Phyfib Essay Research Paper History of Fibre

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 a