S Fiber Taper-Based Fiber Loop Ring-Down Refractometer

A fiber-optic refractive index (RI) sensor based on fiber loop ring-down (FLRD) technique with an S fiber taper (SFT) structure is proposed and experimentally investigated. The SFT is served as an in-line Mach–Zehnder interferometer and inserted into a compensated fiber loop for RI measurement. By optimizing the parameters of the fiber loop, the quality of the ring-down signals is improved effectively. The surrounding refractive index can be conveniently measured by interrogating the ring-down time of the fiber loop. The experimental results demonstrate that the proposed sensor has a high sensitivity of $- 3128.954\,\,\mu \text{s}$ /RIU in the measurement range of 1.3330–1.3682. The proposed RI sensor combines the specialties of SFT interferometer and FLRD technology, which could be a good candidate in applications.