Design of an ultracompact MMI wavelength demultiplexer in slot waveguide structures

Based on a multimode interference (MMI) coupler in slot waveguide structures, an ultracompact wavelength demultiplexer operating at 1.30 and 1.55mum wavelengths is proposed and designed by using a full-vector mapped Galerkin mode solver and a modified three-dimensional full-vector beam propagation method. The tapered waveguide structures are applied to connect the input/output channels and the MMI section for reducing excess loss. The modal characteristics of the slot waveguides are analyzed and the evolution of the injected field in whole device are demonstrated. The results show that a MMI section of 119.8mum in length, which is only 27.5% length of that of the MMI coupler by using conventional rib waveguides, is achieved with the contrasts of 26.03 and 28.14dB at wavelengths 1.30 and 1.55mum, respectively, and the insertion losses are below 0.2dB at both wavelengths.