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Fiber optics

By: Raj

Thesis: Some of the important features of the fiber optics that we are interested are discussed below. Now a days we are using copper wires as they are the most cost effective and reliable interconnect in parallel machines. However as machines grow more powerful, wire density becomes critical making fiber possible alternatives because of their small wire size. Fiber optics are used mainly to use effectively its high bandwidth. On a single fiber lots of information/data can be transmitted concurrently and in parallel. Over 1000 high bandwidth (100-200 Mb/s) independent

channels or busses can be supported on a single optical fiber. Furthermore multiple buses can co-exist on a single fiber. Fiber links allow a number of high speed serial links to replace a large number of electrical lines. The use of fiber is thus space saving. The input and output properties of the fiber are very important. They give us an idea about the nature and working of fiber materials. The fiber cables can transport light signals from one place to another place just similar to the way the metallic conductors transport electric signals. The fiber cables guide light around bends and they are able to carry light for very long distances with very little attenuation. But the transmission characteristics of the fiber are not complete and completely efficient. The fiber cables introduce loss of light and smearing of the modulation imposed on the light signals to represent information. These affects of delay distortion and attenuation limit the distances that can be spanned without electro-optic repeaters and thus limit the information rates which can be carried over long distances as well. Make of fiber optic cables. Turns out they can be made of just glass, glass plus polymers, or just polymers ("plastic optical fibers" -POF-). The most basic optical fiber consist of: a) An inner cylinder with high refractive index, called the core. b) Middle cylinder with a lower refractive index, called the cladding. c) An outer protective polymer layer (usually polyurethane or PVC) called the jacket. For glass optical fibers, the diameter of the core ranges between 10-600 microns, the cladding thickness is between 125-630 microns, and that of the jacket varies between 250-1040 microns. For POF all diameters range between 750-2000 microns. As can be seen, one of the main differences between glass and plastic optical fibers is their diameter. This makes POF easier to handle. The material used for currently commercialized fibers (core and cladding) include pure glass (SiO2), plastic, or a combination of both. The use of one or the other material will be determined by such factors as quality and economics. Plastic optical fibers (POF) have the advantage of being made of cheaper materials than glass and to operate in the visible range of the spectrum. However, they show a high loss, and for that reason their applications are confined to short distance transmission. In spite of this, POF is widely used for medical and industrial instruments, and currently research is carried out about using POF as a replacement of copper wiring for data transmission in automobiles. If you use silica glass for the core, it must be high purity in order to allow the light to be transmitted along the core with minimal loss. Some of the advantages associated with the use of fiber optic cables are: 1) Immunity to Electromagnetic Interference Although fiber optics can solve data communications problems, they are not needed everywhere. Most computer data goes over ordinary wires. Most data is sent over short distances at low speed. In ordinary environments, it is not practical to use fiber optics to transmit data between personal computers and printers as it's too costly. Electromagnetic Interference is a common type of noise that originates with one of the basic properties of electromagnetism. Magnetic field lines generate an electrical current as they cut across conductors. The flow of electrons in a conductor generates a magnetic field that changes with the current flow. Electromagnetic Interference does occur in coaxial cables, since current does cut across the conductor. Fiber optics are immune to this EMI since signals are transmitted as light instead of current. Thus, they can carry signals through places where EMI would block transmission. 2) Data Security Magnetic fields and current induction work in two ways. They don't just generate noise in signal carrying conductors; they also let the information on the conductor to be leaked out. Fluctuations in the induced magnetic field outside a conductor carry the same information as the current passing through the conductor. Shielding the wire, as in coaxial cables can reduce the problem, but sometimes shielding can allow enough signal leak to allow tapping, which is exactly what we wouldn't want. There are no radiated magnetic fields around optical fibers; the electromagnetic fields are confined within the fiber. That makes it impossible to tap the signal being transmitted through a fiber without cutting into the fiber. Since fiber optics do not radiate electromagnetic energy, emissions cannot be intercepted and physically tapping the fiber takes great skill to do undetected. Thus, the fiber is the most secure medium available for carrying sensitive data. 3) Non Conductive Cables Metal cables can encounter other signal transmission problems because of subtle variations in electrical potential. Electronic designers assume that ground is a uniform potential. That is reasonable if ground is a single metal chassis, and it's not too bad if ground is a good conductor that extends through a small building. However, the nominal ground potential can differ by several volts if cables run between different buildings or sometimes even different parts of the same building. Signal levels in semiconductor circuits are just a few volts, creating a problem known as ground loop. When the difference in ground potential at two ends of a wire gets comparable to the signal level, stray currents begin to cause noise. If the differences grow large enough, they can even damage components. Electric utilities have the biggest problems because their switching stations and power plants may have large potential differences. A serious concern with outdoor cables in certain computer networks is that they can be hit by lightning, causing destruction to wires and other cables that are involved in the network. Certain computer companies are aware of this problem and trying to solve it by having protective devices for wire circuits to block current and voltage surges. Any conductive cables can carry power surges or ground loops. Fiber optic cables can be made non-conductive by avoiding metal in their design. These kinds of cables are economical and standard for many indoor applications.



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