Free-space optical communication
Free-space optical communication is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking 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
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. 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 apart.
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 as did British, French, US or Ottoman signals.
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 at daylight and of up to 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 that the German army used in their World War II anti-aircraft defense units, or in bunkers at the Atlantic Wall.
The invention of lasers in the 1960s revolutionized free-space optics. Military organizations were particularly interested and boosted their development. In 1973, while prototyping the first laser printers at PARC, Gary Starkweather and others made a duplex 30 Mbit/s CAN optical link using astronomical telescopes and HeNe lasers to send data between offices; they chose the method due partly to less strict regulations on free-space optical communication by the FCC. However, laser-based free-space optics 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 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 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.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. This is from both independent studies, such as in the Czech Republic, as well as internal studies, such as one conducted by MRV FSO staff.Military based studies consistently produce longer estimates for reliability, projecting the maximum range for terrestrial links is of the order of. 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.
TMEX USA ran two eight-mile links between Laredo, Texas and Nuevo Laredo, Mexico from 1998 to 2002. The links operated at 155 Mbit/s and reliably carried phone calls and internet service.
Extending the useful distance
The main reason terrestrial communications have been limited to non-commercial telecommunications functions is fog. Fog often prevents FSO laser links over 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.Other non-government groups are fielding tests to evaluate different technologies that some claim have the ability to address key FSO adoption challenges., 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:
- 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. The work ultimately failed, and the company was purchased in 2004 for $52 million by Falls Church, Virginia-based YDI, effective June 22, 2004, and used the name Terabeam for the new entity. On September 4, 2007, Terabeam announced it would change its name to Proxim Wireless Corporation, and change its NASDAQ stock symbol from TRBM to PRXM.
- 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.
- LightPointe Communications received $76 million in start-up funds, and eventually reorganized to sell hybrid FSO-RF units to overcome the weather-based challenges.
- The Maxima Corporation published its operating theory in Science, 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.
Extraterrestrial
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 on November 28, 2014. The system is operational and is being used on a daily basis.In December 2023, the Australian National University 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.
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, 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 in 1992.In January 2013, NASA used lasers to beam an image of the Mona Lisa to the Lunar Reconnaissance Orbiter roughly away. To compensate for atmospheric interference, an error correction code algorithm similar to that used in CDs was implemented.
In the early morning hours of October 18, 2013, NASA's Lunar Laser Communication Demonstration transmitted data from lunar orbit to Earth at a rate of 622 megabits per second. LLCD was flown aboard the Lunar Atmosphere and Dust Environment Explorer 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 MB in 20 s. 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, 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 stations, Optical Ground Station -1 and -2, at Table Mountain Observatory in California, and Haleakalā, Hawaii. One of LCRD's first operational users is the Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal, 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 on December 5, 2023.
On April 28, 2023, NASA and its partners achieved 200 gigabit per second throughput on a space-to-ground optical link between a satellite in orbit and Earth. This was achieved by the TeraByte InfraRed Delivery system, mounted on NASA's Pathfinder Technology Demonstrator 3 satellite.