Structures including a waveguide core and methods of fabricating a structure including a waveguide core. The structure comprises a photonics chip including a first chip region, a second chip region, a first waveguide core in the first chip region, and a second waveguide core in the second chip region. The first chip region adjoins the second chip region along a boundary. The first waveguide core includes a first tapered section, and the second waveguide core includes a second tapered section positioned across the boundary from the first tapered section. The first tapered section has a first width dimension that increases with increasing distance from the boundary, and the second tapered section has a second width dimension that increases with increasing distance from the boundary.