Investigation of diffraction-based measurement errors in optical testing of aspheric optics with digital micromirror devices

Abstract

Spatial light modulators (SLMs) have shown to be versatile tools for displays, but also for various applications in optical metrology since light can be individually directed and customized and thus they may serve as flexible masks or holograms. In contrast to SLMs based on liquid crystal on silicon or liquid crystal displays (LCDs), which allow tailoring phase and/or amplitude of a wavefront in transmission or reflection, digital micromirror devices (DMDs) offer highest reflectivity and allow precise as well as fastest guidance of light rays based on the fundamental reflection law. Here, we present and compare different approaches for simulating the diffractive pattern when applying a micromirror device for wavefront readout. The different simulation methods for calculating the diffraction pattern are based on Monte-Carlo simulations in combination with nonsequential ray tracing, on Fourier optics methods (Fourier transform, FT) and on complex digital holographic wavefront propagation. The wavefront measurement concept with the DMDs is based on selecting single subapertures of the wavefront under test and on measuring the wavefront slope consecutively in a scanning procedure. In contrast, in LCD-based approaches already shown in literature, the selection of subapertures and thus the scanning procedure is performed in transmission. The measurement concept and diffraction-based measurement errors of this method will be demonstrated for aspheric optics. Furthermore, different approaches for the prediction and reduction of the diffractive pattern—also based on holographic complex wave front propagation—will be described and characterized.