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Single-shot spectro-temporal characterization of XUV pulses from a seeded free-electron laser

Single-shot characterization of FEL pulses is a challenging task, mainly due to the strong absorption of XUV light in solid-state crystals and to the ultra-short duration of the pulses under scrutiny. Until now, only few effective techniques have been proposed to measure the duration of a femtosecond short-wavelength SASE FEL pulse. In our work, recently published in Nature Communication, we take a step further and demonstrate the possibility to reconstruct, both in the temporal and spectral domains, the envelopes and phases of a pulse generated by a seeded FEL. The proposed method is based on the generation of two FEL pulses and on their characterization through spectral phase interferometry for direct electric-field reconstruction (SPIDER).
In an FEL, the electron-beam energy is generally characterized by a time-dependent profile, E(t).If two seed replicas separated by a time interval toverlap with a quadratic zone of the E(t) curve, two FEL pulses are generated, which are separated by the same temporal distance and shifted in frequency by a shear Wdepending on the quadratic curvature. Moreover, for a sufficiently homogeneous electron beam (i.e., constant emittance, energy spread and current) and a properly tuned FEL, the generated pulses have equal intensities and temporal phases. These are the conditions realized at FERMI and needed for carrying out the SPIDER analysis. A fine-tuning of the accelerator parameters allows the generation of stable and reproducible charge distributions with homogeneous properties, see for example the current profile in Fig.1a, where a plot of the measured electron-beam energy profile is also shown. The highlighted region is characterized by a dominant quadratic dependence of E(t). In such a region, higher-order nonlinear terms are negligible.
The twin FEL pulses yield a spectral interferogram on which the SPIDER algorithm relies. As shown in Fig.1b, the interferograms evolve as a function of the temporal position of the seed replicas with respect to the electron beam. Vertical cuts of the map provide single-shot interferograms, see Fig.1c. The good contrast of the interference fringes is a clear indication of the similarity of the interfering pulses. In the highlighted region of Fig. 1b the fringes in the interferograms do not evolve significantly. This is a nice proof of phase locking between the two pulses and a confirmation of the homogeneity of the electron beam. The measured shift between spectra separated by the temporal distance τ corresponds to the spectral shear Ω. For the reported case, the two seed pulses have a duration of 125 fs (FWHM) and a positive linear frequency chirp.

Figure 1. a) Energy and current time-dependent profiles of the FERMI electron beam (the bunch head comes at smaller times). In the highlighted region, the energy profile is characterized by a dominant and constant quadratic term. b) FEL intereferograms as a function of the temporal position of the leading seed pulse with respect to the electron beam. The region of interest corresponds to the overlap between the seed replicas and the electron-beam quadratic region. Also shown is the definition of the parameters τ and Ω. For the reported experiments, Ω= 1.7 x 1013 Hz. c), Vertical cut of the map, providing a single-shot interferogram.

 

The interferograms in the region of interest were used to carry out the SPIDER analysis of the FEL pulse. Fig. 2 shows the result of the reconstruction in the spectral (Fig.2a) and temporal (Fig.2b) domains for a particular interferogram located in the middle of the region of interest. The obtained spectral bandwidth and pulse duration are, respectively, 6.3 x 10nm and 71 fs (FWHM).
Our results are supported by, and provide support to, previously published theory. The method is independent of the photon energy and decoupled from machine parameters. Therefore, it can be easily implemented on present and future facilities, providing users with the unique possibility to monitor and shape at will the fundamental properties of a seeded FEL pulse. 

Figure 2. a) Reconstruction of the spectral envelope and phase from an interferogram acquired in the middle of the region of interest shown in Fig. 1b. b) Reconstruction of the temporal envelope and phase.

 

This research was conducted by the following research team:

 
G. De Ninno1,2, D. Gauthier1 , B. Mahieu3 , P. Rebernik Ribič1, E. Allaria1, P. Cinquegrana1 , M. B. Danailov1 , A. Demidovich1 , E. Ferrari1,4, L. Giannessi1,5, G. Penco1, P. Sigalotti1, M. Stupar2
 

1 Elettra - Sincrotrone Trieste, Trieste 34149, Italy
2 Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica 5001, Slovenia
3 Laboratoire dOptique Appliquee, UMR 7639, ENSTA-CNRS-Ecole, France
4 Università degli Studi di Trieste, Dipartimento di Fisica, 34100 Trieste, Italy
5 Theory Group ENEA C.R. Frascati, 00044 Frascati, Italy


Contact person:

Giovanni De Ninno, e-mail: govanni.deninno@elettra.eu


Reference

G. De Ninno, D. Gauthier, B. Mahieu, P. Rebernik Ribič, E.Allaria, P. Cinquegrana, M. B. Danailov, A. Demidovich, E. Ferrari, L. Giannessi, G. Penco, P. Sigalotti, M. Stupar “Single-shot spectro-temporal characterization of XUV pulses from a seeded free-electron laser“ Nature Communications 6, 8075 (2015), doi:10.1038/ncomms9075.

 

Last Updated on Tuesday, 22 September 2015 13:58