Abstract

In fiber-optic communication systems, such functions as signal generation, switching, modulation, filtering, and multiplexing will likely be done in a "planar" geometry, consistent with planar photolithographic fabrication methods. The mismatch between the fiber and the typically smaller semiconductor waveguide mode of the planar photonic device can result in substantial insertion loss. Mode matching can be accomplished by tapering the waveguide cross-section. Although modem fabrication methods allow a waveguide cross-section to be tapered in both width and thickness, fabrication is simpler if only the width is tapered. We have shown that, with an appropriate layer structure of the waveguide, control over both transverse dimensions of the mode can be achieved by tapering only the rib width of a rib waveguide. The concept is illustrated with waveguide parameters corresponding to GaAs/AlGaAs waveguides at 1.52 μm wavelength. Calculations of mode shapes based on the Gauss–Seidel finite-difference iteration strategy indicate that such waveguide tapers can reduce coupling losses of single-mode optical fiber-to-semiconductor channel waveguides to the range of 1.0–1.5 dB, as compared with 3 dB or more in coupling directly from the fiber to the tighter guide. Preliminary experiments indicate that the tapers do indeed produce a large change in the near-field spot size.

© 1990 Optical Society of America