31 August 2005 Coherence reduction using optical fibers
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Spatially partially coherent laser beams have shown good transmission properties in free space laser communications, and a variety of methods have been used to generate spatially partially coherent beams including rotating diffusers and modulated acousto-optic cells or liquid crystals. With respect to high data rate laser communications, however, those methods exhibit serious drawbacks, the most serious being their dependence on mechanically or electrically-induced changes in the medium that are relatively very slow when compared to the gigabit data rates. To solve this problem, a nonlinear effect in a single mode fiber is used to reduce the temporal coherence of the transmitted beam, and its high spatial coherence is then reduced by a multimode fiber bundle made of fibers having very small length differences on the order of the beam's coherence length. Using this method, both the temporal and spatial coherence of the fiber output beam are greatly reduced. A comparison of the propagation properties of the beam transmitted through this combination of optical fibers with those of the original coherent beam reveals a significant performance improvement. To our knowledge, producing a spatially partially coherent beam in this way has not been reported previously.
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Qingsong Wang, Qingsong Wang, Michael K. Giles, Michael K. Giles, } "Coherence reduction using optical fibers", Proc. SPIE 5892, Free-Space Laser Communications V, 58920N (31 August 2005); doi: 10.1117/12.620375; https://doi.org/10.1117/12.620375

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