Slight-Offset Tapered Multimode Fiber with High Sensitivity for Low Refractive Index Range

A high-sensitivity optical fiber sensor for low refractive index (RI) measurement is proposed and experimentally demonstrated. It is fabricated by fused splicing a section of multimode fiber (MMF) between two single-mode fibers (SMFs) with a slight offset at two splicing joints, then the MMF is tapered using fused tapering technology. The offset splicing between SMF and MMF can effectively excite more high-order modes. The tapered multimode fiber (TMMF) can enhance the strength of the evanescent field and promote the interplay between the high-order modes and the surrounding medium. The experimental results indicate that the offset value and the waist diameter of the TMMF are two determinants for RI sensitivity enhancement. The RI sensitivity up to 12145 nm/RIU is achieved over the low RI range of 1.3345-1.339. This proposed high-sensitivity fiber-optic RI sensor may provide a low-cost and effective approach for low RI measurement in many chemical and biological applications.