Free-space optical communication involves the use of optical links across the space between two points, either within the Earth's atmosphere, or in outer space.

The optical links usually use laser light, although low-data-rate communication over short distances is possible using LEDs. IrDA is a very simple form of free-space optical communications.

For longer distances, a pair of telescopes is used at each end, with a laser and photosensors mounted in each telescope.

Free-space optical communications has a number of uses:

• as a last mile telecommunications link
• as a LAN link between buildings
• for communications between spacecraft, including elements of a satellite constellation
• for interstellar communication

For terrestrial applications, the limiting factors are the attenuation of the beam by:

• beam dispersion
• atmospheric absorption
• rain (lower attenuation)
• fog (10..~100dB/km attenuation)
• snow (lower attenuation)
• scintillation (lower attenuation)

For space applications, none of these defects applies, and free-space optical communications is potentially usable at interstellar distances.

See also

• Free Space Optics, FSO
• optical window, radio window, Free-space loss, Rayleigh scattering, Mie scattering
• IrDA, RONJA
• Optical telegraph for the early history of optical communication, including semaphore and smoke signals.

References

• Kontogeorgakis, Christos; Millimeter Through Visible Frequency Waves Through Aerosols-Particle Modeling, Reflectivity and Attenuation