Abstract

We have designed a triple grating soft x-ray interferometer, which will directly measure material optical constants over the 1-10-nm range. This instrument uses a forward diffracting geometry, similar to that used in monolithic-crystal Laue x-ray interferometers. Three photolithographically produced gratings act as beam splitter, beam recombiner, and beam analyzer to measure phase shifts induced by a test foil. Each freestanding grating has a 200-nm linewidth and spacing and is 500 nm thick, fabricated in gold. In lieu of an x-ray laser, existing x-ray sources provide limited spatial and temporal coherence. To achieve high contrast fringes serious constraints must be placed both on the interferometer design and on the use of the x-ray source. Grating interferometers are notoriously insensitive to incomplete spatial coherence. In our design, a 100-μm wide source in the diffraction plane is sufficiently small to minimize grating defocus concerns and achieve high fringe visibility. Adequate temporal coherence is assured by limiting the source bandwidth to λ/Δλ = 1000 with an extended range grasshopper monochromator and a synchrotron source. Grating flatness, which could affect path length matching, is shown from existing gratings to be fully adequate.

© 1985 Optical Society of America