A method of fabricating on a substrate an optical detector in an optical waveguide, the method involving: forming at least one layer on a surface of the substrate, said at least one layer comprising SiGe; implanting an impurity into the at least one layer over a first area to form a detector region for the optical detector; etching into the at least one layer in a first region and a second region to form a ridge between the first and second regions, said ridge defining the optical detector and the optical waveguide; filling the first and second regions with a dielectric material having a lower refractive index than SiGe; and after filling the first and second regions with the dielectric material, removing surface material to form a planarized upper surface.