Atmospheric changes in temperature and pressure that affect the refractive index and optical density of the air and, in turn, confound the transmission of optical signals have long been a major hurdle to successful free-space optical (FSO) communications. Atmospheric turbulence corrupts the phase purity of propagating optical beams, making it difficult to perform space-division multiplexing (SDM) and quantum-key distribution (QKD) functions, even for twisted light with orbital angular momentum (OAM) and its multiple modes with different helical phase.
To better understand the hurdles to successful FSO transmission, researchers from the University of Glasgow in Scotland, the Max Planck Institute (Erlangen, Germany), and the Universities of Otago (Dunedin, New Zealand), Ottawa (Ottawa, ON, Canada), and Rochester (Rochester, NY) developed a comprehensive model to study the effects of atmospheric changes on the phase of OAM beams and tested the model’s viability in a 1.6 km FSO link over the city of Erlangen in Germany. The model reveals weaknesses in prior simulations, and concludes that complex adaptive optics would be necessary for successful FSO transmission of OAM beams at lengths >500 m.
Read more at http://www.laserfocusworld.com/articles/print/volume-54/issue-01/world-news/free-space-optical-communications-atmospheric-turbulence-model-highlights-hurdles-to-fso-communications-using-twisted-oam-light.html