Historical Uses and the Importance of Optical Fibres

Glass has been used since Neolithic times. It is a remarkably versatile material, having been used initially as a tool then in jewellery, in glassware, in architecture and most recently telecommunications. It was the basis for Venice's huge commercial empire during the 13th, 14th and 15th centuries when the punishment for anyone divulging the secret of glassblowing was death but today it is present on the cutting edge of technology as the main component of optical fibres. The use that I will be investigatingin this project is optical fibres. This is an interesting area of physics as considerable research is currently underway to develop glass for optical fibres. Such glass must obviously be extremely clear to allow easy passage of light. Attentuation of light travelling along a fibre decreases with wavelength and at a wavelength of only 1.3µm, the clearest glass available absorbs 50% of the light entering it after a distance of 50 km. Thus, wavelengths used for optical fibre transmission are in the near infra-red part of the spectrum.

Major Uses of Optical Fibres

The advent of practicable optical fibres has seen the development of much medical technology. Optical fibres have paved the way for a whole new field of surgery, called laproscopic surgery, commonly known as keyhole surgery, which is usually used for operations in the stomach area such as appendectomies. Keyhole surgery usually makes use of two or three bundles of optical fibres with each bundle containing thousands of individual fibres. The surgeon makes a number of small incisions in the target area and the area can then be filled with air to provide more room. One bundle of optical fibres can be used to illuminate the chosen area, and another bundle can be used to bring information back to the surgeon. Moreover, this can be coupled with laser surgery, by using an optical fibre to carry the laser beam to the relevant spot, which could then be used to cut the tissue or affect it in some other way.

Optical fibres can also be used for the purposes of illumination, often carrying light from outside to rooms in the interiors of large buildings, or in sensors. If a fibre is stretched or squeezed, heated or cooled or subjected to some other change of environment, there is usually a small but measurable change in its light transmission. Hence, a cheap sensor can be made which can be put in a tank of acid, or near an explosion or in a mine and connected back, through perhaps kilometres of fibre, to a safe area where the effects can be measured and examined.

The Importance of Optical Fibres in Telecommunications

The most important use of optical fibres and the one which threatens to have the most impact on our society is their use in telecommunications. A coaxial cable is capable of carrying 2000 simultaneous phone calls. The optical fibres currently under development have a potential capacity of more than 30,000 simultaneous calls per fibre. Coaxial cables are made from copper which is expensive to refine while optical fibres are made from much cheaper material. It is therefore unsurprising that optical fibres are likely to dominate telecommunictions in the future. Except in the case of single-mode fibres, light does not actually follow the curves of an optical fibre, but Total Internal Reflection allows it to travel along the fibre by bouncing repeatedly off the inside of the interface of the glass with the surrounding medium (fibres are designed so that the glass has a higher refractive index than this outside substance, thus allowing TIR).

The Properties of Optical Fibres

Optical fibres are made from glass and very thin for the most part. The more general term for an optical fibre is a light waveguide (the pipes in a plumbing system could be thought of as water waveguides). They are designed to carry light for extremely long distances. Optical fibres are often thought of as long, thin, hollow tubes which have a mirrored inner surface, down which light can bounce. But they are actually not quite that simple. They function in the way they do because of the principle of Total Internal Reflection.

An LED can be used to send pulses of light down an optical fibre. LEDs which emit at a wavelength of 850 nm are used. Because the light intensity from an LED is low, the width of the fibres cannot be made too small or not enough light will be transmitted in each pulse. The main advantages of optical fibres are
The Purpose of Cladding

The field of light which penetrates the fibre surface through quantum tunnelling is called the evanescent field. The intensity of this field decreases exponentially as you move away from the surface, as the light is able to penetrate only a very small distance outside the fibre before it is sucked back in. However, anytime the fibre touches something else, the light in the evanescent field can leak into the new medium or be scatterred away from the fibre. This effect causes a significant leakage of the signal out of the fibre, which drastically affects its efficiency. Even a small amount of dust on the surface would cause a fair amount of leakage.

The solution to this problem was found to be in cladding the optical fibre. This is where the central core (normally glass) of the fibre is surrounded by another material, which has a refractive index lower than that of the core by enough to still cause sufficient Total Internal Reflection. Because the evanescent field surrounding the core dies off very quickly, the energy does not penetrate very far into the cladding, and it is then pulled back into the core. Provided that the cladding is thicker than the effective distance of the evanescent field, it prevents the leakage of light out of the fibre. Cladding also reduces the effect of any imperfections on the surface of the fibre.

This solution allowed optical fibres to be developed which could be applied practically to industry and more specifically telecommunications. In 1970 the first fibre was made which had a transmission of over 1% over 1km. Today, fibres are made which have transmissions of around 95% over 1km. To put this in perspective, if ocean water had an optical transmission of about 79% through each km of depth, the bottom of the world's deepest oceans could be seen with the naked eye.

Step-Index and Graded-Index Fibres

There are two main types of optical fibres, step-index fibres and graded-index fibres. Step-index fibres are those fibres where the refractive index of the core is constant throughout and steps down to the refractive index of the cladding. The light in these fibres travels down the fibres by bouncing off the core-cladding interface. Step index fibres with cores as small as 50 µm create problems due to rays travelling along the axis taking less time than rays undergoing multiple reflections Thus a pulse of light entering at one end spreads out as it travels along the fibre. This is called modal dispersion. Over 1 km, the time difference can be as large as 30ns which creates problems if pulses are being transmitted at frequencies of more than 30 MHz. Therefore, this type of fibre can only be used for short-distance links.

Graded-index fibres have a core with a refractive index which varies quadratically according to the distance out from the centre of the fibre, decreasing as you move further out. Light travelling in these fibres actually follows a curved path due to the continuously varying refractive index. The graded index fibre slows down the waves which are at such an angle that they hit the sides infrequently. This allows all the light travelling through the fibre to have the the same horizontal speed and reduces modal dispersion to less than 1ns per km.

Material Dispersion

The refractive index of glass varies according to the wavelength of light being passed through it in an inverse relationship (refractive index goes down as light wavelength increases). This means that different wavelengths of light are affected differently by the same piece of glass and can travel at different speeds through it. Over a long distance this difference can become marked, and to avoid it single wavelengths of light must be used to transmit signals down optical fibres.

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