| Abstract | Surface grating couplers are an important component for interfacing photonic integrated circuits with optical fibers. However, conventional coupler designs typically provide limited performance due to low directionality and poor fiber-to-grating field overlap. The efficiency can be improved by using non-uniform grating structures at the expense of small critical dimensions complicating the fabrication process. While uniform gratings can alleviate this constraint, they produce an exponentially decaying near-field with the Gaussian fiber mode overlap limited to a theoretical maximum of 80%. In this work, we propose a uniform grating coupler that circumvents this field overlap limitation. This is achieved by leveraging inter-layer mode interference through a virtual directional coupler effect in a hybrid amorphous-silicon (α-Si) on silicon nitride (Si₃N₄) platform. By optimizing the inter-layer gap and grating geometry, a near-Gaussian profile of the out-radiated beam is achieved, resulting in an unprecedented grating-to-fiber overlap of 96%. The full three-dimensional (3D) finite-difference time-domain (FDTD) simulations show a high directionality of 84% and a record coupling loss of −1.27 dB with a 1-dB bandwidth of 20 nm for the uniform grating coupler design. Our device is designed for a wavelength of 950 nm aimed for use in hybrid quantum photonic integrated circuits using III-V quantum dot single photon sources. |
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