Measurement and Characterization of Key Fiber Reliability Attributes

log(failure stress)nd = 24.1 (2PBT)SEE = .0007563.853.803.753.703.65log(V/r)Figure 3 – Results of two-point bending method n dtesting on a fiber with a low modulus inner primary.The same fiber demonstrated highly elevated SEE inaxial tension testing (Figure 2).2.2 SEE Values and n d DeterminationA low SEE value is instrumental to the accurate measurement ofthe dynamic stress corrosion susceptibility. Scatter in the failurestress measurements via either n d measurement method canprovide both erroneously high and low n d values depending on thestress rates at which the scatter is observed. The internationalmeasurement standard, IEC 60793-1-33, requires themeasurement of additional samples when the SEE value exceeds0.0017. That SEE limit, however, does not always ensure that then d value is properly represented. Figure 4 shows the results of n dtesting via the axial tension method testing on two fibers. Onesample has a conforming SEE and n d value of 26.2. The otherfiber has a conforming SEE of less than 0.0017, but scatter at thehigh stress rates caused the slope to be lower, which elevated themeasured n d value. The actual n d value of the second fiber isimproperly represented despite the conforming SEE value.ln(failure stress)6.756.76.656.66.556.56.456.4Dynamic Fatigue - Axial Tension - IEC 60793-1-33nd = 25.2SEE = 0.000493nd = 33.0SEE = 0.001559ln(stress rate)Figure 4 – Axial tension method test results for twofibers with conforming SEE values. The n d value of 33is not an accurate reflection of fiber performancedespite the conforming SEE of less than 0.0017.The requirement for limitations on SEE values is not absoluteamong all measurement standards. While the internationalstandard, IEC 60793-1-33, has a SEE limitation, some domesticstandards may not have a requirement. For example, TIA-455-28C, Measuring Dynamic Strength and Fatigue Parameters ofOptical Fibers by Tension, does not incorporate a SEE limitation 3 .Since this document may be referenced in some industrystandardized fiber specifications, harmonization of the TIAstandard with international standards is desirable. The TIAdocument also does not include the two-point bending method,which should be included as an acceptable alternative dynamicstress corrosion susceptibility measurement method.3. Use of n in Lifetime Estimates3.1 Impact of nThe significance of the stress corrosion susceptibility (n) isrealized in reliability equations. When a fiber is under bend, theouter cladding surface will experience stresses that could be inexcess of the proof test value. The radius of the bend must besufficiently large to ensure that the fiber remains in the extrinsicstrength region of the fiber. Under that bend condition, extrinsicflaws at the surface of the cladding will be susceptible to fatigueand failure when under bend. The allowable bend stress (σ a ) towhich a fiber can be subjected for a specified period of time (t a ,the design lifetime) with a defined failure probability (F) isdefined in Equation 1 4 .σ⎡(n−2)1/ ntpln(1 F) me⎛ ⎞ ⎢⎧− ⎫a = σp⎜⎟ ⎨1− ⎬ −1a ⎢p b⎝ t ⎠⎩⎣L * N * C ⎭⎤⎥⎥⎦1/ nIn Equation 1, σ p is the proof test value, t p is the dwell time for theproof test, N p is the proof test break rate, L*C b is the effectivelength of the fiber that is under bend, and m e is the Weibull slopefor the extrinsic strength region. This equation is typicallyconfigured to represent the lifetime at a specified bendradius/bend stress with a defined part-per-million (ppm) failureprobability. A more meaningful representation of Equation 1expresses the failure probability (F) in terms of the bend stressand design lifetime, which is presented in Equation 2.⎡F = 1−exp⎢⎢L * N⎢⎣p⎧⎪ ⎛ σ* Cb* ⎨1−⎜⎪ ⎝ σ⎩t ⎞+ 1⎟t ⎠na anp pmen−2This form of the equation is more suitable to demonstrate thechange in ppm failure probability at a specified bend radius as afunction of the stress corrosion susceptibility. As expected, theequation indicates that a higher n value will yield a decreasingppm failure probability. There are several key points about thisestimate, however, that must be taken into consideration whenassessing the actual impact of a measured n d change on the longterm reliability of a fiber that is under bend.There are documented concerns associated with the use of themeasured n d from IEC 60793-1-33 for lifetime/failure probabilityestimates. This standard specifically notes in Section 4.2, “Theuse of stress corrosion susceptibility (and proof stress) parametersfor reliability estimates is still under consideration. A method forextrapolating such parameters to service environments differentfrom the default environment specified above has not beendeveloped. It has been observed that the value of n produced bythese tests can change after even brief exposure of the fibre toelevated temperature and humidity.” Therefore, a higher⎫⎤⎪⎥⎬⎥⎪⎭⎥⎦(1)(2)International Wire & Cable Symposium37Proceedings of the 60th IWCS Conference

measured n d value is not assured to translate into enhancedreliability in the deployed system. This is consistent withprevious work that recommended the use of n values based on theevaluation of large flaws at low stress rates and the two-regionpower law theory 5 . Furthermore, the difference in measured n dfrom different industry standardized and approved test methodsdemands that proper engineering judgment must be used whenapplying a measured n d value to lifetime estimates.In some cases, the measured change in n d can be attributed to acause that is based on a fundamental understanding of glassperformance. Glass composition, product enhancements thatpermanently prohibit or significantly reduce the diffusion ofmoisture to the glass surface (e.g., hermetic coatings), andenvironmental conditions are typically accepted as characteristicsthat will cause actual n d to vary. In some cases, the cause for theincrease in the measured n d value is not fully understood. Forexample, reference 6 attributed the increase in n d to thecharacteristics of the coating system. While a potentialmechanism was suggested, the actual cause for the observedchange in measured n d values was not definitively known. Untilthe increase in measured n d can be directly attributed to a specificmechanism that explains the observed improvement in fatigueresistance and industry experts accept the explanation, no benefitshould be assumed in lifetime modeling.Another consideration in the use of measured n d is the magnitudeof the failure probability at a given bend radius. In general, themagnitude of the change in the failure probability with n value isslight. Figure 5 shows the projected failure probability over fortyyears for four different bend diameters. All of the projectedfailure probabilities for a single full turn are two ppm or less.Even at 5-mm bend radius, the projected failure probability isreduced from about 1.5 ppm to about 0.6 ppm for an assumedchange in n from 20 to 30. Larger radius bends have even smallerprojected failure probabilities with this equation. The magnitudeof the failure probability is already exceptionally low whencalculated by applying the n value with the conservatism that isdemanded by the industry standard. The practical benefitassociated with the assumption that higher n values can be appliedis negligible.5Failure Probability versus n d at Small Bend RadiiOne Full Turn for 40 Years3.2 Impact of Extrinsic Strength/N pEquation 2 requires the proof test break rate during themanufacturing process, N p , to estimate a failure probabilityestimate at a given bend radius. Implicit with the use of N p is thatthe proof test break rate accurately represents the extrinsicstrength distribution that is susceptible to breakage at small bendsover the design lifetime of the fiber. Previous work has indicatedthat this assumption is not implicit. Reference 7 noted that theproof test distribution might be multi-modal. As a result, theextrinsic strength distribution is not assured to be represented bythe proof test break rate. Additional measurement work isnecessary to ensure that the proof test break rate will adequatelyrepresent the extrinsic strength distribution.To enable effective lifetime modeling for a fiber under bend, theuse of a long gage length test device that can stress significanttotal lengths of fiber (hundreds of kilometers) to well above theproof test value should be used to capture the extrinsic strengthdistribution of the optical fiber. Figure 6 contains the results oflong gage length testing on several different optical fiber products.The fibers are differentiated by manufacturing process andcladding characteristics. Upon inspection, it is clear that severalof the products have significantly different extrinsic strengthdistributions. The use of modeling tools, such as the two-regionpower law, can be used to process the data and determine if theproof test break rate can be accurately projected. The data inFigure 6 can also be used to determine an extrinsic Weibull slope(m e ), which can provide a more refined estimate of actual failureprobabilities under bend conditions.Failure Probability0.010.0010.00010.00001Product 1Product 2Product 3Product 4Product 520 m Gage Length Failure StrengthFailure Probability (ppm)4321019 20 21 22 23 24 25 26 27 28 29 30 31n d5-mm 7.5-mm 10-mm 15-mmFigure 5 – Plot of failure probability versus n d at 5, 7.5,10, and 15 mm bends. The results show low failureprobabilities and little change in F with varying n d0.000001Failure Stress (kpsi)Figure 6 – Plot of post proof test extrinsic strengthdistribution of different optical fiber products. The fiberwas subjected to long gage length (20 meter) testing ata high stress rate. Disparity in failure rates is observedamong several of the products.3.3 Impact of Proof Test on ReliabilityAnother issue that is related to the extrinsic strength distributionof bend insensitive fibers is the proof test level that is required toenable long term reliability. The standard proof test level, 100kpsi, has been driven primarily by the long term reliabilityrequirements for long lengths of deployed fiber. A higher prooftest level of 200 kpsi has been suggested for bend insensitivefibers. This higher proof test level, however, may simply increasecost with minimal benefit in projected reliability under bend. Forexample, the maximum post-proof test flaw size that will survivea 40 year lifetime at a 10-mm bend radius (minimal bendInternational Wire & Cable Symposium38Proceedings of the 60th IWCS Conference

insensitive fiber bend radius requirement in ITU-TRecommendation G.657.A1) will exceed 200 kpsi. Therefore, theproof test will not eliminate all of the extrinsic flaws that couldyield when subjected to the design bend of a RecommendationG.657.A1 fiber for 40 years. As the minimum bend diameter isreduced, the size of the largest surviving flaw gets smaller. A 200kpsi proof test will not eliminate all of the extrinsic flaws thatcould fail when deployed at those reduced minimum design bendradii for a 40 year lifetime. Therefore, long term reliability insmall bend deployments is driven by the extrinsic flawdistribution. Optical fibers with superior strength distributionsgain little benefit through exposure to a 200 kpsi proof test. Infact, the higher stress test and additional handling may only serveto damage the fiber or fatigue existing extrinsic flaws, whichcould increase the long term failure probability.4. ConclusionsIndustry standards cite two methods for the measurement ofdynamic stress corrosion susceptibility - axial tension and twopointbending. Both can provide reliable trade and commercevalues for n d , but the two-point bending method values areapproximately 2 units higher on average than the reference testmethod, axial tension, when the same fibers are evaluated. Agood SEE value is important to ensure a reliable n d value, but theactual results should be closely reviewed for credibility, even ifconforming SEE values are obtained. The two-point bendingmethod will tend to yield better SEE values due to the shortevaluated gage length, but successful results from the referencetest method should be obtained first to ensure that localizedstrength issues are not masked. Due to the variability in the n dmeasurement method and the disparity in measured n d valuesbetween methods, a conservative n d value is recommended for usein lifetime modeling. The suggestion of any benefit in n d shouldbe attributable to a specific attribute or mechanism that isaccepted by experts within the industry. The extrinsic strengthdistribution of the fiber is a critical attribute in reliabilitymodeling. Actual long gage length testing is required tocharacterize the extrinsic strength distribution and validate the useof proof test break rates and to understand better the long termreliability under small bend conditions.5. AcknowledgementsSpecial thanks to Eric Argentieri for the stress corrosionsusceptibility measurement work that supported this paper.6. References[1] IEC 60793-1-22, Measurement Methods and Test Procedures– Stress Corrosion Susceptibility, First Edition, 08-2001[2] Long Han, Xiaosong Wu, et. al, Characterization of TensileProperties of Optical Fibers Coated with a New GenerationCoating System and the Comparison of Fatigue Behavior byTensile Test and Two Point Bending Technique, 59 th IWCS[3] TIA-455-28C, Measuring Dynamic Strength and FatigueParameters of Optical Fibers by Tension (2005)[4] Pieter Matthijsse, Willem Griffioen, Matching Optical FiberLifetime and Bend-Loss Limits for Optimized Local LoopFiber Storage, Optical Fiber Technology 11 (2005) pp 92-99[5] T. Volotinen, A. Breuls, N. Evanno, K. Kemeter, C.Kurkjian, P. Regio, S. Semjonov, T. Svensson and S.Glaesemann, Mechanical behavior and B-value of anabraded optical fiber, Proc. of 48 th IWCS, (1998), p. 881 -890[6] Xinwei Qian, Ruichun Wang, et. al, ContinuousStrengthening of Optical Fiber over Time – Contribution ofNext Generation Coating System to Dynamic FatigueImprovement, 59 th IWCS[7] A. Paul, G. S. Glaesemann, An Appraisal of MechanicalReliability Predictions for Optical Fibers Based on BreakRates, 46 th IWCS (1997)7. Author BiographiesJeffrey J. Englebert started his career inthe optical fiber industry with Siecor(now Corning Cable Systems) in 1992,where he served in a variety of technicaland supervisory positions. In 1999, hetransferred to Corning’s Center forFiber-Optic Testing (CFT) in Corning,NY where he was assigned Supervisor,Optical/Physical Measurements Groupand Supervisor, CFT. He is currentlythe Manager, Product Engineering/CFTfor Corning Optical Fiber. Prior to hiswork in optical fiber, he served as a nuclear submarine officerwith the US Navy. Jeff received a bachelor’s degree in chemistryfrom Northwestern University.Amy Millett is a senior productengineer for Corning Optical Fiberwhere she is responsible for designcontrol of new and existing opticalfiber product offerings. She joinedCorning in 2007 in their Life Sciencesdivision where she held positions inprocess and quality engineering. Sheassumed her current responsibilities in2010. Prior to joining Corning, Amyworked for Hutchinson Technology,where she held a number of roles inquality, applications, and product and process development. Amyreceived a bachelor’s degree in mechanical engineering fromCornell University.International Wire & Cable Symposium39Proceedings of the 60th IWCS Conference