40-Gb/s DWDM free-space optical transmission link over 4.4 km

We simultaneously transmit 16 separate 2.5 Gb/s wavelength data channels, with a 200 Ghz channel spacing, error-free, over a horizontal free space distance of 4.4 km. We believe this result represents the largest bandwidth transmitted at one time over such a distance, without the use of optical transmission fiber