Scattering-parameter model analysis of side-coupled plasmonic Fabry–Perot waveguide filters

We modeled a compact side-coupled plasmonic Fabry–Perot (FP) waveguide filter using a combination of several fundamental waveguide parts, and compared its characteristics with those obtained using finite-difference time-domain (FDTD) simulations. The equivalent model of the proposed filter was regarded as three cascaded resonators, two symmetric overlapping regions as two directional couplers (DCs), and one side-coupled cavity as an FP resonator. A scattering matrix (S-matrix) analysis was employed to reveal the origin of the spectral responses. The obtained characteristics, such as different peak transmittance (mainly influenced by the DCs’ overlapping length) and wide free spectral range (FSR) between two resonance modes, showed a good agreement with those obtained using FDTD simulations. By suppressing the neighboring resonance mode, through an adjustment of the DCs’ overlapping length, we demonstrated a potential design of a 1 × 2 ultra-wide-FSR wavelength demultiplexing filter.