BACK REFLECTION
THE BEGINNING OF WAVELENGTH DIVISION MULTIPLEXING COMMUNICATIONS IN 1880 BY PHILIP PILGRIM
L
ong, long ago, before commercial AC power, before light bulbs, before lasers, before radio, before optical fibre, just five years after the invention of his telephone, Alexander Graham Bell invented the first optical communication system using light. This device was named the Photophone. Just as in modern-day optical transmission, the photophone used a transmitter that modulated light, and a receiver that converted the received light to an electrical signal. The only available light source of sufficient power at the time was the sun, so Bell’s photophone took on the shape of an astronomical telescope mount in order to track the sun. The sunlight was then shaped into a beam using lens’ then shone upon a mirror which was mounted on a speaker-like device. This modulated the beam of light which then travelled through the open air to the distant receiver. The receiver was a parabolic mirror which focused the light onto a selenium cell photodetector that transformed the light energy back to electrical energy. Today’s new frontier in telecommunication is space and the use of free-space optical connections using lasers. Currently the brightest minds are working on developing platforms that will be on-station just 20 km above populated areas and provide
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