Optical fiber communications systems development in the UK

F OLLOWING the invention of the laser, work commenced during the early 1960s on the use of light waves for telecommunications transmission. Metallic pipe guides and free space propagation were studied but rejected as impractical and, in the mid 1960s, glass fibers were identified as a potential transmission medium. There ensued the now classic paper by C. K. Kao and G. Hockham [l] in the Proceedings of the IEE, describing the monomode optical fiber waveguide. At the time, this appeared to many to be a somewhat absurd proposal because the attenuation of glasses was believed to be far too high for any useful system to emerge. However, shortly after its publication, Kao and co-workers [2] demonstrated that blocks of pure silica were commercially available with attenuations less than 10 dB/km. This was a major step forward because it had earlier been established that to compete with coaxial cable systems, the fiber attenuation should be less than 20 dB/km to allow a repeater spacing of about 2 km or greater. Intensive studies of the attenuation mechanisms in glasses continued during the 1960s, as did studies of fabrication methods for fibers. The major emphasis was on multicomponent glasses made from conventional powder materials and converted into fiber by the double crucible process. Subsequently, this process was developed to the point where attenuation figures of under 5 dB/km could be achieved with good yield on very long continuous runs [3] and it is now being used by a number of manufacturers to make graded index and high Numerical Aperture (NA) fibers. In 1970, the Corning Glass Works announced (at the IEE in London), the achievement of a fiber having an attenuation of 2 0 dB/km. Silica glass had been converted into cored light-guiding fiber and it was only a matter of time before the technology was exploited in the field. The author joined the optical communications group at the British Post Office (now British Telecom) in 1971. It was a time of great excitement and very rapid change as scientists, and engineers in laboratories all over the world started to take the subject seriously. Casual examination of the literature shows a virtual explosion in the numbers of published papers starting at about this time and continuing throughout the 1970s. There were numerous unsolved problems. How to make low attenuation fiber? Would it be strong enough to cable? How to joint it and handle it in a field environment? How to make a long-lived CW semiconductor laser? How to achieve adequate bandwidth in the fiber without using monomode technology? Most of these questions were answered, independently or in collaboration, by laboratories around the world. Thus, by about 1975, a position had been reached whereby all the key components existed to mount a field experiment and in 1977, two were commissioned in the UK.