Linewidth narrowing in silver-coated fiber surface plasmon resonance sensors through waveguide-structure-dependent modal regulation

Broad resonance dips limit the spectral resolution and figure of merit (FOM) of wavelength-interrogated fiber surface plasmon resonance (SPR) sensors. Here, silver-coated no-core fiber (NCF), multimode fiber (MMF), and hollow-core fiber (HCF) probes were compared under matched sensing conditions to clarify the influence of host-fiber waveguide structure on SPR linewidth. Experiments over a refractive-index range of 1.333–1.420 showed similar resonance-shift and sensitivity trends for all three probes, while the measured full width at half maximum consistently followed NCF > MMF > HCF. The HCF probe therefore achieved the highest FOM over most of the tested range. Finite-element analysis further showed that NCF supports SPR-active modes with more diverse field distributions and coupling strengths, whereas the annular silica waveguide of HCF constrains the spatial and polarization characteristics of the surface-sensitive modes. In particular, the radially divergent HCF mode exhibited the strongest resonance loss and the narrowest calculated modal peak. Taken together, these results indicate that the observed linewidth narrowing is closely associated with waveguide-structure-dependent regulation of the dominant modal ensemble, providing a potential route to improve fiber-SPR spectral characteristics without introducing additional multilayer films, gratings, or complex micro/nanostructures.