Effects of normal mode loss in dielectric waveguide directional couplers and interferometers

Theoretical arguments and experimental evidence are presented to show that the two fundamental normal modes of a coupled waveguide structure have different attenuations in traversing such a structure. The effects of this phenomenon on evanescent wave directional couplers and interferometers are derived. Parasitic effects in Mach-Zehnder and Sagnac interferometers utilizing directional couplers are described. An asymmetric output for the recently demonstrated all-single-mode fiber resonator is predicted and compared to experimental results. Some qualitative results are presented for integrated optic directional coupler switches.