Free-space optical communication (FSO) is anoptical communication technology that uses light propagating in free space towirelessly transmit data fortelecommunications orcomputer networking over long distances. "Free space" means air, outer space, vacuum, or something similar. This contrasts with using solids such asoptical fiber cable.The technology is useful where the physical connections are impractical due to high costs or other considerations.
Optical communications, in various forms, have been used for thousands of years. Theancient Greeks used a coded alphabetic system of signalling with torches developed by Cleoxenus, Democleitus andPolybius.[1] In the modern era,semaphores and wireless solartelegraphs calledheliographs were developed, using coded signals to communicate with their recipients.
In 1880,Alexander Graham Bell and his assistantCharles Sumner Tainter created thephotophone, at Bell's newly establishedVolta Laboratory inWashington, DC. Bell considered it his most important invention. The device allowed for thetransmission ofsound on a beam oflight. On June 3, 1880, Bell conducted the world's first wirelesstelephone transmission between two buildings, some 213 meters (699 feet) apart.[2][3]
Its first practical use came in military communication systems many decades later, first for optical telegraphy. German colonial troops usedheliograph telegraphy transmitters during theHerero Wars starting in 1904, inGerman South-West Africa (today'sNamibia) as did British, French, US or Ottoman signals.
During thetrench warfare ofWorld War I when wire communications were often cut, German signals used three types of optical Morse transmitters calledBlinkgerät, the intermediate type for distances of up to 4 km (2.5 mi) at daylight and of up to 8 km (5.0 mi) 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]
A major technological step was to replace the Morse code by modulating optical waves in speech transmission.Carl Zeiss, Jena developed theLichtsprechgerät 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 theAtlantic Wall.[4]
The invention oflasers in the 1960s revolutionized free-space optics.[citation needed] Military organizations were particularly interested and boosted their development. In 1973, while prototyping the firstlaser printers atPARC,Gary Starkweather and others made aduplex 30 Mbit/sCANoptical link using astronomical telescopes andHeNe 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 theFCC.[5][non-primary source needed] However, laser-based free-space optics lost market momentum when the installation ofoptical fiber networks for civilian uses was at its peak.[citation needed]
Many simple and inexpensive consumerremote controls use low-speed communication usinginfrared (IR) light. This is known asconsumer IR 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 usingLEDs.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 theoptical wireless communications applications. Free-space optics can be used for communications betweenspacecraft.[6]
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 (1,300 to 1,600 ft)). This is from both independent studies, such as in the Czech Republic,[7] as well as internal studies, such as one conducted by MRV FSO staff.[8]
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).[9] 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.[10][11]
TMEX USA ran two eight-mile links betweenLaredo, Texas andNuevo Laredo, Mexico from 1998[12] to 2002. The links operated at 155 Mbit/s and reliably carried phone calls and internet service.[13][dubious –discuss][citation needed]
The main reason terrestrial communications have been limited to non-commercial telecommunications functions is fog. Fog often prevents FSO laser links over 500 meters (1,600 ft) 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 betterquality of service.DARPA has sponsored over US$130 million in research toward this effort, with the ORCA and ORCLE programs.[14][15][16]
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[update], 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:[17]
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.[25]
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.
The first gigabit laser-based communication[clarification needed] was achieved by the European Space Agency and called theEuropean Data Relay System (EDRS) on November 28, 2014. The system is operational and is being used on a daily basis.
In December 2023, theAustralian National University (ANU) demonstrated its Quantum Optical Ground Station at itsMount Stromlo Observatory. QOGS uses adaptive optics and lasers as part of a telescope, to create a bi-directional communications system capable of supporting theNASAArtemis program to theMoon.[26]
A two-way distance record for communication was set by the Mercury laser altimeter instrument aboard theMESSENGER spacecraft. It was able to communicate across a distance of 24 million km (15 million mi), 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 (3.7 million mi) in 1992.
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.[27]
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).[28] LLCD was flown aboard theLunar 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'sOPALS instrument successfully uploaded 175 megabytes in 3.5 seconds and downloaded 200–300 MB in 20 s.[29] Their system was also able to re-acquire tracking after the signal was lost due to cloud cover.
On December 7, 2021 NASA launched theLaser Communications Relay Demonstration (LCRD), which aims to relay data between spacecraft ingeosynchronous orbit and ground stations. LCRD is NASA's first two-way, end-to-end optical relay. LCRD uses twoground stations, Optical Ground Station (OGS)-1 and -2, atTable Mountain Observatory in California, andHaleakalā,Hawaii.[30] One of LCRD's first operational users is theIntegrated 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 onSpaceX CRS-29, and achievedfirst light on December 5, 2023.[31][32]
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 theTeraByte InfraRed Delivery (TBIRD) system, mounted on NASA'sPathfinder Technology Demonstrator 3 (PTD-3) satellite.[33]
Varioussatellite constellations that are intended to provide global broadband coverage, such asSpaceXStarlink, employlaser communication for inter-satellite links. This effectively creates a space-basedoptical mesh network between the satellites.
In 2001, Twibright Labs releasedRONJA Metropolis, an open-source DIY 10 Mbit/s full-duplex LED FSO system that can span 1.4 km (0.87 mi).[34][35]
In 2004, avisible light communication consortium was formed inJapan.[36] This was based on work from researchers that used a white LED-based space lighting system for indoorlocal area network (LAN) communications. These systems present advantages over traditionalUHF 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.[37] In January 2009, a task force for visible light communication was formed by theInstitute of Electrical and Electronics Engineers working group for wirelesspersonal area network standards known asIEEE 802.15.7.[38] A trial was announced in 2010, inSt. Cloud, Minnesota.[39]
Amateur radio operators have achieved significantly farther distances using incoherent sources of light from high-intensity LEDs. One reported 278 km (173 mi) in 2007.[40] However, physical limitations of the equipment used limitedbandwidths to about 4kHz. 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. Lasers can reach very high data rates which are comparable to fiber communications.
Projected data rates and future data rate claims vary. A low-costwhite LED (GaN-phosphor) which could be used for space lighting can typically be modulated up to 20 MHz.[41] Data rates of over 100Mbit/s can be achieved using efficientmodulation schemes andSiemens claimed to have achieved over 500 Mbit/s in 2010.[42] Research published in 2009, used a similar system for traffic control of automated vehicles with LED traffic lights.[43]
In September 2013, pureLiFi, the Edinburgh start-up working onLi-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, theLi-1st, maximizes the available optical bandwidth for any LED device, thereby reducing the cost and improving the performance of deploying indoor FSO systems.[44]
Typically, the best scenarios for using this technology are:
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 improvedelectromagnetic interference (EMI) behavior compared to usingmicrowaves.
For terrestrial applications, the principal limiting factors are:
These factors cause an attenuated receiver signal and lead to higherbit 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 largerfade margin (extra power, reserved for rain, smog, fog). To keep an eye-safe environment, good FSO systems have a limited laser power density and supportlaser classes1 or1M. Atmospheric and fog attenuation, which are exponential in nature, limit practical range of FSO devices to several kilometers. However, free-space optics based on1550 nm wavelength, have considerably lower optical loss than free-space optics using830 nm wavelength, in dense fog conditions. FSO using wavelength 1550 nm system are capable of transmitting several times higher power than systems with850 nm and are safe to the human eye (1M class). Additionally, some free-space optics, such as EC SYSTEM,[47] 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.[47]
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