High precision measurements of the groove density of diffraction gratings

The diffraction gratings are widely used to monochromatize and even focus the soft X-ray radiation 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 0.1%. 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.