Free-space optical communication

Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking. "Free space" means air, outer space, vacuum, or something similar. This contrasts with using solids such as optical fiber cable.

The technology is useful where the physical connections are impractical due to high costs or other considerations.

An 8-beam free space optics laser link, rated for 1 Gbit/s. The receptor is the large disc in the middle, the transmitters the smaller ones. At the top right corner is a monocular for assisting the alignment of the two heads.


A photophone receiver and headset, one half of Bell and Tainter's optical telecommunication system of 1880

Optical communications, in various forms, have been used for thousands of years. The Ancient Greeks used a coded alphabetic system of signalling with torches developed by Cleoxenus, Democleitus and Polybius.[1] 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 assistant Charles Sumner Tainter created the photophone, at Bell's newly established Volta Laboratory in Washington, DC. 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 buildings, some 213 meters (700 feet) apart.[2][3]

Its first practical use came in military communication systems many decades later, first for optical telegraphy. German colonial troops used heliograph telegraphy transmitters during the Herero and Namaqua genocide starting in 1904, in German South-West Africa (today's Namibia) as did British, French, US or Ottoman signals.

Germany blinker signal lamp - National World War I Museum - Kansas City, MO - DSC07704
WW I German Blinkgerät

During the trench warfare of World War I when wire communications were often cut, German signals used three types of optical Morse transmitters called Blinkgerät, the intermediate type for distances of up to 4 km (2.5 miles) at daylight and of up to 8 km (5 miles) at night, using red filters for undetected communications. Optical telephone communications were tested at the end of the war, but not introduced at troop level. In addition, special blinkgeräts were used for communication with airplanes, balloons, and tanks, with varying success.

A major technological step was to replace the Morse code by modulating optical waves in speech transmission. Carl Zeiss, Jena developed the Lichtsprechgerät 80/80 (literal translation: optical speaking device) that the German army used in their World War II anti-aircraft defense units, or in bunkers at the Atlantic Wall.[4]

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.

Many simple and inexpensive consumer remote controls use low-speed communication using infrared (IR) light. This is known as consumer IR technologies.

Usage and technologies

Free-space point-to-point optical links can be implemented using infrared laser light, although low-data-rate communication over short distances is possible using LEDs. Infrared Data Association (IrDA) technology is a very simple form of free-space optical communications. On the communications side the FSO technology is considered as a part of the optical wireless communications applications. Free-space optics can be used for communications between spacecraft.[5]

Commercial products

  • In 2008, MRV Communications introduced a free-space optics (FSO)-based system with a data rate of 10 Gbit/s initially claiming a distance of 2 km at high availability.[6] This equipment is no longer available; before end-of-life, the product's useful distance was changed down to 350 m.[7]
  • In 2013, the company MOSTCOM started to serially produce a new wireless communication system[8] that also had a data rate of 10 Gbit/s as well as an improved range of up to 2.5 km, but to get to 99.99% uptime the designers used an RF hybrid solution, meaning the data rate drops to extremely low levels during atmospheric disturbances (typically down to 10 Mbit/s). In April 2014, the company with Scientific and Technological Centre "Fiord" demonstrated the transmission speed 30 Gbit/s under "laboratory conditions".
  • LightPointe offers many similar hybrid solutions to MOSTCOM's offering.[9]

Useful distances

The reliability of FSO units has always been a problem for commercial telecommunications. Consistently, studies find too many dropped packets and signal errors over small ranges (400 to 500 meters). This is from both independent studies, such as in the Czech republic,[10] as well as formal internal nationwide studies, such as one conducted by MRV FSO staff.[11] Military based studies consistently produce longer estimates for reliability, projecting the maximum range for terrestrial links is of the order of 2 to 3 km (1.2 to 1.9 mi).[12] All studies agree the stability and quality of the link is highly dependent on atmospheric factors such as rain, fog, dust and heat.

Extending the useful distance

DARPA ORCA official concept art
DARPA ORCA official concept art created c. 2008

The main reason terrestrial communications have been limited to non-commercial telecommunications functions is fog. Fog consistently keeps FSO laser links over 500 meters from achieving a year-round bit error rate of 1 per 100,000. Several entities are continually attempting to overcome these key disadvantages to FSO communications and field a system with a better quality of service. DARPA has sponsored over US$130 million in research towards this effort, with the ORCA and ORCLE programs.[13][14][15]

Other non-government groups are fielding tests to evaluate different technologies that some claim have the ability to address key FSO adoption challenges. As of October 2014, none have fielded a working system that addresses the most common atmospheric events.

FSO research from 1998–2006 in the private sector totaled $407.1 million, divided primarily among four start-up companies. All four failed to deliver products that would meet telecommunications quality and distance standards:[16]

  • Terabeam received approximately $575 million in funding from investors such as Softbank, Mobius Venture Capital and Oakhill Venture Partners. AT&T and Lucent backed this attempt.[17][18] The work ultimately failed, and the company was purchased in 2004 for $52 million (excluding warrants and options) by Falls Church, Va.-based YDI, effective June 22, 2004, and used the name Terabeam for the new entity. On September 4, 2007, Terabeam (then headquartered in San Jose, California) announced it would change its name to Proxim Wireless Corporation, and change its NASDAQ stock symbol from TRBM to PRXM.[19]
  • AirFiber received $96.1 million in funding, and never solved the weather issue. They sold out to MRV communications in 2003, and MRV sold their FSO units until 2012 when the end-of-life was abruptly announced for the Terescope series.[7]
  • LightPointe Communications received $76 million in start-up funds, and eventually reorganized to sell hybrid FSO-RF units to overcome the weather-based challenges.[20]
  • The Maxima Corporation published its operating theory in Science (magazine),[21] and received $9 million in funding before permanently shutting down. No known spin-off or purchase followed this effort.
  • Wireless Excellence developed and launched CableFree UNITY solutions that combine FSO with millimeter wave and radio technologies to extend distance, capacity and availability, with a goal of making FSO a more useful and practical technology.[22]

One private company published a paper on November 20, 2014, claiming they had achieved commercial reliability (99.999% availability) in extreme fog. There is no indication this product is currently commercially available.[23]


The massive advantages of laser communication in space have multiple space agencies racing to develop a stable space communication platform, with many significant demonstrations and achievements.

Operational systems

The first gigabit laser-based communication was achieved by the European Space Agency and called the European Data Relay System (EDRS) on November 28, 2014. The system is operational and is being used on a daily basis.


NASA's OPALS announced a breakthrough in space-to-ground communication December 9, 2014, uploading 175 megabytes in 3.5 seconds. Their system is also able to re-acquire tracking after the signal was lost due to cloud cover.

In the early morning hours of Oct. 18, NASA’s Lunar Laser Communication Demonstration (LLCD) made history, transmitting data from lunar orbit to Earth at a rate of 622 Megabits-per-second (Mbps). LLCD was flown aboard the Lunar Atmosphere and Dust Environment Explorer satellite (LADEE), whose primary science mission was to investigate the tenuous and exotic atmosphere that exists around the moon.

In January 2013, NASA used lasers to beam an image of the Mona Lisa to the Lunar Reconnaissance Orbiter roughly 390,000 km (240,000 mi) away. To compensate for atmospheric interference, an error correction code algorithm similar to that used in CDs was implemented.[24]

A two-way distance record for communication was set by the Mercury laser altimeter instrument aboard the MESSENGER spacecraft, and was able to communicate across a distance of 24 million km (15 million miles), as the craft neared Earth on a fly-by in May, 2005. The previous record had been set with a one-way detection of laser light from Earth, by the Galileo probe, of 6 million km in 1992. Quote from Laser Communication in Space Demonstrations (EDRS)

Commercial use

Various planned satellite constellations such as SpaceX's Starlink intended to provide global broadband coverage employ laser communication for inter-satellite links between the several hundred to thousand satellites effectively creating a space-based optical mesh network.


Ronja beam Prostejov
RONJA is a free implementation of FSO using high-intensity LEDs.

In 2001, Twibright Labs released Ronja Metropolis, an open source DIY 10 Mbit/s full duplex LED FSO over 1.4 km[25][26] In 2004, a Visible Light Communication Consortium was formed in Japan.[27] This was based on work from researchers that used a white LED-based space lighting system for indoor local area network (LAN) communications. These systems present advantages over traditional UHF RF-based systems from improved isolation between systems, the size and cost of receivers/transmitters, RF licensing laws and by combining space lighting and communication into the same system.[28] In January 2009, a task force for visible light communication was formed by the Institute of Electrical and Electronics Engineers working group for wireless personal area network standards known as IEEE 802.15.7.[29] A trial was announced in 2010, in St. Cloud, Minnesota.[30]

Amateur radio operators have achieved significantly farther distances using incoherent sources of light from high-intensity LEDs. One reported 173 miles (278 km) in 2007.[31] However, physical limitations of the equipment used limited bandwidths to about 4 kHz. The high sensitivities required of the detector to cover such distances made the internal capacitance of the photodiode used a dominant factor in the high-impedance amplifier which followed it, thus naturally forming a low-pass filter with a cut-off frequency in the 4 kHz range. Use of lasers can reach very high data rates which are comparable to fiber communications.

Projected data rates and future data rate claims vary. A low-cost white LED (GaN-phosphor) which could be used for space lighting can typically be modulated up to 20 MHz.[32] Data rates of over 100 Mbit/s can be easily achieved using efficient modulation schemes and Siemens claimed to have achieved over 500 Mbit/s in 2010.[33] Research published in 2009, used a similar system for traffic control of automated vehicles with LED traffic lights.[34]

In September 2013, pureLiFi, the Edinburgh start-up working on Li-Fi, also demonstrated high speed point-to-point connectivity using any off-the-shelf LED light bulb. In previous work, high bandwidth specialist LEDs have been used to achieve the high data rates. The new system, the Li-1st, maximizes the available optical bandwidth for any LED device, thereby reducing the cost and improving the performance of deploying indoor FSO systems.[35]

Engineering details

Typically, best use scenarios for this technology are:

  • LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds
  • LAN-to-LAN connections in a city, a 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
    • Especially powerful in combination with auto aiming systems, this way you could power moving cars or you can power your laptop while you move or use auto-aiming nodes to create a network with other nodes.
  • As a safety add-on for important fiber connections (redundancy)
  • For communications between spacecraft, including elements of a satellite constellation
  • For inter- and intra-chip communication[36]

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 electromagnetic interference (EMI) behavior compared to using microwaves.

Technical advantages

Range limiting factors

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. However the free space optics, based on 1550 nm wavelength, have considerably lower optical loss than free space optics, using 830 nm wavelength, in dense fog conditions. FSO using wavelength 1550 nm system are capable of transmitting several times higher power than systems with 850 nm and are at the same time safe to the human eye (1M class). Additionally, some free space optics, such as EC SYSTEM,[37] ensure higher connection reliability in bad weather conditions by constantly monitoring link quality to regulate laser diode transmission power with built-in automatic gain control.[37]

See also


  1. ^ "Book X". The Histories of Polybius. 1889. pp. 43–46.
  2. ^ Mary Kay Carson (2007). Alexander Graham Bell: Giving Voice To The World. Sterling Biographies. New York: Sterling Publishing. pp. 76–78. ISBN 978-1-4027-3230-0.
  3. ^ Alexander Graham Bell (October 1880). "On the Production and Reproduction of Sound by Light". American Journal of Science, Third Series. XX (118): 305–324. also published as "Selenium and the Photophone" in Nature, September 1880.
  4. ^ "German, WWII, WW2, Lichtsprechgerät 80/80". LAUD Electronic Design AS. Archived from the original on July 24, 2011. Retrieved June 28, 2011.
  5. ^ Schütz, Andreas; Giggenbach, Dirk (10 November 2008). "DLR communicates with TerraSAR-X Earth Observation satellite via laser beam". DLR Portal. Deutsches Zentrum für Luft und Raumfahrt (DLR) - German Aerospace Center. Retrieved 14 March 2018.
  6. ^ "TereScope 10GE". MRV Terescope. Archived from the original on 2014-08-18. Retrieved October 27, 2014.
  7. ^ a b An end-of-life notice was posted suddenly and briefly on the MRV Terescope product page in 2011. All references to the Terescope have been completely removed from MRV's official page as of October 27, 2014.
  8. ^ "10 Gbps Through The Air". Arto Link. Retrieved October 27, 2014. new Artolink wireless communication system with the highest capacity: 10 Gbps, full duplex [..] Artolink M1-10GE model
  9. ^ "LightPointe main page". Retrieved October 27, 2014.
  10. ^ Miloš Wimmer (13 August 2007). "MRV TereScope 700/G Laser Link". CESNET. Retrieved October 27, 2014.
  11. ^ Eric Korevaar, Isaac I. Kim and Bruce McArthur (2001). "Atmospheric Propagation Characteristics of Highest Importance to Commercial Free Space Optics" (PDF). Optical Wireless Communications IV, SPIE Vol. 4530 p. 84. Retrieved October 27, 2014.
  12. ^ Tom Garlington, Joel Babbitt and George Long (March 2005). "Analysis of Free Space Optics as a Transmission Technology" (PDF). WP No. AMSEL-IE-TS-05001. US Army Information Systems Engineering Command. p. 3. Archived from the original (PDF) on June 13, 2007. Retrieved June 28, 2011.
  13. ^ US Federal Employees. "$86.5M in FY2008 & 2009, Page 350 Department of Defense Fiscal Year (FY) 2010 Budget Estimates, May 2009, Defense Advanced Research Projects Agency, Justification Book Volume 1, Research, Development, Test & Evaluation, Defense-Wide, Fiscal Year (FY) 2010" (PDF). Retrieved October 4, 2014.CS1 maint: Uses authors parameter (link)
  14. ^ US Federal Employees. "US$40.5M in 2010 & 2011, page 273, Department of Defense, Fiscal Year (FY) 2012 Budget Estimates, February 2011, Defense Advanced Research Projects Agency, Justification Book Volume 1, Research, Development, Test & Evaluation, Defense-Wide, Fiscal Year (FY) 2012 Budget Estimates". Retrieved October 4, 2014.CS1 maint: Uses authors parameter (link)
  15. ^ US Federal Employees. "US$5.9M in 2012, page 250, Department of Defense, Fiscal Year (FY) 2014 President's Budget Submission, April 2013, Defense Advanced Research Projects Agency, Justification Book Volume 1, Research, Development, Test & Evaluation, Defense-Wide". Archived from the original on October 25, 2016. Retrieved October 4, 2014.CS1 maint: Uses authors parameter (link)
  16. ^ Bruce V. Bigelow (June 16, 2006). "Zapped of its potential, Rooftop laser startups falter, but debate on high-speed data technology remains". Retrieved October 26, 2014.CS1 maint: Uses authors parameter (link)
  17. ^ Nancy Gohring (March 27, 2000). "TeraBeam's Light Speed; Telephony, Vol. 238 Issue 13, p16". Retrieved October 27, 2014.
  18. ^ Fred Dawson (May 1, 2000). "TeraBeam, Lucent Extend Bandwidth Limits, Multichannel News, Vol 21 Issue 18 Pg 160". Retrieved October 27, 2014.
  19. ^ Terabeam
  20. ^ "LightPointe Website". Retrieved October 27, 2014.
  21. ^ Robert F. Service (21 December 2001). "Hot New Beam May Zap Bandwidth Bottleneck". Retrieved 27 October 2014.
  22. ^ "CableFree UNITY Website". Retrieved September 28, 2016.
  23. ^ Fog Optics staff (20 November 2014). "Fog Laser Field Test" (PDF). Archived from the original (PDF) on 2015-04-26. Retrieved 21 December 2014.
  24. ^ "NASA Beams Mona Lisa to Lunar Reconnaissance Orbiter at the Moon". NASA. January 17, 2013. Archived from the original on April 19, 2018. Retrieved May 23, 2018.
  25. ^ "Changelog of Twibright Labs Products". Retrieved 14 March 2018.
  26. ^
  27. ^ "Visible Light Communication Consortium". web site. Archived from the original on April 6, 2004. (Japanese)
  28. ^ Tanaka, Y.; Haruyama, S.; Nakagawa, M.; , "Wireless optical transmissions with white colored LED for wireless home links," Personal, Indoor and Mobile Radio Communications, 2000. PIMRC 2000. The 11th IEEE International Symposium on, vol. 2, no., pp. 1325–1329 vol.2, 2000.
  29. ^ "IEEE 802.15 WPAN Task Group 7 (TG7) Visible Light Communication". IEEE 802 local and metro area network standards committee. 2009. Retrieved June 28, 2011.
  30. ^ Kari Petrie (November 19, 2010). "City first to sign on to new technology". St. Cloud Times. p. 1.
  31. ^ Clint Turner (October 3, 2007). "A 173-mile 2-way all-electronic optical contact". Modulated light web site. Retrieved June 28, 2011.
  32. ^ J. Grubor; S. Randel; K.-D. Langer; J. W. Walewski (December 15, 2008). "Broadband Information Broadcasting Using LED-Based Interior Lighting" (PDF). Journal of Lightwave Technology. 26 (24): 3883–3892. Bibcode:2008JLwT...26.3883G. doi:10.1109/JLT.2008.928525.
  33. ^ "500 Megabits/Second with White LED Light". news release. Siemens. January 18, 2010. Archived from the original on March 11, 2013. Retrieved February 2, 2013.
  34. ^ Lee, I.E.; Sim, M.L.; Kung, F.W.L.; , "Performance enhancement of outdoor visible-light communication system using selective combining receiver," Optoelectronics, IET , vol. 3, no. 1, pp. 30–39, February 2009.
  35. ^ "Pure LiFi transmits data using light". web site. (English)
  36. ^ Jing Xue, Alok Garg, Berkehan Ciftcioglu, Jianyun Hu, Shang Wang, Ioannis Savidis, Manish Jain, Rebecca Berman, Peng Liu, Michael Huang, Hui Wu, Eby G. Friedman, Gary W. Wicks, Duncan Moore (June 2010). "An Intra-Chip Free-Space Optical Interconnect" (PDF). The 37th International Symposium on Computer Architecture. Retrieved June 30, 2011.CS1 maint: Uses authors parameter (link)
  37. ^ a b [1], PragueBest s.r.o. "Free Space optics (FSO) with capacity 10 Gigabits Full Duplex - EC System". Retrieved 14 March 2018.

Further reading

External links

Adaptive optics

Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, in microscopy, optical fabrication and in retinal imaging systems to reduce optical aberrations. Adaptive optics works by measuring the distortions in a wavefront and compensating for them with a device that corrects those errors such as a deformable mirror or a liquid crystal array.

Adaptive optics should not be confused with active optics, which works on a longer timescale to correct the primary mirror geometry.

Other methods can achieve resolving power exceeding the limit imposed by atmospheric distortion, such as speckle imaging, aperture synthesis, and lucky imaging, or by moving outside the atmosphere with space telescopes, such as the Hubble Space Telescope.

Communications receiver

A communications receiver is a type of radio receiver used as a component of a radio communication link. This is in contrast to a broadcast receiver which is used to receive radio broadcasts. A communication receiver receives parts of the radio spectrum not used for broadcasting, that includes amateur, military, aircraft, marine, and other bands. They are often used with a radio transmitter as part of a two way radio link for shortwave radio or amateur radio communication, although they are also used for shortwave listening.

Consumer IR

Consumer IR, consumer infrared, or CIR, is a class of devices employing the infrared portion of the electromagnetic spectrum for wireless communications. CIR ports are commonly found in consumer electronics devices such as television remote controls, PDAs, laptops, and computers.

The functionality of CIR is as broad as the consumer electronics that carry it. For instance, a television remote control can convey a "channel up" command to the television, while a computer might be able to surf the internet solely via CIR. The type, speed, bandwidth, and power of the transmitted information depends on the particular CIR protocol employed.

CIR is the most common type of free-space optical communication.

Free space (disambiguation)

Free space may refer to:

A perfect vacuum, that is, a space free of all matter

In electrical engineering, free space means air (as opposed to a material, transmission line, fiber-optic cable, etc.):

Free-space optical communication is communication by shining light through air

Free-space path loss, the spreading-out of light as it travels through 3D space

Free-space display is a 3D display projected into the air, often with the help of mist

Autonomous free space, community centers in which non-authoritarians enact principles of mutual aid

Social centre the free shared space in a community

Descent: FreeSpace – The Great War, a space combat simulation computer game

The area of a data storage device (for example, a computer disk drive) that is still available for more data storage

Motion planning § Free Space, the subset of a configuration space where a robot will not collide with obstacles

Koruza (technology)

Koruza is a Slovenian open source and open hardware project providing equipment for low-cost free-space wireless optical connections. One can use 3D printing to create their own equipment. It is based on use of existing SFP optical modules which brings the costs of manufacturing down. Because it uses infrared light it is an alternative to Wi-Fi and does not have issues with spectrum congestion and radio interference. It is available in 1 Gbit/s and 10 Gbit/s forms. Connection can be established at up to 100 m.

It is one of the projects funded by Shuttleworth Foundation through their fellows program.


A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "Light Amplification by Stimulated Emission of Radiation". The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

A laser differs from other sources of light in that it emits light coherently. Spatial coherence allows a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over great distances (collimation), enabling applications such as laser pointers and lidar. Lasers can also have high temporal coherence, which allows them to emit light with a very narrow spectrum, i.e., they can emit a single color of light. Alternatively, temporal coherence can be used to produce pulses of light with a broad spectrum but durations as short as a femtosecond ("ultrashort pulses").

Lasers are used in optical disk drives, laser printers, barcode scanners, DNA sequencing instruments, fiber-optic and free-space optical communication, laser surgery and skin treatments, cutting and welding materials, military and law enforcement devices for marking targets and measuring range and speed, and in laser lighting displays for entertainment. They have been used for car headlamps on luxury cars, by using a blue laser and a phosphor to produce highly directional white light.

Laser communication in space

Laser communication in space is free-space optical communication in outer space.

In outer space, the communication range of free-space optical communication is currently of the order of several thousand kilometers, suitable for inter-satellite service. It has the potential to bridge interplanetary distances of millions of kilometers, using optical telescopes as beam expanders.

Li-Fi Consortium

The Li-Fi Consortium is an international organization focusing on optical wireless technologies.

It was founded by four technology-based organizations in October 2011. The goal of the Li-Fi Consortium is to foster the development and distribution of (Li-Fi) optical wireless technologies such as communication, navigation, natural user interfaces and others.

Modulating retro-reflector

A modulating retro-reflector (MRR) system combines an optical retro-reflector and an optical modulator to allow optical communications and sometimes other functions such as programmable signage.Free space optical communication technology has emerged in recent years as an attractive alternative to the conventional radio frequency (RF) systems. This emergence is due in large part to the increasing maturity of lasers and compact optical systems that enable exploitation of the inherent advantages (over RF) of the much shorter wavelengths characteristic of optical and near-infrared carriers:

Larger bandwidth

Low probability of intercept

Immunity from interference or jamming

Frequency spectrum allocation issue relief

Smaller, lighter, lower power

Optical communication

Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information. It can be performed visually or by using electronic devices. The earliest basic forms of optical communication date back several millennia, while the earliest electrical device created to do so was the photophone, invented in 1880.

An optical communication system uses a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal. When electronic equipment is not employed the 'receiver' is a person visually observing and interpreting a signal, which may be either simple (such as the presence of a beacon fire) or complex (such as lights using color codes or flashed in a Morse code sequence).

Free-space optical communication has been deployed in space, while terrestrial forms are naturally limited by geography, weather and the availability of light. This article provides a basic introduction to different forms of optical communication.

Optical mesh network

An optical mesh network is a type of optical telecommunications network employing wired fiber-optic communication or wireless free-space optical communication in a mesh network architecture.

Most optical mesh networks use fiber-optic communication and are operated by internet service providers in metropolitan and regional but also national and international scenarios. They are faster and less error prone than other network architectures and support backup and recovery plans for established networks in case of any disaster, damage or failure. Currently planned satellite constellations aim to establish optical mesh networks in space by using wireless laser communication.

Orbital angular momentum multiplexing

Orbital angular momentum (OAM) multiplexing is a physical layer method for multiplexing signals carried on electromagnetic waves using the orbital angular momentum of the electromagnetic waves to distinguish between the different orthogonal signals.Orbital angular momentum is one of two forms of angular momentum of light. OAM is distinct from, and should not be confused with, light spin angular momentum. The spin angular momentum of light offers only two orthogonal quantum states corresponding to the two states of circular polarization, and can be demonstrated to be equivalent to a combination of polarization multiplexing and phase shifting. OAM on the other hand relies on an extended beam of light, and the higher quantum degrees of freedom which come with the extension. OAM multiplexing can thus access a potentially unbounded set of states, and as such offer a much larger number of channels, subject only to the constraints of real-world optics.

As of 2013, although OAM multiplexing promises very significant improvements in bandwidth when used in concert with other existing modulation and multiplexing schemes, it is still an experimental technique, and has so far only been demonstrated in the laboratory. Following the early claim that OAM exploits a new quantum mode of information propagation, the technique has become controversial; however nowadays it can be understood to be a particular form of tightly modulated MIMO multiplexing strategy, obeying classical information theoretic bounds.

Outline of radio

The following outline is provided as an overview of and topical guide to radio:

Radio – transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space. Information is carried by systematically changing (modulating) some property of the radiated waves, such as amplitude, frequency, phase, or pulse width. When radio waves pass an electrical conductor, the oscillating fields induce an alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information.


RONJA (Reasonable Optical Near Joint Access) is a free-space optical communication system originating in the Czech Republic, developed by Karel Kulhavý of Twibright Labs and released in 2001. It transmits data wirelessly using beams of light. Ronja can be used to create a 10 Mbit/s full duplex Ethernet point-to-point link. It has been estimated that 1000 to 2000 links have been built worldwide The range of the basic configuration is 1.4 km (0.87 mi). The device consists of a receiver and transmitter pipe (optical head) mounted on a sturdy adjustable holder. Two coaxial cables are used to connect the rooftop installation with a protocol translator installed in the house near a computer or switch. The range can be extended to 1.9 km (1.2 mi) by doubling or tripling the transmitter pipe.

Building instructions, blueprints, and schematics are published under the GNU Free Documentation Licence. Only free software tools are used in the development. The author calls this level of freedom "User Controlled Technology". Ronja is a project of Twibright Labs.

Radial polarization

A beam of light has radial polarization if at every position in the beam the polarization (electric field) vector points towards the centre of the beam. In practice, an array of waveplates may be used to provide an approximation to a radially polarized beam. In this case the beam is divided into segments (eight, for example), and the average polarization vector of each segment is directed towards the beam centre.

Radial polarization can be produced in a variety of ways. It is possible to use a liquid crystal device to convert the polarization of a beam to a radial state, or a radially polarized beam can be produced by a laser, or any collimated light source, in which the Brewster window is replaced by a cone at Brewster's angle. Called a "Rotated Brewster Angle Polarizer," the latter was first proposed and put into practice (1986) to produce a radially-polarized annular pupil by Guerra at Polaroid Corporation (Polaroid Optical Engineering Dept., Cambridge, Massachusetts) to achieve super-resolution in their Photon Tunneling Microscope. A metal bi-cone, formed by diamond-turning, was mounted inside a glass cylinder. Collimated light entering this device underwent two air-metal reflections at the bi-cone and one air-glass reflection at the Brewster angle inside the glass cylinder, so as to exit as radially-polarized light. A similar device was later proposed again by Kozawa A related concept is azimuthal polarisation, in which the polarisation vector is tangential to the beam. If a laser is focused along the optic axis of a birefringent material, the radial and azimuthal polarizations focus at different planes. A spatial filter can be used to select the polarization of interest.A radially polarized beam can be used to produce a smaller focused spot than a more conventional linearly or circularly polarized beam, and has uses in optical trapping.It has been shown that a radially polarized beam can be used to increase the information capacity of free space optical communication via mode division multiplexing, and radial polarization can "self-heal" when obstructed.

Semaphore Flag Signaling System

In computer networking, Semaphore Flag Signaling System (SFSS) is a humorous proposal to carry Internet Protocol (IP) traffic by semaphores. Semaphore Flag Signaling System was initially described in RFC 4824, an April Fools RFC issued by the Internet Engineering Task Force edited by J. Hofmueller, et al. and released on April Fool's Day 2007. It is one of several April 1 RFCs.

Telecommunications equipment

Telecommunications equipment (also telecoms equipment or communications equipment) is hardware used for the purposes of telecommunications. Since the 1990s the boundary between telecoms equipment and IT hardware has become blurred as a result of the growth of the internet and its increasing role in the transfer of telecoms data.


Wireless communication, or sometimes simply wireless, is the transfer of information or power between two or more points that are not connected by an electrical conductor. The most common wireless technologies use radio waves. With radio waves distances can be short, such as a few meters for Bluetooth or as far as millions of kilometers for deep-space radio communications. It encompasses various types of fixed, mobile, and portable applications, including two-way radios, cellular telephones, personal digital assistants (PDAs), and wireless networking. Other examples of applications of radio wireless technology include GPS units, garage door openers, wireless computer mice, keyboards and headsets, headphones, radio receivers, satellite television, broadcast television and cordless telephones. Somewhat less common methods of achieving wireless communications include the use of other electromagnetic wireless technologies, such as light, magnetic, or electric fields or the use of sound.

The term wireless has been used twice in communications history, with slightly different meaning. It was initially used from about 1890 for the first radio transmitting and receiving technology, as in wireless telegraphy, until the new word radio replaced it around 1920. The term was revived in the 1980s and 1990s mainly to distinguish digital devices that communicate without wires, such as the examples listed in the previous paragraph, from those that require wires or cables. This became its primary usage in the 2000s, due to the advent of technologies such as mobile broadband, Wi-Fi and Bluetooth.

Wireless operations permit services, such as long-range communications, that are impossible or impractical to implement with the use of wires. The term is commonly used in the telecommunications industry to refer to telecommunications systems (e.g. radio transmitters and receivers, remote controls, etc.) which use some form of energy (e.g. radio waves, acoustic energy,) to transfer information without the use of wires. Information is transferred in this manner over both short and long distances.

Network topology
and switching

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