16.13 Optical Fibres

Today most long distance data and voice telecommunications is carried in optical fibres. They have a much higher information carrying capacity than metal wires, and improvements in attenuation have been rapid. In 1960 the typical attenuation was 1dB over a metre of fibre (1000 dB per km), in 1970 it had become 20 dB (equivalent to a reduction of power to a tenth) per km, by 1980 it reached 20 dB per 40 km and by 1990 per 125 km, making it practical to use undersea cables with repeaters at intervals to regenerate the signal. By 2002, using lasers and very pure glass, Lucent had achieved a 64-channel link with a bandwidth of 40 GB/s per channel over 4000 km (1GB/s represents 1000 typical novels per second!).

Optical fibre bundles

Optical fibre cables and connectors

Optical fibres are also used over short distances for sensors, to deliver laser beams, in automobiles, industrial process control and even for optical computing.

Optical fibres are made from low “dielectric constant” silica-based glass. The lowest loss fibres have a cylindrical structure having a central core with an outer cladding having a slightly lower dielectric constant, achieved by doping the central silica core with germanium oxide.

16.14 Lasers

T H Maiman at Hughes Research Labs developed the first laser in 1960, based on a synthetic single crystal ruby (alumina) doped with chromium. A laser absorbs incoherent light energy and converts it to intense pulses or beams of coherent, single colour (monochromatic) light. Maiman’s laser was powered by a coiled xenon flash tube so produced pulses of light, but in 1962 researchers Nelson and Boyle at Bell Telephone Laboratories used an arc lamp to produce a continuous beam. In 1964 Bell developed an improved version based on neodymium doped (0.5 to 1%) yttrium aluminium garnet (YAG).

Helium/neon laser cutting equipment

Laboratory laser set-up

The first application of lasers was for military range finding, but its uses expanded into mine detection, industrial welding, cutting and drilling metals, drilling holes in diamonds and sapphire for watch bearings and dies for wire drawing.

Subsequently they were integrated into fibre optic communications, material analysis, medical surgery, tunnel alignment and even for cleaning sculpture. A notable future use is the remote monitoring of buried casualties in a major disaster to set recovery priorities, by pinpointing those still alive.