A two-photon correlator has been constructed and successfully operated in measuring the true two-photon correlation of synchrotron radiation in the soft-x-ray region. The strong influence of the nonstationary features of the optical field, which is the so-called accidental correlation induced by the systematic bunched structure of the stored current in a storage ring, has been removed by a coherence time modulation technique. The explicit bunching effect of the measured two-photon correlation shows that synchrotron radiation is a chaotic rather than coherent radiation, indicating a chaotic nature of the stored current. This experimental method provides a way to measure the instantaneous emittance with the time scale of the coherence time ${\tau }_c,$ and to characterize the coherence property of incomplete free-electron lasers, such as self-amplified spontaneous emission.

• Received 18 March 1999

DOI:https://doi.org/10.1103/PhysRevA.60.3262