Often lightguide fiber processing involves steps that may cause degradation of very high strength or flaw-free, perfect fiber. A very obvious type of degradation is the development of abrasion flaws during handling. Also, heating of a fiber to moderate temperatures (~300-600°C), for instance during the soldering of pigtails, has been shown to result in strength degradation of strong fiber. It has been suggested that the use of HF etching may be a reasonable technique for
the elimination of many types of strength-lowering defects. In this paper we discuss early results from the literature on the effects of heating and HF etching on the strength of silica glass and present new results on both.
The strength of optical fiber at low temperature is an important parameter since it approximates the inert strength, i.e. the starting strength of the material before degradation by fatigue. Published data suggest that the fatigue may abruptly slow below some temperature. However, published data are limited to strength vs temperature or fatigue in liquid nitrogen. We report strength and fatigue data for both bare (stripped) and metal coated fused silica optical fiber at temperatures down to 77 K. While fatigue slows as the temperature is reduced (i.e. the stress corrosion parameter increases with falling temperature) fatigue is still measurable at 77 K. This is the case even for hermetic metal coated fiber with extremely low water activity at the glass surface. The results confirm that fused silica exhibits "intrinsic" fatigue, i.e. fatigue in the absence of moisture.
In this paper we present a brief historical review, including a discussion of some of the advances we think have been made in the area of mechanical reliability of lightguide fibers. Fatigue and aging of these fibers are reviewed in detail. It is shown that these processes are fundamental to the silica glass itself, at least under normal environmental conditions. It is suggested that the single most important outstanding issue is the determination of the presence of fatigue and/or aging limits. If these limits are shown to exist in general, or at least under certain conditions, a major simplification of lifetime analysis will have been accomplished.
The mechanical reliability of fused silica glass fibers is significantly influenced by the properties of their polymer coatings. The primary coating, which is in contact with the fiber surface, is expected to control the chemistry there, but the secondary coating does have a considerable effect on the strength and aging behavior of the fiber. This observation is confirmed by data obtained for ten fused silica fibers, having the same primary coatings but ten different secondary coatings. These fibers were aged at 85 degree(s)C in both de-ionized water and 85% relative humidity for up to 6 weeks and the residual strength as a function of aging time was measured. Dynamic fatigue measurements were carried out on as-received and aged fibers using two-point bending. The results show that the secondary coating has a notable effect on the aging behavior and the coating strip force, but does not greatly influence the dynamic fatigue parameter.
Early fiber reliability efforts concentrated on the development of manufacturing processes for the production of long lengths of strong fibers. More recently new devices and packages have been developed. Their successful deployment requires not only engineering procedures for the production of strong fibers, but also a much better understanding of the behavior of these fibers in a variety of situations.
In earlier work, diffusion of moisture through polymer coatings was modeled by using an analytical solution to the diffusion equation and so was only applicable to the simplest cases, e.g. cylindrically symmetric Fickian diffusion. In this work the limitation of the analytical approach is avoided by the use of finite element analysis. However, finite element programs do not usually implement matter diffusion, and therefore it has been modeled by analogy with thermal conduction.
In this paper we present tensile and 2-point bending strength results on 5 cm. (2-inch) sections of mechanically stripped fiber. We show that the ‘weakest-link’ model is apparently not obeyed since the low strength mode of the resulting distributions have different widths, i.e., the Weibull m-values for the tensile and 2-point tests are ~2 and ~7, respectively. This simply means that the two measurements are sampling different flaw populations. The importance of understanding this behavior and applying that understanding in practice is emphasized.
Carbon coatings have been applied to silica lightguide fibers for many years in order to provide hermeticity to both water and H2. While there was a flurry of activity in this technology in the late 80’s and early 90’s, it appears that these coatings are little used today. The reasons for this are the increased cost associated with their use, the perception that they are unnecessary, and perhaps most important, the lack of complete understanding and control of their properties. In this paper we will review the history, preparation, structure and properties of these coatings in an effort to clarify some of the issues surrounding their production and use.
Atomic force microscopy has proven to be of great value in the study of the surface roughness of aged silica fibers. It has been shown that aging in both liquid water and water vapor results in surface roughening which correlates with and is apparently responsible for the strength degradation. In this paper, we show that the application of a new indenting/scratching/imaging tip and associated software allow this tool to be extended to the study of a new range of problems. We illustrate the usefulness of this `nanoindentation/imaging probe' in the study of indents and scratches on silica fiber surfaces which have undergone a variety of treatments as well as in study of coatings on these silica lightguide surfaces.
Since the classic paper of A. A. Griffith in 1920, questions have been asked about the variability of the strength of glass fibers as a function of composition and processing conditions. While Griffith apparently found that the strength of soda-lime-silica fibers increased as their diameter decreased, he also found that quite large-diameter silica fibers had strengths of the order of 6.9 GPa (1 X 106 psi). Since that time, almost all investigators have found very similar strength values for silica regardless of the fiber diameter, raw material or conditions of formation. The history of the strength of fused silica fibers and the effect of various coatings on the strength, fatigue and aging are reviewed in this paper.
A single-layer UV-curable polyacrylate-coated telecommunications grade fused silica fiber was found to have a significant reduction in two-point bending strength after immersion in acetone. The two-point bending and tensile strengths of this fiber as a function of immersion time in acetone were determined, and this strength loss was not seen for 0.5-m gauge length tensile specimens. SEM and optical fractography was performed on the weak specimens, and the cause of the strength reduction is proposed to arise from particles smaller than 3 micrometers in the coating. These particles could cause surface flaws by sliding contact damage incurred during relative motion between the coating and the glass. This sliding could occur either while flexing the fiber in preparation for a bending strength measurement or due to coating elongation. While it is not clear which mechanism is operating, both are consistent with the observation that degradation is only observed for bending strength measurements.
Lightguide technology is now considered to be mature. Indeed, improvements in all aspects of this technology over the years have been most impressive - multimode to singlemode fiber, electronic to optical amplification and the newest and perhaps the most important: the use of WDM and DWDM technologies. In spite of these striking advances, the very nature of the silica lightguide material poses reliability issues which have not yet been fully resolved or in some cases even confronted. These issues by and large have to do with the intrinsic brittle nature of the glass material. In order to assure reliable performance of these lightguide fibers in telecommunications service environments, coating and cabling technologies developed over the past two decades have evolved to give robust fiber optic cables, devices and components. In this presentation, we review key materials issues in the development and use of lightguide technology in telecommunications. Furthermore, we analyze current trends and discuss major materials reliability issues that need to be resolved for further developments in future applications of optical fibers, fiber optic cables and fiber-based components.
The intrinsic strength ((sigma) i) is defined as the strength in the absence of slow crack growth or fatigue. It is of general interest in the study of brittle fracture, but it is of particular importance in the evaluation of lightguide fiber lifetimes since it is required for the calculation of the constant B. In this work we review the existing literature relating to the inert strength, indicate the limits that can be proposed for and suggest possible approaches to more satisfactory estimates of value(s) of (sigma) i. It is suggested that reasonable values of the inert strength may be obtained by taking 85 - 90% of the liquid nitrogen strength.
The strength degradation of lightguide fibers has been studied over a range of elevated temperatures and at room temperature. Using these data we show that accelerated testing can be used to predict ambient temperature behavior. An activation energy of approximately 90 kJ.mol-1 describes the shift in corresponding times.
Hermetic aluminum-coated fused silica fibers can withstand high stress levels without failure for prolonged periods of time in water-containing environments. Aluminum-coated fibers from several sources exhibit differences in strength. The aluminum and silica surfaces have been examined using SEM and AFM in order to understand this variation. Differences in the interfacial interaction between aluminum and glass and in the microstructure of the coatings were considered, but were not unequivocally identified as being responsible for the differences in strength observed for the various aluminum-coated specimens.
The strength of flaw-free silica fibers in the absence of water is of the order of 12-14
GPa and is essentially independent of time. (Formula available in paper). On the other hand,
in the presence of water, the time dependence is very much more serious (n - 20) and
even at relatively short testing times (tf 10 secs), the measured strength is <6 GPa.
The development of a completely satisfactory hermetic coating which will allow the
realization of the above 'water-free'
strengths is not simple, however, while the
advantages of a polymer coating in mechanically protecting the fiber and at the same
time isolating it from microbending losses is well-known, no water-impermeable
viscoelast.ic coating has yet been proposed. Two other types of coatings have been
proposed and each is successful to some extent as an hermetic coating. These materials
and their behavior are quite different and will be described below.
The factors limiting the maximum strength of carbon coated fibers are considered. In spite of the variations in the inert strength, at room temperature the strength of fibers under investigation depends not on the drawing conditions, but on the properties of the carbon coating. The strength of weak carbon coated fibers with melted-in zirconia particles is also investigated. It grows with increasing carbon thickness (i.e. decreasing of fiber electrical resistance). When the carbon coating is thick enough (electrical resistance is less than 10 kOhm/cm), the fiber strength practically does not depend on the coating thickness and environment humidity and is more than two times higher than that of polymer coated fibers.
The most critical variable effecting the strength of silica lightguides is the
availability of water to the fiber surface. At very low temperatures (T < 77 °K) and at
high vacuum (PH2O < i07 torr) the thermodynamic activity of water is so low that
mechanical failure of the fiber occurs by the direct breaking of Si-O-Si bonds. In this
case very high strength (- 12-14 GPa) and very slight time dependence (formula available in paper) are expected. Also an activation energy comparable to the SiO
bond energy is observed (- 100 kcals/mol). On the other hand, under normal
conditions (T - 0- 100°C and normal relative humidity), the strength and time
dependence are controlled by the combination of stress and the reactivity of water with
the fiber surface: SiO2 + H2O = SiOH HOSi. In this case the time dependence of
strength is very much greater (n 20) and the activation energy is approximately 30
kcaI/mol.2 Because of this rather extreme time dependence, the short time tensile
strength (say tf = 10 sees) is only about 5.5 GPa and will be reduced again by a factor
of 2 (to = 2.8 GPa) in about one week. A subject which continues to be discussed
and studied is the proper analytical description of this time dependence. In this regard,
Bubel and Matthewson3 have studied the behavior of several proposed models for
time dependence. They find that the differences in predicted lifetimes from the models
differs significantly. In particular they suggest that the universal use of the optimistic
power law is not appropriate.
Mechanical failure of silica lightguides is almost always initiated at the fiber surface. Because of this it is important to understand those processes which can lead to surface `damage' and thus to reductions in strength. A consideration of such processes, as well as their time dependence, is undertaken here.
The effect of ions in solution on the stress assisted mechanical corrosion of high strength lightguide fibers is compared to the rate of crack growth in bulk silica. This review shows that the mechanical corrosion of bulk silica is apparently insensitive to ions in solution, whereas, fatigue of uncoated lightguide fiber is ion sensitive. These observations are discussed with respect to long term mechanical reliability.
Recent advances in the understanding of reliability of silica optical fiber indicate that the chemical durability of the fiber can control the long duration lifetime both under stress- induced fatigue and zero-stress aging conditions. In particular, dissolution of surface material produces strength degrading surface roughness. These mechanisms are discussed and strategies for improving reliability by inhibiting dissolution are examined. As an example, a modified polymer coating formulation is described that is shown to increase the lifetime of the fiber by up to a factor of thirty-fold. Strategies for improving the strength and durability of non-oxide fibers using a similar approach are also discussed.
This paper is an update of one published with the same title and by the same authors in a special issue of “Optical Engineering” on optical fiber reliability in June 1991. It points out some recent major advances in two areas of importance to fiber strength and reliability. These are (1) the measurement and prediction of small crack velocities and thus long failure times, and (2) a consideration of the minimum fatigue rate possible in an hermetically-coated fiber.
While great progress has been made recently in the development of mechanically satisfactory lightguide fibers, detailed fundamental understanding in a number of areas seems lacking. Although in some cases this understanding may seem practically unimportant, in others it is critical to our ability to improve our product as well as to predict its reliability confidently.