Fabrication tolerant coupling between silicon strip and subdiffraction V-groove waveguides

Abstract

We consider the problem of efficiently coupling light into a recently proposed all-dielectric subdiffraction waveguide exhibiting an ultra-small mode area (An ∼ 10−3 λ 0 2 ), more than one order of magnitude lower than a diffraction limited strip waveguide (An ∼ 10−2 λ 0 2 ). Two prospective coupling solutions are compared with respect to coupling efficiency, fabrication tolerance, and optical bandwidth. The strategy based on adiabatic mode evolution is shown to be superior with respect to fabrication tolerance as it preserves ≥99% efficiency under +/- 10 nm critical dimension (CD) variations, whereas the directional coupling approach achieves only ≥60% efficiency for the same CD errors. Similar results are obtained with respect to optical bandwidth, with the nominal adiabatic mode evolution and directional coupling based designs achieving >95% efficiency over wavelength ranges of >200 nm and ∼50 nm respectively. The superior performance of the adiabatic design requires a coupler length in the range L ≈ 100–250 µm. The results yield a high performance, compact, and straightforward design solution for efficiently interfacing between conventional diffraction limited waveguides and all-dielectric subdiffraction waveguides with an ultra-small mode area.