Step-etched U-bent silica fiber optic probes – Design and optimum geometry for refractive index sensing

Abstract

This study presents a ray-optics based model for the multimode U-bent silica fiber optic refractive index (RI) sensor probes, followed by a detailed experimental investigation to identify the optimum fiber core (d) and bend diameter (D), and the optimal bend ratio (D/d). U-bent probes with different geometries (d = 200, 400 and 600 µm; D = 1.0, 1.5, 2.0, and 2.5 mm) were fabricated and evaluated for their RI sensitivity in terms of refractive losses. Among the bend ratios between 1.7 and 12.5, the U-bent probes with a bend ratio between 3 and 5 exhibit a higher RI sensitivity for a given core diameter. Among the twelve different geometries tested, U-bent probes with 600 µm core and ~2.5 mm bend diameter demonstrate the highest RI sensitivity of ~6 ∆A/RIU, measured as a change in absorbance (∆A) per refractive index unit (RIU). Further investigations to improve light-medium interactions by wet chemical step-etching of the silica fiber core show a gradual improvement in the RI sensitivity until a etched waist diameter of 200 µm, followed by an abrupt drop. At least a two-fold improvement in the RI sensitivity was observed. A compact LED-photodetector based dip-type handheld refractometer was realized as a proof-of-the-concept to demonstrate the feasibility of using these etched probes in reusable or disposable form for RI based analytical applications.