Optical fibres for communications

For Reeves the advent of the laser came as an unexpected boon. His men were already running along the right track when a bandwagon suddenly appeared. They jumped upon it without losing any time while the rest of the world was not even aware that there were any bandwagons on the move. Reeves and his co-workers looked at various guiding structures: the hollow metal tube already mentioned; an array of lenses (called confocal waveguides because the light in them was focussed and defocussed as it propagated from lens to lens, see Fig. 12.2); and the dielectric waveguide which had already existed in the form of bundles of glass fibre and which had been used for medical purposes. A dielectric is nothing more than an insulator, a material that does not conduct electricity. The mechanism by which they could guide electromagnetic waves had been known for a century at least. For a qualitative explanation see Box 12.1.

Armed with the knowledge that coherent light had become available, did people in the field realize that optical communications was around the corner? By that time it was an obvious assumption. In all the previous history of the subject whenever generators of higher and higher frequencies were produced, they were invariably used for communications. The question was not whether, but when. I remember discussions at the time at Standard Telecommunications Laboratories. Most of us believed that the millimetre waveguide would come in the 1970s and optical communications in the 1980s. What form did we think optical communications would take? The answer depended on the background

Fig. 12.2 A confocal waveguide: light propagates by focussing and defocussing.

of those making the prediction. Those without a background in electromagnetic theory thought that a metal pipe with silver deposited on its inner surface might do. Those who came to the problem from the optical end were in favour of the lens array of Fig. 12.2, and finally those with a microwave background thought of dielectric waveguides as the likely winners.

Toni Karbowiak, who was by 1964 in charge of the transmission medium studies at Standard Telecommunications Laboratories, concluded in a paper read to the Institution of Electrical Engineers that of all the guides known to-date the fibre guide appears to hold most promise, if due to advances in material technology, it becomes possible to manufacture cladded fibres having effective loss tangent about two orders of magnitude better than at present.

Cladding was an important part of the set-up. The fibre diameter needed for single mode operation was comparable with the wavelength, that is, of the order of one-thousandth of a millimetre, much thinner than a strand of human hair. It was to be kept in place by the cladding, which also had to have low losses since light penetrated it to a certain extent.

The main problem was losses. The glass fibres available at the time gobbled up the power. The best figure for attenuation was 1 decibel per metre. That meant that the power declined to 10% of its value after travelling one metre in the material. This is quite a sharp decline. After travelling a mere 120 m the light emerging would be only one billionth of that entering. Quite obviously some considerable improvement was necessary before fibre guides could be used for communications.

Karbowiak left STL in December 1964 to take up a professorship in the sunnier climate of Sydney. The remnant of the group, Charles Kao and George Hockham, continued the good work. Looking at the state of the art they concluded that all the likely obstacles (scattering of light by impurities, attenuation, effect of bends) seemed to be surmountable. In particular, there was an encouraging report published in the previous year by Steele and Douglas claiming that an attenuation of better than 2odb/km was achievable if the iron impurities in glass could be reduced to 1 part per million. The situation looked moderately hopeful. The likely reason for the presence of impurities was that nobody ever tried to get rid of them. There had been no motivation to do so since the existing glasses satisfied all demand—but then nobody had wanted to use fibres for communications before.

Millimetre waveguides had attenuation of a few decibels per km at most. For optical fibres to become competitive on trunk lines the same kind of attenuation would have been needed and that was entirely out of the question at the time. There was however another kind of application for which optical fibres were suitable but millimetre waveguides were not. Let us remember that the initial motive for digitalization was the fact that the ducts in the junction lines were full. Capacity between urban exchanges was increased by converting lines carrying one analogue channel to carriers of 24 digital channels, thanks to the technique of pulse code modulation and time division multiplex. Optical fibres could of course do the same thing even better. They were flexible, they could carry digital modulation and would fit nicely into ducts. If the attenuation was low enough (and about 2odb/km was deemed to be sufficient), then the next repeater could be put into the next manhole and the whole system would work beautifully.

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