Dual-parameter measurement of salinity and temperature based on fiber sagnac loop

Abstract

The ocean contains abundant untapped resources, driving ongoing academic research in marine exploration and monitoring. While electronic devices currently dominate ocean monitoring tools, they often come with high costs and electromagnetic interference issues. In contrast, fiber optic sensors have become a research focus due to their small size and anti-corrosion properties. Optical fiber structural designs can effectively capture ocean parameters. However, existing fiber optic ocean sensing research faces challenges, such as weak signal-to-noise ratio and cross-sensitivity, hindering practical implementation. To tackle these problems, this PhD thesis presents the design and development of three advanced fiber optic sensing systems for simultaneous monitoring of ocean salinity and temperature. The research addresses critical challenges in existing fiber optic ocean sensing, such as weak signal-to-noise ratio (SNR), cross-sensitivity, and structural weakness. The first system utilizes a tapered polarization maintaining fiber (PMF) within a Sagnac fiber optic sensor based on a fiber ring laser (FRL). The tapered PMF enables dual sensing of salinity (0.173 nm/%) and temperature (-0.306 nm/°C), while the FRL boosts the SNR up to 50 dB with a narrow linewidth of 0.15 nm. However, this design still suffers from cross-sensitivity limitations. To overcome this, the second system employs a Sagnac loop (SL) with a tapered PMF cascaded with a fiber Bragg grating (FBG). The FBG provides temperature-only sensing, enabling effective compensation of the cross-sensitivity issue. The sensitivities are improved to 0.356 nm/% for salinity and -0.616 nm/°C for temperature. However, the tapered fiber structure still has limited long-term stability. Finally, the third system replaces the tapered fiber with a more robust design using an SL composed of PMF and tilted fiber Bragg gratings (TFBGs). The TFBG can simultaneously detect salinity and temperature, while the PMF's high birefringence provides exceptional temperature sensitivity. By combining these complementary structures and integrating artificial intelligence algorithms, this system achieves high sensitivity, adaptability, and dual-parameter monitoring capabilities. In summary, this thesis contributes three innovative fiber optic sensing architectures that significantly enhance the performance, stability, and functionality of ocean monitoring systems. The research advances the practical application of fiber optic sensing in long-term, remote monitoring of marine environments.