Coherence Properties of Individual Femtosecond Pulses of an X-Ray Free-Electron Laser

Abstract

Measurements of the spatial and temporal coherence of single, femtosecond x-ray pulses generated by the first hard x-ray free-electron laser (FEL), the Linac Coherent Light Source (LCLS), are presented. Single shot measurements were performed at 780 eV x-ray photon energy using apertures containing double pinholes in "diffract and destroy" mode. We determined a coherence length of 17 µm in the vertical direction, which is approximately the size of the focused LCLS beam in the same direction. The analysis of the diffraction patterns produced by the pinholes with the largest separation yields an estimate of the temporal coherence time of 0.6 fs. We find that the total degree of transverse coherence is 56% and that the x-ray pulses are adequately described by two transverse coherent modes in each direction. This leads us to the conclusion that 78% of the total power is contained in the dominant mode.

Determining the polarization state of an extreme ultraviolet free-electron laser beam using atomic circular dichroism

“…Non-linear phenomena and femtosecond time-resolved spectroscopy can be explored using high intensity extreme ultraviolet (XUV) and X-ray beams [1][2][3] . In addition, the ultra-short pulse duration and the high degree of spatial coherence of the X-ray FELs have enabled imaging experiments [4][5][6] aiming to determine the structure of a single biomolecule. Nearly all experiments to date have been carried out with linearly polarized short-wavelength FELs thereby precluding studies at high intensities and on systems where the result might depend on the target sensitivity to the helicity of the electric field.…”

High-resolution three-dimensional partially coherent diffraction imaging

“…All of these effects remain unexplored at the present state of development of CDI because the density maps of the images produced are not very reliable. XFEL sources experience shot to shot variations in the coherence properties 15 , potentially limiting their ability for single-shot diffract and destroy imaging. Furthermore, the XFEL pulse transverse coherence lengths are still comparable in size 15 to many objects of interest to CDI, meaning that methods that both characterize the coherence and image simultaneously are needed.…”

Coherence loss by sample dynamics and heterogeneity in x-ray laser diffraction

“…Single XFEL pulses have been shown to be highly spatially and temporarily coherent [16,17]. As far as the incident pulse is concerned, the assumption of full coherence for CDI experiments is usually justified.…”