Abstract

To resolve the mode field diameter mismatch between silicon waveguide and fiber and to obtain the better alignment tolerances in the horizontal and vertical directions, a novel high coupling efficiency fiber-to-chip edge coupler with four auxiliary thin layers is proposed. The edge coupler consists of two segments. The first section is composed of a straight waveguide and four tapered auxiliary thin layers around the aforementioned waveguide. At the facet, the thin layers are arranged as a closed square, which can capture a large area of light from the fiber to form a superimposed mode. Along the light propagation direction, the four thin layers are taper structures, which facilitates the evolution of the superimposed mode. The second section is an inverted taper silicon waveguide, which is able to transfer the superimposed mode from the first section adiabatically into the silicon waveguide. The coupling efficiency of the edge coupler is determined by both overlap efficiency and mode conversion efficiency. In order to improve the coupling efficiency, we simulate as well as optimize several important parameters by FDE method and EME method. And we calculate the alignment tolerances by moving the position of the fiber in the horizontal and vertical directions. In conclusion, the simulated highest overlap efficiency is 93.9% and the mode conversion efficiency is 98.6%, thus, the total coupling efficiency is 92.6% for fundamental mode and the 3-dB alignment tolerances in horizontal and vertical directions are about ± 3.5 μm, ± 3.2 μm, respectively.

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