An optical device structure includes a first cladding having a first refractive index, a second cladding having a second refractive index, a core disposed between the first and second claddings, wherein the core has a third refractive index, and wherein the first and second refractive indices are relatively lower than the third refractive index, and a diffusion region disposed between the core and one of the first cladding, the second cladding, or both. A method for making the aforementioned optical device structure is provided.