Optical communication is any form of telecommunication that uses light as the transmission medium.

An optical communication system consists of 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.

Forms of optical communication

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There are many forms of non-technological optical communication, including body language and sign language.

Techniques such as semaphore, ship flags, smoke signals, and beacons; fires were the earliest form of technological optical communication.

The heliograph uses a mirror to reflect sunlight to a distant observer. By moving the mirror the distant observer sees flashes of light that can be used to send a prearranged signaling code. Navy ships often use a signal lamp to signal in Morse code in a similar way.

Distress flares are used by mariners in emergencies, while lighthouses and navigation lights are used to communicate navigation hazards.

Aircraft use the landing lights at airports to land safely, especially at night. Aircraft landing on an aircraft carrier use a similar system to land correctly on the carrier deck. The light systems communicate the correct position of the aircraft relative to the best landing glideslope.

Optical fiber is the most common medium for modern digital optical communication.

Free-space optical communication is also used today in a variety of applications.

Optical fiber communication

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Optical fiber is the most common type of channel for optical communications, however other types of optical waveguides are used within communications gear, and have even formed the channel of very short distance (e.g. chip-to-chip) links in laboratory trials. 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. These coherent optical transmission schemes were largely superseded by the introduction of the erbium-doped fiber amplifier, which extended link distances at lower cost.

LEDs are generally restricted to low data rates, up to about 100 Megabits per second (Mb/s). Lasers are used for higher data rates. These devices are often directly modulated, that is the light output is controlled by a current applied directly to the device. For very high data rates or very long distance links, a laser source may be operated continuous wave, and the light modulated by an external device such as an electroabsorption modulator or Mach-Zehnder interferometer. External modulation increases the achievable link distance by eliminating laser chirp, which broadens the linewidth of directly-modulated lasers, increasing the chromatic dispersion in the fiber.

Receivers used in fiber links use p-i-n diodes, avalanche photodiodes, or other semiconductor devices as the optical-electrical converter. The optical-electrical converters is typically coupled with a transimpedance amplifier and limiting amplifier to produce a digital signal in the electrical domain from the incoming optical signal, which may be attenuated and distorted by passing through the channel. Further signal processing such as clock recovery from data (CDR) by a phase-locked loop may also be applied before the data is passed on.

To increase the link distance, early optical fiber links contained repeaters, which were essentially back-to-back receivers and transmitters that converted the signal from the optical to electrical domain and back to an optical signal again and essentially broke a long link into many short ones. Since its invention, however, the erbium-doped fiber amplifier (EDFA), has replaced the repeater in most telecommunication links.

By using different light wavelengths (or colors) multiple communications can be sent optically, in what is known as wavelength division multiplexing (WDM). This requires a wavelength division multiplexer in the transmitting equipment and a wavelength division demultiplexer (essentially a spectrometer) in the receiving equipment. Arrayed waveguide gratings are commonly used for multiplexing and demultiplexing in WDM.

Free-space optical communication

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IrDA is an example of low-data-rate, short distance free-space optical communications using LEDs. RONJA is an example of 10Mbit/s 1.4 km full-duplex optical point-to-point link.

See also

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• Laser
• Optical telegraph
• Telegraph
• Jun-Ichi Nishizawa an inventor of optical communication.

References

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Communication