Monolithically integrated asymmetric graded and step-index couplers for microphotonic waveguides

A monolithically integrated asymmetric graded index (GRIN) or step-index (GRIN) mode converters for microphotonic waveguides are proposed and described. The design parameters and tolerances are calculated for amorphous silicon (a-Si) couplers integrated with silicon-on-insulator waveguides. The GRIN and step-index couplers operate over a wide wavelength range with low polarization dependence, and the lithographic resolution needed is only +/- 1 mu m. Finally, experimental results are presented for a single layer 3 mu m thick step-index a-Si coupler integrated on a 0.8 mu m thick SOI waveguide. The measured variation of coupling efficiency with coupler length is in agreement with theory, with an optimal coupling length of 15 mu m for this device. (c) 2005 Optical Society of America.