Specification and characterization of CGHs for interferometrical optical testing

摘要

Computer-generated holograms (CGHs) are increasingly used in optical shop testing, especially for testing as-pheric surfaces. For precise interferometrical measurements the error influence of the CGHs must be known and needs to be characterized. Different error types of two-level binary CGHs are identified and their contribution to the reconstructed wavefront are discussed: pattern errors of the CGH structure, duty cycle errors, surface figure errors of the CGH-substrate, etching depth variations in phase holograms and thickness variations of the chromium layer of amplitude chrome-type holograms. Methods to determine all these different errors are explained and an error budget of the total CGH error is given. Since the pattern error of a CGH is the most critical error it will be discussed in detail. This error depends on the writing technology (e-beam, laser beam) and on the type of the CGH (inline, off-axis). As examples of the two CGH-types a test method for measuring the pattern errors of Fresnel zone plates and linear gratings is presented. Experimental results of the tests are discussed.