Sensitivity enhancement of RI sensor based on SMS fiber structure with high refractive index overlay

We propose a novel way to enhance the sensitivity of single-mode-multimode-single-mode (SMS) fiber structure in the measurement of surrounding refractive index (RI), which is realized by depositing the multimode fiber section with a high RI overlay. First, as an essential basis of the analysis of the novel structure, characteristics of mode reorganizations in the deposited multimode fiber are discussed. Second, based on full-vector mode expansion, a theoretical analysis of transmission characteristics of the deposited SMS fiber structure is presented. Third, aimed at RI measurement ranges which are of common interest in liquid and gas sensing, the transmission spectra of the structure in response to the surrounding RI are investigated by simulations. To well utilize the collective effect of multiple interference modes in mode reorganizations, the monitoring based on wavelength shifts of valleys and the optimization of the overlay are analyzed for various application requirements. Lastly, largely enhanced sensitivities for RI measurements are demonstrated with properly designed overlay. For RI ranges from 1.31 to 1.35 and 1.0 to 1.03, average sensitivities of 900 and 206 nm/RIU are achieved with overlay thicknesses of 340 and 470 nm, respectively.