A fiber design is proposed that addresses bend loss, high power and handleability concerns for fibers deployed in high power devices and associated pigtails. If bending is tight enough to leak power from the core into the glass cladding, a coating with a sufficiently low index traps this potentially dangerous power in the cladding. The coating is also designed to keep cladding power from damaging downstream devices and terminations. Loss due to tight bends is minimized by use of a glass design with a small mode-field diameter. This glass design further suppresses the high power failure mode. Over the high power coating is a thick layer of a toughened polymer coating which provides a significant increase in mechanical protection over most commercial coatings.
Optical fiber networks are being developed that require higher optical power levels. Examples include long haul communication with Raman amplification and fiber to the premises. Previous studies indicate that tightly bent optical fiber can mechanically fail when exposed to high optical power levels. In an extreme case where fiber is sharply bent and subjected to a power level of 1 to 2 W in the near-infrared wavelength window, optical fiber can fail in minutes. It also has been shown that time to failure decreases with increasing bend stress and optical power. This study is a further investigation of the physical events leading to failure. Previously we demonstrated that the optical signal that escapes the core of bent fiber passes into the coating, where a small amount is absorbed and converted to heat. As a result the coating can be heated to significant temperatures resulting in degradation over time. This paper focuses on several key aspects of the failure kinetics associated with bent fiber under high power. As a result of bending, optical power leaked from the core is distributed in the glass cladding and polymer coating. We have modeled this power distribution and compared it with measured temperature profiles in the coating. The results show that this redistribution of the power is key to establishing the distribution of temperature in the coating and ensuing degradation. This understanding is used to design glass and coating solutions for inhibiting this potential failure mode.
Static fatigue of bare indented fibers in different environments was studied. It was found that the n-value in different pH-solutions did not significantly change and was higher than that for strong fibers and lower than that for bulk samples in similar conditions. All lifetime reduction with a pH increase was due to a change of the B-value. The results obtained were used for evaluation of the lifetime of weak fibers for different service environments.
The failure of tightly bent optical fiber under high optical power is observed dynamically with fine time resolution and explained in terms of the behavior of the polymer coating and underlying glass. An abrupt rise in coating temperature stimulates the viscoelastic deformation of the glass. The abrupt bending of the glass is explained by the ability of highly quenched silica to deform at low temperatures. There is no evidence of thermal runaway of the glass core. Coating decomposition is self limiting with no visible flame.
A novel method for incorporating static fatigue results into dynamic fatigue curves allows one to significantly extend the range of experimental fatigue results for further analysis. One obtains a power-law description of static fatigue data over a narrow range of time-to-failure and subsequently translates the data into this region into dynamic fatigue data. This recalculation method applies to data that does not follow the traditional power law model.
Standard glass optical fiber was intentionally damaged and strength tested in tension for a wide range of failure stress values. The mirror/mist boundary on each specimen was measured and compared to the measured failure stress. When the size of the fracture mirror is small compared to the fiber diameter, the well-known linear square root dependence of strength on mirror size was reaffirmed. However, when the mirror size approaches the fiber diameter, this relationship does not hold. The classical relationship overestimates the failure stress for large mirrors. It was determined that the circular shape of the fiber contributes to, but can not fully account for, departure from the classical strength/mirror relationship. A new empirical relationship was developed to enable predictive determinations of failure stress to levels as low as 20 to 25 kpsi (0.14 to 0.17 GPa).
A technique was developed for obtaining proof-stress level flaws in commercial optical fiber with low variability in strength. It involves a novel method for stabilizing and protecting the round fiber prior to indentation. Indentation was performed in an automated fashion using a nano-indenter equipped with a cube-corner indenter. A Weibull modulus of 50 was achieved with a value of 100 over the lower portion of the distribution. This method will be useful in static fatigue testing of fiber with proof test level flaws.
Comparison of high-speed strength data for weak (abraded, contaminated and indented) and pristine fibers was performed. It was shown that fatigue behavior of abraded fiber practically coincides with that of the fiber contaminated by zirconia powder and is close to that of indented fiber. The fatigue parameters obtained for strong pristine fiber cannot be used to obtain the correct prediction of fiber strength after proof testing. A two-region power law model was used for mathematical description of these results and the fatigue parameters for three types of weak fibers were obtained.
This study presents results from an experiment where Bragg gratings were used to measure the stress on a bare optical fiber placed inside a 3M VF-45 connector. The connector exerts lateral compressive and longitudinal tensile stresses on the fiber, both of which yield a shift in the Bragg wavelength. The axial stress is of interest for mechanical reliability predictions, so additional measurements were performed in order to separate the two effects. This involved simulating the use of an alternate material in the connector to induce differential longitudinal stress on the fiber grating.
It was obtained that (t2Si2)-((sigma) s/Si) and ((sigma) d/Si1)-((sigma) '/Si3) are universal coordinates for presentation of static fatigue and dynamic fatigue data respectively. Usage of these coordinates helps to correctly compare the results of tests of different kinds of fibers (strong and weak) regardless of the initial defect size. presentation of the dynamic fatigue data for pristine and indented fibers in universal coordinates showed a very similar behavior of both fiber types in spite of their difference in strength.
This paper reviews recent advancements in the area of mechanical reliability of optical fiber, building upon previously published reviews in this area.1-5 Of particular significance is progress in the area of establishing reliability programs. This review considers efforts made to reduce early failures and to establish reliability programs with an emphasis on functional reliability.
High speed strength testing of proof test level flaws in optical fiber was accomplished using a conventional belt slide and a piezoelectric load cell. Stressing rates of 10,000 kpsi/s were achieved and fractographic analysis of fracture surfaces confirms the measured strengths. Even at such high stressing rates, little evidence of the inert strength was found. However, significant curvature in the log strength versus log stressing rate data suggests the presence of region II crack velocities.
A ratio of inert to ambient strength >= 1.5 is suggested for large flaws in optical fiber near the proof stress level. Also, a temperature dependent strength at low temperatures, similar to that observed in pristine fibers, was investigated. An increase in fracture toughness or changes to the crack tip geometry at low temperatures did not account for the increase in strength at low temperatures. Inert strength distributions were predicted from strength obtained under ambient conditions for as-manufactured fiber and fibers with handling damage.
It is found that by abrading an optical fiber surface, a controlled strength distribution can be produced that is effective for dynamic fatigue testing of large flaws. Dynamic fatigue testing of 30 kpsi flaws overcoated with carbon yielded n values greater than 100. It is believed that the abraded fiber samples can be utilized as a surrogate for measuring the fatigue parameters of carbon coated splices and flaws near the proof test level. In addition, it was found that overcoating abraded fiber with carbon increased the strength by a factor of two. This increase is more than what would be predicted by the increase in fatigue resistance that comes with carbon overcoating, and suggests deposition of carbon alters the flaw geometry or stress field surrounding the flaw.
An engineering methodology for estimating the mechanical reliability of optical fiber is being developed within a fracture mechanics framework. The
model, one of several currently under development, expresses allowable inservice stresses as a fraction of fiber strength in a fatigue environment for a
range of n values and fiber types. Of particular importance is the need for obtaining strength data on fiber lengths appropriate for the application.
An engineering methodology for the mechanical reliability of optical fiber is developed within a fracture-mechanics framework. The model expresses allowable in-service and installation stresses as a fraction of fiber strength in a fatigue environment for a range of n values and fiber types. Failure probability is incorporated into the model by the measurement of the fiber-strength distribution appropriate to the application. For long-length applications, strength distributions of hundreds to thousands of kilometers of fiber are needed. A 400-km strength distribution captures the beginnings of the truncated portion of the distribution.
A phenomenological formulation of Si-O bond dissociation is utilized to interpret stable crack velocity and static and dynamic fatigue phenomena. The resulting model has an exponential form and is applicable to a wide range of flaw sizes, service stresses, and test environments. Furthermore, it is readily reduced to the power law by retaining the first term of the series expansion of the exponential function. The model provides a sound physical basis for comparing different fiber compositions, service environments, and stress-time histories (static versus dynamic) from the fatigue point of view. The application of the model to silica and titania-doped silica optical fibers provides valuable insight into their relative fatigue behaviors and sheds further light on the fundamental mechanisms controlling such behavior.
A technique for measuring the fiber strength distribution of many kilometers of fiber was developed. The strength distribution of 100 km can be generated in a week's time by a single operator. A strength distribution for 400 km of fiber is shown to depart from the high-strength region around the 1% failure probability level and the data exhibit the beginnings of the truncated portion of the strength distribution. Such data are believed to be useful in making failure probability predictions, process improvements, and aid in the understanding of crack-growth behavior during proof testing.
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