steps: a. the 1 ＝Pχ 1 l 1 S 1 (T)；
Where P is the pressure of the absorption medium and l 1 To absorb the optical path of the medium, S 1 (T) is the absorption line intensity, background gas volume partial absorbance A 1 By converting the light intensity transmittance into absorbance alpha 1 Then integrating in a frequency domain to obtain;
the light intensity transmittance tau (v) is obtained according to the following formula:
wherein tau is light intensity transmittance, upsilon is laser output frequency, upsilon and wavelength form reciprocal relation, I 0 For input of light intensity, I t To output light intensity, S j The intensity of the absorption line of j transition of a single gas molecule, T is the temperature of the absorption medium, χ is the concentration of the absorption medium, l is the optical path of the absorption medium, φ j Is a linear function of the j transition of a single gas molecule;
linear function phi j For the Voigt profile, the Voigt profile is a convolution of the Lorentzian and Gaussian profiles:
wherein phi D Is Gaussian linear form, phi C Of the Lorentzian type, the function of the profile phi for a single transition of a gas molecule j Has a full frequency domain integral of 1;
the absorption line intensity S (T) is:
wherein，υ 0 For transition of the center frequency, T 0 The reference temperature is 296K, the c is the speed of light, the h is the Planckian constant, the K is the Boltzmann constant, the E' is the low-state energy level of the molecular absorption line, the Q is the distribution function, the distribution function Q is only related to the temperature, and the distribution function Q is obtained by discrete point interpolation in a standard database;
background gas volume partial absorbance A 1 The obtaining method comprises the following steps: carrying out multimodal nonlinear least square fitting on the absorbance of the background gas by utilizing a Voigt line type, automatically obtaining the number of absorption peaks by a peak searching algorithm during fitting, and obtaining optimal input parameters by setting absorption peak threshold values and intervals, wherein each absorption comprises four fitting parameters, the fitting parameters are integral absorbance, absorption line central frequency, Gaussian broadening and pressure broadening, and four fitting parameters are added when one absorption line is added;
including the intensity of the absorbed light I of the target medium 2 (t) is:
I 2 (t)＝I 02 (t)·τ(υ 2 (t))；
wherein, I 02 (t) is the signal of the intensity of the unabsorbed light, I 02 (t) is obtained by using segmented L-M fitting, tau is the light intensity transmittance,
is the light-emitting center frequency of the laser, a represents the modulation depth, f is the sine modulation frequency, upsilon 2 (t) and τ (v) 2 (t)) are even functions of time t, including the absorbed light intensity I of the target medium 2 (t) comprises background absorption accumulation and target molecule absorption accumulation;
s2, obtaining a background absorption light intensity signal: obtaining a background absorption light intensity signal I through a molecular absorption model according to an environment input parameter and a background gas dynamic concentration value b2 (t)；
Parameters of the molecular absorption model include: spectral line parameters, flow field parameters of a measuring medium and laser parameters;
obtaining spectral line parameters from a HITRAN database, wherein when the spectral line parameters are spectral line parameters of a single absorption line, the spectral line parameters comprise absorption center frequency, line intensity at a reference temperature, an air broadening coefficient, a self broadening coefficient, an air broadening temperature dependence coefficient and a pressure frequency shift coefficient;
the flow field parameters of the measuring medium comprise the temperature, the background gas dynamic concentration value and the pressure value in the measuring environment, and the background gas dynamic concentration value is obtained in real time according to the step S1;
s3, reducing the target absorption harmonic signal: the absorbed light intensity I of the target medium 2 (t) and background absorbed light intensity signal I b2 (t) simultaneously performing phase-locked demodulation and low-pass filtering by using the same phase-locked demodulation and low-pass filtering, and reducing by using a sub-channel background subtraction method to obtain a target absorption harmonic signal S 2f/1f-bgsub ；
The absorbed light intensity I of the target medium 2 (t) and background absorbed light intensity signal I b2 And (t) transferring the absorption information to a high-frequency part, realizing by superposing high-frequency sinusoidal modulation on a scanning light source of the measuring light path, and obtaining harmonic signals of each order by a double-channel demodulation mode.
The absorbed light intensity I of the target medium 2 (t) and background absorbed light intensity signal I b2 (t) simultaneously obtaining the background harmonic signals of two channels through the same phase-locked demodulation and low-pass filtering
And
double-channel measurement harmonic signal X 1f (t)、Y 1f (t) and X 2f (t)、Y 2f (t) in which
For one measurement of the background signal for the X channel,
one measurement of background signal for Y channel，
The background signal is measured for a second time for the X channel,
for secondary measurement of the background signal of the Y channel, X 1f (t) first measurement harmonic signal of X channel, Y 1f (t) a first measurement harmonic signal of Y channel, X 2f (t) is the second measurement harmonic signal of the X channel, Y 2f (t) is the second measurement harmonic signal of the Y channel;
normalizing the second harmonic to obtain a target absorption harmonic signal S 2f/1f-bgsub ：
Wherein R is 1f For the first measurement harmonic signal of the two channels,
a background signal which is a double channel;
wherein n is 1 or 2.
Example 2
As shown in fig. 1, a method for extracting effective absorption information in a complex background environment is exemplified by extracting effective absorption information of hydrogen sulfide from natural gas.
Besides methane, natural gas also comprises a plurality of gaseous components, wherein hydrogen sulfide is an acidic fatal toxic gas, which not only harms the health of operating personnel, but also has strong corrosive effect on gas transmission pipelines when contacting with water vapor. The hydrogen sulfide measuring method based on the optical principle can realize lossless and non-contact online monitoring, but compared with hydrogen sulfide measurement in the conventional environment, the absorption information of hydrogen sulfide is completely submerged by high-concentration methane background absorption, and background gas is in a continuous change state. Based on the above, a method for extracting effective absorption information in a complex background environment is provided, which accurately extracts a weak absorption signal of hydrogen sulfide, and realizes accurate measurement of concentration, wherein in the embodiment, the ambient temperature is 296K, the pressure is 1atm, the methane concentration is 95%, the hydrogen sulfide content is 1ppm to 10ppm, and the balance is nitrogen.
Through analysis of absorption lines, the absorption center is selected to be 6351.00468cm -1 Is used for the concentration measurement of hydrogen sulfide, but there is a plurality of methane absorptions near this absorption line, and the concentration of methane in natural gas exceeds 90%. 6349.5cm -1 ～6352.5cm -1 The absorbance of methane and hydrogen sulfide in the range is shown in fig. 2, the absorption of methane occupies a dominant position of the total absorption, and the absorption of hydrogen sulfide is very weak, so that in order to realize the accurate extraction of weak absorption signals, the absorption information is transferred to a high-frequency part, and the laser is in a modulation working mode in the waveband.
According to the steps of the invention, the concentration of methane background gas is firstly determined, the spectral information is obtained by utilizing the working mode of direct scanning, and the center frequency is selected to be 6047.95cm -1 On the other hand, the absorption of hydrogen sulfide in the band is negligible and can be used for accurate measurement of methane. The absorbed light intensity signal is shown in figure 3, the background light intensity is obtained by selecting the light intensity of two sections of non-absorption parts to carry out polynomial fitting, the light absorbance is subjected to L-M fitting in the frequency domain by combining the frequency domain information of the wave band, the integral absorbance is obtained as shown in figure 4, the accurate methane concentration is obtained by calculation, and the accurate measurement is realized, wherein the methane concentration is 94.99%.
After the concentration of methane is obtained, the ambient temperature and the pressure are simultaneously used as the ambient input parameters, the non-absorption light intensity signal of the modulation signal is obtained by nonlinear fitting of a plurality of sections of non-absorption signals, but the fitting model is not a polynomial but a polynomial high-frequency sinusoidal modulation model, the frequency domain signal is more complicated to obtain compared with the direct absorption, the non-absorption light intensity signal is obtained by reordering interference peaks and fitting the non-absorption light intensity same model, and finally the light intensity signal of the methane absorption background is obtained, as shown in fig. 5.
The measured signal and the methane absorption background light intensity signal are simultaneously subjected to the same phase locking and low-pass filtering to obtain a double- channel 1 and 2 background harmonic signal and a measured signal, and a background-subtracted target absorption 2 harmonic signal S normalized by 1 harmonic is obtained by calculating a formula (4) in the claim 2f/1f-bgsub As shown in fig. 6. The harmonic signal in the dashed line frame is an electric signal finally used for hydrogen sulfide inversion, the peak value and the concentration of the signal are in a monotone positive correlation relationship, and the concentration and the peak value curve are accurately calibrated, so that the accurate measurement of the concentration of the trace hydrogen sulfide in the background environment of the high-concentration methane is realized, and the curve is shown in fig. 7. And (3) performing first-order polynomial fitting on the curve to obtain a relation that the peak value changes along with the concentration: peak is 1.731e-4 x-1.632 e-7, accords with the linear transmission rule of the harmonic signal Peak value and the concentration under the weak absorption condition, and realizes the accurate extraction of the effective absorption information in the complex background environment, thereby obtaining the accurate gas concentration information.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered as the technical solutions and the inventive concepts of the present invention within the technical scope of the present invention.
Claims (10)
1. A method for extracting effective absorption information in a complex background environment is characterized by comprising the following steps: the method comprises the following steps:
s1, obtaining a light intensity signal containing absorption information: after the first laser and the second laser are continuously scanned, the light intensity I of the absorbed background medium is obtained 1 (t) and the intensity of the absorbed light I containing the target medium 2 (t) obtaining background gas volume fraction absorbance A by polynomial fitting and frequency domain integration 1 Then calculating to obtain the background gas concentration x 1 ；
S2, obtaining a background absorption light intensity signal: inputting parameters according to the environment andobtaining a background absorption light intensity signal I by a background gas dynamic concentration value through a molecular absorption model b2 (t)；
S3, reducing the target absorption harmonic signal: the absorbed light intensity I of the target medium 2 (t) and the background absorbed light intensity signal I b2 (t) simultaneously performing phase-locked demodulation and low-pass filtering by using the same phase-locked demodulation and low-pass filtering, and reducing by using a sub-channel background subtraction method to obtain a target absorption harmonic signal S 2f/1f-bgsub ；
S4, extracting the harmonic signal peak value of the target molecule: extracting the target absorption harmonic signal S 2f/1f-bgsub According to the monotonic curve of the peak value and the concentration, the target molecule concentration chi is obtained 2 And the effective absorption information extraction is completed.
2. The method for extracting effective absorption information from complex background environment according to claim 1, wherein: in step S1, the scanning method of the first laser and the second laser includes: the first laser and the second laser work simultaneously, the first laser is used for directly scanning to acquire background information, the second laser is used for modulating and scanning through a measuring medium, output light of the first laser and output light of the second laser are coupled and then divided into three paths of output light in equal proportion to be emitted, and the first path of output light obtains first laser light-emitting frequency time response upsilon through an etalon 1 (t) and the second laser's optical frequency time response upsilon 2 (t); the second path of output light has an optical path of l 1 Obtaining the light intensity I of the background medium after absorption 1 (t); the third path of output light has an optical path l 2 To obtain the absorbed light intensity I containing the target medium 2 (t)。
3. The method for extracting effective absorption information from complex background environment according to claim 2, wherein: the background gas concentration χ 1 The calculation method comprises the following steps:
A 1 ＝Pχ 1 l 1 S 1 (T)；
where P is the pressure of the absorption medium and l 1 To absorb the optical path of the medium, S 1 (T) is the absorption line intensity, and the background gas volume is divided into absorbance A 1 The light intensity transmittance of the background gas is converted into absorbance, and the absorbance is integrated in a frequency domain to obtain the light intensity transmittance.
4. The method for extracting effective absorption information from complex background environment according to claim 3, wherein:
the light intensity transmittance tau (nu) is obtained according to the following formula:
wherein tau is light intensity transmittance, upsilon is laser output frequency, upsilon and wavelength form reciprocal relation, I 0 For input of light intensity, I t To output light intensity, S j The intensity of the absorption line of j transition of a single gas molecule, T is the temperature of the absorption medium, χ is the concentration of the absorption medium, l is the optical path of the absorption medium, φ j Is a linear function of the j transition of a single gas molecule.
5. The method for extracting effective absorption information from complex background environment according to claim 4, wherein:
said linear function phi j For the Voigt profile, the Voigt profile is a convolution of a Lorentzian profile and a Gaussian profile:
wherein phi D Is of Gaussian linear type, phi C For the Lorentzian profile, the profile function φ is such that for a single transition of a gas molecule j The full frequency domain integral of (1);
the absorption line intensity S (T) is:
wherein, upsilon 0 For a transition of the center frequency, T 0 For a reference temperature, 296K, c the speed of light, h the planckian constant, K the boltzmann constant, E "the low-state energy level of the molecular absorption line, Q the partition function, Q being only temperature dependent, said partition function Q being obtained by discrete point interpolation in a standard database.
6. The method for extracting effective absorption information from complex background environment according to claim 3, wherein: the background gas volume partial absorbance A 1 The obtaining method comprises the following steps: the method comprises the steps of carrying out multimodal nonlinear least square fitting on background gas absorbance by utilizing a Voigt line type, automatically obtaining the number of absorption peaks through a peak searching algorithm during fitting, obtaining optimal input parameters by setting absorption peak thresholds and intervals, wherein each absorption comprises four fitting parameters, the fitting parameters comprise integral absorbance, absorption line central frequency, Gaussian broadening and pressure broadening, and the four fitting parameters are increased when one absorption line is added.
7. The method for extracting effective absorption information for complex background environment as claimed in claim 1, wherein: in step S1, the absorbed light intensity I of the medium containing the target 2 (t) is:
I 2 (t)＝I 02 (t)·τ(υ 2 (t))；
wherein, I 02 (t) is the signal of the intensity of the unabsorbed light, I 02 (t) is obtained by using segmented L-M fitting, tau is the light intensity transmittance,
is the light-emitting center frequency of the laser, a represents the modulation depth, and f isSinusoidal modulation frequency, upsilon 2 (t) and τ (v) 2 (t)) are even functions of time t, said target medium comprising the absorbed light intensity I 2 (t) includes background absorption accumulation and target molecule absorption accumulation.
8. The method for extracting effective absorption information from complex background environment according to claim 1, wherein: in step S2, the parameters of the molecular absorption model include: spectral line parameters, flow field parameters of a measured medium and laser parameters;
the spectral line parameters are obtained from a HITRAN database, and when the spectral line parameters are spectral line parameters of a single absorption line, the spectral line parameters comprise absorption center frequency, line intensity at a reference temperature, an air broadening coefficient, a self broadening coefficient, an air broadening temperature dependence coefficient and a pressure frequency shift coefficient;
the flow field parameters of the measuring medium comprise the temperature in the measuring environment, the dynamic concentration value and the pressure value of the background gas, and the dynamic concentration value of the background gas is obtained in real time according to the step S1;
the laser parameters comprise light intensity and light frequency, the light intensity takes a zero-absorption background intensity signal as an input parameter, and the light frequency is obtained by combining an etalon with an intelligent processing algorithm.
9. The method for extracting effective absorption information from complex background environment according to claim 1, wherein: in step S3, the absorbed light intensity I of the medium containing the target is measured 2 (t) and the background absorbed light intensity signal I b2 And (t) transferring the absorption information to a high-frequency part, realizing by superposing high-frequency sinusoidal modulation on a scanning light source of the measuring light path, and obtaining harmonic signals of each order by a double-channel demodulation mode.
10. The method for extracting effective absorption information from complex background environment according to claim 1, wherein: in step S3, the absorbed light intensity I of the medium containing the target is measured 2 (t)And the background absorbed light intensity signal I b2 (t) simultaneously performing phase-locked demodulation and low-pass filtering to obtain two-channel background harmonic signals
And
dual-channel measurement harmonic signal X 1f (t)、Y 1f (t) and X 2f (t)、Y 2f (t) wherein,
for one measurement of the background signal for the X channel,
for one measurement of the background signal for the Y channel,
the background signal is measured for a second time for the X channel,
for secondary measurement of the background signal of the Y channel, X 1f (t) first measurement harmonic signal of X channel, Y 1f (t) first harmonic signal of measurement for Y channel, X 2f (t) is the second measurement harmonic signal of the X channel, Y 2f (t) is the second measurement harmonic signal of the Y channel;
normalizing the second harmonic to obtain the target absorption harmonic signal S 2f/1f-bgsub ：
Wherein R is 1f For the first measurement harmonic signal of the two channels,
a background signal which is a double channel;
wherein n is 1 or 2.
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