Embodiments herein describe disposing a waveguide adapter onto an SOI device after the components on a silicon surface layer have been formed. That is, the waveguide adapter is disposed above optical components (e.g., optical modulators, detectors, waveguides, etc) formed in a surface layer. In one embodiment, a waveguide in a bottom layer of the waveguide adapter overlaps a silicon waveguide in the surface layer such that the silicon waveguide and the waveguide in the bottom layer are optically coupled. The waveguide adapter also includes other layers above the bottom layer (e.g., middle and top layers) that also contain waveguides which form an adiabatic optical system for transmitting an optical signal. At least one of the waveguides in the multi-layer adapter is exposed at an optical interface of the SOI device, thereby permitting the SOI device to transmit optical signals to, or receive optical signals from, an external optical component.