DEVICE FOR COLLABORATIVE DETECTION OF CARBON AND NITROGEN EMISSIONS AND METHOD THEREOF

Abstract

A device for collaborative detection of carbon and nitrogen emissions and a method thereof are provided. The device includes an integrating cavity, a single beam laser and a dual beam hybrid laser set at opposite ends of the integrating cavity, and an information acquisition and analysis system. The single beam laser and the dual beam hybrid laser are respectively used to continuously emit and transmit laser beams into the integrating cavity. The single beam laser and the dual beam hybrid laser can achieve simultaneous spatio-temporal high-frequency detection of any concentration of CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas in one integrating cavity, and the concentration of CO.sub.2, CH.sub.4, and N.sub.2O can be inverted through the information acquisition and analysis system. This device meets the research demand for simultaneous detection of multiple greenhouse gases under the same path, greatly improving the utilization efficiency of the integrating cavity and saving detection costs.

Claims

1. A collaborative measurement method for carbon and nitrogen emission gas concentration of a carbon and nitrogen emission collaborative detection device, comprising an integrating cavity, a single beam laser and a dual beam hybrid laser set at opposite ends of the integrating cavity, and an information acquisition and analysis system, wherein the single beam laser and the dual beam hybrid laser are used to continuously emit and transmit laser beams to the integrating cavity, respectively; the integrating cavity comprises a gas absorption cell with a CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas, the dual beam mixed laser is located at a left end of the gas absorption cell, and the single beam laser is located at a right end of the gas absorption cell; the information acquisition and analysis system comprises a first concave mirror arranged on an optical path between the dual beam hybrid laser and the gas absorption cell, a second concave mirror arranged on an optical path between the single beam laser and the gas absorption cell, a first optical filter and a first photodetector arranged in sequence on a reflected light path of the first concave mirror, a second optical filter and a second photodetector arranged in sequence on a reflected light path of the second concave mirror, a data acquisition card connected to the first photodetector and the second photodetector respectively, and a computer connected to the data acquisition card; the first concave mirror and the second concave mirror are respectively provided with incident holes, a concave surface of the first concave mirror and a concave surface of the second concave mirror are both inclined relative to the gas absorption cell, the concave surface of the first concave mirror is configured to reflect a laser beam passing through the gas absorption cell onto the first optical filter, and the concave surface of the second concave mirror reflects a laser beam passing through the gas absorption cell onto the second optical filter; the first concave mirror allows a laser of the dual beam hybrid laser to pass through the incident hole and enter the gas absorption cell, the concave surface of the first concave mirror reflects the light reflected by the gas absorption cell to the first optical filter, the laser beam within a band range of the first optical filter passes through the first optical filter and enter the first photodetector, and the laser beam outside the band range of the first optical filter is reflected back into the gas absorption cell according to the original path; the second concave mirror allows the laser of the single beam laser to pass through the incident hole and enter the gas absorption cell, while reflecting the light feedback from the gas absorption cell to the second optical filter; during this process, the laser beam within a band range of the second optical filter passes through the second optical filter and enter the second photodetector, and the laser beam outside band range of the second optical filter is reflected back into the gas absorption cell according to the original path; wavelength ranges of the first optical filter and the second optical filter are different, a wavelength of a laser beam emitted by the dual beam hybrid laser is within the wavelength range of the first optical filter, while a wavelength of a laser beam emitted by the single beam laser is within the wavelength range of the second optical filter; wherein: the computer collects voltage signals from the first photodetector and/or the second photodetector through the data acquisition card, and the computer inverts and calculates the concentrations of CO.sub.2, CH.sub.4, N.sub.2O in the CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas to be measured based on the voltage signals; characterized in that, the method comprises the following steps: step A, determining optimal structural parameters for a curvature radius of the first concave mirror, a curvature radius of the second concave mirror, a distance and an angle between the concave mirrors and a cavity mirror in the integrating cavity, and optimal positions of the first photodetector and the second photodetector through an optical design software; step B, establishing atmospheric transmittance models for CO.sub.2, CH.sub.4, N.sub.2O and their selected isotope molecules, and selecting spectral detection windows and target spectral line combinations suitable for simultaneous detection of CO.sub.2, CH.sub.4, and N.sub.2O greenhouse gases; step C, using the dual beam hybrid laser and the single beam laser respectively receive superimposed signals of high-frequency sine waves and low-frequency triangular waves for operation to generate laser beams separately, wherein the high-frequency sine waves are generated by a sine wave generator, and the low-frequency triangular waves generated by the sawtooth wave generator; a first quantum cascade laser and a second quantum cascade laser with similar wavelengths are placed on the same side of the dual beam hybrid laser, two laser beams from the first quantum cascade laser and the second quantum cascade laser are coupled by a fiber optic coupler, after being collimated by a collimator, then coupled into the integrating cavity at appropriate positions and off-axis angles through the incident hole on the concave mirror on the same side of the integrating cavity; the third quantum cascade laser of the single beam laser is set on the other side of the integrating cavity, after being collimated by a collimator, laser beams from the third quantum cascade laser then coupled into the integrating cavity through the incident hole on the concave mirror on the same side of the integrating cavity at an appropriate position and off-axis angle; the two coupled laser beams enter from both sides of the integrating cavity, then pass through the integrating cavity with the gas to be detected separately, then are focused and reflected by the concave mirror on one side of the exit cavity mirror, filtered out by a narrow band-pass filter of the corresponding wavelength to remove opposing beams, and then are received by photodetectors and demodulated by a lock-in amplifier to obtain two spectral lines corresponding to CO.sub.2, two spectral lines corresponding to CH.sub.4, and three spectral lines corresponding to N.sub.2O; step D, removing background signals for the two second harmonic signals corresponding to CO.sub.2, the two second harmonic signals corresponding to CH.sub.4, and the three second harmonic signals corresponding to N.sub.2O corresponding to the target greenhouse gas obtained to obtain absorption related signals corresponding to the seven spectral lines, and using a wavelet denoising method to reduce noises in a WMS-2f signal measured by the photodetector; step E, based on the pressure and temperature information inside the integrating cavity, obtaining the absorption state signals of CO.sub.2, CH.sub.4, and N.sub.2O corresponding to the second harmonic signals of the spectral lines under the same temperature and pressure environment; step F, performing inversion calculations on peaks of absorption related signals corresponding to CO.sub.2, CH.sub.4, N.sub.2O, and their selected isotopes to obtain the concentrations of CO.sub.2, CH.sub.4, and N.sub.2O, respectively.

2. The collaborative measurement method for carbon and nitrogen emission gas concentration according to claim 1, wherein the selection of target spectral lines in step B follows the standard that is suitable for the spectral detection window and target spectral line combination for simultaneous detection of CO.sub.2, CH.sub.4, and N.sub.2O molecules and isotopic molecules are: step B1, the selected absorption spectral line combinations for CO.sub.2, CH.sub.4, and N.sub.2O molecules and their isotopes are required to distribute within the range of 2 cm.sup.1; step B2, based on the distribution of absorption bands of each molecule and its isotopes, analyzing the leading mixing effect between each band to ensure that the spacing between spectral lines within each combination is not less than 0.1 cm.sup.1; step B3, the strengths of absorption spectral lines of each combination is required as high as possible to ensure that the system has good detection sensitivity; step B4, considering the abundance of each molecular isotope, the absorption depth of each spectral line within the combination is required not differ too much, ensuring that all detection spectral lines have a good signal-to-noise ratio; step B5, within each spectral line combination, ensuring there is not absorption interference from other molecules with higher concentrations in the atmosphere as possible; step B6, the lower energy level of each selected spectral line is required as small as possible, so as to effectively reduce the impact of spectral line strength changes caused by environmental temperature changes on measurement accuracy.

3. The collaborative measurement method for carbon and nitrogen emission gas concentration according to claim 2, wherein the inversion of concentration in step F is as follows: step F1, performing vacuum treatment on the integrating cavity, and using CO.sub.2, CH.sub.4, N.sub.2O, and N.sub.2 to prepare CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gases with different concentrations; step F2, synchronously collecting emitted signals, triangular wave signals, and sine wave signals after multiple reflections of the mixed gas, demodulating the collected signals from the integrating cavity at the corresponding sine wave modulation frequency to obtain the second harmonic signals of the CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas; step F3, calibrating the system: providing the relationship between the peak height of the second harmonic signal and the concentration at the 5 concentrations in step F1; performing polynomial fitting on the peak height at 5 concentrations obtained in step F1 to obtain a calibration formula; based on the calibration formula, calculating the concentration of at least one gas in CO.sub.2, CH.sub.4, and N.sub.2O in the CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas to be measured; step F4, measuring CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas with any concentration, and applying the calibration formula in step F3 to reverse calculate the concentration of at least one gas in in CO.sub.2, CH.sub.4, and N.sub.2O in the CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas to be measured.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] FIG. 1 is a schematic diagram of the structure of a device for collaborative detection of carbon and nitrogen emissions;

[0077] FIG. 2 is a schematic diagram of the noise suppression method used in the dual beam hybrid laser of the present invention.

[0078] FIG. 3 is a schematic diagram of a method for collaborative measurement of carbon and nitrogen emissions of the present invention.

[0079] Reference marks in the figures: 1sine wave generator, 2sawtooth wave generator, 3adder, 4first laser controller, 5second laser controller, 6third laser controller, 7first quantum cascade laser, 8second quantum cascade laser, 9third quantum cascade laser, 10beam combiner, 11collimator, 12first concave mirror, 13gas absorption pool, 14first optical filter, 15first photodetector, 16second concave mirror, 17second optical filter, 18second photodetector, 19data acquisition card, 20computer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0080] To achieve simultaneous detection of all molecules and their isotopes with detection by accurately selecting detection spectral lines using two set of lasers, the present invention provides a carbon and nitrogen emission collaborative detection device that can achieve simultaneous, spatio-temporal, and high-frequency detection of three important greenhouse gases, CO.sub.2 (carbon dioxide), CH.sub.4 (methane), and N.sub.2O (nitrous oxide), within the same integrating cavity.

[0081] The present invention adopts the following technical solution: a carbon and nitrogen emission collaborative detection device that simultaneously monitors three gases: CO.sub.2, CH.sub.4, and N.sub.2O.

[0082] The carbon and nitrogen emission collaborative detection device includes an integrating cavity, a single beam laser and a dual beam hybrid laser set at opposite ends of the integrating cavity, and an information acquisition and analysis system, wherein the single beam laser and the dual beam hybrid laser are used to continuously emit and transmit laser beams to the integrating cavity, respectively. The integrating cavity includes a gas absorption cell with a CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas, the dual beam mixed laser is located at the left end of the gas absorption cell, and the single beam laser is located at the right end of the gas absorption cell. The information acquisition and analysis system includes a first concave mirror arranged on an optical path between the dual beam hybrid laser and the gas absorption cell, a second concave mirror arranged on an optical path between the single beam laser and the gas absorption cell, a first optical filter and a first photodetector arranged in sequence on a reflected light path of the first concave mirror, a second optical filter and a second photodetector arranged in sequence on a reflected light path of the second concave mirror, a data acquisition card connected to the first photodetector and the second photodetector respectively, and a computer connected to the data acquisition card. The first concave mirror and the second concave mirror are respectively provided with incident holes, a concave surface of the first concave mirror and a concave surface of the second concave mirror are both inclined relative to the gas absorption cell, the concave surface of the first concave mirror is configured to reflect a laser beam passing through the gas absorption cell onto the optical filter, and the concave surface of the second concave mirror reflects a laser beam passing through the gas absorption cell onto the second optical filter. The first concave mirror allows a laser of the dual beam hybrid laser to pass through the incident hole and enter the gas absorption cell, the concave surface of the first concave mirror reflects the light reflected by the gas absorption cell to the first optical filter, the laser beam within a band range of the first optical filter passes through the first optical filter and enter the first photodetector, and the laser beam outside the band range of the first optical filter is reflected back into the gas absorption cell according to the original path. The second concave mirror allows the laser of the single beam laser to pass through the incident hole and enter the gas absorption cell, while reflecting the light feedback from the gas absorption cell to the second optical filter. During this process, the laser beam within a band range of the second optical filter passes through the second optical filter and enter the second photodetector, and the laser beam outside band range of the second optical filter is reflected back into the gas absorption cell according to the original path. The wavelength ranges of the first optical filter and the second optical filter are different, a wavelength of a laser beam emitted by the dual beam hybrid laser is within the wavelength range of the first optical filter, while a wavelength of a laser beam emitted by the single beam laser is within the wavelength range of the second optical filter.

[0083] Wherein:

[0084] The computer collects voltage signals from the first photodetector and/or the second photodetector through the data acquisition card, and the computer inverts and calculates the concentrations of CO.sub.2, CH.sub.4, N.sub.2O in the CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas to be measured based on the voltage signals.

[0085] The length of the gas absorption cell is 30 cm.

[0086] Both the first concave mirror and the second concave mirror are silver coated concave spherical mirrors.

[0087] The carbon and nitrogen emission collaborative detection device also includes narrowband filters that can serve as the first optical filter and the second optical filter. The first optical filter corresponding to the wavelength of the opposing laser beam is placed in front of the first photodetector, and the second optical filter corresponding to the wavelength of the opposing laser beam is placed in front of the second photodetector. At the same time, the reflection of the optical filter on the beam outside the bandpass wavelength can be used to reinject and enhance the beam on the local side.

[0088] As a preferred technical solution of the present invention, the first quantum cascade laser, the second quantum cascade laser, and the third quantum cascade laser all use DFB tunable semiconductor lasers.

Embodiment 1

[0089] As shown in FIG. 1 and FIG. 2, the carbon and nitrogen emission collaborative detection device of the present invention includes a laser group, a gas absorption cell, and an information acquisition and analysis system.

[0090] Specifically, the carbon and nitrogen emission collaborative detection device further includes a sine wave generator 1, a sawtooth wave generator 2, an adder 3, a first laser controller 4, a second laser controller 5, a third laser controller 6, a first quantum cascade laser 7, a second quantum cascade laser 8, a third quantum cascade laser 9, a beam combiner 10, a collimator 11, a first optical filter 14 and the second optical filter 17. The sine wave generator 1 is a function generator RIGOL, and the technical parameters of DG1000Z1 are: the frequency of the sine wave is 12 kHz, and a function signal generator model F05 can be selected. The technical parameters of the sawtooth wave generator 2 are: the frequency of sawtooth wave is 30 Hz, and a function signal generator with model F05 can be selected. The technical parameters of the laser controller are: temperature adjustment range is 20-30 C., step size is 0.5 C. with accuracy of 0.005 C., current tuning range is 20-110 mA with accuracy of 0.001 mA, and ITC-4002QCL controller can be selected. The technical parameters of the first quantum cascade laser 7: center wavelength is located at 3.3 m, the maximum output power is 5 mW. The technical parameters of the second quantum cascade laser 8: center wavelength is located at 4.3 m, the maximum output power is 5 mW. The technical parameters of the third quantum cascade laser 9: the center wavelength is located at 4.5 m, and the maximum output power is 5 mW.

[0091] In this embodiment, the gas absorption cell is filled with a mixture of carbon dioxide, methane, nitrous oxide, and nitrogen gas. The concentrations of carbon dioxide, methane, and nitrous oxide can be calculated by the volume ratios of carbon dioxide, methane, and nitrous oxide, respectively. The long-range gas absorption cell 13 includes a cylindrical cavity made of Pyrex glass material, with a volume of 3.2 L. The end of the cylindrical cavity is equipped with a light inlet and a light outlet. The side of the cylindrical cavity is equipped with an exhaust port and an intake port, both of which are equipped with needle valves. A discrete concave mirror is located near each end of the cylindrical cavity, with a distance of 55 cm between the two discrete concave mirrors.

[0092] The information collection and analysis system includes a first photodetector 15, a second photodetector 18, a data acquisition card 19, and a computer 20. The technical parameters of the first photodetector 15 and the second photodetector 18: the response band is 1200-1800 nm, and the 2011 InGaAs photodetector can be selected. The technical parameters of data acquisition card 19: PCI bus is used and supports plug and play, with 2 channels of 12 bit D/A output, 8 channels of non-phase-difference analog input, 1 channel of 16 bit counter, and 16 channels of programmable switch quantity. The acquisition conversion can support multiple triggering forms and can use AC6115 data acquisition card.

Working Principle:

Working Principle of the Present Invention:

[0093] Step F1, performing vacuum treatment on an integrating cavity, preparing different concentrations of carbon dioxide-methane-nitrous oxide-nitrogen (CO.sub.2CH.sub.4N.sub.2ON.sub.2) mixed gases using carbon dioxide gas, methane gas, nitrous oxide gas, and high-purity nitrogen gas, filling the integrating cavity with mixed gases of carbon dioxide-methane-nitrous oxide-nitrogen (CO.sub.2CH.sub.4N.sub.2ON.sub.2) of 450 ppm-2 ppm-0.4 ppm, 600 ppm-3 ppm-0.5 ppm, 700 ppm-4 ppm-0.6 ppm, 800 ppm-5 ppm-0.7 ppm, and 900 ppm-6 ppm-0.8 ppm, respectively; [0094] Step F2, synchronously collecting emitted signals, triangular wave signals, and sine wave signals after multiple reflections of the samples, demodulating the collected signals from the integrating cavity at the corresponding sine wave modulation frequency to obtain the second harmonic signals of the target gas; [0095] Step F3, calibrating the system, providing the relationship between the peak height of the second harmonic signal and the concentration at the 5 concentrations in step F1; performing polynomial fitting on the peak height at all concentrations obtained to obtain a calibration formula; [0096] Step F4, measuring any concentration of CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas, and using the calibration formula in step F3 to invert the concentration of at least one gas of CO.sub.2, CH.sub.4, and N.sub.2O in the CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas to be measured.

[0097] The output terminals of the sine wave generator 1 and the sawtooth wave generator 2 are respectively connected to the input terminals of the adder 3 and the data acquisition card 19. The output terminal of the adder 3 is connected to the input terminal of the first laser controller 4. The output terminal of the first laser controller 4 is connected to the first quantum cascade laser 7, and the output terminal of the second laser controller 5 is connected to the second quantum cascade laser 8. The output lights of the first quantum cascade laser 7 and the second quantum cascade laser 8 pass through the collimator 10, then the light after passing through the collimator 10 passes through the first concave mirror 12, the gas absorption cell 13, the second concave mirror 16, the first optical filter 14, and the first photodetector 15 in sequence. The output terminal of the third laser controller 6 is connected to the third quantum cascade laser 9. After passing through the collimator, the output light of the third quantum cascade laser 9 passes through the second concave mirror 16, the gas absorption cell 13, the first concave mirror 12, the second optical filter 17, and the second photodetector 18 in sequence. The output terminals of the first photodetector and the second photodetector are both connected to the input terminal of the data acquisition card, and the output terminal of the data acquisition card is connected to the input terminal of the computer. The present invention uses frequency division multiplexing technology and wavelength modulation spectroscopy technology. The implementation method is to load a high-frequency sine wave modulation signal into the injection current of the laser, causing the laser output to oscillate sinusoidally near its optical frequency. When demodulating the signal, only narrow bandwidth signals near the second harmonic wave of the modulation frequency are extracted, and the tuning range is achieved near the absorption spectral line of the target gas. This effectively suppresses noise signals in most other frequency bands, greatly improving the signal-to-noise ratio.

[0098] At the same time, a lower frequency sawtooth wave is simultaneously loaded into the injection current of the laser, allowing the center wavelength of the laser output to fully scan the gas absorption spectrum. The two absorbed light signals are converted into voltage signals by the photodetector and collected by the data acquisition card, and finally sent to the computer 20 for processing. Using Lab VIEW to obtain signals through the data acquisition card and send them to digital phase-locked, the signals are demodulated according to the different modulation frequencies of two lasers. Subsequently, the collected signals are subjected to wavelet denoising and Kalman filtering to obtain the second harmonic signals of each detection spectral line.

[0099] Firstly, conducting a series test of CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas spectral lines with different concentrations within the set range of the laser, selecting some spectral line pairs with high correlation, establishing models for these spectral line pairs, then inverting the concentrations of carbon monoxide and methane to obtain calibration formula, and then conducting measurement on any concentration of CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas using the calibration formula, so as to invert the concentrations of CO.sub.2, CH.sub.4, N.sub.2O and other gases to be measured.

[0100] As shown in FIG. 3, a collaborative measurement method for carbon and nitrogen emission gas concentration is provided by the present invention, which includes the following steps: [0101] Step A, determining optimal structural parameters for a curvature radius of the first concave mirror, a curvature radius of the second concave mirror, a distance and an angle between the concave mirror and a cavity mirror in the integrating cavity, and optimal positions of the first photodetector and the second photodetector through an optical design software; [0102] Step B, establishing atmospheric transmittance models for CO.sub.2, CH.sub.4, N.sub.2O and their selected isotope molecules, and selecting spectral detection windows and target spectral line combinations suitable for simultaneous detection of CO.sub.2, CH.sub.4, and N.sub.2O greenhouse gases and their selected isotope molecules; [0103] Step C, using the dual beam hybrid laser and the single beam laser respectively receive superimposed signals of high-frequency sine waves and low-frequency triangular waves for operation to generate laser beams separately, wherein the high-frequency sine waves are generated by a sine wave generator, and the low-frequency triangular waves generated by the sawtooth wave generator, wherein a first quantum cascade laser and a second quantum cascade laser with similar wavelengths are placed on the same side of the dual beam hybrid laser, two laser beams from the first quantum cascade laser and the second quantum cascade laser are coupled by a fiber optic coupler, after being collimated by a collimator, then coupled into the integrating cavity at appropriate positions and off-axis angles through the incident hole on the concave mirror on the same side of the integrating cavity; the third quantum cascade laser of the single beam laser is set on the other side of the integrating cavity, after being collimated by a collimator, laser beams from the third quantum cascade laser then coupled into the integrating cavity through the incident hole on the concave mirror on the same side of the integrating cavity at an appropriate position and off-axis angle; the two coupled laser beams enter from both sides of the integrating cavity, then pass through the integrating cavity with the gas to be detected separately, then are focused and reflected by the concave mirror on one side of the exit cavity mirror, then pass through a narrow band-pass filter of the corresponding wavelength to filter out opposing beams, and then are received by photodetectors and demodulated by a lock-in amplifier to obtain two spectral lines corresponding to CO.sub.2 (These two spectral lines correspond to .sup.16O.sup.12C.sup.16O and .sup.16O.sup.13C.sup.16O, respectively), two spectral lines corresponding to CH.sub.4 (These two spectral lines correspond to .sup.12CH.sub.4 and .sup.13CH.sub.4, respectively), and three spectral lines corresponding to N.sub.2O (These three spectral lines correspond to .sup.14N.sub.2.sup.16O, .sup.14N.sup.15N.sup.16O, and .sup.15N.sup.14N.sup.16O, respectively); [0104] Step D, removing background signals for the two second harmonic signals corresponding to CO.sub.2, the two second harmonic signals corresponding to CH.sub.4, and the three second harmonic signals corresponding to N.sub.2O corresponding to the target greenhouse gas obtained to obtain absorption related signals corresponding to the seven spectral lines, and using a wavelet denoising method to reduce noises in a WMS-2f signal measured by the photodetector; [0105] Step E, based on the pressure and temperature information inside the integrating cavity, obtaining the absorption state signals of CO.sub.2, CH.sub.4, and N.sub.2O corresponding to the second harmonic signals of the spectral lines under the same temperature and pressure environment; [0106] Step F, performing inversion calculations on peaks of absorption related signals corresponding to the selected isotopes of CO.sub.2, CH.sub.4, N.sub.2O to obtain their concentrations, respectively.

[0107] Further, in step A, the optimal structural parameters are determined using the following method:

[0108] Using Matlab to simulate and analyze the intracavity light field using a decentered Gaussian beam, the intracavity light field in bi-directional mode can be described by the sum of the incident light fields on both sides:

[00007]$\begin{array}{c}\hat{E}\left(x,y,z\right)=\hat{E}\left(x_1,y_1,z_1\right)+\hat{E}\left(x_2,y_2,z_2\right)\\ \hat{E}\left(x_i,y_i,z_i\right)={\left(\frac{2}{}\right)}^{1/2}.Math.\frac{{\stackrel{}{q}}_0}{\stackrel{}{q}\left(z_i\right)}\exp \left(-{\mathrm{jkz}}_i-\mathrm{jk}\frac{{\left(x_i-{\stackrel{}{p}}_{x_i}\right)}^2+{\left(y_i-{\stackrel{}{p}}_{y_i}\right)}^2}{2\stackrel{}{q}\left(z_i\right)}\right),\end{array}$

[0109] At this point, the light field of any N times reflected light spots on the cavity mirror can be calculated from the light field matrix M of the first light spot:

[00008]$\left[\begin{array}{cc}A& B\\ C& D\end{array}\right]=M^N{M}^{},M=\left[\begin{array}{cc}1& d\\ 0& 1\end{array}\right]\left[\begin{array}{cc}1& 0\\ -2/r& 1\end{array}\right]\left[\begin{array}{cc}1& d\\ 0& 1\end{array}\right]\left[\begin{array}{cc}1& 0\\ -2/r& 1\end{array}\right],M^{}=\left[\begin{array}{cc}1& d\\ 0& 1\end{array}\right]$

[0110] By calculating the vector sum of all light spots on the cavity mirror, the total light field emitted by the cavity mirror on one side of the integrating cavity can be obtained:

[00009]${\hat{E}}_{\mathrm{out}}\left(x,y\right)=\underset{n=0}{\overset{+}{.Math.}}{\hat{E}}_{0,n}{t}^2.Math.r^{2n}.Math.\exp \left(-\mathrm{jkd}-\mathrm{jk}.Math.2\mathrm{nd}\right).Math.\exp \left(-\mathrm{jk}\frac{{\left(x-{\hat{p}}_{x,n}\right)}^2+{\left(y-{\hat{p}}_{y,n}\right)}^2}{2{\hat{q}}_n}\right),$

[0111] Further, the transmittance spectrum on one side of the integrating cavity is obtained from the output and input power:

[00010]$T_{ca}\left(\right)=\frac{p_{out}\left(\right)}{p_{in}\left(\right)}=\frac{I_{out}dxdy}{I_{in}dxdy}=\frac{{\hat{E}}_{out}{\stackrel{}{E}}_{out}^{*}\mathrm{dxdy}}{{\hat{E}}_{in}{\hat{E}}_{in}^{*}\mathrm{dxdy}},$

[0112] In order to improve computational efficiency, the cavity mirror will be divided into a series of grids during the simulation process. During the calculation, only the light field at the grid points can be considered, and the total power can be obtained by multiplying the irradiance at the grid points by the area of the grid. Here, by changing parameters such as cavity length, cavity mirror curvature radius, incident beam diameter, off-axis angle of incident light on both sides, and incident position, the Airy function will be used to simulate and analyze the intracavity light field and transmittance spectrum under various conditions. Combined with the balance between the optimal optical path length and the optimal signal-to-noise ratio, the optimal structural parameters for bidirectional coupling detection mode will be obtained.

[0113] Furthermore, specifically, the selection of target spectral lines in step B follows the standard that is suitable for the spectral detection window and the target spectral line combination for simultaneous detection of CO.sub.2, CH.sub.4, and N.sub.2O molecules and isotopic molecules are: [0114] Step B1, the selected absorption spectral line combinations for CO.sub.2, CH.sub.4, and N.sub.2O molecules and their isotopes are required to distribute within the range of 2 cm.sup.1, so as to ensure full coverage through a single scan of the laser; [0115] Step B2, based on the distribution of absorption bands of each molecule and its isotopes, analyzing the leading mixing effect between each band to ensure that the spacing between spectral lines within each combination is not less than 0.1 cm.sup.1, so as to ensure that spectral line mixing under normal pressure does not affect measurement accuracy; [0116] Step B3, the strengths of absorption spectral lines of each combination is required as high as possible to ensure that the system has good detection sensitivity; [0117] Step B4, considering the abundance of each molecular isotope, the absorption depth of each spectral line within the combination is required not differ too much, ensuring that all detection spectral lines have a good signal-to-noise ratio; [0118] Step B5, within each spectral line combination, ensuring there is not absorption interference from other molecules with higher concentrations in the atmosphere as possible; [0119] Step B6, the lower energy level of each selected spectral line is required as small as possible, so as to effectively reduce the impact of spectral line strength changes caused by environmental temperature changes on measurement accuracy.

[0120] Further, the inversion of concentration in step F is as follows: [0121] Step F1, performing vacuum treatment on the integrating cavity, and using CO.sub.2, CH.sub.4, N.sub.2O, and N.sub.2 to prepare CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gases with different concentrations, filling the integrating cavity with mixed gases of carbon dioxide-methane-nitrous oxide-nitrogen (CO.sub.2CH.sub.4N.sub.2ON.sub.2) of 450 ppm-2 ppm-0.4 ppm, 600 ppm-3 ppm-0.5 ppm, 700 ppm-4 ppm-0.6 ppm, 800 ppm-5 ppm-0.7 ppm, and 900 ppm-6 ppm-0.8 ppm, respectively; [0122] Step F2, synchronously collecting emitted signals, triangular wave signals, and sine wave signals after multiple reflections of the mixed gas, demodulating the collected signals from the integrating cavity at the corresponding sine wave modulation frequency to obtain the second harmonic signals of the CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas; [0123] Step F3, calibrating the system: providing the relationship between the peak height of the second harmonic signal and the concentration at the 5 concentrations in step F1; performing polynomial fitting on the peak height at 5 concentrations obtained in step F1 to obtain a calibration formula:

[00011]$X=-1.2722+65.3014\mathrm{XP}-504.2734X{P}^2+2444.9629X{P}^3$

[0124] In the formula, X represents the corresponding gas concentration, P represents the peak to peak value of the second harmonic wave, and the peak to peak value of the second harmonic wave represents the peak height of the WMS-2f signal. Parameter obtainment: First, calibrating the system with a set of known gas concentrations to obtain the relationship between the peak height and concentration of the WMS-2f signal (referred to as the calibration formula). During measurement, the inversion of the measured gas concentration is achieved through a calibration formula. The experiment measured five concentrations of CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas and calibrated the system. The relationship between the peak height of the WMS-2f signal and the concentration obtained at the above 5 concentrations was obtained, with a linear correlation of R2=0.991. The peak heights at all measured concentrations were fitted with a cubic polynomial, and the correlation coefficient obtained from the fitting was R2=0.996;

[0125] At this point, the calibration of the system has been completed, and the calibration formula obtained is a cubic polynomial as follows:

[00012]$X=-1.2537+58.4574P-498.1725P^2+2322.4533P^3,$

[0126] Step F4, measuring any concentration of CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas, and using the calibration formula in step F3 to invert the concentration of at least one gas of CO.sub.2, CH.sub.4, and N.sub.2O in the CO.sub.2CH.sub.4N.sub.2ON.sub.2 mixed gas to be measured.

[0127] The method for collaborative measurement of carbon and nitrogen emissions designed in the above technical solution, based on the bidirectional coupling detection mode OA-CEAS, can achieve simultaneous detection of the concentrations of three important greenhouse gases CO.sub.2, CH.sub.4, and N.sub.2O, as well as some isotopic molecular gases, at the ppb level. The various technical features of the above embodiments can be combined in any way. To make the description concise, all possible combinations of the various technical features in the above embodiments have not been described. However, as long as there is no contradiction in the combination of these technical features, those combination should be considered within the scope of this specification.