Dual optical frequency comb spectrum-based open long optical path gas detection method and system

Abstract

The application discloses an open long optical path gas detection method and system based on a double optical frequency comb spectrum, and the method comprises the following steps: acquiring an initial correction reference signal, a background reference signal, a correction reference signal and a detection signal of each laser interference period, performing Fourier transform on each signal to obtain a frequency domain initial correction reference signal, a frequency domain background reference signal, a frequency domain correction reference signal and a frequency domain detection signal; correcting the frequency domain background reference signal and the frequency domain detection signal, and performing multiple coherent accumulation averages and frequency up-conversion on the corrected signals to obtain an optical frequency background signal and an optical frequency average detection signal; sequentially introducing standard gas samples of each target gas component into a calibration light path to obtain the optical frequency calibration spectrum absorption rate signals of each component, performing signal analysis and concentration inversion of each component gas to be measured, and obtaining the concentration of each component gas; and the application has the advantages that the concentration inversion precision is high.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, and specifically to an open long optical path gas detection method and system based on dual optical frequency comb spectroscopy. Background Technology

[0002] Dual-comb spectroscopy is a novel spectral measurement technique that has emerged in recent years alongside the development of femtosecond mode-locked laser technology. Compared to traditional spectral measurement techniques, it offers advantages such as faster measurement speed, wider spectral range, higher spectral resolution, and multi-component detection capabilities, making it a promising area for qualitative and quantitative analysis of gases in environmental and industrial processes. Similar to the widely used Fourier transform spectroscopy (FTIR) technique, dual-comb spectroscopy systems employ two optical frequency combs with slightly different repetition frequencies as interference sources, replacing the mechanical arm of the Michelson interferometer in a Fourier spectrometer. This fully static design significantly improves the stability of the measurement system while also increasing the sampling frequency and enhancing spectral measurement efficiency. For example, Chinese Patent Publication No. CN115165781A discloses a gas parameter measurement method based on phase-locked dual-comb absorption spectroscopy, which utilizes dual-comb spectroscopy.

[0003] The frequency stability of optical comb lasers is a key factor limiting high-precision spectral measurements using dual-comb lasers. Currently, widely used dual-comb spectral systems based on radio frequency references suffer from low frequency locking accuracy and insufficient frequency stability, leading to dynamic shifts and contractions in the frequency spectrum signal. This results in low spectral coherence in continuous measurements, making it impossible to improve spectral signal quality through direct coherent averaging, thus limiting the accuracy of spectral measurements. Optical comb laser sources have high output power and are more suitable for open-space, long-path detection applications compared to existing multi-component gas detection techniques using broadband sources such as Fourier transform spectroscopy (FTIR) and differential absorption spectroscopy (DOAS). However, due to the complex energy spectrum structure of femtosecond optical comb pulses, reconstructing the background signal of dual-comb lasers in absorption spectroscopy measurements is extremely difficult, affecting the accuracy of absorption spectral feature analysis and gas concentration inversion. Furthermore, open long-path detection techniques generally face difficulties in on-site calibration. Current methods mostly utilize gas molecular spectral parameters to construct calibration spectra, which suffers from large spectral data errors leading to low gas concentration inversion accuracy and difficulties in source tracing. Summary of the Invention

[0004] The technical problem to be solved by the present invention is that the existing gas detection methods suffer from dynamic shift and expansion of frequency spectrum signals, difficulty in reconstructing background signals, and difficulty in on-site calibration, resulting in low accuracy of gas concentration inversion.

[0005] This invention solves the above-mentioned technical problems through the following technical means: an open long optical path gas detection method based on dual optical frequency comb spectroscopy, comprising the following steps:

[0006] A. Set up a master optical frequency comb laser and a slave optical frequency comb laser. The beams output by the two are combined and then split into a first beam and a second beam. The first beam is then split into two beams. One beam passes through a first reference absorption cell to a first photodetector, forming a background reference optical path. The other beam passes through a second reference absorption cell to a second photodetector, forming a correction signal reference optical path. The second beam is transmitted as a detection beam to the fiber-coupled transceiver optical end and then returns. The returned beam passes through a calibration absorption cell to a third photodetector, forming a calibration optical path. B. Obtain the initial correction reference signal and the background reference signal, correction reference signal, and detection signal for each laser interference cycle. Perform Fourier transform on each signal to obtain the initial correction reference signal, the background reference signal, the correction reference signal, and the detection signal in the frequency domain. C. Calculate the cross-correlation coefficient between the initial corrected reference signal and the frequency domain corrected reference signal, and take the frequency offset corresponding to the maximum cross-correlation coefficient as the frequency offset. Using frequency offset The frequency domain offset and scaling of the frequency domain background reference signal and the frequency domain detection signal are corrected. The corrected signals are coherently accumulated and averaged multiple times to obtain the frequency domain average background signal and the frequency domain average detection signal. The frequency domain average background signal and the frequency domain average detection signal are upconverted to obtain the optical frequency background signal and the optical frequency average detection signal. D. Standard gas samples of each target gas component are sequentially introduced into the calibration optical path to obtain the optical frequency calibration spectral absorbance signal of each component. The optical frequency background signal, the optical frequency average detection signal, and the optical frequency calibration spectral absorbance signal of each component are used to perform signal analysis and inversion of the gas concentration of each component to be measured, so as to obtain the gas concentration of each component.

[0007] Furthermore, the process of obtaining the initial frequency domain correction reference signal is as follows: Record the initial correction reference signal obtained by the second photodetector for one laser interferometry cycle. , where n is the time-domain sampling point number; for The initial corrected reference signal in the frequency domain is obtained by performing a Fourier transform. w is the radio frequency of the signal point, taken as The center frequency point frequency is The frequency range is 2W.

[0008] Furthermore, the acquisition process of the frequency domain background reference signal, the frequency domain correction reference signal, and the frequency domain detection signal is as follows: Record the background reference signal, correction reference signal, and detection signal output from the photodetectors corresponding to the background reference optical path, correction signal reference optical path, and calibration optical path for each laser interference cycle, denoted as follows: , where i is the sampling period number; for Perform Fourier transforms on each band to obtain the frequency domain background reference signal for the radio frequency band. Frequency domain correction reference signal Frequency domain detection signal .

[0009] Furthermore, the cross-correlation coefficient between the initial corrected reference signal and the frequency domain corrected reference signal is calculated as follows:

[0010] in, This indicates the offset of the radio frequency relative to the current signal point.

[0011] Furthermore, step C includes: use right and The frequency domain offset and scaling are corrected to obtain the corrected frequency domain background reference signal. and corrected frequency domain detection signal The corrected formula is as follows: ) ) The frequency-domain average background signal is obtained by performing multiple coherent cumulative averaging operations on the corrected signal. and frequency domain average detection signal Frequency domain average background signal and frequency domain average detection signal Frequency upconversion is performed to obtain the optical background signal. and optical frequency average detection signal The conversion formula is as follows: ) ) in, This represents the pulse frequency of the master optical frequency comb laser in a dual-frequency comb system. The frequency offset of the pulse frequency of the slave optical frequency comb laser relative to the master optical frequency comb laser in a dual optical frequency comb system.

[0012] Furthermore, the process of acquiring the optical frequency calibration spectral absorbance signals of each component is as follows: Record the output signal of the photodetector in the calibration optical path under conditions without a target gas sample. The processing procedure is the same as in step C. The processed signal is the optical frequency calibration background absorption signal, denoted as . ; Each standard gas sample of known concentration is sequentially introduced into the calibration absorption cell. The output signal of the third photodetector is recorded under gas flow conditions. The processing procedure is the same as in step C. The processed signal is the optical frequency calibration detection signal for each target component gas, denoted as... Where j is the serial number of each gas to be measured, and the concentration of each component standard gas is denoted as . ; The optical frequency calibration spectral characteristic signal is obtained by using the optical frequency calibration detection signal and the optical frequency calibration background absorption signal. The calculation formula is as follows:

[0013] The optical frequency calibration spectral characteristic signal was normalized by the optical frequency calibration background absorption signal to obtain the optical frequency calibration spectral absorbance signal of each component. The calculation formula is as follows: .

[0014] Furthermore, the signal analysis process is as follows: Optical frequency average detection signal Considered as optical frequency background signal The signal is analyzed by linearly superimposing the characteristic signals of the optical frequency calibration spectra of each component, and then using a multiple linear regression fitting algorithm. The formula is as follows:

[0015] in, , These are the fitting parameters.

[0016] Furthermore, the process for retrieving the concentrations of each component gas to be measured is as follows: Optical frequency spectral absorbance signals of each component gas It is calculated by the following formula:

[0017] Optical frequency spectral absorbance signals of each component gas Comparison with the optical frequency calibration spectral absorbance signal of each component It has a linear relationship, and the linear coefficients are... The fitting formula is obtained by the linear least squares fitting algorithm:

[0018] Then the gas concentration of each component It is calculated by the following formula:

[0019] in, The absorption optical path length of the optical end of the fiber-coupled transceiver. To calibrate the gas absorption length of the absorption cell.

[0020] This invention also provides an open long-path gas detection system based on dual-frequency comb spectroscopy, comprising a master frequency comb laser, a slave frequency comb laser, an optical fiber coupled polarization controller, a first optical fiber beam splitter, a second optical fiber beam splitter, a first optical fiber collimating lens, and a third optical fiber collimating lens. Linearly polarized light output from the slave frequency comb laser passes through an optical fiber coupled polarization controller to adjust its polarization direction to be the same as that of the linearly polarized light output from the master frequency comb laser. Then, the beams output from the master frequency comb laser and the optical fiber coupled polarization controller are combined and split through a first optical fiber beam splitter. The first beam output from the first optical fiber beam splitter serves as a reference beam, which is split into two beams by the second optical fiber beam splitter. One beam output from the second optical fiber beam splitter is collimated by the first optical fiber collimating lens and passes through a first reference absorption cell to reach a first photodetector, forming a background reference optical path. A background reference signal is obtained through photoelectric conversion. The first reference absorption cell is filled with nitrogen gas. The other beam output from the second optical fiber beam splitter… The beam is collimated by the second fiber collimating lens and then passes through the second reference absorption cell to reach the second photodetector, forming a correction signal reference optical path. A correction reference signal is obtained through photoelectric conversion. The second reference absorption cell is filled with a gas sample containing the target detection gas component. The second beam output from the first fiber beam splitter serves as the detection beam, transmitted through a single-mode fiber to the fiber-coupled transceiver optical end. A corner reflector is installed on the output optical path of the fiber-coupled transceiver optical end, reflecting the beam output from the end back. The beam collected by the transceiver optical end from the corner reflector returns through a multimode fiber. The returned beam is collimated by the third fiber collimating lens and then passes through the calibration absorption cell to reach the third photodetector, forming a calibration optical path. A detection signal is obtained through photoelectric conversion. The background reference signal, correction reference signal, and detection signal are transmitted to the host computer, which executes the above-described open long-path gas detection method based on dual-frequency comb spectroscopy.

[0021] Furthermore, the optical terminal of the fiber-coupled transceiver integrates a fiber-coupled lens, a reflector, a Fresnel lens, and a fourth fiber-collimating lens. The detection beam is first collimated by the fourth fiber-collimating lens, then reflected by the reflector and output through the center of the Fresnel lens. After passing through the detection area, the output beam reaches the corner reflector placed at the far end of the optical path. After being reflected by the corner reflector, the detection beam returns along the original optical path. The returned beam is collected by the Fresnel lens and converged on the fiber-coupled lens. The fiber-coupled lens couples the returned beam into the multimode fiber and transmits it back to the third fiber-collimating lens.

[0022] The advantages of this invention are: This invention solves the problem of reconstructing the background signal of a dual-comb spectrum under open long-path detection conditions by acquiring the characteristics of the dual-comb spectral background signal through a built-in background reference optical path, thereby improving the accuracy of signal analysis and concentration measurement. By using a built-in correction reference optical path, the dual-comb spectral structure of the target gas is obtained. The cross-correlation between the corrected reference spectral signal obtained synchronously during the detection process and the initial corrected reference signal is used to obtain the shift characteristics of the detected spectral signal, achieving real-time correction of the detected spectrum. This solves the problem of dynamic shift and scaling correction of the frequency spectrum signal caused by insufficient frequency stability of the dual-comb, meets the coherent averaging requirements of the signal during the measurement process, and improves the quality of the spectral signal. Through a built-in calibration optical path and online calibration, a multi-component gas concentration detection calibration signal is obtained under open long-path detection conditions, meeting the needs of accurate spectral analysis and online concentration calibration and source tracing applications. The overall solution improves the accuracy of gas concentration inversion. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the open long optical path gas detection system based on dual optical frequency comb spectroscopy disclosed in the embodiments of the present invention; Figure 2 This is a schematic diagram of the structure of the integrated optical terminal for fiber-coupled transceiver in an open long-path gas detection system based on dual-frequency comb spectroscopy, as disclosed in an embodiment of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] like Figure 1 As shown, the open long optical path gas detection system based on dual optical frequency comb spectroscopy of the present invention includes a dual optical frequency comb light source subsystem, a laser heterodyne beat frequency subsystem, an open long optical path optical subsystem, and a host computer 15.

[0026] The dual-frequency comb light source subsystem includes two near-infrared frequency comb lasers. One of them is selected as the master frequency comb laser 1, and the other is selected as the slave frequency comb laser 2. The slave frequency comb laser 2 shares the radio frequency reference signal of the master frequency comb laser 1. The linearly polarized light output from both the master frequency comb laser 1 and the slave frequency comb laser 2 is transmitted through a single-mode polarization-maintaining fiber.

[0027] The laser heterodyne beat frequency subsystem includes fiber-coupled polarization controllers 3 and 2. 2 Fiber beam splitters 4, 1 The system comprises: 5. Fiber optic beam splitter; 6. First fiber optic collimating lens; 7. Second fiber optic collimating lens; 8. Third fiber optic collimating lens; 9. First reference absorption cell; 10. Second reference absorption cell; 11. Calibration absorption cell; 12. First photodetector; 13. Second photodetector; and 14. The open long-path optical subsystem is a fiber-coupled transceiver optical terminal 16. The host computer 15 is a signal processor.

[0028] The linearly polarized light output from the optical frequency comb laser 2 first passes through a fiber-coupled polarization controller 3, which adjusts its polarization direction to be the same as that of the linearly polarized light output from the main optical frequency comb laser 1. Then, the beams output from the main optical frequency comb laser 1 and the fiber-coupled polarization controller 3 pass through a 2 2. Fiber optic bundle splitter 4 performs bundle combining and 2:8 splitting. The first beam output from fiber beam splitter 4 (a beam with a ratio of 2) is used as a reference beam after passing through 1 2. Fiber optic beam splitter 5 splits the fiber into two beams, 1 One of the beams output from the fiber optic beam splitter 5 is collimated by the first fiber optic collimating lens 6 and then passes through the first reference absorption cell 9 to reach the first photodetector 12, forming a background reference optical path. The background reference signal is obtained through photoelectric conversion. The first reference absorption cell 9 is filled with high-purity nitrogen gas. The other beam output from the fiber optic beam splitter 5 is collimated by the second fiber optic collimating lens 7 and then passes through the second reference absorption cell 10 to reach the second photodetector 13, forming a correction signal reference optical path. After photoelectric conversion, a correction reference signal is obtained. The second reference absorption cell 10 is filled with a high-concentration gas sample containing a target detection gas component; from 2 The second beam (a beam with a ratio of 8) output from the fiber optic beam splitter 4 is transmitted as a detection beam through a single-mode fiber to the fiber-coupled transceiver optical terminal 16. A corner reflector 17 is provided on the output optical path of the fiber-coupled transceiver optical terminal 16. The beam output from the fiber-coupled transceiver optical terminal 16 is reflected back by the corner reflector 17. The beam collected by the fiber-coupled transceiver optical terminal 16 and reflected back from the corner reflector 17 is returned through a multimode fiber. The returned beam is collimated by the third fiber collimating lens 8 and then passes through the calibration absorption cell 11 to reach the third photodetector 14, forming a calibration optical path. The detection signal is obtained through photoelectric conversion. During the calibration process, standard gas samples of each target detection gas component with known concentrations are sequentially introduced into the calibration absorption cell 11 to obtain the calibration signal of each target detection gas.

[0029] Combination Figure 2The fiber-coupled transceiver optical terminal 16 incorporates a fiber-coupled lens 18, a reflector 19, a Fresnel lens 20, and a fourth fiber-coupled collimating lens 21. The detection beam transmitted via single-mode fiber is first collimated by the fourth fiber-coupled collimating lens 21, then reflected by the reflector 19 and output through the center of the Fresnel lens 20. After passing through the detection area, the output beam reaches the corner reflector 17 placed at the far end of the optical path. The detection beam is reflected by the corner reflector 17 and returns along the original optical path. The returned beam is collected by the Fresnel lens 20 and converged onto the fiber-coupled lens 18. The fiber-coupled lens 18 couples the returned beam into the multimode fiber and transmits it back to the laser heterodyne beat frequency subsystem.

[0030] The background reference signal, correction reference signal, and detection signal obtained by the first photodetector 12, the second photodetector 13, and the third photodetector 14 are transmitted to the host computer 15 via a coaxial signal line for signal processing and concentration inversion, thereby obtaining the gas concentration of each target in the detection area.

[0031] The open long-path gas detection system based on dual-frequency comb spectroscopy provided by this invention acquires spectral shift characteristics and background information through multi-path detection, achieving real-time correction and analysis of the dual-frequency comb spectrum. High-precision concentration measurement data of the target gas in the region is obtained through online calibration, resulting in higher spectral analysis accuracy and detection data accuracy. The specific method is as follows: S1. Adjust the laser pulse frequency output from the optical frequency comb laser 2 so that the output pulse frequency of the optical frequency comb laser 2 is relatively lower than the pulse frequency of the main optical frequency comb laser 1. There is a small frequency offset The time interval between the next time-coincident pulse from two optical frequency comb lasers constitutes a laser interference period. .

[0032] S2. As the start of the detection, first record the initial correction reference signal of one laser interference cycle output by the second photodetector 13. , where n is the time-domain sampling point number; for The initial corrected reference signal in the frequency domain is obtained by performing a Fourier transform. w is the radio frequency of the signal point. It is the frequency conversion spectrum of the target detection gas component filled in the second reference absorption cell 10 within the frequency band of the absorption spectrum radio frequency band covered by the optical frequency comb laser wavelength, possessing the spectral characteristic structure of the gas, and taking... The center frequency point frequency is The frequency range is 2W.

[0033] S3. Synchronously record the background reference signal, correction reference signal, and detection signal output by the first photodetector 12, the second photodetector 13, and the third photodetector 14 for each laser interference cycle, and denote them as follows: , where i is the sampling period number.

[0034] S4, to Perform Fourier transforms on each band to obtain the frequency domain background reference signal for the radio frequency band. Frequency domain correction reference signal Frequency domain detection signal Since the three-channel signals are acquired synchronously, therefore Changes in the signal spectrum can reflect , The channel exhibits dynamic shifts and expansion / contraction characteristics in the frequency spectrum signal due to the frequency jitter of the master and slave optical frequency comb lasers.

[0035] S5, with For reference, the spectral characteristic structure of the target detection gas component in the second reference absorption cell 10 is utilized, through... and Cross-correlation calculation can obtain and Center frequency relative frequency offset . and The formula for calculating the cross-correlation coefficient is as follows:

[0036] in, This indicates the offset of the radio frequency relative to the current signal point. yes When taking the maximum value value.

[0037] S6, Utilization right and The frequency domain offset and scaling are corrected to obtain the corrected frequency domain background reference signal. and corrected frequency domain detection signal The corrected formula is as follows: ) ) S7. Perform multiple coherent averaging operations on the corrected signal to obtain the frequency-domain average background signal. and frequency domain average detection signal Frequency domain average background signal and frequency domain average detection signal Frequency upconversion is performed to obtain the optical background signal. and optical frequency average detection signal The conversion formula is as follows: ) ) S8. Acquisition of calibration signals: The system of the present invention has a built-in calibration absorption cell 11 with a gas absorption length of L2. The calibration signals of each target gas component can be obtained by sequentially introducing standard gas samples of each target gas component.

[0038] (1) The calibration absorption cell 11 records the output signal of the third photodetector 14 under the condition of no target gas sample. The processing procedure is the same as the processing steps S2-S7. The processed signal is the optical frequency calibration background absorption signal, denoted as .

[0039] (2) Each high-concentration standard gas sample of the target gas is sequentially introduced into the calibration absorption cell 11. The output signal of the third photodetector 14 is recorded under gas flow conditions. The processing procedure is the same as steps S2-S7. The processed signal is the optical frequency calibration detection signal of each target component gas, denoted as... Where j is the serial number of each gas to be measured, and the concentration of each component standard gas is denoted as . .

[0040] (3) The optical frequency calibration spectral characteristic signal is obtained by using the optical frequency calibration detection signal and the optical frequency calibration background absorption signal, denoted as The calculation formula is as follows:

[0041] (4) The optical frequency calibration spectral characteristic signal is normalized by the optical frequency calibration background absorption signal to obtain the optical frequency calibration spectral absorbance signal of each component, denoted as . The calculation formula is as follows:

[0042] S9. Analysis of the detection signal: Optical frequency average detection signal It can be regarded as optical frequency background signal The signal is analyzed by linearly superimposing the characteristic signals of the optical frequency calibration spectra of each component, and then using a multiple linear regression fitting algorithm. The regression algorithm formula is as follows:

[0043] in , These are the fitting parameters.

[0044] S10. Concentration inversion of each component gas: optical frequency spectrum absorbance signal of each component gas. It is calculated by the following formula:

[0045] Optical frequency spectral absorbance signals of each component gas Comparison with the optical frequency calibration spectral absorbance signal of each component It has a linear relationship, and the linear coefficients are... The fitting formula is obtained by the linear least squares fitting algorithm:

[0046] Then the gas concentration of each component It is calculated by the following formula:

[0047] in, The absorption optical path length of the optical end of the fiber-coupled transceiver. To calibrate the gas absorption length of absorption cell 11.

[0048] Through the above technical solutions, this invention acquires the background signal characteristics of a dual-comb spectrum by incorporating a built-in background reference optical path, solving the problem of reconstructing the background signal of a dual-comb spectrum under open long optical path detection conditions, and improving the accuracy of signal analysis and concentration measurement. By incorporating a built-in correction reference optical path, the dual-comb spectral structure of the target gas is acquired. The cross-correlation between the corrected reference spectral signal obtained synchronously during the detection process and the initial corrected reference signal is used to obtain the shift characteristics of the detected spectral signal, achieving real-time correction of the detected spectrum. This solves the problem of dynamic shift and scaling correction of the frequency spectrum signal caused by insufficient frequency stability of the dual-comb, meets the coherent averaging requirements of the signal during the measurement process, and improves the quality of the spectral signal. Through a built-in calibration optical path and online calibration, a multi-component gas concentration detection calibration signal is obtained under open long optical path detection conditions, meeting the needs of accurate spectral analysis and online concentration calibration for source tracing applications.

[0049] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for detecting open long-path gases based on dual-frequency comb spectroscopy, characterized in that, Includes the following steps: A. Set up a master optical frequency comb laser and a slave optical frequency comb laser. The beams output by the two are combined and then split into a first beam and a second beam. The first beam is then split into two beams. One beam passes through a first reference absorption cell to a first photodetector, forming a background reference optical path. The other beam passes through a second reference absorption cell to a second photodetector, forming a correction signal reference optical path. The second beam is transmitted as a detection beam to the fiber-coupled transceiver optical end and then returns. The returned beam passes through a calibration absorption cell to a third photodetector, forming a calibration optical path. B. Obtain the initial correction reference signal and the background reference signal, correction reference signal, and detection signal for each laser interference cycle. Perform Fourier transform on each signal to obtain the initial correction reference signal, the background reference signal, the correction reference signal, and the detection signal in the frequency domain. C. Calculate the cross-correlation coefficient between the initial corrected reference signal and the frequency domain corrected reference signal, and take the frequency offset corresponding to the maximum cross-correlation coefficient as the frequency offset. Using frequency offset The frequency domain offset and scaling of the frequency domain background reference signal and the frequency domain detection signal are corrected. The corrected signals are coherently accumulated and averaged multiple times to obtain the frequency domain average background signal and the frequency domain average detection signal. The frequency domain average background signal and the frequency domain average detection signal are upconverted to obtain the optical frequency background signal and the optical frequency average detection signal. D. Standard gas samples of each target gas component are sequentially introduced into the calibration optical path to obtain the optical frequency calibration spectral absorbance signal of each component. The optical frequency background signal, the optical frequency average detection signal, and the optical frequency calibration spectral absorbance signal of each component are used to perform signal analysis and inversion of the gas concentration of each component to be measured, so as to obtain the gas concentration of each component.

2. The method for detecting open long optical path gases based on dual optical frequency comb spectroscopy according to claim 1, characterized in that, The process of obtaining the initial frequency domain correction reference signal is as follows: Record the initial correction reference signal obtained by the second photodetector for one laser interferometry cycle. , where n is the time-domain sampling point number; for The initial corrected reference signal in the frequency domain is obtained by performing a Fourier transform. w is the radio frequency of the signal point, taken as The center frequency point frequency is The frequency range is 2W.

3. The method for detecting open long optical path gases based on dual optical frequency comb spectroscopy according to claim 2, characterized in that, The acquisition process of the frequency domain background reference signal, the frequency domain correction reference signal, and the frequency domain detection signal is as follows: Record the background reference signal, correction reference signal, and detection signal output from the photodetectors corresponding to the background reference optical path, correction signal reference optical path, and calibration optical path for each laser interference cycle, denoted as follows: , where i is the sampling period number; for Perform Fourier transforms on each band to obtain the frequency domain background reference signal for the radio frequency band. Frequency domain correction reference signal Frequency domain detection signal .

4. The open long-path gas detection method based on dual-frequency comb spectroscopy according to claim 3, characterized in that, The cross-correlation coefficient between the initial corrected reference signal and the frequency domain corrected reference signal is calculated as follows: in, This indicates the offset of the radio frequency relative to the current signal point.

5. The method for detecting open long optical path gases based on dual optical frequency comb spectroscopy according to claim 3, characterized in that, Step C includes: use right and The frequency domain offset and scaling are corrected to obtain the corrected frequency domain background reference signal. and corrected frequency domain detection signal The corrected formula is as follows: ) ) The frequency-domain average background signal is obtained by performing multiple coherent cumulative averaging operations on the corrected signal. and frequency domain average detection signal Frequency domain average background signal and frequency domain average detection signal Frequency upconversion is performed to obtain the optical background signal. and optical frequency average detection signal The conversion formula is as follows: ) ) in, This represents the pulse frequency of the master optical frequency comb laser in a dual-frequency comb system. The frequency offset of the pulse frequency of the slave optical frequency comb laser relative to the master optical frequency comb laser in a dual optical frequency comb system.

6. The method for detecting open long optical path gases based on dual optical frequency comb spectroscopy according to claim 5, characterized in that, The process of obtaining the optical frequency calibration spectral absorbance signals of each component is as follows: Record the output signal of the photodetector in the calibration optical path under conditions without a target gas sample. The processing procedure is the same as in step C. The processed signal is the optical frequency calibration background absorption signal, denoted as . ; Each standard gas sample of known concentration is sequentially introduced into the calibration absorption cell. The output signal of the third photodetector is recorded under gas flow conditions. The processing procedure is the same as in step C. The processed signal is the optical frequency calibration detection signal for each target component gas, denoted as... Where j is the serial number of each gas to be measured, and the concentration of each component standard gas is denoted as . ; The optical frequency calibration spectral characteristic signal is obtained by using the optical frequency calibration detection signal and the optical frequency calibration background absorption signal. The calculation formula is as follows: The optical frequency calibration spectral characteristic signal was normalized by the optical frequency calibration background absorption signal to obtain the optical frequency calibration spectral absorbance signal of each component. The calculation formula is as follows: 。 7. The method for detecting open long optical path gases based on dual optical frequency comb spectroscopy according to claim 6, characterized in that, The signal analysis process is as follows: Optical frequency average detection signal Considered as optical frequency background signal The signal is analyzed by linearly superimposing the characteristic signals of the optical frequency calibration spectra of each component, and then using a multiple linear regression fitting algorithm. The formula is as follows: in, , These are the fitting parameters.

8. The method for detecting open long optical path gases based on dual optical frequency comb spectroscopy according to claim 7, characterized in that, The process for inverting the concentrations of the components to be measured is as follows: Optical frequency spectral absorbance signals of each component gas It is calculated by the following formula: Optical frequency spectral absorbance signals of each component gas Comparison with the optical frequency calibration spectral absorbance signal of each component It has a linear relationship, and the linear coefficients are... The fitting formula is obtained by the linear least squares fitting algorithm: Then the gas concentration of each component It is calculated by the following formula: in, The absorption optical path length of the optical end of the fiber-coupled transceiver. To calibrate the gas absorption length of the absorption cell.

9. An open long-path gas detection system based on dual-frequency comb spectroscopy, characterized in that, The system includes a master optical frequency comb laser, a slave optical frequency comb laser, an optical fiber coupled polarization controller, a first optical fiber beam splitter, a second optical fiber beam splitter, a first optical fiber collimating lens, and a third optical fiber collimating lens. Linearly polarized light output from the slave optical frequency comb laser passes through an optical fiber coupled polarization controller to adjust its polarization direction to be the same as that of the linearly polarized light output from the master optical frequency comb laser. The beams output from the master optical frequency comb laser and the optical fiber coupled polarization controller are then combined and split by the first optical fiber beam splitter. The first beam output from the first optical fiber beam splitter serves as a reference beam, which is split into two beams by the second optical fiber beam splitter. One beam output from the second optical fiber beam splitter is collimated by the first optical fiber collimating lens and passes through a first reference absorption cell to reach a first photodetector, forming a background reference optical path. A background reference signal is obtained through photoelectric conversion. The first reference absorption cell is filled with nitrogen gas. The other beam output from the second optical fiber beam splitter is collimated by the second optical fiber collimating lens and passes through a second reference absorption cell to reach a second photodetector, forming a correction signal reference optical path. A correction reference signal is obtained through photoelectric conversion. The second reference absorption cell is filled with a gas sample containing a target detection gas component. The second beam output from the first fiber beam splitter is used as the detection beam and transmitted through a single-mode fiber to the fiber-coupled transceiver optical end. A corner reflector is provided on the output optical path of the fiber-coupled transceiver optical end. The beam output from the fiber-coupled transceiver optical end is reflected back by the corner reflector. The beam collected by the fiber-coupled transceiver optical end and reflected back from the corner reflector returns through a multimode fiber. The returned beam is collimated by a third fiber collimating lens and then passes through a calibration absorption cell to reach a third photodetector, forming a calibration optical path. The detection signal is obtained through photoelectric conversion. The background reference signal, the correction reference signal, and the detection signal are transmitted to the host computer, and the host computer executes the method described in any one of claims 1-8.

10. The open long-path gas detection system based on dual-frequency comb spectroscopy according to claim 9, characterized in that, The fiber-coupled transceiver optical terminal has a built-in fiber-coupled lens, a reflector, a Fresnel lens, and a fourth fiber-coupled collimating lens. The detection beam is first collimated by the fourth fiber-coupled collimating lens, then reflected by the reflector and output through the center of the Fresnel lens. After passing through the detection area, the output beam reaches the corner reflector placed at the far end of the optical path. After being reflected by the corner reflector, the detection beam returns along the original optical path. The returned beam is collected by the Fresnel lens and converged on the fiber-coupled lens. The fiber-coupled lens couples the returned beam into the multimode fiber and transmits it back to the third fiber-coupled collimating lens.

Images