View source for Free-space optical communication

{{short description|Communication using light sent through free space}}
{{For|short- to medium-range communication|Optical wireless communications}}
{{Use American English|date=March 2019}}

[[File:FSO-gigabit-laser-link-0a.jpg|thumb|right|250px|An eight-beam free-space–optics laser link, rated for 1&nbsp;Gbit/s. The receptor is the large lens in the middle, the transmitters the smaller ones. At the top right corner is a [[monocular]] for assisting the alignment of the two heads.]]

'''Free-space optical communication''' ('''FSO''') is an [[optical communication]] technology that uses light propagating in free space to [[wireless]]ly transmit data for [[telecommunications]] or [[computer network]]ing over long distances. "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.

==History==
[[File:Photophony1.jpg|thumb|A photophone receiver and headset, one half of Bell and Tainter's optical telecommunication system of 1880]]

[[Optical communication]]s, 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]].<ref>{{cite book |title=The Histories of Polybius |year=1889 |chapter=Book X |pages=43–46 |chapter-url=https://books.google.com/books?id=DPRCAQAAMAAJ&pg=PA45 |author1=Polybius}}</ref> In the modern era, [[semaphore line|semaphores]] and wireless solar [[telegraph]]s called [[heliograph]]s 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 and Bureau|Volta Laboratory]] in [[Washington, DC]]. Bell considered it his most important invention. The device allowed for the [[transmission (telecommunications)|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 {{convert|213|m|ft|abbr=off|sp=us}} apart.<ref>{{cite book |author=Mary Kay Carson |title=Alexander Graham Bell: Giving Voice To The World |publisher=Sterling Publishing |location=New York |year=2007 |series=Sterling Biographies |pages=[https://archive.org/details/alexandergrahamb0000cars/page/76 76]–78 |isbn=978-1-4027-3230-0 |url=https://archive.org/details/alexandergrahamb0000cars |url-access=registration}}</ref><ref>
{{cite journal |author=Alexander Graham Bell |title=On the Production and Reproduction of Sound by Light |journal=American Journal of Science |series=Third Series |volume=XX |number=118 |date=October 1880 |pages=305–324 |doi=10.2475/ajs.s3-20.118.305 |bibcode=1880AmJS...20..305B |s2cid=130048089 |url=https://zenodo.org/record/1450056 |author-link=Alexander Graham Bell}} also published as "Selenium and the Photophone" in ''[[Nature (journal)|Nature]]'', September 1880.</ref>

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 Wars]] starting in 1904, in [[German South-West Africa]] (today's [[Namibia]]) as did British, French, US or Ottoman signals.

[[File:Germany blinker signal lamp - National World War I Museum - Kansas City, MO - DSC07704.JPG|thumb|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 ''{{lang|de|Blinkgerät}}'', the intermediate type for distances of up to {{convert|4|km|mi|abbr=on}} at daylight and of up to {{convert|8|km|mi|abbr=on}} 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.{{citation needed|date=May 2014}}

A major technological step was to replace the Morse code by modulating optical waves in speech transmission. [[Carl Zeiss AG|Carl Zeiss, Jena]] developed the ''{{lang|de|Lichtsprechgerät}} 80/80'' (literal translation: optical speaking device) that the German army used in their World War&nbsp;II anti-aircraft defense units, or in bunkers at the [[Atlantic Wall]].<ref>{{cite web |url=http://www.la6nca.net/tysk2/lispr/lispr2.htm |title=German, WWII, WW2, Lichtsprechgerät 80/80 |publisher=LAUD Electronic Design AS |access-date=June 28, 2011 |url-status=live |archive-url=https://web.archive.org/web/20110724181528/http://www.laud.no/ww2/lispr/lispr2.htm |archive-date=July 24, 2011}}</ref>

The invention of [[laser]]s in the 1960s revolutionized free-space optics.{{cn|date=November 2024}} Military organizations were particularly interested and boosted their development. In 1973, while prototyping the first [[Laser printing#History|laser printer]]s at [[Xerox PARC|PARC]], [[Gary Starkweather]] and others made a [[duplex (telecommunications)|duplex]] 30 Mbit/s [[campus network|CAN]] [[optical link]] using astronomical telescopes and [[HeNe lasers]] to send data between offices; they chose the method due partly to less strict regulations (at the time) on free-space optical communication by the [[FCC]].<ref>{{cite AV media|url=https://www.youtube.com/watch?v=BZFaQiItckU&t=2813s |title=Birth of the Laser Printer |via=YouTube |publisher=[[Computer History Museum]] |author=[[Gary Starkweather]] |time=46:53}}</ref>{{secondary source needed|date=November 2024}} However, laser-based free-space optics lost market momentum when the installation of [[optical fiber]] networks for civilian uses was at its peak.{{cn|date=November 2024}}

Many simple and inexpensive consumer [[remote control]]s 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 [[light-emitting diode|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]].<ref name="TerraSAR-X NFIRE test">{{cite web |url=https://www.dlr.de/content/en/downloads/news-archive/2008/20081110_dlr-communicates-with-terrasar-x-earth-observation-satellite-via-laser-beam_14120.pdf |website=DLR Portal |title=DLR communicates with TerraSAR-X Earth Observation satellite via laser beam |date=10 November 2008 |publisher=Deutsches Zentrum für Luft und Raumfahrt (DLR){{dash}}German Aerospace Center |first1=Andreas |last1=Schütz |first2=Dirk |last2=Giggenbach |access-date=14 March 2018 }}{{Dead link|date=May 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

===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 ({{convert|400|to|500|meter|ft}}). This is from both independent studies, such as in the Czech Republic,<ref>{{cite web |url=https://archiv.cesnet.cz/doc/techzpravy/2007/mrv-terescope-700/ |title=MRV TereScope 700/G Laser Link |publisher=CESNET |author=Miloš Wimmer |date=13 August 2007 |access-date=October 27, 2014}}</ref> as well as internal studies, such as one conducted by MRV FSO staff.<ref>{{cite web |url=http://laseritc.ru/files/files/MRV-WP-FSOAtmosProp.pdf |title=Atmospheric Propagation Characteristics of Highest Importance to Commercial Free Space Optics |publisher=Optical Wireless Communications IV, SPIE Vol. 4530 p. 84 |author=Eric Korevaar, Isaac I. Kim and Bruce McArthur |date=2001 |access-date=October 27, 2014}}</ref>

Military based studies consistently produce longer estimates for reliability, projecting the maximum range for terrestrial links is of the order of {{convert|2|to|3|km|mi|abbr=on}}.<ref>{{cite web |url=http://www.hqisec.army.mil/isec/publications/Analysis_of_Free_Space_Optics_as_a_Transmission_Technology_Mar05.pdf |title=Analysis of Free Space Optics as a Transmission Technology |publisher=US Army Information Systems Engineering Command |author=Tom Garlington, Joel Babbitt and George Long |page=3 |work=WP No. AMSEL-IE-TS-05001 |date=March 2005 |access-date=June 28, 2011 |archive-date=June 13, 2007 |archive-url=https://web.archive.org/web/20070613000248/http://www.hqisec.army.mil/isec/publications/Analysis_of_Free_Space_Optics_as_a_Transmission_Technology_Mar05.pdf}}</ref> All studies agree the stability and quality of the link is highly dependent on atmospheric factors such as rain, fog, dust and heat. Relays may be employed to extend the range for FSO communications.<ref name="kshetrimayum1">{{cite journal |last1=Bhowal |first1=A. |last2=Kshetrimayum |first2=R. S. |title=Outage Probability Bound of Decode and Forward Two Way Relay employing Optical Spatial Modulation over Gamma-Gamma Channels |journal=IET Optoelectronics |volume=13 |issue=4 |date=2019 |pages=183–190 |doi=10.1049/iet-opt.2018.5103|s2cid=115680008 }}</ref><ref name="kshetrimayum3">{{cite journal |last1=Bhowal |first1=A. |last2=Kshetrimayum |first2=R. S. |title=Relay based Hybrid FSO/RF communication employing Hybrid Spatial Modulation and Transmit Source Selection |journal=IEEE Transactions on Communications |volume=68 |issue=8 |date=2020 |pages=5018–5027 |doi= 10.1109/TCOMM.2020.2991054 |s2cid=219041497}}</ref>

TMEX USA ran two eight-mile links between [[Laredo, Texas]] and [[Nuevo Laredo|Nuevo Laredo, Mexico]] from 1998<ref>{{Cite web |title=BNamericas{{dash}}TMEX Launches Mexico-US Switch |url=https://www.bnamericas.com/en/news/tmex-launches-mexico-us-switch |access-date=2024-03-16 |website=BNamericas.com |language=en}}</ref> to 2002. The links operated at 155&nbsp;Mbit/s and reliably carried phone calls and internet service.<ref>{{Cite press release |last= |first= |date=2009-01-09 |title=TMEX USA, Inc. Announces Entry Into Definitive Merger Agreement With Solargen Energy, Inc., 2,001-to-1 Reverse Stock Split, Amend Its Articles of Incorporation, Change Its Name and Change Its Trading Symbol |url=https://www.globenewswire.com/news-release/2009/01/09/390730/11024/en/TMEX-USA-Inc-Announces-Entry-Into-Definitive-Merger-Agreement-With-Solargen-Energy-Inc-2-001-to-1-Reverse-Stock-Split-Amend-Its-Articles-of-Incorporation-Change-Its-Name-and-Change.html |access-date=2024-03-16 |website=GlobeNewswire News Room |language=en}}</ref>{{dubious|date=September 2023}}{{citation needed|date=September 2023}}

===Extending the useful distance===
[[File:DARPA ORCA official concept art.jpg|thumb|DARPA ORCA official concept art created {{circa|2008}}]]

The main reason terrestrial communications have been limited to non-commercial telecommunications functions is fog. Fog often prevents FSO laser links over {{convert|500|meter}} from achieving a year-round availability sufficient for commercial services. 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 toward this effort, with the ORCA and ORCLE programs.<ref>{{cite web |url=http://www.srwolf.com/reports/2010PBDARPAMay2009.pdf |title=Department of Defense Fiscal Year (FY) 2010 Budget Estimates: May 2009: Research, Development, Test And Evaluation, Defense-Wide |access-date=October 4, 2014 |archive-url=https://web.archive.org/web/20141027184129/http://www.srwolf.com/reports/2010PBDARPAMay2009.pdf |archive-date=2014-10-27}}</ref><ref>{{cite web |url=https://www.scribd.com/doc/54142717/MasterJustification-DARPA-PB-2012 |title=Department of DefenseFiscal Year (FY) 2012 Budget Estimates: February 2011: Defense Advanced Research Projects Agency: Research, Development, Test & Evaluation, Defense-Wide |access-date=October 4, 2014}}</ref><ref>{{cite web |url=http://doocument.com/fy/fy-2014-budget-estimates-darpa.html |title=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 |access-date=October 4, 2014 |archive-date=October 27, 2014 |archive-url=https://archive.today/20141027171505/http://doocument.com/fy/fy-2014-budget-estimates-darpa.html |url-status=dead }}</ref>

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 to 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:<ref>{{cite web |url=http://www.utsandiego.com/uniontrib/20060616/news_1b16kaput.html |title=Zapped of its potential, Rooftop laser startups falter, but debate on high-speed data technology remains |author=Bruce V. Bigelow |date=June 16, 2006 |access-date=October 26, 2014}}</ref>
* [[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.<ref>{{cite web |title=TeraBeam's Light Speed; Telephony, Vol. 238 Issue 13, p16 |author=Nancy Gohring |date=March 27, 2000 |url=http://connection.ebscohost.com/c/articles/2965610/terabeams-light-speed |access-date=October 27, 2014 |archive-url=https://web.archive.org/web/20141027191354/http://connection.ebscohost.com/c/articles/2965610/terabeams-light-speed |archive-date=October 27, 2014}}</ref><ref>{{cite web |url=http://connection.ebscohost.com/c/articles/3257227/terabeam-lucent-extend-bandwidth-limits |archive-url=https://web.archive.org/web/20141027191631/http://connection.ebscohost.com/c/articles/3257227/terabeam-lucent-extend-bandwidth-limits |archive-date=October 27, 2014 |title=TeraBeam, Lucent Extend Bandwidth Limits, Multichannel News, Vol 21 Issue 18 Pg 160 |author=Fred Dawson |date=May 1, 2000 |access-date=October 27, 2014}}</ref> The work ultimately failed, and the company was purchased in 2004 for $52 million (excluding warrants and options) by Falls Church, Virginia-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.<ref>[[Terabeam]]</ref>
* 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.<ref name=MRVTerescope>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.</ref>
* LightPointe Communications received $76 million in start-up funds, and eventually reorganized to sell hybrid FSO-RF units to overcome the weather-based challenges.<ref name=lightpointe>{{cite web |url=http://www.lightpointe.com/ |title=LightPointe main page |access-date=October 27, 2014 |archive-url=https://web.archive.org/web/20180314051837/http://www.lightpointe.com/ |archive-date=2018-03-14}}</ref>
* The Maxima Corporation published its operating theory in ''[[Science (magazine)|Science]]'',<ref>{{cite journal |url=https://www.science.org/doi/10.1126/science.294.5551.2454 |title=Hot New Beam May Zap Bandwidth Bottleneck |author=Robert F. Service |journal=Science |date=21 December 2001 |volume=294 |issue=5551 |page=2454 |doi=10.1126/science.294.5551.2454 |pmid=11752548 |s2cid=11584005 |access-date=27 October 2014|url-access=subscription }}</ref> 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.<ref>{{cite web |url=http://www.cablefree.net/unity/ |title=CableFree UNITY Website |access-date=September 28, 2016}}</ref>

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.<ref>{{cite web |url=http://fogoptics.com/wp-content/uploads/2014/12/Fog-Optics-White-PaperTechnical-Field-Test.pdf |title=Fog Laser Field Test |author=Fog Optics staff |date=20 November 2014 |access-date=21 December 2014 |archive-url=https://web.archive.org/web/20150426225922/http://fogoptics.com/wp-content/uploads/2014/12/Fog-Optics-White-PaperTechnical-Field-Test.pdf |archive-date=2015-04-26}}</ref>

===Extraterrestrial===
{{See also|Laser communication in space}}

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.
[[File:Mynaric Condor Mk3.1 Satellite optical communication terminal.jpg|thumb|[[Mynaric]]-built terminal for satellite optical communications]]

====Operational systems====
The first gigabit laser-based communication{{clarify|what is "gigabit communication?|date=December 2023}} 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.

In December 2023, the [[Australian National University]] (ANU) demonstrated its Quantum Optical Ground Station at its [[Mount Stromlo Observatory]]. QOGS uses adaptive optics and lasers as part of a telescope, to create a bi-directional communications system capable of supporting the [[NASA]] [[Artemis program]] to the [[Moon]].<ref>[https://www.abc.net.au/news/2023-12-06/act-new-telescope-anu-quantum-optical-ground-station-mt-stromlo/103192694 New Quantum Optical Ground Station allows Canberra to play starring role in space communications], Emmy Groves, [[ABC News Online]], 2023-12-06</ref>

====Demonstrations====
A two-way distance record for communication was set by the Mercury laser altimeter instrument aboard the ''[[MESSENGER]]'' spacecraft. It was able to communicate across a distance of {{convert|24|e6km|e6mi|abbr=unit}}, 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 {{convert|6|e6km|abbr=unit}} in 1992.

In January 2013, NASA used lasers to beam an image of the Mona Lisa to the Lunar Reconnaissance Orbiter roughly {{convert|390,000|km|mi|abbr=on}} away. To compensate for atmospheric interference, [[Reed–Solomon error correction|an error correction code algorithm similar to that used in CDs]] was implemented.<ref name="NASA Mona Lisa">{{cite web |url=https://www.nasa.gov/mission_pages/LRO/news/mona-lisa.html |title=NASA Beams Mona Lisa to Lunar Reconnaissance Orbiter at the Moon |date=January 17, 2013 |website=NASA |access-date=May 23, 2018 |url-status=live |archive-url=https://web.archive.org/web/20180419005026/https://www.nasa.gov/mission_pages/LRO/news/mona-lisa.html |archive-date=April 19, 2018}}</ref>

In the early morning hours of October 18, 2013, NASA's Lunar Laser Communication Demonstration (LLCD) transmitted data from lunar orbit to Earth at a rate of 622 megabits per second (Mbit/s).<ref>{{cite web |url=https://www.nasa.gov/content/goddard/historic-demonstration-proves-laser-communication-possible/ |title=Historic Demonstration Proves Laser Communication Possible |website=NASA |date=October 28, 2013}}</ref> LLCD was flown aboard the [[LADEE|Lunar Atmosphere and Dust Environment Explorer]] (LADEE) spacecraft, whose primary science mission was to investigate the tenuous and exotic atmosphere that exists around the Moon.

Between April and July 2014 NASA's [[OPALS]] instrument successfully uploaded 175 megabytes in 3.5 seconds and downloaded 200–300&nbsp;MB in 20&nbsp;s.<ref>{{cite news|url=http://www.jpl.nasa.gov/news/news.php?feature=4402|title=OPALS: Light Beams Let Data Rates Soar|last=Landau |first=Elizabeth|date=9 December 2014|work=Jet Propulsion Laboratory|access-date=18 December 2014|publisher=NASA}} {{PD-notice}}</ref> Their system was also able to re-acquire tracking after the signal was lost due to cloud cover.

On December 7, 2021 NASA launched the [[Laser Communications Relay Demonstration]] (LCRD), which aims to relay data between spacecraft in [[geosynchronous orbit]] and ground stations. LCRD is NASA's first two-way, end-to-end optical relay. LCRD uses two [[ground station]]s, Optical Ground Station (OGS)-1 and -2, at [[Table Mountain Observatory]] in California, and [[Haleakalā]], [[Hawaii]].<ref>{{Cite news |url=https://www.spacedaily.com/reports/Getting_NASA_data_to_the_ground_with_lasers_999.html |title=Getting NASA data to the ground with lasers |work=Space Daily |date=October 28, 2021 |first=Katherine |last=Schauer}}</ref> One of LCRD's first operational users is the [[Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal]] (ILLUMA-T), on the International Space Station. The terminal will receive high-resolution science data from experiments and instruments on board the space station and then transfer this data to LCRD, which will then transmit it to a ground station. After the data arrives on Earth, it will be delivered to mission operation centers and mission scientists. The ILLUMA-T payload was sent to the ISS in late 2023 on [[SpaceX CRS-29]], and achieved [[First light (astronomy)|first light]] on December 5, 2023.<ref>[https://www.spacedaily.com/reports/NASAs_First_Two_way_End_to_End_Laser_Communications_System_999.html ''NASA's First Two-way End-to-End Laser Communications System'' Oct 2023]</ref><ref>{{Cite web |last1=Schauer |first1=Katherine |last2=NASA |title=NASA's space station laser comm terminal achieves first link |url=https://phys.org/news/2023-12-nasa-space-station-laser-comm.html |access-date=2023-12-16 |website=phys.org |language=en}}</ref>

On April 28, 2023, NASA and its partners achieved 200 gigabit per second (Gbit/s) throughput on a space-to-ground optical link between a satellite in orbit and Earth. This was achieved by the [[TeraByte InfraRed Delivery]] (TBIRD) system, mounted on NASA's [[Pathfinder Technology Demonstrator#PTD-3|Pathfinder Technology Demonstrator 3]] (PTD-3) satellite.<ref>{{Cite web |last=Tavares |first=Frank |date=2023-05-11 |title=NASA, Partners Achieve Fastest Space-to-Ground Laser Comms Link |url=http://www.nasa.gov/feature/ames/tbird-milestone |access-date=2023-08-26 |website=NASA}}</ref>

====Commercial use====
Various [[satellite constellation]]s that are intended to provide global broadband coverage, such as [[SpaceX]] [[Starlink]], employ [[laser communication in space|laser communication]] for inter-satellite links. This effectively creates a space-based [[optical mesh network]] between the satellites.

==LEDs==
[[File:Ronja beam Prostejov.jpg|thumb|right|250px|[[RONJA]] is a [[GNU Free Documentation License|free]] implementation of FSO using high-intensity [[light-emitting diode|LEDs]].]]

In 2001, Twibright Labs released [[RONJA|RONJA Metropolis]], an open-source DIY 10&nbsp;Mbit/s full-duplex LED FSO system that can span {{cvt|1.4|km}}.<ref>{{cite web |url=http://ronja.twibright.com/changelog.php |title=Changelog of Twibright Labs Products |website=ronja.twibright.com |access-date=14 March 2018}}</ref><ref>{{cite web |url=http://www.bizjournals.com/prnewswire/press_releases/2013/01/17/BR44159 |title=Visible Light Communication (VLC)/Li-Fi Technology & Free Space Optics (FSO) Market (2013–2018){{dash}}By Component (LED, Image Sensor, Opto-couplers), Application (Indoor Networking, Underwater Communication, Location Based Service, ITS) & Geography |date=January 17, 2013 |archive-url=https://web.archive.org/web/20150709163224/http://www.bizjournals.com/prnewswire/press_releases/2013/01/17/BR44159 |archive-date=2015-07-09}}</ref>

In 2004, a [[visible light communication]] consortium was formed in [[Japan]].<ref>{{cite web |title=Visible Light Communication Consortium |website=VLCC |url=http://www.vlcc.net/ |url-status=live |archive-url=https://web.archive.org/web/20040406083532/http://www.vlcc.net/ |archive-date=April 6, 2004 |language=ja}}</ref> 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.<ref>{{cite conference |last1=Tanaka |first1=Y. |last2=Haruyama |first2=S. |last3=Nakagawa |first3=M. |book-title=11th IEEE International Symposium on Personal Indoor and Mobile Radio Communications. PIMRC 2000. Proceedings |title=Wireless optical transmissions with white colored LED for wireless home links |year=2000 |volume=2 |pages=1325–1329 |doi=10.1109/PIMRC.2000.881634 |isbn=0-7803-6463-5 |s2cid=45422597}}</ref> 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#IEEE 802.15.7: Visible Light Communication|IEEE 802.15.7]].<ref>{{cite web |title=IEEE 802.15 WPAN Task Group 7 (TG7) Visible Light Communication |year=2009 |publisher=[[IEEE 802]] local and metro area network standards committee |url=https://www.ieee802.org/15/pub/TG7.html |access-date=June 28, 2011}}</ref> A trial was announced in 2010, in [[St. Cloud, Minnesota]].<ref>{{cite news |title=City first to sign on to new technology |newspaper=St. Cloud Times |first=Kari |last=Petrie |page=1 |date=November 19, 2010 |url=https://pqasb.pqarchiver.com/sctimes/access/2192375711.html?FMT=ABS&date=Nov+19%2C+2010 |access-date=July 6, 2017 |archive-date=June 16, 2013 |archive-url=https://web.archive.org/web/20130616112031/http://pqasb.pqarchiver.com/sctimes/access/2192375711.html?FMT=ABS&date=Nov+19%2C+2010 }}</ref>

[[Amateur radio]] operators have achieved significantly farther distances using incoherent sources of light from high-intensity LEDs. One reported {{convert|278|km|mi|abbr=on}} in 2007.<ref>{{cite web |title=A 173-mile 2-way all-electronic optical contact |url=http://www.modulatedlight.org/optical_comms/optical_qso_173mile.html |work=Modulated light web site |first=Clint |last=Turner |date=October 3, 2007 |access-date=June 28, 2011}}</ref> However, physical limitations of the equipment used limited [[bandwidth (signal processing)|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&nbsp;kHz range. 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 [[light-emitting diode#White|white LED (GaN-phosphor)]] which could be used for space lighting can typically be modulated up to 20&nbsp;MHz.<ref>{{cite journal |author1=J. Grubor |author2=S. Randel |author3=K.-D. Langer |author4=J. W. Walewski |title=Broadband Information Broadcasting Using LED-Based Interior Lighting |journal=Journal of Lightwave Technology |volume=26 |number=24 |pages=3883–3892 |date=December 15, 2008 |doi=10.1109/JLT.2008.928525 |bibcode=2008JLwT...26.3883G |s2cid=3019862 |url=https://zenodo.org/record/891080}}</ref> Data rates of over 100 [[Mbit/s]] can be achieved using efficient [[modulation]] schemes and [[Siemens]] claimed to have achieved over 500&nbsp;Mbit/s in 2010.<ref>{{cite news |title=500 Megabits/Second with White LED Light |work=news release |publisher=Siemens |date=January 18, 2010 |url=https://www.siemens.com/innovation/en/news/2010/500-megabits-second-with-white-led-light.htm |access-date=February 2, 2013 |archive-url=https://web.archive.org/web/20130311005757/http://www.siemens.com/innovation/en/news/2010/500-megabits-second-with-white-led-light.htm |archive-date=March 11, 2013}}</ref> Research published in 2009, used a similar system for traffic control of automated vehicles with LED traffic lights.<ref>{{cite journal |doi=10.1049/iet-opt:20070014 |last1=Lee |first1=I.E. |last2=Sim |first2=M.L. |last3=Kung |first3=F.W.L. |title=Performance enhancement of outdoor visible-light communication system using selective combining receiver |journal= IET Optoelectronics|volume=3 |issue=1 |pages=30–39 |date=February 2009}}</ref>

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.<ref>{{cite web |title=Pure LiFi transmits data using light |website=CNET |url=http://www.cnet.com/uk/news/pure-lifi-transmits-data-using-light-video/}}</ref>

==Engineering details==
Typically, the best scenarios for using this technology are:
* LAN-to-LAN connections on [[campus]]es 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 ([[IT disaster recovery|disaster recovery]])
* As an alternative or add-on to existing wireless technologies
** Especially powerful in combination with automatic aiming systems, in moving vehicles
* 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<ref>{{cite journal |title=An Intra-Chip Free-Space Optical Interconnect |author=Jing Xue |author2=Alok Garg |author3=Berkehan Ciftcioglu |author4=Jianyun Hu |author5=Shang Wang |author6=Ioannis Savidis |author7=Manish Jain |author8=Rebecca Berman |author9=Peng Liu |author10=Michael Huang |author11=Hui Wu |author12=[[Eby Friedman|Eby G. Friedman]] |author13=Gary W. Wicks |author14=Duncan Moore |url=http://www.ece.rochester.edu/users/mihuang/PAPERS/isca10.pdf |journal=The 37th International Symposium on Computer Architecture |date=June 2010 |access-date=June 30, 2011 |archive-date=April 3, 2012 |archive-url=https://web.archive.org/web/20120403001844/http://www.ece.rochester.edu/users/mihuang/PAPERS/isca10.pdf |url-status=dead }}</ref>

The light beam can be very narrow, which makes FSO hard to intercept, improving security. [[Encryption]] can secure the data traversing the link. FSO provides vastly improved [[electromagnetic interference]] (EMI) behavior compared to using [[microwave]]s.

===Technical advantages===
* Ease of deployment
* Can be used to power devices {{citation needed|date=November 2016}}
* [[Broadcast license|License]]-free long-range operation (in contrast with radio communication)
* High [[bit rate]]s
* Low [[bit error rate]]s
* Immunity to [[electromagnetic interference]]
* Full-[[duplex (telecommunications)|duplex]] operation
* [[Communications protocol|Protocol]] transparency
* Increased [[data security|security]] when working with narrow beam(s)<ref>{{cite journal |doi=10.1109/COMST.2014.2329501 |first1=M. A. |last1=Khalighi |first2=M. |last2=Uysal |title=Survey on Free Space Optical Communication: A Communication Theory Perspective |journal=IEEE Communications Surveys & Tutorials |year=2014 |volume=16 |issue=4 |pages=2231–2258 |s2cid=3141460|doi-access=free }}</ref>
* No [[Fresnel zone]] necessary
* Reference open source implementation
* Reduced size, weight, and power consumption compared to RF antennas

===Range-limiting factors===
For terrestrial applications, the principal limiting factors are:
* [[Fog]] (10 to ~100&nbsp;dB/km attenuation)
* [[Dispersion (optics)|Beam dispersion]]
* [[Atmospheric absorption]]
* [[Rain]]
* [[Snow]]
* [[Terrestrial scintillation]]
* Interference from background light sources (including the sun)
* [[Shadow fading|Shadowing]]
* Pointing stability in wind
* Pollution, such as [[smog]]

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 safety|laser class]]es [[Laser safety#Class 1|1]] or [[Laser safety#Class 1M|1M]]. Atmospheric and fog attenuation, which are exponential in nature, limit practical range of FSO devices to several kilometers. However, free-space optics based on [[infrared|1550&nbsp;nm]] wavelength, have considerably lower optical loss than free-space optics using [[infrared|830&nbsp;nm]] wavelength, in dense fog conditions. FSO using wavelength 1550&nbsp;nm system are capable of transmitting several times higher power than systems with [[Infrared|850&nbsp;nm]] and are safe to the human eye (1M class). Additionally, some free-space optics, such as EC SYSTEM,<ref name="auto">{{cite web |url=http://www.ecsystem.cz/en/products/free-space-optic-equipment |title=Free Space optics (FSO) with capacity 10 Gigabits Full Duplex{{dash}}EC System |first=PragueBest s.r.o. |last=praguebest.cz |website=ecsystem.cz |access-date=14 March 2018}}</ref> 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.<ref name="auto"/>

==See also==
* [[Atomic line filter#Laser tracking and communication]]
* [[Extremely high frequency]]
* [[Koruza|KORUZA]]
* [[Laser safety]]
* [[Mie scattering]]
* [[Modulating retro-reflector]]
* [[N-slit interferometer]]
* [[Optical window]]
* [[Radio window]]
* [[Rayleigh scattering]]
* [[Free-space path loss]]

==References==
{{Reflist}}

==Further reading==
* {{cite thesis |author=Christos Kontogeorgakis |title=Millimeter Through Visible Frequency Waves Through Aerosols-Particle Modeling, Reflectivity and Attenuation |publisher=Virginia Polytechnic Institute and State University |date=May 1997 |hdl=10919/37049 |url=https://vtechworks.lib.vt.edu/handle/10919/37049 |type=Thesis}} Master's Thesis
* {{cite book |author=Heinz Willebrand & Baksheesh Ghuman |title=Free Space Optics: Enabling Optical Connectivity in Today's Networks |publisher=SAMS |date=December 2001 |url=http://www.lightpointe.com/opticalbridgesfso.html |archive-url=https://web.archive.org/web/20120622121259/http://www.lightpointe.com/opticalbridgesfso.html |archive-date=2012-06-22}}
* {{cite news |last=Moll |first=Florian |title=Free-space laser system for secure air-to-ground quantum communications |newspaper=SPIE Newsroom |date=December 2013 |doi=10.1117/2.1201311.005189}}
* {{cite book |author=David G. Aviv |title=Laser Space Communications |publisher=ARTECH HOUSE |year=2006 |isbn=978-1-59693-028-5}}

==External links==
{{Commons category|Free-space optical communication}}
* {{usurped|1=[https://archive.today/20130113075910/http://www.hapcos.org/DOCS/wg2/wg2_home.php Free Space Optics on COST297 for HAPs]}}
* [http://ronja.twibright.com/fresnel.php Explanation of Fresnel zones in microwave and optical links]
* {{YouTube|vR4N6MTx_vw|video showing Lichtsprechgerät 80 in use}}
* [http://www.parabolicarc.com/2014/03/13/nasa-laser-iss-experiment/ International Space Station to Beam Video Via Laser Back to Earth], March 2014 NASA's [[OPALS|Optical Payload for Lasercomm Science]] demonstration mission to the ISS
* [https://nbviewer.jupyter.org/github/kirlf/cubesats/blob/master/Optical-ISL-LB.ipynb Wireless Optical Link Budget] (with python examples)

{{Optical telecommunication}}
{{Telecommunications}}

[[Category:Laser communication in space]]
[[Category:Optical communications]]
[[Category:Telecommunications equipment]]
[[Category:Wireless communication systems]]
[[Category:Optical wireless interfaces]]