Refractive index sensor based on ultrafine tapered single-mode nocladding single-mode fiber structure

A refractive index (RI) sensor based on a tapered no cladding fiber (NCF) is proposed theoretically and experimentally. The RI sensor is fabricated by splicing a NCF in the middle of two single mode fibers (SMF), and then the NCF is tapered to get lower waist diameter. Due to the lack of cladding, surrounding refractive index (SRI) can directly affect the light propagation of NCF, and the decrease of diameter can further improve the RI sensitivity by strengthening the evanescent field. The sensitivity with different lengths and waist diameters of NCF is investigated. It is observed that there are different linear relationships in different refractive interval and the RI sensitivity increases obviously with the waist diameter decreasing, especially when it is below 30 μm. RI sensitivity of the sensor reaches 686 nm/RIU when the waist diameter is 13 μm and SRI is at the range of 1.333–1.350. The sensor can be widely used in biosensing for its simple operation and high sensitivity at low SRI.