VLS Grating with a High Line Density

We report the fabrication and measurement of spherical VLS gratings with a central line-density of 3200 mm–1 and a radius of curvature of 23.55 m. The gratings were made by interference lithography at a wavelength of 488 nm of an Ar-ion laser. The optical lithographic configuration was calculated to produce the design interference fringe density distribution across the grating blank. The gratings are intended for a soft X-ray grazing-incidence spectrograph with a 4-cm long flat field spanning the 1–5 nm wavelength range. The resultant groove density distribution was measured in an autocollimation scheme at a wavelength of 532 nm to reveal excellent correspondence to the design distribution. The surface profile of the VLS gratings was measured with AFM technique and their 1st-order efficiency was simulated.