Experimental study of a shearing interferometer concept for at-wavelength characterization of extreme-ultraviolet optics.

We describe the experimental evaluation of a shearing interferometer concept for at-wavelength testing of extreme-ultraviolet optics. The concept is based on the Ronchi test, which has been modified by a new design for entrance and exit gratings to suppress disturbing higher-order interference patterns. The interferometer concept has been tested on an experimental setup, of which all relevant parameters have been scaled from extreme-ultraviolet to visible-light wavelengths. A Twyman–Green interferometer has been integrated into the setup for comparison with the improved Ronchi test. A systematic difference of 7–12 mλ rms has been found between wave fronts measured with the improved Ronchi test and with the Twyman–Green interferometer. Possible error sources have been analyzed. The accuracy of the interferometer is estimated to be 10 mλ rms.