Remote Sensing of Seawater Temperature and Salinity Profiles by the Brillouin Lidar Based on a Fizeau Interferometer and Multichannel Photomultiplier Tube

Abstract

Brillouin spectroscopy is a powerful tool to measure the water temperature and salinity profiles of seawater. Considering the insufficiency of the current spectral measurement methods in real-time, spectral integrity, continuity, and stability, we developed a new lidar system for spectrum measurement on an airborne platform that is based on a Fizeau interferometer and multichannel photomultiplier tube. In this approach, the lidar system uses time-of-flight information to measure the depth and relies on Brillouin spectroscopy as the temperature and salinity indicator. In this study, the system parameters were first optimized and analyzed. Based on the analysis results, the performance of the system in terms of detection depth and accuracy was evaluated. The results showed that this method has strong anti-interference ability, and under a temperature measurement accuracy of 0.5 °C and a salinity measurement accuracy of 1‰, the effective detection depth exceeds 40.51 m. Therefore, the proposed method performs well and will be a good choice for achieving Brillouin lidar application in seawater remote sensing.

Ocean Temperature Profiling Lidar: Analysis of Technology and Potential for Rapid Ocean Observations

“…While this Brillouin lidar technique for measuring ocean water features was proposed nearly a half century ago [30], a recent flurry of work [30][31][32][33][34][38][39][40][41][42][43][44][45] has been done on the ability of using Brillouin lidar to measure temperature, salinity, and sound speed. Various laboratories are developing benchtop instrumentation and testing the capability using select samples of ocean water.…”

“…In addition to the Brillouin wavenumber shift, a second feature of Brillouin scattering is the variability of the linewidth as a function of salinity, temperature, and pressure [41]. Several recent efforts have capitalized on using these two Brillouin scattering measurements, wavenumber shift and linewidth, to measure both temperature and salinity [42][43][44]. Meanwhile, another effort has demonstrated a multi-lidar technique to perform inversions using two wavenumber shift observations [45] to estimate both temperature and salinity.…”

Underwater Temperature and Salinity Measurement by Rayleigh–Brillouin Spectroscopy Using Fizeau Interferometer and PMT Array

“…To achieve high accuracy in temperature and salinity inversion, the system utilizes a seed-injected, high-stability-frequency pulsed laser. Considering that water has a high attenuation of ultraviolet frequencies [19,28], the center frequency of the light source used in this study was 532 nm. The frequency drift of the laser under short-term operating conditions was less than 7 MHz, whereas that under long-term operating conditions was less than 10 MHz, which meets the requirements for detection accuracy.…”

“…Considering the disadvantages of the dual-step FP method, a method combining a Fizeau interferometer with a photomultiplier tube (PMT) array was previously proposed [19]. In this method, the problem of absolute frequency instability can be overcome, because it can accurately determine the relative frequency.…”

Research on Laser Dual-Mode Fusion Detection Method of Ship Wake Bubbles

“…Ship wake detection technology based on laser guidance is a typical application of underwater laser detection technology [1]. The wake of the ship is a dynamic air curtain belt formed at the tail of the ship when the ship is sailing on the water surface due to the cavitation of the propeller, the breaking of the wave, and the involvement of a large amount of air in the water line of the ship [2,3]. The theoretical geometric optics of the scattering characteristics of bubbles in water is in good agreement with the Mie scattering theory.…”