Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to 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 or an optical transmission line. The technology is useful where the physical connections are impractical due to high costs or other considerations.
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Optical communications, in various forms, have been used for thousands of years. The Ancient Greeks polished their shields to send signals during battle. In the modern era, semaphores and wireless solar telegraphs called heliographs were developed, using coded signals to communicate with their recipients.
In 1880 Alexander Graham Bell and his 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 apart.[1][2] Its first practical use came in military communication systems many decades later.
Carl Zeiss Jena developed the Lichtsprechgerät 80 (direct translation: light speaking device) that the German army used in their World War II anti-aircraft defense units.[3]
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 commnication using infrared (IR) light. This known as consumer 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 using LEDs. Infrared Data Association (IrDA) technology is a very simple form of free-space optical communications. Free Space Optics are additionally used for communications between spacecraft. Maximum range for terrestrial links is in the order of 2 to 3 km (1.2 to 1.9 mi),[4] but the stability and quality of the link is highly dependent on atmospheric factors such as rain, fog, dust and heat. Amateur radio operators have achieved significantly farther distances using incoherent sources of light from high-intensity LEDs. One reported 173 miles (278 km) in 2007.[5] 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.
In outer space, the communication range of free-space optical communication is currently in the order of several thousand kilometers,[6] but has the potential to bridge interplanetary distances of millions of kilometers, using optical telescopes as beam expanders.[7] The distance records for optical communications involved detection and emission of laser light by space probes. A two-way distance record for communication was set by the Mercury laser altimeter instrument aboard the MESSENGER spacecraft. This infrared diode neodymium laser, designed as a laser altimeter for a Mercury orbit mission, was able to communicate across a distance of 15 million miles (24 million km), 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, as two ground-based lasers were seen from 6 million km by the out-bound probe, in 1992.[8]
Secure free-space optical communications have been proposed using a laser N-slit interferometer where the laser signal takes the form of an interferometric pattern. Any attempt to intercept the signal causes the collapse of the interferometric pattern.[9] [10] This technique has been demonstrated to work over propagation distances of practical interest[11] and, in principle, it could be applied over large distances in space.[9]
Researchers 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.[12] In 2003, a Visible Light Communication Consortium was formed in Japan.[13] A low-cost white LED (GaN-phospor) which could be used for space lighting can typically be modulated up to 20 MHz.[14] 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.[15] Research published in 2009 used a similar system for traffic control of automated vehicles with LED traffic lights.[16] 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.[17] A trial was announced in 2010 in St. Cloud, Minnesota.[18]
Typically scenarios for use are:
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.
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.
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