Interferometry of attosecond pulse trains in the extreme ultraviolet wavelength region.

The temporal coherence of an attosecond optical field in the extreme ultraviolet wavelength region can be defined in terms of the extent of interference in time domain. We successfully measured this phenomenon both with and without spectral decomposition. We also report the results of using this approach to directly observe both symmetry and symmetry breaking of interference fringes in an attosecond pulse train.