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Design of Compact but Fabrication-Tolerant Vertical Coupler for Active–Passive Integration

We propose a novel scheme of compact but fabric-ation-tolerant vertical coupler based on resonant coupling for active–passive integration. The transfer matrix method is used to carry out the design and optimization of various vertical coupler structures efficiently. Simulations show that transfer efficiency higher than 96% can be achieved for the proposed coupler with a length as short as 55 μm which is about half of a classic adiabatic coupler design. By employing a mode beating section with a length twice the beating length and a relatively steep linear taper, the proposed resonant coupler breaks the strongly periodic behavior of mode beating in conventional resonant couplers, thus dramatically improving the fabrication tolerance. The proposed resonant coupler is predicted with a fabrication tolerance of 380 nm which is about 2.5 times of the conventional resonant coupler, under the condition that the transfer efficiency is higher than 90%. The scheme of this paper can be widely used to design compact and fabrication-tolerant couplers.