Computer-generated holographic null correctors for measuring asphere optics can now be used with commercial interferometers. At the SPIE annual meeting in July (San Diego, CA), scientists from Diffraction International (DI, Minneapolis, MN) and the Department of Energy’s Oak Ridge National Laboratory (ORNL, Oak Ridge, TN) discussed how an alignment computer-generated hologram (CGH) used with a specialized mount make it possible to treat holographic null correctors as external cavity elements (paper #2536-13).
Conventional interferometric testing of aspheres requires null-corrector elements that compensate for aberrations introduced in a spherical wavefront by an aspheric optic. A properly designed null lens used in conjunction with the asphere under test will yield a stigmatic image of a point source. Unfortunately, these test elements can be as time-consuming and expensive to fabricate as the aspheres themselves.
Another approach is to use a CGH as the null-corrector element. The CGH diffracts a spherical or collimated test beam, creating an aspheric wavefront suitable for characterizing the part under test. Computer-generated holographic nulls are extremely sensitive to alignment errors and noise from fabrication errors, however. They are also limited in the amount of aspheric departure they can correct. Perhaps most critical, to be used with a CGH null, an interferometer must be modified, another expensive and difficult process. Such considerations have precluded a wider adoption of this approach.
Optical alignment during interferometric testing is important because any errors are interpreted as figure errors in the test optic. The null corrector and the test optic must be aligned to one another and to the interferometer, a hard and lengthy task when the figure errors of the optic being tested are unknown. Under a Cooperative Research and Development Agreement (CRADA) with ORNL, Steven Arnold of DI developed a precision kinematic CGH mount and an alignment CGH that allow a CGH null to be used with a conventional interferometer. Oak Ridge National Laboratories metrologists led by Curt Maxey have tested the system and successfully evaluated a series of aspheres.
Precision positioning
The CGH null-adaptor fixture is a three-axis mount with an adjustment accuracy on the order of microns. Computer-generated holograms are mounted in frames and attached to the fixture. An alignment CGH that simulates a reflective sphere is placed on the mount, then aligned to the interferometer such that a null interferogram is produced. The alignment CGH is next exchanged with a CGH null, which generates an aspheric wavefront. The asphere under test is aligned to the null element, at which point the system is ready for surface metrology acquisition. No modification of the interferometer is required.
The alignment elements are standardized, but the CGH null must be designed specifically for the element under test. An optical-design program performs a ray trace of the ideal system, determining the fringes produced by the interference of a reference wavefront and the reflected wavefront from the optic under test. A carrier wave added to the pattern ensures separation of the various diffracted orders during the test process, enhancing fringe contrast. Designed and produced by DI, the CGHs are fabricated in chrome on glass using e-beam lithography.
The ORNL team tested a number of aspheres using the DI system in conjunction with a Twyman-Green interferometer (Breault Research Organization, Tucson, AZ) and a Fizeau instrument (Zygo, Middlefield, CT). They obtained good agreement between the CGH data and that obtained using conventional autocollimation methods.
Prior to the establishment of the CRADA with DI, Maxey was unable to find a satisfactory asphere metrology system based on a conventional CGH null configuration. Says Maxey, "Steve [Arnold] has come up with a way to apply CGH nulls as an extracavity element. What I couldn`t buy reliably for under $100,000 five years ago is now available for less than $10,000 to anyone with an interferometer."