Phase Fresnel lens recorded in photo-thermo-refractive glass by selective exposure to infrared ultrashort laser pulses.

A new two-step approach for fabricating phase optical elements in photo-thermo-refractive glass by exposure to IR ultrashort laser pulses followed by thermal development is shown. A binary phase Fresnel lens was designed to focus light at 632.8 nm to a focal length of 400 cm. Conditions of ultrashort pulse irradiation and thermal development were chosen to achieve pi phase shift between zone boundaries. The focusing efficiency of the element was measured to be close to 50%.