Optical Fibres Done by Clement Chia Hui Tien (09) (2A2)
<ul><li>Optical Fibres are thin rods of glass wrapped in a low density plastic and cabling. In modern days, it is used as an instrument in micro surgery to project images from inside the body and help surgeons see in hard to reach places. </li></ul><ul><li>It is also used widely in communications, both in computer networks as a fast Internet connection source and in telecommunications both transcontinentally and transoceanically. </li></ul><ul><li>These will be explained and shown in more detail in the following slides. </li></ul>So what are optical fibres?
<ul><li>In 1870, John Tyndall demonstrated that light follows the curve of a stream of water pouring from a container, it was this simple principle that led to the study and development of applications for this phenomenon. </li></ul><ul><li>John Logie Baird patented a method of transmitting light in a glass rod for use in an early colour TV, but the optical losses inherent in the materials at the time made it impractical to use. In the 1950's more research and development into the transmission of visible images through optical fibres led to some success in the medical world, as they began using them in remote illumination and viewing instruments. </li></ul><ul><li>In 1966, Charles Kao and George Hockham proposed the transmission of information over glass fibre, and they also realised that to make it a practical proposition, much lower losses in the cables were essential. This was the driving force behind the developments to improve the optical losses in fibre manufacturing, and today optical losses are significantly lower than the original target set out by Charles Kao and George Hockham. </li></ul>Brief History of optical fibres
<ul><li>In 1966, Charles Kao and George Hockham proposed the transmission of information over glass fibre, and they also realised that to make it a practical proposition, much lower losses in the cables were essential. </li></ul><ul><li>This was the driving force behind the developments to improve the optical losses in fibre manufacturing, and today optical losses are significantly lower than the original target set out by Charles Kao and George Hockham. </li></ul>Brief History of optical fibres (2)
<ul><li>Because of the low loss, high bandwidth properties of fiber cable they can be used over greater distances than copper cables, in data networks this can be as much as 2km without the use of repeaters. Their light weight and small size also make them ideal for applications where running copper cables would be impractical, and by using multiplexors one fibre could replace hundreds of copper cables. </li></ul><ul><li>The greatest benefits in the data industry are its immunity to Electro Magnetic Interference (EMI), and the fact that glass is not an electrical conductor. Because fibre is non-conductive, it can be used where electrical isolation is needed, for instance between buildings where copper cables would require cross bonding to eliminate differences in earth potentials. </li></ul><ul><li>Fibres also pose no threat in dangerous environments such as chemical plants where a spark could trigger an explosion. Last but not least is the security aspect, it is very, very difficult to tap into a fibre cable to read the data signals. </li></ul>Why do we use optical fibres?
<ul><li>The concepts of reflection and refraction are very important in the design of Optical Fibres and has a key part in how Optical fibres work. </li></ul><ul><li>The law of reflection states that a beam of light striking a flat surface (the incidence ray) would be reflected at the same angle at the normal, the medium dividing the incidence ray and the reflective ray and is perpendicular to the surface of reflection. That would also mean that the angle of incidence (from the incidence ray to the normal) is also equivalent to the angle of reflection (from the reflective ray to the normal). </li></ul>Concepts of Reflection and Refraction in optical fibres
<ul><li>Light travels along a fiber cable by a process called 'Total Internal Reflection' (TIR), this is made possible by using two types of glass which have different refractive indexes. The inner core has a high refractive index and the outer cladding has a low index. This is the same principle as the reflection you see when you look into a pond. </li></ul><ul><li>The water in the pond has a higher refractive index than the air, and if you look at it from a shallow angle you will see a reflection of the surrounding area, however, if you look straight down at the water you can see the bottom of the pond. At some specific angle between these two view points the light stops reflecting off the surface of the water and passes through the air/water interface allowing you to see the bottom of the pond. </li></ul>Concept of TIR in optical fibres (1)
<ul><li>Optical fibres relies heavily on two concepts of physics, the concepts of refraction, refractive indexes, critical angles and Total Internal Reflection. The concepts of refraction states that a ray of light travelling from a medium with a higher refractive index to a medium with a lower refractive index would bend away from the normal. With this in mind, it also states that a critical angle would be reached when the ray of light increases to an angle that will bend it 90 deg. away from the normal. </li></ul><ul><li>The concept of Total Internal Reflection is apparent when the ray of light travelling from a higher refractive index medium to a lower refractive index medium has an angle so great it is able to refract the light greater than the critical angle, resulting in the ray reflecting back into the first high refractive index medium. The main function for optical fibres is to send information through it by transmitting a beam of light from one end to another, trying to have as little quality loss as possible. The design of modern optical fibres reflect this and relies much on the physics involved. </li></ul>Concept of TIR in optical fibres (2)
<ul><li>When transmitting a signal, an L.E.D. or laser is used to send a beam of light in on/off pulses (digital) down the Optical Fibre to be received on the other end. It is either transmitted straight down the Optical Fibre, resulting in longer distance travel, or it is transmitted at an angle which is calculated to be greater than critical angle of the glass to the plastic, resulting in Total Internal Reflection and the ability for the beam of light to turn corners. </li></ul><ul><li>The angle of transmission of the beam into the glass is usually greater than 82 deg, which is required to achieve Total Internal Reflection when the light hits the plastic cladding. If the angle is less than 82 deg, the beam of light would be refracted out of the Optical Fibre. </li></ul>In more detail…
<ul><li>Transferring information and Medical science </li></ul>Examples of the use of Optical Fibres
<ul><li>At the moment, in UK, there are three million kilometres of optical fibre cable in the BT network. Most of BT's trunk network now uses optical fibre cables. In a recent trial in Bishop's Stortford optical fibres were actually laid into homes. This allowed customers to receive cable TV and stereo radio as well as phone and information services. </li></ul><ul><li> Optical fibres could be put into all homes but currently the cost of the system including lasers and detectors would be too high for simple telephone calls. Some companies have a direct optical fibre link if they need to send large quantities of information by phone, for example between computers at different business centres. </li></ul><ul><li> Optical fibre submarine links are in use all around the world. Because of the low loss and high bandwidth of optical fibre systems they are ideal for submarine systems where you want to minimise the amount of complex electronics in regenerators sitting on the sea bed. In fact, the link from the UK to the English Channel Islands is achieved directly without any submerged regenerators. </li></ul>Case Study: British Telecom (BT)
<ul><li>The world's first international optical fibre submarine cable was laid by BT in 1986 between the UK and Belgium. It is 112Km in length and has only 3 regenerators. BT was a major partner in the first transatlantic optical fibre cable system - TAT 8 (Transatlantic Telecommunications cable no 8) which was capable of carrying 40,000 telephone calls at once, or the equivalent in data, facsimile, or TV pictures. </li></ul><ul><li> The second transatlantic cable, TAT 9, which came into service in 1992, has twice that capacity and links five separate landing points in the UK the USA, Canada, France and Spain. The transatlantic optical fibre cable network, completed in 1996, spanned 14,000Km and linked the UK, France and the USA . It could handle up to 320,000 phone calls at one time. </li></ul><ul><li> Undersea cables consist of fibres with a copper coated steel conductor which are covered in protective layers of steel and polypropylene. In shallow waters, e.g. the continental shelf, a submersible remote-controlled plough is used to bury the cables one metre below the seabed, to protect them from damage by trawling and ships' anchors. </li></ul>Case Study: British Telecom (BT)(Continued)
<ul><li>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 (or more commonly, 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. A "bundle" can contain 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. </li></ul><ul><li>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 relevent spot, which would then be able to be used to cut the tissue or affect it in some other way. </li></ul>Optical Fibres in the Medicine Industry