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An 8-beam free space optics laser link, rated for 1 Gbit/s at a distance of approximately 2km. The receptor is the large disc in the middle, the transmitters the smaller ones.To the top and right side a monocular for assisting the alignment of the two heads.

In telecommunications, Free Space Optics (FSO) is an optical communication technology that uses light propagating in free space to transmit data between two points. The technology is useful where the physical connections by the means of fibre optic cables are impractical due to high costs or other considerations.

Contents

History

Optical communications, in various forms, have been used for thousands of years. The Ancient Greeks polished their shields to send signals during battle. In the modern era, semaphores and wireless solar telegraphs called heliographs were developed, using coded signals to communicate with their recipients.

In 1880 Alexander Graham Bell and his then-assistant Charles Sumner Tainter created the photophone, at Bell's newly establish Volta Laboratory in Washington, D.C. Bell considered it his most important invention. The device allowed for the transmission of sound on a beam of light. On June 3, 1880, Bell conducted the world's first wireless telephone transmission between two building rooftops.[1] Its first practical use came in military communication systems many decades later.

The invention of lasers in the 1960s revolutionized free space optics. Military organizations were particularly interested and boosted their development. However the technology lost market momentum when the installation of optical fiber networks for civilian uses was at its peak.

Usage and technologies

Free Space Optics are additionally used for communications between spacecraft. The optical links can be implemented using infrared laser light, although low-data-rate communication over short distances is possible using LEDs. Maximum range for terrestrial links is in the order of 2-3 km,[2] but the stability and quality of the link is highly dependent on atmospheric factors such as rain, fog, dust and heat. Amateur radio operators have achieved significantly farther distances (173 miles in at least one occasion) using incoherent sources of light from high-intensity LEDs. [3] However, the low-grade equipment used limited bandwidths to about 4kHz. In outer space, the communication range of free-space optical communication is currently in the order of several thousand kilometers[4], but has the potential to bridge interplanetary distances of millions of kilometers, using optical telescopes as beam expanders[5]. IrDA is also a very simple form of free-space optical communications.

Secure free-space optical communications have been proposed using a laser N-slit interferometer where the laser signal takes the form of an interferometric pattern. Any attempt to intercept the signal causes the collapse of the interferometric pattern.[6] Although this method has been demonstrated at laboratory distances in principle it could be applied over large distances in space.

Applications

Two solar-powered satellites communicating optically in space via lasers.

Typically scenarios for use are:

  • LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds.
  • LAN-to-LAN connections in a city. example, Metropolitan area network.
  • To cross a public road or other barriers which the sender and receiver do not own.
  • Speedy service delivery of high-bandwidth access to optical fiber networks.
  • Converged Voice-Data-Connection.
  • Temporary network installation (for events or other purposes).
  • Reestablish high-speed connection quickly (disaster recovery).
  • As an alternative or upgrade add-on to existing wireless technologies.
  • As a safety add-on for important fiber connections (redundancy).
  • For communications between spacecraft, including elements of a satellite constellation.
  • For inter- and intra[7]-chip communication.

The light beam can be very narrow, which makes FSO hard to intercept, improving security. In any case, it is comparatively easy to encrypt any data traveling across the FSO connection for additional security. FSO provides vastly improved EMI behavior using light instead of microwaves.

Advantages

RONJA is a free implementation of FSO utilizing high-intensity LEDs.

Disadvantages

For terrestrial applications, the principal limiting factors are:

These factors cause an attenuated receiver signal and lead to higher bit error ratio (BER). To overcome these issues, vendors found some solutions, like multi-beam or multi-path architectures, which use more than one sender and more than one receiver. Some state-of-the-art devices also have larger fade margin (extra power, reserved for rain, smog, fog). To keep an eye-safe environment, good FSO systems have a limited laser power density and support laser classes 1 or 1M. Atmospheric and fog attenuation, which are exponential in nature, limit practical range of FSO devices to several kilometres.

See also

References

Notes
  1. ^ Carson 2007, pg.76-78
  2. ^ Analysis of Free Space Optics as a Transmission Technology, U.S. Army Information Systems Engineering Command, page 3.
  3. ^ A 173-mile 2-way all-electronic optical contact
  4. ^ http://www.esa.int/esaTE/SEMN6HQJNVE_index_0.html
  5. ^ http://silicium.dk/pdf/speciale.pdf Optical Communications in Deep Space, University of Copenhagen
  6. ^ F. J. Duarte, Secure interferometric communications in free space, Opt. Commun. 205, 313-319 (2002).
  7. ^ http://www.cs.utah.edu/cmpmsi/papers09/paper1.pdf CMP-MSI: 3rd Workshop on Chip Multiprocessor Memory Systems and Interconnects held in conjunction with the 36th International Symposium on Computer Architecture, June 2009.
Bibliography
  • Carson, Mary Kay (2007). "8". Alexander Graham Bell: Giving Voice To The World. Sterling Biographies. 387 Park Avenue South, New York, NY 10016: Sterling Publishing Co., Inc.. pp. 76-78. ISBN 978-1-4027-3230-0. OCLC 182527281. http://books.google.ca/books?id=a46ivzJ1yboC. 
  • Bell, A. G.: "On the Production and Reproduction of Sound by Light", American Journal of Science, Third Series, vol. XX, #118, October 1880, pp. 305 - 324; also published as "Selenium and the Photophone" in Nature, September 1880.
  • Kontogeorgakis, Christos; Millimeter Through Visible Frequency Waves Through Aerosols-Particle Modeling, Reflectivity and Attenuation

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