Phyfib Essay, Research Paper
History of Fibre OpticsWhen 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 whichwas focused by means of a reflector and a lens to a device which could be made to vibrate inharmony with speech from a human voice. The light beam was made to vary the focus in andout so that the strength on a selenium detector could be made to activate a telephonereceiver and recreate the original voice. The distance between the transmitter and receiverwere very short, but it was the beginning of communication via light. The method ofchanging the intensity of the light beam is still what we base our communications on, butnow, more often digital communication consisting of “on” and “off” patterns is used. Thesimplest use of optical fibres is that of light pipes. A light source that gives off heatand light transmits the light only through the pipes to give “cold” light. This is howdoctors see inside the body. What is so great about optical fibre? It is a piece of glass that allows light to travelthrough. Actually it is a very fine strand of very special glass which might be only 125microns 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 wireseveral thousand times better than current coaxial cable. Since the mid 1980’s, thousandsof kilometres of optical fibre have been laid in the United States and Japan to carry longdistance telephone communications. Fibre optics are also used in various medicalinstruments designed to examine the interior of the body, since the images transmitted bythese devices can be magnified and rotated for close observation of hollow organs. Opticalfibres are also used in many laser-based computer printers to produce photo quality copies. Glass or plastic filaments are spun to diameters between 5 and 100 micrometers and packedinto 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 betwisted and bent to conduct light and images around corners. In order to protect thefibres, a protective layer is applied. ReflectionFigure 3 Reflection of lightWhen light falls on a medium a percentage is reflected back. The amount of light reflecteddepends on the angle a1 between the incidence ray, and the normal ray. q1=q2RefractionWhen 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’s1 law of refractionapplies: Figure 4 Refraction of lightwhere 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=c1sinbc2With two transparent media, the one with the lower speed is considered to be denser. When light travels in a vacuum at a speed of c0 to a medium with a speed of light c thefollowing applies sina=c0=n sinbcThe ratio of the speed of light in a vacuum and the speed of light in a medium is called therefractive index (more precisely the phase refractive index)For two different mediums with the refractive indexes of n1 and n2 and their speeds oflight c1 and c2, the following applies:c1=c0n1Another form of Snells law is:sina=n2sinbn1Critical AngleIt is possible for the difference between in refractive indexes between two mediums to causerefracted light to have an angle of 90., or parallel to the medium surface.. This angle iscalled the critical angle. The critical angle can be found by:sinqc=n2/n1Figure 5 Total internal reflection of lightTotal Internal reflectionWhen a light ray comes into contact with a medium with a different refractive index, it isrefracted. If the angle of incidence is less than the critical angle, it will be reflectedinside the medium. This is called total internal reflection. It is possible for this rayto 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 FibreLets consider a short piece of cable with two rays entering, A and B. Figure 6 The passage of light through a fibre optic cableRay A enters the fibre at an angle of qA. This ray strikes point C. Some of the light isreflected on to point D, and some of the light is refracted outside. Again at point D, somelight is reflected and some is refracted outside. This will continue until the all theenergy 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 acone of angles that will be reflected. qB=sin-1(n1/n2)Ray C enters the fibre at an angle less than the qc. This ray will continue on foreverbeing totally internally reflected2 . The skip distance is the distance between two reflections and can be found by:Ls=dcotqwhere d is the core diameter. Numerical ApertureIn order to launch light from outside into the core glass, the launch angle between lightray and fibre axis can be found by:sinq=n1sin(90.-a0)n2The greatest launch angle qmax is called the acceptance angle of the fibre. The sine of theacceptance 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 FibresCore/claddingn1n2jcriticalqmaxN.A. 1/LsGlass/air1.501.041.8.90.0.15944Plastic/plastic1.491.39220.127.116.11.0.543866Glass/plastic1.461.4073.5.24.5.0.412962Glass/glass1.481.4618.104.22.168.0.241657BandwidthThe bandwidth is a continuous range of frequencies between a lower and upper limit. Themore complicated a signal is, the greater the range of frequencies needed to represent itare. The output of a FM radio station is much clearer than that of a telephone because agreater frequency range is given to the FM. For example, a telephone conversation normallytakes 4 kHz, where as a FM radio takes 200 kHz. A television station takes 6 MHz ofbandwidth. The potential of the optical fibres is enormous. It is possible to calculatethe 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 3×108 MHz tocarry the information, the ratio is 3×108MHZ/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 bits3 are required to represent the amplitude of an analog signal. A analogsignal is supposed to by sampled at a rate of at least twice as high as its highestfrequency. In the case of a TV channel with a bandwidth of 6 MHz, this means that 2 x 6
MHz, or 12×106 samples must be taken each second. Since each sample is described as using 8bits, the required data rate is 96 Mbps (megabits per second). The data rates are limitedat the moment by fibre distortions, and equipment to transmit this fast. AttenuationWhen the light is absorbed by the fibres, it is called attenuation. There is severalreasons 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 variationsin the index of the refraction of the glass. This gives a uniform loss over the entirefibre. 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 ironbeing left in the fibre when made. It is very difficult to get rid of these trace elementsbecause they are found almost everywhere. These types of attenuation often only absorbslight at certain wavelengths. The other type is when the fibre is bent too much, or fromtiny micro-defects in the fibre. These are called extrinsic losses, and causes the rayangle to be greater than the critical angle, and is not reflected. Attenuation is measured in decibles per kilometer lost, or dB/km. The early cables producedhad 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) microdefect in fibreTransmission of Digital Signals89There are three main components used in fibre optic communications. The first component isthe transmitter. It modulates the electrical energy into light energy. This is the partthat generates the light signals, capable of being switched on and off very quickly. Thefaster that these can be switched on and off, the more information can be sent in a giventime. 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 thefrequencies 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 stationsare set up. These amplify the signal to avoid loss and distortion. Finally the last component is the receiver. This converts back to electrical energy thelight signal. It is made with a detector, which detects the light and turns it back intoelectrical energy. A signal processor that amplifies the signal, filters and changes thesignal into a useable form, ie analogue sound. Digital signals are what are used for communication in a telecommunications and datacommunications. Transmission of Analogue Signals10Analogue signals are continuous. That is they have an infinite number of values. All soundlight we hear and see is also analogue. Analogue is used when data is not beingcommunicated. A endoscope which is used to see inside the body uses analogue. One fibrewill carry white light inside, and another tube will carry back the reflected image. Figure11 Basic diagram of the fibre drawing and coating processCommercial Manufacture of Optical FibresThe main material of optical fibre is ultrapure silica powder. This is heated to a hightemperature until it is molten. A glass rod, or preform, is formed when it is slightlycooled. A fibre is then pulled out and stretched, keeping the heat constant to ensure aneven pull.. The next step is to coat this fibre in either glass or a plastic coating toform the cladding. It is then put through a test using ultraviolet rays to check forimperfections. 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 mixingof the materials needs to be done extremely carefully. A single speck of dust cancontaminate an entire batch of fibre. The glass produced is so pure, that a block onekilometre thick is as clear as a normal window pane. Optical Fibres in the Telecommunications IndustryThe use of optical fibres for telecommunications is by far the biggest use, and probably themost potential. During the next decade or two, almost every house will probably beconnected with optical fibres. Many experts expect that by the year 2020, nearly all homesin America will have fibre optic connections. The first telephone network was tested by Western Electric in 1976. One year later, Bellcarried the first optical cable in Chicago. It covered 2.5km. Figure 12 Some of the possible uses for optical fibresToday, telecommunication companies in America have replaced nearly all the major citieslinks with optical fibres. The cables are about as wide as a closed fist, and conduct asmuch 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 maintrunk rail track to transmit telephone calls between the cities. In the last year, Clearhas entered the business district in Wellington, and now connects some buinesses with fibreoptic cables to the local and international network. Telecom has installed a fibre optic link to carry all telephone messages between the twoislands. There is also a fibre optic link between Australia and Auckland, and Auckland andHawaii. Kapiti Cable, on the Kapiti Coast, is experimenting with connecting houses with an opticalfibre. 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 withaccess to the global computer network, the Internet. This is the most advanced network inNew Zealand at the moment. There are other smaller networks experimenting at the moment inAuckland and Christchurch too. Medical Uses of Optical FibresFigure 13 A tiny probe pierces a cellThere are many new instruments that now use fibre optic cable. The name Endoscopy is givento the instruments that look inside the body. The esophagoscope allows doctors to examinethe oesophagus, the gastroscope is a flexible tube to examine the instruments is used to seeinside 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 slipbetween the membrane of cells. It is the smallest sensor over developed at onlyone-thousandth the width of a human hair. With these aids, it is possible for doctors to perform surgery without having to performmake a cut. Lasers can be aimed through a fibre and can “shoot” gall stones in the bladderor remove blockages in the arteries. Eye surgeons can fix a wide array if problems using lasers and fibre optics. The laser canbe directed to exactly where the problems. Correction of the shape of eyes used to be aserious problem, now most cases can be done under no anethesic, and no overnight stay athospital. Military Uses of Optical FibresThe military was the first to use optical fibres and most of the early research withapplications was done my them. In 1973, the first optical cables were put into operation onAmerican navy shipsWith 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 completevideo 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 technologythat will able computers in the house to replace the phone, TV, radio, and every other kindof electrical communication. Eventually it will be possible to have entire libraries ofinformation “beamed” into the house. 1 Willebrord Snell discovered this important law of refraction in 1621. It refers to howmuch light is bent when moving from one medium to another. 2 Although the energy willeventually be absorbed into the fibre through attenuation 3 A “bit” is either on or off. ??