FIBRE OPTIC TECHNOLOGY INTRODUCTION Interest in the use of light as a carrier for information in the 1960’s with the advent of the laser as a source of coherent light. Initially the transmission distances were very short, but as manufacturing techniques for very pure glass arrived in 1970, it became feasible to use optical fibres as a practical transmission medium. At the same time developments in semi- conductor light sources and detectors meant that by 1980 worldwide installation of fibre optic communication systems had been achieved. What are Optical Fibres? Optical fibres are fibres of glass, usually about 120 micrometers in diameter, which are used to carry signals in the form of pulses of light over distances up to 50 km without the need for repeaters. These signals may be coded voice communications or computer data. Fibre Optic (or “Optical Fibre”) refers to the medium and the technology associated with the transmission of information as light impulses along a glass or plastic wire of fibre. Fibre Optic wire carries much more information than conventional copper wire and is far less subject to electromagnetic interference. Most telephone company long-distance lines are now fibre optic. Transmissions on fibre optic wire requires repeating at distance intervals. The glass fibre requires more protection within an outer cable than copper. For these reasons and because the installation of any new wiring is labor-intensive, few communities yet have fibre optic wires or cables from the phone company’s branch office to local customers. Reflection and Refraction of Light When light traveling in a transparent meets the surface of another transparent material two things happen some of the light is reflected, some of the light is transmitted into the second transparent material. The light which is transmitted usually changes direction when it enters the second material. This bending of light is called refraction and it depends upon the fact that light travels at one speed in one material and at a different material. As a result each material has its own Refractive index that we use to help us calculate the amount of bending which takes place. Refractive index is defined as C n= V Where n is the refractive index. c is the speed of light in a vacuum v is the speed of light in the material Two possibilities cases exist. These are: 1. Where light goes from a material with a low refractive index to one with a high refractive index. 2. Where light goes from a material with a high refractive index to one with a low refractive index. DIAGRAMS Total Internal Reflection In the second case above, n2 is always greater than n1. So, as we increase n1, eventually n2 will reach 900 before n1 does. At this point where n1 has reached a value called critical angle (qc). The transmitted ray now tries to travel in both materials simultaneously for various reasons this is physically impossible so there is no transmitted ray and all the light energy is reflected. This is true for any value of n1, the angle of incidence, equal to or greater than qc. This phenomenon is called Total Internal Reflection (TIR). We can define the two conditions necessary for TIR to occur the refractive index of the first medium is greater than the refractive index of the second one. The angle of incidence, n1, is greater than or equal to the critical angle, qc. The phenomenon of TIR causes 100% reflection. In no other situation in nature, where light is reflected, does 100% reflection occur. So TIR is unique and very useful. OPTICAL FIBRES Structure of Fibre Optical Fibres are very fine fibres of glass. They consist of a glass core, roughly 50 micrometers in diameter, surrounded by glass “optical cladding” giving an outside diameter of about 120 micrometers. They make use of TIR to confine light with the core of the fibre. DIAGRAM The core has a higher refractive index than the cladding. Although the cladding does not carry light, it is nevertheless an essential part of the fibre. The cladding is not just a mere covering. It keeps the value of the critical angle constant throughout the whole length of the fibre. DIAGRAM Optical Fibres are optical waveguides. This means that wherever the fibre goes the light, which is confined to the core of the fibre, also goes. So optical fibres can be used to make light bend round corners. Propogation of light in the fibre The angle q is the Acceptance Angle. Any light entering the fibre at less than this angle will meet the cladding at an angle greater than qc. If light meets the inner surface of the cladding (the core – cladding interface) at greater than or equal to qc then TIR occurs. So all the energy in the ray of light is reflected back into the core and none escapes into the cladding. The ray then crosses to the other side of the core and, because the fibre is more or less straight, the ray will meet the cladding on the other side at an angle which again causes TIR. The ray is then reflected back across the core again and the same thing happens. In this way the light zig zags its way along the fibre. This means that the light will be transmitted to the end of the fibre. DIAGRAM In reality the light which enters the fibre is a focused beam, consisting of many millions of rays behaving in a similar way. They are all zig zag along the core of the fibre, crossing over each other, and filling up the core with light. A pulse of light traveling along the core of the fibre is really a bundle of these rays. Fibre Types There are two main fibre types: 1. Step index (multimode, single mode) 2. Graded index (multimode) Step Index Fibre: Step index is so called because the refractive index of the fibre steps up as we move from the cladding to the core of the fibre. Within the cladding the refractive index is constant, and within the core of the refractive index is constant. DIAGRAM Multimode: Although it may seem from what we have said about total Internal reflection that any ray of light can travel down the fibre, In fact, because of the wave nature of light, only certain ray directions can actually travel down the fibre. These are called the Fibre Mode. In a multimode fibre many different modes are supported by the fibre. DIAGRAM Single Mode: Because its core is so narrow single mode fibre can support only one mode. This is called the Lowest Order Mode. Single mode fibre has some advantages over multimode fibre which we will deal with later. DIAGRAM Graded Index Fibre: Graded Index Fibre has a different core structure from single mode and multimode fibre. Whereas in a step-index fibre the refractive index of the core is constant throughout the core, in a graded index fibre the value of the refractive index changes from the centre of the core onwards. In fact it has what we call a Quadratic Profile. This means that the refractive index of the core is proportional to the square of the distance from the centre of the fibre. DIAGRAM Graded index fibre is actually a multimode fibre because it can support more than one fibre mode. But when we refer to multimode fibre we normally mean step index multimode. PULSE SPREADING PULSE SPREADING The data which is carried in an optical fibre consists of pulses of light energy following each other rapidly. There is a limit to the highest frequency, i.e. how many pulses per second which can be sent into a fibre and be expected to emerge intact at the other end. This because of a phenomenon known as pulse spreading which limits the “bandwidth” of the fibre. Diagram The pulse sets off down the fibre with an nice square wave shape. As it travels along the fibre it gradually gets wider and the peak intensity decreases. Cause of pulse spreading The cause of pulse spreading is dispersion. This means that some components of the pulse of light travel at different rates along the fibre. There are two forms of dispersion. Chromatic dispersion Modal dispersion Chromatic dispersion Chromatic dispersion is the variation of refractive index with the wavelength of the light. Another way of saying this is that each wavelength of light travels through the same material at its own particular speed which is different from that of other wavelengths. For example, when light passes through a prism some wavelengths of light bend more because their refractive index is higher, i.e. they slower this is what gives us the “spectrum” of white light. The “red” and “orange” light travel slowest and so are bent most while the “violet” and “blue” travel fastest and so are bent less. All the other colours lie in between. This means that different wavelengths traveling through an optical fibre also travel at different speeds. This phenomenon is called “chromatic dispersion”. Diagram Modal dispersion In an optical fibre there is another type of dispersion called “Multimode dispersion”. More oblique rays travel a shorter distance. These correspond to rays traveling almost parallel to the centre line of the fibre and reach the end of fibre sooner. The more zig-zag rays take a longer route as they pass along the fibre and so reach the end of the fibre later. Now:- Total dispersion = chromatic dispersion + multimode dispersion Or put simply: for various reasons some components of a pulse of light travelling along an optical fibre move faster and other components move slower. So, a pulse which starts off as a narrow burst of light gets wider because some components race ahead while other components lag behind, rather like the runners in a marathon race. Consequences of pulse spreading Frequency limit(Band width) The further the pulse travels in the fibre the worse the spreading gets Diagram Pulse spreading limits the maximum frequency of signal which can be sent along a fibre. If signal pulse follows each other too fast then by the time they reach the end fibre they will have merged together and become indistinguishable. This is unacceptable for digital systems which depend on the precise sequence of pulses as a code for information. The Bandwidth is the highest number of pulses per second, which can be carried by the fibre without loss of information due to pulse spreading. Distance Limit A given length of fibre, as explained above has a maximum frequency which can be sent along it. If we want to increase the bandwidth for the same type of fibre we can achieve this by decreasing the length of the fibre. Another way saying this is that for a given data rate there is a maximum distance which the data can sent. Bandwidth Distance Product (BDP) We can combine the two ideas above into a single term called the bandwidth distance product (BDP). It is the bandwidth of a fibre multiplied by the length of the fibre. The BDP is the bandwidth of a kilometer of fibre and is a constant for any particular type of fibre. For example, suppose a particular type of multimode firbre has a BDP of 20 MHz. km, then:- 1 km of the fibre would have a bandwidth of 20 MHz 2 km of the fibre would have a bandwidth of 10 MHz 5 km of the fibre would have a bandwidth of 4 MHz 4 km of the fibre would have a bandwidth of 5 MHz 10 km of the fibre would have a bandwidth of 2 MHz 20 km of the fibre would have a bandwidth of 1 MHz The typical B.D.P. of the three types of fibres are as follows:- Multimode 6 – 25 MHz.Km Single Mode 500 – 1000 MHz.km Graded Index 100 – 1000 MHz.km (read as megahertz kilometers). They are not MHz/Km (read as megahertz per kilometers). This is because the quantity is a product (of bandwidth and distance) and not a ratio. Choice of Fibre Multimode Fibre Multimode fibre is suitable for local area networks(LAN’s) because it can carry enough energy to support all the subscribers to the network. In a LAN the distances involved, however, are small. Little pulse spreading can take place and so the effects of dispersion are important. Single Mode Fibre Multimode Dispersion is eliminated by using Single Mode fibre. The core is so narrow that only one mode can travel. So the amount of pulse spreading in a single mode fibre is greatly reduced from that of a multimode fibre. Chromatic dispersion however remains even in a single mode fibre. Thus even in single mode fibre pulse spreading can occur. But chromatic dispersion can be reduced by careful design of the chemical composition of the glass. The energy carried by a single mode fibre , however, is much less than that carried by a multimode fibre . For this reason single mode fibre is made from extremely low less,very pure glass. Single mode low absorption fibre is ideal for telecommunications because pulse spreading is small. GRADE INDEX FIBRE In graded index fibre rays of light follow sinusoidal paths. This means that low order modes, i.e. oblique rays stay close to the center of the fibre, high modes spend more time near the edge of the core. Graded index fibre has the advantage that it can carry the same amount of energy as multi mode fibre. The disadvantage is that this effect takes place at only one wave length. DIAGRAM FIBRE MANUFACTIRE The most common method of making fibre is known as Modified Chemical Vapour Dispersion (MCVD).An outer glass “bait tube” isheated by a traversing burner. Through this tube a mixture of gases is passed at a steady rate , which when heated undergoes a chemical reaction. The gas mix contains compounds of silicon , metal halides ,oxygen and dopant material which will determine the refractive index of the glass core. The solid end products of the reaction are deposited on the interior of the bait tube as soot. This soot will eventually form the core of the fibre while the bait tube will form the cladding . when enough soot is deposited the gas flow is stopped and the heat is turned up so that soot melts to form a sintered glass. This sintered glass is heated to form a solid rod. This rod is held vertically and passed through a oven which softens its ends. This end is now stretched to form a glass fibre. SPLICING Optical fibres have to be joined to make longer lengths of fibre or existing lengths which have been broken have to be repaired. Also the ends of fibre have to be fitted with convenient connectors (terminations) to allow them to be easily plugged into equipment such as power meters, data transmitters ,etc. Splicing is the process of joining the two bare ends of two fibres together. The ends of the fibre must be precisely lined up with each other, otherwise the light will not be able to pass from one fibre across the gap to another to another fibre. There are mainly four alignment of optical fibres. There are : LATERAL AXIAL ANGULAR POOR END FINISH There are mainly two types of splicing : Fusion splicing Mechanical splicing FUSION SPLICING In fusion splicing the ends of the fibre are aligned either manually using micro-manipulators and a microscope system for viewing the slice or automatically either using cameras or by measuring the light transmitted through the slice and adjusting the positions of the fibre. The ends of the fibre are then melted together using a flame or more commonly an electric arc. MECHANICAL SPLICING In mechanical splicing the two fibre ends are held together in a splice. This consists of some device usually made of glass which by its internal design automatically brings the two fibres into alignment. The openings at each ends of the device are usually fluted to allow the fibre to be guided into the capillary where the alignment takes place. AREAS OF APPLICATION TELECOMMUNICATIONS Optical fibre are now the standard point to point cable link between telephone sub stations. A telecommunication link is the simplest of the fibre optic system. It consists basically of a transmitter , a fibre link and a receiver. The transmitter will normally be equipped with a laser diode, usually with an output wave length of 1300nm or 1500 nm. The fibre link will be made of single lengths of single mode optical fibre of length 2km fusion spliced together. The link will be able to carry thousands of telephone conversations simultaneously. LOCAL AREA NETWORKS(LAN’S) Multimode fibre is commonly used as a “back bone” to carry signals between the hubs of LAN’s from where copper coaxial cable takes the data to the desktop. Fibre links to the desk tops are also however common. CABLE TV As mentioned above domestic cable tv networks use optical fibre because of its very low power consumption. CCTV Closed Circuit Television security systems use optical fibre because of its inherent security as well as the other advantages mentioned above. OPTICAL FIBRE SENSORS Many advances have been made in recent years in the use of optical fibres as sensors. Gas concentration , chemical concentration, pressure , temperature , and rate of rotation can all be sensed using optical fibre. Much work in this field is being done at the University Of Strathclyde. ADVANTAGES CAPACITY Optical fibres carry signals with much energy loss than copper cable and with a much higher bandwidth. This means that fibres can carry more channels of information over longer distances and with fewer repeaters required. SIZE AND WEIGHT Optical fibre cables are much lighter and thinner than copper cables with the same bandwidth. This means that much less space is required in underground cabling ducts. Also they are easier for installation engineers to handle. SECURITY Optical fibre are much more difficult ot tap information from undetected; a great advantage for banks and security installations. They are immune to Electro magnetic interference from radio signals, explosive or flammable atmospheres, for example , in the petro chemical industries or munitions sites ,without any risk of ignition. RUNNING COSTS The main consideration in choosing fibre when installing domestic cable tv networks is the electric bill. Although copper coaxial cable can handle the bandwidth requirement over the short distances of a housing scheme, a copper system consumes far more electrical power than fibre , simply to carry the signals. DISADVANTAGES PRICE In spite of the fact that the raw material for making optical fibres and sand is abundant and cheap , optical fibres are still more expensive per meter than copper. Having said this one fibre can carry more signals than a single copper cable and the large transmission distances mean that fewer expensive repeaters are required. SPECIAL SKILLS Optical fibres cannot be joined together as easily as copper cable ,and requires additional training of personnel and expensive precision splicing and measurement equipment CONCLUSION We have looked at how the optic fibres work and how they are made. The usage of these optical fibres is gradually increasing in all the fields. The communication can be passed on more quickly and safely as the usage of these optical fibres increases.
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