Two-dimensional control of mode size in optical channel waveguides by lateral channel tapering

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Abstract
We propose optical channel waveguide structures in which two-dimensional mode size transformation is achieved by lateral tapering only. The layer structure of the waveguide is designed such that tapering the lateral channel width results in a tapering of the vertical size of the mode as well. This is accomplished by providing two waveguide cores in a single-mode system. When the rib is wide, the mode resides in the upper core, tightly confined by the rib. As the rib narrows, the field migrates to the lower core and spreads out in both dimensions, permitting a better match to large-mode structures such as optical fibers. Our calculations show that such waveguide tapers can significantly reduce the losses for coupling single-mode optical fibers to semiconductor channel waveguides.