CN111829980A - Linear nonlinear correction detection system and method based on harmonic technology - Google Patents

Linear nonlinear correction detection system and method based on harmonic technology Download PDF

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CN111829980A
CN111829980A CN202010716706.3A CN202010716706A CN111829980A CN 111829980 A CN111829980 A CN 111829980A CN 202010716706 A CN202010716706 A CN 202010716706A CN 111829980 A CN111829980 A CN 111829980A
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CN111829980B (en
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唐七星
张玉钧
陈东
刘路
廖娟
王玉伟
何莹
郭楠
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Anhui Agricultural University AHAU
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Abstract

The invention discloses a detection system and a method for linear nonlinear correction based on harmonic technology, the system comprises a tunable semiconductor laser, a laser control module, a signal generator, a reference light path, a detection light path, a phase-sensitive detection circuit, a data processor, a display module and the like, and through background noise elimination and zero line offset correction, a standard signal corresponding to second harmonic is obtained by utilizing Hitran database fitting, and then self-adaptive iterative fitting is carried out according to an optimal criterion, so that the asymmetry of the signal is minimized, and the signal after linear asymmetry correction is obtained. The invention can effectively reduce the influence of zero line offset noise fluctuation, line type asymmetry and the like, and improve the accuracy of gas measurement.

Description

Linear nonlinear correction detection system and method based on harmonic technology
Technical Field
The invention relates to the field of environmental optics, in particular to a linear nonlinear correction detection system and method based on a harmonic technology, which are provided for nonlinear influence of a second harmonic signal.
Background
The laser spectrum technology utilizes the fingerprint characteristic of molecular spectrum to carry out quantitative analysis on the gas concentration, and is widely applied to gas detection. The wavelength modulation spectrum technology adopts a high-frequency detection signal, so that background noise can be effectively inhibited, and the detection sensitivity is improved. However, when the open space gas detection is performed by using the wavelength-modulated laser spectrum technology, the laser light intensity is inevitably interfered by the intensity modulation factor, the residual amplitude modulation of the laser and the uncontrollable environmental noise, so that the second harmonic spectrum signal is deformed and shifted, the nonlinearity of the spectrum line is enhanced, and the measurement accuracy is affected. Therefore, the nonlinear influence of spectral lines is effectively reduced, and the measurement accuracy is improved, so that the method has important research significance.
The existing research mainly solves the problem of light intensity modulation of second harmonic from the perspective of reducing RAM, but aims at the fact that the influence of nonlinear error in the actual measurement process is complex and various, and only by reducing the perspective of RAM, other influence factors still exist in a certain sense, and the influence of nonlinearity on a spectrum signal is not fundamentally solved.
Disclosure of Invention
In view of the defects of the prior art, the invention provides a linear nonlinear correction detection system and method based on a harmonic technology, and aims to solve the problem that the nonlinearity of a spectral line is enhanced and the measurement accuracy is influenced due to the deformation, the offset and the like of a second harmonic spectral signal in harmonic detection, so that the nonlinearity of the spectral line is eliminated and the measurement accuracy is improved.
The technical scheme for solving the problems is as follows:
the invention relates to a detection system for linear nonlinear correction based on harmonic technology, which is characterized by comprising the following steps: the device comprises a laser, a control unit, an optical unit and a data processing unit;
the laser and control unit consists of a tunable semiconductor laser, a laser control module and a signal generator;
the laser control module controls the temperature and the current of the tunable semiconductor laser to enable the tunable semiconductor laser to output wavelength near a target absorption spectral line;
the sawtooth scanning signal and the high-frequency sinusoidal signal generated by the signal generator are superposed on the tunable semiconductor laser, so that the output wavelength of the tunable semiconductor laser is scanned and modulated, and a modulated light beam is obtained;
the optical unit consists of a transceiver telescope, a reference light path and a detection light path;
the modulation light beam is divided into reference light and detection light, and the reference light path receives the reference light, converts the reference light into a reference electric signal and transmits the reference electric signal to the data processing unit;
the detection light path receives the detection light, enters the open space through the transceiver telescope, telemeters the atmosphere, converts the atmosphere into a detection electric signal and transmits the detection electric signal to the data processing unit;
the data processing unit consists of a phase-sensitive detection circuit, a data processor and a display module;
the phase-sensitive detection circuit performs harmonic signal detection on the reference electric signal in the ith measurement period to obtain a second harmonic spectrum signal of the reference light path in the ith measurement period or a second harmonic background signal of the reference light path without absorption in the ith measurement period;
the phase-sensitive detection circuit performs harmonic signal detection on the detection electric signal in the ith measurement period to obtain a second harmonic spectrum signal of the detection optical path in the ith measurement period;
and carrying out nonlinear correction on the second harmonic spectrum signal or the second harmonic background signal of the reference light path and the second harmonic spectrum signal of the detection light path in the ith measurement period by the data processor, and then carrying out online concentration inversion, so as to obtain an inversion result in the ith measurement period and sending the inversion result to the display module for display.
The invention relates to a detection method of linear nonlinear correction based on harmonic technology, which is characterized by comprising the following steps:
step 1, acquiring a second harmonic spectrum signal D under the ith measurement periodi(n) second harmonic background signal B without absorption with reference light path in ith measurement periodi(n) and correcting the background noise by using the formula (1) to obtain a spectrum signal D after correcting the background noise in the ith measurement periodi’(n);
D′i(n)=Di(n)-Bi(n)n=0,1,2...N (1)
In the formula (1), i represents a corresponding measurement period, N is a signal corresponding sequence position, and N is a maximum value obtained by the N;
step 2, selecting a spectral signal D under the i-1 th measurement periodi-1' (n) m signals of the side lobe non-absorption region on the right side to the spectral signal D at the ith measurement periodi' (n) m signals of left side lobe non-absorption region, and spectral signal D at i-th measurement periodi' (n) m signals of the side lobe non-absorption region on the right side to the spectral signal D after background noise correction in the (i + 1) th measurement periodi+1' (n) m signals of the unabsorbed region of the left side lobe; and m is more than or equal to 0 and less than or equal to N;
polynomial fitting is carried out on the selected non-absorption area signals by using the formula (2) to obtain zero line offset noise signals Z under the ith measurement periodi(n’);
Zi(n')=a0+a1n'+a2n'2n'=0,1,2...N+2m (2)
In the formula (2), a0,a1,a2Is a fitting coefficient, and n' is a sequence position corresponding to the fitting signal;
and 3. step 3.Zero line offset noise signal Z using equation (3)i(n') intercepting to obtain the intercepted zero line offset noise signal Zi’(n’):
Figure BDA0002598458760000031
In the formula (3), in the formula (1), pi-1、pi、pi+1Respectively are unequal precision weights, n 'of the ith-1 measurement period, the ith measurement period and the (i + 1) th measurement period'maxIs the maximum value obtained by n ', and n'max=N+2m;
Step 4, the spectrum signal D after the background noise is corrected by using the formula (4)i' (n) correcting to obtain a detection spectrum signal D after correcting zero line offset noisei”(n);
D″i(n)=D′i(n)-Z′i(n+m)n=0,1,2…N (4)
Acquiring a second harmonic spectrum signal R under the ith measurement periodi(n) according to the method of the steps 1 to 4, carrying out measurement on the second harmonic spectrum signal R under the ith measurement period of the reference light pathi(n) obtaining a reference spectrum signal R after zero line offset noise correctioni”(n);
Step 5, acquiring relevant parameters from a Hitran database to be used for solving a second order differential of the Gaussian line type, and substituting the obtained differential result into an expression of a second harmonic component so as to obtain a standard signal of a second harmonic;
the optimal criterion for the adaptive fit is set using equation (5):
Figure BDA0002598458760000032
in the formula (5), IminlRepresenting the left trough intensity, I, of the second harmonic lineminrRepresenting the intensity of the right trough of the second harmonic line, IminmRepresents the average of the intensities at the two troughs of the second harmonic line,
Figure BDA0002598458760000033
representing the theoretical left and right side lobe widths, Λ, respectivelyl、ΛrThe left side lobe width and the right side lobe width of a second harmonic spectral line are represented respectively, and xi is a peak-valley asymmetric threshold and a side lobe threshold respectively;
according to the optimal criterion, the reference spectrum signal R after the zero line offset noise is corrected by using the standard signali"(n) and the detection spectral signal Di"(n) performing adaptive iterative fitting to obtain a reference signal XR after linear asymmetry correction in the ith measurement periodi"(n) with the detection signal XDi”(n);
Step 6, according to the known reference light path concentration, the reference signal XR is correctedi"(n) with the detection signal XDiAnd (n) inverting the concentration of the gas to be measured, thereby obtaining an inversion result in the ith measurement period.
Compared with the prior art, the invention has the beneficial effects that:
1. according to the invention, through a 'black box' mode, on the basis of not changing a hardware structure system and not increasing the complexity of hardware, a linear nonlinear correction method is provided for processing, so that nonlinear influence is eliminated, and the measurement accuracy is improved.
2. The invention eliminates the spectrum signal D after the background noise correction by correcting the zero line offset and processing according to unequal precision and weighti' (n) zero line offset noise reduces the offset effect of the second harmonic.
3. According to the invention, the optimal criterion is set, and self-adaptive iterative fitting is carried out, so that the asymmetry of the signal is minimized, and the nonlinear influence is eliminated.
Drawings
FIG. 1 is a schematic diagram of a linear nonlinear correction detection system based on harmonic techniques of the present invention;
FIG. 2 is a flow chart of a non-linear modified detection method of the present invention;
FIG. 3 is a diagram of a standard second harmonic in the prior art;
FIG. 4 is a verification chart before and after correction of the method of the present invention;
reference numbers in the figures: 1. a tunable semiconductor laser; 2. a laser control module; 3. a signal generator; 4. a beam splitter; 5. a collimator of the reference light path; 6. a standard gas reference cell; 7. an InGaAs photodetector in the reference optical path; 8. a collimator that detects the light path; 9. a transceiver telescope; 10. a corner mirror; 11. an InGaAs photodetector for detecting the optical path; 12. a phase sensitive detection circuit; 13. a data processor; 14. and a display module.
Detailed Description
In this embodiment, referring to fig. 1, a linear nonlinear correction detection system based on a harmonic technique includes a laser, a control unit, an optical unit, and a data processing unit;
the laser and control unit consists of a tunable semiconductor laser 1, a laser control module 2 and a signal generator 3; the optical unit consists of a transceiver telescope 9, a reference light path and a detection light path; the data processing unit consists of a phase-sensitive detection circuit 12, a data processor 13 and a display module 14;
the tunable semiconductor laser 1 is used as a detection light source, and the temperature and the current are changed through the laser control module 2, so that the output wavelength of the tunable semiconductor laser 1 is changed; enabling the tunable semiconductor laser 1 to output wavelength near a target absorption spectral line;
the tunable semiconductor laser 1 scans and modulates the output wavelength under the combined action of a sawtooth scanning signal generated by the signal generator 3 and a high-frequency sinusoidal signal, so as to obtain a modulated light beam; the modulated light beam is divided into reference light and detection light by a beam splitter 4, the reference light is collimated by a collimator 5 of a reference light path, and then is subjected to photoelectric conversion by an InGaAs photoelectric detector 7 of the reference light path through a standard gas reference cell 6 to obtain a reference electric signal; the detection light is collimated by a collimator 8 of a detection light path, then is emitted by a transceiver telescope 9, is telemetered to atmosphere through an open space, returns by a corner reflector 10, and is subjected to photoelectric conversion by an InGaAs photoelectric detector 11 of the detection light path to obtain a detection electric signal; both signals are sent to a data processing unit, and harmonic signal detection is carried out by a phase sensitive detection circuit 12;
the phase-sensitive detection circuit performs harmonic signal detection on the reference electric signal in the ith measurement period to obtain a second harmonic spectrum signal of the reference light path in the ith measurement period or a second harmonic background signal of the reference light path without absorption in the ith measurement period; the phase-sensitive detection circuit performs harmonic signal detection on the detection electric signal in the ith measurement period to obtain a second harmonic spectrum signal of the detection optical path in the ith measurement period;
the data processor 13 performs nonlinear correction on the second harmonic spectrum signal or the second harmonic background signal of the reference light path in the ith measurement period and the second harmonic spectrum signal of the detection light path in the ith measurement period, and then performs online concentration inversion, so as to obtain an inversion result in the ith measurement period and send the inversion result to the display module 14 for display.
Referring to fig. 2, in this embodiment, a detection method for linear nonlinear correction based on a harmonic technique is performed according to the following steps:
step 1, acquiring a second harmonic spectrum signal D under the ith measurement periodi(n) second harmonic background signal B without absorption with reference light path in ith measurement periodi(n) and correcting the background noise by using the formula (1) to obtain a spectrum signal D after correcting the background noise in the ith measurement periodi’(n);
D′i(n)=Di(n)-Bi(n)n=0,1,2...N (1)
In the formula (1), i represents a corresponding measurement period, N is a signal corresponding sequence position, and N is a maximum value obtained by the N;
step 2, selecting a spectral signal D under the i-1 th measurement periodi-1' (n) m signals of the side lobe non-absorption region on the right side to the spectral signal D at the ith measurement periodi' (n) m signals of left side lobe non-absorption region, and spectral signal D at i-th measurement periodi' (n) m signals of the side lobe non-absorption region on the right side to the spectral signal D after background noise correction in the (i + 1) th measurement periodi+1' (n) m signals of the unabsorbed region of the left side lobe; and is not more than 0m≤N;
Polynomial fitting is carried out on the selected non-absorption area signals by using the formula (2) to obtain zero line offset noise signals Z under the ith measurement periodi(n’);
Zi(n')=a0+a1n'+a2n'2n'=0,1,2...N+2m (2)
In the formula (2), a0,a1,a2Is a fitting coefficient, and n' is a sequence position corresponding to the fitting signal;
step 3, zero line offset noise signal Z is obtained by using formula (3)i(n') intercepting to obtain the intercepted zero line offset noise signal Zi’(n’):
Figure BDA0002598458760000051
In the formula (3), in the formula (1), pi-1、pi、pi+1Respectively are unequal precision weights, n 'of the ith-1 measurement period, the ith measurement period and the (i + 1) th measurement period'maxIs the maximum value obtained by n ', and n'max=N+2m;
Step 4, the spectrum signal D after the background noise is corrected by using the formula (4)i' (n) correcting to obtain a detection spectrum signal D after correcting zero line offset noisei”(n);
D″i(n)=D′i(n)-Z′i(n+m)n=0,1,2…N (4)
Acquiring a second harmonic spectrum signal R under the ith measurement periodi(n), correcting zero line offset noise according to the method of the steps 1 to 4 to obtain a reference spectrum signal R after correcting the zero line offset noisei”(n);
Step 5, acquiring relevant parameters from a Hitran database to be used for solving a second order differential of the Gaussian line type, and substituting the obtained differential result into an expression of a second harmonic component, so as to obtain a standard signal of a second harmonic, wherein the expression is shown in figure 3;
the optimal criterion for the adaptive fit is set using equation (5):
Figure BDA0002598458760000061
in the formula (5), IminlRepresenting the left trough intensity, I, of the second harmonic lineminrRepresenting the intensity of the right trough of the second harmonic line, IminmRepresents the average of the intensities at the two troughs of the second harmonic line,
Figure BDA0002598458760000062
representing the theoretical left and right side lobe widths, Λ, respectivelyl、ΛrThe left side lobe width and the right side lobe width of a second harmonic spectral line are represented respectively, and xi is a peak-valley asymmetric threshold and a side lobe threshold respectively;
according to the optimal criterion, the reference spectrum signal R after the zero line offset noise is corrected by using the standard signali"(n) and the detection spectral signal Di"(n) performing adaptive iterative fitting to obtain a reference signal XR after linear asymmetry correction in the ith measurement periodi"(n) with the detection signal XDi”(n);
Step 6, according to the known reference light path concentration, the reference signal XR is correctedi"(n) with the detection signal XDiAnd (n) inverting the concentration of the gas to be measured, thereby obtaining an inversion result in the ith measurement period.
In order to verify the effect of the method, the established atmosphere detection system for harmonic detection is used for carrying out experiments, the scanning frequency is set to be 100Hz, the modulation frequency is 50kHz, simulation verification experiments are carried out, a 20m multiple reflection cell is placed on a detection light path in the experiments, and methane with the fixed concentration of 27ppm is measured. The result graphs before and after correction by using the method of the invention are shown in figure 4, the zero line offset detail after correction is inhibited, and the line type asymmetry is eliminated. Therefore, the method provided by the invention can effectively reduce the cost requirement, simultaneously can eliminate the nonlinear influence in harmonic detection, and improves the accuracy of measurement.

Claims (2)

1. A linear nonlinear correction detection system based on harmonic technology, comprising: the device comprises a laser, a control unit, an optical unit and a data processing unit;
the laser and control unit consists of a tunable semiconductor laser (1), a laser control module (2) and a signal generator (3);
the laser control module (2) controls the temperature and the current of the tunable semiconductor laser (1) so that the tunable semiconductor laser (1) outputs a wavelength near a target absorption spectral line;
the sawtooth scanning signal and the high-frequency sinusoidal signal generated by the signal generator (3) are superposed on the tunable semiconductor laser (1), so that the output wavelength of the tunable semiconductor laser (1) is scanned and modulated, and a modulated light beam is obtained;
the optical unit consists of a transceiver telescope (9), a reference light path and a detection light path;
the modulation light beam is divided into reference light and detection light, and the reference light path receives the reference light, converts the reference light into a reference electric signal and transmits the reference electric signal to the data processing unit;
the detection light path receives the detection light, enters open space through a transceiving telescope (9) for telemetering atmosphere, converts the detection light into a detection electric signal and transmits the detection electric signal to the data processing unit;
the data processing unit consists of a phase-sensitive detection circuit, a data processor and a display module;
the phase-sensitive detection circuit performs harmonic signal detection on the reference electric signal in the ith measurement period to obtain a second harmonic spectrum signal of the reference light path in the ith measurement period or a second harmonic background signal of the reference light path without absorption in the ith measurement period;
the phase-sensitive detection circuit performs harmonic signal detection on the detection electric signal in the ith measurement period to obtain a second harmonic spectrum signal of the detection optical path in the ith measurement period;
and carrying out nonlinear correction on the second harmonic spectrum signal or the second harmonic background signal of the reference light path and the second harmonic spectrum signal of the detection light path in the ith measurement period by the data processor, and then carrying out online concentration inversion, so as to obtain an inversion result in the ith measurement period and sending the inversion result to the display module for display.
2. A detection method of linear nonlinear correction based on harmonic technology is characterized by comprising the following steps:
step 1, acquiring a second harmonic spectrum signal D under the ith measurement periodi(n) second harmonic background signal B without absorption with reference light path in ith measurement periodi(n) and correcting the background noise by using the formula (1) to obtain a spectrum signal D after correcting the background noise in the ith measurement periodi’(n);
Di′(n)=Di(n)-Bi(n) n=0,1,2...N (1)
In the formula (1), i represents a corresponding measurement period, N is a signal corresponding sequence position, and N is a maximum value obtained by the N;
step 2, selecting a spectral signal D under the i-1 th measurement periodi-1' (n) m signals of the side lobe non-absorption region on the right side to the spectral signal D at the ith measurement periodi' (n) m signals of left side lobe non-absorption region, and spectral signal D at i-th measurement periodi' (n) m signals of the side lobe non-absorption region on the right side to the spectral signal D after background noise correction in the (i + 1) th measurement periodi+1' (n) m signals of the unabsorbed region of the left side lobe; and m is more than or equal to 0 and less than or equal to N;
polynomial fitting is carried out on the selected non-absorption area signals by using the formula (2) to obtain zero line offset noise signals Z under the ith measurement periodi(n’);
Zi(n')=a0+a1n'+a2n'2n'=0,1,2...N+2m (2)
In the formula (2), a0,a1,a2Is a fitting coefficient, and n' is a sequence position corresponding to the fitting signal;
step 3, zero line offset noise signal is obtained by using formula (3)Zi(n') intercepting to obtain the intercepted zero line offset noise signal Zi’(n’):
Figure FDA0002598458750000021
In the formula (3), in the formula (1), pi-1、pi、pi+1Respectively are unequal precision weights, n 'of the ith-1 measurement period, the ith measurement period and the (i + 1) th measurement period'maxIs the maximum value obtained by n ', and n'max=N+2m;
Step 4, the spectrum signal D after the background noise is corrected by using the formula (4)i' (n) correcting to obtain a detection spectrum signal D after correcting zero line offset noisei”(n);
D″i(n)=D′i(n)-Z′i(n+m) n=0,1,2…N (4)
Acquiring a second harmonic spectrum signal R under the ith measurement periodi(n) according to the method of the steps 1 to 4, carrying out measurement on the second harmonic spectrum signal R under the ith measurement period of the reference light pathi(n) obtaining a reference spectrum signal R after zero line offset noise correctioni”(n);
Step 5, acquiring relevant parameters from a Hitran database to be used for solving a second order differential of the Gaussian line type, and substituting the obtained differential result into an expression of a second harmonic component so as to obtain a standard signal of a second harmonic;
the optimal criterion for the adaptive fit is set using equation (5):
Figure FDA0002598458750000022
in the formula (5), IminlRepresenting the left trough intensity, I, of the second harmonic lineminrRepresenting the intensity of the right trough of the second harmonic line, IminmRepresents the average of the intensities at the two troughs of the second harmonic line,
Figure FDA0002598458750000031
representing the theoretical left and right side lobe widths, Λ, respectivelyl、ΛrThe left side lobe width and the right side lobe width of a second harmonic spectral line are represented respectively, and xi is a peak-valley asymmetric threshold and a side lobe threshold respectively;
according to the optimal criterion, the reference spectrum signal R after the zero line offset noise is corrected by using the standard signali"(n) and the detection spectral signal Di"(n) performing adaptive iterative fitting to obtain a reference signal XR after linear asymmetry correction in the ith measurement periodi"(n) with the detection signal XDi”(n);
Step 6, according to the known reference light path concentration, the reference signal XR is correctedi"(n) with the detection signal XDiAnd (n) inverting the concentration of the gas to be measured, thereby obtaining an inversion result in the ith measurement period.
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