High-sensitivity refractive index sensor based on ultrafine conical no-core optical fiber

A high-sensitivity refractive index (RI) sensor based on a Mach-Zehnder Interferometer (MZI) configuration utilizing ultrafine tapered no-core fiber (Tapered No-Core Fiber, TNCF) is proposed and fabricated. The sensor consists of a single-mode fiber (SMF) - no-core fiber (NCF) - single-mode fiber (SMF) structure, with a tapered configuration formed in the no-core fiber region via the fusion tapering process. Multi-mode interference (MMI) and the excitation of higher-order modes within the tapered region significantly enhance the evanescent field effect of the light beam, thereby improving the sensor's sensitivity to refractive index variations. Three sensors with tapered waist diameters of 60.1 mu m, 41.2 mu m, and 22.1 mu m were fabricated. Experimental results show that, within refractive index ranges of 1.338-1.358 and 1.347-1.405, the refractive index sensitivity of the sensor increases significantly as the waist diameter of the tapered region decreases. When the tapered waist diameter is 22.1 mu m, the refractive index sensitivity reaches 617.60 nm/RIU and 643.36 nm/RIU, with linearity coefficients of R2 = 0.9835 and R2 = 0.9994, demonstrating excellent sensitivity and linear performance. The proposed sensor not only exhibits high sensitivity, structural stability, and a wide measurement range, but also demonstrates suitability for high-precision refractive index measurements in the fields of medicine, biological sensing, and energy.