Abstract: We describe exact representations for partially coherent beams whose spectral degree of coherence remains invariant on propagation. Such beams are obtained by taking a random superposition of well-known coherent Bessel beams, each of which propagates without spreading in the transverse direction. The invariance in coherent properties for such partially coherent beams holds for propagation distances as large as the propagation distances of coherent Bessel beams.!4
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