Avoided-crossing-based ultrasensitive photonic crystal fiber refractive index sensor.

We propose and demonstrate an ultrasensitive photonic crystal fiber refractive index sensor by introducing the avoided-crossing effect in a bent-controlled fluid-filled photonic-bandgap fiber (FF-PBGF). By controlling the bend radius of the FF-PBGF, resonant couplings between the fundamental core mode and the cladding modes are realized and transmission notches are observed in the transmission band of the FF-PBGF. By changing the refractive index or temperature of the bent FF-PBGF, a sensitivity of 32,400 nm per refractive index unit (or 13.1 nm/degrees C) is achieved, which is the highest for a fiber device to date, to our best knowledge.