Line of Sight Optical Wireless Communication Architecture
In this experiment conducted by the National Center for Space Studies (CNES) in Toulouse, France, a group of researchers engineered an Optical Wireless Communication (OWC) link between two buildings to test the efficiency of the laser communications system and the receiver. In order to measure the performance of the OWC link, they send two signals, the first being through the OWC channel and the other through an underground fire optic channel. The fiber optic channel served as a baseline for the OWC signal. The setup for this experiment consisted of a transmitter and a receiver located 265 meters away on a direct line of sight OWC channel and a 715-meter long optical fiber that sent the baseline signal. The transmitter was located 15 meters above the ground and the receiver was located 12 meters off the ground, see figure 1.
The measured signal was a 15 dBm optical signal generated from a 1550 nm NKT Photonics X15 Laser, this signal is passed through two independent phase stabilization systems so the signal is in phase for both the optical wireless channel and optical fiber. This experiment conducted at CNES spanned over a two-week period in late winter, where there were all types of atmospheric turbulence such as winds, snow, rain, etc. As expected the best results were observed where there were favorable weather conditions such s clear skies and low winds. The biggest issue that was seen with this experiment was that the OWC signal would experience high losses when compared to the baseline signal. From figure 1 in the description, it was noted that the principal component of the receiver is the Quad-Photodetector (QPD), and to overcome atmospheric losses especially over long distances, the QPD needs to be improved to increase the sensitivity and be able to detect the same signal at lower power.