Refractive index measurement using photonic crystal fiber-based Fabry-Perot interferometer

We have constructed a novel refractive index (RI) sensor based on a fiber optic Fabry–Perot interferometer (FPI) by splicing a section of hollow core fiber between a single-mode fiber and a photonic crystal fiber (PCF). Owing to the air holes in the cladding of the PCF, various substances, such as liquids and gases with different RI, can enter or leave the in-fiber air cavity, which makes the device usable as a refractometer. In this paper, the fiber optic FPI sensor has been used to monitor the RI changes of air with different pressures, and the experimental results show that such a sensor has an RI sensitivity of $805.1\mathrm{\mu m}/\mathrm{RIU}$, and hysteresis is not observed. Moreover, the easy fabrication method gives the in-fiber refractometer many potential applications in the sensing field.