In this paper, a novel refractive index (RI) sensor is proposed based on the fused tapered special multi-mode fiber (SMMF). Firstly, a section of SMMF is spliced between two single-mode fibers (SMFs). Then, the SMMF is processed by a fused tapering machine, and a tapered fiber structure is fabricated. Finally, a fused tapered SMMF sensor is obtained for measuring external RI. The RI sensing mechanism of tapered SMMF sensor is analyzed in detail. For different fused tapering lengths, the experimental results show that the RI sensitivity can be up to 444.517 81 nm/RIU in the RI range of 1.334 9-1.347 0. The RI sensitivity is increased with the increase of fused tapering length. Moreover, it has many advantages, including high sensitivity, compact structure, fast response and wide application range. So it can be used to measure the solution concentration in the fields of biochemistry, health care and food processing.Optical fiber refractive index (RI) sensor [1,2] is very important in biology, chemistry and medicine fields due to the advantages of high sensitivity, compact size and fast response. It has several kinds of common structures, including the core offset splicing [3,4] , material coating [5,6] , fiber grating [7,8] and so on. For example, Shan Zhu et al [9] introduced the atomic layer deposition technology to fabricate a high sensitivity RI sensor based on an adiabatic tapered optical fiber, where an asymmetric Fabry-Perot like interferometer is constructed along the fiber taper. Peter Tatar et al [10] proposed a modification structure model of in-fiber sensor based on intermodal interference in two core photonic crystal fibers for measuring external RI. Especially, an increase of RI sensitivity to the surrounding medium could be achieved by decreasing the fiber diameter. It is the most common solution to etching the fiber cladding with chemical reagent. However, this method has the disadvantages of low control precision and large repeatability error. So the fused tapered method is proposed and applied in different fiber sensor, mainly including the flame brushing technique and CO 2 laser fusion technology. For example, Le Xu et al [11] proposed a high-temperature sensor based on an abrupt fiber-taper Michelson interferometer in single-mode fiber (SMF) fabricated by a fiber-taper machine and electric-arc discharge, S. A. Ibrahim et al [12] proposed an ammonia sensor composed of a tapered multimode fiber coated with polyaniline nanofibers, and Fu Guangwei et al [13] analyzed an RI sensor of photonic crystal fiber Mach-Zehnder interferometer (MZI) based on CO 2 laser fusion technology. Due to the controllable accuracy and simple preparation of fused tapered method, an RI sensor is proposed based on the fused tapered special multi-mode fiber (SMMF).In this paper, an RI sensor is fabricated by using a section of SMMF, which is spliced between two SMFs, and then tapered down by flame brushing technique. The RI sensing principle is analyzed, and the testing experiments are performed. By changing the tapering len...
