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

We describe the experimental evaluation of a shearing interferometer concep t for at-wavelength testing of extreme-ultraviolet optics. The concept is b ased on the Ronchi test, which has been modified by a new design for entran ce and exit gratings to suppress disturbing higher-order interference patte rns. The interferometer concept has been tested on an experimental setup, o f which all relevant parameters have been scaled from extreme-ultraviolet t o visible-light wavelengths. A Twyman-Green interferometer has been integra ted into the setup for comparison with the improved Ronchi test. A systemat ic difference of 7-12 m lambda rms has been found between wave fronts measu red with the improved Ronchi test and with the Twyman-Green interferometer. Possible error sources have been analyzed. The accuracy of the interferome ter is estimated to be 10 m lambda rms. (C) 2001 Optical Society of America .