An optical device may include a waveguide converter that couples an external light-carrying medium to a waveguide embedded within the optical device. In one embodiment, the optical signal emitted from the light-carrying medium enters the converter which focuses the signal (e.g., shrinks the mode of the optical signal) to better match the physical dimensions of the waveguide. Using the converter may improve transmission efficiency relative to directly coupling (e.g., butt-coupling) the light-carrying medium to the waveguide. Specifically, the converter may enable the light-carrying medium to transmit the optical signal directly into the optical device without the use of any external lenses, even if the waveguide is a sub-micron waveguide.