A fiber-to-waveguide coupler is proposed to efficiently couple light from a single-mode fiber into a submicrometer semiconductor waveguide for integration with optoelectronic circuits. A microring with a specific refractive index is designed on the top of the semiconductor waveguide. The gradual vertical leakage from the microring forms steady coupling into the semiconductor waveguide. Coupling efficiency up to 93% is demonstrated using the three-dimensional finite-difference time-domain method. A tapered-waveguide or microring structure can be used to convert the lateral-mode size for coupling light into a single-mode semiconductor waveguide.
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