Design of a single mesa vertical coupler

Abstract

A new design of vertical coupler (VC) based on only a single mesa is presented. The newly proposed vertical coupler has the potential of improving the fabrication yield, as it requires much simpler fabrication process. The vertical coupler comprises a large underlying rib waveguide to which light is coupled. The lower waveguide is formed by the loading effect provided by the smaller upper ridge waveguide. The upper ridge waveguide contains the actual device (e.g. modulator or photodetector) and is tapered in a way to maximize the vertical transfer of light from the lower waveguide, and thus the vertical coupler can be used as a spot-size converter (SSC) between a single-mode fiber and the actual device.. We investigate the transfer efficiency of the single-mesa vertical coupler with different configurations. The single-mesa design is found to give a lower total loss (i.e., fiber coupling loss plus transfer loss) compared with direct coupling to the actual device. The transfer efficiencies obtained are more than 90% for both TE and TM polarizations, and are polarization independent.