High-Sensitivity Salinity Sensor Based on Helical Long-Period Fiber Grating in Thin-Cladding Fiber

In this article, we demonstrate a detailed investigation of the high-performance helical long-period fiber grating (HLPFG) near dispersion turning point (DTP) based on a thin-cladding fiber (TCF). The LP
₁₇
mode near DTP can be generated by CO
₂
-laser-inscribed TCF-HLPFG, whose transmissive properties have been studied theoretically and experimentally. With grating period of 140 μm, the TCF-HLPFG could be obtained at DTP and presents a high-sensitivity refractive index (RI) sensing ability that the maximum RI sensitivity of −14962.4 nm/RIU can be achieved. Furthermore, temperature sensing characteristics of the proposed TCF-HLPFG are investigated. With temperature changing from 20 to 80°C, the LP
₁₇
mode near DTP demonstrates the highest sensitivity of −1.14 nm/°C for grating period of 137.5 μm and ultra-low wavelength shifts for grating period of 140 μm, respectively. The temperature insensitive property makes it useful for eliminating cross-sensitivity to temperature. Meanwhile, the experimental results demonstrate that a salinity sensitivity of 0.87 nm/‰ can be achieved when the salinity increases from 5‰ to 50‰. The proposed TCF-HLPFG makes it a potential for environmental monitoring.