Abstract

A multimode waveguide that branches into several single-mode guides, all of different propagation constants, can route each mode into a different output channel. Such branching waveguides are important in splitting and recombining optical signals in guided-wave optoelectronic devices. A branching waveguide is fabricated as a sequence of waveguide sections, joined end to end. The distance between the parallel guiding channels is stepped discontinuously at the junctions between sections. Preserving this feature of the fabrication in our model, we first determine the local guided modes (supermodes) in each section. The field in each section is then represented as a superimposition of the guided modes of that section, with mode coefficients determined to satisfy approximately the boundary conditions at the junctions between sections. The step size of the fabrication process and the section length define an angle. We find that to achieve low intermode crosstalk, this angle must be small, of the order of a few milliradians. We find a range of angles over which the crosstalk decreases with decreasing angle, almost independently of the step size. For still smaller angles, there is a section length dependence caused by interference of the intermode scattering off the junctions.

© 1987 Optical Society of America