Modal Interferometer Based on a C-Shaped Ultrathin Fiber Taper for High-Sensitivity Refractive Index Measurement

The ultrathin fiber taper fabricated by adiabatically stretching a heated single-mode optical fiber (SMF) is made in a C-shape bent to form a modal interferometer. The interference fringes due to the mode beating in the multimode taper waist are dependent on the bending radius. Under an optimized bending radius, a maximum 18 dB interference depth can be obtained. The influence of the fiber taper geometry on the interference fringes is discussed. The proposed modal interferometer has a high refractive index (RI) sensitivity of ∼658 nm/RIU (refractive index unit) for RI= 1.333–1.353, which is expected to be useful for precision bio/chemical sensing applications.