Compact and low-loss RGB coupler using mode-conversion waveguides

We developed a silica optical-waveguide-type red-green-blue (RGB) multiplexer called an RGB coupler. It is a two-stage color multiplexer with a novel mode converter. In successive stages green and red light are combined with blue light accumulatively almost without leakage of recoupling. The mode converter has an additional multimode waveguide that is wider than the single-mode waveguides and is sandwiched between the single-mode waveguides of a conventional directional coupler. It converts the propagation mode of the single-mode waveguide into a higher-order mode of the additional multimode waveguide. Using a higher-order mode enhances the isolation of colors and enables us to reduce the circuit length. In this study we fabricated an RGB coupler about 2.5-mm long and 1-mm thick, and for each color the loss, excluding that of input/output routing circuits, is less than 1.2 dB.