A high-resolution seawater salinity sensor, based on a temperature self-compensating resonator, has been developed, which accommodates simultaneous temperature variations in both the reference and sensing arms. The impact of external temperature fluctuations on the optical components affecting the optical path difference is significantly reduced and a salinity sensitivity of 145 pm/PSU and a salinity resolution of 0.0003 PSU are achieved. The sensor utilizes a structure that converts optical rays to spatial light, achieving non-contact measurements which is potentially pivotal for various applications in oceanography and marine studies.

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.

This paper presents a novel optical Fabry-Pérot (FP) interferometric pressure sensor with an extensive measurement range, specifically designed for the high-pressure and high-salinity conditions of the ocean. Utilizing ultra-high-precision demodulation techniques, the sensor adeptly captures minute structural deformations, significantly reducing material fatigue and enhancing overall sensitivity. It demonstrates exceptional performance features, including superior repeatability, excellent linearity over a wide measurement spectrum, and significant scalability—attributes crucial for diverse pressure measurement applications. Optimized for fatigue resistance and stability over extended operational periods, the sensor is positioned as an ideal tool for deep-sea exploration and as a critical tool for multi-parameter optical sensing integration. Additionally, an improved simulation model was developed to include processing and analysis errors, enhancing the system’s predictive accuracy under varied environmental conditions. We describe the working principles of the sensor and present both theoretical analyses and standard experimental comparisons. The experimental results highlight the sensor’s accuracy, long-term stability, expansive measurement capacity, and strong environmental adaptability. Key performance indicators include a dual-range sensitivity of 0.125 pm/bar within the 0–400 bar range and 0.066 pm/bar from 400 to 1200 bar, a pressure resolution of 0.75 bar (0.063% F.S.), and an Allan variance of 1.12 × 10⁻¹¹ nm under a 1000 bar load. Based on the experimental results, an online calibration method for optical measurement chamber length based on Free Spectral Range (FSR) is proposed, addressing the identified measurement challenges and offering constructive insights to improve the adaptability of marine optical sensors to environmental conditions.

A novel seawater density sensor has been developed, offering high resolution and improved accuracy. The sensor employs a Michelson interferometer as the core sensing element, measuring density by directly incorporating seawater samples into the measurement path. To enhance measurement precision, standard seawater is used in the reference path to mitigate temperature and dispersion effects. The sensor system consists of a superluminescent diode and an interrogator, forming a compact, high-sampling-rate wavelength demodulation system that is particularly suitable for measuring seawater density with high spatiotemporal variability. However, the spectrum collected by the interrogator suffers from discontinuity. To address this issue, we designed a high precision multi wavelength demodulation algorithm. This algorithm not only analyzes the movement of each resonance peak with a resolution of 0.0008 nm within a 70nm detection range, but also enables continuous tracking of specific resonance peaks via a dynamic adjustment mechanism. It overcomes the measurement range constraints typically faced by interference-based sensors, which are limited to a single free spectral range. Experimental validations demonstrate the sensor’s capability, achieving a density resolution of 1.79×10 ^-5 kg/m ³ and an extended measurement range exceeding 35 kg/m ³ . The sensor’s average relative error in density measurement is 0.0144%, highlighting its effectiveness and potential for advanced oceanographic research.

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.