A model of a refractive fiber optic sensor sensing element based on MMF-SMF-MMF structure using surface plasmon resonance

This paper presents a mathematical model of the sensitive element of a refractometric fiber-optic sensor the principle of operation of which is based on the phenomenon of surface plasmon resonance. The sensing element design is a sequential connection of a multimode fiber (MMF), a single-mode fiber (SMF), and a multimode fiber forming an MMF-SMF-MMF structure. The SMF site is coated with a thin film of gold. To model the element, the approach used in calculating the classical Kretschmann configuration for volumetric optical structures was applied. The refractive index of the fiber is calculated based on the Sellmeyer equation, and the refractive index of the gold is determined using the Drude model. The simulation results are compared with experimentally obtained transmission spectra of fabricated samples of sensing elements. For approbation of the model, the sensing elements of fiber-optic sensors with the following parameters are made: core diameter of multimode fiber 62.5 μm, core diameter of singlemode fiber 9 μm, coating SMF-segment with 50 nm gold film. Transmission spectra of fiber-optic sensor sensing elements in aqueous glucose solutions of various concentrations were obtained. It is demonstrated that the proposed model describes well the experimentally obtained transmission spectra of sensitive elements based on MMF-SMF-MMF structures in the region of surface plasmon resonance. The proposed model can be used to optimize the design of the sensitive element of refractometric fiber-optic sensors in order to increase the sensitivity. The proposed model implies its use in the development of an algorithm for interrogation of sensing elements based on fiber MMF-SMF-MMF structures.