Inline core-cladding intermodal interferometer based on nano-coated photonic crystal fiber for refractive-index sensing

Abstract

The paper presents a modification of the refractive-index (RI) response of a photonic crystal fiber (PCF) based intermodal interferometer using a thin plasma-deposited silicon nitride (SiN_x) overlay with a high refractive index. We show that the film overlay can effectively change the distribution of the cladding modes and thus tune the RI sensitivity of the interferometer. Thanks to the nano-coating we were able to increase RI sensitivity eightfold in the range required for biosensors (nD~1.33). Due to the extreme hardness of SiN_x films and their excellent adhesion to the fiber surface, we believe that after the deposition the device can still maintain its advantages, i.e. lack of degradation over time or with temperature.