Covariance of the received intensity of a partially coherent laser speckle pattern in the turbulent atmosphere

Effects of residual coherence on the scintillation of a partially coherent beam

Recent results show that partially coherent beams (PCB) can be conveniently generated in a multimode fiber and modulated with data at gigabit per second rates, which makes them attractive for free-space optical communication through turbulent atmosphere. An important feature of these realistic beams in contrast to model ones is the presence of residual coherence between pairs of points spatially separated by more than a few coherence radii on the beam aperture. In the present work we experimentally study the influence of this residual coherence on the scintillation of a partially coherent beam in a laboratory turbulence. It is shown that the total scintillation can be considered as a combination of scintillations of the coherent and incoherent parts of the full beam. When residual coherence is large the scintillation is mostly due to speckle motion on the detector. In the opposite case, the scintillation index settles at a low value pertaining to "ideal" homogeneous PCB.

Scintillations of partially coherent multiple Gaussian beams in turbulence

For an incidence composed of partially coherent multiple Gaussian beams, Huygens-Fresnel principle-based on-axis scintillation index is formulated in a weakly turbulent homogeneous horizontal atmospheric path. Our general formulation is applied to two examples of partially coherent annular and partially coherent flat-topped Gaussian beams. Compared to partially coherent single Gaussian beam scintillations, annular beam scintillations seem to possess higher values for all partial coherence levels, whereas flat-topped Gaussian beam intensity fluctuations are slightly larger, especially at lower coherence levels and at larger source sizes. At the same source partial coherence, annular beams exhibit smaller scintillations for larger ring sizes. For flat-topped Gaussian beams, except for very small and very large source sizes, as the number of Gaussian beams forming the flatness increases, intensity fluctuations also increase, a trend applicable for different degrees of coherence. A trend valid for both single and multiple Gaussian incidence, except for certain annular beams of large primary beam sizes, is that the scintillations decrease as the source becomes less coherent. Being applicable for all degrees of source coherences, for both beams examined, scintillations increase steadily as the Rytov plane wave scintillation index increases.