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

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 thincore fiber. Fiber gratings with periods of 80 mu m, 150 mu m, and 300 mu m were selected for the study and showed maximum refractive indices of 1.4054, 1.4215, and 1.4326, respectively. With an 80 mu m period, the maximum refractive index sensitivity increases significantly with wavelength and reaches 1430 nm/RIU. The temperature sensitivity is 7.6 pm/degrees C between 30 degrees C-430 degrees C and 14.4 pm/degrees C between 430 degrees C-630 degrees C. The sensor is hightemperature 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.