Ocean lidar remote sensing technology based on Brillouin scattering spectrum

The monitoring of marine environmental information is of great importance in the development of marine science, the preservation of marine rights and interests, the development of marine resources and the establishment of the marine industry. Laser remote sensing has become one of the important means of monitoring the marine environment due to its water penetration, strong energy and high vertical resolution.

Ocean laser remote sensing mainly measures environmental information by analyzing scattered echo energy or spectral information. In the power dimension, the return echo contains a variety of scattered signals and noise, and the echo signal-to-noise ratio is low, which limits the measurement accuracy. Besides, the characteristic information of the echo energy is limited which is only used for single parameter inversion.

While different scattering has its own spectral distribution characteristics in the spectral dimension, and the spectrum is not easy to pollute with noise, resulting in a high signal-to-noise ratio. At the same time, the spectrum contains rich information, and multiple environmental elements can be measured by a variety of spectral features. Therefore, the use of lidar spectroscopic detection is an important direction for the development of marine monitoring in the future.

Compared with other scattering spectra, the Brillouin scattering spectrum can be distinguished independently, and the spectrum is stable and has rich information. Simultaneous reversal of sea water temperature and salinity can be achieved by Brillouin spectrum.

In addition, the cross-section of Brillouin scattering is large which makes Brillouin detection with strong scattering signal and depth of detection. Therefore, Brillouin spectrometry-based lidar has great potential in marine multiparameter remote sensing.

At present, Brillouin lidar has fully demonstrated its ability to high-resolution measurement of sea water temperature and vertical salinity from theory, simulation and laboratory experiments.

However, the current Brillouin spectrophotometric technology has requirements for real-time application, safety of spectroscopic detection, rapid and continuous measurement in the application of simultaneous real-time measurement of subsurface seawater temperature and salinity vertical profile. Therefore, breaking through the technical bottleneck of real-time measurement and continuous measurement of the full Brillouin scattering spectrum is an important research topic for enhancing Brillouin lidar application.

According to the actual measurement needs of Brillouin lidar, the research team of Professor Kun Liang of Huazhong University of Science and Technology, together with Beijing Institute of Space Electromechanical Research and University of Electronic Science and Technology, conducted research work using Brillouin spectrum to achieve high-accuracy lateral measurement of underwater temperature and salinity.

The team proposed Brillouin’s method for double-edge spectrometry along with PMT. Based on the notion of scattered reconstruction, the energies of two or more local spectra are measured by means of a multi-edge filter. Then, with the help of the Brillouin scattering spectrum function, the complete Brillouin scattering spectrum is obtained at very high resolution using energies. Finally, the characteristic spectral parameters of the scattering spectrum are extracted and used for the simultaneous reflection of sea water temperature and salinity.

The measurement technology is based on a wide-band multi-channel edge filter to ensure that each channel can transmit large spectral power, which theoretically ensures the ability of the system to measure bathymetry. The entire hyper-resolution spectrum is reconstructed according to the scattering low-resolution narrowband filter, and the high-resolution measurement of the Brillouin spectrum is achieved. Therefore, this technique takes into account the detection depth and measurement accuracy of the system.

In addition, a high-sensitivity, short response time photovoltaic conversion module and a high sampling rate data acquisition module are also used in the system to ensure continuous rapid measurement of seawater temperature and salinity.

According to the principle of Brillouin detection technology, the team developed a lidar test system. This system adopts transceiver coaxial design, and drops the laser into the water through the telescope system to generate Brillouin scattering signal. The backscattered signal received by the telescope system is first passed through an iodine pool to filter out Rayleigh scattering background noise and counter scattering.

Then, the remaining Brillouin scattering light is divided into two parts. One part is collected by the PMT as a reference signal (Ig signal), and the other part is collected after the double edge filter of two Fabry Perot etalon by two PMTs (signals I1 and I2). Finally, based on the obtained relative edge energies I1/Ig and I2/Ig, the corresponding Brillouin scattering spectra with the notion of scattered reconstruction are obtained.

After obtaining the spectrum using the above system, with the processes of data characteristic analyzes, spectral extraction data correction, temperature and salinity inversion model, the system achieves the measurement with a temperature accuracy of 0.5? And the salinity accuracy is 1psu, which has reached the highest level in the world.

Overall, the measurement results demonstrate the potential of the Brillouin spectrum detection method in seawater ecological measurement and oceanographic research and provide theoretical and technical support to enhance the practical application of lidar based on Brillouin dispersion.

search report:Brillouin scattering spectrum for fluid detection and applications in oceanography

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Huazhong University of Science and Technology

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