A more accurate analysis for the coupling between two rectangular dielectric waveguides laid in different layers

The coupling characteristics of crossed-over rectangular dielectric waveguides laid in different layers are analyzed using the coupled-mode theory with the individual waveguide modal fields obtained from solving the corresponding exact boundary-value problem. The results agree with experimental data reported earlier. By comparing these data to previous analyses which employed approximate field expressions. it is found that using more accurate modal fields for the individual waveguides is essential in achieving correct prediction from coupled-mode analysis. In the case considered, the mode overlap integral which describes the crosstalk between the waveguides is found to be small so that the differences in the coupling strength as predicted by the conventional coupled-mode theory and the new theory are negligibly small. It is shown that when the conventional theory is applied, the mode overlap integral has to be included in the calculation of the coupled power in order to better explain the measured results