Cross-talk problems in optical directional couplers

Abstract

Cross-talk in optical directional couplers is usually considered to be limited only by the processing technology. We show that unequal coupling of an input wave to the even and the odd mode of the coupling region gives rise to another kind of cross-talk. The extinction ratio is well approximated by the overlap integral of the fields in the two waveguides. For a given coupling length, the higher the index step a waveguide has, the less the cross-talk is. For straight couplers without bends one way to eliminate the cross-talk is to offset the input wave relative to the coupled guides with the penalty of power loss. A three-waveguide coupler has a different cross-talk problem of comparable magnitude.