How the fibre communications revolution began

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Standard Telecommunication Laboratories

STL DEVELOPS TECHNIQUES AIMED AT COMMUNICATION BY GUIDED LIGHT

British Team’s Work described at IEE Meeting in London

Techniques for guiding light energy along special types of optical conductors are under active investigation at Standard Telecommunication Laboratories Limited, Harlow. Short distance experimental runs of these optical waveguides have been operated satisfactorily. They have exhibited an information-carrying capacity of one Gigacycle, which is equivalent to about 200 television channels or over 200,000 telephone channels.

One type of conductor successfully operated was described by Dr C. Kao at an IEE meeting today (27th January), It consists of a glass core about three or four microns in diameter, clad with a coaxial layer of another glass having a refractive index smaller than that of the core by about one per cent, to a total diameter of three to four hundred microns.

Surface optical waves can then be propagated along the interface between the two types of glass. Such a fibre is relatively strong, can be easily supported, and the guiding surface is protected from external influences. It is capable of carrying 10 mW of power, and has such a low mechanical bending radius as virtually to make it completely flexible.

A limiting factor at present with the fibre guide is that the best readily available low-loss material has a loss of about 1,000 dB/km. However, it is foreseen that the present material can be improved, and the STL team have indications that materials having losses of only tens of dB/km should be forthcoming.

A second approach at STL to the loss problem has been to use an extremely thin film for the guide, supported on U-shaped channel
material about one centimetre across. The wave is then carried at the interface between the film and surrounding air, and if, for an operating wavelength of 1 micron, the film is 0.1 micron thick, then it carries about one per cent of the energy of the wave, so that the effective loss is only about one per cent of that in a fibre guide of similar material.

The techniques for making the film are already available at STL for some materials. Launching and extracting techniques are being closely examined; so also are methods of joining the guides,

Special Notes to Editors’
1. The devices described are not yet commercially available,

2. The potential of coherent light (i.e. the type produced by lasers, lasing gallium arsenide diodes, etc.) as a communication medium is well-known.

The problem of using such light to communicate between remote points, however, remains unsolved. A direct beam may be obstructed by the atmosphere, and is in any case limited by the visible horizon. Balloons, towers, etc., are possible but not probable. Direct beams are therefore likely to be restricted, for long distance work, to outer space.

Much attention is now being given to guiding light along physical conductors. The most promising method is to use propagation methods equivalent to those used in microwave work, where the energy is propagated in a true electromagnetic mode along a waveguide. The surface wave work at STL is in this category.

It should be noted that when these methods are perfected, it will be possible to transmit very large quantities of information (telephone, television, data, etc.) between say, the Americas and Europe, along a single undersea cable.

3, The fibre guide diameter, 300-400 microns (millionths of a metre) is only about twice the diameter of a human hair. The film in the film guide is less than one hundredth of the thickness of a hair.

Geoffrey Charlish

From The STC Press Office, STC House,
190 Strand, London, W.C.2

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