Ion Exchange in Glass for the Fabrication of Continuous-Phase Diffractive Optical Elements

Diffractive phase elements, which only modulate the phase of an incident wavefront, provide high diffraction efficiencies. Such elements can be fabricated in the form of surface-relief profiles by a variety of methods including selective etching or material deposition, and diamond turning. Binary surface-relief elements have diffraction efficiencies of the order of 30–75% depending on the symmetries of the signal, while efficiencies even in excess of 90% are only possible if one employs continuous surface profiles fabricated, e.g., by direct-write laser beam or electron-beam lithography. Then, however, accurate exposure control is needed. Continuous surface profiles can be approximated by multilevel profiles, which may be fabricated by successive lithography steps, where careful mask alignment is then required.