Ocean lidar remote sensing technology based on Brillouin scattering spectrum

A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2023.220016   discusses 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 I_g), 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 / I_g and I₂ / I_g, 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.

Article reference: Wang YQ, Zhang JH, Zheng YC, Xu YR, Xu JQ et al. Brillouin scattering spectrum for liquid detection and applications in oceanography. Opto-Electron Adv 6, 220016 (2023). doi: 10.29026/oea.2023.220016 

Keywords: Brillouin scattering spectrum / double-edge technique / temperature / salinity / oceanography

The team of Professor Kun Liang, School of electronic information and communication, Huazhong university of science and technology, start to focus on the research of Rayleigh Brillouin scattering and lidar remote sensing applications in 2003. In recent years, relying on platforms such as Huazhong university of science and technology and Wuhan National photoelectric research center, the research group has made influential research achievements in Rayleigh Brillouin scattering lidar remote sensing of atmosphere and sea water. At present, the team laboratory has two sets of atmospheric lidar systems for measuring wind, temperature and pressure, and two sets of seawater lidar systems for measuring temperature, salt and target detection. Professor Kun Liang has successively presided over many national projects such as the National Natural Science Foundation of China and the National 863 program. He won 2 first prizes of scientific and technological progress in Hubei Province and applied for 17 invention patents. At present, more than 40 papers have been published, of which more than 30 are included by SCI.

The group of researcher Yun Su of Beijing Space Electromechanical Research Institute is subordinate to the core professional laboratory of space laser information sensing technology of China Academy of space technology. They are mainly engaged in the research work in the fields of space optical remote sensing, ocean remote sensing, space optical system design, computational optics and so on. Researcher Su Yun has presided over many national research projects such as the 863 program, completed the optical design of the domestic multi-type space optical remote sensing load system. She won 2 provincial and ministerial awards, published more than 30 articles in the research field and more than 80 patents.

The research team led by Professor Hai-Feng Lü of University of Electronic Science and technology is mainly engaged in the research of condensed matter physics and the interaction mechanism between laser and matter. In recent years, he has presided over and participated in major projects of state, natural science foundation projects and national key laboratory opening projects. The main research fields of the team include laser irradiation damage mechanism and pretreatment, laser Brillouin scattering spectrum analysis, high-power intrinsic vortex electromagnetic field generation and transmission technology, etc. The research group has published more than 50 papers in international journals such as Phys. Rev. Lett., Phys. Rev. B, Appl. Phys. Lett., Opt. Lett., Appl. Surf. Sci.

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