It is sometimes necessary to couple light into or out of a silicon (Si) optical circuit (for example, from a laser diode or into a fiber or waveguide) in a no-coplanar manner, usually in a direction perpendicular to the plane of the optical circuit. This is conventionally done with a 45° microprism and a separate lens (left). Researchers at National Central University (Jhong-li, Taiwan) and National Dong Hwa University (Hualien, Taiwan) have simplified this arrangement by combining an integrated Si 45° microreflector with a focusing diffractive optical element (DOE) etched on the surface of the microreflector (right).
The angled Si reflector is 216 µm high and is fabricated by anisotropic wet etching. The DOE has a focal length of 150 µm and is created by dry etching, with a minimum 0.71 µm linewidth; a final gold plating increases reflection efficiency. The DOE has an elliptical off-axis design to compensate for astigmatism, producing a 15-µm-diameter spot at best focus and a 20-µm-or-less spot throughout a 600 µm focal depth. Both the reflector and DOE can be monolithically integrated using standard lithographic techniques. Contact Mount-Learn Wu at [email protected].