Realization of triple dispersion turning points in tapered seven-core fiber interferometer for sensitivity enhancement

Seven-core fibers have been widely applied in optical network, astronomy, and sensing applications. In this work, we report and demonstrate the existence of triple dispersion turning points of the super-mode interferences in a seven-core fiber spliced between two single-mode fibers by modifying the diameter of the multi-core fiber. The sensitivities of the interferometer on external variables, i.e., temperature, axial strain, and refractive index (RI), are measured, showing significant enhancement around the dispersion turning points. Compared to the regular multi-core fiber device without dimension modification, the sensitivities of temperature and axial strain are improved by about 60 times and 73 times around the dispersion turning points, respectively. While the regular device has no response to the external refractive index, the magnitude of RI sensitivity can reach as high as 33 459.0 nm/RIU in air, when the fiber diameter is modified to approach the dispersion turning point, enabling the discrimination of the refractive indices of N2 and CO2. The modification method of locating the modes at the dispersion turning points significantly enhances device sensitivity, providing an avenue for integrating ultra-sensitive sensors into commercial fiber networks.