Lasers, light and telephones

Abstract

1. Early forms of optical communication: In 1588, when the Armada was sighted sailing up the English Channel for the invasion of England, a warning signal was sent to the Queen by the lighting of bonfires on a chain of hilltops from Devon to London. This was a common method of sending important messages for many centuries and the hills on which the fires or beacons were made, came to be known as 'Beacon Hill' - name which many of them still retain. Although nowadays it seems a crude and clumsy technique, it represents one of the earliest forms of an optical communication system. Only the simplest possible message could be sent, either a 'Yes' or a 'No' - the absence of the fire meaning 'No, the Armada has not been sighted', changing when the bonfire was lit to 'Yes, the Armada is coming'. In the jargon of the communications engineer only one 'binary digit' or 'bit' of information could be sent at a time and it could not be repeated until the beacon had burnt out and was rebuilt. When one realises that it takes nearly four million bits of information to make one frame of a colour television picture and the frames are flashed on to the screen fifty times every second, it is possible to appreciate just how sophisticated present-day communications systems have become.
An improvement on beacons is the heliograph which consists of a mirror so adjusted as to reflect sunlight to an observer. By tilting the mirror a series of flashes can be sent in the form of Morse code and the rate of transmission is increased from something like one bit of information per day in the case of the beacon to perhaps five bits or flashes per second. The heliograph can only be used if the sun is visible but in the modern version an artificial light source in the form of an electric lamp is provided and by means of a shutter, or a dipping reflector, coded flashes of light can be sent, as in a naval signalling lamp. However, the rate at which messages can be sent is limited by two factors, firstly by the mechanical shutter and secondly by the fact that both the human eye and brain are limited in their ability to respond to and decode messages sent by flashing lights. Thus the signalling lamp can be used to send relatively slow and simple messages, such as those required between ships at sea when radios cannot be used, but it would take a long time, no less than 278 hours or 11.5 days at 24 hours a day to send as much information with a lamp as there is in even a single static colour television picture.

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