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

Abstract

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 (M 2 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.

Imprinting a Focused X-Ray Laser Beam to Measure Its Full Spatial Characteristics

“…In the latter case the beam characterization was performed for the purposes of interaction experiments aimed at radiation damage to X-ray optics induced by intense hard X-rays [24]. Techniques of ablative imprints find applications not only in the short-wavelength spectral domain, e.g, in testing of new focusing optics [25,26] and measurements of coherence properties of FEL beams [27], but also in the longwavelength spectral region [28].…”

Fluorescence-based knife-edge beam diameter measurement to characterize X-ray beam profiles in reflection geometry

“…In order to get a full knowledge of the 2D distribution and allow the reconstruction of the 2D beam profile, one should scan the edge in the same plane along other directions. In this work, as mentioned by Qin [27], the edge was scanned along the horizontal and vertical axis, assuming that the 2D-intensity distribution was a function of two separable variables in these two directions. Finally, the diameter is defined as the Full-Width at Half Maximum (FWHM) of the signal presented in Fig.…”

“…3a by "+" indicate that the number of points was satisfying for extracting a general conclusion from this experiment. We applied the SavitzkyGolay smoothing [26] over 25 points in order to obtain a smoothed signal easier to process as described by Qin [27]. We finally computed the first derivative, then Gaussian fit, in order to retrieve the beam profile along the horizontal axis.…”

Terahertz confocal microscopy with an injection-seeded terahertz parametric generator

“…We employed the knife-edge method to measure the scanning spot size around the focal plane. 13 We recorded the intensity profiles of x and y axes by sequentially blocking the beam with a knife blade. Then we fitted the intensity profiles to the Gaussian function to get the beam widths.…”