Ultrasensitive Refractive Sensor Based on Femtosecond Laser Inscribed Long-Period Fiber Grating With a Few Microcavities

We experimentally demonstrate a single mode fiber (SMF) ultrasensitive refractive index (RI) sensor based on long-period fiber grating (LPFG) with periodic microcavities fabricated by femtosecond laser. The liquid in the microcavities causes a strong RI modulation to excite the high-order cladding modes. The proposed sensor can operate in reflection mode by coating the end face of the fiber with gold film, as a result a cascade LPFG structure is equivalently formed to increase the contrast of the resonance dip. The LPFG has an ultra-high RI sensitivity of -12576 nm/RIU with a high linearity in the range of 1.3335-1.3430 RIU and a low temperature cross talk. The sensor has promising applications in biochemical or biomedical industries.