Double layer beam expander for device-to-fiber coupling

A method of forming an optical device including a double layer beam expander for providing coupling between a semiconductor optical device and an optical fiber, the method comprising the steps of:

• a) forming a semiconductor optical device including an endface facet on a top major surface of a provided substrate;
• b) masking said optical device;
• c) growing a first layer of a high refractive index material on said top major surface of said substrate, said first layer grown to be contiguous with the optical device endface and laterally taper away therefrom to a predetermined beam expander termination point, the thickness of said grown first layer at the optical device being a maximum (“A”) and the thickness of said grown first layer at the expander termination point being a minimum (“B”);
• d) depositing an index-matching layer to cover the top surface of the first layer;
• and
• e) growing a second layer of a low refractive index material to cover said first layer of a high refractive index material and the index-matching layer.