Efficient perfectly vertical grating coupler for multi-core fibers fabricated with 193  nm DUV lithography

We propose a novel high-efficiency, low-reflection, and fabrication-tolerant perfectly vertical grating coupler (PVGC) with a minimum feature size >200  nm to allow for fabrication using 193 nm deep-ultraviolet lithography. The structural parameters of PVGC were optimized by a genetic optimization algorithm. Simulations predicted the coupling efficiency to be -2.0  dB (63.0%) and the back reflections to be less than -20  dB in the wavelength range of 1532-1576 nm. The design was fabricated in a multi-project wafer run for silicon photonics, and a coupling efficiency of -2.7  dB (53.7%) with a 1 dB bandwidth of 33 nm is experimentally demonstrated. The measured back reflection is less than -16  dB over the C-band. The PVGC occupies a compact footprint of 30  μm×24  μm and can be interfaced with the multi-core fibers for future space-division-multiplexing networks.