Fully Etched Low-Back-Reflection and High-Efficiency Silicon Waveguide Grating Couplers With Minimum Feature Size Above 260 nm

We propose and validate a new approach for low-back-reflection and high-efficiency silicon waveguide grating couplers (GC) with a fully etched structure. Enhanced coupling efficiency is achieved by using a slot waveguide structure in the vertical direction. No bottom mirror is needed to improve directionality. The low-refractive-index slot structure enables the grating strength to be engineered by introducing the slot thickness as an additional parameter for mode-matching and low-back-reflection optimization. The resultant grating can achieve excellent mode matching even with a minimum feature size as large as 260 nm, unlike previous grating-period-only apodizations which need a feature size as small as a few tens of nanometers. With this novel structure, FDTD simulations predict the coupling efficiency to be −0.89 dB. The structure is also tolerant to fabrication errors. Preliminary experimental results achieved −1.84 dB coupling efficiency. The design does not require etching of the silicon waveguide layer and thus can achieve ultralow back reflection. The simulations predicted that the minimum back reflection can be suppressed down to −46.7 dB and the experimentally measured back reflection was −35.7 dB with fully etched structure.