Strip-loaded Mach–Zehnder interferometer for absolute refractive index sensing

This paper presents the design, fabrication, and characterization of a Mach–Zehnder interferometer (MZI) on a strip-loaded platform specifically developed for the measurement of refractive index variations in liquids. A novel approach of large opening in the sensing region enhances the sensitivity by using the analyte as the loading strip. This allows surpassing the performances of the conventional perforations and trenched-based MZIs and prevent protecting the reference arm. The implementation of this design resulted in a high confinement factor of approximately 23 \(\%\) $\%$ in the cladding, enabling an effective interaction between the evanescent field and analyte. Ethanol-water solutions with varying concentrations were used as analyte for the characterization of the device. The strip-loaded waveguide was a 200 nm TiO \(_2\) ${}_2$ thin film on a silicon dioxide wafer and a 1.2 \(\upmu \) $\mathrm{\mu }$ m wide for 800 nm-thick nLOF resist served as loading strip. The sensing area featured a 10 \(\upmu \) $\mathrm{\mu }$ m wide open-ing in the loading material with a sensing length of 5 mm. The homogeneous sensitivity was experimentally determined to be 0.128, demonstrating the effectiveness of the proposed design, which enabled a refractive index change monitoring of 10 \(^{-6}\) ${}^{-6}$ .