  Ultraviolet and extreme ultraviolet electric-fields produced by relativistic
electrons in an undulator of a synchrotron light source are characterized by
using spectral phase interferometry for direct electric-field reconstruction
(SPIDER). A tandem undulator with a phase shifter produces a pair of wavelength
shifted wave packets with some delay. The interferogram between the pair of the
wave packets is analyzed with a SPIDER algorithm, which is widely used for
ultrashort pulse characterization. As a result, a 10-cycle square shaped
electric-field is reconstructed. The waveform corresponds to the radiation from
an electron accelerated with the undulator which consists of 10 periods of
permanent magnets.
