Diffractive optics fabrication system for large aspheric surface testing

A computer-generated hologram (CGH) is widely used for null testing of large aspheric surfaces. The problem is every single aspheric surface requires a different CGH. If various surfaces are to be tested, it is necessary to fabricate various CGHs in the laboratory. To address this, we have developed a cylindrical-type laser writing system, which can fabricate a maximum 300-mm-diameter CGH with about a 0.78-mu m spatial resolution in the radial direction. For focusing the writing beam on the material surface, we use the well-known auto-focusing technique with a couple of astigmatic lenses.