(color online) Three-dimensional schematic view of our proposed graphene-based planar band-pass filter consisting of two graphene ribbons as input/output ports and a graphene ring between them as a resonator. The input/output ribbons have a gate voltage value of V_G, and the graphene ring resonator has a gate voltage value of V_G-r. The inner and the outer radii of the ring, the graphene ribbons width, the lengths of the input/output ribbons, the distance between the input/output ribbons, and the distance between the left end of the output ribbon and the center of the ring are R₁, R₂, W, L, h, and d, respectively. Two monitors at the points P_in and P_out measure the input and output optical powers. One dipole point source is placed 2 nm above the input ribbon to excite SPPs. Only the resonance wavelengths of the ring resonator transmit effectively to the output ribbon port. The structure should be located on a dielectric substrate. In our FDTD simulations, we assume the substrate is air.