Accurately measuring a surface by using a computer-generated hologram

Abstract

Abstract: By using a CGH test plate fabricated with our equipment and techniques, we measured a perfect sphere surface. The measurement result is quantified into four parts: the figure error from the spherical surface under test; the figure error from the spherical reference surface; the error from hologram and the adjustment error from misalignment. The measurement result removed from the later three errors, shown excellent agreemen t with Zygo test of the same sphere surface. This verified that the measurement accuracy by using this kind of CGH coul d be very high. Key words : computer-generated hologram; convex aspheric surface; test 1. Introduction It is well known that the use of large and deep convex aspheric surfaces in reflective optical system allows improved performance with fewer elements. However, it is difficult and expensive to measure them accurately with traditional me thods [1-4]. Recently, a valid met hod by using computer-generated hologram (CGH) test plat has been studied [5-8]. In this method, the CGH was constructed using thermally selective oxidization to transfer a CGH pattern onto a metallic film on a curved substrate. However, the resolution and linear profile of the gr atings that compose the CGH was of inferior quality in comparison with that produced by use of lithography. This may decrease significantly surface measurement accuracy. Our lab has he ld the technology to transfer a large CGH pattern onto a concave lens surface with precise alignment [9-12]. The technology combined laser direct writing and lithography. And we have successfu lly test a convex aspheric surface wi th a CGH test plat fabricated with this method [13-14]. However, the measurement error was not analyzed, and the measurement result was not verified by compared with that of other methods. In this paper, we built an optical test system to measure a perfect sphere surface with 100mm of diameter and 279mm of radius of curvature. The system had two illumination lenses and a test plate with a CGH fabricated by using our new technology. We analyzed the errors existing in this test. The measurement result was quantified into four parts. One is the figure error from the spherical surface under test, and it can be gotten with absolute test method by using Zygo interferometer. One is the figure error from the spherical reference surface, and it can also be gotten with absolute test method by using Zygo interferometer. One is the error from hologram, and it can be verified by confirming the ring positions. One is the adjustment error from misalignment, but it can be removed by using the method of Zernike polynomials fitting and measuring twice[15]. The measurement result removed from the later three errors, shown excellent agreemen t with Zygo test of the same sphere surface. 1. The Optical system design