A Coronagraph For The Sensing Of Fabrication Errors In Diffraction Gratings

We describe a coronagraph-like instrument for the characterization of fabrication errors in diffraction gratings and diffractive optics. A diffraction grating under test is illuminated with collimated laser light, and a reflected diffraction order is focused by a lens. A beam stop placed at the focal plane of the lens blocks the primary diffracted beam, passing only the small fraction of light scattered into nearby angles due to fabrication errors. The transmitted light fraction is focused by a second lens onto a digital camera and the light distribution is measured with a sensitive high dynamic range imaging method. Preliminary measurements with the coronagraph setup are compared to focal-plane images made without beam stop.