Structural parameter optimization and mode-dependent sensing performance analysis of a D-shaped multimode fiber SPR sensor

Abstract

The core refractive index of conventional pure SiO2 optical fiber (approximately 1.45) is lower than that of typical industrial lubricating oils (n ≈ 1.47), rendering it incapable of satisfying the phase-matching condition for surface plasmon resonance (SPR) excitation. To address this limitation, this paper proposes a D-shaped SPR refractive index sensor based on a commercial germanium-doped multimode fiber (Ceram Optec Optran Ultra WFGE, core diameter 200 μm, cladding diameter 220 μm, NA = 0.37 ± 0.02, core refractive index about 1.491). The GeO2 doping elevates the core refractive index to approximately 1.491, enabling SPR excitation in high-refractive-index lubricating oil media. Using the COMSOL Multiphysics finite element simulation platform, a simultaneous two-dimensional joint scan of the polishing depth (70–110 μm) and Au film thickness (20–40 nm) is performed with the figure of merit ( FOM) as the optimization objective. The sensing performance is then evaluated over an analyte refractive index range of 1.44–1.48, and four modes (LP01, LP11, LP21, LP02) are systematically compared. Simulation results show that the optimal parameters are a polishing depth of 110 μm and an Au film thickness of 25 nm. With a 10 mm sensing region, the resonance wavelength redshifts from 751 nm to 1483 nm, with the piecewise sensitivity increasing superlinearly from 6200 nm/RIU (1.440–1.445) to 48 600 nm/RIU - 1 (1.475–1.480). The FWHM remains at 34–35 nm over 1.44–1.46, increases to 41.1 nm at n = 1.47, reaches 50.1 nm at n = 1.475, and broadens to 68.5 nm at n = 1.48; the maximum piecewise FOM reaches 820 RIU−1. The LP02 mode achieves the deepest dip (about 2.8 times that of LP01), but undergoes avoided-crossing degradation at n = 1.48, with its FWHM abruptly increasing from 35.7 nm to 146.5 nm. The LP11 and LP21 modes achieve a superior balance between dip depth and bandwidth, maintaining complete coupling across 1.44–1.48.