Theory and numerical design of coupled-resonator optical waveguide sections with bends

We present a theoretical analysis and numerical design of bent and branched sections of coupled resonator optical waveguides (CROWs) composed of side-coupled whispering-gallery (WG) mode microdisk resonators. Our analysis is based on a rigorous Muller boundary integral equations method that enables accurate treatment of CROWs consisting of both identical and different microdisks as well as studying CROW finite-size effects. Differences in WG modes coupling in the vicinity of bends in CROWs composed of optically-large and wavelength-scale microcavities are revealed and discussed. We propose possible ways of pre- and post-fabrication tuning of bent CROW sections. At the pre-fabrication design stage, adjusting the radius of the microdisk positioned at the CROW bend may yield significant reduction of bend losses for any chosen CROW bend angle. Post-fabrication tuning capability of the designed structures is also discussed.