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Cytat
Do celu tam się wysiada. Lec Stanisław Jerzy (pierw. de Tusch-Letz, 1909-1966)
A bogowie grają w kości i nie pytają wcale czy chcesz przyłączyć się do gry (. . . ) Bogowie kpią sobie z twojego poukładanego życia (. . . ) nie przejmują się zbytnio ani naszymi planami na przyszłość ani oczekiwaniami. Gdzieś we wszechświecie rzucają kości i przypadkiem wypada twoja kolej. I odtąd zwyciężyć lub przegrać - to tylko kwestia szczęścia. Borys Pasternak
Idąc po kurzych jajach nie podskakuj. Przysłowie szkockie
I Herkules nie poradzi przeciwko wielu.
Dialog półinteligentów równa się monologowi ćwierćinteligenta. Stanisław Jerzy Lec (pierw. de Tusch - Letz, 1909-1966)
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.14 Loss values versus fiber geometryThe total reflected power is therefore calculated (for = 0°) fromequation (4.11):n= "Preflected = Pincident x + xy2 x2n (4.11)n=0Figure 4.15 Return loss (single junction) 86 Fiber Optic CablingFigure 4.16 Return loss at a jointn1 n3 4n1n3where x = and y =(n1 + n3)2 (n1 + n3)2The summation represents the continuing reflections taking place betweenthe end-faces.Calculating in a rough fashion, using the same values as inthe Fresnel loss calculation above, gives:Preflected = Pincident(0.0375 + 0.0347 + 0.0000488 +.)= Pincident(0.0722)and return loss = 11.41 dBTherefore any joint with an air gap is predicted to exhibit a return lossof approximately 11 dB.Return loss was not a particularly important feature in optical connec-tion theory until high-speed communications using lasers was developed.Lasers do not perform well when subject to high levels of reflected lightand performance suffers both in the short and long term.Methods hadto be found to reduce the reflections from nearby joints by improvingthe return loss figures.As discussed earlier, physical contact between end-faces is nowcommonplace, with the result that little or no air gap remains.Returnloss figures of more than 30 dB are produced.These developments haveresulted in improvements in insertion loss by the removal of Fresnel loss Optical fiber connection theory and basic techniques 87Figure 4.17 Angled face connectorsin the forward direction.Unfortunately the contact between the end-facescarries with it an added responsibility for cleanliness and long-termperformance can suffer if rigorous instructions are not followed.Another method of improving the return loss which is receivingwarranted attention amongst single mode connector applications is theuse of demountable connectors which feature angled ferrule end-faces.These are designed such that the reflections lie beyond the critical angleat the CCI and are removed from the core, as shown in Figure 4.17.These connectors are very effective and in certain applications are theideal solution.SummaryThis chapter has reviewed connection theory and the limitations under-lying any kind of joint.Insertion loss and return loss are the measures of the optical perfor-mance of an optical fiber joint.Their dependence upon core diameter,numerical aperture and misalignment has been discussed and will befurther expanded upon in the next chapter.The methods for the connection of optical fiber vary and all aredesigned to minimize the wastage of light, but the ideal joint with zeropower loss is rarely attainable, since fiber tolerances form the limit ratherthan the joint mechanism itself.This idea may not be in line with the product literature generated bythe manufacturers of the joint components.They naturally attempt topromote their product in a competitive market by offering premiumperformance.Written specifications must therefore be carefully studiedbefore accepting their validity in the real world of cabling designs andinstallations.This is covered in Chapter 5. 5 Practical aspects ofconnection technologyIntroductionThe theoretical analysis of a fiber optic connection with due regard tobasic parameter mismatch, misalignment and the other factors discussedin Chapter 4 is useful to understand the losses within the various typesof optical fiber joint.However, at the practical level this theory is submerged in a sea ofmarketing, standardization and specification jargon.Finally, a goodlyamount of processing is involved in producing any of the joints discussedand the quality of the processing may further muddy the waters which,it is hoped, showed moderate clarity at the end of Chapter 4.This chapterseeks to mark out the true path through this most difficult area in aneffort to enable the reader to determine how joints will perform ratherthan how they can perform.Alignment techniques within jointsThe previous chapter defined two types of joint mechanisms:" fusion splice techniques;" mechanical alignment techniques;For the purpose of the next section it is valid to recategorize all jointsas using either:" relative diameter cladding alignment, or" absolute cladding alignment.Relative cladding diameter alignment refers to those techniques which donot depend upon the absolute value of the reference surface (the claddingdiameter) but rather are based upon the relative values of the diameter. Practical aspects of connection technology 89Figure 5.1 Relative cladding diameterFigure 5.2 Absolute cladding diameter alignmentExamples are fusion splice jointing and V-groove mechanical splicing,where provided that the two cladding diameters are equal (independentof their values) then the core alignment will be purely a function of itsown eccentricity with the cladding (see Figure 5.1).Absolute cladding diameter alignment processes involve the use offerrules as in some mechanical splices and virtually all demountableconnectors.The ferrules have holes along the axis to allow the fibers tobe brought into alignment.The diameter of these holes and the positionof the fiber within them is one further misalignment factor in thecomplex issue of joint loss and its measurement (see Figure 5.2). 90 Fiber Optic CablingThe joint and its specificationThe optical specification of any jointing technique is based on its inser-tion loss and its return loss.As discussed in the previous chapter, the inser-tion loss of a joint is a measure of the core core matching and alignmentwithin the joint.From Figure 5.3 we have equation (5.1):insertion loss = 10 log10 Paccepted dB (5.1)PincidentIt was continually highlighted in Chapter 4 that any forecast of the inser-tion loss should take into account basic parametric mismatches of theoptical fiber as well as the performance of the connection technique.Figure 5.3 Insertion lossThe justification for this stance is simple: in real fiber optic cabling itis normal for a single optical fiber geometry to be adopted.Neverthelessany individual link may comprise several types of optical fiber cable perhaps a direct burial cable jointed to an intra-building cable jointed toan office cable connected via a jumper cable assembly to the transmis-sion equipment (see Figure 5.4).The fiber in each cable will almost alwayshave different origins and batch history despite having the same nominalfiber geometry.All these fibers have to be jointed and therefore it is vitalto understand what a joint will produce (in terms of insertion loss) ratherthan what it can produce (based upon the product literature).The perfor-mance that will be produced will depend upon the quality and dimen-sional tolerance of the two fibers involved as much, if not more, than thesubmicron accuracy of the connection technique.Scepticism is therefore the watchword and careful assessment is theprerequisite for understanding the difference between the figures for inser-tion loss quoted by the joint manufacturers and the potential resultsobtained with a real system. Practical aspects of connection technology 91Figure 5
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