Theoretical modeling and investigations of AZO coated LMR based fiber optic tapered tip sensor utilizing an additional TiO2 layer for sensitivity enhancement

This article presents a theoretical model for lossy mode resonance (LMR) based fiber optic (FO) tapered tip sensor coated with aluminum doped zinc oxide (AZO) and titanium dioxide (TiO2) layer. In LMR based FO tapered tip sensor, sensing signal is measured in terms of retro-reflecting spectra whereas in conventional LMR sensors it is measured in terms of the transmittance. A major advantage of the proposed sensor is that it requires small sample volume. Also its compact tapered tip design makes it more suitable for point sensing. In this work, for the first time, a detailed analysis of AZO coated LMR based FO tapered tip sensor has been carried out. Additionally, the combined effect of AZO and TiO2 thickness ratio along with the fiber parameters such as fiber tip radius, tip length and numerical aperture on the reflectance spectrum has been investigated. The proposed geometry along with appropriate combination of various fiber parameters and the thin films thickness ratio provides approximately sixfold increase in the sensitivity compared to the conventional tapered LMR sensors. Sensitivity of the proposed sensor has also been compared with straight fiber geometry. Refractive index (RI) sensitivity of the proposed sensor is in the range of 1500-9000 nm/RIU.