Ultra-broadband and highly efficient silicon nitride bi-layer grating couplers

The silicon nitride (Si3N4) waveguide platform becomes prominent in silicon photonics due to its ultralow propagation loss, negligible two-photon absorption, tolerance to phase error, and so on. Particularly, Si3N4 photonic circuits provide nonlinear opportunities as thick waveguides enable dispersion engineering, which is fundamental for frequency comb and supercontinuum generation. However, grating couplers based on thick Si3N4 waveguides, as the building block for any device, are prone to higher-order Bloch mode excitation, restricting the coupling efficiency dramatically, thus precluding practical applications that large bandwidth and are power restricted. We propose and demonstrate an 800 nm-thick Si3N4bi-layer fully etched grating coupler which is ultra-broadband and highly efficient. By applying the apodization technique, the bi-layer grating shows a conversion efficiency of −1.44 dB and a 1 dB bandwidth of 135 nm. Further integrated with a bottom reflector, a conversion efficiency of −1.01 dB with a 1-dB bandwidth of 117 nm is achieved. The proposed structures are highly fabrication tolerant and remain great performance regardless large structural parameter variations. It paves the way for a large-volume high-yield production towards fully-integrated frequency comb chip, optical beam forming network, Lidar, etc.