Figure 2

Image: Figure 2| Schematic of Brillouin underwater lidar experimental system for temperature and salinity measurements based on multi-edge technology.
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Credit: OEA

New issue from Advances in optoelectronicsDOI 10.29026/oea.2023.220016 describes a marine lidar remote sensing technique based on Brillouin scattering spectra.

Monitoring marine environmental information is very important for the development of marine science, maintenance of marine interests, development of marine resources, and establishment of marine industry. Laser remote sensing has become one of the important means of marine environment monitoring due to its advantages of water penetration, strong energy and high vertical profile resolution.

Marine laser remote sensing measures environmental information mainly by analyzing backscattered echo energy or spectral information. In the energy dimension, the backscattered echo contains various scattered signals and noise, resulting in a low echo signal-to-noise ratio and limited measurement accuracy. Moreover, echo energy characterization information is limited and only used for inversion of a single parameter. Different scatterers have unique spectral distribution characteristics in the spectral dimension, and the spectrum is less polluted by noise, resulting in a higher signal-to-noise ratio. At the same time, the spectrum contains rich information, and the measurement of multiple environmental elements can be achieved through various spectral functions. Therefore, lidar using spectral sensing is an important direction for future marine monitoring developments.

Compared with other scattering spectra, the Brillouin scattering spectra are independently distinguishable, the spectra are stable and have a wealth of information. Simultaneous inversion of seawater temperature and salinity can be achieved by the Brillouin spectrum. In addition, the Brillouin scattering cross section is large, and Brillouin detection is performed with a strong scattering signal and detection depth. Lidars based on Brillouin spectral measurements therefore have great potential for oceanic multi-parameter remote sensing.

Brillouin lidar has now fully proven its ability in high-precision measurements of seawater temperature and salinity vertical profiles in theory, simulations and laboratory experiments. However, existing Brillouin spectral measurement techniques have application requirements of real-time spectral detection integrity and fast and continuous measurements in applications of real-time synchronous measurement of seawater subsurface temperature and salinity vertical profiles. Therefore, breaking through the technical bottleneck of real-time and continuous measurement of the full Brillouin scattering spectrum is an important research topic for promoting the application of Brillouin lidar.

In response to the actual measurement needs of the Brillouin lidar, the research group of Professor Kun Liang of Huazhong University of Science and Technology, in cooperation with the Beijing Institute of Space Electromechanics and the University of Electronic Science and Technology, carried out research work using the Brillouin spectrum. . It realizes highly accurate profile measurement such as temperature and salinity in water.

The team proposed a double-edge Brillouin spectrometry method combined with PMT. Based on the concept of sparse reconstruction, the energy of multiple local narrowband spectra is measured by multiedge filters. Then, with the help of the Brillouin scattering spectrum feature, the energy is used to obtain the full Brillouin scattering spectrum with ultra-high resolution. Finally, spectral characteristic parameters of the scattering spectrum are extracted and used for synchronous reversal of seawater temperature and salinity.

This measurement technique employs wideband multichannel edge filters to allow each channel to carry a large amount of spectral energy. This theoretically guarantees the sounding capability of the system. The full super-resolution spectrum is reconstructed according to a sparse low-resolution narrowband filter to achieve high-precision measurements of the Brillouin spectrum. Therefore, the detection depth and measurement accuracy of the system are considered in this technique. In addition, a photoelectric conversion module with high sensitivity and short response time and a data acquisition module with high sampling rate are also used in the system, allowing rapid and continuous measurement of seawater temperature and salinity profiles.

Following the principles of Brillouin detection technology, the team developed a lidar test system. The system employs a transceiver coaxial design, with a laser injected into the water through a telescope system to produce a Brillouin scattering signal. The backscattered signal received by the telescope system first passes through an iodine pool to filter out Rayleigh scattering and measure background noise. Then the remaining Brillouin scattered light he splits in two. A portion is collected by the PMT as a reference signal (signal Ig), and the other part after the double-edge filter, which consists of two Fabry-Perot etalons, is collected by two PMTs (signal I1 and me2). Finally, based on the obtained two relative edge energies, I1 / Meg and me2 / Megthe corresponding Brillouin scattering spectrum is obtained with the idea of ​​sparse reconstruction.

After acquiring the spectrum using the above system, data feature analysis, spectral feature extraction data correction, and operation of the temperature/salinity inversion model realized measurement with a temperature accuracy of 0.5°C and a salinity accuracy of 1 psu. World’s highest level. Overall, the measurement results demonstrate the potential of the Brillouin spectral detection method in seawater environmental element determination 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 spectra for liquid detection and applications in oceanography. optoelectronic advance 6, 220016 (2023). Doi: 10.29026/oea.2023.220016

keyword: Brillouin scattering spectrum / double edge method / temperature / salinity / oceanography

In 2003, Professor Kun Liang’s team at the School of Electronics, Information and Communication Technology, Huazhong University of Science and Technology began to focus on the remote sensing applications of Rayleigh Brillouin scattering and lidar. In recent years, it has relied on platforms such as Huazhong University. A research group in Science and Technology and the Wuhan National Photoelectric Research Center has made influential research results in Rayleigh-Brillouin scattering lidar remote sensing of the atmosphere and seawater. Currently, the team’s lab has two sets of atmospheric lidar systems for measuring wind, temperature, and pressure, and two sets of seawater lidar systems for measuring temperature, salinity, and target detection. Professor Kun Liang has presided over many national projects in succession, such as the National Natural Science Foundation of China and the National 863 Program. He has won his two first prizes for science and technology progress in Hubei Province and has applied for 17 invention patents. Currently, over 40 papers have been published, of which over 30 are included in his SCI.

Yun Su’s group, a researcher at the Beijing Institute of Space Electromechanical Research, belongs to the Core Specialized Laboratory of Space Laser Information Sensing Technology, China Academy of Space Technology. I am mainly engaged in research in fields such as space optical remote sensing, ocean remote sensing, space optical system design, and computational optics. Researcher Su Yun presided over many national research projects, such as the 863 program, and completed the optical design of domestic multi-type spatial light remote sensing load system. She has won her two state and ministerial awards, has published over 30 of her papers in her field of research, and has received over 80 patents.

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

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