High-Resolution and Temperature-Insensitive Fiber Optic Refractive Index Sensor Based on Fresnel Reflection Modulated by Fabry–Perot Interference

A temperature-insensitive fiber optic sensor for high-resolution measure of refractive index is proposed and demonstrated. The sensor is based on Fresnel reflection modulated by Fabry-Perot interference, which consists of a section of single-mode fiber (SMF) tip coated with a thin film of SU-8 at the end of the fiber tip. The fringe contrast of the interference spectrum reflected from the sensor head is used to determine the extern RI. The experimental results agree well with the theoretical results and show that the fringe contrast is not sensitive to temperature variation. Such a refractive index sensor can offer the refractive index resolution and accuracy of 5 × 10^-6 and 5 × 10^-5, respectively. By simultaneous tracking the wavelength shift of the interference pattern, the sensor also has the potential capability for the simultaneous measurement of liquid RI and temperature.