Simplest Use Of Optical Fibres example essay topic

History of Fibre Optics When Alexander Graham Bell spoke over a beam of light in the 1880's, he never dreamed ofthe possibilities that modern scientists are dreaming up for light. He used sunlight which was focused by means of a reflector and a lens to a device which could be made to vibrate in harmony with speech from a human voice. The light beam was made to vary the focus in and out so that the strength on a selenium detector could be made to activate a telephone receiver and recreate the original voice. The distance between the transmitter and receiver were very short, but it was the beginning of communication via light.

The method of changing the intensity of the light beam is still what we base our communications on, but now, more often digital communication consisting of "on" and "off" patterns is used. The simplest use of optical fibres is that of light pipes. A light source that gives off heat and light transmits the light only through the pipes to give "cold" light. This is how doctors see inside the body. What is so great about optical fibre It is a piece of glass that allows light to travel through. Actually it is a very fine strand of very special glass which might be only 125 microns in diameter.

It is a glass strand that is about the same thickness as a human hair. Fibre optic technology can simultaneously transmit voice, video, and data over the same wire several thousand times better than current coaxial cable. Since the mid 1980's, thousands of kilometres of optical fibre have been laid in the United States and Japan to carry long distance telephone communications. Fibre optics are also used in various medical instruments designed to examine the interior of the body, since the images transmitted by these devices can be magnified and rotated for close observation of hollow organs. Optical fibres are also used in many laser-based computer printers to produce photo quality copies. Glass or plastic filaments are spun t diameters between 5 and 100 micrometers and packed into bundles of several thousand each.

The bundles may be made as rods, ribbons, or sheets. Because the bundles keep some of the flexibility of the individual fibres, they can be twisted and bent to conduct light and images around corners. In order to protect the fibres, a protective layer is applied. Reflection Figure 3 Reflection of light When light falls on a medium a percentage is reflected back. The amount of light reflected depends on the angle a 1 between the incidence ray, and the normal ray. q 1 = q 2 Refraction When a ray of light with an angle of incidence a enters an optically denser medium for anoptically less dense one, its direction bends toward the angle of refraction b. If a medium has identical properties in all directions, then Snell's 1 law of refraction applies: Figure 4 Refraction of light where the ratio of the angle of incidence and sine of the angle of refraction is equal tothe ratio of the speed of light in one medium to the speed of light in the other. sina = c 1 sin bc 2 With two transparent media, the one with the lower speed is considered to be denser.

When light travels in a vacuum at a speed of c 0 to a medium with a speed of light c the following applies sina = c 0 = n sinbcThe ratio of the speed of light in a vacuum and the speed of light in a medium is called the refractive index (more precisely the phase refractive index) For two different mediums with the refractive indexes of n 1 and n 2 and their speeds of light c 1 and c 2, the following applies: c 1 = c 0 n 1 Another form of Snell law is: sina = n 2 sinn 1 Critical Angle It is possible for the difference between in refractive indexes between two mediums to cause refracted light to have an angle of 90., or parallel to the medium surface... This angle is called the critical angle. The critical angle can be found by: sinc = n 2/n 1 Figure 5 Total internal reflection of light Total Internal reflection When a light ray comes into contact with a medium with a different refractive index, it is refracted. If the angle of incidence is less than the critical angle, it will be reflected inside the medium.

This is called total internal reflection. It is possible for this ray to continue on forever in this manner. Total internal reflection can only occur at an interface where a light ray travels from anoptically denser medium to a optically less dense medium. Transmitted light through an Optical Fibre Lets consider a short piece of cable with two rays entering, A and B. Figure 6 The passage of light through a fibre optic cable Ray A enters the fibre at an angle of qA. This ray strikes point C. Some of the light is reflected on to point D, and some of the light is refracted outside. Again at point D, some light is reflected and some is refracted outside.

This will continue until the all the energy is lost. Ray B enters the fibre at the angle qB. The refracted ray has an angle of 90., parallel tothe side of the medium. This ray is therefore the critical angle and forms the slope of a cone of angles that will be reflected. qB = sin-1 (n 1/n 2) Ray C enters the fibre at an angle less than the qc. This ray will continue on forever being totally internally reflected 2.

The skip distance is the distance between two reflections and can be found by: Ls = dcotqwhere d is the core diameter. Numerical Aperture In order to launch light from outside into the core glass, the launch angle between light ray and fibre axis can be found by: sina = n 1 sin (90. -a 0) n 2 The greatest launch angle q max is called the acceptance angle of the fibre. The sine of the acceptance angle of the fibre is called the numerical aperture. NA = sinqmaxThis quantity has a major importance in launching light into fibres. Characterisation of Several Optical Fibres Core / cladding n 1 n 2 jcriticalqmaxN.A. 1/Ls Glass / air 1.501. 041.8.

90.0. 15944 Plastic / plastic 1.491. 3968.9. 32.5. 0.543866 Glass / plastic 1.461. 4073.5.

24.5. 0.412962 Glass / glass 1.481. 4680.6. 14.0. 0.241657 Bandwidth The bandwidth is a continuous range of frequencies between a lower and upper limit. The more complicated a signal is, the greater the range of frequencies needed to represent i tare.

The output of a FM radio station is much clearer than that of a telephone because greater frequency range is given to the FM. For example, a telephone conversation normally takes 4 kHz, where as a FM radio takes 200 kHz. A television station takes 6 MHz of bandwidth. The potential of the optical fibres is enormous. It is possible to calculate the possible bandwidth of a fibre.

For example, a TV station that uses a 300 MHz carrier, the ratio is 300 MHz/6 MHz, or 50; for an optical fibre using a carrier of 3108 MHz to carry the information, the ratio is 3108 MHZ/6 MHz, or 50,000,000. Much more information can be sent when pulses can be transmitted. This is called binary, and is either on or off. This is what computers, and CD's use as a means of communication. Suppose that 8 bits 3 are required to represent the amplitude of an analog signal. A analog signal is supposed to by sampled at a rate of at least twice as high as its highest frequency.

In the case of a TV channel with a bandwidth of 6 MHz, this means that 2 x 6 MHz, or 12106 samples must be taken each second. Since each sample is described as using 8 bits, the required data rate is 96 Mbps (megabits per second). The data rates are limited at the moment by fibre distortions, and equipment to transmit this fast. Attenuation When the light is absorbed by the fibres, it is called attenuation. There is several reasons for this to occur and they fall into two types, extrinsic and intrinsic loss. Examples of intrinsic loss are Rayleigh scattering which is caused by microscopic variations in the index of the refraction of the glass.

This gives a uniform loss over the entire fibre. OH- absorption happens when molecules of OH- get into the fibre when it is made. Metallic ion absorption is caused from trace elements such as gold, magnesium, and iron being left in the fibre when made. It is very difficult to get rid of these trace elements because they are found almost everywhere.

These types of attenuation often only absorbs light at certain wavelengths. The other type is when the fibre is bent too much, or from tiny micro-defects in the fibre. These are called extrinsic losses, and causes the ray angle to be greater than the critical angle, and is not reflected. Attenuation is measured in deciles per kilometer lost, or dB / km.

The early cables produced had an attenuation of 20 dB / km. Today the cables are being produced with 0.1 dB / km loss. Figure 7 (a) sharp bend in fibre and (b) micro defect in fibre Transmission of Digital Signals 89 There are three main components used in fibre optic communications. The first component is the transmitter.

It modulates the electrical energy into light energy. This is the part that generates the light signals, capable of being switched on and off very quickly. The faster that these can be switched on and off, the more information can be sent in a given time. The light source is usually a LED (light emitting diode) or a LD (laser diode). Itis possible to use other laser sources, but these are the cheapest and most reliable. The second component is the optical fibre which has a high purity, and transparent to the frequencies being transmitted.

It must be able to be spliced and repaired when necessary. To transmit the light a long distance, and to overcome the loss of energy, repeater stations are set up. These amplify the signal to avoid loss and distortion. Finally the last component is the receiver. This converts back to electrical energy the light signal. It is made with a detector, which detects the light and turns it back into electrical energy.

A signal processor that amplifies the signal, filters and changes the signal into a useable form, ie analogue sound. Digital signals are what are used for communication in a telecommunications and data communications. Transmission of Analogue Signals 10 Analogue signals are continuous. That is they have an infinite number of values. All sound light we hear and see is also analogue.

Analogue is used when data is not being communicated. A endoscope which is used to see inside the body uses analogue. One fibre will carry white light inside, and another tube will carry back the reflected image. Figure 11 Basic diagram of the fibre drawing and coating process Commercial Manufacture of Optical FibresThe main material of optical fibre is ultrapure silica powder. This is heated to a high temperature until it is molten. A glass rod, or preform, is formed when it is slightly cooled.

A fibre is then pulled out and stretched, keeping the heat constant to ensure an even pull... The next step is to coat this fibre in either glass or a plastic coating to form the cladding. It is then put through a test using ultraviolet rays to check for imperfections. The fibre is then covered with a plastic coating for protection. Other trace materials can be put in depending on what type of cable is wanted. The mixing of the materials needs to be done extremely carefully.

A single speck of dust can contaminate an entire batch of fibre. The glass produced is so pure, that a block one kilometre thick is as clear as a normal window pane. Optical Fibres in the Telecommunications Industry The use of optical fibres for telecommunications is by far the biggest use, and probably the most potential. During the next decade or two, almost every house will probably be connected with optical fibres. Many experts expect that by the year 2020, nearly all homes in America will have fibre optic connections.

The first telephone network was tested by Western Electric in 1976. One year later, Bell carried the first optical cable in Chicago. It covered 2.5 km. Figure 12 Some of the possible uses for optical fibres Today, telecommunication companies in America have replaced nearly all the major cities links with optical fibres. The cables are about as wide as a closed fist, and conduct as much information as the old copper cables which when put together would be about as wide asa large tractor tyre in diameter! In New Zealand, Clear Telecommunications uses fibre optic cables running along side the main trunk rail track to transmit telephone calls between the cities.

In the last year, Clear has entered the business district in Wellington, and now connects some businesses with fibre optic cables to the local and international network. Telecom has installed a fibre optic link to carry all telephone messages between the two islands. There is also a fibre optic link between Australia and Auckland, and Auckland and Hawaii. Kapiti Cable, on the Kapiti Coast, is experimenting with connecting houses with an optical fibre. There are about 2000 subscribers to this service.

The facilities at the momentavailably ate a dial up video library, several TV channels, and are experimenting with access to the global computer network, the Internet. This is the most advanced network inner Zealand at the moment. There are other smaller networks experimenting at the moment in Auckland and Christchurch too. Medical Uses of Optical Fibres Figure 13 A tiny probe pierces a cell There are many new instruments that now use fibre optic cable.

The name Endoscopy is given to the instruments that look inside the body. The esophagoscope allows doctors to examine the oesophagus, the gastroscope is a flexible tube to examine the instruments is used to seein side the stomach. Most of these instruments use natural openings in the bodies to get in. Scientists at the University of Michigan have invented a fibre that is so small, it can slip between the membrane of cells. It is the smallest sensor over developed at only one-thousandth the width of a human hair.

With these aids, it is possible for doctors to perform surgery without having to perform make a cut. Lasers can be aimed through a fibre and can "shoot" gall stones in the bladder or remove blockages in the arteries. Eye surgeons can fix a wide array if problems using lasers and fibre optics. The laser can be directed to exactly where the problems. Correction of the shape of eyes used to be a serious problem, now most cases can be done under no anethesic, and no overnight stay at hospital. Military Uses of Optical FibresThe military was the first to use optical fibres and most of the early research with applications was done my them.

In 1973, the first optical cables were put into operation on American navy ships With the new cables, it will be possible to do all communications now possible using radio, and telephone from one terminal. It will be possible to have video on demand, with complete video libraries only a few key presses away. All the major telecommunications companies inthe USA are currently looking at these options. IBM is looking at developing technology that will able computers in the house to replace the phone, TV, radio, and every other kind of electrical communication. Eventually it will be possible to have entire libraries of information "beamed" into the house. 1 Willebrord Snell discovered this important law of refraction in 1621.

It refers to how much light is bent when moving from one medium to another. 2 Although the energy will eventually be absorbed into the fibre through attenuation 3 A "bit" is either on or off.