Photonic crystal fibre based high temperature sensor with three-beam path interference

Photonic crystal fibre (PCF) modal interferometers are widely used for sensing applications. Owing to the intrinsic Mach-Zehnder-like dual-mode interference there is a trade-off between sharp spectral response and wide sensing range. A three-beam path interferometer induced by mode-conversion in a single dual mode PCF end face is experimentally demonstrated. Doubled free spectral range is obtained owing to the three-beam path interference, which is suitable in sensing large index change, wide temperature variation and strong applied pressures. As an example, a temperature sensor with the sensitivity of 8.17 pm/ C is demonstrated ranging from room temperature to 520 C. The mechanism and further improvements of the multi-beam path interferometers are also discussed.