Compact Hartmann wavefront sensor with a large field of view 15.2 x 15.2 mm including movements (xy direction and tip/tilt) for alignment mounted behind user chamber
During the last decades, DESY has been developing wavefront sensors (WFS) for the soft X-ray spectral range of approx. 1 - 60 nm, in collaboration with the Institut für Nanophotonik Göttingen e.V. (IFNANO, former Laser-Laboratorium Göttingen e.V.). Wavefront sensors have been shown to be an important and reliable tool for the fast and real-time determination of the wavefront. This is required for the alignment of optical components and beam characterization. Wavefront sensors provide an efficient and accurate determination of the size and position of the focus, as well as an accurate prediction of both. This means, the calculation of beam profiles along the caustic can be achieved without the need for a time, space and labour intensive experiment. The wavefront sensors employed are adapted to the unique beam properties of FLASH, with particular regard to wavelength, pulse energy, and coherence. The Hartmann pinhole plate splits the incident FEL beam into partial beams and illuminates the camera chip. This can be a phosphor-coated CCD or CMOS chip, as well as a backside-thinned CCD chip. The wavefront of single pulses is reconstructed using a modal approach, based on lateral displacements in the beam spot pattern when compared to the reference pattern. A Zernike analysis is a method of assessing wavefront aberrations. Such aberrations can be either inherent in the beam or introduced by the optical system. From the wavefront and the spot intensity distribution (beam profile) second moment beam parameters such as waist position, waist size, beam width, divergence, Rayleigh length, and M2 are calculated. The combination of Fresnel integration and the Schell model facilitates the back-propagation of both fully coherent and partially coherent beams to obtain intensity profiles in any plane along the optical axis. A compact wavefront sensor with a large field of view for the spectral range of approx. 6 nm – 40 nm is available for flexible setup. The beamlines with K-B optics systems (FL23, FL24 and later FL11) are equipped with a new differential pumping unit containing an integrated wavefront sensor located directly after the K-B optics system. This newly designed wavefront sensor (spectral range approx. 2 nm – 30 nm) is located in front of the focus position and permanently installed under an angle on 45° to the beam. The beam is directed to the WFS by means of a Ni mirror, which is adjustable in 6 axes. The straight beam can still be used for experiments, since only the deflecting mirror has to be moved into the beam for wavefront sensor measurements (optics alignment and focus characterization). This is a fast and space-saving method for beam alignment.
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References:
Barbara Keitel et al., Comparison of wavefront sensing and ablation imprinting for FEL focus diagnostics at FLASH2, Optics Express 32(12), 21532-21552 (2024). Article
Barbara Keitel et al., Hartmann wavefront sensors and their application at FLASH, Proc. PhotonDiag2015, J. Synchrotron Rad. 23, 43-49 (2016). Article
B. Keitel et al., Hartmann wavefront measurements at FLASH, Advances in X-ray Free-Electron Lasers II: Instrumentation, Prague, Czech Republic, 04/17/2013 - 04/18/2013 Proceedings of SPIE 8778, 877814-1--1877814-7 (2013). Article
B. Flöter et al., Beam parameters of FLASH beamline BL1 from Hartmann wavefront measurements, Nucl. Instrum. Methods A 635, 108-112 (2011). Article
B. Flöter et al.,EUV Hartmann sensor for wavefront measurements at the Free-electron LASer in Hamburg, New J. Phys. 12, 083015 (2010). Article
