Efficiency improvement in codirectional coupling between stacked waveguides using grating couplers

When two stacked waveguides are coupled by a grating coupler, the coupling efficiency is strongly periodically dependent on the waveguide spacing. In codirectional coupling using a uniform grating coupler, the coupling effici ency is at most 25%. In this paper, an enhancement of the coupling efficien cy of codirectional coupling is discussed. The waveguide spacing dependence of the coupling efficiency is measured. It is experimentally confirmed tha t optimization of the waveguide spacing is indispensable for enhancing effi ciency. We theoretically study a method for improving the coupling efficien cy in which the coupling coefficient is nonuniformly distributed along the propagation direction of the guided wave in the coupling region, so that th e amplitude distribution of the radiation mode associated with the output g uided mode is made to coincide with that of the radiation mode associated w ith the input guided mode. It is shown that an efficiency higher than that obtained in codirectional coupling can be expected. (C) 1999 Scripta Techni ca, Electron Comm Jpn Pt 2, 82(6): 1-7, 1999.