This study introduces the design and analysis of two surface plasmon resonance (SPR) biosensors based on photonic crystal fibers (PCFs) coated with a gold layer, utilizing the finite element method (FEM) in COMSOL Multiphysics. The investigation focuses on two structural configurations: circular and elliptical air holes in the first cladding ring. The \(y\) -polarized mode, due to stronger coupling with the surface plasmon polariton (SPP) mode, provided enhanced sensitivity. Phase matching occurred at ~0.66 µm for an analyte refractive index of 1.37. The elliptical design showed higher confinement loss ( \(131.8\mathrm{ dB}/\mathrm{cm}\) ) compared to the circular one ( \(120.4\mathrm{ dB}/\mathrm{cm}\) ). As the analyte refractive index increased from 1.33 to 1.37, both confinement loss and wavelength sensitivity increased, with a red shift in resonance wavelength. Polynomial fitting showed strong linearity between resonance wavelength and analyte index, with adjusted \({R}^{2}\) values of 0.9923 and 0.9927 for circular and elliptical designs, respectively. Final sensor resolutions were \(3.22\times {10}^{-6}\mathrm{ RIU}\) and \(3.15\times {10}^{-6}\mathrm{ RIU}\) . These results highlight the critical role of hole geometry in optimizing SPR-PCF biosensor performance.
