Simultaneous Multipoint Refractive Index Sensing Using FIB-Fabricated D-Shaped Optical Fiber Sensors Interrogated via OTDR

This study presents the design, fabrication, and experimental validation of a refractive index sensor system based on D-shaped multimode optical fibers integrated with optical time-domain reflectometry (OTDR). The sensor structure was developed by removing the cladding material to expose the fiber core with xenon plasma focused ion beam (Xe-PFIB) milling. Two types of graded-index multimode fibers, with core diameters of 62.5 μm and 50 μm, were used to investigate the influence of core size on sensor performance. In the experiment, the refractive indices of glycol-based solutions, ranging from 1.336 to 1.432, were measured using an OTDR system. The experimental results revealed a linear relationship between the normalized reflected light intensity and the refractive index of a substance. The sensor based on the 62.5-μm-diameter core fiber exhibited higher sensitivity, achieving a maximum of 1.169 a.u./RIU at 850 nm and 0.843 a.u./RIU at 1300 nm. In comparison, the sensor with a 50-μm core diameter reached sensitivities of 0.165 a.u./RIU at 850 nm and 0.285 a.u./RIU at 1300 nm. Simultaneous measurement with multiple sensors along an optical fiber was successfully demonstrated, confirming the feasibility of multipoint sensing with a standard OTDR system. These findings highlight the potential of the proposed system for real-time, remote monitoring applications where simple configuration, scalability, and compatibility with existing OTDR infrastructure are needed.