Abstract: A fiber-to-chip coupler includes a substrate, a waveguide on a top surface of the substrate, an optical fiber axially aligned to the waveguide, and a cap. The waveguide has a uniform region with uniform width and a tapered-waveguide region having a width that adiabatically increases from a minimum width to the uniform width. The optical fiber has a tapered fiber tip having a minimum core diameter, a cylindrical section having a maximum core diameter, and a tapered-fiber section therebetween. The optical fiber is located at least in part above the tapered-waveguide region, and has a core diameter that adiabatically decreases within a taper length of the tapered-fiber section. The cap extends from the tapered fiber tip toward the cylindrical section, is formed of a second material having a cap refractive index that exceeds a refractive index of the optical fiber, and includes a cap-region disposed on the tapered-waveguide region.