CN114325656B - A laser radar and method for detecting the bio-optical characteristic profile of water bodies - Google Patents

Abstract

The present invention discloses a laser radar and method for detecting the bio-optical characteristic profile of a water body, wherein the laser radar includes a laser emitting system, a four-channel signal receiving system and a data acquisition and processing system; the laser emitting system is used to emit laser pulses; the four-channel signal receiving system is used to receive echo signals containing different wavelengths and polarization information from the water body and convert them into electrical signals, and the data acquisition and processing system is used to collect and process the electrical signals received by the four-channel receiving system. The present invention overcomes the deficiency of not considering suspended inorganic matter and CDOM in the classical bio-optical model, and by establishing a bio-optical characteristic profile inversion algorithm suitable for complex water bodies, the bio-optical characteristics of the water body can be detected over a large range with high spatial resolution after a calibration experiment, and the bio-optical characteristic profile of the water body can be obtained.

Description

A laser radar and method for detecting the bio-optical characteristic profile of water bodies

Technical Field

The present invention belongs to the field of laser radar technology, and in particular relates to a laser radar and a method for detecting the bio-optical characteristic profile of a water body.

Background technique

Marine ecosystems play an important role in global climate change and ecological environmental protection. Phytoplankton is the main producer of marine ecosystems, and its net photosynthetic carbon fixation is approximately equal to the sum of the carbon fixation of all terrestrial plants. Lake ecosystems play an important role in regulating regional climate and maintaining regional ecological balance. They are also important freshwater resource reservoirs. Eutrophication of water bodies can lead to the outbreak of algal blooms and pollute the water environment.

Chlorophyll is the main pigment for phytoplankton photosynthesis and is used to characterize the biomass of phytoplankton. Currently, the main means of detecting chlorophyll are biochemical methods, in-situ instrument measurement, passive remote sensing and active remote sensing. Among them, lidar, as an active optical remote sensing device, can be used to detect the bio-optical characteristics profile of water bodies and conduct remote sensing measurements of the vertical distribution structure of the upper ocean. It has the advantages of high temporal and spatial resolution, continuous observation day and night, and global scale measurement. According to the composition of water bodies, water bodies can be divided into Class I water bodies and Class II water bodies. Phytoplankton in Class I water bodies is the main cause of changes in the optical properties of water bodies. At present, combined with the bio-optical model of Class I water bodies, lidar has been successfully applied to the study of biomass detection, phytoplankton layer distribution, etc. However, the optical properties of Class II water bodies are not only affected by phytoplankton and related particles, but also related to inorganic suspended particles and colored dissolved organic matter (CDOM). This makes it difficult to use lidar to invert the bio-optical characteristics profile of water bodies.

A Chinese patent document with publication number CN110673157A discloses a high spectral resolution laser radar system for detecting ocean optical parameters. The system has four channels for detecting polarization information, chlorophyll content, phytoplankton and seawater temperature in the ocean, and can detect these parameters; a Chinese patent document with publication number CN105486664A discloses a device and method for detecting marine phytoplankton biomass and particulate organic carbon. Based on a high spectral resolution laser radar, using a marine phytoplankton biomass and POC inversion method, it can achieve synchronous inversion of phytoplankton biomass and POC. However, the structure of the high spectral resolution laser radar is complex, and when inorganic suspended particles and CDOM are present in the water body, the single-wavelength laser radar cannot accurately obtain the chlorophyll concentration. Existing research proposes to use multi-wavelength laser radar to solve the problem that single-wavelength laser radar cannot accurately invert chlorophyll concentration. For example, the Chinese patent document with publication number CN112034480A discloses a wavelength optimization method for dual-wavelength ocean laser radar detection. Based on the dual-wavelength ocean high-spectral resolution laser radar, the inversion model of chlorophyll concentration and CDOM concentration is established according to the water absorption formula. By establishing an evaluation method for relative error of parameters, the two wavelengths of the laser radar are optimized to obtain the best chlorophyll and CDOM absorption coefficient inversion model. However, the dual-wavelength laser radar discussed in this method has higher requirements for the laser, and the system structure is more complicated.

Therefore, there is an urgent need to provide a laser radar and method that has a simple structure and can detect the bio-optical property profile of water bodies with high precision.

Summary of the invention

In order to solve the deficiencies in the prior art, the present invention provides a laser radar and method for detecting the bio-optical property profile of a water body, which can realize rapid and accurate detection of the bio-optical property profile.

A laser radar for detecting the bio-optical characteristic profile of a water body, characterized in that it comprises a laser emission system, a four-channel signal receiving system and a data acquisition and processing system;

The laser emission system comprises a solid pulse laser, a beam expander and a Glan laser prism; wherein the solid pulse laser is used to emit pulse laser to the water body, the beam expander is used to adjust the pulse laser divergence angle, and the Glan laser prism is used to improve the polarization degree of the emitted pulse laser;

The four-channel signal receiving system includes four detachable receiving channels, each receiving channel includes a telescope, a field aperture, a converging lens, an interference filter, and a photodetector, wherein the first receiving channel and the fourth receiving channel also include a linear polarizer module; the linear polarizer module is used to adjust the polarization direction of the echo signal, the telescope is used to collect the echo light signal from the water body, the field aperture is used to adjust the field angle, the converging lens is used to collimate the received light signal, the interference filter is used to suppress the light signal other than the target wavelength, and the photodetector is used to convert the collected light signal into an electrical signal;

The data acquisition and processing system includes a data acquisition card and a computer; wherein the data acquisition card is used to collect the light trigger signal from the laser emission system and the echo analog electrical signal from the four-channel receiving system, and convert the analog electrical signal into a digital electrical signal, and the computer is used to control each module of the laser radar in real time and to invert the chlorophyll concentration of the water body in real time.

Furthermore, the optical axis of the emission light path of the laser emission system and the optical axes of the four receiving channels of the four-channel signal receiving system are adjacent and parallel; the light-emitting surface of the Glan laser prism is parallel to the horizontal plane, the light-emitting surface of the beam expander is parallel to the light-entering surface of the Glan laser prism, and the light-emitting surface of the solid pulse laser is parallel to the light-entering surface of the beam expander;

In the four-channel signal receiving system, the light incident surface of the telescope is parallel to the horizontal plane, the linear polarization module is arranged below the light incident surface of the telescope of the first receiving channel and the fourth receiving channel, the center of the field aperture is arranged at the focal plane of the telescope and the converging lens, the converging lens is arranged above the field aperture, the interference filter is arranged above the converging lens, and the photoelectric detector is arranged above the interference filter;

In the data acquisition and processing system, the data acquisition card is connected to four photoelectric detectors of the four-channel signal receiving system through coaxial cables, and the computer is connected to the data acquisition card through wires.

Furthermore, the solid pulse laser adopts a solid pulse laser with an operating wavelength of 532nm, a single pulse energy of not less than 5mJ, and a pulse width of not more than 10ns; the beam expander adopts an anti-strong laser beam expander.

The field of view angles of the four receiving channels are set to be no less than 200 mrad; the linear polarization modules in the first receiving channel and the fourth receiving channel can rotate 360° and have scales;

The central wavelength of the interference filter in the first receiving channel and the fourth receiving channel is 532nm, the bandwidth does not exceed 10nm, and the receiving channels are called parallel polarization channel and vertical polarization channel; the central wavelength of the interference filter in the second receiving channel and the third receiving channel is 650nm and 685nm respectively, the bandwidth does not exceed 20nm, and the receiving channels are called Raman channel and fluorescence channel;

The photodetector uses a photomultiplier tube with a spectral response range of 185 to 730 nm, a typical cathode light sensitivity of not less than 120 μA/lm, and a rise time of not more than 2.2 ns;

The sampling rate of the data acquisition card is not less than 400MSa/s, and the quantization bit number is not less than 12 bits.

The present invention also provides a method for detecting the bio-optical characteristic profile of a water body, using the above-mentioned laser radar, wherein the laser radar signal generated by the water body at a depth of z is expressed as follows after being corrected by environmental parameters, system parameters and distance square.

Wherein, B represents the lidar signal after correction of system environmental parameters and distance square, subscripts ⊥, ||, R, f represent the vertical polarization channel, parallel polarization channel, Raman channel and fluorescence channel respectively, β represents the 180° volume scattering function, k _lidar represents the lidar attenuation coefficient, superscripts 650 and 685 represent the corresponding wavelengths, and when there is no superscript, it means the wavelength is 532 nm;

The specific steps include:

Step S1, calibrating the proportional coefficient _A1 of the chlorophyll concentration of the surface layer of the water body detected by the laser radar, performing linear fitting on the ratio of the water surface chlorophyll concentration Chl(0) detected by multiple groups of laboratory fluorescence methods and the laser radar fluorescence channel signal water surface value _Bf (0) and the Raman channel signal water surface value _BR (0) detected in the same area, to obtain the proportional coefficient _A1 ;

Step S2, invert the Raman channel signal of the laser radar using an inversion algorithm to obtain the diffuse attenuation coefficient of particles on the water surface Kd _,p (0), calibrate the Raman channel signal _Bf and the sum of the parallel polarization channel and the vertical polarization channel signal _Bc = B _|| + _B⊥ , and obtain the particle 180° volume scattering function _βp (0) and the proportionality coefficient _A2 , and then obtain the particle laser radar ratio R(0) on the water surface, which is expressed as

Step S3, determining the chlorophyll concentration Chl(0) of the surface layer of the water body in the detection area by using the laser radar fluorescence signal and the Raman signal and the proportionality coefficient _A1 obtained in step S1, and obtaining Kd _,chl (0) according to a type of water body bio-optical model, wherein Kd _,chl is the diffuse attenuation coefficient contributed by chlorophyll;

Step S4, using the diffuse attenuation coefficient of particles on the surface of the water body Kd _,p (0) and the particle lidar ratio R(0) obtained in step S2 as boundary conditions, using the inversion algorithm to obtain Kd _,p (z), _βp (z) and _bbp (z), and completing the detection of the optical characteristic profile of the water body;

Step S5, assuming that the contribution Kd _,chl (z) of chlorophyll in the diffuse attenuation coefficient Kd _,p (z) of particulate matter detected by the lidar does not change with depth, the diffuse attenuation coefficient Kd,chl (z) contributed by chlorophyll is obtained by using the Kd _,p (0) obtained in step S2, the Kd _,chl (0) obtained in step S3 and the diffuse attenuation coefficient Kd _,p (z) of particulate matter obtained in step S5, and a type of water body bio-optical model and Kd _{, chl} (z) are used to obtain the water body chlorophyll concentration Chl (z), thereby completing the detection of the water body biological characteristic profile.

Furthermore, in step S1, near the water surface, the ratio of the fluorescence signal to the Raman signal is approximately a linear function of the chlorophyll concentration, expressed as

Among them, _A1 is a constant related to environmental parameters and system parameters.

In step S2, when the field of view angle is large enough, the laser radar attenuation coefficient k _lidar is approximately equal to the diffuse attenuation coefficient K _d ; the diffuse attenuation coefficient of particles detected by the Raman channel is expressed as

Where K _d,w is the diffuse attenuation coefficient of pure water at 532nm, is the diffuse attenuation coefficient of pure water at 650nm, is the diffuse attenuation coefficient of particles at 650nm;

The particle 180° volume scattering function _βp is approximately proportional to the ratio of Mie scattering and Raman scattering intensities and is expressed as

Among them, _A2 is a constant related to environmental parameters and system parameters.

In step S3, a water bio-optical model is expressed as

K _d,chl (z)＝χChl(z) ^e ,

Among them, χ and e are parameters related to wavelength.

In step S4, the inversion method used is the Fernald forward algorithm, and the diffuse attenuation coefficient K _d (z) is expressed as

Where R _S is the ratio of the particle lidar ratio R to the water molecule lidar ratio R _w , z _c is the boundary depth, and Φ(z) is expressed as

The particle 180° volume scattering function β _p (z) is expressed as

The relationship between the particle backscattering coefficient b _bp and the particle 180° volume scattering function β _p (z) is expressed as

b _bp (z)＝2πχ _p (π)β _p (z),

Wherein, χ _p (π) is the conversion factor when the backscattering angle is 180°.

In step S5, the chlorophyll concentration Chl(z) of the water body is expressed as

Compared with the prior art, the present invention has the following beneficial effects:

The present invention uses a four-channel laser radar working in the blue-green band to establish a bio-optical characteristic profile inversion method suitable for complex water bodies, expand the application scenarios of laser radar remote sensing bio-optical characteristic profiles, and improve the inversion accuracy of chlorophyll in complex water bodies. It overcomes the deficiency of the classical bio-optical model that does not consider suspended inorganic matter and CDOM. By establishing a bio-optical characteristic profile inversion algorithm suitable for complex water bodies, the bio-optical characteristics of water bodies can be detected over a large range and with high spatial resolution after calibration experiments, and the bio-optical characteristic profile of water bodies can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a laser radar for detecting a bio-optical characteristic profile of a water body according to the present invention;

FIG2 is a flow chart of a method for detecting a bio-optical property profile according to the present invention;

FIG3 is a fitting result of the fluorescence Raman ratio and the laboratory result in an embodiment of the present invention;

FIG. 4 is a comparison diagram of the chlorophyll inversion result and the in-situ detection result in an embodiment of the present invention.

Detailed ways

The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

As shown in FIG1 , a laser radar for detecting the bio-optical property profile of a water body includes a laser emitting system 1, a four-channel signal receiving system 2 and a data acquisition and processing system 3. The laser emitting system 1 is used to emit laser pulses; the four-channel signal receiving system 2 is used to receive echo signals containing different wavelengths and polarization information from the water body and convert them into electrical signals; the data acquisition and processing system 3 is used to collect and process the electrical signals received by the four-channel receiving system 2.

The laser emission system 1 includes a solid pulse laser 1-1, a beam expander 1-2 and a Glan laser prism 1-3. The light emitting surface of the Glan laser prism 1-3 is arranged parallel to the horizontal plane, the light emitting surface of the beam expander 1-2 is arranged parallel to the light incident surface of the Glan laser prism, and the light emitting surface of the solid pulse laser 1-1 is arranged parallel to the light incident surface of the beam expander.

The four-channel signal receiving system 2 includes four detachable receiving channels, which are respectively set as the first receiving channel 2-1, the second receiving channel 2-2, the third receiving channel 2-3 and the fourth receiving channel 2-4; wherein the first receiving channel 2-1 includes the first linear polarizer module 2-1-1, the first telescope 2-1-2, the first field aperture 2-1-3, the first converging lens 2-1-4, the first interference filter 2-1-5, and the first photodetector 2-1-6. The second receiving channel 2-2 includes the second telescope 2-2-1, the second field aperture 2-2-2, the second converging lens 2-2-3, the second interference filter 2-2-4, and the second photodetector 2-2-5. The third receiving channel 2-3 includes the third telescope 2-3-1, the third field aperture 2-3-2, the third converging lens 2-3-3, the third interference filter 2-3-4, and the third photodetector 2-3-5. The fourth receiving channel 2-4 includes a second linear polarizer module 2-4-1, a fourth telescope 2-4-2, a fourth field aperture 2-4-3, a fourth converging lens 2-4-4, a fourth interference filter 2-4-5, and a fourth photodetector 2-4-6. The light incident surface of the telescope in each receiving channel is arranged parallel to the horizontal plane, the linear polarization module is arranged below the light incident surface of the telescope of the first receiving channel and the fourth receiving channel, the center of the field aperture is arranged at the focal plane of the telescope and the converging lens, the converging lens is arranged above the field aperture, the interference filter is arranged above the converging lens, the photodetector is arranged above the interference filter, and the optical axis of the receiving channel is parallel to and adjacent to the optical axis of the transmitting channel.

The data acquisition processing system 3 includes a data acquisition card 3-1 and a computer 3-2. In the data acquisition processing system 3, the data acquisition card 3-1 is connected to four photoelectric detectors of the four-channel signal receiving system through a coaxial cable, and the computer 3-2 is connected to the data acquisition card 3-1 through an electric wire.

In this embodiment, the solid pulse laser uses a solid pulse laser with an operating wavelength of 532nm, a single pulse energy of not less than 5mJ, and a pulse width of not more than 10ns, such as the semiconductor pump laser of Montfort Laser of Austria, with an operating wavelength of 531.7nm, a single pulse energy of 10mJ, and a pulse width of 3ns.

The beam expander uses a beam expander that can resist strong lasers, such as the BE02-532 high-power beam expander from Thorlabs, USA, which can expand the beam by 3 times.

The field angles of the four receiving channels are set to be no less than 200 mrad, and appropriate parameters are selected to determine the design of the telescope, field stop, and converging lens.

The linear polarization module uses a linear polarization module that can rotate 360° and has a scale, such as the FLP20-VIS linear thin film polarizer and RM1 rotating mount from China Lubang Technology Co., Ltd.

The first interference filter and the fourth interference filter adopt interference filters with a central wavelength of 532nm and a bandwidth not exceeding 10nm, such as the MBF03-532 filter of China Lubang Technology Co., Ltd., with a central wavelength of 532nm and a bandwidth of 6nm; the second interference filter (2-2-4) and the third interference filter (2-3-4) respectively adopt interference filters with central wavelengths of 650nm and 685nm and a bandwidth not exceeding 20nm, such as the FL650-10 model and FL680-10 model filters of Thorlabs, USA, with central wavelengths of 650nm and 680nm respectively, and bandwidths of 20nm.

The photodetector uses a photomultiplier tube with a spectral response range of 185 to 730 nm, a typical cathode illumination sensitivity of not less than 120 μA/lm, and a rise time of not more than 2.2 ns. For example, the R7518 photomultiplier tube produced by Hamamatsu Photonics Co., Ltd. in Japan has a spectral response range of 185 to 730 nm, a typical cathode illumination sensitivity of 130 μA/lm, and a rise time of 2.2 ns.

The data acquisition card uses a data acquisition card with a sampling rate of not less than 400MSa/s and a quantization bit number of not less than 12 bits, such as the M4i.4481-x8 high-speed data acquisition card of Germany's Spectrum, which has a sampling rate of 400MSa/s and a quantization bit number of 14 bits.

Based on the above-mentioned laser radar for detecting the bio-optical characteristics profile of water bodies, the method flow of detecting the bio-optical characteristics profile is shown in Figure 2. The laser radar emits a pulsed laser to the water body. By setting polarizers in different states and interference filters with different central wavelengths, the receiving system receives the parallel polarization signal B _|| , the fluorescence signal _Bf , the Raman signal _BR and the vertical polarization signal B _|| respectively; further, a calibration experiment is carried out to obtain the ratio _Bf (0)/ _BR (0) of the fluorescence signal Raman signal of the water body surface and the proportionality coefficient _A1 of chlorophyll Chl(0), the ratio _BC (0)/BR(0) of the sum of the parallel polarization channel and the vertical polarization channel signals of the water body surface _BC (0)＝B _|| (0)+ _B⊥ (0) to the Raman signal _BR (0) and the proportionality coefficient _A2 of the 180° volume scattering function _βp (0) of the particle, and the diffuse attenuation coefficient of the particle _at 650nm in the water body surface and the proportional coefficient A ₃ of the 532nm particle diffuse attenuation coefficient K _d,p (0); the boundary conditions of optical parameters K _d,p (0), K _d,chl (0), β _p (0), R (0) are obtained by using the sum of the fluorescence signal, Raman signal, parallel polarization signal and vertical polarization signal of the water surface, and then the bio-optical characteristic profile of the water body is inverted.

The lidar signal generated by the water body at depth z can be expressed as follows after correction by environmental parameters, system parameters and distance square:

Wherein, B represents the lidar signal after correction of system environmental parameters and distance square, subscripts ⊥, ||, R, and f represent the vertical polarization channel, parallel polarization channel, Raman channel, and fluorescence channel, respectively, β represents the 180° volume scattering function, k _lidar represents the lidar attenuation coefficient, and superscripts 650 and 685 represent the corresponding wavelengths. When there is no superscript, it means the wavelength is 532 nm.

The inversion detection of the bio-optical property profile is achieved through the following steps:

The first step, as shown in Figure 3, is to calibrate the proportional coefficient _A1 of the chlorophyll concentration in the surface layer of the water body detected by the lidar. Limited by the signal-to-noise ratio, near the water surface, the ratio of the fluorescence signal to the Raman signal can be approximated as a linear function of the chlorophyll concentration, expressed as

The ratio of the water surface chlorophyll concentration Chl(0) detected by multiple laboratory fluorescence methods and the water surface value _Bf (0) of the laser radar fluorescence channel signal to the water surface value _BR (0) of the Raman channel signal detected in the same area were linearly fitted to obtain the proportionality coefficient _A1 , and the determination coefficient reached 0.9.

In the second step, the ratio of the sum of the parallel polarization channel and the vertical polarization channel signals B _C =B _|| +B _⊥ to the Raman channel signal B _f is calibrated, and the 180° volume scattering function β _p of multiple groups of particles is obtained using an in-situ instrument. It is approximately proportional to the ratio of the Mie scattering and Raman scattering intensities and is expressed as

Diffuse attenuation coefficient for 650nm particles The ratio of the diffuse attenuation coefficient of particles at 532 nm, K _{d,p, is used for calibration. In the same water area, A 3 is} approximately a constant. Multiple groups of / > The ratio of K _d,p is averaged to obtain the proportionality coefficient A ₃ .

The third step is to obtain the boundary conditions of the inversion algorithm. The inversion algorithm is used to invert the Raman channel signal of the lidar. When the field of view angle is large enough, the lidar attenuation coefficient k _lidar is approximately the diffuse attenuation coefficient K _d , and the diffuse attenuation coefficient of particles detected by the Raman channel is expressed as

Where, K _d,w is the diffuse attenuation coefficient of pure water at 532nm, which is 0.045m ^-1 , is the diffuse attenuation coefficient of pure water at 650nm, which is 0.34m ^-1 .

According to formula (3), the 180° volume scattering function β _p (0) of particles on the water surface is obtained. According to formula (4), the diffuse attenuation coefficient K _d,p (0) of particles on the water surface is obtained. Then, the laser radar ratio R (0) of particles on the water surface is obtained, which is expressed as

The laser radar fluorescence signal, Raman signal and proportional coefficient A ₁ are used to determine the chlorophyll concentration Chl(0) of the surface layer of the water body in the detection area. K _d,chl (0) is obtained according to a water body bio-optical model, where K _d,chl is the diffuse attenuation coefficient contributed by chlorophyll. A water body bio-optical model is expressed as

K _d,chl (z)＝χChl(z) ^e , (6)

Among them, χ and e are parameters related to wavelength. At 532nm, χ is 0.0474 and e is 0.6667.

In the fourth step, the diffuse attenuation coefficient of particles in the water surface K _d,p (0) and the particle lidar ratio R(0) are used as boundary conditions, and the Fernald forward algorithm is used to invert K _d,p (z) and β _p (z). The diffuse attenuation coefficient K _d (z) is expressed as

Where R _S is the ratio of the particle lidar ratio R to the water molecule lidar ratio R _w , z _c is the boundary depth, and Φ(z) is expressed as

The particle 180° volume scattering function β _p (z) is expressed as

The relationship between the particle backscattering coefficient b _bp and the particle 180° volume scattering function β _p (z) is expressed as

b _bp (z)＝2πχ _p (π)β _p (z), (10)

Wherein, χ _p (π) is a conversion factor when the backscattering angle is 180°. In this embodiment, χ _p (π) is 1.

In the fifth step, assuming that the contribution of chlorophyll Kd _,chl (z) to the diffuse attenuation coefficient of particulate matter Kd _,p (z) detected by the lidar does not change with depth, the diffuse attenuation coefficient Kd,chl (z) contributed by chlorophyll is calculated using Kd, _p (0) and Kd _,chl (0) obtained in the third step and Kd _{,p (} z) obtained in the fourth step. The water body chlorophyll concentration Chl(z) is obtained using a type of water body bio-optical model and Kd _,chl (z). The water body chlorophyll concentration Chl(z) is expressed as

The chlorophyll concentration obtained by inversion according to the embodiment is shown in FIG4 , wherein the solid line represents the chlorophyll concentration obtained by lidar inversion, and the dotted line represents the chlorophyll concentration detected by the in-situ instrument. The two are well consistent, indicating that the inversion method is effective.

In this embodiment, the laser radar and method for detecting the bio-optical property profile of water bodies of the present invention are used to realize the detection and inversion of the surface optical properties Kd _,p , _βp and chlorophyll concentration profile of the water body, which compares well with the detection results of in-situ instruments, verifying the effectiveness of the present invention.

The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are 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 (8)

1. A method for detecting the bio-optical characteristic profile of a water body, characterized in that the laser radar used comprises a laser emission system (1), a four-channel signal receiving system (2) and a data acquisition and processing system (3); The laser emission system (1) comprises a solid pulse laser (1-1), a beam expander (1-2) and a Glan laser prism (1-3); wherein the solid pulse laser (1-1) is used to emit pulse laser to a water body, the beam expander (1-2) is used to adjust the divergence angle of the pulse laser, and the Glan laser prism (1-3) is used to improve the polarization degree of the emitted pulse laser; The four-channel signal receiving system (2) comprises four detachable receiving channels, each receiving channel comprises a telescope, a field aperture, a converging lens, an interference filter, and a photodetector, wherein the first receiving channel (2-1) and the fourth receiving channel (2-4) further comprise a linear polarizer module; the linear polarizer module is used to adjust the polarization direction of the echo signal, the telescope is used to collect the echo light signal from the water body, the field aperture is used to adjust the field angle, the converging lens is used to collimate the received light signal, the interference filter is used to suppress the light signal other than the target wavelength, and the photodetector is used to convert the collected light signal into an electrical signal; The data acquisition and processing system (3) comprises a data acquisition card (3-1) and a computer (3-2); wherein the data acquisition card (3-1) is used to collect light trigger signals from the laser emission system (1) and echo analog electrical signals from the four-channel signal receiving system (2), and convert the analog electrical signals into digital electrical signals, and the computer (3-2) is used to control each module of the laser radar in real time and to invert the chlorophyll concentration of the water body in real time; In the process of detecting the bio-optical characteristics profile of water bodies, the lidar signal generated by the water body at a depth of z is expressed as follows after being corrected by environmental parameters, system parameters and distance square: Wherein, B represents the lidar signal after correction of system environmental parameters and distance square, subscripts ⊥, ||, R, f represent the vertical polarization channel, parallel polarization channel, Raman channel and fluorescence channel respectively, β represents the 180° volume scattering function, k _lidar represents the lidar attenuation coefficient, superscripts 650 and 685 represent the corresponding wavelengths, and when there is no superscript, it means the wavelength is 532 nm; The method specifically comprises the following steps: Step S1, calibrating the proportional coefficient _A1 of the chlorophyll concentration of the surface layer of the water body detected by the laser radar, performing linear fitting on the ratio of the water surface chlorophyll concentration Chl(0) detected by multiple groups of laboratory fluorescence methods and the laser radar fluorescence channel signal water surface value _Bf (0) and the Raman channel signal water surface value _BR (0) detected in the same area, to obtain the proportional coefficient _A1 ; Step S2, using the Fernald forward algorithm as the inversion algorithm to invert the Raman channel signal of the lidar, obtain the diffuse attenuation coefficient of particles on the water surface Kd _,p (0), calibrate the Raman channel signal _Bf and the sum of the parallel polarization channel and the vertical polarization channel signal _Bc = B _|| + _B⊥ , obtain the particle 180° volume scattering function _βp (0) and the proportionality coefficient _A2 , and then obtain the particle lidar ratio R(0) on the water surface, expressed as Step S3, determining the chlorophyll concentration Chl(0) of the surface layer of the water body in the detection area by using the laser radar fluorescence signal and the Raman signal and the proportionality coefficient _A1 obtained in step S1, and obtaining Kd _,chl (0) according to a type of water body bio-optical model, wherein Kd _,chl is the diffuse attenuation coefficient contributed by chlorophyll; Step S4, using the diffuse attenuation coefficient of particles on the surface of the water body Kd _,p (0) and the particle lidar ratio R(0) obtained in step S2 as boundary conditions, using the inversion algorithm to obtain Kd _,p (z), _βp (z) and _bbp (z), and completing the detection of the optical characteristic profile of the water body; Step S5, assuming that the contribution Kd _,chl (z) of chlorophyll in the diffuse attenuation coefficient Kd _,p (z) of particulate matter detected by the lidar does not change with depth, the diffuse attenuation coefficient Kd,chl (z) contributed by chlorophyll is obtained by using the Kd _,p (0) obtained in step S2, the Kd _,chl (0) obtained in step S3 and the diffuse attenuation coefficient Kd _,p (z) of particulate matter obtained in step S5, and a type of water body bio-optical model and Kd _{, chl} (z) are used to obtain the water body chlorophyll concentration Chl (z), thereby completing the detection of the water body biological characteristic profile. 2. According to the method for detecting the bio-optical property profile of water bodies as described in claim 1, it is characterized in that the optical axis of the emission light path of the laser emission system (1) and the optical axes of the four receiving channels of the four-channel signal receiving system (2) are adjacent and parallel; the light emitting surface of the Glan laser prism (1-3) is parallel to the horizontal plane, the light emitting surface of the beam expander (1-2) is parallel to the light incident surface of the Glan laser prism (1-3), and the light emitting surface of the solid pulse laser (1-1) is parallel to the light incident surface of the beam expander (1-2); In the four-channel signal receiving system, the light incident surface of the telescope is parallel to the horizontal plane, the linear polarizer module is arranged below the light incident surface of the telescope of the first receiving channel (2-1) and the fourth receiving channel (2-4), the center of the field stop is arranged at the focal plane of the telescope and the converging lens, the converging lens is arranged above the field stop, the interference filter is arranged above the converging lens, and the photoelectric detector is arranged above the interference filter; In the data acquisition processing system (3), the data acquisition card (3-1) is connected to four photoelectric detectors of the four-channel signal receiving system (2) via coaxial cables, and the computer (3-2) is connected to the data acquisition card (3-1) via wires. 3. The method for detecting the bio-optical characteristic profile of water bodies according to claim 1 is characterized in that the solid pulse laser (1-1) adopts a solid pulse laser with an operating wavelength of 532nm, a single pulse energy of not less than 5mJ, and a pulse width of not more than 10ns; the beam expander (1-2) adopts an anti-strong laser beam expander; The viewing angles of the four receiving channels are set to be no less than 200 mrad; the linear polarizer modules in the first receiving channel (2-1) and the fourth receiving channel (2-4) can rotate 360° and are scaled; The central wavelength of the interference filter in the first receiving channel (2-1) and the fourth receiving channel (2-4) is 532 nm, the bandwidth does not exceed 10 nm, and the receiving channels are called parallel polarization channel and vertical polarization channel; the central wavelength of the interference filter in the second receiving channel (2-2) and the third receiving channel (2-3) is 650 nm and 685 nm respectively, the bandwidth does not exceed 20 nm, and the receiving channels are called Raman channel and fluorescence channel; The photodetector uses a photomultiplier tube with a spectral response range of 185 to 730 nm, a typical cathode light sensitivity of not less than 120 μA/lm, and a rise time of not more than 2.2 ns; The sampling rate of the data acquisition card (3-1) is not less than 400MSa/s, and the quantization bit number is not less than 12 bits. 4. The method for detecting the bio-optical characteristic profile of a water body according to claim 1, characterized in that in step S1, near the water surface, the ratio of the fluorescence signal to the Raman signal is approximately a linear function of the chlorophyll concentration, expressed as Among them, _A1 is a constant related to environmental parameters and system parameters. 5. The method for detecting the bio-optical property profile of water bodies according to claim 1, characterized in that, in step S2, when the field of view angle is large enough, the laser radar attenuation coefficient k _lidar is approximately equal to the diffuse attenuation coefficient K _d ; the diffuse attenuation coefficient of particles detected by the Raman channel is expressed as Where K _d,w is the diffuse attenuation coefficient of pure water at 532nm, is the diffuse attenuation coefficient of pure water at 650nm, is the diffuse attenuation coefficient of particles at 650nm; The particle 180° volume scattering function _βp is approximately proportional to the ratio of Mie scattering and Raman scattering intensities and is expressed as Among them, _A2 is a constant related to environmental parameters and system parameters. 6. The method for detecting the bio-optical characteristic profile of a water body according to claim 1, characterized in that in step S3, a type of water body bio-optical model is expressed as K _d,chl (z)＝χChl(z) ^e , Among them, χ and e are parameters related to wavelength. 7. The method for detecting the bio-optical characteristic profile of water body according to claim 5, characterized in that in step S4, the inversion method used is Fernald forward algorithm, and the diffuse attenuation coefficient K _d (z) is expressed as Where R _S is the ratio of the particle lidar ratio R to the water molecule lidar ratio R _w , z _c is the boundary depth, and Φ(z) is expressed as The particle 180° volume scattering function β _p (z) is expressed as The relationship between the particle backscattering coefficient b _bp and the particle 180° volume scattering function β _p (z) is expressed as b _bp (z)＝2πχ _p (π)β _p (z), Wherein, χ _p (π) is the conversion factor when the backscattering angle is 180°. 8. The method for detecting the bio-optical characteristic profile of a water body according to claim 5, characterized in that in step S5, the chlorophyll concentration Chl(z) of the water body is expressed as Where χ and e are parameters related to wavelength.