Effect of ageing conditions on performance properties of selected commercial fibers

Transcription

1 Effect of ageing conditions on performance properties of selected commercial fibers Pratik Shah, Long Han, Ed Murphy, Steve Schmid, Daniel Peterson DSM Functional Materials, St. Charles St., Elgin, IL USA Verizon, Richardson, TX USA Abstract The literature contains years worth of reports on the exposure of optical fibers to various ageing environments, yet absent is a universally agreed upon accelerated ageing condition useful in predicting long term fiber performance. During the past + years, several new categories of optical fibers have been introduced, largely based on innovations in glass technology, which include low-water peak, non-zero dispersion and bend insensitive fibers. Also within this period, innovations in optical fiber coating technology have resulted in the introduction of a new generation of coatings that improve both the optical and mechanical performance of optical fiber. To keep pace with these innovations, various standards organizations are discussing improvements and changes in test methods to adequately characterize these innovations. An area of particular interest is accelerated ageing under combinations of temperature and humidity that do not create unrealistic ageing conditions. We propose to introduce a set of realistic temperature and humidity combinations and show how they might be used to accelerate testing in a more reliable manner by introducing microbend evaluations. Results of five-month long accelerated ageing study, involving commercial G.6, G.6 and G.67 fibers, will be discussed and an attempt will be made to correlate changes in coating mechanical properties with changes in optical microbending losses. Keywords: Acrylate; Coating; Ageing; Microbend; Fiber; Measurement; OTDR.. Introduction Previous ageing investigations have focused on the changes in fiber mechanical properties. The power law of reliability was published as a technical report (TR), in International Electrotechnical Commission (IEC) about years ago []. It has now been revised with some minor corrections, but no major technical changes. It provides information about lifetime calculations and fiber fatigue. The most comprehensive work on fiber reliability is found in T. Volotinen et.al. [].This is the final report issued in 999 by COST 6, a European research program. The members of COST 6 came from an international arena. Some were also members of IEC. In Section we will present the current status in IEC about ageing. Then we continue with the chosen fibers in Section. In Section we give details about our motivations and test program. For our study we have used different temperatures and humidity conditions and microbend test method B as described in [], but with some modifications. Section will contain our conclusions. For convenience we have collected an overview of the test results in Annex A and B. In Table below we present the origin/class of the fibers in the study.. Ageing in IEC A number of test methods are standardized in IEC as described in [,,6,7]: - Damp heat 8ºC/8% RH - Dry heat 8ºC - Change of temperature between -6ºC and +8ºC - Water immersion +ºC These tests have been standardized for many years and discussions have started in IEC whether they constitute a realistic set of tests. It has been suggested that water immersion and dry heat ageing conditions may no longer be required. [8]. However, there is reluctance in the industry to give up well experienced test methods. On the other hand it is inescapable not to analyze the present methods in IEC.. Evaluated Fibers The chosen fibers for the study are both old and new and of different classes, as shown in Table. Focus was on classes G.6 and G.67, but G.6 has also been included. Table. Fibers in the study ID Class Coating G G6B old G G6 old F G6 old T G6 old AK G67A new AM G6D new AO G6D new AP G6D new AQ G6 new AR G6 new

2 Another parameter of interest in the study is the MAC-number, i.e. mode field diameter (MFD)/ fiber cutoff. We have only used MAC for nm MFD and for G.6- and G.67-fibers, see Table. A lower MAC-number indicates less macrobend sensitivity. Table. MAC-numbers ID MFD at nm Cutoff - (nm) MAC G G- x 9 x F T x 98 x AK AM AO AP AQ x 8 x AR x 6 x With the combination of old and new coatings, different fiber categories and MAC-numbers we have good tools to analyze the microbend results in this study.. Test program. Damp Heat Methods As was indicated in Section, dry heat and water immersion did not seem to provide any new information in the evaluation of current commercial fibers. Therefore we have introduced a set of realistic temperature and humidity combinations: +ºC/8% RH and +6ºC/8% RH in two different chambers, see Figs,. We also exposed the fibers to +8ºC/8% RH in a third chamber. Each chamber contained a minimum of ten different fiber spools of sufficient length to run a microbend test on a monthly basis. In this way we evaluated the effects of accelerated ageing in a broader scope by also introducing microbend evaluations. Figure Chamber at +6ºC/8% RH. Microbend Test Method In IEC TR6, currently under revision, four types of microbend test methods are described []. For this study, method B was selected. In each microbend test, about meter fiber was wound around a quartz drum coated with sandpaper as shown in Fig. A & B. With this technique it is possible to use the same fiber spool during the entire month program for each different fiber. Other important parameters are also described in [9], such as winding conditions, sandpaper roughness, and OTDR settings. Most importantly, these parameters were carefully controlled for each test occasion as described in [9], see Table below. Table Some important parameters and used values OTDR, Wavelengths:, and 6nm Pulse width/lsa: ns Winding tension: N Winding speed: m/min Winding pitch on sandpaper spool:.mm 6 Measured length on sandpaper: m 7 Sandpaper P grit 8 Sandpaper spool size: quartz drum 8mm X 8mm 9 Temperature/Humidity: C & -% RH Change of Sandpaper: no change of sandpaper on individual quartz drum for entire experiment, all microbending tests are done at room temperature Figure Chamber at +ºC/8% RH

3 The fibers AO and AP have significantly lower values during the whole test period, while fiber AM shows increased values during the same period. In Figure & 6 below we show the results at & 6 nm. Figure A Fiber winding on sandpaper drum... AO AP AP AM AM Figure Microbending results at nm, 6 C and 8% RH, for AO, AP and AM..... Figure B Sandpaper drum to the left. Microbend evaluations The microbend properties were measured for all fibers in the study using method B as described above. Supplementary figures are found in Section 9, Annex A. It is evident that both glass and coating properties are important in determining the microbend sensitivity of a specific fiber. In Figure below we find a simplified graph of microbending values at nm for three currently available commercial G.6.D fibers exposed to 6 C and 8% RH. Annex A gives data for all fibers in the study. Attenuation (db/km).. AO AP AM AO AP AM Figure Microbending results at nm, 6 C and 8% RH, for three different G.6.D fibers: AO, AP, AM AO AP AM Figure 6 Microbending results at 6 nm, 6 C and 8% RH, for AO, AP and AM The trend is similar as for nm. By studying results in Annex A we find similar results for all different temperature/humidity conditions. There is one exception AQ at nm. By analyzing Figures A A8 in Annex A we find some general trends. Surprisingly, most of the fibers show no major increase in attenuation as a function of time during the - mos. test period. Such an increase over time might be realized over a longer test period than that used in this study. Nevertheless, some fibers like AM did show an increase on aging. We did find that microbend attenuation became more pronounced as wavelength increased form nm to nm to 6 nm in virtually all fibers, Finally, the fibers with the lowest attenuation and highest stability were AO (G.6.D), AP (G.6.D) and AK (G.67.A).. Coating mechanical properties The process of ageing has been studied extensively in the past [8, ]. Elevated temperatures have mostly been used in these studies. In the present study we chose a combination of lower temperatures and 8% RH in an effort to find more realistic conditions. At each microbend test, samples were taken for material tests. To improve microbending sensitivity a combination of a lower modulus primary coating in combination with a harder secondary coating is strongly proposed []. All mechanical properties fulfil specifications as given in [, ] after (and during) the environmental testing.

4 Strip force F average:, F av, N Strip force F peak:, F p 8,9 N Stress corrosion susceptibility constant: n d 8 Tensile strength:, GPa Supplementary results are found in Section, Annex B. We also note in Section the in-situ modulus results for primary and secondary coatings.. Conclusion Long term testing, - months, was performed on different fibers in +ºC/8% RH and +6ºC/8% RH. We also exposed the fibers to +8ºC/8% RH (although not thought to be as realistic an ageing exposure for multiple months). The fibers were evaluated for microbend attenuation on a monthly basis. First we conclude that the different temperature and humidity conditions produced similar results. However, we suggest that +6 C/8% might be a more reasonable ageing condition, as harsher conditions could introduce unrealistic aging mechanisms and give misleading results under prolonged testing periods. Another conclusion is that Table MAC-numbers indicate indirectly that coating has a strong influence in our test results. This is obvious in particular for fiber F with high attenuation and AO and AP fibers with low attenuation. On the other hand it seems reasonable that glass properties of AK (G.67) have an influence on microbending as well. Testing was focused on different generations of commercially available G.6 fibers: year old (F), year old (G-) and currently available commercial fibers (AO, AP, AM). Results shed light on what might be seen in actual field conditions. Mechanical data (strip force, stress corrosion susceptibility constant and fiber tensile strength) were always measured within specifications [, ]. Additionally, a single G.67 and a few G.6 fibers were evaluated. The accelerated aging conditions employed in this paper surprisingly show, overall, no defined degradation of microbending performance or strip force reduction as expected. A few isolated field deployments, dating -yrs, have shown both. It is possible that this lack of substantiation of found field results is due to the limited -month-aging on bare fibers only, which is not sufficient to model what happens in the ground over a yr period to the tightly-buffered cable systems that are mechanically coupled to the glass. Tight-buffered cables employed thermoplastic extrusion over the UV acrylate-coated fibers. Possibly the changes in behavior of such fibers has to do with the differential aging mechanisms of the extruded thermoplastic and the UV cured coating. Several field observables have been seen on isolated cases of these cables aged over yrs. Some of these observables include random fiber breaking in hand holes, so-called brittle fiber, which is due to some level of pitting of the cladding material due to fiber coating issues, and seasonal attenuation fluctuations. A general trend has not been observed, but rather isolated cases. It may be interesting to subject the tight-buffered cable system to this same accelerated aging experiment to study effects on microbending and strip force. Telecommunication service providers are always monitoring and modeling the future of the performance and reliability of their networks. So, even though most of these old cables dating back to the late 98s have been retired, many remain in the ground for continued aging studies. Much can be learned by studying these aging cables that may be extrapolated to the future to better understand failure modes of different cable constructions. During such activities, various changes in performance of optical cables have been observed that have driven future designs of these systems. As proposed in [,] we observed that microbend attenuation will decrease as a function of lower primary coating ISM (in-situ modulus). Older generation fibers are generally protected with primary coatings having higher ISM and generally show correspondingly higher microbend sensitivity than currently available commercial fibers. However as seen in Fig. -6, the data indicates that there can be a - fold difference in the microbend sensitivity among current commercial fibers and therefore cablers and system operators need to ensure the proper level of performance when selecting fibers for microbend sensitive applications. In the future, it is possible that with ever increasing bandwidth demands and improvements in hardware that use of the 6nm wavelength beyond the current supervisory channel application may become important. The G.6 fibers have different glass designs (effective area) and the results are a combination of coating and glass properties, which is more difficult to differentiate between as compared to the various G.6 fibers in the study. Finally we conclude that the AO and AP fibers of the G.6 group demonstrated the best microbend performance, benefitting from the on-fiber designed coating properties. 6. Acknowledgments The authors would like to thank DSM Functional Materials for permission to conduct and publish these studies. We would also like to recognize the many associates who have aided with assistance or valuable suggestions; Kate Roberts, Todd Anderson, Loretta Lawrence, Hersh Agnihotri, Mike Scianna, Xiaosong Wu, Marie Lahoud and Olaf Storaasli. We also like to thank Dr. Bertil Arvidsson and his team from Fiberson for some valuable tests and discussion. 7. References [] IEC TR68, Power law of reliability [] T. Volotinen et. al., Reliability of Optical Fibres and Components (Final Report of COST 6), 999 [] IEC TR6, Microbending sensitivity, Ed., [] IEC 679--, Damp heat [] IEC 679--, Dry heat [6] IEC 679--, Change of temperature [7] IEC 679--, Water immersion [8] L.Terruzzi, D.Cuomo, Coating Ageing and Impact on Fibre Performances, IWCS 9

5 [9] B. Arvidsson et. al., Microbend evaluation of selected G6D & G67 fibers and ribbons before cabling, IWCS [] P.A. Högström, Aging of coating materials for optical fiber cables, Licentiate Thesis, Royal Institute of Technology, Stockholm, Sweden, 998 [] S.R. Schmid et. al., Development and characterization of a superior class of microbend resistant coatings for today s networks, IWCS 9 [] J. A. Jay, Optical fiber design to improve microbending resistance, Lightwave May/June [] Generic requirements for optical fiber and optical fiber cable, Telcordia Technologies, GR--CORE, Issue, May 8 [] IEC 679--, Optical fibres Product specifications Sectional specification for class B single-mode fibres 8. Pictures of Authors Ed Murphy Ed Murphy is a Senior Scientist and Project Leader in the Fiber Optic Materials Group of DSM Functional Materials. He has been with the company for 6 years. He has a B.S. Chemistry degree from Duquesne University in 969 and an M.B.A. degree from the University of Pittsburgh in 97. He is a named inventor on 8 issued U.S. or European patents, and coauthor of 6 technical publications. Pratik Shah Pratik Shah is Applications Development and Technical Service (Americas) Manager in the Fiber Optic Materials Group of DSM Functional Materials. He has a B.S. in Polymer engineering from Pune University, MS in Plastics engineering from University of Massachusetts and an M.B.A. degree from Anderson School of Management. He is a winner of a 8 R&D award. He is also a named inventor on U.S. patents and (co)author of publications. Steven R. Schmid Steve Schmid is currently Global Applications Development Manager in the Fiber Optic Materials Group of DSM Functional Materials. Previously, he held positions in research and development management, product management, market development and business management. He holds a B.S. Degree in Chemistry from the University of Illinois, a M.S. Degree in Chemistry from the University of Houston and M.B.A. from IIT. He has over years experience in the UV coatings industry and has authored over a dozen papers, been awarded patents and made several international presentations. He was a co-recipient of an IR Award in 987 and also a co-recipient of DSM s Special Inventor Award in. Long Han Dr. Long Han is an Applications Development & Testing Manager in the Fiber Optic Materials Group of DSM Functional Materials and a six sigma black belt. He joined DSM Desotech in and is involved in liquid and solid rheological characterization, optical fiber properties characterization, statistical analysis and design of experiments. He has a Ph.D. degree in Chemical Engineering from West Virginia University in, an M.S. Degree in Statistics from West Virginia University and a B.S. Degree in Chemical Engineering from Tsinghua University. Daniel Peterson Daniel Peterson is a distinguished member of the technical staff at Verizon. He is an internal advisor on optical technologies for Verizon's ULH network. He received a Ph.D. in electrical engineering from the University of Texas at Dallas. He is a senior member of IEEE.

6 9. Annex A Supplementary Microbending Data 9. Results at 6 C&8% RH. F, G- old generation G6; G-, T old generation G6.. F G- C- AM AO AP AQ AR G- C- T AK Figure A Microbending at nm for all ten fibers Attenuation (db/km)..... F G- C- AM AO AP AQ AR G- C- T AK Figure A Microbending at nm for all ten fibers Attenuation (db/km) F G- C- AO AP AM G- C- T AQ AR AK Figure A Microbending at 6 nm for all ten fibers

7 9. Results at C&8% RH... F C- G- AO AP AM G- C- T AQ AR AK Figure A Microbending at nm for all ten fibers Attenuation (db/km) (db/km) F G- C- AO AP AM G- C- T AQ AR AK Figure A Microbending at nm for all ten fibers F G- C- AO AP AM G- C- T AQ AR AK Figure A6 Microbending at 6 nm for all ten fibers

8 9. Results at 8 C&8% RH.... F G- C- AO AP AM G- C- T AQ AR AK Figure A7 Microbending at nm for all ten fibers..... F G- C- AM AO AP AQ AR T G- C- AK Figure A8 Microbending at nm for all ten fibers Note: 6 nm was not measured for the fibers in 8 C/8% RH.

9 . Annex B Supplementary Mechanical Data nd Value F G- C- AM AO AP AQ AR G- C- T AK Figure B nd values: 6 C & 8 RH Ageing nd value F G- C- AM AO AP AQ AR G- C- T AK Figure B nd values: C & 8 RH Ageing Peak Strip Force (N).... F G- C- AO AP AM AQ AR G- C- T AK Figure B Peak strip force: 6 C & 8 RH Ageing

10 Peak Strip Force (N).... F G- C- AO AP AM AQ AR G- C- T AK Figure B Peak strip force: C & 8 RH Ageing Fiber Strength (GPa) Fiber Strength (GPa) 7 6 F G- AP AO AM AQ AR G- T AK Figure B Fiber strength: 6 C & 8 RH Ageing Fiber Strength (GPa) 7 6 F G- AP AO AM AQ AR G- T AK Figure B6 Fiber strength: C & 8 RH Ageing

11 ISM (MPa)..... F G- C- AM AO AP AQ AR G- C- T AK Figure B7 In-situ modulus: 6 C & 8 RH Ageing ISM (MPa) ISM (GPa) ISM (GPa).. F G- C- AO AP AM G- C- T AQ AR AK Figure B8 In-situ modulus: 6 C & 8 RH Ageing ISM (GPa)