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.This represents an increase over the normal attenuation of thecabled fiber which may be attributed to the impact of radiation incidenton the fiber core.In this way a radiation performance requirement canbe generated:If Dose A is incident upon the fiber for a period B then the resultingattenuation shall not increase by more than C.Some users may accept system failure during irradiation as long as thereis a known recovery state, and this will mean another specification for therecovery time, e.g.If Dose A is incident upon the fiber for a period B and subsequentlyremoved, then after time C the resulting attenuation shall not be morethan D.From the above radiation performance requirements it would appear thatthere is no such thing as radiation hardness as an all-encompassingparameter and as a result a fiber is neither radiation hard nor radiationnon-hard but has a specified performance against given levels of radiation.The optical fibers manufactured from the range of materials andpreform techniques discussed above offer a variety of performance levelsunder the influence of radiation.The radiation performance requirementgenerated for a given system can therefore be matched against the knownperformance of these fiber designs.Pure silica fibers such as PCS and HCS designs exhibit moderate radia-tion hardness.It is the addition to pure silica of dopants such as ger-manium that encourages the formation of colour centres which areresponsible for the attenuation increases observed.This suggests that the higher bandwidth, lower attenuation, fibergeometries manufactured from VDS preforms exhibit lower levels of radiation hardness a feature which is observed in practice.To overcomethis problem, further preform dopants are added during the depositionprocess.The dopants, such as boron and phosphorus, act as bufferspreventing the formation, or speeding the removal, of colour centres.Inthis way a range of VDS-based fibers have been rendered radiation hardagainst a particular radiation performance requirement.As the germanium content is directly linked to the numerical apertureof the fiber it is interesting to note that radiation performance improvesfor lower NA values.It is realistic to expect 8/125 micron single modefibers to exhibit considerably better radiation performance than theirmultimode counterparts.This is observed in practice. 70 Fiber Optic CablingFigure 3.9 Cladding mode transmissionPrimary coating processesIn Figure 3.8 the drawing of optical fiber is shown accompanied by asecondary process the addition of a primary coating which immedi-ately surrounds the cladding and prevents surface degradation due tomoisture and other pollutants.When produced, the optical fiber is stronger than steel in tension butunfortunately humidity can rapidly produce surface defects and cracks onthe silica cladding which can eventually lead to complete failure by crackgeneration if not controlled.The primary coating is normally a thin, ultraviolet cured acrylate layerwith a diameter of around 250 microns, depending upon the fiber geo-metry.The layer is the subject of particular interest because it immedi-ately surrounds the cladding.Its refractive index is of concern due to itsability to trap light between the CCI and the cladding surface.At onetime primary coatings were applied which had lower refractive indicesthan the cladding.Light therefore becomes trapped by TIR as is shownin Figure 3.9.These cladding modes, whilst not impacting overall atten-uation, do cause confusion at the test and measurement stage.Generally these primary coatings have been phased out and they havebeen replaced by mode stripping fibers with high index coatings.Theseabsorb the cladding modes rapidly.The primary coating on modern fiber is an easy-to-strip substance yetproviding an essential part of the mechanical integrity and water resis-tance of the optical fiber structure as a whole. Optical fiber production techniques 71SummaryThe range of optical fiber geometries necessary to meet both current andhistoric demands can be serviced by a variety of manufacturingtechniques.Large core diameter, high NA fibers can be produced from pure silica(HCS), doped silica (VDS) or glass components (double crucible).The lower NA, smaller structured fibers required tight tolerancepreforms and the VDS processes meet this need.The cost base is well defined with large core diameter, high NA fiberscosting significantly more than the high-performance single modeproducts.The end result of all fiber production is primary coated optical fiberwhich is the foundation for all further cabling processes. 4 Optical fiber connectiontheory and basic techniquesIntroductionHaving covered optical fiber theory and production techniques in theprevious two chapters it may seem natural to move to optical fiber cableas the topic of this chapter.However, before leaving optical fiber it isrelevant to cover the connection techniques used to joint optical fiber intemporary, semi-permanent or permanent fashions.The theoretical basisof fiber matching and, more importantly, mismatching, is of vital impor-tance in many areas of cabling design and demands treatment ahead ofthe purely practical aspects of cabling.Connection techniquesThe end result of the fiber production process is primary coated opticalfiber (PCOF).The PCOF is not manufactured in infinite lengths andtherefore must be jointed together to produce long-haul systems.Forshort-haul data communications the PCOF, in its cabled form, may beeither jointed or connected at numerous points.Equally importantly thecabling may have to be repaired once installed and the repair may requirefurther joints or connections to be made.Therefore to achieve flexibility of installation, operation and repair itis necessary to consider the techniques of connection as they apply tooptical fiber.As will be seen the connection techniques are inevitably linkedto PCOF tolerances and acceptable performance of joints and inter-connection is the result of careful design and not pure chance.The major difference between copper connections and optical fiberjoints is that a physical contact between the two cables is not sufficient.The passage of current through a 13 amp mains plug relies purely on Optical fiber connection theory and basic techniques 73good physical (electrical) contact between the wires and the pins of theplug.To achieve satisfactory performance through an optical fiber joint itis necessary to maximize the light throughput from one fiber (input) tothe other (output).Optical fiber connection techniques are frequently of paramount impor-tance in cabling design because, perhaps surprisingly, the amount of trans-mitted power lost through a joint can be equivalent to many hundreds ofmetres of fiber optic cable and is a major contributor to overall attenua-tion.It is therefore important to gain a complete understanding of themechanisms involved in the connection process and their measurement.Connection categoriesThere are many ways to categorize the range of connection techniquesapplicable to optical fiber
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