Ultrahigh-sensitivity liquid-core FMZI enabled by modal dispersion engineering with side-polished hollow-core...

We present an ultrahigh-sensitivity liquid-core (LC) fiber Mach-Zehnder interferometer (FMZI) enabled by modal dispersion engineering and side-polished hollow-core interfaces. The proposed device integrates a 10 µm-core liquid-filled hollow-core fiber (HCF₁₀) between two side-polished large-core HCFs, forming an open microchannel that enables efficient filling with high thermo-optic coefficient (TOC) liquids. The modal dispersion of the liquid core is tailored such that the effective index difference between the core and cladding modes increases with wavelength (λ), resulting in a temperature-induced redshift in the interference spectrum. The optical interference behaviors of the dispersion-engineered structure reverse from those of conventional silica-core FMZIs. To the best of our knowledge, this is the first study to derive an explicit analytical formulation that quantitatively relates temperature sensitivity to the spectral dispersion of the effective index difference between the core and cladding modes under arbitrary refractive index dispersion conditions. Experimental results exhibit excellent agreement with theory, with average temperature sensitivities exceeding +80 nm/°C across multiple interference dips using Cargille liquid (n_D = 1.46) as the core medium. Our findings further demonstrate that temperature sensitivity strongly depends on the interference wavelength and the order of core-cladding mode coupling: shorter wavelengths and lower-order cladding modes produce higher sensitivities. The results of the agreement support the robustness of the proposed dispersion-engineered configuration for realizing predictable and adaptive temperature sensitivity in liquid-core fiber interferometers.