Next-generation optical-communications systems require on-wafer integration of active and passive opto-electronic 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.
R. S. Balmer, J. M. Heaton, J. O. Maclean, S. G. Ayling, J. P. Newey, M. Houlton, P. D. J. Calcott, D. R. Wight, and T. Martin, "Vertically Tapered Epilayers for Low-Loss Waveguide-Fiber Coupling Achieved in a Single Epitaxial Growth Run," J. Lightwave Technol. 21, 211- (2003)