Femtosecond laser direct writing of long period fiber grating sensor with high refractive index sensitivity

Abstract

Long-period fiber grating sensors with high refractive index sensitivity were designed and validated using femtosecond laser direct writing. The device consists of a single-mode fiber and a 2 cm long thin-core fiber fused. The refractive index is changed periodically by direct writing of a femtosecond laser pulse at the core of the thin-core fiber. Fiber gratings with periods of 80 μm, 150 μm, and 300 μm were selected for the study and showed maximum refractive indices of 1.4054, 1.4215, and 1.4326, respectively. With an 80 μm period, the maximum refractive index sensitivity increases significantly with wavelength and reaches 1430 nm/RIU. The temperature sensitivity is 7.6 pm/°C between 30 °C–430 °C and 14.4 pm/°C between 430 °C–630 °C. The sensor is high-temperature resistant, robust, simple to fabricate, and has a wide range of measurable refractive indices. This thin-core long-period fiber grating can better reduce crosstalk due to temperature effects in refractive index testing.