Techniques relating to an improved optical waveguide are described. The optical waveguide includes an upper and lower waveguide that each comprise a first and second layer, in which photons are transferred from the lower waveguide to the upper waveguide. A structured subwavelength coupling region is included, for example, in the first upper waveguide layer. The fill factor of the subwavelength grating coupling region is increased in the direction of light propagation to increase the index of refraction of the structured subwavelength coupling region and therefore improve photon transfer from the lower waveguide. Additionally, the width of the optical waveguide (at least along the structured subwavelength coupling region) remains constant as the fill factor increases.