Improvement of the precision (repeatability and reproducibility) of a test
method to characterize microbending performance of optical fibers
Long Han
1, Pratik Shah
1, Jackie Zhao
2, Xiaosong Wu
1, Steven R. Schmid
11
DSM Functional Materials, 1122 St. Charles St., Elgin, IL 60120, USA
+1-847-608-2532 [email protected]
2
DSM Functional Materials, Zhangjiang High-Tech Park, Shanghai, 201203, China
Abstract
Microbending performance of optical fibers has significant impact on cable manufacturing, network deployment and maintenance. However, no international standard method has been established due to the difficulty of obtaining microbending testing results with good repeatability and reproducibility. A Six Sigma methodology was applied to address the repeatability and reproducibility issues concerning a test procedure which is based on IEC TR 62221 Method B. In this work, the entire microbend testing procedure was treated as a process. Factors from the process inputs and key steps of this process were studied extensively. Effectively addressing these key factors may enable the achievement of repeatable results within a single lab as well as between different labs.
Keywords
: microbending, Six Sigma, IEC TR 622211. Introduction
Microbend testing is a very important method of characterizing the optical performance of optical fibers. Microbend testing has proven to be an effective tool for use in the development of optical fibers, fiber coatings and cables [1-3]. However, no industry standard specification or test method exists. It is believed that “the test repeatability is not sufficient to use these methods in trade and commerce.” IEC TR 62221 [4] introduced four methods to qualitatively evaluate the microbend performance of optical fibers. It states that “the results from the four methods can only be compared qualitatively.”
It is suspected that the heretofore observed deficiencies in the repeatability of microbend testing is mostly related to the two coating layers of the optical fiber, since the glass core and cladding of an optical fiber are quite stable under most testing environments. This study is an effort to improve microbend testing repeatability from the point of view of optical fiber coatings. In this study, improvements to microbend test repeatability were made by understanding and controlling the impact of many factors on the properties (especially mechanical properties) of optical fiber coatings. This study is focused on method B of IEC TR 62221 due to its simple setup and convenience to implement.
A Six Sigma methodology was adopted to help capture all possible factors which affect the test variation of the microbend testing. In addition, this tool helps quantify the impact of each factor. Understanding of the quantitative impact of different factors is the key to understanding and controlling the repeatability (or test variation) of the microbend test.
A SIPOC (Suppliers, Inputs, Process, Outputs, Customers) diagram is a typical tool in Six Sigma methodology employed in studying a process. When a microbend test is treated as a process, a SIPOC diagram (Figure 1) is very helpful in identifying factors which affect the output of the process – microbend test results (including the test variation). One can check the inputs at each step of this process for potential impact on the final results. For example, ambient humidity and temperature conditions are inputs for this process. Therefore, we need to understand the quantitative impact of humidity and temperature on the testing results.
Figure 1. SIPOC diagram of microbending testing process
The following sections of this paper demonstrate the quantitative impact of some selected key factors.
2. Experimental
2.1 Samples
Commercially available optical fibers were chosen for this study. These optical fibers were stored under ambient conditions and then preconditioned for at least 12 hours at 23±2°C and 50±5% relative humidity.
2.2 Microbend testing
The test setup described in the Method B of IEC TR 62221 was used as the starting point of this study. Figure 2 demonstrates the setup for the microbend test. A self adhesive sand paper (P320
grit) was applied to the surface of a quartz drum (length 280mm x outer diameter 280 mm) such that it covered the drum from edge to edge. A length of 450 meter of an optical fiber was wound onto the quartz drum to form a single layer with a winding tension of 100±2 gram, a winding speed of 50±5 m/min, and a pitch of 0.50 mm.
Figure 2. Microbending test setup
After the winding, the quartz drum (wound with optical fiber) was moved to a controlled environment (23±2°C and 50±5% relative humidity). One end of the wound optical fiber was connected to the injection fiber of an OTDR (Optical Time-Domain Reflectometer), which contains at least 2 km of optical fiber. A precision cleaver was used to cut the fiber ends for connection. A digital tension meter with a resolution of 0.1 gram was used for monitoring winding tension. An environmental chamber was used to control the temperature and the humidity effects.
The attenuation loss for a section of 400 meters of optical fiber on the quartz drum was recorded 120 minutes after the winding process commenced. The backscatter technique defined by method C of IEC 60793-1-40 [5] was used to take the measurement. The test is repeated at least twice with new winding, with the average result to determine the final value.
3. Results and discussion
3.1 Humidity and time effect
Figure 3 demonstrates the time and humidity effects for an optical fiber when a test is carried out at 23°C. The reported attenuation loss (or Least Squares Approximation loss) is measured at a wavelength of 1550 nm. The attenuation loss was monitored up to 20 hours after winding. In general, the attenuation loss kept increasing over time. It increased much faster immediately after the winding process. Taking the test results at 76% relative humidity (RH) as the example, attenuation loss increased from 0.70 to 1.90 dB/km 4 hours after the winding, while it only increased from 1.90 to 2.10 dB/km between 4 hours and 20 hours. This suggests a clear time dependency when taking the measurement. Taking into consideration this observation and also from a practical point of
view, 2 hours after the winding was selected as the initial time for taking a reading.
CM-3 1550 nm Humidity effect at 23°C 0.50 0.70 0.90 1.10 1.30 1.50 1.70 1.90 2.10 2.30 0 200 400 600 800 1000 1200 1400 Time elapsed [min]
L SA Los s [dB/km ] 76% RH 49% RH 31% RH
Figure 3. Time and humidity effect on the microbending test results
A strong humidity effect was also observed on the microbending test results. For example, the attenuation loss at 100 minutes after winding was approximately 0.7, 0.9, and 1.5 dB/km at 31%, 49%, and 76% relative humidity respectively. There is a big leap in attenuation loss when the relative humidity is increased from 49% to 76%. This finding has significant practical interest. Humidity levels will vary as a consequence of seasons and geographic locations. It would be impossible to repeat the results if the humidity is not adequately controlled for this test. To align with the typical testing condition for optical fiber, 23±2°C and 50±5% relative humidity were selected as both the preconditioning and testing environment.
3.2 Winding tension effect
CM-1 1550 nm 50 m/min Tension Effect
0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 0 60 120 180
Test Time after Winding [min]
LS A L o ss [d B /k m] 100 gram 105 gram 95 gram
Figure 4. Winding tension effect on the microbend testing results
Method B of IEC TR 62221 calls for a winding tension of 3 N. In the studies reported here, the fiber tended to break on the drum either during or after winding with a winding tension of 3N. At the same time, microbend attenuation loss is very sensitive to the amount of tension applied. Eventually, 100 gram was selected as the standard winding tension.
Figure 4 shows the sensitivity of microbend attenuation loss to slight deviations in winding tension. For the fiber shown in the graph, the attenuation loss is about 1 dB/km with a winding tension of 100 gram, 120 minutes after winding. This attenuation loss increases to 1.05 dB/km with a winding tension of 105 gram at 120 minutes after winding, but decreases to about 0.95 dB/km with a winding tension of 95 gram. In other words, a difference of 5 gram winding tension led to 0.05 dB/km change (5%) in the attenuation loss. Therefore, it is critical to have strict control of the applied winding tension.
Figure 5. Study of the reading of the winding tension with 3 operations and 2 tension meters
The winding tension during the winding process was measured with a tension meter. To ensure that different operators read the results in the same way, a digital style tension meter (with a resolution of 0.1 gram) was implemented to replace a traditional mechanical gauge style tension meter. However, it takes effort and proper measurement procedure to take the right measurement. Figure 5 shows the results of a study to illustrate the variation in readings obtained with 3 operators and 2 digital tension meters. During this study, the winding machine was set at a fixed tension. Each of the 3 operators measured the winding tension twice with random ordering by using each of the 2 tension meters. In general, it is common to see a 2 gram difference in the replicate readings for an operator with one tension meter. However, a 5 gram difference (operator B with tension meter 1) can be seen if the reading was not taken properly.
3.3 OTDR setting effect
A thorough study was carried out to understand the impact of the OTDR setting on the final results. Two additional factors found to be important include the OTDR calculation method of attenuation loss and the pulse width. In this study, a fiber was selected that had been wound on the quartz drum for more than 60 hours. Such a long waiting period was chosen to exclude the time effect from this study. Two calculation methods, 2 point (2pt) loss and Least Squares Approximation (LSA) loss, and seven different pulse widths (10, 50, 100, 200, 500, 1000 and
2000 ns) were selected for this study. Five measurements were made at each combination of calculation method and pulse width. The study results are shown in Figure 6. Note that the calculation of each error bar (95% CI) on the chart was based on the 5 measurements for each combination of factors. We can see from the Figure 5 that, in general, greater testing error is associated with the 2 point loss method than with the LSA test method. In addition, the dependence of the mean of the test results on the pulse width is more pronounced with 2 point loss method compared to the LSA loss method. It is obvious that the LSA loss method yields less test variation. Figure 7 shows the pulse width effect when using the LSA loss method. It essentially shows the right half of Figure 6 in more details. In general, the test error decreases with the increasing pulse width. A pulse width of 200 nanoseconds (ns) was selected as the standard setting for all measurements since the 95% confidence interval (95% CI) was only slightly more than 0.01 dB/km, or 1% of the mean value of 1.25 dB/km. pulse width [ns] LSA l oss [dB/km ] 2pt loss [dB/km] 2000 1000 500 200 100 50 10 2000 1000 500 200 100 50 10 1.35 1.30 1.25 1.20 1.15 1.10 1.05 1.00 D a ta
Interval Plot of 2pt loss [dB/km], LSA loss [dB/km] 95% CI for the Mean
measurements were taken more than 60 hours after winding
Figure 6. OTDR setting (calculation method and pulse width) effect 2000 1000 500 200 100 50 10 1.26 1.24 1.22 1.20 1.18 1.16 1.14 1.12 1.10 pulse width [ns] LS A lo ss [ d B / km ]
95% CI for the Mean
Interval Plot of LSA loss [dB/km] for CM1 pulse width effect
measurements were taken more than 60 hours after the winding
Figure 7. OTDR setting (pulse width) effect
3.4 Verification of improvement on test
repeatability
To evaluate how effectively the test repeatability was improved, two fibers with similar microbending performance were selected to see whether this method could clearly differentiate their microbending performance. Each of these two fibers was wound around the sandpaper covered core 6 different times for measurement under identical conditions. Figure 8 shows the results of those 6 individual trials for each fiber. Figure 9 is the interval plot of microbending loss for these two fibers. The error bars of 95% CI clearly differentiate the microbending performance of these two optical fibers. In fact, a 95% confidence interval of around 0.05 dB/km indicates that excellent repeatability has been achieved with this improved method.
Figure 8. Individual value plot to demonstrate the test repeatability
Figure 8. Interval plot to demonstrate the test repeatability
4. Conclusions
This study demonstrated the quantitative impact of some important factors on the final results of the microbend test. Through careful control of important factors and details surrounding each testing step, we were able to achieve excellent testing repeatability.
We conclude that temperature and humidity must be controlled for fiber preconditioning and testing. Testing result is sensitive to time and 2 hour of waiting time is selected to ensure good repeatability. We also propose a winding tension of 100±2 gram
for these single-mode fibers, since higher values might initiate breaks. The quality of the sand paper is another important factor. The OTDR settings are proposed to be chosen as LSA-loss and pulse width of 200 ns. These values might differ for other fiber classes.
This is an ongoing study and the ultimate goal is to achieve good repeatability and reproducibility not only within one lab but also between different labs.
5. Acknowledgments
This collective study would not be possible without the collaboration of many people. The authors would like to thank Loretta Lawrence, John Holmstrom, Todd Anderson and Mike Scianna for their help in testing.
The authors would also like to express their appreciation to Dr. Bertil Arvidsson (Fiberson AB) and Dr. Huimin Cao for the insight they provided through discussions.
The authors wish to express their appreciation to Margaret Brumm and Dr. Bertil Arvidsson for their review of the paper
6. References
[1] J. A. Jay, “An overview of macrobending and microbending of optical fibers”, White Paper WP1212, Corning, December 2010 (2010).
[2] S.R. Bickham, et al, “Ultimate limits of Effective Area and Attenuation for High data Rate Fibers”, OWA5, OFC2011 [3]
Y. Yamamoto, et al, “A New Class of Optical Fiber to
Support Large Capacity Transmission”, OWA6,
OFC2011
[4] IEC TR 62221 “Optical fibers measurement methods -microbending sensitivity”, 1st Edition, October 2001 (2001). [5] IEC 60793-1-40 “Measurement methods and test procedures
– Attenuation”, 1st Edition, July 2001 (2001).
7. Author Information
Dr. Long Han is Applications Development and Testing Manager in the Fiber Optic Materials Group of DSM Functional Materials and a Six Sigma black belt. He joined DSM in 2005 and is involved in liquid and solid rheological characterization, optical fiber properties characterization, statistical analysis and design of experiments. He has earned a B.S. Degree in Chemical Engineering from Tsinghua University in China, an M.S. Degree in Statistics from West Virginia University and a Ph.D. degree in Chemical Engineering from West Virginia University in 2005.
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 the 2008 R&D 100 award. He is a named inventor on 4 U.S. patents and (co)author of more than 10 publications.Jackie Zhao joined DSM in 2008 as Technical Service Manager- Asia-Pacific region in the Fiber Optics Materials Group. He has earned a B.Sc. in Applied Chemistry from Peking University in China, in 1991. In 2005, he obtained a Master Degree in Material Sciences and Engineering from Shanghai Jiaotong University in China. He worked for Lucent Technologies fiber optics in China for 6 years and worked for Hengtong Alpha fiber optics in China for 8 years as technical department manager with total 14 years experience in optical fiber drawing technology.
Dr. Xiaosong Wu has a B. Sc. in Polymer Chemistry from University of Science and Technology of China in 1993. In 2000, he obtained a Ph.D. in Photochemical Sciences from Bowling Green State University in Ohio. In the same year, he joined DSM Desotech, Inc. (now DSM Functional Materials) as a research scientist. Currently, he holds the position of R&D Manager for Fiber Optical Materials at DSM Functional Materials in Elgin, Illinois.
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 30 years experience in the UV coatings industry and has authored over a dozen papers, is a named inventor on 10 U.S. patents and made several international presentations. He was a co-recipient of an IR100 Award in 1987 and also a co-recipient of DSM’s Special Inventor Award in 2001.
