Based on digital signal processing theory, a simple method of designing interleavers is proposed for MZI with fiber ring coupled structure. The results of different coupled structures, including no-ring, single ring coupled on one arm of MZI, single ring coupled on each arm of MZI, two rings coupled on one arm of MZI while one ring coupled on another arm and two rings coupled on the both arms of MZI structures are given and analyzed. The proposed method is not only simple but also effective. At last, take two vertically cascaded rings coupled on one arm and one ring coupled on another arm for instance, the change of spectral transmittance is investigated for each design parameters deviating from ideal value and the tuning characteristics are discussed as well.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.