An optical device comprises a substrate, a waveguide disposed on the substrate, and a spot size converter (SSC) disposed on the substrate. The waveguide comprises a shoulder and a ridge. The SSC comprises a shoulder and a ridge. The ridge of the waveguide is aligned to a first stage of the ridge of the SSC. The waveguide is made of a first material. The shoulder and the ridge of the SSC are made of a second material. The second material is different from the first material.