Metal-free plano-convex lens-based optical probes for high-sensitivity and chemically resistant refractive index sensing

We report the development of a metal-free optical fiber probe incorporating a plano-convex (PC) lens for refractive index (RI) sensing of liquids. The probe structure employs chemically robust materials, including perfluoroalkoxy alkane (PFA) for the housing and either quartz or sapphire for the lens. Optical fibers were aligned across a 5-mm-diameter PC lens, and the effects of fiber spacing (D1) and lens-to-reflector distance (D2) on RI sensitivity were evaluated. Light intensity varied significantly with D1, with an optimal range of 4.2-4.6 mm, enabling enhanced detection of RI changes in sucrose solutions. The sapphire-based probe exhibited approximately 20-fold higher sensitivity than the quartz-based version and maintained excellent linearity (R² = 0.999) over the RI range of 1.33-1.45. Spectroscopic capabilities were also assessed using potassium permanganate, demonstrating that quartz-based probes more accurately captured the characteristic absorption features. These findings highlight that PC lens-based fiber probes offer a tunable, non-metallic, and chemically durable platform for RI sensing with high linearity and resolution, making them suitable for use in chemically aggressive environments.