Sensing Characteristics of Femtosecond Laser-Induced Long Period Gratings by Filling Cladding Holes in Photonic Crystal Fiber

The sensing properties of long period grating (LPG) fabricated by femtosecond laser-induced filling cladding air holes in photonic crystal fiber (PCF) were investigated. The sensitivity of the LPG to temperature, strain, bending, and external refractive index is studied systematically. Based on the property of pure silica PCF, the proposed LPGs exhibited low-temperature sensitivity, and the measured temperature coefficient is ~10.5 pm/°C for temperature 20-1100 °C. It should be noted that the pure structural waveguide change induced by femtosecond laser can withstand high temperature. The LPG exhibits high strain sensitivity of -1.86 pm/με. Due to the asymmetric modified cladding structure, the LPG exhibits strong directional bend sensitivity, and the maximum bending sensitivity is -1.12 nm·m. The maximum external refractive index sensitivity of LPGs was estimated as -725 nm/RIU. The very weak sensitivity to temperature especially in high temperature situation and strong sensitivities to strain, bending, and external refractive index is of interest as it offers a simple solution to the problem of fiber sensor cross sensitivity. Therefore, the gratings can be used as sensors in a series of application field including high temperature and harsh environments.