A Hybrid Optical Fiber Interferometer Based on Capillary and Three-core Fiber for Refractive Index Detection

In this paper, we proposed and experimentally investigated a hybrid optical fiber interferometer consisting of three-core fiber (TCF) and capillary fiber (CF) for refractive index (RI) detection. The hybrid fiber structure forms from a section of 2 mm TCF fused between two sections of 2 mm CF, in which two single-mode fibers (SMFs) act as lead-in and lead-out fibers, namely, an SMF-CF-TCF-CF-SMF (SCTCS) structure. The sensing part of the structure is only 6 mm in length. The RI sensitivity is measured to be 392.8 nm/RIU and 1844.5 nm/RIU in the region of 1.333–1.4385 and 1.442–1.457, respectively. Moreover, the proposed sensor shows thermal stability against temperature variations. Therefore, the device has good prospects for development in RI measurement in complicated environmental monitoring.