Performance of laminar-type holographic grating for a soft x-ray flat-field spectrograph in the 0.7–6 nm region

Abstract

A laminar-type holographic grating having a groove density of 2400 lines/mm was designed and fabricated for use with a soft x-ray flat-field spectrograph covering the 0.7–6 nm region. The varied-line-spaced grating pattern was generated by use of an aspheric wave front recording system and the laminar-type grooves were formed by a reactive ion-etching method. The performance of the grating was evaluated by using a molybdenum K x-ray generator and a spectrograph with a charge coupled device imaging detector. Also, the absolute diffraction efficiencies of the zeroth, first, second, and third, spectral orders were measured by use of the reflectometers installed at Synchrotron Radiation Beamlines. The experimental results showed that the holographic grating indicated comparable spectral resolution and maximum first-order diffraction efficiency (>2%) at ∼2 nm with a replica grating made from a mechanically ruled varied-line-spacing grating. It also showed higher efficiency in the <1.0 nm region and lower level higher-order light over the whole wavelength range than the replica grating.