Fabrication of Computer Generated Hologram for Aspheric Surface Measurement

Abstract

High-precision aspheric surfaces are generally measured using interferometer with a computer-generated holograms (CGH), which has a wavy line pattern fabricated onto a glass substrate. CGH patterns are generally made using lithographic techniques that was developed for semiconductor industry. Patterns can be subsequently etched into glass substrate using reactive ion or chemical etching. The accuracy of the drawn pattern on a CGH decides the accuracy of the measurement. Draw pattern error mainly includes the line-width deviation and its position error. In this paper, the influences of defocus of drawing laser and the wet-etching processes on the line-width were firstly investigated. On the other hand, the position error under different line-width was obtained by analyzing the relationship of line-width error and the position error. Based on the above-obtained results, a CGH having a diameter of 80 mm and the minimum line-width of 1.8 μm was successfully fabricated. Testing results showed that the wavefront error was only 3.79 nm, significantly higher than the commercial-available ones. The fabricated CGH is expected to use in the high-precision measurement of asphercal surfaces.