Ultimate channel capacity of free-space optical communications [Invited]

The ultimate classical information capacity of multiple-spatial-mode, wideband optical communications in vacuum between soft-aperture transmit and receive pupils is considered. The ultimate capacity is shown to be achieved by coherentstate encoding and joint measurements over entire code words. This capacity is compared with the capacities realized with the same encoding and homodyne or heterodyne detection, which are single-channel-use measurements. Realistic background spectral- radiance values are used to obtain tight bounds on the capacity of single-spatial-mode, narrowband 1.55 mu m wavelength free-space communications in the presence of background light. (c) 2005 Optical Society of America.