ASSESSMENT OF COUPLED-MODE THEORY FOR TAPERED OPTICAL COUPLER

The accuracy and the scope of validity of different coupled-mode formulations for tapered optical waveguide coupler are assessed by comparison with the beam propagation method (BPM). It is demonstrated that the coupled-mode theory (CMT) based on local modes is valid only for small tilt angles, whereas the newly developed improved formulation which takes into account the wavefront-tilt effect is able to yield highly accurate results for relatively large tilt angles. Furthermore, the radiation loss at the input and the output junctions can also be estimated by a simple mode matching method combined with the improved CMT. Finally, the coherent coupling between two junctions, which is neglected in the coupled-mode formulations, is observed in the BPM simulations.