How the fibre communications revolution began

How the optical communication revolution began

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Throughout history we humans have used light to communicate, using fire, the sun’s rays and signal lamps. However, in the 19th century the harnessing of electricity led to the invention of the electrical telegraph, and began an era where information was communicated almost exclusively by electricity.

After the end of the 1939-45 World war, the demand for widespread telephone use encouraged research into better telecommunications network technology. There were plenty of skilled scientists with expertise in electronics and microwaves, and the proliferation of TV created an expectation that we would soon be demanding personal Videophones. It was evident that existing copper wire technology would be incapable of meeting the need so research began into potential alternatives.

The development of Radar during the war inevitably let to the idea of using microwaves for communication. While microwave communication through the air was already practical, It was guided communication that would be necessary, a faster alternative to the copper cables. Many laboratories around the World then began to investigate some form of Long Haul Microwave Waveguide. Bell Labs in the USA and the British Post Office established major research programmes.

The invention of the Laser in 1960 reawakened interest in the possibility of using light as the medium of communication instead of electricity or radio waves. Communication through the air (“free space” communication) was easily achievable, but severely limited by weather and impractical in most network scenarios. At Standard Telecommunication Laboratories in the UK, a small team was established in 1959 with the mission to explore possible optical guiding mechanisms.

As the attenuation of light through glass was so high, all the initial efforts were directed towards solutions in which the optical energy travelled almost entirely in air. Three approaches were investigated in some detail:

1/. The Optical Pipeline

A one inch diameter steel tube with a Silvered inner surface. Although theory predicted low loss (2.5 dB/mile) at the shallow reflecting angles involved, in practice the loss was high: 200 dB/mile. While this alone would have killed the idea, the minimum bend radius that could be tolerated was estimated to be half a mile.

2/. The Confocal Lens system

The basic idea was to use a linear array of lenses spaced by their focal length (~100 metres), each one refocusing the beam of light. In practice the system was very sensitive to temperature fluctuations. Various methods for automatically steering each lens to compensate were proposed. However, it came to nothing.

3/. Thin Film Waveguide

If a dielectric guide is sufficiently thin, it can still guide light, but most of the power is guided in the air on either side. This offered the possibility of a low optical loss optical waveguide using an otherwise lossy dielectric. Planar thin-film ribbon guides were investigated. (A similar idea was investigated using an incredibly thin glass fibre, but a sufficiently thin fibre could not be supported). The film was supported at its edges and this caused loss, though the idea of twisting it to maintain central guidance was proposed.

None of these methods showed significant promise. Charles Kao and his colleagues then began to explore the possibility of a Single Mode glass fibre, and the rest is history!