High-performance D-shaped optical fiber refractive index sensor based on coupling between a silicon subwavelength grating and surface plasmons

Abstract To simultaneously achieve high spectral resolution and efficient fixed-wavelength intensity interrogation in D-shaped fiber refractive-index sensing, a waveguide-coupled D-shaped optical fiber sensor integrating an indium tin oxide (ITO) film, a Cytop coupling layer, and a silicon subwavelength grating (Si-SWG) is proposed. The transmission spectra, electric-field distributions, and refractive-index responses of the proposed sensor are numerically investigated using the finite element method (FEM), with particular emphasis on the effects of the Si-SWG thickness, Cytop coupling-layer thickness, and sensing-region length on the resonance lineshape and figures of merit (FOMs). The analysis shows that appropriate optimization of the waveguiding and coupling-layer dimensions produces a pronounced Fano resonance with well-defined peak–dip characteristics and a steep spectral slope in the near-infrared region. Under their respective optimized structural conditions, the maximum intensity-interrogation figure of merit (FOMI) reaches 3442.17 a.u./RIU, while the maximum wavelength-interrogation figure of merit (FOMλ) reaches 8620.69 RIU⁻¹. To assess the operating range associated with these sensing responses, local refractive-index analysis is performed over ns = 1.3300-1.3303. The wavelength response is approximately linear over this interval, whereas the fixed-wavelength intensity response exhibits a turnover, indicating that high local intensity responsivity does not necessarily ensure a broad linear sensing range. The sensitivity to geometric deviations is further examined through tolerance analysis, which shows that ±2% deviations in the ITO-film and residual-cladding thicknesses produce only small changes in the resonance positions and peak-to-dip separation, whereas deviations in the Si-SWG thickness and duty cycle appreciably affect the sensing figures of merit. These findings support the potential of the proposed compact fiber-optic platform for in-line near-infrared refractive-index sensing using wavelength and fixed-wavelength intensity interrogation, while identifying operating-range and dimensional-control requirements for its implementation.