Temporal and spatial coherence properties of Free Electron Laser pulses in the XUV regime

A. Singer, K. Tiedtke, and I. A. Vartanyants Deutsches Elektronen-Synchrotron, Notkestrasse 85, 22603 Hamburg, Germany (Dated: June 30, 2011) Abstract The average temporal (longitudinal) and spatial (transverse) coherence of free electron laser pulses in the extreme ultraviolet (XUV) at FLASH is measured by interfering two time-delayed partial beams directly on a CCD camera. Wavelengths between λ = 32nm and λ =8 nm are examined. A decrease of the coherence time for the fundamental wavelengths from τc = (6 ± 0.5) fs at 32 nm to τc = (2.9 ± 0.5) fs at 8 nm is measured. At λ = 8 nm the fundamental wavelength and the third harmonic of 24 nm are compared to each other. For 8 nm radiation as third harmonic of 24 nm a coherence time of τc = (2.5 ± 0.5) fs is observed. The spatial coherence of 24 nm and 8 nm fundamental pulses are found to be very similar. The visibility decreases to 50% of the maximum visibility at about 3.2 mm overlap of the partial beams, which corresponds to 42% of the beam diameter (FWHM) at a distance of 90 m from the exit of the undulator. These results are analysed in terms of the Gaussian Schell-model (GSM) resulting in six contributing modes to the total radiation. In addition, the correlation of the visibility between the fundamental radiation at 24 nm and its third harmonic at λ = 8 nm is investigated for identical shots.