Free Space Optics (FSO) is an optical communication technology that uses modulated light propagating through the atmosphere to transmit data wirelessly for telecommunications or computer networking applications. This is in contrast to Fibre Optics which uses “guided” media, namely the fibre, to carry the optical signals
Free Space Optics typically uses transmission in the infrared bands using either Laser or LED devices. Almost all modern long range FSO links use Laser technology which offers superior beam collimation, high output power and high data rates.
Limitations of Free Space Optics are absorption and scatter in the atmosphere by effects such as thick fog, snow and dust storms, which restricts practical reliable link distances to around 1-4km depending on region. In contrast to Microwave and Millimeter Wave links, FSO does not suffer significant attenuation in moderate and heavy rain, making it a complimentary technology and one suited to high rainfall regions.
Modern FSO links offer up to 4km distance and capacities up to 1.5Gbps per link, and feature advanced technologies such as Automatic Transmit Power Control (ATPC) to increase signal margin and availability in adverse weather conditions
Free Space Optics is a Point to Point (P2P) technology due to the narrow beams involved. These narrow beams are typically 0.1 to 0.5 degrees divergence and require stable or solid mounting points such as building structures.
Examples of commercial Free Space Optics links are found here