  We present a direct measurement of the spatiotemporal coherence of parametric
down-conversion in the range of negative group-velocity dispersion. In this
case, the frequency-angular spectra are ring-shaped and temporal coherence is
coupled to spatial coherence. Correspondingly, the lack of coherence due to
spatial displacement can be compensated with the introduction of time delay. We
show a simple technique, based on a modified Mach-Zehnder interferometer, which
allowed us to measure time coherence and near-field space coherence
simultaneously, with complete control of both variables. This technique will be
also suitable for the measurement of second-order coherence, where the main
applications are related to the two-photon spectroscopy.
