22 January 2002 Rayleigh link to overcome fog attenuation
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Abstract
In case of fog ground to ground free space link with a laser is almost impossible because of the strong attenuation. In principle this event is relatively rare and the usual solution is to provide an alternate radio link in such a situation. However, in several practical cases, optical links for the 'last mile' are required in order to avoid to obtain licensing for a radio channel from local authorities. Usually, however, the fog has a vertical thickness that is very limited. It is not uncommon, for instance, that in foggy nights one can easily see 'stars' while a direct vision over the ground is completely forbidden. In this case one can send a modulated beam 'onto the sky' and recover it from its side by means of Rayleigh scattering. In order to achieve competitive data rate, however, one is forced to make on-ship fast tracking of the pulse train, in order to accumulate signal for a significant range of the Rayleigh beacon. A preliminary photon budget shows that such an approach can give interesting data rate with reasonable laser power and affordable optics. A preliminary discussion of potential drawbacks and circumventing possibilities is also reported.
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Roberto Ragazzoni, Roberto Ragazzoni, Emiliano Diolaiti, Emiliano Diolaiti, } "Rayleigh link to overcome fog attenuation", Proc. SPIE 4489, Free-Space Laser Communication and Laser Imaging, (22 January 2002); doi: 10.1117/12.453220; https://doi.org/10.1117/12.453220
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