Soft X-ray grating monochromators as a source of spatial coherence degradation: A wave-optical approach

We present wave-optical simulations and a coherence analysis of the photon beam transported through a soft X-ray beamline, paying particular attention to a focusing varied line spacing (VLS) plane grating monochromator (PGM). We show that this beamline optical element used in several soft X-ray beamlines at synchrotron sources and free-electron lasers can cause a non-negligible spatial coherence degradation. We demonstrate that the origin of this effect arises from the coupling between spatial and spectral properties of the photon beam generated by the grating. The latter implies that space and frequency dependencies are not separable after such a dispersive element. It is shown which parameters are essential for this effect and how they are linked to each other.