Photon-counting detectors for optical communications

Photon counting direct detection allows communications past the classical limit of 2.88 bits/photon, and is an attractive solution for photon starved optical communications links, as in deep space scenarios. High bandwidth communications requires a corresponding high bandwidth detector with a high saturation fluence, especially in the presence of a large background noise flux, as is encountered during daytime optical link operations. Traditional photon counting optical detectors, such as Geiger mode avalanche photodiodes and photomultiplier tubes, have been developed primarily for low rate / low flux applications such as fluorescence spectroscopy. Ground based detectors have an additional requirement of large area since a single spatial mode transmit signal is scattered into many spatial modes by atmospheric turbulence, resulting in a solid angle times area (Lagrange) invariant that must be processed by the optical receiver train. As there is a physical limit to the solid area (2π steradians for single side illumination), there is thereby a corresponding minimum detector area that is proportional to the number of spatial modes.