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Recently, Saldin et al. (NIM A 539, 499 (2005)) introduced a novel scheme to characterize ultra-short electron bunches in a FEL. The method is based on producing an "optical copy" of the electron bunch , which can then be easily analyzed using well-known non-linear optical techniques. To this end, a near-IR laser beam is overlapped with the electron beam in the first undulator of an optical klystron. In the following dispersive section the laser-induced energy modulation is transformed into a density modulation. The modulated electron bunch then produces a strong optical pulse in the second undulator. Analysis of this near-IR pulse (the optical copy) then provides information about the length , the lisce emittance and the slice energy spread of the original electron bunch. We discuss the implementation of such a measurement set-up at the FLASH at DESY and investigate the influence of various parameters on the performance of the device. |