Miniaturized photonic crystal fiber tip sensor for refractive index sensing

We report on the fabrication of a miniaturized photonic crystal fiber (PCF) tip sensor for sensitive liquid refractive index measurement. The PCF tip sensor has a hybrid configuration of Fabry-Perot (FP) and modal interferometer. A short length (less than 1 mm) of PCF is spliced to a standard single mode fiber. An air bubble is created in the splice region forming the FP cavity. The air holes are fully collapsed on both sides of the PCF. The collapsed sections act as multimode fibers which excite and recombine higher order modes experiencing optical path difference (OPD) than that of the core mode and forming modal interference. The end of the PCF is modified to a solid sphere by fusion arc splicer. Experimental results show that the sensitivity of the probe can reach up to 21.4 nm/RIU (refractive index unit) to detect bulk liquid with refractive indices from 1.33 to 1.4.