Novel bent-tapered mode converting multimode optical fiber sensor based on Evanescent Wave Absorption

In this study, design and fabrication of a novel bent-tapered optical fiber sensor for enhanced Evanescent Wave Absorption (EWA) based sensing application is demonstrated. A combination of bending and tapering acts as a mode converter, which results in high penetration depth of evanescent field. In addition, tapered region of the probe increases the coupling efficiency at the detector end by V-number matching and thus helps in improving the signal to noise ratio significantly. Effect of taper ratios (ranging from 0.17 to 1) was investigated using fluorescein isothiocyanate (FITC) dye as a model analyte, binding directly to the functionalize fiber. Taper ratio of 0.37 showed the highest sensitivity towards FITC, and was found to be 2.4-fold better compared to untapered bent probe of similar length. The larger penetration depth coupled with higher sensitivity will be beneficial for sensing application involving larger analytes e.g. bacteria and virus.