Parallel dual‐cavity fiber‐optic F‐P salinity sensor based on hollow core fiber and ceramic ferrule

Abstract

A parallel dual‐cavity fiber‐optic Fabry‐Perot (F‐P) salinity sensor based on hollow core fiber and a ceramic ferrule is proposed. The sensor is comprised of a F‐P reference cavity and a F‐P sensing cavity in parallel. The reference cavity is a closed cavity that is fusion spliced with a segment of single‐mode optical fiber (SMF) and a small piece of silica tube (ST). The sensing cavity is an open liquid cavity that is connected by a ferrule after inserting two single‐mode fibers into a ceramic core, facilitating the replacement of salt water. Variations in the salinity of salt water will affect the change in refractive index of optical path, and then change the output spectral structure. By precisely controlling the length of two cavities, the free spectral range (FSR) of the sensing cavity is approximately equal to the FSR of the reference cavity, resulting in the generation of the Vernier effect, which improves the sensitivity of the sensor to salinity greatly. From the experimental results, it can be seen that the sensor sensitivity to salinity reaches −0.78 nm/‰ (−3809.68 nm/RIU) in the salinity range from 35‰ to 20‰ at room temperature of 25°C, which is 10.8 times higher than the sensitivity of a single sensing cavity. The enhancement factor is generally consistent with theoretical analysis results.