Gas Concentration Detection Method Based on Multi-Harmonic Information Fusion Laser Absorption Spectroscopy Technology

Abstract

A gas concentration detection method based on multi-harmonic information fusion laser absorption spectroscopy technology, which absorbs and utilizes the demodulated second harmonic signal normalized by the fundamental harmonic, the third harmonic signal normalized by the fundamental harmonic, …… the high-order harmonic signal normalized by the fundamental harmonic to obtain gas concentration information, and compares it with the simulation result obtained by the direct absorption spectroscopy technology. If the numerical difference is less than 2%, then the multi-harmonic signal and the simulation result are subjected to least squares, and the final result obtained by the least squares method is the gas concentration. In this method, high-order harmonics are introduced for concentration calculation, effectively improving the measurable accuracy, making the measurement result more accurate, and utilizing the information advantages of multiple harmonic spectral lines in different temperature ranges to significantly broaden the effective sensing range of the absorption spectrum, simply and efficiently improving the measurement accuracy.

Description

Technical Field

[0001] The present invention relates to a method for detecting gas concentration based on multi - harmonic information fusion laser absorption spectroscopy technology, and is specifically applicable to improving anti - interference ability and measurement accuracy. Background Art

[0002] Gas concentration is one of the most important physical and chemical parameters, which is closely related to daily life, safety production and national defense industry. By measuring the concentrations of gases such as water vapor, aldehydes, and benzene, the living environment can be evaluated; by measuring the gas components such as carbon dioxide, oxygen, and nitric oxide in exhaled human breath, the basic metabolism of the human body can be determined; by measuring the methane concentration, early warning can be provided for underground coal mine operations; by measuring the concentrations of multi - component gases, the operating efficiency and emission characteristics of combustion power devices such as power station boilers, industrial furnaces, and aero - engines can be evaluated. Laser absorption spectroscopy technology is a typical non - contact gas detection technology, which is simple to implement, easy to integrate the system, has a fast response speed, high selectivity and sensitivity, and can achieve quantitative measurement. It has been widely used in industrial process detection, environmental monitoring, and high - end instrument manufacturing.

[0003] However, when using laser absorption spectroscopy technology to detect gases in a complex and harsh environment, the stability and reliability of measurement results are easily affected by the external environment, which limits the fidelity of detection results. Although both the primary - harmonic - normalized second - harmonic method and the normalization noise - reduction method of dual - path tunable laser absorption spectroscopy technology can improve the sensitivity of gas detection, since both methods use a single signal to map the measurement results, the improvement of the stability and fidelity of the measurement results is limited. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problem of weak anti - interference ability in the prior art, and provide a gas concentration detection method based on multi - harmonic information fusion laser absorption spectroscopy technology with anti - interference ability.

[0005] To achieve the above purpose, the technical solution of the present invention is as follows:

[0006] A method for detecting gas concentration based on multi - harmonic information fusion laser absorption spectroscopy technology, the detection method is based on the following detection device, including: a laser generating device, an optical path measuring device, and a signal acquisition and analysis device. The laser of the laser generating device is a distributed feedback semiconductor laser, and the center frequency of the laser is equal to the center frequency of the gas to be measured;

[0007] The detection method includes the following steps:

[0008] S1. Using time-division multiplexing technology, injection scanning and modulation currents are applied to the laser. In the upper half cycle, it is a scanning plus modulation signal, and in the lower half cycle, it is a scanning signal. The laser-emitted laser passes through the gas to be measured and is received by the photodetector of the signal acquisition and analysis device. The signal generated by the photodetector is collected by the data acquisition card;

[0009] S2. The signal acquisition and analysis device performs software demodulation and baseline fitting on the absorption signal collected by the data acquisition card in the data post-processing program, respectively obtaining multiple harmonic signals 1f~nf (5≤n≤10) and spectral absorbance DAS data. Then, all the obtained multiple harmonic signals are normalized to obtain the normalized high-order harmonic signals of the first harmonic for experimental measurement.

[0010] S3. The n-1 normalized high-order harmonic spectral signals of the first harmonic and one spectral absorbance obtained in S2 are post-processed, and the component concentration of the target gas is inverted using the spectral fitting optimization algorithm to obtain the concentration values corresponding to the n-1 normalized high-order harmonic spectral signals of the first harmonic;

[0011] S4. According to the current test environment, the signal acquisition and analysis device performs simulation calculations using the original absorption light intensity signal and the absorbance parameters corresponding to the experimental parameters to obtain the normalized high-order harmonic signals of the first harmonic obtained by simulation calculation.

[0012] S5. Using the least squares algorithm to invert and solve the concentration of the target gas to obtain the final gas concentration value.

[0013] In S2, the signal acquisition and analysis device performs software demodulation and baseline fitting on the absorption signal collected by the data acquisition card in the data post-processing program to obtain the first harmonic signal 1f, the second harmonic signal 2f, the third harmonic signal 3f,..., the nth harmonic signal nf (5≤n≤10) and spectral absorbance DAS data;

[0014] The signal acquisition and analysis device normalizes the obtained multiple harmonic signals respectively, and obtains the normalized second harmonic signal of the first harmonic through calculation. The normalized third harmonic signal of the first harmonic ... The normalized nth harmonic signal of the first harmonic

[0015] S3. Post-process the n - 1 normalized higher harmonic spectral signals of the first harmonic and one spectral absorbance obtained in S2, and use the spectral fitting optimization algorithm to invert the component concentration of the target gas, obtaining n - 1 corresponding concentration values Xabs_2f / 1f, Xabs_3f / 1f, …… Xabs_nf / 1f obtained from the normalized higher harmonic spectral signals of the first harmonic and a concentration value Xabs_DAS obtained from the spectral absorbance.

[0016] S4. The signal acquisition and analysis device determines the current temperature, humidity, optical path, and air pressure according to the current test environment, searches for the theoretical absorbance of the corresponding gas in the corresponding environment in the internal database of the signal acquisition and analysis device, and uses the original absorption light intensity signal and the absorbance parameters corresponding to the experimental parameters to perform simulation calculations to obtain the simulated first harmonic normalized higher harmonic signals.

[0017] The simulated first harmonic normalized k - th harmonic signal in S4 is calculated as follows:

[0018] The laser frequency and light intensity nonlinear effects are described by the following formula:

[0019]

[0020]

[0021] where is the center frequency corresponding to the current scanning signal, ω m is the angular frequency of the modulation signal, a [cm -1 is the frequency modulation depth; is the light intensity at the center frequency of the laser output light, i j is the normalized j - th nonlinear intensity modulation amplitude, ψ j is the phase difference between the j - th nonlinear light intensity modulation and frequency modulation;

[0022] When a monochromatic laser beam with a frequency of v passes through the absorption gas to be measured, its absorption law follows the following Beer - Lambert law:

[0023]

[0024] In the formula: τ(v(t)) is the transmittance varying with time, I0 is the incident light intensity, I t is the transmitted light intensity; L is the absorption optical path, S(T) is the line intensity, X is the absorption component concentration, P is the total gas pressure, and the product A of the above four parameters is called the absorption rate integral value. is a linear function and is described by the Voigt function; the transmittance τ(v) in the time domain is expanded into a Fourier series, and we can get:

[0025]

[0026] In the formula: ω m is the laser current modulation frequency, H k and J k are the k-th Fourier coefficients of the transmittance, and their expressions are as follows:

[0027] J0 = 0

[0028]

[0029] Substitute I0 and I in formulas (1) and (4) t into formula (3) and perform Fourier expansion to obtain the Fourier coefficients corresponding to kω m times frequency doubling, which are defined as X kf and Y kf , and these coefficients are the k-th harmonics (2 ≤ k ≤ n) of the transmitted light intensity on the X-axis and Y-axis, and their general term expressions are as follows:

[0030]

[0031]

[0032]

[0033] Then

[0034] Through the above simulation calculation of the first harmonic normalized k-th harmonic signal

[0035] S5 above, using multiple first harmonic normalized high-order harmonic spectral signals, the optimal solution of the concentration is obtained by solving the final concentration through the least squares method: When using the least squares algorithm to inversely solve the target gas concentration, one of the following four methods can be used:

[0036] Method 1: According to the selected n - 1 normalized high-order harmonic spectral signals, the following function is used as the optimization objective function to solve the gas concentration parameter X abs :

[0037]

[0038] Among them, k is the harmonic order (2 ≤ k ≤ n), kf is the k-th harmonic, It represents the absorption peak of the k-th harmonic normalized to the fundamental harmonic. exp represents the experimental value, and sim represents the simulation value;

[0039] Method 2: Based on the selected n - 1 normalized high-order harmonic spectral signals, the following function is used as the optimization objective function to solve for the gas concentration parameter X abs as follows:

[0040]

[0041] where k is the harmonic order (2 ≤ k ≤ n), kf is the k-th harmonic, It represents the absorption value within the characteristic range of the k-th harmonic normalized to the fundamental harmonic. exp represents the experimental value, and sim represents the simulation value;

[0042] Method 3: Based on the selected n - 1 normalized high-order harmonic spectral signals, the following function is used as the optimization objective function to solve for the gas concentration parameter X abs as follows:

[0043]

[0044] where k is the harmonic order (2 ≤ k ≤ n), kf is the k-th harmonic, It represents the absorption peak of the k-th harmonic normalized to the fundamental harmonic, (S kf ) v0 represents the k-th harmonic value at the absorption peak position. exp represents the experimental value, and sim represents the simulation value;

[0045] Method 4: Based on the selected n - 1 normalized high-order harmonic spectral signals, the following function is used as the optimization objective function to solve for the gas concentration parameter X abs as follows:

[0046]

[0047] where k is the harmonic order (2 ≤ k ≤ n), kf is the k-th harmonic, It represents the absorption value within the characteristic range of the second harmonic normalized to the fundamental harmonic, (S kf ) v represents the absorption value within the characteristic range of the k-th harmonic. exp represents the experimental value, and sim represents the simulation value.

[0048] The optical path measurement device is an open measurement optical path or a measurement optical path of a closed gas cell.

[0049] The laser generating device includes: a function generator, a laser controller, a laser, and a collimator. The function generator is used to send scanning and modulation signals to the laser controller. The laser controller is used to control the laser to emit laser light that meets the test conditions according to the received scanning and modulation signals. The collimator is installed at the output end of the laser 3;

[0050] The signal acquisition and analysis device includes: a photodetector, a data acquisition card, a data processing device, and a display. The photodetector is used to collect the laser light emitted by the laser. The data acquisition card is used to collect the drive signal of the function generator and the detection signal of the photodetector and synchronously transmit them to the data processing device. The data processing device analyzes the absorption spectrum of the detection signal according to the drive signal to obtain the concentration value of the gas to be measured.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0052] 1. In the gas concentration detection method based on multi-harmonic information fusion laser absorption spectroscopy of the present invention, the demodulated normalized second harmonic signal of the fundamental harmonic, the normalized third harmonic signal of the fundamental harmonic,..., the normalized high-order harmonic signal of the fundamental harmonic are absorbed and utilized to obtain the gas concentration information, and it is compared with the simulation result obtained by the direct absorption spectroscopy technology. If the numerical difference is less than 2%, then the multi-harmonic signal and the simulation result are subjected to least squares. The final result obtained by the least squares method is the gas concentration. In this method, high-order harmonics are introduced for concentration calculation, effectively improving the measurement accuracy, making the measurement result more accurate, and utilizing the information advantages of multiple harmonic spectral lines in different temperature ranges to significantly broaden the effective sensing range of the absorption spectrum, simply and efficiently improving the measurement accuracy.

[0053] 2. In the gas concentration detection method based on multi-harmonic information fusion laser absorption spectroscopy of the present invention, relying on the differences in the error responses of multiple harmonic information in different temperature ranges, during the actual measurement process, the errors caused by factors such as flow field changes and vibrations are greatly reduced, thereby reducing the influence of experimental system noise on the final result. Therefore, this design can effectively reduce the experimental system noise and improve the measurement accuracy.

[0054] 3. The gas concentration detection method based on multi-harmonic information fusion laser absorption spectroscopy of the present invention has relatively low requirements for the measurement environment, taking the final absorbance of multiple harmonics as the main selection criterion, so as to be applicable to the diagnosis of target fields with different physical scales, flexible and reliable. This characteristic enables this method to be applicable to the vast majority of test scenarios and achieve high-precision detection in complex environments. Therefore, this design can adapt to complex measurement environments and achieve high-precision detection in complex environments.

[0055] 4. In the gas concentration detection method based on multi - harmonic information fusion laser absorption spectroscopy of the present invention, the laser wavelength can be adjusted to be applicable to the wide - range and high - precision measurement of single - component or multi - component target gases at different temperatures and pressures. It has strong anti - interference ability and can realize the measurement of multiple gas concentrations. Therefore, this design has strong anti - interference ability, can realize the measurement of multiple gas concentrations, and has a wide application range. Brief Description of the Drawings

[0056] Figure 1 is the flow chart of the present invention.

[0057] Figure 2 is the structural schematic diagram of the test equipment of the present invention.

[0058] Figure 3 is the comparison chart of test results in Example 3.

[0059] In the figure: function generator 1, laser controller 2, laser 3, collimator 4, optical path measurement device 5, first reflector 6, second reflector 7, photodetector 8, data acquisition card 9, data processing device 10, display 11, simulation processing module 12, multiple - harmonic calculation module 13, concentration solution module 14, environment detection module 15. Detailed Embodiments

[0060] The present invention will be further described in detail below in conjunction with the description of the drawings and specific embodiments.

[0061] See Figures 1 to 2 , for the gas concentration detection method based on multi - harmonic information fusion laser absorption spectroscopy, the detection method is based on the following detection device, including: a laser generating device, an optical path measurement device, and a signal acquisition and analysis device. The laser of the laser generating device is a distributed feedback semiconductor laser, and the center frequency of the laser is equal to the center frequency of the gas to be measured;

[0062] The detection method includes the following steps:

[0063] S1. Using time - division multiplexing technology, a scanning and modulating current is injected into the laser. In the upper half - cycle, it is a scanning plus modulating signal, and in the lower half - cycle, it is a scanning signal. The laser 3 emits laser light that passes through the gas to be measured and is received by the photodetector of the signal acquisition and analysis device. The signal generated by the photodetector is collected by the data acquisition card;

[0064] S2. The signal acquisition and analysis device performs software demodulation and baseline fitting on the absorption signal collected by the data acquisition card in the data post - processing program to respectively obtain multiple - harmonic signals 1f - nf (5 ≤ n ≤ 10) and spectral absorbance DAS data. Then, all the obtained multiple - harmonic signals are normalized to obtain the normalized high - order harmonic signals of the first harmonic of the test measurement

[0065] S3. Post-process the n - 1 primary harmonic normalized higher harmonic spectral signals and one spectral absorbance obtained in S2, and use the spectral fitting optimization algorithm to invert the component concentration of the target gas to obtain the concentration values corresponding to the n - 1 primary harmonic normalized higher harmonic spectral signals.

[0066] S4. According to the current test environment, the signal acquisition and analysis device uses the original absorption light intensity signal and the absorbance parameter corresponding to the experimental parameters to perform simulation calculations to obtain the simulated primary harmonic normalized higher harmonic signal.

[0067] S5. Use the least squares algorithm to invert and solve the target gas concentration to obtain the final gas concentration value.

[0068] In S2, the signal acquisition and analysis device performs software demodulation and baseline fitting on the absorption signals collected by the data acquisition card in the data post - processing program to obtain the primary harmonic signal 1f, secondary harmonic signal 2f, tertiary harmonic signal 3f,..., nth harmonic signal nf (5 ≤ n ≤ 10) and the spectral absorbance DAS data.

[0069] The signal acquisition and analysis device performs normalization processing on the obtained multiple harmonic signals respectively, and obtains the primary harmonic normalized secondary harmonic signal through calculation. The primary harmonic normalized tertiary harmonic signal ... The primary harmonic normalized nth harmonic signal

[0070] S3. Post-process the n - 1 primary harmonic normalized higher harmonic spectral signals and one spectral absorbance obtained in S2, and use the spectral fitting optimization algorithm to invert the component concentration of the target gas to obtain n - 1 concentration values Xabs_2f / 1f, Xabs_3f / 1f,..., Xabs_nf / 1f obtained from the primary harmonic normalized higher harmonic spectral signals and one concentration value Xabs_DAS obtained from the spectral absorbance.

[0071] S4. According to the current test environment, the signal acquisition and analysis device determines the current temperature, humidity, optical path, and air pressure, searches for the theoretical absorbance of the corresponding gas in the corresponding environment in the internal database of the signal acquisition and analysis device, and uses the original absorption light intensity signal and the absorbance parameter corresponding to the experimental parameters to perform simulation calculations to obtain the simulated primary harmonic normalized higher harmonic signal.

[0072] The simulated primary harmonic normalized kth harmonic signal in S4 The calculation method is as follows:

[0073] The non - linear effects of laser frequency and optical intensity are described by the following formula:

[0074]

[0075]

[0076] Wherein, is the central frequency corresponding to the current scanning signal, ω m is the angular frequency of the modulation signal, a [cm -1 is the frequency modulation depth; is the optical intensity at the center frequency of the laser output light, i j is the normalized non - linear intensity modulation amplitude of the j - th order, ψ j is the phase difference between the non - linear optical intensity modulation and the frequency modulation of the j - th order;

[0077] When a monochromatic laser beam with a frequency of v passes through the absorption gas to be measured, its absorption law follows the following Beer - Lambert law:

[0078]

[0079] In the formula: τ(v(t)) is the transmittance varying with time, I0 is the incident light intensity, I t is the transmitted light intensity; L is the absorption optical path, S(T) is the line intensity, X is the concentration of the absorption component, P is the total gas pressure, and the product A of these four parameters is called the absorption rate integral value, is the line - shape function, which is described by the Voigt function; performing a Fourier series expansion on the transmittance τ(v) in the time domain, we can obtain:

[0080]

[0081] In the formula: ω m is the laser current modulation frequency, H k and J k are the k - th order Fourier coefficients of the transmittance, and their expressions are as follows:

[0082] J0 = 0

[0083]

[0084] Substituting I0 and I t in formulas (1) and (4) into formula (3) and performing a Fourier expansion, we can obtain the Fourier coefficients corresponding to the kω m multiple frequency, which are defined as X kf and Y kf , and these coefficients are the k - th harmonics (2 ≤ k ≤ n) of the transmitted light intensity on the X - axis and Y - axis, and their general term expressions are as follows:

[0085]

[0086]

[0087]

[0088] Then

[0089] The first - harmonic normalized k - th harmonic signal obtained through the above simulation calculation

[0090] S5. Using multiple first - harmonic normalized high - order harmonic spectral signals, the optimal solution of the concentration is obtained by solving the final concentration through the least - squares method: When using the least - squares algorithm to inversely solve the target gas concentration, one of the following four methods can be adopted:

[0091] Method 1: According to the selected n - 1 normalized high - order harmonic spectral signals, the following function is used as the optimization objective function to solve the gas concentration parameter X abs for solution:

[0092]

[0093] where k is the harmonic order (2 ≤ k ≤ n), kf is the k - th harmonic, represents the absorption peak corresponding to the first - harmonic normalized k - th harmonic, exp represents the experimental value, and sim represents the simulation value;

[0094] Method 2: According to the selected n - 1 normalized high - order harmonic spectral signals, the following function is used as the optimization objective function to solve the gas concentration parameter X abs for solution:

[0095]

[0096] where k is the harmonic order (2 ≤ k ≤ n), kf is the k - th harmonic, represents the absorption value within the characteristic range of the first - harmonic normalized k - th harmonic, exp represents the experimental value, and sim represents the simulation value;

[0097] Method 3: According to the selected n - 1 normalized high - order harmonic spectral signals, the following function is used as the optimization objective function to solve the gas concentration parameter X abs for solution:

[0098]

[0099] where k is the harmonic order (2 ≤ k ≤ n), kf is the k - th harmonic, represents the absorption peak corresponding to the k-th harmonic normalized by the fundamental harmonic, (S kf ) v0 represents the k-th harmonic value of the absorption peak position, exp represents the experimental value, and sim represents the simulation value;

[0100] Method 4: According to the selected n - 1 normalized high-order harmonic spectral signals, the following function is used as the optimization objective function to solve for the gas concentration parameter X abs for solution:

[0101]

[0102] where k is the harmonic order (2 ≤ k ≤ n), kf is the k-th harmonic, represents the absorption value within the characteristic range of the second harmonic normalized by the fundamental harmonic, (S kf ) v represents the absorption value within the characteristic range of the k-th harmonic, exp represents the experimental value, and sim represents the simulation value.

[0103] The optical path measurement device is an open measurement optical path or a measurement optical path of a closed gas cell.

[0104] The laser generating device includes: a function generator 1, a laser controller 2, a laser 3, and a collimator 4. The function generator 1 is used to send scanning and modulation signals to the laser controller 2. The laser controller 2 is used to control the laser 3 to emit laser light that meets the test conditions according to the received scanning and modulation signals. The collimator 4 is installed at the output end of the laser 3;

[0105] The signal acquisition and analysis device includes: a photodetector 8, a data acquisition card 9, a data processing device 10, and a display 11. The photodetector 8 is used to collect the laser light emitted by the laser 3. The data acquisition card 9 is used to collect the drive signal of the function generator 1 and the detection signal of the photodetector 8 and synchronously transmit them to the data processing device 10. The data processing device 10 analyzes the absorption spectrum of the detection signal according to the drive signal to obtain the concentration value of the gas to be measured.

[0106] The principle of the present invention is described as follows:

[0107] The laser 3 uses an arbitrarily tunable distributed semiconductor laser 3 in the wavelength range of 1 - 12 micrometers.

[0108] The photodetector 8 uses InSb and MCT material photodetectors 8.

[0109] Example 1:

[0110] Gas concentration detection method based on multi-harmonic information fusion laser absorption spectroscopy technology. The detection method is based on the following detection device, including: a laser generating device, an optical path measuring device, and a signal acquisition and analysis device. The laser of the laser generating device is a distributed feedback semiconductor laser, and the central frequency of the laser is equal to the central frequency of the gas to be measured;

[0111] The detection method includes the following steps:

[0112] S1. Using time-division multiplexing technology, inject scanning and modulation currents into the laser. The upper half cycle is a scanning plus modulation signal, and the lower half cycle is a scanning signal. The laser beam emitted by the laser 3 passes through the gas to be measured and is received by the photodetector of the signal acquisition and analysis device. The signal generated by the photodetector is collected by the data acquisition card;

[0113] S2. The signal acquisition and analysis device performs software demodulation and baseline fitting on the absorption signal collected by the data acquisition card in the data post-processing program to obtain multiple harmonic signals 1f to nf (5 ≤ n ≤ 10) and spectral absorbance DAS data respectively. Then, all the obtained multiple harmonic signals are normalized to obtain the experimentally measured first-harmonic normalized high-order harmonic signals

[0114] S3. Post-process the n - 1 first-harmonic normalized high-order harmonic spectral signals and one spectral absorbance obtained in S2, and use the spectral fitting optimization algorithm to invert the component concentration of the target gas to obtain the concentration values corresponding to the n - 1 first-harmonic normalized high-order harmonic spectral signals;

[0115] S4. The signal acquisition and analysis device performs simulation calculations according to the current test environment, using the original absorption light intensity signal and the absorbance parameters corresponding to the experimental parameters to obtain the first-harmonic normalized high-order harmonic signal obtained by simulation calculation

[0116] S5. Use the least squares algorithm to invert and solve the target gas concentration to obtain the final gas concentration value.

[0117] In S2, the signal acquisition and analysis device performs software demodulation and baseline fitting on the absorption signal collected by the data acquisition card in the data post-processing program to obtain the first harmonic signal 1f, the second harmonic signal 2f, the third harmonic signal 3f,..., the nth harmonic signal nf (5 ≤ n ≤ 10) and spectral absorbance DAS data;

[0118] The signal acquisition and analysis device normalizes the obtained multiple harmonic signals respectively, and obtains the first-harmonic normalized second harmonic signal through calculation The first-harmonic normalized third harmonic signal ... The first-harmonic normalized nth harmonic signal

[0119] S3. Post-process the n - 1 normalized higher harmonic spectral signals of the first harmonic and a spectral absorbance obtained in S2, and use the spectral fitting optimization algorithm to invert the component concentration of the target gas to obtain n - 1 corresponding concentration values Xabs_2f / 1f, Xabs_3f / 1f, …… Xabs_nf / 1f obtained from the normalized higher harmonic spectral signals of the first harmonic and a concentration value Xabs_DAS obtained from the spectral absorbance.

[0120] S4. The signal acquisition and analysis device determines the current temperature, humidity, optical path, and air pressure according to the current test environment, searches for the theoretical absorbance of the corresponding gas in the corresponding environment in the internal database of the signal acquisition and analysis device, and uses the original absorption light intensity signal and the absorbance parameter corresponding to the experimental parameters to perform simulation calculations to obtain the simulated first harmonic normalized higher harmonic signal.

[0121] The simulated first harmonic normalized k - th harmonic signal in S4 is calculated as follows:

[0122] Describe the laser frequency and optical intensity nonlinear effect through the following formula:

[0123]

[0124]

[0125] where is the center frequency corresponding to the current scanning signal, ω m is the angular frequency of the modulation signal, a[cm -1 is the frequency modulation depth; is the optical intensity at the center frequency of the laser output light, i j is the normalized j - th nonlinear intensity modulation amplitude, ψ j is the phase difference between the j - th nonlinear optical intensity modulation and frequency modulation;

[0126] When a monochromatic laser beam with a frequency of v passes through the absorption gas to be measured, its absorption law follows the following Beer - Lambert law:

[0127]

[0128] where: τ(v(t)) is the transmittance varying with time, I0 is the incident light intensity, I t is the transmitted light intensity; L is the absorption optical path, S(T) is the line intensity, X is the absorption component concentration, P is the total gas pressure, and the product A of the above four parameters is called the absorption rate integral value. is a linear function and is described by the Voigt function; the transmittance τ(v) in the time domain is expanded into a Fourier series, and we can get:

[0129]

[0130] In the formula: ω m is the laser current modulation frequency, H k and J k are the k-th Fourier coefficients of the transmittance, and their expressions are as follows:

[0131] J0 = 0

[0132]

[0133] Substitute I0 and I in equations (1) and (4) t into formula (3) and perform Fourier expansion, we can get the Fourier coefficients corresponding to kω m times frequency, which are defined as X kf and Y kf , and these coefficients are the k-th harmonics of the transmitted light intensity on the X-axis and Y-axis (2 ≤ k ≤ n), and their general term expressions are as follows:

[0134]

[0135]

[0136]

[0137] Then

[0138] Through the above simulation calculation of the fundamental harmonic normalized k-th harmonic signal

[0139] S5 above, using multiple fundamental harmonic normalized high-order harmonic spectral signals, the optimal solution of the concentration is obtained by solving the final concentration through the least squares method: When using the least squares algorithm to inversely solve the target gas concentration, one of the following four methods can be adopted:

[0140] Method 1: According to the selected n - 1 normalized high-order harmonic spectral signals, the following function is used as the optimized objective function to solve the gas concentration parameter X abs :

[0141]

[0142] where k is the harmonic order (2 ≤ k ≤ n), kf is the k-th harmonic, It represents the absorption peak of the k-th harmonic normalized to the fundamental harmonic. exp represents the experimental value, and sim represents the simulated value;

[0143] Method 2: Based on the selected n - 1 normalized high-order harmonic spectral signals, the following function is used as the optimization objective function to solve for the gas concentration parameter X abs as follows:

[0144]

[0145] where k is the harmonic order (2 ≤ k ≤ n), and kf is the k-th harmonic. It represents the absorption value within the characteristic range of the k-th harmonic normalized to the fundamental harmonic. exp represents the experimental value, and sim represents the simulated value;

[0146] Method 3: Based on the selected n - 1 normalized high-order harmonic spectral signals, the following function is used as the optimization objective function to solve for the gas concentration parameter X abs as follows:

[0147]

[0148] where k is the harmonic order (2 ≤ k ≤ n), and kf is the k-th harmonic. It represents the absorption peak of the k-th harmonic normalized to the fundamental harmonic. (S kf ) v0 represents the k-th harmonic value at the absorption peak position. exp represents the experimental value, and sim represents the simulated value;

[0149] Method 4: Based on the selected n - 1 normalized high-order harmonic spectral signals, the following function is used as the optimization objective function to solve for the gas concentration parameter X abs as follows:

[0150]

[0151] where k is the harmonic order (2 ≤ k ≤ n), and kf is the k-th harmonic. It represents the absorption value within the characteristic range of the second harmonic normalized to the fundamental harmonic. (S kf ) v represents the absorption value within the characteristic range of the k-th harmonic. exp represents the experimental value, and sim represents the simulated value.

[0152] Example 2:

[0153] Example 2 is basically the same as Example 1, except that:

[0154] The optical path measurement device is an open measurement optical path or a measurement optical path of a closed gas cell.

[0155] The laser generating device includes: a function generator 1, a laser controller 2, a laser 3, and a collimator 4. The function generator 1 is used to send scanning and modulation signals to the laser controller 2. The laser controller 2 is used to control the laser 3 to emit laser that meets the test conditions according to the received scanning and modulation signals. The collimator 4 is installed at the output end of the laser 3;

[0156] The signal acquisition and analysis device includes: a photodetector 8, a data acquisition card 9, a data processing device 10, and a display 11. The photodetector 8 is used to collect the laser emitted by the laser 3. The data acquisition card 9 is used to collect the drive signal of the function generator 1 and the detection signal of the photodetector 8 and synchronously transmit them to the data processing device 10. The data processing device 10 analyzes the absorption spectrum of the detection signal according to the drive signal to obtain the concentration value of the gas to be measured.

[0157] Embodiment 3:

[0158] Embodiment 3 is basically the same as Embodiment 2, and the difference is that:

[0159] The laser controller adjusts the laser frequency and power by changing the temperature and current. The laser is a distributed feedback semiconductor laser with a center frequency of 5262.9 cm -1 The output beam diameter of the fiber collimating mirror is 3 mm. The effective photosensitive surface diameter of the photodetector is 3*3 mm 2 The photodetector converts the optical signal into an electrical signal. The multi-channel synchronous sampling data acquisition card synchronously acquires multi-channel signals and transmits them to the computer for storage.

[0160] Using time-division multiplexing technology, a scanning and modulation current is injected into the laser. The trigger time is used for distinction, with 1 s as a cycle. The first 0.5 s is for the scanning plus modulation signal, and the latter 0.5 s is for the scanning signal. Among them, the scanning frequency is 100 Hz, the amplitude is 70 mV, the bias is 250 mV, the phase is 270°, the modulation frequency is 50000 Hz, the amplitude is 30 mV, the bias is 0, and the phase is 270°. The laser emitted by the laser passes through the collimator and is collimated into spatially parallel light, then enters the multi-pass gas cell, passes through the gas to be measured and then exits and is received by the photodetector. The signal generated by the photodetector is collected by the data acquisition card and transmitted to the computer;

[0161] The direct absorption method, the second harmonic method, and the multiple harmonic method of the present invention are respectively used for calculation, and the calculation results are shown in Figure 3 .

Claims

1. A method for detecting gas concentration based on multi - harmonic information fusion laser absorption spectroscopy technology, characterized in that: The detection method is based on the following detection device, including: a laser generating device, an optical path measuring device (5) and a signal acquisition and analysis device. The laser of the laser generating device is a distributed feedback semiconductor laser, and the central frequency of the laser is equal to the central frequency of the gas to be measured; The detection method includes the following steps: S1. Using time - division multiplexing technology, a scanning and modulating current is injected into the laser. In the upper half - cycle, it is a scanning plus modulating signal, and in the lower half - cycle, it is a scanning signal. The laser (3) emits laser light that passes through the gas to be measured and is received by the photodetector of the signal acquisition and analysis device. The signal generated by the photodetector is collected by the data acquisition card; S2. The signal acquisition and analysis device performs software demodulation and baseline fitting on the absorption signals collected by the data acquisition card in the data post-processing program, respectively obtaining multiple harmonic signals 1f to nf (5 ≤ n ≤ 10) and spectral absorbance DAS data. Then, all the obtained multiple harmonic signals are normalized to obtain the high-order harmonic signals normalized by the fundamental harmonic of the test measurement. ; S3. Post - process the n - 1 normalized high - order harmonic spectral signals of the first - order harmonics and a spectral absorbance obtained in S2, and use the spectral fitting optimization algorithm to invert the component concentration of the target gas to obtain the concentration values corresponding to the n - 1 normalized high - order harmonic spectral signals of the first - order harmonics; S4. The signal acquisition and analysis device determines the current temperature, humidity, optical path, and air pressure according to the current test environment, searches for the theoretical absorbance of the corresponding gas in the corresponding environment in the internal database of the signal acquisition and analysis device, and performs simulation calculations using the absorbed original light intensity signal and the absorbance parameter corresponding to the experimental parameters to obtain the normalized high-order harmonic signal of the first harmonic of the simulation calculation ; S5. Use the least - squares algorithm to invert and solve the target gas concentration to obtain the final gas concentration value. When using the least - squares algorithm to invert and solve the target gas concentration, the following method is adopted: Method 1: According to the selected n - 1 normalized high - order harmonic spectral signals, the following function is used as the optimized objective function to solve for the gas concentration parameter X abs as follows: (8) where k is the harmonic order (2 ≤ k ≤ n), kf is the k-th harmonic, represents the absorption peak of the normalized k-th harmonic corresponding to the fundamental harmonic, exp represents the experimental value, and sim represents the simulation value.

2. The method for detecting gas concentration based on multi - harmonic information fusion laser absorption spectroscopy technology according to claim 1, characterized in that: In S2, the signal acquisition and analysis device performs software demodulation and baseline fitting on the absorption signal collected by the data acquisition card in the data post - processing program to obtain the first - order harmonic signal 1f, the second - order harmonic signal 2f, the third - order harmonic signal 3f,..., the n - th order harmonic signal nf (5 ≤ n ≤ 10) and the spectral absorbance DAS data; The signal acquisition and analysis device respectively performs normalization processing on the obtained multiple harmonic signals, and obtains the second harmonic signal normalized by the fundamental harmonic through calculation , the third harmonic signal normalized by the fundamental harmonic , …… the nth harmonic signal normalized by the fundamental harmonic ( ).

3. The method for detecting gas concentration based on multi - harmonic information fusion laser absorption spectroscopy technology according to claim 2, characterized in that: S3. Post - process the n - 1 normalized high - order harmonic spectral signals of the first - order harmonics and a spectral absorbance obtained in S2, and use the spectral fitting optimization algorithm to invert the component concentration of the target gas to obtain n - 1 concentration values Xabs_2f / 1f, Xabs_3f / 1f,..., Xabs_nf / 1f obtained from the normalized high - order harmonic spectral signals of the first - order harmonics and a concentration value Xabs_DAS obtained from the spectral absorbance.

4. The method for detecting gas concentration based on multi - harmonic information fusion laser absorption spectroscopy technology according to claim 1, characterized in that: The calculation method of the normalized k -th harmonic signal of the fundamental harmonic in the simulation calculation in S4 is as follows: The laser frequency and light - intensity nonlinear effect are described by the following formula: (1) (2) Among them, is the center frequency corresponding to the current scanning signal, ω m is the angular frequency of the modulation signal, a [cm -1 is the frequency modulation depth; is the optical intensity at the center frequency of the laser output light, i j is the normalized j-th order nonlinear intensity modulation amplitude, ψ j is the phase difference between the j-th order nonlinear optical intensity modulation and the frequency modulation; When a monochromatic laser beam with a frequency of v passes through the absorption gas to be measured, its absorption law follows the following Beer - Lambert law: (3) where: τ(v(t)) is the transmittance varying with time, I0 is the incident light intensity, and I t is the transmitted light intensity; L is the absorption optical path, S(T) is the line intensity, X is the concentration of the absorbing component, and P is the total gas pressure. The product A of the above four parameters is called the absorption rate integral value. is the line shape function, which is described by the Voigt function; performing a Fourier series expansion on the transmittance τ(v) in the time domain, we can obtain: (4) where: ω m is the laser current modulation frequency, H k and J k are the k-th Fourier coefficients of the transmittance, and their expressions are as follows: (5) Substitute \(I_0\) and \(I_t\) in equations (1) and (4) into formula (3) and perform Fourier expansion to obtain the Fourier coefficients corresponding to \(k\omega\). Define them as \(X\) m and \(Y\) kf . These coefficients are the \(k\)th harmonics of the transmitted light intensity on the \(X\)-axis and \(Y\)-axis (\(2\leq k\leq n\)), and their general term expressions are as follows: kf ​ (6) Then (7) The fundamental harmonic normalized k-th harmonic signal obtained from the above simulation calculation .

5. The method for detecting gas concentration based on multi - harmonic information fusion laser absorption spectroscopy technology according to claim 1, 2, 3 or 4, characterized in that: S5. Using multiple fundamental harmonic normalized higher harmonic spectral signals, the optimal solution of the concentration is obtained by solving the final concentration through the least squares method: When using the least squares algorithm to inversely solve the target gas concentration, one of the following three methods can also be adopted in addition to the Method 1: Method 2: According to the selected n-1 normalized high-order harmonic spectral signals, the following function is used as the optimized objective function to solve for the gas concentration parameter X abs as follows: (9) Among them, k is the harmonic order (2 ≤ k ≤ n), kf is the k-th harmonic, represents the absorption value within the characteristic range of the k-th harmonic normalized to the fundamental harmonic. exp represents the experimental value, and sim represents the simulation value; Method 3: According to the selected n-1 normalized high-order harmonic spectral signals, the following function is used as the optimized objective function to solve for the gas concentration parameter X abs as follows: (10) where k is the harmonic order (2 ≤ k ≤ n), kf is the k-th harmonic, represents the absorption peak value of the k-th harmonic normalized to the fundamental harmonic, (S kf ) v0 represents the k-th harmonic value at the absorption peak position, exp represents the experimental value, and sim represents the simulation value; Method 4: According to the selected n - 1 normalized higher harmonic spectral signals, the following function is used as the optimized objective function to solve the gas concentration parameter Xabs: (11) Among them, k is the harmonic order (2 ≤ k ≤ n), kf is the kth harmonic, represents the absorption value within the characteristic range of the normalized second harmonic corresponding to the fundamental harmonic, (S kf ) v represents the absorption value within the characteristic range of the kth harmonic, exp represents the experimental value, and sim represents the simulation value.

6. The method for detecting gas concentration based on multi - harmonic information fusion laser absorption spectroscopy according to claim 5, wherein: The optical path measurement device is an open - type measurement optical path or a measurement optical path of a closed - type gas cell.

7. The method for detecting gas concentration based on multi - harmonic information fusion laser absorption spectroscopy according to claim 6, wherein: The laser generating device includes: a function generator (1), a laser controller (2), a laser (3), and a collimator (4). The function generator (1) is used to send scanning and modulation signals to the laser controller (2). The laser controller (2) is used to control the laser (3) to emit laser that meets the test conditions according to the received scanning and modulation signals. The collimator (4) is installed at the output end of the laser (3). The signal acquisition and analysis device includes: a photodetector (8), a data acquisition card (9), a data processing device (10), and a display (11). The photodetector (8) is used to collect the laser emitted by the laser (3). The data acquisition card (9) is used to collect the drive signal of the function generator (1) and the detection signal of the photodetector (8) and synchronously transmit them to the data processing device (10). The data processing device (10) analyzes the absorption spectrum of the detection signal according to the drive signal to obtain the concentration value of the gas to be measured.

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