Experimental Demonstration of Efficient and Polarization-Diversity Fiber-Chip Coupling by 2D Grating Couplers with 193-nm DUV Lithography

Abstract: Two-dimensional (2D) diffraction gratings offer a polarization-independent coupling solution between the planar photonic chips and optical fibers, with advantages including placement flexibility, ease of fabrication, and tolerance to alignment errors. In this work, we first proposed and experimentally demonstrated a highly efficient 2D grating coupler enabled by exciting multipolar resonances through bi-level dielectric structures. A 70-nm shallow-etched hole array and a 160-nm-thick deposited polycrystalline silicon tooth array are employed in our proposed 2D grating coupler. Strong optical field confinement and enhanced radiation directionality can thus be attained through the use of 193-nm deep-ultraviolet (DUV) lithography, which is readily accessible from commercial silicon photonics foundries. The measured experimental peak coupling efficiency is -2.54 dB with a minimum feature size of 180 nm. Our design exhibits a 3-dB bandwidth of around 23.4 nm with good positioning tolerance for optical fibers. Due to the benefits of perfectly vertical coupling, the measured polarization-dependent loss in our experiments is below 0.3 dB within the 3-dB working bandwidth. Our proposed 2D grating structure and design method can also be applied to other integrated optics platforms, enabling an efficient and polarization-diversity coupling between optical fibers and photonic chips while reducing requirements on feature size.