Ocean laser radar system and seawater multi-parameter detection method

Abstract

The invention relates to the technical field of ocean detection equipment, and particularly provides an ocean laser radar system and a seawater multi-parameter detection method. The system comprises a laser emission subsystem, an optical receiving subsystem and a signal acquisition and control subsystem. The laser emission subsystem can emit multi-wavelength pulse laser to seawater; the optical receiving subsystem adopts a double-receiving telescope structure, respectively receives scattering and fluorescence signals with different properties, and realizes multi-channel signal separation through light splitting, light filtering and polarization light splitting elements; and the signal acquisition and control subsystem realizes synchronous acquisition and processing of multi-channel data. The system can synchronously invert water optical parameters, water particulate matter depolarization ratio and color ratio, chlorophyll concentration, suspended load concentration and other multi-parameter profiles, and is suitable for marine environment unattended monitoring of a fixed platform and a mobile platform.

Description

Technical Field

[0001] This invention belongs to the field of marine exploration equipment technology, specifically, it relates to a marine lidar system and a method for multi-parameter seawater detection. Background Technology

[0002] The ocean plays an irreplaceable role in Earth's ecosystem, harboring rich biodiversity. Seawater optical parameters are crucial for marine ecology and environmental research. Seawater optical parameters, chlorophyll, and suspended matter are important parameters in marine optics and marine environmental research, used to assess marine ecological environment, primary productivity, and carbon sequestration capacity, and are of great significance for strengthening marine environmental research, protection, and governance. Currently, the main methods for obtaining seawater optical parameters domestically and internationally include in-situ detection, passive remote sensing, and active remote sensing. In-situ detection mainly obtains water optical parameters through direct measurement of the water body, such as the hyperspectral attenuation measurement instrument from the German company TriOS. In-situ detection offers high measurement accuracy, but the entire detection process is time-consuming and inefficient. Passive remote sensing technology mainly utilizes multispectral imagers carried by marine satellites to passively acquire light radiation sources to invert water optical parameters. Its main advantage is the ability to capture data over large areas of the sea, but this technology cannot be applied at night, on cloudy or rainy days, or in high-latitude regions.

[0003] Marine lidar, as an active remote sensing technology, boasts advantages such as high spatiotemporal resolution and all-weather detection. It can be mounted on various platforms including aircraft, ships, and satellites, and can be used to detect optical parameters of ocean water, as well as important ocean color elements such as chlorophyll concentration and suspended matter concentration. It is of great significance for marine resource management, marine ecological environment security, water quality monitoring, and marine research. Marine lidar can achieve multi-parameter water body profiling, compensating for the shortcomings of in-situ detection and passive remote sensing.

[0004] Therefore, a marine lidar system and a multi-parameter seawater detection method are needed to solve the above problems. Summary of the Invention

[0005] In order to overcome the above-mentioned defects of the prior art, embodiments of the present invention provide a marine lidar system and a multi-parameter seawater detection method to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a marine lidar system, comprising a laser emitting subsystem, an optical receiving subsystem, and a signal acquisition and control subsystem; the laser emitting subsystem includes a pulsed laser unit and a trigger module for emitting multi-wavelength pulsed lasers into seawater; the optical receiving subsystem includes a receiving telescope for receiving echo signals generated by the interaction between the laser and seawater, and separating multiple signal channels with different wavelengths and polarization states via a dichroic mirror, a narrowband interference filter, and a polarizing beam splitter; the signal acquisition and control subsystem includes a multi-channel data acquisition unit and an industrial control computer for acquiring, processing, and inverting the echo signals to obtain at least one parameter among seawater optical parameters, particulate matter depolarization ratio and color ratio, chlorophyll concentration, and suspended matter concentration.

[0007] Optionally, the laser emission subsystem further includes an electrically controlled aperture shutter, a Glan-Taylor prism, and multiple mirrors; the pulsed laser unit emits pulsed lasers with wavelengths of 486nm, 532nm, and 1064nm; the electrically controlled aperture shutter is used to independently control the emission of lasers of different wavelengths; the Glan-Taylor prism is used to polarize the 486nm and 532nm lasers to output linearly polarized light with a high extinction ratio.

[0008] Optionally, the optical receiving subsystem further includes a pinhole aperture, a collimating lens, a converging lens, and a photodetector, the photodetector including a photomultiplier tube and an avalanche photodiode; the pinhole aperture is used to adjust the receiving field of view; the collimating lens is used to convert the echo signal into parallel light; and the converging lens is used to converge the light signal to the photosensitive surface of the corresponding photodetector.

[0009] Optionally, the optical receiving subsystem is provided with seven signal receiving channels, including a 486nm parallel polarization channel, a 486nm vertical polarization channel, a 532nm parallel polarization channel, a 532nm vertical polarization channel, a 1064nm channel, a 650nm water Raman scattering channel, and a 685nm chlorophyll fluorescence channel.

[0010] Optionally, the trigger terminal of the multi-channel data acquisition unit is connected to the trigger module to receive a synchronous trigger signal. Its signal receiving terminal is connected to the output terminals of the photomultiplier tube and the avalanche photodiode via a coaxial cable to realize the synchronous acquisition of multi-channel electrical signals.

[0011] Optionally, the system also includes a sealed outer shell and an internal frame, with its optical components fixed by stress-free or low-stress clamping, and each interface is equipped with a sealing strip or sealing ring, and internal temperature control and dehumidification equipment.

[0012] A method for multi-parameter detection of seawater, employing the aforementioned marine lidar system, includes the following steps:

[0013] S1. Emit multi-wavelength pulsed laser into seawater;

[0014] S2. The laser echo signals of different wavelengths and polarization states are received by the receiving telescope and separated by a dichroic mirror, a narrowband interference filter and a polarizing beam splitter.

[0015] S3. Perform background noise removal, smoothing and correction processing on the echo signal;

[0016] S4. Based on the processed signal, retrieve at least one of the following parameters: seawater optical parameters, particulate matter depolarization ratio and color ratio, chlorophyll concentration, and suspended matter concentration.

[0017] Optionally, when inverting the depolarization ratio of water particles, the depolarization ratio value at that wavelength is obtained by correcting the ratio of the intensity of parallel polarized and vertically polarized echo signals at the same wavelength with a correction coefficient.

[0018] Optionally, when retrieving chlorophyll concentration, the chlorophyll concentration is calculated by the ratio of the 685nm chlorophyll fluorescence signal to the 650nm water Raman scattering signal, combined with experimental calibration coefficients.

[0019] Optionally, when inverting the suspended mass concentration, the suspended mass concentration is calculated by the ratio of the 532 nm scattering signal to the 650 nm Raman scattering signal, combined with experimental calibration coefficients.

[0020] The technical effects and advantages of this invention are as follows:

[0021] Compared with existing technologies, this invention achieves selective combination emission of blue, green and near-infrared pulsed lasers by using multi-wavelength laser emission and intelligent switch control. It can flexibly adjust the emission strategy according to different water body types and detection targets, thereby improving detection adaptability and efficiency, and effectively overcoming the limitations of single wavelength in terms of water penetration and signal specificity.

[0022] Compared to existing technologies, this invention uses dual receiving telescopes to receive elastic scattering signals and Raman / fluorescence signals respectively. By combining paraxial and coaxial optical path designs, it avoids signal saturation caused by strong scattering from the sea surface and reduces the detection blind zone of Raman and fluorescence signals, thereby achieving efficient separation and synchronous reception of signals with different optical characteristics.

[0023] Compared to existing technologies, this invention achieves precise separation and reception of seven echo signal channels through a multi-channel optical subsystem and a high extinction ratio polarization beam splitting design, including signals of different wavelengths, polarization states, Raman scattering, and fluorescence, significantly improving the system's ability to simultaneously detect multiple seawater parameters and the purity of the signals.

[0024] Compared to existing technologies, this invention integrates the Collis and Fernald inversion strategies and automatically selects the inversion algorithm based on signal uniformity. This enables more accurate inversion of optical parameters in both uniform and non-uniform water bodies, enhancing the system's robustness in data processing and parameter inversion accuracy in complex marine environments.

[0025] Compared to existing technologies, this invention effectively resists harsh marine environments such as high humidity and high salt spray through a sealed system structure, corrosion-resistant material treatment, and internal environmental control design, ensuring long-term stable operation of optical and electronic components and significantly improving the reliability and service life of the equipment.

[0026] Compared with existing technologies, this invention normalizes chlorophyll fluorescence and Mie scattering signals using Raman signal correction technology, effectively suppressing the effects of laser energy fluctuations and water body attenuation, achieving high-precision and quantitative inversion of chlorophyll and suspended matter concentrations, and improving the accuracy of environmental monitoring data. Attached Figure Description

[0027] Figure 1 : A schematic diagram of the optical path structure of a marine lidar system according to the present invention;

[0028] Figure 2 This invention provides a software functional diagram of a marine lidar system.

[0029] Figure 3 This invention provides a data inversion flowchart for a marine lidar system.

[0030] The attached figures are labeled as follows: 1-1, pulsed laser unit; 1-2, trigger module; 2-1 to 2-3, mirrors mounted on a two-dimensional adjustable optical adjustment frame; 2-4 to 2-6, mirrors; 3-1 and 3-2, receiving telescopes; 4-1 and 4-2, pinhole diaphragms; 5-1 and 5-2, collimating lenses; 6-1 to 6-3, dichroic mirrors; 7-1 to 7-5, narrowband interference filters; 8-1 and 8-2, polarizing beam splitters; 9-1 to 9-7, converging lenses; 10-1 to 10-6, photomultiplier tubes; 11, avalanche photodiodes; 12, multi-channel data acquisition unit; 13, industrial computer; 14, electrically controlled aperture shutter; 15-1 and 15-2, Glan-Taylor prisms. Detailed implementation method:

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the specific apparatus and implementation methods of the present invention will be further described in detail below with reference to the accompanying drawings.

[0032] Example 1

[0033] like Figure 1As shown, a marine lidar system mainly includes a laser emitting subsystem, an optical receiving subsystem, and a signal acquisition and control subsystem.

[0034] The optical system structure of the marine lidar system mainly includes a laser emitting subsystem, an optical receiving subsystem, and a signal acquisition and control subsystem. In this embodiment, the marine lidar system adopts a multi-wavelength transmitting and multi-channel receiving optical system structure, with two receiving telescopes collecting the echo signals.

[0035] The laser emission subsystem mainly includes a pulsed laser unit 1-1, a trigger module 1-2, an electrically controlled aperture shutter 14, Glan-Taylor prisms 15-1 and 15-2, and mirrors 2-1 to 2-3 and 2-4 to 2-6 mounted on a two-dimensional adjustable optical adjustment frame.

[0036] Furthermore, the pulsed laser unit 1-1 is used to emit pulsed lasers in three bands: blue, green, and near-infrared. It primarily utilizes near-infrared wavelength lasers to detect the height between the marine lidar system and the sea surface, and can perform distance correction on lidar data from all channels. Green lasers have better penetration when detecting the optical characteristics of Class II water bodies, while blue lasers have better penetration when detecting the optical characteristics of Class I water bodies.

[0037] This system preferentially uses three-wavelength pulsed lasers of 486nm, 532nm, and 1064nm, with a pulse width of less than 5ns, a single pulse energy greater than 1mJ, and a laser repetition frequency greater than 500Hz. The three-wavelength pulsed lasers are emitted through three laser exit holes, first passing through an electrically controlled aperture shutter 14. This electrically controlled aperture shutter 14 includes three electrically controlled shutters, each located on the emission path of one of the three wavelengths of laser light, used to control the on / off switching of the emitted laser light of different wavelengths. It can control the simultaneous emission of the three wavelengths of laser light, or the emission of only one or two wavelengths of laser light, depending on different detection elements, detection requirements, or detection scenarios. Normally, all three electrically controlled aperture shutters 14 are open to allow simultaneous emission of the three wavelengths of laser light.

[0038] Then, the three wavelength lasers are emitted and passed through mirrors 2-1 and 2-2 mounted on a two-dimensional adjustable optical adjustment frame. The 1064nm laser is directly emitted into the target (seawater), while the 486nm and 532nm lasers are emitted into the target (seawater) after passing through Glan-Taylor prisms 15-1 and 15-2, respectively.

[0039] Furthermore, the reflectors 2-1 to 2-6 are preferably selected as reflectors with a reflectivity greater than 98% for the emitted laser and a diameter of half an inch. Reflectors 2-1 to 2-3 are mounted on a two-dimensionally adjustable optical adjustment frame, while reflectors 2-4 to 2-6 are fixedly installed at a 45° angle to the laser transmission direction. The direction of the emitted laser is adjusted using the two-dimensional optical adjustment frame on which the reflectors 2-1 to 2-3 are mounted, so that the optical axis of the emitted laser is aligned with the optical axis of the receiving telescope, further improving the reception efficiency of the echo signal. The trigger module 1-2 is used to provide a synchronous trigger signal to the multi-channel data acquisition unit 12.

[0040] The optical receiving subsystem is mainly used to receive echo signals from the interaction between laser and ocean water. It adopts a dual receiving field-of-view structure, and the receiving channels mainly include seven echo signal detection channels: 486nm parallel polarization, 486nm vertical polarization, 532nm parallel polarization, 532nm vertical polarization, 650nm water Raman, 685nm fluorescence, and 1064nm.

[0041] The optical receiving subsystem, in order to acquire seven echo signals, mainly includes receiving telescopes 3-1 and 3-2, pinhole apertures 4-1 and 4-2, collimating lenses 5-1 and 5-2, dichroic mirrors 6-1 to 6-3, narrowband interference filters 7-1 to 7-5, polarizing beam splitters 8-1 and 8-2, converging lenses 9-1 to 9-7, photomultiplier tubes 10-1 to 10-6, and avalanche photodiode 11.

[0042] In this embodiment, the collimating lenses 5-1 and 5-2, the narrowband interference filters 7-1 to 7-5, and the converging lenses 9-1 to 9-7 are all 1-inch diameter lenses. The pinhole diaphragms 4-1 and 4-2 are adjustable pinhole diaphragms, which can adjust the field of view of the two receiving fields of view by adjusting the pinhole diameter, further studying the influence of multiple scattering of water on the detection signal. The collimating lenses 5-1 and 5-2 and the converging lenses 9-1 to 9-7 are lenses with a transmittance greater than 98% for the corresponding wavelength. The narrowband interference filters 7-1 to 7-5 are selected with a center wavelength of the corresponding allowable transmittance wavelength, a bandwidth of less than 1 nm, and a center wavelength transmittance greater than 80%. The polarizing beam splitters 8-1 and 8-2 are both installed in a cage-like cube, with corresponding laser linear polarization wavelengths of 486 nm and 532 nm, respectively, used to reflect the vertical polarization component s of the corresponding wavelength while transmitting the parallel polarization component p, with an extinction ratio r. P :r S Greater than 3000:1.

[0043] The receiving telescope 3-1 is used to receive the elastic scattering echo signal from the interaction between the laser and the water body, mainly including 486nm parallel polarization, 486nm vertical polarization, 532nm parallel polarization, 532nm vertical polarization, and 1064nm echo signals. After the background light is compressed by the pinhole aperture 4-1, it passes through the collimating lens 5-1, and the echo signal becomes parallel light before entering the dichroic mirror 6-1. The dichroic mirror 6-1 has a reflectivity greater than 95% for the 486nm wavelength signal and a transmittance greater than 95% for the 532nm and 1064nm wavelength signals, and is used to separate the 486nm signal from the 532nm and 1064nm signals. The 486nm wavelength echo signal is reflected by the dichroic mirror 6-1 and further filtered by a narrowband interference filter 7-1 with a center wavelength of 486nm to remove other wavelengths of light. Then, the 486nm echo signal is split into a parallel polarization component and a perpendicular polarization component by a polarizing beam splitter 8-1. These components then pass through converging lenses 9-1 and 9-2, respectively, and are received by the photosensitive surfaces of photomultiplier tubes 10-1 and 10-2, converting the light signal into an electrical signal. The 532nm and 1064nm wavelength echo signals pass through the dichroic mirror 6-1 and enter the dichroic mirror 6-2. The dichroic mirror 6-2 has a reflectivity greater than 95% for 532nm wavelength signals and a transmittance greater than 95% for 1064nm wavelength signals, thus separating the 532nm and 1064nm signals. The 532nm echo signal, after being reflected by the dichroic mirror 6-2, is filtered by a narrowband interference filter 7-2 with a center wavelength of 532nm to remove other wavelengths of light. Then, the parallel polarization component is transmitted through the polarizing beam splitter 8-2, while the vertical polarization component is reflected. The two polarization components of the 532nm signal pass through converging lenses 9-3 and 9-4, respectively, and are then converted into electrical signals by photomultiplier tubes 10-3 and 10-4. The 1064nm echo signal, after passing through the dichroic mirror 6-2, is first filtered by a narrowband interference filter 7-3 with a center wavelength of 1064nm to remove other wavelengths of light. Then, the converging lens 9-5 focuses the light signal into an avalanche photomultiplier tube 11, where it is converted into an electrical signal.

[0044] The receiving telescope 3-2 is mainly used to receive the Raman signal and chlorophyll fluorescence signal from the interaction between the laser and the water body, primarily including the 650nm Raman signal and the 685nm chlorophyll fluorescence signal. The signal then passes through the pinhole aperture 4-2 to compress the background light before passing through the collimating lens 5-2. The echo signal is then converted into parallel light and enters the dichroic mirror 6-3. The dichroic mirror 6-3 has a reflectivity greater than 95% for the 650nm wavelength signal and a transmittance greater than 95% for the 685nm wavelength signal, used to separate the 650nm and 685nm signals. The 650nm echo signal, after being reflected by the dichroic mirror 6-3, passes sequentially through a narrowband interference filter 7-4 with a center wavelength of 650nm to filter out stray light, and is then converged by the converging lens 9-6 before being converted into an electrical signal by the photomultiplier tube 10-5. The 685nm echo signal passes through the dichroic mirror 6-3 and is filtered out by the narrowband interference filter 7-5 with a center wavelength of 685nm to remove stray light. Then, the converging lens 9-7 converges the light onto the photosensitive surface of the photomultiplier tube 10-6, converting the 685nm echo signal into an electrical signal.

[0045] In this embodiment, the apertures of the receiving telescopes 3-1 and 3-2 are between 100mm and 150mm. The lenses of the receiving telescopes are coated with anti-reflection films corresponding to the received signal wavelength to improve signal transmittance. The receiving optical axis of the receiving telescope 3-1 is placed off-axis with the transmitting optical axis, while the receiving optical axis of the receiving telescope 3-2 is placed coaxially with the transmitting optical axis. The advantage of this arrangement is that when the marine lidar is used for observation on a shipboard platform, the elastic scattering signal from the sea surface is strong. The off-axis placement can increase the detection blind zone and transition zone of the Mie scattering detection channel, preventing signal saturation caused by the strong elastic scattering signal from the sea surface. Furthermore, the coaxial arrangement for acquiring water Raman and fluorescence signals reduces the detection blind zone and effectively obtains the distance the laser travels into the water.

[0046] The signal acquisition and control subsystem mainly includes a multi-channel data acquisition unit 12 and an industrial control computer 13. The trigger terminal of the multi-channel data acquisition unit 12 receives a synchronous trigger signal from the trigger module 1-2 of the laser emission subsystem. The electrical signals output by the photomultiplier tubes 10-1 to 10-6 and the avalanche photodiode 11 are sent to the signal receiving terminal of the data acquisition unit 12 via a coaxial signal cable, and the multi-channel data acquisition unit 12 acquires multi-channel data.

[0047] In this embodiment, the multi-channel data acquisition device has a sampling rate of ≥1 GSPS, a bandwidth of ≥400MHz, and a resolution of 14 bits, which can realize the synchronous acquisition of data from seven channels of this system.

[0048] In this embodiment, the marine lidar system can effectively improve the range resolution of the detection data by using a narrow pulse width laser and a high sampling rate data acquisition unit.

[0049] like Figure 2 As shown, to facilitate unattended automatic observation of the marine lidar field environment, the industrial control computer 13 is equipped with marine lidar system software, which can perform functions such as system control, data acquisition, data inversion, and display. The system control function mainly includes laser control, data acquisition parameter setting and acquisition control, scanning control, and operating environment temperature control. The data acquisition function mainly collects multi-channel echo data, the operating status of the marine lidar system, the angle of laser incident on seawater when the marine lidar system is operating, the attitude (roll, pitch, yaw) and azimuth (latitude and longitude) data of the system during operation, and the height of the marine lidar above the water surface. The data inversion and display function mainly processes and inverts the collected echo data to obtain profile data of various marine water environmental parameters such as water optical parameters, water particulate matter depolarization ratio and color ratio, chlorophyll concentration, and suspended matter concentration. The inverted data results are displayed on the software interface, and the data is stored in the industrial control computer.

[0050] Due to the harsh outdoor environment, including high humidity, frequent fog, and high salinity at sea, the requirements for observation instruments and equipment are very high. This invention's marine lidar adopts a sealed structural design. The overall outer shell and internal frame are constructed from 316L stainless steel, aluminum alloy, and polytetrafluoroethylene (PTFE), with an anti-corrosion coating applied to the surface. Key components undergo polishing-electroplation or baking paint treatment to improve the equipment's corrosion resistance. According to the design requirements of the optical path system, the surfaces of relevant frames and structural components are blackened or treated with a blackening process. Sealing strips are added to each pressed part, and after assembly, the interfaces between the internal and external parts are sealed with glass glue or sealing rings to prevent the intrusion of sand, dust, and salt spray. Simultaneously, temperature control and dehumidification equipment are provided to ensure a suitable and controllable internal environment during lidar operation in the field. To ensure system stability during observation, the marine lidar is internally vibration-isolated, and the internal frame adopts an integrated structural design. During the assembly of optical components, the surface stress of the optical components is monitored in real time to achieve stress-free or low-stress clamping. Concentricity is improved through centering machining and adjustment, and position correction is achieved by adding cutting pads.

[0051] The detection principle of the aforementioned marine lidar is as follows:

[0052] The marine lidar system receives echo signals that carry information about the interaction between the laser beam and the target. By analyzing changes in the echo signals, the characteristic information of the target can be obtained.

[0053] For a marine lidar that emits pulsed lasers, if the receiving field of view is located at the sea surface, the signal power at an underwater distance R can be expressed by the lidar equation:

[0054]

[0055] Where P0 is the average power of the emitted laser, η is the optical efficiency of the lidar system, R corresponds to the detection range, A is the area of ​​the receiving telescope, β is the water backscattering coefficient, ΔR is the range resolution, and T is the underwater transmittance, which is related to the water's diffuse attenuation coefficient of the laser. B λ represents the background noise, and λ is the wavelength of the echo signal. L If the echo signal is an elastically scattered signal when the laser wavelength is detected, then λ = λ L .

[0056] If the lidar operates away from the sea surface, such as when an ocean lidar is used on an aircraft platform, the lidar equation changes as the laser transmission path passes through the atmosphere, the air-sea interface, and the water body:

[0057]

[0058] Where H is the distance between the lidar and the water surface, n is the refractive index of the water, and T is the distance between the lidar and the water surface. atm This refers to the attenuation of the laser in the atmosphere and at the air-sea interface.

[0059] A marine lidar system emits a laser beam into the water. Fluorescent substances in the water (such as chlorophyll) absorb the laser and emit laser-induced fluorescence spectra corresponding to their chemical properties. Laser-induced fluorescence can measure water parameter concentrations based on the principle that there is a direct correlation between the fluorescence intensity at a specific wavelength and the concentration of fluorophores in the excited substance. Under laser excitation, the water simultaneously generates Mie scattering signals, Raman signals, and other fluorescence signals. The power of the Mie scattering signal, Raman scattering signal, and fluorescence signal received by the marine lidar system at point R underwater can be expressed in the following forms:

[0060]

[0061] Where K1, K2, and K3 are the system constants for the Mie scattering channel, Raman channel, and fluorescence channel, respectively; k L k R k F These are the laser, Raman, and fluorescence attenuation coefficients of seawater, respectively. M n R n F σ represents the number density of suspended matter, water molecules, and chlorophyll, respectively. M σ R σ FThese represent the scattering cross sections of suspended matter, water molecules, and chlorophyll, respectively.

[0062] The marine lidar system can acquire data from seven channels. Through data inversion, parameters such as water optical parameters, particulate matter depolarization ratio and color ratio, chlorophyll concentration, and suspended solids concentration can be obtained. The data processing and inversion process is as follows: Figure 3 As shown, comparison Figure 3 The specific process of marine lidar data processing and multi-parameter inversion is introduced as follows:

[0063] After acquiring data from seven channels, the marine lidar system first removes background noise from the signal. Generally, the last 50 points of the signal are averaged to obtain the signal noise, which is then subtracted. Next, the signal is smoothed and corrected. The smoothing is mainly based on the detection requirements, which involves averaging over time or distance. The correction mainly includes overlap correction, starting point correction, and normalization correction. Finally, the signal after background noise removal, smoothing, and correction is used for data inversion.

[0064] Furthermore, when a 1064nm near-infrared laser is incident on the water surface, it produces a strong backscattered signal. This strong backscattered signal can be used to detect the height between the marine lidar system and the sea surface, and distance correction can be performed on lidar data from all channels. Since near-infrared laser light is quickly absorbed in water, this wavelength is not used for detecting water optical parameters.

[0065] Furthermore, this system uses blue light at 486nm and green light at 532nm to detect optical parameters of water bodies. It mainly utilizes the detection advantages of blue light in Class I water bodies and green light in Class II water bodies, and can detect most water bodies. At the same time, the optical parameters of water bodies obtained by the two wavelengths can be used to invert the color ratio of water particles.

[0066] Furthermore, for the inversion of the depolarization ratio of water particles, a linearly polarized pulsed laser emitted into the water body, after interacting with the water particles, produces a backscattered signal consisting of two components: parallel and perpendicular. These two signal components are split by a polarization beam splitter and received by two separate detection channels. The parallel polarized signal is denoted as P. P (λ,R), the vertically polarized signal is denoted as P. S The depolarization ratio δ of particulate matter in water can be obtained by correcting the ratio of the two signal components (λ,R) after correction by the correction coefficient K.

[0067]

[0068] The depolarization ratio can be used to study the morphology of particulate matter in water. Its value is between 0 and 1. The more irregular the particle, the larger the depolarization ratio, while the depolarization ratio of spherical particles is smaller. The marine lidar system of this invention can detect 486nm parallel polarization and vertical polarization signals, and 532nm parallel polarization and vertical polarization signals. Through inversion, the depolarization ratio of water particles at 486nm and 532nm wavelengths can be obtained.

[0069] Furthermore, the main methods for inverting seawater optical parameters are the Collis method and the Fernald method. The Collis method is mainly for homogeneous water bodies, while the Fernald method is for non-homogeneous water bodies. However, in real seawater environments, there is no single water body. This invention proposes an automatic inversion method for seawater optical parameters based on the fusion of the Collis and Fernald methods. First, the effective detection range is determined by the signal-to-noise ratio (SNR) of the echo signal. This invention uses an SNR greater than 3 as the threshold for determining the effective detection range. Then, a linear fit is performed using the effective detection range and the observed data within the effective detection range as the x and y axes, respectively, and the correlation coefficient of the linear fit is judged. If the correlation coefficient of the linear fit is ≥97%, the water body within the effective detection range is considered relatively homogeneous, and the Collis method is used to invert the water body optical parameters. This method mainly assumes that the types and concentrations of suspended matter in the water body within the effective detection range are constant. At this time, the backscattering coefficient β(λ,R) and the lidar attenuation coefficient k L (λ,R) should be a constant over the entire depth range.

[0070] At this point, the lidar equation can be expressed using the distance-squared correction as follows:

[0071] D (λ, R) = P (λ, R)(nH + R) 2 (7)

[0072] S(λ,R)=ln[P(λ,R)(nH+R) 2 ],(8)

[0073] After differentiation, the lidar equation becomes:

[0074]

[0075] Under homogeneous water conditions, β and k L All are constants, that is It then becomes:

[0076]

[0077] By fitting the curve S(R) and finding its slope, the attenuation coefficient in a homogeneous water body can be determined.

[0078] If the correlation coefficient of the linear fit is <97%, the water body is considered non-uniform, and the Fernald method is used to invert the optical parameters of the water body. This method takes into account that the scattering and attenuation of water bodies are mainly caused by water molecules and suspended matter; therefore, β and k... L It can be represented as:

[0079] β = β w + β p (11)

[0080] k L = k w + k p (12)

[0081] β w and k w Let β represent the backscattering coefficient and attenuation coefficient of water molecules, respectively. p and k p The backscattering coefficient and attenuation coefficient of suspended matter in water, according to the detection principle of lidar, the backscattering coefficient here mainly refers to the 180° backscattering coefficient. Water molecule scattering is relatively stable, and its attenuation coefficient and 180° backscattering coefficient can be obtained experimentally. The Fernald method requires the assumption of the relationship β = Ck. L This is based on the premise that the ratio of water molecules to suspended matter in water is constant, i.e., C is constant.

[0082] Based on the recursive iteration, the general solution can be obtained:

[0083]

[0084] In the formula, D(R) is the distance-corrected echo signal. S represents the ratio of lidar to suspended matter in water. p Compared to pure water lidar, S w The ratio of S w Approximately 216sr, R c This is the reference boundary depth.

[0085] Color ratio can be used to reflect the scale characteristics of particulate matter in water. Different scattering characteristics exist between particles of different sizes and the interacting laser wavelengths; lasers of different wavelengths scatter particles of the same size differently. The color ratio is usually expressed as the ratio of optical parameters of longer wavelengths to those of shorter wavelengths. This invention also uses the ratio of backscattered signal intensity across multiple wavelengths to calculate the color ratio χ(z).

[0086] This can be expressed by formula (14):

[0087]

[0088] Its value is usually between 0 and 2. Although the color ratio cannot quantitatively describe the size of water particles, it can characterize the particle size; the larger the color ratio, the larger the relative particle size of the water particles.

[0089] The inversion of chlorophyll concentration in water is mainly based on the fact that chlorophyll in water will produce chlorophyll fluorescence signals under laser induction. The intensity of laser-induced chlorophyll fluorescence in seawater can be approximately considered to be linearly related to chlorophyll concentration. The chlorophyll concentration and fluorescence intensity satisfy the lidar fluorescence equation (5). The marine lidar transmitting subsystem emits pulsed lasers into the seawater to excite the chlorophyll fluorescence of phytoplankton in the seawater. Since Raman scattering of water is an inherent property of water, the Raman scattering signal is related to the number density and cross section of water molecules. The Raman scattering cross section and number density can be assumed to be constants. The strength of the signal is mainly determined by the effective attenuation coefficient of the seawater at the emitted laser and Raman scattering wavelength. Therefore, the Raman signal can be normalized using the Raman signal correction method as an internal standard. The marine lidar system of this invention, after emitting a 532nm laser, interacts with the water to generate a 685nm chlorophyll fluorescence and a 650nm water Raman signal. By comparing the ratio of the chlorophyll fluorescence and water Raman signals, the influence of laser energy fluctuations, optical efficiency, and the effective attenuation coefficient of seawater on the measurement can be eliminated. This ratio is related to the sea surface chlorophyll concentration n. F Proportional. F (R) represents the chlorophyll concentration at depth R, C F These are parameters for experimental calibration. To ensure accurate chlorophyll concentration retrieval, in-situ detection instruments are typically used during experiments to calibrate the calibration coefficients.

[0090]

[0091] For the inversion of suspended sediment concentration in water, since Mie scattering is mainly elastic scattering caused by suspended sediment in the sea, the concentration of suspended sediment in the sea can be obtained simply by measuring the Mie scattering signal. A Raman scattering signal and a Mie scattering signal receiving channel with a certain bandwidth are set up, and Raman signal correction is used to adjust the signal P of the Mie scattering signal receiving channel. M Divided by the integral value P of the Raman scattering signal from the water body R This ratio is related to the suspended solids concentration n. M Proportional. When a laser pulse with a wavelength of 532 nm is incident on seawater, the Mie scattering signal is 532 nm, and the Raman scattering signal is 650 nm. Since the laser has a certain penetration depth in seawater, the suspended matter concentration can be calculated using the ratio of the Mie scattering to the Raman scattering signals using the following formula:

[0092]

[0093] By comparing the signals from the two channels, the concentration of suspended matter in the ocean as a function of depth, n, can be obtained. MC M These are the calibration parameters for the experiment. To ensure accurate inversion of suspended sediment concentration, in-situ detection instruments are usually used during the experiment to calibrate the calibration coefficients.

[0094] Through the above data processing and inversion methods, the multi-wavelength multi-channel marine lidar of this invention can obtain parameters such as water optical parameters, water particulate matter depolarization ratio and color ratio, chlorophyll concentration and suspended matter concentration, providing technical means and research methods for marine environmental monitoring and research.

Claims

1. A marine lidar system, characterized by, The system comprises a laser emission subsystem, an optical receiving subsystem and a signal acquisition and control subsystem; the laser emission subsystem comprises a pulsed laser unit (1-1) and a trigger module (1-2) for emitting multi-wavelength pulsed laser to seawater; the optical receiving subsystem comprises receiving telescopes (3-1, 3-2) for receiving echo signals generated by the interaction of laser and seawater, and separating multiple signal channels of different wavelengths and polarization states via dichroic mirrors (6-1 to 6-3), narrow-band interference filters (7-1 to 7-5) and polarization beam splitter prisms (8-1, 8-2); the signal acquisition and control subsystem comprises a multi-channel data acquisition device (12) and an industrial computer (13) for acquiring, processing and inverting the echo signals to obtain at least one of the following parameters: seawater optical parameters, particle depolarization ratio and color ratio, chlorophyll concentration and suspended matter concentration.

2. A marine lidar system according to claim 1, wherein, The laser emission subsystem further comprises an electrically controlled diaphragm shutter (14), a Glan-Taylor prism (15-1, 15-2) and multiple mirrors (2-1 to 2-6); the pulsed laser unit (1-1) emits pulsed laser including wavelengths of 486 nm, 532 nm and 1064 nm; the electrically controlled diaphragm shutter (14) is used for independently switching control of the emission of laser of different wavelengths; the Glan-Taylor prisms (15-1, 15-2) are used for polarization processing of 486 nm and 532 nm laser, and output linearly polarized light with high extinction ratio.

3. A marine lidar system according to claim 1, wherein, The optical receiving subsystem further comprises pinholes (4-1, 4-2), collimating lenses (5-1, 5-2), converging lenses (9-1 to 9-7) and photoelectric detection devices including photomultiplier tubes (10-1 to 10-6) and avalanche photodiodes (11); the pinholes (4-1, 4-2) are used for adjusting the receiving field of view; the collimating lenses (5-1, 5-2) are used for converting echo signals into parallel light; and the converging lenses (9-1 to 9-7) are used for converging optical signals to the light-sensitive surfaces of corresponding photoelectric detection devices.

4. A marine lidar system according to claim 3, wherein, The optical receiving subsystem is provided with seven signal receiving channels, including a 486 nm parallel polarization channel, a 486 nm vertical polarization channel, a 532 nm parallel polarization channel, a 532 nm vertical polarization channel, a 1064 nm channel, a 650 nm water Raman scattering channel and a 685 nm chlorophyll fluorescence channel.

5. A marine lidar system according to claim 1, wherein, The trigger end of the multi-channel data acquisition device (12) is connected to the trigger module (1-2) to receive a synchronous trigger signal, and the signal receiving end thereof is connected to the output ends of the photomultiplier tubes (10-1 to 10-6) and the avalanche photodiode (11) via a coaxial cable to realize synchronous acquisition of multi-channel electrical signals.

6. A marine lidar system according to claim 1, wherein, The system further comprises a sealed structure shell and an internal frame, optical elements of which are fixed in a stress-free or micro-stress clamping manner, and each interface is provided with a sealing strip or a sealing ring, and the internal frame is equipped with temperature control and dehumidification equipment.

7. A method for detecting multiple parameters of seawater using the ocean lidar system according to any one of claims 1 to 6, characterized in that, The system comprises the following steps: S1, emitting multi-wavelength pulsed laser to seawater; S2, receiving and separating laser echo signals of different wavelengths and polarization states via dichroic mirrors (6-1 to 6-3), narrow-band interference filters (7-1 to 7-5) and polarization beam splitter prisms (8-1, 8-2) by receiving telescopes (3-1, 3-2); S3, performing background noise removal, smoothing and correction processing on the echo signals; S4, based on the processed signals, at least one of the following parameters is retrieved: seawater optical parameters, particle depolarization ratio and color ratio, chlorophyll concentration and suspended matter concentration.

8. The method according to claim 7, wherein, When retrieving the water body particle depolarization ratio, the ratio of the parallel polarization and the vertical polarization echo signal intensity of the same wavelength is obtained after correction by the correction coefficient, and the depolarization ratio value at the wavelength is obtained.

9. The method according to claim 7, wherein, When retrieving the chlorophyll concentration, the ratio of the 685nm chlorophyll fluorescence signal and the 650nm water Raman scattering signal is obtained, and the chlorophyll concentration is calculated by combining the experimental calibration coefficient.

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