History of Optical Fibre

In 1870 John Tyndall demonstrated that a jet of water could be used to guide light by the process of total internal reflection. Very little interest was shown and no further development took place.

In 1880 Bell invented the photophone, which used a diaphragm to modulate sunlight to transmit speech. It was however extremely limited since sunlight is an unpredictable light source and because the transmission medium was free space which allowed only line of sight communication, was susceptible to external influences and suffered from significant dispersion.

In 1956 Kapany used a rod of glass coated with a further layer of glass. This was not used for communication purposes, but was nonetheless the first appearance of the structure we now use as an optical fibre.

It wasn’t until 1966 that these ideas were brought together. Kao and Hockham proposed that a glass fibre could be used as a viable waveguide for light. Unfortunately the losses achievable at the time were extremely high (~1000dB per km). Obviously this order of loss was unacceptable for any useful signal transmission, and so the development of optical fibres (which I shall now refer to simply as ‘fibres’) started.

Why Optical Fibre?

Why has the development of fibres been given so much attention by the scientific community when we have alternatives? The main reason is bandwidth – fibres can carry an extremely large amount of information. I shall discuss the advantages and disadvantages of fibre compared to the four other commonly used media.

Twisted Pair Cable is used for, and is still suitable for, simple telephone links (known as the local loop) from the consumer to the nearest telephone exchange. The bandwidth is low, but is adequate for carrying low quality analogue voice signals. Attenuation of the signal is not significant over the short distances such signals are carried. The main advantage of twisted pair cable is the very low cost.

Coaxial Cable can carry a much larger amount of data – especially by multiplexing (the process of transmitting several signals of different wavelengths along the same cable) analogue signals. Multiplexing however is also possible with fibre, and fibre provides significantly higher bandwidth. Digital signals can be transmitted, but the bandwidth is limited if signal quality is to be maintained. Again, fibre is more expensive for many applications where coaxial cable is still used.

Microwave Surface Links are used for short-distance transmission of digital signals. The bandwidth does not match that of fibre, and only line-of-sight communications are possible, but it is still used for some applications (e.g. distribution of television signals and Wide Area computer Networks – WANs).

Microwave Satellite Links are useful for mobile communications, communicating with remote areas (such as the Antarctic) and for broadcasting the same information to a large number of recipients. Fibres have a higher capacity, don’t require expensive satellites and can be maintained more easily, but are not suitable for one-to-many communication. Another disadvantage of satellite communications is the appreciable delay from the signal transit time (a geostationary satellite is approximately 37000km above the Earth’s surface giving a delay of the order of quarter of a second for a return journey).

There are clearly arguments for different media in different situations, but the cost of fibre is decreasing and so is becoming a viable option for more ‘local’ settings (rather than its traditional role in communication backbones).