Kenneth P. Gross

Interferometric testing of cylinder optics using a computer-generated hologram (CGH)

In this paper we describe an interferometric technique for testing uniformity in surface figure of large cylinder optics. In our laser scanning system, we employ large (1 1 inch) cylinder mirrors for focusing of laser beams over a wide scan range. Performance of the system depends critically on the uniformity of the beam size on the focal plane over this scan range. Departure of portions of the surface figure from the ideal cylindrical surface lead to varying beam size that limit the performance of our systems. Initially we tested these mirrors with a metrology technique that sample beam size on the focal plane at various scan positions. This technique is both time consuming and unreliable. To circumvent these limitations, we developed a fast interferometric technique to qualify these mirrors using a computer generated hologram (CGH) as our reference cylinder optics. The methodology employed simulates the scanning process by subdividing the interferogram of the cylinder mirror into small sections that are of the order of the beam size, analyzing the surface figure over these subsections and then computing a figure of merit (FOM) that is tied to the uniformity of the surface figure over the entire mirror. The correlation between the two independent measurements is found to be very good. In the following we discuss briefly the methodology developed in the Zygo platform, which can be easily implemented as an automated QA procedure.