CN114942235B - 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 used for obtaining a light intensity signal containing absorption information according to a beer-lambert law and a wavelength modulation spectrum technology, obtaining a background harmonic signal through a background gas characteristic absorption line combination absorption model accurate construction method, obtaining target molecular characteristic absorption through a double-channel background subtraction mode, inhibiting the influence of a reference signal primary phase, and obtaining integral absorbance, absorption line center frequency, gaussian broadening and pressure broadening through a nonlinear least square fitting method, wherein the method is used for accurately calculating the concentration of background gas. The method solves the problem of accurate measurement of trace gas in a complex background environment with high concentration and rapid change, applies the weak absorption signal extraction method to the related engineering field, solves the key technical problems, is suitable for severe environments with complex components, serious superposition of absorption spectrum lines, rapid 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 test, in particular to a method for extracting effective absorption information in a complex background environment.
Background
The accurate measurement of molecular concentration and state parameters (such as temperature, pressure, flow rate and the like) in a gas medium has very important significance in various fields of environmental monitoring, industrial process control, combustion flow field diagnosis, breath detection and the like. The tunable semiconductor absorption spectrum (TDLAS) technology adopts a tunable semiconductor laser, and the output wavelength of the laser is tuned by changing the injection current or temperature of the laser, so that the tunable semiconductor laser scans a single or multiple absorption lines of gas molecules to obtain a high-resolution gas absorption spectrum, and the high-resolution gas absorption spectrum is analyzed to obtain gas parameter information.
In practical engineering application, complex background absorption influence often exists in a measuring environment, even if a large number of absorption lines are selected and analyzed, isolated target absorption is difficult to find, particularly, low-concentration gas content is measured under a high-concentration background, absorption of target molecules is submerged by integral absorption, and the concentration of background gas is in a continuous change state, so that greater difficulty is increased for accurately extracting weak absorption signals.
Disclosure of Invention
The invention provides a method for extracting effective absorption information in a complex background environment, which aims to solve the problem that a weak absorption signal cannot be extracted accurately, and comprises the steps of firstly determining concentration information of background absorption by adding a reference light path, and inputting the background concentration as an environment parameter into an accurate absorption model; and creating a background absorption signal through an absorption model, carrying out sub-channel background subtraction on a measurement signal containing background absorption, and carrying out normalization processing to solve the problem of accurate measurement of the concentration of the complex background absorption ambient gas.
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: the method comprises the steps that after continuous scanning is carried out by a first laser and a second laser, light intensity I 1 (t) after absorption of a background medium and light intensity I 2 (t) after absorption of a target medium are obtained, background gas integral absorbance A 1 is obtained through polynomial fitting and integral in a frequency domain, and then background gas concentration χ 1 is obtained through calculation;
s2, obtaining a background absorption light intensity signal: obtaining a background absorption light intensity signal I b2 (t) through a molecular absorption model according to an environment input parameter and a background gas dynamic concentration value;
S3, absorbing harmonic signals by a reduction target: the method comprises the steps that absorbed light intensity I 2 (t) containing a target medium and a background absorbed light intensity signal I b2 (t) are subjected to phase locking demodulation and low-pass filtering simultaneously, and then a multichannel background subtraction method is utilized for reduction to obtain a target absorbed harmonic signal S 2f/1f-bgsub;
S4, extracting harmonic signal peak values of target molecules: and extracting the peak value of the target absorption harmonic signal S 2f/1f-bgsub, and obtaining the target molecular concentration χ 2 according to the monotonic curve of the peak value and the concentration, thereby completing the effective absorption information extraction.
In the method for extracting effective absorption information in a complex background environment, in the preferred mode, in step S1, the scanning methods of the first laser and the second laser are as follows: the first laser and the second laser work simultaneously, the first laser is used for directly scanning to obtain background information, the second laser is used for modulating scanning penetrating through a measuring medium, the output light of the first laser and the output light of the second laser are divided into three paths of output light with equal proportion after being coupled to be emitted, and the first path of output light obtains a first laser light emitting frequency time response v 1 (t) and a second laser light emitting frequency time response v 2 (t) through an etalon; the second path of output light passes through a background medium with an optical path of l 1 to obtain light intensity I 1 (t) after the background medium absorbs; the third path of output light passes through a target absorption medium with an optical path length of l 2 to obtain the light intensity I 2 (t) after absorption of the target medium.
The method for extracting effective absorption information in the complex background environment is characterized in that the method for calculating the background gas concentration χ 1 is as follows: a 1＝Pχ1l1S1 (T);
Wherein P is the pressure of the absorption medium, l 1 is the optical path of the absorption medium, S 1 (T) is the absorption line intensity, and the background gas integral absorbance A 1 is obtained by converting the light intensity transmittance into absorbance alpha 1 and then integrating in the frequency domain.
The invention relates to a method for extracting effective absorption information in a complex background environment, which is characterized in that the light intensity transmittance tau (v) is obtained according to the following formula as a preferable mode:
where τ is the light intensity transmittance, v is the output frequency of the laser, v and wavelength are in inverse relationship, I 0 is the input light intensity, I t is the output light intensity, S j is the absorption line intensity of the 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, and Φ j is the linear function of the j transition of the single gas molecule.
The invention relates to a method for extracting effective absorption information in a complex background environment, which is characterized in that a linear function phi j is a Voigt linear type, and the Voigt linear type is the convolution of a Lorentz linear type and a Gaussian linear type:
Wherein phi D is Gaussian line type, phi C is Lorentz line type, and the total frequency domain integral of the line type function phi j is 1 for single transition of gas molecules;
the absorption line strength S (T) is:
wherein v 0 is the transition center frequency, T 0 is the reference temperature, the reference temperature is 256K, c is the light velocity, 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 partitioning function, Q is only related to temperature, and the partitioning function Q is obtained by interpolation of discrete points in a standard database.
The method for extracting effective absorption information in the complex background environment, which is disclosed by the invention, is used as a preferable mode, and the method for obtaining the integrated absorbance A 1 of the background gas comprises the following steps of: and when the fitting is performed, the number of absorption peaks is automatically obtained through a peak searching algorithm, and optimal input parameters are obtained by setting absorption peak threshold values and intervals, wherein each absorption comprises four fitting parameters, the fitting parameters are integral absorption, absorption line center frequency, gaussian broadening and pressure broadening, and each absorption line is increased by four fitting parameters.
In the method for extracting effective absorption information in a complex background environment, in the step S1, as a preferred mode, the light intensity I 2 (t) after absorption including the target medium is as follows:
I2(t)＝I02(t)·τ(υ2(t))；
Wherein I 02 (t) is a non-absorption light intensity signal, I 02 (t) is obtained by fitting with a segment L-M, τ is light intensity transmittance, The method is characterized in that the laser light emitting center frequency, a represents the modulation depth, f is sinusoidal modulation frequency, v 2 (t) and τ (v 2 (t)) are even functions of time t, and the light intensity I 2 (t) after absorption of a target medium comprises background absorption accumulation and target molecule absorption accumulation.
In the method for extracting effective absorption information in a complex background environment, in the step S2, as a preferred mode, parameters of a molecular absorption model include: spectral line parameters, flow field parameters of a measuring medium and laser parameters;
The spectral line parameters are obtained from an HITRAN database, and when the spectral line parameters are the spectral line parameters of a single absorption line, the spectral line parameters comprise an 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;
Measuring flow field parameters of a medium, wherein the flow field parameters comprise temperature, background gas dynamic concentration value and pressure value in a measuring environment, and the background gas dynamic concentration value is obtained in real time according to the step S1;
The laser parameters comprise light output intensity and light output frequency, wherein the light output intensity uses zero absorption background intensity signals as input parameters, and the light output frequency is obtained by using an standard tool to combine an intelligent processing algorithm.
In the method for extracting effective absorption information in a complex background environment, in the step S3, as an optimal mode, absorption information in the light intensity I 2 (t) after absorption of a target medium and the light intensity signal I b2 (t) of background absorption is transferred to a high-frequency part, the method is realized by overlapping a scanning type light source of a measuring light path with a high-frequency sine modulation mode, and then each-order harmonic signal is obtained by a double-channel demodulation mode.
In the method for extracting effective absorption information in complex background environment, in the preferred mode, in the step S3, the absorbed light intensity I 2 (t) containing the target medium and the background absorption light intensity signal I b2 (t) are simultaneously demodulated and filtered by the same phase lock to respectively obtain the background harmonic signals of the two channelsAnd/>Dual channel measurement of harmonic signals X 1f(t)、Y1f (t) and X 2f(t)、Y2f (t), wherein/>For one measurement of the background signal of the X channel,/>For one measurement of the background signal of the Y channel,/>For the secondary measurement of the background signal of the X channel,/>As a secondary measurement background signal of the Y channel, X 1f (t) is a primary measurement harmonic signal of the X channel, Y 1f (t) is a primary measurement harmonic signal of the Y channel, X 2f (t) is a secondary measurement harmonic signal of the X channel, and Y 2f (t) is a secondary measurement harmonic signal of the Y channel;
Normalizing the second harmonic to obtain a target absorption harmonic signal S 2f/1f-bgsub:
Wherein R 1f is a two-channel first-time measurement harmonic signal, Is a background signal of two channels;
Where n=1 or 2.
A method for extracting effective absorption information in complex background environment includes obtaining light intensity signal containing absorption information according to beer-lambert law and wavelength modulation spectrum technique, covering absorption of target molecule in absorption information by complex background absorption, accurately extracting weak absorption signal of target molecule according to physical transmission process, reducing target absorption harmonic signal by using multichannel background subtraction method, extracting harmonic signal peak value, and calculating gas concentration in absorption medium according to monotonic curve of peak value and concentration:
The intensity of light follows the beer-lambert law through the medium as follows:
Where τ is the light intensity transmittance, v is the laser output frequency (unit: cm -1, in reciprocal relation to wavelength), I 0 and I t are the input and output light intensities, respectively, S j、φj are the absorption line intensity and line type functions of the transition of a single gas molecule j, and T, P, χ, l are the temperature, pressure, mole fraction, and optical path of the absorption medium, respectively. The absorption line and line width of the molecule are mainly due to the fact that the emission or absorption line of the molecule itself is not a single frequency distribution, and the absorption line comprises two widening mechanisms, namely uniform widening and non-uniform widening, wherein the uniform widening is generally represented by Lorentzian line type, and the non-uniform widening is represented by Gaussian line type. In practical gas measurements, the description is more accurate using the Voigt line form, which is a convolution of the Lorentz line form and the Gaussian line form, expressed as follows:
Wherein phi D and phi C are gaussian and lorentz lines, respectively, and the full frequency domain integral of the linear function is 1 for a single transition of the gas molecule. The molecular absorption line intensity is a function of temperature, and the line intensity at temperature T can be expressed as:
where v 0 denotes the transition center frequency, T 0 denotes the reference temperature, typically taken to be 256 k, c denotes 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 partitioning function, and is only temperature dependent, and can be obtained by discrete point interpolation in a standard database.
Complex absorption background requires two steps to acquire:
1) Adding 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.
The reference light path adopts a continuous scanning mode to obtain an absorbed light intensity signal, the scanning range only comprises background absorption, does not comprise 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 calculated, and a Levenberg-Marquardt (L-M) method with higher efficiency is adopted for fitting, so that the method is multi-parameter nonlinear fitting. When fitting, the number of absorption peaks is automatically obtained through a peak searching algorithm, the optimal input parameters are obtained through setting the threshold value and the interval of the absorption peaks, each absorption comprises four fitting parameters including integrated absorbance A, the center frequency upsilon 0 of an absorption line, gaussian broadening delta upsilon D and pressure broadening delta upsilon C, and each absorption line is added with four fitting parameters.
The absorption model, namely a molecular absorption model which can be directly compared with the measured signal, comprises spectral line parameters, flow field parameters of a measuring medium and laser parameters. Wherein the spectral line parameters are obtained from an HITRAN database updated and maintained by Harvard university, and comprise an absorption center frequency, line intensity at a reference temperature (296K), an air broadening coefficient, a self broadening coefficient, an air broadening temperature dependent coefficient and a pressure frequency shift coefficient for a single absorption line; measuring medium parameters including temperature, background gas concentration and pressure value in the measuring environment, the temperature and the pressure are monitored by a sensor in real time, and the background gas concentration is obtained by claim 3 in real time; the laser parameters consist of two parts: firstly, the change of the light emitting intensity along with time is obtained by taking a zero absorption background intensity signal as an input parameter and secondly, the change of the light emitting frequency along with time by utilizing a standard tool to combine an intelligent processing algorithm in order to ensure the accuracy of a model.
In order to achieve accurate extraction of target molecule weak absorption signals, comprehensive absorption information of a background and target molecules is transferred to a high-frequency part, the detection is achieved by overlapping a high-frequency sinusoidal modulation mode with a scanning light source of a measuring light path, and each order harmonic signal is obtained by a two-channel demodulation mode.
The obtained measured harmonic signal is subjected to background subtraction and normalization processing, the normalization is performed by using first harmonic, and finally the obtained harmonic signal expression is as follows:
Wherein X 2f is, Representing the secondary measurement harmonic signal and the background signal of the X channel, Y 2f,/>, respectivelyRepresenting the second measurement harmonic signal and the background signal of the Y channel respectively, R 1f,/>, R 1f Representing the two-channel first measurement harmonic signal and background signal, respectively.
The technical scheme adopted by the invention is as follows: through the time-sharing work of drive current, make two lasers work simultaneously, first laser directly scans and is used for background information to acquire, and second laser modulation scans and passes measuring medium, and two light divide into three routes of equal proportion after coupling and jet out: one path passes through an etalon to obtain the light-emitting frequency time responses upsilon 1 (t) and upsilon 2 (t) of the two lasers; one path passes through an absorption medium with an optical path length of l 1 to obtain the light intensity I 1 (t) after absorption; the last path passes through an absorption medium with an optical path length of l 2 to obtain light intensity I 2 (t);
The light beam transmission of the absorption medium with the optical path of l 1 follows Beer-Lambert law, absorbance can be directly obtained through transformation, and the expression is as follows:
Wherein I 01 (t) is non-absorption background, which is obtained by piecewise fitting the non-absorption part polynomial in I 1 (t), and the linear function phi 1 has normalized characteristics, and the integral absorbance independent of the linear function can be obtained by integrating the absorbance in the frequency domain, and the expression is as follows:
A1＝Pχ1l1S1(T) (2)
And (3) realizing absorbance fitting through an L-M nonlinear fitting algorithm to obtain integrated absorbance, further calculating background absorption gas concentration χ 1, wherein the fitting is performed in a frequency domain, and a frequency domain signal v 1 (t) is obtained from a light path passing through an etalon.
The light beam passing through the absorption medium with the optical path of l 2 is modulated absorption, the absorption accumulation of the background and the target molecules is included, and the light intensity expression after absorption is as follows:
I2(t)＝I02(t)·τ(υ2(t)) (3)
Wherein the method comprises the steps of The method is characterized in that the laser light emitting center frequency, a represents modulation depth, f is sinusoidal modulation frequency, v 2 (t) is an even function of time t, the transmission rate is also an even function of time t, the reference signal is multiplied by the light intensity after absorption, and then the reference signal passes through a low-pass filter, and corresponding harmonic signals are obtained according to different frequency values of the reference signal.
The background subtraction process is shown in fig. 1 and is divided into four steps:
Step one: the background absorption model is created to obtain the light intensity I b2 (t), and four input aspects are involved, including the environmental parameters, the background absorption line parameters, the dynamic concentration value of the background and the laser parameters.
The environmental parameters in the first step are the temperature value and the pressure of the environment, and accurate measurement values are obtained through corresponding sensors;
The background absorption spectrum line parameters in the first step refer to values in an HITRAN database, and 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, so that the influence of all absorption lines in a laser scanning range needs to be considered in order to ensure the accuracy of the finally obtained I b2 (t);
The background concentration value in the first step refers to a gas concentration value which affects a measurement result except for a target component in a measurement environment, the measured absorbance is taken as a target, the Voigt line type is used for carrying out multimodal nonlinear least square fitting, and the influence of light jitter is considered during fitting, namely, a third-order polynomial bias is added in a fitting model, and the expression is as follows:
wherein offset represents the combined effect of optical jitter and computational error, which is a nonlinear function of frequency;
In the first step, the laser parameters comprise a non-absorption light intensity background I 02 (t) and a frequency time response upsilon 2 (t) of a modulation working mode, the non-absorption light intensity signal is directly obtained by using a segmentation L-M fitting, and the frequency time response is accurately obtained by an automatic peak searching algorithm.
Step two: the signal demodulation is realized by a software demodulation mode, and is easier to operate than hardware demodulation, after the parameters of the filter are fixed, the background absorption light intensity I b2 (t) and the measurement absorption light intensity I 2 (t) are subjected to the same treatment, so that two groups of background harmonic signals with double channels are obtainedAnd/>Two sets of dual channel measurement harmonic signals X 1f(t)、Y1f (t) and X 2f(t)、Y2f (t).
Step three: obtaining a normalized 2 nd harmonic signal S 2f/1f-bgsub with a background subtracted, the subtraction method being as described in formula (4) in the claims, wherein
Step four: obtaining the target molecule concentration χ 2, and directly obtaining the monotonic relation between the harmonic signal S 2f/1f-bgsub and the concentration in the third step.
The invention has the following advantages:
(1) According to the method for extracting effective absorption information in the complex background environment, disclosed by the invention, the dynamic background gas concentration information is obtained by adding the reference light path, so that the problem that the dynamic absorption background target molecule weak absorption information cannot be extracted accurately is solved; the method for accurately constructing the absorption model based on the modulation spectrum technology is adopted, an accurate dynamic absorption background is created, the modulation technology transfers the target absorption to a high-frequency part, the low-frequency noise is restrained, the signal to noise ratio is improved, the concentration value is inverted by utilizing the harmonic signal peak value of the target molecular absorption, and the problem of accurately measuring the weak absorption component in the environment of the strong absorption background is effectively solved.
(2) The invention discloses a method for extracting effective absorption information in a complex background environment, which inhibits the influence of the phase value of a reference signal by modulating an absorption light intensity dual-channel demodulation mode; the 1-order harmonic is adopted to carry out 2-order harmonic normalization processing, the influence of gain and light-emitting jitter is eliminated, common mode noise and non-resonance transmission loss of a light source and a detector are restrained, and the measurement accuracy is further improved.
(3) The method for extracting effective absorption information in the complex background environment disclosed by the invention can be used in the fields of complex industrial process control and special measurement, and can widen the application range of spectrum measurement.
Drawings
FIG. 1 is a flow chart of a method for extracting effective absorption information for a complex background environment;
FIG. 2 is an absorbance in the range 26349.5cm -1～6352.5cm-1 for an example method for extracting effective absorbance information for a complex background environment;
FIG. 3 is a direct absorption intensity signal for background concentration measurement of example 2 of a method for extracting effective absorption information for a complex background environment;
FIG. 4 is a graph of the integrated absorbance of the methane absorption fit of example 2 of a method for extracting effective absorption information for a complex background environment
FIG. 5 is a method for extracting effective absorption information for a complex background environment example 2 includes a modulated absorption intensity signal for an absorption background;
FIG. 6 is a normalized 2 nd harmonic signal of the background subtracted from example 2 of a method for extracting effective absorption information in a complex background environment;
Fig. 7 is a linear relationship between peak value and concentration of harmonic signals of method example 2 for extracting effective absorption information in a complex background environment.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all 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, characterized by: the method comprises the following steps:
S1, obtaining a light intensity signal containing absorption information: the method comprises the steps that after continuous scanning is carried out by a first laser and a second laser, light intensity I 1 (t) after absorption of a background medium and light intensity I 2 (t) after absorption of a target medium are obtained, background gas integral absorbance A 1 is obtained through polynomial fitting and integral in a frequency domain, and then background gas concentration χ 1 is obtained through calculation;
The scanning method of the first laser and the second laser comprises the following steps: the first laser and the second laser work simultaneously, the first laser is used for directly scanning to obtain background information, the second laser is used for modulating scanning penetrating through a measuring medium, the output light of the first laser and the output light of the second laser are divided into three paths of output light with equal proportion after being coupled to be emitted, and the first path of output light obtains a first laser light emitting frequency time response v 1 (t) and a second laser light emitting frequency time response v 2 (t) through an etalon; the second path of output light passes through a background medium with an optical path of l 1 to obtain light intensity I 1 (t) after the background medium absorbs; the third path of output light passes through a target absorption medium with an optical path length of l 2 to obtain absorbed light intensity I 2 (t) containing the target medium;
The calculation method of the background gas concentration χ 1 comprises the following steps: a 1＝Pχ1l1S1 (T);
Wherein P is the pressure of the absorption medium, l 1 is the optical path of the absorption medium, S 1 (T) is the strong absorption line, and the background gas integral absorbance A 1 is obtained by converting the light intensity transmittance into absorbance alpha 1 and then integrating in the frequency domain;
The light intensity transmittance τ (v) is obtained according to the following formula:
Wherein τ is light intensity transmittance, v is output frequency of the laser, v and wavelength are in reciprocal relation, I 0 is input light intensity, I t is output light intensity, S j is absorption line intensity of j transition of single gas molecule, T is temperature of the absorption medium, χ is concentration of the absorption medium, l is optical path of the absorption medium, and phi j is linear function of j transition of single gas molecule;
the linear function phi j is a Voigt line, which is the convolution of a Lorentzian line and a Gaussian line:
Wherein phi D is Gaussian line type, phi C is Lorentz line type, and the total frequency domain integral of the line type function phi j is 1 for single transition of gas molecules;
the absorption line strength S (T) is:
Wherein v 0 is the transition center frequency, T 0 is the reference temperature, the reference temperature is 256K, c is the light velocity, 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 partitioning function, Q is only related to the temperature, and the partitioning function Q is obtained by interpolation of discrete points in a standard database;
The method for obtaining the background gas integral absorbance A 1 comprises the following steps: performing multimodal nonlinear least square fitting on the absorbance of the background gas by utilizing 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 center frequency, gaussian broadening and pressure broadening, and the four fitting parameters are added every time one absorption line is added;
The post-absorption intensity I 2 (t) comprising the target medium is:
I2(t)＝I02(t)·τ(υ2(t))；
Wherein I 02 (t) is a non-absorption light intensity signal, I 02 (t) is obtained by fitting with a segment L-M, τ is light intensity transmittance, The method is characterized in that the laser light emitting center frequency, a represents the modulation depth, f is sinusoidal modulation frequency, v 2 (t) and τ (v 2 (t)) are even functions of time t, and the light intensity I 2 (t) after absorption of a target medium 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 b2 (t) through a molecular absorption model according to an environment input parameter and a background gas dynamic concentration value;
parameters of the molecular absorption model include: spectral line parameters, flow field parameters of a measuring medium and laser parameters;
The spectral line parameters are obtained from an HITRAN database, and when the spectral line parameters are the spectral line parameters of a single absorption line, the spectral line parameters comprise an 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;
Measuring flow field parameters of a medium, wherein the flow field parameters comprise temperature, background gas dynamic concentration value and pressure value in a measuring environment, and the background gas dynamic concentration value is obtained in real time according to the step S1;
S3, absorbing harmonic signals by a reduction target: the method comprises the steps that absorbed light intensity I 2 (t) containing a target medium and a background absorbed light intensity signal I b2 (t) are subjected to phase locking demodulation and low-pass filtering simultaneously, and then a multichannel background subtraction method is utilized for reduction to obtain a target absorbed harmonic signal S 2f/1f-bgsub;
the method comprises the steps of transferring absorption information in an absorbed light intensity I 2 (t) and a background absorbed light intensity signal I b2 (t) of a target medium to a high-frequency part, realizing by overlapping a scanning light source of a measuring light path with a high-frequency sinusoidal modulation mode, and obtaining harmonic signals of each order by a double-channel demodulation mode.
The absorbed light intensity I 2 (t) containing the target medium and the background absorbed light intensity signal I b2 (t) are simultaneously demodulated by the same phase lock and filtered by low pass to respectively obtain the background harmonic signals of the double channelsAnd/>Dual channel measurement of harmonic signals X 1f(t)、Y1f (t) and X 2f(t)、Y2f (t), wherein/>For one measurement of the background signal of the X channel,/>For one measurement of the background signal of the Y channel,/>For the secondary measurement of the background signal of the X channel,/>As a secondary measurement background signal of the Y channel, X 1f (t) is a primary measurement harmonic signal of the X channel, Y 1f (t) is a primary measurement harmonic signal of the Y channel, X 2f (t) is a secondary measurement harmonic signal of the X channel, and Y 2f (t) is a secondary measurement harmonic signal of the Y channel;
Normalizing the second harmonic to obtain a target absorption harmonic signal S 2f/1f-bgsub:
Wherein R 1f is a two-channel first-time measurement harmonic signal, Is a background signal of two channels;
Where n=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.
The natural gas contains various gaseous components besides methane, wherein hydrogen sulfide is an acidic fatal toxic gas, which not only endangers the health of operators, but also has strong corrosion effect on gas pipelines when contacted with water vapor. The hydrogen sulfide measuring method based on the optical principle can realize lossless and non-contact on-line monitoring, but compared with the hydrogen sulfide measurement in the conventional environment, the high-concentration methane background absorption completely submerges the absorption information of the hydrogen sulfide, and the background gas is in a continuously-changing state. Based on the above, a method for extracting effective absorption information in a complex background environment is provided, weak absorption signals of hydrogen sulfide are accurately extracted, accurate measurement of concentration is realized, the environment temperature is set to be 296K, the pressure is 1atm, the methane concentration is 95%, the hydrogen sulfide content is 1 ppm-10 ppm, and the balance is nitrogen.
Through analysis of the absorption line, a line with an absorption center of 6351.00468cm -1 was selected for concentration measurement of hydrogen sulfide, but there was a plurality of methane absorption near the absorption line, and the methane concentration in natural gas exceeded 90%. The absorbance of methane and hydrogen sulfide in the range of 6349.5cm -1～6352.5cm-1 is shown in fig. 2, the absorption of methane occupies the dominant position of total absorption, the absorption of hydrogen sulfide is weak, and in order to realize the accurate extraction of weak absorption signals, absorption information is transferred to a high-frequency part, so that the laser is in a modulation working mode in the wave band.
According to the steps of the invention, firstly, the concentration of methane background gas is determined, the spectral information is obtained by utilizing a direct scanning working mode, and the absorption line with the center frequency of 6047.95cm -1 is selected as a research object, on the other hand, the absorption of hydrogen sulfide in the wave band is negligible, and the method can be used for accurately measuring methane. The light intensity signal after absorption is shown in fig. 3, the light intensity of the two sections of non-absorption parts is selected to perform polynomial fitting to obtain background light intensity, the L-M fitting is performed on the absorbance in the frequency domain by combining the frequency domain information of the wave band, the integral absorbance is obtained as shown in fig. 4, the accurate methane concentration is obtained through calculation, and the methane concentration obtained through calculation is 94.99%, so that accurate measurement is realized.
After the concentration of methane is obtained, the ambient temperature and the pressure are taken as the ambient input parameters at the same time, the non-absorption light intensity signal of the modulation signal is obtained by analogy with the direct absorption signal obtaining mode, and 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 any more, but a polynomial high-frequency sinusoidal modulation model, the obtaining of the frequency domain signal is more complex than the direct absorption, the light intensity signal of the methane absorption background is finally obtained by using the same model fitting of non-absorption light intensity through the reordering of interference peaks, as shown in figure 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, 2-order background harmonic signal and a measured signal, and the background-subtracted target absorption 2-order harmonic signal S 2f/1f-bgsub normalized by the 1-order harmonic is obtained through calculation of a formula (4) in the claims, 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 monotonic positive correlation, and the accurate measurement of the concentration of trace hydrogen sulfide in the high-concentration methane background environment is realized through the accurate calibration of the concentration and peak value curve, and the curve is shown in fig. 7. Performing first-order polynomial fitting on the curve to obtain the change relation of the peak value along with the concentration as follows: peak= 1.731 e-4.χ -1.632e-7 accords with the linear transmission rule of the Peak value and the concentration of the harmonic signal under the weak absorption condition, and realizes the accurate extraction of the effective absorption information of the complex background environment, thereby acquiring the accurate gas concentration information.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical scheme of the present invention and the inventive concept thereof, and should be covered by the scope of the present invention.
Claims (10)
1. A method for extracting effective absorption information in a complex background environment, characterized by: the method comprises the following steps:
S1, obtaining a light intensity signal containing absorption information: the method comprises the steps that after continuous scanning is carried out by a first laser and a second laser, light intensity I 1 (t) after absorption of a background medium and light intensity I 2 (t) after absorption of a target medium are obtained, background gas integral absorbance A 1 is obtained through polynomial fitting and integral in a frequency domain, and then background gas concentration χ 1 is obtained through calculation;
s2, obtaining a background absorption light intensity signal: obtaining a background absorption light intensity signal I b2 (t) through a molecular absorption model according to an environment input parameter and a background gas dynamic concentration value;
S3, absorbing harmonic signals by a reduction target: the absorbed light intensity I 2 (t) containing the target medium and the background absorbed light intensity signal I b2 (t) are simultaneously demodulated by the same phase lock and filtered by low pass, and then restored by a multichannel background subtraction method to obtain a target absorbed harmonic signal S 2f/1f-bgsub;
S4, extracting harmonic signal peak values of target molecules: and extracting the peak value of the target absorption harmonic signal S 2f/1f-bgsub, and obtaining the target molecular concentration χ 2 according to a monotonic curve of the peak value and the concentration, thereby completing the effective absorption information extraction.
2. A method for extracting effective absorption information in a complex background environment as defined in claim 1, wherein: in step S1, the scanning method of the first laser and the second laser is as follows: the first laser and the second laser work simultaneously, the first laser is used for directly scanning to obtain background information, the second laser is used for modulating scanning penetrating through a measuring medium, the output light of the first laser and the output light of the second laser are divided into three paths of output light with equal proportion after being coupled to be emitted, and the first path of output light is used for obtaining a first laser light-emitting frequency time response v 1 (t) and a second laser light-emitting frequency time response v 2 (t) through an etalon; the second path of output light passes through a background medium with an optical path of l 1 to obtain light intensity I 1 (t) after the background medium absorbs the light; the third path of output light passes through a target absorption medium with an optical path length of l 2 to obtain the light intensity I 2 (t) after absorption of the target medium.
3. A method for extracting effective absorption information in a complex background environment as defined in claim 2, wherein: the calculation method of the background gas concentration χ 1 comprises the following steps:
A1＝Pχ1l1S1(T)；
Wherein P is the pressure of the absorption medium, l 1 is the optical path of the absorption medium, S 1 (T) is the absorption line intensity, and the integrated absorbance A 1 of the background gas is obtained by converting the light intensity transmittance of the background gas into absorbance and then integrating in a frequency domain.
4. A method for extracting effective absorption information in a complex background environment according to claim 3, wherein:
The light intensity transmittance τ (v) is obtained according to the following formula:
where τ is the light intensity transmittance, v is the output frequency of the laser, v and wavelength are in inverse relationship, I 0 is the input light intensity, I t is the output light intensity, S j is the absorption line intensity of the 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, and Φ j is the linear function of the j transition of the single gas molecule.
5. The method for extracting effective absorption information in a complex background environment of claim 4, wherein:
The linear function phi j is a Voigt line type, which is a convolution of a lorentz line type and a gaussian line type:
Wherein phi D is a gaussian line type, phi C is a lorentz line type, and the full frequency domain integral of the line type function phi j is 1 for single transition of gas molecules;
the absorption line strength S (T) is:
wherein v 0 is the transition center frequency, T 0 is the reference temperature, the reference temperature is 256K, c is the light velocity, 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 partitioning function, Q is only related to the temperature, and the partitioning function Q is obtained by discrete point interpolation in a standard database.
6. A method for extracting effective absorption information in a complex background environment according to claim 3, wherein: the method for obtaining the background gas integral absorbance A 1 comprises the following steps: and when the background gas absorbance is subjected to multimodal nonlinear least square fitting by utilizing Voigt line type, the number of absorption peaks is automatically obtained by a peak searching algorithm, the optimal input parameters are obtained by setting absorption peak threshold values and intervals, each absorption comprises four fitting parameters, the fitting parameters are integral absorbance, absorption line center frequency, gaussian broadening and pressure broadening, and four fitting parameters are added every time one absorption line is added.
7. A method for extracting effective absorption information in a complex background environment as defined in claim 1, wherein: in step S1, the light intensity I 2 (t) after absorption of the target medium is:
I2(t)＝I02(t)·τ(υ2(t))；
Wherein I 02 (t) is a non-absorption light intensity signal, I 02 (t) is obtained by fitting with a segment L-M, τ is light intensity transmittance, The laser light emitting center frequency is represented by a modulation depth, f is a sinusoidal modulation frequency, v 2 (t) and τ (v 2 (t)) are even functions of time t, and the light intensity I 2 (t) after absorption of the target medium comprises background absorption accumulation and target molecule absorption accumulation.
8. A method for extracting effective absorption information in a complex background environment as defined in claim 1, wherein: in step S2, the parameters of the molecular absorption model include: spectral line parameters, flow field parameters of a measuring medium and laser parameters;
The spectral line parameters are obtained from an HITRAN database, and when the spectral line parameters are the spectral line parameters of a single absorption line, the spectral line parameters comprise absorption center frequency, line intensity at a reference temperature, air broadening coefficient, self broadening coefficient, air broadening temperature dependence coefficient and pressure frequency shift coefficient;
the flow field parameters of the measuring medium comprise the temperature in the measuring environment, the dynamic concentration value of the background gas and the pressure value, and the dynamic concentration value of the background gas 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 uses zero absorption background intensity signals as input parameters, and the light-emitting frequency is obtained by combining an intelligent processing algorithm through an etalon.
9. A method for extracting effective absorption information in a complex background environment as defined in claim 1, wherein: in step S3, the absorption information in the absorbed light intensity I 2 (t) containing the target medium and the background absorbed light intensity signal I b2 (t) is transferred to a high-frequency part, and is realized by overlapping a high-frequency sinusoidal modulation mode with a scanning light source of a measuring light path, and then each order harmonic signal is obtained by a dual-channel demodulation mode.
10. A method for extracting effective absorption information in a complex background environment as defined in claim 1, wherein: in step S3, the absorbed light intensity I 2 (t) containing the target medium and the background absorbed light intensity signal I b2 (t) are demodulated and filtered by the same phase lock at the same time to obtain two-channel background harmonic signals respectivelyAndTwo-channel measurement of harmonic signals X 1f(t)、Y1f (t) and X 2f(t)、Y2f (t), where/>For one measurement of the background signal of the X channel,/>For one measurement of the background signal of the Y channel,/>For the secondary measurement of the background signal of the X channel,/>As a secondary measurement background signal of the Y channel, X 1f (t) is a primary measurement harmonic signal of the X channel, Y 1f (t) is a primary measurement harmonic signal of the Y channel, X 2f (t) is a secondary measurement harmonic signal of the X channel, and Y 2f (t) is a secondary measurement harmonic signal of the Y channel;
Normalizing the second harmonic to obtain the target absorption harmonic signal S 2f/1f-bgsub:
Wherein R 1f is a two-channel first-time measurement harmonic signal, Is a background signal of two channels;
Where n=1 or 2.
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