Optical Communication Systems: Free Space Optical Communications

Free space optical (FSO) communication is the wireless transmission of data via a modulated optical beam directed through free space, without fiber optics or other optical systems guiding the light. The fundamental idea goes back to ancient times, as light (or smoke) signals were used to transmit information. From a more modern point of view, Graham Bell's patent on the photophone may mark the onset of modern FSO techniques, as it transmitted audio signals (i.e., voice) via the modulation of sunlight. A renaissance of FSO systems started with the availability of lasers, light sources with high output power and high coherence, which allowed the accurate direction of the light beams over long distances. During the 1970s and 1980s the main proposed application of FSO systems was for secure and long distance (50-1000 km) communication, mainly targeted for ground-satellite or satellite-satellite communication. This focus changed drastically over the last decade as a new market for FSO grew in the establishment of high bandwidth data link and their integration over a locally restricted area.