Bent optical fiber taper for refractive index detection with a high sensitivity

A bent optical fiber taper (BOFT) is proposed for refractive index (RI) detection with a high sensitivity. The BOFT consists of a bent cladding downtaper where the whispering gallery mode (WGM) forms along the curved cladding-external medium interface due to radiation from the bent core. The phase change upon total internal reflection (TIR) of the WGM is a function of external RI, which is utilized for RI sensing. The incident angle of the TIR is decreased due to the tapered cladding, which speeds up the phase change and thus enhances the sensitivity. A high sensitivity of 4000 nm/RIU (RI unit) was experimentally obtained over a RI range of 1.424–1.435.