A scanning multi-field-of-view aerosol lidar

Abstract

The present invention discloses a scanning multi-field-of-view aerosol lidar, which includes a laser, an optical transmitting module, an optical receiving module, and an acquisition, processing and control module; the optical transmitting module, the optical transmitting module includes a beam expander and a laser reflector; the optical receiving module includes a Kepler transmissive structure, a rotating wheel device disposed on the image-side focal plane of the Kepler transmissive structure, a polarization beam splitting cube, and a coupling lens; wherein, the rotating wheel device includes a rotating wheel and a rotating motor, and the rotating wheel adjusts the field-of-view angle of the Kepler transmissive structure under the drive of the rotating motor. The small field-of-view receives the single-scattering dominant signal, which is beneficial to the inversion of the backscattering efficiency; the large field-of-view receives the multiple-scattering signal, which is beneficial to the inversion of the forward scattering phase function, and improves the detection accuracy of the multi-field-of-view lidar.

Description

Technical Field

[0001] The present invention relates to an aerosol lidar in the technical field of lidar technology, and particularly to a scanning multi-field-of-view aerosol lidar. Background Art

[0002] The microphysical parameters of aerosols and clouds (such as extinction coefficient, effective radius, number concentration) are core parameters for studying air pollution, cloud microphysical processes, and climate change. The limitations of traditional single-field-of-view (SFOV) lidar systems are mainly reflected in their reliance on a single scattering signal to infer backscattering and extinction coefficients in the atmosphere. However, such an approach often brings uncertainties and requires the use of additional assumptions or models to obtain more accurate results.

[0003] Multiple field of view (MFOV) lidar systems, i.e., systems that collect data through multiple fields of view, have been developed to address these limitations of traditional systems. By simultaneously capturing signals from different fields of view, MFOV lidars can obtain more independent data, thereby improving the inversion accuracy of aerosol and cloud properties.

[0004] Commercial aerosol radars are mostly single-field-of-view systems, and MFOV radar systems have not been commercialized yet. The main bottlenecks are high hardware complexity and insufficient algorithm robustness. Summary of the Invention

[0005] In order to improve the detection sensitivity of multi-field-of-view lidars, the present invention provides a scanning multi-field-of-view aerosol lidar.

[0006] The present invention is implemented by the following technical solutions: A scanning multi-field-of-view aerosol lidar, the lidar includes a laser, an optical transmitting module, an optical receiving module, and an acquisition, processing, and control module; the laser, the optical transmitting module, the optical receiving module, and the acquisition, processing, and control module are sequentially connected by optical fibers, and the optical transmitting module includes a beam expander and a laser mirror; the beam expander and the laser mirror are sequentially connected by an optical fiber;

[0007] The optical receiving module, the optical receiving module includes a Kepler transmissive structure, a rotating wheel device disposed on the image-side focal plane of the Kepler transmissive structure, a polarization beam splitting cube, and a coupling lens; the Kepler transmissive structure, the polarization beam splitting cube, and the coupling lens are sequentially connected by optical fibers;

[0008] Among them, the rotating turntable device includes a rotating turntable and a rotating motor. The rotating turntable adjusts the field of view angle of the Kepler transmissive structure under the drive of the rotating motor.

[0009] As a further improvement of the above solution, a plurality of small aperture diaphragms with different apertures are evenly distributed on the circumference of the rotating turntable. The rotating turntable makes each of the small aperture diaphragms align with the focal point position of the Kepler transmissive structure in turn under the drive of the rotating motor.

[0010] As a further improvement of the above solution, the rotating motor is electrically connected to the acquisition processing and control module; the rotating turntable device further includes a feedback controller and a limit optoelectronic sensor.

[0011] As a further improvement of the above solution, the optical receiving module further includes a narrowband filter.

[0012] As a further improvement of the above solution, the acquisition processing and control module includes a horizontal polarization signal detector, a vertical polarization signal detector, and an acquisition card; the acquisition card is electrically connected to the horizontal polarization signal detector and the vertical polarization signal detector respectively.

[0013] As a further improvement of the above solution, both the horizontal polarization signal detector and the vertical polarization signal detector are PMT single photon detectors.

[0014] As a further improvement of the above solution, the optical transmitting module further includes a power monitoring module. The power monitoring module includes a laser beam sampling mirror and a power monitor; the power monitoring module is connected to the laser through an optical fiber.

[0015] As a further improvement of the above solution, the lidar includes a scanning pan-tilt head, which is used to carry the laser, the optical transmitting module, the optical receiving module, and the acquisition processing and control module.

[0016] The multi-field-of-view aerosol lidar of the present invention has the following beneficial effects:

[0017] In this application, the laser, the optical transmitting module, the optical receiving module, and the acquisition, processing, and control module are sequentially connected by an optical fiber; the optical receiving module includes a Kepler transmissive structure, a rotating wheel device disposed on the image-side focal plane of the Kepler transmissive structure, a polarization beam splitting cube, and a coupling lens; wherein, the rotating wheel device includes a rotating wheel and a rotating motor, and the rotating wheel adjusts the field of view angle of the Kepler transmissive structure under the drive of the rotating motor. A small field of view receives the single-scattering dominant signal, which is beneficial to accurately invert the backscattering efficiency; a large field of view receives the multiple-scattering signal, which is beneficial to accurately invert the forward scattering phase function, improving the detection accuracy of the multi-field aerosol lidar. Brief Description of the Drawings

[0018] Figure 1 It is a structural block diagram of the multi-field aerosol lidar according to an embodiment of the present invention.

[0019] The reference numerals are as follows: objective lens 1, eyepiece 2, narrowband filter 3, polarization beam splitting cube 4, first coupling lens 5, second coupling lens 6, rotating wheel 7, rotating motor 8, horizontally polarized signal detector 9, vertically polarized signal detector 10, laser 11, beam expander 12, first laser mirror 13, second laser mirror 14, laser beam sampling mirror 15, power monitor 16, and scanning pan-tilt 17. Detailed Embodiments

[0020] In order 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 embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0021] Please refer to Figure 1 , in the embodiment of this application, a specific structure of a scanning multi-field aerosol lidar is proposed. This structure includes a laser, an optical transmitting module, an optical receiving module, and an acquisition, processing, and control module; the laser, the optical transmitting module, the optical receiving module, and the acquisition, processing, and control module are sequentially connected by an optical fiber.

[0022] The laser 11 is a YAG micro-pulse laser for generating detection laser light.

[0023] The optical transmitting module includes a beam expander 12, a first laser mirror 13, and a second laser mirror 14; wherein the beam expander 12 is a 20-fold beam expander, compressing the divergence angle <0.1 mrad to ensure that the laser beam diameter of the detection laser light matches the telescope field of view; the first laser mirror 13 and the second laser mirror 14 are used to reflect the laser beam of the collimated detection laser light and are installed on an adjustable mounting bracket to facilitate adjusting the parallelism of the transmitting and receiving optical axes.

[0024] The optical receiving module includes a Kepler transmissive structure, a rotating wheel device disposed on the image-side focal plane of the Kepler transmissive structure, a polarization beam splitting cube 4, a first coupling lens 5, and a second coupling lens 6; the Kepler transmissive structure, the polarization beam splitting cube 4, and the first coupling lens 5 are sequentially connected by optical fibers; the Kepler transmissive structure, the polarization beam splitting cube 4, and the second coupling lens 6 are also sequentially connected by optical fibers; wherein, the Kepler transmissive structure includes an objective lens 1 and an eyepiece 2, the rotating wheel device is disposed on the image-side focal plane of the Kepler transmissive structure, the rotating wheel device includes a rotating wheel 7 and a rotating motor 8, and the rotating wheel 7 adjusts the field of view angle of the Kepler transmissive structure under the drive of the rotating motor 8.

[0025] In an embodiment of the application, the optical receiving module further includes a narrowband filter 3 for filtering out stray light and background light in other wavelength bands.

[0026] In an embodiment of the application, the acquisition, processing and control module includes a horizontal polarization signal detector, a vertical polarization signal detector, and an acquisition card; the acquisition card is electrically connected to the horizontal polarization signal detector and the vertical polarization signal detector respectively.

[0027] Specifically, the Kepler transmissive structure collects the echo signal after the detection laser is reflected by the atmosphere, collimates the echo signal, filters out stray light and background light in other wavelength bands through the narrowband filter 3, and divides the echo signal into two parts with different polarization components through the polarization beam splitting cube 4. The horizontal polarization signal is coupled into the horizontal polarization signal detector 9 through the first coupling lens 5, and the vertical polarization signal is coupled into the vertical polarization signal detector 10 through the second coupling lens 6; wherein, the horizontal polarization echo signal and the vertical polarization echo signal generate corresponding electrical signals after photoelectric conversion, which are collected by the acquisition card and processed and analyzed by a digital processor.

[0028] Specifically, the processing and analysis process of the digital processor includes:

[0029] First, when the field of view angle of the Kepler transmissive structure is a small field of view, the single-scattering signal equation of the received single-scattering signal is:

[0030]

[0031] Wherein, is the single-scattering signal intensity at height ; is the laser emission power; is the system constant; is the extinction coefficient; is the backscattering cross section; is the backscattering efficiency.

[0032] When the field of view angle of the Kepler transmissive structure is a large field of view, the multiple scattering signal equation of the received multiple scattering signal is as follows:

[0033]

[0034] Among them, is the field of view interval of the multiple scattering signal intensity; is the forward scattering phase function; is the altitude at which the extinction coefficient is located.

[0035] It can be understood that the single scattering signal is not affected by multiple scattering and is suitable for accurately retrieving the extinction coefficient and backscattering efficiency of the atmosphere; the differences between multiple scattering signals can accurately retrieve the forward phase function, and finally, the particle size distribution of the atmosphere can be accurately retrieved based on the extinction coefficient, backscattering efficiency, and forward phase function.

[0036] In the embodiments of the present application, the laser, the optical emission module, the optical reception module, and the acquisition processing and control module are sequentially connected by optical fibers; the optical reception module includes a Kepler transmissive structure, a rotating wheel device disposed on the image-side focal plane of the Kepler transmissive structure, a polarization beam splitting cube, and a coupling lens; among them, the rotating wheel device includes a rotating wheel and a rotating motor, and the rotating wheel adjusts the field of view angle of the Kepler transmissive structure under the drive of the rotating motor. When the field of view angle of the Kepler transmissive structure is a small field of view, the single scattering signal is received; when the field of view angle of the Kepler transmissive structure is a large field of view, the multiple scattering signal is received; and the single scattering signal is not affected by multiple scattering and is suitable for accurately retrieving the extinction coefficient and backscattering efficiency of the atmosphere. Therefore, the small field of view is used to separate single scattering, and the large field of view inversely calculates the forward phase function through the differences between multiple scattering signals, and accurately retrieves the particle size distribution of the atmosphere based on the extinction coefficient, backscattering efficiency, and forward phase function.

[0037] Please continue to refer to Figure 1 , in an embodiment of the application, a plurality of small hole diaphragms with different apertures are uniformly distributed on the circumference of the rotating wheel 7. The rotating wheel 7 is driven by the rotating motor 8 to align each small hole diaphragm with the focal position of the Kepler transmissive structure in sequence; exemplarily, 6 stainless steel small hole diaphragms with different aperture sizes are evenly distributed on the circumference of the rotating wheel 7, corresponding to the full field of view angles of 0.5 mrad, 1 mrad, 4 mrad, 8 mrad, 16 mrad, and 32 mrad respectively.

[0038] Among them, the rotating motor 8 is electrically connected to the acquisition processing and control module; the rotating wheel device further includes a feedback controller and a limit photoelectric sensor; a stepping motor with encoder feedback and a Hall sensor are used to achieve a small hole position positioning error.

[0039] Specifically, after the detection laser pulse is emitted, it is incident on the atmosphere and interacts with aerosol particles, cloud droplets, etc., and multiple scattering occurs to form an echo signal. The echo signal includes a single echo signal and a multiple scattering echo signal. The echo signal is collected by the optical receiving module. Among them, the single scattering signal is mainly received by a small field of view (0.5~1mrad), and the multiple scattering signal is received by other larger fields of view (4~32mrad). The PMT single-photon detector sequentially receives the scattering signals of 6 fields of view, and the acquisition card records the time-domain waveforms of each field of view. The motor controls the rotation of the turntable. According to the preset positioning, when the field of view angle is sequentially switched from small to large to a certain field of view angle, the signal at the current field of view angle is collected within the preset time period. After the collection is completed, it is switched to the next field of view angle, and no signal is collected during the switching process to avoid data confusion.

[0040] In the embodiment of the present application, a single turntable is used to achieve multi-field-of-view switching, replacing multi-detectors or spectroscopic systems, which greatly simplifies the hardware complexity of the aerosol lidar.

[0041] In an embodiment of the application, both the horizontal polarization signal detector 9 and the vertical polarization signal detector 10 are PMT single-photon detectors; among them, the detection laser can use a laser with a wavelength of 532nm; the aerosol Mie scattering cross-section at a wavelength of 532nm is 30%-50% higher than that at 1.06μm; and the PMT in the PMT single-photon detector has a quantum efficiency >25% at 532nm, a dark noise <50Hz, supports single-photon counting under a micro-pulse (100μJ), and the signal-to-noise ratio (SNR) of the aerosol scattering signal can be increased to more than 20dB.

[0042] In the embodiment of the present application, the detection laser with a wavelength of 532nm and the PMT single-photon detector cooperate to significantly enhance the signal-to-noise ratio of the aerosol scattering signal.

[0043] In an embodiment of the application, the optical emission module further includes a power monitoring module, and the power monitoring module includes a laser beam sampling mirror 15 and a power monitor 16; among them,

[0044] The laser beam sampling mirror 15 samples 1-10% of the laser from the incident light through Fresnel reflection of the uncoated optical surface based on the polarization direction of the incident detection laser for power monitoring; the power monitor 16 receives the trace sampled laser beam emitted by the laser beam sampling mirror 15 to monitor the light power fluctuation of the laser.

[0045] In an embodiment of the application, the lidar includes a scanning pan-tilt 17, and the scanning pan-tilt 17 is used to carry the laser, the optical emission module, the optical receiving module, and the acquisition processing and control module.

[0046] Specifically, the scanning pan-tilt 17 is driven by a servo motor, and a closed-loop control without cumulative error is achieved through a harmonic reducer and an encoder. Additionally, the scanning data of the horizontal azimuth angle and the pitch angle of the scanning pan-tilt are synchronously acquired with the scattered signal. Motion artifacts are eliminated through a spatio-temporal registration algorithm, and the three-dimensional extinction coefficient and particle size distribution of aerosols and clouds are obtained through a programmed scanning path, breaking through the limitations of traditional vertical fixed observations.

[0047] In the embodiment of the present application, a scanning device is added to the lidar, and a single device can replace traditional multi-site network observations, reducing the hardware deployment cost, and is particularly suitable for urban environmental monitoring networks and field stations.

[0048] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A scanning multi-field-of-view aerosol lidar, the lidar comprising a laser, an optical transmitting module, an optical receiving module, and an acquisition, processing and control module; the laser, the optical transmitting module, the optical receiving module, and the acquisition, processing and control module are sequentially connected by optical fibers, and is characterized in that, the optical transmitting module includes a beam expander and a laser reflector; the beam expander and the laser reflector are sequentially connected by an optical fiber; the optical receiving module includes a Kepler transmissive structure, a rotating wheel device disposed on the image-side focal plane of the Kepler transmissive structure, a polarization beam splitting cube, and a coupling lens; the Kepler transmissive structure, the polarization beam splitting cube, and the coupling lens are sequentially connected by optical fibers; wherein, the rotating wheel device includes a rotating wheel and a rotating motor, and the rotating wheel adjusts the field of view angle of the Kepler transmissive structure under the drive of the rotating motor; a plurality of small aperture diaphragms with different apertures are evenly distributed on the circumference of the rotating wheel, and the rotating wheel under the drive of the rotating motor makes each of the small aperture diaphragms sequentially align with the focal point position of the Kepler transmissive structure; when the field of view angle of the Kepler transmissive structure is a small field of view, single-scattering signals are received; when the field of view angle of the Kepler transmissive structure is a large field of view, multi-scattering signals are received.

2. The scanning multi-field-of-view aerosol lidar according to claim 1, wherein the rotating motor is electrically connected to the acquisition, processing and control module; the rotating wheel device further includes a feedback controller and a limit optoelectronic sensor.

3. The scanning multi-field-of-view aerosol lidar according to claim 1, wherein the optical receiving module further includes a narrowband filter.

4. The scanning multi-field-of-view aerosol lidar according to claim 1, characterized in that, the acquisition, processing and control module includes a horizontal polarization signal detector, a vertical polarization signal detector, and an acquisition card; the acquisition card is electrically connected to the horizontal polarization signal detector and the vertical polarization signal detector respectively.

5. The scanning multi-field-of-view aerosol lidar according to claim 4, wherein both the horizontal polarization signal detector and the vertical polarization signal detector are PMT single-photon detectors.

6. The scanning multi-field-of-view aerosol lidar according to claim 1, wherein the optical transmitting module further includes a power monitoring module, the power monitoring module includes a laser beam sampling mirror and a power monitor; the power monitoring module is connected to the laser by an optical fiber.

7. The scanning multi-field aerosol lidar according to claim 1, characterized in that, the lidar includes a scanning pan-tilt head, and the scanning pan-tilt head is used to carry the laser, the optical transmitting module, the optical receiving module, and the acquisition, processing and control module.

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