The variation of global coherence of light is examined in the framework of coherent mode representation. Given the coherent mode representation of the field in input plane, we seek the possible changes of mode functions in subsequent plane as the field propagates. We introduce the entropic measure to describe global coherence quantitatively. It is explicitly shown that the global coherence may in general be enhanced, be reduced, or remain unchanged on propagation, however, when the mode functions form a complete set and each of them equally contributes to the field correlation in input plane, the field necessarily acquires some amount of global coherence in subsequent plane. The physical implication of this result is analyzed and the possible connection to van Cittert-Zernike theorem is discussed.
© 2007 Optical Society of AmericaPDF Article