Highly sensitive detection of water salinity and surface height using a double fiber grating system fabricated by femtosecond laser

Abstract

A double fiber grating sensor system is proposed and experimentally demonstrated for a highly sensitive measurement of water salinity and surface height simultaneously. The system consists of one horizontal and one vertical fiber grating, which are fabricated by the femtosecond laser direct writing. The horizontal fiber with a 20-mm-long grating structure can realize an 18.3-dB extinction ratio and a 300-nm/RIU sensitivity for refractive index sensor, which can be used for water salinity measurement with a 375-pm/% sensitivity. As for the vertical fiber with similar grating structure, it can realize a 0.62-dB/mm sensitivity for the surface height sensing with a resolution of 0.125 mm, which can be used for the water surface height monitoring. The presented sensor system is easy fabrication and cost effective, which would have a great application potential for water environment monitoring.