Large dynamic range wavefront error metrology bench (Conference Presentation)

Abstract

We present an optical metrology instrument for measuring the transmitted wavefront error (TWE) of coated or uncoated optics having large aberrations. Fizeau interferometry is a standard technique to measure TWE. However, the dynamic range is limited and measuring large aberrations requires the use of computer-generated holograms or null lenses, which increases both cost and time dedicated to precise alignment. The chosen solution is based on a quadriwave lateral shearing interferometer (QWLSI) wavefront sensor. QWLSI is an achromatic technique, meaning that it measures optical path differences at any wavelength without any need for recalibration at specific wavelengths. Moreover, QWLSI directly measures phase derivatives unlike Fizeau interferometers that measure phase. Therefore, QWLSI has, by design, a better wavefront error dynamic range for TWE and also reflected wavefront error measurements. The proposed optical solution was characterized using samples previously measured using an interferometer from Zygo. Comparisons were made on samples having 5 µm defocus P-V and 3 µm astigmatism. Finally, measurements were made on optics presenting more than 20 µm peak-to-valley (P-V) of defocus aberration. Measuring this sample with a classical interferometer is not possible and our solution provided TWE measurements without using any relay optics.