Large Holograms

Meter-class telescopes comprising adaptive and segmented optics have ushered in a new “golden age” in observational astronomy, and highly light weighted versions of these telescopes for spaceborne applications are under active development. Optical testing of these complex optical systems presents their own set of unique challenges. One of which is the need to generate aspheric optical wavefronts with precision on the order of nanometers, maintained over meters of spatial extent, to serve as the reference standard in interferometric “optical null” testing. The critical optic in this type of testing is a Computer-Generated Hologram (CGH), a lithographically patterned diffractive optical element that transforms a plane wavebeam into the desired spherical or aspherical wave that perfectly matches the design of the mirror or system under test.

To address the need for larger CGH’s than are available from commercial sources, MDL developed new electron-beam lithography capability to fabricate CGH’s up to 230 mm in diameter with pattern placement accuracy in the tens of nanometers. This new capability required fabrication of a new e-beam cassette, measurement of e-beam placement accuracy, correction of e-beam placement errors, and development of new e-beam exposure and resist processing techniques. The larger CGH’s will enable our partners to fabricate large mirrors and align segmented mirror systems to tighter tolerances than ever before. These large-aperture optical systems will enable new astronomical discoveries from ground and space.