Wavelength characteristics of (2*2) optical channel-type directional couplers with symmetric or nonsymmetric coupling structures

The wavelength dependence of waveguide-type directional couplers is investigated with respect to waveguide pattern symmetry in the coupling region. A rough approximation of the dependence is determined based on simplified transfer matrix multiplications, followed by a more detailed computer calculation using the beam propagation method (BPM). Wavelength-flattened coupling characteristics over a wide range, for example, from 1.33-1.55 mu m, are predicted for several types of directional coupler with waveguide width asymmetry or a tapered width structure in the coupling region.>