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. "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. History
Optical communications, 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 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 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 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, 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. 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 communication using infrared light. This
is known as consumer IR technologies. A recently declassified 1987 Pentagon
report reveals free-space lasers have been mounted on Israeli F-15 fighter
jets for the purposes of surveillance, missile-tracking, and targeted weaponry.
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, but this has not
been put into practice. = Current Market Demands =
The demand for a high-speed and long range FSO system is apparent in the
market place. In 2008, MRV Communications introduced a
free-space optics-based system with a data rate of 10GB/s initially claiming a
distance of 2 km at high availability. This equipment is no longer available;
before end-of-life, the product's useful distance was changed down to 350m.
In 2013, the company MOSTCOM started to serially produce a new wireless
communication system that also had a data rate of 10GB/s as well as an
improved range of up to 2.5 km, but to get to 99.99% up-time the designers used
an RF hybrid solution, meaning the data rate drops to extremely low levels
during atmospheric disturbances. LightPointe offers many similar hybrid
solutions to MOSTCOM's offering. = 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 formal internal nationwide 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 2 to 3 km. All studies agree
the stability and quality of the link is highly dependent on atmospheric factors
such as rain, fog, dust and heat. = Extending the Useful Distance =
The main reason terrestrial communications have been limited to
non-commercial telecommunications functions is fog. Fog consistently keeps
FSO laser links over 500 meters from achieving a year-round bit error rate of
99.999%. 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 $130 million USD in research towards
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. As
of October 2014, none have fielded a working system that addresses the most
common atmospheric events. FSO research from 1998-2006 in the
private sector totaled $407.1 million, divided primarily among 4 start-up
companies. All four failed to deliver products that would meet
telecommunications quality and distance standards:
Terabeam received approximately $226 million in funding. AT&T and Lucent
backed this attempt. The work ultimately failed, and the company reorganized in
2004. 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. One private company published a paper on
Nov 20,2014, claiming they had achieved commercial reliability in extreme fog.
There is no indication this product is currently commercially available.
= 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. To date, no laser communication system is in use in space.
See Laser communication in space Demonstrations in Space:
The first gigabit laser-based communication was achieved by the
European Space Agency and called the European Data Relay System on November
28, 2014.[4] The initial images have just been demonstrated, and a working
system is expected to be in place in the 2015-2016 time frame.
NASA's OPALS announced a breakthrough in space-to-ground communication December
9, 2014, uploading 175 megabytes in 3.5 seconds. Their system is also able to
re-acquire tracking after the signal was lost due to cloud cover.[5]
In January 2013, NASA used lasers to beam an image of the Mona Lisa to the
Lunar Reconnaissance Orbiter roughly 390,000 km away. To compensate for
atmospheric interference, an error correction code algorithm similar to
that used in CDs was implemented.[6] 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 diode-pumped
infrared neodymium laser, designed as a laser altimeter for a Mercury orbit
mission, was able to communicate across a distance of 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.[7] Quote from Laser Communication in Space Demonstrations
LEDs In 2004, a Visible Light Communication
Consortium was formed in Japan. This was based on work from researchers that used
a white LED-based space lighting system for indoor local area network
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 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.
Amateur radio operators have achieved significantly farther distances using
incoherent sources of light from high-intensity LEDs. One reported 173
miles 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.
Projected data rates and future data rate claims vary. A low-cost white LED
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 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. Engineering Details
Typically, best use scenarios for this technology 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 Reestablish high-speed connection
quickly As an alternative or upgrade add-on to
existing wireless technologies Especially powerful in combination with
auto aiming systems, this way you could power moving cars or you can power your
laptop while you move or use auto-aiming nodes to create a network with other
nodes. As a safety add-on for important fiber
connections 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 behavior compared to using microwaves.
= Technical Advantages = Ease of deployment
Can be used to power devices License-free long-range operation
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
= Range Limiting Factors = For terrestrial applications, the
principal limiting factors are: Fog
Beam dispersion Atmospheric absorption
Rain Snow
Terrestrial scintillation Interference from background light
sources Shadowing
Pointing stability in wind Pollution / smog
These factors cause an attenuated receiver signal and lead to higher bit
error ratio. 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. 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. See also
Laser communication in space Optical Wireless Communications
Applications of atomic line filters in laser tracking and communication
Extremely high frequency IrDA
RONJA Laser safety
List of laser articles Mie scattering
Modulating retro-reflector N-slit interferometer
Semaphore line Optical window
Photophone Radio window
Rayleigh scattering Smoke signal
Terabeam Visible light communication
Li-Fi References
Further reading Christos Kontogeorgakis. Millimeter
Through Visible Frequency Waves Through Aerosols-Particle Modeling, Reflectivity
and Attenuation. Virginia Polytechnic Institute and State University.
Master's Thesis Heinz Willebrand & Baksheesh Ghuman.
Free Space Optics: Enabling Optical Connectivity in Today's Networks. SAMS.
Moll, Florian. "Free-space laser system for secure air-to-ground quantum
communications". SPIE Newsroom. David G. Aviv. Laser Space
Communications. ARTECH HOUSE. ISBN 1-59693-028-4.
External links Free Space Optics on COST297 for HAPs
Explanation of Fresnel zones in microwave and optical links
video showing Lichtsprechgerät 80 in use on YouTube
International Space Station to Beam Video Via Laser Back to Earth, March
2014 NASA's Optical Payload for Lasercomm Science demonstration mission
to the ISS.