The three-dimensional structure of oceanographic parameter profiles, particularly temperature and salinity, has persistently posed a significant challenge in remote sensing detection. While traditional ocean color satellites and microwave radiometers can achieve sea surface temperature measurements with accuracies better than 0.5 K and salinity measurements with monthly averages of 0.1 psu, these techniques are fundamentally limited to two-dimensional planar observations. In situ measurements from ships and buoys, though capable of vertical profiling, yield discrete, asynchronous, and non-global data that cannot satisfy the requirements of ocean science, environmental monitoring, marine economy, and national defense. Conventional oceanographic lidar systems, which rely solely on echo intensity detection, are unable to simultaneously measure multiple parameters like temperature and salinity. To address this critical gap, this paper proposes a novel remote sensing system for multi-parameter oceanographic detection based on Brillouin laser spectroscopy.

The proposed system leverages the laser non-elastic scattering effect, specifically spontaneous Brillouin scattering, which occurs when photons interact with thermal acoustic phonons in seawater. The Brillouin frequency shift and linewidth are functions of seawater temperature and salinity, as they depend on physical properties including sound velocity, refractive index, density, and viscosity coefficients. A two-parameter inversion model is established by solving coupled equations that relate Brillouin shift and linewidth to temperature and salinity. Through regression fitting of numerical data generated across temperature ranges of 0–30 °C and salinity ranges of 0–35 ‰, empirical formulas are derived to directly calculate temperature and salinity from measured Brillouin parameters.

The detection system comprises a transmitter system, a receiver system, and an integrated control and data processing system. A frequency-stabilized narrow-linewidth laser employing injection seeding and iodine molecular frequency stabilization technology serves as the light source, with a center frequency stability of 6 MHz, wavelength output at 532.29 nm, single pulse energy of 1 mJ, and pulse width of 7.6 ns. The core spectroscopic detection module consists of a high-precision Fizeau interferometer coupled with a highly sensitive 16-channel photomultiplier tube (PMT) array. The Fizeau interferometer is designed with a plate spacing of 6 mm to achieve a free spectral range of 25 GHz, a mirror reflectivity of 0.873 to yield an instrumental function linewidth of 1.079 GHz, and a plate angle of 6.65 μrad with a clear aperture of 40 mm. When the laser beam passes through the Fizeau interferometer, the Rayleigh and two Brillouin components form three distinct transmission peaks at different positions on the PMT array. The multi-channel signals are processed through baseline correction, abnormal signal rejection, noise suppression, and cumulative averaging. A nonlinear least-squares fitting algorithm is then employed to reconstruct the discrete Brillouin spectrum, extracting the frequency shift and linewidth from the measured data.

Simulation analyses reveal that to achieve temperature retrieval accuracy ≤1 K and salinity accuracy ≤2 psu for typical seawater conditions (temperature 10-25 °C, salinity 30-35 psu), the measurement uncertainty of both Brillouin shift and linewidth must be controlled within 20 MHz. This requirement drives the stringent design specifications of the system components. The PMT channel number selection is optimized based on simulation results showing that 8 or more channels satisfy the detection requirements, with 16 channels selected for the actual system implementation.

Laboratory experiments were conducted in a controlled water tank environment using two separate tanks to simulate layered water body slices, with independently controllable temperature and salinity conditions. Tank 1 was maintained at 23.1 °C and 33.8 psu, while Tank 2 was maintained at 23.4 °C and 34.8 psu. The laser pulses sequentially passed through both tanks, and the backscattered signals were received and processed. After data accumulation of 8000 pulses, the system achieved temperature measurement accuracy of 0.37 °C and salinity accuracy of 0.79 psu, with maximum errors of 0.72 °C and 1.52 psu respectively over five repeated measurements. The relationship between accumulation counts and measurement accuracy was investigated, showing that accuracy improves with increasing accumulation counts and converges to the system's optimal performance at approximately 8000 accumulations.

This paper successfully demonstrates a Brillouin spectroscopy-based remote sensing system capable of simultaneous temperature and salinity profiling in seawater. The system combines a frequency-stabilized laser, a custom-designed Fizeau interferometer with optimized parameters, and a multi-channel PMT array for high-resolution spectral detection. The two-parameter inversion model effectively derives temperature and salinity from measured Brillouin shift and linewidth. Laboratory validation confirms that the system achieves temperature accuracy of 0.37 °C and salinity accuracy of 0.79 psu, exceeding the design targets of 1 K and 2 psu respectively. These results establish the feasibility of using Brillouin lidar for remote sensing of oceanographic vertical profiles, providing a promising new approach to address the long-standing challenge of three-dimensional ocean parameter measurement. Future work will focus on field deployment and validation under real oceanic conditions.