Longitudinally shifting waveguide junctions can achieve high efficiency light coupling from external silica waveguides into photonic crystal waveguides (PCWs) without requiring taper PCW structures in the waveguide junctions. Due to the periodic field distribution of PCWs along the propagation direction, maximum and minimum coupling efficiency occur when waveguide junctions are located near the peak and valley of the longitudinal mode profile, and coupling efficiency depends on both the localization of mode fields in PCWs and the group velocities of eigenmodes. Coupling efficiency of 86% between silicon/silica PCWs and external silica waveguides can be achieved without the need for special design in the junction region for the reduced-rod PCW. Due to large Bragg reflection, coupling of the band edge modes at the flat dispersion regions that possess small group velocities is inefficient. (C) 2004 Elsevier B.V. All rights reserved.
