Time-dependent FEL wavefront propagation calculations: Fourier optics approach

Numerical examples of time-dependent FEL wavefront propagation calculation and coherence analysis are presented. The calculations are performed using the Synchrotron Radiation Workshop (SRW)—wave-optics computer code optimized for synchrotron radiation, and the 3D FEL simulation code GENESIS 1.3. In the examples described, the electric field obtained at the exit of an FEL undulator after running GENESIS in time-dependent mode is transformed from time to frequency domain, and then propagated, using Fourier-optics methods implemented in SRW, through interferometer-type optical schemes. Time-averaged intensity patterns, obtained in image planes of the interferometers as a result of these simulations, allow for straightforward coherence characterization by visibility of interference fringes. The presented examples show that Fourier optics-based wavefront propagation method, as implemented in the SRW, can be effectively used for detailed analysis of characteristics of FEL radiation in time and in frequency domains. Besides, due to its high accuracy and CPU-efficiency, this method is very well suited for optimization of optical beamlines for fourth generation synchrotron radiation sources, for which accurate treatment of wave-optical phenomena, aimed to preserve radiation coherence and time structure, is of paramount importance.