Adiabatic and Ultracompact Waveguide Tapers Based on Digital Metamaterials

Adiabatic waveguide taper is one of the fundamental building blocks in photonic integrated circuits, but it is challenging to achieve high efficiency and compact size simultaneously. Here we demonstrate efficient, compact, and adiabatic on-chip spot-size converters using inversely designed tapers based on digital metamaterials. Ultracompact tapers with only 3-μm length between single-mode and few-mode waveguides are demonstrated with 90% efficiencies over 80 nm wavelength range. For another scenario of interconnection between a 0.5- μm-wide waveguide and a 10-μm-wide slab waveguide, an efficiency of 76% is experimentally achieved by 5-μm long taper. Using such a compact taper, the grating coupler footprint is reduced to even smaller than a focusing coupler. The nanostructured tapers have mode rejection ratios of >20 dB and fabrication tolerance to ±5 nm variations of the pixel dimension without efficiency drop.