Three-dimensional finite difference simulation of coupling behavior and loss in multimode interference devices

A three-dimensional finite difference method scheme for simulation of multimode interference couplers with arbitrary waveguide cross section is presented. With this method we investigate the influence of the multimode waveguide spectrum and of the access waveguide position on 3-dB coupling. These effects, which were not considered in previous theories on multimode interference structures, are shown to be important for the design of dielectric multimode waveguide devices. Taking these effects into account, fabrication tolerant 3-dB couplers with low insertion loss can be designed.<>