Device and method for detecting CO2 concentration based on electron resonance enhanced coherent Raman spectroscopy

Abstract

A device and method for detecting CO2 concentration based on electron resonance enhanced coherent Raman spectroscopy. In the device, a circularly polarized femtosecond laser ionizes CO2 molecules, establishes the vibration coherence of CO2+, and simultaneously excites N2 molecules in the air to generate narrow-linewidth air lasers with a wavelength of 337.1nm. The wavelength of the air laser resonates with the electronic state transition of CO2+, and can be used as a detection light to induce the coherently excited CO2+ to generate vibration Raman scattering enhanced by electron resonance. Under laboratory conditions, the intensities of the Raman signals collected at several different CO2 concentrations are calibrated, and the relationship between the gas concentration and the Raman signal intensity and the fitting coefficient are obtained by square fitting. By measuring the Raman signal intensity of the gas to be measured, the CO2 concentration in the gas to be measured is inferred based on this relationship. The present invention has the characteristics of simple detection device, high sensitivity and suitability for remote measurement, and can be used for rapid identification of CO2 isotopes.

Description

Technical Field

[0001] The invention relates to the field of nonlinear ultrafast spectroscopy, and in particular to a device and method for detecting CO2 concentration based on electron resonance enhanced coherent Raman spectroscopy. Background Art

[0002] Carbon dioxide (CO2) is an important greenhouse gas in the atmosphere, affecting global climate, ecosystem carbon cycle, human food supply and living environment. Therefore, the precise measurement of CO2 concentration in the atmosphere and the spatiotemporal resolution of CO2 isotopes are of great significance for studying the carbon cycle process, tracing the source and sink of carbon emissions, and promoting my country to achieve the strategic goals of "carbon peak" and "carbon neutrality" as soon as possible. At present, coherent Raman spectroscopy technology has been widely used in gas concentration detection, atmospheric temperature measurement, etc. with good directionality and high intensity (see S.Roy et al., J.RamanSpectrosc.41,1194-1199 (2010) and M.Schenk et al., Appl.Optics 44,6526-6536 (2005)). However, due to the relatively large Raman frequency shift of CO2 molecules, it is difficult to achieve efficient coherent Raman excitation using traditional multi-photoperiod femtosecond laser pulses, which greatly limits the intensity and detection sensitivity of CO2 coherent Raman scattering. Electron resonance enhancement can significantly increase the polarizability of coherent Raman scattering, and thus becomes one of the important means to increase the intensity of Raman signals (see S. Druet et al., Prog. Quantum Electron. 7, 1-72 (1981)). However, to achieve electron resonance enhanced coherent Raman scattering, the incident laser wavelength must be close to the CO2 or Therefore, it places high demands on the laser light source and is difficult to achieve with a single femtosecond laser. These problems limit the effective application of electron resonance enhanced coherent Raman spectroscopy in CO2 concentration detection. Summary of the invention

[0003] The purpose of the present invention is to solve the above technical problems by using nitrogen laser produced naturally in the atmosphere The electronic state transition resonance characteristics are used to achieve the resonance enhancement of coherent Raman scattering, and provide a device and method for detecting CO2 concentration. The present invention not only utilizes the advantages of long-distance transmission of femtosecond laser filamentation, but also gives full play to the advantages of the resonance of nitrogen laser and CO2+ electron transition induced by femtosecond laser filamentation, and has the characteristics of simple device, high sensitivity and suitability for remote measurement. In addition, due to the narrow spectrum width of nitrogen laser, only The electronic resonance of resonance, thus providing an effective means for rapid identification of CO2 isotope gases.

[0004] The technical solution of the present invention is as follows:

[0005] A CO2 concentration detection device based on electron resonance enhanced coherent Raman spectroscopy is characterized in that it includes a near-infrared femtosecond laser, a quarter-wave plate, a focusing lens, an air cavity, a collimating lens, a filter, a collecting lens and a grating spectrometer;

[0006] The air cavity is filled with standard air (N2: 80%, O2: 20%) at 1 atmosphere pressure and CO2 gas of a certain concentration;

[0007] The near-infrared femtosecond laser outputs a linearly polarized femtosecond laser, which is converted into a circularly polarized femtosecond laser by a quarter-wave plate and incident into the air cavity by a focusing lens; the circularly polarized femtosecond laser ionizes CO2 molecules in the air cavity to establish The vibration coherence of N2 molecules simultaneously excites the air laser with a wavelength of 337.1nm. The air laser is used as the probe light and The electronic state transition resonance induces the vibration Raman scattering enhanced by the electronic resonance; the light emitted from the air cavity passes through the collimating lens, the filter and the collecting lens in sequence, and is focused to the grating spectrometer.

[0008] The filter is a narrow-band filter with high transmittance to the air laser band, used to obtain air laser signals; or a narrow-band filter with high transmittance to the electron resonance enhanced Raman scattering signal band, used to obtain electron resonance enhanced Raman scattering signals.

[0009] Preferably, the CO2 gas concentration in the gas cavity is 0 to 6.25% of the total gas volume.

[0010] The method for detecting CO2 concentration using the above device comprises the following steps:

[0011] ① Close the air valve of the air cavity, evacuate it to vacuum, open the air valve, fill it with a certain concentration of CO2 gas and standard air at 1 atmosphere, and control the CO2 concentration in the cavity by adjusting the air valve;

[0012] ② Turn on the near-infrared femtosecond laser;

[0013] ③ Adjust the angle of the quarter-wave plate to convert the linearly polarized femtosecond laser into a circularly polarized femtosecond laser;

[0014] ④ The circularly polarized femtosecond laser is focused into the air cavity by the focusing lens and then passes through the collimating lens;

[0015] ⑤ Insert a filter after the collimating lens to obtain air laser signal or electron resonance enhanced Raman scattering signal;

[0016] ⑥ The air laser signal or the electron resonance enhanced Raman scattering signal is focused to the slit of the grating spectrometer through the collecting lens, and the grating spectrometer collects and spectrally analyzes the two signals respectively;

[0017] ⑦ Record the intensity of the air laser signal and the electron resonance enhanced Raman scattering signal under a specific CO2 concentration, measure multiple times and take the average value; adjust the gas valve to gradually change the CO2 concentration in the chamber, and repeat the above steps;

[0018] ⑧ Perform square fitting on the above multiple experimental data to obtain the relationship between the intensity of air laser signal and electron resonance enhanced Raman scattering signal and CO2 concentration, and determine the fitting coefficient;

[0019] ⑨Measure the air laser signal intensity and the electron resonance enhanced Raman scattering signal intensity of the gas to be tested, substitute them into the fitting relationship, and calculate the CO2 concentration in the gas to be tested.

[0020] The technical effects of the present invention are as follows:

[0021] ① The present invention utilizes 337.1nm nitrogen laser and By taking advantage of the characteristics of the electronic state transition resonance, an electronic resonance-enhanced coherent Raman scattering signal was obtained, thereby improving the detection sensitivity of CO2 concentration.

[0022] ② The present invention utilizes the characteristics of long-distance transmission of femtosecond optical filaments and their ability to generate nitrogen lasers in the atmosphere, and takes advantage of the directionality of coherent Raman scattering signals, making it suitable for remote measurement of CO2 concentration.

[0023] ③ Due to the narrow spectrum width of nitrogen laser, it can only achieve The electronic resonance of Therefore, the present invention also provides an effective means for quickly identifying CO2 isotope gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of a CO2 concentration detection device based on electron resonance enhanced coherent Raman spectroscopy.

[0025] Among them: 1 is a near-infrared femtosecond laser, 2 is a quarter wave plate, 3 is a focusing lens, 4 is an air cavity, 5 is a collimating lens, 6 is a first filter, 7 is a second filter, 8 is a collecting lens, and 9 is a grating spectrometer. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments, but the protection scope of the present invention should not be limited thereto.

[0027] See also Figure 1 , Figure 1 It is a schematic diagram of a detection device for CO2 concentration based on electron resonance enhanced coherent Raman spectroscopy of the present invention. As shown in the figure, it includes a near-infrared femtosecond laser 1, a quarter wave plate 2, a focusing lens 3, an air cavity 4, a collimating lens 5, a first filter 6, a second filter 7, a collecting lens 8 and a grating spectrometer 9. Among them, the quarter wave plate 2 is used to convert linearly polarized light into circularly polarized light. Circularly polarized femtosecond laser can significantly enhance the intensity of air laser, effectively reduce the background of supercontinuum (white light), and improve the signal-to-noise ratio of the signal. The focal length of the focusing lens 3 can be optimized to obtain the strongest electron resonance enhanced Raman scattering signal.

[0028] The first filter 6 is a narrow-band filter with high transmittance to the air laser band, and is used to obtain the air laser signal; the second filter 7 is a narrow-band filter with high transmittance to the electron resonance enhanced Raman scattering signal band, and is used to obtain the electron resonance enhanced Raman scattering signal.

[0029] The principle of the device of the present invention for detecting CO2 concentration is as follows:

[0030] Near-infrared femtosecond laser outputs linearly polarized light, which is converted into circularly polarized light after passing through a quarter-wave plate. Circularly polarized femtosecond laser can be used as pump light and anti-Stokes light to establish The vibration coherence of the gas can generate hot electrons, which can generate narrow-linewidth air lasers with a wavelength of 337.1nm through electron collision excitation. The frequency ω0 of the air laser radiation is exactly the same as of State and The transition resonance between the states, so the air laser is used as the probe light to act on the coherently excited It can produce electronic resonance enhanced coherent Raman scattering. This Raman signal has the advantages of higher intensity and better directionality, with a frequency of ω0-Ω, where Ω corresponds to of The frequency difference between the (000) state and the (000) state.

[0031] Theoretical analysis shows that the electron resonance enhanced Raman scattering signal intensity I is related to the air laser signal intensity The relationship between the ratio and the CO2 gas concentration ρ is

[0032]

[0033] α is the coefficient.

[0034] Under laboratory conditions, the intensity of the air laser signal was measured at several different CO2 concentrations. The value of coefficient α can be obtained by fitting the data using the above formula and the electron resonance enhanced Raman scattering signal intensity I. In practical applications, the air laser signal intensity of the gas to be tested is measured. Substituting the electron resonance enhanced Raman scattering signal intensity I' into the calibrated formula, the CO2 concentration ρ' in the gas to be measured can be obtained by reverse deduction.

[0035] The steps of the embodiment of the present invention are as follows:

[0036] ① Close the air valve of air cavity 4, evacuate it to vacuum, open the air valve, fill it with a certain concentration of CO2 gas and standard air at 1 atmosphere, and control the CO2 concentration in the cavity by adjusting the air valve;

[0037] ② Turn on the near-infrared femtosecond laser 1;

[0038] ③ Adjust the angle of the quarter wave plate 2 to convert the linearly polarized femtosecond laser into a circularly polarized femtosecond laser;

[0039] ④ The circularly polarized femtosecond laser is focused into the air cavity 4 through the focusing lens 3 and then passes through the collimating lens 5;

[0040] ⑤ Insert the first filter 6 after the collimating lens 5 to obtain the air laser signal; remove the first filter 6 and insert the second filter 7 after the collimating lens 5 to obtain the electron resonance enhanced Raman scattering signal;

[0041] ⑥ The air laser signal and the Raman signal are focused to the slit of the grating spectrometer 9 through the collecting lens 8, and the grating spectrometer 9 collects and performs spectral analysis on the two signals respectively;

[0042] ⑦Record the air laser signal intensity at a specific CO2 concentration The electron resonance enhances the Raman scattering signal intensity I, and the average value is obtained by multiple measurements; the gas valve is adjusted to gradually change the CO2 concentration in the chamber, and the above steps are repeated;

[0043] ⑧Use the formula The least squares method is used to fit the above multiple sets of experimental data to obtain the value of coefficient α;

[0044] ⑨Measure the air laser signal intensity of the gas to be tested The electron resonance enhanced Raman scattering signal intensity I′ is measured multiple times, and the average value is substituted into the above formula to reversely obtain the CO2 concentration ρ′ in the measured gas.

Claims

1. A method for detecting CO2 concentration using a CO2 concentration detection device based on electron resonance enhanced coherent Raman spectroscopy, the device comprising: Near-infrared femtosecond laser (1), quarter wave plate (2), focusing lens (3), air cavity (4), collimating lens (5), filter, collecting lens (8) and grating spectrometer (9); The air cavity (4) is filled with standard air at 1 atmosphere pressure and CO2 gas of a certain concentration; wherein the standard air contains 80% N2 and 20% O2; The near-infrared femtosecond laser (1) outputs linearly polarized femtosecond laser light, which is converted into circularly polarized femtosecond laser light through a quarter wave plate (2) and incident into an air cavity (4) through a focusing lens (3); the circularly polarized femtosecond laser light ionizes CO2 molecules in the air cavity (4), establishes the vibration coherence of CO2+, and simultaneously excites N2 molecules to generate air laser light with a wavelength of 337.1 nm. The air laser light serves as a detection light, resonates with the electronic state transition of the coherently excited CO2+, and induces vibration Raman scattering enhanced by electronic resonance; the light emitted from the air cavity (4) passes through a collimating lens (5), a filter, and a collecting lens (8) in sequence, and is then focused onto a grating spectrometer (9); The method is characterized in that it comprises the following steps: ① Close the air valve of the air cavity (4), evacuate it to a vacuum, open the air valve, fill it with a certain concentration of CO2 gas and standard air at 1 atmosphere pressure, and control the CO2 concentration in the cavity by adjusting the air valve; ② Turn on the near-infrared femtosecond laser (1); ③ adjusting the angle of the quarter wave plate (2) so that the linearly polarized femtosecond laser is converted into a circularly polarized femtosecond laser; ④ The circularly polarized femtosecond laser is focused into the air cavity (4) through the focusing lens (3) and then passes through the collimating lens (5) after being emitted; ⑤ Inserting a filter after the collimating lens (5) to obtain an air laser signal or an electron resonance enhanced Raman scattering signal; ⑥ The air laser signal or the electron resonance enhanced Raman scattering signal is focused onto the slit of the grating spectrometer (9) through the collecting lens (8), and the grating spectrometer (9) collects and performs spectral analysis on the two signals respectively; ⑦ Record the intensity of the air laser signal and the electron resonance enhanced Raman scattering signal under a specific CO2 concentration, measure multiple times and take the average value; adjust the gas valve to gradually change the CO2 concentration in the chamber, and repeat the above steps; ⑧ Perform square fitting on the above multiple experimental data to obtain the relationship between the intensity of air laser signal and electron resonance enhanced Raman scattering signal and CO2 concentration, and determine the fitting coefficient; ⑨Measure the air laser signal intensity and the electron resonance enhanced Raman scattering signal intensity of the gas to be tested, substitute them into the fitting relationship, and calculate the CO2 concentration in the gas to be tested.

2. The CO2 concentration detection device based on electron resonance enhanced coherent Raman spectroscopy according to claim 1 is characterized in that: The filter is a narrow-band filter with high transmittance to the air laser band, used to obtain air laser signals; or a narrow-band filter with high transmittance to the electron resonance enhanced Raman scattering signal band, used to obtain electron resonance enhanced Raman scattering signals.

3. The CO2 concentration detection device based on electron resonance enhanced coherent Raman spectroscopy according to claim 1 or 2, characterized in that: The CO2 gas of a certain concentration is that the CO2 gas accounts for 0 to 6.25% of the total volume of the gas.

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