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

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 mu m; 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 mu m core and -2.5 mm bend diameter demonstrate the highest RI sensitivity of -6 Delta A/RIU, measured as a change in absorbance (Delta 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 mu m, 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.

Automated summary

This study presents a ray-optics based model for the multimode U-bent silica fiber optic refractive index (RI) sensor probes. Experimental investigation was conducted to determine the optimum fiber core and bend diameter, as well as the optimal bend ratio. The results showed that U-bent probes with a specific core diameter exhibited higher RI sensitivity within a certain range of bend ratios. Further investigations using wet chemical step-etching of the silica fiber core showed improved RI sensitivity until a certain waist diameter, followed by a drop. A compact LED-photodetector based dip-type handheld refractometer was also demonstrated for potential analytical applications.