High precision measurements of the groove density of diffraction gratings

摘要

The diffraction gratings are widely used to monochromatise and even focus the soft X-ray radiaiton produced by the high brilliance third generation synchrotron radiation sources. Nevertheless, the final performance of an instrument that uses a diffraction grating is sensitive to any figure error and to any undesired groove density variation along the surface of the grating itself. Therefore, typical requirements are 0.1-0.2 arcsec (even less) on the residual slope errors (after proper shape subtraction), while the groove density is required to be constant along the surface with a percentage error below 0.1percent. Vice-versa, sometimes groove density variation is required along the surface to correct spherical aberrations or to change the focal properties of a grating. Since the gratings, in the soft X-ray region, work in grazing incidence mode, the ideal instrument to measure it is a mono-dimensional profilometer. At ELETTRA, the Italian third generation synchrotron radiation source, we have an in house modified version of the Long Trace Profiler (LTP). This instrument measures directly the local slope of the surface under test with a precision of about 0.1 arcsec rms and a reproducibility better than 0.02 arcsec on optics up to 1 meter in length. Using this instrument, we have developed a method to measure not only the groove density (with an offset of about 0.1percent), but also the groove density variation along the surface with a precision better then 0.005percent on the whole surface. The method is based on the Littrow mounting configuration, and will be discussed in detail.