Controlling accuracy of fabricating computer-generated holograms on circular and X-Y laser writing systems

Controlling an accuracy of fabricating computer-generated holograms (CGH) is actual task. Such holograms are usually used for generating reference wavefront for interferometric testing of aspherical surfaces. The influence of external factors on the positioning systems of the writing system occurs during fabricating the holograms and leads to microstructure errors that affect the quality of the wavefront formed by such elements. Fabrication errors of CGH affect the accuracy and reliability of interferometric measurements. Controlling these errors allows determining the quality of the manufactured element and evaluating the accuracy of the wavefront which it forms. This paper presents the experimental results of using CGH error testing methods for laser writing systems that operate in a polar or cartesian coordinate system. These methods are based on writing of series of embedded small marks with gratings having 2-5 mu m period and following measurement of light intensities in curtain diffraction orders. These marks consist of two parts, one of which is quickly formed before the fabrication of CGH and the second one during writing the pattern of the main CGH. The shift between the first and second segments of the mark makes it possible to determine the CGH writing errors caused by external influences on the positioning system for both circular and X-Y laser writing systems. To determine writing errors can be use simple optical diffractometer.