A guided-mode-selective focusing grating coupler

Guided-mode-selective focusing grating coupler, which couples out not TE
₀
mode but TE
₁
mode to a free space wave, was designed at around 850-nm wavelength for constructing a wavelength-division-multiplexing chip-to-chip optical interconnection board. The coupler was fabricated in a slab waveguide consisting of a Ge-SiO
₂
main-guiding core, a SiO
₂
subguiding core, a Si-N grating layer, and a high-reflection layer. Coupling efficiencies for TE
₀
and TE
₁
modes were obtained to be 5% and 46%, respectively, with 0.1-mm interaction length. The focus spot size of 3×1 μm was obtained for TE
₁
mode.