Dual Fabry-Perot interferometric fibre-optic vernier temperature and salinity sensor based on femtosecond laser etching

In this research, dual Fabry Perot interferometric (FPI) vernier sensor have been developed to enable highly sensitive measurements of temperature and salinity. Utilizing femtosecond laser technology, a U-shaped open microcavity is etched on hollow core fiber (HCF) to serve as the salinity cavity, allowing for full flow of seawater into the microcavity. Simultaneously, a rectangular square groove is etched on another HCF to serve as the temperature cavity for rapid filling with ultraviolet (UV) adhesive material. To our knowledge, this is the first time that U-type optical fibers are etched to form a fiber optic vernier sensor for simultaneous measurement of seawater temperature and salinity based on femtosecond laser etching. The developed vernier sensor can accurately measure temperature and salinity with the sensitivity of -3.602 nm/degrees C and 1.533 nm/parts per thousand, according to experimental results. In stability tests, maximum shifts in interferometric wavelengths for the salinity and temperature cavities are recorded at 0.58 nm and 0.78 nm respectively. Furthermore, positions of interfering wavelengths remain consistently aligned across six repeatability measurement experiments conducted under identical conditions of salinity and temperature. The proposed structure has excellent stability, sensitivity, and cost-effectiveness. It may be used as a novel form of sensor to monitor seawater salinity and temperature with high resolution.