Crosstalk and switching characteristics in directional couplers

The normal mode approach is used to calculate the fundamental limits on crosstalk and switching characteristics in directional coupler switches of the stepped delta beta type. The normal modes are calculated numerically as well as analytically using a method for computing the eigenmodes of two coupled, identical waveguides with an antisymmetric index perturbation. The results of the two methods are compared to one another as well as to the results of coupled-mode theory. It is shown that the theoretical minimum for the crosstalk level is very low for the stepped delta beta structure. Nonzero crosstalk was found in some cases where coupled-mode theory predicts zero crosstalk. This is attributed to the improper handling of discontinuities in coupled-mode theory