Femtosecond-Laser-Processed Three-Beam Interference Fiber Fabry–Pérot Microcavity Sensor for Simultaneous Measurement of Seawater Salinity and Temperature

This study proposes a three-beam interference fiber Fabry-Pérot (F-P) microcavity sensor for simultaneous measurement of seawater salinity and temperature. The sensor is made by etching an open microcavity on the end face of a single-mode fiber (SMF) using femtosecond laser processing technology. By systematically comparing the performance of three sensors with different cavity lengths (40μm, 70μm, and 100μm), theoretical simulation revealed that longer cavity length would lead to higher sensitivity. The sensor with the 100μm cavity length demonstrated a salinity sensitivity of 0.157nm/‰ and a temperature sensitivity of -0.085nm/°C. Experimental observation showed great consistency, as the actual sensitivity of the sensor (100μm) reached 0.129nm/‰ in the salinity range of 0‰ to 60‰, and -0.084nm/°C in the temperature range of 25°C to 65°C. Moreover, simultaneous monitoring of seawater salinity and temperature was realized by constructing a sensitivity matrix. The sensor enables plug-and-play operation without complex packaging, allowing rapid deployment and easy operation to significantly enhance real-time monitoring efficiency. With its highly integrated design, it exhibits excellent stability and repeatability and can provide long-term and reliable data output. It provides a real-time, efficient and reliable sensing solution for marine environmental monitoring, and has broad application prospects.