Optical communication
Free-space optical communication use lasers to transmit signals in space, while terrestrial forms are naturally limited by geography and weather.
Visual techniques such as smoke signals, beacon fires, hydraulic telegraphs, ship flags and semaphore lines were the earliest forms of optical communication.
Aircraft pilots often use visual approach slope indicator (VASI) projected light systems to land safely, especially at night.
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 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.
[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).
For example, Britain and Sweden adopted systems of shuttered panels (in contradiction to the Chappe brothers' contention that angled rods are more visible).
[citation needed] 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.
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.
The light gun's lamp has a focused bright beam capable of emitting three different colors: red, white and green.
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.
The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th century.
[5] In the present day a variety of electronic systems optically transmit and receive information carried by pulses of light.
The need for periodic signal regeneration was largely superseded by the introduction of the erbium-doped fiber amplifier, which extended link distances at significantly lower cost.
The commercial introduction of dense wavelength-division multiplexing (WDM) in 1996 by Ciena Corp was the real start of optical networking.
[25] 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.
