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

• A LPFG sensor based on bend insensitive SMF has been proposed and investigated for the measurement of seawater salinity, and its sensitivity also has been comprehensively optimized by fully combining MT and reducing cladding diameter. • Compared with the LPFG based on traditional SMF, the proposed sensor achieved a low curvature sensitivity of 0.889 nm/m−1 in the curvature range of 0–2.25 m−1, which makes it have a strong resistance to bending interference in the process of seawater salinity measurement. • The maximum RI sensitivity reaches to 954.18 nm/RIU, which is about 5.9 times higher than that of the unoptimized LPFG, and an average sensitivity of 163.299 pm/‰ can be achieved when seawater salinity changes from 5.001 to 39.996 ‰ due to the contribution of reducing cladding diameter and MT. 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 (TiO 2) 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. [ABSTRACT FROM AUTHOR]