Design and Fabrication of Twin Computer-generated Holograms for Testing Concave Aspherical Surfaces

Based on the principle of testing aspherical surfaces with Computer-generated Holograms (CGH), a binary phase twin CGH is proposed and designed. It consists of two segments, namely main CGH and alignment CGH. Main CGH, fabricated on the inner part of a substrate, is used for testing an aspherical surface. The outer segment is alignment CGH, used for aligning the main CGH accurately by eliminating errors such as defocus, decentration and tilt on the premise that they are concentric with each other. A fabricating method which is able to manufacture twin CGHs with two different phase depth is introduced. Both holograms are fabricated on one mask simultaneously. After being exposed, the substrate is etched twice. Finally, a design example of twin CGH for testing a paraboloidal mirror (Φ140、F2) is presented. The validity of design method and the feasibility of fabrication process are demonstrated by comparing the test result with that of the autocollimation test.