Ultracompact tapered coupler for the Si/III–V heterogeneous integration

An ultracompact tapered coupler, which is suitable for mode transformation between a 220 nm high silicon wire waveguide and a Si/III–V hybrid waveguide, is proposed for Si/III–V heterogeneous integration. The tapered coupler is composed of three sections. Since the tapered coupler avoids exciting the unwanted high-order modes in the III–V waveguide, the length of the tapered coupler can be dramatically shortened. In the proposed structure, the total length of the trisectional tapered coupler can be as short as 8 μm with a fundamental mode-coupling efficiency of over 95% in a bandwidth of over 100 nm. The alignment tolerance of the proposed structure is also analyzed.