LMR and SPR induced Fano resonance in a planar waveguide-coupled D-shaped optical fiber for enhanced refractive index sensing in the Vis–NIR region

This study examines the performance of an indium tin oxide coated D-shaped optical fiber (ITO-DOF) sensor and a planar waveguide-coupled indium tin oxide coated D-shaped optical fiber (PWG-DOF) sensor for inline refractive index sensing applications. The ITO-DOF sensor enables lossy mode resonance in the visible region and surface plasmon resonance in the near-infrared region. The PWG-DOF sensor enables the simultaneous generation of Fano resonance in both regions by utilizing lossy mode resonance and surface plasmon resonance effects across the visible and near-infrared regions. It is observed that the PWG-DOF sensor achieves a higher figure of merit than the ITO-DOF sensor due to its narrower full-width at half-maximum. Also, the penetration depth of the Fano resonance mode is recorded at 127.99 nm in the visible region and 500.81 nm in the near-infrared region, surpassing lossy mode resonance (126.75 nm) and surface plasmon resonance (499.91 nm). These values increase with film thickness, highlighting the Fano resonance as a superior sensing signal. Given its improved figure of merit and penetration depth, this study suggests that Fano resonance can enhance the sensitivity and performance of refractive index sensors beyond conventional lossy mode resonance and surface plasmon resonance techniques.