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 (RI) sensor system based on D-shaped multimode optical fibers (MMFs) integrated with optical time-domain reflectometer (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 and $50~\mu $ m, 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 RI of a substance. The sensor based on the 62.5- $\mu $ m-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- $\mu $ m 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 the existing OTDR infrastructure are needed.