<![CDATA[Background: Surface Plasmon Resonance (SPR) is an optical sensing method with high sensitivity to refractive index changes near a metal surface. Integrating SPR with D-shaped optical fibers provides compact dimensions, flexibility, resistance to electromagnetic interference, and real-time sensing capabilities. Sensor performance is affected by the plasmonic material, metal thickness, and incorporation of two-dimensional materials such as graphene oxide (GO). Objective: This study analyzes the characteristics of a D-shaped fiber-optic SPR sensor with a double-layer Cu/GO structure for detecting refractive index changes in NaCl solutions using the Finite-Difference Time-Domain (FDTD) method in Ansys Lumerical. Methods: The study compared Ag and Cu as plasmonic materials, evaluated the effect of GO addition, optimized the Cu thickness from 30–60 nm with a fixed GO thickness of 5 nm, and optimized the GO thickness from 0.5–30 nm. Sensor performance was evaluated based on the resonance wavelength, sensitivity, full width at half maximum (FWHM), and figure of merit (FOM). Results: Cu produced better resonance characteristics than Ag in the investigated configuration. Addition of GO induced a redshift in the resonance wavelength and enhanced the sensor response to refractive index changes. Variations in Cu and GO thickness demonstrated that sensor performance depends on their combined configuration. Optimization identified a configuration of 45 nm Cu and 5 nm GO provided a favorable balance among sensitivity, resonance sharpness, FWHM, and FOM. Conclusion: Optimized Cu/GO configuration shows potential as a basic structure for D-shaped fiber-optic SPR sensors with improved sensitivity for refractive index detection and future biosensing applications.]]>
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