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Plastic path to information

Plastic path to information IN Tins age of information, telecommunications is evolving at a breakneck pace. While a few years ago, even movie channel through a TV cable was something of a novelty, the technological pundits are today close to realising their vision of sending 500 cable channels into each home, simultaneously.

Though this was possible using high performance glass 6bre-optic cables capable of transmitting massive amounts of data in the form of pulses of light, the cost of taking these fibres from the curb to each house was a prohibitive factor. Now, a group of Japanese researchers say that they have developed a new type of plastic cable that would be cheaper than glass optical fibres and yet be capable of transmitting huge amounts of data over short distances (Science, Vol 267, No 5206).

Glass fibre-optic cables are poorly suited for 'short-hop' application. Each is thinner than a hair, and expensive junction equipment is required to align the optical beam so that it carrjump from one 'trunk' fibre to another.

In contrast, the copper wires that are conventionally used for cable TV are cheap andsimple but they cannot transmit nearly as much data as the fibres do. The solution can be found in one word: plastics, say researchers - from the Kcio Univer- Highest sity in Yokohama.

Led by material sci- Graded-index entist Yashuhim Koike, more information the team of researchers copper wire has devised a new type of plastic optical fibre Carrier to transmit light pulses from a red semiconductor laser at a blinding Copper wire 2.5 billion bits per sec- Step-index plastic ond - that's a transmission rate, or hand-width, 25 times greater than that of a copper cable.

"The performance is more than enough to handle the transmission demands of most short-hop applications for the foreseeable future," says Robert Steele, a plastic-fibre specialist with Delphi gackard Electric in Warren, Ohio. Plastic-optical fibres have been in use for over a decade now for wiring homes and also for carrying information between sensors and processors in robots.

But the traditional plastic fibres - known as step-index fibres - were not suitable for high speed transmission of data. Reason: Photons in each light pulse travelling down the fibre spread apart as they move. As a result the pulses, which constitute the signals, must be spaced far apart or they overlap. That limits bandwidth to 100 million bits per second over 100 metres of fibre.

This problem, however, does not afflict normal glass fibres because their extremely narrow confines ensure that photons travel only in one path.

The fibre developed by Koike and his colLeagues is what is called a graded-index fibre, OUW which sharply reduces pube the scattering of signals (we diagrarn). The fibre consists of polymethyl methacrylate (PMMA) a common fibre polymer - with continuously graded concentrations of some highly refractive dopants molecules which when added to the solid PMMA fibre help reduce scattering of light pulses.

The researchers, however, concede that the new fibre does not dispense with the degradation problem altogether. "Optical signals still get knocked down by a factor of around 30 after they pass through 100 metres of the fibre," says-Harry Lockwood, an opto-electronic consultant with the Lockwood group in Newton, Massachussetts. 'But it's still usable, because 100 metres is ample for wiring most homes or local Computer networks within offices," he adds.

Highest grade
Graded-index plastic fibre can carry 25 times more information than the conventional copper wire
Carrier Bandwidth over 100 m
(Megabits per second)
Copper wire 100
Step-index plastic fibre 100
Granded-index plastic fibre 2500

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