Ocean lidar remote sensing technology based on Brillouin scattering spectrum

The monitoring of marine environmental information is of great significance to the development of marine science, the maintenance of marine rights and interests, the development of marine resources and the establishment of marine industry. Laser remote sensing has become one of the important means of marine environmental monitoring because of its advantages of penetrating water, strong energy and high vertical profile resolution.

Ocean laser remote sensing mainly measures environmental information by analyzing the backscattered echo energy or the spectral information. In the energy dimension, the backscattered 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 echo energy is limited which is just used for single parameter inversion.

While different scatterings have their own spectral distribution characteristics in the spectral dimension, and the spectrum is not easy to be contaminated by noise, leading to high signal-to-noise ratio. At the same time, the spectrum contains rich information, and the measurement of multiple environmental elements can be realized through a variety of spectral features. Therefore, lidar using spectral detection is an important direction for the development of marine monitoring in the future.

Compared with other scattering spectra, Brillouin scattering spectrum can be distinguished independently, and the spectrum is stable and has rich information. The simultaneous inversion of seawater temperature and salinity can be realized by Brillouin spectrum.

In addition, the Brillouin scattering cross section is large making Brillouin detection with strong scattering signal and detection depth. Therefore, lidar based on Brillouin spectrum measurement has great potential in marine multi-parameter remote sensing.

At present, Brillouin lidar has fully proved its ability in high-precision measurement of seawater temperature and salinity vertical profile in theory, simulation and laboratory experiments.

However, the existing Brillouin spectral measurement technology has the application requirements of real-time, spectral detection integrity, and fast and continuous measurement in the application of real-time synchronous measurement of seawater subsurface temperature and salinity vertical profile. Therefore, breaking through the technical bottleneck of real-time and continuous measurement of complete Brillouin scattering spectrum is an important research topic to promote the application of Brillouin lidar.

According to the actual measurement needs of Brillouin lidar, the research group of Prof. Kun Liang from Huazhong University of Science and Technology, together with Beijing Space Electromechanical Research Institute and University of Electronic Science and Technology, carried out the research work of using Brillouin spectrum to realize high-precision profile measurement of underwater temperature and salinity.

The team proposed the Brillouin spectrum measurement method of double edge combined with PMT. Based on the idea of sparse reconstruction, the energies of two or more local narrow-band spectra are measured by multi-edge filter. Then, with the help of the function of Brillouin scattering spectrum, the complete Brillouin scattering spectrum with ultra-high resolution is obtained by using the energies. Finally, the spectral characteristic parameters of the scattering spectrum are extracted and used for synchronous inversion of seawater temperature and salinity.

The measurement technique adopts a wide-band multi-channel edge filter to ensure that each channel can transmit large spectral energy, which theoretically ensures the bathymetric ability of the system. The complete super-resolution spectrum is reconstructed according to the sparse low-resolution narrow-band filter, and the high-precision measurement of Brillouin spectrum is realized. Therefore, this technique considers the detection depth and measurement accuracy of the system.

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

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

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

After obtaining the spectrum by using the above system, with the operations of data feature analysis, spectral feature extraction data correction and temperature and salinity inversion model, the system realizes the measurement with temperature accuracy of 0.5 ? and salinity accuracy of 1psu, which has reached the highest level in the world.

All in all, the measurement results show the potential of Brillouin spectrum detection method in seawater environmental element measurement and oceanographic research and provide theoretical and technical support for promoting the practical application of lidar based on Brillouin scattering.

Research Report:Brillouin scattering spectrum for liquid detection and applications in oceanography

Related Links
Huazhong University of Science and Technology
Space Technology News - Applications and Research

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