Performance evaluation of photonic crystal ring resonators based optical channel add-drop filters with the aid of whispering gallery modes and their Q-factor

As a basic type of linear defects, the PhC ring resonators are considered the most fascinating elements to be used in photonics integrated circuits for applications such as dense wavelength division multiplexing and Optical filtering that are among the most important components of the telecommunication systems. This article proposes two different optical channel add-drop filters (CDFs) based on rod-type two-dimensional square-lattice all-circular photonic crystal ring resonator. In the studied ring-type PhC cavity, there are some modes that are analogous to whispering gallery (WG) modes. For the proposed all-circular PhC ring resonator, the WG-like mode with the azimuthal mode number \(m=10\), couples out from cavity to the drop waveguide. Although because of the absence of perfectly circular symmetry, these WG-like modes are not exactly degenerate but they form a close doublet. The normalized frequencies (\(a/\lambda \)) of the doublets of \(m=10\) are \(a/\lambda =0.3684\), and 0.3645 and their \(Q\)-factors are 1050, 866 respectively. By selecting appropriate coupling distance between the PhC ring resonator and side-coupled \(W_{1}\) waveguide, the CDFs are formed. For a TM polarized Gaussian source, the drop efficiency of both filters is more than 99.8 % in the 1.535–1.625 \(\upmu \hbox {m}\) wavelength interval. The photonic bandgap and the WG-like modes of the PhC ring resonator are calculated using the PWE method, and the \(Q\)-factor of modes and the transmission spectra of CDFs are calculated using 2D-FDTD method.