Apodization of coupled resonator optical waveguide devices through a longitudinal offset technique

摘要

In this paper, a novel technique to set the coupling constant between cells of a coupled resonator optical waveguide (CROW) device, in order to tailor the filter response, is presented. It is known that using the same K value for all the couplers produces filtering responses with significant side-lobes for the side-coupled integrated spaced sequence of resonators (SCISSOR) or significant ripples in the pass-band for the direct coupled microrings (CROW). It is also known that the side-lobes/ripples can be reduced, and the pass/reject bands can be made wider, by apodizing the K value of each individual coupler in the structure, starting from a nominal K value (either increasing or decreasing it). This technique consists on changing the effective length of the coupling section by applying a longitudinal offset between the resonators. On the contrary, the conventional techniques are based in the transversal change of the distance between the ring resonators, in steps that are commonly below the current fabrication resolution step (nm scale), leading to strong restrictions in the designs. The technique has been experimentally demonstrated employing a racetrack ring resonator geometry. The proposed longitudinal offset technique allows a more precise control of the coupling and presents an increased robustness against the fabrication limitations, since the needed resolution step is two orders of magnitude higher. Both techniques are compared in terms of the transmission response of CROW devices, under finite fabrication resolution steps. The offset technique presented is sufficient by itself for apodization, and optimized CROW's can be produced with a fixed distance between the rings, solely by changing the offsets.