Combination of phase and amplitude diffractive structures in computer-generated holograms for aspheric optical testing

At the present time, computer-generated holograms (CGH) are state-of-the-art components in interferometric systems for testing aspherical surfaces. When using CGH, there is a problem of assembling the control scheme. Even an error of ~ 10 microns in the exposure of scheme elements distorts the interference pattern and makes it impossible to further adjust the optical elements to control the aspherical surface. This problem is solved by the introduction of additional diffractive structures that form spatial labels for rough positioning of control scheme elements. In this paper, we give an overview of peculiarities of combination of amplitude and phase diffractive structures at designing of high precision CGHs fabricated by means of circular laser-writing system (CLWS) and direct laser thermochemical writing technology . The problem of optimizing the writing strategy and different technological steps is discussed in order to make easy alignment procedure and to reduce errors. The limiting parameters of the control scheme related to the characteristics of the created diffractive structures, limited by the features of the technological stages of manufacturing CGH, are also discussed. We also present practical results in CGHs fabrication using modernized laser-writing system at the IA&E SB RAS and combination of different types of diffractive structures.