A passive optical waveguide is solely built on a Si substrate while still maintaining high optical quality. Two side-by-side diamond shaped cavities may be etched into the Si wafer and oxide grown on the inner walls of the cavities until the oxide meets at opposing inner vertices of the diamond shaped cavities. An optical waveguide is formed by the inverted, generally triangular cross-sectional, portion of silicon remaining between the top surface of the wafer and the opposing inner vertices.