Wide bandwidth and high coupling efficiency Si_3N_4-on-SOI dual-level grating coupler

Abstract
We propose and experimentally demonstrate fiber-to-chip grating couplers with aligned silicon nitride (Si3N4) and silicon (Si) grating teeth for wide bandwidths and high coupling efficiencies without the use of bottom reflectors. The measured 1-dB bandwidth is a record 80 nm, and the measured peak coupling efficiency is −1.3 dB, which is competitive with the best Si-only grating couplers. The grating couplers are integrated in a Si3N4 on silicon-on-insulator (SOI) integrated optics platform with aligned waveguides in both the Si3N4 and Si, and we demonstrate a 1 × 4 tunable multiplexer/demultiplexer using the Si3N4-on-SOI dual-level grating couplers and thermally-tuned Si microring resonators.