Novel Planar Waveguide-Coupled D-Shaped Optical Fiber Sensor to Generate Fano Resonance for Enhanced Refractive Index Sensing Applications

In this article, the generation of Fano resonance (FR) in a novel optical fiber platform, which addresses a significant challenge within the scientific community, is theoretically investigated. The proposed sensor is designed with a D-shaped surface plasmon resonance (SPR) fiber coupled with a three-layer planar waveguide (PWG) structure for inline and enhanced refractive index (RI) sensing applications. Our analysis demonstrates that an optimum thickness of low index dielectric material, i.e., cytop fluoropolymer, as a coupling layer is required to generate FR in association with SPR. It is observed that the FR demonstrates a significant enhancement in the figure of merit (FOM), achieving 6383 RIU<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for wavelength interrogation and 13 195 a.u./RIU for intensity interrogation at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d} _{f} \,\, =520$ </tex-math></inline-formula> nm and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d} _{c} \,\, =700$ </tex-math></inline-formula> nm. These values greatly surpass the FOM of conventional SPR-based sensors, which are 34.90 RIU<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 39.96 a.u./RIU. Also, the FOM increases by increasing the thickness of coupling layer. Furthermore, FWHM of the FR is consistent with the length of D-shaped region, whereas FWHM of SPR increases as the length of D-shaped region increases. The penetration depth of FR mode’s evanescent field in the sensing region also increases with the film layer thickness, consistently exceeding the penetration depth of SPR (122.47 nm). Hence, the FR mode is proposed as the sensing signal instead of conventional SPR mode because it offers superior performance compared in terms of FOM and penetration depth.