A signal-enhanced extremely weak absorptive gas imaging system

Abstract

The present invention discloses an extremely weak absorbent gas imaging system with signal enhancement, comprising: a spectral acquisition module capable of acquiring optical signals with enhanced signal-to-noise ratio in the observation area; a beam splitting optical fiber that splits the acquired optical signals and simultaneously transmits them to a dual-temperature-controlled spectrometer module and an acousto-optic tunable filter detection module; the dual-temperature-controlled spectrometer module performs spectral processing to reduce dark current and spectral shift, obtaining a spectral image that retains more spectral information of extremely weak absorbent gases; the acousto-optic tunable filter detection module performs filter detection to prevent cross-interference of absorption signals of different atmospheric components, obtaining a wavelength fingerprint image with characteristic wavelengths for multiple extremely weak absorbent gases; the control and calculation module analyzes the concentration of extremely weak absorbent gases in the plume area 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 extremely weak absorbent gases, thus realizing imaging of extremely weak absorbent gases.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical remote sensing, and particularly relates to an imaging system for extremely weakly absorbent gases with signal enhancement. Background Art

[0002] Optical remote sensing technology has been widely applied in the detection of atmospheric components, capable of remotely and real-time monitoring various components in the atmosphere. Especially for trace gases such as nitrogen dioxide, sulfur dioxide, formaldehyde, etc., which have obvious absorption spectral characteristics and absorption spectral signals, current technologies can effectively detect and trace them. However, it is very difficult for existing optical remote sensing systems to detect and image extremely weakly absorbent gases such as other aldehydes, ketones, lipids, benzene series compounds, etc. Most of these gases are irritating and toxic. Aldehydes, ketones, and lipid compounds, such as the decomposition products of formaldehyde, acetone, and vegetable oil, can irritate the eyes, nose, and throat, and may also cause headache, nausea, and difficulty in breathing at high concentrations. Benzene series compounds such as benzene, toluene, and xylene are known carcinogens.

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

[0004] Furthermore, the difficulty of existing optical remote sensing systems in detecting extremely weakly absorbent gases will lead to difficulty in imaging, specifically, the plume boundary cannot be identified, and the plume emission flux cannot be calculated. In practical applications, it is impossible to confirm whether the exhaust port emits extremely weakly absorbent polluting gases. Without traceability and imaging, targeted prevention and control cannot be carried out, which allows highly polluting emission enterprises to take advantage of the loopholes and also tears a gap in the prevention and control of air pollution. Summary of the Invention

[0005] In view of the above, the purpose of the present invention is to provide an imaging system for extremely weakly absorbent gases with signal enhancement, which can realize the imaging of extremely weakly absorbent gases, and further realize the identification of the plume boundary of extremely weakly absorbent gases and the calculation of the plume emission flux.

[0006] To achieve the above-mentioned invention object, an extremely weak absorbent gas imaging system with signal enhancement provided by an embodiment includes a spectral acquisition module, a beam splitting optical fiber, a dual-temperature-controlled spectrometer module, an acousto-optic tunable filter detection module, and a control and calculation module;

[0007] The spectral acquisition module has a function of enhancing the signal-to-noise ratio in acquisition, and can acquire the solar scattered light signal with enhanced signal-to-noise ratio in the observation area;

[0008] The beam splitting optical fiber is used to split the acquired solar scattered light signal into two beams and simultaneously transmit them to the dual-temperature-controlled spectrometer module and the acousto-optic tunable filter detection module;

[0009] The dual-temperature-controlled spectrometer module is used to perform spectral acquisition on the input optical signal to reduce dark current and spectral shift, and obtain a spectral image that retains more spectral information of extremely weak absorbent gases;

[0010] The acousto-optic tunable filter detection module is used to perform filter detection on the input solar scattered light signal to prevent cross-interference of absorption signals of different atmospheric components, and obtain a wavelength fingerprint image with characteristic wavelengths for multiple extremely weak absorbent gases;

[0011] The control and calculation module is used to control the spectral acquisition module to perform swing-scanning observation to acquire the solar scattered light signal, analyze the concentration of extremely weak absorbent gases in the plume area based on the observed spectral image and wavelength fingerprint image, and calculate the plume identification, plume concentration, and emission flux based on the concentration of extremely weak absorbent gases.

[0012] Preferably, the spectral acquisition module includes a first aperture diaphragm, an off-axis parabolic mirror, a mirror thermostat, a focusing lens group, a band-pass filter, a light homogenizer, and an optical fiber probe arranged in sequence along the optical path. Except for the first aperture diaphragm that transmits light to the outside, the rest of the components are all sealed;

[0013] The first aperture diaphragm is used to limit the diameter of the input beam and block the outer beam with large distortion;

[0014] The off-axis parabolic mirror is used to turn the optical path of the input light passing through the first aperture diaphragm and then converge the light to the focusing transmission group. Among them, the off-axis parabolic mirror is coated, and the coating band covers the absorption characteristic band of the extremely weak absorbent gas to be detected. The RMS value of the mirror surface shape accuracy should be less than λ / 8, λ the optical wavelength used when testing the surface shape accuracy of the mirror;

[0015] The mirror thermostat is used to control the off-axis parabolic mirror to be in a constant temperature state;

[0016] The focusing lens group is used to adjust the system focal length and shape the light beam, so as to achieve imaging of extremely weakly absorbing gases with the best field of view range and spatial resolution, reduce aberration and shrink the image spot diameter;

[0017] The band-pass filter is used to block the absorption bands of non-extremely weakly absorbing gases in the output light of the focusing lens group, preventing stray light in other bands from entering the beam splitting optical fiber. The band-pass band of the band-pass filter depends on the characteristic absorption band of the extremely weakly absorbing gas to be detected. If multiple extremely weakly absorbing gases are detected, the union of multiple gas characteristic absorption bands needs to be taken;

[0018] The light homogenizer is used to equalize the light intensity of the output light of the band-pass filter, so as to equalize the image spot before the light beam enters the fiber optic probe and keep the spectral information consistent;

[0019] The fiber optic probe is used to receive the output light of the band-pass filter and transmit it to the dual-temperature-controlled spectrometer module and the acousto-optic tunable filter detection module through the beam splitting optical fiber.

[0020] Preferably, the first aperture stop adopts a three-layer structure, one layer is a shutter-type mechanical structure for changing the aperture size of the aperture stop, and two layers are double-layer planar thin fused silica glasses for sealing and heat insulation.

[0021] Preferably, the beam splitting optical fiber contains multiple cores and the front end is arranged in a planar close-packed manner for connecting with the fiber optic probe included in the spectral acquisition module. The beam splitting optical fiber is also divided into two strands. One strand is arranged in a line at the end and aligned with the spectrometer slit of the dual-temperature-controlled spectrometer module, and the other strand is arranged in a planar close-packed manner at the end and aligned with the input optical axis of the acousto-optic tunable filter detection module.

[0022] Preferably, the dual-temperature-controlled spectrometer module includes a spectrometer, a heat insulation housing, a support pad, a spectrometer temperature controller, a cooling fan, and multiple temperature sensors. Among them, the spectrometer includes a spectrometer slit, a collimating mirror, a diffraction grating, a converging mirror, a CCD detector, and a CCD detector cooler that are arranged in sequence along the optical path and installed on the spectrometer housing.

[0023] 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 mirror, the diffraction grating, and the converging mirror in sequence to obtain a spectral image that retains more spectral information of extremely weakly absorbing gases. The CCD detector is simultaneously cooled by the CCD detector cooler to reduce its temperature to the optimal working temperature of the CCD and keep it constant;

[0024] The spectrometer is supported within a sealed heat-insulating housing by support pads. The space between the spectrometer and the heat-insulating housing is filled with dry inert gas. The inner layer of the heat-insulating housing 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 ultraviolet-resistant material that can protect the spectrometer. A plurality of temperature sensors are arranged on at least some of the optical components within the spectrometer or on the spectrometer housing. The spectrometer thermostat is closely attached to the heat-insulating housing. Based on the feedback temperature from the temperature sensors, the spectrometer is evenly temperature-controlled through the inner heat-conducting material of the heat-insulating housing and the filled nitrogen gas, while preventing water vapor condensation. A cooling fan is used to dissipate heat from the spectrometer thermostat.

[0025] Preferably, the acousto-optic tunable filter detection module 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 arranged in sequence along the optical path.

[0026] The optical signal input through the beam-splitting optical fiber is collimated by the collimating lens group and the monochromatic aberration is reduced, and then input to the second aperture stop. The second aperture stop limits the beam diameter to match the beam entrance of the acousto-optic tunable filter.

[0027] The beam passing through the second aperture stop enters the acousto-optic tunable filter through the polarizer. After Bragg diffraction, spectral splitting is achieved. The separated narrowband beam has a very narrow full width at half maximum. 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 absorption gas. The acousto-optic tunable filter with an extremely short response time can detect the characteristic wavelengths of various extremely weak absorption gases.

[0028] The output light of the acousto-optic tunable filter enters the digital imaging device through the analyzer and the converging lens group to obtain a wavelength fingerprint image with characteristic wavelengths for the extremely weak absorption gas. By rotating the analyzer, the light intensity in different polarization directions can be measured. The converging lens group can reduce chromatic aberration and the image spot of optical imaging.

[0029] Preferably, both the focusing lens group and the converging lens group adopt doublet lenses.

[0030] Preferably, analyzing the concentration of extremely weak absorption gases in the plume region based on the observed spectral image and wavelength fingerprint image includes:

[0031] Determining the types of extremely weak absorption gases in the plume region according to the spectral image obtained by the dual-temperature-controlled spectrometer module, detecting the corresponding wavelength fingerprint image based on the type by the acousto-optic tunable filter detection module, and then calculating the concentration of extremely weak absorption gases in the plume region based on the spectral information of the wavelength fingerprint image of the extremely weak absorption gas.

[0032] Preferably, plume identification and plume concentration calculation based on the concentration of extremely weak absorption gases include:

[0033] Based on the concentration of the extremely weakly absorbent gas, the high-concentration part is distinguished as the plume shape of the gas to achieve plume recognition;

[0034] Calculate the concentration of the entire observed plume based on the plume slices detected by a single pitch angle sequence. Specifically: approximate the slices as circles, and obtain the spatial position of the projection height of the plume with respect to the observation point according to the azimuth angle of the spectral acquisition module and the distance from the spectral acquisition module to the emission source. Connect the upper and lower boundaries of the plume shape identified by the plume to the observation point respectively. The slice circle is tangent to the connection line. Combine the spatial position of the projection height to obtain the spatial position and size of the plume slice. Calculate the plume slices of the pitch angle sequence at each azimuth angle to form the specific spatial shape and concentration of the entire observed plume, where the concentration is in one-to-one mapping correspondence with the pixel concentrations on the plume shape.

[0035] Preferably, the emission flux calculation includes:

[0036] According to the concentration of the plume cross-section of each extremely weakly absorbent gas C , time difference , wind speed v and crosswind speed , calculate the emission flux of the exhaust port:

[0037] ;

[0038] where z represents the height of the center of the extremely weakly absorbent gas slice from the ground.

[0039] Compared with the prior art, the beneficial effects of the present invention at least include:

[0040] The optical signal in the observation area is collected with enhanced signal-to-noise ratio by the spectral acquisition module to reduce noise interference. On this basis, the optical signal is divided into two beams. One beam of optical signal is subjected to spectral acquisition through the dual-temperature-controlled spectrometer module. By dual-temperature control, the dark current and spectral shift in the spectral acquisition process are reduced, so that the obtained spectral image retains more spectral information of the extremely weakly absorbent gas. At the same time, the other beam of optical signal is subjected to the screening of characteristic wavelengths through the acousto-optic tunable filter detection module. Specifically, the characteristic wavelengths of the extremely weakly absorbent gas are screened through the acousto-optic tunable filtering process that prevents cross-interference of absorption signals of different atmospheric components, and then a wavelength fingerprint image for the extremely weakly absorbent gas is obtained. Finally, analyze the concentration of the extremely weakly absorbent gas in the plume area based on the observed spectral image and wavelength fingerprint image, and perform plume recognition, plume concentration calculation, and emission flux calculation based on the concentration of the extremely weakly absorbent gas, so as to realize imaging of the extremely weakly absorbent gas. Description of the Drawings

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0042] Figure 1 is a schematic structural diagram of an extremely weak absorbent gas imaging system with signal enhancement provided by an embodiment;

[0043] Figure 2 is a schematic structural diagram of a dual-temperature-controlled spectrometer module provided by an embodiment;

[0044] Figure 3 is a schematic structural diagram of an acousto-optic tunable filter detection module provided by an embodiment;

[0045] Figure 4 is a schematic diagram of plume profile recognition and calculation of pollutant gas emission flux provided by an embodiment;

[0046] Figures 5 - 7 is the imaging result of the exhaust plume using the extremely weak absorbent gas imaging system with signal enhancement provided by an embodiment. Detailed implementation manners

[0047] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific implementation manners described herein are only used to explain the present invention and do not limit the protection scope of the present invention.

[0048] The inventive concept of the present invention is as follows: In order to realize the detection and imaging of extremely weak absorbent atmospheric pollutants (abbreviated as extremely weak absorbent gases), the embodiments of the present invention provide an extremely weak absorbent gas imaging system with signal enhancement, aiming to enhance the signals of the spectral acquisition part and the spectral processing part, improve the signal-to-noise ratio, solve the problem of cross-interference between the absorption signals of different atmospheric components, and finally use the plume profile recognition technology and the pollutant gas emission flux algorithm to realize the imaging of extremely weak absorbent atmospheric pollutants and the calculation of the plume emission flux.

[0049] As Figure 1As shown in the figure, a very weak absorptive gas imaging system 1 provided by an embodiment includes a spectral acquisition module, a beam splitting optical fiber 2, a dual-temperature-controlled spectrometer module 3, an acousto-optic tunable filter detection module 4, and a control and calculation module 5. These components cooperate to achieve the imaging of very weak absorptive gases. Specifically, the spectral acquisition module 1 is used to collect solar scattered light, and perform operations such as folding, focusing, and shaping on 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 simultaneously transmits it into the dual-temperature-controlled spectrometer module 3 and the acousto-optic tunable filter detection module 4. The dual-temperature-controlled spectrometer module 3 is used to detect the spectral information of the solar scattered light, and this spectral information includes the spectral information of very weak absorptive gases and strongly absorptive atmospheric pollutants (referred to as strongly absorptive gases), forming a spectral image; the acousto-optic tunable filter detection module 4 is used to detect the characteristic wavelength information of very weak absorptive gases; the control and calculation module 5 detects the concentration of very weak absorptive gases, images the plume shape of very weak absorptive gases, and calculates the emission flux.

[0050] In the embodiment, the spectral acquisition module 1 has a signal-to-noise ratio enhanced acquisition function, and can collect solar scattered light signals with enhanced signal-to-noise ratio in the observation area, such as Figure 1 As shown in the figure, it specifically includes a first aperture diaphragm 11, an off-axis parabolic mirror 12, a mirror thermostat 13, a focusing lens group 14, a band-pass filter 15, a light homogenizer 16, and an optical fiber probe 17 arranged in sequence along the optical path.

[0051] Among them, the first aperture diaphragm 11 is used to limit the diameter of the input light beam, and to a certain extent reduce the optical distortion of the entire optical system. Figure 1 The light beam shown in light gray in the figure is the outer light beam with relatively large distortion, and this 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 the outside from entering the spectral acquisition module 1 and affecting the system life, in actual use, the entire spectral acquisition module 1 is fully sealed, and only the first aperture diaphragm 11 transmits light to the outside. 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 for changing the aperture size, and two layers are relatively thin double-layer planar fused silica glasses for sealing and heat insulation.

[0052] The off-axis parabolic mirror 12 is used to fold the optical path and converge the light beam, and observes solar scattered light at different angles by rotating the optical axis of the focusing optical path of the off-axis parabolic mirror 12. The off-axis parabolic mirror 12 needs to be coated, and the coating band covers the absorption characteristic band of the very weak absorptive gas to be detected. Generally, for VOCs, a UV-enhanced aluminum film is used, and for greenhouse gases, a gold film is used. The advantages of choosing an off-axis parabolic mirror are as follows:

[0053] 1. Aberration: Compared with the transmission combination of a right-angle prism and a lens, an off-axis parabolic mirror can focus the beam without chromatic aberration, eliminating the phase delay and absorption loss introduced by transmission optical elements, which is beneficial for wide-band spectral measurement;

[0054] 2. Surface accuracy: The surface accuracy of an off-axis parabolic mirror can reach a very high level, and the RMS value of the surface accuracy should be less than λ / 8. λ The optical wavelength is used when testing the surface accuracy of the mirror to ensure high-quality focusing of the beam;

[0055] 3. Degree of freedom: The off-axis parabolic mirror has a low degree of freedom. Its reflection function and focusing function are combined, reducing the difficulty and workload of adjusting the optical path.

[0056] The defect of the off-axis parabolic mirror 12 is that its temperature stability is not as good as that of the transmission combination. The deformation of the mirror caused by temperature has a greater impact on the optics. And the system of the present invention is mainly used for outdoor monitoring, where the temperature changes greatly. Therefore, a mirror thermostat 13 is used to keep the temperature of the off-axis parabolic mirror 12 stable and maintain the best function. The mirror thermostat 13 includes a micro temperature sensor, a semiconductor cooler and a cooling fan, and uses the metal material of the off-axis parabolic mirror 12 itself for heat conduction.

[0057] The off-axis parabolic mirror 12 with a smaller focal length can detect a larger field of view angle range, but the spatial resolution will be reduced. Therefore, in order to image extremely weak absorptive gases in different situations, it is necessary to adjust the focal length of the optical system as needed. Due to the high processing difficulty and cost of the off-axis parabolic mirror 12, it is impossible to achieve focusing with only a single off-axis parabolic mirror 12. Therefore, a focusing lens group 14 is added to adjust the system focal length and shape the beam to achieve imaging of extremely weak absorptive gases with the best field of view angle range and spatial resolution in different situations. In order to further reduce aberration and reduce the spot diameter, the focusing lens group 14 needs to use a doublet lens to reduce chromatic aberration, and multiple lenses are combined to reduce the spot of optical imaging. By moving the focusing lens group 14 and the fiber optic probe 17, the change of the focal length of the entire optical system is realized.

[0058] The band-pass filter 15 is used to block the absorption bands of non-extremely weak absorptive gases to prevent stray light in other bands from entering the optical fiber and generating noise. The band-pass band of the band-pass filter 15 depends on the characteristic absorption band of the extremely weak absorptive gas to be detected. If multiple extremely weak absorptive gases are to be detected, the union of the characteristic absorption bands of multiple gases needs to be taken. If there are many extremely weak absorptive gases to be detected and the band covers the coating band of the off-axis parabolic mirror, the band-pass filter 15 can be not used.

[0059] The light homogenizer 16 is used to equalize the light intensity of the output light of the band-pass filter 15. Since aberration is inevitable, in order to make the light intensity and spectral information entering different cores of the splitting optical fiber 2 consistent, a relatively thin light homogenizer 16 needs to be placed close to the optical fiber.

[0060] The optical fiber probe 17 is used to receive the solar scattered light output by the light homogenizer 16 and conduct the light into the optical fiber.

[0061] In the embodiment, the splitting optical fiber 2 is used to split the collected optical signal and simultaneously transmit it to the dual-temperature-controlled spectrometer module 3 and the acousto-optic tunable filter detection module 4. As Figure 2 shown, the splitting optical fiber 2 includes multiple cores, for example, 19 cores. The diameter of each core is 0.2 mm. The front end 21 of the cores is in planar close packing 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 respectively. Among them, one strand containing 12 cores 22 is arranged in a straight line at the end and aligned with the spectrometer slit 31 of the dual-temperature-controlled spectrometer module 3; the other strand containing 7 cores 23 is arranged in planar close packing at the end and aligned with the input optical axis of the acousto-optic tunable filter detection module 4. Figure 1 In [the figure], one strand of 12 cores 22 is represented in white, and one strand of 7 cores 23 is represented in gray. Their arrangement at the front end 21 of the cores should be as even as possible.

[0062] In the embodiment, the dual-temperature-controlled spectrometer module 3 is used to perform spectral acquisition on the input optical signal to reduce dark current and spectral shift, and obtain a spectral image that retains more spectral information of extremely weak absorptive gases. As Figure 2 shown, it specifically includes a heat-insulating housing 37, a support pad 38, a spectrometer thermostat 39, a cooling fan 310, a spectrometer 311, and multiple temperature sensors ( Figure 2 not shown in the figure). Among them, the spectrometer 311 includes a spectrometer slit 31, a collimating mirror 32, a diffraction grating 33, a converging mirror 34, a CCD detector 35, and a CCD detector cooler 36 that are sequentially arranged along the optical path and installed on the spectrometer housing.

[0063] Among them, the spectrometer slit 31 is used to receive the solar scattered light transmitted through the splitting optical fiber completely without gaps. Specifically, the spectrometer slit 31 is connected to the 12 cores 22 arranged in a straight line, and the slit width is the core diameter. During installation, it is necessary to ensure that the 12 cores 22 are aligned with the spectrometer slit 31. The narrow slit and the core arrangement method are on the one hand to make full use of the optical signal in the optical fiber, and on the other hand to improve the spectral resolution as much as possible, and to capture the spectral absorption peaks of extremely weak absorptive atmospheric pollutants as much as possible during spectral analysis.

[0064] The light beam input into the spectrometer through the spectrometer slit 31 is divergent. It is reflected and collimated into approximately parallel light by the collimating mirror 32. The approximately parallel light is then reflected and dispersed by the diffraction grating 33. After that, the light beam is converged by the converging mirror 34 and irradiates on the CCD detector 35 to obtain a spectral image that retains more spectral information of extremely weak absorptive gases. The function of the CCD detector cooler 36 is to cool the CCD detector 35, reduce the detector temperature to the optimal operating temperature of the CCD, and keep it constant within a very small temperature range to reduce thermal noise, improve the signal-to-noise ratio and sensitivity.

[0065] The heat-insulating housing 37 wraps the spectrometer. The heat-insulating housing 37 adopts a multi-layer structure. The inner layer uses a heat-conducting material for heat conduction, such as a metal material; the middle layer is a heat-insulating material; the outer layer uses a material with high strength and ultraviolet resistance to protect the spectrometer, such as white fiberglass nylon to protect the spectrometer. The heat-insulating housing 37 and the spectrometer are only connected by a rubber support pad 38 to support the spectrometer in the sealed heat-insulating housing 37. The rubber material has both shock-absorbing and heat-insulating functions. The space between the spectrometer and the heat-insulating housing 37 is filled with a dry inert gas (such as nitrogen). Multiple temperature sensors are arranged on at least some optical components inside the spectrometer or on the spectrometer housing. The spectrometer temperature controller 39 is closely attached to the heat-insulating housing 37. When it controls the temperature of the spectrometer based on the feedback temperature of the temperature sensors, the temperature controller cools the inner layer metal junction material of the heat-insulating housing 37, and then the whole spectrometer is evenly cooled through the heat transfer of the inert gas. At the same time, the filling gas does not contain moisture, which can effectively prevent water vapor from condensing on the optical devices inside the spectrometer. The cooling fan 310 is used to dissipate heat from the spectrometer temperature controller 39.

[0066] In the embodiment, multiple temperature sensors are arranged on the spectrometer slit 31, collimating mirror 32, diffraction grating 33, converging mirror 34, 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. They 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.

[0067] In the embodiment, the acousto-optic tunable filter detection module 4 is used to perform filter detection on the input optical signal to prevent cross-interference of absorption signals of different atmospheric components, and obtain a wavelength fingerprint image with characteristic wavelengths for multiple extremely weak absorptive gases, as Figure 3 shown, specifically including a collimating lens group 43, a second aperture stop 42, a polarizer 43, an acousto-optic tunable filter 44, an analyzer 45, a converging lens group 46, and a digital imaging device 47 arranged in sequence along the optical path.

[0068] Among them, the collimating lens group 41 collimates the divergent light beams of the seven optical fibers 23 into approximately parallel light. The collimating lens group 41 uses a doublet lens to reduce chromatic aberration, and multiple lenses are combined to reduce monochromatic aberration. The second aperture stop 42 is used to limit the diameter of the light beam so that it matches the light beam entrance of the acousto-optic tunable filter 44. After the light beam is collimated, it passes through the polarizer 43, which is used to turn the light beam into linearly polarized light.

[0069] After the polarized light enters the acousto-optic tunable filter 44, it undergoes Bragg diffraction to achieve spectral splitting. The separated narrowband light beam has a very narrow full width at half maximum. The frequency of the high-frequency drive electrical signal of the acousto-optic tunable filter 44 is adjusted according to the characteristic wavelength of the extremely weakly absorbing gas. Generally speaking, at least two wavelengths of intensity are required for a gas to determine the gas concentration. The acousto-optic tunable filter 44 with an extremely short response time can detect the characteristic wavelengths of multiple extremely weakly absorbing gases.

[0070] The light output from the acousto-optic tunable filter 44 passes through the analyzer 45 and the converging lens group 46 and then enters the digital imaging device 47 to obtain a wavelength fingerprint image with characteristic wavelengths for the extremely weakly absorbing gas. Among them, the analyzer 45 is used to detect the polarization state of the light beam. When the polarization state does not need to be detected, the polarization angle of the analyzer 45 is the same as that of the polarizer 43. When detecting the polarization state, the analyzer 45 is rotated to measure 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 doublet lens to reduce chromatic aberration, and multiple lenses are combined to reduce the image spot of optical imaging.

[0071] In the embodiment, the control and calculation module 5 is used to control the spectral acquisition module to perform swing-scanning observation to collect the solar scattered light signal, analyze the concentration of the extremely weakly absorbing gas in the plume area based on the observed spectral image and wavelength fingerprint image, and perform plume identification, plume concentration calculation, and emission flux calculation based on the concentration of the extremely weakly absorbing gas, as Figure 4 shown, which is specifically abbreviated as spectral observation, spectral analysis, plume identification, plume concentration calculation, and emission flux calculation.

[0072] For the spectral observation step, the spectral acquisition module is controlled to perform pendulum-sweeping scanning observation and acquisition to obtain the solar scattered light signals of different pitch angle sequences and different azimuth angle sequences. Specifically, three components, namely the aperture diaphragm 11, the off-axis parabolic mirror 12, and the mirror thermostat 13, are rotated to achieve observations of different pitch angle sequences, and the entire spectral acquisition module 1 is rotated to achieve observations of different azimuth angle sequences. After determining the observation area and observation resolution, the pitch angle and azimuth angle sequences are determined. After one pitch angle sequence observation is completed, the azimuth angle is changed to perform the next pitch angle sequence observation 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 the background spectrum during spectral analysis calculation. The spectrum of the actual observed plume area is subtracted from the background area spectrum to obtain the superimposed spectrum of the absorption spectra of all absorption characteristic gases in the plume.

[0073] After the spectrum is observed, it enters the spectral analysis step. The goal of this step is to obtain the concentration of extremely weakly absorptive gases in the plume area. In order to separate the spectra of extremely weakly absorptive gases from those of strongly absorptive gases and calculate the concentration of extremely weakly absorptive gases according to the Lambert-Beer law, the dual-temperature-controlled spectrometer module 3 and the acousto-optic tunable filter detection module 4 are required. The dual-temperature-controlled spectrometer module 3 can minimize dark current and spectral shift as much as possible compared with ordinary spectrometers, retain more spectral information of extremely weakly absorptive gases, and greatly increase the possibility of successful detection. The concentrations of all strongly absorptive gases and some weakly absorptive gases can be analyzed according to the absorption characteristics of different gases. Since the cross-interference between the absorption signals of different atmospheric components will greatly affect the detection of extremely weakly absorptive gases, some extremely weakly absorptive gases with relatively close spectral characteristics cannot be distinguished, and the acousto-optic tunable filter detection module 4 can solve this problem. The types of extremely weakly absorptive gases that are difficult to distinguish are selected as the analysis objects of the acousto-optic tunable filter detection module 4, and the characteristic wavelengths of these gases are selected and analyzed. On the one hand, the acousto-optic tunable filter detection module 4 uses ultrasonic waves to generate spatial periodic modulation in the crystal, so as to achieve more refined targeted wavelength screening than traditional filters. On the other hand, by adjusting the ultrasonic frequency, the wavelength can be quickly switched to provide a super-resolution spectral signal to achieve the analysis of the fine structure of the molecular absorption cross-section. With the supplementary analysis of the acousto-optic tunable filter detection module 4, the spectral observation step can accurately identify the absorption signals of the characteristic fingerprints of different gases and achieve the synchronous imaging of extremely weakly absorptive gas VOCs such as aldehydes, ketones, esters, and benzene series.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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:

[0078] ;

[0079] Where z represents the height of the center of the extremely weakly absorbing gas slice above the ground.

[0080] Figures 5 - 7The imaging results of the plume at the chimney outlet by the extremely weak absorbent gas imaging system with signal enhancement are shown, and the measured plumes of aldehydes, ketones and benzene series are presented. It can be seen that under single measurement, the plumes of the three are not exactly the same, which is related to their physical and chemical properties, and also indicates that the extremely weak absorbent gas imaging system with signal enhancement does identify and image different gas plumes, demonstrating the effectiveness of this method.

[0081] The extremely weak absorbent gas imaging system with signal enhancement provided by the present invention can image and calculate the flux of the plume of extremely weak absorbent atmospheric pollutants, and relevant factories and relevant departments can carry out precise monitoring and efficient prevention and control based on the calculation results.

[0082] The above-described specific embodiments have elaborated on 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 used to limit the present invention. Any modifications, supplements, equivalent replacements, etc. made within the principle scope of the present invention shall be included within 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 enhanced acquisition function, and can acquire solar scattered light signals with enhanced signal-to-noise ratio in the observation area, wherein the spectrum acquisition module includes a first aperture diaphragm, 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, and except for the first aperture diaphragm which is transparent to the outside, the other components are all sealed; 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 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 passing through the first aperture stop and converge the light to the focusing transmission group, wherein the off-axis parabolic reflector is coated, and 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, where λ is the wavelength of light used to test 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 1, characterized in that: The focusing lens group adopts a double cemented lens.

8. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 6, characterized in that: The converging lens group adopts a double cemented lens.

9. 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.

10. 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.

11. The signal-enhanced extremely weakly absorbing gas imaging system according to claim 1, characterized in that: Emission flux calculations, including: According to the concentration C, time difference Δt, wind speed v and lateral wind speed v of each extremely weakly absorbing gas plume section x , calculate the discharge flux Φ of the outlet: Φ=∫∫C·v·Δt·dv x ·dz Where z represents the height of the center of the extremely weakly absorbing gas slice above the ground.

Images