Vertically tapered epilayers for low-loss waveguide-fiber coupling achieved in a single epitaxial growth run

Next-generation optical-communications systems require on-wafer integration of active and passive optoelectronic components to increase operating speed and reduce packaging costs. Increased coupling efficiencies between semiconductor waveguides and optical fibers are of particular interest. A simple and cost-effective method of fabricating a mode-size converter monolithically integrated with a semiconductor waveguide is presented. An on-wafer mode-size converter reduces the number of interfaces in an opto-electronic circuit and improves the coupling efficiency between semiconductor waveguide and optical fiber. Vertically tapered epilayers are deposited in a single epitaxial growth run using shadow-masked growth by chemical-beam epitaxy, avoiding complex and expensive processing and regrowth stages. Waveguides that taper vertically and horizontally over ~ 1 mm for gradual expansion of the mode size are demonstrated. Waveguide loss measurements showed that there was negligible loss across the tapered regions. A loss of <2 dB/interface was achieved compared with ~8 dB/interface for a butt-coupled discrete device.