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History of Fibre Optics

Light has been used as a means of communication for many hundreds, even millions of years and humans are not the only creatures to use it. Bioluminesent creatures such as fireflys and glow-worms have been using light for millions of years to communicate.

Humans have been using light since before the days of electricity. One of the earliest forms of long distance communication used fires on hilltops and oil lit lighthouses on coasts.

In 1621 Dutch mathematician Willebrord Snell discovered the mathematical formula for refraction. This is the phenomenon of light bending as it passes from one medium (material) to another. This is because light travels at different speeds in different materials. This is why when you put a straw into a glass of water it appears to bend. Snell gave different types of material a refractive index which describes the speed at which light travels through it. How much the light bends when it passes into the material depends on its refractive index.

In 1840 Daniel Colladon and Jacques Babinet demonstrated the principle of Total Internal Reflection - the phenomenon by which light could be guided through something without it escaping. Their demos used jets of water to produce decorative light displays but gained little attention until a few years later when John Tyndall performed the demo to the Royal Society.

Total Internal Reflection occurs when light is passing through a material with a higher refractive index (such as a jet of water) than the material it is next to (the air), this causes the light to bounce off the boundary and stay within the water jet even if the jet is curved. Fibre optics relies on the same principle. The cable consists of two materials (usually two types of glass) called the core (which carries the light) and the cladding (which acts a bit like insulation on an electric cable). The point where the two meet is called the boundary.

Cross section of fibre optic cable showing core and cladding

Cross section of typical fibre optic cable

Total Internal Reflection works when the angle at which the light strikes the boundary is greater than the critial angle. In the digagram below the critical angle C is shown in yellow. There are two beams of light. The blue beam of light hits the boundary at angle A which is greater than the critical angle hence it is totally internally reflected. The purple beam hits at angle B which is less than the critical angle, hence it escapes through the cladding.

Total Internal Reflection depends
					 on the angle at which the light strikes the boundary between one material and anothrt

Total Internal Relection

How it came about

The inventor of the phone Alexander Graham Bell was the first to demonstrate that light could be used to carry complex signals. In 1880, just four years after the invention of the telephone. The photophone used sunlight captured by a mirror which vibrated as the user spoke into the mouthpiece. This vibration modulated (altered) the sunlight being reflected by the mirror and was picked up by another mirror upto 200 metres away. The receiving mirror was coated with selenium and produced a varying (analogue) electrical signal in unison with the variations in light which was fed to a speaker. Despite pioneering two future technologies for the first time - optical and wireless communication, the fact that it combined the two meant it was prone to interference and limited to a novelty show piece.

The first people to put the principle of Total Internal Reflection to use for something other than decorative water jets were doctors trying to find ways to see inside the body and several novel inventions were created during the 1880s and 90s to illuminate internal organs and allow dentists better views of teeth.

A British physics teacher Charles Vernon Boys started trying to produce thin fibres by using a crossbow to fire a needle through molten silica which produced fibre strands far thinner than anything produced before.

A big breakthrough came in the 1920s when an engineer working for RCA doing research on the development of TV created a system using parallel fibres to transmit an image over a short distance. This was further developed a few years later by a medical student Heinrich Lamm who was able to transmit a rough analogue image a short distance through a fibre cable.

The biggest problem they encountered was that light leaked out from the fibre because the fibre itself didn't have a cladding with a lower refractive index.

The problem occurred again when the Dutch government tried to use a similar system to create better periscopes for submarines. The Dutch turned to American scientist Brian O'Brien who suggested coating the fibre with a cladding with a lower refractive index, the result was a success and is a technique used today. The Dutch used a liquid plastic cladding which although successful was further improved on by scientists at The University of Michigan who used a new type of glass from Corning which had a lower refractive index than the glass they were using for the fibre core. By fusing a tube of the new Corning glass around the outside of the core they produced a fibre with a light carrying ability far superior to anything before.

Although these creations were well suited to medical uses for looking inside the body, their use for telecommunications was still impractical. The signal degraded at 1dB per metre (1000dB per km) meaning long distance transmission was out of the question.

Telecommunications gained a big boost when the laser was invented in 1960 and Charles Kao at Standard Telephones & Cables (STC) London, began looking into the problems with optical fibre for telecommunications. He discovered that the signal loss problems were not inherent in the glass itself but with the manufacturing process used to make the fibres proposing that improved manufacturing could reduce signal loss to 20db per kilometre providing the ability to carry upto 200,000 telephone calls on one fibre. Soon Kao's ideas were in production and fibre with a loss of 20db/km was available. By the mid 70's that had been refined further to a loss of 0.5db/km and by the end of the 70's it was down to 0.2db/km. Long distance high bandwidth communication had arrived.