TiO2-Modified SPR Fiber-Optic Sensor for High-Sensitivity Salinity and Temperature Detection in Low-Temperature Environments

Global climate change has led to significant fluctuations in ocean salinity and temperature, especially at higher latitudes, which have severely affected natural ecosystems and human production and life. This has placed higher demands on real-time and precise hydrological detection. This article utilizes Ag-based surface plasmon resonance (SPR) optical fiber sensors modulated by TiO2 films of different thicknesses to achieve simultaneous detection of two parameters within a wide temperature range ( $- 40~^{\circ }$ C to $100~^{\circ }$ C) and a wide salinity range (0%–25%). The Ag/thin-layer TiO2 structure used for salinity measurement can effectively enhance the sensitivity of salinity sensing and the oxidation resistance of the Ag film. The Ag/thick-layer TiO2/PDMS composite film structure used for temperature measurement can broaden the refractive index (RI) range and measurement range by enhancing the local electric field and improving the equivalent RI. The integration of PDMS can improve the spectral response and probe stability at low temperatures. The cascaded probe structure enables the simultaneous and distinguishable measurement of the two parameters at different working wavelengths. Experimental results show that the maximum salinity sensitivity is 7.2 nm/% and the maximum temperature sensitivity is 12.8 nm/°C. This study demonstrates the path of using semiconductor thickness modulation to expand the SPR bandwidth and achieve simultaneous sensing of multiple parameters, which avoids the complexity of multimaterial structure integration and the risk of stress cracking. It provides technical reserves for in situ hydrological detection in high-altitude or complex water environments in the future.