Ocean salinity plays an important role in oceanographic research as one of the fundamental parameters. An optical salinometer based on the Michelson interferometer (MI) suitable for in situ measurement in deep-sea environments is proposed in this work, and it features real-time calibration and multichannel multiplexing using the frequency modulated continuous wave (FMCW) technique. The symmetrical sapphire structure used to withstand deep-sea pressure can not only achieve automatic temperature compensation, but also counteract the changes in optical path length under deep-sea pressure. A model formula suitable for optical salinity demodulation is proposed through the nonlinear least squares fitting method. In vertical profile testing, the optical salinometer demonstrated remarkable tracking performance, achieving an error of less than 0.001 psu. The sensor displays a stable salinity demodulation error within ±0.002 psu during a three-month long-term test at a depth of 4000 m. High stability and resolution make this optical salinometer have broad development prospects in ocean observation.

Accurate pressure measurement is crucial for understanding ocean dynamics in marine research. However, pressure sensors based on strain measurement principles are significantly affected by temperature variations, impacting the accuracy of depth measurements. This study investigates the SBE37-SM sensor and presents an improved calibration method based on a constant-pressure, variable-temperature scheme that effectively addresses temperature-induced deviations in pressure measurement. Experiments were conducted across a pressure range of 2000 dbar to 6000 dbar and a temperature range of 2 °C to 35 °C, establishing a comprehensive pressure–temperature calibration grid. The results show that, at a pressure of 6000 dbar, temperature-induced variations in readings for brand new SBE37-SM sensors can reach up to 9 dbar, while, for used sensors, they exceed 12 dbar, following a U-shaped trend. After applying a polynomial regression model for calibration, these variations were reduced to within ±0.5 dbar, significantly reducing the measurement uncertainty of the sensors in complex marine environments. This method underscores the necessity of further optimizing the CTD system’s temperature compensation mechanism during calibration and highlights the importance of regular calibration to minimize measurement uncertainty.

Accurate ocean temperature measurement is critical for climate studies and ocean circulation modeling. Thermistor-based sensors, valued for high sensitivity and stability, require precise calibration due to their nonlinear resistance-temperature relationship. We evaluated six calibration equations (Basic, Hoge-1–4, S&H) across 273.15–308.15 K, and compared performance at calibration and non-calibration points. Furthermore, a novel temperature-distribution-optimized calibration strategy was proposed to enhance efficiency. Results showed that higher-order equations (Hoge-2, Hoge-3, Hoge-4, S&H) achieved sub-mK residuals with 13 calibration points, while lower-order equations failed. Extrapolation errors remained <1 mK within 2 K. With minimal calibration points, Hoge-2, Hoge-4, and S&H maintained <0.1 mK residuals at calibration points but exhibited increased residuals (~0.5 – 1 mK) in low-temperature non-calibration regions. The proposed strategy paired with the Hoge-2 equation reduced calibration time/cost by 50% compared to traditional 5 K-interval methods, without compromising accuracy. This work advances high-precision ocean temperature sensing, offering insights for in-situ calibration and climate model refinement.

This paper presents a high-precision temperature sensing system for in-situ monitoring of the deep-sea environments. The system utilizes a silicon-based Fabry-Perot interferometer as its sensing probe and employs a current-modulated distributed feedback laser to generate a linearly periodic wavelength-varying light source. The absorption line of H13C14N gas cell is used as a reference for real-time laser wavelength calibration. Laboratory test results indicate that the sensor has a sensitivity of 75 pm/°C and a resolution of 6.2×10-5°C with a dynamic range of 0 to 35°C. A watertight and pressure-resistant packaging structure was designed and fabricated, capable of enduring hydrostatic pressures up to 115 MPa, as verified through laboratory-simulated pressure tests, thus guaranteeing the system's adaptability to deep-sea environments. Field tests were conducted in the South China Sea, encompassing both short-term profile tests and long-term bottom-sitting tests, with comparative experiments performed against an SBE37 Conductivity-Temperature-Depth (CTD) sensor. The test results demonstrated consistent temperature measurement details between the sensor and the CTD, exhibiting excellent long-term stability and reliability. The system features a simple structure, high resolution, rendering it a promising tool for oceanographic research, resource exploration, and deep-sea environmental monitoring.

A novel Michelson salinometer is designed and experimentally demonstrated based on laser frequency-modulated continuous wave (FMCW) technique and cross-correlation algorithm. By modulating the driving current of the distributed feedback laser with FMCW signal, the lasing wavelength changes periodically and scans through the reference and sensing probes. The absorption peak of HF gas is used as reference to calibrate the drift of the laser wavelength in real time to improve the measurement stability. Standard seawater was diluted by titration to evaluate its salinity sensing performance. Experimental results show that the sensor has a resolution of 5×10 ^-4 g/kg with a measuring range from 26 to 40 g/kg. This sensor, boasting a simple and robust structure, possesses potential application in deep-ocean environments.