Characterization of non-Gaussian mid-infrared free-electron laser beams by the knife-edge method

We report the characterization of mid-infrared free-electron laser (FEL) beams at the wavelength of 11 μm by the knife-edge method. From the knife-edge data we find that the FEL beam has a non-Gaussian shape. To represent the non-Gaussian beam shape we employ two methods: fitting the knife-edge data to some analytical functions with a few free parameters and numerical smoothing of the knife-edge data. Both methods work equally well. Using those data we can reconstruct the two-dimensional (2D) beam profiles at different positions around the focus by assuming that the 2D intensity distribution function is separable in x (horizontal) and y (vertical) directions. Using the 2D beam profiles at different positions around the focus, we find that the beam propagation factor (M2 factor) is ∼1.1 in both x and y directions. As a cross-check, we also carry out the burn pattern experiment to find that the behavior of the focused FEL beam along the propagation is consistent with the results obtained by the knife-edge method.