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 or an optical transmission line. 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. 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. 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 1904/05 Herero Genocide in German South-West Africa, today's Namibia as did British, French, US or Ottoman signals. During the trench warfare in 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 at daylight and of up to 8 km 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, ballons, 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.
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.
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. Free Space Optics are additionally used for communications between spacecraft. Maximum range for terrestrial links is of the order of 2 to 3 km (1.2 to 1.9 mi), 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. 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. From the other side use of lasers radiation source allows to reach very high data rates which are comparable to fiber communications. For example, in 2008 the Company MRV Communications has introduced the free-space optics (FSO)-based TereScope TS-10GE system with data rate 10 Gbps on a distance till 350 m. Now this equipment is not presented on the FSO market. In 2013 the company MOSTCOM started the serially production of the new wireless communication system Artolink M1-10G with data rate 10 Gbps and the distance up to 2.5 km. In outer space, the communication range of free-space optical communication is currently of the order of several thousand kilometers, but has the potential to bridge interplanetary distances of millions of kilometers, using optical telescopes as beam expanders. In January 2013, NASA used lasers to beam an image of the Mona Lisa to the Lunar Reconnaissance Orbiter roughly 240,000 miles away. To compensate for atmospheric interference, error correction code algorithm similar to that used in CDs was implemented. 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.
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.  This technique has been demonstrated to work over propagation distances of practical interest and, in principle, it could be applied over large distances in space.
Visible light communication
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. In 2003, a Visible Light Communication Consortium was formed in Japan. A low-cost white LED (GaN-phosphor) which could be used for space lighting can typically be modulated up to 20 MHz. 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. Research published in 2009 used a similar system for traffic control of automated vehicles with LED traffic lights. 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. A trial was announced in 2010 in St. Cloud, Minnesota.
Typically scenarios for use 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
- 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.
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.
- Ease of deployment
- License-free long-range operation (in contrast with radio communication)
- High bit rates
- Low bit error rates
- Immunity to electromagnetic interference
- Full duplex operation
- Protocol transparency
- Increased security when working with narrow beam(s)
- No Fresnel zone necessary
For terrestrial applications, the principal limiting factors are:
- Beam dispersion
- Atmospheric absorption
- Fog (10..~100 dB/km attenuation)
- Interference from background light sources (including the Sun)
- Pointing stability in wind
- Pollution / 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 classes 1 or 1M. Atmospheric and fog attenuation, which are exponential in nature, limit practical range of FSO devices to several kilometres.
An in-space laser communication experiment was one of four science instruments launched with the NASA Lunar Atmosphere and Dust Environment Explorer (LADEE) mission on 7 Sept 2013. After a month-long transit to the Moon, and a 40-day spacecraft checkout, the laser comm experiments will occur over an approximately three-month period of time, in late 2013 to early 2014.
Lunar Laser Communication Demonstration—2013
Initial data returned from the Lunar Laser Communication Demonstration (LLCD) equipment on LADEE set a space communication bandwidth record in October 2013 when early tests using a pulsed laser beam to transmit data over the 385,000 kilometres (239,000 mi) between the Moon and Earth passed data at a "record-breaking download rate of 622 megabits per second (Mbps)", and also demonstrated an error-free data upload rate of 20 Mbps from an Earth ground station to LADEE in Lunar orbit. The LLCD is NASA's first attempt at two-way space communication using an optical laser instead of radio waves, and is expected to lead to operational laser systems on NASA satellites in future years.
- Applications of atomic line filters in laser tracking and communication
- Extremely high frequency
- RONJA (Reasonable Optical Near Joint Access)
- Laser safety
- List of laser articles
- Mie scattering
- Modulating retro-reflector
- N-slit interferometer
- Semaphore line
- Optical window
- Radio window
- Rayleigh scattering
- Smoke signal
- Visible light communication
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