Signal-enhanced extremely-weak absorptive gas imaging system

Abstract

The invention discloses a signal-enhanced extremely-weak absorptive gas imaging system. The system comprises a spectrum acquisition module which can acquire an optical signal with an enhanced signal-to-noise ratio in an observation area; the beam splitting optical fiber splits the collected optical signal and transmits the split optical signal to the double-temperature-control spectrometer module and the acousto-optic tunable filtering detection module at the same time; the double-temperature-control spectrograph module is used for carrying out spectrum processing for reducing dark current and spectrum deviation to obtain a spectrum image which retains multipolar weak absorptive gas spectrum information; the acousto-optic tunable filtering detection module is used for carrying out filtering detection for preventing cross interference of absorption signals of different atmospheric components to obtain wavelength fingerprint images with characteristic wavelengths for various extremely weak absorptive gases; and the control and calculation module analyzes the concentration of the extremely-weak absorptive gas in the plume region based on the observed spectral image and wavelength fingerprint image, and performs plume identification, plume concentration calculation and emission flux calculation based on the concentration of the extremely-weak absorptive gas, so that the imaging of the extremely-weak absorptive gas is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of optical remote sensing, and in particular relates to a signal-enhanced extremely weak absorption gas imaging system. Background Art

[0002] Optical remote sensing technology has been widely used in the detection of atmospheric components. It can monitor various components in the atmosphere in real time over long distances, especially for trace gases such as nitrogen dioxide, sulfur dioxide, and formaldehyde, which have obvious absorption spectral characteristics and absorption spectral signals. Current technology can effectively detect and trace the source. However, it is very difficult for existing optical remote sensing systems to detect and image extremely weakly absorbing gases such as other aldehydes, ketones, lipids, and benzene series. Most of these gases are irritating and toxic. Aldehydes, ketones, and lipid compounds, such as formaldehyde, acetone, and the decomposition products of vegetable oils, can irritate the eyes, nose, and throat, and may also cause headaches, nausea, and difficulty breathing at high concentrations. Benzene series such as benzene, toluene, and xylene are known carcinogens.

[0003] The reason why current optical remote sensing systems have difficulty detecting extremely weakly absorbing gases is their weak absorption in the spectrum. During actual detection, their spectral absorption signals will blend with each other and be submerged in the noise. Only by improving the signal-to-noise ratio of the detection method can the effective absorption spectrum signal be separated from the noise. On the other hand, the actual emission source is often a mixed emission of multiple gases, and the collected absorption spectrum is also the result of the superposition of multiple gas absorption spectra. Especially after improving the signal-to-noise ratio, the effective spectrum will fuse multiple strong absorption gas signals and multiple weak absorption gas signals. In order to separate each weak absorption gas signal separately, it is necessary to add characteristic wavelength recognition technology on the basis of the traditional spectral processing system, and judge the specific concentration of the gas by extracting the absorption characteristic wavelength of the extremely weak absorption gas.

[0004] Furthermore, the difficulty of existing optical remote sensing systems in detecting extremely weakly absorbing gases leads to imaging difficulties, specifically, the inability to identify plume boundaries and calculate plume emission fluxes. In practical applications, it is impossible to confirm whether the outlet is emitting extremely weakly absorbing pollutant gases. Without source tracing and imaging, targeted prevention and control is impossible, which leaves high-pollution emission companies with loopholes to exploit and also opens a gap in air pollution prevention and control. Summary of the invention

[0005] In view of the above, an object of the present invention is to provide a signal-enhanced extremely weakly absorbing gas imaging system, which can realize imaging of extremely weakly absorbing gases, and further realize plume boundary identification and plume emission flux calculation of extremely weakly absorbing gases.

[0006] To achieve the above-mentioned purpose of the invention, an embodiment provides a signal-enhanced extremely weak absorption gas imaging system, comprising a spectrum acquisition module, a beam splitting optical fiber, a dual temperature-controlled spectrometer module, an acoustic-optically tunable filtering detection module, and a control and calculation module; The spectrum acquisition module has a signal-to-noise ratio enhancement acquisition function, and can acquire solar scattered light signals with enhanced signal-to-noise ratio in the observation area; The beam splitting optical fiber is used to split the collected solar scattered light signal into two beams and transmit them to the dual temperature controlled spectrometer module and the acoustic-optic tunable filtering detection module at the same time; The dual temperature-controlled spectrometer module is used to collect the spectrum of the input light signal to reduce dark current and spectrum shift, and obtain a spectrum image that retains more spectrum information of extremely weakly absorbing gases; The acoustic-optic tunable filtering detection module is used to filter and detect the input solar scattered light signal to prevent cross-interference of absorption signals of different atmospheric components, and obtain wavelength fingerprint images with characteristic wavelengths for a variety of extremely weak absorption gases; The control and calculation module is used to control the spectrum acquisition module to perform swing scanning observation to collect solar scattered light signals, and analyze the concentration of extremely weakly absorbing gases in the plume area based on the observed spectral image and wavelength fingerprint image, and perform plume identification and plume concentration calculation and emission flux calculation based on the extremely weakly absorbing gas concentration.

[0007] Preferably, the spectrum acquisition module comprises a first aperture stop, an off-axis parabolic reflector, a reflector thermostat, a focusing lens group, a bandpass filter, a light homogenizer, and an optical fiber probe, which are sequentially arranged along the optical path. Except for the first aperture stop which is transparent to the outside, the other components are all sealed; The first aperture stop is used to limit the diameter of the input light beam and block the outer light beam with large distortion; The off-axis parabolic reflector is used to deflect the input light through the first aperture diaphragm and converge the light to the focusing transmission group, wherein the off-axis parabolic reflector is coated, the coating band covers the absorption characteristic band of the extremely weak absorption gas to be detected, and the RMS value of the reflector surface accuracy should be less than λ / 8, λ The wavelength of light used when testing the surface accuracy of the reflector; The reflector thermostat is used to control the off-axis parabolic reflector to be in a constant temperature state; The focusing lens group is used to adjust the focal length of the system and shape the light beam, so as to achieve imaging of the extremely weakly absorbing gas with an optimal field angle range and spatial resolution and reduce aberrations and image spot diameter; The bandpass filter is used to block the absorption band of non-extremely weakly absorbing gases in the output light of the focusing lens group, and prevent stray light of other bands from entering the beam splitting optical fiber. The bandpass band of the bandpass filter depends on the characteristic absorption band of the extremely weakly absorbing gas to be detected. If multiple extremely weakly absorbing gases are to be detected, it is necessary to take the union of the characteristic absorption bands of multiple gases; The light homogenizer is used to balance the light intensity of the output light of the bandpass filter, so as to balance the image spot before the light beam enters the optical fiber probe and keep the spectral information consistent; The optical fiber probe is used to receive the output light of the bandpass filter so as to transmit the light to the dual-temperature-controlled spectrometer module and the filter detection module based on acoustic-optical tunability through the beam splitting optical fiber.

[0008] Preferably, the first aperture diaphragm adopts a three-layer structure, one layer is a shutter-type mechanical structure for changing the aperture opening size, and two layers are double-layer flat thin fused quartz glass for sealing and heat insulation.

[0009] Preferably, the splitting optical fiber comprises a plurality of fiber cores and the front end is arranged in a planar close-packed manner for connection with the optical fiber probe included in the spectral acquisition module. The splitting optical fiber is further divided into two strands, one end of which is arranged in a straight line and aligned with the spectrometer slit of the dual-temperature-controlled spectrometer module, and the other end of which is arranged in a planar close-packed manner and aligned with the input optical axis of the acoustic-optically tunable filtering detection module.

[0010] Preferably, the dual temperature-controlled spectrometer module comprises a spectrometer, a heat-insulating housing, a support pad, a spectrometer temperature controller, a cooling fan, and a plurality of temperature sensors, wherein the spectrometer comprises a spectrometer slit, a collimating reflector, a spectroscopic grating, a converging reflector, a CCD detector, and a CCD detector cooler which are sequentially arranged along the optical path and mounted on the spectrometer housing. The spectrometer slit is used to receive the solar scattered light transmitted through the beam splitting optical fiber completely without gaps, and the solar scattered light is received by the CCD detector after passing through the collimating reflector, the beam splitting grating, and the converging reflector in sequence to obtain a spectral image retaining more spectral information of the extremely weakly absorbing gas, and the CCD detector is simultaneously cooled by the CCD detector refrigerator to reduce its temperature to the optimal working temperature of the CCD and keep it constant; The spectrometer is supported in a sealed insulating shell by a support pad, and a dry inert gas is filled between the spectrometer and the insulating shell. The inner layer of the insulating shell is made of a heat-conducting material, the middle layer is made of a heat-insulating material, and the outer layer is made of a high-strength and UV-resistant material that can protect the spectrometer. A plurality of temperature sensors are arranged on at least part of the optical components in the spectrometer or on the spectrometer shell. A spectrometer temperature controller is closely attached to the insulating shell. Based on the feedback temperature of the temperature sensor, the spectrometer is uniformly temperature-controlled through the inner layer of the insulating shell and the filled nitrogen, while preventing water vapor condensation. A cooling fan is used to dissipate heat from the spectrometer temperature controller.

[0011] Preferably, the acoustic-optic tunable filtering detection module comprises a collimating lens group, a second aperture stop, a polarizer, an acoustic-optic tunable filter, an analyzer, a converging lens group, and a digital imaging device, which are sequentially arranged along the optical path. The optical signal input through the beam splitting optical fiber is collimated by the collimating lens group and the monochromatic aberration is reduced before being input to the second aperture stop, and the second aperture stop limits the beam diameter to match the beam entrance of the acousto-optic tunable filter; The light beam passing through the second aperture diaphragm enters the acousto-optic tunable filter through the polarizer, and is split after Bragg diffraction. The separated narrow-band light beam has a very narrow half-height width. The frequency of the high-frequency electrical signal of the acousto-optic tunable filter is adjusted according to the characteristic wavelength of the extremely weak absorbing gas. The acousto-optic tunable filter with an extremely short response time can detect the characteristic wavelengths of a variety of extremely weak absorbing gases. The output light of the acousto-optic tunable filter passes through the analyzer and the converging lens group and enters the digital imaging device to obtain a wavelength fingerprint image with a characteristic wavelength for extremely weakly absorbing gases. The light intensity in different polarization directions can be tested by rotating the analyzer, and the converging lens group can reduce chromatic aberration and image spots of optical imaging.

[0012] Preferably, the focusing lens group and the converging lens group are both double-cemented lenses.

[0013] Preferably, analyzing the concentration of extremely weakly absorbing gases in the plume region based on the observed spectral image and wavelength fingerprint image includes: The type of extremely weakly absorbing gas in the plume area is determined based on the spectral image obtained by the dual-temperature controlled spectrometer module. The wavelength fingerprint image of the extremely weakly absorbing gas is detected specifically according to the type based on the acoustic-optical tunable filter detection module. Then, the concentration of the extremely weakly absorbing gas in the plume area is calculated based on the spectral information of the wavelength fingerprint image of the extremely weakly absorbing gas.

[0014] Preferably, plume identification and plume concentration calculation based on the very weakly absorbing gas concentration include: According to the concentration of extremely weakly absorbing gas, the high-concentration part is distinguished as the plume shape of the gas to achieve plume identification; The concentration of the entire observed plume is calculated based on the plume slices detected by a single pitch angle sequence. Specifically, the slices are approximated into a circle, and the spatial position of the projection height of the plume relative to the observation point is obtained according to the azimuth of the spectrum acquisition module and the distance from the spectrum acquisition module to the emission source. The upper and lower boundaries of the plume shape identified by the plume are connected to the observation point respectively, and the slice circle is tangent to the connecting line. The spatial position and size of the plume slice are obtained in combination with the spatial position of the projection height. The plume slices of the pitch angle sequence at each azimuth are calculated to form the specific spatial shape and concentration of the entire observed plume, where the concentration is mapped one-to-one with the concentration of the pixels on the plume shape.

[0015] Preferably, the emission flux calculation includes: Based on the concentration of each very weakly absorbing gas in the plume section C , time difference , wind speed v and lateral wind speed , calculate the discharge flux of the outlet: ; Where z represents the height of the center of the extremely weakly absorbing gas slice above the ground.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The spectral acquisition module is used to enhance the signal-to-noise ratio of the optical signal in the observation area to reduce noise interference. On this basis, the optical signal is divided into two beams. One beam of optical signal is collected by a dual-temperature controlled spectrometer module. The dark current and spectral offset in the spectral acquisition process are reduced by dual temperature control, so that the obtained spectral image retains more spectral information of extremely weakly absorbing gases. At the same time, the other beam of optical signal is screened for characteristic wavelengths by a filter detection module based on tunable acoustic and optical filters. Specifically, the characteristic wavelengths of extremely weakly absorbing gases are screened through an acoustic and optical tunable filtering process that prevents cross-interference of absorption signals of different atmospheric components, thereby obtaining a wavelength fingerprint image for extremely weakly absorbing gases. Finally, the concentration of extremely weakly absorbing gases in the plume area is analyzed based on the observed spectral image and wavelength fingerprint image, and plume identification, plume concentration calculation and emission flux calculation are performed based on the concentration of extremely weakly absorbing gases, so that imaging of extremely weakly absorbing gases can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0018] Figure 1 is a schematic structural diagram of a signal-enhanced extremely weakly absorbing gas imaging system provided in an embodiment; Figure 2 is a schematic structural diagram of a dual temperature-controlled spectrometer module provided in an embodiment; Figure 3 is a schematic structural diagram of a filter detection module based on acoustic-optic tunable filtering provided in an embodiment; Figure 4 is a schematic diagram of plume profile identification and pollutant gas emission flux calculation provided by an embodiment; Figure 5-Figure 7This is the exhaust plume imaging result of the extremely weak absorption gas imaging system using signal enhancement provided in the embodiment. DETAILED DESCRIPTION

[0019] To make the purpose, technical solution 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. It should be understood that the specific implementation methods described herein are only used to explain the present invention and do not limit the scope of protection of the present invention.

[0020] The inventive concept of the present invention is: in order to realize the detection and imaging of extremely weakly absorbing atmospheric pollutants (referred to as extremely weakly absorbing gases), the embodiment of the present invention provides a signal-enhanced extremely weakly absorbing gas imaging system, which aims to enhance the signals of the spectral acquisition part and the spectral processing part, improve the signal-to-noise ratio, and solve the problem of cross-interference between the absorption signals of different atmospheric components, and finally use the plume contour recognition technology and the polluted gas emission flux algorithm to realize the imaging of extremely weakly absorbing atmospheric pollutants and the calculation of the plume emission flux.

[0021] like Figure 1 As shown, a signal-enhanced extremely weakly absorbing gas imaging system 1 provided in the embodiment includes a spectrum acquisition module, a beam splitting optical fiber 2, a dual temperature-controlled spectrometer module 3, a filter detection module 4 based on acoustic and optical tunability, and a control and calculation module 5. These components cooperate to realize the imaging of extremely weakly absorbing gases. Specifically, the spectrum acquisition module 1 is used to collect solar scattered light, and perform operations such as folding, focusing, and shaping the scattered light, and finally send a high-quality light beam into the beam splitting optical fiber 2; the beam splitting optical fiber 2 splits the solar scattered light signal and transmits it to the dual temperature-controlled spectrometer module 3 and the filter detection module 4 based on acoustic and optical tunability at the same time. The dual temperature-controlled spectrometer module 3 is used to detect the spectral information of solar scattered light, which includes the spectral information of extremely weakly absorbing gases and strongly absorbing atmospheric pollutants (referred to as strongly absorbing gases) to form a spectral image; the filter detection module 4 based on acoustic and optical tunability is used to detect the characteristic wavelength information of extremely weakly absorbing gases; the control and calculation module 5 detects the concentration of extremely weakly absorbing gases, and images the plume shape of extremely weakly absorbing gases and calculates the emission flux.

[0022] In the embodiment, the spectrum acquisition module 1 has a signal-to-noise ratio enhancement acquisition function, and can acquire solar scattered light signals with enhanced signal-to-noise ratio in the observation area, such as Figure 1 As shown, it specifically includes a first aperture stop 11, an off-axis parabolic reflector 12, a reflector thermostat 13, a focusing lens group 14, a bandpass filter 15, a light homogenizer 16 and a fiber optic probe 17 which are sequentially arranged along the optical path.

[0023] The first aperture stop 11 is used to limit the diameter of the input light beam, thereby reducing the optical distortion of the entire optical system to a certain extent. Figure 1The light gray shown in the figure is the outer light beam with greater distortion. The outer light beam is blocked by the first aperture diaphragm 11 and does not enter the subsequent optical system. In order to prevent moisture, dust, etc. from entering the spectrum acquisition module 1 and affecting the life of the system, in actual use, the entire spectrum acquisition module 1 is fully sealed, and only the first aperture diaphragm 11 is transparent to the outside world. In order to prevent moisture and dust from entering the system from the first aperture diaphragm 11, the first aperture diaphragm 11 adopts a three-layer structure, one layer is a shutter-type mechanical structure used to change the size of the aperture opening, and the other two layers are thinner double-layer flat fused quartz glass for sealing and heat insulation.

[0024] The off-axis parabolic reflector 12 is used to deflect the light path and converge the light beam. The off-axis parabolic reflector 12 is rotated to focus the optical axis of the light path to observe the scattered light from the sun at different angles. The off-axis parabolic reflector 12 needs to be coated, and the coating band covers the absorption characteristic band of the extremely weak absorption gas to be detected. Generally, UV enhanced aluminum film is used for VOCs, and gold film is used for greenhouse gases. The use of off-axis parabolic reflectors has the following advantages: 1. Aberration: Compared with the transmission combination of right-angle prism and lens, the off-axis parabolic reflector can focus the light beam without chromatic aberration, eliminating the phase delay and absorption loss introduced by the transmission optical element, which is beneficial for wide-band spectral measurement; 2. Surface accuracy: The surface accuracy of the off-axis parabolic reflector can reach a very high level, and the surface accuracy RMS value should be less than λ / 8. λ The wavelength of light used when testing the surface accuracy of the reflector to ensure high-quality focusing of the light beam; 3. Degree of freedom: The degree of freedom of the off-axis parabolic reflector is low. Its reflection function and focusing function are integrated, which reduces the difficulty and workload of adjusting the optical path.

[0025] The defect of the off-axis parabolic reflector 12 is that the temperature stability is not as good as that of the transmission combination, and the deformation of the reflector caused by temperature has a greater impact on the optics. The system of the present invention is mainly used for monitoring outdoor, and the temperature changes greatly. Therefore, a reflector thermostat 13 is used to keep the temperature of the off-axis parabolic reflector 12 stable and maintain the best function. The reflector thermostat 13 includes a micro temperature sensor, a semiconductor refrigerator and a cooling fan, and uses the metal material of the off-axis parabolic reflector 12 itself to conduct heat.

[0026] The off-axis parabolic reflector 12 with a smaller focal length can detect a larger field angle range, but the spatial resolution will be reduced. Therefore, in order to image extremely weak absorbing gases under different conditions, the focal length of the optical system needs to be adjusted as needed. Since the off-axis parabolic reflector 12 is difficult to process and has a high processing cost, focusing cannot be achieved using only a single off-axis parabolic reflector 12. Therefore, a focusing lens group 14 is added to adjust the focal length of the system and shape the light beam to achieve imaging of extremely weak absorbing gases with the best field angle range and spatial resolution under different conditions. In order to further reduce aberrations and reduce the image spot diameter, the focusing lens group 14 needs to use a double-cemented lens to reduce chromatic aberration, and a combination of multiple lenses to reduce the image spot of optical imaging. The focal length of the entire optical system is changed by moving the focusing lens group 14 and the optical fiber probe 17.

[0027] The bandpass filter 15 is used to block the absorption band of non-extremely weakly absorbing gases to prevent stray light from other bands from entering the optical fiber and generating noise. The bandpass band of the bandpass filter 15 depends on the characteristic absorption band of the extremely weakly absorbing gas to be detected. If multiple extremely weakly absorbing gases are to be detected, the union of the characteristic absorption bands of multiple gases needs to be taken. If there are many extremely weakly absorbing gases to be detected, and the band covers the coating band of the off-axis parabolic reflector, the bandpass filter 15 can be omitted.

[0028] The light homogenizer 16 is used to balance the light intensity of the output light of the bandpass filter 15. Since aberration cannot be completely avoided, in order to make the light intensity and spectral information of different cores entering the splitting optical fiber 2 consistent, a thinner light homogenizer 16 needs to be placed close to the optical fiber.

[0029] The optical fiber probe 17 is used to receive the solar scattered light output by the light homogenizer 16 and guide the light into the optical fiber.

[0030] In the embodiment, the splitting optical fiber 2 is used to split the collected optical signal and transmit it to the dual temperature-controlled spectrometer module 3 and the acoustic-optic tunable filter detection module 4 at the same time. Figure 2 As shown, the beam splitting optical fiber 2 includes multiple cores, for example, 19 cores, each with a diameter of 0.2 mm, and the front end 21 of the core is densely packed in a plane and connected to the optical fiber probe 17. The optical fiber is split into two strands in the middle, for example, 12 cores 22 and 7 cores 23. Among them, one strand including 12 cores 22 is arranged in a straight line at the end, aligned with the spectrometer slit 31 of the dual temperature-controlled spectrometer module 3; the other strand including 7 cores 23 is arranged in a densely packed plane at the end, aligned with the input optical axis of the filter detection module 4 based on acousto-optic tunable. Figure 1 In the figure, one strand of the 12 fiber cores 22 is represented by white, and one strand of the 7 fiber cores 23 is represented by gray, and their arrangement at the fiber core front end 21 needs to be as uniform as possible.

[0031] In the embodiment, the dual temperature-controlled spectrometer module 3 is used to collect the spectrum of the input light signal to reduce the dark current and spectral shift, and obtain a spectral image that retains more spectral information of extremely weakly absorbing gases. Figure 2 As shown, it specifically includes a heat-insulating housing 37, a support pad 38, a spectrometer temperature controller 39, a cooling fan 310, a spectrometer 311, and a plurality of temperature sensors ( Figure 2 ), wherein the spectrometer 311 includes a spectrometer slit 31, a collimating reflector 32, a spectroscopic grating 33, a converging reflector 34, a CCD detector 35, and a CCD detector cooler 36 which are sequentially arranged along the optical path and mounted on the spectrometer housing.

[0032] Among them, the spectrometer slit 31 is used to receive the solar scattered light transmitted through the splitting optical fiber without any gaps. Specifically, the spectrometer slit 31 is connected to the 12 fiber cores 22 arranged in a straight line, and the slit width is the fiber core diameter. During installation, it is necessary to ensure that the 12 fiber cores 22 are aligned with the spectrometer slit 31. The narrower slit and fiber core arrangement is to make full use of the optical signal in the optical fiber on the one hand, and on the other hand, to maximize the spectral resolution and capture the spectral absorption peaks of extremely weakly absorbing atmospheric pollutants as much as possible during spectral analysis.

[0033] The light beam input into the spectrometer through the spectrometer slit 31 is divergent, and is reflected and collimated into approximately parallel light by the collimating reflector 32. The approximately parallel light is then reflected and split by the spectroscopic grating 33, and then the light beam is converged by the converging reflector 34 and irradiated on the CCD detector 35 to obtain a spectral image that retains more spectral information of extremely weakly absorbing gases. The function of the CCD detector cooler 36 is to cool the CCD detector 35, reduce the detector temperature to the optimal working temperature of the CCD, and keep it constant in a very small temperature range to reduce thermal noise and improve the signal-to-noise ratio and sensitivity.

[0034] The heat-insulating shell 37 wraps the spectrometer. The heat-insulating shell 37 adopts a multi-layer structure. The inner layer adopts a heat-conducting material for heat conduction, such as a metal material for heat conduction; the middle layer is a heat-insulating material; the outer layer adopts a high-strength and UV-resistant material that can protect the spectrometer, such as white glass fiber nylon to protect the spectrometer. The heat-insulating shell 37 and the spectrometer are connected only by a rubber support pad 38 to support the spectrometer in the sealed heat-insulating shell 37. The rubber material has both shockproof and heat insulation. The space between the spectrometer and the heat-insulating shell 37 is filled with dry inert gas (such as nitrogen). Multiple temperature sensors are arranged on at least part of the optical components or the shell of the spectrometer in the spectrometer. The spectrometer temperature controller 39 is close to the heat-insulating shell 37. When the temperature of the spectrometer is controlled based on the feedback temperature of the temperature sensor, the temperature controller cools the inner metal junction material of the heat-insulating shell 37, and then the heat transfer of the inert gas makes the spectrometer uniformly cooled as a whole. At the same time, the filling gas does not contain moisture, which can effectively prevent water vapor from condensing on the optical devices in the spectrometer. The cooling fan 310 is used to dissipate heat from the spectrometer temperature controller 39 .

[0035] In the embodiment, a plurality of temperature sensors are arranged on the spectrometer slit 31, the collimating reflector 32, the spectroscopic grating 33, the converging reflector 34, the CCD detector 35 and the spectrometer housing to monitor the temperature information of each part. The CCD detector cooler 36 and the spectrometer temperature controller 39 are two temperature control devices of the spectrometer, which operate independently and are responsible for the cooling of the CCD detector 35 and the constant temperature control of the internal optical path structure of the spectrometer, respectively.

[0036] In the embodiment, the acoustic-optic tunable filtering detection module 4 is used to filter the input optical signal to prevent cross-interference of absorption signals of different atmospheric components to obtain wavelength fingerprint images with characteristic wavelengths for a variety of extremely weak absorption gases, such as Figure 3 As shown, it specifically includes a collimating lens group 43, a second aperture diaphragm 42, a polarizer 43, an acousto-optic tunable filter 44, an analyzer 45, a converging lens group 46, and a digital imaging device 47 which are sequentially arranged along the optical path.

[0037] The collimating lens group 41 collimates the divergent light beams of the seven fiber cores 23 into approximately parallel light. The collimating lens group 41 uses a double cemented lens to reduce chromatic aberration and a combination of multiple lenses to reduce monochromatic aberration. The second aperture stop 42 is used to limit the diameter of the light beam to match the light beam entrance of the acousto-optic tunable filter 44. After the light beam is collimated, it passes through the polarizer 43 to convert the light beam into linearly polarized light.

[0038] After the polarized light enters the acousto-optic tunable filter 44, it undergoes Bragg diffraction to achieve light splitting. The separated narrowband light beam has a very narrow half-height width, and the frequency of the high-frequency driving electrical signal of the acousto-optic tunable filter 44 is adjusted according to the characteristic wavelength of the extremely weak absorbing gas. Generally speaking, the intensity of at least two wavelengths of a gas is required to determine the gas concentration. The acousto-optic tunable filter 44 with an extremely short response time can detect the characteristic wavelengths of a variety of extremely weak absorbing gases.

[0039] The light output by the acousto-optic tunable filter 44 passes through the analyzer 45 and the converging lens group 46 and enters the digital imaging device 47 to obtain a wavelength fingerprint image with a characteristic wavelength for extremely weakly absorbing gases. Among them, the analyzer 45 is used to detect the polarization state of the light beam. When there is no need to detect the polarization state, the polarization angle of the analyzer 45 is consistent with that of the polarizer 43. When detecting the polarization state, the analyzer 45 is rotated to test the light intensity in different polarization directions. The converging lens group 46 is used to focus the approximately parallel light beam on the digital imaging device 47. The converging lens group 46 uses a double cemented lens to reduce chromatic aberration, and a combination of multiple lenses to reduce the image spot of optical imaging.

[0040] In the embodiment, the control and calculation module 5 is used to control the spectrum acquisition module to perform swing scanning observation to collect solar scattered light signals, and analyze the concentration of extremely weakly absorbing gases in the plume area based on the observed spectrum image and wavelength fingerprint image, and perform plume identification and plume concentration calculation and emission flux calculation based on the extremely weakly absorbing gas concentration, such as Figure 4 As shown, it is specifically referred to as spectral observation, spectral analysis, plume identification, plume concentration calculation and emission flux calculation.

[0041] For the spectral observation step, the spectral acquisition module is controlled to perform swing-sweep scanning observation and acquisition to obtain solar scattered light signals of different pitch angle sequences and different azimuth angle sequences. Specifically, the aperture diaphragm 11, the off-axis parabolic reflector 12 and the reflector thermostat 13 are rotated to achieve observation of different pitch angle sequences, and the observation of different azimuth angle sequences is achieved by rotating the entire spectral acquisition module 1. After determining the observation area and the observation resolution, the pitch angle and azimuth angle sequences are determined. After one pitch angle sequence is observed, the azimuth angle is changed to observe the next pitch angle sequence until the entire observation area is observed. Among them, the observation area includes an uncontaminated background area and a contaminated plume area. The role of the background area is to serve as a background spectrum when calculating the spectral analysis. The spectrum of the background area is deducted from the actual observed plume area to obtain the superposition spectrum of the absorption spectra of all absorption characteristic gases in the plume.

[0042] After observing the spectrum, the spectrum analysis step is entered, and the goal of this step is to obtain the concentration of extremely weak absorbing gases in the plume area. In order to separate the spectrum of extremely weak absorbing gases from that of strongly absorbing gases, and calculate the concentration of extremely weak absorbing gases according to the Lambert-Beer law, it is necessary to use a dual-temperature controlled spectrometer module 3 and a filter detection module 4 based on tunable acoustic and optical sound. Compared with ordinary spectrometers, the dual-temperature controlled spectrometer module 3 can reduce dark current and spectral offset as much as possible, retain more spectral information of extremely weak absorbing gases, and greatly increase the possibility of successful detection. According to the absorption characteristics of different gases, the concentrations of all strongly absorbing gases and some weakly absorbing gases can be analyzed. Since the cross-interference between the absorption signals of different atmospheric components will greatly affect the detection of extremely weak absorbing gases, some extremely weak absorbing gases with similar spectral characteristics will be indistinguishable, and the filter detection module 4 based on tunable acoustic and optical sound can solve this problem. Select the types of extremely weak absorbing gases that are difficult to distinguish as the analysis objects of the filter detection module 4 based on tunable acoustic and optical sound, and pick out the characteristic wavelengths of these gases and analyze them. The filter detection module 4 based on acoustic-optic tunable technology uses ultrasound to generate spatial periodic modulation in the crystal, thereby achieving more precise targeted wavelength screening than traditional filters. On the other hand, it adjusts the ultrasound frequency and quickly switches the wavelength to provide super-resolution spectral signals, thereby achieving the analysis of the fine structure of the molecular absorption cross section. Under the supplementary analysis of the filter detection module 4 based on acoustic-optic tunable technology, the spectral observation step can accurately identify the absorption signals of the characteristic fingerprints of different gases, and achieve synchronous imaging of extremely weakly absorbing gases such as aldehydes, ketones, lipids, and benzene series.

[0043] The specific implementation is as follows: the type of extremely weakly absorbing gas in the plume area is determined based on the spectral image obtained by the dual-temperature controlled spectrometer module, the wavelength fingerprint image is detected specifically according to the type based on the acoustic-optical tunable filter detection module, and then the concentration of the extremely weakly absorbing gas in the plume area is calculated based on the wavelength fingerprint image of the extremely weakly absorbing gas.

[0044] The plume identification step and the spectral analysis step can be performed simultaneously. Since the spectral analysis is for the entire plume area, after obtaining the concentration of each gas in the plume area, the high-concentration part in the area can be determined, and this part is the plume shape of the gas. Then, the plume shape is smoothed to make the different parts of the plume more continuous and smooth, and the plume shape is more in line with the actual situation.

[0045] The plume concentration calculation step is performed based on the spectral analysis results and the plume identification results. The plume slice is obtained according to the detection of a single pitch angle sequence. The slice is approximated into a circle. According to the azimuth of the spectral acquisition module and the distance between the instrument and the emission source, the spatial position of the projection height of the plume for the observation point is obtained. The upper and lower boundaries of the plume obtained in the plume identification step are connected to the observation point respectively, and the slice circle is tangent to the connection line. The spatial position and size of the plume slice can be obtained by combining the position of the projection height, and the plume slice of the pitch angle sequence under each azimuth is calculated to form the specific spatial shape and concentration of the entire plume, wherein the concentration is mapped one-to-one with the concentration of the pixel points on the plume shape. This step also needs to be combined with the meteorological information at the time of observation to correct the results, including but not limited to wind speed, wind direction, light, haze level, etc. The spatial position of the plume is adjusted according to the wind direction, the position between different slices is adjusted according to the wind speed, and the effectiveness and confidence of the final imaging detection are relaxed or tightened according to the light and haze level.

[0046] The emission flux calculation steps are performed based on the real meteorological information and plume concentration. Under the influence of the actual meteorological environment, the plume is generally discharged from the outlet and drifts upward obliquely, so the spatial position of each slice is different. There is a certain time difference between each elevation sequence, so according to the concentration of each extremely weakly absorbing gas plume section C , time difference , wind speed v and lateral wind speed , the discharge flux of the outlet can be calculated: ; Where z represents the height of the center of the extremely weakly absorbing gas slice above the ground.

[0047] Figure 5-Figure 7 The results of imaging chimney outlet plumes using a signal-enhanced extremely weak absorption gas imaging system are shown, showing the measured plumes of aldehydes, ketones and benzene series. It can be seen that the plumes of the three are not exactly the same in a single measurement, which is related to their physical and chemical properties. It also shows that the signal-enhanced extremely weak absorption gas imaging system does identify and image different gas plumes, which shows the effectiveness of this method.

[0048] The signal-enhanced extremely weakly absorbing gas imaging system provided by the present invention can image and calculate the flux of extremely weakly absorbing atmospheric pollutant plumes, and relevant factories and relevant departments can carry out precise monitoring and efficient prevention and control based on the calculation results.

[0049] The specific implementation methods described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A signal-enhanced extremely weakly absorbing gas imaging system, characterized in that: It includes a spectrum acquisition module, a beam splitting optical fiber, a dual temperature-controlled spectrometer module, an acoustic-optical tunable filter detection module, and a control and calculation module; The spectrum acquisition module has a signal-to-noise ratio enhancement acquisition function, and can acquire solar scattered light signals with enhanced signal-to-noise ratio in the observation area; The beam splitting optical fiber is used to split the collected solar scattered light signal into two beams and transmit them to the dual temperature controlled spectrometer module and the acoustic-optic tunable filtering detection module at the same time; The dual temperature-controlled spectrometer module is used to collect the spectrum of the input light signal to reduce dark current and spectrum shift, and obtain a spectrum image that retains more spectrum information of extremely weakly absorbing gases; The acoustic-optic tunable filtering detection module is used to filter and detect the input solar scattered light signal to prevent cross-interference of absorption signals of different atmospheric components, and obtain wavelength fingerprint images with characteristic wavelengths for a variety of extremely weak absorption gases; The control and calculation module is used to control the spectrum acquisition module to perform swing scanning observation to collect solar scattered light signals, and analyze the concentration of extremely weakly absorbing gases in the plume area based on the observed spectral image and wavelength fingerprint image, and perform plume identification and plume concentration calculation and emission flux calculation based on the extremely weakly absorbing gas concentration.

2. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 1, characterized in that: The spectrum acquisition module comprises a first aperture stop, an off-axis parabolic reflector, a reflector thermostat, a focusing lens group, a bandpass filter, a light homogenizer, and an optical fiber probe, which are sequentially arranged along the optical path. Except for the first aperture stop which is transparent to the outside world, the other components are all sealed; The first aperture stop is used to limit the diameter of the input light beam and block the outer light beam with large distortion; The off-axis parabolic reflector is used to deflect the input light through the first aperture diaphragm and converge the light to the focusing transmission group, wherein the off-axis parabolic reflector is coated, the coating band covers the absorption characteristic band of the extremely weak absorption gas to be detected, and the RMS value of the reflector surface accuracy should be less than λ / 8, λ The wavelength of light used when testing the surface accuracy of the reflector; The reflector thermostat is used to control the off-axis parabolic reflector to be in a constant temperature state; The focusing lens group is used to adjust the focal length of the system and shape the light beam, so as to achieve imaging of the extremely weakly absorbing gas with an optimal field angle range and spatial resolution and reduce aberrations and image spot diameter; The bandpass filter is used to block the absorption band of non-extremely weakly absorbing gases in the output light of the focusing lens group, and prevent stray light of other bands from entering the beam splitting optical fiber. The bandpass band of the bandpass filter depends on the characteristic absorption band of the extremely weakly absorbing gas to be detected. If multiple extremely weakly absorbing gases are detected, it is necessary to take the union of the characteristic absorption bands of multiple gases; The light homogenizer is used to balance the light intensity of the output light of the bandpass filter, so as to balance the image spot before the light beam enters the optical fiber probe and keep the spectral information consistent; The optical fiber probe is used to receive the output light of the bandpass filter so as to transmit the light to the dual-temperature-controlled spectrometer module and the filter detection module based on acoustic-optical tunability through the beam splitting optical fiber.

3. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 2, characterized in that: The first aperture diaphragm adopts a three-layer structure, one layer is a shutter-type mechanical structure for changing the size of the diaphragm opening, and two layers are double-layer flat thin fused quartz glass for sealing and heat insulation.

4. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 1, characterized in that: The splitting optical fiber includes multiple fiber cores and the front end is arranged in a planar close-packed manner for connection with the optical fiber probe included in the spectrum acquisition module. The splitting optical fiber is further divided into two strands, one end of which is arranged in a straight line and aligned with the spectrometer slit of the dual-temperature controlled spectrometer module, and the other end is arranged in a planar close-packed manner and aligned with the input optical axis of the filter detection module based on acousto-optic tunable.

5. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 1, characterized in that: The dual temperature control spectrometer module comprises a spectrometer, a heat insulation shell, a support pad, a spectrometer temperature controller, a cooling fan, and a plurality of temperature sensors, wherein the spectrometer comprises a spectrometer slit, a collimating reflector, a spectroscopic grating, a converging reflector, a CCD detector, and a CCD detector cooler which are sequentially arranged along the optical path and installed on the spectrometer shell. The spectrometer slit is used to receive the solar scattered light transmitted through the beam splitting optical fiber completely without gaps, and the solar scattered light is received by the CCD detector after passing through the collimating reflector, the beam splitting grating, and the converging reflector in sequence to obtain a spectral image retaining more spectral information of the extremely weakly absorbing gas, and the CCD detector is simultaneously cooled by the CCD detector refrigerator to reduce its temperature to the optimal working temperature of the CCD and keep it constant; The spectrometer is supported in a sealed insulating shell by a support pad, and a dry inert gas is filled between the spectrometer and the insulating shell. The inner layer of the insulating shell is made of a heat-conducting material, the middle layer is made of a heat-insulating material, and the outer layer is made of a high-strength and UV-resistant material that can protect the spectrometer. A plurality of temperature sensors are arranged on at least part of the optical components in the spectrometer or on the spectrometer shell. A spectrometer temperature controller is closely attached to the insulating shell. Based on the feedback temperature of the temperature sensor, the spectrometer is uniformly temperature-controlled through the inner layer of the insulating shell and the filled nitrogen, while preventing water vapor condensation. A cooling fan is used to dissipate heat from the spectrometer temperature controller.

6. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 1, characterized in that: The filter detection module based on acousto-optic tunable includes a collimating lens group, a second aperture stop, a polarizer, an acousto-optic tunable filter, an analyzer, a converging lens group, and a digital imaging device, which are sequentially arranged along the optical path. The optical signal input through the beam splitting optical fiber is collimated by the collimating lens group and the monochromatic aberration is reduced before being input to the second aperture stop, and the second aperture stop limits the beam diameter to match the beam entrance of the acousto-optic tunable filter; The light beam passing through the second aperture diaphragm enters the acousto-optic tunable filter through the polarizer, and is split after Bragg diffraction. The separated narrow-band light beam has a very narrow half-height width. The frequency of the high-frequency electrical signal of the acousto-optic tunable filter is adjusted according to the characteristic wavelength of the extremely weak absorbing gas. The acousto-optic tunable filter with an extremely short response time can detect the characteristic wavelengths of a variety of extremely weak absorbing gases. The output light of the acousto-optic tunable filter passes through the analyzer and the converging lens group and enters the digital imaging device to obtain a wavelength fingerprint image with a characteristic wavelength for extremely weakly absorbing gases. The light intensity in different polarization directions can be tested by rotating the analyzer, and the converging lens group can reduce chromatic aberration and image spots of optical imaging.

7. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 2 or 6, characterized in that: The focusing lens group and the converging lens group both adopt double cemented lenses.

8. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 1, characterized in that: The concentration of extremely weakly absorbing gases in the plume area is analyzed based on the observed spectral images and wavelength fingerprint images, including: The type of extremely weakly absorbing gas in the plume area is determined based on the spectral image obtained by the dual-temperature controlled spectrometer module. The wavelength fingerprint image of the extremely weakly absorbing gas is detected specifically according to the type based on the acoustic-optical tunable filter detection module. Then, the concentration of the extremely weakly absorbing gas in the plume area is calculated based on the wavelength fingerprint image of the extremely weakly absorbing gas.

9. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 1, characterized in that: Plume identification and plume concentration calculation based on very weakly absorbing gas concentrations, including: According to the concentration of extremely weakly absorbing gas, the high-concentration part is distinguished as the plume shape of the gas to achieve plume identification; The concentration of the entire observed plume is calculated based on the plume slices detected by a single pitch angle sequence. Specifically, the slices are approximated into a circle, and the spatial position of the projection height of the plume relative to the observation point is obtained according to the azimuth of the spectrum acquisition module and the distance from the spectrum acquisition module to the emission source. The upper and lower boundaries of the plume shape identified by the plume are connected to the observation point respectively, and the slice circle is tangent to the connecting line. The spatial position and size of the plume slice are obtained in combination with the spatial position of the projection height. The plume slices of the pitch angle sequence at each azimuth are calculated to form the specific spatial shape and concentration of the entire observed plume, where the concentration is mapped one-to-one with the concentration of the pixels on the plume shape.

10. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 1, characterized in that: Emission flux calculations, including: Based on the concentration of each very weakly absorbing gas in the plume section C , time difference , wind speed v and lateral wind speed , calculate the discharge flux of the outlet: ； Where z represents the height of the center of the extremely weakly absorbing gas slice above the ground.

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