Collapse and revival of spatial coherence on free-space propagation

We demonstrate analytically and numerically that the transverse spatial coherence (the spectral degree of coherence) of the field radiated by two partially correlated electromagnetic sources oscillates as a function of propagation distance in free space. In particular, it is possible for its magnitude to completely collapse to zero on propagation, and then return again to unity at a further distance. Our results are a direct demonstration of the wave character of the optical correlation functions. A similar oscillatory effect occurs for the degree of polarization.