An In-Fiber and End-Reflective Optical Seawater Temperature-Salinity-Pressure Sensor Based on SPR and FPI

An integrated optical fiber sensor for simultaneously measuring seawater temperature, salinity, and pressure (TSP) is firstly presented by combining dual surface plasmon resonance (SPR) effect and Fabry-Perot (FP) interference effect. By coating gold film on a single mode fiber (SMF) and partially coating polydimethylsiloxane (PDMS) on the surface of the gold film, dual SPR effect can be excited, which were sensitive to the salinity and temperature. Then, a section of hollow core fiber (HCF) was welded with the SMF to generate FP interferometer (FPI), which was sensitive to pressure. Besides, the FPI with closed cavity had minimal sensitivity to temperature and salinity. By combining a $3\times 3$ sensitivity matrix for simultaneous detection of TSP, the cross-sensitivity between TSP parameters could be effectively reduced. The experimental results show that the sensor behaves good hysteresis and stability, and high sensitivities of 0.4058 nm/‰, −2.7270 nm/°C, and −1.3606 nm/MPa to TSP, respectively. In addition, its compact silicon-based structure ensures excellent mechanical properties and enables reliable measurements in high-pressure deep-sea environment.