Compact high-efficiency perfectly-vertical grating coupler on silicon at O-band.

A compact, high-efficiency grating coupler is demonstrated for interfacing a silicon waveguide and a perfectly-vertical fiber at O-band. The grating lies on a tilted silicon membrane for minimizing the reflections. Circular grating lines are adopted to shorten the overall device length to about 60μm. 57% peak coupling efficiency and >28nm 1-dB coupling bandwidth are obtained experimentally. Back reflections of 1% to the silicon waveguide and the single mode fiber are theoretically estimated. The processing flow to realize the proposed structure is discussed in detail. The fabrication control over the tilted angle of the silicon membrane is investigated. The approach by applying an oxide cladding to improve the stability of the membrane is also introduced. The present grating coupler is compatible to common fabrication processes for silicon photonic chips.