A method to improve the accuracy of lidar remote sensing seawater profile data

Abstract

This invention relates to a method for improving the accuracy of seawater profile data obtained by lidar remote sensing. It employs a laser transmitter and an optical receiver to emit laser light and receive Raman and elastic scattered light. A processor calculates the ratio of the elastic scattered main wave signal to the Raman scattered signal at the elastic main frequency, thus largely eliminating the influence of path extinction in the observed values. By measuring seawater Raman scattering, this invention eliminates the effects of scattering and absorption along the light path, increasing the accuracy of lidar measurements of the ocean's vertical profile and improving our understanding of the marine environment.

Description

Technical Field

[0001] This invention relates to the field of marine monitoring technology, specifically to a method for improving the accuracy of lidar remote sensing seawater profile data. Background Technology

[0002] Blue-green lidar is a commonly used technology for observing vertical ocean profiles using lasers. However, when using lidar to measure the composition of seawater and underwater topography or targets, various particles and water molecules along the optical path scatter and absorb the incident light and the reflected echo. This scattering and absorption cause strong attenuation of the incident and reflected light in both the incident and echo directions, preventing the lidar receiving system from obtaining reliable optical signals from the target. This attenuation along the light path is also why existing lidar systems cannot measure deeper layers of seawater.

[0003] To solve this problem, in addition to using high-power blue-green lasers, developing larger receiving mirrors, and considering using blue-green channels on the Fraunhofer dark line (i.e., the spectral band where sunlight is completely absorbed by the sun's own matter) to eliminate the influence of sunlight noise on lidar signals, a method is also needed to eliminate the influence of path attenuation by simultaneously observing one channel for elastic scattering and two channels for Raman scattering. Summary of the Invention

[0004] In order to solve the problems of the prior art, the present invention provides a method for improving the accuracy of seawater profile data by lidar remote sensing.

[0005] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution: a method for improving the accuracy of lidar remote sensing seawater profile data, characterized in that the specific method is as follows:

[0006] Step 1: A laser emitter is used to emit laser light into the seawater, which then causes elastic scattering and Raman scattering.

[0007] Step two: An optical receiver is used to receive Raman scattered light and elastic scattered light, and the received signals are processed.

[0008] Step three: Using a processor, the ratio of the elastic scattering main wave signal to the "Raman scattering signal at the elastic main frequency" is calculated. Specifically, taking advantage of the symmetrical distribution of Raman scattering Stokes lines and anti-Stokes lines on both sides of the elastic scattering main frequency, the average value of the two Raman scattering frequency signals is used to represent the "Raman scattering signal at the elastic main frequency". This "Raman scattering signal at the elastic main frequency" will have a path extinction function that is almost exactly the same as the elastic scattering main wave. Therefore, the ratio of the received elastic scattering main wave signal to the "Raman scattering signal at the elastic main frequency" will be basically free from the influence of path extinction.

[0009] Preferably, in a method for improving the accuracy of seawater profile data by lidar remote sensing, the wavelength of the emitted laser in step one is 488 nm, the wavelength of the Stokes line Raman scattered light received in step two is 585 nm, and the wavelength of the anti-Stokes line Raman scattered light is 418 nm.

[0010] Preferably, in a method for improving the accuracy of seawater profile data from lidar remote sensing, step three involves the processor calculating the ratio of the elastic scattering main wave signal to the "Raman scattering signal at the elastic main frequency," which is used to eliminate the effects of scattering and absorption of light along its path.

[0011] The beneficial effects of this invention are as follows:

[0012] 1. This invention, by measuring the optical signal of Raman scattering in seawater, can eliminate the effects of light scattering and absorption along the path, increase the measurement accuracy of lidar, facilitate the discovery of deeper conditions, and improve the understanding of the marine environment. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the principle of the marine lidar with dual Raman scattering channels of the present invention.

[0014] Figure 1 In the middle: 1. Laser; 2. Vertically polarized elastic scattering channel receiver; 3. Horizontally polarized elastic scattering channel receiver; 4. Raman scattering Stokes line receiver; 5. Raman scattering anti-Stokes line receiver; 6. Sea surface; 7. Emitting laser; 8. Laser echo. Detailed Implementation

[0015] Example: Figure 1 The method shown here aims to improve the accuracy of seawater profile data obtained through lidar remote sensing. It employs a dual-channel Raman lidar with a transmission wavelength of 488 nm. At this wavelength, the Stokes line Raman scattering wavelength is 585 nm, and the anti-Stokes line Raman scattering wavelength is 418 nm. The received information is then processed and analyzed. This data can be used for the classification and identification of marine material composition, providing more accurate measurement results.

[0016] When a laser beam is incident at high intensity on a substance, including seawater, the incident laser light is scattered by molecules or other particles distributed within the substance. Most of this scattered light has the same wavelength as the incident laser, a phenomenon known as elastic scattering. Elastic scattering by molecules is called Rayleigh scattering, while scattering by larger particles is called Mie scattering. Scattering from even larger objects can only be calculated using geometrical optics. However, a very small portion of the scattered light has a different wavelength than the incident light; this change in wavelength is determined by the molecular structure of the scattering substance. This portion of the scattered light is called Raman scattering.

[0017] Both elastic scattering and Raman scattering are caused by energy level transitions of electrons in excited states of atoms. The structure of molecules, such as the vibrations or rotations of the hydrogen-oxygen bonds (HO) within a water molecule, can interfere with energy level transitions corresponding to the incident light frequency, causing insufficient or excessive transitions and thus altering the frequency of the scattered light. This frequency difference is called the Raman shift, which is the difference Δv between the scattered light frequency and the excitation light frequency. Raman scattering produces two different frequency shifts: spectral lines with frequencies lower than the incident light frequency (v0) are called Stokes lines (v0 - Δv), and spectral lines with frequencies higher than the incident light frequency are called anti-Stokes lines (v0 + Δv). Stokes lines are generally stronger than anti-Stokes lines because, according to the Boltzmann distribution, the number of particles in the vibrational ground state (E0) is much greater than the number of particles in the vibrational excited state (E1). Therefore, the number of particles transitioning back from E0 + hv0 to E1 is much greater than the number transitioning back from E1 + hv0 to E0. Plotting frequency as the independent variable, the Raman scattered light intensity, as a function of frequency, is symmetrically distributed on both sides of the elastically scattered light, but its intensity is much weaker than Rayleigh scattered light, typically only 10 times that of Rayleigh light. -6 -10 -9 This places higher demands on the accuracy and dynamic range of instruments used for Raman scattering measurements.

[0018] It should be noted that the ratio of the arithmetic mean of the elastic scattering signal to the Stokes-Raman and anti-Stokes-Raman signals cannot completely eliminate the effects of path attenuation as ideally would. This method will also have errors, but these errors are much smaller than the uncertainty caused by light attenuation along the path. Furthermore, this error can be corrected by performing pre-calculations for different water environments and creating an error correction lookup table to process the measured data. However, if only the elastic scattering signal is used to invert the 180° backscattered signal, the inversion error may reach tens of times the true value, depending on the lidar ratio selected.

[0019] It's important to note that because only water molecules produce Raman scattering in the ocean, and we can know the Raman scattering signal of water molecules in advance, the ratio of the elastic scattering main wave signal to the "Raman scattering signal at the elastic main frequency," multiplied by the Raman scattering signal of water molecules, gives the true value of the optical signals of the material composition at various layers in seawater. Simultaneously, because path interference on the signal is largely eliminated, lidar can increase its measurement depth, facilitating the discovery of deeper environments.

Claims

1. A method for improving the accuracy of laser radar remote sensing seawater profile data, characterized in that, The specific method is as follows: Step 1: A laser emitter is used to emit laser light into the seawater, which then causes elastic scattering and Raman scattering. Step two: An optical receiver is used to receive Raman scattered light and elastic scattered light, and the received signals are processed. Step three: Using a processor, the ratio of the elastic scattering main wave signal to the "Raman scattering signal at the elastic main frequency" is calculated. Specifically, taking advantage of the symmetrical distribution of Raman scattering Stokes lines and anti-Stokes lines on both sides of the elastic scattering main frequency, the average value of the two Raman scattering frequency signals is used to represent the "Raman scattering signal at the elastic main frequency". This "Raman scattering signal at the elastic main frequency" will have a path extinction function that is almost exactly the same as the elastic scattering main wave. Therefore, the ratio of the received elastic scattering main wave signal to the "Raman scattering signal at the elastic main frequency" will be basically free from the influence of path extinction.

2. The method for improving the accuracy of lidar remote sensing seawater profile data according to claim 1, characterized in that: The wavelength of the emitted laser in step one is 488nm, the wavelength of the Stokes line Raman scattered light received in step two is 585nm, and the wavelength of the anti-Stokes line Raman scattered light is 418nm.

3. The method for improving the accuracy of lidar remote sensing seawater profile data according to claim 1, characterized in that: In step three, the processor calculates the ratio of the elastic scattering main wave signal to the "Raman scattering signal at the elastic main frequency". This ratio is used to eliminate the effects of scattering and absorption of light along the path.

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