Differential Phase Measuring Deflectometry for On-Machine Metrology of Ultrafast Laser Stress Figuring

Abstract

Ultrafast laser stress figuring (ULSF) currently allows for the figuring of large aspect ratio fused silica mirrors with an accuracy of <20 nm RMS over 6 orders of Zernike polynomials. While similar technologies are capable of this accuracy, ULSF has the potential to be orders of magnitude faster. ULSF is an iterative process in which (1) an optic is figured, (2) optical metrology is used to measure the surface figure, and that measurement is used as feedback to repeat this closed-loop process and further figure the mirror until the mirror has reached the desired figure. We present an in-situ, on-machine optical metrology system that measures the mirror surface figure using differential phase measuring deflectometry (DPMD). After an absolute measurement of the surface figure is done using interferometry prior to any figuring, our differential deflectometry system can measure the change in surface figure after each laser stress figuring process. This eliminates the need to remove the mirror from its mounting, which can induce non-repeatable surface figure errors during the metrology step. The differential deflectometry system is also used to calibrate the ULSF process prior to the figuring of the mirror. We utilize and characterize the in-situ differential deflectometry system during the surface figuring of a 25.4 mm fused silica mirror and summarize the results in this proceeding. © 2022 SPIE.