A long period fiber grating seawater salinity sensor based on bend insensitive single mode fiber

In this work, we proposed a long period fiber grating (LPFG) seawater salinity sensor based on a bend insensitive single mode fiber (SMF). The average curvature sensitivity of LPFG in the most sensitive direction is 0.889 nm/m−1 in curvature range of 0 m−1–2.25 m−1. A comprehensive design method was proposed to optimize the sensitivity of seawater salinity by fully combining mode transition (MT) and reducing cladding diameter. The enhancing effects of MT and reducing cladding on surrounding refractive index (SRI) sensitivity were discussed, respectively. After reducing the cladding diameter by hydrofluoric acid (HF) etching, a layer of titanium dioxide (TiO2) nanofilm was deposited on the surface of LPFG by electrostatic self-assembly technology. The average seawater salinity sensitivity of 163.299 pm/‰ can be achieved in salinity range of 5.001 ‰–39.996 ‰. The experimental results are well agreement with theoretical analysis.