Techniques for forming a facet optical coupler that includes a waveguide formed over a trench of a silicon substrate are described. The trench is formed in a silicon substrate and then filled with a dielectric material. The waveguide is patterned on the dielectric material over the trench such that the waveguide is disposed a distance from the first surface. A first end of the waveguide has a first size and a second end of the waveguide distal the first end has a second size different than the first size. A material of the waveguide and the first size define a mode size of the waveguide.