Increasing the sensitivity of a refractive fiber-optic sensor based on surface plasmon resonance and MMF-SMF-MMF fiber using dielectric coatings

This paper presents theoretical and experimental research on improving the sensitivity of a refractive fiber-optic sensor operating on the principle of surface plasmon resonance (SPR). The sensitive element consists of a multimodesinglemode-multimode (MMF-SMF-MMF) fiber structure. To induce the SPR effect, the single-mode fiber section is sequentially coated with metal (Cu) and dielectric layers (Al 2 O 3 , TiO 2 ), which results in narrower resonance peaks. This enhances wavelength shift detection and increases sensor sensitivity. Mathematical modeling of the sensitive element with a multilayer surface structure was conducted using characteristic matrices. Each layer of the sensitive element was individually characterized, followed by the formulation of the overall characteristic matrix to calculate the transmission coefficient. Based on the simulation results, optimal dielectric coatings and layer thicknesses were selected to achieve the narrowest resonance peak. To validate the simulation findings, sensitive element samples with dielectric coatings of Al 2 O 3 (60 nm and 100 nm) and TiO 2 (50 nm and 100 nm) were fabricated. Transmission spectra were obtained in air, water, and ethanol. The results demonstrate that the proposed coating increases the sensitivity of the fiber-optic SPR sensor threefold compared to an uncoated sensitive element. The proposed approach not only enhances sensor sensitivity but also shifts the resonance peaks into the infrared spectral region. Additionally, the study highlights the feasibility of using more accessible fiber-optic components for the investigated sensor system.