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Description
This study presents a systematic investigation into the chemical etching kinetics, geometric optimization, and evanescent field dynamics of unclad multimode optical fibers for refractive index sensing applications. Cladding removal was controlled using hydrofluoric acid (HF) at distinct concentrations of 30.6% and 49%. Experimental results demonstrate that the etching rate is highly dependent on both acid concentration and fiber core dimensions; a 49% HF concentration significantly accelerated the cladding removal process down to 3 minutes for a 105 µm core fiber, compared to 20 minutes for a 50 µm core and 32 minutes for a 9 µm core fiber. The etching deceleration observed near the core boundary is primarily attributed to the structural composition and presence of dopants within the fiber core. Furthermore, mathematical modeling of the evanescent wave penetration depth within U-shape fiber configurations was conducted at a baseline wave-length of 1310 nm. Analytical assessments indicate that larger core diameters (105 µm versus 50 µm) yield an enhanced penetration depth, increasing from 0.218 µm to 0.236 µm at a launch height of h = 0 µm, thereby improv-ing core-ambient field interactions. Conversely, smaller core diameters optimize the spatial reflection frequency along the interaction zone. The penetration depth expanded significantly as the surrounding refractive index approached the core index, yielding intentional, measurable optical power leakage. These findings offer a comprehensive structural framework for designing highly optimized, geometrically tailored evanescent field fiber sensors.
