An adiabatic optical coupler can include: a top tapered region that includes a top taper having two top tapered sides that taper from a first end region to a top tip region, the top taper having a first length; and a bottom tapered region under the top tapered region, wherein the bottom tapered region includes a bottom taper having two bottom tapered sides that taper from the first end region to a bottom tip region, the bottom taper having a second length that is longer than the first length. Another adiabatic optical coupler can include: a tapered region that includes a taper having two tapered sides that taper from an end region to a tip region; and a sub-wavelength grating (SWG) optically coupled with the tip region. Another adiabatic optical coupler can include a combination of these two adiabatic optical couplers.