Optical communication

Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information. It can be performed visually or by using electronic devices. The earliest basic forms of optical communication date back several millennia, while the earliest electrical device created to do so was the photophone, invented in 1880.

An optical communication system uses a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal. When electronic equipment is not employed the 'receiver' is a person visually observing and interpreting a signal, which may be either simple (such as the presence of a beacon fire) or complex (such as lights using color codes or flashed in a Morse code sequence).

Free-space optical communication has been deployed in space, while terrestrial forms are naturally limited by geography, weather and the availability of light. This article provides a basic introduction to different forms of optical communication.

US Navy 020623-N-5329L-007 Signalman 2nd Class Eric Palmer signals the U.S. Navy mine hunter coastal ship USS Raven (MHC 61
A naval signal lamp, a form of optical communication that uses shutters and is typically employed with Morse code (2002)

Forms

Visual techniques such as smoke signals, beacon fires, hydraulic telegraphs, ship flags and semaphore lines were the earliest forms of optical communication.[1][2][3][4] Hydraulic telegraph semaphores date back to the 4th century BCE Greece. Distress flares are still used by mariners in emergencies, while lighthouses and navigation lights are used to communicate navigation hazards.

The heliograph uses a mirror to reflect sunlight to a distant observer.[5] When a signaler tilts the mirror to reflect sunlight, the distant observer sees flashes of light that can be used to transmit a prearranged signaling code. Naval ships often use signal lamps and Morse code in a similar way.

Aircraft pilots often use visual approach slope indicator (VASI) projected light systems to land safely, especially at night. Military aircraft landing on an aircraft carrier use a similar system to land correctly on a carrier deck. The coloured light system communicates the aircraft's height relative to a standard landing glideslope. As well, airport control towers still use Aldis lamps to transmit instructions to aircraft whose radios have failed.

In the present day a variety of electronic systems optically transmit and receive information carried by pulses of light. Fiber-optic communication cables are now employed to send the great majority of the electronic data and long distance telephone calls that are not conveyed by either radio, terrestrial microwave or satellite. Free-space optical communications are also used every day in various applications.

Semaphore line

OptischerTelegraf
A replica of one of Chappe's semaphore towers (18th century).

A 'semaphore telegraph', also called a 'semaphore line', 'optical telegraph', 'shutter telegraph chain', 'Chappe telegraph', or 'Napoleonic semaphore', is a system used for conveying information by means of visual signals, using towers with pivoting arms or shutters, also known as blades or paddles. Information is encoded by the position of the mechanical elements; it is read when the shutter is in a fixed position.[2][6]

Semaphore lines were a precursor of the electrical telegraph. They were far faster than post riders for conveying a message over long distances, but far more expensive and less private than the electrical telegraph lines which would later replace them. The maximum distance that a pair of semaphore telegraph stations can bridge is limited by geography, weather and the availability of light; thus, in practical use, most optical telegraphs used lines of relay stations to bridge longer distances. Each relay station would also require its complement of skilled operator-observers to convey messages back and forth across the line.

The modern design of semaphores was first foreseen by the British polymath Robert Hooke, who first gave a vivid and comprehensive outline of visual telegraphy in a 1684 submission to the Royal Society. His proposal (which was motivated by military concerns following the Battle of Vienna the preceding year) was not put into practice during his lifetime.[7][8]

The first operational optical semaphore line arrived in 1792, created by the French engineer Claude Chappe and his brothers, who succeeded in covering France with a network of 556 stations stretching a total distance of 4,800 kilometres (3,000 mi). It was used for military and national communications until the 1850s.

Many national services adopted signaling systems different from the Chappe system. For example, Britain and Sweden adopted systems of shuttered panels (in contradiction to the Chappe brothers' contention that angled rods are more visible). In Spain, the engineer Agustín de Betancourt developed his own system which was adopted by that state. This system was considered by many experts in Europe better than Chappe's, even in France.

These systems were popular in the late 18th to early 19th century but could not compete with the electrical telegraph, and went completely out of service by 1880.[1]

Semaphore signal flags

020118-N-6520M-011 Semaphore Flags
A naval signaler transmitting a message by flag semaphore (2002).

Semaphore Flags is the system for conveying information at a distance by means of visual signals with hand-held flags, rods, disks, paddles, or occasionally bare or gloved hands. Information is encoded by the position of the flags, objects or arms; it is read when they are in a fixed position.

Semaphores were adopted and widely used (with hand-held flags replacing the mechanical arms of shutter semaphores) in the maritime world in the 19th century. They are still used during underway replenishment at sea and are acceptable for emergency communication in daylight or, using lighted wands instead of flags, at night.

The newer flag semaphore system uses two short poles with square flags, which a signaler holds in different positions to convey letters of the alphabet and numbers. The transmitter holds one pole in each hand, and extends each arm in one of eight possible directions. Except for in the rest position, the flags cannot overlap. The flags are colored differently based on whether the signals are sent by sea or by land. At sea, the flags are colored red and yellow (the Oscar flags), while on land, they are white and blue (the Papa flags). Flags are not required, they just make the characters more obvious.

Optical fiber

Optical fiber is the most common type of channel for optical communications. The transmitters in optical fiber links are generally light-emitting diodes (LEDs) or laser diodes. Infrared light, rather than visible light is used more commonly, because optical fibers transmit infrared wavelengths with less attenuation and dispersion. The signal encoding is typically simple intensity modulation, although historically optical phase and frequency modulation have been demonstrated in the lab. The need for periodic signal regeneration was largely superseded by the introduction of the erbium-doped fiber amplifier, which extended link distances at significantly lower cost.

Signal lamps

TC with light gun
An air traffic controller holding a signal light gun that can be used to direct aircraft experiencing a radio failure (2007).

Signal lamps (such as Aldis lamps), are visual signaling devices for optical communication (typically using Morse code). Modern signal lamps are a focused lamp which can produce a pulse of light. In large versions this pulse is achieved by opening and closing shutters mounted in front of the lamp, either via a manually operated pressure switch or, in later versions, automatically.

With hand held lamps, a concave mirror is tilted by a trigger to focus the light into pulses. The lamps are usually equipped with some form of optical sight, and are most commonly deployed on naval vessels and also used in airport control towers with coded aviation light signals.

Aviation light signals are used in the case of a radio failure, an aircraft not equipped with a radio, or in the case of a hearing-impaired pilot. Air traffic controlers have long used signal light guns to direct such aircraft. The light gun's lamp has a focused bright beam capable of emitting three different colors: red, white and green. These colors may be flashing or steady, and provide different instructions to aircraft in flight or on the ground (for example, "cleared to land" or "cleared for takeoff"). Pilots can acknowledge the instructions by wiggling their plane's wings, moving their ailerons if they are on the ground, or by flashing their landing or navigation lights during night time. Only 12 simple standardized instructions are directed at aircraft using signal light guns as the system is not utilized with Morse code.

Photophone

The photophone (originally given an alternate name, radiophone) is a communication device which allowed for the transmission of speech on a beam of light. It was invented jointly by Alexander Graham Bell and his assistant Charles Sumner Tainter on February 19, 1880, at Bell's 1325 'L' Street laboratory in Washington, D.C.[9][10] Both were later to become full associates in the Volta Laboratory Association, created and financed by Bell.

On June 21, 1880, Bell's assistant transmitted a wireless voice telephone message of considerable distance, from the roof of the Franklin School to the window of Bell's laboratory, some 213 meters (about 700 ft.) away.[11][12][13][14]

Bell believed the photophone was his most important invention. Of the 18 patents granted in Bell's name alone, and the 12 he shared with his collaborators, four were for the photophone, which Bell referred to as his 'greatest achievement', telling a reporter shortly before his death that the photophone was "the greatest invention [I have] ever made, greater than the telephone".[15]

The photophone was a precursor to the fiber-optic communication systems which achieved popular worldwide usage starting in the 1980s.[16][17][18] The master patent for the photophone (U.S. Patent 235,199 Apparatus for Signalling and Communicating, called Photophone), was issued in December 1880,[13] many decades before its principles came to have practical applications.

Free-space optical communication

Free-space optics (FSO) systems are employed for 'last mile' telecommunications and can function over distances of several kilometers as long as there is a clear line of sight between the source and the destination, and the optical receiver can reliably decode the transmitted information.[19] Other free-space systems can provide high-data-rate, long-range links using small, low-mass, low-power-consumption subsystems which make them suitable for communications in space.[20] Various planned satellite constellations intended to provide global broadband coverage take advantage of these benefits and employ laser communication for inter-satellite links between the several hundred to thousand satellites effectively creating a space-based optical mesh network.

More generally, transmission of unguided optical signals is known as optical wireless communications (OWC). Examples include medium-range visible light communication and short-distance IrDA, using infrared LEDs.

Heliograph

Australian Heliograph in Egyptian Desert 1940
Heliograph: Australians using a heliograph in North Africa (1940).

A heliograph (Greek: Ἥλιος helios, meaning "sun", and γραφειν graphein, meaning "write") is a wireless solar telegraph that signals by flashes of sunlight (generally using Morse code) reflected by a mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter.

The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th century. Its main uses were in military, surveys and forest protection work. They were standard issue in the British and Australian armies until the 1960s, and were used by the Pakistani army as late as 1975.[5]

See also

References

Citations

  1. ^ a b Chapter 2: Semaphore Signalling ISBN 978-0-86341-327-8 Communications: an international history of the formative years R. W. Burns, 2004
  2. ^ a b Telegraph Vol 10, Encyclopædia Britannica, 6th Edition, 1824 pp. 645-651
  3. ^ "Nation Park Service Fire History Timeline".
  4. ^ "Lewis and Clark Journals, July 20, 1805".
  5. ^ a b Harris, J.D. Wire At War - Signals communication in the South African War 1899–1902. Retrieved on 1 June 2008. Note a discussion on the heliograph use during the Boer War.
  6. ^ Telegraph, Volume 17 of The Edinburgh encyclopaedia, pp. 664-667, 1832 David Brewster, ed.
  7. ^ Calvert, J.B. The Origin of the Railway Semaphore, Boston University, 15 April 2000, Revised 4 May 2007.
  8. ^ McVeigh, Daniel P. An Early History of the Telephone: 1664-1865, Part 2, Columbia University in The City of New York, Institute For Learning Technologies, 2000.
  9. ^ Bruce 1990, pg. 336
  10. ^ Jones, Newell. First 'Radio' Built by San Diego Resident Partner of Inventor of Telephone: Keeps Notebook of Experiences With Bell Archived 2006-09-04 at Archive.today, San Diego Evening Tribune, July 31, 1937. Retrieved from the University of San Diego History Department website, November 26, 2009.
  11. ^ Bruce 1990, pg. 338
  12. ^ Carson 2007, pg. 76-78
  13. ^ a b Groth, Mike. Photophones Revisted, 'Amateur Radio' magazine, Wireless Institute of Australia, Melbourne, April 1987 pp. 12–17 and May 1987 pp. 13–17.
  14. ^ Mims 1982, p. 11.
  15. ^ Mims 1982, p. 14.
  16. ^ Morgan, Tim J. "The Fiber Optic Backbone", University of North Texas, 2011.
  17. ^ Miller, Stewart E. "Lightwaves and Telecommunication", American Scientist, Sigma Xi, The Scientific Research Society, January–February 1984, Vol. 72, No. 1, pp. 66-71, Issue Stable URL.
  18. ^ Gallardo, Arturo; Mims III, Forrest M.. Fiber-optic Communication Began 130 Years Ago, San Antonio Express-News, June 21, 2010. Accessed January 1, 2013.
  19. ^ Clint Turner (October 3, 2007). "A 173-mile 2-way all-electronic optical contact". Modulated light web site. Retrieved June 28, 2011.
  20. ^ Wilson, K. (2000-01-04). "Recent Development in High-Data Rate Optical Communications at JPL". Jet Propulsion Laboratory. NASA Technical Reports Server. hdl:2014/18156.

Bibliography

Further reading

Arrayed waveguide grating

Arrayed waveguide gratings (AWG) are commonly used as optical (de)multiplexers in wavelength division multiplexed (WDM) systems. These devices are capable of multiplexing a large number of wavelengths into a single optical fiber, thereby increasing the transmission capacity of optical networks considerably.

The devices are based on a fundamental principle of optics that light waves of different wavelengths interfere linearly with each other. This means that, if each channel in an optical communication network makes use of light of a slightly different wavelength, then the light from a large number of these channels can be carried by a single optical fiber with negligible crosstalk between the channels. The AWGs are used to multiplex channels of several wavelengths onto a single optical fiber at the transmission end and are also used as demultiplexers to retrieve individual channels of different wavelengths at the receiving end of an optical communication network.

Container glass

Container glass is a type of glass for the production of glass containers, such as bottles, jars, drinkware, and bowls. Container glass stands in contrast to flat glass (used for windows, glass doors, transparent walls, windshields) and glass fiber (used for thermal insulation, in fiberglass composites, and optical communication).

DBm

dBm (sometimes dBmW or decibel-milliwatts) is unit of level used to indicate that a power ratio is expressed in decibels (dB) with reference to one milliwatt (mW). It is used in radio, microwave and fiber-optical communication networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form compared to dBW, which is referenced to one watt (1,000 mW).

Since it is referenced to the watt, it is an absolute unit, used when measuring absolute power. By comparison, the decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio.

The dBm is also dimensionless but since it compares to a fixed reference value the dBm rating is an absolute one.

In audio and telephony, dBm is typically referenced relative to a 600-ohm impedance, while in radio-frequency work dBm is typically referenced relative to a 50-ohm impedance.

Finisar

Finisar Corporation is a manufacturer of optical communication components and subsystems. In 2008, Finisar merged with Optium Corporation.

Finisar's products include optical transceivers, optical engines, active optical cables, optical components, optical instrumentation, ROADM & wavelength management, optical amplifiers, and RF-over-Fiber. Their products enable high-speed voice, video and data communications for networking, storage, wireless, and cable TV applications.

In November 2018, II-VI Incorporated acquire Finisar.

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.

The technology is useful where the physical connections are impractical due to high costs or other considerations.

GSAT-29

GSAT-29 is a high-throughput communication satellite developed by the Indian Space Research Organisation (ISRO). The mission aims at providing high-speed bandwidth to Village Resource Centres (VRC) in rural areas. The two Ku and Ka operational payloads will provide communication services to Jammu and Kashmir and Northeast India under Digital India programme. GSAT-29 was the heaviest satellite, weighing 3,423 kg (7,546 lb), that was placed in orbit by an Indian launch vehicle.

Ginrei

Ginrei or ShindaiSat was a 400x400x450mm cube-like microsatellite intended to text experimental visible light communication. The satellite is made in Shinshu University (Japan). The ground station was completed by 18 March 2014 and attempts to communicate with satellite have started the same day. 2-way optical communication with ground station is planned. Also, advanced attitude control using visible light communication is planned as well.

Heliograph

A heliograph (helios (Greek: ἥλιος), meaning "sun", and graphein (γράφειν), meaning "write") is a wireless telegraph that signals by flashes of sunlight (generally using Morse code) reflected by a mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter. The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th century. Its main uses were military, survey and forest protection work. Heliographs were standard issue in the British and Australian armies until the 1960s, and were used by the Pakistani army as late as 1975.

Infrared Data Association

The Infrared Data Association (IrDA) is an industry-driven interest group that was founded in 1993 by around 50 companies. IrDA provides specifications for a complete set of protocols for wireless infrared communications, and the name "IrDA" also refers to that set of protocols. The main reason for using the IrDA protocols had been wireless data transfer over the "last one meter" using point-and-shoot principles. Thus, it has been implemented in portable devices such as mobile telephones, laptops, cameras, printers, and medical devices. Main characteristics of this kind of wireless optical communication is physically secure data transfer, line-of-sight (LOS) and very low bit error rate (BER) that makes it very efficient.

John Rarity

John G. Rarity is professor of optical communication systems in the department of electrical and electronic engineering at the University of Bristol, a post he has held since 1 January 2003. He is an international expert on quantum optics, quantum cryptography and quantum communication using single photons and entanglement. Professor Rarity is a member of the Quantum Computation and Information group and quantum photonics at the University of Bristol.

Laser communication in space

Laser communication in space is free-space optical communication in outer space.

In outer space, the communication range of free-space optical communication is currently of the order of several thousand kilometers, suitable for inter-satellite service. It has the potential to bridge interplanetary distances of millions of kilometers, using optical telescopes as beam expanders.

Mode partition noise

Mode partition noise: In an optical communication link, is phase jitter of the signal caused by the combined effects of mode hopping in the optical source and intramodal distortion in the fiber.

Mode hopping causes random wavelength changes which in turn affect the group velocity, i.e., the propagation time. Over a long length of fiber, the cumulative effect is to create jitter, i.e. mode partition noise. The variation of group velocity creates the mode partition noise.

On-off keying

On-off keying (OOK) denotes the simplest form of amplitude-shift keying (ASK) modulation that represents digital data at the presence or absence of a carrier wave. In its simplest form, the presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a binary zero. Some more sophisticated schemes vary these durations to convey additional information. It is analogous to unipolar encoding line code.

On-off keying is most commonly used to transmit Morse code over radio frequencies (referred to as CW (continuous wave) operation), although in principle any digital encoding scheme may be used. OOK has been used in the ISM bands to transfer data between computers, for example.

OOK is more spectrally efficient than frequency-shift keying, but more sensitive to noise when using a regenerative receiver or a poorly implemented superheterodyne receiver.

For a given data rate, the bandwidth of a BPSK (Binary Phase Shift keying) signal and the bandwidth of OOK signal are equal.

In addition to RF carrier waves, OOK is also used in optical communication systems (e.g. IrDA).

In aviation, some possibly unmanned airports have equipment that let pilots key their VHF radio a number of times in order to request an Automatic Terminal Information Service broadcast, or turn on runway lights.

Physical optics

In physics, physical optics, or wave optics, is the branch of optics that studies interference, diffraction, polarization, and other phenomena for which the ray approximation of geometric optics is not valid. This usage tends not to include effects such as quantum noise in optical communication, which is studied in the sub-branch of coherence theory.

Plate glass

Plate glass, flat glass or sheet glass is a type of glass, initially produced in plane form, commonly used for windows, glass doors, transparent walls, and windscreens. For modern architectural and automotive applications, the flat glass is sometimes bent after production of the plane sheet. Flat glass stands in contrast to container glass (used for bottles, jars, cups) and glass fibre (used for thermal insulation, in fibreglass composites, and optical communication).

Flat glass has a higher magnesium oxide and sodium oxide content than container glass, and a lower silica, calcium oxide, and aluminium oxide content. (From the lower soluble oxide content comes the better chemical durability of container glass against water, which is required especially for storage of beverages and food).

Most flat glass is soda–lime glass, produced by the float glass process. Other processes for making flat glass include:

Rolling (rolled plate glass, figure rolled glass)

Overflow downdraw method

Blown plate method

Broad sheet method

Window crown glass technique

Cylinder blown sheet method

Fourcault process

Machine drawn cylinder sheet method

Plate polishing

Return-to-zero

Return-to-zero (RZ or RTZ) describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive 0s or 1s occur in the signal. The signal is self-clocking. This means that a separate clock does not need to be sent alongside the signal, but suffers from using twice the bandwidth to achieve the same data-rate as compared to non-return-to-zero format.

The "zero" between each bit is a neutral or rest condition, such as a zero amplitude in pulse amplitude modulation (PAM), zero phase shift in phase-shift keying (PSK), or mid-frequency in frequency-shift keying (FSK).

That "zero" condition is typically halfway between the significant condition representing a 1 bit and the other significant condition representing a 0 bit.

Although return-to-zero (RZ) contains a provision for synchronization, it still has a DC component resulting in “baseline wander” during long strings of 0 or 1 bits, just like the line code non-return-to-zero.

Signal lamp

A signal lamp (sometimes called an Aldis lamp, after Arthur Cyril Webb Aldis, who invented a widely used design, or a Morse lamp) is a visual signaling device for optical communication, typically using Morse code. Modern signal lamps are focused lamps which can produce a pulse of light. In large versions, this pulse is achieved by opening and closing shutters mounted in front of the lamp, either via a manually operated pressure switch, or, in later versions, automatically. With hand held lamps, a concave mirror is tilted by a trigger to focus the light into pulses. The lamps were usually equipped with some form of optical sight, and were most commonly used on naval vessels and in airport control towers (using color signals for stop or clearance). In manual signaling, a signalman would aim the light at the recipient ship and turn a lever, opening and closing the shutter over the lamp, to emit flashes of light to spell out text messages in Morse code. On the recipient ship, a signalman would observe the blinking light, often with binoculars, and translate the code into text.

Sterlite Technologies

STL, the brand name of Sterlite Technologies Limited designs, builds and manages "smarter networks". STL develops & delivers optical communication products, network & system integration services and software solutions for telecoms globally. The company is listed on the Bombay Stock Exchange and the National Stock Exchange of India. The company changed its name to Technologies Limited' from Dec.2006. It is India's only integrated Optical Fiber producer and one of the largest suppliers of Optical Fibers to overseas markets in China, Europe and South East Asia.STL is partially owned by Sterlite Industries (India) Limited, which is in turn 77%-owned by Vedanta Resources.

The company has an optical fiber manufacturing plant located at Aurangabad, India and Telecom Cable & Power Transmission Conductor plants at Silvassa, Pune& Haridwar India.

Waveguide (optics)

An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber and rectangular waveguides.

Optical waveguides are used as components in integrated optical circuits or as the transmission medium in local and long haul optical communication systems.

Optical waveguides can be classified according to their geometry (planar, strip, or fiber waveguides), mode structure (single-mode, multi-mode), refractive index distribution (step or gradient index) and material (glass, polymer, semiconductor).

Optical telecommunication
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