Optical communication
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Optical communication is any form of telecommunication that uses light as the transmission medium.
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 signalling 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.
Modern optical communications
In modern optical communications, lasers and light-emitting diodes are usually used as light sources. Often the light is in the infrared spectrum, rather than being visible light, because the glass fibers transmit those frequencies better. LEDs are generally restricted to low-data-rate use, with lasers being used for higher data rates.
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.
The bulk of the world's high speed data communications backbone networks now use optical communications over optical fiber.
By using different light frequencies (or colours) multiple communications can be sent optically, in what is known as wavelength division multiplexing.