A SOI device may include a waveguide adapter that couples light between an external light source—e.g., a fiber optic cable or laser—and a silicon waveguide on the silicon surface layer of the SOI device. In one embodiment, the waveguide adapter is embedded into the insulator layer. Doing so may enable the waveguide adapter to be formed before the surface layer components are added onto the SOI device. Accordingly, fabrication techniques that use high-temperatures may be used without harming other components in the SOI device—e.g., the waveguide adapter is formed before heat-sensitive components are added to the silicon surface layer.