An in-line optical fiber refractometer with porous thin film coating

An in-line refractive-index sensor is proposed and demonstrated. The sensor is based on Fresnel reflection modulated by Fabry-Perot interference, which consists of a section of multi-mode fiber (MMF) tip coated with porous dielectric multilayer. The measure is conducted by monitoring the shift of the interference fringe, which is correlated to different refractive indices. When the external refractive index (RI) changes from 1.333 to 1.443, the RI sensitivity is 135.5 nm/RIU with good linearity. The miniature size, simple physical structure, small temperature dependence, linear response and ability for batch fabrication make the device attractive for chemical and biological sensing.