Silver-Coated tapered No-core fiber Mach–Zehnder interferometer for enhanced refractive index sensing

A high-performance refractive index (RI) sensor based on a silver-coated tapered no-core fiber (NCF) Mach–Zehnder interferometer (MZI) is proposed and experimentally demonstrated. Unlike conventional bare-fiber configurations that suffer from spectral instability and limited evanescent-field interaction, our approach synergistically combines controlled NCF tapering with a nanoscale silver film deposited via vacuum thermal evaporation. The tapering process effectively excites higher-order cladding modes and enhances the evanescent field penetration while the silver coating not only suppresses optical leakage to stabilize the interference spectrum but also improves the mechanical robustness of the sensing structure. To provide practical design guidance, we systematically compare two sensor variants with taper lengths of 4000 µm and 6000 µm. Experimental characterizations conducted over a biologically and environmentally relevant RI range of 1.338 to 1.361 yield sensitivities of 161.28 nm/RIU and 180.72 nm/RIU, respectively, with excellent spectral stability and repeatability. The superior performance, combined with the simple fabrication process and low-cost materials, positions the proposed sensor as a promising candidate for real-time biochemical detection, water quality monitoring, and food-safety analysis, where accurate, stable, and cost-effective RI measurement is critically needed.