Photon information efficient communication through atmospheric turbulence

摘要

High photon-efficiency (many bits/photon) optical communication is possible with pulse-position modulationand direct detection, and high spectral efficiency (many bits/sec-Hz) optical communication is possible withquadrature-amplitude modulation and coherent detection. These high efficiencies, however, cannot be achievedsimultaneously unless multiple spatial modes are employed. Previous work for the vacuum-propagation channelhas shown that achieving 10 bits/photon and 5 bits/sec-Hz is impossible with coherent detection, and it requires189 low diffraction-loss spatial modes at the ultimate Holevo limit, and 4500 such modes at the Shannon limitfor on-off keying with direct detection. For terrestrial propagation paths, however, the effects of atmosphericturbulence must be factored into the photon and spectral efficiency assessments. This paper accomplishesthat goal by presenting upper and lower bounds on the turbulent channel’s ergodic Holevo capacity for M-modesystems whose transmitters use either focused-beam, Hermite-Gaussian (HG), or Laguerre-Gaussian (LG) modes,and whose receivers do M-mode detection either with or without adaptive optics. The bounds show that use ofadaptive optics will not be necessary for achieving high photon efficiency and high spectral efficiency throughatmospheric turbulence, although receivers which do not use adaptive optics will need to cope with considerablecrosstalk between the spatial patterns produced in their entrance pupils by the M-mode transmitter. Thebounds also show the exact theoretical equivalence of the HG and LG mode sets for this application, generalizinga result previously established for the vacuum-propagation channel. Finally, our results show that the FB modesoutperform the HG and LG modes in operation with and without adaptive optics.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.