Spatial resolution requirements in the fabrication of branching optical waveguides

A multimode optical waveguide trunk that branches into several single-mode waveguides, all of different cross sections, can route each of the modes of the trunk into a different output channel. Such branching waveguides (branches) are important in splitting and recombining optical signals in guided-wave optoelectronic devices. In practical optical waveguide branches, however, the channel separation is not achieved continuously, but rather by discrete increases in the distance between the branching channels. We show that this finite resolution in the pattern of branching waveguides may give rise to an increased intermode crosstalk, which severely deteriorates the mode routing features of these devices. This increased crosstalk arises from constructive interference of the scatterings which occur at the discontinuities along the branch. Our analysis provides design guidelines for minimizing this intermode crosstalk.