Mode-Dependent Radiation Loss in Y Junctions and Directional Couplers

Calculation of radiation loss in symmetric dielectric optical waveguide Y-junction power dividers and evanescent-field directional couplers is presented. The loss is calculated as a function of the separating angle for the odd and even exciting normal modes in several test structures. It is shown that for most practical cases, the even normal mode is more lossy than the odd normal mode. The difference increases with the separating angle and with the guidance strength. This radiation-loss difference is, in general, smaller for directional couplers than for Y-junction power dividers. Introducing a proper curvature to the output waveguides of a structure can serve to minimize this difference.