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.As theprimary technical mission was for improvements in bandwidth then the 34 Fiber Optic Cablingrefinement of processing techniques was totally dominated by develop-ments in this area.This refinement led to the production of graded indexfibers.Step index and graded index fibersSo far in the treatment of optical fiber it has been assumed that the coreand cladding comprise a two-level refractive index structure: a high-indexcore surrounded by cladding with a lower refractive index.This type offiber is defined as stepped or step index because the refractive indexprofile is in the form of a step (see Figure 2.19).This design of fiberconstruction is still used for large core diameter, large NA, fibers asdiscussed earlier in this chapter where applications required a high levelof light acceptance.Figure 2.19 Stepped refractive index profileAs an alternative, the concept of a graded index fiber was considered.Graded index optical fibers are manufactured in a comparativelycomplex manner and they feature an optical core in which the refractiveindex varies in a controlled way.The refractive index at the central axisof the core is made higher than that of the material at the outside of thecore.The effect of the lower refractive index layers is to accelerate thelight as it passes through.The higher-order modes, which spend proportionally more time awayfrom the centre of the core, are therefore speeded up with the intention ofnarrowing the time dispersion and hence increasing the operating bandwidthof the fiber.The refractive index layers are built up concentrically duringthe manufacturing process.Careful design of this profile enabled not onlythe acceleration effect mentioned above but also had a secondary but no lessimportant function.Treating the profile as shown in Figure 2.20 it is clearthat the light no longer travels as a straight line but is curved, although atthe microscopic level this curve is composed of a series of straight lines. Optical fiber theory 35Figure 2.20 Graded refractive index profileThe ideal graded index profile balances the additional path lengths ofhigher-order modes with the increased speeds of travel within thosemodes, thereby reducing intermodal dispersion considerably.This wasachieved by producing a profile defined by equation (2.17):n(r) = n1 1 2s(r2) (2.17)d2Where s is given by (2.18):s = (n2 n2)/n2 (2.18)1 2 2Increases in bandwidths of the order of tenfold were achieved and at thisstage it was felt that optical fiber had reached the point of acceptabilityas a carrier of high-speed data.Manufacturing tolerances have continually been improved and profileshave been more closely controlled.As a result operating bandwidths ofgraded index fibers have gradually increased to the point where littlefurther improvement is possible, although the manufacturing process canbe tuned and product selected to give some outstanding performances inexcess of 1000 MHz.km.The majority of optical-based data communications networks todayutilize graded index (GI) profiles.However, 97% of the world market foroptical fiber (year 2000) is for single mode, sometimes known asmonomode.Single mode fiber is addressed in more detail later in thischapter.The use of GI profiles modifies the equations detailed earlier in thischapter.In particular equation (2.7):(n2 n2)0.5 (2.7)1 2 36 Fiber Optic CablingA graded index fiber with an ideal profile will have a modified NA asshown in (2.19):(0.5(n2 n2))0.5 H" numerical aperture, graded index (2.19)1 2It can be seen then that the NA of a graded profile fiber will be lowerthan that of an equivalent stepped index fiber.Once again it is logical toexpect large core diameter, high NA (high light acceptance) fibers tofeature stepped index core structures whereas the higher-bandwidth, lowattenuation fibers will feature smaller core diameters and will utilize aslow an NA value as possible; normally achieved by the use of a gradedindex core structure.Modal conversion and its effect upon bandwidthIn the previous sections we have treated the fiber core as being able tosupport a large but finite number of transmission modes ranging from thezero-order mode (travelling parallel to the axis of the core) to the highest-order mode (travelling along the fiber at the critical angle).This assump-tion allows the calculation of attenuation and bandwidth dependencies ashas already been shown.Needless to say this ideal model is far from thetruth.It is unlikely, not to say impossible, to manufacture and cable a fibersuch that propagation of the modes would continue in such an orderlyfashion.In practice there is a continual process of modal conversion (see Figure2.21) changing zero-order modes to higher orders and vice versa.In anyfiber it is fair to assume that bandwidth performances will exceed theoret-ical models outlined here, be they of stepped or graded index profile, dueto the equalization of path lengths.The practical improvements in bandwidths due to modal conversionmay be as much as 40%, which pushed fiber bandwidths into the regionof 1000 MHz.km.However, these levels were not sufficient for the tele-communications market with their huge cabling infrastructure requiringFigure 2.21 Modal conversion Optical fiber theory 37to be as future-proof as possible.One further technical leap was neces-sary: single mode (or monomode) optical fiber.Differing multimode bandwidths when using lasers and LEDsUp until around 1996 multimode fiber was generally used with lightemitting diodes (LEDs) as the transmitting devices.LEDs are low cost andtheir ability to send up to 200 Mb/s over 2 kilometres of multimode fiberwas seen as more than adequate.The bandwidth of multimode fiber wasmeasured using the output conditions of an LED which uniformly illumi-nates the core of the fiber and excites hundreds of modes in the core.This is known as overfilled launch (OFL) and the resulting bandwidth,when measured with such an input device, is known as overfilled launchbandwidth (OFL-BW).This is shown in Figure 2.22.Overfilled launch conditionCore of fiber fullyLED lightfilled with lightsourceSpot of lightmuch smallerthan theLaser lightmultimodesourcecoreRestricted mode launchFigure 2.22 Overfilled and restricted mode launch conditionsUnfortunately LEDs can only be modulated up to a few hundreds ofmegahertz, and so are not generally suitable for gigabit speed transmis-sion.In November 2000 a Japanese laboratory demonstrated a 1 Gb/ssuperluminescent InGaAs/AlGaAs LED operating at 880 and 930 nm.This device, however, remains in the lab for the present and the vastmajority of gigabit transmission equipment will need lasers to achievegigabit speeds.The problem with lasers is that traditionally they are very expensive,having been developed with long-haul telecommunications in mind.Thetwo styles of laser used in single mode telecommunications systems areknown as distributed feedback (DFB), and Fabry Perot (FP)
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